Deformation and fracture of PMMA with application to nanofoaming and adhesive joints

This thesis contributes to the understanding of the deformation and fracture of methyl methacrylate (MMA)-based polymers in the context of void growth. The first part of the thesis focuses on the prediction of void growth during solid state nanofoaming of polymethyl methacrylate (PMMA). These predictions may contribute to the development of polymeric foams with a thermal conductivity close to that of air. The second part of the thesis explores the fracture behaviour of structural adhesive (e.g. MMA)-based joints. An adhesive layer within such a joint is prone to defects such as (micro)voids and (micro)cracks. The ability to accurately predict the failure strength of adhesive joints as a function of pore or crack size is essential in order to design reliable structures based on adhesive bonding technology. A one dimensional void growth model is developed to simulate cavity expansion during solid-state nanofoaming of PMMA by CO$_2$ in the first part of the thesis. To that end, tensile tests on two PMMA grades of markedly different molecular weight are conducted close to the glass transition temperature and over two decades of strain rate. The void growth model makes use of fitted constitutive laws for each PMMA grade and the effect of dissolved CO$_2$ is accounted for by a shift in the glass transition temperature of the PMMA. Solid-state nanofoaming experiments are performed on the two PMMA grades to validate the void growth model. The morphology of the foams (and the limit in attainable porosity) is found to be sensitive to the molecular weight. The measured porosity versus foaming time curves are in good agreement with those predicted by the model, for porosities below the maximum observed porosity. The observed limit of achievable porosity is interpreted in terms of cell wall tearing; it is deduced that the failure criterion is sensitive to cell wall thickness. The tensile strength of a centre-cracked elastic layer, sandwiched between two elastic substrates, and subjected to remote tensile stress, is predicted in the second part of the thesis. An analytical theory is developed by making use of a cohesive zone at the crack tip to predict the strength of the joint as a function of the relative magnitude of crack length, layer thickness, plastic zone size, specimen width, and elastic modulus mismatch ratio. Joint design maps are constructed, revealing competing regimes of fracture. The analytical theory is verified by finite element calculations, and validated by means of two experimental case studies.EPSRC, SABI

The mechanics of solid-state nanofoaming.

Solid-state nanofoaming experiments are conducted on two polymethyl methacrylate (PMMA) grades of markedly different molecular weight using CO2 as the blowing agent. The sensitivity of porosity to foaming time and foaming temperature is measured. Also, the microstructure of the PMMA nanofoams is characterized in terms of cell size and cell nucleation density. A one-dimensional numerical model is developed to predict the growth of spherical, gas-filled voids during the solid-state foaming process. Diffusion of CO2 within the PMMA matrix is sufficiently rapid for the concentration of CO2 to remain almost uniform spatially. The foaming model makes use of experimentally calibrated constitutive laws for the uniaxial stress versus strain response of the PMMA grades as a function of strain rate and temperature, and the effect of dissolved CO2 is accounted for by a shift in the glass transition temperature of the PMMA. The maximum achievable porosity is interpreted in terms of cell wall tearing and comparisons are made between the predictions of the model and nanofoaming measurements; it is deduced that the failure strain of the cell walls is sensitive to cell wall thickness

Micronuclei in cord blood lymphocytes and associations with biomarkers of exposure to carcinogens and hormonally active factors, gene polymorphisms, and gene expression: The NewGeneris cohort

Background: Leukemia incidence has increased in recent decades among European children, suggesting that early-life environmental exposures play an important role in disease development. Objectives: We investigated the hypothesis that childhood susceptibility may increase as a result of in utero exposure to carcinogens and hormonally acting factors. Using cord blood samples from the NewGeneris cohort, we examined associations between a range of biomarkers of carcinogen exposure and hormonally acting factors with micronuclei (MN) frequency as a proxy measure of cancer risk. Associations with gene expression and genotype were also explored. Methods: DNA and protein adducts, gene expression profiles, circulating hormonally acting factors, and GWAS (genome-wide association study) data were investigated in relation to genomic damage measured by MN frequency in lymphocytes from 623 newborns enrolled between 2006 and 2010 across Europe. Results: Malondialdehyde DNA adducts (M1dG) were associated with increased MN frequency in binucleated lymphocytes (MNBN), and exposure to androgenic, estrogenic, and dioxin-like compounds was associated with MN frequency in mononucleated lymphocytes (MNMONO), although no monotonic exposure-outcome relationship was observed. Lower frequencies of MNBN were associated with a 1-unit increase expression of PDCD11, LATS2, TRIM13, CD28, SMC1A, IL7R, and NIPBL genes. Gene expression was significantly higher in association with the highest versus lowest category of bulky and M1dG-DNA adducts for five and six genes, respectively. Gene expression levels were significantly lower for 11 genes in association with the highest versus lowest category of plasma AR CALUX® (chemically activated luciferase expression for androgens) (8 genes), ERα CALUX® (for estrogens) (2 genes), and DR CALUX® (for dioxins). Several SNPs (single-nucleotide polymorphisms) on chromosome 11 near FOLH1 significantly modified associations between androgen activity and MNBN frequency. Polymorphisms in EPHX1/2 and CYP2E1 were associated with MNBN. Conclusion: We measured in utero exposure to selected environmental carcinogens and circulating hormonally acting factors and detected associations with MN frequency in newborns circulating T lymphocytes. The results highlight mechanisms that may contribute to carcinogen-induced leukemia and require further research

Data for 'The mechanics of solid-state nanofoaming'

This data set contains two items: 1) Measurement data of the solid-state nanofoaming experiments at different foaming times (60s, 180s, 300s, 600s) and foaming temperatures (25deg, 40deg, 60deg, 80deg). Reported are: relative density rr, porosity p, cell nucleation density N0, average cell size f, and observed standard deviation SD in cell sizes. All measurements methods are summarised in the main manuscript. More details are given in the open access publication (https://doi.org/10.3390/polym8070265) of the Cellmat group at the University of Valladolid. 2) Code files for the void growth simulations in Matlab The theoretical framework is summarised in the main manuscript. More details can be found in the thesis of Van Loock (https://doi.org/10.17863/CAM.41889). See readme.txt file for further details

Implementation and calibration of a mesoscale model for amorphous plasticity based on shear transformation dynamics

A mesoscale numerical model based on shear transformation zone (STZ) theory is implemented in a commercial finite element software. The model is designed to predict the (visco)plastic deformation response of amorphous solids at the nano- and micro-scale. The theoretical framework relies on earlier models developed by Bulatov and Argon (1994a) and of Homer and Schuh (2009). We justify the potential of the computational model by conducting reference calculations for model metallic and polymeric glasses in plane strain compression. Emphasis is placed on the effect of time and space discretisation on the predicted macroscopic response. The dependence of the predicted yield strength upon the values of the fundamental model parameters is analysed via a mean-field approximation. The mean-field approximation is validated based on a series of simulations in model parameter space. We provide guidelines for a straightforward but consistent parameter identification method via the mean-field approximation while starting from experimental data