On the reinforcement of rubber by fillers and strain-induced crystallization

Abstract

This thesis is dedicated to reinforcement mechanisms in filled and strain-crystallizing elastomers. First of all, a constitutive model for filled elastomers is presented. It is based on the non-affine tube model whose length measure is heterogeneously constraint by the filler particles. The model successfully describes a variety of compounds for different deformation modes, speeds and temperatures. When increasing crosslink density or amount of filler systematically, the parameters evolve in a physically reasonable manner. Moreover an extension and simplification of the model is presented, greatly improving its performance in Finite-Element applications. To further support the modeling hypothesis the interaction of different industrial grade carbon blacks and silicas with polymer-analogue gases is analyzed by means of static gas adsorption. The isotherms are deconvolved into a surface energy distribution using a specifically designed algorithm. Probing the samples with different gases reveals the different polar- and dispersive interaction capabilities of silica and carbon black. Chain desorption from the fillers surface is identified as a possible origin of mechanical hysteresis at large strains. Self reinforcement due to strain-induced crystallization (SIC) in natural rubber is analyzed by comparing the measured temperature increase upon deformation with a hypothetical temperature calculated from the mechanical response. The difference of both temperatures is attributed to crystallization. The method is applied to differently crosslinked and filled compounds and is shown to deliver fast, easy and reproducible results. Incorporation of filler changes the behavior of crystallization in accordance with the assumption of heterogeneous strain amplification. Cyclic loading at elevated strains reveals vanishing hysteresis in the degree of crystallinity, which is indicative of a different crystallization mechanism. Additionally, a theory of SIC is derived which relates mechanical response, crystal size and degree of crystallization using a minimal set of parameters. It takes into account the entropy loss due to attaching a chain to a crystal and naturally explains the constant crystal length and constant crystallization strain onset observed by many authors. The reinforcing effect of SIC is attributed to an intrinsic strain regulation mechanism.