Specific Refractive Index Increment Measurements on Macromolecules Using a Waters R401 Differential Refractometer

Getting Closer to Absolute Molar Masses of Technical Lignins

Determination of molecular weight parameters of native and, in particular, technical lignins are based on size exclusion chromatography (SEC) approaches. However, no matter which approach is used, either conventional SEC with a refractive index detector and calibration with standards or multi-angle light scattering (MALS) detection at 488 nm, 633 nm, 658 nm, or 690 nm, all variants can be severely erroneous. The lack of calibration standards with high structural similarity to lignin impairs the quality of the molar masses determined by conventional SEC, and the typical fluorescence of (technical) lignins renders the corresponding MALS data rather questionable. Application of MALS detection at 785 nm by using an infrared laser largely overcomes those problems and allows for a reliable and reproducible determination of the molar mass distributions of all types of lignins, which has been demonstrated in this study for various and structurally different analytes, such as kraft lignins, milled-wood lignin, lignosulfonates, and biorefinery lignins. The topics of calibration, lignin fluorescence, and lignin UV absorption in connection with MALS detection are critically discussed in detail, and a reliable protocol is presented. Correction factors based on MALS measurements have been determined for commercially available calibration standards, such as pullulan and polystyrene sulfonate, so that now more reliable mass data can be obtained also if no MALS system is available and these conventional calibration standards have to be resorted to.

Determination of molar masses of macromolecules by size exclusion chromatography-light scattering not requiring knowledge of refractive index increments

A new approach for the calibration of SEC-light scattering (SEC-LS) setups is proposed, which requires solely the molar mass of a reference polymer. Neither the specific refractive index increment of the calibrant nor of the analyte is required. Comparison of the molar masses derived in different solvents for a large number of chemically different polymers shows that the new approach yields the same molar masses as if molar masses were derived using dn/dc to calibrate the light scattering setup. The approach therefore allows easier determination of molar masses by SEC-LS.

Specific refractive index increment (∂n/∂c) of polymers at 660 nm and 690 nm

The specific refractive index increment (∂n/∂c) is an essential datum for the accurate quantitation of molar mass averages and distributions (inter alia) of macromolecules when refractometry, static light scattering, and/or viscometry detection are coupled on-line to size-based separation techniques. The latter include methods such as size-exclusion and hydrodynamic chromatography, and asymmetric and hollow-fiber flow field-flow fractionation. The ∂n/∂c is also needed for accurate determination of the weight-average molar mass of polymers by off-line, batch-mode multi-angle static light scattering. However, not only does ∂n/∂c differ among chemical species, it also depends on experimental conditions such as solvent, temperature, and wavelength. For the last seventeen years, the author's laboratories have measured the ∂n/∂c of a variety of natural and synthetic polymers, at both 690 nm and, more recently, 660 nm, under a variety of solvent and temperature conditions. In all cases, this has been done by off-line, batch-mode differential refractometry, not by assuming 100% analyte column recovery and 100% accurate peak integration. Results of these determinations are presented here, along with the relevant experimental data.

Consequences of on-line dialysis on polyelectrolyte molar masses determined by size-exclusion chromatography with light scattering detection

Size-exclusion chromatography with light scattering detection experiments conducted on poly(acrylic acid) neutralized to different degrees or using hydroxides with different counterions suggest that the same counterion and degree of neutralization is observed at the detector, irrespective of salt concentration, degree of neutralization and counterion at the time of injection. This strongly supports that during the chromatographic experiment the counterions of the polyelectrolyte are exchanged with those of the eluent, resulting in an effective dialysis of the polyelectrolyte solution during the size-exclusion chromatography experiment. Consequently, the refractive index increment determined by a refractive index detector equals the refractive index increment obtained after excessive dialysis against the pure eluent. Therefore, the species detected and characterized by light scattering coupled to size-exclusion chromatography are not identical to the species injected into the chromatographic system. Despite this structural change during the chromatographic experiments, the correct molar mass for the injected species is obtained by size-exclusion chromatography with light scattering detection.

Characterization of copolymers and blends by quintuple-detector size-exclusion chromatography

The properties imparted, oftentimes synergistically, by the different components of copolymers and blends account for the widespread use of these in a variety of industrial products. Most often, however, processing and end-use of these materials (especially copolymers) is optimized empirically, due to a lack of understanding of the physicochemical phase-space occupied by the macromolecules. Here, this shortcoming is addressed via a quintuple-detector size-exclusion chromatography (SEC) method consisting of multiangle static light scattering (MALS), quasi-elastic light scattering (QELS), differential viscometry (VISC), ultraviolet absorption spectroscopy (UV), and differential refractometry (DRI) coupled online to the separation method. Applying the SEC/MALS/QELS/VISC/UV/DRI method to the study of a poly(acrylamide-co-N,N-dimethylacrylamide) copolymer in which both monomer functionalities absorb in the same region of the UV spectrum, we demonstrate how to determine the chemical heterogeneity, molar mass averages and distribution, and solution conformation of the copolymer all in a single analysis. Additionally, through the various mutually independent conformational and architectural metrics provided by combining the five detectors, including the fractal dimension (derived from two different detector combinations), two different dimensionless size parameters, the chemical heterogeneity, and the persistence length, it is shown that the monomeric arrangement is more alternating than random at lower molar masses, thus causing the copolymer to adopt a more extended conformation in solution in this molar mass (M) regime. At high M, however, the copolymer is shown to be and to behave more like a random coil homopolymer, after passing through a 250 kg mol(-1)-broad region of intermediate chain flexibility. Thus, the combination of five detectors provides a unique means by which to determine absolute properties of the copolymer, solution-specific physical behavior, and the underlying chemical basis of the latter. The quintuple-detector method was also extended to the study of blends of polyacrylamide and poly(N,N-dimethylacrylamide) homopolymers to quantitate their molar mass, solution conformation, and chemical heterogeneity and to shed light on the breadth of the distributions of the component species. The method presented should be applicable to the study of copolymers and blends in which either one or both component moieties or polymers absorb in the UV region and can be implemented using not only SEC but other size-based separation methods as well.