The mechanical properties of films. III. Dilato‐metric observations of multiple transitions

Evaluation of dilatometric data on cast films by the normal graphical method indicated the presence of multiple transitions. An alternative method is developed in which the coefficients of the polynomial which approximates to the given set of point data have been calculated using a least‐squares criterion and a programme written for a Ferranti Mercury computer. The peak values of the second derivative of this curve corroborated the transition temperatures obtained by the graphical method. Preliminary experiments suggested that the highest transition is associated with oxidative ageing conditions; this transition is absent in specimens prepared in non‐oxidising atmospheres. The free volume expansion coefficient αF approximates very closely to the Williams, Landel & Ferry predicted value of 4.8 × 10−4 c.c./°c/c.c. provided the contributions from all transitions are included. Experimental values less than this have invariably led to the detection of a further transition or transitions when the temperature range has been extended.