Shrinking Kinetics of Polymer Gels with Alternating Hydrophilic

Article pubs.acs.org/Macromolecules

Shrinking Kinetics of Polymer Gels with Alternating Hydrophilic/ Thermoresponsive Prepolymer Units Hiroyuki Kamata, Ung-il Chung, and Takamasa Sakai* Department of Bioengineering, School of Engineering, The University of Tokyo, 7-3 1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ABSTRACT: The volume phase transition kinetics of polymer gels with alternating hydrophilic/thermoresponsive prepolymer units is investigated. We succeeded in fabricating polymer gels with tuning the ratio of thermoresponsive (poly(ethyl glycidyl ether), PEGE) to hydrophilic (poly(ethylene glycol), PEG) prepolymers (hereafter called TetraPEG-PEGE gels). Even upon the temperature jump across the phase transition temperature, no skin layer or heterogeneous deformation was observed. Tetra-PEG-PEGE gels had the advanced shrinking kinetics, which obeyed the theoretical prediction for homogeneous shrinking. The introduction of alternating hydrophilic/thermoresponsive prepolymer units will be a versatile method to improve the volume phase transition kinetics of conventional gels.

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INTRODUCTION Since the discovery of “volume phase transition” of polymer gels by Tanaka et al.,1 the phase transition behavior has been extensively studied due to the fact that their ability to swell and shrink in response to external conditions, such as solvent composition, temperature, and pH, has the potential impact in terms of designing new intelligent materials.2−5 Although the kinetics of swelling and shrinking has been investigated from both theoretical and experimental point of view,6,7 the validity of the theoretical model for shrinking has rarely been confirmed upon a temperature jump across the lower critical solution temperature (LCST, Tc). This is mainly because conventional gels form a dense skin layer on the surface at an initial stage, and the transition proceeds with heterogeneous deformation e.g. bubble formation,8 which extremely decelerates further shrinking. Much attention has been paid to this issue, and recent experimental studies showed that designing special gels, such as those with heterogeneous macroporous structure,9,10 surfactant moieties,11,12 dangling chains,13 or mobile cross-links,14,15 leads to rapid shrinking. In particular, Hirotsu et al. developed a new class of PNIPAAm gel by incorporating hydrophilic polymer segments that work as a pathway for water molecules during the shrinking process and demonstrated that the gel shrunk rapidly compared to other conventional PNIPAAm gels.16 The polymer network structure proposed by Hirtosu et al. was an amphiphilic conetwork (APCN) type structure, which has a segmented structure comprising two components with different hydrophobicity.17−20 We have also reported a novel APCN type hydrogel, Tetra-PEG-PEGE gel, composed of a controlled APCN structure with equimolar amounts of tetra-armed hydrophilic and thermoresponsive polymer units, i.e., tetrapoly(ethylene glycol) (Tetra-PEG) and tetra-poly(ethyl glycidyl ether) (Tetra-PEGE).21 Although the APCN type © XXXX American Chemical Society

hydrogels exhibited a rapid and homogeneous shrinking and showed no obvious skin layer or bubble formation, the structural conditions that inhibited heterogeneous deformations were not verified. In this paper, we examined the shrinking behavior of TetraPEG-PEGE gels with tuned APCN structures. We tuned the ratio of hydrophilic to thermoresponsive segments (r) with maintaining the equilibrium swelling degree below Tc. This design, r-tuned Tetra-PEG-PEGE gels, was ideal for examining the true effect of APCN structure on the gel shrinking behavior. First, we investigated the swelling isotherms of r-tuned TetraPEG-PEGE gels and measured the r-dependence of Tc. Then, we investigate the shrinking kinetics upon a sudden temperature change across Tc.

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EXPERIMENTAL SECTION

Synthesis of Tetrafunctional Prepolymers. Tetraamine-terminated PEG (Tetra-PEG227-NH2) and tetrasuccinimidyl-terminated PEG (Tetra-PEG227-OSu) were prepared from tetrahydroxyl-terminated PEG (Tetra-PEG227-OH). The conversion of each end group was ∼100%. The detailed preparation methods were reported elsewhere.22 Tetrasuccinimidyl-terminated PEGE (Tetra-PEGE215OSu) was synthesized as reported previously with the conversion ratio of ∼100%,21 and tetraamine-terminated PEGE (Tetra-PEGE215NH2) was prepared by the following steps. In a flask, Tetra-PEGE215OSu (1 g, 0.046 mmol) was dissolved in CH2Cl2 (40 mL) while ethylenediamine (1.22 mL, 18.2 mmol, 396 equiv) and CH2Cl2 (10 mL) were placed in another flask. With stirring, the polymer solution was dropwisely injected via syringe into the flask in which the ethylenediamine solution was prepared. The mixed solution was allowed to stir for 3 h. After the reaction, the solution was washed with Received: April 2, 2013 Revised: May 7, 2013

A

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Figure 1. Schematics of Tetra-PEG-PEGE gels with controlled APCN structures composed of hydrophilic PEG and thermoresponsive PEGE prepolymer units. water and brine before being dried over MgSO4. Finally, dichloromethane was removed in vacuo, and the viscous yellow transparent polymer Tetra-PEGE215-NH2 was recovered after lyophilization from benzene. 1H NMR (CDCl3): δ 4.14 (s, 6.5H*, −O−CH2−CO− NH−), δ 3.3−3.75(m, 1506H, −C−CH2−O−, −CO−NH−CH2− and EGE), δ2.82 (t, 7H*, −CH2−CH2−NH2), δ1.17 (t, 642H, −O− CH2CH3). *The conversion ratio from succinimidyl group to amine group was ∼0.88. Fabrication of Tetra-PEG-PEGE Gels. Tetra-PEG-PEGE gels with different PEGE ratios (r) were prepared by means of mixing of tetrafunctional prepolymer solutions (Figure 1). The polymerization degree of Tetra-PEG and Tetra-PEGE was ∼220. Briefly, for PEG-rich hydrogels (r = 0−0.5), tetrasuccinimidyl-terminated prepolymers (i.e., Tetra-PEG227-OSu and Tetra-PEGE215-OSu) were mixed in cyclohexanone to obtain the precursor solutions with the ratios of TetraPEG to Tetra-PEGE (0:10, 3:7, 5:5, 7:3, and 10:0), while TetraPEG227-NH2 was separately prepared as a cyclohexanone solution. Likewise, for PEGE-rich hydrogels (r = 0.65−1), tetraamineterminated prepolymers (i.e., Tetra-PEG227-NH2 and Tetra-PEGE215NH2) were mixed to prepare cyclohexanone solutions with the ratios of Tetra-PEG to Tetra-PEGE (7:3, 5:5, 3:7, and 0:10), while TetraPEGE215-OSu was separately prepared as a cyclohexanone solution. The same volume of the tetrasuccinimidyl and tetraamine precursor solutions were then mixed and poured into glass capillaries with inner diameter 640 μm. The solutions were incubated at least for 24 h for

the completion of the gelation reaction. The polymer concentration was fixed at 10 mM, and the ratio of amine and succinimidyl groups was set to 1 throughout the gelation process. The cylindrical gels obtained were immersed in the mixture solvent of cyclohexanone and methanol with increasing the methanol ratio from 0.25, 0.5, 0.75 to 1.0. Subsequently, the gels swollen in methanol were immersed in the mixture solvent of methanol and H2O with increasing the H2O ratio from 0.25, 0.5, 0.75 to 1.0. Finally, we obtained Tetra-PEG-PEGE hydrogels with the different PEGE ratios (r = 0, 0.15, 0.25, 0.35, 0.5, 0.65, 0.75, 0.85, and 1). The final reaction conversion was ∼75% regardless of r, which was determined by IR measurement using the reported procedure.23 Swelling Degree Measurement. Tetra-PEG-PEGE hydrogels were first immersed in distilled water. The temperature was then increased at the rate of 1 °C/20 min. At each temperature, we recorded the change in diameter after we confirmed that the hydrogels were in their equilibrium. The equilibrium volume swelling ratio (V/ V0) was calculated by the diameter change (V/V0 = (d/d0)3), where d is the diameter at each temperature, and d0 is the gel diameter at the preparation stage. Temperature Jump Experiment. Tetra-PEG-PEGE hydrogels were incubated in a thermostated chamber filled with distilled water at 3 °C until the gels reached their equilibrium state. The temperature jump experiments were carried out by putting the gel sample into another chamber, of which the temperature was set to 40 °C. The time B

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required for the temperature jump was about 1 s. Images of gels during the temperature jump experiment were captured by a microscope in order to record the diameter change.

the similar swelling ratios imply that PEG and PEGE have the similar Flory’s interaction parameters (χ).24 The swelling ratio changed dramatically around specific temperatures (Tc), showing the volume phase transition behavior. The volume change at the highest r (=0.85) was ∼10 times. Figure 2b shows the ratio of the volume of water (Vw) to V at each temperature. Vw/V in the collapsed state (≈0.3) is similar to that of conventional PNIPAAm gels.25 Thus, we can conclude that the structure of the polymer network changed drastically across Tc. The r-dependence of Tc was observed in this system; Tc decreased linearly with an increase in r (Figure 3a). This

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RESULTS AND DISCUSSION LCST of Tetra-PEG-PEGE Gels. In order to tune the fraction of the thermoresponsive prepolymer units (r), we fabricate a series of polymer gels from different compositions of Tetra-PEG-OSu, Tetra-PEG-NH2, Tetra-PEGE-OSu, and Tetra-PEGE-NH2 prepolymers with maintaining the same molar concentrations of amine and succinimidyl end groups. Since we used AB-type cross-link coupling, which prohibits a coupling between the same type of prepolymers (or the same end groups), the prepolymer units were homogeneously mixed in the resultant polymer network. We tuned r from 0 to 1, corresponding to the change from the single-component TetraPEG gel to the single-component Tetra-PEGE gel through Tetra-PEG-PEGE gels. First, we measured the temperature (T) dependence of the swelling ratio (V/V0), where V0 is the volume of the as-prepared specimens and V is the volume of the specimens in the equilibrium-swollen state at each temperature (Figure 2a). In the swollen state at around 3 °C, the specimens with r ≤ 0.75 had similar swelling ratios. Here, it should be noted that the final reaction conversion of the hydrogels was ∼75% regardless of r, which suggests that the cross-link densities of gels were also constant. Taken together,

Figure 3. (a) Volume phase transition temperature (Tc) of TetraPEG-PEGE gels as a function of PEGE segment ratio (r). (b) Volume phase transition temperature (Tc) of both Tetra-PEGE aqueous solutions and Tetra-PEG-PEGE gels as a function of PEGE concentration. The PEGE concentration of gel samples (=CPEGE) was calculated as CPEGE = WPEGE/Vgel, where WPEGE is the weight of PEGE units weighed at the preparation stage and Vgel is the volume of hydrogels at 3 °C.

behavior is related to the phase transition characteristic of PEGE molecules. The decrease in LCST with an increase in PEGE concentration was also observed in our previous measurement for PEGE aqueous solution.21 Figure 3b shows the PEGE concentration dependence of LCST for PEGE solution and Tetra-PEG-PEGE gels. The concentration of PEGE in Tetra-PEG-PEGE gel is calculated from the concentration of PEGE in the equilibrium-swollen state at 3 °C. In PEGE solutions, Tc shifted from 14 to 6 °C with an increase in PEGE concentration from 0.01% to 2%, and the solubility limit of PEGE was around 2%. On the other hand, the PEGE concentrations of Tetra-PEG-PEGE gels were much higher than the solubility limit in the aqueous solution, and the value was up to 5.4% (r = 0.85). This increment in the

Figure 2. (a) Equilibrium swelling ratio (V/V0) of cylindrical TetraPEG-PEGE gels as a function of temperature with different PEGE ratio (r) shown as different symbols. V0 is the volume of the asprepared specimens, and V is the volume of the specimens in the equilibrium-swollen state at each temperature. (b) Water content of Tetra-PEG-PEGE gels (Vw/V) in the equilibrium state, where Vw is the water volume at each temperature. C

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Figure 4. (a) Diameter change of Tetra-PEG-PEGE gels (d) after a sudden temperature jump from 3 to 40 °C. The symbols represent PEGE ratios (r). (b−e) Shrinking behavior of Tetra-PEG-PEGE gels with different PEGE ratios upon a sudden temperature jump from 3 to 40 °C.

Figure 5. (a−d) Semilog plots of dn against elapsed time. The relaxation time was determined from the slope (−1/τ) of the dashed line. The Pearson’s correlation coefficients of the fits were (a) −0.970, (b) −0.994, and (c) −0.973.

specimens with the diameter of ∼1 mm is completed within several seconds. Upon a temperature jump, Tetra-PEG-PEGE gels (r = 0.15−0.75) shrank homogeneously without forming any surface skin layer or apparent phase separation and reached their equilibrium state within 10 min (Figure 4). Because the time for the volume change is much longer than that for the thermal equilibrium, the effect of the thermal equilibrium is negligible. On the other hand, in the case of r = 0.85, the gel shrunk with bubble formation, similar to the volume phase transition behavior of conventional thermoresponsive polymer gels. Tetra-PEG-PEGE gel (r = 1), i.e. Tetra-PEGE gel, formed a dense skin layer on the gel surface immediately after the temperature jump, and further shrinkage did not proceed at all

solubility limit suggests the solubilization effect of PEG unit existing around the PEGE unit. Similarly, Tc shifted from 20 to 9 °C with an increase in the PEGE concentration. Although the invariance of LCST in another block copolymer with hydrophilic/thermoresponsive sequences has been reported so far,26,27 this result indicates the hydrophilic component affects the LCST of thermoresponsive component. The detailed mechanism is not clear at this stage. Shrinking Kinetics. We investigated the shrinking kinetics of Tetra-PEG-PEGE gels with different r upon a temperature jump. The temperature jump was set to be from 3 to 40 °C in order to cross the LCSTs of all the specimens. According to the thermal diffusion equation, the temperature change of the D

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at temporal scales of minutes. After 3 weeks, the specimen partly shrunk to V/V0 ≈ 0.63 (data not shown), which is still far from the equilibrium state (V/V0 ≈ 0.16). These results suggest that hydrophilic PEG prepolymer units incorporated into the gels act as a pathway through which water can escape outside the gel during the shrinking process, and approximately 1/4 of the PEG prepolymer is necessary to make the percolated water path. In order to investigate this phenomenon in detail, we analyzed the shrinking kinetics quantitatively. Tanaka et al. first predicted the kinetics of spherical polymer gel as the diffusion process of polymer network characterized by the cooperative diffusion coefficient (D) as ⎤⎫ ∂u ∂ ⎧1⎡∂ = D ⎨ 2 ⎢ (d 2u)⎥⎬ ⎣ ⎦⎭ ∂t ∂d ⎩ d ∂d

(1)

where u is the normalized displacement and d is the diameter. Theoretically, eq 1 holds regardless of the magnitude and manner of deformation, and D is given by Mos/f, where Mos and f are the osmotic modulus and the friction coefficient of the network.8 It is confirmed that D is identical to that obtained by the dynamic light scattering (DLS).6 For the homogeneous deformation process, only the boundary condition on the surface is applied and the following solution is obtained. dn =

d∞ − d(t ) ⎛t ⎞ 6 = exp⎜ ⎟ ⎝τ⎠ π d∞ − d0

(t > > τ )

(2) Figure 6. (a) Variation of relaxation time of Tetra-PEG-PEGE gels during the shrinking process (τsh) as a function of PEGE ratio (r). (b) Variation of collective diffusion coefficient of Tetra-PEG-PEGE gels estimated from the relaxation time of the shrinking process as a function of PEGE ratio (r).

where dn is the normalized size of gel, d(t) is the gel diameter at time t, d0 is the diameter of the initial state, d∞ is the diameter in the equilibrium state, and τ (= d∞2/D for the spherical object) is the characteristic time. Although the validity of this model was confirmed for the moderate swelling process,28 it was expected that the application of eq 2 should be restricted to the linear response regime,8 where the volume change is small enough so that the collective diffusion coefficient is constant throughout the swelling or shrinking process. In 1995, Doi et al. proposed the extended model for the heterogeneous deformation process with coexistence of the swollen with collapsed phases; they introduced the other boundary condition between the swollen and collapsed phases. This model well predicted the characteristic behaviors of real system including hysteresis and incubation period. Here, we applied eq 2 to the shrinking process upon a temperature jump across T c , since no heterogeneous deformation was observed in Tetra-PEG-PEGE gels (r = 0.15−0.75). We plotted the time variation of dn against elapsed time after a temperature jump (Figure 5). It should be noted again that the shrinking kinetics of conventional PNIPAAm gels after a sudden temperature jump across their Tc, in general, cannot be described by using eq 2 due to heterogeneous deformation or a skin layer formation.29 Nevertheless, for Tetra-PEG-PEGE gels, the data points were well predicted by eq 2, except for those with higher PEGE prepolymer unit ratios (r ≥ 0.85). The characteristic time (τ) was estimated from the slope (−1/τ) of semilog plot of dn against elapsed time (Figure 5). As for the gels of r = 0.85 and 1, we gave up conducting the same analysis because of the heterogeneous shrinking. The values of τ are shown against r in Figure 6a. τ was a decreasing function of r; larger deformation process had faster shrinking kinetics. For rod-shaped gel specimens, τ is related to D as follows:6,8

D=

3d∞2 8π 2τ

(3)

Figure 6b shows D as a function of r. Surprisingly, the values were constant against r and around 7 × 10−7 cm2 s−1, well corresponding to that obtained from DLS for Tetra-PEG gel in the as-prepared state (r = 0, DDLS ≈ 6.5 × 10−7 cm2 s−1). It should be noted that Tetra-PEG-PEGE gels with the higher r are not in the linear regime, where eq 2 is expected to break. The constant D, regardless of r, and correspondence with DDLS indicate that the shrinking kinetics of Tetra-PEG-PEGE gel is governed by the cooperative diffusion coefficient in the asprepared state, regardless of magnitude of volume change. The macroscopic shrinking kinetics may obey the shrinking kinetics of the PEGE prepolymer unit because the PEG prepolymer unit accept and leak the water molecules expelled from the PEGE segment. In contrast to the invariance of D in r ≤ 0.75, the shrinking process in r ≥ 0.85 changed drastically; the skin layer formed and heterogeneous deformation occurred, suggesting the existence of the critical fraction of PEG prepolymer unit (r*) that forms the path leaking the water molecules around r* ≈ 0.8.

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CONCLUSION

We carried out the shrinking experiments upon a temperature jump across Tc on a series of r-tuned Tetra-PEG-PEGE gels with different ratios of PEGE prepolymer units. In the case of r ≤ 0.75, even with the condition that the gel was exposed to a sudden temperature jump across T c , the gel shrunk E

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(15) Bin Imran, A.; Seki, T.; Ito, K.; Takeoka, Y. Macromolecules 2010, 43, 1975. (16) Hirotsu, S. Jpn. J. Appl. Phys. 2 1998, 37, L284. (17) Erdodi, G.; Kennedy, J. P. J. Polym. Sci., Polym. Chem. 2005, 43, 4953. (18) Ivan, B.; Haraszti, M.; Erdodi, G.; Scherble, J.; Thomann, R.; Mulhaupt, R. Macromol. Symp. 2005, 227, 265. (19) Rikkou-Kalourkoti, M.; Loizou, E.; Porcar, L.; Matyjaszewski, K.; Patrickios, C. S. Polym. Chem. 2012, 3, 105. (20) Erdodi, G.; Kennedy, J. P. Prog. Polym. Sci. 2006, 31, 1. (21) Kamata, H.; Chung, U.; Shibayama, M.; Sakai, T. Soft Matter 2012, 8, 6876. (22) Sakai, T.; Matsunaga, T.; Yamamoto, Y.; Ito, C.; Yoshida, R.; Suzuki, S.; Sasaki, N.; Shibayama, M.; Chung, U. I. Macromolecules 2008, 41, 5379. (23) Akagi, Y.; Katashima, T.; Katsumoto, Y.; Fujii, K.; Matsunaga, T.; Chung, U.; Shibayama, M.; Sakai, T. Macromolecules 2011, 44, 5817. (24) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (25) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (26) Kokufuta, E.; Wang, B. L.; Yoshida, R.; Khokhlov, A. R.; Hirata, M. Macromolecules 1998, 31, 6878. (27) Chen, G. H.; Hoffman, A. S. Nature 1995, 373, 49. (28) Shibayama, M.; Takeuchi, T.; Nomura, S. Macromolecules 1994, 27, 5350. (29) Matsuo, E. S.; Tanaka, T. J. Chem. Phys. 1988, 89, 1695.

homogeneously and did not accompany a skin layer formation. Furthermore, we confirmed that the shrinking kinetics of TetraPEG-PEGE gels can be described by the theoretical model proposed by Tanaka et al. We also found that D was constant and corresponds to that measured by DLS, regardless of r (r ≤ 0.75), suggesting that the macroscopic shrinking kinetics obeyed the shrinking kinetics of PEGE prepolymer unit. There was the critical fraction of PEG prepolymer unit that forms the path leaking the water molecules and the value is around r* ≈ 0.8. In the region above r*, the skin layer formed and heterogeneous deformation occurred. Taking into account these data, we can conclude that the hydrogels with alternating hydrophilic/thermoresponsive prepolymer units have drastically improved the volume phase transition kinetics of conventional hydrogels. This methodology will be a versatile method to improve the volume phase transition kinetics of conventional gels.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (T.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science (JSPS) through the Grants-in-Aid for Scientific Research, the Center for Medical System Innovation (CMSI), the Graduate Program for Leaders in Life Innovation (GPLLI), the International Core Research Center for Nanobio, and the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST program); the Ministry of Education, Culture, Sports, Science, and Technology in Japan (MEXT) through the Center for NanoBio Integration (CNBI); the Japan Science and Technology Agency (JST) through the S-innovation program; and Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (no. 25708033 to T.S. and no. 24240069 to U.C.).

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REFERENCES

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