Physics-Based Constitutive Modelling of Thermoplastic Elastomers

弹性体构成了一大类软聚合材料，用于各种工程应用，如密封件、粘合剂、模压柔性部件、吸能器和软机器人。传统的弹性体，如硫化橡胶，由长的聚合物链组成，通过永久性的交联剂连接在一起。这些交联物具有很高的弹性、强度和韧性，但也防止了弹性体被熔融重新加工。因此，在迈向更可持续的未来的现代世界中，它们正变得越来越没有吸引力。相比之下，热塑性弹性体(TPE)提供了类似弹性体的机械性能和热塑性特性：它们可以作为熔体加工，使许多传统和新兴的制造工艺(如3D打印)和回收利用成为可能。人们对用聚酯等天然来源的聚合物制造可生物降解的热塑性弹性体的兴趣也越来越大。在这种背景下，迫切需要可靠的力学模型和计算工具来将TPE的复杂力学行为与其潜在的分子结构联系起来。热塑性弹性体和热塑性弹性体的结合源于其聚合物网络的暂时性。与传统弹性体不同，热塑性弹性体中的聚合物链之间的交联点可以在外加力或热的作用下断裂，然后重新形成。在低温或低力下，热塑性弹性体的行为类似于柔软的弹性固体，而在高温或高力下，它们可以塑性变形。它们的机械反应也高度依赖于速率和温度。虽然橡胶弹性理论较好地描述了传统弹性体的本构响应，但到目前为止，很少有人尝试提出热塑性弹性体的本构模型。现有的模型要么局限于特定的加载条件(如单轴松弛试验)，要么在很大程度上是唯象的，因此预测能力有限。该项目的总体目标是开发热塑性弹性体的本构模型，该模型可以明确地将非弹性、大变形响应与微观结构的关键参数(组成、相对分子质量、形态)联系起来。这将通过对微观结构效应的仔细分析和物理理解来实现。具体研究目标如下：1)建立具有暂态网络结构的弹性体的通用本构框架。该框架将基于连续介质力学、不可逆热力学和橡胶弹性理论。2)开发强大的计算工具，能够在任意载荷条件下对热塑性弹塑性体结构进行有限元模拟。3)制定和验证具有工业意义的热塑性弹性体的具体模型，即苯乙烯嵌段共聚物(SBS，SIS)，其实验数据可在文献中获得。4)随后，将该方法推广到其他热塑性弹性体，包括热塑性聚氨酯和可生物降解的聚酯型热塑性弹性体。这将涉及与C.Siviour教授和C.Williams教授(化学)的小组进行新的合作，他们在他们的实验室开发和表征这些材料。5)制定分子设计规则，以达到目标性质。该项目产生的模型和设计工具将使开发新型聚合物的研究人员以及对TPE在各种应用的软结构中的应用感兴趣的工程师受益。该项目属于EPSRC工程研究领域。

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.