Studies of structures of poly-ε-caprolactam/montmorillonite nanocomposite: Part 2. Tests of annealed specimens

Using the novel version of thermomechanical analysis (TMA) method, a poly-ε-caprolactam (PA6) and its nanocomposite (PNC) containing 1.6 wt.% of montmorillonite (MMT) were examined. Several disc specimens of those as investigated in the first part of this article were then melted, annealed, and sheared at a rate of 0.5 s⁻¹ engendering ca. 140% strain at 240 °C in a rotational rheometer, and next solidified in ambient air. In PNC as in PA6 specimens prepared in the same way, an amorphous isotropic structure in the surface layer up to 0.5 mm thick was identified, with topological regions differing in thermal expansion properties and related a state of order. This finding differs from other tests, which detect a high level of crystallinity. Probably, it is related with small thickness of the tested layer of material and kinetics of solidification in ambient air.An increase in the free volume fraction, V_f^TMA, evaluated as equal to 3ΔαT_g (Δα is the difference in linear thermal expansion coefficients below and above the glass transition temperature T_g) and resulted from melting, annealing, shearing, and next solidification in ambient air was observed. It suggests that these operations introduce into the specimen some amount of gases (e.g., evaporated water absorbed when cooling) what increases thermal expansion over high-temperature transition of the high-temperature topological region. It means that V_f^TMA is a sum of all voids within a specimen tested (not only a real free volume) independently on their origin. Because of this, it is better to term it as micro or nanoporosity.     

Influence of a cooling rate on a structure of PA6

The structure of the plate specimens obtained from the molten PA6 that was cooled at rates between 2 and 2000 °C/min have been studied. The cooling rate of 2000 °C/min did not ensure a complete amorphization of the specimens. The amorphous phase created by supercooling is unstable and at room temperature undergoes a noticeable cold crystallization. The access of water to the specimen from the surrounding air accelerates this process.Variations in the cooling rate of the melt reflect in rearrangement of the amorphous phase of PA6. Owing to the effect of interaction with the crystalline phase, the amorphous regions undergo changes in molecular weight and MWD of the chain segments between junctions in topological regions, temperatures of the glass transition and β-relaxation, compaction, etc.     

Synthesis and thermal behaviour of polyamide 6/bentonite/ammonium polyphosphate composites

In order to achieve acceptable levels of flame retardancy of polymers, phosphorus-based flame retardant (FR) additives at about 20-30% w/w are required which is too high for conventional synthetic fibres. To know whether more finely sized particles of conventional FRs with or without nanoclay are more effective at the same concentration, composites of PA6 with bentonite and ammonium polyphosphate (APP) have been prepared by melt processing in a twin-screw extruder. XRD peaks and TEM images of PA6/Org-bentonite composite show partially ordered intercalation and ordered exfoliation. Thermal analysis in He shows that thermal stability of PA6 nanocomposite has increased by 18 °C compared with pure PA6 during degradation after 425 °C but it has decreased by 100 °C on inclusion of APP in PA6/nanoclay composites. The char yield is increased by 20% in PA6/bentonite/APP composites. No effect on thermal stability or char yield is observed by reducing the particle size of APP.     

Photoluminescence properties of BaMoO4 amorphous thin films

BaMoO₄ amorphous and crystalline thin films were prepared from polymeric precursors. The BaMoO₄ was deposited onto Si wafers by means of the spinning technique. The structure and optical properties of the resulting films were characterized by FTIR reflectance spectra, X-ray diffraction (XRD), atomic force microscopy (AFM) and optical reflectance. The bond Mo-O present in BaMoO₄ was confirmed by FTIR reflectance spectra. XRD characterization showed that thin films heat-treated at 600 and 200 °C presented the scheelite-type crystalline phase and amorphous, respectively. AFM analyses showed a considerable variation in surface morphology by comparing samples heat-treated at 200 and 600 °C. The reflectivity spectra showed two bands, positioned at 3.38 and 4.37 eV that were attributed to the excitonic state of Ba²⁺ and electronic transitions within MoO²⁻₄, respectively. The optical band gaps of BaMoO₄ were 3.38 and 2.19 eV, for crystalline (600 °C/2 h) and amorphous (200 °C/8 h) films, respectively. The room-temperature luminescence spectra revealed an intense single-emission band in the visible region. The PL intensity of these materials was increased upon heat-treatment. The excellent optical properties observed for BaMoO₄ amorphous thin films suggested that this material is a highly promising candidate for photoluminescent applications.     

Hydrolytic aging of crystallizable shape memory poly(ester urethane): Effects on the thermo-mechanical properties and visco-elastic modeling

The shape memory functionality of a segmented poly(ester urethane) and its hydrolytically aged specimens has been studied by cyclic thermo-mechanical measurements with an imposed strain of 100%. The shape memory effect was triggered by a melting transition in the soft segment phase. Aging was enforced by immersion in hot de-ionized water. In the course of the immersion the tensile properties (secant moduli, stress and strain at yield and break) were impaired by hydrolysis. Advanced specimen embrittlement finally led to rupture during the first thermo-mechanical cycle. This happened after 68 days of aging at 55 °C and correspondingly after 8 days at 80 °C. The residual strain after the first cycle, which was about 25%, increased significantly with aging time. Therefore, the total strain recoverability became ever smaller: aged specimens needed conditioning by at least two cycles for a full development of shape recoverability. Likewise the recovery force decreased continuously. Despite these degradation effects, it was observed that the shape fixity and the cycle-related shape recovery of appropriately conditioned specimens (number of cycles N > 2) remained on a constant high level (at round 100% and between 90% and 100%, respectively) throughout the whole aging period. These observations are discussed within the framework of a simplified model of the behavior of crystallizable shape memory polymers. The amorphous state of the polymer is described by the equation of the linear visco-elastic solid. As for the semi-crystalline state the material is assumed to react elastically with respect to deviations from the configuration, which was frozen up under constraint conditions. The curves of the dependence of the material behavior on aging time at 55 °C match perfectly those at 80 °C when the time axis is adjusted by a factor of 8.5, from which the apparent activation energy for hydrolytic aging in the amorphous state of 82 kJ mol⁻¹ could be deduced.     

Optimization of the photocatalytic activity of CdO + CdTiO3 coupled oxide thin films obtained by sol-gel technique

In the preparation of CdO + CdTiO₃ polycrystalline thin films by the sol-gel method, the optical, structural and crystalline properties, as well as the photocatalytic activity (PA) depends strongly on the sintering temperature (Ts) of the films and of the Ti/Cd ratio used in the precursor solution. In this work, CdO + CdTiO₃ thin films were prepared using a Ti/Cd constant ratio in the precursor solution. The films were sintered at six different Ts in the 450-550 °C range, in an open atmosphere. The structure of the films was characterized by X-ray diffraction and the PA was evaluated by the photobleaching of methylene blue in an aqueous solution using a UV-vis spectrometer. The relative intensity of the diffraction peaks associated with CdO and CdTiO₃, change with the Ts. The better photocatalytic activities were obtained for the films sintered at 490 °C and 550 °C. When the CdO was removed from the films by chemical etching the PA decreased, showing the importance of coupling both oxides.     

Formation of γ-Fe2O3 nanoparticles and vacancy ordering: An in situ X-ray powder diffraction study

The formation of maghemite, γ-Fe₂O₃ nanoparticles has been studied by in situ X-ray powder diffraction. The maghemite was formed by thermal decomposition of an amorphous precursor compound made by reacting lauric acid, CH₃(CH₂)₁₀COOH with Fe(NO₃)₃·9H₂O. It has been shown that cubic γ-Fe₂O₃ was formed directly from the amorphous precursor and that vacancy ordering starts about 45 min later at 305 °C resulting in a tripled unit cell along the c-axis. The kinetics of grain growth was found to obey a power law with growth exponents n equal to 0.136(6) and 0.103(5) at 305 and 340 °C, respectively. Particles with average sizes of 12 and 13 nm were obtained in 86 and 76 min at 305 and 340 °C, respectively. The structure of cubic and vacancy ordered phases of γ-Fe₂O₃ was studied at 305 °C by Rietveld refinements.     

Ageing properties of polyamide-12 pipes exposed to fuels with and without ethanol

The chemical and physical properties of polyamide-12 (PA12) fuel pipes/lines, aged for ≤2400 h at 110 °C, have been investigated. The pipes, containing fuel with or without ethanol, were either of a single PA12 layer, or of two PA12 layers surrounding a poly(vinylidene fluoride) barrier layer. The molar mass of the inner surface region obtained by size-exclusion chromatography was reduced during ageing, and optical microscopy revealed a surface that was partly dissolved in the presence of ethanol. Infrared spectroscopy revealed a rapid loss of plasticiser, especially in the presence of ethanol, and the fuel contained plasticiser and other polymer related components. Immersion tests at 60 °C showed that the swelling of the pipe and the amount of dissolved material were greatest for the fuels with intermediate ethanol content (50 vol.%). Aged samples experienced an increase in melting point, presumably, to a large extent, due to the loss of plasticiser and/or PA12-related components. In addition, for several samples, the crystallinity seemed to increase with ageing.     

Effects of reprocessing on the structure and properties of polyamide 6 nanocomposites

PA6 based nanocomposites (NCs) were reprocessed by repeated injection moulding to find out whether reprocessing is possible in these materials by means of the observation of the changes in the structure and mechanical properties. The studied variables were (a) the number of cycles (1-5), (b) the origin of the NC: either laboratory mixed or commercial and (c) the processing temperature (230 °C and 270 °C). Neat PA6 was also reprocessed as a reference material. In spite of the colour change, the Young's modulus, the solid state characteristics and the dispersion level were preserved upon reprocessing. The lack of change of chemical nature observed by FTIR, and the observed decreases in viscosity indicated that the main effect of reprocessing was a decrease in the molecular weight. At 230 °C the decreases in viscosity were smaller after reprocessing, and almost no change was seen in the structural parameters and properties. The decrease in the molecular weight after reprocessing at 270 °C leads to lower ductility and mainly to a decrease in the ability of the nPA6 matrix to cold draw. However, no change of the interphase conditions or agglomeration of the OMMT was detected and the NCs remained clearly ductile; thus, revealing a lack of deterioration of the interface and the ability of the NCs for recycling.     

Miscibility and hydrolytic degradation in alkaline solution of poly(l-lactide) and poly(p-vinyl phenol) blends

Poly(l-lactide) (PLLA) was melt-blended with poly(p-vinyl phenol) (PVPh) using a two-roll mill, and the miscibility between PLLA and PVPh and degradation of the blend films were investigated. It was found that PLLA/PVPh blend has miscibility in the amorphous state because only single T_g was observed in the DSC and DMA measurements. The T_g of the PLLA/PVPh blend could be controlled in the temperature range from 55 °C to 117 °C by changing the PVPh weight fraction. In alkaline solution, degradation rate of PLLA/PVPh blends was faster than that of neat PLLA because PVPh could dissolve in alkaline solution. The surface morphology of degraded PLLA and PLLA/PVPh blend were observed by SEM. The surface morphology of degraded PLLA/PVPh blend was finer than that of PLLA. Young's modulus of PLLA/PVPh blend increased with increasing PVPh content. Yield stress of PLLA/PVPh blends whose PVPh content was less than 30 wt% kept the level of about 55 MPa and that of PLLA/PVPh blend whose PVPh content was 40 wt% is much lower than that of neat PLLA.