Nucleation Of iPP Research Articles

Estimating the Nucleation Ability of Various Surfaces Towards Isotactic Polypropylene via Light Intensity Induction Time Measurements

Crystallization of isotactic polypropylene (iPP) at the interface with crystalline films of two commercially employed nucleating agents (sodium benzoate (NaBz) and sodium 2,2’-methylene bis-(4,6-di-tert-butylphenyl)phosphate (NA-11)) and with a glass fiber (GF) was investigated using a polarized optical microscope. The analysis of the light intensity evolution during the crystallization process enabled the successful estimation of the time at which the crystal growth began, i.e., the induction time (ti), at various crystallization temperatures. Meaningful differences in the ti values were observed between the investigated systems. Moreover, the ti data have been analyzed according to different nucleation models proposed in the literature, which consider either the time to form the first crystalline layer in contact with the substrate or the time required to grow a cluster of critical size. It has been found that the two models are applicable in different temperature ranges depending on the efficiency of the given substrate. Therefore, in order to obtain the value of the surface free energy difference function, Δσ, which is directly related to the nucleation energy barrier and useful for the definition of a universal nucleating efficiency scale, a model that considers both the above-mentioned times was fitted to the overall data. The values of Δσ for the nucleation of iPP on the surface of the different substrates are thus obtained and discussed in the framework of the literature results.

Unique crystallization behavior of isotactic polypropylene in the presence of l‐isoleucine and its inhibition and promotion mechanism of nucleation

ABSTRACTl‐Isoleucine (l‐Ile) was identified as an efficient anti‐nucleating agent for isotactic polypropylene (iPP). At 0.08 wt %, l‐Ile could significantly decrease the peak crystallization temperature (Tcp) of iPP by up to 8 °C at a cooling rate of 20 °C/min. Furthermore, l‐Ile exhibited both anti‐nucleation and pro‐nucleation abilities; i.e., a low content of l‐Ile inhibited iPP crystallization, whereas a high content promoted iPP crystallization. The unique crystallization behavior of iPP in the presence of l‐Ile was investigated by differential scanning calorimetry, polarized optical microscopy (POM), and rheological measurement. According to POM, a low content of l‐Ile completely dissolved in the iPP melt, whereas a high content of l‐Ile did not. Therefore, a mechanism by which l‐Ile inhibits and promotes the nucleation of iPP was proposed. Dissolving l‐Ile molecules in the iPP melt hindered the homogeneous nucleation of iPP as a “dilution effect”; however, as the content increases, l‐Ile could not be completely dissolved in molten iPP, and the residual crystals of l‐Ile thus provided heterogeneous nucleation sites for iPP and further promoted its crystallization. Experimental evidence from rheology and POM supported this mechanism. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45956.

The Crystallization Behavior of Isotactic Polypropylene Induced by a Novel Antinucleating Agent and Its Inhibition Mechanism of Nucleation

The addition of foreign substances is well-known to be able to improve the crystallization of isotactic polypropylene (iPP) following a nucleation-promoting mechanism. In the present work, however, we found that zinc salt of 1, 4, 5, 6, 7, 7-hexachlorobicyclo [2.2.1] hept-5-ene-2, 3-dicarboxylic acid (HCHD-Zn) unexpectedly decreased the crystallization temperature (Tcp) and crystallization rate of iPP, generating remarkable antinucleation effects. The results of differential scanning calorimetry (DSC) and polarized optical microscopy (POM) showed Tcp of iPP decreased more than 4 °C and nucleation densities of iPP decreased nearly 70% upon the addition of the “anti-nucleating agent” HCHD-Zn. Isothermal crystallization kinetics indicated HCHD-Zn lowered the crystallization kinetic constant k of iPP but had similar Avrami exponent with neat iPP, which revealed HCHD-Zn hindered the crystallization of iPP but did not change the type of nucleation and growth geometries of iPP. We proposed a mechanism of antinucleation of iPP upon the addition of HCHD-Zn from a structural perspective. Hydrogens on the tertiary carbons in the melt of iPP may react with the chlorine in the HCHD-Zn particles dispersed within. The removal of hydrochloride left a 3D network of iPP chains cross-linked by HCHD-Zn particles. Therefore, the restricted movement of polymer chains inhibited the nucleation of iPP.

Recrystallization of shish-kebab structures induced by self-seeding nucleation

This article reports melting and recrystallization of shish-kebab (cylindrite) structures in isotactic polypropylene containing high-content (15%wt) of ultra-high molecular weight polyethylene (UHMWPE). UHMWPE molecules can decrease the free volume of iPP molecules, which result in a great improvement of thermodynamic stability of shish. Therefore, some oriented structures can reappear due to inducement of two different kinds of shish (self-seeding nucleation of iPP), which are found with different morphology: One is threadlike, the other is nuclei-row. The way they absorb epitaxial growth of kebab is also different. It can be concluded that two different mechanisms of shish-kebab recrystallization exist. This work provides a new understanding of the formation of shish-kebab structure.

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

Thermoreversible DMDBS Phase Separation in iPP: The Effects of Flow on the Morphology

The crystallization rate of isotactic polypropylene (iPP) is often enhanced with sorbitol-based nucleating agents like 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol or DMDBS. In the concentration range from the eutectic point (∼0.1 wt %) to 1 wt %, DMDBS crystallizes before the polymer, building a network of nanofibrils. The surface of this network hosts a large number of tailored nucleation sites that promotes the epitaxial nucleation of iPP. The formation of DMDBS fibrils corresponds to a liquid−solid phase separation that occurs on cooling, at a temperature TcDMDBS. On heating, DMDBS fibrils dissolve in iPP at a temperature TmDMDBS > TcDMDBS. For DMDBS contents less than 1 wt %, both TcDMDBS and TmDMDBS depend on the composition. We show that in the temperature window TcDMDBS < T < TmDMDBS a percolated network of DMDBS fibrils is retained. This network can be broken with a shear flow that aligns the fibrils. At these high temperatures, iPP does not crystallize. Upon cooling, in the early stages of nucle...

Crystallization under shear in isotactic polypropylene containing nucleators

We report results from time-resolved small/wide angle X-ray scattering of well-characterized isotactic polypropylene (iPP), in its pure form as well as loaded with nucleating additives of different shapes (isotropic, elongated, platelet-like). Isothermal crystallization of these systems has been studied both under quiescent conditions and after application of a shear pulse. A quantitative analysis is presented that couples the applied shear field and the specific additive to the nucleation of iPP and the subsequent kinetics of crystallization. The effect of flow is only relevant if its contribution to the nucleation density is at least of the same order of magnitude as the nucleation density under quiescent conditions. Anisotropic additives show strong interactions with the flow field leading to extra nucleating sites.

Flow Induced Crystallization in Isotactic Polypropylene−1,3:2,4-Bis(3,4-dimethylbenzylidene)sorbitol Blends: Implications on Morphology of Shear and Phase Separation

Nucleation is the limiting stage in the kinetics of polymer crystallization. In many applications of polymer processing, nucleation is enhanced with the addition of nucleating agents. 1,3:2,4-Bis(3,4-dimethylbenzylidene)sorbitol or DMDBS is a nucleating agent tailored for isotactic polypropylene (iPP). The presence of DMDBS changes the phase behavior of the polymer. For high enough temperatures, the system iPP−DMDBS forms a homogeneous solution. However, in the range of concentration spanning from 0 to 1 wt % of DMDBS, the additive can phase separate/crystallize above the crystallization temperature of the polymer, forming a percolated network of fibrils. The surface of these fibrils hosts a large number of sites tailored for the nucleation of iPP. The aim of this paper is to investigate the combined effect of flow and DMDBS phase separation on the morphology of iPP. To this end, we studied the rheology of phase separated iPP−DMDBS systems and its morphology with time-resolved small-angle X-ray scattering (SAXS). The effect of flow is studied combining rheology, SAXS, and a short-term shear protocol. We found that, with phase separation, DMDBS forms fibrils whose radius (∼5 nm) does not depend on the DMDBS concentration. The growth of these fibrils leads to a percolated network with a mesh size depending on DMDBS concentration. Compared to the polymer, the relaxation time of the network is quite long. A shear flow, of 60 s-1 for 3 s, is sufficient to deform the network and to produce a long-lasting alignment of the fibrils. By design, lateral growth of iPP lamellae occurs orthogonally to the fibril axis. Therefore, with crystallization, the preorientation of DMDBS fibrils is transformed into the orientation of the lamellae. This peculiarity is used here to design thermomechanical histories for obtaining highly oriented iPP morphologies after shearing well above the melting point of the polymer (i.e., without any undercooling). In contrast, when shear flow is applied prior to DMDBS crystallization, SAXS showed that iPP crystallization occurs with isotropic morphologies.

Experimental study of flow-induced crystallization in the blends of isotactic polypropylene and poly(ethylene -co-octene)

The crystallization behavior of the blends of isotactic polypropylene ( iPP) and poly(ethylene -co-octene) (PEOc) under quiescent condition and shear flow were studied by differential scanning calorimetry (DSC) and rheology, respectively. The DSC curves of the iPP phase in the blends showed similar crystallization exothermic peaks to that of pure iPP itself, indicating that the addition of PEOc up to a percentage of 30 in weight almost had no influence on the crystallization behavior of iPP at quiescent condition. The rheological results of isothermal flow-induced crystallization (FIC) of iPP in the blends showed that the crystallization kinetics of iPP was enhanced with the increase of shear rate, similar to that of pure iPP, but the presence of PEOc enhanced the effect of shear on the crystallization kinetics of iPP significantly in the cases of shear rates larger than 0.2 s −1, which was due to that PEOc played an important role to promote the nucleation of iPP. The rheological results also implied that the characteristic relaxation times of blends were longer than that of pure iPP during the FIC process, indicating a different relaxation mechanism which might be related to the occurrence of interface relaxation and chain relaxation of the PEOc phase in the blends.

Isothermal crystallization behavior of isotactic polypropylene blended with small loading of polyhedral oligomeric silsesquioxane

The isothermal crystallization kinetics and morphology development of isotactic polypropylene (iPP) blended with small loading of nanostructure of polyhedral oligomeric silsesquioxane (POSS) were studied with differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD). The crystallization behaviors of iPP/POSS composites presented an unusual crystallization behavior during isothermal and nonisothermal crystallization conditions. The exothermic morphologies of isothermal and nonisothermal crystallization of iPP/POSS composites changed remarkably with increasing POSS. Moreover, the developments of spherulitic morphology for iPP/POSS composites showed that the major dispersed POSS molecules became nanocrystals first and then aggregated together forming thread- or network-like morphologies, respectively, depending on POSS content, which was observed. It implies that these major POSS nanocrystals' morphologies appeared as an effective nucleating agent and promoted the nucleation rate of iPP, whereas the minor dispersed POSS molecules that had slight miscibility between iPP retarded the nucleation and growth rates of iPP in the remaining bulk region. Therefore, the isothermal crystallization showed a single exothermic peak at pure iPP and POSS-1.0, whereas at POSS-2.0 and POSS-3.0, displayed the multi-exothermic peaks during isothermal crystallization. These faces indicated that POSS molecules were both influence on the transport of iPP chain in the melted state and on the free-energy of formation the critical nuclei of iPP assisted by the POSS structures were observed. Therefore, we postulated that the crystallization mechanisms of multi-exothermic peaks in isothermal crystallization may proceed to combine the “ nucleating agent inducing nucleation of iPP event assisted by the POSS domains” that the nucleation of iPP does occur preferentially on the surfaces of the POSS “threads” or “networks” structures, and “ nucleation and growth of iPP in the remaining bulk melted iPP region retarded by dispersed POSS molecules”. Therefore, effects of POSS content on the isothermal and nonisothermal crystallization behaviors of iPP/POSS composites due to the POSS molecules partially miscible with iPP, at very small loading of POSS molecules, promoted or retarded the rates of nucleation and growth of iPP depending on the POSS content and crystallization temperature were discussed.

Shear-induced crystallization of isotactic polypropylene within the oriented scaffold of noncrystalline ultrahigh molecular weight polyethylene

Shear-induced crystallization of isotactic polypropylene (iPP) within the oriented scaffolds of noncrystalline ultrahigh molecular weight polyethylene (UHMWPE) was investigated by means of in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). The study was carried out using iPP/UHMWPE blends under isothermal crystallization at 145°C (i.e., above the melting point of polyethylene) and step shear (shear rate=60s−1, duration=5s) conditions. The oriented and isotropic iPP crystalline phases were extracted from the 2D WAXD pattern, and their kinetics data were evaluated with the Avrami equation. The dominant component in the oriented iPP phase was a kebab structure, whose nanostructure dimensions were determined by a novel SAXS analysis scheme. The minor non-crystalline but oriented UHMWPE component played a key role in the nucleation of iPP, which could be explained in terms of mutual diffusion at the interface, resulting in a significant increase in the relaxation time of iPP chains. As a result, after shear, the interfacial iPP chains could also retain their orientation and formed oriented nuclei to initiate the kebab growth.

Isothermal crystallisation of iPP/Vectra blends by DSC and simultaneous SAXS and WAXS measurements employing synchrotron radiation

The isothermal crystallisation behaviour and morphology of blends of isotactic polypropylene, iPP, and a liquid crystal polymer, Vectra A950, has been studied using differential scanning calorimetry, optical microscopy and simultaneous WAXS and SAXS in real-time measurements using synchrotron radiation. It has been observed that Vectra domains act as sites for the nucleation of iPP, and the rate of crystallisation is enhanced with increasing Vectra content in the blend. The presence of the α crystalline form in pure iPP, and both α and β forms for iPP in iPP/Vectra blends has been found. The SAXS patterns for iPP/Vectra blends containing β iPP are characterized by two different long period values that were related to the α and β lamellae. The secondary crystallisation mechanism has been investigated by SAXS/WAXS experiments. It is shown that, in contrast to primary crystallisation, secondary crystallisation of iPP is not affected by the presence of the thermotropic liquid crystalline polymer. As already known from pure iPP, the main process of secondary crystallisation is the growth of new lamellar stacks within remaining amorphous regions in the iPP spherulites.

A comparison study on the homogeneity and heterogeneity fiber induced crystallization of isotactic polypropylene

The crystallization behavior of iPP in composites with PET, Nylon-6 and its own fibers under various conditions was studied using an optical microscope equipped with a hot stage. The results show that the nucleation capacity of PET and Nylon-6 fibers towards the iPP matrix is mainly controlled by the shear flow of the iPP matrix during sample preparation. When the composites were prepared at a temperature where the iPP was kept in its supercooled state, the nucleation of iPP on the PET and Nylon-6 fiber surfaces was enhanced due to the shearing of the iPP melts caused by introduction of the fibers. The nucleation was markedly reduced by keeping the composites at the fiber introduction temperature for a short time to relax the shear flow of the iPP matrix. The nucleation of iPP on its own fiber, however, is mainly related to the nature of the iPP fiber itself. No detectable morphological change of iPP on its own fiber can be identified under all thermal conditions used in this study.

Interfacial crystallization of isotactic polypropylene molded against the copper surface with various surface roughnesses prepared by an electrochemical process

The influence of surface topography of copper (Cu) sheet on the heterogeneous nucleation of isotactic polypropylene (iPP) at the iPP/Cu interface had been investigated mainly using polarized optical microscope with hot stage. Various textures of Cu surfaces prepared by sandpaper polishing and electrochemical anodizing had been made and they were utilized to induce interfacial nucleation of iPP upon supercooling. This process enables us to change the topological feature of the copper surface without changing their chemical compositions. The pretreated surfaces were quantitatively characterized by a surface texture instrument in terms of RMS roughness (Ra). Copper surface with higher surface roughness induced more nuclei of iPP and led to a thicker transcrystalline layer in the interfacial region upon supercooling over the temperature range 128°C < Tc < 134°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/Δσbulk) for the polished surface (Ra = 0.09 μm) is 2.89, implying the transcrystallization growth is unfavorable from a thermodynamic point of view. The ratio of ΔσTCL/Δσbulk becomes smaller as the current density for anodizing increases, indicating the transcrystallization growth is getting favorable. Moreover, induction times and nucleation rates were also measured to characterize quantitatively the nucleating ability of various Cu surfaces.

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