Date of Award

5-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Committee Member

Dr. Hongseok Choi, Committee Chair

Committee Member

Dr. Georges Fadel

Committee Member

Dr. Oliver J. Myers

Abstract

Thermoplastic polymers have been widely used in the industry due to their high toughness and impact resistance along with their remolding capabilities as compared to thermoset polymers, which also makes them an attractive choice for polymer composites. Many desirable features of thermoplastic polymers like ease of processing, low weight and cost, and corrosion resistance make thermoplastics a viable option for applications in the automotive, aerospace, sporting goods and many other industries. To further increase the mechanical, thermal and electrical properties of thermoplastic polymers, nanomaterials are added to the polymer matrix that can improve these properties at very low loading levels as compared to conventional fillers. In this work, Polypropylene (PP), a semi-crystalline thermoplastic known for its balance of strength, modulus and chemical resistance has been used as the polymer matrix. It is one of the most widely used thermoplastic polymers in several industries as it shows a good combination of stiffness, toughness and creep resistance along with being light-weight and cost effective. Carbon Nanofibers have been used as the nanomaterial in this work for understanding the effect of nanomaterials to mechanical properties of the polymer matrix. Thermoplastics, including Polypropylene, exhibit varying mechanical properties based on the different loading rates that they are subjected to. Polymers experience stress relaxation at constant strains and creep under constant load due to their viscoelastic nature, i.e. they exhibit properties both of an elastic solid and a viscous liquid. The stress relaxations are distinct for different polymers and are divided into unique processes that lead to a strain-rate dependency of the semi-crystalline polymer which has been studied in this work. To fabricate these nanocomposites, ultrasound-assisted mixing has been used to reduce the processing time, utilizing the well-known dispersive qualities of ultrasound in solutions. Ultrasound-assisted mixing, as a processing technique for directly manufacturing polymer matrix nanocomposites has not been studied much in the literature. Ultrasonication in polymer solutions can also be responsible for polymer degradation due to its cavitation effects. To understand the effects of dispersion and polymer degradation caused by ultrasonication, mechanical mixing of polymer solutions has also been used as a counterpart to the ultrasonication process. For studying the effects of processing and nanomaterial addition on the strain-rate dependency of the polymer matrix, tensile tests were conducted using injection molded dog-bone samples made as per the ASTM D638 V standards. For manufacturing these dog-bone samples, an injection mold was designed and manufactured based on the statistical analysis of simulations conducted using Moldex 3D, a polymer melt-flow simulating software. Tensile strength, elongation at break and the tensile modulus values have been used as the basis for comparison. To understand the strain rate dependency of the polymer and its nanocomposite, quasi-static strain rates varying from 10-4 to 10-1 s-1 have been utilized. Thermal Gravimetric Analysis (TGA) has also been conducted for studying the effects on the thermal properties of the polymer. Polypropylene has shown a visible response to varying strain rates as expected, as the strength and modulus of the polymer increases with increasing strain rate, while the elongation decreases. Ultrasonically processed polymer and its nanocomposites also show a similar linearity in the strain-rate dependency as the pure polymer. The effects of ultrasonication on the polymer degradation have been presented along with the effects of addition of nanomaterials. Mechanical and thermal properties have been discussed based on the tensile tests and TGA. Conclusions and future recommendations are presented based on the observations done.

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