Poly stress-strain behavior

FIGURE 14.9 Influence of temperature on the stress-strain behavior of a sample of poly(methyl methacrylate). (Modeled after Carswell, T.S. and Nason, H.K. Effects of Environmental Conditions on the Mechanical Properties of Organic Plastics, 1944. Copyright, ASTM, Philadelphia, PA. With permission.)... 

This also results in a strong temperature dependence of stress/strain behavior. Consider poly(methyl methacrylate), for example (Figure 13-44). At temperatures around room temperature or less, it is a typical glassy polymer, failing in a brittle fashion. 

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). 

Fig. 5(b). Initial portion of the stress-strain behavior of poly ether polyurethaneurea elastomers (strain rate = 1.2%/sec.). 

Figure 11.12 Stress-strain behavior of bimodal poly(dimethyl siloxane) networks. Each curve is labeled with the mol% of the short chains. The area under each curve represents the energy required for rupture (22).

Characterization of stress-strain behavior of poly(ether ester)s is of a great interest from both practical and fundamental point of view. Poly(ether ester)s exhibit a high tensile strains comparable to chemically crosslinked rubbers ranging from 500 to 800 %, while their tensile stress is higher than that of vulcanized rubbers, i.e. 20 to 50 MPa... 

Boyce, M.C., Socrate, M. and Liana, P.G. (2000) Constitutive model for the finite deformation stress-strain behavior of poly(ethylene terephthalate) above the glass transition. Polymer, 41, 2183. 

In addition, the mechanical characteristics of polymers are much more sensitive to temperature changes near room temperature. Consider the stress-strain behavior for poly(methyl methacrylate) (Plexiglas) at several temperatures between 4 C and 60°C (40°F and 140°F) (Figure 15.3). Increasing the temperature produces (1) a decrease in elastic modulus, (2) a reduction in tensile strength, and (3) an enhancement of ductility—... 

For the phenol-formaldehyde (Bakelite) poly-mer whose stress-strain behavior can be observed... 

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. 

Uniaxial tensile tests of poly (ethylene terephthalate) (PET)/montmorillonite(MMT) nanocomposites were preformed over a temperature range of 85°C-105°C and stretch rate of 7.5mm/s-12.5mm/s. The stress-strain curves consisted of three regions the hnear visoelasticity, the rubbery plateau and the strain hardening. The effects of temperature and stretch rate on stress-strain behavior were discussed. The results of differential scanning calorimetry (DSC) measurements indicated that the stretch lead the increase of the crystallinity degree of specimens. The wide angle X-ray diffraction (WAXD) measurements revealed that the more perfect crystal structures were obtained with the increase of temperature and oriented along the stretch direction. 

Summary In this chapter, a discussion of the viscoelastic properties of selected polymeric materials is performed. The basic concepts of viscoelasticity, dealing with the fact that polymers above glass-transition temperature exhibit high entropic elasticity, are described at beginner level. The analysis of stress-strain for some polymeric materials is shortly described. Dielectric and dynamic mechanical behavior of aliphatic, cyclic saturated and aromatic substituted poly(methacrylate)s is well explained. An interesting approach of the relaxational processes is presented under the experience of the authors in these polymeric systems. The viscoelastic behavior of poly(itaconate)s with mono- and disubstitutions and the effect of the substituents and the functional groups is extensively discussed. The behavior of viscoelastic behavior of different poly(thiocarbonate)s is also analyzed. 

The mechanical properties of a craze were first investigated by Kambour who measured the stress-strain curves of crazes in polycarbonate (Lexan, M = 35000) which had first been grown across the whole cross-section of the specimen in a liquid environment and subsequently dried. Figure 25 gives examples of the stress-strain curves of the craze determined after the 1st and 5th tensile loading cycle and in comparison the tensile behavior of the normal polymer. The craze becomes more and more elastic in character with increasing load cycles and its behavior has been characterized as similar to that of an opencell polymer foam. When completely elastic behavior is observed the apparent craze modulus is 25 % that of the normal poly-... 

The most common type of stress-strain tests is that in which the response (strain) of a sample subjected to a force that increases with time, at constant rate, is measured. The shape of the stress-strain curves is used to define ductile and brittle behavior. Since the mechanical properties of polymers depend on both temperature and observation time, the shape of the stress-strain curves changes with the strain rate and temperature. Figure 14.1 illustrates different types of stress-strain curves. The curves for hard and brittle polymers (Fig. 14.1a) show that the stress increases more or less linearly with the strain. This behavior is characteristic of amorphous poly-... 

Figure 5. The stress-strain dUatational behavior of a filled high-density poly-...

Poly(g-methyl- D-glutamate) Chloroform, DCA, MC, FA, TCE Solvent controls the degree of a, B-configuration, or random coil contents in the film, its mechanical behavior and morphology. Rheovibron, Stress-strain measurements, WAXS, SAXS, Polarizing photomicrography, IR [Mohadger and Wilkes, 1976]... 

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]. 

Surprisingly, relatively few data have been published on stress-strain, creep, or fatigue behavior. Auskern and Horn (1971) and Manning and Hope (1971) have reported stress-strain relationships for samples of concrete containing up to 12 vol. % poly(methyl methacrylate) see Figure 11.10 (Auskern and Horn, 1971). More comprehensive stress-strain data have been... 

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