取代度和金属平衡离子对羧甲基纤维素热行为的影响

The Effects of the Degree of Substitution and Counterions on Thermal Behavior of CMC

低取代度(DS<0.8)的NaCMC具有纤维素(Ⅱ)型的晶型结构,随DS增加结晶度迅速下降,DS>1时为无定形结构。其T_(?)随DS增加而降低,但DS=1.55的CMC的T_(?)则稍有提高。其起始分解温度(T_(?))随DS增加而增加,而分解活化能则与DS没有规律性的关系。CMC链上的平衡离子H^+、Na^+、Mg^+、Ca^+和Al^(3+)等对其结晶形态的破坏能力不同,对相同价数的离子,随离子半径增大,破坏力增强,而对半径相近的离子,高价的比低价的破坏作用明显。NaCMC的分解活化能E=251～293 kJ/mol,而高价平衡离子的CMC的E=105 kJ/mol左右。

The relationship of the crystallinity, glass transition temperature (Tg) and thermal stability of carboxymethylcellulose (CMC) with substitution degree (DS) from 0. 4 to 1. 55 were studied, by using WAXS, DSC and TGA methods. The crystallinities.rapidly decrease with the increase of DS. In the meanwhile, Tg of CMC also decreases with increasing DS, except the sample with DS=1. 55. The activation energies of the thermal degradation for the main chain of CMC lie between 251 and 293 kJ/ mol, similar to precursor cellulose. However, the initial temperature of the thermal degradation (Td) of CMC is influenced by the -CH2COONa contents on the chains, which increase somewhat with the increase of DS. This may be attributed to the interaction of metal ions and-carboxylate groups, on the polymer chains.The effects of counterions: H+, Na2+, Mg2+, Ca2+and Al3+ on the crystallinity of CMC are quite different. The crystallinity decreases in either case when the radiis of counterions with the same valence increase or when the valences of counterions with the similar radius increase. The effects of the counterions on the thermal behavior of CMC are quite complicate, therefore only a preliminary discussion is presented.