Broadband nanoindentation of glassy polymers: Part II. Viscoplasticity

Abstract

The relationship between hardness and flow stress in glassy polymers is examined. Materials studied include poly(methylmethacrylate), polystyrene, and polycarbonate. Properties are strongly rate dependent, so broadband nanoindentation creep (BNC) is used to measure hardness across a broad range of indentation strain rates (10 À4 to 10 s À1 ). Molybdenum (Mo) is also studied to serve as a "control" whose rate-dependent hardness properties have been measured previously and whose flow stress, unlike the polymers, is pressure insensitive. The BNC hardness data are converted to uniaxial flow stress using two methods based on the usual Tabor-Marsh-Johnson correlation. With both methods the resulting BNC-derived uniaxial flow stress data agree closely with literature compression uniaxial flow stress data for all materials. For the polymers, the BNC hardness data depend on initial rate of loading, indicating that the measured properties are path dependent. Path dependence is not detected in Mo.

Spontaneous Phase Transformation and Exfoliation of Rectangular Single-Crystal Zinc Hydroxy Dodecylsulfate Nanomembranes

“…Using the standard Oliver-Pharr analysis on the final unloading segment, a 3.2 GPa hardness is estimated for the ZnO nanosheet, which is within the range of previous Berkovich nanoindentation of bulk ZnO single crystals . ZHDS nanomembranes are much softer with a 0.2 GPa hardness, more typical of a structural polymer such as polycarbonate …”

“…39 ZHDS nanomembranes are much softer with a 0.2 GPa hardness, more typical of a structural polymer such as polycarbonate. 40 Exfoliation and Dehydration of ZHDS Sheets to ZnO Nanosheets. Removing the dodecylsulfate ions from the ZHDS sheets and preparing ZnO nanosheets is necessary to fulfill the full potential of this 2-D nanostructure.…”

Room-Temperature Creep Behavior and Activation Volume of Dislocation Nucleation in a LiTaO3 Single Crystal by Nanoindentation

“…The strain rate rapidly decreased from 10 −3 s −1 to 10 −5 s −1 in the transient stage and tended to be stable about 3.5 × 10 −5 s −1 in the steady-state stage. As it was indicated that strain rate under nanoindentation was one order of magnitude lower than that in conventional tension [22]. Thus, the localized brittle deformation could be suppressed under low strain rate (10 −4 s −1 –10 −6 s −1 ) during creep flow.…”

Thermomechanical Properties of Lignin-Based Electrospun Nanofibers and Films Reinforced with Cellulose Nanocrystals: A Dynamic Mechanical and Nanoindentation Study

“…Using the same experimental methods, no H indentation size effect was observed in the glassy polymers poly(methyl methacrylate), polystyrene, and polycarbonate over a similar range of indent sizes. 35 The magnitude of the indentation size effect, as indicated by the slope of a straight line fit to the H data in Figure 7, dH/dA 0 1/2 , is also included in Table 2. For comparison, the average H assessed at A 0 1/2 values from 3 to 5 μm was used (Table 2).…”

“…Nanoindentation has been used to study nanofibers, − biological materials, , polymer composites, − nanocellulose, − and composite membranes consisting of poly(vinyl alcohol) and CNCs . Despite the common use of nanoindentation to study polymer-based materials, proper experimental protocols and the meaning of E s NI and H are still debated in polymer science. − Standard nanoindentation protocols (i.e., the Oliver–Pharr method) were developed for ceramic and metal materials that exhibit behavior different from that of polymers. Issues surrounding the assessment of contact area and viscoplasticity during unloading, the so-called “nose” effect, were recently addressed for polymers by Jakes and co-workers .…”