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Acceleration of deswelling for poly(N-isopropylacrylamide) gel prepared on the hydrophobic surface of the matrix
Colloids and Surfaces A: Physicochemical and Engineering Aspects 189 (2001) 189– 195 www.elsevier.com/locate/colsurfa
Acceleration of deswelling for poly(N-isopropylacrylamide) gel prepared on the hydrophobic surface of the matrix Norihiro Kato *, Yukie Oohira, Yasuzo Sakai, Fujio Takahashi Department of Applied Chemistry, Faculty of Engineering, Utsunomiya Uni6ersity, 7 -1 -2 Yoto, Utsunomiya 321 -8585, Japan Received 4 September 2000; accepted 24 January 2001
Abstract The surface property of vessels (matrices) for gelation affected the deswelling process of thermosensitive poly(N-isopropylacrylamide) (NIPAAm) gels. Cracks were frequently observed on the gel surface in the process of deswelling. The outbreak of cracks caused a change of the deswelling rates. The deswelling rate was accelerated in the case of the gel prepared in a matrix made of hydrophobic polytetrafluoroethylene (PTFE) or silicone, while no acceleration for deswelling was observed in the case of the gel prepared on hydrophilic glass. The probability of cracking and the deswelling rate increased with increasing hydrophobicity of the matrix surface. The storage moduli (E%) of the swollen gels (22°C) prepared on glass and PTFE as the matrices were 50 and 18 kPa, respectively. The deswelling time for gels prepared in PTFE matrix decreased from 400 (gels prepared on glass) to 50 min without cracking. The hydrophobic matrix was suitable for preparation of the fast-responsive poly(NIPAAm) gel. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Acceleration of deswelling; Hydrophobic surface; Poly(N-isopropylacrylamide) gel
1. Introduction The fast deswelling of a gel is useful for a drug-delivery system or other application. There have been several papers on the treatments such as g-ray irradiation and comb-type grafting to make the response of thermosensitive gel fast [1–4]. We have investigated the deswelling kinetics of poly(N-isopropylacrylamide) (NIPAAm) gel with respect to the rate-determining step of the * Corresponding author. Tel.: + 81-28-6896157; fax: +8128-6896009. E-mail address: [email protected] (N. Kato).
deswelling [5]. The occurrence of cracks was very frequent on the surface of conventional gel on increasing the temperature of the gel [6,7], the deswelling rate of which was accelerated. Fastdeswelling of poly(NIPAAm) gel was also studied in connection with the theory of partitioned-polymer-networks in the gel obtained by the freeze-dry treatment [8]. Osada et al. reported distinctive features on the surface of poly[2-(acrylamido)-2-methylpropanesulfonic acid] gel prepared on a variety of matrices, which exhibited a hydrophobic or hydrophilic property [9]. It is plausible that the surface of a polymer gel is constructed by differ-
0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 0 5 7 9 - 9
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N. Kato et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 189–195
ent polymer network resulting from the surface of matrix in the process of gelation. Conventional gels have usually been synthesized in hydrophobic silicone tubes in our previous reports [5,8]. The purpose of this study is to obtain a polymer gel that is deswollen as fast as possible without cracks. Accordingly, thermal deswelling profiles of poly(NIPAAm) gels were evaluated in order to choose a suitable matrix for the preparation of the polymer gel. The relationship between the storage or loss moduli of poly(NIPAAm) gels and the contact angles of matrices was investigated in order to elucidate the elasticity of gel in relation to the hydrophobicity of matrices and the cracks on the gel surface. Silicone, polytetrafluoroethylene (PTFE), polyethylene (PE), and glass were used as matrices, which exhibit different hydrophobicity.
2. Materials and methods Fig. 1. Deswelling profiles of the cylindrical poly(NIPAAm) gels in water. The temperature of gels equilibrated at 22°C was increased to 40°C. The gels were cross-linked by 3 mol% MBA. The gel surfaces cracked at the arrows indicated in figures. The gels deswelled with ( ) and without () cracks. The gels were prepared in the tubes of (a) glass and (b) PTFE.
2.1. Materials NIPAAm was purchased from Wako Pure Chemical Ind. Ltd. All other chemicals were of guaranteed grade or the best commercially available.
2.2. Preparation of cylindrical gels
Fig. 2. Relationship between the contact angle of the matrices with water and the deswelling rate of the gel. The deswelling rates for the gels with ( ) and without () cracking were calculated from the slopes of the lines described in Fig. 1.
The cylindrical poly(NIPAAm) gels crosslinked by N,N%-methylenebisacrylamide (MBA) [MBA/NIPAAm = 3/97 (mol/mol)] were prepared with water in the tubes of glass, PE, PTFE, and silicone (2 mm each in inner diameter of the matrix), as described in the previous papers [5,8]. All tubes containing pre-gel solution were immersed into approximately 0.1 M sodium sulfite solution at 6°C in the treatment of gelation to avoid termination reaction caused by oxygen during the radical polymerization. In the case of Section 3.4, poly(NIPAAm) gels [MBA/NIPAAm =5/95 (mol/mol)] were prepared in the same way as described above in the tubes of glass and PTFE.
N. Kato et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 189–195
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Fig. 3. SEM microphotographs of the gel surfaces. The cylindrical gels were synthesized in the tubes of (A) glass, (B) PTFE, and (C) silicone.
2.3. Deswelling profiles
2.4. Preparation of slab-shaped gels
The deswelling rate of the gel was determined as following. The gel rod (the initial length: Lo = 60 mm at 22°C) was transferred into water at 40°C for rapid change of the gel temperature from 22 to 40°C. Changes in the gel length (L) were measured with time, and then L/Lo was calculated.
The gel plate (60×30×3 mm3) of MBA/NIPAAm = 3/97 was prepared by putting pre-gel solution between a glass plate (top) and a different kind of plate (bottom): silicone, PTFE, PE, and glass. The slab-shaped gel (4× 4× 3 mm3) was prepared by cutting off the side of the gel plate (up to more than 10 mm apart from the side
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wall) in order to avoid effects of the side surface. The slab-shaped gel was washed with water and swelled in water at 22°C.
3. Results and discussion
3.1. Effect of the 6essel (matrix) surface on the deswelling rate of gel
2.5. SEM After swelling gels in water (22°C), the cylindrical gels were frozen at − 30°C and then freezedried. SEM microphotographs of gel surfaces were taken with S-4500 (Hitachi).
2.6. Dynamic mechanical measurements Dynamic mechanical measurements were carried out with a thermally mechanical analyzer TMA-SS6100 (Seiko Instrument Co.). The slabshaped gels used were prepared as described in Section 2.4. A synthesized wave strain (0.1 Hz) was loaded on the gel. The strain was kept within 1% of the gel thickness. The compressive modulus, E*= E% +iE¦, was obtained, where E% and E¦ are the storage and loss moduli, respectively. All measurements were performed in water at 22°C. E% and E¦ were plotted against each contact angle of the matrix with water. The contact angles were determined with the contact angle measurering system (KRU8 SS, G2).
Gel rods were prepared on a variety of matrix surfaces. The gel rods at the swelling-state (22°C) were heated to 40°C [above a lower critical solution temperature, LCST, of the poly(NIPAAm) gel]. Length changes of the gel rods were measured with time. Fig. 1 shows the representative results of deswelling profiles on the gel rods prepared in the matrices of glass and PTFE. No matter what matrices were used, gel rods cracked at the surface of the gel frequently under heating to deswell. The arrows in Fig. 1 showed the time at cracking. The deswelling rate did not increase despite cracks occurring on the surface of the gel rods prepared on the glass matrix. On the contrary, the deswelling was accelerated drastically after cracking in the case of the PTFE matrix (Fig. 1b). As described in our previous papers [5,7], the occurrence of cracks made the deswelling rate faster for the gels prepared on the silicone matrix. The skin layer at the outer surface of the gel was elucidated to act as a hydrophobic barrier for
Fig. 4. Photographs and illustrations for the gel surfaces with cracking. The gels were prepared in the tubes of (a) glass and (b) PTFE. The swelling gels at 22°C were heated to 40°C for (a) 12, and (b) 1 h, respectively.
N. Kato et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 189–195
Fig. 5. (a) Relationship between E% or E¦ for the swollen gel and the contact angle of the matrices with water. E% (storage modulus) and E¦ (loss modulus) were determined under 0.1 Hz of synthesized wave strain at 22°C. (b) Relationship between tan l and the contact angle of the matrices with water.
Fig. 6. Deswelling profiles of the gels cross-linked by 5 mol% MBA. The temperature of gels equilibrated at 22°C was increased to 40°C. The gels were prepared in the tubes of (a) glass and (b) PTFE.
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water release from the gel. Park and Hoffman described that the process of water release through the hydrophobic skin layer made the rate-determining step for the conventional poly(NIPAAm) gel [10]. Cracks made new surfaces on the gel. The occurrence of cracks caused a remarkable increase of the deswelling rate. It was considered from Fig. 1b that water was released faster through the new surface of the gel rather than the outer surface of the non-cracked gel. Osada and co-workers reported that the surface energy of the matrix was lowered if the polymer networks of gel surface became heterogeneous. This means that the polymer density is higher inside the gel than near the outer surface of the gel prepared on the hydrophobic matrix [9]. According to Osada’s theory, the use of PTFE or silicone as the hydrophobic matrices makes heterogeneous polymer networks possible. The lower polymer density on the outer surface of the gel causes the gel to release water more easily in the process of deswelling. However, there might be no difference in the case of glass matrix on the polymer networks between the outer surface of the gel and the new surface generated by cracking.
3.2. Cracking of the gel during deswelling The deswelling rate, − d(L/Lo)/dt, was determined from the slope of the line drawn in Fig. 1. Fig. 2 shows the relationship between the deswelling rate and the contact angle (q) of the matrix with water. Because q inside the tubes could not be determined experimentally, the deswelling rates were plotted against q, as measured using plates of glass, PE, PTFE, and silicone as matrices used for the preparation of slab-shaped gels. The measured value of q for glass, PE, PTFE, and silicone were 3, 81, 100 and 112°, respectively. The deswelling rates without cracks remained low whichever matrix was used, while those with cracks increased abruptly above q=80°, as shown in Fig. 2. The occurrence of cracking was too frequent to obtain the deswelling profile without cracking in the case of silicone, of which q is the highest.
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The SEM microphotographs on surfaces of cylindrical gels were taken as shown in Fig. 3. Fig. 3 shows macropores formed during freezing gels as described in our previous paper [8]. The thickness of the gel wall comprising the macropores was determined from the SEM to be around 3–5, 0.5–2, and less than 0.2 mm, respectively, for the gel prepared in the matrices of glass, PTFE, and silicone. The wall thickness decreased with increasing hydrophobicity of matrices. The shapes of cracks were characteristic for each gel prepared in different matrices, as shown in the photographs, and also illustrated in Fig. 4. In the case of the glass matrix, it looked as if there were scales scraping off at the surface of the gel. In the cases of PTFE and silicone, the crack ran on the straight and reached to the center of cylindrical gel. It is plausible that deswelling pressure may deeply split the soft gels composed of low polymer density.
3.3. Relationship between gel elasticity and deswelling rate Fig. 5 shows the q dependency on E% or E¦ of the gel prepared on different matrices. E% decreased drastically above q =90°. In other words, E% decreased when the gel was prepared on hydrophobic matrices. It is considered that the decrease of E% corresponds to the increasing heterogeneity of polymer networks and fast release of water from the gel with cracking. E% is known to express the elasticity of gel. Therefore, the decrease of elasticity gives rise to the increase of cracks on the surface of the gel. As shown in Fig. 5b, tan l ( =E¦/E%) was not controlled by q. Judging from tan l =0.1 –0.2, the poly(NIPAAm) gel is considered to exhibit elastic behaviour below the LCST. It was known that the poly(NIPAAm) gel became viscoelastic above the LCST [11]. This result corresponds to Tanaka’s theory, where gel swelling is discussed with respect to the gel elasticity [12].
on the surface of the gel constructed by heterogeneous polymer networks, which could squeeze water out easily under heating (Figs. 2 and 5). However, we reported previously that a polymer gel released water more easily without cracking by increasing the MBA content as a cross-linker for the synthesis of poly(NIPAAm) [7]. Therefore, it was expected that it would be possible to obtain a gel exhibiting a reversible volume-change property for the deswelling–swelling cycle without any cracking. Fig. 6 shows the deswelling profiles of gels cross-linked by 5 mol% MBA prepared on matrices of glass and PTFE. The deswelling rate was accelerated without cracking by using the PTFE matrix for gel preparation. The deswelling time of the gel, which was 400 min when prepared in a glass tube, decreased to 50 min for a gel in the PTFE tube with heating to 40°C. In the case of the silicone matrix, cracking occurred frequently for the gel cross-linked with 6 mol% MBA [7] and did not occur in the case of 10 mol% MBA [7,8]. However, the increase of the cross-linker depressed the length change of the gel from L/Lo = 0.6–0.7 (5 mol%) to L/Lo = 0.7–0.8 (10 mol%). This is a disadvantage for the gel from the viewpoint of the length change in the swelling– deswelling cycle.
4. Conclusion Poly(NIPAAm) gel was prepared with a 3 and 5 mol% cross-linker on PTFE as hydrophobic matrix. The polymer networks of the gel surface are heterogeneous. The deswelling occurred in the cases of 3 mol% cross-linked polymer gel with frequent cracking, but the 5 mol% gel released water without cracking. The deswelling time for 5 mol% cross-linked gel decreased from 400 min (glass matrix) to 50 min (PTFE matrix).
Acknowledgements
3.4. Preparation of fast-responsi6e poly(NIPAAm) gel The hydrophobic matrix caused frequent cracks
This work was partly supported by Utsunomiya University Satellite Venture Business Laboratory (SVBL).
N. Kato et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 189–195
References [1] O. Hirasa, Y. Morishita, R. Onomura, H. Ichijo, A. Yamauchi, Kobunsi Ronbunshu 146 (1989) 661. [2] R. Kishi, O. Hirasa, H. Ichijo, Polym. Gels Networks 5 (1997) 145. [3] B.G. Kabra, S.H. Gehrke, Polym. Commun. 32 (1991) 322. [4] R. Yoshida, K. Uchida, Y. Kaneko, K. Sakai, A. Kikuchi, Y. Sakurai, T. Okano, Nature 374 (1995) 240. [5] N. Kato, F. Takahashi, Bull. Chem. Soc. Jpn. 72 (1999) 357. [6] N. Kato, F. Takahashi, Bull. Chem. Soc. Jpn. 71 (1998) 1299.
.
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[7] N. Kato, H. Hasegawa, F. Takahashi, Bull. Chem. Soc. Jpn. 73 (2000) 1089. [8] N. Kato, F. Takahashi, Bull. Chem. Soc. Jpn. 70 (1997) 1289. [9] A. Kii, J. Xu, K. Okawa, X. Zhang, J. Gong, Y. Osada, Polymer Preprints Japan 48 (1999) 464. [10] T.G. Park, A.S. Hoffman, J. Appl. Polym. Sci. 52 (1994) 85. [11] M. Shibayama, M. Morimoto, S. Nomura, Macromolecules 27 (1994) 5060. [12] T. Tanaka, L.O. Hocker, G.B. Benedek, J. Chem. Phys. 59 (1973) 5151.
Acceleration of deswelling for poly(N-isopropylacrylamide) gel prepared on the hydrophobic surface of the matrix Norihiro Kato *, Yukie Oohira, Yasuzo Sakai, Fujio Takahashi Department of Applied Chemistry, Faculty of Engineering, Utsunomiya Uni6ersity, 7 -1 -2 Yoto, Utsunomiya 321 -8585, Japan Received 4 September 2000; accepted 24 January 2001
Abstract The surface property of vessels (matrices) for gelation affected the deswelling process of thermosensitive poly(N-isopropylacrylamide) (NIPAAm) gels. Cracks were frequently observed on the gel surface in the process of deswelling. The outbreak of cracks caused a change of the deswelling rates. The deswelling rate was accelerated in the case of the gel prepared in a matrix made of hydrophobic polytetrafluoroethylene (PTFE) or silicone, while no acceleration for deswelling was observed in the case of the gel prepared on hydrophilic glass. The probability of cracking and the deswelling rate increased with increasing hydrophobicity of the matrix surface. The storage moduli (E%) of the swollen gels (22°C) prepared on glass and PTFE as the matrices were 50 and 18 kPa, respectively. The deswelling time for gels prepared in PTFE matrix decreased from 400 (gels prepared on glass) to 50 min without cracking. The hydrophobic matrix was suitable for preparation of the fast-responsive poly(NIPAAm) gel. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Acceleration of deswelling; Hydrophobic surface; Poly(N-isopropylacrylamide) gel
1. Introduction The fast deswelling of a gel is useful for a drug-delivery system or other application. There have been several papers on the treatments such as g-ray irradiation and comb-type grafting to make the response of thermosensitive gel fast [1–4]. We have investigated the deswelling kinetics of poly(N-isopropylacrylamide) (NIPAAm) gel with respect to the rate-determining step of the * Corresponding author. Tel.: + 81-28-6896157; fax: +8128-6896009. E-mail address: [email protected] (N. Kato).
deswelling [5]. The occurrence of cracks was very frequent on the surface of conventional gel on increasing the temperature of the gel [6,7], the deswelling rate of which was accelerated. Fastdeswelling of poly(NIPAAm) gel was also studied in connection with the theory of partitioned-polymer-networks in the gel obtained by the freeze-dry treatment [8]. Osada et al. reported distinctive features on the surface of poly[2-(acrylamido)-2-methylpropanesulfonic acid] gel prepared on a variety of matrices, which exhibited a hydrophobic or hydrophilic property [9]. It is plausible that the surface of a polymer gel is constructed by differ-
0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 0 5 7 9 - 9
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ent polymer network resulting from the surface of matrix in the process of gelation. Conventional gels have usually been synthesized in hydrophobic silicone tubes in our previous reports [5,8]. The purpose of this study is to obtain a polymer gel that is deswollen as fast as possible without cracks. Accordingly, thermal deswelling profiles of poly(NIPAAm) gels were evaluated in order to choose a suitable matrix for the preparation of the polymer gel. The relationship between the storage or loss moduli of poly(NIPAAm) gels and the contact angles of matrices was investigated in order to elucidate the elasticity of gel in relation to the hydrophobicity of matrices and the cracks on the gel surface. Silicone, polytetrafluoroethylene (PTFE), polyethylene (PE), and glass were used as matrices, which exhibit different hydrophobicity.
2. Materials and methods Fig. 1. Deswelling profiles of the cylindrical poly(NIPAAm) gels in water. The temperature of gels equilibrated at 22°C was increased to 40°C. The gels were cross-linked by 3 mol% MBA. The gel surfaces cracked at the arrows indicated in figures. The gels deswelled with ( ) and without () cracks. The gels were prepared in the tubes of (a) glass and (b) PTFE.
2.1. Materials NIPAAm was purchased from Wako Pure Chemical Ind. Ltd. All other chemicals were of guaranteed grade or the best commercially available.
2.2. Preparation of cylindrical gels
Fig. 2. Relationship between the contact angle of the matrices with water and the deswelling rate of the gel. The deswelling rates for the gels with ( ) and without () cracking were calculated from the slopes of the lines described in Fig. 1.
The cylindrical poly(NIPAAm) gels crosslinked by N,N%-methylenebisacrylamide (MBA) [MBA/NIPAAm = 3/97 (mol/mol)] were prepared with water in the tubes of glass, PE, PTFE, and silicone (2 mm each in inner diameter of the matrix), as described in the previous papers [5,8]. All tubes containing pre-gel solution were immersed into approximately 0.1 M sodium sulfite solution at 6°C in the treatment of gelation to avoid termination reaction caused by oxygen during the radical polymerization. In the case of Section 3.4, poly(NIPAAm) gels [MBA/NIPAAm =5/95 (mol/mol)] were prepared in the same way as described above in the tubes of glass and PTFE.
N. Kato et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 189–195
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Fig. 3. SEM microphotographs of the gel surfaces. The cylindrical gels were synthesized in the tubes of (A) glass, (B) PTFE, and (C) silicone.
2.3. Deswelling profiles
2.4. Preparation of slab-shaped gels
The deswelling rate of the gel was determined as following. The gel rod (the initial length: Lo = 60 mm at 22°C) was transferred into water at 40°C for rapid change of the gel temperature from 22 to 40°C. Changes in the gel length (L) were measured with time, and then L/Lo was calculated.
The gel plate (60×30×3 mm3) of MBA/NIPAAm = 3/97 was prepared by putting pre-gel solution between a glass plate (top) and a different kind of plate (bottom): silicone, PTFE, PE, and glass. The slab-shaped gel (4× 4× 3 mm3) was prepared by cutting off the side of the gel plate (up to more than 10 mm apart from the side
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N. Kato et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 189–195
wall) in order to avoid effects of the side surface. The slab-shaped gel was washed with water and swelled in water at 22°C.
3. Results and discussion
3.1. Effect of the 6essel (matrix) surface on the deswelling rate of gel
2.5. SEM After swelling gels in water (22°C), the cylindrical gels were frozen at − 30°C and then freezedried. SEM microphotographs of gel surfaces were taken with S-4500 (Hitachi).
2.6. Dynamic mechanical measurements Dynamic mechanical measurements were carried out with a thermally mechanical analyzer TMA-SS6100 (Seiko Instrument Co.). The slabshaped gels used were prepared as described in Section 2.4. A synthesized wave strain (0.1 Hz) was loaded on the gel. The strain was kept within 1% of the gel thickness. The compressive modulus, E*= E% +iE¦, was obtained, where E% and E¦ are the storage and loss moduli, respectively. All measurements were performed in water at 22°C. E% and E¦ were plotted against each contact angle of the matrix with water. The contact angles were determined with the contact angle measurering system (KRU8 SS, G2).
Gel rods were prepared on a variety of matrix surfaces. The gel rods at the swelling-state (22°C) were heated to 40°C [above a lower critical solution temperature, LCST, of the poly(NIPAAm) gel]. Length changes of the gel rods were measured with time. Fig. 1 shows the representative results of deswelling profiles on the gel rods prepared in the matrices of glass and PTFE. No matter what matrices were used, gel rods cracked at the surface of the gel frequently under heating to deswell. The arrows in Fig. 1 showed the time at cracking. The deswelling rate did not increase despite cracks occurring on the surface of the gel rods prepared on the glass matrix. On the contrary, the deswelling was accelerated drastically after cracking in the case of the PTFE matrix (Fig. 1b). As described in our previous papers [5,7], the occurrence of cracks made the deswelling rate faster for the gels prepared on the silicone matrix. The skin layer at the outer surface of the gel was elucidated to act as a hydrophobic barrier for
Fig. 4. Photographs and illustrations for the gel surfaces with cracking. The gels were prepared in the tubes of (a) glass and (b) PTFE. The swelling gels at 22°C were heated to 40°C for (a) 12, and (b) 1 h, respectively.
N. Kato et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 189–195
Fig. 5. (a) Relationship between E% or E¦ for the swollen gel and the contact angle of the matrices with water. E% (storage modulus) and E¦ (loss modulus) were determined under 0.1 Hz of synthesized wave strain at 22°C. (b) Relationship between tan l and the contact angle of the matrices with water.
Fig. 6. Deswelling profiles of the gels cross-linked by 5 mol% MBA. The temperature of gels equilibrated at 22°C was increased to 40°C. The gels were prepared in the tubes of (a) glass and (b) PTFE.
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water release from the gel. Park and Hoffman described that the process of water release through the hydrophobic skin layer made the rate-determining step for the conventional poly(NIPAAm) gel [10]. Cracks made new surfaces on the gel. The occurrence of cracks caused a remarkable increase of the deswelling rate. It was considered from Fig. 1b that water was released faster through the new surface of the gel rather than the outer surface of the non-cracked gel. Osada and co-workers reported that the surface energy of the matrix was lowered if the polymer networks of gel surface became heterogeneous. This means that the polymer density is higher inside the gel than near the outer surface of the gel prepared on the hydrophobic matrix [9]. According to Osada’s theory, the use of PTFE or silicone as the hydrophobic matrices makes heterogeneous polymer networks possible. The lower polymer density on the outer surface of the gel causes the gel to release water more easily in the process of deswelling. However, there might be no difference in the case of glass matrix on the polymer networks between the outer surface of the gel and the new surface generated by cracking.
3.2. Cracking of the gel during deswelling The deswelling rate, − d(L/Lo)/dt, was determined from the slope of the line drawn in Fig. 1. Fig. 2 shows the relationship between the deswelling rate and the contact angle (q) of the matrix with water. Because q inside the tubes could not be determined experimentally, the deswelling rates were plotted against q, as measured using plates of glass, PE, PTFE, and silicone as matrices used for the preparation of slab-shaped gels. The measured value of q for glass, PE, PTFE, and silicone were 3, 81, 100 and 112°, respectively. The deswelling rates without cracks remained low whichever matrix was used, while those with cracks increased abruptly above q=80°, as shown in Fig. 2. The occurrence of cracking was too frequent to obtain the deswelling profile without cracking in the case of silicone, of which q is the highest.
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The SEM microphotographs on surfaces of cylindrical gels were taken as shown in Fig. 3. Fig. 3 shows macropores formed during freezing gels as described in our previous paper [8]. The thickness of the gel wall comprising the macropores was determined from the SEM to be around 3–5, 0.5–2, and less than 0.2 mm, respectively, for the gel prepared in the matrices of glass, PTFE, and silicone. The wall thickness decreased with increasing hydrophobicity of matrices. The shapes of cracks were characteristic for each gel prepared in different matrices, as shown in the photographs, and also illustrated in Fig. 4. In the case of the glass matrix, it looked as if there were scales scraping off at the surface of the gel. In the cases of PTFE and silicone, the crack ran on the straight and reached to the center of cylindrical gel. It is plausible that deswelling pressure may deeply split the soft gels composed of low polymer density.
3.3. Relationship between gel elasticity and deswelling rate Fig. 5 shows the q dependency on E% or E¦ of the gel prepared on different matrices. E% decreased drastically above q =90°. In other words, E% decreased when the gel was prepared on hydrophobic matrices. It is considered that the decrease of E% corresponds to the increasing heterogeneity of polymer networks and fast release of water from the gel with cracking. E% is known to express the elasticity of gel. Therefore, the decrease of elasticity gives rise to the increase of cracks on the surface of the gel. As shown in Fig. 5b, tan l ( =E¦/E%) was not controlled by q. Judging from tan l =0.1 –0.2, the poly(NIPAAm) gel is considered to exhibit elastic behaviour below the LCST. It was known that the poly(NIPAAm) gel became viscoelastic above the LCST [11]. This result corresponds to Tanaka’s theory, where gel swelling is discussed with respect to the gel elasticity [12].
on the surface of the gel constructed by heterogeneous polymer networks, which could squeeze water out easily under heating (Figs. 2 and 5). However, we reported previously that a polymer gel released water more easily without cracking by increasing the MBA content as a cross-linker for the synthesis of poly(NIPAAm) [7]. Therefore, it was expected that it would be possible to obtain a gel exhibiting a reversible volume-change property for the deswelling–swelling cycle without any cracking. Fig. 6 shows the deswelling profiles of gels cross-linked by 5 mol% MBA prepared on matrices of glass and PTFE. The deswelling rate was accelerated without cracking by using the PTFE matrix for gel preparation. The deswelling time of the gel, which was 400 min when prepared in a glass tube, decreased to 50 min for a gel in the PTFE tube with heating to 40°C. In the case of the silicone matrix, cracking occurred frequently for the gel cross-linked with 6 mol% MBA [7] and did not occur in the case of 10 mol% MBA [7,8]. However, the increase of the cross-linker depressed the length change of the gel from L/Lo = 0.6–0.7 (5 mol%) to L/Lo = 0.7–0.8 (10 mol%). This is a disadvantage for the gel from the viewpoint of the length change in the swelling– deswelling cycle.
4. Conclusion Poly(NIPAAm) gel was prepared with a 3 and 5 mol% cross-linker on PTFE as hydrophobic matrix. The polymer networks of the gel surface are heterogeneous. The deswelling occurred in the cases of 3 mol% cross-linked polymer gel with frequent cracking, but the 5 mol% gel released water without cracking. The deswelling time for 5 mol% cross-linked gel decreased from 400 min (glass matrix) to 50 min (PTFE matrix).
Acknowledgements
3.4. Preparation of fast-responsi6e poly(NIPAAm) gel The hydrophobic matrix caused frequent cracks
This work was partly supported by Utsunomiya University Satellite Venture Business Laboratory (SVBL).
N. Kato et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 189–195
References [1] O. Hirasa, Y. Morishita, R. Onomura, H. Ichijo, A. Yamauchi, Kobunsi Ronbunshu 146 (1989) 661. [2] R. Kishi, O. Hirasa, H. Ichijo, Polym. Gels Networks 5 (1997) 145. [3] B.G. Kabra, S.H. Gehrke, Polym. Commun. 32 (1991) 322. [4] R. Yoshida, K. Uchida, Y. Kaneko, K. Sakai, A. Kikuchi, Y. Sakurai, T. Okano, Nature 374 (1995) 240. [5] N. Kato, F. Takahashi, Bull. Chem. Soc. Jpn. 72 (1999) 357. [6] N. Kato, F. Takahashi, Bull. Chem. Soc. Jpn. 71 (1998) 1299.
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