A comprehensive experimental investigation of the nonlinear viscoelastic properties of PMMA and other glassy polymers
Abstract
A comprehensive set of non-linear viscoelastic data was obtained for a single well characterized glass forming polymeric material - a lightly crosslinked bulk polymerized poly(methylmethacrylate) PMMA. The data set includes pressure-volume-temperature data in both rubbery and glassy state, where the effect of the formation pressure on the property of resulting glass has been studied. The data set also includes enthalpy relaxation, stress relaxation, creep and recovery from creep, stress-strain behavior with yield, post-yield softening and flow in both uniaxial tension and compression, where the effects of temperature, temperature history, aging, deformation rate and deformation history have been studied. The data set is intended to serve as a test bed for critical evaluation of existing and future constitutive models of glassy polymers. The experiments included in the data set were systematically chosen to challenge the assumptions of the existing constitutive models. Stress relaxation was used to probe the instantaneous state of mobility in the glassy polymeric material during deformation at various points along the stress-strain curve before, during, and after yield, where it was demonstrated that the mobility is the highest during post-yield softening, which is not predicted by the existing constitutive models. Effects of the thermal and deformation history on the non-linear creep and recovery from creep were studied. At the same load, aging dramatically decreases the compliance at a given creep time; however, when an aged sample is allowed to creep for a longer time to reach the same value of strain as the unaged sample, the effect of aging is completely erased. The initial rate of recovery upon removal of the load is insensitive to the strain reached during creep - an observation inconsistent with the accelerated aging idea incorporated in many a constitutive model. Effect of aging time on the yield stress in uniaxial deformation in both tension and compression was studied, where the yield stress and the magnitude of post-yield softening increase dramatically with aging - effect not predicted by NLVE model. The transition from the glassy asymptotic behavior to the rubbery asymptotic behavior during constant rate heating was studied via enthalpy response and volume response. It was found that the glass transition temperature values observed using these two techniques at the same heating rate differ by up to 7 to 8°C in case of a thermoplastic (PMMA) and thermoset (Epon825 - 3,3'DDS epoxy) materials - a result not captured by existing constitutive models. The implications of these experimental findings critically evaluating exists and for developing new physically based constitutive model of glassy polymers are discussed.
Degree
Ph.D.
Advisors
Caruthers, Purdue University.
Subject Area
Polymer chemistry|Chemical engineering
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