Volume Change in Polymerization

An important part of the optimization process is the stabilization of the monomer-template assemblies by thermodynamic considerations (Fig. 6-11). The enthalpic and entropic contributions to the association will determine how the association will respond to changes in the polymerization temperature [18]. The change in free volume of interaction will determine how the association will respond to changes in polymerization pressure [82]. Finally, the solvent s interaction with the monomer-template assemblies relative to the free species indicates how well it will stabilize the monomer-template assemblies in solution [16]. Here each system must be optimized individually. Another option is simply to increase the concentration of the monomer or the template. In the former case, a problem is that the crosslinking as well as the potentially nonselective binding will increase simultaneously. In the... 

Dilatometry utilizes the volume change that occurs on polymerization. It is an accurate method for some chain polymerizations because there is often a high-volume shrinkage when monomer is converted to polymer. For example, the density of poly(methyl methacrylate) is 20.6% lower than that of its monomer. Polymerization is carried out in a calibrated reaction vessel and the volume recorded as a function of reaction time. Dilatometry is not useful for the usual step polymerization where there is a small molecule by-product that results in no significant volume change on polymerization. 

It is obvious from Fig. 12 that crack formation does not start at emergent imperfections which has been confirmed by transmission electron microscopy (see Sect. IV.b.)59. The scanning electron micrographs of poly-DSP and poly-P2VB44) crystals are quite different. The difference may be reflected by a crystal volume change during polymerization in which the DSP crystals shrink while the P2VB crystals expand. 

The corresponding H-NMR spectra are shown in Fig. 8.5. The volume changes during polymerizations, as calculated from densities of monomer and polymers, are given in the Table 8.11 47). 

A similar approach has been used more recently by Miyata et al (2006) to prepare tumour marker-responsive hydrogels. These polymeric networks, prepared by biomolecular imprinting, were conjugated with lectins and antibodies so that they exhibited volume changes in response to antifreeze glycoprotein (AFP), a glycoprotein which is widely used for the serum diagnosis of primary hepatoma. 

The techniques discussed may be viewed as a fonn of dilatometry, and a glass dilatometer was, in fact, used by Laita (23) to study the polymerization of etl lene in benzene at pressures which were not recOTded, but must have been as h%h as about 50 atm in some cases. Here, too, recourse had to be taken to gravimetric calibration of the volume change of polymerization, particularly, since a liquid and a gas phase must have coexisted. 

Methacrylate polymerizations are accompanied by the Hberation of a considerable amount of heat and a substantial decrease in volume. Both of these factors strongly influence most manufacturing processes. Excess heat must be dissipated to avoid uncontrolled exothermic polymerizations. Volume changes are particularly important in sheet-casting processes where the mold must compensate for the decreased volume. In general, the percent shrinkage decreases as the size of the alcohol substituent increases on a molar basis, the shrinkage is relatively constant (35). 

---