Size and pH effect on electrical and conformational behavior of poly(acrylic acid): Simulation and experiment
ABSTRACT Monte Carlo simulations, experimental titrations and fluorescence correlation spectroscopy experiments were used to investigate the conformational and electrical properties of polyacrylic acids (PAA). On the one hand, titration curves were calculated to get an insight into the role of pH on the degree of ionization and conformation of PAA chains. On the other hand, experimental potentiometric titrations of PAA were also achieved for different PAA molecular weights and compared to the calculated titration curves obtained by Monte Carlo coarse grained simulations. It was found that for a large range at intermediate PAA ionizations, a good correlation is obtained between experimental and simulations data thanks to the prominence of electrostatic interactions in this domain. The effect of ionic concentration and PAA molecular weight on the titration curves was also investigated. In order to get a better understanding of PAA conformational behavior, we also investigated PAA diffusion properties in aqueous solutions as a function of pH and ionic strength by fluorescence correlation spectroscopy (FCS), thanks to its high sensitivity to measure diffusion coefficients of tracer solutes. Good qualitative agreements were observed between experimental diffusivities and polymer properties calculated from MC simulations. It was shown that the high molecular weight PAA chains display more significant changes in diffusivity in agreement with the ionization degrees and conformational changes observed in the simulations.
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ABSTRACT: The behavior of pH-responsive polymers poly(N-methacryloyl-L-valine)(P1), poly(N-methacryloyl-L-phenylalanine)(P2), and poly(N-methacryloylglycyne-L-leucine)(P3) has been studied in the presence of the non-ionic surfactant Brij98. The pure polymers phase-separate in an acidic medium with critical pHtr values of 3.7, 5.5, and 3.4, respectively. The addition of the surfactant prevents phase separation and promotes reorganization of polymer molecules. The nature of the interaction between polymer and surfactant depends on the amino acid structure in the side chain of the polymer. This effect was investigated by dynamic light scattering, isothermal titration calorimetry, electrophoretic measurements, small-angle neutron scattering, and infrared spectroscopy. Thermodynamic analysis revealed an endothermic association reaction in P1/Brij98 mixture, whereas a strong exothermic effect was observed for P2/Brij98 and P3/Brij98. Application of regular solution theory for the analysis of experimental enthalpograms indicated dominant hydrophobic interactions between P1 and Brij98 and specific interactions for the P2/Brij98 system. Electrophoretic and dynamic light scattering measurements support the applicability of the theory to these cases. The specific interactions can be ascribed to hydrogen bonds formed between the carboxylic groups of the polymer and the oligo(ethylene oxide) head-groups of the surfactant. Thus, differences in polymer-surfactant interactions between P1 and P2 polymers result in different structures of polymer-surfactant complexes. Specifically, small-angle neutron scattering revealed pearl-necklace complexes and "core-shell" structures for P1/Brij98 and P2/Brij98 systems, respectively. These results may help in the design of new pH-responsive site-specific micellar drug-delivery systems or pH-responsive membrane-disrupting agents.Langmuir 09/2014; 30(38). DOI:10.1021/la5031262 · 4.38 Impact Factor
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ABSTRACT: The advantage of fluorescence correlation spectroscopy to study single chain behavior of polyelectrolytes has been demonstrated by checking the coil-to-globule transition of poly 2-vinylpyridine with the change of pH value in aqueous solution. The ultra-high sensitivity of FCS allows measurement at extreme dilution where the effect of electrostatic interaction between the chains is greatly suppressed. The results exposed first-order conformation transition of P2VP as detected by FCS while inter-chain aggregation occurred in the experiments of dynamic light scattering.Chinese Journal of Chemistry 09/2012; 30(9). DOI:10.1002/cjoc.201200596 · 0.92 Impact Factor
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ABSTRACT: The high sensitivity at single molecule level of fluorescence correlation spectroscopy (FCS) enables measurements of charged polymer molecules at extreme dilution condition, which has been difficulty for conventional techniques as dynamic light scattering. By FCS, the conformation transition of single chain of polyelectrolytes is clearly exposed: the first-order conformation transition by pH of poly 2-vinylpyridine, the scaling analysis of polystyrene sulfonate shows a change from rod-like conformation to random coil conformation with the increase of the molecular weight. Photon count histogram (PCH) helps to determine the electric potential of single charged polymer molecules, explaining the important contribution of the counterion distribution: the breakdown of “counterion condensation.” FCS is a very important method for the study of charged polymers and bio-macromolecules.Conference on Single Molecule Spectroscopy and Superresolution Imaging; 02/2013