Measurement of Concentration Dependence of Índice de refracción to Enhance Gel Permeation Chromatography for Polymer Characterization

Autor: Gabor Kiss
Cita: Octubre de 2019
Antecedentes:
One of the workhorse techniques for polymer characterization is Gel Permeation Chromatography (GPC) which is used to measure molecular weight, molecular weight distribution, degree of branching, radius of gyration, solvent power, etc. The instrument relies on small changes in índice de refracción due to the presence of polymer in a solvent (called the “mobile phase”) to generate an “elution curve” as solution is pumped through a porous medium. The determinant parameter is the change of refractive index with concentration, called dn/dC. This parameter varies widely among polymer/solvent systems. It is possible to use GPC if one does not know dn/dC, by assuming 100% “recovery ratio”, i.e. that no polymer is lost due to absorption within the GPC column. However, if one knows dn/dC, this assumption is not required, so measurements are more precise. Indeed, by knowing dn/dC the Recovery Ratio becomes a figure of merit, the closer to 100% the better, indicating valid measurement results, and thus enhancing the Gel Permeation Chromatography technique.
El desafío:
Chromis Technologies manufactures polymer optical fibers which are used in a line of Active Optical Cables (AOC) for datacenter and audiovisual applications. The polymer is a specialized perfluorinated amorphous polymer. In order to expand the application space addressed by these polymers into separations, coatings, and others, Chromis embarked on a polymer development program. GPC was one of the polymer characterization techniques brought to bear in this program.
Un paso clave en el proceso de síntesis de polímeros es el post-procesamiento de un polímero precursor. Chromis se enfrentó a la cuestión de si la intensidad del post-procesamiento afecta al dn/dC. En caso afirmativo, entonces las curvas de elución de GPC de polímeros con diferentes grados de post-procesamiento no podrían compararse directamente y las mediciones de MW / MWD serían menos significativas.
In order to answer this question, Chromis engaged with La investigación analítica de Rudolph to measure the refractive index of dilute solutions over a range of concentrations. This was an extremely demanding application for three reasons:
1) Gel Permeation Chromatography requires that solutions be dilute to the point that polymer molecules do not interact. Since polymers are large molecules, the solutions are extremely dilute so refractive index changes are small, on the order of 0.05% over the range of concentrations studied.
2) El disolvente perfluorado necesario para esta aplicación tiene un valor extremadamente bajo de índice de refracción, fuera del rango de los refractómetros de la competencia
3) El sistema particular de polímeros/solventes tiene un bajo dn/dC, por lo que la medición es aún más difícil
As shown below, the Rudolph J457 Laboratory Refractometer ...enfrentó este desafío de manera brillante.
Resultados:
Los cuatro gráficos que figuran a continuación muestran mediciones del índice de refracción en una gama de concentraciones que van de 0 (disolvente puro) a unos 15 mg/mL para varios polímeros con un número diferente de ciclos de posprocesamiento.

Note the extremely narrow range spanned by these data (1.2658 – 1.2665 = 0.0008 or about 0.06%). Despite that, the reproducibility at each concentration is excellent and all data fall on beautiful straight lines with correlation coefficient very close to 1. The dn/dC calculated for each polymer is very close. Even more compelling, when all of the data are plotted together, it is clear that dn/dC does not depend on intensity of post-processing for the dilute solutions used by the Gel Permeation Chromatography.
$Gel Permeation Chromatography - Refractive Index$
Conclusión:
By using the La investigación analítica de Rudolph J457 Laboratory Refractometer, Chromis was able to easily and quickly obtain the data required to answer a key question in their polymer development program and thereby maximize the benefit from their investment in a sophisticated GPC apparatus.
Cualquiera que utilice la cromatografía de permeación en gel (GPC) se beneficiaría de este enfoque, que utiliza mediciones del índice de refracción para obtener un parámetro clave de GPC. Esto mejora la precisión y exactitud de las mediciones de GPC más allá de lo que la GPC por sí sola puede proporcionar.