Lu Lu, Liu Xinxing, Dai Lin, Tong Zhen
Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China.
Biomacromolecules. 2005 Jul-Aug;6(4):2150-6. doi: 10.1021/bm050126u.
The sol-gel transition in aqueous alginate solutions induced by chelation with calcium cations from in situ release has been investigated with viscoelastic methods. Two alginate samples having different molecular weights (MW) were used over the concentration C(Alg) of 2 approximately 6 wt % with different mole ratio f of Ca2+ to the alginate repeat unit. The gel point f(gel) and relaxation critical exponent n were determined according to Winter's criterion, the later agrees well with that obtained from the relaxation modulus. The results indicate that the power law is valid for the dynamic relaxation at the gel point and the critical gel possesses the self-similarity in structure. With increasing C(Alg), f(gel) for the alginate with lower MW decreases dramatically and n is almost constant of about 0.71. In contrast, f(gel) for the higher MW alginate with is almost a constant and n decreases from 0.72 then levels off at 0.37 with increasing C(Alg), indicating that the concentration dependence of n varies with MW of alginate in the starting solution. The fractal dimension d(f) estimated from n suggests a denser structure in the critical gel of higher MW alginate. Either n or d(f) has been found to follow one curve for the two samples if plotted against the number of cross-link junctions per polymer chain, which is proportional to the alginate MW.
采用粘弹性方法研究了原位释放的钙阳离子与海藻酸钠水溶液螯合引发的溶胶-凝胶转变。使用了两种不同分子量（MW）的海藻酸钠样品，浓度范围为2%至6%（重量），钙与海藻酸钠重复单元的摩尔比f不同。根据温特准则确定凝胶点f(gel)和弛豫临界指数n，后者与从弛豫模量获得的结果吻合良好。结果表明，幂律在凝胶点处的动态弛豫中有效，临界凝胶在结构上具有自相似性。随着海藻酸钠浓度C(Alg)的增加，低分子量海藻酸钠的f(gel)急剧下降，n几乎恒定在约0.71。相反，高分子量海藻酸钠的f(gel)几乎恒定，n随着C(Alg)的增加从0.72下降，然后稳定在0.37，这表明n的浓度依赖性随起始溶液中海藻酸钠的分子量而变化。由n估计的分形维数d(f)表明高分子量海藻酸钠临界凝胶中的结构更致密。如果将n或d(f)与每条聚合物链上的交联节点数作图，发现两个样品遵循一条曲线，交联节点数与海藻酸钠MW成正比。
