Samples of very low molecular-weight fractions of isotactic polypropylene have been crystallized by means of a solvent-evaporation technique as well as by melt recrystallization. The crystalline entities obtained by these methods have been examined in terms of their internal structure and associated morphology. The particular polymorphic form observed for these fractions is dependent on both the length of the molecule and the particular crystallization conditions employed. The morphological characteristics of the samples have been found to be qualitatively similar to those previously reported for unfractionated polypropylene and high-molecular-weight fractions, in spite of the fact that the molecular lengths of the samples investigated are sufficiently short to prevent chain folding. Selected-area electron-diffraction patterns have been obtained from the γ form and from mixed α and γ form of the isotactic polymer. The diffraction data have resulted in a modification of the previously proposed triclinic unit cell for the γ phase, and a conclusion that the γ→α transformation in isotactic polypropylene is martensitic in nature. The mechanism of this solid-solid transformation involves relative motion between the (040) planes which are found to be invariant in the two crystal phases.

Polypropylene aggregates, consisting of interwoven arrays of single-crystal lamellae, have been obtained by crystallization of samples of isotactic polypropylene from dilute solutions held at constant temperatures between 85° and 115°C. The solvent used was α-chloronaphthalene with polymer concentrations ranging from 0.4% to 0.01% by weight. The resulting crystalline aggregates were examined by means of phase contrast and electron microscopy. Similar crystal aggregates have been studied by Khoury and found to be incipient spherulites. These aggregates tend to occur in the form of open weave structures, with characteristic cusps. Lath-like lamellar crystals, with well-defined faces, are found at the edges of these structures. These edge lamellae produce a single-crystal electron-diffraction pattern corresponding to the monoclinic structural form of isotactic polypropylene. On the basis of the experimental findings, it is suggested that folding occurs in monoclinic polypropylene only along a given set of parallel fold planes, viz. {010}. Preliminary results of a study of the fracture characteristics of the crystal lamellae tend to support the suggested hypothesis.

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Fatigue tests in reversed tension‐compression have been carried out on samples of polyethylene and polystyrene of widely varying molecular weights, extending up to 2, 000, 000. All tests on polystyrene specimens were made at 1600 rpm. For polyethylene, tests speeds had to be reduced to 100 rpm in order to avoid serious temperature effects. For both materials, increasing molecular weight leads to improved resistance to alternating loading. For polystyrene, this improvement in ultimate properties even continued well beyond molecular weight values where Tg, becomes effectively independent of molecular weight. For polyethylene, samples of high molecular weight did not fail even after 107 cycles of alternating loading at a stress level of 3000 psi.

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