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.     

Finite strain behavior of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG)

Uniaxial and plane strain compression experiments are conducted on amorphous poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG) over a wide range of temperatures (25-110 °C) and strain rates (.005-1.0 s⁻¹). The stress-strain behavior of each material is presented and the results for the two materials are found to be remarkably similar over the investigated range of rates, temperatures, and strain levels. Below the glass transition temperature (θ_g=80 °C), the materials exhibit a distinct yield stress, followed by strain softening then moderate strain hardening at moderate strain levels and dramatic strain hardening at large strains. Above the glass transition temperature, the stress-strain curves exhibit the classic trends of a rubbery material during loading, albeit with a strong temperature and time dependence. Instead of a distinct yield stress, the curve transitions gradually, or rolls over, to flow. As in the sub-θ_g range, this is followed by moderate strain hardening and stiffening, and subsequent dramatic hardening. The exhibition of dramatic hardening in PETG, a copolymer of PET which does not undergo strain-induced crystallization, indicates that crystallization may not be the source of the dramatic hardening and stiffening in PET and, instead molecular orientation is the primary hardening and stiffening mechanism in both PET and PETG. Indeed, it is only in cases of deformation which result in highly uniaxial network orientation that the stress-strain behavior of PET differs significantly from that of PETG, suggesting the influence of a meso-ordered structure or crystallization in these instances. During unloading, PETG exhibits extensive elastic recovery, whereas PET exhibits relatively little recovery, suggesting that crystallization occurs (or continues to develop) after active loading ceases and unloading has commenced, locking in much of the deformation in PET.     

Synthesis and characterization of copolymers of poly(m-xylylene adipamide) and poly(ethylene terephthalate) oligomers by melt copolycondensation

Poly(m-xylylene adipamide)/poly(ethylene terephthalate)(MXD6/PET) copolymers are synthesized by melt copolycondensation with 1–5 wt% low molecular weight PET oligomers into the MXD6 oligomers at 260 °C.FR-IR and1 H NMR analysis results indicate that the interchange reaction has occurred between MXD6 oligomers and PET oligomers. The thermal behavior of copolymers shows that the melting temperature of MXD6/PET copolymers decreases with the increasing of amount of PET oligomers, while the crystallization temperature accordingly increases. And the equilibrium temperature Tm0 is evaluated to be 251.8 °C for the copolymers with5 wt% PET oligomer adding, which is very close to that of neat MXD6. The tensile and impact strength of MXD6/PET copolymers are significantly improved than that of pure MXD6 by mechanical properties test, and the microfibril structure in the impact fracture sample's surface reveals the feature of ductile fracture.     

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.     

Blends of poly(ethylene terephthalate) and poly(butylene terephthalate)

Commercial grade poly(ethylene terephthalate), (PET, intrinsic viscosity = 0.80 dL/g) and poly(butylene terephthalate), (PBT, intrinsic viscosity = 1.00 dL/g) were melt blended over the entire composition range using a counterrotating twin‐screw extruder. The mechanical, thermal, electrical, and rheological properties of the blends were studied. All of the blends showed higher impact properties than that of PET or PBT. The 50:50 blend composition exhibited the highest impact value. Other mechanical properties also showed similar trends for blends of this composition. The addition of PBT increased the processability of PET. Differential scanning calorimetry data showed the presence of both phases. For all blends, only a single glass‐transition temperature was observed. The melting characteristics of one phase were influenced by the presence of the other. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 75–82, 2005     

Transesterification in poly(ethylene terephthalate) and poly(ethylene naphthalate 2,6-dicarboxylate) blends: model compounds study

Kinetics of transesterification reaction in poly(ethylene terephthalate)-poly(ethylene naphthalate 2,6-dicarboxylate), PET-PEN, blends resulting from melt processing was simulated using model compounds of ethylene dibenzoate (BEB) and ethylene dinaphthoate (NEN). The exchange reaction between BEB and NEN was followed by ¹H NMR spectroscopy using signals from the aliphatic protons of ethylene glycol moieties at 4.66 and 4.78 ppm, respectively. The first-order kinetics was established under pseudo-first-order conditions for both reactants. Thus, the overall transesterification reaction was second order reversible. The reversibility was confirmed experimentally by heating a mixed sequence of 1-benzoate 2-naphthoate ethylene (BEN) under similar conditions. Both forward reaction of the equimolar amounts of the reagents and reverse reaction came to equilibrium at the same molar ratio of the reactants and reaction products of roughly 0.25:0.50:0.25 for BEB, BEN, and NEN, respectively. The rate equation for the transesterification reaction in the model system was modified using half-concentration of BEN, which is the only effective in the intermolecular exchange. Direct ester-ester exchange was deduced as a prevailing mechanism for the transesterification reaction under the conditions studied, and the values of equilibrium and rate constants, as well as other basic thermodynamic and kinetic parameters were determined. The use of Zn(OAc)₂ as a catalyst resulted in a significant decrease in the activation enthalpy of transesterification, which might be due to the partial switch of the reaction mechanism from primarily pseudo-homolytic to more heterolytic where Zn^II acts as a Lewis base which binds to the ester carbonyl oxygen.     

Experimental studies and molecular modelling of the stress-optical and stress-strain behaviour of poly(ethylene terephthalate). Part II: Molecular modelling of birefringence of poly(ethylene terephthalate) networks

The Monte-Carlo (MC) approach of Paper I is developed to predict the birefringence of PET. An extension of the modelling of polarisability is used that accounts for chain flexibility and for structural units containing several types of bonds. The rotational-isomeric-state (RIS) model for PET chains in melts is employed to calculate the polarisability of the terephthaloyl segment and of each of the 27 possible conformations of the five skeletal bonds of the glycol segment. These polarisabilities then enable the birefringent properties of drawn PET melts to be predicted. A method of pre-averaging the individual glycol polarisabilities that greatly reduces the length of the calculation is shown to be valid.It is found that shorter PET chains produce higher values of birefringence (Δñ) for a given deformation. This trend is due to greater proportions of the chains reaching higher conformational extensions and therefore becoming more oriented. In disagreement with Kuhn and Grün theory, Δñ is not linearly related to λ² − λ⁻¹. This non-linear behaviour is related to the non-linear behaviour of the orientation functions of the terephthaloyl and glycol segments, 〈P₂(cos ζ_ter)〉 and 〈P₂(cos ζ_gly)〉, with λ² − λ⁻¹. The non-linear behaviour of 〈P₂(cos ζ_ter)〉 with λ² − λ⁻¹ was confirmed experimentally in Paper I, where the measured values of 〈P₂(cos ζ_ter)〉 were found to be closely predicted by the present MC modelling.In the present paper, the predicted values of Δñ, calculated according to various published values of bond polarisabilities are presented and discussed. They will be used in Paper III to model the measured birefringence of drawn PET.     

Multiple melting behaviour of poly(ethylene terephthalate)

Differential scanning calorimetry (DSC) and temperature modulated DSC (MTDSC) have been used to investigate the melting behaviour of poly(ethylene terephthalate) (PET). Multiple melting endotherms were observed even at high heating rates, e.g. 160 K min⁻¹ and these have been attributed to the presence of two different distributions of lamella thickness and re-crystallisation (reorganisation) on heating. This has been confirmed by MTDSC—the presence of endotherms and an exotherm in the reversing component of the heat flow during heating. Examination of the endotherms of samples heating stepwise indicated that further crystallisation took place above the isothermal crystallisation temperature (T_c). Some part of this was associated with lamella thickening and crystal perfecting. The multiple melting endotherms observed are a consequence of the balance between the melting and re-crystallisation and the lamella thickness distribution existing within the sample, prior to heating. The triple melting endotherms observed are attributed to the melting of secondary and primary lamellae produced on crystallisation and to thickened lamellae produced during heating to the melting point.     

Crystallization characteristics of weakly branched poly(ethylene terephthalate)

A series of branched poly(ethylene terephthalate) (BPET) samples were prepared from melt polycondensation by incorporation of various amount (0.4-1.2 mol%) of glycerol as a branching agent. These polymers were characterized by means of H¹ NMR, intrinsic viscosity. The general crystalline and melting behavior was investigated via DSC. It was found that the crystalline temperature T_cc from the melt shifted to high temperature and the T_hc from the glass got low for BPETs while the melting temperatures of BPETs kept almost unchanged. The kinetics of isothermal crystallization was studied by means of DSC and POM. It was found that the present branching accelerated the entire process of crystallization of BPETs, although prolonged the induced time. In addition, branching reduced nucleation sites; hence the number of nucleates for BPET got smaller. Therefore, more perfect geometric growth of crystallization and greater radius of spherulites could develop in BPET due to less truncation of spherulites.     

Study of secondary relaxations of poly(ethylene terephthalate) by photoluminescence technique

The α and β relaxation processes in two types of poly(ethylene terephthalate) with different degrees of crystallinity were studied by means of three methods, differential scanning calorimetry, dynamic-mechanical analysis and fluorescence spectroscopy. Information provided is complementary in the mean that every method sense phenomena that may occur at different times and length scales. Several probes, Coumarin 152 (C152), Coumarin 153 (C153), Coumarin 337 (C337) and 4′-dimethylamino-4-nitrostilbene (DMANS), were adsorbed in polymer films, and their fluorescence analysed over the temperature range from −150 to 150 °C. In general, a decrease in fluorescence intensity of probes as temperature increase was observed. This behaviour has been explained as a consequence of the enhancement of the free volume fraction that favoured the radiationless process of the lowest excited singlet state. Plots of fluorescence intensity versus temperature showed changes around the secondary relaxation temperatures. Therefore, good correlations between fluorescence and dynamic mechanical and calorimetric analysis were established. The obtained results indicated that the fluorescence from the probes incorporated to the material was dependent on the crystallinity of polymer. It would indicate that the fluorescence emission from those probes can be used to analyse annealing processes in semicrystalline polymers.     

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.     

Aspects of the chemistry of poly(ethylene terephthalate): 5. Polymerization of bis(hydroxyethyl)terephthalate by various metallic catalysts

Bis(hydroxyethyl)terephthalate (BHET) was polymerized to poly(ethylene terephthalate) (PET) in the presence of various metallic catalysts. The influence of the nature and concentration of these catalysts on the rate of polymerization has been investigated. The effect of the reaction temperature has also been studied. The order of decreasing catalytic influence of various metal ions, on the polymerization of BHET was found to be: Ti>Sn>Mn>Zn>Pb>No.     

Synthesis, characterization, and properties of poly(ethylene terephthalate)/poly(1,4-butylene succinate) block copolymers

Reactive blending at 290 °C of a series of mixtures of poly(ethylene terephthalate) (PET) and poly(1,4-butylene succinate) (PBS) led to the formation of block PET/PBS copolyesters. The block lengths of the resulting copolymers decreased with the severity of the treatment. Copolyesters with PET/PBS molar compositions of 90/10, 80/20, 70/30, and 50/50 were prepared by this method and their composition and microstructure were characterized by ¹H and ¹³C NMR, respectively. The T_g, T_m, and crystallinity of the copolymers decreased as the content in PBS and the degree of randomness increased. The elastic modulus and tensile strength of the copolymers decreased with the content of PBS, whereas, on the contrary, the elongation at break increased. The PET/PBS copolymers exhibited a pronounced hydrolytic degradability, which increased with the content in 1,4-butylene succinic units. Hydrolysis mainly occurred on the aliphatic ester groups.     

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.     

Optically transparent self-reinforced poly(ethylene terephthalate) composites: molecular orientation and mechanical properties

Self-reinforced composites have been fabricated by compaction of oriented polyethylene terephthalate (PET) fibers under pressure at temperatures near, but below, their melting point. The originally white fiber bundles, which were about 40% crystalline, show increased crystallinity (55%) but optical translucency after processing. Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) were used to study the crystallization and orientation of the fibers, revealing that the degree of crystallinity was somewhat insensitive to compaction conditions while the melting point increased substantially with increasing compaction temperature. Crystalline orientation, gauged using the Hermans orientation parameter from WAXD data, indicated that no significant loss in orientation of the crystalline fraction occurs due to compaction. Mechanical characterization revealed a stepwise decrease in flexural modulus (9.4-8.1 GPa) and concomitant increase in transverse modulus and strength on increasing the compaction temperature from 255 to 259 °C. This transition in behavior was accompanied by a loss of optical transparency and a change in the distribution of amorphous fraction from fine intrafibrillar domains to coarse interfibrillar domains as seen with electron microscopy. We argue then that the mechanical properties of PET compactions are influenced more by orientation of the amorphous phase than that of the crystalline phase. The impact properties of compacted materials, characterized using an unnotched Charpy test method, showed remarkable impact resistance after compaction, with impact toughness decreasing as compaction temperature was increased.     

Sorption of lower alcohols in poly(ethylene terephthalate)

This paper presents equilibrium sorption and kinetics of lower alcohols in a 1.5 μ thick, biaxially oriented PET film. Methanol, ethanol, n-propanol and iso-propanol have been studied for the solubility and sorption kinetics in this film to understand how these properties change with penetrant size and branching. It is observed that n-propanol shows dual mode characteristics at all activities whereas the other three penetrants show Flory-Huggins uptake at high activities. Infinite dilution solubility is estimated and compared with that of esters, ketones and other hydrocarbons previously reported. The dispersive solubility parameter, δ_d is found to correlate well with the solubility of penetrants with the same functional group. The hydrogen bonding parameter, δ_h is observed to influence the solubility of penetrants with the same carbon number but different functional groups. This correlation with the solubility parameters may be extended to other functional groups and used to predict the infinite dilution solubility of larger penetrants in PET. Diffusion coefficients in the Fickian kinetics regime and Berens-Hopfenberg parameters in the non-Fickian kinetics regime have been evaluated. Diffusivity increases with concentration and decreases with size. Diffusion coefficient of iso-propanol is an order of magnitude lower than that of n-propanol due to branching effects.     

Carbon nanotubes induced crystallization of poly(ethylene terephthalate)

We have investigated the crystallization characteristics of melt compounded nanocomposites of poly(ethylene terephthalate) (PET) and single walled carbon nanotubes (SWNTs). Differential scanning calorimetry studies showed that SWNTs at weight fractions as low as 0.03 wt% enhance the rate of crystallization in PET, as the cooling nanocomposite melt crystallizes at a temperature 10 °C higher as compared to neat PET. Isothermal crystallization studies also revealed that SWNTs significantly accelerate the crystallization process. WAXD showed oriented crystallization of PET induced by oriented SWNTs in a randomized PET melt, indicating the role of SWNTs as nucleating sites.     

Preparation and hydrolytic degradation of sulfonated poly(ethylene terephthalate) copolymers

A series of poly(ethylene terephthalate-co-ethylene 5-sodiosulfoisophthalate) copolyesters containing from 1 up to 50 mol% of sulfonated units was prepared by melt polycondensation from ethylene glycol and mixtures of dimethyl terephthalate and dimethyl 5-sodiosulfoisophthalate. The resulting copolymers had a random microstructure and contained oligo(ethylene glycol) units in amounts increasing with the content in sulfonated isophthalate units. Copolyesters with more than 20 mol% of 5-sodiosulfoisophthalic units were amorphous and easily soluble in water. The hydrodegradability of the copolyesters was very high as compared to poly(ethylene terephthalate), and increased with the content in sulfonated units. It was demonstrated that the susceptibility to acidic hydrolysis of these copolymers is mainly due to the presence of the sodium sulfonate groups, the influence of the oligo(ethylene glycol) units in this regard being noticeable but limited.     

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.