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A series of linear segmented poly(urethane acyl-semicarbazides) of different compositions were synthesized from polypropylene glycol (Mn ∼ 1000 and 2000), hexamethylene diisocyanate and cyclohexanedicarboxylic acid 1,4 dihydrazide. The polymers exhibited thermoreversible gelation at a minimum gelation concentration of 25 mg/mL in DMSO in about 25 min. Among the comparisons of varying NCO/OH ratios, PPG1000 based polyurethanes with NCO/OH ∼ 2.5 and 3 and PPG2000 based systems with NCO/OH ∼ 3.0 and 3.5 exhibited gelation. IR studies on the solutions and gels (in DMSO) revealed the extensive H-bonding between the chains in gel state. Scanning electron micrographs of the xerogel revealed a 3D-network structure – higher hard segment content (P1000-1:3.5) led to a fibrous morphology and lower hard segment content (P2000-1:3) led to a microsphere like architecture. The phase separation induced gelation mechanism of self-assembly and effect of the stoichiometry on the morphology were explained based on the correlation between DSC, WAXS and FE-SEM data. The xerogels of the self-assembled polymers described in this work demonstrate a remarkable ability to remove dye molecules selectively, rapidly and efficiently. The gels retain their original structure and performance even after they are recycled 20 times which was quantified by UV–vis studies.

Herein we report the anion tuning and dye adsorbing properties of aromatic bis(acyl-semicarbazides) carrying aromatic core and substituents of varying alkyl chain length. Gelator with a specific spacer length (C10) was chosen to study the effect of stimuli such as temperature, ultrasound and anions on gel strength, mode of interaction and morphology. A three dimensional network structure composed of fibers of varying diameters were observed for gels obtained by thermal treatment whereas a lamellar aggregation resulted upon ultrasound treatment as noticed in SEM studies. Ultrasound induced gels (USIG) exhibited higher gel-sol transition temperature and strength than temperature induced gel as observed from dynamic oscillatory studies, FT-IR and XRD due to the differences in the modes of aggregation. The gels could be tuned in the presence of anions at different concentrations and addition of 5 equivalents of F− and AcO− induced gel-sol transition. In addition, these organogels adsorbed dyes such as crystal violet and rhodamine-B from water every effectively which was confirmed by visual method and quantified by UV–vis spectrophotometry.

Segmented poly(urethane acyl semicarbazides) carrying aromatic groups in the hard segments were synthesized by two step polymerization technique using commercially available, cost effective raw materials. The resulting polymers with specific ratios of NCO/OH were found to gel organic solvents like dimethylformamide, dimethylacetamide and N -methylpyrrolidone and their corresponding gels were thermoreversible. Tuning of the gelation behaviour and gel properties have been achieved by varying the polyol molecular weight (polypropylene glycol - 1000 and 2000) and NCO/OH ratio. Influence of hard segment content on the gel-sol transition as studied by DSC showed that increasing hard segment content led to stronger gelator-gelator interactions and hence higher T gel . Broad tanδ peaks of films of xerogels reflected a network kind of structure due to physical crosslinks in the self-assembled state in DMA analysis. The films of xerogels of PPG 1000 based gelators exhibited higher tensile strength but lower elongation compared to PPG 2000 based gelators. Viscoelastic nature of the supramolecular elastomeric gels was confirmed by dynamic oscillatory studies and stability of the gel was found to increase with increase in hard segment content. H-bonding between the urethane moieties as confirmed by IR spectroscopy and hydrophobic interactions between the PPG units in the soft segments lead to a phase separated structure. Hence phase separation induced gelation mechanism has been proposed and supported by SEM images and XRD data. Both the gelators and xerogels had capability of adsorbing crystal violet from their aqueous solutions and desorbing in organic solvents like acetone and methanol.

Low molecular weight biscarbamate organogelators (LMWBGs) with simple molecular structures that could gel different types of organic solvents were synthesized. The LMWBGs were composed of a long hydrophobic tail of linear fatty alcohol (C8–C18) and an aromatic core and could gel organic solvents such as xylene, toluene, NMP, cyclohexanol and chlorobenzene at a concentration of 10–15 mg/mL. Gelation studies indicated that the alkyl chain length residues did not affect the gelation time which was generally around 10–15 min leading to opaque gels in all the solvents. The gels in p-xylene were studied by FTIR spectroscopy and differential scanning calorimetry (DSC) for the nature of the interactions between the gelators and the solvent. FTIR spectroscopic studies revealed that hydrogen bonding and van der Waals interactions were the driving forces for the formation of the gels, while solid-state UV–visible studies revealed the existence of π–π interaction in the xerogel. The gel melting temperatures were found to decrease and then increase with increasing alkyl chain length as observed in DSC. The microscopic observations (FE-SEM) suggested that the gelator molecules self-assembled leading to nano- and microfibres in the turbid gel, and as the alkyl chain length increased, the width of the fibres increased. Rheological studies evinced the viscoelastic nature of the soft gels viscoelasticity increased with increasing gelator concentration, while fluidity increased with increasing chain length. The X-ray diffraction analysis revealed that in the xerogels from p-xylene the molecules aggregated into a layered structure. This study on the influence of alkyl side chains shows that hydrogen bonding and van der Waals interactions play a significant role in the morphology of such self-assembled structures.

The current study reports the one-step synthesis and gelation properties of cyclohexane-based bis(acyl-semicarbazide) gelators with an additional -NH group incorporated into urea moieties and carrying hydrophobic chains of varying length (C8-C18). The gels exhibited thermoreversibility and could be tuned in the presence of anions at different concentrations in addition their the ultrasound-responsive nature, thus making them multi-stimuli-responsive. The combined experimental and computational study on these gels reveals that the balance between two noncovalent interactions, viz., hydrogen bonding between the amide groups in acyl-semicarbazide moieties and van der Waals forces between long hydrocarbon tails, is found to be the determining factor in the process of organogelation. A systematic increase in alkyl chain length leads to equilibrium between these two types of noncovalent forces that is manifested in the spectral and thermal properties of the gels. The H-bonding interactions dominated up to a certain chain length, and further increases in the alkyl chain length led to increased van der Waals interactions as observed by IR, XRD, and thermal studies. Computational calculations were carried out on dimer structures of C8-C18 to understand the variation in noncovalent forces responsible for aggregate formation in the gel state as a function of the alkyl chain length. The results indicate that both intermolecular and intramolecular hydrogen bonding stabilize the aggregate structures. Supramolecular aggregation in the gel state led to the viscoelastic nature of the gels, and the addition of anions led to the disruption of self-assembly, which was studied by rheology.