Viscoelasticity of Dense Suspensions of Thermosensitive Microgel Mixtures Undergoing Colloidal Gelation
Viscoelasticity of Dense Suspensions of Thermosensitive Microgel Mixtures Undergoing Colloidal Gelation
复制标题
胶体凝胶化过程中热敏微凝胶混合物致密悬浮液的粘弹性
作者：
Minami;S.;Watanabe;T.;Suzuki;D.;Urayama;K.
温度（T）敏感的聚（N-异丙基丙烯酰胺）（N）和聚（N-异丙基甲基丙烯酰胺）（NM）微凝胶混合物的致密悬浮液具有不同的体积转变温度（分别为TNc和TNMc; TNc < TNMc），由于T诱导的颗粒间相互作用类型以及每种凝胶的体积分数的变化，其表现出特征性的T依赖性粘弹性。在T < TNc的范围内，其中具有排斥性颗粒间相互作用的溶胀微凝胶被密集堆积，平衡模量（G）在加热时完全由于堆积效应而降低，即，微凝胶的总体积分数（μ g）的降低。在T > TN c时，脱水和疏水微凝胶之间出现有吸引力的颗粒间相互作用，由于收缩的微凝胶的网络状絮凝（胶体凝胶化），悬浮液显示出固体状的弹性性质，即使当微凝胶化变得显著低于随机紧密堆积的阈值时。G的T-依赖性在特征温度（TB; TB > TNc）处表现出最小值，这是由于颗粒间的排斥相互作用和疏水相互作用之间的竞争所致。N/NM混合悬浮液中的TB随着混合物中N含量（XN）的降低而向更高的值移动，并伴随着在特定XN值（XN*）处的不连续样变化。在XN的每个值下的TB与其中有吸引力的疏水微凝胶的总体积分数为0.3的温度大致一致，而不管微凝胶类型（N或NM）如何。TB在XN* 处的不连续样变化反映了网状絮凝颗粒的变化，从高XN状态中仅吸引N微凝胶到中等XN状态中吸引N和NM微凝胶混合物。使用PNIPAM-co-富马酸（NF）微凝胶悬浮液在不同pH下也证明了具有适当强度的排斥静电力对于网络状絮凝的稳定性的要求。
Dense suspensions of temperature (T)-sensitive poly(N-isopropyl acrylamide) (N) and poly(N-isopropyl methacrylamide) (NM) microgel mixtures with different volume transition temperatures (TNc and TNMc, respectively; TNc < TNMc) exhibit a characteristic T-dependent viscoelasticity due to T-induced changes in the type of interparticle interaction as well as the volume fraction of each gel. In the range of T < TNc, where the swollen microgels with repulsive interparticle interactions are densely packed, the equilibrium modulus (G) decreases upon heating due entirely to the packing effect, i.e., a reduction in the total volume fraction of the microgels (ϕ). At T > TNc where the attractive interparticle interactions between dehydrated and hydrophobic microgels emerge, the suspensions show solid-like elastic properties due to the network-like flocculation of the shrunken microgels (colloidal gelation), even when ϕ becomes considerably lower than the threshold for randomly close packing. The T-dependence of G shows a minimum at a characteristic temperature (TB; TB > TNc) due to the competition between the repulsive interparticle interactions from the packing effect and electrostatic force, and the attractive interactions from the hydrophobicity. The TB in N/NM mixture suspensions shifts to a higher value with a decrease in N content in the mixtures (XN), accompanied by a discontinuous-like change at a specific value of XN (XN*). The TB at every value of XN agrees approximately with the temperature where the total volume fraction of the attractive hydrophobic microgels is 0.3 regardless of microgel type (N or NM). The discontinuous-like variation in TB at XN* reflects the change in the network-like flocculation particles, from only attractive N microgels in the high XN regime, to the attractive N and NM microgel mixtures in the moderate XN regime. The requirement of the repulsive electrostatic force with an appropriate strength for the stability of the network-like flocculation is also demonstrated using the PNIPAM-co-fumaric acid (NF) microgel suspensions at various pH.