SMP Seminar: Prof. John Sweeney (University of Bradford), Shape memory polymers: physical origins and applications

Prof. John Sweeney
IRC in Polymer Science and Technology, School of Engineering, Faculty of Engineering and Digital Technologies, University of Bradford, Bradford BD7 1DP, UK.

Time: Wednesday 7th February, 2-3pm

Location: William Bragg SR (GR.18)

Shape memory polymers: physical origins and applications

Abstract: We have performed shrinkage restraint force measurements on three shape memory polymers of PET, PMMA and a blend of the two in equal proportions at a range of temperatures. Observations are made of the change in stress during temperature rise, hold and cooling. Once temperatures surpass the trigger threshold, all materials show an increase in stress during rise and hold, but on cooling the three materials behave differently; the PET shows a drop in stress, the PMMA a rise and the blend a much smaller rise. This behaviour correlates with the reversible thermal dimensional change at below the shrinkage threshold temperature; the expansion coefficients are negative for PET, positive for PMMA and positive at a lower order of magnitude for the blend. We model the behaviour by supposing that the shrinkage forces are created by a network of prestressed strains effective at long range, acting within a matrix of shorter chains effective at short range. The coefficients of expansion are features of the short-range matrix. The total stress is the sum of the shrinkage stress and the thermal stress in the matrix. On this basis the drops in stress on cooling are modelled successfully using an elastic analysis based on measured elastic moduli and thermal expansion coefficients. For the blend, downward jumps in temperature produce small transient increases in the total stress observed, leaving it effectively unperturbed. This phenomenon, together with the success generally of the elastic model for the stress drops in all the materials, imply that the shrinkage stress from the long-range chain network is largely unaffected by the temperature change, and so is not entropic.

These materials were developed in response to the need for a source of high compressive forces to heal cracks in concrete structures. This application area will be discussed.