Polymorphic and Morphological Aspects of Recrystallized Cellulose as a Function of Molecular Weight

Abstract

The polymorphism and morphology of cellulose precipitated from solutions in amine oxide by the slow diffusion of water vapors, was investigated, as functions of the temperature of recrystallization and the degree of polymerization(DP) of the material to be recrystallized. At temperatures around 90°C, low DP cellulose crystallized almost exclusively as cellulose IV II , whereas higher DP material was found in the form of cellulose II. Substantial differences were also found in the morphologies of the various samples: with cellulose II, rod-like crystals were obtained with low DP material while a crystalline fibrillar gel precipitated when high DP samples were recrystallized. In all cases, cellulose IV II was obtained as a granular precipitate.The crystallization behaviour of polysaccharides is influenced not only by their chain conformation but also by the multiple possibilities of inter and intra molecular hydrogen bonding (1,2). This multiplicity, in particular, explains why several polymorphic forms can be obtained when different crystallization conditions are selected for a given specimen.A survey of the various parameters which have a role in directing the crystallization of a given polysaccharide toward one or another polymorph, presents the temperature of crystallization as being the most influential: polysaccharides such as cellulose (3), mannan (4), dextran (5,6), etc. are particularly sensitive to changes in crystallization temperatures as they yield completely different crystals at low or high temperature.In other instances, it is the solvent of crystallization which plays a decisive role in orienting the crystallization toward one or the other polymorph. Such solvent dependant crystallization is well documented in the case of amy lose (7) where minute changes in solvent/precipitant ratio have a dramatic effe ct on shifting the recrystallized amylose among three polymorps: amylose A,Β and V.

Structural Aspects in Semicrystalline Samples of the Mannan II Family

“…Thus, it seems that when allowed to crystallize as ribbons in the mannan II family, a low molecular weight mannan will crystallize in one allomorph, whereas a high molecular weight samples will be able to crystallize in another form. Such dependence of the polymorphism on the molecular weight has already been reported for the occurrence of mannan I and mannan II in mannan-based polysaccharides. , It has also been demonstrated in recrystallized cellulose and for (1→3)-β- d -glucan. , So far however, no satisfactory explanation has been given for such a phenomenon. In light of earlier work 12,13 and the present observations, mannan and the related glucomannans appear to be quite versatile in their crystallization mode.…”

The chemical-free production of nanocelluloses from microcrystalline cellulose and their use as Pickering emulsion stabilizer

“…Moreover, crystallization held at intermediate temperature conditions yielded hybrid interlinked cellulose II/IVII crystals, while the precipitation of a higher DP cellulose results in distorted shapes and loss of crystal structure 31,32 . Furthermore, a different study concluded that low DP cellulose crystallized almost exclusively as cellulose IVII, whereas higher DP material precipitated as cellulose II 38 . Figure 2 summarizes some of the literature findings regarding the crystallization of LMWC into single crystals of cellulose II and IVII.…”

“…In addition, it contains the first cross-team cooperation with the University of Valladolid and reconciliation of experimental results from that research group during the same period 29 . Furthermore, the results on LMWC precipitation were analyzed in light of the extensive and high-quality corpus of literature from the 1970s and 1980s on cellulose crystallization [31][32][33][34][35][36][37][38] . Finally, significant effort was put in characterizing the LMWC produced by supercritical water treatment and to compare it with microfibrillated nanocellulose produced from MCC by mechanical means 39,40 .…”

“…The cellulose IV family, composed of the IVI and IVII allomorphs, is the preferred arrangement for cellulose precipitated or regenerated at higher temperatures 38 . The classic preparation procedure of cellulose IVI involves heating cellulose IIII in glycerol at 260 °C 91 .…”