Page 159 - Photoreactive Organic Thin Films

ZOUHEIR SEKKAT AND WOLFGANG KNOLL

of the effect of pressure on water as a pressure medium were performed
without polymer film, and the refractive index of water, n water) increased at
pressure. n water is known for each pressure value and taken into account for
the determination of the optical constants of the polymer film. The refractive
index of water is considerably smaller than that of PMMA-DR1 at all
pressures (for example at 100 MPa, «SS" = 1.345±0.003 and WPMMA-DRI =
1.630±0.003). The increase of n PMMA_ DR1 up to 1.636 at 150 MPa (vide infra]
cannot be due to the absorption of water by the polymer, otherwise n PMMA_ DK
should decrease rather than increase. Even though water molecules are smalL
they do not penetrate into the polymer film studied. Figure 4.24 shows that
most photo-orientation is suppressed at 150 MPa. Assuming that pressure
does not affect noticeably the lateral dimensions of the films, the thickness
variation mentioned previously implies a - 2.4% volume change at that
pressure value. A volume fraction, i.e., free volume, is necessary for the
isomeric and reorientational movement of most of the azo chromophores in
PMMA-DR1. This near 2.4% volume change is due to a change in density
that couples to a change in the refractive index of the material. It can be
rationalized by the following Clausius-Mosotti equation: 68

2
2
M- (" -l(" + 2) Ap (4J)
where «, and p are the isotropic refractive index and the density of the
material, respectively, and An and Ap are the corresponding changes induced
§
by pressure. We found experimentally that «, i.e., «PMMA-D#IJ i equal to
1.636 at 150 MPa for a 633 -nm probe light and that consequently Equation
4.1 predicts a An value of 0.019 for a 2.4% change in density, whereas the
experimentally measured value for An at 150 MPa is ~ 0.018. This value is
2
obtained by multiplying the slope, i.e., ~ 0.012*10~ /MPa, of An versus pressure
by 150 MPa. The Clausius-Mosotti equation well supports the claimed ~ 2,4%
change in the sample's density at 150 MPa; it also supports the arguments we
put forth concerning free-volume reduction by pressure. The free volume size
necessary for one DR1 molecule isomerization in PMMA-DR1 is discussed next.
The gradual reduction of DR1 photo-orientation with increased pressure
implies a distribution of local free- volume elements of different sizes available
to the trans isomers in PMMA-DR1, a concept that has theoretical
49 69 70
support, ' ' and that is experimentally observed and discussed in the
13 19
49
literature for photoisomerization in polymers and poled PMMA-DR1. '
Photo-orientation in the glassy state requires a minimum, critical size of local
free volume in the vicinity of the chromophore — the photo-orientation activation
volume, i.e., the volume swept by the chromophore during photoisomerization
71
and photo-orientation geometrical rearrangement (vide infra). During the
early time evolution, the cis concentration is negligible, and the observed
anisotropy is dictated by the trans orientation. In fact, the rate k, i.e., the
slope, of the early time photo-orientation is proportional to the quantum
yield of the trans— >cis photoisomerization, which, as far as friction effects are
concerned, is the only material parameter that can be pressure dependent in
72
k. Reaction rates at high pressure are theoretically and experimentally
63 66
rationalized by: "