Formvar support films

Saran (Dow polyvinylidene dichloride) is a tough, chemically resistant plastic available in a variety of forms that are useful in the laboratory. Saran pipe or tubing can easily be welded to itself or sealed to glass and is useful for handling corrosive solutions. Thin Saran film, available commercially as a packaging material, is useful for windows, support films, etc. Mylar (du Pont polyethylene terephthalate) film and other polyester films are also useful for these purposes. Mylar is chemically inert and has excellent electrical properties for electrical insulation and for use as a dielectric medium in capacitors. Much thinner than these are films that can be made in the laboratory by allowing a dilute ethylene dichloride solution of Formvar (polyvinyl acetal) to spread on a water surface and dry. 

Since most materials are opaque to the electron beam, even when only a few hundred nanometers thick, special problems arise in the production of suitable mounted specimens. Specimen support films are usually made of plastic or carbon, though other materials have also been used. Suitable film solutions may be made up of 2% w/v formvar (polyvinyl formal) in ethylene dichloride or chloroform. 

The most commonly used support films are either ultrathin carbon films or Formvar films stabilized with carbon. Essentially the carbon is vaporized in a vacuum chamber by passing a high current through two carbon rods. The carbon is deposited on to a... 

Powders are suspended in a fluid, mixed and dropped onto the plastic or carbon coated support grid. If the powders do not disperse by this method, the powder can be rolled onto a plastic support film on a glass slide, and cut, scored, floated off and picked up. Another variation is that powders may be mixed with the collodion or formvar and a film cast with the material held in the support film. 

Scanning transmission electron microscopy (STEM), coupled with EDX, has been used to determine metal particle sizes. The specimens for STEM are prepared by dispersing the sample ultrasonicaUy in methanol and placing one drop of the suspension onto a Formvar film supported on a copper grid. 

Extensive documentation on the preparation of holey film substrates for cryo-TEM sample preparation is available in the literature. Holey films or nets have been produced [10,25-27] from a glycerol-water mixture in a solution of a mixture of formvar and triafol in dichloromethane spread on mica. After floating the thin film off the mica support, the films were treated with ethyl acetate to transform pseudoholes into holes or nets. 

Procedure. The material to be studied is obtained by the microspreading method for SCs (see Section V). The support used is copper grids (100-200 mesh) covered with Formvar film (0.8% in 1,2-dichloroethane (Pelco International, Redding, CA). The plastic-covered grids are stabilized by evaporation of a thin carbon film (light brown color in the porcelain or filter paper used as control) in a shadow-cast unit (vacuum evaporator). 

Plastic, carbon and metal films (Section 4.7) are used as specimen supports on TEM grids. There are two plastic support materials in use collodion, 0.5% solution of nitrocellulose in amyl acetate and formvar, 0.25% polyvinyl formal in ethylene dichloride. These polymers are available as powders, solutions or prepared films on TEM grids. Formvar films, especially holey ones, are used as substrates for the formation of holey carbon films. Today, collodion is not used too often, as it is not as stable in the electron beam as formvar or carbon films. 

Premixed solutions of formvar and collodion are recommended for high quality film supports as the powders take several days to dissolve and are less uniform. Collodion is generally film cast on a water surface. A large Petri dish filled... 

---