Ionization Characteristics and Structural Transitions of Alternating Maleic Acid Copolymer Films

Abstract

Alternating maleic acid copolymers dissolved in aqueous solutions exhibit characteristic two-step dissociation profiles and pH-induced structural transitions in dependence on the comonomer unit. In extension of these findings, we analyzed a set of thin films of maleic acid copolymers (poly(octadecenealt-maleic acid), POMA; poly(propylene-alt-maleic acid), PPMA; poly(styrene-alt-maleic acid), PSMA) randomly attached to planar glass surfaces by covalent binding. Streaming potential/streaming current measurements with a microslit electrokinetic setup (MES) were applied to determine zeta potential (ζ) and surface conductivity (K σ ) data of the polymer layers in aqueous solutions of varied pH. The results confirmed the two-step dissociation behavior for all immobilized copolymers. A dramatic increase of K σ was observed for PPMA and PSMAsbut not for POMAslayers at alkaline pH values, indicating that electrostatic repulsion of ionized groups controls the extension of the confined layers as long as hydrophobic interactions between the comonomers do not inhibit this effect. The isoelectric points (IEPs) show that for PPMA and PSMA layers the acidic functions dominate the interfacial charging and exhibit an enhanced acidity which can be explained by hydrogen bonds between the anion formed in the first dissociation and the proton of the adjacent undissociated group. In contrast, the layered POMA exhibits significantly less acidic IEP values due to the large comonomer. Hysteresis effects of the ζ versus pH profiles were observed for dissociation and association of the copolymers. This was explained by the reduced dynamics of structural transitions in the immobilized layers as compared to the dissolved molecules.

Electrostatic stretching of grafted maleic acid copolymer chains

“…Hence, the change in the layer thickness can be attributed to the partial protonation of the carboxylic acid groups. As reported in an earlier study [14] the pK a values of the two adjacent carboxylic acid groups of maleic acid (pK a1 = 3 and pK a2 = 7) explain an only partial dissociation of the acidic groups at pH 4. Therefore, the electrostatic repulsion along (and between) the grafted polymer chains is lowered at pH 4 which leads to the reported change from good solvent conditions to bad solvent conditions [7].…”

“…As found in earlier studies [14] switching from low pH (pH 4) to high pH (pH 7) increases the charge density along the polymer chain due to an increased dissociation of the carboxylic acid groups. The charged moieties on the polymer chains are shielded by counterions in the solution over distances (Debye screening lengths) that exponentially decrease with increasing ionic strength of the solution.…”

“…The swelling kinetics of polymer films readily accounted for the variation in hydrophilicity and polarity of the polymer chains due to the variation of the comonomer and were in excellent agreement with swelling studies of maleic anhydride copolymers in solution [11]. The response of the copolymer films to varying properties of aqueous environments is of crucial importance for intended further applications in biointerfacial science [14]. In addition to the variation of the pH changes of the ionic strength often occur and need to be considered when applying thin polymer films e.g.…”

Immobilized enzymes affect biofilm formation

“…The properties of the films were investigated using a wide range of techniques, details can be found elsewhere (Pompe et al 2003;2005;Osaki and Werner 2003).…”

“…Upon determination of 4 the thickness of all layers below the protein layer, a five-model approximation (Si/SiO 2 /silane + MA film/protein/air) was used to determine the thickness of the protein layer. The refractive index for the MA copolymer film was taken from Osaki and Werner (2003). The refractive index of the protein layers was maintained fixed for all protein-containing surfaces at 1.465 (Fischer et al 2010).…”