Higham Alina K, Bonino Christopher A, Raghavan Srinivasa R, Khan Saad A
Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA.
Soft Matter. 2014 Jul 21;10(27):4990-5002. doi: 10.1039/c4sm00411f. Epub 2014 Jun 3.
We examine the gelation of alginate undergoing ionic crosslinking upon ultraviolet (UV) irradiation using in situ dynamic rheology. Hydrogels are formed by combining alginate with calcium carbonate (CaCO3) particles and a photoacid generator (PAG). The PAG is photolyzed upon UV irradiation, resulting in the release of free calcium ions for ionic crosslinking. The viscous and elastic moduli during gelation are monitored as a function of the UV irradiation intensity, exposure time, alginate concentration, and the ratio between alginate and calcium carbonate. Gel time decreases as irradiation intensity increases because a larger concentration of PAG is photolyzed. Interestingly, dark curing, the continuing growth of microstructure in the absence of UV light, is observed. In some instances, the sample transitions from a solution to a gel during the dark curing phase. Additionally, when exposed to constant UV irradiation after the dark curing phase, samples reach the same plateau modulus as samples exposed to constant UV without dark curing, implying that dark curing does not affect the gelation mechanism. We believe the presence of dark curing is the result of the acidic environment persisting within the sample, allowing CaCO3 to dissociate, thereby releasing free Ca(2+) ions capable of binding with the available appropriate ionic blocks of the polymer chains. The growth of microstructure is then detected if the activation barrier has been crossed to release sufficient calcium ions. In this regard, we calculate a value of 30 J that represents the activation energy required to initiate gelation.
我们使用原位动态流变学研究了海藻酸盐在紫外线（UV）照射下进行离子交联时的凝胶化过程。水凝胶是通过将海藻酸盐与碳酸钙（CaCO₃）颗粒和光酸发生器（PAG）混合形成的。PAG在紫外线照射下发生光解，导致释放出用于离子交联的游离钙离子。凝胶化过程中的粘性和弹性模量作为紫外线照射强度、曝光时间、海藻酸盐浓度以及海藻酸盐与碳酸钙比例的函数进行监测。随着照射强度的增加，凝胶时间缩短，这是因为更多的PAG发生了光解。有趣的是，观察到了暗固化现象，即在没有紫外线的情况下微观结构仍在持续生长。在某些情况下，样品在暗固化阶段从溶液转变为凝胶。此外，在暗固化阶段之后暴露于恒定紫外线照射时，样品达到的平台模量与未经过暗固化而暴露于恒定紫外线的样品相同，这意味着暗固化不影响凝胶化机制。我们认为暗固化的存在是由于样品内部酸性环境持续存在，使得CaCO₃解离，从而释放出能够与聚合物链上可用的合适离子基团结合的游离Ca²⁺离子。如果越过了激活势垒以释放足够的钙离子，那么就会检测到微观结构的生长。在这方面，我们计算出一个值为30 J，它代表引发凝胶化所需的活化能。
