BI-MwA; Real-time Molecular Weights
Breaking New Ground in Polymer Science with Molecular Weight Analysis by Wayne F. Reed
The Physics Department at Tulane University (New Orleans, LA) is an acknowledged center for experimental research into solid state, atomic, optical, and polymer physics. Materials science and biophysics are also specialties. A nine-person group led by the author 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, the group uses techniques such as size exclusion chromatography (SEC), batch dynamic and static light scattering, X-ray diffraction, and electron microscopy. They have published two widely cited articles on the coupling of SEC to light scattering, refractometric arid Viscometric detectors, and subsequent data analysis. One of the main focuses is the investigation of nonequilibrium characterization by means oon-line monnitoring of polymerization reactions. The Tulane Physics group is believed to be the first in the world to have developed A fully automated, continuous on-line means of monitoring absolute molar mass during polymerization reactions alongside the associated monomer conversions, evolution of viscosity, and polydispersity.
One of the main focuses is the investigation of nonequilibrium characterization by means of online monitoring of polymerization reactions.
Prototype Molecular Weight Analyzer
Although still at the developmental stage, this proyotype molecular weight analyzer has already attracted state, federal, and private sector funding (Figures 1 and 2). The author was recently granted a patent that covers both the light scattering instrumentation used in this methodology and the continous dilution and on-line monitoring technique. Two other patents related to this and further extensions of light scattering instrumentation and applications see pending.
Figure 1. Close-up of scattering chamber, Laser beam enters from the left.
Initial work on the analyzer began in the summer Of 1996. The following year, Brookhaven Instruments Corp. (BIC, Holtsville, NY), a company known for its work in colloid and particle characterization, became involved as a supplier of cells and photodiode detectors. Since then, the two teams have worked closely together. In 1998, the analyzer was used in the first demonstration of the absolute on-line monitoring method1. Further demonstrations have followed2, including more comprehensive approaches for on-line determination of evolving polydispersity3. More recently, the analyzer has been used in conjunction with the Tulane group's automatic dilution technique to provide continous, automated batch characterization of polymers and colloids4,5c). The extension of the system to the measurerment of both difficult solutions, such as those contaminated with dust and other large particles, and those solutions in which an integral colloid phase coexists with the polymers, formed the basis of a 1999 article introducing the HTDSLS (heterogeneous time-dependent static light scattering) technique. Current manuscripts in preparation focus further on the on-line determination of polydispersity, monitoring of new types of polymerization reactions, and determination of polymerization kinetics and mechanisms. The ability to monitor such reactions successfully will bring greatly improved understanding of the reaction kinetics and mechanisms that drive different types of chain growth during polymerization and copolymerization as well as the role of chemical and UV initiation. The analyzer and associated methods are beginning to shed light on such reactions.
Figure 2. The BIC Molecular Weight Analyzer.
The light scattering chamber itself is economical, rugged, and relatively simple to build. It consists of a flow-through chamber made of a cylindrical piece of Delrin (DuPont, Wilmington, DE) (although other plastics, metals, or ceramics could also be used) with opposing windows for the entrance and exit of the laser beam. Optical fibers are mounted integrally to the cell, the part of the chamber containing the sample to be analyzed, and are slightly recessed from the internal circular bore of the chamber: The nominal sample volume is 90 uL with a scattering volume of append. 20 nL. A vertically polarized 25 mW diode laser operating at 677 nm is used as the incident beam for scattering.
Standard HPLC flow fittings at each end of the chamber are used to pass sample through the cells. The direction of sample flow is perpendicular to that of the laser beam. The optical fibers carry scattered light to an external detector. So far, high-senisitivity, low-noise photodiode detectors supplied by BIC have been used. However, the company is planning to replace these with a charge-couple-device (CCD). As it stands, the device can be used to make traditional batch light scattering characterizations of polymer solutions and colloidal suspensions, including use as a light scattering detector for SEC (gel permeation chromatography, GPC), as well as certain nonequilibrium measurements. Auxiliary components for following on-line polymerization include a mixing pump, refractometer, UV photometer, and viscometer. These developments in polymer science have enhanced the Tulane Physics Department's reputation and visibility, attracted students and collaborators, and increased external funding. The group at Tulane and BIC are continuing their close collaboration, aiming to have a commercial version of the molecular weight analyzer available in the very near future.
The group is also involved in monitoring other, non-equilibtium processes such as polymer degradation, phase separation, aggregation, microcrystallization, and gelation. It has produced pioneering theoretical and experimental papers on the use of time-dependent static light scattering (TDSLS) to monitor and quantify degradation reactions. Characterization reactions for the polymer, pharmaceutical, food, and other industries are frequently undertaken.
One of the author's recent inventions concerns simtultaneous multiple sampling light scattering, with applications foreseen in multiple, parallel sampling of industrial reactors, combinatorial chemistry applied to polymers and colloids, and multiplexed analytical laboratory instruments. The TDSLS technique allows the simultaneous monitoring of coexisting populations of polymers and colloids (for example, bacteria and the polysaccharides they produce in biotechniology reactors). The technique holds promise for a wide variety of applications. 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. (of the nine current group members, two are from Turkey and two are from Brazil. All four are taking sabbatical leave from their own organizations to further their understanding of the techniques being developed at Tulane. All of this research demands the very best in monitoring techniques; therefore, new instrumentations is constantly being developed to keep up with the advances in technology. At the same time, the group is deveping 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.
1. Florenzano F., Strelitzki R. Reed W.F., Absolute online monitoring of polymerization reactions. Macromlecules 1998;317226-38
. 2. Catal-Giz H, Brousseau JL., Giz A, Alb A., Reed WF,. On-line measurements of polyacrylamide polymerization reactions. Macromolecules, submitted.
3. Reed WF. A method for online determinatoin of polydispersitv during polymerization reactions. Macromolecules, submitted.
4. Strelitzki R. Reed WF: Automated batch characterization of polymer solutions by static light scattering and viscometry. J. Appl. Polym. Sci. 1999; 73:2359-68.
5. Bayly E, Brousseau JL, Reed WF: Continuous monitoring of the effect of changing solvent conditions on polyelectrolyte conformations and interactions. Int. J. Poly Char Anal, submitted.
6,. Schimanowski R, Strelitzki R. Reed WF. Heterogeneous time dependent static light scattering. Macromolecules 1999; 32: 7055-63.