The effect of polyacid microstructure on hydrogen-bonded complexation and ion-association properties

Porter Clarke Shannon

Abstract

This dissertation elucidates the microstructural differences between poly(acrylic acid) (PAA) and hydrolyzed poly(ethylene-maleic anhydride) (HPEMA) in two specific areas. First it examines the hydrogen-bonded complexation of these two acids with poly(vinyl pyrrolidone) (PVP), and secondly it examines the microstructural effect on ion association. Additionally the dissertation examines the nature of biexponentiality of $\sp{23}$Na NMR, and develops a further understanding of the prephase separation transition in PAA/Ca++ systems. There is some debate over the mechanism of aqueous polyacid-polybase complexation. To elucidate this mechanism, we studied complexes of PAA and HPEMA with PVP at low pHs (4.00-5.00) through $\sp{23}$Na NMR, fluorescence, potentiometry, and phase behavior. It was hypothesized that if the complexes are formed by a blocky mechanism, the head to tail microstructure of the PAA would be expected to complex more efficiently with PVP than the vicinal microstructure of HPEMA. We draw three principal conclusions from this study. First, the vicinal microstructure of HPEMA does not complex with PVP as well as PAA, which supports the blocky idea of these associations. Second, complexation increases sodium ion/carboxylate interactions and release of counterions is not a significant entropic driving force for complexation. Finally, we have evidence indicating that as PAA is complexed with PVP, carboxylate groups are excluded into microdomains. There is very little difference in the ion association behavior of the ionized form of HPEMA and PAA as evidenced from $\sp{23}$Na NMR and potentiometry. NMR results show that the alkali monovalent counterion order of binding for each polymer is consistent with literature values for polycarboxylates: Li$\sp+ >$ Na$\sp+ >$ K$\sp+$. Two amine salts were also investigated; ammonium chloride (NH$\sb4\sp+$) and triethanolamine chloride (TEA$\sp+$). These two amine counterions both showed a stronger affinity for the polycarboxylates than the alkaline counterions; however NH$\sb4\sp+$ showed preferential binding over TEA$\sp+$. This difference was attributed to greater hydrophobicity of the TEA$\sp+$ cation. For divalent ions, the affinity order was the same for each polyelectrolyte Ca$\sp{++}>$ Mg$\sp{++}$ from potentiometric results. Neither counterion seemed to display a specific affinity for either polycarboxylate. Unusual behavior was noted in the Be$\sp{++}$ system, an effect which may be due to its acidity. The prephase separation of the Ca$\sp{++}$/PAA system was found to be a general phenomenon as evidence of it was observed in all HPEMA systems and lower molecular weight Ca$\sp{++}$/PAA systems. It is thought to be due to Na$\sp+$ residing within the collapsing polymer and becoming entrenched in a lower dielectric environment. Finally we were able to produce experimental evidence supporting the model for biexponential behavior of the $\sp{23}$Na R$\sb2$. Halle's model of biexponentiality is based on counterion diffusion between chains in dilute solution. By correlating the polymer concentration regime with the degree of biexponential behavior of the transverse relaxation rate constant data, we show evidence correlating biexponentiality with the dilute solution expansion regime of polyelectrolytes, indicating that biexponential behavior is a consequence of counterions interacting with chains during the time scale of the experiment, and counterions residing between polyions.