Homopolymer induced shape transition in BCP self-assemblies : underlying mechanism and implications on the resultant nanostructures

Abstract

Block copolymers (BCP) are known to be able to self-assemble into a myriad of morphologies in solution. Having control over these morphologies is extremely important since the shape of the BCP nanostructures plays an essential role in determining the properties and application of the final construct. BCP morphologies are typically regulated by varying the relative chain length of the hydrophilicity and hydrophobicity blocks on the copolymer. However, this entails judicious tuning of the polymer block lengths which is neither a trivial nor broadly accessible option. Over the years, there is increasing awareness of factors beyond hydrophilicity-hydrophobicity length ratio that can affect the overall BCP self-assembly and researchers have started using these factors as alternative ways to manipulate the BCP morphologies. One such approach is through the use of homopolymer additives, which can be easily included in the self-assembling system to influence the final nanostructure. Although there have been reports that investigated how different homopolymer additives affect BCP self-assembly in solution, the effects of non-associating hydrophilic homopolymer additives remain poorly understood. This poses a knowledge gap in the understanding of BCP self-assembly and precludes researchers from taking advantage of a potentially facile method of tuning BCP morphology. This thesis aims to address this issue by examining the effects of non-associating hydrophilic homopolymer additives on BCP self-assembly in solution, using a model system comprising of poly(ethylene glycol) (PEG) homopolymer and poly(ethylene glycol)-b-poly(lactide) (PEG-PLA) copolymer. The first part of this thesis studies the mechanism by which the non-associating hydrophilic PEG homopolymer additive was able to influence the morphology of PEG-PLA BCP self-assemblies in water. The subsequent portions of this thesis explore how the effects of the PEG homopolymer additives can be strategically combined with the inherent properties of the PEG-PLA copolymer to generate complex nanostructures with unique behaviours such as stimuli-responsiveness and controlled shape transitions. The work here contributes to the overall understanding of self-assembling systems and how next-generation materials can be designed and engineered.