Crosslinking stress-strain properties

Swollen tensile and compression techniques avoid both of these problems since equilibrium swelling is not required, and the method is based on interfacial bond release and plasticization rather than solution thermodynamics. The technique relies upon the approach to ideal rubberlike behavior which results when lightly crosslinked polymers are swelled. At small to moderate elongations, the stress-strain properties of rubbers... 

Plasticised PVC sheets were surface modified by nucleophilic substitution of chlorine by azide in aqueous media under phase transfer conditions. The azidated PVC surface was then irradiated by UV light to crosslink the surface. It was found that considerable reduction in the migration of the plasticiser di-(2-ethylhexyl phthalate) could be achieved by this technique, depending on the extent of azidation of the PVC surface and the irradiation dose. After surface modification, there was around 30% reduction in the stress-strain properties of the PVC sheets but these values were still well above the minimum prescribed for PVC used in biomedical applications. 19 refs. 

The mechanical and thermal properties of a range of poly(ethylene)/po-ly(ethylene propylene) (PE/PEP) copolymers with different architectures have been compared [2]. The tensile stress-strain properties of PE-PEP-PE and PEP-PE-PEP triblocks and a PE-PEP diblock are similar to each other at high PE content. This is because the mechanical properties are determined predominantly by the behaviour of the more continuous PE phase. For lower PE contents there are major differences in the mechanical properties of polymers with different architectures, that form a cubic-packed sphere phase. PE-PEP-PE triblocks were found to be thermoplastic elastomers, whereas PEP-PE-PEP triblocks behaved like particulate filled rubber. The difference was proposed to result from bridging of PE domains across spheres in PE-PEP-PE triblocks, which acted as physical crosslinks due to anchorage of the PE blocks in the semicrystalline domains. No such arrangement is possible for the PEP-PE-PEP or PE-PEP copolymers [2]. 

The phantom network can account qualitatively for many properties of crosslinked elastomers, but the quantitative explanation of basic properties is wrong. For example, stress-strain properties, especially in simple extension, show departures from the phantom network results even at extension ratios covered by the Gaussian chain model. The explanation of these departures, phenomenologically described by the famous Mooney-Rivlin Eq. (1)... 

In the study of swollen networks, two problems are of major importance The dependence of the stress-strain properties on the solvent or polymer fraction and the mking contributions to the free energy of the network or the elastic contribution to the chemical potential. Latest research seems to provide an improved insight into some special effects which are typical for swollen and completely crosslinked networks, and for unswollen (and swollen) incompletely crosslinked networks. The relaxation on the deformation dependence of topological constraints, which leads to a constraint release, is one of them. 

Fig. 3. Stress-strain properties of 50% w/w TEA plasticized NaCAS films crosslinked with formaldehyde (HCHO/e-NHj molar ratios reported on each curve) at 53% RH.

Recent work has focused on a variety of thermoplastic elastomers and modified thermoplastic polyimides based on the aminopropyl end functionality present in suitably equilibrated polydimethylsiloxanes. Characteristic of these are the urea linked materials described in references 22-25. The chemistry is summarized in Scheme 7. A characteristic stress-strain curve and dynamic mechanical behavior for the urea linked systems in provided in Figures 3 and 4. It was of interest to note that the ultimate properties of the soluble, processible, urea linked copolymers were equivalent to some of the best silica reinforced, chemically crosslinked, silicone rubber... 

The comparison of the mechaiucal properties of the UPy samples and the PEG controls demonstrates that the introduction of our biomimetic module into the network dramatically enhanced the polymer mechanical properties. As shown in the stress-strain curves (Fig. 10.8), the network containing the biomimetic crosslinker has significantly higher modulus, tensile strength, and toughness than the... 

Fisher124,125 has studied the stress-strain and optical properties of three SBS block copolymers containing, respectively, 31,40 and 49% polystyrene as a function of temperature. He has shown that these materials are two phase systems in which the polybutadiene chains form an elastomeric phase and the polystyrene chains a glassy phase acting as physical crosslinks. Fisher126 has also obtained electron micro-... 

In an attempt to simplify the discussion, we ignore the fact that the modulus and properties of bone are dependent on the testing direction and mineral content. A typical stress-strain curve for cortical bone is illustrated in Figure 6.7. Mineralized ECMs show a much higher modulus and UTS, and the strain at failure is markedly decreased. In the same manner that increased crosslinking increases the UTS of unmineralized tissue, mineral deposition acts as a crosslink and improves the UTS and the modulus of bone. The UTS for cortical bone varies from 100 to 300 MPa, the modulus varies from several to more than 20GPa, and the strain at failure falls to only 1 to 2%. 

Before concluding this discussion of cell walls, we note that the case of elasticity or reversible deformability is only one extreme of stress-strain behavior. At the opposite extreme is plastic (irreversible) extension. If the amount of strain is directly proportional to the time that a certain stress is applied, and if the strain persists when the stress is removed, we have viscous flow. The cell wall exhibits intermediate properties and is said to be viscoelastic. When a stress is applied to a viscoelastic material, the resulting strain is approximately proportional to the logarithm of time. Such extension is partly elastic (reversible) and partly plastic (irreversible). Underlying the viscoelastic behavior of the cell wall are the crosslinks between the various polymers. For example, if a bond from one cellulose microfibril to another is broken while the cell wall is under tension, a new bond may form in a less strained configuration, leading to an irreversible or plastic extension of the cell wall. The quantity responsible for the tension in the cell wall — which in turn leads to such viscoelastic extension — is the hydrostatic pressure within the cell. 

Stress is related to strain through constitutive equations. Metals and ceramics typically possess a direct relationship between stress and strain the elastic modulus (2) Polymers, however, may exhibit complex viscoelastic behavior, possessing characteristics of both liquids and solids (4.). Their stress-strain behavior depends on temperature, degree of cure, and thermal history the behavior is made even more complicated in curing systems since material properties change from a low molecular weight liquid to a highly crosslinked solid polymer (2). ... 

The stress-strain behavior of thermosets (glassy polymers crosslinked beyond the gel point) is not as well-understood as that of elastomers. Much data were analyzed, in preparing the previous edition of this book, for properties such as the density, coefficient of thermal expansion, and elastic moduli of thermosets [20,21,153-162]. However, most trends which may exist in these data were obscured by the manner in which the effects of crosslinking and of compositional variation were superimposed during network formation in different studies, by... 

This discovery culminated in the commercial production and the announcement (41) in 1965 of thermoplastic elastomers from block polymers of styrene and butadiene (S-B-S) and of styrene and isoprene (S-I-S). To rubber scientists and technologists the most outstanding property of S-B-S and S-I-S was the unvulcanized tensile strength compared to that of vulcanized NR and vulcanized SBR carbon black stocks. Stress-strain curves, to break, of these latter materials are compared to that of S-B-S in Figure 2. It was pointed out that the high strength of S-B-S must be due to physical crosslinks. 

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). 

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