In situ synthesis of calcium pimelate as a highly dispersed β-nucleating agent for improving the crystallization behavior and mechanical properties of isotactic polypropylene

The reaction precursors pimelic acid (Pi) and calcium stearate (CaSt) were added in situ during extrusion of isotactic polypropylene (iPP) to generate self-dispersed calcium pimelate (CaPi[IS]). The results reveal that the nucleating agents (NAs) synthesized in situ obviously affected the nucleation effect in iPP. The crystallization, mechanical properties and melting behavior of nucleated iPP were investigated. The crystallization temperature (T_c) of iPP modified with CaPi(IS) increased by 4–5°C when compared to that of pure iPP. Especially, when the addition amount of CaPi(IS) in iPP was 0.30 wt%, the relative β-crystal concentration of iPP/CaPi(IS) reached the highest level of 96.47%, 22.71% higher than that of iPP/CaPi. However, Pi and CaSt has hardly impact on the nucleation effect in iPP. The mechanical properties of iPP show that CaPi and CaPi(IS) have excellent toughening effect on iPP while Pi and CaSt greatly improved the stiffness of iPP. Furthermore, the dispersion of these NAs in iPP was investigated by scanning electronic microscope (SEM). It can be clearly seen that the CaPi(IS) particles are uniformly distributed in the iPP after magnification.     

Facile Synthesis and Chiral Resolution of Expanded Helicenes with up to 35 cata-Fused Benzene Rings**

Expanded helicenes are expected to show enhanced chiroptical properties as compared to the classical helicenes but the synthesis is very challenging. Herein, we report the facile synthesis of a series of expanded helicenes Hn (n=1–4) containing 11, 19, 27 and 35 cata-fused benzene rings through Suzuki coupling-based oligomerization followed by Bi(OTf)₃-mediated regioselective cyclization of vinyl ethers. Their structures were determined by X-ray crystallographic analysis. Enantiopure H2 , H3 , and H4 can be isolated by chiral HPLC and they all exhibit strong chiroptical responses with high absorption dissymmetry factor (|g_abs|) values (0.020 for H2 , 0.021 for H3 , and 0.021–0.024 for H4 ).     

Assessment of Compatibility in Polypropylene/Poly(lactic acid)/Ethylene Vinyl Alcohol Ternary Blends: Relating Experiments and Molecular Dynamics Simulation Results

Two systems of polypropylene (PP), poly(lactic acid) (PLA) and ethylene vinyl alcohol copolymer (EVOH) ternary blends having different compositions were extruded in a co-rotating twin screw extruder. The first system was PP/PLA (75/25) with various EVOH contents, the second one was PP/EVOH (75/25) having various PLA contents. The effects of composition on the morphology and the tensile and impact properties of the blends were investigated. There were increases in the tensile modulus and tensile strength with an increase in the EVOH and PLA contents in the first and second systems, respectively. A molecular dynamics (MD) simulation was used to investigate the compatibility between the components. Prediction of the miscibility of the blends was carried out by determining the interaction parameters (χ), mixing energies (ΔH_mix), phase diagrams and Gibbs free energies. The MD simulation showed a UCST behavior for the components. Moreover, the simulation results showed a compatibilizer effect for the EVOH component. The experimental values of the dynamic mechanical thermal analysis (DMTA) and mechanical properties were correlated to the MD results. There was a good correlation between the MD and DMTA results. The modulus values using the parallel and Davis models were near to the experimental ones. A good fitting to the mixture law with addition of EVOH confirmed a good compatibilzing effect of it between the PP and PLA components.     

Relationship between Peroxide Initiators and Properties of Styrene Grafted Polypropylene via Reactive Extrusion

To understand the relationship between the initiators and the properties of grafted polypropylene (PP), and provide guidance for designing polymers with different performance through selecting appropriate initiators, a series of styrene (St) grafted PP was prepared by modifying commercial linear PP via reactive extrusion using two different peroxide initiators, dicumyl peroxide (DCP) and benzoyl peroxide (BPO). Fourier transform infrared spectra indicated that the use of DCP led to a higher St grafting degree compared to the system using BPO. The melt flow index and rheological characteristics suggested the existence of short chain branching (SCB) structures in the St grafted PP using DCP, and long chain branching (LCB) structures in the St grafted PP using BPO. Differential scanning calorimetry and polarized optical microscopy results showed that the degradation of the PP chains and the introduction of SCB structures hindered the crystallization process of the St grafted PP using DCP, and the existence of the LCB structures accelerated the crystallization process of the St grafted PP using BPO. We suggest this research can contribute to the understanding of methods to prepare grafted PP with special properties via reactive extrusion by using proper initiators.     

Isothermal Crystallization of Isotactic Polypropylene Nucleated with a Novel Aromatic Heterocyclic Phosphate Nucleating Agent

The incorporation of a nucleating agent into isotactic polypropylene (iPP) is one of the most important and widely used methods to improve performance in the polypropylene industry. Aromatic heterocyclic phosphate salt is a kind of highly effective nucleating agent for iPP and one of the typical products is a compound nucleating agent based on 2, 2-methylene-bis (4, 6-di-tert-butylphenyl) phosphate hydroxyl aluminum (commercial product name: ADK NA-21). In this paper the isothermal crystallization kinetics of iPP nucleated with the α-nucleating agent NA-21, investigated using differential scanning calorimetry (DSC), is described with the crystallization data being analyzed by using the classic Avrami method. During isothermal crystallization the addition of nucleating agent NA-21 dramatically shortened the crystallization half time (t_1/2) of iPP under the same conditions and the crystallization activation energy, ΔE, decreased from 422 kJ/mol for virgin iPP to 369 kJ/mol with the addition of NA-21. Thus, the addition of NA-21 significantly increased the crystallization rate of iPP.     

Characterization of annealed isotactic polypropylene in the solid state by 2D time‐domain 1H NMR

Two‐dimensional time‐domain ¹H NMR was used to investigate annealed isotactic polypropylene in the solid phase. The spin–lattice relaxation in the laboratory frame and in the rotating frame were correlated with the shape of the free induction decay to identify and characterize relaxation components over the temperature range −120 to 120 °C. Several phase transitions were observed, and three distinct solid phases, with different chain mobilities, were detected. Two of these phases were identified as regions with different mobilities within the crystalline phase. The third phase was characterized by a high degree of isotropy in molecular motion. This phase, identified as the amorphous phase, appeared as the polymer was heated above a low‐temperature (−45 °C) phase transition. All transitions observed at higher temperatures occurred exclusively in this phase. About one‐third of the polymer chains reside between crystalline lamellae, whereas the majority form amorphous regions outside fibrils of multilamellar structure. Furthermore, the glass‐to‐rubber transition, occurring above −15 °C, consists of three stages. During the first stage, between −15 °C and 15 °C, regions with an increased segment mobility (labeled intermediate phase) appear gradually within the amorphous phase. At 15 °C, the intermediate phase consists of ∼10% of the polymer units, or one‐third of the polymer units constituting the amorphous phase. Between 15 °C and 25 °C, the intermediate phase increases rapidly to 18%. This is associated with the appearance of semiliquid and liquid regions, likely within the intermediate phase. Polymer chain segments (and possibly entire chains) involved in the liquidlike phases exhibit heterogeneous molecular motion with a correlation frequency higher than 10⁶ Hz. These two stages of glass‐to‐rubber transition occur within amorphous regions outside multilamellar structures. The third stage of the glass transition, appearing above 70 °C, is associated with the upper glass transition and occurs within the interlamellar amorphous phase. Finally, on a timescale of 100 ms or less, spin diffusion does not couple the amorphous regions outside fibrils with crystalline and amorphous regions within multilamellar fibrils. However, on a timescale of hundreds of milliseconds to seconds, all different regions within isotactic polypropylene are partially coupled. It is proposed that the relative magnitude of the crystalline magnetization, as observed in the T_1ρ experiment, is a good measure of polymer crystallinity. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2487–2506, 2000     

Nucleation and growth of gas barrier aluminium oxide on surfaces of poly(ethylene terephthalate) and polypropylene: effects of the polymer surface properties

The nucleation and initial stages of growth of aluminium oxide deposited on two different polymer surfaces [poly(ethylene terephthalate), (PET) and amorphous polypropylene, (PP)] have been studied by atomic force microscopy (AFM). The permeation of water vapor and oxygen through the films has been measured. The initial stages of the growth of the oxide consisted of separated islands on the polymer surface. Further growth of oxide depends strongly on the surface morphology and chemical nature of the polymer surface. Growth on PET follows a layer‐by‐layer mechanism that maintains the native surface roughness of the polymer substrate. Growth on PP, however, follows an island mode, which leads to an increase in surface roughness. This may be due to a lack of chemical bonding between the polymer and the arriving metal–oxygen particles. The oxide layer on PET grows more densely than on PP, providing superior barrier to gas permeation. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3151–3162, 2000     

Functionalized polystyrene as a versatile support for olefin polymerization catalysts

Heterogeneous polyolefin catalysts based on metallocenium salts of weakly coordinating anions can be prepared via a series of simple reactions from lightly crosslinked chloromethylated polystyrene beads. Catalytic sites are distributed homogeneously throughout the polystyrene particles. The nonporous nature of these catalysts affords a high degree of control over the olefin uptake rate, avoiding problems of premature catalyst fragmentation that often plague high‐surface‐area heterogeneous catalysts based on highly reactive species. The choice of amine as the means of binding or templating allows catalysts based on a wide variety of metallocenes to be readily prepared by the same synthetic approach. The dative interactions between the metallocene cation and the amine functionality of the support material are sufficient to prevent extraction under polymerization conditions to yield excellent particle morphology of the polyolefin product, but they are not so strong as to affect the nature of the polyolefin produced. The polymer‐supported catalysts have been used effectively for the polymerization of ethylene and polypropylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2979–2992, 2000     

Electron spin resonance study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent. V. Comparison with polypropylene and ethylene‐propylene copolymer

We studied the chemical reaction process of polypropylene (PP), ethylene‐propylene copolymer (EPM), and ethylene‐propylene‐diene copolymer (EPDM) crosslinking induced by dicumyl peroxide (DCP) using electron spin resonance (ESR). Free radicals appeared at an elevated temperature of around 120 °C and the behavior and kinetics of the reaction process were observed at 180 °C. The radical species detected in PP were alkyl type radicals, formed by the abstraction of hydrogen atoms from the tertiary carbon of polymer chains. For EPDM containing a diene component, the radicals were trapped at double bonds in this diene component to form allyl radicals. The resolutions of these spectra were extremely clear; hence, isotropic spectra of these polymer radicals were obtained. We measured the ESR at high temperatures and confirmed that the process of crosslinking induced by DCP was a free radical reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3383–3389, 2000