Evaluating the molecular weight distribution of ultrahigh molecular weight polypropylene through rheology

This work investigates the possibility of obtaining the molecular weight distribution (MWD) of linear ultrahigh molecular weight (UHMW) polypropylene (PP) through rheology. To this end, the linear viscoelastic response of a set of UHMWPP samples is measured over the largest possible frequency range. The terminal relaxation is achieved by running creep experiments and converting the compliance in dynamic moduli. A time-temperature concentration principle, recently validated for UHMW polyethylene, is also applied to obtain the terminal relaxation of the sample with the largest molecular weight. The linear rheological response is correlated with gel permeation chromatography (GPC) results by means of the mixing rule based on the relaxation modulus. The implementation of such a rule requires the knowledge of some material parameters governing the stress relaxation of the polymer. Since they are unknown in literature for PP, they are estimated from the comparison between the viscoelastic spectra and the GPC distributions of three lab-made UHMWPPs with narrow polydispersity. Such parameters are then used as a basis to predict the MWDs of two UHMWPP samples with large polydispersity. The variability of the parameters upon molecular weight and polydispersity is assessed by applying the mixing rule to two different PP samples with lower molecular weights, one with narrow polydyspersity and another one with broad polydispersity. As the GPC curves of the samples are available, first the direct problem of estimating the rheological response from MWD and then the inverse problem of obtaining the MWD from the rheological data are solved. An overall satisfactory agreement is found between the calculated and measured MWD for the two samples, with both the direct and inverse approach.

Automated summary

This study examines the possibility of determining the molecular weight distribution (MWD) of linear ultrahigh molecular weight (UHMW) polypropylene (PP) using rheology. Various UHMWPP samples are analyzed using linear viscoelastic measurements and creep experiments to calculate the terminal relaxation. The results are compared with gel permeation chromatography (GPC) data using a mixing rule based on the relaxation modulus. Material parameters governing the stress relaxation of PP are estimated from lab-made UHMWPP samples, and this information is used to predict the MWD of other samples. The direct and inverse problems of obtaining rheological responses from MWD and vice versa are solved, showing satisfactory agreement between calculated and measured MWD.