The Cast Film Method

Casting a film can refer to picking people to be in a movie, but here it refers to a method of preparing polymers for infrared transmission analysis. In this technique, the polymer is first dissolved in a volatile solvent, which is required because the solvent is evaporated later. Examples of solvents that work well include acetone, methyl ethyl ketone, chloroform, methylene chloride, tetrahydrofuran, and toluene. The polymer can be in any form, be it a sheet, formed part, chunk, or pellet. As long as the polymer will dissolve in a volatile solvent, in theory its spectrum can be measured. Thermoset polymers, which consist of three-dimensional cross-linked polymer networks, do not dissolve in solvents and hence cannot be analyzed using this technique. The amount of polymer and solvent used can be eyebaUed, but more consistent results are obtained if the amount of polymer and solvent are measured. 

FIGURE 4.13 The process of building up a cast polymer film by placing drops of solution on an IR transparent window. The hot plate is used to speed the evaporation of the solvent. 

FIG U RE 4.14 The infrared spectrum of a polystyrene film cast from a methyl ethyl ketone solution. A KBr window was used. 

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A third method of sample preparation, not quite as common as the two described above, is the cast film method. In this technique a solution of the sample is evaporated to dryness and the IR spectrum of the deposited solid film is recorded. This has the advantage that no extraneous matrix is required however, a satisfactory film may not form in all cases. The cast film method is discussed in more detail in the following references (Alpert et al., 1964 Parker, 1971 Price, 1972). 

With a few exceptions (MacCarthy et al., 1975 MacCarthy and Mark, 1975) all infrared spctra of humic substances have been measured on dried solid samples, and the pressed-pellet method has been used almost exclusively. The mull technique has been used to a very limited extent in the study of humic substances (Ceh and Hadzi, 1956 Orlov et al., 1962 Wagner and Stevenson, 1965), and a few workers have also used the cast film method (MacCarthy and Mark, 1975 Wershaw and Pinckney, 1980). Typical infra-... 

Immobilization of the bilayer membranes as thin solid films is required when the bilayer membranes are used as novel functional materials. Casting method is a simple way to immobilize the bilayer membrane on a solid support from an aqueous solution by drying. Polymer film is easily prepared when the cast film of polymerizable bilayer membrane is polymerized. A free standing polymer film prepared by photo polymerization of the cast film of diacetylene amphiphiles was reported by O Brien and co-workers [34]. Composition with macromolecular materials is another way of polymer film preparation. Bilayer membranes are immobilized as polymer composites by the following physical methods ... 

Precipitation of the cast liquid polymer solution to form the anisotropic membrane can be achieved in several ways, as summarized in Table 3.1. Precipitation by immersion in a bath of water was the technique discovered by Loeb and Souri-rajan, but precipitation can also be caused by absorption of water from a humid atmosphere. A third method is to cast the film as a hot solution. As the cast film cools, a point is reached at which precipitation occurs to form a microporous structure this method is called thermal gelation. Finally, evaporation of one of the solvents in the casting solution can be used to cause precipitation. In this technique the casting solution consists of a polymer dissolved in a mixture of a volatile good solvent and a less volatile nonsolvent (typically water or alcohol). When a film of the solution is cast and allowed to evaporate, the volatile good solvent evaporates first, the film then becomes enriched in the nonvolatile nonsolvent, and finally precipitates. Many combinations of these processes have also been developed. For example, a cast film placed in a humid atmosphere can precipitate partly because of water vapor absorption but also because of evaporation of one of the more volatile components. 

Particularly, the nonsolvent immersion, that is, the Loeb-Sourirajan preparation method is an important methodology. In this method, a polymer solution is cast into a film and the polymer precipitated by immersion into water [10,144], The nonsolvent (water) quickly precipitates the polymer on the surface of the cast film, producing an extremely thin, dense-skin layer of the membrane [10,144], The polymer under the skin layer precipitates gradually, ensuing in a more porous polymer sublayer [145], Following polymer precipitation, the membrane is usually annealed in order to improve solute rejection [10,144]. 

A new approach was proposed for making effective helmets which could replace the former British army steel helmet. Essentially the new helmet used modified phenolic resins reinforced with nylon, and the crown cap inside was thermoformed from polyethylene. Formerly the crown cap was attached to the steel by rivets—not an appropriate method for fixing polyethylene to reinforced plastics. Instead a method was developed with a hot-melt adhesive based on ethylene-vinyl acetate copolymers cast as film on release paper. For assembly, the cast film is cut in advance to match the intricate shape required and activated by heat to bond under light pressure subsequently, a further heat activation is employed to fix the crown cap in place (Figure 52 illustrates this). 

Park et al. (60) studied dispersion characteristics of MWCNT-PMMA composites synthesized by in-situ bulk polymerization using AIBN as free radical initiator. In their method, CNTs in varying amounts such as 0.001, 0.01 and 0.1 wt% with respect to MMA were first dispersed in MMA monomer by ultrasonication before polymerization. Experimental evidence such as molecular weight of free PMMA prepared via in-situ polymerization with and without CNTs, diameter of pristine MWCNT and diameter of MWCNT in composite, FTIR and SEM studies confirmed the role of AIBN and MWCNT in polymerization. The induced radicals on MWCNT by AIBN were found to trigger grafting of PMMA on to CNT surface. Solvent cast film of the composite was transparent and showed a better nanoscopic dispersion without aggregates compared to the cast film prepared from direct mixing of MWCNT and PMMA. 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

The most popular method of processing is based on limited evidence that the reduced or de-doped form is soluble. It is widely applied to PANi based on extensive experimental studies by MacDiarmid and his group. In brief, a solution of the de-doped and neutral form of the CP (the emeraldine base) in an organic solvent is used to coat a substrate and the cast film is chemically doped after drying. 

Molecular composites in terms of synthesis The reinforcements are also formed by the in-situ reaction in the matrix polymers. Such methods are another way to pursue the limit of molecular dispersion of reinforcing materials. One example is found in the in-situ precipitation of reinforcing silica in polydimethyl siloxane networks with the sol-gel methods [21], Another example is the direct polycondensation of p-aminobenzoic acid or p-hydroxybenzoic acid from their monomers in solutions of polyarylate [22]. Mechanical properties of the cast films indicated increase in modulus and tensile strength at elevated temperatures. 

As also seen in Figrue 5, the initial resistance of the electrospim nanofiber mat was higher than that of the cast film, although they have been made from the same starting solution. The lower conductivity values for the electrosprm nanofiber mats than those of the cast films can be attributed to the porous nature of the non-woven nanofiber mats, as the present method meastrres the volttme resistivity rather than the conductivity of an individual fiber. 

Alternative preparation methods involve working from the melt [70], the pre-cross-linked state [71] or by milling the solid constituents together at low temperatures followed by hot pressing [72]. Removal of final traces of solvent (where appropriate) and/or of moisture (either present in trace amounts, perhaps in an adventitious form, or in substantial amounts in the case when the casting solvent is water itself) from the cast film can be achieved by heating under vacuum. 

The monomers and polymers were characterized by infrared (IR) and ultraviolet-visible (UV-vis) spectroscopy, H and - C nuclear magnetic resonance (NMR), element analyses, differential scanning calorimetry (DSC), and polarizing optical microscopy. The molecular weights of the polymers were evaluated by gel permeation chromatography (GPC) using polystyrene standards, and electrical conductivities upon iodine doping for the cast film of the polymers were measured by the four-probe method. 

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