The mechanical relaxation behavior and microstructure of a series of novel norbornyl-POSS organic-inorganic copolymers have been investigated. We have examined the influence on physical properties of both the weight fraction of POSS-norbornyl monomer and the corner group composition. POSS refers to the polyhedral oligomeric silsesquioxane inorganic/organic macromer, which is composed of an inorganic Si 8O12 spherical core surrounded by seven inert organic corner groups and one reactive norbornyl moiety. It was observed that POSS copolymerization enhances the R-relaxation temperature, TR, in proportion to the weight fraction of the POSS-norbornyl comonomer. Interestingly, however, the magnitude of this dependence is larger for the POSS-norbornyl comonomer possessing cyclohexyl corner groups (CyPOSS) than for the copolymer with cyclopentyl corner groups (CpPOSS). Although POSS copolymerization yields only slight enhancement of the room temperature storage modulus, at temperatures lower than a strong mechanical relaxation, identified as a -relaxation, and near T ) -

ABSTRACT:We report on the viscoelastic behavior of linear thermoplastic nonpolar hybrid inorganic-organic polymers. These materials have been synthesized through copolymerization of an oligomeric inorganic macromer with 4-methylstyrene where the inorganic portion of the material is a well-defined polyhedral oligosilsesquioxane (POSS), R 7 (Si 8 O 12 )(CH 2 CH 2 C 6 H 4 CH|CH 2 ), with R Å c-C 6 H 11 or c-C 5 H 9 . A series of 4-methyl styrene copolymers with approximately 4, 8, and 16 mol % POSS macromer incorporation were investigated. Rheological measurements show that the polymer dynamics are profoundly affected as the percent of POSS increases. In particular, a high-temperature rubbery plateau develops (where a terminal zone is not observed), despite the fact that the parent poly 4-methylstyrene is unentangled. It is also observed that the thermal properties are influenced as the percent of POSS incorporation increases, with increases in the glass and decomposition temperatures. The results suggest that interchain interactions between the massive inorganic groups are responsible for the retardation of polymer chain motion, a mechanism similar to the ''sticky reptation'' model conceived for hydrogen-bonded elastomers and developed by Leibler et al. [Macromolecules, 24, 4701 (1991)]. Control over the interchain interactions would also give rise to the observed increases in glass transition and the establishment of a rubbery plateau.

A direct rheo-optical characterization of the flow-induced isotropic-nematic (I-N) transition in a semiflexible thermotropic liquid-crystalline polymer (TLCP) was investigated, using a specially constructed apparatus enabling in-situ optical microscopic observations at elevated temperatures, along with cone-and-plate rheometry. For the investigation, an aromatic polyester, poly[(phenylenesulfonyl)-p-phenylene 1,10-decamethylenebis(4-oxybenzoate)] (PSHQ10), was synthesized via solution polymerization. Above the equilibrium isotropic-nematic transition temperature for this polymer, T ) 170.5°C, application of steady-state shear flow above a certain critical value of shear rate, γ c, produces a firstorder I-N transition, with γ c increasing with temperature. This transition is evidenced by the formation of elongated nematic (birefringent) domains in the isotropic matrix, accompanied by a drastic decrease in shear viscosity (η). Remarkably, the nematic domains that form for γ > γ c are optically uniform under cross-polarized optical microscopy; i.e., they are apparently free of disclinations (defects), typical of textured TLCPs. The flow-induced I-N transition in PSHQ10 is found to be reversible; i.e., upon cessation of shear flow, the domains melt to the original isotropic phase and the dynamic moduli rise toward the pretransition values. The observed flow-induced I-N transition may find important applications, such as envisaging new routes for processing TLCPs with better mechanical properties and helping to understand bioprocesses such as silk thread spinning.

In situ optical microscopy, small-angle light-scattering and X-ray-scattering techniques have been applied to study flow-induced textures in main-chain thermotropic liquid-crystalline polymers (LCPs). In order to gain more insight into the underlying mechanism leading to the formation of microstructure, the effect of molecular weight has been investigated. The well-known 'banded' texture, commonly observed in fibres and sheared thin films of LCPs, is observed to develop after cessation of shear. Furthermore, the banded-texture formation is molecular-weight dependent, in that it occurs only above a certain 'critical' molecular weight,M c w . For polymers with M w >M c w , the banded texture forms within a few seconds of cessation of shear, relaxing to a disordered 'tight' texture. On the other hand, for polymers withM w

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