Narrow fractions of polystyrene molecules in the form of uncatenated rings (cycles) were synthesized by reacting bifunctional living linear precursors with an appropriate coupling agent at very low concentrations. The cyclic molecules were separated from the simultaneously formed linear polycondensates by fractionate precipitation. The molecular weights of the cycles ranged from 11100 to 185000, thus encompassing the critical molecular weight for entanglements in linear polystyrene. The ring-like nature of these fractions has been investigated by a variety of techniques, including the limiting viscosity number in a good and in a solvent as well as neutron scattering in deuteriated cyclohexane. These measurements, part of which are reported here in some detail, display a gratifying agreement with the theoretical predictions reported earlier for uncatenated cyclic polymers. The zero-shear melt viscosities of these cyclic fractions and several others prepared by Fetters and Hostetter to extend the molecular weight range to 390000 were measured over a wide range of temperatures and compared with the viscosities of linear polystyrenes of similar molecular weights. Above the critical molecular weight for entanglement coupling, no major differences were found between the temperature dependence or the molecular weight dependence of the cyclic polymers and those of their linear counterparts. For the same molecular weight, however, the cycles exhibit somewhat lower melt viscosity values than do the linear molecules. The results are critically compared with those reported by Roovers on similar polystyrene cycles and briefly discussed in terms of recent molecular theories based on snake-like motion (reptation) of chains along a curvilinear tube formed by the constraints of the surrounding entangled matrix.

No abstract

Characteristic features of isothermal growth, thickening, and melting behavior of melt grown single crystals of low molecular weight poly (ethylene oxide) fractions are presented and discussed in terms of their relevance to basic concepts of polymer crystal growth. Measurements of growth (G) and thickening rates of once folded chain crystals were extended (up to the melting point) into a temperature range where usually only extended chain crystals grow. Monolayer extended and folded chain crystals melt by lateral shrinkage, the rate of which (G−) depends not only on temperature, but also on thermal history. G− increases exponentially with the degree of superheating, and the value of the temperature coefficient d log G−/dT is proportional to the lamellar thickness. In the narrow temperature interval where transition from once folded to extended chain growth occurs, the crystal morphology displays some spectacular features. These may be accounted for in terms of the crystal habit and the rate of completion, g. of a molecular layer in the particular (hk0) prism face considered. Morphological analysis shows that g is of the same order of magnitude as G(hkO), leading to the conclusion that the growth of these PEO crystals is controlled by multiple surface nucleation rather than by deposition of a single surface nucleus.

No abstract