Polymeric volume change

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). 

Crystal Structure. The crystal stmcture of PVDC is fairly well estabhshed. Several unit cells have been proposed (63). The unit cell contains four monomer units with two monomer units per repeat distance. The calculated density, 1.96 g/cm, is higher than the experimental values, which are 1.80—1.94 g/cm at 25°C, depending on the sample. This is usually the case with crystalline polymers because samples of 100% crystallinity usually cannot be obtained. A dkect calculation of the polymer density from volume changes during polymerization yields a value of 1.97 g/cm (64). If this value is correct, the unit cell densities may be low. 

In some cases, in particular for lattice and polymeric systems, volume change moves may be hard to perform, but particle insertions and deletions may be relatively... 

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. 

PNIPAM microsphere gels with diameter of 100-200 jim were prepared by emulsion polymerization [21]. The gel containing 12 mole % benzo[18]crown-6 was immersed in water and the diameter change of the gel was measured during heating at a rate of 0.3 °C/min. The gel was swollen below 25 °C. In the absence of metal ions, it started to shrink at 26 °C and showed a sharp volume change at 28.4 °C. Finally, the volume decreased by as much as 10 times the original volume. 

Emulsion Polymerization The polymerization apparatus was a dilatometer which was similar to though smaller than the one used before.(10) A graduated 10 ml column (adapted from a 10 ml pipette) was connected to a 50 ml volumetric flask with three indentations in it set at 120°. Latex volume changes were read by taking the emulsion to a reference point in the column and reading the height on a precision bore side tube. 

Polymerization frequently is performed in gas-phase reactors at intermediate pressures. The role of heterogeneous catalysts and the interaction between reaction kinetics and mass transfer can only be understood if sorption effects, solubilities of gases in solids, volume changes, and diffusivities at reactor conditions are known. 

Monomers opening at least two rings in linear chains formation polymerize either without any volume change, or with a small expansion. ... 

The presence of a carbon-halogen bond is not absolutely essential for the occurrence of tranfer to monomer. Moore et al. [37] studied styrene polymerization with y-irradiation. They measured thermodynamic quantities, particularly the Gibbs energy, enthalpy, entropy and volume changes by the method of rotating sectors and found that transfer to monomer is negli-... 

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. 

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