Cure State Distributions in Rubbers by Dynamic Nano-Indentation

Abstract

Abstract A depth-sensing and frequency-specific dynamic indentation technique was used to determine the spatial distributions of dynamic moduli, including storage (E′) and loss moduli (E″), in poly-cis-1,4-butadiene (BR) and brominated poly(isobutylene-co-p-methylstyrene) (BIMS) rubbers with varying degrees of cure and with or without carbon black fillers as a function of penetration depth. The BR rubbers were cured with sulfur and a sulfur accelerator and the BIMS rubbers were cured using zinc oxide and stearic acid. The dynamic modulus reached a constant bulk value after the Berkovich indentor penetrated into rubbers far beyond the surface. Spatially averaged bulk dynamic moduli thus determined using the nano-indentation technique were compared with the tensile extensional moduli and the frequency-dependent dynamic tensile moduli both measured from the bulk rubbers. Spatial distributions of cure and its dependence on penetration depth were evaluated as functions of cure level, filler loading, and sample surface preparation. The ultimate goal is to provide quantitative cure distribution maps in rubber blends and compounds through a combination of Atomic Force Microscopy (AFM) and dynamic nano-indentation characterization.