The University of Massachusetts Amherst
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PhD Defense: Kiran Subramaniam Iyer, "Kinetics of the Crystal-Melt Phase Transformation in Semicrystalline Polymers"


Tuesday, December 17, 2019 - 3:00pm


A110/A111 Conte



Polymers are widely utilized as materials owing to their tunable mechanical, chemical, and electronic properties. This has led to their use as commodity materials, specialist materials or as materials under active research for futuristic purposes such as solar cells and organic electronics. A key feature that affects these material properties is the crystallinity of the polymer, for which there are very few unifying concepts despite investigations across several decades. This is because of the considerable conformational entropy of the polymers, which competes with the intermolecular attractions, and drives the crystallization process. Thus, any quantity that can control the conformational entropy of the polymer, whether through equilibrium thermodynamic factors or through kinetic factors, can also direct the crystallization of the polymer. This thesis studies the effect of kinetic factors on the crystallization of semicrystalline polymers primarily using computer simulation methods.

In the first work, the effect of chain ends is studied by comparing the crystallization behavior of ring polymers with that of linear polymers. Recently, experimental understanding of the semicrystalline state of ring polymers has been made possible due to the development of the Grubbs Catalyst which generates high purity ring polymers, although several experimental features cannot still be reconciled because of mutual disagreements. An attempt has been made to gain a fundamental understanding of the crystallization of ring polymers using coarse-grained Langevin dynamics simulations on model polymeric systems. Various features of both ring and linear polymers have been studied, including their melting behavior, equilibrium shapes of the crystals, and kinetics of the growth process. Contrary to expectations from equilibrium thermodynamics, the single ring polymers melt at lower temperatures than the linear polymers. They crystallize into several metastable lamellar thicknesses, with their growth process proceeding through several free energy barriers. The results of these simulations are in qualitative agreement with several experiments and have resolved long-standing disagreements in the determination of equilibrium melting points and crystalline morphologies.

In the second work, we investigate the process of melting of polymer crystals. The mechanism of melting is influenced by the annealing temperature and the annealing time as seen in experimental literature. Some of the intriguing experimental observations are about the spontaneity, and the multiple processes that take place during melting. However, these experiments do not provide a precise molecular understanding as they investigate bulk systems. On the other hand, simulations provide clear details of the molecular processes that occur during melting and show that polymer crystals go through a globular metastable state at low melting temperatures before transforming to completely molten chains at higher melting temperatures. To quantify these metastable states, a free energy landscape is computed using parallel tempering Langevin dynamics simulations. The computed free energy landscape confirms the existence of metastable states, with the preference for the metastable state reducing with increasing melting temperature. Our results add to the understanding of how semicrystalline polymers melt, and answer some of the intriguing molecular questions posed by experimental literature.

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