New method to measure mass distribution of polyolefins. (News Briefs).

A new technique for measuring the molecular mass distribution of polyolefins has been developed by researchers at NIST. The new method is based on mass spectrometry, a long-standing technique for characterizing low mass substances, but one that is difficult to use on high mass polymers owing to the requirement of producing intact charged molecules in the vapor state. In the past decade a variation of mass spectrometry, termed matrix assisted laser desorption/ionization (MALDI), has become a powerful tool to characterize biopolymers, such as proteins and nucleic acids. Most-biopolymers and some types of synthetic polymers are readily charged. However, polyolefins lack the chemical composition for charging by the usual methods. NIST researchers have developed a method of chemically modifying polyethylene to a readily ionized molecule. The method has been demonstrated on polyethylene of masses to 15 ku, a mass limit that far exceeds what had been previously reported in the scientific literature. The first report of the NIST accomplishment appeared in the journal Polymer. A manuscript describing optimization of the method has been accepted by Macromolecules.

The method addresses a major need of polymer producers and users for rapid and reliable methods of measuring the molecular mass distribution of polymers. The need is especially urgent for polyolefins, examples are polyethylene and polypropylene that dominate the commercial market, where advances in catalysis have led to a plethora of new molecular variations. Owing to the way synthetic polymers are synthesized they possess a range of molecular masses. The distribution of the mass is critical to how the resins are processed into useful objects as well as the resultant properties. The methods currently used yield only averages of the mass distribution; chromatographic methods produce the mass distribution, if calibrated properly with polymer mass standards. All these methods require dissolution of the polymer in a solvent, for polyolefins this usually means using aggressive solvents at elevated temperature. This is one reason why few mass standards are available for polyolefins. The innovative mass spectrometr ic method provides a powerful direct approach for measuring mass distribution in polyethylene at a time of accelerated introduction of new molecular architectures.

CONTACT: William Wallace, [phone]; [e-mail].