Dingcai Wu,*,† Fei Xu,† Bin Sun,† Ruowen Fu,† Hongkun He,‡ and Krzysztof Matyjaszewski*,‡ †Materials Science Institute, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, People's Republic of China ‡Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States

Design and Preparation of Porous Polymers

Superabsorbent hydrogels based on cellulose for smart swelling and controllable delivery

Superabsorbent polymers are hydrophilic networks that can absorb and retain huge amounts of water or aqueous solutions. They are currently used in many areas including hygienic and bio-related uses (particularly in disposable diapers), agricultural uses (e.g., water reserving in soil, soil conditioning, and controlled release of agrochemicals), pharmaceutical dosage forms, separation technology, fibers/textiles, water-swelling rubbers, soft actuators/valves, electrical, construction, packaging, artificial snow, sludge/coal dewatering, fire-extinguishing gels, etc. Addition of clays and organo-modified clays into superabsorbent formulation has funded new generations of these hydrogels, i.e., superabsorbent hydrogel composites (SHCs) and superabsorbent hydrogel nanocomposites (SHNCs). New superabsorbent hydrogels are cheaper in price, and possess superior mechanical properties than non-composite counterparts. This review introduces the SHC and SHNC polymers with the focus on different minerals and humic additives, synthetic methods, the hydrogel characteristics and their applications. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers

Superabsorbent hydrogel composites and nanocomposites: A review

Thermoset polymers and composite materials are integral to today’s aerospace, automotive, marine and energy industries and will be vital to the next generation of lightweight, energy-efficient structures in these enterprises, owing to their excellent specific stiffness and strength, thermal stability and chemical resistance1–5. The manufacture of high-performance thermoset components requires the monomer to be cured at high temperatures (around 180 °C) for several hours, under a combined external pressure and internal vacuum
 6
 . Curing is generally accomplished using large autoclaves or ovens that scale in size with the component. Hence this traditional curing approach is slow, requires a large amount of energy and involves substantial capital investment6,7. Frontal polymerization is a promising alternative curing strategy, in which a self-propagating exothermic reaction wave transforms liquid monomers to fully cured polymers. We report here the frontal polymerization of a high-performance thermoset polymer that allows the rapid fabrication of parts with microscale features, three-dimensional printed structures and carbon-fibre-reinforced polymer composites. Precise control of the polymerization kinetics at both ambient and elevated temperatures allows stable monomer solutions to transform into fully cured polymers within seconds, reducing energy requirements and cure times by several orders of magnitude compared with conventional oven or autoclave curing approaches. The resulting polymer and composite parts possess similar mechanical properties to those cured conventionally. This curing strategy greatly improves the efficiency of manufacturing of high-performance polymers and composites, and is widely applicable to many industries. Frontal polymerization of dicyclopentadiene is used to generate thermoset polymers and composite materials with much lower energy requirements and cure times than are needed in conventional oven or autoclave curing approaches.

Rapid energy-efficient manufacturing of polymers and composites via frontal polymerization

Ampholytic hydrogels with pH and salt responsive properties have been synthesized by cross-linking quaternized cellulose (QC) and carboxymethyl cellulose (CMC) with epichlorohydrin (ECH) in NaOH aqueous solution. The swelling behaviors of the QC/CMC hydrogels were studied as a function of the polymer composition, pH, and salt concentration. The equilibrium swelling ratio of the hydrogel in ultrapure water strongly depended on the composition, and increased dramatically from 8.6 to 498 g/g with the change of the weight ratio of QC to CMC from 3:1 to 1:3 (w/w). The hydrogel (Gel32) consisted of QC and CMC in the ratio of 3:2 (w/w) had the minimum swelling ratio, and it was electrically neutral. The results from ζ-potential experiments were in good agreement with the theoretically calculated stoichiometry for balanced charge, confirming that the weight ratio of QC and CMC was 1.5, corresponding to Gel32. Furthermore, all hydrogels exhibited excellent pH sensitivity in the range of pH from 1 to 13 and shrunk ...

Swelling Behaviors of pH- and Salt-Responsive Cellulose-Based Hydrogels

Recently, a considerable amount of research has centered on uniquely structured polymers synthesized through self-propagating frontal polymerization. The obtained polymer materials have better features than those obtained by using the classical batch route. The additional advantages are short reaction times and low cost. This work describes the first frontal polymerization synthesis of a graft copolymer superabsorbent hydrogel of acrylic acid onto starch at high monomer and initiator concentration. The effects of varying the relative amounts of the reaction components on the most relevant parameters relating to frontal polymerization were explored. The front velocity dependence on initiator concentration could be fit to a power function. The temperature profiles were found to be very sharp with a maximum temperature below 150 degrees C, which was responsible for high monomer conversion. The ultimate properties of the product appear to depend on the polymerization front velocity and the temperature. The high-temperature and rapid temperature increase at the polymerization front led to products with interconnected porous structures caused by the evaporation of water. So, a fast-swelling, highly absorbing hydrogel with respect to batch polymerization was obtained.

Frontal copolymerization synthesis and property characterization of starch-graft-poly(acrylic acid) hydrogels.

Polyacrylamide hydrogels with defined porous structure were synthesized through frontal polymerization (FP) in the presence of NaHCO3 as a foaming agent Pore properties were analyzed using scanning electron microscopy and mercury intrusion porosimetry The as-prepared hydrogels displayed a small cell diameter of ca 2 µm The dissolved foaming agent dispersing at the level of molecules and the polymerization front propagating step by step should be responsible for the small uniform cell structure The pore volume varied from 063 to 365 cm3 g−1 and the bulk density changed from 048 to 028 g cm−3 for a foaming agent content from 0 to 18% The hydrogels synthesized by FP exhibited higher swelling rate and swelling ratio with respect to conventional batch polymerization The highest swelling ratio and rate were obtained at a foaming agent concentration of 12% based on monomer Copyright © 2007 Society of Chemical Industry

Preparation of porous polyacrylamide hydrogels by frontal polymerization

The frontal polymerization process was used to produce superab- sorbent hydrogels based on acrylic acid monomers grafted onto starch. Using a simple test tube which was nonadiabat- ic and permitted contact with air, the effects of initial temperature and tube size on the propagating front of graft- ing copolymerization and the proper- ties of hydrogels were explored. The unrestricted access of the reaction mix- ture to oxygen delayed the formation of self-propagating polymerization front. The ignition time was markedly lengthened with the increasing of tube size attributed to the formation of large amounts of peroxy radicals. The front velocity dependence on initial temperature could be fit to an Arrhe- nius function with the average appa- rent activation energy of 24 kJ mol � 1 , and on tube size to a function of higher order. The increase of the initial tem- perature increased the front tempera- ture, which lead to more soluble oligomers and higher degree of cross- linking. The interplay of two opposite effects of oligomer and crosslinking de- termined the sol and gel content. An increase in tube size had two effects on the propagating front. One was to reduce heat loss. The other effect was to increase the number of escaping gas bubbles. The combined action of the two effects resulted in a maximum value of front temperature, an increase in sol content and a reduction in gel content with tube size. The highest swelling capacity of hydrogels was ob- tained when the initial temperature or tube size favored a formation of porous microstructure of hydrogels.

Frontal polymerization synthesis of starch-grafted hydrogels: Effect of temperature and tube size on propagating front and properties of hydrogels.

A frontal polymerization method is used to produce highly porous polymer monoliths. The method is an approach to polymer synthesis that exploits the heat produced by the reaction itself. This heat triggers polymerization of neighboring monomer molecules, leading to a self-sustaining hot front, which propagates along the reacting vessel. Dissolved or microencapsulated foaming agents are decomposed only at the fronts, synchronizing the polymerization and the foaming. The ultimate pore structures appear to depend on the polymerization-front velocity and temperature. The resultant materials are porous, exhibiting tunable pore volume and a multimodal pore size distribution. No organic solvents or high-pressure equipment are used in the process, and no solvent residues are left in the resulting materials. Specifically, this route allows for the synthesis of large-scale samples with the additional advantages of high velocity, low energy cost, and the avoidance of multiple process steps. Substitution of hydrophilic acrylamide, N-isopropylacrylamide, with hydrophobic styrene and methyl methacrylate also leads to porous monolithic materials, suggesting that frontal polymerization represents a powerful and facile method for an exothermic polymerization reaction and the creation of porous polymers.

Guo Dong Lu

Papers

Frontal copolymerization synthesis and property characterization of starch-graft-poly(acrylic acid) hydrogels.

Preparation of porous polyacrylamide hydrogels by frontal polymerization

Frontal polymerization synthesis of starch-grafted hydrogels: Effect of temperature and tube size on propagating front and properties of hydrogels.

Frontal Polymerization Synthesis of Monolithic Macroporous Polymers