Microporous films of polyolefins, namely, polyethylene and polypropylene, have been prepared using the process based on the extrusion of the melt with the subsequent annealing, uniaxial extension,...

The crystallization of flexible-chain molecules under conditions of molecular extension is considered. The molecular orientation was characterized by a parameter β that is the ratio of end-to-end distance to the contour length of macromolecule. Calculations were performed for two types of crystals: folded-chain crystals and extended-chain crystals. Thermodynamical analysis shows that the decrease of conformational entropy owing to the drawing of the molecule into a crystal makes an essential contribution to the free-energy change. This value is of great importance in the definition of the equilibrium state of a system and so it is the reason why it is impossible to get the degree of crystallinity equal to 1. The values of molecular orientation at which each of two mechanisms of crystallization takes place are evaluated. It is shown that there is a critical value β* as a boundary between two ranges of extensions: at β < β*, the crystallization with the formation of folded-chain crystals is thermodynamically preferable; if β > β* can be obtained in the melt, extended-chain crystals are formed. The dependence of the degree of crystallinity and the melting temperature on β for each type of crystal are calculated. The data obtained agree with experimental results.

A new interpretation of the mechanism of the polyvinylidene fluoride (PVDF) membrane formation using the nonsolvent-induced phase separation (NIPS) method based on an analysis of the complete experimental phase diagram for the three-component mixture PVDF–dimethyl acetamide (DMAc)–water is proposed. The effects of the precipitation bath’s harshness and thermodynamic affinity of the polymer’s solvent on the morphology, crystalline structure, transport and physical–mechanical properties of the membranes are investigated. These characteristics were studied via scanning electron microscopy, wide-angle X-ray scattering, liquid–liquid porosimetry and standard methods of physico-mechanical analysis. It is established that an increase in DMAc concentration in the precipitation bath results in the growth of mean pore size from ~60 to ~150 nm and an increase in permeance from ~2.8 to ~8 L m⁻² h⁻¹ bar⁻¹. It was observed that pore size transformations are accompanied by changes in the tensile strength of membranes from ~9 to ~11 and to 6 MPa, which were explained by the degeneration of finger-like pores and appearance of spherulitic structures in the samples. The addition of water to the dope solution decreased both the transport (mean pore size changed from ~55 to ~25 nm and permeance reduced from ~2.8 to ~0.5 L m⁻² h⁻¹ bar⁻¹) and mechanical properties of the membranes (tensile strength decreased from ~9 to ~6 MPa). It is possible to conclude that the best membrane quality may be reached using pure DMAc as a solvent and a precipitation bath containing 10–30% wt. of DMAc, in addition to water.