Crystallization behaviour of isotactic polypropylene/linear low density polyethylene blends

Crystallization behaviour of isotactic polypropylene/linear low density polyethylene (iPP/LLDPE) blends has been investigated by optical microscopy and DSC. Crystallization of iPP depends upon blend composition and thermal history. When blended with LLDPE, the crystallization temperature of iPP, T_c, decreased slightly. Crystallinity did not change in the range 0-80wt% LLDPE; there were only slight changes in the crystalline structure, but LLDPE seemed to resist forming the β type of spherulites. Below 80 wt% of LLDPE, iPP was a continuous phase. The iPP spherulite growth rate was almost constant; however, overall crystallization decreased due to decreasing primary nuclei density.     

Rheological studies on the quasi-quiescent crystallization of polypropylene nanocomposites

Isothermal crystallization of isotactic polypropylene (iPP)/organic montmorillonite (OMMT) binary nanocomposite and iPP/OMMT/poly(ethylene-co-octene) (PEOc) ternary nanocomposites has been investigated by polarized optical microscopy (POM), rheometry and scanning electron microscopy (SEM). At the stage of nucleation the heterogeneous nucleation effect of OMMT was much greater than the concentration fluctuation assisted nucleation effect in the ternary nanocomposite. Besides, PEOc played a role of inhibitor of OMMT nucleation agents at the nucleation stage because many of OMMT layers were distributed around PEOc-rich domains. At stage II of the crystal growth process, the entanglement effect of PEOc greatly affected the rheological response (storage modulus (G′) and its growth rate) due to the long side chains of PEOc component. In stage III of the growth process, OMMT layers and the entanglement of PEOc chains limited the motion of polypropylene chains. So the growth rate of G′ was slowed down. During the shrinkage and cooling process after isothermal crystallization, some fibril links between the spherulites, consisting of PEOc chains and iPP chains, were formed from the amorphous phases surrounding the spherulites.     

Morphology of a melt crystallized iPP/HDPE/hydrogenated hydrocarbon resin blend

The structure, phase structure, morphology, crystallization and melting behavior of isotactic polypropylene (iPP) blended with a master batch (MB), formed by high density polyethylene and hydrogenated hydrocarbon resin (iPP/MB), have been in details investigated by using X-ray diffraction, optical microscopy and differential scanning calorimetry. It was found that the structure and morphology depend on crystallization conditions. A new family of α spherulites of iPP (type I spherulites) can be activated using appropriate crystallization conditions. Nucleation of these spherulites has been explained by using the approach of nucleus migration in polymer blends. Type I spherulites present specific morphological, kinetic and thermal behaviors. In particular it was found that the growth rate of type I spherulites, at a given T_c, is higher than the growth rate of spherulites grown from plain iPP.     

Temperature dependence of the nucleation effect of sorbitol derivatives on polypropylene crystallization

The temperature dependence of the nucleation effect of three sorbitol derivatives on the crystallization of isotactic polypropylene (iPP) was studied by means of isothermal crystallization kinetic analysis. Isothermal crystallization thermograms obtained by differential scanning calorimetry (DSC) were analyzed based on the Avrami equation. The Avrami analysis for the nucleated iPP was carried out with DSC data collected to 35% relative crystallinity, and the rate constants were corrected assuming the heterogeneous nucleation and three dimensional growth of iPP spherulites. A semi-empirical equation for the radial growth rate of iPP spherulites was given as a function of temperature and was used to determine the number of effective nuclei at different temperatures. The number of effective nuclei in the nucleated samples was estimated to be 3 × 10² ∽ 10⁵ times larger than that in the neat iPP. The logarithmic numbers of the effective nuclei decreased linearly with decreasing degree of supercooling in the range of crystallization temperatures tested. The temperature dependence of the effect of the nucleating agents on iPP crystallization was given quantitatively in terms of the deactivation factor defined as a fraction of the particles that are active at a particular temperature but inert at the temperature one degree higher. The nucleation activity and its temperature dependence are considered to be cooperative effects of many factors, including the dispersion and the physical or chemical nature of the agent as well as the interaction between the agent and the polymer. It is suggested that the temperature dependence of the effect of a nucleating agent should be treated as a characteristic of a given polymer/ nucleating agent mixture.     

Isothermal crystallization kinetics and morphology development of isotactic polypropylene blends with atactic polypropylene

The crystallization kinetics and morphology development of pure isotactic polypropylene (iPP) homopolymer and iPP blended with atactic polypropylene (aPP) at different aPP contents and the isothermal crystallization temperatures were studied with differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized optical microscopy. The spherulitic morphologies of pure iPP and larger amounts of aPP for iPP blends showed the negative spherulite, whereas that of smaller amounts of aPP for the iPP blends showed a combination of positive and negative spherulites. This indicated that the morphology transition of the spherulite may have been due to changes the crystal forms of iPP in the iPP blends during crystallization. Therefore, with smaller amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends increased with increasing aPP and presented a lower degree of perfection of the γ form coexisting with the α form of iPP during crystallization. However, with larger amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends decreased and reduced the γ‐form crystals with increasing aPP. These results indicate that the aPP molecules hindered the nucleation rate and promoted the molecular motion and growth rate of iPP with smaller amounts of aPP and hindered both the nucleation rate and growth rate of iPP with larger amounts of aPP during isothermal crystallization. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1093–1104, 2007     

Morphology,crystallization and melting behaviour of films of isotactic polypropylene blended with ethylene-propylene copolymers and polyisobutylene

The crystallization, the morphology and the thermal behaviour of thin films of isotactic polypropylene (iPP) blended with elastomers such as random ethylene-propylene copolymers (EPM) with different ethylene content and polyisobutylene (PiB) were investigated by means of optical microscopy, differential scanning calorimetry and wide angle X-ray diffractometry. During crystallization EPM copolymers are ejected on the surface of the film forming droplet-like domains. A different morphology is observed in iPP/PiB blends. For these mixtures the elastomers separate from the iPP phase forming spherical domains that are incorporated in the iPP intraspherulitic regions. Both EPM and PiB elastomers act as nucleant agents for iPP spherulites. This nucleation efficiency is strongly dependent on the chemical structure and molecular mass of the elastomers. The addition of EPM causes an elevation of the observed and equilibrium melting temperature of iPP. This unusual effect may be accounted for by assuming that the elastomers are able to extract selectively the more defective molecules of iPP. The depression of the growth rate of spherulites and the observed and equilibrium melting temperature of iPP, noted in iPP/PiB blends, suggests that these two polymers have a certain degree of compatibility in the melt.     

Spherulite nucleation in blends of isotactic polypropylene with isotactic poly(butene-1)

The phase morphology and the influence of composition on the primary nucleation of isotactic polypropylene in isotactic polypropylene/isotactic poly(butene-1) (iPP/iPB) blends were investigated by electron and light microscopy and small-angle light scattering. It was found that iPP and iPB are miscible but the thermal treatment induces partial phase separation of components and the formation of iPP-rich and iPB-rich phases. The complete phase separation needs high temperatures and/or a long time of melt annealing. In samples crystallized isothermally at low undercooling the heterogeneous primary nucleation in blends is depressed as compared to plain iPP. In blends the less active heterogeneities lose their activity because of an increase of the energy barrier for critical size nucleus formation due to phase separation of blend components during crystallization. For the same reason the rate of homogeneous nucleation in blends decreases, as observed in samples crystallized at very high undercooling. At very high undercooling iPP and iPB are able to crystallize with similar rates, which results in the formation of a fraction of iPB spherulites in addition to iPP spherulites. Consequently the number of spherulites in the blend is larger than that in plain iPP, in spite of the decrease in the homogeneous nucleation rate of iPP in the blend. © 1994 John Wiley & Sons, Inc.     

Properties of thin films of isotactic polypropylene blended with polyisobutylene and ethylene-propylene-diene terpolymer rubbers

An experimental study was carried out to investigate the kinetic, morphological and thermodynamic properties of thin films of isotactic polypropylene (iPP) blended with several elastomers such as ethylene-propylene-diene terpolymer (EPDM) and three samples of polyisobutylene (PIB) with different molecular masses. The addition of the rubber to iPP causes drastic modifications in the morphology, nucleation density, spherulite growth rate and thermal behaviour of iPP. Such modifications depend strongly on the chemical and molecular mass of the added elastomer and on the composition of the blend. All the elastomers studied seem to act as nucleating agents for the iPP spherulites. The addition of PIB to iPP results in a reduction of the spherulite growth rate G, whereas the addition of EPDM does not seem to have a great influence. For the iPP/PIB_HM iPP/PIB_MM and iPP/EPDM blends a depression of the equilibrium melting temperature T_m, with respect to that of pure iPP, is observed. This depression is increased for the blend containing 20% rubber. This effect is probably related to phenomena of partial miscibility in the melt and to the coexistence of processes such as molecular fractionation and preferential dissolution of the more defective molecules.     

Isothermal crystallization of PP-g-GMA copolymer

The overall isothermal crystallization kinetics for neat polypropylene and grafted polypropylene systems were investigated. The rate constants were corrected assuming the heterogeneous nucleation and three dimensional growth of polypropylene spherulites. A semiempirical equation for the radial growth rate of polypropylene spherulites was developed as a function of temperature, and was used to determine the number of effective nuclei of different temperatures. The number of nuclei in grafted samples was estimated to be 10²–10³ times larger than that of neat polypropylene. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1565–1569, 1997     

Nucleation effect of inclusion complexes with different polyolefin as guest molecules on the crystallization of polypropylene

The γ‐cyclodextrin (γ‐CD) inclusion complexes (ICs) with four kinds of polyolefin (PO) as guest molecules were prepared. The crystallization behavior of isotactic polypropylene (iPP) blended with the γ‐CD and γ‐CD–PO ICs was investigated by differential scanning calorimetry, polarized optical microscopy, and light scattering. The iPP blended with the ICs was found to exhibit higher crystallization temperature (T_C), smaller spherulites, and faster crystallization rate than those of neat iPP, indicating that the ICs play a role of nucleating agent on the crystallization of iPP and induce accelerated crystallization. The IC with PO having higher T_C as guest molecules showed higher nucleation effect than the IC with PO having lower T_C as guest molecules. The results suggest that the nucleation effect of these ICs was affected by the kinds of the guest molecules. The higher T_C guest molecules could result in higher nucleation effect. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The influence of rubber-matrix interfaces on the crystallization kinetics of isotactic polypropylene blended with ethylene-propylene-diene terpolymer (EPDM)

Blends of isotactic polypropylene with amorphous and slightly crystalline ethylene-propylene-diene terpolymer (EPDM), prepared by solution blending, have been investigated by optical microscopy and differential scanning calorimetry. Nucleation and crystallization kinetic parameters, such as nucleation rates, nucleation half times, Avrami-exponents and spherulitic growth rates, have been determined. It has been found that the dispersion of crystalline EPDM in iPP is different from that of amorphous EPDM. Both EPDMs are incorporated into the spherulites, causing a decrease of the maximum growth rate of the iPP spherulites. The surface free energy of the iPP crystals is diminished on adding EPDM to iPP and is accompanied by a higher secondary nucleation rate. From the decrease observed in the Avrami exponent with increasing EPDM concentration in the blend, it has been concluded that nucleation becomes predominantly heterogeneous, as there is a proportional increase in the interfacial area between the two components.     

Crystallization behaviour of fractions of isotactic polypropylene with different degrees of stereoregularity

The influence of the degree of stereoregularity on the overall rate of crystallization and on the melting behaviour of fractions of isotactic polypropylene has been investigated. Fractions with different stereoregularity were obtained by extraction methods. The stereochemical pentad population was determined by ¹³C n.m.r. analysis. The results show that at constant T_c the overall rate constant of crystallization decreases with the increase of configurational chain defects whereas at a given ΔT the contrary is observed. An evident phenomenon of secondary crystallization is observed only in fractions of IPP with a higher concentration of defects. Such observation may explain the double peaked endotherm of fusion of more defective IPP fractions. The free energy of formation of a nucleus of critical dimensions has been calculated for each fraction and its value correlated with the degree of stereoregularity.     

Enhanced spherulite growth rates of isotactic polypropylene in incompatible blends

Isothermal radial growth rates (G) of isotactic polypropylene (iPP) spherulites in immiscible blends with a polyethylene based ionomer (1) partially neutralized with Zn⁺², (2) completely acidified, and (3) completely esterified have been determined at crystallization temperatures between 118 and 124°C by time-lapse photomicroscopy. In these blends iPP spherulites grow with increased G as compared to that of pure iPP. This behavior is attributed to increases in the rate of transport, due to limited miscibility induced by mechanical mixing. However, subtle differences in the melting behavior suggest that changes in Spherulite morphology may also be a contributing factor.     

Crystallization behaviors in the isotactic polypropylene/graphene composites

Crystallization behaviors and kinetics of iPP in an in-situ prepared isotactic polypropylene/graphene (iPP/G) composites were studied in this paper. In samples used in this study, the graphene fillers were well dispersed, and the interfacial adhesion exhibited enhanced features between graphene and iPP components. The thermal stability of the composites was improved by about 100 °C compared to the pristine iPP. It was found that the crystallization morphology, crystallization rate and kinetics of the iPP/G composites were significantly influenced by the presence of graphene. The nucleation and epitaxial growth of iPP on the graphene surface were observed and studied in detail. It was observed that the nucleation of iPP favored to occur at the wrinkles and edges due to the good match of the lattice parameters and the weak spatial hindrance compared to the smooth surface. Numerous nuclei epitaxially formed and the size of the crystals was very small. The schematic diagram was also proposed for the nucleation and growth process of iPP on the graphene surface in the iPP/G composites. Meanwhile, the overall crystallization kinetics and crystals growth were analyzed through Avrami equation. The obtained Avrami index n decreased with the graphene loadings and was close to 2 for the iPP/G composites, which implied that the growth of iPP in the composites was in two-dimension. And this was caused by the structure of graphene and the spatial confinement effect of graphene platelets in the iPP/G composites.     

The effect of propylene–ethylene copolymers with different comonomer content on melting and crystallization behavior of polypropylene

The effect of propylene–ethylene copolymers (PEc) with different ethylene‐unit contents on melting and crystallization behaviors of isotactic‐polypropylene (iPP) were investigated by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The results show that the addition of PEc decreases significantly crystallization temperature (T_c) of iPP, but slightly affects melting temperature (T_m). With increasing the ethylene‐unit content of the propylene–ethylene copolymers, the decrease in crystallization temperature of iPP is smaller. The PLM results show that the spherulite growth rate decreases with increasing crystallization temperature for iPP and iPP/PEc blends. The higher the ethylene‐unit content of the copolymers is, the lower the spherulite growth rate (G) of iPP/PEc blends is. The influence of the PEc on nucleation rate constant (K_g) and fold surface energy (σ_e) of iPP was examined by nucleation theory of Hoffman and Lauritzen. The results show that both K_g and σ_e of iPP/PE20(80/20) and iPP/PE23(80/20) blends are higher than those of iPP, demonstrating that the overall crystallization rate of iPP/PEc blends decreased as compared to that of iPP, resulting from the decrease of the nucleation rate and the spherulite growth rate of iPP. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Influence of two liquid crystalline polysiloxanes with different average molecular weight as new β-nucleator on crystallization structure of isotactic polypropylene

Two liquid crystalline polymers (LC-N1 and LC-N2) with different average molecular weight were designed and synthesized as a new β-nucleator in isotactic polypropylene (iPP). The main aim of this work was to investigate the influence of LC-N1 and LC-N2 on the crystallization structure and β nucleating activity of the iPP with wide angle X-ray diffraction and polarized optical microscopy. The β nucleation activity not only depended on the nucleator content, mesogenic molecular structure, and thermal processing history, but also on average molecular weight of the liquid crystalline polymers. LC-N1 or LC-N2 has been found to be an effective β-nucleator for the iPP. The experimental results indicated the relative content of β-crystal first increased and then decreased with increasing the nucleator content or crystallization temperature. The addition of LC-N1 or LC-N2 could provide a large number of nuclei, enhance the crystallization rate, reduce the spherulite size, and lead to a more uniform morphology; moreover, the colorful β-crystal was also induced. In addition, LC-N2 with higher average molecular weight is more effective than LC-N1 in inducing β heterogeneous nucleation under same crystallization conditions.     

Thermal and morphological characteristics of polypropylene/smectic polyester blends

Several blends of isotactic polypropylene, iPP, and the liquid crystalline polymer poly(triethylene glycol p,p′-bibenzoate), PTEB, have been prepared with different compositions, and analyzed by scanning electron microscopy, polarizing optical microscopy, infrared spectroscopy, X-ray diffraction with synchrotron radiation, and thermal methods (thermogravimetric analysis and differential scanning calorimetry). The results show a good adhesion matrix-dispersed phase when a maleic anhydride-grafted PP is used as compatibilizer, MA-g-PP. The calorimetric and synchrotron experiments indicate higher crystallization temperatures for the iPP component in the blends when compared with those of pure iPP. The higher crystallization temperature seems to be associated with a crystallinity increase. Optical microscopy results point out the nucleating effects of both PTEB and MA-g-PP on the iPP spherulites. The synchrotron experiments reveal that, on cooling from the isotropic melt, a SmA mesophase is formed first from the PTEB component, followed by its transformation into a tilted SmC mesophase. The isotropic-SmA transition is clearly affected in the blends with higher contents in iPP.     

Isothermal crystallization behavior of metallocene‐catalyzed isotactic polypropylene

Isothermal crystallization behavior of isotactic polypropylene (iPP) synthesized using metallocene catalyst was investigated in this work. The isotacticity of the polypropylene was characterized by ¹³C‐NMR spectroscopy. It was found that the melting temperature (T_m) of the iPP is 123.51°C and the crystallization temperature (T_c) is 93°C. The iPP synthesized in this work did not show a general increase of T_m with an increase of crystallization temperature T_c, due to the short crystallization time of 20 min and low molecular weight (number average molecular weight = 6,300). The iPP showed a tendency of increasing heat of fusion (ΔH_f) with decreasing crystallization temperature. All the spherulites of iPP samples showed negative birefringence. For the iPP sample crystallized at the highest T_c (= 123°C, just below T_m), the spherulite showed a pronounced Maltese Cross and a continuous sheaf‐like texture aligning radially, which suggests that R‐lamellaes are dominant in this spherulite. The crystalline structure of the iPP was also investigated by X‐ray diffraction. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 231–237, 2005     

Investigation of the crystallization behavior of isotactic polypropylene polymerized with different Ziegler‐Natta catalysts

Detailed characterization of the crystallization behavior is important for obtaining better structure property correlations of the isotactic polypropylene (iPP), however, attributed to the complexity in ZN‐iPP polymerization, the relationship between crystallization behavior and the stereo‐defect distribution of iPP is still under debate. In this study, the crystallization kinetics of the primary nucleation, crystal growth and overall crystallization of two iPP samples (PP‐A and PP‐B) with nearly same average isotacticity but different stereo‐defect distribution (the stereo‐defect distribution of PP‐B is more uniform than PP‐A) were investigated. The results of isothermal crystallization kinetics showed that the overall crystallization rate of PP‐A was much higher than that of PP‐B; but the analysis of self‐nucleation isothermal crystallization kinetics and the polarized optical microscopy (POM) observation indicated that the high overall crystallization rate of PP‐A was attributed to the high primary nucleation rate of the resin. The stereo‐defect distribution plays an important role in determining both the nucleation kinetics and crystal grow kinetics, and thus influence the overall crystallization kinetics. A more uniform distribution of stereo‐defects restrains the crystallization rate of iPP, moreover, it has more influence on nucleation kinetics, comparing with the crystal growth. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of silicon dioxide on crystallization and melting behavior of polypropylene

The crystallization and melting behavior of isotactic polypropylene (iPP) and polypropylene copolymer (co‐PP) containing silicon dioxide (SiO₂) were investigated by differential scanning calorimeter (DSC). SiO₂ had a heterogenous nucleating effect on iPP, leading to a moderate increase in the crystallization temperature and a decrease in the half crystallization time. However, SiO₂ decreased the crystallization temperature and prolonged the half crystallization time of co‐PP. A modified Avrami theory was successfully used to well describe the early stages of nonisothermal crystallization of iPP, co‐PP, and their composites. SiO₂ exhibited high nucleation activity for iPP, but showed little nucleation activity for co‐PP and even restrained nucleation. The iPP/SiO₂ composite had higher activation energy of crystal growth than iPP, indicating the difficulty of crystal growth of the composite. The co‐PP/SiO₂ composite had lower activation energy than co‐PP, indicating the ease of crystal growth of the composite. Crystallization rates of iPP, co‐PP, and their composites depended on the nucleation. Because of its high rate of nucleation, the iPP/SiO₂ composite had higher crystallization rate than iPP. Because of its low rate of nucleation, the co‐PP/SiO₂ composite had lower crystallization rate than co‐PP. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1889–1898, 2006