Intermolecular friction

The viscosity of a fluid is an important property in the analysis of liquid behavior and fluid motion near solid boundaries. Viscosity is the fluid resistance to shear or flow and is a measure of the adhesive/cohesive or frictional fluid property. The resistance is caused by intermolecular friction exerted when layers of fluids attempt to slide by one another. 

Thermal effects (dielectric heating) can result from dipolar polarization as a consequence of dipole-dipole interactions of polar molecules with the electromagnetic field. They originate in dissipation of energy as heat, as an outcome of agitation and intermolecular friction of molecules when dipoles change their mutual orientation at each alternation of the electric field at a very high frequency (v = 2450 MHz) [10, 11] (Scheme 3.1). 

Single-stage simulations reveal that intermolecular friction forces do not lead to reverse diffusion effects, and thus the molar fluxes calculated with the effective diffusion approach differ only slightly from those obtained via the Maxwell-Stefan equations without the consideration of generalized driving forces. This result is as expected for dilute solutions and allows one to reduce model complexity for the process studied (143). 

Acceleration force = converted momentum change partial pressure gradient + external forces - thermal diffusion force + shearing force + intermolecular friction force... 

A major goal in the physics of polymer melts and concentrated solutions is to relate measurable viscoelastic constants, such as the zero shear viscosity, to molecular parameters, such as the dimensions of the polymer coil and the intermolecular friction constant. The results of investigations to this end on the viscosity were reviewed in 1955 (5). This review wiU be principaUy concerned with advances made since in both empirical correlation (Section 2) and theory of melt flow (Section 3). We shall avoid data confined to shear rates so high that the zero shear viscosity cannot be reliably obtained. The shear dq endent behavior would require an extensive review in itself. 

The lubricity theory explains the resistance of a polymer to deformation. Stiffness and rigidity are explained as the resistance of intermolecular friction. The plasticizer acts as a lubricant to facilitate movement of macromolecules over each other, thus giving the resin an internal lubricity. The gel theory is applied to predominantly amorphous polymers. It proposes that their rigidity and resistance to flex are due to an internal three-dimensional honeycomb structure or gel. The spatial dimensions of the cell in a brittle resin are small because their centers of attraction are closely spaced and deformation cannot be accommodated by internal movement in the cell-locked mass. Thus, the elasticity limit is low. Conversely, a thermoplastic or thermosetting polymer with widely separated points of attachment between its raacroraolecules is flexible without plasticization. 

From their results on molecular reorientation in pure and mixed tin tetrahalides. Sharp and Tolan [94] discuss the origin of the intermolecular friction. Since the rate of reorientation varies in a regular way with the composition of the molecule it could be argued that molecular shape or dipolar interactions are relatively unimportant and that instead intermolecular friction is closely related to the magnitudes of dispersion forces. 

The Lubrication Theory. The lubrication theory is based on the assiunp-tion that the rigidity of the resin arises from intermolecular friction binding the chains together in a rigid network. On heating, these firictional forces are weakened to allow the plasticizer molecules to lubricate the chains. Once incorporated into the polymer, the plasticizer molecules shield the chains from each other, thus preventing the reformation of the rigid network (1). 

The inhibitor contributes to increasing mobility and to the macromolecules alignment on the direction of the field of forces. In this way, intermolecular frictions are diminished, which is confirmed by the values recorded for the extrusion forces. Table 3.94. 

In Eq. (27.24), a is the hydrodynamic interaction parameter. The parameter m accounts for the effects of collective motion it increases the term (a + tti) in the square bracket (correction for nonideal intermolecular friction) in the limit of infinitely dilute solution (Q —> 0 nondraining coils). Substituting the above equations into Eq. (27.22) and simplification leads to ... 

In addition to the intramolecular forces, the chain is subjected to a Brownian force g n,t) due to random collisions with other chains, and an intermolecular frictional force, x(n, t) where < is an atomic friction coefficient. Defining the following Fourier transforms... 

In ultrasonic (US) welding, mechanical vibrations are used to generate heat via intermolecular friction. These same US vibrations are also used in many other processing functions, including mixing. 

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