Morphological studies on polybutylene terephthalate

The morphology of poly(butylene terephthalate) (PBT) crystallized from the melt at various temperatures was studied by small-angle light scattering, polarizing microscopy, and wide-angle x-ray diffraction. Spherulites with a maltese cross at 45° to the polars formed at lower temperatures while spherulites having an apparently higher melting point with a maltese cross along the polars (0°–90°) formed at higher temperatures. The spherulite size and crystallinity increased with increasing crystallization temperature. The H_v scattering patterns arising from the spherulites formed at lower temperature showed intensity maxima at azimuthal angles of 0° and 90°, while those obtained at higher temperatures showed the more common 45° intensity maxima. Microtomed samples from molded PBT bars showed spherulites with a 45° maltese cross which changed to a 0°–90° maltese cross upon heating just prior to melting. The skin-core effect due to varying thermal histories in these molded bars was clearly observed. Solvent crystallized films contained positive 0°–90° spherulites. Some changes occurring upon uniaxial stretching of PBT films are also discussed.     

Crystallization and thermoreversible gelation of poly(butylene terephthalate)–epoxy mixtures

The thermoreversible gelation of solutions of poly(butylene terephthalate) (PBT) and a liquid diglycidyl ether of bisphenol-A epoxy has been investigated. The morphology of the gels and the conditions under which they form have been characterized by optical microscopy, thermal analysis, and x-ray scattering. Gels were found to form under two different conditions and with different morphologies. Gels formed after a considerable delay when homogenous PBT-epoxy solutions were cooled to slightly below the dissolution temperature of crystalline PBT. These gels contained large, irregular PBT spherulites and smaller birefringent interspherulitic matter. The melting of these gels and the onset of macroscopic flow coincided with the melting of the interspherulitic matter, and occurred before the melting of the large spherulites. Thermoreversible gels formed very quickly when PBT-epoxy solutions were self-nucleated by heating a dispersion of crystalline PBT in epoxy slightly and briefly above the dissolution temperature and then cooling. These gels displayed only a weak background birefringence and were molten when the weak birefringence disappeared. In both cases, gelation occurred by the formation of a three-dimensional PBT network in the epoxy liquid, and the nodes of the network were crystalline PBT particles. $ 1994 John Wiley & Sons, Inc.     

Calorimetric study of blends of poly(butylene terephthalate) and liquid crystalline copolyester

A calorimetric study of blends of poly(ethylene terephthalate-co-p-oxybenzoate), PET/PHB, with poly(butylene terephthalate), PBT has been carried out in the form of as-spun and drawn fibres. DSC melting and crystallization results show that PBT is compatible with LCP and the crystallization of PBT decreases by the addition of LCP in the matrix. The crystallization behaviour of blend fibres is investigated as a function of temperature of crystallization. A detailed analysis of the crystallization course has been made utilizing the Avrami expression. The isothermal calorimetric measurements provide evidence of decrease of rate of crystallization of PBT on addition of the liquid crystalline component up to about 50% by weight. The values of the Avrami exponents change in the temperature range from 200° to 215°C. Dimensionality changes in crystallization could be due to LCP mesophase-transition.     

La cristallisation du polydioxolanne

Crystallization kinetics have been studied for two samples of polydioxolan (molecular weights 10.000 and 30.000). The crystallization was followed at temperatures between 0 and 21°C in a DSC calorimeter. The results obey Avrami's equation. The Avrami exponent was found to be two indicating a spontaneous and probably heterogeneous mode of nucleation. Over the temperature range studied, the crystallization lead to the growth of two-dimensional spherulites.The morphology of polydioxolan samples was studied by optical microscopy and small-angle light scattering. Samples crystallized in liquid nitrogen are made of small spherulites of the order of 5 μm. Samples crystallized between 0 and 21°C are made of large spherulites, of the order of 1 mm. Samples crystallized at 25 and 35°C show large and “abnormal” spherulites, made of two optical phases corresponding to the centre and the perimeter.No difference was seen between the morphologies of the two samples studied. In both cases, the time of half-crystallization was the same when plotted as a function of the degree of supercooling. Equilibrium melting points of 79 and 85°C were found for the low and high molecular weight samples, respectively.     

Influence of microcrystalline cellulose fiber (MCCF) on the morphology of poly(3-hydroxybutyrate) (PHB)

Biopolymer composites were prepared from poly(3-hydroxybutyrate) (PHB)/microcrystalline cellulose fiber (MCCF)/plastiziers/poly(vinyl acetate) by melt extrusion. The morphology, crystal structure, and non-isothermal crystallization of these composites were investigated by polarized optical microscopy (POM), differential scanning calorimetry, Fourier transform infrared spectrometer, and wide-angle X-ray diffraction. The results of DSC indicate that the addition of small amount of MCCF improved the crystallization rate. Non-isothermal crystallization shows that the composites 1 and 2 have lower crystallization half time (t _{0 .5}) than that of pure PHB. Higher MCCF contents in PHB (composite 4) lead to a decrease in the crystallization rate. POM micrographs show that the MCCF were well dispersed in the PHB matrix and served as a nucleating agent with a strong change in PHB morphology. Increasing the isothermal crystallization temperature above 120 °C, leads to the formation of banded spherulites with large regular band spacing. Decreasing the isothermal crystallization temperature below 100 °C produces more and small spherulites.     

Study of crystallization kinetics of trans-1,4-polyisoprene

The concentrations and the growth rates of high- and low-melting type spherulites of trans-1,4-polyisoprene were measured in the temperature range 39–49°C. It was shown that above about 40°C., the crystallization rate of trans-1,4-polyisoprene is determined primarily by the radial growth rate of high-melting form (HMF) spherulites, whereas the predominance of the low-melting form (LMF) crystals below 40°C. can be attributed to the high rate of formation of LMF primary nuclei at lower crystallization temperatures. Temperature-independent rate parameters were calculated from optical and dilatometric measurements and were found to be in good agreement. Both the change in nucleation habit and spherulite growth rate with temperature can be explained on the basis of a lower end surface free energy of LMF crystals of trans-1,4-polyisoprene compared to that of the HMF crystals.     

Inter-spherulitic/inner-spherulitic localization of PBSU during crystallization of PVDF in PVDF/PBSU blend

In this work, the miscibility effect on the localization of poly(butylene succinate) (PBSU) during the crystallization of PVDF in their blend has been investigated. After annealed at 200 °C and 240 °C, homogeneous and phase-separated structures can be obtained respectively, which was followed by isothermal crystallization at 141 °C. In the case of 200°C, PBSU tends to enrich in inner-spherulitic regions because of the excellent miscibility of the blend and the higher growth rate of PVDF crystals. When the specimen was annealed at 240 °C, phase separation produces PVDF and PBSU domains. Upon cooling to 141 °C, one part of PBSU is miscible with , while the other part of it remains as phase-separated domains due to the high viscosity and slow relaxation of them. The former accounts for the distribution of PBSU in inner-spherulitic regions. In the latter, however, phase-separated structures depress the diffusion of PVDF during its crystallization, leading to the lower magnitude of growth rate of spherulites. Both of them contribute to the localization of PBSU in inter-spherulitic regions. The distribution of PBSU among PVDF spherulites has been validated by long periods, pore size, and mechanical performance of the porous PVDF membranes.     

Influence of graphene nanosheets on stereocomplex crystallization behaviors of star‐shaped poly (D(L)‐lactide) stereoblock copolymer

The nanocompsites of star‐shaped poly(D‐lactide)‐co‐poly(L‐lactide) stereoblock copolymers (s‐PDLA‐PLLA) with two‐dimensional graphene nanosheets (GNSs) were prepared by solution mixing method. Crystallization behaviors were investigated using differential scanning calorimetry, polarized optical microscopy, and wide angle X‐ray diffraction. The results of isothermal crystallization behaviors of the nanocompsites clearly indicated that the GNS could remarkably accelerate the overall crystallization rate of s‐PDLA‐PLLA copolymer. Unique stereocomplex crystallites with melting temperature about 207.0°C formed in isothermal crystallization for all samples. The crystallization temperatures of s‐PDLA‐PLLAs shifted to higher temperatures, and the crystallization peak shapes became sharper with increasing GNS contents. The maximum crystallization temperature of the sample with 3 wt% GNS was about 128.2°C, ie, 15°C higher than pure s‐PDLA‐PLLA. At isothermal crystallization processes, the halftime of crystallization (t_0.5) of the sample with 3 wt% GNS decreased to 6.4 minutes from 12.9 minutes of pure s‐PDLA‐PLLA at 160°C.The Avrami exponent n values for the nanocomposites samples were 2.6 to 3.0 indicating the crystallization mechanism with three‐dimensional heterogeneous nucleation and spherulites growth. The morphology and average diameter of spherulites of s‐PDLA‐PLLA with various GNS contents were observed in isothermal crystallization processes by polarized optical microscopy. Spherulite growth rates of samples were evaluated by using combined isothermal and nonisothermal procedures and analyzed by the secondary nucleation theory. The results evidenced that the GNS has acceleration effects on the crystallization of s‐PDLA‐PLLA with good nucleation ability in the s‐PDLA‐PLLA material.     

A modulated dsc study on the in situ polymerization of cyclic butylene terephthalate oligomers

The polymerization of a cyclic butylene terephthalate (CBT) oligomer was studied as a function of temperature (T=200 and 260°C, respectively) by modulated DSC (MDSC). The first heating was followed by cooling after various holding times (5, 15 and 30 min) prior to the second heating which ended always at T=260°C. This allowed us to study the crystallization and melting behavior of the resulting polybutylene terephthalate (PBT), as well. In contrary to the usual belief, the CBT polymerization is exothermic and the related process is superimposed to that of the CBT melting. The melting behavior of the PBT was affected by the polymerization mode (performed below or above the melting temperature of the PBT product) of the CBT. Annealing above the melting temperature of PBT yielded a product featuring double melting. This was attributed to the presence of crystallites with different degrees of perfection. The crystals perfection which occurred via recrystallization/remelting was manifested by a pronounced exothermic peak in the non-reversing trace.     

Thermally induced phase separation of poly(vinylidene fluoride)/diluent systems: Optical microscope and infrared spectroscopy studies

Thermally induced phase separation (TIPS) has been developed to prepare porous membranes. The porous structures are mainly dependent on diluents adopted in the TIPS process. We obtained two typical morphologies of poly(vinylidene fluoride) (PVDF) membranes using cyclohexanone (CO) and propylene carbonate (PC) as diluents, respectively. SEM observation displays that porous spherulites are formed from PVDF/CO system, whereas smooth particles result from PVDF/PC system. The TIPS processes of these two systems have been investigated in detail by optical microscope observation and temperature‐dependent FTIR combined with two‐dimensional infrared correlation analysis. Rapid crystallization of PVDF can be seen around 110 °C in the PVDF/CO system, which is consistent with the results of temperature‐dependent FTIR spectra. The spectral evolution indicates a transform of PVDF from amorphous to α‐phase after 110 °C. The ν_s(C?O) band at 1712 cm^?1 narrows and the ν_s(C? F) band at 1188 cm^?1 shifts to 1192 cm^?1 before crystallization, which implies the destruction of interaction between PVDF and CO. In contrast, the PVDF/PC system shows slow crystallization with all‐trans conformation assigned to β‐phase and γ‐phase below 60 °C but no obvious change of polymer?diluent interaction. We propose two mechanisms for the different phase behaviors of PVDF/CO and PVDF/PC systems: a solid?liquid phase separation after destruction of polymer?diluent interaction in the former, and a liquid?liquid phase separation process coupled with rich‐phase crystallization in the later. This work may provide new insight into the relationship among morphologies, crystal forms, and phase separation processes, which will be helpful to adjust membrane structure. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1438–1447     

Phase behavior and morphology in epoxy resin/poly(L-lactide) blends. Comparison with epoxy resin/poly(L,D-lactide) blends

Thermoset/thermoplastic blends were prepared with epoxy–aromatic diamine mixtures and poly(L-lactide) (PLLA), as semicrystalline thermoplastic, in concentrations ranging from 4 to 25 wt.%. In some cases, poly(L,D-lactide) (PDLLA), an amorphous thermoplastic, was used instead for comparative purposes. Diglycidyl ether of bisphenol-A (DGEBA) was employed as epoxy resin and 4,4′-diaminodiphenylmethane (DDM) as curing agent. Phase behavior and morphology were studied during curing at 140 °C. Initially, all blends were homogeneous; however, the curing reaction of the epoxy resin caused a liquid–liquid phase separation. A co-continuous morphology was formed at the beginning of the phase separation in all the considered blend compositions. Blends evolved to a particle/matrix structure or to a phase-inverted structure depending on the initial blend composition. At 140 °C, crystallization only occurred in blends with 16 and 25 wt.% PLLA. This crystallization originates changes in the surface of the epoxy-rich droplets developed with the phase separation.     

ABA triblock copolymer containing two crystallizable components

A triblock copolymer of the ABA type in which both components were crystallizable was synthesized. The A block was poly(ethylene oxide), PEO, and the B block, poly(dimethyl siloxane), PDMS. Upon cooling from the melt to liquid nitrogen temperature, the PEO block crystallized at around 40°C. When the copolymer was heated from ?170°C after quenching, glass transition, crystallization and melting of the PDMS middle block were identified in the thermogram at ?117°C, ?74°C and ?42°C, respectively. The degree of crystallinity of the PDMS block was estimated from the heat of fusion to be about 27%. The growth rates of the PEO spherulites were reduced by the presence of the middle block.     

Calorimetric study of blends of poly(butylene terephthalate) and a liquid crystalline polyester

The thermal properties of solution-prepared blends of poly(butylene terephthalate) (PBT) and liquid crystalline poly(biphenyl-4,4′-ylene sebacate) (PB8) have been studied by differential scanning calorimetry (DSC). The smectic-to-isotropic (s → i) transition of the mesomorphic component is observed at temperatures slightly increasing with the PB8 content, in the 270–280°C range; on cooling, the mesophase formation (i → s transition) takes place at temperatures that decrease markedly with decreasing PB8 content. The temperatures of the crystal-to-smectic and smectic-to-crystal transitions of PB8 are almost invariant with blend composition. The melting and crystallization temperatures of the PBT phase decrease on increasing the content of the liquid crystalline component. These results, together with those of isothermal calorimetry of both the crystallization of the PBT component and the mesophase formation of the PB8 component of the blends, indicate that the two polymers are not immiscible in the isotropic state. In this latter state, however, the two polyesters undergo transesterification, which can be followed through changes in the DSC scans. The effect of the thermal history on the properties of the blends has been studied with particular attention. Both the dynamic and the isothermal calorimetric measurements provide evidence of an increase of the degree of crystallinity of PBT on addition of the liquid crystalline component up to about 35–50% by weight.     

Synthesis,morphology, and nonisothermal crystallization behavior of poly(trimethylene terephthalate)/poly(propylene glycol) segmented random copolymers

Poly(trimethylene terephthalate)/poly(propylene glycol) (PTT/PPG) segmented random copolymers were synthesized by melt copolycondensation. The weight fraction of PPG blocks was ranged from 12.1 to 33.4 wt%, which was confirmed by ¹H NMR spectroscopy. The result of wide‐angle X‐ray diffractometer indicated that all copolymers had the same crystal structure of PTT homopolymer at room temperature. At a determined crystallization temperature, ring‐banded spherulites could be observed in all copolymers samples, and the band spacing increased with the increase of PPG content. Morphologies of copolymers after nonisothermal crystallization process were strongly depended on the cooling rate. Well‐defined ring‐banded spherulites can be observed only at moderate cooling (20°C/min), while it was really hard to be observed at too low (2.5°C/min) or too high (by air‐quenching) cooling rate. Moreover, the size of spherulites decreased with the increase of cooling rate. Finally, different nonisothermal crystallization kinetics were adopt to analyze this copolymer system, and only the Mo method was suitable to describe this copolymer system. Copyright © 2016 John Wiley & Sons, Ltd.     

Free poly(l‐lactic acid) spherulites grown from thermally induced phase separation and crystallization kinetics

Free poly(L‐lactic acid) (PLLA) sheaves and spherulites were prepared by thermally induced phase separation method from its tetrahydrofuran solution without the assistance of other additives. The effects of variables such as polymer concentration, quenching temperature and time on the morphology of PLLA spherulites were studied. The morphology, size, degree of crystallinity, and crystal structure of spherulites were characterized by SEM, DSC and XRD, and so forth. No obvious sheaves or spherulites were observed at quenching temperature of 8 and 0 °C, whereas sheaves composed of fluffy nanofibers with diameter of about 250 nm were formed at quenching temperature range of ?10 to ?40 °C. With increasing quenching time, the PLLA morphology changed from small sheaves to big sheaves (cauliflower‐like) to spherulites. Low concentration (3 and 5 wt %) solutions were favorable for the formation of sheaves, whereas high concentration (7 wt %) solution as good for the formation of spherulites. The mechanism for the formation of PLLA sheaves or spherulites was examined by the isothermal and nonisothermal crystallization of PLLA/tetrahydrofuran solutions using DSC. The Avrami equation was used to analyze the data and good linear double‐logarithmic plots were obtained. The Avrami exponent n and rate constant K indicated the crystal growth mechanism was intermediate between completely instantaneous and completely sporadic types of nucleation and growth, and the spherulites were there dimensional. Compared to the spherulites embedded in the bulky film obtained from the melt processing, this study provided a feasible technique for the fabrication of free PLLA spherulites. The PLLA spherulites composed of fluffy nanofibers with a high porosity (≥90%) may be potentially applied as functional materials such as catalyst support, adsorption and biomedical materials, and so forth. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1476–1489     

Effects of miscible diluent of poly(ether imide) on ring-banded morphology of poly(trimethylene terephthalate)

The melting, crystallization, and self-packed ring patterns in the spherulites of miscible blends comprising poly(trimethylene terephthalate) (PTT) and poly(ether imide) (PEI) were revealed by optical, scanning electron microscopies (PLM and SEM) and differential scanning calorimetry (DSC). Morphology and melting behavior of the miscible PTT/PEI blends were compared with the neat PTT. Ringed spherulites appeared in the miscible PTT/PEI blends at all crystallization temperatures up to 220 °C, whereas at this high temperature no rings were seen in the neat PTT. A postulation was proposed, and interrelations between rings in spherulites and the multiple lamellae distributions were investigated. The specific interactions and the segregation of amorphous PEI were discussed for interpreting the morphological changes of 220 °C-melt-crystallized PTT/PEI samples. Interlamellar segregation of PEI might be associated with multiple lamellae in the spherulites of PTT/PEI blends; therefore, rings were more easily formed in the PTT/PEI blends at all crystallization temperatures. A postulated model of uneven lamellar growth, coupled with periodical spiraling, more properly describes the possible origin of ring bands from combined effects of both interactions and segregation between the amorphous PEI and PTT in blends.     

Rapid crystallization and thermoreversible gelation of poly(ethylene terephthalate) in polymer/oligomer binary system

Poly(ethylene terephthalate) (PET) was rapidly crystallized through thermoreversible gelation in a liquid ethylene glycol oligomer or in epoxy resin. The solutions formed gel rapidly on cooling. Polarized light microscopy and small-angle light scattering showed that these gels contain large, regular PET spherulites. The gels may be formed by two consecutive processes: the phase separation and crystallization, and gelation by formation of a three-dimensional PET network in the oligomer solvents, where the nodes of the network are PET spherulites. The crystallinity of PET recovered from polymer/oligomer gels is near 72% measured by wide-angle X-ray diffraction method, which is about 20% higher than PET samples crystallized by solution crystallization in small molecule solvent, high temperature annealing, and stretching techniques. It takes only a few minutes to form the highly crystalline phase PET in the PET/oligomer system, and the crystallinity of the dried gel is independent of the concentration of the original solution. Excimer-fluoresence and Raman spectroscopic studies indicated that PET recovered from the gels are in an ordered state with few chain entanglements. The entanglement density of the recovered PET recovered from a 20 wt % solution in ethylene glycol oligomer is as low as that of freeze-extracted PET from a 0.5 wt % solution in phenol. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1219–1225, 1998     

Nonisothermal bulk crystallization studies of high density polyethylene using light depolarizing microscopy

The quiescent nonisothermal bulk crystallization kinetics of two high-density polyethylene resins were investigated by a modified light-depolarizing microscopy (LDM) technique. The technique allows studies at average cooling rates up to 2500°C/min. The polymer was found to crystallize at a pseudo-isothermal temperature even at these very high cooling rates. The overall bulk crystallization rate increased rapidly as the cooling rate and supercooling increased. Crystallization kinetics was analyzed by Avrami analysis. Avrami exponents near 3 suggested spherical growth geometry and instantaneous nucleation at predetermined sites. Observation of spherulites by optical microscopy together with a number density of spherulites that changed little with increase in cooling rate or supercooling supported this model of crystallization behavior. Analysis of the half-time of crystallization based on the Lauritzen and Hoffman secondary nucleation theory indicated that the regime II-III transition was found to occur at a degree of supercooling of approximately 22°C. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 681–692, 1998     

High melting thermoplastic/epoxy resin blends: Influence of the curing reaction on the crystallization and melting behavior

The influence of the cure process and the resulting reaction‐induced phase separation (RIPS) on the crystallization and melting behavior of polyoxymethylene (POM) in epoxy resin diglycidylether of bisphenol A (DGEBA) blends has been studied at different cure temperatures (180 and 145 °C). The crystallization and melting behavior of POM was studied with DSC and the simultaneous blend morphology changes were studied using OM. At first, the influence of the epoxy monomer on the dynamically crystallized POM was investigated. Secondly, a cure temperature above the melting point of POM (T_cure = 180 °C) was applied for blends with curing agent to study the influence of resulting phase morphology types on the crystallization behavior of POM in the epoxy blends. Large differences between particle/matrix and phase‐inverted structures have been observed. Thirdly, the cure temperature was lowered below the melting temperature of POM, inducing isothermal crystallization prior to RIPS. As a consequence, a distinction was made between dynamically and isothermally crystallized POM. Concerning the dynamically crystallized material, a clear difference could be made between the material crystallized in the homogeneous sample and that crystallized in the phase‐separated structures. The isothermally crystallized POM was to a large extent influenced by the conversion degree of the epoxy resin. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2456–2469, 2007     

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