Copolymer Crystallization

 

Controlling the crystallization rate using the “melt memory” effect for the fabrication of medical devices.

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In this research work, we studied the effect of the topological structure of the copolymer melt on the subsequent crystallization rate. We are especially discovered in probing if melt-memory in random ethylene copolymers follows the dictates observed in the homopolymer, or if the need for selecting long crystallizable sequences within the entangled random coils, those additionally make more segmental constraints in the phase structure that prevent fast homogenization upon melting. We found a strong melt memory in random copolymer melts at temperatures even above equilibrium, and is absent in linear polyethylene fractions. To understand the origin and nature of the melt memory, and establish the critical value for the temperature of the melt above which crystallization kinetics are reproducible, we have studied the topology and their effect on melt memory experimentally in detail. We found that the strength of melt memory (number of seeds that remain above the equilibrium melt temperature) is molar mass dependent and requires a relatively high degree of initial crystallinity. These features support the dynamic nature of the memory effect. We have established that the origin of the self-seeds comes from the need to select suitable crystallizable sequences and transport them through the entangled melt to a crystal site. This process leaves an amorphous chain topology constrained by knots, loops, ties and other entanglements, such that upon further melting, diffusion of the partitioned sequences back to the homogeneous randomized melt state is a very slow process, hindered by this topology. For many copolymers, diffusion to the randomized melt requires temperatures well above their equilibrium melt temperature for a longer time.

 

In the second part of this work, we extend the study of the strong melt memory effect on crystallization to two types of ethylene−1-alkene copolymers (LLDPEs). Within the first type, copolymers were synthesized with a single-site metallocene catalyst leading to a relatively narrow unimodal interchain comonomer content distribution. Studies on this series display a strong memory effect of crystallization above their equilibrium melting point akin to the melt memory effect observed in model ethylene−1-butene copolymers. The second type of copolymer is commercial LLDPEs synthesized with either a Ziegler−Natta or with a metallocene catalyst, but with bimodal and broad interchain comonomer content distribution and broad molar mass distribution. This type of copolymer displays an inversion of the crystallization rate in a range of melt temperatures where narrow copolymers show a continuous acceleration of the rate. To conclude this peculiar behavior of the inversion, we remark on the onset of a self-seed assisted liquid−liquid phase separation (LLPS) between comonomer-rich and comonomer-poor molecules. The interplay between the numbers of self-seeds and chain diffusion during LLPS causes a decrease in the crystallization rate with decreasing melt temperature. The effect in nucleation density and in the overall crystalline morphology of crystallization from one-phase homogeneous melts (region A), one-phase heterogeneous melts (region B), and two-liquid-phase melts (region C) was followed by polarized optical microscopy, transmission electron microscopy, and atomic force microscopy.

 

In the third part of this work, we explored our expectation that upon melting, diffusion of crystalline sequences back to the randomized state will be easier if the level of crystallinity is lower than when the copolymer is fully crystallized. Thus, it was necessary to address how this strong melt memory effect can be correlated with the initial crystallinity level and whether there is a crystallinity threshold below which melt memory is no longer observed above the equilibrium melting temperature of the copolymer. We discovered a critical threshold level of crystallinity of 6–12% to observe the effect of melt memory on the subsequent crystallization rate of the copolymer studied in this research. To compare dynamic cooling and isothermal conditions, we determine the crystallinity threshold (≈6%) for dynamic cooling and (≈12%) for isothermal crystallization conditions. We have concluded that a faster development of the initial crystallinity may have more efficiently trapped knots and loops around the crystallites, leading to a lower crystallinity threshold than for slow or isothermally crystallized copolymers. The results help establish a correlation between the content of melt memory (as the number of self-nuclei) and the initial level of crystallinity of narrow LLDPEs.

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The following research articles are published from this project.

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[BO Reid, M Vadlamudi, A Mamun, H Janani, H Gao, W Hu, RG Alamo, “Strong memory effect of crystallization above the equilibrium melting point of random copolymers”, Macromolecules (ACS Publisher), 2013]

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[A Mamun, X Chen, RG Alamo, “Interplay between a Strong Memory Effect of Crystallization and Liquid–Liquid Phase Separation in Melts of Broadly Distributed Ethylene–1-Alkene Copolymers”, Macromolecules (ACS Publisher), 2014]

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[X Chen, A Mamun, RG Alamo, “Effect of Level of Crystallinity on Melt Memory Above the Equilibrium Melting Temperature in a Random Ethylene 1‐Butene Copolymer”, Macromolecular Chemistry and Physics (Macromolecular Publications), 2015]