Abstract
Molar masses, Mark-Houwink-Sakurada (MHS) exponents, and refractive index increments (dn/dc) for three lignins were determined without derivatization by multi-detector gel permeation chromatography (GPC) in dimethylformamide (DMF) with 0.05 M lithium bromide (LiBr). The lack of effectiveness of fluorescence filters on molar mass determination by GPC-multi-angle laser light scattering (MALS) was confirmed for softwood kraft lignin (Indulin AT) and revealed for mixed hardwood organosolv lignin (Alcell) as well as soda straw/grass lignin (Protobind 1000). GPC with viscometry detection confirmed that these lignins were present as compact molecules. The MHS exponent α for Indulin AT and Alcell was in the order of 0.1. Additionally, the intrinsic viscosity of Protobind 1000 for a given molar mass was much lower than that of either Alcell or Indulin AT. This is the first report of dn/dc values for these three lignins in DMF with 0.05 M LiBr.
Highlights
The molar mass of lignin is a key parameter which governs its reactivity and physical properties
This study aims to fill this gap by employing multi-detector gel permeation chromatography (GPC) to determine molar masses as well as MHS exponent α and dn/dc values for a softwood (G-type) kraft lignin (Indulin AT), a hardwood (S- and G-type) organosolv lignin (Alcell), and a soda straw/grass (S, G and H-type) lignin (Protobind 1000) (Constant et al, 2016)
The number average molar masses (Mn) of Indulin AT increased with increasing sample concentration from 490 g mol−1 at 0.3 mL−1 to 1410 g mol−1 at 6.0 mL−1
Summary
The molar mass of lignin is a key parameter which governs its reactivity and physical properties. As reported by Cathala et al, THF is a poor solvent even for acetylated lignins, and new systems for molar mass determinations of lignins are desirable (Cathala et al, 2003). As an alternative to THF, the use of N,N-dimethylformamide (DMF) with and without the addition of lithium bromide (LiBr) has been reported in literature for molar mass determination of lignin (Tolbert et al, 2014; Gosselink et al, 2004; Cathala et al, 2003; Connors et al, 1980; Dong & Fricke, 1995). Many lignins are readily soluble in DMF, and derivatization such as acetylation, acetobromination, and benzoylation of the lignin is not required. As DMF is well-known for promoting association effects resulting in bimodal elution profiles (Cathala et al, 2003; Connors et al, 1980), the addition of lithium chloride or bromide to DMF was recommended to minimize aggregation and to obtain monomodal elution curves (Cathala et al, 2003)
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