Cellulose (2019) 26:959–970
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equipped with glass microfiber filters. Extractions were performed with a Dionex Corp. accelerated solvent extractor (ASE200) using a mixture of acetone:water:acetic acid (94:5:1 v/v). The samples were extracted consecutively three times for 5 min, at 100 C and 13.8 MPa, and the extract from each extraction cycle was combined into one. The total volume of the extract was adjusted to exactly 50 mL at room temperature. The concentration of extractives was determined by gas chromatography using both long and short column techniques (O¨ rsa˚ and Holmbom 1994). Exactly 10 mL of the extracts were pipetted into test tubes. Exactly 2 mL of internal standard, containing 0.02 mg/mL of heneicosanoic acid (21:0), betulinol or cholesterol, cholesteryl heptadecanoate (Ch17), and 1,3-dipalmi- toyl-2-oleyl glycerol (triglyceride standard) in MTBE, was added to the samples. The samples were dried at 40 C under a stream of nitrogen gas. The samples were dried to complete dryness in a vacuum oven (40 C). The dry samples were silylated by addition of a 1:4:1 mixture of pyridine, N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA), and trimethylchlorosi- lane (TMCS). The silylation reaction took place in a 70 C oven for 45 min. Analysis of different groups of extractives was conducted on a short column with a PerkinElmer Clarus 500 GC. The column was a HP-1, 7 m 9 0.53 mm, with film thickness of 0.15 mm. Hydrogen was used as carrier gas (see e.g. Strand et al. 2011). The injection volume was 0.5 l L, on-column, and the detector used was a flame ionization detector (FID). Analysis of individual components was conducted on a long column with a PerkinElmer AutosystemXL GC. The used column was a HP-1, 25 m, 0.2 mm, with film thickness of 0.11 mm (see e.g. Strand et al. 2011). Hydrogen was used as carrier gas. The injection volume was 1 l L, split 24:1, and the detector used was an FID.
disordered material in the starting pulps was applied in this study, i.e. acid methanolysis. It is known that the acid methanolysis procedure cannot hydrolyze crys- talline cellulose, and it is commonly used to analyze the hemicelluloses and pectins in various ligno- cellulosic materials (Sundberg et al. 1996a). By performing acid methanolysis on these three kraft pulps, it was possible to get an approximation of the amount of amorphous and para-crystalline carbohy- drate regions in the raw materials. The dominating sugar units detected in eucalyptus and birch kraft pulp were xylose and glucose (Table 1). The dominating sugar units detected in the softwood kraft pulp were glucose, xylose and mannose. A large part of the glucose was thought to originate from amorphous cellulose regions. By performing acid methanolysis on the kraft pulps, the content of easily hydrolysable carbohydrates was determined as roughly 25–29% by weight depending on raw material. The reaction conditions used in this study were not able to cleave all fiber residue into CNCs, and a significant fraction of partly hydrolyzed fiber frag- ments remained in the suspension. It has been reported that insufficient hydrolysis will result in larger parti- cles with lower surface charge (Dong et al. 1998; Beck-Candanedo et al. 2005). This type of large particles have previously been described as partially hydrolyzed cellulosic residue (Chen et al. 2015). The CNC suspensions were allowed to sediment prior to flow cytometry (FCM) analyses, in order to circum- vent the presence of large particles. Only the non- sedimenting particles in the supernatants were used as representative CNC samples for the different kraft pulps. Flow cytometry of the CNCs prepared from the three different kraft pulps revealed clear particle populations (Fig. 1). The forward scattering (FSC) intensities of the CNCs were plotted against their side scattering (SSC) as logarithmic density plots. The graphs revealed single particle population for all of the three different CNCs as a function of their light- scattering ability. The shapes of the CNC populations were slightly different depending on the raw material used, which indicated that the choice of raw material influenced the resulting CNCs. Previously, FCM measurements of polyelectrolyte complexes (PECs) highlighted that the FSC and SSC offers quite different information about particles in suspension (Strand et al. 2018). It was theorized that the SSC gives information about the amount of dense and light scattering surfaces
Results and discussion
Previously, the weight loss upon hydrolysis prior to level-off degree of polymerization (LODP) has been used to assess the amount of disordered material in pulps of different origins (Ha˚kansson and Ahlgren 2005). Another method to determine the amount of
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