Crystallization and impact energy of polypropylene/CaCO3 nanocomposites with nonionic modifier

Isotactic polypropylene (PP) and calcium carbonate (CaCO₃) nanocomposites were prepared by melt extrusion in a twin screw extruder. The commercial CaCO₃ nanoparticles had a poor dispersion in PP matrix. The addition of a small amount of a nonionic modifier during melt extrusion greatly improved the dispersion of CaCO₃ nanoparticles. The influence of CaCO₃ nanoparticles on the crystallization of PP was studied by wide angle X-ray diffraction and polarized optical microscopy. The introduction of CaCO₃ particles resulted in small and imperfect PP spherulites, decreased spherulite growth rate and induced formation of β-form PP. The yield strength of PP decreased gradually while its Young's modulus increased slightly with increasing CaCO₃ loading. By adding 1.5 wt% of nonionic modifier to PP/CaCO₃ (85/15) nanocomposite these tensile properties were not changed much but the notched Izod impact energy of the composites was significantly increased.     

Crystallization behaviors in the isotactic polypropylene/graphene composites

Crystallization behaviors and kinetics of iPP in an in-situ prepared isotactic polypropylene/graphene (iPP/G) composites were studied in this paper. In samples used in this study, the graphene fillers were well dispersed, and the interfacial adhesion exhibited enhanced features between graphene and iPP components. The thermal stability of the composites was improved by about 100 °C compared to the pristine iPP. It was found that the crystallization morphology, crystallization rate and kinetics of the iPP/G composites were significantly influenced by the presence of graphene. The nucleation and epitaxial growth of iPP on the graphene surface were observed and studied in detail. It was observed that the nucleation of iPP favored to occur at the wrinkles and edges due to the good match of the lattice parameters and the weak spatial hindrance compared to the smooth surface. Numerous nuclei epitaxially formed and the size of the crystals was very small. The schematic diagram was also proposed for the nucleation and growth process of iPP on the graphene surface in the iPP/G composites. Meanwhile, the overall crystallization kinetics and crystals growth were analyzed through Avrami equation. The obtained Avrami index n decreased with the graphene loadings and was close to 2 for the iPP/G composites, which implied that the growth of iPP in the composites was in two-dimension. And this was caused by the structure of graphene and the spatial confinement effect of graphene platelets in the iPP/G composites.     

Influence of crystal form and morphological characteristics of CaCO3 particles on kinetic of combustion gases desulfurization

Influence of crystal form and morphological characteristics of CaCO₃ samples with different structural (calcite, aragonite, low-crystalline and amorphous) and morphological (medium particle dimensions and shapes) characteristics on the kinetic of time-limited desulfurization process of coal combustion gases between 1030 and 1150 K was determined. Kinetic parameters (activation energies and pre-exponential factors of sulfurization reaction) were established. Correlations of the above-mentioned kinetic parameters of the reaction with size of particles (within the same crystal phase), morphology of particles or particle structure were explained by the difference in distribution of energy of active centers within border phase of interaction.     

PP/CaCO3/PP-g-AA复合材料形态与性能研究

采用双螺杆挤出机液体进料熔融接枝制备了丙烯酸接枝聚丙烯(PP-g-AA),研究了聚丙烯(PP)/碳酸钙(CaCO3)/PP-g-AA复合材料的力学性能,并采用红外光谱、差示扫描量热仪、扫描电子显微镜等对复合材料进行了表征分析。结果表明,用此PP-g-AA作为PP/CaCO3的相容剂时,能有效提高其力学性能,且当CaCO3的含量为10%(质量分数,下同),接枝率为2.204%的PP-g-AA的含量为10%时,效果最佳。     

纳米碳酸钙改性聚丙烯

研究了不同牌号的聚丙烯（PP）与纳米CaCO3复合材料的力学性能,考察了双螺杆挤出和密炼混合工艺及相容剂含量对PP／纳米CaCO3复合材料力学性能的影响,采用透射电子显微镜观察了纳米CaCO3在PP中的分散情况。结果表明,纳米CaCO3对不同牌号的PP均有增韧作用,对基材韧性较好的共聚PP增韧效果较显著．冲击强度提高了81％;双螺杆挤出和密炼混合均能使纳米CaCO3粒子在PP中达到较好的分散。     

Unusual crystallization behavior of isotactic polypropylene and propene/1-alkene copolymers at large undercoolings

During the investigation of the crystallization of metallocene isotactic polypropylene and copolymers with low amount of 1-butene and 1-hexene at large undercoolings, an unexpected behavior has been found. Random copolymers crystallize faster than the homopolymer between 80 and 40 °C, while at high temperatures the overall crystallization rates follow the expected trend. On the basis of structural and morphological evidences we suggest that the overall structuring kinetics of the homopolymer is slowed down by the concomitant formation of mesophase and monoclinic structures. This effect is absent in the copolymers because the branched counits retard the development of mesophase.     

Crystallization behaviour of fractions of isotactic polypropylene with different degrees of stereoregularity

The influence of the degree of stereoregularity on the overall rate of crystallization and on the melting behaviour of fractions of isotactic polypropylene has been investigated. Fractions with different stereoregularity were obtained by extraction methods. The stereochemical pentad population was determined by ¹³C n.m.r. analysis. The results show that at constant T_c the overall rate constant of crystallization decreases with the increase of configurational chain defects whereas at a given ΔT the contrary is observed. An evident phenomenon of secondary crystallization is observed only in fractions of IPP with a higher concentration of defects. Such observation may explain the double peaked endotherm of fusion of more defective IPP fractions. The free energy of formation of a nucleus of critical dimensions has been calculated for each fraction and its value correlated with the degree of stereoregularity.     

The effect of the structural order of isotactic polypropylene containing magnetically aligned nickel particles on its electrical resistivity

The effect of the high order structure of an isotactic polypropylene (PP) composite on the resistivity of composites containing magnetically aligned Ni particles was studied. Only a small amount of particles needed to be added for the composite material to become conducting after heating while in a magnetic field. The Ni columns formed on applying the field were distorted by the formation of large PP spherulites. Changes to the crystallization process due to the addition of a nucleating agent gave rise to changes in the columnar structure, resulting in large changes in the resistivity of the composite material. Controlling the high order structure of the polymer matrix including its morphology is very important in order to be able to control the magnetically aligned Ni structure.     

Structural deformation behavior of isotactic polypropylene with different molecular characteristics during hot drawing process

Structural evolution in hot drawing process of isotactic polypropylene (iPP) films with different molecular weight distribution (MWD) and isotacticity (IT) was investigated by in situ time-resolved measurements of synchrotron-sourced wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Any significant difference was not recognized among different molecular characteristics as for the changes in the WAXD patterns, indicating that the deformation behavior viewed at the crystal lattice scale was almost the same among these samples. On the other hand, the deformation behavior of lamellar stacking structure was found to be significantly dependent on the molecular characteristics of the sample used. For iPP sample with narrower MWD and higher IT, only the lamellar stacking structure with c-axis crystallites oriented along the drawing direction was detected at the deformation stage after necking, but the oriented fibrillar structure was observed in addition to the lamellar stacking structure for the iPP sample with broader MWD and IT distribution. The structural deformation models were presented for both the samples with different molecular characteristics, and these models were reasonably related with the difference in the stress-strain curve.     

Effect of chain disentanglement on melt crystallization behavior of isotactic polypropylene

Isotactic polypropylene (iPP) with “disentangled” chains was generated through crystallization of iPP from its mineral oil solution. TGA test assured complete removal of mineral oil from iPP precipitates. Time sweep rheological measurements showed the modulus build-up with time indicating the formation of “disentangled” chains in iPP after the sample disentanglement treatment. The “disentangled” chains could preserve for a certain time before completely re-entangled during melting. Crystallization kinetics of iPP with “disentangled” chains was studied by using polarized optical microscope. The growth rate of spherulites in “disentangled” iPP was faster than that in the entangled one.     

Morphology, reorganization and stability of mesomorphic nanocrystals in isotactic polypropylene

The morphology and thermodynamic stability of crystals of isotactic polypropylene (iPP) were analyzed as a function of the path of crystallization by atomic force microscopy (AFM) and differential scanning calorimetry (DSC). Samples were melt-crystallized at different rates of cooling using a “controlled rapid cooling technique”, and subsequently annealed at elevated temperature. Mesomorphic equi-axed domains with a size less than 20 nm were obtained by fast cooling from the melt at a rate larger about 100 K s⁻¹. These domains stabilize on heating by growing in chain direction and cross-chain direction, to reach a maximum size of about 40-50 nm at a temperature of 433 K, with the quasi-globular shape preserved. Annealing at 433 K additionally triggers formation of different types of lamellae. It is suggested that these lamellae either develop by coalescence of nodules, or by recrystallization from the melt. The transition from the disordered mesomorphic structure, evident at ambient temperature after fast crystallization, to monoclinic structure on heating at about 340 K occurs at local scale within existing crystals, and cannot be linked to complete melting of mesomorphic domains and recrystallization of the melt. The temperature of melting of initial mesomorphic domains, after reorganization at elevated temperature, is identical to the temperature of melting of rather perfect lamellae, obtained by initial slow melt-crystallization, followed by annealing. The close-to-identical temperatures of melting of these crystals of largely different shapes are confirmed by model calculations, using the Gibbs-Thomson equation. Modeling of the melting temperature reveals that nodular crystals, stabilized by annealing at high temperature, exhibit a similar fold-surface as lamellar crystals.     

Preparation of nanocoatings in the presence of precipitated CaCO3 fillers by UV-curing

Various methods were employed to prepare precipitated calcium carbonate (PCC) particles. Particle size and morphology were dependent on reaction conditions as well as temperature, reagents and stirring, either mechanical or by ultrasound probe sonication. Sonication helped to reduce the size of CaCO₃ particles as well as the precipitation time. Sonication was also used to disperse PCC particles in the UV-curable formulations as well as for preparation of the PCC. Formulations containing precipitated calcium carbonates and ground calcium carbonates in various grades were cured by a Mini UV-Cure device. Mechanical and thermal properties of films were characterized with a Dynamic Mechanical Analyzer (DMA). SEM analysis was also used to determine the shape and the size of the PCCs. Sonication decreased the precipitation time of calcium carbonate at least four times more than that of conventional precipitation procedures and the dispersion of particles in the formulations increased to a great extent.     

Influence of structural organization on tensile properties in mesomorphic isotactic polypropylene

The effects of annealing on the structure and mechanical properties of mesomorphic isotactic polypropylene have been investigated using wide-angle and small-angle X-ray scattering and rheo-optics in addition to tensile tests. Young's modulus of mesomorphic phase was estimated to be 5 GPa using Takayanagi model. The α-crystallitic iPP prepared by annealing the quenched mesomorphic iPP was transparent because of the absence of spherulitic structure. It was found that the mechanical yielding of α-crystallitic iPP is dominated by the plastic flow of crystalline structural units whereas the yield process of α-spherulitic iPP quenched at 80 °C is caused by the fracture or fragmentation of crystalline structural units.     

Ductile-brittle transition controlled by isothermal crystallization of isotactic polypropylene and its blend with poly(ethylene-co-octene)

The correlation between crystalline morphology development and tensile properties of isotactic polypropylene (iPP) and its blend with poly(ethylene-co-octene) (PEOc) was investigated to study the ductile-brittle transition (DBT) in fracture modes. The sample processing strategy and the scientific observations have never been reported previously. The samples were first isothermally crystallized at 130 °C, 123 °C or 115 °C for a wide range of crystallization times, and then quenched to 35 °C for characterization. It was found that the crystallization conditions including crystallization temperature and time governed the crystalline morphology and even the tensile properties of iPP and the iPP/PEOc (80/20) blend. The lower the crystallization temperature, the shorter the crystallization time was needed for the occurrence of DBT, and the sharper the transition would be. The addition of the elastomer component delayed the DBT occurrence for the iPP/PEOc blend in terms of the crystallization time, owing to the fact that the existence of PEOc domains between the iPP lamellar stack regions or at the iPP spherulitic boundaries enhanced the ductility of the blend. The X-ray diffraction results displayed the oriented and destroyed crystalline structure characterizing the ductile fracture, while unoriented structure describing the brittle failure. The DBT is closely related to the crystal perfection, and factors such as the crystallization temperature and time and the compositions have been proven to be significant variables in determining the DBT occurrence.     

From stiff plastic to elastic polypropylene: Polymorphic transformations during plastic deformation of metallocene-made isotactic polypropylene

An analysis of the mechanical properties and polymorphic transformations occurring during plastic deformation of isotactic polypropylene with variable stereoregularity, containing only rr stereo-defects, is presented. Thermoplastic materials showing high stiffness, or high flexibility, or elastic properties can be produced depending on the concentration of defects. Relationships between the different mechanical behavior and the different observed polymorphic transformations occurring during tensile deformation are discussed. The elastic behavior of the poorly isotactic samples and the values of the corresponding mechanical parameters are related to the structural transformations occurring during stretching. The γ-form present in the unstretched sample transforms by stretching into the α-form, which in turn, transforms into the mesomorphic form at very high deformations. The mesomorphic form transforms back into the crystalline α-form upon releasing the tension and elastic recovery is observed. The crystallization of the mesomorphic form into the α-form upon releasing the tension is not observed in the case of flexible or stiff-plastic samples, which do not show elastic behavior. This indicates that in the elastomeric samples elasticity is probably partially due to the enthalpic contribution associated with the crystallization of the mesomorphic form into the α-form.     

The effect of annealing on the time-dependent behavior of isotactic polypropylene at finite strains

Four series of tensile relaxation tests are performed on isotactic polypropylene at elongations up to the necking point. In the first series of experiments, injection-molded samples are used without thermal pre-treatment. In the other series, the specimens are annealed for 24 h prior to testing at 110, 120 and 130 °C, respectively. Results of mechanical experiments are compared with DSC measurements.A constitutive model is derived for the time-dependent response of semicrystalline polymers at finite strains. A polymer is treated as an equivalent temporary network of macromolecules bridged by junctions (physical cross-links, entanglements and crystalline lamellae). At random times chains separate from their junctions and merge with new ones (the viscoelastic response), whereas junctions slip with respect to their positions in the bulk material (the viscoplastic behavior). The network is thought of as an ensemble of active meso-regions with various potential energies for detachment of chains from temporary nodes and passive meso-domains, where separation of chains is prevented by surrounding radial and tangential lamellae.Experimental data demonstrate that the content of active meso-domains increases with elongation ratio driven by the release of constrained amorphous phase induced by fragmentation of lamellae. In the sub-critical region of deformation (relatively small strains), the growth of the concentration of active meso-regions is associated with breakage of subsidiary (thin) lamellae developed at annealing. In the post-critical region (large strains), an increase in the fraction of active amorphous domains is attributed to disintegration of primary (thick) lamellae.     

Investigation of induction period and morphology of CaCO3 fouling on heated surface

The accumulation of unwanted crystalline deposits (fouling) reduces the efficiency of heat exchanger considerably. In order to mitigate fouling, many measures have been taken including the use of low-energy surface and antifoulant. In this investigation, the CaCO₃ fouling experiments in both cooling water and pool-boiling systems were performed, the induction period as well as the removal of fouling was studied, and the fouling morphology was also investigated by scanning electron microscopy and atomic force microscopy (AFM). Compared with the copper surface, the self-assembled monolayers low-energy surface can prolong the induction period of fouling in the cooling water system. The induction period increases with decreasing initial surface temperature and fluid velocity. When the heat flux is fixed in different experiments, an increase in the fluid velocity will result in a decrease in the initial surface temperature. Under this condition, owing to the interactional effects between surface temperature and fluid velocity, the induction period increases with increasing fluid velocity. The removal experiments were carried out both in the induction period and in the post-induction period. The results show that only in the induction period can the fouling resistance be reduced owing to the weaker adhesion strength of fouling. In the presence of antifoulant polyacrylic acid (PAA), the crystal forms are changed and the fractal dimensions of CaCO₃ morphologies increase for both the cooling water and the pool-boiling systems. AFM images show that the steps are bunched for CaCO₃ formed in the pool-boiling system, and in the presence of PAA, the step spacing is widened compared to the case in the absence of PAA.     

Shish-kebab-like cylindrulite structures resulted from periodical shear-induced crystallization of isotactic polypropylene

In this study, isotactic polypropylene (iPP) samples were prepared by conventional injection molding (CIM) and pressure vibration injection molding (PVIM), in which a periodical shear field was imposed on the iPP melt during the cooling solidification. The distribution of supermolecular structures of samples was investigated by Polarized Light Microscopy (PLM) and Scanning Electron Microscopy (SEM). Results show that the through-the thickness-morphology of sample prepared by CIM features a typical skin-core structure, as a result of general shear-induced crystallization. This structure can be divided into three layers, including a skin layer in which the shish-kebab structure was found, a transition region with deformed spherulite structure and a core layer with spherulitic structure. However, the morphology of the sample prepared by PVIM, as a result of periodical shear-induced crystallization, features a richer and fascinating supermolecular structure and can not be roughly divided into three layers. A region full of shish-kebab-like cylindrulite structures was found between the transition region and the core layer, which is rare to be seen in conventional injection molding. Based on their various core structures, two kinds of shish-kebab-like cylindrulites were defined: one is multi-fibril-core cylindrulite of which core is an assembly of multiple fibrils, and the other is single-fibril-core cylindrulite of which the core just contains a single fibril. Based on the investigated results, a schematic illustration is proposed to depict the through-the thickness-distribution of supermolecular structure of iPP sample prepared by PVIM. The mechanism of the formation of the two kinds of shish-kebab-like cylindrulite structures is also depicted by a schematic illustration, and it was discussed in terms of periodical shear-induced crystallization.     

CaCO3粒子及其增韧母料对聚丙烯材料力学性能的影响

采用扫描电镜、红外光谱和材料力学性能试验等方法研究了表面处理剂品种、CaCO3粒径及含量对均聚PP／CaCO3、共聚PP／CaCO3和PP／弹性体／CaCO3共混材料力学性能的影响。结果表明，采用平均粒径在1．0μm左右并经端噁唑啉聚醚、烷氧焦磷酸酯型钛酸酯等复合处理的CaCO3或其增韧母料增韧PP，可使均聚PP、共聚PP的缺口冲击强度提高至2．5～3倍，弯曲模量提高至1．2～1．4倍，可以在较少的弹性体用量条件下较大幅度地提高PP/弹性体共混材料的缺口冲击强度，并同时保持较高的弯曲模量和熔体流动性。红外光谱和扫描电镜分析表明，端噁唑啉聚醚等复合偶联助剂在CaCO3表面的作用及其弹性包覆层的形成是加强界面相的粘结、缓冲基体成型收缩应力、减少界面处微裂缝形成和提高材料缺口冲击强度的关键。