Melt crystallization of syndiotactic polypropylene in nanolayer confinement impacting structure

Layer multiplying coextrusion technique was used to fabricate films with hundreds of alternating layers of a crystallizable polymer, syndiotactic polypropylene (sPP), and an amorphous polymer, polycarbonate (PC). Atomic force microscopy and wide-angle X-ray scattering revealed the absence of any oriented crystal morphology of sPP in the extruded layered films. An approach of isothermal melt recrystallization of sPP nanolayers revealed the formation of oriented lamellae under the rigid confinement of hard glassy PC layers. X-ray scattering data showed that sPP crystallized as stacks of single crystal lamellae oriented parallel to the layers at high crystallization temperatures. As the crystallization temperature decreased, on-edge lamellar orientation was preferred. Formation of in-plane lamellae was attributed to heterogeneous bulk nucleation, while nucleation of on-edge lamellae was initiated at substrate interface. It was observed that as the layers thickness reduced, the orientations of both in-plane and on-edge lamellae became sharper.Detailed analysis of crystal orientations in 30 and 120 nm sPP layers was carried out. Melt recrystallization of 30 nm layers revealed formation of in-plane lamellae above 90 °C and mainly on-edge lamellae below 70 °C. At intermediate temperatures, formation of mixed crystals was reported. In 120 nm layers, crystallization temperature of 100 °C was required to form in-plane crystals, while on-edge lamellae were formed below 90 °C.We also investigated crystallization onset for on-edge and in-plane lamellar nucleation. Although, the two crystal fractions were significantly affected as a function of crystallization temperature, it was noticeable that both crystal habits were initiated at the same time. The results suggested that the relative growth rates of in-plane and on-edge crystal orientations was responsible for different fractions of the two crystal orientations at a given crystallization temperature.Oxygen transport properties of melt recrystallized sPP layers were measured. When the melt recrystallization temperature increased from 85 to 105 °C in 120 nm sPP layers, at least one order of magnitude enhancement in the barrier properties was observed. It was evident from the X-ray data that the amount of in-plane crystal fraction increased with increasing crystallization temperature. In-plane crystals acted as impermeable platelets to oxygen flux resulting in improved gas barrier properties. A similar effect was observed in 30 nm sPP layers over a temperature range of 60-105 °C. A correlation between in-plane crystal fraction and the oxygen permeability was obtained from X-ray and oxygen transport data analysis. It was shown that the permeability decreased exponentially with increasing in-plane crystal fraction.     

Stability of form II of syndiotactic polypropylene confirmed by cold and melt crystallization in supercritical carbon dioxide

The form II of syndiotactic polypropylene (sPP) has been found more thermodynamically stable than form I when melt crystallized at pressures above 150 MPa, while the reverse occurs below 150 MPa. In the present study, through the cold and melt crystallization in supercritical CO₂ the stability of various polymorphic forms of sPP, especially form II, was confirmed by using Fourier-transform infrared spectroscopy and wide-angle X-ray diffraction. Compared with the formation of pure form I at high temperatures under ambient condition, a mixture of forms I and II was formed by both the cold and melt crystallization in supercritical CO₂. This atmosphere changed the relative stability of forms I and II, and made the form II more thermodynamically stable than form I. The increased solubility parameters of the surroundings, at which the form II was formed, also confirmed the stability of form II over form I in supercritical CO₂. The incubation pressure was the key factor affecting the formation and amount of form II. Supercritical CO₂ provides a combining severe condition to obtain the form II crystal, although its pressure was much lower than the elevated pressures (>150 MPa) reported before.     

Influence of structural organization on tensile properties in mesomorphic isotactic polypropylene

The effects of annealing on the structure and mechanical properties of mesomorphic isotactic polypropylene have been investigated using wide-angle and small-angle X-ray scattering and rheo-optics in addition to tensile tests. Young's modulus of mesomorphic phase was estimated to be 5 GPa using Takayanagi model. The α-crystallitic iPP prepared by annealing the quenched mesomorphic iPP was transparent because of the absence of spherulitic structure. It was found that the mechanical yielding of α-crystallitic iPP is dominated by the plastic flow of crystalline structural units whereas the yield process of α-spherulitic iPP quenched at 80 °C is caused by the fracture or fragmentation of crystalline structural units.     

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.     

Evidence for a mechanically active high temperature relaxation process in syndiotactic polypropylene

Frequency- and temperature-dependent dynamic mechanical tests on syndiotactic-polypropylene (s-PP) show the existence of a relaxation process at the high temperature or low frequency-side of the dynamic glass process. Its frequency is about 10⁻¹ Hz at 100 °C and varies with temperature in an unusual manner, non-Arrhenius like, with an upward curvature with increasing temperature. The blocky substructure found for the crystalline lamellae of s-PP and the strain control of the deformation behavior, which needs more degrees of freedom than offered by stacks of internally stiff crystal lamellae, favors the assignment of the new process to intralamellar block slips. The assignment finds support in the observed retardation of the process accompanying a crystal thickening and the isotropy of the associated mechanical relaxation strength in an oriented sample.     

Normal mode analysis of syndiotactic polypropylene (T2G2T6G2) form

Syndiotactic polypropylene of all-trans form is transformed to (T₂G₂T₆G₂) form (new form) on exposing to xylene vapor under tension. This new form was characterized by infrared spectra and normal mode analysis. By increasing the gauche component from the all-trans to the new form, the infrared intensities of some bands increase. The potential energy distributions (PED) and L vectors for six monomers, which compose the repeating unit, were investigated. As a result, the PED and L vectors of the conformation-sensitive bands were found to be localized at the gauche position.     

Mechanically and thermally induced chain conformational transformations between helical form I and trans-planar form III in syndiotactic polypropylene using FT-IR and Raman spectroscopic techniques

The mechanically as well as the thermally induced chain conformational transformations in syndiotactic polypropylene (sPP) were monitored using both Raman and FT-IR spectroscopic techniques. Samples of syndiotactic polypropylene of helical crystalline conformations were elongated at different elongations up to 600%, while Raman and infrared spectra were simultaneously recorded. Quantitative analysis of these spectra using the ratio of helical to trans-planar content known as the ‘Transformation Index’ calculated from both Raman and infrared methods revealed that the relative proportion of the helical to the trans-planar conformation dramatically alters when the 100% elongation is reached, while any residual helical proportion has vanished above 500% elongation. Furthermore, we report for the first time an overall comprehensive Raman study of the helical to the trans-planar transformation of sPP during elongation up to 600% and consequent relaxation. In addition, a study of the thermal treatment of an sPP sample in the trans-planar conformation also took place. Furthermore, molecular orientation studies with the use of polarized infrared spectroscopy have been performed in order to monitor the changes in the orientation of the molecular chains during the application of stress. Both Raman and infrared experiments have demonstrated that when the tension is removed, a mixed stage of conformations is observed. Finally, the use of the above-mentioned techniques allows the monitoring of the thermally induced phase transformation from trans-planar configurations to helical in the temperature range between 60 and 90 °C. These spectroscopic findings are in agreement with differential scanning calorimetry thermal studies, which also confirm the thermally induced phase transformation.     

Stress-induced structure transition of syndiotactic polypropylene via melt spinning

Syndiotactic polypropylene (sPP) fiber was prepared by melt spinning with the taken-up velocity of 200-700 m/min, the conformation and crystallization of which were systematically investigated by a combination of Fourier transform infrared (FTIR) spectroscopy, wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The results indicated that sPP fibers consist of form I crystal with helical conformation at the spinning velocity of 200-300 m/min, and the crystallinity and orientation are improved with the increase of spinning velocity in this range. As the spinning velocity exceeds 300 m/min, sPP fibers contain mainly mesophase with trans-planar conformation and the content of form I decreases correspondingly. The crystallization behavior of sPP fiber with spinning velocity is different from that of most other crystalline polymers, i.e., the theory of orientation-induced crystallization is not well conformed to. For sPP, form I comprising of helical conformation is thermodynamically stable, though extensional stress can lead to transition from helical to trans-planar conformation, which is not favorable for the crystallization of form I.     

Variations of regular conformation structures in melt of syndiotactic polypropylene

Regular structures in polymer melt play important roles during crystallization and subsequently influence performances of polymer materials. In the present work, differential scanning calorimetry (DSC) and variable temperature Fourier transform infrared spectroscopy (VT-FTIR) were used to characterize the variations of regular structures in melt of syndiotactic polypropylene (sPP) during heating. It was found that during heating, the structural transition in sPP melt undergoes four stages: (1) destruction of long regular structures; (2) formation of short regular structures; (3) transition from short regular structures to isotropic state; and (4) isotropic state. These regular structures in sPP melt have distinct memory effects on the crystallization behaviors. By using Flory's RIS model, it was found that in final isotropic state, the most common conformers are ttgg and tttt, corresponding to helical and planar-zigzag conformations, respectively. These findings are helpful to deeply understand the essence of structures in sPP melt and microstructure variations during melting.     

Crystallization and phase behavior of crystalline syndiotactic 1,2-polybutadiene/isotactic polypropylene blends in solution-cast thin films

Crystallization and phase behavior in solution-cast thin films of crystalline syndiotactic 1,2-polybutadiene (s-1,2-PB) and isotactic polypropylene (i-PP) blends have been investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM) techniques. Thin films of pure s-1,2-PB consist of parallel lamellae with the c-axis perpendicular to the film plane and the lateral scale in micrometer size, while those of i-PP are composed of cross-hatched and single-crystal-like lamellae. For the blends, TEM and AFM observations show that with addition of i-PP, the s-1,2-PB long lamellae become bended and i-PP itself tends to form dispersed convex regions on a continuous s-1,2-PB phase even when i-PP is the predominant component, which indicates a strong phase separation between the two polymers during film formation. FESEM micrographs of both lower and upper surfaces of the films reveal that the s-1,2-PB lamellae pass through i-PP convex regions from the bottom, i.e. the dispersed i-PP regions lie on the continuous s-1,2-PB phase. The structural development is attributed to an interplay of crystallization and phase separation of the blends in the film forming process. With solvent evaporation, s-1,2-PB would crystallize first forming the continuous phase, while the segregated i-PP phase accumulated on s-1,2-PB and crystallized subsequently, forming dispersed i-PP regions.     

Novel morphology of isotactic polypropylene crystal generated by a rapid temperature jump method

Novel morphology of isotactic polypropylene (iPP) crystal, which is quite different from that of usual spherulite, has been observed by scanning electron microscope (SEM). The crystals show ‘bamboo leaf-like (BL)’ shape with α-monoclinic high crystallinity. The BL crystals are formed by neither melt nor glass crystallization, but by a complicated annealing process that goes through mesomorphic phase of iPP. Substrates are not essential for the formation of BL crystals, since the BL crystals are formed both on glass surface and free surface as well as in bulk. Along with the annealing process, a possible explanation for the mechanism of the formation of the BL crystal is proposed.     

Melt compounding of syndiotactic polypropylene nanocomposites containing organophilic layered silicates and in situ formed core/shell nanoparticles

Syndiotactic polypropylene (sPP) compounds containing organophilic layered silicates were prepared by means of melt extrusion at 220 °C using a corotating twin screw extruder in order to examine the influence of the silicate modification and the addition of maleic-anhydride-grafted isotactic polypropylene (iPP-g-MA) as compatibilizer on morphology development and mechanical properties. Synthetic sodium fluoromica was used as water-swellable layered silicate, which was rendered organophilic by means of cation exchange with protonated octadecylamine. Only compounding of the modified silicate in conjunction with iPP-g-MA afforded exfoliation and dispersion of individual silicate layers, encapsulated in an iPP-g-MA shell, within the polypropylene matrix. Interlayer distance increased with increasing content and increasing molecular weight of the compatibilizer. The Young's modulus of the nanocomposite increased fivefold from 490 to 2640 MPa. This was attributed to silicate nanoreinforcement and nucleation of sPP crystallization via the iPP-g-MA shell of the dispersed organophilic silicate nanoparticles. The yield stress was increased to 29 MPa with respect to 16 MPa for the bulk sPP. Morphology and mechanical properties were examined as a function of the silicate—and compatibilizer content.     

Nonisothermal melt crystallization kinetics of syndiotactic polypropylene/alumina nanocomposites

Nonisothermal melt crystallization kinetics of syndiotactic polypropylene (sPP)/alumina nanocomposites were investigated via differential scanning calorimetry. The addition of alumina nanoparticles significantly increases the number of nuclei and promotes the crystallization rate of sPP. Nonisothermal melt crystallization kinetics was analyzed by fitting the experimental data to a Nakamura model using Matlab. The average values of Avrami exponent n are 1.7 for both sPP and sPP/Al₂O₃ nanocomposites during slow cooling, which implies a two‐dimensional growth is the predominant mechanism of crystallization following a heterogeneous nucleation. The two nanocomposites give n values equal to 2.3 during faster cooling, indicating that the main growth type taking place for sPP/alumina nanocomposites is also the two‐dimensional growth. The subsequent melting behavior shows that the presence of alumina nanoparticles changed both the cold crystallization and the recrystallization of sPP. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Isothermal melt crystallization and melting behaviour of syndiotactic polypropylene

The lamellar morphological information and subsequent melting behaviour of syndiotactic polypropylene (s‐PP) samples isothermally crystallized at crystallization temperatures ranging from 30 to 95 °C have been investigated using a combination of wide‐angle X‐ray diffraction (WAXD), small‐angle X‐ray scattering (SAXS) and differential scanning calorimetry (DSC) techniques. Three known methods for determining the equilibrium melting temperature T_m°, namely the Gibbs–Thomson extrapolation, the linear Hoffman–Weeks extrapolation and the non‐linear Hoffman–Weeks extrapolation, have been employed to evaluate this important thermodynamic parameter, and the results obtained are compared. Finally, an estimate of the equilibrium melting temperature for a perfect s‐PP sample (T_m°)_100% is given. © 2000 Society of Chemical Industry     

Morphology of syndiotactic polypropylene/alumina nanocomposites

The effect of the addition of 50 nm spherical alumina nanoparticles with hydrophilic or hydrophobic surfaces on the morphology of syndiotactic polypropylene (sPP) was investigated. The filler content in the nanocomposites was 3 wt%. Polarized Optical Microscopy and Small Angle Light Scattering (SALS) studies showed that sPP and the nanocomposites form hedrites. The addition of alumina nanoparticles significantly increased the number of hedrites. A higher number of nucleation sites in the nanocomposites promote a higher crystallization rate, and thus hedritic growth was stopped at the early stage of crystallization. Quantitative evaluations by SALS analysis show that the object size is decreased by not only the crystallization conditions but also the presence of Al₂O₃ nanoparticles. A small amount of Al₂O₃ nanoparticles did not noticeably affect the crystallinity of sPP, but increased the melting point. Transmission Electron Microscopy images showed that the lamellar thickness did not change significantly with the incorporation of nanoparticles. The lamella thickness, however, depends on cooling rates. X-ray diffraction characterization indicated that the sPP and the nanocomposites were crystallized in disordered “form I”. The incorporation of alumina nanoparticles had a small effect on the crystal structure of syndiotactic polypropylene.     

Influence of syndiotacticity and annealing temperature on the double melting peak behaviour of syndiotactic polypropylene

Fractionated syndiotactic polypropylene (sPP) samples with homogeneous tacticity were annealed at different temperatures. The influence of syndiotacticity and annealing temperature on the double melting peak phenomena were investigated. It is found that all fractions show double melting peaks at 75°C annealing temperature, while the low peak disappears when the fractions with higher syndiotacticity are annealed at 85°C and above. The fraction with the lowest syndiotacticity remains the same at any annealing temperature. In combination with wide-angle X-ray diffraction experiments, the double peaks are believed to correspond to the melting of cell II and III. The results indicate that higher temperature and syndiotacticity are the external conditions and internal structural factor that permit cell II to transform into cell III. © 1999 Society of Chemical Industry     

Crystalline memory effects in isothermal crystallization of syndiotactic polypropylene

Isothermal crystallization behavior after partial or complete melting of syndiotactic polypropylene was investigated by differential scanning calorimetry (DSC). On partial melting, the total concentration of predetermined nuclei was found to decrease with increasing fusion temperature and increasing time period that the sample spent at a specific fusion temperature. A significant effect of the rate of heating to the fusion temperature was also observed. On complete melting, the total concentration of predetermined nuclei was found to approach a constant value, which is the concentration of infusible heterogeneous nuclei (e.g., impurities, catalyst residues) present originally in the sample. At a specific fusion temperature, the concentration of predetermined athermal nuclei was found to decrease exponentially with the time period spent in the melt. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 337–346, 2000     

Effects of silver nanoparticles on the dynamic crystallization and physical properties of syndiotactic polypropylene

The effects of silver (Ag) nanoparticles on the physical properties of syndiotactic PP (sPP) were investigated concentrating on the isothermal melt crystallization behavior under shear. sPP with 5 wt % Ag nanoparticles presented higher crystallization temperature (T_c) and heat of crystallization (ΔH_c) than pure sPP. At 90°C, the Ag nanoparticles had little effect on the induction time of crystallization but a little increased the half‐time (t_1/2) for the crystallization. At 100°C, however, the induction time was decreased with increasing the Ag content and the t_1/2 was decreased up to the Ag content of 0.5 wt %. DSC melting endotherms exhibited double melting peaks when crystallized at 90°C under shear but a single melting peak when crystallized at 100°C. The WAXD patterns exhibited that the presence of Ag nanoparticles did not produce any change in the crystal structure of sPP. The tensile strength of sPP is little changed up to the Ag content of 0.1 wt % but it was decreased with further addition. In addition, the introduction of less than 0.1 wt % Ag increased the elongation at break, but further addition decreased it abruptly. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Unbiased evaluation of literature data on equilibrium melting temperature and enthalpy of fusion of perfectly syndiotactic polypropylene

The thermodynamic properties of highly syndiotactic polypropylene (PP) were reevaluated based on the data taken from the literature. The thermodynamic equilibrium melting temperature of a perfectly syndiotactic PP, which was estimated based on the Flory theory for the depression of the melting point in random copolymers, was 168.0°C. However, it was found to be 174.2°C when a linear extrapolation was attempted on a plot of the observed equilibrium melting temperature against the syndiotacticity level. The thermodynamic enthalpy of fusion of a perfect crystal of fully syndiotactic PP was estimated to be 8.7 kJ mol⁻¹, and the average value of the literature data was 7.8 kJ mol⁻¹. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1603–1609, 2001     

Nonisothermal melt‐crystallization kinetics of syndiotactic polypropylene compounded with various nucleating agents

Nonisothermal melt‐crystallization behavior of syndiotactic polypropylene (sPP) compounded with 5% by weight (wt %) of some inorganic fillers [i.e., kaolin, talcum, marl, titanium dioxide (TiO₂), and silicon dioxide (SiO₂)] and 1 wt % of some organic fillers, which are some sorbital derivatives (i.e., DBS, MDBS, and DMDBS) was investigated and reported for the first time. It was found that the ability of these fillers to nucleate sPP decreased in the following sequence: DBS > talcum > MDBS > SiO₂ ～ kaolin ～ DMDBS > marl > TiO₂, with DBS being able to shift the crystallization exotherm by ～ 18°C on average, while TiO₂ was able to shift the crystallization exotherm by only ～ 6°C on average, from that of neat sPP. The Avrami analysis revealed that the Avrami exponent for sPP compounds varied between 2.9 and 4.3, with the values for neat sPP varying between 3.1 and 6.8. Lastly, the Ziabicki's crystallizability of sPP compounds was greater than that of neat sPP, suggesting an increase in the crystallization ability of sPP as a result of the presence of these fillers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 245–253, 2005