Bulk fatigue damage evolution in polyamide‐6 and in a polyamide‐6 nanocomposite

The mechanical response of a polyamide‐6 montmorillonite clay nanocomposite and of a polyamide‐6 was monitored during axial fatigue tests performed at R‐ratios of 0.1 and −1. For both materials, two transitions were usually observed in the evolution of all the stress‐strain‐time parameters studied after similar numbers of loading cycles, suggesting interrelationships between the mechanisms of molecular reorganization. Fatigue test monitoring indicated an initial decrease in the storage modulus and a subsequent trend for this modulus to increase, especially in polyamide‐6. During all tests, a partially recoverable strain was accumulated because of viscoelastic deformation. Nanoparticles reduced this strain in the initial cyclic straining regime but not in the last regime, probably because such particles cannot inhibit viscoelastic events constrained in a volume larger than their interaction volume within the matrix. Based on the accumulated volume variation measured, the nucleation and growth of microvoids can be expected to occur in the last cyclic straining regime. POLYM. COMPOS., 26:636–646, 2005. © 2005 Society of Plastics Engineers     

Rheological behaviour of acrylate/montmorillonite nanocomposite latexes and their application in leather finishing as binders

The paper describes the rheological behaviour of nanocomposite latexes based on butylacrylate–co-methylmethacrylate–co-acrylamide terpolymers including various commercial nanoclays obtained via in situ emulsion polymerization. Rotational, oscillatory, emulsion stability and thixotropy tests (recovery) were performed to evaluate the influence of clay incorporation, type of clay and also emulsifier content in the composition of these nanomaterials. It was observed that the viscosities of the nanocomposite latexes were increased by clay incorporation at low shear rates, while a shear thinning effect was observed at higher shear rates. Oscillatory tests indicated a dominant elastic behaviour and high physical stabilities for all the nanocomposites. The hydrophobic character of the clay and emulsifier content also influenced the viscosity and dynamic modulus of the emulsions.     

Comparing the Continuous Chaotic and Shear Mixing-Induced Morphology Development of Polyblend and its Nanocomposites

Continuous chaotic and shear mixing-induced morphology development of 60/40 w/w polypropylene/polyamide 6 (PP/PA6) blend and its nanocomposite with 5 wt% clay was investigated. PP and PP nanocomposite were mixed with PA6 in a single-screw extruder, respectively. Two screw geometries were used to induce chaotic and shear mixing, respectively. It was demonstrated that for PP/PA6 blend, the PA6 domains were transformed from large particles to short striations and to small droplets finally in shear mixing, whereas morphology of PA6 phase developed from lamellar layers to a partial continuous structure finally in chaotic mixing. For (PP/clay)/PA6 blend nanocomposite, the PA6 domains were deformed gradually from droplets to irregular fibrils in shear mixing and from short layers to thin fibrils in chaotic mixing. The PA6 fibrils formed finally in the latter were much thinner and uniform than those in the former. The dynamic storage moduli of samples prepared in both shear and chaotic mixing with these PA6 fibrils presented a solid-like response at lower frequencies. Moreover, the clay platelets dispersed initially in the PP phase migrated into PA6 phase finally in both shear and chaotic mixing and the exfoliation of clay platelets in PA6 phase was obviously improved in the latter.     

Deformation and fracture behavior of polyamide nanocomposites: The effect of clay dispersion

This study describes the effect of the clay content and its dispersion on deformation and fracture behavior of polyamide nanocomposites. Two nanocomposite systems, intercalated and exfoliated nanocomposites containing layered silicate, were compared. They were prepared by melt‐compounding of polyamide with sodium montmorillonite or organophilized montmorillonite. It has been shown that while the exfoliated structure imparts to the nanocomposite higher stiffness and strength, the toughness is inferior to the intercalated nanocomposite. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The Effect of Viscoelasticity of Polymeric Adhesives on Shear Stress Distribution in a Single-Lap Joint

In this paper, the effect of viscoelasticity of adhesives on shear stress distribution in the adhesive layer of a single-lap joint under shear load is studied. The joint comprises two elastic single isotropic adherend layers joined by a viscoelastic adhesive polymer. A three-parameter viscoelastic solid model is used to deduce the governing differential equation in the Laplace domain, which is solved using residue theorem. Results show that for an impulse load of 100 N, the maximum shear stress in the adhesive layer is reduced to 39% of its initial value. Also, the ratio of viscous modulus of the adhesive to its shear modulus has an adverse effect on the peak shear stress. An increase in the thickness of the adhesive layer reduced the induced peak shear stress in the joint. Moreover, the shear strain in the adhesive layer reached its steady value after 1000 seconds.     

PA6/MMT/SGF复合材料的流变性能

利用毛细管流变仪和动态流变仪分别测试了纯尼龙(PA)6以及PA6/蒙脱土(MMT)和PA6/MMT/短玻璃纤维(SGF)复合材料的剪切粘度、复数粘度、储能模量和损耗模量,并用Han曲线分析了材料的粘弹特性.结果表明,在低剪切速率下,PA6/MMT和PA6/MMT/SGF复合材料的剪切粘度大于纯PA6的剪切粘度;在高剪切速率下,PA6/MMT和PA6/MMT/SGF复合材料剪切粘度低于纯PA6的剪切粘度.动态流变测试显示,PA6/MMT/SGF复合材料在低频区呈类固响应,其储能模量和复数粘度明显提高.Han曲线表明,PA6/MMT/SGF复合材料的粘性响应占主导地位.     

Constitutive modeling of HDPE polymer/clay nanocomposite foams

A constitutive model for tensile behavior of high density polyethylene (HDPE)/clay nanocomposite foams was proposed. The elastic modulus of HDPE/clay nanocomposite was developed using micromechanics theory, and the modulus for foams was obtained by using representative volume element (RVE) concept. In order to describe the tensile behavior of the foams, a constitutive equation obtained from a viscoelastic model was proposed. The constitutive model was expressed in terms of microstructural properties of polymer, and physical properties of the foams. The effects of the material parameters and processing conditions on the foam morphologies and mechanical properties of HDPE/clay nanocomposite foams were investigated. Microcellular closed-cell nanocomposite foams were manufactured with HDPE, where the nanoclay loadings of 0.5, 1.0, and 2.0 wt% were used. The effect of clay loading and foaming conditions on the volume expansion ratio, elastic modulus, tensile strength, and elongation at break was investigated. Except for the elongation at break, the mechanical properties were improved with nanoclay loading. The tensile experimental data of the foams were compared with the prediction by the theoretical model. It was demonstrated that the tensile behaviors of HDPE/clay nanocomposite foams were well described by the constitutive model.     

Ageing under oscillatory stress: Role of energy barrier distribution in thixotropic materials

In this work the ageing dynamics of soft solids of aqueous suspension of laponite has been investigated under the oscillatory stress field. We observed that, at small stresses elastic and viscous moduli showed a steady rise with the elastic modulus increasing at a faster rate than the viscous modulus. However, at higher stresses both the moduli underwent a sudden rise by several orders of magnitude with the onset of rise getting shifted to a higher age for a larger shear stress. We believe that the observed behavior is due to interaction of barrier height distribution of the potential energy wells in which the particle is trapped and strain induced potential energy enhancement of the particles. Strain induced in the material causes yielding of the particles that are trapped in the shallower wells. Those trapped in the deeper wells continue to age enhancing the cage diffusion timescale and consequently the viscosity, which lowers the magnitude of strain allowing more particles to age. This coupled dependence of strain, viscosity and ageing causes forward feedback for a given magnitude of stress leading to sudden rise in both the moduli. Changing the microstructure of the laponite suspension by adding salt affected the barrier heights distribution that showed a profound influence on the ageing behavior. Interestingly, this study suggests a possibility that any apparently yielded material with negligible elastic modulus may get jammed at a very large waiting time.     

Thermal and mechanical properties of PA12/C30B nanocomposites in relationship with nanostructure

This work focuses on the effect of nanoclay mass fraction on the properties of polyamide 12 matrix. Relationships between mechanical, thermal, and structural properties of polyamide 12/Cloisite^® 30B nanocomposites were studied. The material structure, previously described from XRD and TEM experiments, was more thoroughly characterized in the present work using SEM and FTIR techniques. The FTIR results clearly showed that clay galleries are intercalated by PA chains, which leads to a partially exfoliated nanostructure, confirming the TEM observations and the XRD analysis. However, a few micrometric aggregates are evidenced by SEM analysis, particularly at high clay fractions. TGA and DTA measurements showed that the thermal stability of PA12 matrix is slightly modified by the Cloisite^® 30B content. Viscoelastic properties of the nanocomposites in solid‐state were analyzed as functions of strain, frequency, and temperature. The extent of the linear response regime of the material is shown to be sensitive to the amount of clay: nonlinearities appear at lower strain values as the clay mass fraction increases. Both relative dynamical moduli also increase with increasing clay mass fraction, with frequency dependence for the viscous modulus and without frequency dependence for the elastic modulus. Finally, similarities have been pointed out between viscoelastic properties of the nanocomposites in solid and melt states. For example, the percolation threshold is highlighted at the same clay mass fraction, ～2%, in both states. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41938.     

Nanoindentation and morphological studies on nylon 66/organoclay nanocomposites. II. Effect of strain rate

Strain rate effects on surface deformation behavior of exfoliated nylon 66 (PA66)/organoclay nanocomposites have been explored by nanoindentation in present study. Sharp indenter (Berkovich) has been used to indent on the surfaces of polymer/clay nanocomposite with different strain rates. Significant strain-rate hardening has been found consistently existing in both neat PA66 and its nanocomposite systems from surface to subsurface (a few micron deep into the bulk). However, strain rate shows almost no effect on the elastic moduli of the neat system and the nanocomposites. The elastic modulus and hardness increase with the indentation depth due to inhomogeneous distributions of the crystalline morphology as well as clay concentration for the case of the nanocomposites along the indentation direction. The mechanical properties observed are correlated with the inhomogeneous microstructures of the studied systems. The plastic index of PA66 and the nanocomposites have been evaluated as a function of strain rate.     

Quantification of surface forces of thermoplastic elastomeric nanocomposites based on poly(styrene‐ethylene‐co‐butylene‐styrene) and clay by atomic force microscopy

Atomic force microscopy was used for qualitative phase morphological mapping as well as quantitative investigation of surface forces measured at constituting blocks and clay regions of a thermoplastic elastomeric nanocomposite based on triblock copolymer: poly(styrene‐ethylene‐co‐butylene‐styrene) (SEBS) and organically modified nano‐clay. The roughness and power spectral density analyses of surface topography provided the increment in random roughness of the nanocomposite surface compared to pristine SEBS surface. The same surfaces were examined by means of single point force‐distance, and force‐volume measurements. Large adhesive force of 25 nN and contact force of 260 nN were found in soft polyethylene (PEB) segments and higher cantilever deflection of 210 nm was found for clay regions of SEBS‐clay nanocomposite. Mapping of elastic modulus of the glassy and rubbery blocks and clay regions was probed by employing Hertzian and JKR model from respective approaching and retracting parts of force‐distance curves. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Moisture sorption and permeation in polyamide 6/clay nanocomposite films

The moisture transport properties of a polyamide 6/clay nanocomposite have been studied through moisture sorption and permeation rate measurements. The temperature and concentration dependence of the steady state moisture uptake and permeation rate from the two types of experiment are distinctly different. The activation energy of moisture permeation obtained from the sorption experiment is lower than that derived from the permeation measurement. The interaction and contribution of the diffusion parameter and solubility parameter shows a complex moisture transport behavior in these nanocomposite films. Copyright © 2004 Society of Chemical Industry     

The influence of matrix viscosity and composition on the morphology,rheology, and mechanical properties of thermoplastic elastomer nanocomposites based on EPDM/PP

The morphological and rheological properties of thermoplastic elastomer nanocomposites (TPE nanocomposites) were studied using different viscosities of polypropylene (PP) and ethylene‐propylene‐diene monomer (EPDM) rubber content (20, 40, 60 wt%). The components, namely EPDM, PP, Cloisite 15A, and maleic anhydride‐modified PP as compatibilizer, were compounded by a one‐step melt mixing process in a laboratory internal mixer. The structure of the nanocomposites was characterized with X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, and rheometry in small amplitude oscillatory shear. The distribution state of the clay between the two phases (PP and EPDM) was found to be dependent on the viscosity ratio of PP to EPDM. In the nanocomposites prepared based on low viscosity PP (LVP) and EPDM, the clay was mostly dispersed into the PP phase and the size of the dispersed rubber particles decreased in comparison with unfilled but otherwise similar blends. However, the dispersed elastomer droplet size in the high viscosity PP (HVP) blends containing 40 and 60% EPDM increased with the introduction of the clay. For TPE nanocomposites, the dependence of the storage modulus (G′) on angular frequency (ω) followed a clear nonterminal behavior. The increase in the storage modulus and the decrease in the terminal zone slope of the elastic modulus curve were found to be larger in the LVP nanocomposite in comparison with the HVP sample. The yield stress of nanoclay‐filled blends prepared with LVP increased more than that of HVP samples. The tensile modulus improved for all nanocomposites but a higher percentage of increase was observed in the case of LVP samples. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Melt rheology of organoclay and fumed silica nanocomposites

The objective of the present work is to investigate, from the open literature, the recent developments in the rheology of silica and organoclay nanocomposites. In particular, this paper focuses on general trends of the linear viscoelastic behaviour of such nanocomposites. Hence, the variations of the equilibrium shear modulus and critical strain (limit of linearity), which depend on power laws of the volume fraction of particles, are discussed as filler fractal structure. In the third section, the strong nonlinearity behaviour (Payne effect) of filled polymers has been discussed in terms of filler nature. Typically two mechanisms arise to depict the linear solid-like behaviour and the Payne effect: particle–particle interactions is the dominant mechanism in fumed silica nanocomposites whereas particle–polymer interaction is the dominant one in colloidal silica nanocomposites at identical filler concentrations. However, these interactions are balanced in each nanocomposite systems by the silica surface treatments (chain grafting, silane modification) and the molecular weight of the matrix. Finally, we aim to unify the main findings of the literature on this subject, at least from a qualitative point of view.We finally report on the thixotropy and modulus recovery after a large deformation in steady and dynamic shear conditions. Following this, the nonlinear rheological properties of nanocomposite materials have been discussed. The discussion is particularly focused on the effect of flow history (transient shear experiments) on the orientation–disorientation of clay platelets. Actually, the linear and nonlinear rheological properties are consistent with a network structure of a weakly agglomerated tactoids. As far as exfoliated clay nanocomposites are concerned, the inter-particle interaction is the dominant effect in the nonlinearity effect.     

Dynamic rheological study of paraffin wax and its organoclay nanocomposites

Dynamic rheological data for paraffin wax and its organoclay nanocomposites are reported. Dynamic mechanical analysis of paraffin wax for temperatures ranging from ?40 to 55°C showed a decrease of several orders of magnitude in the dynamic moduli and a significant shift toward viscous behavior, which resulted from the occurrence of two solid–solid phase transitions. In both the crystalline and mesophase regions, the dispersion of organoclay platelets in paraffin wax via ultrasonication increased the storage modulus, whereas the effect on the loss modulus was temperature‐dependent. The melt rheology data of the wax–clay nanocomposites at 70°C showed that the complex viscosities increased monotonically with clay addition and demonstrated shear‐thinning behavior for frequencies between 0.1 and 100 rad/s. The complex viscosity versus angular frequency data were well fit by a power‐law function for which the shear‐thinning exponent provides a gauge for the extent of clay exfoliation. The nanocomposites exhibited low‐frequency solid behavior, which indicated good exfoliation of the organoclay in the wax matrix. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Preparation and characterisation of polyamide–polyimide organoclay nanocomposites

BACKGROUND: Ternary nanocomposites containing an organomodified layered silicate polyimide additive within a polyamide matrix have been investigated to gain greater insight into structure–property relationships and potential high‐temperature automotive applications. RESULTS: Polyamide nanocomposite blends, containing 3 wt% of organoclay, were prepared and compared with organoclay‐reinforced polyamide and neat polyamide. Nanoclay addition significantly increased heat distortion temperature, as well as both the tensile and flexural moduli and strength. The addition of polyimide demonstrated further increases in heat distortion temperature, glass transition temperature and the flexural and tensile moduli by about 17, 21 and 40%, respectively. The tensile and flexural strengths were either unaffected or decreased modestly, although the strain‐to‐failure decreased substantially. Morphological studies using transmission electron microscopy (TEM) and X‐ray diffraction showed that the nanoclay was dispersed within the ternary blends forming highly intercalated nanocomposites, regardless of the presence and level of polyimide. However, TEM revealed clay agglomeration at the polyamide–polyimide interface which degraded the mechanical properties. CONCLUSIONS: A range of improvements in mechanical properties have been achieved through the addition of a polyimide additive to a polyamide nanocomposite. The decrease in ductility, arising from the poor polyamide–polyimide interface and nanoclay clustering, clearly requires improving for this deficiency to be overcome. Copyright © 2008 Society of Chemical Industry     

Indentation of an oriented transparent polyamide

In this study, the effect of orientation on the indentation hardness and energy absorption of an oriented transparent Trogamid polyamide was investigated with a spherical indentation methodology. It was found that the orientation significantly improved the indentation hardness and energy absorbed by plastic deformation. From the indentation hardness measurement, the elastic modulus, yield stress, and strain hardening exponent were derived from both the elastic and plastic regions of the indentation load–displacement curves. The elastic modulus was found to remain the same with orientation; the yield stress and the strain hardening exponent increased with orientation. The increase in the strain hardening exponent was the primary reason for the improved indentation hardness and energy absorption in the oriented samples. The mechanical properties from indentation measurements were compared to values obtained from tension true stress/true strain measurements. Good agreement was observed between the results from the indentation and tension tests. The effect of temperature on the mechanical properties was also studied. It was found that the modulus and yield stress were higher at a lower temperature; however, the strain hardening exponent remained unaffected. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Melt compounding of various polymers with organoclay by shear flow

We studied melt compounding of polymer/organoclay composites by shear flow in a rotating cylindrical mixer to investigate an effect of shear stress on the dispersion state of clay. The commercial organoclay, which is intercalated by dimethyl benzyl stearyl ammonium ion between clay platelets, and four kinds of commercial polymer (polystyrene (PS), poly(lactic acid) (PLA), polyamide(PA6), and poly(butylene succinate) (PBS)) were used in this study. According to the TEM photographs, there is exfoliated clay in PA6/organoclay composite, but the exfoliated clay cannot be seen in other polymer/organoclay composites. We calculated the value of clay dispersion from the low magnification TEM photograph, called the dispersion coefficient to consider the micro level dispersion of the clay in a polymer matrix. Generally, the dispersion coefficient increases with shear stress. However, the dispersion coefficients in case using PA6 and PBS as the matrix, whose melts have low viscosity, are larger than those in case using PS and PLA, whose melts have high viscosity. According to XRD results, d₀₀₁ peak originated from organoclay mostly shifted to the low value of angle for PA6/organoclay composite and began shifting to the low value of angle for PBS/organoclay composite. However, there is a peak for PS and PLA based organoclay composites. From the cases of PA6 and PBS in the DMA results, the storage modulus increases with an addition of organoclay. These results imply that low viscosity polymer is typical easy to get the composites with intercalated clay by melt compounding. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Constitutive modeling for intercalated PMMA/clay nanocomposite foams

A constitutive model for tensile behavior of PMMA/clay nanocomposite foams was developed in this study. The elastic modulus of the nanocomposites is affected by the form of clays embedded in the polymer matrix. The reinforcing effect by intercalation of the clays and the detrimental effect by clay agglomeration were considered for the determination of the elastic modulus of the nanocomposites. A viscoelastic model was adapted for the tensile behavior of the material. The developed constitutive equation is expressed in terms of clay morphology and material properties. The aspect ratio of clays and the expansion of clay layer spacing in the intercalated clay clusters were proved to play a vital role in the reinforcing mechanism. For the verification of the constitutive model, Poly(methyl‐methacrylate) (PMMA)/clay nanocomposite foams were manufactured by batch process method and their uniaxial tensile test results were compared with theoretical predictions. Compared with the experimental results, the proposed constitutive equation showed agreement with the experimental test results. POLYM. ENG. SCI. 46:1787–1796, 2006. © 2006 Society of Plastics Engineers.     

Shear and extensional rheometry of PA6 reinforced with polyacrylic nanoparticles

Shear and extensional rheometry was carried out on polyamide 6 (PA6) reinforced with crosslinked polyacrylic nanoparticles (PNPs) with mean size of 8 nm. The PNPs were dispersed into a commercial, injection grade, PA6 matrix by melt extrusion, at a concentration of 3 wt%. Thermal analysis showed that the PNPs did not influence the melting and decomposition temperature of the polymer matrix. However, grafting of maleic anhydride to the PNPs (denoted PNP-g-MA) increased the decomposition temperature. On the other hand, X-ray scattering and small-angle light scattering showed that the degree of crystallinity and crystal size were reduced, relative to the neat PA6, i.e., the PNPs disrupted the ordering of the polymer chains. The shear rheological properties showed that the PA6/PNP nanocomposites exhibited a linear viscoelastic behavior. Small-strain oscillatory shear showed that PA6 exhibited a predominantly viscous behavior. However, addition of PNPs induced a predominantly elastic behavior, as measured by the mechanical damping tan δ (=G″/G′), and increased the zero-shear viscosity. The increase in melt elasticity and viscosity was greater for the PA6/PNP-g-MA nanocomposite. Extensional rheometry experiments demonstrated that when PNPs were added to PA6, they induced smaller extensional viscosity, η _ext, values in the matrix, at low strain rates. However, at higher strain rates the PNPs induced a strain hardening behavior. Whereas the neat polymer melt rapidly broke under extensional flow, the PA6/PNP nanocomposites first exhibited lower η _ext than the neat PA6, and then η _ext rapidly increased before breaking, i.e. a strain hardening behavior. The higher melt elasticity of the molten PA6 nanocomposites appears to arise from a jamming effect promoted by the PNPs.