Thin Film Morphology of Symmetric Semicrystalline Oxyethylene/Oxybutylene Diblock Copolymers on Silicon

Abstract

Thin film morphologies of four symmetric semicrystalline oxyethylene/oxybutylene diblock copolymers (E76B38, E114B56, E155B76, and E224B113) were investigated by tapping mode atomic force microscopy (AFM). The effects of chain length, annealing, and surface property were examined. It is found that the lamellar orientation of the nascent block copolymer thin film varies with chain length. For low molar mass block copolymers (E76B38, E114B56, and E155B76), the lamellar orientation is parallel to the silicon surface, but for the high molar mass block copolymer, E224B113, the lamellar orientation is perpendicular to the silicon surface. The nascent block copolymer thin films of the low molar mass block copolymers are composed of multiple polymer layers with mixed thicknesses L ≈ L0 and L ≈ 1/2L0, where L0 is the long period of the block copolymers in the bulk. The existence of polymer layers with L ≈ 1/2L0 besides the first layer on the substrate surface indicates that the crystalline domain consists of double crystalline layers, and parts of the poly(oxyethylene) block can also be located at the polymer/air interface. Annealing leads to the disappearance of the metastable polymer layer of thickness L ≈ L0/2 via a morphological transformation involving different polymer layers and slight increase in lamellar thickness. The heterogeneities on the substrate surface can nucleate crystallization of block copolymer thin film during annealing when the lamellar structure is perpendicular to the surface, leading to formation of spherulites. The silicon surface was modified by grafting a monolayer of hydrophobic silane. On the modified surface, the lamellar structure of E224B113 becomes parallel to the surface, and the nucleation effect of the heterogeneities on crystallization of poly(oxyethylene) block is suppressed.