Characterising pH response in phase separated hydrogels using small angle X-ray scattering.

A range of (methyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate)block-
poly(methyl methacrylate) [PMMA-block-PDEA-block-PMMA] and poly(methyl
methacrylate )-block-poly(methacrylic acid)-block-poly(methyl methacrylate) [PMMA-blockPMAA-
block-PMMA] triblock copolymers have been prepared via group transfer and
anionic polymerisation respectively. These hydrophilic-hydrophobic block copolymers
were characterized with respect to their chemical composition and molecular weights by 1 H
and by gel permeation chromatography respectively. Morphological development in
solvent cast films were followed using SAXS by monitoring changes in the structure peak.
[PM MA-block-PMAA-block-PM MA] triblocks were found to be mechanically weak and
therefore not suitable as chemical actuators.
Using SAXS the "static" equilibrium molecular response of phase separated films of
[PMMA-block-PDEA-block-PMMA] triblock copolymers to changes in pH, ionic strength
and salt identity was evaluated. Changes in pH were effected using a controlled set of
buffers, namely citric acid, sodium phosphate and ethanol amine. The ionic strength of the
buffers was fixed using a series of salts from the Hofmeister series, namely NaAce, NaCl,
NaBr, Nal, NaN03 and NaSCN.
At a fixed pH and ionic strength the equilibrium expansion ratio of the polymer was
found to be highly dependent on the identity of the salt. The extent of swelling was
correlated with the surface charge density of the anionic component of the salt. Hydrogels
swollen in solution containing more polar anions were found to have a larger expansion
ratio. This was explained in terms of the water perturbing effect of the ion. The swelling of
the polymer was also monitored as a function of pH. A drastic collapse of the polymer was
observed at a specific pH corresponding to the apparent pKa of the PDEA block. The
apparent pKa of [PMMA-block-PDEA-block-PMMA] copolymers was found to be
.. dependent on the overall molecular weight, where higher molecular weight material had a
lower apparent pKa. Below the apparent pKa the extent of swelling in solutions prepared at
0.1 M ionic strength was found to be dependent on the pH. This has been attributed to the
. complex interplay of citrate species in the buffer. The effect of ionic strength on the
"swelling was also investigated at fixed pH and salt identity using buffer and simple mineral
acid. A modified Donnan theory has been used to interpret the results. SAXS was found to
be a highly sensitive technique for measuring the "static response" of the hydrogels.
The use of SAXS was also investigated as a technique for measuring the swelling
"kinetics" of [PMMA-block-PDEA-block-PMMA] hydrogels. Thin annealed films of hydrogel
were swollen in a range of control buffered solutions. The in-situ nature of the technique
afforded high resolution data. The kinetics of swelling was monitored by following the
change in the structure peak position and its half height peak width, which gave the
expansion ratio and diffusive behavior respectively. Swelling stresses induced in the
samples made measuring the kinetics problematic and this limited the technique.
Finally, to avoid the expense and limited availability of SAXS beam lines a new
technique was used to monitor the response of the hydrogels. A diffraction grating was
imprinted onto the surface of the hydrogel, and "static" and "kinetic" response was
measured by monitoring the change in the diffraction pattern. This technique was found to
offer similar accuracy as SAXS, at a Significantly reduced cost.