Materials Science - Rheology

Materials Science/Rheology

Unique properties allow silicon-based materials to be used in a wide variety of applications. These applications include:

• Electronics
• Automotive
• Construction
• Photonics
• Life sciences

Many of these unique properties are derived from the flexibility of the siloxane bond. What distinguishes siloxane polymers from their organic counterparts is a very flexible backbone comprised of alternating silicon and oxygen atoms. This structure allows exceptional mobility and orientation.		The siloxane molecule has a flexible backbone of alternating silicon and oxygen atoms.

Low Glass-Transition Temperature
The molecule’s extra mobility results in a low glass-transition temperature for siloxanes.
Poly(dimethylsiloxane), or PDMS, has a glass-transition temperature of less than -120ºC. Typical use temperatures range from below -40ºC to greater than 150ºC for many products.

Viscosity
By increasing the molecular weight of a linear PDMS polymer, the viscosity at room temperature can be varied from less than 0.65 cP to greater than 100,000,000 cP. At low molecular weight, these materials behave essentially as Newtonian liquids (i.e., their viscosity is not a function of shear rate) to shear rates in excess of 1,000 s-1. At higher molecular weights, they become entangled and exhibit a viscoelastic response and shear thinning behavior at modest shear rates. Because of their range of rheological responses and their exceptional stability over time, siloxane polymers are often used as rheological calibration standards.

Molecular weight dependence of low shear rate viscosity for poly(dimethylsiloxanes). UTC = Silicon Compounds: Register and Review, 5th ed., United Chemical Technologies, Pennsylvania (1993); Lee et al = C.L. Lee, K.E. Polmanteer, E.G. King, “Flow Behavior of Narrow-Distribution Polydimethylsiloxane,” J. Polym. Sci., A-2, 8, p1909-1916 (1970); Barry = A.J. Barry, “Viscometric Investigation of Dimethylsiloxane Polymers,” J. Appl. Phys., 17, p1020-1024 (1946); Mills = N.J. Mills, “The Rheological Properties and Molecular Weight Distribution of Polydimethylsiloxane,” Eur. Polym. J., 5, p675-695 (1969); Rahalkar et al = R.R. Rahalkar, J. Lamb, G. Harrison, A.J. Barlow, “Viscoelastic Studies of Reptational Motion of Linear Polydimethylsiloxanes,” Faraday Symp. Chem. Soc., 18, p103-114 (1983); El Kissi = N. El Kissi, J.M. Piau, P. Attané, G. Turrel, “Shear Rheometry of Polydimethylsiloxanes. Master Curves and Testing of Gleissle and Yamamoto Relations,” Rheol. Acta, 32, p293-310 (1993); Dow Corning = Dow Corning fluids product information.

Typical behavior of random coil polymers as they transition from Newtonian to shear thinning flow.

Smaller Activation Energies
An additional advantage from the exceptional mobility of the siloxane backbone is that the activation energies required for viscous flow are smaller than that of many common organic polymers. This results in the viscosity being less temperature dependent – particularly for PDMS. The low glass-transition temperature – coupled with a less temperature-dependent viscosity and high thermal stability – makes siloxanes very useful for applications with a broader range of temperatures.

Activation energy for viscous flow of linear polymers, showing the dependence on energy for viscous flow.  PE=poly(ethylene), PB= poly(butane), PP = poly(propylene), PIB = poly(isobutylene), PET = poly(ethyleneterapthalate), PVAc = poly(vinylacetate), PS = poly(styrene), and PVC = poly(vinylchloride).

Beyond Linear Materials
This discussion is limited to linear siloxane materials. But by combining structural changes like branching, cross-linking, and substituting different organic functionalities on the silicon in the backbone, a wide variety of materials and rheological behaviors can be generated – ranging from solids…to viscoelastic materials…to Newtonian liquids. In many cases, the rheological properties are tuned to meet the needs of specific applications.