Clay Loading and Dispersion Effects on the Rheolog

ical Properties of Unsaturated Polyester NanocompositesClay Loading and Dispersion Effects on the Rheological Properties of Unsaturated Polyester Nanocomposites
Tony Nguyen
(Mentor: Abbas A. Zaman, College of Engineering)
ABSTRACT
The objective of this work is to characterize the influence of clay loading and dispersion effects on the rheological properties of unsaturated polyester composites. Toughened unsaturated polyester (UPE) composites were synthesized by the blending of delaminated clay with unsaturated polyester. Rheological behavior is shown to be strongly influenced by clay loading and the extent of clay dispersion in the polymer matrix. Transition from liquid-like behavior to solid-like behavior shifts to significantly higher solids loading at higher shear rates which may be due to the alignment of the particles in the direction of flow at high shear rates. SEM micrographs are used to display the extent of intercalation and dispersion of the clay within the polymer matrix.

INTRODUCTION AND BACKGROUND
1.1 Definition
Polymer/clay nanocomposites display a change in composition and structure over a nanometer length scale and have been shown to present considerable property enhancements relative to conventionally scaled composites. Layered silicates dispersed as a reinforcing phase in an engineering polymer matrix are one of the most important of such “hybrid organic-inorganic nanocomposites” 1. Polymer-layered silicate nanocomposites containing low levels of exfoliated clays, such as montmorillonite and vermiculite have a structure consisting of platelets with at least one dimension in the nanometer range. One of the most important features of polymeric materials is the possibility of controlling their macroscopic physical properties by tailored manipulation of their structures at a nanoscopic scale. To influence the interactions that govern the mechanical properties of polymers, specific nanoscopic scale reinforcement is efficient and beneficial. For example, montmorillonite clay provides such reinforcement through the interaction of polymer chains with the charged surfaced of clay lamellae 2.

The use of organoclays as precursors to nanocomposite formation has been extended into various polymer systems including epoxies, polyurethanes, polyimides, nitrile rubber, polyesters, polypropylene, polystyrene and polysiloxanes, among others. Even a variety of inorganic materials, such as glass fibers, talc, calcium carbonate, and clay minerals, have been successfully used as additives or reinforcements to improve the various properties of polymers 3-10.

1.2 Structure
The optimal properties of nanocomposites arise as the clay nanolayers are uniformly dispersed (exfoliated) in the polymer matrix, as opposed to being aggregated or phase separated as tactoids or simply intercalated. As nanolayer exfoliation becomes achieved, there is a trend in the improvement in desired properties that is manifested as an increase in tensile properties, enhancement of barrier properties, a decrease in solvent uptake, an increase in thermal stability and flame retardance, among others 11-12. The complete dispersion of clay nanolayers in a polymer optimizes the number of available reinforcing elements for carrying an applied load and deflecting cracks. The coupling between the and the polymer matrix facilitates stress transfer to the reinforcement phase, allowing for tensile and toughening improvements. Conventional polymer-clay composites containing aggregated nanolayers tactoids ordinarily improve rigidity, but they often sacrifice strength, elongation and toughness. However, exfoliated clay nanocomposites, have to the contrary shown improvements in all aspects of their mechanical performance 3.

1.3 Preparation and Synthesis
The preparation of nanocomposites requires extensive delamination of the layered clay structure and complete dispersal of the resulting platelets throughout the polymer matrix. Nanocomposite synthesis by conventional polymer processing operations therefore requires strong interfacial interaction between the polymer matrix and the clay in order to generate shear forces of sufficient strength. This is readily achieved with high surface energy polymers such as polyamides, where polarity and hydrogen-bonding capacity generates considerable adhesion between the polymer and clay phases. However, low-energy materials such as polyethylene and polypropylene interact only weakly with mineral surfaces, making the synthesis of polyolefin nanocomposites by melt compounding considerably more difficult 13. Several studies exist for examining behavior of polymer/clay nanocomposites with weak adsorbing parts 14. Common methods to synthesize polymer nanocomposites are: 1) intercalation of a suitable monomer followed by polymerization, 2) polymer intercalation from solution, 3) and direct polymer melt intercalation 14-19.
EXPERIMENTAL PROCEDURE
2.1 Material and Methods
The polymer used in this study was unsaturated polyester (UPE). The silicate clays used is referred to as C1. C1 has a surface area of 16m2/g, as measured with the Quanta Chrome NOVA 1200. Particle size analysis was performed on C1 using a Coulter LS230 laser diffraction apparatus and the experimentally measured volume average (d50) particle diameter is 4 μm. Figure 1 is an image of the C1 clay particles at 50X objective captured with the Olympus BX60 Optical Microscope with SPOT RT Digital Camera.


Figure 1. Delaminated, dispersed C1 clay particles.


Measured quantities of UPE were mixed with the clay in a custom-built high/low shear blender. After sufficient mixing of the polymer and clay, an initiator was added to induce polymerization and further blending was provided. While in the melt state, data for steady-shear viscosity and storage modulus were obtained using parallel plate geometry on a Paar Physica UDS 200 rheometer. The diameter of the upper disk was 50 mm, and the gap distance between the two plates was 0.3 mm. The sample temperature was kept constant at room temperature (25žC 0.1žC) using water as the heat transfer fluid. SEM micrographs with a JEOL JSM6330F cold field emission scanning electron microscope were taken and is used to visually evaluate the surface dispersion of the clay within polymer matrix.
RESULTS AND DISCUSSION
3.1 Rheological Analysis of UPE/Clay Nanocomposites
For the UPE/C1 nanocomposite system, Figure 2 shows that viscosity increases with solids loading, and decreases with shear rate. The pure polymer system (0 wt% clay) has much lower viscosity than the nanocomposites, indicating a lack of matrix reinforcement that would exist with the presence of clay. At low shear rates, the dependency of viscosity on solids loading is more significant. There is an indication of Newtonian behavior at low shear rates, a shear thinning region at intermediate shear rates, and a second Newtonian plateau at higher shear rates.
Figure 2. Viscosity as a function of shear rate for UPE/C1 composites at different solid loadings (25žC).
For all clay loading weight percentages, the data shows decreasing viscosity with increasing shear rate. There is significant decrease in the viscosity at high shear rates for all clay loading percentages, including the pure polymer. With increasing shear rate the conformations of the intercalated chains are expected to change as silicate layers align parallel to the flow field, thus showing a shear thinning effect, especially for higher shear rates 20, 21. Figure 3 shows the viscosity behavior of the nanocomposites system as a function of clay solids loading at two different shear rates. Limiting viscosities are significantly affected by solids fraction at low shear rates. With increasing clay loading the viscosity increases and the point at which the viscosity approaches infinity may be considered the point of maximum packing fraction. With increasing shear rate, the conformations of the intercalated chains are expected to change as silicate layers align parallel to the flow field, and therefore transition from liquid-like behavior to solid-like behavior occurs at significantly higher solids loadings 13.
Figure 3. Viscosity as a function of % solids loading at high and low shear rates for UPE/C1 composites (25žC).


Figure 4. Effect of % solids loading on storage modulus at 25žC for UPE/C1 composites.

Figure 4 represent plot of storage modulus as a function of clay loadings and frequency for the samples used in this study. It can be observed that the storage modulus increases as a function of frequency and solids loading. This is evidence that improvements in terms of enhanced reinforcement potential of the nanocomposites occur with increasing solids loading. Previous research has shown that high storage modulus at low frequencies are exhibited for intercalated nanocomposites due to the reinforcement effect of a well-dispersed, or exfoliated clay in the polymer matrix 22. Enhanced moduli over the entire frequency range are expected for exfoliated nanocomposites.
3.2 Surface Structure of UPE/Clay Nanocomposites
The surface of the nanocomposites with 5 wt% loading of C1 was observed via a scanning electron microscope (SEM). Figures 5 and 6 show the SEM images of the nanocomposites at 5 wt%. The dark entities are regions of polymer matrix and the light colored shapes are surface fractures, clay particles, or areas of agglomerated clay layers. From the surface of the nanocomposites the clay particles appear to be not uniformly dispersed throughout the polymer matrix. The clay particles are coagulated together like conventional fibers. This is likely to affect the rheological and tensile properties of the nanocomposite samples. A study of the rheology of polyethylene oxide/organoclay nanocomposites showed that different surfactants adsorbed to the exterior surface of the platelet domains mediate differences in the attractive interparticle interactions that give rise to the nanocomposite structure 23. Some methods commonly employed to obtain exfoliation where dispersion is difficult include the addition of a compatibilizing agent and/or surface treatment. Future work will attempt to address these possibilities.


Figure 5. SEM micrograph of 5wt% UPE/C1 composites at 10,000 magnification.


Figure 6. SEM micrograph of 5wt% UPE/C1 composites at 50,000 magnification.

SUMMARY AND CONCLUSION
In this work, nanocomposites with C1 clay and unsaturated polyester were prepared for rheological testing. Rheological tests show a shear thinning behavior for the pure polymer system and for varying loadings of clay. SEM micrographs show non-uniform dispersion of the C1 clay in the UPE polymer matrix. Viscosity versus shear rate data show a shear thinning effect at high shear rates and also a convergence to a similar viscosity which is attributed to the alignment and orientation of the clay particles to the flow field at high shear rates. There is strong indication that rheological behavior of the nanocomposites is related to clay loading and the extent of clay dispersion in the polymer matrix. Surface treatment may be employed to bring about exfoliation of the particles in the polymer matrix. Further testing to be conducted on the nanocomposites made with C1 clay are XRD and tensile stress/strain tests.
ACKNOWLEDGEMENTS
The authors are grateful for the financial support provided by the University of Florida Particle Engineering Research Center (NSF Grant No. EEC-94-02989) and the industrial partners of the PERC. Useful discussions with Professor C.L. Beatty and his graduate student Mr. Ajit Bhaskar is greatly acknowledged. Assistance from Ms. Kerry Siebein is also greatly acknowledged. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect those of the National Science Foundation.


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