Thermoresponsiveness of PDMAEMA. Electrostatic and

Article pubs.acs.org/Macromolecules

Thermoresponsiveness of PDMAEMA. Electrostatic and Stereochemical Effects Jukka Niskanen,† Cynthia Wu,‡ Maggie Ostrowski,‡ Gerald G. Fuller,‡ Sami Hietala,† and Heikki Tenhu*,† †

Laboratory of Polymer Chemistry, Department of Chemistry, University of Helsinki, P.O Box 55, Helsinki FIN-00014 HU, Finland Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States

‡

S Supporting Information *

ABSTRACT: Isotactic triads are introduced into poly(dimethylaminoethyl methacrylate) (PDMAEMA) when a Lewis acid yttrium(III)trifluoromethanesulfonate, Y(OTf)3, is present during the ATRP polymerization. The changes in the tacticities of the polymers are modest. However, the tacticity affects the phase separation process but in a different way in two studied cases, at pH 8 and 9. The pH, and thus the charge of the polymer, affects the balance between electrostatic and stereochemical effects. Upon the chain collapse, the zeta potential of the polymer decreases discontinuously at pH 9, whereas at pH 8 the potential keeps almost constant. However, even in the latter case the influence of the isotactic segments on the thermal transition may be observed. Increasing isotacticity is suggested to decrease the flexibility of the polymer chain. It also causes the polymers to adsorb in a more organized manner to the air/water interface than the atactic ones do. The change in the thermoresponsive behavior due to the changing tacticity of the polymer has been studied at the interface by observing the surface tension and by surface rheology and in the solution by conventional rheology. Differences in the elastic and viscous moduli owing to the different tacticities of the polymers are compared to those attributed to different molar masses and to varying pH.

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INTRODUCTION In studies of thermo- and pH-responsive polymers, atactic polymers have mainly been used due to the ease of the synthesis. However, the tacticity of a polymer is known to have a profound effect on its properties, as seen e.g. in crystallinity and solubility.1 Important studies on how an increase in isotacticity affects polymer solution properties have been conducted during the past decades on poly(methacrylic acid), PMAA,2,3 a “hydrophobic poly(carboxylic acid)”, the solubility of which in water is strongly affected by the stereoregularity of the chain.4 Thermoresponsive properties of poly(N-isopropylacrylamide), PNIPAM, have recently also been shown to be affected by the tacticity.5,6 Isotactic PMAA4 has so far most often been prepared by the hydrolysis of isotactic poly(methyl methacrylate) (PMMA).4 Isotactic PMMA can be prepared e.g. by anionic polymerization of methyl methacrylate using phenylmagnesium bromide7 or nbutyllithium8 as an initiator. In these cases the tacticity of PMMA may be controlled by the polymerization temperature. At low temperatures (−20 °C), isotactic polymers are formed, whereas isotactic-rich polymers are obtained at room temperature.7 Isotactic-rich poly(methacrylic acid) has recently been synthesized also by free radical polymerization. Kaneko et al. showed that the polymerization of methacrylic acid calcium salt in dimethylformamide (DMF) or toluene produced PMAA with a 29−65% isotactic triad content.9 Isotactic and atactic © 2013 American Chemical Society

PMAAs have very different solubility in water. Atactic PMAA is soluble in water as such, but the solubility of isotactic PMAA depends on the degree of neutralization.10 Isotactic PMAA forms thermoreversible elastic gels when cooling a 10% aqueous solution (degree of neutralization, α, 0.28) to 5 °C.4 Isotactic and atactic poly(methyl methacrylates) have different conformations, and it has been shown that the isotactic PMMA has a helical structure. In the solid state, isotactic PMMA may even exist as double-stranded helices.11 Isotactic PMAA is also more or less helical, and this affects its properties in aqueous solutions. It has been suggested that isotactic PMAA is less hydrated (more hydrophobic) than the syndiotactic one, owing to the helicity and higher stiffness of the isotactic chain.10 PNIPAM is one of the most studied thermoresponsive polymers still worth consideration, being an exceptionally wellcharacterized one. Atactic PNIPAM produced by radical polymerization has a ∼45−47% content of isotactic diads, and the polymer undergoes a phase transition at ∼32 °C.12 The isotactic content of PNIPAM can be increased e.g. by conducting the polymerization in the presence of a coordinating Lewis acid.5,13 The obtained degree of isotacticity for PNIPAM is somewhat lower (57−92%13) than that Received: December 26, 2012 Revised: February 27, 2013 Published: March 8, 2013 2331

dx.doi.org/10.1021/ma302648w | Macromolecules 2013, 46, 2331−2340

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Table 1. Summary of the Polymerizations

a

denotation

a23

Mn PDI initiator (mmol) Y(OTf)3 (mmol) Y(OTf)3/Mona monomer (mmol) CuBr (mmol) CuBr2 (mmol) Cu (mmol) solvent (mL) ligand (mmol) temp (°C) time (h) polymer (g)

23 200 1.3 DMDBHD (0.14)

56 200 1.5 CPDT (0.05)

84 900 1.6 CPDT (0.05)

22.9 0.14 (CuCl)

29.7 0.05 0.01 8 DMF (5) PMDETA (0.03) 60 2 1.30

59.3 0.05 0.01 7 DMF (10) PMDETA (0.03) 60 3 4.03

6 isopropanol (10) PMDETA (0.14) 60 2 2.68

a56

a85

i52

i46

52 000 1.8 DMDBHD (0.03) 1.9 1/8 14.8 0.06

45 700 1.5 DMDBHD (0.03) 3.7 1/4 14.8 0.05

2 isopropanol (5) bipyridyl (0.10) 60 4 1.52

6 isopropanol (7) bipyridyl (0.10) 60 5 0.83

Lewis acid (Y(OTf)3) to monomer molar ratio.

reported for anionically polymerized isotactic PMMA7 and varies with the type and amount of Lewis acid used.14 However, even a moderate increase from 45 to 57% in the isotacticity of PNIPAM has an effect on the phase transition temperature, this decreasing from 31.1 to 27.7 °C. Further increasing the isotactic content up to 66% lowers the phase separation temperature below room temperature (17 °C), and PNIPAM turns insoluble in water when the isotactic content is 72%.14 Interestingly, it has been shown by Chang et al. that upon increasing the syndiotactic content of PNIPAM to 61% the phase separation temperature rises to as high as 43 °C.15 The finding seems to be consistent with the conclusions on the hydration of PMAAs with different tacticities.10 Isotactic-rich PNIPAM (64% isotacticity) forms temperature-sensitive gels in 3% aqueous solutions. A clear solution obtained at 4 °C forms a clear gel when heated to 20 °C. Upon further heating, above the phase transition temperature (24−27 °C) the gel turns turbid but does not collapse.16 Radical polymerization reactions, even the controlled ones (controlled radical polymerization, CRP), generally yield atactic polymers, owing either to the chaotic nature of the reactions or to the high temperatures used.17 The tacticities of certain polymers with polar side groups, such as PNIPAM and poly(dimethylacrylamide) (PDMAAM), can be influenced by adding a coordinating component to the reaction, such as a Lewis acid18 or a polar solvent. Fluorinated alcohols and solvent mixtures have been successfully tested.19,20 The Lewis acid or the fluorinated alcohol coordinates with the end group of the growing polymer chain and the approaching monomers, thus influencing the tacticity of the polymers.21 Yet another way to introduce stereoregularity in polymers is by utilizing monomers with bulky substituents. The polymerization of monomers such as silyl methacrylates where the bulkiness of the silyl group can be varied, leads to atactic, syndiotactic, or isotactic polymers depending on the silyl group. The silyl group can later be removed to obtain the desired polymer, for example syndiotactic or isotactic PMAA.22 CRP methods, such as atom transfer radical polymerization (ATRP),21 nitroxidemediated polymerization (NMP),21 and reversible addition− fragmentation chain transfer polymerization (RAFT),5 have been used to prepare isotactic or isotactic-rich polymers. Poly(dimethylaminoethyl methacrylate) (PDMAEMA) is a pH and thermoresponsive polymer, the phase separation temperature of which can easily be tuned by adjusting the

pH of the aqueous solution.23 It has been shown that also the presence of negatively charged surfaces, such as clay24 or cellulose,25 can affect the phase transition of PDMAEMA. Přad́ ný et al. prepared isotactic PDMAEMA (∼70% isotactic triads) by anionic polymerization and studied the reactivity of the polymer as well as the viscosity26 and the potentiometric behavior27 of aqueous solutions of both isotactic and atactic PDMAEMA. They found that the intrinsic viscosity of isotactic PDMAEMA in a water/ethanol mixture (23.2 wt % ethanol) is higher than that of an atactic PDMAEMA. Both polymers behaved as polyelectrolytes; that is, the reduced viscosity increased with decreasing polymer concentration.26 The addition of salt suppressed the polyelectrolyte effect in both cases. The observed differences in the intrinsic viscosities of PDMAEMAs with different stereoregularities may indicate differences in the chain stiffness. However, the molar masses of the polymers in the cited work differed considerably, and the question on how the high isotactic content affects the chain conformation was left open. In this report, we consider the possibility of preparing PDMAEMA with increased isotacticity by ATRP in the presence of a Lewis acid yttrium(III)trifluoromethanesulfonate, Y(OTf)3. We shall show that the isotacticity indeed can be increased and that the solution properties of the polymers can be changed even by small differences in their stereoregularity. The changes in isotacticity obtainable by adding a Lewis acid in the ATPR polymerization reactions were observed to be modest, however. DMAEMA is structurally different from NIPAM, and the coordination of the monomers reacting with the growing chain is weaker. Because of this, the present report will not present the full picture of the tacticity effects on thermal behavior of PDMAEMA. We shall show, instead, that even subtle changes in the chain stereostructure have an interesting impact on the behavior of aqueous PDMAEMA. The thermoresponsive properties at molecular scale have been studied by microcalorimetry and zeta potential measurements. Because amphiphilic polymers have a tendency to concentrate at air−water interfaces,28 we have studied the surface tensions of the aqueous PDMAEMA solutions. In addition, rheological properties of the polymers have been studied both at the interface and by cone and plate geometry, which gives information on polymer interactions at a macroscopic scale. The polymers have been studied in borax buffer solutions with pH 8 and 9. To minimize the effect of the salts in the buffers on 2332


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(r) −CH2−, 54.5 −N−CH2−, 59.7 −O−CH2− and 175.4 (mm), 176.0 (mr) and 176.7(rr) ⟩CO. PDMAEMA a23. One more atactic PDMAEMA was synthesized by ATRP according to the method reported above for the isotactic-rich polymers but without the presence of yttrium(III) trifluoromethanesulfonate. Buffer Solutions and Sample Preparation. The buffers were made by dissolving 5.031 g of anhydrous borax in 410 mL (pH 8) or 92 mL (pH 9) of 0.1 M HCl, and both solutions were diluted to 2 L each with UHQ water.34 The ionic strengths of these buffers were 0.058 M (pH 8) and 0.038 M (pH 9); thus, they were further diluted with UHQ water to obtain the desired ionic strength of 0.02 M. Samples for the micro-DSC, surface tension, solution rheology, and zeta potential measurements were made as follows. 150 mg of polymer was dissolved in 6 mL of undiluted buffer solution. 5 mL of this solution was placed in a 5 mL dialysis capsule and dialyzed against the buffer with ionic strength 0.02 M for 3 days. The buffer was changed once during the preparation. The final volume of the solution was determined, and the solution was diluted to 10 mL using the buffer used in the dialysis. This way a stock solution with 12.5 mg/mL polymer concentration was obtained. The solutions for each measurement were prepared by dilution of this stock solution with the buffer solution from the dialysis. The solutions were stored at 7 °C prior to the measurements. For interfacial surface rheometry (ISR), solutions with concentration 10 mg/mL were prepared by dialysis as described above, by dissolving 100 mg of polymer in 5 mL of buffer and dialyzing against the buffer. Finally, the solution was diluted to 10 mL. Characterization and Instrumentation. Surface Tension Measurements. The surface tensions of the polymer solutions were measured with a KSV Sigma 703 surface tension balance using a DuNoyu ring and an external thermostat regulating the temperature at 22 °C. Solutions with concentrations 12.5, 1.0, 0.1, 0.01, and 0.001 mg/mL were used in the measurements. Six repetitions of each measurement were made, and the average value is reported together with the standard deviation. NMR spectra were recorded with a 500 MHz Bruker Avance III spectrometer at room temperature. The 1H decoupled carbon spectra of at least 3000 scans were collected using 30° pulse and pulse delay of 3 s. Inverse gated 13C spectra to determine possible NOE effects were collected for the atactic sample, and the sample with highest expected isotacticity with 30° pulse, 10 000 scans, and pulse delay of 6 s. Sample concentrations were 100 mg/mL in DMSO-d6 with tetramethylsilane as the reference in all measurements. The tacticity of the polymers was determined from the main chain methylene peaks which were the best resolved of the polymer main chain peaks (Figure 2). 1H spectra were collected at 60 °C. Size exclusion chromatography (SEC) was used to determine the molar masses of the polymers. PMMA standards from PSS Polymer Standards Service GmbH were used for calibration. Eluent was THF with tetrabutylammonium bromide (1 mg/mL). The apparatus included the following instruments: Biotech model 2003 degasser, Waters 515 HPLC pump, Waters 717plus autosampler, Viscotek 270 dual detector, Waters 2487 dual λ absorbance detector, Waters 2410 refractive index detector, and the OmnisecTM software from Viscotek. Styragel HR 1, 2, and 4 columns and a flow rate of 0.8 mL/min were used in the measurements. Calorimetric analyses of the polymers were conducted with a MicroCal VP DSC microcalorimeter, equipped with a pressure perturbation (PPC) accessory. Polymer solutions (5 mg/mL in buffer) were degassed before measurement. Thermograms were measured from 5 to 100 °C with 60 °C/h heating and cooling rate, repeating the heating−cooling cycle three times. The pre-equilibration time was 60 min before each heating cycle. For the samples at pH 8, the second heating scans were used for analysis. Because of aggregation and noise, the first heating scans were used for analysis of the samples at pH 9. The enthalpy values were normalized to repeating DMAEMA unit. The integration of asymmetric peaks in the thermograms is challenging and imprecise. By evaluating different

the phase transition of PDMAEMA, the ionic strengths of the buffers were adjusted to 0.02 M. For comparison, a series of atactic PDMAEMAs with varying molar masses have also been investigated. Thus, we can compare the effects of molar mass and tacticity on the rheological and thermoresponsive properties of PDMAEMA.

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MATERIALS

Anisole (99%, Aldrich), azobis(isobutyronitrile) (>98%, Fluka), bipyridyl (Aldrich), borax (anhydrous, Fluka), chloroform-D + 0.003% TMS (Euriso-Top), copper(I) bromide (Aldrich), 2-cyano-2propyldodecyl trithiocarbonate (CPDT, Aldrich), dimethylformamide (DMF for HPLC, Lab-Scan), dimethyl-d6 sulfoxide (Euriso-Top), 1,4dioxane (puriss., Aldrich), dimethyl 2,6-dibromoheptanediotate (DMDBHD, Aldrich), hydrogen chloride (0.1 M, FF Chemicals OY), hydrogen chloride (37%, VWR), isopropanol (for HPLC, VWR), potassium hydroxide (>85%, Fluka), sodium hydroxide (0.1 M, FF Chemicals OY), tetrabutylammonium bromide (puriss., Fluka), tetrahydrofuran (for HPLC, VWR), tetramethylsilane (TMS, puriss., Fluka), and yttrium(III) trifluoromethanesulfonate (>97%, Aldrich) were used a received. Copper chips were washed with acetone prior to use. N,N-Dimethylaminoethyl methacrylate (DMAEMA, Acros Organics) was passed through a basic aluminum oxide column (anhydrous, Merck) to remove inhibitor. N,N,N′,N′,N″-Pentamethyldiethylenetriamine (PMDETA, Aldrich) was distilled under reduced pressure prior to use. Synthesis (Table 1). Atactic PDMAEMA (a85 and a56). Two atactic PDMAEMAs were synthesized by the ATRP-RAFT method.29,30 In a typical synthesis DMAEMA (5 mL, 45.8 mmol), 2-cyano2-propyldodecyl trithiocarbonate (17 μL, 0.05 mmol), Cu(I)Br (7.2 mg, 0.05 mmol), Cu(II)Br2 (1.4 mg, 0.01 mmol), a copper chip (8.0 mg), and dimethylformamide (5 mL) were placed in a flask, which was sealed with a rubber septum. The solution was then purged with nitrogen for 20 min. The ligand (PMDETA, 0.013 mL, 0.06 mmol) was added as a 10% solution in DMF. The flask was placed in an oil bath at 60 °C. The nitrogen purge was stopped after 5 min of heating. The reaction was stopped after 2 h (conversion 29%) by pouring the reaction mixture into hot water. The polymer (1.3 g) was separated and freeze-dried from dioxane. Removal of the Dodecyl End Group. The dodecyl end group was removed with AIBN as reported by Willcock et al.31 The polymer (1.3 g, 24 mmol) was dissolved in DMF (15 mL), and 80.5 mg (0.49 mmol) of AIBN was added to the solution. The flask was sealed and purged with nitrogen for 20 min. Next, the flask was placed in an oil bath and heated at 85 °C for 3 h. The reaction was stopped by opening the flask, and the polymer was purified by dialysis against water and freeze-dried. 1.1 g of polymer was obtained. Isotactic-Rich PDMAEMA (i46 and i52). Isotactic-rich PDMAEMA was synthesized using ATRP in the presence of yttrium(III) trifluoromethanesulfonate.18,21,32,33 In a typical synthesis yttrium(III) trifluoromethanesulfonate (2 g, 3,7 mmol) was dissolved in isopropanol (7 mL). Then DMAEMA (2.5 mL, 22.9 mmol), dimethyl 2,6-dibromoheptanediotate (6 μL, 0.03 mmol), Cu(I)Br (7.6 mg, 0.05 mmol), and a copper chip (6.0 mg) were added to flask. The flask was sealed with a rubber septum, and the solution was purged with nitrogen for 20 min, after which the ligand (bipyridyl, 16 mg, 0.1 mmol) was added through the septum as a 10% solution in isopropanol. The flask was placed in an oil bath at 60 °C, and the nitrogen purge was stopped after 5 min of heating. After 5 h the septum was opened and the reaction mixture diluted with THF, and a precipitate was obtained. The precipitate was dissolved in acidic water, and the polymer was reprecipitated from hot water by addition of KOH. This was repeated once. Then the polymer was dissolved in acidic water and dialyzed against acidic water and later pure water until the water was neutral. The purified polymer (830 mg) was obtained by freeze-drying. The tacticity of the polymers were determined by 13C NMR spectroscopy using TMS as a reference. 13C NMR (ppm(tacticity)): 16.9 (rr), 18.5 (mr) and 21.1 (mm) −CH3, 42.8 N−CH3, 44.1 (rr), 44.4 (mr) and 45.0 (mm) quaternary C, 51.1 (m) and 52.5 2333


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baseline corrections, the error in ΔH values was estimated to be less than 10%. PPC measurements were conducted at 5−99 °C with 15 min equilibrating time before the run and 2 min equilibration time at each temperature point. The pulse time was set to 80 s to give the system enough time to stabilize after the pressure drop before the increase of pressure. For these calculations the specific volume of the solute (in this case the polymer) needs to be known. Zana et al. have shown that the partial specific volumes V̅ of polymers can be estimated by group contribution theory within an accuracy of 2%. In their paper the value of V̅ for PDMAEMA is reported as 0.854 mL/g.35 Measuring the heat flow at constant temperature and repeating the measurement at various temperatures allow one to determine the coefficient of thermal expansion of the partial volume of the polymer (eq 1). This is based on the Maxwell relation (eq 2). From the data one can calculate the average value for the change of the hydration volume ΔV/V, which is of high importance in the case of thermally responsive aqueous polymers. It is generally accepted that the change in the hydration volume is not a result of the association of the polymer chains as such, but of the disruption of the hydration layer.36−38 From the data obtained, the thermal expansion coefficient (α) of the partial volume of the polymer can be calculated using eq 1. These measurements were repeated four times and an average value for the change of the hydration volume ΔV/V was obtained.36−38 α=

1 ⎛⎜ ∂V ⎞⎟ V ⎝ ∂T ⎠ P

⎛ ∂S ⎞ ⎛ ∂V ⎞ ⎜ ⎟ = −⎜ ⎟ ⎝ ∂P ⎠T ⎝ ∂T ⎠P

the heating could only be turned on or off. Because of this, accurate stepwise heating of the sample chamber was not possible, and continuous heating was chosen as the heating method. The samples were kept at 7 °C before the measurement and placed in the sample cell just before the measurement. The sample was left to stabilize (∼20−30 min) until it reached the temperature of 20 °C. The PTFE cell was heated continuously until the temperature of 40 °C was reached. The average heating rate was determined as 0.8 °C/min. Measurements were made with 1 min intervals, and the temperature of the solution was recorded at each point. The dynamic temperature sweeps were made with a strain amplitude of 1% and frequency of 0.6283 rad s−1. To ensure that the measurements were within the linear viscoelastic regime, a dynamic strain sweep was conducted prior the measurements. Polymer concentrations were 10 mg/mL, and the solutions were prepared by dialysis as described previously. At least duplicate measurements were made for all the solutions to demonstrate reproducibility.

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RESULTS AND DISCUSSION Increasing the Isotacticity of PDMAEMA. A significant increase in the isotactic diad (triad) content was observed in the polymers prepared in the presence of Y(OTf)3 (see Table 2). A 1/8 molar ratio of Y(OTf)3 to monomer yielded a Table 2. Summary of the Polymers and the Tacticities of the Polymers

(1)

(2)

Zeta potential (ζ) of the polymers was measured as a function of temperature using a Zetasizer Nano-ZS ZEN3600 from Malvern Instruments Ltd. The instrument is equipped with a 4 mW He−Ne laser operating at λ0 = 633 nm. The measurements were performed at a scattering angle of 17° using both slow and fast electric field reversal. The samples were heated from 20 to 70 °C and back with a 30 min equilibrating time before the measurement series and a 15 min equilibration time at each temperature point. Three measurements were conducted at each temperature, and an average of these measurements is reported. The concentrations of the samples were 0.25 mg/mL. Rheological measurements were made using the stock solutions with polymer concentration 12.5 mg/mL after the dialysis. A TA AR2000 stress-controlled rheometer equipped with a 40 mm aluminum 2° cone and a Peltier heated plate was used for the oscillatory measurements. Temperature was increased from 5 to 45 °C (pH 9) or from 5 to 90 °C (pH 8) with a heating rate of 2 °C/min. Oscillatory frequency and temperature sweeps were made using strains within the linear viscoelastic regime. Dynamic temperature sweeps were made with a heating rate of 2 °C/min, 10% strain amplitude, and 6.283 rad s−1 frequency. A similar heating protocol was used in the interfacial rheology measurements. Interfacial rheology measurements of PDMAEMA at the air/water interface were obtained using the double wall-ring method.39,40 A stress-controlled rheometer, TA Instruments ARG2 (TA Instruments), was outfitted with a Du Nouy ring with a radius of 20 mm, suspended from the upper geometry mount. The ring was placed concentrically within the gap of a PTFE sample cell that was machined to provide a double Couette effect of interfacial shear on either side of the ring. The PTFE sample cell was equipped with four heating filaments and one temperature sensor on the outside of the cell. The heating filaments were connected with heat conductive copper tape to make the heat distribute more evenly. The outside of the cell was then insulated with insulating tape, leaving the bottom of the cell bare, thus maximizing the heating efficiency. A thermocouple was placed inside the sample cell to monitor the temperature of the sample solution. The thermocouple sensor was placed along the outer wall of the sample cell so that it was as far away from the DuNuoy ring as possible. The power source and heating filament setup was such that

denotation

Mn

PDI

a23 a56 a85 i52 i46

23 200 56 200 84 900 52 000 45 700

1.3 1.5 1.6 1.8 1.5

Y(OTf)3/Mona

mm/mr/rrb

m/rc

1/8 1/4

0/33/67 0/35/65 0/34/66 3/40/57 16/37/47

17/83 18/82 17/83 23/77 35/65

a Lewis acid (Y(OTf)3) to monomer ratio. bIsotactic triad content of the polymers. cDiad content of the polymers.

polymer with Mn 52 000 g/mol and an isotactic diad content of 23% (3% isotactic triad content), whereas increasing the ratio to 1/4 resulted in a 45 700 g/mol polymer with 35% isotactic diads (16% isotactic triads). In a typical atactic PDMAEMA, the isotactic diad content was 17%. The isotactic triads (mm) can be clearly observed in the 13C NMR spectra presented in Figure 1, in the methyl, methylene, and quaternary carbon peaks of the backbone and also in the carbonyl signal.41 Worth mentioning

Figure 1. Carbonyl region of 13C NMR spectra of atactic and isotacticrich PDMAEMA. 2334


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is that the NMR spectra for the atactic polymers only show peaks assignable to the racemo (rr) and syndiotactic (mr) triads. The nuclear Overhauser effect (NOE) was investigated by measuring the most isotactic and one atactic sample with inverse gated pulse sequences. Proton decoupled 13C spectra gave mm triad content of 16% and the inverse gated spectra a mm triad content of 13%. In both cases the peaks were equally resolved (spectra for comparison are presented as Supporting

Figure 3. Main chain methyl region of 1H NMR spectra of atactic and isotactic-rich PDMAEMA (top) and a proton decoupled 13C spectrum (black, mm = 16%) compared with an inverse gated 13C spectrum (red, mm = 13%) (bottom).

Figure 2. Alkane region of 13C NMR spectra of atactic and isotacticrich PDMAEMA.

nonionic thermoresponsive polymers.44 Water solubility of atactic PDMAEMA is strongly pH dependent. To keep the polymers well solvated, the samples were studied in two dilute buffers with pH 8 and 9.

Information, Figure S1). The isotactic content calculated from 1 H spectra was also 16% for the polymer with highest isotacticity content (Figure 3). All the spectral lines could be assigned for PDMAEMA, and thus it can be concluded that no side reactions between DMAEMA and the solvent or Y(OTf)3 had occurred during the polymerization. The finding excludes the occurrence of transesterification which may take place when the polymerization is conducted in an alcohol solvent.42 The coordination effect of Y(OTf)3 is weaker for DMAEMA than what has been reported for polyacrylamides as PNIPAM and PDMAAM.5,21,18 In the case of polyacrylamides, Y(OTf)3 effectively coordinates with the amide in the monomers and the growing polymer chain. With methacrylates, however, the effect is not as pronounced. Isobe et al. studied the polymerization of methacrylates in the presence of Lewis acids and obtained isotacticities of 3−14%.43 Since the nitrogen in DMAEMA is further apart from the double bond than it is in NIPAM and DMAAM, the coordination effect of the Lewis acid is similar as with methacrylates in general. Thus, the coordination effect is weaker for the growing PDMAEMA chain than it is for PNIPAM and PDMAAM. High amounts of Y(OTf)3 were needed (see Table 2) to influence the tacticity. Further increasing the amount of Y(OTf)3 inhibited the polymerization. The triad tacticities of the polymers were quantified using the signals from the methyl substituents of the main chain. The meso (m) and racemo (r) diad contents were determined from the triad peaks and are presented in Table 2. Thermoresponsive Properties and Interactions at the Molecular Scale. Interactions between water molecules and PDMAEMA are more complex than those in the case of most

Figure 4. Microcalorimetry thermograms of PDMAEMA solutions at pH 8.

Microcalorimetry on aqueous PDMAEMA solutions gives information not only on the phase separation temperature and the enthalpy change associated with the process but also on the hydration layer around the polymer. Data on the latter one is a net sum of multiple interactions, as will be seen below. Therefore, when discussing the hydration of the polymer, we try to avoid the concepts of “hydrophobicity” and “hydrophilicity”.45 The thermograms of aqueous polymer solutions at pH 9 (Figure 5) show that the onset temperature and the enthalpy of 2335


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magnitude higher than at pH 9. In the case of atactic polymers, the higher charging leads to an opposite molar mass effect than what was observed at pH 9. Tmax of the phase transition now increases from 51.8 to 60.3 °C when the molar mass increases from 23 to 85 kg/mol. The increasing protonation of the polymer by lowering the pH changes the balance between the stereochemical and electrostatic effects. In spite of the fact that tacticity clearly changes Tmax, the differences in ΔH and ΔV/V values in Table 3 do not show any clear trend. ΔV/V values at pH 8 are of the same order of magnitude as earlier observed in pure water for a noncharged poly(vinyl caprolactam),37 but lower than those observed for PNIPAM.36 At pH 9 the changes of the hydration volume are very low but measurable. This observation calls for further investigation, which will follow below. As already noted, in the case of thermoresponsive polymers the ΔV/V is an important measure because the changes in the polymer conformation upon heating are due to the disruption of the hydration layer. It is worth noting that the onset of the phase transition for PDMAEMA at pH 9 resembles that of PNIPAM;36,46 that is, the transition starts with a sharp increase in the Cp. At pH 8 the thermograms show a pretransition possibly originating from the charge of the polymer. Similar observations were recently made by Sun et al. in their studies of charged copolymers of PNIPAM, (PNIPAM-b-PAA)2−(PVP)2.46 To understand the differences in thermal behavior of the polymers, the zeta potentials for aqueous PDMAEMA solutions (0.25 mg/mL) with pH 8 and 9 were determined as functions of temperature. The measured values for the solutions with pH 8 (Figure 6) are slightly higher than those reported for

Figure 5. Microcalorimetry thermograms of PDMAEMA solutions at pH 9.

the phase transition changes with the molar mass of the polymers. With increasing the molar mass of the atactic polymer the transition gets sharper and occurs at lower temperatures. For the shortest polymer (a23) the onset is at 38.7 °C, while for the polymer with the highest molar mass (a85) it is at 34.5 °C. Calorimetric data are collected in Table 3. Table 3. Calorimetric Data for the Polymers in 5 mg/mL Aqueous Solutions at pH 8 and 9 denotation

pH

a23 a56 a85 i52 i46

8.3 8.1 8.1 8.1 8.0

a23 a56 a85 i52 i46

9.3 9.1 9.1 9.1 9.1

ΔH (kJ/mol) pH 8 1.85 1.76 2.53 1.77 1.92 pH 9 1.64 1.30 1.16 0.92 1.01

Tmax

Tonset

ΔV/V

51.8 56.5 60.3 56.3 58.3

48.1 54.8 51.9 47.2 44.3

0.64 0.80 0.62 0.81 0.61

42.3 36.4 35.4 35.7 32.6

38.7 35.3 34.5 33.3 31.7

0.04 0.07 0.07 0.06 0.05

The enthalpy of the transition is somewhat higher (1.64 kJ/ mol) for a23 than for a85 (1.16 kJ/mol). This is well in line with studies by Plamper et al., where linear and star-shaped PDMAEMAs with varying molar masses were compared. These authors found that the cloud point temperatures decreased with increasing molar mass.23 The polymers with increased isotacticity have almost equal enthalpies (0.92 and 1.01 kJ/ mol) of the phase transition, but the values are somewhat lower than those for atactic PDMAEMA. Increasing the isotactic content lowers the onset temperature from 33.3 to 31.7 °C. Interestingly, the changes in the hydration volumes (ΔV/V) obtained from the PPC data are almost equal for all the polymers and not noticeably correlated with either the molar mass or the tacticity. The hydration volume is more affected by the degree of protonation and thus the charge of the polymer, as we shall now see by comparing the results at pH 9 with those at pH 8. The effect of pH on ΔV/V was also observed in our previous work.24 Lowering the pH from 9 to 8 results in a higher charging of the polymer, and this is clearly seen in the calorimetric data. Phase transition temperatures of all samples are now higher, and the change in the hydration volume is an order of

Figure 6. Zeta potentials of PDMAEMA in solutions with pH 8 as a function of temperature. The onset temperature region of the phase transitions (determined by microcalorimetry) is marked with a dashed box.

PDMAEMA grafted to silica particle surfaces (8−5 mV)47 but close to those reported for polyplexes of PDMAEMA and DNA. The zeta potential of a PMAEMA−DNA complex depends on the ratio of the two components. When the amount of PDMAEMA in the polyplex is high enough, the potential reaches a constant value around 18−26 mV and is unaffected by further addition of PDMAEMA.48−50 In solutions with pH 8, the zeta potential remains almost constant upon heating though for atactic samples it decreases slightly, as was also observed by Dong et al.47 The polymers with isotactic triads have the highest zeta potentials in the whole group of polymers. In the solutions with pH 9, a clear drop in the zeta potential is observed upon heating for all samples, starting at 2336


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the onset temperature of the phase transition. Some hysteresis is observed in the heating−cooling cycles (cooling curves not shown), but the changes in the zeta potentials are reversible (see Supporting Information, Figure S2). A change in the molar mass has no clear effect on the zeta potential of the polymers. The isotactic triads have an effect which, however, is visible only in the solutions with pH 8.

Figure 8. Schematic illustration, based on the combined results from calorimetric and zeta potential measurements, of PDMAEMA chains at pH 8 and 9 before and after the chain collapse. pH affects the number of counterions and correspondingly the swelling of the chain. The counterions reside inside the collapsed coil at low pH but diffuse out of it at higher pH.

at pH 8 than the other polymers with comparable molar mass (a56 and i52). Sun et al. studied the zeta potentials of (PNIPAM-b-PAA)2− (PVP)2 copolymers at pH 7 and 10 as a function of temperature. They found that at pH 7 the zeta potential drops discontinuously from −1 to −10 mV during the collapse of the copolymer. At pH 10 the potential is lower (−22 mV) and slowly decreases to −27 mV with increasing temperature. At this pH, no clear step was observed upon the contraction of the polymer.46 It may be concluded that the zeta potential of the (PNIPAM-b-PAA)2−(PVP)2 copolymers is affected both by the pH of the solution (the charge of the PAA blocks) and by the phase transition of PNIPAM. Reddy et al. observed that whereas the change of zeta potential of aqueous PNIPAM upon heating was continuous, an addition of a zwitterion (trimethylamine N-oxide, TMAO) turned it more stepwise.52 These findings are similar to what has been observed in the present study. Polymer Interactions at Macroscopic Scale. Microcalorimetry and zeta potential measurements give information about the properties of polymers at molecular level, the level where the charging of the polymer, the counterions, and the water molecules in the solvation layer are the dominating factors governing the solution properties. Rheology, on the other hand, is sensitive to the sizes of and the interactions between the polymer coils and aggregates. It has been shown recently with PNIPAM that the thermal collapse and the thermally induced aggregation change rheological properties in both solutions53,54 and the air−water interface.55 For solutions of atactic PDMAEMA with pH 8 (12.5 mg/ mL), only two polymers with the highest molar masses (a56 and a85) show a change in the viscous (G″) and elastic (G′) moduli during the phase transition (see Figure 9). For the isotactic-rich polymers (i46 and i52), only small changes in G″ are observed. With increasing temperature the moduli slightly decrease up to the phase separation temperature, above which they increase again. No increase in the elastic response G′ was recorded, however, for neither of the isotactic-rich polymers. In solutions with pH 9 PDMAEMA is less charged than at pH 8 and more prone to aggregation during heating. Most of the polymers show an increase in both G′ and G″ (Figures 11 and 12) upon the phase transition. Only the polymer with the lowest molar mass (a23) shows no rheological response to

Figure 7. Zeta potentials of PDMAEMA in solutions with pH 9 as a function of temperature. The onset temperature region of the phase transitions (determined by microcalorimetry) is marked with a dashed box.

The observations so far are the following. In higher pH the zeta potentials drop several tens of millivolts upon the thermal collapse, but the calorimetrically measured change in the hydration volume (ΔV/V, see Table 3) is small. At pH 8, the zeta potential keeps constant or only slightly decreases for the atactic polymers, but the volume change is considerable. The findings may be rationalized by taking into account the weak basicity of PDMAEMA. The pKa for PDMAEMA is 7.4, and thus, the polymer is significantly less charged in solutions with pH 9 than in solutions with pH 8.51 At pH 9, the osmotic pressure due to the counterions inside the polymer coil is lower than it is at pH 8 due to the lower degree of protonation and, correspondingly, a lower number of counterions. The experimental data support the conclusion that although in lower pH the polymer is swollen at room temperature and the volume change upon the thermal collapse is large, the counterions still reside inside the polymer coil at elevated temperatures. In higher pH the situation is reversed. The counterions are fewer, and they seem to be mobile; thus, the counterions diffuse out from the coil interior to the interfacial layer upon the subtle collapse. The two cases are schematically described in Figure 8. In borate buffers the borate ions exist as B(OH)4− and B4O72−, and especially the divalent anion is likely to bind strongly to PDMAEMA at pH 8. The suggested effect of the counterions would also explain why increasing the molar mass increases the phase transition temperature of atactic polymers at pH 8, as seen in Figure 4. The longer the chains, the more charges there are to resist the collapse of the extended polymer coil. This observation does not perfectly match the one by Plamper et al.,23 probably because of different ionic strengths. An increase in the isotactic content of the chains may extend the polymers even more, as is discussed by Přad́ ný et al. in their studies on isotactic and atactic PDMAEMA.26,27 Thus, the isotactic-rich polymer with 35% mesodiads (i46) has a higher phase transition temperature 2337


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Figure 12. Viscous (G″) and elastic (G′) modulus plotted as a function of temperature for 12.5 mg/mL isotactic-rich PDMAEMA solutions with pH 9.

Figure 9. Viscous (G″) and elastic (G′) modulus plotted as a function of temperature for 12.5 mg/mL atactic PDMAEMA solutions with pH 8.

this indicating that the stereoregular polymers associate more strongly than the atactic polymer. In lower pH, however, the high degree of charging of the polymer dominates over the possible effects the isotactic triads could have on G′ and G″. Most water-soluble polymers adsorb readily to the air−water interface, causing the surface tension of the solution to decrease. For PNIPAM the adsorption to the interface is fast and has been reported to occur in minutes.55,56 The surface tension of PNIPAM solution decreases rapidly from ∼73 to ∼43 mN/m when increasing the polymer concentration from 0.001 to 10 mg/mL.57 It has also been shown that the surface tension properties are independent of molar mass58 and the presence of isotactic segments in stereoblock polymers of NIPAM.59 In the present study, similar observations have been made on the surface tension of PDMAEMA solutions at 25 °C at pH 9. For the lowest polymer concentration (0.001 mg/mL) the surface tension is close to that of pure water (72 mN/m), but with increasing the PDMAEMA concentration the surface tension drops to ∼40 mN/m (see Figure 13). Neither the

Figure 10. Viscous (G″) and elastic (G′) modulus plotted as a function of temperature for 12.5 mg/mL isotactic-rich PDMAEMA solutions with pH 8.

Figure 11. Viscous (G″) and elastic (G′) modulus plotted as a function of temperature for 12.5 mg/mL atactic PDMAEMA solutions with pH 9. Figure 13. Surface tensions of aqueous PDMAEMA solutions at 25 °C as functions of concentration.

temperature. G′ and G″ start to increase around the onset temperature of the phase transition, indicating the association of the chains and increasing interactions between the polymer chains and aggregates. The associations are, however, broken when temperature is further raised: both moduli reach maxima and start to decrease. The polymer with the highest molar mass (a85) is the most entangled one, and G′ and G″ retain their values above the phase transition temperature. Importantly, at pH 9 the polymers i46 and i52 show a stronger elastic response than their atactic counterpart (a56),

molar mass nor the incorporation of isotactic triads has any effect on the surface tension of the solutions. The surface tensions of the solutions with 12.5 mg/mL PDMAEMA were slightly higher at pH 8 than at pH 9 (Table 4). Recently, Alvarez et al. have studied PDMAEMA and PDMAEMA grafted nanoparticles and their adsorption to the air−water interface in aqueous solution with pH 7.5 and 0.01 M NaCl. They report that the surface tension of the PDMAEMA 2338


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stereoblock polymers of PNIPAM.59 In the present study, all rheological measurements have been conducted with similar heating programs. In ISR, the differences between the isotactic-rich and atactic PDMAEMA are small, since the interface did not have time to stabilize, but they are clearly observable. In the surface tension measurements the interface was equilibrated, but no differences were observed between the aqueous isotactic-rich and atactic PDMAEMAs. The observation is important because it shows the sensitivity of the dynamic ISR measurement and well reveals the differences in the behaviors of the polymers. The observations are in accordance with our previous ones on aqueous stereoblock PNIPAMs, that the stereostructure of the polymers did not affect the surface tension of the aqueous solutions. However, the stereostructure had a noticeable effect on the interfacial surface rheology.59

Table 4. Surface Tensions of 12.5 mg/mL Aqueous Solutions of PDMAEMA at pH 8 and 9 at 25 °C denotation

pH

γ (mN/m)

pH

γ (mN/m)

a23 a56 a85 i52 i46

8.2 8.2 8.2 8.2 8.1

43.1 38.8 39.5 41.5 41.1

9.0 9.1 9.1 9.1 9.1

41.9 37.4 38.2 40.6 38.5

solutions and PDMAEMA grafted nanoparticles dispersions decrease to 45−50 mN/m.60 Our findings are well in line with these values. Interfacial surface rheology (ISR) measurements were conducted for PDMAEMA solutions with pH 9. The polymer with the highest content of isotactic triads was compared with two atactic polymers. Dynamic measurements in the temperature range 20−40 °C reveal that the polymer with isotactic parts is more associated than either of the atactic ones, as could be expected from the above-described results. This is seen as a moderate increase in G″ starting prior to the onset temperature of the polymer in question. The atactic polymers show only a slight increase in G″ after the phase transition, which is at the limits of detection (Figure 14). The elastic response, G′, was

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CONCLUSIONS

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ASSOCIATED CONTENT

The isotacticity of PDMAEMA can be increased by conducting the polymerization in the presence of yttrium(III) trifluoromethanesulfonate. The isotactic diad content was increased from 17% to 35%. In comparison, the polymers obtained without the presence of Y(OTf)3 had no isotactic triads at all, whereas the isotactic-rich polymer had 16% isotactic triads. The isotactic-rich polymers were studied and compared with atactic ones, both at molecular scale by microcalorimetry and zeta potential measurements and at the scale of macroscopic aggregates by rheology and interfacial surface rheology. The temperature-induced phase transition is affected by the molar mass and tacticity of PDMAEMA, but these changes are also affected by the pH of the aqueous solutions. At pH 9 the phase transition shifts to lower temperatures with increasing molar mass and isotacticity, but in solutions with pH 8 both the effects of molar mass and increased isotacticity are opposite. The zeta potential of the polymers in solutions with pH 9 decreases considerably during the phase transition, indicating that when the chain contracts the counterions are released from the coil interior to the surface of the collapsed coil. At pH 8, the polymers are considerably more charged and the polymer coils are more extended. During the phase transition the counterions are not released from the polymer coil, and thus, the zeta potential hardly changes with temperature. With increasing the molar mass the total number of charges in a polymer coil increases. The charges resist the contraction of the polymer, thus shifting the phase separation to higher temperatures. The isotactic segments may be expected to extend the coils even more and also increase the phase transition temperature. Rheological measurements showed that pH has a greater influence on the aggregation behavior of the polymers than the molar mass or the isotactic segments. For solutions with pH 8 only the polymers with highest molar masses showed an increase in G′ and G″ due to the phase transition. However, at pH 9 both G′ and G″ increased upon the phase transition and increasingly so with increasing the molar mass and isotacticity. The isotactic segments also enhanced the accumulation of PDMAEMA to the air−water interface with increasing temperature.

Figure 14. Interfacial surface rheology data on isotactic-rich PDMAEMA compared with two atactic PDMAEMAs.

not observed at all. This indicates that the PDMAEMA aggregates do not concentrate at the air−water interface in the same manner as PNIPAM does. Some accumulation of polymers at the interface is observed with increasing temperature. The isotactic segments enhance the accumulation and also the interactions between the polymer chains. The amount of adsorbed polymer on the interface is far too low to form any network structures, and therefore no elastic response is observed at the air−water interface of the PDMAEMA solutions in question. Apart from the nature of PDMAEMA, we believe one of the reasons for the lack of polymer accumulation at the air−water interface is the heating method used during the measurements. It has been shown for PNIPAM that the heating procedure used in ISR measurements influence greatly the rheological properties of the interface. Stepwise heating and equilibration at each measuring point gives the interface time to accumulate polymer chains, thus increasing both the G′ and G″.55 In the present case the heating was continuous, and thus the interface did not have time to accumulate polymer chains after the phase transition. Continuous heating, without any equilibrium periods, has been used earlier also in studies on aqueous

S Supporting Information *

Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org. 2339


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AUTHOR INFORMATION

Corresponding Author

*E-mail heikki.tenhu@helsinki.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by a 7th EU framework project MUST (Multi-Level Protection of Materials for Vehicles by “Smart” Nanocontainers) project. Ville Lovikka and Zhe Zhang are acknowledged for their assistance in polymer synthesis and characterization.

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Thermoresponsiveness of PDMAEMA. Electrostatic and

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