Role of Methyl in the Phase Transition of Poly(N

Jun 26, 2008 - change (ΔH) and entropy change (ΔS) than PiPA chains during the phase transition, indicating that PiPMA chains have smaller conformat...
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J. Phys. Chem. B 2008, 112, 8447–8451

8447

ARTICLES Role of Methyl in the Phase Transition of Poly(N-isopropylmethacrylamide) Yecang Tang, Yanwei Ding, and Guangzhao Zhang* Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei, China ReceiVed: December 9, 2007; ReVised Manuscript ReceiVed: March 15, 2008

Poly(N-isopropylmethacrylamide) (PiPMA) has one more methyl group at each monomeric unit than poly(Nisopropylacrylamide) (PiPA). By use of laser light scattering (LLS) and ultrasensitive differential scanning calorimetry (US-DSC) we have investigated the association and dissociation of PiPMA chains in water. LLS studies reveal that PiPMA chains form larger aggregates at a temperature above its lower critical solution temperature (LCST) as the chain molar mass (Mw) decreases. In comparison with PiPA aggregates, PiPMA aggregates show a larger ratio of average radius of gyration to average hydrodynamic radius (〈Rg〉/〈Rh〉), indicating that PiPMA aggregates are looser. US-DSC studies show PiPMA chains have smaller enthalpy change (∆H) and entropy change (∆S) than PiPA chains during the phase transition, indicating that PiPMA chains have smaller conformational change. Our experiments demonstrate that the additional methyl groups in PiPMA chains restrain the intrachain collapse and interchain association, leading the phase transition to occur at a higher temperature. Introduction Thermally responsive polymers in aqueous solution usually undergo phase transition with a lower critical solution temperature (LCST). Such polymers have attracted considerable interest in the past decades because of their potential applications in catalysts, drug delivery, chemical separation, and immobilized enzyme reactors.1,2 It is generally accepted that the LCST is determined by the balance between hydrogen bonding and hydrophobic interactions in the system. The introduction of hydrophilic or hydrophobic moieties into the polymer chain can either increase or decrease the LCST, respectively.2,3 Besides the monomer structure, the distribution of the hydrophilic or hydrophobic moieties also has a heavy effect on the phase transition. Poly(N-isopropylacrylamide) (PiPA) is a well-known thermally responsive polymer with an LCST at ∼32 °C in water.4–9 In comparison with PiPA, poly(N-isopropylmethacrylamide) (PiPMA) has an additional hydrophobic methyl group at each monomeric unit. In terms of the structure, we may take it for granted that PiPMA should have an LCST below 32 °C in water. In fact, its LCST (∼45.0 °C) is higher than that of PiPA.10–19 One explanation is that the hydrophobic moieties cannot associate in the most favorable manner due to the steric hindrance of methyl groups.12 Recent ultra-small-angle X-ray scattering studies reveal that the hydrogen-bonding constraint related to the rotation of methyl groups is probably responsible for the higher LCST.16 So far, the physics behind the phase transition is still an open question. In the present work, we have prepared PiPMA samples with chain molar mass ranging from 2.6 × 104 to 1.6 × 106 g/mol. By use of laser light scattering (LLS) and ultrasensitive * To whom correspondence should be addressed. E-mail: gzzhang@ ustc.edu.cn.

differential scanning calorimetry (US-DSC), we have investigated the effects of chain molar mass and scanning rate on the association and dissociation of PiPMA chains in water. Our aim is to understand the role of the methyl group in the phase transition. Experimental Section Sample Preparation. Methacryloyl chloride, isopropylamine, and triethylamine were distilled prior to use. Methylene chloride was dried over calcium hydride and distilled. 2,2′-Azobis(isobutyronitrile) (AIBN) was recrystallized three times from methanol. Other regents were used as received. The details about the preparation of N-isopropylmethacrylamide can be found elsewhere.20 The crude product was recrystallized from benzene/ hexane to yield a white crystal. 1H NMR (300 MHz, D2O): δ 5.31-5.52 (s, 2H, dCH2), δ 3.91 (m, 1H, -CH), δ 1.83 (s, 3H, -CH3), δ 1.10 (d, 6H, -CH3). PiPMA was prepared via radical polymerization in benzene at 50 °C using AIBN as initiator. The sample was fractionated by successive dissolution/precipitation in a mixture of acetone and n-hexane at 26.0 °C. The weight-average molar mass (Mw) was measured by static light scattering (SLS) in water at 25.0 °C. The polydispersity (Mw/Mn) was estimated from the relative line width distribution in dynamic laser light scattering (DLS).21 For fractions P1600K, P1000K, P200K, P82K, and P26K, Mw values are 1.6 × 106, 1.0 × 106, 2.0 × 105, 8.2 × 104, and 2.6 × 104 g/mol; Mw/Mn values are 1.3, 1.5, 1.5, 1.5, and 1.4, respectively. LLS Measurements. All LLS measurements were conducted on an ALV/DLS/SLS-5022F spectrometer with a multi-τ digital time correlation (ALV5000) and a cylindrical 22 mW UNIPHASE He-Ne laser (λ0 ) 632 nm) as the light source. The weight-average molar mass (Mw), the root-mean-square radius of gyration 〈Rg2〉z1/2 (or written as 〈Rg〉), and the second virial

10.1021/jp711581h CCC: $40.75  2008 American Chemical Society Published on Web 06/26/2008

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Figure 2. Temperature dependence of the apparent molar mass (Mw,a) of PiPMA chains in one heating-and-cooling cycle, where Mw ) 1.6 × 106 g/mol and C ) 2 × 10-5 g/mL.

Figure 1. Temperature dependence of the average hydrodynamic radius (〈Rh〉) and average radius of gyration (〈Rg〉) of PiPMA chains in one heating-and-cooling cycle, where Mw ) 1.6 × 106 g/mol and C ) 2 × 10-5 g/mL.

coefficient A2 were obtained from the angular dependence of the absolute excess time-average scattering intensity or Rayleigh ratio Rvv(q) in SLS.22 In DLS,21 the Laplace inversion of each measured intensity-intensity time correlation function G(2)(q, t) in the self-beating mode can lead to a line width distribution G(Γ). For a diffusive relaxation, Γ is related to the translational diffusion coefficient D by (Γ/q2)Cf0,qf0 f D so that G(Γ) can be converted into transitional diffusion coefficient distribution G(D) and further into hydrodynamic radius distribution f(Rh) via the Stokes-Einstein equation, Rh ) kBT/(6πηD), where kB, T, and η are the Boltzmann constant, the absolute temperature, and the solvent viscosity, respectively. All DLS measurements were performed at a scattering angle (θ) of 20°. Each PiPMA solution with a concentration (C) of 2.0 × 10-5 g/mL was clarified with a 0.45 µm Millipore Millex-LCR filter. The refractive index increment (dn/dC) was measured using a precise differential refractometer.23 US-DSC Measurements. PiPMA solutions were measured on a VP-DSC microcalorimeter from MicroCal with deionized water as the reference. Before heating, each PiPMA solution (C ) 1.0 × 10-3 g/mL) was degassed at 25.0 °C for 0.5 h and equilibrated at 10.00 °C for 2 h. In the cooling process, the solution was equilibrated at 80.00 °C for 2 h to eliminate the effect of thermal history. The phase transition temperature (Tp) was taken as the peak of specific heat capacity (Cp) during the transition. Note that there is some difference between Tp and LCST. The former was measured with a certain scanning rate, whereas the latter was determined in equilibrium. The enthalpy of the transition (∆H) was calculated from the area under each peak. Results and Discussion Figure 1 shows the temperature dependence of 〈Rg〉 and 〈Rh〉 of PiPMA chains in one heating-and-cooling cycle. In the heating process, both 〈Rg〉 and 〈Rh〉 decrease with temperature at a temperature below 44.5 °C. Meanwhile, the apparent molar mass (Mw,a) measured at C ) 2 × 10-5 g/mL does not change (Figure 2), indicating that only intrachain contraction occurs. In the range of 44.5-45.2 °C, the sharp increases in 〈Rg〉, 〈Rh〉, and Mw,a indicate the interchain association. Namely, the phase transition occurs. At a temperature above 45.2 °C, 〈Rh〉 and 〈Rg〉 slightly decrease but Mw,a levels off. The facts indicate that the interchain association stops, but the chains inside each aggregate

Figure 3. Temperature dependence of the ratio of the average radius of gyration to average hydrodynamic radius (〈Rg〉/〈Rh〉) of PiPMA chains in one heating-and-cooling cycle, where Mw ) 1.6 × 106 g/mol and C ) 2 × 10-5 g/mL.

still undergo some intrachain contraction. Finally, both interchain association and intrachain contraction stop, reflecting in the invariant 〈Rg〉, 〈Rh〉, and Mw,a. Note that no precipitation occurs even when the solution was heated up to 50.0 °C for 20 h, indicating that the aggregates are mesoglobules.24 In the cooling process, as the temperature decreases to the LCST, 〈Rg〉, 〈Rh〉, and Mw,a sharply decrease, indicating the dissociation of the aggregates. Figure 2 also shows the phase transition temperature in the cooling process is 0.4 °C lower than that in the heating process (Figure 1). This is indicative of a hysteresis. A similar phenomenon has been observed for PiPA, which has been attributed to the additional hydrogen bonds formed in the collapse state of chains at temperatures above the LCST.7,25 Figure 3 shows the temperature dependence of 〈Rg〉/〈Rh〉. The conformation of polymer chains or structure of particles can be described by 〈Rg〉/〈Rh〉. For a random coil, hyperbranched cluster or micelle, and uniform sphere, 〈Rg〉/〈Rh〉 are 1.5-1.8, 1.0-1.2, and ∼0.774, respectively.26 Here, 〈Rg〉/〈Rh〉 is ∼1.6 at a temperature below LCST in either the heating or cooling process, indicating that PiPMA chains are random coils. In the range of 44.5-45.2 °C, 〈Rg〉/〈Rh〉 decreases from ∼1.6 to ∼0.9, further indicating that the random coils aggregate into mesoglobules. At the temperature above LCST, PiPMA aggregates have 〈Rg〉/〈Rh〉 (∼0.9) larger than PiPA aggregates (∼0.8).24,27 This indicates that PiPMA chains form less compact mesoglobules. Namely, the presence of methyl groups leads the chains to pack more loosely. Figure 4 shows the PiPMA chain molar mass (Mw) dependence of 〈Rg〉 and 〈Rh〉 of the aggregates at 50.0 °C. As Mw increases, 〈Rg〉 and 〈Rh〉 decrease. The phenomenon has also been observed in the case of PiPA.27 As shown in the inset, for the same concentration in terms of grams per milliliter, the population of long chains is less than that of shorter chains in

Role of Methyl in the Phase Transition of PiPMA

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Figure 4. Effect of chain molar mass (Mw) on average hydrodynamic radius (〈Rh〉) and average radius of gyration (〈Rg〉) of PiPMA aggregates at 50.0 °C, where C ) 2 × 10-5 g/mL. The inset shows the effect of Mw on the average aggregation number (Na), where Na ) Mw,a/Mw with Mw,a being the apparent molar mass of the aggregates.

Figure 6. Temperature dependence of specific heat capacity (Cp) of PiPMA chains with different chain molar mass (Mw), where both the heating and cooling rates are 1.00 °C/min.

Figure 5. Effect of chain molar mass (Mw) on the ratio of 〈Rg〉/〈Rh〉) of PiPMA aggregates at 50.0 °C, where C ) 2 × 10-5 g/mL.

the same volume. Because longer chains have more chance to undergo intrachain contraction instead of interchain association, they form smaller aggregates. That is why 〈Rg〉 and 〈Rh〉 decrease with Mw. Figure 5 shows 〈Rg〉/〈Rh〉 (∼0.9) is independent of the chain molar mass, indicating that the aggregates formed by either long chains or shorter chains have the same density distribution. Cheng et al.24 reported that PiPA aggregates are swollen but not dissociated when temperature is close to the LCST in the cooling process since the additional hydrogen bonds formed in the collapse state cannot be dissolved at the temperature. In the present study, PiPMA aggregates are dissociated once the temperature approaches the LCST. This further indicates the methyl groups restrain the chains from forming additional hydrogen bonds. Figure 6 shows the temperature dependence of specific heat capacity (Cp) of PiPMA with different chain molar masses (Mw) in one heating-and-cooling cycle, where both the heating and cooling rates are 1.00 °C/min. The narrow and relatively symmetric peaks of specific heat capacity indicate that PiPMA chains undergo a sharp phase transition. In the heating process, as Mw increases, Tp shifts to lower temperature. It is known that the LCST is proportional to the critical value of the Flory-Huggins interaction parameter (χc) which decreases with the ratio (r) of molar volume of the polymer to that of the solvent.28,29 Thus, it is understandable that the phase transition temperature decreases with Mw because r increases with the chain molar mass. The microcalorimetric measurements also show that the hysteresis becomes more obvious as Mw decreases. As discussed above, shorter chains form larger aggregates with more additional hydrogen bonds. When the solution is cooled down, the aggregates are more difficult to dissociate, leading to a larger hysteresis. Note that the specific heat capacity peak becomes

Figure 7. Effects of scanning rate on specific heat capacity (Cp) of PiPMA, where Mw ) 1.6 × 106 g/mol. The scanning rate is 0.08 (1), 0.17 (2), 0.25 (3), 0.50 (4), 0.75 (5), and 1.00 °C/min (6).

broader and more asymmetrical as Mw decreases in the heating process. This is because the intrachain contraction and interchain association in the formation of a large aggregate do not occur simultaneously. For the same reason, in the cooling process, shorter chains do not dissociate at the same time and the transition becomes broader and more asymmetrical. Figure 7 shows the effect of scanning rate on the specific heat capacity (Cp) of PiPMA with Mw ) 1.6 × 106 g/mol. As the heating rate increases from 0.08 to 1.00 °C/min, Tp increases from 45.7 to 46.2 °C. In addition, the US-DSC curves become more asymmetrical. As discussed before,24 the chains undergo intrachain contraction and interchain association in the heating process. A fast heating leads the intrachain contraction and interchain association to occur at different times, whereas a slow heating allows the polymer chains to collapse and associate simultaneously. That is why the curve is more asymmetrical at a faster heating rate. It has also been reported PiPA exhibits a bimodal exothermic peak when the cooling rate is lower than ∼0.30 °C/min.25 In the present study, PiPMA does not demonstrate such a bimodal transition even when the cooling rate is as low as 0.08 °C/min. The fact also indicates PiPMA

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Figure 8. Scanning rate dependence of phase transition temperature (Tp) of PiPMA, where Mw ) 1.6 × 106 g/mol.

Figure 10. Scanning rate dependence of enthalpy change (∆H) with different chain molar masses (Mw).

Figure 9. Effect of heating history on the enthalpy change (∆H) of PiPMA chains, where Th is the highest temperature in one heatingand-cooling cycle. The heating and cooling rates are 1.00 °C/min, and Mw ) 1.6 × 106 g/mol.

chains form looser aggregates in the heating process, which can be readily swollen and dissociated in the cooling process. Figure 8 shows that Tp linearly increases with the heating rate. On the other hand, Tp linearly decreases with the cooling rate. Besides, as the scanning rate decreases, the hysteresis becomes smaller. The extrapolation of Tp to zero scanning rate leads to Tp,0 ) 45.7 and 44.6 °C in heating and cooling processes, respectively. The difference suggests that the additional hydrogen bonds cannot be completely removed within the cooling time. Note that the heat history has a great effect on the dissociation of the aggregates. In the above microcalorimetric experiments, PiPMA solution was heated to a certain temperature (Th). Then, the solution was cooled from Th to 10.0 °C at a certain rate. Namely, Th is the highest temperature in one heating-and-cooling cycle. Figure 9 shows the Th effect on the enthalpy change (∆H) of PiPMA chains, where Mw ) 1.6 × 106 g/mol. Clearly, ∆H does not have Th dependence in the heating process. However, ∆H linearly increases with Th in the cooling process. Moreover, when Th < 68.0 °C, ∆H in the cooling process is smaller than that in heating process. This is probably because the additional hydrogen bonds are not disrupted completely in the time we investigated. When Th > 68.0 °C, ∆H in the cooling process is larger than that in the heating process. As we know, ∆H in the heating process relates to the dehydration due to the intrachain collapse and interchain association. At Th > 68.0 °C, the chains should be overdehydrated and overcollapsed so that some chain knotting occurs. A higher Th leads to more dehydration and knotting. Thus, a cost of conformational entropy is required in the cooling process so that ∆H in the cooling process is larger than that in the heating process. Figure 10 shows that ∆H linearly decreases as the scanning rate increases in either the heating or cooling process. It has been reported that ∆H of PiPA linearly increases with the heating rate but is nearly independent of the cooling rate.25 For

Figure 11. Chain molar mass (Mw) dependence of enthalpy change (∆H0) at zero scanning rate.

PiPMA chains, the reduction in ∆H indicates the decrease of dehydration in the heating process. This is probably because the methyl groups restrain the intrachain collapse and interchain association. Since the formation of additional hydrogen bonds is exothermic, the presence of methyl groups, which decreases the number of additional hydrogen bonds, would increase ∆H. Consequently, the absolute value of ∆H decreases with the heating rate. In the cooling process, the swelling and dissociation related to hydration are exothermic, while the disruption of the additional hydrogen bonds is endothermic. As the cooling rate increases, the aggregates can be more quickly cooled so that more additional hydrogen bonds are disrupted. This explains why ∆H decreases with the increasing cooling rate. Figure 11 shows the chain molar mass (Mw) dependence of the enthalpy change (∆H0) at zero scanning rate. In either the heating or cooling process, when Mw < 2.0 × 105 g/mol, ∆H0 increases with Mw. As discussed before,30 the enthalpy change for intrachain contraction is larger than that for interchain association because the former involves a larger change in the chain conformation and more stress inside. As Mw increases, the weighting of intrachain contraction increases, but the weighting of interchain association decreases. This is why ∆H0 increases with the chain molar mass. At Mw > 2.0 × 105 g/mol, ∆H0 is no longer dependent on the chain molar mass because the weighting of intrachain contraction or interchain association does not vary when Mw is above a critical value. For PiPMA chains with Mw ∼ 1.6 × 106 g/mol, when the heating rate ranges from 0.08 to 1.00 °C/min, the ∆H is

Role of Methyl in the Phase Transition of PiPMA 53-48 J/g. In contrast, ∆H of PiPA chains with a close molar mass is 60-71 J/g.25 The smaller ∆H of PiPMA chains further indicates that they form looser aggregates. By use of the equation ∆G ) ∆H - T∆S, where ∆G, ∆S, and T are the Gibbs free energy change, entropy change, and phase transition temperature, respectively, we have the ∆S for the phase transition which directly relates to the conformational change of the polymer chains. For PiPMA and PiPA, ∆S values are ∼0.15-0.17 and ∼0.20-0.23 J/g/K, respectively. Namely, PiPMA chains have smaller conformational changes. Clearly, the additional methyl groups in the PiPMA chain restrain its conformational change. It is known that the glass transition temperature (Tg) of poly(methyl methacrylate) is higher than that of poly(methyl acrylate) and the Tg of poly(R-methyl styrene) is higher than that of polystyrene. The reason is that the presence of methyl increases the kinetic rigidity.31,32 The increases in Tg and LCST should follow the same mechanism. Namely, the presence of methyl restrains the chain collapse and increases the persistence length so that the polymer chains are difficult to undergo intrachain collapse (swelling) and interchain association (dissociation) at a lower temperature. Consequently, the phase transition or glass transition occur at a higher temperature. Conclusion The present studies lead to the following conclusions. In comparison with PiPA chains, PiPMA chains form looser aggregates during the phase transition. The additional methyl groups in PiPMA chains restrain the chain conformational change so that PiPMA chains have a higher LCST than PiPA. PiPMA chains can be overdehydrated at temperatures higher than the LCST. Acknowledgment. The financial support of the National Natural Science Foundation of China (20725414 and 20611120039) and the Ministry of Science and Technology of China (2007CB936401) is acknowledged. References and Notes (1) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163–249. (2) Zhang, G. Z.; Wu, C. AdV. Polym. Sci. 2006, 195, 101–176. (3) Okhapkin, I. M.; Makhaeva, E. E.; Khokhlov, A. R. AdV. Polym. Sci. 2006, 195, 177–210.

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