Melt Structure and Crystallization of Random Ethylene Copolymers

Linear low density polyethylenes (LLDPEs are substantially linear polyethylenes [(C2H4)n] with randomly distributed 1-alkyl short chain branches. The presence of branches on the ethylene backbone sharply reduces the crystallinity level of LLDPEs and retards the crystallization rate compared to the homopolymer. With respect the linear chain, LLDPEs are more flexible, ductile, and more transparent; hence, they are widely used for film applications. It is of great interest to find avenues to increase the crystallization rate of LLPDEs, or increase production cycles, due to its huge global demand. This work covers a comprehensive study of the correlation between melt topology and phase structure with the crystallization rates of LLDPEs. Narrowly distributed random ethylene 1-alkene copolymers display strong memory of crystallization in the melt, even at temperatures above their equilibrium melting point, as indicated by a sharp increase in the re-crystallization temperature. Melt-memory is associated with ethylene sequences from prior crystallites that remain in close proximity forming clusters that do not dissolve till very high temperatures. The melt memory strength first increases with branch content due to a topologically more complex inter-crystalline region formed by diffusing a larger number of crystalline sequences and then decreases due to depleted crystallinity level. The observed MM strength is independent of alkyl branch length up to octyl. Increasing the crystallinity level makes the inter-crystalline region topologically more complex, retarding the dissolution of the clusters in the melt. The dissolution process is thermally activated and becomes extremely slow in the melt of high molar mass copolymers (> 50 kDa) with ~2.2 mol% ethyl branches. Broadly distributed random ethylene 1-alkene copolymers, display a special melt memory effect: the crystallization rate increases and then decreases with lowering temperature the melt is cooled from. This inversion of the expected trend of the crystallization rate is associated with the onset of liquid-liquid phase separation (LLPS) between lowly branched molecules and highly branched molecules. Upon decreasing melt temperature and crossing the binodal, the thermodynamic drive of phase separation enhances chain dynamics and accelerates dissolution of melt memory. Strong evidence of LLPS is provided in both, solid and melt states using TEM, AFM, SANS, SALS and BF-OM. The inversion of crystallization rate is enhanced in systems with a broader distribution of branch content due to stronger thermodynamic drive of phase separation.