Molecular weight distributions of industrially-produced poly-(ε-caprolactams) by gel permeation chromatography

Abstract

Gel permeation chromatography with differential refractometry is used to obtain molecular weight distributions (MWD) of poly-(ε-caprolactams). Elution is carried out using an m-cresol-chlorobenzene mixture (50:50, v/v) at 50°C. MW values are obtained by a Hamielec-based calibration method, using broad-MWD poly-(ε-caprolactam) standards with the same chemical nature and similar MWD to the samples. Relative errors for the number-average MW (M n) using this calibration method range from 0.4% (in the low polyamide MW range) to 20% (in the high polyamide MW range). These values are much lower than those obtained from narrow-MWD polystyrene calibration, which range from 39% to 78%. Similar values have been obtained for the other usual average MW parameters. The ability to obtain repeatability parameters for a given confidence interval and the utilization of statistical criteria for chromatogram rejection allow this method to be used in quality control for MWD of poly-(ε-caprolactams). Thus, production variables are related to polyamide-6 behavior in its ulterior treatment. Typical relative standard deviation percentages (for n = 6) of a polyamide sample range from 1.9% (for Mn) to 3.3% (for Mz + 1).