Side group motion

In the temperature range where side group motions occur, leading to a secondary transition like the ft transition of PMMA, these motions soften the surrounding of the moving units and, consequently, allow local conformational changes of the main chain under a lower stress than in the low temperature case. In the case of the ft transition of PMMA, on the low temperature side, the ester group n-flips are localised and the mechanism above described operates. 

Below Tg, structural relaxation is too slow to be observable, but secondary processes persist, which determine, e.g., the mechanical and dielectric properties of glasses. These processes have been extensively studied for polymers, where they are usually associated with polymer-specific dynamics such as side-group motion. From the point of view of glass physics, it is more interesting to investigate secondary relaxation processes in glasses comprised of rigid molecules, i.e., molecules without... 

It appears that the reaction mechanism and the intermediates involved in the solid-state polymerization of diacetylenes are reasonably well understood. However, experimental results obtained with special monomers should not be generalized. It is not possible to design a monomer with desired properties. Inspection of Table 1 shows that on the basis of the crystallographic data and the monomer packing the absolute reactivity and the polymerization kinetics caimot be quantitatively predicted, e.g. it is not possible, to date, to explain why certain diacetylenes can be polymerized thermally whereas others with equal packing are thermally inactive. A more realistic kinetic model should include the various energy transport processes and the complex side group motions which are connected to the reaction. 

Figure 12.11 Activation energy as a function of the temperature associated with the peak maximum of the P relaxation at 1 Hz. Filled symbols, main-chain motions open symbols, side group motions crosses, motions within molecules dissolved in the polymer matrix. (From Ref. 11.)...

Buffeteau, T., Natansohn, A., Rochon, R, Pezolet, M. (1996). Study of cooperative side group motions in amorphous polymers by time dependent infrared spectroscopy. Macromolecules 29, 8783-8790. 

Anisotropy of side group motion of poly(fluoroalkyl acrylates) crosslinked with allyl methacrylate and ethylene dimethacrylate [129]. 

Now, we focus on polymeric materials. Most of polymers have monomeric dipoles classified as type-A, type-B, and type-C. The type-A and type-B dipoles are directly attached to the chain backbone in the directions parallel and perpendicular to the backbone, respectively cf. Figure 3.2. The dielectrically observed fluctuation of these dipoles is activated only by the motion of the chain backbone. In contrast, the type-C dipole (not shown in Figure 3.2) is attached to the side groups and fluctuates through the side group motion even in the absence of the backbone motion. 

This study compared methacrylate and acrylate polymers to structural analogs with fluorinated ester groups. Two types of relaxations were characterized, the primary relaxation associated with the glass transition and secondary relaxations associated with side group motion and localized segmental motion. Dielectric analysis was used to characterize the response of dipoles to an electric field as a fimction of temperature. Mechanical properties were analyzed via dynamic mechanical analysis and stress relaxation measurements. Relaxation behavior was interpreted in terms of intermolecular and intramolecular mechanisms. 

The small length seale of RIB relaxation eould imply that it is of the nature of P-relaxation due to side group motions. However, the P-relaxation relaxation times are mueh shorter than 1 s at 20 °C and above, while the RIB relaxation times ean exceed 10 ° s, comparable to that of the cooperative segmental relaxations (a-relaxation). This effectively... 

Energies required for main-chain and side-group motions can be obtained by determining the effect of frequency on the maximum temperatures of the loss or tan 8 peaks. The temperature at the peak maxima, Tmax, increases with increasing frequency and the activation energy, of the relaxational process may be determined from the slope of a semilog plot of frequency (/) versus reciprocal peak-temperature (l/Tmax) as... 

Einfeldt, J., Kwasniewski, A., Klemm, D., Dicke, R., and Einfeldt, L. 2000. Analysis of side group motion in O-acetyl-starch using regioselective 2-O-acetyl-starches by means of dielectric spectroscopy. Polymers... 

Measurements of the frequency and temperature dependence of the H T, in poly(dimethyl siloxane) revealed relaxations due to methyl rotation and segmental motions and also an oxygen impurity effect The experimental data could not be fitted using thermally activated Arrhenius behaviour, as was also true of backbone motions in poly(vinyl chloride). " Multiple side-group motions have also been observed in poly (diethyl siloxane) and poly(L-histidine). Backbone motions have been observed in poiy(diethyl siloxane), poly(oxymethylene), poly(ethylene terephthalate), poly(p-phenylene sulphide), aromatic polyamides, and PTFE. A close similarity between the effects of entanglements and radiation cross-linking on the of cis-polyisoprene has been found. ... 

Inelastic neutron scattering is used for the smdy of transmission or absorption neutron energy spectra, particularly the side-group motion in polymers. All data reported so far for polymers have been concerned with symmetric top molecules. Three spectrometries are available at present (1) slow neutron spectrometry, which studies slow neutron excitation functions with continuous-energy neutron sources (2) fast neutron spectrometry, which smdies the spectra of neutrons produced in nuclear reaction and (3) monoenergetic slow neutron spectrometry, which smdies the spectra of neutrons corresponding to the inelastic scattering from atoms in solids or fluids. 

---