Chen-Yang Liu

Chemical structure and physical properties of cyclic olefin copolymers (IUPAC Technical Report)

Cyclic olefin copolymers comprise a new class of polymeric materials showing properties of high glass-transition temperature, optical clarity, low shrinkage, low moisture absorption, and low birefringence. There are several types of cyclic olefin copolymers based on different types of cyclic monomers and polymerization methods. In this work, we have analyzed the chemical structure of the currently commercialized cyclic olefin copolymers by 13C NMR, and investigated their glass-transition temperatures and surface characteristics. It was observed that the glass-transition temperature, Tg, of cyclic olefin copolymers depended on the bulkiness of the main chain, and the number of rings had an important role in increasing the bulkiness of cyclic olefin copolymers. Cyclic olefin copolymers with polar substituents such as ester or ether groups showed high surface energy per area and peel strength.

Stable dispersions of reduced graphene oxide in ionic liquids

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.

Thermal degradation studies of cyclic olefin copolymers

The influences of the chemical composition and microstructure on the degradation behaviors of 3-series cyclic olefin copolymers (COCs) were investigated by using non-isothermal thermogravimetric analysis (TGA). Kinetic parameters of degradation were evaluated by using the Flynn–Wall–Ozawa iso-conversional method and the pseudo first-order method. Compared with conventional polyolefins, e.g. HDPE, COCs have lower peak temperatures of degradation, narrower degradation temperature ranges and higher amount of residual weights at the end of the degradation, which should be attributed to the chemical structure and microstructure features of COCs including the branching effect and the steric effect. The values of the reaction order of COCs determined by the Kissinger method are close to 1 in the non-isothermal degradation process. Although the values of Ea in region II calculated by using the pseudo first-order method are much higher than those calculated by using the Flynn–Wall–Ozawa method, there is a similar change trend of Ea between these two methods. However, there is a good correlation between the Ea in region II and the peak temperature of degradation for COCs. The theoretical weight loss versus temperature curves, generated by using the estimated kinetic parameters, well fit the experimental data, which indicates that the analysis method used in this work is valid. # 2003 Elsevier Science Ltd. All rights reserved.

Nanoscale ionic materials based on hydroxyl-functionalized graphene

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.

Thermoplastic High Strain Multishape Memory Polymer: Side-Chain Polynorbornene with Columnar Liquid Crystalline Phase

A thermoplastic high strain multishape memory polymer can be fabricated using a hemiphasmid side-chain polynorbornene (P1) with hexagonal columnar liquid crystalline (ΦH ) phase. Without any chemical crosslinks, P1 can memorize multiple temporary shapes with high strain and exhibit excellent shape fixity and shape recovery. As the building blocks of ΦH , the multichain columns in P1 act as robust physical crosslinks.

Toward highly compressible graphene aerogels of enhanced mechanical performance with polymer

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.

An ABA triblock containing a central soft block of poly[2,5-di(n-hexogycarbonyl)styrene] and outer hard block of poly(4-vinylpyridine): synthesis, phase behavior and mechanical enhancement

We designed and synthesized a series of ABA triblock copolymers containing an inner soft poly[2,5-di(n-hexogycarbonyl)styrene] (PHCS) block and an outer hard poly(4-vinylpyridine) (P4VP) block, wherein the PHCS block has the potential to form a columnar liquid crystalline phase at high temperatures and the P4VP block can complex with metal salt or organic molecules via non-covalent interactions. Using an atom transfer radical polymerization method, we prepared successfully the triblock samples with controlled molecular weights (MWs) and compositions. The triblocks were characterized with various experimental techniques including differential scanning calorimetry, dynamic mechanical analysis, small-angle X-ray scattering, and transmission electron microscopy. The experimental results indicate the occurrence of microphase separation. With the P4VP volume fraction lower than 20%, the triblocks exhibit the phase morphologies of cylindrical or spherical hard P4VP domains arranged in the soft PHCS matrix. Mechanical property testing reveals that the tensile strength and maximum elongation at break can be tuned by varying the total MW and composition of the samples, indicating that the triblock copolymer can be used as thermoplastic elastomer while carrying functional blocks. Applying the metal–ligand coordination interaction between P4VP and Zn2+, we prepared the hybrid of triblock and zinc perchlorate. Domain spacing expansion and phase morphology change induced by adding the metal salt are detected. Furthermore, because of the hybrid formation and the glass transition temperature of hard phase increased dramatically by adding only a small amount of the salt, the rubbery plateau of the hybrid is extended, indicating a better thermal stability than that of the pure triblock.

Effect of chemical structure of polycarbonates on entanglement spacing

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.

Probing the Foundations of Tube Theory: Comparisons Between Model and Experimental Scalings for the Rheology of Linear Entangled Polymers

Tube models have achieved spectacular success for predictions of the linear rheology of entangled polymers. Quantitative agreement has been achieved at the price of significant complexity increase as compared to the original de Gennes picture, but the models have retained their high internal coherence (only two fundamental scaling parameters, one for time and one for stress). However, very essential questions have yet to receive a fully satisfactory answer. We summarize in this paper recent findings about three issues : 1. the molecular weight dependence of the plateau modulus, 2. the molecular weight dependence of the terminal relaxation time and 3. the experimental evidence of monomer redistribution in the tube.