The influence of crystal structures of nucleating agents on the crystallization behaviors of isotactic polypropylene

The crystals of N, N′-dicyclohexyl-2,6-naphthalenedicarboxamide (DC26NDCA) and N,N′-dicyclohexyl-1,4-naphthalenedicarboxamide (DC14NDCA) were selected as nucleating agents to induce the crystallization of isotactic polypropylene (i-PP). The influence of the crystal structures of the nucleating agents on the crystallization behaviors and morphologies was studied by means of differential scanning calorimeter, wide-angle X-ray diffraction, polarized light microscopy, and atomic force microscopy (AFM). The results show that DC26NDCA is a selective β-nucleating agent for the i-PP crystallization, while DC14NDCA only has a very weak α-nucleating effect for the i-PP crystallization. The dynamic growth processes reveal that i-PP crystals grow with different crystallization behaviors in the presence of the nucleating agents. The i-PP lamellar structures on the crystal surfaces of the nucleating agents were observed by AFM in details, which suggested that the nucleating agents induced the epitaxial growth of the i-PP lamellae.     

Different influences of nucleating agents on crystallization behavior of polyethylene and polypropylene

The crystallization behavior of polyethylene (PE) and polypropylene (PP), including the neat ones and the ones nucleated with the same nucleating agent (NA), was studied by DSC. It was found that the nucleating agent decelerated the PE nonisothermal crystallization process. NA did enhance the nucleating rates for both PE and PP, but the linear growth rate dominated the volumetric growth rate for PE, and the volumetric growth rate dominated the overall crystallization rate. That is why PE nucleated with NA had a slower overall crystallization rate than the neat one.     

The effect of nucleating agents on the crystallization kinetics of polypropylene

Compositions of neat polyproylene (PP), PP–PE (polyethylene) blend and PP–PE–DBS (dibenzylidene sorbitol) were studied with respect to their crystallization kinetics by means of differential scanning calorimetry in isothermal mode. A modified Avrami equation was applied to obtain the crystallization parameters of PP, PP–PE and PP–PE–DBS. Optical and hot-stage microscopy and dynamical analysis were used for structure determination. Experimental results have indicated that PE addition inhibits PP crystallization rate and acts as a plasticizer, while addition of DBS changes the crystallization kinetics. A detailed study of the DBS nucleation effects indicates that owing to the high surface energy, nuclei are formed during primary crystallization, leading to a fine PP crystalline structure and improved mechanical properties.     

Crystallization and melting behaviors of isotactic polypropylene nucleated with compound nucleating agents

Crystallization and melting behaviors of isotactic polypropylene (iPP) nucleated with compound nucleating agents of sodium 2,2′‐methylene‐bis (4,6‐di‐tert‐butylphenyl) phosphate (hereinafter called as NA40)/dicyclohexylterephthalamide (hereinafter called as NABW) (weight ratio of NA40 to NABW is 1:1) were studied by differential scanning calorimetry and wide‐angle X‐ray diffraction (WAXD), the relative β‐amount of iPP nucleated with these compound nucleating agents was also calculated in Turner‐Jones equation by using wide‐angle X‐ray diffraction data. Under isothermal crystallization, there exists a temperature range favorable for formation of β‐iPP. When the concentration of compound nucleating agents is 0.2 wt %, the temperature range is from 100 to 140 °C. While in nonisothermal crystallization, lower cooling rate is favorable for form of β‐iPP and the relative β‐amount of iPP increases with the decreasing of cooling rate in crystallization process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 911–916, 2008     

Shear-induced crystallization of isotactic polypropylene based nanocomposites with montmorillonite

Shear-induced isothermal crystallization in iPP based nanocomposites with organo-modified montmorillonite was followed by light depolarization technique. Prior to the crystallization, samples were sheared at 1 or 2 s⁻¹ for 10 s in a plate-plate system at crystallization temperature of 136 °C. Structure of the solidified specimens was investigated by light microscopy and electron microscopy, X-ray techniques and IR spectroscopy. Strong enhancement of the nucleation and crystallization after shearing was observed in the compatibilized nanocomposites with the clay. Clay exfoliation was found to accelerate strongly the shear-induced nucleation and overall crystallization. However, the sheared samples exhibited only weak orientation of α crystals with (0 4 0) crystallographic planes parallel to shearing direction that resulted probably from a small population of oriented crystals that formed due to shear-induced orientation of iPP chains and served as nuclei for further nearly isotropic growth.     

Crystallization behavior of polypropylene with or without sodium benzoate as a nucleating agent

The crystallization kinetics of polypropylene (PP) with or without sodium benzoate as a nucleating agent were investigated by means of DSC and polarized optical microscopy in isothermal and nonisothermal modes. A modified Avrami equation was applied to the kinetic analysis of isothermal crystallization. The addition of the nucleating agent up to its saturation concentration increased the crystallization temperature by 15 °C and shortened both the isothermal and nonisothermal crystallization half‐times. It was concluded that the sodium benzoate acted as a good nucleating agent for α‐form PP. By adding the nuclefier to PP, adequately controlled spherulites increased the mechanical properties including especially the Izod impact strength and shortened cycle time of PP. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1001–1016, 2001     

Nonisothermal crystallization behaviors of polypropylene with α/β nucleating agents

In this work, the nonisothermal crystallization and subsequent melting behaviors of polypropylene (PP) nucleated with different nucleating agents (NAs) have been studied. α‐phase NA 1,3:2,4‐bis (3,4‐dimethylbenzylidene) sorbitol (DMDBS, Millad 3988), β‐phase NA aryl amides compound (TMB‐5), and their compounds were introduced into PP matrix, respectively. The results show that the nonisothermal crystallization behaviors and crystalline structures of PP with compounded NAs are dependent on the composition of NAs. In the sample of PP with 0.1 wt % DMDBS and 0.1 wt % TMB‐5, the nucleation efficiency (NE) of TMB‐5 is much higher than that of DMDBS and PP crystallizes mainly nucleated by TMB‐5, and in this condition, β‐phase PP is the main crystallization structure. For the sample of PP with 0.2 wt % DMDBS and 0.2 wt % TMB‐5, 0.2 wt % DMDBS has higher NE than 0.2 wt % TMB5, and α‐phase is the main crystalline structure. The cooling rate is proved to be very important in controlling the nonisothermal crystallization behavior and the final crystalline structure of nucleated PP. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1853–1867, 2008     

Carbon nanoparticles as effective nucleating agents for polypropylene

The effect of four nucleating agents on the crystallization of isotactic polypropylene (iPP) was studied by differential scanning calorimetry (DSC) under isothermal and non-isothermal conditions. The nucleating agents are: carbon nanofibers (CNF), carbon nanotubes (CNT), lithium benzoate and dimethyl-benzylidene sorbitol. Avrami?s model is used to analyze the isothermal crystallization kinetics of iPP. Based on the increase in crystallization temperature (T _c) and the decrease in half-life time (τ_½) for crystallization, the most efficient nucleating agents are the CNF and CNT, at concentrations as low as 0.001 mass%. Sorbitol and lithium benzoate show to be less efficient, while the sorbitol needs to be present at concentrations above 0.05 mass% to even act as nucleating agent.     

Effect of nucleating agent supported on zeolite via the impregnation on the crystallization ability of isotactic polypropylene and its mechanism

Addition of an α‐nucleating agent is the simple and effective method to increase nucleation efficiency of isotactic polypropylene (iPP). However, severe agglomeration and poor dispersibility of sodium 2,2′‐methylene‐bis(4,6‐di‐tertbutylphenyl) phosphate (NA11) decrease the nucleation efficiency in the iPP, and much more nucleating agent is needed to maintain the nucleating property. As a result, it becomes the key how to decrease the size of NA11 and increase the nucleating property. In this paper, zeolite 4A (Z4A) was firstly supported by NA11 through solution impregnation, and NA11 was dispersed by Z4A depending on the dispersion of zeolite as carrier for the second component. Then, the dispersed NA11 system (NA11‐Z4A) exhibited a superior nucleation behavior during the crystallization of the iPP matrix when it was used with iPP together. The isothermal and nonisothermal crystallization kinetics indicated that the NA11‐Z4A/iPP system had the best crystallization effect. Polarized optical microscopy (POM) and scanning electron microscopy (SEM) analyses showed that the size of NA11 decreased obviously when it was adsorbed on the surface of Z4A, which leads a better dispersibility of the nucleating agent and thus an accelerated nucleation process in the iPP matrix. In the end, the mechanism for the excellent dispersibility of NA11‐Z4A, which was based on hydrogen bonding between NA11 and Z4A, was confirmed by Fourier‐transform infrared spectroscopy (FTIR). Based on the research work, the solution impregnation strategy can potentially be applied to other systems to inhibit the agglomeration and improve the dispersibility of additives in iPP.     

Effect of functionalization on non-isothermal crystallization behavior of polypropylene

The non-isothermal crystallization kinetics of three functionalized polypropylenes (PPs; polypropylenes-g-acrylic acid [PP-g-AA], polypropylenes-g-glycidyl methacrylate [PP-g-GMA], polypropylenes-g-maleic anhydride [PP-g-MAH]) at different cooling rates were investigated by differential scanning calorimetry, using the Jeziorny method, Ozawa method, and Mo method. The result showed that Mo method can adequately describe the non-isothermal crystallization kinetics of pure PP and functionalized PPs, and at a given relative crystallinity, the crystallization rate obtained using Mo method followed an order of PP-g-AA > PP-g-GMA > PP > PP-g-MAH. The crystallization activation energy for these samples was calculated using Kissinger's method, which indicated that the introduction of monomers had a confinement effect on the motion of PP chains.     

Specificity and versatility of nucleating agents toward isotactic polypropylene crystal phases

Nucleating agents with an ≈6.5 Å lattice parameter induced the α phase of isotactic polypropylene (iPP, α‐iPP). A 6.5 Å periodicity is also involved in the nucleating agents for the β phase of iPP (β‐iPP). The similarity in substrate periodicities suggests that some nucleating agents may induce either the α or β phase under different crystallization conditions. 4‐Fluorobenzoic acid, dicyclohexylterephthalamide, and γ‐quinacridone (the latter two are known as β‐iPP nucleators) were tested over a wide range of crystallization temperatures [up to crystallization temperature (T_c) > 145 °C]. The two former nucleating agents induce exclusively α‐iPP and β‐iPP, respectively. γ‐Quinacridone on the contrary is a versatile agent with respect to the crystal phase generated. More specifically, the same crystal face of γ‐quinacridone induces either β‐iPP or α‐iPP when T_c is below or above ≈140 °C. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2504–2515, 2002     

The influences of α/β compound nucleating agents based on octamethylenedicarboxylic dibenzoylhydrazide on crystallization and melting behavior of isotactic polypropylene

The influences of α/β compound nucleating agents based on octamethylenedicarboxylic dibenzoylhydrazide on crystallization and melting behavior of isotactic polypropylene (iPP) were analyzed. It is found that the crystallization temperatures of nucleated iPP were increased by above 11.0°C and the relative contents of β‐crystals (K_β ) in iPP reached above 0.40 after addition of compound nucleating agents. The K_β values depend on cooling rate, crystallization temperature in isothermal crystallization, and the difference between the crystallization temperatures of iPP nucleated by two individual nucleating agents. The nonisothermal crystallization kinetics were studied by Caze method and Mo method, respectively. The effective activation energy was calculated by the Friedman's method. The results illustrate that the half crystallization time was shortened and the crystallization rate was increased obviously after addition of nucleating agents, and the effective activation energy was increased with the relative crystallinity.     

Attapulgite and ultrasonic oscillation induced crystallization behavior of polypropylene

In this article, the effect of ultrasonic oscillation on the dispersed morphology of attapulgite in polypropylene (PP) and crystallizing kinetics of PP/attapulgite composites prepared through extrusion in the presence and the absence of ultrasonic oscillation were studied. X‐ray diffraction analysis results showed that ultrasonic oscillation did not change attapulgite crystal structure during extrusion in PP/attapulgite composites. On the other hand, scanning electron microscopy and transmission electron microscope photographs indicated that ultrasonic oscillation promoted the dispersion of attapulgite particles in PP matrix. The dispersed morphology of attapulgite and ultrasound oscillation affected the crystalline form, nucleation rate, crystallization temperature, crystallinity, and spherulite size of PP crystals. PP transcrystals were formed on the attapulgite particle surface. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2300–2308, 2007     

Non-isothermal crystallization of polyethylene blends with bimodal molecular weight distribution

Non-isothermal crystallization of polyethylene (PE) blends with bimodal molecular weight distribution (MWD) was investigated by differential scanning calorimetry (DSC) at various scanning rates. The bimodal PE blends were prepared by blending two unimodal polyethylenes with large difference in molecular weigh in different ratio in xylene solution. Different kinetic parameters such as the half-time of crystallization (t_1/2), crystallization rate constant (Z_c), F(T) and the effective activation energy were determined. Some complicated relationships between these parameters and the average molecular weight were found. The crystallization rate first increased and reached a maximum then decreased, and also correlated with the MWD. The Avrami index under non-isothermal conditions was analyzed with a method developed by Harnisch and Muschik; the results indicated that homogeneous nucleation and spherulitic growth regimes were present in all samples studied.     

The variable role of clay on the crystallization behavior of DMDBS-nucleated polypropylene

The effect of clay on the nucleating behavior of 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol(DMDBS) in cryatallization of isotactic polypropylene(iPP) was investigated by means of differential scanning calorimetry(DSC), dynamic rheology and polarized light microscopy(PLM).It is interesting to note that the incorporation of layered clay nanoparticles into DMDBS-nucleated iPP may induce a synergetic nucleation effect while the DMDBS content is below 0.1 wt%,otherwise it restricts the crystallization rate prominently as the DMDBS content increases up to 0.3 wt%,which has exceeded the content threshold to yield a nucleating agent(NA) network.As shown by dynamic rheological investigations, the clay nanoparticles demonstrate an obstructive effect of disturbing the consistency of DMDBS fibrils network.Moreover, to further demonstrate the importance of NA network formation in the crystallization of iPP,we used another NA named HPN-20e,which can not form network structure at all over the concentration studied,for comparison.In this case,the nucleated-crystallization rate is independent on the addition of clay nanoparticles,as the nucleating mechanism is an individual nuclei manner without NA network forming.     

The influence of thermoelastomers on the crystallization behavior of isotactic polypropylene under shear

The crystallization behaviors of isotactic polypropylene (iPP) and its blends with thermoelastomers have been investigated with in situ X‐ray scattering and optic microscopy. At quiescent condition, the crystallization kinetics of iPP is not affected by the presence of elastomers; while determined by the viscosity, the differences are observed on sheared samples. With a fixed shear strain, the crystallization rate increases with increasing the shear rate. The fraction of oriented lamellar crystals in blends is higher than that in pure iPP sample, while the percentage of β phase is reduced by the presence of the elastomers. On the basis of experimental results, no direct correlation among the fraction of oriented lamellae, the percentage of β phase, and growth rate can be deduced. The evolution of the fraction of oriented lamellae supports that shear field promotes nucleation rather than growth process. Shear flow induces the formation of nuclei not only with preferring orientation but also with random orientation. The total density of nuclei, which determines the crystallization kinetics, does not control the ratio between nuclei with and without preferring orientation, which determines the fraction of oriented lamellae. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1188–1198, 2006     

Effects of toughening propylene/ethylene graft copolymer on the crystallization behavior and mechanical properties of polypropylene random-copolymerized with a small amount of ethylene

Linear low-density polyethylene (LLDPE) was grafted onto the backbone chains of isotactic polypropylene (iPP) during reactive melt-extrusion to produce a novel toughening modifier, propylene/ethylene graft copolymer (PEGC), to improve the properties of iPP random(-copolymerized with a small amount of ethylene) (PPR). The crystallization behavior as well as the non-isothermal crystallization kinetics of the PEGC modified PPRs were investigated via differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). The fractured surface topography was characterized using scanning electron microscopy (SEM), and the mechanical properties through notched impact and tensile testing as well as dynamic mechanical thermal analysis (DMTA). The results show that, at a PEGC content of 8 wt%, notched impact strength of the PEGC modified PPR increased by 30.6% at low temperature (−25 °C). As regards crystalline morphology, the PEGC, as an effective heterogeneous nucleating agent, fostered nucleation of the PPR to elevate its crystallization temperature as well as rate of crystallization, thus refining the PPR (iPP) spherulites and improving the interfacial structure between iPP spherulites. The Jeziorny approach was unsatisfactory for simulation of the non-isothermal crystallization process of the PEGC modified PPRs; however, the Mo method described consistently the crystallization kinetics over the entire isothermal process.     

Nucleation characteristics of the α/β compounded nucleating agents and their influences on crystallization behavior and mechanical properties of isotactic polypropylene

Nucleation characteristics of isotactic polypropylene (iPP) nucleated by the α/β compounded nucleating agents (NAs) were investigated by wide‐angle X‐ray scattering, differential scanning calorimetry and mechanical testing. The results showed that the nucleation effect of the α/β compounded NAs depends on not only nucleation efficiency (NE) of individual β and α NAs and their ratios but also the processing conditions, especially the cooling rates. The nucleating characteristics of the α/β compounded NAs can be illustrated by competitive nucleation. The NA with high NE played a leading role during iPP crystallization even at a low weight ratio and at different cooling rates. The stiffness and toughness of iPP can be simultaneously improved by using suitable compositions at the appropriate ratios. Finally, the nonisothermal crystallization kinetics of iPP nucleated with the α/β compounded NAs was described by Caze method and the crystallization activation energy of nucleated iPP was calculated by Kissinger equation. The result indicated that the crystal growth pattern of nucleated iPP was heterogeneous nucleation followed by three‐dimension spherical growth. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 653–665, 2010     

On the crystallization behavior of cold-drawn syndiotactic polypropylene

The influence of the chain conformation on the crystallization behavior of cold-drawn syndiotactic polypropylene (sPP) has been investigated. The conformational and structural changes depending on drawing conditions and thermal treatments has been observed by x-ray diffraction, infra-red spectroscopy and modulated differential scanning calorimetry. A nucleation and crystal growth model is introduced, which explains the low crystallinity of cold-drawn sPP.     

Isothermal crystallization behaviors of isotactic polypropylene nucleated with α/β compounding nucleating agents

Sodium 2,2′‐methylene‐bis(4,6‐di‐tert‐butylphenyl) phosphate (NA40) and N,N‐dicyclohexylterephthalamide (NABW) are high effective nucleating agents for inducing the formation of α‐isotactic polypropylene (α‐iPP) and β‐iPP, respectively. The isothermal crystallization kinetics of iPP nucleated with nucleating agents NABW, NA40/NABW (weight ratio of NA40 to NABW is 1:1) and NA40 were investigated by differential scanning calorimetry (DSC) and Avrami equation was adopted to analyze the experimental data. The results show that the addition of NABW, NA40/NABW and NA40 can shorten crystallization half‐time (t_1/2) and increase crystallization rate of iPP greatly. In these three nucleating agents, the α nucleating agent NA40 can shorten t_1/2 of iPP by the largest extent, which indicates that it has the best nucleation effect. While iPP nucleated with NA40/NABW compounding nucleating agents has shorter t_1/2 than iPP nucleated with NABW. The Avrami exponents of iPP and nucleated iPP are close to 3.0, which indicates that the addition of nucleating agents doesn't change the crystallization growth patterns of iPP under isothermal conditions and the crystal growth is heterogeneous three‐dimensional spherulitic growth. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 590–596, 2007