Crystallization Behavior Of Isotactic Polypropylene Research Articles

Shear‐Induced Orientation and Structure Development in Isotactic Polypropylene Melt Containing Modified Carbon Nanofibers

The shear‐induced crystallization behavior of isotactic polypropylene (iPP) nanocomposite melt containing modified carbon nanofibers (MCNFs) was investigated by rheo‐SAXS (small‐angle X‐ray scattering) and rheo‐WAXD (wide‐angle X‐ray diffraction) techniques using synchrotron radiation. Under quiescent conditions, the nucleating effect of MCNFs on crystallization of iPP was pronounced and the system exhibited a remarkably low saturation point (ca. 0.05 wt% of MCNF). In‐situ SAXS and WAXD results showed the development of shear‐induced crystalline structures and lamellar morphology in nanocomposite melts. Under the same shear conditions, the filled system exhibited notably faster kinetics compared with the unfilled system. The oriented crystalline fraction was found to decrease with the MCNF loading, indicating the competition between oriented crystals (induced by shear) and unoriented crystals (due to the nucleating effect of MCNF). At the early stages of crystallization, the amount of the oriented crystals increased with the MCNF concentration, suggesting that the nanofiller hindered the motion of polymer chains after the cessation of flow resulting in the delayed relaxation of stretched polymer segments. Dedicated to Prof. Phillip H. Geil's seventy‐fifth birthday.

The influence of thermoelastomers on the crystallization behavior of isotactic polypropylene under shear

AbstractThe crystallization behaviors of isotactic polypropylene (iPP) and its blends with thermoelastomers have been investigated with in situ X‐ray scattering and optic microscopy. At quiescent condition, the crystallization kinetics of iPP is not affected by the presence of elastomers; while determined by the viscosity, the differences are observed on sheared samples. With a fixed shear strain, the crystallization rate increases with increasing the shear rate. The fraction of oriented lamellar crystals in blends is higher than that in pure iPP sample, while the percentage of β phase is reduced by the presence of the elastomers. On the basis of experimental results, no direct correlation among the fraction of oriented lamellae, the percentage of β phase, and growth rate can be deduced. The evolution of the fraction of oriented lamellae supports that shear field promotes nucleation rather than growth process. Shear flow induces the formation of nuclei not only with preferring orientation but also with random orientation. The total density of nuclei, which determines the crystallization kinetics, does not control the ratio between nuclei with and without preferring orientation, which determines the fraction of oriented lamellae. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1188–1198, 2006

In situ optical microscope study of the shear-induced crystallization of isotactic polypropylene

The shear-induced crystallization behavior of isotactic polypropylene (i-PP) has been investigated by in situ optical microscope under various thermal and shear histories. A shish-kebab structure during growth was observed under a well controlled, however, weak shear field. According to our results under these weak shear and thermal history, a modified model was proposed for the growing process of melt shear-induced crystallization of i-PP. Furthermore, many physical insights were provided on several still unsettled issues such as extended chain crystals, row nuclei, and smectic ordering questions.

Influence of CaCO3 nanoparticles shape on thermal and crystallization behavior of isotactic polypropylene based nanocomposites

The influence of calcium carbonate nanoparticles with different shapes (spherical and elongated) on the thermal properties and crystallization behavior of isotactic polypropylene was investigated. CaCO3 nanoparticles were covered by an appropriate coating agent to improve the interfacial adhesion between the filler and the polyolefin matrix. The nanocomposites were prepared by melt mixing and subsequent compression molding. A remarkable effect of CaCO3 on the thermal properties of iPP was observed. Moreover, the analysis of crystallization kinetics showed that CaCO3 nanopowder coated with PP-MA are efficient nucleating agents for iPP, and the overall crystallization rate results higher than plain iPP.

Isothermal crystallization behavior of metallocene‐catalyzed isotactic polypropylene

AbstractIsothermal 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 (Tm) of the iPP is 123.51°C and the crystallization temperature (Tc) is 93°C. The iPP synthesized in this work did not show a general increase of Tm with an increase of crystallization temperature Tc, 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 (ΔHf) with decreasing crystallization temperature. All the spherulites of iPP samples showed negative birefringence. For the iPP sample crystallized at the highest Tc (= 123°C, just below Tm), 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

Influence of Surface Carbon Deposition on the Bulk Crystallization of Isotactic Polypropylene

AbstractSummary: The effect of a surface‐vacuum‐evaporated ultrathin carbon layer on the bulk crystallization behavior of isotactic polypropylene (iPP) films was investigated by means of polarized optical microscopy, differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction. Results obtained from both isothermal crystallization and constant cooling experiments indicate that surface carbon deposition on an iPP film can remarkably increase the crystallization ability of it during the melt recrystallization process. Optical microscopy observations show that the melt recrystallization of iPP with a carbon coating starts about 10 °C earlier than the pure iPP during a re‐cooling process of 1 °C · min−1. Under isothermal crystallization conditions, a much shorter induction period for nucleation is required for the carbon‐coated iPP than the non‐carbon‐coated one. The wide‐angle X‐ray diffraction and DSC analyses further demonstrate a crystallinity increment of the carbon‐coated iPP with respect to its non‐carbon‐coated counterpart. This clearly indicates that the strong influence of the interfacial interaction between the vacuum‐evaporated carbon and the iPP film on its bulk crystallization can be extended deep into the bulk material.An OM micrograph of a hot‐pressed partially carbon‐coated (top half) iPP film.magnified imageAn OM micrograph of a hot‐pressed partially carbon‐coated (top half) iPP film.

Effect of the vibration shear flow field in capillary dynamic rheometer on the crystallization behavior of polypropylene

The self-made capillary dynamic rheometer was adopted to study the relationship between the crystallization behavior of isotactic polypropylene (iPP) and the vibration shear conditions, namely, vibration amplitude and vibration frequency. The crystalline structure of iPP under different vibration conditions was characterized by using differential scanning calorimeter (DSC) and wide-angle X-ray diffractometer (WAXD) techniques. The samples extruded under vibration shear conditions had a higher melting temperature (from DSC). A new shoulder-shape peak appeared at ca. 162 °C under low frequency or low amplitude conditions, which was engulfed by the main melting peak with the increase of the vibration amplitude or frequency. This was probably an indication that more perfect crystals had formed [Polym Eng Sci 38 (1998) 1–20]. The WAXD demonstrated that crystalline form of iPP extruded was not changed but the average crystalline size decreased, according to the Scherrer formula [Analytical methods of polymer materials, China Petrochemical Press, Beijing, 1997]. This proved a large increase in the number of small crystals.

The Effect of Nucleants on the Crystallization Behavior of Isotactic Polypropylene

The Effect of Nucleants on the Crystallization Behavior of Isotactic Polypropylene

Isothermal crystallization behavior and properties of polypropylene/EPR blends nucleated with sodium benzoate

AbstractThe effect of sodium benzoate on the isothermal crystallization behavior of isotactic polypropylene (iPP)/ethylene–propylene rubber(EPR) blends was investigated using differential scanning calorimetry. Dynamic mechanical and physical properties of the iPP/EPR blends nucleated with sodium benzoate were also measured. It was found that the crystallization behavior and physical properties such as heat deflection temperature (HDT), flexural modulus, and impact strength were strongly affected by the competition between the nucleating effect of EPR attributed to its partial compatibility with iPP and the simple addition of the amorphous component, as well as the nucleating effect of sodium benzoate. High impact strength was achieved by addition of EPR and sodium benzoate to iPP. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 201–211, 2002

Influence of the order of polymer melt on the crystallization behavior: II. Crystallization kinetics of isotactic polypropylene

The crystallization behavior of isotactic polypropylene (iPP) melts with a high order has been carefully examined by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The experimental results show that the helically ordered iPP melt crystallizes by heterogeneous nucleation with two-dimensional diffusion controlled growth and the Avrami exponent is about 2. The data available both from our DSC and PLM experiments and from the literature indicate that the order of a polymer melt can speed up the crystallization process. When some unmelted materials exist in the ordered melt, the crystallization will become more rapid.

STUDIES OF KINETICS OF NONISOTHERMAL CRYSTALLIZATION OF i-POLYPROPYLENE BY WIDE-ANGLE AND SMALL-ANGLE SCATTERING OF X-RAY SYNCHROTRON RADIATION

Crystallization behavior of isotactic polypropylene (i-PP) in nonisothermal conditions was studied by wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) using monochromatized X-rays from synchrotron radiation. It was demonstrated that the crystallization rate during continuous cooling with a constant rate is not only a function of the actual, momentary temperature, but also depends on the cooling rate. Except for temperatures close to the melting temperature, the measured values of the crystallization rate are much smaller than those evaluated from isothermal crystallization. SAXS studies revealed two regions of changes in lamellar morphology occurring during the initial and final periods of the process.

Interaction of self‐nucleation and the addition of a nucleating agent on the crystallization behavior of isotactic polypropylene

AbstractThe relationship between heterogeneous or homogeneous nucleation and self‐nucleation of polypropylene (PP) and PP nucleated by an organic phosphate salt (PPA) was studied by DSC. For pure PP, it homogeneously nucleated during cooling after melting at the selected temperature (Ts) of 170–200°C for 3 min, but at the Ts of 160–168°C self‐nucleation occurred; PPA only nucleated heterogeneously at the Ts of 168–200°C, and there existed self nucleation at the Ts of 160–168°C. The double melting peaks of PP and PPA at the Ts of 162°C were observed. Once the self‐nucleation occurred, the change of the crystallization temperature and heat of fusion of PP is more significant than that of PPA with the change of the Ts, depending upon the crystallization conditions. Results were explained by homogeneous nucleation, heterogeneous nucleation, self‐nucleation, and annealing crystallization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 78–84, 2001

Effect of the surface roughness of sulfuric acid-anodized aluminum mold on the interfacial crystallization behavior of isotactic polypropylene

The influence of the surface topography of aluminum alloy (Al) on the heterogeneous nucleation of isotactic polypropylene (iPP) at the iPP/Al interface has been investigated using a polarized optical microscope (POM) with a hot stage. Different textures of the Al surface were prepared by electrochemical processes, including polishing and anodizing, and utilized to induce interfacial nucleation upon supercooling. This process enabled the topological features of the aluminum surface to be controlled without altering their chemical composition by such a procedure. The pretreated surfaces were investigated by scanning electron microscopy and quantitatively characterized by a surface texture instrument in terms of RMS roughness (R a). The Al surface with a higher surface roughness induced more nuclei of iPP and led to a transcrystalline layer (TCL) in the interfacial region upon supercooling over the temperature range 128°C < T c < 154°C. Based on the theory of heterogeneous nucleation, it was found that the induction time correlates well with the nucleation rate in determining the interfacial free energy difference function Δσ of iPP. The ratio of Δσ at the interface to that in the bulk matrix (ΔσTCL/Δσ bulk) for the polished surface (R a = 0.38 μm) is 4.45, implying that transcrystallization growth is unfavorable from a thermodynamic point of view. On the other hand, the Δσ TCL/Δσ bulk ratio decreases as the current density for anodizing increases, indicating that transcrystallization growth becomes favorable. The induction times and nucleation rates were also measured to characterize quantitatively the nucleating ability of various Al surfaces. The oxide porosity was filled in when sealing treatment by hydration was carried out. This resulted in Δσ TCL/Δσ bulk being slightly higher as the surface roughness decreased.

Lamellar Thickening in Isotactic Polypropylene with High Tacticity Crystallized at High Temperature

Crystallization behavior of isotactic polypropylene (iPP) with extremely high isotacticity has been studied over a very wide range of crystallization temperature Tc (145 °C ≤ Tc ≤ 166 °C). Optical polarized microscopy observation reveals that morphologies obtained before and after Tc = 157.0 °C are different in nature. Differential scanning calorimetric measurement shows that the melting temperature (Tm) increases significantly with increase of both Tc and crystallization time t. A sudden jump in the slope of Tm versus log t plot is observed at Tc = 157.0 °C, which suggests a kind of order−disorder transition taking place at high Tc (≥157.0 °C) to allow better chain sliding diffusion. Such a disordering has been confirmed by means of X-ray measurements and will be reported in another paper of this series. It was also convinced that annealing at high temperature subsequent to lower temperature crystallization promotes much more rapid lamellar thickening than crystallization alone does at the same high temperature. The lamellar thickness is determined by transmission electron microscopy and also shows a discontinuous upward shift at Tc = 157.0 °C. The lamella with the thickness up to 66 nm is reported for the first time.

Effect of transcrystalline morphology on interfacial adhesion in cellulose/polypropylene composites

A study was conducted on the effect of cotton cellulose fibres on the crystallization behaviour of isotactic polypropylene (PP) from the melt and the resulting morphology. When the PP was allowed to crystallize isothermally at 131° C, the cotton fibres acted as nucleating agents and a transcrystalline phase was created around the fibres. Quench cooling of the melt prevented the occurrence of such a phase. Transcrystalline layers of different thicknesses were created by interrupting the isothermal crystallization at certain intervals and quenching the melt. The effect of these morphologies on interfacial shear stress transfer was investigated using the single-fibre fragmentation test. It was found that the transcrystalline morphology at the fibre/matrix interface improved the shear transfer considerably when a tensile load was applied in the fibre direction. One mechanism is proposed to be particularly responsible for this increase: slow cooling favours the kinetics of the approach of PP molecules, and hence interfacial adsorption, which yields an ordered transcrystalline PP interphase having a high density of intermolecular secondary bonds with the cellulose surface. An increase in the shear transfer efficiency with increasing thickness of the transcrystalline layer was also observed.

Crystallization behaviour of isotactic polypropylene blended withtrans-octenylene rubber

The crystallization behaviour of isotactic polypropylene (iPP) blended withtrans-octenylene rubber (TOR) has been investigated by optical microscopy. It is found that crystallization kinetical parameters like nucleation densities, Avrami exponents and spherulitic growth rates are strongly dependent on the TOR concentration in the blend. While at 10% TOR content the nucleation density passes through a maximum, both the Avrami exponent and the spherulitic growth rate are minimal. Due to an increased dispersion of TOR in iPP at 10% TOR concentration and the subsequent formation of interfaces, the nucleation changes from preferentially homogeneous to preferentially heterogeneous. The concentration oftrans double bonds in the TOR chain has no influence on the crystallization behaviour of the samples.

Effects of EPDM rubber on crystallization behaviour and morphology of iPP/HDPE blend

The effects of EPDM rubber on the crystallization behaviour of isotactic polypropylene (iPP) in the ternary iPP/HDPE/EPDM blends were studied by means of DSC and X-ray diffraction. Analysis of the crystallization exotherm peaks in terms of crystallization nucleation and growth rates and crystallinity revealed variations in the morphology of the iPP component in the blends as a function of the EPDM content. The DSC and X-ray diffraction results showed that the overall crystallinity decreased as the weight percentage of EPDM was increased in the iPP/HDPE blends. The morphology of these blends was studied by scanning electron microscopy, which revealed a random distribution of EPDM throughout the iPP matrix. The size and number of these rubber particles increased with increase of the EPDM weight percentage in the ternary iPP/HDPE/EPDM blends. The probable existence of composite inclusions of EPDM-HDPE in an iPP matrix is suggested.

High pressure d.t.a. study on the melting and crystallization of isotactic polypropylene

The melting and crystallization behaviour of isotactic polypropylene (iPP) in the α ( monoclinic) and γ (triclinic) crystalline modifications has been studied by differential thermal analysis (d.t.a.) under high pressure. The d.t.a. melting thermograms of the sample prepared by slow cooling from the melt at atmospheric pressure (α form) and high pressure (γ form) show single endothermic peaks up to about 4600 bar in isobaric measurements. Two isolated melting point curves, corresponding to the melting of the α and γ modifications, were obtained and the melting points were fitted to a quadratic equation as a function of the pressure (bar). The slope of the curve at atmospheric pressure has been determined accurately for each crystalline form. Two curves were observed for the pressure dependence of the crystallization peak temperature in the d.t.a. curve. The origin of the separated crystallization curves is discussed in relation to the degradation of the iPP melt-crystallized under high pressure.