Effect of nucleating duality on the formation of γ‐phase in a β‐nucleated isotactic polypropylene copolymer

BACKGROUND: It is a challenge for polymer processing to promote the formation of γ‐phase under atmospheric conditions in isotactic polypropylene (iPP) copolymer containing chain errors. Incorporation of an α‐nucleator in iPP copolymer seems reasonable since it can enhance non‐isothermal crystallization. Up to now, however, the issue regarding a β‐nucleated iPP copolymer still remains unclear, which is the subject of this study. RESULTS: The results indicate that the γ‐phase indeed occurs in a β‐nucleated random iPP copolymer with ethylene co‐unit (PPR) sample and becomes predominant at slow cooling rates (e.g. 1 °C min^?1) where the formation of the β‐form is suppressed to a large extent. With detailed morphological observations the formation of γ‐phase in the β‐nucleated PPR sample at slow cooling rate is unambiguously attributed to the nucleating duality of the β‐nucleator towards α‐ and β‐polymorphs. The α‐crystals, induced by the β‐nucleator, serve as seeds for the predominant growth of the γ‐phase. Moreover, the presence of the β‐nucleator, acting as heterogeneous nuclei, promotes the formation of γ‐phase in the nucleated PPR sample, at least to some extent. CONCLUSION: The findings in this study extend our insights into the formation of γ‐phase in β‐nucleated iPP copolymer and, most importantly, provide an alternative route to obtain iPP rich in γ‐phase. Copyright © 2008 Society of Chemical Industry     

Negative effect of stretching on the development of β‐phase in β‐nucleated isotactic polypropylene

On the premise that shear in the slit die of an extruder was minimized as far as possible, β‐nucleated isotactic polypropylene (iPP) was extruded. Simultaneously, once the extrudate (in the melt state) left the die exit, it was stretched at various stretching rates (SRs). For iPP with a low content of β‐nucleating agent (β‐NA), the crystallinity of β‐phase (X_β) initially increases with increasing SR, and then decreases slightly with further increase in SR. However, for iPP containing a higher content of β‐NA, with increasing SR, X_β decreases monotonically, indicating a negative effect of SR on β‐phase formation. Small‐angle X‐ray scattering and polarized optical microscopy experiments reveal that, when SR is less than 30 cm min^?1, the increasing amount of row nuclei induced by increasing SR is mainly responsible for the increase of X_β. In contrast, when SR exceeds 30 cm min^?1, the overgrowth of shish structures unexpectedly restrains the development of β‐phase, and spatial confinement is considered as a better explanation for the suppression of β‐phase. Copyright © 2011 Society of Chemical Industry     

Isothermal crystallization kinetics and subsequent melting behavior of β‐nucleated isotactic polypropylene/graphene oxide composites with different ordered structure

The effects of ordered structure on isothermal crystallization kinetics and subsequent melting behavior of β‐nucleated isotactic polypropylene/graphene oxide (iPP/GO) composites were studied using differential scanning calorimetry. The ordered structure status was controlled by tuning the fusion temperature (T_f). The results showed that depending on the variation of crystallization rate, the whole T_f range could be divided into three regions: Region I (T_f > 179 °C), Region II (170 °C ≤ T_f ≤ 179 °C) and Region III (T_f < 170 °C). As T_f decreased from Region I to Region III, the crystallization rate would increase substantially at two transition points, due to the variation of the ordered structure status. Calculation of Avrami exponent n indicated that the ordered structure induced the formation of two‐dimensional growing crystallites rather than three‐dimensional growing crystallites. Moreover, in the case of isothermal crystallization, the ordered structure effect (OSE) can also greatly increase the relative content of β‐phase (β_c). In Region II, OSE took place, resulting in evident increase of β_c, achieving 92.4% at maximum. The variation of the isothermal crystallization temperature (T_iso) had little influence on the T_f range (Region II) of the OSE. The higher T_f in Region II was more favorable for the formation of higher β_c. The ordered structure was favorable for the improvement of the nucleating efficiency of β‐nucleating agent (β‐NE), and was more effective for the improvement of lower β‐NE. © 2018 Society of Chemical Industry     

Variation of non‐isothermal crystallization behavior of isotactic polypropylene with varying β‐nucleating agent content

The non‐isothermal crystallization behavior, the crystallization kinetics, the crystallization activation energy and the morphology of isotactic polypropylene (iPP) with varying content of β‐nucleating agent were investigated using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The DSC results showed that the Avrami equation modified by Jeziorny and a method developed by Mo and co‐workers could be successfully used to describe the non‐isothermal crystallization process of the nucleated iPPs. The values of n showed that the non‐isothermal crystallization of α‐ and β‐nucleated iPPs corresponded to a tridimensional growth with homogeneous and heterogeneous nucleation, respectively. The values of crystallization rate constant showed that the rate of crystallization decreased for iPPs with the addition of β‐nucleating agent. The crystallization activation energy increased with a small amount (less than 0.1 wt%) of β‐nucleating agent and decreased with higher concentration (more than 0.1 wt%). The changes of crystallization rate, crystallization time and crystallization activation energy of iPPs with varying contents of β‐nucleating agent were mainly determined by the ratio of the content of α‐ and β‐phase in iPP (α‐PP and β‐PP) from the DSC investigation, and the large size and many intercrossing lamellae between boundaries of β‐spherulites for iPPs with small amounts of β‐nucleating agent and the small size and few intercrossing bands among the boundaries of β‐spherulites for iPPs with large amounts of β‐nucleating agent from the SEM examination. Copyright © 2010 Society of Chemical Industry     

Structure and properties of a β‐nucleated polypropylene impact copolymer

The effect of a β‐nucleating agent (β‐NA) on the properties and structure of a commercial impact polypropylene copolymer (IPC) was investigated. The effect of selected β‐NAs on the impact resistance, stress and strain behaviour of the IPC is reported. In addition, the IPC was fractionated according to crystallinity by preparative temperature rising elution fractionation. Fractions with varying chemical composition and crystallinity were treated with a two‐component β‐NA to investigate the effect of the β‐NA on the various fractions. The results indicate that the efficacy of the β‐NA is dependent on the chemical composition of the polymer that crystallises, more specifically on the sequence length of crystallisable propylene units. The effect of the addition of β‐NAs on the overall morphology of the IPC was also investigated, and in particular the size and distribution of the rubbery particles in these complex reactor blends were probed. © 2014 Society of Chemical Industry     

Effect of syndiotactic polystyrene on the crystallization behavior of isotactic polypropylene/syndiotactic polystyrene blends with and without β‐nucleating agent

Blends of isotactic polypropylene (PP) and syndiotactic polystyrene (sPS) with and without β‐nucleating agent were prepared using a twin‐screw extruder at 290 °C. Blends of PP/sPS with β‐nucleating agent mainly show β crystalline form, irrespective of high (20 °C min^?1) or low (2 °C min^?1) previous cooling rates. This suggests that the cooling rates have little effect on the polymorphic composition of PP in PP/sPS blends. The effect of sPS on the crystallization of PP is compared with that of polyamide 6 (PA6). The increase in crystallization temperature of PP is smaller in the presence of sPS than in the presence of PA6; the fold surface free energy of PP/sPS is larger than that of PP/PA6 blends. These results reveal that compared with PA6, sPS has much weaker α‐nucleation effect on the crystallization of PP. The weak α‐nucleation effect of sPS is attributed to the high lattice mismatch between PP and sPS crystals.     

α‐ and β‐crystalline forms of isotactic polypropylene investigated by nanoindentation

Under special crystallization conditions from the melt, both α‐ and β‐forms of isotactic polypropylene were produced simultaneously. The α‐ and β‐spherulites of polypropylene were differentiated under optical microscope, allowing the nanoindentation of individual spherulites of each crystallographic form. Elastic modulus and hardness of β‐spherulites were found to be 10 and 15% respectively lower than in α‐spherulites. The higher stiffness of α may be related to the particular interlocked structure with cross‐hatched lamellae, and to a lower molecular mobility in the crystallites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 195–200, 1999     

β‐Nucleation of pimelic acid supported on metal oxides in isotactic polypropylene

To investigate the nucleation of metal pimelate for isotactic polypropylene (iPP) crystallization, iPP filled with a series of metal oxides with and without metal pimelate on their surface was prepared. There was a chemical reaction between pimelic acid (PA) and metal oxides MgO, CaO, BaO or ZnO, but not TiO₂. The corresponding metal pimelate formed by the chemical reaction between PA and MgO, CaO, BaO or ZnO had a different influence on the crystallization behavior and melting characteristics of iPP. Addition of metal oxides increased the crystallization temperature of iPP and mainly formed α‐phase due to the heterogeneous α‐nucleation of metal oxides. The α‐nucleation of CaO could be easily changed into β‐nucleation using CaO‐supported PA, and 90.1% β‐phase was obtained. The β‐nucleation of BaO could be markedly enhanced by barium pimelate formed using supported PA. However, no β‐phase was observed for iPP filled with MgO‐ or ZnO‐supported PA. The various metal oxides with supported PA had a different influence on the crystallization behavior and melting characteristics of iPP due to the different structure of metal pimelate formed by chemical reaction between PA and the metal oxides. Copyright © 2012 Society of Chemical Industry     

Nonisothermal crystallization and melting behavior of β‐nucleated isotactic polypropylene and polyamide 66 blends

A highly novel nano‐CaCO₃ supported β‐nucleating agent was employed to prepare β‐nucleated isotactic polypropylene (iPP) blend with polyamide (PA) 66, β‐nucleated iPP/PA66 blend, as well as its compatibilized version with maleic anhydride grafted PP (PP‐g‐MA), maleic anhydride grafted polyethylene‐octene (POE‐g‐MA), and polyethylene‐vinyl acetate (EVA‐g‐MA), respectively. Nonisothermal crystallization behavior and melting characteristics of β‐nucleated iPP and its blends were investigated by differential scanning calorimeter and wide angle X‐ray diffraction. Experimental results indicated that the crystallization temperature (T) of PP shifts to high temperature in the non‐nucleated PP/PA66 blends because of the α‐nucleating effect of PA66. T of PP and the β‐crystal content (K_β) in β‐nucleated iPP/PA66 blends not only depended on the PA66 content, but also on the compatibilizer type. Addition of PP‐g‐MA and POE‐g‐MA into β‐nucleated iPP/PA66 blends increased the β‐crystal content; however, EVA‐g‐MA is not benefit for the formation of β‐crystal in the compatibilized β‐nucleated iPP/PA66 blend. It can be relative to the different interfacial interactions between PP and compatibilizers. The nonisothermal crystallization kinetics of PP in the blends was evaluated by Mo's method. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Highly active thermally stable β‐nucleating agents for isotactic polypropylene

Calcium salts of suberic (Ca‐Sub) and pimelic (Ca‐Pim) acids were synthesized and implemented as in different grades of isotactic polypropylene (iPP). Propylene homopolymer, as well as random and block copolymers containing these additives, crystallized iPP into pure or nearly pure β modification in the isothermal and nonisothermal crystallization experiments. Recently, Ca‐Sub proved to be the most effective β‐nucleating agent of iPP. The Ca‐Sub nucleating agent widens the upper crystallization temperature range of pure β‐iPP formation up to 140°C. In this study the effect of the these additives on the crystallization, melting characteristics, and structure of the PP were studied. The degree of crystallinity of β‐iPP was markedly higher than that of α‐iPP. A widening in the melting peak of the samples crystallized in a high temperature range was first observed and discussed in regard to literature results of the same phenomenon for α‐iPP. The morphology of the β‐iPP samples was revealed by scanning electron microscopy. Independent of the type of polymer or nucleating agent, hedritic structures were found in the early stages of growth of the β‐spherulites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2357–2368, 1999     

The effects of crystallization condition on the microstructure and thermal stability of istactic polypropylene nucleated by β‐form nucleating agent

This article deals with the crystallization behaviors of original (prepared in a torque rheometer), DSC crystallization and mold crystallization (quenching and slow nonisothermal crystallization) of isotactic polypropylene (iPP) mixed with β‐form nucleating agent. The microstructure and thermal stability of these samples were investigated. The wide angle X‐ray diffraction (WAXD) results indicate that fast cooling is favorable for β‐form iPP formation. With slower cooling rate and higher concentration of nucleating agent, the lamellar thickness and stability of crystal0s were enhanced. Polarized optical microscopy (POM) and scanning electron microscopy (SEM) both showed that rapid crystallized samples gave rise to tiny spherulites, whereas under slow crystallization condition, nucleated samples could be fully developed in the form of dendritic or transcrystalline structures, depending on the nucleating agent concentration. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A novel β‐nucleating agent for isotactic polypropylene

The nucleating ability of p‐cyclohexylamide carboxybenzene (β‐NA) towards isotactic polypropylene (iPP) was investigated by differential scanning calorimetry, X‐ray diffraction, polarized optical microscopy and scanning electron microscopy. β‐NA is identified to have dual nucleating ability for α‐iPP and β‐iPP under appropriate kinetic conditions. The formation of β‐iPP is dependent on the content of β‐NA. The content of β‐phase can reach as high as 96.96% with the addition of only 0.05 wt% β‐NA. Under non‐isothermal crystallization the content of β‐iPP increases with increasing cooling rate. The maximum β‐crystal content is obtained at a cooling rate of 40 °C min^–1. The supermolecular structure of the β‐iPP is identified as a leaf‐like transcrystalline structure with an ordered lamellae arrangement perpendicular to the special surface of β‐NA. Under isothermal crystallization β‐crystals can be formed in the temperature range 80–140 °C. The content of β‐crystals reaches its maximum value at a crystallization temperature of 130 °C. © 2012 Society of Chemical Industry     

The effect of specific β‐nucleation on morphology and mechanical behavior of isotactic polypropylene

The commercial grade of isotactic polypropylene was modified by a specific β‐nucleating agent in a broad concentration range. The supermolecular structure of the specimens prepared by injection molding was characterized by X‐ray scattering and correlated with mechanical behavior. It was found that at a critical nucleant concentration of 0.03 wt % the content of the β‐modification virtually reaches a saturation level. With further addition of the nucleant, the β‐phase content increases only slightly. The long period passes through a distinct maximum at the same nucleant concentration. This singularity in structure remarkably correlates with a minimum of the yield stress and maxima of strain at break and fracture toughness. Such general behavior is also reflected in the correlation between the β‐phase concentration and fracture toughness profiles along the injection‐molded bars. It is suggested that in the critically nucleated material an optimum thickness of the amorphous interlayer with connecting chains between the β‐crystallites is established, rendering the material the highest possible ductility and toughness. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1174–1184, 2002     

Effect of polystyrenes with different architectures on the β‐nucleating efficiency and toughening of isotactic polypropylene

The effect of polystyrenes (PSs) with different architectures (three‐arm star‐shaped polystyrene (sPS), comb‐like branched polystyrene (cPS) and linear polystyrene) on their β‐nucleating efficiency for isotactic polypropylene (iPP) during crystallization and final impact and the tensile properties of iPP/PS blends were investigated by dynamic rheological measurements, SEM, DSC, polarized optical microscopy, wide angle X‐ray diffraction and mechanical property measurements. The results show that the architecture of PS has marked influence on its dispersibility in iPP and β‐nucleating efficiency. For iPP/cPS blend, plenty of short side chains reduce the probability of cPS chain entanglements, facilitating the interdiffusion between iPP and cPS chains. A favorable interfacial interaction results in good dispersibility, high β nucleating efficiency and an excellent toughening effect of cPS on iPP. However, the relatively high chain entanglement degree of sPS may not be in favor of chain diffusion between iPP and sPS and therefore relatively poor dispersibility and toughening effect are obtained. The elongation at break and impact strength of iPP were dramatically improved, especially with the addition of 1 wt% cPS. The toughening mechanism of PS on iPP is the dissipated energy caused by cavitation and the β‐nucleating effect of PS. © 2018 Society of Chemical Industry     

Influence of thermal treatments,molecular weight,and molecular weight distribution on the crystallization of β‐isotactic polypropylene

Six samples of isotactic polypropylene were examined to study the influence of the thermal treatments and the molecular weights and their distribution on the β‐crystallization of the polymer. The highest amount of the β‐phase was obtained by isothermal crystallization and in correspondence of high average molecular weights and wide molecular weight distributions. Small‐angle X‐ray scattering pointed out that a partial β‐crystallization seems not to influence the lamellar morphology parameters. Differential scanning calorimetry measurements, at different heating rates, allowed us to confirm that the multiple melting endotherms behavior of the β‐phase is to be correlated to a melting–recrystallization–melting mechanism. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1008–1012, 2004     

Effect of carbon nanotube‐supported β nucleating agent on the thermal properties,morphology, and mechanical properties of polyamide 6/isotactic polypropylene blends

A new kind of β nucleating agent, multi‐wall carbon nanotube (MWCNT)‐supported calcium pimelate was introduced into polyamide 6 (PA 6)/isotactic polypropylene (iPP; 10/90 by weight) blend and the thermal properties, morphology, and mechanical properties were investigated. The results showed that β‐iPP appeared at low content of MWCNT‐supported calcium pimelate which surmounted the α‐nucleating effect of PA 6 for iPP, and the content of β‐iPP increased with increasing content of MWCNT‐supported calcium pimelate. The impact strength, elongation at break, and flexural modulus were improved with increasing content of MWCNT‐supported calcium pimelate without significantly deteriorating the tensile strength. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Melting characteristic and β‐crystal content of β‐nucleated polypropylene/polyamide 6 alloys prepared using different compounding methods

BACKGROUND: The distribution of nucleating agents in different phases is still an open question in general, and how to control conditions to prepare alloys rich in β‐crystals of polypropylene (PP) is hardly reported. The main goal of this study was to find out the factors influencing the β‐crystal content in β‐nucleated PP/polyamide 6 (PA6) alloys and determine the best preparation conditions to obtain β‐nucleated PP/PA6 alloys rich in β‐crystals. RESULTS: The compounding methods had little influence on the crystallization temperature of both PP and PA6. However, the melting characteristic and β‐crystal content in β‐nucleated PP/PA6 alloys not only depended upon the compounding methods, but also on the temperature at which the nucleating agent was added. A higher β‐crystal content can be obtained by adding the nucleating agent at a temperature below 190 °C, which is also dependent on the mixing time. CONCLUSION: It is proved by etching the alloys with sulfuric acid that the nucleating agent mainly disperses in the PA6 phase and/or the interface between PP and PA6 when blended at high temperature. Copyright © 2009 Society of Chemical Industry     

A novel highly efficient β‐nucleating agent for polypropylene using nano‐CaCO3 as a support

In order to increase the isotactic content of β‐nucleated polypropylene (β‐iPP) and decrease the cost of its production, the investigation and development of novel highly efficient β‐nucleators are important issues. Nano‐CaCO₃ was used as a support to prepare a supported β‐nucleator, nano‐CaCO₃‐supported calcium pimelate. Fourier transform infrared spectral analysis shows that an in situ chemical reaction takes place between nano‐CaCO₃ and pimelic acid. Differential scanning calorimetry results indicate that the crystallization and melting temperatures of β‐phase in supported β‐nucleator‐nucleated iPP are higher than those of calcium pimelate‐nucleated iPP. The β‐nucleating ability of the supported β‐nucleator is little influenced by the cooling rate and crystallization temperature over a wide range. The decreased content of pimelic acid in the supported β‐nucleator slightly decreases the crystallization temperature of iPP but it has no influence on the content of β‐phase in nucleated iPP. A novel supported β‐nucleator has been successfully synthesized via pimelic acid supported on the surface of CaCO₃. The crystallization temperature of iPP and melting temperature of β‐phase in iPP nucleated using the supported β‐nucleator are higher than those of iPP nucleated using calcium pimelate. The concept of a supported nucleator will provide a new way to increase the efficiency of polymer additives and to decrease the amounts of them that need to be used by using nanoparticles as supports. Copyright © 2010 Society of Chemical Industry     

Cationic copolymers of α,β‐poly‐(N‐2‐hydroxyethyl)‐DL‐aspartamide (PHEA) and α,β‐polyasparthylhydrazide (PAHy): synthesis and characterization

In the present study the derivatization of two water‐soluble synthetic polymers, α,β‐poly(N‐2‐hydroxyethyl)‐DL ‐aspartamide (PHEA) and α,β‐polyasparthylhydrazide (PAHy), with glycidyltrimethylammonium chloride (GTA) is described. This reaction permits the introduction of positive charges in the macromolecular chains of PHEA and PAHy in order to make easier the electrostatic interaction with DNA. Different parameters affect the reaction of derivatization, such as GTA concentration and reaction time. PHEA reacts partially and slowly with GTA; on the contrary the reaction of PAHy with GTA is more rapid and extensive. The derivatization of PHEA and PAHy with GTA is a convenient method to introduce positive groups in their chains and it permits the preparation of interpolyelectrolyte complexes with DNA. © 2000 Society of Chemical Industry