Crystallization Behavior Of Isotactic Polypropylene Research Articles

Investigation of crystal habit of N,N′-dicyclohexylterephthalamide and its influence on the crystallization behavior of isotactic polypropylene

Investigation of crystal habit of N,N′-dicyclohexylterephthalamide and its influence on the crystallization behavior of isotactic polypropylene

Effects of process conditions and nano-fillers on the cell structure and mechanical properties of co-injection molded polypropylene-polyethylene composites

Compared with traditional injection foaming, co-injection foaming serves as a method to prepare products with high-quality surfaces and enhanced mechanical properties, demonstrating excellent technical advantages and economic benefits. The effects of high-density polyethylene (HDPE) content, foaming agent content, melting temperature of core material, and nano-organic montmorillonite (OMMT) content on the cell structure, mechanical properties, and crystallization behavior of isotactic polypropylene (iPP)/HDPE composite foam prepared by co-injection foaming were systematically studied. At a foaming agent content of 2 wt.%, HDPE content of 30 wt.%, OMMT content of 3 wt.%, and the melting temperature of core material of 190 °C, the composite foam exhibited a cell size measuring 85 μm, a cell density of 2.5 × 105 cells/cm3, a tensile strength of 25.8 MPa, and an impact strength of 12.5 kJ/m2. The addition of HDPE reduced the crystallinity of iPP matrix, while enhancing the solubility of carbon dioxide within the matrix. The non-miscible interface between HDPE and iPP helped reduce the energy barrier for cell nucleation. Furthermore, the introduction of OMMT as a nucleating agent in iPP/HDPE composite material dramatically decreased the energy barrier for cell nucleation. These synergistic effects result in foam with the smaller cell size and the higher cell density, exhibiting excellent comprehensive mechanical properties. This work provides a feasible method for preparing composite foam with excellent comprehensive performance.

Model of Crystallization Behavior of Isotactic Polypropylene: The Role of Defects

AbstractThe crystallization behavior of propene–ethylene (iPPC2), propene–butene (iPPC4), propene–pentene (iPPC5), and propene–hexene (iPPC6) isotactic copolymers, prepared with single center metallocene catalysis, is described. Thanks to the perfectly random distribution of comonomers along the macromolecules, these copolymers can be considered as model for the crystallization behavior of isotactic polypropylene (iPP) that is mainly defined by the average length of the regular propene sequences. A model of the crystallization behavior of iPP is described based on the definition of a double effect of defects (stereodefects and comonomers), the interruption effect of defects excluded from the crystals that shortens the length of the crystallizable propene sequences inducing crystallization of the γ form, and the effect of inclusion of defects in the crystals of α form, γ form or δ form that induces the crystallization of the form whose lattice is less disturbed by the defect.

Reversible nucleation behavior of N, N′-dicyclohexylterephthalamide to polypropylene with the aid of polyamide 12

Through the hydrogen bonding between polyamide 12 (PA12) and N, N′-dicyclohexylterephthalamide (DCHT) nucleator, the intentionally controlled self-assembly of DCHT and the crystallization behavior of isotactic polypropylene (iPP) matrix are investigated. The introduction of PA12 could severely postpone the condensation of DCHT vapor in iPP matrix due to the reduced vapor concentration. As a result, the nucleating efficiency of DCHT at lower concentration for example iPP/PA12/0.05DCHT even failed at a relatively higher final heating temperature Tf. Such a failure of nucleating efficiency could be recoverable after a step-cycle treatment at lower Tf, where the sublimation of DCHT can hardly occur. The changes from a cluster-like structure to a network structure driven by hydrogen bonding between DCHT and PA12 could be a reason of nucleation failure and recovery. This means that the nucleating efficiency of DCHT and crystallization of iPP could be tuned by controlling the absorption and release of DCHT vapor at the surface of PA12 particles. For iPP/PA12/0.5DCHT with a higher concentration of DCHT, the absorption of PA12 could only reduce the DCHT vapor concentration in the iPP melt and thus, the dendrite structure of DCHT shows more and thinner branches, which results in better nucleating efficiency of DCHT than that in iPP/0.5DCHT.

Modification of the Crystallization Behavior of Isotactic Polypropylene by Grafting Styrene Segments

As a typically semicrystalline polymer, the physical properties of isotactic polypropylene (iPP) are directly related to its crystallization. In this work, styrene (St) is grafted onto iPP chains by reactive processing to modify its crystallization behavior. The St grafting segments disrupt the stereoregularity of the iPP chain, leading to the appearance of a γ-crystal. More interestingly, the presence of St grafting segments can remarkably increase the crystallization temperature and crystallization rate of iPP and reduce the size of crystals. On the one hand, the rigid structure of benzene rings in St grafting segments can squeeze easily iPP chains to form the relatively ordered structure, which can be further stabilized by the mutual π–π conjugation between the benzene rings and the helix chains of iPP. On the other hand, the St grafting segments locating in the amorphous phase can hinder the growth of crystals, which leads to finer and more crystals.

Crystallization behavior of isotactic polypropylene containing a fibrous nucleating agent in a flow field

Crystallization behavior of isotactic polypropylene containing a fibrous nucleating agent in a flow field

Crystallization of Propene–Pentene Isotactic Copolymers as an Indicator of the General View of the Crystallization Behavior of Isotactic Polypropylene

The crystallization from the melt in isothermal conditions of metallocene random propene–pentene isotactic copolymers (iPPC5) has been studied. All samples with pentene concentration between 0.5 and 10 mol % crystallize at any crystallization temperature in mixtures of α and γ forms of isotactic polypropylene (iPP) and the amount of γ form increases with increasing crystallization temperature up to a maximum (fγ(max)), which depends on pentene concentration. Pentene defects produce a shortening of the regular propene sequences that in turn induces crystallization of the γ form. At concentrations higher than 6–7 mol %, pentene units are incorporated to a high extent in the crystals of α and trigonal forms, which are stabilized over the γ form, and fγ(max) decreases. The maximum fraction of γ form is, therefore, related to the average length of regular propene sequences and the degree of incorporation of defects in the crystals of α and γ forms. The values of fγ(max) that develop in iPPC5 copolymers have been compared with those that develop in copolymers of iPP with ethylene (iPPC2), butene (iPPC4), and hexene (iPPC6) and in stereoirregular iPPs reported in the literature. Stereoirregular iPPs and iPPC2 copolymers give the same relationship between fγ(max) and the average length of regular propene sequences (LiPP), whereas iPPC4, iPPC5, and iPPC6 copolymers show different behaviors. In particular, iPPC5 copolymers exhibit a behavior intermediate between those of iPPC4 and iPPC6 copolymers. The relationship between fγ(max) and LiPP of iPPC5 copolymers fits perfectly between the relationships found for iPPC4 and iPPC6 copolymers, in agreement with the different types and sizes of comonomers and the different efficiencies of their interruption and inclusion effects. These data give evidence of the general view of the crystallization behavior of iPP, based on the definition of a double role exerted by defects, the interruption effect that shortens the regular propene sequences and favors crystallization of γ form, and the effect of incorporation of defects into the crystalline unit cells of α and γ forms, which favors crystallization of the form that better accommodates the defect into crystals. The relative efficiency of these two effects depends on the type and size of the defect. The different relationships between fγ(max) and LiPP are a result of the equilibrium between interruption and inclusion effects achieved by each defect and confirm that the crystallization of γ form is a perfect indicator of the length of the regular propene sequences and may provide very detailed information on the molecular structure of iPP.

Investigation on the Effects of MXene and β-Nucleating Agent on the Crystallization Behavior of Isotactic Polypropylene.

The effects of MXene on the crystallization behavior of β-nucleated isotactic polypropylene (iPP) were comparatively studied. The commonly used MXene Ti3C2Tx was prepared by selective etching and its structure and morphology were studied in detail. Then MXene and a rare earth β-nucleating agent (NA) WBG-II were nucleated with iPP to prepare samples with different polymorphic compositions. The crystallization, melting behavior, and morphologies of neat iPP, iPP/MXene, iPP/WBG-II, and iPP/MXene/WBG-II were comparatively studied. The crystallization behavior analysis reveals that a competitive relationship exists between MXene and WBG-II when they were compounded as α and β nucleating agents. In the system, the β-nucleation efficiency (NE) of WBG-II is higher than α-NE of MXene. The β-phase has relatively low thermal stability and would transform to α-phase when cooled below a critical temperature.

Nucleation and crystallization behavior of isotactic polypropylene induced by zinc tetrahydrophthalate

Nucleation and crystallization behavior of isotactic polypropylene induced by zinc tetrahydrophthalate

An effective nucleating agent for isotactic polypropylene (iPP): Zinc bis- (nadic anhydride) double-decker silsesquioxanes

The zinc bicyclo [2.2.1]-heptane-2, 3-dicarboxylate-double-decker silsesquioxanes (Zn2(DDSQ-ND)) was firstly synthesized and the crystallization behavior of isotactic polypropylene (iPP) filled with different amount of Zn2(DDSQ-ND) (0–1 wt %) was investigated in detail. With the addition of Zn2(DDSQ-ND), the crystallization temperature enhanced significantly, indicating that it acted novel and effective nucleating agent in the iPP system. The morphology of spherulites, nucleation density, and the crystal structure of the samples were characterized using POM and WAXD. Besides, ultra-fast scanning calorimetry (FSC) was employed to study the effects of Zn2(DDSQ-ND) on the non-isothermal and isothermal crystallization behaviors of the sample iPP/Zn2(DDSQ-ND) under the high cooling rates and wide crystallization temperature ranges, as well as the cold crystallization and recrystallization behavior during the heating process. The data suggested that Zn2(DDSQ-ND) accelerated the formation of α-phase iPP and acted as a novel α-nucleating agent in the iPP system.

Concentration Effect of a Bis-amide Nucleating Agent on the Shear-Induced Crystallization Behavior of Isotactic Polypropylene

The concentration of nucleating agent (NA) and flow fields are both important factors affecting the crystalline behavior and final properties of semicrystalline polymers. In this work, the concentration effect of N,N′-dicyclohexylterephthalamide (DCHT), an effective β-NA for isotactic polypropylene (iPP), on the shear-induced crystallization of iPP was investigated by comparing the crystallization temperatures (Tcs) of iPP/DCHT under unsheared, shear-unrelaxed, and relaxed conditions via rheological measurement. For iPP with an extremely low concentration of 0.01 wt %, Tc greatly improved after shearing and distinctly reduced after relaxation of 30 min, which was similar to that of pure iPP. For iPP with moderate concentrations of 0.05–0.3 wt %, the Tc could still increase after shearing but remained nearly constant after relaxation. In this concentration range, the increment in Tc induced by shear gradually decreased with increasing concentration. Interestingly, for iPP with high concentrations of 0.5 and 1 wt %, the Tc hardly changed regardless of applying shear or relaxation. The results of Fourier transform infrared suggested that DCHT existed in different forms including “free” molecules only and the coexistence of “free” DCHT and the associated assembly in various iPP/DCHT samples, and polarized optical microscopy observation showed that the DCHT assembly exhibited diverse morphologies at different concentrations. Combining the roles of “free” DCHT molecules and the associated assembly with diverse morphology in the shear-induced crystallization of iPP, a possible mechanism explaining the effect of DCHT concentration was proposed and experimentally verified.

Non-isothermal crystallization behavior of isotactic polypropylene/copper nanocomposites

The Non-isothermal crystallization behavior of isotactic polypropylene (iPP)/copper nanocomposites with four different mass percentages (0.5, 1.0, 2.0 and 4.0 mass%) of copper nanoparticles (nCu) were study by means of differential scanning calorimeter (DSC) at 2.5, 5, 10 and 20 °C min−1. The nanostructure of PP/nCu nanocomposites was also studied by WAXD, SEM and optical microscopy, while their hierarchical crystalline morphologies (e.g., spherulites, lamellas and unit cell) were studied during the crystallization progress through polarized optical microscopy and in situ X-ray scattering at small and wide angle, observing that the spherulite size was not influenced by the nCu’s, although the lamellas size and X-ray diffraction intensity increased with the incorporation of nanoparticles. The PP/nCu nanocomposites presented a mixture morphology with well-dispersed oxidized nanoparticles and some agglomerates, which were larger at higher concentration of particles. The crystallization results obtained by DSC indicated a displacement in the peak crystallization temperature at higher values. These results in combination with the kinetic of crystallization analyzed by the Jeziorny method clearly indicated an acceleration in the crystallization process. Additionally, the crystallization activation energy decreases for all nanoparticle’s concentration, indicating a possible nucleating effect. However, the nucleation activity analyzed by the Dobreva and Gutzow’s method showed that nCu could not be considered such as conventional heterogeneous nucleating agent of iPP. Conversely, the incorporation of nCu in the iPP matrix increased significantly the thermal conductivity of nanocomposites helping to the thermal dissipation from the melted macromolecules to the nCu’s, accelerating the non-isothermal crystallization process.

Effect of different nucleating agent on crystallization kinetics and morphology of polypropylene

AbstractThe effects of phosphate nucleating agent (NA), carboxylate nucleating agent (MD), rosin type nucleating agent (WA) and sorbitol nucleating agent (NX) on crystallization behavior of isotactic polypropylene were investigated by fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The results showed that different structure nucleating agents significantly affected the crystallization kinetics, rate and temperature of polypropylene. Among them, half crystallization time of NX nucleating agent was the shortest, which was 53.4 seconds, and the crystallization temperature was the highest, reaching 129.8°C.

Influence of β-nucleating agent dispersion on the crystallization behavior of isotactic polypropylene

In this article we report an in-situ catalytic strategy for homogeneous dispersion of β-nucleating agent N,N′-dicyclohexyl-terephthalamide (DCTPA) into isotactic polypropylene (iPP) as well as morphology control of the DCTPA/iPP composites. This strategy is based on catalyst to polymer morphology replication approach, in which by embedding Ziegler-Natta catalyst species on DCTPA surface, the fresh iPP chains first grew on the DCTPA tiny crystals and then wrapped about them; thus the interaction between DCTPA crystals and iPP matrix was enhanced, and the thermodynamic barrier of dispersing DCTPA in the inert iPP was avoided. The dispersion morphology and the self-assembly/recrystallization morphology of DCTPA in iPP were investigated; the relationship between DCTPA/iPP composites micro-structures and their crystallization behaviors was compared with that of the melting-blended DCTPA/iPP systems. Due to the improvement of DCTPA dispersion and the enhancement of interaction between DCTPA and iPP melts, the synthesized samples presented characteristics of fast crystallization initiation and slow spherulites growth rate, which is different from the general melting-blended composites.

Effect of Self-Assembly of Oxalamide Based Organic Compounds on Melt Behavior, Nucleation, and Crystallization of Isotactic Polypropylene.

We report on the effect of an aliphatic oxalamide based nucleating agent (OXA3,6) on the melt and crystallization behavior of isotactic polypropylene (iPP) under defined shear conditions. Through polarized optical microscopy, we demonstrate that OXA3,6 self-assembles from the iPP melt into rhombic crystals whereas their size and distribution proved highly dependent on the employed cooling rates. The presence of 0.5 wt % of OXA3,6 in iPP results in a significant suppression in iPP melt viscosity, which could not be explained via molecular modeling. A possible cause for the drop in viscosity in the presence of OXA3,6 is attributed to the interaction (absorption) of high molecular weight iPP chains with the nucleating agent, thereby suppressing their contribution to the viscoelastic response of the melt. This proposed mechanism for the suppression in melt viscosity appears similar to that encountered by the homogeneous distribution of nanoparticles such as CNTs, graphene, and silica. Shear experiments, performed using a slit flow device combined with small-angle X-ray diffraction measurements, indicate that crystallization is significantly enhanced in the presence of OXA3,6 at relatively low shear rates despite its lowered sensitivity to shear. This enhancement in crystallization is attributed to the shear alignment of the rhombic OXA3,6 crystals that provide surface for iPP kebab growth upon cooling. Overall, the suppression in melt viscosity in combination with enhanced nucleation efficiency at low as well as high shear rates makes this self-assembling oxalamide based nucleating agent a promising candidate for fast processing.

Shear‐Induced Skin‐Core Structure of Molten Isotactic Polypropylene and the Formation of β‐Crystal

AbstractThe study of crystallization behavior and crystalline morphology of polymer melt under shear flow is of great interest due to the strong effect of flow field on the final properties of polymer products in the practical processing. In this respect, the shearing hot stage provides a unique tool which monitors sensitively the changes in crystalline structure induced by precise experimental conditions. Herein, the impacts of both melting temperature and shear rate on the crystallization behavior of isotactic polypropylene (iPP) melt are investigated. Under static conditions, there are only random spherulite structures. Once shear is involved, the cylindrite‐layers appear near both surfaces of the sample, which is consistent with the skin‐core structure in the injection molded parts. Meanwhile, the β‐crystals can be developed and are related to the molecular orientation, depending on the applied melting temperatures and shear rates. More interestingly, the crystallinity of β‐crystal in the pure iPP can reach 15%. The above results indicate that the melting temperature and shear rate are important factors in determining the β‐form crystal development of iPP matrix.

Effect of Flowing Preformed Spherulites on Shear-Induced Melt Crystallization Behaviors of Isotactic Polypropylene

The effects of flowing preformed spherulites on shear-induced melt crystallization behaviors of isotactic polypropylene (iPP) had been investigated by using a Linkam shearing cell coupled with an optical microscope, atomic force microscope, and rheometer. By quenching iPP melt to 130 °C for a short time, preformed spherulites were intentionally obtained. While iPP films were heated up to temperatures below the nominal melting point of iPP (164 °C), various shear conditions were applied, and then the crystalline morphologies during crystallization at the isothermal crystallization temperature of 135 °C were traced by alternately using phase contrast optical microscopy and polarized optical microscopy modes. Two distinct highly orientated thread- and cylinder-like crystalline morphologies were induced by shearing with preformed spherulites present, which were dependent on the applied shear rate and temperature. The distinct crystalline morphologies were attributed to relative movements between shear flowing...

Effect of benzoic acid surface modified alumina nanoparticles on the mechanical properties and crystallization behavior of isotactic polypropylene nanocomposites.

The effect of benzoic acid (BA) surface modified alumina (Al2O3) nanoparticles (NPs) on the mechanical properties and crystallization behavior of isotactic polypropylene (iPP) nanocomposites was studied. Characterization of the modified Al2O3 NPs (BA-Al2O3) by FTIR and XRD analyses confirmed that benzoic acid molecules chemisorb on the surface of the NPs, forming benzene groups-rich microstructures. A considerable increase in the tensile strength, flexural modulus, and toughness was observed for the nanocomposites with only 0.2 wt% BA-Al2O3. Enhanced interfacial adhesion with the matrix was achieved, which enabled effective reinforcement of the nanocomposites. The higher crystallization temperature along with shorter crystallization halftime indicated the higher nucleation activity of BA-Al2O3. Furthermore, the interchain conformational ordering of iPP was significantly accelerated in the presence of the BA-Al2O3 NPs. The CH–π interaction between the polymer and BA-Al2O3 NPs was considered to facilitate the attachment of the iPP chains and stimulate conformational ordering, crystallization, as well as mechanical properties of nanocomposites.

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.

Effects of cyclic carboxylate nucleating agents on nucleus density and crystallization behavior of isotactic polypropylene

Isothermal crystallization behavior of isotactic polypropylene (iPP) nucleated with three cyclic carboxylate nucleating agents, namely, sodium bicyclic[2,2,1] heptane dicarboxylate (HPN-68), sodium benzoate (Be-Na), and sodium salt of hexahydrophthalic acid (HHPA-Na), was investigated by using differential scanning calorimetry. The classical Avarmi method was used to evaluate the isothermal crystallization behavior of iPP. The nucleus density was obtained by the calculation model of Lamberti based on the Avrami method of crystallization kinetics from calorimetric crystallization curves recorded under isothermal conditions. This method was able to measure nucleus density and growth rate of nucleated iPP, for which the nucleation phenomenon was substantially heterogeneous. The addition of nucleating agents shortened the crystallization halftime (t 1/2) and increased the crystallization rate of iPP under isothermal crystallization, eventually increasing the nucleus density of iPP nucleated with nucleating agents for finishing crystallization under higher crystallization temperature, which was larger than that of virgin iPP. The results of polarized optical microscopy also showed that the nucleus density and crystallization rate were greatly increased with addition of nucleating agents. In these three nucleating agents, HPN-68 had the best nucleation effect, and iPP nucleated with HPN-68 had shorter crystallization halftime and higher nucleus density than iPP nucleated with Be-Na and HHPA-Na.