The glass transition and crystallization of ball milled cellulose
The glass transition and crystallization of ball milled cellulose
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DOI：
10.1007/s10570-010-9425-7
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发表时间：
2010-08-01
期刊：
影响因子：
5.7
通讯作者：
Mitchell, John R.
中科院分区：
文献类型：
--
作者：
Paes, Sabrina S.;Sun, Shaomin;Mitchell, John R.
以桉树和针叶木(云杉/松木)为原料，采用球磨法制备了球磨纤维素样品。在水含量为5-42%db(db=干基；水占总固体的百分比)的范围内，通过动态蒸汽吸附和饱和盐溶液上的平衡获得了水的吸附等温线。用口袋技术进行的动态力学分析表明，在相同的温度下，两种纸浆样品都发生了与水分有关的热转变，这被解释为玻璃化转变。用Couchman-Karasz关系式对数据进行拟合，预测了干燥纤维素的T(G)值约为478K，这与之前报道的其他干燥多糖的值相似。量热法没有观察到明显的玻璃化转变，尽管在约333K测得吸热，这在聚合物中通常被归因于热弛豫，但这一吸热不依赖于水含量，这表明一些弱缔合的熔融，如残留的氢键，可能是更可信的解释。在加热时也观察到一个放热，这取决于水分含量，这归因于纤维素的部分结晶。广角X射线衍射和交叉极化魔角旋转(13)碳核磁共振(CPMAS)证实了这一点。重结晶主要是形成第一种形式的纤维素。这被认为是由于少量残留的形式I(可能不到5%)作为结晶材料的模板造成的。差示扫描量热法再热曲线表明，在水含量高于20%时出现可冻结水，这是水在结晶后向无定形相转移的结果。CPMAS核磁共振弛豫也证实了结晶后纤维素刚性的增加。低分辨质子核磁共振T(2)弛豫表明存在质子水/纤维素交换，该交换在含水率为20%及以上时活跃。
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 X-ray diffraction and cross polarization magic angle spinning (13)C 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 above.