Understanding the phase transition in dilute solutions of short linear polymer chains is a fundamental problem in the field of polymer science. There are different factors contributing to such transformation like temperature, pressure, solvent quality, chain length and chemistry. As a first step toward understanding the mechanism of undergoing transformations, in this work we studied the effect of temperature and chain length on the conformational changes of isolated poly(ethylene) chains infinitely diluted in water. Systems considered are isolated short oligomers of polyethylene (C20,C30 and C40) in the temperature range of 300 to 360 K. Careful analysis of the radius of gyration showed that both swollen-extended conformation and collapsed globule conformations are possible for all chain lengths at all temperatures studied. As the free energy landscape indicates, the C20 chain tends to adopt the extended conformation, while the C40 chain more toward the collapsed conformation. As reported before, the C30 chain is bi-stable between both conformations with a small free energy barrier.  Detailed analysis of the chain’s shape, using the moment of inertia and a size independent shape factor, reveals that there is an expressive difference between the shapes of the collapsed states at different chain lengths. While shorter chains develop a two-dimensional disk shape structure, longer chains are more three dimensional and globule in shape. Data at different temperatures confirmed that, within the range of chain lengths considered, PE in water was not thermo-sensitive as free energy landscapes did not change by temperature significantly.  Regarding the mechanism of the conformational transition, the proximity water analysis and internal energy partitioning were done. The results demonstrated that the transition for shorter chains is mostly induced by the chain’s internal degrees of freedom, namely the dihedral angles within the backbone. On the other hand, as longer chains are more flexible and larger effect on water hydrogen bonding network, the conformational changes are mostly driven by proximity water or polymer/water interactions.

On the conformational changes of isolated, short poly(ethylene) chains in water

251st National Meeting (2016)

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