Polymer creep

Figure 5.26. Running along the centre of the tube is a primitive chain. This is the shortest path down the tube. The deviations of the polymer chain from this path can be considered as defects. The motion of these kinks or defects in the chain away from the primitive path allows the chain to move within the tube. The polymer creeps through the tube, losing its original constraints and gradually creating a new portion of tube. This reptilian-like motion of the chain was named by de Gennes from the Latin reptare, to creep, hence reptation.

Alterations by moisture exposure are weak shrinkage and coefficients of thermal expansion are low, as for other amorphous polymers creep resistance is rather good at room temperature. 

Alterations by moisture exposure are fair. Shrinkage and coefficient of thermal expansion are anisotropic with values typical of crystalline polymers. Creep resistance is good. 

When dash pot and spring elements are connected in parallel they simulate the simplest mechanical representation of a viscoelastic solid. The element is referred to as a Voigt or Kelvin solid, and it is shown in Fig. 3.10(c). The strain as a function of time for an applied force for this element is shown in Fig. 3.11. After a force (or stress) elongates or compresses a Voigt solid, releasing the force causes a delay in the recovery due to the viscous drag represented by the dash pot. Due to this time-dependent response the Voigt model is often used to model recoverable creep in solid polymers. Creep is a constant stress phenomenon where the strain is monitored as a function of time. The function that is usually calculated is the creep compliance/(f) /(f) is the instantaneous time-dependent strain e(t) divided by the initial and constant stress o. ... 

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. 

There is strong interest to analytically describe the fzme-dependence of polymer creep in order to extrapolate the deformation behaviour into otherwise inaccessible time-ranges. Several empirical and thermo-dynamical models have been proposed, such as the Andrade or Findley Potential equation [47,48] or the classical linear and non-linear visco-elastic theories ([36,37,49-51]). In the linear viscoelastic range Findley [48] and Schapery [49] successfully represent the (primary) creep compliance D(t) by a potential equation ... 

In both polymers, creep of compression-molded specimens is caused mainly by crazing, with shear processes accounting for less than 20% of the total time-dependent deformation. Crazing is associated with an increasing creep rate and a substantial drop in modulus. The effects of stress upon creep rates are described by the Eyring equation, which also offers an explanation for the effects of rubber content upon creep kinetics. Hot-drawing reduces creep rates parallel to the draw direction and increases the relative importance of shear mechanisms. 

ASA Polymers. Creep tests were performed on Luran S 757R at stresses from 19-26 MN/m2. Over this range, the time to reach 5% ex-... 

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

As with the Tenax system, there is evidence here that the sorbent may be perturbed by the supercritical fluid. Wang and coworkers (T5) have shown that pressurized CO2 can severely plasticize polymers. Creep tests ( ) indicate that appreciable changes in bulk polymer moduli as well as differential changes in the glass transition temperature (up to 50 to 70 C) occur at pressures under 100 atmospheres in polystyrene. Additional dilation data for carbon dioxide absorbing into polystyrene (37) support the above study and indicate that the superimposition of a polymer phase change may also influence the retentive capacity of the polymeric sorbent. 

In many applications, plastic parts carry reasonably constant mechanical loads over periods up to few years. The polymer will creep during the lifetime of the part. At moderate load levels, long-term prediction of creep from short-term tests is possible, because the viscoelastic response of polymers (creep, stress relaxation) measured at different temperatures superimpose when shifted along the time axis [24]. 

It will be necessary to describe the definition and measurement of the parameters used to quantify viscoelastic effects, in the sense that elastic moduli allow one to quantify elastic effects. For example how do you design the size of a torsional shaft when the polymer creeps for a plastic bolt after it is tightened, how do you compute the rate of stress decay or, for the damping of mechanical vibrations what parameters would you require to design the polyurethane foam used to control and dampen vibrations and resonances in a hi-fi tone arm ... 

Keywords Complex polymer systems Deformation kinetics geneity Glass transition anomalies Laser interferometry morphology Polymer creep Relaxation dynamics... 

Reliable detection and detailed studying of the jump-like character of polymer creep on the submicro-, micro-, and meso-scale levels, and the successful looking... 

Below we consider the results of our systematic research of deformation kinetics for glassy polymers over the wide ranges of temperatures and deformations, using the laser-interferometric technique under consideration [11,278,280-287], This research allowed us (1) to study the dependencies of kinetic parameters of creep on these factors, (2) to reveal the regular relations between the activation parameters of polymer creep, (3) to demonstrate their intimate connection with the parameters of relaxation transitions, and (4) to confirm directly the intermolecular physical nature of potential barriers of polymer plasticity. ... 

In Fig. 17.18, the MEAs reduce their thicknesses in the whole range of contact pressure during the first cycle and the following reduction of the contact pressure leads to an increase of the MEAs thickness. Nevertheless, the MEAs do not recover back to their initial thickness. In the second cycle, the MEAs thickness is not recovered again to the same value, but their behavior is similar to the previously observed. The GDL loses most of its elasticity in the first cycles and approaches a constant value over time [90], MEA thickness change consists of a reversible and an irreversible part [17,21], Thus, the loss of elasticity in the MEA is owing to the irreversible destruction of GDL pores. Electrolyse reallocation in CL and GDL and/or even membrane/polymer creeping due to compression forces are other phenomena that can... 

There are three fundamental test methods for characterization of the viscoelastic behavior of polymers creep, stress relaxation, and dynamic mechanical analysis. Although the primary focus for this chapter is DMA, it is useful first to discuss the fundamentals of creep and stress relaxation, not only because they are conceptually simpler but because most DMA instruments also are capable of operating in either a creep or stress relaxation mode. All three of the methods are related, and numerical techniques are available for calculating creep and stress relaxation data from dynamic mechanical data (Ferry 1980). 

Peschanskaya, N. N., Bershtein, V. A., Stepanov, V. A. (1978). The Connection of Glassy Polymers Creep Activation Energy with Cohesion Energy. Fizika Tverdogo Tela, 20(11), 3371-3374. 

The effect of high atmospheric temperatures or heating from direct exposure to the sun can be particularly severe on thermoplastic polymers. Creep or distension of the pol5winyl chloride and polyethylene plastics will occur readily unless provision is made to prevent overheating or stressing of the materials. 

Clustering of sulfonic acid groups and water helps elucidate the large and dramatic changes in proton conduction, water transport, elastic modulus, polymer creep, and stress relaxation that occur with Nafion at low water activity and high temperature. The identification of a clustering transition facilitates the choice of processing conditions to erase the memory of Nafion and better control the mechanical and transport properties. 

Flexural creep occurs with half the specimen in tension and half in compression. Thus, the compliance for flexural creep for polymeric solids (no reinforcement) should be less than that for tension, since the polymer creeps more slowly in compression than in tension, as previously discussed. 

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