Browsing by Subject "Polyethylene"
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Item Modeling and simulation of linear thermoplastic thermal degradation(2012-05) Bruns, Morgan Chase; Ezekoye, Ofodike A.; Ganesan, Venkat; Howell, John R.; Koo, Joseph H.; Nyden, Marc R.; Ruoff, Rodney S.Thermal degradation of linear thermoplastics is modeled at several scales. High-density polyethylene (HDPE) is chosen as an example material. The relevant experimental data is surveyed. At the molecular scale, pyrolysis chemistry is studied with reactive molecular dynamics. Optimization is used to calibrate several pyrolysis mechanisms with thermogravimetric analysis (TGA) data. It is shown that molecular scale physics may be coupled to continuum scale transport equations through a population balance equation (PBE). A PBE solution method is presented and tested. This method has the advantage of preserving detailed information for the small species in the molecular weight distribution with minimal computational expense. The mass transport of these small species is modeled at the continuum scale with a bubble loss mechanism. This mechanism includes bubble nucleation, growth, and migration to the surface of the condensed phase. The bubble loss mechanism is combined with a random scission model of pyrolysis to predict TGA data for HDPE. The modeling techniques developed at these three scales are used to model two applications of engineering interest with a combined pyrolysis and devolatilization PBE. The model assumes a chemically consistent form of the random scission pyrolysis mechanism and an average, parameterized form of the bubble loss mechanism. This model is used to predict the piloted ignition of HDPE. Predictions of the ignition times are reasonable but the model over predicts the ignition temperature. This discrepancy between model and data is attributed to surface oxidation reactions. The second application is the prediction of differential scanning calorimetry (DSC) data for HDPE. The model provides detailed information on the energy absorption of the thermally degrading sample, but the literature data is too variable to validate the model.Item Oxidative degradation of polyethylene(Texas Tech University, 1971-05) Spore, Richard LawrenceNot availableItem Phase behavior of ethylene/low-density polyethylene mixtures(Texas Tech University, 1976-08) Harmony, Stephen CNot availableItem Stretch-induced wrinkling of thin sheets(2013-08) Nayyar, Vishal; Huang, Rui, doctor of civil and environmental engineering; Ravi-Chandar, K.Thin sheets and membrane structures are widely used in space applications such as solar sails, sunshields and membrane optics. Surface flatness over a large area is one of the key requirements for many applications using the flexible thin structures. However, wrinkles are commonly observed in thin sheets. It is thus important to understand the mechanics of thin sheets for practical applications that require reliable control of surface wrinkles. In this study, a model problem of stretch-induced wrinkling of thin sheets is considered. First, a two-dimensional (2-D) finite element model was developed to determine stretch-induced stress distribution patterns in hyperelastic thin sheets, assuming no wrinkles. As a prerequisite for wrinkling, development of compressive stresses in the transverse direction was found to depend on both the length-to-width aspect ratio of the sheet and the applied tensile strain. Next, an eigenvalue analysis was performed to predict the critical conditions for buckling of the elastic sheet under the prescribed boundary conditions, followed by a nonlinear post-buckling analysis to simulate evolution of stretch-induced wrinkles. Experiments were conducted to measure stretch-induced wrinkling of polyethylene thin sheets, using the three-dimensional digital image correlation (3D-DIC) technique. It was observed that the wrinkle amplitude first increased and then decreased with increasing nominal strain, in agreement with finite element simulations for a hyperelastic thin sheet. However, unlike the hyperelastic model, the stretch-induced wrinkles in the polyethylene sheet were not fully flattened at high strains (> 30%), with the residual wrinkle amplitude depending on the loading rate. The hyper-viscoelastic and the parallel network nonlinear viscoelastic material models were adopted for finite element simulations to improve the agreement with the experiments, including the wrinkle amplitude, residual wrinkles and rate dependence. Finally it is noted that wrinkling is sensitive to defects and material inhomogeneity in thin sheets. By varying the elastic stiffness in a narrow region, numerical simulations show drastically different wrinkling behavior, including the critical strain and evolution of wrinkle amplitude and wavelength. In conclusion, a comprehensive understanding of stretch-induced wrinkling is established, where geometry, material, and boundary conditions all play important roles.Item Thermodynamics of some concentrated solutions of polyethylene oxide.(Texas Tech University, 1974-05) Chang, Yen-huangGas-liquid chromatography (GLC) was used to study the complex solution thermodynamics of concentrated solution of an important water-soluble polymer, polyethylene oxide (PEO). Experimental data from 70°C to 150°C at essentially atmospheric pressure are presented when the solute in PEO solution is at both infinite-dilution and finite concentration. The infinite-dilution volatilites of 14 organic solutes and water in PEO were obtained at each temperature. Activity coefficients of benzene in PEO were also obtained at temperatures from 70°C to 150°C over the polymer concentration range 90 to 100 weight per cent. The corresponding-states theory of polymer solutions initiated by Prigogine and developed by Flory and Patterson was used to correlate our experimental results. Excellent agreement was obtained. Comparison of our infinite-dilution results with the earlier work of Cheng and Bonner (1974) shows some discrepancy. The discrepancy is ascribed to differences in column preparation used in the work of Cheng and Bonner (1974) and in this work. In this work, we account for the effect of PEO degradation of GLC results.