Dhruba P. Chatterjee

Amphiphilic poly(N-vinyl pyrrolidone) grafted graphene by reversible addition and fragmentation polymerization and the reinforcement of poly(vinyl acetate) films

The reversible addition and fragmentation (RAFT) polymerization of vinyl pyrrolidone (VP) from graphene oxide (GO) is used to produce GO-g-PVP (GP) and the grafting is confirmed from Fourier transformed infrared (FTIR) and nuclear magnetic resonance spectra. The average thickness of GP (8.2 nm) obtained from atomic force microscopy is higher than that of GO (1.2 nm), indicating the wrapping of grafted PVP on the GO sheets. Transmission electron microscopy of GP exhibits swollen domains (white spots) characterizing the grafted PVP chains from the GO surface. The dispersibility of the GP sheets becomes greatly improved over that of GO and they are dispersible in the solvents of Hansen solubility parameter (δp + δH) range 6.3–58. Three nanocomposites GP1, GP3 and GP5, produced by mixing with 1, 3 and 5 (w/w)% GP with poly(vinyl acetate) (PVAc), produce a stable dispersion in dimethyl formamide, although mixtures of GO and PVAc do not. The field emission scanning electron microscopy of the GP5 sample indicates a good homogeneous dispersion of GP sheets within the PVAc matrix, although both GO and PVP are individually immiscible with PVAc. The FTIR data indicates a specific interaction between GP and PVAc. The glass transition temperature (Tg) of the pure PVAc increases in the GP composites, but in the GO composite it remains unchanged. In the GPP5 hybrid containing the GO, PVP and PVAc mixture produced at the same composition as in GP5, an increase of Tg is seen to a lesser degree than that of GP, indicating that GO acts as a compatibilizer of a PVP and PVAc immiscible blend. The mechanical properties of PVAc exhibit a strong reinforcement and the Youngs modulus & tensile strength data show a 190% and 169% increase over PVAc in the GP5 sample due to the homogenous dispersion and unidirectional (parallel) orientation of GP sheets in the composite film.

Water soluble polythiophenes: preparation and applications

This review describes the synthesis of different water soluble polythiophenes and their versatile applications. Solubility in water is essential for developing sensors for different bio-molecules and polythiophene derivatives are excellent candidates due to their important optoelectronic properties. A pristine polythiophene chain is hydrophobic and it exhibits aqueous solubility after attachment/grafting of ionic pendent groups or hydrophilic polymer chains on its backbone. A concise account of the different synthetic procedures of preparing water soluble polythiophene is described and all the specific techniques relevant to the synthesis of water soluble polythiophene using cationic, anionic pendent groups and grafting of hydrophilic polymers are discussed. Different grafting processes e.g. “grafting from” and “grafting to” techniques using click chemistry and atom transfer radical polymerization (ATRP) are described in detail. Detections of different bio-molecules such as DNA, RNA, polypeptides, polysaccharides, ATP, UDP and ADP from the excellent opto-electronic properties of aqueous polythiophenes are discussed. The reports on fluorescence based specific sensing of metal ions, and nitro-aromatics using water soluble polythiophenes are also embodied with an up-to-date description of the optoelectronic device applications such as logic gates, molecular thermometers, photovoltaic cells etc. Finally, a summary and outlook is presented discussing the future scope of research on this important subject.

Thermo and pH responsive water soluble polythiophene graft copolymer showing logic operation and nitroaromatic sensing

A polythiophene based water soluble thermo and pH responsive graft copolymer is prepared by anchoring the initiator moiety (2-bromoisobutyryl bromide) on 3-thiophene ethanol and polymerizing using ferric chloride to produce the 2,5-poly(3-[1-ethyl-2-(2-bromoisobutyrate)]) thiophene macroinitiator (PTI), followed by polymerization with a mixture of varying composition of diethyleneglycol methylether methacrylate (MeO2MA) and N,N-dimethyl aminoethyl methacrylate (DMAEMA) at 30 °C using copper based atom transfer radical polymerization (ATRP). The polymers are characterized by gel permeation chromatography (GPC) and by 1H NMR spectroscopy. Polythiophene-g-P(MeO2MA-co-DMAEMA) (PTDM) exhibits considerable water solubility but due to the lower critical solution temperature (LCST) of aqueous PMeO2MA at ∼26 °C the particle sizes observed by dynamic light scattering (DLS) show a sharp increase in the region 25–30 °C only for a pH value of 9.2. However, at lower pH values (pH 4 or 7), in the LCST region of PMeO2MA there is no increase of particle size. The TEM micrographs of PTDM indicate core–shell morphology at pH4 and pH7 with a gradual decreasing of the size (with PT at the core and P(MeO2MA-co-DMAEMA) at the shell), and at pH 9.2 no core–shell morphology is observed due to the absence of protonation at the –NMe2 groups of the PDMAEMA segments. The fluorescence intensity of the PTDM solution at pH 9.2 also shows a sharp increase in the temperature range 22–29 °C, but remains almost without change at pH 4 and 7. Using the pH and temperature as inputs and the fluorescence intensity as an output, the system functions as a fully polymeric AND logic gate, and this is the first report using polythiophene as the fluorescence probe. Also PTDM in the solid/solution state exhibits considerable quenching of fluorescence intensities in the presence of nitroaromatics such as picric acid, dinitro phenol, etc. and may be used for sensing nitroaromatics.

Polythiophene-g-poly(dimethylaminoethyl methacrylate) doped methyl cellulose hydrogel behaving like a polymeric AND logic gate

The variation of photoluminescence (PL) property of polythiophene-g-poly(dimethylaminoethyl methacrylate) (PT-g-PDMA, PD) with temperature and pH is used to develop a fully polymeric fluorescent AND logic gate type material using methyl cellulose (MC) hydrogel. The PL intensity gradually increases with increasing temperature of the PD doped aqueous MC solution and with increasing pH of the medium. In contrast, the PL-intensity of the PD solution decreases with increase in temperature for all of the pH values studied here due to collapsing of PDMA chains on the PT core, signifying that the PL intensity increases in the MC gel after compensating for the above decrease. The truth table suggests that it acts as an AND fluorescent molecular logic gate type system with fluorescence as output and temperature and pH as inputs. The maximum sensitivity of this logic gate is at higher pH (pH 9.2) than at neutral or acidic pH (pH 4) and at 45 °C. The reason is discussed from the viewpoint of the change in polarity at the microenvironment of the polythiophene chain in PD in the MC gel due to the change in temperature and pH.

Temperature triggered antifouling properties of poly(vinylidene fluoride) graft copolymers with tunable hydrophilicity

A water soluble polymer poly(diethylene glycol methyl ether methacrylate) (PMeO2MA) is grafted on a poly(vinylidene fluoride) (PVDF) backbone via coupled atom transfer radical coupling (ATRC) followed by atom transfer radical polymerization (ATRP). The PVDF-g-PMeO2MA copolymers are designated as PD-24, PD-16, etc. depending on the polymerization time and are characterized using 1H NMR spectroscopy, FTIR spectroscopy and gel permeation chromatography. AFM images indicate a change in morphology from spherulitic PVDF to the self-organized nano-sphere morphology with hairy PMeO2MA chains at the surface corona. The TGA data of PD graft copolymers indicate a two-stage degradation whose temperatures are higher compared to those of the components. The glass transition temperatures of the PD graft copolymers are higher than that of PMeO2MA, and both melting point & crystallinity decrease progressively with an increase of graft conversion. Dynamic light scattering (DLS) data indicate that PD graft copolymers possess a lower critical solution temperature (LCST) at ∼30 °C which can be tuned by changing the composition of the graft copolymer. The antifouling properties of the PD-24 film, produced specifically by water treatment at 15 °C (PD-24-15) and 37 °C (PD-24-37), are tested with bovine serum albumin (BSA) at below and above the LCST and a lower protein adsorption is noticed at 37 °C indicating a temperature triggered antifouling property of the PD graft co-polymers. The surface hydrophilicity of the graft copolymer, measured from the contact angle measurement, is higher in the PD graft co-polymer than that of PVDF and the contact angle decreases more significantly for the PD-24-15 film than that for the PD-24-37 film with time. The filtration of BSA solution using these two films monitored through fluorescence intensity indicates ∼60% protein absorption during filtration through the PD-24-15 film but PD-24-37 does not exhibit any change of fluorescence intensity, indicating superior antifouling properties.

Nano-structured poly(3-hexyl thiophene) grafted on poly(vinylidene fluoride) via poly(glycidyl methacrylate)

Poly(vinylidene fluoride)-g-poly(glycidyl methacrylate)-g-poly(3-hexyl thiophene) (PGHT) co-polymer was synthesized using atom transfer radical polymerization (ATRP) of glycidyl methacrylate (GMA) on a poly(vinylidene fluoride) (PVDF) backbone in ethylene carbonate (EC), followed by the oxidative polymerization of 3-hexyl thiophene (3-HT) from the anchored thiophene unit in nitromethane. The poly(vinylidene fluoride)-g-poly(glycidyl methacrylate (PG) and PGHT graft co-polymers are characterized by 1H NMR, FTIR and GPC analysis. The PG graft co-polymer exhibits an open spherulitic morphology which further worsens with increasing polymerization time. In PGHT, P3HT exhibits nanosphere morphology of diameter 2.9–5.5 nm that decreases with increased PG polymerization time. The lamellar structure of PVDF deteriorates with the progress of PG polymerization, however, upon further grafting with P3HT the lamellar structure of PVDF reappears. In the PG co-polymers PVDF exists in the α-polymorph but in PGHT, it transforms into the piezoelectric β-polymorph. Both the PG and PGHT graft co-polymers exhibit high thermal stability. The PVDF melting point in the PG co-polymers has decreased by 12–19°. However, in PGHT the PVDF melting point remains the same and the P3HT melting point increases. In PGHT, the π–π* transition peak shows a small red shift emitting at 14–18 nm lower wavelength than that of pristine P3HT. The above spectral shift is attributed to the self organized structure of grafted P3HT chains in PGHT forming a nanosphere morphology. The dc conductivity of PGHT is lower than that of P3HT.

Supramolecular assembly of polythiophene-g-polymethacrylic acid-doped polyaniline with interesting morphological and opto-electronic properties

Polythiophene-g-poly(methacrylic acid) (PTMA) is prepared by atom transfer radical polymerization (ATRP) of tert-butyl methacrylate upon a polythiophene backbone followed by hydrolysis of tert-butyl groups using trifluoroacetic acid. PTMA is characterised by 1H NMR and gel permeation chromatography (GPC). It acts as both a template and a dopant for the synthesis of polyaniline (PANI) nanostructures, and the PTMA concentration is varied to tune the physical properties of PTMA–PANI (PTPA) hybrids. Field-emission scanning electron microscopy (FESEM) exhibits the formation of nanorod morphology till 1 : 10 composition (PTMA–PANI; w/w) and for the PTPA120 hybrids helical nanorods are produced along with some small size spheroids. UV-vis spectra of the PTPA hybrids indicate the formation of PTMA-doped PANI; moreover, with increase in the concentration of dopant PTMA, the π band to the polaron band transition peak is red-shifted. Circular dichroism (CD) spectra of the hybrid exhibit a broad positive peak at 310–470 nm, indicating the presence of single-handed chirality in all the PTPA hybrids. Fourier transformed infrared (FTIR) spectra also confirm the doped nature of PANI in the PTPA hybrids, and X-ray diffraction (XRD) data indicates that there is self-organization of PTMA and PANI due to π–π interaction. Thermogravimetric analysis data indicate an increase of thermal stability of PTPA hybrids compared to that of PTMA. The PTPA hybrids are semiconducting in nature (dc-conductivity 10−2 to 10−5 s cm−1), and they exhibit reproducible photo-conductivity by alternate “On” and “Off” switching of white light illumination. Analysis of impedance results suggests the presence of an equivalent circuit with a capacitance value of 3 × 10−10 F and a bulk resistance of 5.8 × 104 Ω.

Water soluble stimuli-responsive star copolymers with multiple encapsulation and release properties

A series of three arm star shaped (random/block) and linear water soluble copolymers are synthesised by atom transfer radical polymerization (ATRP) using di(ethylene glycol) methyl ether methacrylate (DEGMA) and 2-(dimethylamino) ethyl methacrylate (DMAEMA). The structure and composition of the block and random copolymers are characterized by 1H NMR spectra and gel permeation chromatography (GPC). The self-assembly of these copolymers, investigated by dynamic light scattering (DLS), exhibits that below the lower critical solution temperature (LCST) of pDEGMA all the copolymers are soluble in water and possess lower particle size but above its LCST particle size increases particularly in a basic medium. On addition of 8-anilino-1-naphthalenesulfonic acid (ANS) the particle size increases by ∼10 times below the LCST. Both the DLS and fluorescence studies using a hydrophobic fluorescent dye, exhibit temperature triggered encapsulation and pH triggered release. All the copolymers exhibit highly reversible multiple aggregation at different temperature and pH conditions and require increased temperature for the aggregation with decrease in pH of the medium. The random star copolymer [3-arm-p(DEGMA40-co-DMAEMA18)] exhibits aggregate formation under physiological conditions (37 °C and pH 7.5) and with decreasing the pH to 6.5 the aggregates dissociate. The MTT assay and cell morphology indicate that the three arm star and linear random copolymers have lower cytotoxicity against normal CHO-K1 cells having lower positive zeta potential values.

Phase Behavior of Poly(vinylidene fluoride)-graft-poly(diethylene glycol methyl ether methacrylate) in Alcohol-Water System: Coexistence of LCST and UCST.

A thermoresponsive polymer poly(diethylene glycol methyl ether methacrylate) (PMeO2MA) is grafted from poly(vinylidene fluoride) (PVDF) backbone by using a combined ATRC and ATRP technique with a high conversion (69%) of the monomer to produce the graft copolymer (PD). It is highly soluble polymer and its solution property is studied by varying polarity in pure solvents (water, methanol, isopropanol) and also in mixed solvents (water-methanol and water-isopropanol) by measuring the hydrodynamic size (Z-average) of the particles by dynamic light scattering (DLS). The variation of Z-average size with temperature of the PD solution (0.2%, w/v) indicates a lower critical solution temperature (LCST)-type phase transition (T(PL)) in aqueous medium, an upper critical solution temperature (UCST)-type phase transition (T(PU)) in isopropanol medium, and no such phase transition for methanol solution. In the mixed solvent (water + isopropanol) at 0-20% (v/v) isopropanol the TPL increases, whereas the T(PU) decreases at 92-100% with isopropanol content. For the mixture 20-90% isopropanol, PD particles having larger sizes (400-750 nm) exhibit neither any break in Z-average size-temperature plot nor any cloudiness, indicating their dispersed swelled state in the medium. In the methanol + water mixture with methanol content of 0-30%, T(PL) increases, and at 40-60% both UCST- and LCST-type phase separations occur simultaneously, but at 70-90% methanol the swelled state of the particles (size 250-375 nm) is noticed. For 50 vol % methanol by varying polymer concentration (0.07-0.2% w/v) we have drawn a quasibinary phase diagram that indicates an approximate inverted hourglass phase diagram where a swelled state exists between two single phase boundary produced from LCST- and UCST-type phase transitions. An attempt is made to understand the phase separation process by temperature-dependent (1)H NMR spectroscopy along with transmission electron microscopy.

Surfactant-Triggered Fluorescence Turn “on/off” Behavior of a Polythiophene-graft-Polyampholyte

Polythiophene-graft-polyampholyte (PTP) is synthesized using N,N-dimethylaminoethyl methacrylate and tert-butyl methacrylate monomers by grafting from polythiophene backbone, followed by hydrolysis. The resulting polymer exhibits aqueous solubility via formation of small-sized miceller aggregates with hydrophobic polythiophene at the center and radiating polyionic side chains (cationic or anionic depending on the pH of the medium) at the outer periphery. The critical micelle concentration of PTP in acidic solution (0.025 mg/mL, pH = 2.7) is determined from fluorescence spectroscopy. PTP exhibits reversible fluorescence on and off response in both acidic and basic medium with the sequential addition of differently charged ionic surfactants, repeatedly. The fluorescence intensity of PTP at pH 2.7 increases with the addition of an anionic surfactant, sodium dodecyl benzenesulfonate (SDBS), due to the self-aggregation forming compound micelles. The fluorescence intensity of these solutions again decreases on addition of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), because of assembling of SDBS with CTAB, thus deassembling the PTP-SDBS aggregates. At pH 9.2, these turn on and turn off responses are also shown by PTP with the sequential addition of cationic surfactant (CTAB) and anionic surfactant (SDBS), respectively. This result shows that PTP has potential for surfactant-induced reversible fluorescence turn on and off using ionic surfactant (SDBS and CTAB) through self-assembling and deassembling of the ionic aggregates. The reversible aggregation and disaggregation process of PTP with the surfactants at both acidic and basic pH is supported from dynamic light scattering and Fourier transform infrared spectroscopy. The morphology of the above systems studied by transmission and scanning electron microscopy also supports the above aggregation and disaggregation process.