Physical properties of dense amorphous poly(vinyl alcohol) as revealed by molecular dynamics simulation

• Dense amorphous poly(vinyl alcohol) (PVAL) built from a chain with 145 repeating units was studied by constant number of particles, pressure, and temperature-molecular dynamics simulations at temperatures between 400 and 527K. The results of the simulations were compared with pressure-volume-temperature data, solubility parameter, x-ray scattering pattern, and data for the characteristic ratio. The fractional free-volume distribution was computed and the diffusion characteristics of water in the polymer were studied. Experimental water desorption data were obtained and they were compared with the results obtained from the simulations. The AMBER-force field was initially adopted to generate the dynamics, but this force field yielded structures with too high specific volumes. The size of the oxygen atoms was then decreased until the maximum difference between experimental and simulated specific volumes was within 2.2%. This modification of the force field resulted in satisfactory agreement between experimental and simulated data for the volume expansion coefficient and the compressibility in the temperature range from 402 to 502K. The essential features in the local packing of the atoms as revealed by wide-angle x-ray scattering were also present in the simulated structures. Simulated values for the characteristic ratio were in agreement with earlier reported experimental values. The solubility parameter showed only minor deviations from reported experimental values. Water-molecule trajectories yielded diffusivity in accordance with the experimental zero-concentration diffusivity. The water molecule makes well-resolved jumps (cage-like penetrant motion) even at the highest temperature of simulation, 510K. The water molecule rotates much faster than it makes jumps in the PVAL matrix. The lifetimes of the hydrogen bonds between polymer chain segments were longer than for those of the hydrogen bonds formed between water and polymer chain segments. This finding suggests that the primary rate-controlling factor for water diffusion in this polymer is the constraining effect of hydrogen bonds formed between polymer chain segments.

Nyckelord

poly(vinyl alcohol)

molecular dynamics simulation

physical properties

water

diffusion

polymer-water interaction

diffusion

polyethylene

hydrogels

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