DETERMINATION OF THE REFRACTIVE INDEX INCREMENT OF NATURAL AND SYNTHETIC POLY(CIS-1,4-ISOPRENE) SOLUTIONS AND ITS EFFECT ON STRUCTURAL PARAMETERS

Abstract

The refractive index increment (dn/dc) of a solution is a constant that indicates the variation of the refractive index with the solute concentration. It is used in the multi-angle light scattering technique to determine the concentration and the weight-average molar mass of polymers. It depends on many parameters, including the structure of the polymer, the solvent, the wavelength of the light and the temperature of the solution. To obtain accurate results, it is necessary to determine the dn/dc at the same condition as for the multi-angle light scattering measurement. In this work, the dn/dc at 633 nm of standard synthetic monodispere poly(cis-1,4-isoprene) (PI) and natural rubber (NR) solutions in THF were determined at 25 and 40°C using an Optilab DSP refractometer. The mean value of the dn/dc obtained for NR and PI was 0.13 mL/g. No significant difference between different types of samples and temperatures (25°C and 40°C) were observed. The nanoaggregates in NR solution that were not retained after filtration through 1 μm filter had no effect on the dn/dc. The intercept value of the conformation plot increased in line with the dn/dc, but the Flory exponent remained unchanged.

Keywords:

• natural rubber
• polyisoprene
• refractive index increment
• size-exclusion chromatography with multi-angle light scattering

ACKNOWLEDGMENT

We thank Ms. Valérie Bonniol of the European Membrane Institute, University of Montpellier 2, for her assistances.

Notes

^a Value published in Polymer Handbook was referred to a wrong source.

^b Ribbed smoked sheet natural rubber.

^c Industrial synthetic poly(cis-1,4-isoprene).

^d Guayule rubber.

^e Values obtained by extrapolation from other types of solvents using Eq. (1).

^f Values obtained by extrapolation from other wavelengths using Eq. (2).

^a Results obtained from this work.

^b The sample lipids were extracted using chloroform/methanol mixture (1/2).

^a Weight-average molar mass, the theoretical values were provided by supplier.

^b The theoretical concentration were those prepared for SEC-MALS injections.

^c  s or ns: the difference between theoretical and calculated values were significant (P < 0.01) or not significant.