•TGA method was developed to accurately determine the imidization process.•The interplay between the solvent evaporation and the imidization was clarified.•An optimized condition was selected to fabricate high-quality polyimide films.•The methods developed here can be used to investigate other polyimide systems.The thermal imidization process of 1,2,4,5-benzenetetracaboxylic (PMDA)/4,4’-diamnodiphenyl ether (4,4′-ODA) solutions in N-methyl-2-pyrrolidone (NMP) from polyimide (PI) precursor, poly (amic acid) (PAA), was systematically investigated through a series of characterization methods, such as Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). At the low-temperature drying stage, mechanical properties and Tg of polymer films were increased as a result of solvent removal. TGA method was used to quantitatively monitor the evaporations of hydrogen-bonded solvents on PAA and of the dehydration during the imidization reaction that occurred above 150 °C. During the imidization stage, the interplay between the solvent evaporation and the imidization was the key factor that determined the enhancement of the mechanical properties and Tg. The degree of imidization approached 94% when the temperature was increased to 250 °C. A “complete” imidization was achieved by annealing between 350 and 400 °C. The enhancement in the mechanical properties of final PI films may result from the increased Tg and the crystallized structure formed during the annealing stage.

•Self-healing PDMS elastomers based on acylhydrazone groups were prepared.•A reversible transition was observed around 80 °C.•Role of H-bonds on self-healing properties was elucidated.A PDMS elastomer based on acylhydrazone groups with both acid- and heat-assisted self-healing properties was successfully prepared from tetra-acylhydrazine-terminated PDMS and terephthalaldehyde through solution casting. The good healing performance was obtained with catalytic acetic acid for 24 h at 25 °C or by annealing at 120 °C for 2 h. The elastomer exhibited a reversible transition near 80 °C observed by rheological measurements and variable-temperature FTIR, which corresponded to the dissociation and reconstruction of hydrogen bonds between acylhydrazone groups. Since the non-equimolar sample presented similar behaviors with the equimolar sample, it verifies that the reversible dissociation/reformation of hydrogen bonds dominates the heat-assisted self-healing process. This finding will enable better understanding of the contribution of hydrogen bonding interactions in acylhydrazone self-healing systems, thus promoting the development of self-healing bulk materials based on acylhydrazone groups.Download high-res image (321KB)Download full-size image

In this study, we developed a convenient way to prepare highly compressive durable graphene aerogels (GAs) with enhanced strength by combining the freeze-casting process with the binding effect of polymers. It was revealed that poly(acrylic acid) (PAA) or poly(ethylene oxide) (PEO) had no noticeable influences on the chemical reduction process of GO and the formation of the aerogel structure. However, they showed different effects on the mechanical performances of the resulting hybrid aerogels. When PAA was in the appropriate feeding content range (∼30 wt%), PAA could significantly improve the strength of GAs with a 200–300% increase and simultaneously preserve their elasticity. In contrast, PEO showed a weaker reinforcing effect, and the hybrid aerogels completely lost their elasticity when the PEO feeding ratio was greater than 20 wt%. Furthermore, hybrid aerogels had slightly reduced electrical conductivity compared with that of GA. After the thorough analysis of the microstructure of hybrid aerogels (e.g., WAXD, XPS, Raman), we proposed that PAA chains could bridge and bind the edges of different graphene layers together, thus strengthening the network structure of hybrid aerogels. The combination of a functional polymer (i.e., PAA) with the freeze-casting process to prepare more mechanically robust GAs is simple, and the process is scalable and economical.

For simply and accurately determining molecular weight of cellulose, an ionic liquid mixed with a co-solvent, 1-butyl-3-methylimidazolium acetate/dimethyl sulfoxide (BmimAc/DMSO) (1:1, w/w) was used and dissolved cellulose well at ambient temperature. During the dissolution process no degradation of cellulose was observed, and all the resultant cellulose/BmimAc/DMSO solutions were transparent and stable. These advantages make it as an ideal solvent system to build a new characteristic method of cellulose’s molecular weight by the measurement of the intrinsic viscosity [η], which is significantly better than the currently used solvent systems. [η] of solutions of nine cellulose samples was measured by using rheometer with cylinder fixture and Ubbelohde viscometer, respectively. The [η] values obtained by these two methods were well consistent. The degree of polymerization (DP) of these cellulose samples was determined by Copper (II) ethylenediamine method. Then the molecular weight and its distribution of representative samples were cross-checked by gel permeation chromatography for soluble derivatives of cellulose. As a result, a relationship DP = 134 [η]1.2 was built, suitable for DPs in the range of 220–1400. The uncertainty of this relationship was estimated to be 5 %. This work provided a simple, accurate and reliable method for determining [η] and the molecular weight of cellulose.

Polymer and ionic liquid (IL) mixtures have attracted an increasing amount of attention due to their unique properties and potential applications. The interactions between poly(ethylene oxide) (PEO) and imidazolium ILs of different cation alkyl lengths and anion structures have been investigated by measuring melting points (Tm), contact angles, and rheological properties. Tm of crystalline PEO dramatically decreased when it was blended with ILs. Similarly, the contact angles of different ILs on a PEO surface proportionally decreased. The interaction energy, as calculated from melting point depression using the Flory equation, increased with the length of imidazolium alkyl cations and the size of anions. The different anionic structures had a more significant influence on the interaction energy than the alkyl chain lengths of cations. These trends accorded with the solubility obtained by high-energy X-ray diffraction and swelling ratio measurements of PEO in different ILs [Asai Macromolecules 2013, 46, 2369−2375] and the solubility of poly(methyl methacrylate) in different ILs [Ueno Langmuir 2014, 30, 3228−3235]. The rheological behavior of PEO in three different anionic ILs has also been studied to determine the effect of the anions on PEO conformations. The molecular weight dependence of the intrinsic viscosity of PEO in ILs revealed that the solvent quality of ILs (from poor solvents to good solvents) is highly influenced by anionic structures, which was consistent with the results of the melting point depression and contact angle.

Poly(octamethylene carbonate) (POMC), as the eighth member of the newly developed biodegradable aliphatic polycarbonate family, demonstrates a reversible crystal–crystal transition, which is highly similar to Brill transition extensively studied in the nylon family. With the dipole–dipole interaction in POMC much weaker than the hydrogen bonding, POMC exhibits its “Brill transition” temperature at around 42 °C, much lower than nylons. The two crystalline structures of POMC at below and above the transition temperature can be identified. The transition of POMC is largely associated with the reversible conformation change of methylene sequences from trans-dominated at low temperatures to trans/gauche coexistence at high temperatures.

A macroscopic organohydrogel hybrid was prepared by fast adhesion between the hydrogel and organogel which often repel each other. The two original gels were prepared by condensation of two poly(ethylene glycol) (PEG) gelators in anisole and water, respectively. Reversible acylhydrazone bonds formed in the condensation act as linking points of the polymer networks in the gels. When the two gels were brought into contact, a robust hybridized gel was obtained in 10 min. An emulsion layer formed at the interface between the two gels and dynamic chemistry of acylhydrazone bonding are key factors in rapid adhesion of the two inherently different gels. We hope this finding will enable the development of intelligent soft objects whose macroscopic water and oil phases contain different functional components.

Polymer-tethered nanoparticles with different geometric shapes are very useful fillers of polymer nanocomposites. Herein, a universal approach for the fabrication of such nanoparticles with precisely controlled shape and composition is reported. By microphase separation of poly(3-(triethoxysilyl)propyl methacrylate)-block-polystyrene (PTEPM-b-PS) in the presence of oligomers, o-TEPM (oT) and/or o-S (oS), followed by cross-linking and dispersion in PS solvent, precisely tailored PS-grafted nanoparticles were prepared. These particles include those with varied shapes but identical PS shells, particles with varied core sizes but the same PS shell, and particles with fixed shapes but varied PS shells. These particles are ideal model nanofillers to study the dynamics and reinforced mechanism of polymer nanocomposites.

Nanoscale ionic materials (NIMs) are novel organic–inorganic hybrid materials consisting of inorganic nanocore covalently attached with charged corona that is electrostatically coupled to oppositely charged canopy. In this study, graphene-based NIMs were prepared from hydroxyl-functionalized graphene (G-OH) that acquired via nitrene chemistry. The obtained G-OH-based NIM exhibited fluidity at its equivalence point (pH 6.3) at room temperature; in contrast, the graphene oxide (GO)-based NIM appeared as a black solid at its equivalence point because of the relatively low –OH density on GO. X-ray photoelectron spectroscopy and thermogravimetric analyses revealed grafting densities for G-OH and GO-based NIMs of ca. one polymer chain per 21 and 94 graphene carbon atoms, respectively. Microstructure analyses indicated the even dispersion of graphene nanosheets in NIMs. Rheological properties of G-OH-based NIMs could be adjusted over a wide range through variation of the volume fractions of canopy (Jeffamine M-2070 polyetheramine). G-OH-based NIMs also showed different viscoelastic behaviours from that of a G-OH–canopy physical mixture with similar graphene content. Thermal analyses showed that the crystallization temperature of canopy in G-OH-based NIMs decreased compared to that in physical mixtures. Cold crystallization was apparent during the heating cycle for G-OH-based NIMs, which did not exist for the physical mixtures. Furthermore, G-OH-based NIMs showed even dispersion and months-long stability in water and many organic solvents, indicating its amphiphilic nature. The unique properties of graphene–NIMs hold great potential for applications employing graphene-based materials.

In concentrated polymer solutions, the concentration (ϕ) dependence of the terminal relaxation time τd reflects ϕ-dependent changes of several factors, the monomeric friction ζ0(ϕ), the entanglement length, a(ϕ), and the correlation length, ξ(ϕ). Usually, the effect of the latter two factors on τd can be cast in a simple power law, τd ∼ ϕv. This power law form is to be examined for τd after correction of the changes of ζ0 with ϕ, but this iso-ζ0 correction is an unsettled problem. The correction based on the concept of “iso-free-volume” has been attempted in literatures. This study focused on four groups of solutions with different local frictional environments to examine universal validity of this correction. The isothermal data of τd were rheologically measured, and then corrected to the iso-frictional (iso-ζ0) state. After this correction, τd of most solutions in small molecule solvents showed the power law behavior τd ∼ ϕv with exponent of v = 2.0 ± 0.2, irrespective of the solvent type, either neutral small molecules or an ionic liquid (organic salt), and of the difference of the glass transition temperatures of the solvent and polymer. In contrast, the exponent became smaller (v ≈ 1.3) for the solutions in an oligomeric solvent. These results are discussed within the frame of the two-length scaling theory that considers changes of ξ and a with ϕ.

Dynamic polymer hydrogels with an environmental adaptive self-healing ability and dual responsive sol–gel transitions were prepared by combining acylhydrazone and disulfide bonds together in the same system. The hydrogel can automatically repair damage to it under both acidic (pH 3 and 6) and basic (pH 9) conditions through acylhydrazone exchange or disulfide exchange reactions. However, the hydrogel is not self-healable at pH 7 because both bonds are kinetically locked, whereas the hydrogel gains self-healing ability by accelerating acylhydrazone exchange with the help of catalytic aniline. All of the self-healing processes are demonstrated to be effective without an external stimulus at room temperature in air. The hydrogel also displays unique reversible sol–gel transitions in response to both pH (HCl/triethylamine) and redox (DTT/H2O2) triggers.

Rheological properties of cellulose dissolved in two ionic liquids (ILs), 1-allyl-3-methylimidazolium chloride (AmimCl) and 1-butyl-3-methylimidazolium chloride (BmimCl), with co-solvent dimethylsulfoxide (DMSO), are studied in the concentration range of cellulose from 0.070 to 6.0 wt%. The viscosities of ILs are exponentially decreased by adding DMSO in the concentration range of 0–100 wt%. The co-solvent DMSO decreases the monomer friction coefficient in cellulose solutions and has no significant change for the entanglement state of cellulose, thus results in the reduced solution viscosity, shortened relaxation time and unchanged moduli of the cross-over point. For cellulose solutions, dilute regime, semidilute unentangled regime and semidilute entangled regime were determined by steady shear experiments. In semidilute entangled regime, the specific viscosities ηsp, relaxation time τ, and plateau modulus GN, exhibit concentration dependences as ηsp ∼ C4.4, τ ∼ C2.2, andGN ∼ C1.9, respectively, in AmimCl-DMSO (80/20 w/w); and ηsp ∼ C4.3, τ ∼ C2.0, and GN ∼ C2.1, respectively, in BmimCl–DMSO (80/20 w/w). Therefore, the rheological properties of cellulose/IL/DMSO solutions are approximately of IL-independence in this study. The dependence of ηsp upon cellulose concentration shows that the IL–DMSO mixture is more like a θ solvent for cellulose, and the thermodynamic properties of IL–DMSO mixtures are similar with those of ILs for cellulose at 25 °C. The conformation of cellulose in ILs would not be changed with the addition of DMSO not only in the dilute regime but also in the entanglement regime.Graphical abstract

The master curves of a series of aliphatic polycarbonates (APCs) with different lengths of methylene segments in the repeat unit were obtained by dynamic rheological measurements. The plateau modulus and entanglement molecular weight were determined and cross-checked by different methods. Though having distinct difference in chemical structure of repeat units, both APCs and bisphenol-A polycarbonates have the similar entanglement weight and entanglement spacing. On the other side, the plateau modulus decreases with increasing the length of the side group of aliphatic polycarbonates with different side-chain lengths in the literature. The packing length model can explain the relationship between chain structure and entanglements.

In our previous work [Macromolecules2010, 43, 1191–1194], we synthesized dynamic covalent cross-linked polymer gels through condensation of acylhydrazines at the chain ends of poly(ethylene oxide) (A2) and aldehyde groups in tris[(4-formylphenoxy)methy]ethane (B3) and reported reversible sol–gel transition and self-healing properties of the gels. For those dynamic gels, this paper examines the gelation kinetics and rheological behavior in pre- and postgelation stages and discusses the molecular mechanism underlying the mechanical and self-healing properties. The results showed that the condensation reaction before the critical gelation point can be treated as the pseudo-second-order reaction. The scaling exponent n (=0.75) for the frequency dependence of the complex moduli at the critical gel point, the exponent γ (=1.5) for the concentration dependence of the viscosity in the pregel regime, and the exponent z (=2.5) for the concentration dependence of the equilibrium modulus in the postgel regime were found to not exactly obey the relationship for covalent gels, n = z/(z + γ), possibly because of the dynamic nature of the gels. The terminal relaxation of the dynamic gels at high temperature (125 °C) accorded with the Maxwellian model, as often observed for transient associating networks. In contrast, at low temperature (25 °C) where this transient network reorganization was essentially quenched in a time scale of experiments (∼50 s), the uniaxial stress–strain behavior of the gel was well described by the classical model of rubber elasticity σeng = G(λ – 1/λ2) up to 300% stretch (as similar to the behavior of usual gels chemically cross-linked in a swollen state). Ultimately, the gel cut into two pieces was found to exhibit self-healing under ambient conditions in 8 and 24 h, respectively, when the edges of those pieces were coated and not coated with acid (catalyst for dynamic covalent bond formation).

Torsional compliance of the torque transducer can be an important issue in linear viscoelastic (LVE) measurements when the sample stiffness is high relative to the instrument stiffness. We evaluated compliance effects of the ARES 2K-FRT on LVE measurements by systematically comparing the results of the frequency sweep mode obtained with 25 and 8 mm plates, respectively. In addition to the transducer, the test fixtures do contribute significantly to the system compliance. Without correction, the upper limit for the complex modulus ∣ G ∗  ∣ is approximately close to 4 × 105 Pa at 10% uncertainty, when using 25 mm plates. This limit is lower than the plateau modulus \(\left( {G_N^0 } \right)\) of most polymers. Therefore, instrument compliance can lead to significant errors for \(G_N^0 \) and wrong scaling for G″ in the plateau and Rouse regions. The respective roles of transducer and tool compliances are discussed. The FRT transducer compliance is corrected in real time in the instrument firmware. Tool compliance is a common problem for all rheometers when measuring stiff samples.

Starting with graphene oxide, we successfully prepared stable dispersions of reduced graphene oxide (RGO) in three hydrophilic ionic liquids (ILs) at relatively high concentration without using any surfactants/stabilizers.

A novel class of dumbbell-shaped dendritic molecules with a p-terphenylene core was synthesized, and their self-assembling properties were investigated. The incorporation of bulky dendritic wedges to the central stiff aromatic scaffolds could finely tune their solubility in many organic solvents. Unlike the self-assembly behavior of p-terphenylen-1,4′′-ylenebis(dodecanamide), the p-terphenylene cored different generation dendritic molecules could form gels in several kinds of organic solvents through a cooperative effect of the π−π stacking, hydrogen-bonding, and van der Waals forces. Interestingly, significant fluorescence enhancement was observed after gelation. Extensive investigations with atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), rheological measurements, UV−vis absorption spectroscopy, FT-IR spectroscopy, 1H NMR, and X-ray powder diffraction (XRD) revealed that these dendritic molecules self-assembled into elastically interpenetrating one-dimensional nanostructures in organogels.

Starting with graphene oxide, we successfully prepared stable dispersions of reduced graphene oxide (RGO) in three hydrophilic ionic liquids (ILs) at relatively high concentration without using any surfactants/stabilizers.

Nanoscale ionic materials (NIMs) are novel organic–inorganic hybrid materials consisting of inorganic nanocore covalently attached with charged corona that is electrostatically coupled to oppositely charged canopy. In this study, graphene-based NIMs were prepared from hydroxyl-functionalized graphene (G-OH) that acquired via nitrene chemistry. The obtained G-OH-based NIM exhibited fluidity at its equivalence point (pH 6.3) at room temperature; in contrast, the graphene oxide (GO)-based NIM appeared as a black solid at its equivalence point because of the relatively low –OH density on GO. X-ray photoelectron spectroscopy and thermogravimetric analyses revealed grafting densities for G-OH and GO-based NIMs of ca. one polymer chain per 21 and 94 graphene carbon atoms, respectively. Microstructure analyses indicated the even dispersion of graphene nanosheets in NIMs. Rheological properties of G-OH-based NIMs could be adjusted over a wide range through variation of the volume fractions of canopy (Jeffamine M-2070 polyetheramine). G-OH-based NIMs also showed different viscoelastic behaviours from that of a G-OH–canopy physical mixture with similar graphene content. Thermal analyses showed that the crystallization temperature of canopy in G-OH-based NIMs decreased compared to that in physical mixtures. Cold crystallization was apparent during the heating cycle for G-OH-based NIMs, which did not exist for the physical mixtures. Furthermore, G-OH-based NIMs showed even dispersion and months-long stability in water and many organic solvents, indicating its amphiphilic nature. The unique properties of graphene–NIMs hold great potential for applications employing graphene-based materials.