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.     

Homogeneous nucleation and mesophase formation in glassy isotactic polypropylene

Differential fast scanning chip calorimetry has been employed to study nucleation/ordering during annealing the glass of quenched isotactic polypropylene. Initially non-ordered samples were annealed below the glass transition temperature for different periods of time, and the change of structure during isothermal annealing was then analyzed by monitoring the exchange of latent heat on heating. Primary result of this work is the proof of homogeneous nucleation of ordering and mesophase formation in the glassy state. It is suggested that only local non-cooperative mobility of molecular segments is required to form small, ordered domains, and that the classical nucleation theory, which restricts nucleation of the crystallization/ordering process of polymers to temperatures between the equilibrium melting temperature and the glass transition temperature, needs modification.     

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.     

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     

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.     

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.     

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     

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     

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     

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     

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.     

Nucleating activity of the quinacridone and phthalocyanine pigments in polypropylene crystallization

The nucleating ability of two organic pigments, quinacridone and phthalocyanine, in nonisothermal polypropylene (PP) crystallization was investigated. The investigations were carried out using DSC and polarizing microscopy. The efficiency of pigments in the nucleation process was determined in a simple test comparing the crystallization temperature of the PP‐containing pigments with respect to the pure polymer. Both pigments revealed good nucleating activity. In the presence of pigments the increment of the crystallization temperature and the increase of nucleation rate were observed. The nucleating efficiency of pigments was confirmed in investigations of the obtained structure. The nucleating activity of pigments was explained by their features fulfilling several requirements for good nucleating agents. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3957–3964, 2003     

The correlation between crystal structure and nucleation efficiency of a lithium (I) complex on isotactic polypropylene

In this work, 2,2′-methylene-bis-(4,6-di-t-butylphenylene)phosphate lithium (NA03) was synthesized and its crystal structural characterization was obtained by single crystal X-ray diffusion. The crystal data showed geometrically the cell parameter of NA03 matched with isotactic polypropylene (iPP), the a cell dimension was about two times to the value of cell edge of (010)_iPP. The disregistry was 2.89%, which was under the upper limit between the lattice matching spacing of host and guest crystals. Then NA03 was proved to be a highly effective nucleating agent for iPP through studying crystallization behaviors, crystallization morphologies, and mechanical properties of iPP nucleated with NA03. The outstanding nucleation efficiency could be attributed to the lattice matching between nucleating agent and iPP. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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。     

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.     

Thermal characteristics and crystallinity of Ziegler–Natta isotactic polypropylene/metallocene isotactic polypropylene polyblended fibers

Ziegler–Natta isotactic polypropylene (ZN‐iPP) and metallocene isotactic polypropylene (m‐iPP) were extruded (in ratios of 75/25, 50/50, and 25/75) from one melt twin‐screw extruder to produce three ZN‐iPP/m‐iPP polyblended polymers and, subsequently, spin fibers. In this study, we examined the rheology of the ZN‐iPP/m‐iPP polyblended polymers and the thermal characteristics and crystallinity of the ZN‐iPP/m‐iPP polyblended fibers using gel permeation chromatography, rheometry, differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction, density gradient analysis, and extension stress–strain measurement. The apparent melt viscosity of the ZN‐iPP/m‐iPP polyblended polymers revealed positive‐deviation blends. The 50/50 blend of ZN‐iPP/m‐iPP had the highest apparent melt viscosity. For five samples, the complex melt viscosity decreased with the angular frequency, which represented typical non‐Newtonian behavior. The Cole–Cole plot, which consisted of the imaginary part of the complex melt viscosity versus the real part of the complex melt viscosity plot, of the ZN‐iPP/m‐iPP polyblended polymers showed a semicircular relationship with the blend ratios. It indicated that the ZN‐iPP/m‐iPP polyblended polymers were miscible. We analyzed the shear modulus data (G′ vs G″) by plotting them on a log–log scale. The plot revealed almost the same slopes for the ZN‐iPP/m‐iPP polyblended polymers, which indicated a good miscibility between the ZN‐iPP and m‐iPP polymers. The experimental DSC results demonstrate that the ZN‐iPP and m‐iPP polymers constituted a miscible system. The crystallinity and tenacity of the ZN‐iPP/m‐iPP polyblended fibers initially increased and then fell as the m‐iPP content increased. Meanwhile, the 50/50 blend of ZN‐iPP/m‐iPP had the highest crystallinity and tenacity. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

聚丙烯复合材料的非等温结晶动力学

采用差示扫描量热仪研究了β成核剂和水滑石(LDH)/β成核剂复配的成核剂对聚丙烯(PP)非等温结晶动力学及熔融行为的影响。结果表明:加入成核剂后,PP中晶体分布不均匀且分散度增大。莫志深方法采用F(θ)表征聚合物在单位时间内达到某一结晶度时所需的冷却(或加热)速率,结晶度达到40%时,纯PP的F(θ)为3.82,加入β成核剂的PP的F(θ)为3.30,加入LDH/β成核剂的PP的F(θ)为2.49。与纯β成核剂相比,LDH/β成核剂能更好地提高PP的结晶温度、结晶速率,增强PP的β晶熔融峰,减弱β晶和α晶共存熔融峰和α晶熔融峰。     

固相氯化法氯化等规聚丙烯结构分析

借助ＤＳＣ,ＩＲ,＾１Ｈ－ＮＭＲ方法对搅拌式固相氯化制备的氯化等规聚丙烯的宏观氯原子分布和微观氯原子分布进行了分析。结果表明,搅拌式固相氯化法能够迅速破坏等规聚丙烯的结晶,从而得到宏观氯原子分布均匀的ＣＩＰＰ。此方法制备的ＣＩＰＰ主要为等丙烯分子链上的仲氢原子被取代的产物。     

The physical aging of isotactic polypropylene

The microstructural changes in isotactic polypropylene (PP), subsequent to quench from the melt to around T_g were studied using dynamic mechanical loss, volume dilatometry, small and wide angle X-ray scattering, infrared and NMR spectroscopy. The β-transition loss tangent and the specific volume decreased linearly with logarithm of aging time. Dynamic mechanical loss and NMR spectroscopy results established that amorphous chain mobility reduced during aging. X-ray and IR techniques showed that the crystallinity, the crystalline density, and the average chain conformation do not change during aging. Tensile tests indicated that diffusion of air or moisture into the polymer is not a competitive mechanism for the aging phenomenon. A simple free-volume model is quantitatively consistent with these observations. The fraction of the material which cannot age increased as the quench temperature decreased. Further, at lower temperatures a portion of the nonageable fraction is shown to reside in the amorphous fraction.