An unexpected plasticization phenomenon and a constant of the change rate of viscoelastic properties for polymers during nanoindentation test

Abstract

AbstractThe effect of loading force, loading rate and unloading rate on the viscoelastic behavior of three representative polymers: poly(methyl methacrylate) (PMMA, amorphous polymer), polyvinylidene fluoride (PVDF, semicrystalline polymer), and epoxy (crosslinked polymer) have been investigated using nanoindentation. The results showed that the maximum indentation depth increased with the increase of loading force, and the relationship between loading force and depth became linear when the loading force is beyond 3000 μN. At the beginning, the plasticity index changed substantially with the increase of loading force, and after reaching a critical loading force, the plasticity index almost remained constant. The maximum indentation depth decreased with the increase of loading rate, which followed a power law curve. With the increase of loading rate, a plasticization phenomenon happened, and a possible reason is that the heat may accumulate and raise the local temperature. The plasticity index initially followed the power law with the increase of unloading rate and then almost remained constant. A constant, the change rate of viscoelastic properties with the unloading rate, for the three representative polymers studied in this research, around −0.033, has been obtained, which may be another manifestation of the phenomenon that many polymers have similar time/temperature shifts and that their WLF equation constants are approximately the same. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011