Constitutive Model For Soft Materials Based On Conformational Statistics Of Molecular Chains

Soft material is a kind of polymer materials with multi-scale structure.Soft materials have been widely used in many fields,such as flexible sensors,soft robots,flexible electronic devices and human-computer interaction.In the process of using and designing the function and structure of soft materials,mechanical properties is extremely important.Characterizing and describing the mechanical constitutive behaviors of soft materials are very essential for their design and application.Currently,most of the hyperelastic constitutive models for soft materials are phenomenological models.A few models are based on microstructures,and some of them have material parameters without physical meaning,and others do not take the entanglement between molecular chains into account,so they fail to describe the constitutive behavior of soft materials well under complex deformation.For viscoelastic models,most of them are phenomenological ones.A few constitutive models based on microstructures only consider reptation of free chains,ignoring the relaxation of contour length.Based on conformational statistics and reptation dynamics,in this dissertation,we developed the hyperelastic and visco-hyperelastic model respectively.(1)A new constitutive model based on microstructures has been proposed,which has only three parameters.The model is available for various complicated load conditions.We obtain the energy of the network by considering the deformation of a micro-sphere,and construct the entanglement energy by using the concept of tube model[1].The model can successfully predict the stress-stretch curves of various elastomers,such as vulcanized rubber,natural rubber,TPE,silicone rubber and Tera-PEG gel.(2)We developed a new physically-based model to capture the viscoelastic behavior of soft materials by decomposing molecular chains into the hyperelastic network,which can be further decomposed into the cross-linked network,the entanglement network,and free chains.Furthermore,we assume the rate dependence of stress comes from the free chains,which can be further additively decomposed into untangled cross-linked free chains' network and the free chains' entangled network,each possessing different relaxation time.We use the model to predict the mechanical behavior of VHB 4910 and describe the rate dependent behaviors of DMs.The results indicate that our model can give an excellent quantitative description of the viscoelastic behaviors for soft materials.Especially,We compare the stress responses from disentanglement and contour relaxation to demonstrate that the contour relaxation plays an important role in the large stretch and the disentanglement plays an important role in softening of materials.