The study of filled rubber composites with rheology and neutron scattering

Abstract

Polymer nanocomposites represent a growing and important field from academic, environmental, and commercial viewpoints. Filled rubber composites of car tyres are a prime example, as improvement of the material properties and reduction in the carbon footprint are both of interest. However, even with continued research into this subject, questions remain concerning the causes of the observed properties of these composites, namely the non-linear reinforcement and the presence of significant strain softening.
This report documents the examination of polybutadiene rubber composites with multiple techniques to characterise and determine the cause of the non-linear phenomena. The macroscopic material properties were characterised with rheological measurements, while the nanoscale composite structure and polymer dynamics were examined with small angle neutron scattering and quasi-elastic neutron scattering, respectively. Different filler compounds, carbon black, precipitated silica, and Stöber silica, were utilized to provide additional data to build an improved picture of the composite systems. A hydroxyl end functionalised polybutadiene chain known to segregate to silica substrates was used for examination of polymer interfacial dynamics and changes to composite material properties when used as an additive.
Reinforcement of the rubber with the addition of all the fillers was noted and analysed to determine the causes. A significant observation was the change in the mechanical properties of the silica filler composite with the presence of the end functionalised polybutadiene; decreased reinforcement and changes to behaviour under significant strains were noted. Changes to the nanoscale correlations of the silica filler with the presence of the end functionalised polybutadiene was observed with small angle neutron scattering, and determined to be from steric stabilisation of the silica particles. Hindered polymer dynamics near the silica filler surface were noted with quasi elastic neutron scattering. Neither the particle correlations nor the polymer dynamics were found to shift with the application of strain to the composite samples, in contrast to the alterations noted in the mechanical properties.
From the information gathered on the composite microscopic and macroscopic properties a model for the composite behaviour was devised. The reinforcement of the rubber with the addition of filler material was determined to be a result of the networking of the filler particles. The strain softening was found to be caused by the breakages of filler bridging contacts and thus involved only a small fraction of the total material.