Charles Y.-C. Lee

Moisture absorption and hygrothermal aging in a bismaleimide resin

Moisture diffusion in a BMI resin has been investigated by submerging thin specimens in water. The weight gain of the neat resin deviates significantly from Ficks law in that the uptake continues to slowly increase over an extended time scale. A two-stage diffusion model can successfully fit the experimental data. This model considers the structural relaxation induced by absorbed moisture, which is a good plasticizer of the resin. Desorption and re-absorption experiments indicate that the structural relaxation is irreversible upon desorption of water molecules. Dynamic mechanical analysis results demonstrate the plasticizing and rejuvenating effect of water.

Waveguiding in substrate supported and freestanding films of insoluble conjugated polymers

We demonstrate that waveguiding is possible in substrate supported and freestanding films of insoluble conjugated polymers. Photoresist gratings were used to couple 1.06–1.53 μm radiation into the waveguides allowing the refractive indices, the birefringence, and the linear losses to be measured.

Third order non-linear optical properties of poly-p-phenylene benzobisthiazole and its novel composite with Zytel processed via methane sulphonic acid solution extrusion

Abstract It was found that improved optical quality films of poly-p-phenylene benzobisthiazole (PBZT) and of a novel composite of PBZT and nylon (Zytel 330) can be produced by extruding below critical concentration solutions in methane sulphonic acid. The linear refractive indices and their dispersion behaviour in these films have been determined by analyses of the interference fringes observed in the optical transmission spectra. A relatively large value of the linear refractive index is obtained for PBZT. The third-order non-linear optical susceptibility, χ(3), is determined for the films of PBZT and PBZT/Zytel composite using subpicosecond degenerate four-wave mixing. The χ(3) value observed for this PBZT film is almost an order of magnitude larger than that previously reported for PBZT films of a rather inferior quality processed through polyphosphoric acid solution. This result emphasizes the importance of polymer processing to produce better optical quality films for evaluation of intrinsic χ(3) values of polymers. The response time of the non-linearity is limited by the laser pulse width, indicating that the non-linear response is derived from the coherent electronic χ(3). The χ(3) value of the PBZT/Zytel composite is lower than that for pure PBZT because of the reduced number density. However, determination of the optical loss, α, in these films yields a much lower value of α for the PBZT/Zytel composite film, resulting in an improved value of the often used device figure of merit χ (3) α . This result suggests that composite structures may be useful for optimizing the χ (3) α ratio.

Influence of molecular structure on processing conditions and mechanical properties of graft rigid-rod copolymers

Abstract Dynamic and tensile mechanical properties of a series of graft rigid-rod copolymers were studied. The copolymers were single-component rigid-rod molecular composites consisting of a rigid-rod poly( p -phenylenebenzobisthiazole) backbone grafted with flexible-coil poly(oxy-1,3-phenylenecarbonyl-1,4-phenylene) side-chains. Dynamic storage moduli of the graft copolymers showed three characteristic regions: an initial plateau region, a transition zone and a second plateau region. The peak transition temperatures of the dynamic loss modulus curves were independent of the rigid-rod backbone length but decreased with increasing side-chain lengths, suggesting that the transition was a secondary transition associated with localized motions of the flexible side-chains. The graft rigid-rod copolymers were compression moulded into tensile test specimens at temperatures above their peak transition temperatures. Although the specimens appeared well consolidated as evidenced by the scanning electron micrographs of tensile-fractured surfaces, their tensile properties were relatively poor compared with those reported for rigid-rod molecular composite blends. The significance of poor tensile properties is discussed based on the structural characteristics of the graft copolymers, namely the length of rigid-rod backbone, the frequency of graft sites, and the average contour length of flexible-coil side-chains.

Phase behaviour of rigid-rod/coil/solvent ternary systems with rod—coil interaction

Abstract This paper reports the phase diagrams of PBZT/nylon/MSA, PBZT/NaAMPS polymer/MSA, PBZT/HAMPS monomer/MSA and PBZT/PPTA-PS/MSA, with PBZT = poly( p -phenylene benzobisthiazole), MSA = methanesulphonic acid, NaAMPS = sodium 2-acrylamido-2-methylpropanesulphonate, HAMPS = 2-acrylamido-2-methylpropanesulphonic acid and PPTA-TS = poly( p -phenyleneterephthalamido)propanesulphonate). The ternary phase diagram of rigid-rod polymer PBZT and amorphous nylon in MSA solvent was found to follow Florys prediction of a rod/coil/solvent ternary system very well. Florys theory assumes no interaction between the rod and the coil, and the PBZT/nylon system fits this model nicely. When sulphonated polymers, such as NaAMPS polymer and PPTA-PS, were used instead of nylon in a ternary solution, it was found that the phase behaviour deviated from Florys theory. This deviation could be due to the possible coil—rod intermolecular interaction in the ternary solution. Florys formalism was used to analyse these data. It was found that for the PBZT/nylon/MSA system, the X 3 value was consistent with the contour length of the coil molecules as predicted by Florys theory. However, both PBZT/HAMPS monomer/MSA and PBZT/NaAMPS polymer/MSA systems showed an X 3 value of unity. The ternary phase diagram of the PPTA-PS system showed an X 3 value between 1 and 5. This suggests that the degree of intermolecular interaction between rod and coil plays an important role in determining the phase behaviour of a rod/coil/solvent ternary system.

Novel Ternary Molecular Composites Prepared by a Sol–Gel Process and Their Conversion into Microcellular Foams

Abstract A carboxylated polysulfone (C‐PSF) was reacted with 3‐(glycidoxypropyl)trimethoxysilane (GPS) in a sol–gel reaction to form hybrid networks. Specifically, GPS was grafted to the C‐PSF chains at their epoxy ends by ring‐opening reactions and then the methoxy silane groups at their other ends were crosslinked through the usual hydrolysis and condensation reactions. These C‐PSF/GPS cross‐linked binary networks are of interest in their own right, but this type of network was also used in the incorporation of polybenzimidazole (PBI) rigid‐rod polymer to form a C‐PSF/PBI/GPS ternary composite. In this case, the network structures obtained by carrying out the same reaction in the presence of PBI chains successfully suppressed their undesirable segregation within the composite. The transparencies of the preparative solutions and the dried films indicated that the dispersion of PBI was maintained in these environments, presumably at the nano‐ or molecular level. The ternary composites were characterized using Fourier‐transform infrared (FT‐IR) spectroscopy, scanning electron microscopy (SEM), thermal mechanical analysis (TMA), and thermogravimetric analysis (TGA). Some of these composites were converted to closed‐cell microcellular foams, which were then characterized in a preliminary way with regard to their morphologies.