Fractional crystallization and homogeneous nucleation of confined PEG microdomains in PBS-PEG multiblock copolymers

Fractional crystallization, homogeneous nucleation of poly(ethylene glycol) (PEG) segment, and self-nucleation behavior of PEG segment within miscible double crystalline poly(butylene succinate)-poly(ethylene glycol) (PBSEG) multiblock copolymers with different composition and segment chain length were studied by differential scanning calorimetry (DSC). Surface morphology of PBSEG10K with different PEG content was investigated by atomic force microscope (AFM). Different from di- or triblock copolymers, the microstructure and confinement of PEG dispersed phase in PBS matrix phase highly depends on chain length and sequence as well as segment content. The transition point of the PEG segment content from heterogeneous to homogeneous nucleation mechanism decreased from 50 to 39 wt % with PEG segment chain length increasing from 1000 to 2000 g/mol. When PEG segment chain length increased further to 6000 and 10000 g/mol, homogeneous nucleation phenomenon took place at much lower PEG content and fractional crystallization was observed at 29 and 24 wt %, respectively. Homogeneous nucleation mechanism of PBSEG(1K-36), PBSEG(2K-26), PBSEG(6K-19), and PBSEG(10K-12) was evidenced by the large supercoolings needed for crystallization, as well as first-order crystallization kinetics obtained. Self-nucleation behaviors of PEG segment still rely on the composition of PBSEGs. In the case of heterogeneous nucleation crystallization, self-nucleation behaviors of PEG segment showed standard self-nucleation behavior with classical three self-nucleation domains. When the crystallizable chains were confined into isolated microdomains, however, self-nucleation domain (domain II) disappeared. The absence of III(A) was observed in PBSEG(2K-39), while PBSEG(6K-29) had both III(A) and III(SA). Furthermore, AFM morphology studies still indicated the confined degree of PEG segment by previous PBS crystals was profoundly influenced by segment fraction. The confinement of the PEG segment by previous PBS edge-on lamellae was observed in the sample which displays a homogeneous nucleation crystallization behavior.