D. R. Paul

Nylon 6 nanocomposites : the effect of matrix molecular weight

Organoclay nanocomposites based on three different molecular weight grades of nylon 6 were prepared by melt processing using a twin screw extruder. Mechanical properties, transmission electron microscopy, wide-angle X-ray diffraction, and rheological measurements were used to characterize the three types of composites. Tensile modulus and yield strength were found to increase with increasing concentration of clay, while elongation at break decreased. Izod impact strength was relatively independent of clay content for the higher molecular weight composites, but slightly decreased with increasing clay content for the lowest molecular weight polyamide. In general, nanocomposites based on the higher molecular weight polyamides yielded superior composite properties, having higher degrees of clay exfoliation, higher stiffness and yield strength values, and marginal loss of ductility as compared to nanocomposites based on the low molecular weight polyamide. Differences in properties between the three types of composites were attributed to differences in melt rheology. Capillary and dynamic parallel plate data revealed sizeable differences in the levels of shear stress between each nanocomposites system. A mechanism for exfoliation during melt mixing is outlined.

NYLON 6 NANOCOMPOSITES BY MELT COMPOUNDING

Abstract Nylon 6–organoclay nanocomposites were prepared via direct melt compounding using a conventional twin screw extruder. The mechanical properties and morphology of these nanocomposites were determined and compared to similar materials made by an in situ polymerization process. The organoclay was well exfoliated into the nylon 6 matrix when compounded with the twin screw extruder but use of a single screw extruder was far less effective. The mechanical properties of the organoclay nanocomposites were significantly increased with marginal decrease of ductility and showed much greater values than glass fiber composites.

Effect of melt processing conditions on the extent of exfoliation in organoclay-based nanocomposites

Polymer layered silicate nanocomposites have been studied for nearly 50 years, but few references deal with the importance of how the organoclay was processed into the plastic of choice. Many articles focus on the importance of the chemistry used to modify the surface of the clay, usually montmorillonite, without including the role of processing. This paper demonstrates the importance of both the chemistry of the clay surface and how the clay was melt processed into the thermoplastic. Two different clay treatments were added to polyamide 6 using four different types of extruders with multiple screw designs. The mixtures of organoclay and polyamide 6 were characterized by X-ray diffraction, transmission electron microscopy, and mechanical property tests. The degree of dispersion is interpreted in terms of the residence time distribution in the extruder and the intensity of shear. A model for organoclay delamination in a polymer melt is proposed that envisions the role of both shear and time.

Modeling properties of nylon 6/clay nanocomposites using composite theories

Abstract The reinforcement of nylon 6 by layered aluminosilicates (LAS) and glass fibers was examined using the composite theories of Halpin–Tsai and Mori–Tanaka. Theoretical comparisons show that exfoliated LAS offer superior reinforcement to glass fibers owing to the fillers high modulus, high aspect ratio, and its ability to reinforce in two directions. The effect of incomplete exfoliation of simple stacks of LAS on nanocomposite modulus was also examined. Increasing the number of platelets per stack and the gallery spacing between platelets results in a dramatic decrease in reinforcing efficiency. The predictions were benchmarked against experimental data for nylon 6 nanocomposites based on organically modified montmorillonite and glass fibers. The quantitative determination of the morphology of the nanocomposites is non-trivial due to the large distribution of filler shapes and sizes present. Thus, a detailed experimental procedure for determining the aspect ratio of the nanocomposites is reported. The composite theories satisfactorily capture the stiffness behavior of both types of composites. Furthermore, experimental heat distortion temperatures and those predicted from modeling the dynamic mechanical properties of nanocomposites are in reasonable agreement.

Crystallization behavior of nylon 6 nanocomposites

Abstract 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 organoclay structure on nylon 6 nanocomposite morphology and properties

Abstract 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.

Elucidating the structure of poly(dopamine)

Herein we propose a new structure for poly(dopamine), a synthetic eumelanin that has found broad utility as an antifouling agent. Commercially available 3-hydroxytyramine hydrochloride (dopamine HCl) was polymerized under aerobic, aqueous conditions using tris(hydroxymethyl)aminomethane (TRIS) as a basic polymerization initiator, affording a darkly colored powder product upon isolation. The polymer was analyzed using a variety of solid state spectroscopic and crystallographic techniques. Collectively, the data showed that in contrast to previously proposed models, poly(dopamine) is not a covalent polymer but instead a supramolecular aggregate of monomers (consisting primarily of 5,6-dihydroxyindoline and its dione derivative) that are held together through a combination of charge transfer, π-stacking, and hydrogen bonding interactions.

A Binary Interaction Model for Miscibility of Copolymers in Blends

Abstract Miscibility windows often exist in polymer blend systems when the chemical structure of one of the components is systematically varied, e.g. a random copolymer may be miscible with another polymer when neither limiting homopolymer is. A binary interaction model is developed which explains such behaviour. From this prediction, the general notion is advanced that many cases exist where the net exothermic heat of mixing required for miscibility of high molecular weight polymer mixtures may result from appropriate considerations of both intermolecular and intramolecular interactions of component units without an exothermic interaction existing between any individual pair of units. However, it is shown that for a net exothermic mixing the individual interaction parameters for the pairs of units must differ from those predicted by solubility parameter theory. Moreover, the departures from the geometric mean assumption of the solubility parameter theory need not be large to achieve conditions for miscibility. Several examples of the use of such a model are given including one where the homologous series of aliphatic polyesters is treated as ‘copolymers’ by considering their CH x and COO constituents as the ‘monomers’.

Correlation and prediction of gas permeability in glassy polymer membrane materials via a modified free volume based group contribution method

Over the past decade or more an extensive amount of data on the permeation of gases such as helium, hydrogen, oxygen, nitrogen, methane, and carbon dioxide in a wide array of glassy polymers has been published. Much of this work has been motivated by the search for materials with high permeability and high selectivity for potential use as gas separation membranes. This paper attempts to develop a method for correlating this data in a way that permits prediction of permselectivity behavior of other polymer structures. The method used involves an empirical modification of a free volume scheme that has been used in the past with some success. The previous method requires an experimental density of the polymer and an estimate of occupied volume from a group contribution method developed by Bondi. The present method actually predicts the density and uses a refined estimate of occupied volume specific to each gas. The parameters in the model were deduced from a database including over one hundred polymers. The new method significantly improves the accuracy of correlation and of prediction.

Sorption and transport of various gases in polycarbonate

The solubility, permeability, and diffusion time lag for CO2, CH4, Ar, N2, and He in polycarbonate are reported at 35°C for pressures ranging from 1 atm to 20 atm. The solubility data are well-described by a model which postulates that gas sorption in glassy polymers results from both a Henrys law and a Langmuir mechanism. The time lags for all gases except helium decreased with increasing upstream pressure. The permeability of all gases decreased with increasing upstream pressure, although the effect was barely larger than the experimental error for helium. The data have been analyzed in terms of a transport model which attributes separate mobilities to the Henrys law and Langmuir species. The resulting parameters of the model are correlated as functions of characteristics of the penetrant gases, and the possible significance of these correlations is discussed.