The influence of thermal treatment on the morphology in differently prepared films of a oligodepsipeptide based multiblock copolymer

Abstract

Degradable multiblock copolymers prepared from equal weight amounts of poly(ε-caprolactone)-diol (PCL-diol) and poly[oligo(3S-iso-butylmorpholine-2,5-dione)]-diol (PIBMD-diol), named PCL-PIBMD, provide a phase-segregated morphology. It exhibits a low melting temperature from PCL domains (Tm,PCL) of 38 ± 2 °C and a high Tm,PIBMD of 170 ± 2 °C with a glass transition temperature (Tg,PIBMD) at 42 ± 2 °C from PIBMD domains. In this study, we explored the influence of applying different thermal treatments on the resulting morphologies of solution-cast and spin-coated PCL-PIBMD thin films, which showed different initial surface morphologies. Differential scanning calorimetry results and atomic force microscopy images after different thermal treatments indicated that PCL and PIBMD domains showed similar crystallization behaviors in 270 ± 30 µm thick solution-cast films as well as in 30 ± 2 and 8 ± 1 nm thick spin-coated PCL-PIBMD films. Existing PIBMD crystalline domains highly restricted the generation of PCL crystalline domains during cooling when the sample was annealed at 180 °C. By annealing the sample above 120 °C, the PIBMD domains crystallized sufficiently and covered the free surface, which restricted the crystallization of PCL domains during cooling. The PCL domains can crystallize by hindering the crystallization of PIBMD domains via the fast vitrification of PIBMD domains when the sample was cooled/quenched in liquid nitrogen after annealing at 180 °C. These findings contribute to a better fundamental understanding of the crystallization mechanism of multi-block copolymers containing two crystallizable domains whereby the Tg of the higher melting domain type is in the same temperature range as the Tm of the lower melting domain type.