Effects of solvent and thermal annealing on the dielectric properties and morphology of dimethyl siloxane/α-methylstyrene block copolymers

The dielectric and morphological properties of a series of 9/1 poly(dimethyl siloxane) PDMS)/poly(α-methylstyrene) (PαMS) block copolymers and a 6/4 PDMS/PαMS block copolymer have been determined as a function of solvent casting and thermal treatment. Transmission electron microscope (TEM) results show better phase separation as a function of thermal annealing and casting from cyclohexane, a PDMS preferential solvent. Dielectric studies in the temperature region of the PDMS glass transition are consistent with the TEM results and are interpreted in terms of a most probable distribution of PDMS/PαMS mixed states. When the PDMS segment molecular weight is less than the critical molecular weight, thermal annealing of the solvent cast samples produces a phase separated sample exhibiting the T_g of PDMS as well as a mixed phase. Thermal annealing of samples with the PDMS segment molecular weight greater than the critical molecular weight produces little change in the mixed structure and the dielectric data. All samples have PαMS segment molecular weights less than the critical molecular weight. The observed changes are interpreted in terms of kinetic effects associated with polymer melt viscosity (critical molecular weight) and expected mixing of the two components.     

Surface morphology and Flory-Huggins interaction strength in UCST blend system comprising poly(4-methyl styrene) and isotactic polystyrene

The surface morphology and polymer-polymer interaction parameter (χ₁₂) of UCST blend systems comprising isotactic polystyrene and poly(4-methyl styrene) (P4MS) were investigated using atomic-force microscopy (AFM) and differential scanning calorimetry (DSC). From the measured glass transition temperature and the specific heat increments (ΔC_p) at T_g, it was found that the P4MS dissolved more easily in the iPS rich-phase than did the iPS in the P4MS rich-phase. AFM result also supported that the compatibility increased more in the regions of P4MS-rich compositions than in the regions of PS-rich compositions of the PS/P4MS blends. From the measured T_g’s and apparent weight fractions of iPS and P4MS dissolved in each phase, the values of the Flory-Huggins interaction parameter (χ₁₂) were determined to be 0.0163-0.0232 depending on the composition. These results indicate that the χ₁₂ is quite dependend on the apparent volume fraction of the polymers dissolved in each phase. The values of χ₁₂ calculated from this work (method based on T_g’s of phases) were lower than those estimated using an earlier method based on the UCST or clarity temperatures. All values of χ₁₂ are greater than the values of interaction parameter at the critical point (χ₁₂)_c. This fact indicates that the iPS/P4MS blend are immiscible for all blend compositions. The surface of the phase-separated blend system was mostly covered with the P4MS rich-phase owing to its lower surface free energy in comparison with that of the neat iPS. The mechanism of surface phase separation for the P4MS blends with aPS or iPS is governed by two factors: (1) difference in the solubility of the two polymers in the solvent and (2) surface free energy.     

Synthesis,characterization and thermal behaviors of polymers containing organosilicon groups

Various bis(silyl)ethenyl groups were attached to the aromatic ring of poly(α-methylstyrene) via Peterson olefination reaction of (RMe₂Si)₃CLi (R = H, Me and Ph) with formylated poly(α-methylstyrene) (Pα-MS-CHO) to give poly(α-methylstyrene)-co-[2,2-bis(silyl)ethenyl(α-methylstyrene)] as functionalized poly(α-methylstyrene) (Pα-MS-SiMe₂H, Pα-MS-SiMe₃ and Pα-MS-SiMe₂Ph). The trimethylsilyl groups of Pα-MS-SiMe₃ have been converted to 2,2-dibromoethenyl and epoxybis(silanes) groups via NBS-based bromodesilylation and MCPBA-based epoxidation respectively. The thermal degradation behaviors of the polymers were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).     

Comparison of surface mechanical properties among linear and star polystyrenes: Surface softening and stiffening at different temperatures

In a previous study, we reported both surface softening and stiffening for poly(α-methylstyrene) (PαMS) over a temperature range from room temperature to 21 °C above the glass transition temperature (T_g) using the spontaneous particle embedment technique. In the present study, we have explored the surface mechanical responses of a linear polystyrene (PS), a 3-arm star PS, and an 8-arm star PS emphasizing the range of temperature from T_g −10 °C to T_g +10 °C, similar to that in which the prior study had shown a transition from surface softening (increased mobility) to stiffening (decreased mobility). We used atomic force microscopy for particle embedment depth estimation and the isochronal form of the Lee and Radok (LR) model for surface compliance determination. We observed both surface softening and stiffening for the linear PS and 3-arm star PS for the studied temperature range. However, surface softening was observed at all temperatures in this study for the 8-arm star PS. The surface compliance results for the different PS molecular architectures are compared with those for the linear PαMS from the previous study.     

Differences in enthalpy recovery of gradient and random copolymers of similar overall composition: styrene/4-methylstyrene copolymers made by nitroxide-mediated controlled radical polymerization

Styrene/4-methylstyrene (S/MS) random and gradient copolymers were synthesized by nitroxide-mediated controlled radical polymerization (NM-CRP) and compared to random copolymers made by conventional free radical polymerization (ConvFRP). The gradient copolymers have molecular weight (MW) values approaching 85,000 g/mol, making these some of the higher MW gradient copolymers reported to date. Due to the proximity of the glass transition temperatures (T_g) of polystyrene (PS) and poly(4-methylstyrene) (PMS), there is no significant difference in T_g between the gradient and random copolymers, with both copolymer types yielding single T_gs that typically increase slightly with increasing MS content. While enthalpy relaxation studies demonstrate similarity in random copolymers made by NM-CRP and ConvFRP, they reveal significant differences between random and gradient copolymers. Gradient copolymers exhibit broad enthalpy recovery peaks, whereas random copolymers exhibit narrower enthalpy recovery peaks. The maxima in the enthalpy recovery peaks are at substantially lower temperature, as much as 17 °C, in the gradient copolymers as compared to random copolymers of equal overall composition. While random and gradient copolymers of a given overall composition exhibit similar enthalpy recovery values at a common physical aging time and quench depth relative to T_g, the major differences in the enthalpy recovery peaks indicate that differences in sequence distribution along the chain length can lead to unusual behavior in gradient copolymers relative to random copolymers.     

Regenerable solid CO2 sorbents prepared by supercritical grafting of aminoalkoxysilane into low-cost mesoporous silica

The present work examines the functionalization of silica supports via supercritical CO₂ grafting of aminosilanes, which is an important step in the preparation of materials used as solid sorbents in CO₂ capture. Four materials have been considered as solid supports: two commercially available silica gels (4.1 and 8.8 nm pore diameter), the mesoporous silica MCM-41 (3.8 nm pore diameter) and a microporous faujasite of the Y type. Mono- and di-aminotrialkoxysilane were chosen for this study. The optimal operating conditions required to have free aminosilane in solution were first evaluated by studying the phase behavior of the system scCO₂/aminosilane at different pressures and temperatures. FTIR spectroscopy was used to determine the chemical structure of the grafted species. Aminosilane uptake was estimated by thermogravimetric and elemental analysis. Densities up to 3–4 molecules of monoaminosilane per nm⁻² were reached by using a small amount of a cosolvent together with the supercritical CO₂. The samples were characterized in regards of thermal stability, showing that aminosilane groups were covalently attached to the amorphous silica surface in the mesoporous supports, but not in the microporous zeolite. Low temperature N₂ and ambient temperature CO₂ isotherms were recorded to establish the adsorptive behavior of the prepared hybrid materials. The amine functionalized MCM-41 and the 8.8 nm silica gel exhibited a significant higher uptake of CO₂ at low pressures compared with the bare supports. On the contrary, for the 4 nm silica gel and the zeolite the adsorption decreased after impregnation.     

Enhancement in interfacial reactive compatibilization by chain mobility

This study investigates the role of polymer chain mobility, through the addition of a plasticizer, N-butylbenzene sulfonamide (BBSA), on the overall blending dynamics and interfacial copolymer chain formation in the reactive blending of brominated poly(isobutylene-co-p-methylstyrene) (BIMSM) and polyamide (PA). Compared to a non-reactive poly(isobutylene-co-p-methylstyrene) (IMSM)/PA blend, the reactive BIMSM/PA blend demonstrates a rapid, order of magnitude increase in viscosity, and attains a plateau viscosity at higher temperatures. The addition of a BBSA plasticizer (5–10 wt%) surprisingly increases the viscosity even further, suggesting an enhancement in interfacial reaction due to increased chain mobility. It is also found to increase the level of grafting from about 46 to 57% and has a significant influence on the morphology of the blend system. It is shown that the reactive grafting has an onset at lower temperatures for the blend system with 10 and 43% plasticizer. The morphologies of the blends with 5–10% plasticizer have a more uniform distribution (d_v/d_n) with a reduced d_v for the dispersed BIMSM particles. At 43 wt% plasticizer the blends show an increased phase size resulting from the significantly enhanced phase coalescence. These results indicate that enhanced chain mobility through plasticizer usage can impact interfacial compatibilization and phase coalescence and that a threshold level of 5–10 wt% plasticizer provides an optimum level of chain mobility for this reactive blending system.     

Synthesis of linear isotactic-rich poly(p-methylstyrene) via cationic polymerization coinitiated with AlCl3

Cationic polymerizations of p-methylstyrene (pMS) with H₂O/AlCl₃/triphenylamine (TPA) or triethylamine (TEA) initiating system were carried out in mixed solvents of n-hexane and dichloromethane at ?80 ～ ?50 °C. The effects of TPA or TEA concentration, solvent polarity, polymerization temperature and time on monomer conversion, number-average molecular weight (M_n), molecular weight distribution (MWD, M_w/M_n), stereoregulatity and crystallinity of poly(p-methylstyrene) (PpMS) were investigated. The stereospecific cationic polymerization of p-methylstyrene could be achieved and high molecular weight (M_n = 116,000 ～ 436,000 g mol^?1) polymers with isotactic-rich segments (more than 75% of meso dyad) along macromolecular chains could be successfully synthesized. A possible mechanism for stereospecific cationic polymerization of pMS was proposed. The propagation proceeded via the dominant back-side attack and insertion of monomer from the growing ion paired species. The steric course of propagation was mainly determined by the tightness of the growing ion paired species and steric hindrance in counteranion. The resulting isotactic-rich PpMS could form crystal morphology with 10 ～ 30 μm in size by flow-induced crystallization under pressure at 180 °C. A possible model for the aligning mechanism was sketched to describe crystallization and to explain the multi-melting peaks and lower glass transition temperatures of PpMS. This is the first example of stereospecific cationic polymerization of p-methylstyrene to get crystallizable polymers with such high molecular weights and isotacticity.     

Electrochemical Determination of Food Preservative Nitrite with Gold Nanoparticles/p-Aminothiophenol-Modified Gold Electrode

Due to the negative impact of nitrate and nitrite on human health, their presence exceeding acceptable levels is not desired in foodstuffs. Thus, nitrite determination at low concentrations is a major challenge in electroanalytical chemistry, which can be achieved by fast, cheap, and safe electrochemical sensors. In this work, the working electrode (Au) was functionalized with p-aminothiophenol (p-ATP) and modified with gold nanoparticles (Au-NPs) to manufacture the final (Au/p-ATP-Au_nano) electrode in a two-step procedure. In the first step, p-ATP was electropolymerized on the electrode surface to obtain a polyaminothiophenol (PATP) coating. In the second step, Au/p-ATP-Au_nano working electrode was prepared by coating the surface with the use of HAuCl₄ solution and cyclic voltammetry. Determination of aqueous nitrite samples was performed with the proposed electrode (Au/p-ATP-Au_nano) using square wave voltammetry (SWV) in pH 4 buffer medium. Characteristic peak potential of nitrite samples was 0.76 V, and linear calibration curves of current intensity versus concentration was linear in the range of 0.5–50 mg·L⁻¹ nitrite with a limit of detection (LOD) of 0.12 mg·L⁻¹. Alternatively, nitrite in sausage samples could be colorimetrically determined with high sensitivity by means of p-ATP‒modified gold nanoparticles (AuNPs) and naphthylethylene diamine as coupling agents for azo-dye formation due to enhanced charge-transfer interactions with the AuNPs surface. The slopes of the calibration lines in pure NO₂⁻ solution and in sausage sample solution, to which different concentrations of NO₂⁻ standards were added, were not significantly different from each other, confirming the robustness and interference tolerance of the method. The proposed voltammetric sensing method was validated against the colorimetric nanosensing method in sausage samples.     

Effect of steric restrictions on the enhancement of miscibility through hydrogen bonding in blends of modified poly(α-methyl styrene) with polymers containing acceptor sites

Two series of copolymers have been prepared from α-methylstyrene and α-methylstyrene units modified by the incorporation of the hydrogen-bond donor units, methyl carbinol and trifluoromethyl carbinol, designated PαMS ( I ) and PαMS ( II ), respectively. It has been observed that if the concentration of the donor unit exceeds 4 mol %, one-phase blends can be prepared with a series of polymers containing hydrogen-bond acceptor sites such as poly(vinyl acetate), poly(vinyl pyrrolidone), poly(4-vinyl pyridine), and some poly(alkyl acrylate)s, none of which are miscible with the unmodified PαMS. All the one-phase blends formed are stable at temperatures above the glass transition temperature, T_g, of the blend, but eventually phase separate when the temperature is increased sufficiently. These lower critical cloud-point curves have been measured for a wide range of the blends and act as an indicator of the effectiveness of the hydrogen bonds in enhancing one-phase blend formation. The T_g's of the blends are in many cases higher than those calculated from the simple rule of mixtures and reflect the reduction in chain mobility in the blends caused by extensive hydrogen bonding. The results demonstrate the effect that specific interactions have in enhancing miscibility in binary polymer blends. © 1993 John Wiley & Sons, Inc.     

Controlled atom transfer radical polymerization of MMA onto the surface of high-density functionalized graphene oxide

We report on the grafting of poly(methyl methacrylate) (PMMA) onto the surface of high-density functionalized graphene oxides (GO) through controlled radical polymerization (CRP). To increase the density of surface grafting, GO was first diazotized (DGO), followed by esterification with 2-bromoisobutyryl bromide, which resulted in an atom transfer radical polymerization (ATRP) initiator-functionalized DGO-Br. The functionalized DGO-Br was characterized by X-ray photoelectron spectroscopy (XPS), Raman, and XRD patterns. PMMA chains were then grafted onto the DGO-Br surface through a ‘grafting from’ technique using ATRP. Gel permeation chromatography (GPC) results revealed that polymerization of methyl methacrylate (MMA) follows CRP. Thermal studies show that the resulting graphene-PMMA nanocomposites have higher thermal stability and glass transition temperatures (T_g) than those of pristine PMMA.     

Enhancement of thermal conductivity of alumina/epoxy composite using poly(glycidyl methacrylate) grafting and crosslinking

Polymer grafting is a well-known method for surface modification. It improves the interfacial adhesion of the particle with the epoxy. In this study, aluminum oxide (Al₂O₃) was functionalized with vinyltriethoxysilane (VTES) to graft the carbon double bond for further polymerization. Then, glycidyl methacrylate (GMA) was polymerized to graft poly(glycidyl methacrylate) (PGMA) onto the surface of Al₂O₃ using the carbon double bond grafted in the previous step. Finally, 4.4′-diaminodiphenylmethane (DDM) was applied to bridge epoxide groups contained in the PGMA and the matrix. Consequently, the thermal conductivity of the composites increased about 320%.     

Evidence of sidewall covalent functionalization of single-walled carbon nanotubes and its advantages for composite processing

Single-walled carbon nanotubes (SWCNTs) were functionalized in a three-step procedure. The first step is a radical reaction creating a covalent bond between the carbon nanotube surface and grafted p-methoxyphenyl functional groups. In a second step, a deprotection of the methoxy functions generates free alcohol groups and in the final step an esterification is done in order to install a double bond for further polymerization. Evidence that functionalization has actually occurred on the SWCNT sidewalls is furnished through investigations involving several complementary techniques (visual dispersion tests, transmission electron microscopy, thermal gravimetric analysis and adsorption volumetry). We show that surface properties of SWCNTs are changed throughout the chemical treatments and that the obtained level of functionalization is low. Incorporation of functionalized SWCNTs in a polymer (poly(methyl methacrylate)) matrix was done through an in situ polymerization process. Observations of the obtained composites using scanning and transmission electron microscopy illustrate that interactions between the SWCNT surface and the polymer matrix are improved.     

Investigation of adsorption and inhibitive effect of 2-mercaptothiazoline on corrosion of mild steel in hydrochloric acid media

The inhibition effect of 2-mercaptothiazoline (2MT) on the corrosion behavior of mild steel (MS) in 0.5 M HCl solution was studied in both short and long immersion times (120 h) using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) techniques. For long-term tests, hydrogen gas evolution (VH₂−t) and the change of the open circuit potential with immersion time (E_ocp − t) were also measured in addition to the former three techniques. The surface morphology of the MS after its exposure to 0.5 M HCl solution with and without 1.0 × 10⁻² M 2MT with the different immersion times was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The thermal stability of the inhibitor film was investigated by thermogravimetric analysis (TGA). The value of activation energy (E_a) for the MS corrosion and the thermodynamic parameters such as adsorption equilibrium constant (K_ads), free energy of adsorption (ΔG_ads), adsorption heat (ΔH_ads) and adsorption entropy (ΔS_ads) values were calculated and discussed. The potential of zero charge (PZC) of the MS in inhibited solution was studied by the EIS method, and a mechanism for the adsorption process was proposed. The results showed that 2MT performed excellent inhibiting effect for the corrosion of the MS. Finally, the high inhibition efficiency was discussed in terms of adsorption of inhibitor molecules and protective film formation on the metal surface. TGA results also indicated that the inhibitor film on the surface had a relatively good thermal stability.     

A molecular dynamics simulation study of the influence of free surfaces on the morphology of self-associating polymers

Molecular dynamics simulations of thin films and bulk melts of model self-associating polymers have been performed in order to gain understanding of the influence of free surfaces on the morphology of these polymers. The self-associating polymers were represented by a simple bead-necklace model with attractive groups (stickers) at the chain ends (end-functionalized polymer) and in the chain interior (interior-functionalized polymer). The functionalized groups were found to form clusters in the melt whose size is representative of that found experimentally in many ionomer melts. While the size distribution and shape of the clusters in the thin films were found to be relatively unperturbed compared to their corresponding bulk melts, the morphology of the self-associating melts was found to be significantly perturbed by the free surfaces. Specifically, a strong depletion of stickers near the interface and the emergence of clearly defined layers of stickers parallel to the surface was observed. Increased bridging of clusters by the functionalized polymers was also observed near the free surface. We conclude that these effects can be associated with a high free energy for stickers in the low-density interfacial regime: stickers prefer to be in the higher-density interior of the film where relatively unperturbed sticker clusters can form.     

Neutron scattering, electron microscopy and dynamic mechanical studies of carbon nanofiber/phenolic resin composites

Carbon nanofiber (CNF)/resole phenolic resin (Hitco 134A) composites exhibited very large increases of bending storage modulus above the glass transition temperature and had higher glass transition temperatures with increasing CNF weight percentage. Small angle neutron scattering (SANS) from dilute suspensions of surface-oxidized CNF in D₂O exhibited a Guinier plateau in the q range examined, indicating that isolated scatterers exist. The CNF dispersion, average fiber diameter, average core diameter and polydispersity within the composites and in D₂O were quantified by approximating the small angle neutron scattering data with a hollow tube model. The scattering from CNF/phenolic resin composites exhibited a q⁻⁴ power law behavior, indicating the presence of sharp interfaces between fibers and phenolic resin. Surface-oxidized (PR-19-PS) CNF nanocomposites exhibited lower surface to volume ratio values and larger shell thickness compared with heat-treated (PR-19-HT) CNF composites. However, carbon nanofibers, with and without oxygenated surface groups, exhibited some agglomerates with fractal dimensions within the phenolic resin composites. Fiber surface treatment with nitric acid appears to promote dispersion and results in looser bundles of nested fiber networks.     

Adsorption of triethanolamine at the solution/mercury and the solution/air interface

Comparison of adsorption of triethanolamine at the Hg/solution and the air/solution interface has been carried out by means of interfacial and surface tension and differential capacity measurements. Adsorption is stronger at the metal/solution interface than at the free surface of the solution. This is explained in terms of some metal—adsorbate specific interaction. Adsorption parameters for the Hg/solution interface indicate that the adsorbate molecules interact with neighbouring water molecules. While the flat orientation of the large organic molecule does not change with electrode charge, the functional OH groups, slightly turned to the solution, are very likely to rotate under the action of the electrode potential thus giving rise to a field dependent inner layer capacity, C₁.     

Low-temperature reactive coupling at polymer-polymer interfaces facilitated by supercritical CO2

Supercritical CO₂ (scCO₂) has been used to facilitate reactions in thin film bilayers between functionalized polystyrene and poly(methyl methacrylate) at temperatures far below the glass transition temperatures of the respective polymers. Secondary ion mass spectrometry (SIMS) is used to monitor the reaction progression directly by measuring the interfacial excess of deuterated PS. Complementary X-ray reflectometry (XR) yields the interfacial width and surface roughness of bilayer films for reactive systems with and without the addition of scCO₂, and comparisons are made with unreactive reference systems. From XR and SIMS analyses, the interfacial width and roughness have been found to be effectively independent of the reaction conditions employed here, and the primary impact of incorporated scCO₂ is enhanced mobility of the reactive polymer chains. The use of scCO₂ can change polymer mobility significantly enough over a very small temperature range (ΔT∼15 °C) so that both diffusion- and reaction-controlled type behavior can be observed for otherwise identical systems.     

The rotational and translational dynamics of molecular hydrogen physisorbed in activated carbon: A direct probe of microporosity and hydrogen storage performance

We have measured Incoherent Inelastic Neutron Scattering (IINS) spectra of H₂ physisorbed in high purity chemically activated carbon (AC) at different surface coverage and at temperatures near the triple point of bulk hydrogen. Our experimental results and DFT calculations show that at low surface coverage, due to the very low corrugation of the adsorption potential, and in the absence of H₂-H₂ lateral interactions, the adsorbed molecules are practically free to translate in the 2D plane parallel to the surface. Model calculations show that a complete mixing between the sub-states of the J = 1 manifold occurs on the free surface. The J = 0-to-1 rotational transition should split if the H₂ molecule is adsorbed in a slit type pore. Rotational splitting of up to 13 meV is found in the narrowest pores of around 6 Å investigated. The calculated isosteric heat of adsorption for molecules adsorbed on the free surface, at different sites and molecule orientations, range between −39 and −42 meV/H₂ at 77 K. In the optimum size slit pores, these numbers double up. Micropore volume of 0.34-0.45 ml/g carbon, and an upper limit of 4 wt% hydrogen storage is anticipated for the investigated material.