Poly stress/strain studies

Table 8.3. Stress-Strain Studies on 80/20 Polyurethane/Poly(methyl methacrylate) SINs ...

Figure 3. Stress/strain studies on 20 Mrad 7 irradiation crosslinked poly(VPAVG), designated as X o-poly-(VPAVG), in H2O. (A) At 25 C, the elastic modulus is 1.5 X 1Q3 dynes/cm2. (g) 35 C, the elastic...

Studies have been made of the elastic (time-independent) properties of single-phase polyurethane elastomers, including those prepared from a diisocyanate, a triol, and a diol, such as dihydroxy-terminated poly (propylene oxide) (1,2), and also from dihydroxy-terminated polymers and a triisocyanate (3,4,5). In this paper, equilibrium stress-strain data for three polyurethane elastomers, carefully prepared and studied some years ago (6), are presented along with their shear moduli. For two of these elastomers, primarily, consideration is given to the contributions to the modulus of elastically active chains and topological interactions between such chains. Toward this end, the concentration of active chains, vc, is calculated from the sol fraction and the initial formulation which consisted of a diisocyanate, a triol, a dihydroxy-terminated polyether, and a small amount of monohydroxy polyether. As all active junctions are trifunctional, their concentration always... 

Studies have been conducted on poly (tetramethylene oxide )-poly-(tetramethylene terephthalate) -segmented copolymers that are identical in all respects except for their crystalline superstructure (66,67,68). Four types of structures—type I, II, and III spherulites (with their major optical axis at an angle of 45°, 90°, and 0° to the radial direction, respectively), and no spherulitic structure—were produced in one segmented polymer by varying the sample-preparation method. Figures 10 and 11 show the stress-strain and IR dichroism results for these samples, respec-... 

Another example of the use of polarized radiation in imaging studies is the analysis of poly(vinylidene fluoride)(PVDF) films, which have been uniaxially elongated at different temperatures. Depending on the thermal, mechanical and electrical pretreatment, PVDF can exist in different modifications [59]. The crystal structure of the cmmpled 11(a) modification can be converted into the aU-tra s 1(P) form by tensile stress below 140°C (see Figure 9.27a). Figure 9.27b shows the stress-strain diagrams of PVDF films in the 11(a) form which have been elongated to 400 % strain at 100 and 150°C. The observed decrease in stress upon elevation of the... 

Although the dynamic mechanical properties and the stress-strain behavior iV of block copolymers have been studied extensively, very little creep data are available on these materials (1-17). A number of block copolymers are now commercially available as thermoplastic elastomers to replace crosslinked rubber formulations and other plastics (16). For applications in which the finished object must bear loads for extended periods of time, it is important to know how these new materials compare with conventional crosslinked rubbers and more rigid plastics in dimensional stability or creep behavior. The creep of five commercial block polymers was measured as a function of temperature and molding conditions. Four of the polymers had crystalline hard blocks, and one had a glassy polystyrene hard block. The soft blocks were various kinds of elastomeric materials. The creep of the block polymers was also compared with that of a normal, crosslinked natural rubber and crystalline poly(tetra-methylene terephthalate) (PTMT). 

Nematic monodomains have been prepared using a variant of the method introduced by Kiipfer and Finkelmann [10, 18] instead of keeping the stress constant in the second crosslinking step the strain was held constant in the second crosslinking step [53]. As for the case studied by Kupfer and Finkelmann, it is found that the monodomain character is retained after heating the sample into the isotropic phase upon return into the nematic phase. Stress strain relation has been measured for two classes of poly-siloxane elastomers. 

Static mechanical measurements to evaluate the stress-strain relationship in cholesteric sidechain LCEs have been described [71, 72]. In [72] it has been found, for example, thatfor0.94nominal stress Cn is nearly zero as the poly domain structure must be converted first into a monodomain structure. For deformations A < 0.94, the nominal stress increases steeply. Similar results have also been reported elsewhere [71]. The nominal mechanical stress as a function of temperature for fixed compression has also been studied for cholesteric sidechain elastomers [71]. It turns out that the thermoelastic behavior is rather similar as that of the corresponding nematic LCE [2, 5]. 

Wan mimics the mechanical behavior of cardiovascular tissues, such as aorta and heart valve leaflets. The stress-strain properties for porcine aorta are matched by microbial cellulose-poly(vinyl alcohol) nanocomposite in both the circumferential and the axial tissue directions. Relaxation properties of the nanocomposite, which are important for cardiovascular applications, were also studied and found to relax at a faster rate and to a lower residual stress than the tissues they might replace. The study showed that this nanocomposite is a promising material for cardiovascular soft tissue replacement applications. The aim of a study by Mohammadi et al. was to mimic not only the nonlinear mechanical properties displayed by porcine heart valves, but also their anisotropic behavior, by applying a controlled strain to the samples while imdergoing low-temperature thermal cycling, in order to induce oriented mechanical properties. 

Differential dynamic measurements have also been made with other kinds of oscillating deformations. In studies by Painter of small dynamic shear deformations superimposed on large static shear strains in the same direction, the dynamic storage modulus G of cross-linked natural rubber and poly(dimethyl sil-oxane) at 24 Hz was found to increase with increasing static strains in excess of 721 = 0.2. Here 721 is defined as u jxt in the notation of Chapter 1. After a history of large static strain, however, the change in G with static strain in a second (and subsequent) sequence of experiments was much smaller. These history-dependent effects are no doubt related to the behavior in repeated stress-strain cycles in extension mentioned in Section 3 above. Some experiments on torsional deformations of stretched rubber strips have been reported. " ... 

Cail, J.J., Stepto, R.F.T. and Ward, I.M. (2007) Experimental studies and molecular modelling of the stress-optical and stress-strain behaviour of poly(ethylene terephthalate). Part 111 Measurement and quantitative modelling of birefringence-strain, stress-strain and stress-optical properties. Polymer, 48, 1379. 

Wen J, Mark JE. Torsion studies of thermoelasticity and stress-strain isotherms of unimodal, bimodal, and filled networks of poly(dimethylsiloxane). Polym J 1994 26 151-7. 

Ranjan et al. [25] studied the stress-strain behavior of dibu-tyltin diacetate (DBTDA) and dibutyltin dilaurate (DBTDL) in poly(dimethylsiloxane) nanocomposites. They noted that the ultimate tensile strengths and Young s moduli increase with higher silica loading for both types of composites. Elongation at break remains almost the same as that of the unfilled network except for the 14.2 wt% DBTDA-filled composite. 

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