In a previous post, I have presented some of our results obtained during stress-free shape memory recovery of amorphous polymer networks submitted to large deformation torsion tests. These tests introduce large deformation but moderate strain. In order to provide a rather complete set of data on the strain recovery and `stress recovery’ for amorphous polymers, including large strain, uniaxial tension tests were chosen. Therefore an amorphous acrylate network was heated above its glass transition, submitted to a uniaxial tension (temporary strain), the specimen was then cooled down while maintaining its deformed length. Once cooled, the uniaxial compression (due to the cooling) was released by opening the bottom clamp. Then two options were possible: 1) the STRAIN RECOVERY test – heat the sample while recording the strain recovery, 2) the STRESS RECOVERY test – close the bottom clamp, maintain the specimen length constant and record the stress while heating the sample. Strain and stress recoveries were then recorded according to the ramp of heating, the temperature of applied strain, the applied temporary strain, the strain rate and relaxation when applying the temporary strain, the ramp of heating, the number of shape memory cycles.
The glass transition of the acrylate network measured by DSC at 10 °C/min was 46 °C. The stored strain was applied either at 65 °C in the rubbery state or at 45 °C at the beginning of the glass transition.
STRAIN RECOVERY RESULTS
– The strain recovery kinematic does not depend on the temperature of applied strain
– It does not depend on the amount of strain applied at high temperature (the temporary strain)
– It does not evolve with the number of shape memory cycles
– It depends on the heating ramp
STRESS RECOVERY RESULTS
– The stress recovery kinematic strongly depends on the temperature of applied strain
– It depends on the amount of strain applied at high temperature
– It does not evolve with the number of shape memory cycles
– It depends on the heating ramp
– It depends on the molecular relaxation during application of the storage strain
The results presented here are part of the Ph. D. thesis of Sebastian Arrieta. For more details and data on the topic see:
S. Arrieta, J. Diani, P. Gilormini, 2014. Cyclic and monotonic testing of free and constrained recovery properties of a chemically cross linked acrylate, Journal of Applied Polymer Science 131, 39813.
S. Arrieta, J. Diani, P. Gilormini, 2014. Experimental characterization and thermoviscoelastic modeling of strain and stress recoveries of an amorphous polymer network, Mechanics of Materials, 68, 95-103.