Polypropylene Melt Research Articles

Ultrasonic Degradation Studies and Its Effect on Thermal Properties of Polypropylene

In this article, the ultrasonic degradation of polypropylene (PP) melt was conducted in a specially designed reactor. Dynamic stress rheometer and gel permeation chromatography (GPC) were used to study the degradation behavior of PP melt. Thermal properties of ultrasonically treated PP were further discussed. The results showed the decrease of complex viscosity, zero shear viscosity, representative relaxation time, viscoelastic moduli as well as cross-over modulus, but an increase of cross-over frequency with introduction of ultrasonic irradiation, due to reduction of weight average molecular weight and increase of molecular weight distribution index of the polymer. The ultrasonic chain scissions of PP melt mainly occurred at the initial 3 min of irradiation and subsequently tended to terminate under our experimental conditions. In addition, the crystallization temperature and melting temperature of PP together got decreased in the presence of ultrasonic irradiation due to the occurrence of lower molecular weight components and some chemical irregularities in PP chain, along with the slight increase of crystallinity.

Measurement of gas solubility in linear/branched PP melts

AbstractA magnetic suspension balance was employed in experiments to measure gas solubility in the polymer melts. The gas solubilities of CO2 and N2 in both linear and branched Polypropylene (PP) were investigated. The swollen volumes predicted by the Sanchez–Lacombe equation of state (EOS) and Simha–Somcynsky EOS were applied to incorporate the buoyancy effect, which is essential for the accurate measurement of solubility data. The effects of the branched structure on the swollen volume and gas solubility were discussed. It was observed that the long chain branched PP exhibited less expandability than the linear PP, due to the entangled molecular chain structure. Therefore, the total amount of gas that was able to dissolve into the long chain branched PP turned out to be less. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2497–2508, 2007

Shear-Induced Conformational Ordering in the Melt of Isotactic Polypropylene

College of Polymer Science and Engineering and State KeyLaboratory of Polymer Materials Engineering, SichuanUniVersity, Chengdu, China, and National SynchrotronRadiation Lab and Department of Polymer Science andEngineering, UniVersity of Science and Technology of China,Hefei, ChinaReceiVed May 5, 2007ReVised Manuscript ReceiVed May 23, 2007Introduction. Flow-induced order and crystallization ofpolymers are fundamental (but poorly understood) nonequilib-rium properties of these systems. As moreover flow fields areinevitably applied during polymer processing, the developmentof order under such conditions has attracted considerableinterest.

Characterization of phase morphology of polymer melts (PP/PE blends) via rheology

AbstractThe viscoelastic behavior of high impact polypropylene (PP) melts, a multiphase system with an isotactic PP matrix and inclusions consisting of mainly amorphous ethylene–propylene copolymer (EP), is strongly influenced by the size of small inclusions. The dynamic moduli of such two‐phase systems (reactor products) are well described by Palierne's emulsion model. For this analysis the reactor product is separated into matrix and dispersed phase via the different solubility in xylene. This analysis also provides information on the ratio interfacial tension to particle size of the dispersed phase. With morphology data (particle size) of the solidified heterophasic samples (Transmission Electron Microscopy), we estimate the interfacial tension between PP and EP copolymer via the emulsion model of Palierne and from the relaxation time spectra according to Gramespacher and Meissner. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Synthesis of lamellar isobutyl silicates and dispersion in polypropylene melts

AbstractA new synthetic clay, iC4‐LMS, which is a lamellar mesostructured silicate with isobutyl groups covalently attached to silicate sheets, was synthesized with the goal to increase the compatibility of the inorganic sheets with polypropylene (PP) in a melt‐blending process. The lamellar morphology of iC4‐LMS was confirmed using X‐ray diffraction and transmission electron microscopy. Based on 29Si and 13C{1H} CP‐MAS NMR spectra, isobutyl functional groups were attached to at least 10 mol % of silicate tetrahedral sites in the inorganic layers. These surface groups mimic the subunits in the PP chains. Samples of iC4‐LMS were mixed with several organic solvents and sonicated. The solvent most like PP, tetramethylpentadecane, had the highest viscosity, forming a gel which indicates very good dispersion of the clay. However, when iC4‐LMS was melt‐blended with PP, it did not show significant increase in rheology. This modest effect on rheology may arise from fracture of iC4‐LMS layers as a result of the shear stresses during melt blending produced by the viscous PP. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Aggregative structure of the 1‐octadecyl‐3‐methylimidazolium cation in the interlayer of montmorillonite and its effect on polypropylene melt intercalation

AbstractA series of organomontmorillonites were synthesized from pristine sodium montmorillonite via ion exchange in toluene with 1‐octadecyl‐3‐methylimidazolium chloride, a kind of ionic liquid with high thermal stability, as a modifier. The amount and aggregative state of the 1‐octadecyl‐3‐methylimidazolium cation ([C18mim]+) adsorbed in the interlayer of organomontmorillonite (MMT) were investigated with thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy, X‐ray diffraction (XRD), and differential scanning calorimetry. XRD patterns showed that [C18mim]+ was successfully intercalated into the interlayer of MMT, and the interlayer distances of the clays increased dramatically. On the basis of the TGA results, three types of arrangement models of [C18mim]+ could be seen, and they were related to the loading level of [C18mim]+ in the gallery of MMT. The effects of the aggregative structure of [C18mim]+ in the interlayer of MMT on polypropylene melt intercalation was also examined. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1252–1259, 2007

Segmental and Chain Dynamics of Isotactic Polypropylene Melts

The properties of isotactic polypropylene (iPP) melts are investigated via atomistic molecular dynamics (MD) simulations in the isothermal−isobaric (NPT) ensemble. A fully flexible model is developed and validated by comparing predicted volumetric and thermodynamic behavior with available experimental data. Atomic-level packing in the simulated polymer melt is examined through the calculation of an X-ray diffraction pattern. Segmental dynamics is investigated through the reorientation of the methylene C−H bonds and the decorrelation function of torsion angles over a wide range of temperatures and pressures. Predicted correlation times are in reasonable agreement with experimental values derived from 13C NMR, QENS, and dielectric spectroscopy measurements. The temperature (T) and pressure (P) effects on the relaxation times are compared by calculating the /H* ratio, which provides a quantitive measure of the relative importance of P and T on the dynamics. By consistently mapping the atomistic trajectories ...

Evaluation of the crystallization kinetics and melting of polypropylene and metallocene‐prepared polyethylene blends

AbstractThe kinetics of the isothermal crystallization of a polypropylene (PP) random copolymer containing 5 mol % ethylene, a metallocene linear low‐density polyethylene (m‐LLDPE) with 3.3 mol % hexene‐1 as a comonomer, and three blends were studied with differential scanning calorimetry at temperatures sufficiently high to prevent any crystallization of m‐LLDPE. The analysis was carried out with the Avrami equation. The overall crystallization rate and the equilibrium melting temperature of the PP copolymer decreased with increasing amounts of m‐LLDPE in the blends. The former was attributed to the effect of m‐LLDPE in reducing the number of primary nuclei, and the latter was attributed to a lowering of the fold surface energy due to the limited partial miscibility of the blend components. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 634–640, 2007

Shear-Induced Smectic Order in Isotactic Polypropylene Revisited

We provide evidence that earlier reported smectic ordering in the melt of isotactic polypropylene is not intrinsic to the system but results from the presence of small quantities of calcium stearate. No causal relation exists between this effect and the crystallization properties of iPP.

Interfacial effects in nano-silica/polypropylene composites fabricated by in-situ chemical blowing

By mixing macromolecular blowing agent grafted nano-SiO2 with polypropylene (PP) melt, the nanoparticle agglomerates can be pulled apart due to the in-situ bubble-stretching resulting from gasification of the side foaming groups on the grafted polymer. The present work evaluated the interfacial effect in the PP based nanocomposites prepared using the aforesaid technique through introducing rubbery components to the backbone of the grafted polymer chains. The results indicated that deagglomeration of the nanoparticles was not bound to yield the highest properties of the composites. The positive effect of the nanoparticles was brought into full play because of the joint contributions of particles dispersion sta- tus and interfacial interaction. An interlayer with proper flexibility ensured an overall enhancement of mechanical proper- ties, especially impact strength, of the nanocomposites.

Monte Carlo simulations of the crystallization of isotactic polypropylene

AbstractWe model the temperature dependent conformational changes and orientational and translational ordering of isotactic polypropylene melt in the vicinity of its melting temperature. We use the coarse grained second nearest neighbor lattice model, which has previously been shown to properly model the liquid state thermodynamics of this material. Equilibrium Monte Carlo (MC) simulations show the existence of two characteristic temperatures. Above T = 450 K the chains assume standard Gaussian statistics, but below this temperature we find that helix formation is thermodynamically favored even though the melt is still disordered. Both the helix length and fraction of chain monomers involved in helices increase monotonically with decreasing temperature, and below T = 385 K this material undergoes crystallization. Our results are consistent with recent simulations, which suggest that isothermal crystallization is temporally preceded by the formation of a smectic phase: we argue that the helices are long enough under these conditions to support liquid crystallinity. The crystallinity then evolves in these liquid crystalline “droplets”. Further, we can understand the recent results of de Jeu et al. who had found that isotactic polypropylene chains formed a smectic‐like phase under the action of a planar shear even at temperatures above the equilibrium melting point. In summary, these findings support the following physical picture: the unusual spinodal decomposition phenomena seen in the early stages of isothermal crystallization, which have been assigned to the formation of small liquid crystal droplets, have equilibrium signatures above the melting point where locally rigid chain sequences become thermodynamically stable. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3453–3460, 2006

The effect of compatibilizers on the crystallisation, melting and polymorphic composition of β-nucleated isotactic polypropylene and polyamide 6 blends

Non-compatibilized and compatibilized blends of isotactic polypropylene (iPP) and polyamide 6 (iPP/PA6) as well as their β-nucleated versions were prepared using maleic anhydride functionalized iPP (MAPP) with different anhydride contents as compatibilizer. Ca-suberate, a highly efficient and selective β-nucleating agent was added to the blends in order to promote the formation of the β-modification of iPP. The melting and crystallisation characteristics, as well as the polymorphic composition of the blends were studied by differential scanning calorimetry (DSC). The supermolecular and phase structure of the blends were studied by polarised light microscopy (PLM). iPP and PA6 form blends with heterogeneous phase structure; the PA6 component is dispersed in the iPP matrix in the concentration range studied. The compatibilizer promotes the dispersion of PA6 resulting in smaller particles than without MAPP. In the non-compatibilized β-nucleated blends, an iPP matrix consisting mainly of the α-modification was formed already at low PA6 content. On the contrary, predominantly β-iPP matrix developed in the presence of MAPP compatibilizers. The formation of α-iPP matrix in the absence of compatibilizer is related to the selective encapsulation of the nucleating agent in the polar PA6 phase. The influence of the blending technique on the polymorphic composition of the matrix supports the hypothesis of selective encapsulation. Compatibilizers, besides their traditional benefits assist the distribution of the β-nucleating agent between both phases of the blends and promote the formation of a matrix rich in β-iPP. In the presence of β-nucleating agent MAPP with low anhydride content and blends of iPP containing maleated polypropylene crystallise predominantly in the β-form.

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.

Study of the rheological properties of melts of polypropylene and poly(butylene terephthalate) blends

The viscosity properties of melts of fibre-forming polypropylene (PP) and poly(butylene terephthalate) blends were investigated in the entire range of ratios at different stresses and shear rate gradients at 230–250°C. It was shown that melts of fibre-forming PP and PBT blends are weakly crosslinked systems. The effect of the mass ratio of PP and PBT on the apparent activation energy of viscous flow of melts of the blends was investigated at different shear stresses and shear rate gradients. It was hypothesized that this blend can be assigned to the group of limitedly compatible systems. The probability of compatibility of the polymers in the melt appears when up to 20% PBT is incorporated in the PP. The blends are not compatible for the remaining ratios of polymers in the investigated system.

Melting and crystallization behaviors of isotactic polypropylene/acrylonitrile–butadiene rubber blends in the presence and absence of compatibilizers and fillers

AbstractThe melting and crystallization behaviors of isotactic polypropylene (PP)/acrylonitrile–butadiene rubber (NBR) blends were investigated with differential scanning calorimetry. The samples were scanned at a heating rate of 20°C/min in a nitrogen atmosphere. The effects of blend ratio, compatibilizer, and filler addition on the melting and crystallization characteristics of the blends were analyzed. Analysis showed that blend ratio had a predominant effect on the values of onset of crystallization and crystallization temperature, although the heat of fusion (ΔHf), onset of melting, and melting temperature were unaffected. The presence of compatibilizer in the blend had an appreciable influence on the crystallization behavior. ΔHf, heat of crystallization, and percentage crystallinity of the compatibilized blends were higher than those of the uncompatibilized blends. Fillers had little impact on the melting behavior of the blends. The morphology of the blends were analyzed with scanning electron microscopy. Hot‐stage polarizing optical microscopy was used to study the spherulitic morphology of PP on the addition of NBR. The addition of a few percentage NBR significantly reduced the average spherulite size of PP in the blend followed by a marginal decrease. Blend ratio had a pronounced impact on the growth rate of PP spherulites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2067–2080, 2006

Mechanical properties and morphological behavior of calcium carbonate‐filled polypropylene in dynamic injection molding

AbstractA custom‐made electromagnetic dynamic injection molding machine was adopted to study the mechanical properties and morphological behavior of calcium carbonate‐filled polypropylene (PP) in a dynamic injection molding process. The influence of vibration amplitude and frequency on the mechanical properties and morphological behavior of samples was investigated using tensile tests, notched Izod impact tests, differential scanning calorimetry, and scanning electronic microscopy. The tensile stress and the impact stress for all samples investigated were found to increase in a nonlinear manner with increasing vibration amplitude and frequency. The tensile stress reached a maximum value at about 8 Hz and 0.15 mm for neat PP and PP filled with 3, 20, and 30 wt% CaCO3. For PP filled with 40 wt% CaCO3, the tensile stress reached a maximum value at about 12 Hz and 0.2 mm. The impact stress reached a maximum value at about 12 Hz. From DSC experiments it was shown that the melting temperature slightly increased, but no new polymeric crystalline peak appeared under the vibration force field. The CaCO3 particles were diffused easily and distributed evenly in the PP melt under the vibration force field, so it is very useful in improving the quality of injection products. Copyright © 2006 Society of Chemical Industry

Dynamic rheological behavior of polypropylene melts with pulsatile pressure flow in a dynamic capillary rheometer

AbstractA self‐made dynamic capillary rheometer (DCR) was designed to investigate the dynamic viscoelastic characteristic of polypropylene (PP) melt during the pulsatile pressure extrusion. A vibration force field was parallel superposed upon steady shear flow in this DCR by means of a vibration driven piston. During the pulsatile pressure extruding process in DCR, the PP melt displayed apparent viscoelasticity. The experiment results proved the pressure pulsatile extrusion could reduce the viscosity of polymer melts effectively. The phase difference between the shear stress and the shear rate decreased with the superposed vibration. But, at large amplitude conditions, the viscosity has an increasing tendency. This maybe illuminated that large amplitude could be harmful for the vibration‐assistant polymer processing. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1834–1838, 2006

Importance of elongational properties of polymer melts for film blowing and thermoforming

AbstractTwo polyethylene and two polypropylene melts were characterized in uniaxial elongational flow. They exhibit significant differences with respect to strain hardening. For the polypropylenes it was shown that the elongational behavior found in uniaxial elongation is qualitatively reflected in biaxial deformation too. From the polyethylenes, films were blown using laboratory equipment, and the polypropylenes were processed into beakers by thermoforming. For both materials it could be shown that strain hardening is of advantage for the geometrical uniformity of the processed items. POLYM. ENG. SCI., 46: 1190–1195, 2006. © 2006 Society of Plastics Engineers

Fibril Formation of 1,3:2,4-Di(3,4-dimethylbenzylidene) Sorbitol in a Polypropylene Melt

Binary mixtures of 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS) within the melt of polypropylene (PP) were studied at DMDBS contents of 0.4 and 1.0 wt %. DMDBS serves as a nucleating agent in PP crystallization by formation of a nanofibrillar network. The kinetics of the DMDBS solidification process within the PP melt and the ensuing nanofibrillar structure were studied by in situ small-angle X-ray scattering (SAXS) analysis combined with imaging by electron microscopy. The dynamic lag of the fibrillar structure formation kinetics and its temperature dependence indicate a nucleation and growth mechanism, controlled by the rate of nucleation. Investigation of the fibrillar structure by electron microscopy indicates a complex structure in which long and thin fibrils (less than 100 nm in cross-section) are composed of thinner nanofibrils (less than 10 nm in cross-section).

Melt Elasticity Phenomenon of Multicomponent (Talc and Calcium Carbonate) Filled Polypropylene

The compound of single-component filled polypropylene (PP) (consists of either talc or CaCO3) and multicomponent filled PP (talc/CaCO3/PP) are prepared by melt mixing using Brabender Plasticorder internal mixer. All systems exhibit good stability and normal flow in the molten state. The multicomponent filled PP has shown an intermediate degree of die swell and flow instabilities. The defect of extrudate in the extrusion process in the form of melt fracture could only be evidenced in the semisolid state of PP melt. The result shows that composite with highest content of talc exhibits the least melt fracture.