Polymers film tension

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. 

FIG. 20 (a) Density profiles p(z) vs z for e = —2 and four average bulk densities (f> as indicated, (b) Surface excess vs density in the bulk for four choices of e. (c) Profiles for the diagonal components of the pressure tensor and of the total pressure for (p = l.O and e = —2. Insert in (c) shows the difference between P, and Px to show that isotropic behavior in the bulk of the film is nicely obtained, (d) Interfacial tension between the polymer film and the repulsive wall vs bulk density for all four choices of e. Curve is only a guide for the eye [18]. 

Fogging is formation of small water droplets (visible condensation) on the surface of a polymer film. Undesirable effects may result from fog formation, such as reduction of clarity and dripping. Incorporation of antifogging agents eliminates the reduction of transparency by migration to the surface and increases the polymer surface critical wetting tension. This results in... 

The few examples of deliberate investigation of dynamic processes as reflected by compression/expansion hysteresis have involved monolayers of fatty acids (Munden and Swarbrick, 1973 Munden et al., 1969), lecithins (Bienkowski and Skolnick, 1974 Cook and Webb, 1966), polymer films (Townsend and Buck, 1988) and monolayers of fatty acids and their sodium sulfate salts on aqueous subphases of alkanolamines (Rosano et al., 1971). A few of these studies determined the amount of hysteresis as a function of the rate of compression and expansion. However, no quantitative analysis of the results was attempted. Historically, dynamic surface tension has been used to study the dynamic response of lung phosphatidylcholine surfactant monolayers to a sinusoidal compression/expansion rate in order to mimic the mechanical contraction and expansion of the lungs. 

A very common and useful approach to studying the plasma polymerization process is the careful characterization of the polymer films produced. A specific property of the films is then measured as a function of one or more of the plasma parameters and mechanistic explanations are then derived from such a study. Some of the properties of plasma-polymerized thin films which have been measured include electrical conductivity, tunneling phenomena and photoconductivity, capacitance, optical constants, structure (IR absorption and ESCA), surface tension, free radical density (ESR), surface topography and reverse osmosis characteristics. So far relatively few of these measurements were made with the objective of determining mechanisms of plasma polymerization. The motivation in most instances was a specific application of the thin films. Considerable emphasis on correlations between mass spectroscopy in polymerizing plasmas and ESCA on polymer films with plasma polymerization mechanisms will be given later in this chapter based on recent work done in this laboratory. 

Gas permeabilities for 02, N2 and CO2 were realised with Lyssy GPM 20 apparatus. Surface tensions of modified and unmodified polymers films were derived from contact angle measurements which were evaluated with six liquids (H20, HCONH2, Hg, CH2I2, tricresyl-phosphate and 1 -bromonaphtalene). 

The films of organic polymers like polypropylene orpolyethyleneterephta-late, when subjected to mechanical tensions, develop systems of micro- and mesopores that make them transparent for the diffusion of solutions in solvents not dissolving the polymers itself. The counterflow diffusion of solutions ofM02(0C2H40Me)2 (M = Mo, W) in MeOC2H4OH (from one side) and water solutions (from the other side) permitted composites to be obtained where the metal (Mo, W) oxohydroxide particles were formed in situ inside such organic polymer films and remained there after the removal of the solvents [171],... 

The dilational rheology behavior of polymer monolayers is a very interesting aspect. If a polymer film is viewed as a macroscopy continuum medium, several types of motion are possible [96], As it has been explained by Monroy et al. [59], it is possible to distinguish two main types capillary (or out of plane) and dilational (or in plane) [59,60,97], The first one is a shear deformation, while for the second one there are both a compression - dilatation motion and a shear motion. Since dissipative effects do exist within the film, each of the motions consists of elastic and viscous components. The elastic constant for the capillary motion is the surface tension y, while for the second it is the dilatation elasticity e. The latter modulus depends upon the stress applied to the monolayer. For a uniaxial stress (as it is the case for capillary waves or for compression in a single barrier Langmuir trough) the dilatational modulus is the sum of the compression and shear moduli [98]... 

One of the main drivers in moving to plastic substrates is that it opens up the possibility of roll-to-roll processing and the process and economic advantages that this brings. Under these conditions a winding tension will clearly be present and polymer film substrates with low moduli will be susceptible to internal deformation, particularly at elevated process temperatures. Figure 7.8 shows a comparison between poly(ethylene terephthalate) and poly(ethylene naphthalate) films. 

Kollicoat IR is a unique polymer for pharmaceutical applications prepared by a graft polymerization process of polyethylene glycol (25%) with polyvinyl alcohol (75%). Kollicoat IR dissolves quickly in water and aqueous solutions of acid and alkali and reduces the surface tension of aqueous solutions to allow the solutions to have high spray rates. The polymer film is very flexible, not tacky, and easily colored. The polymer can be used as instant release coating, pore former, binder, protective colloid, etc. 

When a dry polymer film is immersed into liquid water, an opposite process to what described above takes place. Hydrophilic groups migrate to the interface, and hydrophobic moieties move to the inner part of the surface state of a polymer. The main driving force for these movements is the interfacial tension. The greater the interfacial tension, the greater is the surface configuration change. 

Like surface tension, viscosity is a manifestation of the physical interaction between molecules. The uniquely shallow slope of the viscosity-temperature curve of PDMS (20) is due in part to low intermolecular forces. The interfacial viscosity of PDMS ( 10 xg/s) is the lowest known for a polymer film (9) and is also partly the result of low methyl-methyl interactions dominating polymer-polymer interactions in PDMS. 

Table XIV. Surface Tensions of Plasma Polymer Films Deposited...

Polymer Film Thickness pm Tensile Strength (MPa) Tension at Break (MPa) Elongation % Puncture Strength (N)... 

Factors influencing the rate of monomer diffusion are surface tension distribution coefficient of monomers between the two phases rate of monomer transfer from phase to phase solvent power of organic phase for the polymer permeability of polymer film to monomers adsorption of monomers by the polymer film viscosity of the system. 

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