Determination of molecular weight distribution from rheological measurements

An important characteristic of a polymer is its molecular weight distribution (MWD). The MWD affects both the processing and solid state performance properties of polymeric materials. Currently the analytical method for evaluating MWD involves the use of gel permeation chromatography (GPC). This is a labor and time intensive procedure, with the accuracy of the results depending strongly on the skill and experience of the equipment operator. Furthermore, GPC is not sensitive to small amounts of high molecular weight material that can have an important effect on processing and product properties. Thus GPC is far from ideal as a tool for industrial quality control. It is known that the rheological properties depend on the MWD, and it has been proposed that complex viscosity data be used to infer the MWD for commercial polymers. There has been some degree of controversy as to whether this is feasible, but recent results of Shaw, Tuminello and their coworkers indicate that for linear polymers this is, in theory, possible. However limitations in the accessible range of frequency, as well as inevitable experimental errors pose serious barriers to the use of such a procedure. We have investigated the severity of these baniers and have shown that within certain, reasonable constraints complex viscosity data can be used to infer a realistic MWD and that this procedure could be used for routine quality control in a production facility