Gary Price

Polymer/layered silicate nanocomposites as high performance ablative materials

The ablative performance of poly(caprolactam) (nylon 6) nanocomposites is examined. A relatively tough, inorganic char forms during the ablation of these nanocomposites resulting in at least an order-of-magnitude decrease in the mass loss (erosion) rate relative to the neat polymer. This occurs for as little as 2 wt.% (∼0.8 vol.%) exfoliated mica-type layered silicate. The presence of the layers does not alter the first-order decomposition kinetics of the polymer matrix. Instead, the nanoscopic distribution of silicate layers leads to a uniform char layer that enhances the ablative performance. The formation of this char is only minutely influenced by the type of organic modification on the silicate surface or specific interactions between the polymer and the aluminosilicate surface, such as end-tethering of a fraction of the polymer chains through ionic interaction to the layer surface. Thus, the enhancement in ablative performance should be general for the class of exfoliated layered silicate/polymer nanocomposites.

Morphology and properties of rigid-rod poly(p-phenylene benzobisoxazole) (PBO) and stiff-chain poly(2,5(6)-benzoxazole) (ABPBO) fibres

Abstract The morphology of two new high performance polymer fibres, rigid-rod poly( p -phenylene benzobisoxazole) (PBO) and stiff-chain poly(2,5(6)-benzoxazole) (ABPBO), has been examined by wide angle X-ray scattering and scanning and transmission electron microscopy. Heat treatment of as-spun PBO and ABPBO fibres produces transversely broadened crystallites, which are unlike uniaxially elongated crystallites in heat treated fibres of rigid-rod poly( p -phenylene benzobisthiazole) (PBT) or stiff-chain Kevlar, i.e. poly( p -phenylene terephthalamide) (PPTA). Heat treated fibres of PBO have greater three-dimensional crystallinity and long range crystallite ordering than axially disordered PBT, but much less than well ordered PPTA. Fibres of both as-spun and heat treated PBO have a crystalline c -axis with high orientation and low paracrystalline disorder, but heat treatment of as-spun fibres of ABPBO significantly increases c -axis orientation and decreases c -axis paracrystallinity, due to additional extension of the stiff-chain ABPBO molecules. Eventual improvements in orientation and ordering of PBO fibre through improvements in synthesis and processing may, based upon theoretically predicted values, result in up to triple the strength and modulus of commercial PPTA fibre.

Mechanical properties of intercalated cyanate ester -layered silicate nanocomposites

Abstract Cyanate ester resins are among the most important engineering thermosetting polymers and have received attention because of their outstanding physical properties such as low water absorptivity and low outgassing. However, like most thermosets their main drawback is brittleness. Nanocomposites of cyanate esters were prepared by dispersing organically modified layered silicates (OLS) into the resin. Inclusion of only 2.5% by weight of OLS led to a marked improvement in physical and thermal properties (Coefficient of thermal expansion, Tg and effective thermal stability). Most impressively, a 30% increase in both the modulus and toughness was obtained.

Morphology and mechanical properties of a phase separated and a molecular composite 30% PBT/70% ABPBI triblock copolymer

Abstract The morphology of a triblock copolymer of 30% rigid-rod poly(p-phenylene benzobisthiazole) (PBT) and 70% semi-flexible coil poly(2,5(6)benzimidazole) (ABPBI) was examined by wide angle X-ray scattering and scanning and transmission electron microscopy. Samples that were vacuum cast from a solution formed a microphase separated film with 0.1 μm particles and platelets of well-oriented 10 nm PBT crystallites in a ductile ABPBI matrix. Fibres were dry-jet/wet-spun from an optically homogeneous solution into a water coagulation bath to inhibit large scale phase separation. Heat-treated fibre contained crystallites of PBT and ABPBI with lateral dimensions no larger than 3 nm, demonstrating that PBT molecular segments were well dispersed and that a rigid-rod, molecular level composite had been achieved. The molecular level dispersion and high orientation in the ‘molecular composite’ fibre resulted in excellent mechanical properties with a modulus of 100 GPa and a tensile strength of 1.7 GPa which were about an order of magnitude greater than for the vacuum cast copolymer film.

Solvent cast thermoplastic and thermoset rigid-rod molecular composites

Acid–base interaction-mediated compatibilization between rigid-rod and matrix polymer components facilitated the formation and processing of solvent cast aromatic heterocyclic rigid-rod thermoplastic as well as thermoset molecular composites above the critical concentration (Ccr) of the rigid-rod polymer in solution, without phase separation. The blends were solvent cast by the mechanism of ionic interchange between a sulfonic acid-pendent poly(p-phenylenebenzobisimidazole) (SPBI), solubilized in alcohol as its triethylammonium salt and basic thermoplastics such as poly(vinylpyridine)s or secondary or tertiary amines with thermosettable phenylethynyl, nadimide and bisbenzoxazine functionalities. Morphological characterization, utilizing SEM, WAXS and SAXS of as cast as well as annealed/thermally cured optically clear film composites of a broad range of compositions revealed homogeneous microstructures with no observable phase-separated domains, indicating high miscibility, ascribable to the favorable negative enthalpy of the ionic association between the rod-matrix components. A preliminary dynamic mechanical study of compression molded rigid-rod thermoplastic blends with relatively low rod contents showed significant enhancement in thermomechanical properties vis-a-vis the pristine matrix.

Rod aggregation in graft rigid-rod copolymers for single-component molecular composites

Graft copolymers of rigid-rod polymers, specially synthesized for single-component molecular composites, were investigated by wide-angle X-ray scattering. The copolymers consist of a flexible poly(ether ketone) (PEK) side chain attached to a rigid-rod poly(p-phenylene benzobisthiazole) (PBZT). Emphasis was especially placed on the extent of rod aggregation in the copolymers before and after heat treatments. Results were compared among the graft copolymers of various structural characteristics and the blend of PBZT and PEK homopolymers. The results confirmed that the flexible side chains effectively suppress the thermally induced rod aggregation. The frequency of the graft sites along the rigid-rod backbone was found to be the key structural parameter limiting the extent of rod aggregation. A certain minimum frequency of the graft sites appeared to be required for the copolymers to be stable against the thermal aggregation. The side-chain length showed only a minimal effect, while the rigid-rod backbone length exhibited no effect on the stability of the copolymer morphology in association with the thermal aggregation.

Synthesis, Properties and Potential Applications of Sulpho-Pendent Poly(Arylene Ether Ketone)s

High molecular weight sulpho-pendent poly(arylene ether ketone) homopolymers and copolymers were synthesized with inherent viscosities ranging from 0.94 dl g−1 to 1.20 dl g−1 and glass transition temperatures (T gs) in the range 190–200 °C. Their potential use as transparent matrix hosts for second-order NLO chromophores was explored from the point of view of obtaining monodisperse guest–host systems mediated by specific interaction between the sulphonic acid pendant of the polymer host and the basic functionality of the chromophore structure. Homogeneously dispersed, optically clear thin film composites were obtained for aromatic heterocyclic chromophores with electron-rich thienyl, N,N-dialkylamino or N,N-diphenylamino donors and a pyridyl acceptor in their molecular structures.

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.

Solid state coagulation of rigid-rod polymer thin films

Abstract A novel coagulation processing scheme has been developed to fabricate optical quality polymer thin films for non-linear optical applications. In this process, termed solid state coagulation, an optically isotropic polymer solution is rapidly frozen into a solid mixture to preserve the molecular dispersion of the polymer in the solvent. It is then coagulated at a cryogenic temperature in a proper coagulant to extract the frozen solvent without disrupting the molecular distribution of the polymer. Thin films of rigid-rod poly(p-phenylene benzobisthiazole) (PBZT) prepared by this technique showed an optical attenuation coefficient of about 300 cm−1 (or 1300 db cm−1), significantly lower than the 990 cm−1 (or 4300 db cm−1) obtained from the thin films prepared by the conventional solution-state coagulation process. Wide angle X-ray scattering confirmed that the domain structures in the solid-state coagulated PBZT thin films are smaller and less ordered than those in the solution-state coagulated thin films. This solid state coagulation processing scheme is particularly valuable for polymer systems, such as rigid-rod polymers, ladder polymers, crystalline polymers and polymer blends of incompatible systems, which cannot be processed with significant optical clarity by melt processing or solution casting.

1-(4-Nitrophenyl)-4-piperidinol

The title compound, C 11 H 14 N 2 O 3 , is a non-linear optical chromophore. The piperidinol ring is in a chair conformation. The C-N-C fragment of the piperidinol moiety is nearly coplanar with the nitrophenyl ring system. The molecular stacking allows hydrogen bonding between the piperidinol hydroxy group and the nitro group.