Study on the effect of dispersion phase morphology on porous structure of poly (lactic acid)/poly (ethylene terephthalate glycol-modified) blending foams

A methodology for blending foam of poly (lactic acid) (PLA)/poly (ethylene terephthalate glycol-modified) (PETG) was proposed. PLA/PETG blends were prepared through a melt blending method, using multiple functionality epoxide as reactive compatibilizer. The effects of blending ratio and compatibilizer content on the dispersion morphology, molecular structure, mechanical properties, and rheological behavior of PLA/PETG blends were studied. Then PLA/PETG blends were foamed using supercritical CO₂ as physical blowing agent, and their porous structure, pore size, as well as pore density were investigated. The results showed that the mechanical properties and rheological parameters such as melt strength and melt elasticity, as well as the porous structure of the foams dispersion morphology of PLA/PETG blends were affected strongly. The melt elasticity of PLA/PETG blends increased with increasing compatibilizer content. Dispersion phase morphology of PLA/PETG blends also had a significant effect on the pore density of all the samples. The results indicated that homogeneous and finer porous morphology of PLA/PETG foams with high expansion ratio could be achieved with a proper content of compatibilizer in the blends.     

The role of nanoclay in the generation of poly(ethylene terephthalate) fibers with improved modulus and tenacity

The effect of nanoclay concentration on the molecular orientation and drawability of poly(ethylene terephthalate) PET was examined using thermal and vibrational spectroscopic analysis. Although drawability at 83 °C in hot air increased by the addition of nanoclay, the maximum draw ratio was independent of nanoclay concentration. The average molecular orientation of the PET chain was found to mimic the trend in mechanical property improvements. Both Young's modulus and tenacity (i.e. strength) showed the maximum improvement at a 1 wt% loading of clay, which was shown to coincide with the maximum amount of molecular orientation. Nanoclay was shown to intercalate with PET and enhanced amorphous orientation that led to modulus and strength improvements. However, at higher concentrations of nanoclay the presence of large agglomerates prevented efficient orientation to the fiber axis and acted as stress concentrators to aid in cavitation and failure during testing. Raman spectroscopy showed that the as-spun unfilled PET fibers possessed significantly more trans rotamer content of the ethylene glycol moiety than the nanocomposite fibers.     

Irreversible structural changes during PET recycling by extrusion

Steric exclusion chromatography (SEC) and rheometry were used to study irreversible structural changes occurring during poly(ethylene terephthalate) (PET) extrusion. It appeared that polymer gelation could occur after three successive extrusions, showing the existence of a crosslinking process. This latter occurs only in the presence of oxygen at low oxygen pressures (typically P_O2<9×10³ Pa) as a result of coupling of alkyl radicals generated by the oxidation chain process. A kinetic model based on this assumption but taking also into account chemical reactions occurring in nitrogen, generates kinetic curves of melt (Newtonian) viscosity changes in good agreement with experimental ones.     

Fragility index of drawn or annealed poly(ethylene terephthalate) films studied by thermally stimulated depolarisation currents

According to the ‘strong-fragile’ glass forming liquid concept, amorphous poly(ethylene terephthalate) (PET) must be classified as a ‘fragile’ supercooled liquid. In this work we show that the value of the fragility index of the glassy material (m_g) can be obtained by means of Thermally Stimulated Depolarisation Currents measurements. Experiments were carried out on different PET for which the molecular mobility of the vitreous domain was constrained by the existence of a crystalline phase. This crystalline phase was obtained in two different ways, i.e. by thermal cold crystallisation or by a uniaxial hot-drawing method. Depending on the degree of crystallinity, but whatever the nature of the crystalline morphology (spherulitic or oriented crystallites), it is found that the initial fragile character becomes stronger as the degree of crystallinity increases.     

Morphological consequences of interchange reactions during solid state copolymerization in poly(ethylene terephthalate) and polycarbonate oligomers

Poly(ethylene terephthalate) (PET) (IV:0.15 dL/g) oligomer was obtained by depolymerisation of high molecular weight PET. Polycarbonate (PC) oligomer (IV: 0.15 dL/g) was synthesized by standard melt polymerization procedure using bisphenol A and diphenyl carbonate in the presence of a basic catalyst. Blends of varying compositions were prepared by melt blending the chemically distinct PET and PC oligomers. The copolymer, poly(ethylene terephthalate-co-bisphenol A carbonate) was synthesized by simultaneous solid state polymerization and ester-carbonate interchange reaction between the oligomers of PET and PC. The reaction was carried out under reduced pressure at temperatures below the melting temperature of the blend samples. DSC and WAXS techniques characterized the structure and morphology of the blends, while ¹NMR spectroscopy was used to monitor the progress of interchange reactions between the oligomers. The studies have indicated the amorphisation of the PET and PC crystalline phases in solid state with the progress of solid-state polymerization and interchange reaction.     

On the nature of multiple melting in poly(ethylene terephthalate) (PET) and its copolymers with cyclohexylene dimethylene terephthalate (PET/CT)

The multiple melting behavior of poly(ethylene terephthalate) (PET) homopolymers of different molecular weights and its cyclohexylene dimethylene (PET/CT) copolymers was studied by time-resolved simultaneous small-angle X-ray scattering/wide-angle X-ray scattering diffraction and differential scanning calorimetry techniques using a heating rate of 2 °C/min after isothermal crystallization at 200 °C for 30 min. The copolymer containing random incorporation of 1,4-cyclohexylene dimethylene terephthalate monomer cannot be cocrystallized with the ethylene terephthalate moiety. Isothermally crystallized samples were found to possess primary and secondary crystals. The statistical distribution of the primary crystals was found to be broad compared to that of the secondary crystals. During heating, the following mechanisms were assumed to explain the multiple melting behavior. The first endotherm is related to the non-reversing melting of very thin and defective secondary crystals formed during the late stages of crystallization. The second endotherm is associated with the melting of secondary crystals and partial melting of less stable primary crystals. The third endotherm is associated with the melting of the remaining stable primary crystals and the recrystallized crystals. Due to their large statistical distribution, the primary crystals melt in a broad temperature range, which includes both second and third melting endotherms. The amounts of secondary, primary and recrystallized crystals, being molten in each endotherm, are different in various PET samples, depending on variables such as isothermal crystallization temperature, time, molecular weight and co-monomer content.     

Observations of structure development during crystallisation of oriented poly(ethylene terephthalate)

Two types of SAXS and WAXS experiments have been made using synchrotron radiation to observe the transformation from smectic to crystalline phases in oriented poly(ethylene terephthalate) (PET). In step-anneal experiments, PET was drawn slowly at 30 °C and then observed after annealing at 5 °C steps up to 100 °C. In the other experiments, time-resolved observations were made while drawing at 90 °C at rates up to 10 s⁻¹. Up to 70 °C the WAXS data in the step-anneal experiments showed the smectic meridional reflection reducing in lateral width, indicating an increase in lateral long range order with annealing. Between 70 and 100 °C, there was a reduction in the intensity of the smectic reflection which correlated with an increase in the intensity of crystalline reflections. The SAXS from the step-anneal experiments showed an intense equatorial streak which has a correlation peak around 20 nm and which diminishes with annealing above 70 °C. It is concluded that this feature is a characteristic of the presence of the mesophase in oriented PET and is due to elongated domains of smectic mesophase with a length >75 nm and with an interdomain spacing of around 20 nm. Between 70 and 100 °C the SAXS data showed additional diffuse diffraction which correlated quantitatively with the crystalline phase and evolved from a cross-like appearance to a well resolved four-point pattern. The time-resolved drawing experiments were limited by the time resolution of the SAXS detector. They showed the same development of four-point diffuse SAXS patterns as was observed in the step-anneal experiments and a very weak equatorial streak. Differences in phase transformation kinetics between the two types of experiment are attributed to the different chain relaxation processes available under different conditions.     

Nanoindentation and nanoscratch testing of uniaxially and biaxially drawn poly(ethylene terephthalate) film

Nanoindentation and nanoscratch testing have revealed large differences in nanomechanical behaviour on uniaxially and biaxially drawn poly(ethylene terephthalate) films. Differences can be ascribed to the processing history of the film. The biaxial material exhibited significantly higher hardness and elastic modulus than the uniaxial film, presumably due to increased crystallinity from the second draw. The biaxially drawn material was also less susceptible to creep deformation. The plasticity index, the ratio of the dissipated energy to the total indentation energy, was greater on the uniaxial film, indicating that it exhibits less plastic deformation than the biaxially stretched film. The differences in processing also affected the resistance of the films to nanoscratching wear. The wear resistance of the films correlated with the ratio of the hardness to the modulus.     

Recycled poly(ethylene terephthalate)/layered silicate nanocomposites: morphology and tensile mechanical properties

Various amounts (1, 3 and 5 wt%) of a non-modified natural montmorillonite clay (Cloisite^® Na⁺) or of an ion-exchanged clay modified with quaternary ammonium salt (Cloisite^® 25A) were dispersed in a recycled poly(ethylene terephthalate) matrix (rPET) by a melt intercalation process. Microphotographs of composite fracture surfaces bring evidence that particles of Cloisite^® 25A are much better dispersed in the rPET matrix than those of Cloisite^® Na⁺. Moreover, WAXS measurements indicate that the lamellar periodicity of Cloisite^® 25A is increased in the composites, which evidences intercalation of rPET between silicate layers (lamellae) of the clay. In the case of Cloisite^® Na⁺, a very small thickening of lamellae due to mixing with rPET indicates only minute intercalation.Uniaxial tensile tests show that both clays increase the modulus of the rPET composites; more effective Cloisite^® 25A accounts for a 30% increase at loading of 5 wt%. Yield strength remains practically unaffected by the used fractions of the clays while tensile strength slightly decreases with the clay content; in parallel, strain at break dramatically drops. Tensile compliance of the composites is virtually independent of applied stress up to 26 MPa. Essential part of the compliance corresponds to the elastic time-independent component, while the viscoelastic component is low corresponding only to a few percent of the compliance even at relatively high stresses. The compliance of the composites is only slightly lower than that of the neat rPET, the reinforcing effect of Cloisite^® 25A being somewhat stronger. Both clays have beneficial effect on the dimensional stability of the composites since—in contrast to the neat rPET—the creep rate does not rise at long creep periods.     

Thermo-mechanical coupling and self-excited oscillation in the neck propagation of PET films

The self-excited oscillation of neck propagation during cold drawing of polymer films has been examined experimentally. On the basis of Barenblatt's model considering a thermo-mechanical coupling at the neck, the temperature rise at the neck has been studied with an infrared camera. The temperature began to rise in a range showing a negative velocity dependence of the applied load. The behavior is consistent with the view of thermo-mechanical coupling. The temperature rise was up to 80°C (>T_g) and explains the occurrence of crystallization for faster drawing rates. It has also been confirmed that the temperature rise follows the oscillation of stress due to the coupling.     

Experimental studies and molecular modelling of the stress-optical and stress-strain behaviour of poly(ethylene terephthalate). Part III: Measurement and quantitative modelling of birefringence-strain, stress-strain and stress-optical properties

Experimental measurements and Monte-Carlo (MC) modelling of the birefringence-strain, stress-strain and stress-optical behaviour of poly(ethylene terephthalate) (PET) are used, together with the analysis of orientation-strain and conformation-strain behaviour reported in Paper I, to give a detailed, quantitative interpretation and characterisation of its deformation-related properties. The difference between the stress-strain and stress-optical behaviour of PET that had been reported previously is confirmed. Except for the stress, the measured values of all the properties studied are in agreement with those calculated using the MC modelling, which suggests that not all of the junctions or the chains in the entangled PET network are elastically active.The results given by Kuhn and Grün theory are compared with those given by the MC modelling. The expected shortcomings of Kuhn and Grün theory are found. However, distinct from the behaviour reported previously for polyethene, the theory can be used to evaluate, semi-empirically, the stress-optical coefficient of PET.     

DSC and DMTA study of annealed cold-drawn PET: a three phase model interpretation

Cold-drawn poly(ethylene terephthalate) (PET) samples annealed at different undercoolings are studied by means of differential scanning calorimetry and dynamic mechanical thermal analysis. When heating from room temperature, the onset of the glass transition region in cold-drawn, un-annealed samples is found to be significantly lower than in the case of un-oriented PET. On the contrary, the presence of crystalline lamellae in oriented PET cause a shift (and spread out) of the glass transition region towards higher temperatures. The crystal thickening process caused by heating above the annealing temperature, is suggested to take place after a rigid amorphous phase linked to the basal surface of the lamellae has softened. It is found that the low-temperature (between 100 and 140 °C) annealed samples have a glass dispersion region ranging significantly above the annealing temperature itself. This circumstance leads to envisage vitrification as a possible mechanism able to limit lamellar thickening during the annealing process at these low temperatures.     

Effect of antimony catalyst on solid-state polycondensation of poly(ethylene terephthalate)

The effect of antimony trioxide (Sb₂O₃) catalyst on the solid-state polycondensation (SSP) of poly(ethylene terephthalate) (PET) has been rigorously studied. It has been determined that the rate constant increases, while the activation energy decreases, linearly with increasing catalyst concentration within the range of 0-100 ppm Sb. The SSP rate reaches its maximum value at about 150 ppm Sb. The activation energies are 30.7 and 23.3 kcal/mol respectively for the uncatalyzed and fully catalyzed SSP. The frequency factor decreases with increasing catalyst concentration due to the decreased mobility of catalyzed end groups. A mechanism of Sb catalysis has been proposed to explain these observations.     

Role of thermal history on quiescent cold crystallization of PET

The cold crystallization of poly(ethylene terephthalate) (PET) has been studied as a function of the initial structure of the glass using density, microhardness, wide angle X-ray scattering, small angle X-ray scattering and DSC measurements. Glassy PET samples varying from slightly crystalline to completely amorphous phase were investigated. Results reveal that differences in the inner structure of the starting glassy material induce different crystallization rates from the glassy amorphous state. Thus, it is observed that crystallization rate decreases with the increasing cooling rate used to quench the samples. Results have been analyzed using the Kolmogroff-Avrami-Evans theory. A good agreement between theoretical and experimental data is obtained providing accurate values for kinetic constants. The different crystallization rates obtained are explained in terms of differences in nucleation density.     

Experimental studies and molecular modelling of the stress-optical and stress-strain behaviour of poly(ethylene terephthalate). Part I: Infra-red spectroscopic investigation and modelling of chain conformation and orientation changes on drawing

Quantitative infra-red data on oriented poly(ethylene terephthalate) (PET) films have been used to determine the changes in the proportions of the trans and gauche conformers of the glycol residues and the development of molecular orientation of the terephthaloyl groups as functions of draw ratio. The results are compared with predictions for the stretching of a rubber-like network based on rotational-isomeric-state (RIS) Monte Carlo (MC) modelling. It is shown that both sets of data are consistent with stretching an entangled molecular network of about 10 PET monomer units per network chain. The onset of crystallisation at a draw ratio of 2.5 affects the glycol trans-gauche conformer contents but has no detectable effect on terephthaloyl orientation.     

Multiple thermosetting of semicrystalline polymers: Polyamide 66 and poly(ethylene terephthalate) fibers

Polyamide 66 fibers were thermoset in a torsion-bending deformation at various temperatures up to 240 °C. Some of the fibers were heat-set at constant length prior to the deformation at presetting temperatures of 150 °C and 200 °C to vary the structural state of the starting material. Fractional recovery was measured after various combinations of temperature and time. It was found that heat setting of PA66 is dominated by time-dependent stress relaxation exhibiting time-temperature equivalence. Increased crystallinity, and/or other molecular rearrangements occurring during presetting, impose additional constraints on molecular mobility, which delay onset of the flow regime and increase the time constant of relaxation at a given temperature. The thermosetting characteristics of PA66 fibers are very similar to those of poly(ethylene terephthalate) fibers. For both polymers, superposing the curves of fractional recovery vs. setting time at different temperatures produce satisfactory master curves, without the need for vertical shifting of the data. Arrhenius plots yield approximate activation energies for the thermosetting flow process of 35-65 kcal/mol in PA66 and 95-115 kcal/mol in PET.     

Melt crystallization of poly(ethylene terephthalate): Comparing addition of graphene vs. carbon nanotubes

Poly(ethylene terephthalate) (PET)-based nanocomposites with graphene or multi-wall carbon nanotubes (MWCNT) were prepared by melt mixing. Aspect ratio, A_f, and interparticle distance, λ, of graphene in the nanocomposites were obtained from melt rheology and transmission electron microscopy respectively. λ of PET/graphene nanocomposites was much smaller than λ in PET/MWCNT. For PET/graphene with highest A_f, λ became <1 μm at more than 0.5 wt% graphene. Non-isothermal crystallization behavior from the melt was investigated by differential scanning calorimetry. The crystallization temperatures suggest that the nucleation effect of graphene was stronger than that of MWCNT. The half crystallization time of PET/graphene became longer than PET/MWCNT with increasing graphene loading, suggesting that confinement by graphene suppressed the crystal growth rate. XRD analysis indicated that smaller crystals formed in PET/graphene than in PET/MWCNT. From Raman spectroscopy, the π–π interaction between PET and graphene was stronger than that between PET and MWCNT. This stronger interaction in PET/graphene appears to result in formation of crystals with higher perfection.     

Design of a laboratory experiment on heat transfer in an agitated vessel

A novel teaching laboratory experiment is described, which demonstrates heat transfer under agitation. The experiment involves a simple and inexpensive apparatus with necessary basic components. The laboratory deals with the experimental determination of the heat transfer rates and the overall heat transfer coefficient between steam and water in an agitated vessel. The apparatus can be operated under both static and flow conditions, which affords the student an improved understanding of heat transfer during transient and steady-state modes. Further, student is trained to study the effect of the impeller speed and water flow rate on the rate of heat transfer. By performing the experiment, the student is able to determine the overall heat transfer coefficients experimentally, and compare the results with those obtained from theoretical calculations using correlations available in the literature. In addition, while working in groups, students develop team work and technical writing skills in preparing a comprehensive laboratory report.