Simultaneously improving tensile strength and toughness of melt‐spun β‐nucleated isotactic polypropylene fibers

Polymer processing methods generally play a crucial role in determining the development of microstructure in the fabricated product. In this study, isotactic polypropylene (iPP) melt containing 0.05 wt % β‐nucleating agent (β‐NA) was extruded via a melt flow rate indicator. The molten extrudate was stretched into a fiber upon various take‐up velocities (TVs). The microstructures of the fiber were investigated by differential scanning calorimeter, two‐dimensional wide‐angle X‐ray diffraction, and small‐angle X‐ray scattering. Also, its tensile properties (including tensile strength, modulus, elongation at break, and toughness) were measured by tensile test. Interestingly, the tensile strength (135.0 MPa) of a melt‐spun β‐nucleated iPP fiber fabricated at 400 cm/min was enhanced by 115.2%, compared with that (62.7 MPa) prepared at 100 cm/min, with a considerable increment in toughness (from 661 to 853 MJ/m³). The enhancement mechanism for tensile properties was discussed based on the microstructures. This work offers a simple approach to prepare β‐nucleated iPP fibers with excellent strength and toughness. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43454.     

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     

Structural characterization of α‐ and β‐nucleated isotactic polypropylene

Modification of isotactic polypropylene (iPP) with two nucleation agents, namely 1,3:24‐bis(3,4‐dimethylobenzylideno) sorbitol (DMDBS) (α‐nucleator) and N, N′‐dicyclohexylo‐2,6‐naphthaleno dicarboxy amide (NJ) (β‐nucleator), leads to significant changes of the structure, morphology and properties. Both nucleating agents cause an increase in the crystallization temperature. The efficiency determined in a self‐nucleation test is 73.4 % for DMDBS and 55.9 % for NJ. The modification with NJ induces the creation of the hexagonal β‐form of iPP. The addition of DMDBS lowers the haze of iPP while the presence of NJ increases the haze. Copyright © 2004 Society of Chemical Industry     

Effects of mineral additives on the β‐crystalline form of isotactic polypropylene

Nineteen kinds of minerals, alone and in bicomponent mixtures with LaC (a mixed ternary complex of trivalent lanthanum stearate and stearin), were added to isotactic polypropylene (iPP). The influences of the minerals on the crystallographic forms of iPP were investigated. A wide‐angle X‐ray diffraction examination demonstrated that no mineral or LaC acting alone could induce the occurrence of the hexagonal β‐form, whereas the bicomponent mixtures could when the mineral was a calcium compound or contained calcium compounds, whether the calcium compounds had a hexagonal crystallographic form or not. We surmise that the actual β‐iPP substrate in such a system might be some binuclear complexes of calcium and rare earth elements with some specific ligands. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1742–1748, 2002     

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     

β-nucleators and β-crystalline form of isotactic polypropylene

Different levels of β-form crystallinity in isotactic polypropylene (iPP) were produced by blending a series of additives. X-ray diffraction analyses showed that the Kx values varied from 0 to 0.05. Indigosol Brown IRRD, Indigosol Red Violet IRH, Cibantine Orange HR, Indigosol Pink IR, Cibantine Blue 2B, Indigosol Golden Yellow IGK, and Indigosol Grey IBL were found to be effective β-nucleators. The respective Kx values of the iPP samples containing the above seven β-nucleators of 0.05 wt % are 0.54, 0.68, 0.82, 0.82, 0.86, 0.92, and 0.95. All the β-nucleators possess fused benzene rings or heterocycles in their molecular structures. X-ray diffraction studies on them revealed a mutual character in that the strongest or the second-strongest reflections almost locate at the d-spacing of 2.83 Å. The magnitude is about half of that of the β-spherulites possess a bright color and that their Maltese Crosses display some concentric banding of a rather spiky, jagged character. Their lamellae exhibit a correlated and dendritic structure seen by a scanning electron microscope. © 1995 John Wiley & Sons, Inc.     

Shear‐induced β‐form polypropylene in long chain branching isotactic polypropylene

Long chain branching polypropylene (LCBPP) with different long chain branching (LCB) contents were prepared by reactive extrusion in the presence of styrene and benzoyl peroxide, and their shear‐induced crystallization behaviors were investigated. The results indicated that the LCB structure extended the relaxation time of LCBPP in the molten state, which led to the formation of β‐form polypropylene under shear and high cooling rate. The content of β‐form (K_β) increased with the increase of LCB content, shearing rate and cooling rate. The K_β value of LCBPP3 whose weight average molecular weight was 920,000 g mol^?1 could be up to 52.0% with a shear rate of 60 s^?1 associated with a cooling rate of 280°C min^?1. This study is expected not only to have a deeper understanding of the shear‐induced crystallization behavior of LCBPP, but also provide a new strategy to obtain high level β‐form polypropylene. POLYM. ENG. SCI., 56:240–247, 2016. © 2015 Society of Plastics Engineers     

A highly active and selective β‐nucleating agent for isotactic polypropylene and crystallization behavior of β‐nucleated isotactic polypropylene under rapid cooling

Zinc adipate (Adi‐Zn) was observed to be a highly active and selective β‐nucleating agent for isotactic polypropylene (iPP). The effects of Adi‐Zn on the mechanical properties and the β‐crystals content of nucleated iPP were investigated. The impact strength of iPP nucleated with 0.2 wt % Adi‐Zn was 1.8 times higher than that of neat iPP. In addition, wide‐angle X‐ray diffraction analysis indicated that the content of β‐crystals in nucleated iPP (k_β value) reached 0.973 with 0.1 wt % Adi‐Zn, indicating that Adi‐Zn is a highly active and selective β‐nucleating agent for iPP. Furthermore, fast scanning chip calorimetry (FSC) studies using cooling rates from 60 to 13,800 °C min^?1 revealed that the formation of β‐crystals significantly depended on the cooling rates. At cooling rates below 3000 °C min^?1, only β‐crystals existed. However, at cooling rates above 6000 °C min^?1, β‐crystals failed to form. Moreover, a lower critical crystallization temperature that corresponded to the generation of β‐crystals was investigated using cooling‐induced crystallization, and the results are in good agreement with those of a previous study. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43767.     

Effect of N,N′‐diphenyl adipamide on the formation of the β‐crystalline form in isotactic polypropylene

The polymorphic compositions and mechanical properties of isotactic polypropylene (iPP) samples nucleated by a selective β‐nucleating agent [N,N′‐diphenyl adipamide (DPA)] were investigated with wide‐angle X‐ray diffraction, polarized light microscopy, scanning electron microscopy, and mechanical tests. It was found that β‐phase crystals emerged with the addition of DPA, and the relative proportion of the β‐crystalline form reached the maximum value of 0.97 with the addition of 0.1 wt % DPA. The curved lamellae in the β spherulites were like flowers. The β spherulites were etched more easily than α spherulites because amorphous regions were distributed inside the β spherulites. The Izod notched impact strength increased sharply with the addition of DPA and attained the maximum value of 7.30 kJ/m² (the value of blank iPP was 3.13 kJ/m²) with the addition of 0.1 wt % DPA. An analysis of the misfit factors between DPA and β‐iPP showed that β‐iPP could epitaxially crystallize on the DPA crystal well. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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     

Study of phenyl‐terminated hyperbranched polyester as a special β‐nucleating agent on the toughness of isotactic polypropylene

The effect of phenyl‐terminated hyperbranched polyester (HBP‐Bz) with different generation (the first generation and the fourth generation) as a special β‐nucleating agent on the toughness of isotactic polypropylene (iPP) was investigated by dynamic rheological measurements, scanning electron microscopy, differential scanning calorimetry, wide‐angle X‐ray diffraction, polarized optical microscopy, and mechanical properties measurements. The results show that the β nucleating activity of HBP‐Bz significantly depends on its concentration and molecular structure. The relative content of β‐crystal form (K_β) increases with the increasing HBP‐Bz percentage reaches a maximum and then decreases as HBP‐Bz percentage further increases. The K_β values of iPP/HBP‐G1‐1% and iPP/HBP‐G4‐1% blends are 26.52% and 20.80%, respectively. When compared with HBP‐G4, HBP‐G1 has incompact molecular structure, facilitating the π–π interaction between phenyl‐terminated groups and the helix chains of iPP crystallize on it, and therefore relatively good dispersibility, high β nucleating activity and excellent toughening effect are obtained. The impact strength of iPP was dramatically improved, especially with addition of 1 wt% HBP‐G1. POLYM. ENG. SCI., 59:E133–E143, 2019. © 2018 Society of Plastics Engineers     

Electrical β-relaxation of die-drawn isotactic polypropylene

Measurements of a.c. impedance were carried out in the frequency range 1 Hz to 10⁶ Hz at room temperature on isotropic and die-drawn isotactic polypropylene. It was found that the a.c. impedance decreases with increasing draw ratio in the frequency range 1 Hz to 10⁴ Hz and then becomes independent of draw ratio with increasing frequency. A dramatic drop in the a.c. impedance was observed in the frequency range 10⁴ to 10⁶ Hz, which is associated with a β-relaxation process. The intensity of the β-relaxation peak decreases with increasing draw ratio and nearly disappears for a draw ratio of 16·8. The Cole Cole plots show good semicircles for isotropic and die-drawn samples indicating that a certain polarization mechanism is dominant in polypropylene.     

On the differences in micro‐deformation mechanism between isotactic polypropylene and β‐nucleated isotactic polypropylene as revealed by the confocal laser scanning microscopy

To understand the toughness enhancement of β‐nucleated isotactic polypropylene (iPP) in comparison with iPP, the differences in the micro‐deformation mechanisms between the neat iPP and β‐nucleated iPP were visualized using the confocal laser scanning microscopy (CLSM). Structure of the α‐ and β‐spherulites situated close to the tip of the sharp starter crack has been investigated during tensile deformation in the viewing field of the CLSM. In the α‐spherulite of the neat iPP, highly localized inter‐ and intra‐spherulitic micro‐shear bands have been observed. In the β‐nucleated iPP, relatively uniform distribution of diffuse shear bands has been observed in the β‐phase, while the α‐phase remained relatively undeformed exhibiting only narrow intra‐spherulitic shear bands in the direction perpendicular to the loading direction. Delocalization of plastic deformation into diffuse shear bands in β‐nucleated iPP can explain its enhanced crack resistance compared with the neat iPP exhibiting highly localized shear banding. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

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     

A novel highly efficient β‐nucleating agent for isotactic polypropylene

A novel highly efficient β‐nucleating agent for isotactic polypropylene (iPP), hexahydrophthalic barium (HHPA‐Ba), was found and its effects on the mechanical properties, the β‐phase content, and crystallization behavior of iPP were investigated, respectively. The results show that the β‐phase content of nucleated iPP (k_β value) can reach 80.2% with 0.4 wt % HHPA‐Ba. The impact strength and crystallization peak temperature of nucleated iPP are greatly increased. Compared with pure iPP, the impact strength of nucleated iPP can increase 2.4 times. Meanwhile, the spherulite size of nucleated iPP is dramatically decreased than that of pure iPP. The Caze method was used to investigate the nonisothermal crystallization kinetics of nucleated iPP and the crystallization active energy was achieved by Kissinger method. © 2011 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     

Controllable reinforcement of stiffness and toughness of polypropylene via thermally induced self‐assembly of β‐nucleating agent

An easy approach was reported to achieve the simultaneous reinforcement and toughening of polypropylene (PP) via thermally induced self‐assembly of β‐nucleating agent (TMB‐5). The results showed that the processing temperatures dictated the solubility and self‐assembly of TMB‐5 in the polymer melts to determine the subsequent morphology development of PP. At low processing temperature, TMB‐5 did not dissolve into the polymer melt but remained original shape to induce PP to crystallize into spherulites so that it only promoted the formation of β‐form crystals to enhance the toughness of the samples. At high processing temperature, TMB‐5 gradually dissolved into the polymer melts. On cooling, the dissolved nucleating agent self‐assembled into high aspect ratio fibrils through intermolecular hydrogen‐bonding interactions. Due to a favorable matching between PP and TMB‐5, PP preferred to nucleate and grow orthogonally to the fibril axis and into oriented hybrid shish‐kebab morphology with rich β‐form crystals. Compared with isotropic spherulites, the anisotropic structure exhibited excellent properties of the β‐form crystal and shish‐kebab morphology to simultaneously improve the strength and toughness of TMB‐5‐modified PP samples. With the increasing processing temperature, more dissolved TMB‐5 was involved in the self‐assembly procedure to generate longer fibrils and induce more lamellae to grow on the surface. As a consequence, the anisotropy of the PP samples increased further, bringing out more improvements of the tensile strength. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40605.     

Comparative study on the crystallization behavior of β‐isotactic polypropylene nucleated with different β‐nucleation agents —effects of thermal conditions

In this study, the crystallization behavior of the β‐isotactic polypropylene (β‐iPP) samples nucleated by a rare earth based β‐nucleating agent (β‐NA) WBG‐II and a metal salts compound β‐NA NAB83 (denoted as WPP and NPP, respectively) under different cooling conditions were comparatively investigated. The thermal conditions such as the cooling rate, isothermal crystallization temperature, isothermal crystallization time, and the subsequent cooling to room temperature. The results of WAXD, SEM, and nonisothermal crystallization reveal that under the same processing conditions, the crystallite size of NPP is smaller, which arrange more compactly as compared with WPP. Meanwhile, NPP has shorter crystallization rate and higher β‐nucleation selectivity, but WPP can crystallization at wider temperature range. The results of isothermal crystallization showed that NPP has higher selectivity and higher β‐nucleation efficiency, which favors the formation of high proportion of β‐phase at the isothermal crystallization temperature of 110–130°C with and without subsequent cooling; WPP has lower selectivity, which can only induce high content of β‐phase under isothermal crystallization without subsequent cooling to 25°C. In tuning the crystallization behavior and the properties of β‐PP, the joint influence of the efficiency and selectivity of the β‐NA, and the thermal conditions should be taken into consideration. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40115.     

Effect of magnesium malonate on the formation of the β crystalline form in isotactic polypropylene

Magnesium malonate (MgMt) is used to induce the β crystalline form in isotactic polypropylene (iPP) for the first time. It is shown by wide angle X‐ray diffraction, differential scanning calorimetry, and polarized light microscopy that the content of the β crystalline form increases markedly with the addition of MgMt and attains the maximum value at an MgMt content of 0.10wt%. The tensile strength and flexural modulus of the doped iPP sample decrease somewhat; the tensile strain at break increases slightly; and the Izod notched impact strength increases significantly with the addition of MgMt, as compared with the values for the blank iPP sample. The β‐nucleation mechanism of MgMt is analyzed by the dimensional lattice match criterion. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers.     

The formation of β‐polypropylene crystals in a compatibilized blend of isotactic polypropylene and polyamide‐6

Compatibilized polypropylene (PP)/polyamide (PA6) blends with and without β nucleating agent (β‐NA) are prepared, and are designated as Blend‐0.3 and Blend‐0, respectively. The melting and crystallization characteristic of the blends crystallized under different cooling rates and different crystallization temperatures are studied. It is observed that high β‐PP content can be developed in Blend‐0.3 only at slow cooling rates (<5°C/min), whereas high α‐PP content is formed at fast cooling rates. Isothermal crystallization analysis of Blend‐0 indicates that PA6 is an effective NA for α‐PP in the lower temperature range, whereas the α‐nucleating effect disappears in the higher temperature range. Blend‐0.3 can, therefore, be viewed as a system containing both α‐ and β‐NAs, simultaneously. PA6 is competing with β‐NA in inducing PP crystallization. Under the normal injection of Blend‐0.3, the melt will be cooled through the higher temperature that favors the effectiveness of β‐NA rapidly because of the faster cooling rate. However, the α‐nucleation effect from PA6 predominate at the lower temperature. This explains the difficulty in obtaining high β‐PP content in Blend‐0.3 from injection molding. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers