Polymers containing metastable-state photoacids
Florida Institute of Technology
Metastable-state photoacid (mPAH) can be used to reversibly alter proton concentration with visible light. Most of its applications reported in the past years used solutions of mPAHs. For the development of photoresponsive material, solid polymers containing mPAHs are often desirable. We studied the behaviours of merocyanine-type and TCF-type mPAHs in polymer films. Results showed that low dark acidity and fast reverse reaction are important for an mPAH to be used in solid polymer materials and the polymer media needs to allow efficient proton transfer. Photochromic films with different colour changes were developed by loading mPAHs and acidochromic dyes into polymer films. Local pH pulses in PBS buffer were demonstrated by irradiating a hydrogel or micro-meter polymer thin film containing an mPAH with visible light. Synthesis of polymers grafted with mPAHs will also be presented.
HIERARCHICAL FRACTAL ASSEMBLIES FORMED BY POLY(ETHYLENE OXIDE-b-LYSINE-b-LEUCINE)
Craig A. Machado, Kyle C. Bentz, Daniel A. Savin*
George & Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science & Engineering, Department of Chemistry, University of Florida, Gainesville, FL 32611, USA.
Poly(ethylene oxide-b-lysine-b-leucine) was synthesized via ring-opening polymerization of N-carboxyanhydrides using amine-terminated poly(ethylene oxide) as a macroinitiator, with polymer blocks produced by sequential monomer addition. Infrared spectroscopy indicates the block polymer forms an -helix in the solid state. In solution, this polymer self-assembled into spherical and wormlike micelles with hydrodynamic radii of approximately 90 nm and radii of gyration between 220 and 310 nm in water at concentrations ranging from 0.05 to 0.20 wt% and pH values between 2 and 6.5. Upon preparation of transmission electron microscopy grids, the micelles underwent a hierarchical assembly to produce fractal assemblies. Zeta potential measurements show a slight decrease in surface charge with increasing pH, which results in more densely packed fractals due to decreased electrostatic repulsions. This system provides a facile route to nanostructured surfaces which can be used for applications such as modulating cell adhesion or promoting growth of neurons.
Acid-Degradable Segmented Hyperbranched Polymers by RAFT Polymerization of Imine-Containing Crosslinkers
Michael B. Sims, Kush Y. Patel, Mallika Bhatta, Soma Mukherjee, Brent S. Sumerlin
George & Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science & Engineering, Department of Chemistry, University of Florida, PO Box 117200, Gainesville, FL 32611-7200, USA.
The degradation of higher-order polymeric architectures is an important environmental and biological consideration for practical applications of these systems. Both the variety of pH-sensitive functional groups and the ubiquity of pH gradients in nature make the incorporation of acid- and base-reversible linkages an effective method for facilitating this degradation. Imines are particularly attractive candidates due to their facile preparation and tunable acid stability depending on the identity of the N-substituent. To this end, segmented hyperbranched polymers containing acid-labile imine crosslinks were synthesized by reversible addition-fragmentation chain transfer polymerization of three novel divinyl compounds containing oxime, semicarbazone, and acyl hydrazone moieties. The branched polymers degraded into well-defined linear polymers under aqueous acidic conditions, and the rate of degradation depended on the hydrolytic stability of the imine crosslinks. Furthermore, precise control over the degradation rate was achieved through the incorporation of different imines at various ratios into the same branched polymer.
Structure and Dynamics of Thermo-reversible Colloidal Gels near the Gel Boundary
Divya Bahadur1, Qingteng Zhang2, Eric Dufresne2, Piotr Maj3, Suresh Narayanan2, Subramanian Ramakrishnan1, Alec Sandy2
1 Department of Chemical and Biomedical Engineering, FAMU - FSU College of Engineering, Tallahassee, FL
2 Advanced Photon Source, Argonne National Laboratory, Lemont, IL
3 AGH University of Science and Technology, Kraków, Poland
We examined gel formation and dissolution processes for thermo-reversible colloidal gels by studying the structure, dynamics and mechanical properties of the gel as it traverses the gel boundary via x-ray photon correlation spectroscopy (XPCS) and rheology. Octadecyl silica particles in decalin (f = 0.2) at three different particle sizes (56 nm, 82 nm and 110 nm) were made to reversibly gel upon cooling. Gels were formed by rapidly quenching the system at a range of temperatures close to the gel point (ΔT ~ 2K) and the dynamics and mechanical properties were probed as they evolved in time. A slow heating ramp from a well formed gel state was carried out to identify the gel transition temperature. The transition is characterized by a slowing down of dynamics upon gel formation in the intermediate scattering function, emergence of low q up - turns in the structure factor as well as a rapid increase in the modulus a function of quench depth and aging time while the inverse is observed upon melting. The transition times and temperatures observed via XPCS are in good agreement with rheological measurements. Similar length and time scales emerge upon both gel formation and melting which were scaled using universal scaling criteria suggested by the mode coupling theories.