Conformational Asymmetry And Entropy-Driven Phase Transitions In Polymer Systems

In this dissertation, by combining single-chain molecular theory, statistical theory, Monte Carlo simulations and molecular dynamic simulations, we study some important issues regarding the effect of conformational asymmetry in polymer systems. Confor-mational asymmetry can cause immiscibility and de-mixing in polymer systems. This effect is found to be entropy-driven and only notable in molecular-crowding systems. Although much have been known for entropic effect in colloid and liquid crystal sys-tems, the entropic effect in polymer systems is still a mystery, which is not only a fundamental question in polymer physics but also has significant implications in un-derstanding the molecular-crowding phenomena in cell, especially for chromosome in nucleus. This dissertation mainly tackles this problem from three aspects, namely, the conformational (helix-coil) transition, external force (stretching) and the chirality.We first study the crowding-induced cooperativity in DNA surface hybridization. High density DNA brush is not only used to model cellular crowding, but also has a wide application in DNA sequence and DNA-functionalized materials. Experiments have shown complicated cooperative hybridization/melting phenomena in these sys-tems, raising the question that how molecular crowding influences DNA hybridization. In this work, a theoretical modeling including all possible inter and intramolecular interactions, as well as molecular details for different species, is proposed. We find that molecular crowding can lead to two distinct cooperative behaviors:negatively co-operative hybridization marked by a broad transition width, and positively cooperative hybridization with a sharp transition, well reconciling the experimental findings. More-over, a phase transition as a result of positive cooperativity is also found. Our study provides new insights in crowding and compartmentation in cell, and has the potential application in controlling surface morphologies of DNA functionalized nano-particles.The second topic of this dissertation is the stretching-induced immiscibility in non-monodisperse polymer systems. The behavior of polymer chains under stretch-ing is a classical problem in polymer science. However, a fundamental question still in mist, is how the stretching affects the interactions between polymer chains, espe-cially when the tensions on the chains are unequal, which is very common in many non-monodisperse polymer systems. In this work, we combine statistical theory and molecular simulations to study the influence of this tension disparity on the miscibility of athermal polymer systems. Through a minimal model, we demonstrate that when polymer chains of different lengths are under the same stretching, disparate tension states among polymer chains can lead to either macroscopic or microscopic phase sep-aration, depending on whether their ending points are mobile or not. Generally, the immiscibility found here is an entropic effect arising from conformational asymmetry between unequally stretched polymer chains. Our findings provide a new mechanism to explain the flow-induced de-mixing in polymer blends and indicate that heteroge-nous structure can occur during stretching, simply as a result of non-monodispersity in elastic polymer materials.The third topic of this dissertation is the geometry-controlled chiral interaction and complex phase behaviors in helices mixture systems. chirality is a unique property of structured objects, ranging from galaxy to elementary particles. How the chiral-ity affects the inter-objects interaction is a mysterious yet fundamental question. In this work, by studying a simple racemic mixture which possesses equal numbers of opposite-handed helices theoretically, we demonstrate that chiral interaction can in-duce complex mixing-demixing behaviors and liquid-solid transitions in the classical thermo-equilibrium region. What’s more surprised is that, these phase behaviors are found to be entropy-driven, totally controlled by the geometry of helices. Our work not only provides intuitive pictures to understanding more complex chiral phenomena, but also demonstrate that chirality can be a powerful self-assembling tool.At last, we give a summary and outlook regarding the conformational asymmetry effect of polymer chains and the resultant entropy-driven phase behaviors.