Multidimensionally stimuli-responsive phase transition of aqueous solutions of poly((N,N-dimethylamino)ethyl methacrylate) and poly(N,N-dimethyl-N-(methacryloyl)ethyl ammonium butane

Multidimensionally Stimuli-Responsive PhaseTransition of Aqueous Solutions of Poly((N,N-dimethylamino)ethyl methacrylate) and Poly(N,N-dimethyl-N-(methacryloyl)ethyl ammoniumbutane sulfonate)

Jun Gao, Guangqun Zhai, Yan Song, Bibiao Jiang

Section of Soft Matter and Macromolecular Science, Key Laboratory of Polymeric Materials of Changzhou City,Department of Materials Science and Engineering, Jiangsu Polytechnic University, Changzhou, 213164,People’s Republic of China

Received 1 December 2006; accepted 6 March 2007DOI 10.1002/app.26683Published online 5 December 2007 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Free radical polymerization of N,N-(dime-thylamino)ethyl methacrylate and N,N-dimethyl-N-(metha-cryloyloxy)ethyl ammonium butane sulfonate was carriedout to prepare PDMAEMA and PDMABS. Proton nuclearmagnetic resonance spectroscopy indicated that bothPDMAEMA and PDMABS exhibited an electrolyte-respon-sive conformational dynamics in D2O of different ionicstrength. PDMAEMA, as a polybase, and PDMABS, as apolysulfobetaine, exhibited a series of multidimensionalstimuli-responsive phase transition behaviors. Adding

NaCl would decrease the phase transition temperature(TPT) of their aqueous mixtures, because of polyelectrolyteeffect for PDMAEMA and anti-polyelectrolyte effect forPDMABS, respectively. For PDMAEMA, a low pH wouldfacilitate the dissolution; on the other hand, for PDMABS,a maximum TPT was achieved in neutral media. � 2007Wiley Periodicals, Inc. J Appl Polym Sci 107: 3548–3556, 2008

Key words: LCST; UCST; phase transition; pH-dependent;electrolyte-responsive; thermoresponsive

INTRODUCTION

The physicochemical properties of hydrosoluble po-lymers and their aqueous solutions have attractedmuch attention from both academia and industry.1

Typical water-soluble polymers embrace polyacryl-amide (PAM), polyacrylic acid (PAAc) and its salts,poly(vinylpyridine) (PVP) and its N-alkylated deriv-atives, poly(ethylene imine) (PEI), polyoxazoline(POXZ), poly((N,N-dimethylamino)ethyl methacry-late) (PDMAEMA) and its N-alkylated derivatives,poly(ethylene oxide) (PEO), etc. They have been uti-lized for flocculation, dispersion, stabilization, visco-sification, moisture sorption, metal recovery, as wellas other application potentials.1

The behavior of polymers in a given mediumreflects the balance interaction among its own seg-ments and the surrounding molecules.2,3 For some

polymers, their aqueous solutions could undergoobservable phase transition in response to the exter-nal factors, viz. pH, ionic strength, and temperature.1

In particular, polyampholytes, i.e. the copolymers ofanionic monomers and cationic monomers, couldform a homogeneous solution when dissolved inacidic or alkaline aqueous media to achieve a highcharge asymmetry, characteristic of common poly-electrolyte.4–6 However, the phase transition from asolution to an emulsion could be triggered by adjust-ing the aqueous pH to around isoelectric points(IEP), in which there is no net charge in polyampho-lytic chains.4–6 The typical polyampholytes, includ-ing poly(acrylic acid)-co-poly(vinylpyridine) andpoly(acrylic acid)-co-poly((N,N-dimethylamino)ethylmethacrylate), have been intensively investigated fora detailed understanding on their phase transitionand conformational change.4–8

Also termed as inner salt,9 polybetaines, contain-ing both positive and negative charges in individualrepeat units, represent another class of water-solublepolymers, in which the phase transition is triggeredby ionic strength. When dispersed in electrolyte-freeaqueous media, because of the intrachain and inter-chain electrostatic attraction as well as the hydropho-bic interaction of the main chains, polybetaines

Correspondence to: G. Q. Zhai ([email protected]).Contract grant sponsor: Natural Science Foundation of

China; contract grant numbers: 20574033, 20674033.Contract grant sponsor: Natural Science Foundation of

Jiangsu Province; contract grant number: BK2005404.

Journal of Applied Polymer Science, Vol. 107, 3548–3556 (2008)VVC 2007 Wiley Periodicals, Inc.

assume a globular conformation, and the matricesform a de facto cross-linked network, leading to a tur-bid emulsion. However, such electrostatic attractioncould be shielded by the addition of NaCl, leadingto a globule-to-coil transition in the chain conforma-tion. As a result, the polybetaine network is dis-rupted finally to achieve a homogeneous aqueous so-lution.10–16

Although the lower critical solution temperature(LCST)-type phase transition of polystyrene/cyclo-hexane system has been noted,17,18 thermoresponsivephase transitions primarily occur and have beeninvestigated in aqueous systems of both nonionichydrogen-bondable systems and polybetaines.Poly(N-isopropylacrylamide) (PNIPAm) is the mostpopularly studied member of the former class thatpossess an inverse aqueous solubility upon heating.3

Hydrogen bonding interaction of amide moieties ofPNIPAm with water facilitates its dissolution atlower temperature. Upon heating, enhanced hydro-phobic effect of isopropyl moieties and weakenedhydrogen bonding reduce the water solubility ofPNIPAm. At a certain critical temperature, negativeexcess entropy of mixing is achieved, favorable to aphase transition from solutions to emulsions. It hasalso been noted that the phase diagram of aqueousPNIPAm solution is rather flat, indicating that transi-tion temperature of aqueous PNIPAm solution shownegligible concentration dependence, helpful toexperimentally determine LCST using cloud pointmeasurement, differential scanning calorimetry, lightscattering, viscometry, and so on.3

On the other hand, this PNIPAm-specific phenom-enon has probably given rise to a misunderstandingamong many researchers that the phase transitiontemperature (TPT) equals to the LCST or vice versa. Infact, for a given system in which the properties ofpolymer (chemical structure including MW andMWD, monomer composition, terminal functionality,etc.) and the solvent (chemical nature including com-position, pH, ionic strength, etc.) are fixed, LCST hasbeen accordingly specified; while the TPT is, in addi-tion to chemical natures of both the polymer and sol-vent, additionally, depending on the composition ofthe polymer/solvent binary system, i.e. the polymerconcentration.

Diametrically opposite to aqueous PNIPAm solu-tions, those of polybetaines exhibit a upper criticalsolution temperature (UCST)-type phase transition;i.e. they are water-insoluble at lower temperaturedue to the electrostatic attraction. Heating promotesthe water molecules to diffuse into the network,leading to the disruption of the network at a criticaltemperature. As a result, the solution undergoes anemulsion–dissolution phase transition.10,11

Besides PNIPAm, poly((N,N-dimethylamino)ethylmethacrylate) (PDMAEMA) is also known for its

LCST-type phase transition behavior, since the terti-ary amine moieties can hydrogen-bond with water,while the gemini methyl groups exert hydrophobiceffects. What is more, the amines could undergo N-alkylation (quaternization or betainization), to resultin cationic polyelectrolytes or zwitterionic polybe-taines, respectively. In addition, since PDMAEMAitself could be deemed as a weak polyelectrolyte to acertain extent, the effect of pH and ionic strength ofaqueous media may impose significant influence onthe phase transition of its aqueous solution. There-fore, the aqueous PDMAEMA systems might exhibita multidimensional stimuli-responsive behavior.However, there have been few systematic investiga-tions for such a multidimensional stimuli-responsiveaqueous polymer mixtures, especially vis-a-vis PNI-PAm.

In this article, we report on the phase transitionbehaviors of aqueous solutions of PDMAEMA andpoly(N,N-dimethyl-N-(methacryloyloxy)ethyl ammo-nium butane sulfonate) (PDMABS) prepared fromthe sulfobetaine derivative of DMAEMA. Both ofthem were found to exhibit an electrolyte-responsiveconformational dynamics using nuclear magneticresonance spectroscopy. Thus, their phase transitiontemperature (TPT), as a function of polymer concen-tration, pH, ionic strength, etc., was studied in-depth. It might help to clarify the multidimensionalstimuli-responsive behaviors of both PDMAEMAand polybetaines.

EXPERIMENTAL SECTIONS

Materials

(N,N-dimethylamino)ethyl methacrylate (DMAEMA)was purchased from Xinyu Fine Chemicals, Wuxi,China. The inhibitor was removed by distillationin vacuo. 1,4-butane sultone was from FengfanChemical, Wuhan, China. Ammonium persulfate(APS) and 2,20-azoisobutyrynitrile (AIBN) was fromShanghai Chemical Reagent, Shanghai, China. Theywere recrystallized before use.

Sulfobetainization of DMAEMA (DMABS)

DMAEMA (0.127 mol) and 1,4-butane sultone (0.125mol) were added into a 50 mL single-necked round-bottom flask to form a transparent and colorless so-lution. The solution was sealed to react at roomtemperature overnight to produce white powders,N,N-dimethyl-N-(methylmethacryloyl ethyl) ammo-nium butane sulfonate (DMABS). The powderswere washed by acetone and recovered by filtrationand dried in vacuum overnight. The yield wasabout 85%.

STIMULI-RESPONSIVE PHASE TRANSITION OF AQUEOUS SOLUTIONS 3549

Journal of Applied Polymer Science DOI 10.1002/app

Homopolymerization of DMAEMA (PDMAEMA)

DMAEMA (63.5 mmol), AIBN (0.3 mmol), tetrahy-drofuran (THF, 20 mL) were added into a flask. Af-ter sealing, the system was allowed to react at 708Cfor 56 h. After the reaction was completed, thePDMAEMA was precipitated in petroleum ether. Af-ter being washed by petroleum ether several times,the polymer was dried under reduced pressure at1008C for about 24 h to yield white blocks.

Homopolymerization of DMABS (PDMABS)

DMABS (34.1 mmol) and APS (0.2 mmol) wereadded into deionized water (20 mL) to form a color-less solution. The system was allowed to react at708C for 24 h. After the reaction, the solution wascooled down to room temperature and poured intoacetone. The polymer was recovered by drying invacuum at about 1008C for overnight to yield whiteor slightly yellow loose blocks, which are highly hy-groscopic when exposed to atmosphere.

Fourier-transform infrared and proton nuclearmagnetic resonance spectroscopy

Fourier-Transform Infrared (FTIR) spectra of mono-mers and polymers were recorded on Nicolet Avatar370 spectrophotometer (4000–400 cm21) using KBr asthe matrix. Each spectrum was collected by cumulat-ing 10 scans at a resolution of 4 cm21. The protonnuclear magnetic resonance (1H NMR) spectroscopywas carried out on a Bruker 300 NMR spectrometerat room temperature. For the monomers, D2O wasused as the solvent. For the polymers, D2O and 0.5MNaCl/D2O solution were used as the solvents.

Gel permeation chromatography

The number–average MW and MWD of PDMAEMAwas determined on a Waters-150 Gel PermeationChromatography (GPC) system at 258C, usingHPLC-grade THF as eluent at a flow rate of 1.0 mL/min and PS as the standards.

Differential scanning calorimetry

The thermal analysis of PDMABS was carried out onthe Perkin–Elmer Pyris-1 differential scanning calorim-eter. The sample was heating from 40 to 2508C underN2 atmosphere, at a heating rate of 208C/min, afterwhich the sample was cooled rapidly to room temper-ature and then heated to 2508C at the same rate.

Determination of TPT

TPT was determined in conventional cloud-pointapproach. Predetermined amounts of PDMAEMA or

PDMABS were added into aqueous media to achievea specific polymer concentration, in the range from0.0001 to 0.05 g/mL. HCl, NaOH, and NaCl wereused to adjust pH value and ionic strength of aque-ous solutions. The aqueous systems were graduallyheated by oil bath at a rate of 1–28C/min until thephase transition occurs; the temperature at whichthe phase transition occurred was recorded as TPT.

RESULTS AND DISCUSSION

Sulfobetainization of DMAEMA (DMABS)

DMAEMA has a tertiary amine group that couldundergo protonation, quaternarization, and betaini-zation.6,9,10,11,15,19,20 Typical carbobetainizing reagentsinclude lactone, acrylic acid, and haloalkycarboxy-late, while sulfobetainizing reagents embrace sul-tone, vinyl sulfonylchloride, and haloalkysulfo-nate.6,9,10,11,15,19,20 In this study, 1,4-butane sultonewas chosen to sulfobetainize DMAEMA to produceDMABS. The chemical structure of DMABS was ana-lyzed by FTIR and 1H NMR, respectively.

Figure 1(a,b) show the FTIR spectra of DMAEMAand DMABS. The absorption band at wavenumberof 1635 cm21 was assigned to the C¼¼C doublebond. After the betainization, the absorption band at1165 and 1023 cm21, associated with the tertiaryC��N bond of DMAEMA, disappeared, and a strongabsorption of quaternary ammonium appeared at1043 cm21; meanwhile, the new absorption bandappeared at 1177 cm21, probably attributable to��SO3

2 groups.The formation of DMABS was also confirmed by

1H NMR spectroscopy. Figure 2(a,b) show the 1HNMR spectra of DMAEMA and DMABS using D2O

Figure 1 FTIR spectra of (a): DMAEMA (b): DMABS.

3550 GAO ET AL.


as solvent, respectively. In common, the peaks at d ��5.7 and 6.1 ppm were assigned to two protons in theCH2�� moieties of both DMAEMA and DMABS,respectively. For the spectrum of DMAEMA, the peaksat d �� 1.8, 2.2, 2.6, and 4.2 ppm were assigned to theproton in the ¼¼C��CH3 N��CH3, ��CH2��N andO��CH2�� moieties, respectively. The molar ratio of[CH2��] : [¼¼C��CH3] : [O��CH2��] : [��CH2��N] :[N��CH3], determined from the integral peak areas, isabout 2 : 2.9 : 2 : 2 : 6, in a good agreement to the theo-retical value of 2 : 3 : 2 : 2 : 6, indicating a high purity ofDMAEMA.

For the DMABS, in addition to those associatedwith the CH2�� moieties, the peaks at d �� 1,7, 1.9,2.8, 3.1, 3.3, 3.7, and 4.5 ppm were assigned to theprotons in the ��CH2��SO3

2, ��C��CH3, N1��CH2

��CH2��CH2��, CH3N1, N1��CH2��CH2��CH2��,

O��CH2��CH2��N1 and O��CH2��CH2 moieties,respectively. On the basis of the integral peak areas,their molar ratio is well consistent with the theoreti-cal value. Therefore, both the FTIR and NMR resultsconfirmed that DMAEMA was betainized intoDMABS.

Electrolyte-responsive conformational dynamicsof PDMAEMA and PDMABS

PDMAEMA and PDMABS were prepared via thefree radical polymerization. Although DMAEMA iswater-soluble, THF, instead, was chosen as solventto avoid the probable cross-linking. PDMABS wasprepared using H2O as solvent. The apparent num-ber–average molecular weight (MW) and its distribu-tion (MWD) of PDMAEMA were determined byGPC to be about 1.73 3 104 and 1.81, respectively.Because of negligible solubility of PDMABS in THFand the absence of aqueous GPC setup, we failed todetermine the exact MW and MWD of PDMABS.

Since PDMAEMA and PDMABS were deemed asweak polyelectrolyte and polysulfobetaine, respec-tively, it was expected that the ionic strength of theaqueous media could impose significant influence ontheir chain conformations.1,19–23 1H NMR spectros-copy could provide inherent information on the con-formational dynamics, as adopted by Butun,20

Armes and coworkers,24 and Einsenberg and co-

Figure 2 1H NMR spectra of DMAEMA and DMABS.

Figure 3 1H NMR spectra of PDMAEMA: (a) D2O, (b)0.5M NaCl/D2O.



workers.25 In this study, it was used to confirm thechemical structures and conformational changes ofthe polymers in response to ionic strength.

Figure 3(a,b) show the 1H NMR spectra ofPDMAEMA in D2O and NaCl/D2O, respectively. It isdiscernible that ionic strength holds an important roleon the chain conformation; all the intensity of thepeaks associated with PDMAEMA chains diminishedremarkably with the addition of NaCl. Such a changewas attributed to the electrolyte-responsive conforma-tional change of PDMAEMA. The amine moieties ofthe PDMAEMA chains are partly protonated in D2O,so electrostatic repulsion exists readily, leading to arather expansive conformation, corresponding toenhanced chain mobility, and thus an enhanced NMRsignal intensity.20,24,25 Upon the addition of NaCl, asstated earlier, the screening effect leads to the collapseof PDMAEMA chain into a compact state, a reducedchain mobility and a reduced NMR signal.

Figure 4(a,b) compare the 1H NMR spectra ofPDMABS in D2O and NaCl/D2O, respectively. Com-

paring the figures, the signals appeared much stron-ger in D2O than in NaCl/D2O. Commonly, it wasattributed to the screening effect of small ions to thepolybetaine chains. In contrast to PDMAEMA struc-ture, the PDMABS chains contain both cations andanions, among which the electrostatic attractiondominated the intrachain and interchain interaction,so PDMABS chains aggregated into compact glob-ules with significantly restricted chain mobil-ity.20,24,25 The addition of low molecular weight elec-trolytes greatly shielded the electrostatic interactions,leading to less restricted chain mobility and anintensified NMR signal. These results further con-firmed the low molecular weight electrolytes imposesignificant influence on the conformational dynamicsof PDMABS.

The thermal properties of PDMAEMA andPDMABS were analyzed by differential scanning cal-orimetry (DSC). Because of the flexible side chains,PDMAEMA homopolymer has a low glass transitiontemperature (Tg) of about 198C. Figure 5 shows theDSC curve of PDMABS. For run one, besides a glasstransition at about 1608C, the endothermal processsteadily occurred at about 80–1008C, attributable tothe evaporation of moisture residues. After the ther-momechanical history being removed, a Tg of about1758C was detected. Such a high Tg was primarilydue to the presence of zwitterionic side chains, sigi-nificantly improving the intrachain interaction.

Phase transition behavior of aqueousPDMAEMA media

In comparison with those of PNIPAm, PHEMA,PEO, etc., aqueous PDMAEMA media were expected

Figure 4 1H NMR spectra of PDMABS: (a) D2O, (b) 0.5MNaCl/D2O.

Figure 5 DSC curve of PDMABS: (a) first run and (b) sec-ond run.

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to undergo a multidimensional phase behavior inresponse to temperature, pH and ionic strength inthe presence of the amine moieties. In this study,TPT of the aqueous PDMAEMA systems as a func-tion of polymer concentration, ionic strength, pH,etc., was investigated in details.

Figure 6 shows the TPT of the aqueousPDMAEMA systems with different polymer concen-tration and ionic strength. In common, TPT increasedwith the decreasing polymer concentration, typicalfor the LCST-type phase behavior in the dilute re-gime.17,18 Because the intrachain and interchainhydrophobic effect in dilute solutions is ratherweaker than that in concentrated solution, a highertemperature is required to militate the hydrogenbonding between the PDMAEMA and water, givingrise to a higher TPT.

It could also be observed from Figure 6 that anincrease in ionic strength also exerts observable effecton the TPT. As reported for aqueous PNIPAm media,3

a high ionic strength facilitated the phase transitionfrom solution to emulsion, resulting in a lower TPT.As mentioned earlier, PDMAEMA chain is weaklycharged into a weak polycation in water. The electro-static repulsion among the repeating units drives thePDMAEMA chains to adopt a more extensive confor-mation. With the addition of NaCl, the repulsion isshielded in such a degree that a more coiled confor-mation is assumed. As a result, it enables the aqueousPDMAEMA solution of a higher ionic strength tophase separate at a lower temperature. However, it isalso noteworthy that such an effect was not suffi-ciently pronounced, probably ascribable to the lowcharge density of the PDMAEMA chains.

With PDMAEMA a polybase (pKa of 6.6 or 8),26

pH plays a pivotal role, via protonation and hydro-gen bonding, on the chemical nature, conformationalchange and phase behavior of aqueous PDMAEMAsystems. Figure 7 shows the variation of TPT of theaqueous PDMEMA systems as a function of the me-dium pH, while the ionic strength and polymer con-centration were fixed at 0.1M and 0.001 g/mL,respectively. For the solution with a pH < 3, nophase transition was observed to occur. Clearly, inhighly acidic media, PDMAEMA units are highlyprotonated, giving rise to a cationic polyelectrolytechains. The electrostatic repulsion has overcome thecontractive potential induced by hydrophobic inter-action among the backbones. For the solvent with apH of 3, the mixture exhibited a TPT of 548C, indicat-ing a fact that because of a low proton concentration,much less repeating units were protonated so thatthe hydrophobic interaction outweighed the electro-static repulsion and the hydrogen bonding, givingrise to a phase separation at elevated temperature.

Similarly, with pH increasing to 4 and 5, respec-tively, TPT was detected to be about 348C and 338C,respectively. With pH shifting from neutral to alka-line, the TPT of the aqueous PDMAEMA mixturewas about 30–318C, approximately independent onthe solvent pH. To the extreme, when the mediumpH was adjusted to 13, PDMAEMA cannot be welldissolved to form a homogenous solution even atroom temperature, indicating that the hydrophobicinteraction has dominated the intramolecular andintermolecular interaction, or, in other words, thehydrogen bonding has been too weak to well solubi-

Figure 6 The dependence of TPT of the aqueousPDMAEMA mixtures on polymer concentration and ionicstrength.

Figure 7 The dependence of TPT of the aqueousPDMAEMA mixtures on pH; ionic strength and polymerconcentration fixed at 0.1 mol/L and 0.0001 g/mL, respec-tively.



lize the PDMAEMA in the aqueous media at roomtemperature.

Phase transition behavior of aqueousPDMABS systems

Aqueous polybetaine systems form heterogeneousemulsions at lower temperature. Upon heating oradding NaCl, as a result, the systems undergo aphase transition to homogenous solution. It has beenalso reported that the phase behavior of aqueouspolybetaine systems also depends on polymer con-centration, ionic strength, pH, etc. Therefore, a seriesof experiments were conducted to correlate thesefactors.

Effects of ionic strength

In our study, the aqueous PDMABS system main-tained turbid even when the system was heatedclose to about 1008C. Besides, for the systems with ahigher PDMABS load, PDMABS failed to be welldispersed during short period. Thus, NaCl wasadded into the system to adjust the ionic strength,and the polymer concentration was set at 0.001–0.05g/mL.

Figure 8 shows TPT of aqueous PDMABS systemas a function of polymer concentration and ionicstrength. In general, the addition of NaCl led to anobvious decrease in TPT, a typical behavior associ-ated with the anti-polyelectrolyte effect. On the otherhand, the polymer composition exerted a dual in-fluence on TPT. In the dilute regime, the increase ofthe polymer concentration gave rise to a remarkableincrease in TPT. Nonetheless, after achieving a maxi-

mum at a polymer concentration of about 0.01 g/mL,TPT decreased steadily with the further increase inthe polymer concentration. Such a parabolic changein TPT with respect to the polymer concentrationindicates that, for the systems studied in this section,their UCST might approximately equal to TPT at apolymer concentration of 0.01 g/mL.

Figure 9 shows the variation of UCST, as derivedearlier, of these systems as a function of ionicstrength of the aqueous media. In contrast to theaqueous PDMAPS mixture,14 it is evident that anincrease in ionic strength could lead to a significantdecrease in UCST of the aqueous PDMABS system.For example, an increase in ionic strength from 0.051to 0.137M, corresponds to a steady decrease in UCSTfrom 95 to 858C. Furthermore, an increase in ionicstrength to 0.171M led to a significant decrease ofUCST to 608C. What is more, in our study, a furtherincrease in ionic strength may result in a UCSTbelow room temperature; i.e. the aqueous PDMABSsystems with any PDMABS load and ionic strengthhigher than 0.2M, could maintain homogeneous andtransparent, even at room temperature. On the otherhand, the decrease in ionic strength would enhancethe UCST of the aqueous PDMABS systemadversely. In our study, heating these mixtures withvery low ionic strength to even about 1008C failed toobtain a highly transparent solution.

Effect of pH value

It has been believed that since sulfonic acid is astrong acid, no hydrolysis or association occurs foraqueous polysulfobetaine systems. Consequently,pH-sensitive properties could only exist in aqueous

Figure 8 TPT of aqueous PDMABS system with differentionic strength and polymer concentration.

Figure 9 The dependence of UCST of aqueous PDMABSsystems on ionic strength.

3554 GAO ET AL.


polycarbobetaine systems and it was absent, orrather weak at least, in aqueous polysulfobetainesystems.9–11,15 However, Armes, McCormick et al.have systematically investigated the pH-sensitiveproperties of aqueous polybetaine systems. It wasrevealed then that even aqueous polysulfobetainesystems can exhibit a series of pH-sensitive solutionproperties.23,24,27 In this section, the TPT of aqueousPDMABS systems was correlated to the solvent pHat a fixed polymer concentration and ionic strength.

Figure 10 shows the dependence of TPT of aqueousPDMABS systems on medium pH, with polymer con-centration and ionic strength fixed at 0.001 g/mL and0.1M, respectively. In acidic or basic media, the mix-tures could be homogenized into transparent solutionsupon heating to slightly above temperature. In general,with the medium pH shifting to the neutral range, TPT

increases gradually. For example, TPT achieved a maxi-mum up to 1258C in a medium with pH of 5.

Such a pH-sensitive TPT variation was probablyattributed to the site-binding ability of ammoniumand sulfonate ions. Although the protonation of sul-fonate anions is rather difficult.9,10,15 in acidic media,a high proton concentration renders it possible thatprotons, replacing ammonium cations, preferablydiffuse into the vicinity of sulfonate anions, resultingin a militated electrostatic attraction. In the highlybasic media, hydroxide anions exclude the sulfonateanions close to ammonium cations. In the neutralmedia, both proton cations and hydroxide anions aresparsely dispersed, rarely reducing electrostaticattraction. As a result, the intrachain and interchainelectrostatic attraction reached the maximum, whichneeds a higher medium temperature to disrupt.

CONCLUSIONS

TPT of various aqueous mixtures of PDMAEMA andPDMABS, as two multidimensional stimuli-respon-sive water-soluble polymers, were determined as afunction of polymer concentration, pH and ionicstrength to study their LCST- and UCST-type phasetransition behaviors, respectively.

For aqueous PDMAEMA systems, a decrease inpolymer concentration leads to a higher TPT in thedilute regime while a higher ionic strength promotedthe phase separation at lower TPT probably due tothe polyelectrolyte effect. pH directly controlled thesolubility of PDMAEMA. In highly acidic media, theelectrostatic repulsion retards the conformationalcontraction; a gradually militated protonation gaverise to a steady decrease in TPT to even below roomtemperature.

Aqueous PDMABS mixtures exhibited classicalUCST-type phase transition behaivor. Their UCSTdecreases monotonically with the increase in ionicstrength of the aqueou media because of anti-poly-electrolyte effect. The phase transition behavior ofaqueous PDMABS systems also showed a pH-de-pendent character: TPT achieved a maximum in theneutral media and decreased significantly when themedia shifted to highly acidic or basic. Such a pH-dependent variation was attributed to the site-bind-ing ability of ammonium and sulfonate ions, respec-tively.

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Documents

Multidimensionally stimuli-responsive phase transition of aqueous solutions of poly((N,N-dimethylamino)ethyl methacrylate) and poly(N,N-dimethyl-N-(methacryloyl)ethyl ammonium butane