Authors: Sabrina S Paes Shaomin Sun William MacNaughtan Roger Ibbett Johannes Ganster Timothy J Foster John R Mitchell
Publish Date: 2010/05/27
Volume: 17, Issue: 4, Pages: 693-709
Abstract
Samples of ball milled cellulose were prepared by ball milling pulps from eucalyptus and softwood spruce/pine Water sorption isotherms were obtained by both dynamic vapor sorption and equilibration over saturated salt solutions in the water content range of 5–42 db db = dry basis water as a age of total solids Dynamic mechanical analysis using a pocket technique showed a water content dependent thermal transition occurring at the same temperature for the two pulp samples which was interpreted as a glass transition Fitting the data to a Couchman–Karasz relationship predicted a value for T g of the dry cellulose of approximately 478 K which was similar to values previously reported for other dry polysaccharides No clear glass transition could be observed calorimetrically although an endotherm at approximately 333 K was measured which in polymers is normally attributed to enthalpic relaxation however the lack of dependence of this endotherm on water content suggests that the melting of some weak associations such as residual hydrogen bonds could be a more credible explanation An exotherm was also observed on heating which was dependent on water content and which was attributed to partial crystallization of the cellulose This was confirmed by Wide angle Xray diffraction and cross polarization magic angle spinning 13C NMR CPMAS NMR The recrystallisation was predominantly to form I of cellulose This was thought to be caused by a small amount of residual form I probably less than 5 acting as a template for the crystallizing material Differential scanning calorimetry reheat curves showed the appearance of freezable water for water contents higher than 20 as a result of a transfer of water to the amorphous phase following crystallization The increase in cellulose rigidity following crystallization was also confirmed by CPMAS NMR relaxation Low resolution proton NMR T 2 relaxation suggested the presence of proton water/cellulose exchange which was active at water contents of 20 and aboveWe acknowledge the help of Tim Benson Advanced Magnetic Resonance Ltd in the setting up and maintenance of the NMR equipment We thank Dr Andreas Bohn Fraunhofer Institute Germany for preparing the ball milled softwood cellulose and molecular weight determination
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