Crystallization of Weakly Segregated Poly(styrene-b-ε-caprolactone) Diblock Copolymer in Thin Films

Abstract

The surface morphology and crystallization behavior of a weakly segregated symmetric diblock copolymer, poly(styrene-b-ε-caprolactone) (PS-b-PCL), in thin films were investigated by optical microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy (AFM). When the samples were annealed in the molten state, surface-induced ordering, that is, relief structures with uniform thickness or droplets in the adsorbed monolayer, were observed depending on the annealing temperature. The polar PCL block preferred to wet the surface of a silicon wafer, while the PS block wet the air interface. This asymmetric wetting behavior led to the adsorbed monolayer with a PCL block layer having a thickness of around 4.0 nm. The crystallization of PCL blocks could overwhelm the microphase-separated structure because of the weak segregation. In situ observation of crystal growth indicated that the nucleation process preferred to occur at the edge of the thick parts of the film, that is, the relief structures or droplets. The crystal growth rate was presented by the time dependence of the distance between the tip of crystal clusters and the edge. At 22 and 17 °C, the average crystal growth rates were 55 ± 10 and 18 ± 4 nm/min, respectively. Nevertheless, at a low crystallization temperature (17 °C), the formation of a primary nucleus and subsequent growth in the adsorbed monolayer with a crystalline PCL layer of 4.0 nm were directly observed by real-time AFM. It was concluded that the primary nucleus formed at the polymer/substrate interface and the average growth rate for individual edge-on lamellae at 17 °C was about 12 nm/min. The nucleation mechanism in the adsorbed monolayer was discussed, and the influence of the polymer/substrate interface and weak segregation accounted for the nucleation process.