Crystallization Of Isotactic Polypropylene Research Articles

In-situ synchrotron X-ray investigating crystallization of isotactic polypropylene with β-nucleating agent during injection molding

The coupling effects of mold temperature and packing pressure fields on structure evolution of isotactic polypropylene with β-nucleating agent (iPP/β) during injection molding were systematically investigated using high-throughput in-situ synchrotron X-ray diffraction/scattering technology for the first time. A small number of β-crystals are generated in pure iPP and iPP/β due to the shear flow introduced by the filling stage. The formation of β-crystals induced by β-nucleating agent (β-NA) in iPP/β commenced after about 1.8 s of melt filling. The two-stage growth of β-crystals in iPP/β exhibits differences in growth time and orientation. Both the decrease in the lattice spacing and the β-α transition in iPP/β occur at the packing stage and are suppressed at high mold temperatures (≥ 90 °C). The polymorphism competitive growth of iPP/β is revealed, and the effects of coupling external fields on it are well elucidated by combining finite element simulation and classical nucleation theory. Additionally, the scattering intensity and orientation of iPP/β lamellae initially increase to a maximum value and subsequently decrease until reaching the equilibrium with time. Our current work lays a solid foundation to precisely customize the crystal structure of iPP/β in practical processing and thus to produce high-performance iPP products.

How open‐stage melt crystallization affects tensile and shrinkage properties of 3D printed polypropylene

AbstractMelt crystallization of isotactic Polypropylene (PP) on open platform during 3D processing is critical and often challenging. Numerous factors that influence the melt extrusion of PP directly influence its crystallization and hence impact the physico‐mechanical properties of the printed parts. The present article represents a detailed investigation of the influence of 3D processing parameters on tensile and shrinkage properties of PP subject to respective melt‐crystallization behaviors. Moreover, the article also demonstrated the best experimental conditions that can produce mechanically stronger parts (even stronger than compression molded specimens used for comparison) with minimum warping. Influence of print temperature, bed temperature, print layer thickness (when the layer thickness varies by a margin of four folds), print orientation (horizontal or vertical), and infill rates was found to be highly sensitive towards melt crystallization of the PP dumbbell (which was printed as a product) hence brought about the highest variation in tensile and shrinkage properties. A print head temperature of above 220°C and a bed temperature of 60°C produced stronger dumbbells when printed with a higher layer thickness (0.20 mm/0.25 mm) in horizontal conditions with a 100% infill rate. Such specimens possessed higher tensile strength, and elongation at break values than the compression molded sample.

An Unconventional Route for Synthesis and Solid-State Processing of Low-Entangled Ultra-High Molecular Weight Isotactic Polypropylene.

Introducing the reverse micelle formation during polymerization, and thus avoiding the catalyst support, aggregated single crystals of ultra-high molecular weight isotactic polypropylene having spherical morphology are obtained. The ease in flowability of the spherical nascent morphology, having a low-entangled state in the non-crystalline region of the single crystals in the semi-crystalline polymer, allows the sintering of the nascent polymer in the solid state without melting. Thusmaintains a low-entangled state, and facilitates the translation of macroscopic forces to macromolecular length scale, without melting, leading to the formation of uniaxially drawn objects having unprecedented properties that can be used in the development of one component, high-performance, easy-to-recycle composites. Thus having the potential of replacing difficult-to-recycle hybrid composites.

Nanolignin as Nucleating Agents Promoting Crystallization of Isotactic Polypropylene

Lignin is the second most abundant pollution-free biomass material. However, most of lignin is discarded as waste in river or burned as fuel, which results in serious environmental pollution problems and low utilization efficiency of lignin at present. Thus, high-value utilization of lignin has become a hot research field. Herein, nanolignin (nano-Lig) is prepared successfully by the self-assembly method, and then nano-Lig and isotactic polypropylene (iPP) are mixed to prepare a series of nano-Lig/iPP composites by the solution blending method. Nano-Lig not only enhance thermostability of iPP but also improve crystallization properties of iPP. When 1.0 wt% nano-Lig was added in iPP, the crystallinity of iPP increased by 6.39% compared to iPP. Nano-Lig can increase the crystallization rate of iPP through investigation of kinetics of nonisothermal crystallization, suggesting nano-Lig can be used as a nucleating agent for iPP.

Morphological evolution ofTMB‐5/ethylene–vinyl alcohol copolymer blends under biaxial‐stretching field and its effect on the crystallization and gas barrier properties of isotactic polypropylene

AbstractThe morphological evolution of TMB‐5/ethylene–vinyl alcohol (EVOH) copolymer blends under biaxial‐stretching field and its effect on the crystallization and gas barrier properties of isotactic polypropylene (iPP) were investigated. iPP/TMB‐5/EVOH ternary blends with TBM‐5 selectively distributed in the EVOH phase were prepared through a multistage stretching extrusion, which combined an assembly of layer multiplying elements (LMEs) with an extruder. Scanning electron microscopy revealed that the biaxial‐stretching field in LMEs transformed the TMB‐5/EVOH phase into platelets along the extrusion direction. Differential scanning calorimetry and X‐ray diffraction results indicated that the TMB‐5 particles were distributed in the EVOH phase and noβcrystals formed in as‐extruded samples. Especially, the TMB‐5 was released from the EVOH phase into iPP in the annealing process and induced the formation ofβcrystals. Compared with that of pure iPP, the oxygen permeability coefficient of iPP containing aligned TMB‐5/EVOH platelets decreased more than three times. The enhanced barrier properties resulted from the increased tortuosity of the diffusion pathway caused by the orientated high‐aspect‐ratio platelets. © 2023 Society of Industrial Chemistry.

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.

High-pressure crystallization of iPP nanocomposites with montmorillonite and carbon nanotubes

High pressure facilitates crystallization of isotactic polypropylene (iPP) in the orthorhombic γ-form, differing in structure and properties from the α-form, in which iPP crystallizes under common processing conditions. In the study, the effect of high pressure on crystallization of iPP nanocomposites was examined. The iPP nanocomposites with 1-5 wt.% of well dispersed either o-MMT (PP/MT) or MWCNT (PP/CN) were prepared by mixing molten iPP with the nanofillers. The nanocomposites and neat iPP were crystallized during cooling under pressures up to 300 MPa. Regardless of crystallization pressure, the crystallization temperatures were similar whereas the sizes of the polycrystalline aggregates were only slightly smaller in PP/MT compared to those of neat iPP. On the contrary, the presence of MWCNT elevated the crystallization temperature of PP/CN by 8-13 K compared to that of neat iPP, and strongly decreased the grain size. This evidenced the nucleating activity of MWCNT during high-pressure crystallization of iPP in the γ-form. The higher crystallization temperature allowed a larger portion of the polymer to crystallize in the γ-domain before the α-domain was reached. As a result, PP/CN, especially crystallized under 50-100 MPa, contained more γ-form than neat iPP and PP/MT.

The role of oxidized polyethylene wax in processing and crystallization of isotactic polypropylene in WPC composites

The role of oxidized polyethylene wax in processing and crystallization of isotactic polypropylene in WPC composites

Isothermal crystallization behaviors of isotactic polypropylene with ultrasonically‐modified calcium sulfate whisker as β‐nucleating agent

AbstractIn this work, the effect of calcium sulfate whisker (CSW) and ultrasonically‐modified CSW (UCSW) on the crystal structure and isothermal crystallization of isotactic polypropylene (iPP) is studied by using differential scanning calorimetry (DSC), X‐ray diffraction (XRD), and polarized optical microscopy (POM). UCSW obviously increases heterogeneous nucleation. The isothermal crystallization kinetics of pure iPP, iPP/CSW, and iPP/UCSW composites are analyzed by the Avrami method. Although the Avrami exponent (n) of these samples only changed slightly, the half crystallization time (t1/2) indicates that CSW and UCSW decrease the overall crystallization time and increase the crystallization rate (Rc). POM results show that most crystals are β‐iPP shaped and the spherulite size decreases significantly with the addition of UCSW. The apparent equilibrium temperature of iPP/UCSW composites is lower than that of pure iPP and iPP/CSW composites. The values of β‐iPP fold surface free energy (σe) and the nucleation parameter (Kg) are much lower than that of α‐iPP for these iPP composites. The content of β‐crystal in iPP composites increases due to the addition of UCSW.

The effect of masterbatch pigments on the crystallisation, morphology, and shrinkage behaviour of Isotactic Polypropylene

Variations in mould shrinkage when using organic and inorganic pigments in semicrystalline polymers is a well-known phenomenon within industry. These differences in mould shrinkage are thought to be caused by the presence of the pigments acting as nucleating agents, altering the crystallisation of semicrystalline polymers. These shrinkage variations can give rise to problems in obtaining the correct interference fit between parts and can cause issues in automated equipment such as filling lines. It has been previously reported that the onset temperature of crystallisation measured via DSC (differential scanning calorimetry) can be used to predict shrinkage when a variety of neat pigments are added to un-nucleated PP (polypropylene). However, the shrinkage and crystallisation behaviour of masterbatch pigments, which are widely used industrially is poorly understood. To better understand the influence of masterbatch pigments on crystallisation and shrinkage behaviour, injection moulded samples were prepared using variety of reds, whites, and purple commercial-masterbatch pigments with PP. The crystallisation kinetics and crystallinity were studied using DSC, LPOM (Linkam hot stage polarising optical microscopy), XRD (X-ray diffraction), and FTIR (Fourier transform infrared spectroscopy). The morphology was investigated via LPOM and SEM (scanning electron microscopy). A clear correlation was observed between the crystallisation onset temperature measured using DSC and the recorded shrinkage. A strong relationship was also observed between the percentage crystallinity measured using FTIR and shrinkage. Quinacridone and pyrrole based red and purple pigments were found to act as strong nucleating agents, with the pyrrole based red pigment also acting as β nucleator in PP. The white pigments were found to have less influence on the nucleation behaviour. For the pigments which induced the largest variation in shrinkage, a higher rate of nucleation and proportionally smaller spherulitic diameter was observed by DSC, SEM, and LPOM.

Quantitative analysis of chiral transitions and its implication for non-classical crystallization in isotactic polypropylene

Molecular dynamics simulations were used to quantify chiral transitions in an isolated isotactic polypropylene (iPP) chain on a nucleating agent substrate during the early stages of ordering. The effects of temperature, dihedral rigidity, and epitaxy of a single iPP chain placed on α-iPP, β-iPP, and TPDT surfaces were investigated. Using the adaptive biasing force (ABF) method, potentials of mean force (PMF) were computed for iPP segments to explain the selective stabilization of helical segments on various epitaxies. Whereas, chiral selection of helices from the extended-coil conformations was found to be more efficient, the selection of helices from coiled segments was poor. For α-iPP and β-iPP substrates, the stability of the extended-coils and formation of helices from extended-coils were positively correlated with each other. In contrast, a negatively correlation was found for the TPDT substrate, showing that a helix existed as a competitive conformation with the extended-coils when selective stabilization was absent. These observations support the validity of multi-stage nucleation routes during polymer crystallization, Further, they show that selective stabilization of helices by an epitaxy plays a vital role in the dynamics of multi-stage nucleation.

Strain dependent crystallization of isotactic polypropylene during solid-state stretching

In this study, the crystallization behavior of oriented isotactic polypropylene during solid-state stretching was investigated by in-situ wide-angle X-ray scattering. Results showed that a new arc appeared on the (040) reflection with the previous arcs remained when the stretching direction was inclined to the initial orientation direction, as a result of stretching induced crystallization. The crystallization process occurred later than the yielding but earlier than the strain hardening. The crystallinity of the newly formed crystal showed a linear dependence on the Hencky strain, and the slope of the plot was more sensitive to the stretching angle rather than the stretching temperature. Lastly, the stress relaxation test showed that the crystallinity was linear correlated to the amorphous phase orientation. The Pearson correlation coefficient was as high as −0.99, suggesting that the crystallization process is originated from the orientation of the chain network during stretching.

Influence of chain length, particle size, and thermal treatment of dicarboxylic acid-functionalized titanium dioxide filler in polypropylene

The influence of size and thermal treatment of titanium dioxide (TiO2) functionalized with dicarboxylic acids of different length chains on the crystallization of isotactic polypropylene (iPP) has been investigated. The functionalization and crystallization were analyzed by FTIR, DSC, XRD, and mechanical tests. The chain length is crucial, while glutaric acid favored α-nucleation, pimelic and azelaic promoted β-crystals. Pimelic acid was the best by increasing β-crystals up to 54.1% compared with pure iPP (4.7%). Regarding particle size, the smallest dimension increased β-nucleation by ~ 32%. While thermal treatment was applied, β-crystal content increased by ~ 22.7%. Regarding mechanical properties, the fillers presence decreased tensile stress and stiffness since the high content of β-crystals decrease elastic modulus. These results offer evidence that it is possible to prepare fillers able of selecting the type of crystallization since functionalized-TiO2 can promote α- or β-crystals by varying particle size, functionalization agent, and applying a thermal treatment. However, more detailed studies are needed to confirm these observations.

Effect of N,N′-Dicyclohexyldicarboxamide Homologues on the Crystallization and Properties of Isotactic Polypropylene

Application of nucleating agents is the most versatile and industrially applied way to manipulate the crystalline structure of isotactic polypropylene (iPP). Various materials possess a nucleating effect, but from the viewpoint of dispersibility, the partially soluble ones are the most advantageous. Our objective was to synthesize new N,N′-dicyclohexyldicarboxamide homologues and study their applicability as nucleating agents in iPP. Carbon-13 nuclear magnetic resonance (13C NMR) and infrared spectroscopy were used to prove that the synthesis reactions were successful. Thermal stability of the compounds was investigated with simultaneous thermal analysis. Nucleating efficiency and solubility were characterized by polarized light microscopy and differential scanning calorimetry. Polarized light microscopy was also applied to study the effect of novel additives on the morphology of iPP. The properties, important from the viewpoint of applicability, were also investigated. Tensile tests were performed to characterize the main mechanical properties, and standard haze measurements were performed to characterize optical properties. It can be concluded that the investigated compounds are partially soluble nucleating agents and influence the crystalline structure of iPP. Most of the studied compounds have a moderate nucleating efficiency, but a very interesting dendritic structure develops in their presence. Two of them proved to be non-selective β-nucleating agents, which result in a remarkable improvement of impact resistance and higher opacity.

Effect of the lanthanum and cerium phenylphosphonates on the crystallization and mechanical properties of isotactic polypropylene

In this article, the lanthanum phenylphosphonates (LaPPA2) and cerium phenylphosphonates (CePPA2) were synthesized and the effect on the crystallization conducts and mechanical performances of isotactic polypropylene (iPP) were studied. After adding 0.05wt% of LaPPA2 and CePPA2, the crystallization temperature enhanced appreciably. The polarized optical microscopy (POM) imagines indicated that the LaPPA2 and CePPA2 could accelerate the crystallization speed and reduce the spherulites sizes significantly. The presence of LaPPA2 and CePPA2 decreased appreciably the non-isothermal crystallization kinetics parameter F(T). Meanwhile, the nucleation efficiency of LaPPA2 and CePPA2 was similar to industrial standard NAs NA-11, HPN20E and HPN68L. In addition, the mechanical properties of iPP have been improved by adding LaPPA2 and CePPA2. According to the investigation of nucleated iPP viscoelasticity, it can be confirmed that the CH/π interplay between iPP and the aromatic groups of LaPPA2 and CePPA2 promoted the attachment of iPP helix and following nucleation.

Enhancement of crystallization and mechanical properties in isotactic polypropylene via adding liquid-phase exfoliated graphene

In this work, liquid-phase exfoliated graphene (G) was introduced into isotactic polypropylene (iPP) via melt blending. Wide-angle X-ray diffraction measurements revealed that liquid-phase exfoliated graphene is a strong α-nucleating agent, making the iPP crystal grow preferentially on the (040) crystal plane. Differential scanning calorimetry and polarized optical microscopy results showed that the increase in both the nucleation density and crystal growth rate brings about the remarkable elevation of overall crystallization rate of iPP with the addition of liquid-phase exfoliated graphene. Moreover, remarkable enhancements in mechanical properties especially the toughness were obtained at low graphene loading. With the addition of 0.5 wt.% graphene, the impact strength and elongation at break of iPP increased by 103 % and 510 %, respectively. The significant enhancement of crystallization and mechanical properties of the composites could be attributed to favorable dispersion of liquid-phase exfoliated graphene in the iPP matrix. A possible toughening mechanism was proposed.

Heterogeneous nucleation and self-nucleation of isotactic polypropylene with addition of nano-ZnO

The influence of nano-zinc oxide (nano-ZnO) on the crystallization and melting behaviors of isotactic polypropylene (iPP) was investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarized optical microscopy (POM). The results indicated that nano-ZnO is an efficient β-nucleating agent for iPP. The relative content of β-crystal rises at first and then goes down with increasing nano-ZnO concentration and can reach 95.2% while incorporating 3% nano-ZnO during isothermal crystallization at 132 °C. The β-nucleating efficiency of nano-ZnO depends on its concentration and dispersibility in the iPP matrix. The lattice constants of nano-ZnO are a = b = 0.325 nm and c = 0.521 nm, and 0.325 nm is exactly half of the unit cell parameter in the c-direction of β-iPP. This lattice matching facilitates the formation of β-crystal. Moreover, the relationship between heterogeneous nucleation and self-nucleation of iPP with addition of nano-ZnO was also explored by adjusting the fusion temperature Tf. DSC measurements confirmed that the ordered structures of iPP inhibit β-crystallization induced by nano-ZnO. Meanwhile, the introduction of nano-ZnO also reduces the α-nucleating activity of the ordered structures. A possible interaction mechanism was proposed.

El papel de los grupos funcionales alcoxisilanos para la modificación de la superficie de nanopartículas de TiO2 en la cristalización no isotérmica de compositos de polipropileno isotáctico

Functionalization of fillers provides advantages in non-isothermal crystallization of isotactic polypropylene (iPP); therefore, kinetic properties iPP composites filled with alkoxysilane-functionalized titanium dioxide (TiO2) were investigated to understand the role of functional groups. The surface modification of TiO2 nanoparticle was carried out with three different alkoxysilanes: 3-aminopropyltrimethoxysilane (APTMS), 3-chloropropyltrimethoxysilane (CPTMS) and 3-glycidoxypropyltrimethoxysilane (GPTMS); after, composites were prepared 0.5% by weight in iPP. The results of the X-Ray Diffraction and Nuclear magnetic resonance analyses showed the characteristic signals of each chemical species that confirm the success of surface modification of the oxide with alkoxysilanes. The study by Differential scanning calorimetry at different cooling rates allowed to observe that the non-isothermal crystallization of iPP composites is described by Jeziorny’s and Mo’s equations; and that the addition of fillers of TiO2 modified with the alkoxysilanes changed the crystallization process thanks to the functional groups and their interface interactions with the polymer matrix; for example, the presence of CPTMS and GPTMS caused recrystallization of iPP and therefore, increased the crystallization process rate. Finally, the activation energy of the composites varied depending on the alkoxysilane used, since less energy was needed for some cases; derived from better dispersion and that the particles acted as nucleation centers.

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.

The mechanical properties, crystallization and rheological behavior of isotactic polypropylene with nucleating agent supported on polyhedral oligomeric silsesquioxanes (POSS)

The influence of the nucleating agent that supported on polyhedral oligomeric silsesquioxanes (supported NA) on the properties of iPP was investigated in detail. It was found that the supported NA have higher nucleating efficiency than the original NA powder in the iPP system. The flexural modulus of iPP increased by 11.6% with 0.04 wt.% original NA powder and increased by 20.5% when supported NA added. The sizes of supported NA were investigated by scanning electron microscope (SEM) coupled with energy dispersive spectrometer (EDS). The crystal structure and morphology of the iPP samples was studied using wide-angle X-ray diffraction (WAXD) and polarized optical microscope (POM), respectively. The nucleating efficiency of supported NA and the dry mix of NA and POSS was also analyzed. The non-isothermal crystallization behaviors were analyzed using Mo’s model. Besides, the sample with supported NA exhibited lower complex viscosity (η*) values, indicting supported NA can act as a lubricant in system. A possible crystallization mechanism of supported NA in the iPP was proposed.