Crystallization kinetics of poly(ethylene terephthalate) with thermotropic liquid crystalline polymer blends

The isothermal and dynamic crystallization behaviors of polyethylene terephthalate (PET) blended with three types of liquid crystal polymers, i.e., PHB60–PET40, HBA73–HNA27, [(PHB60–PET40)–(HBA73–HNA27) 50 : 50], have been studied using differential scanning calorimetry (DSC). The kinetics were calculated using the slope of the crystallization versus time plot, the time for 50% reduced crystallinity, the time to attain maximum rate of crystallization, and the Avrami equation. All the liquid crystalline polymer reinforcements with 10 wt % added accelerated the rate of crystallization of PET; however, the order of the acceleration effect among the liquid crystalline polymers could not be defined from the isothermal crystallization kinetics. The order of the effect for liquid crystalline polymer on the crystallization of PET is as follows: (PHB60–PET40)–(HBA73–HNA27) (50 : 50); HBA73–HNA27; PHB60–PET40: This order forms the dynamic scan of the DSC measurements. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1383–1392, 1998     

Studies on blends of a thermotropic liquid crystalline polymer and polybutylene terephthalate

Summary Structure-property relationships of blends of a thermotropic polyester-type main-chain LCP and polybutylene terephthalate (PBT) were investigated. LCP was melt blended with three different PBTs and the blends were processed by injection moulding or extrusion. Mechanical and thermal properties of the blends were determined and the blend structure was characterized by scanning electron microscopy (SEM). LCP acted as mechanical reinforcement for PBT and improved also its dimensional and thermal stability. The stiffness of PBT increased with increasing LCP content, but at the same time the blends became more brittle. In extrusion the orientation of LCP phases could be further enhanced by additional drawing, which led to significant improvements in strength and stiffness at LCP contents of 20–30 wt.-%.     

Studies of polycarbonate — poly (butylene terephthalate) blends

The composition and microstructure of a blend of bisphenol-A polycarbonate (PC) and poly (butylene terephthalate) (PBT) have been established by a variety of physical methods. The composition was established by solvent extraction and infra-red spectrophotometry, while the microstructure was determined by these and the additional methods of differential scanning calorimetry and dynamic mechanical thermal analysis. The PBT retained its crystallinity in the commercial blend, (Xenoy CL-100), but blending reduced the main glass-rubber transition of the PC from 147°C to approximately 100°C. Conditioning of the blend at high temperatures resulted in progressive transesterification: 3 minutes at 240°C gave a small but significant effect, while 30 minutes at 270°C yielded large changes in the structure. These findings are important in respect of processing the material, and the limitations which might be incurred in plant recycling of scrap.     

Rubber-Toughened polymer blends of polycarbonate (PC) and poly (ethylene terephthalate (PET)

The intrinsically impact brittle nature of the PC/PET blends can be effectively toughened by incorporating butylacrylate core-shell rubber. The rubber-modified PC/PET blend possess both excellent low temperature impact properties and reduced notch sensitivity. The ductile-brittle transition temperature of the blend decreases with the increase of rubber content. The presence of rubber in the PC/PET blend does not relieve the strain rate induced yield stress increase. Two separate modes, localized shear yielding and mass hear yielding, work simultaneously in the rubber toughening mechanism. The plane-strain localized shear yielding dominates the toughening mechanism at lower temperature and results in brittle failure. At higher temperature, the planestress mass shear yielding dominates the toughening mechanism and results in ductile failure. The critical plastic zone volume can be used to interpret the observed phenomenon.     

Thermal and mechanical properties of injection molded blends of a liquid crystalline polymer and poly(butylene terephthalate)

The microstructure and the thermal and mechanical properties of injection molded samples of different blends of Vectra (LCP) and poly(butylene terephthalate) (PBT) have been studied. Differential scanning calorimetry and hot-stage polarized light microscopy showed that the crystallization of PBT was unaffected by the presence of LCP. X-ray diffraction showed that the PBT component was always unoriented in the injection molded samples. Blends with less than 28 vol% LCP exhibited the same stiffness and the same coefficient of linear thermal expansion as PBT. Blends containing more than 38 vol% LCP contained an oriented LCP phase and had a stiffness in accordance with the upper-bound composite equation. The coefficients of linear thermal expansion for these blends were close to that of pure LCP.     

Morphological studies of blends containing liquid crystalline polymers with poly(ethylene terephthalate)

The investigation involved the structure–property behavior of extruded cast films prepared from blends of thermotropic liquid crystalline copolyesters with poly(ethylene terephthalate) (PET). Data were obtained which showed not only the temperature dependence of the moduli and stress–strain behavior but also the orientation effects that must be prevalent in order to explain the differences between the moduli measured parallel and perpendicular to the extrusion direction. Only at high liquid crystal polymer (LCP) composition is the modulus particularly increased. The modulus enhancement with lower LCP content and utilization of process variables are discussed with respect to the induced morphological textures and nature of the process equipment. Specifically, the process variable extruder gear pump speed did not enhance Young's modulus at the same LCP content as extensively as did the process variable of extruder screw speed.     

Thermal properties of blends of poly(ethylene terephthalate) and a liquid crystalline copolyester

Summary A thermotropic liquid crystal copolyester (CHQ/BP/TA/IA; 40/10/40/10) (LCP), and melt blends of poly (ethylene terephthalate) (PET) with LCP have been studied for thermal transition and crystallization behaviour. The LCP has a mesophase transition (KM) in the temperature range of 295–315°C. The endothermic peak showing mesophase to Isotropic (MI) transition is observed around 420°C. These transitions are supported by hot stage polarizing microscopy. In blends of PET/LCP, the mesomorphic transition is observed at temperature around 314°C, along with the melting transition of PET around 274°C. The dynamic calorimetric measurements reveal that the two polymers are at least partially miscible.     

Properties of ultrasound-assisted blends of poly(ethylene terephthalate) with polycarbonate

This study investigated the effect of ultrasound irradiation on blends of polyethylene terephtalate (PET) and polycarbonate (PC). The blends of PET/PC were prepared by a twin-screw extruder with an attached ultrasonic device. Thermal, rheological, and mechanical properties and morphology of the blends with and without sonication have been analyzed. The two distinct T_gs of the blends measured by DSC showed immiscibility over all compositions. The theoretical PET content that is miscible in PC-rich phase calculated using the Fox equation showed that ultrasonic waves made the blends more miscible. From mechanical test results, when sonication was not applied, the 20/80 blend was the most miscible composition. At that composition, the impact strength of sonicated blend was surprisingly high. It was believed to be due to the enhancement of compatibility by a reaction such as transesterification. The results from the morphology of the 20/80 sonicated blend were in agreement with DSC and impact test results.     

Mechanical properties and morphology of polymer blends of poly(ethylene terephthalate) and semiflexible thermotropic liquid crystalline polyesters

Two thermotropic liquid crystalline polyesters (TLCPs) with long flexible spacer groups in the main chain were prepared by melt polymerization: one was a homopolymer with only decane groups (LCPHO) and the other was a copolymer with hexane or decane groups (LCPCO) between mesogen units. These polyesters were blended with a matrix polymer of poly(ethylene terephthalate) (PET). Scanning electron microscopy (SEM) revealed the excellent interfacial adhesion between polyester and PET, and the large aspect ratio of polyester microfibrils in the blend fiber made by extruding and drawing the blend through a die. The aspect ratio was estimated by using the modified Halpin-Tsai equation. The fiber with LCPHO showed more extensive fibril formation than that with LCPCO.     

The vibration welding of poly(methyl methacrylate) to itself and to polycarbonate,poly(butylene terephthalate), and modified poly(phenylene oxide)

The weldability of poly(methyl methacrylate) (PMMA) to itself and to polycarbonate (PC), poly(butylene terephthalate) (PBT), and modified poly(phenylene oxide) (M-PPO) is assessed through 120 and 250 Hz vibration welds. Weld strengths equal to those of the base resin have been demonstrated in welds of PMMA and M-PPO to themselves. In welds of PMMA to PC and to M-PPO, weld strengths equal to those of PC and M-PPO, respectively, have been demonstrated. PMMA does not weld well to PBT; the highest weld strength obtained was 21% of the strength of PBT resin.     

Reinforcement of mechanical properties of a poly(ethylene terephthalate) and polycarbonate blend by the addition of thermotropic liquid crystalline polymer

Mechanical properties of the ternary blends of poly(ethylene terephthalate) (PET), polycarbonate (PC), and thermotropic liquid crystalline (TCLP, Vectra A950) were investigated. The ternary blends were prepared by varying the amount TLCP but fixing the ration of PET and PC. The fiber fallen freely through the capillary die had the highest initial modulus (1.46 GPa)/tensile strength (73 MPa) when 10% of TLCP was added. Above this TLCP content, however initial modulus and tensile strength decreased. The scanning electron microscope (SEM) micrographs of the TLCP phase which was extracted by dissolving PET/PC matrix from the blend showed the fine fibrils formed at 5 and 10% of TLCP, while the aggregated TLCP phases at 20 and 30% of TLCP. It was suggested that the decrease of the mechanical properties of the resulting blend was caused by the aggregation of TLCP phase above 10% of TLCP. A high draw ratio gave a rise to the formation of highly oriented fibrils of TLCP phase in the PET/PC matrix and the improvement of mechanical properties of the ternary blend.     

Structural properties and impact fracture behavior of injection molded blends of liquid crystalline copolyester and modified poly(phenylene oxide)

Blends of a thermotropic liquid crystalline polymer (LCP) with modified poly (phenylene oxide) (PPO) were injection molded. The morphology, tensile properties and dynamic mechanical behavior of the blends have been studied as a function of LCP content. Furthermore, the impact performance of these blends has been investigated by the instrumented Izod and Charpy falling weight tests. The critical strain energy release rate (G_IC) of the blends were determined and the G_IC values were found to be dependent on the LCP content. The results are discussed and explained in terms of materials morphology.     

Injection-moulded blends of a thermotropic liquid crystalline polymer with polyethylene terephthalate,polypropylene, and polyphenylene sulfide

The properties of various blends of a polyester-type thermotropic liquid crystalline polymer (LCP) with polyethylene terephtalate, polypropylene, and polyphenylene sulfide were investigated. The polymers were blended in a twin screw extruder after which samples were injection moulded. The mechanical properties, morphology, and thermal properties of the blends are discussed.     

Extruded blends of a thermotropic liquid crystalline polymer with polyethylene terephthalate,polypropylene, and polyphenylene sulfide

Structure–property relationships were investigated for blends of a polyester-type thermotropic liquid crystalline polymer (LCP) with polyethylene terephthalate (PET), polypropylene (PP), and polyphenylene sulfide (PPS). The polymers were melt blended in a twin-screw extruder and the blends were extruded to strands of different draw ratios. Tensile properties of the blends were determined as a function of LCP content and draw ratio and compared with the results of morphological and rheological analyses. In general, the strength and stiffness of the matrix polymers were improved with increasing LCP content and draw ratio. At a draw ratio of 11, the blends of PET/30 wt % LCP exhibited a tensile strength about three times and an elastic modulus nearly four times that of pure PET. All blends exhibited a skin/core morphology with thin fibrils in the skin region. The formation and the sizes of the fibril-like LCP domains in the matrices were found to depend on LCP content and the viscosity ratio of the blend components.     

Izod impact fracture morphology of rubber-toughened polysulfone and poly(phenylene sulfide) blends

Blends of polysulfone (PSF) and poly-phenylene sulfide (PPS) exhibit ductile behavior, below 35% by weight PPS, under tensile loading conditions. However, the blends are notch sensitive to Izod impact. The use of a core-shell type rubber-modifier effectively toughens the blends. Notched Izod impact strength rises, from ～ 50 J/m to about 900 j/m, by increasing rubber content from 0% to 10–15%. It remains constant at a rubber content > 10–15%. Scanning electron microscopy (SEM) is used to study the morphology of the fracture surfaces. At low modifier content (5%), smooth or mesa-like fracture surfaces are observed. Voids and interfacial debonding are revealed. With a higher concentration of toughening agent (> 10%), some crazing is evidence but not consistent. However, matrix yielding and extensive plastic flow of the PSF/PPS matrix are seen throughout, with a higher level of rubber modifier.     

Mechanical properties of the rubber-toughened polymer blends of polycarbonate (PC) and poly(ethylene terephthalate) (PET)

The intrinsically impact-brittle PC/PET blends can be effectively toughened, in terms of lower ductile brittle transition temperature (DBTT) and reduced notch sensitivity, by incorporating butylacrylate core-shell rubber. The rubber particles are distributed exclusively in the PC phase. Varying the PC melt flow rate (MFR) is more important than varying the PET I.V. to vary the low temperature toughness of the blends. PC with MFR = 3 is essential to produce the toughest PC/PET/rubber blend. The presence of rubber slightly relieves the strain rate sensitivity on yield stress increase. Lower MFR PC in the blend results in smaller activation volume and, therefore, higher strain rate sensitivity, because a greater number of chain segments are involved in the cooperative movement during yielding. Two separate modes, localized and mass shear yielding, work simultaneously in the rubber toughening mechanism. The plane-strain localized shear yielding dominates the toughening mechanism at lower temperatures and brittle failure, while the plane-stress mass shear yielding dominates at higher temperatures and ductile failure. The critical precrack plastic zone volume has been used to interpret the observed phenomenon. © 1994 John Wiley & Sons, Inc.     

Morphological studies on the blends of poly(butylene terephthalate) and bisphenol-A polycarbonate

The morphology of the blends of poly(butylene terephthalate) (PBT) and bisphenol-A polycarbonate (PC) crystallized from the melt were studied by density measurement and small angle light scattering techniques. Rate of crystallization of these blends was found to be slower with increasing amount of PC. The Avrami exponent n for the blends was calculated and related to the shape of superstructures formed. H_v and V_v scattering patterns as seen by small angle light scattering were analyzed to determine the nature, shape, and size of the superstructures. The changes in the superstructure formation with increasing amount of PC in these blends have been analyzed.     

Calorimetric and dielectric study on poly(trimethylene terephthalate)/polycarbonate blends

Poly(trimethylene terephthalate) (PTT)/polycarbonate (PC) blends with different compositions were prepared by melt blending. The miscibility and phase behavior of melt-quenched and cold-crystallized blends were studied using differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy. The blends of all compositions display only one glass transition (T _g ) in both states. The melting temperature and the crystallinity of PTT in the blend decrease with increasing PC content. The dielectric results for the melt-quenched blends, for PC content up to 60 wt.%, exhibited two merged relaxation peaks during the heating scan; the lower temperature relaxation peak represent the normal glass-transition (α) relaxation of the mixed amorphous phase and the higher temperature relaxation due to the new-constrained mixed amorphous phase after crystallization. Cold-crystallized blends displayed only one glass transition α-relaxation whose temperatures varied with composition in manner similar to that observed by DSC. The dielectric α-relaxation of cold crystallized blends has been analyzed. Parameters relating to relaxation broadening, dielectric relaxation strength, and activation energy were quantified and were found to be composition dependent. The PTT/PC blends could be considered as two-phase system, a crystalline PTT phase and a mixed amorphous phase consisting of a miscible mixture of the two polymers. However, the crystallinity was only detected for blends containing greater than 40 wt.% PTT.     

Injection molding of poly(ethylene terephthalate) reinforced with pregenerated thermotropic liquid crystalline polymer microfibrils

This work was concerned with the injection molding of poly(ethylene terephthalate) (PET) reinforced with pregenerated thermotropic liquid crystalline polymer (TLCP) fibrils, where the TLCP had a higher melt processing temperature than PET. These composites, referred to as pregenerated microcomposites, were produced using a two step processing scheme. First, a novel dual extrusion process was used to spin strands of PET reinforced with nearly continuous TLCP fibrils. Second, these strands were subsequently chopped into pellets and injection molded below the melt processing temperature of the TLCP but above that of PET. This allowed the high modulus TLCP fibrils generated in the spinning step to be retained in the injection molded samples. TLCP concentration and strand draw ratio were varied in the composite strands to determine how they affected mechanical properties. It was shown that the best properties were obtained using strands containing 50 weight percent TLCP with draw ratios greater than 50, which were diluted to the desired loading level with a low viscosity injection molding grade of PET. Specifically, these composites had tensile moduli as high as 5.7 GPa when reinforced with 30 weight percent HX1000. Also, it was determined that pregenerated microcomposites had smoother surfaces than glass-filled PET.     

Kinetics of isothermal and non‐isothermal crystallization of poly(butylene terephthalate)/ liquid crystalline polymer blends

The melting behavior and isothermal and non‐isothermal crystallization kinetics of poly(butylene terephthalate) (PBT)/thermotropic liquid crystalline polymer (LCP), Vectra A950 (VA) blends were studied by using differential scanning calorimetry. Isothermal crystallization experiments were performed at crystallization temperatures (T_c), of 190, 195, 200 and 205°C from the melt (300°C) and analyzed based on the Avrami equation. The values of the Avrami exponent indicate that the PBT crystallization process in PBT/VA blends is governed by three‐dimensional morphology growth preceded by heterogeneous nucleation. The overall crystallization rate of PBT in the melt blends is enhanced by the presence of VA. However, the degree of PBT crystallinily remains almost the same. The analysis of the melting behavior of these blends indicates that the stability and the reorganization process of PBT crystals in blends are dependent on the blend compositions and the thermal history. The fold surface interfacial energy of PBT in blends is more modified than in pure PBT. Analysis of the crystallization data shows that crystallization occurs in Regime II across the temperature range 190°C‐205°C. A kinetic treatment based on the combination of Avrami and Ozawa equations, known as Liu's approach, describes the non‐isothermal crystallization. It is observed that at a given cooling rate the VA blending increases the overall crystallization rate of PBT.