Connecting macroscopic properties to microstructure of block copolymer materials through simulation

Abstract

Due to their molecular topology, block copolymers exhibit rich and unique physical properties that make them of interest for use in a wide variety of applications. In this thesis, we first discuss the methodology and development of software for performing self-consistent field theory calculations. We then investigate the properties of block copolymers and their microscopic origin in two distinct contexts. First, we consider the equilibrium states of materials consisting solely of block copolymers or of majority block copolymer. Block copolymers are well-known to self-assemble into ordered microstructures. These microstructures constitute a balance of entropic and enthalpic contributions to the free energy. The particular microstructure that forms depends on the block copolymer chemistry, the temperature, and if it is a multi-component system, the blending fraction. In addition to the long-established classical phases such as lamellae or hexagonal cylinders, near the turn of the century, theoretical efforts led to predictions of the stability of more complicated phases in block copolymer melts such as the Frank-Kasper A15 phase might be stable in block copolymer melts. Furthermore, Frank-Kasper phases, originally discovered in metals in the 1950s, had been reported to form in other soft matter systems such as lipid-containing micelles. They nonetheless remained elusive in block copolymer materials until 2010 when the Frank-Kasper σ phase was serendipitously discovered. This discovery was followed by the discovery of many more complex ordered microstructures in polymer melts and blends, leading to questions about what drives these phases to be stable. In this work, we calculate structures of diblock copolymer melts using self-consistent theory and perform a novel analysis that connects free energy to crystal structure. We find that the transition from the body-centered cubic (bcc) phase to the σ phase is driven by a decrease in enthalpy, and that this decrease is a result not of a decrease in contact area but instead of a sharpening of the interface. This contradicts previous explanations employing strong-stretching theory. We then transition to investigate the properties of immiscible polymer blends compatibilized with block copolymers as small weight-fraction additives. Due to vanishing entropy of mixing, all commercially relevant polymers are thermodynamically immiscible. In blends, this manifests as large phase-separated domains with weak separating interfaces across which there is a very low degree of entanglement between dissimilar polymers. Thus, blends of immiscible polymers exhibit poor mechanical properties. However, these blends are of interest because of their potential relevance in multi-functional materials or in recycling of highly heterogeneous waste streams. Early efforts attempted to improve blend properties without any additives by refining the structure of the interface to promote entanglement. In the past two decades, however, attention has turned to block copolymers as potential additives to achieve the two goals of compatibilization: reduction of domain size and improvement of mechanical properties. Block copolymers are known to act in a surfactant-like fashion by reducing interfacial tension at homopolymer-homopolymer interfaces. Furthermore, they have been found to improve mechanical strength through anchoring mechanisms such as entanglement and co-crystallization. Diblock copolymers were first investigated for their performance as “compatibilizers”, and recent work has shown other architectures such as linear multiblock copolymers to significantly outperform diblock copolymers. In this work, we investigate the thermodynamic and mechanical properties of polymer blends compatibilized by linear multiblock copolymers using coarse-grained molecular dynamics. We find that the density and uniformity of interfacial crossings determined the reduction of interfacial tension. Furthermore, we identify the existence of an optimal loading of copolymer that maximizes toughness and strain-at-break, and examine the mechanism of failure of a glassy compatibilized blend under uniaxial elongation. We show that failure occurs via a two-step mechanism that involves cavitation at the interface followed by simultaneous re-densification and chain pullout. This mechanism is qualitatively different from failure of a single-component polymer glass, but leads to a nearly identical stress-strain response.