The melt compatibility of blends of polypropylene and ethylene-propylene copolymers

Blends of polypropylene with ethylene-propylene copolymers of various compositions have been studied by small angle neutron scattering with regard to their compatibility at room temperature and in the melt. It has long been known that such blends separate into distinct phases at lower temperatures due to the crystallinity of the isotactic polypropylene, The work described herein has shown that these blends are also immiscible in the melt, even where the ethylene content of the copolymer is as low as 8 percent. Moreover, the separated phase domains grew rapidly at melt temperatures. Blends of atactic polypropylene with isotactic polypropylene did become miscible upon melting.     

AC breakdown of melt-crystallized isotactic polypropylene

The effect of polymer morphology on the dielectric breakdown of isotactic polypropylene was investigated under the influence of a ramped ac voltage. Samples were prepared by melt-pressing polypropylene beads into plaques and subsequently quenching or isothermally cyrstallizing these plaques. Some of the quenched samples were annealed at different temperatures to induce further crystallinity. The plaques were then characterized by wide-angle X-ray diffraction, density measurements, and differential scanning calorimetry. The 60 Hz, ac breakdown voltages of the samples were determined at 7°C, 23°C, and 45°C in a specially designed test cell. There was a lack of any significant effect on the ac breakdown strength of isotactic polypropylene of either the overall crystallinity or the crystallite thickness. This result can be understood in light of previous studies relating the electrical breakdown of polymers to morphology. It is postulated that the degree of crystallinity would influence the breakdown characteristics only when the amorphous regions constitute a small fraction of the total volume, that is, at very high crystallinity values—values beyond the range of the present study.     

Stereoblock polypropylene/isotactic polypropylene blends. II. Mechanical behavior

The mechanical properties of stereoblock polypropylene/isotactic polypropylene blends have been analyzed at different temperatures and at large deformations. The samples at a lower content of isotactic polypropylene show good elastic properties, both in terms of deformation reversibility and in terms of energy dissipation in the hystersis cycles. Considering the dependence of the elastic behavior on the temperature, the model that can be suggested is based on the presence of a physical network in which the cross-linking is due to cocrystallization between the matrix (the stereblock polypropylene) and the crystalline domains of isotactic polypropylene. © 1995 John Wiley & Sons, Inc.     

Interferometric determination of the physical properties of annealed isotactic polypropylene fibers

Automated variable wavelength interference microscope was used to study the effect of annealing process on the physical properties of isotactic polypropylene fibers (4:1 draw ratio, 515 tex, Bolton UK). The isotactic polypropylene (iPP) fibers were annealed at temperatures ranging from 60°C to 140°C. The spectral dispersion curves of refractive indices and birefringence of iPP fibers were determined at different annealing temperatures. The resulting data were used to determine the optical orientation function, orientation angle, degree of crystallinity, and dispersion parameters of the annealed samples. The study indicates that, the measured birefringence, orientation function, and the dispersion of crystallinity of iPP fibers have been improved with the increasing of annealing temperature. POLYM. ENG. SCI., 59:35–41, 2019. © 2018 Society of Plastics Engineers     

Some aspects of isotactic polypropylene crystallization in an ethylene–propylene–diene rubber matrix

Crystallization of isotactic polypropylene (iPP) in an ethylene–propylene–diene rubber matrix (EPDM), crosslinked with dicumyl peroxide (DCP), has been studied. A discrepancy concerning the degree of crystallinity of the blends determined using different experimental techniques, has been discovered and an effort to explain it is made. iPP was found to recrystallize in EPDM in a manner, depending on the amount of plastomer added. Nucleation, being predominantly homogeneous for ‘pure’ iPP, becomes predominantly hetero-geneous for its blends with EPDM. The smaller the polypropylene content the more a defect crystalline phase is formed. The defects, as well as an orientation were shown to be responsible for the higher, in comparison with other methods, degree of crystallinity determined by wide-angle X-ray scattering. ©1997 SCI     

Self‐interference flow of an isotactic polypropylene melt in a cavity during injection molding. II. Morphology and crystallinity

This article is principally concerned with the morphology and crystallinity of isotactic polypropylene (iPP) parts molded by injection molding, during which a self‐interference flow (SIF) occurs for the melt in the cavity. Scanning electron microscopy shows that a transverse flow takes place in SIF samples. Wide‐angle X‐ray diffraction and differential scanning calorimetry show that SIF moldings exhibit a γ phase, in addition to α and β phases, and high crystallinity. Meanwhile, the results for iPP moldings made by the conventional flow process, that is, conventional injection molding, are reported for comparison. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2791–2796, 2003     

Character of crystallization nuclei in isotactic polypropylene

Examination of the isothermal crystallization and the effect of melting conditions on samples of isotactic polypropylene and its composite with talc, combined with electron-microscopic observation, has shown two types of heterogeneous nuclei effective in the crystallization process: (1) metastable nuclei, representing the unmelted crystalline phase of polypropylene, stabilized by solid heterogeneities, and operating after melting at relative low melting temperature and/or short melting time; and (2) stable crystallization nuclei, associated with solid heterogeneities, being probably catalyst residues. On the surface of these nuclei isotactic polypropylene tends to crystallize in an ordered fashion.     

Polypropylene Blends with m-EPR Copolymers: Structure,Morphology and Thermal Properties

The effects of different contents of two metallocene propylene-based m-EPR elastomers on structure, morphology, thermal, and dynamic mechanical properties of the isotactic polypropylene/m-EPR blends were investigated. The both m-EPR copolymers have been built in isotactic polypropylene matrix as amorphous phase. However, the nucleation effect at lowest addition (2.5?vol%) and the solidification effect along with increased m-EPR’s additions have caused changes of the crystallinity degree and the size of spherulites in the isotactic polypropylene matrix. Higher degree of miscibility/compatibility of the isotactic polypropylene/m-EPR2, with lower viscosity has been observed. Homogeneous dispersion of m-EPR particles as well as their radial distribution has been observed.     

Quantification of the Young's modulus for polypropylene: Influence of initial crystallinity and service temperature

In this study, the mechanical properties of isotactic polypropylene (iPP) materials with different crystallinities at room and elevated temperatures were investigated. In order to obtain samples with a certain range of crystallinity, and to ensure a uniform microstructure of these samples, the iPP samples obtained by injection molding required melt compression molding and controlled annealing. In the macromechanical studies, the experimental results showed that the storage modulus and Young's modulus of polypropylene were sensitive to the service temperature. The crystallinity also had a great influence on this relationship. A function was proposed to evaluate the dependence of the Young's modulus of polypropylene on initial crystallinity and service temperature, and tested based on experimental data. The Young's modulus of iPP is reduced by about 90% when the service temperature rises from 25 to 125 °C. Moreover, the reduced value in Young's modulus between polypropylene having the highest and lowest crystallinity was reduced from 214.55 to 56.75 MPa. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48581.     

Novel morphology of isotactic polypropylene crystal generated by a rapid temperature jump method

Novel morphology of isotactic polypropylene (iPP) crystal, which is quite different from that of usual spherulite, has been observed by scanning electron microscope (SEM). The crystals show ‘bamboo leaf-like (BL)’ shape with α-monoclinic high crystallinity. The BL crystals are formed by neither melt nor glass crystallization, but by a complicated annealing process that goes through mesomorphic phase of iPP. Substrates are not essential for the formation of BL crystals, since the BL crystals are formed both on glass surface and free surface as well as in bulk. Along with the annealing process, a possible explanation for the mechanism of the formation of the BL crystal is proposed.     

Stereoblock polypropylene/isotactic polypropylene blends. IV. Cocrystallization and phase separation

The blending process of stereoblock isotactic polypropylene and isotactic polypropylene (sbiPP/iPP) including a well distribution of the components (by solvent mixing), posterior hot pressing (shear mixing), and a final quenching into ice water freezing these blends at room temperature in a particular flow oriented cocrystallized structure. This structure gives rise to a birefringent pattern which is stable up to 170°C when it disappears by melting. In the molten state the two components segregate destroying the original flow oriented structure. In the following cooling the iPP crystallizes in a spherulitic form and the sbiPP phase crystallizes only partially disturbed by the restrictions imposed by the iPP crystals. © 1996 John Wiley & Sons, Inc.     

Crystallization and melting behaviour of photodegraded polypropylene - I. Chemi-crystallization

An investigation has been conducted into the effects of photodegradation on the crystallinity and melting behaviour of isotactic polypropylene (PP). PP samples having different structural characteristics were prepared and exposed to ultraviolet radiation (u.v.) in the laboratory for periods of up to 48 weeks. The changes in crystallinity during exposure were followed by X-ray diffraction and differential scanning calorimetry (d.s.c.), whereas the chemical degradation of the specimens sampled was evaluated by gel permeation chromatography (g.p.c.) and Fourier transform infrared spectroscopy (f.t.i.r.). An increase in fractional crystallinity during u.v. exposure was noted for all types of samples studied, and the gain in crystallinity was usually between 6% and 7% and was virtually independent of the initial structure of the polymer. Measurements conducted at different depths within the test bars indicated that the fractional crystallinity increased during u.v. exposure due to crystal growth using molecule segments released by the scission of molecules (probably taut molecules). This process, called chemi-crystallization, is restricted by the chemical defects introduced into the molecules by the photodegradation. Possible mechanisms for the chemi-crystallization process are discussed. The melting thermograms of most types of samples exhibited single peaks with melting range increasing with exposure time. The broadening of the melting thermograms occurred during the period between the commencement and the completion of the chemi-crystallization. Secondary crystallization proceeded much more slowly in the interior of the test bars because of the limited oxygen supply, but the final crystallinity was the same as the plateau value, which was attained more rapidly when the material in the interior was exposed by machining away the surface prior to u.v. exposure. © 1997 Elsevier Science Ltd.     

Transcrystallinity phenomena in fiber-reinforced polypropylene. II: Morphology,thermal and mechanical properties relationships

The effect of short-length carbon and Kevlar fibers on the crystallization of isotactic polypropylene (iPP) in composites prepared by compression molding has been investigated. The tendency of carbon and Kevlar fibers to nucleate the iPP during isothermal and nonisothermal crystallization has been evaluated by differential scanning calorimetry. The influence of different thermal histories used to prepare the unreinforced and reinforced samples on the crystallization parameters of iPP was examined. In addition, the tensile behavior was related to the resulting morphologies of the samples. It was observed that the crystallinity content, obtained by using different thermal treatments (slowly cooling or quenching), gives rise to different morphologies by influencing the mechanical behavior of materials as well. Moreover, the composites obtained by slow cooling seem to present a better fibber/matrix adhesion then that found in quenched samples. Possible underlying microstructures, which can explain the properties and the morphological characteristics, are also discussed.     

Effect of flexibility of ethylene vinyl acetate and crystallization of polypropylene on the mechanical properties of i‐PP/EVA blends

Polymer blend technology has been widely used for the past several years for the modification or enhancement of mechanical properties of polymers to obtain an overall balance of properties over those of the constituents. Despite its interesting mechanical and thermal properties, the impact strength of polypropylene leaves wide scope for improvement. A series of blends of ethylene vinyl acetate (EVA) copolymer with an impact grade of isotactic polypropylene (i‐PP) were prepared by single screw extrusion at 0–0.32 volume fraction of the dispersed phase. The mechanical properties such as tensile behavior, elongation‐at‐break, and impact strength of these blends systems as well as crystallinity were evaluated. Crystallinity data have been used in greater depth to support the mechanical properties. Differential scanning calorimetry studies conducted to study the modification in crystallinity of the crystalline component, i‐PP, of the blend revealed that the rubber component of the blend enhanced the crystallinity of i‐PP phase by providing sites for nucleation. Tensile modulus and strength decreased while the impact strength and breaking elongation enhanced with blending elastomer concentration. The improved properties of these PP/EVA blends are encouraging for carrying out further work on this system (composites) and suggest potential high impact strength applications for PP. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Deformation behavior of isotactic polypropylene crystallized via a mesophase

The mechanical behavior of semicrystalline isotactic polypropylene (iPP) of different crystallinity, crystal morphology and superstructure was investigated by standard tensile stress–strain analysis, dynamic-mechanical analysis, and in situ observation of the deformation by atomic force microscopy (AFM). Emphasis is put on the comparison of the mechanical characteristics of specimens containing either non-isometric lamellae, being arranged in spherulites, or nodular isometric domains, which are not organized in a superstructure. The formation of lamellae/spherulites and of nodules was controlled by the conditions of crystallization. The replacement of cross-hatched monoclinic lamellae and a spherulitic superstructure by randomly arranged isometric nodules leads to a distinct increase of the ductility and toughness, even if the crystallinity is identical. The modulus of elasticity and the yield stress increase as expected with increasing crystallinity. Slightly lower values of Young’s modulus and yield strength are detected if samples contained non-lamellar crystals in a non-spherulitic superstructure, proving an effect of the crystal shape on the deformation behavior. For the first time, tensile deformation of semicrystalline iPP which contains nodular ordered domains instead of lamellae has been followed by in situ AFM.     

Morphology, phase structure and thermal behaviour of films of isotactic polypropylene/hydrogenated oligocyclopentadiene blends: 1. Extruded isotropic films

The effect of hydrogenated oligocyclopentadiene (HOCP) on the structure and morphology of isotropic isotactic polypropylene (iPP) films obtained by melt extrusion has been studied using wide-angle X-ray scattering, thermal analysis and electron microscopy. It was found that the addition of HOCP causes the formation of the smectic phase of iPP at temperatures where bulk iPP crystallizes only in the monoclinic form. The amount of smectic phase present in the blend is dependent on the blend composition. The spherulitic morphology of iPP is completely modified by the presence of HOCP. For blends containing up to 10% HOCP, the iPP spherulite dimensions are drastically reduced. In the case of blends of higher HOCP content, the film crystallinity is reduced, no spherulites are visible and the film surface appears smoother.     

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.     

Recrystallization kinetics of isotactic polypropylene (α-form)

The recrystallization kinetics of isotactic polypropylene (i-PP) (α-form) in the usual melting temperature region (155°–170°C) has been studied by calorimetric and dilametric techniques. The recrystallization is possible over a large range of temperatures also when the residual crystallinity is very low ($\text{? 1\%}$). Low values of the Avrami exponents independent of T_r (n ≤ 1.6), have been found, although the residual crystallinity shows a large variation (1–35%).     

Effect of structure on light transmission in isotactic polypropylene and random propylene-1-butene copolymers

The structure of semi-crystalline isotactic polypropylene and random isotactic copolymers of propylene and 1-butene at the nanometer and micrometer scales is controlled by the pathway of melt-crystallization, and the concentration of chain defects. Rapid cooling of the melt results in formation of mesomorphic nodules, being not organized in a higher order superstructure. Slow melt-crystallization, in contrast, allows formation of lamellae. These lamellae arrange within spherulites of a size which is decreased in copolymers. Analysis of the light transmission reveals a distinctly higher transparency in non-spherulitic preparations. Spherulitic samples exhibit a lower transparency, increasing with decreasing size of spherulites. Annealing of quenched preparations at elevated temperature leads to an increase of the crystallinity and of the dimensions of crystals, without affecting their habit, and their higher order organization. The transparency is only slightly decreased in these specimens. It can be demonstrated that samples of largely different transparency but identical crystallinity can be generated without the use of optical clarifiers/nucleation agents.     

Effect of the structure at the micrometer and nanometer scales on the light transmission of isotactic polypropylene

The spherulitic superstructure, crystallinity, and structure and morphology of crystals of isotactic polypropylene were controlled by the conditions of melt crystallization and related to the transmittance of visible light. Spherulitic samples, which contained monoclinic lamellae, were prepared by slow cooling of the quiescent melt at rates lower than 10 K/s and by isothermal melt crystallization at temperatures between 373 and 413 K. Nonspherulitic specimens, which contained nonlamellar mesomorphic domains, in contrast, were obtained by rapid cooling of the melt with rates faster than 100 K/s. The crystallinity and the size of crystals were furthermore fine‐tuned by subsequent annealing at elevated temperatures. Analysis of such films of different structure by ultraviolet–visible spectroscopy revealed that the light transmission was independent of (1) the fraction, (2) the internal structure, and (3) the size of the crystals. In contrast, the light transmission increased with decreasing size of spherulites and finally exceeded 90% in films of 100 μm thickness when spherulites were completely absent. The crystallinity and the structure and size of the crystals of the films of isotactic polypropylene could be adjusted within wide limits without affecting the light transmission. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010