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Bibliography: pages 89-93. / A laboratory wear testing facility has been developed to generate wear rate data for polymeric materials sliding at constant velocity against a hardened stainless steel base. The polymers investigated were ultra high molecular weight polyethylene (UHMWPE), ultra high molecular weight polyethylene-with a friction reducing additive (UHMWPE/FILL), polyoxymethylene (POM), poly(ethylene terephthalate) (PETP), molybdenum disulphide filled polyamide 6 (PA6/MoS₂) and graphite filled poly(amide-imide) (P(A-I)/GR). Testing was carried out as a function of sliding velocity between 0.13 to 2.27 ms⁻¹, loads of 1, 3 and 5 MP a and counterface roughnesses which varied from 1 micrometre to 0.25 micrometres. An increase in the counterface roughness resulted in a variable increase in the wear rate of the individual polymers except for the filled UHMWPE. These changes in the wear rate have been explained in terms of the mechanism of material removal. A progressive increase in sliding velocity has been shown to result in an initial increase in the wear rate followed by a decrease and finally a rapid increase for all materials under the majority of applied conditions. Explanations for such behaviour have been advanced in terms of the viscoelastic response of the polymers to strain rate and temperature. Low modulus materials however showed a significant drop in wear rate under low loads above a critical velocity which is believed to be due to a transition from boundary to partial el astohydrodynamic lubrication. Generally an increase in load gave an increase in wear rate for all polymers except for UHMWPE and filled UHMWPE at a counterface roughness of 1 micrometre. These conditions have been discussed with reference to the materials response to thermal effects and counterface interactions.

The purpose of this research was first, to investigate a polymerization method to produce polylactides of desired molecular weights. The polymers PLA-L and PLA-DL produced were then utilized to make blends of semicrystalline/amorphous components. The compatibility of the blends was investigated by determining their Tg with a DSC. However, because of the proximity of the Tg's of the two components, a new approach , which utilized the effects of the "physical" aging on the the Tg peaks obtained by DSC was considered. The results obtained by this method showed that only three blends containing low molecular weight species were compatible. All the others, while incompatible, possessed a certain degree of partial miscibility. The crystallization kinetics were investigated on the pure PLA-L and the blends. Two separate studies were conducted, the first on the kinetics of the overall crystallization by means of an Avrami analysis, the second on the growth rates of the individual spherulites. The kinetics of crystallization were found to decrease as the molecular weight of the semicrystalline component increased and as the PLA-DL content in the blend increased. In some particular cases, however, a spectacular increase of the growth rates was observed for high DL content blends. Optical microscopy showed that under these conditions, the spherulites grown, contained large inclusions of amorphous material. / M.S.

A constant-strain wear test apparatus was built and used to test polyethyleneterephthalate. The results of the constant-strain wear tests were compared to results from a constant-strain fatigue test. The results of the wear test showed that the polymer did not fatigue below the surface where the distortion energy was calculated to be a maximum. Instead, the polymer fatigued in an interfacial region at the surface due to stress concentrations resulting from surface irregularities, inhomogeneities, and localized adhesive forces. The adhesive forces were seen to increase as the number of cycles of the test increased eventually becoming the dominant frictional force. / M.S.

Structure-property relationships were investigated for the bis-glycidyl methacrylate deriva-tives of bisphenol-A crosslinked by electron beam irradiation. This material, commonly called bis-GMA, is a viscous liquid at room temperature which crosslinks to form a glassy network when a 3 to 5 mil coating is irradiated with sufficient energy. The major parameters which were systematically varied in this study were radiation dosage, dose rate, aging time after irradiation, and post-cure annealing at higher temperatures. Measurements were conducted first to quantify the crosslinking reaction, then to characterize the physical properties of the resulting networks. Extraction by a solvent was done to deter-mine the degree of network formation through the equilibrium swelling ratio and the gel weight fraction after drying. Another method utilized FTIR to monitor the disappearance of double bonds as the crosslinking reaction proceeded. In order to characterize the physical properties, differential scanning calorimetry (DSC) and dynamic mechanical spectroscopy were done to determine the glass transition temperature. It was found that the network density or amount of cure is proportional to the irradiation dosage, with an upper limit reached above some critical dosage. Dose rate was not found to influence the degree of cure greatly. The crosslinking reaction often became diffusion limited as vitrification occurred. These phenomena were discussed in terms of the well-known Time-Temperature-Transformation diagram. Free radicals trapped in these networks exhibited a finite lifetime. Post-curing can be achieved by annealing at a temperature above the T₉ of the network, as shown by the increase of the glass transition temperature from DSC and dynamic mechanical results. Bis-GMA was mixed with rubbery modifier materials with acrylate and methacrylate functional ends in order to toughen the bis-GMA networks. It was observed that the acrylate end groups were more reactive in EB cured systems than analogous methacrylates probably due to their higher polymerization enthalpy and less steric hindrance. Phase separation, which would provide rubber toughening without depressing the high glass transition temperature, was not achieved by irradiation with the modifying materials at the molecular weights used in this study, but the mixtures were toughened as shown by the dynamic mechanical data. / M.S.

Four methods of biaxial deformation were considered in an attempt to obtain biaxial orientation and properties in two thermotropic copolyesters: hydroxybenzoic acid-co-polyethylene terephthalate (HBA/PET) and hydroxybenzoic acid/2, 6 hydroxynaphthoic acid (HBA/HNA). They consisted of solid phase forming, extrusion blow molding, biaxial stretching, and film blowing. Some measure of biaxial properties and orientation was obtainable in all four processes, from the layered structure apparent in the blow molded bottles to the uniplanar orientation in the stretched film. Tensile and flexural properties were exceptional in the blown film in comparison to the other processes considered: tensile strengths of 300 to 400 MPa and moduli of 13 GPa were observed in the axial direction, for example. This result is believed due to the absence of a skin-core texture in which the core often remains relatively unoriented. A second objective of this work was to determine the feasibility of using Doi's theory for anisotropic solutions of rigid rods to predict the structure developed during the deformation processes for these thermotropic systems. Doi's theory of nematics, although limited to monodomains and containing numerous mathematical approximations in order to obtain a closed form for the kinetic equation, was used to predict the orientation development in the blown film process. Although qualitative agreement with experiment was found, quantitative agreement could not be expected. Since Doi's theory was developed by examining an isolated rigid rod in solution and modifying the resultant equations to account for interactions of neighbors, transient predictions on a lyotropic solution were also made and compared to experimental results. In particular, stress growth on the inception of a steady simple shear flow was examined for the system of poly(p-phenylene terephthalamide)/ 100% sulfuric acid. Comparisons to Doi's theory were made for both isotropic and anisotropic solutions. Quantitatively the theory was found to be unacceptable, particularly at higher shear rates in which the equilibrium stress was at least an order of magnitude lower than experimental results showed. It did, however, predict an overshoot in the shear stress, which increases with deformation rate. / Ph. D.

The crystallization behaviour of poly (p-phenylene sulfide) (PPS) has been studied. lsothermal rates of bulk crystallization, spherulitic radial growth rates, and nucleation densities were analyzed as functions of molecular weight of PPS, branching agent concentration, and chemical nature of the endgroup counter-atom. The overall rate of bulk crystallization was described by an Avrami equation with exponent ~3. As molecular weight increased in the 24,000-63,000 range, the rates of spherulitic growth and bulk crystallization decreased by a factor of 4. The nucleation density of PPS with <M_w>=24,000 was 32-fold lower than those of the higher molecular weight polymers. Introduction of 0.2% by weight of branching agent trichlorobenzene in the synthesis vessel reduced the spherulitic growth rate to 1/2, and the rate of bulk crystallization and the nucleation density to 1/4 of those of the linear polymer of the same molecular weight. The decreasing order of crystal growth rates for the endgroup counter-atoms is Ca>H>Zn>Na with calcium providing growth rates two·foId higher than the sodium counter-atom. The rates of overall bulk crystallization (K) followed the decreasing trend H>Zn>Ca>Na. For hydrogen, K was 17-fold higher than that of sodium counter-atom. The nucleation densities presented a different trend, H> Na>Zn>Ca. The spherulitic growth rates were analyzed in terms of equations proposed to explain the molecular weight dependence. The logarithm of the crystal growth rate followed a linear function of the logarithm of the number average molecular weight proposed by Cheng and Wunderlich. Since most processing operations occur under non-isothermal conditions, the non-isothermal crystallization of PPS was also studied. Differential scanning calorimetry data folIowed a model proposed by Ozawa for non-isothermal kinetics of crystallization. This model allowed the determination of the Avrami exponent under non-isothermal conditions. The values of the Avrami exponent obtained were in good agreement with values determined isothermally. The morphological textures of semicrystalline poly (p-phenylene sulfide) (PPS) were investigated utilizing two etching techniques to enhance the finer details. One technique was based on the partial attack of chromic acid solutions at high temperature (100°C), whereas the other used suspensions of anhydrous aluminum chloride at lower temperature (35°C). The aluminum chloride approach was shown to be more appropriate and successful in enhancing the fine details in textures of PPS. This reagent selectively attacked amorphous PPS enhancing the crystalline textures to a greater extent than the chromic acid approach. This technique allowed the observation of the spherulitic growth habit and the “interconnectivity” between impinged spherulites. The morphology of PPS in composite materials containing few carbon fibers was analyzed. Low crystallization temperatures, slow cooling rates, low molecular weight polymer and local orientation favored the nucleation of transcrystallinity at the carbon fiber surface. T300U carbon fibers were the most effective in nucleating transcrystallinity. Fiber anodization in H₂SO₄ and NaOH electrolytes decreased the nucleating ability of the fibers possibly due to an increase of polarity at the fiber surface. Utilization of the aluminum chloride etching technique in conjunction with scanning electron microscopy conürmed that PPS was nucleating at the fiber surface and showed the radial growth habit and cylindrical symmetry of the transcrystalline regions. / Ph. D.

A Research Report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfillment of the requirements for the degree of Master of Science in Engineering(Mechanical Engineering 30 January 2017 / An extensive variety of experimental techniques exist to determining residual stresses, but few of these techniques is suitable, however, for finding the residual stresses that exist in orthotropic or anisotropic layered materials, such as carbon-fibre reinforced polymers (CFRP). Among these techniques, particularly among the relaxation techniques, the incremental hole-drilling technique (IHD) has shown to be a suitable technique to be developed for this purpose. This technique was standardized for the case of linear elastic isotropic materials, such as the metallic alloys in general. However, its reliable application to anisotropic and layered materials, such as CFRP materials, needs to be better studied. In particular, accurate calculation methods to determine the residual stresses in these materials based on the measured in-depth strain relaxation curves need to be developed. In this work, existing calculation methods and already proposed theoretical approaches to determine residual stresses in composite laminates by the incremental hole-drilling technique are reviewed. The selected residual stress calculation method is implemented using MATLAB. For these calculations, specific calibration coefficients have to be numerically determined by the finite element method, using the ANSYS software. The developed MATLAB scripts are then validated using an experimental procedure previously developed. This experimental procedure was performed using CFRP specimens, with the stacking sequence [0o, 90o]5s and, therefore, this composite laminate was selected as case study in this work. Some discrepancies between the calculated stresses using the MATLAB scripts and those imposed during the experimental calibration procedure are observed. The errors found could be explained considering the limitations inherent to the incremental hole-drilling technique and the theoretical approach followed. However, the obtained results showed that the incremental hole-drilling can be considered a promising technique for residual stress measurement in composite laminates. / MT2017

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Bibliography: leaf 42. / by Eliot H. Frank. / B.S.