Nanoindentation creep and glass transition temperatures in polymers

Abstract

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 = εeln(εrt + 1), where t is the loading time and εe and εr are material constants. Creep was determined in this way for a variety of polymers at Texper = 301.7 K. Some of the materials studied were far above, some far below and some near their glass transition temperatures Tg. The creep rate εr was plotted as a function of y = (Tg − Texper); a single curve was obtained in spite of a large variety of chemical structures of the polymers. The εr = εr(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 εr. 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

Accuracy in locating glass transitions: aging and gamma sterilization of vulcanized thermoplastic elastomers

“…For instance, TPV-4 shows large decreases in T g after both sterilization and aging, thus significant effects on properties. As another example, nanoindentation creep is at a minimum at T g while going away from the glass transition in either direction increases the creep [41]. The reasons are understandable: far above T g we are in the liquid state while far below T g we have significant brittleness.…”

A study of low temperature mechanical properties and creep behaviour of polypropylene using a new sub-ambient temperature nanoindentation test platform

“…As described in reference [19], reported glass transition temperatures for polypropylene are influenced by a wide range of factors, such as sample tacticity, sample crystallinity and chain length, as well as the measurement technique used. In Dynamic Mechanical Analysis measurements (DMA), the peak in tan delta is typically offset to a few degrees higher than the T g determined from the inflexion point in the storage modulus versus temperature graph [6,30]. It has been suggested previously that, based on previous ambient and elevated temperature measurements on semi-crystalline and amorphous polymers, changes in A/d(t h =0) are correlated with changes in the tan delta peak [12].…”

“…When the testing temperature is higher than T g , the free volume mechanism of creep was implicated as there is more free volume available for molecular motions [6,45,46]. At lower temperatures (e.g.…”

“…Normally, these tests are carried out under ambient conditions that may not be close to the actual working conditions for many applications. For example, temperature can play an important role in materials properties; indeed, many mechanical properties are strongly temperature-dependent, for instance a system can operate safely at one temperature and fail when the temperature is changed [6][7][8][9]. To address this problem, several approaches have been used to probe the surface properties of materials under their actual service conditions.…”

Poly(methyl acrylate) plus Mesoporous Silica Nanohybrids: Mechanical and Thermophysical Properties

“…Among PBMs with reinforcement, nanohybrids seem interesting where the filler or reinforcing phase of the polymer consists of inorganic particles with dimensions < 100 nm; the resulting PBM escapes the traditional classification of materials into inorganic and organic [13]. Given the growing popularity of nanoscale materials, techniques are being developed by which structures at the nm level can be evaluated, such as nanoindentation [14,15].…”