Monitoring elasticity and orientation in syndiotactic polypropylene

The effect of temperature on the elasticity and structure of syndiotactic polypropylene (sPP) is investigated using a combination of WAXD and rheo-optical FTIR spectroscopy. sPP has a rich crystal structure, which leads to unique mechanical behavior. Beyond yield point, it exhibits elastic response associated with deformation-induced structure-structure transformation. The structure and orientation were measured both during and after uniaxial tensile stretching of films (up to 200%) as a function of temperature (25-70 °C). Our WAXD and rheo-FTIR results suggest that as the temperature increases, there is a reduction in the extent of helical to trans-planar conformational change upon stretching. When the strain is released, there is partial transformation of trans-planar conformation back to helical. The presence and orientation of the trans-planar conformation plays a key role on the elastic behavior of sPP beyond the yield point. Rheo-optical FTIR dichroism studies provide further insights into the elasticity in syndiotactic polypropylene. Different proportions of helical and trans-planar conformations orient to different extents. The helical conformation does not orient appreciably at higher temperature though they are present beyond the yield point. In contrast, the trans-planar chains show a significant increase in dichroism beyond the yield point, suggesting that there is a difference in mobility (orientation) of the helical and trans-planar chains. This further supports the importance of trans-planar chains on the elastic behavior.     

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.     

Structure of syndiotactic propylene-ethylene copolymers: Effect of the presence of ethylene units on the structural transitions during plastic deformation and annealing of syndiotactic polypropylene

Syndiotactic propylene-ethylene copolymers have been synthesized with a single-center C_s-symmetric syndiospecific metallocene catalyst. A study of the effect of the presence of ethylene comonomeric units on the polymorphic behavior of syndiotactic polypropylene (sPP) and on the structural transitions occurring during stretching is reported. For copolymer samples with low ethylene contents, in the range 2-7 mol%, crystals of the helical form I, present in the melt-crystallized samples, transform into the trans-planar form III by stretching at high deformation. Form III transforms in part into the helical form II by releasing the tension, as it occurs for sPP. Samples with ethylene contents in the range 8-10 mol% are crystallized from the melt as a mixture of crystals of helical form I and form II. Both helical forms transform by stretching at low values of deformation (lower than 300%) into the trans-planar mesomorphic form, which transforms into the trans-planar form III by further stretching at higher deformations (higher than 500%). For these samples form III transforms back into the mesomorphic form, rather than into the helical forms, by releasing the tension. Unoriented samples of copolymers with ethylene content in the range 13-18 mol% are mainly crystallized in the helical form II, which transforms into the trans-planar mesomorphic form by stretching. Upon releasing the tension the mesomorphic form remains stable and no polymorphic transition is observed. The presence of ethylene comonomeric units stabilizes the trans-planar forms in fibers of the copolymer samples. This has been confirmed by the result that for high ethylene contents the trans-planar form III and the mesomorphic form do not transform in helical forms by annealing of fibers stretched at high deformations.     

Chain conformations of syndiotactic poly(m-methylstyrene) in the crystalline state

A conformational energy analysis of the isolated chain of syndiotactic poly(m-methylstyrene) under the constraint of a crystalline field is reported. Two different minimum energy conformations having similar energy have been found; the trans-planar conformation with tcm symmetry and the two-fold helical conformation with s(2/1)2 symmetry, according with the observed polymorphic behavior of this polymer. The calculated chain axes are in agreement with the experimental axes of 5.1 and 7.9 Å found for the different polymorphic forms of syndiotactic poly(m-methylstyrene). However, only a metastable disordered modification (form III) having chains in trans-planar conformation has been described. This indicates that, even though the trans-planar conformation is, in the isolated chain as stable as the helical conformation, the packing of the chains in helical conformation is probably more efficient than that of the trans-planar chains.     

In situ FTIR spectroscopic study of the conformational change of syndiotactic polypropylene during the isothermal crystallization

The isothermal crystallization of syndiotactic polypropylene (sPP) was investigated by in situ Fourier transform infrared spectroscopy (FTIR) and wide angle X-ray diffraction (WAXD). It was found that the ordered helical structure developed during the induction period of the isothermally crystallization of sPP. Moreover, the normalized intensity profiles of the characteristic FTIR absorption bands of the sPP helical conformation at 1005, 867 and 812 cm^?1 are not synchronized during the induction period of the sPP crystallization, which suggest that these bands should be corresponding to the helical chain with various persistence lengths. The non-zero value of the initial normalized intensity for the band at 867 cm^?1 indicates that there are sPP chains in the short helical conformation in the initial amorphous state. However, the helical chains with longer persistence length can only be observed with increasing the annealing time in the induction period as suggested by the intensity changes of the bands at 1005 and 812 cm^?1. Particularly, the intensity of the characteristic absorbance band at 1005 cm^?1 starts to increase at an earlier time than that of 812 cm^?1. These observations are discussed in terms of the critical length by Doi theory. It can be estimated that the sPP melt system is stable when the persistence length of helical sequences is less than 16 monomer units. The results could be helpful on the understanding the pathway of polymer chains packing in the early stage of the crystallization of semi crystalline polymers.     

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.     

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.     

Oriented crystallization in fiber formation: Inferences from the structure and properties of melt spun syndiotactic polypropylene filaments

In processes, such as melt spinning, the crystallization behavior of syndiotactic polypropylene (sPP) is found to be substantially different from that of most other linear polymers. The anisotropic stress field in such processes leads invariably to extension as well as alignment (orientation) of the chains in the melt, both of which contribute usually to dramatic enhancement in the rate of crystallization. However, since the primary structure of the sPP chain in its most preferred crystal form is comprised of a “coiled helical,” ? (T₂G₂)₂? , sequence, stress‐induced chain extension can lead to conformational sequences that are not favorable for crystallization in this form. As a consequence, process conditions that generate higher stress levels can cause a diminution in the rate of crystallization of this polymer. Such conformation‐related aspects of oriented crystallization of sPP have been addressed through an analysis of the structure and properties of melt‐spun fibers, produced over a range of spinning speeds. The results serve to identify a refinement that is needed in current models of oriented crystallization and also a mechanism to promote the nucleation of crystallization of sPP. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2305–2317, 2001     

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     

Nanocomposites of syndiotactic polypropylene: Phase behavior and morphology

Through differential scanning calorimetry, the crystallization kinetics of syndiotactic polypropylene (sPP) nanocomposites with organically modified layered silicates has been investigated in both nonisothermal and isothermal conditions. A strong influence on the thermal behavior is exerted by the compatibilizer agent (low molecular weight isotactic polypropylene‐g‐maleic anydrhyde, iPP‐g‐MA), which increases the crystallization rate of sPP. A further strong increase in the crystallization rate is observed upon increasing the OLS content in the sample. The kinetics data have been analyzed through the Ozawa, Avrami, and Tobin methods, the first of them resulted ineffective in describing our data. Morphological analyses have been performed by polarized optical microscopy and atomic force microscopy. Spherulitic morphology is observed for neat sPP; for the sample containing only the compatibilizer the spherulites grow around the iPP‐g‐MA particles, revealing their nucleating effect on sPP crystallization; for the nanocomposites the spherulitic morphology is lost: the texture appears globular, compact; inside the globules lamellae are present, parallel one to the other. POLYM. ENG. SCI., 46:1433–1442, 2006. © 2006 Society of Plastics Engineers     

DSC Study of Syndiotactic Polypropylene/Organoclay Nanocomposite Fibers: Crystallization and Melting Behavior

The effect of anisotropic particles of organophilic layered silicate on the crystallization and melting behavior of prepared nanocomposite systems was studied. The matrix was syndiotactic polypropylene (sPP). Organophilic layered silicate M-QDA filler was prepared by modification of hectorite SOMASIF ME 100 with octadecyl amine. The compatibilizer was isotactic polypropylene (iPP) grafted with maleic anhydride (iPP-g-MA). The silicate was exfoliated in situ within the sPP during the melting process to produce anisotropic nanoparticles. The sPP/M-ODA nanocomposite was spun at different drawing ratios. The resulting fibers were examined by differential scanning calorimetry. It was found that neither the spinning process nor the presence of nanofiller affected the crystallinity of the sPP matrix of the nanocomposite in comparison with the neat sPP. At a raised drawing ratio of the fibers slightly increased crystallinity of matrix was observed; however, it was still lower than the neat sPP fibers prepared at the same drawing ratio. The presence of M/ODA nanofiller in sPP matrix increased the melting temperature of the fibers.     

The role of nanoclay in the generation of poly(ethylene terephthalate) fibers with improved modulus and tenacity

The effect of nanoclay concentration on the molecular orientation and drawability of poly(ethylene terephthalate) PET was examined using thermal and vibrational spectroscopic analysis. Although drawability at 83 °C in hot air increased by the addition of nanoclay, the maximum draw ratio was independent of nanoclay concentration. The average molecular orientation of the PET chain was found to mimic the trend in mechanical property improvements. Both Young's modulus and tenacity (i.e. strength) showed the maximum improvement at a 1 wt% loading of clay, which was shown to coincide with the maximum amount of molecular orientation. Nanoclay was shown to intercalate with PET and enhanced amorphous orientation that led to modulus and strength improvements. However, at higher concentrations of nanoclay the presence of large agglomerates prevented efficient orientation to the fiber axis and acted as stress concentrators to aid in cavitation and failure during testing. Raman spectroscopy showed that the as-spun unfilled PET fibers possessed significantly more trans rotamer content of the ethylene glycol moiety than the nanocomposite fibers.     

Evaluation of mechanical,morphological, and biodegradable properties of hybrid natural fiber polymer nanocomposites

Hybrid natural fiber polymer nanocomposites were prepared using various natural fibers (kenaf, coir, and wood), polypropylene, and montmorillonite nanoclay through the hot compression method. The effects of fiber hybridization and nanoclay content on the physico‐mechanical and biodegradable properties of the synthesized composites were investigated. Fourier‐transform infrared and scanning electron microscopic analyses indicated that the structure and surface morphology of composites were transformed after fiber hybridization and the subsequent nanoclay incorporation. X‐ray diffraction pattern revealed that the percent crystallinity of hybrid nanocomposites significantly increased. Furthermore, the tensile strength and tensile modulus also significantly improved for the hybrid nanocomposites due to the addition of montmorillonite nanoclay. The biodegradability and water absorption tests were conducted. The results show that biodegradability of the nanocomposites decreased and water absorption increased due to the addition of montmorillonite nanoclay. POLYM. COMPOS., 38:583–587, 2017. © 2015 Society of Plastics Engineers     

Toughening of syndiotactic polypropylene with chalk

We hypothesized that polymer crystal anisotropy is advantageous for toughening of polymer composites involving easy slip network of oriented crystalline layers around filler particles. To this end, composites of syndiotactic polypropylene (sPP) with high concentration of submicrometer calcium carbonate particles were prepared and examined because usual sPP crystals exhibit high packing anisotropy. The specific orientation of sPP lamellae around chalk grains was found, which is supposed to facilitate the plastic deformation of polymer matrices. The compression molded bars of the composite exhibited markedly higher Izod impact strength than those of neat sPP. Toughening was even enhanced in the injection molded composite, for which 4.5‐fold increase in the impact strength was achieved. Injection‐induced orientation of the disordered form I sPP crystals was enhanced in the composite. The injection molded tensile specimens exhibited also a good drawability. Debonding at chalk–sPP interface occurred both during the impact and tensile tests facilitating the plastic deformation of sPP matrix. Chalk did not have any significant influence on the thermal properties of the composites but it affected the rheological behavior, increasing the loss and storage moduli, and the viscosity. Highly filled sPP composite exhibited solid‐like behavior in a molten state with the storage modulus exceeding the loss modulus in the entire frequency range. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43651.     

Influence of the quenching temperature on the crystallization of the trans-planar mesomorphic form of syndiotactic polypropylene

The crystallization from the melt of the trans-planar mesomorphic form of syndiotactic polypropylene is investigated at different quenching temperatures. The formation of the trans-planar mesomorphic form at −5, 0 and 6 °C is followed as a function of the residence time at these temperatures by X-ray diffraction and FTIR spectroscopy. The quenching temperature influences the rate of formation of the mesomorphic form as well as the maximum amount of the obtained mesomorphic form. By increasing the quenching temperature, in the examined range between −5 and +6 °C, an increase in the rate of formation of the mesomorphic form is observed. The maximum amounts of mesomorphic form obtained at 6 and −5 °C are lower than the amount achieved at 0 °C, which corresponds to nearly 100% of the total crystalline phase.     

Vibrational dynamics and heat capacity in syndiotactic poly(propylene) form II

Normal modes and their dispersion are obtained for planar-zigzag form II (tttt) of syndiotactic polypropylene (sPP) in the reduced zone scheme using Urey-Bradley force field and Wilson's GF matrix method as modified by Higgs. It is observed that this all trans backbone conformation can be characteristized by a band at 1233 cm⁻¹ (calculated at 1239 cm⁻¹). A comparison is made with the spectra of its isotactic and helical form. Characteristic features of the dispersion curves such as crossing, repulsion and von Hove type singularities (regions of high density-of-states) have been explained. Heat capacity obtained from the density-of-states agrees with the experimental data up to 250 K at which the glass transition sets in and the experimental curve exhibits a marked change in slope.     

Morphology,Rheological Properties,and Crystallization Behavior of Polypropylene/Clay Nanocomposites

Polypropylene/clay nanocomposites (PP/I.44P, PPCNs) were prepared in a twin-screw extruder using maleic anhydride grafted polypropylene (MAPP) as a compatibilizer. The intercalation of polypropylene into nanoclay particles was studied using X-ray diffraction. Rheological properties of the nanocomposites were investigated using a rheometer. The enhanced complex viscosity at low frequency regime indicated that the melt elasticity and melt strength of the nanocomposites were improved by adding nanoclay. The non-isothermal crystallization behavior of the nanocomposites was studied using differential scanning calorimetry (DSC) at various cooling rates and was analyzed with the Avrami method. It was found that the nanoclay acted as a heterogeneous nucleating agent resulted in higher crystallization temperature and higher crystallization rate than neat PP. Polarized optical microscopy revealed that the spherulites in the nanocomposites were finer than in the neat system.     

Fabrication and characterization of functionally graded nanoclay/glass fiber/epoxy hybrid nanocomposite laminates

In this work, hardness, tensile, impact, bearing strength and water absorption tests were performed to study the mechanical properties of stepwise graded and non-graded hybrid nanocomposites. Three different stepwise graded nanocomposites and one non-graded (homogeneous) nanocomposite with the same geometry and total nanoclay content of 10 wt% were designed and prepared. Moreover, one neat glass fiber laminate was manufactured. The results of the tests indicated that addition of the graded and non-graded nanoclay improves hardness over neat glass fiber reinforcement. The maximum increase in hardness of about 53% over neat specimen is obtained for specimens that have the highest weight percentage (2 wt%) of the clay nanoparticles on its surface (S-specimen and the side of F-specimen that reinforced with 2 wt% nanoclay). The gradation process results in an increase in hardness of about 11% compared with non-graded (homogeneous) specimen. In addition, an improvement of 11.9% in strain-to-failure is achieved with specimen having greatest amount of nanoclay in the middle over neat glass fiber/epoxy composite. The other nanoclay-filled glass fiber composites have strain-to-failure close to neat glass fiber/epoxy. The addition of nanoclay reinforcement has insignificant effect on ultimate tensile strength, tensile modulus, water absorption, bearing strength and impact strength compared with neat glass fiber/epoxy.     

Crystal structures and orientation development in tubular film extrusion of syndiotactic polypropylene and isotactic polypropylene

The differences in behavior of isotactic polypropylene (iPP) and syndiotactic polypropylene (sPP) in tubular film extrusion are qualitatively described. The crystalline form and orientation in the films were characterized using wide‐angle X‐ray diffraction (WAXD) patterns, pole‐figure analysis and birefringence. The sPP films had the crystalline form of the disordered Form I and the a‐crystallographic axis was found to be preferentially oriented in the film normal direction (ND) under the conditions of biaxial stresses. High transverse orientations were developed in the sPP films. In the iPP films, the monoclinic crystalline form was found and the b‐crystallographic axis was preferentially oriented in the ND. The birefringence of the films showed trends very similar to the crystalline orientations characterized by WAXD in both iPP and sPP films.