The understanding of size-dependent properties is key to the implementation of nanotechnology. One controversial and unresolved topic is the influence of characteristic size on the glass transition temperature (T(g)) for ultrathin films and other nanoscale geometries. We show that T(g) does depend on size for polystyrene spherical domains with diameters from 20 to 70 nm which are formed from phase separation of diblock copolymers containing a poly(styrene-co-butadiene) soft block and a polystyrene hard block. A comparison of our data with published results on other block copolymer systems indicates that the size dependence of T(g) is a consequence of diffuse interfaces and does not reflect an intrinsic size effect. This is supported by our measurements on 27 nm polystyrene domains in a styrene-isobutylene-styrene triblock copolymer which indicate only a small T(g) depression (3 K) compared to bulk behavior. We expect no effect of size on T(g) in the limit as the solubility parameters of the hard and soft blocks diverge from each other. This strongly segregated limiting behavior agrees with published data for dry and aqueous suspensions of small polystyrene spheres but is in sharp contrast to the strong influence of film thickness on T(g) noted in the literature for free standing ultrathin polystyrene films.

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A series of 1,4-polybutadienes of varying molecular weight, both monodisperse and having broad or bidisperse molecular weight distributions, were studied using dielectric relaxation. The glass transition temperature, T g, and the T g-normalized temperature dependence of the segmental relaxation times, τα, varied monotonically with number-average molecular, M n. Polydispersity significantly affects neither T g nor the shape of the loss peak; the segmental relaxation dispersion is determined solely by M n, even when the distribution of chain lengths spans molecular weights over which T g varies. However, there is a small but significant influence of polydispersity on the T-dependence of the relaxation times, manifested as greater fragility in samples having bimodal molecular weight distributions. Properties of the prominent Johari−Goldstein (JG) secondary relaxation in 1,4-polybutadiene were measured and found to be qualitatively in accord with predictions of the coupling model. These results underscore the link between the JG and segmental processes, consistent with the JG relaxation functioning as the precursor to structural relaxation.

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