Viscosity relation, spin coating

These films can be prepared by a variety of routes, only a few of which are mentioned here. The original references should be consulted for more practical details. Titanium dioxide is used as an illustrative example below. First, colloidal solutions are prepared, e.g., from titanium isopropoxide. The resultant sol is concentrated under vacuum at room temperature until its viscosity increases. Then it is spin-coated on to suitable supports (e.g., conducting glass) and fired in an oven. The firing temperature critically controls the morphology of the resultant film as discussed elsewhere [300-303]. Films up to several micrometers thick can be prepared by this simple version of the sol-gel technology [304]. Aerosol or spray pyrolysis is a somewhat related approach [305, 306]. 

The possibihty of being able to change easily and controllably the film thickness by varying the ccaicentration of the solution used for spin-coating opened up a whole field of research on questions related to the chain-like nature of these macromolecules. In particular, at that time it was unclear (and partially still is ) if polymer properties like viscosity, chain conformation (as expressed for example by the radius of gyration), chain orientation, and interdiffusion rate, or mechanical properties and Tg change once the thickness of the film decreases below the diameter of Gaussian polymer coils in bulk samples [1]. 

Certain important properties of polymers are directly related to the average chain length and the distribution. One of particular importance is the viscosity. This affects the flowability of the polymer and the kinds of applications where it can be used (injection molding, blow molding, fiber spinning, sheet formation, coating, etc.). 

---