Hydrogels PNIPAM

Fig. 19.7 Oscillating change of temperature (a) and time-dependent pulsatile release of BSA (b) PNIPAM hydrogel in phosphate buffer, pH = 7.4 (1) and water (2) at 25°C and 40°C...

In contrast to these approaches based on nonspecific interactions, Zhang and coworkers described a molecularly imprinted hydrogel based on the ther-moresponsive PNIPAM [184], This hydrogel was prepared by copolymerization of a metal chelate monomer iV-(4-vinyl)-benzyl iminodiacetic acid, which formed a coordination complex with the template protein in the presence of Cu ions, A-isopropylacrylamide, acrylamide, and IV.lV-methylenebisacrylamide as crosslinker. The interaction of the imprinted thermoresponsive hydrogel with the protein could be switched between coordination effects and electrostatic attraction by addition or omission of Cu ions. Furthermore, this imprinted hydrogel allowed switching of lysozyme adsorption by changing the temperature. 

The hydrogel coating on the nanoparticles has also been achieved by radical polymerization. For poly (isobutyl cyanoacrylate) (PIBCA), the monomer was emulsified in an aqueous solution containing PEG that acts as a nucleophile initiator of polymerization through its hydroxyl terminal groups. Once the aqueous pH is adjusted to 1, polymerization is initiated, thus forming PEG-coated PIBCA nanoparticles. PNIPAM has been coated on the nanoparticles by radical polymerization in the presence of hydrophobic... 

By polymerizing poly(A -isopropylacrylamide) (PNIPAM) [55] or poly (2-(diethylamino)ethyl methacrylate) (PDEAEMA) [56] as a stimuli responsive polymer/hydrogel layer around a colored nanoparticle of PS-co-PMMA, the local refractive index and consequently the color intensity of the latex could be switched by the temperature [55] or pH [56]. 

In contrast, neither acrylamide nor N-aUcylacrylamides could be anionically polymerized, due to proton abstraction from their acidic amide protons. Among such monomers, N-isopropylacrylamide (NIPAM) is the most often used, with recent interest in its polymer - poly(N-isopropylacrylamide) (PNIPAM) - having increased in exponential fashion due to its possible use as hydrogels, in drug-delivery devices, in biomedicine, and in permeation membranes - all of which reflect the polymer s water-solubiUty and thermoresponsive nature (Tc = 32 °C). 

PNIPAM miCTOgels have been insetted in the powder state into the hydrogel chamber as schematically shown in Figure 36(a). 

Hydrogel-based colloidal crystals were designed for sensing applications. Asher and co-workers polymerized NIPAM in the colloidal crystal fabricated from PS particles. The ther-moresponsivity of the PNIPAM chains allows regulating the interparticle distance in the colloidal crystal and the position of Bragg peak. 

The double-amphiphilic PNIPAM-PMPC-PNIPAM triblock copolymer that resulted exhibits interesting thermosensitive properties due to the PNIPAM building blocks. At low temperatures, the copolymer is molecularly dissolved, whereas above the LCST of PNIPAM, a reversible hydrogel is formed. 

Figure 2.12 Thermal sensitive hydrogels of poly(N-isopropyl acrylamide) (PNIPAm).

Our work employs electrochemically initiated polymerization (EIP) which is an easy and flexible method to produce surface coatings of various kinds. The technique makes use of the decomposition of an electro-active initiator at an electrode to start a free-radical polymerization. The polymer is formed directly at the electrode surface. As a consequence, the films adhere tightly to the surface. Adhesion is based on physisorption of the hydrogel to the metal. Note in this context that EIP is different from electrografting of conductive polymers. In EIP, the initiator is the electro-active species, rather than the monomer. Recently, we reported on the formation of thermoresponsive poly-A -isopropylacrylamide (pNIPAm) hydrogel coatings on gold surfaces based on this approach. ... 

A thermochromic hydrogel based on Bragg reflection has been reported (16). The described system consists of crystalline colloidal arrays of monodisperse polystyrene spheres embedded in a poly(iV-isopropyl acrylamide) (PNIPAM) hydrogel, which was prepared by first dispersing highly charged and monodisperse... 

Mohammad S, Fatemeh S. Swelhng behavior of carboxymethyl cellulose-g-PMAAm-co-PNIPAm superabsorbent hydrogels. Res J Chem Environ 2012 16 125-132. 

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