Polyhedral Oligomeric Silsesquioxanes

Table 1 How POSS Structure Affects Density and Refractive Index

While all of these cage molecules generated excitement and did indeed turn out to have unique properties, large commercial applications have not emerged for three reasons. It has proven difficult to synthesize larger amounts and to produce the diverse chemistries needed to facilitate adoption in a wide range of applications. Furthermore, the prices of these materials are too high to justify their use. Even though performance advantages may have been seen in some cases, they were simply not enough to make these new materials competitive with those already incumbent in the market.

Therefore, we can conclude that it is not enough that a molecule is intriguing or different. In order to have commercial success it must fulfill one or both of these criteria:

• • • • Provide performance levels unattainable by any other route
      • Lower the final cost (holistic, total system cost, not materials cost alone

Furthermore, it is my experience that the best breakthrough technologies on the market today took 15-20 years to come to fruition. When one discovers a new material, it is both a blessing and a curse. Certainly, the material is likely to exhibit new properties but one must first find those properties, then work out how that can bring value and who the potential customers might be. Testing ensues, alliances are made and with luck, eventually a product reaches the market.

Chevron Technology Ventures’ interest in the diamondoids has fizzled out and while academic research continues on those and hyperbranched polymers, dendrimers and of course C₆₀, there is little prospect for commercial applications. Looking at all these materials, it is my view that the POSS family of chemicals is, in fact, the most promising of all. They offer the whole range of chemistries and can be tuned easily to attain a spectrum of properties. Equally important, they can be made on a large scale using conventional chemical reactors, resulting in prices far lower than for those other materials.

The organic side groups on POSS span the whole range of chemistries: fluoroalkyl, alkyl, phenyl, alcohols, thiols, amines, vinyl, carboxylic acid, sulfonic acid, PEG, acrylate, methacrylate, epoxides, halides, imides, silanes and silanols. Interestingly, some organosilanes starting materials convert readily to POSS whereas other POSS types simply cannot be synthesized. As one example octaisooctyl POSS is available in bulk whereas octa n-octyl POSS cannot be made at all. There is at present no understanding as to why some kinds can be made and others cannot.

Table 2 How POSS Compares to Other Molecular Fillers Regarding Availability and Price

As a comparison, a starburst dendrimer with 8 functional (amino) groups per molecule costs $181 for 2.0g of a solution in methanol where the concentration of dendrimer is just 20 weight %. Thus, the effective price for the dendrimer is ~$450/g. Contrast that to an octafunctional (vinyl) POSS, also a spherical molecule of well-defined structure with pricing of $2/g, POSS octaamic acid for $24/g or the acrylates and epoxy POSS at only 60 cents per gram. Not to mention that those are the R&D, small order prices for the POSS which drop very significantly for larger orders.

Probably due to the diversity and relatively low POSS prices, some commercial POSS applications do exist and they continue to grow slowly as is the way with such new technologies. However, companies are often hesitant to use any technology that has only one supplier because it can lead to unfavorably high pricing and instability of supply. It could well be that such considerations have slowed the application of POSS.

Having set the scene in terms of what POSS is and how it fits into the landscape of other high profile molecules, we will now look in more detail at POSS types and how they can be used to modify the properties of polymers. Normally, scientific reviews simply look at what can be achieved. As stated in the preface, the aim of the POSS Handbook is to take a more pragmatic view. Thus, not only will the effect of the POSS be examined but also whether it offers an attractive solution compared to those already known or on the market.

Is POSS a Molecule or a Particle?

POSS appear spherical or cubic to the eye and somehow this makes the viewer think that POSS are particles. They are often described in the literature as nanoparticles with all the good and the bad associations that come with that nomenclature. Nano has, in recent years been a buzzword and research on all things nano has been popular and well-funded. On the downside, the term nano now invokes concerns over safety and lack of commercialization means nano has lost much of its shine.

As mentioned earlier, POSS are in fact molecules and that can be proven categorically in several ways. POSS can be analyzed by the following techniques which only apply to molecules:

• • • • Solution NMR spectroscopy (¹H or ²⁹Si)
      • HPLC
      • Mass spectroscopy
      • Dielectric spectroscopy
      • Some POSS types are liquid at room temperature
      • POSS can be melted and recrystallized
      • POSS can be recrystallized from solution

POSS dissolves molecularly in solvents if the polarity of the solvent is matched to that of the POSS. The POSS can then be recrystallized from solution to purify it, just as with other solid chemical substances. Suitable solvents for POSS can be predicted using the Solubility Parameter concept, which is also useful for example when one needs to predict the solubility of an as yet unsynthesized POSS type. POSS can be dissolved in solvents in high concentrations, approaching 100 weight percent in some cases. POSS can also be dissolved in polymers but in that case the solubility limit is normally around 5 weight percent.

There are four main types of POSS configuration:

• • • Eight identical side groups
    • A statistical mixture of two different ligands totaling eight
    • Seven identical units with one corner of the cage open exposing three silanol groups
    • Seven identical groups with one different group inserted in one corner of the cage

So, POSS are molecules that just look reminiscent of particles and that is why they are sometimes referred to as “molicles” to reflect their dual nature. The fact that POSS are molecules is a great advantage because it means they dissolve. In contrast, nanoparticles are notoriously difficult to disperse and that has held back their use in commercial applications. Furthermore, as POSS are not nanoparticles they avoid the safety uncertainties associated with nano materials.

Properties of POSS Polyhedral Oligomeric Silsesquioxanes

Naturally, with well over 200 POSS types in existence, one cannot detail the properties for each and every one. It is more convenient to take an overview (Table 3).

Table 3 POSS Properties Overview

When trying to identify applications for POSS, it is appropriate to start by looking at the unusual and unique properties arising from the POSS molecular structure. These can be summarized as follows where each of the features results in consequences in terms of properties:

• • • Rigid cage structure
    • High thermal stability
    • Relatively high molecular weight
    • Fully tunable solubility
    • Fully tunable reactivity

Special Properties and Applications

In order to have a chance in the marketplace, a product needs to present an attractive value proposition to the customer. Specifically, the following three criteria must be fulfilled:

• • • The product must produce an effect
    • The effect must be useful in some way
    • The product must be competitive in terms of cost and performance compared to products incumbent in the marketplace

In order to have a metric of the viability of each proposed application for POSS, these three criteria will be applied in each case. Some applications are mentioned in the section below. The POSS Handbook gives more insight into each possible application, specifically how effective POSS is and whether the application has commercial potential.