Mechanical Behavior of Poly(Ethylene 2,6-Naphthalene-Dicarboxylate) (PEN) Fibres near the Glass-Rubber Transition Temperature

Abstract

The mechanical properties of as-spun poly(ethylene 2,6-naphthalene-dicarboxylate) (PEN) fibres were studied in order to characterize this relatively new material near its glass-rubber transition.

Tensile tests were carried out on amorphous (low-speed spun) PEN filaments. The temperature range of 90°C up to 160°C was covered using increments of 10°C. A transition from necking and cold drawing to rubber-like behavior was observed in the stress-strain relationship. Dynamic mechanical experiments were carried out on PEN yarns spun at speeds from 500 to 4000 m min^?1. Both temperature and frequency were varied. The maxima in loss modulus depend on spinning speed. Tensile behavior and dynamic mechanical behavior of PEN fibres demonstrate that the glass-rubber transition temperature of PEN is approximately 125°C.     

Extrusion drawn amorphous and semicrystalline poly(ethylene terephthalate): 3. Linear thermal expansion analysis

Thermal expansion analysis of uniaxially-oriented poly(ethylene terephthalate) (PET) films has been carried out from 123 K up to the PET glass transition temperature, T_g. The films are prepared by solidstate co-extrusion, from different premorphologies (amorphous, 33% and 50% crystalline), to draw ratios (EDR) up to 4.4, over a wide temperature range (T_ext). The coefficients of linear thermal expansion exhibit anisotropy: normal to the draw direction (α_⊥) it increases, whereas along the draw direction (α_∥) it always decreases with draw, independent of the initial morphology. Results are interpreted by treating PET as a simple two-phase composite structure. Tie-molecules occurring in the amorphous domains and bridging adjacent crystallites have a major influence. At EDR=4.4, a significant number of taut tiemolecules are developed, resulting in α_∥ becoming negative (α^max_∥=?1.0×10^?5°C^?1) at temperatures below ambient. This appears to be the first report of a negative α for a polymer of relatively low crystallinity. Temperature for extrusion draw significantly affects α. Normal to the draw direction, α_⊥ decreases with T_ext whereas α_∥ increases. The results show the thermal expansion of PET depends primarily on orientational effects. Only in the absence of anisotropy does per cent crystallinity have a dominant influence. In addition, the TMA thermograms display sharp transitions which are attributed to irreversible shrinkage of the oriented films. The shrinkage temperature (T_s) shows a strong dependence on both orientation and crystallinity, and it is discussed in association with T_g.     

Carbon nanofibres produced from electrospun cellulose nanofibres

Cellulose nanofibres have been fabricated by electrospinning of a cellulose acetate solution followed by deacetylation. The cellulose nanofibres were then carbonized using temperatures in the range 800–2200 °C and the resulting carbon nanofibres (CNFs) were characterized using transmission electron microscopy, X-ray diffraction and Raman spectroscopy. A graphitic structure was observed for CNFs treated at a relatively low temperature of 1500 °C, with no obvious skin-core heterogeneity observed for fibres treated up to 2200 °C, suggesting a possible advantage of using nano-scale precursors. The effective Young’s modulus of the CNFs was assessed using an in situ Raman spectroscopic technique following the shift in the position of the G′ (2D) band located at ∼2660 cm⁻¹ and relating this to a calibration established for a range of other carbon fibres. Using this approach the moduli of the CNFs were found to be ∼60 and ∼100 GPa for samples carbonized at 1500 and 2200 °C, respectively.     

Sol-gel preparation and thermo-mechanical properties of porous xAl2O3–ySiO2 coatings on SiC Hi-Nicalon fibres

Porous thin films of mixed aluminium silica oxides were elaborated by the sol-gel process from aluminium tri-sec-butoxide and tetraethylorthosilicate on Hi-Nicalon fibres. The porosity was generated by addition of a surfactant, namely cetyltrimethylammonium bromide (CTAB). SiC Hi-Nicalon fibres were coated by the dip-coating technique. After annealing in air (500–1200 °C) crack-free coatings were observed, with a thickness in the range 100–1000 nm. The fibres were tensile tested and results were analysed by the Weibull statistic. They showed good mechanical properties compared to the commercial fibres. The systems were characterized by thermal gravimetry, differential scanning calorimetry, X-ray diffraction, BET and scanning electron microscopy. The powders obtained in the same conditions as the coatings were highly porous with surface areas in the range 540–150 m²/g depending on the annealing temperature (400–1000 °C).     

Mechanical and dielectric relaxations in polytrifluoroethylene

Molecular motions in polytrifluoroethylene are studied by dynamic mechanical and dielectric measurements. Two absorption peaks (α_a, β) have been observed in the frequency range from 3 Hz to 300 kHz and in the temperature range from ?150° to 130°C. The molecular mechanisms for these absorptions are discussed, and the results are compared with ultrasonic, heat capacity and n.m.r. measurements. It is concluded that the α_a absorption located at 45°C (10 Hz) is associated with the large-scale motion of the main chains and the β absorption at ?35°C (10 Hz) is attributed to the limited motion of a few segments of the frozen main chains. In contrast to polychlorotrifluoroethylene the α_c absorption associated with crystalline regions has not been observed in polytrifluoroethylene. At high temperatures a steep rise in dielectric absorption is observed, which has been ascribed to a d.c. ionic conduction process.     

Infiltrated La0.5Ba0.5CoO3-δ in La0.8Sr0.2Ga0.8Mg0.2O2.8 scaffolds as cathode material for IT-SOFC

The electrochemical response of infiltrated La_0.5Ba_0.5CoO_3-δ (LBC) in porous La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 (LSGM) has been investigated. The thermal expansion coefficient (TEC) of the resulting electrode was measured, obtaining α?=?12.5?×?10^?6 K^?1, a value similar to that of LSGM. The polarization resistance (Rp) and the processes involved in the oxygen reduction reaction (ORR) for the new electrode were studied and analyzed through complex impedance spectroscopy measurements as a function of temperature and oxygen partial pressure (pO₂), using a symmetrical cell. The value of Rp for the infiltrated LBC turned out to be lower than that measured for an electrode prepared with a composite LBC-LSGM (1:1?wt%) by an order of magnitude, for the temperature range 750?°C ≤ T?≤?900?°C, and about 5 times lower for the temperature range 450?°C≤ T?≤?650?°C. At 600?°C, the LBC infiltrated cathode exhibits a polarization resistance R_p =?0.22?Ω?cm², in air. The complex impedance spectra show two processes, one identified as low frequency (LF),with a characteristic frequency of 10?Hz, and the other as intermediate frequency (IF), with a range between 0.05 and 2000?Hz. The LF process could be associated to the diffusion of oxygen in the gas phase through the pores of the electrode. Its resistance, R_LF =?0.01?Ωc?m²_, was found to be independent of the temperature and half of that obtained for the LBC composite cathode. On the other hand, the IF process is related to charge transfer at the electrode surface and the electrode-electrolyte interface. The LBC cobaltite infiltrated in the LSGM scaffolds offers an adequate thermal expansion coefficient and good electrocatalytic activity for the ORR.     

Mechanical anisotropy in oriented polychlorotrifluoroethylene

The dynamic tensile modulus (E) and loss (tan δ) of various oriented samples of polychlorotrifluoroethylene have been measured at 0°, 45° and 90° to the draw direction over the range of ? 100 to 160°C. For the cold-drawn samples the mechanical anisotropy at the lowest temperature is determined by the overall chain orientation resulting in E₀ >E₄₅ >E₉₀. Above the γ relaxation (20°C) a shear process is activated leading to a change in the anisotropy pattern to E₀ >E₉₀ >E₄₅. Cold-drawing followed by annealing gives rise to further changes so that E₉₀ >E₀ >E₄₅ above the β relaxation (120°C). The effect of annealing has been attributed to the relaxation of the amorphous regions and the development of lamellar texture, and the anisotropy in tan $\text{δ}\text{E}$ at the β relaxation is found to be consistent with the interlamellar shear model.     

Electron-microscope studies of structural heterogeneity in pan-based carbon fibres

Thin longitudinal and transverse sections of Type I (2500°C), Type II (1500°C), and Type III (1000°C) PAN-based carbon fibres have been examined by dark-field and lattice-fringe electron microscopy. Skin/core heterogeneity has been found in Type I, but not in Type II or Type III fibres. Time of oxidation (5 hr or more) had no effect on the extent of the skin, which generally comprised about 5% of the fibre and was 100–150 nm in thickness. No evidence for the popular concept of a thick circumferentially organized sheath surrounding a radially arranged core has been found in our specimens. Crystallite organisation was parallel to the surface in the skin, and random in the core, thus the c axes of the layer planes are normal to the surface in the skin. Transverse sections show the convoluted nature of the surface region and the extent of layer folding in the transverse direction, but there was no evidence for discrete fibrils. Characterization parameters have been evaluated from selected-area electron-diffraction patterns after the application of a peak-resolution procedure. The mean defect-free distance D_c (perfect L_v) was 10.0 nm in the skin and 3.3 nm in the core, for a 3 denier fully stabilized fibre (Type I); lattice distortion was 0.6% and 1.5% and preferred orientation 14° and 26°, in skin and core respectively. The change in stacking size with aximuthal angle has been measured for all fibre types and is a function of heat-treatment temperature. Large misoriented crystallites have been observed in the skin of Type I fibres and are considered to be features limiting the intrinsic strength of this material.     

Study on phase separation of PET/PEN blends by dynamic rheology

Blends of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) were processed into biaxially drawn films, and samples taken from the bi‐oriented films were then investigated by dynamic rheology experiments in the melt state. Storage modulus G′ and loss modulus G″ were determined in the frequency range of 10^?2–10² rad/s at temperatures between 260 and 300°C. Although the time–temperature superposition (TTS) principle was found to hold in the high frequency regime, a breakdown of TTS was observed at low frequencies, and the terminal behavior of the storage modulus G′ of the blends departs drastically from the terminal behavior observed for the blend components. This is caused by interfacial surface tension effects. The results indicate that despite the effect of transesterification reactions, the PET/PEN blend systems investigated consist of a microseparate phase of PEN platelets in a matrix of PET. This morphology is produced when the blends are processed into biaxially oriented PET/PEN films, and droplets of PEN are deformed into a lamellar structure consisting of parallel and extended, separate layers. The large interfacial surface area of the bi‐oriented PET/PEN blends leads to remarkably strong interfacial tension effects in dynamic rheology measurements. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of uniaxial stretching on the tensile impact test for poly(ethylene terephthalate)

In order to correlate impact strength with the structure of polymeric materials, a tensile impact test was carried out using a uniaxially stretched poly(ethylene terephthalate) (PET) sheet. The structural factors were the draw ratio (λ) and the cut-out angle (θ) under the conditions of constant impacting speed (10.5 m/sec = 1.8 × 10⁴%/sec) and temperature (20° ± 1°C). The occurrence of structural anisotropy by stretching of isotropic amorphous quench-rolled PET was classified into three drawing stages: λ = ?1.5 or 2.0, ?3.5, and over 3.5. This is related to a deformation mechanism of masses with dimensions a few tenths of a μm and their boundaries. The boundaries are not clear until adiabatic deformation (mechanical impact) is applied. The area supporting the mechanical strength was limited to just a small section. Its character was affected by macroscopic deformation only in the direction parallel or near to the stretching direction, since the mechanical properties were not changed for the range of the cut-out angle θ = 45–90°.     

Characterization of poly(amic acid)s and resulting polyimides bearing azobenzene moieties including investigations of thermal imidization kinetics and photoinduced anisotropy

Azobenzene‐containing poly(amic acid)s and resulting polyimides were synthesized. Polymers differed in the polymer backbone structure and the position at which the azobenzene group was attached to the polymer chain. Polymers were characterized and evaluated using Fourier transform infrared, ¹H NMR and UV‐visible spectroscopies, X‐ray diffraction, differential scanning calorimetry (DSC) and thermogravimetric analysis. Polyimides revealed glass transition temperatures in the range 160–265 °C, and thermal stability with decomposition temperatures in the range 336–444 °C. The thermal imidization kinetics of poly(amic acid)s was investigated by means of DSC. Kinetic parameters, such as the activation energy (E) and the frequency factor, were estimated using the Ozawa and Kissinger models. The values of E, determined using both methods, were in the range 142.52–200.92 kJ mol^?1. The photoinduced optical anisotropy (POA) was studied in the obtained azopolymers using a holographic grating recording technique. Surface relief gratings, which appeared after light exposure, were observed using atomic force microscopy. For the first time to the best of our knowledge, photoinduced anisotropy in poly(amic acid)s was studied and compared with POA in their polyimide analogues. © 2014 Society of Chemical Industry     

Isoconversional kinetic analysis of DSC data on nonisothermal crystallization: Estimation of Hoffman-Lauritzen parameters and thermal transitions in PET/MMT nanocomposites

The differential isoconversional method of Friedman is applied to non-isothermal melt crystallization DSC data to obtain effective activation energy ΔE. In comparison to neat PET, ΔE of intercalated 93A MMT clay nanocomposites (PCNs) decreased and highly dependent on the clay content. Hoffman-Lauritzen (H–L) secondary nucleation theory parameters, K_g and U* were evaluated using isoconversional approach of Vyazovkin and Sbirrazzuoli. Crystallization regime transition from regime III is observed at 190°C for neat PET, which shifted to higher temperature range 200°C–208 °C for PCNs. The K_g parameters for both regimes for PET are consistent with our isothermal experiments and reported in literature. However, the K_g values of nanocomposites are highly sensitive to the temperature dependent nucleation activity and ΔE of PET chain motion in melt and are not very much comparable with our isothermal results. Nevertheless, the observed results clearly indicate that the 93AMMT clay layers predominantly act as heterogeneous nucleating sites.     

A comparative study of rheological behavior of PET and PET/carbon black composite masterbatch

In order to explore the process of masterbatch dilution technique, the performance differences between poly(ethylene terephthalate) (PET) and PET/carbon black (CB) masterbatch were investigated. Scanning electron microscopy images of PET/CB masterbatch display a homogeneous dispersion status of CB particles in PET matrix. Besides, the improvement in heat resistance of masterbatch was confirmed by thermogravimetric analysis. The rheological measurement demonstrates that compared with PET the shear thinning effect of PET/CB masterbatch is more remarkable. The non-Newtonian index (n), activation energy of viscous flow (?E) and structural viscosity index (?η) can provide much important information for understanding the blending mechanism between PET and the masterbatch, which is valuable for spinning composite fiber with masterbatch dilution technique. According to the results of rheological measurement, the temperature is optimized to 280 °C and simultaneously the shear rate should be exceed to 1,150 s^?1 on the basis of the theory of similarity and intermiscibility.     

Unit cell variations of polyethylene crystal with temperature and pressure

The unit cell dimensions for the drawn and the extended chain crystal samples of polyethylene (PE) have been measured at temperatures between 20° and 130°C under hydrostatic high pressures up to 4000 kg/cm², by the use of high pressure and high temperature X-ray diffraction apparatus. A clear difference was found in the variation of a, b and c dimensions of orthorhombic unit cell in PE with temperature and pressure. The temperature changes of linear compressibility for each axis direction of the unit cell and for cell volume were determined on the bases of the variation of cell dimensions. For the a-axis direction, a drastic increase of the compressibility was observed above ～90°C but for the b- and c-axis directions, it was constant for all the temperature region of the measurement. The values of Grüneisen constant, γ, were evaluated at various temperatures from the compressibility data for both samples. The value of γ of the ECC sample was nearly constant below 90°C and gradually increased above 90°C; in the drawn sample, however, a rather steep increase was observed above 90°C.     

General crystallization behaviour of poly(l-lactic acid)

A study has been made of the general crystallization behaviour of poly(L-lactic acid), PLLA, and is intended to be the basis for further work on fibre formation processes. PLLA is shown to be a semicrystalline polymer that may crystallize from the melt and from solution, and that may form fibres. Spherulites grown from the melt were negatively birefringent and grew at a rate similar to that for polypropylene. The equilibrium melting point and the glass transition temperature were found to be about 215°C and 55°C, respectively. Solution crystallization resulted in lamellar crystals about 10 nm thick and electron diffraction revealed a hexagonal unit structure with dimensions smaller than reported earlier. The equilibrium dissolution temperature in p-xylene and the end surface free energy at the fold surface amounted to 126.5°C and about 0.075 J m^?2 respectively. Fibres of PLLA were formed by precipitation in a non-solvent; some of the fibres were highly porous with a pore size in the range 0.1–0.6 μm.     

Nuclear spin—lattice relaxation and molecular motions in isotactic poly(but-1-ene) and (but-1-ene)—propylene copolymers in solution

¹³C spin—lattice relaxation times have been measured at 25.15 MHz on poly(but-1-ene) and (but-1-ene)—propylene copolymers, as a function of molecular weight and concentration in 1,2,4-trichlorobenzene, and over the 50°–150°C temperature range. Maximum nuclear Overhauser enhancement was observed for all carbons. Independence of T₁ from molecular weight and bulk sample viscosity for solutions less concentrated than 50% w/v, indicates that relaxation is dominated by local conformational rearrangements rather than by overall tumbling of the macromolecule. Relaxation studies on copolymers show that this segmental motion affects only short sequences of the chain. No influence of stereochemical configurations is observed. Relaxation data for backbone carbons was interpreted in terms of a single τ isotropic motional model. Internal rotations of side CH₂ and CH₃ groups were analysed using the two and three correlation times Woessner models, and show little dependence on an eventual anisotropy of chain motion. Similar temperature dependence (18–21 kJ/mol) is observed for chain and side carbons, and even if internal reorientation of side CH₃ groups is much more rapid, it appears interlinked with segmental motion.     

Temperature-induced fibre/matrix interactions in porous alumino silicate ceramic matrix composites

The thermal stability of alumino-silicate fibre (Nextel 720)/porous mullite matrix composites was investigated in the temperature range between 1300 and 1600°C. In the as-prepared state the fibres consist of mullite plus α-Al₂O₃, while the porous mullite matrix includes minor amounts of a SiO₂-rich glass phase. Temperature-controlled reactions between the silica-rich glass phase of the matrix and α-Al₂O₃ at the rims of the fibres to form mullite have been observed. At the end of this process, virtually all glass phase of the matrix is consumed. Simultaneously, alumina-free layers about 1 μm thick are formed at the periphery of the fibres. The mullite forming process is initiated above about 1500°C under short time heat-treatment conditions (2 h) and at much lower temperature (1300°C) under long-term annealing (1000 h). Subsequent to annealing below the thermal threshold, the composite is damage tolerant and only minor strength degradation occurs. Higher annealing temperatures, however, drastically reduce damage tolerance of the composites, caused by reaction-induced gradually increasing fibre/matrix bonding. According to this study, the thermal stability of alumino silicate (Nextel 720) fibre/mullite matrix composites ranges between 1500°C in short-term and 1300°C in long-term heat-treatment conditions.     

Structure determination and microwave dielectric properties of La(MgSn)0.5O3 ceramics

La(MgSn)_0.5O₃ (LMS) has been prepared by solid-state reaction method. A single perovskite phase with super lattice reflections corresponding to octahedral tilting, A-site cation displacement and 1:1 B-site ordering was observed in X-ray diffraction pattern. The Rietveld analysis was carried out with the initial model obtained from Structure Prediction and Diagnostics Software (SPuDS). The structure is determined to be monoclinic with space group P2₁/n corresponding to a⁻a⁻c⁺ tilting system. Long range ordering parameter (LRO) between Mg²⁺ and Sn⁴⁺ ions is calculated to be 89%. Dielectric characterization at microwave frequencies is also carried out. The dielectric constant (ɛ_r), product of quality factor and resonant frequency (Q.f) and temperature coefficient of resonant frequency (τ_f) of the sample sintered at 1600 °C are 20.1, 63,000 (GHz) and −78 (ppm/°C), respectively.     

Influence of processing conditions on the ionic conductivity of holmium zirconate (Ho2Zr2O7)

Nanopowders of holmium zirconate (Ho₂Zr₂O₇) synthesised through carbon neutral sol-gel method were pressed into pellets and individually sintered for 2 h in a single step sintering (SSS) process from 1100 °C to 1500 °C at 100 °C interval and in a two step sintering (TSS) process at (I) −1500 °C for 5 min followed by (II) - 1300 °C for 96 h. Relative density of each of the sintered pellet was determined using the Archimedes’ technique and the theoretical density was calculated from crystal structure data. Grain size was obtained from SEM micrographs using ImageJ. Pellets processed by TSS have been found to be denser (98 %) with less grain growth (1.29 μm) as compared to the pellets processed using SSS process. Ionic conductivity of Ho₂Zr₂O₇ pellets sintered by two different processes was measured using ac impedance spectroscopy technique over the temperature range of 350 °C–750 °C in the frequency range of 100 mHz–100 MHz for both heating and cooling cycles. The temperature dependence of bulk (2.67⨯10⁻³ Scm⁻¹) and grain boundary (2.50⨯10⁻³ Scm⁻¹) conductivities of Ho₂Zr₂O₇ prepared by TSS process are greater than those processed by SSS process suggesting the strong influence of processing conditions and grain size. Results of this study, indicates that the TSS is the preferable route for processing the holmium zirconate as it can be sintered to exceptionally high densities at lower temperature, exhibits less grain growth and enhanced ionic conductivity compared with the samples processed by SSS process. Hence, holmium zirconate can be considered as a promising new oxide ion conducting solid electrolyte for intermediate temperature SOFC applications between 350 °C and 750 °C temperature range.     

Kinetic and thermodynamic study of methanolysis of poly(ethylene terephthalate) waste powder

Depolymerization of poly(ethylene terephthalate) waste (PETW) was carried out by methanolysis using zinc acetate in the presence of lead acetate as the catalyst at 120–140 °C in a closed batch reactor. The particle size ranging from 50 to 512.5 µm and the reaction time 60 to 150 min required for methanolysis of PETW were optimized. Optimal percentage conversion of PETW into dimethyl terephthalate (DMT) and ethylene glycol (EG) was 97.8% (at 120 °C) and 100% (at 130 and 140 °C) for the optimal reaction time of 120 min. Yields of DMT and EG were almost equal to PET conversion. EG and DMT were analyzed qualitatively and quantitatively. To avoid oxidation/carbonization during the reaction, methanolysis reactions were carried out below 150 °C. A kinetic model is developed and the experimental data show good agreement with the kinetic model. Rate constants, equilibrium constant, Gibbs free energy, enthalpy and entropy of reaction are also evaluated at 120, 130 and 140 °C. The methanolysis rate constant of the reaction at 140 °C (10.3 atm) was 1.4 × 10^?3 g PET mol^?1 min^?1. The activation energy and the frequency factor for methanolysis of PETW were 95.31 kJ mol^?1 and 107.1 g PET mol^?1 min^?1, respectively. © 2003 Society of Chemical Industry