Determining Molecular Weights

Breaking new ground in polymer science

A long-standing collaboration between Brookhaven Instruments Corporation and the Physics department at Tulane University in New Orleans, USA, has broken new ground in the understanding of polymer characterization. This has resulted in the development of the Brookhaven BI-MwA, a molecular weight analyzer for online monitoring of polymerization reactions. Professor Wayne Reed explained the pivotal role of his group in this area of research

The Physics Department at Tulane University is an acknowledged center for experimental research into solid state, atomic, optical and polymer physics. Materials science and biophysics are also specialties. A nine-strong group led by Professor Reed within the Physics Department is having particular success with research into the characterization of polymers in solution and colloids in suspension. This group has international stature in characterizing the equilibrium and non-equilibrium properties of polymer solutions and colloidal suspensions.

In the area of equilibrium characterization, they use techniques such as size exclusion chromatography (SEC), batch dynamic and static light scattering, x-ray diffraction and electron microscopy. They have two widely cited articles on the coupling of SEC to light scattering, refractometric and viscometric detectors, and subsequent data analysis.

The main focus of the group’s work is the investigation of non-equilibrium characterization by means of online monitoring of polymerization reactions. The Tulane Physics group is believed to be the first in the world to have developed a fully automated, continuous online means of monitoring the absolute molar mass during polymerization reactions alongside the associated monomer conversions, evolution of viscosity and polydispersity.

The developmental of the BI-MwA

Initial work on the analyzer began in the summer of 1996 after attracting state, federal and private sector funding. The following year Brookhaven Instruments Corporation (BIC), a company known for its expertise in colloid and particle characterization, became involved as a supplier of cells and photodiode detectors. In 1998, the analyzer was used in the first-ever demonstration of the absolute online monitoring method. Further demonstrations followed, including more comprehensive approaches for online determination of evolving polydispersity. More recently, the analyzer has also been used in conjunction with the Tulane group’s automatic dilution technique to provide continuous, automated batch characterization of polymers and colloids.

The extension of the system to the measurement of both ‘difficult’ solutions, such as those contaminated with dust and other large particles, and those solutions where an integral colloid phase co-exists with the polymers, formed the basis of a 1999 article introducing the HTDSLS (heterogeneous time-dependent static light scattering) technique. Articles since have focused further on the online determination of polydispersity, monitoring of new types of polymerization reactions, and determination of polymerization kinetics and mechanisms.

The ability to monitor such reactions successfully brings a greatly improved understanding of the reaction kinetics and mechanisms that drive different types of chain growth during polymerization and co-polymerization as well as the role of chemical and UV initiation. Professor Reed’s group and BIC have closely collaborated for a number of years to produce the commercial version of the analyzer – the BI-MwA.

The technology

The BI-MwA is a light scattering instrument that is uniquely adapted to real-time monitoring of processes occurring in polymer solutions. The instrument comprises a flow-through, seven angle, static light scattering device with special features and software that allow the user to monitor polymerization as it occurs, plotting molecular weight, radius of gyration and conversion as a function of time.

The design of the BI-MwA avoids many of the pitfalls that are inherent in this type of work. Theoretically, a single low angle of measurement is sufficient to determine molecular weight and at least two angles are needed to determine the radius of gyration. Therefore three angles of measurement are an absolute minimum for a least squares fit. But this does not take into account the distortion of angular intensity that can occur due to the presence of dust.

To overcome these caveats, the standard optical configuration of the BI-MwA has seven angles. By ensuring the sample flow path is vertical, rather than horizontal, trapped air bubbles can be eliminated, while the conical shape of the inside of the cell helps the flushing of previous solutions. The cell is also constructed to withstand up to 3.5Mpa of pressure, minimizing the possibility of breakage. Originally, Brookhaven supplied high-sensitivity, low noise photodiode detectors for the prototype but these have since been replaced with ultra sensitive CCD detectors to give spatial uniformity. Coupled with a microcontroller, the instrument can also make automatic gain adjustments over its lifetime.

The BI-MwA can be used in batch or flow mode, as a chromatography detector, or for following the kinetics of polymerization using time-dependent static light scattering (TDSLS). The Tulane group produced the pioneering theoretical and experimental papers on the use of TDSLS to monitor and quantify degradation reactions, and the technique allows the simultaneous monitoring of co-existing populations of polymers and colloids, for example bacteria and the polysaccharides they produce in biotechnology reactors.

Other research

Over the last few years, Professor Reed has pioneered a number of new inventions concerning simultaneous multiple sampling light scattering, with applications seen in multiple, parallel sampling of industrial reactors, combinatorial chemistry applied to polymers and colloids, and multiplexed analytical laboratory instruments.

Other interests of the Tulane group include characterizing and analyzing both natural product biopolymers and synthetic polymers. Strong collaborative links have been forged between Tulane and universities and businesses in France, Germany and Brazil. Links are also being made with Turkish organizations. It is not uncommon for current group members to be scientists taking sabbatical leave from their own organizations to further their understanding of the techniques being developed at Tulane.

This breaking of new ground in polymer science has enhanced the Tulane Physics Department’s reputation and visibility, attracted students and collaborators, and increased external funding. All this research demands the very best in monitoring techniques, so new instrumentation is constantly being developed to keep up with the advances in technology. At the same time, the group is developing its own theoretical models, modifying those that already exist and developing software simulations to ensure that they remain ahead of the field in polymer characterization.

Applications: Polymers
Instruments: BI-MwA