Chyi-Shan Wang

Processing of bulk (p-phenylene benzobisthiazole)nylon-6,6 molecular composites

Abstract A novel process for the fabrication of rigid-rod polymer reinforced thermoplastic molecular composites into bulk articles is presented. The as-processed poly( p -phenylene benzobisthiazole) (PBT)/nylon-6,6 molecular composites, depending on their PBT content, showed significantly higher tensile properties than nylon-6,6 and maintained an excellent bulk integrity even above the melting temperature of nylon. The Youngs moduli of these molecular composites were compared to those predicted from the rule of mixtures for three-dimensionally isotropic fibre composites. By assuming PBT/nylon-6,6 molecular composites to be stiff fibre composites, the reinforcement efficiency of PBT was estimated to achieve 50–70% that of the maximum.

Axial tensile and compressive properties of high-performance polymeric fibres

Abstract Axial tensile moduli, compressive moduli and compressive strengths of rigid-rod poly( p -phenylene benzobisoxazole) and stiff-chain poly( p -phenylene terephthalamide), Kevlar™, fibres were measured with a Tecam Micro-tensile Testing Machine. This machine was configured to allow for testing of single fibres in direct tension and compression at extremely small gauge lengths. The appropriate gauge lengths were estimated based on the discussion of Euler buckling and non-uniform stress distribution in anisotropic materials. The measured tensile and compressive moduli were analysed for corrections to machine compliance and possible gauge length error. The corrected compressive moduli were slightly lower than the corrected tensile moduli, probably due to fibre misalignment under compression and the fibrillar nature of the fibres. The fibre axial compressive strengths measured by direct compression were comparable to those measured from recoil and composite tests but lower than those from cantilever beam and elastica loop tests.

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.

Multidimensional benzobisoxazole rigid-rod polymers. II. Processing, characterization, and morphology

A high-torque rheometer was used to facilitate the polycondensation of 4-[5-amino-6-hydroxybenzoxazol-2-yl]benzoic acid (ABA) with trimesic acid and 1,3,5,7-tetrakis(4-carboxylatophenyl)adamantane to yield two- and three-dimensional benzobisoxazole polymers, respectively. Although the resultant polymer dopes exhibited improved homogeneity compared to polymer dopes previously prepared in glassware, improved polymer solution viscosities were not achieved. Fibers spun from the two- and three-dimensional polymers did not show a significant increase in compressive strength compared to fibers of the linear or one-dimensional benzobisoxazole polymer derived from the homopolymerization of ABA. Morphological studies of the polymer fibers and films by wide-angle X-ray scattering and scanning electron microscopy strongly indicated more lateral disorder and a more isotropic character for the three-dimensional structures compared to the one-dimensional structures.

Processing of optical quality rigid-rod polymer thin films

A novel process for the fabrication of optical quality thin films of rigid-rod polymers by coagulation from an isotropic solution is reported. This process consists of the following steps: (1) extrusion or doctor-blading of an isotropic rigid-rod polymer solution into a uniform thin solution layer; (2) relaxation of the solution in an atmosphere containing the vapour of a non-solvent for an appropriate length of time; and (3) coagulation of the solution in a non-solvent bath. Thin films of rigid-rod poly(p-phenylene benzobisthiazole) (PBZT) thus prepared do not show the large voids or line defects commonly observed in the extruded and subsequently coagulated PBZT thin films. A processing window based on the solution thickness and relaxation duration is outlined for a 1 wt% PBZT solution in methanesulfonic acid to fabricate void-free PBZT thin films up to 6 μm in thickness at room temperature. Optical spectra and micrographs show that the PBZT thin films prepared by the current process compare favourably with those extruded and then coagulated in terms of low optical loss and fewer defects.

Novel anisotropic texture of rigid-rod poly(p-phenylene benzobisthiazole) film

Abstract Poly(p-phenylene benzobisthiazole) (PBZT) polymer film, prepared by a doctor-blading technique, was studied by wide-angle X-ray scattering. Emphasis was placed on the anisotropic arrangement of the extended rigid-rod chains against the macroscopic axes. Two X-ray patterns, one measured with the X-ray beam perpendicular to the plane of the film surface and the other with the beam parallel to the surface plane, revealed a unique morphological feature: the large molecular plane of the PBZT chain is lying perpendicular to the film surface plane with the long polymer axis being random but lying in the plane of the film surface.

Single-longitudinal-mode fiber laser with passive multiple ring cavity and its application for video transmission

We have demonstrated an Er-doped fiber laser, in which the unwanted side-mode frequencies can be removed beyond 1 GHz by the multiple ring cavity (MRC) method. The ability of this fiber laser as an AM transmitter source has been verified by the experiment of 10 NTSC-channel transmission with a high CNR of >52 dB and excellent video quality.

Weak localization, electron-electron interaction, and metal-insulator transition in ion-implanted polymers

Abstract Kr + ion implanted rigid rod PBO, PBT and ladder BBL polymers show similar transport and optical behavior, with room temperature conductivities increasing from 10 −12 S/cm to ∼ 10 2 S/cm after implantation. A metal-insulator transition is observed at T c ∼ 30 K. Above T c , the results of a negative magnetoresistance ( ΔR / R ( H , T )), a weakly temperature dependent conductivity ( σ ( T ) α T p , 0.5 p T -dependent thermopower ( S ( T )), a Pauli spin susceptibility ( χ Pauli ), and a large microwave dielectric constant ( e mw ) resemble those of disordered metals in which weak localization and e-e interactions dominate the charge transport. Below T c , the changed behavior of positive ΔR / R , stronger T -dependent σ , and 1/ T -dependent S with a continued large and positive e mw all suggest an increased e-e interaction effect which opens up a ‘Coulomb’ gap (∼2meV), likely due to the enhanced localization. The density of states (DOS) is zero only at the center of the gap in accord with the theoretical prediction of the ‘Coulomb’ gap.

Processing, Properties and Structure of Bulk Poly(P-Phenylene Benzobisthiazole)/Poly(Ether Ether Ketone) Molecular Composites

The wet-state-consolidation process was used to fabricate bulk rigid-rod/thermoplastic molecular composites of poly(p-phenylene benzobisthiazole) and poly(ether ether ketone). The composites were molded into bar-shaped specimens. The morphology of those specimens was studied throughout the entire process using X-ray scattering and scanning electron microscopy to correlate processing conditions and properties. The mechanical properties of the bulk composites were comparable to those of other rigid-rod/thermoplastic molecular composite systems.

Void formation in coagulated rigid-rod polymer thin films

Large voids of about 100μm are often found in rigid-rod polymer thin films coagulated from isotropic solutions. To understand the mechanism of this void formation so as to improve the optical quality of coagulated rigid-rod polymer thin films, surface and internal structures of various coagulated poly(p-phenylene benzobisthiazole) solutions were investigated on freeze-dried films by high resolution scanning electron microscopy. The films coagulated immediately from doctor-bladed solutions exhibited surface crinkles and large internal voids, while the films coagulated from the doctor-bladed solutions that had undergone a surface treatment in an atmosphere containing the vapour of a non-solvent showed a surface network structure but no large internal voids. Results support that the large voids were formed due to a large-scale coagulant incursion during coagulation. The absence of the large voids in the films coagulated from the surface-treated solutions is attributed to the surface network structure being strong enough to regulate the incursion of coagulant during the film forming process at coagulation.