Rheological and thermal study of delayed crystallization in poly(butylene terephthalate)/ethylene‐co‐ethyl acrylate blends

The effects of the composition and resulting morphology on the crystallization and rheology of blends containing poly(butylene terephthalate) (PBT) and an ethylene‐co‐ethyl acrylate (EEA) copolymer, two immiscible polymers, were studied over the entire range of volume fractions. Differential scanning calorimetry (DSC) thermograms recorded during cooling showed important differences, mainly in terms of the PBT crystallization temperatures, depending on the blend composition. In addition to the classical crystallization peaks of PBT and EEA, a third crystallization peak appeared for blends containing less than 60% PBT. This peak was attributed to a delayed crystallization of PBT. This phenomenon was examined in terms of homogeneous crystallization. Linear viscoelastic measurements allowed the delayed crystallization behavior in these polymer blends to be displayed. Indeed, the variation of the storage modulus with the temperature showed increasing steps during cooling. These sudden increases appeared at temperatures very close to those at which the crystallization peaks were observed in the DSC experiments. This behavior was verified for different blend compositions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 714–721, 2004     

Investigations of transesterification in PC/PBT melt blends and the proof of immiscibility of PC and PBT at completely suppressed transesterification

The miscibility of polycarbonate PC and poly(butylene terephthalate) PBT is controversially discussed in the literature. Partial miscibility has been generally found in melt blends of the two polymers. However, in solution cast blends they were found to be immiscible. It is known that the transesterification takes place in the melt. Copolyesters formed by the transesterification change the compatibility of PC and PBT. In this work PC/PBT melt blends of various composition were investigated in dependence on the copolyester content by means of DSC and NMR. It can be shown that the time regime of the thermal treatment in the melt determines the transesterification degree. The PBT crystallization behavior is strongly influenced by both the PC and copolyester content. The glass transition temperatures of the PBT-rich and PC-rich phase approach each other with the increasing copolyester content. The analysis of the glass transition behavior permits the conclusion that PC and PBT are inherently immiscible provided that the copolyester content is exactly zero. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2161–2168, 1997     

Characterization of PBT/POSS nanocomposites prepared by in situ polymerization of cyclic poly(butylene terephthalate) initiated by functionalized POSS

Novel poly(butylene terephthalate) (PBT)/polyhedral oligomeric silsesquioxane (POSS) nanocomposites were synthesized by ring‐opening polymerization of cyclic poly(butylene terephthalate) initiated by functionalized POSS with various feed ratios. The impact of POSS incorporation on melting and crystallization behaviors of PBT/POSS nanocomposites was investigated by means of X‐ray diffraction and differential scanning calorimetry. It was found that the novel organic–inorganic association result in the significant alterations in the melting and crystallization behavior of PBT. Thermal studies confirmed that the incorporation of POSS can enhance the thermal stability of the polymers, and the copolymer glass transition temperature increased with the increasing of POSS macromonomer content. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1853–1859, 2010     

Miscibility and crystallization behavior of PBT/epoxy blends

The miscibility and the isothermal crystallization kinetics for PBT/Epoxy blends have been studied by using differential scanning calorimetry, and several kinetic analyses have been used to describe the crystallization process. The Avrami exponents n were obtained for PBT/Epoxy blends. An addition of small amount of epoxy resin (3%) leads to an increase in the number of effective nuclei, thus resulting in an increase in crystallization rate and a stronger trend of instantaneous three‐dimensional growth. For isothermal crystallization, crystallization parameter analysis showed that epoxy particles could act as effective nucleating agents, accelerating the crystallization of PBT component in the PBT/Epoxy blends. The Lauritzen–Hoffman equation for DSC isothermal crystallization data revealed that PBT/Epoxy 97/3 had lower nucleation constant K_g than 100/0, 93/7, and 90/10 PBT/Epoxy blends. Analysis of the crystallization data of PBT/Epoxy blends showed that crystallization occurs in regime II. The fold surface free energy, σ_e = 101.7–58.0 × 10^?3 J/m², and work of chain folding, q = 5.79–3.30 kcal/mol, were determined. The equilibrium melting point depressions of PBT/Epoxy blends were observed and the Flory–Huggins interaction parameters were obtained. It indicated that these blends were thermodynamically miscible in the melt. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1320–1330, 2006     

Heat capacity of poly(butylene terephthalate)

The low‐temperature heat capacity of poly(butylene terephthalate) (PBT) was measured from 5 to 330 K. The experimental heat capacity of solid PBT, below the glass transition, was linked to its approximate group and skeletal vibrational spectrum. The 21 skeletal vibrations were estimated with a general Tarasov equation with the parameters Θ₁ = 530 K and Θ₂ = Θ₃ = 55 K. The calculated and experimental heat capacities of solid PBT agreed within better than ±3% between 5 and 200 K. The newly calculated vibrational heat capacity of the solid from this study and the liquid heat capacity from the ATHAS Data Bank were applied as reference values for a quantitative thermal analysis of the apparent heat capacity of semicrystalline PBT between the glass and melting transitions as obtained by differential scanning calorimetry. From these results, the integral thermodynamic functions (enthalpy, entropy, and Gibbs function) of crystalline and amorphous PBT were calculated. Finally, the changes in the crystallinity with the temperature were analyzed. With the crystallinity, a baseline was constructed that separated the thermodynamic heat capacity from cold crystallization, reorganization, annealing, and melting effects contained in the apparent heat capacity. For semicrystalline PBT samples, the mobile‐amorphous and rigid‐amorphous fractions were estimated to complete the thermal analysis. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4401–4411, 2004     

Relationships between the molecular architecture,crystallization capacity,and miscibility in poly(butylene terephthalate)/polycarbonate blends: A comparison with poly(ethylene terephthalate)/polycarbonate blends

Poly(butylene terephthalate) (PBT)/polycarbonate (PC) samples, prepared via reactive blending in the presence of Ti‐ and Sm‐based catalysts, resulted in block copolymers whose block length decreased as the mixing time increased. A single homogeneous amorphous phase occurred when the blocks had monomeric sequences shorter than 10 units. Otherwise, a crystalline phase of PBT developed. Also, in poly(ethylene terephthalate) (PET)/PC blends previously studied, the miscibility was strictly correlated with the crystallizability of the system. Therefore, the miscibility of the PBT/PC and PET/PC blends was compared with respect to the tendency of the PBT and PET blocks to crystallize under isothermal conditions. The crystallization rate of the PBT/PC copolymers was faster than that of the PET/PC copolymers with similar block lengths. Accordingly, the minimum crystallizable sequence length of the PBT blocks was shorter than that of the PET blocks (18 vs 31 monomeric unit sequences). This behavior was interpreted as an effect of the more flexible PBT units, which had a greater tendency to fold and crystallize than the PET units. Therefore, PBT, the blocks of which tended to crystallize even if they were very short and phase‐separated, was characterized by a poorer compatibility with PC than that of PET. As a result, the block size had a fundamental role in determining the crystallizability and, therefore, phase behavior of the semicrystalline block copolymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2821–2832, 2004     

Isothermal crystallization kinetics of poly(butylene terephthalate)/attapulgite nanocomposites

Poly(butylene terephthalate) (PBT)/organo‐attapulgite (ATT) nanocomposites containing 2.5 and 5 wt % nanoparticles loadings were fabricated via a simple melt‐compounding approach. The crystal structure and isothermal crystallization behaviors of PBT composites were studied by wide‐angle X‐ray diffraction and differential scanning calorimetry, respectively. The X‐ray diffraction results indicated that the addition of ATT did not alter the crystal structure of PBT and the crystallites in all the samples were triclinic α‐crystals. During the isothermal crystallization, the PBT nanocomposites exhibited higher crystallization rates than the neat PBT and the varied Avrami exponents when compared with the neat PBT. At the same time, the regime II/III transition was also observed in all the samples on the basis of Hoffman‐Laurizten theory, but the transition temperature increased with increasing ATT loadings. The fold surface free energy (σ_e) of polymer chains in the nanocomposites was lower than that in the neat PBT. It should be reasonable to treat ATT as a good nucleating agent for the crystallization of PBT, which plays a determinant effect on the reduction in σ_e during the isothermal crystallization of the nanocomposites, even if the existence of ATT could restrict the segmental motion of PBT. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2112–2121, 2006     

Isothermal crystallization studies of poly(butylene terephthalate) composites

Different crystallization kinetic models (Avrami and Tobin) have been applied to study the crystallization kinetics of virgin poly(butylene terephthalate) (PBT) and filled PBT systems under isothermal experimental conditions. The experimental data have been analyzed with a nonlinear, multivariable regression program. The kinetic parameters for the isothermal crystallization have been determined. The analysis results indicate that both models satisfactorily represent the isothermal crystallization kinetics. PBT crystallizes most slowly. The presence of nanoclays or nanofibers, added as fillers, enhances the crystallization rate of PBT composites. An analysis of the kinetic data with the Avrami and Tobin models has shown little change in the crystallization exponent compared with that of virgin PBT. The crystallization rate constant decreases with a rise in the temperature for the two models. This trend has been observed for similar polyester systems reported in the literature. The dispersion of the clay layers in the PBT nanocomposites has been characterized with wide‐angle X‐ray diffraction and transmission electron microscopy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1344–1353, 2007     

Poly(butylene terephthalate)/clay nanocomposite compatibilized with poly(ethylene‐co‐glycidyl methacrylate). II. Nonisothermal crystallization

Melting behaviors and nonisothermal crystallization of poly(butylene terephthalate)/poly(ethylene‐co‐glycidyl methacrylate) (PBT/PEGMA), PBT/commercial modified montmorillonite clays (PBT/Clay), and PBT/exfoliated silicates (PBT/PEGMA/Clay) nanocomposites were studied by wide‐angle X‐ray diffraction and differential scanning calorimeter. PEGMA is used as a compatibilizer. For both isothermally and nonisothermally crystallized samples, PEGMA facilitates the recrystallization of PBT during the heating scans, and leads to a less degree of perfection of the crystals. However, the clay hinders the recrystallization growth during heating scans, and increases perfection of the crystals. Nonisothermal crystallization kinetics was described by kinetic models and undercooling was taken into account. The PEGMA would lead to an increase of the blend viscosity, rendering the chains less mobile and lower the crystallizability of PBT in PBT/PEGMA. The well‐dispersed exfoliated silicates in PBT/PEGMA/Clay cause a large number of nuclei to precede crystallization. The fold surface free energy (σ_e) and activation energy also supported the interpretation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 564–576, 2008     

Transesterification and crystallization behavior of poly(butylene succinate)/poly(butylene terephthalate) block copolymers

The block copolymers of poly(butylene succinate) (PBS) and poly(butylene terephthalate) (PBT) were synthesized by melt processing for different times. The sequence distribution, thermal properties, and crystallization behavior were investigated over a wide range of compositions. For PBS/PBT block copolymers it was confirmed by statistical analysis from ¹H-NMR data that the degree of randomness (B) was below 1. The melting peak (T_m) gradually moved to lower temperature with increasing melt processing time. It can be seen that the transesterification between PBS and PBT leads to a random copolymer. From the X-ray diffraction diagrams, only the crystal structure of PBS appeared in the M1 copolymer (PBS 80 wt %) and that of PBT appeared in the M3 (PBS 50 wt %) to M5 (PBS 20 wt %) copolymers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 147–156, 1998     

Isothermal melt-crystallization and melting behavior for three linear aromatic polyesters

Isothermal crystallization and subsequent melting behavior for three different types of linear aromatic polyester, namely poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT), and poly(butylene terephthalate) (PBT), were investigated (with an emphasis on PTT in comparison with PET and PBT). These polyesters were different in the number of methylene groups (i.e. 2, 3, and 4 for PET, PTT, and PBT, respectively). Isothermal crystallization studies were carried out in a differential scanning calorimeter (DSC) over the crystallization temperature range of 182-208 °C. The wide-angle X-ray diffraction (WAXD) technique was used to obtain information about crystal modification and apparent degree of crystallinity. The kinetics of the crystallization process was assessed by a direct fitting of the experimental data to the Avrami, Tobin, and Malkin macrokinetic models. It was found that the crystallization rates of these polyesters were in the following order: PBT>PTT>PET, and the melting of these polyesters exhibited multiple-melting phenomenon. Lastly, the equilibrium melting temperature for these polyesters was estimated based on the linear and non-linear Hoffman-Weeks (LHW and NLHW) extrapolative methods.     

Physicochemical properties of alkali-treated new cellulosic fiber from cotton shell

The aim of this present investigation was to identify a new natural fiber from one of the cotton plant’s byproducts, which is chemically modified by alkaline treatment. Its characteristics were examined for the preparation of natural fiber–reinforced polymer composites. The cotton shell fibers (CSFs) were extracted from the cotton shell and its degree of crystallinity, crystallite size, chemical constituents group, and thermal stability were determined by X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis. The alkali treatment of CSFs is optimized at 5% (w/v) NaOH aqueous solution with 45 min soaking time.     

Preparation and characterization of poly(butylene terephthalate)/poly(ethylene terephthalate) copolymers via solid‐state and melt polymerization

To increase the T_g in combination with a retained crystallization rate, bis(2‐hydroxyethyl)terephthalate (BHET) was incorporated into poly(butylene terephthalate) (PBT) via solid‐state copolymerization (SSP). The incorporated BHET fraction depends on the miscibility of BHET in the amorphous phase of PBT prior to SSP. DSC measurements showed that BHET is only partially miscible. During SSP, the miscible BHET fraction reacts via transesterification reactions with the mobile amorphous PBT segments. The immiscible BHET fraction reacts by self‐condensation, resulting in the formation of poly(ethylene terephthalate) (PET) homopolymer. ¹H‐NMR sequence distribution analysis showed that self‐condensation of BHET proceeded faster than the transesterification with PBT. SAXS measurements showed an increase in the long period with increasing fraction BHET present in the mixtures used for SSP followed by a decrease due to the formation of small PET crystals. DSC confirmed the presence of separate PET crystals. Furthermore, the incorporation of BHET via SSP resulted in PBT‐PET copolymers with an increased T_g compared to PBT. However, these copolymers showed a poorer crystallization behavior. The modified copolymer chain segments are apparently fully miscible with the unmodified PBT chains in the molten state. Consequently, the crystal growth process is retarded resulting in a decreased crystallization rate and crystallinity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 882–899, 2007.     

Crystallization and morphology of polymerized cyclic butylene terephthalate

The crystallization behavior and morphology of polymerized cyclic butylene terephthalate (pCBT) were investigated by thermal differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The spherulite growth rate was analyzed based on the Hoffman and Lauritzen theory to better understand the crystallization behavior. We found four typical morphologic features of pCBT corresponding to the crystallization temperature spectrum: usual negative spherulite, unusual spherulite, mixed birefringence spherulite coexisting with boundary crystals, and highly disordered spherulitic crystallites. The Avrami crystallization kinetics confirmed the occurrence of combined heterogeneous nucleation accompanied by a change in the spherulitic shape of pCBT, which also agreed with the PLM results. The equilibrium melting temperature and glass transition temperature of pCBT were 257.8 °C and 41.1 °C, respectively. A regime II–III transition occurred at 200.9 °C, which was 10 °C lower than that reported for poly(butylene terephthalate) (PBT). Coinciding with and attributed to the regime transition, the boundary crystal disappeared at temperatures above 200 °C and the morphology changed from the mixed type to highly disordered spherulitic crystallites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1127–1134, 2010     

Reactive compatibilization of polyamide‐6 (PA 6)/polybutylene terephthalate (PBT) blends by a multifunctional epoxy resin

A multifunctional epoxy resin has been demonstrated to be an efficient reactive compatibilizer for the incompatible and immiscible blends of polyamide‐6 (PA 6) and polybutylene terephthalate (PBT). The torque measurements give indirect evidence that the reaction between PA and PBT with epoxy has an opportunity to produce an in situ formed copolymer, which can be as an effective compatibilizer to reduce and suppress the size of the disperse phase, and to greatly enhance mechanical properties of PA/PBT blends. The mechanical property improvement is more pronounced in the PA‐rich blends than that in the PBT‐rich blends. The fracture behavior of the blend with less than 0.3 phr compatibilizer is governed by a particle pullout mechanism, whereas shear yielding is dominant in the fracture behavior of the blend with more than 0.3 phr compatibilizer. As the melt and crystallization temperatures of the base polymers are so close, either PA or PBT can be regarded as a mutual nucleating agent to enhance the crystallization on the other component. The presence of compatibilizer and in situ formed copolymer in the compatibilized blends tends to interfere with the crystallization of the base polymers in various blends. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 23–33, 2000     

Preparation of small poly(butylene terephthalate) spheres by crystallization from solution

The crystallization of poly(butylene terephthalate) (PBT) from moderately dilute solutions of PBT in a diglycidyl ether of bisphenol-A epoxy has been investigated. PBT dissolves in this epoxy approximately 35°C below its usual melting temperature of 227°C to form a one-phase solution. Cooling this solution below 165°C leads to rapid crystallization of the PBT. The resulting mixture of liquid epoxy and crystalline PBT has a low viscosity and contains highly birefringent, individual PBT spherulites. The PBT spherulites have a narrow size distribution and a high surface-to-volume ratio. These particles are suggested to arise from a rapid crystallization that follows liquid–liquid phase separation. © 1994 John Wiley & Sons, Inc.     

Chemical degradation of an uncrosslinked pure fluororubber in an alkaline environment

The chemical degradation of an uncrosslinked pure fluoroelastomer (FKM; Viton A) in an alkaline environment (10% NaOH and 80 °C) was investigated. Scanning electron microscopy images showed that on a microscopic level, significant degradation substantially increased the surface roughness after prolonged exposure (e.g., 12 weeks). The molecular mechanisms of the chemical degradation processes at the surface were evaluated with X‐ray photoelectron spectroscopy and attenuated total reflectance/Fourier transform infrared spectroscopy. The results revealed that the early degradation proceeded primarily via dehydrofluorination reactions, creating double bonds in the rubber backbone. This further accelerated the degradation after longer exposure times. Furthermore, the resulting double bonds underwent nucleophilic attack by an aqueous NaOH solution to form several oxygenated species. All these species ultimately recombined to form crosslinks, as evidenced by the increase in the gel fraction and surface hardness (Shore A). The pronounced effect of chemical degradation through a reduction in the thermal stability of the pure FKM rubber upon exposure was also evident from thermogravimetric analysis and differential thermogravimetry. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6216–6229, 2004     

Melting behavior in blends of isotactic polypropylene and a liquid crystalline polymer

The influence of low contents of a liquid crystalline polymer on the crystallization and melting behavior of isotactic polypropylene (iPP) was investigated using electron and optical microscopy, differential scanning calorimetry, and X-ray diffraction. In pure iPP, the α modification was found, whereas for iPP/Vectra blends at Vectra concentration <5%, both α and β forms were observed. The amount of β phase varied from 0.23 to 0.16. Optical microscopy showed that Vectra was able to nucleate both α and β forms. Non-isothermal crystallization produces a material with a strong tendency for recrystallization of the α and β forms (αα′ and ββ′ recrystallization) leading to double endotherms for both crystalline forms in DSC thermograms. Melting thermograms after isothermal crystallization at low temperatures showed a similar behavior. At values of T_c > 119 °C for the α form and T_c > 125 °C for the β form, only one melting endotherm was observed because enough perfect crystals, not susceptible to recrystallization, were obtained. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1949–1959, 2004     

Block copolymers of poly(ethylene terephthalate–polybutylene terephthalate). I. Preparation and crystallization behavior

Block copolymers of two crystallizable compounds, poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), were developed with PET as the major component and the amount of PBT varying from 1.0 to 20.0 wt %. These block copolymers were prepared by end-group coupling of preformed oligomers. All polymers prepared were of equivalent molecular weight as determined by the intrinsic viscosity method. Thermal properties were determined by differential thermal analysis (DTA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). With increasing PBT content, the block copolymers showed a general decrease in the values of glass transition temperature, melting temperature, initial decomposition temperature, and maximum decomposition temperature. The heat of fusion and heat of crystallization first increased and then decreased slightly. Rates of crystallization were determined by measuring density as a function of time of isothermal crystallization carried out at 95°C. It was found that small amounts of PBT increased the crystallization rate considerably over that of PET. Random copolymers did not show this phenomenon and behaved more like pure PET. The crystallization behavior of block copolymers was analyzed by the Avrami equation and Avrami exponents were determined. Results were explained on the basis that the faster-crystallizing PBT blocks crystallized first and provided built-in nucleation sites for the subsequent crystallization of PET, thus resulting in a relatively fast-crystallizing copolyester.     

Nonisothermal cold‐crystallization kinetics of poly(trimethylene terephthalate)

The nonisothermal cold‐crystallization kinetics and subsequent melting behavior of poly(trimethylene terephthalate) (PTT) were investigated with differential scanning calorimetry. The Avrami, Tobin, and Ozawa equations were applied to describe the kinetics of the crystallization process. Both the Avrami and Tobin crystallization rate parameters increased with the heating rate. The Ozawa crystallization rate increased with the temperature. The ability of PTT to crystallize from the glassy state at a unit heating rate was determined with Ziabicki's kinetic crystallizability index, which was found to be about 0.89. The effective energy barrier describing the nonisothermal cold‐crystallization process of PTT was estimated by the differential isoconversional method of Friedman and was found to range between about 114.5 and 158.8 kJ mol^?1. In its subsequent melting, PTT exhibited double‐melting behavior for heating rates lower than or equal to 10 °C min^?1 and single‐melting behavior for heating rates greater than or equal to 12.5 °C min^?1. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4151–4163, 2004