Impact of Crystallization on the Development of Statistical Self-Bonding Strength at Initially Amorphous Polymer-Polymer Interfaces

To investigate the mechanisms of the adhesion (self-bonding) strength (sigma) development during the early stages of self-healing of polymer-polymer interfaces and fracture thereof, it is useful to operate not only with the average sigma value but with the sigma distribution as well. The latter has been shown to obey Weibull's statistics for such interfaces. However, whether it can also follow the most widely used normal (Gaussian) distribution is currently unclear. Moreover, a more complicated self-healing case, when the sigma development at an initially amorphous interface is accompanied by its crystallization, has not been investigated yet in this respect. In order to address these two important issues, 10 pairs of amorphous poly(ethylene terephthalate) (PET) samples were kept in contact for various periods of time from 5 min to 15 h at a temperature (T) of 94 degrees C (preserving the amorphous state) or T = 150 degrees C (giving rise to cold crystallization), or both Ts. Thereafter, the as-formed amorphous and semi-crystalline PET-PET auto-adhesive joints were shear fractured in tension at ambient temperature. For the first time, the statistical distributions of a number of the measured sigma data sets were analyzed and discussed using both Weibull's and the Gaussian model, including several normality tests.