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

Abstract

Mechanical properties and creep behaviour of an atactic-polypropylene (aPP) have been studied in the vicinity of its glass transition temperature (-18ºC) via a nanoindentation platform integrated with a sub-ambient temperature capability. All low temperature tests were validated by measurements on a fused silica reference sample from 25ºC to -30ºC. The fused silica results showed virtually invariant elastic modulus with temperature over this range consistent with literature measurements by sonic resonance. Hardness and elastic modulus of aPP increased as the test temperature decreased and the amorphous regions went through the glass transition. The creep behaviour was analysed using two approaches: (i) a logarithmic method, and (ii) the Boltzmann integral method. The results showed that the creep extent decreased as the temperature was reduced, and for the time constants obtained there were upper-limit values at -10ºC, about 8ºC above the quoted glass transition temperature. The strain rate sensitivity obtained by the logarithmic method also showed a maximum at -10ºC.

Advanced Nanomechanical Test Techniques

“…Using dual cooling control Chen et al reported the development of a prototype cold stage accessory for the NanoTest demonstrating its ability to investigate the local mechanical properties and the creep behavior of atactic polypropylene down to -30ºC [135]. The cooling system used incorporated a purging chamber to eliminate condensation during cooling and two Peltier coolers to achieve the isothermal contact.…”

Probing polymer chain constraint and synergistic effects in nylon 6-clay nanocomposites and nylon 6-silica flake sub-micro composites with nanomechanics

“…Equation 2 has proved successful in interpreting loading rate effects and in determination of the strain rate sensitivity, a dimensionless creep parameter that exhibits some correlation with tan delta, 20,28,29 and in determination of glass transition temperatures occurring below 0 °C. 19 …”

“…19,29 The experimental creep depth data are closely fitted by the logarithmic Equation 2 enabling subtle changes in behavior to be determined. Tests at room and elevated temperatures on a range of amorphous to the improved initial creep compliance, and constraint did not occur.…”

“…sliding wear) than bulk tensile testing. In addition to the determination of mechanical properties, such as hardness (H) and elastic modulus (E), it is possible to assess time-dependent behavior 14,[19][20][21] and the response of the composites to high-strain rate impact and repetitive contact so that fatigue can be directly studied at the nanoscale. 22 In the majority of studies 13, there has been a focus on reporting H and E with the time-dependent behavior being an unwanted complication.…”

“…22 In the majority of studies 13, there has been a focus on reporting H and E with the time-dependent behavior being an unwanted complication. However, with suitable experimental design 14,[19][20][21] the creep behavior of the polymeric materials can be investigated at the same time as the determination of hardness and elastic modulus.…”