Strain-rate effects on hardness of glassy polymers in the nanoscale range.Comparison between quasi-static and continuous stiffness measurements

Strain rate effects on Hardness and Young's modulus of two glassy polymers, poly(diethylene glycol bis allyl carbonate) (CR39) and bisphenol-A polycar bonate (PC), were studied in the nanoscale range. Before analyzing material behaviors, we focused on a particular phenomenon prevailing at the first s tage of the contact between the surface of these polymers and the Berkovitc h diamond tip used in the experiments, leading to an apparent increase of t he tip defect (i.e., the missing tip of the diamond from having a shape equ ivalent to a perfect cone). The common methods based on calibration functio ns of the tip appear to be inaccurate to calculate correctly the contact ar ea at the nanoscale range for these polymers. A new method based on Loubet et al.'s model to calculate the contact area by taking account of the appar ent tip defect is proposed. The hardness values obtained this way were comp ared to the compressive yield stress using Tabor's relationship. A hardness -yield stress ratio close to 2.0, as expected on such polymers, was found. A strain-rate effect on the load-depth curve for these two polymers is inte rpreted as an increase of the hardness with the strain rate. The results fr om quasi-static and dynamic (the continuous stiffness method) measurements are compared. The strain-rate effect on Young's modulus in dynamic conditio ns should be taken into account in the hardness calculation.