Transient dynamic mechanical data

Transient dynamic mechanical data on the DGEBA-TETA and high performance M-5208 epoxy based systems have been obtained and compared with "equilibrium" data.. The transient data have demonstrated that moisture can act not only to plasticize an epoxy network but also to restrict and stiffen molecular chain movement. The behavior observed was explained by examining the synergistic effects that moisture and temperature have on the particular epoxy network structure. 

Figure 6 plots transient Isothermal tan 6 dynamic mechanical data for a 25 PHR-DDS N-5208 epoxy sample. This sample was Initially exposed to a dry 50 C environment. This temperature was selected since It coincides with the vicinity of the dynamic mechanical u transition. Hence, differences between properties In the dry and wet states could be maximized. Behavior of the Initial dry to wet state transient cycle was previously discussed for DGEBA-TETA epoxy sample of Figure 5. Similar behavior Is noted for this N-5208 epoxy sample. There Is an Initial rise In the tan 5 followed by a "blocking" and gradual reduction. After the tan 6 appeared to approach a stable value, the environment In the sample chamber was switched from one of a 50 C liquid environment to a 50°C desiccated environment. Once again, a rapid Increase In the mobility of the system occurred. After the sharp Increase In tan 5, a gradual decrease followed.

The second type of dynamic mechanical experiment involved collection of "transient" -Rheovibron data at a fix frequency as a function of time after a change in either the moisture or thermal environment. The experimental apparatus designed for this purpose is depicted in Figure 1. 

Tan 5, storage compliance, and loss compliance values for these experiments are plotted as a function of time in Figure 8. This transient temperature cycle data illustrates interactions between the dynamic mechanical plasticization and blocking behavior just discussed as well as the epoxy s equilibrium moisture uptake behavior (3), and the temperature behavior of dynamic mechanical properties observed for this epoxy in Figure 2a. Perhaps the easiest comparison to consider involves the relationship between transient temperature cycling data of Figure 8 and the thermal behavior observed for N-5208 epoxy tan 6 data of Figure 2a. 

Based on "equilibrium" dynamic mechanical results of Figure 2a, tan 6 properties associated with network thermal behavior at 20 C should be greater than comparable behavior at the 50 C thermal state. The difference in relative magnitudes stems from the relative positions of the 20 C and 50 C thermal states with respect to the low temperature 8 transition for this epoxy. This difference for the transient data is best observed in Figure 8. Equilibrated 20°C tan 5 and loss property values for the 20°C hygrothermal state are greater than the subsequent 50°C hygrothermal state measurements. 

Glass transition temperature, Tg, and storage modulus, E , were measured to explore how the pigment dispersion affects the material (i.e. cross-link density) and mechanical properties. Both Tg and E were determined from dynamic mechanical analysis method using a dynamic mechanical thermal analyzer (DMTA, TA Instruments RSA III) equipped with transient testing capability. A minimum of 3 to 4 specimens were analyzed from each sample. The estimated uncertainties of data are one-standard deviation. 

We have already referred to various kinds of data on mechanical behavior of polymers. We are now going to consider methods of acquisition of such information. The most fi equently used are the so-called quasistatic methods which involve relatively slow loading. Tension, compression, and flexure belong here. The quasistatic methods have to be distinguished from so-called transient tests which include stress relaxation and creep. There are also impact tests and dynamic mechanical procedures which will be defined later. 

In a very extensive study of both stress relaxation and dynamic mechanical properties in simple extension, on single crystal mats of fractions of linear polyethylene, Takayanagi and collaborators were able to combine data at different temperatures by reduced variables over most of the range from 16°C up to the temperature of crystallization and also to show that the dynamic and transient data corresponded fairly closely, provided the latter were corrected for nonlinear behavior by an extrapolation procedure to zero strain. It is characteristic of crystalline polymers that departures from linear viscoelastic behavior appear at very small strains, and are sometimes significant in stress relaxation even at a tensile strain of = 0.001. In dynamic measurements, the strains are usually small enough to fall within the linear range. 

Although, relevant information about ferrous hemeproteins kinetics, dynamics and ligand photodissociation pathways has been obtain, less is known about ferric hemeproteins photophysic processes. Recent studies performed with Hbl-CN and Mb-CN at ultrafast time scale, have suggested that some of the transients intermediaries observed after ferrous complexes ligand photodissociation are observed in ferric Mb and Hbl [7], However, time-resolved infrared data shows that the complex remained six coordinated after photoexcitation. In this work we present ultrafast data on ferric Hbl-NO, HM-N3, HM-H2S and metHbl complexes that suggest a mechanism for the photoinduced reduction of Hbl species. 

There are many experimental techniques for studying interfacial relaxations of soluble adsorption layers. Except for the wave damping techniques, these methods are developed and used only by individual research groups. Up to now, no commercial set-up exists and therefore, relaxation experiments are not so wide spread. New developments in this field will probably increase the number of investigators studying the dynamic and mechanical properties of adsorption layers, since instruments are easy to construct and data handling is relatively simple. In this section, wave damping and other harmonic methods as well as transient relaxation techniques will be described. 

One of the most important and impressive applications of transient RR and TR studies deals with investigation of the strutural dynamics and mechanism of action of the terminal oxidase in aerobic respiration namely, cytochrome-c oxidase (CcO). In contrast to the previous section which dealt with the dynamics involved in protein structural rearrangements, the following TR studies to be summarized for CcO deal with the detection and characterization of the complex, multistep proton and electron transfer reactions which follow O2 binding by CcO. Whereas in the case of Hb structural dynamics the essential data consist of frequency shifts of key modes, the TR spectra acquired during the multistep redox reaction involved in CcO function reveal several distinct sets of transient spectral features which are characteristic of specific intermediates. The... 

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