Formation Of Hydrophobic Domains Research Articles

A study of hydrophobic domain formation of polymeric drug precipitation inhibitors in aqueous solution

Despite the widespread use of polymers as precipitation inhibitors in supersaturating drug formulations, the current understanding of their mechanisms of action is still incomplete. Specifically, the role of hydrophobic drug interactions with polymers by considering possible supramolecular conformations in aqueous dispersion is an interesting topic. Accordingly, this study investigated the tendency of polymers to create hydrophobic domains, where lipophilic compounds may nest to support drug solubilisation and supersaturation. Fluorescence spectroscopy with the environment-sensitive probe pyrene was compared with atomistic molecular dynamics simulations of the model drug fenofibrate (FENO). Subsequently, kinetic drug supersaturation and thermodynamic solubility experiments were conducted. As a result, the different polymers showed hydrophobic domain formation to a varying degree and the molecular simulations supported interpretation of fluorescence spectroscopy data. Molecular insights were gained into the conformational structure of how the polymers interacted with FENO in solution phase, which apart from nucleation and crystal growth effects, determined drug concentrations in solution. Notable was that even at the lowest polymer concentration of 0.01 %, w/v, there were polymer-specific solubilisation effects of FENO observed and the resulting reduction in apparent drug supersaturation provided relevant knowledge both from a mechanistic and practical perspective.

A smart superwetting PET-Anthocyanin membrane for pH monitoring in water and emulsion separation

Smart superwetting membranes with anti-oil pollutions and process water pH monitoring show great potential before downstream processing. Herein, a flexible intelligent monitoring membrane with strong anti-oil fouling properties was developed by utilizing a PET-Anthocyanin mixed solution through electrospinning. Specifically, using waste PET plastic bottles as raw materials, a dual-solvent system is used to dissolve the PET, and then anthocyanins are added to blend to form an electrospinning solution. The obtained A/PET showed excellent porous morphology and wettability, and its underwater oil contact angle could reach up to 150°. The introduction of anthocyanins (5 %) rich in hydroxyl functional groups reduced the formation of hydrophobic domains and promoted the hydration layer on the membrane surface, showing excellent oil resistance property. Furthermore, the stress–strain test of the A/PET5 membrane shows that the strain is significantly enhanced after the yield phenomenon, which shows that the membrane has a flexible mechanical property. Most importantly, based on the pH-sensitivity of anthocyanins, it can be intuitively found that color response changes from the original pink to light pink, and then to white, showing excellent monitoring the pH of the continuous phase. Moreover, the developed membrane possesses water flux of 7440 L m-2h−1 and oil rejection up to 99.4 % for O/W emulsions, surpassing that of the superhydrophilic PET base membrane (956 L m-2h−1, 99.3 %). Briefly, this work provides a novel multifunctional smart membrane with excellent anti-fouling, flexible mechanical, and outstanding separation performance, which can visually monitor the separation process of oily wastewater.

Toward a Better Understanding of the Gelation Mechanism of Methylcellulose via Systematic DSC Studies.

A methylcellulose (MC) is one of the materials representatives performing unique thermal-responsive properties. While reaching a critical temperature upon heating MC undergoes a physical sol-gel transition and consequently becomes a gel. The MC has been studied for many years and researchers agree that the MC gelation is related to the lower critical solution temperature (LCST). Nevertheless, a precise description of the MC gelation mechanism remains under discussion. In this study, we explained the MC gelation mechanism through examination of a wide range of MC concentrations via differential scanning calorimetry (DSC). The results evidenced that MC gelation is a multistep thermoreversible process, manifested by three and two endotherms depending on MC concentration. The occurrence of the three endotherms for low MC concentrations during heating has not been reported in the literature before. We justify this phenomenon by manifestation of three various transitions. The first one manifests water–water interactions, i.e., spanning water network breakdown into small water clusters. It is clearly evidenced by additional normalization to the water content. The second effect corresponds to polymer–water interactions, i.e., breakdown of water cages surrounded methoxy groups of MC. The last one is related to the polymer–polymer interactions, i.e., fibril hydrophobic domain formation. Not only did these results clarify the MC crosslinking mechanism, but also in the future will help to assess MC relevance for various potential application fields.

The structuring effect of the alkyl domains on the polar network of ionic liquid mixtures: a molecular dynamics study.

By using molecular dynamics simulations, we investigate the structural and dynamic properties of mixtures of 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, [C1C1im][Tf2N] and 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C12C1im][Tf2N] (also C1 and C12 in short). Such mixtures feature an imidazolium bistriflimide salt with a very short alkyl chain, not giving rise to any nano-segregation as a pure component, with another one with a longer alkyl chain that exhibits a substantial nano-segregation as a pure liquid. As the mole fraction of the long-chain component C12 is increased, the so-called pre-peak of the structure factor S(q), occurring in the region 1-3 nm-1, shows a shift to higher values of the wavevector q, mirroring a decrease of the corresponding correlation length. Moreover, the intensity of the pre-peak strongly increases with the C12 concentration. These results are in very good agreement with experimental X-ray scattering data in the literature. On the other hand, the diffusion of the ions is found to exhibit a simple behaviour consistent with the increased viscosity of the mixture, and these results are also in good agreement with NMR experimental data from the literature. The simulation results are rationalized as caused by a structuring effect, similar to the hydrophobic effect, of the alkyl domains of the C12 component dissolved in the "solvent" represented by the C1 cation, the Tf2N- anion and the C12 cation head. In short, the exclusion of the alkyl chains from the polar network, a process mostly governed by electrostatic interactions, favours the formation of hydrophobic domains, which in turn exert a structuring effect on the ions of the polar domains, favouring a stronger ionic interaction. This is finally reflected in a shorter correlation length and a higher intensity of the pre-peak of the structure factor S(q) as the C12 mole fraction is increased. At variance with the microscopic structure, the diffusion of all three types of ions is not strongly influenced by the nano-segregation and is essentially dependent on the viscosity of the mixture.

Origins of Structural Elasticity in Metal-Phenolic Networks Probed by Super-Resolution Microscopy and Multiscale Simulations.

Metal-phenolic networks (MPNs) are amorphous materials that can be used to engineer functional films and particles. A fundamental understanding of the heat-driven structural reorganization of MPNs can offer opportunities to rationally tune their properties (e.g., size, permeability, wettability, hydrophobicity) for applications such as drug delivery, sensing, and tissue engineering. Herein, we use a combination of single-molecule localization microscopy, theoretical electronic structure calculations, and all-atom molecular dynamics simulations to demonstrate that MPN plasticity is governed by both the inherent flexibility of the metal (FeIII)-phenolic coordination center and the conformational elasticity of the phenolic building blocks (tannic acid, TA) that make up the metal-organic coordination complex. Thermal treatment (heating to 150 °C) of the flexible TA/FeIII networks induces a considerable increase in the number of aromatic π-π interactions formed among TA moieties and leads to the formation of hydrophobic domains. In the case of MPN capsules, 15 min of heating induces structural rearrangements that cause the capsules to shrink (from ∼4 to ∼3 μm), resulting in a thicker (3-fold), less porous, and higher protein (e.g., bovine serum albumin) affinity MPN shell. In contrast, when a simple polyphenol such as gallic acid is complexed with FeIII to form MPNs, rigid materials that are insensitive to temperature changes are obtained, and negligible structural rearrangement is observed upon heating. These findings are expected to facilitate the rational engineering of versatile TA-based MPN materials with tunable physiochemical properties for diverse applications.

Surface-controlled galvanic replacement for the development of Pt-Ag nanoplates with concave surface substructures

Galvanic replacement, a spontaneous redox reaction arising from contact between a nanotemplate and a replacing ion, has been widely used for its convenience, stability, and ability to form various complex nanostructures. In this study, the two-dimensional nanosurface adsorption of polymeric micelles is performed for substructure formation through advanced galvanic replacement strategies. Polymeric micelles adsorbed on the nanotemplate surface form a nano-array structure of hydrophobic and hydrophilic domains, resulting in surface controlled galvanic replacement and the formation of concave surface substructures. The hydrophobic domain formation and blockage region expansion due to nanotemplate curvature and solvent composition contributes greatly to the surface substructure formations. The results can be summarized as a successful attempt to design the nanostructure by coordinating all potential factors involved in conventional galvanic replacement to create an unprecedented high-complexity nanostructure. Manufactured Pt-Ag nanoplates with concave surface substructures exhibited the successful hydrogen evolution electrocatalytic efficiency as demonstration. The Pt-Ag nanoplates with concave surface substructures exhibit the enhanced HER activity in strong acidic solutions with a low Tafel slope. The enhancement of the HER performance can be attributed to concave surface substructures, which greatly improve the edge sites and defects, resulting in a high density of exposed active sites. The galvanic replacement-based material design strategy demonstrated here exemplifies a novel and versatile approach to developing electrocatalysts with high activity and durability for advanced electrochemical water-splitting systems.

Formation of Hydrophobic Domains on the poly(MPC-co-Dodecyl Methacrylate)-Coated Surface Recognized by Macrophage-like Cells

Copolymers comprising 2-methacryloyloxyethyl phosphorylcholine (MPC) and hydrophobic methacrylic esters were used as biomembrane-mimetic polymers to provide blood compatibility. In the present study, we compared the surfaces coated with two MPC polymers with different alkyl groups, namely, poly(MPC-co-butyl methacrylate) (PMB) and poly(MPC-co-dodecyl methacrylate) (PMD), to clarify the effect of their hydrophobic units. Various substrates, such as poly(ethylene terephthalate), polycarbonate, polypropylene, acrylonitrile-butadiene-styrene copolymer, and stainless steel, were coated with ethanol solutions containing various concentrations ofPMD or PMB. The solubility of PMD in ethanol changed depending on the water content. Scanning probe microscopy and rhodamine 6G staining revealed heterogeneous microstructures on the PMD-coated surface but not on the PMB-coated surface. Adhesion of various cells was efficiently suppressed by the PMD coating at lower concentration than the PMB coating, except regarding the adhesion of macrophage-like RAW264.7 cells. Our results suggest that the dodecyl groups in PMD increased its affinity for the substrates and simultaneously induced the formation of hydrophobic domains recognized by RAW264.7 cells.

Dynamics of thermoresponsive conetwork gels composed of poly(ethylene glycol) and poly(ethyl glycidyl ether-co-methyl glycidyl ether)

The dynamics of thermoresponsive conetwork gels consisting of poly(ethylene glycol) (PEG) and poly(ethyl glycidyl ether-co-methyl glycidyl ether) (PEMGE) (PEG-PEMGE gel) was systematically investigated as a function of temperature and the mole fraction of the thermoresponsive modules (PEMGE) r using dynamic light scattering (DLS). The PEG-PEMGE gels were prepared by end-linking of four-armed hydrophilic modules (Tetra-PEG) and four-armed lower critical solution temperature (LCST)-type modules (Tetra-PEMGE) in water by the molar ratio of (1 − r): r, where r was varied from 0.1, to, 0.2, 0.3, and 0.4. The dynamics of the conetwork gels was discussed with the ensemble-average of the field correlation function, gE(1)(τ) , which was classified to three temperature regions: at low temperatures below LCST of PEMGE (≈23 °C) (region I), the dynamics was insensitive to r. On the other hand, strong r-dependence was observed for temperatures ≥ LCST (regions II and III). By approaching the LCST (region II), the slow dynamics component in gE(1)(τ) became more pronounced due to formation of hydrophobic domains. Additionally, the power-law exponent of the slow mode decreased in region II (near the LCST). In region III, the slow dynamics disappeared as a result of the further growth and immobilization of the hydrophobic domains.

The supramolecular polymer complexes with oppositely charged calixresorcinarene: hydrophobic domain formation and synergistic binding modes.

The association of branched polyethyleneimine (PEI) with a series of octacarboxy-calixresorcinarenes bearing different low-rim substituents leads to the formation of nanosized supramolecular complexes. The PEI-macrocycle complexes have fine-tunable sizes regulated by variations in the self-association capacity of the calixresorcinarenes. In the supramolecular complexes, hydrophobic fragments of the polymer and calixresorcinarenes form cooperative hydrophobic domains which provide synergistic enhancement of guest molecule binding. The formation of the supramolecular complexes was investigated by NMR FT-PGSE, NMR 2D NOESY, DLS and TEM methods. In addition, fluorimetry and UV-vis methods were used with the help of optical probes, namely water-soluble Crystal Violet and water-insoluble Orange OT. The investigation demonstrates the first example of the formation of cooperative hydrophobic domains in supramolecular polyelectrolyte-macrocycle complexes which enhance the binding of both water-soluble and water-insoluble organic compounds. The presented supramolecular systems have potential as sensory and drug delivery systems.

Thermoresponsive (star) block copolymers from one-pot sequential RAFT polymerizations and their self-assembly in aqueous solution

A series of hydrophobically end-capped linear triblock copolymers as well as of three-arm and four-arm star block copolymers was synthesized in a one-pot procedure from N,N-dimethylacrylamide (DMA) and N,N-diethylacrylamide (DEA). The sequential reversible addition-fragmentation chain transfer (RAFT) polymerization of these monomers via the R-approach using bi-, tri- and tetrafunctional chain transfer agents (CTAs) bearing hydrophobic dodecyl moieties proceeded in a well-controlled manner up to almost quantitative conversion. Polymers with molar masses up to 150 kDa, narrow molar mass distribution (PDI ≤ 1.3) and high end group functionality were obtained, which are thermoresponsive in aqueous solution showing a LCST (lower critical solution temperature) transition. The temperature-dependent associative behavior of the polymers was examined using turbidimetry, static and dynamic light scattering (SLS, DLS), and small angle neutron scattering (SANS) for structural analysis. At 25 °C, the polymers form weak transient networks, and rather small hydrophobic domains are already present for polymer concentrations of 5 wt%. However, when heating above the LCST transition (35–40 °C) of the PDEA blocks, the enhanced formation of hydrophobic domains is observed by means of light and neutron scattering. These domains have a size of about 12–15 nm and must be effectively physically cross-linked as they induce high viscosity for the more concentrated samples. SANS shows that these domains are ordered as evidenced by the appearance of a correlation peak. The copolymer architecture affects in particular the extent of ordering as the four-arm star block copolymer shows much more repulsive interactions compared to the analogous copolymers with a lower number of arms.

Aggregation behaviour of hydrophobically modified polyacrylate – Variation of alkyl chain length

The aggregation behaviour in aqueous solution of hydrophobically modified polyacrylates, synthesized by Atomic Transfer Radical Copolymerisation (ATRP) of mixtures of alkyl acrylate and t-butyl acrylate and subsequent hydrolysis of the t-butyl acrylate, was investigated by a combination of static and dynamic light scattering with small-angle neutron scattering (SANS). The degree of amphiphilicity was varied by the percentage of alkyl chains and the length of the alkyl chain (butyl to dodecyl), and, in addition, depends strongly on pH via the ionization of the polyacrylate backbone. SANS shows the formation of hydrophobic domains whose size scales with the length of the alkyl chain. The tendency for domain formation increases with the length of the alkyl chains and is much more pronounced for lower pH, while at high pH the electrostatic charging suppresses the formation of hydrophobic domains for chains shorter than octyl. Then only relatively large and loosely connected aggregates are formed. These hydrophobically modified copolymers show a pronouncedly pH dependent aggregation behaviour that is controlled by the length and percentage of hydrophobic modification and this widely tuneable aggregation behaviour could be interesting for the transport and controlled release of hydrophobic cargo molecules.

Extraction of oil from oil sands using thermoresponsive polymeric surfactants.

Several thermoresponsive block copolymers constituted of a poly(ethylene glycol) (PEG) and a poly(2-(2-methoxyethoxy) ethyl methacrylate) (PMEO2MA) block were prepared by atom transfer radical polymerization (ATRP) and their ability to extract oil from oil sands was evaluated. The chemical composition of the PEG113-b-PMEO2MAX block copolymers was determined by (1)H NMR and gel permeation chromatography (GPC) with X-values ranging between 48 and 80. Aqueous solutions of block copolymers showed a cloud point of 34 ± 1 °C as determined by turbidimetry and dynamic light scattering (DLS) measurements. DLS experiments indicated that these polymers formed stable block copolymer micelles due to association of the PMEO2MA blocks at temperatures greater than 45 °C with a unimodal distribution of hydrodynamic diameters. Since characterization of the block copolymer solutions as a function of temperature indicated the formation of hydrophobic domains in water for T > 45 °C, extractions of oil from oil sands with the block copolymers were conducted at T = 45 and 50 °C. At these temperatures, 15 mL of a 1 mg/mL PEG113-b-PMEO2MA77 aqueous solution extracted 100% of the oil trapped in 1 g of oil sand if 60 mg of toluene was added to the mixture. When the extraction was conducted under the same experimental conditions without block copolymer, a poor oil recovery of less than 30% was achieved. Starting with a 1 mg/mL block copolymer concentration, the block copolymer aqueous solution could be recycled up to five successive extractions while maintaining satisfying oil recovery. Each extraction cycle led to a 22% mass loss of block copolymer, certainly due to association with the toluene, oil, and sand particles. Together these experiments demonstrate that thermoresponsive block copolymers can be powerful aids to enhance the oil recovery of oil sands.

The effect of sugar stereochemistry on protein self-assembly: the case of β-casein micellization in different aldohexose solutions.

Protein self-assembly applications, such as nanoencapsulation of drugs and nutraceuticals, require deep understanding of the parameters governing the micellization process, including the effects of ionic and non-ionic co-solutes, like salts and sugars respectively, which is often overlooked. Herein, with the aim of shedding light on the effect of nonionic cosolute stereochemistry on protein self-assembly, we studied the ternary system of water-protein-sugar by examining the concentration-dependent effects of three aldohexoses, d-glucose (Glu), d-galactose (Gal) and d-mannose (Man) and that of urea, on the micellization of beta casein (β-Cas), using pyrene as a fluorescent probe for the formation of hydrophobic domains. Pyrene's excitation spectra were recorded for several sets of samples with rising protein concentration (0-5 mg ml(-1)), each set with a different co-solute type and concentration. Critical micellization concentration (CMC) and cooperativity of micellization were evaluated according to changes in pyrene spectra as it partitioned from the aqueous environment to the hydrophobic cores of β-Cas micelles. All sugars examined lowered the CMC of β-Cas with increasing sugar concentration and with a diminishing degree of effectiveness (Glu > Gal > Man) which correlated well with the sugars' dynamic hydration number, defined by Uedaira, and correlated negatively with their hydrophobic to hydrophilic molecular surface ratio. These results support the hypothesis that sugars affect protein self-assembly through both changes in water structure and by hydrophobic interactions, both of which are evidenced to be highly sensitive to sugar stereochemistry.

Photoactive graft amphiphilic polyelectrolyte: Facile synthesis, intramolecular aggregation and photosensitizing activity

Simple and versatile two-step procedure for synthesis of graft amphiphilic polyelectrolytes has been developed. A photoactive copolymer of poly(vinylnaphthalene) grafted from poly(sodium 2-acrylamido-2-methyl-1-propanesulfonate) (PAMPS-graft-PVN) was obtained and characterized. By separating the processes of generation of grafting sites on the precursor main chain and subsequent nitroxide-mediated polymerization (NMP), the formation of the unwanted homopolymer could be avoided. Combination of charged polymer backbone with short hydrophobic side chains in PAMPS-graft-PVN leads to spontaneous intramolecular aggregation of the macromolecule in an aqueous solution with formation of hydrophobic domains consisting of highly packed naphthalene chromophores arranged in a “pearl-necklace” structure as shown using atomic force microscopy. The presence of fluorophores in the polymer structure is beneficial for the studies of such macromolecular system as it allows application of emission spectroscopy, which is very sensitive to structural arrangement of the fluorophores. The domains formed within the polymer chain in an aqueous solution were also characterized using fluorescent molecular probes. Studies on photophysical properties of PAMPS-graft-PVN indicated that the polymer can be an efficient photosensitizer in photoinduced electron transfer processes. The proposed here synthetic approach can be easily applied for syntheses of other graft copolymers that are otherwise difficult to be obtained.

Spontaneous Formation of Hydrophobic Domains in Isolated Peptides

Aromatic amino acids are known for their hydrophobicity and the active role they play in protein folding. Here, we investigate the intrinsic propensity of small peptides to form hydrophobic domains in the absence of solvent water molecules. The structures of three aromatic-rich isolated peptides, Ac-Phe-Phe-NH2 (FF), Ac-Trp-Tyr-NH2 (WY), and Ac-Phe-Phe-Phe-NH2 (FFF), all in the gas phase, have been studied by infrared-ultraviolet (IR/UV) double resonance laser spectroscopy, aided by dispersion-corrected density functional theory (DFT-D) calculations. Spontaneous formation of hydrophobic domains is systematically observed, whatever the secondary structure adopted by the backbone. Various types of aromatic-aromatic arrangements have been identified and associated to specific secondary structures, illustrating the interplay between the hydrophobic clusters and the backbone. Backbone NH amide groups surrounded by aromatic rings have also been evidenced and are found to contribute significantly to the stabilization of aromatic pairs. These results suggest that the formation of aromatic clusters involving contiguous residues might be a very efficient process leading to the formation of hydrophobic domains in the early stages of protein folding, well before a hydrophobic collapse into the tertiary structure.

Gel Formation Driven by Tunable Hydrophobic Domain: Design of Acrylamide Macromonomer with Oligo Hydrophobic Segment

Nowadays, biomaterials with amphiphilic properties are undergoing remarkable development. Here, we present one such development, in which we prepared amphiphilic graft copolymers, with a main chain composed of hydroxyethyl acrylamide (HEAA), to introduce hydrophilicity, and a side chain composed of poly(trimethylene carbonate) (PTMC) to introduce tunable hydrophobicity. These macromonomers were created with a novel molecular design, which introduced a ring-opening polymerization by the hydroxyl end group of HEAA in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene, and were analyzed by (1)H NMR and gel permeation chromatography. The amphiphilic graft copolymers were shown to form a hydrogel, the swelling ratio of which was greatly influenced by the number of trimethylene carbonate units. These copolymers also exhibited the Tyndall phenomenon in aqueous solution; they aggregated spontaneously due to hydrogen bonding and hydrophobic interactions, and a sodium 8-anilino-1-naphthalenesulfonate (ANS) fluorescence probe was introduced into the hydrophobic domain. The solution property of ANS in the polymer solution was analyzed by fluorescence measurement and (1)H NMR. The maximum fluorescence wavelength of ANS shifted to shorter wavelengths as the degree of polymerization of the hydrophobic PTMC, the composition of the macromonomer, and the concentration of the copolymer increased. The resulting copolymer formed a polymer micelle structure due to the tunable hydrophobic domain formation in selected solvents. Therefore, these amphiphilic graft copolymers containing a PTMC segment are excellent candidates for use as hydrophobic drug delivery carriers.

Scanning of Silicon Wafers in Contact with Aqueous CTAB Solutions below the CMC

Hydrophilic silicon wafers are studied against aqueous solutions of hexadecyl trimethyl ammonium bromide (CTAB) at concentrations between 0.05 mM up to 1 mM (CMC). AFM studies show that nanobubbles are formed at concentrations up to 0.4 mM. From 0.5 mM upward, no bubbles could be detected. This is interpreted as the formation of hydrophobic domains of surfactant aggregates, becoming hydrophilic at about 0.5 mM. The high contact angle of the nanobubbles (140-150° through water) indicates that the nanobubbles are located on the surfactant domains. A combined imaging and colloidal probe AFM study serves to highlight the surfactant patches adsorbed at the surface via nanobubbles. The nanobubbles have a diameter between 30 and 60 nm (after tip deconvolution), depending on the surfactant concentration. This corresponds to a Laplace pressure of about 30 atm. The presence of the nanobubbles is correlated with force measurements between a silica probe and a silicon wafer surface. The study is a contribution to the better understanding of the short-range attraction between hydrophilic surfaces exposed to a surfactant solution.

Nanostructure formation in aqueous solution of amphiphilic copolymers of 2-( N, N-dimethylaminoethyl)methacrylate and alkylacrylate: Characterization, antimicrobial activity, DNA binding, and cytotoxicity studies

Three amphiphilic random copolymers poly(2-(dimethylaminoethyl)methacrylate-co-alkylacrylate) (where, alkyl = hexyl, octyl, dodecyl) with 16 mol% hydrophobic substitution were synthesized. Surface tension, viscosity, fluorescence probe, dynamic light scattering (DLS), as well as transmission electron microscopic (TEM) techniques were utilized to investigate self-assembly formation by the hydrophobically modified polymers (HMPs) in pH 5. Formation of hydrophobic domains through inter-polymer chain interaction of the copolymer in dilute solution was confirmed by fluorescence probe studies. Average hydrodynamic diameter of the copolymer aggregates at different polymer concentration was measured by DLS studies. The copolymer with shorter hydrophobic chain exhibits larger hydrodynamic diameter in dilute solution, which decreased with either increase of concentration or increase of hydrophobic chain length. TEM images of the dilute solutions of the copolymers with shorter as well as with longer hydrophobic chain exhibit spherical aggregates of different sizes. The antimicrobial activity of the copolymers was evaluated by measuring the minimum inhibitory concentration value against one Gram-positive bacterium Bacillus subtilis and one Gram-negative bacterium Escherichia coli. The copolymer with the octyl group as pendent hydrophobic chain was found to be more effective in killing these microorganisms. The interaction of the cationic copolymers with calf-thymus DNA was studied by fluorescence quenching method. The polymer-DNA binding was found to be purely electrostatic in nature. The hydrophobes on the polymer backbone were found to have a significant influence on the binding process. Biocompatibility studies of the copolymers in terms of cytotoxicity measurements were finally performed at different concentrations of the HMPs to evaluate their potential application in biomedical fields.

Synergistic interactions during thermosensitive chitosan-β-glycerophosphate hydrogel formation

Time-evolved gelation behavior of a chitosan-β-glycerophosphate (CS/β-GP) system was elucidated from rheological investigation, NMR analysis, fluorescence measurement, and morphology observation. Urea and isobutanol were selected to assess the interactions between the two components during the gelation process of the CS/β-GP system. Urea was found to be detrimental to the gelation process by both disrupting hydrogen bonding and retarding the formation of hydrophobic domains. On the contrary, the addition of isobutanol accelerated the sol–gel transition by strengthening the hydrophobic interactions. These results reveal that both hydrogen bonding and hydrophobic interactions within chitosan or between chitosan and β-glycerophosphate molecules are the main reasons for gel formation. The results also indicate that firstly the formation of hydrogen bonds makes the hydrophobic sites more accessible, and then the synergistic hydrogen bonding and hydrophobic interactions lead to the final formation of CS/β-GP gel network. This work demonstrates the possibility of tuning the gelling ability as well as the mechanical properties of CS/β-GP hydrogels. Thus, it gives the opportunities to optimize the performance of this promising natural thermosensitive material for practical applications in the future.

Synthesis and characterisation of temperature responsive poly (2-ethyl-2-oxazolines

A bifunctional Poly(2-ethyl-2-oxazolines) containing hydrophobic end groups at both end of the polymer chains were synthesised by terminating a living cationic polymerisation with 4- bromomethyl benzoic acid. The results obtained for the clouding point measurement using UVvisible spectrophotometer shows that bifunctional polymer containing 20 monomer units (DP 20) exhibited a lower critical solution temperature (LCST) near 49oC due to formation of hydrophobic domains by the polymer end-groups. In contrast, the polymer with 50 monomer units does not exhibit any LCST between 20 to 70oC due to the imbalance in the hydrophilic-hydrophobic interaction in this polymer.