Effect of the Molecular Weight on the Dynamics of the Conformational Transition of Poly(methacrylic acid)

Abstract

Stop flow pH jump experiments were carried out on poly(methacrylic acid), PMA, solution of various molecular weights. The fluorescence changes of the probe pyrene, which can locate in the hydrophobic PMA coil or in the aqueous phase, were used to monitor pH-induced events in the PMAwater system. At least two well-separated time regions for the pH-induced conformational change of PMA were observed: one with τ ) 1-2 s and another with τ ) 50-100 s. A theoretical model to quantitatively explain the observed kinetic changes is given. This is based on a model developed by Schwarz for the helix-coil transition of polypeptides and can be briefly described as follows. Any configuration of the polymer is characterized by a statistical distribution of segments in polar and hydrophobic regions. Then a conformational transition involves the change of a number of segments from one state to the another, and its kinetics corresponds to a relaxation process with the pH jump as the initial perturbation. Several details pertinent to unambiguous interpretation are also described briefly. This includes the rate of neutralization of the acid groups on PMA and the rate of solution of small pyrene crystals in a colloidal system. The above confusing effects were established and thus could be avoided in the actual pH-induced conformational changes of PMA.

Monitoring pH-dependent conformational changes in aqueous solutions of poly(methacrylic acid)-b-polydimethylsiloxane copolymer based on fluorescence spectra of pyrene and 1,3-bis(1-pyrenyl)propane

“…Consequently, the excimer emission was not observed for the diluted polymer systems. We have to mention, however, that in a more recent study of Olea et al [26] an increasing excimer band was observed at pH . 5 for 0.01 mol dm 23 PMA/ 2 mmol dm 23 P solutions containing 0.1 mol dm 23 NaCl, and the I 1 =I 3 ratio in the high pH range was lower than in the absence of salt.…”

Optical Properties of Polyelectrolytes

“…[16]) or post-synthesis, a polyelectrolyte solution can be doped with fluorescent material [6,[17][18][19][20][21]. A luminescent species may be incorporated into the macromolecule either during polymerization (see e.g., Ref.…”

“…To achieve the latter, spectroscopists have exploited the fact that when in a coiled conformation in aqueous solution, certain polyelectrolytes can solubilize [6,12,[17][18][19][20][21] low molar organic molecules. On changing the pH, the chain expands releasing this material into the aqueous phase.…”

“…Because poly(carboxylic acid)s have received particular attention in the literature in respect of their fluorescence behavior in aqueous solution [6,12,13,[16][17][18][19][20][21][22][23][24]26,[29][30][31][32][33][34][43][44][45][46] and have been comprehensively studied by classical methods [2][3][4][47][48][49][50][51][52] , this important class of polyelectrolyte will be reviewed initially before moving on to other examples. The active repeat unit in poly(methacrylic acid) (PMAA) is the carboxylic acid function, which dissociates upon increasing pH to form negatively charged carboxylate anions.…”

“…PMAA can be classed as an amphiphilic polymer containing both hydrophobic (water insoluble) and hydrophilic (water soluble) units [17] dependent on the degree of ionization. The presence of the (hydrophobic) backbone methyl unit induces additional coiling, known as hypercoiling [48], which results in a more compact structure under these conditions [2][3][4]12,[16][17][18][19]21,22,24,26,[46][47][48][51][52][53][54][55][56][57][58][59]. The macromolecular backbone is hydrophobic at all pH values and, at higher degrees of protonation of PMAA, the carboxylic acid groups can also be considered as hydrophobic.…”