Gelation network structure

Gelation and the attendent insolubility mentioned above are encountered in all of the nonlinear polymerizations listed in Table I and in many others likewise. Naturally these characteristics have been attributed to the restraining effects of three-dimensional, or space, network structures of infinite size within the polymer. This is the feature which distinguishes most nonlinear from linear polymers. 

CaCl2 formed wide but relatively short strands without forming apparent continuous network structures in most cases (Fig. 6.13D). For i-carra-geenan, the addition of or Ca formed localized networks through side-by-side aggregation between helices, which was consistent with the thermal hysteresis between sol-to-gel and gel-to-sol transitions. However, this interhelical aggregation was not necessarily a prerequisite for gelation. 

At large strain, the liquid shows shear thinning in some poorly understood fashion. Shearing causes breakage of the fragile network structure near LST, which has been observed as an apparent delay in gelation. 

Controlled sample preparation is difficult near the gel point where the rate of property change is largest. Physical gelation usually proceeds too rapidly so that the material near the gel point eludes the experiment or the application. However, chemical gelation is most suitable for controlling the evolving network structure. Several approaches have been explored in industrial applications and in research laboratories ... 

It is shown that model, end-linked networks cannot be perfect networks. Simply from the mechanism of formation, post-gel intramolecular reaction must occur and some of this leads to the formation of inelastic loops. Data on the small-strain, shear moduli of trifunctional and tetrafunctional polyurethane networks from polyols of various molar masses, and the extents of reaction at gelation occurring during their formation are considered in more detail than hitherto. The networks, prepared in bulk and at various dilutions in solvent, show extents of reaction at gelation which indicate pre-gel intramolecular reaction and small-strain moduli which are lower than those expected for perfect network structures. From the systematic variations of moduli and gel points with dilution of preparation, it is deduced that the networks follow affine behaviour at small strains and that even in the limit of no pre-gel intramolecular reaction, the occurrence of post-gel intramolecular reaction means that network defects still occur. In addition, from the variation of defects with polyol molar mass it is demonstrated that defects will still persist in the limit of infinite molar mass. In this limit, theoretical arguments are used to define the minimal significant structures which must be considered for the definition of the properties and structures of real networks. 

Kinetic Gelation Modeling of Crosslinked Network Structure.197... 

Despite its limitations, kinetic gelation modeling is still a very useful tool in simulating network structure in highly crosslinked systems. While kinetic gelation models have gained widespread use in the polymer science field, the application of these models to dental materials and their development appears to be an area appropriate for further exploration. 

The gelation process that leads to the network structures required for rubber-like elasticity have been extensively studied, by experiments, theory, and simulations.245-249 In some case, the gelation can be made to be reversible.250... 

In fully reacted polymer networks, practically all constituent units are covalently bonded into an infinite three-dimensional structure. It means that during polymerization or crosslinking the system evolves from a collection of molecules of finite size to an infinite network, proceeding through the gel point at which the infinite network structure appears for the first time. This transformation is called gelation. 

Network structures are still determined by nodes and strands when long chains are crosslinked at random, but the segmental spacing between two consecutive crosslinks, along one chain, is not uniform in these systems which are currently described within the framework of bond percolation, considered within the mean field approximation. The percolation process is supposed to be developed on a Cayley tree [15, 16]. Polymer chains are considered as percolation units that will be linked to one another to form a gel. Chains bear chemical functions that can react with functions located on crosslinkers. The functionality of percolation units is determined by the mean number f of chemical functions per chain and the gelation (percolation) threshold is given by pc = (f-1)"1. The... 

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