Tailor-made Polyhedral Oligomeric Silsesquioxane (POSS) Molecules for Thermosets

Bruce X. Fu*, Chris DeArmitt, Joseph Schwab

Hybrid Plastics, Inc. 55 W.L. Runnels Industrial Drive

Hattiesburg, MS 39475

Presented at a meeting of the Thermoset Resin Formulators Association at the Hilton Suites Chicago Magnificent Mile in Chicago, Illinois, September 15 through 16, 2008.

This paper is presented by invitation of TRFA. It is publicly distributed upon request by the TRFA to assist in the communication of information and viewpoints relevant to the thermoset industry. The paper and its contents have not been reviewed or evaluated by the TRFA and should not be construed as having been adopted or endorsed by the TRFA.

Abstract

Polyhedral oligomeric silsesquioxanes (POSS) are a class of unique hybrid organic-inorganic molecules commercialized by Hybrid Plastics, Inc.. The inorganic cage structure is thermally and chemically stable. The organic substituents can be tailor-made through chemical reactions to enhance many properties of thermosetting polymers. Most POSS molecules are readily soluble in common thermoset ingredients. It was found that the incorporation of POSS molecules enhances the thermomechanical properties by significantly improving the rubbery plateau modulus. Multifunctional POSS molecules can improve the scratch resistance of a coating. POSS molecules have also been found to improve resistance to various kinds of environmental degradation, including moisture, corrosion, radiation, etc.. POSS molecules having silanol and mercapto groups can be used as dispersants due to their strong bonding to the filler surface. Examples of POSS applications in various thermosetting polymers are provided in the paper.

Introduction

Polyhedral oligomeric silsesquioxane (POSS) molecules are a unique class of materials that can be used to enhance the properties of thermosetting polymers. POSS molecules have an empirical formula of (RSiO1.5)n. A typical fully condensed POSS molecule has a cage structure comprised of 8, 10, or 12 silicon atoms. They have a hybrid organic-inorganic composition with an inorganic cage surrounded by a number of organic substituents [Figure 1]. The inorganic cage provides thermal and chemical stability while the organic substitutes can be tailor-made to have desired functionalities. The cages can be functionalized with non-reactive groups for enhancing compatibility to the polymer matrix, or functionalized with groups for chemically reacting with a polymer matrix [1].

Figure 1. Schematic drawing of a POSS molecule

POSS molecules are nanoscopic in size. A typical POSS cage has a diameter of 1.5 nanometers. The overall diameter of the molecule ranges from 1-3 nanometers if the organic substitutes are included. Unlike nanoscopic fillers, POSS molecules can have different physical forms such as crystalline solids, waxes, liquids. In general POSS molecules can be easily dispersed at the molecular level if the organic substitutes on the cage are similar in chemical structure to the polymer matrix (i.e., they are compatible). In many cases, a good dispersion of POSS molecules can be achieved by simple stirring. POSS molecules can be used as reactive ingredients for formulators to design new polymers, or as inert additives to impact desired properties. A single POSS molecule can be tailored to have 1 to 12 functionalized groups depending on the size of the cage. These functionalized groups include alcohols, phenols, acids, esters, amines, acrylates & methacrylates, epoxides, halides, imides, norbornenys, olefins (vinyl), silanols, sulfonates, and thiols.

Because of the organic-inorganic hybrid composition, POSS molecules can bring many benefits to a thermosetting polymer. For instance, POSS can be used to design high use temperature polymers due to its ability to thermal stability. Due to its bulky size, a chemically incorporated POSS molecule can restrict chain mobility and thus increase the glass transition and heat distortion temperature of the resin. Another important aspect of POSS is the ability to resist environmental degradation from oxidation, corrosion, radiation, and solvents.

POSS Enhanced Epoxies

POSS epoxides and POSS amines can be used to formulate epoxy adhesives/coatings. For POSS epoxides, they can be mono-functional, tri-functional, and multi-functional (up to 12). Monofunctional epoxide has been found to increase the modulus under mechanical stress [2]. Multi-functional POSS epoxides have drawn increasing attention lately due to their high compatibility with common epoxy resins. As a matter of fact, the glycidyl POSS cage mixture and epoxycyclohexyl POSS cage mixture can be mixed at any ratio with most of the aliphatic epoxies, aromatic epoxies, and epoxy diluents. The POSS cage mixtures are a mixture of octa, deca and dodeca cage sizes. Mixed cage sizes are desirable as this affords a melting point depression relative to pure octameric cages. The glycidyl-POSS cage mixture has relatively low viscosity at room temperature (48 poise typical). This makes it useful for vacuum assisted resin transfer molding (VARTM) of glass fiber and carbon fiber composites. The glycidyl POSS cage mixture can be cured with aliphatic or aromatic amines. The epoxycyclohexyl POSS cage mixture has relatively high viscosity at room temperature but can be diluted with common epoxy diluents. It can be cured with anhydrides at low temperature.

The dynamic mechanical properties of a glycidyl POSS enhanced resin are shown in Figure 2. In this formulation the glycidyl POSS was mixed with a common novolac epoxy (Epon 162 from Hexion) at different weight ratios. The resin is then cured with 2-methylpentamethylenediamine (Dytek A from Invista) at ambient temperature for 3 days. Dynamic mechanical tests were performed in 3-point bending mode at 1Hz frequency at a temperature ramp rate of 2oC/min. As seen in Figure 2, the epoxy control has a glass transition temperature around 65oC. Here we use the tanδ peak to determine the glass transition temperature. Significant changes were observed after adding POSS to the formulation. The glassy plateau modulus (i.e., the modulus before the glass transition) was the same for all samples except the 80% and 100% POSS samples, which showed slightly lower modulus. However, the addition of POSS significantly increases the higher rubbery plateau modulus (i.e., the modulus after glass transition). For example, the flexural modulus of the epoxy control at 120oC is about 10 MPa. The 100% POSS sample, however, has a flexural modulus of 200 MPa, a 20 fold

improvement. As a result, the tanδ peak height decreases with increasing POSS content (tanδ equals the loss modulus divided by Young’s modulus). For the 100% POSS sample, the tanδ curve is nearly flat, indicating that the sample retains most of its stiffness long after the glass transition point has been reached. A further interesting observation is that the glass transition temperature decreases with higher POSS loading. This might be due to a slightly lower density of crosslinking caused by steric hindrance of POSS molecules.

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Figure 2: Dynamic mechanical analysis of a glycidyl POSS cured epoxy system

POSS molecules are useful for formulating composite materials due to its ability to retain modulus at elevated temperatures. Table 1 shows the mechanical properties of POSS enhanced glass fiber composites made via vacuum assisted resin transfer molding (VARTM). In these formulations, glycidyl POSS was first mixed with a low viscosity bisphenol F resin (Epon 862, Hexion) by stirring at room temperature for 20 minutes. The resin was then added to tetraethylenepentamine (TEPA) in a stoichiometric ratio and mixed for 5 minutes. The mixture was then vacuum pulled into glass fiber woven sheets as in a typical VARTM process. The composites were allowed to cure at room temperature for one week. As seen in Table 1, 30% glycidyl POSS has similar viscosity to the control. At 70%, the viscosity is higher compared to the epoxy control. This resulted in a small amount of visible bubbles trapped in the matrix post infusion. We expect that a more efficient vacuum system would have completely

eliminated all voids. The residual voids may have contributed to the lower flexural modulus of the 70% POSS sample at room temperature (8.3 GPa) relative to the control (10.1 GPa). The 70% POSS sample also shows a lower tensile strength and tensile modulus again attributed to the void inclusion. For the 30% POSS sample, the tensile strength at room temperature is the same as control. It has a lower tensile modulus but higher flexural modulus. The strain at break (tensile) is similar for all samples. The POSS samples show somewhat higher elongation at break in flexural strength which may indicate an improvement in toughness.

Table 1: Mechanical properties of POSS enhanced glass fiber composites

In Table 1, the rubbery plateau moduli of the POSS glass fiber composites are all higher than the epoxy control. This is consistent to our previous findings for the pure resin. All of the composite samples show a glass transition temperature around 60oC. At 100oC, the remaining modulus of the epoxy control is about 16% of that at room temperature. The 30% POSS retained 29% of its initial modulus at 100oC, compared to a retention of 46% for the 70% loaded POSS sample. This clearly shows that POSS molecules can raise the high temperature performance of the composites. By retaining most of the modulus and strength after glass transition temperature, POSS molecules can effectively increase the heat distortion temperature and thus increase the upper use temperature without sacrificing the VARTM ability at room temperature.

POSS Enhanced Acrylates and Methacrylates

Multifunctional POSS acrylates and methacrylates are particularly useful in UV curable coatings. They are soluble in most of the common ingredients in a typical UV coating. Similar to multifunctional epoxies, POSS acrylates are made into cage mixtures that can have 8, 10, or 12 functional groups and corresponding distribution of cage sizes. They have a fast cure speed and low viscosity at room temperature. Due to their high functionality, POSS acrylates and methacrylates can significantly increase the scratch resistance of a coating. Another use is to improve stain resistance and hydrophobicity of resin which corresponds to commercial value in dental restorations. Similar to POSS epoxies, POSS acrylates and methacrylates can also be used to increase the service temperature and enhance the overall mechanical properties of a resin.

Figure 3. Chemical structure of an octa-functional POSS acrylate molecule

POSS Enhanced Silicones

POSS has been used in two-part silicone adhesives/coatings to enhance radiation and atomic oxygen resistance for space vehicle applications. POSS silicones have low outgassing properties and are well suited for space applications. POSS cages are particularly efficient at stopping material erosion caused by atomic oxygen. Under the initial bombardment of atomic oxygen, the cage-organic groups are burned off and the POSS cages fuse together to form thin, adherent, amorphous silica layer. The process is commonly called “glassification” because of the resulting amorphous silica layer has the same chemical composition as glass. Unlike glass, this amorphous layer is not rigid and can be bent and stretched just like the underlyng silicone. A typical thickness range for the glassified surface is 50-100 nanometers. Once formed, the amorphous silica layer protects virgin resin underneath, and component devices, by stopping further oxidation from atomic oxygen, ozone, and oxygen plasma. The POSS silicones have also exhibited good resistance to proton and vacuum ultraviolet radiation [3].

Figure 4. Glassification process of POSS containing polymers under oxidation

POSS cages are also found to improve the solvent resistance of silicones. Table 2 compares the solvent resistance of several POSS silicones to a control sample and a space grade silicone (DC93-500, Dow Corning). The samples were soaked in acetone for 7 days. For non-POSS silicones, the weight gain is around 25%. After the incorporation of 20wt% norbornenyl POSS, the weight gain is reduced to 19%. At 60wt%, the weight gain is only 10%.

Table 2. Solvent resistance of POSS silicones compared to non-POSS silicones.

POSS Enhanced Polyimide

A colorless and transparent polyimide (Corin, Mantech International) was developed by incorporation of POSS molecules into the polyimide backbone and resulting in greatly reducing the normal orange coloration of polyimide. Polyimides have been used in many applications because of their excellent thermomechanical properties. They can be made into sheets, rods, or coatings/enamels. Polyimide usually has an orange color which is problematic in applications that require optical clarity. An example is solar cell panels that require a very high transmittance of light. As seen in Figure 5, This POSS polyimide shows over 90% transmittance of visible light. Depending on the thickness, the 50% transmittance UV cutoff is near 350nm (0.1mil thick) to be near 390nm for 1mil thick films. The colorless POSS polyimide also has a glass transition temperature at 250°C and is resistant to scratch and abrasion.

The colorless POSS polyimide can be directly used as a conformal coating because of its solubility in ketone solvents. A simple spray process can yield a re-workable conformal coating that offers excellent scratch resistance and mechanical properties. The colorless POSS polyimide also has superb hydrophobicity because of the POSS and fluorinated components. Similar to POSS silicones, this POSS polyimide also has outstanding resistance to various radiations such as atomic oxygen, proton, and vacuum UV.

Figure 5. UV-Vis spectrum of a 0.9 mil colorless polyimide sample

POSS as Dispersants

POSS molecules, particularly silanols and thiols, are excellent dispersants for fillers, pigments, and nanoparticles. The silanol or mercapto groups covalently bond to the surface of the filler particle and thereby reduce particle-particle interactions. It was reported by Chen, et al. that the POSS molecules can even be used to disperse multi-

wall nanotubes [4]. Another example is the use of POSS molecules to disperse nano-metal particles [5].

Conclusion

POSS molecules are versatile reinforcements for thermosetting polymers. The versatility comes from the unique chemical structure of the POSS cage and its well defined molelcular attributes, where the inorganic cage is surrounded by a number of organic substituents. By altering the chemistry of the organic substituents, POSS molecules can be tailor-made for different resin systems and product applications. Many POSS molecules have been used in thermosets, particularly epoxies, UV curable acrylates, polyimides and silicones. POSS was found to significantly enhance the rubbery plateau modulus of the polymers, which makes POSS well suited for high temperature applications. POSS molecules also have robust resistance to environmental degradation such as oxidation, corrosion, and various types of radiation. A number of products have been developed aiming at combining the thermal and mechanical properties of the inorganic cage with the compatibility and processibility of the organic substituents.

References

1. Lichtenhan, J.D., Comments Inorg Chem, 1995, 17, 115

2. Lee, A., Lichtenhan, J.D., Macromolecules, 1998, 31, 4970

3. Wells, B., Brandhorst, H., Isaacs-Smith, T., Lichtenhan, J.D., Fu, B.X., Proceedings of AIAA IECEC07 conference June25-29

4. Chen, G.X., Shimizu, H., Polymer, 2008, 49, 943

5. Carroll, J.B., Frankamp, B.L., Srivastava, S., Rotello, V.M., J. Mater. Chem., 2004, 14, 690