Influence of a synthetic ureido nucleating agent on crystallization behavior and mechanical properties of polyamide 6

A synthetic ureido mixture prepared from the reaction of 4,4′‐diphenylmethane disocynanate (MDI) and cyclohexylamine without using any harmful organic solvents, has been used as a nucleating agent (PNA) for polyamide 6 (PA6). The effect of PNA on the crystallization and mechanical properties of PA6 has been studied by means of differential scanning calorimetry (DSC), polarized optical microscopy (POM), tensile test, melt flow index (MFI), and X‐ray diffraction (XRD). The results show that PNA is an effective nucleation agent for PA6. PNA affects the nucleation mechanism of PA6, and substantially accelerates the crystallization rate of PA6 and gives rise to smaller crystal size. In comparison with PA6, the crystallization temperature (T_c) of PA6/PNA (100/0.5) increases 21.3°C and the degree of sub‐cooling (ΔT_c) decreases 23.7°C. Furthermore, because of the heterogeneous nucleation induced by PNA, the spherulites of PA6 become even and tiny based on POM observation. Polymorph transform has been obtained from XRD analysis. The virgin PA6 is free of γ‐phase crystals, presented as α‐phase crystals in this study, but γ‐phase crystal appears after the introduction of PNA. The mechanical and thermal properties of PA6 are obviously improved by the addition of PNA. POLYM. ENG. SCI., 55:2011–2017, 2015. © 2015 Society of Plastics Engineers     

Effect of Melt‐Memory on the Crystal Polymorphism in Molded Isotactic Polypropylene

The influence of γ‐quinacridone as a β‐crystal nucleating agent in injection molded isotactic polypropylene (iPP) is discussed. Samples are injection molded and characterized via polarized‐light optical microscopy and X‐ray diffraction. Mold‐filling simulation is used to understand the shear and cooling processes during sample preparation. The cooling rate associated with the quench near the mold wall is estimated to be greater than 600 K s^?1 using simulation, confirming previous studies that β‐crystal growth is not supported at that cooling rate. The non‐nucleated samples form β‐crystals at a distance of 100–300 µm from the skin and in the core of the sample, which is not expected based on quiescent cooling data. Since the mold‐filling simulation does not predict shear in the core, the formation of the β‐crystals formed in this region is attributed to shear‐induced crystallization effects in the injection unit of the molding machine that are not modeled in flow simulation, as they are typically excluded from any molding simulation analysis. This “melt‐memory” effect has shown to be significant, and it is suggested that the prediction of final properties of injection moldings requires understanding and knowledge of the entire shear history of the material including that of the injection unit.     

Processing–mechanical property relationships in injection moldings of polybutylene

Two unfilled nonpigmented extrusion grades of polybutylene have been injection-molded into a tensile bar mold under a wide range of barrel and mold temperatures. The overall structure of the moldings has been determined and correlated with processing conditions. The short term tensile mechanical properties of the moldings have been ascertained and correlated with molding structure. For low mold temperatures, the Young's modulus and tensile strength of injection moldings of polybutylene are controlled by the extent of and structure within the highly oriented skin. Low barrel temperatures can give rise to highly crystalline thick skins that treble the Young's modulus and fracture stress, when compared to high barrel temperature moldings. Increasing the mold temperature introduces a brittle response in polybutylene injection moldings. Modulus is controlled, at the high mold temperatures, by the skin thickness and by the crystallinity of the material comprising the core of the molding.     

The morphology evolution and crystallization behavior of microinjection molded polyoxymethylene/molybdenum disulfide nanocomposite

Microinjection molding was carried out on polyoxymethylene (POM)/molybdenum disulfide (MoS₂) nanocomposite prepared by solid state shear milling (S³M) technology. The morphology evolution and crystallization behavior were then investigated under different conditions of mold temperature and injection speed. The comparison between the microinjection molded micropart with conventional injection molded macropart was also conducted. Results showed that the higher mold temperature could improve the filling property and replication quality. The MoS₂ particles played a heterogeneous nucleation role and remarkably affected the crystallization process of POM. Meanwhile, the crystallization orientation and skin‐core structure induced by the shear stress presented the similar evolution tendency under different microinjection conditions. The multi‐melting behaviors of microparts occurring under different microinjection conditions were attributed to the formation of shish‐kebab structures. The increase of crystallinity and the reduction of mechanical property occurring in macropart are related to its crystallization morphology formed, which is different from that of micropart. The results of this work would lay a foundation for preparation of POM/MoS₂ transmission microparts with good comprehensive performance. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44625.     

The crystallization morphology evolution of polyoxymethylene/poly(ethylene oxide) blend micropart prepared under microinjection molding conditions

This article is the first study on the microinjection molding and the effects of the microprocessing parameters on the crystallization and orientation of polyoxymethylene/poly(ethylene oxide) (POM/PEO) blend, which has better toughness and self‐lubricity compared with the neat POM and therefore is a better candidate material for making microparts like microgears with higher performances. The crystalline and phase morphologies were investigated by polarized light microscope (PLM), differential scanning calorimeter (DSC) and scanning electron microscope (SEM). The crystalline orientation of the microparts was evaluated by two‐dimensional wide‐angle X‐ray diffraction (2D‐WAXD) and Herman's orientation function. The experimental results showed that both POM and POM/PEO microparts prepared by microinjection molding exhibited three distinct layers, i.e., skin layer, shear layer and core layer, while the latter had thicker shear layer but thinner skin layer and core layer. PEO was well dispersed in POM matrix. The spherulite size, the melting point as well as the crystallinity of POM in the POM/PEO blend decreased due to the interference of PEO in the crystallization of POM. A shish‐kebab structure was observed in the shear layers of the POM/PEO microparts. The effects of processing parameters on the thicknesses of different layers of the POM/PEO microparts were investigated. With increase of the injection speed or decrease of the mold temperature, the skin layer and the core layer became thicker, while the shear layer and the oriented region became thinner. However, the influence of the injection pressure was not obvious. Also, the processing parameters affected the crystalline orientation of the POM/PEO microparts. With increase of the injection speed or decrease of the mold temperature, the orientation function f decreased, indicating a lower degree of orientation. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40538.     

The formation of β‐polypropylene crystals in a compatibilized blend of isotactic polypropylene and polyamide‐6

Compatibilized polypropylene (PP)/polyamide (PA6) blends with and without β nucleating agent (β‐NA) are prepared, and are designated as Blend‐0.3 and Blend‐0, respectively. The melting and crystallization characteristic of the blends crystallized under different cooling rates and different crystallization temperatures are studied. It is observed that high β‐PP content can be developed in Blend‐0.3 only at slow cooling rates (<5°C/min), whereas high α‐PP content is formed at fast cooling rates. Isothermal crystallization analysis of Blend‐0 indicates that PA6 is an effective NA for α‐PP in the lower temperature range, whereas the α‐nucleating effect disappears in the higher temperature range. Blend‐0.3 can, therefore, be viewed as a system containing both α‐ and β‐NAs, simultaneously. PA6 is competing with β‐NA in inducing PP crystallization. Under the normal injection of Blend‐0.3, the melt will be cooled through the higher temperature that favors the effectiveness of β‐NA rapidly because of the faster cooling rate. However, the α‐nucleation effect from PA6 predominate at the lower temperature. This explains the difficulty in obtaining high β‐PP content in Blend‐0.3 from injection molding. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Crystallization and melting behavior of multi‐walled carbon nanotube‐reinforced nylon‐6 composites

The crystallization and melting behavior of neat nylon‐6 (PA6) and multi‐walled carbon nanotubes (MWNTs)/PA6 composites prepared by simple melt‐compounding was comparatively studied. Differential scanning calorimetry (DSC) results show two crystallization exotherms (T_{CC, 1} and T_{CC, 2}) for PA6/MWNTs composites instead of a single exotherm (T_{CC, 1}) for the neat matrix. The formation of the higher‐temperature exotherm T_{CC, 2} is closely related to the addition of MWNTs. X‐ray diffraction (XRD) results indicate that only the α‐phase crystalline structure is formed upon incorporating MWNTs into PA6 matrix, independently of the cooling rate and annealing conditions. These observations are significantly different from those for PA6 matrix, where the increase in cooling rate or decrease in annealing temperature results in the crystal transformation from α‐phase to γ‐phase. The crystallization behavior of PA6/MWNTs composites is also significantly different from those reported in PA6/nanoclay systems, probably due to the difference in nanofiller geometry between one‐dimensional MWNTs and two‐dimensional nanoclay platelets. The nucleation sites provided by carbon nanotubes seem to be favorable to the formation of thermodynamically stable α‐phase crystals of PA6. The dominant α‐phase crystals in PA6/MWNTs composites may play an important role in the remarkable enhancement of mechanical properties. Copyright © 2005 Society of Chemical Industry     

Impact properties and microhardness of double‐gated glass‐reinforced polypropylene injection moldings

Injection moldings with weld lines were produced in glass reinforced polypropylene grades differing in filler content using a two‐gated hot runner injection mold. The skin‐core microstructure developed during injection molding was qualitatively analyzed by means of optical and scanning electronic microscopy techniques. The load bearing capacity of the moldings was assessed by uniaxial tensile‐impact and biaxial instrumented falling dart impact tests. Microhardness was also used to ascertain the possibility of using it as a simple nondestructive technique for characterizing glass fiber‐reinforced injection moldings. The properties were monitored at various points to evaluate their variation at the bulk and the knit region. The biaxial impact test highlights the 10‐fold reduction of the impact strength caused by the weld line. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Study on structure and property relations of α‐iPP during uniaxial deformation via in situ synchrotron SAXS/WAXS and POM investigations

Micro‐ and meso‐scale structure changes of α‐form isotactic polypropylene (α‐iPP) during uniaxial stretching is studied by time‐resolved synchrotron small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray scattering (WAXS). The structure/property relations are investigated at different temperatures, and the effects of isothermal crystallization are also studied with POM. The X‐ray scattering results show that the long period increased and the lamellar oriented along the stretching direction in the elastic deformation stage. The lamellar and crystals start destructing after yielding. And from it POM images it can be seen that with higher crystallization temperature the spherulites connected to form a crystalline network, on which the stress is mainly loaded. It turns out different environment temperatures affect mostly the amorphous domains. And samples exhibit different yielding mechanisms with different thermal histories. A hypothetical structural mechanism is proposed based to explain the observed relationship between the processing parameters, thermal history and the structure/property relations of α‐iPP. POLYM. ENG. SCI., 58:160–169, 2018. © 2017 Society of Plastics Engineers     

Crystallization and thermal behavior of microcellular injection‐molded polyamide‐6 nanocomposites

This article presents the effects of nanoclay and supercritical nitrogen on the crystallization and thermal behavior of microcellular injection‐molded polyamide‐6 (PA6) nanocomposites with 5 and 7.5 wt% nanoclay. Differential scanning calorimetry (DSC), X‐ray diffractometry (XRD), and polarized optical microscopy (POM) were used to characterize the thermal behavior and crystalline structure. The isothermal and nonisothermal crystallization kinetics of neat resin and its corresponding nanocomposite samples were analyzed using the Avrami and Ozawa equations, respectively. The activation energies determined using the Arrhenius equation for isothermal crystallization and the Kissinger equation for nonisothermal crystallization were comparable. The specimen thickness had a significant influence on the nonisothermal crystallization especially at high scanning rates. Nanocomposites with an optimal amount of nanoclay possessed the highest crystallization rate and a higher level of nucleation activity. The nanoclay increased the magnitude of the activation energy but decreased the overall crystallinity. The dissolved SCF did not alter the crystalline structure significantly. In contrast with conventionally injection‐molded solid counterparts, microcellular neat resin parts and microcellular nanocomposite parts were found to have lower crystallinity in the core and higher crystallinity near the skin. POLYM. ENG. SCI., 46:904–918, 2006. © 2006 Society of Plastics Engineers     

The thermomechanical environment and the mechanical properties of injection moldings

Axisymmetric specimens were injection molded in a propylene copolymer with systematic variations of the melt and mold temperatures and the injection flow rate, in a total of 15 different processing conditions. From computer simulations of the mold filling stage using commercially available software packages, two thermomechanical indices were calculated. They aim at evaluating the level of orientation of the skin and the degree of crystallinity of the core layers. Assuming that these morphological features determine the mechanical response of the moldings, the thermomechanical indices were weighted by the relative thickness of the skin and core layers. The tensile behavior of the moldings was assessed at two velocities of 3.3 × 10^?5 (2 mm/min) and 3 m/s. The mechanical properties studied were the initial modulus, the yield stress and the strain at break. The relationships between the weighted thermomechanical indices and these mechanical properties were analyzed from 3D response surfaces obtained by polynomial fittings. Globally, a marked effect of the strain rate on the mechanical response along with a distinct sensitivity on the weighted thermomechanical indices was found. At high strain rates the microstructural differences were enhanced. The dependence of the yield stress on the thermomechanical indices was not significantly affected by the strain‐rate. However, the strain‐rate dependence of the other mechanical properties was strongly influenced by the initial microstructural state. Furthermore, the maximization of different mechanical properties could not be made simultaneously due to their distinct microstructural dependences. The concept of the thermomechanical indices is evidenced as a simple, valid and valuable tool to establish straightforward relationships between the processing and the mechanical behavior. Polym. Eng. Sci. 44:1522–1533, 2004. © 2004 Society of Plastics Engineers.     

Theoretical and experimental studies of anisotropic shrinkage in injection moldings of semicrystalline polymers

A novel approach to predict anisotropic shrinkage of semicrystalline polymers in injection moldings was proposed using flow‐induced crystallization, frozen‐in molecular orientation, elastic recovery, and PVT equation of state. The anisotropic thermal expansion and compressibility affected by the frozen‐in orientation function and the elastic recovery that was not frozen during moldings were introduced to obtain the in‐plane anisotropic shrinkages. The frozen‐in orientation function was calculated from amorphous and crystalline contributions. The amorphous contribution was based on the frozen‐in and intrinsic amorphous birefringence, whereas the crystalline contribution was based on the crystalline orientation function, which was determined from the elastic recovery and intrinsic crystalline birefringence. To model the elastic recovery and frozen‐in stresses related to birefringence during molding process, a nonlinear viscoelastic constitutive equation was used with temperature‐ and crystallinity‐dependent viscosity and relaxation time. Occurrence of the flow‐induced crystallization was introduced through the elevation of melting temperature affected by entropy production during flow of the viscoelastic melt. Kinetics of the crystallization was modeled using Nakamura and Hoffman‐Lauritzen equations with the rate constant affected by the elevated melting temperature. Numerous injection molding runs on polypropylene of various molecular weights were carried out by varying the packing time, flow rate, melt temperature, and mold temperature. The anisotropic shrinkage of the moldings was measured. Comparison of the experimental and simulated results indicated a good predictive capability of the proposed approach. POLYM. ENG. SCI., 46:712–728, 2006. © 2006 Society of Plastics Engineers     

Theoretical and experimental studies of anisotropic shrinkage in injection moldings of various polyesters

A novel approach to predict anisotropic shrinkage of slow crystallizing polymers in injection moldings was proposed, using the flow‐induced crystallization, frozen‐in molecular orientation, elastic recovery, and PVT equation of state. In the present study, three different polyesters, polyethylene terephthalate, polybutylene terephthalate, and polyethylene‐2,6‐naphthalate (PEN), are used. The anisotropic thermal expansion and compressibility affected by the frozen‐in orientation function and the elastic recovery that was not frozen during moldings were introduced to obtain the in‐plane anisotropic shrinkages. The frozen‐in orientation function was calculated from the amorphous contribution based on the frozen‐in and intrinsic amorphous birefringence and crystalline contribution based on the crystalline orientation function determined from the elastic recovery and intrinsic crystalline birefringence. To model the elastic recovery and frozen‐in stresses related to birefringence during molding process, a nonlinear viscoelastic constitutive equation was used with the temperature‐dependent viscosity and relaxation time. Occurrence of the flow‐induced crystallization was introduced through the elevation of melting temperature affected by entropy production during flow of the viscoelastic melt. Kinetics of the crystallization was modeled using Nakamura and Hoffman‐Lauritzen equations with the rate constant affected by the elevated melting temperature. Numerous injection molding runs were carried out by varying the packing time, packing pressure, flow rate, melt and mold temperature, and anisotropic shrinkage of moldings were measured. The experimental results were compared with the simulated data and found in a fair agreement. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3526–3544, 2006     

Morphology and mechanical properties of injection molded poly(ethylene terephthalate)

This work reports on the relationships between processing, the morphology and the mechanical properties of an injection molded poly(ethylene terephthalate), PET. Specimens were injection molded with different mold temperatures of 30°C, 50°C, 80°C, 100°C, 120°C, 150°C, while maintaining constant the other operative processing parameters. The thermomechanical environment imposed during processing was estimated by computer simulations of the mold‐filling phase, which allow the calculation of two thermomechanical indices indicative of morphological development (degree of crystallinity and level of molecular orientation). The morphology of the moldings was characterized by differential scanning calorimetry (DSC) and by hot recoverable strain tests. The mechanical behavior was assessed in tensile testing at 5 mm/min and 23°C. A strong thermal and mechanical coupling is evidenced in the injection molding process, significantly influencing morphology development. An increase in the mold temperature induces a decrease of the level of molecular orientation (decrement in the hot recoverable strain) and an increment in the initial crystallinity of the moldings (decrement in the enthalpy of cold crystallization), also reflected in the variations of the computed thermomechanical indices. The initial modulus is mainly dependent upon the level of molecular orientation. The yield stress is influenced by both the degree of crystallinity and the level of molecular orientation of the moldings, but more significantly by the former. The strain at break was not satisfactorily linked directly to the initial morphological state because of the expected morphology changes occurring during deformation. Polym. Eng. Sci. 44:2174–2184, 2004. © 2004 Society of Plastics Engineers.     

Co-injection molding of reinforced polymers

Co-injection molding of calcium carbonate filled polypropylene, short glass-fiber-filled polypropylene, or unfilled high-density polythylene melts is studied using a mediumsize injection-molding machine and a center-gated disc mold. Injection molding is carried out under non-isothermal conditions. Order of injection of the melts, injection speed, and mold temperature is changed in order to understand the mold filling in general and to investigate the type of skin/core structure and mechanical interlocking of the phases in the moldings. It is found that the order of injection is not significant in obtaining a skin/core structure but it is important in obtaining extensive phase interlocking, which is reduced if the flow rafe and the mold temperature are low. Presence of fillers appears to result in more mechanical interlocking of the phases.     

Comparison of structure development in injection molding of isotactic and syndiotactic polypropylenes

A comparative study of the crystallization and orientation development in injection molding isotactic and syndiotactic polypropylenes was made. The injection molded samples were characterized using wide angle X‐ray diffraction (WAXD) techniques and birefringence. The injection molded isotactic polypropylene samples formed well‐defined sublayers (skin, shear and core zones) and exhibited polymorphic crystal structures of the monoclinic α‐form and the hexagonal β‐form. Considerable amounts of β‐form crystal were formed in the shear and core zones, depending on the injection pressure or on the packing pressure. The isotactic polypropylene samples had relatively high frozen‐in orientations in the skin layer and the shear zone. The injection molded syndiotactic polypropylene exhibited the disordered Form I structure, but it did not appear to crystallize during the mold‐filling stage because of its slow crystallization rate and to develop a distinct shear zone. The core zone orientation was greatly increased by application of high packing pressure. The isotactic polypropylene samples exhibited much higher birefringence than the syndiotactic polypropylene samples at the skin and shear layers, whereas both materials exhibited similar levels of crystalline orientation in these layers.     

Modeling and experimental study of birefringence in injection molding of semicrystalline polymers

The prediction of birefringence developed in injection moldings is very important in order to satisfy required specification of molded products. A novel approach for the numerical simulation of the flow-induced crystallization and frozen-in birefringence in moldings of semicrystalline polymers was proposed. The approach was based on the calculation of elastic recovery that becomes frozen when the flow-induced crystallization occurred. The flow effect on the equilibrium melting temperature elevation due to the entropy reduction between the oriented and unoriented melts was incorporated to model crystallization. To find the entropy reduction and the frozen-in elastic recovery during crystallization, a non-linear viscoelastic constitutive equation was used. From the ultimate elastic recovery the crystalline orientation function was calculated. The crystalline and amorphous contributions to the overall birefringence were obtained from the crystalline orientation function and the flow birefringence, respectively. The birefringence profiles were measured and predicted in moldings of polypropylenes of different molecular weights obtained at various melt temperatures, injection speeds, holding times and mold temperatures. The resulting predictions were in fair agreement with corresponding experimental data.     

Flow‐induced crystallization in the injection molding of polymers: A thermodynamic approach

The prediction of the crystallinity and microstructure that develop in injection molding is very important for satisfying the required specifications of molded products. A novel approach to the numerical simulation of the skin‐layer thickness and crystallinity in moldings of semicrystalline polymers is proposed. The approach is based on the calculation of the entropy reduction in the oriented melt and the elevated equilibrium melting temperature by means of a nonlinear viscoelastic constitutive equation. The elevation of the equilibrium melting temperature that results from the entropy reduction between the oriented and unoriented melts is used to determine the occurrence of flow‐induced crystallization. The crystallization rate enhanced by the flow effect is obtained by the inclusion of the elevated equilibrium melting temperature in the modified Hoffman–Lauritzen equation. Injection‐molding experiments at various processing conditions were carried out on polypropylenes of various molecular weights. The thickness of the highly oriented skin layer and the crystallinity in the moldings were measured. The measured data for the microstructures in the moldings agree well with the simulated results. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 502–523, 2005     

Organoclay‐reinforced dynamically vulcanized thermoplastic elastomers of polyamide‐6/polyepichlorohydrin‐co‐ethylene oxide

Dynamically vulcanized thermoplastic elastomers (TPVs) based on polyamide‐6 (PA‐6) and poly(epichlorohydrin‐co‐ethylene oxide) (ECO) and their nanocomposites were prepared via melt‐blending process. The unfilled and organoclay (OC)‐filled TPVs were characterized using X‐ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry, thermal gravimetric analysis (TGA), and mechanical tests. XRD and TEM results showed that the OC particles were well exfoliated into the samples with high rubber content while both intercalated and exfoliated structures were found in the samples with low rubber content. The mechanical properties showed that ECO improved the elongation at break and the presence of OC increased the Young's modulus. Also, wide angle XRD analysis showed an increase in α‐crystals of PA‐6 with addition of ECO rubber. Moreover, it was found that by increasing OC content, crystallization temperature increased but the degree of crystallinity decreased. TGA showed that increasing ECO content decreased thermal stability of the samples, while the presence of OC did not have any considerable effect on the thermal stability. POLYM. COMPOS., 38:1948–1956, 2017. © 2015 Society of Plastics Engineers     

The effect of pressure and clay on the crystallization behavior and kinetics of polyamide‐6 in nanocomposites

The crystallization kinetics of polyamide‐6 (PA‐6) and its nanocomposite (PNC) with 2% clay were studied, using a pressure dilatometer (50 MPa to 200 MPa) to follow the volume changes associated with the crystallization process. Isobaric experiments were carried out to evaluate the effect of pressure and clay on melting temperature (T_m) and crystallization temperature (T_a) of PA‐6. The melting temperatures of PA‐6 in the PNC were very close to those of PA‐6 alone at comparable pressures, but the crystallization temperatures in the PNC were lower than those of PA‐6 alone. The materials exhibited two crystallization zones in isothermal/isobaric experiments. The initial zone involved both the γ‐form and the α‐form of PA‐6, while in the latter zone the γ‐form was dominant. The Avrami equation was used to fit the isothermal/isobaric crystallization data. The Avrami exponent n was between 1.0 and 3.2 for the γ‐form of unfilled PA‐6, between 0.9 and 2.6 for the γ‐form in PNC and for the γ‐form of PA‐6 alone, n was between 1.0 and 2.1 and in PNC between 1.2 and 2.6. The Avrami rate constants (K) for PA‐6 and PNC depend on the experimental crystallization temperature as well as pressure. The rate of crystallization under similar conditions was higher for PNC. Infrared studies on compression molded PA‐6 and PNC samples, cooled from melt at different rates, confirm the formation of the γ‐form in the initial stages of crystallization, as well as its transformation into the α‐form at later stages. In the case of PNC, the γ‐form stabilized when the sample was quenched from melt.