Dynamic mechanical spectroscopy behavior

Another possibility of determining the gel point with the help of rheological methods is dynamical mechanical spectroscopy. Analysis of change of dynamic mechanical properties of reactive systems shows that the gel point time may be reached when tan S or loss modulus G" pass a miximum [3,4,13], Some authors proposed to correlate the gel point with the intersection point of the curves of storage and loss moduli, i.e., with the moment at which tan 5 = 1 [14-16], However, theoretical calculations have shown that the intersection point of storage modulus and loss modulus meets the gelation conditions only for a certain law of relaxation behavior of the material and the coincidence erf the moment of equality G = G" with the gel point is a particular case [17]. The variation of the viscosity... 

Characterization by DMS and DSC. Although characterization of small-strain viscoelastic and stress-strain behavior is not yet complete, preliminary dynamic mechanical spectroscopy (DMS) and differential scanning calorimetry (DSC) data were obtained for the blends having the highest and lowest molecular weights. 

Effects of addition of a compatibilizing block copolymer, poly(styrene-b-methyl methacrylate), P(S-b-MMA) on the rheological behavior of an immiscible blend of PS with SAN were studied by dynamic mechanical spectroscopy [Gleisner et al., 1994]. Upon addition of the compatibilizer, the average diameter of PS particles decreased from d = 400 to 120 nm. The data were analyzed using weighted relaxation-time spectra. A modified emulsion model, originally proposed by Choi and Schowalter [1975], made it possible to correlate the particle size and the interfacial tension coefficient with the compatibilizer concentration. It was reported that the particle size reduction and the reduction of occur at different block-copolymer concentrations. 

Dynamic mechanical spectroscopy using a device described in [157] at 100 Hz frequency and over a 243-533 K temperature interval was used to investigate the viscous-elastic behavior of OMC. Cured... 

This section will examine some of the characteristic features of IPN s from a physical and mechanical point of view. Emphasis will be on relating the glass transition behavior to corresponding aspects of morphology. The principal instrumentation employed in the studies discussed here includes a torsional tester for creep-type studies (Section 8.3.1) and a fixed-frequency vibrating unit for dynamic mechanical spectroscopy (see Section 8.3.2). In addition, stress-strain, tensile, and Charpy impact strength values will be briefly discussed. 

The mechanical properties and glass transition behavior of these polymer alloys were compared. The kinetics of polymerization of the component pol3rmers were measured and varied by changing the concentration of catalysts in order to determine the effect of polymerization rates on the morphology of the IPN s. Electron microscopy and dynamic mechanical spectroscopy were also carried out. Several theoretical models predicting the modulus of... 

Dynamic Mechanical Spectroscopy The determination of dynamic mechanical behavior over a range of frequency or temperature. 

The mechanical properties of a series of main-chain and side-chain LC elastomers that possessed Sc, Sc, N, cholesteric, and isotropic phases were studied using dynamic mechanic spectroscopy [25], Around Tg, the polymers exhibit value s variation for storage modulus above and below the transition. In the nematic state, G is below the value observed in the isotropic state. In the smectic phase, the layered organization produces a kind of network that causes a G plateau. A summary of this behavior is shown in Figure 9.9. 

Several instruments are employed to measure the dynamic mechanical spectroscopy (DMS) behavior (see Table 8.5). The Rheovibron (30) requires a sample that is self-supporting and that yields absolute values of the storage modulus and tan S. The value of E" is calculated by equation (8.17). Typical data are shown in Figure 8.11 (31). Although the instrument operates at several fixed frequencies, 110 Hz is most often employed. The sample size is about that of a paper match stick. This method provides excellent results with thermoplastics (30) and preformed polymer networks (31). 

The three methods described thus far are simple methods for measming the loss of the fluidity of sol and the appearance of elasticity [21]. However, they merely observe apparent phenomena. In other words, as viscosity and elasticity of polymeric materials depend on measurement time, it is necessary to observe the loss of fluidity and appearance of elasticity of materials with long relaxation time, such as gels, over a long period of time. In this sense, dynamic mechanical spectroscopy is useful. Readers are referred to monographs on dynamic mechanical spectroscopy [22-24]. In this section, a few examples of viscoelastic behaviors observed at the gel point will be discussed. 

A more selective approach consists in trying to influence the kinetics of formation of at least one network in this case, the two networks are formed more or less simultaneously, and the resulting morphology and properties can be expected to vary to some extent without changing the overall composition. The same system as previously studied, PUR/PAc, has been utilized in order to prepare a series of in situ simultaneous IPNs (SIM IPNs), by acting essentially on two synthesis parameters the temperature of the reaction medium and the amount of the polyurethane catalyst. Note that the term simultaneous refers to the onset of the reactions and not necessarily to the process. The kinetics of the two reactions are followed by Fourier transform infra-red (FTIR) spectroscopy as described earlier (7,8). In this contribution, the dynamic mechanical properties, especially the loss tangent behavior, have been examined with the aim to correlate the preceding synthesis parameters to the shape and temperature of the transitions of the IPNs. 

The activation energy of secondary relaxation in polymers can be measured by means of dynamic mechanical or dielectric relaxation spectroscopy. Dynamic mechanical or dielectric relaxation spectra of polymers can be obtained as a function of temperature at different frequencies. As an example, the dielectric relaxation behavior of the secondary relaxation in some AB cross-linked polymers is shown in Figure 4.63. 

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