Viscoelastic transitions

During our early experiments on chemical gels, when first observing the intermediate state with the self-similar spectrum, Eq. 1-5, we simply called it viscoelastic transition . Then, numerous solvent extraction and swelling experiments on crosslinking samples showed that the viscoelastic transition marks the transition from a completely soluble state to an insoluble state. The sol-gel transition and the viscoelastic transition were found to be indistinguishable within the detection limit of our experiments. The most simple explanation for this observation was that both phenomena coincide, and that Eqs. 1-1 and 1-5 are indeed expressions of the LST. Modeling calculations of Winter and Cham-bon [6] also showed that Eq. 1-1 predicts an infinite viscosity (see Sect. 4) and a zero equilibrium modulus. This is consistent with what one would expect for a material at the gel point. 

The presence of a low-viscosity interfacial layer makes the determination of the boundary condition even more difficult because the location of a slip plane becomes blurred. Transitional layers have been discussed in the previous section, but this is an approximate picture, since it stiU requires the definition of boundary conditions between the interfacial layers. A more accurate picture, at least from a mesoscopic standpoint, would include a continuous gradient of material properties, in the form of a viscoelastic transition from the sohd surface to the purely viscous liquid. Due to limitations of time and space, models of transitional gradient layers will be left for a future article. 

Thus, perturbation measurements are best made when (1) highest accuracy is not necessary and (2) a substantial perturbation only to the region between IDTs is anticipated or (3) such a large perturbation is anticipated that it would be difficult to differentiate between a mode hop and the perturbation itself (e.g., the transition from a dry surface to one covered with liquid) or (4) such a large change in insertion loss is anticipated over the course of the measurement (e.g., a viscoelastic transition in a polymer layer) that it is not possible to set up and maintain a stable oscillator circuit. 

Fig. 9 Dynamic moduli vs area isotherm for DSPE-PEG2000, with k — A isotherm also shown, pointing out that the viscoelastic transition point, where storage modulus Gs = loss modulus Gs" is only slightly above the plateau of the high-pressure transition [2] (reproduced with permission from the American Chemical Society)...

Fig. 10 Storage modulus, Gs, and loss modulus, G/, of DSPE-PEG750, DSPE-PEG1000, DSPE-PEG2000, DSPE-PEG3000, and DSPE-PEG5000 plotted vs area per molecule. All lipopolymers show a viscoelastic transition [11] (reproduced with permission from the American Chemical Society)...

Fig. 19 a, b Viscoelastic response of the DSPE-PEG2000 monolayer as a function of amount of DMPC incorporated. Loss modulus a and storage modulus b are shown vs Aiipo, and are essentially independent of amount of phospholipids incorporated for mol% lipopolymer >40%. No viscoelastic transition occurs for mol% lipopolymer <40% [42] (reproduced with permission from the American Chemical Society)... 

Fig. 22 7i - A isotherms of mixtures of DMPC/DSPE-PEG2000 from 10 to 100mol% DSPE-PEG2000, where the film balance transitions are plotted for each isotherm and the viscoelastic transitions are plotted for 40-100 mol%. The transitions only coincide at 100% DSPE-PEG2000. The different trends underlie the fact that the high pressure transition and viscoelastic transition signify different physical phenomena [42] (modified, with permission from the American Chemical Society)...

For these catalyzed systems, two main viscoelastic transitions were observed in the dynamic mechanical data as described in our previous publication (5) and shown in Figure 1(a) for tan 6. The low temperature, 3s transition provides information concerning the amine-epoxide reactions. Since the... 

Polymers undergo deformation under an applied stress over their lifetime some deformations which are irrecoverable once the source of stress is removed are referred to as creep. An understanding of the mechanical response of a polymer can be obtained by investigating the viscoelastic properties using creep experiments, where the behaviour is monitored under small deformational stress. Creep behaviour is an important consideration if the properties and dimensions are to be maintained. Experimental creep behaviour can be quantified using the four-element model with some limitations evident in the viscoelastic transitional region. ... 

Finally, it can be noted that the coefficient a in Equation (11.10) depends on the temperature of measurement and increases markedly near a viscoelastic transition. Briscoe and Tabor [26] have pointed out that a is equivalent to the coefficient of friction in sliding friction, and show that there is good numerical agreement between values of fx and the values of a obtained from yield stress/pressure measurements. 

Figure 5.12. Schematic of the storage and loss moduli from a DMA frequency scan for an amorphous polymer showing multiple viscoelastic transitions T = constant).

APPLICATIONS OF DYNAMIC MECHANICAL ANALYSIS 5.4.1. Viscoelastic Transitions or Relaxations... 

Figure 9.13 Model of morphology of oriented and annealed sheets of low-density polyethylene. This photograph shows the structure of the be sheet a, b and c axes indicate the crystallographic directions in the crystalline regions. (Reproduced from Stachurski, Z.H. and Ward, I.M. (1968) Anisotropy of viscoelastic transitions in oriented polyethylenes.. Polym. Sci. A2, 6, 1817. Copyright (1968) John Wiley Sons, Ltd.)...

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