Ph.D. thesis (pdf) - dirac

196 Details on the samples
electric setup in Orsay (see section 5.1.1) because the alpha relaxation does not enter
the experimental window of frequencies at lower than 1 MHz within the pressure
and temperature range accessible. By using liquid nitrogen we did however obtain
a few spectra at 300 MPa in the temperature range 180 K - 195 K. The results are
consistent with the analysis above which is illustrated in figure A.1.
A.2 PIB
Polyisobutylene (PIB) is a polymer, with a unit consisting of a backbone of carbons
with two methyl groups on every second carbon (-CH 2 -C(CH3) 2 -) n
n
PIB does as other polymers have molecular weight dependent thermodynamic and
dynamics properties. In this work we consider 4 different samples PIB680, PIB1190,
PIB3580 and PIB500k. PIB680 (Mw=680 g/mol, Mw/Mn=1.06) PIB3580 (Mw=3580
g/mol, Mw/Mn=1.23) are from Polymer Standard service, PIB500k (Mw=500.000
g/mol, Mw/Mn=2.5) is from Sigma Aldrich while the sample PIB1190 (Mw=1190
g/mol, Mw/Mn=1.08) is supplied by Alexei Sokolov. The polydispersity is higher
for PIB500k because it is more difficult to separate and characterize high molecular
weight polymers. We believe that this is of minor importance, because the molecular
weight dependence of different properties saturates at high molecular weight,
meaning that a precise characterization is less important.
The density at ambient conditions increases weakly as a function of molecular weight.
The high molecular weight PIB has a density of 0.92 g/mL at room temperature
and atmospheric pressure.
The T g increases with about 10% from the lowest to the highest molecular weight,
with the Mw-dependence saturating close to 10000 g/mol where it reaches a value
of T g ≈ 205 K [Ding et al., 2004; Chauty-Cailliaux, 2003].
PIB has a low dipole moment which makes it inadequate to study it by dielectric
spectroscopy.
The fragility of high molecular weight PIB is reported by Plazek and Ngai [1991] to
be m = 46 based on shear mechanical creep data.