A shape memory polymer (SMP) is stimuli-responsive with the fantastic capacity to “memorize” a temporary shape under certain conditions and to recover to its permanent shape upon exposure to certain external stimulus (e.g. heat, light, electromagnetic field). In the past few decades, various SMPs have been investigated and applied in the area of aerospace, biomedicine, and textiles, etc. Recently, a special type of SMP called a ‘two-way reversible shape memory polymer’ or ‘reversible shape memory polymer’ (RSMP) capable of transitioning between two temporary shapes without the need for reprogramming after each change has attracted the attention of many researchers. In this class of polymer, the semicrystalline RSMP was studied considerably due to the various chain structures produced by relatively simple synthesis routes. The crystallization-induced elongation (CIE) and melting-induced contraction (MIC) of the oriented crystal domains has been theorized as the main mechanism of semicrystalline RSMP. However, most RSMPs are predominantly thermosets, which implies significant drawbacks regarding reprocessing and recycling.
This thesis focuses on the development of RSMP based on polyolefin materials, especially novel high-performance polyolefin elastomers, due to the advantages of high crystallizability, varying chain structures, tunable and broad melting transitions, and low cost. The thesis starts off by demonstrating the reversible shape memory effect (RSME) of the thermoplastic ethylene/1-octene diblock copolymer, which contains the ethylene-rich hard segments and the 1-octene-rich soft segments. The delicately designed chain structure exhibited a broad melting transition and strong physical crosslinks, which contributed to the resulting RSME and the CIE/MIC effect at load-free conditions. Furthermore, the commercially available polyolefin elastomer blends demonstrated the RSME. The utilization of commercial products and simple processing method to achieve a thermoplastic RSMP offers easy production in large scale and low costs. The second part of the thesis developed a polyolefin-based RSMP with reconfigurable network by introducing a transesterification catalyst into a crosslinked poly(ethylene-co-vinyl acetate). The network reconfiguration achieved a dynamic covalent polymer network by breaking the ester bonds and reconnecting. The third part of the thesis explored a new RSMP foam material developed by utilizing polyolefins. The polyolefin elastomers of differing compositions were blended and foamed to fabricate the porous structure. The RSME in a load-free condition was then demonstrated successfully. This thesis represents significant progress in the development of polyolefin-based RSMPs, outlining new structural design, processability improvements, and potential applications. / Thesis / Doctor of Philosophy (PhD) / Shape memory polymer (SMP) is stimuli-responsive capable of “memorizing” a temporary shape and yet recovering to its permanent shape upon a certain external trigger. SMPs are widely studied and applied in the areas of aerospace, biomedicine, textile, etc. On the other hand, a reversible shape memory polymer (RSMP) is a new type of SMP that can shift back and forth between two different temporary shapes without the need of reprogramming between transitions, and has been applied in soft actuators, microrobotics, and artificial muscles. In this thesis, unique polyolefin-based RSMP were developed with good reprocessability and shown in new application scenarios. Firstly, a thermoplastic semicrystalline polymer was demonstrated to exhibit the reversible shape memory effect (RSME) by using a lab-designed ethylene/1-octene diblock copolymer and commercial polyolefin elastomer blends. Subsequently, the reprocessability of a crosslinked poly(ethylene-co-vinyl acetate) (PEVA) RSMP was improved by introducing a dynamic covalent polymer network. Finally, transitional changes between shapes was amplified by developing a RSMP foam by utilizing polyolefin elastomer blends. This thesis represents significant progress in the study of polyolefin-based RSMPs.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/25067 |
| Date | January 2019 |
| Creators | Gao, Yuan |
| Contributors | Zhu, Shiping, Thompson, Michael, Chemical Engineering |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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