Poly(N-isopropylacrylamide) (PNIPA) hydrogels with varied degree of crosslinking (DC) were synthesized by using poly(ethylene glycol) (PEG) as an additive. A phase separated ("macroporous") morphology was formed when using PEG contents of > or = 20 wt.-%. Temperature-dependent degrees of swelling had been measured, and average mesh sizes of the swollen polymer network had been calculated. The loading of the hydrogels with labelled dextrans with various molar masses and bovine serum albumin (BSA)-via swelling of the shrunken gel in a cold solution-and their subsequent unloading-via immersion in hot water-were studied in detail. The loading efficiencies were close to zero for PNIPA prepared at PEG contents of < or = 10 wt.-%, and they increased sharply to about 100% for PNIPA prepared with PEG contents of > or = 20 wt.-%. A complete unloading was achieved as well. For macroporous PNIPA prepared at 40 wt.-% PEG content, the loading efficiency was a function of the DC, and the "cut-off" observed as a function of dextran or protein size correlated with the mesh size of the hydrogel. The function of these "smart" hydrogels can be explained by the temperature-induced "pumping" of the solution into the gel bulk via the permanent pores, along with an uptake into the adjacent hydrogel network. Those materials could be used as matrices for the efficient and reversible immobilization of (bio)macromolecules.
This paper focuses on a method to determine the swelling pressure of polymeric hydrogels. The method is based on determining the swelling pressure at a defined volume of swelling that is smaller than the volume the gel can attain by free swelling to equlibrium. Data for one type of polymer particles made from weakly cross-linked and partly neutralized poly(acrylic acid) are presented for swelling in deionized water. For polymer volume fractions (φ P ) in the range of 0.05-0.30, swelling pressures in the range of 0.25-4.31 MPa were obtained. The kinetic data are discussed with a combined diffusion-relaxation model for low and medium polymer volume fractions. A novel model is presented for higher polymer volume fractions. The mechanism of the gel-blocking effect that is evident in this φ P range is explained and considered in the novel model, in the form of a blocking factor. The results obtained may be used for the design of improved sealing concepts that are based on "super absorbing polymers".
Superabsorbent polymers (SAPs) are well known for their ability to absorb and hold high water amounts accompanied by a high volume expansion. In this work we show the benefits of this underlying property of SAPs to induce underwater crack closure with subsequent barrier restoration in damaged protective coatings. For the proof of concept, three layer epoxypolyester (EP) powder coating systems were developed and applied on carbon steel. In these systems the middle EP layer (also called functional layer) contained crosslinked acrylamide/acrylic acid copolymer SAPs in different amounts ranging from 0 to 40 wt%. The capability of the SAPs to close damages and extend barrier and corrosion protection was evaluated by electrochemical impedance spectroscopy (EIS), NaCl aqueous solution immersion test and optical microscopy. It was found that coatings loaded with a 20 wt% SAP led to the best overall corrosion protection for the studied systems. In order to proof the potential use of this extrinsic healing concept for multiple healing events wet-dry cycles on scratched systems
BACKGROUND: This paper focuses on the development of temperature induced phase transition hydrogels based on poly(Nisopropylocrylamide (PNIPA) copolymers and their application as an immobilization matrix for biocatalysts.
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