Dynamic mechanical relaxation

In the following section the results on dynamic mechanical relaxation of various polybibenzoate series are summarized. In general, all the polybibenzoates display three dynamic mechanical relaxations, called a, /3, and y in order of decreasing temperature. 

Among these three polybibenzoates, PTEB has a smectic mesophase stable during several days at any temperature below its isotropization point, although the transformation into a three-dimensional crystal can be attained by annealing at the appropriate temperatures, thus making it possible to analyze the effect of the thermal history on the dynamic mechanical relaxations of PTEB [27]. 

In the case of dynamic mechanical relaxation the Zimm model leads to a specific frequency ( ) dependence of the storage [G ( )] and loss [G"(cd)] part of the intrinsic shear modulus [G ( )] [1]. The smallest relaxation rate l/xz [see Eq. (80)], which determines the position of the log G (oi) and log G"(o>) curves on the logarithmic -scale relates to 2Z(Q), if R3/xz is compared with Q(Q)/Q3. The experimental results from dilute PDMS and PS solutions under -conditions [113,114] fit perfectly to the theoretically predicted line shape of the components of the modulus. In addition l/xz is in complete agreement with the theoretical prediction based on the pre-averaged Oseen tensor. 

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. 

Figure 11.9 Dynamic mechanical storage ( ) and loss ( ") moduli of isotropic PTT at 11 Hz showing the three relaxations [61]. From Dynamic mechanical relaxations of polyterephthalates based on trimethylene glycols, Gonzalez, C. C., Perena, J. M. and Bello, A., J. Polym. Sci., Polym. Phys. Ed., 26,1397-1408 (1988), Copyright (1988 John Wiley Sons, Inc.). Reprinted by permission of John Wiley Sons, Inc...

Dynamic Mechanical Relaxations in Glassy Polymers 3.1 Dynamics of Relaxations... 

The a Relaxation. One of the principal findings in the earlier study of the PPO-PS mixtures (11) was that samples which appeared to be homogeneous when studied calorimetrically, inasmuch as they indicated only a single Tg> showed a dynamical mechanical relaxation which could be resolved into two components. These were interpreted in terms of PS-rich (lower Tg) and PPO-rich (upper Tg) phases. It is clear from... 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

Hardy, L., Stevenson, I., Boiteux, G., Seytre, G., and Schonhals, A. (2001). Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6 naphthalene dicarboxyl-ate). I. Amorphous films. Polymer 42(13), 5679-5687. 

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. 

Dynamic mechanical relaxation tests were carried out by use of a Piezotron apparatus. Thin reedlike samples were subject to alternating tension at low applied stress and at a fixed frequency of 3 Hz. From the recorded observations, made as a function of temperature at a controlled rate of 2 °C/min, the storage modulus, E, the loss modulus, E", and the loss tangent, tan 6 = E"/E were determined. 

However, with dynamic mechanical relaxation domains of about 100 A can be detected. The dynamic mechanical spectrum of a polymer prepared by system 3 ("Figure 3 ) indeed displays two separate loss peaks one at about -62 °C owing to the glass transition of polyol... 

A lower temperature discontinuity in the Inip vs. T plots is observed for all of the systems studied is apparent at ca. 140K. In view of the data furnished by polarization data (vide supra) and of the problems associated in dynamic mechanical relaxation with separation of the a- and 3- transitions of the polymer(1 4) it is suggested that this "transition" is photophysical in origin as discussed below. 

Of particular importance for detection of chemical or physical change in polymer materials are mobility filters, which are sensitive to differences in the numbers of molecules within a given window of correlation times. Within reasonable approximation such filters are relaxation filters. Here, Tj filters are sensitive to differences in the fast motion regime while T2 and Tip filters are sensitive to the slow motion regime. Which time window is of importance can be seen from Fig. 5.7 [101]. It shows a double-logarithmic plot of the mechanical relaxation strengths Hi(t) for two carbon-black filled styrene-butadiene rubber (SBR) samples as a function of the mechanical relaxation time T. They have been measured by dynamic mechanical relaxation spectroscopy. In terms of NMR, the curves correspond to spectral densities of motion. But the spectral densities relevant to NMR are mainly those referring... 

The temperature dependency of the apparent energy of activation of the bleeching process as exhibited by the curvature in the Arrhenius plot (Fig. 3) is typically found for, e. g., dynamic mechanical relaxation processes (17), which leads to the connection whith the free volume theory. The latter processes are best described by the WLF-equation (18), log a =Cj(T-Tg)/(C +T-Tg), i. e., a master plot is obtained when plotting the logarithm of... 

The dynamic mechanical relaxations in the high temperature region are very weak and the glass transition was indistinguishable from the melting point (Fig. 4). However, the mechanical properties of polyurethanes with chemically crosslinked hard segments were quite different from uncrosslinked polyurethanes. In the linear adhesives (73-14 and 73-15), the rubbery plateau ends at the melting point of the... 

The fact that crystals can be formed and annealed at room temperature in these systems can also be cited as evidence that the glass transition temperature (Tg) of these materials is sufficiently below room temperature. If the amorphous material were glassy at room temperature, there could not be sufficient large-scale motion to allow the formation of such crystals. This information can be applied in the interpretation of the dynamic mechanical relaxations as will be seen. No indication of a glass-to-rubber relaxation was found in the DSC scans of these materials. 

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. 

In Chapter 5 we cited dynamic mechanical relaxation data for polymethyl methacrylate (PMMA). There it was shown that PMMA possesses two mechanical relaxation regions over the temperature range - 50° to 160°C at low frequencies. These were labeled a for the relaxation accompanying the glass transition and / for a secondary relaxation that has generally been associated with motions of the ester side group. PMMA has a predominantly nonpolar... 

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)... 

R.G. Matthews, I.M. Ward, and G. Capaccio, Relationship between the dynamic mechanical relaxations and the tensile deformation behaviour of polyethylene, Journal of Materials Science, 34 (12), 2781-2787,1999. 

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