Glass Transition and its Characteristic Length for Thin Crosslinked Polystyrene Shells of Rodlike Capsules

Rodlike capsules consisting of a calcium carbonate core and a crosslinked polystyrene shell were synthesized, and the glass transition temperature (Tg) and characteristic length of the glass transition ξ(Tg) for the thin outer shells were investigated by temperature-modulated differential scanning calorimetry. The shell thickness ranged from 20 to 129 nm. The ratio of the Tg for the outer shell to the bulk Tg increases with decreasing shell thickness d. The d-dependence of Tg is interpreted in terms of a simple two-layer model which assumes that an immobile layer exists near the core-shell interface. Shells of hollow capsules unexpectedly exhibit a similar d-dependence of Tg to that for the filled capsules. This is characteristic of the crosslinked polymeric shells, and is attributed to certain spatial heterogeneity of crosslink distribution, and/or to the unstable configuration in the ultrathin shell that does not undergo relaxation due to the crosslink. The latter idea is based on the assumption that unstable configurational state is responsible for the Tg shift from the bulk value observed for nanosized polymeric materials. The ratio of the characteristic length for the shell of the filled capsule to that of the bulk ξf(Tg) /ξb(Tg) decreases with decreasing d. The results are interpreted in terms of the configurational entropy, and it is also suggested that the configurational state of network polymer chains in the shell affects the characteristic length.