On the feasibility of determining polymer chemical heterogeneity by SEC with continuous off-line Raman detection
Examined here is the feasibility of employing Raman spectroscopy as a detection method in size-exclusion chromatography (SEC) and related macromolecular separations, for the purposes of determining the chemical heterogeneity of copolymers. To this effect, heart-cutting fractions from the SEC peak of a gradient random copolymer of styrene (S) and methyl methacrylate (MMA) were collected, at concentrations resembling those of eluting slices in an analytical SEC experiment, and subsequently analyzed by off-line Raman. The amount (weight or mole percent) of styrene in each fraction was quantitated by Raman by applying a calibration curve constructed through the analysis of well-characterized block, alternating, and random S-MMA copolymers. The weight percentages of S (w% S) obtained from SEC with off-line Raman analysis of the gradient copolymer were compared to results previously obtained using SEC with on-line multi-angle static light scattering, differential refractometry, and ultraviolet absorption detection. Not only did Raman rank the w% S of the fractions in the correct order but, on average, Raman results differed by 3% or less as compared to the values obtained via multi-detector SEC. The results from this study provide proof of principle of both the feasibility of continuous off-line Raman detection for macromolecular separations and of the possibility of employing for this purpose the same or similar hardware to that currently used for continuous off-line Fourier transform infrared detection.