Cellulose nanocrystals (CNCs) were modified with natural di-and tricarboxylic acids using two concurrent acid-catalyzed reactions including hydrolysis of amorphous cellulose segments and Fischer esterification, resulting in the introduction of free carboxylic acid functionality onto CNC surfaces. CNC esterification was characterized by Fourier Transform Infrared Spectroscopy, 13 C solid state magic-angle spinning (MAS) and conductometric titration experiments. Average degree of substitution values for malonate, malate, and citrate CNCs are 0.16, 0.22 and 0.18, respectively. Despite differences in organic acid pKa, optimal HCl cocatalyst concentrations were similar for malonic, malic and citric acids. After isolation of modified CNCs, residual cellulose co-products were identified that are similar to microcrystalline cellulose based on SEM and XRD analysis. As proof of concept, recycling experiments were carried to increase the yield of citrate CNCs. The by-product was then recycled by subsequent citric acid/HCl treatments that resulted in 55% total yield of citrate CNCs.The crystallinity, morphology, and substitution of citrate CNCs from recycled cellulose coproduct is similar to modified citrate CNCs formed in the first reaction cycle. Thermal stability of all modified CNCs under air and nitrogen resulted in T 10% and T 50% values above 256 °C and 323 °C, respectively. Thus, they can be used for melt-processing operations performed at moderately high temperatures without thermal decomposition. Nanocomposites of polyvinyl alcohol with modified CNCs (1 wt% malonate-, malate-, citrate and unmodified CNCs) wereprepared. An increase in the thermal decomposition temperature by almost 40 °C was obtained for PVOH-citrate modified CNC nanocomposites.Furthermore, since TGA determined weight loss up to 150°C is attributed to loss of bound water, this provides a tool to determine CNC water affinity. By 150°C, the weight loss of modified and non-modified CNCs is 6% and 2%, respectively. Consequently, modified CNCs with surface carboxylate groups have higher water affinity than non-modified CNCs. Moreover, significant differences in the thermal stability are observed as a function of the di-or triacid used for CNC modification. Based on the peaks of the derivative thermogravimetric curve (DTG, T 50% ), modified CNCs have the following thermal stability: malonate = HCl > malate > citrate CNCs.Corresponding values for T 50% are 366°C, 365°C, 350°C and 345°C, respectively. The amount of residual char at 600°C for HCl, malonate, malate and citrate CNCs is 8, 13, 16 and 20%, respectively. Hence, it follows that increasing the T 50% for di-and tri-acid modified CNCs results in correspondingly lower char formation. Increased char amounts is likely due to CNC functionalizations that lead to relatively larger number of cross-linking events at elevated temperatures.Since the exclusion of air during melt processing is generally not practical, the effect of CNC modification on thermal stability in air was also determined and the correspo...
This paper demonstrated the feasibility of conducting an enzymatic ring-opening polymerisation by reactive extrusion (REX) at high shear and temperature conditions. Using immobilized Candida antarctica Lipase B (CALB) as catalyst at temperatures ranging from 90 to 130°C, ω-pentadecalactone (PDL) was converted (>99%) by REX at 60 RPM for 15 min to PPDL with ≥M w 163 000 g mol −1 .The majority of current polymerisation methods use metal catalysts. Residual metal catalysts are often undesirable in materials used for biomedical and electronic applications. 1 Immobilized enzyme-catalysts have been shown to have distinct advantages relative to most metal catalysts including: (1) naturally derived, (2) low toxicity, (3) high chemo-and regioselectivity, (4) activity at relatively low temperatures and (5) no need for strict exclusion of water and oxygen. 2-6 The most commonly employed lipase for enzyme-catalysed ring opening polymerisations (eROP) and polycondensations is the immobilized lipase form of Candida antarctica Lipase B (CALB). Of the many lactonic substrates for which CALB is an active polymerisation catalyst, CALB efficiently catalyses ROP's of larger lactones (e.g. ω-pentadecalactone, PDL). 3 The immobilized CALB catalyst used in ref. 4 and herein is Novozyme 435 (N435). ROP of larger lactones are known to be difficult for many organometallic catalysts because the polymerisations are primarily entropy-driven. 7 Nevertheless, recent progress has resulted in a number of chemical catalysts that successfully convert PDL to high molecular weight polymers. Examples of these catalysts are aluminium salen and terdentate phonoxyimine-amine aluminium. Problems encountered with these catalysts are as follows: (i) synthesis from expensive ligands, (ii) requiring inert reaction conditions (e.g. performed in a glove box) and (iii) the use of solvents. 8,9 The ability of lipases to catalyse ring-opening and condensation polymerisations at relatively low temperatures (e.g. 70-90°C) is advantageous to reduce energy input and to preserve thermally sensitive chemical moieties. However, when high molecular weight polymer synthesis is desired, corresponding diffusional constraints must be overcome by either running reactions at higher temperatures (e.g. 150-220°C), which is generally regarded as not feasible for enzyme-catalysts that denature under such conditions, 10 or by adding solvent. For example, for N435-catalyzed synthesis of high molecular weight (M n > 50 000 g mol −1 ) poly(ω-pentadecalactone), PPDL, and poly(ε-caprolactone, PCL), the viscosity was lowered by the addition of toluene. Subsequently, the final polymer products are obtained by precipitation into a non-solvent such as methanol. 11-13 Solvent-based processes reduce the volumetric productivity of reactions and also require solvent recycling.Reactive extrusion (REX) is an industrially relevant technique because it combines polymerisation and processing into a single step. 14 REX has been used to overcome the aforementioned problems of bulk polymerisations, mai...
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