aPMAA_WALS_Dimensions_Kogej_JPhysChem_B_2004 - Unusual...

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Unformatted text preview: Unusual Behavior of Atactic Poly(methacrylic acid) in Aqueous Solutions Monitored by Wide-Angle Light Scattering Ksenija Kogej,* ,²,‡,§ Hugo Berghmans, ‡ Harry Reynaers, ‡ and Sergio Paoletti ² Department of Chemistry, Catholic Uni V ersity of Leu V en, Celestijnenlaan 200F, B-3001 He V erlee, Belgium, and Department of Biochemistry, Biophysics, and Macromolecular Chemistry, Uni V ersity of Trieste, I-34127 Trieste, Italy Recei V ed: March 26, 2004; In Final Form: September 16, 2004 Wide-angle light scattering studies were performed on the atactic poly(methacrylic acid), HPMA, in 0.002 and 0.1 M HCl at 25 and 55 ° C in a wide concentration range. The weight-average degree of polymerization of a single poly(methacrylate), PMA, chain N h w ( ) 1528) was estimated from scattering experiments on poly- (sodium methacrylate) in 0.1 M NaCl. The intermolecular association between PMA chains in carefully prepared acidic aqueous solutions is evidenced in both angular and concentration dependencies of scattered light. The dependence of the reciprocal reduced scattering intensity on the square of the scattering vector is heavily curved at high angles, and it exhibits a minimum as a function of concentration. In 0.002 M HCl, the associated forms deaggregate on a time scale of several days. This process is accompanied by a decrease of the second virial coefficient, A 2 , and by chain expansion; the radius of gyration, R g , increases from about 17 to about 28 nm after 0.17 and 576 h, respectively. The intermolecular association in 0.1 M HCl is stronger than in 0.002 M HCl and seems to be stable on a much longer time-scale. The time-average R g value (about 18 nm) of the associated HPMA form in 0.1 M HCl indicates a more compact nature of entanglements; this fact decisively contributes to the overall stability of intermolecular aggregates. For HPMA dissolved in 0.1 M HCl, an increase in temperature from 25 to 55 ° C brings about no increase in R g . The changes in thermodynamic functions of association point to a process driven by hydrophobic interactions. Introduction Many biological systems involving water-soluble polymers have an ability to induce thickening or gelation of solutions that is driven by aggregation of some ordered forms of the polymer chain. 1- 7 The phenomenon can be induced by various physicochemical factors, i.e., by change in temperature, presence of suitable counterions, and/or sufficient ionic strength, and it usually involves a conformational transition of the macro- molecular chain. 1,4- 7 The complicated structure of many bio- colloids makes it difficult to determine the forces and mecha- nisms that are operative during the gelation process. Therefore, one tries to study simpler model systems....
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This note was uploaded on 07/20/2011 for the course EMA 6165 taught by Professor Brennan during the Spring '08 term at University of Florida.

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aPMAA_WALS_Dimensions_Kogej_JPhysChem_B_2004 - Unusual...

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