Samples of isotactic polypropylene (PP) were zone‐solidified in temperature gradients up to 300°C/cm at growth rates down to 3 μm/min. Oriented α‐type spherulites were obtained only by nucleation. While β nucleation is extremely rare, the β phase is easily initiated by growth transformations along the oriented α front. Since the β phase was found to grow considerably faster than the α phase, the α‐to‐β transformation points diverge across the sample, interrupting growth of the oriented α fibrils. This causes subsequent nucleation to yield teardrop‐shaped α spherulites. Differential scanning calorimetry (DSC) studies of zone‐solidified PP show the β‐phase to be favored by slow growth rates, high temperature gradients, and large degrees of superheat in the melt—all of which tend to suppress nucleation. Differential thermograms of largely β‐PP obtained at a heating rate of 1°C/min show the actual melting and recrystallization of the β spherulites into the α form.

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Aliphatic polyamides (Nylon‐66 and a Nylon‐6, ‐66, ‐610 terpolymer) were isothermally precipitated from formic acid solution by immersing in an aqueous nonsolvent bath or a solvent/nonsolvent mixture. Depending on the composition of the polymer solution and nonsolvent bath, phase separation by nucleation and growth can be initiated for a liquid‐liquid phase separation process, a crystallization process or a combined process. Under certain conditions, crystallization of Nylon‐66 results in a membrane with a uniform skinless microporous structure that was rapidly wetted by water. In contrast, liquid‐liquid phase separation produces a polyamide film with largely unconnected cellular voids that is as a result not wetted by water. © 1995 John Wiley & Sons, Inc.

ABSTRACT:The equilibrium phase behavior of water (nonsolvent)-DMF (solvent)-PVDF system at 25°C was investigated via both theoretical and experimental approaches. Using binary interaction parameters, ij , obtained previously, the theoretical phase boundaries were computed and were found to match closely the measured binodal and crystallization-induced gelation data. Membranes were prepared using the isothermal immersion-precipitation processes in various dope and bath conditions. The formed membranes demonstrated a broad spectrum of morphologies: At one extreme, asymmetric structure was obtained featuring a continuous tight skin and a sublayer composed of parallel macrovoids and cellular pores; at the other limit, skinless microporous membrane was produced with spherical particles packed into a bi-continuous structure. The crystalline characters of PVDF gels and membranes were examined by small angle light scattering, scanning electron microscopy, and differential scanning calorimetry techniques. In addition, diffusion trajectories and concentration profiles in the membrane solution before precipitation were calculated for the immersion process. These results predicted reasonably the various morphologies observed in the membranes.

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