8 October 2014 | 09:00
By Dr Daniel Burnett

The use of nanomaterials as composite reinforcing materials has shown significant interest in recent years. Both carbon nanotubes and clay nanoparticles have been studied as a means to improve composite properties [1-5]. The quality and performance of nanocomposites depend strongly on the interaction of the components at their interface. Filler-matrix interactions are commonly described by adhesion and cohesion phenomena. Both properties depend on the surface energetic situation of the materials commonly expressed by the surface energy. Surface energies of different multi-walled carbon nanotubes (MWCNT) and nanoclays with different surface treatments were determined by IGC SEA. Nanofiller-matrix interactions have been calculated by means of the thermodynamic work of adhesion from the surface energy values and correlated with composite mechanical properties.

Table 1. Thermodynamic

Table 1. Thermodynamic works of adhesion cohesion of nano filler/PU composites

Dispersive, acid-base and total surface energy values were measured for MWNT, oxidized MWNT, nanoclay, isocyanate functionalized nanoclay, and polyurethane (PU) samples. These values were then used to calculate the thermodynamic work of adhesion values (Wad; nanofiller-PU interaction) and work of cohesion values (Wcoh; nanofiller-nanofiller interaction). In addition, the Wad/Wcoh ratios were determined. A summary of these results are shown in Table 1. High Wcoh values indication strong filler-filler interactions and lower Wad/Wcoh ratios. The Oxidized MWNT shows a higher Wcoh value compared to the pure MWNT, indicating stronger filler-filler interactions which could lead to a decrease in composite performance. For the nanoclay samples, the pure Nanoclay sample has a significantly higher Wcoh value than the Isocyanate Nanoclay, suggesting stronger nanoclay-nanoclay interactions and weaker nanoclay-PU adhesion.

Table 2. Mechanical properties of Nanofiller/PU composites

Table 2. Mechanical properties and Tg of the nanofiller/PU composites.

Mechanical properties of composites made from the nanofiller materials and PU were determined using dynamic mechanical thermal analysis (DMTA). These results are summarized in Table 2.

The Oxidized MWNT-PU composite has a lower tensile modulus than the pure MWNT-PU, indicating a decrease in composite strength, as predicted by the surface energy and Wad/Wcoh values. Likewise, the pure Nanoclay-PU composite has poorer mechanical properties (lower tensile modulus and tensile strength) compared to the Isocyanate-Nanoclay composite, as suggested by the Wad/Wcoh values determined by IGC-SEA. Therefore, surface energy and works of adhesion/cohesion values measured on individual nanocomposite materials can give valuable and predictive information on composite performance.

In addition, nanomaterials are often energetically inhomogeneous, exhibiting various surface sites, such as structural defects or specific functional groups. Therefore, a surface energetic heterogeneity profile can provide more comprehensive information on the nature and population of these surface sites [6]. Despite the potential importance of heterogeneity profiles, until now, there has been little emphasis on the characterization of the surface energy distribution of nanomaterials. Recent advances of IGC surface energy methodology allow for the determination of the aforementioned surface energy distribution [6,7].

iGC Surface Energy Analyzer (Surface Measurement Systems Ltd, London, UK) adopts this new approach and is equipped with the state-of-the-art injection technology which allows the precise control of the injection size.

The surface energetic heterogeneity of commercial multi-walled CNTs was measured, relating to the effects of annealing and oxidation. Figures 1 and 2 display the dispersive and acid-base surface energy profiles for these samples, respectively.

Figure 1

Figure 1. Dispersive surface energy profiles

Figure 2

Figure 2. Specific (acid-base) surface energy profiles

Dispersive surface energy profiles in Figure 1 show that all MWCNT samples were energetically heterogeneous, but the degree of energetic heterogeneity was found to depend on the modification treatment. Oxidised MWCNTs were found to be energetically most active and most heterogeneous. It is also well recognised that energetic surface heterogeneity can have a significant impact on the thermodynamic characterisation of carbon surfaces and often this is discussed in the context of wetting hysteresis. As shown in Figure 2, oxidised MWCNTs had relatively higher concentration of Lewis acid-base functional groups on the surfaces, with almost 80% increment in acid-base surface energy values comparing to the as-received MWCNTs. Acid-base surface energy of as-received MWCNTs was marginally higher than that of annealed MWCNTs. Results presented here clearly indicate that energetic heterogeneity and homogeneity of the as-received and surface modified MWCNT samples can be easily distinguished by IGC technique. This is very important to differentiate any subtle differences in surface physical and/or chemical conditions of a wide range of solid materials.

In summary, IGC was proven as a powerful and sensitive technique for assessing the surface energy and surface chemistry of various nanomaterials. Surface energy values measured on nanofillers can be predictive of nanofiller-composite mechanical performance. Also, this work demonstrates the importance of determining surface energy profiles, in order to fully characterise the often pronounced difference in energetic heterogeneity of real solid surfaces. For more details on these studies,
please see Application Note 226 and Case Study 609.

References
[1] E. P. Giannelis, Polymer layered silicate nanocomposites, Adv. Mater., 1996, 8:29-35.
[2] C. Zilg, R. Thomann, R. Mulhaupt, J. Finter, Polyurethane nanocomposites containing laminated anisotropic nanoparticles derived from organophilic layered silicates, Adv. Mater., 1999, 11:49-52.
[3] D.K. Chattopadhyay and K.V.S.N. Raju, Structural engineering of polyurethane coatings for high performance applications, Prog. Polym. Sci., 2007, 32:352:418.
[4] M. Moniruzzaman, F. Du, N. Romero, K. Winey, Polymer, 2005, xx pp1-6.
[5] J. Yang, J. Hu, C. Wang, Y. Qin, z. Guo, Macrom. Mater. Eng. 2004, 289, pp 828-832.
[6] Yla-Maihaniemi, P.P. et al. (2008) Inverse gas chromatographic method for measuring the dispersive surface energy distribution for particulates. Langmuir, 24, p9551-9557. [7] Thielmann, F., Burnett, D.J. and Heng, J.Y.Y. (2007) Determination of the surface energy distributions of different processed lactose. Drug Dev. Ind. Pharm. 33, 1240-1253.

Author
Dan Burnett is the Director of Science Strategy for Surface Measurement Systems, Ltd. located at the North American headquarters in Allentown, PA. His team works on research studies and science applications for many prominent companies. He received his bachelor’s degree in Professional Chemistry from Eastern Michigan University in 1997 and Ph.D. degree in Chemical Engineering from the University of Michigan in 2001. Since joining Surface Measurement Systems in 2001, he has continued his interests in sorption science and surface chemistry on a range of materials including: pharmaceutical powders, natural and man-made fibers, polymers, films, and food/flavor systems. Dr Burnett has authored or co-authored over 25 papers in peer-reviewed journals and presented at numerous national and international conferences.

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