The effect of melt history,pressure, and crystallization temperature on spherulite size in bulk isotactic polypropylene

Melt history, pressure, and crystallization temperature are three variables that may be used to vary spherulite size in polymer systems. In this study, bulk polypropylene samples were given various melt treatments and then isothermally crystallized under constant pressure. Spherulite size was found to increase with increasing severity (i.e., increased temperature or time at temperature) of melt treatment, explained by the thermal deactivation of nucleation sites. Spherulite size also increases with increasing crystallization temperature, owing to a smaller driving force for nucleation and the deactivation of increasing numbers of nuclei at higher crystallization temperatures. An analogous effect of pressure was also found, and a simple model to compare increased pressure and decreased crystallization temperature was derived.     

Nodular structure of isotactic polypropylene crystallizes from the melt

This article highlights the melt crystallization behavior of different grades of isotactic polypropylene (iPP) using a hot‐stage polarizing optical microscopy. iPP samples were heated up at a heating rate of 10°C/min passing the melting temperature and then kept for 3 min at a temperature range of 175–200°C before they cooled rapidly at 40°C/min to crystallize isothermally at a range of 130–145°C. It has been found that the temperature at which the samples were kept has a strong effect on the crystallization mode; for samples heated up and kept at temperatures below 190°C, the crystallization started with thin and long rods or nodules, which grew in the circumferential direction only while their lengths remain unchanged as the time passed. The shape of the nodules can be straight, circular, branched, or entangled, and they can grow parallel to each other or they can be crossed or in a random way. This phenomenon disappeared completely for samples melted and kept at temperatures above 195°C. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of chain disentanglement on melt crystallization behavior of isotactic polypropylene

Isotactic polypropylene (iPP) with “disentangled” chains was generated through crystallization of iPP from its mineral oil solution. TGA test assured complete removal of mineral oil from iPP precipitates. Time sweep rheological measurements showed the modulus build-up with time indicating the formation of “disentangled” chains in iPP after the sample disentanglement treatment. The “disentangled” chains could preserve for a certain time before completely re-entangled during melting. Crystallization kinetics of iPP with “disentangled” chains was studied by using polarized optical microscope. The growth rate of spherulites in “disentangled” iPP was faster than that in the entangled one.     

Influence of crystallization temperature on the morphologies of isotactic polypropylene single-polymer composite

The supermolecular structures of iPP fiber/matrix composites as a function of crystallization temperature were studied by means of optical microscopy. The results show that, even though partial melting of the iPP fibers is in favor of initiating the β-iPP crystal growth, the interfacial morphology of iPP single-polymer composites induced by its own fiber depends strongly on the crystallization temperature. It was found that transcrystalline structures of negative radial β_III-iPP or banded β_IV-iPP can be produced within the crystallization temperature range 105-137 °C, while transcrystallization zone of pure negative radial α_II-iPP crystals is observed at higher crystallization temperature, e.g. 141 °C. On the other hand, the surrounding iPP spherulites grown from the bulk are composed of α-iPP in the whole crystallization temperature range. However, the optical character of the spherulites is controlled by the thermal condition.     

The effect of talc on the crystallization of isotactic polypropylene

Isotactic polypropylene (iPP) has been crystallized in the presence of talc under the quiescent state and shear flow of injection molding. The resulting morphology has been investigated by means of polarizing microscopy, transmission electron microscopy, and wide angle X‐ray diffraction. In the quiescent state, the iPP lamellae grew from the surface of talc and the transcrystalline region was formed at the interface between iPP melt and the talc. The nucleation of iPP was very frequent on the cleavage plane of talc. The X‐ray diffraction pattern of the transcrystal showed a*‐axis orientation to the crystal growing direction. In injection‐molded samples of the talc‐filled iPP, the morphology of lamella growing from talc appeared as same as that of the transcrystal. However, the crystalline orientation of injection‐molded talc‐filled iPP, in which the b axis was oriented to the thickness direction and the a* and the c axis was oriented to the flow direction, was quite different from that of the transcrystal. This b‐axis orientation results from the orientation of the plate plane of talc, which induces the nucleation and the crystallization under shear flow. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1693–1703, 2001     

Substrate effect on the crystallization of isotactic polypropylene

The evolution of storage modulus measured by a rotational rheometer shows that the isothermal crystallization of isotactic polypropylene (iPP) melts in contact with aluminum plates (PP-Al) are considerably faster than that with stainless-steel plates (PP-SS). The difference is bigger at higher temperatures, and this behavior is opposite to that expected by our numerical simulation considering uniform bulk phase transition and substrate's ability to remove the latent heat. Polarized optical observations and surface energy evaluations via contact angle measurement indicate that surface energy of the substrates, including the effects of submicrometer morphology and roughness, should be the key factor to affect the crystallization of iPP. Transcrystallization zones, in which the nucleation density is controlled by the surface energy of substrates, were observed to grow toward the bulk with the thickness of about 0.2 mm for iPP to affect the global crystallization behavior. The critical value of surface energy of substrate to promote the interfacial crystallization of a polymer melt is derived, in terms of which the aluminum and stainless steel as well as optical glass, promote the surface nucleation with respect to the bulk nucleation of iPP. As a consequence, the conventional differential scanning calorimetry measurement mainly gives the heat fluxes of interfacial crystallization rather than the bulk crystallization due to the large surface-to-volume ratio of the specimen and the aluminum pan used which is a high surface energy substrate. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Dependence of bulk viscosity of polypropylene on strain,strain rate,and melt temperature

The relations between bulk viscosity, strain, strain rate, and melt temperature of polypropylene are investigated in uniaxial compression similar to the measurement of PVT diagram of a polymer. A new and more practical bulk viscosity model for the simulation of injection molding is proposed. The bulk viscosity increased with the increase of compression deformation and decreased when the compression speed and the melt temperature are higher. This dependency is attributed to the orientation and occupancy of the molecule within the free interstitial space. In addition, the magnitude of the bulk viscosity was found to be significantly greater than the shear viscosity at high compression deformation and low strain rate. Thus, not only shear viscosity but also the bulk viscosity should be considered in the simulation of the packing‐holding stage of the plastic injection molding process. POLYM. ENG. SCI., 57:830–837, 2017. © 2016 Society of Plastics Engineers     

The influence of stem conformation on the crystallization of isotactic polypropylene

The influence of stem conformation on the crystallization of i-PP is studied by growing α-form lamellae in melts of β-form lamellae at different temperatures. The melting of β-form lamellae and the crystallization of α-form lamellae is observed in situ at the interface of α- and β-form spherulites by AFM. The growth rate of α-form lamellae in the melt of β-form lamellae is much lower than that in the isotropic melt due to the stem conformation barrier, which originates from the difference in the α and β unit cell packing models.     

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.     

Effect of polycyclopentene on crystallization of isotactic polypropylene

The application of some polymers as nucleating agents for polypropylene has been examined. Among various polymeric nucleating agents, polycyclopentene was found to be a superior nucleating agent to typical organic nucleating agents. When polycyclopentene was added to polypropylene, the crystallization temperature and the degree of crystallinity of polypropylene increased. In addition, the crystallization rate and the number of spherulites increased whereas the size of spherulites decreased remarkably. As a result of polycyclopentene addition, the transparency of polypropylene film could be improved considerably. © 1994 John Wiley & Sons, Inc.     

Critical crystallization temperature for the occurrence of epitaxy between high-density polyethylene and isotactic polypropylene

The crystallization behavior of high-density polyethylene (HDPE) on highly oriented isotactic polypropylene (iPP) at elevated temperatures (e.g., from 125 to 128°C), was studied using transmission electron microscopy and electron diffraction. The results show that epitaxial crystallization of HDPE on the highly oriented iPP substrates occurs only in a thin layer which is in direct contact with the iPP substrate, when the HDPE is crystallized from the melt on the oriented iPP substrates at 125°C. The critical layer thickness of the epitaxially crystallized HDPE is not more than 30 nm when the HDPE is isothermally crystallized on the oriented iPP substrates at 125°C. When the crystallization temperature is above 125°C, the HDPE crystallizes in the form of crystalline aggregates and a few individual crystalline lamellae. But both the crystalline aggregates and the individual crystalline lamellae have no epitaxial orientation relationship with the iPP substrate. This means that there exists a critical crystallization temperature for the occurrence of epitaxial crystallization of HDPE on the melt-drawn oriented iPP substrates (i.e., 125°C). © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2029–2034, 1997     

Effects of thermal history on isotactic polypropylene

The effect of past thermal history on the melting behavior of isotactic polypropylene is investigated in some detail. It is shown that a series of stepwise annealing treatments at steadily increasing temperatures will raise the final melting point and will result in a double endothermic peak if the final anneal temperature is at or close to 160°C. It is also shown that a series of stepwise annealing treatments at steadily decreasing temperature will lead to multiple DSC peaks. The number of such separate peaks is equal to or greater than the number of annealing steps. Even low-temperature anneals (100–130°C) affect the melting endotherm, while high-temperature anneals have a marked effect on both the degree of crystallinity of the sample and the final melting temperature. For a 3-min anneal, the highest degree of crystallinity is produced by an anneal temperature of 155°C. The highest melting temperature (～182°C) is produced by a 30-min, or longer, anneal at about 160°C. The implications of these results in terms of crystal thickening and perfection are discussed.     

Effect of vibration on crystal morphology and structure of isotactic polypropylene in nonisothermal crystallization

An experimental study on crystal structure and morphology of isotactic polypropylene (iPP) subjected to vibration was carried out on a laboratory apparatus. Crystallite size, crystal structure, and crystallinity of iPP under vibration or nonvibration were investigated through differential scanning calorimeter (DSC), wide angle X‐ray diffraction (WAXD), and polarized optical microscopy (POM). The results reveal that at high cooling rate, the crystallinity of samples under vibration decreases, and at low cooling rate it remains constant because of chain relaxation. On the other hand, the sizes of the iPP spherulites under vibration decrease as compared with those without vibration. Taking the relaxation of the iPP chain into consideration, we believe that the influence of vibration conditions on the main α‐form of the iPP crystal is rather complex. An obvious increase of β‐form content in the crystal phase results from the imposition of vibration. The results indicate that the content of β‐iPP is dependent on vibration amplitude and time. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2187–2195, 2004     

The influence of structure and processing conditions on engineering mechanical properties in bulk crystallized isotactic polypropylene

A study of the influence of processing conditions and structure on engineering mechanical properties was conducted in bulk isotactic polypropylene. The influence of one processing parameter, undercooling, defined so as to account for both pressure and temperature effects, was particularly studied. Improved mechanical properties were found with increased undercooling. At low undercoolings, brittle failure without yield occurred, presumably the result of a sparsity of intercrystalline links under these conditions. As undercooling was increased, failure occurred after yielding as failure stress elevated dramatically, apparently because of greater link density. A modest improvement in yield stress with increased undercooling was attributed to the increasingly crosshatched lamellar structure produced at higher undercoolings, a structural trend confirmed by electron microscopy. Spherulite size, varied by altering melt history (melt temperature and time at melt temperature) at constant undercooling, was found to have no effect on engineering yield stress. This result indicates that apparent yield stress–spherulite size effects found by several earlier investigators were probably caused by structural variations other than spherulite size.     

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     

Rheological and structure investigation of shear‐induced crystallization of isotactic polypropylene

In the present work, the influence of well‐defined simple shear flow histories on the isothermal crystallization of an isotactic polypropylene (i‐PP) has been investigated. At first, the research of the flow conditions in terms of temperature, shear rate ( ) and shear strain (γ) has been performed by means of the rheological technique. The continuous shearing analysis enabled us to build the flow curve at 144°C showing a Newtonian region as well as a shear‐thinning zone. Indeed, for above a critical value, the molecular orientation occurring during flow provides a kinetic promotion of the crystallization process. In the rheological step‐shear flow analysis, an increase of the flow sensitivity parameter, k_S/k_Q, with increasing the shear rate at a constant strain (γ = 150) is observed. The structure of the crystallized samples has been investigated by differential scanning calorimetry (DSC) and wide angle X‐ray scattering (WAXS) methods. In agreement with the DSC, the WAXS results show that crystals with a certain bimodal distribution are generated in the samples crystallized under step‐shear flow conditions. A small orientation of the (110) plane of the i‐PP α‐phase crystals is also detected. POLYM. ENG. SCI., 45:153–162, 2005. © 2005 Society of Plastics Engineers.     

Hydrogenated petroleum resin effect on the crystallization of isotactic polypropylene

The 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     

Comparison of structure development in quiescent crystallization,die extrusion and melt spinning of isotactic polypropylene and its compounds containing fillers and nucleating agents

The influences of fillers and nucleating agents on crystalline structure and stress induced crystallization of isotactic polypropylene were studied under a range of cooling and processing conditions, including die extrusion and melt spinning. Continuous cooling transformation curves were determined for polypropylene and various polypropylene filler compounds. The influence of spinline stress on crystallization was studied. The experiments reveal that under quiescent conditions, the kinetics and crystalline forms produced by the crystallization of polypropylene are dominated by nucleating fillers and impurities. The crystalline orientation‐spinline stress relationship, on the other hand, was found to be the same for polypropylene and its compounds. At high uniaxial stresses, kinetics and orientation development are dominated by homogeneous orientation crystallization.     

The effects of shear and temperature history on the crystallization of chocolate

Experiments have been carried out on the tempering of chocolate using a temperature-controlled shearing rig with a concentric cylinder geometry. This design maximizes uniformity of shear rate during tempering in contrast to most tempering devices where shear is often concentrated in a small part of the chocolate mass. Samples were subsequently cooled in a differential scanning calorimeter (DSC) to monitor how the sample crystallized, and then reheated to gain further information on the melting points of the polymorphs formed. The results can be interpreted using established theories on the crystallization mechanism. It was found that at least two polymorphic forms could be generated. The higher melting form predominated provided the shear rate was high enough, the temper time long enough, the rewarm temperature low enough, and the DSC scan rate slow enough. For parameters affecting the nucleation stage (temper time and shear rate), it was found that the transition was a sharp one, reflecting the notion that seed crystals need to grow past a threshold point in order to be stably formed. Raising the rewarm temperature had the effect of destroying seed nuclei, but this was a more gradual process. The bimodal nature observed of the melting points compared to the more spread-out behavior of crystallization temperatures reflects the kinetic constraints found in crystallization which are not found on melting.     

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 (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 ( 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 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.