Oguz Okay

Macroporous copolymer networks

Macroporous copolymer networks form as a result of the phase separation during the free-radical crosslinking copolymerization of vinyl and divinyl monomers in the presence of an inert diluent. In this article, the develop- ments achieved in the field of macroporous networks over the last few years are presented. Special attention is paid to the preparation techniques of macroporous networks in an effort to highlight the new synthesis strategies developed in the recent years. It has been demonstrated that a variety of porous structures can be obtained during or after the cross linking process by changing the independent variable of the network synthesis, i.e. the extent of the polymer-(diluent 1 monomer) interactions, the amount of the crosslinker and the diluent as well as the initiator concentration or the polymerization temperature. The reaction system leading to macroporous networks is a (quasi)ternary system composed of a polymer network, soluble polymers, and low molecular compound (monomers and diluent). All concentrations and properties of the components of the system change continuously during the crosslinking process. It will be shown that the theoretical models developed recently correctly predict the phase separation condition during the crosslinking process as well as the total porosity of the resulting macroporous networks. q 2000 Elsevier Science Ltd. All rights reserved.

Microgels-Intramolecularly Crossünked Macromolecules with a Globular Structure

In polymer science and technology, linear, branched and crosslinked structures are usually distinguished. For crosslinked polymers, insolubility and lack of fusibility are considered as characteristic properties. However, insoluble polymers are not necessarily covalently crosslinked because insolubility and infusibility may be also caused by extremely high molecular masses,strong intermolecular interaction via secondary valency forces or by the lack of suitable solvents. For a long time, insolubility was the major obstacle for characterization of crosslinked polymers because it excluded analytical methods applicable to linear and branched macromolecules. In particular, the most important structural characteristic of crosslinked polymers, the crosslink density, could mostly be determined by indirect methods only [1],or was expressed relatively by the fraction of crosslinking monomers used in the synthesis.

Acrylamide/2-acrylamido-2-methylpropane sulfonic acid sodium salt-based hydrogels: synthesis and characterization

Relationships between the formation mechanism and the swelling behavior of acrylamide (AAm)/2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS)-based hydrogels were studied. The hydrogels were prepared by free-radical crosslinking copolymerization of AAm and AMPS at 408C in the presence of N,N 0 -methylenebis(acrylamide) (BAAm) as the crosslinker. Both the crosslinker ratio (mole ratio of crosslinker to monomer) and the initial monomer concentration were fixed at 1/82 and 0.700 M, respectively, while the AMPS content in the monomer mixture was varied from 0 to 100 mol%. It was found that the copolymer composition is equal to the monomer feed composition, indicating that the monomer units distribute randomly along the network chains of the hydrogels. The monomer conversion versus time histories as well as the growth rate of the gel during the polymerization were found to be independent of the amount of AMPS in the initial monomer mixture. It was shown that the reaction system separates into two phases at the gel point and the gel grows in a heterogeneous system. The equilibrium degree of swelling of the final hydrogels increases with increasing AMPS content until a plateau is reached at about 10 mol% AMPS. Between 10 and 30 mol% AMPS, the equilibrium gel swelling in water as well as in aqueous NaCl solutions was independent on the ionic group content of the hydrogels. Further increase in the AMPS content beyond this value increased the gel swelling continuously up to 100 mol%. The polyelectrolyte theories based on the counterion condensation cannot explain the observed swelling behavior of AAm/AMPS hydrogels. The swelling curves of the hydrogels in water and in aqueous NaCl solutions were successfully reproduced with the Flory‐Rehner theory of swelling equilibrium including the ideal Donnan equilibria, where the effective charge density was taken as an adjustable parameter. Scaling rules were derived for the ionic group content and the effective excluded volume of the hydrogels. q 2000 Elsevier Science Ltd. All rights reserved.

Formation and structure of polyacrylamide gels

SYNOPSIS Acrylamide and N,N-methylenebis(acry1amide) (AAm-Bis) copolymerization has been investigated in water at a monomer concentration of 1.8 w/v %. Conversion of monomer and pendant vinyl groups was measured as a function of the reaction time up to the onset of macrogelation. Experimental results indicate that 80% of pendant vinyl groups are consumed by cyclization reactions. When the monomer concentration was kept constant at 1.8%, the critical conversion at the gel point shows a minimum at 7.5 mol % Bis. The equilibrium degree of swelling of the polyacrylamide (PAAm) gels is independent of their crosslinker content. Calculation results show that the average reactivity of pendant vinyl groups for intermolecular links decreases as the Bis concentration increases. All these results suggest formation of PAAm microgels prior to the onset of macrogelation. As the reaction proceeds, microgels are connected to a macrogel through their peripheral pendant vinyls and radical ends, whereas those in their interior remain intact. The microgels seem to act as the junction points of the final inhomogeneous networks. 0 1996 John Wiley & Sons, Inc.

Boron pollution in the Simav River, Turkey and various methods of boron removal

MgO can be used effectively at a Mg/B mo[ ratio of 20 for three cleaning cycles wl.h 85,,,0 removal efficiency but must be discharged and refreshed after the third cycle. Consumption of 18,100 tons of MgO per year in the removal system makes this process uneconomical. Boron-specific resin Amberlite IRA-743 in free-amine form is also uneconomical due to expensive regeneration steps. The capacity of this resin in salt-form was found to be 1.5 mg boron ml - ~ resin at 10Yo boron leakage and at a flow rate of 161 h - q - t. The possibility of reuse of 90Yo of the regenerant and production of 4400 tons of boric acid in the removal system with salt-form boron-specific resin, compensate the annual cost of chemicals and also bring in some profit.

Charge density dependence of elastic modulus of strong polyelectrolyte hydrogels

The mechanical behavior of a series of strong polyelectrolyte hydrogels based on acrylamide and 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) was investigated. The hydrogels were prepared at a fixed crosslinker ratio and monomer concentration, but at various charge densities, i.e. AMPS contents between 0 and 100 mol%. The elastic modulus of the hydrogels after their preparation first increases with increasing charge density but then decreases continuously. Investigation of the swollen state properties of the hydrogels shows existence of a large number of ionic groups inside the gel that are ineffective in gel swelling. The results indicate two opposite effects of charged groups on the elastic modulus of the hydrogels: formation of multiplets acting as additional crosslinks in the gel increases the elastic modulus of ionic hydrogels, whereas the effect of the electrostatic interaction of charged groups on elastic free energy decreases the modulus.

Kinetics of Emulsifier-Free Emulsion Polymerization of Methyl Methacrylate

The influences of polymerization temperature, initiator and monomer concentrations, ionic strength of the aqueous phase, as well as ethylene glycol dimethacrylate (EGDM) co-monomer, on the kinetics of the emulsifier-free emulsion polymerization of methyl methacrylate (MMA) and on the properties of the resulting poly(methyl methacrylate) (PMMA) lattices were studied. The polymerizations were carried out using potassium persulfate (KPS) as the initiator. Monodisperse PMMA lattices with particle diameters varying between 0.14–0.37 μm and polymer molecular weights of the order 0.4 × 106 to 1.2 × 106 g/mol were prepared. The initial rate of polymerization increases with increasing temperature, KPS-MMA mole ratio, EGDM content, or with decreasing ionic strength of the aqueous phase. It was shown that the bead size can be limited by reducing the monomer concentration or by using the cross-linking agent EGDM. The ionic strength of the aqueous phase has a dominant effect on final particle diameter and polymer molecular weight. The uniformity of the latex particles increases as the temperature increases or as the initiator concentration decreases. The experimental results can be reasonably interpreted by the homogeneous nucleation mechanism of the emulsifier-free emulsion polymerization of MMA.

Swelling behavior of anionic acrylamide-based hydrogels in aqueous salt solutions: Comparison of experiment with theory

A series of hydrogels were prepared from acrylamide and 2-acrylamido-2- methylpropanesulfonic acid (AMPS) monomers with 0 - 80 mol % AMPS and using N,N9-methylenebis(acrylamide) as the crosslinker. The swelling capacities of hydrogels were measured in water and in aqueous NaCl solutions. The volume swelling ratio qv of hydrogels in water increases sharply when the mole fraction fc of AMPS increases from 0 to 0.06. At higher values of fc from 0.06 up to 0.18, no change in the swelling capacities of hydrogels was observed; in this range of fc, qv becomes nearly constant at 750. However, as fc further increases, qv starts to increase again monotonically over the entire range of fc. At a fixed value of fc, the swelling ratio of hydrogels decreases with increasing salt concentration in the external solution. The results of the swelling measurements in aqueous salt solutions were compared with the predictions of the Flory-Rehner theory of swelling equilibrium. It was shown that the theory correctly predicts the swelling behavior of hydrogels up to 80 mol % charge densities. The method of estimation of the network parameters was found to be unimportant in the prediction of the experimental swelling data. The network parameters used in the simulation only correct the deficiency of the swelling theory.

Temperature sensitive poly(N-t-butylacrylamide-co-acrylamide) hydrogels: synthesis and swelling behavior

A series of temperature sensitive hydrogels was prepared by free-radical crosslinking copolymerization of N-t-butylacrylamide (TBA) and acrylamide in methanol. N,N 0 -methylenebis(acrylamide) was used as the crosslinker. It was shown that the swelling behavior of the hydrogels can be controlled by changing the amount of TBA units in the network chains. Hydrogels immersed in dimethylsulfoxide (DMSO)– water mixtures exhibited reentrant swelling behavior, in which the gels first deswell then reswell if the DMSO content of the solvent mixture is continuously increased. In water over the temperature range of 2 – 64 8C, hydrogels with less than 40% TBA by mole were in a swollen state while those with TBA contents higher than 60% were in a collapsed state. Hydrogels with 40 – 60% TBA exhibited swelling – deswelling transition in water depending on the temperature. The temperature interval for the deswelling transition of 60% TBA gel was found to be in the range from 10 to 28 8C, while for the 40% TBA gel, the deswelling started at about 20 8C and continued until the onset of the hydrolysis of the network chains at around 64 8C. It was shown that the Flory– Rehner theory of swelling equilibrium provides a satisfactory agreement to the experimental swelling data of the hydrogels, provided that the sensitive dependence of the x parameter on both temperature and polymer concentration is taken into account. q 2002 Elsevier Science Ltd. All rights reserved.

Effect of hydrolysis on spatial inhomogeneity in poly(acrylamide) gels of various crosslink densities

The spatial inhomogeneity in poly(acrylamide) (PAAm) gels of various crosslink densities has been investigated with the static light scattering measurements. The gels were prepared using N,N 0 -methylenebis(acrylamide) (BAAm) as a crosslinker at a fixed initial monomer concentration but at various crosslink densities. Ammonium persulfate-N,N,N 0 ,N 0 -tetramethylethylenediamine (TEMED) redox initiator system was used to initiate the polymerization reactions as well as to create charged groups in the aged gels. The gels were characterized by elasticity tests and by light scattering measurements at a gel state just after their preparation. Elasticity measurements show that 91 ‐ 94% of the crosslinker molecules used in the hydrogel preparation were wasted in ineffective crosslinks. Debye ‐ Bueche analysis of the light scattering data indicates frozen concentration fluctuations within the gel samples, which increase continuously with increasing crosslink density of the hydrogels. This phenomenon was explained with the multiple crosslinking reactions leading to the formation of highly crosslinked regions in the final hydrogel. The extent of concentration fluctuations was found to decrease drastically with increasing time of aging of gels in the synthesis reactor, indicating that the hydrolysis of amide groups into carboxylate anions facilitates the homogenization of the gel samples. A thermodynamic model was developed to explain the experimental observations in terms of the osmotic pressure of counterions in the aged gels. q 2003 Elsevier Ltd. All rights reserved.