The influence of molecular weight on the properties of polyacetal/hydroxyapatite nanocomposites. Part 2. In vitro assessment

A series of polyoxymethylene (POM)/hydroxyapatite (HAp) nanocomposites for long-term bone implants were prepared by melt processing using POM with different molecular weight. Bioactivity was examined by incubating the samples in simulated body fluid. Moreover, in vitro stability, wettability, thermal stability, and formaldehyde release process were investigated. Results of in vitro investigations show that an increase in HAp content in POM nanocomposites facilitates the formation of an apatite layer on the sample surface. There is no significant influence of HAp concentration on the contact angle values as revealed by wettability studies. Thermogravimetric analysis results indicate that with an increase in HAp content the thermal stability of POM matrix decreases significantly depending upon the POM molecular weight. Finally, it was found the amount of formaldehyde leached out of the samples during the incubation period depends on the HAp content: amounts in excess of 3 ppm were emitted for the samples containing only 10% HAp.     

Crystallization behavior of polymer/montmorillonite nanocomposites. Part III. Polyamide-6/montmorillonite nanocomposites, influence of matrix molecular weight, and of montmorillonite type and concentration

Several series of polyamide-6 (PA-6) nanocomposites, differing in montmorillonite (MMT) type and content and PA-6 matrix molecular weight, were prepared by melt-extrusion and the associated PA-6 crystallization behavior and morphology was evaluated using (synchrotron) X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The nucleating ability of silicate layers is poor in PA-6 nanocomposites made by melt-extrusion because highly active, stable PA-6 crystallization precursors are generated during melt-extrusion. In most of the studied PA-6/MMT nanocomposites the dispersed silicate layers act as impurities and decrease rather than increase the overall crystallization kinetics of PA-6, especially at high MMT contents. Furthermore, at a given MMT concentration, the crystal growth retardation inflates with increasing degree of exfoliation, which dependents on the MMT type and which increases with increasing PA-6 molecular weight. One of the considered MMT types leads to a poorly exfoliated nanomorphology and as a result no retardation of crystal growth is observed. Furthermore, the disturbed crystal growth does not alter the PA-6 semicrystalline stack morphology. Moderate nucleation effects due to the presence of MMT can be observed when the particle load is low (low amount of MMT and/or poor degree of exfoliation) and provided the supercooling is sufficiently large.     

Interfacial reaction and its influence on phase morphology and impact properties of modified‐polyacetal/thermoplastic polyurethane blends

The cationic polymerization of 1,3,5‐trioxane, 1,3‐dioxolane and a small amount of 2‐hydroxyacetic acid (HAA) was carried out, and the resulting modified‐polyacetal (POM) was blended with thermoplastic polyurethane (TPU) in melt. The results of ¹H NMR analysis indicated that HAA was almost incorporated in the modified‐POM, and that the resulting carboxyl end‐group and hydroxyl end‐group in the modified‐POM reacted with TPU during the melt blending. There were many boundary layers between the cavities and matrix in the modified‐POM/TPU (82/18 by weight) blend that was etched with tetrahydrofuran (THF), and the diameter of the cavities became ～0.3–1 μm long when the blending time reached 10 min. The results of scanning electron microscopic (SEM) observation and dynamic mechanical analysis (DMA) indicated that the modified‐POM/TPU blend had a good compatibility because of the interfacial reaction between the modified‐POM and TPU phase in the blend. The modified‐POM/TPU blend exhibited higher Charpy impact strength when compared with a normal‐POM/TPU blend; the toughness of the modified‐POM/TPU blend attributed to the good compatibility between the two phases. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4375–4382, 2006     

The effect of molecular weight on fatigue crack propagation in nylon 66 and polyacetal

The influence of molecular weight on fatigue and fracture behavior in nylon 66 (N66) and polyacetal (PA) is examined. Fatigue crack propagation (FCP) resistance and apparent fracture toughness (K_cf) in these two semicrystalline polymers increase with increasing molecular weight in a manner consistent with that reported for another semicrystalline polymer (HDPE) as well as for several amorphous polymers. The improved FCP resistance with increasing molecular weight is attributed to the development of a molecular entanglement network that more effectively resists cyclic-load-induced breakdown. A type of discontinous crack growth is identified in PA at 100Hz and in N66 (2.6% H₂O) at 50 Hz and compared with that observed in amorphous polymers.     

TPO based nanocomposites. Part 1. Morphology and mechanical properties

The relationship between morphology and the mechanical properties of thermoplastic olefin (TPO) materials that are reinforced with organoclay fillers and prepared by melt processing is reported. Nanocomposites based on blends of polypropylene and elastomer and using an organoclay masterbatch were prepared in a twin-screw extruder. Transmission electron microscopy, atomic force microscopy and wide-angle X-ray scattering were employed to carry out a detailed particle analysis of the morphology of the dispersed clay and elastomer phases for these nanocomposites. The improvement in mechanical properties, e.g. stiffness enhancement as evaluated by stress-strain analysis and impact strength obtained from notched Izod impact tests, were successfully explained in terms of morphological changes induced by the presence of the clay and elastomer particles. Quantitative analyses of TEM micrographs and AFM images revealed a decrease in the aspect ratio of the clay particles and a reduction in the size of elastomer particles with increasing clay content. In addition, WAXD scans indicated a skin-core effect for the injection molded specimens in terms of both polypropylene crystal orientation and clay filler orientation. This information is essential for the understanding of the mechanism of mechanical property enhancement in nanocomposite materials.     

Polycarbonate nanocomposites. Part 1. Effect of organoclay structure on morphology and properties

Polycarbonate nanocomposites were prepared by melt processing from a series of organoclays based on sodium montmorillonite exchanged with various amine surfactants. To explore the effects of matrix molecular weight on dispersion, an organoclay was melt-mixed with a medium molecular weight polycarbonate (MMW-PC) and a high molecular weight polycarbonate (HMW-PC) using a twin screw extruder. The effects of surfactant chemical structure on the morphology and physical properties were explored for nanocomposites formed from HMW-PC. Wide angle X-ray scattering, transmission electron microscopy, and stress-strain behavior were employed to investigate the nanocomposite morphology and physical properties. The modulus enhancement is greater for nanocomposites formed from HMW-PC than MMW-PC. This trend is attributed to the higher shear stress generated during melt processing. A surfactant having both polyoxyethylene and octadecyl tails shows the most significant improvement in modulus with some of the clay platelets fully exfoliated. However, the nanocomposites formed from a range of other organoclays contained both intercalated tactoids and collapsed clay particles with few, if any, exfoliated platelets.     

Nanomechanical and tribological behavior of hydroxyapatite reinforced ultrahigh molecular weight polyethylene nanocomposites for biomedical applications

A hydroxyapatite (HA) particulate reinforced ultrahigh molecular weight polyethylene (UHMWPE) nanocomposite is fabricated by internal mixer at 180°C and using of paraffin oil as a processing aid to overcome the high viscosity of melted UHMWPE. The reinforcing effects of nano‐HA are investigated on nanomechanical properties of HA/UHMWPE nanocomposites by nanoindentation and nanoscratching methods. Results show that the nanocomposite with 50 wt % nano‐HA exhibits a Young's modulus and hardness of 362.5% and 200% higher, and a friction coefficient of 38.86% lower than that of pure UHMWPE, respectively. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42052.     

P(VDF-TrFE)-layered silicate nanocomposites. Part 1. X-ray scattering and thermal analysis studies

X-ray scattering and thermal analyses were used to investigate the effects of organically modified layered silicates (OMS) on the paraelectric and ferroelectric phase transitions in poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)]/OMS nanocomposites. Nanocomposites comprising a 75/25 P(VDF-TrFE) random co-polymer with either Nanomer I.30TC or Lucentite STN OMS were prepared with compositions ranging from 2 to 25 wt% OMS. Wide-angle X-ray scattering (WAXS) studies show that the silicate gallery spacing increases modestly in the nanocomposites compared to the neat OMS powder, indicating a level of co-polymer intercalation. Thermogravimetric analysis indicates that thermal stability is improved in nanocomposites with higher OMS contents: they have substantial increase in weight remaining, both at 500 °C and at 1000 °C, compared to that predicted from the behavior of the neat co-polymer and OMS. Differential scanning calorimetry (DSC) and WAXS results show that thermal transitions in the nanocomposites depend on OMS content. Nanocomposites with 2% OMS exhibited a crystal nucleating effect, which results in significant increase in the amount of ferroelectric crystals formed during cooling. For greater OMS additions (10-25%), the amounts of para- and ferroelectric crystals are reduced. The larger OMS additions depress the melt-to-paraelectric transition temperature, while an increase in the paraelectric-to-ferroelectric transition temperature is observed for all compositions. Upon reheating, the ferroelectric phase transition shows significant hysteresis. We conclude that the addition of either Lucentite or Nanomer OMS to 75/25 P(VDF-TrFE) causes an increase in the temperature stability range for the ferroelectric phase.     

Surface modification of MWCNT and its influence on properties of paraffin/MWCNT nanocomposites as phase change material

Multiwalled carbon nanotubes (MWCNTs) were modified by an organo-silane in order to improve their dispersion state and stability in paraffin wax. A family of paraffin-based phase change material (PCM) composites filled with MWCNTs was prepared with different loadings (0, 0.1, 0.5, and 1 wt%) of pristine MWCNTs and organo-silane modified MWCNTs (Si-MWCNT). Structural analyses were performed by means of Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and rheological studies using temperature sweeps. Moreover, phase change transition temperatures and heat of fusion as well as thermal and electrical conductivities of the developed PCM nanocomposites were determined. The SEM micrographs and FTIR absorption bands appearing at approximately 1038 and 1112 cm⁻¹ confirmed the silane modification. Differential scanning calorimetery (DSC) results indicate that the presence of Si-MWCNTs leads to slightly favorable enhancement in the energy storage capacity at the maximum loading. It was also shown that the thermal conductivity of the PCM nanocomposites, in both solid and liquid phases, increased with increasing the MWCNT content independent of the kind of MWCNTs by up to about 30% at the maximum loading of MWCNTs. In addition, the modification of MWCNTs made the samples completely electrically nonconductive, and the electrical surface resistivity of the PCMs containing pristine MWCNTs decreased with increasing MWCNTs loading. Furthermore, the rheological assessment under consecutive cyclic phase change demonstrated that the samples containing modified MWCNTs are more stable compared to the PCM containing pristine MWCNTs. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48428.     

High density polyethylene/ultra high molecular weight polyethylene blend. II. Effect of hydroxyapatite on processing,thermal, and mechanical properties

Hydroxyapatite (HA) is part of bone mineral composition. Several attempts have been made to incorporate HA into high density polyethylene (HDPE) to produce bone replacement biomaterials since neat HDPE is not suitable as bone replacement. The blending of HDPE with ultra high molecular weight polyethylene (UHMWPE) up to 50% by weight was performed with the aim of improving the toughness of composites. Reinforcement of blend with HA of up to 50% by weight was carried out. Methods of characterizing the composites included density, differential scanning calorimetry, thermal gravimetric analysis, ash content, and morphological examination using scanning electron microscope. For the mechanical properties of the composites, tensile, flexural, and impact tests were carried out. Incorporation of HA into HDPE has resulted in the brittleness of the composites. Blending of HDPE with UHMWPE in the presence of HA was found to improve the mechanical properties and promote a ductile failure of the resulting composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3931–3942, 2006     

Microstructural and thermal properties of fluoroelastomer/acidic surface modified carbon nanotube nanocomposites

Acid surface modified carbon nanotube (MCNT)‐, carbon nanotube (CNT)‐filled fluoroelastomer (FE), and unfilled‐FE were prepared (MCNT/FE, CNT/FE, and FE). Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) showed that the glass transition temperature (T_g) of MCNT/FE is less than that of CNT/FE and FE. The storage modulus of CNT/FE is higher than that of MCNT/FE over the whole temperature range. Storage modulus in rubbery state is higher in the case of MCNT/FE when compared to FE, however, in glassy state, it is slightly higher. For MCNT/FE at glassy and rubbery state, loss modulus is less than that of CNT/FE. Loss modulus in rubbery states is higher in the case of MCNT/FE compared to FE and in glassy states for both are the same. The DSC, DMA, and X‐ray diffraction (XRD) results confirmed that the FE and filler/FE is partially crystalline mostly in γ‐form. MCNT and CNT together with shear and high temperature, induced γ‐crystallinity in the FE, γ‐phase melting of MCNT/FE is higher than that of CNT/FE and FE. Most probably MCNT/FE obeys behaviors of thin films with high surface energy substrate interactions, while in CNT/FE the substrate surface energy is lower. XRD and DSC showed that MCNT causes more γ‐crystallinity in FE compared to others. Tetrafluoroethylene segments crystals have the degree of crystallinity in the order of CNT/FE > MCNT/FE > FE. The γ–phase crystal size is in the order of FE > CNT/FE > MCNT/FE. XRD, DSC, and DMA results for FE and filler/FE confirmed each other. POLYM. COMPOS., 37:3341–3353, 2016. © 2015 Society of Plastics Engineers     

Studies on the influence of molecular weight and isotacticity of polypropylene on the formation of mesomorphic phase

Polypropylene (PP) blends with various molecular weight and isotacticity were prepared through solution blending and subjected to rapid melt quenching. Structural changes in the PP matrix during mesomorphic phase formation were measured by FTIR spectroscopy and wide‐angle X‐ray diffraction measurements. The blends with different molecular weight and isotacticity provided the pathway to understand their influence on mesomorphic phase formation. It is observed that low molecular weight PP with low isotacticity forms mesomorphic phase, whereas high molecular weight and low isotactic PP does not lead to the formation of mesomorphic phase. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

The influence of molecular weight distribution of industrial polystyrene on its melt extensional and ultimate properties

We analyze the linear viscoelastic behavior and the strain‐rate dependence of nonlinear viscoelastic as well as the ultimate extensional properties of industrially relevant linear polystyrene mixtures (PS). The studied materials comprise different miscible binary mixtures of a well entangled matrix and unentangled diluent resulting in bimodal molar mass distribution (MWD). We also analyze the effect of the diluent weight average molar mass (M_w) by comparison with a mixture having broad but monomodal MWD. We show that the dilution effect on linear rheological properties is in agreement with the theoretical value of unity for the dilution exponent. We further show that the processing window, expressed as the ability of the material to withstand a given load without loss of homogeneity during elongation or ultimate loss of cohesion, is affected differently depending on the diluent M_w and concentration. Finally, we conclude that the existence of strain hardening is not sufficient for complete characterization of extension dominated operations. Our results demonstrate that significant enhancement of strain hardening achieved by adding small‐M_w diluents is often accompanied by trade‐off with respect to failure behavior of these mixtures. POLYM. ENG. SCI. 56:1012–1020, 2016. © 2016 Society of Plastics Engineers     

The influence of molecular weight on the melt rheology of polypropylene

Melt viscosity and melt elasticity data were obtained over a broad range of temperatures and shear rates on a series of four polypropylenes of different molecular weight but approximately the same molecular weight distribution. The superposition technique was used with both temperature and molecular weight to shift flow curves for all four materials at three temperatures each along the shear rate axis to generate a master flow curve at a given temperature and molecular weight. For polypropylenes of this type, and molecular weight distribution shift, factors which can be used to extend the useful range of experimentally obtained flow data were determined. The dependency of apparent viscosity on weight average molecular weight at shear stresses as high as 10⁶ dynes/cm² is shown. The dependency of melt elasticity on molecular weight and temperature is discussed.     

Functionalization of low molecular weight atactic polypropylenePart II. Thermal and molecular weight studies

Summary Number average molecular weight (M_n), intrinsic viscosity ([η]) and glass transition temperature (T_g) of low molecular weight atactic polypropylene (APP) grafted with 4-allyl-1,2-dimethoxybenzene (DMAB), 4-allyl-2-methoxyphenol (eugenol) and 4-propenylanisole (trans-anethole) in the presence of dicumyl peroxide (DCP) at 170°C were determined. The influences of reaction time, concentration and extent of grafting on M_n, [η] and T_g of APP were examined. The data were discussed with the mechanism of grafting reactions. Received: 6 June 2000/Revised version: 23 July 2000/Accepted: 4 September 2000     

Preparation and properties of epoxy nanocomposites. Part 2: The effect of dispersion and intercalation/exfoliation of organoclay on mechanical properties

The effect of the dispersion and intercalation/exfoliation of organoclay on the mechanical properties of epoxy nanocomposites was studied. The epoxy resin was EPON828 and the hardener was Jeffamine D‐230. The organoclay Cloisite 30B was used. Nanocomposites were prepared by different mixing devices that can generate different shear forces, such as a mechanical stirrer, a microfluidizer, and a homogenizer. The results indicate that the modulus increases almost linearly with the clay loading and also is improved with the quality of microdispersion, although the latter plays a less important role. On the other hand, only good dispersion can improve the strength, while poor dispersion results in loss of strength. The strength levels off above 4 wt% organoclay loading. It can be concluded that finer and more uniform dispersion increases the clay surface area available for interaction with the matrix and reduces stress concentration in the large aggregates that initiate the failure under stress. It is also observed that the presence of C30B does not significantly affect the glass transition (T_g) of the epoxy systems regardless of the level of clay dispersion and clay loading. Dynamic mechanic analysis (DMA) shows the positive effect of dispersion and intercalation/exfoliation on the storage modulus of epoxy nanocomposites (ENCs). POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Exploring interfacial interactions,dielectric, ferroelectric and piezoelectric properties of ultrahigh molecular weight polyethylene/graphene oxide nanocomposites

Graphene oxide (GO) incorporated ultra-high molecular weight polyethylene (UHMWPE) nanocomposites were prepared by encapsulating GO by UHMWPE in an aqueous media via high-shear mixing, which were subsequently dried and compression molded. Morphological characterizations via scanning electron microscopy revealed the intercalation of UHMWPE chains in the graphitic stacks corresponding to GO. Further, dielectric permittivity of UHMWPE/GO nanocomposite of 1 wt% GO showed a drastic increase (~61) as compared to pure UHMWPE (~2) due to an enhanced interfacial polarization. A significantly higher value of remnant polarization (~10 nC/cm²) and coercive field (~3 kV/cm) was observed in UHMWPE/GO nanocomposite of 1 wt% GO, which showed a strong hysteresis loop of polarization versus electric field plot as compared to pure UHMWPE, which displayed a very weak hysteresis loop. The piezoelectric coefficient (d₃₃) of ~9.5 pm/V was estimated in UHMWPE/GO nanocomposite of 1 wt% GO via piezoresponse force microscopy. Nanocomposite sensor devices were also fabricated and piezoelectric output voltage of ~6 V was recorded in UHMWPE/GO nanocomposite of 1 wt% of GO. We report here for the first time the unique ferroelectric and piezoelectric properties displayed by UHMWPE/GO nanocomposites.