Nonisothermal crystallization and multiple melting behaviors of β‐nucleated impact‐resistant polypropylene copolymer

As a substitute of isotactic polypropylene in applications requiring excellent fracture resistance, impact‐resistant polypropylene copolymer (IPC) has attracted much attention in recent years. In this study, a highly effective β‐form nucleating agent (β‐NA; an aryl amide compound) was introduced into IPC, and our attention was focused on the nonisothermal crystallization and subsequent melting behaviors of the nucleated samples. The nonisothermal crystallization behaviors were investigated on the basis of the different cooling rates and different concentrations of β‐NA with differential scanning calorimetry, wide‐angle X‐ray diffraction (WAXD), and polarized optical microscopy. The results show that both the cooling rate and concentration of β‐NA greatly determined the nonisothermal crystallization process and subsequent multiple melting behaviors. Further results show that the multiple melting behaviors were related to the transition in β crystallites and those between the β and α crystallites. The morphologies of the dispersed particles and the supermolecular structure of the matrix were characterized with scanning electron microscopy. Finally, the effect of the β‐NA concentration on the fracture resistance of IPC was evaluated by measurement of the notched Izod impact strength. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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

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

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     

Effects of rosin‐type cocrystal nucleating agents on the crystallization process and the properties of polypropylene

A new kind of rosin‐type nucleating agent for polypropylene (PP), the cocrystal of dehydroabietic acid, potassium dehydroabietate, and sodium dehydroabietate, was prepared, and the effects of the nucleating agents on the mechanical and crystallization properties of PP were also studied. The results of differential scanning calorimetry and X‐ray diffraction proved that the cocrystal of dehydroabietic acid and compound alkali dehydroabietate was formed rather than a simple blend of dehydroabietic acid and single alkali dehydroabietate. When it was added to PP, the size of the PP spherulite decreased; the mechanical properties, crystallization temperature, and transparency of PP were substantially improved. Thus, the cocrystal of dehydroabietic acid, potassium dehydroabietate, and sodium dehydroabietate acted as a more effective nucleating agent for PP. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2137–2141, 2003     

Charge storage behavior of isotropic and biaxially‐oriented polypropylene films containing α‐ and β‐nucleating agents

A study on the influence of the crystal modification (α and β) of isotactic polypropylene (i‐PP) films on the resulting electret properties is presented. Two commercial nucleating agents, sodium 2,2′‐methylene‐bis(4,6‐di‐tert‐butylphenyl)‐phosphate (NA11) and N,N′‐dicyclo‐hexyl‐2,6‐naphthalene‐dicarbox‐amide (NU100), were employed in this investigation. Isothermal charge decay was measured at 90°C. In hot pressed isotropic polypropylene films, no significant differences in the charge storage properties were observed for α‐ and β‐nucleated specimens. In addition, the article presents the influence of the nucleating agents at different concentrations on the PP‐film morphology of biaxially stretched films with respect to electret features. It was possible to prepare elongated cavities with the virtually insoluble NA11 additive during stretching, even at concentrations below 0.3 wt %. These films displayed slightly improved electret properties in comparison to stretched neat PP films due to generated cavities acting as barriers for the drift of charges. Various draw ratios were also studied for i‐PP films with 0.15 wt % NA11. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 650–658, 2006     

Crystal morphology and structure of the β‐form of isotactic polypropylene under supercooled extrusion

A supercooled melt of isotactic polypropylene (iPP) was extruded through a capillary die. Polarized light microscopy (PLM), wide‐angle X‐ray diffraction (WAXD), and differential scanning calorimetry (DSC) were used to investigate the effects of the relatively weak wall shear stress (σ_w), extrusion temperature (T_e), and crystallization temperature (T_c) on the structure and morphology of β‐form isotactic polypropylene (β‐iPP). β‐cylindrites crystals could be observed by PLM in the extruded specimen even at a lower σ_w's (0.020 MPa), and the β‐iPP content increased with decreasing T_e. Under a given T_e of 150°C, the increase in σ_w positively influenced the β‐iPP content. The DSC and WAXD results indicate that the total crystallinity and β‐iPP content increased when T_c was set from 105 to 125°C; the other experimental parameters were kept on the same level. Although T_c was above 125°C, the β‐iPP content obviously decreased, and the total crystallinity continued to increase. On the basis of the influences of σ_w, T_e, and T_c on the β‐iPP crystal morphology and structure, a modified model is proposed to explain the growing of shear‐induced β‐iPP nucleation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Interaction of self‐nucleation and the addition of a nucleating agent on the crystallization behavior of isotactic polypropylene

The relationship between heterogeneous or homogeneous nucleation and self‐nucleation of polypropylene (PP) and PP nucleated by an organic phosphate salt (PPA) was studied by DSC. For pure PP, it homogeneously nucleated during cooling after melting at the selected temperature (T_s) of 170–200°C for 3 min, but at the T_s of 160–168°C self‐nucleation occurred; PPA only nucleated heterogeneously at the T_s of 168–200°C, and there existed self nucleation at the T_s of 160–168°C. The double melting peaks of PP and PPA at the T_s of 162°C were observed. Once the self‐nucleation occurred, the change of the crystallization temperature and heat of fusion of PP is more significant than that of PPA with the change of the T_s, depending upon the crystallization conditions. Results were explained by homogeneous nucleation, heterogeneous nucleation, self‐nucleation, and annealing crystallization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 78–84, 2001     

Effects of rosin‐type nucleating agent and low density polyethylene on the crystallization process of polypropylene

In this article, the influence of rosin‐type nucleating agent (Nu–Na) and low density polyethylene (LDPE) on the crystallization process of polypropylene (PP) from the melt state was studied by differential scanning calorimeter and polarization microscope. It was found that LDPE obstructed the crystallization of PP, decreased the crystallization rate of PP. The rosin‐type nucleating agent Nu–Na substantially improved the rate of crystallization, and decreased the size of spherulites also. The cooperative effect of LDPE and Nu–Na made the crystallization rate of PP increase greatly, the spherulites of PP became much smaller and dispersed more uniformly, and the transparency of PP was further improved evidently. The crystallization temperature (T_c) and melting temperature (T_m) of PP and LDPE in PP/LDPE/Nu–Na (97:3:0.5) were not affected by the number of mixed passes—the nuclei migration from PP to PE had not happened in the mixed passes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2804–2809, 2003     

Analysis of the isothermal crystallization of isotactic polypropylene nucleated with sorbitol derivatives

The isothermal crystallization kinetics of isotactic polypropylene (iPP) and iPP nucleated with the sorbitol derivatives 1,3:2,4‐bis(4‐methyldibenzylidene)sorbitol and 1,3:2,4‐bis(3,4‐dimethylbenzylidene)sorbitol was studied, along with the subsequent melting behavior, as a function of the nucleating agent concentration. The influence of the agents on the crystallization rate, crystallization temperature, and crystallization range was examined. The isothermal crystallization temperature increased, along with the crystallization rate, with increasing nucleating agent concentration. The maximum effect of the additives occurred at concentrations of 0.3% or greater. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2261–2274, 2003     

Crystallization behavior and mechanical properties of polypropylene copolymer by in situ copolymerization with a nucleating agent and/or nano‐calcium carbonate

The crystallization behavior of polypropylene (PP) copolymer obtained by in situ reactor copolymerization with or without a nucleating agent and/or nano‐CaCO₃ particles was investigated both by thermal analysis and by polarized light microscopy. The Avrami model is successfully used to describe the crystallization kinetics of the studied copolymer. The results of the investigation show that a dramatic decrease of the half‐time of crystallization t_1/2, as well as a significant increase of the overall crystallization rate, are observed in the presence of the nucleating agent. These effects are further promoted in the presence of the nano‐CaCO₃ particles. The incorporation of the nucleating agent and nano‐CaCO₃ particles into PP copolymer remarkably improved the mechanical properties and heat distortion temperature. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 431–438, 2004     

Nonisothermal crystallization behavior of glass‐bead‐filled polypropylene

The effects of the glass‐bead content and size on the nonisothermal crystallization behavior of polypropylene (PP)/glass‐bead blends were studied with differential scanning calorimetry. The degree of crystallinity decreased with the addition of glass bead, and the crystallization temperature of the blends was marginally higher than that of pure PP at various cooling rates. Furthermore, the half‐time for crystallization decreased with an increase in the glass‐bead content or particle size, implying the nucleating role of the glass beads. The nonisothermal crystallization data were analyzed with the methods of Avrami, Ozawa, and Mo. The validity of various kinetic models for the nonisothermal crystallization process of PP/glass‐bead blends was examined. The approach developed by Mo successfully described the nonisothermal crystallization behavior of PP and PP/glass‐bead blends. Finally, the activation energy for the nonisothermal crystallization of pure PP and PP/glass‐bead blends based on the Kissinger method was evaluated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2026–2033, 2006     

Nonisothermal crystallization,melting behavior,and morphology of polypropylene/metallocene‐catalyzed polyethylene blends

The nonisothermal crystallization, melting behavior, and morphology of blends of polypropylene (PP) and a metallocene‐catalyzed polyethylene (mPE) elastomer were studied with differential scanning calorimetry, scanning electron microscopy, polarized optical microscopy, and X‐ray diffraction. The results showed that PP and mPE were partially miscible and could form some cocrystallization, although the extent was very small. A modified Avrami analysis and the Mo method were used to analyze the nonisothermal crystallization kinetics of the blends. The values of the Avrami exponent indicated that the crystallization nucleation of the blends was homogeneous, the growth of the spherulites was three‐dimensional, and the crystallization mechanism of PP was not affected by mPE. The crystallization activation energy was estimated with the Kissinger method. Interesting results were obtained with the modified Avrami analysis and Mo and Kissinger methods, and the conclusions were in good agreement. The addition of less mPE increased the overall crystallization rate of PP. The relationship between the composition and morphology of the blends was examined. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1203–1210, 2004     

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     

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     

Combined effect of shear and nucleating agent on the multilayered structure of injection‐molded bar of isotactic polypropylene

Molten polymers are usually exposed to varying levels of shear flow and temperature gradient in most processing operations. Many studies have revealed that the crystallization and morphology are significantly affected under shear. A so‐called “skin‐core” structure is usually formed in injection‐molded semicrystalline polymers such as isotactic polypropylene (iPP) or polyethylene (PE). In addition, the presence of nucleating agent has great effect on the multilayered structure formed during injection molding. To further understand the morphological development in injection‐molded products with nucleating agent, iPP with and without dibenzylidene sorbitol (DBS) were molded via both dynamic packing injection molding (DPIM) and conventional injection molding. The structure of these injection‐molded bars was investigated layer by layer via SEM, DSC, and 2 days‐WAXD. The results indicated that the addition of DBS had similar effect on the crystal size and its distribution as shear, although the later decreased the crystal size more obviously. The combination of shear and DBS lead to the formation of smaller spherulites with more uniform size distribution in the injection‐molded bars of iPP. A high value of c‐axis orientation degree in the whole range from the skin to the area near the core center was obtained in the samples molded via DPIM with or without DBS, while in samples obtained via conventional injection molding, the orientation degree decreased gradually from the skin to the core and the decreasing trend became more obvious as the concentration of DBS increased. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Activity of a β‐nucleating agent for isotactic polypropylene and its influence on polymorphic transitions

The influence of a nonpigmenting β‐nucleating additive in the crystallization of isotactic polypropylene (iPP) is investigated by differential scanning calorimetry and X‐ray diffraction. It is found that this additive induces the formation of a very high level of the trigonal modification of iPP. The crystallization and melting behavior of the nucleated systems are studied as a function of the cooling and heating rates and the control of the final temperature during the cooling process. The nucleating agent exerts an important effect on the crystallization temperatures and the polymorphic transitions of iPP, delaying the β–α recrystallization process through an increase in the stability of the trigonal crystals. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 531–539, 2002     

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.     

Isothermal crystallization kinetics and morphology development of isotactic polypropylene blends with atactic polypropylene

The crystallization kinetics and morphology development of pure isotactic polypropylene (iPP) homopolymer and iPP blended with atactic polypropylene (aPP) at different aPP contents and the isothermal crystallization temperatures were studied with differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized optical microscopy. The spherulitic morphologies of pure iPP and larger amounts of aPP for iPP blends showed the negative spherulite, whereas that of smaller amounts of aPP for the iPP blends showed a combination of positive and negative spherulites. This indicated that the morphology transition of the spherulite may have been due to changes the crystal forms of iPP in the iPP blends during crystallization. Therefore, with smaller amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends increased with increasing aPP and presented a lower degree of perfection of the γ form coexisting with the α form of iPP during crystallization. However, with larger amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends decreased and reduced the γ‐form crystals with increasing aPP. These results indicate that the aPP molecules hindered the nucleation rate and promoted the molecular motion and growth rate of iPP with smaller amounts of aPP and hindered both the nucleation rate and growth rate of iPP with larger amounts of aPP during isothermal crystallization. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1093–1104, 2007     

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     

Stress‐induced crystallization of biaxially oriented polypropylene

The hot stretching of thick, extruded sheets at high temperatures is a very important process in the production of finished biaxially oriented polypropylene (BOPP) films with special inner structures. Through a simulation of hot stretching in the machine direction (MD) of the processing of BOPP films, it was found that at high temperatures, the stretching ratio greatly influenced the obtained crystalline structure, as observed by differential scanning calorimetry (DSC). Also, in MD hot stretching, the crystallinity increased by an average of 20%. Wide‐angle X‐ray diffraction patterns of extruded sheet samples with and without stretching confirmed the structural changes shown by DSC, and the results proved that β‐crystal modification did not occur during the MD hot‐stretching process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 686–690, 2003