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, dc -1 , but with different slopes and different limiting temperatures for dc -1 f 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 (dc -1 vs Tc) show up, indicating the occurrence of two different initial states. On the other hand, only one common melting line (Tf vs dc -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 Rg of the chains in the melt with the crystal thickness dc and propose that the change in the crystallization mechanism could be due to a change from foldedchain to chain-extended crystallization, taking place when dc gets larger than Rg.

Lamellar crystal thickness transition of melt-crystallized isotactic polybutene-1 observed by small-angle X-ray scattering

“…We observed two different dependences of l c on 1/ÁT and the transition between them. This is consistent with the observation by Fu et al (2001). On the other hand, l c depends linearly on 1/ÁT in each of the low and high supercooling ranges, which is in accordance with the nucleation theory.…”

“…4. This is in agreement with the observation reported by Fu et al (2001). At 1/ÁT = 0.017, l c lies in the middle of the two linearities, which suggests a transition state between the two linear relationships.…”

“…If the crystal thickness is smaller than the chain dimensions in the melt, Fu et al (2001) assumed that crystallization does not necessary need a chain disentangling; the entanglements can be shifted into the amorphous regions, where they can be accumulated together with the other noncrystallizable chain parts such as end groups and stereodefects. An estimation of the chain dimensions in the melt is the radius of gyration R g .…”

“…Recently, Fu et al (2001) observed a deviation from the nucleation theory in the temperature dependence of the crystal thickness of isotactic polybutene-1 (it-PB1) determined from electron density correlation analysis of small-angle X-ray scattering (SAXS) profiles. They observed that two different relationships exist between crystal thickness l c and crystallization temperature T, and that a transition from one relationship to the other occurs, i.e., l c increases discontinuously accompanied by a morphological change of the crystals when l c becomes comparable to the chain dimensions.…”

“…To elucidate the influence of chain dimensions on the resultant lamellar long period structure, we have reinvestigated this transition in terms of more simple methods, using it-PB1 samples with a different molecular weight distribution from that used in the study by Fu et al (2001). We have studied the temperature dependence of the first-and second-order long periods and lamellar crystal thicknesses of it-PB1 tetragonal crystals grown in the melt by SAXS experiments and crystallinity measurements.…”