Crystallization Of Isotactic Polypropylene Research Articles

Influence of the cooling rate on the non-isothermal crystallization of isotactic polypropylene modified with sorbitol derivative and silsesquioxane

Summary — The influence of 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS) and tetra-silanolphenyl silsesquioxane (phPOSS) on the modification of isotactic polypropylene ( i PP) wasinvestigated. In particular, dependency of crystallization behavior on the cooling rate was deter-mined. To understand these effects the results of differential scanning calorimetry (DSC) measure-ments and the crystallization curves were analyzed. Keywords : isotactic polypropylene, silsesquioxane, sorbitol derivative, crystallization.OCENAWP£YWU SZYBKOŒCI CH£ODZENIANAPRZEBIEG NIEIZOTERMICZNEJ KRYSTA-LIZACJI IZOTAKTYCZNEGO POLIPROPYLENU MODYFIKOWANEGO POCHODN¥ SORBI-TOLU I SILSESKWIOKSANEM Streszczenie — W niniejszym artykule opisano modyfikacje izotaktycznego polipropylenu zapomoc¹ 1,3:2,4-bis(3,4-dimetylobenzylideno)sorbitolu (DMDBS) oraz tetrasilanolofenylosilses-kwioksanu (phPOSS). W szczegolnoœci zbadano zale?noœae przebiegu procesu krystalizacji odszybkoœci ch³odzenia w przypadku polipropylenu modyfikowanego uk³adem DMDBS/phPOSS.W tym celu przeprowadzono badania metod¹ skaningowej kalorymetrii ro?nicowej (DSC) orazanalizowano przebieg krzywych krystalizacji.

Shear-induced crystallisation of molten isotactic polypropylene within the intertube channels of aligned multi-wall carbon nanotube arrays towards structurally controlled composites

A melt processing route was successfully applied in the manufacturing of aligned multi-wall carbon nanotube/isotactic polypropylene (MWCNT-iPP) composites. As high as 25wt% content of the catalytic Chemical Vapour Deposition (c-CVD) derived nanotube filler could be introduced into the iPP matrix. The composite was characterised by a practically retained nanotube alignment as well as an enhanced crystallisation of iPP. Analysis of the phase iPP composition in the composite using XRD and DSC revealed a dominating α-phase with polymer molecules uniaxially oriented in the same direction as the nanotube alignment. Furthermore, kinetic in situ Synchrotron XRD studies verified that the enhanced crystallisation was induced mainly by shear stress occurring in the initial stage of infiltration MWCNT arrays by flowing molten polymer and, to a lesser extent, by the template-based growth of iPP crystallites.

The influence of inertia and elastic retraction on flow-induced crystallization of isotactic polypropylene

The influence of nonquiescent relaxation of isotactic polypropylene on flow-induced crystallization (FIC) is investigated by a combination of particle tracking velocimeter (PTV) with a cone-plate shearing geometry and synchrotron radiation microbeam wide-angle X-ray diffraction (SR-μWAXD), which is aimed to correlate real flow profile and distribution of crystal orientation. With PTV technique, we observed that flow remains homogeneous during shear, while postshear movement and delayed fracture take place after a step strain when large shear rates and strains were imposed. Delayed fracture slices samples into several layers which move either forward or backward after the cessation of shear imposed externally. SR-μWAXD measurements reveal that the layers moving forward keep high crystal orientations while the layers moving backward show low orientations, which gives an inhomogeneous distribution of crystal orientation across the thickness of sheared samples. The correlation between moving direction and cry...

Influence of a particulate nucleating agent on the quiescent and flow-induced crystallization of isotactic polypropylene

Flow induced crystallization of commercial isotactic polypropylene (iPP) and its blends with sodium 2,2′-methylene bis-(4,6-di-tert-butylphenyl) phosphate (also known as NA11) is studied by means of in-situ time resolved small-angle X-ray scattering (SAXS). The isothermal crystallization at 145 °C (i.e. well below melting temperature of polymer) is performed after the application of steady shear to probe the anisotropic structure formation. In order to separate the influence of shear rate and shear time on polymer crystallization, four different shear conditions (60 s−1 for 1 s, 30 s−1 for 2 s, 15 s−1 for 4 s and 6 s−1 for 10 s) are applied while maintaining the same imposed strain in the polymer melt. Further the effect of different concentration of nucleating agent on the crystallization kinetics of iPP is examined both under quiescent and shear flow conditions. For instance, under quiescent condition, the crystallization half-time (τ1/2) decreases with the increasing concentration of nucleating agent in the polymer. Under shear flow conditions, our observations are as follows: In the case of neat iPP, τ1/2 decreases significantly at higher shear rates (≥30 s−1). Compared to the neat iPP, for the same concentration of NA11 in the NA11/iPP blends differences in τ1/2 with the increase in applied shear rates are significantly smaller. In other words, the crystallization kinetics is dominated by the amount of nucleating agent in the NA11/iPP blends as opposed to shear rates in the neat iPP. The present study shows that the critical value of shear rate required for chain orientation in the molten polymer is lower in the presence of the nucleating agent compared to neat iPP. The self-nucleation process investigated with the aid of differential scanning calorimetry (DSC), indicates that the nucleating efficiency of NA11 on iPP is around 60%.

Effect of nano-scaled styrene butadiene rubber based nucleating agent on the thermal, crystallization and physical properties of isotactic polypropylene

• The effect of a SBR based β-NA on the properties iPP was investigated. • The addition of β-NA led to higher population of nuclei and smaller spherulites. • β to α phase transformation was observed when re-extrusion process was applied. • Impact strength was increased when the β-NA was added from 0.10 to 0.20 wt%. The influence of a specific nano-scaled styrene butadiene rubber based β-nucleating agent (β-NA) on the properties of isotactic polypropylene (iPP) was investigated in the current research. β-NA was applied at the concentration ranged from 0.05 to 0.50 wt%. Microscopic observation revealed that the neat iPP crystals grew very slowly; they ranged in size from 100 to 200 μm. The addition of β-NA led to higher population of nuclei and smaller spherulites than those found in neat iPP. The addition of only 0.05 wt% β-NA significantly decreased the sizes of the spherulites down to 5 μm; the crystal grew very rapidly, leading to extremely fine morphology. Analysis by X-ray diffraction (XRD) confirmed that iPP/β-NA constituted mainly of β-crystal structure. The transformation of β to α phase was observed upon re-extrusion, it was verified by the lowered fraction of the β-crystalline phase (K β ) although the total degree of crystallinity remained unchanged. A significant improvement in the impact strength of the iPP/β-NA was observed when the β-NA was employed from 0.10 to 0.20 wt%, leading to the formation of tough β-crystals in the β-NA nucleated iPP. The color measurement implied that the iPP nucleated with β-NA was superior in terms of whiteness but it was less transparent, as was evident by the increased haze.

Solution Crystallization and Dissolution of Polyolefins as Monitored by a Unique Analytical Tool: Solution Crystallization Analysis by Laser Light Scattering

For the first time, the solution crystallization and dissolution behavior of polyolefins in a variety of solvents was investigated by using a recently developed crystallization based analysis technique, solution crystallization analysis by laser light scattering (SCALLS). SCALLS results provide clear evidence that crystallization and dissolution of linear polyethylene (PE) and isotactic polypropylene (iPP) are greatly influenced by the type of solvent used. It was demonstrated for a blend of PE and iPP that cocrystallization effects are minimal in solvents such as TCB and o-DCB and are significantly more pronounced in xylene and decalin. Surprisingly, in xylene, individual dissolution curves (bimodal SCALLS profile) for both PE and iPP with minimal codissolution effects were observed while in TCB, o-DCB, and decalin both components dissolve simultaneously. These findings provide a novel and facile approach to understand the effect of solvents on cocrystallization and codissolution of chemically dissimilar components in preparative fractionations such as prep TREF (which normally uses xylene), by using TCB as the crystallization solvent and xylene as the eluent.

High-Stress Shear-Induced Crystallization in Isotactic Polypropylene and Propylene/Ethylene Random Copolymers

Crystallization of an isotactic polypropylene (iPP) homopolymer and two propylene/ethylene random copolymers (RACO), induced by high-stress shear, was studied using in situ synchrotron wide-angle X-ray diffraction (WAXD) at 137 °C. The “depth sectioning” method (Fernandez-Ballester Journal of Rheology 2009, 53 (5), 1229−1254) was applied in order to isolate the contributions of different layers in the stress gradient direction and to relate specific structural evolution to the corresponding local stress. This approach gives quantitative results in terms of the specific length of fibrillar nuclei as a function of the applied stress. As expected, crystallization becomes faster with increasing stress—from the inner to the outer layer—for all three materials. Stress-induced crystallization in a RACO with 7.3 mol % ethylene content was triggered at only 1 °C below its nominal melting temperature. The comparison of iPP and RACO’s with 3.4 and 7.3 mol % ethylene monomer reveals the effect of ethylene defects on high-stress shear induced crystallization at 137 °C. It is found that, for a given applied stress, the specific nuclei length formed by flow increases with ethylene content—which is attributed to a greater high molecular weight tail. However, the linear growth rate is significantly reduced by the presence of ethylene comonomers and it is found that this effect dominates the overall crystallization kinetics. Finally, a time lag is found between development of parent lamellae and the emergence of daughter lamellae, consistent with the concept of daughter lamellae nucleated by homoepitaxy on the lateral faces of existing parent lamellae.

Comparison Study on the Heterogeneous Nucleation of Isotactic Polypropylene by Its Own Fiber and α Nucleating Agents

The crystallization of isotactic polypropylene (iPP) in the presence of its own fiber or/and α nucleating agents, i.e., DBS and NA11, was studied. The results show that both nucleating agents exhibit nucleation ability toward iPP and accelerate iPP melt crystallization. This is manifested by an upward shift of crystallization temperature by up to 10 °C compared with its bulk crystallization. The iPP fiber exhibits also high nucleation ability toward the iPP matrix as demonstrated by the formation of transcrystalline layers. The crystallization of iPP in the nucleated iPP matrix/fiber system shows that the nucleating agents do not affect the transcrystallization of iPP surrounding its fiber. This indicates that the fiber exhibits higher nucleation ability toward iPP than the nucleating agents. On the basis of the exact same chemical and crystallographic structures of the iPP fiber and matrix, the results demonstrate the importance of structure similarity and matching between the crystallographic structures...

Slippage and shear inhomogeneity in shear-induced crystallization of isotactic polypropylene on metal substrates

Substrate effect, slippage, and shear inhomogeneity in the shear-induced crystallization of an isotactic polypropylene are studied by rheological and optical experiments. Significant wall slip that reflects the polymer chain desorption from the wall was observed for the supercooled isotactic polypropylene (iPP) melt at 131 \(^{\circ }\)C in both the pre-shear and the subsequent small-amplitude oscillatory shear that monitor the crystallization of iPP. Crystallization of the iPP melt on aluminum substrate is faster than that on stainless-steel substrate with the same pre-shearing condition. Because the surface energy of aluminum plate is higher than that of stainless-steel plate, when using the aluminum plates, the slip during the pre-shearing is smaller; thus, the real shear rate (or shear strain, shear work) exerted to the melt is higher and so is the shear-induced nucleation density. By using the observed nucleation density and the estimated nucleus growth rate, the Kolmogoroff equation can yield correct orders of magnitudes of crystallization rates of iPP. Assuming that higher shear rate induces higher nucleation density in the iPP melt, the shear inhomogeneity during the pre-shearing can be inferred based on the optical observation on the crystallized iPP samples.

Hydrogenated petroleum resin effect on the crystallization of isotactic polypropylene

AbstractThe influence of the hydrogenated petroleum resin P125 on the crystallization behavior, crystallization kinetics, and optical properties of polypropylene (PP) were investigated. The results of differential scanning calorimetry, successive self‐nucleation, and annealing fractionation demonstrated that P125 reduced the interaction between the PP molecules, decreased the crystallization, prevented PP from forming thick lamellae, and encouraged the formation of thin lamellae. The isothermal crystallization kinetics, self‐nucleation isothermal crystallization kinetics, and polarized optical microscopy observations showed that P125 slightly decreased the nucleation rate, significantly decreased the crystal growth rate, generally reduced the overall crystallization rate, and effectively deceased the crystallite sizes of PP. The optical properties studies showed that P125 effectively decreased the haze and increased the surface glossiness and yellowness index of PP. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Crystallization of isotactic polypropylene containing beta-phase nucleating agent at rapid cooling

The crystallization behavior of isotactic polypropylene containing a β-phase nucleating agent has been investigated, focusing on evaluation of the effects of cooling rate and/or supercooling of the melt on the generation of different polymorphs. It has been found that β-crystals only form on cooling the melt at rates lower than about 50Ks−1 while cooling at rates between 50 and 300Ks−1 leads to formation of α-crystals; even faster cooling is connected with mesophase formation or vitrification of the entire melt. Fast scanning chip calorimetry revealed different mechanisms of nucleation at low and high supercooling. In comparison to non-nucleated iPP the presence of the β-phase nucleating agent only affects the crystallization kinetics at low supercooling, supporting the idea that ordering at high supercooling is governed by homogeneous nucleation. β-crystals, formed initially on slow cooling, melt below about 420K on slow heating, followed by formation of few α-crystals on continuation of heating. The mesophase initially formed on fast cooling and aging at ambient temperature, in contrast, re-crystallizes directly into α-structure. The results of the present work provide comprehensive information about the condition of formation and the stability of different polymorphs in isotactic polypropylene containing a β-phase nucleating agent.

β-Crystal in the iPP melt encapsulated by transcrystallinity and spherulites: effect of molecular weight

The crystallization in isotactic polypropylene (iPP) melt inclusions, encapsulated by transcrystallinity and spherulites, was investigated by polarized light microscope. Generally, owing to the proceeding of crystallization in melt inclusions, a negative pressure will be built up. Up to a critical value, the negative pressure is released by the formation of cavitation in melt. Unexpectedly, β-crystals with different morphologies emerge at different locations in the melt inclusions. For low molecular weight iPP, only smaller β-crystals around bubbles are discernable. Turn to higher molecular weight iPP, apart from the smaller β-crystals, fan-shaped ones can be developed under suitable conditions (e.g., lower than 131 °C) at the growth front of spherulites and transcrystallinity. Considering the characteristics of molecular weight and morphology, it is proposed that β-crystals are induced by different mechanisms. That is, smaller β-crystals are resulted from the stress caused by the appearance of bubbles while the fan-shaped ones are derived due to the temperature gradient during the release of negative pressure.

Largely enhanced crystallization of semi-crystalline polymer on the surface of glass fiber by using graphene oxide as a modifier

Interfacial crystallization of polymer on the surface of filler not only offers crystallography interest but also has a potential to improve the interfacial interaction, which is a key for the preparation of high-performance polymer/filler composites. In this work, a new method is proposed to improve the interfacial crystallization between semi-crystalline polymer and glass fiber (GF) by introducing graphene oxide (GO) to the surface of amorphous GF. The coating of GO on GF surface is realized via electrostatic self-assembling of the oppositely charged GO and amino coupling agent modified GF (GF-NH2). After the thermal reduction of the coated GO (RGO), RGO coated GF (GF-RGO) is obtained. The interfacial crystallization of isotactic polypropylene (iPP) and poly(l-lactide) (PLLA) on the surface of raw GF, GF-NH2, and GF-RGO are investigated using Polarized light microscope (PLM). It is found that raw GF and GF-NH2 has almost no nucleation ability on the polymers crystallization. However, transcrystalline structure can be successfully induced at the polymers/GF-RGO interface, indicating a significantly improved nucleation ability of GF-RGO for polymer crystallization. This work could provide a new way to control interfacial crystallization, thus the interfacial adhesion of polymer/filler composites, and could also find a new application for GO as well.

Temperature dependence of crystallization of nano-oriented crystals of iPP and the formation mechanism

We have studied the melt-elongational crystallization of isotactic polypropylene. In 2010, we observed that ‘nano-oriented crystals (NOCs)’ were formed when the elongational strain rate ( ) exceeded a critical value ( ). In this study, we have clarified the formation mechanism of NOCs using the crystallization temperature (Tc) dependence of the formation of NOCs and structural studies using small angle X-ray scattering, SAXS, wide angle X-ray scattering, WAXS, and optical microscopy. We propose a ‘chain reaction model’ for the formation of NOCs; the locally oriented melt is formed by the melt elongation, which significantly accelerates nucleation. The generated nuclei accelerate the formation of the locally oriented melt, which further accelerates the nucleation. We formulate a relationship between and the nucleation rate (I) based on the above model. We have confirmed that NOCs are formed in the range of Tc=150–167 °C. We have obtained the degree of supercooling (ΔT) dependence of I from the Tc dependence of . Here, we estimated an equilibrium melting temperature in the oriented melt Tm0=220 °C from the observation of NOC formation, where the error in Tm0 was a few tens of K. The observed I was well-fitted with the well-known equation of for homogeneous nucleation from classical nucleation theory (CNT), where C is a constant. Therefore, we conclude that the formation of NOCs is primarily controlled by a homogeneous nucleation process, which confirms the chain reaction model. We studied the formation mechanism of ‘nano-oriented crystals (NOCs)’ of isotactic polypropylene in elongational crystallization through crystallization temperature dependence of NOCs formation. We proposed a ‘chain reaction model’ of NOCs formation: The local oriented melt is formed by the melt elongation, which significantly accelerates nucleation; and, generated nuclei accelerate formation of local oriented melt that accelerates nucleation again. We obtained the nucleation rate (Iobs) of NOCs against the degree of supercooling (ΔT). Here, an equilibrium melting temperature in the oriented melt Tm0=220 °C is estimated from observation of NOCs formation. Iobs was well fitted by well-known equation of for homogeneous nucleation in classical nucleation theory, where C is a constant. Therefore, we concluded that NOCs formation is mainly controlled by the homogeneous nucleation process.

Quantification of non-isothermal, multi-phase crystallization of isotactic polypropylene: The influence of shear and pressure

Key issue in studying the crystallization process of semi-crystalline polymers, is the need for controlled (or known) boundary and initial conditions. Here dilatometry (PVT) is used to reveal the crystallization kinetics and the resulting morphology of isotactic polypropylene homopolymer as a consequence of the combination of non-isothermal cooling at elevated (isobaric) pressure and application of shear flow. Based on the flow strength the crystallization kinetics can be classified in three regimes; quiescent crystallization, flow enhanced point nucleation and flow-induced creation of oriented structures. Using ex-situ wide angle X-ray diffraction and small angle X-ray scattering we revealed the details of the (oriented) crystalline structures for the different regimes including the different phases (α-, β-, and γ-phase) that can occur in iPP. Some unique oriented structures of combined α- and γ-phase were found. The distribution of the crystal phase content is explained by the interplay of nucleation density and crystal phase dependent growth rate, which are functions of both temperature and pressure.

Effect of Shear Stress on Crystallization of Isotactic Polypropylene from a Structured Melt

Effect of Shear Stress on Crystallization of Isotactic Polypropylene from a Structured Melt

Crystallization of Isotactic Polypropylene under the Spatial Confinement Templated by Block Copolymer Microdomains

We investigate the crystallization behavior of isotactic polypropylene (iPP) under the influence of nanoscale confinement templated by the microphase-separated structure of an iPP-based diblock copolymer system, isotactic polypropylene-block-atactic polystyrene (iPP-b-aPS). Three types of iPP microdomains, i.e., lamellae, cylinder, and sphere, were generated by controlling the composition of the diblock. The effect of microdomain morphology on the nucleation mechanism, crystallization kinetics, self-nucleation behavior, the population of the helical sequence of iPP block in the melt state, and crystal orientation have been systematically studied. It was found that the crystallization rate of iPP was predominantly controlled by homogeneous nucleation when the crystallization process was largely confined within the individual cylindrical and spherical microdomains. Such a nucleation mechanism and the highly frustrated crystal growth in the isolated microdomains led to the absence of Domain II and atypical crystallization kinetics in Domain III in the self-nucleation study. The population of the longer helical sequence of iPP block revealed by infrared spectroscopy was found to be affected by temperature but not by the spatial confinement, chain stretching, and junction point constraint imposed by the microdomains. Finally, the orientation of α-form iPP crystals in the lamellae-forming iPP-b-aPS was identified over a broad range of crystallization temperatures (T(c)). Different from other crystalline-amorphous diblocks, a lamellar branching of α-form iPP was observed in the lamellar microdomains at T(c) lying between 15 and 80 °C, where the daughter lamellae developed from the perpendicularly orientated parent iPP crystals with a specific angle of 80° or 100°. Once the sample was crystallized at T(c) ≤ 10 °C, the iPP crystals became randomly oriented.

Effect of β-Nucleating Agent on the Crystallization, Mechanical Properties, and Heat Resistance of Injection-Molded Isotactic Polypropylene

Injection-molded β-isotactic polypropylene (β-iPP) was prepared with a commercial β-nucleating agent (NT-A). The effect of NT-A on the crystallization, mechanical properties, and heat resistance of β-iPP was investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), polarized light microscopy (PLM), and mechanical and heat deflection tests. DSC and WAXD analysis showed that the content of β-crystals in the nucleated iPP was higher than that of pure iPP, and the content of β-crystals of the core was higher than that of the skin. PLM observations showed that injection-molded iPP had an obvious skin-core structure. NT-A induced abundant β-crystals and resulted in small spherulites which improved the Izod notched impact strength. When the content of NT-A was 0.075wt%, the Izod notched impact strength reached a maximum, 2.6 times more than that of pure iPP. The heat distortion temperature was also improved by NT-A.

Isothermal and nonisothermal crystallization of isotactic polypropylene/graphene oxide nanosheet nanocomposites

Isothermal and nonisothermal crystallization of isotactic polypropylene (iPP)/graphene oxide nanosheet (GONS) nanocomposites were investigated by differential scanning calorimetry, polarizing optical microscopy and synchrotron wide-angle X-ray diffraction. Nonisothermal crystallization measurement revealed that GONSs significantly elevated crystallization peak temperature of iPP. Nucleating activity of the nanocomposites containing 0.05 wt% and 0.1 wt% GONSs was 0.61 and 0.56, respectively, indicating strong nucleation ability of GONSs. Induction period and half crystallization time of nanocomposites was greatly reduced during isothermal crystallization process. Moreover, only 0.05 wt% and 0.1 wt% caused less temperature dependence of crystallization kinetics in a wide temperature range of 132–140 °C. A very low GONS loading provided sufficient surface area for crystallite nucleation. Polarized optical microscopy showed that nucleation density of nanocomposites was much larger than that of neat iPP, accelerating overall spherulite growth rate. Crystal structure of iPP was not affected by GONSs, confirming the α-nucleating ability of GONSs.

Quantification of non-isothermal, multi-phase crystallization of isotactic polypropylene: The influence of cooling rate and pressure

The structure of semi-crystalline polymers is strongly influenced by the conditions applied during processing and is of major importance for the final properties of the product. A method is presented to quantify the effect of thermal and pressure history on the isotropic and quiescent crystallization kinetics of four important structures of polypropylene, i.e. the α-, β-, γ- and mesomorphic phase. The approach is based on nucleation and growth of spherulites during non-isothermal solidification, described by the Schneider rate equations combined with the Komogoroff-Avrami expression for space filling. Using an optimization routine the time-resolved multi-phase structure development is accurately described using crystal phase dependent growth rates and an overall nucleation density, all as function of temperature and pressure. It is shown that the maximum growth rate of the α-, and γ-phase increases with applied pressure, while it decreases for the mesomorphic phase. Addition of β-nucleation agent is interpreted as a secondary nucleation density with a coupled β-phase growth. This complete crystallization kinetics characterization of isotactic polypropylene allows prediction of the multi-phase structure development for a wide range of quiescent processing conditions.