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Langmuir 1995,11, 2301-2302
Notes Temperature-DependentContact Angles of Water on Poly(N-isopropylacrylamide)Gels Ju Zhang,? Robert Pelton,* and Yulin Deng McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering, McMaster University, Hamilton, Canada L8S 4L7 Received August 23, 1994. I n Final Form: January 17, 1995
Introduction This communication describes results of the first reported contact angle measurements on water swollen cross-linked poly(N-isopropylacrylamide), polyNIPAM, gels which show unique behavior as a function of temperature. Many papers in the last decade have discussed the swelling properties of cross-linked polyNIPAM gels in water.l This is an interesting system because the gels are highly swollen (typically 90 wt % water) up to 32 "C whereas a t higher temperatures the gels expel most of the water to leave about 2 water molecules per amide unit. Temperature-sensitive swelling has been observed in both macroscopic2and microscopic colloidal microgels3 and the volume phase transition temperature (i.e., the temperature corresponding to the greatest change in volume), T,, is in the range 31-35 "C. This behavior reflects the fact that linear polyNIPAM has a lower critical solution temperature in water a t 32 "C.4 Figure 1shows the advancing contact angle of water as a function of temperature. The behavior was remarkable. At 25 "C when the gel contained about 90% water, the contact angle was 42". Around the T,(about 36 "C) the contact angle jumped to 90" which corresponds to values for water on alkane surfaces. Very recently Takei and co-workers have reported similar temperature-dependent contact angles for polyNIPAM grafted onto glass.5 Similarly, Yamada et al. reported a contact angle of 48" for water on polyNIPAM-treated glass at 37 "C.6 The arrows in Figure 1show the direction in which the temperature was changed. Although a minimum of 30 min was given for equilibration, we believe the scatter reflects incomplete swelling or deswelling of the gel. Slow deswelling kinetics have been reported by other^.^ The results can most obviously be interpreted by assuming that propyl groups tend to concentrate a t the airlwater interface at all temperatures. At high temperatures the result is an alkane-like surface of packed propyl groups whereas at low temperature there are some water or amide groups mixed in a t the interface. Attempts to measure a contact angle by the captive bubble method with the gel immersed in water were unsuccessful. Below T,, the bubbles did not adhere + Visiting
Scientist from Shandong Institute of Medical Instru-
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Temperature(C)
Figure 1. The advancingcontactangle ofwater on cross-linked polyNIPAM gel swollen with water. The initial measurement was made at 25 "C and the arrows indicate the direction of temperature change.
indicating a contact angle of zero. At higher temperatures the results were erratic; in some cases the captive bubble would adhere to give a contact angle whereas in repeated measurement at the same location the air bubble did not displace water from the surface. Perhaps with immersed gels the more polar amide groups were facing the water phase. The concept of polymers and gels orienting to give the minimum interfacial tension has been proposed by others.8 High contact angles for water on other types of hydrogels has been reported. For example, Holly and Refojo reported advancing contact angles between 60" and 85" for crosslinked poly(2-hydroxyethyl methacrylate) swollen containing 30-40% water.g Similarly Chappuis et al. showed that saline solution had an advancing contact angle of 100 "C on animal cartilage.1° Therefore, we conclude that the polyNIPAM results are reasonable. The swelling behavior of polyNIPAM gels is influenced by solvent composition, surfactant, and the presence of acrylamide or other co-monomers. We speculate that the contact angle will also be sensitive to these factors and work is in progress to verify this. In summary, we believe the results in Figure 1are the first reported contact angles for water on polyNIPAM gels and we do not know of any other system where the contact angles are such a strong function of temperature.
Experimental Section NIPAM (Kodak) was purified by dissolution in toluene and recrystallizationwith n-hexane. Methylenebidacrylamide) (Aldrich), potassium persulfate (BDH, analytic grade), and sodium bisulfate (BDH,laboratory reagent)were used as received. Milli Q water (Millipore Corp.) was used for all solutions. To a 100 x 50 crystallizing dish (Pyrex) was added 4 g of
5,1989,816. (4) Heskin, M.; Guillet, J. E. J. Macromol. Sci., Chem. 1968,AZ(8),
NIPAM and 0.1g of methylenebis(acry1amide)in 40 mL of water. The dish was sealed with a rubber stopper and oxygen was removed from the mixture by nitrogen bubbling. With the temperature controlled at 18 "C, the mixture was stirred with a magnetic bar and polymerization was initiated by the addition
1441. (5)Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, Y. Macromolecules 27,1994,6163. (6) Yamada, N.; Okano, T.; Sakai, H.; Karikusa, F.; Sawasaki, U.; Sakurai, Y. Makromol. Chem., Rapid Commun. 11, 1990,571. (7) Chiklis, C. K.;Grasshoff, J. M. J. Polym. Sci.,A-2 1970,8, 1617.
(8)Yasuda. H.: Sharma, A. K.: Yasuda, T. J. Polym. Sci., Polym. Phys. Ed. 1981,19, 1285. (9) Holly, F. J.; Refojo, M. F.; J. Biomed. Res. 1976,9 , 315. (10) ChaDDUiB. J.: Sherman, I. A,: Neumann, A. W. Ann. Biomed. Eng. 1986,*il, 435.
ments. (1)Schild, H. G. Prog. Polym. Sci. 17, 1992,163. (2) Dong, L. C.; Hoffman, A. S.J. Controlled Release 13, 1990,21. (3) Pelton, R. H.; Pelton, H. M.: Morfesis, A Rowell, R. L. Langmuir
0743-746319512411-2301$09.00/0 0 1995 American Chemical Society
Notes
2302 Langmuir, Vol.' 11, No. 6, 1995 of 5 mg of sodium bisulfite and 5 mg of potassium persulfate. After 10 min the stirring and nitrogen flow were stopped and the system was sealed. After 2 h the transparent, elastic crosslinked polyNIPAM gel was removed and extensively washed in water. The cleaned gel was cut into 12 x 4 x 2 mm samples which were stored in water. Advancing contact angles were measured by the sessile drop method with a Rame-Hart Model 100-00contact angle goniometer fitted with a Model 100-07environmental chamber. Gel samples were positioned with the side cast against the crystallizing dish facing upward, maintaining water around the substrate base to keep the vapor-phase saturated. The chamber was closed and conditioned with a Brinkman Model RMS-6 water bath set at 25 "C for about 1h, after which the chamber was opened and any excess water on top of the gel was blotted with filter paper. Ten minutes later, a 0.5 p L sessile drop of water was placed on the gel and the advancing contact angle was measured when the three-phase boundary of the drop stopped moving. Angles on
both sides of each drop were measured to assure symmetry. The gel was allowed 30 min to equilibrate after each temperature change. Two batches of gel were polymerized and both showed the same contact angle behavior. We recognize that contact angle measurements on gels are subject to difficulties arising from the deformation of the gel." However,the changes in contact angle with temperature observed in this work were sufficiently high that small errors due to deformation were unlikely to influence the conclusions. This work was partially supported by the Canadian Natural Science and Engineering Research Council. The authors also acknowledge the Provincial Government of Shandong Province of the People's Republic of China for financial support of J. Zhang. LA940663S (11)Andrade, J.D.;King, R. N. J.Polym. Sci., Polym. Symp. 1979, 66, 313.
