Physics-Based Constitutive Modelling of Thermoplastic Elastomers

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Elastomers constitute a broad class of soft polymeric materials that are used in a myriad of engineering applications, such as seals, adhesives, moulded flexible parts, energy absorbers and soft robotics. Traditional elastomers, such as vulcanised rubber, consist of long polymer chains held together by permanent crosslinks. These crosslinks give rise to high elasticity, strength and toughness, but also prevent elastomers from being re-processed by melting. Therefore, they are becoming increasingly unattractive in a modern world moving towards a more sustainable future. In contrast, thermoplastic elastomers (TPEs) proffer elastomer-like mechanical behaviour together with thermoplastic characteristics: they can be processed as a melt, enabling many conventional as well as emerging manufacturing processes (such as 3D printing) and recycling. There is also a growing interest in making biodegradable thermoplastic elastomers from naturally-sourced polymers, such as polyesters. In this context, there is a critical need for reliable mechanical models and computational tools that can link the complex mechanical behaviour of TPEs to their underlying molecular architecture. The combination of elastomeric and thermoplastic properties of TPEs stems from the transient nature of their polymer network. In contrast to traditional elastomers, the crosslinking points between the polymer chains in TPEs can be broken - and subsequently reformed - under an applied force or heat. At low temperature or under low force, TPEs behave like soft, elastic solids, whereas at high temperature or under high force, they can deform plastically. Their mechanical response is also highly rate- and temperature-dependent. While the constitutive response of traditional elastomers is relatively well described by rubber elasticity theory, there have been very few attempts so far at proposing constitutive models for TPEs. Existing models are either limited to specific loading conditions (e.g. uniaxial relaxation tests), or are largely phenomenological and therefore have limited predictive capability. The general objective of this project is to develop constitutive models for thermoplastic elastomers that can explicitly relate the inelastic, large-deformation response to key parameters of the microstructure (composition, molecular weight, morphology). This will be done through careful analysis and physical understanding of microstructural effects. Specific research aims are the following: 1) To develop a general constitutive framework for elastomers with transient network structure. The framework will be based on continuum mechanics, irreversible thermodynamics, and rubber elasticity theory. 2) To develop robust computational tools enabling finite element simulations of TPE structures under arbitrary loading conditions. 3) To formulate and validate specific models for TPEs of industrial importance, namely styrenic block copolymers (SBS, SIS), for which experimental data are available in the literature. 4) Subsequently, to extend the methodology to other TPEs, including thermoplastic polyurethanes (TPUs) and biodegradable polyester-based TPEs. This will involve new collaborations with the groups of Prof. C. Siviour and Prof. C. Williams (Chemistry), who develop and characterise these materials in their labs. 5) To formulate molecular design rules to achieve targeted properties. The models and design tools generated by this project will benefit researchers developing new polymers, as well as engineers interested in the application of TPEs in soft structures for various applications. This project falls within the EPSRC Engineering research area.