Doping of polyvinylidene difluoride with cobalt nitrate: Structural,electrical, and magnetic properties

Polyvinylidene difluoride (PVDF) doped with a few percentage of cobalt nitrate hexahydrate induces transition into the ferroelectric β‐phase. Infrared spectra show peaks indicative of β‐phase PVDF at 1277 cm^?1 and loss of the α‐phase peak at 790 cm^?1 while polarization measurements show loops consistent with relaxor ferroelectric behavior. Electronic spectroscopy shows that the Co²⁺ ion is in a six‐coordinate environment, and this is confirmed by magnetic susceptibility. Thermal measurements show that the PVDF melts near 155 °C, depending on the Co²⁺ content, and that there are several other thermal features that are associated with the hydrated cobalt nitrate. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Thin and surface adhesive ferroelectric poly(vinylidene fluoride) films with β phase‐inducing amino modified porous silica nanofillers

We report an efficient route for ferroelectric polar β phase generation in poly(vinylidene fluoride) (PVDF) through incorporation of amine functionalized, porous silica (MCM‐41 and fumed silica) based nanofillers. These porous highly functionalized surfaces exhibit the efficient secondary interaction with polymer chain via hydrogen bonding. Structural analysis through FTIR, XRD, and TEM confirm high degree of ferroelectric polar β phase generation of PVDF through incorporation of amino modified porous silica nanofillers. Optimized loading (5 wt %) of amine functionalized, porous silica in PVDF matrix enhances relative intensity of β phase up to 75%. Disappearance of spherulite structure of PVDF with amino modified porous silica nanofillers, as confirmed through POM, TEM, SEM and AFM studies also supports the above conclusion. The P‐E hysteresis loop at sweep voltage of ±50 V of a thin PVDF‐amino modified porous nanofiller film shows excellent ferroelectric property with nearly saturated high remnant polarization 2.8 µC.cm^?2 owing to its large proportion of β PVDF, whereas, a nonpolar pure PVDF thin film shows unsaturated hysteresis loop with 0.6 µC.cm^?2 remnant polarization. PVDF films with the nanofillers exhibit strong adhesive strength over different metallic substrates making them have edge over PVDF in various thin film applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2401–2411     

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 f_c = 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 P_r of 90 mC/m², and the electromechanical coupling factor k_t 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     

Preferential formation of electroactive crystalline phases in poly(vinylidene fluoride)/organically modified silicate nanocomposites

The role of organically modified silicate (OMS), Lucentite STN on the formation of β‐crystalline phase of poly(vinylidene fluoride) (PVDF) is investigated in the present study. The OMS was solution blended with PVDF and cast on glass slide to form PVDF‐OMS nanocomposites. Solution cast samples were subjected to various thermal treatments including annealing (AC‐AN), melt‐quenching followed by annealing (MQ‐AN), and melt‐slow cooling (MSC). Fourier‐transform infrared spectroscopy (FT‐IR), wide angle X‐ray diffraction (WAXD), and differential scanning calorimetry (DSC) were used to investigate the crystalline structure of thermally treated samples. As a special effort, the combination of in situ thermal FT‐IR, WAXD, and DSC studies was utilized to clearly assess the thermal properties. FT‐IR and WAXD results of MQ‐AN samples revealed the presence of β‐phase of PVDF. Ion‐dipole interaction between the exfoliated clay nanolayers and PVDF was considered as a main factor for the formation of β‐phase. Melt‐crystallization temperature and subsequent melting point were enhanced by the addition of OMS. Solid β‐ to γ‐crystal phase transition was observed from in situ FT‐IR and WAXD curves when the representative MQ‐AN sample was subjected to thermal scanning. Upon heating, β‐phase was found to disappear through transformation to the thermodynamically stable γ‐phase rather than melting directly. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2173–2187, 2008     

Phase transformation and enhancement of toughness in polyvinylidene fluoride by onium salts

We study the effect of onium salts (benzyl triphenyl phosphonium chloride, BTPC; tetrabutyl ammonium perchlorate, TAP) on the crystallization of polyvinylidene fluoride (PVDF) from a melt. The β phase of PVDF crystals is polar and has excellent piezoelectric properties, in contrast to the nonpolar α phase. Processing of PVDF results in the formation of predominantly α‐phase crystals. Different amounts of the onium salts were melt‐mixed into PVDF using a micro‐compounder. PVDF containing about 0.5 wt % of BTPC is found to have predominantly β‐phase crystals in compression molded PVDF films, with an increase in the melting temperature by about 7 °C. A significant increase in the toughness of PVDF is obtained by the addition of BTPC. Polarizing microscopy indicates that the onium salts act as nucleating agents and result in significant reduction in spherulite size. Similar results were observed for samples prepared with TAP. The results of the study indicate a facile method for producing β‐phase PVDF films. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1339–1344, 2011     

Thin poly(ionic liquid) and poly(vinylidene fluoride) blend films with ferro‐ and piezo‐electric polar γ‐crystals

Ferro‐ and piezo‐electric poly(vinylidene fluoride) (PVDF) thin film is reported to be obtained by using a poly(ionic liquid) (PIL) [poly(2‐(dimethylamino)ethyl methacrylate) methyl chloride quaternary salt] through solution route. The short range interactions between localized cationic ions of PIL and polar >CF₂ of PVDF are responsible for modified polar γ‐PVDF (T₃GT₃Ḡ) formation. Modification in chain conformation of PVDF is confirmed by FTIR, XRD, and DSC studies suggesting the miscible PVDF–PIL (PPIL) blend. Up to 40 wt % loading of PIL in PVDF matrix enhances relative intensity of γ‐phase up to 50% in the entire crystalline phase. The P‐E hysteresis loop of PVDF‐PIL blends at 25 wt % PIL loading (PPIL‐25) thin film at sweep voltage of ±50 V shows excellent ferroelectric property with nearly saturated high remnant polarization ∼6.0 µC cm⁻² owing to large proportion of γ‐PVDF. However, non‐polar pure PVDF thin film shows unsaturated hysteresis loop with 1.4 µC cm⁻² remnant polarization. The operation voltage decreases effectively because of the polar γ‐phase formation in PPIL blended film. High‐sensitivity piezo‐response force microscopy shows electromechanical switching property at low voltages in PPIL‐25 thin films through local switching measurements, making them potentially suitable as ferroelectric tunnel barriers. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 795–802     

Facile deposition of high quality ferroelectric poly(vinylidene fluoride) thin films by thermally modulated spin coating

Direct deposition of high quality ferroelectric PVDF thin films using a modulated temperature spin coating method is demonstrated. The method is qualitatively guided from Flory‐Huggins theory of polymeric solutions and is general in applicability. Ferroelectric PVDF films with similar high structural and dielectric quality are deposited in environments ranging from 20 to 80% relative humidity on polar and non‐polar surfaces. The films do not show the presence of the non‐ferroelectric α‐phase. Resultant films have rms roughness values lower than 16 nm and remnant polarizations up to 6.5 µC cm^?2. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 221–227     

Synthesis of PVDF/BiFeO3 nanocomposite and observation of enhanced electrical conductivity and low‐loss dielectric permittivity at percolation threshold

A novel two‐phase polymer nanocomposite film comprising of polyvinylidene fluoride (PVDF) and nanocrystalline (～90 nm) semiconducting multiferroic BiFeO₃ (BFO) have been fabricated by hot‐molding technique. Such flexible thick nanocomposite films, semicrystalline in nature, exhibited extraordinarily high effective dielectric permittivity ε_eff ～ 10³ (compared with that of pure PVDF) near the low percolation threshold (f_c = 0.12) at room temperature (RT) and the films also possessed low dielectric loss (～0.18). The polarization‐electric field (P‐E) hysteresis loops are displayed at RT, which indicate ferroelectric like behavior of PVDF still persists in the percolative nanocomposite. There is also large increase of remanent polarization of BFO in the composite indicating improvement of the multiferroic behavior of BFO embedded in the PVDF polymer. The sample also indicates good fatigue endurance. Formation of microcapacitors and percolative behavior are correlated to explain the obtained results based on the special geometry of the BFO nanofillers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Crystal forms and ferroelectric properties of poly(vinylidene fluoride)/polyamide 11 blends prepared by high‐shear processing

Nanostructured poly(vinylidene fluoride) (PVDF)/polyamide 11 (PA11) blends have been melt‐processed using a high‐shear extruder. Uniaxially oriented blended films were fabricated by hot rolling to prepare ferroelectic films. The effects of rolling temperature and draw ratio on the crystal forms of both PVDF and PA 11 were investigated by means of Fourier transform infrared spectra (FTIR) and wide‐angle X‐ray diffraction (WAXD). It was shown that hot rolling in the range of 25–110 °C results in the crystal form transformation from the nonpolar α‐form into the polar β‐form for PVDF. The content and orientation function of β‐crystallites are strongly dependent upon the rolling temperature and the draw ratio. The highest content of well‐oriented β‐crystallites was achieved with a draw ratio of 4.0 upon rolling at 80 °C. At the same time, the content of the α‐form of PA11 in the blend was also found to decrease by hot rolling. The ferroelectric properties (D‐E hysteresis) of the oriented blended films were measured. The remanent polarization of the PVDF/PA11 = 90/10 blend is as high as 91 mC/m², which is about 1.2 times higher than that of pure PVDF. The D‐E hysteresis curves and the temperature dependence of the piezoelectric stress coefficients of the high‐shear‐processed sample suggested that the formation of nano‐dispersed structures resulted in the improvement of the remanent polarization and thermal characteristics at a temperature higher than 80 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2707–2714, 2007     

Enhanced dielectric performance and energy storage of PVDF‐HFP‐based composites induced by surface charged Al2O3

In order to enhance dielectric properties and energy storage density of poly(vinylidene fluoride‐hexafluoro propylene) (PVDF‐HFP), surface charged gas‐phase Al₂O₃ nanoparticles (GP‐Al₂O₃, with positive surface charges, ε’ ≈ 10) are selected as fillers to fabricate PVDF‐HFP‐based composites via simple physical blending and hot‐molding techniques. The results show that GP‐Al₂O₃ are dispersed homogeneously in the PVDF‐HFP matrix and the existence of nanoscale interface layer (matrix‐filler) is investigated by SAXS. The dielectric constant of the composites filled with 10 wt % GP‐Al₂O₃ is 100.5 at 1 Hz, which is 5.6 times higher than that of pure PVDF‐HFP. The maximum energy storage density of the composite is 4.06 J cm^?3 at an electrical field of 900 kV mm^?1 with GP‐Al₂O₃ content of 1 wt %. Experimental results show that GP‐Al₂O₃ could induce uniform fillers’ distribution and increase the concentration of electroactive β‐phase as well as enhance interfacial polarization in the matrix, which resulted in enhancements of dielectric constant and energy storage density of the PVDF‐HFP composites. This work demonstrates that surface charged inorganic‐oxide nanoparticles exhibit promising potential in fabricating ferroelectric polymer composites with relatively high dielectric constant and energy storage. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 574–583     

Development of oriented structure and properties on drawing of poly(vinylidene fluoride) by solid‐state coextrusion

Gel films of poly(vinylidene fluoride) (PVDF) consisting of α‐form crystals were drawn uniaxially by solid‐state coextrusion to extrusion draw ratios (EDR) up to 9 at an optimum extrusion temperature of 160 °C, about 10°C below the melting temperature (T_m). The development of an oriented structure and mechanical and electrical properties on coextrusion drawing were studied as a function of EDR. Wide‐angle X‐ray diffraction patterns showed that the α crystals in the original gel films were progressively transformed into oriented β‐form crystals with increasing EDR. At the highest EDR of 9 achieved, the drawn product consisted of a highly oriented fibrous morphology with only β crystals even for the draw near the T_m. The dynamic Young's modulus along the draw direction also increased with EDR up to 10.5 GPa at the maximum EDR of 9. The electrical properties of ferroelectricity and piezoelectricity were also markedly enhanced on solid‐state coextrusion. The D–E square hysteresis loop became significantly sharper with EDR, and a remanent polarization P_r of 100 mC/m² and electromechanical coupling factor along the thickness direction k_t of 0.27 were achieved at the maximum EDR of 9. The crystallinity value of 73–80% for the EDR 9 film, estimated from these electrical properties, compares well with that calculated by the ratio of the crystallite size along the chain axis to the meridional small‐angle X‐ray scattering (SAXS) long period, showing the average thickness of the lamellae within the drawn β film. These results, as well as the appearance of a strong SAXS maximum, suggest that the oriented structure and properties of the β‐PVDF are better explained in terms of a crystal/amorphous series arrangement along the draw axis. Further, the mechanical and electrical properties obtained in this work are the highest among those ever reported for a β‐PVDF, and the latter approaches those observed for the vinylidene fluoride and trifluoroethylene copolymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1371–1380, 2001     

Metal Salt‐Induced Ferroelectric Crystalline Phase in Poly(vinylidene fluoride) Films

In this Communication, the effect of varying mass fractions (0–20 wt.‐%) of calcium chloride (CaCl₂) salt on the α‐ and β‐phase content of poly(vinylidene fluoride) (PVDF) as‐cast films were investigated. Spectral and X‐ray studies revealed the maximum ferroelectric β‐phase for the addition of 15 wt.‐% of CaCl₂ in PVDF compared to neat PVDF samples. The dense β‐phase dominant PVDF–CaCl₂ (15 wt.‐%) thick film used as a ferroelectric insulator in one‐capacitor (1C) type random access memory device exhibited a remnant polarization of 3.1 µC · cm², and is a good indication that the unoriented PVDF–CaCl₂ films can be used in electronic applications without further stretching process.

     

Conformational changes and phase transformation mechanisms in PVDF solution‐cast films

The supramolecular crystal structure in poly(vinylidene fluoride) (PVDF) solution‐cast films is studied through changing crystallization conditions in two solvents of different structures and polarities. The crystalline‐state chain conformations of isothermally solution‐crystallized PVDF in N, N‐dimethylacetamide (DMAc), and cyclohexanone are studied through the specific FTIR absorption bands of α, β, and γ phase crystals. There are no changes in the FTIR spectra of cyclohexanone solution‐crystallized films in the temperature range of 50–120 °C. In the case of DMAc solution‐crystallized films, low temperature crystallization mainly results in formation of trans states (β and γ phases), whereas at higher temperatures gauche states become more populated (α phase). This is due to the variations in solvent polarity and ability to induce a specific conformation in PVDF chains, through the changes in chain coil dimensions. This indicates that in spite of cyclohexanone solutions, the intermolecular interactions between PVDF and DMAc are temperature‐sensitive and more important in stabilizing conformations of PVDF in crystalline phase than temperature dependence of PVDF chain end‐to‐end distance <r²>. The high‐resolution ¹⁹F NMR spectroscopy also showed little displacement in PVDF characteristic chemical shifts probably due to changes in PVDF chain conformation resulting from temperature variations. Upon uniaxial stretching of the prepared films under certain conditions, contribution of trans state becomes more prominent, especially for the originally higher α phase‐containing films. Due to formation of some kink bands during film stretching and phase transformation, α phase absorption bands are still present in infrared spectra. Besides, uniaxial stretching greatly enhances piezoelectric properties of the films, maybe due to formation of oriented β phase crystals, which are of more uniform distribution of dipole moments. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3487–3495, 2004     

Effect of annealing on the structure and properties of poly(vinylidene fluoride) β‐form films

Oriented poly(vinylidene fluoride) (PVDF) films consisting of β crystals were prepared by the solid‐state coextrusion (SC) of a gel film near the melting temperature (T_m) and by conventional cold tensile drawing (TD) of a melt‐quenched film. These films were annealed over the temperature range of 75–190 °C (below and above the static T_m) while the sample length was kept constant or constant loads were applied. After annealing with the sample length kept constant, the dynamic Young's modulus markedly decreased because of the relaxation of oriented amorphous chains, as shown by infrared spectroscopy. In contrast, annealing under a constant load improved the chain orientation in both the crystalline and amorphous regions, resulting in an increase in the modulus from an initial 10.5 to 14.3 GPa for the SC and from an initial 3.3 to 4.8 GPa for the TD. The SC, annealed at 190 °C with a constant load corresponding to an initial tension of 200 MPa, exhibited an extreme crystalline‐chain orientation of 0.998 and a modulus of 14.3 GPa, among the highest values ever reported for PVDF. Although the remanent polarization (P_r) of the TD increased slightly from the initial 62 to 68 mC/m², P_r of the SC stayed constant at 100 mC/m² independently of the annealing conditions. This suggests that the P_r value of 100 mC/m² approached the equilibrium value for this PVDF sample containing 3.5 mol % structural defects. Therefore, although the modulus and P_r of the TD increased slightly with annealing, the maximum values achieved by annealing were markedly lower than those of the SC and annealed SC. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1701–1712, 2003     

New technique of processing highly oriented poly(vinylidene fluoride) films exclusively in the β phase

Oriented β‐phase films were obtained by utilizing two different techniques: conventional uniaxial drawing at 80 °C of predominantly α‐phase films, and by drawing almost exclusively β‐phase films obtained by crystallization at 60 °C from dimethylformamide (DMF) solution with subsequent pressing. Wide angle X‐ray diffraction (WAXD) and pole figure plots showed that with the conventional drawing technique films oriented at a ratio (R) of 5 still contained about 20% of phase α, a crystallinity degree of 40% and β‐phase crystallographic c ‐axis orientation factor of 0.655. Drawing at 90 °C and with R = 4 of originally β‐phase films results in exclusively β‐phase films with crystallinity degree of 45% and orientation factor of 0.885. Crystalline phase, crystallinity degree, and crystallographic c‐axis orientation factor of both phases were also determined for α‐phase oriented films obtained by drawing α‐phase films at 140 °C. For films drawn at 140 °C the α to β phase transition drops to about 22%. Reduction in crystallinity degree with increasing R is more pronounced at draw temperature of 140 °C compared with 80 °C. Moreover, for both phases the c ‐axis orientation parallel to the draw direction is higher at draw temperature of 140 °C than at 80 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2793–2801, 2007     

Tailoring poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) microstructure and physicochemical properties by exploring its binary phase diagram with dimethylformamide

Poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) (PVDF‐CTFE) membranes were prepared by solvent casting from dimethylformamide (DMF). The preparation conditions involved a systematic variation of polymer/solvent ratio and solvent evaporation temperature. The microstructural variations of the PVDF‐CTFE membranes depend on the different regions of the PVDF‐CTFE/DMF phase diagram, explained by the Flory‐Huggins theory. The effect of the polymer/solvent ratio and solvent evaporation temperature on the morphology, degree of porosity, β phase content, degree of crystallinity, mechanical, dielectric, and piezoelectric properties of the PVDF‐CTFE polymer were evaluated. In this binary system, the porous microstructure is attributed to a spinodal decomposition of the liquid‐liquid phase separation. For a given polymer/solvent ratio, 20 wt % , and higher evaporation solvent temperature, the β phase content is around 82% and the piezoelectric coefficient, d₃₃, is ? 4 pC/N © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 761–773     

Computational analysis of bending strain in single chains of β‐PVDF

Hartree–Fock calculations on 20 carbon atom chains of β‐phase poly(vinylidene fluoride) (PVDF) were done as a function of bending strain. The results can be modeled in terms of a classical energy versus strain curve resulting in a pseudomodulus (310 GPa) comparable to the Young's modulus calculated for stretching along carbon atom chain (199 GPa). The model also shows that the minimum energy state of a single chain of the polymer is not linear in the all‐trans geometry and that a significant strain energy is stored in natural thin films. This suggests that energy can be captured from bending motions in β‐PVDF. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1493–1495, 2011     

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     

Oriented crystallization of poly(L‐lactic acid) in uniaxially oriented blends with poly(vinylidene fluoride)

This article describes the oriented crystallization of poly(L ‐lactic acid) (PLLA) in uniaxially oriented blends with poly(vinylidene fluoride) (PVDF). Uniaxially drawn films of PLLA/PVDF blend with fixed ends were heat‐treated in two ways to crystallize PLLA in oriented blend films. The crystal orientation of PLLA depended upon the heat‐treatment process. The crystal c‐axis of the α form crystal of PLLA was highly oriented in the drawing direction in a sample cold‐crystallized at T_c = 120 °C, whereas the tilt‐orientation of the [200]/ [110] axes of PLLA was induced in the sample crystallized at T_c = 120 °C after preheating at T_p = 164.5–168.5 °C. Detailed analysis of the wide‐angle X‐ray diffraction (WAXD) indicated that the [020]/ [310] crystal axes were oriented parallel to the drawing direction, which causes the tilt‐orientation of the [200]/ [110] axes and other crystal axes. Scanning electron microscopy (SEM) suggested that oriented crystallization occurs in the stretched domains of PLLA with diameters of 0.5–2.0 μm in the uniaxially drawn films of PVDF/PLLA = 90/10 blend. Although the mechanism for the oriented crystallization of PLLA was not clear, a possibility was heteroepitaxy of the [200]/[110] axes of the α form crystal of PLLA along the [201]/[111] axes of the β form crystal of PVDF that is induced by lattice matching of d₁₀₀(PLLA) ≈ 5d₂₀₁(PVDF). © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1376–1389, 2008     

Specificity and versatility of nucleating agents toward isotactic polypropylene crystal phases

Nucleating agents with an ≈6.5 Å lattice parameter induced the α phase of isotactic polypropylene (iPP, α‐iPP). A 6.5 Å periodicity is also involved in the nucleating agents for the β phase of iPP (β‐iPP). The similarity in substrate periodicities suggests that some nucleating agents may induce either the α or β phase under different crystallization conditions. 4‐Fluorobenzoic acid, dicyclohexylterephthalamide, and γ‐quinacridone (the latter two are known as β‐iPP nucleators) were tested over a wide range of crystallization temperatures [up to crystallization temperature (T_c) > 145 °C]. The two former nucleating agents induce exclusively α‐iPP and β‐iPP, respectively. γ‐Quinacridone on the contrary is a versatile agent with respect to the crystal phase generated. More specifically, the same crystal face of γ‐quinacridone induces either β‐iPP or α‐iPP when T_c is below or above ≈140 °C. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2504–2515, 2002