Investigations of the equilibrium melting temperature in PBT and PC/PBT blends

The melting behavior of poly(butylene terephthalate) and its blends with bisphenol-A polycarbonate was investigated with differential scanning calorimetry. The aim of this work was to determine the equilibrium melting temperature and its dependence on the blend composition using the Hoffman-Weeks plots. It is shown that the critical analysis of various influences on the melting peak is necessary for the reorganization processes and crystallized content of blends. The experimental conditions and the corrections of measured temperatures were derived and discussed. It was found that the use of the extrapolated onset temperature T_m,o of the melting peak is more efficient than the maximum temperature T_m for the Hoffman-Weeks plots. The equilibrium values of pure PBT are determined to be T^o_m,o = 501 K and T^o_m = 506 K. The equilibrium temperatures of the blends do not show a depression with increasing PC content. Using the Nishi-Wang relation, the results can be qualitatively interpreted with a polymer-polymer interaction coefficient χ ≥ 0 between both components. A weak increase in the equilibrium temperature with increasing PC content was observed. A hypothesis to explain this is based on the possibility of a changed population of the different spherulites with various melting temperatures in dependence on PC content. © 1996 John Wiley & Sons, Inc.     

Isothermal and nonisothermal crystallization kinetics of poly(propylene terephthalate)

The kinetics of crystallization of poly(propylene terephthalate) (PPT) samples of different molecular weights were studied under both isothermal and nonisothermal conditions. The Avrami and Lauritzen–Hoffmann treatments were applied to evaluate kinetic parameters of PPT isothermal crystallization. It was found that crystallization is faster for low‐molecular‐weight samples. The modified Avrami equation, and the combined Avrami–Ozawa method were found to successfully describe the nonisothermal crystallization process. Also, the analysis of Lauritzen–Hoffmmann was tested and it resulted in values close to those obtained with isothermal crystallization data. The nonisothermal kinetic data were corrected for the effect of the temperature lag and shifted alone with the isothermal kinetic data to obtain a single master curve, according to the method of Chan and Isayev, testifying to the consistency between the isothermal and corrected nonisothermal data. A new method for ranking of polymers, referring to the crystallization rates, was also introduced. This involved a new index that combines the maximum crystallization rate observed during cooling with the average crystallization rates over the temperature range of the crystallization peak. Furthermore, the effective energy barrier of the dynamic process was evaluated with the isoconversional methods of Flynn and Friedmann. It was found that the energy barrier is lower for the low‐molecular‐weight PPT. The effect of the catalyst remnants on the crystallization kinetics was also investigated and it was found that this is significant only for low‐molecular‐weight samples. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3775–3796, 2004     

Multiple melting behavior of poly(butylene succinate). II. Thermal analysis of isothermal crystallization and melting process

The multiple melting behavior of poly(butylene succinate) (PBSu) was studied with differential scanning calorimetry (DSC). Three different PBSu resins, with molecular weights (MWs) of 1.1 × 10⁵, 1.8 × 10⁵, and 2.5 × 10⁵, were isothermally crystallized at various crystallization temperatures (T_c) ranging from 70 to 97.5 °C. The T_c dependence of crystallization half‐time (τ) was obtained. DSC melting curves for the isothermally crystallized samples were obtained at a heating rate of 10 K min⁻¹. Three endothermic peaks, an annealing peak, a low‐temperature peak L, and a high‐temperature peak H, and an exothermic peak located between peaks L and H clearly appeared in the DSC curve. In addition, an endothermic small peak S appeared at a lower temperature of peak H. Peak L increased with increasing T_c, whereas peak H decreased. The T_c dependence of the peak melting temperatures [T_m(L) and T_m(H)], recrystallization temperature (T_re), and heat of fusion (ΔH) was obtained. Their fitting curves were obtained as functions of T_c. T_m(L), T_re, and ΔH increased almost linearly with T_c, whereas T_m(H) was almost constant. The maximum rate of recrystallization occurred immediately after the melting. The mechanism of the multiple melting behavior is explained by the melt‐recrystallization model. The high MW samples showed similar T_c dependence of τ, and τ for the lowest MW sample was longer than that for the others. Peak L increased with MW, whereas peak H decreased. In spite of the difference of MW, T_m(L), T_m(H), and T_re almost coincided with each other at the same T_c. The ΔH values, that is crystallinity, for the highest MW sample were smaller than those for the other samples at the same T_c. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2039–2047, 2005     

Nonisothermal crystallization behavior of SPS/APS blends

The nonisothermal crystallization behavior of syndiotactic polystyrene (SPS)/atactic polystyrene (APS) blends was studied with differential scanning calorimetry to investigate the effects of APS on the crystallization behavior of SPS. Polarized optical microscopy and wide‐angle X‐ray diffractometry were also used to observe the morphology of the SPS crystalline structure. From a cyclic heating/cooling temperature program, we obtained found that APS retarded the crystallization (and recrystallization) of SPS and made the crystal less perfect, but the ultimate crystallinity of SPS did not change with the addition of APS. We also observed that APS was disposed in the interfibrillar region of SPS spherulites and did not change the crystalline form of SPS. This result will be helpful for improving SPS applications through blending with rubber‐toughened APS. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3001–3008, 2000     

Miscibility and hydrogen‐bonding interactions in blends of carbon dioxide/epoxy propane copolymer with poly(p‐vinylphenol)

The miscibility and hydrogen‐bonding interactions of carbon dioxide and epoxy propane copolymer to poly(propylene carbonate) (PPC)/poly(p‐vinylphenol) (PVPh) blends were investigated with differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy and X‐ray photoelectron spectroscopy (XPS). The single glass‐transition temperature for each composition showed miscibility over the entire composition range. FTIR indicates the presence of strong hydrogen‐bonding interassociation between the hydroxyl groups of PVPh and the oxygen functional groups of PPC as a function of composition and temperature. XPS results testify to intermolecular hydrogen‐bonding interactions between the oxygen atoms of carbon–oxygen single bonds and carbon–oxygen double bonds in carbonate groups of PPC and the hydroxyl groups of PVPh by the shift of C_1s peaks and the evolution of three novel O_1s peaks in the blends, which supports the suggestion from FTIR analyses. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1957–1964, 2002     

Multiple melting endotherms in isothermally melt-crystallized poly(butylene terephthalate)

The melting behavior of poly(butylene terephthalate) crystallized isothermally for various times was examined using differential scanning calorimetry. After short crystallization times, the DSC analysis gave two melting peaks, but after longer times, the analysis gave three peaks. The latter triplet of DSC peaks can be denoted as low, middle, and high, starting with the lowest temperature endotherm. The DSC peaks were simulated using a measured recrystallization rate and behavior for PBT and an assumed initial melting point distribution. The low and middle peaks represent the original melting peaks arising from isothermal crystallization. The high melting peak arises from recrystallization during the DSC heating scan. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1757–1767, 1998     

Crystallization behavior of dynamically cured polypropylene/epoxy blends

Dynamically cured polypropylene (PP)/epoxy blends compatibilized with maleic anhydride grafted PP were prepared by the curing of an epoxy resin during melt mixing with molten PP. The morphology and crystallization behavior of dynamically cured PP/epoxy blends were studied with scanning electron microscopy, differential scanning calorimetry, and polarized optical microscopy. Dynamically cured PP/epoxy blends, with the structure of epoxy particles finely dispersed in the PP matrix, were obtained, and the average diameter of the particles slightly increased with increasing epoxy resin content. In a study of the nonisothermal crystallization of PP and PP/epoxy blends, crystallization parameter analysis showed that epoxy particles could act as effective nucleating agents, accelerating the crystallization of the PP component in the PP/epoxy blends. The isothermal crystallization kinetics of PP and dynamically cured PP/epoxy blends were described by the Avrami equation. The results showed that the Avrami exponent of PP in the blends was higher than that of PP, and the crystallization rate was faster than that of PP. However, the crystallization rate decreased when the epoxy resin content was greater than 20 wt %. The crystallization thermodynamics of PP and dynamically cured PP/epoxy blends were studied according to the Hoffman theory. The chain folding energy for PP crystallization in dynamically cured PP/epoxy blends decreased with increasing epoxy resin content, and the minimum of the chain folding energy was observed at a 20 wt % epoxy resin content. The size of the PP spherulites in the blends was obviously smaller than that of PP. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1181–1191, 2004     

The crystallization of polypropylene with reduced density of entanglements

The crystallization of polypropylene with different density of macromolecular entanglements was studied in isothermal and non‐isothermal conditions. The growth rate of spherulites increased with reduced concentration of entanglements. Reduction of entanglements shifted the temperature of transition between Regimes II and III, which means that more regular growth of crystals was possible at lower temperature. The range of temperatures at which polypropylene cavitated in regions of melt occluded by spherulites was limited to 137–139°C, with weak dependence on entanglements density. DSC studies showed that isothermal crystallization is faster in less entangled polymers, however the crystallinity degree and long period of structure (by SAXS) were similar for studied materials. When the crystallization was completed during fast cooling, the differences between individual samples were more significant. The partial disentangling, overcoming some limitation for movements of macromolecules, made possible easier crystallization, even at low temperature of Regime III. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 748–756     

Poly(butylene terephthalate)/poly(ε-caprolactone) blends: Influence of PCL molecular mass on PBT melting and crystallization behavior

The isothermal crystallization kinetics and melting behavior of poly(butylene terephthalate) (PBT) in binary blends with poly(ε-caprolactone) (PCL) was investigated as a function of PCL molecular mass by differential scanning calorimetry and optical microscopy. The components are miscible in the melt when oligomeric PCL (M_w = 1250) is blended with PBT, whereas only partial miscibility was found in mixtures with higher molecular mass (M_w = 10,000 and 50,000). The equilibrium melting point of PBT in the homopolymer and in blends with PCL was determined through a non-linear extrapolation of the T_m = f(T_c) curve. The PBT spherulitic growth rate and bulk crystallization rate were found to increase with respect to plain PBT in blends with PCL1250 and PCL10000, whereas addition of PCL50000 causes a reduction of PBT solidification rate. The crystallization induction times were determined by differential scanning calorimetry for all the mixtures through a blank subtraction procedure that allows precise estimation of the crystallization kinetics of fast crystallizing polymers. The results have been discussed on the basis of the Hoffman-Lauritzen crystallization theory and considerations on both the transport of chains towards the crystalline growth front and the energy barrier for the formation of critical nuclei in miscible and partially miscible PBT/PCL mixtures are widely debated.     

Epoxy resin/poly(ϵ‐caprolactone) blends cured with 2,2‐bis[4‐(4‐aminophenoxy)phenyl]propane. I. Miscibility and crystallization kinetics

Crystalline thermosetting blends composed of 2,2′‐bis[4‐(4‐aminophenoxy)phenyl]propane (BAPP)‐cured epoxy resin (ER) and poly(?‐caprolactone) (PCL) were prepared via the in situ curing reaction of epoxy monomers in the presence of PCL, which started from initially homogeneous mixtures of diglycidyl ether of bisphenol A (DGEBA), BAPP, and PCL. The miscibility of the blends after and before the curing reaction was established with differential scanning calorimetry and dynamic mechanical analysis. Single and composition‐dependent glass‐transition temperatures (T_g's) were observed in the entire blend composition after and before the crosslinking reaction. The experimental T_g's were in good agreement with the prediction by the Fox and Gordon–Taylor equations. The curing reaction caused a considerable increase in the overall crystallization rate and dramatically influenced the mechanism of nucleation and the growth of the PCL crystals. The equilibrium melting point depression was observed for the blends. An analysis of the kinetic data according to the Hoffman–Lauritzen crystallization kinetic theory showed that with an increasing amorphous content, the surface energy of the extremity surfaces increased dramatically for DGEBA/PCL blends but decreased for ER/PCL blends. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1085–1098, 2003     

Miscibility and isothermal crystallization behavior of polyamide 6/poly(vinyl alcohol) blend

An investigation of miscibility and isothermal crystallization behavior of Polyamide 6 (PA6)/Poly(vinyl alcohol) (PVA) blends was conducted. Fourier transform infrared spectra (FTIR) analysis indicated that the interactions between the carbonyl groups of PA6 and hydroxyl groups of PVA increase as the weight ratios of PA6 to PVA of PA6/PVA specimens increase. This interaction between PA6 and PVA leads to their miscibility in the amorphous region and even some extent effects on their crystal phase, respectively. Further isothermal crystallization behavior of PA6/PVA indicate that the miscibility of PVA in PA6 leading difficulty in crystallization of PA6. Several kinetics equations are employed to describe the effects of PVA on the crystallization properties of PA6 in PA6/PVA blends in detail. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1360–1368, 2008     

Miscibility and crystallization behavior of solution-blended PEEK/PI blends

Miscibility and crystallization behavior of solution-blended poly(ether ether ketone)/polyimide (PEEK/PI) blends were investigated by using DSC, optical microscopy and SAXS methods. Two kinds of PIs, YS-30 and PEI-E, which consist of the same diamine but different dianhydrides, were used in this work. The experimental results show that blends of PEEK/YS-30 are miscible over the entire composition range, as all the blends of different compositions exhibit a single glass transition temperature. The crystallization of PEEK was hindered by YS-30 in PEEK/YS-30 blends, of which the dominant morphology is interlamellar. On the other hand, blends of PEEK/PEI-E are immiscible, and the effect of PEI-E on the crystallization behavior of PEEK is weak. The crystallinity of PEEK in the isothermally crystallized PEEK/YS-30 blend specimens decreases with the increase in PI content. But the crystallinity of PEEK in the annealed samples almost keeps unchanged and reaches its maximum value, which is more than 50%. The spherulitic texture of the blends depends on both the blend composition and the molecular structure of the PIs used. The more PI added, the more imperfect the crystalline structure of PEEK. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2267–2274, 1998     

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     

The multiple melting endotherms from poly(butylene terephthalate)

The melting behavior of poly(butylene terephthalate) (PBT) has been investigated, and a simulation has been performed to determine whether the multiple melting endotherms observed during the thermal analysis of PBT can be explained by the simultaneous melting and recrystallization of an initial distribution of crystal melting temperatures that contains only one maximum and two inflection points. Specimens that were cooled at constant rates from the melt showed between one and three melting endotherms upon heating in a differential scanning calorimeter (DSC). The position and breadth of the crystallization exotherms upon cooling from the melt and small-angle x-ray scattering showed that as the cooling rate is increased, the distribution of melting temperatures broadens and shifts to lower temperatures. By combining temperature-dependent recrystallization with an initial distribution of melting temperatures, simulated DSC curves were produced that agreed well with experimental DSC curves. In instances of triple peaked curves, the high temperature peak was due to crystals formed during the scanning process, and the middle and low temperature peaks were due to crystals originally present in the material. Satisfactory agreement between the experimental and simulated curves was found without considering additional crystallization from the amorphous regions during the scanning process.     

Competition between supernucleation and plasticization in the crystallization and rheological behavior of PCL/CNT‐based nanocomposites and nanohybrids

PCL was blended with pristine multiwalled carbon nanotubes (MWCNT) and with a nanohybrid obtained from the same MWCNT but grafted with low molecular weight PCL, employing concentrations of 0.25 to 5 wt % of MWCNT and MWCNT‐g‐PCL. Excellent CNT dispersion was found in all samples leading to supernucleation of both nanofiller types. Nanohybrids with 1 wt % or less MWCNTs crystallize faster than nanocomposites (due to supernucleation), while the trend eventually reverses at higher nanotubes content (because of plasticization). Rheological results show that yield‐like behavior develops in both nanocomposites, even for the minimum content of carbon nanotubes. In addition, the MWCNT‐g‐PCL family, when compared with the neat polymer, exhibits lower values of viscosity and modulus in oscillatory shear, and higher compliance in creep. These rheological differences are discussed in terms of the plasticization effect caused by the existence of low molecular weight free and grafted PCL chains in the nanohybrids. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1310–1325     

Isothermal melt and cold crystallization kinetics of poly(aryl ether ketone ether ketone ketone)

The isothermal melt and cold crystallization kinetics of poly(aryl ether ketone ether ketone ketone) are investigated by differential scanning calorimetry over two temperature regions. The Avrami equation describes the primary stage of isothermal crystallization kinetics with the exponent n ≈ 2 for both melt and cold crystallization. With the Hoffman–Weeks method, the equilibrium melting point is estimated to be 406 °C. From the spherulitic growth equation proposed by Hoffman and Lauritzen, the nucleation parameter (K_g) of the isothermal melt and cold crystallization is estimated. In addition, the K_g value of the isothermal melt crystallization is compared to those of the other poly(aryl ether ketone)s. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1992–1997, 2000     

CO2‐delayed crystallization of isotactic polypropylene: A kinetic study

The effect of CO₂ on the nonisothermal crystallization of isotactic polypropylene (iPP) was studied with high‐pressure differential scanning calorimetry at cooling rates of 0.2–5 °C/min. CO₂ significantly delayed the melt crystallization of iPP, and both the crystallization temperature and the heat of crystallization decreased with increasing CO₂ pressure. The crystallization rate of iPP, as characterized by the half‐time, was also prolonged by the presence of CO₂. With a modified Ozawa model developed by Seo, the Avrami crystallization exponent n of iPP was calculated. This value was depressed by the addition of CO₂ and was strongly dependent on the CO₂ pressure at low cooling rates. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1518–1525, 2003     

Differential scanning calorimetric study on the curing behavior of epoxy resin/diethylenetriamine/organic montmorillonite nanocomposite

The curing mechanisms and kinetics of diglycidyl ether of bisphenol A with diethylenetriamine as the curing agent and different amounts of organic montmorillonite were examined with isothermal and dynamic scanning calorimetry. The modified Avrami equation was used to calculate the activation energy and reaction orders in the isothermal experiment. A single peak was observed in each dynamic scan. The curing mechanism and kinetics of the curing reaction were also analyzed by two kinds of methods—Kissinger and Flynn–Wall–Ozawa. The results obtained from those methods under dynamic measurement agreed with those obtained from the modified Avrami equation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 378–386, 2003     

Nonisothermal crystallization behavior and crystalline structure of poly(3‐hydroxybutyrate)/layered double hydroxide nanocomposites

X‐ray diffraction method and differential scanning calorimetry analysis have been used to investigate the nonisothermal crystallization of poly(3‐hydroxybutyrate) (PHB)/poly(ethylene glycol) phosphonates (PEOPAs)‐modified layered double hydroxide (PMLDH) nanocomposites. Effects of cooling rates and PMLDH contents on the nonisothermal crystallization behavior of PHB were explored. These results show that the addition of 2 wt % PMLDH into PHB caused heterogeneous nucleation increasing the crystallization rate and reducing the activation energy. By adding PMLDH into the PHB probably hinder the transport ability of the molecule chains and result in a decreasing crystallity of PHB, thus increasing the activation energy. The correlation among melting behavior, apparent crystallite size, and paracrystalline distortion of PHB/PMLDH nanocomposites has been also discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 995–1002, 2007     

Thermodynamic and kinetic thermal analyses on dual crystal forms in polymorphic poly(heptamethylene terephthalate)

Thermal analyses were performed for determining the equilibrium melting temperatures T of the respective α‐ and β‐crystal in melt‐crystallized polymorphic poly(heptamethylene terephthalate) (PHepT) using both linear and nonlinear Hoffman‐Weeks (H‐W) methods for comparison of validity. These two crystals in PHepT do not differ much in their melting temperatures. The equilibrium melting temperatures of the α‐ and β‐crystal as determined by the linear H‐W method are 98 °C and 100.1 °C, respectively; but the nonlinear H‐W method yielded higher values for both crystals. The equilibrium melting temperatures of the α‐ and β‐crystal according to the nonlinear H‐W method are 121 °C and 122.5 °C, respectively. Both methods consistently indicate that T of the β‐crystal is only slightly higher than that of the α‐crystal. Such small difference in T between the α‐ and the β‐crystal causes difficulties in judging the relative thermodynamic stability of these two crystals. Thus, kinetics of these two crystals was compared using the Avrami and Ozawa theory. The crystallization produced by quenching from T_max = 110 °C and 150 °C shows a heterogeneous and homogeneous nucleation mechanism, respectively. The lower T_max = 110 °C leads to heterogeneous nucleation and only α‐crystal in PHepT, whose crystallization rates at same T_c are much higher than crystallization rates by quenching from T_max = 150 °C leading to either α‐ or β‐crystal with homogeneous nucleation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1839–1851, 2009