Effect of Chain Ends and Entanglement on Segmental Mobility of Polymers Under Confinement

It was suggested that segregation of chain ends to the free surface, and reduced entanglement of a polymer chain near an interface might lead to enhanced mobility of the polymer in thin films with thickness comparable to the unperturbed size of the polymer molecule, R_g. Variable Angle Spectroscopic Ellipsometry (VASE) was used to investigate the glass transition temperature, T_g of polystyrene (PS) thin films deposited on silicon covered with native oxide. We studied the thickness (d) dependence (1 to ~ 20R_g) of the T_g of the polymers at two different weight averaged molecular weights, M_w = 13.7K and 550K Da - one above and one below the entanglement molecular weight (M_w ≃ 20K Da), as well as the M_w dependence (M_w = 13.7K to 2.3M Da) of the T_g at two different thickness, d = 15 and 50 nm. For the 13.7K and 550K samples, T_g decreases monotonically with decreasing d, with negligible difference between the two. This result shows that the density of chain ends and degree of entanglement hardly affects the amount confinement modifies segmental mobility of polymers in thin films. On the other hand, M_w dependence of T_g weakens noticeably with decreasing d. We hence deduced that the free volume associated with chain ends was reduced by as much as 40% when d was reduced to 15 nm. Our finding undermines the effect of chain ends in causing the observed reduction in T_g under confinement.