Design And Synthesis Of Novel Soft Composites From Physical Gels And Nanomaterials

Abstract

The present thesis entitled “Design and Synthesis of Novel Soft Composites from Physical Gels and Nanomaterials” deals with soft materials derived from low molecular weight gels and nanomaterials. Chapter 1 gives a general introduction and overview of the low molecular weight gel (LMOG) which forms the basis of the work. It delves with the history of research in physical gel field, design of different types of gelator molecules, their interesting self-assembly patterns, potential applications of these gelator molecules as well as challenges to design new gelator molecules. It also encompasses the relatively recent area of two component gel system to conveniently bypass the cumbersome synthetic protocol. The aspect of liquid crystallinity in the gel phase is also discussed to throw light on the pattern of assembly and potential uses of these materials. Towards the end there is a comprehensive discussion on the smart nanocomposites derived from LMOGs and nanomaterials. The design, synthesis and numerous applications of inorganic-organic hybrid composites are discussed. Chapter 2A describes the synthesis and characterization of a variety of fatty acid amides of different naturally occurring L-amino acids whose molecular structures are shown in Chart 2A.1. Some of them were found to form gels with various hydrocarbons. The gelation properties of these compounds were studied by a number of physical methods including FT-IR spectroscopy, X-ray diffraction, scanning electron microscopy (SEM), differential scanning calorimetry, rheology and it was found that gelation was critically dependent on the fatty acid chain length and nature of the amino acid. Among them, L-alanine based gelators were found to be the most efficient and versatile as they self-assemble into a layered structure to form the gel network. Mechanisms for the assembly and formation of gels from these molecules are discussed. (Structural formula) Chart 2A.1. Molecular structures of various fatty acid amides of different amino acids. Chapter 2B describes efficient gelation of both aliphatic and aromatic hydrocarbon solvents by a fatty acid amide, n-lauroyl-L-alanine (Chapter 2B.1). In addition, this compound was found to gelate the binary solvent mixtures comprised of aromatic hydrocarbon e.g. toluene and aliphatic hydrocarbon e.g. n-heptane. SEM and AFM showed that the fiber thickness of the gel assembly increases progressively in the binary mixture of n-heptane and toluene with increasing percentage of toluene. The self- Chart 2B.1. Molecular structure of the gelator. assembly patterns of the gels in individual solvents, n-heptane and toluene are however, different. The toluene gel consists of predominantly one type of morphological species while n-heptane gel has more than one species leading to polymorphic nature of the gel. The n-heptane gel is thermally more stable than the toluene gel as evident from the measurement using differential scanning calorimetry. The thermal stability of the gels prepared in the binary mixture of n-heptane and toluene is dependent on the composition of solvent mixture. Rheology of the gels shows that they are shear-thinning material and show characteristic behavior of soft viscoelastic solids. For the gels prepared from binary solvent mixture of toluene and n-heptane, with incorporation of more toluene in the binary mixture, the gel becomes a more viscoelastic solid. The time sweep rheology experiment demonstrates that the gel made in n-heptane has faster gel formation kinetics than that prepared in toluene. Chapter 2C describes lyotropic mesophase formation by organogels of different fatty acid amides of L-alanine in aromatic solvents. The helical assembly, characteristic of the cholesteric mesophase was found to exhibit reflection bands in circular dichroism spectra. The reflection bands corresponded to the pitch of the helical arrangement of the gelator molecules in the aromatic solvent. Transmission Electron Microscopy (TEM) showed presence of twist in the gel fibres. Polarising optical microscopy of the organogel exhibited weak birefringence confirming lyotropic nature of the assembly. Chapter 3 deals with synthesis and characterization of a new class of molecules with molecular structures shown in Chart 3.1. Among a variety of amino acid based molecules only alanine and serine based molecules were found to form translucent gels in aliphatic hydrocarbons such as n-heptane. TEM showed presence of fiber like structures for alanine whereas serine based gelator produces unique network like structures. SEM of the dried gels exhibited presence of three dimensional fibrous networks to spongy globular cauliflower like structures depending on the molecular structure of the gelators. Rheological studies of the organogels showed that they behave like typical LMOG gels. The oscillatory rheological studies demonstrated that the L-serine based gelator, 5 formed more viscoelastic solid like gel than that of L-alanine based gelator, 1 in n-heptane. Chart 3.1. Molecular structures of different amino acid derivatives from 3,4,5-tri-dodecyloxybenzoic acid scaffold. Chapter 4A presents design and properties of new nanocomposites from LMOG and metal nanoparticles (Chart 4A.1). The profound influence of nanoparticle (NP) incorporation into physical gels was evident from various microscopic and bulk properties. The interaction of nanoparticles with the gelator assembly was found to depend critically on the capping agent coating the nanoparticles. TEM showed long range Chart 4A.1. Molecular structures of the gelator and various AuNPs synthesized. directional assembly of the certain AuNPs along the gel fibers. SEM of the dried gels and nanocomposites indicated that the morphological transformation in the composite microstructures depended profoundly on the capping agent of the nanoparticle. Differential Scanning Calorimetry showed that gel formation from sol postponed to lower temperature with incorporation of AuNPs having capping agents which were able to interact with the gel fibers. Rheological studies indicated that the gel-nanoparticle composites exhibit greater rigidity as compared to the naked gel only when the capping agents were able to interdigitate into the gelator assembly. Also, very low percentage of the AuNPs incorporation could switch the cholesteric mesophase of gel assembly, as evident from circular dichroism. We have been able to define a relationship between materials and molecular properties via manipulation of the molecular structures of NP capping agents. Chapter 4B discusses the design and preparation of novel organogel-carbon nanotube composites by incorporation of single-walled carbon nanotubes (SWNT) into physical gels formed by an L-alanine based Low Molecular Mass Organogelator (Chart 4B.1). The gelation process and the properties of the resulting nanocomposites were found to depend on the kind of SWNTs incorporated in the gels. With pristine SWNTs, only a limited amount could be dispersed in the organogels. Attempted incorporation of higher amounts of pristine SWNTs led to precipitation from the gel. To improve their solubility in the gel matrix, a variety of SWNTs functionalized with different aliphatic and aromatic chains were synthesized (Chart 4B.1). Scanning electron microscope images of the nanocomposites showed that the texture and organization of the gel aggregates were altered upon incorporation of SWNTs. The microstructures of nanocomposites were found to depend on the kind of SWNTs used. Incorporation of functionalized SWNTs into the organogels depressed the sol to gel transition temperature, with the n-hexadecyl chain functionalized SWNTs being more effective than the n-dodecyl chain functionalized counterpart. Rheological investigations of pristine SWNT containing gels indicated that the flow of nanocomposites became resistant to applied stress at a very low wt-% of SWNT incorporation. Again, more effective control of flow behavior was achieved with functionalized SWNTs possessing longer hydrocarbon chains. This happens presumably via effective interdigitation of the pendant chains with the fatty acid amides of L-alanine in the gel assembly. Also, the helical cholesteric mesophase formed by the toluene gel could be switched to a layer stacked assembly by doping functional SWNT. Remarkably, by using a near IR laser irradiation at 1064 nm for a short duration (1 min) at room temperature, it was possible to selectively induce a gel-to-sol phase transition of the nanocomposites, while prolonged irradiation (30 min) of the organogel under identical conditions did not cause gel melting. Chart 4B.1. Molecular structures of the gelator and different functionalized SWNT synthesized. Chapter 5A presents design of two component hydrogels and their potential utilization as a template for metal nanoparticle synthesis. Among a variety of acids and amines (Chart 5A.1) only stearic acid or eicosanoic acid when mixed with di- or oligomeric amines in specific molar ratios form stable gels in water. The formation of such hydrogels depends on the hydrophobicity of the fatty acid, and also on the type of amine used. The gelation properties of these two component systems were investigated using electron microscopy, FTIR, 1H NMR spectroscopy, differential scanning calorimetry (DSC) and both single crystal and cast film X-ray diffraction. FTIR spectral analysis suggests salt formation during gelation. 1H NMR of the gels indicates that the fatty acid chains are immobilized in the gel state and when the gel is melted, these chains regain their mobility. Analysis of DSC data indicates that increase in spacer length in the di-/oligomeric amine lowers the gel melting temperature. Two of these gelator salts developed into crystals and structural details of such systems could be secured by single-crystal X-ray diffraction analysis. The structural information of the salts thus obtained was compared with the XRD data of the self-supporting films of those gels. Such analyses provided pertinent structural insight on the supramolecular interactions that prevail within these gelator assemblies. From the crystal structure it is confirmed that the multilayered lamellar aggregates exist in the gel and it also showed that only one plane of symmetry is present in the gel state. Finally, the hydrogel was used as a medium for the synthesis of silver nanoparticles. The nanoparticles were found to position themselves on the fibers and produce a long ordered assembly of gel-nanoparticle composite (Figure 5A.1). Chart 5A.1. Structures and abbreviations of different acids and amines checked for gelation. Figure 5A.1. TEM images of gel-Ag-NP composite. (a) Ag-NP synthesized in hydrogel of SA-IBPA (1:3.5), (b) Magnified images of Ag-NP preferentially residing on gel fibers. Chapter 5B demonstrates the aptitude of supramolecular hydrogel formation using simple bile acids e.g. lithocholic acid (LCA) in aqueous solution containing di- or oligomeric amines (Chart 5B.1). By variation of the choice of the amines in such mixture the hydrogelation properties could be modulated. However, replacement of LCA by cholic acid or deoxycholic acid resulted in no hydrogelation. FT-IR studies show that the carboxylate and ammonium residues of the two components are primarily involved in salt formation. This promotes further assembly of the components reinforced by continuous Chart 5B.1. Structures and abbreviations of different bile acids and amines checked for gelation. hydrogen bonded network leading to gelation. Electron microscopy shows that the morphology of the gels of two component systems which also depends strongly on the amine part. Variation of amine component from the simple ethanediamine (EDA) to oligomeric amine with lithocholic acid changes the morphology of the assembly from long one dimensional nanotubes to three dimensional complex structures. Single crystal X-ray diffraction analysis with one of the amine-LCA complexes suggested the motif of fiber formation where the amines participate with the carboxylate and hydroxyl moiety through H-bonding and electrostatic forces. The rheological properties of this class of two component system provide clear evidence that this system is a shear-sensitive hydrogel and the flow behavior can be modulated varying the acid-amine ratio. From small angle neutron scattering study, it becomes clear that loose gel from LCA-EDA shows scattering oscillation due to the presence of non interacting nanotubules while for gels of LCA with oligomeric amine the individual fibers come together to form complex three dimensional structures of higher length scale.(For structural formula pl refer the pdf file)