Stiff, highly damping thermoplastic polyurethane nanolaminates

10 September 2009
Erik Dunkerley and Daniel Schmidt
A new class of high-clay-content nanocomposites not only combines transparency, stiffness, and good damping properties but are also easier to test than their conventional counterparts.

When creating components, designers are often forced to choose between materials that provide structural stiffness and those that provide vibrational damping. Consequently, many assemblies must include multiple components made from materials displaying one property or the other, adding bulk, weight, and complexity of manufacture. Materials featuring both properties would avoid these disadvantages.

The field of nanocomposites has yielded new materials displaying combinations of properties not normally seen in ceramics, polymers, or conventional composites. Studies of highly organized natural materials, such as nacre, highlight the connection between organization, connectivity, phase morphology (i.e., the structure of uniform regions), and properties.1 Artificial materials with similar morphology and levels of connectivity and organization can possess unique combinations of mechanical properties as well.2 Termed ‘nanolaminates,’ these materials are polymer/nanoclay composites with a highly ordered brick-and-mortar structure in which the clay silicate layers are the bricks, and the polymer the mortar. Systems with high concentrations of silicate layers exhibit high stiffness, while the polymer acts both to transfer stress from layer to layer and also to allow energy dissipation through interfacial slip, viscoelasticity, etc.

Efficient production of materials that exhibit all of these desirable features requires new processing techniques. Here, we describe an approach that relies on the tendency of silicate layers dispersed in a fluid medium to self-assemble parallel to one another as the medium is removed. With proper optimization, this method makes it possible to fabricate highly organized structures in a short period of time.

For these experiments, thermoplastic polyurethane and dimethylditallowammonium-modified montmorillonite were combined in a blend of toluene and isopropyl alcohol and spray-deposited onto a release film using a commercial spray gun. The morphology of the resulting materials was characterized by scanning electron microscopy (SEM) and wide-angle x-ray diffraction (XRD). SEM images (see Figure 1) reveal an ordered structure consisting of stacks of parallel layers. XRD patterns (see Figure 2) confirm the existence of such a structure throughout the sample and indicate polymer intercalation (i.e., between the layers of silicate) with increasing polymer content, and concomitant expansion of the interlayer spacing by ~1nm.


Scanning electron microscope image of (a) 100/0 volume % modified clay to volume % polymer ratio (×3.00k magnification), (b) 90/10 (×3.00k), (c) 80/20 (×3.00k), (d) 70/30 (×3.00k), (e) 70/30 (×15.0k), and (f) 90/10 (×2.50k).


X-ray-diffraction patterns for all samples. a.u.: Arbitrary units.

Dynamic mechanical analysis (DMA) was used to demonstrate the simultaneous existence of high stiffness and high damping capacity. As shown in Figures 3 and 4, maximum storage and loss values of 1200 and 550MPa, respectively, were achieved close to the composition at which just enough polymer is present to intercalate all of the silicate layers. The loss modulus (an index of damping properties) peak observed at −25°C is assigned to the polymer glass-transition temperature, while that at 43°C is assigned to the transition of the alkylammonium silicate modifiers. While we believe this to be the first observation of such a transition via DMA, the phenomenon has been observed before by other techniques.3,4 Following studies by several groups, the general consensus5,6 is that the alkylammonium chains are probably transitioning from a quasi-crystalline to an amorphous state.


Storage-modulus data plotted vs. temperature and modified clay.


Loss-modulus data plotted vs. temperature and modified clay.

The values obtained for the composites were greater than the sum of the two components at the same temperatures, implying a synergistic effect most likely due to the structure. As these materials resemble intercalated stacks found in conventional low-silicate-content nanocomposites, understanding their properties is expected to be relevant for a wide range of nanocomposite systems. In particular, to date the mechanical properties of these stacks have been thought to behave according to micromechanical models that assume a combination of the bulk properties of the polymer and the silicate layers. Our results suggest that other morphological affects could be present and may have a significant impact on the mechanical properties of the assemblies.

Although the study of nanolaminates is still in its infancy, the findings described here show that these materials can be made to possess both high damping and stiffness. Moreover, those properties can be predicted and controlled with a proper understanding of component characteristics and morphological influences. With further development and understanding, such hybrids may one day yield both new solutions for structural applications and a better grasp of the mechanical properties of nanocomposites already in use. Future work will concentrate on examining the impact of the various polymer and clay properties on the thermal mechanics, morphology, and barrier characteristics of the composites.


Authors

Erik Dunkerley
University of Massachusetts Lowell

Daniel Schmidt
University of Massachusetts Lowell


References

  1. K. S. Katti and D. R. Katti, Why is nacre so tough and strong?, Mater. Sci. Eng. 26, pp. 1317-1324, 2006.

  2. Z. Tang, N. A. Kotov, S. Magonov and B. Ozturk, Nanostructured artificial nacre, Nat. Mater. 2, pp. 413-418, 2003.

  3. J. D. Jacobs, Dynamics of alkyl ammonium intercalants within organically modified montmorillonite: dielectric relaxation and ionic conductivity, J. Phys. Chem. B 110, pp. 20143-20157, 2006.

  4. H. Hongping, Thermal characterization of surfactant-modified montmorillonites, Clays Clay Min. 53, pp. 287-293, 2005.

  5. H. Heinz, R. A. Vaia and B. L. Farmer, Relation between packing density and thermal transitions of alkyl chains on layered silicate and metal surfaces, Langmuir 24, pp. 3727-3733, 2008.

  6. H. Heinz, H. J. Castelijns and U. W. Suter, Structure and phase transitions of alkyl chains on mica, J. Am. Chem. Soc. 125, pp. 9500-9510, 2003.

DOI:  10.2417/spepro.000064