Role of tie molecules in the yielding deformation of isotactic polypropylene

We examined the effects of the tie‐molecule fraction on the yielding behavior of two isotactic polypropylenes, one having little ethylene content and the other as the homopolymer with no ethylene. The tie‐molecule fraction of the samples used in this study was controlled by blending ethylene‐α‐olefin of an α‐olefin content above 50 mol % in the blend of which the copolymers were incorporated into the amorphous regions of polypropylene (PP). An excellent linear relationship was observed between the measured yield stress and the tie‐molecule fraction estimated from the Huang–Brown model, suggesting that the tie‐molecule fraction and lamellar stiffness determine whether the lamellar fragmentation is easily activated or not, depending on the PP composition. Furthermore, an extended Huang–Brown model predicts a lamellar cluster connecting about five lamellae, which has a potential to account for morphological transformation of the spherulitic structure into a fibrillar one. Comparing the immiscible blends showing a phase‐separated morphology with the partially miscible blends mentioned above, the yield stress was lowered by the presence of rubber phase, apparently in a similar manner; but the yielding processes were clearly discriminated between both cases when the yield stress was plotted against the tie‐molecule fraction. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 357–368, 1999     

Morphological study of fatigue-induced damage in isotactic polypropylene

This article reports initial results of an investigation whose aim is to characterize fatigue damage induced in semicrystalline polymers subjected to uniaxial high cycle fatigue. Herein we report results obtained from fatiguing tensile bars of high molecular weight compression-molded alpha-phase iPP. Samples were fatigued for up to one million cycles at a frequency of 2 Hz. During fatigue, in situ measurements of dynamic mechanical response and energy densities were recorded. Postmortem morphological studies were also conducted using SEM of etched surfaces and TOM. The results show that damage formation occurs in a regularly spaced array of crazes. This damage, its evolution, and energetics are discussed as they relate to the overall fatigue life of the material. A methodology to isolate the energy consumption for the formation of a single craze is given. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2751–2760, 1998     

Shear-induced crystallization of isotactic polypropylene based nanocomposites with montmorillonite

Shear-induced isothermal crystallization in iPP based nanocomposites with organo-modified montmorillonite was followed by light depolarization technique. Prior to the crystallization, samples were sheared at 1 or 2 s⁻¹ for 10 s in a plate-plate system at crystallization temperature of 136 °C. Structure of the solidified specimens was investigated by light microscopy and electron microscopy, X-ray techniques and IR spectroscopy. Strong enhancement of the nucleation and crystallization after shearing was observed in the compatibilized nanocomposites with the clay. Clay exfoliation was found to accelerate strongly the shear-induced nucleation and overall crystallization. However, the sheared samples exhibited only weak orientation of α crystals with (0 4 0) crystallographic planes parallel to shearing direction that resulted probably from a small population of oriented crystals that formed due to shear-induced orientation of iPP chains and served as nuclei for further nearly isotropic growth.     

Tensile yield of isotactic polypropylene in terms of a lamellar‐cluster model

In this study, the structural factors controlling the yield in isotactic polypropylene materials were theoretically investigated. To describe the yielding behavior of spherulitic polypropylenes, we introduced a new structural unit, lamellar clusters, which are several stacked lamellae bound by tie molecules. It was shown that tie molecules between adjacent lamellar clusters produce a concentrated load acting on the cluster surface, leading to the bending deformation of the lamellar clusters. The yielding behavior can be explained if one assumes that the disintegration of the lamellar clusters occurs when the elastic‐strain energy stored by the bending deformation reaches a critical value. By applying the fracture theory of composites to a system consisting of lamellar clusters and tie molecules, we found the yield stress σ_y to be proportional to , in which E_Y is the Young's modulus and U_y is the yield energy. The proportional coefficient between σ_y and depends only on the cluster size and tie‐molecule density, so this proportionality is expected to be true for other spherulitic semicrystalline polymers such as polyethylenes, being independent of temperature and tensile rate. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1037–1044, 2000     

A bimodal structure of solution-grown isotactic polypropylene with orthogonally crossed lamellae

Isotactic polypropylene (iPP) was crystallized from solution on a uniaxially-oriented iPP film. Small-angle x-ray scattering patterns obtained from the sample showed two perpendicularly crossed lameliae 9.3 nm thick that overgrew flat-on and edge-on on the substrate. In the through wide-angel x-ray diffraction pattern (taken with incident x-rays normal to the iPP film surface), strong hkO reflections were arranged in an hkO net pattern indicating that the a-axis of the monoclinic α unit cell was oriented parallel to the chain direction of the substrate. From this, it was concluded that the flat-on lamellae grew with the a-axis parallel to the chain axis of the substrate and with the b-axis parallel to its surface. In the edge wide-angle x-ray diffraction pattern (X-rays incident on the edge of the film), arced, strong 110 and 220 reflections from overgrown crystals were observed on the equator of the fiber pattern of the substrate. This indicated that the edge-on lamellae epitaxially grew with the c-axis aligned parallel to the chain axis of the substrate. The homoepitaxy explains the correlated growth mode between the orthogonally crossed lamellae: they grew epitaxially, the a-axis of one lamella coinciding with the c-axis of the other and the {010} planes in contact. © 1993 John Wiley & Sons, Inc.     

Thermal behavior of isotactic polypropylene freeze-extracted from solutions with varying concentrations

Isotactic polypropylene (iPP) with narrow molecular mass distribution was freeze-extracted from n-octane solutions with varying concentrations. The recovered samples were characterized by differential scanning calorimetry. It is found that the sample recovered from the very dilute solution exhibits the higher non-isothermal crystallization temperature, faster isothermal crystallization rate, and smaller Avrami index. And there should exist a critical concentration corresponding with the critical overlap concentration proposed by de Gennes in the polymer solutions. In the solution well below the critical concentration, the iPP chains were isolated from each other, resulting in an acceleration of melt crystallization for the recovered samples. It seems that the chain entanglement is a barrier to the melt crystallization of polymer.     

The effect of specific nucleation on tensile mechanical behaviour of isotactic polypropylene

Commercial-grade isotactic polypropylene was modified with a specific β-nucleation agent NJ-Star (N,N^′-dicyclohexylnaphthalene-2,6-dicarboxamide) in concentrations 0.03, 0.10 wt.% and with a specific α-nucleating agent Millad 3988 (1,2:3,4-bis-O-(3,4-dimethylbenzylidene)sorbitol) in a concentration of 1.0 wt.%. Specimens for mechanical studies were prepared by injection moulding. Two types of tensile mechanical testing were performed at room temperature: (1) stress-strain test encompassing the plastic behaviour well behind the yield point and (2) tensile creep in the region of non-linear viscoelasticity. The results derived from the stress-strain traces show a distinct decrease in Young's modulus and yield stress for samples containing the crystalline β-phase as compared with non-nucleated and α-nucleated samples. This decrease was more pronounced with samples containing the lower β-nucleant concentrations (0.03 wt.%). Higher compliance of specimens containing the β-phase was also manifested in their creep behaviour. However, the creep rate of the specimen with the higher nucleant content (0.10 wt.%) did not rise with time so that its creep curve intersected the creep curves of non-nucleated and α-nucleated samples. Thus, at creep times longer than 1000 min, the sample with 0.1 wt.% of the β-nucleant showed a lower compliance than non-nucleated polypropylene and at 10 000 min reached the compliance of the α-nucleated sample. The different softening effect of the β-phase in the high-strain and low-strain regions has been ascribed to a specific structure of the amorphous interlayer induced by the presence of the β-crystallites.     

Stability and phase transformations of the mesomorphic form of isotactic polypropylene in stereodefective polypropylene

A study of the thermodynamic stability and the related polymorphic transformations induced by thermal treatments of the mesomorphic form that crystallizes in stereodefective metallocene isotactic polypropylene (iPP) is presented. We show that the mesomorphic form of the more isotactic samples is stable at room temperature, whereas the mesomorphic form crystallizing in the more stereoirregular sample is unstable and crystallizes at room temperature in the crystalline α form. In any case, the mesomorphic form transforms during heating or by annealing at temperatures higher than 60–80 °C always in the α form, regardless of the stereoregularity, even in the case of stereoirregular samples generally crystallizing from the melt in the γ form. These data confirm the proposed model of structure of the mesomorphic form as small aggregates of chains in three-fold helical conformation packed with lateral correlations similar to the α form of iPP.     

Crystallization and melting behaviors of isotactic polypropylene nucleated with compound nucleating agents

Crystallization and melting behaviors of isotactic polypropylene (iPP) nucleated with compound nucleating agents of sodium 2,2′‐methylene‐bis (4,6‐di‐tert‐butylphenyl) phosphate (hereinafter called as NA40)/dicyclohexylterephthalamide (hereinafter called as NABW) (weight ratio of NA40 to NABW is 1:1) were studied by differential scanning calorimetry and wide‐angle X‐ray diffraction (WAXD), the relative β‐amount of iPP nucleated with these compound nucleating agents was also calculated in Turner‐Jones equation by using wide‐angle X‐ray diffraction data. Under isothermal crystallization, there exists a temperature range favorable for formation of β‐iPP. When the concentration of compound nucleating agents is 0.2 wt %, the temperature range is from 100 to 140 °C. While in nonisothermal crystallization, lower cooling rate is favorable for form of β‐iPP and the relative β‐amount of iPP increases with the decreasing of cooling rate in crystallization process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 911–916, 2008     

Crystallization and melting of isotactic polypropylene in response to temperature modulation

Isotactic polypropylene (iPP) was crystallized using temperature modulation in a differential scanning calorimeter (DSC) to thicken the crystals formed on cooling from the melt. A cool-heat modulation method was adopted for the preparation of the samples under a series of conditions. The effect of modulation parameters, such as temperature amplitude and period was monitored with the heating rate that followed. Thickening of the lamellae as a result of the crystallization treatment enabled by the cool-heat method lead to an increase in the peak melting temperature and the final traces of melting. For instance, iPP melting peak shifted by up to 3.5°C with temperature amplitude of 1.0°C while the crystallinity was increased from 0.45 (linearly cooled) to 0.53. Multiple melting endotherms were also observed in some cases, but this was sensitive to the temperature changes experienced on cooling. Even with a slower underlying cooling rate and small temperature amplitudes, some recrystallization and reorganization occurred during the subsequent heating scan. The crystallinity was increased significantly and this was attributed to the crystal perfection that occurred at the crystal growth surface. In addition, temperature modulated differential scanning calorimetry (TMDSC) has been used to study the melting of iPP for various crystallization treatments. The reversing and non-reversing contribution under the experimental time scale was modified by the relative crystal stability formed during crystallization. Much of the melting of iPP was found to be irreversible.This revised version was published online in November 2005 with corrections to the Cover Date.     

Nucleation of crystallization in isotactic polypropylene and polyoxymethylene with poly(tetrafluoroethylene) particles

Nucleation of crystallization of isotactic polypropylene (iPP) and polyoxymethylene (POM) with dispersed submicron particles of another polymer - poly(tetrafluoroethylene) (PTFE) was studied. The polymers were mixed with various contents of PTFE particles, in the range from 0.005 to 0.5 wt.%. iPP and POM with PTFE particles are all-polymer systems with enhanced nucleation of crystallization. PTFE particles with sizes below 300 nm added to POM and iPP efficiently decreased sizes of polycrystalline aggregates. Moreover, nonisothermal crystallization temperature of iPP by increased by up to 14 °C. iPP and POM with PTFE exhibited the elastic modulus slightly higher, by up to 10-13%, than that of the neat polymers. Other mechanical properties remained unchanged, with the exception of reduced elongation at break of POM with PTFE.     

Wide and small-angle X-ray scattering study of isotactic polypropylene gamma irradiated in bulk

Gamma irradiated isotactic polypropylene (IPP) has been studied by means of wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS). The skin layer has been investigated by WAXS reflection mode while the core layer underwent WAXS studies by transmission mode. β-IPP has been found solely in the skin layer. An increase in the β-phase has been observed as result of the irradiation. A phase transitions and decrease of crystallite sizes have been also observed. All of the parameters show a sharp change at a critical dose of 100 kGy. At this point the system parameters reverse. Radiation processes proceeding up to 100 kGy called radiation annealing are related to the improved crystallite perfection and thus emphasize the phase boundary. The processes provoke at higher radiation doses, up to 2000 kGy, damage in both crystal and amorphous lamellar parts. The values of the crystal and amorphous densities get closer and the process is similar to the partial radiation melting.     

AFM imaging of interfacial morphologies in carbon-fiber reinforced isotactic polypropylene

 The morphology of high-modulus carbon-fiber (HM-CF) reinforced isotactic polypropylene (iPP) was investigated for the first time by atomic force microscopy (AFM) using chemically etched specimens. The images exhibited α-transcrystalline morphology for samples crystallized from quiescent melts, nucleated by HM-CF. In melts sheared by fiber pulling, αβ-cylindritic columnar morphology was observed in agreement with earlier thermo-optical studies. AFM images in the interfacial region of the β-cylindrites unveiled fine morphological details of α-row nuclei. Based on the observations, we concluded that in β-cylindrites, the lamellar growth in α-row nuclei occurs during epitaxial crystallization on bundles of extended iPP chains which form during shearing of the polymer matrix by fiber pull. Received: 25 June 1996 Accepted: 17 October 1996     

The β‐crystalline form of isotactic polypropylene in blends of isotactic polypropylene and polyamide‐6/clay nanocomposites

Blends of isotactic polypropylene and polyamide‐6/clay nanocomposites (iPP/NPA6) were prepared with an internal batch mixer. A high content of the β‐crystalline form of isotactic polypropylene (β‐iPP) was observed in the injection‐molded samples of the iPP/NPA6 blends, whereas the content of β‐iPP in the iPP/PA6 blends and the iPP/clay composite was low and similar to that of neat iPP. Quiescent melt crystallization was studied by means of wide‐angle X‐ray diffraction, differential scanning calorimetry, and polarized optical microscopy. We found that the significant β‐iPP is not formed during quiescent melt crystallization regardless of whether the sample used was the iPP/NPA6 blend or an NPA6 fiber/iPP composite. Further characterization of the injection‐molded iPP/NPA6 revealed a shear‐induced skin–core distribution of β‐iPP and the formation of β‐iPP in the iPP/NPA6 blends is related to the shear flow field during cavity‐filling. In the presence of clay, the deformation ability of the NPA6 domain is decreased, as evidenced by rheological and morphological studies. It is reasonable that the enhanced relative shear, caused by low deformability of the NPA6 domain in the iPP matrix, is responsible for β‐iPP formation in the iPP/NPA6 blends. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3428–3438, 2004     

In situ fibrillation of polyamide 6 in isotactic polypropylene occurring in the laminating‐multiplying die

The polyamide 6 (PA6)/isotactic polypropylene (iPP) in situ fibrillation composites are prepared by a novel extrusion die with an assembly of laminating‐multiplying elements (LMEs). The scanning electron micrographs illustrate that the dividing‐multiplying processes in LMEs elongate, break, and stabilize the dispersed PA6 phase in the iPP matrix along the flowing direction (FD). The morphology development of PA6 with different LME numbers greatly affects the rheological properties, crystalline behaviors, and mechanical properties. The dynamic rheological test performed at 195°C shows that the increased spatial restriction of the high‐aspect‐ratio PA6 particles increases the viscoelastic moduli, complex viscosity, and relaxation time. The crystalline analysis reveals that the heterogeneous nucleation becomes predominant and the transcrystalline morphology is observed in those samples produced by more LMEs. The tensile tests indicate that both, breaking strength and elongation, enhanced simultaneously because of the fibrillation of dispersed phase and the improvement in interfacial adhesion between the fibers and the matrix. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical investigation on the model active site for isotactic polypropylene in heterogeneous Ziegler-Natta catalyst: A density functional study

The stereoselectivity of the model active site formed by the adsorption of Ti₂Cl₇ on the (1 0 0) surface of MgCl₂ was investigated by density functional calculations. The analysis of energetics for successive propylene insertions into the model active site reveals that the insertion of propylene into the model active site is energetically more favorable when a growing chain and one chlorine atom (that makes the active site chiral) are on the opposite side rather than on the same side. From this result, it is realized that the model active site is highly stereoselective. It is also observed that the Cl atoms near the growing chain significantly affect the activation energy barrier through the interaction with the growing chain.     

Ordered long‐helical conformation of isotactic polypropylene obtained in constrained environment of nanoclay

Special crystallization event of isotactic polypropylene (iPP) in a constrained environment, a layered clay network, was followed by in situ Fourier transform infrared (FTIR) spectroscopy during the cooling process. Before occurrence of nucleation/crystallization, a conformationally ordered phase, which consists of significant amounts of long 3₁ helices with 14 monomeric units, has been identified for the first time. More importantly, it was found that the long‐ordering helices could play a more important role than short ones for the confined crystallization. It could be tentatively explained as due to the existence of constrained regions in the proximity of the nanoclay platelets or tactoids and the heterogeneous nucleation capability of the surface of nanoclay. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of conformational-segmental mobility in linear polyethylene and isotactic polypropylene by the spin probe method

A general version of the Kusumoto-Bullock free-volume model for reorientation dynamics of spin probe in solid polymers is described. Its application enables one to obtain simultaneously the size parameter of a segment and the kinetic-energy parameter of segmental mobility. These data, along with theoretical knowledge of the conformational microstructure and conformational dynamics, allow a discussion of molecular mobility in terms of concrete conformation-segmental motion mechanisms. Participation of one-barrier transitions of the typePptqQPpg ^± qQ, and two-barrier transition of the type gauche-migration, creation and annihilation of kink sequence was observed for linear polyethylene. In agreement with the conformational microstructure, it was found for isotactic polypropylene that Schatzki's crankshaft motions of four-member segments might be involved.     

Thermal stability of shear-induced precursor structures in isotactic polypropylene by rheo-X-ray techniques with couette flow geometry

Combined in situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) studies using couette flow geometry were carried out to probe thermal stabilty of shear-induced oriented precursor structure in isotactic polypropylene (iPP) at around its normal melting point (162 °C). Although SAXS results corroborated the emerging consensus about the formation of “long-living” metastable mesomorphic precursor structures in sheared iPP melts, these are the first quantitative measures of the limiting temperature at which no oriented structures survive. At the applied shear, rate = 60 s⁻¹ and duration t_s = 5 s, the oriented iPP structures survived a temperature of 185 °C for 1 h after shear, while no stable structures were detected at and above 195 °C. Following Keller's concepts of chain orientation in flow, it is proposed that the chains with highly oriented high molecular weight fraction are primarily responsible for their stability at high temperatures. Furthermore, the effects of flow condition, specifically the shear temperature, on the distributions of oriented and unoriented crystals were determined from rheo-WAXD results. As expected, at a constant flow intensity (i.e., rate = 30 s⁻¹ and duration, t_s = 5 s), the oriented crystal fraction decreased with the increase in temperature above 155 °C, below which the oriented fraction decreased with the decrease in temperature. As a result, a crystallinty “phase” diagram, i.e., temperature versus crystal fraction ratio, exhibited a peculiar “hourglass” shape, similar to that found in many two-phase polymer–polymer blends. This can be explained by the competition between the oriented and unoriented crystals in the available crystallizable species. Below the shear temperature (155 °C), the unoriented crystals crystallized so rapidly that they overwhelmed the crystallization of the oriented crystals, thus depleting a major portion of the crystallizable species and increasing their contribution in the final total crystalline phase. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3553–3570, 2006     

Rules of supermolecular structure formation in sheared isotactic polypropylene melts

Shear-induced crystallization of isotactic polypropylene (iPP) homo-, block, and random copolymers was studied and compared to that in quiescent melt. It was evidenced by means of the thermo-optical technique that melt-shearing, caused by fiber pulling, is associated with the development of α-row-nuclei. The surface of the in situ formed α-row-nuclei may induce the growth of the β-modification of iPP resulting in a cylindrite of polymorphous composition. The polymorphous composition is controlled by the temperature-dependent relative growth rate of the α- and β-iPP for which a model explanation was given. The β-nucleation ability of the α-row-nuclei is lost by melt-shearing at high temperature or remelting. This was attributed to a coverage of the β-nuclei by the α-phase. The structural memory of the supermolecular structures was studied in repeated melting and crystallization cycles and discussed. It was found that the quality of the fiber did not influence the mechanisms concluded. The shear-induced crystallization of propylene block copolymers was highly analogous to the homopolymers. In case of the random copolymers, however, crystallization in sheared melt resulted in an α-cylindritic structure, because for propylene random copolymers the growth rate of the α-modification is always higher than that of the β. It was also demonstrated that the mechanism of shear-induced crystallization was unaffected when the crystallizing PP melt contained selective β-nucleants. © 1996 John Wiley & Sons, Inc.