Gpc Calibration for the Molecular Weight Measurement of Butyl Rubbers

Abstract

Size Exclusion Chromatography (SEC) or Gel Permeation Chromatography [GPC] was used to determine the molecular weight and molecular weight distribution of butyl rubbers. Quantitative data were obtained by viscometry, and by the use of narrow distribution polyisobutylene (PIB) standards for calibration. The application of the Universal Calibration Principle, which uses readily available narrow distribution polystyrene (PS) standards, relies on the knowledge of the Mark-Houwink-Sakurada constants, K and α, for PIB or butyl rubber (the copolymer of isobutylene and isoprene containing 1–3 mol% of the latter). Comparison of narrow PIB and narrow PS calibrations led us to the selection of KPIB=2×10−4 dl/g and α=0.67, the constants published by the American Standards Corporation for PIB in tetrahydrofuran (THF) at 30°C, as best fit values. The weight average molecular weight [Mw] of various samples of commercially produced butyl rubber determined by using the Universal Calibration Principle with the selected constants, agreed well with those measured by viscometry. Other published K and α values yielded a poorer agreement