Frequency dependence polymerization

Most polymers are applied either as elastomers or as solids. Here, their mechanical properties are the predominant characteristics quantities like the elasticity modulus (Young modulus) E, the shear modulus G, and the temperature-and frequency dependences thereof are of special interest when a material is selected for an application. The mechanical properties of polymers sometimes follow rules which are quite different from those of non-polymeric materials. For example, most polymers do not follow a sudden mechanical load immediately but rather yield slowly, i.e., the deformation increases with time ( retardation ). If the shape of a polymeric item is changed suddenly, the initially high internal stress decreases slowly ( relaxation ). Finally, when an external force (an enforced deformation) is applied to a polymeric material which changes over time with constant (sinus-like) frequency, a phase shift is observed between the force (deformation) and the deformation (internal stress). Therefore, mechanic modules of polymers have to be expressed as complex quantities (see Sect. 2.3.5). 

Polymer properties are very often dependent on the polymer preparation. So, a good monitoring of the polymerization process is the key step to obtaining good and reproducible materials. The extent of the polymerization can be controlled in different ways. IR is the most usual [27,30] but is not very accurate and requires the extraction of samples to analyze. Recently, an in situ monitoring of PMR-15 processing has been provided by means of frequency-dependent dielectric measurements [33,34]. This non-destructive technique allows the characterization of all the steps of the curing process and thus they can be optimized. 

Implications. These results have an important implication concerning the use of Fourier analysis of DC transients in polymeric materials to extract the frequency-dependence of the dielectric response (12)- In order for the principle of superposition to apply the electric field inside the material being measured must be time- and space-invariant. This critical condition may not be met in polymers which contain mobile ionic impurities or injected electrons. Experimentally, we can fix only the average of the electric field. Moreover, our calculations demonstrate that the bulk field is not constant in either time or space. Thus, the technique of extracting the dielectric response from the Fourier components of the transient response is fundamentally flawed because the contribution due to the formation of ionic and electronic space-charge to the apparent frequency-dependent dielectric response can not generally be separated from the dipole contribution. 

The spin dynamics of ESR was studied in polythiophene doped with C10, prepared by electrochemical polymerization at 300 K [296]. Heavily doped PT is known to show a metallic temperature dependence on ESR linewidth caused by the Elliott mechanism, characteristic of metals, as will be mentioned in section 7 [254,258,287,295,296]. In addition to this, a line broadening due to the spin dynamics is expected. Mizoguchi et al. reported the frequency dependence of the ESR linewidth in PT-CIO4 as shown in Figure 6.51 [296]. The filled circles do not behave simply following the prediction of (6.20) and (6.21) of Q-l-D spin motion, but there is a broadening mechanism other than... 

Okamoto et al. [2000] preintercalated Na-MMT with oligo(propylene glycol) diethyl methyl-ammonium chloride, or methyl-trioctyl-ammonium chloride (SPN and STN, respectively), dispersed the organoclays thus obtained in methyl methacrylate (MMA) or styrene (St) and then polymerized. The interlayer spacing indicated intercalation. The frequency sweeps showed frequency-independent dynamic moduli, indicating a three-dimensional solid for MMA/STN and St/SPN systems. In a better dispersed MMA/SPN system, strong frequency dependence was observed (i.e., exfoliation apparently eliminated the three-dimensional structures). 

The a transition, which involves motion in long segments of the main polymer chain, is related to the Tg. The P transition involves rotation of short-chain ester side groups in PMMA and therefore occurs below the Tg. The frequency dependency of the p-Tg can be used to calculate the activation energy for the molecular motion, which provides important information for characterising the structure and predicting the performance of polymeric materials. In a dielectric experiment, the calculated activation energy for the P transitions in PMMA was 17.7 kcal/mol. This correlates well with the values calculated from DMA and creep experiments. 

Temperature and other environmental factors affect the mechanical behavior. Thus, polymers can show all features of a glassy brittle solid, an elastic rubber, or a viscous liquid depending on the temperature and time scale of measurements. At low temperatures or high frequencies, a polymeric... 

Whereas elastic modulus at a single temperature (as measured by a static-mode instrument) is indicative of the properties and quality of metals, this is not necessarily true of polymeric materials in which molecular relaxations dramatically influence the temperature and time (reciprocal frequency) dependences of material properties. 

Frequency response analysis (FRA 6.3.2) lOOpHz-lOMHz A sinusoidal voltage signal is applied and the analyzer measures the frequency dependence of the complex impedance between the electrical ports of a system under test, at a given temperature Extremely precise results for any form of polymeric materials (hquid, sohd, thin flhn, etc.) The turnkey BDS concept 10,20, or 40 systems, available from Novocontrol the first two are economical versions of BDS 40, but the sample cell has to be ordered separately... 

---