New Aspects on Melting and Annealing Behaviour of Polymers

Poly( 1-butene) (PB) crystallizes from the melt in a metastable modification II (mod. II) which slowly transforms into the stable modification I (mod. I). X-ray wide angle (WAXS) measurements show that in mod. I the size of the microparacrystallites (mPC’s) in chain direction, D̅012, the polydispersity gD of the size distribution in this direction, the lateral size D̅110 and the paracrystalline g110-value do not change upon annealing at temperatures up to the melting point. In mod. II, however, the sizes D̅012 and D̅110 increase with rising annealing temperature Tann. At a certain Tann and beyond a sufficient annealing time tann the size D̅012 shows a logarithmic increase with tann whereas D̅110 stays constant. Measuring melting points Tm of mod. I-samples, we found a linear relationship between Tm and 1/D̅012 according to the Thomson equation resulting in a melting point for an infinite crystal of Tm∞ (mod. I) = 139 °C and a mean surface free energy of σ̅e′̅ (mod. I) = 47 ergs/cm2. T m versus 1/D̅012 for mod. II is linear only for high D̅012-values yielding Tm∞ (mod. II) = 130 °C and σ̅e′̅ (mod. II) = 29 ergs/cm2. However, a partially molten and afterwards quenched sample of mod. I with small mPC’s shows a mod. II-peak which fits the straight line extrapolated from the large D̅012-values. The DTA curves of mod. I-samples shift to higher temperatures and narrow after annealing although the crystallite sizes and size distributions remain as well as the paracrystalline distortions the same. X-ray and DTA measurements eliminate therefore surface premelting and selective melting of thinner and more distorted lamellae in mod. I. Upon annealing this modification, σ̅e′̅ decreases from 47 ergs/cm2 to 15 ergs/cm2 and the distribution of σe′ narrows. The latter determines predominantly the shape of the DTA curve. The Thomson equation therefore, applied to different samples links only the average crystallite size and the mean surface free energy with the melting point. In mod. I partial melting occurs independent of D̅012 and starts mainly at those mPC’s which have exposed surfaces with high σe′. At the beginning only single mPC’s or single lamellae melt, but no bundles of lamellae. The logarithmic increase of D̅012 in mod. II with tann can be explained according to Hosemann’s model of “lateral melting” also by a partial melting of mPC’s with unprotected lateral surfaces and by a consecutive solid state diffusion of their chainsegments into the two mPC’s adjacent in chain direction, increasing the averaged sizes of the long period and the lamellae thickness.