Molecular dynamics simulations using umbrella sampling methods to sample the free energy of stretching a poly(dimethylsiloxane) (PDMS) oligomer (M w = 904 g/mol) in a melt of PDMS oligomers were performed at 300 K. The free energy of stretching and restoring force were found to be mainly a result of the changes in entropy of the chain as the chain was contracted or stretched, and only at severe extensions do energetic contributions due to deformation of internal bends make a significant contribution to the free energy. The changes in entropy with chain extension were found to be due to restrictions in the polymer dihedral sequences, where the average torsional distribution did not change appreciably until severe extensions. Because the PDMS chain studied is only between 3 and 4 statistical segments in length, the force and free energy could not be fit with the exact solutions of the Gaussian or freely jointed chain models over the region of moderate to high stretching, but the wormlike chain model fit well with deviations only at severe extensions. Excellent agreement, even at severe extensions, was achieved using an extensible wormlike chain model wherein the changes in the chain length smaller than the persistence length (determined from the simulations) were used to adjust the end-to-end distances of the chain. Results are compared with previous PDMS stretching simulations in the literature.
All Science Journal Classification (ASJC) codes
- Organic Chemistry
- Polymers and Plastics
- Inorganic Chemistry
- Materials Chemistry