Aqueous Solution‐Processable,Flexible Thin‐Film Transistors Based on Crosslinked Chitosan Dielectric Thin Films

The hydrophilic character of chitosan (CS) limits its use as a gate dielectric material in thin‐film transistors (TFTs) based on aqueous solution‐processable semiconductor materials. In this study, this drawback is overcome through controlled crosslinking of CS and report, for the first time, its application to aqueous solution‐processable TFTs. In comparison to natural CS thin films, crosslinked chitosan (Cr‐CS) thin films are hydrophobic. The dielectric properties of Cr‐CS thin films are explored through fabrication of metal–insulator–metal devices on a flexible substrate. Compared to natural CS, the Cr‐CS dielectric thin films show enhanced environmental and water stabilities, with a high breakdown voltage (10 V) and low leakage current (0.02 nA). The compatibility of Cr‐CS dielectric thin films with aqueous solution‐processable semiconductors is demonstrated by growing ZnO nanorods via a hydrothermal method to fabricate flexible TFT devices. The ZnO nanorod‐based TFTs show a high field‐effect mobility (linear regime) of 10.48 cm² V^?1 s^?1. Low temperature processing conditions (below 100 °C) and water as the solvent are utilized to ensure the process is environmental friendly to address the e‐waste problem.     

Post and prepolymerization strategies to develop novel photochromic poly(esterimide)s

This paper reports novel photoresponsive, processable poly(esterimide)s functionalized with the azobenzene and tricyanovinylene groups. Post and prepolymerization strategies were utilized for preparation of the new photochromic polymers with imide rings. The postpolymerization method of azopolymer synthesis included a functionalization of precursor poly(esterimide) containing dialkylaminophenyl groups with diazonium salt of aniline. The precursor poly(esterimide) was obtained from synthesized 2,2′[N‐phenylethyloaniline‐di(4‐estro‐1,2‐dicarboxylic)]anhydride and 4,4′‐methylene bis(2,6‐dimethylaniline). The prepolymerization strategy based on polycondesation of 2,4‐diaminoazobenzene with two dianhydrides, that is, with or without the azobenzene group. The new dianhydride with azobenzene unit was obtained. The reaction of post‐tricyanovinylation was carried out for substitution of prepared polymers with tricyanovinylene groups. The synthesized polymers were characterized and evaluated by FTIR, ¹H NMR, X‐ray, UV‐Vis spectroscopies, and DSC. The polymers exhibited glass transition temperatures in the range of 119–208°C and good solubilities in common organic solvents, providing optical‐quality films. Thermal stability of the polymers films was investigated by UV‐Vis spectroscopy. Preliminary investigations of the optical anisotropy in chosen poly(esterimide)s were carried out by photoinduced birefringence measurements. Relatively high photoinduced birefringence (Δn = 0.01) was measured for polyimide with covalently bonded azobenzene substituted with strong acceptor group. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Transparent poly(methyl methacrylate‐co‐butyl acrylate) nanofibers

Nanofibers of n‐Butyl Acrylate/Methyl Methacrylate copolymer [P(BA‐co‐MMA)] were produced by electrospinning in this study. P(BA‐co‐MMA) was synthesized by emulsion polymerization. The structural and thermal properties of copolymers and electrospun P(BA‐co‐MMA) nanofibers were analyzed using Fourier transform infrared spectroscopy–Attenuated total reflectance (FTIR–ATR), Nuclear magnetic spectroscopy (NMR), and Differential scanning calorimetry (DSC). FTIR–ATR spectra and NMR spectrum revealed that BA and MMA had effectively participated in polymerization. The morphology of the resulting nanofibers was investigated by scanning electron microscopy, indicating that the diameters of P(BA‐co‐MMA) nanofibers were strongly dependent on the polymer solution dielectric constant, and concentration of solution and flow rate. Homogeneous electrospun P(BA‐co‐MMA) fibers as small as 390 ± 30 nm were successfully produced. The dielectric properties of polymer solution strongly affected the diameter and morphology of electrospun polymer fibers. The bending instability of the electrospinning jet increased with higher dielectric constant. The charges inside the polymer jet tended to repel each other so as to stretch and reduce the diameter of the polymer fibers by the presence of high dielectric environment of the solvent. The extent to which the choice of solvent affects the nanofiber characteristics were well illustrated in the electrospinning of [P(BA‐co‐MMA)] from solvents and mixed solvents. Nanofiber mats showed relatively high hydrophobicity with intrinsic water contact angle up to 120°. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4264–4272, 2013     

Photoactive polyamideimides synthesized by the polycondensation of azo‐dye diamines and rosin derivative

New rosin‐based photoactive polyamideimides (PAI‐a and PAI‐b) with a large second‐order nonlinear optical coefficient and photoinduced birefringence were synthesized by the polycondensation of rosin–maleic anhydride adduct (RMA) with azo‐dye diamines, 2‐(2‐{ethyl‐[4‐(4‐nitrophenylazo)phenyl]aminoethoxy)benzene‐1,4‐diamine (4a) or 2‐(2‐{ethyl‐[4‐(4‐methylsulfonylphenylazo)phenyl]amino}ethoxy)benzene‐1,4‐diamine (4b). Due to the bulky alicyclic structure of the rosin, these polymers were highly soluble in organic solvents and could easily be processed by spin‐coating into thin films of high optical quality. Fabricated polymer films were very smooth, tough, adhesive to the substrate, and optically clear. Second‐order nonlinear optical coefficients (d₃₃) of the corona‐poled PAI‐a and PAI‐b films were 83 and 54 pm/V, respectively, at the fundamental wavelength of 1064 nm. Birefringences as high as 0.072 for PAI‐a and 0.035 for PAI‐b could be induced and erased repeatedly by the irradiation of linearly polarized and circularly polarized 532‐nm light, respectively. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 687–695, 2001     

Studies of solvent effect on the conductivity of 2‐mercaptopyridine‐doped solid polymer blend electrolytes and its application in dye‐sensitized solar cells

Solvents and electrolytes play an important role in the fabrication of dye‐sensitized solar cells (DSSCs). We have studied the poly(ethylene oxide)‐poly(methyl methacrylate)‐KI‐I₂ (PEO‐PMMA‐KI‐I₂) polymer blend electrolytes prepared with different wt % of the 2‐mercaptopyridine by solution casting method. The polymer electrolyte films were characterized by the FTIR, X‐ray diffraction, electrochemical impedance and dielectric studies. FTIR spectra revealed complex formation between the PEO‐PMMA‐KI‐I₂ and 2‐mercaptopyrindine. Ionic conductivity data revealed that 30% 2‐mercaptopyridine‐doped PEO‐PMMA‐KI‐I₂ electrolyte can show higher conductivity (1.55 × 10^?5 S cm^?1) than the other compositions (20, 40, and 50%). The effect of solvent on the conductivity and dielectric of solid polymer electrolytes was studied for the best composition (30% 2‐mercaptopyridine‐doped PEO‐PMMA‐KI‐I₂) electrolyte using various organic solvents such as acetonitrile, N,N‐dimethylformamide, 2‐butanone, chlorobenzene, dimethylsulfoxide, and isopropanol. We found that ac‐conductivity and dielectric constant are higher for the polymer electrolytes processed from N,N‐dimethylformamide. This observation revealed that the conductivity of the solid polymer electrolytes is dependent on the solvent used for processing and the dielectric constant of the film. The photo‐conversion efficiency of dye‐sensitized solar cells fabricated using the optimized polymer electrolytes was 3.0% under an illumination of 100 mW cm^?2. The study suggests that N,N‐dimethylformamide is a good solvent for the polymer electrolyte processing due to higher ac‐conductivity beneficial for the electrochemical device applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42489.     

Thermal and electrical properties of copoly(1,3,4‐oxadiazole‐ethers) containing fluorene groups

A series of aromatic copolyethers containing 1,3,4‐oxadiazole rings and fluorene groups was prepared by nucleophilic substitution polymerization technique of 9,9‐bis(4‐hydroxyphenyl)fluorene, 1 , or of different amounts of 1 and an aromatic bisphenol, such as 4,4′‐isopropylidenediphenol or phenolphthalein, with 2,5‐bis(p‐fluorophenyl)‐1,3,4‐oxadiazole. The polymers were easily soluble in polar solvents like N‐methylpyrrolidone, N,N‐dimethylacetamide, N,N‐dimethylformamide, and chloroform and can be cast from solutions into thin flexible films. They showed high thermal stability, with decomposition temperature being above 425°C. The polymers exhibited a glass‐transition temperature in the range of 195–295°C, with a reasonable interval between glass‐transition and decomposition temperature. Electrical insulating properties of some polymer films were evaluated on the basis of dielectric constant and dielectric loss and their variation with frequency and temperature. The values of the dielectric constant at 10 kHz and 20°C were in the range of 3.16–3.25. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Silicon‐containing heterocyclic polymers and thin films made therefrom

Thin films, in the range of tens of micrometers thickness, have been prepared by casting onto glass plates the chloroform or N‐methylpyrrolidone solutions of polyimides or poly(imide‐amide)s containing silicon and phenylquinoxaline units in the main chain. The polymers have been synthesized by solution polycondensation reaction of aromatic diamines having preformed phenylquinoxaline rings with bis(3,4‐dicarboxyphenyl)dimethylsilane dianhydride or with a diacid chloride resulting from the reaction of this dianhydride with p‐aminobenzoic acid. The polymers were easily soluble in polar aprotic solvents and showed high thermal stability. The free‐standing films exhibited good mechanical properties with tensile strengths in the range of 48–86 MPa, tensile modulus in the range of 1.25–2.22 GPa and elongation at break in the range of 3–37%. Electrical insulating properties of some polymer films were evaluated on the basis of dielectric constant and dielectric loss and their variation with frequency and temperature. The values of the dielectric constant at 10 kHz were in the range of 2.94–3.08 for polyimides and 3.89–4.49 for poly(imide‐amide)s. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3062–3068, 2006     

Synthesis and study of new fluorinated poly(imide‐amide)s

New fluorinated poly(imide‐amide)s have been synthesized by solution polycondensation of various aromatic diamines containing a naphthalene unit with diacid chlorides having both imide and hexafluoroisopropylidene (6F) groups. These polymers are soluble in polar aprotic solvents, such as N‐methylpyrrolidone (NMP) or N,N‐dimethylformamide (DMF), and can be cast into flexible thin films from solutions. They show high thermooxidative stability with decomposition temperatures being above 425°C and glass transition temperatures being in the range of 235–305°C. The polymer films exhibit a low dielectric constant and tough mechanical properties.     

Vapor phase photografting of methyl methacrylate on polymer films: Accelerating effects of solvent mixed with monomer

The effects of solvent mixed with monomer on benzopheneone-sensitized photografting of methyl methacrylate on polymer films were investigated in vapor phase at 60°C. The grafting on nylon 6 film was promoted by adding a pertinent quantity of solvent, such as phenol, m-cresol, benzyl alcohol, and formic acid, to the monomer. The similar effects accelerating the grafting on polyester film were observed for solvent, such as m-cresol, phenol, o-chlorophenol, and nitrobenzene. Such an accelerating function of the solvents is supposed to originate in the swelling of the film substrates due to solvents. On the other hand, solvents such as n-decane, decalin, and p-xylene retarded the grafting on low density polyethylene and polypropylene films. The differences in the solvent effects by the types of film substrates are discussed in terms of photoinduced radicals on the substrates containing solvents.     

Effect of various plasticizers on the transport properties of hexanoyl chitosan‐based polymer electrolyte

Hexanoyl chitosan that exhibited solubility in tetrahydrofuran was prepared by acyl modification of chitosan. Films of hexanoyl chitosan‐based polymer electrolyte were prepared by the technique of solution casting. Ethylene carbonate, propylene carbonate, and diethyl carbonate with different dielectric constants were employed as the plasticizing solvents and lithium trifluoromethanesulfonate (LiCF₃SO₃) was used as the salt. The importance of dielectric constant affecting conductivity and transport properties of hexanoyl chitosan:LiCF₃SO₃ electrolytes have been examined in the present study. An enhancement in the ionic conductivity has been found on plasticization, and the magnitude of conductivity increment strongly depended on the dielectric constant of the plasticizer. Transport properties such as activation energy and charge carrier concentration have been calculated to obtain information that may be used to elucidate the mechanism of conductance. In addition to conductivity studies, thermal studies and transference number measurements were performed to correlate the phase structure and diffusion phenomena to the conductivity behavior of hexanoyl chitosan‐based polymer electrolyte. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101:4474–4479, 2006     

Organosoluble and light‐colored fluorinated polyimides based on 2,2‐bis[4‐(4‐amino‐2‐trifluoromethylphenoxy)phenyl]propane and aromatic dianhydrides

A novel fluorinated diamine monomer, 2,2‐bis[4‐(4‐amino‐2‐trifluoromethylphenoxy)phenyl]propane (2), was prepared through the nucleophilic substitution reaction of 2‐chloro‐5‐nitrobenzotrifluoride with 2,2‐bis(4‐hydroxyphenyl)propane in the presence of potassium carbonate, followed by catalytic reduction with hydrazine and Pd/C. Polyimides were synthesized from diamine 2 and various aromatic dianhydrides 3a–f via thermal imidization. These polymers had inherent viscosities ranging from 0.73 to 1.29 dL/g. Polyimides 5a–f were soluble in amide polar solvents and even in less polar solvents. These films had tensile strengths of 87–100 MPa, elongations to break of 8–29%, and initial moduli of 1.7–2.2 GPa. The glass transition temperatures (T_g) of 5a–f were in the range of 222–271°C, and the 10% weight loss temperatures (T₁₀) of them were all above 493°C. Compared with polyimides 6 series based on 2,2‐bis[4‐(4‐aminophenoxy)phenyl]propane (BAPP) and polyimides 7 based on 2,2‐Bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane (6FBAPP), the 5 series showed better solubility and lower color intensity, dielectric constant, and lower moisture absorption. Their films had cutoff wavelengths between 363 and 404 nm, b* values ranging from 8 to 62, dielectric constants of 2.68–3.16 (1 MHz), and moisture absorptions in the range of 0.04–0.35 wt %. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 922–935, 2005     

Enhanced dielectric performances of polypropylene films via polarity adjustment by maleic anhydride‐grafted polypropylene

Maleic anhydride‐grafted polypropylene (MA‐g ‐PP) was hybridized with pure PP to form hybrid films with the aim to enhance the dielectric performances of the PP film via polarity adjustment. The changes of microstructure and crystallinity of PP matrix by MA‐g ‐PP incorporation were studied, and the polarity change was identified by the surface wettability. The dielectric behaviors of the hybrid films were explored. Increasing polarity of PP film leads to increase in dielectric constant but decrease in breakdown strength, and a balance is achieved in 10 vol % MA‐g ‐PP/PP hybrid film, with the maximum discharge energy density reaching 1.96 J cm^?3 and charge–discharge efficiency as high as 96%. The ultralow loss is attributed to the dense and homogeneous microstructure together with increased crystallinity induced by incorporation of MA‐g ‐PP. These PP‐based films with increased polarity not only show enhanced dielectric performances but also provide a type of matrix that would be compatible with polar fillers to further increase dielectric properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45029.     

Ion‐transport study in nanocomposite solid polymer electrolytes based on chitosan: Electrical and dielectric analysis

In this study, (1.1111 ? x)(0.9CS–0.1NaTf)? xAl₂O₃(0.02 ≤ x ≤ 0.1) [where CS is chitosan, NaTf is sodium triflate (NaCF₃SO₃), and Al₂O₃ is aluminum oxide] nanocomposite solid polymer electrolyte (SPE) films based on CS were prepared by a solution casting technique. X‐ray diffraction and scanning electron microscopy analysis revealed that the alumina nanoparticles had a great effect on the structural and morphological behavior of the CS–NaTf (90:10) polymer electrolyte. An investigation of the electrical and dielectric parameters of the nanocomposite SPE films was conducted. Electrical impedance spectroscopy was carried out for this purpose. The relationships between the electrical and dielectric parameters were used to interpret and understand the ion‐conduction mechanism. We observed that the direct‐current conductivity (σ_dc) and dielectric constant followed the same trend with salt concentration. σ_dc versus temperature showed the Arrhenius and Vogel‐Fulcher‐Tammann (VTF) regions. The drops of σ_dc at high temperatures were observed for all of the samples. The ion relaxation dynamics were studied from Argand plots. For the first time, we confirmed the existence of a strong experimental relationship between the high‐frequency semicircle of the impedance plots and the high‐frequency dispersion regions of the alternating‐current conductivity (σ_ac). The dispersion regions of σ_ac were used to study the ion‐conduction mechanism. The behavior of the frequency exponent as a function of the temperature was used to interpret σ_dc versus the temperature. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41774.     

Effects of solvents on electrospun polymeric fibers: preliminary study on polystyrene

Six solvents [acetic acid, acetonitrile, m‐cresol, toluene, tetrahydrofuran (THF) and dimethylformamide (DMF)] with different properties (eg density, boiling point, solubility parameter, dipole moment and dielectric constant) were used to prepare electrospun polystyrene (PS) fibers. Fiber diameters were found to decrease with increasing density and boiling point of the solvents. A large difference between the solubility parameters of PS and the solvent was responsible for the bead‐on‐string morphology observed. Productivity of the fibers (the numbers of fiber webs per unit area per unit time) increased with increasing dielectric constant and dipole moment of the solvents. Among the solvents studied, DMF was the best solvent that provided PS fibers with highest productivity and optimal morphological characteristics. The beadless, well‐aligned PS fibers with a diameter of ca 0.7 µm were produced from the solution of 10 % (w/v) of PS in DMF at an applied electrostatic field of 15 kV/10 cm, a nitrogen flow rate of 101 ml min^?1 and a rotational speed of the collector of 1500 rev min^?1. Copyright © 2004 Society of Chemical Industry     

In situ encapsulation of hydrogel in ultrafine fibers by suspension electrospinning

Composite ultrafine fibers of hydrogel/polylactide copolymer were successfully electrospun from water‐in‐oil suspensions. Effects of the suspension composition on the morphology and microstructure of the obtained fibers were investigated, including both aqueous phase and oily phase, that is, hydrogel (chitosan or gelatin), polylactide copolymers [poly(ethylene glycol)‐b‐poly (L ‐lactide‐co‐caprolactone) (PELCL) or poly(L ‐lactide‐co‐glycolide) (PLGA)], organic solvents and surfactants. Scanning electron micrographs showed that mixed solvents of chloroform and N,N‐dimethyl formamide or 2,2,2‐trifluoroethanol were preferred to form beads‐free ultrafine fibers with diameter in the range of 230–470 nm. With ethyl acetate as organic solvent, compared with chitosan hydrogel/PELCL composite fibers, chitosan hydrogel/PLGA fibers showed narrower distribution of diameter in 230–590 nm. Different hydrogel and surfactants used in this experiment had slight effects on the morphology of the obtained fibers, whereas transmission electron micrographs exhibited chitosan and gelatin hydrogel could be in situ encapsulated in the fibers discontinuously. This method may promise a new aqueous reservoir for encapsulation and controlled release of bioactive agents. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Transparent and organosoluble cardo polyimides with different trans/cis ratios of 1,4‐diaminocyclohexane via aromatic nucleophilic substitution polymerization

Two series of cardo polyimides were prepared from 1,4‐bis(4‐fluorophthalimide)cyclohexane with different trans/cis ratios and phenolphthalein/o‐cresolphthalein via aromatic nucleophilic substitution reaction. The inherent viscosities of the synthesized polymers were found to be 0.55–0.66 dL g^?1 in N,N′‐dimethylacetamide. The cardo polyimides showed excellent solubility in organic solvents, high glass transition temperatures (T_g) of 275–312 °C and moderate thermal stability with 5% weight loss temperatures (T_d5%) of 415–441 °C in nitrogen and 370–436 °C in air. The polyimide films exhibited high optical transparency with cut‐off wavelengths of 350–355 nm and moderate mechanical properties. The different properties of the polymers caused by trans and cis configurations of 1,4‐diaminocyclohexane were also investigated. It was found that with an increasing content of trans configuration of 1,4‐diaminocyclohexane in the polyimide backbone, T_g of the polyimides increased as well as T_d5%, while the solubility gradually decreased. The polyimide films had good optical transparency regardless of trans/cis configuration. © 2018 Society of Chemical Industry     

Three‐dimensional dielectric characterization of polymer films

Polymer films are widely used in electronic packaging applications due to their low dielectric constant and ease of fabrication. These films often exhibit anisotropic electrical and thermomechanical properties, due to orientation of polymer chains, which need to be evaluated for performance and reliability modeling of electronic packages. This article presents a dual‐capacitor technique to measure the anisotropic dielectric permittivities of such films. Results are reported for in situ measurements for several spin‐coated polymeric films, some of which exhibit different permittivities in the in‐plane (x and y) and the through‐plane (z) directions (transversely isotropic), and free‐standing liquid crystalline Vectran films which exhibit orthotropic permittivities. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2328–2334, 2001     

Dielectric properties of polymers containing dispersed TCNQ salts

Tetracyanoquinodimethane salts have been incorporated into insulating polymers to raise their dielectric constants. Cast films prepared from mixtures of insulating polymers and tetracyanoquinodimethane salts in a solvent show a high dielectric constant (>20). The dielectric constant of the films varies with the tetracyanoquinodimethane salts. High values were attained with the films containing high-conducting tetracyanoquinodimethane salts. These values remain constant over a range of frequencies (10²–10⁴ Hz). Dissipation factors and electric strength values of the films are also reported.     

Dielectric relaxation and optical properties of polyvinyl chloride/lead monoxide nanocomposites

Sol–gel synthesized lead monoxide (PbO) nanoparticles were mixed with polyvinyl chloride (PVC). The dielectric characteristics of PVC/PbO nancomposite films were studied in the temperature range 303–408 K and frequency range 30 kHz–3 MHz. There is a sharp increase in the dielectric constant (ε′), after the glass transition temperature. According to the temperature and frequency dependence of the dielectric loss modulus (M″), the observed α_a‐relaxation peak is due to the micro‐Brownian motion of the polymer main chains. The behavior of σ_ac (f) for the composite films indicate that the homogenous distribution of PbO nanoparticles allows the formation of conductive three‐dimensional networks throughout the nanocomposite films which assisting the charge carriers to hop from conducting clusters to neighbors. The influences of PbO content on the direct optical band gap and the refractive index of the films are also discussed and compared with those of previous studies of PVC composites. POLYM. COMPOS., 34:2031–2039, 2013. © 2013 Society of Plastics Engineers     

Structural and mechanical characterization of poly(ether ether ketone) (PEEK) and sulfonated PEEK films: Effects of thermal history,sulfonation, and preparation conditions

In this work, virgin and sulfonated poly(ether ether ketone) films (PEEK and SPEEK, respectively) have been studied by dynamic mechanical analysis, modulated differential scanning calorimetry, wide‐angle X‐ray diffraction, birefringence, and optical microscopy. The properties of the unmodified polymer have been addressed to assess the original morphological characteristics and the changes induced by sulfonation. In general, the introduction of ionic groups in the polymer backbone alters dramatically the intrinsic properties of the parent material. The particular thermomechanical response exhibited by PEEK and SPEEK samples, characterized by a hysteresis loop, can be explained by the reversible and irreversible relaxation–orientation of the microstructure, even in the sub‐T_g region. The results showed that the preparation conditions largely determine the nonequilibrium morphological features of both compression‐molded PEEK films and solvent‐cast SPEEK membranes. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 756–774, 2006