Thickening of Isolated and Multilayered Crystals and Its Implications on Melting Kinetics; The Role of Inter‐ and intra‐crystalline Regions

Abstract Melting of crystalline region in a semicrystalline polymer is a complex phenomenon, which is strongly influenced by the topological constraints present in the noncrystalline region. In an isolated crystal, detachment of chain stems from its crystalline surface can occur in succession or in cluster. The difference in detachment of the chain stems strongly depends on the topological constraints of the chain segments on the fold surface. Successive detachment of the chain stems, observed on melting of crystals while annealing at the onset of the peak melting temperature, appears to occur when chain folding is facilitated by adjacent reentry. In the annealing process, no crystal thickening in isolated crystals, having molar mass sufficiently high to suppress the effect of chain ends, is observed, as the crystal thickening will require cooperative chain mobility of the chain stems within the crystal linked by the chain segments in the fold. Retrospectively, because of the possibility of successive chain detachment, melting temperature decreases with decreasing heating rate, contradicting the assumption of increase in melting temperature on annealing. In the isolated crystals, having complex reentrant chains on the fold surface, successive detachment of chains from a crystal surface is frustrated, and cluster melting is promoted. Such crystals, though isolated, do not show melting on annealing. However, in contrast to the isolated crystals, in multilayered crystals instead of melting on annealing at the onset melting temperature, crystal thickening will occur that would cause increase in the melting temperature. This thickening process is facilitated, as it requires the localized motion only rather than the cooperative motion of the chain stems within the crystals. Thickening in the isolated crystals, if observed during polymerization or due to the appearance of a transient mobile phase, is referred as the primary thickening, whereas the crystal thickening in the multilayered crystals is called the secondary thickening.