Co-reporter:Tomasz Modzelewski;Ian Hotham
Journal of Applied Polymer Science 2015 Volume 132( Issue 13) pp:
Publication Date(Web):
DOI:10.1002/app.41741
ABSTRACT
A number of polyphosphazenes with negatively charged β-alanine (β-Ala) and γ-amino butyric acid (GABA) side groups were synthesized and studied for their ability to initiate the growth of hydroxyapatite (HAp) during exposure to simulated body fluid (SBF). All the polymers were hydrolytically sensitive, with the final hydrolysis rate dependent on the specific active side groups (GABA > β-Ala). These systems also underwent extensive mineralization, with calcium phosphate deposited across their entire surface during exposure to SBF (up to 115 wt % gain after 4 weeks). This degree of deposition is a major advance over previously reported polyphosphazene systems, which underwent a maximum of 27 wt % gain after immersion in SBF for 4 weeks. The extent of mineralization over the surface was monitored using environmental scanning electron microscopy (ESEM) coupled with energy dispersive X-ray spectroscopy (EDS). In addition, X-ray diffraction (XRD) was used to determine the identity of the mineralized material. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41741.
Co-reporter:Jessica L. Nichol, Ian T. Hotham, Harry R. Allcock
Polymer Degradation and Stability 2014 Volume 109() pp:92-96
Publication Date(Web):November 2014
DOI:10.1016/j.polymdegradstab.2014.05.015
Hydrolytically sensitive polymers that degrade into benign products are of interest as biomedical materials. The aim of this study was to determine the properties and hydrolytic characteristics of both poly(diethoxyphosphazene) and related derivatives with both ethoxy and hydrophobic co-substituent groups in a near-1:1 M ratio such as 2,2,2-trifluoroethoxy, phenoxy, or p-methylphenoxy units. These hydrophobic cosubstituents served as models for bioactive counterparts. The hydrolytic sensitivity of the ethoxyphosphazene units was so pronounced that even hydrophobic or bulky O-linked co-substituents failed to counteract the hydrolysis behavior during a twelve-week hydrolysis study. This work illustrates a pathway for the development of a new class of useful bioerodible polymers.
Co-reporter:Nicole L. Morozowich, Jordan O. Lerach, Tomasz Modzelewski, Lauren Jackson, Nicholas Winograd and Harry R. Allcock
RSC Advances 2014 vol. 4(Issue 38) pp:19680-19689
Publication Date(Web):15 Apr 2014
DOI:10.1039/C3RA47205A
Synthetic bone grafts that promote the natural mineralization process are excellent candidates for the repair and replacement of bone defects. In this study, a series of phosphoester and phosphonic acid containing polyphosphazenes were examined for their ability to mineralize hydroxyapatite (HAp) during exposure to a solution of simulated body fluid (SBF) for a period of four weeks. Although all the polymers showed an initial mineralization response, the amount of deposition and the time scale were dependent upon the side group chemistry of the polymers. After exposure to SBF for one week, all polymers mineralized HAp. After three weeks in SBF, polymers containing phosphoester substituents showed no significant change, with a weight gain of <1%, while polymers containing phosphonic acid substituents underwent a significant increase in the amount of mineralized HAp, with weight gains between 5–10%. The morphology of mineralized features was observed with Environmental Scanning Electron Microscopy (ESEM). However, due to the structural complexity of the mineralized polymers, the identity of the mineralized phase could not be definitively identified using traditional characterization techniques such as energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), or X-ray photoelectron spectroscopy (XPS). Time-of-flight secondary ion mass spectrometry (ToF-SIMS), a technique not previously explored for this type of application, successfully reveals details of the chemistry associated with the mineralized phase not possible to achieve with XRD analysis.
Co-reporter:Zhicheng Tian, Chen Chen, and Harry R. Allcock
Macromolecules 2014 Volume 47(Issue 3) pp:1065-1072
Publication Date(Web):February 3, 2014
DOI:10.1021/ma500020p
The design and assembly of new organophosphazene polymeric materials based on supramolecular “host–guest” interactions was accomplished by linkage of supramolecular coupling units to either the main-chain terminus or the side-chains of the parent phosphazene polymer. Noncovalent interactions at the main chain terminus were used to produce amphiphilic palm-tree like pseudoblock copolymers via host–guest interactions between an adamantane end-functionalized polyphosphazene and a 4-armed β-cyclodextrin (β-CD) initiated poly[poly(ethylene glycol) methyl ether methacylate] branched-star type polymer. Moreover, noncovalent interactions involving polymer side-chains were achieved between polyphosphazenes with β-CD pendant units and other polyphosphazene molecules with adamantyl moieties on the side-chains. These new organo–phosphazene structures based on noncovalent “host–guest” interactions generate new opportunities for the macromolecular modification of polyphosphazenes. The resultant materials demonstrated useful properties including self-aggregation, supramolecular gelation, and stimulus-responsive behavior.
Co-reporter:Tomasz Modzelewski and Harry R. Allcock
Macromolecules 2014 Volume 47(Issue 19) pp:6776-6782
Publication Date(Web):October 3, 2014
DOI:10.1021/ma501927u
We report the synthesis and characterization of a new linear polyphosphazene architecture in which rigid, bulky side units provide the possibility of interdigitation with their counterparts on neighboring chains to generate noncovalent cross-links and distinct elastomeric properties. The bulky side groups are cyclotriphosphazene rings substituted with trifluoroethoxy groups connected to the main chain via aryloxy spacers. These bulky units are distributed along the polymer backbone and separated from each other by trifluoroethoxy units linked directly to the main chain. Compared to the well-known poly[(bis-2,2,2-trifluoroethoxy)phosphazene], [NP(OCH2CF3)2]n, which is a microcrystalline film- and fiber-forming polymer, several of the new materials are elastomers with properties that arise partly from interactions of the protruding cyclotriphosphazene side units with those on nearby polymer chains. Specific elastomers are capable of regaining up to 89% of their original shape when elongated to high strain (up to 1000%) over four elongation cycles and show even longer elongations at break (>1600%). The overall physical properties depend on the ratios of the cyclic trimeric side units to main chain linked trifluoroethoxy side groups. The polymers were characterized using 1H, 31P NMR, DSC, TGA, X-ray diffraction, GPC, and stress–strain techniques.
Co-reporter:Zhicheng Tian;Xiao Liu;Abdellatif Manseri;Bruno Ameduri
Polymer Engineering & Science 2014 Volume 54( Issue 8) pp:1827-1832
Publication Date(Web):
DOI:10.1002/pen.23729
Mixed-substituent fluoroalkoxyphosphazene polymers bearing ∼15% 1H,1H,2H,2H-perfluorooctan-1-oxy or 1H,1H,2H,2H-perfluorodecan-1-oxy side groups together with trifluoroethoxy cosubstituent groups were synthesized. The low reactivity of the long-chain fluoroalkoxides and their limited solubility in organic solvents prevented higher levels of substitution. Moreover, the sodium alkoxides with two methylene residues adjacent to the oxygen proved to be unstable in solution due to elimination of NaF and precipitation of side products, and this limited the time available for chlorine replacement reactions. The resulting cosubstituent polymers were characterized by proton nuclear magnetic resonance (1H-NMR), 31P-NMR, 19F-NMR, gel-permeation chromatography, and differential scanning calorimetry. Unlike homo- or mixed-substituent fluoroalkoxyphosphazene polymers, such as [NP(OCH2CF3)2]n (a microcrystalline thermoplastic, Tg ∼ −63°C, Tm ∼ 242°C) or [NP(OCH2CF3)(OCH2(CF2)xCF2H)]n (PN-F, a rubbery elastomer, Tg ∼ −60°C, but no detectable Tm), the new polymers are gums (Tg ∼ −50°C, but no detectable Tm) with molecular weights in the 105 g/mol rather than the 106 g/mol range. POLYM. ENG. SCI., 54:1827–1832, 2014. © 2013 Society of Plastics Engineers
Co-reporter:Nicole L. Morozowich;Ryan J. Mondschein
Journal of Inorganic and Organometallic Polymers and Materials 2014 Volume 24( Issue 1) pp:164-172
Publication Date(Web):2014 January
DOI:10.1007/s10904-013-9989-4
Phosphazene polymers with N-linked and O-linked amino acid side groups are of biomedical interest, especially for their ability to bioerode under physiological conditions. Polyphosphazenes containing serine and threonine substituents, which contain two different functional sites for attachment to a polyphosphazene backbone (N- and O-terminus), were synthesized using an improved technique, and their hydrolysis behavior was investigated. In aqueous media the solid polymers yield hydrolysis products phosphate, ammonia, the amino acid, and ethanol and have the potential to be used in several different biomedical applications ultimately determined by their hydrolysis behavior. A hydrolysis study in deionized water revealed that all the polymers are hydrolytically sensitive, regardless of the type of linkage to the polyphosphazene backbone, although the hydrolysis rates may be different. Polymers with amino acid ester side groups linked through the N-terminus underwent solid phase hydrolysis between 16 and 60 % within a 6-week period. This is the fastest reported solid state hydrolysis of any amino acid ester substituted polyphosphazene. The mechanism of hydrolysis is by bulk erosion as monitored by environmental scanning electron microscopy. Polymers with the amino acid units linked through the O-terminus are soluble in water; thus their solid state erosion profile in aqueous media could not be determined. However, 31P NMR spectroscopy confirmed their hydrolytic sensitivity in aqueous solution and the formation of phosphorus-containing oligomeric species, the concentrations of which increased during the 6-week hydrolysis period. Complete hydrolysis did not occur within 6 weeks. The O-linked species are possible starting points for bioerodible hydrogel formation.
Co-reporter:Andrew Hess, Greg Barber, Chen Chen, Thomas E. Mallouk, and Harry R. Allcock
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 24) pp:13029
Publication Date(Web):December 4, 2013
DOI:10.1021/am403924t
A homologous series of fire-retardant oligoalkyleneoxy-phosphates was synthesized for evaluation as liquid or gel-type electrolyte media for dye-sensitized solar cells (DSSCs) and secondary lithium batteries. Unoptimized DSSC electrolyte formulations for DSSCs achieved ionic conductivities as high as 5.71 × 10–3 S·cm–1 and DSSC test-cell efficiencies up to 3.6% as well as Voc, Jsc, and ff up to 0.81 V, 8.03 mA·cm–2, and 0.69, respectively. Poly(bis-(2-(2-methoxyethoxy)ethoxy)phosphazene)-based Li+-conducting gel electrolytes plasticized with the best performing phosphate had conductivities as high as 9.9 × 10–4 S·cm–1 at 30 °C. All the liquids have boiling points higher than 197 °C. The results show that the viscosity, glass transition temperatures, and conductivity of the phosphates are dependent mainly on the length of the longest alkyleneoxy chain.Keywords: dye-sensitized solar cell; electrolyte; fire-retardant; lithium batteries; phosphate;
Co-reporter:Xiao Liu, Zhicheng Tian, Chen Chen and Harry R. Allcock
Polymer Chemistry 2013 vol. 4(Issue 4) pp:1115-1125
Publication Date(Web):15 Nov 2012
DOI:10.1039/C2PY20825C
UV-cleavable star polymers composed of a well-defined 6-arm amphiphilic block copolymer and a UV-cleavable core with photolabile o-nitrobenzyl groups have been synthesized and characterized. The core of the star polymer is a cyclotriphosphazene, which can biodegrade to phosphate and ammonium ion. The resultant unimolecular micelles can be dissociated in a controlled manner by UV irradiation. The inner lipophilic poly(methyl methacrylate) (PMMA) and the outer hydrophilic poly[poly(ethylene glycol) methyl ether methacrylate] (PPEGMA) were grafted by atom transfer radical polymerization (ATRP), leading to the formation of star-PMMA-PPEGMA with different compositions. The effects of various factors, such as molecular weight, solution concentration, solvents (THF, toluene, and water) and monomers (MMA, PEGMA, and 2-(dimethylamino)ethyl methacrylate (DMA)) on the photodegradation rate of the star polymers were studied by gel permeation chromatography (GPC). The micellar behavior of star-PMMA-PPEGMA was unambiguously demonstrated by dynamic light scattering (DLS), fluorescence techniques and transmission electron microscopy (TEM). The formation of nanoparticles (star-AGG) from the aggregation of the two star-PMMA-PPEGMA polymer micelles in aqueous solution was detected by DLS with hydrodynamic radii of 86 and 111 nm. The critical aggregation concentration (CAC) of star-AGG-2 from star-PMMA179-PPEGMA89-2 was 0.0026 g L−1 and 0.022 g L−1 before and after UV-irradiation indicating the reduced stability of the polymer micellar structures after UV-irradiation due to the detachment of the amphiphilic arms from the cyclotriphosphazene core. As a result, spontaneous dissociation of cleaved micelles can be induced by the dilution effect in the human body for stimulus-controlled drug release.
Co-reporter:Zhicheng Tian, Yufan Zhang, Xiao Liu, Chen Chen, Mark J. Guiltinan and Harry R. Allcock
Polymer Chemistry 2013 vol. 4(Issue 6) pp:1826-1835
Publication Date(Web):21 Dec 2012
DOI:10.1039/C2PY21064A
Novel polyphosphazenes containing the fluoroquinolone antibiotic substituents, ciprofloxacin or norfloxacin, were synthesized and characterized. Nano/microfibers of several selected polymers were prepared by an electrospinning technique. The sensitivity to hydrolysis, pH behavior, and antibiotic release profile of all the polymers as films and the selected polymers as nano/microfibers were investigated. The hydrolysis release behavior was further studied by an in vitro antibacterial test against E. coli. Protection and deprotection reactions of ciprofloxacin and norfloxacin were carried out before and after polymer synthesis to prevent cross-linking of multi-functional reagents with polymers. No more than 25 mol% of antibiotics can be introduced to poly(dichlorophosphazene) which was synthesized by a thermal ring opening polymerization of hexachlorocyclotriphosphazene. Various amino acid ethyl esters (glycine, alanine, and phenylalanine) were linked to the polymers as co-substituents to increase the solubility as well as to control the rate of antibiotic release. Depending on the polymer compositions, 5–23% weight loss and 4–30% antibiotic release were observed in a six week hydrolysis study at 37 °C. The corresponding nano/microfibers showed a much faster degradation and antibiotics release due to a substantially larger surface area. The in vitro antibacterial tests showed an antibacterial effect as long as ciprofloxacin or norfloxacin was released from the polymers. Meanwhile, all the polymers gave a near-neutral hydrolysis environment with the pH ranging from 5.9 to 6.8.
Co-reporter:Jessica L. Nichol, Nicole L. Morozowich and Harry R. Allcock
Polymer Chemistry 2013 vol. 4(Issue 3) pp:600-606
Publication Date(Web):08 Oct 2012
DOI:10.1039/C2PY20631E
New polyphosphazenes were designed, synthesized, and characterized to determine their potential as scaffolds for ligament and tendon tissue engineering. The carboxylic acid moiety of the amino acids L-alanine and L-phenylalanine were protected with alkyl esters with increasing chain length from 5 to 8 carbon atoms. This combined the hydrolytic sensitivity of the amino acid ester polyphosphazenes with the elastomeric characteristics induced by the long chain alkoxy polyphosphazenes. Test side group substitution reactions were performed on the cyclic small molecule model, hexachlorocyclotriphosphazene (NPCl2)3, to determine if steric hindrance would inhibit the degree of chlorine replacement by the amino acid ester units. Counterpart polymers were then synthesized by replacement of the chlorine atoms in poly(dichlorophosphazene) (NPCl2)n by the same amino acid esters. The glass transition temperatures of the polymers decreased with increasing alkyl ester chain length, ranging from 11.6 to −24.2 °C. Polymer hydrolysis was studied for solid samples in deionized water at physiological temperature for 12 weeks. The starting pH was 6.3 and the final pH ranged between 5.2 and 6.8. Polymer film mass decreased between ∼8.7 and 26 percent during the 12 week period, while the molecular weights decreased ∼57 to 99 percent.
Co-reporter:Harry R. Allcock
Applied Organometallic Chemistry 2013 Volume 27( Issue 11) pp:620-629
Publication Date(Web):
DOI:10.1002/aoc.2981
Polyphosphazenes are macromolecules with a chain of alternating phosphorus and nitrogen atoms and with two (usually) organic side groups linked to each phosphorus. Although the earliest poly(organophosphazenes) were essentially linear polymers each bearing only one type of side group along the chain, research in our program in recent years has widened the scope to include mixed-substituent polymers with two or more different side groups, block copolymers, comb structures, stars, and dendrimers, all with a wide variety of different properties. In addition, supramolecular systems have also been synthesized including vesicles, microspheres, and a variety of composite systems. This review provides a summary of these developments together with an introduction to the widening array of developing uses. Copyright © 2013 John Wiley & Sons, Ltd.
Co-reporter:Zhicheng Tian, Chen Chen, and Harry R. Allcock
Macromolecules 2013 Volume 46(Issue 7) pp:2715-2724
Publication Date(Web):March 22, 2013
DOI:10.1021/ma4004314
Biodegradable poly(organophosphazenes) containing side chains of various oligo(ethylene glycol) methyl ethers (mPEGs) and glycine ethyl ester units were synthesized and characterized. Novel supramolecular-structured hydrogel systems based on the inclusion complex between the mPEG grafted polyphosphazenes and α-cyclodextrin were prepared in aqueous media. The gelation time depended on the length of the mPEG side chains, the molar ratio between mPEG repeat units and α-cyclodextrin, and the concentration of the polymeric gel precursors. The rheological measurements of the supramolecular hydrogels indicate a fast gelation process and flowable character under a large strain. The hydrogel systems demonstrate unique structure-related reversible gel–sol transition properties at a certain temperature due to the reversible supramolecular assembly. The formation of a channel-type inclusion complex induced gelation mechanism was studied by DSC, TGA, 13C CP/MAS NMR, and X-ray diffraction techniques. The strong potential of the system for injectable drug delivery applications was explored with the use of bovine serum albumin as a model protein for in vitro release studies. All the supramolecular hydrogels studied showed disintegration by dethreading of the α-cyclodextrin. Polymers with longer poly(ethylene glycol) side chains had better stability and slower protein release profiles. The molecular weights of the polymers were monitored by GPC to show the biodegradability of the hydrogel system.
Co-reporter:Nicole L. Morozowich, Jessica L. Nichol, and Harry R. Allcock
Chemistry of Materials 2012 Volume 24(Issue 17) pp:3500
Publication Date(Web):August 28, 2012
DOI:10.1021/cm3022825
Synthetic bone grafts that promote the natural mineralization process would be excellent candidates for the repair or replacement of bone defects. In this study, a series of antioxidant-containing polyphosphazenes were evaluated for their ability to mineralize apatite during exposure to a solution of simulated body fluid (SBF). All polymers contained ferulic acid (antioxidant), cosubstituted with different amino acid esters linked to the polyphosphazene backbone. Differences in the side groups determined the hydrophobicity or hydrophilicity of the resulting polymers. All of the polymers mineralized monocalcium phosphate monohydrate, a type of biological apatite. However, the mineralization process (the amount of deposition and length of time) was dependent on the hydrophilicity or hydrophobicity of the polymers. The polymer–apatite composites were examined by electron scanning microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravametric analysis. Weight gain data were also obtained. To verify that the nucleation process was due to the presence of calcium and phosphate, two standard solutions were prepared: one solution (NaCl solution) contained only sodium chloride, and the second solution (mSBF) was similar to SBF except without known crystal growth inhibitors such as Mg2+ and HCO3–. No mineralization occurred when the polymers were exposed to the NaCl solution, but mineralization took place upon exposure to mSBF. The apatite phase produced was hydroxyapatite (HAp). The mineralization process in mSBF was much more extensive, with all samples gaining more weight following exposure to SBF. A similar trend was also found (as in the case of SBF), with the amount of deposition and length of deposition time depending on the hydrophilicity/hydrophobicity of the polymer. These results suggest that the nucleation process is due to calcium and phosphate, and the absence of crystal growth inhibitors allows for the rapid nucleation of HAp. In both cases, the mineralization process was favored on hydrophilic surfaces (static water contact angle of 56–65°) versus hydrophobic surfaces (71–86°).Keywords: antioxidant; apatite; biodegradable; bone tissue engineering; matrix hydrophobicity/hydrophilicity; polyphosphazene;
Co-reporter:Xiao Liu, Hua Zhang, Zhicheng Tian, Ayusman Sen and Harry R. Allcock
Polymer Chemistry 2012 vol. 3(Issue 8) pp:2082-2091
Publication Date(Web):02 May 2012
DOI:10.1039/C2PY20170D
Densely grafted star- and comb-shaped molecular brushes composed of poly[2-(dimethylamino)ethyl methacrylate] were prepared via atom transfer radical polymerization (ATRP) using either cyclotriphosphazenes or polyphosphazenes as initiators. The grafting conditions were optimized in order to obtain well controlled brush structures. The kinetics of the reactions were first-order with respect to the monomer concentration in both cyclotriphosphazene and polyphosphazene systems. The resultant star- and comb-shaped brushes were quaternized with iodomethane, iodobutane, iodoheptane, iododecane and iodododecane, and the products were characterized by NMR, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). Electrospinning of the quaternized brushes gave rise to microfibers with diameters in range of 700 nm to 1.1 μm, evidenced by scanning electron microscopy (SEM). The antibacterial activity of the quaternized brush species in both aqueous solution and as fibrous solids against Escherichia coli (E. coli) has been evaluated. In aqueous solution, star-shaped brushes quaternized with iodoheptane showed the best antibacterial effect, with a minimum inhibitory concentration (MIC) as low as 250 μg mL−1. In the fibrous solid state, more than 99% of E. coli were killed within 2 h after contacting 100 mg of microfibers electrospun from the star-shaped brush polymers quaternized with either iododecane or iodododecane.
Co-reporter:Nicole L. Morozowich, Jessica L. Nichol, Ryan J. Mondschein and Harry R. Allcock
Polymer Chemistry 2012 vol. 3(Issue 3) pp:778-786
Publication Date(Web):19 Jan 2012
DOI:10.1039/C2PY00570K
Polyphosphazenes were synthesized that contained the antioxidant, ferulic acid, and amino acid esters as co-substituents. The synthesis protocol utilized the replacement of chlorine atoms in poly(dichlorophosphazene) by ferulic acid with either glycine, alanine, valine, or phenylalanine ethyl ester. Ferulic acid protects cells from free radical damage, and its steric characteristics have the potential to generate polymeric materials with high mechanical strength for hard tissue engineering scaffolds. Incorporation of the amino acid esters allows for control over the polymer hydrolysis while releasing the antioxidant at different rates. Macromolecular substitution reactions were carried out utilizing the allyl ester of ferulic acid to prevent side reactions during halogen replacement. The allyl protecting group was then removed using mild conditions. The polymers were characterized by 1H and 31P NMR, GPC, and DSC techniques. Static water contact angles were measured to monitor the change in hydrophobicity/hydrophilicity. Before deprotection, the water contact angles were 82–88° and after deprotection the water contact angles were 56–71°. A pH-dependent hydrolysis study revealed that the polymers are hydrolytically sensitive and decomposed ∼5–25% over 8 weeks yielding a final pH between 6.0–6.8. Polymer hydrolysis resulted in the release of ferulic acid. The polymers were photocrosslinked by a [2 + 2] cycloaddition of the ferulic acid moieties induced by exposure to long-wave UV light. The crosslinking was monitored by UV-spectroscopy via the apparent decrease in the 320 nm absorbance, attributed to the cyclization of the ferulic acid moieties. Increased UV exposure time led to an increase in the level of crosslinking. After 60 s, the polymers were crosslinked ∼41–62%. Hydrolysis experiments of the crosslinked materials showed only a 0–5% weight loss after 8 weeks with a pH between 6.6–7.0.
Co-reporter:Harry R. Allcock and Nicole L. Morozowich
Polymer Chemistry 2012 vol. 3(Issue 3) pp:578-590
Publication Date(Web):16 Nov 2011
DOI:10.1039/C1PY00468A
An account is presented of the development, evaluation, and current status of an unusual series of polymers optimized specifically for biomedical applications. The polymers are based on the polyphosphazene platform with side groups chosen for their ability to sensitize the polymers to hydrolysis to benign small molecules that can be metabolized or excreted from the body. The largest class of these polymers consists of macromolecules with amino acid ester side groups and these are the main focus of the article. However, a variety of polymers with other side groups also show promise as bioerodible species, and these are mentioned later in the article.
Co-reporter:Xiao Liu, Jonathan P. Breon, Chen Chen, and Harry R. Allcock
Inorganic Chemistry 2012 Volume 51(Issue 21) pp:11910-11916
Publication Date(Web):October 5, 2012
DOI:10.1021/ic301808v
Side-group-exchange reactions have been studied for short-chain linear oligomeric phosphazenes, (RO)4P[N═P(OR2)]nOR (n = 6, 10, 20, and 40) as models for the corresponding linear high polymers (n ∼ 15000). Specifically, the exchange behavior of oligomers where OR = OCH2CF3, OC6H5, OC6H4CHO-p, OC6H4CN-p, and OC6H4NO2-p with sodium trifluoroethoxide was examined. The ease of aryloxy group replacement by trifluoroethoxy increased with the electron-withdrawing character in the order OR = OC6H5 ≪ OC6H4CHO-p < OC6H4CN-p < OC6H4NO2-p, but the reaction was efficient only if the phosphazene contained no more than 20 repeating units. However, attempts to force slower reactions by the use of excess sodium trifluoroethoxide induced secondary reactions at the trifluoroethoxy units already introduced to produce CF3CH2OCH2CF3 and generate −O–Na+ units in their place. The longest chain model underwent side-group-exchange reactions preferentially at the end units. These results are significant for the synthesis of phosphazene high polymers with fluoroalkoxy and aryloxy side groups.
Co-reporter:Harry R. Allcock
Soft Matter 2012 vol. 8(Issue 29) pp:7521-7532
Publication Date(Web):12 Jun 2012
DOI:10.1039/C2SM26011E
Nearly all soft materials are based on organic polymer molecules. In other words they are derived from macromolecules constructed around the chemistry of carbon. Yet there are roughly 100 other elements in the periodic table that could in principle provide the building blocks for polymers and for soft materials. A few “inorganic” elements are incorporated into gels and related materials. Examples include silicate, aluminate, and titanate sol–gel substances, but the opportunities for diversification within these systems are relatively limited. The poly(organosiloxanes) (silicones) were the first hybrid inorganic–organic polymers to be discovered and they continue to play a major role in research and technology. Nevertheless, a need exists for additional systems that have the structural diversity found in the field of organic polymers, combined with the attributes of inorganic skeletal elements rather than carbon. It is with this objective in mind that we have devoted several decades to exploring the synthesis and diversification of the polyphosphazenes.
Co-reporter:Xiao Liu, Zhicheng Tian, Chen Chen, and Harry R. Allcock
Macromolecules 2012 Volume 45(Issue 3) pp:1417-1426
Publication Date(Web):January 18, 2012
DOI:10.1021/ma202587z
A series of densely grafted star- and comb-shaped molecular brushes composed of polystyrene, poly(tert-butyl acrylate), and poly(N-isopropylacrylamide) were prepared by atom transfer radical polymerization (ATRP) using either cyclotriphosphazenes or polyphosphazenes as initiators. The initiators were prepared by the introduction of a free hydroxyl group into the side chains of a phosphazene cyclic trimer and polymer, followed by esterification with 2-bromopropionyl bromide. The grafting conditions were optimized for various monomers. The kinetics of the reaction were first-order with respect to the monomer concentration in both cyclotriphosphazene and polyphosphazene systems. The molecular weights of the resulting polymers were estimated by gel permeation chromatography (GPC). The side chains of the brush polymers were cleaved from the backbone and analyzed by GPC to confirm the synthesis of well-defined polymer brushes. Brushes based on poly(tert-butyl acrylate) were subjected to hydrolysis to yield negatively charged brushes. In addition, the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) brush polymers was measured by both dynamic light scattering (DLS) and differential scanning calorimetry (DSC), exhibiting a sharp phase transition at 33 °C. Furthermore, star- and comb-block copolymers with a hard polystyrene core and a soft poly(tert-butyl acrylate) shell were also synthesized.
Co-reporter:Chen Chen, Andrew R. Hess, Adam R. Jones, Xiao Liu, Greg D. Barber, Thomas E. Mallouk, and Harry R. Allcock
Macromolecules 2012 Volume 45(Issue 3) pp:1182-1189
Publication Date(Web):January 30, 2012
DOI:10.1021/ma202619j
Novel polyelectrolytes were synthesized by quaternization of the backbone of poly(alkoxy- and aryloxyphosphazenes) with strong alkylation reagents. As models for the synthesis of these polymers, similar quaternization reactions were also carried out on small-molecule alkoxy and aryloxy cyclotriphosphazenes. The quaternized small molecules and high polymers were characterized by 1H NMR, 31P NMR, DSC, TGA, and AC impedance studies. The quaternized poly(alkoxyphosphazenes) showed ionic conductivities of 2.58 × 10–4 S·cm–1 at 25 °C and 2.09 × 10–3 S·cm–1 at 80 °C, which are among the highest values for known solvent-free ionically conducting polymers.
Co-reporter:Zhicheng Tian, Xiao Liu, Chen Chen, and Harry R. Allcock
Macromolecules 2012 Volume 45(Issue 5) pp:2502-2508
Publication Date(Web):February 28, 2012
DOI:10.1021/ma300139z
A number of amphiphilic diblock copolymers based on poly[bis(trifluoroethoxy)phosphazene] (TFE) as the hydrophobic block and poly[(dimethylamino)ethyl methacrylate] (PDMAEMA) as the hydrophilic block were developed. The TFE block was synthesized first by the controlled living cationic polymerization of a phosphoranimine, followed by replacement of all the chlorine atoms using sodium trifluoroethoxide. To allow for the growth of the PDMAEMA block, 3-azidopropyl-2-bromo-2-methylpropanoate, an atom transfer radical polymerization (ATRP) initiator, was grafted onto the end-cap of the TFE block via the “click” reaction followed by the ATRP of 2-(dimethylamino)ethyl methacrylate (DMAEMA). Once synthesized, micelles were formed by a standard method, and their characteristics were examined using fluorescence techniques, dynamic light scattering, and transmission electron microscopy. The critical micelle concentrations of the diblock copolymers as determined by fluorescence techniques using pyrene as a hydrophobic probe were between 3.47 and 9.55 mg/L, with the partition equilibrium constant of pyrene in these micelles ranging from 0.12 × 105 to 1.52 × 105. The diameters measured by dynamic light scattering were 100–142 nm at 25 °C with a narrow distribution, which were also confirmed by transmission electron microscopy.
Co-reporter:Xiao Liu, Jonathan P. Breon, Chen Chen, and Harry R. Allcock
Macromolecules 2012 Volume 45(Issue 22) pp:9100-9109
Publication Date(Web):November 13, 2012
DOI:10.1021/ma302087a
Side group exchange reactions have been studied for linear high polymeric organophosphazenes, [N═P(OR2)]n (n ∼ 15 000). Specifically, the exchange behavior of polymers was examined where OR = OCH2CF3, OCH2CF2CF2CF2CF2H, OCH2Cl3, OC6H4CHO-p, OC6H4CN-p, and OC6H4NO2-p with sodium trifluoroethoxide. No aryloxy group replacement by trifluoroethoxy was detected, probably due to the well-protected reactive sites of the polyphosphazenes shielded by aryloxy side groups. For the exchange behavior of [N═P(OCH2CF3)2]n and [N═P(OCH2CF2CF2CF2CF2H)2]n with NaOCH2CF2CF2CF2CF2H and NaOCH2CF3, partial substituent exchange was achieved for both reactions. Furthermore, under conditions with excess nucleophile these side group exchange reactions are followed by reactions that introduce sodium–oxo groups attached to phosphorus in place of organic substituents, and this is a mechanism for subsequent hydrolysis and molecular weight decline in the presence of water. Thus, the relative instability of high polymeric [N═P(OCH2CF3)2]n in the presence of excess NaOCH2CF3 emerged as an explanation for problems with the synthesis of [NP(OCH2CF3)2]n and [NP(OCH2CF3)x(OCH2(CF2)mCF2H)2–x]n, polymers that have attracted wide fundamental and practical interest. However, substituent exchange reactions do appear to be an alternative synthetic approach for the synthesis of some polyphosphazenes, especially mixed-substituted elastomers and the recently reported trichloroethoxy/trifluoroethoxy-containing cosubstituted polyphosphazenes.
Co-reporter:Nicole L. Morozowich, Tomasz Modzelewski, and Harry R. Allcock
Macromolecules 2012 Volume 45(Issue 19) pp:7684-7691
Publication Date(Web):September 19, 2012
DOI:10.1021/ma301679k
Phosphonate and phosphonic acid containing polymers are of interest for bone tissue engineering because these species have the ability to bind hydroxyapatite [Ca10(PO4)6(OH)2], which comprises 70 wt % of bone. The synthesis of phosphoester [−PO(OEt)2] and phosphonic acid [−PO(OH)2] functionalized polyphosphazenes is described. These polymers could mimic the natural bone healing mechanism, making them excellent candidates for implantable bone grafts. Two synthetic protocols have been developed to obtain the polymers, herein referred to as prior- and post-side-group assembly. Prior assembly required the synthesis of a phosphonate-containing side group before attachment to the polyphosphazene backbone through nucleophilic substitution, whereas post-assembly required the synthesis of a polyphosphazene containing free amino groups to which the phosphonate can be coupled by Michael addition after polymer synthesis. The final step for both routes required the deprotection of the phosphoester to the corresponding phosphonic acid. The polymers were characterized by 1H and 31P NMR, GPC, and DSC techniques. A six week hydrolysis study using phosphate buffered saline (PBS) determined their hydrolytic sensitivity. All the polymers were hydrolytically sensitive, as required for this purpose, and decomposed ∼2–50% by week six. The hydrolysis products were analyzed by UV–vis techniques, and their release was monitored over the course of the experiment. These results are in agreement with percent solid mass loss data. In general, all the phosphonic acid polymers hydrolyzed at a faster rate than their corresponding phosphoester derivatives.
Co-reporter:Chen Chen, Xiao Liu, Zhicheng Tian, and Harry R. Allcock
Macromolecules 2012 Volume 45(Issue 22) pp:9085-9091
Publication Date(Web):November 5, 2012
DOI:10.1021/ma301822m
High polymeric organophosphazenes have been synthesized with trichloroethoxy side groups either as the sole substituents or as cosubstituents with trifluoroethoxy, phenoxy, or methoxyethoxyethoxy groups. Initially, small-molecule model compound studies were carried out between sodium trichloroethoxide and hexachlorocyclotriphosphazene at ambient temperature to yield the fully substituted product hexakis(trichloroethoxy)cyclotriphosphazene as a prelude to the synthesis of high polymeric trichloroethoxy homo- and cosubstituted phosphazenes. The cyclic trimeric and polymeric species were characterized by 1H and 31P NMR, GPC, and DSC techniques. Physical property comparisons were made with the long-established elastomers that contain both trifluoroethoxy and longer chain fluoroalkoxy side groups by the use of DSC, TGA, and limited oxygen index tests. The introduction of trichloroethoxy side groups further improves the resistance of these polymers to combustion.
Co-reporter:Shih-To Fei, Seung-Hyun Anna Lee, Sean M. Pursel, James Basham, Andrew Hess, Craig A. Grimes, Mark W. Horn, Thomas E. Mallouk, Harry R. Allcock
Journal of Power Sources 2011 Volume 196(Issue 11) pp:5223-5230
Publication Date(Web):1 June 2011
DOI:10.1016/j.jpowsour.2011.01.052
We report here a study of phosphazene polymer and oligomer electrolyte infiltration into high surface area titanium dioxide electrodes and its effect on the performance of dye-sensitized solar cells. The effects of different cell assembly procedures on the electrochemical properties are examined, as well as the infiltration of electrolytes based on poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (MEEP), hexakis(2-(2-methoxyethoxy)ethoxy)cyclotriphosphazene (MEE trimer), and a linear short chain analogue into conventional titanium dioxide electrode mesoporous (nanosphere) films, microcolumns and nanowires. The effects of temperature, co-solvents, and the order of addition of the electroactive components are found to affect both the conductivity of the electrolytes and the electrochemical performance of the cells. Cross-sectional scanning electron microscopy (SEM) imaging is employed to examine the degree of electrolyte infiltration into the nanostructured electrodes as a function of filling conditions. Using these techniques, conditions are identified for achieving a high degree of pore filling by the three electrolyte systems. Increased power conversion efficiency is obtained when iodine is introduced after the heating and evacuation procedures required for maximum infiltration.Graphical abstractThe results of a study of phosphazene polymer and oligomer electrolyte infiltration into high surface area titanium dioxide electrodes are described together with its effect on the performance of dye-sensitized solar cells. The influence of different cell assembly procedures on the electrochemical properties was studied as well as the infiltration of the electrolytes into various electrode designs such as conventional mesoporous films, microcolumns, and nanowires. The conditions required for increased power conversion efficiency were examined.Research highlights► Heating under vacuum is required for full infiltration of MEEP type electrolytes. ► Some iodine is volatilized during heat treatment of unsealed cells. ► Column type electrodes are infiltrated more readily than conventional electrodes.
Co-reporter:David K. Lee, Tomonori Saito, Alan J. Benesi, Michael A. Hickner, and Harry R. Allcock
The Journal of Physical Chemistry B 2011 Volume 115(Issue 5) pp:776-783
Publication Date(Web):January 10, 2011
DOI:10.1021/jp106757b
2H T1 NMR relaxation was used to characterize the molecular motion of deuterated water (2H2O) in Aquivion E87-05, Nafion 117, and sulfonated-Radel proton-exchange membranes. The presence of bound water with solid character was confirmed by the dependence of the 2H T1 relaxation on the magnetic field of the spectrometer. By comparing the 2H T1 relaxation times of the different membranes that were equilibrated in varying humidities, the factors that influence the state of water in the membranes were identified. At low levels of hydration, the molecular motion of 2H2O is affected by the acidity and mobility of the sulfonic acid groups to which the water molecules are coordinated. At higher levels of hydration, the molecular motion of 2H2O is affected by the phase separation of the hydrophilic/hydrophobic domains and the size of the hydrophilic domains.
Co-reporter:Nicole L. Morozowich, Arlin L. Weikel, Jessica L. Nichol, Chen Chen, Lakshmi S. Nair, Cato T. Laurencin, and Harry R. Allcock
Macromolecules 2011 Volume 44(Issue 6) pp:1355-1364
Publication Date(Web):February 14, 2011
DOI:10.1021/ma1027406
Novel polyphosphazenes containing various vitamin substituents were synthesized and characterized, and their sensitivity to hydrolysis and pH behavior was investigated. Vitamins L1, E, and B6 were used because of their biocompatibility, their importance in a variety of biological functions, and their potential to increase the mechanical properties of the resulting polymers, thus making these materials promising candidates for hard tissue engineering scaffolds. Chlorine replacement reactions were carried out initially with the small molecule, hexachlorocyclotriphosphazene, as a model for high polymeric poly(dichlorophosphazene). Because of the steric hindrance generated by vitamin E as a substituent, co-substituted polymers were synthesized with either glycine ethyl ester or sodium ethoxide as the second substituent. Similarly, vitamin B6 was co-substituted with glycine ethyl ester or phenylalanine ethyl ester to favor biodegradability. To prevent cross-linking via multifunctional reagents, the hydroxyl groups in vitamin B6 were protected and subsequently deprotected under acidic conditions after side group linkage to the polymer backbone. The glass transition temperatures of the polymers ranged from −24.0 to 44.0 °C. Hydrolysis of the polymers in deionized water at 37 °C was used as an initial estimate of their hydrolytic sensitivity. Different solid polymers underwent 10−100% weight loss in 6 weeks with the generation of a broad pH range of ∼2.5−9. The weight loss during preliminary hydrolysis experiments was attributed to cleavage of the polymer backbone and/or the polymers becoming soluble in the aqueous media during hydrolytic reactions.
Co-reporter:Shih-To Fei, Harry R. Allcock
Journal of Power Sources 2010 Volume 195(Issue 7) pp:2082-2088
Publication Date(Web):2 April 2010
DOI:10.1016/j.jpowsour.2009.09.043
The current highly flammable configurations for rechargeable lithium batteries generate safety concerns. Although commercial fire retardant additives have been investigated, they tend to decrease the overall efficiency of the battery. We report here ionically conductive, non-halogenated lithium battery additives based on a methoxyethoxyethoxyphosphazene oligomer and the corresponding high polymer, which can increase the fire resistance of a battery while retaining a high energy efficiency. Conductivities in the range of 10−4 S cm−1 have been obtained for self-extinguishing, ion-conductive methoxyethoxyethoxyphosphazene oligomers. The addition of 25 wt% high polymeric poly[bis(methoxyethoxyethoxy)phosphazene] to propylene carbonate electrolytes lowers the flammability by 90% while maintaining a good ionic conductivity of 2.5 × 10−3 S cm−1.
Co-reporter:Mark D. Hindenlang, Anton A. Soudakov, Gregory H. Imler, Cato T. Laurencin, Lakshmi S. Nair and Harry R. Allcock
Polymer Chemistry 2010 vol. 1(Issue 9) pp:1467-1474
Publication Date(Web):30 Jul 2010
DOI:10.1039/C0PY00126K
The first poly(organophosphazenes) with iodinated side groups for possible use as radio-opaque biomaterials are reported. Depending on the types of side groups present, these polymers may be biostable or bioerodible. Single-substituent polymers with 4-iodophenoxy or 4-iodophenylanaline ethyl ester units as the only side groups were prepared. Although a single-substituent polymer with 3,5-diiodotyrosine ethyl ester groups was difficult to synthesize, probably because of steric hindrance, mixed-substituent polymers that contained the non-iodinated ethyl esters of glycine, alanine, or phenylalanine plus a corresponding iodinated substituent could be synthesized. 31P, 1H, and 13C NMR spectroscopy techniques were used to follow the linkage of the side groups to the phosphazene skeleton and to establish the ratios of the different side groups. Hydrolysis of the iodo-amino acid/non-iodinated amino acid side group species in deionized water at 37 °C followed a bulk hydrolysis profile, with the rates dependent on the structure of the side groups. The effectiveness of these polymers as X-ray opaque materials was examined by the use of the poly(organophosphazenes) and conventional organic polymers as filters for copper Kα or rhenium–tungsten–molybdenum radiation. The phosphazene polymers that contained iodine in the side groups were opaque to X-rays, whereas the conventional organic polymers were essentially transparent to the same radiation.
Co-reporter:Arlin L. Weikel, Song Yun Cho, Nicole L. Morozowich, Lakshmi S. Nair, Cato T. Laurencin and Harry R. Allcock
Polymer Chemistry 2010 vol. 1(Issue 9) pp:1459-1466
Publication Date(Web):10 Aug 2010
DOI:10.1039/C0PY00150C
This is the first reported synthesis of completely hydrolysable polyphosphazene block co-polymers with bioerodible polyesters. Specifically, poly[(lactic acid)-co-poly(bis-alanine ethyl ester phosphazene)], poly[(lactic acid)-co-poly(bis-valine ethyl ester phosphazene)], and poly[(lactic acid)-co-poly(bis-phenylalanine ethyl ester phosphazene)] block copolymers have been prepared. These block co-polymers are blend compatibilizers used to form miscible polymer systems between poly(lactic-co-glycolic acid) (PLAGA) (50:50) or PLAGA (85:15) and poly[(bis-alanine ethyl ester phosphazene)], poly[(bis-valine ethyl ester phosphazene)], or poly[(bis-phenylalanine ethyl ester phosphazene)]. The resultant combined systems were characterized using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) to investigate polymer miscibility. The rates of hydrolysis and pH of the hydrolysis media were also examined.
Co-reporter:David K. Y. Lee, Anne-Martine S. Jackson, Toshiki Fushimi, Hemant Yennawar and Harry R. Allcock
Dalton Transactions 2010 vol. 39(Issue 22) pp:5341-5348
Publication Date(Web):04 May 2010
DOI:10.1039/B925734A
Two novel cyclic phosphazenes with asymmetric spiro rings were synthesized via reactions of hexachlorocyclotriphosphazene with chiral amino alcohol residues. The reactions showed preferential formation of the cis isomer possibly due to the delocalization of the lone pair electrons of the spirocylic nitrogen, which reduces its ability to solvate protons. Crystals of these phosphazenes were analyzed by X-ray crystallography which confirmed the formation of cis isomers and showed their ability to include guest molecules within the crystal lattices. The selective inclusion of epoxides by one of the phosphazenes was an effective method for the separation of thermally sensitive guest molecules.
Co-reporter:Toshiki Fushimi and Harry R. Allcock
Dalton Transactions 2010 vol. 39(Issue 22) pp:5349-5355
Publication Date(Web):04 May 2010
DOI:10.1039/B926069B
Novel cyclotriphosphazenes with sulfur-containing spirocyclic side groups were synthesized and polymerized to cyclomatrix materials for potential optical applications. The cyclotriphosphazenes were designed to give a high content of the phosphazene unit and sulfur atoms, as well as the capability for polymerization by ring- opening of the side groups. The chemical structures of the monomers were confirmed by NMR spectrometry, mass spectrometry, and single-crystal X-ray diffraction. Transparent solids were obtained by thermal bulk polymerization, and these were analyzed by the use of DSC, infrared spectroscopy, and mass spectrometry. One of the resultant cyclomatrix polyphosphazenes had a refractive index at 589 nm of 1.6465 and an Abbe number of 39. The contribution of the phosphazene unit to the refractive index is discussed.
Co-reporter:Nicholas R. Krogman;Arlin L. Weikel;Nhu Q. Nguyen;Katherine A. Kristhart;Syam P. Nukavarapu;Lakshmi S. Nair;Cato T. Laurencin
Journal of Applied Polymer Science 2010 Volume 115( Issue 1) pp:431-437
Publication Date(Web):
DOI:10.1002/app.31057
Abstract
New biomedically erodible polymer composites were investigated. Polyphosphazenes containing the dipeptide side groups alanyl–glycine ethyl ester, valinyl–glycine ethyl ester, and phenylalanyl–glycine ethyl ester were blended with poly(lactide-co-glycolide) (PLGA) with lactic to glycolic acid ratios of 50 : 50 [PLGA (50 : 50)] and 85 : 15 [PLGA (85 : 15)] with solution-phase techniques. Each dipeptide ethyl ester side group contains two NH protons that are capable of hydrogen bonding with the carbonyl functions of PLGA. Polyphosphazenes that contain only the dipeptide ethyl ester groups are insoluble in organic solvents and are thus unsuitable for solution-phase composite formation. To ensure solubility during and after synthesis, cosubstituted polymers with both dipeptide ethyl ester and glycine or alanine ethyl ester side groups were used. Solution casting or electrospinning was used to fabricate polymer blend matrices with different ratios of polyphosphazene to polyester, and their miscibilities were estimated with differential scanning calorimetry and scanning electron microscopy techniques. Polyphosphazenes with alanyl–glycine ethyl ester side groups plus the second cosubstituent were completely miscible with PLGA (50 : 50) and PLGA (85 : 15) when processed via solution-casting techniques. This suggests that the hydrogen-bonding protons in alanyl–glycine ethyl ester have access to the oxygen atoms of the carbonyl units in PLGA. However, when the same pair of polymers was electrospun from solution, the polymers proved to be immiscible. Solution-cast miscible polymer blends were obtained from PLGA (50 : 50) plus the polyphosphazene that was cosubstituted with valinyl–glycine ethyl ester and glycine ethyl ester side groups. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
Co-reporter:Harry R. Allcock
Applied Organometallic Chemistry 2010 Volume 24( Issue 8) pp:600-607
Publication Date(Web):
DOI:10.1002/aoc.1636
Abstract
Although more than 700 different polyphosphazenes are known, with a wide range of physical and chemical properties, a further expansion of materials properties can be accomplished by combining these macromolecules with other materials such as organic polymers, ceramics or semiconductors. These combinations lead to uses in the fields of biomedicine, energy generation and energy storage, and the formation of nano-particles such as micelles. This article is a review of recent developments in the author's laboratory to combine polyphosphazenes with other materials in order to generate new combinations of useful properties. Copyright © 2010 John Wiley & Sons, Ltd.
Co-reporter:Arlin L. Weikel, Steven G. Owens, Toshiki Fushimi and Harry R. Allcock
Macromolecules 2010 Volume 43(Issue 12) pp:5205-5210
Publication Date(Web):May 27, 2010
DOI:10.1021/ma1007013
The preparation of phosphazenes that possess reversible cross-linking groups to control mechanical stability and hydrolysis has been accomplished using cysteine and methionine amino acid side groups. Small molecule models and linear polymeric phosphazenes that contain methionine ethyl ester and cysteine ethyl disulfide ethyl ester side groups were synthesized. Protection of the free thiol groups was carried out to circumvent unwanted cross-linking of the phosphazenes through the cysteine ethyl ester N- and S-termini. Cyclic trimeric cysteine ethyl disulfide ethyl ester model compounds were deprotected by S−S bond cleavage using β-mercaptoethanol, dithiothreitol (DTT), and zinc in aqueous hydrochloric acid. For the high polymeric derivatives, the extent of S−S bond cleavage varied depending on the deprotection method used. With the exception of the Zn/HCl method, the resultant deprotected polymers were soluble in common organic solvents and underwent minimal chain cleavage during the reaction sequence. The protected or deprotected high polymers are candidates for reversible cross-linking in drug delivery systems and for cross-link stabilization of tissue engineering scaffolds.
Co-reporter:Arlin L. Weikel, Steven G. Owens, Nicole L. Morozowich, Meng Deng, Lakshmi S. Nair, Cato T. Laurencin, Harry R. Allcock
Biomaterials 2010 31(33) pp: 8507-8515
Publication Date(Web):
DOI:10.1016/j.biomaterials.2010.07.094
Co-reporter:David K. Lee, Harry R. Allcock
Solid State Ionics 2010 Volume 181(39–40) pp:1721-1726
Publication Date(Web):20 December 2010
DOI:10.1016/j.ssi.2010.09.051
Lithium trifluoromethanesulfonate (LiTf), lithium bis(trifluoromethanesulfonyl)imidate (LiTFSI), magnesium trifluoromethanesulfonate (MgTf2) and magnesium bis(trifluoromethane sulfonyl)imidate (MgTFSI2) were dissolved in poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (MEEP) to compare the effect on solvent-free polymer ionic conductivity of monovalent versus divalent cations, and two anions with different degrees of dissociation. The polymer electrolytes with the bis(trifluoromethanesulfonyl)imidate anion had higher ionic conductivities even though the glass transition temperatures, which reflected polymer molecular motion, were higher than those of their counterparts with the trifluoromethanesulfonate anion. Furthermore, polymer electrolytes with magnesium salts achieved their maximum conductivity at lower salt concentrations than their counterparts with lithium salts. The temperature dependence of the ionic conductivity of the solid solutions was fitted to the Vogel–Tamman–Fulcher (VTF) equation. The pseudo-activation energy term, B, of the VTF equation showed a strong dependence on the anion present. This result suggests that the dominant mobile species is the anion, while the cation remains relatively bound to the polymer.
Co-reporter:Marta M. Guron, Xiaolan Wei, Daniel Welna, Nicholas Krogman, Myung Jong Kim, Harry Allcock and Larry G. Sneddon
Chemistry of Materials 2009 Volume 21(Issue 8) pp:1708
Publication Date(Web):April 2, 2009
DOI:10.1021/cm900304r
Simple blends of the poly(norbornenyldecaborane) (PND) boron-carbide preceramic polymer with either of the commercial poly(methylcarbosilane) (PMCS) or allylhydridopolycarbosilane (AHPCS) silicon-carbide preceramic polymers have been found to provide excellent processable precursors to boron-carbide/silicon-carbide ceramic composite materials. The blends exhibited good char yields with tunable ceramic compositions. Certain compositions of the PND/AHPCS derived ceramics also exhibited significant oxidation resistance. Nonwoven mats of PND/PMCS polymer composite fibers were readily obtained by the electrostatic spinning of polymer blend solutions. The pyrolytic ceramic conversion reactions of the PND/PMCS polymer fibers then produced mats of micro- and nanodiameter boron-carbide/silicon-carbide ceramic composite fibers.
Co-reporter:Toshiki Fushimi and Harry R. Allcock
Dalton Transactions 2009 (Issue 14) pp:2477-2481
Publication Date(Web):16 Feb 2009
DOI:10.1039/B819826H
Two types of cyclotriphosphazenes with sulfur-containing side groups were synthesized for an examination of their optical properties including refractive index, optical dispersion, and absorption in the UV-vis region. In the first type, the residue from ethanethiol was introduced through P–S bonding. In the second type, the residues from alcohols that contain alkyl sulfide units were introduced through P–O bonding. A high content of sulfur and phosphorus atoms and thus a high refractive index of 1.6161 were achieved in the first case: however, the optical dispersion increased due to the absorption by the P–S bonds. This increase in optical dispersion was avoided in the second case, where the phosphazene skeleton showed no absorption down to 230 nm. The molar refraction and the molar volume of the –PN– repeating unit were evaluated, and these demonstrated the advantage of the cyclotriphosphazene skeleton as a building block for optical materials with high refractive indices. In addition, the transparency of the phosphazene skeleton is beneficial for low optical dispersion.
Co-reporter:Youngkyu Chang;Song Yun Cho;Lee B. Steely
Journal of Polymer Science Part A: Polymer Chemistry 2009 Volume 47( Issue 3) pp:692-699
Publication Date(Web):
DOI:10.1002/pola.23180
Abstract
Amphilic triblock copolymers with varying ratios of hydrophilic poly[bis (methoxyethoxyethoxy)phosphazene] (MEEP) and relatively hydrophobic poly(propylene glycol) (PPG) blocks were synthesized via the controlled cationic-induced living polymerization of a phosphoranimine (Cl3PNSiMe3) at ambient temperature. A PPG block can function as either a classical hydrophobic block or a less hydrophobic component by varying the nature of a phosphazene block. The aqueous phase behavior of MEEP-PPG-MEEP block copolymers was investigated using fluorescence techniques, TEM, and dynamic light scattering (DLS). The critical micelle concentrations (cmcs) of MEEP-PPG-MEEP block copolymers were determined to be in the range of 3.7–16.8 mg/L. The mean diameters of MEEP-PPG-MEEP polymeric micelles, measured by DLS, were between 31 and 44 nm. The equilibrium constants of pyrene in these micelles ranged from 4.7 × 104 to 9.6 × 104. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 692–699, 2009
Co-reporter:Arlin L. Weikel, Nicholas R. Krogman, Nhu Q. Nguyen, Lakshmi S. Nair, Cato T. Laurencin and Harry R. Allcock
Macromolecules 2009 Volume 42(Issue 3) pp:636-639
Publication Date(Web):January 13, 2009
DOI:10.1021/ma802423c
The dipeptides alanyl-glycine ethyl ester, valinyl-glycine ethyl ester, and phenylalanyl-glycine ethyl ester were synthesized by mixed anhydride solution-phase peptide reactions. The free N-terminus was used as a reactive site for nucleophilic replacement of the chlorine atoms in poly(dichlorophosphazene). The C-terminus was protected with an ethyl ester to prevent side reactions and cross-linking. The alanyl-glycine ethyl ester replaced all the chlorine atoms in poly(dichlorophosphazene). However, replacement of all the chlorine atoms in poly(dichlorophosphazene) by valinyl-glycine ethyl ester or phenylalanyl-glycine ethyl ester polyphosphazenes was prevented by the insolubility of the partially substituted intermediates. To circumvent this problem, cosubstitution was carried out using the valinyl- or phenylalanyl esters with glycine ethyl ester or alanine ethyl ester in a 1:1 ratio. Cosubstituted polyphosphazenes with alanyl glycine ethyl ester and glycine ethyl ester or alanine ethyl ester were also synthesized with a side group ratio of 1:1. The polymer structures and physical properties were studied using multinuclear NMR, DSC, and GPC techniques. Heterophase hydrolysis experiments in aqueous media at different pH values were carried out to estimate the hydrolytic sensitivity of these polymers. All the polymers were less sensitive to hydrolysis under neutral or basic (pH, 10.0) conditions than at pH 4.0, where rapid hydrolysis occurred.
Co-reporter:Song Yun Cho and Harry R. Allcock
Macromolecules 2009 Volume 42(Issue 13) pp:4484-4490
Publication Date(Web):May 13, 2009
DOI:10.1021/ma900567n
Hydrophobic block copolymers with adamantyl polyphosphazene and polystyrene blocks were synthesized via the controlled cationic living polymerization of a phosphoranimine at ambient temperature. β-Cyclodextrins (β-CDs) were then complexed with the adamantyl side groups in aqueous media to generate amphiphilic block copolymers. These underwent micelle formation in an aqueous environment. The micellar behavior of these complexes was monitored using fluorescence techniques, transmission electron microscopy (TEM), and dynamic light scattering. The critical micelle concentration of the adamantyl polyphosphazene-polystyrene block copolymer complexes was 0.925 mg/L. TEM imaging revealed spherically shaped micelles. A mean diameter of 193 nm was measured by dynamic light scattering. It was possible to control micelle formation by changing the amount of β-CD in the aqueous medium at constant block copolymer concentration.
Co-reporter:Nicholas R. Krogman, Arlin L. Weikel, Nhu Q. Nguyen, Lakshmi S. Nair, Cato T. Laurencin and Harry R. Allcock
Macromolecules 2008 Volume 41(Issue 21) pp:7824-7828
Publication Date(Web):October 10, 2008
DOI:10.1021/ma801961m
Polyphosphazenes that contain serine and threonine side groups have been synthesized by the macromolecular substitution technique, and their structure and properties were determined. Serine and threonine have two different sites for covalent linkage to the polyphosphazene backbone. For one series of polymers, serine ethyl ester and threonine ethyl ester units were linked to the polyphosphazene skeleton via the N-terminus. The free hydroxyl groups on each side group were then used to graft poly(l-lactide). Graft densities of 1:0.5, 1:2, and 1:4 grafts per repeat unit were controlled through the stoichiometric ratios of sodium naphthalenide to polyphosphazene side groups during the synthetic procedures. For the second series of macromolecules, the hydroxyl function of serine or threonine was used as the site for covalent attachment to the polyphosphazene backbone, a process that required protection of both the N- and C-termini. Secondary reactions to remove the protective groups then yielded side units with free amino and carboxylic acid functions. These polymers (with serine or threonine groups linked via the hydroxyl function) are water-soluble over a broad pH range but have the ability to form ionic cross-links in the presence of calcium ions.
Co-reporter:Nicholas R. Krogman, Mark D. Hindenlang, Lakshmi S. Nair, Cato T. Laurencin and Harry R. Allcock
Macromolecules 2008 Volume 41(Issue 22) pp:8467-8472
Publication Date(Web):October 24, 2008
DOI:10.1021/ma8008417
We report here the first examples of poly(organophosphazenes) with side groups derived from the purines guanine and adenine and the pyrimidine cytosine. Polymers with these purines or pyrimidines as the only side groups proved difficult to synthesize by macromolecular substitution techniques because of the insolubility of the intermediate products. Therefore, cosubstitution reactions of the poly(dichlorophosphazene) with glycine ethyl ester, alanine ethyl ester, or diethylene glycol methyl ether, followed by the respective purine or pyrimidine, were utilized. Each pair of side groups was incorporated into the polyphosphazene in a 1:1 ratio. 31P NMR spectroscopy verified the replacement of all the chlorine atoms, while 1H and 13C NMR techniques confirmed the presence and ratio of the different side group. DRIFT spectroscopy indicated that the attachment of the purines or pyrimidines was via the primary amino functionality. Glass transition temperatures ranged from −28 to −15 °C for the mixed-substituent polyphosphazenes with both purine or pyrimidine side groups together with glycine ethyl ester or alanine ethyl ester units. Other mixed-substituent polymers with purine- or pyrimidine-substituted polyphosphazenes with the cosubstituent diethylene glycol methyl ether yielded block type side group distributions, which were revealed by 31P NMR and DSC methods. The polymers appeared to be thermally stable to at least 200 °C, with 10% mass loss at temperatures as high as 263 °C. The hydrolysis of the amino acid ester/purine or pyrimidine cosubstituted polyphosphazenes in deionized water at 37 °C followed a bulk hydrolysis profile with the initial reaction being a cleavage of the amino acid ester units from the skeleton, followed by subsequent detachment of the purine or pyrimidine. Significant hydrolytic weight loss and molecular weight declines were detected after 14 days for the amino acid/purine or pyrimidine cosubstituted polyphosphazenes.
Co-reporter:Lee B. Steely;Qixiu Li;John V. Badding
Polymer Engineering & Science 2008 Volume 48( Issue 4) pp:683-686
Publication Date(Web):
DOI:10.1002/pen.20995
Abstract
Three dimensional foams of fluorinated polyphosphazenes were prepared using liquid CO2 to solvate polymer films, which were then expanded by volatilization of liquid CO2 within the polymer matrix. The polymers dissolved in CO2 at 1.5 × 107 Pa (2200 psi) and room temperature. Materials were examined using Raman spectroscopy before and during high pressure CO2 exposure. The solubility in liquid CO2 is attributed to interactions between the solvent and the fluorine atoms in the polymer side groups. Fluoroalkoxy polyphosphazenes are known for their hydrophobicity and fire retardant properties, and these characteristics are retained in the foams. Scanning electron microscopy, water swelling, and preliminary fire retardance tests were conducted to determine the morphology and properties of the foams. These materials are possible candidates for a number of different engineering applications. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers
Co-reporter:Song Yun Cho and Harry R. Allcock
Chemistry of Materials 2007 Volume 19(Issue 25) pp:6338
Publication Date(Web):November 14, 2007
DOI:10.1021/cm702066k
Novel cyclophosphazenes containing aryl trifluorovinyl ether functional units together with the corresponding perfluorocyclobutane (PFCB) phosphazene polymers have been synthesized and characterized. These polymers have desirable properties for low optical loss waveguide applications. The monomers used in this study were prepared from sodium 4-(trifluorovinyloxy)phenoxides and cyclophosphazenes with fluorinated alkoxy or aryloxy side groups. The resulting polymers, produced by 2π + 2π cyclopolymerization of the aromatic trifluorovinyl ether moieties, showed good chemical stabilities and high thermal stabilities (Td up to 330 °C). Tough and transparent thin films of these polymers were readily prepared by solvent-free processes using spin-coating of the monomers. By adjusting the ratio of monomers, the refractive index of the polymers could be controlled from 1.4528 to 1.5187 at 1550 nm, with exceptionally low birefringence of Δn = 0 to 0.0003 ± 0.0002. A propagation optical loss lower than ∼0.25 dB/cm at 1550 nm was determined from the polymer films, for the TE polarization by measuring the scattered light intensity along the slab waveguide length. Such good thermal and optical properties demonstrate that these novel PFCB-based phosphazene polymers are promising candidates for optical waveguide or optical device materials.
Co-reporter:Harry R. Allcock
Journal of Inorganic and Organometallic Polymers and Materials 2007 Volume 17( Issue 2) pp:349-359
Publication Date(Web):2007 June
DOI:10.1007/s10904-007-9134-3
New methods are described for the incorporation of cyclophosphazene rings into macromolecules. These include polynorbornenes with cyclophosphazene side units produced by ROMP reactions, polystyrenes linked to phosphazene rings by azide coupling processes, and cyclolinear polymers by ADMET and Heck coupling reactions. These last species are wavelength-emission tailored electroluminescent materials.
Co-reporter:Shih-To Fei, Mwita V. B. Phelps, Yang Wang, Eric Barrett, Farhan Gandhi and Harry R. Allcock
Soft Matter 2006 vol. 2(Issue 5) pp:397-401
Publication Date(Web):14 Mar 2006
DOI:10.1039/B516972K
We report here an electrochemically responsive polymer hydrogel based on ionic crosslinking. The crosslinking by metal cations and anionic carboxylic acid side groups can be controlled by redox reactions. The crosslinks dissociate when the cation crosslinker is reduced to a lower oxidation state and reform following oxidation, which leads to a reversible and localized swelling–contraction. By choosing biocompatible components and miniaturization designs, the system has potential in microrobotic and biomedical applications.
Co-reporter:Harry R Allcock;Lee B Steely;Anurima Singh
Polymer International 2006 Volume 55(Issue 6) pp:
Publication Date(Web):29 MAR 2006
DOI:10.1002/pi.2030
Hydrophobic polymers play a crucial role in many biomedical and commercial applications. Hydrophobic polyphosphazenes offer opportunities for the tuning of surface properties that are not found for many conventional hydrophobic materials. Thus, changes in the side groups linked to the polyphosphazene skeleton allow the surface character to be changed from highly hydrophilic to hydrophobic. The hydrophobic side groups range from fluoroalkoxy groups to aryloxy and organosilicon units. Moreover, the polymer architectures can be varied from single-substituent species to mixed substituent polymers or to block or comb copolymer structures. Superhydrophobicity, with contact angles to water as high as 159°, has been achieved by electrospinning a fluoroalkoxy derivative to nanofiber mats. Copyright © 2006 Society of Chemical Industry
Co-reporter:Youngkyu Chang;David A. Stone
Journal of Polymer Science Part A: Polymer Chemistry 2006 Volume 44(Issue 1) pp:69-76
Publication Date(Web):10 NOV 2005
DOI:10.1002/pola.21140
A new type of cyclolinear polymer, poly(phenylene vinylene-alt-cyclotriphosphazene), was synthesized through Heck-type coupling reactions to produce π-conjugated macromolecules with excellent solubility and precise electronic control of the band-gap energy. This synthesis method is capable of producing well-defined alternating polymers. The method is highly adaptable and can be readily used for other chromophore systems. The resulting polymers were also capable of accommodating a wide variety of substituents on the cyclophosphazene rings with minimal effect on the electronic properties. The band gap and electron affinities of the polymer were varied through the manipulation of the π-conjugated unit located between the insulating phosphazene rings. Each chromophore matched the intended conjugation length consistently throughout the macromolecules. The polymers were good film formers because of the chosen substituents on the phosphazene rings. The absorbance of the polymers indicated minimal spectral shift from the monomer absorbance. This suggested an effective insulation of each chromophore unit from its neighbors by the phosphazene rings. Solution photoluminescence efficiencies were found to be up to 44.1%. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 69–76, 2006
Co-reporter:Robert V. Morford, Daniel T. Welna, Clay E. Kellam III, Michael A. Hofmann, Harry R. Allcock
Solid State Ionics 2006 Volume 177(7–8) pp:721-726
Publication Date(Web):15 March 2006
DOI:10.1016/j.ssi.2006.01.014
The influence of an organophosphosphate additive on poly(ethylene oxide) lithium bis(trifluoromethylsulfonyl)imide-based gel polymer electrolytes for secondary lithium battery applications is described. Tris(2-(2-methoxyethoxy)ethyl)phosphate, is compared to the well known gel-battery component, propylene carbonate, through a study of complex impedance analysis, differential scanning calorimetry, and limiting oxygen index combustion analysis. The conductivities of the gels at low concentrations of tris(2-(2-methoxyethoxy)ethyl)phosphate (1.9–4.2 mol%) are higher to those of propylene carbonate-based systems with the same concentration. Despite micro-phase separation at high concentrations of tris(2-(2-methoxyethoxy)ethyl)phosphate (7.0–14.9 mol%), the conductivities remain comparable to systems that use propylene carbonate. The addition of tris(2-(2-methoxyethoxy)ethyl)phosphate to poly(ethylene oxide) gives increased fire retardance, while the addition of propylene carbonate to poly(ethylene oxide) results in increased flammability.
Co-reporter:Harry R. Allcock, Youngkyu Chang, Daniel T. Welna
Solid State Ionics 2006 Volume 177(5–6) pp:569-572
Publication Date(Web):February 2006
DOI:10.1016/j.ssi.2005.11.017
Silicate sol–gel precursors of poly[bis(methoxyethoxyethoxy)phosphazene] and their corresponding hybrid networks were synthesized by hydrolysis and condensation reactions. Conversion of the precursor polymers to covalently interconnected hybrid networks with controlled morphologies and physical properties was achieved. Thermal analyses showed no melting transitions for the networks and low glass transition temperatures that ranged from approximately − 38 to − 67 °C. Solid solutions with lithium bis(trifluoromethanesulfonyl)amide in the network showed a maximum ionic conductivity value of 7.69 × 10− 5 S/cm, making these materials interesting candidates for dimensionally stable solid polymer electrolytes.
Co-reporter:Harry R. Allcock;Richard M. Wood
Journal of Polymer Science Part B: Polymer Physics 2006 Volume 44(Issue 16) pp:2358-2368
Publication Date(Web):13 JUL 2006
DOI:10.1002/polb.20864
New proton-conductive fuel cell membranes have been produced by the linkage of acidic functional groups to aryloxy side units attached to a polyphosphazene macromolecular chain. The functional groups include sulfonic acid, phosphonic acid, and sulfonimide units. The different functional groups impart different water retention, proton conduction, and methanol resistance to the membranes. Special advantages are foreseen for uses in direct methanol fuel cells. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2358–2368, 2006
Co-reporter:D. T. Welna;J. D. Bender;X. Wei;L. G. Sneddon;H. R. Allcock
Advanced Materials 2005 Volume 17(Issue 7) pp:
Publication Date(Web):30 MAR 2005
DOI:10.1002/adma.200401257
Pyrolysis of poly(norbornenyldecaborane) that has been electrostatically spun provides a route to non-woven mats of boron-carbide/carbon ceramic nanofibers with narrow distributions and controllable dimensions (see Figure). This approach allows the fabrication of composite ceramic fibers with varying composition, which could be tailored to suit specific applications.
Co-reporter:Youngkyu Chang;Eric S. Powell
Journal of Polymer Science Part A: Polymer Chemistry 2005 Volume 43(Issue 13) pp:2912-2920
Publication Date(Web):20 MAY 2005
DOI:10.1002/pola.20748
Amphiphilic diblock copolymers that contained hydrophilic poly[bis(potassium carboxylatophenoxy)phosphazene] segments and hydrophobic polystyrene sections were synthesized via the controlled cationic polymerization of Cl3PNSiMe3 with a polystyrenyl–phosphoranimine as a macromolecular terminator. These block copolymers self-associated in aqueous media to form micellar structures which were investigated by fluorescence spectroscopy, dynamic light scattering, and transmission electron microscopy. The size and shape of the micelles were not affected by the introduction of different monovalent cations (Li+, K+, Na+, and Cs+) into the stable micellar solutions. However, exposure to divalent cations induced intermicellar crosslinking through carboxylate groups, which caused precipitation of the ionically crosslinked aggregates from solution. This micelle-coupling behavior was reversible: the subsequent addition of monovalent cations caused the redispersion of the polystyrene-block-poly[bis(potassium carboxylatophenoxy)phosphazene] (PS–KPCPP) block copolymers into a stable micellar solution. Aqueous micellar solutions of PS–KPCPP copolymers also showed pH-dependent behavior. These attributes make PS–KPCPP block copolymers suitable for studies of guest retention and release in response to ion charge and pH. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2912–2920, 2005
Co-reporter:Andrew E. Maher;Catherine M. Ambler;Eric S. Powell
Journal of Applied Polymer Science 2004 Volume 92(Issue 4) pp:2569-2576
Publication Date(Web):11 MAR 2004
DOI:10.1002/app.20223
A series of poly(fluoroalkoxyphosphazenes) containing a mixture of 2,2,3,3,4,4,5,5-octafluoropentoxy and 2,2,2-trifluoroethoxy substituents was synthesized. The series included polymers with 25–94% incorporation of the trifluoroethoxy substituent, as well as single-substituent polymers with 100% octafluoropentoxy or trifluoroethoxy side groups. Polymers were analyzed by multinuclear NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis, and were subjected to limiting oxygen index and microtensile testing. It was found that the Tg and Td values among the cosubstituted polymers varied little with changes in composition, but the mechanical properties varied over a wide range. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2569–2576, 2004
Co-reporter:Y. Chang;H.R. Allcock
Advanced Materials 2003 Volume 15(Issue 6) pp:
Publication Date(Web):20 MAR 2003
DOI:10.1002/adma.200390126
Co-reporter:Harry R Allcock, Michael A Hofmann, Catherine M Ambler, Serguei N Lvov, Xiangyang Y Zhou, Elena Chalkova, Jamie Weston
Journal of Membrane Science 2002 Volume 201(1–2) pp:47-54
Publication Date(Web):31 May 2002
DOI:10.1016/S0376-7388(01)00702-5
A method for the synthesis of phenyl phosphonic acid functionalized poly[aryloxyphosphazene] membranes is described. Membranes of polymers with total ion-exchange capacities between 1.17 and 1.43 meq. g−1 were cast from N,N-dimethyl-formamide and found to have proton conductivities between 10−2 and 10−1 S cm−1. Equilibrium water swelling values were determined to be between 11 and 32% depending on the polymer structure. Methanol diffusion coefficients for both radiation cross-linked and non-cross-linked membranes in 3 M aqueous methanol at 80 °C and 2.8 bar were found to be at least 12 times lower than for Nafion 117, and 6 times lower than for a radiation cross-linked sulfonated polyphosphazene membrane.
Co-reporter:Robert V Morford, E.Clay Kellam III, Michael A Hofmann, Richard Baldwin, Harry R Allcock
Solid State Ionics 2000 Volume 133(3–4) pp:171-177
Publication Date(Web):2 August 2000
DOI:10.1016/S0167-2738(00)00741-4
The development of a small-molecule organophosphorus gel polymer electrolyte additive for use in rechargeable lithium batteries is described. This organophosphorus additive is less volatile and more resistant to ignition than propylene carbonate, a well known component of gel polymer electrolytes. Moreover, the ionic conductivities of gels containing the organophosphorus additive at low molar concentrations (11–21%) exceed the ionic conductivities of gels containing equimolar amounts of propylene carbonate (5.3×10−5 versus 4.8×10−5 S/cm). At higher molar concentrations (31–52%) there is evidence of phase separation, but conductivities remain comparable to propylene carbonate systems.
Co-reporter:Toshiki Fushimi and Harry R. Allcock
Dalton Transactions 2010 - vol. 39(Issue 22) pp:NaN5355-5355
Publication Date(Web):2010/05/04
DOI:10.1039/B926069B
Novel cyclotriphosphazenes with sulfur-containing spirocyclic side groups were synthesized and polymerized to cyclomatrix materials for potential optical applications. The cyclotriphosphazenes were designed to give a high content of the phosphazene unit and sulfur atoms, as well as the capability for polymerization by ring- opening of the side groups. The chemical structures of the monomers were confirmed by NMR spectrometry, mass spectrometry, and single-crystal X-ray diffraction. Transparent solids were obtained by thermal bulk polymerization, and these were analyzed by the use of DSC, infrared spectroscopy, and mass spectrometry. One of the resultant cyclomatrix polyphosphazenes had a refractive index at 589 nm of 1.6465 and an Abbe number of 39. The contribution of the phosphazene unit to the refractive index is discussed.
Co-reporter:David K. Y. Lee, Anne-Martine S. Jackson, Toshiki Fushimi, Hemant Yennawar and Harry R. Allcock
Dalton Transactions 2010 - vol. 39(Issue 22) pp:NaN5348-5348
Publication Date(Web):2010/05/04
DOI:10.1039/B925734A
Two novel cyclic phosphazenes with asymmetric spiro rings were synthesized via reactions of hexachlorocyclotriphosphazene with chiral amino alcohol residues. The reactions showed preferential formation of the cis isomer possibly due to the delocalization of the lone pair electrons of the spirocylic nitrogen, which reduces its ability to solvate protons. Crystals of these phosphazenes were analyzed by X-ray crystallography which confirmed the formation of cis isomers and showed their ability to include guest molecules within the crystal lattices. The selective inclusion of epoxides by one of the phosphazenes was an effective method for the separation of thermally sensitive guest molecules.
Co-reporter:Toshiki Fushimi and Harry R. Allcock
Dalton Transactions 2009(Issue 14) pp:NaN2481-2481
Publication Date(Web):2009/02/16
DOI:10.1039/B819826H
Two types of cyclotriphosphazenes with sulfur-containing side groups were synthesized for an examination of their optical properties including refractive index, optical dispersion, and absorption in the UV-vis region. In the first type, the residue from ethanethiol was introduced through P–S bonding. In the second type, the residues from alcohols that contain alkyl sulfide units were introduced through P–O bonding. A high content of sulfur and phosphorus atoms and thus a high refractive index of 1.6161 were achieved in the first case: however, the optical dispersion increased due to the absorption by the P–S bonds. This increase in optical dispersion was avoided in the second case, where the phosphazene skeleton showed no absorption down to 230 nm. The molar refraction and the molar volume of the –PN– repeating unit were evaluated, and these demonstrated the advantage of the cyclotriphosphazene skeleton as a building block for optical materials with high refractive indices. In addition, the transparency of the phosphazene skeleton is beneficial for low optical dispersion.
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