Nanoindentation creep and glass transition temperatures in polymers

The nanoindentation creep behaviour of several different polymers has been investigated. The extent of creep ε is represented by the Chudoba and Richter equation: ε = ε e ln(ε r t + 1), where t is the loading time and ε e and ε 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 ε r was plotted as a function of y = ( T g − T exper ); a single curve was obtained in spite of a large variety of chemical structures of the polymers. The ε r = ε r ( y ) diagram can be divided into three regions according to the chain mobility. At large negative y values, the creep rate is high due to the liquid‐like behaviour. At large positive y values in the glassy region, the creep rate is higher than that in the negative y ‐value region; the creep mechanism is assigned to material brittleness and crack propagation. In the middle y range there is a minimum of ε 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. Copyright © 2007 Society of Chemical Industry