Trans- and Dimethyl quinacridone nucleation of isotactic polypropylene

Isotaclic polypropylene (iPP) was nucleated specifically for structural and mechanical studies, especially of impact resistance. Nucleation was accomplished by two red pigments: a linear trans-quinacridone dye (as β-phase nucleator) and a dimethyl quinacridone resulting in the formalion of a pure α-phase structure. This procedure allowed the comparison of both the crystal structure and the morphology of the polymer. For the β-nucleated iPP, by means of the WAXS measurements in the transmission mode, a maximum of the k-value (fraction of β-phase) was found for a concentration of 5 × 10^?5 wt% of the trans-quinacridone. By means of polarized light microscopy (PLM), the mold temperature and pigment concentration dependent spherulites distribution on the cross section of the samples was observed. DSC permitted confirmation of the WAXS measurements, giving a characteristic double melting peak for the β polypropylene. By isothermal WAXS measurements the temperature of 135°C as the upper limit of the β phase was detected. For both nucleating agents (linear trans-quinacridone and dimethyl quinacridone) the dependence of the mechanical properties (modulus of elasticity and brittleness by impact resistance) on the pigment concentration was observed. For the β-phase iPP the highest ductility by impact testing corresponds to the highest values of the k parameter. The explanation is based on the SEM observations, which permitted the detection of spherulites with cross-hatched lamellae (for the α-spherulites) and with simple radial growth lamellae for the β-spherulites.     

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.     

Influence of β nucleation on the mechanical properties of isotactic polypropylene and rubber modified isotactic polypropylene

The tensile and fracture behaviour of neat α and β nucleated isotactic polypropylene and rubber-modified α and β nucleated isotactic polypropylene has been investigated at test speeds of 0.0001-10 ms⁻¹ in the temperature range −30 to +60 °C. The presence of the β phase had little effect at low temperature. However, at +25 and +60 °C, it increased the speeds corresponding to the ductile-brittle transition in the neat polymer by more than three decades. This behaviour has been linked to changes in microdeformation mechanisms observed at the lamellar and spherulitic level, an increase in cavitational deformation in tensile tests and an increase in the strength of the β relaxation in dynamic mechanical spectra. In the blends, the presence of the β phase led to somewhat higher energy dissipation in regimes of ductile fracture. However, the ductile-brittle transitions were not significantly affected. The modifier phase was therefore inferred to control the initiation and propagation of the plastic zone ahead of the crack tip during fracture.     

Mesomorphic form of isotactic polypropylene in stereodefective polypropylene: Solid mesophase or liquid-crystal like structure

A study of the crystallization of the mesomorphic form of isotactic polypropylene (iPP) in samples of different stereoregularity prepared with metallocene catalysts is presented. Contrary to what claimed in the literature, we have found that the mesomorphic form can be obtained by quenching of the melt at 0 °C also in the case of low stereoregular samples, provided that the samples are kept at 0 °C for long time. The key is the formation of mesomorphic bundles with chains in ordered 3/1 helical conformation of size large enough to be stable and inhibit the crystallization of the α form at room temperature. For stereoirregular samples the concentration of long ordered helical stretches is low and this requires long residence time at 0 °C for the formation of mesomorphic aggregates of size larger than the critical size. This result provides evidence that the mesomorphic aggregates are not formed as a result of a cooperative process driven by entropy, as in the case of liquid crystals, but rather they form via a nucleation and growth mechanism, as in the normal crystallization processes, according to the idea that the mesophase of iPP is a highly defective crystalline form.     

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.     

High‐pressure induced formation of isotactic polypropylene mesophase: Synergistic effect of pressure and pressurization rate

The crystallization induced by high pressures up to 2 GPa was studied for isotactic polypropylene (iPP) using wide angle X‐ray diffraction, small angle X‐ray scattering, and atomic force microscopy methods. The iPP mesophase can be induced at 200°C by high pressures with fast pressurization rate. A pressure–pressurization rate kinetic phase diagram of mesophase and γ‐phase was established to systematically analyze the influences of pressure and pressurization rate on crystallization polymorphism. Under pressure not higher than 0.75 GPa, only γ‐phase is formed, even the pressurization rate reaches 5 × 10³ MPa/s. With increasing pressure to 1.0 GPa, the mesophase is possible to be generated. As pressure further increases to 1.5 GPa and higher, the pure mesophase system can be obtained for pressurization rate above the critical values (about 10 MPa/s). Moreover, the structure of iPP mesophase obtained by rapid pressurization was analyzed and compared with that prepared by temperature quenching. Also interestingly, the pressure‐induced mesophase can transform into γ‐phase by annealing under pressure at 200°C, and the rate of this phase transition is slowed down with increasing annealing pressure. POLYM. ENG. SCI., 59:439–446, 2019. © 2018 Society of Plastics Engineers     

Spherulite nucleation in isotactic polypropylene based nanocomposites with montmorillonite under shear

The crystallization of nanocomposites of isotactic polypropylenes with organo modified montmorillonite compatibilized by maleic anhydride grafted polypropylene was studied by light microscopy in isothermal conditions in quiescent state and in shear. The isothermal and nonisothermal crystallization of the composites was also investigated by DSC method. Only weak nucleation activity of montmorillonite was observed during crystallization in static conditions. The clay nucleation activity was greatly enhanced in shear-induced crystallization and resulted in a drastic decrease of spherulite sizes. In nanocomposite films crystallized isothermally the intense nucleation of isotactic polypropylene spherulites was observed when the polymer was forced to flow to compensate the volume shrinkage due to crystallization. It was also found that the presence of a glass support enhances spherulite nucleation in nanocomposites which is caused possibly by shear due to a difference in thermal shrinkage of a polymer matrix and a glass support.     

Primary nucleation behaviour in isotactic polypropylene/ethylene-propylene random copolymer blends

Primary nucleation of spherulites in blends of isotactic polypropylene (iPP) with ethylene-propylene random copolymer (EPM) has been investigated using optical microscopy. The number of spherulites generally increases with increasing EPM content. It is shown that this increase is caused by migration of heterogeneous nuclei across interface boundaries from EPM to iPP during melt-mixing. The migration was observed in the blends with the nucleating agent initially added to EPM before blending with iPP. It is suggested that the interfacial free energy difference between nuclei and the molten components of the blend is responsible for the migration of nuclei. It is also shown that self-seeded nucleation becomes damped in the blends due to partial solubility of the components and that the degradation of the blends during melt-annealing depresses the primary nucleation.     

Relationship between molecular structure and nucleation of benzylidene acetals in isotactic polypropylene

Relationship between molecular structure and nucleation of benzylidene acetals in isotactic polypropylene was investigated by differential scanning calorimeter and two‐dimensional correlative infrared spectra. The results indicated that benzylidene acetals can dissolve into isotactic polypropylene at high temperature and form nuclei to induce the crystallization of isotactic polypropylene at low temperature. The solubility of benzylidene acetals depend on the number of hydroxyl, but the nucleation of benzylidene acetals depend on the number of benzylidene, which could interact with methyl of isotactic polypropylene. The latter intermolecular interaction makes the infrared absorbance of methyl change before that of skeleton C‐C and methine of isotactic polypropylene in the induction period of crystallization. The order is opposite in the induction period of crystallization of virgin isotactic polypropylene. Then, new efficient nucleating agents for isotactic polypropylene can be designed by changing the number of hydroxyl and benzylidene of benzylidene acetals. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Crystal morphology and transcrystallization mechanism of isotactic polypropylene induced by fibres: interface nucleation versus bulk nucleation

Using polarized optical microscopy (POM) equipped with a hot stage, morphological investigations of an isotactic polypropylene (iPP) matrix, induced by a homogeneous iPP fibre and heterogeneous pure/modified nylon 6 fibres, were carried out. With respect to transcrystallization related to heterogeneous nucleation on the surface of the fibre, the nucleation mode was found to be different for iPP fibres and nylon 6 fibres. An iPP fibre can serve as a macroscopic linear nucleus, similarly to the shish‐type structure formed in stress‐induced crystallization, to induce kebab‐like growth of lamellae, whereas numerous closely packed spherulites along nylon 6 fibres resulted in macroscopic transverse growth to form a transcrystallite owing to the limitation along the fibre axis. The difference in nature between these two transcrystallites can be further demonstrated by their optical characters related to the lamellar arrangement inside the transcrystallite. As for homogeneous iPP composites, the formation of transcrystallites results from lattice matching, in consideration of the same chemical structure and lattice parameters between fibre and matrix. The incorporation of calcium chloride into a nylon 6 fibre—to destroy its crystal structure—confirmed the role of lattice matching between nylon 6 fibre and iPP matrix. The addition of atactic polypropylene (aPP) in order to enhance the nucleation ability of the iPP matrix also greatly weakened transcrystallization. Our work demonstrates that transcrystallization is just a matter of competition between interface nucleation and bulk nucleation, namely, if interface nucleation is faster than bulk nucleation, transcrystallization will develop. If not, it will be suppressed. Copyright © 2006 Society of Chemical Industry     

Deformation and pore formation mechanism under tensile loading in isotactic polypropylene

Four different polypropylene (PP) samples were prepared through isothermally crystallizing at 0 °C (PP‐Q), 80 °C (PP‐80), 100 °C (PP‐100) and 120 °C (PP‐120). The results of differential scanning calorimetry, wide‐angle X‐ray diffraction, polarized light microscopy and tensile testing indicate that the spherulite structure gradually improves with increasing isothermal crystallization temperature. Meanwhile, the interface between spherulites becomes more obvious due to the larger dimension and the higher strength of spherulites. Therefore, the trend of interfacial debonding during stretching is enhanced distinctly. In addition, based on the structural characterization of samples at different draw ratios, two completely distinct morphological changes are demonstrated. There are no defects generated after longitudinal stretching within PP‐Q, because intra‐spherulitic deformation predominates, which is caused by the imperfect spherulites of PP‐Q. As a result, no microporous structure is produced after sequential biaxial stretching. And the improvement of the crystalline structure makes interfacial debonding more likely to occur. Therefore, fully developed crazes and cracks disperse between microfibril structures after longitudinal stretching. Furthermore, numerous microporous structures are produced through debonding of fully developed crazes and cracks after sequential biaxial stretching. Meanwhile, the quantity, dimension and uniformity of the microporous structures and the porosity are gradually improved. © 2017 Society of Chemical Industry     

Influence of the conformational order of glassy isotactic polypropylene on its crystallization and melting behaviour

The influence of conformational order of glassy isotactic polypropylene (iPP) on its crystallization and melting behaviour was studied by wide‐angle X‐ray diffraction (WAXD), Fourier‐transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The glassy iPP samples with various conformational orders were prepared by freeze‐drying procedure from very dilute solution. WAXD and FTIR results suggest that although freeze‐dried samples are non‐crystalline, their conformational order is rather high and increases with decreasing concentration of parent solutions. With increasing conformational order of glassy samples, the cold crystallization peak shifts to lower temperature, indicating acceleration of crystallization. At low fusion temperature, the conformational order of melt obtained from freeze‐dried samples is high so that the relaxation time of single‐ and pauci‐chains to re‐entangle is long and the interpenetration between single‐ and pauci‐chain collective particles is slow. However, at high fusion temperature, the interpenetration process can be completed very quickly. Copyright © 2004 Society of Chemical Industry     

Combined effect of specific nucleation and rubber dispersion on morphology and mechanical behavior of isotactic polypropylene

This study aims to explore the joint effects of specific β‐nucleation and rubber dispersion on morphology and mechanical behavior of materials derived from isotactic polypropylene. A β‐nucleator (N,N′‐dicyclohexylnaphthalene‐2,6‐dicarboxamide) and an amorphous EPM rubber were used for the modification of isotactic polypropylene. Four samples were investigated: neat polypropylene, the same polymer modified with 0.03 wt % of β‐nucleator or with 15 wt % of dispersed rubber particles, and finally polypropylene containing both the β‐nucleator and the rubber particles. Tensile and impact behavior were followed at room and cryogenic temperatures. It has been observed that the β‐nucleation and rubber modification have brought about a similar macroscopic softening effect on the tensile mechanical behavior. Microscopy of fracture surfaces, however, has shown different toughening mechanisms caused by specific nucleation on one hand and by rubber dispersion on the other. While a distinct synergy effect of nucleation and rubber modification on the resulting toughness was found at low temperature, no such cooperative effect manifested itself at room temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3539–3546, 2007     

Physical aging in isotactic polypropylene

Physical aging studies were made using commercial and laboratory samples of isotactic polypropylene. Linear and nonlinear viscoelastic responses were measured after quenching the glass from above the glass transition temperatures to below the glass transition. Results show that aging is not eliminated by large mechanical deformations; rather, the time required for the glass to age into equilibrium is independent of the applied stress. Results obtained are purely kinetic and interpreted as the effects accompanying the process of glass formation in a semicrystalline polymer. © 1996 John Wiley & Sons, Inc.     

Fabrication and morphology development of isotactic polypropylene nanofibers from isotactic polypropylene/polylactide blends

The excellent characteristics of polymeric nanofibers with diameters less than 1 μm such as the enormous specific surface result in a dramatic increase in a variety of functional applications. In this article, polymer blends of isotactic polypropylene (iPP) and polylactide (PLA) were fabricated through a twin‐screw extruder. The extrudates were prepared at various processing conditions and the iPP nanofibers were obtained by removal of the PLA matrix from the drawn samples. The influences of drawing ratio, the processing temperature, and the blend ratio of iPP/PLA on the morphology development of iPP phase were investigated by scanning electron microscopy. It was found that the uniformed iPP nanofibers with averaged diameters less than 500 nm were fabricated by the suitable processing parameters. Otherwise, the processing immiscibility and rheological behavior of iPP/PLA blends were studied by means of dynamic mechanical analysis and capillary rheometer. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The effect of nanoclays on the nucleation,crystallization, and melting mechanisms of isotactic polypropylene

The influence of organomodified nanoclay (montmorillonite) on the crystallization and melting mechanisms of isotactic polypropylene (iPP) was studied. Films of both neat polymer and clay nanocomposites were prepared after molecular weight optimization through melt extrusion. Products exhibited the tactoidlike morphology since no compatibilizers were used. The effect of introduction of nanoclay within the polymer was studied through isothermal crystallization and linear heating. The results indicated that low nanoclay concentrations induce the formation of the β‐crystalline structure, its formation being inhibited with high nanoclay contents. Dynamic nonisothermal bulk crystallization experiments indicated that nanoclays act as nucleating agents of iPP. Isothermal, bulk crystallization studies indicated that the crystallization process (induction time and crystallization kinetics) is proportionally accelerated by the nanoclay presence. There was also an accelerated formation of secondary crystallization when nanoclays were present in high concentrations. POLYM. ENG. SCI., 47:1889–1897, 2007. © 2007 Society of Plastics Engineers     

Interphase layer formation in isotactic polypropylene/ethylene–propylene rubber blends

The influence of the interphase layer, formed by the introduction of an oil in ethylene–propylene rubber (EPR), on the structure and properties of isotactic polypropylene (iPP)/EPR blends was studied. The dispersity of the rubber phase in the iPP matrix did not depend on presence of oil. The melting temperature of iPP decreased with increasing content of oil‐extended EPR, and it did not change if the oil was absent. The compatibility parameter was determined from the dependency of the iPP melting point on the rubber content with the Nishi–Wang equation. A negative value of the parameter indicated a limited compatibility of iPP with oil‐extended EPR. The latter also reduced the temperature and heat of crystallization of iPP. The mechanical properties of iPP/EPR blends were investigated as functions of temperature and elongation rate. It appeared that elastic modulus and yield stress of the blends linearly depended on the logarithm of the elongation rate. Activation volumes, calculated with the Eyring equation, increased with increasing content of elastomer; moreover, this increase was more pronounced for the oil‐extended elastomer. It is suggested that the oil influenced the structure of the interphase layer and, accordingly, the characteristics of the iPP/EPR blends. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 249–257, 2003     

SAXS study on structure formation from the uniaxially oriented glass in isotactic polypropylene

The structure formation of isotactic polypropylene from the uniaxially oriented glass into the smectic mesophase has been studied by small-angle X-ray scattering. Remarkable anisotropy between the directions parallel and perpendicular to the chain axis is found in the development of structure formation. Below −20 °C is observed a structure with density variation having the wave vector, perpendicular to the chain axis with a period of 5 to 6 nm. Between −20 and +25 °C, the density variations with both parallel and perpendicular to the chain axis develop. Above +25 °C, the density variation with perpendicular to the chain axis develops but disappears eventually, while that parallel to the chain axis keeps on growing.     

低等规聚丙烯的研制Ⅴ.粘性聚合物的分离技术

采用液相本体法生产的粘性低等规聚丙烯在排放输送和分离回收时易堵塞阀门和管道。模试结果表明：控制聚合时丙烯转化率为20％～50％,同时预先在闪蒸釜中加人表面分散剂质量分数约0．3％的液体介质及预先充人丙烯单体可以有效地解决这一问题。介绍了采用乙醇-水溶液作为分散介质的分离技术特征,并可用深度冷冻法或吸附极性分子法从回收体系中除去乙醇。     

Luminescence emission of isotactic polypropylene

In order to elucidate the origin of luminescent species in unstabilized commercial isotactic polypropylene (IPP), effects of hexane extraction and exposure of the extracted IPP film to the atmosphere on both fluorescence and phosphorescence emissions were examined. Fluorescence emission from the unextracted IPP film disappeared by the extraction, and only a very weak phosphorescence emission was found. During the exposure of the extracted IPP film to the atmosphere, both fluorescence and phosphorescence emissions appeared again. The intensity of the spectra increased with the exposure time and finally almost reached that from the unextracted IPP film. The results indicated that main fluorescent species in IPP are foreign impurities originating from an ambient atmosphere, and that contribution of a very minor amount of phosphorescence species such as carbonyl groups incorporated with the polymer was scarce.