H. G. Zachmann

Kinetics of crystallization and melting behaviour of poly(ethylene naphthalene-2,6-dicarboxylate)

Abstract The crystallization and melting of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) has been investigated by differential scanning calorimetry and by wide angle X-ray scattering using synchrotron radiation. The material can be crystallized in two different crystal modifications called α and β. At crystallization temperatures up to 200°C only the α-modification is formed. Above this temperature, the β-modification is obtained if the material had been molten at 280°C, while the α-modification is formed if the temperature of the melt was raised to 320°C. The half times of crystallization as a function of temperature show a broad minimum ranging from 180 to 240°C. Under usual conditions, both modifications melt at 272°C. However, by applying a special annealing procedure, the melting points of both modifications can be raised to up to 290°C.

Glass transition temperature of copolyesters of PET, PEN and PHB as determined by dynamic mechanical analysis

Abstract Binary and ternary copolymers of poly(ethylene terephthalate) (PET), poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) and poly( p -hydroxybenzoic acid) (PHB) were synthesized and dynamic mechanical measurements at temperatures ranging from −140°C up to the melting point were performed. A linear dependence of the glass transition temperature of the isotropic materials on composition was obtained. Copolymers containing more than about 30 mol% PHB are partly liquid-crystalline, and those containing more than 50 mol% PHB are completely liquid-crystalline. The PET/PEN/PHB ternary copolyester containing equal parts of each of the three components can be obtained at room temperature as a liquid-crystalline glass and as an isotropic glass, depending on thermal history. The glass transition temperature T g of the material in the liquid-crystalline state was found to be about 30°C lower than that of samples in the isotropic state. By extrapolation to 100 mol% PHB, a glass transition temperature of about 120°C was estimated for the PHB homopolymer. Samples containing more than 70 mol% PHB show an additional peak in tan σ, which is attributed to a relaxation process in a disturbed hexagonal crystalline phase. At temperatures below T g in all copolymers investigated several β relaxation maxima were observed. The temperature positions of these maxima do not depend on the composition.

Influence of molar mass and catalysts on the kinetics of crystallization and on the orientation of poly(ethylene terephthalate)

Abstract Poly(ethylene terephthalate) with different molar masses and different catalysts and additives (calcium acetate, manganese acetate, triphenylphosphate) was synthesized. The influence of the molar mass distribution, and of the additives used, on the rate of crystallization was studied. Also the dependence of the orientation obtained during drawing at elevated temperatures on the drawing conditions, on the average molar mass, and on the additives was investigated. It is shown that, under the same drawing conditions, an increase in molar mass leads to greater orientation. Also, at very small draw rates the orientation in the samples containing calcium acetate is greater than in those containing manganese acetate.

Intermolecular cross-polarization nuclear magnetic resonance studies of the miscibility of poly(ethylene naphthalene dicarboxylate)/poly(ethylene terephthalate) blends

Abstract Cross-polarization/magic-angle spinning 13 C nuclear magnetic resonance measurements on blends of poly(ethylene terephthalate) (PET), poly(ethylene naphthalene dicarboxylate) (PEN) and copolyesters of PET and p -hydroxybenzoic acid (PHB) were performed using different dephasing times of the dipolar decoupling. In the case in which the spectra of the two components of the blends are different, information on miscibilities is obtained by measurements with full dipolar decoupling. In the case of similar spectra, however, this method cannot be applied. It is shown that, under this condition, information on phase separation is obtained if the measurements are performed with delayed decoupling and no decoupling. While blends of PEN and copolyesters of PEN are miscible, phase separation is obtained if one component contains PEN and the other one PET.

Real time dielectric relaxation of poly(ethylene terephthalate) during crystallization from the glassy state

Abstract The crystallization of poly(ethylene terephthalate) has been followed in real time by measuring the dielectric complex permittivity. The measurements have been discussed by assuming the contribution to the dielectric losses of two types of amorphous phases: amorphous regions filling the interspherulitic space and amorphous regions located between crystalline lamellae within the spherulites. The deviation of the measurements from the calculated values assuming a simple two-phase model has been interpreted as being due to changes in the nature of the amorphous regions as crystallization proceeds. A phenomenological description of the experiments in terms of the Havriliak-Negami description permits the changes of the relaxation time distribution functions upon crystallization to be followed. The evolution with crystallization time of the derived dipole moment time correlation functions are discussed in the light of different models.

Studies of miscibility, transesterification and crystallization in blends of poly(ethylene terephthalate) and poly(ethylene-2,6-naphthalene dicarboxylate)

Blends of poly(ethylene terephthalate) (PET) and poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) were obtained by coprecipitation from solution followed by melt-pressing for different timestmand quenching in iced water. When the melt-pressing time was 0.2 and 0.5 min, two glass transition temperaturesTgwere observed by means of dynamic mechanical analysis (DMA), indicating that there are two phases present, a PEN-rich phase and a PET-rich phase. The differential scanning calorimetry (DSC) curves show two crystallization peaks and two melting peaks which, according to wide-angle x-ray scattering (WAXS) measurements, can be attributed to PET and PEN, respectively. In the case oftm=2 min or longer, a single value ofTgand thus a single phase is found to exist. Fortm=10 min and 45 min no crystallization and melting at all is observed during heating with 10°C/min, indicating that a copolyester of PET and PEN has been formed by transesterfication during melt-pressing.Time-resolved WAXS measurements during isothermal crystallization show that, in the blend, the half-time of crystallization of PET is different from that of PEN, and not the same as that which is found in the pure polymer.

Mechanical properties and structure of glassy and semicrystalline random copolymers of poly(ethylene terephthalate) and poly(ethylene naphthalene-2,6-dicarboxilate)

The microhardness,H, of random copolymers of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalene-2,6-dicarboxilate) (PEN) was determined over a wide range of compositions. It is shown that microhardness of the materials is strongly affected by the composition. The mechanical property,H, of the quenched amorphous copolyester films is discussed in terms of a simple model given by the additivity values of the single componentsHaPET andHaPEN. In materials containing up to 30% PEN, crystals of PET are found after annealing at temperatures 10 °C below their melting points. In materials containing ∼ 80% PEN, after annealing at about 20 °C below the melting point, crystals of PEN are formed. The observed deviation ofH for the crystallized films from the additive behaviour of the single components can be quantitatively related to two factors: the changes occurring in the crystallinity value and in the thickness of PET and PEN crystals.

Investigation of the high speed spinning process of poly(ethylene terephthalate) by means of synchrotron X-ray diffraction

Measurements of wide-angle diffraction during high speed spinning of poly(ethylene terephthalate) (PET) have been performed using synchrotron radiation. The experimental set-up has been improved so that it became possible to determine the degree of crystallinity, crystallite orientation and fiber diameter along the spin line. For take-up speeds of 3600 m/min and higher, the crystal reflections appeared at a distance of 40 cm from the spinneret, just after the necking region. The crystallization speed increases proportional to increasing take-up speed. For the take-up speeds investigated, this results in a constant crystallinity profile when measured as a function of the distance from the spinneret. The final fibers exhibited an extraordinarily high degree of crystallinity, which has been attributed to the high molecular weight of the polymer. Below 3500 m/min, no crystallization and no necking could be detected up to a distance of 90 cm from the spinneret. The results showed that necking and crystallization were closely coupled. No orientation of the amorphous matrix prior to crystallization could be detected by means of X-ray diffraction. Moreover, the orientation of the crystallites was constant along the spin line.

On-line measurements of orientation induced crystallization of PET during high speed spinning

An experimental set-up in the synchrotron radiation beam was constructed for performing wide angle X-ray scattering (WAXS) on-line measurements, during high speed spinning of poly(ethylene terephtalate) (PET). In order to detect the very weak scattering of the thin filaments (= 26 μm diameter) the capton windows of the vacuum tubes were removed and the air was replaced by helium. In this way it became possible to measure the development of crystal reflections as a function of the distance from the spinneret. A comparison of the results with corresponding on-line measurements of the fibre diameter indicates that crystallization starts at the beginning of the necking zone.

Determination of the temperature and time dependence of the absolute small-angle X-ray scattering intensity of partially crystalline polymers employing synchrotron radiation

A method was developed to correct the influence of variations of the cross section of the synchrotron radiation beam on the intensity of small-angle X-ray scattering (SAXS). By means of this method the change of the SAXS intensity of poly(ethylene terephthalate) during heating and cooling was quantitatively evaluated. Three different effects could be distinguished: (1) changes in Δϱ, the difference between the densities of the crystalline and the amorphous regions, (2) partial melting and (3) recrystallization. By separation of the three effects, it becomes possible to measure the kinetics of partial melting and recrystallization. By comparing the change of the SAXS power Q with the change of the degree of crystallinity as determined by wide-angle X-ray scattering, it was proved that partial melting and recrystallization takes place within the spherulites without changing the spherulitically crystallized volume of the sample. The measurements of Δϱ indicate that an orientation of the chains by drawing leads to an increase of the glass transition temperature and to a decrease of the thermal expansion coefficient of the amorphous regions.