Polymer Modification with Multifunctional Additives and Unique Processing Methods

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Ware, Polina R
polymer composites; organic/inorganic thermoplastic elastomers; crystallizable additives; solid state deformation; semi-crystalline polymers; Polymer and Organic Materials
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This dissertation describes the investigation of unconventional methods to enhance polymer properties either by using unique processing methods, such as solid-state deformation, or by using multifunctional additives that can simultaneously provide a number of property enhancements. The three main research areas of this dissertation are self-reinforced glassy thermosets, melt-processeable organic/inorganic thermoplastic elastomers and semi-crystalline thermoplastics with improved ductility due to a unique solid-state process treatment.Various thermosets, including epoxies, are used in systems with fibers or other additives to improve stiffness. However such systems are difficult to process due to increased viscosity. This work investigates a new method to reinforce thermoset materials with low molecular weight (LMW) crystallizable additives. Desoxyanisoin, which is a pre-cursor to a flame-retardant molecule 4,4’-bishydroxydesoxybenzoin that is engineered to promote char, is used as a LMW organic crystallizable compound. At processing temperatures, desoxyanisoin is miscible with the matrix/epoxy thereby lowering the process viscosity. Upon cooling or reaction, desoxyanisoin phase separates and crystallizes providing unique in-situ reinforced composites. The mechanical and flammability properties of these desoxyanisoin-epoxy thermoset composites are described.Another approach to enhancement of flammability properties is demonstrated by fabricating organic/inorganic thermoplastic elastomers (TPEs) based on isotactic polypropylene (iPP) and polydimethylsiloxane (PDMS) materials by reactive melt mixing in the presence of peroxides. These polyolefin based thermoplastic elastomers are similar to the dynamically vulcanized elastomer-thermoplastic blends of polyolefins with ethylene-propylene diene copolymer (EPDM). Herein PDMS is used as an EPDM equivalent. Furthermore, not only PDMS is dynamically cross-linked, but there are some cross-links with the amorphous iPP. Varying the content of PDMS allows for design of TPEs with a wide range of mechanical and physical properties.Lastly, an approach to enhance the toughness and ductility of semi-crystalline polymers by deforming the material in a solid state to a strain-softening regime is described. This treatment imposes changes in the post yield behavior by process of compression and shear in the solid state at low temperature, low pressures and unconfined geometry. Crystal-structure alteration due to orientation and fragmentation of the lamellae during this treatment leads to the enhancement in non-linear ductility. The fundamental principles governing this approach are investigated.