iPP Samples Research Articles

The effects of vibration field on crystal structures and orientation of isotactic polypropylene injection samples

In this study, isotactic polypropylene (iPP) samples were prepared by self-made pressure vibration injection device, in which a periodical shear field was imposed on the iPP melt during the filling and packing stages. The crystal structures and orientation of samples were investigated by wide-angle X-ray diffraction and polarizing light microscopy (PLM). The β form and γ form were found in vibration injection molding samples. Compared with the static samples, the orientation of iPP chains within lamellae was observed in the transition layer (at the depth of 1.5 mm from the surface) of the vibration injection molding samples, and the orientation was found even in the core layer. The PLM observation indicates the β form exists in the transition and core region of the vibration sample. Besides, two types of β-PP could be observed simultaneously in the core region under high magnification. The bright β crystallites (βIII) are fan-shaped and lie perpendicularly to the flow direction. In the other hand, the ringed β crystallites (βIV) almost remain their spherulite shape. Moreover, fiber crystal was observed in vibration sample, even in the core region. Some fiber crystals in the transition region reappeared in the cooling circle after the sample was held at 180 °C for 2 min. This could be explained in structure memory effect and its high thermal stability.

Carbon Dioxide Induced Crystallization for Toughening Polypropylene

This study proposes a novel process for significantly toughening isotactic polypropylene (iPP) by finely tuning and controlling the structure and morphology of iPP. The toughness of injection-molded iPP specimens can be significantly improved by controlled shearing, CO2-induced recrystallization, and adequate cooling without loss of strength. The distribution and structure of the iPP samples before and after toughening were characterized by polarized optical microscopy, Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, small-angle X-ray scattering, scanning electron microscopy, and differential scanning calorimetry, to investigate the structure–property relationship. Under shear, a high degree of orientation can be obtained with “shish-kebab” crystals formed in the shear zone. During the subsequent CO2 treatment, a crystal network morphology can be formed as a result of an increase in the numbers of primary lamellae and crosshatched subsidiary lamellae, which leads to an increase in toughness. Wide-angle X-ray diffraction patterns indicate that quenching in ice–water of scCO2-treated iPP promotes the formation of nanosized mesomorphic phase domains in the shear zone, which further toughens the iPP. The impact strength of the best toughened iPP sample was found to be over 12 times that of the original sample without loss in tensile strength and modulus.

Nodular structure of isotactic polypropylene crystallizes from the melt

AbstractThis article highlights the melt crystallization behavior of different grades of isotactic polypropylene (iPP) using a hot‐stage polarizing optical microscopy. iPP samples were heated up at a heating rate of 10°C/min passing the melting temperature and then kept for 3 min at a temperature range of 175–200°C before they cooled rapidly at 40°C/min to crystallize isothermally at a range of 130–145°C. It has been found that the temperature at which the samples were kept has a strong effect on the crystallization mode; for samples heated up and kept at temperatures below 190°C, the crystallization started with thin and long rods or nodules, which grew in the circumferential direction only while their lengths remain unchanged as the time passed. The shape of the nodules can be straight, circular, branched, or entangled, and they can grow parallel to each other or they can be crossed or in a random way. This phenomenon disappeared completely for samples melted and kept at temperatures above 195°C. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Shish–kebab-like cylindrulite structures resulted from periodical shear-induced crystallization of isotactic polypropylene

In this study, isotactic polypropylene (iPP) samples were prepared by conventional injection molding (CIM) and pressure vibration injection molding (PVIM), in which a periodical shear field was imposed on the iPP melt during the cooling solidification. The distribution of supermolecular structures of samples was investigated by Polarized Light Microscopy (PLM) and Scanning Electron Microscopy (SEM). Results show that the through-the thickness-morphology of sample prepared by CIM features a typical skin-core structure, as a result of general shear-induced crystallization. This structure can be divided into three layers, including a skin layer in which the shish–kebab structure was found, a transition region with deformed spherulite structure and a core layer with spherulitic structure. However, the morphology of the sample prepared by PVIM, as a result of periodical shear-induced crystallization, features a richer and fascinating supermolecular structure and can not be roughly divided into three layers. A region full of shish–kebab-like cylindrulite structures was found between the transition region and the core layer, which is rare to be seen in conventional injection molding. Based on their various core structures, two kinds of shish–kebab-like cylindrulites were defined: one is multi-fibril-core cylindrulite of which core is an assembly of multiple fibrils, and the other is single-fibril-core cylindrulite of which the core just contains a single fibril. Based on the investigated results, a schematic illustration is proposed to depict the through-the thickness-distribution of supermolecular structure of iPP sample prepared by PVIM. The mechanism of the formation of the two kinds of shish–kebab-like cylindrulite structures is also depicted by a schematic illustration, and it was discussed in terms of periodical shear-induced crystallization.

Isotactic Polypropylene with (3,1) Chain-Walking Defects: Characterization, Crystallization, and Melting Behaviors

Using two living nickel α-diimine complexes (rac-1 and rac-4) activated with methylaluminoxane (MAO), isotactic polypropylene (iPP) samples containing five types of regiodefects were synthesized. Both isolated and successive groups of (2,1) and (3,1) enchainments were identified by solution-state 13C NMR spectroscopy. The extended monomer and bulky nature of these defects add some restraints to the crystallinity level that can be achieved in these polymer samples. The complex based on a cumyl-derived ligand, rac-4, produces higher molecular weight iPP with a higher content of bulky defects than rac-1. Melting temperatures and crystallinity levels of rac-4-derived iPP are accordingly lower than the polymers obtained from rac-1. Although the nature of the chain-walking defect is similar to the addition of the ethylene unit, the difference in polymer properties is profound. At equivalent point defects per total monomers (XB), iPP samples with chain-walking defects display lower melting temperatures and much lower degrees of crystallinity than random 1-alkene copolymers, including those with comonomers excluded from the crystal lattice such as the 1-hexene and 1-octene counits. Furthermore, iPP containing (3,1) regiodefects develops significantly higher contents of the γ-polymorph than any other iPP or random copolymer with a matched XB composition. The experimental evidence is consistent with shorter crystallizable sequences for iPP with chain-walking defects based on (3,1) enchainments. Taking crystallizable and noncrystallizable total units as the basis to compute point defects, the properties of (3,1) iPP adhere to the basis of exclusion equilibrium theory, indicating that the defects are random-bulky or generated in a random fashion but of a defined extended/multimonomer nature.

Translation and Rotation of Spherulites during the Crystallization of Isotactic Polypropylene with Reduced Chain Entanglements

The isothermal crystallization of isotactic polypropylene (iPP) spherulites was observed under chain entanglements, reduced chain entanglements, and unconfined thick film conditions using polarized optical microscopy (POM). While spherulite translations and rotations were not observed in entangled iPP samples, the growing spherulites in the entanglements-reduced crystallizing melt conspicuously translated and/or rotated, even when their sizes exceeded the sample film thicknesses. Our entanglements-reduced samples were prepared by first slowly crystallizing a commercial entangled iPP in mineral oil (and then extracting the mineral oil with hexane) and by a direct polymerization method, respectively. The purity of the entanglements-reduced iPP samples and the extent of chain entanglements were examined by thermogravimetric analysis (TGA), gel permeation chromatography (GPC), and rheological measurements. Our findings seem to provide new clues into the crystallization of high molecular mass polymers.

3D refractive index profile for the characterization of necking phenomenon along stretched polypropylene fibres

The phenomenon of neck formation in polymers has attracted considerable attention. During the cold-drawing process an initial undrawn material is transformed into anisotropic one across a narrow transition region called ‘neck’. The Video Opto-Mechanical (VOM) device attached with multiple-beam Fizeau fringes techniques are used to stretch polypropylene (PP) fibres. A iPP sample is stretched to have a neck at room temperature. The optical properties of the deformed material over the necking region are examined. Another PP sample is stretched (without necking deformation) at room temperature and the optical properties are also examined. The task of this study is to characterize and assess the necking phenomenon along cold-drawn polypropylene (iPP) fibre axis. The effect of necking on the optical properties of the fibre is confirmed by the determination of the 3D refractive index profile at different regions along the deformed PP fibre. Also the orientation function is calculated for the necked sample. The contour lines of microinterferograms are given for illustrations.

Properties of isotactic polypropylene irradiated in various atmospheres

In this paper, changes in structure and physical properties of stabilized isotactic polypropylene (iPP) were created by gamma irradiation, up to a dose of 700 kGy, in different media: air, deionized distilled (DD) water and acetylene. Two main effects occur when polyolefins, such as iPP, are subjected to ionizing radiation: crosslinking and scission of macromolecules. The domination of one or the other of these competitive processes is determined by both the structural peculiarities of the polymers and the experimental irradiation conditions. Gel and infrared (IR) spectroscopy measurements were used to determine the changes in the degree of network formation and oxidative degradation, respectively. Sol-gel analysis was studied in detail using the Charlesby-Pinner (C-P) equation. The radiation-induced changes in the structure and evolution of oxygen-containing species were also studied through dielectric loss (tan ?) analysis in a wide temperature and/or frequency range. Evolution of low temperature dielectric relaxations with gamma irradiation was investigated. The results showed that degradation was the major reaction in the initial step of irradiation, no matter what the atmosphere was. The C-P equation seemed applicable when stabilized iPP was irradiated within a certain dose range in various atmospheres. The iPP irradiated in acetylene/air had the lowest/highest values for oxidation level, dielectric losses, Dg and G(s)/G(x) values. The calculated Dg values are 1.5 and 5 times larger for the irradiation in DD water and air than for the acetylene. Furthermore, our data confirm that oxidation strongly affects the gel point but has a much lower effect on the G(S)/G(X) ratio. In the case of dielectric relaxation measurements, the connection between the oxidative degradation and dielectric properties is well established and is in good agreement with IR spectroscopy measurements. The amount of carbonyl, hydroperoxide and other polar groups is much higher for the irradiation in air than in other media, leading to higher dielectric losses. Disappearance of low temperature (? and ?) relaxations with gamma radiation confirmed great sensitivity of iPP structure to radiation-induced changes. Complete ?vanishing? of the ? relaxation in iPP samples irradiated in air is connected with a large radiation-induced oxidative degradation in this medium. Similar crosslinking, oxidation and dielectric behaviour was observed for the samples irradiated in water and acetylene, indicating DD water as a good crosslinking medium.

Influence of Annealing on Microstructure and Mechanical Properties of Isotactic Polypropylene with β-Phase Nucleating Agent

The microstructure and mechanical properties of β-nucleated iPP before and after being annealed at different temperatures (90−160 °C) have been analyzed. Annealing induced different degrees of variation in fracture toughness of β-nucleated iPP samples, namely, slight enhancement at relatively low annealing temperatures ( 140 °C) has been observed. The variation of fracture toughness of β-nucleated iPP is observed to be dependent on the content of β-NA. Experiments, including scanning electronic microscope (SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and dynamic mechanical analysis (DMA), are performed to study the variations of microstructures as well as the toughening mechanism of the β-nucleated iPP after being annealed. The results indicate that the decreased number of chain segments in the amor...

Effect of Metallic Salts of Glutaric Acid on the Formation of β-crystalline Form in Isotactic Polypropylene

The effects of glutaric acid and the lithium, sodium, potassium, magnesium, calcium, strontium, barium, zinc, and cadmium salts of glutaric acid on the formation of β-crystalline form in isotactic polypropylene (iPP) at the crystallization temperatures 120 and 130°C have been investigated. It is found that glutaric acid and the sodium, potassium, and zinc salts of glutaric acid inhibit the formation of β-crystalline form in polypropylene. Lithium glutarate has a limited positive effect on the formation of β-crystalline form. Magnesium, calcium, strontium, barium salts of glutaric acid exhibit increased β-nucleation abilities with the increasing atomic radii of the combined metals. Cadmium glutarate is a good β-nucleating agent and the formation of β-crystalline form is favored at the crystallization temperature of 120°C. The increases in the orientation of b-axes of the nucleated iPP samples are due to the presences of sodium and zinc salts of glutaric acid.

Effect of network relaxation on void propagation and failure in isotactic polypropylene at large strain

AbstractThe important role of network relaxation in the voiding and fracture toughness of isotactic polypropylene (iPP) has been explored with video‐aid tensile tests, two‐dimensional small‐angle X‐ray scattering (2D‐SAXS) measurements, and morphological observations. The results indicated that macroscopic volume dilatation related to voiding became lower during large deformation of iPP sample with rich γ phase (denoted as γ‐iPP) cooled at 1°C/min, compared with one with exclusive α form (referred to α‐iPP) quenched in air. Furthermore, void propagation perpendicular to the tensile direction, demonstrated by Guinier approximation analysis of 2D‐SAXS results, was suppressed to a large extent in such a γ‐iPP sample. Less network relaxation, resulted from its peculiar crystalline and amorphous phases, was responsible for lower volume dilatation and slower void propagation in the γ‐iPP sample. Meanwhile, less network relaxation and suppressed transverse void propagation contributed to higher toughness in the γ‐iPP sample. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Influence of microstructure and semi-crystalline morphology on the β and γ mechanical relaxations of the metallocene isotactic polypropylene

In this work, the characteristics of the β and γ mechanical relaxations, i.e., temperature and relative intensity, of a series of metallocene iPP samples (MPP) are analysed. The hypothesis that the temperature and the intensity of the glass transition (β relaxation) and local sub- T g motions (γ relaxation) are related mainly to chain parameters and morphology has been corroborated. On the one hand, it has been found a critical average isotactic length ( n 1) around 30 propylene units, under which the β and γ dynamics are promoted with respect to the α relaxation. On the other hand, it is apparent that the features which determine the degree of constraint within the inter-lamellar region, i.e., the fraction of low- T m crystals, drive the relative intensities of the α, β and γ relaxation processes.

Study on transparent poly(propylene) nucleated by the “in situ” reaction products of dehydroabietic acid and sodium stearate

AbstractDehydroabietic acid (DHA), sodium stearate (NaSt), DHA/NaSt (1/1 wt/wt) mixture, the reaction products of DHA/NaSt (1/1 wt/wt) mixture after heating at 220°C for 10 min and sodium dehydroabietate (DHAA‐Na) were qualitatively analyzed by Fourier transform infrared spectrometry (FTIR) and wide angle X‐ray diffractometry (WAXD). The crystallization behavior, spherulitic morphology, optical and mechanical properties of iPP samples nucleated by DHA, NaSt, DHA/NaSt (1/1 wt/wt) mixture, and DHAA‐Na under the same concentration (0.3 wt %) were investigated by differential scanning calorimetry (DSC), polarized light microscope (PLM), optical and mechanical measurements. The results showed that an “in situ” reaction occurred between DHA and NaSt, and an effective transparent nucleating agent (DHAA‐Na) was produced during the melting blending of iPP and DHA/NaSt (1/1 wt/wt) mixture in the twin screw extruder. The crystallization onset and peak temperatures, the gloss and the mechanical properties were improved markedly; the crystallization half‐time, the spherulitic size, and the haze were reduced substantially when iPP samples are nucleated by DHA/NaSt (1/1 wt/wt) mixture. The optimum composition ratio of DHA/NaSt should be kept at ≈ 1/1 wt/wt and above 0.4 wt %/0.4 wt %. The DHA/NaSt bi‐component nucleator was effective for iPP with different MFR values. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Improved performance of isotactic polypropylene/titanium dioxide composites: Effect of processing conditions and filler content

Titanium dioxide (TiO 2) filled isotactic polypropylene (iPP) with various content of TiO 2, which was used as filler, were first single-extruded by an extruder, and then double-molded by compression molding. Scanning electron micrographs show a better adhesion between iPP and filler in the extrusion cum compression-molded samples than the extrusion-molded ones. X-ray diffraction and IR spectral studies reveal a structural change from a three-phase (α, β and γ) crystalline system of the neat iPP sample to only α-form due to inclusion of fillers. Microhardness increases rapidly and then levels off with increasing filler content and also shows variations with respect to molding conditions. A slight decrease of melting temperatures and a considerable increase of degradation temperatures of the samples with addition of filler are also observed. The dc electrical resistivity is observed to decrease with increasing TiO 2 content and temperature. Both the thermal and electrical properties are also found to affect by processing conditions. Based on these results, effect of processing conditions and filler content on changing morphologies and properties of the composites is described.

Effect of N,N′‐diphenyl adipamide on the formation of the β‐crystalline form in isotactic polypropylene

AbstractThe polymorphic compositions and mechanical properties of isotactic polypropylene (iPP) samples nucleated by a selective β‐nucleating agent [N,N′‐diphenyl adipamide (DPA)] were investigated with wide‐angle X‐ray diffraction, polarized light microscopy, scanning electron microscopy, and mechanical tests. It was found that β‐phase crystals emerged with the addition of DPA, and the relative proportion of the β‐crystalline form reached the maximum value of 0.97 with the addition of 0.1 wt % DPA. The curved lamellae in the β spherulites were like flowers. The β spherulites were etched more easily than α spherulites because amorphous regions were distributed inside the β spherulites. The Izod notched impact strength increased sharply with the addition of DPA and attained the maximum value of 7.30 kJ/m2 (the value of blank iPP was 3.13 kJ/m2) with the addition of 0.1 wt % DPA. An analysis of the misfit factors between DPA and β‐iPP showed that β‐iPP could epitaxially crystallize on the DPA crystal well. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

The β - and γ -Form Distributions of Isotactic Polypropylene Injection Molding Controlled via Melt Vibration

The effect of melt vibration on the crystal form of isotactic Polypropylene (iPP) samples prepared by low-frequency Vibration-Assisted Injection Molding (VAIM) was investigated. The distribution of crystal forms through the thickness was investigated under different VAIM processing conditions, with Conventional Injection Molding (CIM) for comparison. Wide-Angle X-ray Diffraction (WAXD) showed that the content of β -form crystals decreases from the surface to the core region in the CIM and VAIM samples. The β -form distribution through the thickness is controlled by the VAIM processing conditions. The occurrence of γ -form crystals was found in the VAIM samples produced at relatively low vibration frequency and relatively large pressure vibration amplitude.

Effect of calcium malonate on the formation of β crystalline form in isotactic poly(propylene)

AbstractCalcium malonate (Camt) is used to induce the β crystalline form in isotactic polypropylene (iPP) for the first time. The relative content of the β crystalline form (K value) increases markedly with the addition of Camt and attains the maximum value of 73.48% at 0.40% Camt. The amounts of bright and colorful β spherulites increase and the spherulitic sizes decrease with the increase in Camt contents. The tensile strength and the Izod notched impact strength of the nucleated iPP samples increase with the addition of Camt; in particular, the latter almost doubles at 0.40% Camt compared to that of the blank iPP sample. The β nucleation mechanism of Camt is analyzed by the dimensional lattice match criterion. Copyright © 2008 John Wiley & Sons, Ltd.

Changes of crystallinity and spherulite morphology in isotactic polypropylene after rolling and heat treatment

The spherulite morphology of the rolled and subsequent heat-treated isotactic polypropylene (iPP) was observed by polarized microscopy, and the crystallinity evolution of materials was also measured by the wide angle X-ray scattering (WAXS). Rolling led to the oblate spherulites in the deformed iPP samples. The sheared crystalline lamellae broke apart into sets of crystalline blocks during rolling. As a result, the crystallinity of the iPP samples was greatly reduced during deformation, which induced the unclear spherulites and spherulite boundaries. Subsequent heat treatment resulted in the strong recrystallization of the rolled iPP samples. But the recrystallization in this work only meant the rearrangement of the macromolecule along the unbroken crystalline lamellae and the existing small crystalline blocks in the deformed spherulites. Heat treatment did not change the shape of the spherulites formed during deformation. The recrystallization also resulted in very clear spherulites and spherulite boundaries.

Effect of magnesium malonate on the formation of the β crystalline form in isotactic polypropylene

AbstractMagnesium malonate (MgMt) is used to induce the β crystalline form in isotactic polypropylene (iPP) for the first time. It is shown by wide angle X‐ray diffraction, differential scanning calorimetry, and polarized light microscopy that the content of the β crystalline form increases markedly with the addition of MgMt and attains the maximum value at an MgMt content of 0.10wt%. The tensile strength and flexural modulus of the doped iPP sample decrease somewhat; the tensile strain at break increases slightly; and the Izod notched impact strength increases significantly with the addition of MgMt, as compared with the values for the blank iPP sample. The β‐nucleation mechanism of MgMt is analyzed by the dimensional lattice match criterion. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers.

Structure and properties of new reversibly crosslinked iPP/LDPE blends

AbstractReversibly crosslinked blends of isotactic polypropylene and low density polyethylene (iPP/LDPE) were prepared in the presence of crosslinking agents using reactive extrusion. The structure and properties of the modified blends were investigated by means of wide‐angle X‐ray scattering (WAXS), differential scanning calorimetry (DSC), and macro‐ and micro‐mechanical measurements. The crystallinity of the modified samples (LDPE, iPP, and their blends) does not seem to be so much affected by the crosslinking process. Results show that the microhardness of the iPP/LDPE blends notably increases with the iPP content. The micromechanical properties of the modified blends only improve slightly as a consequence of the crosslinking process. In the iPP samples, and also in the iPP/LDPE blends, this process gives rise to the appearance of new, crystalline ethylenic chains, as evidenced by the calorimetric measurements. Furthermore, the impact strength of the modified materials is improved as compared with that of the original ones, while some of the crosslinked blends show a ductile fracture behavior. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008