Molecular characteristics of room-temperature soluble fractions of low-density polyethylene film resins

The molecular characteristics of the room-temperature Soluble fractions (RT solubles) of three low-density polyethylene film resins were characterized by size-exclusion chromatography (SEC), SEC combined with FTIR (SEC-FTIR), and nuclear magnetic resonance spectroscopy (NMR). The high-molecular-weight components of the RT solubles were found to be highly branched components with uniform short-chain branching (SCB) profiles. For the low-molecular-weight components, however, SCB content was a function of molecular weight (MW), increasing with an increase in MW. When the chain ends were considered as SCB equivalents, the distribution of the sum of SCB and chain ends across the Molecular weight distribution was practically flat, Suggesting that the driving force for polymer chains remaining in solution at RT was the length of the undisrupted methylene sequence in the backbone, or methylene run length, which was too short to form crystal lamellae with a melting temperature above RT, regardless of the molecular weight of the polymer. Moreover, the NMR results revealed that the polymer components of the RT solubles had "superrandom" SCB distributions, that is, the fraction of comonomer clusters in the polymer chains of the RT solubles was lower than that predicted by Bernoullian statistical analysis, indicating that the probability of adding a comonomer to a comonomer-ended propagating chain was lower than that of adding a comonomer to an ethylene-ended one, presumably because of an unfavorable steric effect. Furthermore, contrary to the common belief that RT solubles are mainly low-molecular-weight polymers, high-molecular-weight components were found in high concentrations in the RT solubles, with a cutoff MW as high as 1,000,000 g/mol. The proportion of RT solubles in the film resins was found to depend on the type of resin. (c) 2006 Wiley Periodicals, Inc.