An exploration of shape memory polymers and (nano) composites for packaging applications

• Main Author: Barwood, Michael
• Other Authors: Breen, Christopher ; Clegg, Francis
• Published: Sheffield Hallam University 2012
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.741412

Shape memory polymers, SMP, are gaining considerable interest in research and industry due to their novel properties and vast potential. SMP belong to an emerging class of materials known as 'smart polymers', and have the intriguing ability to be moulded into a temporary shape, store that temporary shape and, when activated by an external stimulus, recover its original shape. SMP have currently been used for a wide range of applications including SMP stents, catheters, smart clothing, composite tooling and biodegradable sutures. This project aims to develop an understanding of how SMP can be of use in the packaging industry, exploring several types of thermoset and thermoplastic based SMP for packaging applications. The fundamental aspects of SMP, the methods to tailor the mechanical and thermal properties and the quantification of the shape memory effect are also considered. Initially photopolymerised thermoset shape-memory networks, class I SMP, with tailorable thermomechanics were investigated. Tertiary butylacrylate (tBA) and polyethylene glycol dimethacrylate (PEGDMA) were copolymerized to create networks with adjustable glass transition temperatures (Tg) and rubbery modulus values ranging from -18 to 39 °C and 0.7 to 25.0 MPa, respectively. The networks were characterised for homogeneity, thermal, mechanical and shape memory properties. The adjustable Tg had a great impact on the shape memory effect however the varying rubbery modulus had no significant effect. Previously incorporation of clay into polymers has shown to dramatically increase the mechanical, barrier and fire resistant properties of a wide range of polymers. At addition levels of 1 to 5 wt% no significant changes occurred to the mechanical abilities of the tBA-co-PEGDMA networks, although there was a minor improvement of the rubbery modulus at 1 wt% clay loading. There was a significant increase in initial shape memory recovery, at 1 wt% clay loading, but no improvement in total shape memory recovery. Alternatives to tBA-co-PEGDMA were also studied; these include i-bornylacrylate, IBoA, i-bornylmethacrylate, IBoMA, and tertiary butylmethacrylate, tBMA, in co-PEGDMA networks, which were also explored for their SMP packaging potential. The IBoA, IBoMA and tBMA-co-PEGDMA networks also had adjustable Tg and rubbery modulus values ranging from -20 to 53 °C and 2.0 to 22.0 MPa, -11 to 93 °C and 4.0 to 30.0 MPa and -11 to 74 °C and 1.0 to 30.0 MPa, respectively. Thermoplastic polyurethane, PU, PU-co-PVC blends, class IV SMP, and their nanocomposites were also evaluated as SMP packaging materials. As clay content was increased, 1 to 5 wt%, in the SMP PU, there was a significant shift of the soft segment (SS) Tg from -29 to -10 °C, respectively, but only a minor improvement in the rubbery modulus. There was no significant effect on the shape memory ability. As PVC was blended with PU the SS Tg shifted from -29 to 1 °C, and the PU initial modulus dropped dramatically from 2414 to 493 MPa. On any addition of PVC the shape memory recovery time was extended. As clay was increased, 1 to 5 wt%, in the PU-co-PVC blend the overall modulus was reduced with no other significant effects apart from a minor increase in intensity and shift to lower temperatures of the PU/PVC/Lap 1 tan 6 peak. As clay increased the time for the initial shape recovery of the polymer blends decreased, from 12.5 to 8.0 seconds, however there was no effect on the overall return time, ca. 15 seconds.