Browsing by Subject "polymer degradation"

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    Numerical Modeling of Stress Corrosion Cracking in Polymers
    (2015-12) Ge, Hanxiao
    Polymeric materials have been increasingly used for structural purposes in civil infrastructures. However, stress corrosion cracking has been a critical issue that affects the service lifetime of polymer components. My preliminary study showed that polyethylene may be severely corroded in an oxidizing environment and lose its fracture resistance property. Experimental methods have been primarily adopted to investigate stress corrosion cracking in polymers; however, these approaches are expensive to apply, and may fail to account for certain aspects of this chemo-mechanical process. Therefore, a numerical approach is needed to investigate this issue. A unified chemo-mechanical model is developed to predict the stress corrosion cracking (SCC) of a viscoplastic polymer. This model is applied to the specific case of high density polyethylene (HDPE) exposed to a chlorinated environment at a constant stress load. This chemo-mechanical model is comprised of three components, each capturing a critical aspect of SCC. An elastic-viscoplastic constitutive model is adopted to predict the time-dependent creep behavior of HDPE, and the model parameters have been calibrated through tensile testing. This constitutive model has been implemented in finite element analysis by using a user-defined material subroutine. The polymer fracture property is considered to be dependent on the extent of corrosion, and this dependence is implemented with a cohesive zone model. A chemical kinetics and diffusion model is utilized to predict the degradation of fracture properties in the material as a result of reactions and migration of chemical substances. The coupled chemo-mechanical simulation is accomplished by integrating the chemical reaction calculation into finite element analysis via user defined subroutines. Two modes are considered for failure of the polymer: excessive plastic deformation or catastrophic unstable crack growth. At high stresses, the failure is primarily due to excessive plastic deformation. At low stresses, chemical reactions and diffusion are the dominant factors leading to failure. In addition, two distinct patterns of crack growth (reaction-driven or diffusion-driven) are revealed at various disinfectant concentrations at low stress levels. In reaction-driven crack growth, material degradation is localized at the crack tip, and crack growth rate is a constant throughout the simulated lifetime. However, when diffusion dominates, the entire specimen ligament may be severely degraded, and crack growth accelerates at the end of component lifetime. The current simulation framework allows exploring the interaction of various factors in stress corrosion cracking, such as disinfectant concentration, loading, and temperature. The framework is also general enough to be implemented for other polymeric materials and corresponding corrosion mechanisms. In the future, the proposed chemo-mechanical modeling approach may be expanded to analyze the performance of a variety of materials under stress corrosion cracking. In addition, a stochastic methodology may be incorporated to account for the variances in loading, as well as material properties.
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    Synthesis and Physiochemical Characterization of Biobased, Compostable Polymers Containing Lignin
    (2016-09) Harris, Stephanie
    Biobased and compostable materials have gained in popularity as sustainable solutions for reducing waste and minimizing environmental impacts. Polylactide (PLA) continues to be a popular biobased polymer, but has limited use due to its brittleness, high vapor and ultraviolet (UV) light permeability. This study addresses these shortfalls through copolymerization of lactide (LA) with organosolv lignin from switchgrass (OSL) and delta-valerolactone (DVL) to produce 100 % biobased polymers with improved properties. Incorporation of hydrophobic OSL into poly(L-lactide) (PLLA), even in small quantities (up to 0.26%) resulted in a considerable decrease in water vapor transition rate (WVTR) of up to 64 %, nearly a 20 % decrease in UV light transmission, and slowing of hydrolytic degradation. Unfortunately, lignin appears to stop the chain propagation and effectively reduces overall M ̅_w of the copolymer. Young’s modulus for these copolymers is affected little as stress and strain decreased proportionally with the addition of OSL, resulting in a copolymer that is nearly equal to PLLA in stiffness. To improve elasticity, terpolymers of PLLA-DVL-OSL were explored. The addition of DVL to the polymerization was found to increase UV transmission rate, an effect that could be counteracted through OSL addition. PLLA-DVL co and terpolymers showed low Young’s modulus, characteristic for polymers with elastomeric properties. WVTR was seen to decrease with the addition of DVL and was even further reduced through addition of OSL, resulting in an overall WVTR reduction of up to 79 %.