Photoresponsive , Amphiphilic Copolymers of Azobenzene and N,N-Dimethylacrylamide
Smeet Deshmukh, Lev Bromberg, Kenneth A. Smith, and T. Alan Hatton. Chemical Engineering, MIT, 25 Ames Street, Cambridge, MA 02139
The present study was motivated by our interest in the design and synthesis of polymers that undergo significant photosensitive conformational and structural changes upon irradiation with light of different wavelengths for potential use as matrices in separation processes, in particular in capillary and gel electrophoresis. Photoinduced changes in polymer conformation are very attractive as triggers for modulating the separation medium or gel properties as they are not affected by any of the external mass transfer limitations associated with pH-, redox-, and other chemical triggers. Another perceived advantage of these gels is the potential to establish sharp, spatially well-defined regions of modified gel properties, which may have potential benefits for the dynamic control and modulation of the separation processes; it would be difficult to sustain such spatially-resolved structural features using temperature-responsive polymers owing to dissipative heat transfer effects and the inevitable establishment of temperature gradients between the heated and non-heated, or cooled, regions within the gel. Copolymers of 4-methacryloyloxyazobenzene and N,N-dimethylacrylamide (MOAB-DMA) hence synthesized were found to aggregate strongly in aqueous solution to give concentration-dependent aggregate size distributions and well-defined boundaries between the dilute and semi-dilute regimes, determined by dynamic light scattering, surface tension and probe solubilization experiments. The copolymers were strongly surface active, and exhibited pronounced photoviscosity effects at higher concentrations. The concentration dependence of the kinetic parameters for the reversible polymer rearrangement upon photoisomerization, is attributed to steric hindrances to the molecular conformational rearrangement in the constrained environment. Trans-to-cis isomerization under UV light leads to partial dissociation of the azobenzene aggregates that cross-link the polymers, significantly affecting the polymer solution rheology, with a consequent loss of viscoelasticity (~ 10-20 fold) upon irradiation, especially in concentrated polymer solutions. Dynamic changes in the aggregation behavior of polymers enable novel opportunities for the control of viscosity as well as solute transport properties for the potential use as dynamically and spatially tunable electrophoretic matrix.