Effects of surface wetting induced segregation on crystallization behaviors of melt-miscible poly(l-lactide)-block-poly(ethylene glycol) copolymer thin film

The effects of wetting induced surface segregation on the crystal nucleation, growth rate, morphology and crystal orientation of melt-miscible crystalline-crystalline diblock copolymer of poly(l-lactide)-block-poly(ethylene glycol) (PLLA-b-PEG) thin films (∼90 nm) were investigated in this work with reflection optical microscopy (ROM), atomic force microscopy (AFM), and two-dimensional grazing incident wide angle X-ray scattering (2D GIWAXS) methods. The XPS results demonstrated that the wetting induced surface enrichment of PLLA block on the free surface of PLLA-b-PEG thin films occurred when the thin films annealed at 160 °C (molten state). When the PLLA-b-PEG thin film annealed at 160 °C for 30 min, the fraction of PLLA block (fPLLA) on the free surface could reach to ∼90%. The ROM, AFM and 2D GIWAXS results indicated that the nucleation, crystal growth, morphology and crystal orientation of PLLA block in PLLA-b-PEG thin films were significantly influenced by prior wetting process. For the PLLA-b-PEG thin film with wetting time shorter than 15 min (∼52% < fPLLA  <∼75%), the O-shaped flat-on nucleus preferentially formed on the free surface of thin films. However, for the thin film with wetting time longer than 15 min (fPLLA>∼  75%), the S-shaped edge-on nucleus preferentially formed on the free surface at early stage of crystallization and then grew into tilted flat-on lamella. Furthermore, the overall spherulite growth rate of PLLA in PLLA-b-PEG thin films decreased with increase of wetting time. We found that the crystallization behavior of PLLA on the free surface in the PLLA-b-PEG thin films was strongly influenced by surface wetting induced changes of surface energy, composition and chain alignments. Our results demonstrated that the crystallization kinetics, morphology and crystal orientation of crystalline block copolymer thin films can be controlled by prior wetting process.