Responsive polymers for dynamic modulation of bio-macromolecular transport properties

Abstract

Responsive self-assembling polymers are used in wide range of applications in the food, pharmaceutical, agricultural, electronic and environmental industries, as well as in the biomedical field. The proper design of such polymers is critical for the particular applications being considered. In this thesis, different matrices that can be modulated dynamically by the application of appropriate stimuli were designed and used for two applications: electrophoretic separation and gene transfection. Light represents an attractive trigger to change the properties of a polymer solution because it enables structural transitions to be induced under isothermal conditions without the addition of other chemical species to the solution, and is externally reversible and hence amenable to device design and automation. Amphiphilic copolymers with azobenzene moieties are of interest because the azobenzene can undergo reversible trans-cis photoisomerization leading to conformational isomers with significantly dissimilar dipole moments and hydrophobicities and thus different propensities to aggregate into nanoscale structures in aqueous media. Copolymers of 4methacryloyloxyazobenzene (MOAB) and N,N-dimethylacrylamide aggregate strongly in aqueous solutions with concentration-dependent aggregate size distributions and welldefined boundaries between the dilute and semi-dilute regimes. The copolymers are strongly surface active, an uncommon observation for random copolymers, and exhibit pronounced photoviscosity effects at higher concentrations. Trans-to-cis isomerization under UV light leads to partial dissociation of the azobenzene aggregates that form physical crosslinks, thereby significantly affecting the polymer solution rheology, with a consequent ten-fold loss of viscoelasticity upon irradiation, especially in concentrated polymer solutions. Photo-responsive poly(N,N-dimethylacrylamide-co-methacryloyloxyazobenzene) (MOAB-DMA) and temperature-responsive Pluronic F127 (PF127) copolymers were blended to obtain mixed micellar systems that were responsive to both stimuli.

(cont.) The azobenzene groups of DMA-MOAB in the trans conformation self-associate and the interactions with PF127 are less pronounced when compared to those with cis conformation of the azobenzene groups. The cis- isomer of the MOAB-DMA copolymer self-associates less strongly than does the trans conformation, and thus the copolymer micelles dissociate upon UV irradiation. These polymeric unimers can form mixed micelles with the PF127 present. This causes the sol-gel transition temperature of the MOAB-DMA/PF127 blend to be 2-6 degrees lower upon UV irradiation than under dark conditions depending on the molar ratio of the two polymers. It has been found that aqueous blends of PF127 (20 wt%) and DMA-MOAB (5 wt%) possess a low viscosity at room temperature when equilibrated in the dark and undergo a sol-gel transition when UV irradiated, with a 1000-fold viscosity increase. Such a transition strongly alters the transport properties of solutes such as proteins, DNA and the like within the polymer solutions. The electrophoretic mobility of proteins was measured in both the sol and the gel state obtained in dark and under UV irradiation, respectively. A fifty to seventy percent decrease in the electrophoretic mobility was obtained by UV irradiation responsive gelation, depending on the size of the protein molecules. Dynamic changes in the aggregation behavior of photoresponsive polymers enable novel opportunities for the control of viscosity and transport properties for the polymer solutions to be used as a matrix in separation processes. The binary protein mixture separations that were carried out in dynamically modulated electrophoresis experiments were found to have improved resolution as compared to the conventional electrophoresis operations. The improvement in resolution and yield, which depends on three dimensionless parameters, can be predicted using numerical simulations if the operating conditions, electrophoretic mobility of solutes and the change in electrophoretic mobility on gelation are known.

(cont.) A facile, one-step synthesis of cationic block copolymers of poly(2-N (dimethylaminoethyl) methacrylate) (pDMAEMA) and copolymers of poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO) has been developed. The PEO-PPO-PEOpDMAEMA (L92-pDMAEMA) and PEO-pDMAEMA copolymers were obtained via free radical polymerization of DMAEMA initiated by polyether radicals generated by cerium(IV). Over 95% of the copolymer fraction was of molecular mass ranging from 6.9 to 7.1 kDa in size, indicating the prevalence of the polyether-monoradical initiation mechanism. The L92-pDMAEMA copolymers possess parent surfactant-like surface activity. In contrast, the PEO-pDMAEMA copolymers lack significant surface activity. Both copolymers can complex with DNA. Hydrodynamic radii of the complexes of the L92-pDMAEMA and PEO-pDMAEMA with plasmid DNA ranged in size from 60 to 400 nm, depending on the copolymer/DNA ratio. Addition of Pluronic P123 to the L92pDMAEMA complexes with DNA masked charges and decreased the tendency of the complex to aggregate, even at stoichiometric polycation/DNA ratios. The transfection efficiency of the L92-pDMAEMA copolymer was by far greater than that of the PEOpDMAEMA copolymer. An extra added Pluronic P123 further increased the transfecton efficacy of L92-pDMAEMA, but did not affect that of PEO-pDMAEMA.