Macromolecules, Vol. 38, No. 23, 2005
Lamellar Liquid Single Crystal Hydrogels 9775
crude product of the preceding reaction was dissolved in
ethanol. 3 g of pyridinium p-toluenesulfonic acid was added.
The pH of the solution was 3. The mixture was refluxed for 3
h, and then the ethanol was removed in a vacuum. The crude
product was dissolved in chloroform and washed with water.
After drying with MgSO4 and removal of the solvent, the
product was used in the next step without further purification.
4-(2-{2-[2-(2-Hydroxyethoxy)ethoxy]ethoxy}ethoxy)benzoic acid
(7) was obtained by deprotection of the carbonic acid. The crude
product of the preceding reaction was dissolved in 600 mL of
a water/methanol mixture (2:1). 10 g (0.41 mol) of lithium
hydroxide was added. The mixture was refluxed for 3 h. The
methanol was removed by vacuum distillation. Hydrochloric
acid was added, and the product was extracted with chloro-
form. The solution was dried with MgSO4 and the chloroform
removed. The product was purified by multiple recrystalliza-
tion from ethyl acetate/cyclohexane (1:1) [yield: 10 g of a
colorless wax, 25% with respect to 4 after three reaction steps].
1H NMR (300 MHz, CDCl3): δ [ppm] ) 3.7 (t, 3 H, CH2-
CH2-OH), 3.8 (m, 10 H, glycol), 4.0 (t, 2 H, COO-CH2-CH2),
4.2 (t, 2 H, COO-CH2), 7.0 (d, 2 H, Ar-H), 8.1 (d, 2 H, Ar-
H).
4-[2-(2-{2-[2-(2-Methylacryloyloxy)ethoxy]ethoxy}ethoxy)ethoxy]-
benzoic acid (8) was prepared by functionalization of 7 with
methacrylic acid. A mixture of 7.5 g (0.024 mol) of 7, 10 g (0.12
mol) of methacrylic acid, and 0.2 g of p-toluenesulfonic acid in
chloroform was refluxed in a Dean-Stark apparatus for 72 h.
The chloroform and the excess methacrylic acid were removed
by vacuum distillation. The crude product was purified by
column chromatography (silica gel F60, ethyl acetate) [yield:
6.3 g (68%) of a colorless wax].
types of samples were aligned in an NMR magnet (11 T for
the spherical samples and 7 T for the stripes) to obtain a
uniform director orientation. For the stripes of length z and
width x the layer normal (director) is parallel to z. The lamellar
phase (LR) was reached by slowly heating or cooling from the
isotropic phases (L1 or L2, cf. Figure 1) and subsequent
annealing in the biphasic region. The process of orientation
was followed by 2H NMR. At the starting temperature a
narrow single peak is observed in the isotropic phase. In the
two-phase region the quadrupolar doublet grows at the
expense of the central isotropic peak. The lamellar phase
shows a pure doublet of narrow lines. Within the highly viscous
mesophase, the orientation of the molecules is stable, and the
sample can be removed from the magnetic field for the
polymerization, which was carried out in a Kulzer Dentacolor
XS UV lamp equipped with two xenon lamps. The irradiation
wavelength was 320-520 nm. The exposure time was 120 s.
A home-built cooling device which uses a liquid nitrogen cooled
gas stream allows us to control the temperature within the
polymerization chamber of the UV-lamp. The temperature was
kept at 293 K during the polymerization process.
The stripe-shaped hydrogels were stored in the swollen state
in bidistillated water. The stripes are stable against slight
mechanical deformation in all directions of space. The water
can be removed from the stripes without damaging them. If
they are penetrated with water again, they regain their initial
appearance. The dry elastomers exhibit common rubber
elasticity.
Measurements. X-ray Measurements. The hydrogel samples
were placed in water in a 2 mm diameter glass capillary.
Diffraction studies were performed with a rotating anode
generator (6.4 kW) in a point-collimated Kiessig camera with
Ni-filtered Cu KR radiation (wavelength λ ) 1.542 Å) at a
distance of 180 mm. Two-dimensional patterns were recorded
with an image plate system (Schneider, Freiburg).
1H NMR (300 MHz, CDCl3): δ [ppm] ) 1.9 (s, 3 H, CH2d
C(CH3)-COO), 3.6 (m, 10 H, glycol), 3.9 (t, 2 H, COO-CH2-
CH2), 4.2 (t, 2 H, COO-CH2), 4.3 (t, 3 H, CH2-CH2-OOC-
C(CH3)dCH2), 5.5 (s, 1 H, CH2dC(CH3)-COO), 6.1 (s, 1 H,
CH2dC(CH3)-COO), 6.9 (d, 2 H, Ar-H), 8.0 (d, 2 H, Ar-H).
2,5-Dihydroxybenzoic acid 2-(2-methyl-acryloyloxy)ethyl ester
(3) was prepared using the procedure described by Amigo´-
Melchior.3
2
NMR Measurements. H NMR spectra were recorded on a
Bruker Avance 500 spectrometer (11.7 T) at a frequency of
76.773 MHz. Orientation-dependent measurements were per-
formed using a goniometer probe with a 5 mm solenoid coil.
The sample can be rotated about an axis perpendicular to the
external magnetic field. Orientation of the monodomains used
for hygroelastic measurements were performed on a Bruker
MSL spectrometer (7 T), operating at a frequency of 46.073
MHz. The polymerization mold was placed in a 15 mm saddle
coil. The sample temperature was kept constant within 1 K
by a Eurotherm thermostat.
2,5-Di-(4-[2-(2-{2-[2-(2-Methylacryloyloxy)ethoxy]ethoxy}-
ethoxy)ethoxy]benzoic acid ester)benzoic acid 2-(2-methyl-acry-
loyloxy)ethyl ester (2) was obtained by linking 8 and 3. All
substances were vacuum-dried. 2.5 g (0.0065 mol) of 8, 0.9 g
(0.0033 mol) of 3, and 80 mg of p-(dimethylamino)pyridine
were dissolved in dry methylene chloride. The mixture was
cooled in an ice bath. 1.3 g (0.0065 mol) of N,N′-dicyclohexyl-
carbodiimide (DCC) was added. The mixture was allowed to
warm to room temperature and stirred for 3 h. The precipitate
which was formed during the reaction was filtered off. The
solution was washed with HCl solution and sodium bicarbon-
ate solution consecutively. The solution was dried with MgSO4
and the solvent removed by distillation. The cross-linker was
purified by column chromatography (Sephadex LH 20, metha-
nol). To prevent spontaneous polymerization, the substance
was stored in cryo-tubes using liquid nitrogen as cooling
medium [yield: 1 g (30%) of a colorless, viscous liquid].
1H NMR (300 MHz, CDCl3): δ [ppm] ) 2.1 (s, 9 H, CH2d
C(CH3)-COO), 3.9 (m, 20 H, glycol), 4.1 (t, 4 H, Ar-O-CH2-
CH2), 4.35 (m, 6 H, Ar-COO-CH2-CH2 and Ar-O-CH2-
CH2), 4.5 (t, 4 H, CH2dC(CH3)-COO-CH2-glycol), 4.6 (t, 2 H,
CH2d(CH3)-COO-CH2-CH2-OOC-Ar), 5.7 (s, 3 H, CH2d
C(CH3)-COO), 6.3 (s, 3 H, CH2dC(CH3)-COO), 7.1 (dd, 2 H,
Ar-H), 7.4 (d, 1 H, Ar-H), 7.6 (dd, 1 H, Ar-H), 8.05 (d, 1 H,
Ar-H), 8.3 (d, 2 H, Ar-H).
Synthesis of LSCH Samples. To produce the hydrogels
monomer, cross-linker, deuterium oxide, and photoinitiator
(Ciba Irgacure 2959 and Ciba Darocure 1173 1:1) were mixed
in a Teflon capsule using a Perkin-Elmer vibrating mill. Two
different types of samples were prepared. For the NMR
measurements the thoroughly homogenized mixture was filled
through a capillary of 0.5 mm inner diameter into a small
sphere (4 mm diameter) of quartz glass. The sample was then
sealed by melting of the capillary. For all other investigations
the mixture was filled in a Teflon mold of a stripe form (20
mm × 5 mm × 0.4 mm), covered by a quartz glass plate. Both
Pulsed Gradient Diffusion NMR. Pulsed-field gradient
measurements of the self-diffusion coefficient D were per-
formed in a Bruker high-resolution probe with a singly tuned
saddle coil (∼5 µs 90° pulses) using the pulsed gradient
stimulated-echo technique (PGSTE). The standard method of
Steijskal and Tanner23,24 and its basic theory are extensively
covered in the literature.25 A matched pair of trapezoidal
ramped gradient pulses, separated by the diffusion time ∆,
was inserted in the evolution delays t2 of the stimulated echo
sequence 90°-t2-90°-t1-90°-t2-aq. The attenuation of the
spin-echo intensity due to diffusion is given by
S
ln
) -4π2q2D(∆ - δ/3)
(1)
(S )
0
where a correction for the finite duration of the gradient
pulses, δ, is included. S0 is the signal intensity with gradient
amplitudes of zero. The “wave vector” q ) γδg/(2π) further
depends on the magnetogyric ratio γ and the gradient ampli-
tude g. During one diffusion experiment all delays are kept
constant, and the gradient amplitude g is incremented.
Gradients were calibrated to the known diffusion coefficient
of D2O.26 Diffusion parallel to the layer normal was measured
with the Bruker Diff30 gradient system (which has a stronger
gradient), whereas diffusion perpendicular was measured with
the Bruker Micro5 imaging system with three orthogonal
gradient coils.