Β-form Crystals Research Articles

Melting and crystallization behavior of poly(trimethylene 2,6-naphthalate)

The melting and crystallization behavior of poly(trimethylene 2,6-naphthalate) (PTN) are investigated by using the conventional DSC, the temperature-modulated DSC (TMDSC), wide angle X-ray diffraction (WAXD) and polarized light microscopy. It is observed that PTN has two polymorphs (α- and β-form) depending upon the crystallization temperature. The α-form crystals develop at the crystallization temperature below 140 °C while β-form crystals develop above 160 °C. Both α- and β-form crystals coexist in the samples crystallized isothermally at the temperature between 140 and 160 °C. When complex multiple melting peaks of PTN are analyzed using the conventional DSC, TMDSC and WAXD, it is found that those arise from the combined mechanism of the existence of different crystal structures, the dual lamellar population, and melting–recrystallization–remelting. The equilibrium melting temperatures of PTN α- and β-form crystals determined by the Hoffman–Weeks method are 197 and 223 °C, respectively. When the spherulitic growth kinetics is analyzed using the Lauritzen–Hoffmann theory of secondary crystallization, the transition temperature of melt crystallization between regime II and III for the β-form crystals is observed at 178 °C. Another transition is observed at 154 °C, where the crystal transformation from α- to β-form occurs.

FABRICATION OF ORIENTATION-CONTROLLED MEROCYANINE J-AGGREGATES FROM VAPOR PHASE

The absorption spectrum of merocyanine (BTET) film deposited on a NaCl substrate showed a broad absorption band in the wavelength region between 450 nm and 550 nm and a narrow peak at 640 nm. The former bands were assigned to the absorption bands of α- and β-form crystal, the latter peak was assigned to J-band. BTET film was composed of square-shaped and rectangular crystals grown epitaxially. The square-shaped crystal was composed of α-form and the rectangular crystal was composed of α-form and J-aggregate. The formation mechanism of J-aggregate in film was discussed.

Crystallization and Orientation Development in Melt Spinning Isotactic PP of Varying Tacticity

Abstract We investigated the effects of tacticity on crystallization and orientation development in melt spinning isotactic polypropylene (iPP) through air. Melt-spun fibers were characterized by wide-angle X-ray diffraction (WAXD), birefringence and differential scanning calorimetry (DSC). The melt-spun fibers usually exhibited the monoclinic α-crystalline form, but the mesomorphic form was found under certain processing conditions. The mesomorphic structure was formed more readily in low tacticity fibers. We found certain amounts of hexagonal β-crystalline form in all of the α-form fiber samples. The amouns were generally negligibly small, but the low tacticity fibers spun at high draw-down ratios exhibited significant amounts of β-form crystals. At low spinline stress levels, some low tacticity fibers exhibited lower birefringence and chain orientation than high tacticity fibers due to a less perfect crystal structure. At certain high spinline stress levels, low tacticity fibers exhibited much higher birefringence and chain-axis orientation levels than expected. This was attributed to a reduction in epitaxial lamellar branching (a*-axis oriented group in bimodal orientation). However, the extent of lamellar branching in low tacticity fibers appeared to increase again at higher stress levels. This led to a significant decrease in birefringence and crystalline orientation.

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

AbstractA 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.

Structural properties and mechanical behavior of injection molded composites of polypropylene and sisal fiber

AbstractComposites based on isotactic polypropylene (PP) and sisal fiber (SF) were prepared by melt mixing and injection molding. The melt mixing characteristics, thermal properties, morphology, crystalline structure, and mechanical behavior of the PP/SF composites were systematically investigated. The results show that the PP/SF composites can be melt mixed and injection molded under similar conditions as the PP homo‐polymer. For the composites with low sisal fiber content, the fibers act as sites for the nucleation of PP spherulites, and accelerate the crystallization rate and enhance the degree of crystallinity of PP. On the other hand, when the sisal fiber content is high, the fibers hinder the molecular chain motion of PP, and retard the crystallization. The inclusion of sisal fiber induces the formation of β‐form PP crystals in the PP/SF composites and produces little change in the inter‐planar spacing corresponding to the various diffraction peaks of PP. The apparent crystal size as indicated by the several diffraction peaks such as L(110)α, L(040)α, L(130)α and L(300)β of the α and β‐form crystals tend to increase in the PP/SF composites considerably. These results lead to the increase in the melting temperature of PP. Moreover, the stiffness of the PP/SF composites is improved by the addition of sisal fibers, but their tensile strength decreases because of the poor interfacial bonding. The PP/SF composites are toughened by the sisal fibers due to the formation of β‐form PP crystals and the pull‐out of sisal fibers from the PP matrix, both factors retard crack growth.

Effects of crystallization temperature on the polymorphic behavior of syndiotactic polystyrene

The influence of crystallization temperature on formation of the α- and β-form crystals of syndiotactic polystyrene (sPS) was investigated by X-ray diffraction and non-isothermal differential scanning calorimetry analysis. For sPS samples without any thermal history, the crystallization temperature must be the intrinsic factor controlling the formation the α and β-form crystals. Being crystallized at different cooling rate from the melt, sPS forms the β-form crystal until the temperature cooled down to about 230°C, and α-form crystal can only be obtained when the temperature was below about 230°C.

Crystal polymorphism of poly(butylene-2,6-naphthalate) prepared by thermal treatments

The crystal polymorphisms of the poly(butylene-2,6-naphthalate) (PBN) prepared by various thermal treatments in bulk state were investigated. Crystalline forms obtained from different thermal treatments were characterized using wide-angle X-ray diffraction (WAXD) measurements. The α-form crystal is obtained by annealing the quenched PBN sample in the solid state or by crystallizing from the static melt at a relatively lower temperature (T C <205 ° C). On the other hand, an exclusive β-form crystal is generated through unique thermal treatment by non-isothermal crystallization at a cooling rate of 0.1 °C min −1 from 280 to 25 °C. Additionally, both α- and β-form crystals appear in the sample melt-crystallized at a relatively higher temperature, and the relative ratio of the α- and β-forms depends on the predetermined crystallization temperature. In this study, it was found that the crystalline form transition never occurs in the solid state. Optical microscopic observations indicate that the α-form crystal exhibits a typical spherulite morphology, while the β-form crystal displays a dendritic spherulite morphology due to the extremely slow spherulite growth rate. Fourier transform infrared spectroscopy was used to examine the crystal structures of both α- and β-forms of the PBN. It is proposed that the main difference between these two crystal structures of PBN prepared by different thermal treatments lies in the packing efficiency of the crystal chains, rather than in the conformation change of the glycol residue, with the β-form possessing tighter packing efficiency than the α-form.

Crystal Modification in Polypropylene Fibers Containing .BETA.-form Nucleating Agent.

Sample fibers were prepared with a multi-filament melt-spinning machine, from polypropylene (hereafter PP) to which β-form nucleating agent had been added. The relations between melt-spinning conditions and crystal modification of obtained fibers were studied. Wide-angle X-ray diffraction measurement of fibers yielded the following results: The diffraction peaks of α-form crystal were clearly observed in undrawn fiber to which β-form nucleating agent had been added in an amount of 0.05 wt%. The diffraction peaks of β-form crystal were observed in undrawn fiber to whichβ-form nucleating agent had been added in an amount of more than 0.10wt%. Also, the effects of spinning temperature on PP to which β-form nucleating agent had been added in an amount of 0.10 wt% were studied. Only crystalline reflections of α-form were found in the sample produced at a spinning temperature lower than 250°C. However, the reflections of β-form appeared when spinning temperature was increased to 280°C. When the draft ratio was more than 3100, the diffraction peaks of β-form crystal were not observed. When the samples were drawn with a heated roller at 110°C, a few diffraction peaks of β-form crystal were found when the draw ratio was set at 4.0, but the peaks were never found when the draw ratio exceeded 5.0. The results indicate that crystal transformation from β-form to α-form takes place in samples for which the crystallite-orientation factor ofβ-form, fC(β), reaches about 0.788.

Atomic force microscopic observation of phase transformation process of bis(1,2-benzoquinonedioximato)platinum(II)

Bis(1,2-benzoquinonedioximato)platinum(II) [Pt(bqd) 2] single crystal shows polymorphism and the polymorphic phase transformation occurs by heat treatment. By transmission electron microscopy, Pt(bqd) 2 is found to grow epitaxially as fine needle-like crystals on alkali halide single crystals that exhibit a polymorphic structure of α-form which has a longer Pt–Pt distance than the known α-form. The epitaxy is discussed from the point-on-line coincidence. The β-form crystals transform into the α-form by heating at approximately 170°C. Such the morphological changes of the β-form crystals were studied with an atomic force microscope and it is concluded that this phase transformation could be proceeded via a gas phase, but not an ordinary solid–solid transition.

Melting behavior and equilibrium melting temperatures of syndiotactic polystyrene in α and β crystalline forms

Syndiotactic polystyrene (sPS) specimens with neat α- and β-form crystals were prepared by using press molding under different conditions. The crystal form of the as-molded specimens was verified using wide angle X-ray diffraction and Fourier transform infrared spectroscopy techniques. Isothermal crystallization of α- and β-form sPS was conducted at various temperatures Tc and periods tc. The corresponding melting enthalpies and temperatures upon subsequent heating were determined using differential scanning calorimetry (DSC). At given Tcs, the melting temperatures at zero crystallinity were deduced and used for the determination of equilibrium melting temperature, Tm0, for both crystal forms. Based on the linear Hoffman–Weeks (HW) plots, the values of Tm0 for α- and β-form sPS are obtained to be 281 and 291°C, respectively. The corresponding thickening coefficients are ca. 1.89 and 1.58, which seem unexpectedly high in consideration of the limited growth of lamellae at the early stage of crystallization. When a recently developed MX plot that is based on a non-linear HW approach is applied, reasonable thickening coefficients but much larger Tm0 values are derived; being 294 and 320°C for α- and β-form sPS. On the basis of Avrami plot, the β-form crystals show three-dimensional growth, which is different from that for α-form crystals showing one-dimensional growth. For specimens with mixed α/β-crystals, the details of crystal development during the isothermal crystallization at 240°C is provided to account for the observation of the triple melting peaks which is normally detected by DSC heating scan.

Enzymatic degradation of bacterial homo-poly(3-hydroxybutyrate) melt spun fibers

The effect of the higher order structure on the degradation behavior of bacterial homo-poly(3-hydroxybutyrate) (PHB) melt spun fibers by a PHB depolymerase from Commamonas testosteroni was investigated. After the enzymatic degradation, fibrillar structure remained at the surface of the drawn and annealed fibers, while a spongy structure appeared in the as-spun fiber. Similar spongy structure to the degraded as-spun fiber was observed in the core part of some drawn and annealed fibers after the fibrillar surface disappeared. The mechanical properties deteriorated as enzymatic degradation proceeded. After the amorphous region was degraded, β-form crystal, which seems to have less ordered structure, degraded in advance to the α-form crystal as revealed by WAXS measurements. However, these tendencies were less pronounced for the fiber annealed at high temperature under high tension. This result suggests that the β-form crystal in the fiber annealed at high temperature under high tension would have more ordered crystalline structure, which withstands against the attack by the enzyme.

Structural Characterization of Wool by Thermal Mechanical Analysis of Yarns

A thermomechanical analysis of yarns in a tensile mode is used to characterize wool fibers modified to show the β-crystal form in comparison with native untreated wool retaining the α-crystal form. The TMA curves show that fiber extension causes significant structural modification of the wool samples. In particular, melting of the α-form crystallites in native wool can be detected at around 215°C, with supercontraction prior to effective crystallite melting. But extended wool yarn shows no thermal mechanical event corresponding to melting of the β-form crystallites. Extended wool has a thermal mechanical behavior similar to that of silk yarn.

A scanning tunneling microscopy study on the monolayer epitaxy of [cyano(ethoxycarbonyl) methylene]-2-ylidene-4,5-dimethyl-1,3-dithiole on graphite and its dynamic feature

The monolayer film of [cyano(ethoxycarbonyl)methylene]-2-ylidene-4,5-dimethyl-1,3-dithiole (CEMYDD) vacuum-deposited on a (0001) graphite surface was investigated by scanning tunneling microscopy (STM). High-resolution STM images showed that all of the molecules formed the same two-dimensional unit cell in each domain and that the unit cell was almost equivalent to that in the (011) plane of the second-harmonic-generation active β-form crystal. Therefore the CEMYDD molecules were arranged as polar domains with a size of several hundred nm. The crystallographic orientation of the unit cell in the film with respect to the graphite surface was found to have commensurate epitaxial coincidence. This commensurate structure was stable under STM measurements, except near the domain boundaries where the moiré-like, long-range, periodic contrast modulation due to the commensurate coincidence often varied while scanning with the STM tip. Although the arrangement of molecular images themselves moved only slightly, such a collective dynamic feature at boundaries could be detected from changes in the moiré-like contrast modulation.

Effect of shearing on crystallization behavior of poly(ethylene naphthalate)

The effect of shear history on the isothermal crystallization behavior of poly(ethylene naphthalate) (PEN) was investigated by rheological and morphological measurements. Time sweep measurements of storage modulus (G′) and dynamic viscosity (η′) were carried out on the molten PEN by Advanced Rheometric Expansion System (ARES) in the parallel-plate geometry at several different temperatures and frequencies, followed by structural analysis by differential scanning calorimeter (DSC), X-ray diffractometer, and polarizing microscopy for the shear-induced crystallized PEN specimens in the ARES measurements. The rate of isothermal crystallization of PEN was notably affected by temperature, while the shear rate has an important effect on the structures of the resultant crystals. At a constant shear rate, the rate of crystallization by shear-induced structuring mechanism was increased with lowering temperature over the temperature range 230–250°C. The rate of crystallization was increased with increasing shear rate at a given temperature. An increase in shear rate increased both nucleation and number of crystallites. Further, it increased the content of the α-form crystal in the specimen. On the other hand, lower shear rate offered more favorable conditions for forming the β-form crystal. DSC analysis exhibited that the β-form crystal had higher melting temperature (Tm) than the α-form crystal. The wide angle X-ray diffraction (WAXD) patterns also ascertained that higher content of the α-form crystal was produced in the PEN specimen crystallized at higher frequency.

Study of multiple melting behaviour of syndiotactic polystyrene in ?-crystalline form

A series of syndiotactic polystyrene (SPS) samples in β-crystalline form were prepared by cooling from the melt at various rates. The effects of cooling rate from the melt, DSC heating rate and annealing on the multiple melting behaviours of β crystals were investigated by differential scanning calorimetry (DSC) and temperature modulated differential scanning calorimetry (TMDSC), from which the nature of the multiple melting behaviour was determined. The two melting endotherms of β-form crystals were considered to arise from the occurrence of simultaneous melting, recrystallization and remelting processes in the melting region. It is suggested that the lower melting endotherm is due to the melting of imperfect β crystals originally present in the sample, whereas the higher melting endotherm comes from the melting of recrystallized SPS crystals, ie more perfect β crystals that formed during the DSC scanning process. © 2000 Society of Chemical Industry

Toughening of PP/EPDM blend by compatibilization

To improve the mechanical properties of blends of polypropylene (PP) and terpolymer of ethylene–propylene–diene (EPDM), a triblock copolymer, (PP-g-MAH)-co-[PA-6,6]-co-(EPDM-g-MAH), was synthesized by coupling reaction of maleic anhydride (MAH)-grafted PP (PP-g-MAH), EPDM-g-MAH, and PA-6,6. The newly prepared block copolymer brought about a physical interlocking between the blend components, and imparted a compatibilizing effect to the blends. Introducing the block copolymer to the blends up to 5 wt % lead to formation of a β-form crystal. The wide-angle X-ray diffractograms measured in the region of 2θ between 10° and 50° ascertained that incorporating the block copolymer gave a new peak at 2θ = 15.8°. The new peak was assigned to the (300) plane spacings of the β-hexagonal crystal structure. In addition, the block copolymer notably improved the low-temperature impact property of the PP/EPDM blends. The optimum usage level of the compatibilizer proved to be 0.5 wt %. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1267–1274, 2000

An Interpretation of the Formation of α- and β-Form Crystals in Bulk Syndiotactic Polystyrene

The crystallization of the α- and β-form crystals of syndiotactic polystyrene (sPS) is determined by the crystallization temperature. Being crystallized at various temperatures from the melt, sPS forms the β form crystal at high temperatures above about 230 °C, α form crystal at low temperatures below about 170 °C and a mixture of the α and β form at intermediate temperatures.

Enhanced electrical properties of highly oriented poly(vinylidene fluoride) films prepared by solid-state coextrusion

Oriented poly(vinylidene fluoride) (PVDF) films with β-form crystals have been commonly prepared by cold drawing of a melt-quenched film consisting of α-form crystals. In this study, we have successfully produced highly oriented PVDF thin films (20 µm thick) with β-crystals and a high crystallinity (55–76%), by solid-state coextrusion of a gel film to eight times the original length at an established optimum extrusion temperature of 160°C, some 10°C below the melting temperature. The resultant drawn films had a highly oriented (orientation function fc = 0.993) fibrous structure, showing high mechanical properties of an extensional elastic modulus of 8.3 GPa and tensile strength of 0.84 GPa, along the draw direction. Such highly oriented and crystalline films exhibited excellent ferroelectric and piezoelectric properties. The square hysteresis loop was significantly sharper than that of a conventional sample. The sharp switching transient yielded the remnant polarization Pr of 90 mC/m2, and the electromechanical coupling factor kt was 0.24 at room temperature. These values are about 1.5 times greater than those of a conventional β-PVDF film. Thus, solid-state coextrusion near the melting point was found to be a useful technique for the preparation of highly oriented and highly crystalline β-PVDF films with superior mechanical and electrical properties. The morphology of the extrudate relevant to such properties is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2549–2556, 1999

Structural Change during Uniaxial Drawing of Unoriented Poly (tetramethylene succinate) Films.

Unoriented films of poly (tetramethylene succinate) [PTMS] were uniaxially drawn at a given temperature (basically at room temperature and occasionally at elevated temperatures). In their drawing process, stress-strain measurements and time-resolved wide-angle X-ray diffraction [WAXD] experiments were carried out.In the drawing process, at first the chain axes (c-axes) of α-form crystallites were gradually oriented in the drawing direction via necking, and then the solid-state phase transformation from α- to β-form crystals occurred increasingly by further drawing. The stress at which the transformation took place was always greater than the yielding stress that was recorded just before the onset of necking. Even at diffrent drawing temperatures ranging from room temperature to 110°C, the transformation occurred at the approximately same strain (or draw ratio), but the stress at which the tranformation took place decreased with increasing temperature.After a run of uniaxial drawing, the sample strip was held still at the final length for more than 120min, but its WAXD pattern displayed only the β-form reflections during such a holding process. In the case of the strip held at a constant strain of 640% for 120min, the stress thereby decreased by 25%, but no α-form reflections were observed in the WAXD pattern. When the stress was fully released, the reflections of β-form disappeared completely and those of α-form appeared again: Finally the uniaxially oriented film of PTMS consisting of α-form crystals was obtained.

Polymorphic crystal forms and morphology of syndiotactic polystyrene in miscible states

X-ray diffraction and optical microscopy characterization were performed to evaluate the phenomenon of alteration of polymorphism of syndiotactic polystyrene (s-PS) in the presence of other blending miscible polymers: poly(2,6-dimethyl-p-phenylene oxide) (PPO) or atactic polystyrene (a-PS). Both α and β crystal forms were observed in the neat s-PS sample, but only β-form crystal was found in miscible blends of s-PS with a-PS or PPO. The order and neighboring chain segments of neat s-PS are different from those of s-PS/PPO or s-PS/a-PS blends; thus, it is plausible that the greater randomness in the melt state of s-PS/a-PS or s-PS/PPO blends might be unfavorable for formation of α-form crystals from melts. The final spherulitic morphology the s-PS/a-PS or s-PS/PPO blends suggests that the amorphous-state miscibility of does not change much the spherulitic structure of s-PS. The radial growth rate is, in general, depressed with the presence of blending miscible polymers in s-PS of equal Tg or PPO of higher Tg. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2725–2735, 1998