Amorphous PEN Research Articles

Flexural and low velocity impact characterization of thermoplastic composites based on PEN and high performance woven fabrics

Poly(Ethylene 2,6‐Naphthalate)—PEN has been used as matrix for laminates with the aim to develop woven fiber reinforced composites with a service temperature as high as 100°C. Three different classes of fibers have been considered as reinforcement in woven fabric form: carbon, polymers (Twaron and Vectran) and mineral (basalt). Composites were manufactured by using the film stacking technique, alternating layers of balanced plain weave fabrics (0/90) and films of amorphous PEN. The manufacturing process was optimized to obtain a good impregnation of fibers and very low void content. Static flexural tests, dynamic mechanical analysis and low velocity impacts were used to characterize the mechanical behavior of the composites. The flexural modulus and strength of laminates resulted to be very high, in particular for the PEN/Carbon and PEN/Basalt systems. The storage modulus from the dynamic mechanical analysis indicated that 100°C is a suitable service temperature for all configurations. The impact resistance of the composites showed a different trend with respect to static tests, since PEN/Basalt and PEN/Aramid exhibited the best performances. These results prove that PEN is a viable option for the production of structural composites as replacement of thermosetting matrices due to its high performance/cost ratio, in particular when combined with basalt fibers. POLYM. COMPOS., 39:2942–2951, 2018. © 2017 Society of Plastics Engineers

Effect of temperature on static and low velocity impact properties of thermoplastic composites

Abstract In this work, thermoplastic composites based on poly(ethylene 2,6-naphthalate) (PEN) have been investigated with the aim to elucidate the effect of temperature on static and impact properties. The matrix was reinforced with four different high performing woven fabrics based on carbon, Twaron, Vectran, basalt fibres. Composites were manufactured by using the film stacking technique, alternating layers of balanced plain weave fabrics (0/90) and films of amorphous PEN, keeping the fibre volume fraction around 40%. The compression moulding process was optimized to obtain optimal fabric impregnation and to keep the void content lower than 1%. The structural response was evaluated at 20 °C, 60 °C and 100 °C by means of static flexural and low velocity impact tests. Dynamic mechanical scans (DMA) were also performed to evaluate the stiffness of the laminates at temperatures ranging from – 80 °C to 230 °C. The flexural modulus and strength of laminates resulted to be very high in proportion with the fibre stiffness. The flexural behaviour was affected by the temperature but a limited reduction of the stiffness (lower than 20%) was exhibited by all but PEN/Vectran up to temperatures of 100 °C. PEN/carbon resulted the best performing system at each testing temperature (flexural modulus equal to 38.8 GPa, flexural strength equal to 714 MPa at 20 °C). Results from DMA tests demonstrated a satisfactory agreement with static tests (differences within ±13%, except that for PEN/Vectran). The impact resistance of the composites was found to depend on the fibre type, temperature and interface strength. PEN/basalt resulted the best performing composite followed by PEN/Twaron (perforation energy at 20 °C: 44.3 J and 38.2 J, respectively). The PEN/carbon laminate exhibited the lowest impact performance but, unlike PEN/basalt and PEN/Vectran, the impact resistance improved with the increasing temperature.

Mechanical behavior of solid and foamed polyester/expanded graphite nanocomposites

Poly(ethylene 2,6-naphthalate)- PEN is a thermoplastic polyester characterized by a high glass transition temperature (125°C), comparable to that of polyetheretherketone (143°C), but with a significantly lower melting temperature (265°C). Its physical and chemical properties are very promising for applications in transport industry and aeronautics. Nanocomposite matrices based on PEN and expanded graphite were developed to be used as matrix for foams. Expanded graphite was melt blended with the polymer by means of extrusion process and its effects on the foaming properties were investigated through solid state foaming process. Graphite nanoparticles increased the crystallization kinetics of the polymer, inducing the formation of small crystals but lowering the total amount of crystalline phase. Transmission electron microscopy analysis showed a good dispersion of the nanofiller but some aggregates were still present, as also confirmed by graphite peak in the X-ray diffraction patterns of all nanocomposites. The elastic modulus of nanocomposites with amorphous matrix increased with respect to the neat amorphous PEN, while the modulus of crystallized nanocomposites decreased. Nanocomposite foams were successfully prepared, and an higher cell density was obtained when compared to the neat PEN. In the latter case, a strong increase in both yield and strain at break was measured. Furthermore, the elastic modulus and compressive yield stress of foamed PEN nanocomposites increased with the expanded graphite.

Mechanical deformation and electrical breakdown of PEN at high electric fields

AbstractIn this work, samples of amorphous and semicrystalline PEN (polyethylene‐2,6‐naphthalene dicarboxylate) were subjected to DC electric fields for different durations up to their dielectric breakdown, with or without depolarization. The field‐induced mechanical strains in the samples were studied by means of a nondestructive optical technique without resorting to any physical contact. The results showed that amorphous samples were more vulnerable to the field‐induced mechanical deformation when compared with similar semicrystalline samples. However, both morphologies showed in some samples contradictory behavior at prebreakdown field strength, some caused yielding of the polymer while the others seemed to have the opposite effect. In the latter category, yet some showed the maximum yield point at a particular critical field starting from which the deformation began to diminish significantly to reach a total cancellation at the breakdown field. This critical field quantified at 260 kV/mm is probably a pronouncement of critical ageing and breakdown in PEN. The deformation seems to be an evolution very similar to the evolution of DC current at high fields. Furthermore, the varying response of the samples implies further work to be undertaken to validate a clear mechanism. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Establishing a three‐dimensional diagram with solubility parameter representing the general behavior of crystallization for amorphous poly(ethylene 2,6‐naphthalate)

AbstractSupercritical fluids having different solubility parameters were obtained by changing the parameters of supercritical CO2 and adding a cosolvent (methanol). The crystallization behavior of amorphous poly(ethylene 2,6‐naphthalate) (PEN) in these supercritical fluids covering a wide range of solubility parameter was investigated using wide‐angle X‐ray diffraction and differential scanning calorimetry. A three‐dimensional diagram of crystallization versus temperature, pressure and solubility parameter (i.e. solvent concentration) was established to represent the general behavior of crystallization for amorphous PEN. Supercritical fluids with a higher overall solubility parameter plasticized the PEN chains more effectively and thus provided moderate conditions to induce the crystallization of amorphous PEN. Copyright © 2007 Society of Chemical Industry

Uniaxially stretched poly(ethylene naphthalene 2,6-dicarboxylate) films studied by broadband dielectric spectroscopy

The thermal behaviors of amorphous PEN films uniaxially stretched above and below T g have been characterized by Temperature Modulated DSC. When drawing is performed above T g, the degree of crystallinity and the glass transition temperature T g increase with increasing drawing ratio. PEN samples stretched below T g are more crystalline than when stretched above T g. Moreover the cold crystallization phenomenon disappears with drawing as a result of the mechanically induced crystallization. Broadband dielectric spectroscopy has also been used to study the influence of the orientation on the molecular mobility of PEN to gain insight into the microscopic origin of the relaxation processes in the transverse direction i.e. when the electric field is perpendicular to the stretching direction. Dielectric relaxation phenomena are thus discussed as a function of the microstructure (orientation, crystallinity). The α-relaxation dynamics associated with the glass transition are largely slowed down by the drawing process and the relaxation strength decreases. This can be related to a transformation of the amorphous phase into (i) crystalline, (ii) rigid amorphous fraction (RAF) and/or constrained amorphous phase with lower molecular mobility due to the crystallization induced by the orientation process. In contrast, the dynamics of the β*-relaxation are apparently increased and the activation energy for the uniaxially stretched samples decreases compared to the unstretched isotropic amorphous state. If the β*-relaxation is assigned to the presence of naphthalene aggregates, this leads to the conclusion that stretching is structurally altering the naphthalene aggregates (creation of defective aggregates that induces a higher molecular mobility) or orientation is selecting the faster motions active for β*. The β-relaxation which corresponds to more local motions implying reorientation of the ester groups seems to be broad and composed of two components as in the case of PET: the β 1- and β 2-relaxations. While the high frequency β 1-component relaxation strength is increasing significantly with the deformation below T g, that of the β 2-process is more related to the presence of water.

Influence of morphology on electric conductance in poly(ethylene naphthalene‐2,6‐dicarboxylate)

AbstractPolymers find an important field of application in the electrical industry because of their utilization as insulators and dielectrics. The challenge over the last several years has been to make electrical compounds smaller in size. In that way the material morphology is of strategic importance in the performance of new components in relation to their geometry. Therefore, it is important to know the influence of different types of stress on electric properties of the materials.We have studied the influence of the crystallinity on electric conductance of poly(ethylenenaphthalene‐2,6‐dicarboxylate) (PEN), which is increasingly replacing some dielectrics (polypropylene and PET) in certain electric applications.Starting from an amorphous PEN, samples with various crystallinities were obtained and analyzed using DSC in order to determine their thermal characteristics.Moreover, measurements of charge and discharge currents were carried out on PEN samples to characterize their electric properties. The steady state current observed after 120 min, in response to the electric field, shows an increase in the insulator resistivity with the crystallinity. The increase in the crystallinity favors a decrease in the free volume, so the movement of charges is restricted especially by the species of large sizes.

Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6-naphthalene dicarboxylate). II. Semicrystalline oriented films

The dielectric and dynamic mechanical behaviour of bi-stretched non-treated and annealed semicrystalline poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) films are studied as a function of different morphologies obtained by thermal treatments at temperatures close to the melting temperature of a semicrystalline film. Differential scanning calorimetry (DSC) shows that the glass transition temperatures do not change significantly with the thermal treatment for bi-stretched films. However, the melting temperatures and the degree of crystallinity increase with the value of annealing temperature. Both dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA) display three relaxation processes. In order of decreasing temperature, can be observed: the α-relaxation due to the glass transition, the β ∗-process assigned to cooperative molecular motions of the naphthalene groups which aggregate and the β-relaxation due to local fluctuations of the carbonyl groups. The α-relaxation process shifts to higher temperatures for the 250 and 260 °C treated bi-stretched semicrystalline samples compared to the sample thermally treated at 240 °C according to DRS data but shifts to lower temperatures according to the DMA measurements for the three annealed samples. This discrepency results from the different sensitivity of each methods with regards to the release of orientation. At a fixed frequency the temperature associated to β ∗-relaxation is lower for the non-treated bi-stretched semicrystalline samples than for the treated ones using DMA but no difference can be seen in DRS. The associated apparent activation energies are rather high which suggest cooperative motions. It is assumed that the orientation of the samples prevents coupling between the naphthalene groups due to the stretched chain configuration in the amorphous phase. The activation energy for the β-process given by DRS is independent of the thermal treatment and the value agrees with those found for poly(ethylene terephthalate) (PET) and amorphous PEN. Evidence of the decrease of orientation in the sample with thermal treatment can be seen via the onset of mobility, both by DRS and DMA. Thus, the orientation induces a greater change of properties compared to the crystalline samples obtained from the thermal treatment of an amorphous sample. Finally, a three phase model is proposed since there is evidence of a rigid amorphous phase present in PEN biaxially stretched samples which was favoured by the dependence of dielectric relaxation strengths on the degree of crystallinity for the β ∗- and α-relaxation.

Structural development of poly(ethylene 2,6-naphthalene dicarboxylate) film upon solvent drawing

To investigate the effect of “chemical energy” by solvent on structural development, initially molded amorphous poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) films were drawn in various solvent systems and at the optimum temperatures deliberately determined, and the resulting fine structure and properties were analyzed by using several experimental techniques. Water, 1-butanol, 1,4-dioxane/water (20 : 80 by volume), and 1,4-dioxane/1-butanol (20 : 80 by volume) were chosen for drawing solvent. For this study, in view of chemical structure and solubility parameter, though, these are nearly immiscible or partially miscible liquids against PEN, with just the plasticizing actions more or less the primary concern here. Interesting enough, the temperature range near the main endothermic peak corresponding to the depressed boiling point of liquid caused by unfavorable interaction between PEN and solvent in mixtures of amorphous PEN film with various solvent systems, as determined by the DSC scans, was found to coincide with the optimum drawable range for each solvent system, over which several experiments, including the determination of maximum draw ratio (DRmax) have been carried out throughout the study. The experimental results obtained from this study could be explained, through qualitatively, in terms of the interactions between PEN and solvents. Hence, it is expected to control the fine structure and properties of PEN films resulting from the low-temperature solvent drawing via a suitable choice of liquids and the corresponding draw temperature ranges. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1357–1365, 2003

An X-ray diffraction study of poly(ethylene-2,6-naphthalate), PEN

X-ray diffraction techniques have been applied to study the crystallization of poly(ethylene-2,6-naphthalate), PEN. Uniaxial and biaxial orientation of amorphous cast PEN films resulted in stress-induced crystallization of the triclinic α-PEN polymorph. Annealing of the amorphous PEN samples in the range of 160–240 °C produced thermally induced crystallization of α-PEN. Slow cooling of amorphous PEN from the melt state generated α-PEN as well as the triclinic β-PEN polymorph.

Molecular mobility in amorphous and partially crystalline PEN after isothermal crystallization

Poly(ethylene-2,6-naphthalene dicarboxylate) (PEN), a new aromatic polyester, presents high-performance physical and chemical properties and may be considered as a worthy substitute for polyethylene terephthalate (PET). Dynamic dielectric measurements were performed under isochronal conditions for twelve different frequencies between 1 and 10/sup 5/ Hz. The aim was to follow the dielectric properties and their dependence on temperature, ranging from -100 to 200/spl deg/C, with a heating rate of 2/spl deg/C/min. Based on experimental considerations by using differential scanning calorimeter (DSC), a thermal cycle of crystallization was carried out. Different specimens were obtained in this way starting with as-received amorphous polymers. Crystallinity saturation, accompanied by a microstructure change, was observed with a second melting peak and a dual lamellar stack model was adopted. Amorphous and semi-crystalline PEN samples were compared. The thermal instability of PEN may be shown through dielectric relaxation before melting. A study was undertaken to understand the different dielectric relaxations present in PEN and the effect of thermal treatment on these relaxations. PEN mobility has been characterized by the presence of four relaxations: /spl beta/, /spl beta/*, /spl alpha/ and /spl rho/. When frequency increased, the /spl beta/ and /spl alpha/ relaxations moved towards higher temperature, while the /spl beta/* process disappeared gradually under the a maximum. The two secondary relaxations B and /spl beta/* were found to obey Eyring's law while the primary one obeyed the empirical Vogel-Fulcher-Tamman (VFT) relation. Characteristic of the amorphous PEN relaxation is the presence of the /spl rho/ peak, at a temperature>T/sub g/. In this temperature range, it was concluded that this peak, not observed in semicrystalline specimens, was due to cold crystallization of the amorphous structure. To assign the occurrence of this peak to the mechanism of detrapping of free charges in the material seems inappropriate.

Crystallization kinetics of poly(ethylene naphthalene-2,6-dicarboxylate) as revealed by microhardness

The microhardness technique has been used to follow the crystallization of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) from the glassy state. The crystallization temperatures investigated ranged from 423 to 448K. Results are analyzed on the basis of the Avrami equation. Avrami parameters are discussed in the light of the morphology developed during the isothermal crystallization of PEN, as revealed by preceding electron microscopic studies [Baltá Calleja et al. J Macromol Sci Phys 1998;37:411]. The temperature dependence of microhardness for amorphous PEN has been also investigated from room temperature up to ∼30K above the glass transition temperature, Tg, of the material. From these measurements, the Tg value has been derived in agreement with preceding calorimetric data [Buchner et al. Polymer 1989;30:480].

Physical Aging of Amorphous PEN: Isothermal, Isochronal and Isostructural Results

The sub-glass-transition viscoelastic and physical aging responses of an amorphous poly(ethylene naphthalate) (PEN) have been studied using uniaxial tension stress relaxation experiments. It is known that PEN exhibits a strong β relaxation that overlaps the α relaxation in the experimental time and temperature ranges studied. In prior work, we had shown that both amorphous and semicrystalline PEN exhibit thermorheologically complex behaviors in that neither time−temperature nor time−aging time superposition apply to the materials. Here we compare results from two-step aging experiments in which the material is first annealed at a temperature near the nominal glass transition temperature of 120 °C. In the second step, the viscoelastic response of the material is tested at a lower temperature, following classical sequential aging techniques. We find, for samples first annealed at 100 °C, that the amorphous PEN shows time−temperature superposition behavior for constant annealing times. The results are interpreted in terms of an isostructural or constant fictive temperature glass. When the responses are compared isothermally for different fictive temperatures, as found previously, thermorheological simplicity breaks down. Possible reasons for this behavior are discussed.

Comparative study of amorphous and partially crystalline poly(ethylene-2,6-naphthalene dicarboxylate) by TSDC, DEA, DMA and DSC

A comparative study of the relaxational behavior of amorphous and partially crystalline poly(ethylene-2,6-naphthalene dicarboxylate) (PEN), has been carried out by thermally stimulated depolarization currents (TSDC), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and dynamic electric analysis (DEA). As received, PEN (partially crystalline) shows, in the temperature range from −150 to 200°C, four relaxations located, in increasing temperature order, around −70°C ( β), 60°C ( β ∗), 130°C ( α) and 170°C ( ρ). Amorphous PEN has been crystallized thermally heating up to different temperatures between 170 and 200°C. The DSC measurements of these samples show a small endothermic premelting peak, once the crystallization of the sample is completed. This peak increases and shifts towards higher temperatures as the sample is further thermally treated. Associated with the presence of this endothermic peak, the ρ relaxation passes through a maximum and presents a sharp decrease as it is further thermally treated. The α-relaxation, detected by dynamic mechanical as well as dielectric (ac) measurements, shifts to higher temperatures and broadens as the crystallinity degree increases. The kinetic parameters of the observed relaxations have been determined fitting experimental data to standard models.

Structure and properties of naphthalene containing polyesters, 1. High resolution solid state13C NMR study of poly(ethylene naphthalene-2,6-dicarboxylate)

Poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) crystallizes in two different modifications depending on the temperature of crystallization. The high resolution solid state 13 C NMR spectra of the α modification, β modification and amorphous PEN have been measured by employing cross polarization with magic angle spinning and high power proton decoupling. By application of both a short contact time pulse sequence and an interrupted decoupling pulse sequence all resonance lines, some of which are overlapped in the original 13 C CP/MAS NMR spectra of both amorphous and β modification PEN, are separated. The α modification PEN, β modification PEN, amorphous PEN and PEN in solution show different chemical shifts for each individual carbon atom. The chemical shift difference between amorphous PEN and PEN in solution is much smaller than the difference between either modification crystalline PEN and PEN in solution. The resonance lines of the naphthalene carbon atoms of crystalline PEN have been found to be at a lower field shift than the corresponding lines of PEN in solution. Conversely, the resonance lines of the other carbon atoms on crystalline PEN are shifted upfield from the ones of PEN in solution. A significant difference between the spectra of α and β form PEN can be observed in resonances of the protonated naphthalene carbon atoms C 1 , C 5 , C 4 , and C 8 . They differ by 1.10 ppm. These chemical shift differences are explained in terms of different conformation and chain packing in the samples.

Time-Resolved Fluorescence Measurements on Poly(ethylene naphthalene-2,6-dicarboxylate)

The time-resolved fluorescence behavior of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) has been investigated by synchrotron radiation. Solutions of different concentrations of PEN dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (10-4 and 10-2 M) have been excited at a wavelength of 340 nm, and the emission has been monitored in the wavelength range from 360 to 480 nm in 10 nm steps. The measurements have been performed at two different temperatures (288 and 298 K). Nearly all decay curves can be fitted by a sum of three exponential functions. These three exponential decay time components seem to be parameters which give the main values of a distribution of decay times. We obtained such distribution functions by assuming a simple model on the basis of excitation energy migration from monomer units (donors) to excimer sites (traps) by a Förster type mechanism. These distribution functions can be fitted by a sum of three exponential functions with reliable confidence parameters. The fits on the calculated distribution functions are in good agreement with the fits obtained from the measured data. We compared these results with data obtained on amorphous PEN films excited at the same energy. Again three exponential functions are needed to fit the measured data. The measured fluorescence decay curves can also be explained by our model.

Infrared spectroscopic studies of melting behavior of poly(ethylene 2,6-naphthalenedicarboxylate)

The FTIR method was used to study the melting behavior of poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) samples of different thermal history of crystallization. With subtraction of the spectra of PEN samples of different crystallinities and amorphous PEN, the spectra of crystalline trans conformers, amorphous trans conformers, and amorphous gauche conformers were obtained. By following the changes in intensity of various gauche and crystalline bands during heating, the phenomenon of multiple melting endothermic peaks was shown to be due to the melting of imperfect crystalline conformation formed during crystallization. For two-step annealed samples, the crystalline conformational defects change into a more perfect one during annealing at higher temperatures.

A study on the structural changes during uniaxial drawing and/or heating of poly(ethylene naphthalene-2,6-dicarboxylate) films

The structural changes in the uniaxial drawing process of an unoriented amorphous film of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) and in the heating process of an oriented amorphous film of PEN were studied using X-ray diffraction apparatus equipped with imaging plates. This apparatus allows us to record a time-resolved series of two-dimensional patterns of wide-angle X-ray diffraction in rapid succession, and, therefore, we can follow the structural changes during uniaxial deformation and/or thermal treatment processes. The results are as follows: (1) When an unoriented amorphous PEN film was stretched below Tg (= 117 °C), it could be elongated up to a draw ratio (DR) of 4–5 via neck formation and became an oriented amorphous film. (2) In the heating process of the oriented amorphous film (DR = 3.6, 65 °C), crystalline reflections started to appear near 120 °C, accompanied by streaks on the off-equatorial layer lines. The existence of these streaks on such layer lines indicates a lattice distortion due to the axial shift of neighbouring chains along the chain axis with respect to one another. Finally the film exhibited a fibre structure (transverse isotropy). (3) When an unoriented amorphous PEN film was drawn at 150 °C, the film exhibited a fibre structure accompanied by a lattice distortion that is similar to that mentioned above, and thereafter frequently showed so-called double orientation (uniplanar axial orientation: (10)[001]) in which naphthalene rings in the main chain are preferentially aligned parallel to the film surface despite the fact that the film was deformed in the mode of uniaxial free-width drawing.

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.