A comparison between different ways to obtain molecular weight distributions of PVC and a study on the anomalies of PVC in tetrahydrofuran solutions

Classical methods for determinations of molecular weight distribution of PVC have been compared with gel permeation chromatography (GPC) measurements. Two PVC samples representing extreme values of polydispersity for common commercial resins, have been characterized by GPC and precipitation fractionation, using the tetrahydrofuran/H2O system. The concept of universal calibration and a correction for incomplete resolution have been applied to GPC data. Intrinsic viscosities and molecular weight averages have been determined both for the fractions and the original polymers. Different ways to calculate molecular weight distributions from the fractions have been compared. It was found that the modified BEALL method using the SCHULZ distribution function for the fractions, gives the most accurate molecular weight distribution if spurious peaks could be eliminated. Among the methods using molecular weight averages for the whole polymer in combination with a distribution function, the SCHULZ distribution function showed the best agreement with GPC. Distribution functions based on one molecular weight average only seemed to give less satisfactory results. Association of PVC molecules in tetrahydrofuran causes anomalous behaviour during the fractionation procedure and with other characterization methods. It was found that the amount of aggregates increases with syndiotacticity. To avoid the undesired influence of such supermolecular structures a heat treatment of the PVC solutions was used. The deviation from linearity in log [η] vs. log Mw plots was thus found to be due to aggregation and not to long chain branching.