Polymorphism and Mixing Phase Behavior in Ternary Mixture Systems of SOS–SSO–OSO: Formation of Molecular Compound Crystals

This paper reports the phase behavior of ternary mixtures of saturated and cis‐monounsaturated mixed‐acid triacylglycerols (TAG) of SOS (1,3‐distearoyl‐2‐oleoyl‐glycerol), SSO (1,2‐distearoyl‐3‐oleoyl‐rac‐glycerol), and OSO (1,3‐dioleoyl‐2‐stearoyl‐glycerol) examined with X‐ray diffractometry and a differential scanning calorimeter. The ternary mixtures were crystallized by cooling from melt (60 °C) to 5 °C, and the crystals were then stabilized by storing the mixture samples at 28 °C for 10 days. The following results were obtained. (1) Molecular compound (MC) crystals of stable β polymorph having a double‐chain‐length structure (β‐2) were formed in mixtures of SOS/SSO/OSO in which the concentration of SOS was 50% with varying concentrations of SSO/OSO. This is in contrast to the fact that the stable polymorphic forms of the component TAG are β‐3 for SOS and OSO and β′‐3 for SSO. (2) When the concentration of SOS deviated from 50%, immiscible mixtures of β‐2 MC made of SOS/SSO/OSO and the component TAG (β‐3 of SOS and OSO and β′‐3 of SSO) were formed. Therefore, ternary mixtures of SOS/(SSO + OSO) = 50/50 with different concentrations of SSO and OSO are miscible mixtures of β‐2 of SOS/SSO and SOS/OSO.     

Multi-Length-Scale Elucidation of Kinetic and Symmetry Effects on the Behavior of Stearic and Oleic TAG. II: OSO and SOO

The phase composition of triacylglycerols (TAG) is determined by chemical structure and is greatly affected by kinetics. Positional isomerism and unsaturation are two key structural elements that govern, for a large part, the phase behavior of TAG during crystallization. Polymorphism, thermal properties, and microstructure of dioleoyl‐stearoyl isomers (OSO, SOO) were investigated at different cooling rates with XRD, DSC and PLM techniques, respectively. The physical properties of the symmetrical TAG were found to predictably vary with cooling rate; whereas, the properties of the asymmetrical TAG remained relatively constant. This was explained to be mainly due to the extra steric hindrance caused by asymmetry and the disturbances introduced at the “terrace” level via methyl‐end group interactions. The findings underscore the intricate contribution of saturation and symmetry to the phase trajectories of diunsaturated TAG. The knowledge gained will help understand the behavior of more complex materials and can be used for the manipulation of fat structures at different length scales and direct the manufacture of novel food systems and other relevant materials.     

Crystallization of model fat blends containing symmetric and asymmetric monounsaturated triacylglycerols

In this study, the crystallization and melting properties of four different fat blends with the same saturated fat content (30%) but with different ratios of symmetric and asymmetric monounsaturated triacylglycerols were investigated using pNMR, DSC and polarized light microscopy. Blends were either palmitic (P) or stearic (S) based, and were combinations of SatOSat‐rich (Sat = saturated, O = oleic) and SatSatO‐rich vegetable oils with high‐oleic sunflower oil. The DSC results demonstrate that there was almost no difference in crystallization mechanism and crystallization rate between the two P‐based blends. Both blends showed a two‐step crystallization, which can be explained by polymorphism. Stop‐and‐return DSC results suggested an initial crystallization into an unstable polymorph followed by polymorphic transition during the crystallization. For the S‐based blends there was a clear difference between the SOS‐rich and the SSO‐rich blend, with a slower crystallization for the SSO‐rich blend. Possibly, this can be explained by fractional crystallization. The microstructure did not differ greatly between the blends. Directly after crystallization, the crystals of the SSO‐rich blend were slightly larger than the crystals of the SOS‐rich blend.     

Elucidation of Kinetic and Symmetry Effects on the Viscosity and Flow Behavior of Stearic and Oleic Triacylglycerols

The flow behavior and viscosity of TAGs containing oleic and stearic acids were examined in the liquid phase as well as at temperatures close to the onset of crystallization by means of a temperature‐controlled rheometer. Gelling and crystallization transitions were unambiguously identified by singularities in the viscosity versus temperature curves of the TAGs. An additional transition between the gelling and onset of crystallization temperatures, attributed to the reorganization of the gel, was observed in the symmetrical TAGs only. The effect of the cooling rate, ranging between 0.1 and 5 °C/min, was investigated. The flow behavior of the studied TAGs was shown to be strongly determined by structural parameters (symmetry, degree of unsaturation). The flow behavior was also affected significantly by the cooling rate, suggesting that to certain extents the flow behavior determined by structural parameters could be changed by manipulating the cooling rate. The study confirmed that TAG crystallization is initiated by and depends, at least in part, on complex molecular short range order transformations occurring in the melt. The findings of the study may be valuable for evaluating systems and equipment that are involved in the storage, handling and processing of materials incorporating these TAGs.     

Rheometry and polymorphism of cocoa butter during crystallization under static and stirring conditions

In this work the association between polymorphism and the crystal network structure developed by the TAG of cocoa butter (CB) was investigated under static and stirring crystallization conditions using a dynamic mechanical spectrometer. The results obtained showed that parameters obtained through oscillatory rheometry (i.e., phase shift angle, δ) followed the polymorphism of CB during static crystallization. Although standard DSC was not capable of differentiating the α to β′ phase transformation from the direct β′ crystallization from CB melt, δ rheograms measured these two processes separately. Additionally, through oscillatory rheometry, we followed the dimensionality of the crystal network during CB crystallization. Within this context, the pre-exponential term (In γ) from the weak-link regime equation for colloidal dispersions was much more sensitive than the fractal dimension (D) to differences in crystal size, spatial distribution of the crystal network, and melting temperature of the β′ phase of CB. On the other hand, torque measurements obtained during CB crystallization under stirring conditions showed a shear rate effect that favored TAG development in the β phase at temperatures of 19, 22, and 26.5°C, particularly at shear rates of 120 and 400 rpm. In contrast, under static conditions CB did not develop in the β phase at any of the crystallization temperatures investigated (i.e., 18 to 26.5°C).     

Synthesis of a Cocoa Butter Equivalent by Enzymatic Interesterification of Illipe Butter and Palm Midfraction

This study aims to synthesize a cocoa butter equivalent (CBE)‐structured lipid from a blend of illipe butter (IB) and palm midfraction (PMF) by means of enzymatic interesterification using Rhizomucor miehei sn‐1,3 specific lipase, Lipozyme^® RM IM (Novozymes North America, Inc., Franklinton, NC, USA) as the biocatalyst. Physical and chemical attributes of the CBE and cocoa butter (CB) were analyzed. The synthesized CBE matched the triacylglycerol (TAG) profile range of a commercial CB and is therefore hypothesized to show similar physical and chemical characteristics to CB. The TAG profile, fatty‐acid constituents, melting and cooling behavior, polymorphism, and crystal morphology were determined using high‐performance liquid chromatography, gas chromatography, differential scanning calorimetry, X‐ray diffraction (XRD), and polarized light microscopy, respectively. Four enzymatically interesterified blends of IB:PMF at different weight ratios were analyzed for their TAG profiles, and a ratio of IB:PMF 10:3 (%, w/w) at 5% enzyme load and a reaction time of 30 min gave similar TAG results to CB. The TAG values of the IB:PMF 10:3 interesterified product (IP) were 1,3‐dipalmitoyl‐2‐oleoylglycerol at 19.1 ± 1.0%, 1‐palmitoyl‐2‐oleoyl‐3‐stearoylglycerol at 42.7 ± 1.0%, and 1,3‐distearoyl‐2‐oleoylglycerol at 29.9 ± 0.3%. The melting and the cooling profile of IP and CB showed no significant difference. XRD of IP and CB displayed similar dominant peaks at 4.6 Å, representing a β polymorph. Both CB and IP have similar granular spherulitic crystals.     

The avrami index and the fractal dimension in vegetable oil crystallization

The behavior of the Avrami plot during TAG crystallization was studied by DSC and rheological measurements in oil blends of palm stearin (26 and 80%) in sesame oil, using different crystallization temperatures (T _Cr ^o) attained under several cooling rate conditions (1, 10, and 30°C/min). In the same way, the relationship between the growth mechanisms of TAG, measured by the Avrami index (n), and the mass fractal dimension (D) of the crystal network was investigated. This last parameter was measured as TAG crystallized in the oil blend under isothermal conditions. Results showed that TAG crystallization in a vegetable oil involves the process of TAG lamellar development, nucleation, and crystal growth. Each event occurred at a different rate and extent as affected by cooling rate and T _Cr ^o, and as a function of crystallization time under isothemal conditions at a given cooling rate. Within this framework, we proposed that n calculated from the second region of the Avrami plot is a parameter mainly associated with crystal growth, whereas n from the first region is associated more with nucleation. On the other hand, changes in D values followed the different polymorphic states developed by TAG as a function of T _Cr ^o. Additionally, it was shown that, independent of the concentration of palm stearin in the oil blend, at cooling rates of 1 and 10°C/min the increase in n from ∼3 to ∼4 produced a curvilinear increase in D from ∼1.75 to ∼3.0. The growth mechanism of the TAG crystals (i.e., n), also affected the magnitude of D. However different behavior was observed in the n-D relationship when n<2.7 and at 30°C/min.     

Phase behavior of a binary lipid shortening system: From molecules to rheology

The phase behavior of fully hydrogenated canola oil in soybean oil was investigated using iso-solid lines from temperature-controlled pulse-NMR along with DSC data, with the rate of cooling of crystallized samples kept constant. The molecular diversity within the fat system was investigated using HPLC and GC. The microstructure of the fats was determined using a temperature-controlled polarized light microscope, and the polymorphism of the solidified fat structures was determined via a temperature-controlled X-ray diffractometer. Hardness was measured by a temperature-controlled Instron mechanical analyzer with a penetration cone. The phase behavior predicted by the DSC and iso-solid lines did not account for the hardness trends observed, as the microstructure and polymorphism of the fat also played a significant role. The addition of hard fat to a system did not consistently increase the hardness of the fat system. Furthermore, the solution behavior demonstrated by the iso-solid line diagram did not account for all trends in melting behavior, as both intersolubility and polymorphic changes occurred simultaneously. it was found that variations in hardness can be inferred from structural changes, although the structural level causing variation differs.     

Effect of CO2 Bubbles on Crystallization Behavior of Anhydrous Milk Fat

A study was designed to observe the effect of bubbles created from dissolved CO₂ (0–2000 ppm) on crystallization and melting behavior, fat polymorphs, microstructure, and hardness of anhydrous milk fat (AMF) under nonisothermal crystallization conditions. Calculated amounts of dry ice were added to generate 2000 ppm CO₂ at low partial pressure, and an ultrasound (205 kHz, 10 s; US) treatment was delivered at 35 °C through a noncontact metal transducer on the molten AMF to generate bubbles (~500 nm) of CO₂. The generated CO₂ bubbles were found to induce a higher onset of crystallization temperature during cooling from 35 to 5°C at the rate of 0.5°C min⁻¹. The changes in crystallization behavior owing to the generation of a smaller and significant number of TAG crystals also increased the hardness of the AMF at room temperature and refrigerated conditions. The work suggested the potential use of CO₂ nanobubbles derived from the dry ice with the emission of low power US to control the crystallization behavior and thereby the physical properties of milk fat-containing dairy products.     

Effect of process parameters on phase behavior and particle size of aspirin during freeze concentration

Abstract

The present work investigated effect of process parameters on phase behavior of aspirin (ASP) solution and ASP particle size during freeze concentration. ASP solution (3% w/w in 40% w/w TBA in water) was studied using differential scanning calorimetry (DSC) and cold stage polarized light microscopy (CSPLM) at cooling rates of 0.1, 0.5, 1, 3, 5, 10, 20, and 30?°C/min. Significant reduction in D₉₀ of ASP crystals (from 45.6?µm to 3.0?µm) was observed for ASP crystals in frozen solution with increase in cooling rate. Non-isothermal crystallization kinetics (NITC) of ASP in frozen solution using CSPLM revealed linear increase and an exponential increase in Avrami exponent and crystallization rate constant, respectively. NITC results indicated generation of higher nucleation sites and localized multidimensional crystallization with increase in cooling rate. Moreover, the high degree of supercooling and higher supersaturation achieved with increasing the cooling rate further supported the generation of more nuclei and smaller ASP crystals. Finally, the implication of phase behavior was established using lab scale freeze dryer, where particle size (D₉₀) of 41.8, 14.8, and 7.1?µm was obtained with cooling rate of 1?°C/min, 15?°C/min and quench cooling, respectively. The proposed strategy can be applied for development of lyophilized product that can generate parenteral suspension upon reconstitution.     

Effect of processing conditions on crystallization kinetics of a milk fat model system

Crystallization behavior of three blends of 30, 40, and 50% of high-melting fraction (MDP=47.5°C) in low-melting fraction (MDP=16.5°C) of milk fat was studied under dynamic conditions in laboratory scale. The effect of cooling and agitation rates, crystallization temperature, and chemical composition of the blends on the morphology, crystal size distribution, crystal thermal behavior, polymorphism, and crystalline chemical composition was investigated by light microscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and gas chromatography (GC). Different nucleation and growth behavior were found for different cooling rates. At slow cooling rate, larger crystals were formed, whereas at fast cooling rate, smaller crystals appeared together. Slowly crystallized samples had a broader distribution of crystal size. Crystallization temperatures had a similar effect as cooling rate. At higher crystallization temperatures, larger crystals and a broader crystal size distribution were found. Agitation rate had a marked effect on crystal size. Higher agitation rates lead to smaller crystal size. Cooling rate was the most influential parameter in crystal thermal behavior and composition. Slowly crystallized samples showed a broader melting diagram and an enrichment of long-chain triacylglycerols. Crystallization behavior was more related to processing conditions than to chemical composition of blends.     

The effect of heat treatment and cooling rate on the properties of lightweight aggregates

Lightweight aggregates (LWA) were produced from clay in the laboratory. After firing different heat treatments and cooling rates were applied and the resulting material was investigated with respect to strength and microstructure. Fast cooling led to the formation of micro cracks and weakened the material whereas slow cooling enhanced the strength of LWA. The residence time at temperatures between 700 °C and 900 °C led to differences in average oxidation state of iron in the matrix phase leading to substantial changes in thermal behaviour of the matrix phase. The combination of a highly oxidized shell and a reduced core proved to enhance the strength of LWA. A two hour heat treatment at 800 °C in air combined with a subsequent slow cooling rate (0.7 °C/min) applied to LWA produced in an industrial rotary kiln led to a strength increase of 114% compared to material of the normal production without changing any other property.     

Effect of processing conditions on the yield and failure response of an aliphatic polyketone terpolymer

The effect of processing conditions on the yield and failure behavior of an aliphatic polyketone terpolymer was studied. Testing and characterization were performed on samples that were extruded in the form of hollow cylinders. We performed the extrusion process at different shear rates and at different cooling rates to assess the effect that process conditions had on the polymer properties. We performed biaxial testing on the samples to characterize the failure envelopes, including the ductile–brittle transition condition for each process condition. The effect of shear rate was negligible, whereas the cooling rate significantly affected the failure behavior. To explain these differences in behavior, we performed characterization via differential scanning calorimetry, wide‐angle X‐ray diffraction, attenuated total reflection IR spectroscopy, dynamic mechanical thermal analysis, scanning electron microscopy, and residual stress measurements. A broad glass transition was found for all samples at temperatures higher than previously reported for this material. Alteration of the processing conditions did not influence the crystalline phase (percentage crystallinity, crystalline orientation, crystallite size, etc.). A change in spherulitic structure was also observed with altered cooling rate and is suggested to have contributed to the change in failure behavior. Residual stresses also affected the behavior of all samples. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 318–334, 2002; DOI 10.1002/app.10334     

Determining the relative amounts of positional isomers in complex mixtures of triglycerides using reversed-phase high-performance liquid chromatography-tandem mass spectrometry

A reversed-phase HPLC-tandem mass spectrometry (RP-HPLC-MS-MS) method was refined for the positional analysis of complex mixtures of TAG. This method has the advantages of speed, ease of automation, and specificity over traditional digestion-based methods for the positional analysis of TAG. Collision-induced dissociation (CID) of ammoniated TAG in an ion-trap mass spectrometer produced spectra that were dependent on the FA position. Dominant DAG fragments were formed from the loss of a FA moiety from the ammoniated TAG species. The loss of FA in the outer positions was favored over their loss in the central position. The combination of RP-HPLC and CID produced spectra that were free of the isotope effects that can complicate spectral interpretation in existing methods. The combination also provided selectivity based on the chromatographic fractionation of TAG, in addition to the selectivity inherent in the CID process. Proof-of-concept experiments were performed with binary mixtures of TAG from the SOS/SSO, OSO/OOS, and the PSO/POS/SPO positional isomer systems (where S is 18∶0, stearic acid; O is 18∶1 (cis-9), oleic acid; and P is 16∶0, palmitic acid). Plots of fractional DAG fragment intensities vs. fractional composition of the binary mixtures were linear. These plots were used to determine the fractional composition of each of these isomeric systems in a variety of vegetable oils and animal fats. Current limitations, future developments, and applications of this method are discussed.     

Cholesterol vehicle in experimental atherosclerosis. 23. Effects of specific synthetic triglycerides

Earlier work has shown that increasing concentration of palmitic acid at the sn-2 position of a fat enhances the atherogenic properties of that fat. This effect has been observed with lard, tallow, cottonseed oil, and palm oil. In the experiment reported here, we have studied the atherogenic effects of four synthetic fats fed to rabbits as 58% (w/w) of the total fat (15%) (w/w) of a semipurified diet containing 0.05% cholesterol. The fats being tested were: 1,3-stearoyl-2-oleoylglycerol (SOS); 1,2-stearoyl-3-oleoylglycerol (SSO); 1,3-palmitoyl-2-oleoylglycerol (POP); and 1,2-palmitoyl-3-oleoylglycerol (PPO). After 20 wk on diet there were no differences among the groups in weight gain, liver weight, serum, or liver lipids. These data are consistent with our previous findings. There were significant differences in atherosclerosis. The most severe atherosclerosis was observed in group PPO and the least in groups SSO and POP. Severity of atherosclerosis was graded visually on a 0–4 scale. The average atherosclerosis [(aortic arch and thoracic aorta)÷2] was: SOS-1.35; SSO-0.97; POP-0.83; and PPO-1.80. Fecal fat excretion (an indicator of fat absorption) was higher in the two groups fed the stearic acid-rich fats and lower in groups fed the palmitic acid-rich fats. There were no differences in low density lipoprotein particle size. The results confirm previous findings concerning the increased atherogenicity of fats bearing palmitic acid at the sn-2 position. The mechanism underlying these observations is moot but may, in part, reflect greater absorption of the atherogenic fat.     

Cooling rate and dilution affect the nanostructure and microstructure differently in model fats

The effects of cooling rate and solid mass fraction on the polymorphism, nano and microstructure, thermal and rheological properties of binary mixtures of fully hydrogenated canola oil and canola oil at 20°C have been studied. The β‐polymorph was observed in fully hydrogenated canola oil (FHCO) when crystallized at slow cooling rates (0.1C°/min), however crystallization at higher cooling rates (0.7 and 10°C/min) resulted in the formation of the α form. The β‐polymorph was detected in all the binary mixtures of FHCO/canola oil and was not affected by crystallization at different cooling rates. Melting thermograms obtained from 100% FHCO displayed three melting peaks, associated with the development of the β‐polymorph via α→ β′→ β‐polymorphic transition in the DSC pan. Some solubilization of solid FHCO into canola oil was observed and the solubility was proportionally higher with increasing liquid oil fraction. The strong influence of the matrix concentration on micro/nanoscale structure was demonstrated by characterization of crystal size using cryogenic transmission electron (Cryo‐TEM) and polarized light microscopy (PLM). Crystallization under higher cooling rates lead to formation of smaller nano and meso‐structural elements. Furthermore, oscillatory rheology showed the influence of structural elements' size and polymorphism on material strength. The shear storage modulus (G′) of the mixtures was higher when crystallized at fast cooling rates (10°C/min). In contrast, for pure FHCO, G′ increased by lowering the cooling rate and the highest storage modulus was observed after crystallization at 0.1°C/min.     

Fabrication of transparent Tb3Al5O12 ceramics by hot isostatic pressing sintering

Tb₃Al₅O₁₂ (TAG) transparent ceramics were prepared by a reactive sintering method using presintering in a muffle furnace combined with hot isostatic pressing (HIP) sintering. The dilatometric, differential scanning calorimetry‐thermogravimetric (DSC‐TG) curves and optical quality were investigated. The microstructure evolution of the TAG ceramic samples was clarified. Two successive transformations were found to generate a TAG phase, as observed in the dilatometric and DSC‐TG curves and XRD patterns of TAG ceramics sintered at different temperature. The changes in average grain size and densification suggest that a 1600°C presintering temperature is suitable for HIP. The optical transmittance of the obtained 0.4 wt% TEOS:TAG transparent ceramics, which were fabricated by a new two‐step sintering of presintering at 1600°C in a muffle furnace followed by HIP at 1650°C, can reach above 80% in the visible (vis) and near‐infrared (NIR) regions. Its transmittance was very close to the theoretical limit. To the best of our knowledge, this is the first time that TAG transparent ceramics with ideal optical quality were obtained without vacuum sintering.     

Utilization of Olive-Pomace Oil for Enzymatic Production of Cocoa Butter-like Fat

Refined olive-pomace oil (ROPO) was utilized as a source for the production of a cocoa butter (CB)-like fat. Immobilized sn-1,3 specific lipase-catalyzed acidolysis of ROPO with palmitic (PA) and stearic (SA) acids was performed at various substrate mole ratios (ROPO:PA:SA) to produce major triacylglycerols (TAGs) of CB. Products obtained for various substrate mole ratios were compared to a commercial CB in terms of TAG content, melting profile, solid fat content (SFC) and microstructure. The fat produced at a substrate mole ratio of 1:2:6 was the most similar to CB. The product contained 11% POP, 21.8% POS, 15.7% SOS while commercial CB contained 18.9% POP, 33.1% POS and 24.7% SOS. The product had a melting peak of 29.9 °C while CB had one of 28.5 °C. Polarized light microscope (PLM) images showed that fat crystal network microstructures of this product and CB were very similar.     

Phase transitions and polymorphism of cocoa butter

The polymorphism and phase transitions of cocoa butter (CB) have been reexamined separately by differential scanning calorimetry (DSC) and X-ray diffraction as a function of temperature (XRDT) at scanning rates between 0.1 to 5°C/min and 0.1 to 2°C/min, respectively. A new instrument, which allowed simultaneous DSC and XRDT recordings from the same sample by taking advantage of the high-energy flux of a synchrotron, was employed for characterization of the intermediate phase transitions. These techniques allowed us to confirm the existence of the six polymorphic forms of CB (called I to VI) by in situ characterization of their formation in the DSC + XRDT sample holder. A detailed study of Form I structure led us to propose a liquid-crystal organization in which some of the chains displayed sharp long-spacing lines (d₀₀₁=52.6±0.5 Å) and a β′ organization (4.19 and 3.77 Å), while the others remained unordered with broad scattering (maxima at about 112 and 36.5 Å). The organization of this liquid crystalline phase is compared to that of fat and oil liquids. This liquid crystalline phase progressively transformed on heating into a more stable phase (Form II, α type, d₀₀₁=48.5±0.5 Å and short-spacing at 4.22 Å). Form III was only observed in a sharp temperature domain through its specific short-spacings. The existence of the six species has been essentially related to the crystallization of monounsaturated triacylglycerols (TAG), while trisaturated species were found partly solid-soluble in these six polymorphic forms. An insoluble fraction crystallized independently of the polymorphism of the monounsaturated TAG in a separate phase with long-spacings that were either of the α (49.6±0.5 Å) or β (44.2±0.5 Å) form. In mixture with Form V, this fraction melts and solubilizes in the liquid phase at 37.5°C. Isolation of these high-melting crystals shows a melting point of about 50°C. High-performance liquid chromatography analysis of this fraction confirmed an increase from 3.0 to 11.3% of saturated TAG and their association with part of the 1,3-stearoyl-2-oleoylglycerol preferentially to 1-palmitoyl-2-oleoyl-3-stearoylglycerol and (1,3-palmitoyl-2-oleoylglycerol).     

The effects of annealing (solid and melt) on the time evolution of the polymorphic structure of polyamide 6

To understand whether and how the thermal history, especially the melting annealing, affects the polymorphism and thermal property of polyamide 6 (PA6), the temperature‐modulated differential scanning calorimetry technology was used to investigate the effects of thermal histories, including annealing temperature, annealing time, and cooling rate, on the polymorphic behavior and thermal property of PA6. It was found that longer annealing time and faster cooling rate favored the formation of α crystal when PA6 samples were annealed in the solid‐state at 175°C. As the annealing temperature was elevated to 195°C, faster cooling rate also favored the formation of α crystal, whereas longer annealing time was more favorable for the formation of γ‐form crystals. When PA6 samples were annealed in the melt‐state (245°C), however, although the α crystal was dominating crystalline phase, the formation of γ crystal was greatly enhanced with increasing annealing time and cooling rate. Moreover, a small endothermic peak was observed in the low‐temperature region in PA6 samples annealed at 175°C and 195°C, which might be related to the melting of microcrystals formed in the amorphous regions during annealing. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010