|Title:||Study of bio-compatible semi-crystalline polyurethane networks showing two way shape memory effect|
|Advisors:||Hu, Jinlian (ITC)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Shape memory polymers
Shape memory effect
|Department:||Institute of Textiles and Clothing|
|Pages:||xxiii, 165 pages : color illustrations|
|Abstract:||Most shape memory polymers are one way shape memory polymers. The word "one way" means that the shape changing is irreversible by stimulus. In other words, one stimulus triggers one shape recovery. Another shape recovery needs additional "programming process". Two way shape memory polymers are developed recently. They can change between two shapes reversibly by stimulus. Amongst the two way shape memory polymers is the chemically crosslinked semi-crystalline polymers. Prior arts believed that the chemically crosslinking structure is the prerequisite for the semi-crystalline polymers to achieve two way shape memory effect. One standpoint believe that high gel content is needed for semi-crystalline polymers to exhibit two way shape memory effect. A series of linear semi-crystalline shape memory polyurethane-ureas were fabricated using diamine as the chain extenders. There are two NH groups in every urea moiety and one NH group in every urethane moiety. Thus double hydrogen bonds are formed between the two NH groups and one carbonyl group in every two urea groups whereas single hydrogen bond is formed between one NH group and one carbonyl group in every two urethane groups. So the hydrogen bonding in polyurethane-urea is much stronger than that in polyurethane. Two way shape memory effect was found in the linear shape memory polyurethane-urea due to the strong molecular interaction. It was found that the degree of swelling rather than the gel content determine whether semi-crystalline polymers show two way shape memory effect. Adjustment of the melting temperature of the semi-crystalline SMPUs is very meaningful. For example, adjusting the Tm to near body temperature is very important in some cases, however it is also very difficult. The reason may be the poor phase separation in the synthesized polyurethanes while using low molecular weight crystalline polyol, e.g. Mn~2000g/mol, as the soft segment, which is essential to lower the Tm to body temperature. A simple and cost effective method is proposed to fabricate body temperature sensitive SMPUs, while using commercially available PCL2000 as the soft segment. The body temperature sensitive SMPUs have Tm around 32℃ and can be fixed to the temporary shape at low temperature, e.g. 5℃, and the temporary shape can be maintained under room temperature, i.e. 25℃. Body temperature, i.e. 32℃, can trigger the shape recovery. Two methods, namely PU-acrylate and PU-silicone techniques, are introduced to produce the chemically crosslinked semi-crystalline polymers. Compared to previous reported crosslinked semi-crystalline polymers showing 2W-SME, these two kinds of SMPUs have controllable and well defined molecular structure. Both the two polymers show two way shape memory effect. Meanwhile a chemically crosslinked semi-crystalline PU-acrylate network with high degree of swelling does not exhibit two way shape memory effect, although it have high gel content.|
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