Intra‐ and extra‐gallery reactions in tri‐functional epoxy polymer layered silicate nanocomposites

Achieving a high degree of exfoliation in epoxy‐based polymer layered silicate (PLS) nanocomposites is crucial to their successful industrial application, but has hitherto proved elusive. In this work, a system is presented which shows significant promise in this respect. The isothermal cure of PLS nanocomposites based upon a tri‐functional epoxy resin (TGAP) has been studied by DSC, and displays two exothermic peaks. The first peak, very rapid, relates to a homopolymerization reaction within the intra‐gallery regions, while the second peak reflects the bulk crosslinking reaction. The occurrence of the intra‐gallery reaction before the bulk reaction enhances the degree of exfoliation in the cured nanocomposite. Furthermore, pre‐conditioning the resin/clay mixture before adding the curing agent and effecting the isothermal cure also allows a greater extent of intra‐gallery reaction to occur before the extra‐gallery epoxy‐amine reaction. Consequently, this system results in a high degree of exfoliation, as revealed by transmission electron microscopy. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Epoxy‐montmorillonite clay nanocomposites: Synthesis and characterization

Nanocomposites of epoxy resin with montmorillonite clay were synthesized by swelling of different proportions of the clay in a diglycidyl ether of bisphenol‐A followed by in situ polymerization with aromatic diamine as a curing agent. The montmorillonite was modified with octadecylamine and made organophilic. The organoclay was found to be intercalated easily by incorporation of the epoxy precursor and the clay galleries were simultaneously expanded. However, Na‐montmorillonite clay could not be intercalated during the mixing or through the curing process. Curing temperature was found to provide a balance between the reaction rate of the epoxy precursor and the diffusion rate of the curing agent into the clay galleries. The cure kinetics were studied by differential scanning calorimetry. The exfoliation behavior of the organoclay system was investigated by X‐ray diffraction. Thermogravimetric analysis was used to determine the thermal stability, which was correlated with the ionic exchange between the organic species and the silicate layers. The morphology of the nanocomposites was evaluated by scanning electron microscopy. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2201–2210, 2004     

Processing–structure–properties relationships of mechanically and thermally enhanced smectite/epoxy nanocomposites

Mechanically reinforced and thermally enhanced smectite/epoxy nanocomposites were synthesized using “direct” (without solvent) and “solvent” processing techniques. The molecular dispersion of smectite clay in the epoxy resin was investigated for its role in the rheology, structure formation, and properties of nanocomposites. The effects of three types of organic modifiers on the dispersion structure were compared. The use of solvent during processing assists in the enhancement of clay exfoliation. Rheology was used as a method to compare the degree of clay delamination in the resin matrix, as well as to estimate the suspension structure. The critical volume fraction (Φ*) and maximal packaging of smectites were determined and used for prediction of the viscosity. The qualitative changes in the nanostructure of suspensions above Φ*, due to flocculation of exfoliated clay layers, were compared with the alteration of the properties of nanocomposites, related to the structure formation and morphology. The curing kinetics were found to depend on both the organic modifier and solvent, but the extent of curing was roughly equivalent for the pure epoxy resin and the nanocomposites. The structure of the nanocomposites, either intercalated or exfoliated, produced by the direct processing technique was controlled by the organic modifier. By using solvent processing, the effect of the solvent dominates that of the organic modifier, presumably leading to exfoliated nanocomposites. The mechanical and thermal properties are strongly enhanced above the Φ* of smectites, and they are significantly dependent on the type of nanocomposite structure and the use of solvent. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2499–2510, 2005     

Development of high‐performance epoxy/clay nanocomposites by incorporating novel phosphonium modified montmorillonite

Novel organoclays were synthesized by several kinds of phosphonium cations to improve the dispersibility in matrix resin of composites and accelerate the curing of matrix resin. The possibility of the application for epoxy/clay nanocomposites and the thermal, mechanical, and adhesive properties were investigated. Furthermore, the structures and morphologies of the epoxy/clay nanocomposites were evaluated by transmission electron microscopy. Consequently, the corporation of organoclays with different types of phosphonium cations into the epoxy matrix led to different morphologies of the organoclay particles, and then the distribution changes of silicate layers in the epoxy resin influenced the physical properties of the nanocomposites. When high‐reactive phosphonium cations with epoxy groups were adopted, the clay particles were well exfoliated and dispersed. The epoxy/clay nanocomposite realized the high glass‐transition temperature (T_g) and low coefficient of thermal expansion (CTE) in comparison with those of neat epoxy resin. On the other hand, in the case of low‐reactive phoshonium cations, the dispersion states of clay particles were intercalated but not exfoliated. The intercalated clay did not influence the T_g and CTE of the nanocomposite. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Analysis of the cure of epoxy based layered silicate nanocomposites: Reaction kinetics and nanostructure development

The cure reaction kinetics of epoxy resin, with organically modified montmorillonite loadings of up to 20 wt % and with stoichiometric conditions, has been studied by differential scanning calorimetry with a view to understanding further the fabrication of epoxy‐based polymer layered silicate nanocomposites. The kinetic analysis of isothermal and nonisothermal cure shows that the autocatalytic model is the more appropriate to describe the kinetics of these reactions, and it is observed that a dominant effect of the montmorillonite is to catalyze the curing reaction. However, it was not possible to model the reactions over the whole range of degrees of conversion, in particular for nonisothermal cure. This attributed to the complexity of the reactions, and especially to the occurrence of etherification by cationic homopolymerization catalyzed by the onium ion of the organically modified montmorillonite. The homopolymerization reaction results in an excess of diamine in the system, and hence in practice the reaction is off stoichiometric, which leads to a reduction in both the heat of cure and the glass transition temperature as the montmorillonite content increases. Small angle X‐ray scattering of the cured nanocomposites shows that an exfoliated nanostructure is obtained in nonisothermal cure at slow heating rates, whereas for nonisothermal cure at faster heating rates, as well as for isothermal cure at 70°C and 100°C, a certain amount of exfoliation is accompanied by the growth of d‐spacings of 1.4 nm and 1.8 nm for dynamic and isothermal cure, respectively, smaller than the d‐spacings of the modified clay before intercalation of the resin. A similar nanostructure, consisting of extensive exfoliation accompanied by a strong scattering at distances less than the d‐spacing of the modified clay, is also found for resin/clay mixtures, before the addition of any crosslinking agent, which have been preconditioned by storage for long times at room temperature. The development of these nanostructures is attributed to the presence of clay agglomerations in the original resin/clay mixtures and highlights the importance of the quality of the dispersion of the clay in the resin in respect of achieving a homogeneous exfoliated nanostructure in the cured nanocomposite. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Dynamic mechanical and rheological behavior of fluoroelastomer‐organoclay nanocomposites obtained from different preparation methods

A comparison among different preparation methods of fluoropolymer/clay nanocomposites based on a fluoroelastomeric matrix (Tecnoflon® P959) and organomodified montmorillonite clay (Cloisite®20A) is reported. While melt blending leads to intercalated structures, the X‐ray diffraction patterns of solution blended nanocomposite suggest a better delamination of the clay platelets within the fluoropolymeric matrix (no diffraction peaks) if the solvent evaporation step is carried out very slowly (72 h) at ambient pressure and moderate temperature. For the solution blended‐slow evaporation nanocomposite, dynamic mechanical analysis and dynamic rheological measurements show a strong increase in G′, a lower damping peak at T_g, and a pseudosolid like behavior in the terminal flow region, suggesting a likely exfoliation of the organomodified clay in the fluoroelastomeric matrix. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4484–4487, 2006     

Preparation and characterization of modified‐clay‐reinforced and toughened epoxy‐resin nanocomposites

Epoxy–clay nanocomposites were prepared by the dispersion of an organically modified layered clay in an epoxy resin (diglycidyl ether of bisphenol A) and curing in the presence of methyl tetrahydro acid anhydride at 80–160°C. The nanometer‐scale dispersion of layered clay within the crosslinked epoxy‐resin matrix was confirmed by X‐ray diffraction and transmission electron microscopy, and the basal spacing of the silicate layer was greater than 100–150 Å. Experiments indicated that the hydroxyethyl groups of the alkyl ammonium ions, which were located in the galleries of organically modified clay, participated in the curing reaction and were directly linked to the epoxy‐resin matrix network. Experimental results showed that the properties of epoxy were improved, evidently because of the loading of organically modified clay. The impact strength and tensile strength of the nanocomposites increased by 87.8 and 20.9%, respectively, when 3 wt % organic clay was loaded, and this demonstrated that the composites were toughened and strengthened. The thermal‐decomposition and heat‐distortion temperatures were heightened in comparison with those of pure epoxy resin, and so were the dynamic mechanical properties, including the storage modulus and glass‐transition temperature. Moreover, experiments showed that most properties of the composites were ameliorated with low clay contents. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2649–2652, 2004     

Isothermal and nonisothermal cure kinetics of an epoxy/poly(oxypropylene)diamine/octadecylammonium modified montmorillonite system

The effect of an octadecylammonium‐exchanged montmorillonite on the curing kinetics of a thermoset system based on a bisphenol A epoxy resin and a poly(oxypropylene)diamine curing agent were studied with differential scanning calorimetry (DSC) in isothermal and dynamic (constant‐heating‐rate) conditions. Montmorillonite and the prepared composites were characterized by X‐ray diffraction analysis and simultaneous DSC and thermogravimetric analysis. The analysis of the DSC data indicated that the intercalated octadecylammonium cations catalyzed the epoxy–amine polymerization. A kinetic model, arising from an autocatalyzed reaction mechanism, was applied to the DSC data. Fairly good agreement between the experimental data and the modeling data was obtained. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1765–1771, 2006     

Preparation and relaxation dynamics of ethylene–propylene–diene rubber/clay nanocomposites with crosslinking interfacial design

Ethylene–propylene–diene rubber (EPDM)/clay nanocomposites with crosslinking bonding at the interface were fabricated through the intercalation method involving double‐bond functional groups. For comparison, an organoclay modified with an intercalation agent without double bonds was also prepared. X‐ray diffraction indicated that the EPDM intercalated into the galleries of the nanoclay due to crosslinking with the organic intercalation reagent containing double bonds. According to the dielectric relaxation spectra, the segmental relaxation of EPDM was greatly confined, due to the strong filler/polymer interfacial interaction. And a new relaxation appeared at higher temperature and lower frequency than segmental relaxation when the content of clay with double bonds reached 10 phr; the new relaxation is attributed to interfacial relaxation. Whereas the new relaxation did not appear by adding ordinary organoclay, the dynamic mechanical analysis loss peak of EPDM, corresponding to the glass transition, moved to a higher temperature due to covulcanization. The presence of crosslinking in the EPDM/clay nanocomposites can play a significant role in improving their mechanical properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45553.     

Effect of blending sequence on the morphologies of poly(butylene terephthalate)/epoxy/clay nanocomposites by a rheological approach

Epoxy resin was used as a compatilizer to prepare poly(butylene terephthalate)/clay nanocomposites via melt intercalation. Three different mixing sequences were attempted in the present work: (1) to mix poly(butylene terephthalate) (PBT), epoxy, and organoclay in one step; (2) first to mix epoxy and organoclay, and then mix it with PBT; (3) to prepare PBT/organoclay hybrid first, and then mix it with epoxy to get the final nanocomposites. The results from X‐ray diffraction (XRD) reveal that all these hybrids present an intercalated structure. However, it can be observed that there are distinct differences in the amount and average size of clay tactoids dispersed in those hybrids from the transmission electron microscope (TEM) photographs, which are further confirmed successfully by the rheological measurements. On the basis of the strain overshoot intensity and the low‐frequency solid‐like response level in the small amplitude oscillatory shear (SAOS) measurements, it can be concluded that the blending sequence (2) is the best way to obtain a nice dispersion of clay in the matrix, forming a percolated tactoids network with highest density and intensity. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 340–346, 2006     

Preparation of polyacrylate‐clay nanocomposites by in‐situ polymerization and heterocoagulation

Polyacrylate‐clay nanocomposites were prepared by an in situ polymerization method followed by heterocoagulation. In the heterocoagulation method, a cationic polyacrylate emulsion was prepared by emulsion polymerization using a cationic initiator in the presence/absence of free surfactant, cetyl trimethylammonium bromide (CTABr), followed by mixing with an aqueous clay slurry. WAXD results and TEM images suggest that morphologies of these nanocomposites depend on preparation method, mixing method, and the amount of free surfactant. TG‐DTG analyses demonstrate the improvement in thermal stability of these nanocomposites, while DSC results indicate no significant changes in glass transition temperature of these nanocomposites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3850–3855, 2006     

Preparation and thermal properties of resole‐type phenol resin–clay nanocomposites

Resole‐type phenol resin–clay nanocomposites have been prepared successfully by melt compounding phenol resin with organophilic clay. In the resulting phenol resin–clay nanocomposite, the silicate layers of the clay were exfoliated and dispersed as monolayers. The nanocomposite exhibited higher long‐term heat resistance when compared with unmodified phenol resin. It was surmised that the silicate layers of the clay acted as barriers to oxygen penetration into the resin, as the degree of heat degradation of the nanocomposite was much lower than that of the straight phenol resin. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3236–3240, 2006     

Preparation and characterization of rubber–clay nanocomposites

Rubber–clay nanocomposites were prepared by two different methods and characterized with TEM and XRD. The TEM showed clay had been dispersed to one or several layers. The XRD showed that the basal spacing in the clay was increased. It was evident that some macromolecules intercalated to the clay layer galleries. The clay layer could be uniformly dispersed in the rubber matrix on the nanometer level. The mechanical tests showed that the nanocomposites had good mechanical properties. Some properties exceeded those of rubber reinforced with carbon black, so the clay layers could be used as an important reinforcing agent as the carbon black was. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1879–1883, 2000     

Highly exfoliated nanostructure in trifunctional epoxy/clay nanocomposites using boron trifluoride as initiator

Epoxy/clay nanocomposites based upon a trifunctional epoxy resin, triglycidyl p‐amino phenol (TGAP), have been prepared by intercalating an initiator of cationic homopolymerization, a boron trifluoride monoethylamine (BF₃·MEA) complex, into the montmorillonite clay galleries before the addition of the TGAP and the curing agent, 4,4‐diamino diphenyl sulfone (DDS), and effecting the isothermal curing reaction. The BF₃·MEA enhances the intragallery cationic homopolymerization reaction, which occurs before the extragallery cross‐linking reaction of the TGAP with the DDS, and which hence contributes positively to the mechanism of exfoliation of the clay. The effects of isothermal cure temperature and of BF₃·MEA content have been studied, in respect of both the reaction kinetics, monitored by differential scanning calorimetry, and the nanostructure, as identified by small‐angle X‐ray scattering and transmission electron microscopy. It is shown that the use of BF₃·MEA in this way as an initiator of intragallery homopolymerization significantly improves the degree of exfoliation in the cured nanocomposites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40020.     

Polypropylene nanocomposite film: A critical evaluation on the effect of nanoclay on the mechanical,thermal, and morphological behavior

Polypropylene (PP)/clay nanocomposites prepared by melt blending technique using different percentages of clay with and without maleic anhydride grafted PP (MA‐PP) were studied. The intercalated and exfoliated structure of nanocomposites was characterized by X‐Ray Diffraction (XRD) and transmission electron microscopy (TEM). Because of the typical intercalated and exfoliated structure, the tensile modulus of the nanocomposites were improved significantly as compared to virgin PP. The viscoelastic behavior of the nanocomposites was studied by dynamical mechanical analysis (DMA) and the results showed that with the addition of treated clay to PP there was substantial improvement in storage modulus increases. The thermal stability and crystallization of the PP nanocomposites as studied by differential scanning calorimeter (DSC) and thermo gravimetric analysis (TGA) were also improved significantly compared to PP. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of clay addition on the morphology and thermal behavior of polyamide 6

The nanostructure, morphology, and thermal properties of polyamide 6 (PA6)/clay nanocomposites were studied with X‐ray scattering, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The wide‐angle X‐ray diffraction (WAXD) and TEM results indicate that the nanoclay platelets were exfoliated throughout the PA6 matrix. The crystallization behavior of PA6 was significantly influenced by the addition of clay to the polymer matrix. A clay‐induced crystal transformation from the α phase to the γ phase for PA6 was confirmed by WAXD and DSC; that is, the formation of γ‐form crystals was strongly enhanced by the presence of clay. With various clay concentrations, the degree of crystallinity and crystalline morphology (e.g., spherulite size, lamellar thickness, and long period) of PA6 and the nanocomposites changed dramatically, as evidenced by TEM and small‐angle X‐ray scattering results. The thermal behavior of the nanocomposites was investigated with DSC and compared with that of neat PA6. The possible origins of a new clay‐induced endothermic peak at high temperature are discussed, and a model is proposed to explain the complex melting behavior of the PA6/clay nanocomposites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1191–1199, 2007     

Preparation and characterization of polystyrene–clay nanocomposites by free‐radical polymerization

The polymerizable cationic surfactant, vinylbenzyldimethylethanolammouium chloride (VBDEAC), was synthesized to functionalize montmorillonite (MMT) clay and used to prepare exfoliated polystyrene–clay nanocomposites. The organophilic MMT was prepared by Na⁺ exchanged montmorillonite and ammonium cations of the VBDEAC in an aqueous medium. Polystyrene–clay nanocomposites were prepared by free‐radical polymerization of the styrene containing intercalated organophilic MMT. Dispersion of the intercalated montmorillonite in the polystyrene matrix determined by X‐ray diffraction reveals that the basal spacing is higher than 17.6 nm. These nanocomposites were characterized by differential scanning calorimetry (DSC), transmission electron micrograph (TEM), thermal gravimetric analysis (TGA), and mechanical properties. The exfoliated nanocomposites have higher thermal stability and better mechanical properties than the pure polystyrene. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1370–1377, 2002     

The effect of shear forces on the microstructure and mechanical properties of epoxy–clay nanocomposites

The objective of this work is to understand the effect of shear force on the properties of epoxy–clay nanocomposites. The shear force was controlled by changing the revolutions per minute on a mechanical mixer. Differences in the aspect ratio of clay layers and differences of clay particle distribution in the epoxy matrix were caused by shear force. Shear force mechanism on epoxy–clay nanocomposites' intercalation/exfoliation were compared with the other mechanism already suggested. X‐ray diffraction, transmission electron microscopy, and scanning electron microscopy were utilized to investigate the degree of exfoliation and morphology. The mechanical and thermal properties were also studied to demonstrate the effect of shear force. This study revealed that appropriate shear force and mixing time on nanocomposite preparation was required to achieve the desired properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3465–3473, 2006     

E‐beam‐cured layered‐silicate and spherical silica epoxy nanocomposites

This research demonstrates that an epoxy nanocomposite can be made through electron beam (e‐beam) curing. The nanofillers can be two‐dimensional (layered‐silicate) and zero‐dimensional (spherical silica). Both the spherical silica epoxy nanocomposite and the layered‐silicate epoxy nanocomposite can be cured to a high degree of curing. The transmission electron microscopy (TEM) and small‐angle X‐ray scattering of the e‐beam‐cured layered‐silicate epoxy nanocomposites demonstrate the intercalated nanostructure or combination of exfoliated and intercalated nanostructure. The TEM images show that the spherical silica epoxy nanocomposite has the morphology of homogeneous dispersion of aggregates of silica nanoparticles. The aggregate size is ～ 100 nm. The dynamic mechanical analysis shows that the storage modulus of the spherical silica nanocomposite has been improved, and the glass transition temperature can be very high (～ 175°C). © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Homopolymerization effects in polymer layered silicate nanocomposites based upon epoxy resin: Implications for exfoliation

Exfoliation of polymer layered silicate nanocomposites based upon epoxy resin has previously been reported to be enhanced by allowing some homopolymerization of the resin to occur, catalyzed by the onium ion of the organically modified clay, before the addition of the cross‐linking agent and the curing of the nanocomposite. In this work we examine the effects of homopolymerization induced by pre‐conditioning the resin/clay mixtures by storing them at various temperatures, from room temperature to 100°C, prior to curing. It is found that pre‐conditioning results in similar increases in both the epoxy equivalent (EE) and the glass transition temperature (T_g) of the resin as a consequence of homopolymerization, with a linear relationship between EE and T_g that depends on the pre‐conditioning temperature. This is attributed to two different homopolymerization reaction mechanisms, activated monomer (AM) and activated chain end (ACE), the former dominating at high temperature and the latter at low temperature. The effects of these homopolymerization reactions on the network and nanostructure of the nanocomposite are discussed, the important aspect emerging being that the ACE mechanism is the one that most significantly enhances the exfoliation process. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009