Elsevier

Polymer

Volume 43, Issue 22, 2002, Pages 5915-5933
Polymer

Effect of organoclay structure on nylon 6 nanocomposite morphology and properties

https://doi.org/10.1016/S0032-3861(02)00400-7Get rights and content

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.

Introduction

Interest in polymer layered silicate nanocomposites is driven by the possibility of exceptional physical property enhancements, e.g. increased modulus and improved barrier properties, at low filler levels. The key to such performance rests in the ability to exfoliate and disperse individual, high-aspect ratio silicate platelets within the polymer matrix. Recent studies have explored achieving exfoliated structures from organoclays in a wide range of polymer matrices using a variety of processing routes [1]. To date, well-exfoliated composites have been achieved in only a selected number of polymers, viz., nylon 6 [2], [3], [4], [5], polystyrene [6], certain polyimides [7], [8], [9] and epoxies [10], [11], [12]. Of these, nylon 6 is particularly unique since exfoliated nanocomposites have been formed by more than one processing technique, i.e. in situ polymerization [2], [3] and melt processing in a well-designed twin screw extruder [4], [5], [13]. The technique of melt processing is particularly attractive due to its versatility and compatibility with existing processing infrastructure and is beginning to be used for commercial applications [14], [15], [16].

Recent work in this laboratory has focused on the effects of processing conditions, polymer structure and rheology, and the structure of the organic modifier of the clay on the formation of polymer nanocomposites via melt processing. For nylon 6, the extruder must be optimized with regard to both shear intensity and residence time to achieve high levels of clay platelet exfoliation [17]. Use of high molecular weight nylon 6 leads to much better exfoliation owing to the high shear stresses in the extruder that result from its high melt viscosity [18], [19]. Limited results suggest that the nature of the organic component, or ‘surfactant’, on the organoclay may also be a key factor.

Reichert et al. investigated how the length of the alkyl group on the amine used to modify sodium fluoromica and addition of maleated polypropylene (PP-g-MA) affected the morphology and mechanical properties of polypropylene nanocomposites formed by melt processing [20]. A critical alkyl length of 12 carbons or more was found necessary for promoting exfoliation in conjunction with PP-g-MA. Increasing the anhydride functionality of PP-g-MA also promoted exfoliation.

There is also evidence that surfactant structure affects the extent of exfoliation achieved by the in situ polymerization technique. For example, Usuki et al. showed that swelling of montmorillonite modified with ω-amino acids by ε-caprolactam increased significantly when the carbon number of the amino acid was greater than 8 [21], [22]. This ultimately led to the selection of the proper amino acid needed to form the exfoliated nanocomposite described in the literature as NCH [2], [3], which is now a commercial product of Ube Industries Ltd (Japan). In addition, Usuki et al. compared four types of inorganic silicates: montmorillonite, synthetic mica, saponite, and hectorite. They discovered that clays having higher exchange capacities, i.e. montmorillonite and synthetic mica, lead to more efficient exfoliation of the silicate platelets.

Lan et al. described how the structure of the organic modifier of the clay, as well as the clay itself, influenced exfoliation in epoxy nanocomposites [23]. They found that use of alkyl ammonium cations with chain lengths longer than eight carbons, high acidity of the onium ion, and clays with low to intermediate charge density led to greater extents of exfoliation. The latter observation is counter to other results [22] and may be due to differences in the nature of the bond between polymer segments and the organic surfactant, i.e. secondary bonding as opposed to covalent bonding, respectively. At this point, the available literature does not allow one to generalize the relationship between organoclay structure, nanocomposite morphology, and the processing route.

The objective of this paper is to examine the relationship between the structure of the organic cation on the organoclay and the morphology and properties of nylon 6 nanocomposites formed by melt processing. Specific comparisons among organic amine surfactants that are potentially available from commercial sources will be made by addressing structural variations one issue at a time. Wide angle X-ray scattering (WAXS), transmission electron microscopy (TEM), and stress–strain behavior are used to evaluate nanocomposite morphology and physical properties. A subsequent paper will focus on color formation and polymer matrix molecular weight degradation issues associated with the organic component.

Section snippets

Nylon 6 materials

Two commercial grades of nylon 6 from Honeywell (formerly AlliedSignal), a high molecular weight grade (Capron® B135WP, with Mn=29 300) and low molecular weight grade (Capron® 8202 with Mn=16 400), were used in this study. Further details of these materials are reported elsewhere [18], [19]. The high and low molecular weight polyamides will be referred to as HMW and LMW, respectively.

Melt processing

Melt blended nanocomposites were formed using a Haake co-rotating twin screw extruder with the barrel

Organic modifier structure and characterization

Fig. 1(a) shows the various amine compounds that were exchanged for the sodium ion of native montmorillonite clay (CEC=92 mequiv./100 g clay). A simple nomenclature system for the organic cation is used in Fig. 1(a) to describe its chemical structure in a concise manner. The letters M, H, and (HE) represent methyl, hydrogen, and 2-hydroxy-ethyl substituents, while C, T, R, and HT, represent units derived from natural products that consist of a distribution of saturated and unsaturated long chain

Mechanical property analysis

Comparison of properties of nanocomposites formed from the different organoclays should be made at a fixed montmorillonite content, rather than at a fixed organoclay content, since the silicate portion of the organoclay is the reinforcing component. Therefore, mechanical property data are plotted versus weight percent MMT. Fig. 4 shows the relationship between modulus and montmorillonite content for all the nanocomposites prepared in this study based on the high molecular weight nylon 6

Organoclay physical structure

There are suggestions in the literature that the extent of exfoliation of an organoclay in a polymer matrix should relate to the basal spacing of the organoclay. For example, Reichert et al. indirectly showed a gradual increase in tensile modulus with organoclay d-spacing, caused by the increase in the carbon number of the alkyl substituent on the onium ion, for a PP/PP-MA/modified MMT system [20]. They found a gradual increase in tensile modulus as alkyl chain length was increased from a

Organoclay structure–LMW nanocomposite property relationships

In a recent publication, we showed that higher molecular grades of nylon 6 were more effective at exfoliating (HE)2M1R1 than a low molecular weight grade, designated as LMW [18], [19] (Section 2.1). This effect was attributed to the higher melt viscosity and consequently higher shear stresses during extrusion compounding associated with the higher molecular weight polyamides. The materials described earlier were made from the highest molecular weight nylon 6 in this series; thus, the effect of

Conclusions

Structure–property relationships for nanocomposites formed by melt processing from a series of organically modified montmorillonite clays and high and low molecular weight grades of nylon 6 are presented here. The structure of the alkyl ammonium on the clay was systematically varied to determine how specific groups affect polyamide nanocomposite morphology and physical properties. As seen by WAXS, galleries of the organoclays expand in a systematic manner to accommodate the molecular size and

Acknowledgements

This work was supported by the Texas Advanced Technology Program under grant number 003658-0067 and by the Air Force Office of Scientific Research. The authors would like to especially thank Randy Chapman and Ryan Dennis of Southern Clay Products and Peggy Miller of The University of Texas Health Science Center in San Antonio for their help with WAXS and TEM analyses.

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