Selda  DURMAZ and Oguz OKAY*
Istanbul Technical University, Department of Chemistry, 80626 Maslak, Istanbul, Turkey and TUBITAK Marmara Research Center, Department of Chemistry, Gebze, Kocaeli, Turkey

SYNOPSIS
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 400C in the presence of N,N’-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 0.700M, 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 on 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 succesfully 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.
 

Keywords: acryamide, 2-acrylamido-2-methylpropane sulfonic acid, hydrogel formation, swelling, Flory-Rehner theory, counterion condensation.

Phase Separation During the Formation of Poly(acrylamide) Hydrogels
Selda  DURMAZ and Oguz OKAY*
Istanbul Technical University, Department of Chemistry, 80626 Maslak, Istanbul, Turkey and TUBITAK Marmara Research Center, Department of Chemistry, Gebze, Kocaeli, Turkey

SYNOPSIS
The crosslinking polymerization of acrylamide (AAm) was investigated at a monomer concentration of 5 w/v % in water with N,N’-methylenebis(acrylamide) (BAAm) as the crosslinker.  The crosslinker ratio (mole ratio of BAAm to AAm) was set to 1/82.  The fractional monomer conversion x, the weight fraction of gel Wg, and the equilibrium weight swelling ratio qw of the hydrogels were measured as a function of the reaction time.  Experimental results indicate that the first gel molecule in the reaction system appears after a reaction time tc = 4 ± 1 min or after a monomer conversion of  xc = 0.07 ± 0.02.  At this critical point the homogeneous reaction system turned into a heterogeneous one consisting of gel and separated solution phases.  The network formation process took place in this heterogeneous system within the 60 min of the reaction time.  Experimental x, Wg, and qw data indicate that, during the gel growth process, the growing hydrogel molecule cannot occupy the entire polymerization system, which is responsible for the observed phase separation.  It was shown that the volume of the separated liquid phase decreases as the polymerization and crosslinking reactions proceed and finally, the reaction system becomes homogeneous again due to the decreasing dilution degree of the hydrogel.

(Keywords: crosslinking polymerization, acrylamide, poly(acrylamide), phase separation, heterogeneity)
 

Phase Separation in Free-Radical Crosslinking Copolymerization: Formation of Heterogeneous Polymer Networks

Oðuz OKAY

Istanbul Technical University, Department of Chemistry, 80626 Maslak, Istanbul, Turkey, and TUBITAK Marmara Research Center, Department of Chemistry, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey
 

SYNOPSIS
A model combining both the thermodynamic and kinetic aspects of free-radical crosslinking copolymerization (FCC) is presented to predict the formation conditions and the properties of heterogeneous (porous) networks.  The model involves thermodynamic equations describing the phase equilibria between the network and separated phases during FCC of vinyl/divinyl monomers and kinetic equations giving the concentration of reacting species and the polymer properties as a function of the monomer conversion.  Calculation results were presented for styrene - m-divinylbenzene (S - m-DVB) copolymerization system in the presence of inert diluents.  S - m-DVB copolymerization system at a high m-DVB concentration, or, at a low monomer concentration phase separates at the gel point and results in the formation of a microgel solution.  The calculation results also show that the heterogeneity in S - m-DVB copolymer networks increases on increasing DVB or diluent concentration, or, on decreasing the solvating power of the diluent, in accord with the experimental data published previously.  The model also predicts correctly the equilibrium volume swelling ratio of heterogeneous networks in solvents.

Keywords: phase separation, heterogeneous networks, kinetic-thermodynamic modeling, free-radical crosslinking copolymerization.
 
 
 

Gel Growth in Free Radical Crosslinking Copolymerization:  Effect of Inactive Gel Radicals

Oguz OKAY
Istanbul Technical University, Department of Chemistry, 80626 Maslak, Istanbul, and TUBITAK Marmara Research Center, 41470 Gebze, Kocaeli, Turkey.  Fax: (0212) 2856386. E-mail: okayo@itu.edu.tr

SUMMARY
Taking into account the trapped radical centers in gel forming systems, a gel fraction equation was derived using the method of moments.  It was shown that the weight fraction of sol, Ws, relates to the number of crosslinked units per weight-average primary molecule, ?, through the equation    where n = 2 for Flory’s most probable molecular weight distribution, and n = 3 for primary molecules formed by radical combination.  Calculation results demonstrate that the trapped radicals significantly affect the growth rate of the gel molecule. It increases the total radical concentration and accelerates the gel growth.  The difference in the predictions with and without considering the trapped radicals becomes significant as the crosslinker concentration decreases or, as the vinyl group reactivity on the crosslinker or on the polymer decreases.
 
 
 
 
 
 
 

Free-radical Crosslinking Copolymerization of Styrene and Divinylbenzene:
Real Time Monitoring of the Gel Effect Using Fluorescence Probe

Oðuz OKAY
Istanbul Technical University, Department of Chemistry, Maslak, 80626 Istanbul, and TUBITAK Marmara Research Center, P. O. Box 21, 41470 Gebze, Kocaeli, Turkey

Demet KAYA and Önder PEKCAN*
Istanbul Technical University, Department of Physics, Maslak, 80626 Istanbul, Turkey

SYNOPSIS
 The steady-state fluorescence and dilatometric techniques were used to study the free-radical crosslinking copolymerization of styrene (S) and commercial divinylbenzene (DVB) in bulk.  Pyrene (Py) was used as a fluorescence probe for the in situ polymerization experiments.  The time required for a sudden increase in the fluorescence intensity of Py was recorded for various DVB content and temperature.  The monomer conversions and the gel points were recorded by dilatometry.  To interpret the experimental data, a kinetic model was developed for S - DVB copolymerization system. It was shown that both the pendant vinyl groups and Py molecules are affected by the reaction medium in a similar way; their mobility decreases as the DVB concentration increases or the temperature decreases.   The reaction time at which the Py intensity in the fluorescence spectra exhibits a sudden increase corresponds to the reaction time at which the rate of polymerization becomes maximum due to the gel effect.  The results show that the fluorescence technique can be used to follow the onset of the gel effect in S - DVB copolymerization.

Keywords: steady-state fluorescence technique, free-radical crosslinking copolymerization, gel effect, pendant vinyl group reactivity, kinetic modeling.
 
 

Formation of Macroporous Styrene - Divinylbenzene Copolymer Networks:  Theory versus Experiments

Oðuz OKAY
Istanbul Technical University, Department of Chemistry, 80626 Maslak, Istanbul, and TUBITAK Marmara Research Center, P.O. Box 21, 41470 Gebze, Kocaeli,
Turkey

ABSTRACT: A kinetic - thermodynamic model is presented to predict the total porosities of macroporous copolymer networks formed by free-radical crosslinking copolymerization of styrene (S) and commercial divinylbenzene (DVB, a mixture of meta and para DVB isomers and ethylstyrene).  The kinetic part of the model predicts, based upon the method of moments, the concentration of the reacting species, the gel and sol properties as a function of the monomer conversion.  The thermodynamic part of the model describes the phase equilibria between the gel and separated phases during the S - DVB copolymerization and predicts the volume of the separated phase, which is the pore volume of the crosslinked material, as a function of the monomer conversion.  Calculation results show that the porosity of S - DVB networks increases as the polymer - diluent interaction parameter increases, or, as the initial monomer concentration decreases.  Porosity also increases on increasing the DVB content of the monomer mixture.  Both the polymerization temperature and the initiator concentration affect significantly the kinetics of S - DVB copolymerization.  However, the final porosity of S - DVB copolymers is largely insensitive to the amount of the initiator and to the polymerization temperature.  All calculation results are in accord with the experimental data published previously.

Keywords: porosity formation, phase separation, macroporous styrene - divinylbenzene copolymer networks, kinetic-thermodynamic modeling, free-radical crosslinking copolymerization.
 

Contract grant sponsor:  Scientific and Technical Research Council of Turkey (TUBITAK), contract grant number: TBAG - 1561.
 
 
 
 

Pore-memory of Macroporous Styrene - Divinylbenzene Copolymers

Erol ERBAY1,2 and Oðuz OKAY1,3 *
1 Kocaeli University, Department of Chemistry, 41300 Izmit, Kocaeli, Turkey,
2 PETKIM Petrochemicals Holding Inc., Research Center, P. O. Box 9, 41740 Korfez, Kocaeli, Turkey,
3 TUBITAK Marmara Research Center, Department of Chemistry, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey.

ABSTRACT: The variation of the pore structure of styrene-divinylbenzene (S-DVB) copolymer beads depending on the drying conditions was investigated.  Macroporous S-DVB copolymer beads with various DVB contents were prepared in the presence of toluene - cyclohexanol mixtures as a diluent.  It was found that the pores of 101 nm in radius, corresponding to the interstices between the microspheres, collapse upon drying of the copolymers from toluene.  The collapsed pores reexpand again if the copolymers were dried from methanol.  The collapse - reexpansion process of the pores was found to be reversible, indicating that the actual pore structure formed during the crosslinking copolymerization is memorized by the copolymer network.  The magnitude of the pore structure variation increased on worsening the polymer - diluent interactions during the gel formation process due to the simultaneous increase in crosslink density distribution.
Key words: macroporous styrene-divinylbenzene copolymers, pore structure, pore-memory, pore structure variation, crosslink density distribution.

____________________________
* To whom correspondence should be addressed. Present address: Istanbul Technical University, Department of Chemistry, 80626 Maslak, Istanbul, Turkey.
 

Swelling Behavior of Poly(acrylamide-co-sodium acrylate) Hydrogels in Aqueous Salt Solutions: Theory versus experiments
 

Oðuz OKAY1*,   Safiye B. SARIISIK2
 

1 Istanbul Technical University, Department of Chemistry, 80626 Maslak, Istanbul, and, TUBITAK Marmara Research Center, P. O. Box 21, 41470 Gebze, Kocaeli, Turkey

2 Kocaeli University, Department of Chemistry, 41300 Izmit, Kocaeli, Turkey,
 
 

____________________________
* To whom correspondence should be addressed.
    Fax: 0262-6412300
    E-mail: oguz@mam.gov,tr
 

ABSTRACT:  The predictions of the Flory-Rehner theory of swelling equilibrium including the ideal Donnan equilibria were compared with the experimental swelling data obtained from poly(acrylamide-co-sodium acrylate) hydrogels swollen in water and in aqueous salt (NaCl) solutions.  For this comparison, the fraction of counterions which are effective in the gel swelling was taken into account.  The ionic hydrogels were prepared from acrylamide and sodium acrylate (NaAc) monomers with 0 to 5 mol % NaAc and using N,N’-methylenebis(acrylamide) as the crosslinker.   As expected, at a fixed crosslinker ratio, the volume swelling ratio of hydrogels in water increases sharply when the mole fraction of NaAc increases or the NaCl concentration in the external solution decreases.  Taking into account the wasted counterions  within the hydrogels, the theory correctly predicts the swelling behavior of hydrogels in water and in aqueous salt solutions.  Not predicted by the theory is the increased swelling ratio of hydrogels with less than 4 mol % NaAc content with increasing salt concentration from 10-1M to 100M.
 

Key words: anionic hydrogels, polyacrylamide, sodium acrylate, swelling, swelling theory.
 
 
 
 
 

Macroporous Styrene - Divinylbenzene Copolymers: Formation of Stable Porous Structures During the Copolymerization
 
 

Erol Erbay1,2 and Oðuz Okay1,3*
 

1Kocaeli University, Department of Chemistry, 41300 Izmit, Kocaeli, Turkey,
2PETKIM Petrochemicals Holding Inc., Research Center, P. O. Box 9, 41740
  Korfez, Kocaeli, Turkey,
3TUBITAK Marmara Research Center, Department of Chemistry, P. O. Box 21,
  41470 Gebze, Kocaeli, Turkey.

 _________________________
* To whom correspondence should be addressed at Istanbul Technical
   University, Department of Chemistry, Maslak 80626, Istanbul, Turkey.

 
Summary
The variation of the pore structure of styrene-divinylbenzene (S-DVB) copolymer beads depending on the polymerization time was investigated.  Macroporous S-DVB copolymer beads with 10 mol % DVB content were prepared in the presence of cyclohexanol as a diluent.  It was found that the stable pores in S-DVB copolymers mainly form at an early stage of the copolymerization, i.e., at the gel point.  Thus, the early phase separated portions of the network, where the crosslink density is locally high, do not collapse on drying and illustrate the stable part of the porosity of S-DVB copolymers.  The number of stable pores does not change much during the whole course of the copolymerization.  The maximum porosity first decreases on rising the post-gelation time due to the decreasing degree of dilution of the gel phase.  Then, it increases continuously due to the increasing crosslink density of the gel.  The pores formed at a later stage of the copolymerization are unstable and they collapse during the drying process.  This is due to the lower crosslink density of the network regions forming later.
 

Swelling Behavior of Anionic Acrylamide-based Hydrogels in Aqueous Salt Solutions:  Comparison of Experiment with Theory

Oðuz OKAY1,2,   Safiye B. SARIIÞIK1,   and   Sibel D. ZOR1
1Kocaeli University, Department of Chemistry, 41300 Izmit, Kocaeli, Turkey,
2TUBITAK Marmara Research Center, Department of Chemistry, P. O. Box 21, 41400 Gebze, Kocaeli, Turkey

ABSTRACT: A series of hydrogels were prepared from acrylamide and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) monomers with 0 to 80 mol % AMPS and using N,N’-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.
Key words: anionic hydrogels, polyacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, swelling, swelling theory.
 
 

Heterogeneities in polyacrylamide gels immersed in acetone-water mixtures

Oðuz OKAY1, 2  and  Ufuk AKKAN3

1 Istanbul Technical University,  Department of Chemistry, 80626 Maslak,  Istanbul, Turkey,
2 TUBITAK Marmara Research Center, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey,
3 Kocaeli University, Department of Chemistry, 41300 Izmit, Kocaeli, Turkey.
 

Summary
Immersion of polyacrylamide gels in acetone-water mixtures with a high content of acetone leads to the formation of heterogeneous structures.  The extent of heterogeneity increases as the initial swelling degree of the gel increases, or, as the polarity of the nonsolvent decreases.  The nature of the deswelling curves indicates that the polymer-rich regions of the gel formed by the collapse transition block the solvent diffusion outside the gel.  This prevents the equilibration of the gel volume with surroundings.
 
 
 

Swelling of Polyacrylamide Gels in Polyacrylamide Solutions
Nilhan Kayaman1,  Oðuz Okay1,2*  and  Bahattin M. Baysal1,3
1 TUBITAK Marmara Research Center, Department of Chemistry,  P.O. Box 21, 41470
   Gebze, Kocaeli, Turkey,
2 Kocaeli University, Department of Chemistry, 41300 Izmit, Kocaeli, Turkey,
3 Boðazici University, Department of Chemical Engineering, 80815 Bebek, Istanbul,
 Turkey.

ABSTRACT: Swelling behavior of polyacrylamide (PAAm) and polyacrylamide-co-polyacrylic acid (PAAm-co-PAAc) gels was investigated in aqueous solutions of monodisperse PAAm’s with molecular weights ( ) ranging from 1.5x103 to 5x106 g/mol.  The volume of the gels decreases as the PAAm concentration in the external solution increases.  This decrease becomes more pronounced as the molecular weight of PAAm increases.  The classical Flory - Huggins (FH) theory correctly predicts the swelling behavior of non-ionic PAAm gels in PAAm solutions.  The polymer-polymer interaction parameter   was found to decrease as the molecular weight of PAAm increases.  The swelling behavior of PAAm-co-PAAc gels in PAAm solutions deviates from the predictions of the FH theory.  This is probably due to the change of the ionization degree of AAc units depending on the polymer concentration in the external solution.

Keywords: swelling, polyacrylamide gels, swelling in polymer solutions, polymer-polymer interaction parameter.

_______________________________________
*To whom correspondence should be addressed.
 
 

Effects of Cyclization and Electrostatic Interactions on the Termination Rate of Macroradicals in Free-Radical Crosslinking Copolymerization
 
 

Muzaffer Keskinel and Oðuz Okay*
 

Kocaeli University, Department of Chemistry, 41300 Izmit, Kocaeli, Turkey, and
TUBITAK Marmara Research Center, Department of Chemistry, P. O. Box 21,
  41470 Gebze, Kocaeli, Turkey
 
Summary
Effects of cyclization and ionic group contents on the termination rate of macroradicals formed at zero monomer conversion were investigated.  For this purpose, the pregel regime of free-radical methyl methacrylate / ethylene glycol dimethacrylate (MMA/EGDM) and acrylamide / N,N’-methylenebisacrylamide (AAm/BAAm) copolymerization systems was studied by means of the dilatometric technique. To eliminate the chain-length dependent variation of the termination rates, different sets of experiments were carried out each at a fixed monomer and initiator concentration.  At low crosslinker contents, the termination rate of zero-conversion macroradicals was enhanced in crosslinking copolymerizations compared to linear polymerization. This is due to the cyclization reactions which reduce the size of the macroradical coils and thus, enhance the termination rates due to the lowering of the thermodynamic excluded volume effect.  As the amount of the crosslinker increases, an enhancement in the initial rate of polymerization is observed in all series of experiments, indicating that steric effects on segmental diffusion dominate at high crosslinker contents. The results also indicate a slower rate of termination of ionic macroradicals compared to the non-ionic radicals of the same molecular weight and points the significance of the thermodynamic excluded volume effect on rising the ionic group content.
 
 

MICROGELS - INTRAMOLECULARLY CROSSLINKED MACROMOLECULES WITH A GLOBULAR STRUCTURE

By W. Funke1, B. Joos-Müller2, O. Okay3 and U. Seitz4
 

1  II. Institut für Technische Chemie, Universität Stuttgart. D 70569, Stuttgart
2  Forschungsinstitut für Pigmente und Lacke e.V., D 70569 Stuttgart
3  Marmara Research Center, 41470 Gebze-Kocaeli, and Kocaeli University,
Dept. of Chemistry, Turkey
4  Fachhochschule für Technik (FHTE), PLZ? Esslingen
 
 
 
 

 CONTENTS

1 HISTORY 6
2 DEFINITIONS 10
3 GENERAL ASPECTS OF MICROGEL SYNTHESIS 12
4 MICROGEL FORMATION IN EMULSION 15
4.1 MACROEMULSION POLYMERIZATION 15
4.2 MICROEMULSION POLYMERIZATION 20
4.3 CHARACTERISTIC PROPERTIES OF MICROGELS 20
4.4 EXPANDED (PRESWOLLEN) AND HETEROGENEOUS (POROUS) MICROGELS 22
5 MICROGELS BY EMULSION COPOLYMERIZATION OF SELF-EMULSIFYING UNSATURATED POLYESTERS AND COMONOMERS 24
5.1 UNSATURATED POLYESTERS AS SELF-EMULSIFYING COMPONENTS OF COPOLYMERIZATION 25
5.1.1 Solubilization of the monomer mixture 26
5.1.2 Critical micelle concentration of unsaturated polyesters 27
5.1.3 Micelles and microemulsion droplets 28
5.2 EMULSION COPOLYMERIZATION OF SELF-EMULSIFYING UNSATURATED POLYESTERS AND COMONOMERS 28
5.2.1 Molar mass and diameter of microgels 30
5.2.2 Viscosity 31
5.3 CHARACTERIZATION AND PROPERTIES OF MICROGELS FROM SELF-EMULSIFYING UNSATURATED POLYESTERS AND COMONOMERS 32
5.3.1 Viscosity and hydrodynamic diameter 33
5.3.2 Reactive groups 34
5.3.3 Rheological properties of EUP/comonomer-microgels 35
6 MICROGEL FORMATION IN SOLUTION BY FREE-RADICAL CROSSLINKING (CO)POLYMERIZATION 35
6.1 THEORETICAL CONSIDERATIONS 35
6.2 EXPERIMENTAL EVIDENCES OF INTRAMOLECULAR CROSSLINKING 38
6.3 MICROGEL SYNTHESIS BY RADICAL COPOLYMERIZATION 41
6.4 CHARACTERISTICS OF MICROGELS 45
7 MICROGEL FORMATION BY ANIONIC POLYMERIZATION 46
7.1 1,4-DIVINYLBENZENE (1,4-DVB) 47
7.2 1,3-DIVINYLBENZENE (1,3-DVB) 52
7.3 ETHYLENEGLYCOL DIMETHACRYLATE (EDMA) 53
7.4 MICROGELS FROM OTHER DIVINYL MONOMERS 55
8 OTHER TECHNIQUES FOR MICROGEL SYNTHESIS 56
9 SURFACE MODIFICATION OF MICROGELS 57
9.1 REACTIONS FOR MODIFYING AND CHARACTERIZING SURFACES OF MICROGELS 58
9.1.1 Characterization of divinylbenzene microgels 58
9.2 AGING OF DIVINYLBENZENE MICROGELS 59
9.3 INTRODUCTION OF OTHER FUNCTIONAL GROUPS IN MICROGELS 60
9.3.1 Surface modification  by hydroxy groups 60
9.3.2 Surface modification by epoxy groups 60
9.3.3 Surface modification by ozone 61
9.3.4 Surface modification by dye molecules 61
9.3.5 Modification by polymer analogous esterification 61
9.4 SYNTHESIS AND MODIFICATION OF MICROGELS FOR BIOCHEMICAL PURPOSES 62
9.4.1 Functional comonomers 62
9.4.2 Copolymerization in a homogeneous-aqueous solution 63
9.4.3 Copolymerization in an aqueous emulsion 63
10 APPLICATIONS OF MICROGELS 64
10.1 ORGANIC COATINGS 65
10.2 MICROGELS AS CARRIERS FOR DYES 68
10.3 MICROGELS AS SUBSTRATES FOR BIOMEDICAL AND DIAGNOSTICAL PURPOSES 69
10.4 MICROGELS AS FILLERS 70
11 CONCLUDING REMARKS 70
12 REFERENCES 72

Structure and Protein Separation Efficiency of Poly(N-isopropylacrylamide) Gels: Effect of Synthesis Conditions

Nilhan Kayaman1,  Dilek Kazan1,  Altan Erarslan1,2,  O?uz Okay1,2*,
and Bahattin M. Baysal1,3

1 TUBITAK Marmara Research Center, Research Institutes for Basic Sciences and
     Genetic Engineering & Biotechnology, P.O.Box 21, 41470 Gebze, Kocaeli,
     Turkey;
2 Kocaeli University, Faculty of Arts and Sciences, Department of Chemistry, 41300
    Izmit, Kocaeli, Turkey;
3 Bo?aziçi University, Department of Chemical Engineering, 80815 Bebek, Istanbul,
    Turkey.
 

Keywords: Poly(N-isopropylacrylamide), temperature-sensitive gels, gel swelling, protein separation efficiency, gel extraction.
 

_______________________________________
*To whom correspondence should be addressed.
 
SYNOPSIS
Crosslinked poly(N-isopropylacrylamide) (PNIPA) gels with different crosslink densities in the form of rods and beads were prepared by free-radical crosslinking copolymerization.  Solution and inverse suspension polymerization techniques were used for the gel synthesis.  The gels were utilized to concentrate dilute aqueous solutions of penicillin G acylase (PGA), bovine serum albumin (BSA), and 6-aminopenicillanic acid (6-APA).  The discontinuous volume transition at 34?C observed in the gel swelling was used as the basis of concentrating dilute aqueous protein solutions.  PNIPA gels formed below 18oC were homogeneous whereas those formed at higher temperatures exhibited heterogeneous structures.  The water absorption capacity of PNIPA gels in the form of beads was much higher and their rate of swelling was much faster than the rod-shaped PNIPA gels.  It was also found that the polymerization techniques used significantly affect the properties of PNIPA gels.  The separation efficiency decreased when the protein molecules PGA or BSA in the external solution were replaced with small molecular weight compounds such as 6-APA.  The protein separation efficiency by the gel beads increased to 100% after coating the bead surfaces with BSA.
 
 

Effects of Cyclization and Pendant Vinyl Group Reactivity on the Swelling Behavior of Polyacrylamide Gels
 

Oðuz Okay1,2,  Nurgül K. Balýmtaþ1, and Hamid J. Naghash2
 

1Kocaeli University, Department of Chemistry, 41300 Izmit, Kocaeli, Turkey
2TUBITAK Marmara Research Center, Department of Chemistry, P. O. Box 21,
  41470 Gebze, Kocaeli, Turkey
 
 

Summary
A series of polyacrylamide (PAAm) gels were prepared by free-radical crosslinking copolymerization of acrylamide and N,N’-methylenebis(acrylamide) (BAAm) in water at various crosslinker (BAAm) and chain transfer agent (isopropyl alcohol, IPA) concentrations.  It was shown that only 5 % of the crosslinker used in the feed forms effective crosslinks in the final hydrogels.  At BAAm contents as high as 3 mole %, the equilibrium swelling ratio of the gels in water is independent of the crosslinker content in the feed.  This is due to the prevailing multiple crosslinking reactions during the gel formation process.  At a fixed crosslinker content, the onset of gelation is shifted towards higher conversions and reaction times as the amount of IPA increases.  Addition of IPA in the monomer mixture also increases the equilibrium swelling ratio of PAAm gels.  It was shown that the gel crosslinking density increases on rising IPA concentration in the feed due to the increasing rate of intermolecular crosslinking reactions.
 
 

Swelling of Polyacrylamide Gels in Aqueous Solutions of Ethylene Glycol Oligomers

Nilhan Kayaman1, Oðuz Okay1,2, and Bahattin M. Baysal1,3*

1 TUBITAK Marmara Research Center, Research Institute for Basic Sciences, Department
 of Chemistry,  P.O. Box 21, 41470 Gebze, Kocaeli, Turkey,
2 Kocaeli University, Department of Chemistry, 41300 Izmit, Kocaeli,
3 Boðazici University, Department of Chemical Engineering, 80815 Bebek, Istanbul,
 Turkey.
ABSTRACT
Addition of a small amount of ethylene glycol oligomers (OEG) with number of repeat units y = 2 - 4 in the aqueous solution leads to the contraction of both linear and crosslinked poly(acrylamide-co-acrylic acid) chains. The results present clear evidence for the screening effect of OEG on the ionic groups of the polymer chains.  Measurements indicate that the preswollen poly(acrylamide-co-acrylic acid) gels immersed in aqueous OEG solutions are at equilibrium.  However, as the molecular weight of OEG increases, these gels cannot attain their equilibrium swelling ratio due to the appearance of nonequilibrium structures.  The stability of these structures increases as the initial swelling degree of the gels  increases.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Real Time Monitoring of Polymerization Rate of Methyl Methacrylate Using
Fluorescence Probe
Ö. Pekcan* and Y. Y?lmaz
Istanbul Technical University, Department of Physics, Maslak, 80626 Istanbul, TURKEY
O. Okay
TUBITAK Marmara Research Center and Kocaeli University,  Department of Chemistry, P.O. Box 21, 41470 Gebze, Kocaeli, TURKEY

SYNOPSIS
 The steady-state fluorescence technique was used to study the polymerization rate and the autoacceleration due to the gel effect in free-radical polymerization of methyl methacrylate. Pyrene was used as a fluorescence probe for the in situ polymerization experiments. The times required for the onset of the gel effect were recorded for various polymerization temperatures. A simple kinetic model was used to interpret the experimental data. The results show that the fluorescence technique can be used to follow the onset of the gel effect and the activation energy during the polymerization processes.
Keywords: steady-state fluorescence technique, free-radical polymerization, gel effect, activation energy
 
 

ABSTRACT: Photoinitiated free-radical crosslinking copolymerization of methyl methacrylate and ethylene glycol dimethacrylate has been investigated in toluene at a monomer concentration of 35 w/v %. The photoinitiators used were benzoin, benzoin methyl ether, benzoin ethyl ether, and 2,2-dimethoxy-2-phenyl-acetophenone. Conversion of monomer and pendant vinyl groups as well as the size of the pregel polymers were measured as a function of the reaction time up to the onset of macrogelation. Approximately 30 % of the pendant vinyl groups were found to be consumed by cyclization reactions. The fraction of units in cycles is independent on the EGDM concentration. Due to the cyclization reactions, the accessibility of the radical centers and the pendant vinyl groups for other polymer molecules are strongly reduced. As a result, both termination and crosslinking reactions are controlled by the segmental diffusion of the polymer radicals. Calculations indicate a 1 - 2 orders of magnitude decrease in the average reactivity of pendant vinyls for intermolecular reactions compared to the monomeric vinyls. This drastic decrease in the pendant reactivity is mainly responsible for the delay in the gel point. Contrary to the gelation theories, the size distribution curves of the pregel polymers change from monomodal to bimodal distributions as polymerization proceeds. This finding confirms the coagulation type gelation mechanism of compact primary particles and indicates the shape polydispersity of the pregel polymers.
(Keywords: photopolymerization, microgel formation, cyclization, pendant reactivity, termination rate.)
 
 
 

Phase Transition of Polyacrylamide Gels in PEG Solutions
Nilhan Kayaman1, O?uz Okay1,2, and Bahattin M. Baysal1,3*
1 TUBITAK Marmara Research Center, Research Institute for Basic Sciences, Department
of Chemistry, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey,
2 Kocaeli University, Department of Chemistry, Izmit, Kocaeli,
3 Bo?azici University, Department of Chemical Engineering, 80815 Bebek, Istanbul,
Turkey.
 
ABSTRACT
Non-ionic polyacrylamide gels immersed in aqueous solutions of poly(ethylene glycol) (PEG) exhibit a continuous volume change upon continuous increase of the PEG concentration in the external solution. The volume change becomes discontinuous when ionizable groups are incorporated into the network. As the proportion of ionizable groups increases or, as the molecular weight of PEG decreases, the critical concentration of PEG required for a discontinuous volume collapse rises. The distribution of PEG inside and outside the gel phase changes with gel volume; it also exhibits a discontinuous change at the phase transition. Flory - Huggins theory gives a qualitative description of the phenomenon.
Keywords: phase transition, swelling, acrylic acid - acrylamide copolymer gels, poly(ethylene glycol)
 
 
 
 

SYNOPSIS
 The steady-state fluorescence technique was used to study the sol-gel transition in free-radical crosslinking copolymerization of methyl methacrylate and ethylene glycol dimethacrylate (EGDM). Pyrene methyl pivalate was used as a fluorescence probe for the in situ polymerization experiments. The times required for the onset of gelation tc and the critical exponent ??were recorded for various EGDM contents and at different polymerization temperatures. A simple kinetic model was used to interpret the experimental gel point data. The results show that the fluorescence technique can be used to measure the critical exponent ?, the gel point tc and the activation energy during sol-gel phase transition processes.
Keywords: steady-state fluorescence technique, free-radical crosslinking copolymerization, gel point, critical exponent, activation energy.
 
 
 
 

Size Distribution of Polymers During the Photoinitiated Free-Radical Copolymerization of Methyl Methacrylate and Ethylene Glycol Dimethacrylate
Hamid J. Naghash1,3, O?uz Okay1,2*, and Yusuf Ya?c?3
1TUBITAK Marmara Research Center, Department of Chemistry, P. O. Box 21,
41470 Gebze, Kocaeli, Turkey,
2Kocaeli University, Department of Chemistry, 41300 Izmit, Kocaeli, Turkey,
3Istanbul Technical University, Department of Chemistry, Maslak, 80626 Istanbul,
Turkey
 
 ___________________________________
*To whom correspondence should be addressed.
 
Summary
Photoinitiated methyl methacrylate - ethylene glycol dimethacrylate (MMA / EGDM) copolymerization has been investigated in toluene at a monomer concentration of 35 w/v %. Diphenyl-2,4,6-trimethylbenzoylphosphine oxide (DPTPO) was used as a photo-initiator at 0.3 wt % concentration. Monomer conversions and the size distribution of the polymer molecules were measured as a function of the reaction time up to the onset of macrogelation. Compared to the photoinitiators benzoin or benzoin derivatives, gelation process proceeds at much higher rates in the presence of DPTPO. The size distribution curves obtained by size-exclusion chromatography (SEC) change from monomodal to bimodal distributions as the polymerization proceeds. Strongly bimodal SEC curves were obtained in the close vicinity of the gel point. This finding confirms the coagulation type gelation mechanism of compact primary particles. It also indicates that the present gelation theories cannot describe the structure dependent kinetics of free-radical crosslinking copolymerization.
 
 
 

Kinetics of Emulsifier-Free Emulsion Polymerization of Methyl Methacrylate
Taner Tanr?sever1, O?uz Okay2*, and Inci Çetin Sönmezo?lu3
1 Balikesir University, Necatibey Education Faculty, Department of Chemistry
Education, 10100 Bal?kesir, Turkey,
2 TUBITAK Marmara Research Center and Kocaeli University, Department of
Chemistry, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey,
3 Y?ld?z Technical University, Department of Chemistry, 80270 Istanbul, Turkey.

SYNOPSIS
The influences of polymerization temperature, initiator and monomer concentrations, ionic strength of the aqueous phase as well as ethylene glycol dimethacrylate (EGDM) comonomer on the kinetics of the emulsifier-free emulsion polymerization of methyl methacrylate (MMA) and on the properties of the resulting poly(methyl methacrylate) (PMMA) latices were studied. The polymerizations were carried out using potassium persulfate (KPS) as the initiator. Monodisperse PMMA latices with particle diameters varying between 0.14 - 0.37 ?m and polymer molecular weights of the order 0.4x106 to 1.2x106 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 crosslinking agent EGDM. The ionic strength of the aqueous phase has 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 reasonably interpreted by the homogeneous nucleation mechanism of the emulsifier-free emulsion polymerization of MMA.
Keywords: emulsifier-free emulsion polymerization, methyl methacrylate, ethylene glycol dimethacrylate, polymethyl methacrylate latices, ionic strength.
________________________________________________
* To whom correspondence should be addressed.
 
 
 
 

 
Formation and Structure of Polyacrylamide Gels
HAMID J. NAGHASH1 AND O?UZ OKAY1,2*
1 TUBITAK Marmara Research Center, Department of Chemistry, P.O. Box 21,
41470 Gebze, Kocaeli, Turkey,
2 Kocaeli University, Department of Chemistry, Izmit, Kocaeli, Turkey

SYNOPSIS
Acrylamide and N,N'-methylenebis(acrylamide) (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 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.
Keywords: polyacrylamide gels, microgels, cyclization, gelation kinetics, pendant vinyl groups.
*To whom correspondence should be addressed.
 
 
 
 

 
 Teknoloji mi üretelim? Temel Araþtýrma mý yapalým?

Oðuz Okay*
    Ülkemizde önemi giderek anlaþýlmaya baþlayan bilimsel araþtýrmalar artýk güncel tartýþmalar arasýna girdi.  Cumhuriyet Bilim Teknik dergisinin katkýlarý ile bilimsel araþtýrmanýn önemi, ne gibi kýstaslarý olduðu, ülkemizde araþtýrmaya verilen deðer ve dünyadaki durumumuz güzel tartýþmalara neden oldu.  Bu konularda çýkan yazýlarý keyifle okuduk.  Görülen o ki, üzerinde uzlaþýlan nokta bilimsel araþtýrmada sesimizi dünyaya duyurmak için büyük bir istek olduðu.  Artýk dünyada geçerli olan bilimsel araþtýrma kriterleri de gözardý edilemiyor ve uluslararasý hakemli dergilerde çýkan yayýnlar, bu yayýnlarýn dünyada uyandýrdýðý yanký tartýþmasýz bir kýstas olarak kabul görüyor.  Diðer ortaya çýkan bir gerçek ise ülkemizde en az üniversitelerin sayýsý kadar "bilimsel" dergi çýktýðý, ancak bunlarýn hiçbirinin Science Citation Index veya Current Contents gibi ciddi uluslararasý yayýn tarama indekslerine giremediði.  Yani bu dergilerde araþtýrma sonuçlarýný yayýnlayan araþtýrýcý bilerek yada bilmeyerek kendini genel kriterlerin dýþýnda tutuyor.

    Yukarýdaki konularýn dýþýnda bence çok önemli bir konudan bu tartýþmalarda pek söz edilmedi.  Temel araþtýrma mý yapalým?  Uygulamalý araþtýrma yapýp teknoloji mi üretelim?  Yoksa temel  - uygulamalý araþtýrma ayrýmý yapmadan bilimsel araþtýrma mý yapalým?

     Bu konuda genellikle politikacýlarýn ve idarecilerin görüþü temel araþtýrmanýn ülkemiz için bir lüks olduðu ve uzun bir sürece gereksinim duyduðu. Bu görüþe göre (1. görüþ) araþtýrma fonlarý tümüyle ülkenin gereksinim duyduðu alanlara teknoloji üretmek üzere kaydýrýlmalý ve bu þekilde temel araþtýrma için gereksiz vakit, insan gücü ve kaynak harcanmamalý.  Bu 1. görüþü benimseyenler genellikle Japonya örneðini veriyorlar ve endüstri ile bilim arasýnda Japon-usülü-kooperasyon ile kýsa bir süreç sonunda teknoloji üretmeyi tasarlýyorlar.

    Ýkinci bir görüþ, temel araþtýrmanýn yapýlabileceði ancak asýl önemli olan ve kaynaklarýn aktarýlmasý gereken konunun uygulamalý araþtýrma ve teknoloji üretimi olduðu.  Bu görüþe göre temel araþtýrma ve teknoloji üretimi tamamen ayrý konular.  Diðer yandan yukarýda özetlenen her iki görüþe göre teknoloji üreten bilim elemanlarýnýn uluslararasý dergilerde yayýn yapma gibi bir gereksinimleri de yok.

    Son bir görüþ ise, ülkemiz þartlarýnda, ilk önce bilgi birikimi oluþturulmasý ve temel araþtýrma - teknoloji üretimi ayrýmýnýn yapýlmamasý.  Temel araþtýrma yapýlmadan yani yeterli bilgi birikimi olmadan teknoloji üretilemeyeceði ve ancak kritik bir kütle oluþturduktan sonra teknolojiye sýra geleceði belirtiliyor.

    Son on sene bence bu görüþlerin rekabeti ile geçti ve 1. görüþ en çok taraftar topladý.  80'li yýllarýn sonlarýnda Stuttgart Üniversitesinde misafir araþtýrýcý olarak çalýþýrken bir Alman meslektaþýmýn dikkatini çekmiþ, sordu: "Çalýþtýðýnýz yer hep TÜBÝTAK - Gebze olduðu halde, neden yayýnlarýnýzdaki adres sürekli olarak deðiþti?"  Gerçekten de 1985-1993 yýllarý arasýnda çýkan yayýnlarýmda ki adresime baktýðýmda posta adresinin ayný (P.K. 21, Gebze, Kocaeli) ancak araþtýrma merkezinin isminin sürekli deðiþtiði görülüyordu:
1) TÜBÝTAK MBEAE (Marmara Bilimsel ve Endüstriyel Araþtýrma Enstitüsü)
2) TÜBÝTAK TBAE (Temel Bilimler Araþtýrma Enstitüsü)
3) TÜBÝTAK TÜGAM TBAE (Gebze Araþtýrma Merkezi Temel Bilimler Araþtýrma Enstitüsü)
4) TÜBÝTAK MBEAM (Marmara Bilimsel ve Endüstriyel Araþtýrma Merkezi)
5) TÜBÝTAK MAM  TBAE (Marmara Araþtýrma Merkezi Temel Bilimler Araþtýrma Enstitüsü)

Bu isim deðiþiklikleri aslýnda temel - uygulamalý bilim çekiþmesinin ve ülkemize özgü süreksizliðin bir sonucu idi; gerçek araþtýrýcýlarýn çalýþmalarý sürekli devam etti, ancak antetli kaðýtlar, tabelalar, yöneticiler deðiþti durdu.

Ne yapmalý?
    Avrupa'da 300 - 400 yýllýk bir sürecin ürünü olan bilim ve teknolojiyi 70 yýllýk bir süreçte ülkemizde oluþturmaya çalýþan 1. görüþ sahipleri bence ya hiç bilim tarihi okumadýlar, ya da yayýný pek olmayan ancak teknoloji üreteceklerini iddia eden bazý her-devrin-adamlarýný dinliyorlar.  Science dergisinin editörü Daniel E. Koshland'ýn geçtiðimiz yýl Ocak ayýnda çýkan bir yazýsýnda belirttiði gibi "temel araþtýrma bize X - ýþýnlarýný, penisilini, çocuk felci aþýsýný, hafif polimerleri, bilgisayarlarý,... getirdi; bunlar dünyayý devrimci bir þekilde deðiþtiren buluþlarýn sadece birkaçýdýr" (1).  Diðer yandan Alman Araþtýrma Kurumu DFG' nin Baþkaný Prof. Dr. Wolfgang Frühwald geçenlerde idarecilerden yakýnýyor ve onlarýn acil olarak temel araþtýrmanýn önemi ve deðeri üzerine özel derslere ihtiyacý olduðunu  vurguluyordu.  Frühwald'e göre "onlarýn çoðu röntgen ile tanýnýn X ýþýnlarý keþfedilmeden, radyo - TV teknolojilerinin yaygýnlaþmasýnýn Herz dalgalarý bulunmadan gerçekleþtiðine inanýyorlar" (2).

    20'li yýllarýn Almanya' sýna gidebilseydik Hermann Staudinger adlý bir Alman kimyacýnýn (1953 Kimya Nobel Ödülü) yüksek molekül aðýrlýklý bazý maddelerin fiziksel özelliklerini hararetle araþtýrdýðýný ve bulgularýný yayýnladýðýný görürdük.  40' lý yýllarýn Amerikasýnda ise Paul J. Flory adlý genç bir kimyacý (1974 Kimya Nobel Ödülü) polimer çözelti termodinamiði, kauçuk elastisitesi gibi konularda temel araþtýrmalar yapýyordu.  20' li yýllarýn bu "uygulama alaný hiç olmayan, lüks ve 1. görüþ sahiplerince deðersiz" çalýþmalarý, takip eden yýllarda dünyamýzda büyük bir devrim yarattý, plastik çaðýný getirdi.  2000 yýlýnda dünyada kiþi baþýna madde tüketiminin 287 litreye ulaþacaðý ve bunun % 80 inin plastikler olacaðý belirtiliyor.  Bu konuda daha pek çok örnek verilebilir.  Dünyada bilim ve teknolojinin geliþmesinde hep ayný uzun yolu görüyoruz:  Her sene uluslararasý dergilerde çýkan binlerce yayýn ve bu yayýnlarýn onbinlercesinin bir araya gelmesi ile oluþan bilgi birikimi, ardýnda gözükmeye baþlayan bir uygulama alaný ve nihayet teknoloji üretimi.  Her yayýn ileriye, bilinmeyene doðru atýlan sadece bir adýmdýr; ancak bu adýmlar bizi teknoloji üretimi için gereken kritik bilgi birikim noktasýna götürebilir.

    "Ben sizden yayýn deðil buluþ istiyorum" diyen ortaçaðdan kalma köhnemiþ zihniyet, yayýný bir gecede hazýrlanan bir yazý olarak algýlýyor, buluþu ise örneðin Newton bir aðaç altýnda boþ boþ oturup uyuklarken kafasýna düþen elmanýn yarattýðý bir tesadüf olarak görüyor.  Acaba bu zihniyete sahip kimselerin kafalarýna tonlarca elma atsak ses gelirmi dersiniz?

    Dünyaca ünlü teknoloji üreten firmalara baktýðýmýzda, yaptýklarý yayýnlar ve çýkardýklarý patentler ile üniversitelerle hemen hemen baþabaþ gittiklerini görüyoruz.  Diðer yandan 1. görüþcülerin Japonya örneðine en güzel cevap Sayýn Prof. Dr. Cahit Arf'ýn Cumhuriyet gazetesinde 80'li yýllarda çýkan bir yazýsýnda veriliyor "....Japonlar, bizden farklý olarak, iþe, yabancý þirketlere kurma ve iþletme imtiyazlarý vererek, giriþmediler.  Bunun aksine, Batý Avrupalýlarýn bilgilerini, onlarý yeniden üretircesine öðrenerek uygulamaya giriþtiler" (3).  Zaten 1981 - 1990 yýllarý arasýnda Science Citation Index'te görülen yayýnlarýn sayýlarýna baktýðýmýzda Japonyanýn hemen A.B.D. nin ardýndan 557 440 yayýnla ikinci sýrada geldiðini görüyoruz (ülkemiz için bu sayý 7065 dir) (4).

    Avrupa veya Amerika' da hangi profesörle konuþsak her birinin istisnasýz olarak belirli bir konuda derinlemesine bilgi sahibi olduðunu ve o konuda sürekli yayýn yaptýðýný görüyoruz.  Ülkemizde ise maalesef kendi konusunda çýkan yayýnlarý dahi takip eden veya anlayan öðretim üyesi ve araþtýrýcý sayýsý son derece azdýr.  Durum böyle iken, bilgi odaklarý az olan ülkemizde, bilgi üretebilen kritik bir kütleye ulaþmadan, araþtýrmada öncelikli konularý saptamak ve o konulara kanalize olunmasýný istemek doðru deðildir.  Eðer yeterli araþtýrma düzeyine ulaþmýþ olsaydýk  belki böyle bir durum söz konusu olabilirdi.  Ancak þu anki durumumuzda bütün bilim elemanlarýnýn kendi konularýnda çýkan yayýnlarý anlýyor seviyeye gelmeleri ve yayýn yapmalarý istenmelidir.  Diðer yandan sanayileþmiþ ülkelerde bile tartýþýlan hedeflere-göre-yönetim-tarzýný gündeme getirmek, belli hedefler saptamak ne derece doðrudur?   Kaldý ki geçtiðimiz 10 sene zarfýnda çeþitli dönemlerde saptanan hedeflerin deðiþen iktidarlar ile nasýl deðiþtiði ve sonuçta ne teknoloji ne de yayýn getirdiði ortadadýr.

    Sonuç olarak temel araþtýrmalar bizi hiç ilgilendirmeyen saf bilimsel araþtýrmalardýr tezi bilim ve teknolojinin geliþim tarihine tamamen ters düþmektedir.  Ülkemiz þartlarýnda, temel - uygulamalý araþtýrma ayrýmý yapýlmaksýzýn, her konudan gelen ve özgün bilgi üretmeye yönelik araþtýrma teklifleri desteklenmelidir.  Ancak, bu desteðin karþýlýðý her sene yapýlacak uluslararasý yayýnlar olmalýdýr ve bunu üretemeyen araþtýrmalar desteklenmemelidir.  Temel araþtýrmayý atlayýp büyük ve gösteriþli projeler ile teknoloji üretimine soyunmak piyango bileti çekmekten farklý deðildir.  Nasýl bilgi olmadan bilgisayar sadece oyun oynamaya yararsa, temel araþtýrma yapýlmadan teknoloji üretmeye çabalamak da araþtýrmaya ayrýlan kaynaðýn heba olmasýna neden olacaktýr ve bunu bugüne kadar çok kez yaþadýk.

    Eleþtirileri ile bu yazýnýn hazýrlanmasýnda emeði geçen arkadaþlarým, TÜBÝTAK araþtýrýcýlarý Doç. Dr. Altan Erarslan, Doç. Dr. Nezhun Gören, Prof. Dr. Ýsmail Hakký Duru ve eþim Oya Okay'a teþekkür ederim.

(1) D. E. Koshland, Science, 259, 291 (1993)
(2) W. Frühwald, Forschung; Mitteilungen der DFG, 1/93, VCH, sayfa: 3 (1993)
(3) C. Arf, "Bilim Adamlarýnýn Acýlarý ve TÜBÝTAK - Konuk Yazar", Cumhuriyet
      Gazetesi, sayý 22669, 5.10.1987
(4) Ö. Bekaroglu, Konferans, Ege Üniversitesi, 30.4.1992
 

Free-radical Crosslinking Copolymerization:
Effect of Cyclization  on Diffusion-controlled Termination at Low Conversion

Oguz Okay ,  Hamid Javaherian Naghash  and  Ignac Capek+
TUBITAK Marmara Research Center, Department of Chemistry, P.O.Box 21, Gebze, Kocaeli, Turkey

ABSTRACT:  The pre-gelation period of free-radical methyl methacrylate / ethylene glycol dimethacrylate (MMA/EGDM)  and acrylamide / N,N'-methylenebisacrylamide (AAm/BAAm) copolymerization systems was studied by means of dilatometric technique.  The amount of cyclization in MMA/EGDM copolymers was estimated using proton nuclear magnetic resonance and analytical titration methods.  The results suggest that, in MMA/EGDM copolymerization and at a monomer concentration of 220 g l-1, about half of the pendant double bonds are consumed by cyclization.  It was found that the average termination rate constant of macroradicals at zero conversion, kto, is dependent on the crosslinker concentration.  kto increases first but then decreases continuously as the amount of crosslinker increases.   Moreover, termination rate constants become less sensitive to conversion in the presence of small amounts of crosslinker.  All of these phenomena can be explained in terms of  cyclization as it decreases the size of macroradical coils by producing intramolecular links.

Abbreviated title:  Effect of cyclization on diffusion-controlled termination
(Keywords:  Cyclization;  crosslinking;  termination rate; polyacrylamide; polymethyl methacrylate; gels; diffusion control)
________________________________________
   To whom correspondence should be addressed.
+ Present address: Polymer Institute, Slovak Academy of Sciences, CS-842 36, Bratislava, Slovakia.

1.Introduction
 Polymer gels usually have several mistakes such as cycles, inhomogeneous distribution of crosslinks, ... etc.1  These mistakes appear during the gel formation process, for example, during the free-radical crosslinking copolymerization (FCC) of vinyl and divinyl monomers.  FCC has been widely used to synthesize polymer gels having a variety of applications.  In FCC, the reactions are initiated by primary radicals formed by the decomposition of the initiator molecules.  Primary radicals attack to the vinyl groups to produce monomeric radicals which can grow further to polymer radicals.  The polymer radicals stop growing if two radical centers encounter each other and terminate.  Since the lifetime of a radical is usually less than 1 second, radicals are continuously generated and terminated; at any instant the reaction mixture contains monomers, dead (terminated) polymers and a negligible amount of radicals.  The reactions between the radicals and vinyl groups pendant to polymer chains lead to the formation of crosslinks between the primary chains.  Gelation occurs after a certain conversion, i.e., at the gel point at which a three-dimensional network of infinitely large size forms.
 
  In order to characterize gel forming systems, an important property is the distribution of molecular weights of molecules.  Let us call Pr  the number of molecules of r units, normalization gives    with i = 0, 1, 2, ...., where represents the ith moment.  The ith average molecular weight of molecules is defined as  , where   is the molecular weight of the repeating unit.  For example, the second average of the molecular weight distribution called weight - average molecular weight   is given as follows:

         (1)

 During the gel formation process    goes to infinity at a critical point called the gel point.  Before the gel point only soluble chains (sol) are present. Gel starts to form at the gel point and beyond this point, both sol molecules and gel exist.  Several theories have been proposed to predict the properties of the gel forming systems, including statistical and kinetic methods, and simulation in n-dimensional space, such as the percolation method.  Statistical theories originate from Flory2 and Stockmayer3,4 and they introduce the following assumptions: 1) equal reactivity of the functional groups (vinyl groups), 2) absence of cyclization reactions, and 3) random formation of crosslinks.  Other assumptions originate from the mean-field nature of these theories such as neglect of excluded volume effects which may be important in the vicinity of the gel point.  Compared to the statistical theories, kinetic approaches can take into account all the kinetic features of the reactions and so may offer a more realistic approach to the mechanism of the network formation process.1,5-12  Although kinetic theories also belong to the mean-field category, they may account for spatial correlations if the rate constants are defined properly.  Another type of theories called non-mean-field theories such as the percolation theory bases on simulation in n-dimensional space.13-15  The percolation theory can take spatial correlations into account but at present the result of this theory is unrealistic due to the difficulty of introduction of realistic mobilities.  The main domain of application of percolation techniques to polymer networks seems to be near the sol-gel transition called the critical region in which the system specific parameters are not important.

 In this paper, we analyze the gel forming systems by FCC in detail.  We begin our analysis in the next section by showing the non-ideal features of the gel formation process by FCC.  In section 3 a kinetic model proposed recently is summarized and the model predictions are compared to the experimental data.  It will be shown that the non-classical exponents observed by experiments can be predicted using the kinetic theory and, the conversion-dependent kinetic of FCC is mainly responsible for the deviations from the classical exponents
 

Nilhan Kayaman1,  Oðuz Okay1,2* and Bahattin M. Baysal1,3
1 TUBITAK Marmara Research Center,  Department of Chemistry, P. O. Box 21,
  41470 Gebze, Kocaeli, Turkey,
2 Kocaeli University, Department of Chemistry, 41300 Ýzmit, Kocaeli,
3 Boðaziçi University, Department of Chemical Engineering, 80815 Bebek, Ýstanbul,
  Turkey.

ABSTRACT:
Hydrolyzed polyacrylamide gels immersed in aqueous solutions of poly(ethylene glycol) (PEG) of molecular weight 300 exhibit a discontinuous volume change upon continuous increase of the PEG concentration in the external solution.  As the duration of hydrolysis increases, that is, as the proportion of the ionic groups on the network chains increases, the critical concentration of PEG required for a discontinuous volume change rises and also, the magnitude of the volume collapse becomes larger.  Experimental results indicate that the gel collapse in PEG solution is due to the osmotic deswelling of the ionic gel rather than due to a complex formation between polyacrylamide and PEG chains.
____________________
* To whom correspondence should be addressed.
 

SYNOPSIS
A series of strong polyelectrolyte hydrogels based on acrylamide (AAm) and 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) or [(Methacrylamido)propyl]trimethyl ammonium chloride (MAPTAC) were prepared by free-radical crosslinking copolymerization. Their swelling behavior was investigated in a good solvent (water), in polymer melt, as well as in aqueous polymer solutions.  Poly(ethylene glycol) (PEG) of various molecular weights was used as the linear polymer in the swelling experiments. The equilibrium volume swelling ratio qv of the hydrogels in water scales with the network charge density f  by the relation  .  The volume of the hydrogels in polymer melt decreases as the number of segments y on the polymer increases.  It was shown that this deswelling  occurs, for the non-ionic hydrogels, due to the mixing entropy of the linear polymer in the external solution, whereas, for the ionic gels, due to trapping of counterions caused by the decreased polarity of the medium.  The network - polymer interaction parameter ?23  was found to vary with y  by the relation  .    The hydrogels also deswell in aqueous polymer solutions as the polymer concentration increases.  This deswelling occurs smoothly or jumpwise, as a first order phase transition, depending on the ionic group content of the hydrogels. The FH theory correctly predicts the swelling and collapsing behavior of the hydrogels in polymer solutions, if the variation of the effective charge density depending on the medium polarity is taken into account.

Keywords: hydrogels, acrylamide, 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS), [(Methacrylamido)propyl]trimethyl ammonium chloride (MAPTAC),  poly(ethylene glycol), swelling in polymer solution, phase transition.

_______________________________________
*To whom correspondence should be addressed.
 
 

 CONTENTS
1 INTRODUCTION 3
1.1 DEFINITIONS 6
2 GENERAL ASPECTS OF MACROPOROUS COPOLYMER SYNTHESIS 9
2.1 PREPARATION OF MACROPOROUS COPOLYMER NETWORKS 9
2.2 CHARACTERIZATION OF MACROPOROUS COPOLYMER NETWORKS 18
3 FORMATION OF POROUS STRUCTURES DURING CROSSLINKING 22
4 POROUS STRUCTURE VERSUS SYNTHESIS PARAMETERS 32
4.1 EFFECT OF THE DILUENT 32
4.2 EFFECT OF THE CROSSLINKER 38
4.3 EFFECT OF THE TEMPERATURE AND THE INITIATOR 41
5 RELATION BETWEEN THE SWELLING DEGREE AND THE POROSITY 42
6 VARIATION OF THE POROUS STRUCTURE AFTER THE GEL PREPARATION 50
7 THEORETICAL CONSIDERATIONS 58
7.1 CONDITION OF PHASE SEPARATION DURING CROSSLINKING POLYMERIZATION 59
7.2 CONVERSION - DEPENDENT PHASE EQUILIBRIA 61
7.3 KINETICS OF GEL FORMATION AND GROWTH DURING FREE-RADICAL CROSSLINKING COPOLYMERIZATION 66
7.4 THEORETICAL PREDICTIONS 73
8 CONCLUDING REMARKS 78
9 REFERENCES 80
 
 

HETEROGENEITIES DURING THE FORMATION OF POLY(SODIUM ACRYLATE) HYDROGELS
 

Oguz OKAY1,3 ,  Ya?ar YILMAZ2 ,  Demet KAYA2 ,  Muzaffer KESKINEL3, and Önder PEKCAN2
1 Istanbul Technical University, Department of Chemistry, Maslak, 80626 Istanbul,
2 Istanbul Technical University, Department of Physics, Maslak, 80626 Istanbul,
3 TUBITAK Marmara Research Center, P. O. Box 21, 41470 Gebze, Kocaeli, Turkey

Summary
In situ fluorescence and light scattering techniques were used to study the formation of heterogeneities during the free-radical crosslinking polymerization of sodium acrylate (NaAc) in the presence of N,N'-methylene(bisacrylamide) (BAAm) as a crosslinker. Pyranine was used as a fluorescence probe in the experiments.  The reactions were carried out at room temperature and at a monomer concentration of 20.6 w/v% in water. It was found that the gel forming system became turbid at the start of the reaction; continuing polymerization and crosslinking reactions turned the opaque gels into slightly opaque and finally transparent gels. Two peaks were observed both in the scattered and emitted light intensities versus the reaction time plots.  The first peak corresponds to the formation and growth of the phase separated domains whereas the second peak reflects the macrogelation point in the reaction system.
 
 
 

Oguz OKAY 1,2, Selda DURMAZ 1, and Burak ERMAN 3 *

1 TUBITAK Marmara Research Center, Department of Chemistry, P.O. Box 21, 41420 Gebze, Kocaeli, Turkey
2 Istanbul Technical University, Department of Chemistry, 80626 Maslak, Istanbul, Turkey,
3 Sabanci University, Faculty of Engineering and Natural Sciences, Orhanl?, 81474 Tuzla, Istanbul, Turkey.

 ABSTRACT
A strong concentration dependence of the solvent - polymer interaction parameter ? is known to be a requirement for the polymer collapse in hydrophobic gels. However, in practice, a phase transition in hydrophobic matrices has never been observed. Literature values indicate a strong concentration dependent ? parameter for poly(isobutylene) (PIB) in benzene which makes the PIB gels suitable canditates for the investigation of their swelling behavior in solvent - nonsolvent mixtures. In this work, we prepared several series of PIB gels starting from butyl rubber in dilute toluene solutions using sulfur monochloride as a crosslinking agent. By using solution and suspension crosslinking techniques, PIB gels in the form of rods as well as beads in the size range of 0.1 to 2 mm were prepared and subjected to swelling experiments in solvent - nonsolvent mixtures. The swelling capacity of the gels in toluene increased with decreasing concentration of sulfur monochloride or butyl rubber in the organic phase. Solubility parameter of the solution crosslinked PIB gels was found to be 16.8  0.8 (MPa)0.5, in accord with the reported value for bulk crosslinked gels. In toluene - methanol mixtures, PIB gels with 4% crosslinker content exhibited an abrupt deswelling if the toluene content of the external solution is decreased from 97 to 90 %. By using the theory of equilibrium swelling, it was shown that the gel passes through the critical condition as the quality of the solvent becomes poorer.
 

THE EFFECT OF PREPARATION TEMPERATURE ON THE SWELLING BEHAVIOR OF
POLY(N-ISOPROPYLACRYLAMIDE) GELS
        Çigdem SAYIL1 and Oguz OKAY1,2*
 
1 TUBITAK Marmara Research Center, Department of Chemistry, P. O. Box 21, 41470 Gebze, Kocaeli, Turkey
2 Istanbul Technical University, Department of Chemistry, Maslak, 80626 Istanbul, Turkey
 
 Summary
The role of the preparation temperature of poly(N-isopropylacrylamide) (PNIPA) gels on their swelling behavior in water and in aqueous solutions of sodium dodecylbenzenesulfonate was investigated. PNIPA gels were prepared by free-radical crosslinking copolymerization of N-isopropylacrylamide and N,N’-methylene(bis) acrylamide in water at fixed monomer and crosslinker concentrations. The equilibrium swelling ratio of the gels increases first slightly up to about 200C then rapidly with increasing gel preparation temperature. Magnitude of the collapse transition in water at 340C becomes larger as the gel preparation temperature increases. Calculations indicate a decrease in the effective crosslink density of PNIPA gels with increasing preparation temperature. The gels prepared at temperatures higher then 200C were heterogeneous consisting of highly crosslinked regions interconnected by the PNIPA chains.

 
IN SITU PHOTON TRANSMISSION TECHNIQUE FOR MONITORING PHASE SEPARATION IN REAL TIME DURING GELATION
 
Selim KARA1, Oguz OKAY 2,3* , and Önder PEKCAN 2
1 Department of Physics, Trakya University, 22030 Edirne, Turkey
2 Departments of Chemistry and Physics, Istanbul Technical University, Maslak, 80626 Istanbul, Turkey
3 TUBITAK Marmara Research Center, P. O. Box 21, 41470 Gebze, Kocaeli, Turkey
 
Summary
In situ, real time photon transmission technique was used to monitor the formation of heterogeneities during the free-radical crosslinking polymerization of acrylamide in the presence of N,N’-methylenebis(acrylamide) (BAAm) as a crosslinker. Two different gelation experiments were performed with various BAAm and water contents separately at room temperature. It was observed that the transmitted photon intensity, Itr decreases dramatically at a certain time, in both type of experiments during the gel formation process from interconnected microgels. Decrease in Itr was interpreted in terms of phase separation during gelation. The extent of phase separation was measured depending on the crosslinker content and on the degree of monomer dilution during the gel formation process.
 

Çigdem SAYIL1 and Oguz OKAY1,2*
1 TUBITAK Marmara Research Center, Department of Chemistry, P. O. Box 21, 41470 Gebze, Kocaeli, Turkey
2 Istanbul Technical University, Department of Chemistry, Maslak, 80626 Istanbul, Turkey

Abstract
The swelling and shrinking behavior of a series of poly(N-isopropylacrylamide) (PNIPA) hydrogels was studied in aqueous solutions of sodium dodecylbenzenesulfonate (SDBS). 2-acrylamido-2-methylpropanesulfonic acid sodium salt (AMPS) was used as an ionic comonomer in the hydrogel synthesis between 0 and 3 mol %. It was shown that the collapsed PNIPA gels in water at 520C start to swell above a critical SDBS concentration in the external solution. This critical concentration decreases as the ionic group content of PNIPA gel increases. Comparison of swelling and shrinking experiments in SDBS solutions indicates a strong hysteresis behavior of PNIPA gels. More diluted solution was required to make a swollen gel start to shrink again than to cause the gel swelling. The results show strong attractive forces between the isopropyl groups of PNIPA network and the dodecylbenzene groups of SDBS molecules.
 
 
 

Demet MELEKASLAN1 and Oguz OKAY1,2*

1Istanbul Technical University, Department of Chemistry, 80626 Maslak, Istanbul, Turkey, Fax: 0212-2856386, E-mail: okayo@itu.edu.tr
2 TUBITAK Marmara Research Center, Department of Chemistry, P.O. Box 21, 41470
Gebze, Kocaeli, Turkey,
 
SUMMARY
The swelling behavior of a series of strong polyelectrolyte hydrogels based on N-isopropylacrylamide (NIPA) and 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) was investigated in aqueous solutions of poly(ethylene glycol)'s (PEG) of molecular weights 300 and 400 g/mol. Non-ionic hydrogels or hydrogels with 1 mol % AMPS deswell with increasing PEG - 300 volume fraction ? in the external solution up to ? = 0.6. As ??is further increased, the hydrogels start to swell up to ? = 1. The distribution of PEG inside and outside the gel phase changes with the gel volume; PEG chains first move from the gel to the solution phase from ? = 0 to 0.6, while at higher ? values they again penetrate the gel phase. In PEG - 400 solutions, the contraction of the gels is jumpwise first-order phase transition at a critical ?, while their reswelling occurs smoothly at higher values of ?. Calculations using the Flory - Huggins theory indicate attractive interactions between PEG and PNIPA segments, which are responsible for the observed reentrant transition behavior.