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) measure­ ments show that in mod. I the size of the microparacrystallites (mPC’s) in chain direction, Z>oi2 * the polydispersity gj> of the size distribution in this direction, the lateral size D u o and the paracrystalline <7no-value do not change upon annealing at temperatures up to the melting point. In mod. II, however, the sizes D012 and D n o increase with rising annealing temperature T annAt a certain T &nn and beyond a sufficient annealing time £ann the size A) 12 shows a loga­ rithmic increase with fann whereas Ihio stays constant. Measuring melting points T m of mod. Isamples, we found a linear relationship between T m and I/.D012 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 ae' (mod. I) = 47 ergs/cm2. T m versus 1/A)i2 for mod. II is linear only for high Z>oi2-values yielding T m°° (mod. II) = 130 °C and ae' (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 A)i2-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, ae' decreases from 47 ergs/cm2 to 15 ergs/cm2 and the distribution of ae' 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 D012 and starts mainly at those mPC’s which have exposed surfaces with high ae'. At the beginning only single mPC’s or single lamellae melt, but no bundles of lamellae. The logarithmic increase of D012 in mod. II with /ann 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.