Syndiotactic Polypropylene Research Articles

Nonisothermal melt crystallization kinetics of syndiotactic polypropylene/alumina nanocomposites

AbstractNonisothermal 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

Adhesion enhancement of polyolefins by diamond like carbon coating and photografting polymerization

Polyolefins, such as polyethylene (PE) and polypropylene (PP) are widely used materials with low adhesion property. Diamond-like carbon (DLC)-deposited polyolefins are especially attractive for the effective control of the surface and the mechanical properties of the polyolefins, where the adhesion between the DLC and the polyolefins is the key parameter that determines the major properties. Recently we have found that the low-adhesion surfaces of some polyolefins such as high-density PE (HDPE) and linear low-density PE (LLDPE), could be drastically modified to show a good adhesion after diamond-like carbon (DLC) coating, although some other polyolefins such as low-density PE (LDPE), isotactic polypropylene (iPP), and syndiotactic polypropylene (sPP) could not be surface-modified for higher adhesion property by DLC coating, which was due to the low adhesion between polyolefins and DLC. To overcome the problem, photografting polymerization was introduced to improve the low adhesion property of LDPE, iPP, and sPP. The adhesive strengths between the polyolefins and the DLC films were remarkably increased by photografting acrylic acid (AA) onto the polyolefin surface before DLC coating. In addition, the fracture of the base polymer was observed for AA-grafted polyolefins synthesized at the grafting time of ~15–30min, indicating the strong adhesive force between photografted polyolefins and DLC.Furthermore, the adhesion forces between DLC and other widely-used polymer materials, such as polystyrene (PS), polymethylmethacrylate (PMMA), polyethyleneterephthalate (PET), polyurethane (PU), and polydimethylsiloxane (PDMS) which were all rather not easy to directly deposit DLC, were also found remarkably increased by photografting AA onto the polymer surfaces before DLC coating.

Inhomogeneous deformation of crystalline skeleton of syndiotactic polypropylene under uniaxial stretching

Structure changes of syndiotactic polypropylene (sPP) under uniaxial stretching are studied with the combination of micro-tensile tester and in situ wide angle X-ray diffraction (WAXD) measurement. Lamellae stacked “vertically” and “parallel” to the stretching direction (defined as “V” and “P” part) are separated on the basis of two-dimensional WAXD patterns. For all samples with different lamellar thickness, two critical points named as b1 and b2 were found in the stress–strain curves, while b1 and b2 points are the onsets of the rotation for the lamellae of “V” part and “P” part, respectively. The corresponding true stress and true strain for b2 point are bigger than that of b1, demonstrating that for samples with initial isotropic lamellar orientation, inhomogeneous deformation of crystalline skeleton induced by uniaxial stretching is universal. And after b1 point, “stress-induced melting” always occurs simultaneously with lamellar slips. Furthermore, the relationship between lamellar thickness and the true stress for b1 and b2 point was also studied, illustrating a linear correlation between ln σ and 1/l (σ is the corresponding true stress, l is the lamellar thickness), consistent with Young’s model. However, the critical true strains for these two points did not change with the varying thickness.

Living syndiospecific polymerization of propylene promoted by C 1 -symmetric titanium complexes activated by dried MAO

A series of C-1-symmetric titanium complexes with both salicylaldiminato and beta-enaminoketonato as the ligands have been synthesized and investigated as the catalysts for propylene polymerization. In the presence of dried methylaluminoxane (dMAO), the complex with bulky substituent tert-butyl ortho to alkyl oxygen can promote living polymerization of propylene with improved catalytic activity at ambient temperature, producing high molecular weight syndiotactic polypropylenes (rrrr 90.2%) with narrow molecular weight distributions (M-w/M-n = 1.071.22), via a propagation of 1,2-insertion of monomer and chain-end control of stereoselectivity. The propagation of polymer chain is completely different from that mediated by FI catalysts (the titanium complexes with phenoxy-imine chelate ligands) which favor 2,1-insertion of monomer. The interaction between a fluorine and a beta-hydrogen of a growing polymer chain, negligible chain transfer to monomer and dMAO without any free AlMe3 were responsible for the achievement of living propylene polymerization. The substituent ortho to alkyl oxygen determined the stereo structure of the resultant polypropylene. In the case of less steric congested complexes with two nonequivalent coordination positions, the growing polymer chain might swing back to the favorite coordination position (site-epimerization), forming m dyads regioirregular units. (c) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 638-648, 2012

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.

Effect of organic solvent on morphology and mechanical properties of electrospun syndiotactic polypropylene nanofibers

We study the stress–strain behaviors of the electrospun sPP single nanofibers as well as nonwoven mats, which were electrospun from sPP solutions using two different solvents (decalin and cyclohexane) by electrospinning. The effects of organic solvents were explored on the morphologies and the mechanical properties of the corresponding electrospun sPP single nanofibers and nonwoven mats. It was found that the nature of organic solvents dramatically affected the surface morphologies, the circular and looping deposition of the electrospun sPP fibers, and the mechanical properties. The tensile strength of both electrospun sPP single nanofibers and nonwoven mats prepared from decalin-base solution was stronger than that of cyclohexane-base solution.

Surface Reaction of Ozone at High Concentration with Isotactic And Syndiotactic Polypropylene

The surface reaction of isotactic and syndiotactic polypropylene (I-PP and S-PP) films with ozone was studied with FT-IR spectroscopy with differential scanning calorimetry (DSC) and making use of 2,2,4-trimethylpentane as a model compound of PP chain segment. I-PP and S-PP were found reactive toward ozone, and it was found that I-PP being more crystalline than S-PP, reacts more slowly with O3. In fact, the pseudo first rate constant of I-PP with ozone was k’I-PP = 8.0 × 10−6 s−1 while that of S-PP with ozone k’S-PP = 1.3 × 10−5 s−1. However, the DSC study has shown that the ozone attack on I-PP or S-PP films is not selective and simultaneously destroys the amorphous and the crystalline phases. The ozone treated I-PP and S-PP films are much less resistant to the combined thermal and oxidative degradation reaction than the unexposed films. Thus, ozone is a naturally occurring pollutant at the soil level which accelerates the ageing of I-PP and S-PP materials.

Interface adhesion properties of syndiotactic polypropylene/cellulose group composite: Relationship between chemical structure of coupling agent and reactivity for cellulose group

AbstractReinforcement of interfacial adhesion between syndiotactic polypropylene (SPP) and fibrous pure cellulose (FC) was performed by the FC silanizations with 3‐aminopropyltrimethoxysilane (APTMS), with hexyltrimethoxysilane (HTMS) and by an addition of a SPP grafted with dimethyl itaconate (SPP‐g‐DMI: number‐average molecular weight = 3.3 × 104, DMI cont. = 0.11 wt %), respectively. The adhesion and the morphology were improved by them, respectively. However, their behavior was considerably different. The hexyl group in the HTMS converted the FC surface into hydrophobicity, leading to the hindrance of the interhydrogen bonding in the FC. The silanized FC was embrittled with the increase of the degree of the silanization. In the case of the APTMS, the inter hydrogen bonding was kept because of the existence of the amino group. Whereas, the SPP‐g‐DMI was unable to penetrate into the cellulose because of its higher molecules, and the interhydrogen bonding was not hindered. The keeping the inter hydrogen bonding was important for the reinforcement of the tensile properties such as Young's modulus and tensile strength in the composite. Furthermore, the interfacial adhesion between the SPP and holocellulose (HC) was studied as well as the SPP/FC. Although the tensile properties were slightly improved by the APTMS silanization and by the addition of the SPP‐g‐DMI, respectively, the reactivity of the HC was much less than that of the FC. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Nucleator-Enhanced Molecular Orientation and Crystallization in Melt-Spun Monofilaments of Syndiotactic Polypropylene

Different from other semi-crystalline polymers, the crystallization of syndiotactic polypropylene (sPP) into helical sequences under flow is largely inhibited due to favorable formation of trans-planar conformations. The results presented in this study, using micro-FTIR, 2D-WAXS, and 2D-SAXS characterization, indicate that molecular orientation and crystallization in melt-spun sPP monofilaments can be dramatically enhanced by a sorbitol-based nucleator. It is suggested that, similar to the oriented crystallization in the nucleated isotactic polypropylene (iPP), the phase-separated nanofibrils, aligned under flow prior to sPP crystallization, act as shish structure (primary nuclei) to mediate the molecular orientation and crystallization of sPP.

The role of microstructure in the pyrolysis of polypropylene. A preliminary study on the syndiotactic stereoisomer

The pyrolysis of two syndiotactic polypropylene samples with different molar mass and microstructure has been studied by means of TGA. The volatiles evolved have been analysed as a whole by mass spectrometry. The relative content of the different low mass alkenes, alkanes and dienes provides a fingerprint which has been used to asses the similarity of the mechanism. The changes in the apparent E a with conversion have been shown by means of the Friedman’s method. The results show that, even though the chemical pattern is identical in both samples, important differences in the E a trends are found. Such disparity in the energy requirements for the pyrolysis to take place can be reasonably attributed to the different microstructure of the samples.

Phase Separation of DMDBS from PP: Effect of Polymer Molecular Weight and Tacticity

We report an unexpected dependence of DMDBS phase separation temperature on the molecular weight of the matrix isotactic polypropylene (iPP). DMDBS crystallizes out at lower temperatures for iPP with decreasing molecular weight (and correspondingly lower tacticity). All the iPPs in our study are reasonably high molecular weight samples and there is no molecular weight dependence of their solubility parameter. Therefore, the decrease in DMDBS phase separation temperature for lower molecular weights cannot be rationalized using thermodynamic arguments. This molecular weight dependence appears to be unique to isotactic polypropylene and is not observed for either syndiotactic polypropylene or for random copolymers of isotactic polypropylene containing ethylene comonomer.

An investigation of the potential of polypropylene and its blends for use in recyclable high voltage cable insulation systems

Most modern extruded high voltage cables employ cross-linked polyethylene (XLPE) as the insulation material. XLPE has excellent thermo-mechanical properties, is relatively cheap and has a low dielectric loss, which make it an ideal material for this application. Unfortunately, XLPE is not easily recycled at the end of its lifetime leading to questions concerning its long-term sustainability. A previous investigation in this series considered the potential of a range of ethylene-based systems to provide suitable recyclable alternatives to XLPE. Whilst blending could allow systems having similar thermo-mechanical and electrical properties to XLPE to be designed, it was not possible to obtain better performance than XLPE using these systems. Polypropylene offers, potentially, a route to improved insulation systems by virtue of its higher melting point and excellent dielectric properties. However, traditional isotactic polypropylenes have always had the problem of being too brittle for inclusion into practical cable designs. Recently a broad range of propylene co-polymers having improved ductility have become available, which may prove more suitable. The current study compares traditional isotactic and syndiotactic polypropylenes to a range of commercially available propylene co-polymers and focuses on their morphology, thermal, thermo-mechanical and electrical properties. These parameters were then taken together to identify the most suitable candidate materials for future cable applications. The use of blending as a means to further optimise the various material properties was also explored.

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.

Single site metallorganic polymerization catalysis as a method to probe the properties of polyolefins

Single-site metallorganic catalysts for the polymerization of olefins has allowed the synthesis of new polymeric materials having molecular structures that cannot be obtained with conventional Ziegler–Natta catalysts, with exceptional control over the chain architecture, including control of the concentration of stereo- and regio-defects and efficient uniform placement of comonomeric units along the chains. The cases of isotactic and syndiotactic polypropylenes are illustrated as examples of a fine tuning of the physical and mechanical properties of polymeric materials achieved through the controlled incorporation of stereo- and regio-defects, and the rational choice of the catalytic system. Novel isotactic polypropylenes with controlled properties, intermediate between those of stiff-plastic and elastomeric materials have been produced through the design of the catalysts. For syndiotactic polypropylene, a new class of thermoplastic elastomers with finely controlled mechanical properties have been produced by using metallorganic catalysts of different stereoselectivity. Depending on the concentration of stereodefects, elastomers showing entropic or unconventional enthalpic elasticity, can be produced.

Preparation and Characteristics of Electrospun Polypropylene Fibers: Effect of Organic Solvents

Recently, we have reported that syndiotactic polypropylene (sPP) fibrous membrane could be successfully prepared from a multi component solvent system via electrospinning at room temperature. The base solvent of multi component solvent system was cyclohexane. As demonstrated SEM images, the formation of rough surface morphologies and the microholes on electrospun sPP fibers were observed. In this study, we successfully prepared sPP nanofiber webs from a different base solvent was decalin. SEM study demonstrated that the sPP nanofibers prepared from decalin based solvent had smoother surface morphology unlike cyclohexane based solvent, suggesting that the surface morphology of electrospun sPP nanofibers depended on the boiling point of each solvent used. Moreover, to investigate the crystal and molecular structures of electrospun sPP fibers from a different solvent system (i.e., decalin and cyclohexane based solvents), WAXD and FT-IR analysis were carried out. The results showed that the different solvent systems caused the different surface morphologies but the crystal and molecular structure of the electrospun sPP fibers didn’t change.

Helical Mesophase of Syndiotactic Polypropylene in Copolymers with 1-Hexene and 1-Octene

Random copolymers of syndiotactic polypropylene (sPP) with hexene and octene comonomeric units have been synthesized in a wide range of comonomer concentrations with a single-center syndiospecific Cs-symmetric metallocene catalyst. The effects of the presence of hexene and octene on the crystallization behavior of sPP and in particular on the stress-induced transformations have been analyzed. All copolymer samples crystallize from the melt in the stable helical form I of sPP. At very low concentrations of hexene or octene (around 1−2 mol %) the behavior of these copolymers is similar to that of sPP; that is, the stable helical form I of the melt-crystallized samples transforms into the trans-planar form III by stretching at high deformations. For hexene or octene concentrations in the range 2−5 mol % the stretching produces transformation of the helical form I into the trans-planar mesophase. Finally, for hexene or octene concentrations higher than 4−5 mol % the helical form I transforms by stretching at high deformation into a different mesomorphic form, characterized by chains in the ordered 2/1 helical conformation, and disorder in the lateral packing of the chains. This is a first evidence of the crystallization of a solid mesophase of sPP with chains in 2-fold helical conformation, never observed before in the sPP homopolymer.

Elution behavior of polyethylene and polypropylene standards on carbon sorbents

The elution behavior of linear polyethylene and isotactic, atactic and syndiotactic polypropylene was tested using three different carbon column packings: porous graphite (Hypercarb), porous zirconium oxide covered with carbon (ZirChrom-CARB), and activated carbon TA 95. Several polar solvents with boiling points above 150 °C were selected as mobile phases: 2-ethyl-1-hexanol, n-decanol, cyclohexylacetate, hexylacetate, cyclohexanone, ethylene glycol monobutyl ether and one non-polar solvent, n-decane. Polyethylene standards were completely or partially adsorbed in all tested sorbent/solvent systems. Polypropylene standards were partially adsorbed on Hypercarb and carbon TA95, but did not adsorb on ZirChrom-CARB. ZirChrom-CARB retained polyethylene pronouncedly when 2-ethyl-1-hexanol, cyclohexylacetate or hexylacetate were used as mobile phases at temperature 150 or 160 °C, while all three basic stereoisomers of polypropylene eluted in size exclusion mode in these sorbent/solvent pairs. This is very different from the system Hypercarb/1-decanol, which separated polypropylene according to its tacticity. The opposite elution behavior of polyethylene and polypropylene in system ZirChrom-CARB/2-ethyl-1-hexanol (polypropylene eluted, polyethylene fully adsorbed) enabled to realize separation of blends of polyethylene and polypropylene. Ethylene/1-hexene copolymers were separated according to their chemical composition using system Hypercarb/2-ethyl-1-hexanol/1,2,4-trichlorobenzene.

Hard nanocomposite comprising stereoregular syndiotactic polypropylene with chemically bonded gold nanoparticles

AbstractAn effective method to synthesize directly a hard composite material containing uniformly dispersed nanogold particles chemically bonded with a stereospecific, crystalline polymer matrix has been developed. Syndiotactic polypropylene was synthesized and functionalized to have a hydroxyl terminal group (sPPOH) via a metallocene catalysis with a selective chain transfer. Next, sPPOH was activated to react with ethylene sulfide forming the thiol‐terminated polymer, sPPSH. sPPSH was then chemically bonded to gold nanoparticles (AuNPs) formed in situ via a reduction of HAuCl4. The bonding between thiol and AuNP stabilized the AuNPs and led to the formation of sPPAuNPs composite containing uniformly‐dispersed AuNPs of a 19–40 nm size without noticeable aggregation. Furthermore, the chemical bonding of AuNPs has afforded sPPAuNPs a thermal degradation temperature (TD) 49.4 °C higher than the pristine sPP or sPPOH and 25.7 °C higher than sPPSH without any adverse effect on the crystalline temperature and melting temperature. In addition, the characteristic UV‐Vis absorption wavelength of sPPAuNPs remains the same at various temperatures, thus indicating the independence of optical property on temperature as well as the good thermal stability of the sPPAuNPs composite. 1H NMR, 13C NMR, FESEM, STEM, XPS, TGA and DSC were used to investigate the molecular structure, morphology and thermal properties of the resulting sPPAuNPs nanocomposite. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Effect of chemical structure of silane coupling agent on interface adhesion properties of syndiotactic polypropylene/cellulose composite

AbstractTo improve interaction between syndiotactic polypropylene (SPP) and fibrous cellulose (FC), effects of chemical structure of silane coupling agent on the reactivity for the surface hydroxyl group on the FC were studied by X‐ray photoelectron spectroscopy (XPS) measurement. Among the three kinds of the silane coupling agent, the 3‐aminopropyltrimethoxysilane (APTMS) showed the highest reactivity with the surface hydroxyl group on the FC, and the linear silane compound with methoxyl group was found to be suitable for the reaction. Although the morphology of the SPP/FC composite is hardly affected by the difference in the kinds of the silane coupling agent, the tensile properties were considerably different. In particular, in the case of using higher silane coupling agent solution (over 3 wt %), the chemical structure of silane coupling agent certainly affected the tensile properties of the SPP/silanized FC composite. It was found that the tensile properties were distinctly affected by the reactivity between the surface hydroxyl group on FC and the silane coupling agent. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Synchrotron investigation on mesomorphic structure of sPP and poly(ethylene-co-octene) in their initial crystallization stage

Using synchrotron SAXS and WAXS, we investigated crystalline structures including crystallite size, lamellar thickness, long spacing of syndiotactic polypropylene (sPP) and poly(ethylene-co-octene) (PEcO) that crystallized from the sheared melt. The independence of the shear effects on the crystalline structures and the shear related lamellar orientation indicate the stable mesophase. Experiments on the different shear effects on polymer various crystalline structures confirm the multi-step polymer crystallization process.