Physical ageing studies in semicrystalline poly(ethylene terephthalate)

The physical ageing of semicrystalline poly(ethylene terephthalate) (c-PET) of different crystallinities and morphological structures was studied using differential scanning calorimetry. Samples of c-PET of crystallinity content _c = 0.12, crystallized at low temperatures (105 °C for 13 min), submitted to physical ageing in a temperature range between 50 and 65 °C for different periods of time, showed two endothermic peaks. The first peak (P₁) of higher intensity, appeared at a temperature close to the glass transition temperature, T _g, of the amorphous PET, and the other peak (P₂) of lower intensity, merged as a shoulder of the first one, at a higher temperature. These peaks have been attributed to the enthalpy relaxation process of two different amorphous regions: one amorphous phase outside the spherulitic structure (interspherulitic amorphous region) and another amorphous phase inside the spherulites (interlamellar amorphous region). The separation between P₁ and P₂ indicates that DSC, via enthalpy relaxation, is a good technique to detect the real double glass transition of the semicrystalline PET. However, the physical ageing of a semicrystalline PET of _c = 0.32, crystallized at 114 °C during 1 h, showed a main endothermic peak shifted to a higher temperature, which probably corresponds to the enthalpy relaxation of the more restricted interlamellar amorphous region, and a small endothermic peak at lower temperature which could be a reflection of the hindered interspherulitic amorphous region.     

Physical ageing and glass transition in amorphous polymers as revealed by microhardness

Microhardness (MH) data as a function of temperature for two amorphous polymers [poly(methylmethacrylate) and poly(vinylacetate)] and two semicrystalline polymers [poly(ethyleneterephthalate) and poly(arylether ether ketone)] quenched into the amorphous state are presented. It is shown that MH can conveniently detect the glass transition temperature (T _g) for the above mentioned polymers. Molecular rearrangements taking place above and belowT _g, such as physical ageing leading to a more compact molecular packing, and thermal expansion can also be followed by means of MH measurements. Finally, the presence of a crystalline phase in these materials has been shown to shift theT _g value towards higher temperatures.     

Mechanical and thermal properties of poly(butylene terephthalate)/poly(ethylene naphthalate), and Nylon66/poly(ethylene naphthalate) blends

The tensile modulus, tensile strength and impact strength of melt blends of (a) poly(ethylene naphthalate) (PEN) and poly(butylene terephalate) (PBT) with 30, 40, 50, 60 and 70 wt% PEN, (b) Nylon66 and PEN with 30, 50 and 70 wt% Nylon66 were measured, and thermal/thermomechanical properties were analysed by differential scanning calorimetry and dynamic mechanical thermal analysis. Scanning electron microscopy was used for examination of the fracture surfaces of the blends.All PBT/PEN blends show two glass transitions corresponding to the presence of two phases: the glass transition temperature, T _g, of the phase with the lower T _g increases with increasing PEN content, and T _g for the phase with higher T _g decreases with increasing PBT content. The implication is that the two polymers are partially miscible, and scanning electron microscopy of fracture surfaces reveals a very small (sub-micron) domain size. Nylon66/PEN blends also show two phases, but the domain size is of the order of m and there is no evidence of partial miscibility.Up to 50 weight proportions PBT does not lower the tensile strength of PBT/PEN blends, and the tensile strength lies between values predicted by the rule of mixtures and a modified rule of mixtures. Incorporation of at least 40% PEN in PBT increases impact strength, but blending with smaller proportions of PEN decreases impact strength. By contrast, blending of Ny66 and PEN results in reduction of tensile strength for all blend compositions.     

Mechanical study of poly(vinylidene fluoride)-poly(methyl methacrylate) amorphous blends

Amorphous films of poly(vinylidene fluoride)-poly (methyl methacrylate) were prepared by initial precipitation from a solvent and rapid solidification at ≈ 15 °C from the molten state. The PVDF/PMMA compositions studied were 25/75, 45/55, 50/50, 55/45, 60/40 and 75/25. X-ray scattering analysis suggests that mixture of the two components throughout the composition range studied occurs at a molecular level. The parallel decrease of the microhardness, which obeys a simple expression:H _blend =H _PMMA (1−φ) (φ being the PVDF concentration) and the glass transition temperature,T _g, following the predictions, of Gordon and Taylor, reveals that the depression of microhardness is caused by the shift ofT _g towards lower temperatures. It is pointed out that the effect of PVDF molecules is to act as a softening agent within the PMMA component.     

Quasi-static and dynamic compressive behaviour of poly(methyl methacrylate) and polystyrene at temperatures from 293 K to 363 K

Flow stress, Young’s Modulus, energy and strain of fracture of poly(methyl methacrylate) (PMMA) and polystyrene (PS) were studied under compressive loading at strain rates of 10⁻⁴–10 s⁻¹ and temperatures from 293 K to temperatures ∼20 K below T _g. It was found that the energy of fracture shows an increase in the quasi-static strain rate (10⁻⁴–10⁻³ s⁻¹) region and becomes constant in the low strain rate (10⁻²–10 s⁻¹) region, while the strain of fracture shows a slow decrease with rate over the strain rate range tested. The activation energies and volumes of PMMA and PS at yield stress, 20% and 30% strain were evaluated using Eyring’s theory of viscous flow. ΔG was found to be constant for all strain rates and strains for both PMMA and PS. The activation volume for both materials increased as a function of strain.     

Miscible blends from rigid poly(vinyl chloride) and epoxidized natural rubber

Miscible blends of rigid poly(vinyl chloride), PVC, and epoxidized natural rubber (ENR) having 50 mol % epoxidation level, are prepared in a Brabender Plasticorder by the melt-mixing technique. Changes in Brabender torque and temperature, density, dynamic mechanical properties and DSC thermograms of the samples are studied as a function of blend composition. The PVC-ENR blends behave as a compatible system as is evident from the singleT _g observed both in dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The moderate level broadening of theT _g zone in blends is due to microinhomogeneity, which may arise from the particle structures of PVC perturbing the molecular level mixing of PVC and ENR. Scanning electron microscopic studies were conducted on nitric acid-etched samples and the results showed continuous structures of blend components as well as the occurrence of solvent-induced cracks in high PVC blends.     

Electrical conductivity of modified poly(vinyl chloride)

Chemical modification of poly(vinyl chloride) (PVC) by dehydrochlorination with ethanolic KOH is found to yield modified PVC with conjugated polyene sequence. The semiconducting nature of ethoxide-modified PVC is illustrated with temperature dependence of conductivity (σ). The relative ratios (r) of conductivity,σ _modifiedpvc /σ _{unmodifiedpvc} , are greater than unity in the temperature range 50° to 180°C,r being maximum in the vicinity of glass-transition temperature (T _g).T _g inferred from conductivity-temperature profiles is found to be greater for modified PVC relative to unmodified PVC, which is explicable in terms of restricted free rotation limiting segmental motion. For comparison with the conductivity andT _g of ethoxide-modified PVC, LiCl-modified PVC and (aniline + S₂O ₈ ²⁻ )-modified PVC have also been studied.     

Sensitivity of damage to microstructure evolution occurring during long-term high-temperature annealing in a semi-crystalline polymer

This study aimed to highlight the role of microstructure evolutions induced by high temperature annealing on damage in a semi-crystalline polymer during long-term applications. It was based on a polar form of poly(vinylidene fluoride) (T _g = −40 °C; T _m = 170 °C) and the long term annealing context dealt with burst resistance tests performed for 1000 h in a temperature range from 95 °C up to 140 °C. A secondary crystallization was observed after annealing by Differential Scanning Calorimetry and a consistent phenomenology was evidenced in Dynamic Mechanical Analysis. A decrease of the amorphous phase mobility and a weak reorganization of primary crystals were observed at the same time. Tensile tests on annealed specimens pointed out modulus and yield stress reinforcement, partial disentanglement in the amorphous phase and a raise of the volume strain. Thermally-induced microstructure evolutions were shown to enhance cavitation and slow down crack opening displacement kinetics. This last effect would result from both a raise of the yield stress in primary crystals and secondary crystallization.     

Synthesis and characterization of a novel multiblock copolyester containing poly(ethylene succinate) and poly(butylene succinate)

Multiblock copolyester (PBS-b-PES) containing poly(butylene succinate) (PBS) and poly(ethylene succinate) (PES) was successfully synthesized by chain-extension of dihydroxyl terminated PBS (HO-PBS-OH) and PES (HO-PES-OH) using 1,6-hexmethylene diisocyanate (HDI) as a chain extender. The chemical structures, molecular weights, crystallization behaviors, thermal and mechanical properties of the copolyesters were characterized by proton nuclear magnetic resonance spectroscopy (¹H NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetry analysis (TGA), wide-angle X-ray diffraction (WAXD), tensile testing and hydrolytic degradation. High-molecular-weight copolyesters with M_w more than 2.0 × 10⁵ g mol⁻¹ were easily obtained through chain-extension. The copolyesters showed a single glass transition temperature (T_g) which increased with PES content. The melting point temperature (T_m) and relative degree of crystallinity (X_c) of the copolyesters decreased first and then increased with PES content. The copolyesters manifested excellent mechanical properties, for example, PBS₅-b-PES₅ had fracture stress of 61.8 MPa and fracture strain of 1173%. The chain-extension reaction provided a very effective way to produce high molecular weight multiblock copolyesters.     

Interphase transfer of tackifier between poly(butadiene) and poly(styrene-co-butadiene)

The interphase transfer behavior of poly(styrene-co-α-methyl styrene), which is known as a tackifier in rubber industry, is investigated using poly(butadiene) rubber (BR) and poly(styrene-co-butadiene) rubber (SBR). The films of pure rubbers and the blends with the tackifier were prepared by a solution-cast method using toluene as a solvent. Two sheets composed of different rubbers, in which the tackifier was mixed at least in one rubber sheet, were piled together and annealed beyond the glass transition temperature T _g of the tackifier. The transfer phenomenon of the tackifier between the sheets was evaluated by the peak shift in the tensile loss modulus E″ curve, ascribed to T _g of the rubbers, which was measured by the dynamic mechanical analysis. Moreover, crystallization temperature of BR was also employed as a measure of the transfer, because the tackifier retards the crystallization of BR. It is found that the tackifier moves from one rubber to another during annealing procedure to reduce severe localization. When the content is the same in each rubber, the tackifier immigrates from BR to SBR, suggesting a better miscibility with SBR. This behavior is attributed to the small difference in the solubility parameter between SBR and the tackifier as compared to that between BR and the tackifier.     

Effects of thermal history on crystal structure of poly(phenylene sulphide)

The lattice parameters of film-grade poly(phenylene sulphide), PPS, have been studied at room temperature as a function of thermal history. Effects of crystallization temperature and annealing time for films crystallized from the rubbery amorphous state were investigated using wide- and small-angle X-ray diffraction, bulk density and thermal analysis techniques. The dimensions of the crystal lattice are found to depend upon prior thermal treatment conditions. As the cold crystallization temperature, T _c, increases, or the annealing time at fixed temperature increases, the bulk density, degree of crystallinity, and crystal perfection increase. With an increase in annealing time at fixed temperature, lattice a, b, and c decrease leading to an increase in lattice density. As the cold crystallization temperature increases, lattice density also increases as a result of a systematic decrease in lattice parameters a and b.     

Physical ageing of polypropylene in glassy state

Physical ageing behaviour of a semicrystalline polymer, polypropylene, has been studied at a pressure-temperature state (P=2.5 kbar,T=15° C) for which the amorphous region is in the glassy state. Polypropylene contains 57% crystallinity and its glass transition temperature at atmospheric condition is −20° C. The ageing behaviour was monitored by measuring the Young's modulus (E) from the stress-strain curves as a function of ageing time up to 200 h. The Young's modulus of the rapidly pressure quenched samples increased at 0.4% per ageing hour of the initial value for up to 30 h and tapered off thereafter. The glassy polypropylene also exhibited a memory behaviour when it was given a pressure perturbation.     

Thermomechanical properties of poly(vinyl alcohol) plasticized with varying ratios of sorbitol

The objective of this work is to study the thermal and mechanical properties of films based on blends of poly(vinyl alcohol) (PVA) with different weight percent of sorbitol. Solid-state PVA/sorbitol polymer membranes were prepared by a solution casting method. The characteristic properties of these polymer membranes were examined by thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), nanoindentation methods and by Fourier Transform Infrared (FTIR) spectroscopy. It was found that the thermal properties (glass transition, T_g, melting point, T_m and decomposition temperature, T_d) for PVA blends showed a decrease proportional to the sorbitol concentrations. The hardness and elastic modulus obtained from nanoindentation test were also found to decrease with increase in plasticizer concentration. FTIR confirmed the reduction in hydrogen bonding between PVA chains in favour of formation new bonding between the plasticizer and the PVA chains.     

The Potential of Small-Scale Fusion Experiments and the Gordon-Taylor Equation to Predict the Suitability of Drug/Polymer Blends for Melt Extrusion

The aim of this study was to investigate the use of small-scale fusion experiments and the Gordon-Taylor (GT) equation to predict whether melt extrusion of a drug with an amorphous polymer produces a stable amorphous dispersion with increased drug dissolution. Indomethacin, lacidipine, nifedipine, piroxicam, and tolbutamide were used as poorly soluble drugs. Drug/polyvinylpyrrolidone (PVP) blends were prepared at a 1:1 mass ratio. Small-scale fusion experiments were performed in a differential scanning calorimeter (DSC) and in stainless steel beakers. Extrusion was performed in a Brabender Plasti-corder. The glass transition temperatures T_g were determined by DSC. Taking an average T_g from the DSC melt, beaker melt, and GT equation accurately predicted the extrudate T_g. Physical stability of beaker melt and extrudate samples was tested by X-ray powder diffraction (XRPD) and DSC after storage at 30°C (beaker melt) or 25°C (extrudate) and less than 10%, 60%, and 75% relative humidity (RH). Beaker melts were amorphous, apart from some residual crystallinity. Extrudates were amorphous after preparation. Except for indomethacin/PVP, which remained amorphous, the crystallinity of beaker melts and extrudates increased only at 75% RH. Recrystallization occurred even when the T_g of the sample was well above the storage temperature. Chemical stability of the beaker melts and extrudates was tested by capillary electrophoresis and high-performance liquid chromatography (HPLC). Stability was slightly improved in the extrudate compared to the beaker melt. In general, the order for rate of dissolution was crystalline drug was less than the physical mixture, which was less than the drug/PVP beaker melt, which was approximately equal to the extrudate. The use of beaker melts allows a conservative estimate of the potential to melt extrude a drug. To predict physical stability, analysis of the T_g must be combined with physical stability experiments.     

Volume relaxation in amorphous and semicrystalline PET

Two types of polyethyleneterephthalate (PET) were investigated, one nearly amorphous and the other highly crystallized. DSC analysis and mercury-in-glass dilatometry were used to determine the effect of crystalline phase content on the thermal behavior of amorphous phase. Increasing portion of crystals caused an increase in glass transition temperature (T _g) and broadening of the transition zone. Thermal expansion coefficient and specific heat decreased. The amount of rigid amorphous fraction, RAF, was calculated to be around 21–26%. Volume relaxation measurements initiated by temperature down-jump from the equilibrium above T _g to several temperatures in the vicinity of T _g showed considerably reduced relaxation rate for semicrystalline PET.     

Effects of composition and transesterification catalysts on the physico-chemical and dynamic properties of PC/PET blends rich in PC

Melt blending of polycarbonate (PC)/poly(ethylene terephthalate) (PET) rich in PC at absence/present of different type of tranesterification catalysts was carried out by using reactive extrusion method. The thermal, dynamic, and morphological properties were studied. It was found that all blends are formed by a PC matrix and a semicrystalline (12–20% of crystallinity) of PET dispersed phase. The addition of a catalyst in the mixing process promotes a refined and homogeneous dispersion of PET, as well as it enhances the dynamicmechanical behavior of PC/PET blends compared with PC. These effects are attributed to the emulsifying effect of the PC–PET copolymer generated by transesterification. Additionally, this copolymer contributes to the miscibility between phases as demonstrated by the glass transition (T _g) shift of PC phase and PET phase.     

Miscibility, morphology and fracture toughness of tetrafunctional epoxy resin/poly (styrene-co-acrylonitrile) blends

Poly(styrene-co-acrylonitrile) (SAN) was found to be miscible with the tetraglycidylether of 4,4'-diaminodiphenylmethane (TGDDM), as shown by the existence of a single glass transition temperature (T _g) over the whole composition range. However, SAN was found to be immiscible with the 4,4-diaminodiphenylmethane (DDM)-cured TGDDM. Dynamic mechanical analysis (DMA) shows that the DDM-cured TGDDM/SAN blends have two T _gs. A scanning electron microscopy (SEM) study revealed that all the DDM-cured TGDDM/SAN blends have a two-phase structure. The fracture toughness K _IC of the blends increased with SAN content and showed a maximum at 10 wt% SAN content, followed by a dramatic decrease for the cured blends containing 15 wt% SAN or more. The SEM investigation of the K _IC fracture surfaces indicated that the toughening effect of the SAN-modified epoxy resin was greatly dependent on the morphological structures.     

Solid heat-expandable polylactide-poly(methyl methacrylate) foam precursors prepared by immersion in liquid carbon dioxide

Solid heat-expandable foam precursors were prepared by impregnating melt-blended poly(d,l-lactide) (PDLLA)-poly(methyl methacrylate) (PMMA) blends with liquid carbon dioxide (CO₂). The phase behavior of these blends was strongly dependent on the processing steps, but impregnation with liquid CO₂ led to phase separation regardless of the prior thermomechanical history, and crystallization in blends containing a low-D grade of PDLLA suppressed subsequent expansion. On the other hand, blends containing nominally amorphous high-D PDLLA were found to be unstable with respect to expansion under ambient conditions when saturated with CO₂. It was therefore necessary to reduce the overall CO₂ content by allowing it to desorb partially at 10 °C immediately after impregnation. Under these conditions, the amorphous PDLLA-50 wt% PMMA precursors were stable at ambient temperature and pressure, and showed peak expansion ratios at significantly higher temperatures than pure PDLLA, thanks to the increase in effective glass transition temperature with increasing PMMA content. It was hence demonstrated that blending with PMMA may provide a convenient means of tailoring the process window for heat-expandable polylactide foams, as well as improved heat stability.     

The effect of thermal ageing on carbon fibre-reinforced polyetheretherketone (PEEK)

The static and dynamic mechanical properties of carbon fibre-reinforced PEEK (APC-2) laminates subjected to long-term thermal ageing and cycling treatments have been studied using three-point bend flexure tests. Results are discussed with respect to morphological changes and degradation analysis. S/N curves were modelled using fatigue modulus degradation data. Ageing laminates at high temperatures, for long time periods, between the glass transition temperature, T _g, and the melting temperature, T _m, caused a significant reduction in mechanical properties. However, for short ageing periods, a crystal-perfection process occurs which enhanced the low stress level fatigue resistance of both laminate geometries.     

A new crystal morphology of straight-stalk dendrites in blends of poly(butylene adipate) with amorphous poly(vinyl acetate)

Discontinuity in crystallization rates and a new and unusual morphology consisting of thickened straight-stalks crystal lamellae with also straight branches radiating out from a common nucleus were observed in blends of poly(vinyl acetate) (PVAc) with poly(1,4-butylene adipate) (PBA). The discontinuity in the crystal growth and mechanisms of straight-dendrite morphology of the PVAc/PBA blends were analyzed using polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). The discontinuity in crystallization rate and dendritic morphology occurred only at or near PVAc/PBA 10/90 blend composition upon crystallization at high-temperature regimes of T_c = 30-33 °C. By comparison, when crystallized at the same temperatures, PVAc/PBA blends of amorphous polymer loading greater than 15% or the neat PBA (amorphous polymer loading = 0) developed no dendrites but only typical Maltese-cross spherulites. Mechanism of straight dendrites in the blends is preliminarily expounded. Detailed interpretation requires further analyses.