Seminars and thesis defenses
Predicting the deformation response of polymeric glasses via shear transformation zone (STZ) dynamics
By Frederik Van Loock (iMMC / IMAP)
The observed deformation behaviour of glassy polymers is actually notoriously complex. In the absence of failure, the measured uniaxial stress-strain curve of a polymeric glass deformed at a temperature below the glass transition temperature typically has an initial linear, (visco-)elastic part, followed by yield, strain softening, plastic flow, and strain hardening. In addition, the deformation response is sensitive to temperature and rate of deformation, physical ageing, and processing and/or deformation history. A vast amount of sophisticated, three-dimensional (visco)elastic-(visco)plastic models are available to simulate this large deformation response. They generally give good fits to measured uniaxial stress-strain curves. However, they are predominantly phenomenological, give limited insight into the micromechanical nature of deformation (and failure), and require the calibration of a large number of fitting parameters. Molecular dynamics (MD) simulations have also been used to shed light on the role of molecular deformation mechanisms driving the deformation of glassy polymers. These computations confirm that the inelastic deformation occurs through thermally activated molecular rearrangements and conformational changes of a collection of polymer chains. However, MD simulations are restricted to short time and length scales, limiting their use when attempting to predict the response of glassy polymers in bulk or when confined between stiff fibres in fibre-reinforced polymers.