Realizing Enhanced Toughness in Block Copolymer Modified Brittle Plastics
2016-08
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Realizing Enhanced Toughness in Block Copolymer Modified Brittle Plastics
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2016-08
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The great commercial importance of several brittle plastics continuously drives research efforts to be devoted to fabricating well defined structures in these materials for effectively toughening them. Amphiphilic block copolymers can be appropriately designed to generate nanometer scaled structures in a brittle plastic matrix at relatively low loadings (< 5% by weight). The resultant nanostructured plastics exhibit significant toughness enhancement without sacrificing other desirable properties such as transparency, stiffness and use temperature. The goal of this dissertation is to understand the nanostructure formation of block copolymers and the consequent toughening effect under various conditions. In this work we designed different types of block copolymer modifiers in concert with several commercially important brittle plastics, including epoxy thermosets and poly(lactide) (PLA) thermoplastics. The block copolymer toughening strategy was first established in bulk epoxies as well as in epoxy coatings through a model system study with the Jeffamine resin. Two distinct types of diblock copolymers formed spherical micelles in cured bulk epoxies and 15 micrometer thick coatings, but the process of solvent-casting affected the micelle size and distribution in the coating. The toughness enhancement observed in bulk epoxies (up to 5-fold increase in the critical strain energy release rate GIc) successfully translated to coatings, as evidenced by the over 40% increase in the coating abrasive wear resistance with only 5 wt.% of modifiers. Transmission electron microscopy (TEM) revealed that similar toughening mechanisms as those in bulk epoxies (micelle cavitation and matrix shear yielding) still held in thin coatings. Moreover, the hardness, modulus, transparency and glass transition temperature (Tg) of modified coatings were not appreciably affected compared to unmodified ones. Based on this model system study, we proceeded to investigate the commercially viable Cardolite resin system that is more complex thermodynamically but industrially relevant. A series of poly(ethylene oxide)-b-poly(butylene oxide) (PEO-PBO) diblock copolymers were synthesized at fixed composition (31% PEO by volume) and varying molecular weight expanding on a commercial product under the tradename Fortegra™ 100. Direct application of this product resulted in little improvement of the poor fracture toughness of the cured material. Modification of the resin formulation and curing protocol led to the development of well-defined spherical and branched wormlike micelles in cured resins. Thermodynamic interactions and the curing reaction together controlled the micelle formation as evidenced by small angle x-ray scattering (SAXS) measurements. A 9-fold increase in GIc over the neat bulk epoxy, and an over 30% improvement in the coating abrasive wear resistance over the unmodified coating were achieved at 5 wt.% loading of wormlike micelles. We then took one step further to explore the toughening efficacy of block copolymer micelles in hybrid composite systems in the presence of a second type of modifier, rigid graphene fillers with amine-functionalization. Both types of modifiers were well dispersed in cured epoxies with no observable interactions under TEM. The crosslink density of the epoxy network strongly affected the toughening effect. In the matrix with the lowest crosslink density, the combination of micelles and graphene drastically enhanced the GIc value to 19 times that of the neat material with no reduction in the elastic modulus and Tg. Additionally, hybrid ternary composites exhibited a synergistic toughening effect, revealing some positive mutual interference to the toughening mechanisms noted for micelles and graphene particles. Lastly, we extended the block copolymer toughening strategy to the PLA thermoplastic matrix. A low molar mass PEO-PBO diblock copolymer was uniformly dispersed as short cylindrical micelles in a commercial high molecular weight glassy PLLA plastic. This structure formation resulted from the negative Flory-Huggins interaction parameter (X) between PEO and PLLA. Those micelles could effectively toughen the matrix through concurrent cavitation, crazing and shear yielding. At only 5 wt.% of loading, micelles led to a greater than 10-fold increase in the tensile toughness and notched Izod impact strength over the neat PLLA in the glassy state. This toughening effect was retained in plastic films prepared with modified blends via a film blowing process.
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University of Minnesota Ph.D. dissertation.August 2016. Major: Material Science and Engineering. Advisors: Frank Bates, Lorraine Francis. 1 computer file (PDF); xiv, 269 pages.
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Li, Tuoqi. (2016). Realizing Enhanced Toughness in Block Copolymer Modified Brittle Plastics. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/182801.
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