Polymer Support Films, both Carbon Coated and Uncoated
There is a choice of different polymer support films and the polymer film support most often asked for is Formvar®. We are able to make continuous films of Formvar as well as holey Formvar, even in specific hole sizes and also lacey Formvar. The final films are of an excellent quality and very stable under the electron beam in most instances. The specific grids and prices listed on our website are all for 3.05 mm diameter grids, however we can also coat without difficulty 2.3 mm diameter grids as well (however we might need additional lead time).
There are a number of other polymeric support films some of which are essentially the same or similar polyvinyl formal polymer as Formvar where as others are clearly different polymers and with different properties. One should also be aware that most of these resins are commercial grades that have been carefully selected, sometimes purified beyond commercial purities and in some instances, we are blending two or more grades of commercial materials. That is why we present the resins by name only without reference to a specific grade or molecular weight. What is used is that specific formulation that we have determined to be the very optimum for the filming of grids for this kind of application. In any case, we present the following information to help reduce some of the mystery related to the different coating resins:
* Formvar® and Vinylec® are all polyvinyl formal and while there might be some chemical differences, it is our opinion that from a grid making standpoint, they would be indistinguishable. Formvar is our standard resin, however upon special request, we would make grids from any specifically requested resin, but possibly at a slightly higher price.
* Parlodion® and Collodion, sometimes also known as "gun cotton" or nitrocellulose are also used as support films. Our standard product is based on the use of Parlodion because of its well known consistency and virtually zero batch to batch variation in properties.
* Pioloform® and Butvar®, which generically are polyvinyl butyral are also sometimes used, however even though they are not standard support film resins for SPI, we will coat with Pioloform or Butvar upon special request
Both Parlodion and Collodion in amyl acetate will float on water, while Formvar and Vinylec preparations sink. As a result, a Formvar or Vinylec film will have the structure of the glass on which it was formed, while a Parlodion or Collodion film can be made "structureless" and "featureless" by forming the films on water. This is an essential step in making the structureless carbon films on which DNA is studied.
So far we have addressed only the issue of the polymer and not the issue of whether it should be carbon coated. The general rule is this: If possible, carbon coat, because carbon coated grids will be more stable than uncoated support films. However, the addition of the carbon coating has the disadvantage that it can also impart some discernible structure to the support film system, and for those working with either nanoparticles or other fine structures for example, in sections, the structure from the carbon film could be misinterpreted as being structure coming from the sample. We find today the mix is about 50/50 between those who want their support films carbon coated vs. those who want them uncoated. To give an example of how important that carbon film could be in terms of film stability, for the filming of slot grids, the cost to produce Formvar only is about 50% higher than the cost to produce carbon coated Formvar. Why? Very simple: The yield is greater for the carbon coated Formvar grids. Remember, only SPI Supplies is batch checking by TEM our entire production and when a batch just won't stand up in the beam uncoated, we can not proceed and make such a shipment to our customers.
The SPI produced carbon only and carbon coated Formvar grids are known for their high beam stability and absence of structure. Indeed, it is generally recognized that our ability to make stable support films of the highest quality generally exceeds that of the typical user making them on an occasional basis and also exceeds the quality consistent from other commercial providers who do not have their own in-house TEM for quality checking and leave the Quality Control function up to their customers. As a result, SPI experience almost never the request by a customer to return unstable or defective grids. However, carbon films show a significant phase grain, presumably from C60 microcrystals.
Finally there is the question of using "holey" or "no holes" carbon or carbon coated polymeric support films and this depends on the kinds of samples being studied and the need to have completely unobstructed regions of sample with the effect of any substrate effects. In those instances, data is taken only from those portions of the sample essentially suspended over "holes", or in the case of lacey films, over the open areas of the lacey network. This kind of coated grid is preferred for example by those doing EELS (electron energy loss spectroscopy).
Note: If one is contemplating the application the sample from solvents (that would dissolve the support films) or examination (or treatment prior to examination) at elevated temperatures, one just can not use polymeric support films. So read on!
Carbon support films, continuous (no holes), holey carbon, and lacey carbon of the very highest quality are produced in an SPI Vacu Prep™ Table Top Vacuum Evaporator. The grain present is about the smallest possible in a vacuum evaporator and is very stable in the electron beam. When holey carbon films are needed, a holey Formvar filmed grid is then carbon coated and the polymer then dissolved away, leaving only the holey carbon film. Similarly, when lacey carbon films are needed, a similar procedure is followed.