Polymer creep behavior

The concentration of hydrogen in the polymer during irradiation is low, on the order of 10"6 mole per cc. This is far lower than the concentrations of plasticizers required to cause any significant changes in polymer creep behavior. 

Fig. 3. Typical creep behavior for rubber-modified styrene polymers.

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

Creep behavior is similar to viscous flow. The behavior in Equation 14.17 shows that compliance and strain are linearly related and inversely related to stress. This linear behavior is typical for most amorphous polymers for small strains over short periods of time. Further, the overall effect of a number of such imposed stresses is additive. Non-creep-related recovery... 

Einaga,Y., Osaki,K., Kurata,M, Tamura,M. Creep behavior of polymer solutions. II. Steady-shear compliance of concentrated polystyrene solutions. Macromolecules 4, 87-92 (1971). 

The same techniques were used in the present work to study the effects of orientation and rubber content upon the creep behavior of rubber-toughened SAN polymers at room temperature. As in previous work, the tests were conducted at low strain rates and were terminated at longitudinal strains between 5 and 6%. 

The creep behavior of Luran S 776S is similar to that of 757R, except that creep rates are higher. Tests were made at stresses between 19 and 24 MN/m2 the times to reach 5% extension varied from 4700 to 20 sec. Increased creep rates are to be expected in view of the higher rubber content of the polymer. 

The creep behavior of the isotropic ASA and ABS polymers is similar to that previously reported for HIPS (9). The time-dependent part of the deformation is dominated by crazing which begins slowly, accelerates,... 

The observed results shown in Figures 7 and 8 are in general agreement with the predictions of Buknall and Drinkwater (8). They suggested, based on the influence of stress on the creep behavior of ABS polymers that crazing (cavitation) should be the dominant factor under impact conditions. Their predictions were based on low strain (< 5% ) observations and are clearly substantiated by the data shown in Figures 7 and 8. 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

Similarly, Figure 6 summarizes the creep behavior of glass-and mineral-filled polyphenylene sulfide under three sets of conditions 24°C/5,000 psi flexural load, 66°C/5,000 psi, and 1210C/3,000 psi. Table III compares the per cent loss In apparent creep modulus at 1,000 hours and at 10,000 hours for each of these conditions using the apparent creep modulus at one hour as a basis. These data Indicate that the creep resistance of the glass- and mineral-filled polymer Is similar to that of the 40% glass-filled resin. 

For a crossllnked rubber sample, one simple parameter which can be used to roughly characterize the material is the crosslink density (v) or the average molecular weight between crosslinks (Mg a 1/v). It should be clear that this single parameter cannot completely represent a network in general. Nevertheless, it is well known that the viscoelastic behavior of a polymer network will vary with crosslink density as schematically depicted in Figure 1 for the creep behavior of a polymer at two crosslink densities < Vq. Here the kinetic theory of rubber elasticity... 

Figure 1. Schematic of the creep behavior of a polymer network at two cross-Unk densities vt < v . Foints a and b denote two possible initial states immediately...

The creep behavior of the polymers used here is approximated by the following equations for the case of large X. The stretch ratio X (t) is given by... 

The peeling off of a hook fitted with a pressure sensitive adhesive and attached to a ceramic or glass surface can be regarded as a typical example for the creep behavior of an adhesive layer. In particular, thermoplastic adhesives that, to a great extent, also include pressure-sensitive adhesives (Section 5.6) tend to creep under high strain. A reason for this behavior is the time-related failure of individual bonds between the polymer molecules due to the strain imposed from outside. The application of adhesives with a higher crosslink ratio can reduce the adhesive layers tendency to creep. 

Figure 6.8 Creep compliance curve showing polymer creep-response behavior.2...

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). 

Practical consequences of Eg modification in polymer films include significant changes of dissolution, diffusional and etching characteristics, mechanical creep behavior, and adhesion. Figure 17.30 shows a plot of the effective diffusion coefficient of perfluorooctane sulfonate photoacid as a function of film thickness of partially protected poly(4-t-butyloxycarbonyloxstyrene). The profile shows asymptotic behavior at 600 A, below which diffusion slows down remarkably, probably due to interfacial and confinement effects. Clearly, the interaction of the first few hundred angstroms of the film with the substrate determines its adhesion and can alter its electrical and optical properties as well as its topographical and surface characteristics. ... 

The restraining influence of the crystallite alters the mechanical behavior by raising the relaxation time T and changing the distribution of relaxation and retardation times in the sample. Consequently, there is an effective loss of short T, causing both the modulus and yield point to increase. The creep behavior is also curtailed and stress relaxation takes place over much longer periods. Semicrystalline polymers are also observed to maintain a relatively higher modulus over a wider temperamre range than an amorphous sample. 

Beckmann, J., McKenna, G. B., Landes, B. G., Bank, D. H., and Bubeck, R. A., Physical aging kinetics of syndiotactic polystyrene as determined from creep behavior, Polym. Eng. ScL, 37,1459-1468 (1997). 

In vitro mechanical tests should also focus on the dynamic behavior of a given polymer scaffold, which is very crucial for applications like knee/hip joint repair and vascular grafts. Certain polymers like polypropylene show creep behavior, i.e., exhibit dimensional changes nnder continuous load and cannot be used to make vascular grafts [15]. 

In general, creep behavior of ceramics is similar to that of metals. However, in ceramics it usually occurs at higher temperatures, typically >0.5 Tni. In comparison, creep is a consideration in aluminum alloys at 100°C and in polymers at room temperature. Creep is particularly important in ice, which creeps extensively at low temperatures. The creep of ice is responsible for the movement of glaciers and the spreading of the Antarctic ice cap. 

This theory also explains plasticization by nonsolvents (softeners). When introduced into the polymer mass, these molecules act by holding apart the polymer molecules and so breaking some unions between active centers on the polymer. It was also explained why internally plasticized systems behave worse with the temperature than the externally plasticized, since molecules of a separate plasticizer are free to solvate and desolvate the active centers on the resin macromolecules to a given extent, determined by the concentration, the temperature and the equilibrium involved in the system. Permanently bound side chains have no such freedom. Other properties such as the tear strength or the creep behavior of plasticized systems were also explained. 

Fig. 6.10 Illustration of a series connection of the Maxwell model and the Kelvin model fw the four-element model to describe the viscoelastic creep behaviors of polymers...

A series crmnection of the Maxwell and Kelvin models makes the four-element model, known as the Burger s model (Burgers 1935), which can describe the viscoelastic creep behaviors of polymers, as given by... 

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