2574
Macromolecules 1996,28, 2574-2576
Thermoreversible Transition of the Tensile Modulus of a Hydrogel with Ordered Aggregates
2.5~10~ -j
25
Y. Tanaka, Y. Kagami, A. Matsuda, and Y. Osada* Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan Received October 18, 1994 Revised Manuscript Received January 23, 1995
Introduction Water-swollen hydrogels are generally amorphous in nature and have no particular ordered structure in molecular level except for some biological gels where higher-ordered aggregates are observed in the "junction zones".l We have previously reported a water-swollen hydrogel prepared by copolymerizing acrylic acid (AA)with a hydrophobic long alkyl side group-n-stearyl acrylate (SA)has a molecularly-orderedstructure and undergoes reversible order-disorder transition with change in temperat~re.~!~ This paper is concerned with the thermomechanical properties of the poly(SA-co-AA) gel in a crystalamorphous transition region. We have found that the gel abruptly decreases the tensile modulus E from lo8 to lo6 dyn/cm2 when temperature changes from 40 to 50 "C. This drastic change in E is reversible and was associated with the order-disorder transition of the organized structures in the gel. We have also found that the water-swollen poly(SA-co-AA) gel shows a shape memory effect upon cyclical temperature change. The mechanism has been briefly discussed. Experimental Section Materials. Stearyl acrylate (SA) (Tokyo Kasei Kogyo Co. Ltd.) was repeatedly recrystallized from an ethanol solution. Acrylic acid (AA)(Tokyo Kasei Kogyo Co. Ltd.) was distilled at 313 K under reduced pressure before use. a,a'-Azobisisobutyronitrile (NBN) (Tokyo Kasei Kogyo Co. Ltd.), used as a radical initiator, and NJV-methylenebisacrylamide (MBAA) (Wako Pure Chemical Industries Ltd.), used as cross-linking agent, were recrystallized from ethanol solution. Preparation. Poly(SA-co-AA)gel with F = 0.50,0.25,and 0.15 ( F is defined as the mole fraction of SA in the total monomer) was synthesized by radical copolymerization in ethanol. The total monomer concentration was kept a t 3.0 mol dm-3 and that of MBAA was 3.0 x and 9.0 x mol dm-3. The degree of cross-linkage (DCL) was simply calculated as a molar ratio of cross-linking agent to the total monomer. Polymerization was carried out at 323 K for 24 h t o give the chemically-cross-linkedpolymer gel. After polymerization, the gel was immersed in pure ethanol to remove unreacted substances and then in pure water. Measurement. Tensile modulus, E, of poly(SA-co-AA)gel was determined using a tensile tester (TOM-500 Shinko Communication Industry Co. Ltd.). A sheet of poly(SA-co-AA) gel (0.77 cm wide, 0.22 cm thick, 4.2 cm long) was extended with a constant rate of 1 m d m i n in water of various temperatures and established stress-strain relations. The value of E was determined as the slope of the stress-strain curve at the strain of 0.015. Extension test of the gel under various temperatures were made using a thermomechanical analyzer (TMA7 PerkinElmer). A constant load of 20 or 200 mN was applied to the specimen (0.18 cm wide, 0.05 cm thick, 0.75 cm long), and the size change with varying temperature was measured. An unloaded experiment was also made. With the use of this
* To whom correspondence should be addressed.
0
10
20
strain
30
4 0 1~0.3
Figure 1. Stress-strain curves of a poly(SA-co-AA)gel ( F = 0.25). Numbers denote the experimental temperature ("C). The experiment was carried out in water.
apparatus, a three-point bending test applying a loading of 15 mN was made to measure the flexure of the swollen and dry samples under various temperatures.
Results Figure 1 shows the stress-strain curves for a poly(SA-co-AA)gel of F = 0.25, DCL = 1%,measured at various temperatures. The gel moderately swelled in water at 25 "C and exhibited typical hard elastic behavior. As temperature was raised the slopes of the curves decreased with some positive deviations from the linear relations. Above 49 "C the gel exhibited extremely small stresses showing typical rubber-like elastomers. If the slope of these curves at the strain of 0.015 is plotted as a function of temperature, we can get temperature dependence of tensile modulus E. The values of E are shown in Figure 2 for the copolymer gel of F = 0.50, F = 0.25, and F = 0.15 with a constant DCL. E values of the gel of F = 0.25 below 40 "C were lo8 dyn/cm2which is 2 orders magnitude less than the values of some of conventional plastics like polyethylene and poly(methy1 methacrylate) but almost equivalent to amorphous polypropylene4 and poly(viny1 chloride) containing dibutyl phthalate as a pla~ticizer.~ An abrupt decrease in E for the gel of F = 0.25 occurs in a temperature range of 47-49 "C and E values above 50 "C become as low as 2 x lo6 dyn/cm2which is the value of typical soft elastomer. On lowering temperature the gel recovered the tensile modulus achieving lo8 dyn/cm2 around 42 "C. A significant hysteresis was observed on this process. An abrupt change in tensile modulus has been observed for the gel of F = 0.50. On the other hand, the value of E for the gel of F = 0.15 was 2 x lo7 dyn/cm2 at 30 "C, which is 1 order magnitude less than the value for the gel of F = 0.25.E values gradually decrease during raising temperature and become as low as 8 x lo5 dynlcm2, and no clear transition of the tensile modulus has been observed. Temperature dependence of E for the copolymer gel with DCL's of 1%and 3% are shown in Figure 3, the mole fraction of SA was kept constant at F = 0.25. A similar but less of a decrease in E occurs also for the gel of DCL = 3%in the same temperature range as that for DCL = 1%,but E values increased due to increase in DCL above 50 "C. We have made the flexure test for both the swollen and the dried gel by applying a load of 15 mN under various temperatures. As shown in Figure 4 the swollen gel showed an abrupt change in flexure at 49 "C while
0024-929719512228-2574$09.00/0 0 1995 American Chemical Society
Notes 2575
Macromolecules, Vol. 28, No. 7, 1995 0
io9
a)
E .
-0.1
c
C
-0.2
Q)
E -0.3 0)
0
5 eL
.-0
-0.4
u)
lo520 30 40 50 60 70 80
-0.8 30
Temperature I OC
Temperature / "C c)
1o'O
'0-0
M
'0520 30 40 50 60 70 80 Temperature / OC Figure 2. Temperature dependence of the tensile moduli for a poly(SA-co-AA)gel of F = 0.15 (a), F = 0.25 (b), and F = 0.50 (c). DCL = 1%.Open symbols indicate heating, filled symbols, cooling.
io9,
io5
20
40
50
60
70
80
Temperature I 'C Figure 4. Flexural test of a poly(SA-co-AA)gel (F = 0.25): (-) water-swollensample (0.50 cm wide, 0.40 cm deep, 0.091 cm thick);(- - -) dry sample (0.50 cm wide, 0.40 cm deep, 0.055 cm thick). The load applied is 15 mN.
io9
b)
-0.5
I
30
40
5@
60
70
80
Temperature / 'C Figure 3. Temperature dependence of the tensile moduli during heating for a poly(SA-co-AA)gel of DCL = 1%(circle) and DCL = 3% (square). F = 0.25.
the dry sample showed only gradual and monotonous change in a whole experimental temperature range.
Discussion We have previously reported2 that the long alkyl side chains of SA units contained in this copolymer gel form organized layers a t a room temperature, with a thickness of ca. 5 nm and making a tail to tail alignment perpendicular to the main chains. The disintegration of such organized layers due to heating above 50 "C has been confirmed through DSC measurements. Therefore the discontinuous and substantial decrease in the
tensile modulus in a relatively narrow temperature range is associated with the order-disorder transition of the SA units in the copolymer gel. From the results of a wide-angle X-ray diffraction study, it is not until the F value becomes above 0.25 that such a organized arrangement clearly appears. The gradual change in E for the gel of F = 0.15 during heating may come from a lack of arrangement. It is interesting that the thermoreversible transition of tensile modulus is observed for a chemically-cross-linked gel, although such a transition of mechanical properties has been reported for synthetic bulk polymer^.^,^ The layers distance (dz) of the swollen gel was nearly 0.8 nm larger than that of their dry state, suggesting that water molecules in the polymer gel are preferentially adsorbed between two organized layers of stearyl groups. It should be noted that the described abrupt change in E can be observed only for the water-swollen hydrogel and not for dry samples as shown in Figure 4. The behavior can be explained in terms of the hydration of AA sequences in the swollen hydrogel. The AA units of the swollen gel are strongly hydrophilic in nature, capable of adsorbing large amounts of water. When the crystalline aggregates of SA units are transferred t o disordered structure, the AA units are strongly hydrated and the gel becomes totally soften and shows the elasticity. In contrast when the gel is dry, the AA units which have a high glass transition temperature (Tgof polyacrylic acid is 379 KY can sustain the mechanical toughness and do not show any abrupt change in the mechanical property. Thus, the E value of the swollen copolymer gel is exclusively dominated by the SA units below the transition temperature. Actually E value increases as the mole fraction of SA. Interestingly, if the sample is subjected to heat above 49 "C and stretched with 200 mN, the gel extends more than 200%. However, the gel did not return to the initial size upon cooling and kept the deformed shape. When the gel was heated to 46 "C once again without loading, the gel returned to the initial size and showed shape memory effect. In general, the deformation of swollen polymer gels is controlled by a network of cross-linked flexible polymer chains involving the return of polymer chains. Polymer gels containing crystalline layer aggregates, owing to their strong interaction of stearyl group, can lock the return of the segments and restoring force below the transition temperature. If the swollen gel is heated once again, the aggregates transfer to disordered structure and unlock the restoring force, and the gel returns to the original shape and size.
2576 Notes
Acknowledgment. This research was supported in part by a Grant-in-Aid for the Experimental Research Project “Electrically Driven Chemomechanical Polymer Gels as Artificial Muscle” from the Ministry of Education, Science, and Culture (035551881, Japan. The authors also acknowledge to the Agency of Science and Technology, Minister of International Trade and Industry (MITI), for financial support. References and Notes (1) Clark, A. H.; Ross-Murphy, S. B. Structural and Mechanical
Properties of Biopolymer Gels. Adu. Polym. Sei. 1987,83, 57.
Macromolecules, Vol. 28, No. 7, 1995 (2) Matsuda, A.; Okuzaki, H.; Sato, J.; Yasunaga, H.; Osada,
Y. Macromolecules 1994,26,7695. (3) Uchida, M.; Kurosawa, M.; Osada, Y. Macromolecules, in
press. (4) Faucher, J. A. Trans. SOC.Rheol. 1959,3,81. ( 5 ) Schmieder, K.; Wolf, K. Kolloid 2.1952,127, 65. (6) Felisberti, M. I.; Freitas, L. L. d. L.; Stadler, R. Polymer 1990,31,1441. (7) Rosch, J.; Freitas, L. L. d. L.; Stadler, R. Colloid Polym. Sei. 1994,272,261. ( 8 ) Polymer Handbook; Brandrup, J., Immergut, E. H., Eds.; John Wiley & Sons: New York, 1977; Vol. VI, p 215. MA946001R
