Direct Observation of the Relief Structure Formation in the Nearly Symmetric Poly(styrene)-block-poly(ε-caprolactone) Diblock Copolymer Thin Film

Abstract

The relief structure formation associated with microphase separation and dewetting of the nearly symmetric poly(styrene)-block-poly(epsilon-caprolactone) diblock copolymer thin film was studied in this work, for which a suite of complementary methods, namely atomic force microscopy, optical microscopy, and X-ray photoelectron spectroscopy combined with hot stages, were applied. Through control of the microphase separation strength, indicated by the chi(N) (where chi and N indicate the Flory-Huggins interaction parameter and the total degree of polymerization, respectively), varied relief structures were observed. On one side, when there was no microphase separation (chi(N) = 3.9), typical droplets resulting from autophobic dewetting were revealed. On the other side, when there was intermediate microphase separation (chi(N) >= 14.0), superimposed lamellae except for droplets was discerned. Moreover, with continual heating, the formation of the superimposed lamellae and its dynamic transition to ordered droplets were first revealed with the in situ AFM scanning. On the basis of these findings, we conclude that the superimposed lamella is a metastable structure, resulting from the coupling of dewetting and microphase separation, and it finally reaches the equilibrium droplets. The formation of superimposed lamellae was attributed to that the microphase separation strength was forced to yield to the minimization tendency of surface tension.