Analysis of the Degree of Reversibility of Crystallization and Melting in Poly(ethylene-co-1-octene)

Abstract

The degree of reversibility of crystallization and melting of poly(ethylene-co-1-octene) containing 7.3 mol % (24 mass %) of 1-octene was measured with temperature-modulated differential scanning calorimetry. The sample was crystallized at different cooling rates after excluding self-nuclei of the crystals. The sample was also examined by X-ray scattering techniques, infrared spectroscopy, and standard differential scanning calorimetry. The cooling rates ranged from 0.1 K min -1 to quick quenching. The low-temperature enthalpy remained constant in all cases, by increasingly creating a pseudohexagonal crystallinity of lower heat of fusion instead of a smaller amount of orthorhombic crystals. The crystallization during cooling from the nonseeded melt is characterized by a steplike crystal growth, a characteristic not seen on slow, seeded, crystallization. The initial growth is followed by a continuous increase of the crystallinity down to the glass transition. The onset of crystallization shifts to lower temperatures with increasing cooling rates. With increasing cooling rate, the perfection and the size of the initially formed crystals decrease. On slow crystallization orthorhombic crystals grow; on fast crystallization, these are increasingly replaced by the less-ordered, pseudohexagonal crystals. These changes are paralleled by a systematic increase of the reversible crystallization and melting by about 10% when increasing the cooling rate from 1 to 10 K min -1 . For the first time these surprising observations have been discussed on the basis of structural information.

Phase Transitions of Bulk Statistical Copolymers Studied by Dynamic Monte Carlo Simulations

“…The short sequences without the necessity of chain folding at the edge of lamellae, or in the granular crystallites, fringed micelles, or clusters, may pack quite loosely 4 and contribute to pseudohexagonal crystallinity in ethylene-1-octene copolymers. 60 However, in the case of our heterogeneous copolymers, the attained absolute crystallinity seems sensitive to the comonomer content. In association with the molecular segregation in the melt, this implies that the maximal probability of crystallization of the monomer sequences is only in homogeneous phases.…”

“…The decay of excess heat capacity as a function of time should be attributed to the optimization of crystallographic structure, especially near the fold surface, and without any increase in the lamellar thickness. 35 The excess heat capacity appears fully reversible under temperature modulations and seems not to be associated with the fully melting and recrystallization of granular domains, 60 which should show a hysteresis since their sizes are larger than the critical nucleus during primary nucleation. Actually, this hysteresis has been observed in the low-temperature regions of Figures 2 and 4.…”

Properties of poly(ethylene 1‐octene)‐g‐maleic anhydride copolymers prepared via solvothermal process

“…24,26 The physical structure and the properties of inter-and intramolecularly homogeneous poly(ethylene-co-1-octene) are mainly controlled by the constitution of the macromolecule. 27 Because the short-chain branches-MAH existed, the rigidity of the chains was improved, packing of the chains within the crystal was strongly hindered, and the physical parameters like degree of crystallinity and crystallization temperature were strongly influenced. 28 Therefore, the crystal temperature decreases with the MAH concentration.…”

Communication: Theory of melt-memory in polymer crystallization

“…Such self-nucleation from remnant crystalline domains has been qualitatively assigned as the cause of the melt-memory effects. [6][7][8][9][10][11]15,16 However, there have been several investigations where any possible memory from such remnant crystalline domains is deliberately wiped out, but still observing melt-memory effects during second crystallization. 5,[12][13][14][17][18][19][20][21] As a classic example, Schultz observed that the maximum crystallization rate of linear polyethylene can be significantly lower if the sample was prepared at higher melt temperatures.…”