polymer melts (Fig. 7). This observation was carried out on several ordinary
polymers (polyacrylates, polymethacrylates, polybutadienes, etc.) tested from
smaller to higher molecular weights (unentangled and entangled) up to high
macroscopic scales (up to 0.5 mm gap thickness) and at about 100
C away from
the glass transition and is in agreement with piezorheometer measurements carried
at lower thicknesses (Derjaguin 1983; Derjaguin et al. 1989; Badmaev et al. 1983;
Collin 2002).
From Surface-Induced Solidification to the Identification
of the Submillimeter Shear Elasticity
Low-frequency elastic behaviors have been reported at several occasions. Since
1991, Granick and coworkers (Hu et al. 1991; Demirel and Granick 2001; Zhu and
Granick 2004) measured, at the molecular scale or a multiple of that, a solid-like
component at nanoscales by SFA (Surface Force Apparatus) using mica surfaces.
Close to a surface, these results were generally interpreted as resulting from
surface-induced effects. The disappearance of the solid-like response at larger
thicknesses is usually interpreted by a transition from surface to bulk properties.
However the Mica (muscovite) surface does not provide strong liquid-substrate
interactions (partial wetting) with organic fluids. Such boundary conditions are
similar to those for conventional viscoelastic measurements. The very first analysis
in terms of physical property is due to Derjaguin (Badmaev et al. 1983; Derjaguin
et al. 1989) revealing in different fluids including polymers and liquid water, an
elastic response at the micron scale that is interpreted as an intrinsic property
generated by intermolecular interactions. At a larger scale, Collin et al. reported,
using treated glass surfaces and small strains delivered by a piezorheometer, on a
gel-like response up to 50 μm thicknesses in low molecular weight polystyrene
melts that was interpreted as a reminiscence of the glass transition, i.e., clusters of
finite size (Collin and Martinoty 2002). Since 2006 the use of optimized fluid/
substrate wetting conditions has facilitated the measurement of the low shear
elasticity, generalizing its identification to ordinary liquids and confirming the
probable origin due to intermolecular forces. The wetting protocol opens an easier
access to the solid-like response usually hidden in conventional measurements of
viscoelastic fluids.
Figure 8 illustrates the response obtained using full wetting conditions for an
H-bond oligomer poly(propylene glycol) (PPG4000) at room temperature. The flow
behavior obtained conventionally on metallic substrate is here replaced by an elastic
response (G
0 and G
00 nearly independent of the frequency and with G
0
> > G
00 ). The
signal analysis indicates that the shear stress wave is superposed to the strain wave,
confirming the instant transmission of the stress characterizing a solid-like behavior.
This result is coherent with the conclusions in terms of elastic contribution carried
out on the same liquid on the basis of the dynamic of the capillary waves (Chushkin
et al. 2008).
260
L. Noirez
polymers (polyacrylates, polymethacrylates, polybutadienes, etc.) tested from
smaller to higher molecular weights (unentangled and entangled) up to high
macroscopic scales (up to 0.5 mm gap thickness) and at about 100
C away from
the glass transition and is in agreement with piezorheometer measurements carried
at lower thicknesses (Derjaguin 1983; Derjaguin et al. 1989; Badmaev et al. 1983;
Collin 2002).
From Surface-Induced Solidification to the Identification
of the Submillimeter Shear Elasticity
Low-frequency elastic behaviors have been reported at several occasions. Since
1991, Granick and coworkers (Hu et al. 1991; Demirel and Granick 2001; Zhu and
Granick 2004) measured, at the molecular scale or a multiple of that, a solid-like
component at nanoscales by SFA (Surface Force Apparatus) using mica surfaces.
Close to a surface, these results were generally interpreted as resulting from
surface-induced effects. The disappearance of the solid-like response at larger
thicknesses is usually interpreted by a transition from surface to bulk properties.
However the Mica (muscovite) surface does not provide strong liquid-substrate
interactions (partial wetting) with organic fluids. Such boundary conditions are
similar to those for conventional viscoelastic measurements. The very first analysis
in terms of physical property is due to Derjaguin (Badmaev et al. 1983; Derjaguin
et al. 1989) revealing in different fluids including polymers and liquid water, an
elastic response at the micron scale that is interpreted as an intrinsic property
generated by intermolecular interactions. At a larger scale, Collin et al. reported,
using treated glass surfaces and small strains delivered by a piezorheometer, on a
gel-like response up to 50 μm thicknesses in low molecular weight polystyrene
melts that was interpreted as a reminiscence of the glass transition, i.e., clusters of
finite size (Collin and Martinoty 2002). Since 2006 the use of optimized fluid/
substrate wetting conditions has facilitated the measurement of the low shear
elasticity, generalizing its identification to ordinary liquids and confirming the
probable origin due to intermolecular forces. The wetting protocol opens an easier
access to the solid-like response usually hidden in conventional measurements of
viscoelastic fluids.
Figure 8 illustrates the response obtained using full wetting conditions for an
H-bond oligomer poly(propylene glycol) (PPG4000) at room temperature. The flow
behavior obtained conventionally on metallic substrate is here replaced by an elastic
response (G
0 and G
00 nearly independent of the frequency and with G
0
> > G
00 ). The
signal analysis indicates that the shear stress wave is superposed to the strain wave,
confirming the instant transmission of the stress characterizing a solid-like behavior.
This result is coherent with the conclusions in terms of elastic contribution carried
out on the same liquid on the basis of the dynamic of the capillary waves (Chushkin
et al. 2008).
260
L. Noirez