Fast Deswelling of Microporous Cellulose Ether Gel Prepared by Freeze-drying

Transcription

1 Fast Deswelling of Microporous Cellulose Ether Gel Prepared by Freeze-drying N. Kato, 1 H. Suzuki, 1 Y. Sakai, 1 and S. H. Gehrke 2 1 Department of Applied Chemistry, Faculty of Engineering, Utsunomiya Univ., Yoto, Utsunomiya , Japan Phone: (+81) Facsimile: (+81) katon@cc.utsunomiya-u.ac.jp 2 Department of Chemical and Petroleum Engineering, The Univ. of Kansas, 4006 Learned Hall, 1530 W. 15 th Street, Lawrence, KS , USA Phone: (+1) Facsimile: (+1) shgehrke@ku.edu For most applications of responsive gels, control of the response rate is one of the most important factors for technological success. Since economic viability of these applications depends on the rate of solvent sorption/desorption, a variety of protocols for creating microporous gels have been investigated to increase the rate of swelling/shrinking. 1-3 A freeze-drying and subsequent hydration was reported in our previous papers to make thermosensitive gels microporous and fast-responsive. 1,3 In this presentation, fast-shrinking of hydroxypropyl cellulose (HPC) gel, a thermally responsive gel, will be discussed in connection with the microporous structure of the polymer network. HPC gel disk was prepared by crosslinking with Fig. 1 Relationship between log T 1/2 and equilibrium swelling degree Q before the freezing. The time for half deswelling T 1/2 was determined just after temperature was jumped from 295 to 333 K. divinyl sulfone. Weight change of the HPC gel disk (1.5 mm thick) was determined with time, when the temperature was jumped between 295 and 333 K. To evaluate the shrinking rate of gels, the time for half shrinking of gel, T 1/2, was determined by heating above a lower critical solution temperature (LCST). log T 1/ Before freeze-drying Q Rapid shrinking of HPC gel was observed after a treatment of freeze-drying and water hydration; The T 1/2 was reduced to 3±1 s, while it took approximately 240 s for the conventional gel without the freeze-drying treatment. SEM images showed the microporous structures formed in the freezing process of the gels. We hypothesize that the free water froze in the gel, causing the polymer chains to gather and condense. After sublimation of the ice, a heterogeneous polymer network remained. The equilibrium swelling degree (Q) as the ratio of the swollen gel weight to the dry weight of the gel can be reversibly controlled by controlling the temperature. The gel disks with different Q s were freeze-dried and re-hydrated in water. SEM images clearly indicated that the sizes of the pore and the polymer strut are easily controlled with altering water content within the gels just before freezing. Also, the shrinking profiles given by the temperature-jump above the LCST showed that the T 1/2 depended on the water content of the gel just before freezing. Figure 1 showed the relationship between log T 1/2 and Q just before freezing. Shrinking time increased with decreasing Q below 20, and log T 1/2 reached 2.4 (T 1/2 =240 s) at Q = 2. Water content below Q = 20 is supposed to be insufficient to generate micropores in the gel. The freeze-drying and subsequent hydration of gels is a simple and effective technique to control the microstructure of the polymer network and the shrinking rate of the HPC gels. References: (1) Kato, N.; Sakai, Y.; Shibata, S. Macromolecules, 2003, 36, 961. (2) Kabra, B. G.; Gehrke; S. H.; Spontak, R. J. Macromolecules, 1998, 31, (3) Kato, N.; Takahashi, F. Bull. Chem. Soc. Jpn. 1997, 70, 1289.

2 Fast deswelling of Microporous Cellulose Ether Gel Prepared by Freeze-Drying Norihiro Kato 1, Haruyoshi Suzuki 1, Yasuzo Sakai 1, and Stevin H. Gehrke 2 1 Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto, Utsunomiya , JAPAN katon@cc.utsunomiya-u.ac.jp 2 Department of Chemical and Petroleum Engineering, The University of Kansas, Lawrence, KS , USA

3 Introduction For most applications of responsive gels, control of the response rate is one of the most important factors for technological success. 1-5) The freeze-drying and subsequent re-hydration of thermosensitive gels alters the microstructural properties of the gels in a way that leads to rapid shrinking rates. 6,7) Microporous hydroxypropyl cellulose (HPC) gels were created by this method to investigate the effects on the shrinking time of the gels in response to temperature changes. Stimulus-responsive gel Control of response rates Medical & pharmaceutical field Responsive drug delivery system Superabsorbent Diaper etc. Tissue Engineering etc. Recyclable absorbents to dewater waste sludge, coal slurries, etc.

4 Freeze-drying method Simple and effective method to accelerate the gel shrinking freezing hydration -20 C Thermosensitive hydrogel ice Microporous, fast response gel Rapid shrinking of poly(n-isopropylacrylamide) gel, pnipa gel, was observed after a treatment of freeze-drying and re-swelling. 6,7) We hypothesize that the free water froze in gels, causing the polymer chains to gather and condense. After sublimation of the ice, a heterogeneous polymer network remained. The freeze-drying method will be applied to other thermosensitive gels to make the response rate fast. In this research, rapid shrinking of the microporous hydroxypropyl cellulose (HPC) gel, of which LCST is approximately 39 C, will be discussed.

5 Objectives Microporous, fast response hydroxypropyl cellulose (HPC) gel is created by the freeze-drying method. The purpose of this research is to control the shrinking time of HPC gel as desired, and to demonstrate the relationship between the water amount of gels during the freezing process and shrinking time of gels in response to temperature changes. 100 LCST CH 2 OR 80 (39-40 C) OR O O 60 Heating: 0.1 C/ min O CH 2 40 CH 3 -CH n 20 OH Hydroxypropyl cellulose Temperature / C Transmittance / % Temperature dependence of the optical transmittance of the aqueous HPC polymer solution. The polymer solution was stirred during measurement to avoid the precipitation of the insoluble polymers: 500 nm, 1 wt% HPC (Mw 100,000).

6 Materials & Methods Gel synthesis and analysis of shrinking kinetics were carried out in the same way as described in the previous papers. 3,8-11) Hydroxypropyl cellulose, HPC, (average molecular weight: 100,000) was purchased from Acros Organics (Belgium). Divinyl sulfone, DVS, (Kanto Kagaku, Japan) was used as a cross-linker of HPC. The conventional HPC gel was prepared as follows. Silicone rubber gasket (1.5 mm thick) Glass plate Glass plates HPC was dissolved in ph12 NaOH soln. After addition of DVS, the solution was stirred vigorously for 30 s. The reaction mixture was poured between two glass plates separated by silicone rubber gasket. The crosslinking reaction was allowed to proceed for 24 h at 22 C. HPC gel sheet The gels were washed with dilute HCl solution and water repeatedly. Freeze-drying & Hydration

7 The equilibrium gravimetric technique was carried out for gels which weight change was faster than 20 min. After separation of the swollen gel at 22 C, the gel was blotted and weighed. The gel was transferred to water bath at 60 C. After a given time t, the gel was removed from water, blotted, and weighed. The gel was placed back to the original bath at 22 C and allowed to reach equilibrium. The gel was placed in the bath at 60 C again, and time t was increased by a small increment ( t + t). The cycle was repeated until the mass of the gel became independent of time. Equilibrated gel at 22 C 60 C t t blot-and-weigh The dynamic gravimetric technique was carried out for gels which weight change was slower than 20 min. The swollen gel at 22 C was transferred to water bath at 60 C. After a given time t, the gel was removed from water, blotted, and weighed. The gel was placed back to the bath at 60 C. The cycle was repeated until the mass of the gel no longer changed appreciably with time.

8 Results & Discussion Q=30 Q=14 Q=1.5 Freeze-drying To control the microstructure of gels, gels containing different amount of water (equilibrium swelling degree Q = 30 ~ 1.5 ) were freeze-dried and re-swollen in water. Re-swelling Equilibrium swelling degree: Q = M wet M dry Microporous HPC gels with a variety of microstructure

9 1.0 Conventional gel (M t M ) / (M 0 M ) Freeze-dried gel at Q = Time / s Swollen gel Freeze-drying treatment Fast response Formation of porous structure Fig. Weight change kinetics for the HPC gels with and without the freeze-drying treatment. Shrinking kinetics of the gel sheet (1.5 mm thick) was determined by a temperature jump between 22 and 60 C. De-swollen gel Freeze-drying treatment No effect for shrinking rates

10 Before freeze-drying Rapid shrinking was observed after a treatment of freeze-drying and water hydration; T 1/2 was reduced to 3±1 s, while it took approximately 240 s for the conventional gel without the FD-treatment. log T 1/ Shrinking time increased with decreasing Q, and log T 1/2 reached 2.4 (T 1/2 = 240 s), of which value is almost the same as the conventional gel Q Control of the water mass inside gels Fig. Relationship between log T 1/2 and equilibrium swelling degree Q before the freezing. The time for half deswelling T 1/2 [ s ] was determined just after temperature was jumped from 295 to 333 K. Freeze-drying treatment Control of the shrinking time between 2 min and 2 h

11 Void volume : (effective porosity of microporous gels) 9 Mechanical pressure (10 kpa) Glass plate Tissue papers M M wet M wet HPC gel Measurement of the effective porosity The effective porosity ε was defined as the fraction of total gel volume occupied by interconnected pores which water can flow. 9) The gel sheet equilibrated at 22 C was blotted and weighed (M wet ). Then the gel sheet was placed between 10 layers of lint-free tissue papers. Approximately 10 kpa of the mechanical pressure was applied to the assembly sandwiched between two glass plates for 1 min. The expelled water was absorbed by tissue papers. The mass of the gel (M) was measured after applying the mechanical pressure. It appears that the freeze-drying treatment does not affect the effective porosity of gels; The effective porosity remained very low (ε 0.06). This result means that pore water can not expel through the generated pores with convective flow at the isothermal condition of 22 C. The possible explanation for fast shrinking is that the pores come to connect in the beginning of shrinking process, and drastic increase of shrinking rates was caused by the convective flow through the temporarily formed channels.

12 Summary A simple and effective method of preparing fast-response gels is developed. Control of the swelling degree of gels freezing Various micro-structure of gels Control of the shrinking time : 2 min ~ 2 h

13 References 1) S. H. Gehrke, Synthesis and properties of hydrogels for used for drug delivery ; G. L. Amidon, P. I. Lee, and E. M. Topp (Eds.), in Transport Processes in Pharmaceutical Sciences, Marcel Dekker, New York, p473, (2000). 2) S. H. Gehrke, J. P. Fisher, J. F. McBride, S. M. O Connor, and H. Zhu, Gel coated catheters as drug delivery systems ; S. Dinh and J. DeNuzzio (Eds.), in Intelligent Materials and Novel Concepts for Controlled Release Technologies, ACS Symposium Series, Washington D. C., 728, p43 (1999). 3) B. G. Kabra and S. H. Gehrke, Rate limiting steps for solvent sorption and desorption by microporous stimuli-sensitive absorbent gels, F. L. Buchholz and N. A. Peppas (Eds.), in Superabsorbent Polymers: Science and Technology, ACS Symposium Series, 573, p76 (1994). 4) S. H. Gehrke, Synthesis, equilibrium swelling, kinetics, permeability and applications of environmentally responsive gels ; K. Dusek (Ed.), in Advances in Polymer Science Series, Springer- Verlag, Heidelberg, 110, p81 (1993). 5) S. H. Gehrke and P. I. Lee, Hydrogels for drug delivery ; Praveen Tyle, (Ed.), in Specialized Drug Delivery Systems, Manufacturing and Production Technology, Marcel Dekker, New York, p333 (1990). 6) N.Kato and F.Takahashi, Bull. Chem. Soc. Jpn., 70, 1289 (1997). 7) N.Kato, Y. Sakai, S. Shibata, Macromolecules, 36, 961 (2003). 8) D. C. Harsh and S. H. Gehrke, J. Controlled Release, 17, 175 (1991). 9) B. G. Kabra, S. H. Gehrke, R. J. Spontak, Macromolecules, 31, 2166 (1998). 10) N. Kato and S. H. Gehrke, Rate-limiting steps in the volume change kinetics of fast responsive microporous gel ; Eds. N. Yui and K. Park, in Flash polymers and hydrogels: Understanding and designing fast-responsive polymeric systems, CRC Press, Boca Raton, FL., in press. 11) N. Kato and S. H. Gehrke, Colloids Surfaces B: Biointerfaces, submitted 12) Éder D.Oliveira, Samuel G. Hirsch, Richard J. Spontak, and Stevin H. Gehrke, Macromolecules, 36, 6189 (2003).