effects of pore-forming agents and polymer composition on the properties of novel poly( n , n...

Published: May 25, 2011

r 2011 American Chemical Society 8295 dx.doi.org/10.1021/ie102349p | Ind. Eng. Chem. Res. 2011, 50, 8295–8303

ARTICLE

pubs.acs.org/IECR

Effects of Pore-Forming Agents and Polymer Composition on theProperties of Novel Poly(N,N-Dimethylaminoethyl MethacrylateSulfate-co-N,N-dimethylacrylamide) HydrogelsG€ulten G€urda�g* and Ays-eg€ul G€okalp

Department of Chemical Engineering, Faculty of Engineering, Istanbul University, 34320, Avcilar, Istanbul, Turkey

ABSTRACT: Homopolymer of N,N-dimethylaminoethyl methacrylate sulfate (DMAEMASA) and its copolymer with N,N-dimethylacrylamide (DMAm) [P(DMAEMASA-co-DMAm)] were synthesized in the presence and absence of pore-forming agentsNaHCO3, poly(ethylene glycol) 2000 (PEG), and sucrose (SUC). The polymers were characterized by equilibrium swellingmeasurements (ESVs) in distilled water (20�60 �C) and buffer solutions (I = 0.1M, pH = 2.2�10.0, 20 �C), FTIR, DSC, and SEMmethods. The presence of DMAm in monomer feed and the use of pore-forming agents during the polymerization enhanced theswelling of polymers. ESVs of both porous and nonporous PDMAEMASA and P(DMAEMASA-co-DMAm) gels decreased withpH, and displayed a phase transition at pH = 5. Among the pore-formers, NaHCO3made the highest contribution to the swelling ofpolymers, but poly(ethylene glycol) and sucrose slightly affected the swelling values of gels. While glass transition temperature (Tg)of nonporous DMAEMASA homopolymer was determined to be 168.4 �C, Tgs of nonporous copolymer with 20- and 40-mol %DMAmwere found to be 154.5 and 147.8 �C, respectively. Pore-formers decreased the Tg of homopolymer in the order NaHCO3 <PEG < SUC. In case of copolymers, NaHCO3 and PEG had nearly no effect on Tgs, but sucrose led to approximately 7 �C decreasein Tgs of copolymers.

1. INTRODUCTION

Hydrogels are three-dimensional cross-linked hydrophilicpolymers, and they absorb large amounts of water dependingon their chemical structure (hydrophilic or hydrophobic) andcross-link density. Smart hydogels respond to external stimulisuch as temperature, pH, ionic strength, and electric/magneticfield, etc., as a reversible phase transition (swelling or shrinking).These kinds of materials are finding applications in many fieldssuch as drug delivery,1 separation processes,2 enzyme immobili-zation,3 and chemical sensor,4 etc. The response rate of a non-porous gel to any stimuli is slow, and it depends on the size,porosity, and cross-link density of the gel. The response rate canbe increased minimizing the size of the gel since the diffusionlength of water is shortened. An alternative method to obtain fastresponse is to create porosity in the gel. Porous gels absorb fasterand more water than nonporous gels since in the swelling ofnonporous gels, the rate-determining step is the diffusion ofwater into the polymer gel. Porous gels contain open andinterconnected pores; in the swelling of these gels, the capillaryeffect has more dominant effect than the diffusion of water. Theycan be prepared by methods such as porogen leaching, phaseseparation, particulate cross-linking, and freeze-drying. In theporogen leaching technique,5 porous hydrogels are prepared inthe presence of dispersed water-soluble pore-forming agents,such as sodium chloride, PEG, and sucrose which can beremoved by washing with water during the purificiation. In aprevious work,6 we have prepared porous copolymer gels fromN-isopropylacrylamide (NIPAM) and N-hydroxymethyl acryla-mide (NHMAAM) by using poly(ethylene glycol) 400 and 2000(PEG 400 or PEG 2000) as pore-forming agents. While the useof pore-forming agents PEG 400 and PEG 2000 during thepolymer synthesis favored the formation of porous gels and

increased the swelling values, it led to decrease in the compres-sive elastic moduli of PNIPAM and P(NIPAM-co-NHMAAM)gels. Yıldız et al.7 used PEG 4000 as porosity generator in order toenhance the thermosensitivity of P(NIPAM-co-N-hydroxy-methyl acrylamide) (P(NIPAM-co-NHMAAM)) gel, but ithad no effect on the shrinking rate due to the dominant effectof skin formation during the collapse of P(NIPAM-co-NHMAAM) gel. Kabiri et al.8 reported that acetone and sodiumbicarbonate enhanced the swelling rate of superabsorbent hydro-gel from partially neutralized acrylic acid as high as 43�55% and111�131%, respectively. In addition, they8 also observed that thetime and sequence of addition of the porogens, as well as thegelation time of the polymerization, determined the efficiency ofthe porogens. Lee and Yeh9 used CaCO3 and poly(ethyleneglycol) 8000 (PEG 8000) as porogen in the emulsion polymer-ization of N-isopropylacrylamide (NIPAM) with hydrophobicmonomers such as 2,2,3,3,4,4,5,5-octafluoropentyI methacrylate(OFPMA) and n-butyl methacrylate and determined thatCaCO3 is a more efficient pore-former than PEG 8000. Forthe preparation of macroporous PNIPAM gels, the use ofaqueous solution of sucrose as polymerization medium insteadof water was reported by Zhang et al.10 Lee and Chiu11 reportedthat the increase in molecular weight of poly(ethylene glycol)gave porous PNIPAM copolymer gels with larger pore sizes.C-aykara et al.12 synthesized thermoresponsive poly(2-diethyla-mino ethylmethacrylate-co-N,N0-dimethylacrylamide) gels inthe presence of NaHCO3 as porogen, and determined that ESVs

Received: November 22, 2010Accepted: May 25, 2011Revised: May 19, 2011

8296 dx.doi.org/10.1021/ie102349p |Ind. Eng. Chem. Res. 2011, 50, 8295–8303

Industrial & Engineering Chemistry Research ARTICLE

of gels increased with the use of NaHCO3 as pore-former andDMAm content of copolymer.

Very little has been published on the polymerization ofDMAEMASA13,14 monomer, in contrast to those published onN,N0-dimethylaminoethyl methacrylate (DMAEMA) polymeriza-tion. Bune et al.13 copolymerized DMAEMASA with acrylamide(AAM), and determined the reactivity ratios to be rDMAEMASA =1.9 ( 0.2 and rAAM = 0.52 ( 0.05. In another work on theDMAEMASA monomer, Shirshin et al.14 investigated theeffects of initial concentrations of monomers and initiators inthe copolymerization of DMAEMASA and acrylonitrile (AN).

The novel gels poly(N,N0-dimethylaminoethyl methacrylatesulfate) (PDMAEMASA) and P(DMAEMASA-co-DMAm) wereprepared in the absence and presence of pore-forming agentsNaHCO3, poly(ethylene glycol) 2000, and sucrose, and theywere characterized. A comparison between the propertiesof porous and nonporous DMAEMASA homopolymer andDMAEMASA-DMAm copolymers was also performed for thefirst time in this work.

2. EXPERIMENTAL SECTION

2.1. Materials. DMAm monomer and the cross-linkerN,N0-methylene bisacrylamide (NMBA) were purchased fromFluka (Sigma-Aldrich) and Merck (Hohenbrunn, Germany),respectively. DMAEMASA was prepared from DMAEMA(Merck, Hohenbrunn-Germany) and sulphuric acid (Merck,Hohenbrunn-Germany). The initiator ammonium persulfate(APS) and the accelerator N,N,N0,N0-tetramethylethylenedia-mine (TEMED) were purchased from Riedel-de H€aen (Seelze,Germany) and Serva Electrophoresis GmbH (Heidelberg,Germany), respectively. Pore-forming agents NaHCO3, sucrose(SUC), and poly(ethylene glycol) 2000 (PEG) were Riedel-deH€aen (Seelze, Germany), Carlo Erba (Milan, Italy), and Merck(Hohenbrunn, Germany) products, respectively. Buffer solu-tions were prepared15 by using potassium hydrogen phthalate,potassium dihydrogen phosphate, HCl (37% aqueous solution),sodium bicarbonate, sodium chloride, and sodium hydroxide.Buffer chemicals were Merck (Hohenbrunn, Germany) pro-ducts, except sodium bicarbonate and sodium chloride, whichwere obtained from Riedel-de H€aen, and sodium hydroxide,which was produced by JT Baker. Distilled water was used for thepreparation of hydrogels and buffer solutions, and for swellingmeasurements.2.2. Synthesis of DMAEMASA Monomer13. An appropriate

amount of DMAEMA monomer was placed in a glass tube thatwas immersed in an ice bath in order to keep the temperatureof the reaction mixture below 15 �C, as the reaction of DMAE-MA with H2SO4 is exothermic.13 A stoichiometric amountof H2SO4 (98%) was then added to DMAEMA monomer dropby drop with magnetic stirring. The quaternary sulfate salt(DMAEMASA) of DMAEMA monomer was a white solid thatwas used without further treatment. Please see our previouspaper16 for further information about DMAEMASA monomer.2.3. Synthesis of Hydrogels. A series of cationic copolymer

gels of N,N0-dimethylaminoethyl methacrylate sulfate (DMAEM-ASA) with N,N-dimethyl acrylamide (DMAm) P(DMAEMASA-co-DMAm) and DMAEMASA homopolymer were preparedin aqueous solution at 40 �C for 24 h in the presence or absenceof pore-forming agents; NaHCO3, sucrose (SUC), and poly-(ethylene glycol) 2000 (PEG) using N,N0-methylene bis acryla-mide (NMBA) as the cross-linker. The feed compositions of the

hydrogels are presented in Table 1. In polymer codes, DMAE-MASA is represented by DESA. The numerals before DESA inpolymer codes show the mol percentage of DMAEMASA mono-mer in monomer mixture, and the rest is DMAm content. Thenumerals, i.e., 5 or 10 before the pore-formers SUC, PEG, andNaHCO3 show the amounts of pore-forming agents in weightpercentage of total monomer amount. For example, 100 DESAand 100 DESA 2B show the homopolymers of DMAEMASAprepared in the presence of 1 and 2 mol % of NMBA (B) as cross-linker, respectively.Polymerization reactions were performed in glass tubes with

an inner diameter of 1.3 cm and a length of 15 cm. An aqueoussolution of ammonium persulfate (APS) (4.25 g APS/100 mLwater) and N,N,N0,N0-tetramethylethylenediamine (TEMED)were used as initiator and accelerator, respectively. Hydrogelswith different feed compositions were prepared by varying theDMAm content of the monomer mixture from 20 and 40 mol %at a constant total initial monomer concentration of 1 M, whilethe cross-linking agent was kept constant at 1 mol % of the totalmonomer content for porous gels, but 1 and 2 mol % fornonporous gels. Before the polymerization, the monomer solu-tion containing cross-linker and pore-forming agent (PEG orSUC) was bubbled with nitrogen gas for 15 min, followed by theaddition of the initiator and accelerator. Porogen NaHCO3 was

Table 1. Polymer Codes and Feed Compositions of Porousand Non-porous DMAEMASA and DMAEMASA-DMAmGels

polymer code

DMAEMASA

(mol %)

DMAm

(mol %)

pore-formera

(wt %)

NMBAb

(mol %)

60DESA 60 40 1

60DESA 2B 60 40 2

60DESA 5SUC 60 40 5 1

60DESA 10SUC 60 40 10 1

60DESA 5PEG 60 40 5 1

60DESA 10PEG 60 40 10 1

60DESA 5NaHCO3 60 40 5 1

60DESA 10NaHCO3 60 40 10 1

80DESA 80 20 1

80DESA 2B 80 20 2

80DESA 5SUC 80 20 5 1

80DESA 10SUC 80 20 10 1

80DESA 5PEG 80 20 5 1

80DESA 10PEG 80 20 10 1

80DESA 5NaHCO3 80 20 5 1

80DESA 10NaHCO3 80 20 10 1

100DESA 100 1

100DESA 2B 100 2

100DESA 5SUC 100 5 1

100DESA 10SUC 100 10 1

100DESA 5PEG 100 5 1

100DESA 10PEG 100 10 1

100DESA 5NaHCO3 100 5 1

100DESA 10NaHCO3 100 10 1a Pore-forming agents were used in 5 and 10 wt % of total monomeramounts. bNMBA was used in mol percentages of total monomercontent. Initiator APS was used in 1 mol percentages of total monomercontent. Accelerator TEMED was used in the equal amount of APSin gram.



added to the reaction mixture after the addition of the initiatorand accelerator in order to prevent the earlier decomposition ofporogen. The tubes were then sealed and immersed for 24 h in awater bath at 40 �C ((0.01 �C). At the end of the reactionperiod, the tubes were broken carefully without destroying thehydrogels. The cylindrical hydrogel was then sliced into smalldiscs (0.6�0.8 cm in length), which were stored in distilled waterfor one week. The water was changed twice daily to remove thesol fraction (the residual unreacted monomers and linear poly-mers). The resulting swollen hydrogel discs were dried, first in airand then in a vacuum oven at 40 �C.

2.4. Optical Appearances of Hydrogels. The optical pic-tures of the hydrogels were recorded by a digital camera(Canon, A610) in order to observe the effects of porogenson the microheterogeneity, and thus on the transparency ofthe hydrogels. The pictures of the hydrogels are given inFigure 1a and b.2.5. FTIR Measurements. FTIR spectra of the dry gels were

recorded using an ATR technique (Perkin-Elmer SpectrumOne). FTIR spectra of DMAEMASA homopolymer and DMAE-MASA-DMAm gels are presented in Figure 2a and b.2.6. Surface Morphology of the Polymers. The morphol-

ogies of DMAEMASA homopolymer and P(DMAEMASA-co-DMAm) gels were investigated using a scanning electron micro-scope (JEOL, JSM 5600). Before investigation, the hydrogelswere dried at�45 �C and 38.10�3 mmHg using a vacuum freeze-dryer (Armfield SB4), following which their surfaces weresputter-coated with gold. A few representative SEMmicrographsare given in Figure 3 for the investigation of the interior matrixstructure of DMAEMASA homopolymer and P(DMAEMASA-co-DMAm) gels synthesized in the absence or presence of pore-forming agents.2.7. Effect of Temperature on the Equilibrium Swelling

Values.The ESVs of DMAEMASA homopolymer and DMAE-MASA-DAAm copolymer gels were determined at 20, 30, 40, and60 �C in distilled water by the gravimetric method. A givenamount of dry gel (Wd) was immersed in distilled water at thedesired temperature for 24 h in order to attain the swellingequilibrium. At the end of the swelling period, swollen gel pieceswere taken out of water, blotted with a piece of filter paper toremove excess water, and weighed (Ws). ESVs of the gels were

Figure 1. (a) Optical appearances of DMAEMASA-DMAm hydrogelsin glass tubes just after preparation. (b) Optical appearances of swollenDMAEMASA and DMAEMASA- DMAm gels in distilled water.

Figure 2. FTIR spectra of nonporous (a) and porous (b) DMAEMASAhomopolymer and copolymer gels.



determined by the following equation:

Equilibrium Swelling Value ðESVÞ ðg H2O=g polymerÞ¼ ðW s �WdÞ=Wd ð1Þ

2.8. Effect of pH of the Swelling Medium on the Equilib-rium Swelling Values. A given amount of dry gel (Wd) wasimmersed in buffer solutions with different pH values andconstant ionic strength (I = 0.1 M) at 20 �C for 24 h in orderto investigate the effects of pH and pore-forming agents on theESVs of the gels. Different formulations of the buffer solutions15

in a wide range of pH values were prepared, e.g., potassiumhydrogen phthalate�HCl at pH 2.2 and 2.8, potassium hydrogenphthalate�NaOH at pH 5.0, potassium dihydrogen phosphate�NaOH at pH 7.0, diethanolamine�HCl at pH 8.5, and sodiumbicarbonate�NaOH at pH 10.0. After the preparation of buffersolutions, their pH values were checked by a pH meter (ThermoScientific ORION 3 STAR), and no unsuitability was detectedbetween the experimental and theoretical pH values of buffersolutions. The ionic strength of each buffer solution was keptconstant at 0.1 M by the addition of NaCl as necessary. ESVs ofgels in buffer solutions were determined using eq 1.2.9. Elemental Analysis of Copolymers. Elemental analysis

of dry DMAEMASA homopolymer and DMAEMASA-DMAmcopolymer gels was performed using a Flash EA 1112 model of aThermo Finnigan instrument. DMAEMASA content of DMAE-MASA-DMAm gels in weight percentage was calculated17 byS (sulfur) contents of DMAEMASA monomer and copolymergels using eq 2.

DMAEMASA ðwt %Þ ¼ ðS=SoÞ � 100 ð2Þwhere S and So are the sulfur contents (in wt %) of copolymerand DMEMASAmonomer, respectively, determined by elemen-tal analysis.2.10. Differential Scanning Calorimetry (DSC) Measure-

ments. Glass transition temperatures (Tg) of porous ornonporous DMAEMASA and DMAEMASA-DMAm gels weredetermined using 15�20 mg polymer by a SETARAM 131instrument under a nitrogen atmosphere with a heating rateof 10 �C/min.

3. RESULTS AND DISCUSSION

3.1. Synthesis and Characterization of Novel Porous/Nonporous DMAEMASA and P(DMAEMASA-co-DMAm)Gels. A few representative photographs of DMAEMASA homo-polymer and P(DMAEMASA-co-DMAm) gels are shown inFigure 1a and b. They show the hydrogels in glass tubes justafter preparation, andCO2 bubbles are also seen in the pictures ofhydrogels prepared in the presence of NaHCO3. The swollenhomopolymer and copolymer slices that were purified by theremoval of sol fraction are seen Figure 1b. Both porous andnonporous hydrogels for all monomer compositions are highlyelastic and transparent in appearance, implying the absence ofimpurities such as porogens and microheterogeneity in thepolymer structure. Ozmen et al.18 have reported that if theamount of water in the polymerization mixture is higher thanthe swelling capacity of the gel, it cannot absorb all the water inthe polymerization medium, thus a phase separation occursduring the polymer formation. Finally, an opaque gel is obtained.They also concluded that if the volume of swollen gel is higherthan the volume of the just prepared-gel, no phase-separation isobserved. Otherwise, the system will be two-phased, and opaquein appearance. It was also reported that the ionization of polymerhydrogels suppresses the inhomogeneities.19 Because DMAE-MASA homopolymer and its copolymer withDMAMare ionic innature, the transparency of these porous gels may be attributed tosuppression of inhomogeneities.FTIR spectrum of DMAEMASA monomer was given in our

previous paper.16 Characteristic bands in FTIR spectrumof DMAEMASA were seen at 3048 cm�1 (the stretching vibra-tion of C�H in CdC double bond of CH2dC(CH3)— group),at about 2500 cm�1 (the NR3H

þ group which is characteristicfor protonated DMAEMASA monomer20), at 1719 cm�1

(νCdO),21 at 1470 cm�1 (asymmetric in-plane bending ofC�H bond in �CH2 and �CH3 groups of monomer andC�N stretching,21�23 and at 1030 cm�1 (strong) and 1228 cm�1

(weak) (symmetric and asymmetric �SO2 stretchings,24,25 re-spectively). Asymmetric and symmetric C�O�C stretchings21

were represented by two bands at 1145 cm�1 (strong) and1298 cm�1 (weak), respectively.16

Figure 3. SEM micrographs of porous and nonporous DMAEMASA and DMAEMASA-DMAM gels.



FTIR spectra of DMAEMASA homopolymer (100 DESA)and its copolymers with 20 and 40 mol % DMAm (80 DESA and60 DESA) are given in Figure 2a.FTIR spectrum of nonporous homopolymer (100 DESA)

contains all the characteristic bands of DMAEMASA monomerexcept that for CdCdouble bond. The appearance of a new bandat 1627 cm�1 (amide I: νCdO) in the spectrum of 80 DESAconfirms the presence of DMAm in copolymer structure. Theintensity of that band increased with DMAm content as canbe seen in the spectrum of 60 DESA (1623 cm�1) (Figure 2a).One of the changes that occurs due to DMAm incorporation inthe polymer structure is the decrease in the intensity of the banddue to NR3H

þ structure. The shift in amide I band (νCdO)from 1627 cm�1 (80 DESA) to 1623 cm�1 (60 DESA) confirmsthe presence of hydrogen bondings between carbonyl group ofDMAm and �OH and �NH groups of DMAEMASA. It isknown that there are hydrogen bondings between the polymerscontaining hydroxyl groups and tertiary amide polymers. Thiskind of interaction is confirmed by the shift of carbonyl band inFTIR spectrum to lower frequencies.26 The intensity of the bandat 1143 cm�1 increased with the incorporation and contentof DMAm.There is no difference between the FTIR spectra of porous

and nonporous DMAEMASA-DMAm copolymers (Figure 2b).This finding confirms that the pore-forming agents (NaHCO3,sucrose, and poly(ethylene glycol) 2000) are extracted by waterfrom the copolymer during the removal of sol fraction in thepurification step, and they have no interaction with the polymer.The structural morphology of porous and nonporous DMAE-

MASA and (DMAEMASA-co-DMAm) gels was investigated bySEM, and some representative SEM micrographs are given inFigure 3. Both DMAEMASA homopolymer and DMAEMASA-DMAm copolymers prepared in the presence of pore-formingagents have apparently more porous structure than the polymerssynthesized in the absence of porogens.Sulfur and DMAEMASA contents of porous and nonporous

DMAEMASA homopolymer and DMAEMASA-DMAm copo-lymers are given in Table 2. Sulfur content of DMAEMASAmonomer (So) was found to be 10.45 wt %, and it is 83%of theoretical amount. When the contents of C, H, and N ofDMAEMASA monomer are evaluated together with S contents,it can be concluded that DMAEMASA contains a small amountof DMAEMA that is not converted to quaternary sulfate salt.Analysis results for C, H, and N contents of polymers are 3�13%higher than those of theoretical values (not provided). Thisdifference can be assigned to the presence of water which couldnot be removed from the gel during drying. In contrast to theresults for C, H, and N contents, experimental DMAEMASAcontents of the polymers are approximately 70% of theoreticalDMAEMASA values. This result indicates that DMAEMASA-DMAmgel containsDMAEMAunits too, and that the former is aterpolymer. The effect of the use of pore-forming agents on the Snamely DMAEMASA contents of polymers is given Figure 4 andTable 2. The use of pore-forming agent during the polymersynthesis decreases DMAEMASA content of polymer. Thisdecrease in DMAEMASA content is apparent especially forporous 60 DESA and 80 DESA copolymers. Among the poro-gens NaHCO3, sucrose, and poly(ethylene glycol) 2000, thehighest decrease in DMAEMASA content of polymer wascreated by NaHCO3. This higher effect of NaHCO3 on DMAE-MASA content of polymer can be attributed to the neutralizationof some part of acidic DMAEMASA monomer with NaHCO3.

The pore-formers PEG and SUC have nearly no effect onDMAEMASA content of homopolymer, but they decreasedDMAEMASA content of 80 DESA and 60 DESA copolymersat the same amount.3.2. Differential Scanning Analysis (DSC) of DMAEMASA

Homopolymer and DMAEMASA-DMAm Copolymers. Glasstransition temperatures (Tg) of porous and nonporous DMAE-MASA homopolymer and DMAEMASA-DMAm copolymersare given in Figure 5 and Table 2. Tg values of DMAm homo-polymer are given as 11826 and 122.4 �C.27 Bae et al.28 synthe-sized copolymers from DMAm and 2-(N-ethyl-perfluorooctane-sulfonamido) acrylamide (FOSA) with up to 5mol % FOSA, andreported that the Tg values of these copolymers were 110�120 �C. Whereas Tg values of DMAEMA homopolymer aregiven as 20,29 14,30 1931,32 and 12 �C,33 no DSC analysis eitherfor DMAEMASA homopolymer or DMAEMASA-DMAm co-polymer has been reported in literature until now since they arenovel polymers.Tg values of DMAEMASA homopolymer (100DESA) and the

copolymers 80 DESA and 60 DESA were found to be 168.4,154.6, and 147.8 �C, respectively (Figure 5). This findingindicates that the introduction of DMAm in polymer structureand further increase in its content lead to decrease in Tg ofpolymer. As can be understood from the molecular formulas ofmonomers used in the polymer preparation, DMAEMASAhomopolymer has hydrogen atoms which are capable of makinghydrogen bondings, but it is not the case for DMAm homo-polymer. Therefore, Tg of the former polymer (168.4 �C) ishigher than that of the latter given as about 120 �C inliterature.26,27 Therefore, the copolymerization of DMAEMASAwith DMAm reduced the Tg of copolymer. In general, thepresence of pore-forming agents such as NaHCO3 and PEG in

Table 2. Glass Transition Temperatures (Tg) and Results forElemental Analysis of Porous and Nonporous DMAEMASAHomopolymer and DMAEMASA-DMAm Copolymer Gels

polymer code

sulfur content

(S) (wt %)

DMAEMASA

content (wt %) Tg (oC)

60DESA 6.7 64.1 147.8

60DESA 5SUC 5.7 54.5 148.5

60DESA 10SUC 5.2 49.8 140.8

60DESA 5PEG 5.1 48.8 149.0

60DESA 10PEG 5.7 54.5 147.2

60DESA 5NaHCO3 5.0 47.9 148.2

60DESA 10NaHCO3 4.9 46.9 145.7

80DESA 8.5 81.3 154.6

80DESA 5SUC 7.3 69.8 146.8

80DESA 10SUC 7.4 70.8 148.5

80DESA 5PEG 7.1 67.9 153.3

80DESA 10PEG 6.7 64.1 156.6

80DESA 5NaHCO3 6.5 62.2 156.5

80DESA 10NaHCO3 6.5 62.2 155.7

100DESA 8.4 80.4 168.4

100DESA 5SUC 8.8 84.2 156.3

100DESA 10SUC 8.8 84.2 158.5

100DESA 5PEG 10.1 96.7 160.2

100DESA 10PEG 8.4 80.4 167.3

00DESA 5NaHCO3 7.6 72.7 161.8

100DESA 10NaHCO3 7.3 69.9 165.1



polymerizationmedium had nearly no effect on theTgs of porous80 DESA and 60 DESA copolymers, but sucrose decreased theTgs of both homopolymer and copolymers. Highest decreasingeffect of sucrose on Tg of polymers was observed for those ofhomopolymer (100 DESA polymer), and approximately 11 �Cdecrease in Tg of homopolymer was determined due to use ofsucrose as pore-former. The decreases in Tgs of copolymers80 DESA and 60 DESA due to the pore-former sucrose wereabout 7 and 3 �C, respectively. Among the pore-formers, thishigher effect of sucrose inTgs of polymers can be explained by thedifficulty in its removal during the purification of polymers.3.3. Equilibrium Swelling Values of DMAEMASA Homo-

polymer and P(DMAEMASA-co-DMAm) Gels Depending onTemperature of the Swelling Medium. The variation ofequilibrium swelling values (ESVs) of DMAEMASA homo-polymer (100 DESA) and DMAEMASA-DMAm copolymerswith temperature depending on cross-linker content is given inFigure 6. The increase in cross-linker (NMBA) content inmonomer mixture from 1 to 2 mol % decreased the ESVsof both homopolymer and copolymers. In addition, the

introduction of DMAm in polymer structure and further increasein its content in monomer mixture increased the ESVs ofpolymers. As noted before, intermolecular hydrogen bondingsin 100 DESA polymer decrease the swelling capacity of polymer.In nonporous copolymer (80 DESA and 60 DESA) structure, thepresence of DMAm decreases the concentration of hydrogenatoms which are capable of making intra- or intermolecularhydrogen bondings because it has no hydrogen atom bound tonitrogen or oxygen atom. Thus, the higher ESVs of copolymerscan be explained by the decrease in hydrogen bondings betweenpolymer chains. Bae et al.28 have also observed that the incor-poration of DMAm is more efficient on the water absorptionvalues of NIPAM terpolymers (NIPAM/AAm/FOSA and NI-PAM/DMAm/FOSA) than that of acrylamide (AAm). Thisfinding is consistent with our finding. The effects of pore-formingagents on the ESVs of DMAEMASA homopolymer and DMAE-MASA-DMAm copolymers in distilled water at various tempera-tures are given in Figure 7 a�c. The swelling capacity of 100DESA slightly increased with the increase in temperature from 20to 60 �C as can be expected (Figure 7a). The use of pore-formingagents such as NaHCO3, sucrose, and PEG enhanced the waterabsorption capacity of homopolymer. Furthermore, higher im-provements in ESVs of homopolymer were determined with theincrease in the content of pore-former from 5 to 10%. Among thepore-forming agents used in this work, the highest enhancementon the ESV of homopolymer was performed by NaHCO3.Similar findings on the ESV’s of copolymers are seen in Figures 7b and c. Twice increase in PEG and sucrose amounts had nearlythe same and comparatively low amount of improvement on theswelling of both homopolymer and copolymers in distilled water.When NaHCO3 amount in polymerization medium doubled, anapparently higher amount of improvement in the swelling ofporous gels was observed in comparison to the effects of PEGand sucrose.3.4. Equilibrium Swelling Values of DMAEMASA Homo-

polymer and DMAEMASA-DMAm Gels Depending on pH ofthe Swelling Medium. The variation of ESVs of porous andnonporous DMAEMASA homopolymer (100 DESA) and co-polymers (80 DESA and 60 DESA) with pH of swelling mediumare given in Figure 8a�c. While nonporous DMAEMASA homo-polymer has lowest ESVs in buffer solutions at pH range of2�10, homopolymer synthesized with 10% NaHCO3 (100DESA 10 NaHCO3) displayed highest amount of swelling in

Figure 4. DMAEMASA contents (in wt %) of porous and nonporousDMAEMASA homopolymer and DMAEMASA-DMAm copolymer gels.

Figure 5. Glass transition temperatures (Tg) of porous and nonporousDMAEMASA homopolymer and DMAEMASA-DMAm copolymers.

Figure 6. Equilibrium swelling values of nonporous DMAEMASA andDMAEMASA-DMAm gels with two different cross-linker contents indistilled water as a function of temperature.



buffer solutions at all pH values (Figure 8a). The increase inpore-formers’ content led to an apparent improvement in ESV of100 DESA. ESV of homopolymer in acidic medium at pH = 2 ishigher than those at neutral and basic pHs, and it decreases withthe increase in pH up to 5. The decrease in ESV of DMAEMASAhomopolymer with the increase in pH from 2 to 5 can beexplained by the decrease and dissappearance in protonation ofpolymer. In general, cross-linked cationic polymers have higherwater absorption capacities at low pHs due to repulsion forcesbetween the positive-charged polymer chains. They displaylower swelling at neutral and alkaline pHs, because the polymeris in noncharged structure at these pHs. The increase in ESVof homopolymer with pH after pH = 5 may be attributed tothe partial neutralization of DMAEMASA unit (or pendantgroup of DMAEMASA) of polymer to DMAEMA with NaOH

in buffer solutions with pH = 7 and 10 during the swelling.As seen, ESVs of copolymers decrease with the increase inDMAEMASA content and the homopolymer has lowest waterabsorption capacity due to inter- or intramolecular hydrogenbondings between polymer chains. The interaction of DMAE-MASA unit of polymer with a constituent of buffer solutionduring swelling is confirmed by the decrease in the increases inswelling of polymer with the decrease in DMAEMASA content(60 DESA copolymer) of polymer at pH range of 5�10. Thesame swelling and pH relationship are valid for 80 DESA and 60

Figure 7. Equilibrium swelling values of DMAEMASA and DMAE-MASA-DMAm gels in distilled water as a function of temperature.

Figure 8. Equilibrium swelling values of DMAEMASA and DMAE-MASA-DMAm gels in buffer solutions (20 �C) as a function of pH ofswelling medium.



DESA copolymers in buffer solutions at pH range of 2 and 10.The decrease in ESV of polymer with DMAEMASA content wasalso observed for DMAEMASA-NIPAM copolymers.16 Zhanget al.10 reported that the swelling of porous NIPAM gelsincreased with the increase of pore-formers (sucrose) content,consistent with our findings. In the swelling of both homopoly-mer and copolymer gels with pH range of 2�10 (Figure 8a�c),the increase in NaHCO3 and sucrose content from 5 to 10 wt %showed nearly the same amount of effect on the swellingvalues, but doubling the PEG content slightly affected theswelling of the copolymers such as 80 DESA 5 PEG and60 DESA 5 PEG. As to the effect of NaHCO3 on the swellingof both homopolymer and copolymers, the lowest increasein ESV due to NaHCO3 was observed for those of homo-polymer. During the synthesis of porous polymer with the useof NaHCO3 as porogen, gelation rate, and the concentrationsof initiator, monomer, and cross-linker are important parametersaffecting the porosity of polymer gel as well as the amount ofporogen. If the gelation occurs in a longer time (slow gelation),and the evolution of CO2 through decomposition of NaHCO3

takes place fast, porous gel can not be obtained. Bune et al.13

reported that the polymerization rate of DMAEMASA withacrylamide (AAm) decreases with DMAEMASA content inmonomer feed up to 40 mol %. The lower contribution to theswelling of homopolymer with twice increase in NaHCO3

content in comparison to the effect on those of copolymersmay be explained by the following reasons:1. The rate of gelation in the formation of DMAEMASA

homopolymer will be slower than that of copolymerizationof DMAEMASA and DMAm due to repulsion forces ofpositive-charged pendant group of DMAEMASA.

2. In homopolymer structure, the formation of inter- orintramolecular hydrogen bondings is more favored in com-parison to the copolymer. These hydrogen bondings play arole as additional cross-links, and they decrease the ESV ofpolymer.

4. CONCLUSIONS

Novel DMAEMASA homopolymer and DMAEMASA-DMAmcopolymers were synthesized in the presence and absence ofpore-forming agents NaHCO3, sucrose, and poly(ethylene glycol)2000. The effects of kinds and contents of pore-forming agentson the swelling of polymers were determined by equilibriumswelling measurements in distilled water and buffer solutions,FTIR, elemental analysis, DSC, and SEMmethods. The elementalanalysis results showed that DMAEMASA homopolymer and itscopolymer with DMAm are a copolymer and a terpolymer, respec-tively, containingDMAEMAaswell. The presence ofDMAEMA inhomopolymer and copolymermay be attributed to the high affinityof DMAEMASA to water, and the partial neutralization of DMAE-MASA by the accelerator TEMED and pore-former NaHCO3

during the polymerization.DMAEMASA homopolymer has the lowest ESVs both in

distilled water and buffer solutions due to intermolecular hydro-gen bondings. DMAm constituent of the copolymer decreasedthe formation of intermolecular hydrogen bondings, and thus theESV of copolymer increased with DMAm content.

Both homopolymer and copolymers displayed highestamount of water absorption at low pH due to repulsion forcesbetween the protonated polymer chains. ESVs of polymersdecreased with pH up to 5, and then they increased slightly with

further increase in pH, most probably due to the partial conver-sion of DMAEMASA unit of polymer to DMAEMA by itsneutralization with NaOH in buffer solution.

Glass transition temperature of DMAEMASA homopolymergel was determined to be 168.4 �C. DMAm decreased the Tg ofcopolymer due to the decrease in intermolecular hydrogenbondings. Porogens decreased the Tg of homopolymer too inthe order of NaHCO3 < PEG < SUC, and among them, sucroseled to 11 �C decrease in Tg of homopolymer. In case of copoly-mers, NaHCO3 and PEG had no apparent effect on Tgs ofcopolymers, but sucrose led to 7 �C decrease in glass transitiontemperature of copolymer. NaHCO3 has highest effect on theswelling of both DMAEMASA homopolymer and DMAEMA-SA-DMAm copolymer gels in comparison to PEG and sucrose.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected]. Phone: þ90 212 473 70 00.Fax: þ90 212 473 71 80.

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effects of pore-forming agents and polymer composition on the properties of novel poly( n , n...

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