Tailored Nanocomposite Structures for Improved Polymer

Chapter 22

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Tailored Nanocomposite Structures for Improved Polymer Performance 1, 2

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Ian MacDonald , Milena Ginic-Markovic , Stephen Clarke , Janis Matisons , and Dong Yang W u 1

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School of Chemistry, Physics, and Earth Sciences, Nanomaterials Research Group, Flinders University of South Australia, G. P. O. Box 2100, Adelaide South Australia 5001, Australia CSIRO Manufacturing & Infrastructure Technology, Melbourne, Australia 2

The effect of clay, modified with pyridinium cations of systematically increasing chain lengths, on Polyamide 6 based nanocomposites prepared via melt extrusion was investigated. The structure and morphology of the nanocomposites were examined using X-ray diffraction and transmission electron microscopy respectively. It was found that increasing alkyl chain length of the organic modifier leads to a more clay exfoliation which causes an increase in mechanical strength of the resulting nanocomposites. The crystalline structure, obtained by DSC and XRD studies, changes significantly throughout the samples, from the skin layer containing only γcrystals, to the core region, containing both the α and γ forms. Furthermore, a significant increase in the crystallinity of nanocomposites was noted with increasing alky chain length of clay modifier. The shift in the Si-0 band at 1047 cm- , present in the modified clay, to 1042 cm- , in the nanocomposite, is indicative of Clay/Matrix interactions. 1

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© 2008 American Chemical Society

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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262 Introduction Polymer clay nanocomposites have, for some time now, been the subject of extensive research into improving the properties of various matrices and clay types. It has been shown repeatedly that with the addition of organically modified clay to a polymer matrix, either in-situ (1) or by melt compounding (2), exfoliation of the clay platelets leads to vast improvements in fire retardation (J), gas barrier (4) and mechanical properties (5, 6) of nanocomposite materials, without significant increases in density or brittleness (7). There have been some studies on the effect of clay modification and melt processing conditions on the exfoliation in these nanocomposites as well as various studies focusing on their crystallisation behaviour (7-70). Polyamide-6 (PA-6)/montmorillonite (MMT) nanocomposites are the most widely studied polymer/clay system, however a systematic study relating the structure of the clay modification cation to the properties of the composite has yet to be reported. In order to produce highly exfoliated nanocomposites, the clay used must be modified to change its hydrophilic surface to an organophillic one by an ion exchange reaction with a suitable cationic species. Since the first reported production of PA-6/MMT nanocomposites by researchers at Toyota™ (77), the standard modifier has been an ammonium cation with a long alkyl chain. Previous work has shown that use of a cation with less than 12 C H groups in the chain does not produce well exfoliated composites. Other clay modifications include increasing the number of alkyl chains and the use of various end groups, such as COOH (10). The crystallisation behaviour of PA-6/MMT nanocomposites is complicated to analyse because its polymorphic nature produces a monoclinic α and psuedohexagonal γ structure, both of which are affected by the presence of clay particles in the matrix (9). The clay particles themselves have been shown to act as nucleating agents, increasing the crystallisation process in some instances while retarding crystal growth in others. Clearly, the cause of these phenomena must be understood so some measure of control can be employed to produce useful nanocomposite materials with desired properties. To date, there has been few studies on nanocomposites using modifying cations other than ammoniums. Pozgay et al. (12) reported the use of hexadecylpyridinium as a modifying cation for use in PP/clay nanocomposites. Pozgays' work compared the exfoliation level in nanocomposites prepared with hexadecylpyridinium modified clay. The journal focused only on the effect of surface coverage of the clay particles by the organic modifier. Their work showed that exfoliation occurs only above a certain gallery spacing determined by the orientation of the organic modifier. In this work, the effect of alkyl chain length on PA-6/MMT nanocomposites has been studied in a systematic manner and characterised by a range of techniques. The length of the alkyl chain length of the pyridinium cation has 2


263 been systematically increased from 8 to 18 and used to produce polymer/clay nanocomposites by melt compounding. The resultant nanocomposites were then characterised by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Differential Scanning Calorimetry (DSC), PhotoAccoustic InfraRed Spectroscopy (PAIR) and mechanical testing.

Experimental Materials Two types of virgin PA-6, F132-E and F223-D, were purchasedfromDSM Engineering Plastics. Melting and peak decomposition temperatures were determined to be 222°C and 425°C respectively. F132-E is a high viscosity extrusion grade polymer and F223-E a medium/low viscosity injection moulding grade polymer. These polymers are referred to as F132-X and F223-X, where χ denotes the additive type. Na Cloisite was purchased from Southern Clay Products and modified by ion-exchange reaction with synthesised pyridinium cations (scheme 1). Cations were synthesised by reaction of pyridine and the appropriate alkyl chloride under reflux conditions. All synthesised structures were confirmed by NMR. +

Scheme 1. Synthesis ofpyridinium cations

The nanocomposites prepared using modified clay are referred to as oPC, dPC, DdPC, TdPC, HdPC and OdPC denoting increasing alkyl chain length of 8, 10, 12, 14, 16 and 18 respectively and is shown in Table 1. As a reference, a nanocomposite with unmodified clay was also used and denoted UM. Extruded PA-6 and PA-6 nanocomposites were prepared using a Haake twin-screw extruder with a feed rate of 2kg/hour, barrel temperature of 250°C and screw speed of 150rpm. PA-6 was fed into the extruder using a metered hopper and clay was added separately via a pre-calibrated microfeeder to a level of 3% by weight for each sample. The value of 3% was chosen from previous experiments with clay loading. At this level, excellent improvements in mechanical strength and toughness are obtained without the sample becoming too brittle, which can occur at higher clay loading levels. Clay content was confirmed by weighing remaining MMT ash after ThermoGravimetric analysis.


264 Samples were then injection moulded into tensile testing bars at 245°C. Before processing, all samples were dried in an oven for at least 24 hours at 65°C.

Table 1. Composite Designations and Alkyl chain Lengths Clay modification Cation

Designation Alkyl Chain Length

None

UM

0

octylpyridinium

oPC

8

decylpyridinium

dPC

10

Dodecylpyridinium

DdPC

12

Tetradecylpyridinium

TdPC

14

Hexadecylpyridinium

HdPC

16

Octadecylpyridinium

OdPC

18

X-Ray Diffraction (XRD) XRD measurements were performed on a Philips PW1720 with a Co anode running at 40kV and 20mA. Each experiment was conducted from 2.5° to 40° at 0.3°/min on injection-moulded tensile bars both at the skin and the core of each sample, as well as on organoclay powder.

Transmission Electron Microscopy (TEM) Samples were cut from injection moulded tensile testing bars and ultra thin sections were cut using a cryogenically cooled ultramicrotome. Samples were imaged on a Jeol 1200ex TEM operating at an accelerating voltage of 80kV.

Differential Scanning Calorimetry (DSC) DSC measurements were performed on a TA Instruments 2190 DSC with temperature and enthalpy calibrations performed using an indium reference. Experiments were performed under a nitrogen atmosphere with a flow rate of 50ml/min. Extruded granules of nanocomposites were heated at 2°C/min to 250°C, held isothermally for 5mins, then cooled to room temperature at 2°C/min. All samples were heated twice, first to examine the properties post extrusion and secondly to examine the preferred crystal structures with slow cooling.


265 PhotoAccoustic InfraRed Spectroscopy (PAIR) All experiments were performed on a Nicolet 870 FTIR with a photoaccoustic attachment under a helium atmosphere. Clay samples were dried overnight at 60°C and examined in powder form, allowing five minutes to purge the cell with helium. 10mm PA-6 and nanocomposite discs were cut from tensile specimens and allowed three minutes for the cell to purge. Photoaccoustic spectra were collected at a resolution of 4cm" with a velocity of 0.1581 and 256 scans. All samples were tested in duplicate. !

Mechanical Testing Physical performance of the injection moulded tensile testing bars of nanocomposite materials were performed according to ASTM D638 for tensile properties, and ASTM D790 for flexural tests. Tensile modulus and yield strength were determined using an extensometer at a crosshead speed of 0.51 cm/min. Flexural modulus was determined at a span of 50mm with an applied load rate of 1.3mm/min. The results from the tensile tests were averaged over five samples and the standard deviation found to be within the accepted limits.

Results and Discussion Nanocomposite Morphology The structural order of the polyamide 6 nanocomposites was studied by XRD. Figure 1 shows the XRD patterns of PA-6 for the F132-E based nanocomposites containing organically modified clay with alkyl chain lengths from eight to fourteen carbons. Above fourteen carbon units the XRD pattern remains essentially the same so have been omitted from this graph. F223 nanocomposites are not shown as the trends are the same for both materials. The characteristic peak of unmodified clay (not shown) occurs at 20 = 8.8° and with the increasing alkyl chain lengths in the modified clay, shifts up to a maximum 5.1°, which corresponds to an increase in gallery spacing from 11.6 Â to 17.0 Â (determined through the Bragg equation). From the figure, peak broadening and shift to smaller angles as the length of the alkyl chain on the clay modifier increases, is observed in nanocomposites. This can be ascribed to greater levels of clay exfoliation occurring in the polymer matrix as the observed peaks are related to the spacing between individual platelets. Pozsgay et al. (12) showed, using XRD and TEM, the nanocomposite structure achieved by compounding HdPC modified clay and polypropylene, and


266 identified increased gallery spacing as an essential prerequisite for exfoliation to occur. With an increase in the organic modifier alkyl chain length , the clay gallery spacing increases, hence, the polymer is able to better move between the individual layers. This results in a greater dispersion of clay nanoparticles and increased range of spacing which is evident on the XRD scans. The more pronounced peaks for oPC, dPC and DdPC indicate a mixed morphology comprising of a decreasing content of intercalated clay tactoids and an increasing content of individual clay particles. From TdPC to OdPC (HdPC and OdPC not shown) the peaks completely disappear and become less pronounced which corresponds to clay-clay platelet spacings of 30A and higher. Ahmadi et al. (13) described the exfoliation process, by showing the XRD shift to lower angles of the characteristic clay peak as the level of exfoliation and dispersion increases, confirming the results described above.

Figure 1. XRD spectra of F132 Nanocomposites oPC, dPC, DdPC and TdPC

The morphology of the nanocomposites were also determined using TEM. Morgan and Gilman (14) described clearly the use of TEM in analysing polymer clay nanocomposites. The TEM images in Figure 2 show the increasing amount of clay exfoliation occurring in UM

Tailored Nanocomposite Structures for Improved Polymer

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