Thermo-mechanical behavior of uniaxially drawn and crystallized poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films

Thermo-mechanical properties of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films in the amorphous state, uniaxially stretched and thermally crystallized were compared and show different microstructure–property relationships. The glass transition temperature shifts to higher temperatures when films are stretched above T_g at 160 °C, in relation with a crystallinity increase and subsequent confinement effects of the amorphous phase, but films stretched below T_g at 100 °C show an opposite trend while drawing. This is discussed in relation with the highly mobile structural state arising from plastic deformation of the amorphous phase. The confinement effects were particularly assessed by the estimation of a ‘rigid amorphous fraction’ (RAF). Crystallinities and RAF were estimated from DSC data for the uniaxially drawn and thermally crystallized films respectively. It was observed that the RAF was higher in thermally crystallized films than in uniaxially drawn samples in relation to a lamellar semi-crystalline morphology. In addition to this detailed thermo-mechanical characterizations, low stress creep measurements were carried out during heating scans. They can be related to the microstructure and thermal transitions of the films. In particular, for the uniaxially stretched films, the measured shrinkage decreases when the draw ratio increases because the crystalline entities developed during the draw process are locking the chain motions.     

Monitoring the cold crystallization of poly(3-hydroxy butyrate) via dielectric spectroscopy

Dielectric spectroscopy has been used to monitor the cold crystallization kinetics of poly(3-hydroxy butyrate), PHB, just above the glass transition temperature of the amorphous chains. Although the polymer shows a relatively complex dielectric scenario, an easy and fast analysis of the crystallization kinetics was performed by choosing an appropriate temperature range in which the structural relaxation is the only process present in the spectra of the amorphous samples. It was possible to monitor in real time the crystallization kinetics by analysing the changes in the intensity of the α-relaxation process. A recently proposed methodology to overcome some systematic deviation of the Avrami approach, was applied to treat the experimental data.     

Synthesis and structure of bis(2,6-dimethylpyrazine)(THF)copper(II) nitrate

Attempts to crystallize Cu(2,6-dmpz)(NO₃)₂ (2,6-dmpz = 2,6-dimethylpyrazine) by reaction of 2,6-dmpz with Cu(NO₃)₂·3H₂O in THF resulted in the synthesis of Cu(2,6-dmpz)₂(NO₃)₂·THF. The compound crystallized in the monoclinic space group C2/c and exists as isolated molecules with distorted square pyramidal geometry: a = 11.609(4) Å, b = 10.415(3) Å, c = 18.256(6) Å, = 108.332(4)°. The 2,6-dmpz molecules coordinate to the Cu ions through the distal (remote from the methyl) nitrogens. The molecules are arrayed in stacks parallel to the b-axis.     

Effect of ethylene carbonate in poly (methyl methacrylate)-lithium tetraborate based polymer electrolytes

Thin films composed of poly (methyl methacrylate) (PMMA), lithium tetraborate (Li₂B₄O₇) and ethylene carbonate (EC) were prepared by solution casting method. The highest ionic conductivity at room temperature was achieved for the composition PMMA:Li₂B₄O₇:EC (42:18:40) with the value 1.29 × 10⁻⁵ S cm⁻¹. The presence of plasticizer in the polymer complex is crucial in improving the ionic conductivity by increasing the concentration of free mobile ions through the structural conversion from crystalline to amorphous phase. This conversion lowers the viscosity of the polymer complex. Conductivity-temperature plots were found to obey Williams-Landel-Ferry (WLF) mechanism. Dielectric data was analyzed using the dielectric permittivity (ε′) and dielectric modulus (M′) of the samples. Fourier transform infrared (FTIR) studies confirmed that complexation occurs between PMMA, Li₂B₄O₇ and EC. Thermal stability of the polymer complex, which decreases with the addition of plasticizer (EC), was determined using thermal gravimetric analysis (TGA).     

Tribologic properties and crystallization behaviour of filled poly(tetrafluoroethylene) resins

The coefficient of friction and wear rate of poly(tetrafluoroethylene) filled with molybdenum disulphide, graphite, lead oxide, and basalt are determined. It is found that basalt decreases the wear rate 2000 times. The degree of crystallinity and other nucleation characteristics are also studied. It is estimated that the nucleation activity of basalt is 0.9. The specific surface energy at the melt/crystal interface is found to be 9 · 10⁻³ J/m² and the adhesion energy between poly(tetrafluoroethylene) and basalt is 2 · 10³ J/m².     

Dielectric and mechanical relaxation behavior in poly(butyl methacrylate) isomers

Dielectric and mechanical spectroscopies have been used to study the dynamics in poly(butyl methacrylate) isomers (namely PnBMA, PiBMA and PtBMA). This analysis has revealed four relaxation processes: δ, γ, β and α and has allowed the observation of the evolution of molecular relaxation phenomena with the lateral chain steric hindrance. The α relaxation is associated with the glass transition of the isomers and depends strongly on the lateral chain structure. The secondary relaxation phenomena are very similar for the three isomers. Arrhenius diagrams have been plotted for the three isomers. The activation parameters have been calculated from Arrhenius equation for the sub-glass processes δ and γ whereas the dynamics are characterized by a Vogel–Fulcher–Tamman law for the α relaxations. With a deconvolution procedure for the α and β processes, a departure from the Arrhenius law of the β process in the vicinity of the merging between the α and β phenomena is observed for each isomer.     

Peculiarities of dielectric relaxation in poly(vinylidene fluoride) with different thermal history

A method of broad-band dielectric relaxation spectroscopy (10⁻¹–10⁷ Hz) has been used to study the relaxation processes in the region of melting in two isotropic poly(vinylidene fluoride) films prepared by way of crystallization from melts under different conditions. The intensive relaxation process observed is attributed to manifestation of a space charge formed by extrinsic carriers. The relaxation time and the intensity of the process reduce when the crystallinity increases. This is related to the increase of extrinsic carriers’ mobility in the amorphous areas. It is supposed that at isothermal crystallization there are more chemical defects (end groups, branches, HHTT-type defects) in amorphous areas. Such defects may impede compact chain packing in the amorphous domains and provide conditions for a higher level of mobility of free carriers.     

Effect of AC and DC voltage on dielectric properties of CeO2 films

The capacitance-voltage characteristics have been studied for both AC and DC fields of Al/Cerium Oxide/Al thin film capacitor structures. Compared to DC fields, these structures can withstand higher AC fields. Capacitance increases with increase in AC and DC fields. The former behaviour is explained on the basis of presence of oxygen vacancies and the latter one is attributed for the Joule heating phenomena. The presence of oxygen vacancies were confirmed from the thermoluminescence studies on these films.     

Self-assembly and confinement behaviors of poly(ethylene glycol) distearate within lamellar-mesostructured silica

Novel lamellar-mesostructured poly(ethylene glycol) distearate (PEGDS)/silica hybrids have been synthesized successfully through self-assembly with a sol–gel technology, in which the PEGDS is imbedded within silica interlayers through a micelle-templating technique by seeding rod micelles of the PEGDS as surfactant aggregate during the condensation of the silica sol performed by the hydrolysis of organosilane precursor. The well-ordered lamellar mesostructure was confirmed by X-ray diffraction patterns, transmission electronic microscopy, and N₂ adsorption–desorption isotherms. Studies on confined behaviors of the PEGDS within silica matrix indicated that the molecular chains of the PEGDS with two-dimensional degree of freedom could still crystallize, though they were stranded one-dimensionally between the silica interlayers. However, the confinement effects of the lamellar mesostructure caused a significant decrease in both the melting and crystallization temperatures of the PEGDS.     

Thermal characterization, melting behaviors, dynamic mechanical properties, and morphology in the polymorphism of syndiotactic polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide)

The thermal characterization, melting behaviors, dynamic mechanical properties, and morphology of syndiotactic polystyrene and its blends with poly(2,6-dimethyl-1,4-diphenylene oxide) were examined by differential scanning calorimetry, dynamical mechanical analyzer, and scanning electron microscope. With differential scanning calorimetry and scanning electron microscope results, two melting endothermic peaks and blending morphology were observed. We suggest that the lower melting endothermic peak is due to the melt of syndiotactic polystyrene, however, the higher melting endothermic peak comes from the melt of recrystallized syndiotactic polystyrene crystals during the differential scanning calorimetry scan. With dynamical mechanical analyzer results, we find that addition of poly(2,6-dimethyl-1,4-diphenylene oxide) into syndiotactic polystyrene can increase the soft temperature and toughness. Moreover, the experimental results also reveal that crystallization of syndiotactic polystyrene in the syndiotactic polystyrene/poly(2,6-dimethyl-1,4-diphenylene oxide) blend is impeded in the presence of poly(2,6-dimethyl-1,4-diphenylene oxide) and the melting behaviors of syndiotactic polystyrene/poly(2,6-dimethyl-1,4-diphenylene oxide) blends are homogeneous.     

Molecular dynamics in thin films of isotactic poly(methylmethacrylate) – revisited

The molecular dynamics in thin films of isotactic poly(methylmethacrylate) is investigated by broadband dielectric spectroscopy. No shifts in the mean relaxation time of the dynamic glass transition are detected down to a film thickness of 6.6 nm. These results are compared to previous measurements on thin films of isotactic poly(methylmethacrylate) [L. Hartmann et al., Eur. Phys. J. E 8 (2002) 145]. We show that in the latter case, the shifts in the dynamic glass transition are caused by an erroneous analysis of the experimental results.     

Charge carrier generation,transport and trapping in amorphous poly (N-vinylcarbazole) films

Studies on hole photoinjection in films of poly (N-vinylcarbazole) under space-charge-free conditions by the time-of-flight technique have revealed the presence of deep hole traps. The trap density is estimated to be 5–10 × 10¹⁸ m^?3. The voltage and thickness dependence of the initial current in the space-charge-free photocurrent transient is explained in terms of the field dependence of the carrier generation and transport processes.     

Isothermal crystallization behavior and crystal structure of poly(ethylene terephthalate‐co‐1,4‐cyclohexylene dimethylene terephthalate) (P(ET/CT)) copolyesters

Poly(ethylene terephthalate) (PET) and a series of poly(ethylene terephthalate‐co‐1,4‐cyclohexylene dimethylene terephthalate) (P(ET/CT)) copolyesters with different molar ratios of ethylene glycol (EG) to 1,4‐cyclohexanedimethanol (1,4‐CHDM) were investigated by their isothermal crystallization behavior, transparency, crystal structure and morphology changes in different isothermal crystallization process using differential scanning calorimetry (DSC), digital photos, wide‐angle X‐ray diffraction (WAXD) and polarized optical microscopy (POM). The results revealed that P(ET/CT) copolyesters with ≤ 15 mol% 1,4‐CHDM and ≥ 50 mol% 1,4‐CHDM, such as P(ET/CT)(85/15), P(ET/CT)(50/50) and P(ET/CT)(30/70), were crystallizable, while that with 30 mol% 1,4‐CHDM, namely P(ET/CT)(70/30), was amorphous. The crystallization rate, crystallinity and transparency of these copolyesters underwent the same isothermal crystallization process, i.e. it first decreased and then increased remarkably with the increase of 1,4‐CHDM content. Accordingly, the crystal structure of these copolyesters changed from PET‐type lattice to PCT‐type lattice when the copolyester with around 30 mol% 1,4‐cyclohexylene dimethylene terephthalate (CT) unit. Interestingly, the small incorporation of 1,4‐CHDM into PET could lead to the formation of larger spherulite crystals than that of PET. But small and grainy crystallites appeared with further increase in the 1,4‐CHDM content.     

Studies on the structural heterogeneities of poly(dimethylsiloxane) networks modified with poly(phenylsilsesquioxane)s using small-angle X-ray scattering and dynamical mechanical analysis

In the present investigation the morphological study of self-supported translucent films, constituted of semi-inorganic polymeric materials prepared by sol–gel process from poly(phenylsilsesquioxane) (PPSQ) and poly(dimethylsiloxane) (PDMS), modified by diphenylsilanediol (DPS), phenyltriethoxysilane (PTES) and/or tetraethoxysilane (TEOS), is reported. Small-angle X-ray scattering evidenced phase segregation between PPSQ and PDMS in the PPSQ/PDMS film (M1). The addition of TEOS to the sol, constituted by PPSQ and PDMS, led to morphological changes, characterized by nanoparticles of PPSQ and/or SiO₂ embedded in the PDMS-rich matrix. On its course, the use of diphenylsilanediol and phenyltriethoxysilane mixture, as an additive, led to more homogeneous films in comparison to PPSQ/PDMS.     

Spectral and barrier properties of heterocomposites based on poly (para-phenylene) disposed between the silicon films

ABSTRACT

Within the framework of the methods of the electron density functional and the ab initio pseudopotential, we have obtained the spatial distributions of the valence electrons density, the electron energy spectra and the Coulomb potential for heterocomposites based on poly (para-phenylene) and carbon nanotube disposed between the silicon films.

?It was revealed that the maximal value of the potential barrier was noticed in a composite material from polyparaphenylene filaments placed between silicon films and completed with carbon nanotubes perpendicularly to the surface of the silicon films and along them.     

The structural transformation and properties of spin-on poly(silsesquioxane) films by thermal curing

In this study, the structural transformation and properties of five commercially available poly(silsesquioxanes) by thermal curing were investigated, including poly(hydrogen silsesquioxanes) (HSQ and T12), and poly(methylsilsesquioxanes) (MSQ, T7 and T9). These materials with a different cage/network ratio and side groups (Si-H and Si-CH₃). The FTIR spectra show that the poly(silsesquioxane) films have different contents of the Si-O-Si cage and network structures, which significantly affects the refractive index and dielectric constant. The shifting of the Si-O-Si network band in the FTIR spectra can be correlated with their molecular structures. The refractive indices and dielectric constants of the studied poly(silsesquioxane) films increase with increasing the Si-O-Si network content. The retention of the Si-H or Si-CH₃ side group suggests the existence of the cage structures in the poly(silsesquioxane) films. The Si-O-Si cage structure results in a larger free volume than the Si-O-Si network structure in the poly(silsesquioxane) films and thus reduces the refractive index and dielectric constant. It is supported by the porosity result. The order of the refractive index in the studied poly(silsesquioxanes) films is T12>HSQ for the Si-H side group and T7>T9>MSQ with the Si-CH₃ side group, which can be correlated with the Si-O-Si network content. The poly(silsesquioxane) film with the Si-CH₃ side group has a lower refractive index than the Si-H side group at the same Si-O-Si network content, which is probably due to the steric hindrance effect of the CH₃ group.     

Structural and spectroscopic characterization of poly(styrene sulfonate) films doped with neodymium ions

This work reports the structural and spectroscopy characterization of poly(styrene sulfonate) (PSS) films doped with neodymium (Nd) ions. Nd–PSS films were processed using the acid of poly(styrene sulfonate) – H–PSS and neodymium nitrate – Nd(NO₃)₃; the maximum incorporation of Nd ions in the polymeric matrix was equal 19.3%. The absorption in the UV–Vis–NIR spectral region presents typical electronic transitions of Nd³⁺ ions, with well resolved peaks. The infrared spectra present the transition bands of PSS with characteristic line shape broadening, and the presence of vibrational modes of N–O groups in the range of 1400–720 cm^?1, prove the permanence of Nd(NO₃)_x, with x = 1, 2 and/or 3, in the H–PSS matrix. UV–Vis site selective photoluminescence data indicate that the incorporation of Nd³⁺ introduces a blue shift in PSS emission (325–800 nm), decreasing the interaction between adjacent PSS lateral groups (aromatic rings). Nd³⁺ reabsorption and energy transfer effects between the PSS matrix and Nd³⁺ were also observed. The IR emission of Nd–PSS films at 1076 nm (⁴F_3/2 → ⁴I_11/2) present constant efficiency, independent on Nd³⁺ concentration. The Judd–Ofelt theory was employed to analyze radiative properties. The excitation spectra prove the energy transfer between the polymeric matrix and Nd³⁺. Complex impedance data was used to probe relaxation processes during the charge transport within the polymeric matrix.     

Influence of the macro and micro-porous structure on the mechanical behavior of poly(l-lactic acid) scaffolds

The development of macroporous biodegradable polymeric materials with three-dimensional pore structure is an important research field in tissue engineering. Structural scaffolds not only provide the cells with a mechanical support, but also perform an interactive physico-chemical role in tissue regeneration, thus it becomes important to be able to tune their mechanical properties to deliver appropriate mechanical signals to adhered cells for proper tissue regeneration. This work presents two series of poly(l-lactic acid) (PLLA) scaffolds in which we modulated the mechanical properties by systematically changing two synthesis parameters: polymer/solvent ratio and polymer-solution/porogen percentage. The peculiarity of the constructs is the presence of a double porosity: micropores generated by dioxane solvent using a freeze extraction technique and macropores produced by the leaching of macroporogen spheres. An increase in the PLLA/dioxane ratio decreases the micropores size and also influences to some extent the macropores size, due to the ability of dioxane to swell macroporogen particles. On the other hand, an increase in the amount of macroporogen increases the porosity by increasing the dimension of pore the throats connecting the macropores. Consequently, the increase in the PLLA/dioxane ratio produces a significant decrease in the permeability, and an increase in the apparent compression Young's modulus and aggregate modulus. When increasing the amount of macroporogen the permeability significantly increases and a decrease in the mechanical properties of the scaffolds is observed. Summarizing, with a systematic change of two fabrication parameters (amount of dioxane and macroporogen) the structural characteristics of the scaffolds were modulated and thereby their mechanical and transport properties were controlled.     

Processing and characterization of a novel nonporous poly(vinilidene fluoride) films in the β phase

Poly(vinylidene fluoride) (PVDF) has remarkable properties leading to electro-optics, electro-mechanical and biomedical applications. In particular, its piezo- and pyroelectric properties provide possibilities for many technological applications. The semicrystalline nature of PVDF, combined with the occurrence of at least four crystalline phases implies a complicated physical microstructure. The most frequently described and important phase is the β phase. The piezo- and pyroelectric properties mainly depend on this phase, so increasing the β phase content has always been a great concern. It is possible to obtain films in the β phase by solution but this material presents a high porosity leading to an opaque appearance and a decrease of the mechanical and electrical properties. In this work, porous films in the β phase were obtained directly from the solution at 60 °C. After applying pressure perpendicular to the surface of the film at elevated temperature, the pores in the original sample are eliminated. The changes on the morphology and crystallinity associated to the pressure treatment were also studied.