We report the effect of molecular weight and comonomer content on melt crystallization of model random ethylene 1-butene copolymers. A large set of narrowly distributed linear polyethylenes (PE) was used as reference of unbranched molecules. The samples were crystallized from a melt state above the equilibrium melting temperature and cooled at a constant rate. The exothermic peaks of the melt-solid transition are reported as the crystallization temperatures (T c ). Following expectations, the T c of unbranched PE samples was constant and independent of the initial melt temperature. The same independence was observed for copolymers (2.2 mol % ethyl branches) with molar mass below 4500 g/mol. Moreover, the T c of copolymers with higher molar mass depends on the temperature of the initial melt, T c increases as the temperature of the melt decreases. We attribute the increase in T c to a strong crystallization memory in the melt above the equilibrium melting, and correlate this phenomenon with remains in the melt of the copolymer’s crystallizable sequence partitioning. Albeit molten, long crystallizable sequences remain in the copolymer’s melt at a close proximity, lowering the change in free energy barrier for nucleation. The residual sequence segregation in the melt is attributed to restrictions of the copolymer crystalline sequences to diffuse upon melting and to reach the initial random topology of the copolymer melt. Erasing memory of the prior sequence selection in copolymer melts requires much higher temperatures than the theoretical equilibrium value. The critical melt temperature to reach homogeneous copolymer melts (T onset ), and the comonomer content at which melt memory above the equilibrium melting vanishes are established. The observed correlation between melt memory, copolymer crystallinity and melt topology offers strategies to control the state of copolymer melts in ways of technological relevance for melt processing of LLDPE and other random olefin copolymers.

Recently, experiments reported a strong memory effect of crystallization in model ethylene-based homogeneous random copolymers after being annealed at temperatures higher than the equilibrium melting point of copolymers. By means of dynamic Monte Carlo simulations of random copolymers, we reproduced this phenomenon in the similar model copolymer systems. We attributed this phenomenon to the sequence-length segregation upon first-time crystallization. The resulting heterogeneous melt of copolymers survives upon annealing below the critical demixing point that could be much higher than the equilibrium melting point of copolymers. Therefore, the local high concentration of long sequences raises the local melting point to accelerate primary crystal nucleation upon second-time crystallization. This source of memory effects demonstrates how crystallization can be influenced by the substantial trend of demixing between different sequences in homogeneous random copolymers.

Extremely fast crystallization of high-density polyethylene and random copolymers of ethylene with up to 16 mol % 1-octene was observed for the first time by ultrafast scanning calorimetry. In order to account for the inherently high crystallization rate of polyethylenes, in nonisothermal and isothermal crystallization experiments cooling rates up to 1 000 000 K/s and crystallization times as short as 10 μs, respectively, were employed. It was possible to supercool the melt of high-density polyethylene down to 57 °C and the melt of a random ethylene/1-octene copolymer with 16 mol % 1-octene down to −33 °C, without prior crystallization. At these temperatures, the characteristic time of the primary crystallization process is of the order of magnitude of 100 μs. Complete vitrification of the liquid would require cooling even faster than 1 000 000 K/s. Compared to the homopolymer, the cooling-rate dependence of the crystallization temperatures and the temperature dependence of the characteristic time of primary crystallization of random ethylene/1-octene copolymers both are nearly parallel shifted to lower temperatures. Fast crystallization under conditions of reduced linear crystal growth rate is possibly caused by boosting homogeneous nuclei density up to 10 27 m −3 and urgently requires further investigation.

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The bivariate, or cross branching distribution of a gas-phase produced, film-grade ethylene 1-hexene copolymer with enhanced Elmendorf tear in machine direction, MD, and in transverse direction, TD, (> 400 g/mil) and high dart impact has been characterized through the analysis of fractions obtained by molecular weight and 1-hexene composition. The molecular weight fractions, obtained by a solventnon-solvent fractionation technique, are each mixtures of molecules with at least two different 1-hexene compositions, one component with a constant relatively high density ($1 mol% hexene) and a second of a lower density broadly distributed along the molecular weight fractions. The content of the low density component increases with increasing molecular weight of the fraction while the level of 1-hexene decreases. The mixed compositional character of these fractions is easily inferred by their high crystallization rates and both high melting and crystallization temperatures compared to the values of model random ethylene copolymers. The set of compositional fractions obtained by TREF display an increasing 1-hexene concentration with increasing molecular weight, and except for the highest molecular weight components (Mw > 150,000 g/mol) their melting and crystallization behavior followed the random pattern. Higher than expected melting temperatures and a constancy of the high melting temperature peak with increasing crystallization temperature, suggests that the intra-chain 1-hexene distribution of the highly branched, high molecular weight fraction deviates strongly from the random behavior. These structural features and the bimodal character of the composition distribution of this resin, that contains high molecular weight chains with both low and high 1-hexene contents, are correlated with the enhanced key film properties.

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Applications of cyclic olefin copolymers (COC) are manifold, and developing a complete understanding of the characteristics of the glass transition behavior of these materials is essential to their processing and use in a variety of those applications. Fast scanning calorimetry is employed to investigate the glass transition of three commercial TOPAS COCs of different norbornene content between 37 and 54 mol% at cooling rates varying from 0.01 to 100 000 K s −1 . The glass transition temperatures as a function of cooling rate are determined as limiting fictive temperatures from reheating scans at 100 000 K s −1 . The data, covering seven orders of magnitude in cooling rate, allows the determination of the dynamic fragilities. The dynamic fragilities of TOPAS 6013, 6015, and 8007 are in the range of 50-60 and increase with increasing norbornene content. Compared to literature data of other polymers, the dynamic fragilities are rather low, and the COCs are classified as relatively strong glass formers. In other words, relating this information to processability, they have a relatively large "working range" and are classified as "long" glasses. Such classification, borrowed from glass making (blowing) technologies, may also help to describe the processability of polymeric glass formers in general.

The glass transition is a kinetic phenomenon. Its basic characteristics-the glass transition temperature, T g , and the width, δT g , of the glass transition interval-depend significantly on cooling and heating rates as it is observed experimentally by standard DSC and fast scanning calorimetry. The knowledge of these and related correlations is of outstanding importance both for a theoretical understanding of vitrification and devitrification processes and their control in a variety of technological applications. By these reasons, general kinetic criteria of vitrification and, based on them, theoretical expressions for the glass transition temperature and the width of the glass transition range in dependence on rates of change of temperature have been derived in previous papers. These results are advanced here by establishing a direct correlation between these two quantities, T g and δT g . The ratio δT g /T g is shown, for arbitrary cooling and heating rates, to be a function of an appropriately defined index being a straightforward generalization of the definition introduced by Angell. The theoretical results are tested and confirmed by experiment. The methods employed here can be utilized generally for the description of vitrification caused also by a variation of other external control parameters like pressure. K E Y W O R D S glass transition, glasses J E L C L A S S I F I C A T I O N 64.70.kj Glasses; 64.70.Q-Theory and modeling of the glass transition How to cite this article: Schmelzer JWP, Tropin TV, Fokin VM, et al. Correlation between glass transition temperature and the width of the glass transition interval. Int J Appl Glass Sci. 2019;10:502-513. https ://