Monomer solution, shrinkage

Fig. 5.28 Drying of silica gels aged in monomer solution aging times are indicated for xerogels gray lines indicate levels of volumetric shrinkage (adapted from Einarsrud (1998)).

An interesting modification involves expandable monomers. These materials, synthesized by Dr. William J. Bailey and coworkers at University of Maryland, are spiro monomers, double rings which open and expand on polymerization. They can be useful in adhesives which are based on polymer/monomer solution, such as structural acrylics, radiation-cured acrylates, etc. These monomers can counteract shrinkage on curing, which is detrimental to adhesion. 

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical (72—74) and radiation-initiated (75) polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of acrylic monomers is first order with respect to monomer concentration and one-half order with respect to the initiator concentration. Rate data for polymerization of several common acrylic monomers initiated with 2,2 -azobisisobutyronittile (AIBN) [78-67-1] have been determined and are shown in Table 6. The table also includes heats of polymerization and volume percent shrinkage data. 

Polystyrene can be polymerised by foam processes, mass polymerisation, solution polymerisation, suspension polymerisation and emulsion polymerisation. Each contributes to the final characteristics of the plastics and the likely residues. A limit for styrene monomer may be imposed. Styrene can be readily analysed down to a 0.01% level. Polystyrene exhibits low shrinkage, hence is an excellent moulding material. 

The polymerization is accompanied by approximately 6% shrinkage (calculated from densities of monomer and polymer at 25 °C). Upon heating the polymer solution in the presence of trifluoromethanesulfonic acid the substituted 1,3-dioxolane ring opens and linear polymer is formed. This is accompanied by a 9.7% expansion in volume. 

Another possibility might be to use solutions of acrylic polymer dissolved in acrylic monomer and to harden the glue in situ by heat-activated, free-radical-initiated polymerization. Such a glue should have acceptable shrinkage properties and, if the components are properly chosen, could remain fairly tough. Further, if non-cross-linking components are used, it should be a reversible system. 

The determinination of the rate of polymerization of vinyl acetate in solution has been carried out in a mercury recording dilatometer. In this procedure, the solution of the monomer and 2,2 -azobis(2,4-dimethylvaleronitrile) is placed in the dilatometer, degassed, and sealed at a pressure of approximately 10 mm Hg. The dilatometer is maintained at 50.0 0.02°C. The total shrinkage is calculated using 0.892 gm/ml as the density of the monomer and 1.166 gm/ml as that of the polymer. The rate of polymerization is usually determined from the rate of shrinkage in the conversion range of 5-7% [62]. 

The study of unconjugated diolefin polymerizations is usually considered quite difficult because complex, three-dimensional networks are set up at very low conversion. Such cross-linked materials are generally intractable. The situation is quite different in the case of diallyl esters, especially in the case of diallyl phthalates. Both diallyl o-phthalate and diallyl m-phthalate may be converted to prepolymers, which are soluble in a variety of solvents. These two prepolymers are commercially availabe. When dissolved in their respective monomers and heated in the presence of typical initiators, such solutions are converted to cross-linked resins. Most of the shrinkage related to the conversion of monomers to pol)mers has taken place when the pre-polymer was formed originally. Therefore solutions containing relatively high levels of diallyl o-phthalate prepolymer shrink little on polymerization. 

Several general disadvantages of bulk polymerization (removal of the reaction heat, shrinkage, nonsolubility of the resulting polymer in the monomer, side reactions in highly viscous systems such as the Trommsdorff effect or chain transfer with polymer) are responsible for the fact that many polymerization processes are carried out in the presence of a solvent. A homogeneous polymerization occurs when both monomer and polymer are soluble in the solvent. When the polymer is insoluble in the solvent, the process is defined as solution precipitation polymerization. Other heterogeneous polymerization reactions in liquid-solid or liquid-liquid systems such as suspension or emulsion polymerizations are described later. Conventional solution polymerization is compared with solution precipitation polymerization for the synthesis of acrylic resins in Ref. [34]. 

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