Confinement Effects on the Crystallization of Poly(3-hydroxybutyrate)

Poly(3-hydroxybutyrate) (PHB) is taken as an example to explore (i) whether the confined crystallization occurring in anodized aluminum oxide (AAO) nanopores is the same as that in ultrathin films, and (ii) whether the interfacial effect of curve surface is the same as flat surface. The crystallization behavior of PHB in AAO and thin films (sandwiched between two plates) has been compared. A curvature-dependent crystallization behavior of PHB is identified. Stable intermediate structures of PHB confined in narrow pores (diameter <100 nm), which have never been observed in bulk, are obtained for the first time. In larger pores (pore diameter >100 nm), crystallization occurs both in the center and interfacial regions in contact with AAO inner wall. This implies that the strength of interfacial layer weakens with decreased curvature and has been further proved by the crystallization behavior of its sandwiched ultrathin film. It is found that the highly restricted interface layer incapable of crystallization is about 30 nm for film, which is much thinner than that for nanorods (approximately 100 nm). We have also identified two different relaxations of PHB nanorods corresponding to the interfacial effect and spatial confinement, respectively. While the relaxation correlated to the interfacial effect with a slower relaxation time strengthens, the relaxation corresponding to spatial confinement with a faster relaxation time (bulklike alpha-relaxation) weakens with decreased pore size. This is completely different from that of sandwiched thin film, where only a thickness-independent alpha-relaxation same as bulk is observed 1 (Macromolecules 2006, 39, 5967). Therefore, the reduced crystallization kinetics of thin film is attributed to the reduction of long-range chain mobility. By contrast, the inhibited crystallization of PHB nanorods in AAO is attributed to the reduced segmental dynamics and the reduced chain mobility of PHB layer at interface.