Small-angle x-ray scattering and rheological characterization of alginate gels. 3. Alginic acid gels
Small-angle x-ray scattering and rheological characterization of alginate gels. 3. Alginic acid gels
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DOI：
10.1021/bm034105g
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发表时间：
2003-11-01
影响因子：
6.2
通讯作者：
Kajiwara, K
中科院分区：
文献类型：
--
作者：
Draget, KI;Stokke, BT;Kajiwara, K
采用小角X射线散射和流变学方法研究了海藻酸凝胶的组成和分子量对凝胶弹性和分子结构的影响。海藻酸凝胶通过在整个样品中均匀降低pH来制备。三个不同的化学组成和序列，和两个到三个不同的分子量的每个样品的藻酸盐进行了检查。采用了三种藻酸盐样品，其具有0.39（LoG）、0.50（InG）和0.68（HiG）的古罗糖醛酸残基分数，覆盖了市售藻酸盐的范围。海藻酸凝胶的过量散射强度I比离子交联的海藻酸凝胶大约1个数量级，并且表现出更强的朝向低q的曲率。假设海藻酸凝胶是由聚集的多结点和单链组成，I（q）被分解为两个组分。时间分辨实验表明，在凝胶化开始后的第一个2小时内，聚集体的平均尺寸和它们的重量分数有很大的增加，这也与最明显的流变学变化相吻合。在平衡时，很少或没有观察到分子量的影响，而在相当的分子量，增加的散射强度与增加的含量的古洛糖醛酸残基记录，可能是因为一个更大的表观分子量的域。结果表明，在初始阶段形成准有序的结区，随后组装这些区，形成约50埃的畴。由稳定G-嵌段和不稳定交替（MG）嵌段的相对分数控制的初始接合区的平均长度决定最终随机聚集体的密度。因此，与InG或LoG系统中由松散堆积的较短连接区组成的结构域相比，高G藻酸盐产生具有更高聚集体密度的藻酸凝胶。
Alginic acid gels were studied by small-angle X-ray scattering and rheology to elucidate the influence of alginate chemical composition and molecular weight on the gel elasticity and molecular structure. The alginic acid gels were prepared by homogeneous pH reduction throughout the sample. Three alginates with different chemical composition and sequence, and two to three different molecular weights of each sample were examined. Three alginate samples with fractions of guluronic acid residues of 0.39 (LoG), 0.50 (InG), and 0.68 (HiG), covering the range of commercially available alginates, were employed. The excess scattering intensity I of the alginic acid gels was about 1 order of magnitude larger and exhibited a stronger curvature toward low q compared to ionically cross-linked alginate. The I(q) were decomposed into two components by assuming that the alginic acid gel is composed of aggregated multiple junctions and single chains. Time-resolved experiments showed a large increase in the average size of aggregates and their weight fraction within the first 2 h after onset of gelling, which also coincides with the most pronounced rheological changes. At equilibrium, little or no effect of molecular weight was observed, whereas at comparable molecular weights, an increased scattering intensity with increasing content of guluronic acid residues was recorded, probably because of a larger apparent molecular mass of domains. The results suggest a quasi-ordered junction zone is formed in the initial stage, followed by subsequent assembling of such zones, forming domains in the order of 50 Angstrom. The average length of the initial junction zones, being governed by the relative fraction of stabilizing G-blocks and destabilizing alternating (MG) blocks, determines the density of the final random aggregates. Hence, high-G alginates give alginic acid gels of a higher aggregate density compared to domains composed of loosely packed shorter junction zones in InG or LoG system.