Crystallization of the β-Form of Polypropylene from the Melt with Reduced Entanglement of Macromolecules

The influence of decreasing the entanglement density of macromolecules on the crystallization of the β-form of polypropylene was investigated. Polypropylene with seven times less entanglement was obtained from a solution in xylene, and its properties were compared with those of fully entangled polypropylene. To obtain a high β-phase content, the polymer was nucleated using calcium pimelate. In non-isothermal crystallization studies, accelerated growth of β-crystals was found, increasing the crystallization temperature. Also, the isothermal crystallization was fastest in the nucleated, partially disentangled polypropylene. Increased growth rate of spherulites and enhanced nucleation activity in the presence of more mobile macromolecules were responsible for the high rate of melt conversion to crystals in the disentangled polypropylene. It was also observed that the equilibrium melting temperature of β-crystals is lower after disentangling macromolecules. Better conditions for crystal building after reduction of entanglements resulted in enhanced crystallization according to regime II.

β-Nucleated Polypropylene: Preparation, Nucleating Efficiency, Composite, and Future Prospects

The β-crystals of polypropylene have a metastable crystal form. The formation of β-crystals can improve the toughness and heat resistance of a material. The introduction of a β-nucleating agent, over many other methods, is undoubtedly the most reliable method through which to obtain β-PP. Furthermore, in this study, certain newly developed β-nucleating agents and their compounds in recent years are listed in detail, including the less-mentioned polymer β-nucleating agents and their nucleation characteristics. In addition, the various influencing factors of β-nucleation efficiency, including the polymer matrix and processing conditions, are analyzed in detail and the corresponding improvement measures are summarized. Finally, the composites and synergistic toughening effects are discussed, and three potential future research directions are speculated upon based on previous research.

Competing crystallization of α- and β-phase induced by β-nucleating agents in microdroplets of isotactic polypropylene

Crystallization of heterogeneously nucleated isotactic polypropylene microdroplets in an immiscible polystyrene matrix allows the estimation of intrinsic nucleating efficiency of nucleating agents promoting the formation of different polymorphs.

Crystalline Modification of Isotactic Polypropylene with a Rare Earth Nucleating Agent Based on Ultrasonic Vibration

In this paper, the crystalline modification of isotactic polypropylene (PP) with a rare earth β nucleating agent (WBG) with different ultrasound conditions was investigated by scanning electron microscopy (SEM), wide-angle X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The relationship between the ultrasound conditions and the crystalline structure, as well as the mechanism for the behavior, were revealed. SEM showed that the dispersion of the nucleating agent in the PP matrix was better at shorter ultrasound distances. In addition, the higher the water cooling temperature, the better the nucleating agent was dispersed in the PP matrix. The results of XRD and DSC showed that the crystallinity and the relative content of the β-crystal were increased with nearer ultrasound distance, as well as increased in higher water cooling temperatures. In particular, under the same conditions, the crystallinity and the relative content of the β-crystal after ultrasonic treatment were much higher than those without ultrasound.

Modeling and Simulation of Crystallization of Metal–Organic Frameworks

Metal–organic frameworks (MOFs) are the porous, crystalline structures made of metal–ligands and organic linkers that have applications in gas storage, gas separation, and catalysis. Several experimental and computational tools have been developed over the past decade to investigate the performance of MOFs for such applications. However, the experimental synthesis of MOFs is still empirical and requires trial and error to produce desired structures, which is due to a limited understanding of the mechanism and factors affecting the crystallization of MOFs. Here, we show for the first time a comprehensive kinetic model coupled with population balance model to elucidate the mechanism of MOF synthesis and to estimate size distribution of MOFs growing in a solution of metal–ligand and organic linker. The oligomerization reactions involving metal–ligand and organic linker produce secondary building units (SBUs), which then aggregate slowly to yield MOFs. The formation of secondary building units (SBUs) and their evolution into MOFs are modeled using detailed kinetic rate equations and population balance equations, respectively. The effect of rate constants, aggregation frequency, the concentration of organic linkers, and concurrent crystallization of organic linkers are studied on the dynamics of SBU and MOF formation. The results qualitatively explain the longer timescales involved in the synthesis of MOF. The fundamental insights gained from modeling and simulation analysis can be used to optimize the operating conditions for a higher yield of MOF crystals.

Crystalline modification of a rare earth nucleating agent for isotactic polypropylene based on its self-assembly

In this paper, the crystalline modification of a rare earth nucleating agent (WBG) for isotactic polypropylene (PP) based on its supramolecular self-assembly was investigated by differential scanning calorimetry, wide-angle X-ray diffraction and polarized optical microscopy. In addition, the relationship between the self-assembly structure of the nucleating agent and the crystalline structure, as well as the possible reason for the self-assembly behaviour, was further studied. The structure evolution of WBG showed that the self-assembly structure changed from a needle-like structure to a dendritic structure with increase in the content of WBG. When the content of WBG exceeded a critical value (0.4 wt%), it self-assembled into a strip structure. This revealed that the structure evolution of WBG contributed to the K_β and the crystallization morphology of PP with different content of WBG. In addition, further studies implied that the behaviour of self-assembly was a liquid-solid transformation of WBG, followed by a liquid-liquid phase separation of molten isotactic PP and WBG. The formation of the self-assembly structure was based on the free molecules by hydrogen bond dissociation while being heated, followed by aggregation into another structure by hydrogen bond association while being cooled. Furthermore, self-assembly behaviour depends largely on the interaction between WBG themselves.

Competition between α and β Crystallization in Isotactic Polypropylene: Effect of Nucleating Agents Composition

Based on the different nucleation ability, different contents and proportions of the mixed nucleating agents were melt compounded with pure isotactic polypropylene resins employed in this work, selective β nucleating agent was a compound of pimelic acid and calcium stearate, the α nucleating agent was a type of dibenzylidene sorbitol derivative. The competition attributed to the different type of interactions between a nucleating agent and isotactic polypropylene molecular chains during crystallization was investigated in this research. For a low content of the mixed nucleating agents used, α nucleation dominated the crystallization behavior of isotactic polypropylene, while with an increase of the content of the mixed nucleating agents, the situation was reversed and β nucleation became dominant. In the β dominated region, both of the molecular chain and segmental motions were various with different nucleating agent proportion. Thus, the content and proportion of the components of the nucleating agents were the critical factors in the competition during crystallization.

Crystallization Kinetics for PP/EPDM/Nano-CaCO3 Composites – The Influence of Nanoparticles Distribution

The primary aim of this paper is to provide an insight on the effect of the distribution of calcium carbonate nanoparticles (nano-CaCO3) on the isothermal crystallization kinetics in Polypropylene (PP)/ethylene-propylene-diene terpolymer (EPDM)/nano-CaCO3 composites prepared by different compounding procedures. PP/EPDM/CaCO3 composites were prepared by two compounding procedures (direct compounding: mixed all three solid materials together; multistep compounding: the melted EPDM/CaCO3 master batch in a single screw extruder blended with melted PP in twin-screw extruder via injecting into the twin-screw extruder from a lateral port at the melting section of the twin-screw extruder). Morphological observation showed that abundant CaCO3 particles concentrated around EPDM dispersed phase in the multistep compounding composite, essentially different from the respectively dispersed morphology of CaCO3 particles and EPDM domains in the matrix for the direct compounding composite. Moreover, better dispersion of CaCO3 particles in the multistep compounding composite was observed comparing to the direct compounding composite. Futhermore, a pronounced improvement of the crystallization half time (t1/2), rate of crystallization (G) was achieved in the multistep compounding composite, which may originate from the better dispersion of CaCO3 particles providing larger nucleation density and the collaborative nucleation of EPDM and CaCO3 during the isothermal crystallization.

Theoretical investigation of macrodipoles in supramolecular columnar stackings

Supramolecular columnar assemblies are known to form intrinsic macrodipoles, which play an important role in intercolumnar interactions and govern the self-assembly on the mesoscale. A prominent class that provides this feature are trisamide derivatives, namely, 1,3,5-benzenetrisamides and 1,3,5-cyclohexanetrisamides. The understanding of how subtle changes in the chemical structure influence the columnar order and consequently the macrodipole formation is of fundamental interest. Here we report on the theoretical investigation of trisamide derivatives and how the formed macrodipole is related to the properties of the columnar aggregates. Calculations were carried out on a semiempirical level using the PM6 approximation, which is able to treat weak interactions like hydrogen bonding and dispersion forces with a sufficient accuracy. We have compared the influence of a benzene core with a cyclohexane core on the macrodipole formation. It was revealed that columnar aggregates based on 1,3,5-cyclohexanetrisamides have much higher dipole moments than those formed with aromatic cores. A cooperative effect was found during aggregation, as longer aggregates show stronger hydrogen bonding, thereby facilitating the addition of the next molecule. We have also investigated the influence of the amide connection on the strength of the formed macrodipole. The trends observed for the macrodipole strength correlate with the calculated heat of formation. If the amide groups are inverted, the strength of the macrodipole is reduced and the negative heat of formation is increased. HOMO-LUMO gaps were correlated with the inverse of the dipole moment per monomer unit, thus indicating that the macrodipole might act as a perturbation to the supramolecular assemblies.

Influence of the molecular structure and morphology of self-assembled 1,3,5-benzenetrisamide nanofibers on their mechanical properties

The influence of molecular structure on the mechanical properties of self-assembled 1,3,5-benzenetrisamide nanofibers is investigated. Three compounds with different amide connectivity and different alkyl substituents are compared. All the trisamides form well-defined fibers and exhibit significant differences in diameters of up to one order of magnitude. Using nanomechanical bending experiments, the rigidity of the nanofibers shows a difference of up to three orders of magnitude. Calculation of Young's modulus reveals that these differences are a size effect and that the moduli of all systems are similar and in the lower GPa range. This demonstrates that variation of the molecular structure allows changing of the fibers' morphology, whereas it has a minor influence on their modulus. Consequently, the stiffness of the self-assembled nanofibers can be tuned over a wide range--a crucial property for applications as versatile nano- and micromechanical components.