Structural Transformations During Network Formation

An usual way to generate a strong acid as an initiator of cationic polymerizations is by the UV decomposition of a complex aromatic salt of a Lewis acid. Cycloaliphatic epoxy monomers are used in this reaction because they exhibit higher reactivities than those of glycidylether epoxies such as DGEBA. These formulations are used in photopolymerization processes whose main advantage apart from the fast reaction rate is the insensitivity to oxygen (contrary to free-radical polymerizations). 

The reaction of an alkoxide group with a cyclic anhydride is much faster than the reaction of a carboxylate group with an epoxy ring. 

Several epoxy formulations are cured by both step-growth and chain-growth polymerizations occurring sequentially or in parallel. For example, BF3 complexes or tertiary amines may be added as catalysts of an amine-epoxy reaction, leading to different reaction mechanisms taking place whose relative significance depends on the cure temperature (or thermal cycle) and the initial stoichiometry. The structure and properties of the resulting polymer networks depend on the relative contribution of both mechanisms. 

Gelation occurs at a conversion where percolation of a giant molecule takes place throughout the system. At this critical 

1 Stepwise Polymerizations For stepwise polymerizations of stoichiometric formulations of comonomers with / and g active sites per molecule exhibiting an ideal behavior (equal reactivity of functional groups, absence of both substimtion effects and intramolecular cycles), the gel conversion (Xg i) is given by  

---

Our objective is thus to characterize the structure of the polymer network formed during coagulation in the spinning process, as well as the structure obtained by slow coagulation, and to consider the formation of such structures in terms of the phase transformations in rigid polymer solutions. 

The chemical structure of the epoxy matrix constituent as well as processing are reported to strongly influence 11 -I3> the thermoset network and hence the properties and durability of the crosslinked polymer 11 ,4-16). The cure of a reactive prepolymer involves the transformation of low-molecular-weight reactive substances from liquid to rubber and solid states as a result of the formation of a polymeric network by chemical reaction of some groups in the system. Gelation and vitrification are the two macroscopic phenomena encountered during this process which strongly alter the viscoelastic behavior of the material. 

Depending on the voliune filHng factor of the matrix material, substantial shrinkage of the porous network accompanied by crack formation may occur for low filHng fractions during template removal. Furthermore, annealing the repHca material at elevated temperatures may lead to a transformation from one modification or crystal phase to another that is thermodynamically more stable at these temperatures, as shown for the thermal conversion of amorphous titania or its anatase phase to the rutile structure [79]. 

During the preparation of macroporous materials by crosslinking copolymerization in the presence of precipitants, phase separation takes place within a relatively short period of time. This fast phase separation naturally results in the formation of microdroplets of the rejected porogen. Since the conversion of comonomers at that moment is very low, the rapidly growing polymeric network fixes in the gel the emerging liquid droplets. The nonequilibrium microsyneresis thus transforms into the stable form of phase separation within a heterogeneous system. Thus, the fast arrival at the unstable local polymer-solvent relationship, as compared with the slow rates of solvent macrosyneresis and of network relaxation, leads to the formation of gel-included microdroplets and, finally, to a permanent macroporous structure of copolymers. 

---