Molecular weight distribution by stirring‐induced fractional precipitation

Abstract

Ultrahigh molecular weight polyethylenes GUR, UTEC 2540, and UTEC 3540 were fractionated by precipitation from cooling of solutions with stirring [stirring‐induced fractional precipitation (SIFP)]. The samples were dissolved in decalin and precipitated over the temperature range of 100‐78°C. Three types of procedures were investigated. From fractionation data, the integral and differential molecular weight distribution (MWD) of the polymer were calculated. The MWD and average molecular weights, $\bar M_w$ and $\bar M_n$, by size exclusion chromatography (SEC) and the limiting viscosity number of the fractions and whole polymer were determined. From these data, the Mark‐Houwink constants were calculated. The ‘K’ and ‘aη’ values obtained were 6.70 × 10−4 and 0.69, respectively. The MWD curves calculated from SIFP technique by using these constants are in accordance with the MWD by SEC. Fractionation depended on the chemical nature of the stirrer and the cooling rate of the solution during precipitation. The methodology is reproducible. Faster fractionation was observed compared with other methods. © 1995 John Wiley & Sons, Inc.