Journal of Polymer Science: Polymer Symposia最新文献

Propylene is the only monomer which can be polymerized both isospecifically and syndiospecifically with the same family of catalysts, i.e., organometallic compounds of the Ziegler–Natta type. Whereas isotactic poly(propylene) can be manufactured by reasonable polymerization conditions to highly valued thermoplastics, the corresponding syndiotactic polypropylene has remained a laboratory product. However, theoretical considerations show that syndiotactic polypropylene may possess some ultimate properties which surpass those of isotactic polypropylene. Propylene was thus polymerized with various catalyst systems and the resulting polymers characterized by carbon-13 nuclear magnetic resonance, infrared spectroscopy, X-ray diffraction, gel permeation chromatography, differential scanning calorimetry, and stress/strain measurements [1,2]. Several polymers were fractionated. The combined evidence points toward stereoblocks in syndiotactic polypropylenes [1]. The highest syndiotacticities achieved are, however, still too low to permit the ultradrawing to highly oriented polymers, which would allow approaching the ultimate properties. This result is probably due to a stereostatistic peculiar to the catalyst system, i.e., a symmetric Markov statistic of the first order [3]. This statistic predicts the experimentally found linear dependence of triad fractions on diad fractions [4], a characteristic restricted diad range, and an unusual dependence of the heterotacticity index on the diad contents. The same formalism also describes the features of the so-called heterotactic polymerizations.

The molding of fiber-reinforced thermoset components is a complex process involving a highly exothermic chemical reaction which takes place in the presence of flow and thermal gradients. The flow in turn is complicated by time and temperature-dependent rheological characteristics. The industry has grown largely on the basis of empirical developments. However, numerous problems do remain which required increased understanding for solution. This report reviews work underway in numerous laboratories to characterize the flow and cure of SMC and phenomena associated with these processes such as orientation, mechanical properties, residual internal stresses, mold fill, and surface defects.

The Brownian motion of semiflexible polymers in networks and in concentrated solutions is discussed. If the polymer is assumed to move only along itself by reptation, various time correlation functions are calculated rigorously. The results show that the Brownian motion is independent of the mesh size a over a wide range. It is then argued that the observed rodlike behavior of semiflexible polymers in concentrated solutions is due to the rapid bending fluctuation of the polymer.

Paraxylylene bisdialkyl sulfonium salts can be polymerized in several different ways, yielding addition polymers as well as condensation polymers. The chemistry leads to a variety of polymers including poly(p-xylylene), poly(p-xylylidene), poly(xylylene sulfide)s, poly(xylylene oxide)s, and various α-substituted polyxylylenes such as poly(p-xylylene-α-dialkylsulfonium chloride). Addition polymers form via xylylene diradical intermediates. Condensation polymerization can be effected either by coupling with aldehydes or by nucleophilic displacement reactions.

Cellular polymers are important products accounting for about 10% of the global markets for thermoplastic and thermoset materials. During his industrial research career, Turner Alfrey made important contributions to the science and technology of various foams. His studies have not been published elsewhere, but they were pioneering concepts that had a major impact upon the understanding of the continuous extrusion process for polystyrene foams. The included paper entitled “Compression Behavior of Open-Celled Polyurethane Foams” was intended for presentation at the 1980 Gordon Conference on Foams. This author believes this to be the last scientific paper written by Turner Alfrey.

Enhancement of polymer properties by controlling supramolecular structure during fabrication was of major interest to Turner Alfrey. Supramolecular structure, i.e., the geometric arrangement of macromolecules in a specimen, is established during mechanical processing and can have a profound effect on properties. Several examples of Alfrey's contributions to fabrication of polymers are reviewed which demonstrate the Alfrey method of analyzing complicated fabrication processes.

Kinetic studies of reactions in systems containing polymers have led to the following observations: (1) In dilute solution the thermal and the photochemical isomerization of azobenzene residues is not impeded by their incorporation into a polymer backbone. In the glassy state, the kinetics of the isomerization of azobenzene residues in polymer side chains or backbones reflect the distribution of free volume and volume relaxation. (2) Reactions of polymers with low molecular weight reagents may be affected by neighboring group effects and by the contribution of the polymer backbone to the polarity of its microenvironment. (3) Attack of an enzyme on a polymer side chain depends sharply on the spacing of the susceptible bond from the chain backbone. (4) The expected “kinetic excluded volume effect” in reactions between two flexible chain molecules was not observed. (5) Reactions involving groups attached to a swollen crosslinked gel may reflect spatial variations of the local polarity. (6) Attack of water-soluble reagents on latex particles may be restricted to the surface of the particle. (7) The presence of a polyion exerts various effects on reactions involving two small ionic species.