Mechanically deforming polymer

OOLee Lee, J., Yee, A. F. Fracture of glass bead/epoxy composites on micro-mechanical deformations. Polymer 41 (2000) 8363-8373. 

These observations were the basis for the proposal that polymers, like ionic crystals, exhibit shock-induced polarization due to mechanically induced defects which are forced into polar configurations with the large acceleration forces within the loading portion of the shock pulse. Such a process was termed a mechanically induced, bond-scission model [79G01] and is somewhat supported by independent observations of the propensity of polymers to be damaged by more conventional mechanical deformation processes. As in the ionic crystals, the mechanically induced, bond-scission model is an example of a catastrophic shock compression model. 

The mechanical properties of polymers are of interest in all applications where they are used as structural materials. The analysis of the mechanical behavior involves the deformation of a material under the influence of applied forces, and the most important and characteristic mechanical property is the modulus. A modulus is the ratio between the applied stress and the corresponding deformation, the nature of the modulus depending on that of the deformation. Polymers are viscoelastic materials and the high frequencies of most adiabatic techniques do not allow equilibrium to be reached in viscoelastic materials. Therefore, values of moduli obtained by different techniques do not always agree in the literature. 

The above models consider only one spatial variable which is the bonding distance. It is clear that, for a molecule anything more complex than diatomic, many parameters are needed to define even approximately the potential energy surface. The enormous advances in computational chemistry during the last few years have allowed quantum mechanical calculations on fairly large size molecules. The first attempt to apply quantum mechanics on deformed polymer chains was made... 

An uniaxial mechanical deformation provokes drastic changes in the identation pattern of drawn polymers. Some typical results illustrating the dependence of MH on draw ratio for plastically deformed PE are shown in Fig. 19 a. The quoted experiments 12) refer to a linear PE sample (Mw 80.000) prepared in the usual dumbbell form drawn at a rate of 0.5 cm/min at atmospheric pressure. Identations were performed longitudinally along the orientation axis. Before the neck (A = 1), the... 

If the ordered, crystalline regions are cross sections of bundles of chains and the chains go from one bundle to the next (although not necessarily in the same plane), this is the older fringe-micelle model. If the emerging chains repeatedly fold buck and reenter the same bundle in this or a different plane, this is the folded-chain model. In either case the mechanical deformation behavior of such complex structures is varied and difficult to unravel unambiguously on a molecular or microscopic scale. In many respects the behavior of crystalline polymers is like that of two-ph ise systems as predicted by the fringed-micelle- model illustrated in Figure 7, in which there is a distinct crystalline phase embedded in an amorphous phase (134). 

While conductivities of nanocarbons dispersed in polymers fall short of those of metals, a variety of applications can be unlocked by turning an insulating matrix into a conductor, which requires only small volume fractions that can therefore keep the system viscosity at a level compatible with composite processing techniques. Of particular interest are novel functionalities of these conductive matrices that exploit the presence of a conductive network in them, such as structural health monitoring (SHM) based on changes in electrical resistance of the nanocarbon network as it is mechanically deformed [30]. 

Mauritz et al., motivated by these experimental results, developed a statistical mechanical, water content and cation-dependent model for the counterion dissociation equilibrium as pictured in Figure 12. This model was then utilized in a molecular based theory of thermodynamic water activity, aw, for the hydrated clusters, which were treated as microsolutions. determines osmotic pressure, which, in turn, controls membrane swelling subject to the counteractive forces posed by the deformed polymer chains. The reader is directed to the original paper for the concepts and theoretical ingredients. 

Despite the similarities in brittle and ductile behavior to ceramics and metals, respectively, the elastic and permanent deformation mechanisms in polymers are quite different, owing to the difference in structure and size scale of the entities undergoing movement. Whereas plastic deformation (or lack thereof) could be described in terms of dislocations and slip planes in metals and ceramics, the polymer chains that must be deformed are of a much larger size scale. Before discussing polymer mechanical properties in this context, however, we must first describe a phenomenon that is somewhat unique to polymers—one that imparts some astounding properties to these materials. That property is viscoelasticity, and it can be described in terms of fundamental processes that we have already introduced. 

The potential applications of FT-IR to the determination of polymer structure are many. In a number of areas, the impact of FT-IR has been very significant including the structure of polymer blends, polymer surfaces and the structural changes induced by mechanical deformation. These topics will be discussed in detail below. 

Mechanical deformation induces orientation into the polymer samples and polarized infrared can be used to characterize this orientation either by direct measurement of the dichroic ratio 287,289) or by spectral subtraction 286), three dimensional sample tilting 68,286), or internal reflection spectroscopy 130). 

With 7 = 0.072 Nm-1 and 5 = 10 nm the effective pressure is of the order of P = 72 x 10s Pa. Such a high pressure can change the surface structure, cause mechanical deformation at the moving wetting line [250], and can lead to contact angle hysteresis [251-253], especially on soft polymer surfaces. 

Strobl G (2005) Symposium Deformation Mechanisms in Polymers , Halle/Germany, May 19-20... 

For soft materials such as polymers and biomolecules, mechanical deformation of the sample caused by the tip is a point for serious concern. The consequences of the deformation are rather severe. First, it increases contact area and therefore reduces resolution. Second, the height profile of the surface can be underestimated. Third, the original structure of the sample can be destroyed irreversibly. Furthermore, the tip itself can be deformed when imaging hard samples [239]. 

For very thin films V/t Newtonian melt being film cast nonisothermally has been treated by Pearson [J. R. A. Pearson, Mechanics of Polymer Processing, Elsevier, New York, 1985], (a) For thicker films the deformation of the melt can be considered as one where only h = h(z) and w = w0. Use the continuity and z-momentum equations, neglecting inertial terms, gravity, and air-drag forces, to obtain the following expressions for h(z) and v- (z) ... 

In practice, many fabrication processes take place under non-isothermal, non-quiescent and high-pressure conditions. Mechanical deformation and pressure can enhance the crystallisation as well as the crystal morphology, by aligning the polymer chains. This leads to pressure-induced crystallisation and to flow-induced or stress-induced crystallisation, which in fact is the basis for fibre melt-spinning (see Sect. 19.4.1)... 

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