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Modelling the effects of confinement on the glass transition temperature and segmental mobility

An off-lattice simulation of the effect of confinement geometry on segmental mobility and the glass transition temperature has been conducted using Monte Carlo techniques. The off-lattice random coil Monte Carlo model represents the polymer chains, N − 1 units long, as a string of N impenetrable spheres. The segmental mobility is characterized by monitoring the mean square displacement of a bead over two different time intervals, one and fifty Rouse relaxation times, as the temperature is lowered. The change in volume fraction of occupied space, a measure of density, is monitored to detect the glass transition temperature Tg. Simulations have been conducted on freely jointed polymer chains, which have been spatially confined by one dimension (between parallel plates) and by three dimensions (in a sphere). For both environments the value of Tg decreases as the confining dimension decreases in length. The onset of the decrease occurs first when the confinement is 3D and the magnitude of the effect is largest for 3D confinement. For 3D spherical confinement, throughout the cooling cycle, the segmental beads have a significantly higher mobility in the outer spherical region compared to those near the centre of the confinement space. For the 1D confinement, between two plates, any differences in mobility for regions near the plates compared to the centre region disappear as the temperature is lowered.