Non-isothermal crystallization of UHMWPE filled during polymerization with inorganic fillers

The non-isothermal crystallization of UHMWPE, filled with different inorganic fillers during the polymerization on the catalyst system TiCl₄/(C₂H₅)₂AlCl/(C₆H₅)₂Mg has been studied by DSC and polarization microscopy. The melting conditions of UHMWPE have been established before the crystallization experiments. It is shown that the fillers act as nucleating agents only when the crystallization is carried out from a melt, obtained at temperature above the flow temperature of UHMWPE, and at slow cooling rate. It was established that the efficiency of nucleation passes through maximum at 50 wt.% of the filler. It is assumed that this is due to the reduction of the heterogeneous sites, because of the aggregation of the filler with high concentration. The non-isothermal crystallization kinetics is studied by Harnisch and Muschik method. The Avrami exponents do not change in the presence of a filler with concentration up to 90 wt.%.     

Characterization of Polyethylene Nascent Powders Synthesized with TpTiCl2(OR) Catalysts

Summary: Different kinds of polyethylene and ethylene-1-hexene copolymers were synthesized with TpTiCl₂(OR) (Tp = hydrotris(pyrazolyl)borate; R = Et, i-Pr, n-Bu) catalysts with and without H₂. The polymers were characterized by ¹³C NMR, capillary viscosimetry or GPC, and DSC. The homopolymers showed properties characteristic of ultra-high molecular weight polyethylenes (UHMWPE) with linear structure and high density polyethylenes (HDPE) with molecular weights in the range of commercial grades under hydrogen atmosphere. The copolymers showed a 1-hexene incorporation up to 6 mol-%. Important differences in the thermal properties were observed between the first DSC (nascent powders) and the second DSC heatings (melt-crystallized samples), which evidenced the molecular weights influence on the melt-crystallized samples.     

Non-isothermal crystallization kinetics of UHMWPE composites filled by oligomer-modified CaCO3

The processability of ultrahigh molecular weight polyethylene (UHMWPE) improved by oligomer-modified calcium carbonate (CaCO₃) was observed in our previous work. In order to understand the effect of oligomer-modified CaCO₃ on the crystallization of UHMWPE, the non-isothermal crystallization behavior and crystallization kinetics of UHMWPE composites filled by oligomer-modified CaCO₃ was studied by differential scanning calorimetry in this work. Jeziorny and Mo methods were used to describe the non-isothermal crystallization kinetics of UHMWPE composites. The effect of modified filler content and cooling rate on the crystallization temperature and crystallization rate was discussed. The heterogeneous nucleation of modified CaCO₃ slightly increases the crystallization temperature of UHMWPE. The crystallization enthalpy of UHMWPE composites is significantly higher than that of UHMWPE. The crystallization rate of UHMWPE composites depends on the filler contents and cooling rate.

     

Effect of Surface Treatment of Titanium Dioxide Nanoparticles on Non-Isothermal Crystallization Behavior,Viscoelastic Transitions and Cold Crystallization of Poly(Ethylene Terephthalate) Nanocomposites

The effect of untreated and tri-n-octylphosphine oxide (TOPO) surface-treated TiO₂ nanoparticles when included as filler in poly(ethylene terephthalate) on its compatibility, non-isothermal crystallization behavior, viscoelastic transitions and cold crystallization has been studied. The effectiveness of the surface treatment has been studied using infrared spectrophotometry (FTIR) and thermogravimetric analysis (TGA). The effect of the untreated and surface-treated nanofiller content in the polymer, added by an extrusion process, on the non-isothermal crystallization has been studied by differential scanning calorimetry (DSC). The influence on the viscoelastic transitions and cold crystallization of PET nanocomposites has been studied through thermomechanical analysis (TMA). The surface treatment and the concentration of nanofiller influence the non-isothermal crystallization behavior, the viscoelastic transitions and the cold crystallization of the PET nanocomposites, enables us to evaluate the compatibility and the level of dispersion/aggregation of the nanofiller in the poly(ethylene terephthalate).     

Isothermal crystallization kinetics and melting behaviour of PET/ATO nanocomposites prepared by in situ polymerization

Neat poly(ethylene terephthalate) (PET) and PET/antimony doped tin oxide (ATO) nanocomposites were prepared by in situ polymerization. The study of the isothermal crystallization behaviors of neat PET and PET/ATO nanocomposites was carried out using differential scanning calorimetry (DSC). The crystallization kinetics under isothermal conditions could be described by the Avrami equation. For neat PET and PET/ATO nanocomposites, the Avrami exponent n both decreased with increasing crystallization temperature. In addition, for the same crystallization temperature, the value of n increased with increasing ATO content. These suggested that the crystallization types related to the values of n in the Avrami theory could not be suitable for the crystallization of PET and its nanocomposites. The change of the n values indicated that the addition of ATO resulted in the increase of the crystallizing growth points. That is a heterogeneous nucleating effect of ATO on crystallization of PET. In the DSC scan after isothermal crystallization process, multiple melting behavior was found. And the multiple endotherms could be attributed to melting of the recrystallized materials or the secondary lamellae produced during different crystallization processes. The equilibrium melting temperature of PET in the nanocomposites increased with increasing the ATO content. Surface free energy of PET chain folding for crystallization of PET/ATO nanocomposites was lower than that of neat PET, confirming the heterogeneous nucleation effect of ATO.     

Investigation of structural and thermal characteristics of PVDF/ZnO nanocomposites

The structural and thermal behavior of PVDF/ZnO nanocomposites have been investigated by employing scanning electron microscopy (SEM),TEM, DSC, powder X-ray diffraction (XRD), thermally stimulated discharge current (TSDC), and transient current techniques. SEM/TEM observation indicated the homogeneous dispersion of functionalized ZnO nanoparticles throughout PVDF matrix. DSC shows that the crystallinity is influenced by the presence of ZnO nanoparticles in the PVDF matrix because the filler acts as efficient nucleating agent to facilitate PVDF crystallization. DSC results indicated the enhancement of the glass transition temperature (T _g), melting temperature (T _m) and crystallization temperature (T _c) of nanocomposites compared to pristine PVDF. XRD shows that the full-width at half maximum decreases with increasing ZnO content, which is attributed to the improvement in crystallinity. The incorporation of ZnO nanoparticles influences the modification of polarization process in PVDF as observed by means of TSDC and transient current study.     

The Influence of Bridge Type on the Activity of Supported Metallocene Catalysts in Ethylene Polymerization简

Ethylene polymerization was carried out by immobilization of rac-ethylenebis(1-indenyl)zirconium dichloride(Et(Ind)2 ZrCl2) and rac-dimethylsilylbis(1-indenyl)zirconium dichloride(Me2 Si(Ind)2 ZrCl2) preactivated with methylaluminoxane(MAO) on calcinated silica at different temperatures. Polymerizations of ethylene were conducted at different temperatures to find the optimized polymerization temperature for maximum activity of the catalyst. The Me2 Si bridge catalyst showed higher activity at the lower polymerization temperature compared to the Et bridge catalyst. The highest catalytic activities were obtained at temperatures about 50 °C and 70 °C for Me2 Si(Ind)2 ZrCl2 /MAO and Et(Ind)2 ZrCl2 /MAO catalysts systems, respectively. Inductively coupled plasma-atomic emission spectroscopy results and polymerization activity results confirmed that the best temperature for calcinating silica was about 450 °C for both catalysts systems. The melting points of the produced polyethylene were about 130 °C, which could be attributed to the linear structure of HDPE.     

Thermal Degradation Behavior and Kinetic Analysis of Ultra High Molecular Weight Polyethylene Based Multi-Walled Carbon Nanotube Nanocomposites Prepared Via in-situ Polymerization

Thermal degradation behavior of multi-wall carbon nanotubes (MWCNTs)/ultra high molecular weight polyethylene (UHMWPE) nanocomposites, with different nanotubes contents (0.5, 1.5 and 3.5 wt%) prepared via in-situ polymerization technique have been investigated using thermal gravimetric analysis (TGA). TGA spectra revealed that these nanocomposites had enhanced thermal stability and no significant mass loss (<0.4 wt%) occurred up to 350°C. Thermal degradation of these UHMWPE/MWCNT nanocomposites was investigated in terms of parameters such as the onset temperature of degradation (T₁₀), the decomposition temperature at 50% wt loss (T₅₀), the degradation temperature of maximum rate of the weight loss (T_m), and the residual yields (W_R) from TGA. The degradation activation energies (E) of virgin UHMWPE and its nanocomposites were estimated using the Friedman, the Ozawa, Flynn, and Wall (OFW), and the Kissinger's methods. Results indicated that the degradation activation energy for the virgin UHMWPE was 281.3 kJ/mol. The activation energy increased with increasing nanotube loading up to 1.5 wt% indicating that MWCNTs had a stabilizing effect on the degradation of the matrix. However, loadings of 3.5 wt% of nanotube or more could slightly decrease the activation energy. The decrease in the activation energy for degradation of nanocomposites with higher MWCNT concentrations might be attributed to the catalytic effects of nanotubes and polymerization catalyst residues. The “model fitting” method indicated a mechanism of n ^th-order auto-catalysis from the form of the conversion curves for UHMWPE/MWCNTs nanocomposites prepared via in-situ polymerization.     

Polyethylene/graphite nanocomposites obtained by in situ polymerization

The synthesis of polyethylene/graphite nanocomposites by in situ polymerization was achieved using the catalytic system Cp₂ZrCl₂ (bis(cyclopentadienyl)zirconium(IV) dichloride)/methylaluminoxane (MAO). Graphite with nano dimensions, previously treated with MAO, was added into the reactor as filler at percentages of 1, 2, and 5% (w/w). XRD analysis showed that the chemical and thermal treatments employed preserve the structure of the graphite sheets. The formation of graphite nanosheets and nanocomposites was confirmed by TEM and AFM. TEM micrographics showed that the polyethylene grew between the graphene nanosheets, giving intercalated and exfoliated graphite nanocomposites. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 692–698, 2010     

A novel method for preparing poly(ethylene terephthalate)/BaSO4 nanocomposites

We developed a novel method for preparing poly(ethylene terephthalate)/BaSO₄ nanocomposites, which were synthesized by in situ polymerization of terephthalic acid (TPA), ethylene glycol (EG) and BaSO₄ nanoparticles prepared by reacting H₂SO₄ with Ba(OH)₂ in ethylene glycol (EG). It was shown that the addition of BaSO₄ would not influence the synthesis of PET. The structure of the nanocomposites was characterized by transmission electron microscopy (TEM), and the nanoscale dispersion of BaSO₄ particles in the PET matrix was observed when the BaSO₄ content is below 4 wt%. Moreover, the thermal properties of the nanocomposites were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The results suggest that the degree of dispersion of BaSO₄ particles in the PET matrix has important effect on the thermal properties of the nanocomposites. The existence of BaSO₄ nanoparticles enhances the crystallization rate of PET. Besides, it was found that the thermal stability of PET was improved by the addition of the BaSO₄ nanoparticles.     

Ethylene polymerization using novel titanium catalytic precursors bearing N,N‐dialkylcarbamato ligands

Polymerization catalysts based on N,N‐dialkylcarbamato complexes of titanium(IV) appear particularly interesting, because these novel catalytic precursors are rather cheap and easy to synthesize and handle. This contribution reports ethylene polymerization behavior of titanium(IV) complexes of general formula Ti(O₂CNR₂)₄ R = Me ( I ) and Et ( II ) and TiCl₂(O₂CNMe₂)₂ ( III ). These precursors in conjunction with methylaluminoxane resulted active catalysts for the polymerization of ethylene, affording high‐density polyethylene with limited branch content. The influence of the polymerization parameters was studied with particular reference to the type of catalyst components, solvent, temperature, monomer concentration, and Al/Ti ratio. The nature of the solvent appears crucial for catalytic performances: when toluene was replaced by chlorobenzene, a significant increase of the productivity was ascertained. The obtained polymers were characterized by DSC, size exclusion chromatography, FTIR, and NMR techniques. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Ethylene Polymerization by Metallocene Catalysts Supported over Siliceous Materials with Bimodal Pore Size Distribution

Summary: As known, the pore structure of the catalytic support plays a decisive role during the polymerization reactions determining intra-particle mass and heat transport phenomena. In this work, several ethylene polymerizations have been carried out by using as catalytic support different mesostructured materials with unimodal and bimodal pore size distributions in order to evaluate the influence of this pore size distribution on the catalytic behavior. Calcined mesoporous materials were impregnated with the catalytic system MAO/(nBuCp)₂ZrCl₂ and used for ethylene polymerization and ethylene/1-hexene copolymerizations, at 70 °C and 5 bar of ethylene pressure. Polyethylenes obtained were characterized by GPC, DSC and Crystaf. Results indicate that porous structure of the support has a significant influence on polymerization activity and polymer properties. Despite the catalyst bimodal pore size distribution, only ethylene/1-hexene copolymers presented a bimodal chemical composition distribution.     

Thermal properties of poly(lactic acid)/organo-montmorillonite nanocomposites

Poly(lactic acid)/organo-montmorillonite nanocomposites were prepared by melt intercalation technique. Maleic anhydride-grafted ethylene propylene rubber (EPMgMA) was added into the PLA/OMMT in order to improve the compatibility and toughness of the nanocomposites. The samples were prepared by single screw extrusion followed by compression molding. The effect of OMMT and EPMgMA on the thermal properties of PLA was studied. The thermal properties of the PLA/OMMT nanocomposites have been investigated by using differential scanning calorimeter (DSC) and thermo-gravimetry analyzer (TG). The melting temperature (T _m), glass transition temperature (T _g), crystallization temperature (T _c), degree of crystallinity (χ_c), and thermal stability of the PLA/OMMT nanocomposites have been studied. It was found that the thermal properties of PLA were greatly influenced by the addition of OMMT and EPMgMA.     

Thermal behaviors of poly(ethylene terephthalate)/SiO2 nanocomposites prepared by cryomilling

The thermal behaviors of poly (ethylene terephthalate) (PET)/SiO₂ nanocomposites prepared by cryomilling were studied, by comparing with the cryomilled PET and the raw PET. Cryomilling induced amorphization of crystalline PET matrix and the decrease of PET molecular weight. Cryomilled PET/SiO₂ nanocomposites have two abnormal phenomena in the DSC 1st heating thermogram due to the stored energy induced by cryomilling. During DSC cooling process, the dispersed nanometer SiO₂ particles play a role of nucleating agent in PET matrix, and the hot crystallization ability of PET/SiO₂ nanocomposites improves a lot. Besides, the heat stability of cryomilled PET/SiO₂ nanocomposites improves more much during reheating. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1351–1356, 2006     

Surface modification of multiwalled carbon nanotubes with biocompatible polymers via ring opening and living anionic surface initiated polymerization. Kinetics and crystallization behavior

Multiwalled carbon nanotubes (MWNTs) were functionalized with 2‐hydroxyethyl benzocyclobutene (BCB‐EO) through a Diels–Alder cycloaddition reaction. The functionalized MWNTs were utilized for the surface initiated ring opening (ROP) catalyzed and anionic polymerization of ε‐caprolactone (ε‐CL) and ethylene oxide (EO), respectively. The kinetics of the ROP of ε‐CL was monitored through thermogravimetric analysis (TGA) which revealed that the polymerization proceeds very fast as compared to that of EO and that both polymerizations could be controlled with time. ¹H NMR, Raman and FTIR spectroscopy, TGA, DSC, and transmission electron microscopy (TEM) were employed for the characterization of these polymer/CNT hybrids. DSC results showed that a remarkable nucleation effect is produced by MWNTs that reduced the supercooling needed for crystallization of both PεCL and PEO. Furthermore, the isothermal crystallization kinetics of the grafted PεCL and PEO was substantially accelerated compared to the neat polymers. The strong impact on the nucleation and crystallization kinetics is attributed to the covalent MWNT‐polymer bonding. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4379–4390, 2009     

Dynamics and Crystallization in Polydimethylsiloxane Nanocomposites

Several routes were used to achieve silicon nanocomposites. The first and second one are the melt intercalation of polydimethylsiloxane (PDMS), which is a mechanical blending of the polymer in the molten state with the untreated inorganic filler or intercalated nanoparticles. The last one is an in situ polymerization, which previously requires the intercalation of hexamethylcyclotrisiloxane (D₃) followed by a subsequent polymerization step. We used synthetic mineral oxide HTiNbO₅ as nanofiller. These systems were investigated by differential scanning calorimetry (DSC) and solid state NMR in order to better understand the relation between the nanocomposites dynamics, and crystallisation. The efficiency of grafting reactions was studied by ²⁹Si CP/MAS NMR. The nature of the interfacial interactions seems to play the major role. Indeed, the nanocomposites 1 and 2 for which only physical interactions are expected do not exhibit any T_g deviation whereas the nanocomposite 3, for which chemical grafting is achieved, increases strongly the T_g. Crystallization is more sensitive to density and strength of interfacial interactions which are maximum for the pristine filler.     

Novel antimicrobial polyethylene composites prepared by metallocenic in situ polymerization with TiO2‐based nanoparticles

Polyethylene (PE) composites with titanium oxide (TiO₂) nanoparticles were produced via in situ polymerization representing a novel route to obtain antimicrobial polymeric materials. The TiO₂ nanoparticles synthesized by the sol–gel method were used either as‐synthesized or modified organically with hexadecyltrimethoxysilane (Mod‐TiO₂). These particles were added, together with the catalytic system (formed by a metallocenic catalyst and methylaluminoxane as cocatalyst), directly to the reactor, yielding in situ PE composites with 2 and 8 wt % content of nanofiller. The catalytic polymerization activity presented a slight decrease with the incorporation of the TiO₂ and Mod‐TiO₂ nanoparticles compared to polymerization without filler. Regarding the properties of the composites, crystallinity increased slightly when the different nanofillers were added, and the elastic modulus increased around 15% compared to neat PE. PE/TiO₂ nanocomposites containing 8 wt % of TiO₂ exposed to UVA irradiations presented antimicrobial activity against Escherichia coli. The PE/Mod‐TiO₂ nanocomposite with 8 wt % filler killed 99.99% of E. coli, regardless of light and time irradiation. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Tensile modulus of polymer/CNT nanocomposites by effective volume fraction of nanoparticles as a function of CNT properties in the network

In this article, the effects of filler network and interphase between polymer matrix and nanoparticles on the tensile modulus of polymer/carbon nanotubes (CNT) nanocomposites are assumed by the effective volume fraction of nanoparticles. By this approach, the Takayanagi model is developed for polymer/CNT nanocomposites above percolation threshold. Also, the effective factors for filler network including the number (N ), aspect ratio (α ) and percolation threshold (? _p ) of CNT are correlated to three main parameters. The developed model is evaluated for some reported samples from previous papers, and the influences of main parameters on the modulus are examined. The acceptable predictability of the developed model for modulus of nanocomposites is illustrated by experimental results. The “α ” and “N ” parameters play positive roles in the modulus, while an inverse relation is observed between the modulus and the percolation threshold. The reasonable effects of these parameters on the tensile modulus of polymer/CNT nanocomposites are also discussed. Copyright © 2017 John Wiley & Sons, Ltd.     

High polymerization of methyl methacrylate with organonickel/MMAO systems

A series of nickel complexes, including Ni(acac)₂, (C₅H₅)Ni(η³‐allyl), and [NiMe₄Li₂(THF)₂]₂, that were activated with modified methylaluminoxane (MMAO) exhibited high catalytic activity for the polymerization of methyl methacrylate (MMA) but showed no catalytic activity for the polymerization of ethylene and 1‐olefins. The resulting polymers exhibited rather broad molecular weight distributions and low syndiotacticities. In contrast to these initiators, the metallocene complexes (C₅H₅)₂Ni, (C₅Me₅)₂Ni, (Ind)₂Ni, and (Me₃SiC₅H₄)₂Ni provided narrower molecular weight distributions at 60 °C when these initiator were activated with MMAO. Half‐metallocene complexes such as (C₅H₅)NiCl(PPh₃), (C₅Me₅)NiCl(PPh₃), and (Ind)NiCl(PPh₃) produced poly(methyl methacrylate) (PMMA) with much narrower molecular weight distributions when the polymerization was carried out at 0 °C. Ni[1,3‐(CF₃)₂‐acac]₂ generated PMMA with high syndiotacticity. The NiR(acac)(PPh₃) complexes (R = Me or Et) revealed high selectivity in the polymerization of isoprene that produced 1,2‐/3,4‐polymer at 0 °C exclusively, whereas the polymerization at 60 °C resulted in the formation of cis‐1,4‐rich polymers. The polymerization of ethylene with Ni(1,3‐tBu₂‐acac)₂ and Ni[1,3‐(CF₃)₂‐acac]₂ generated oligo‐ethylene with moderate catalytic activity, whereas the reaction of ethylene with Ni(acac)₂/MMAO produced high molecular weight polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4764–4775, 2000     

Carbon nanoparticles as effective nucleating agents for polypropylene

The effect of four nucleating agents on the crystallization of isotactic polypropylene (iPP) was studied by differential scanning calorimetry (DSC) under isothermal and non-isothermal conditions. The nucleating agents are: carbon nanofibers (CNF), carbon nanotubes (CNT), lithium benzoate and dimethyl-benzylidene sorbitol. Avrami?s model is used to analyze the isothermal crystallization kinetics of iPP. Based on the increase in crystallization temperature (T _c) and the decrease in half-life time (τ_½) for crystallization, the most efficient nucleating agents are the CNF and CNT, at concentrations as low as 0.001 mass%. Sorbitol and lithium benzoate show to be less efficient, while the sorbitol needs to be present at concentrations above 0.05 mass% to even act as nucleating agent.