Polydispersity, lignins

Tables 8.2.2 and 8.2.3 give typical results for two series of polydisperse lignin fractions obtained from acidic organosolv delignification of black cottonwood (Pla et al. 1986) and from alkaline delignification of western hemlock (Dolk et al. 1986). In both cases, LALLS allows accurate determination of low molecular weight values. The nearly identical dn/dc values for a given series of lignin fractions indicate the good reproducibility and accuracy of the technique. However, the second virtal coefficients, A2, vary considerably depending upon the fraction measured.

The polydispersity of softwood milled wood lignin, as measured by M /M = 2.5, is high compared with that of cellulose and its derivatives. 

Other lignins show different polydispersity as demonstrated by high pressure size exclusion chromatograms (47). The polydispersity of lignosulfates is much greater, with M ratios in the range of 6—8 (48). 

Application of Unical software also requires the selection of chromatographic baselines, thus selecting the specific data taken for further analysis. In the studies reported here, we choose to analyze only the polydisperse envelope from lignin elution, so that the distinct component which often elutes near V (the total column volume) was not included in the analysis. 

Table I illustrates the molecular weight averages found from universal calibration (narrow standards) for four aspen lignins and two quinonemethide-derived polymers using the Unical software. The poly-dispersities were the same within the experimental errors for all lignin samples. In contrast, the polydispersities found for the quinonemethide-derived polymers by universal calibration were near 1.1. The molecular weight averages found for the four acetylated lignins studied by universal calibration were substantially larger than those determined from previous work using conventional GPC (e.g., AESE and BM lignins from refs. 7 and 12 had approximately one-third those values found by HPSEC-DV in the present study).

This experiment was designed to examine the possible bias the broad polydispersities of the lignin samples may have on estimation of MWD by universal calibration. These data indicate that no such contribution exists, since the summation of the individual fractions of narrow(er) dispersity lead to values of MWD similar to those found using Unical software for the unfractionated lignin sample. 

Lignins are polydisperse macromolecules whose average molecular weights are rather low. Their determination by means of absolute methods becomes now easy owing to improvements in techniques, and this in spite of many problems inherent in the polymer itself. 

Preparation of the Living" Polystyrene. 18 g of the living polymer was prepared by standard anionic polymerization using n-butyl lithium. The reaction was carried out by the dropwise addition of 20 ml of styrene to 5 ml of the initiator solution in 150 ml of neat THF at -78°C. The styrene drip was adjusted to take approximately 30 min for completion and then the reaction was allowed to stir for two hours before the grafting reaction with mesylated lignin was carried out. The number average molecular weight of the polystyrene, as determined by HPSEC, was 9500 with polydispersity of 1.2. 

For polydisperse solutes, M2 is the number-average molecular weight. The calibration constant is commonly determined using a substance of known molecular weight at various concentrations. As a rule, AR is measured at several lignin concentrations a plot of AR/cw against cw is a straight line. If K is known under the same experimental conditions, the intercept at infinite dilution is Mn. 

Lignins. Lignins are amorphous, cross-linked phenolic polymers that occur uniquely in vascular plants and comprise 20-30% of most wood. Lignins isolated from wood are polydisperse, with molecular weights in the range of thousands to hundreds of thousands (4). Lignins are produced almost exclusively from three cinnamyl alcohols, whose structures are shown in Chart I. These structural units have propylphenyl carbon skeletons and differ... 

Previous observations regarding of characteristics of lignin molecules in solution were included the intrinsic viscosity, branching parameter and the degree of polydispersity. 

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