Syndiotactic Polypropylene Research Articles

Evidence of Nodular Morphology in Syndiotactic Polypropylene from the Quenched State

Evidence of Nodular Morphology in Syndiotactic Polypropylene from the Quenched State

Mechanical Property of Polypropylene Gels Associated with That of Molten Polypropylenes.

This study aims to understand the fundamental mechanical relationship between polypropylene (PP)-gels and solid PPs without solvent through mechanical and thermal analyses, by which the mechanical similarities between molten PPs and PP gels were found, leading to the reliable estimate of the mechanical properties of semi-crystalline gels. The gelation of syndiotactic and isotactic polypropylenes (sPP and iPP) was found when PPs were dissolved in 1,2,3,4-tetrahydronaphthalene (tetralin). Interestingly, it was found that the storage modulus of sPP-gel became higher than that of iPP-gel at low PP concentration (<~40 wt%). The result was distinctly different from the result of neat solid PPs (without solvent), where the modulus of solid sPP is generally significantly lower than that of solid iPP. Such inversion behavior in the mechanical property of semi-crystalline gels had not been reported and discussed before. By further investigation of the storage moduli of neat sPP and iPP, it was found that the storage modulus of sPP became higher than that of iPP above the melting points of PP, which was similar to the behavior of the storage moduli observed in the diluted PP-gels. Such similarity between PP-gels and PP melts was also observed within iPP samples with different molecular weights.

Mechanical Properties and Elastic Behavior of Copolymers of Syndiotactic Polypropylene with 1-Hexene and 1-Octene

The mechanical properties of copolymers of syndiotactic polypropylene (sPP) with 1-hexene and 1-octene have been studied and correlated with the structural transformations occurring during stretching and relaxation. These copolymers crystallize in disordered modifications of form I of sPP, with crystallinity, melting, and glass-transition temperatures that decrease with increasing comonomer content. All copolymers show remarkable mechanical properties and elastic behavior, with great improvement of flexibility and ductility with respect to the sPP homopolymer already at small comonomer concentrations. For low comonomer content up to 4–5 mol %, the elastic recovery is associated to a reversible conformational phase transition between the trans-planar form III or the trans-planar mesophase and the helical form II, which gives an enthalpic contribution to the elasticity. As the comonomer content increases, the trans-planar conformation is progressively destabilized and the observed elastic properties are not associated with any conformational transformation during stretching and relaxation. In these cases, copolymers behave as conventional thermoplastic elastomers where elasticity is purely entropic and crystals only act as topological constraints of the elastomeric network. These copolymers represent an exemplary case of the possibility to combine in the same materials desired and apparently incompatible physical properties of elasticity, crystallinity, and strength by controlling the crystallization behavior, achieved by tailoring the chemical structure in the synthetic process.

Highly conductive anion exchange membranes based on polymer networks containing imidazolium functionalised side chains

Two novel types of anion exchange membranes (AEMs) having imidazolium-type functionalised nanofibrous substrates were prepared using the facile and potentially scalable method. The membranes’ precursors were prepared by graft copolymerization of vinylbenzyl chloride (VBC) onto syndiotactic polypropylene (syn-PP) and polyamide-66 (PA-66) nanofibrous networks followed by crosslinking with 1,8-octanediamine, thermal treatment and subsequent functionalisation of imidazolium groups. The obtained membranes displayed an ion exchange capacity (IEC) close to 1.9 mmol g–1 and ionic (OH-) conductivity as high as 130 mS cm–1 at 80 °C. This was coupled with a reasonable alkaline stability representing more than 70% of their original conductivity under accelerated degradation test in 1 M KOH at 80 °C for 360 h. The effect of ionomer binder on the performance of the membrane electrode assembly (MEA) in AEM fuel cell was evaluated with the optimum membrane. The MEA showed a power density of as high as 440 mW cm−2 at a current density is 910 mA cm−2 with diamine crosslinked quaternized polysulfone (DAPSF) binder at 80 °C with 90% humidified H2 and O2 gases. Such performance was 2.3 folds higher than the corresponding MEA performance with quaternary ammonium polysulfone (QAPS) binder at the same operating conditions. Overall, the newly developed membrane was found to possess not only an excellent combination of physico-chemical properties and a reasonable stability but also to have a facile preparation procedure and cheap ingredients making it a promising candidate for application in AEM fuel cell.

Thermorheological evidence and structure of heterogeneity in syndiotactic polypropylene melts with strong memory effects

The memory effect of semi-crystalline polymers is usually related to the heterogeneity in self-nucleated (SN) melts. In this study, the strong memory effects of syndiotactic polypropylene (sPP) and the heterogeneity in SN melts were systematically investigated in a wide temperature range even above its equilibrium melting temperature (Tmo). The rheological measurements revealed that the SN melts exhibited thermorheologically complex behaviors, indicating that the presence of the heterogeneity in the SN melts. From the rheological plots of phase angle (δ) with the complex modulus (G*), the parameter Δ ln G* δ=45° values were determined and used to compare the change of the degree of thermorheological complexity in the SN melts during the heating process. It was found that the memory effect's efficiency to promote the crystallization process improved as the increase of Δ ln G* δ=45° values, which suggested that the memory effect depended on the heterogeneity in the SN melts. After purifying the sample, the strong memory effect still existed, which indicated that it was not caused by the stabilization effect of impurities. The results of wide/small-angle X-ray scattering and infrared spectroscopy revealed that there were no partial residual crystallites or regular chain conformation in our investigated SN melts. Based on the complex chain structure of our investigated sPP, we speculated that the heterogeneity in sPP melts was caused by the separation between the long crystallizable sequence and the atactic sequence.

Synthesis, structure and properties of copolymers of syndiotactic polypropylene with 1-hexene and 1-octene

Incorporation of long branches, such as 1-hexene or 1-octene, in syndiotactic polypropylene gives novel elastomeric materials, whose crystallization behavior and elastic properties can be easily tailored through tuning of the branches concentration.

Effect of sPP Content on Electrical Tree Growth Characteristics in PP-Blended Cable Insulation.

This paper aims at investigating the electrical tree characteristics of isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends for thermoplastic cable insulation. PP blended samples with sPP contents of 0, 5, 15, 30, and 45 wt% are prepared, and electrical treeing experiments are implemented under alternating current (AC) voltage at 50, 70, and 90 °C. Experimental results show that with the incorporation of sPP increasing to 15 wt%, the inception time of electrical tree increases by 8.2%. The addition of sPP by 15% distinguishes an excellent performance in inhibiting electrical treeing, which benefits from the ability to promote the fractal dimension and lateral growth of branches. Further increase in sPP loading has a negative effect on the electrical treeing resistance of blended insulation. It is proved by DSC and POM that the addition of sPP promotes the heterogeneous crystallization the of PP matrix, resulting in an increasing density of interfacial regions between crystalline regions, which contains charge carrier traps. Charges injected from an electrode into a polymer are captured by deep traps at the interfacial regions, thus inhibiting the propagation of electrical tree. It is concluded that the modification of crystalline morphology by 15 wt% sPP addition has a great advantage in electrical treeing resistance for PP-based cable insulation.

Polar Isotactic and Syndiotactic Polypropylenes by Organozirconium‐Catalyzed Masking‐Reagent‐Free Propylene and Amino–Olefin Copolymerization

AbstractPolar functionalized isotactic and syndiotactic polypropylenes (PPs) are synthesized by direct, masking‐reagent‐free propylene and amino–olefin (AO, CH2=CH(CH2)xNnPr2, x=2, 3, 6) copolymerizations using the activated precatalysts rac‐[Me2Si(indenyl)2]ZrMe2 and [Me2C(Cp)(fluorenyl)]ZrMe2, respectively. Polymerization activities at 25 °C are as high as 4208 and 535 kg/(mol h atm) with AO incorporation up to 4.0 mol % and 1.6 mol %, respectively. Remarkably, introducing the amino‐olefin comonomers significantly enhances stereoselection for both isotactic (mmmm: 59.5 %→91.0 %) and syndiotactic (rrrr: 66.3 %→81.3 %) products.

Polar Isotactic and Syndiotactic Polypropylenes by Organozirconium-Catalyzed Masking-Reagent-Free Propylene and Amino-Olefin Copolymerization.

Polar functionalized isotactic and syndiotactic polypropylenes (PPs) are synthesized by direct, masking-reagent-free propylene and amino-olefin (AO, CH2 =CH(CH2 )x Nn Pr2 , x=2, 3, 6) copolymerizations using the activated precatalysts rac-[Me2 Si(indenyl)2 ]ZrMe2 and [Me2 C(Cp)(fluorenyl)]ZrMe2 , respectively. Polymerization activities at 25 °C are as high as 4208 and 535 kg/(mol h atm) with AO incorporation up to 4.0 mol % and 1.6 mol %, respectively. Remarkably, introducing the amino-olefin comonomers significantly enhances stereoselection for both isotactic (mmmm: 59.5 %→91.0 %) and syndiotactic (rrrr: 66.3 %→81.3 %) products.

Predicting the Young's modulus and the impact strength of immiscible polyolefin blends

This paper focuses on the prediction of Young's modulus and impact strength of immiscible polyethylene/polypropylene blends, prepared in the melt, on the whole composition range, starting from the composition, interfacial tension, rheological properties and processing conditions. For this, the model proposed by Veenstra and a tentative model relating respectively the modulus and the impact strength to the composition were used. They were combined with a micro-rheological model developed previously to predict the limits of dispersed phase and co-continuous morphologies, and the characteristic sizes of the phase. The predictions were compared with experimental data for blends of isotactic and syndiotactic polypropylenes and low- and high-density polyethylenes originating from different polymerization process, especially regarding the catalyst systems. The effect of the catalyst systems and the type of morphology on the properties were analyzed, showing better properties for blends of metallocene polyolefins in comparison to those obtained using Ziegler-Natta catalysts.

Investigating the Nucleation Effect of DMDBS on Syndiotactic Polypropylene from the Perspective of Chain Conformation

The mechanism of nucleating agents (NAs) accelerating the crystallization of semi-crystalline polymers has received continuous attention due to the extreme importance in academic research and industry application. In this work, the nucleation effect and probable mechanism of 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) on promoting the crystallization of syndiotactic polypropylene (sPP) was systematically investigated. Our results showed that DMDBS could significantly accelerate the crystallization process and did not change the crystalline form of sPP. The in situ infrared spectra recorded in the crystallization process showed that in pristine sPP the tttt conformers decreased and the ttgg conformers increased subsequently. In sPP/DMDBS system, DMDBS could promote the increase of ttgg conformers rather than the decrease of tttt conformers. The further analysis by 2D-IR spectra revealed that ttgg conformers increased prior to the decrease of tttt conformers in the sPP/DMDBS system comparing with pristine sPP. Considering that ttgg conformers were basic elements of helical conformation of Form I crystal for sPP, we proposed a probable nucleation mechanism of DMDBS for sPP:DMDBS could stabilize the ttgg conformers which induced these ttgg conformers to pre-orientate and aggregate into helical conformation sequences as initial nuclei quickly and early to promote the sPP crystallization. Our work provides some new insights into the nucleation mechanism of NAs for sPP.

Stereoblock Polypropylenes Prepared by Efficient Chain Shuttling Polymerization of Propylene with Binary Zirconium Catalysts and iBu3Al

Stereoblock polypropylenes bearing isotactic, atactic, and syndiotactic polypropylene segments were successfully prepared by dry methyl aluminoxane activated binary catalysts system, Ph2CFluCpZrCl2 and {Me2Si(2,5-Me2-3-(2-MePh)-cyclopento[2,3-b]thiophen-6-yl)2}ZrCl2, in the presence of iBu3Al as a chain shuttling agent. By studying the catalyst activity, chain transfer efficiency, and reversibility of chain transfer reaction of each catalyst system, as well as the molecular weight and polydispersity of the resulting polymers, the alkyl exchange reactions between the zirconium catalyst and different main-group metal alky were estimated, respectively. Based on the optimized react condition, the chain shuttling polymerization was conducted by binary catalyst system in the presence of iBu3Al under both atmospheric and high pressure. Resultant polymers were identified as stereoblock polypropylenes according to microstructure and physical properties analyses by 13C{1H}-NMR, DSC, and GPC.

Correlation between Parameters of Extrusion Technique and Both of Optical and Physical Characteristics of Reinforced Polypropylene Composites

This research was an attempt to illustrate the correlation between parameters of extrusion technique (screw speed) which was used to produce conventional and hybrid reinforced polypropylene composites and their optical and physical characteristics. The obtained results demonstrated the responses of hybrid reinforced composites were more stable and higher than those of conventional reinforced composites. Furthermore; the density and optical traits of hybrid composites were influenced extremely by the screw speed variation (reduction). The syndiotactic polypropylene was used as a matrix while talc and graphite were used as reinforcing agents (fillers) to prepare diverse weight percentages of reinforced polypropylene composites.

Generation of well relaxed all atom models of stereoregular polymers: a validation of hybrid particle-field molecular dynamics for polypropylene melts of different tacticities

ABSTRACT The tacticity of vinyl polymer chains strongly affects the physical properties of polymeric materials, as example the chain conformations and stiffness. In the present work we tested how the hybrid Particle-Field Molecular Dynamics (PF-MD) technique is capable to describe conformational differences of polymer chains as function of the tacticity. In particular, we focus on tacticity effect of atactic, isotactic, and syndiotactic polypropylene (PP) homopolymer melts. We found that PF-MD simulations exhibit dependence of Flory’s Characteristic Ratio from the fraction of racemo diads along the PP chains in qualitative agreement with Small Angle Neutron Scattering (SANS) experiments and theoretical previsions. Finally, we calculated and compared the packing length parameter on very high stereoregular syndiotactic PP systems with rheological measurements. A qualitative agreement between the calculated and experimental packing length is found.

Retention of polypropylene stereoisomers in solvent gradient interaction chromatography on porous graphitic carbon as influenced by temperature and mobile phase composition

The behavior of isotactic, syndiotactic and atactic polypropylene stereoisomers on porous graphitic carbon (PGC) at different column temperatures is investigated with 1-decanol, decalin and decane as the adsorption promoting solvents using gradient interaction chromatography (SGIC). Column temperatures between 120 – 180 °C are investigated for the three adsorption promoting solvents with 1,2,4-trichlorobenzene (TCB) as the desorption promoting solvent. Owing to the different stereochemistry of the isomers, their interaction with the atomic level flat surface (ALFS) of porous graphitic carbon is observed to be different when injected from the three different adsorption promoting solvents. Atactic and isotactic polypropylene are not separable when 1-decanol is used as the adsorption solvent but can be separated from syndiotactic PP at column temperatures of between 120 – 180 °C. The three stereoisomers have almost similar elution volumes when decalin is used as the adsorption promoting solvent between 120 – 170 °C. Decane allows for both adsorption and desorption of the stereoisomers at distinct peak elution volumes at the studied column temperatures of 120 – 160 °C. Furthermore, it is shown that increasing the column temperature while maintaining other chromatographic conditions can either decrease or increase retention depending on the adsorption promoting solvent. More importantly, retention is influenced by adsorption conditions (temperature and adsorption promoting solvent) which may affect macromolecular conformations upon injection onto the PGC stationary phase. This is the first study on PP stereoisomers highlighting this behaviour.

Carbon Dioxide Adsorption on Grafted Nanofibrous Adsorbents Functionalized Using Different Amines

Of late, the demand for new CO2 adsorbents with high adsorption capacity and stability is growing very fast. Nanofibrous adsorbents are potential materials for such application with most attempts made on carbon nanofibers. In this study, a series of electrospun nanofibrous adsorbents containing amines were prepared using a 3-stage promising approach and tested comparatively for CO2 capture. The preparation of adsorbents involved electrospinning of syndiotactic polypropylene (s-PP) solution, radiation-induced grafting (RIG) of glycidyl methacrylate (GMA) onto electrospun nanofibers, and functionalization of poly-GMA grafted s-PP nanofibrous mats with different amines, including ethanolamine (EA) diethylamine (DEA) and triethylamine (TEA). The effect of different amination parameters: namely, amine concentration, reaction time, temperature, and degree of grafting (DG) on the degree of amination (DA), was evaluated. The nanofibrous mats containing amine were tested for CO2 adsorption in a fixed bed column operated under various parameters such as amine density, amine type, initial CO2 concentration and temperature. The adsorbents recorded CO2 adsorption capacities of 2.87, 2.06 and 0.94 mmol/g for EA-, DEA- and TEA-containing adsorbents, respectively, at 30° C using initial CO2 concentration of 15%. This was coupled with the same order of high amine efficiency of 75, 57, and 31%. Results demonstrated that the nanofibrous adsorbent containing amine had strong potential for CO2 capture application.

Dielectric relaxation and trap-modulated DC breakdown of polypropylene blend insulation

This work focuses on the dielectric relaxation of isotactic polypropylene (iPP), syndiotactic polypropylene (sPP) and atactic polypropylene (aPP) blended insulation. The relationship between dielectric relaxation and trap characteristics, as well as their effects on the breakdown properties of polypropylene insulation are investigated. Frequency domain spectroscopy (FDS) is employed to characterize the dielectric relaxation of PP blends at 50, 70 and 90 °C. Isothermal surface potential decay (ISPD) measurements are carried out to obtain the trap characteristics, and the dc breakdown strength is tested; thus, the mechanisms of trap characteristics on the dielectric relaxation and breakdown performance are analysed. The dielectric relaxation processes α and δ of PP blends can be attributed to segmental motions and carrier hopping, respectively. The α relaxation strength (Δεα) decreases with the addition of sPP, since the intermolecular interaction factor (g) between PP molecules is weakened. The activation energy of δ relaxation is proportional to the shallow trap level, since the increased shallow trap level can pose higher potential barriers for carrier hopping. The higher crystallinity of PP blends can induce more deep traps in the boundaries of spherulites, and it can reduce the number of shallow traps that are mainly distributed in amorphous regions. The increased deep trap density serves a function to reduce the mean free path of charges and restrain impact ionization, resulting in an enhancement in the breakdown field.

Discharge Capacity Improvement of Zinc-Air Battery Using Electrospun Polypropylene Nanofibrous Separator

Among metal-air flow batteries, zinc-air offers the advantages of low cost, high safety, environmental friendliness and high theoretical energy density. An electrolyte, as one of the main components in zinc-air batteries, are responsible for the transport of hydroxide ions between the cathode and the anode. A favorable electrolyte for alkaline zinc-air batteries should have high ionic conductivity, low zincate ion crossover, good electrochemical stability in caustic solutions and high electrical resistance. Commercially, microporous films of polyolefins, namely polyethylene and polypropylene, saturated with electrolyte are widely used as the separator. However, using the microporous commercial films with ordered structures in zinc-air batteries usually leads to low performances. Thus, non-ordered and layered-fibrous structures are suggested to improve the performance. Herein, separators based on electrospun polypropylene nanofibers were investigated. Electrospun syndiotactic polypropylene (s-PP) nanofibrous mat was fabricated using electrospinning technique. Granular s-PP was dissolved in a solvent mixture of decalin, acetone and DMF to the concentration of 7.5 wt%. The resulting solution was then electrospun under a controlled condition (Potential: 10.5 kV; Distance: 15 cm; flow rate: 0.8 ml/h). To examine electrochemical characteristics of electrospun s-PP nanofibers and Whatman filter paper as benchmark separator, a three-electrode configuration cell with a zinc working electrode and a platinum counter electrode was used. Furthermore, the performance of the separators was evaluated using a home-made tubular zinc-air battery with stagnant KOH electrolyte. Cyclic voltammetry elucidated higher and broader peak for zinc dissolution for the battery using nanofiber-based separator as shown in Figure 1. The galvanostatic discharge results showed that the battery using s-PP nanofibers exhibited more than 40% improvement in the discharge capacity compared to the Whatman filter paper. The results of this study show the potential of electrospun nanofibrous structures as separators in various types of battery systems. Figure 1

Shape Memory Effect, Shock- and Energy-Absorption Capability of Critically Cross-Linked Syndiotactic Polypropylene

Abstract Critical cross-linking, a light cross-linking right at the borderline between thermoplastic and elastomer, was applied to syndiotactic polypropylene (sPP) to gain at least a shape memory effect for storing preferably high strains. Beside the expected shape memory effect with maximum storable strains of 550%, critically cross-linked syndiotactic polypropylene (x-sPP) exhibits further amazing properties. For example, amorphous x-sPP is one of a few “cold programmable” shape memory polymers. At room temperature, it is easily deformable by hand and forms strain-induced crystals, which stabilize it at strains of up to 340%. Thermal triggering of cold-programmed x-sPP exhibits a triple-shape memory effect with an intermediate shape that depends on the heating rate applied for triggering. Thus, cold-programmed x-sPP is capable of reacting to a temperature increase by adapting its shape change according to the applied heating rate. Since a material with such property can be used e.g. to predict a system failure when used in a defined environment, we called it “predictive material”. Since amorphous x-sPP is easily deformable, transforms itself from a compliant, elastomeric into a rigid, semi-crystalline state upon stretching to large elongations and retains the latter after removing the stretching force, we explored its suitability as a shock- and energy absorber. To this end, thin safety lines of x-sPP were prepared and tested by falling weight experiments to determine characteristic physical quantities such as maximum impact force, maximum jerk (first derivative of the deceleration) and energy absorption capability. It was found that x-sPP is excellently suited for cushioning the impact force, vibration and chatter, which occur when a safety line abruptly tightens after the free fall in a falling weight experiment.

SANS study on the nano-crystalline network structure of elastic physical gels made of syndiotactic polypropylene.

The structure-property relationship of an elastic physical gel, obtained by simply quenching syndiotactic polypropylene (sPP)/decahydronaphthalene solution with liquid nitrogen, was investigated based on small-angle neutron scattering (SANS) analysis. The SANS analysis revealed that sPP nanocrystals with a constant radius of 4-5 nm existed in the sPP gels regardless of the sPP concentration, whereas the correlation length of the nanocrystals drastically decreased from ∼130 to ∼20 nm upon increasing the sPP concentration from 2 to 12 wt%. The volume fraction and the number density of the sPP nanocrystals increased monotonously with the increase in the sPP concentration. The rheological properties and the melting behavior of the quenched sPP gels were highly consistent with the number density of the nanocrystals calculated from the SANS analysis, strongly suggesting that the sPP nanocrystals actually worked as crosslinking points by inducing elasticity in the quenched sPP gels.