A. M. Mathur

Methods for Synthesis of Hydrogel Networks: A Review

Abstract Hydrogels are macromolecular networks that swell, but do not dissolve, in water. The ability of hydrogels to absorb water arises from hydrophilic functional groups attached to the polymeric backbone, while their resistance to dissolution arises from crosslinks between network chains. Many materials, both naturally occurring and synthetic, fit the definition of hydrogels. Crosslinked dextrans and collagens are examples of natural polymers that are modified to produce hydrogels. Classes of synthetic hydrogels include poly(hydroxyalkyl methacrylates), poly(acrylamide), poly(N-vinyl pyrrolidone), poly(acry1ic acid), and poly(vinyl alcohol).

Polymeric emulsifiers based on reversible formation of hydrophobic units

Emulsions consist of mixtures of immiscible liquids where one liquid is finely dispersed within the continuous phase of another. They are generally not thermodynamically stable: the dispersion tends to separate over time. Aqueous emulsions, widely used in food, pharmaceutical, and many other industries, are often stabilized by block copolymers containing alternating hydrophilic and hydrophobic segments (typically based on ethylene oxide/propylene oxide diblock and triblock systems) that penetrate into the oil and aqueous phase, respectively,. Here we describe a conceptually new type of emulsifier whose hydrophobic blocks are formed spontaneously and reversibly by the complexation of hydrophilic segments, thereby allowing the stabilizing properties of the system to be switched on and off. We illustrate this approach using a comb-type graft copolymer containing a poly(methacrylic acid) backbone and short grafts of poly(ethylene glycol). The uncomplexed polymer is hydrophilic, but acidic conditions induce the formation of hydrogen-bonded hydrophobic complexes between parts of the backbone and the grafts. As a result, the grafted copolymer forms alternating blocks of hydrophilic (uncomplexed) and hydrophobic (complexed) segments that stabilize acidic emulsions. An increase in pH suppresses complex formation and thus leads to the breakup of the emulsion. Emulsion tests show that although the performance of the grafted copolymers is not yet competitive with existing emulsifiers, this approach provides an efficient strategy for the design of fully reversible emulsifiers.

Equilibrium swelling of poly(methacrylic acid-g-ethylene glycol) hydrogels: Effect of swelling medium and synthesis conditions

Responsive hydrogel networks comprising of poly(methacrylic acid) (PMAA) backbone and oligomeric ethylene glycol (PEG) grafts were synthesized by free-radical solution polymerization and their equilibrium swelling properties were characterized in aqueous solutions of a homologous series of alcohols. These hydrogels are known to exhibit swelling transitions in response to external stimuli which lead to formation or disruption of hydrogen-bonded complexes between the backbone and the grafts. Swelling studies performed in aqueous mixtures of methanol, ethanol and propanol revealed that the effectiveness of an alcohol in breaking the PMAA/PEG complexes increased as the aliphatic segment length of the alcohol was increased. These results confirm the importance of hydrophobic interactions for stabilizing the complexes. Studies performed to determine the effect of the synthesis conditions on the equilibrium swelling properties revealed that the equilibrium degree of swelling increased as the solvent fraction during synthesis was increased. Finally, molecular stimulations revealed that it is sterically possible to form complexes with a 1:1 stoichiometry between chains of poly(methacrylic acid) and poly(ethylene glycol) with essentially no additional bond strain.

Characterization of hydrogels using nuclear magnetic resonance spectroscopy.

Literature relevant to characterization of hydrogels and cross-linked polymer networks using nuclear magnetic resonance (NMR) spectroscopy has been extensively reviewed. After a brief introduction to the fundamentals of NMR spectroscopy, a variety of NMR techniques are considered, including 13C NMR of swollen polymer networks, end-group studies by 13C NMR with labelled initiators, spin-spin and spin-lattice relaxational studies to distinguish species based upon mobility, and characterization of specific interactions using the nuclear Overhauser effect. Finally, a brief treatment of the characterization of polymer structural quantities such as composition, tacticity and sequence distribution by NMR spectroscopic studies is presented. Although our discussion is representative rather than exhaustive, we are confident that this review will demonstrate the utility of NMR spectroscopy for characterization of hydrogel networks which have applications as biomaterials.

Synthesis and Ion-Binding Properties of Polymeric Pseudocrown Ethers - a Molecular-Dynamics Study

Abstract A novel method for producing inexpensive polymeric pseudocrown ethers in situ during free-radical polymerizations was investigated using molecular dynamics simulations. This scheme is based upon a template ion and exploits the tendency of oligomeric ethylene glycol diacrylates to form intramolecular cycles during polymerization. In the scheme, a template ion is used to induce the poly(ethylene glycol) diacrylates (PEGDA) to assume cyclic structures before polymerization with a comonomer. Experimental studies demonstrated that certain salts that were insoluble in nonpolar solvents were solubilized upon the addition of oligomeric poly(ethylene glycol) (PEG) due to complexation. Further evidence of cation binding by oligomeric PEG was obtained by 1H NMR studies of PEG and its complexes with metal salts. To optimize the template ion synthesis approach, molecular dynamics simulations were performed on PEGDA containing between two and ten ethylene glycol repeating units, with and without the presence o...

Synthesis and Ion-Binding Properties of Polymeric Pseudocrown Ethers. III. Ion-Binding Studies

ABSTRACT An ion-induced templatization strategy was used to prepare crosslinked polymeric pseudocrown ethers (PCEs) in situ during a one-step free-radical polymerization. These studies bring to a conclusion the fundamental studies on the templatization process reported in the first two papers of this series. For the PCEs, with nickel as the templating ion and hydroxyethyl methacrylate as the monovinyl comonomer, the ion-binding studies revealed a significant enhancement in the ion-binding capacity associated with the inclusion of the template ion during synthesis. For example, a PCE synthesized with a monomer-to-solvent ratio of 20/80 exhibited an ion-binding capacity that was 7.5 times higher than a control polymer produced under identical conditions except for the absence of the template cations. A PCE synthesized with a monomer-to-solvent ratio of 20/80 exhibited a capacity of 0.3 mg/g polymer for Ni2+ cations. Finally, it was observed that the column could be fully regenerated by elution with a small ...

Synthesis and Ion-Binding Properties of Polymeric Pseudocrown Ethers II: Template Ion Induced Cyclization of Oligomeric Ethylene Glycol Diacrylates

Abstract Inexpensive polymeric pseudocrown ethers have been synthesized based upon a “template ion” effect in which oligomeric ethylene glycol diacrylates are induced to assume a circular conformation in which the unsaturated end-groups are in proximity. This synthetic scheme has considerable potential for the development of inexpensive materials for binding of target cations. In this contribution we report a spectroscopic study of the induced cyclization caused by ion-dipole interactions between the templating cation and the electron lone pairs of the ethylene glycol ether linkages. Fluorescence spectroscopy was used to provide insight into the templatization process by examining excimer formation in pyrene end-labeled oligomeric ethylene glycol both in the absence and in the presence of the templating ion. Pyrene end-labeled tetraethylene glycol and pentaethylene glycol were synthesized and excimer fluorescence enhancement was studied with the introduction of cations such as nickel, chromium and tin in ...