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

The crystallization behavior of poly(1-butene) (P1b) was investigated by po larized light microscopy (PLM), atomic force microscopy (AFM), wide-angle X -ray scattering (WAXS), dilatometry, and also by time- and temperature-reso lved small-angle X-ray scattering experiments (SAXS). Observations in the P LM indicate a temperature-dependent change in the mechanism of crystallizat ion. When crossing a certain critical crystallization temperature, the morp hology changes from spherulites to quadratic, platelike single crystals. In vestigations of samples with different molar mass show that the transition temperature is molar mass-dependent; on decreasing the molar mass the trans ition shifts to lower temperatures. As proved by WAXS, both the spherulites and the single crystals are of the metastable form II. The morphological c hange 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 l eads to variations in the temperature dependence of the crystallization rat e 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 crystalli zation temperature, T-c, and the melting temperature, Tf, are linearly depe ndent on the reciprocal crystalline layer thickness, d(c)(-1), but with dif ferent slopes and different limiting temperatures for d(c)(-1)-->0. The obs ervations are again indicative for a crystal development in two steps: Firs t an initial form appears which then transforms into the final lamellar cry stallites. As a new feature, in direct correspondence to the two different crystallization mechanisms observed microscopically, two different crystall ization lines (d(c)(-1) vs T-c) show up, indicating the occurrence of two d ifferent 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 mechanis ms produce crystallites with similar surface free energies. We discuss the peculiar crystallization properties of P1b by comparing the radius of gyrat ion R-g of the chains in the melt with the crystal thickness d(c) and propo se that the change in the crystallization mechanism could be due to a chang e from folded-chain to chain-extended crystallization, taking place when d( c) gets larger than R-g.