文档介绍：Gelation
Gelation is a process according to which a sol, or a solution, transforms to a gel. It consists of establishing links between the sol particles, or the solution molecules, so as to form a 3-demensional work
A GEL is an porous 3-dimensionally interconnected work that expands in a stable fashion throughout a liquid medium and is only limited by the size of the container.
Gel Point
Gelation occurs when the extent of polymerization reactions reaches a critical value. This precise critical stage, when for the first time a polymer of infinite size is formed, parison with the molecular scale, defines the Gel Point.
1. Viscous liquid to a material with elastic properties
2. A certain Viscosity increases toward infinity
3. An elastic modulus can be measured and its value starts from zero at the gel point
Flory-Stockmayer Model
凝胶点几率定义为高分子链末端支化单元上一给定的官能团连接到另外一高分子链的支化单元的几率。因而,凝胶化发生的条件是,从支化点长出的(z-1)条链中,至少有一条能够与另一支化点相连接。即
Z is coordination number, functionality Z
Pc is the critical probability to form gel
c extent of reaction at gel-point
Diffusion Limited Aggregation Models
The fractal dimensions is
The fractal dimensions is
Critical electrolyte concentration for gelation
凝胶作用的经典模型
胶凝过程看作是小胶粒不断聚合,形成无穷大胶粒簇的过程,由x个单体(monomer)组成的胶粒称为x-mer。
胶凝作用可以看成是x-mer不断变大的过程,当x时(形成充满整个容器大的簇),凝胶开始形成,该点称为凝胶点。
凝胶过程研究中受到关注的问题有:
1. 凝胶点的计算(或预测)
2. 凝胶过程中(尤其是凝胶点附近)凝胶的尺寸分布,重量分布和平均尺寸的变化
3. 凝胶过程中体系物理性质如粘度、弹性模量的变化
4. 胶粒的几何形状等
经典Flory-Stockmeryer理论
基本原理:x-mer结构
X=1, Z=3
X=3, Z=3
X=7, Z=3
假设:体系中所有键的活性相同,即成键的概率相同;键只在聚合体之间形成,而不形成闭合环。
需要解决的问题:一个聚合体系要形成无限大的聚合簇之前,在该体系全部的配位键中,需要有多少部分成键,即考虑体系的临界成键几率Pc。
Pc的计算
考虑连接一个无穷大的胶粒,则在每一个结点上,去连接下一个结点时,均有z-1种可能的选择。要连接上所有的分子,则每个结点至少形成一个键,即
临界点时:
例如对于z=2的分子,Pc=1,即所有键均要连接起来,形成一个长的分子键,这符合直观的想象。