Nanoindentation creep and glass transition temperatures in polymers

The nanoindentation creep behaviour of several different polymers has been investigated. The extent of creep e is represented by the Chudoba and Richter equation: epsilon = epsilon(e), 1n(epsilon(r)t + 1), where t is the loading time and epsilon(e) and epsilon(r) are material constants. Creep was determined in this way for a variety of polymers at T-exper = 301.7 K. Some of the materials studied were far above, some far below and some near their glass transition temperatures T-g. The creep rate epsilon(r) was plotted as a function of gamma = (T-g - T-exper); a single curve was obtained in spite of a large variety of chemical structures of the polymers. The epsilon(r) = epsilon(r)(gamma) diagram can be divided into three regions according to the chain mobility. At large negative gamma values, the creep rate is high due to the liquid-like behaviour. At large positive gamma values in the glassy region, the creep rate is higher than that in the negative gamma-value region; the creep mechanism is assigned to material brittleness and crack propagation. In the middle gamma range there is a minimum of epsilon(r). These results can be related to glassy and liquid structures represented by Voronoi polyhedra and Delaunay simplices. The latter form clusters; in the glassy material there is a percolative Delaunay cluster of nearly tetrahedral high-density configurations. The creep mechanism here is related to crack propagation in brittle solids. In the liquid state there is a different percolative Delaunay cluster formed by low-density configurations, which, as expected, favour high creep rates. (c) 2007 Society of Chemical Industry