Title: Resonance method for measuring the complex elastic modulus of polymeric films.
Author: prof. Alberto Rovetta N. BF-1

The knowledge of the mechanical properties, and particularly of the elastic modulus and dissipation factors of films, is quite important from a practical point of view. Recently, some apparatus has been set up for measuring the dynamic moduli by the method of constant- frequency forced vibrations. Resonance methods are unfit by the usual apparatus for direct measurements, the films always exhibiting a resonance frequency.

It consists of a small metal sheet clamped to one end; the film to be examined is glued to the top of the other end by an epoxy resin standing both high and low temperatures. The film is bent to 90 in respect of the sheet and fixed in the clamp below. The film clamp is mobile in the vertical direction after loosing and blocking the side screw through a micrometric movement adjusted by a screw placed below the supporting framework. Horizontal movements by blockable sliding guides are allowed. The system is inserted in a cell where vacuum may be obtained and where an electrode, put above the metal sheet unclamped end, excites forced vibrations. By the clamp vertical moment it is possible to adjust the initial static stress in the film. This may be evaluated on the basis or the sheet end deflection (e. g. by a cathetometer) prior to performing the dynamic measurements. Thermostatting is accomplished by inserting the cel1 into a thermostat, where temperature may be varied.

The experimental tests show the condition of maximum vibration amplitude and allow the measurement of the system resonance frequency.
Then the frequencies corresponding to different vibration amplitudes have been measured; they were chosen as convenient fractions of the maximum amplitude. The experimental values of nine points have usually been determined. In this way, the experimental resonance curves of the system are plotted.
Such plots indicate which is the best range of values of the parameters, on which to operate in order to detect the different contributions of the system components.

Future possible developments
The described measurements lead to the conclusion that the indirect measurement method proposed here supplies reliable results both with regard to the values of moduli , and with regard to the possibility of detecting transitions even with low modulus variations.