Polymer under compression

Two simple examples comparing the properties of ideal and real chains are discussed in this section uniaxial and biaxial compression. A related of triaxial confinement shall be discussed in Section 3.3.2 for the 

We consider first biaxial compression corresponding to squeezing of a chain into a cylindrical pore of diameter D. The diameter of the pore defines a natural compression blob size. On length scales smaller than D, sections of the chain do not know that it is compressed and their statistics are still the same as the statistics of an undeformed chain  

These equations can be solved for the number of monomers g in a compression blob of size D  

The above relations are identical to the corresponding equations for tension blobs (Section 3.2.1) because in both examples the conformational statistics are unperturbed on the shortest scales. 

The length of a tube 7 occupied by an ideal chain can be estimated as a random walk of Njg compression blobs along the contour of the tube  

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Ductile Failure of Brittle Polymers under Compressive Shear Stresses... 

Tphe literature is replete with examples showing that the application of hydrostatic pressure enhances the ductile behavior of strained amorphous polymers. In this paper we present a possible explanation of this effect and two experiments demonstrating the enhanced ductility of polymers under compressive shear stresses applied orthogonally to the plane of shear. 

In a final chapter a closely related phenomenon, the formation of shear bands in semi-crystalline polymers under compressive load will be described. It is attempted to discuss under which conditions shear bands are formed in semi-crystalline materials and how they interact with each other or with certain microstructural features, finally leading to crack initiation and shear fracture of the bulk polymer. 

Composition profiles of adsorbed polymers under compression... 

Figure 18.13 A stress-strain behavior of semicrystalline polymers under compression.

This behavior of the density has not been observed in laboratory experiments in fact the density often increases shghtly at yield." Also it is worth recalling the well known experimental result that yield can also occur in amorphous polymers under compression. Simulations of model polymers under compression have yet to be reported. 

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

The systematic study of piezochromism is a relatively new field. It is clear that, even within the restricted definition used here, many more systems win be found which exhibit piezochromic behavior. It is quite possible to find a variety of potential appUcations of this phenomenon. Many of them center around the estimation of the pressure or stress in some kind of restricted or localized geometry, eg, under a localized impact or shock in a crystal or polymer film, in such a film under tension or compression, or at the interface between bearings. More generally it conveys some basic information about inter- and intramolecular interactions that is useful in understanding processes at atmospheric pressure as well as under compression. 

The piezoelectric polymer investigations give new physical insight into the nature of the physical process in this class of ferroelectric polymers. The strong nonlinearities in polarization with stress are apparently more a representation of nonlinear compressibility than nonlinear electrical effects. Piezoelectric polarization appears to be linear with stress to volume compressions of tens of percent. The combination of past work on PVDF and future work on copolymers, that have quite different physical features promises to provide an unusually detailed study of such polymers under very large compression. 

It hag been shown that transition of a backbone carbon from the sp to sp state is promoted by tensile stresses and inhibited by compressive strains (10,44). The acceleration of the process of ozone oxidation of the polymers under load is not associated with the changes in supramolecular structure or segmental mobility of the chain. The probably reason of this effect is a decreasing of the activation energy for hydrogen abstraction (44). The mechanism of initial stages of the reaction of ozone with PP can be represented as ... 

For higher temperature operation, a polybenzimid-azole-based polymer electrolyte may be preferred. The PEMFC structures have good mechanical integrity under compression and expansion from differential temperature and pressure gradients that occur during operation. This system has minimal materials problems, except for the cost and operation characteristics of the membrane. The PEMFC operates at 1 A/cm at 0.7 V. The electrode reactions in acidic media have been discussed above. 

In order to find the influence of compression on collapse of the polymer networks, the experiments on the swelling of the deformed gels of AA-SMA in water-methanol and water-dioxane mixtures were performed [29]. It was shown that uniaxial compression of the gel really affects the swelling curves and that, in a good agreement with the theory, the region of stability of the collapsed state increases and the sharpness of collapse decreases under compression. 

Microfabrication was done by means of excimer laser ablation in a polymer substrate [141], This substrate was covered by a thin polymer sheet by thermal bonding under compression. 

Fig. 32. Shear modulus, G, vs. ratio of undercooling, T/T, (temperature in Kelvin), for various polymers. The modes of deformation which can predominate under compression are indicated...

The formation of shear bands under compression is found in crystalline polymers when loaded at temperatures lower than 0.75 T. Under such a condition the shear bands interact with certain morphological features such as spherulite boundaries or lamellar arrangements inside the spherulites. The band initiation stress, ct, increases and the strain at break, Cp, decreases with decreasing temperature and increasing stiffness of the tested polymer, i.e. increasing degree of crystallinity. 

The behavior of polymeric systems under compression (high pressures) has also been studied insufficiently. Till now the equilibrium under normal pressure has been mainly considered. The experiments on the formation of the ordered state of polyethylene under the pressure above 3-4 kbar revealed that there is much to be done in this field to extend the concept to polymers in general (see ). 

For macroscopically isotropic polymers, the Tresca and von Mises yield criteria take very simple analytical forms when expressed in terms of the principal stresses cji, form surfaces in the principal stress space. The shear yield surface for the pressure-dependent von Mises criterion [Eqs (14.10) and (14.12)] is a tapering cylinder centered on the applied pressure increases. The shear yield surface of the pressure-dependent Tresca criterion [Eqs (14.8) and (14.12)] is a hexagonal pyramid. To determine which of the two criteria is the most appropriate for a particular polymer it is necessary to determine the yield behavior of the polymer under different states of stress. This is done by working in plane stress (ct3 = 0) and obtaining yield stresses for simple uniaxial tension and compression, pure shear (di = —CT2), and biaxial tension (cti, 0-2 > 0). Figure 14.9 shows the experimental results for glassy polystyrene (13), where the... 

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