A temperature- and molar mass-dependent change in the crystallization mechanism of poly(1-butene): Transition from chain-folded to chain-extended crystallization?

Abstract

The crystallization behavior of poly(1-butene) (P1b) was investigated by polarized light microscopy (PLM), atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), dilatometry, and also by time- and temperature-resolved small-angle X-ray scattering experiments (SAXS). Observations in the PLM indicate a temperature-dependent change in the mechanism of crystallization. When crossing a certain critical crystallization temperature, the morphology changes from spherulites to quadratic, platelike single crystals. Investigations of samples with different molar mass show that the transition temperature is molar mass-dependent; on decreasing the molar mass the transition shifts to lower temperatures. As proved by WAXS, both the spherulites and the single crystals are of the metastable form II. The morphological change is also observed in AFM images obtained after a rapid cooling of the samples to room temperature; the difference in the morphological appearance is preserved through the transformation from form II to form I. According to dilatometric measurements, the change in the crystallization mechanism leads to variations in the temperature dependence of the crystallization rate and also to a steplike increase in the crystallinity. The results of SAXS experiments show that the formation of P1b crystallites is governed by the same general laws as for other polymers studied before. Both the crystallization temperature, T-c, and the melting temperature, Tf, are linearly dependent on the reciprocal crystalline layer thickness, d(c)(-1), but with different slopes and different limiting temperatures for d(c)(-1)-->0. The observations are again indicative for a crystal development in two steps: First an initial form appears which then transforms into the final lamellar crystallites. As a new feature, in direct correspondence to the two different crystallization mechanisms observed microscopically, two different crystallization lines (d(c)(-1) vs T-c) show up, indicating the occurrence of two different initial states. On the other hand, only one common melting line (T-f vs d(c)(-1)) is found, which means that the two crystallization mechanisms produce crystallites with similar surface free energies. We discuss the peculiar crystallization properties of P1b by comparing the radius of gyration R-g of the chains in the melt with the crystal thickness d(c) and propose that the change in the crystallization mechanism could be due to a change from folded-chain to chain-extended crystallization, taking place when d(c) gets larger than R-g.