Synergistic effect of carbon nanotubes and layered double hydroxides on the mechanical reinforcement of nylon-6 nanocomposites

Synergistic effect in network formation of nylon-6 （PA6） nanocomposites containing one dimensional （ID） multi-walled carbon nanotubes （CNTs） and two dimensional （2D） layered double hydroxide （LDH） platelets on improving the mechanical properties has been studied. Mechanical tests show that, with incorporation of 1 wt% LDHs and 0.5 wt% CNTs, the tensile modulus, the yield strength as well as the hardness of the ternary composite are greatly improved by about 230%, 128% and 110% respectively, as compared with neat PA6. This is mainly attributed to the unique, strong interactions between the CNTs and the LDHs as well as the jammed network-like structure thus formed between the nanofillers, as confirmed by the morphological observations. As compared with the binary nanocomposites, a much enhanced solid-like behavior in the terminal region of the rheological curves can clearly be observed for the ternary system, which also indicates the formation of a percolating filler network.     

LOW DENSITY POLYETHYLENE/CLAY NANOCOMPOSITES MODIFIED BY ETHYLENE COPOLYMERS： EFFECTS OF FUNCTIONALIZED SEGMENTS ON MORPHOLOGY

Melt extrusion was used to prepare binary nanocomposites of ethylene copolymers and organoclay and trinary nanocomposites of low-density polyethylene （LDPE）, ethylene copolymer and organoclay. X-ray diffraction （XRD） and transmission electron microscopy （TEM） were used to analyze the structure of the clay phase and the morphology of the nanocomposites. Influences of the comonomer in the copolymer and the content of the copolymer on the morphology of the resulting nanocomposites were discussed. The binary and the trinary composites may form intercalated or exfoliated structures depending on the interaction between the copolymer and the clay layers and the content of the copolymer.     

DISPERSION AND TENSILE BEHAVIOR OF POLYPROPYLENE/MONTMORILLONITE NANOCOMPOSITES PRODUCED VIA MELT INTERCALATION

Most of the anicles on polymer nanocomposites focus on the importance of chemistry used to modify the surfaceof the clay, usually montmorillonite (MMT), and characterization of the nano-scale structure obtained. The role andimportance of processing were also discussed recently. However, few papers concerning the correlation between morphologyof MMT and mechanical properties were published. In order to understand the tensile behavior of PP/Montmorillonite(MMT) nanocomposites better, and to further improve the reinforcement efficiency, we first prepared the PP nanocompositesvia direct melt intercalation using conventional twin-screw extrusion. The dispersion and tensile property of the compositeswere then investigated by SEM, XRD, TEM and a video-controlled tensile set-up. The macroscopic and microscopicdispersion of MMT in PP matrix was verified by XRD and TEM, combined with SEM. The tensile properties were obtainedby video-controlled tensile set-up, which gives true stress-strain curve. It was found that a partly intercalated and partlyexfoliated structure (also called incomplete exfoliation) existed in the system. Though the tensile strength of PPnanocomposites is not much improved in engineering stress-strain curves, more than 20% increase of true stress was found ina true stress-strain experiment at high true strain, which indicates that only oriented silicate layers can have a big effect ontensile properties. Not only orientation of silicate platelets but also the degree of exfoliation is a key factor to determine thereinforcement efficiency. The reinforcement efficiency of MMT has been discussed based on the "continuum" Halpin-Tsaiequations. A good agreement was found between experimental data and theoretical prediction by changing N value (number of platelets per stack) which corresponding to different state of the dispersion of MMT in PP matrix.     

Polymer-clay nanocomposites: chemical grafting of polystyrene onto Cloisite 20A

An account of the experiments on preparing polystyrene(PS) nanocomposites through grafting the polymer onto organophilic montmorillonite is reported.Cloisite 20A was reacted with vinyltrichlorosilane to replace the edge hydroxyl groups of the clay with a vinyl moiety.Because the reaction may liberate HC1,it was performed in the presence of sodium hydrogencarbonate to prevent the exchange of quaternary alkylammonium cations with H~+ ions.Only the silanol groups on the edge of the clay react with vinyltrichlorosilane.The radical polymerization of the product with styrene as a vinyl monomer leads to chemical grafting of PS onto the montmorillonite surface.The homopolymer formed during polymerization was separated from the grafted organoclay by Soxhlet extraction.Chemical grafting of the polymer onto Cloisite 20A was confirmed by infrared spectroscopy.The prepared nanocomposite materials and the grafted nano-particles were studied by XRD.Exfoliated nanocomposites may be obtained for 0.5 wt%-l wt%clay content.The nanocomposites were studied by thermogravimertic analysis(TGA) dynamic thermal analysis(DTA) and dynamic mechanical analysis (DMTA).     

Rheology of Poly(vinyl butyral) Solution Containing Fumed Silica in Correlation with Electrospinning

The rheological properties in question are influenced by many factors, ranging from the characteristics of the given polymer or solvent to the flowing conditions. The primary focus of this study is to analyse the rheological behaviour of poly(vinyl butyral)—Mowital B 60 H—(PVB) solutions dissolved in methanol and a blend of these with fumed silica nanoparticles. The preparation of the nanofibrous web and the quality of nanofibres were correlated with the rheology of the polymer solution. It was discerned that drastically intensifying shear viscosity and the elasticity of the solution exerted a negligible effect on the formation of fibres, a finding which has rarely been discussed in the literature. The morphologies and structures of the PVB/silica nanofibrous membranes were investigated by scanning electron microscopy, thermogravimetric analysis and Fourier transform infrared spectroscopy.     

Stress-Structure Relationship of the Reversible Associating Polymer Network under Start-up Shear Flow

We adopt Langevin dynamics to explore the stress-structure relationship of telechelic reversible associating polymer gel during startup shear flow, with shear strengths varying from Wi=12.6 to Wi=12640. At weak shear flow Wi=12.6, the shear stress proportionally increases with shear strain at short times, followed by a strain hardening behavior and then passes through a maximum(σmax, γmax) and finally decreases until it reaches the steady state. During the evolution of stress, the gel network is only slightly broken and essentially maintains its framework, and the strain hardening behavior originates from the excessive stretching of chains. On the other hand, the stress-strain curve at intermediate shear flow Wi=505.6 shows two differences from that at Wi=12.6, namely, the absence of strain hardening and a dramatic increase of stress at large strains,which is caused by the rupture of gel network at small strains and the network recovery at large strains, respectively. Finally, at very strong shear flow Wi=6319.7, the gel network is immediately broken by shear flow and the stress-strain curve exhibits similar behaviors to those of classical polymeric liquids.     

STUDY ON INTERMITTENT SHEAR FLOW AND RELAXATION BEHAVIOR OF THERMOTROPIC LIQUID CRYSTALLINE POLYMER

Intermittent shear flow including start-up flow and small oscillatory amplitude time sweep or stress relaxation after cessation of shear flow was used to study the rheological behavior and internal structure of thermotropic liquid crystalline polymer (TLCP). There are two kinds of intermittent shear flow: all start-up flows are in the same direction (intermittent flow forward: IFF) and start-up flows change their directions alternately (intermittent flow reversal: IFR). The results show that the stress of start-up flow of IFF and IFR in the test process is not superposed, indicating different changes of internal structure of thermotropic LCP (TLCP). Two main factors affect structure changes in the experimental time scale. One relates to long-term texture relaxation process, the other is an interchain reaction that becomes important after 30 min. The two factors raise the stress of IFF, but express complex effects for the stress of IFR. The latter factor becomes very important at long time annealing process. The relaxation behavior was also studied by the application of wide range relaxation spectrum calculated from the combined dynamic modulus, which gave three characteristic relaxation times (0.3, 10 and 600 s) ascribable to the relaxations of less-phase orientation, domain orientation, and domain deformation, respectively. The result also shows that the domain coalescence (texture relaxation), a long relaxation time, is a much slow process and lasts beyond 2400 s of the test time.     

EFFECTS OF COUPLING AGENTS ON THE RHEOLOGICAL BEHAVIOR OF KAOLIN FILLED POLYAMIDE 6

An experimental study was carried out to investigate the effects of coupling agents on the rheological properties of kaolin filled polyamide 6(PA6). We have investigated the state of dispersion and interfacial interaction of the filled systems, using 'h:anning electron microscopy (SEM) and Molau test, respectively. It is found that the addition of the coupling agents to the PA6/ kaolin (20 wt percent) significantly decreases the melt viscosity and the melt elasticity (first normal stress difference). Moreover, the states of dispersion and the polymer/filler interactions have significant influences on the rheological properties of kaolin/PA6 systems. The rheological behavior of KH550 kaolin /PA6 system is different from that of KH560 kaolin/PA6 system, although chemical reactions have taken place between the surface of KH550 kaolin (or KH560 kaolin) and PA6 matrix during melt processing. This is attributable, in part, to the differences in the state of dispersion of kaolins in PA6 matrix and, to a great extent, to the differences in the extent of chemical reactions that have taken place between the filler and polymer matrix.     

The effect of particle shape on the structure and rheological properties of carbon-based particle suspensions

The structure and rheological properties of carbon-based particle suspensions, i.e., carbon black(CB), multi-wall carbon nanotube(MWNT), graphene and hollow carbon sphere(HCS) suspended in polydimethylsiloxane(PDMS), are investigated. In order to study the effect of particle shape on the structure and rheological properties of suspensions, the content of surface oxygen-containing functional groups of carbon-based particles is controlled to be similar. Original spherical-like CB(fractal filler), rod-like MWNT and sheet-like graphene form large agglomerates in PDMS, while spherical HCS particles disperse relatively well in PDMS. The dispersion state of carbon-based particles affects the critical concentration of forming a rheological percolation network. Under weak shear, negative normal stress differences(ΔN) are observed in CB, MWNT and graphene suspensions, while ΔN is nearly zero for HCS suspensions. It is concluded that the vorticity alignment of CB, MWNT and graphene agglomerates under shear results in the negative ΔN. However, no obvious structural change is observed in HCS suspension under weak shear, and accordingly, the ΔN is almost zero.     

The variable role of clay on the crystallization behavior of DMDBS-nucleated polypropylene

The effect of clay on the nucleating behavior of 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol(DMDBS) in cryatallization of isotactic polypropylene(iPP) was investigated by means of differential scanning calorimetry(DSC), dynamic rheology and polarized light microscopy(PLM).It is interesting to note that the incorporation of layered clay nanoparticles into DMDBS-nucleated iPP may induce a synergetic nucleation effect while the DMDBS content is below 0.1 wt%,otherwise it restricts the crystallization rate prominently as the DMDBS content increases up to 0.3 wt%,which has exceeded the content threshold to yield a nucleating agent(NA) network.As shown by dynamic rheological investigations, the clay nanoparticles demonstrate an obstructive effect of disturbing the consistency of DMDBS fibrils network.Moreover, to further demonstrate the importance of NA network formation in the crystallization of iPP,we used another NA named HPN-20e,which can not form network structure at all over the concentration studied,for comparison.In this case,the nucleated-crystallization rate is independent on the addition of clay nanoparticles,as the nucleating mechanism is an individual nuclei manner without NA network forming.     

STRUCTURING & RHEOLOGY OF MOLTEN POLYMER/CLAY NANOCOMPOSITES

The evolution and the origin of "solid-like state" in molten polymer/clay nanocomposites are studied. Using polypropylene/clay hybrid (PPCH) with sufficient maleic anhydride modified PP (PP-MA) as compatibilizer, well exfoliation yet solid-like state was achieved after annealing in molten state. Comprehensive linear viscoelasticity and non-linear rheological behaviors together with WAXD and TEM are studied on PPCH at various dispersion stages focusing on time,temperature and deformation dependencies of the "solid-like" state in molten nanocomposites. Based on these, it is revealed that the solid-structure is developed gradually along with annealing through the stages of inter-layer expansion by PP-MA,the diffusion and association of exfoliated silicate platelets, the formation of band/chain structure and, finally, a percolated clay associated network, which is responsible for the melt rigidity or solid-like state. The network will be broken down by melt frozen/crystallization and weakened at large shear or strong flow and, even more surprisingly, may be disrupted by using trace amount of silane coupling agent which may block the edge interaction of platelets. The solid-like structure causes characteristic non-linear rheological behaviors, e.g. residual stress after step shear, abnormal huge stress overshoots in step flows and, most remarkably, the negative first normal stress functions in steady shear or step flows. The rheological and structural arguments challenge the existing models of strengthened entangled polymer network by tethered polymer chains connecting clay particles or by chains in confined melts or frictional interaction among tactoids. A scheme of percolated networking of associated clay platelets, which may in band form of edge connecting exfoliated platelets, is suggested to explain previous experimental results.     

Rheology of nanocomposites

In this study, a methodology is developed for the quantitative characterisation of the nanofiller network in polymer nanocomposites via dynamic rheometry. Nanoclay-reinforced poly(ε-caprolactone) (PCL) nanocomposites were prepared by melt mixing. Frequency sweep experiments in the melt state display at low frequencies a solid-like elastic response that can be attributed to the formation of a physical nanofiller network. Combining a semi empirical model and the time–temperature superposition principle permits a reliable determination of the zero shear modulus that characterises the solid-like response of nanocomposites at low frequency, and which is related to the nanofiller dispersion.     

Clay network in ABS-graft-MAH nanocomposites: Rheology and flammability

A melt blending method was used to prepare ABS/clay and ABS-g-MAH/clay nanocomposites. Cone calorimeter and advanced rheological extension system (ARES^©) were employed to measure flammability and dynamic rheological properties. The main aim is to establish a relationship between the clay network structure and flammability properties of polymer nanocomposites. From the results of dynamic rheological measurements, it was found that the clay network structure was formed in ABS-g-MAH/clay nanocomposites, which strongly affected the flammability properties of the nanocomposites. The clay network improves the melt viscosity and results in restraint on the mobility of the polymer chains during combustion, which leads to significant improvement of flame retardancy for the nanocomposites.     

Molecular dynamics simulations of nanocomposites based on poly(epsilon-caprolactone) grafted on montmorillonite clay

Intercalated and exfoliated models of polymer nanocomposites based on poly(epsilon-caprolactone) and functionalized montmorillonite clay are studied by means of molecular dynamics simulations. Intercalated and exfoliated models are considered for probing the structural characteristics of the corresponding nanocomposites prepared by melt intercalation and in situ polymerization, respectively. In the exfoliated system, the organization of the polymer chains onto the clay surface is examined in terms of the density profiles and the order parameter function. A layered structure can clearly be seen to form near the surface with density maxima higher than in amorphous poly(epsilon-caprolactone). This can be viewed as an increase in effective particle thickness, which can contribute to the outstanding gas barrier properties of the exfoliated nanocomposites. The comparison of the structures and energetics of the intercalated model with those of a nanocomposite model based on a nonfunctionalized clay indicates nearly similar characteristics. Nevertheless, the slight differences observed for the interfacial polymer density and clay- and surfactant-polymer binding energies can account for the differences in rheological measurements. The results also suggest that the difference in morphology obtained for the nanocomposites prepared by the two synthetic approaches can be ascribed to both a difference in interfacial polymer density and the formation of bridging polymer chain structures that hinder the exfoliation process.     

In situ polymerized poly(butylene succinate)/silica nanocomposites: Physical properties and biodegradation

A series of poly(butylene succinate)/silica (PBS/silica) nanocomposites were prepared by in situ polymerization. Solid-state ²⁹Si NMR and FTIR analysis indicated that silanol-bonded carbonyl groups are established within PBS/silica nanocomposite materials. Rheological effects inherent to the silica filler were evaluated by melt rheological analysis as a function of shear force in the molten state. Despite high shear force, PBS/silica nanocomposites maintained a relatively high melt viscosity, attributable to a network structure resulting from covalent bonding between silica and the polymer chain. Nanocomposite material containing 3.5 wt% silica exhibited greatly improved mechanical properties. The tensile strength at break and elongation were ca. 38.6 MPa and 515%, while those of the parent PBS were 26.3 MPa and 96%, respectively. PBS/silica nanocomposites showed composition dependency on biodegradation ascribable to reduced crystallinity and preferential microbial attack.     

Effect of flocculated structure on rheology of poly(butylene terephthalate)/clay nanocomposites

The microstructure and rheological property of poly(butylene terephthalate) (PBT)/epoxy/montmorillonite nanocomposites (PCNs) were investigated. For the study, PCNs were prepared by melt intercalation in clay content of 4 wt % and, epoxy loadings were varied from 2 to 4 wt %. The intercalated PCNs are characterized by different techniques such as transmission electron microscopy, Fourier transform infrared and rheology. It is interesting that the percolated tactoids network in the ternary hybrids becomes insensitive to the shear deformation with the addition of epoxy in contrast to that in the sample without epoxy, which can be attributed to the formation of a flocculated structure of clay tactoids because of the chain‐extension reactions between PBT matrix and epoxy and possible hydrogen bonding. The flocculated structure has influence on the rheological behavior of the hybrids remarkably, strengthening the percolated strong‐associated‐tactoids network and reducing the percolation threshold, while not changing the strain‐scaling. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2807–2818, 2005     

Nanocomposites with Biodegradable Polycaprolactone Matrix

Summary: Biodegradable polymer/clay nanocomposites and/or composites based on poly(ε-polycaprolactone) (PCL) were prepared by conventional melt mixing. Three kinds of clays, organomodified Cloisite 15A and Cloisite 10A with different ammonium cations located in the silicate gallery and unmodified Cloisite with Na cations were used for composites preparation. The degree of dispersion of silicate layers in the matrix was determined by X-ray diffraction and transmission electron microscopy. Oscillatory rheological measurements were used for characterization of the physical network formed by the filler. The presence of intercalated and exfoliated structures were observed for the composites PCL/Cloisite 15A and PCL/Cloisite 10A, indicating that nanocomposite structure was formed. Changes of viscoelastic properties to more solid-like behavior, especially in the low frequency range were explained by formation of silicate network structure, which can be detected by modified Cole-Cole plots.     

An observation of accelerated exfoliation in iPP/organoclay nanocomposite as induced by repeated shear during melt solidification

A well‐exfoliated morphology is usually observed for polar polymer/clay nanocomposites via dynamic melt processing techniques, whereas only an intercalated or a partially intercalated/partially exfoliated morphology is often obtained for nonpolar polymer/clay nanocomposites, even though some polar compatibilzer is used. In this study, an accelerated exfoliation effect was observed for the first time in iPP/organoclay nanocomposites prepared through so‐called dynamic packing injection molding, in which the specimen is forced to move repeatedly in a chamber by two pistons that move reversibly with the same frequency as the solidification progressively occurs from the mold wall to the molding core part. The disordered level and exfoliated degree of clay was found to dramatically increase from the skin to the core of the prepared samples and eventually the WAXD reflections of interlayer d‐spacing diminished in the core. The changed degree of exfoliation was also proved directly by TEM observation. The prolongation of processing time, the gradual growth of solidification front, the increased melts viscosity, and the shear amplification effect were considered to explain the higher degree of exfoliation in the center zone of mold chamber. Our result suggests that a critical shear force may be needed to break down clay into exfoliated structure. This can be also well used to explain at least partially the intercalated morphology, which is commonly observed for nonpolar polymer/clay nanocomposites via conventional processing. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2005–2012, 2005     

Microscopic morphology of chlorinated polyethylene-based nanocomposites synthesized from poly(epsilon-caprolactone)/clay masterbatches

Chlorinated polyethylene (CPE) nanocomposites were synthesized by melt blending clay-rich/poly(epsilon-caprolactone) (PCL) masterbatches to CPE matrices. The masterbatches were prepared following two synthetic routes: either PCL is melt-blended to the clay or it is grafted to the clay platelets by in situ polymerization. The microscopic morphology of the nanocomposites was characterized by X-ray diffraction, atomic force microscopy, transmission electron microscopy, and modulated temperature differential scanning calorimetry. When using free PCL, intercalated composites are formed, with clay aggregates that can have micrometric dimensions and a morphology similar to that of the talc particles used as fillers in commercial CPE. PCL crystallizes as long lamellae dispersed in the polymer matrix. When using grafted PCL, the nanocomposite is intercalated/exfoliated, and the clay stacks are small and homogeneously dispersed. PCL crystallizes as lamellae and smaller crystals, which are localized along the clay layers. Thanks to the grafting of PCL to the clay platelets, these crystalline domains are thought to form a network with the clay sheets, which is responsible for the large improvement of the mechanical properties of these materials.     

聚合物基粘土纳米复合材料的流变行为研究

聚合物基粘土纳米复合材料具有与常规颗粒填充体系类似的流变特性 :在整个频率范围内 ,储能模量和损耗模量均随粘土含量的增加而变高 ,其频率依赖性会表现出非未端行为 :且当粘土含量超过临界值以后 ,储能模量会在低频区表现出似固体的平台发展。但与之不同的是前者在低粘土含量的条件下 (<10 % (wt) )就会表现出似固体行为或非末端行为。这些流变特性还会受到粘土的径厚比、化学特性、聚合物基体的分子结构参数和粘土与基体间的相互作用强度等因素的影响。聚合物基粘土纳米复合材料的流变行为是与其微观结构的形成和演化以及聚合物分子链在特定环境下的粘弹松弛过程紧密联系在一起的。本文综述了插层型、剥离型和聚合物分子链一端受限剥离型聚合物基粘土纳米复合材料在力场作用下的流变特性和粘弹松弛机理方面的研究进展。