Architecture of Hydrated Multilayer Poly(methacrylic acid) Hydrogels: The Effect of Solution pH

Abstract

We report on the evolution of the internal structure of dry and hydrated poly(methacrylic acid) (PMAA) hydrogels by quantifying the extent of layer interdiffusion in hydrogen-bonded (HB) films and upon subsequent cross-linking and hydration. These hydrogels are produced by ethylenediamine (EDA)-assisted cross-linking of PMAA in spinassisted (SA) and dipped HB PMAA/poly(N-vinylpyrrolidone) (PVPON) multilayers followed by complete release of PVPON at pH 8 due to severing of hydrogen bonds with the PMAA network. Internal hydrogel architecture was monitored by neutron reflectometry using deuterated dPMAA marker layers. We found that even in the highly stratified SA HB films, layer interdiffusion extends over three (PMAA/PVPON) bilayers. Cross-linking of this film induces marker layer interpenetration more deeply into the surrounding material, extending over five layers. The volume fraction of dPMAA at the nominal center of a marker layer decreased from 0.65 to 0.51 after cross-linking. Hydrated SA hydrogels preserve well-organized layering and exhibit a persistent differential swelling with two distinct swelling ratios corresponding to MAA cross-link-rich and cross-link-poor strata. In contrast, layer organization in dipped films decays rapidly with distance from the silicon substrate. Both types of hydrogel swelled by factors of two and four times their dry total thicknesses at pH 5 and 7, respectively, and exhibited elevated surface roughness upon hydration. To fit the neutron reflectometry data, a self-consistent model was developed wherein the amount of PMAA initially deposited was preserved through subsequent chemical modification and hydration. Our results open opportunities for the development of thin hydrogels with a regulated structure, which can be utilized for efficient sensing, protection, activation, and rapid response in an aqueous environment. The internal morphological hierarchy of these multilayer hydrogels affords a means of fine-tuning their response to pH, temperature, or light to a degree rarely possible for randomly cross-linked responsive networks or brushes.

Free-Standing Thin Hydrogels: Effects of Composition and pH-Dependent Hydration on Mechanical Properties

“…Polymers were dissolved in phosphate buffer (0.01 M, pH = 2.5) at a concentration of 1 mg mL –1 and deposited using a SA LbL technique (spin-coater acquired from Laurell Technologies) on silicon wafers with a thick (300 nm) SiO 2 layer. Prior to the deposition of hydrogen-bonded multilayers, silicon wafers were thoroughly cleaned as described elsewhere . An initial PGMA layer was spin-cast from a 0.089 mg mL –1 chloroform solution onto the cleaned silicon wafer, followed by heating at 110 °C for 1 h and washing with chloroform, resulting in a 2 nm PGMA monolayer when dry.…”

“…A PMAA layer was then spin-cast onto the PGMA-primed wafers from a 1 mg mL –1 methanol solution and crosslinked at 80 °C for 40 min, followed by rinsing with DI water. (PMAA/PVPON) n LbL films were spin-casted onto PGMA-PMAA primed wafers as described previously . Briefly, PVPON ( M w = 58 kDa) or PMAA ( M w = 100 kDa) 1 mg mL –1 solutions in 0.01 M phosphate buffer at pH = 2.5 were alternately deposited in 3 mL aliquots onto the wafers and rotated for 30 s at 3000 rpm on a spin-coater; then rinsed twice for 30 s with the phosphate buffer solution before the deposition of the next layer.…”

“…The surface-anchored PMAA networks obtained by chemical crosslinking of PMAA in spin-assisted (SA) hydrogen-bonded PMAA/PVPON multilayers and loaded with Cu­(II) ions demonstrated selective sensing of amino acids . In earlier studies, we showed that surface-attached PMAA hydrogels obtained from SA multilayers had a more organized internal structure and much higher pH-triggered swelling than dipped multilayer hydrogels. , …”

Complete pH-Dependent Shape Recovery in Cubical Hydrogel Capsules after Large Osmotic Deformations

“…Hydrogen-bonded (PVPON/PMAA–NH 2 ) n multilayers, where n denotes the number of polymer bilayers, were assembled on silicon wafers (University Wafer) using dipping LbL. First, a PGMA/PMAA precursor was covalently attached to the wafer surface to anchor the multilayer hydrogel to the surface . For that, PGMA chloroform solution (0.09 mg mL –1 ) spin-coated onto a wafer was heated for an hour at 110 °C in an oven.…”

“…First, a PGMA/PMAA precursor was covalently attached to the wafer surface to anchor the multilayer hydrogel to the surface. 43 For that, PGMA chloroform solution (0.09 mg mL −1 ) spin-coated onto a wafer was heated for an hour at 110 °C in an oven. Unattached PGMA was rinsed off using chloroform three times.…”