Mahdi Abdollahi

Novel three-dimensional, conducting, biocompatible, porous, and elastic polyaniline-based scaffolds for regenerative therapies

The aim of this study is the fabrication of two novel three-dimensional, conducting, biocompatible, porous, and elastic scaffolds composed of hyperbranched aliphatic polyesters (HAPs), polyaniline (PANI), and poly(e-caprolactone) (PCL) for tissue engineering applications. First, HAPs (G3 and G6) were synthesized via a melt polycondensation reaction from tris(methylol)propan, and 2,2-bis(methylol)propionic acid. The synthesized HAPs were further reacted with p-anthranilic acid to afford phenylamine-functionalized aliphatic hyperbranched polyester macromonomers (PhAG3M and PhAG6M). The synthesized macromonomers were subsequently employed in both chemical and electrochemical oxidation copolymerizations with aniline monomer to produce two star-shaped PANIs (S-PANIs) with HAPs cores. The solutions of the chemically synthesized S-PANIs were electrospun with PCL solution to produce uniform conductive nanofibers. The biocompatibility of the electrospun nanofibers were evaluated by assessing the adhesion and proliferation of the mouse fibroblast L929 cell line, and in vitro degradabilities. From the results obtained for the conductivities, biocompatibilities, hydrophilicites, and mechanical properties of the fabricated scaffolds it is suggested that the nanofibers are potentially suitable for use in tissue engineering.

Kinetic study of radical polymerization v. determination of reactivity ratio in copolymerization of acrylonitrile and itaconic acid by 1H-NMR

Many reports exist in the literature about the application of 1H and 13C‐NMR techniques to analyze the copolymer structure and composition and also determination of reactivity ratios. In this work, on‐line 1H‐NMR spectroscopy has been applied to identify reactivity ratios of itaconic acid and acrylonitrile in the solution phase (DMSO as the solvent) and in the presence of AIBN as the radical initiator. All the peaks corresponding to the existing protons were assigned quietly. Therefore, the kinetics of the copolymerization reaction was investigated by studying the variation of integral of two characteristic peaks regarding each monomer. The obtained data were used to find the reactivity ratios of acrylonitrile and itaconic acid by linear least‐squares methods such as Finemann‐Ross, inverted Finemann‐Ross, Mayo‐Lewis, Kelen‐Tudos, extended Kelen‐Tudos and Mao‐Huglin. In addition, a non‐linear least‐square method (Tidwell‐ Mortimer) was used at low conversions. Extended Kelen‐ Tudos and Mao‐Huglin were applied to determine reactivity ratio values at high conversions as well.

Kinetic investigation of the reversible addition-fragmentation chain transfer polymerization of 1,3-butadiene

Reversible addition-fragmentation chain transfer (RAFT) polymerization of 1,3-butadiene using a trithiocarbonate chain transfer agent, S-1-dodecyl-S′-(r,r′-dimethyl-r″-acetic acid)trithiocarbonate (DDMAT), was investigated. For the first time, comprehensive kinetic study of solution RAFT polymerization of 1,3-butadiene is reported. Effect of some factors such as RAFT agent and initiator concentration and initial molar ratio of monomer to both RAFT agent and initiator on the rate of polymerization and molecular weight distribution (MWD) were examined experimentally and discussed quantitatively. The validity of quasi-steady state approximation (QSSA) was shown and the rate of polymerization’s equation was obtained. Good compatibility with the experimental and theoretical values of molecular weights was obtained. Also, polybutadiene samples with narrow molecular weight distribution were synthesized.

Kinetic Study of Atom Transfer Radical Copolymerization of Methyl Acrylate and Methyl Methacrylate Initiated with Poly(vinyl acetate) Macroinitiator

Atom transfer radical bulk copolymerization of MA and MMA was performed in the presence of CuCl/PMDETA as catalyst system and trichloromethyl‐terminated poly(vinyl acetate) macroinitiator at 80°C. The overall monomer conversion was followed gravimetrically and chemical composition of the copolymer was determined by 1H‐NMR spectrometry. The results have been used to calculate monomer reactivity ratios by linear and nonlinear methods. Reactivity ratios calculated were in the range of 0.3766–0.4988 and 1.8832–2.0963 for MA and MMA, respectively. These values were in good agreement with the values reported for a similar system in the free radical copolymerization. It was observed that the copolymerization system tends to produce a random copolymer with longer sequences of MMA than MA in the initial stage of polymerization before any significant decrease of the concentration of MMA in the monomer mixture. Copolymer microstructures in this study indicated that radical stabilization capability of MMA compared to MA is higher. The accuracy of the reactivity ratios were confirmed by 95% joint confidence limits. It was found that considering the effect of conversion in these methods makes the calculation result more accurate. The theoretical composition drifts in the comonomer mixture and copolymer as a function of the overall monomer conversion are also reported.

Kinetic study of radical polymerization. VIII. A comprehensive study of solution copolymerization of vinyl acetate and methyl acrylate by 1H-NMR spectroscopy

Radical copolymerization reaction of vinyl acetate (VA) and methyl acrylate (MA) was performed in a solution of benzene‐d6 using benzoyl peroxide (BPO) as the initiator at 60°C. Kinetic studies of this copolymerization reaction were investigated by on‐line 1H‐NMR spectroscopy. Individual monomer conversions vs. reaction time, which was followed by this technique, were used to calculate the overall monomer conversion, as well as the monomer mixture and the copolymer compositions as a function of time. Monomer reactivity ratios were calculated by various linear and nonlinear terminal models and also by simplified penultimate model with r 2(VA)=0 at low and medium/high conversions. Overall rate coefficient of copolymerization was calculated from the overall monomer conversion vs. time data and k p . k t −0.5 was then estimated. It was observed that k p . k t −0.5 increases with increasing the mole fraction of MA in the initial feed, indicating the increase in the polymerization rate with increasing MA concentration in the initial monomer mixture. The effect of mole fraction of MA in the initial monomer mixture on the drifts in the monomer mixture and copolymer compositions with reaction progress was also evaluated experimentally and theoretically.

A comprehensive study on kinetics of free-radical solution copolymerization of vinyl acetate and dibutyl maleate in chloroform

Free radical solution copolymerization of vinyl acetate (VAc) and dibutyl maleate (DBM) initiated by AIBN was performed in the presence of chloroform as both solvent and chain transfer agent at various temperatures, comonomer mixture composition and initiator concentrations. Structures of the copolymers were characterized by FT-IR and 1H-NMR techniques. Effect of the above-mentioned variables on the copolymerization rate was investigated via data obtained from 1H-NMR spectra. Value of the “lumped” kinetic parameter, i.e. kp.kt−0.5, was calculated for various initial mole fractions of the comonomers. Reactivity ratios of the VAc and DBM were calculated for the first time from medium/high conversion data to be 0.1102, 0.0421, respectively, by extended KT method and 0.1135 and 0.0562, respectively, by MH method. A good fitting between the theoretical and experimental drifts in the comonomer mixture and copolymer compositions with conversion was observed, indicating accuracy of the reactivity ratios calculated for the present pair comonomers.

Synthesis of polyacrylamides hydrophobically modified with butyl acrylate using a nanoclay with interlayer spaces for butyl acrylate aggregation: studies on the microstructure and aqueous solution viscosity

Aqueous solution free-radical copolymerization of acrylamide with hydrophobic butyl acrylate (BA) was performed by potassium persulfate as an initiator in the presence of nanoclay. The effect of two nanoclays with different natures, i.e. hydrophilic Cloisite Na+ and hydrophobic Cloisite 30B, on the microstructure and aqueous solution viscosity of the synthesized copolymers were studied. It was found from microstructural studies by NMR that copolymerization with Cloisite Na+ may proceed via a mechanism similar to the heterogeneous mechanism, while those with Cloisite 30B may proceed simultaneously with both micellar and heterogeneous mechanisms with a relatively high tendency toward the micellar method. These findings were further confirmed by the water solubility, XRD, TEM, dynamic light scattering (DLS) and viscosity analyses. Significant intercalation of the chains into the clay galleries was observed only with Cloisite Na+. The results of the DLS analysis as well as the aqueous solution viscosity versus copolymer composition, NaCl concentration and temperature revealed intermolecular aggregation of the BA groups especially for multiblock structured copolymers synthesized with an emulsifier or Cloisite 30B.

Structure and properties of NR/BR blend/clay nanocomposites prepared by the latex method

Rubber blend/clay nanocomposites based on the 50/50 (wt %) natural rubber/butadiene rubber was prepared by the latex method via mixing the latex of 50/50 NR/BR blend with different amounts of the aqueous sodium montmorillonite (Na-MMT) dispersion and co-coagulating the mixture. XRD and TEM were used to characterize structure of the nanocomposites. It was found that fully exfoliated structure could be obtained by this method only when the low loading of layered silicate (up to 5 phr) is used. With increasing the clay content, both non-exfoliated (stacked layers) and exfoliated structures can be observed simultaneously in the nanocomposites. Nanocomposites showed mechanical properties better than the clay-free volcanizate. Moreover, modulus, tensile strength, elongation at break and tear strength increased significantly by increasing the clay amount up to 5 phr and then remained almost constant by further increasing the clay content. Improvement in the mechanical properties by increasing the clay loading up to 5 phr was attributed to the nano-reinforcement effect of Na-MMT. TGA results indicated an improvement in the main decomposition temperature by increasing the clay amount.

Structure and properties of styrene-butadiene rubber/ pristine clay nanocomposites prepared by latex compounding method

Abstract Styrene- butadiene rubber (SBR)/ clay nanocomposites were prepared by mixing the SBR latex with aqueous clay dispersion and co-coagulating the mixture. Tapping mode AFM and XRD were applied to characterize the structure of nanocomposites. It was found that fully exfoliated structure could be obtained by this method only when the low loading of layered silicate (< 10 phr) is used. With increasing the clay content, both non-exfoliated (stacked layers) and exfoliated structures can be observed simultaneously in the nanocomposites. The results of mechanical tests on the vulcanized pure SBR and SBR/ clay nanocomposites showed that the nanocomposites presents better mechanical properties than clayfree SBR vulcanizate. Furthermore, initial modulus, tensile strength, tensile strain at break, hardness (shore A) and tear strength increased with increasing the clay content, indicating the nanoreinforcement effect of clay on the mechanical properties of SBR/ clay nanocomposites. Compared to the clay free SBR vulcanizate, the nanocomposite vulcanizates exhibit a lower tanδ peak value, higher storage modulus and higher tanδ value at the rubbery region (0-60 °C) which indicate that the elastic responses of pure SBR towards deformation are strongly influenced by the presence of nanodisperced natural sodium montmorillonite layers especially completely exfoliated silicate layers.

Effect of monomer/nanoclay interaction on the kinetics of atom transfer radical homo- and copolymerization of styrene and methyl acrylate

Atom transfer radical homo- and copolymerization of styrene and methyl acrylate initiated with CCl3-terminated poly(vinyl acetate) macroinitiator were performed at 90°C in the presence of nanoclay (Cloisite 30B). Controlled molecular weight characteristics of the reaction products were confirmed by GPC. It was shown that nanoclay slightly decreased the rate of styrene polymerization, while it significantly enhanced the rate of methyl acrylate polymerization and its copolymerization with styrene. The reactivity ratios of the monomers in the presence and in the absence of nanoclay were calculated (rSt = 1.002 ± 0.044, rMA = 0.161 ± 0.018 by extended Kelen-Tudos method and rSt = 1.001 ± 0.038, rMA = 0.163 ± 0.016 by Mao-Huglin method), confirming that the presence of nanoclay has no influence on monomer reactivity. The enhancement in the homopolymerization rate of methyl acrylate as well as its copolymerization rate with styrene was attributed to nanoclay effect on the dynamic equilibrium between active (macro)radicals and dormant species. Dipole moments of the monomers were successfully used to predict structure of the polymer/clay nanocomposites prepared via in situ polymerization.