ABSTRACT
Kinetics of hydrolysis induced swelling as well as pure swelling of a novel class of pH responsive biocompatible polymer gels was studied. Poly(aspartic acid) gels were prepared by hydrolysis of chemically cross-linked poly(succinimide) networks. The volume change of spherical gels of different size, measured at constant pH, proceeds in three distinct processes: solvent exchange, hydrolysis and swelling. It turned out that pure swelling as well as the swelling coupled with hydrolysis can be described by the Tanaka-Fillmore-Peters-Candau theory. The relaxation times as well as the cooperative diffusion coefficient of the network chains were determined. It was found that the initial condition of the swelling makes its influence felt neither on the relaxation time, nor on the cooperative diffusion coefficient.
Subject(s)
Aspartic Acid/analogs & derivatives , Gels/chemistry , Peptides/chemistry , Aspartic Acid/chemistry , Biocompatible Materials/chemistry , Hydrogen-Ion Concentration , Hydrolysis , KineticsABSTRACT
In order to obtain nontoxic functional polymer gels for biomedical applications, chemically crosslinked poly(aspartic acid) gels have been prepared using 1,4-diaminobutane as crosslinker. The presence of COOH and amino groups on the network chains renders these gels pH sensitive. Due to the specific hydrophobic-hydrophilic balance, these gels show a significant volume transition at a well-defined pH close to the pK value of uncrosslinked poly(aspartic acid). Since the magnitude of volume change critically depends on the degree of crosslinking, it is an important task to determine the topological characteristics of these networks. A novel method based on potentiometric acid-base titration has been developed to assess the crosslinking ratio, excluding physical crosslinks and entanglements. It turned out that only 25% of all crosslinker molecules forms real crosslinks between the poly(aspartic acid) chains; the rest react with one of its functional groups and forms short pendant side chains. At a nominal crosslinking ratio of 0.1, the number average molecular mass between crosslinks is found to be M(c) = 2300.
Subject(s)
Biocompatible Materials/chemistry , Cross-Linking Reagents/chemistry , Electrochemistry/methods , Gels/chemistry , Materials Testing/methods , Peptides/chemistryABSTRACT
Chemically cross-linked poly(aspartic acid) (PASP) gels were prepared by the hydrolysis of poly(succinimide) (PSI). The latter was prepared by thermal polycondensation of aspartic acid. The PSI chains were cross-linked by natural amines and amino acid derivatives such as putrescin, spermine, spermidine, lysine and cystamine to obtain biodegradable, biocompatible, amino acid-based hydrogels. The volume of the synthesized unhydrolyzed PSI gels changes abruptly at a well-defined pH that results in ring opening, while the hydrolyzed gels show a volume phase transition around the pK values of PASP. The unidirectional stress-strain behavior of the gels as well as the dependence of equilibrium swelling degree on the pH was carefully studied and the most important network parameters were determined by a modified version of the Brannon-Peppas-Peppas theory.
