An essential requirement for engineering materials is that they must possess both high stiffness and toughness. These two properties, however, are mutually exclusive, meaning that enhancing one typically leads to a reduction in the other. For example, rigid, glassy polymers can be toughened by incorporating soft, rubbery materials, but the inclusion of rubbery materials significantly reduces their strength and stiffness.

This dissertation aims to develop novel toughening agents that considerably enhance both the tensile toughness and impact strength of glassy polymers while preserving their strength and stiffness. High aspect ratio nanofibrils of three different thermoplastic elastomers (i.e., thermoplastic polyester elastomer, polyether block amide, and thermoplastic polyurethane) are developed and incorporated into poly(methyl methacrylate) (PMMA) to fabricate polymer-polymer composites that are not only highly tough but also stiff and transparent. PMMA was chosen as the matrix material because of its brittle nature and tremendous commercial value. This thesis explores the toughening mechanisms in polymer-polymer composites under different deformation speeds, as well as the role of interfacial adhesion and its impact on the toughening performance of the elastomeric nanofibrils. Additionally, the effects of incorporating elastomeric nanofibrils on various properties of PMMA, including mechanical behavior, light transmittance, rheology, glass transition temperature, scratch resistance, creep behavior, and mechanical recyclability, were investigated. It is found that the inclusion of only 3 weight percent (wt%) of high aspect ratio nanofibrils of thermoplastic elastomers can induce a brittle-to-ductile transition in PMMA. Since the toughened PMMA composites contain just 3 wt% of soft, rubbery material, their strength, stiffness, and transparency are nearly identical to those of unmodified PMMA. The major toughening mechanisms are identified as multiple crazing at slow deformation speeds and cavitation followed by subsequent ductile shear yield of the surrounding matrix at high deformation speeds.

This thesis advances the fundamental understanding of the processing-structure-property relationships in polymer-polymer composites, the toughening mechanisms in nanofibril-toughened polymers, and the role of interfacial adhesion in toughness enhancement. The elastomeric nanofibrils developed in this thesis can also be used to toughen other glassy or pseudo-ductile matrices. Finally, the fabricated composites have potential applications in diverse industries, including automotive, transportation, construction, lighting, furniture, and biomedical sectors, serving as alternatives to traditional PMMA.