MOLECULAR-WEIGHT DETERMINATION IN DEGRADED OXIDIZABLE AND HYDROLYZABLE POLYMERS GIVING DEVIATION FROM ACCURATE USING CALIBRATION AND THE MARK-HOUWINK-SAKARUDA (MHS) EQUATION

We present results which show that thermally degraded LDPE and UV-aged PP containing recycled material have a changing relationship between the molecular weight and the retention volume during degradation givin g misleading values in the molecular weights using the Mark-Houwink-Sa karuda equation for undegraded polyolefins, The M-n value calculated b y the MHS equation for undegraded polyethylene is only about 40% of th e value obtained for degraded polyethylene. This deviation resembles t hat seen when comparing chain branched polymers with linear ones. The polydispersity for LDPE increased in both biotic and abiotic environme nt. The increase was largest in the photo-oxidative environment (i.e. H = 4.90 undegraded LDPE to H = 20.65 for LDPE photo-oxidized for 280 h). It has been reported that false values are obtained by the MHS equ ation if the polymer in question has a higher polydispersity than that of the standard used during construction of the calibration curve. Th e Mark-Houwink-Sakaruda (Mi-IS) equation offers a convenient means of determining the molecular weight of a polymer soluble in an organic so lvent (eta = K-m x M-a). For many linear polymer-solvent systems the M I-IS equation parameters K-m, and a are constant over a wide range of molecular weights, although the equation is not always applicable in t he low molecular weight region. The present paper shows that the same trend as was presented for LDPE is not observed in (bio)degraded poly( epsilon-caprolactone), although the polydispersity increased from 1.34 in undegraded PCL to 1.99 in biodegraded samples and to 2.25 in an al kaline environment at elevated temperature. In the context of SEC, com plex polymers are one group for which a unique relationship between th e hydrodynamic volume and the MW does not exist. in that respect degra ded polyethylenes resemble e.g. heterogenuous copolymers or homo- and copolymers with long-chain branching. The observed deviation in the de graded polyolefins is dependent on both the effect of chain-branching and that of increasing polydispersity with prolonged degradation. (C) 1997 Elsevier Science Limited.