Microstructure and fracture behavior of semicrystalline polymer–clay nanocomposites

The relationships between the microstructure and the fracture behavior of three polymer/clay nanocomposites were studied. Two different polymer matrices were chosen, namely polyamide‐6 and polyethylene (compatibilized with PE‐g‐MA or PE‐g‐PEo), to reach very different clay dispersion states. The microstructure was characterized in terms of polymer crystallinity, orientation of the polymer crystalline lamellae, clay dispersion state, and orientation of the clay tactoids. The mechanical behavior was characterized by tensile tests. The essential work of fracture (EWF) concept was used to determine the fracture behavior of the nanocomposites. Both tensile and EWF tests were performed in two perpendicular directions, namely longitudinal and transversal. It is shown that the fracture behaviors of the matrices mainly depend on the polymer crystalline lamellae orientation. For the nanocomposites, the relationships between the matrix orientation, the clay dispersion states, the values of the EWF parameters (w_e and βw_p), and their anisotropy are discussed. The results show that the lower the average clay tactoid thickness, the lower is the decrease of fracture performance for the nanocomposite and the more consumed energy as longer the path of the crack. Besides, a linear dependence of the anisotropy of the EWF parameters of the nanocomposites on the average clay aspect ratio is found. The more exfoliated the structure is, the less pronounced the anisotropy of the EWF parameters. Interestingly, it is thought that the average clay aspect ratio is the parameter representing the clay dispersion state that governs the fracture anisotropy of the nanocomposites (as the elastic properties determined by tensile tests). © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1820–1836, 2008     

Clay‐reinforced polyamide: Preferential orientation of the montmorillonite sheets and the polyamide crystalline lamellae

Intercalated and exfoliated nanocomposites were prepared by the extrusion and injection of polyamide‐6 and highly swollen or slightly swollen montmorillonite, respectively. The microstructure of pure compounds was characterized. The basal spacing was more homogeneous in swollen montmorillonite than in nontreated montmorillonite. This was attributed to the presence of a surfactant. Surfactant crystallites were observed in swollen montmorillonite and in the nanocomposites. Their distribution is discussed. The nanocomposites exhibited anisotropic properties attributed to the orientation of the montmorillonite sheets. The preferential orientation of the montmorillonite sheets was studied in detail by small‐angle X‐ray scattering (SAXS). It was found to be related to the injection direction. An average distance between two sheets was also measured from the SAXS spectra. The crystallographic state of the polyamide matrix was then analyzed. The orientation of the polymer lamellae relative to the montmorillonite sheets is discussed. The orientation of the montmorillonite sheets and the polyamide lamellae is expected to play a major role in the mechanical properties of nanocomposites. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1360–1370, 2001     

Phase behavior of modified montmorillonite– poly(ϵ‐caprolactone) nanocomposites

The thermal behavior and overall isothermal crystallization kinetics of a series of organophilic modified montmorillonite–poly(?‐caprolactone) nanocomposites were investigated. In general, the thermal behavior was influenced more by the type of dispersion than by the clay content. For nanocomposites in which silicate platelets were predominantly dispersed in the polymer matrix to give exfoliated structures, the thermal properties were improved with respect to those of neat poly(?‐caprolactone), whereas in those cases in which simply intercalated structures were attained, the thermal properties regularly decayed as the clay content increased. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1321–1332, 2004     

Diffusion behavior in polymer–clay nanocomposites

This paper reviews our previous studies on the diffusion behavior in polymers clay nanocomposites. A geometric model for predicting the effective diffusivity through this type of systems as a function of clay sheets orientation, volume fraction, polymer clay interaction, and aspect ratio is proposed. Model predictions are compared to the effective diffusivity generated using random walk simulations as well as with predictions obtained from already existing theoretical models. Fair agreement is found between the model prediction and the results of numerical simulations. With respect to the already existing theoretical models, the present mathematical derivation seems more adequate to describe diffusion behavior in conventional nanocomposites systems (i.e. when fillers present very low values of volume to surface ratio). Experimental diffusion tests are discussed and interpreted with the aid of the proposed model. In addition to the aspect ratio and clay concentration, the polymer clay interactions as well as the sheets orientation are the factors controlling the barrier properties of polymer‐layered silicate nanocomposites. Good agreement was found in the case of samples containing exfoliated clay, whereas the model fails in the case of micro‐composites, in which the inorganic lamellae are agglomerated in clusters. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 265–274, 2006     

Long‐lived layered silicates‐immobilized 2,6‐bis(imino)pyridyl iron (II) catalysts for hybrid polyethylene nanocomposites by in situ polymerization: Effect of aryl ligand and silicate modification

Heterogeneous‐layered silicate‐immobilized 2,6‐bis(imino)pyridyl iron (II) dichloride/MMAO catalysts, in which the active polymerization species are intercalated within sodium‐ and organomodified‐layered silicate galleries, were prepared for producing hybrid exfoliated polyethylene (PE) nanocomposites by means of in situ polymerization. The inorganic filler was first treated with modified‐methylaluminoxane (MMAO) to produce a supported cocatalyst: MMAO reacts with silicates replacing most of the organic surfactant, thus modifying the original crystallographic clay order. MMAO anchored to the nanoclay was able to activate polymerization iron complexes initiating the polymer growth directly from the filler lamellae interlayer. The polymerization mechanism taking place in between the montmorillonite lamellae separates the layers, thus promoting deagglomeration and effective clay dispersion. Transmission electron microscopy revealed that in situ polymerization by catalytically active iron complexes intercalated within the lower organomodified clay led to fine dispersion and high exfoliation extent. The intercalated clay catalysts displayed a longer polymerization life‐time and brought about ethylene polymerization more efficiently than analogous homogeneous systems. PEs having higher molecular masses were obtained. These benefits resulted to be dependent more on the filler nature than on the ligand environment around the iron metal center and the experimental synthetic route. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 548–564, 2009     

Effects of spacing between the exfoliated clay platelets on the crystallization behavior of polyamide‐6 in polyamide‐6/clay nanocomposites

Exfoliated polyamide‐6 (PA6)/organoclay nanocomposite films with planar‐oriented clay platelets were prepared by the simple hot pressing of melt‐extruded nanocomposite pellets. The average distance between the neighboring clay platelets was controlled by changes in the clay loading content in the nanocomposites. The effects of the clay platelet spacing on the crystallization behavior of PA6 were investigated with transmission electron microscopy and wide‐angle X‐ray diffraction. The crystal lamellae were found to be mainly perpendicular to the clay surface for the nanocomposites with large spacing between the clay sheets at low clay loading contents. This perpendicular orientation morphology was attributed to the strong interactions between the PA6 molecular chain and the clay surface. In contrast, the crystal lamellae were found to be parallel to the clay surface when the spacing between the neighboring clay platelets was less than 30 nm. It was concluded that the confinement crystallization of PA6 within the nanoscale channels formed by clay sheets resulted in this parallel orientation texture. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 284–290, 2006     

Influence of α‐ZrP fillers and process conditions on the morphology and the gas barrier properties of filled polyamide 6 films

Composite films based on polyamide 6 and lamellar unmodified α‐ZrP nanofillers have been prepared for low filler amounts (less than 2 wt %) using cast process or blowing process. Whatever be the filler content and the film process conditions, the lamellar nanofillers were not intercalated by the polymer chains and microcomposites were obtained. On the other hand, the matrix crystalline structure highly depended on the presence of fillers and on the film process conditions. The nature of the crystalline phase and its orientation were shown to play a major role on the film barrier properties to helium. For instance, the presence of γ crystalline phase associated to a specific orientation of the crystalline lamellae parallel to the film surface could significantly contribute to enhance barrier properties. This specific morphology was favored by the presence of α‐ZrP in the formulation but depended also on the process conditions. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1734–1746, 2008     

Deformation in lamellar and crystalline structures: in situ simultaneous small‐angle X‐ray scattering and wide‐angle X‐ray diffraction measurements on polyethylene terephthalate fibers

Changes in the lamellar and crystalline structures were followed as a function of applied stress to understand the influence of the interactions between the crystalline and amorphous domains on the fiber properties. We observed a reversible transformation from a structure giving a four‐point small‐angle pattern to a structure giving a two‐point pattern; these structures corresponded to the lamellae with oblique and normal lamellar surfaces, respectively. The characteristics of these two structures such as the stack diameter, stack height, and tilt angle were different and were determined by the processing conditions and did not change when the fiber was elastically deformed. The structure giving a two‐point pattern was probably the load‐carrying lamellar entity in these fibers. The diameter of the lamellar stacks, tilt angle of the lamellae, and the strain in the lamellar spacing appeared to have the most influence on properties such as tenacity and dimensional stability. The long‐spacing strain, which is about the same as the fiber strain, determined the modulus at low elongation as well as ultimate elongation. These indicate that the lamellar stacks have at least as much influence as the interfibrillar chains on fiber properties. Structural features that determine the performance in semicrystalline polymers were investigated by analyzing four generations of polyethylene terephthalate fibers. Some of the fiber properties correlate with changes in the crystalline domains such as the crystalline orientation, size, and unit cell dimensions. Fibers in which the crystalline strain was large because of their strong linkages to the amorphous chains, and better load transfer, had the highest modulus and lowest ultimate elongation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1538–1553, 2003     

The effect of nanoparticle shape on polymer‐nanocomposite rheology and tensile strength

Nanoparticles can influence the properties of polymer materials by a variety of mechanisms. With fullerene, carbon nanotube, and clay or graphene sheet nanocomposites in mind, we investigate how particle shape influences the melt shear viscosity η and the tensile strength τ, which we determine via molecular dynamics simulations. Our simulations of compact (icosahedral), tube or rod‐like, and sheet‐like model nanoparticles, all at a volume fraction ? ≈ 0.05, indicate an order of magnitude increase in the viscosity η relative to the pure melt. This finding evidently can not be explained by continuum hydrodynamics and we provide evidence that the η increase in our model nanocomposites has its origin in chain bridging between the nanoparticles. We find that this increase is the largest for the rod‐like nanoparticles and least for the sheet‐like nanoparticles. Curiously, the enhancements of η and τ exhibit opposite trends with increasing chain length N and with particle shape anisotropy. Evidently, the concept of bridging chains alone cannot account for the increase in τ and we suggest that the deformability or flexibility of the sheet nanoparticles contributes to nanocomposite strength and toughness by reducing the relative value of the Poisson ratio of the composite. The molecular dynamics simulations in the present work focus on the reference case where the modification of the melt structure associated with glass‐formation and entanglement interactions should not be an issue. Since many applications require good particle dispersion, we also focus on the case where the polymer‐particle interactions favor nanoparticle dispersion. Our simulations point to a substantial contribution of nanoparticle shape to both mechanical and processing properties of polymer nanocomposites. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1882–1897, 2007     

Influence of organic modifiers on morphology and crystallization of poly(ϵ‐caprolactone)/synthetic clay intercalated nanocomposites

ε‐caprolactone was polymerized in the presence of neat montmorillonite or organomontmorillonites to obtain a variety of poly(ε‐caprolactone) (PCL)‐based systems loaded with 10 wt % of the silicates. The materials were thoroughly investigated by different X‐ray scattering techniques to determine factors affecting structure of the systems. For one of the nanocomposites it was found that varying the temperature in the range corresponding to crystallization of PCL causes reversible changes in the interlayer distance of the organoclay. Extensive experimental and literature studies on this phenomenon provided clues indicating that this effect might be a result of two‐dimensional ordering of PCL chains inside the galleries of the silicate. Small angle X‐ray scattering and wide angle X‐ray scattering investigation of filaments oriented above melting point of PCL revealed that polymer lamellae were oriented perpendicularly to particles of unmodified silicate, while in PCL/organoclay systems they were found parallel to clay tactoids. Calorimetric and microscopic studies shown that clay particles are effective nucleating agents. In the nanocomposites, PCL crystallized 20‐fold faster than in the neat polymer. The crystallization rate in nanocomposites was also significantly higher than in microcomposite. Further research provided an insight how the presence of the filler affects crystalline fraction and spherulitic structure of the polymer matrix in the investigated systems. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2350–2367, 2007     

Morphology and gas barrier properties of polyethylene‐based nanocomposites

Nanocomposites were processed by melt blending two reference matrices, a metallocene polyethylene and a low density maleic anhydride‐grafted polyethylene with an organo‐modified montmorillonite. It was shown that the introduction of a maleated polyethylene compatibilizer was required to improve the clay nanoplatelet dispersion in the metallocene polyethylene‐based nanocomposites. Increasing the montmorillonite content led to a significant increase of the barrier properties. Interfacial agents such as oxidized paraffins were shown to be more effective to reduce the gas permeability than maleated polyethylene and the dependence of the gas transport properties was discussed not only as a function of the clay dispersion but also as a function of the clay/compatibilizer and compatibilizer/matrix interactions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 431–440, 2006     

Investigation of clay modifier effects on the structure and rheology of supercritical carbon dioxide‐processed polymer nanocomposites

Superior property enhancements in polymer–clay nanocomposites can be achieved if one can significantly enhance the nanoclay dispersion and polymer–clay interactions. Recent studies have shown that nanoclays can be dispersed in polymers using supercritical carbon dioxide (scCO₂). However, there is need for a better understanding of how changing the clay modifier affects the clay dispersability by scCO₂ and the resultant nanocomposite rheology. To address this, the polystyrene (PS)/clay nanocomposites with “weak” interaction (Cloisite 93A clay) and “strong” interaction (Cloisite 15A clay) have been prepared using the supercritical CO₂ method in the presence of a co‐solvent. Transmission electron microscopy images and small‐angle X‐ray diffraction illustrate that composites using 15A and 93A clays show similar magnitude of reduction in the average tactoid size, and dispersion upon processing with scCO₂. When PS and the clays are coprocessed in scCO₂, the “dispersion” of clays appears to be independent of modifier or polymer–clay interaction. However, the low‐frequency storage modulus in the scCO₂‐processed 15A nanocomposites is two orders of magnitude higher than that of 93A nanocomposites. It is postulated that below percolation (solution blended composites), the strength of polymer–clay interaction is not a significant contributor to rheological enhancement. In the scCO₂‐processed nanocomposites the enhanced dispersion passes the percolation threshold and the interactions dictate the reinforcement potential of the clay–polymer–clay network. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 823–831, 2010     

Imidazolium‐modified clay‐based ABS nanocomposites: a comparison between melt‐blending and solution‐sonication processes

Acrylonitrile–butadiene–styrene (ABS) nanocomposites containing imidazolium‐modified montmorillonite have been prepared by melt‐blending (MB) and solution‐sonication in order to study the effects of processing on the morphology and properties of the polymer/clay composites. The structure‐property relationships of the prepared composites have been studied by means of X‐ray diffraction (XRD), transmission electron microscopy (TEM), mechanical testing, dynamic‐mechanical analyses (DMA), thermal gravimetrical analyses (TGA), fluorescence probe confocal microscopy, and fluorescence spectroscopy (FS). X‐Ray and TEM show that both nanocomposites have a mixed intercalated/exfoliated structure. Fluorescence probe confocal microscopy reveals that the sonicated sample has a more homogeneous dispersion: this result is confirmed by the values of elongation at break and flexural elastic modulus measured for the composites. Fluorescence spectroscopy has also been used to investigate the distribution of clay in the composites and results indicate that clay layers in ABS are preferentially located in the styrene‐acrylonitrile (SAN) phase, independent of the dispersion process used. Published in 2008 by John Wiley & Sons, Ltd.     

Mechanical properties of polymer‐blend nanocomposites with organoclays: Polystyrene/ABS and high impact polystyrene/ABS

Thirty‐three polystyrene (PS)/acrylonitrile‐butadiene‐styrene (ABS) and high impact PS/ABS polymer blends with organoclay and copolymer additives were prepared by melt processing using different mixing sequences in order to test the putative capability of clay to perform a compatibilizing role in polymer blends. In general, the addition of clay increased the tensile modulus and had little effect on tensile strength. For the blends studied in this work, the addition of organoclays caused a catastrophic reduction in impact strength, a critical property for commercial viability. The polymer‐blend nanocomposites adopted a structure similar to that for ABS/clay nanocomposites as determined by X‐ray diffraction and transmission electron microscopy. It is suggested that clay reinforcement inhibits energy absorption by craze formation and shear yielding at high strain rates. Simultaneous mixing of the three components provided nanocomposites with superior elongation and energy to failure compared to sequential mixing. The clay pre‐treated with a benzyl‐containing surfactant gave the best overall properties among the various organoclays tested and of the two clay contents studied 4 wt % was preferred over 8 wt % addition. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Morphology and mechanical properties of a novel amorphous polyamide/nanoclay nanocomposite

A novel amorphous polyamide/montmorillonite nanocomposite based on poly(hexamethylene isophthalamide) was successfully prepared by melt intercalation. Wide angle X-ray diffraction and transmission electron microscopy showed that organoclay containing quaternary amine surfactants with phenyl and hydroxyl groups was delaminated in the polymer matrix resulting in well-exfoliated morphologies even at high montmorillonite content. Differential scanning calorimetry results indicated that clay platelets did not induce the formation of a crystalline phase in this amorphous polymer. Tensile tests demonstrated that the addition of nanoclay caused a dramatic increase in Young's modulus (almost twofold) and yield strength of the nanocomposites compared with the homopolymer. The nanocomposites exhibited ductile behavior up to 5 wt % of nanoclay. The improvement in Young's modulus is comparable with semicrystalline aliphatic nylon 6 nanocomposites. Both the main chain amide groups and the amorphous nature of the polyamide are responsible for enhancing the dispersion of the nanofillers, thereby, leading to improved properties of the nanocomposites. The structure-property relationship for these nanocomposites was also explored. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2605–2617, 2008     

Influence of the compatibilizer polarity and molar mass on the morphology and the gas barrier properties of polyethylene/clay nanocomposites

In this study, different modified polyethylenes with different molar masses and different modification rates were examined as compatibilizers to prepare high density polyethylene/organoclay nanocomposites. Nanocomposites having 5 wt % organo-modified clay and 20 wt % interfacial agent were prepared by melt blending. The effect of compatibilizer molar mass and polarity was investigated on the clay dispersion and on the gas barrier properties. It was observed that the amount of large and dense fillers aggregates was considerably reduced by introduction of an interfacial agent. The nanocomposite final morphology was governed by a diffusion/shear mechanism. A high degree of clay delamination was obtained with the high molar mass compatibilizers, whereas highly swollen clay aggregates resulted from the incorporation of the low molar mass interfacial agents. In the investigated nanocomposites series, the barrier properties could not be directly related to the clay dispersion state but resulted also from the matrix/clay interfacial interactions. A gas transport mechanism based on these both parameters was proposed to explain the barrier properties evolution in these low polar nanocomposites series. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2593–2604, 2008     

Flow‐induced particle orientation and rheological properties of suspensions of organoclays in thermoplastic resins

The influence of nano‐scale particles on the viscoelastic properties of polymer suspensions is investigated. We have developed a simulation technique for the particle orientation and polymer conformation tensors to study various features of the suspensions. The nano‐particles are modeled as thin rigid oblate spheroid particles and the polymers as FENE‐P type viscoelastic and Newtonian fluid. Both interparticle and polymer‐particle interactions have been taken into account in our numerical computations. The nonlinear viscoelastic properties of nanocomposites of layered silicate particles in non‐Newtonian fluids are examined at the start‐up of shear flow and are interpreted using the model to examine the effects of model parameters as well as flow conditions on particle orientation, viscosity, and first normal stress difference of the suspensions. We have studied the microstructure of polymer‐clay nanocomposites using X‐ray diffraction (XRD) scattering and transmission electron microscopy (TEM). The rheology of these nanocomposites in step‐shear is shown to be fairly well predicted by the model. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2003–2011, 2010     

Synthesis and characterization of a series of thermotropic liquid crystalline copolyester nanocomposites

A series of aromatic thermotropic liquid crystalline copolyester (TLCP) nanocomposites were prepared by the in situ intercalation polymerization of p‐acetoxybenzoic acid (ABA), terephthalic acid (TPA), and diacetoxynaphthalene (DAN) isomers in the presence of the organoclay. The DAN isomers used in this study were 2,3‐ and 2,7‐naphthylene. We examined the variation of the liquid crystallinity, morphology, and thermal properties of the nanocomposites with organoclay content in the range 0–10 wt %. All the polymer nanocomposites were fabricated with a molar ratio of ABA:TPA:DAN = 2:1:1; they were shown to consist of a nematic liquid crystalline phase for low organoclay contents (≤5 wt %), whereas the hybrids with a higher concentration of organoclay (≥10 wt %) were found not to be mesomorphic. By using transmission electron microscopy, the clay layers in the 2,3‐DAN copolyester hybrids were found to be better dispersed in the matrix polymer than those in the 2,7‐DAN copolyester hybrids. The introduction of an organoclay into the matrix polymer was found to improve the thermal properties of the 2,3‐DAN copolyester hybrids. However, the thermal properties of the 2,7‐DAN copolyester hybrids were found to be worse than those of the pure matrix polymer for all organoclay compositions tested. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 387–397, 2006     

Impact of the clay organic modifier on the morphology of polymer–clay nanocomposites prepared by in situ free‐radical polymerization in emulsion

Poly(styrene‐co‐butyl acrylate) copolymers were prepared by free‐radical random copolymerization of styrene and butyl acrylate in emulsion in the presence of 10% of surface‐modified sodium montmorillonite (Na‐MMT). The objective of this work was to evaluate the impact of the clay organic modifier in terms of its chemical structure, its degree of interaction within the clay galleries surface, and its ability to copolymerize with monomers, on the morphology and properties of the final nanocomposite prepared. Na‐MMT was modified using different organic modifiers, namely: sodium 1‐allyloxy‐2‐hydroxypropyl (Cops), 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid (AMPS), N‐isopropylacrylamide (NIPA), and sodium 11‐methacryloyloxy‐undecan‐1‐yl sulfate (MET), respectively. The morphology and properties of the nanocomposites obtained were found to be dependant on the clay organic modifier. X‐ray diffraction (XRD) and transmission electron microscopy indicated that, nanocomposites at 10% clay loading with Cops‐, NIPA‐, and MET‐modified clays, yielded intercalated to partially exfoliated structures, whereas AMPS‐modified clay gave a nanocomposite with a fully exfoliated structure. All polymer–clay nanocomposites were found to be more thermally stable than neat poly(S‐co‐BA) as were determined by TGA. However, nanocomposites with intercalated structures exhibited greater thermal stability relative to fully exfoliated ones. Furthermore, nanocomposites with exfoliated structures exhibited higher storage moduli (G^I) than partially exfoliated once, whereas intercalated structure showed the lowest G^I values. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3619–3628, 2008