(217ab) Conformations, Shapes, and Viscoelastic Properties of 1, 4 Polybutadiene Using a Rotational Isomeric State Approach

Molecular modeling was used to study the size and shape distribution and characteristics of cis and trans 1,4-polybutadiene chains. Computations were inspired from Flory’s Rotational Isomeric State approach in which energy distribution is considered over discrete rotational isomeric states. The Rotational Isomeric State approach was chosen because it allowed generating a large number of polybutadiene chains in a computationally cheap manner using less resources and computation time. The chains were considered under unperturbed condition. Characteristic ratios of both cis and trans chains were found to be in good agreement with experimental values. Increase in the characteristic ratio with increase in temperature was attributed to increased numbers of ‘taut’ conformations with increase in temperature, while the distributions of the most probable middle sized conformations was similar at the different temperatures. The probability distribution of short, medium and long chains followed the Gaussian model well until longer chain conformations. Chain shape parameters such as asphericity (deviation from spherical shape), acylindricity (deviation from cylindrical shape) and relative shape anisotropy were studied based on the squared radius of gyration for both cis and trans chains of different lengths and at different temperatures. Different shape distributions were observed for chains of the same average structure and size. The trans chains, of the same degree of polymerization, were found to be more spherical at lower temperatures than at higher temperatures, whereas cis chains were more spherical at lower temperatures; then with increase in temperature from 275 K till around 343 K, the chains became less spherical and further increase in temperature beyond 343 K resulted in the chains becoming more spherical again. The changes in shape with temperature were more pronounced for trans chains than for cis chains. The trans chains were found to be less spherical for chains of shorter length and gradually became more spherical as the length of chains increased whereas the cis chains were found to be more spherical in shape for shorter length chains and became less spherical as the chain length increased. Similar cylindrical shape comparisons were made for cis and trans chains.  Current steps involve computing effects of deformation forces on the existing chains under unperturbed condition and the changes in probability distribution. Changes in number and distribution of conformations lead to a change in entropy, which in turn can allows calculating viscoelastic properties such as storage and loss moduli.