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Author ORCID Identifier
Open Access Dissertation
Doctor of Philosophy (PhD)
Polymer Science and Engineering
Year Degree Awarded
Month Degree Awarded
H. Henning Winter
Complex Fluids | Nanoscience and Nanotechnology | Polymer and Organic Materials | Polymer Science
The self-assembly of block copolymers (BCP) into microphase separated structures is an attractive route to template and assemble functional nanoparticles (NP) into highly ordered nanocomposites and is central to the “bottom up” fabrication of future materials with tunable electronic, optical, magnetic, and mechanical properties. The optimization of the co-assembly requires an understanding of the fundamentals of phase behavior, intermolecular interactions and dynamics of the polymeric structure. Rheology is a novel characterization tool to investigate these processes in such systems that are not accessible by other means. With the combination of X-ray scattering techniques, structure-property relationships are determined as a function of NP loading in self-assembled hybrid composites.
This thesis examines two classes of BCP templates used for nanocomposite assembly. First, low molecular weight, disordered (low χN) BCP surfactants are considered. The addition of NPs with enthalpically favored interactions between the NP and one of the BCP domains boosts the phase segregation strength and drives self-assembly, resulting in highly filled nanocomposites (φNP ~ 30 vol.%) with small domain spacing (d0 ~ 10 nm) due to the low N. The effect of NPs on the self-assembly dynamics, material properties, and temperature dependent phase transitions are considered in the high loading regime. Oscillatory shear rheology reveals a transition from liquid-like to solid-like behavior with increasing NP content. The addition of stiff NPs to a soft polymer matrix, along with favorable intermolecular interactions, produces a x103 increase in the magnitude of G*. Phase transitions are investigated by correlating time-resolved rheology and time-resolved SAXS. Structure development and viscoelasticity scale with the NP content, and a general master curve describing behavior across all NP loadings is constructed. The access to new material properties and transitional phenomenon provides further insight into the complex structure-property relationships of this class of nanocomposites.
The second BCP template is microphase separated bottlebrush block copolymers (BBCP), macromolecules with discrete blocks of densely grafted side chains tethered to a molecular backbone. Highly extended backbone conformations and significant repulsion between grafted side chains are believed to suppress chain entanglements, resulting in rapid self-assembly (order of minutes) into large nanostructures (d0 > 100 nm) advantageous for optically active materials. A systematic study of model poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO) BBCPs with short side chains below entanglement molecular weight is conducted. We measure dynamic moduli G’(ω) and G”(ω) over a wide range of timescales. The scaling relationships in dynamic data show distinct power law behavior analogous to critical gels. The relaxation mechanisms are a consequence of the reduced entanglements and mobile microstructure. This interplay of high molecular mobility and rapid self-assembly contrasts the viscoelasticity of linear BCP materials with comparable microstructure.
The role that applied shear plays on the directed alignment of microphase separated lamellae in bulk BBCP samples is considered. The periodic structures are found to align at exceptionally low strain amplitudes and mild processing temperatures as confirmed by SAXS. Alignment over several mm3 is realized by high throughput synchrotron experiments and we hypothesize that this method can be applied as a means of fabricating and processing BCP-based hybrid materials with exceptional long-range order.
Building on the understanding from the highly loaded NP/BCP composites, similar considerations are taken towards the investigation of phase behavior, morphology, and rheological response in NP/BBCP hybrids. The goal is to understand how NPs and intermolecular interactions impacts the unique relaxation processes inherent to BBCP melts. From oscillatory shear rheology measurements, systematic transitions in the long-time relaxations towards solid-like behavior is observed with increasing NP loading, suggesting the NP inhibits the highly mobile microstructure and rapid side chain relaxations. The structure-property relationships realized by both rheology and SAXS lay the groundwork as we explore future manipulation and processing of these diverse structures for both well-established and emergent applications.
Yavitt, Benjamin, "Rheological Investigations of Self-Assembled Block Copolymer Nanocomposites with Complex Architectures" (2019). Doctoral Dissertations. 1544.