Phase Transitions and Fluctuations in Block Copolymer–Based Soft Materials

Abstract

Block polymers have sustained the interest of both academic and industrial researchers for decades as the preeminent uniquely tunable self-assembling soft materials, finding practical utility in a diverse array of existing commercial products and emerging technologies. This thesis describes an experimental investigation of a class of materials we call short diblock copolymers—block polymers that self–assemble into ordered mesophases even at low molecular weight as a result of high thermodynamic incompatibility between the constituent blocks. The low molecular weight of short diblock copolymers enables accessing the ever-smaller features sizes required by applications such as nanolithography. However, the behavior of short diblock copolymers is dominated by fluctuations and consequently is not well described by the classical mean-field theoretical framework used to understand block polymer phase behavior. This thesis involves the synthesis of a series of model short diblock copolymers and detailed characterization of their thermodynamics and dynamics using a variety of experimental tools including small-angle scattering, rheology, and thermal analysis techniques. The phase behavior is documented, including the discovery of an ordered phase with dodecagonal quasicrystalline symmetry. Taking advantage of opportunities afforded by the low molecular weight of short diblock copolymers, an expansion of the experimental toolkit for block polymer characterization was achieved through the adaptation and application of relaxation calorimetry in conjunction with the development of new approaches for exploiting established tools like differential scanning calorimetry. These tools provide new insight into the central role of composition fluctuations on the order–disorder phase transition in block polymers. In addition, the unusually comprehensive set of experimental characterization data for a single volumetrically symmetric short diblock copolymer is compared to the predictions of the recently developed renormalized one-loop theory and simulation results.