Soft polymers and nanocomposites in adhesives
Soft polymers are used to make tacky films that function as pressure-sensitive adhesives (PSAs). The soft polymer is able to wet and make close contact with nearly any surface, so that van der Waals bonds hold the surfaces together. Pulling the adhesive away from the surface requires energy to strain the polymer.
With the right balance of viscous and elastic properties, the polymer can be drawn in fibrils during the debonding process, which dissipates a significant amount of energy. This energy dissipation makes polymers better for applications in adhesives than sticky liquids.
The Soft Matter Group studies environmentally-friendly adhesives made from polymer colloids in water (called latex). When the latex is cast onto a surface, no organic solvents or volatile organic compounds (VOCs) are given off into the atmosphere. The VOC-free adhesives are better for the environment and for workers who used them.
Polymer colloids also offer the ability to tailor the structure of adhesives at nanometer length scales. In research directed by Prof. Joseph Keddie, the Soft Matter Group studies how the structure of adhesives affects their mechanical and adhesive properties, and they propose new strategies to increase the adhesiveness.
Nanocomposite adhesive can be created by blending different types of nanoparticle, such as large and small, or elastic and viscous. Two-phase nanocomposite particles can also be employed to introduce structure at even shorter length scales. The Soft Matter Group has blended carbon nanotubes into adhesives as a way to increase their adhesion energy. Carbon nanotubes also introduce electrical conductivity, which make the adhesives useful for electronics and display applications. The Group has also used hard clay nanoparticles to tune the elastic and viscous components of the mechanical properties. They found that a greater amount of energy is dissipated when debonding the adhesive.
Our cover article in Soft Matter reported how supracolloidal particles, with a soft polymer core encapsulated with hard clay platelets, increased the tack adhesion energy of nanocomposite PSAs.
See: T. Wang, P. J. Colver, S. A. F. Bon, and J. L. Keddie, “Synergistic Effects between Clay and a Soft Polymer in a Supracolloidal Structure Leading to Increased Tack Energy in Pressure-Sensitive Adhesives,” Soft Matter (2009) 5, 3842
Nanocomposite pressure-sensitive adhesives made by the Soft Matter Group have optical clarity and are electrically conductive. These novel materials were first reported in this publication: T. Wang, C.H. Lei, A.B. Dalton, C. Creton, Y. Lin, K.A.Shiral Fernando, Y.-P. Sun, M. Manea, J.M. Asua and J.L. Keddie, “Waterborne, nanocomposite pressure-sensitive adhesives with high tack energy, optical transparency and electrical conductivity,” Advanced Materials, 18 (2006) 2730-34.
This paper was highlighted in the “Editor’s Choice” section of Science magazine. (Science, 134 (2006), p. 1051).
In a probe-tack experiment, the force to lift a probe from a tacky surface is measured as a function of distance.
In a probe-tack measurement, the force to lift a probe from a surface is measured as a function of distance. The area under the stress-strain curve is proportional to the tack adhesion energy. Here the effect of small molecules at the interface between carbon nanotubes and the soft polymer is shown. You can read more about the effect of the nanotube/polymer interface on adhesion and mechanical properties in these two publications:
T. Wang, A.B. Dalton and J. L. Keddie, “Importance of molecular friction in a soft polymer-nanotube nanocomposite,” Macromolecules (2008) 41, 7656 - 7661.
T. Wang, C.-H. Lei, D. Liu, M. Manea, J. M. Asua, C. Creton, A. B. Dalton and J. L. Keddie, “A Molecular Mechanism for Toughening and Strengthening Waterborne Nanocomposites,” Advanced Materials 20 (2008) 90.
Fibrils are drawn during the de-bonding of a probe from the surface of a pressure-sensitive adhesive. The process dissipates energy. This time-series of images shows a side view as the fibrils are extended and then pull off of the probe.