Characterization of free volume during vulcanization of styrene butadiene rubber by means of positron annihilation lifetime spectroscopy and dynamic mechanical test.

A J Marzocca,L Gonzalez,S Cerveny,W Salgueiro,A Somoza

doi:10.1103/physreve.65.021801

Abstract

An experimental investigation was performed to study the effect on the free volume of the advance of the cross-linking reaction in a copolymer of styrene butadiene rubber by sulfur vulcanization. The dynamic modulus and loss tangent were evaluated over samples cured for different times at 433 K by dynamic mechanical tests over a range of frequencies between 5 and 80 Hz at temperatures between 200 and 300 K. Using the William-Landel-Ferry relationship, master curves were obtained at a reference temperature of 298 K and the coefficients c(0)(1) and c(0)(2) were evaluated. From these parameters the dependence of the free volume on the cure time is obtained. Positron annihilation lifetime spectroscopy was also used to estimate the size and number density of free volume sites in the material. The spectra were analyzed in terms of continuous distributions of free volume size. The results suggest an increase of the lower free volume size when cross linking takes place. Both techniques give similar results for the dependence of free volume on the time of cure of the polymer.

Highlights

It is known that, on increasing the cure level in sulfur vulcanized elastomers, the mechanical dynamic response, estimated by means of the variation of the dynamic moduli with frequency, shows a shift of the transition zone to lower frequencies1–3͔
The results suggest an increase of the lower free volume size when cross linking takes place
It can be concluded that the presence of cross links decreases the volume of holes in the sample

Summary

INTRODUCTION

On increasing the cure level in sulfur vulcanized elastomers, the mechanical dynamic response, estimated by means of the variation of the dynamic moduli with frequency, shows a shift of the transition zone to lower frequencies1–3͔. In the case of natural rubber vulcanizates this shift is due mainly to the presence of side reactions and secondarily to the increase in cross-link density4 – 6͔ Direct consequences of this are the hindrance of chain mobility and a broadening of the mechanical relaxation curves7,8͔. In the case of dynamic mechanical measurements, the William-Landel-FerryWLFrelationship9͔ is one of the most popular methods based on the timetemperature principle to evaluate the free volume fraction It can be estimated by means of the shift factor aT evaluated from analysis of the dependence of the storage modulus on frequency and temperature by applying the WLF relationship. The values of log10(aT) as a function of temperature were evaluated and the parameters of the WLF relationship With this information it is possible to correlate the fractional free volumes obtained using PALS with those of the free volume theory and confirm its validity

EXPERIMENTAL TECHNIQUES

RESULTS AND DISCUSSION

CONCLUSIONS