Structural relaxation in poly(ethylene terephthalate) studied by positron annihilation lifetime spectroscopy

Positron lifetime technique was used to study the physical ageing process in poly(ethylene terephthalate) (PET). Positron lifetime results show that the structural relaxation processes in PET encompass two different time regimes, one short and the other long. The relaxation function constructed from the measured o‐Ps intensity I₃ exhibits non‐exponential character, which can be best fitted with two additive exponentials. The Narayanaswamy model (Kohlraush‐William Watt (KWW) function) is invoked to extract the stretching parameter β indicating the extent of deviation from exponential relaxation. Based on the relaxation times, the activation energies calculated seem to label the different kinetic units of PET structure participating in the relaxation process. © 2002 Society of Chemical Industry     

Solvent effects on microwave dielectric relaxation in poly(ethylene glycols)

This paper reports the results of a systematic study of microwave dielectric relaxation times of poly(ethylene glycols), average molecular weight 200–9000, in dilute solutions of benzene at 9·83GHz. These results are compared with the values of relaxation times obtained earlier in carbon tetrachloride solutions. This shows that the average relaxation times τ₀ and the relaxation time corresponding to segmental reorientation τ₁ are influenced by the solvent environment. The variation in chain flexibility in these polymers with the increase in degree of polymerization and formation of intra- and inter-molecular hydrogen bonding in benzene and carbon tetrachloride solutions is discussed with the help of relaxation data. The relaxation time τ₂ corresponding to group rotations has been determined. It is found that the τ₂ value is independent of solvent environment and degree of polymerization, and may be attributed to the rotation of chain −OH end-groups around the C−O bonds in dynamic equilibrium, with the formation of a five-membered ring due to intra-molecular hydrogen bonding at the end of the chain. © 1998 SCI.     

Dielectric relaxation spectroscopy of poly(ethylene terephthalate)

Contour maps of dielectric loss tangent within the ranges 0.1 Hz to 3 MHz and ?175 °C to +190 °C are presented for a commercial poly(ethylene terephthalate) (PET) in two initial states of crystallinity. Individual absorption regions resemble those for poly(butylene terephthalate) and are attributed to carbonyl‐driven α‐ and β‐relaxation processes and to Maxwell–Wagner–Sillars polarizations. Possible causes are considered for the asymmetry and structure apparent in the α‐peak of partially crystalline PET. © 2001 Society of Chemical Industry     

Microwave dielectric relaxation in binary mixtures of poly(ethylene glycols) in solution

The average relaxation time τ₀, relaxation times corresponding to segmental motion τ₁ and group rotations τ₂, of a series of binary mixtures of poly(ethylene glycols) (PEG 200+PEG 300, PEG 400+PEG 600, PEG 1500+PEG 4000, PEG 6000+PEG 9000, PEG 200+PEG 1500, PEG 300+PEG 4000, PEG 400+PEG 6000 and PEG 600+PEG 9000) have been carried out in dilute solutions of benzene and carbon tetrachloride at 9·83GHz. The effect on chain flexibility due to inter- and intra-molecular association in these binary mixtures is discussed by comparing relaxation times of these mixtures with their individual relaxation times in solution. It is inferred that the extent of hydrogen bonding in different binary mixtures is not similar and is influenced by solvent environment, but there is correlation between τ₀ and τ₁ values in all these binary mixtures, which may be because of hydrogen bonding. The observed τ₂ values in all these mixtures suggests that the chain-ends remain excluded from the intermolecular association and τ₂ is independent of the polymer constituents of the mixture and solvent. It is also equal to the τ₂ values of the individual polymers. © 1998 SCI.     

Molecular relaxation in cross-linked poly(ethylene glycol) and poly(propylene glycol) diacrylate networks by dielectric spectroscopy

The molecular relaxation characteristics of rubbery amorphous crosslinked networks based on poly(ethylene glycol) diacrylate [PEGDA] and poly(propylene glycol) diacrylate [PPGDA] have been investigated using broadband dielectric spectroscopy. Dielectric spectra measured across the sub-glass transition region indicate the emergence of an intermediate “fast” relaxation in the highly crosslinked networks that appears to correspond to a subset of segmental motions that are more local and less cooperative as compared to those associated with the glass transition. This process, which is similar to a distinct sub-T_g relaxation detected in poly(ethylene oxide) [PEO], may be a general feature in systems with a sufficient level of chemical or physical constraint, as it is observed in the crosslinked networks, crystalline PEO, and PEO-based nanocomposites.     

Studies of post drawing relaxation phenomena in poly(ethylene terephthalate) by infrared spectroscopy

In this paper, we study the relaxation behavior of initially amorphous poly(ethylene terephthalate) (PET) films drawn, at 80°C using a draw rate of 2 cm/min, to a draw ratio (λ) from 1 to 5 and then quenched to room temperature. These films were then heated at different temperatures from 68 to 80°C for different times and their orientation determined. The orientation measurements were performed by transmission infrared spectroscopy and the bands used for the determination of orientation were those at 1340 and 970 cm^?1 for the trans conformers, normalized using the 1410 cm^?1 benzene ring vibration. The crystallinity was determined by thermal analysis. It is shown that when PET is drawn to λ values up to 2–2.5 (before stress-induced crystallization), the orientation relaxes rapidly at temperatures close to the glass transition temperature of PET. For λ values of 3 or higher, the orientational relaxation of the amorphous regions is hindered. This effect is ascribed to the development of strain-induced crystallites, which are believed to act as pseudo-crosslinks.     

Structural development in ion‐irradiated poly(ether ether ketone) as studied by dielectric relaxation spectroscopy

Structural alterations to amorphous poly(ether ether ketone) (PEEK) produced by ion irradiation (11.2 MeV H⁺ and 25.6 MeV He²⁺ ions) were investigated by dielectric relaxation spectroscopy. The analysis in terms of the Havriliak–Negami (HN) equation and the scaling model showed an increase in the intermolecular correlation with increasing irradiation dose. The dynamic fragility index (m) was estimated from Vogel–Fulcher–Tammann analysis. Ion irradiation not only elevated the glass‐transition temperature (T_g) but interestingly decreased m of the PEEK chains around T_g. This was due to increasing polar interaction and better packing efficiency of the irradiated samples compared with those of amorphous PEEK. The average size of the cooperative rearranging region decreased in line with decreasing m and indicated an increase in the rigid amorphous phase fraction after irradiation. The analysis of the direct‐current conductivity confirmed that there was a strong coupling between the macroscopic ion transport and concerted segmental motion. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39929.     

Anisotropy of dielectric relaxation in poly(ethylene terephthalate) fibres

Dielectric measurements have been made on poly(ethylene terephthalate) (PET) fibres in the frequency range 0.5–10 kHz in temperature range ?120°?+30° C with the applied electric field parallel and perpendicular to the fibre axis. Considerable directional anisotropy was observed in the β-relaxation process, which was independent of frequency. The observed dielectric anisotropy has been related to the structural parameters thus leading to a value 〈cos²ψ〉 = 0.21, where ψ is the angle between the dipole moment vector responsible for this relaxation and the molecular chain axis. Activation energy for this process in PET fibres (9 kcal/mol) was lower than that reported for PET films and extruded rods.     

Enthalpy relaxation in poly(ethylene terephthalate) and related polyesters

Enthalpic relaxation data are presented on poly(ethylene terephthalate), poly(ethylene naphthalate) and their copolymers. Analysis of these data allows the determination of the amount of energy absorbed at the glass transition, Q_t, and the location of the enthalpic recovery peak, T_max, as a function of the time of ageing of the samples. Ageing measurements were carried out for periods of up to 2016 h and at temperatures between 40 °C and 110 °C, depending upon the chemical composition of the system being investigated. The enthalpic relaxation rates are influenced by the chemical structure and reflect the effects of local order pinning the chains and influencing the rate of enthalpic recovery. © 2000 Society of Chemical Industry     

Crystallinity in poly(aryl-ether-ketone) plaques studied by multiple internal reflection spectroscopy

Summary Infrared reflection spectroscopy has been used to investigate a series of poly(aryl-ether-ether-ketone) (PEEK) plaques of differing crystallinity. Correlations have been observed between intensity changes in the reflection spectra and the crystallinity as measured by wide angle X-ray scattering (WAXS). These correlations enable estimates to be made of the crystallinity close to the surface and complement the bulk data obtained by WAXS.     

Surface crystallization of poly(ethylene terephtalate) studied by atomic force microscopy

Poly(ethylene terephtalate) (PET) crystallization was shown by atomic force microscopy (AFM) to occur at 85 °C in the first few nanometers near the polymer-air interface. The surface was fully transformed into spherulites after 30 min, while no signs of bulk crystallization were observed by FTIR. All the observed spherulites presented a nucleation centre, indicating that the crystallization process started at the surface of the film. Tapping mode AFM confirmed that the spherulites were not covered by an amorphous layer. The most probable explanation is a decrease of T_g near the surface. Due to the poor crystallization conditions, the constitutive units of the spherulites were small crystalline blocks. By changing the annealing time, it was possible to produce PET surfaces with different surface fractions consisting of semi-crystalline material (spherulites) and amorphous matrix. This provided a controlled surface heterogeneity on the submicrometer scale, with a contrast in terms of stiffness, roughness and swelling by organic solvents.     

Segmental mobility in poly(isoprene) rubber studied by deuterium-carbon NMR correlation spectroscopy

Summary A HMQC based deuterium-carbon correlation method is used for the first time to study the segmental mobility in a fully deuterated poly(isoprene) (PI) rubber network. The isotropic J _CD couplings can be utilized for polarization transfer between ²H and ¹³C spins in a mobile solid polymer. This makes it possible to correlate the ²H resonances with the ¹³C chemical shifts of the attached carbon in a two-dimensional (2D) experiment, and thus allow the extraction of the individual ²H signals. The experimental data obtained from the 2D correlation spectrum indicates differences in the segmental mobility of the C-D vectors in perdeuterated PI. The conclusions are fully consistent with ²H and ¹³C T ₁ relaxation data. The results shown in this paper demonstrate the potential of ²H-¹³C NMR correlation spectroscopy in solid systems which produce overlapped 1D ²H spectra. Received: 11 December 2000/Revised version: 1 March 2001/Accepted: 1 March 2001     

Raman spectroscopy of flowing poly(ethylene terephthalate) melts

Studies have been made of the Raman spectrum of a poly(ethylene terephthalate) melt being extruded through a glass die. The effects observed in the spectrum are interpreted in terms of changes in molecular shape which vary with shear rate and melt temperature.     

Stress relaxation behavior of biaxially oriented poly(ethylene terephthalate)

The stress relaxation behavior of biaxially oriented semicrystalline poly(ethylene terephthalate) was studied by thermomechanical analysis. Experimental techniques were developed for thin films. Relaxation moduli were measured as a function of stress, time, and temperature. The relaxation modulus was shown to be independent of stress over the range tested. Rate of loss of the relaxation modulus was found to be a nonlinear function of time and temperature up to about 100°C, encompassing the T_g for the polymer. Over the temperature range of 100–120°C it was primarily temperature-dependent. An empirical time—temperature superposition showed that significant losses in modulus can occur at very short times. At temperatures above the T_g these losses can result in significantly reduced film physical properties.     

End‐group determination in poly(ethylene terephthalate) by infrared spectroscopy

A complete infrared (IR) spectroscopy experimental procedure was used to determine end‐group concentrations of poly(ethylene terephthalate) (PET). The correlations of the hydroxyl and carboxyl units were derived independently for accurate calibration results. The intermediate monomer of PET, bis(hydroxyethyl terephthalate), was used to prepare hydroxyl end‐group standards and titration measurements were used to determine the carboxyl content for the carboxyl end‐group standards. A double‐Gaussian form equation was defined to account for the interference between the hydroxyl and the carboxyl absorbance peaks in the PET IR spectrum. Some deviation was found from the assumption traditionally used for end‐group determination, stating that carboxyl and hydroxyl are the only end‐group units available in PET. © 2002 Society of Chemical Industry     

Surface of poly(ethylene terephthalate/isophthalate) copolyesters studied by atomic force microscopy

An atomic force microscope (AFM) operating in Tapping™ and contact modes has been used to study the surface topography and the molecular organization of poly(ethylene terephthalate) (PET) films containing 2% (PET‐2I) and 10% (PET‐10I) isophthalate, and of injection/blow molded bottles containing 2.6% (PET‐2.6I) and 10% isophthalate. Large‐scale (15‐μm × 15‐μm) AFM images have shown that both surfaces are fairly flat and heterogeneous in nature, often containing inclusions. Whereas the PET surface appears to be formed mainly by microfibrils, isophthalic acid (IPA) incorporation at the 2–10 mol % level gives the surface a granular appearance. The IPA‐containing PET surfaces are frequently coated by a lacelike film consisting of submicron “beads” joined together by filaments. These “strings of beads” form bundles and can also connect bundles. AFM images of PET‐2I closely resemble those generated for PET films. By contrast, the lacelike structure becomes a dominant feature of the PET‐10I surface. The level of inclusions observed on film surfaces appears to correlate with the levels of extractable oligomers present in the polymers. Nanometer‐scale AFM images of PET‐10I exhibit surfaces composed of short stacks of plates or rods, with 30–50‐nm voids or pores between these stacks. Whereas surface deposits of what we suggest is most likely an oligomer correlates with isophthalate concentration, we see no gross structural features in PET‐2I and PET‐10I that explain the observed improvement in gas diffusion barrier in these polymers. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 750–762, 2001     

Proton spin-lattice and spin-spin relaxation times in annealed poly(ethylene terephthalate)

Proton spin-lattice, T₁, and spin-spin, T₂, relaxation times of poly(ethylene terephthalate) film annealed at various temperatures were measured using a broad line pulse spectrometer. The value of T₁ is closely related to the crystallinity of the sample and only one T₁ was observed for each sample, indicating that the spin diffusion is effectively operating. Even in the amorphous sample there are some nuclear spins, the motion of which is strongly restricted.     

Aggregation properties of amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer studied by cyclic voltammetry

The cyclic voltammetric behaviors at a platinum electrode of an amphiphilic block copolymer [poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (F127)] in aqueous solutions were investigated. The mechanism of the electrochemical reaction of F127 at a platinum electrode was deduced. The diffusion coefficients of different-shaped aggregates formed by F127 were determined on this basis. The first and second critical micelle concentrations, corresponding to the formation of spherical micelles and the transition of the spherical to rod-like micelles, were 3.72×10⁻⁴ mol·L⁻¹ and 1.49×10⁻³ mol·L⁻¹, respectively, which could be confirmed by the fluorescent anisotropy of pyrene in the F127 aggregates and the morphology of F127 micelles observed by freeze-fracture transmission electron microscopy.