Refractive index increment (dn/dc) using GPC for the α‐methyl styrene/methyl methacrylate copolymer at 670 nm in tetrahydrofuran

Abstract

Published online in Wiley InterScience (www.interscience.wiley.com).Although much work has been published about the dn/dc values for homopolymers, including ␣-methyl styrene (AMS) and methyl methacrylate (MMA) 1,2 there has been little in the way of published work to determine the dn/dc values for copolymers. It is generally accepted that the dn/dc value for a homopolymer is almost entirely dependent on the monomer and weakly dependent on (or independent of) molecular weight. 2Copolymers may be very different, however. Because molecular weight and composition can be closely linked (especially for systems with depropagating monomers like AMS and to a lesser extent MMA), some correlation may be present. Another factor that affects molecular weight, besides the reaction conditions, is the feed fraction of monomers. Will this have an effect on the dn/dc values obtained?From the Polymer Handbook 2 there is a historical recording of dn/dc values primarily at wavelengths below 633 nm, with new data appearing for 633 nm He-Ne lasers, but with none at the wavelength that current equipment uses: 670 nm. Another incentive to embark on this work is to show whether the assumption of a linear relationship between dn/dc and copolymer composition is valid 2-6 and whether the relationship works better for the mole or weight fraction composition.To determine the dn/dc of the copolymers and of the two homopolymers, a GPC setup, including a Waters solvent/ sample delivery system (Waters, Milford, MA) with an inline degasser (model AF), 515 HPLC pump, and 717plus autosampler, was used. The detectors on the system were from Viscotek (Houston, TX), contained in the TDA 302 quad detector package that incorporates RALS/LALS (670 nm), differential refractive index (DRI), viscometer, and UV (model 2501) detectors. The analysis software OmniSEC v2.0 (Viscotek) was included with the detectors.The samples analyzed in this study were produced by free-radical polymerizations at temperatures ranging from 100 to 140°C. Consequently, the AMS content in the copolymer reached a maximum of about 20%. To produce a polymer with a higher AMS content we would have to greatly sacrifice both yield and molecular weight, making the polymer undesirable for practical applications.To determine dn/dc it is possible to use this equipment along with the following expression based on the DRI response:where RI cal is the calibration constant for the RI detector, established by analyzing samples of known dn/dc and known concentrations; RI i is the measured response from the detector; C i is the sample concentration; and n 0 is the refractive index of the solvent. Because we already have the RI cal , and we know C i and n 0 , then the dn/dc can be calculated for the sample. Alternatively, the dn/dc can be calculated by plotting the RI area (integrated from the RI signal) versus C i . Because RI area is assumed directly proportional to the refractive index of the sample in solution, the slope of this line will be dn/dc. For the work discussed here, this second techni...

Propensity of Lignin to Associate: Light Scattering Photometry Study with Native Lignins

“…39 Copolymers can be very different, however, because the molecular weight and composition can be closely linked. 28 In almost all cases, the dn/dC that is once determined for a polymer is assumed to be constant and is used for all ensuing measurements. However, dn/dC changes due to chemical modifications have been reported once.…”

“…It is generally accepted that the d n /d C value for a homopolymer is almost entirely dependent on the monomer and is weakly dependent on (or independent of) molecular weight . Copolymers can be very different, however, because the molecular weight and composition can be closely linked . In almost all cases, the d n /d C that is once determined for a polymer is assumed to be constant and is used for all ensuing measurements.…”

“…Differential refractometry (DR) is a widely used technique for the determination of specific refractive index increments, d n /d C , of macromolecular solutions . Although much work has been published about the d n /d C values for homopolymers, there has been little in the way of published work that determines the d n /d C values for heteropolymers …”

“…27 Although much work has been published about the dn/dC values for homopolymers, there has been little in the way of published work that determines the dn/dC values for heteropolymers. 28 The dn/dC needs to be measured accurately to get a precise molecular weight value. There is no previous study of the dn/ dC for lignins in THF.…”

Polymer characterization by size-exclusion chromatography with multi-angle light scattering (SEC-MALS): a tutorial review

“…For interested readers, several studies describe precise d n /d c measurements of various copolymers. For example, Coto and coworkers conducted a thorough evaluation of ethylene–propylene copolymers; 50 Penlidis and coworkers determined d n /d c values in copolymers of alpha-methyl-styrene and methyl methacrylate; 51 Rudin and coworkers measured d n /d c values of ethylene-vinyl acetate copolymers; 52 and Chen et al studied the effects of polybutadiene microstructure on d n /d c values. 53…”