Transport properties of electrically conducting nylon 6/foliated graphite nanocomposites

In this work, we analyze the conductivity data of the nylon 6/FG nanocomposites using the normalized percolation equations and the general effective equation. From the interpretations of the derived results, we demonstrate that the microstructure of the nanocomposites can be readily deduced. Taking several factors into account, it turns out that the tunneling mechanism should be responsible for the observed non-universality of the critical exponents. Experimental evidences show that the existence of the tunneling conduction should be attributed to the particular structure of the prepared materials.     

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.     

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.     

High temperature behaviour of the crystalline phases in unfilled and clay-filled nylon 6 fibers

Temperature-induced crystalline phase transitions in neat nylon 6 fibers as well as nylon 6/montmorillonite nanocomposite fibers have been studied by means of wide-angle X-ray scattering. Both types of melt spun fibers only consist of the γ crystalline phase that does not display any transition during heating up to the melt. In contrast, fibers drawn up to the maximum draw ratio at 140 °C display the single α phase with a high degree of chain orientation. During the temperature increase, the α phase undergoes a gradual structural disordering but preserves its monoclinic character up to melting. The structural evolution of the α form turned out sensitive to the thermal and mechanical treatment of the fibers. Annealing the unfilled drawn fibers at 150 °C prior to the WAXS experiment improves the thermal stability of the α form due to healing of the processing-induced crystalline defects. The montmorillonite-filled fibers display both the α and the γ crystals, which readily turn into α crystal form only upon drawing. Due to the matrix shearing between the MMT platelets, the H-bonded sheets display a higher thermal stability as compared with unfilled drawn fibers. Upon cooling from the melt, the first signs of crystallization are of γ form in the MMT-PA6 fibers, but the α form rapidly turns predominant. Crystallization kinetics considerations are put forward to account for this finding.     

Isothermal crystallization kinetics and melting behavior of nylon/saponite and nylon/montmorillonite nanocomposites

DSC thermal analysis and X‐ray diffraction have been used to investigate the isothermal crystallization behavior and crystalline structure of nylon 6/clay nanocomposites. Nylon 6/clay has prepared by the intercalation of ε‐caprolactam and then exfoliating the layered silicates by subsequent polymerization. The DSC isothermal results reveal that introducing saponite into the nylon structure causes strongly heterogeneous nucleation induced change of the crystal growth process from a two‐dimensional crystal growth to a three dimensional spherulitic growth. But the crystal growth mechanism of nylon/montmorillonite nanocomposites is a mixed two‐dimensional and three‐dimensional spherulitic growth. The activation energy drastically decreases with the presence of 2.5 wt % clay in nylon/clay nanocomposites and then slightly increases with increasing clay content. The result indicates that the addition of clay into nylon induces the heterogeneous nucleation (a lower ΔE) at lower clay content and then reduces the transportation ability of polymer chains during crystallization processes at higher clay content (a higher ΔE). The correlation among crystallization kinetics, melting behavior, and crystalline structure of nylon/clay nanocomposites is also discussed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2196–2204, 2004     

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.     

Morphology and mechanical properties of glass fiber reinforced Nylon 6 nanocomposites

Nylon 6 composites containing both an organoclay and glass fibers as fillers were prepared by melt processing. The aspect ratios of the glass fibers and the clay platelets were determined by electron microscopy techniques. The aspect ratio of each type of filler decreased as filler loading increased. A two particle population model for the tensile modulus was constructed based on the Mori-Tanaka composite theory. The experimental levels of reinforcement appear to be reasonably consistent with model predictions when changes in particle aspect ratios are accounted for. The tensile strength increases and elongation at break decreases as the content of either filler increases according to expected trends. Izod impact strength increased with glass fiber content but decreased with clay content.     

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.     

Nylon 6/Na-montmorillonite nanocomposites prepared by compounding Nylon 6 with Na-montmorillonite slurry

We report a novel compounding process using Na-montmorillonite water slurry for preparing novel nylon 6/Na-montmorillonite nanocomposites. In this compounding process, the Na-montmorillonite slurry was blended with melting nylon 6 using an extruder, followed by removing the water. The Na-montmorillonite silicate layers were found to be exfoliated and dispersed homogeneously at nanometer level in the nanocomposites with an electron transmission microscope. The exfoliated Na-montmorillonite silicate layers were fixed in the nylon 6 matrix almost as they were in water. The nylon 6/Na-montmorillonite nanocomposites, loaded with only 1.6 wt% clay silicate layers, exhibited high strength, high modulus, high heat distortion temperature and low gas permeability compared to neat nylon 6. The properties of the nylon 6/Na-montmorillonite nanocomposites were nearly equal to those of conventional nylon 6/clay nanocomposites prepared by dry-compounding nylon 6 and organophilic clay ion-exchanged with alkylammonium ions.     

Crystallization kinetics and morphology of nylon 1212

The isothermal and non-isothermal crystallization processes of nylon 1212 were investigated by polarized optical microscopy. The crystal growth rates of nylon 1212 measured in isothermal conditions at temperatures ranged from 182 to 132 °C are well comparable with those measured by non-isothermal procedures (cooling rates ranged from 0.5 to 11 °C/min). The kinetic data were examined with the Hoffman-Lauritzen nucleation theory on the basis of the obtained values of the thermodynamic parameters of nylon 1212. The classical regime I→II and regime II→ III transitions occur at the temperatures of 179 and 159 °C, respectively. The crystal growth parameters were calculated with (100) plane assumed to be the growth plane. The regime I →II→ III transition is accompanied by a morphological transition from elliptical-shaped structure to banded spherulite and then non-banded spherulite. The development of morphology during isothermal and non-isothermal processes shows a good agreement.     

Isothermal crystallization and melting behaviors of nylon 11/nylon 66 alloys by in situ polymerization

Nylon 11/nylon 66 alloys were prepared by in situ polymerization. Analysis of the isothermal crystallization behaviors of nylon 11/nylon 66 alloys was carried out using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The crystallization kinetics of the primary stage under isothermal conditions could be described by the Avrami equation. The crystal morphology observed by means of polarized optical microscope (POM). In the DSC scan after isothermal crystallization process, the multiple melting behaviors were found and each melting endotherm has a different origin. The real-time XRD measurements confirmed that no crystalline transition existed during the isothermal crystallization process. The multiple endotherms were experimentally evidenced due to melting of the recrystallizated materials or the lamellae produced under different crystallization processes. The equilibrium melting point of samples for isothermal crystallization was also evaluated.     

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.     

Non-isothermal crystallization kinetics of reactive microgel/nylon 6 blends☆

The non-isothermal crystal ization kinetics of reactive microgel/nylon 6 blends was investigated by differential scanning calorimetry (DSC). The Mo equation was employed to analyze the non-isothermal crystallization data. The crystallization activation energies were also evaluated by the Kissinger method. The results show that the crystallization onset temperature (Tonset) and crystallization peak temperature (Tp) decrease with the increase of the content of reactive microgel, whileΔT (Tonset–Tp), the crystallization half-time (t1/2) and the crystal ization enthalpy (ΔHc) increase. The required cooling rates of blends are higher than that of neat nylon 6 in order to achieve the same relative crystallinity in a unit of time. The crystallization activation energies of the reactive microgel/nylon 6 blends are greater than those of the neat nylon 6. When the content of reactive microgel is 30%, the relative crystallinity (Xt) reaches the maximum.     

Crystallization kinetics and melting behavior of nylon 10,10 in nylon 10,10–montmorillonite nanocomposites

The crystallization kinetics and melting behavior of nylon 10,10 in neat nylon 10,10 and in nylon 10,10–montmorillonite (MMT) nanocomposites were systematically investigated by differential scanning calorimetry. The crystallization kinetics results show that the addition of MMT facilitated the crystallization of nylon 10,10 as a heterophase nucleating agent; however, when the content of MMT was high, the physical hindrance of MMT layers to the motion of nylon 10,10 chains retarded the crystallization of nylon 10,10, which was also confirmed by polarized optical microscopy. However, both nylon 10,10 and nylon 10,10–MMT nanocomposites exhibited multiple melting behavior under isothermal and nonisothermal crystallization conditions. The temperature of the lower melting peak (peak I) was independent of MMT content and almost remained constant; however, the temperature of the highest melting peak (peak II) decreased with increasing MMT content due to the physical hindrance of MMT layers to the motion of nylon 10,10 chains. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2181–2188, 2003     

Rubber toughening of nylon 6 nanocomposites

The rubber toughening of nylon 6 nanocomposites prepared from an organoclay was examined as a means of balancing stiffness/strength versus toughness/ductility. Nine different formulations varying in montmorillonite, or MMT, and maleated ethylene/propylene rubber or EPR-g-MA rubber content were made by mixing of nylon 6 and organoclay in a twin screw extruder and then blending the nanocomposites with the rubber in a single screw extruder. In this sequence, the MMT platelets were efficiently dispersed in the nylon 6 matrix. The MMT platelets did not penetrate into the rubber phase. The addition of clay affected the dispersion of the rubber phase resulting in larger and more elongated rubber particles. The tensile properties and impact strength of these toughened nanocomposites are discussed in terms of the MMT and rubber contents and morphology. There is a clear trade-off between stiffness/strength versus toughness/ductility.     

Studies on crystallization kinetics of nylon 6-polyoxypropylene-nylon 6 copolymers

The crystllization kinetics of anionic-prepared nylon6-poly(oxypropylene) 1000-nylon 6 (NPN) block copolymers containing 1.20 to 8.76 wt% poly(oxypropylene)(POP) were studied. The thermograms of isothermal and nonisothermal differential scanning calorimetry of NPN block copolymers obtained were used for the study. The Avrami equation was used to analyze the isothermal crystallization of NPN nylon block copolymers. The Avrami exponent n obtained in the temperature range of 180 to 200 °C was 2.0 to 2.5. It was not similar to that for nylon 6 reported in literature. The activation energies of crystallization for the nylon block copolymers were smaller than that of nylon 6, and showed a minimum with POP content. The equilibrium melting point increased as the POP content decreased. For the nylon block copolymers with lower POP content, the slopes of T_c vs. T_m plots were higher than the values reported elsewhere. The Ozawa plot was used to analyze the data of nonisothermal crystallization. The obvious curvature in the plot indicated that the Ozawa model could not fit our system well, and there was an abrupt change of the slope in the Ozawa plot at a critical cooling rate.     

Melting behaviors, isothermal and non-isothermal crystallization kinetics of nylon 1212

Isothermal crystallization, subsequent melting behavior and non-isothermal crystallization of nylon 1212 samples have been investigated in the temperature range of 160-171 °C using a differential scanning calorimeter (DSC). Subsequent DSC scans of isothermally crystallized samples exhibited three melting endotherms. The commonly used Avrami equation and that modified by Jeziorny were used, respectively, to fit the primary stage of isothermal and non-isothermal crystallizations of nylon 1212. The Avrami exponent n was evaluated, and was found to be in the range of 1.56-2.03 for isothermal crystallization, and of 2.38-3.05 for non-isothermal crystallization. The activation energies (ΔE) were determined to be 284.5 KJ/mol and 102.63 KJ/mol, respectively, for the isothermal and non-isothermal crystallization processes by the Arrhenius' and the Kissinger's methods.     

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.     

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     

Study on the thermal behaviors and the morphology in PVP and nylon 6 blends

In this report, polyamides were solution blended in the formic acid with poly(vinyl pyrrolidone)(PVP), an amorphous polar polyamide. The thermal behaviors and morphological change in the blends of Nylon 6 (PA6) and PVP were investigated in details via WAXD, DSC, FT‐IR and POM methods. The equilibrium melting temperatures for PA6 in the blends were estimated based on the linear and nonlinear Hoffman‐Weeks (LHW and NLHW) extrapolative methods. With increasing the mass ratio of PVP to PA6, T_m (melting temperature) and T_c (crystallization temperature) of PA6 in blends both decreased as well as that of the spherulite size of PA6. The interaction mode between PVP and PA6 was investigated by FT‐IR spectroscopy, and the spectral changes indicated that the carbonyl groups of PVP had formed hydrogen bonding with the N? H groups of PA6 molecules in the molten state, which resulted in the variation of the morphology and thermal behaviors. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011