Design Driven Multi-scale Mechanical Property Characterization of Polymer Composites

Eaton, Matthew David

doi:https://doi.org/10.21985/n2-3x13-6720

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Design Driven Multi-scale Mechanical Property Characterization of Polymer Composites

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There is no group of materials as diverse, complex, and ubiquitous as polymers. From plastic bags, to rubber tires, electronics, food packaging, water filtration and even aerospace applications, the penetration of polymer materials into all aspects of life make them very important materials throughout all engineering fields. However, this breadth of application means that many different variables can affect the properties of polymer materials, such as the chemical and physical structure of the polymer, the addition of any filler material or additive, or the effect of external stimuli like temperature, strain rate, and humidity. Thus, the development of new polymer materials can be an expensive and time consuming task. Computational simulations can be used to help predict properties and test a range of polymer formulations, but efficient models require robust experimental data to verify the accuracy of the models used. Smart experimental design is therefore imperative to efficiently develop new polymer materials. This thesis takes a design-driven approach to polymer materials characterization to establish and develop experimental methods to connect local properties and morphology to bulk mechanical behavior and bridging the gap between experiments and computational models. Given the breadth of polymer materials, three polymer systems will be explored in more detail, crosslinked rubber blends and nanocomposites, epoxies for fiber reinforced composite matrices, and thermoplastic starch. Each of these polymer material systems had unique motivations and characterization challenges, and so the experimental approaches had to be tailored for each system for efficient materials design and development. In rubber nanocomposites, chemical and physical interactions between fillers and rubber can result in interphase regions that have properties distinct from the constituent materials, which can complicate predictive models. Therefore, a novel atomic force microscopy (AFM) viscoelastic property characterization technique is developed and validated through comparison to bulk dynamic mechanical analysis (DMA). It is applied to multi-phase systems such as crosslinked rubber blends carbon black-rubber composites, and shows great potential for better understanding the time and temperature dependence of these local properties. Additionally, finite element simulations are paired with AFM indentation experiments for rubber composite systems to better understand the effect of indentation artifacts that can lead to artificial increases in modulus near a rigid particle. Pairing the results from FEA simulation and AFM experimentation leads to better interpretation of experimental data and initial results show potential for improved predicitve computational viscoelastic rubber composite models. For fiber-reinforced epoxy composite systems, applications for extreme, cryogenic temperature conditions are complicated by the thermal expansion differences between fibers and polymer matrices, resulting in large thermal stresses and microcracking, and therefore matrix materials with strong fracture toughness are desired to prevent catastrophic failure. This lead to the investigation of a set of model epoxy systems with nanoscale heterogeneity, that has previously been reported to have interesting ballistic properties. In this thesis, the temperature dependent mechanical and fracture properties of the model epoxy system are further characterized using a custom set-up force-displacement analyzer attached to a temperature control chamber and performing Vicker's indentation hardness and single edge notched bend (SENB) toughness tests. Distinct correlations between temperature dependent mechanical and fracture behavior, fracture surface morphology, and nanoscale heterogeneity are made. Additionally, these model epoxy systems are further investigation by fabricating and testing semi interpenetrating polymer networks (IPNs) of the epoxy with thermoplastic poly(methylmethacrylate), which initial data shows the potential for improved mechanical properties over a wider temperature range. Finally, due to their low cost, abundance and biodegradability, thermoplastic starch (TPS) materials have emerged as potential replacements for petrochemical-based single use plastics. However, these materials are very sensitive to atmospheric humidity, and current methods characterizing the effect of water content on TPS materials are labor intensive and time consuming. This thesis will present a non-destructive, efficient method to measure the full range of humidity dependence of viscoelastic mechanical properties and moisture absorption of thermoplastic starch (TPS) films using a quartz crystal microbalance (QCM). It will also be used to evaluate the effect of different plasticizers on TPS properties, and lays the groundwork for future investigations into efficient TPS materials development. From the demonstration of a diverse set of design driven mechanical property characterization approaches on multiple polymer systems, the work presented in this thesis can improve materials design and optimization in polymer materials development.

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