Structure-Property Relations of Thermoreversible Macromolecular Hydrogels

Abstract

Thermoreversible hydrogels of agarose and gelatin have been studied as models for more complex polysaccharide and protein biogels. In addition a study is made of the gelation of poly(vinyl alcohol) (PVA) in ethylene glycol-water mixtures. The structures are characterized by means of optical rotation and low-angle light scattering. The time-dependent and equilibrium mechanical behavior is measured in unilateral compression. Gelatin gels are found to be randomly cross-linked by the formation of "collage-fold like" junction zones. The viscoelastic relaxation and equilibrium stress are both rubberlike, provided junction breakdown and bacterial degradation is eliminated by a suitable extrapolation procedure. Agarose gels are not rubberlike in their mechanical behavior. Their structure consists of a regular array of micron-sized spherical polymer-rich regions probably formed by a nucleation free-(spinodal) phase separation. Hysteresis in the optical rotation indicates the existence of a wide spectrum of junction zones, present in agarose but absent in gelatin gels. The PVA gels are formed via an initial liquid-liquid, nucleated phase separation, followed by syneresis caused by the formation of small crystallites in the polvmer-rich regions. The findings show that the structural makeup of hydrogels is strongly reflected in their time-dependent and equilibrium mechanical behavior, and that a structural model based on randomly cross-linked Gaussian chains is often times inadequate.Macromolecular gels that occur in nature are frequently reversible, i.e., the chains are not chemically cross-linked but exhibit certain junction zones, depending on pH, temperature, and electrolyte concentration. It is the purpose of the present article to investigate the relation between the structure and mechanical properties of such hydrogels, by considering three model systems, viz., gelatin, agarose, and poly(vinyl alcohol). In all three systems an aqueous polymer solution is brought to gelation by lowering the temperature. We will show that the resulting gels vary widely in structure and properties.Gelatin is prepared by partial hydrolytic degradation and disorganization of collagen.28 In the native state collagen occurs in organized fibrils of triple helices of collagen molecules, each molecule being about 100,000 in molecular weight.2b The molecules are rich in glycine, proline, and hydroxyproline sequences and are twisted together in the triple-helical "collagen fold" in which the individual molecules occur in the poly(L-proline II) (trans)

Manufacturing of agarose-based chromatographic adsorbents – Effect of ionic strength and cooling conditions on particle structure and mechanical strength

“…The gel is formed when an infinite extensive three-dimensional network of agarose fibres (consisting of helices) develops [4]. At this state the material is characterized by a co-continuous macroporous morphology with polymer-rich regions (fibres) and solvent-rich regions (pores holding water), and therefore the gelation can be interpreted as a phase separation [5].…”

“…The gel is formed when an infinite extensive three-dimensional network of agarose fibres (consisting of helices) develops [4]. At this state the material is characterized by a co-continuous macroporous morphology with polymer-rich regions (fibres) and solvent-rich regions (pores holding water), and therefore the gelation can be interpreted as a phase separation [5].Because of its physical and chemical stability, neutral charge, hydrophilic character, open structure [6, 7] and biocompatibility, agarose has been extensively used in chromatographic separation of biomolecules [8], cell microencapsulation [9] and food additives [10]. Following functionalization with the appropriate chemistry a selective porous adsorbent can be produced for the purification of biomolecules both in packed beds and, after inclusion of densifiers, in fluidized beds [11].…”

Molecular structures in biopolymers sols and gels

“…The present knowledge of the molecular structure of polysaccharides either in the sol state or in the gel state has been gained from polarimetry [1,2] together with X-ray diffraction [3,4]. Polarimetry experiments led to the conclusion that the chains had to behave as random flexible coils in the sol state.…”

Structure and dynamics of poly(vinyl alcohol) gels in mixtures of dimethyl sulfoxide and water

“…[7][8][9][10][11][12][13] Hydrogels of PVA can be obtained from the aqueous solutions and are often used for tissue engineering. 14,15 The properties of PVA gels are well known to depend on the solvents and preparation methods.…”