The Dow Chemical Company, Midland, MI, USA.
Biomacromolecules. 2012 Oct 8;13(10):3355-69. doi: 10.1021/bm301123a. Epub 2012 Sep 20.
Cold, semidilute, aqueous solutions of methylcellulose (MC) are known to undergo thermoreversible gelation when warmed. This study focuses on two MC materials with much different gelation performance (gel temperature and hot gel modulus) even though they have similar metrics of their coarse-grained chemical structure (degree-of-methylether substitution and molecular weight distribution). Small-angle neutron scattering (SANS) experiments were conducted to probe the structure of the aqueous MC materials at pre- and postgel temperatures. One material (MC1, higher gel temperature) exhibited a single almost temperature-insensitive gel characteristic length scale (ζ(c) = 1090 ± 50 Å) at postgelation temperatures. This length scale is thought to be the gel blob size between network junctions. It also coincides with the length scale between entanglement sites measured with rheology studies at pregel temperatures. The other material (MC2, lower gel temperature) exhibited two distinct length scales at all temperatures. The larger length scale decreased as temperature increased. Its value (ζ(c1) = 1046 ± 19 Å) at the lowest pregel temperature was indistinguishable from that measured for MC1, and reached a limiting value (ζ(c1) = 450 ± 19 Å) at high temperature. The smaller length scale (ζ(c2) = 120 to 240 Å) increased slightly as temperature increased, but remained on the order of the chain persistence length (130 Å) measured at pregel temperatures. The smaller blob size (ζ(c1)) of MC2 suggests a higher bond energy or a stiffer connectivity between network junctions. Moreover, the number density of these blobs, at the same reduced temperature with respect to the gel temperature, is orders of magnitude higher for the MC2 gels. Presumably, the smaller gel length scale and higher number density lead to higher hot gel modulus for the low gel temperature material.
在升温时，已知甲基纤维素（MC）的冷、半稀、水相溶液会发生热可逆凝胶化。本研究集中于两种 MC 材料，它们的凝胶化性能（凝胶温度和热凝胶模量）有很大的不同，尽管它们的粗粒化学结构的度量（甲氧基取代度和分子量分布）相似。进行小角中子散射（SANS）实验以探测水相 MC 材料在预凝胶和后凝胶温度下的结构。一种材料（MC1，较高的凝胶温度）在后凝胶温度下表现出单一的几乎对温度不敏感的凝胶特征长度尺度（ζ(c) = 1090 ± 50 Å）。这个长度尺度被认为是网络节点之间的凝胶团块大小。它也与流变学研究在预凝胶温度下测量的缠结位点的长度尺度相吻合。另一种材料（MC2，较低的凝胶温度）在所有温度下都表现出两个明显的长度尺度。较大的长度尺度随着温度的升高而减小。其在最低预凝胶温度下的值（ζ(c1) = 1046 ± 19 Å）与 MC1 测量的值相同，并在高温下达到一个极限值（ζ(c1) = 450 ± 19 Å）。较小的长度尺度（ζ(c2) = 120 至 240 Å）随着温度的升高略有增加，但仍处于预凝胶温度下测量的链贯穿长度（130 Å）的量级。MC2 的较小的团块尺寸（ζ(c1)）表明键能更高或网络节点之间的连接更硬。此外，对于 MC2 凝胶，在相同的相对于凝胶温度的还原温度下，这些团块的数密度要高几个数量级。大概，较小的凝胶长度尺度和较高的数密度导致了低凝胶温度材料的较高的热凝胶模量。
