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.     

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.     

Chain conformational transformations in syndiotactic polypropylene/layered silicate nanocomposites during mechanical elongation and thermal treatment

Polymer nanocomposites prepared by melt-mixing syndiotactic polypropylene (sPP) with a quaternary modified montmorillonite have been studied with FT-IR and XRD spectroscopic techniques. FT-IR spectroscopic analysis has shown that the addition of the nanoclay results in a higher helical content for the syndiotactic polypropylene matrix. Furthermore, FT-IR spectroscopy showed that the presence of the nanoclay hinders the polymeric chains from achieving the degree of transformation from helical to trans-planar form during the application of mechanical stress compared to the neat sPP case. Accordingly, the sPP nanocomposites show a higher tendency relative to neat sPP to return to the initial helical conformation upon either releasing the applied mechanical tension or upon exposing to heat at 120 °C. Additionally, XRD patterns provided evidence that the use of low concentration of nanoclay (1%) resulted in partially exfoliated nanocomposites, while only intercalated nanostructures were produced at high nanoclay contents (10%). However, the application of stress can improve the degree of exfoliation of an sPP nanocomposite. In addition, linear dichroic infrared measurements which allow the monitoring of the influence of the nanoclay on the orientation of the polymeric chains during the application of stress showed that the trans-planar infrared bands exhibit lower orientation in comparison to the same bands in neat sPP, while the addition of nanoclay has no particular influence on the orientation of the infrared bands that are related to helical conformations. Finally, dynamic mechanical analysis (DMA) verified the enhanced mechanical properties of the sPP nanocomposites relative to neat sPP, whereas differential scanning calorimetry (DSC) depicted a slight increase in the glass transition temperature of the polymeric matrix in these nanocomposites, especially for low clay concentrations.     

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.     

Morphology and mechanical behavior of isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends and fibers

The crystallization morphologies and mechanical behaviors of iPP/sPP blends and the corresponding fibers were investigated in the present work. For all the investigated iPP/sPP blends, the starting crystallization temperature of sPP during cooling process was significantly increased with increasing iPP content. The iPP/sPP blends are strongly immiscible at the conventional melt processing temperatures, in consistence with the literature results. As isothermally crystallized at 130 °C, sPP still keeps melt state, while iPP component is able to crystallize and the spherulites become imperfect accompanied by decreasing of the crystallite size as sPP content increases. The addition of sPP decreases the crystallinity of iPP/sPP blends and fibers. The storage modulus, E′, of the iPP/sPP blends is higher than that of sPP homopolymer in the temperature range from −90 to 100 °C. The iPP/sPP fibers can be prepared favorably by melt-spinning. As sPP content exceeds 70%, the elastic recovery of the iPP/sPP fibers is approximately equal to that of sPP homopolymer fiber. The drawability of the as-spun fiber of iPP/sPP (50/50) is better than that of sPP fiber, which improves the fiber processing performance and enhances the mechanical properties of the final product. The drawn fiber of sPP presents good elastic behavior within the range of 50% deformation, whereas the elastic property of the iPP/sPP (50/50) fiber slightly decreases, but still much better than that of iPP fiber.     

Crystallization and orientation development in fiber and film processing of polypropylenes of varying stereoregular form and tacticity

We investigated the crystallization and orientation development in melt spinning and tubular blown film extrusion of several different types of polypropylenes, including conventional high tacticity isotactic polypropylenes (iPP) and metallocene catalyst low tacticity iPPs and syndiotactic polypropylenes (sPP). The fiber and film samples were characterized by wide‐angle X‐ray diffraction (WAXD), birefringence and differential scanning calorimetry (DSC). In melt spinning iPP, we found that the mesomorphic structure of iPP is more readily formed in lower tacticity fibers, and significant amounts of hexagonal β‐form crystals are found in low tacticity iPP fibers spun at high draw‐down ratios. Low tacticity iPP fibers exhibited a significant decrease in the crystalline chain‐axis orientation at high draw‐down ratios, resulting from increased epitaxially branched lamellae. Melt‐spun sPP fibers exhibit Form I helical structure at low spinning speeds and Form III zigzag all trans structure at high spinning speeds. We found that the level of spinline stress is the governing factor for this structural change. Melt‐spun sPP fibers exhibit much higher chain‐axis (c‐axis) orientation factors (f_c) and lower birefringence than iPP fibers spun at the same spinline stresses. In tubular blown sPP films, the a‐axis of Form I unit cell tends to orient perpendicular to the film surface, while the b‐axis of monoclinic α unit cell does so in iPP blown films.     

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     

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.     

Orientation of multiwalled carbon nanotubes in composites with polycarbonate by melt spinning

A conductive polycarbonate (PC) composite containing 2 wt% multiwalled carbon nanotubes (MWNT) and pure PC were melt spun using a piston type spinning device. Different take-up velocities up to 800 m/min and throughputs leading to draw down ratios up to 250 were used. The composite material of PC with MWNT was prepared by diluting a PC based masterbatch consisting of 15 wt% MWNT by melt mixing in an extruder. The alignment of the nanotubes within melt spun fibers with draw down ratios up to 126 was investigated by TEM and Raman spectroscopy. The nanotubes align in their length axis along the fiber axis increasingly with the draw down ratio, however, the curved shape of the nanotubes still exist in the melt spun fibers. At higher draw down ratios, the MWNT started to align by reducing their curvature. Polarized Raman spectroscopy indicated that the D/D and G/G ratios parallel/perpendicular to the fiber axis increase for both MWNT bands in a similar manner with the draw down ratio. Interestingly, with increasing alignment electrical conductivity of the fibers is lost. Mechanical investigations revealed that at low spinning speeds elongation at break and tensile strength of the composite are lower than those of the pure PC. However, at the highest take-up velocity of 800 m/min the elongation at break is higher and true stress at break of the composite fiber is comparable to the pure PC fiber.     

Crystallization process of poly(?-caprolactone)-poly(ethylene oxide)-poly(?-caprolactone) investigated by infrared and two-dimensional infrared correlation spectroscopy

The crystallization of poly(?-caprolactone)-poly(ethylene oxide)-poly(?-caprolactone) (PCL-PEO-PCL) triblock copolymer was studied using FTIR and 2D FTIR spectroscopies. The weight ratio of PCL/PEO in the investigated sample was about 20:1. Although it is such a low amount of PEO that it cannot form any crystals, the PEO block undergoes some structural change in the cooling process. It was established that the PCL constituent crystallized quickly, and then forced the noncrystallizable PEO constituent to form a tighter structure (helical conformation) from the trans zigzag conformation. Besides, through the 2D IR analysis, more exact and detailed assignments of the overlapped CH₂ bands have been made - the 1193 cm⁻¹ band is attributed to methylene next to the carbonyl group, whereas the 1162 cm⁻¹ and 1295 cm⁻¹ bands are assigned to other common methylenes.     

Fiber structure formation in ultra-high-speed melt spinning of poly(ethylene 2,6-naphthalene dicarboxylate)

The high-speed melt spinning of poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) was performed up to the take-up velocity of the ultra-high-speed region, 9 km/min. From the investigations of the structure and physical properties of the as-spun fibers, the high-speed spinning of PEN was divided into three regions in terms of the mechanism of fiber structure formation. The first region is the take-up velocity of up to 2.5 km/min and the birefringence of up to 0.08 where only a slight increase in molecular orientation was attained. At the take-up velocity of 2.5–4.5 km/min and the birefringence of 0.08–0.25, although some experimental evidences indicated that the orientation-induced crystallization did not occur, there was an increase in the fiber density which suggested the formation of some ordered structure. At the take-up velocity > 4.5 km/min and birefringence > 0.25, the orientation-induced crystallization occurred. The fibers obtained in this region were characterized by the formation of the crystalline structure dominated by the β form. The presence of the necklike deformation in the spinning line was also confirmed. The solidification temperature of the spinning line analyzed from the diameter profile suggested that the formation of β modification crystals occurred at relatively low crystallization temperatures in comparison with that in an isotropic state. Therefore it was indicated that the presence of elongational stress in the spinning line promoted the formation of the β modification crystals. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1415–1427, 1997     

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.     

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.     

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.     

Conformation transition and crystalline phase variation of long chain branched isotactic polypropylenes (LCB-iPP)

The changes of conformation and crystalline structure of long chain branched isotactic polypropylene (LCB-iPP) under different crystallization temperatures and the effects of their special molecular architecture on the crystallization behavior were investigated by a combination of Fourier transform infrared spectroscopy (FT-IR), wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). In these polymers, long chain branching was introduced via in situ polymerization of polypropylene and an asymmetric diene monomer using the metallocene catalyst technology. Through the characterization of the specific IR band variation, it was proved that the conformational orders of helical sequences of LCB-iPP show great changes in different crystallization temperature ranges. In lower crystallization temperature range (100-130 °C), the intensities of all regular helical conformation bands of LCB-iPP increase with the increasing crystallization temperature and the regular helical conformation bands with more monomer units increase faster than that with less monomer units. In higher crystallization temperature range (130-150 °C), the intensities of all regular helical conformation bands of LCB-iPP decrease with the increasing crystallization temperature and the regular helical conformation bands with more monomer units decrease faster than that with less monomer units. The results of WAXD and DSC showed that LCB-iPP crystallizes from the melt as a mixture of α and γ forms. The content of the γ form increases with the increasing crystallization temperature, reaches a maximum value at 130 °C, and then decreases with a further increase of the temperature. At the same time, the crystallization of γ form is favored by the presence of the LCB structure of iPP. Moreover, the transitional temperatures of different helical conformations and crystallization structures of LCB-iPP show obvious correlations.     

Improving graphitization degree of mesophase pitch-derived carbon fiber by solid-phase annealing of spun fiber

The solid-phase annealing of the mesophase pitch spun fiber was examined between the glass transition (T_g) and softening (T_s) temperatures of the pitch to improve the graphitization degree of the graphitized fiber through recovering or further improving the stacking height of the mesogen molecules in the spun fiber, since the rapid spinning reduced markedly stacking height in the as-spun fiber. A naphthalene mesophase pitch as received carried stacking height of 2.9 nm which was markedly reduced to 1.7 nm by spinning at 230 m/min, giving Lc=40 nm for its graphitized fiber. Annealing at 206 °C improved the stacking height of the spun fiber to 2.4 nm and Lc(002) of the graphitized fiber to 54 nm. Annealing of the methylnaphthalene mesophase pitch fiber at 200 °C was much more effective in improving the stacking height from 3.5 to 5.0 nm and its graphitized fiber to Lc=91 from 40 nm. Such an improved graphitization degree led to improved thermal conductivity and tensile modulus of the graphitized fiber. It must be noted that the annealing of the spun fiber reduced its stabilization rate, indicating densification of molecular stacking in the fiber. The transformation scheme of mesophase pitch into graphite fibers is discussed to clarify the roles of molecular stacking in the clusters and their arrangement in the mesophase pitch fiber during the carbon manufacturing process.     

Structure and dynamic mechanical properties of poly(ethylene terephthalate-co-4,4′-bibenzoate) fibers

Poly(ethylene terephthalate-co-4,4′-bibenzoate) (PETBB) fibers containing 5, 15, 35, 45, 55, and 65 mol% bibenzoate (BB) were melt spun. Fiber structure has been determined using wide angle X-ray diffraction, birefringence, and FTIR spectroscopy. When drawn to their respective maximum draw ratios, the structures and properties of high BB containing fibers (PETBB45, 55 and 65) are significantly different than those of PET and low BB containing fibers (PETBB5, 15, and 35). For example, 90% of the ethylene glycol units in high BB containing fibers are in the trans conformation, while only 80% of these units are in trans conformation in PET and low BB containing fibers. Overall orientation of the high BB containing fibers is higher (orientation factor f > 0.85) than those of PET and low BB containing fibers (f < 0.6). Orientation of the crystalline regions is quite high (f_cr ∼ 0.95) for both groups of fibers, while orientation of the amorphous regions (f_am) of high BB containing fibers is higher (∼0.8) than those of the PET and low BB containing fibers (∼0.4). High BB containing fibers exhibit much higher storage modulus and modulus retention with temperature than low BB containing fibers. Glass transition temperature determined from the dynamic loss tangent peak decreased with increasing BB content, while this transition completely disappeared in the high BB containing fibers. The magnitude of the secondary transition, observed at about −50 °C, decreased with increasing BB content. Another secondary transition, not observed in PET, was observed at about 70 °C in high BB containing fibers. These dynamic mechanical results have been rationalized in terms of the observed structural parameters.     

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

Normal modes of vibration of syndiotactic polypropylene (sPP) and their dispersion are obtained in the reduced zone scheme for helical form I having the conformational sequence (t₂g₂) using Urey-Bradley force field and Wilson's GF matrix method as modified by Higgs. Optically active frequencies corresponding to the zone center and zone boundary are assigned and characteristic features of the dispersion curves are discussed. In general the dispersion in the helical form is less as compared to the planar form. Heat capacity has been calculated via density-of-states using Debye relation in the temperature range 10-460 K and compared with the experimental measurements.     

Internal fine structures in the high-speed-spun fibers of poly(ethylene 2,6-naphthalene dicarboxylate)

Morphological study on the poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) fibers, which were prepared at spinning speeds ranging from 2 to 10 km/min, was performed by combining the surface etching techniques with the surface observation methods. From the results by scanning electron microscopy (SEM) of the etched fiber surfaces, it was clarified that well-developed fibrillar structure which runs continuously along the fiber axis exists in the high-speed-spun (HSS) PEN fibers spun at a spinning speed beyond 4 km/min. Fine network structure was observed on the lateral surface of each fibril by SEM. From the results by atomic force microscopy (AFM), it is proposed that several crystalline lamellae are closely stacked in each of the constituents which form the fine network structure.     

Crystallization from the melt of α and β forms of syndiotactic polystyrene

The crystallization of trans-planar α and β forms of syndiotactic polystyrene is studied through X-ray diffraction and DSC analyses of melt-crystallized samples. The factors controlling the crystallization of the two forms are analyzed. Pure α and β forms of syndiotactic polystyrene can be easily obtained setting the maximum temperature at which the melt is heated and the permanence time of the melt at this temperature. The crystallization of the α and β forms does not depend on the crystallization temperature, at least in the range of accessible crystallization temperatures, between 240 and 270 °C, but only depends on the presence of the ‘memory’ of the α form in the melt. The most important factors are, indeed, the crystalline form of the starting material used in the melt crystallization experiments and the maximum temperature of the melt. Relevant recrystallization phenomena, occurring during the melting of the samples crystallized from the melt at low crystallization temperatures, are responsible for the complex melting behavior of the α and β forms. The recrystallization involves only lamellar thickening of the crystals of the same form (α or β) and not structural transformation.