Introduction to liquid crystals and organogelators
1.13. The Gel state
In general, gels are viscoelastic solid-like materials comprised of an elastic cross-linked network and a solvent, which is the major component. Thus by weight, the gels are mostly liquid, but they behave like solids due to a three dimensional cross-linked network within the liquid. In other words, the gels are a dispersion of molecules of a liquid within a solid in which the solid is the continuous phase and the liquid is the discontinuous phase. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state. The three-dimensional cross-linked network within the liquid may result from physical bonds (physical gels) or chemical bonds (chemical gels). Gelation relies on a balance between solvent-gelator and gelator-gelator interactions.36c,37b-g The non-covalent interactions responsible for gelation are mainly the H-bonding, π-π stacking, donor-acceptor36 interactions, metal coordination, solvophobic
(a)
(b)
(c)
(d)
TH-1882_136122026
Chapter 1
forces (hydrophobic forces for gels in water) and van der waals interactions. Most of the naturally occurring gelators are macromolecular and they form gels by physical cross- linking (usually H-bonding37). Such macromolecules include gelatin, collagen, agar, starch etc. Gels derived from synthetic compounds can be subdivided on the basis of their constitution into macromolecular (polymer) and molecular. When the gels are formed by strong chemical bonds, they cannot be redissolved and are thermally irreversible (e.g.
polyester, polyamide, poly(vinyl alcohol), polyethylene) whereas gels formed by weak noncovalent interactions (physical entanglements) are reversible (e.g. polyacrylate, polymethacrylate). Depending on the dispersion medium or the sol, gel can be classified as hydrogels, organogels and aerogels (Fig. 1.11). Since this thesis deals with organogels, our discussion is restricted to this topic only.
Organogels or organic gelators are non-crystalline, non-glassy, thermoplastic solid materials composed of a liquid organic phase entrapped in a three-dimensionally, cross- linked network. The liquid can be, for example, an organic solvent, mineral oil or vegetable oil. The solubility and particle dimensions of the structures are important characteristics for the elastic properties and firmness of the organogel. Organogels are frequently found in food processing, pharmaceuticals and cosmetics.
Figure 1.11. Schematic representation of general classification of gels.
Gels
Natural gel Artificial gel
Hydrogel Aerogel
Source
Organogel
Chemical gel Physical gel
Constitution
Macromolecular Supramolecular
Crosslinking
Medium
Chapter 1
21 Gels of a low molecular mass compound are usually prepared by heating the gelator in an appropriate solvent and cooling the resulting isotropic supersaturated solution to room temperature. When the hot solution is cooled, the molecules start to condense and three situations are possible: (i) a highly ordered aggregation giving rise to crystals, (ii) a random aggregation resulting in an amorphous precipitate or (iii) an aggregation process intermediate between these two, yielding a gel (Fig. 1.12). The process of gelation involves self-association of the gelator molecules to form long, polymer-like fibrous aggregates, which get entangled during the aggregation process forming a matrix that traps the solvent mainly by surface tension. This process prevents the flow of solvent under gravity and the mass appears like a solid. Determination of the sol-to-gel phase transition temperature (TSG or Tgel) is necessary.38d It is generally determined by a “dropping-ball method” where a small ball will be placed on top of the gel in a test tube and slowly heated in an oil bath at a fixed heating rate. Tg is defined as the temperature, when the ball had reached the bottom of the test tube.38 Another important factor is critical gelation concentration (CGC) which means the minimum concentration is required to form a stable gel at room temperature38band it is well known in literature that when the CGC is below or equal to 1 weight% it is called a supergelator.39 Xerogels are solids formed from gels that do not suffer from shrinkage in the process. If the liquid system is almost entirely or wholly removed from the gel, then it will be called as a xerogel. Xerogels generally retain a high degree of porosity. Rubber and gelatin are examples of xerogels.
Figure 1.12. Schematic representation of the self-assembly of low molecular weight gelators into one dimensional aggregates and the subsequent formation of an entangled network.43c
Cool Heat
Solution Assembly Gel
Gelator Solvent Aggregates Fibers
TH-1882_136122026
Chapter 1
LCs can be gelated with chemical or physical 3D network strucutures. The LC chemical gels40 have been prepared by polymerisation of LC or non-LC monomers in liquid crystals. Liquid-crystalline physical gels are obtained by the self-assembly of fibrous solid networks of gelators in liquid crystals (Fig.1.13).41 Liquid crystals become soft solids by gelation, keeping their stimuli-responsive properties. The formation of anisotropic phase-separated structures leads to the induction of new functions and the enhancement of the properties. The LC physical gels are normally formed by thermal processes, in which two independent transitions, the sol-gel transition of a gelator and the isotropic-anisotropic transition of a LC, are observed.
Due to the combination of two components which form phase separated anisotropic structures, LC physical gels exhibit enhanced or induced electronic, photochemical and electro-optical properties.41 Furthermore, these gels can be exploited as a material for electro-optical memory,41f which can be attained by controlling reversible aggregation process under electric fields. The enhanced hole mobility41g was reported with the introduction of fibrous aggregates into triphenylene-based columnar liquid crystals. Similarly, the incorporation of photochromic azobenzenes or electroactive tetrathiafulvalenes into the chemical structures of gelators leads to the preparation of ordered functional materials.41h
Figure 1.13. Schematic illustration of (a) chemical gels; (b) physical gels; (c) liquid-crystalline (LC) chemical gels and (d) LC physical gels (Reproduced from reference [46] with the permission from Royal Chemical Society).
Solvent
Gelator Liquid crystal
(a) (b)
(c)
Covalently bonded polymer (d)
Chapter 1
23 An efficient methodology to elucidate the nature of an organogel is to begin with a characterization of its rheological and thermodynamic properties. Rheology is the study of the flow of matter, primarily in a liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force. Rheology offers a convenient method to measure the flow properties of gel like materials and facilitates the classification between “strong” and
“weak” gel types.42 Dynamic rheological measurements are done to establish the elastic nature of the gel samples that involved strain sweep, angular frequency sweep and step- strain measurements and it is characterized by G*(ω) = G’(ω) + iG’’(ω), where G’ is the storage modulus, G’’ is the loss modulus and ω is the angular frequency. At the microscopic level, the structures and morphologies of supramolecular gels have been investigated by conventional imaging techniques such as scanning electron microscope (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) while thermal and mechanical studies are used to understand the interactions between these structures. However, at the nanoscale, X-ray diffraction, small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS) are required to elucidate the structures of supramolecular gels.
Application of organogels in drug delivery can be illustrated with the example of active enzymes and bacteria entrapped in apolar gels of gelatin. Highly concentrated oil- in-water (O/W) emulsions and various gels have been used as aviation fuels and in cosmetics while highly concentrated water-in-oil (W/O) emulsions are used in some explosives technologies.43a Many aqueous gels43b are employed for macromolecular separations, protein crystallization etc. While there are wide ranging applications for organogels in general, π-gels (formed due to π-π stacking interaction mainly) are of particular interest with respect to electronic and photonic applications.44a,b The special interests in π-gels are associated with their inherent electronic properties such as fluorescence,45 charge carrier mobilities,44d electronic conductivities,44d etc. Therefore, in recent years, considerable effort has been put in by the scientific community to the design of π-gels for specific application in the field of advanced materials.44,45 The present thesis also deals with the π-gels formed by some of the polycatenar mesogens.
TH-1882_136122026
Chapter 1