Christopher J. Ellison

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Name: Ellison, Christopher J.

Organization: The University of Texas at Austin , USA

Department: McKetta Department of Chemical Engineering

Title: Assistant(PhD)

TOPICS

Material Chemistry

Graphene & Graphite

Polymer

POSS Stimuli Responsive Polymers Atom Transfer Radical Polymerization Photopolymerization Stimuli Responsive Polymers Block Copolymer Photo Responsive Polymers

Chemicals
References

Orthogonally Spin-Coated Bilayer Films for Photochemical Immobilization and Patterning of Sub-10-Nanometer Polymer Monolayers

Co-reporter:Dustin W. Janes;Chae Bin Kim;Michael J. Maher

Langmuir July 12, 2016 Volume 32(Issue 27) pp:6940-6947

Publication Date(Web):2017-2-22

DOI:10.1021/acs.langmuir.6b01560

Versatile and spatiotemporally controlled methods for decorating surfaces with monolayers of attached polymers are broadly impactful to many technological applications. However, current materials are usually designed for very specific polymer/surface chemistries and, as a consequence, are not very broadly applicable and/or do not rapidly respond to high-resolution stimuli such as light. We describe here the use of a polymeric adhesion layer, poly(styrene sulfonyl azide-alt-maleic anhydride) (PSSMA), which is capable of immobilizing a 1–7 nm thick monolayer of preformed, inert polymers via photochemical grafting reactions. Solubility of PSSMA in very polar solvents enables processing alongside hydrophobic polymers or solutions and by extension orthogonal spin-coating deposition strategies. Therefore, these materials and processes are fully compatible with photolithographic tools and can take advantage of the immense manufacturing scalability they afford. For example, the thicknesses of covalently grafted poly(styrene) obtained after seconds of exposure are quantitatively equivalent to those obtained by physical adsorption after hours of thermal equilibration. Sequential polymer grafting steps using photomasks were used to pattern different regions of surface energy on the same substrate. These patterns spatially controlled the self-assembled domain orientation of a block copolymer possessing 21 nm half-periodicity, demonstrating hierarchical synergy with leading-edge nanopatterning approaches.

Large area fabrication of graphene nanoribbons by wetting transparency-assisted block copolymer lithography

Co-reporter:Reika Katsumata, Maruthi Nagavalli Yogeesh, Helen Wong, Sunshine X. Zhou, Stephen M. Sirard, Tao Huang, Richard D. Piner, Zilong Wu, Wei Li, Alvin L. Lee, Matthew C. Carlson, Michael J. Maher, Deji Akinwande, Christopher J. Ellison

Polymer 2017 Volume 110(Volume 110) pp:

Publication Date(Web):10 February 2017

DOI:10.1016/j.polymer.2016.12.034

•Wetting transparency assisted block copolymer lithography for graphene nanoribbons.•Graphene nanoribbons from 13 to 51 nm wide were produced over large areas.•The sample sizes were ∼1 cm2 without major defects over sampled areas ∼9 μm2.•This protocol could be useful for high-throughput production of plasmonic devices.Patterning graphene into nanoribbons (graphene nanoribbons, GNR) allows for tunability in the emerging fields of plasmonic devices in the mid-infrared and terahertz regime. However, the fabrication processes of GNR arrays for plasmonic devices often include a low-throughput electron beam lithography step that cannot be easily scaled to large areas. In this study, we developed a GNR fabrication method using block copolymer (BCP) lithography that takes advantage of the wetting transparency of graphene. One major advantage of this method is that the self-assembled domains of the polystyrene-block-poly(methyl methacrylate) BCP are oriented perpendicularly directly on top of the graphene where they can later serve as an etch mask. Large area (cm2 scale, 3 μm × 3 μm defect-free area) 13–51 nm wide GNR arrays were successfully fabricated using this scalable protocol. This wetting transparency-assisted GNR fabrication method could be useful for high-throughput production of various plasmonic devices, including biosensors, and photodetectors.Download high-res image (419KB)Download full-size image

Development of kinetic parameters for polyurethane thermal degradation modeling featuring a bioinspired catecholic flame retardant

Co-reporter:B.C. Roberts, A.R. Jones, O.A. Ezekoye, C.J. Ellison, M.E. Webber

Combustion and Flame 2017 Volume 177(Volume 177) pp:

Publication Date(Web):1 March 2017

DOI:10.1016/j.combustflame.2016.12.014

A recently developed flame retardant (FR) nanocoating of polydopamine (PDA) was applied to flexible polyurethane foam (PU) and thermogravimetrically analyzed (TGA). Thermal degradation kinetics were described by a simplified multi-component, Arrhenius expression coupled with a first-order reaction model. Kinetic parameters were then extracted via an optimization solver. By limiting the number of optimized parameters, a mesh adaptive direct search algorithm was employed to extract meaningful kinetic parameters that better simulate the TGA data compared to graphical methods. Through TGA, it was shown that the effect of the PDA nanocoating on PU degradation differs between oxidative (78 vol% nitrogen (N2) and 21 vol% oxygen) and inert (100% N2) environments. In nitrogen, the mass loss is delayed and diminished in the first PU reaction, which is the opposite effect of a traditional halophosphate FR. In an oxidative environment, the first reaction of PU is greatly delayed by the PDA coating, but once the reaction begins, it becomes accelerated.

Polyhedral Oligomeric Silsesquioxane-Containing Thiol–ene Fibers with Tunable Thermal and Mechanical Properties

Co-reporter:Yichen Fang, Heonjoo Ha, Kadhiravan Shanmuganathan, and Christopher J. Ellison

ACS Applied Materials & Interfaces 2016 Volume 8(Issue 17) pp:11050

Publication Date(Web):April 8, 2016

DOI:10.1021/acsami.6b01692

Polyhedral oligomeric silsesquioxanes (POSS) are versatile inorganic–organic hybrid building blocks that have potential applications as reinforcement nanofillers, thermal stabilizers, and catalyst supports for metal nanoparticles. However, fabrication of fibrous materials with high POSS content has been a challenge because of the aggregation and solubility limits of POSS units. In this paper, we describe a robust and environmentally friendly fabrication approach of inorganic–organic hybrid POSS fibers by integrating UV initiated thiol–ene polymerization and centrifugal fiber spinning. The use of monomeric liquids in this approach not only reduces the consumption of heat energy and solvent, but it also promotes homogeneous mixing of organic and inorganic components that allows integration of large amount of POSS (up to 80 wt %) into the polymer network. The POSS containing thiol–ene fibers exhibited enhanced thermomechanical properties compared to purely organic analogs as revealed by substantial increases in residual weight and a factor of 4 increase in modulus after thermal treatment at 1000 °C. This simple fabrication approach combined with the tunability in fiber properties afforded by tailoring monomer composition make POSS containing thiol–ene fibers attractive candidates for catalyst supports and filtration media, particularly in high-temperature and harsh environments.Keywords: cross-linked fiber; enhanced thermomechanical properties; inorganic−organic hybrid fiber; POSS; reactive centrifugal spinning; thiol−ene photopolymerization

Ultrasmooth Polydopamine Modified Surfaces for Block Copolymer Nanopatterning on Flexible Substrates

Co-reporter:Joon Hee Cho, Reika Katsumata, Sunshine X. Zhou, Chae Bin Kim, Austin R. Dulaney, Dustin W. Janes, and Christopher J. Ellison

ACS Applied Materials & Interfaces 2016 Volume 8(Issue 11) pp:7456

Publication Date(Web):March 4, 2016

DOI:10.1021/acsami.6b00626

Nature has engineered universal, catechol-containing adhesives which can be synthetically mimicked in the form of polydopamine (PDA). In this study, PDA was exploited to enable the formation of block copolymer (BCP) nanopatterns on a variety of soft material surfaces. While conventional PDA coating times (1 h) produce a layer too rough for most applications of BCP nanopatterning, we found that these substrates could be polished by bath sonication in a weakly basic solution to form a conformal, smooth (root-mean-square roughness ∼0.4 nm), and thin (3 nm) layer free of large prominent granules. This chemically functionalized, biomimetic layer served as a reactive platform for subsequently grafting a surface neutral layer of poly(styrene-random-methyl methacrylate-random-glycidyl methacrylate) to perpendicularly orient lamellae-forming poly(styrene-block-methyl methacrylate) BCP. Moreover, scanning electron microscopy observations confirmed that a BCP nanopattern on a poly(ethylene terephthalate) substrate was not affected by bending with a radius of ∼0.5 cm. This procedure enables nondestructive, plasma-free surface modification of chemically inert, low-surface energy soft materials, thus overcoming many current chemical and physical limitations that may impede high-throughput, roll-to-roll nanomanufacturing.Keywords: block copolymer (BCP); conformal coating; flexible substrates; nanopatterning; polydopamine (PDA); self-assembly; solution deposition; surface modification

Photopatterning of Block Copolymer Thin Films

Co-reporter:Austin P. Lane, Michael J. Maher, C. Grant Willson, and Christopher J. Ellison

ACS Macro Letters 2016 Volume 5(Issue 4) pp:460

Publication Date(Web):March 21, 2016

DOI:10.1021/acsmacrolett.6b00075

Block copolymers are potentially useful materials for large-area 2-D patterning applications due to their spontaneous self-assembly into sub-50 nm domains. However, most thin film engineering applications require patterns of prescribed size, shape, and organization. Photopatterning is a logical choice for manipulating block copolymer features since advanced lithography tools can pattern areas as small as a single block copolymer domain. By exposing either the block copolymer or a responsive interfacial surface to patterned radiation, precise control over placement, orientation, alignment, and selective development of block copolymer domains can be achieved. This Viewpoint highlights some of the recent research in photopatterning block copolymer thin films and identifies areas of future opportunity.

Marangoni Instability Driven Surface Relief Grating in an Azobenzene-Containing Polymer Film

Co-reporter:Chae Bin Kim, James C. Wistrom, Heonjoo Ha, Sunshine X. Zhou, Reika Katsumata, Amanda R. Jones, Dustin W. Janes, Kevin M. Miller, and Christopher J. Ellison

Macromolecules 2016 Volume 49(Issue 18) pp:7069-7076

Publication Date(Web):September 2, 2016

DOI:10.1021/acs.macromol.6b01848

The Marangoni effect describes fluid flow near an interface in response to a surface tension gradient. Here, we demonstrate that the Marangoni effect is the underlying mechanism for flow driven feature formation in an azobenzene-containing polymer film; features formed in azobenzene-containing polymers are often referred to as surface relief gratings or SRGs. An amorphous poly(4-(acryloyloxyhexyloxy)-4′-pentylazobenzene) was synthesized and studied as a model polymer. To isolate the surface tension driven flow from the surface tension pattern inscription step, the surface tension gradient was preprogrammed via photoisomerization of azobenzene in a glassy polymer film without forming topographical features. Subsequently, the latent image was developed in the absence of light by annealing above the glass transition temperature where the polymer is a liquid. The polymer flow direction was controlled with precision by inducing different surface tension changes in the exposed regions, in accordance with expectation based on the Marangoni effect. Finally, the height of the formed features decreased upon extensive thermal annealing due to capillary leveling with two distinct rates. A scaling analysis revealed that those rates originated from dissimilar capillary velocities associated with different azobenzene isomers.

Designing Intrablock Attractions To Increase the χ Parameter of a Symmetric Diblock Copolymer

Co-reporter:Sunshine X. Zhou, Dustin W. Janes, Chae Bin Kim, C. Grant Willson, and Christopher J. Ellison

Macromolecules 2016 Volume 49(Issue 21) pp:8332-8340

Publication Date(Web):October 26, 2016

DOI:10.1021/acs.macromol.6b01382

Block copolymer (BCP) lithography is capable of forming features on the order of tens of nanometers, and this size is desirable for numerous applications, including data storage devices, microprocessors, and membranes. BCPs must be oriented to form device-relevant structures, and poly(styrene-block-methyl methacrylate) (PS–PMMA) is the most widely studied BCP due to its ability to form perpendicularly oriented features when simply heated on an energetically nonpreferential substrate. However, the smallest practical feature sizes attainable by PS–PMMA are about 11 nm. In this work, we incorporate a self-interacting monomer, vinylnaphthalene, into the styrenic block of PS–PMMA to effectively increase its Flory–Huggins interaction parameter. Introducing 35 mol % of vinylnaphthalene into the BCP more than doubled its χ parameter, resulting in a BCP structure that is capable of forming features as small as 6.3 nm. We also demonstrate that like PS–PMMA, this new poly((styrene-random-vinylnaphthalene)-block-methyl methacrylate) (PSVN–PMMA) BCP can be oriented vertically with only thermal annealing.

A comparative parameter study: Controlling fiber diameter and diameter distribution in centrifugal spinning of photocurable monomers

Co-reporter:Yichen Fang, Austin R. Dulaney, Jesse Gadley, Joao Maia, Christopher J. Ellison

Polymer 2016 Volume 88() pp:102-111

Publication Date(Web):6 April 2016

DOI:10.1016/j.polymer.2016.02.029

•Fibers with tunable diameters were produced via UV-reactive centrifugal spinning.•Factors controlling diameter in reactive and solution spinning were compared.•Average fiber diameter and diameter distribution can be tailored independently.•Crosslinked thiol-ene fibers with average diameter as small as 1.5 μm were made.In this study, the key factors for controlling the average fiber diameter and diameter distribution of fibers made via simultaneous centrifugal spinning and UV initiated polymerization are elucidated. Through systematic investigation, it was found that the average fiber diameter has a strong dependence on monomer delivery rate through the orifice, which can be intuitively linked to both the orifice diameter and monomer mixture viscosity. On the other hand, the breadth of the fiber diameter distribution can be controlled by the spin speed of the rotating spinneret. Carefully tuning these process parameters allows near independent control of average fiber diameter and its distribution, which could provide access to a widely tailorable range of fibers appropriate for different applications. Finally, under optimized process conditions, crosslinked fibers with average diameters of approximately 1.5 μm can be produced, which are one to two orders of magnitude smaller than photocured fibers fabricated in previous reports and comparable with the smallest melt blown nonwoven fibers produced commercially. Coupled with the advantages of cross-linked fibers made by in-situ photopolymerization, the capability to produce small fibers with tailored diameter distributions by centrifugal spinning could further establish this technology as a competitive alternative to existing approaches.

Synthesis and gas permeability of highly elastic poly(dimethylsiloxane)/graphene oxide composite elastomers using telechelic polymers

Co-reporter:Heonjoo Ha, Jaesung Park, KiRyong Ha, Benny D. Freeman, Christopher J. Ellison

Polymer 2016 Volume 93() pp:53-60

Publication Date(Web):14 June 2016

DOI:10.1016/j.polymer.2016.04.016

•Synthesized polymer/graphene oxide (GO) composites using telechelic polymers.•Solid, mechanically robust elastomers by incorporating less than 1 vol % of GO.•Elastomers showed ∼45% reduction in various gas permeabilities with 0.43 vol % of GO.•These materials could find application as gas barrier or separation membranes.This study illustrates that amine functional groups on the ends of telechelic poly(dimethylsiloxane) (PDMS) can undergo post-processing reactions with surface epoxy groups on graphene oxide (GO) to form a robust elastomer during simple heating. In these materials, GO acts both as a nanofiller which reinforces the mechanical properties and participates as a multifunctional crosslinker, thereby promoting elastic properties. Experiments indicate that the telechelic PDMS/GO elastomer is highly crosslinked (e.g., more than 75 wt % is a non-dissolving crosslinked gel) but highly flexible such that it can be stretched up to 300% of its original length. Finally, the PDMS/GO elastomer was tested as a single gas barrier membrane and gas permeability was decreased ∼45% by incorporating 1 wt % (0.43 vol %) of GO, thereby highlighting its potential use in practical applications.

Thermal stimuli-responsive behavior of pyrene end-functionalized PDMS through tunable interactions

Co-reporter:Heonjoo Ha;Kadhiravan Shanmuganathan;Yunping Fei

Journal of Polymer Science Part B: Polymer Physics 2016 Volume 54( Issue 2) pp:159-168

Publication Date(Web):

DOI:10.1002/polb.23805

ABSTRACT

Pyrene end-functionalized, telechelic poly(dimethyl siloxane) (PDMS) materials were synthesized and their response to different thermal stimuli was evaluated. The incorporation of pyrene end groups introduces strong π–π interactions that facilitated a broad range of thermally responsive properties, in some circumstances forming pyrene nanocrystals that serve as physical crosslinks leading to elastic materials. By synthesizing different chain lengths, samples exhibiting a 7 orders of magnitude change in storage modulus in response to thermal stimuli were produced by modifying only the end-groups (0.6 wt % of all polymer segments). Repeated thermal cycling during rheological experiments revealed that π–π interaction and crystallization/melting kinetics of pyrene chain-ends plays a key role in their thermal responsiveness. The properties of these materials were tuned by adding free pyrene, neat PDMS, or graphene oxide (GO) nanoparticles, making them attractive for many applications (e.g., tunable damping materials, heat/light sensors, conductive gels, or light repositionable adhesives). For example, nanocomposites containing 1 wt % GO caused the melting temperature for pyrene crystal domains to more than double, and even induced pyrene end-group crystallization in samples that did not exhibit crystals in neat form. It is hypothesized that these features originate from π–π interactions between pyrene ends and GO surfaces. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 159–168

Bioinspired Catecholic Flame Retardant Nanocoating for Flexible Polyurethane Foams

Co-reporter:Joon Hee Cho, Vivek Vasagar, Kadhiravan Shanmuganathan, Amanda R. Jones, Sergei Nazarenko, and Christopher J. Ellison

Chemistry of Materials 2015 Volume 27(Issue 19) pp:6784

Publication Date(Web):September 9, 2015

DOI:10.1021/acs.chemmater.5b03013

An efficient, environmentally friendly, and water-applied flame retardant surface nanocoating based on polydopamine (PDA) was developed for foamed materials such as polyurethane (PU). The PDA nanocoating, deposited by simple dip-coating in an aqueous dopamine solution, consists of a planar sublayer and a secondary granular layer structure that evolve together, eventually turning into a dense, uniform, and conformal layer on all foam surfaces. In contrast to flexible PU foams that are known to be highly flammable without flame retardant additives, micro combustion calorimetry (MCC) and thermogravimetric analysis (TGA) confirm that the neat PDA is relatively inflammable with a strong tendency to form carbonaceous, porous char that is highly advantageous for flame retardancy. By depositing nanocoatings of PDA onto flexible PU foams, the flammability of the PU foam was significantly reduced with increasing coating thickness. For the thickest coating (3 days of PDA deposition), the foam quickly self-extinguished and its original shape was completely preserved after exposure to a flame in a torch burn test. In addition to the char forming ability of PDA, it is hypothesized that its catechol units likely scavenge nearby radicals that typically evolve additional fuel for the fire as they attack surrounding materials. This multiple flame retardancy action of PDA (i.e., char formation + radical scavenging) enables flame retardant foams with a peak heat release rate (P-HRR) that is significantly reduced (up to 67%) relative to control foams, representing much better performance than many conventional additives reported in the literature at comparable or higher loadings.

Bidirectional Control of Flow in Thin Polymer Films by Photochemically Manipulating Surface Tension

Co-reporter:Chae Bin Kim, Dustin W. Janes, Sunshine X. Zhou, Austin R. Dulaney, and Christopher J. Ellison

Chemistry of Materials 2015 Volume 27(Issue 13) pp:4538

Publication Date(Web):June 3, 2015

DOI:10.1021/acs.chemmater.5b01744

The Marangoni effect causes liquids to flow toward localized regions of higher surface tension. In a thin film, such flow results in smooth thickness variations and may represent a practically useful route to manufacture topographically patterned surfaces. An especially versatile material for this application should be able to be spatially programmed to possess regions of higher or lower relative surface tension so that the direction of flow into or out of those areas could be directed with precision. To this end, we describe here a photopolymer whose melt-state surface tension can be selectively raised or lowered in the light exposed regions depending on the wavelength and dose of applied light. The direction of Marangoni flow into or out of the irradiated areas agreed with expected surface tension changes for photochemical transformations characterized by a variety of spectroscopic techniques and chromatographic experiments. The maximum film thickness variations achieved in this work are over 200 nm, which developed after only 5 min of thermal annealing. Both types of flow patterns can even be programmed sequentially into the same film and developed in a single thermal annealing step, which to our knowledge represents the first example of harnessing photochemical stimuli to bidirectionally control flow.

Mechanically Stable Thermally Crosslinked Poly(acrylic acid)/Reduced Graphene Oxide Aerogels

Co-reporter:Heonjoo Ha, Kadhiravan Shanmuganathan, and Christopher J. Ellison

ACS Applied Materials & Interfaces 2015 Volume 7(Issue 11) pp:6220

Publication Date(Web):February 25, 2015

DOI:10.1021/acsami.5b00407

Graphene oxide (GO) aerogels, high porosity (>99%) low density (∼3–10 mg cm–3) porous materials with GO pore walls, are particularly attractive due to their lightweight, high surface area, and potential use in environmental remediation, superhydrophobic and superoleophilic materials, energy storage, etc. However, pure GO aerogels are generally weak and delicate which complicates their handling and potentially limits their commercial implementation. The focus of this work was to synthesize highly elastic, mechanically stable aerogels that are robust and easy to handle without substantially sacrificing their high porosity or low density. To overcome this challenge, a small amount of readily available and thermally cross-linkable poly(acrylic acid) (PAA) was intermixed with GO to enhance the mechanical integrity of the aerogel without disrupting other desirable characteristic properties. This method is a simple straightforward procedure that does not include multistep or complicated chemical reactions, and it produces aerogels with mass densities of about 4–6 mg cm–3 and >99.6% porosity that can reversibly support up to 10 000 times their weight with full recovery of their original volume. Finally, pressure sensing capabilities were demonstrated and their oil absorption capacities were measured to be around 120 g oil per g aerogel–1 which highlights their potential use in practical applications.Keywords: aerogels; environmental remediation; graphene; nanocomposites; poly(acrylic acid)

Water-Based Melanin Multilayer Thin Films with Broadband UV Absorption

Co-reporter:Tyler Guin, Joon Hee Cho, Fangming Xiang, Christopher J. Ellison, and Jaime C. Grunlan

ACS Macro Letters 2015 Volume 4(Issue 3) pp:335

Publication Date(Web):March 6, 2015

DOI:10.1021/acsmacrolett.5b00080

Natural melanin is difficult to process due to its poor solubility and poorly understood structure. Synthetic melanin has been produced more recently, which is dispersible in mildly alkaline water and has many of the same properties of natural melanin. In this study, thin films of synthetic melanin and poly(allylamine hydrochloride) were deposited layer-by-layer from dilute aqueous solutions in ambient conditions. This is likely the first time melanin has been deposited from water to produce a functional nanocoating. These films display broadband UV light absorption, absorbing over 63% of incident light that is most damaging to human eyes with a thickness of 108 nm. In an effort to demonstrate the utility of these melanin-based nanocoatings, a 30 bilayer film is shown to increase the useful life of a conductive poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) film by 550%. This novel method of depositing melanin should open the door to a variety of useful applications.

Manipulating characteristic timescales and fiber morphology in simultaneous centrifugal spinning and photopolymerization

Co-reporter:Yichen Fang, Austin D. Dulaney, Jesse Gadley, Joao M. Maia, Christopher J. Ellison

Polymer 2015 Volume 73() pp:42-51

Publication Date(Web):2 September 2015

DOI:10.1016/j.polymer.2015.07.020

•Fibers were fabricated using in-situ photopolymerization and centrifugal spinning.•Material and process parameters crucial for fiber morphologies were identified.•Timescales associated with various morphologies were quantified.•A predictive operating diagram was developed for this reactive spinning system.In this study, the fabrication of crosslinked nonwoven fibers via simultaneous thiol-ene photopolymerization and spinning of monomer jets has been demonstrated in centrifugal Forcespinning for the first time. We observed that simultaneous Forcespinning and photopolymerization resulted in a wide variety of fiber morphologies including beads, beads-on-string, uniform fiber, fused fibers, and well-cured fibers. To elucidate the underlying mechanisms and parameter interactions that give rise to these morphologies, we systematically varied the light intensity, solution elasticity, and spin speed of the Forcespinning process. From these experimental results, an operating diagram was constructed based on the measured process parameters, their respective timescales, and observed effects on fiber morphology. While numerous parameters can individually affect fiber formation and morphology, the interplay between curing kinetics, solution viscoelasticity, and orifice-to-collector processing time window is also crucial in this process. Smooth and well-cured fibers were formed only when the photopolymerization occurred sufficiently quickly, before both the breakup of fibers into droplets due to a surface tension driven Rayleigh instability and the deposition of fibers on the collector. Our findings can serve as a predictive guideline for the fabrication of crosslinked fibers with desired morphology, the implementation of the in-situ polymerization and spinning concept into other commercial fiber manufacturing technologies, and the adaptation of other functional or high performance monomer systems.

Nanocomposites from styrene–butadiene rubber (SBR) and multiwall carbon nanotubes (MWCNT) part 2: Mechanical properties

Co-reporter:S.K. Peddini, C.P. Bosnyak, N.M. Henderson, C.J. Ellison, D.R. Paul

Polymer 2015 Volume 56() pp:443-451

Publication Date(Web):15 January 2015

DOI:10.1016/j.polymer.2014.11.006

•Uncoiling of curved discreet MWCNT resulted in higher values of elongation and tensile stresses at break.•Enhanced tear energy for MWCNT filled SBR-CB composites is due to crack bridging mechanism.•Good interfacial adhesion between MWCNT and SBR caused the reduction in swelling ratios in toluene.Due to their high aspect ratio, strength, and modulus, multiwall carbon nanotubes (MWCNT) have attracted interest as a reinforcing filler in the automotive tire industry. In part 1 of this study, we demonstrated that styrene–butadiene rubber (SBR) composites containing up to 15 wt. % of well-dispersed, discreet MWCNTs can be prepared using MWCNTs with a specific surface modification and controlled aspect ratios. The melt rheology of the composites with discreet MWCNT was best described in terms of an effective aspect ratio and by considering the discrete MWCNT to be flexible rather than rigid rods. In this work, the effect of tensile strains, up to values of 6, for cured SBR composites containing discreet MWCNT concentrations up to 12% by weight were investigated. The deformation behavior indicates good adhesion between these MWCNT and the SBR. Mooney–Rivlin plots derived from the composite tensile stress–strain data displayed a dramatic change in mechanical behavior as the MWCNT loading exceeded about 5 wt. % attributed to a combined reinforcing effect of tubes on SBR plus overlap of curved or coiled MWCNT. Beyond tensile strains of about 1.7, strain hardening increases dramatically at MWCNT loading greater than 5 wt. % that is attributed straightening of the initially curved nanotubes such that they behave as rigid rods or fibers. Mechanical hysteresis and swelling in toluene on cured composites samples revealed that MWCNTs are in fact well bonded to the SBR. Studies with SBR and a combination of carbon black and discreet MWCNT demonstrate dramatically improved resistance to fracture by tearing.

Modulating Solubility and Enhancing Reactivity of Photo-Cross-Linkable Poly(styrene sulfonyl azide-alt-maleic anhydride) Thin Films

Co-reporter:Dustin W. Janes, Michael J. Maher, Gregory T. Carroll, David M. Saylor, and Christopher J. Ellison

Macromolecules 2015 Volume 48(Issue 22) pp:8361-8368

Publication Date(Web):November 3, 2015

DOI:10.1021/acs.macromol.5b01875

To formalize our understanding of indiscriminate grafting chemistries as they pertain to cross-linkable polymers and emerging patterning technologies, we designed a new polymer, poly(styrene sulfonyl azide-alt-maleic anhydride) (PSSMA). By modulating its solubility, it can be deposited into smooth, ultrathin films atop polar and nonpolar polymers. Upon heating above 120 °C or exposure to UV light, highly reactive nitrene intermediates are generated from the azide groups which form covalent adducts and cross-link the PSSMA. Azide photolysis and polymer gelation were studied in the context of a statistical model to gain insight into the network outcomes of nitrenes in a polymer film. For every azide group converted to a nitrene in ambient atmosphere, it has an 11% likelihood of grafting to another chain and a 5% chance of causing a scission. These values can be increased over 3-fold by reducing the O2 content by 85%. Alternatively, the effects of quenching by ground-state O2 can be mitigated by adding Michler’s ketone (MK) to the film. PSSMA/MK blend films possess a 39% (±13) likelihood for grafting and 29% (±10) for scission. The higher ratio of scission to grafting is a consequence of the sensitized azides producing triplet-state nitrenes, which favor hydrogen abstraction. These broadly generalizable considerations will be useful to others who wish to maximize light sensitivity in related polymer systems.

Highly Stretchable Thermoset Fibers and Nonwovens Using Thiol–ene Photopolymerization

Co-reporter:Kadhiravan Shanmuganathan, Steven M. Elliot, Austin P. Lane, and Christopher J. Ellison

ACS Applied Materials & Interfaces 2014 Volume 6(Issue 16) pp:14259

Publication Date(Web):July 30, 2014

DOI:10.1021/am503563q

In this report, we describe the preparation and characterization of a new class of thermoset fibers with high elongation and elastic recovery. Integrating UV-activated thiol–ene photopolymerization and electrospinning, we demonstrate an environmentally friendly single step approach to convert small monomeric precursor molecules into highly elastic fibers and nonwoven mats. The fibers were derived by in situ photopolymerization of a trifunctional vinyl ether monomer and a tetrafunctional thiol. Although thermosets often offer good chemical and thermal stability, these fibers also have a high average elongation at break of 62%. The elastomeric nature of these vinyl-ether based fibers can be partly attributed to their subambient Tg and partly to the cross-link density, monomer structure, and resulting network homogeneity. Nonwoven mats of these fibers were also stretchable and exhibited a much higher elongation at break of about 85%. These thermoset stretchable fibers could have potential applications as textile, biomedical, hot chemical filtration, and composite materials.Keywords: elastomers; electrospinning; fiber; photopolymerization; thermoset; thiol−ene

Synthesis of Amphiphilic Naturally-Derived Oligosaccharide-block-Wax Oligomers and Their Self-Assembly

Co-reporter:Julia D. Cushen, Kadhiravan Shanmuganathan, Dustin W. Janes, C. Grant Willson, and Christopher J. Ellison

ACS Macro Letters 2014 Volume 3(Issue 9) pp:839

Publication Date(Web):August 11, 2014

DOI:10.1021/mz500389g

Self-assembly characteristics of amphiphilic macromolecules into micelles, nanoparticles and vesicles has been of fundamental interest for many applications including designed nanoscale therapeutic delivery systems and enzymatic reactors. In this work, a class of amphiphilic block oligomers was synthesized from naturally occurring oligosaccharides and aliphatic alcohol precursors, which are all currently prominent in the pharmaceutical, food, and supplement industries. These block oligomer materials were synthesized by functionalization of the precursor materials followed by subsequent coupling by azide–alkyne cycloaddition and their bulk self-assembly was investigated after solvent vapor annealing. Self-assembly of the amphiphilic materials into liposomes in aqueous solution was also investigated after preparing solutions using a nanoprecipitation method. Encapsulation of hydrophobic components was demonstrated and verified using dynamic light scattering, transmission electron microscopy, and fluorescence spectroscopy experiments.

Multilayer coextrusion of rheologically modified main chain liquid crystalline polymers and resulting orientational order

Co-reporter:Zhenpeng Li, Zheng Zhou, Shannon R. Armstrong, Eric Baer, Donald R. Paul, Christopher J. Ellison

Polymer 2014 Volume 55(Issue 19) pp:4966-4975

Publication Date(Web):15 September 2014

DOI:10.1016/j.polymer.2014.08.009

•Modified the rheological properties of a liquid crystalline polymer (LCP).•Successfully fabricated high quality LCP multilayer films through coextrusion.•LCP chains were perpendicularly aligned to the extrusion direction.Liquid crystalline polymers (LCP) are attractive candidates for use as barrier films in packaging and high performance membrane applications due to their extraordinary barrier properties and chemical resistance. However, LCPs usually display extremely low viscosities, which makes processing them into multilayer films using coextrusion a challenge due to the mismatch of their viscosities with other film components. In this study, a commercial grade diepoxy reagent (Heloxy 67) was used to modify the rheological properties of a thermotropic main chain LCP. The effects of diepoxy concentration and reaction time on the melt viscosity and liquid crystal structures of the LCP were investigated. The addition of small amounts of diepoxy (e.g., 1.5 wt. %) increased the viscosity of the LCP nearly 15-fold. However, addition of excessive diepoxy (e.g., 2 wt. % or greater) led to cross-linking of the LCP. More importantly, these modified samples were thermally stable when melt reprocessed at temperatures of up to 250 °C in air. High quality multilayer films were prepared by coextruding the rheologically modified LCP with polypropylene-graft-maleic anhydride. The orientational order of the LCP in the multilayer films was studied by polarized infrared spectroscopy as well as x-ray diffraction. Interestingly, instead of aligning along the shear direction, the LCP chains tended to orient perpendicular to the extrusion direction, presumably due to a ‘log-rolling’ effect during processing.

Tin fluorophosphate nonwovens by melt state centrifugal Forcespinning

Co-reporter:Yichen Fang;Matthew Herbert;David A. Schiraldi

Journal of Materials Science 2014 Volume 49( Issue 24) pp:8252-8260

Publication Date(Web):2014 December

DOI:10.1007/s10853-014-8534-3

This report describes the direct melt processing of inorganic tin fluorophosphate (TFP) glass fibers with average diameters ranging from 2 to 4 µm via centrifugal Forcespinning. This was accomplished by using a TFP glass with low glass transition temperature (Tg) and the melt processing capability of Forcespinning. The thermal behavior of TFP glass fibers was investigated by differential scanning calorimetry and thermogravimetric analysis, while the compositional evolution of the fibers was studied using energy-dispersive spectrometry and Fourier-transform infrared spectroscopy. These fibers exhibited excellent thermal stability after thermal post-treatment at 300 °C. The Tg of the thermally treated fibers increased by 100 °C compared to the bulk material. The fibers were found to undergo dehydration and loss of fluorine during thermal treatment, resulting in a rigid and crosslinked glass network with enhanced thermal stability and increased Tg. The enhanced thermal stability demonstrated the potential of TFP fibers for high temperature catalysis and chemical filtration applications.

A Photochemical Approach to Directing Flow and Stabilizing Topography in Polymer Films

Co-reporter:Joshua M. Katzenstein, Chae Bin Kim, Nathan A. Prisco, Reika Katsumata, Zhenpeng Li, Dustin W. Janes, Gregory Blachut, and Christopher J. Ellison

Macromolecules 2014 Volume 47(Issue 19) pp:6804-6812

Publication Date(Web):September 17, 2014

DOI:10.1021/ma5010698

Coatings and substrates with topographically patterned features will play an important role in efficient technologies for harvesting and transmitting light energy. In order to address these applications, a methodology for prescribing height profiles in polymer films is presented here. This is accomplished by photochemcially patterning a solid-state, sensitized polymer film. After heating the film above its glass transition temperature, melt-state flow is triggered and directed by the chemical pattern. A second light exposure was applied to fully activate a heat-stable photo-crosslinking additive. The features formed here are thermochemically stable and can act as an underlayer in a multilayered film. To exemplify this capability, these films were also used to direct the macroscopic film morphology of a block copolymer overlayer.

Surface energy gradient driven convection for generating nanoscale and microscale patterned polymer films using photosensitizers

Co-reporter:Chae Bin Kim;Dustin W. Janes;Dana L. McGuffin

Journal of Polymer Science Part B: Polymer Physics 2014 Volume 52( Issue 18) pp:1195-1202

Publication Date(Web):

DOI:10.1002/polb.23546

ABSTRACT

The Marangoni effect describes how fluid flows in response to gradients in surface energy. This phenomenon could be broadly harnessed to pattern the surface topography of polymer films if generalizable techniques for programming surface energy gradients existed. Here, a near UV–visible light (NUV–vis) photosensitizer, 9,10-dibromo-anthracene (DBA), was doped into thin films of a model polymer, poly(isobutyl methacrylate). After exposure to light through a photomask and heating above the glass transition, thermolysis of photo-oxidized DBA and grafting to the polymer promoted flow of the film material into the exposed regions. This mechanism did not significantly alter the molecular weight of PiBMA or the film's glass transition temperature, but resulted in an increase in film surface energy as indicated by a decrease in water contact angle. Film height variations of 580 nm were produced using a mask with 12.5 μm features; a mask with 800 nm features was also employed to generate topographic features of corresponding width without expensive contacting equipment. Due to the broad absorbance spectra of DBA, highly accessible and/or unconventional light sources may be employed in this process; this advantage was demonstrated by patterning with sunlight. The nonspecific radical-mediated nature of the DBA grafting reaction makes this a promising approach for many classes of polymers. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1195–1202

Ordering poly(trimethylsilyl styrene-block-D,L-lactide) block copolymers in thin films by solvent annealing using a mixture of domain-selective solvents

Co-reporter:Julia D. Cushen;Lei Wan;Gunja Pav;Indranil Mitra;Gila E. Stein;Venkat Ganesan;Ricardo Ruiz;C. Grant Willson

Journal of Polymer Science Part B: Polymer Physics 2014 Volume 52( Issue 1) pp:

Publication Date(Web):

DOI:10.1002/polb.23408

ABSTRACT

Controlling the morphology, domain orientation, and domain size of block copolymer (BCP) thin films is desirable for many applications in nanotechnology. These properties can be tuned during solvent annealing by varying the solvent choice and degree of swelling which affect the effective miscibility and volume fraction of the BCP domains. In this work, we demonstrate with a bulk lamellae-forming BCP, poly(4-trimethylsilylstyrene-block-D,L-lactide) (PTMSS-b-PLA), that varying the composition of a mixture of solvent vapors containing cyclohexane (PTMSS-selective) and acetone (PLA-selective), enables formation of perpendicularly oriented lamellae with sub-20-nm pitch lines. The BCP domain periodicity was also observed to increase by 30%, compared to bulk, following solvent annealing. Furthermore, solvent annealing alone is shown to induce a transition from a disordered to an ordered BCP. We rationalize our observations by hypothesizing that the use of a combination of domain selective solvent mixtures serves to increase the effective repulsion between the blocks of the copolymer. We furnish results from self-consistent field theory calculations to support the proposed mechanism. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 36–45

Bioinspired Catecholic Copolymers for Antifouling Surface Coatings

Co-reporter:Joon Hee Cho, Kadhiravan Shanmuganathan, and Christopher J. Ellison

ACS Applied Materials & Interfaces 2013 Volume 5(Issue 9) pp:3794

Publication Date(Web):April 1, 2013

DOI:10.1021/am400455p

We report here a synthetic approach to prepare poly(methyl methacrylate)-polydopamine diblock (PMMA-PDA) and triblock (PDA-PMMA-PDA) copolymers combining mussel-inspired catecholic oxidative chemistry and atom transfer radical polymerization (ATRP). These copolymers display very good solubility in a range of organic solvents and also a broad band photo absorbance that increases with increasing PDA content in the copolymer. Spin-cast thin films of the copolymer were stable in water and showed a sharp reduction (by up to 50%) in protein adsorption compared to those of neat PMMA. Also the peak decomposition temperature of the copolymers was up to 43°C higher than neat PMMA. The enhanced solvent processability, thermal stability and low protein adsorption characteristics of this copolymer makes it attractive for variety of applications including antifouling coatings on large surfaces such as ship hulls, buoys, and wave energy converters.Keywords: antifouling; coatings; melanin; PMMA; polydopamine;

Light-Activated Replication of Block Copolymer Fingerprint Patterns

Co-reporter:Dustin W. Janes, Christopher J. Thode, C. Grant Willson, Paul F. Nealey, and Christopher J. Ellison

Macromolecules 2013 Volume 46(Issue 11) pp:4510-4519

Publication Date(Web):May 16, 2013

DOI:10.1021/ma400065t

A strategy to replicate fingerprint patterns formed by the self-assembly of lamella-forming block copolymer (BCP) was investigated. To accomplish this, liquid conformal layers were placed between the surfaces of a “master” BCP film and a transparent “replica” substrate that solidified and covalently bonded to the BCP upon exposure to light. The benzophenone-containing conformal layer enabled pattern replication over areas limited only by the size of the samples and exposure field. The replication step is light activated, occurs below the glass transition of the BCP, and takes less than 1 h. This demonstration used a poly(styrene-b-methyl methacrylate) BCP with a bulk domain periodicity of 42 nm, but it is possible that the chemistry may be generalized to many other BCPs. Control experiments conducted with alternative conformal layer compositions indicate that interfacial photosensitization of the BCP by excited benzophenone, followed by propagation to residual acrylate groups present in the conformal layer, is the primary mechanism by which pattern replication takes place.

Directing convection to pattern thin polymer films

Co-reporter:Dustin W. Janes;Joshua M. Katzenstein;Kadhiravan Shanmuganathan

Journal of Polymer Science Part B: Polymer Physics 2013 Volume 51( Issue 7) pp:535-545

Publication Date(Web):

DOI:10.1002/polb.23262

Abstract

Convection can be harnessed in elegant ways to pattern surfaces, often using uncomplicated equipment and materials, providing an interesting platform for future technological developments in thin film topographic assemblies. This manuscript contains a brief review of thin polymer film patterning methods that rely on directing convection, such as “coffee ring” patterning, lithographically induced self-assembly, and electrohydrodynamic patterning. These techniques are described in the context of a recent approach explored in our group for generating topographic patterns by photochemically directing Marangoni flow in thin polymer films with subtle gradients in surface energy. Aspects unique to this process are highlighted so that they may facilitate new developments in manufacturing technologically impactful patterned surfaces. For example, the features produced by photochemically directed Marangoni-driven flow are preprogrammed in the solid state, thermally developed and can be formed in multilayer films. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

Patterning by Photochemically Directing the Marangoni Effect

Co-reporter:Joshua M. Katzenstein, Dustin W. Janes, Julia D. Cushen, Nikhil B. Hira, Dana L. McGuffin, Nathan A. Prisco, and Christopher J. Ellison

ACS Macro Letters 2012 Volume 1(Issue 10) pp:1150

Publication Date(Web):September 10, 2012

DOI:10.1021/mz300400p

Polystyrene (PS) that has been exposed to ultraviolet light (UV) undergoes partial dehydrogenation of the alkane polymer backbone which increases its surface energy. Exploiting this photochemistry, we exposed polystyrene films to UV light using a photomask to induce a patterned photochemical reaction producing regions in the film with differing surface energy. Upon heating the solid polymer film with the preprogrammed surface energy pattern to a liquid state, the polymer flows from the low surface energy unexposed regions to high surface energy exposed regions. This flow creates three-dimensional topography by the Marangoni Effect, which describes convective mass transfer due to surface energy gradients. The topographical features can be permanently preserved by quenching the film below its glass to liquid transition temperature. Their shape and organization are only limited by the pattern on the photomask.

Soybean Oil Based Fibers Made Without Solvent or Heat

Co-reporter:Dustin W. Janes, Kadhiravan Shanmuganathan, Daniel Y. Chou, and Christopher J. Ellison

ACS Macro Letters 2012 Volume 1(Issue 9) pp:1138

Publication Date(Web):September 4, 2012

DOI:10.1021/mz300325g

Thiol–ene chemistry was harnessed to enable production of thermochemically stable thermoset fibers containing 50–87 wt % acrylated epoxidized soybean oil and 49–72% biobased carbon without using solvent or heat. In this demonstration, the fibers were made by simultaneous electrospinning and photocuring of a liquid monomer mixture, which could be translated to other fiber manufacturing processes such as melt blowing or Forcespinning. Scanning electron micrographs illustrate the fiber quality and an average diameter of about 30 μm. Photochemical conversion kinetics of functional groups during light exposure were measured by real-time Fourier transform infrared spectroscopy, providing insight into the advantages of using high-functionality monomers and thiol–ene chemistry in this application.

Solventless High Throughput Manufacturing of Poly(butylene terephthalate) Nanofibers

Co-reporter:Kadhiravan Shanmuganathan, Yichen Fang, Daniel Y. Chou, Sarah Sparks, Jarett Hibbert, and Christopher J. Ellison

ACS Macro Letters 2012 Volume 1(Issue 8) pp:960

Publication Date(Web):July 13, 2012

DOI:10.1021/mz3001995

Nanofibers possess high surface area to volume ratios and are particularly attractive for a variety of applications including tissue regeneration, drug delivery, fiber-reinforced composites, filtration, and protective clothing. Though the production of nanofibers from common thermoplastic polymers is relatively well-demonstrated, processing constraints have limited high throughput manufacturing of nanofibers from high performance polymers. This has in turn limited broad technological exploitation of polymer nanofibers in areas such as hot chemical filtration or high-performance lightweight composites for aerospace and defense applications. We report here that nanofibers can be produced in a solventless high throughput process from polymers such as poly(butylene terephthalate) (PBT) using a newly developed technology termed “Forcespinning” that employs centrifugal force to attenuate fibers. Our investigations also show that these nanofibers have a high crystallinity and enhanced molecular orientation which is important for realizing desirable physical and chemical properties of many high-performance polymer fibers.

Oligosaccharide/Silicon-Containing Block Copolymers with 5 nm Features for Lithographic Applications

Co-reporter:Julia D. Cushen, Issei Otsuka, Christopher M. Bates, Sami Halila, Sébastien Fort, Cyrille Rochas, Jeffrey A. Easley, Erica L. Rausch, Anthony Thio, Redouane Borsali, C. Grant Willson, and Christopher J. Ellison

ACS Nano 2012 Volume 6(Issue 4) pp:3424

Publication Date(Web):March 28, 2012

DOI:10.1021/nn300459r

Block copolymers demonstrate potential for use in next-generation lithography due to their ability to self-assemble into well-ordered periodic arrays on the 3–100 nm length scale. The successful lithographic application of block copolymers relies on three critical conditions being met: high Flory–Huggins interaction parameters (χ), which enable formation of <10 nm features, etch selectivity between blocks for facile pattern transfer, and thin film self-assembly control. The present paper describes the synthesis and self-assembly of block copolymers composed of naturally derived oligosaccharides coupled to a silicon-containing polystyrene derivative synthesized by activators regenerated by electron transfer atom transfer radical polymerization. The block copolymers have a large χ and a low degree of polymerization (N) enabling formation of 5 nm feature diameters, incorporate silicon in one block for oxygen reactive ion etch contrast, and exhibit bulk and thin film self-assembly of hexagonally packed cylinders facilitated by a combination of spin coating and solvent annealing techniques. As observed by small angle X-ray scattering and atomic force microscopy, these materials exhibit some of the smallest block copolymer features in the bulk and in thin films reported to date.Keywords: block copolymer; lithography; nanopatterning; oligosaccharide; poly(trimethylsilyl styrene); thin films

Modification of rheological properties of a thermotropic liquid crystalline polymer by melt-state reactive processing

Co-reporter:Zhenpeng Li, Paola A. Gonzalez Garza, Eric Baer, Christopher J. Ellison

Polymer 2012 Volume 53(Issue 15) pp:3245-3252

Publication Date(Web):6 July 2012

DOI:10.1016/j.polymer.2012.05.008

Thermotropic main-chain liquid crystalline polymers typically have very low melt viscosity with strong temperature dependence compared to other common thermoplastics. While this is beneficial in some processing applications, such as injection molding, it presents challenges for others, such as coextrusion. In this study, the rheological properties of a thermotropic main-chain liquid crystalline polymer (Vectra A950) were enhanced by melt-state reactive processing with triphenyl phosphite (TPP), which can react with up to three polymer chain-ends through their chain-end functionalities. The influence of processing time and TPP content on the shear viscosity and other important material properties were investigated. Optimal conditions, which increased the shear viscosity by nearly a factor of 20 over the neat polymer, were found to be 4 wt% TPP and 30 min of reaction time at 290 °C. Further results from differential scanning calorimetry, wide-angle X-ray diffraction and polarized optical microscopy confirmed that coupling with TPP did not affect the microstructure, melting/crystallization behavior or liquid crystallinity. The stability of TPP-modified samples was also studied at 80 °C in air and following melt reprocessing at 290–300 °C under N2 or air. Samples were stable (as measured by shear viscosity) for more than one month at 80 °C in air or when reprocessed in N2 at 290 °C for up to 10 min. However, when reprocessed at 300 °C in air, the viscosity enhancement was partially reversed due to scission of P–O bonds that were formed during the initial reaction between the polymer chain-ends and TPP.Graphical abstract

Nanoconfined Self-Diffusion of Poly(isobutyl methacrylate) in Films with a Thickness-Independent Glass Transition

Co-reporter:Joshua M. Katzenstein, Dustin W. Janes, Haley E. Hocker, Justin K. Chandler, and Christopher J. Ellison

Macromolecules 2012 Volume 45(Issue 3) pp:1544-1552

Publication Date(Web):January 23, 2012

DOI:10.1021/ma202362j

Even though the physics of nanoconfined polymers have been extensively studied for years, diffusion of polymer chains along confining interfaces has not been widely studied, likely because there are few experimental techniques available for these measurements. Here a fluorescence recovery after patterned photobleaching (FRAPP) technique is developed using an epifluorescence microscope that allows for direct, in situ, visualization of polymer diffusion over several periods of a photobleached array. This visualization approach is more robust compared to measuring fluorescence intensity alone and also significantly increases the experimental throughput. Using this technique, self-diffusion of poly(isobutyl methacrylate) (PiBMA) was investigated at 80 °C (29 °C above the glass transition temperature, Tg) and was found to be film thickness independent down to 30 nm (∼14Rg, where Rg is the radius of gyration) with a diffusion coefficient well predicted by the Rouse model (1.05 × 10–12 cm2/s). PiBMA is an ideal polymer for this study because it exhibits a film thickness-independent Tg down to 15 nm (∼7Rg) as measured by spectroscopic ellipsometry. Since the diffusion coefficient of polymers depends strongly on the proximity of diffusion temperature to Tg, this attribute allows a straightforward measure of nanoconfined diffusion without superimposed influence from Tg nanoconfinement effects.

Thin Film Self-Assembly of Poly(trimethylsilylstyrene-b-d,l-lactide) with Sub-10 nm Domains

Co-reporter:Julia D. Cushen, Christopher M. Bates, Erica L. Rausch, Leon M. Dean, Sunshine X. Zhou, C. Grant Willson, and Christopher J. Ellison

Macromolecules 2012 Volume 45(Issue 21) pp:8722-8728

Publication Date(Web):2017-2-22

DOI:10.1021/ma301238j

Integrating block copolymer self-assembly with existing lithography processes to enhance their patterning capability is a promising approach for manufacturing a variety of semiconductor devices and next-generation magnetic storage media. Sub-10 nm block copolymer domains are specifically targeted in many of these applications, yet there are relatively few block copolymers that can achieve these dimensions. Here the synthesis and self-assembly characteristics of a new block copolymer poly(trimethylsilylstyrene-b-d,l-lactide) (PTMSS-b-PLA) capable of forming domains as small as ∼5 nm are described. Several lamellar and cylinder forming diblocks were synthesized with bulk domain periodicities of 12–15 nm which are among the smallest domains yet reported for any neat block copolymer. Such small domains are possible because this new material has a large segment–segment interaction parameter which is an order of magnitude higher than poly(styrene-b-methyl methacrylate) (PS-b-PMMA) and twice as large as poly(styrene-b-dimethylsiloxane) (PS-b-PDMS), two commonly studied polymers for these applications. Furthermore, the PTMSS-b-PLA blocks have glass transitions well above room temperature with a large reactive ion etch rate contrast between them (∼28) which is at least 4 times greater than PS-b-PMMA due to incorporation of a trimethylsilyl group into the styrene monomer.

Thiol–Ene Chemistry: A Greener Approach to Making Chemically and Thermally Stable Fibers

Co-reporter:Kadhiravan Shanmuganathan, Robert K. Sankhagowit, Prashanth Iyer, and Christopher J. Ellison

Chemistry of Materials 2011 Volume 23(Issue 21) pp:4726

Publication Date(Web):October 12, 2011

DOI:10.1021/cm2015093

Fibers of micrometer and submicrometer diameters have been of significant interest in recent years owing to their advanced applications in diverse fields such as optoelectronics, regenerative medicine, piezoelectrics, ceramic materials, etc. There are a number of processes to make thin fibers including electrospinning, melt blowing, and recently developed Forcespinning. However, use of solvents or heat to lower viscosity for processing is common to all existing polymer fiber manufacturing methods, and a greener approach to making fibers remains a challenge. Interestingly, nature has engineered spiders and silkworms with a benign way of making mechanically strong and tough fibers through an intricate self-assembly of protein constituents during the fiber formation process. Comprehending the biosynthetic process and precisely replicating it has been a challenging task. However, we find that extruding small functional segments into solid fibrillar structures, through mediation of chemical interactions between the subunits, is a design approach that can be broadly adapted from nature to realize a greener fiber manufacturing process. Using the robust chemistry of thiol–ene photopolymerization, we demonstrate here that a photocurable mixture of a multifunctional acrylate, a tetrafunctional thiol, and a photoinitiator can be processed into continuous fibers by in situ photopolymerization during electrospinning under ambient conditions. The fibers are mechanically robust and have excellent chemical and thermal stability. While electrospinning has been used to demonstrate this concept, the chemistry could be broadly adapted into other fiber manufacturing methods to produce fibers without using solvents or heat.Keywords: electrospinning; fibers; green chemistry; photopolymerization; silk; thiol−ene;

Polymeric Cross-Linked Surface Treatments for Controlling Block Copolymer Orientation in Thin Films

Co-reporter:Christopher M. Bates, Jeffrey R. Strahan, Logan J. Santos, Brennen K. Mueller, Benjamin O. Bamgbade, Jonathan A. Lee, Joshua M. Katzenstein, Christopher J. Ellison, and C. Grant Willson

Langmuir 2011 Volume 27(Issue 5) pp:2000-2006

Publication Date(Web):January 7, 2011

DOI:10.1021/la1042958

The orientation of cylinder-forming poly(styrene-block-methyl methacrylate) [P(S-b-MMA)] was investigated on two sets of polymeric surface treatments: 10 para-substituted polystyrene derivatives with <10 mol % poly(4-vinylbenzyl azide) and a series of poly(styrene-random-4-vinylbenzyl azide) [P(S-r-VBzAz)] copolymers with 5−100 mol % poly(4-vinylbenzyl azide). The copolymers were spin-coated to form thin films and then cross-linked by heating. The resulting films exhibited a range of surface tensions from 21 to 45 dyn/cm. Perpendicular orientation of P(S-b-MMA) cylinders was achieved with poly(p-bromostyrene) and all the [P(S-r-VBzAz)] copolymer surface treatments, most notably the homopolymer of poly(4-vinylbenzyl azide). Films made from these simple copolymers are as effective as random terpolymer alignment layers commonly made from both block monomers and a cross-linkable monomer.

Thermooxidative Stabilization of Polymers Using Natural and Synthetic Melanins

Co-reporter:Kadhiravan Shanmuganathan, Joon Hee Cho, Prashanth Iyer, Steven Baranowitz, and Christopher J. Ellison

Macromolecules 2011 Volume 44(Issue 24) pp:9499-9507

Publication Date(Web):November 22, 2011

DOI:10.1021/ma202170n

Melanin is a biopolymer well-known for its intriguing chemical structure and physiological functions including photoprotection, radical scavenging, and metal-ion chelation. Although it has a suite of properties not common to many known organic materials, efforts to exploit those properties in technologically relevant materials have been few compared to other biopolymers such as cellulose, chitin, or collagen. Besides its natural presence in many animals including humans, melanin is also commonly consumed by humans in soups, sauces, and pastas and is widely available in large quantities from a variety of natural sources, suggesting it could serve as a nontoxic additive for enhancing the properties of common polymers. To this end, we report for the first time the potential of natural and synthetic melanins as thermal stabilizers for common polymers by evaluating the addition of melanin to several model polymers with well-known degradation pathways. When added to poly(methyl methacrylate) (PMMA) in very low amounts (0.5–5 wt %), synthetic melanin-like polymers significantly altered the radical initiated chain scission behavior of PMMA and caused a dramatic increase (by about 50–90 °C) in its onset decomposition temperature in both inert and air atmospheres. Moreover, PMMA samples with up to 1 wt % melanin achieved nearly the maximum enhancement level yet retained more than 80% light transmission from 350 to 800 nm in 100 μm thick films. Natural melanin extracted from the ink sac of Sepia officinalis (commonly known as cuttlefish) also displayed significant thermal stabilization effects on PMMA and polypropylene at similar loadings. From molecular weight characterization studies, the associated delay in the molecular weight decrease of PMMA and other polymers at elevated temperature could be potentially beneficial for high-temperature processing or increasing their upper use temperature in demanding applications. It is likely that the thermal stabilization benefits of melanin could be realized in many polymers due to the diversity of its known radical scavenging capabilities in both living systems and the polymers presented in this article. Since natural and synthetic melanin additives are macromolecules, they are also less likely to leach from the base polymer in the same way that small molecule additives often do.