Structural characterization of the δ-clathrate forms of syndiotactic polystyrene with n-alkanes

The δ-clathrate forms of syndiotactic polystyrene (s-PS) containing a homologous series of guests characterized by an increasing molecular volume (n-alkanes ranging from n-pentane to n-dodecane) have been investigated. The present analysis allows concluding that the volume of the guest plays a crucial role in the choice of the type of δ-clathrate that can be obtained. As a matter of fact, longer n-alkanes, such as n-octane, n-nonane or n-decane, turned out to give rise triclinic δ-clathrates presenting a guest/monomeric-unit ratio of 1/8, while δ-clathrates containing shorter alkanes, like n-hexane or n-heptane, instead, present a monoclinic or a triclinic structure with a guest/monomeric-unit ratio of 1/4. These results confirm the validity of a simple criterion to predict the type of s-PS δ-clathrate with a certain guest on the basis of the experimental d₀₁₀, as described in Macromolecules 2010, 43, 8549-8558. Moreover, our analysis confirms previous solid state ¹³C NMR and Raman spectroscopy data, indicating that shorter n-alkanes molecules, like n-hexane, are hosted in the cavities with an all-trans conformation while longer alkanes, like n-nonane, present bent conformations. The effect of n-alkanes molecules on the crystallization of s-PS amorphous samples has been investigated too.     

Crystalline structures of intercalate molecular complexes of syndiotactic polystyrene with two fluorescent guests: 1,3,5-Trimethyl-benzene and 1,4-dimethyl-naphthalene

The crystal structures of two molecular complex phases of syndiotactic polystyrene (s-PS) with 1,3,5-trimethyl-benzene (TMB) and 1,4-dimethyl-naphthalene (DMN) have been described. These structures present a monoclinic unit cell in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P2₁/a and unit cell constants: a=17.3 Å, b=15.4 Å, c=7.8 Å and γ=95.7° for s-PS/TMB and a=17.4 Å, b=17.2 Å, c=7.8 Å and γ=116.4° for s-PS/DMN. Both structures can be described as intercalates, since they present ac layers of polymer helices alternated to layers of contiguous guest molecules and a guest/monomeric-unit molar ratio of 1/2, as recently observed only for s-PS/norbornadiene molecular complex. On the basis of a comparison between crystalline structures and X-ray diffraction data of several s-PS molecular complexes, a simple criterion to anticipate their clathrate or intercalate nature has been suggested.     

FTIR spectra of pure helical crystalline phases of syndiotactic polystyrene

By quantitative evaluation of the spectral changes associated with guest removal from clathrate films as well as by infrared dichroism measurements on uniaxially oriented films, three infrared absorbance peaks (at 1379, 1154 and 841 cm⁻¹) prevailingly associated with the amorphous phase have been located for syndiotactic polystyrene (s-PS) samples with helical crystalline phases. A detailed study relative to the infrared peaks in the wavenumber range 870-820 cm⁻¹, corresponding to phenyl C-H out-of-plane bending, has shown that the 841 cm⁻¹ peak is fully associated with conformationally disordered sequences only for samples including trans-planar crystalline phases (α and β) but not for samples including helical crystalline phases (γ, δ and clathrates). This result has been rationalized on the basis of different intermolecular interactions between phenyl rings, occurring for trans-planar and helical s-PS crystalline phases. Moreover, a preparation procedure leading to fully unoriented films has been described for helical semicrystalline films, which generally present different kinds of uniplanar orientation. FTIR spectra of these fully unoriented semicrystalline films, by subtracting out the amorphous phase contribution with a procedure based on the 1379 cm⁻¹ peak, have allowed a straightforward isolation of the spectra of the helical s-PS crystalline phases and the quantitative determination of the amorphous content.     

Polymorphism of syndiotactic poly(p-fluoro-styrene)

Syndiotactic poly(p-fluoro-styrene) (s-PPFS) has been prepared with a polymerization procedure which allows reaching high average molecular masses and satisfactory yields. The polymorphic behavior of the polymer has been mainly studied by X-ray diffraction, calorimetric and infrared analyses. The main crystalline phase of s-PPFS, obtained by melt processing or cold-crystallization, exhibits trans-planar chains, is orthorhombic (a = 9.5 Å, b = 28.7 Å, c = 5.1 Å) and melts at nearly 320 °C. The X-ray analysis shows a strict analogy of this orthorhombic phase with the β phase of s-PS, also as for the occurrence of two limit ordered (β″) and disordered (β′) modifications, which differ for the intensity of reflections characterized by h + k = 2n + 1. A metastable crystalline phase, also exhibiting trans-planar chains, has been observed for as-polymerized samples as well as for amorphous samples crystallized by sorption of toluene or 1,4-difluoro-benzene. Mainly on the basis of solvent sorption and desorption experiments, it is suggested that this metastable phase is a co-crystalline phase with the low-molecular-mass guest molecules.     

Polymorphism of syndiotactic polystyrene: γ phase crystallization induced by bulky non-guest solvents

The crystallization of amorphous syndiotactic polystyrene (s-PS) films when induced by bulky solvents, whose molecules are too big to be enclosed as guest of s-PS clathrate phases, generally leads to the formation of γ-phase. This kind of solvent induced crystallization in amorphous films generates a preferential (200) uniplanar orientation of the γ-crystalline phase. Moreover, the presence of highly boiling solvents in the amorphous phase of these γ-form samples can induce a γ→β phase transition, as a consequence of thermal annealing procedures at atmospheric pressure. A comparison with literature data suggests that the formation of the thermodynamically stable β-phase is generally favored with respect to the formation of the kinetically favored α-phase, by any s-PS dilution.     

Polymorphism and mechanical properties of syndiotactic polystyrene films

The dynamic-mechanical behaviour and the tensile moduli of unstretched and stretched semicrystalline s-PS films, presenting different polymorphic forms (α, γ, δ and clathrate) but similar crystallinity and orientation, have been compared. The main aim is to elucidate the possible influence of different crystalline phases, being largely different in chain conformation and density, on mechanical properties of s-PS semicrystalline samples. For unstretched films presenting a preferential perpendicular orientation of the chain axes, the highest elastic modulus is observed for films with the high density γ phase while for uniaxially oriented films the highest modulus is observed for films with the trans-planar α phase. As for the clathrate films, the guest molecules when only included into the crystalline clathrate phase, have no plasticizing effect.     

Nascent morphology of syndiotactic polystyrene synthesized over silica-supported metallocene catalyst

The nascent morphology of semi-crystalline syndiotactic polystyrene (sPS) polymerized over silica-supported pentamethyl cyclopentadienyl titanium trimethoxide (Cp^∗Ti(OCH₃)₃) catalyst in a liquid slurry polymerization has been investigated under various reaction conditions. The scanning electron microscopic analysis of nascent polymers reveals that sPS molecules grow as long nanofibrils of 30-50 nm diameter and X-ray diffraction analysis shows the co-crystalline phases including both sPS and low molecular weight guest molecules of monomer and diluent. The energy dispersive X-ray spectroscopy also shows that the disintegration of silica primary particles occurs during the polymerization as evidenced by the uniform dispersion of silicon and aluminum in a polymer particle. The fibrous growth of the polymer inside a polymer particle leads to the shape replication of the original silica particles.     

Structure and properties of hard carbon films depending on heat treatment temperatures via polymer precursor

Phenylcarbyne polymer films were coated on silicon substrates and heat treated in 1 atm pressure of argon at various temperatures. The structural changes occurring during the heat treatment process of the polymer were investigated by Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The Raman and FTIR spectra features of the polymer showed a dependence on the heat treatment temperatures. At low temperatures (below 400°C), the Raman and IR spectra of the polymer were similar to those of the original polymer. The hardness and Young's modulus of the polymer films were below 1 and 50 GPa, respectively. With increasing temperature (above 400°C), thermal decomposition of the polymer occurred, resulting in structural changes of the polymer from soft amorphous hydrocarbon (400–600°C) phases to hard carbon phases (above 600°C). The hardness and Young's modulus increased from 1.5 and 65 GPa at 600°C to 9 and 120 GPa at 1000°C, respectively. It is assumed that the hard carbon film converted from the polymer might contain sp² and sp³ carbon phases; high temperature of heat treatment resulted in increasing sp² (glassy) carbon phase in the films.     

The crystallization characterization of bulk syndiotactic polystyrene sample: immediate evidence from IR spectroscopy

The crystallization characterization of bulk syndiothactic polystyrene (s-PS) sample is thoroughly studied using the Fourier transform infrared spectroscopy (FTIR). The WAXD is further used to identify the s-PS crystal formation to confirm the specific absorbance in FTIR spectra. Both melt and cold-crystallization behavior are quantitatively determined using FTIR spectra ranging from 870 to 820 cm⁻¹ at 264 °C. Fitting curves to IR spectra provides direct evidence of bulk s-PS crystallization behavior in quantification. The melt-crystallization process yields the β-form only; while the cold-crystallization process yields both the α and the β-form crystal in bulk s-PS sample. The β-form crystal is generated from the phase-transformation of the α-form crystal by cold-crystallization process, the α-form crystal is the initial phase. The activity energy of the α-form formation is lower that that of the β-form, suggesting that the α-form crystal is kinetically favorable while the β-form crystal is thermodynamically favorable.     

Polymer electrolytes based on a ternary miscible blend of poly(ethylene oxide), poly(bisphenol A-co-epichlorohydrin) and poly(vinyl ethyl ether)

Ternary blends of poly(ethylene oxide) (PEO), poly(bisphenol A-co-epichlorohydrin) (PBE) and poly(vinyl ethyl ether) (PVEE) were obtained as films and characterized by differential scanning calorimetry (DSC) and vibrational spectroscopy (FTIR). From the DSC results, phase diagrams for the ternary blends were determined, where the variation of the viscoelastic phase extent as a function of the polymers composition was determined. The DSC results also indicated miscibility of the system, exhibiting only one glass transition temperature (T_g) and decrease in the crystallinity of the system, as well as decrease in the crystallinity of PEO present in the blends. Vibrational spectroscopy (FTIR) provided information on the intermolecular interactions between the pairs PBE/PEO and PBE/PVEE, via hydrogen bond interaction. From the FTIR analyses, molecular model systems of equilibrium among the interacting structures were proposed as a molecular basis for the miscibility of the system.Polymer electrolytes based on the ternary blend containing 60/25/15 (PEO/PBE/PVEE) mass percent and lithium perchlorate (LiClO₄) were obtained and characterized by DSC, FTIR, optical microscopy and electrochemical impedance spectroscopy (EIS). Solid electrolytes containing up to 10 wt% LiClO₄ exhibited a single-phase behavior, evidenced by the DSC results. For these electrolytes, FTIR spectra indicated the formation of polymer-ion complexes, in which the cation (Li⁺) acts favoring the polymer-polymer miscibility. Electrolytes containing LiClO₄ higher than 10 wt% exhibit a multiple phase behavior, in which a PEO-rich, salt-containing phase is present in equilibrium with PBE or PVEE-rich phases. Maximum ionic conductivity at room temperature, for the electrolyte containing 20 wt% LiClO₄, reached 4.23 × 10⁻³ Ω⁻¹ cm⁻¹, while all samples exhibited conductivity of approximately 10⁻¹ Ω⁻¹ cm⁻¹ at 80 °C.     

Synthesis and characterization of novel monomers and polymers containing chiral (−)-menthyl groups

To investigate the steric effects of chiral menthyl groups on the induction of cholesteric liquid crystals and the sensitivity of the photoisomerizable azobenzene derivatives, a series of chiral monomers and a photoisomerizable chiral azobenzene derivative with various spacers end-capped with (−)-menthyl group were synthesized. The structures of the novel chiral compounds synthesized in this investigation were identified using ¹³C NMR, FTIR, and elemental analysis. The phase transition temperatures of the chiral compounds were investigated using X-ray diffraction, differential scanning calorimetry, and polarizing optical microscopy. The thermogravimetric characteristics, the glass-transition temperatures (T_g) and the weight-average molecular weights (M_w) of the homopolymers were also evaluated. Polymers containing chiral menthyl groups with a biphenyl segment were found to reveal high thermal resistance. However, the existence of the steric hindered menthyl group disturbed the arrangement of chiral monomers leading to the disappearance of liquid crystal phases. The specific optical rotation of the synthesized monomers and polymers were also evaluated. The effect of the synthesized chiral compounds, monomers and photoisomerizable azobenzene derivative on the induction of the cholesteric liquid crystal films was investigated. The morphological network structure of the polymer matrix inside a liquid crystal cell was studied using a scanning electron microscope (SEM). The phototuning ability of the AzoM on the cholesteric liquid crystals was also established.     

EPR NMR,and XPS study of LaAlO3 phase transition with enhanced magnetic and Faraday rotation properties in glass-ceramics

The phase transition of LaAlO₃ in heavy metal oxide lead-tellurite glass was obtained and verified, along with its influence on glass structure, magnetic, and Faraday rotation properties were characterized through various techniques such as nuclear magnetic resonance (NMR), X-ray neutron diffraction, transmission electron microscopy, X-ray photoelectron microscopy, vibrating-sample magnetometer, and electron paramagnetic resonance (EPR). Cubic LaAlO₃ nanocrystals were synthesized by one-pot hydrothermal technique and doped at 1, 5, 10, and 15 mol% into glasses, respectively. The LaAlO₃ nanocrystals in as-casted glasses were characterized to be around 17 nm in cubic phase belonging to the Pm3m space group. After crystallization treatment at 360 °C for 2 h, cubic LaAlO₃ phases were changed to 14–27 nm-rhombohedral phases due to the temperature-induced viscosity and strain in the melt. The phase transition yielded great modification to the glass structure through converting TeO₄→TeO₃, AlO₆→AlO_4, BO₄→BO₃, and producing non-bridging oxygen. The LaAlO₃ doping enhanced the diamagnetism of as-casted glasses, but after phase transition, the weak diamagnetic nature was changed to strong ferromagnetism and the magnetization significantly increased with the LaAlO₃ doping amount (M_s =55 emu/g). The mechanism behind magnetic property enhancement was discussed through ²⁷Al NMR and EPR of glasses. The Verdet constant of glass with 10% LaAlO₃ was greatly improved to 175 rad/ T.m at 633 nm which is much higher than the documented values in the literature.     

Ionomeric poly(urethane semicarbazides) with azobenzene groups in the main chain – studies on photoswitching behaviour and mechanical properties

Poly(urethane semicarbazide) anionomers containing azobenzene chromophores in the main chain were prepared by chain extending the isocyanate terminated prepolymers with chromophoric dihydrazide. Variations were done with respect to the hard segment content and the nature of the diisocyanate and the effect of the variations on the thermal, mechanical properties and wettability were studied. The polymers in solution showed reversible photoisomerization behaviour evinced by UV/vis spectroscopy in which irradiation of polymer solutions with UV light was observed as a decrease in intensity of absorbance corresponding to trans-form and increase in the absorbance of cis-form. The back relaxation took place in the presence of visible light. Photoswitching or the wettability of the films induced by the reversible cis–trans isomerization of the azobenzene chromophores was experimentally shown from water contact angle measurements. Irradiation of the films with 365 nm light effected a decrease in the water contact angle. X-ray diffraction results indicated dense arrangement of crystallized hard segments in compositions containing higher hard segment content. Thermal stability up to 300 °C was exhibited by the polymers as shown by thermogravimetric analysis and the phase separated morphology was confirmed by dynamic mechanical analysis. Tensile strength measurements showed that the films with increasing hard segment content exhibited increasing tensile strength and modulus but decreasing values of elongation.     

Electro-optical and electrochemical properties of an ionic polyacetylene derivative with azobenzene anisole moieties

An ionic polyacetylene derivative with azoanisole moieties was prepared by the activated polymerization of 2-ethynylpyridine by using 4-[4-(9-bromononyloxy)phenylazo]anisole without any additional initiator or catalyst in high yield. The chemical structure of poly[2-ethynyl-N-(p-methoxyphenylazophenyl)oxynonylpyridinium bromide] (PEMPB) was characterized by instrumental methods such as NMR (¹H and ¹³C), IR, UV–vis spectroscopies to have a conjugated polymer backbone system with the designed azobenzene moieties as substituents. This polymer was completely soluble in organic solvents and well processible. The electrochemical and electro-optical properties of PEMPB were studied. The cyclovoltammograms of this polymer exhibited the irreversible electrochemical behaviors between the oxidation and reduction peaks. The oxidation current density of PEMPB versus the scan rate is approximately linear relationship in the range of 30 mV/s–120 mV/s. The exponent of scan rate, x value of PEMPB, is found to be 0.6. The absorption spectrum starts around 700 nm and shows a characteristic absorption band at visible region of 464 nm due to the π → π* interband transition of the polymer backbone, which is a peak of the conjugated polyene backbone. The photoluminescence spectrum showed that the PL peak is located at 539 nm corresponding to the photon energy of 2.30 eV.     

Polymeric dispersions of model azobenzene dyes

New host-guest systems were prepared by using poly(vinyl acetate) and different ethylene-vinyl acetate copolymers as polymeric hosts because of their good compatibility with polar molecules. Two symmetric azobenzene diesters, that is 4,4′-dicarboxyethylazobenzene and 4,4′-di(2-ethylhexyloxycarbonyl)azobenzene were selected as guests. Both solution casting and melt processing were adopted for the preparation of dispersion films that were thoroughly characterized by thermal analysis, UV-vis and FT-IR spectroscopy, and by polarized optical microscopy. The reported results indicate that solution casting afforded heterogeneous dispersions of dye microcrystals at guest concentrations larger than 0.3% whereas homogeneous colored films were obtained at lower dye contents. On the other hand, both melt processing and the presence of branched diester groups favored the dye dispersion within the polymer matrix. At 0.1% dye content, the adopted preparation technique did not appreciably affect the film properties. After 4-8 fold stretching, the host-guest films were analyzed by polarized light spectroscopy. Some of the investigated films displayed interesting polarization efficiency, potentially suited for the preparation of thin film polarizers.     

Plasma polymerized ferrocene films

This communication describes the formation of high index of refraction polymer thin films using a novel plasma polymerization deposition process. A flowing afterglow plasma reactor was modified to enable sublimation of solid samples into the gas phase for subsequent plasma polymerization. Thin films of plasma polymerized ferrocene were deposited on substrates and subsequently characterized. The refractive index as a function of processing conditions was obtained. Relatively high values of n (∼1.73 at 589 nm) were obtained. The chemical nature of the polymer thin films was characterized using FTIR and XPS spectroscopy. This work demonstrates that plasma polymerization is an enabling technology for the fabrication of photonic thin films that utilize solid state precursors.     

Morphological,structural and ellipsometric investigations of Cr doped TiO2 thin films prepared by sol–gel and spin coating

Pure and chromium doped titanium dioxide (TiO₂) thin films at different atomic percentages (0.5%, 1.3% and 2.9%) have been elaborated on ITO/Glass substrates by sol–gel and spin–coating methods using titanium (IV) isopropoxide as a precursor. The surface morphology of films was investigated by scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM), the structure was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and high resolution transmission microscopy (HRTEM). SEM and HRTEM show homogenous and polycrystalline films. XRD patterns indicate a phase transition from anatase to anatase-rutile leading to expand the absorption band of TiO₂ molecules around 520 cm⁻¹ in FTIR spectra. The optical constants such as the refractive index (n), the extinction coefficient (K) and the band gap (E_g) as well as the film thickness are determined using spectroscopic ellipsometry technique and Fourouhi–Blommer dispersion model. Results show three major changes; (i) the thickness of pure TiO₂ layer is 54 nm, which linearly decreases when the layer is doped with chromium and reaches 33 nm for a doping concentration of 2.9%, (ii) the band gap energy (E_g) is also linearly reduced from 3.24 eV to 2.80 eV when the Cr-doping agent increases, and, (iii) a phase transition from anatase to anatase-rutile is observed causing an increase in values of n(λ) for wavelength greater than 350 nm.     

Methane oxidation by lattice oxygen of CeNbO4+δ

The reactivity of methane with lattice oxygen of cerium niobate, CeNbO_4+δ, was studied by temperature-programmed reduction (TPR) in dry CH₄ flow at 523–1073 K. Phase transformations and reduction of cerium niobate at 900–1023 K lead to a massive release of hyperstoichiometric oxygen, in amounts determined by the intermediate-temperature phase composition dependent on thermal history. In this temperature range, CH₄–TPR shows prevailing formation of carbon monoxide and steam, suggesting that the synthesis gas generation occurs in parallel with extensive oxidation of H₂ on the cerium niobate surface. At 1073 K when δ → 0, the reaction of methane with CeNbO_4+δ selectively yields synthesis gas with H₂/CO ratio close to two.     

Conformation transition and molecular mobility of isolated poly(ethylene oxide) chains confined in urea nanochannels

Inclusion compounds formed from host small molecules and guest polymers have provided a novel platform to study the behavior of isolated polymer chains confined in nanochannels. In this article, the PEO chain conformation in the metastable poly(ethylene oxide) (PEO)-urea inclusion compound (IC) and its transition was characterized via a combination of different analytical methods. Based on the FTIR and Raman spectroscopy results, PEO chains in the metastable tetragonal IC are tentatively assigned to the tgg′ conformation. The structural changes of the metastable tetragonal IC to the stable trigonal form were observed via in situ FTIR and ex situ WAXD. The transformation is a kinetic solid-solid process and can even occur at room temperature. The activation energy of about 222 kJ/mol indicates that the transition occurred via cooperative disruption of several hydrogen bonds. Measurement of the laboratory frame spin-lattice relaxation time T₁ (¹³C) shows that molecular motions of the nanoconfined PEO chains are more intensive than the neat crystalline PEO but weaker than those of the neat amorphous PEO. Second harmonic generation microscopy demonstrates that the trigonal IC exhibits stronger nonlinear optical activity than the tetragonal IC. The intermolecular hydrogen bonding is attributed to the driving force for the transformation of the metastable tetragonal IC into the stable trigonal form.     

Ferrite ceramic filled poly-dimethylsiloxane composite with enhanced magnetic-dielectric properties as substrate material for flexible electronics

Improving magnetic-dielectric properties of polymer materials, through filler of functional ceramics, provides feasibility to develop high-frequency flexible electronics. Poly-dimethylsiloxane (PDMS), an inert silicone with low elasticity modulus and high transparency, has been considered a promising candidate for flexible electronics. Current PDMS matrix used in high-frequency devices suffers from unsatisfactory properties due to very low dielectric constant. In this study, using ultrasonic stirring and vacuum-pumping process, we prepare a series of xCo₂Z/PDMS (x = 2; 4; 6; 8; 10) composite films, which are consisted of PDMS matrix and different quantity of micro-sized ferrite particles. XRD pattern indicates that the obtained ferrite particles include Co₂Z main phase and BaM second phase. We demonstrate that 4Co₂Z/PDMS film has improved magnetic-dielectric properties at 800 MHz (μ' = 1.49; ε' = 4.54 tanδ_μ = 0.058; tanδ_ε = 0.008). Also, the film has high saturation magnetization (σ_s = 17.51 emu/g). Furthermore, SEM micrographs show that using ultrasonic stirring and fast curing, the micro-sized ferrite particles are well dispersed in PDMS matrix. Our study, which provides a simple method to improve high-frequency magnetic and dielectric properties of PDMS matrix, could pave the way for development of high-frequency flexible electronics.