Browsing by Subject "Block copolymers"

Now showing 1 - 3 of 3
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    Block copolymer-based ion gels as solid polymer electrolytes
    (2012-10) Zhang, Sipei
    Block copolymer-based (BCP) ion gels are a class of interesting solid polymer electrolytes (SPEs) in electrochemical applications. This thesis aims to systematically study the mechanical and electrical properties of BCP-based ion gels formed by the selfassembly of ABA triblock copolymers in an ionic liquid, and find ways to enhance the properties of the gels, in order to formulate optimal designs in terms of the triblock for applications to electrochemical devices. Two particular target applications are organic transistors and electrochemical capacitors, due to the very large specific capacitance (on the order of F/cm2) of these electrolytes and therefore low voltage operation and potentially desirable energy storage. To study the effect of the BCP on the properties of ion gels, BCPs with different midblocks and end-block lengths were prepared, and the viscoelastic and electrical properties of the ion gels were investigated over large composition and temperature ranges. The gels were formed by the self-assembly of poly(styrene-b-methyl methacrylate-b-styrene) (SMS) and poly(styrene-b-ethylene oxide-b-styrene) (SOS) in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]). The end-blocks associate into cross-links, while the midblocks are well-solvated by this ionic liquid. In terms of viscoelastic properties, it was found that the plateau modulus of the gels depends primarily on concentration and the molecular weight of the mid-block, while high temperature behavior is controlled by the length of the end-blocks. A body-centered cubic (BCC) structure was observed at elevated temperatures only for gels with short end-blocks due to end-block pull-out from the cross-linking cores, while the relaxation of the end-blocks are within the cores for gels with long end-blocks. In terms of electrical properties, the double-layer capacitance of the gels was found to be fairly insensitive to polymer content and identity, whereas the ionic conductivity varies significantly especially at polymer concentrations of more than 20 wt%. It was also found that the presence of the end-blocks primarily obstructs the ion paths without much effect on ion number density. In terms of materials design, a flexible, low molecular weight mid-block is desirable. Generally, there is a trade-off between ionic conductivity and shear modulus for this type of gels. To enhance the mechanical properties of the gels, a novel ion gel based on poly[(styrene-r-vinylbenzyl azide)-b-ethylene oxide-b-(styrene-r-vinylbenzyl azide)] (SOS-N3) with chemically cross-linkable end-blocks was prepared. The gel with 10 wt% polymer goes through two transitions as temperature increases: solid (physically crosslinked network) --> liquid --> solid (chemically cross-linked network). The modulus and ionic conductivity was found to remain fairly constant after chemical cross-linking, while the toughness is more than 8 times higher. This demonstrates a promising approach to improve the mechanical properties of a moderately dilute gel without interfering with the high ionic conductivity. Overall, BCP-based ion gels are promising SPEs for transistor and capacitor applications. Through judicious selection of the triblocks, the properties of the gels can be tuned to fulfill different requirements.
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    Data for Chain and Structural Dynamics in Melts of Sphere-Forming Diblock Copolymers
    (2024-07-22) Chawla, Anshul; Bates, Frank S; Dorfman, Kevin D; Morse, David C; dorfman@umn.edu; Dorfman, Kevin D; Dorfman Research Group
    Processed simulation data appearing in the related manuscript
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    Toughness in block copolymer modified epoxies
    (2014-09) Declet-Perez, Carmelo
    One of the major shortcomings preventing the widespread use of epoxy resins in engineering applications is the inherent brittleness of these materials. The incorporation of small amounts of amphiphilic block copolymers into the formulation is one of the most promising strategies to toughen epoxies. These molecules are known to form nanostructures in the epoxy resin that can be preserved upon curing. This strategy is very attractive since significant enhancements in toughness can be obtained without detrimental effects on other properties of the matrix. Despite many examples of successful implementation, an in-depth understanding of the factors that lead to toughness in block copolymer modified epoxies is still elusive. The goal of this dissertation is to understand, first, the deformation mechanisms leading to toughness and, second, how different formulation parameters affect these processes.In this work we used two types of block copolymer modifiers, which produced nanostructures with different physical properties. These block copolymers self-assembled into well-dispersed spherical micelles with either rubbery or glassy cores in various epoxy formulations. Both of these modifiers toughened different epoxy formulations, although to different extents. The rubbery core micelles consistently outperformed the glassy core micelles by roughly a factor of two. While the toughening afforded by the rubbery core micelles was consistent with the current understanding of toughening, the results from the glassy core micelles could not be explained with the same reasoning.In order to understand the deformation mechanisms leading to different levels of toughness, we performed small-angle x-ray scattering experiments while simultaneously deforming our material. This combination of techniques, referred to as in-situ SAXS, allowed us to monitor changes in the structure of the block copolymer micelles as a result of the applied load. With this technique, we showed that the rubbery core micelles undergo a dilatational process while the glassy core micelles deform with constant volume. These results provide definitive evidence of cavitation in rubbery nanodomains, a result anticipated by theoretical calculations. The notion of cavitation is useful in understanding the toughness enhancement of the rubbery core micelles; however, it does not explain the toughening from the glassy core micelles. To explain the toughening afforded by the glassy core micelles we proposed the idea of network disruption in the region spanned by the corona block. We suggested that this mechanism is also capable of initiating plastic deformation of the matrix, although to a lesser extent than cavitation. Accordingly, the main toughening mechanism in block copolymer modified epoxies is plastic deformation of the matrix initiated by either cavitation of rubbery domains or by the zone of disrupted network depending on the properties of the micelle core. Having established that the matrix is responsible for dissipating the most amount of energy during fracture, we also investigated the effect of varying the crosslink density and flexibility of the network by means of in-situ SAXS. In networks formulated with different crosslink densities, but the same type of molecules, we found a correlation between different levels of toughness provided by either, rubbery or glassy core micelles, and differences in deformability of the epoxy network. In networks formulated with a different crosslinker, which incorporates flexible groups into the matrix, we found that the properties of the network strongly influence the type of deformation the block copolymer micelles undergo. In conclusion, this work has established a connection between different extents of toughening enhancement, the physical properties of the block copolymer micelles, and the properties of the epoxy network. Judicious selection of all of these formulation parameters is needed to obtain an optimal toughening effect.