Melts of supra-molecular polymers: A study of their thermodynamics, structure and dynamics by Monte Carlo simulations
Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
Final Report Abstract
The aim of this project was to improve our understanding of the aggregation statistics of polymers with hydrogen-bonding stickers in the melt state and its influence on the dynamic response of this melt. Also, dynamics and mechanical response of such melts have been shown to be rather complex due to the interplay of topological constraints like entanglements and the bonding dynamics of the hydrogen bonding stickers. We developed a coarse-grained model for supra-molecular precursors of linear polyethylene-glycol (PEG) and polybutylene-glycol (PBG) chains with hydrogen bonding moieties at both ends. We showed that this model well reproduces the conformational statistics of the precursors on large scales. In a series of publications we addressed the single-chain thermo- dynamics and conformational statistics, the thermodynamics and structural transitions in solution and the dynamic behavior of a dense melt of these precursors. The statistical properties were obtained using Stochastic Approximation Monte Carlo (SAMC) simulations, a powerful at histogram Monte Carlo simulation approach allowing to calculate the density of states of a model numerically exactly. This yields thermodynamic and structural properties for all temperatures. The melt simulations used a parallelized Molecular Dynamics code. Single precursor chains show a ring-forming transition upon cooling which is a continuous transition at which two conformational states (closed and open ring) coexist. The transition temperature depends non-monotonously on chain length due to chain stiffness effects. In solution, there exist two conformational transitions. At high temperatures, the solutions consists of a gas of isolated precursor chains. Lowering the temperature, the precursors aggregate into linear chains, and, finally, these linear chains undergo ring formation. The aggregation temperature increases with concentration, whereas the transition temperature of ring formation decreases with concentration. Both trends can be understood from effective free energy arguments. As a reference for the analysis of the dynamics of supra-molecular PEG melts we first analyzed our coarse-grained homopolymer PEG model. This showed, that the simple model captures the transition from Rouse behavior to reptation behavior nicely, reproducing the experimentally known entanglement molecular weight. We compared this to simulations of melts of precursor chains of length N = 16 which form entangled aggregates at room temperature. However, they also form (mostly small) rings, and these rings lead to additional entanglements in the system. The life-time for the ring forming hydrogen bonds is dominating the relaxation time of the Rouse modes of the linear supra-molecular chains.
Publications
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Thermodynamics of single polyethylene and polybutylene glycols with hydrogen-bonding ends: A transition from looped to open conformations, J. Chem. Phys. 148, 084905 (2018)
E. Lee, W. Paul
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Morphology and thermodynamics of polymers with monofunctional hydrogen bonding ends in dilute and semidilute concentration, Phys. Rev. E 100, 012502 (2019)
E. Lee, W. Paul
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Additional Entanglement Effect Imposed by Small Sized Ring Aggregates in Supramolecular Polymer Melts: Molecular Dynamics Simulation Study, Macromolecules, 53, 1674 (2020)
E. Lee, W. Paul
