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Poor solvent as a nucleating agent to induce poly(ε-caprolactone) ultrathin film crystallization on poly(vinylpyrrolidone) substrate

Guo Caixia,Huo Hong

The poor solvent (hexanol)-induced crystallization of poly(ε-caprolactone) (PCL) ultrathin film on poly(vinylpyrrolidone) (PVPY) substrate is studied by using atomic force microscopy (AFM). PCL single crystals with screw dislocations melt and wet on PVPY substrate, forming a PCL ultrathin film with continuously changed film thickness. The effect of film thickness on the PCL crystallization morphology is directly observed by using the wetted PCL ultrathin film. Morphology transition from compact seaweed (CS) to fractal dendrite (FD) occurs with the decrease of the film thickness. A lot of dispersed FDs appear when PCL ultrathin film recrystallizes in the presence of minute amount of hexanol. We denote it as a “multi-nuclei” phenomenon. The content of residual hexanol is regulated by changing sample drying time under vacuum or sample melting temperature. Results show that the condition to obtain the highest nuclei density (or crystal density) is that the sample is dried under vacuum at room temperature for 24 h during sample preparation and melts at 200 °C for 10 min before recrystallization. If hexanol evaporates completely, the large area of FD grows continuously and no multi-nuclei phenomenon occurs. The interaction between PCL and PVPY substrate is excluded as the reason of the induction of multi-nuclei phenomenon. PVPY which blended in the PCL ultrathin film during the sample preparation is immiscible with PCL and inhibits the nucleation and crystallization of PCL. In this work, poor solvent provides us a new method to find nucleating agent for polymer thin films.