Experiments modulus recovery

Experiments on recovery of dynamic functions after the application of large strain amplitude perturbation were performed to understand the modulus recovery kinetics. To determine the recovery kinetics, samples underwent the following test sequences (a) frequency sweep, (b) strain sweep, (c) relaxation time of 2 min, (d) frequency sweep, (e) strain sweep, (f) relaxation time of 2 min, (g) frequency sweep, and (h) strain sweep [50]. Figure 7 shows the comparative subsequent strain sweep results performed immediately after a relaxation time of... 

Figure 8.1 and Table 8.1 reveal that a significant recovery occurs in the post-fire stiffness (that is, the post-fire stiffness is higher than the stiffness during fire exposure). Furthermore, based on the two DMA tests performed on the same specimen, it was found that, if cooled down from temperatures between glass transition and decomposition, the E-modulus can recover almost to its initial value (see Figure 8.2). In the modeling of the post-fire stiffness, the decomposed material (with the content a j) has no stiffness, while the material after glass transition but before decomposition (with the content a ) experiences a recovery. Thereby, for the modeling of the post-fire stiffness, the E-modulus model (Eq. (5.6)) can be transformed to [12] ...

The modulus recovery experiments allowed measuring the terminal relaxation time of reptation motion of bulk and surface immobilized chains, supporting the hypothesis that theie is no interphase per se when nano-scale is considered. In order to bridge the gap between the continuum interphase on the microscale and the discrete molecular structure of the matrix consisting of freely reptating chains in the bulk and retarded reptatiug chains in contact with the inclusions, higher order elasticity combined with a suitable molecular dynamics model could be utilized [151-155]. 

Figure 7.7. Simple approach combining the reptation dynamics and percolation model to describe the retarded reptation of chains in the vicinity of solid nanosized inclusions representing the iianoscale interphase [144]. (a) The dependence of the tenniiial relaxation time from modulus recovery experiment plotted as a function of the specific filler-matrix interface area. Squares and circles represent experimental data for platelet and spherical particles, respectively, (b) Visualization of the molecular structure of the interphase (upper) and the 2-phase simplified structure of the uauoparticles with interphase considered in the percolation model.

A wide variety of tests is performed in TMA, which are adapted from physical tests that were used before the instrument became commonly available. These tests may also be modeled or mimicked in TMA, such as heat distortion (Fig. 9) and softening points. Methods to obtain the modulus, compressive viscosity, and penetrative viscosity have been developed. Many of these methods, such as ASTM D648 for example, will specify the stress the sample needs to be exposed to during the run. In D684, a sample is tested at 66 and 264 psi. Most TMAs on the market today have software available that allows them to generate stress—strain curves and to run creep—recovery experiments. Some are also capable of limited types of stress relaxation studies (for example a constant gauge length test " ). 

The gel was first pre-sheared at a shear strain rate of 5s for two minutes in the flow mode. Shearing was then stopped and the dynamic mode (frequency 6Hz and shear stress IPa) inunediately commenced allowing the recovery in the storage modulus to be measured as flmction of time. The advantage of the dynamic experiment over the flow experiment is that the shear strain is sufficiently small to prevent significant structure breakdown. 

The age-related viscoelastic properties of the ocular lens have not been fully characterized. Most of the attempts have been at elucidating only the elastic modulus, since the lens has been treated as an elastic substance (19,26). The process of accommodation however is mechanically analogous to a stress-relaxation experiment, where the stress is allowed to decay at constant strain (refractive power). Hence, the lens is truly viscoelastic. Researchers investigating the viscoelastic characteristics of the lens performed creep-recovery or frequency scan techniques ex-vivo ( 1 8). Ejiri et al. (28) investigated creep properties of a decapsulated dog lens by compression and fitted the time-displacement curve with three Kelvin units. The time constants for the three units were 0.09 s, 7.0 s, and 106 s. The elastic modulus could not be obtained, as the applied stress was unknown due to the aspheric geometry of the lens. In this article, we have investigated the creep behavior of cylindrical disc shaped hydrogels in order to obtain the time constants as well as the elastic modulus of the viscoelastic units. 

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