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Structure and swelling behavior of novel amphoteric cryogels

Sarkyt Kudaibergenov, Zhansaya Sadakbayeva, Dmitriy Berillo, Igor Galaev

Last modified: 2010-05-17

Abstract


Structure and swelling behavior of novel amphoteric cryogels

 

Zhansaya Sadakbayeva1,2, Dmitriy Berillo2, Igor Galaev3,

Sarkyt Kudaibergenov1,2

 

1Laboratory of Engineering Profile, Kazakh National Technical University, Satpaev Str. 22, 050013, Almaty, Kazakhstan, ipmt-kau@usa.net

2Institute of Polymer Materials and Technology, Panfilov Str. 52/105, 050004, Almaty, Kazakhstan, skudai@mail.ru

3Biotechnology Division, DSM Company, the Netherlands

 

Introduction

 

Cryogels are gel matrices that are formed in moderately frozen solutions of monomeric and polymeric precursors [1-3]. The morphology of macroporous cryogels is determined by solvent crystallization when the temperature is kept below the freezing point of solvent. According to cryopolymerization concept the freezing of the initially homogeneous system results in crystallization of pure solvent (water) and accumulation of monomers and initiators in unfrozen micro zones (so-called “cryo-concentration”). The polymerization reaction proceeds in this non-frozen part of the reaction mixture. Water crystals grow in the course of freezing and interconnections with other crystals take place until a continuous system of porous is formed. Thawing of the system leads to formation of a monolithic gel matrix with continuous macroporous channels filled with liquid solvent. The gel has a sponge-like morphology and pore size of 10-100 µm. A system of large interconnected pores is a main characteristic feature of cryogels. The pore system in such sponge-like gels ensures unhindered convectional transport of solutes within the cryogels, contrary to diffusion of solutes in traditional homophase gels. In the present communication we report for the first time the synthesis and characterization of amphoteric cryogels that will have potential applications for the encapsulation of cells, immobilization of enzymes, protein (or metal ions) separation and as drug delivery systems.

 

Materials and Methods

 

Monomers and initiators – acrylamide (AAm, 99% purity), allylamine (AA, 99% purity), methacrylic acid (MAA, 99% purity), N,N,N’,N’-tetramethylethylenediamine (TMED), ammonium persulfate (APS), and crosslinking agent N,N’-methylenbisacrylamide (MBAA) were purchased from Aldrich and used without further purification. Amphoteric cryogels were synthesized as follows. Mixture of AA, MAA, and AAm containing various amounts of MBAA was dissolved in 5 mL of deionized water and degassed under vacuum for about 5 min to eliminate the dissolved oxygen. After addition of TMED the solution was cooled in an ice bath for 4-5 min. Then aqueous solution of APS cooled in an ice bath for 4-5 min was added and the reaction mixture was stirred for 1 min. Then the reaction mixture was placed into plastic 5 mL syringe with closed outlet at the bottom. The solution in syringe was frozen within 10 min at –12 °C and was kept frozen during 48 h. After completion of the reaction the sample was thawed at room temperature. The prepared cryogel sample was washed out by distilled water then dried in vacuum to constant weight at room temperature. Thus a series of amphoteric cryogels with molar ratio of AAm:AA:MAA = 80:10:10; 60:20:20, 40:30:30, 20:40:40 and 0:50:50 mol/mol/mol were synthesized. The swelling capacity of cryogel samples as a function of pH was evaluated from the height measurements. The microstructure of the samples was investigated with Scanning Electron Microscopy (JEOL, JSM5800). The flow-rate of water passing through the cryogel samples was determined by the procedure described in [4].

 

Results and Discussion

 

A series of novel amphoteric cryogels based on allylamine (AA), methacrylic acid (MAA) and acrylamide (AAm) have been synthesized to our knowledge for the first time. They were characterized by potentiometric titration, IR spectroscopy and SEM. Cross section of cryogels shows porous structure with size ranging from 50 to 200 mm. Longitudinal section of cryogels shows the interconnected channels. The interconnected system of large pores makes amphoteric cryogels promising materials as new tailor-made matrix for encapsulation and immobilization of drugs, enzymes, cells for application in medicine and biotechnology, chromatographic matrices for purification and separation of proteins [5,6]. Dynamics of water flowing through cryogel samples with diameter 4-5 mm and height 8-9 mm were determined (Table 1).  It is seen that increasing of the content of acidic and basic monomers leads to exponential decreasing of water flowing. This is probably connected with decreasing of pore size of cryogels due to formation of ionic contacts between oppositely charged groups of macromolecules. For samples AAm-AA-MAA (80:10:10) crosslinked at [MBAA] = 10, 6 and 4 weight % exponential increase of water flowing is observed that reveals increasing of pore size of cryogels with decreasing of crosslinker concentrations.

 

Table.1 Dynamics of water flow-rate through amphoteric cryogels

Amphoteric cryogels based on AAm:AA:MAA, mol.%

Water flow-rate,

mL/min

80:10:10

1.2

60:20:20

0.35

40:30:30

0.18

20:40:40

0.05

0:50:50

0.02

 

Amphoteric hydrogels exhibit the isoelectric points (IEPs) that are specific for amphoteric macromolecules [7]. Swelling dynamics of amphoteric cryogels have been determined as a function of pH. It was shown that the swelling degree of amphoteric cryogels is minimal at the IEPs.

 

References

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