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.     

Nanoindentation and morphological studies on injection-molded nylon-6 nanocomposites

The nanoindentation behavior and morphology of the injection-molded specimens of nylon-6 (PA6)/clay nanocomposites prepared by melt-compounding have been studied in present study. The elastic and plastic properties as well as creep behavior of PA6 and its nanocomposites are comparatively evaluated as the function of clay loading by using nanoindentation technique. The anisotropic characteristics in mechanical properties are studied by indenting the injection-molded specimens in two different directions (i.e. parallel and perpendicular to the injection direction). The uneven distribution of both the clay nanofiller and the crystallinity of the polymeric matrix induced by melt-processing leads to the variation of the mechanical property of the nanocomposites in certain directions and locations within the molded specimens. The microstructural and morphological changes of PA6 upon incorporating with clay nanofiller are evidenced by transmission electron microscopy and small-angle X-ray scattering, which are closely correlated with the anisotropy of the mechanical properties observed by nanoindentation.     

Effect of sodium montmorillonite source on nylon 6/clay nanocomposites

The effect of sodium montmorillonite source on the morphology and properties of nylon 6 nanocomposites was examined using equivalent experimental conditions. Sodium montmorillonite samples acquired from two well-known mines, Yamagata, Japan, and Wyoming, USA, were ion exchanged with the same alkyl ammonium chloride compound. The resulting organoclays were extruded with a high molecular weight grade of nylon 6 under the same processing conditions. Quantitative analysis of TEM photomicrographs of the two nanocomposites reveal a slightly larger average particle length and a slightly higher degree of platelet exfoliation for the Yamagata based nanocomposite than the Wyoming version, thus, translating into a higher particle aspect ratio. The stress-strain behavior of the nanocomposites appears to reflect the nanocomposite morphology, in that higher stiffness and strengths are attainable with the increased particle aspect ratio. Moreover, the trends in stiffness behavior between the two types of nanocomposites may be explained by conventional composite theory.     

Polymer matrix degradation and color formation in melt processed nylon 6/clay nanocomposites

Nylon 6 nanocomposites based on various quaternary alkyl ammonium organoclays were prepared by melt processing using a twin screw extruder. Dilute solution viscosity techniques were used to evaluate the level of polymer molecular weight degradation experienced during nanocomposite compounding; whereas colorimeter techniques were used to document color formation. In general, a significant reduction in nylon 6 matrix molecular weight was observed, which is believed to stem, in part, from reaction(s) between the surfactant of the organoclay and the polyamide chains. The level of degradation depends on both the type of nylon 6 material used and the surfactant chemistry in the organoclay. For a given organoclay, nanocomposites based on high molecular weight nylon 6 materials experience more matrix degradation, as well as color formation, than those based on low molecular weight materials; this is believed to arise from increased exposure of the organoclay surface to the nylon 6 owing to increased platelet exfoliation. Different organoclays lead to different levels of polymer degradation and color formation, depending upon the level of unsaturation present in the organic surfactant; the higher the number of double bonds the greater the degradation and the deeper the color formation. The primary mechanism of degradation is believed to be thermo-oxidative. Melt mixing of nylon 6 with model compounds, long-chain alkenes, shows that the same mode of degradation i.e. via double bonds can be replicated. In addition to unsaturation effects, the presence of hydroxyl-ethyl groups, opposed to methyl groups, in the organoclay surfactant, results in more color. Isothermal thermogravimetric analysis (TGA) was conducted on the organoclays to determine if thermal stability was a cause of molecular weight degradation; although, this relationship does not seem to exist, a direction correlation is observed between the organoclay degradation and nanocomposite modulus, or indirectly level of exfoliation. Use of antioxidant was found to reduce the amount of molecular weight loss. All evidence suggests that morphology and physical properties of nanocomposites formed from nylon 6 are not measurably affected by the reactions that lead to molecular weight degradation or color formation.     

Crystallization behavior of nylon 6 nanocomposites

The crystallization behavior of nylon 6 nanocomposites formed by melt processing was investigated. Nanocomposites were produced by extruding mixtures of organically modified montmorillonite and molten nylon 6 using a twin screw extruder. Isothermal and non-isothermal crystallization studies involving differential scanning calorimetry (DSC) were conducted on samples to understand how organoclay concentration and degree of clay platelet exfoliation influence the kinetics of polyamide crystallization. Very low levels of clay result in dramatic increases in crystallization kinetics relative to extruded pure polyamide. However, increasing the concentration of clay beyond these levels retards the rate of crystallization. For the pure nylon 6, the rate of crystallization decreases with increasing the molecular weight as expected; however, the largest enhancement in crystallization rate was observed for nanocomposites based on high molecular weight polyamides; this is believed to stem from a higher degree of platelet exfoliation in these nanocomposites. Wide angle X-ray diffraction (WAXD) and DSC were further used to characterize the polymer crystalline morphology of injection molded nanocomposites. The outer or skin layer of molded specimens was found to contain only γ-crystals; whereas, the central or core region contains both the α and γ-forms. The presence of clay enhanced the γ-structure in the skin; however, the clay has little effect on crystal structure in the core. Interestingly, higher levels of crystallinity were observed in the skin than in the core for the nanocomposites, while the opposite was true for the pure polyamides. In general, increasing the polymer matrix molecular weight resulted in a lower degree of crystallinity in molded samples as might be expected.     

Comparison of nanocomposites based on nylon 6 and nylon 66

Nylon 6 and nylon 6,6 organoclay nanocomposites were prepared by melt processing using a twin screw extruder. The effects of polyamide type and processing temperature on the mechanical properties and the morphology of the nanocomposites were examined. Mechanical properties, transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), percentage crystallinity and isothermal thermo-gravimetric analysis (TGA) data are reported. A particle analysis was performed to quantitatively characterize the morphology; these results are later employed in modeling the modulus of these materials using composite theory. No significant difference was observed in the mechanical properties and morphology of PA-6 nanocomposites processed at two different temperatures. PA-6 nanocomposites had superior mechanical properties than those made from PA-66. The tensile strength of PA-66 nanocomposites deviated from linearity at high levels of MMT. WAXD and TEM results show that the PA-6 nanocomposites are better exfoliated than the PA-66 nanocomposites, which exhibit a mixture of intercalated and exfoliated structures. Mechanical properties were consistent with the morphology. DSC reveals a higher percentage of crystallinity in the PA-66 samples. Isothermal TGA shows only a 5% difference in the degradation of the organic modifier on the organoclay processed at 240 °C versus 270 °C. Particle analysis shows a higher average particle length and thickness, and a lower average particle density and aspect ratio in nanocomposites based on PA-66 versus PA-6. The Halpin-Tsai and Mori-Tanaka composite theories predict satisfactorily the behavior of the PA-6 nanocomposites, while the PA-66 nanocomposites were predicted acceptably up to a certain volume fraction where the non-linear behavior takes effect. All the results indicate that there is a lower degree of exfoliation in the nanocomposites produced with a PA-66 matrix apparently stemming from the chemical differences between PA-6 and PA-66.     

Preparation and characterization of nylon 11/organoclay nanocomposites

Nylon 11/organoclay nanocomposites have been successfully prepared by melt-compounding. X-ray diffraction and transmission electron microscopy indicate the formation of the exfoliated nanocomposites at low clay concentrations (less than 4 wt%) and a mixture of exfoliated and intercalated nanocomposites at higher clay contents. Thermogravimetric and dynamic mechanical analyses as well as tensile tests show that the degree of dispersion of nanoclay within polymer matrix plays a vital role in property improvement. The thermal stability and mechanical properties of the exfoliated nylon 11/clay nanocomposites (containing lower clay concentrations) are superior to those of the intercalated ones (with higher clay contents), due to the finer dispersion of organoclay among the matrix.     

聚乙烯和马来酸酐接枝聚乙烯对尼龙66性能的影响

采用熔融共混法制备了高密度聚乙烯/尼龙66(HDPE/PA66)和马来酸酐接枝聚乙烯/尼龙66(PE-g-MAH/PA66)复合材料,对其力学性能和熔体流动速率进行了测试,对共混物形貌进行了扫描电镜观察。研究表明,与不相容HDPE/PA66共混物比较,PE-g-MAH更能有效改善尼龙66的冲击韧性和加工性能,同时使保持PA66较高的拉伸强度。其原因是基于PE-g-MAH相的细微分散以及与PA66之间存在较强的界面粘附,有利于应力的有效传递。     

Aggregation structure and molecular motion of (glass-fiber/matrix nylon 66) interface in short glass-fiber reinforced nylon 66 composites

Aggregation structure and thermal molecular motion of an adhered polymer layer on a glass-fiber (GF) surface after a removal of nylon 66 from a short glass-fiber reinforced nylon 66 were studied on the basis of photoacoustic spectroscopy-infrared spectroscopy (PAS-IR), pyrolysis-gas chromatography (Py-GC), X-ray photoelectron spectroscopy (XPS) and scanning viscoelasticity microscopy (SVM). PAS-IR, Py-GC and XPS measurements of the GF surface showed the presence of strongly adhered nylon 66 layer on the surface of aminosilane-treated GF. The glass transition temperature, T_g, of the adhered nylon 66 layer on the glass-fiber surface was directly evaluated on the basis of SVM measurement. In the case of the GF treated with an aminosilane coupling agent and a sizing agent, the magnitude of T_g at the (GF/nylon 66) interfacial layer was higher than that of the matrix nylon 66 due to the effective restriction of thermal molecular motion of nylon 66 at the (GF/nylon 66) interfacial layer. It is reasonable to consider that the sizing agent affects the strong interfacial interaction between a glass-fiber surface and matrix nylon 66 with covalent bond formation accompanying the network structure formation.     

Rheological and mechanical comparative study of in situ polymerized and melt-blended nylon 6 nanocomposites

The rheological and mechanical properties of commercial neat nylon 6 and nylon 6 nanocomposites containing organically-modified montmorillonite (organoclays) produced by either in situ polymerization or melt-blending were investigated. The dynamic and steady shear, capillary and extensional viscosity of the neat nylon 6 and nylon 6 nanocomposite melts were studied, as well as the tensile properties of the solid material. X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that the organoclays were largely very well exfoliated, although the lateral size scale of the platelets was different for each material. The in situ polymerized nanocomposite exhibited higher melt viscosity and higher tensile ductility than the melt-blended nanocomposite which was related to improved dispersion and polymer-silicate interactions for this material. Scanning electron microscopy confirmed that the nanocomposite failure surfaces showed more evidence of brittle behavior than the failure surfaces of neat nylon 6, and also that agglomerates of organoclay could be seen easily in the fracture surface of the melt-blended nanocomposite, but not to the same degree as in the in situ polymerized nanocomposite. This is in addition to very fine, individually-dispersed silicate laminates that form in each case.     

Toughened nylon66/nylon6 ternary nanocomposites by elastomers

The elastomer toughening of PA66/PA6 nanocomposites prepared from the organic modified montmorillonite (OMMT) was examined as a means of balancing stiffness/strength versus toughness/ductility. Several different formulations varying in OMMT content were made by mixing of PA6 and OMMT as a master‐batch and then blending it with PA66 and different elastomers in a twin screw extruder. In this sequence, the OMMT layers were well exfoliated in the nylon alloy matrix. The introduction of silicate layers with PA6 induced the appearance of the γ crystal phase in the nanocomposites, which is unstable and seldom appears in PA66 at room temperature and it further affected the morphology and dispersion of rubber phase resulting in much smaller rubber particles. The incorporation of POE‐g‐MA particles toughened the nanocomposites markedly, but the tensile modulus and strength were both reduced. Conversely, the use of OMMT increased the modulus but decreased the fracture toughness. The nanocomposites exhibited balanced stiffness and toughness. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of organoclay structure on nylon 6 nanocomposite morphology and properties

A carefully selected series of organic amine salts were ion exchanged with sodium montmorillonite to form organoclays varying in amine structure or exchange level relative to the clay. Each organoclay was melt-mixed with a high molecular grade of nylon 6 (HMW) using a twin screw extruder; some organoclays were also mixed with a low molecular grade of nylon 6 (LMW). Wide angle X-ray scattering, transmission electron microscopy, and stress-strain behavior were used to evaluate the effect of amine structure on nanocomposite morphology and physical properties. Three surfactant structural issues were found to significantly affect nanocomposite morphology and properties in the case of the HMW nylon 6: decreasing the number of long alkyl tails from two to one tallows, use of methyl rather than hydroxy-ethyl groups, and use of an equivalent amount of surfactant with the montmorillonite, as opposed to adding excess, lead to greater extents of silicate platelet exfoliation, increased moduli, higher yield strengths, and lower elongation at break. LMW nanocomposites exhibited similar surfactant structure-nanocomposite behavior. Overall, nanocomposites based on HMW nylon 6 exhibited higher extents of platelet exfoliation and better mechanical properties than nanocomposites formed from the LMW polyamide, regardless of the organoclay used. This trend is attributed to the higher melt viscosity and consequently the higher shear stresses generated during melt processing.     

Phase transition in nylon 6/clay nanocomposites on annealing

The γ→α crystalline phase transition in nylon 6/clay nanocomposite prior to melting was investigated by X-ray diffraction. The phase transition in the nanocomposite took place at 160 °C, 40 °C higher than that of nylon 6 at 120 °C. The transition extent in the nanocomposite was lower than that in nylon 6. This could be caused by the strongly confined spaces between layers, and the favorable environment for the formation of the γ phase in the existence of clay. Besides, the less grown crystallites of the α phase transformed from the γ phase in the nanocomposite began to melt at much lower temperature than its normal melting temperature.     

Effect of different types and loadings of modified nanoclay on mechanical properties and adhesion strength of EPDM-g-MAH/nylon 66 systems

In this study, the effect of different levels and loadings of modified nanoclay (NC), nanoclay 1 CEC, 2 CEC and 4 CEC, cation exchange capacity on mechanical properties and adhesion strength of maleic anhydride grafted ethylene-propylene-diene terpolymer (EPDM-g-MAH)/nylon 66 systems were investigated. Fourier transform infrared (FTIR) data confirmed the reaction mechanism between maleic anhydride in the polymer backbone and the organomodifier of the nanoclay. Dynamic mechanical analysis (DMA) results showed that on increasing the levels of nanoclay modification, the storage modulus (Eʹ) increased as well as the glass transition temperature (T_g) was slightly shifted to lower temperature and the height of the damping property (tan δ) peaks decreased. The results revealed that the use of the three levels of modified clay with EPDM-g-MAH had significant effects on the tensile strength and elongation at break, especially at 5 parts per hundred rubber by weight (phr) filler content. Whereas in the case of lower nanoclay filler contents (i.e. 1 and 3 phr) the results clarified that they had little effect on tensile and elongation at break values. Pull-out adhesion tests showed that the adhesion force of NC 2 CEC nanocomposite was approximately twice that of the virgin polymer while the nanocomposite NC 4 CEC showed inferior adhesion values, especially at 5 phr filler content. Scanning electron microscopy (SEM) clarified that good wettability of elastomer took place, especially in case of NC 2 CEC which in turn led to an enhancement of the adhesion force between the elastomer and the nylon 66 cord.     

氯化钙对尼龙6/66结晶态结构的影响

本文采用示差扫描量热法(DSC)、傅立叶变换红外光谱法(FTIR)研究了CaCl2对尼龙6/66结晶态结构的影响.DSC测试结果表明,随着尼龙6/66中CaCl2含量的增加,尼龙6/66的结晶温度、熔融温度降低,结晶度逐渐减小直至变为无定型态.红外光谱研究结果证实了钙离子能够插入尼龙6/66分子链,与尼龙6/66分子链上的羰基发生络合作用,破坏尼龙6/66原有的氢键,阻碍尼龙6/66分子链的自由运动,从而使其无法结晶.     

Polymorphism in polyamide 66/clay nanocomposites

Polyamide 66/clay nanocomposites (PA66CN) were prepared via melt compounding method by using a new kind of organophilic clay, which was obtained through co-intercalation of epoxy resin and quaternary ammonium into Na-montmorillonite. The silicate layers were dispersed homogeneously and nearly exfoliated in polyamide 66 (PA66) matrix. The introduction of silicate layers induced the appearance of the γ phase in PA66CN at room temperature, more clay loadings would amplify this phenomenon; the addition of clay also changed the structure of the α crystalline phase. The presence of silicate layers increased the crystallization rate and had a strong hetero phase nucleation effect on PA66 matrix. The lower Brill transition temperature of PA66CN can be attributed to the strong interaction between polyamide chains and surfaces of silicate layers.     

Transcrystallinity in aramid and carbon fiber reinforced nylon 66: determining the lamellar orientation by synchrotron X-ray micro diffraction

This paper presents the structural details of nylon 66 transcrystallinity induced by aramid (kevlar 29, 49 and 149) and carbon (pitch based) fibers, as determined by high spatial resolution X-ray diffraction. Using stepwise scanning, the orientation of the lamellae in the transcrystalline layer was measured as a function of distance from the fiber. The main observation is that this orientation is distinct for each system and almost independent of distance from the fiber. Of particular interest is the bi-layered transcrystallinity formed on a surface treated kevlar 49 fiber, in which the lamellar a^* axis is nearly perpendicular and at an angle of ∼12° to the fiber in the outer and inner layer, respectively. The crystallographic analysis generates grids of oriented lamellae with respect to the fiber axis.     

Influence of thermal processing on the perfection of crystals in polyamide 66 and polyamide 66/clay nanocomposites

A study of the changes in crystal perfection of polyamide 66 (PA66) and polyamide 66/clay nanocomposites (PA66CN) due to different thermal processing was carried out. We designed three series of thermal processing including melt-quench (MQ), post-annealing MQ sample (MQA), and melt–slow cooling–annealing (MSA). The annealing temperature was set as 180 or 210 °C, which is within Brill temperature range of PA66. Fourier transform infrared (FT-IR) spectroscopy and wide angle X-ray diffraction (WAXD) were employed to characterize the perfection in short-range order and long-range order structures, respectively. The results showed that the crystal perfection of PA66 and PA66CN with different thermal processing is quite different, and the changing fashions with thermal processing for different ordered structures are not similar. In this work, MSA is optimal thermal processing for high crystallinity and crystal perfection. Exfoliated nanoclay layers exert considerable impact on the perfection of long-range ordered structures, but little on that of short-range ordered ones.     

尼龙66基复合材料的研究进展

综述了近年来尼龙66(PA66)复合材料的研究状况和技术进展,重点讨论了PA66合金、PA66无机纳米复合材料和PA66填充增强改性,并对PA66复合材料的制备方法进行了介绍。同时对PA66复合材料的发展趋势及应用前景进行了分析和展望。     

Dispersion characterization in layered double hydroxide/Nylon 66 nanocomposites using FIB imaging

Layered double hydroxides (LDHs), a newly emerging 2D host material, consist of cationic brucite-like layers and exchangeable interlayer anions. In this work, the morphology and dispersion of LDH particles in LDH/Nylon 66 (salt) nanocomposites has been investigated using focused ion beam (FIB) techniques, transmission electron microscopy (TEM) and X-ray diffraction (XRD). The FIB images show that LDHs are present in the polymer phase dispersed to different degrees, with partial intercalation, exfoliation, and aggregation all being observed. The most even dispersion was achieved in nanocomposites with the lowest loading (0.5 wt % LDH). Residual tactoids and agglomerates were most common in the samples made with the highest concentration of LDHs studied here (5 wt %). The dispersion observed using FIB was consistent with TEM and XRD analysis, yet this technique had significant benefits in terms of time and simplicity over these “conventional” technologies. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008