Β-nucleated Isotactic Polypropylene Research Articles

Ethylene Comonomer-Directed Epitaxial Nucleation and Growth of β-Nucleated Isotactic Polypropylene

We present a molecular-level investigation of the epitaxial crystallization of β-nucleated isotactic polypropylene associated with the minor and local topographic modification at the interface, i.e., the inclusion of an ethylene comonomer. The experimental results show that the relative fraction of β-crystal (Kβ) decreases progressively with increasing the amount of ethylene comonomer, which originates from a simultaneous reduction in the β-nucleation ability of a β-nucleating agent (N,N′-dicyclohexylterephthalamide, DCHT) toward propylene–ethylene random copolymers (PERs) and the growth rate of the β-crystal relative to the α-crystal. With the aid of molecular mechanics simulation, we demonstrate that such a topographic modification weakens the intermolecular interactions at the nucleation and growth interfaces, thereby modifying the epitaxial nucleation of the β-crystal and the growth kinetics of β- and α-crystals. The study reveals that the polymorph selection of the PERs/DCHT system is not solely determined by the lattice match but also profoundly influenced by the topography characteristics dependence of molecular interaction at the crystalline interfaces. Our fundamental insight could have important implications in the control of the polymorphism of polymers through the careful design of the topography of the nucleation and growth interfaces.

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

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

Influences of Molecular Structure on the Non-Isothermal Crystallization Behavior of β-Nucleated Isotactic Polypropylene

In order to explore the effect of molecular structure on the crystallization and polymorphic behavior of β-nucleated isotactic polypropylene (β-iPP), four commercial iPP samples with different average isotacticities, molecular weights and isotactic distributions were used. The relationship between the molecular structure characteristics and β-crystallization behavior were investigated by means of xylene solvent fractionation (XS), successive self-nucleation and annealing fractionation (SSA), carbon-13 nuclear magnetic resonance spectrometry (13C NMR), gel permeation chromatography (GPC), melt index (MI) and differential scanning calorimetry (DSC). Results of molecular structure characterization revealed that their isotacticity can be ranged as F401 > 5014L > F03G ≈ T38F, while the order of their isotactic distribution width is 5014L ≈ T38F > F401 ≈ F03G. Results of crystallization kinetics revealed that the molecular weight of the samples has little effect on their crystallization and polymorphic behavior. Meanwhile, crystallization activation energy Ec can be ranged as 5014L < F401 < F03G < T38F, indicating that when the molecular weight was above a certain degree, the isotacticity is the main factor affecting the crystallization ability of β-type iPP (β-iPP). The higher the isotacticity and the wider the distribution is, the stronger the β-crystallization ability is. Moreover, the β-crystallization ability of the samples is closely related to the isotacticity of the sample, which can be effectively controlled by tuning the fraction of high isotacticity with high molecular weight components.

Investigation on the Roles of β-Nucleating Agents in Crystallization and Polymorphic Behavior of Isotactic Polypropylene

In this study, four types of widely used commercial β-nucleating agents (β-NAs) were used to prepare β-nucleated isotactic polypropylene (β-iPP). Differential scanning calorimetry (DSC), polarizing optical microscopy (PLOM), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) were used to comparatively study the roles of β-NA in the β-nucleation efficiency, isothermal crystallization kinetics and crystalline morphology of the samples. Results revealed that the geometric shapes and thermal stabilities of the β-NAs are quite different from each other, moreover, the β-nucleation efficiencies (β-NE) of the β-NAs are quite different. The β-NE of NAB83 and PACA are very high, while those of TMB-5 and WBG‑II are relatively low. The critical concentrations of the β-NAs were obtained as follows: NAB83 = 0.02 wt %, PACA = 0.02 wt %, TMB-5 = 0.15 wt %, WBG = 0.3 wt %. Morphological studies of SEM and PLOM indicated that for different β-NAs, the β-phase fraction, crystalline sizes and morphology of the formed β-phase are quite different. Isothermal crystallization kinetics provided detailed kinetic information on the different β-NAs.

Influences of molecular structure on the isothermal crystallization behavior and mechanical properties of β-nucleated isotactic polypropylene

ABSTRACT To investigate the influence of molecular structure of isotactic polypropylene (iPP) on the crystallization behavior and mechanical properties of its β-nucleated product, in this study, four iPP samples were selected. Results of molecular structure characterizations revealed that the molecular weight of the iPP resins are close, while the average isotacticities and the isotacticity distributions of the samples are quite different from each other. Isothermal crystallization kinetics study suggested that due to their different average isotacticities and isotacticity distributions, the crystallinity difference of the four samples is as high as 5.3%, and the crystallinity of β-crystal difference is as high as 7.2%. Generally, the higher the average isotacticity of the iPP and the wider its distribution are, the stronger the β-crystallization ability of the sample is. Moreover, the mechanical measurement revealed that high average isotacticity and the wide isotacticity distribution of the iPP resulted in the strong the β-crystallization ability and thus better the toughness.

Investigation on the Effect of Hyperbranched Polyester Grafted Graphene Oxide on the Crystallization Behaviors of β-Nucleated Isotactic Polypropylene.

In this article, hyperbranched polyester grafted graphene oxide (GO) was successfully prepared. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM) were performed for its characterizations. On the other hand, differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) were also performed to study its influences on non-isothermal crystallization behaviors of β-nucleated isotactic polypropylene (β-iPP). The grafting ratios of hyperbranched polyester with different supermolecular structures were calculated to be 19.8–24.0 wt %, which increase with the degree of branching. The results showed that the grafting of hyperbranched polyester was advantageous in increasing the crystallization peak temperature Tp and decreasing the crystallization activation energy ΔE of β-iPP/GO composites, which contributed to the iPP’s crystallization process. Moreover, under all cooling rates (2, 5, 10, 20, 40 °C/min), crystallinities of β-iPP/GO were greatly improved after being grafted with hyperbranched polyester, because of the increase of the relative contents of α-phase αc and the average α-crystal sizes.

Thermal Stability of Shear-Induced Precursors and Their Effect on the Crystalline Structure of β-Nucleated Isotactic Polypropylene

The thermal stability and lifetime of shear-induced precursors under various annealing temperatures, as well as the influence of their relaxation on the crystalline modification in β-nucleated isotactic polypropylene (iPP), are investigated using an ARES rheometer. The wide-angle X-ray diffraction results show that the β-crystal content of sheared β-nucleated iPP samples gradually increases with thermal treatments. The relaxation of shear-induced precursors during annealing which caused the decrease of shear nuclei may restrain the counteraction effect between the shear flow and β-nucleation agent as well as result in the increase of β-crystal content. At the early stage of relaxation, the relaxation degree is closely related to the increase of β-crystals, for which the deeper relaxed shear-induced precursors result in the more restoration of β-crystals. However, when the relaxation degree exceeded a certain limitation, where the β-crystals reached the maximum, the relaxation of shear-induced precursors will no longer influence the crystal structure of β-nucleated iPP.

Effect of β-crystals on the mechanical and electrica properties of β-nucleated isotactic polypropylene

Polypropylene has great potential as a thermoplastic insulation material for high voltage direct current (HVDC) cable. However, its mechanical properties restrict its application, particularly at low temperature. In this paper, isotactic polypropylene (iPP) with the β-nucleating agent WBG was prepared by the melt blending method. The crystallinity, β-crystal proportion and crystalline morphology were analyzed by wide-angle X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and polarized optical microscopy. The results showed that the crystallinity and β-crystal proportion increased as the WBG concentration increased. Three types of β-crystal morphology, namely β-spherulite, flower-like agglomerate and β-quasi-spherulite, were sequentially obtained with increasing WBG concentration. The mechanical properties, space charge distribution, thermally stimulated discharge current and DC electrical breakdown strength were investigated. Compared with pure iPP, the Izod impact strength and DC breakdown strength of iPP/0.1%WBG were improved by 238 and 16%, respectively, and the fracture energy and brittleness temperature of iPP/0.3%WBG were improved by 420 and 80%, respectively. In addition, the space charge accumulation was suppressed in the range of 0.1–0.5 wt% WBG. Chain coupling between the β-crystals and the amorphous phase, particularly in the flower-like agglomerates, was considered to play an important role in improving the mechanical and electrical performance.

Effect of β-crystals on the mechanical and electrica properties of β-nucleated isotactic polypropylene

Effect of β-crystals on the mechanical and electrica properties of β-nucleated isotactic polypropylene

Evolution of Polymorphic Structure in β-Nucleated Isotactic Polypropylene under a Certain Pressure: Effects of Temperature and Flow

Isotactic polypropylene (iPP) is one of the most intensively investigated polymeric materials for a long time because of its special polymorphism. In this work, the effects of temperature and flow ...

Enhanced beta to alpha recrystallization in beta isotactic polypropylene with different thermal histories

A small but meaningful increase in the content of β-crystals was observed in β-nucleated isotactic polypropylene (iPP) samples isothermally crystallized at high temperature and then cooled to room temperature. The secondary crystallization of these crystals could also be linked to a distinct exothermic effect revealed by differential scanning calorimetry upon cooling. These crystals constituted of thinner lamellae certainly show earlier melting upon reheating, with respect to the isothermally crystallized fraction. As such, the enhancement in βα recrystallization in iPP cooled below 100°C is attributed to the formation of a self-nucleated or partially ordered melt upon melting of these crystals, which induces the fast nucleation and growth of the α-phase, because of larger undercooling with respect to the β-phase at elevated temperatures. On the other hand, the recrystallization of the β-phase from partially molten β-iPP samples is tentatively assigned to homo-epitaxial nucleation on the surface of thicker β-phase lamellae.

Effects of β-nucleating agent and graphene oxide on the crystallization and polymorphic composition of isotactic polypropylene / graphene oxide composites for bridge pavement

To control the polymorphic and crystallization behavior of β-nucleated isotactic polypropylene / graphene oxide (β-iPP/GO) composites for bridge pavement, the roles of β-nucleating agent (β-NA) concentration and GO content in the polymorphic crystallization behavior, morphology and crystallization kinetics were studied by means of differential scanning calorimentry (DSC), wide-angle X-ray diffraction (WAXD), scanning electronic microscopy (SEM). Results revealed that with the increase of β-NA concentration, the crystallization temperature, the percentage of β-phase βc and relative degree of crystallinity Xc increased evidently. Moreover, the crystallite sizes decreased, and the crystallization activation energy △E decreased, reflecting the strong nucleation effect of the β-NA to the composites; On the other hand, as the GO content increased, the crystallization temperatures and relative degree of crystallinity Xc also increased slightly. Interestingly, GO exhibited strong α-nucleation effect, which was not favorable for formation of β-phase. When the GO content was 2.0 wt%, unusual small banding patterned crystals were observed. In general, to control the polymorphic behavior and morphology of the β-iPP/GO composites, both β-NA concentration and GO content were very important factors.

Transformation of β to α phase of isotactic polypropylene nucleated with nano styrene butadiene rubber-based β-nucleating agent under microwave irradiation

This paper investigates the effect of microwave irradiation on the β to α phase transformation of the β-nucleated isotactic polypropylene (iPP). Ten microwave irradiation cycles was applied to the iPP and iPP modified with 0.3 wt% and 0.5 wt% β-NA, and the data at 2nd, 4th, 6th, 8th and 10th irradiation were reported. As expected, the sample temperature was found to increase with the irradiation time, by more than 130 °C, due to high frequency of microwave processing. This was the major factor that induced the β-phase transformation and structural change. Both the differential scanning calorimetry (DSC) and X-ray diffraction (XRD) results indicated that β-phase was mainly transformed to α-phase and partially converted to the amorphous section. It was reflected as 1) the reduction of the enthalpy of β-crystal melting (ΔHmβ), 2) the increased enthalpy of α-crystal melting (ΔHmα), 3) the decreased β-crystalline phase fraction (Kβ) and 4) the decrease of the overall degree of crystallinity (Xall). Under impact force, neat iPP showed a slight increase in the impact strength with the irradiation time, due to the increase of amorphous region. For the β-iPP, it decreased due to the reduction of the β-phase content.

In situ generation of a self-dispersed β-nucleating agent with increased nucleation efficiency in isotactic polypropylene

Addition of a β-nucleating agent (β-NA) is the most effective method of preparing β-nucleated isotactic polypropylene (β-iPP); however, the poor dispersion and agglomeration of β-NAs limit nucleation efficiency. To solve this problem, a self-dispersing β-NA strategy was developed based on in situ preparation of β-NA (instead of pre-addition) during the processing of iPP. Zinc adipate (ZnAA), a typical β-NA, was chosen; self-dispersed ZnAA (ZnAA(IS)) was prepared in situ from its reaction precursors adipic acid (AA) and zinc oxide (ZnO) during extrusion of iPP. In situ preparation of ZnAA(IS) led to a significantly higher nucleation efficiency than pre-addition of ZnAA. The β-crystal content (kβ value) of nucleated iPP prepared with ZnAA(IS) reached 0.99, significantly higher than iPP prepared with ZnAA. Moreover, the impact strength of 0.1 wt % ZnAA(IS)/iPP composites was nearly 5.3-fold higher than neat iPP with a net gain in reinforcement (157%) compared to pre-addition of ZnAA. To explain these phenomena, we proposed mechanism for the self-dispersion of β-NA in the context of in situ generation of β-NA during iPP processing; the mechanism was confirmed by Fourier Transform Infrared Spectroscopy (FTIR), Polarized Optical Microscopy (POM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) data. Therefore, this strategy and method of preparing β-NA in situ offers unique advantages that will advance the production and application of β-iPP. Furthermore, this strategy could potentially be extended to solve issues of poor dispersion and agglomeration for other additives.

The effect of thermal history on crystalline structure and mechanical properties of β-nucleated isotactic polypropylene

The effect of thermal history on β-nucleated iPP was systematically investigated by comparing the variance of crystalline microstructures and mechanical property of stepwise crystallized sample and annealed sample, which experienced different thermal history. The mechanical property tests exhibit that that the toughness of stepwise crystallized sample and annealed sample were both decreased compared to control sample, while the tensile strength of the stepwise crystallized sample increased slightly. Structure investigation showed that the α-relaxation peak, which is related to the assignment of chains in rigid amorphous phase, moved to the high temperature region for stepwise crystallized sample, while it moved to the low temperature region for annealed sample. The results indicated the weakening in rigid amorphous fraction (RAF) and the increase in lamellar thickness of β-iPP after stepwise crystallization treatment. For annealed sample, the RAF strengthened and lamellar thickness decreased slightly after thermal treatment. A mechanism of crystalline microstructures evolution of restricted area between the main lamellar under different treatments was proposed.

Mechanical enhancement of melt-stretched β-nucleated isotactic polypropylene: The role of lamellar branching of β-crystal

Melt-stretched isotactic polypropylene (iPP) filled with different weight fractions (0, 0.1, 0.2 and 0.4 wt %) of β-nucleating agent (β-NA) was extruded via a single screw extruder with a slit die and immediately melt-stretched at the die exit. The microstructure and mechanical properties (i.e., monotonic tensile, step-cycle and stress relaxation behavior) of these samples were analyzed. Unexpectedly, the lamellar branching of β-crystal, which is similar to lamellar branching of α-crystal, is observed in melt-stretched β-nucleated iPP. More interestingly, it is found that, compared with other samples, iPP containing 0.2 wt % of β-NA has the best mechanical properties. Through thorough analysis of various structural characterizations, the improved tensile strength obtained from step-cycle tests and viscous force obtained from stress relaxation measurements for iPP with 0.2 wt% of β-NA is ascribed to lamellar branching of β-crystal. The study is an example of preparing high performance polymers via practical polymer processing methods.

Effect of oscillatory shear field on the morphology and mechanical properties of β-nucleated isotactic polypropylene

The aim of this study is to investigate the effects of extra oscillatory shear field on morphology and mechanical properties of β-nucleated isotactic polypropylene by using a homemade vibration-assisted extrusion apparatus, in which oscillatory shear field was introduced into the extrusion head driven by crank and link mechanism. The results show that the extra oscillatory shear filed induces a significantly enhanced orientation of β-crystal besides causing a better homogeneous dispersion of β-nucleant, and the oriented β-crystal has a high ratio of daughter lamellae to parent lamellae orientation, which was rarely reported in other literatures. The vibration increased the overall crystallinity and changed the relative content of β-crystal. For the higher level nucleation agent content iPP(0.4 wt%), the relative contents of β-crystal just changed a little or basically unchanged after vibration. However, for iPP contenting 0.2 wt% nucleation agent, the content of β-crystal decreased. All the changes in the morphology, crystallinity, dispersion of β-nucleant and β-crystal content had a great influence on the mechanical properties, and that result was the improvement of yield stress and decrement of the elongation at break. POLYM. ENG. SCI., 57:838–845, 2017. © 2016 Society of Plastics Engineers

Influences of isotactic poly(4-methylpentene-1) on the crystallization and morphology of the β-nucleated isotactic polypropylene

ABSTRACTThe β-nucleated isotactic polypropylene (β-iPP)/isotactic poly(4-methylpentene-1) (TPX) composites were prepared using a β-nucleating agent (TMB-5). The crystallization behavior and polymorphic morphology of β-iPP with various concentration of TPX were investigated by differential scanning calorimetry, wide-angle X-ray diffraction, and scanning electronic microscopy. The results showed that the addition of TPX increases the content and the spherulite sizes of β-phase but decreases the crystallization temperature and reduces the crystallization energy barriers as well. For melting behavior, it was found that the β-phase content of samples greatly depend on the cooling rate applied and the appearance of TPX.

Influences of molecular weight on the non-isothermal crystallization and melting behavior of β-nucleated isotactic polypropylene with different melt structures

Previous studies reported that by tuning the fusion temperature T f (i.e., controlling the melt structure), the β-nucleation efficiency of β-nucleated isotactic polypropylene (β-iPP) can be greatly enhanced, which was called ordered structure effect (OSE). The aim of this study is to investigate the roles of melt structures and molecular weight in non-isothermal crystallization behavior of β-iPP. Five samples with different molecular weights were prepared, and their melt structures were controlled using differential scanning calorimetry (DSC) by tuning T f (T f = 200 or 170 °C represent the case of without or with OSE, respectively). Results revealed that for all the samples, the lower the PP molecular weight, the lower the crystallization peak temperature T c, and the higher the activation energy ΔE, but the occurrence of OSE behavior (i.e., T f = 170 °C) can elevate T c, reduces ΔE and encourages crystallization. The decrease of PP molecular weight decreases β-phase proportion no matter OSE occurs or not; moreover, for PP with high molecular weight, OSE behavior not only enhances the β-phase proportion, but also increases the thermal stability of the β-phase; however, for samples with low molecular weight, OSE behavior enhances the β-phase proportion in a larger extent, and decreases the thermal stability of the β-phase. This study provided important understandings in the effect of OSE, cooling rate and PP molecular weight the β-crystallization of iPP.

Influence of injection rate on crystallization of injection-molded β-nucleated isotactic polypropylene

It is widely believed that β-nucleating agent is beneficial for effectively toughening isotactic polypropylene (iPP). However, for the injection molding process, the shearing and thermo-mechanical conditions make the nucleation and crystallization process complicated. In this paper, the effects of injection rate on crystallization of β-nucleated iPP were studied by scanning electron microscope (SEM), two-dimensional wide-angle X-ray diffraction and differential scanning calorimetry (DSC). It is observed that with increasing injection rate, the content of β-crystals exhibits different tendencies in the skin, intermediate layers, and core zone. Specifically, for the intermediate layer, the β-crystals content first increases with increasing injection rate to 85 cm·s−1, and begins to decrease afterward. By simulating the injection process, the most likely explanation for the β-crystal change is the comparatively high shear rate and low shearing time that the melt experienced. Variations in β-form content are mainly responsible for the mechanical properties of β-nucleated iPP. The results of this study provide a valuable way to control the iPP toughness in the injection molding process. POLYM. ENG. SCI., 57:172–182, 2017. © 2016 Society of Plastics Engineers