Structural Transitions in Asymmetric Poly(styrene)-block-Poly(lactide) Thin Films Induced by Solvent Vapor Exposure

Abstract

Successive structural transitions in thin films of asymmetric poly(styrene)-block-poly(lactide) (PS-PLA) block copolymer samples upon exposure to tetrahydrofuran (THF) vapors have been monitored using atomic force microscopy (AFM) and both in situ and ex situ grazing incidence small-angle X-ray scattering (GISAXS). A direct link was established between the structure in the swollen state and the morphology formed in the dried state post solvent evaporation. This was related to the high incompatibility between the constituting blocks of the copolymer that thwarted the system from reaching the homogeneous disordered state in the swollen state under the specific conditions utilized in this study. Upon rapid solvent removal, the morphologies formed in the swollen state were trapped due the fast evaporation kinetics. This work provides a better understanding of the mechanisms associated with block copolymer thin film morphology changes induced by solvent vapor annealing.

Simulation methods for solvent vapor annealing of block copolymer thin films

“…40,41 GISAXS (grazing incidence small-angle X-ray scattering) studies have characterized the average morphology as a function of annealing time for several BCP systems. 25,[42][43][44][45][46] These experiments showed that ordering began during the swelling stages of the thin film and that the final structures were independent of the casting method. If films are swollen too much or if the BCP has a low w, the order is lost at a critical swelling thickness during the anneal as the system exceeds the ODT.…”

“…The pre-stage determines initial film thickness D 0 , but assuming a thermodynamic equilibrium is reached during annealing, the memory of the as-cast morphology is lost. 46 Swelling refers to the ingress of solvent into the thin film and the resulting increase in film thickness, and quenching or deswelling refers to solvent egress and the resulting decrease in film thickness. The film is usually assumed to collapse in the out-of-plane direction, preserving in-plane microdomains.…”

“…The film is usually assumed to collapse in the out-of-plane direction, preserving in-plane microdomains. 45,46 Annealing refers to the diffusion of polymer chains to form ordered structures in the presence of the solvent, and can occur during swelling and quenching.…”

“…If films are swollen too much or if the BCP has a low w, the order is lost at a critical swelling thickness during the anneal as the system exceeds the ODT. 45,46 In these cases ordered structures reappear during quenching and are highly dependent on the quench dynamics. However, if the system is quenched from an ordered state, little lateral (in-plane) shrinkage is observed and the film collapses in the out-of-plane direction.…”

“…However, if the system is quenched from an ordered state, little lateral (in-plane) shrinkage is observed and the film collapses in the out-of-plane direction. 46 This implies that the ordered structures formed in the swelled state are essentially preserved apart from an out-of-plane contraction as the solvent escapes. Thus the morphologies formed during SVA are a function of solvent removal rate, swollen solvent concentration, and swollen film thickness, but not the original as cast film morphology.…”