Gels in Good Solvents

Let us start with a solution of chains (polymerization index A ) in a good solvent (excluded volume parameter i = a (1 - 2 ) 0). The chains repel each other, and this is reflected in the existence of a positive osmotic pressure II. 

We now begin to attach the chains together, for example by reaction of the chain ends with certain z-functional molecules (z being equal to 3, 4, etc.), and we let them choose their density. They would like to separate from each other as much as possible however, each coil must remain in contact with its neighbors because of the crosslinks. The net result is shown in Fig. V. 15. 

What we have is a set of closely packed coils sealed together by the crosslinks. The situation is reminiscent of the overlap threshold in semi-dilute solutions (Chapter III). Thus, the gel automatically nuummns a concentration c proportional to c.  

In his original derivation of equations similar to eq. (V.25) Floiy assumed gaussian statistics for the chains plus a mean field estimate for the repulsive energies. His theory is successful the scientific community has 

Flory s calculation is quite similar to his discussion of a single chain in a good solvent, which we analyzed in Section 1.3. In this case we saw that an excellent result came fiom a cancellation between two serious approximations—one related to the use of gaussian statistics, and one due to the neglect of correlations between chains. The same cancellation occurs for swollen gels, and the success of the theory does not tell us that the chains are gaussian. 

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In this chapter, the emphasis is on swollen gels in good solvents (assuming that they have been prepared under similar conditions). However, we also describe briefly the effects of a decrease in the quality of the solvent and the resulting microsyneresis. 

Molecular weights of PVDC can be determined directly by dilute solution measurements in good solvents (62). Viscosity studies indicate that polymers having degrees of polymerization from 100 to more than 10,000 are easily obtained. Dimers and polymers having DP < 100 can be prepared by special procedures (40). Copolymers can be more easily studied because of thek solubiUty in common solvents. Gel-permeation chromatography studies indicate that molecular weight distributions are typical of vinyl copolymers. 

The chemical gel point defines the instant of LST of chemically crosslinking polymers. Before the crosslinking polymer has reached its gel point it consists of a distribution of finite clusters. It is called a sol since it is soluble in good solvents. Beyond the gel point, it is called a gel . The gel is an infinitely large... 

If an inert good solvent is used in solution polymerization, the gel thus obtained will have a supercoiled (expanded) structure (Gel B). Gel B swells in good solvents much more than Gel A which is synthesized in bulk. If the amount of the crosslinking divinyl monomer in the reaction mixture is increased while the amount of solvent remains constant, highly crosslinked networks are formed that cannot absorb all solvent molecules present in the reaction mixture and a heterogeneous structure results (Gel C). A part of the solvent separates from the gel phase during polymerization and the formed Gel C consists of two continuous phases, a gel and a solvent phase. If the amount of solvent is further increased, a... 

Depending on the conditions of synthesis, copolymerization of divinyl/vinyl-monomers in the presence of an inert solvent leads to the formation of expanded (preswollen) or heterogeneous (porous) structures [54,99,100]. If the solvent remains in the network (gel) phase throughout the copolymerization, expanded networks are formed. If the solvent separates from the network phase the network becomes heterogeneous. According to Dusek et al., heterogeneities may appear in poor solvents due to the polymer-solvent incompatibility (x-induced syneresis), while in good solvents due to an increase in crosslink density (v-induced syneresis) [99]. 

Fig.l Concentration dependence of retention volume for polystyrene in good solvents on polystyrene gel columns ... 

De Gennes proposed a description of the properties of swollen polymer networks based on the analogy found between the swollen network and semidilute polymer solutions (p theorem) [105]. The fully swollen gel is expected to maintain a polymer volume fraction, which is proportional to the overlap concentration. In good solvent condition... 

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