Polymeric matrices interpenetrating

The degree of polymerization of hard clusters increases with evolution of the system as a whole. The hard clusters already exist in pregel molecules. Before the macroscopic gel point of the system is reached they remain usually small. Later on, the hard clusters grow faster and eventually a gel point (percolation threshold) of the hard structure is reached. Below this point, clusters are embedded in the soft matrix beyond the percolation threshold, the hard and soft structures interpenetrate (Figure 5.7). Below the percolation threshold, hard clusters are essentially dendritic when the percolation threshold is surpassed, circuits (cycles) develop within the hard structure. 

Extensive research has been conducted and reviewed on interpenetrating polymer networks during the past decade (31-33). IPNs can be categorized into Semi-l-IPN and Semi-2-IPN. Semi-2-IPNs are those in which crosslinkable oligomers or materials are polymerized into the linear matrix. Semi-interpenetrating networks could have combined polymers that have same or different repeating units. 

Interpenetrated Mall Matrix. Ion-exchange hollow libers can be produced by polymerizing an ionic monomer within (he porous wall matrix of a hollow fiber. Requirements of such a fabrication are th the monomers should not dissolve or plastici/c die polymer from which the fibers arc made. f2i the heat generated during the polymerization and contraction prior to the formation of new interpenetrating polymer should be minimized and ( ) the polymerization should not occur within the lumen (and hence cause plugging of the fiber). One drawback of such fibers is brittleness. 

The examples summarized above are but indicative of the increasing body of evidence in support of the chemical bonding theory, and of the role of the silane organofunctional group in the formation of covalent bonds at the coupling agent/ matrix interphase by reaction or by co-polymerization with the formation of an interpenetrating polymer network. 

Some of the earliest sol-gel hybrids were achieved by monomer infiltration into previously formed silica gel. Starting with a dried silica gel (xerogel), the porous shape is filled with monomer. The monomer is polymerized in situ. This is shown schematically in Figure 5. Interpenetration is achieved when the impregnating monomer polymerizes in the open pores of the rigid silica matrix. 

There are several techniques for producing polymer mixtures for further processing into articles by the reactive (chemical) molding methods simultaneous formation of two polymeric systems with mutual entanglement of their chains and the appearance of so-called interpenetrating networks synthesis of the second polymer within an expanded network of the initial polymer incorporation of oligomers into a polymer matrix as a temporary plasticizer followed by polymerization. 

There are numerous cited examples of blends where at least one polymeric component is vul-canizable. Such systems are quite difficult to analyze after processing. In some cases, formation of an interpenetrating network (IPN) may be mistaken for covalently bonded copolymer. This is particularly so when no deliberate effort has been made to promote vulcanization and its occurrence was adventitious. Selective solvent extraction experiments to detect copolymer can be misinterpreted if some polymer is physically occluded in a matrix of the second polymer. 

Although polymers and monomers in any form such as latexes, water-soluble polymers, liquid resins, and monomers are used in cement composites such as mortar and concrete, it is very important that both cement hydration and polymer phase formation (coalescence of polymer particles and the polymerization of monomers) proceed well to yidd a monolithic matrix phase widi a network structure in which the hydrated cement phase and polymer phase interpenetrate. In the polymer-modified mortar and concrete structures, aggregates are bound by such a co-matrbc phase, resulting in the superior properties of polymer-modified mortar and conoete compared to conventional. 

A special type of polymeric blends is interpenetrating networks (IPN), which represent a system formed in the course of building up one crosslinked polymer inside the ready-made network-matrix of another under conditions of no chemic d reaction between the networks. Such IPN-based adhesive compounds are noted for considerable long-term strength, which is explained by featiu es of the deformation processes that occur in the IPN layer when it is loaded. 

Figure 1 Molecular imprinting of scaffolding monomer having (a) noncovalent bond and (b) covalent bonds between monomer and template, and (c) scaffolding polymers with polymer matrix formation (MF) using interpenetration (IP), cross-linking (C) and aggregation interaction (AI) for matrix formation. P - - C is for polymerization and cross-linking processes and H is for hydrolysis of covalent bonds. E and B stand for extraction and binding of substrate, respectively.

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