Gradient Measurements of Phase Behavior in Block Copolymer-Homopolymer Blends

Abstract

AB diblock copolymer/homopolymer blends are of great interest because the addition of homopolymer can dramatically affect the phase behavior of the block copolymer. The miscibility of A homopolymer in AB block copolymer is limited by the molecular weight of the homopolymer; to circumvent this constraint, we employ AB block copolymer and C homopolymer blends, where the C homopolymer exothermically mixes with the minority A block. Another challenge of blending studies is that the phase space can be tedious to characterize due to the need to make many different discrete blends that span all the blend ratios of interest. In this thesis, I investigated the use of a “high throughput” method of studying these blends by flow coating a homopolymer film with a thickness gradient and subsequently spin coating a uniform layer of block copolymer on top. This creates a range of blend ratios along the film—the resulting morphologies are then probed using atomic force microscopy. Using this gradient blending technique, I studied three triblock copolymers (Kraton G1651, G1652, G1657), poly(styrene-ethylene/butylene-styrene). These block copolymers were blended with poly(2,6-dimethyl 1,4-phenylene oxide) (PXE), which mixes exothermically with polystyrene. The sphere-forming G1657 exhibited a miscibility limit with PXE before reaching the sphere-to-cylinder phase transition. Thin G1657 films also exhibited PS domain swelling with PXE at the core of the spherical microdomains. The cylinder-forming G1652 underwent a cylinder-to-lamellae phase transition and showed no miscibility limit for PXE. G1652 also exhibited coexistence of oppositely oriented cylinders or lamellae (i.e. microdomains oriented both in-plane and out-of-plane with the substrate) in a single film. Another cylinder-former G1651 did not show any blending with PXE; we hypothesized that this was due to slow diffusion caused by the block copolymer’s high molecular weight. Based on these observations, we concluded that this gradient blending technique can provide rapid insight into a variety of interesting phase behaviors and blending physics, such as phase transitions and miscibility limits, in thin films that may be difficult to observe otherwise.