Dynamic mechanical relaxational behavior

The y relaxation at very low temperatures is a narrow mechanical absorption with an activation energy of 13 kcal mol-1. 

At higher temperatures a new relaxation process appears. In this case the drop in the storage modulus is higher, by nearly a decade, and from this point of view this mechanical absorption has characteristic intermediate between an absorption associated with a Tg and a secondary relaxation. 

In the high temperature tail of the p relaxation and as a shoulder, there is evidence of a new relaxation which can be seen clearly at higher frequencies. As it was previously reported, dielectric measurements carried out on the same polymer show evidence of a new relaxation which cannot be observed directly, because of the high 

As a general comment about the dynamic mechanical relaxational behavior of this polymer, the results are consistent with dielectric data [210] and with the fact that no glass transition phenomenon is observed, at least in the range of temperature studied. This is striking in an amorphous polymer. It is likely that the residual part of the molecule mechanically active above the temperature of the ft relaxation is only a small one, and this is the reason for the low loss observed in the a zone. 

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M.E. De Rosa, W.W. Adams, T.J. Bumring, H. Shi, S.H. Chen, Dynamic mechanical relaxation behavior of low molecular weight side chain cyclic liquid crystalline compormds near the glass transition temperatare. Macromolecules 29, 5650-5657 (1996)... 

Attempts to improve the mechanical properties have focused on biocompatible plasticizers. Blending with PEG, the conventional name for low molecular weight (<20 000) PEG, improves elongation at break and softness of PLA. At ambient temperature, the desired mechanical properties are achieved by blending PLA with 30 wt% PEG Table 4.2 shows the effect of PEG content on the thermal and mechanical properties of quenched PLA/PEG blends. However, there is evidence that the blend is not stable and the attractive mechanical properties are lost over time. The dynamic mechanical relaxation behavior and tensile stress-strain behavior of PLA and PLA/PEG blends are shown in Figs. 4.19 and Fig. 4.20 respectively. 

On macroscopic length scales, as probed for example by dynamic mechanical relaxation experiments, the crossover from 0- to good solvent conditions in dilute solutions is accompanied by a gradual variation from Zimm to Rouse behavior [1,126]. As has been pointed out earlier, this effect is completely due to the coil expansion, resulting from the presence of excluded volume interactions. 

Y Relaxation. Unlike the other dynamic mechanical relaxations observed in this study, the Y relaxation does not have an analog in the dynamic mechanical behavior of polyethylene, hydrogenated PP s, or other ionomer systems. In addition, it displays no definite trends in changing temperature or magnitude as the level of sulfonation and thermal history are altered. Coupled with the fact that these systems are known to contain water as well as nitrogen, it is not possible to assign this relaxation to any specific phase or mechanism. Additional studies are necessary before this task can be approached adequately. 

As with the dynamic mechanical relaxations, it is also possible to check the dielectric behavior of the sample. In this case the thermal analysis is carried out measuring the dielectric constant, dissipation factor, loss index, and phase angle as a function of temperature and frequency. In order to see a dielectric effect, a dipole must be connected with the molecular motion. In this way dielectric relaxation may be more specific than DMA. A combination of DMA, dielectric measurements, and DSC is often needed for a detailed interpretation of the properties of the materials. ... 

Dynamic mechanical analysis (DMA) was performed to determine the influence of the polymer constitution on tensile modulus and mechanical relaxation behavior. For this purpose, a Perkin Elmer DMA-7 was run in tensile mode at an oscillation frequency of 1 Hz with a static stress level of 5 x lO Pa and a superposed oscillatory stress of 4 x 10 Pa. With this stress controlled instrument, the strain and phase difference between stress and strain are the measured outputs. Typically, the resulting strain levels ranged from 0.05% to 0.2% when the sample dimensions were 8 mm x 2 mm x 0.1 mm. A gaseous helium purge and a heating rate of 3°C min" were employed. The temperature scale was calibrated with indium, and the force and compliance calibrations were performed according to conventional methods. 

Transitions. Samples containing 50 mol % tetrafluoroethylene with ca 92% alternation were quenched in ice water or cooled slowly from the melt to minimise or maximize crystallinity, respectively (19). Internal motions were studied by dynamic mechanical and dielectric measurements, and by nuclear magnetic resonance. The dynamic mechanical behavior showed that the CC relaxation occurs at 110°C in the quenched sample in the slowly cooled sample it is shifted to 135°C. The P relaxation appears near —25°C. The y relaxation at — 120°C in the quenched sample is reduced in peak height in the slowly cooled sample and shifted to a slightly higher temperature. The CC and y relaxations reflect motions in the amorphous regions, whereas the P relaxation occurs in the crystalline regions. The y relaxation at — 120°C in dynamic mechanical measurements at 1 H2 appears at —35°C in dielectric measurements at 10 H2. The temperature of the CC relaxation varies from 145°C at 100 H2 to 170°C at 10 H2. In the mechanical measurement, it is 110°C. There is no evidence for relaxation in the dielectric data. 

Relaxations of a-PVDF have been investigated by various methods including dielectric, dynamic mechanical, nmr, dilatometric, and piezoelectric and reviewed (3). Significant relaxation ranges are seen in the loss-modulus curve of the dynamic mechanical spectmm for a-PVDF at about 100°C (a ), 50°C (a ), —38° C (P), and —70° C (y). PVDF relaxation temperatures are rather complex because the behavior of PVDF varies with thermal or mechanical history and with the testing methodology (131). 

Material properties at a critical point were believed to be independent of the structural details of the materials. Such universality has yet to be confirmed for gelation. In fact, experiments show that the dynamic mechanical properties of a polymer are intimately related to its structural characteristics and forming conditions. A direct relation between structure and relaxation behavior of critical gels is still unknown since their structure has yet evaded detailed investigation. Most structural information relies on extrapolation onto the LST. 

In a subsequent communication, Elliott and coworkers found that uniaxially oriented membranes swollen with ethanol/water mixtures could relax back to an almost isotropic state. In contrast, morphological relaxation was not observed for membranes swollen in water alone. While this relaxation behavior was attributed to the plasticization effect of ethanol on the fluorocarbon matrix of Nafion, no evidence of interaction between ethanol and the fluorocarbon backbone is presented. In light of the previous thermal relaxation studies of Moore and co-workers, an alternative explanation for this solvent induced relaxation may be that ethanol is more effective than water in weakening the electrostatic interactions and mobilizing the side chain elements. Clearly, a more detailed analysis of this phenomenon involving a dynamic mechanical and/ or spectroscopic analysis is needed to gain a detailed molecular level understanding of this relaxation process. 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

Summary In this chapter, a discussion of the viscoelastic properties of selected polymeric materials is performed. The basic concepts of viscoelasticity, dealing with the fact that polymers above glass-transition temperature exhibit high entropic elasticity, are described at beginner level. The analysis of stress-strain for some polymeric materials is shortly described. Dielectric and dynamic mechanical behavior of aliphatic, cyclic saturated and aromatic substituted poly(methacrylate)s is well explained. An interesting approach of the relaxational processes is presented under the experience of the authors in these polymeric systems. The viscoelastic behavior of poly(itaconate)s with mono- and disubstitutions and the effect of the substituents and the functional groups is extensively discussed. The behavior of viscoelastic behavior of different poly(thiocarbonate)s is also analyzed. 

Keywords Viscoelasticity Glass transition temperature Relaxational processes Dielectric behavior Dynamic mechanical behavior Poly(methacrylate)s Poly (itaconate)s Poly(thiocarbonate)s Spacer groups Side chains Molecular motions... 

Another way to get information about the relaxational behavior of these materials can be performed by dynamic mechanical calculations. In order to get information about the origin of the secondary y relaxation, molecular dynamic (MD) calculations over the repeating unit were performed. By this way considering the axial and equatorial equilibrium on the cyclohexyl group and the interconversion of these two... 

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