POSS Polymer Nanocomposites: Thermal Stability and Design
POSS polymer nanocomposites occupy a deliberate middle ground between organic polymers and inorganic solids, and the literature of the past two decades has established both what the cages contribute and why the contribution varies so widely from one system to another. Melt blending of octameric silsesquioxane cages into polypropylene, to take the most frequently cited example, raised the temperature of maximum degradation rate measured in air from 330 °C for the neat polymer to 341 °C at a 5 % loading of octamethyl POSS, and to 379 °C when octaphenyl POSS was used instead. The result illustrates a general principle. A molecular silica core carried into a polymer matrix on eight tunable organic arms converts an ordinary polyolefin into a thermally protected hybrid material.
The T8 Silsesquioxane Core: Structure and Polymer Compatibility
Silsesquioxanes occupy the ground between organic and inorganic chemistry, and polyhedral oligomeric silsesquioxane (POSS) cages are the most widely exploited members of the family. The archetypal T8 structure is built on an Si8O12 core, a rigid inorganic polyhedron whose eight silicon vertices each carry a single organic substituent. Because almost any group can be installed as a side arm, one inorganic core can be adapted to a wide range of polymer matrices, and the resulting composites acquire characteristics intermediate between those of organic polymers and those of inorganic solids such as silica. That intermediate position is what makes the design of specialized materials with unusual property combinations possible. The features that recommend these cages as fillers are consistent across the family and include high mechanical strength, solubility in most organic solvents, thermal stability and considerable resistance to weathering, together with an absence of volatility and odor and a lack of toxicity.
Thermal Stabilization of Polypropylene by Octamethyl and Octaphenyl POSS
The route to such materials can be as direct as melt processing. Alkyl-substituted octameric silsesquioxanes bearing methyl, isobutyl and isooctyl groups have been introduced into polypropylene by melt blending, and the additive improved the resistance of the polyolefin to thermo-oxidative conditions. Thermogravimetric analysis in air placed the temperature of maximum degradation rate of the polypropylene used to prepare the composites at 330 °C. Introduction of 5 % octamethyl POSS (me-POSS) raised that value to 341 °C, a gain of 11 °C, whereas octaphenyl POSS (ph-POSS) shifted the maximum to 379 °C, an improvement of 49 °C over the neat polymer. The comparison establishes that the identity of the substituent, and not merely the presence of the inorganic core, governs the magnitude of the stabilization.
Fig. 1. Temperature of maximum degradation rate of polypropylene and of its POSS composites, determined by thermogravimetric analysis in air. The neat polymer degrades most rapidly at 330 °C, a 5 % loading of octamethyl POSS (me-POSS) shifts the maximum to 341 °C, and octaphenyl POSS (ph-POSS) shifts it to 379 °C. Values from Fina et al., Polymer 2005, 46, 7855–7866 (Elsevier).
Protective Mechanism: SiO₂ Char Layer and Substituent Dependence
The protective mechanism follows from the sequence in which the cage decomposes. The lateral organic arms are consumed in the first stage, after which the silsesquioxane core forms, on the surface of the degrading material, a carbonized insulating layer containing silicon dioxide (SiO2) that impedes further combustion. Since the arms are lost first, their chemical nature sets both the onset of that first stage and the quality of the residue that follows, which is consistent with the advantage recorded for the aromatic cage over its methyl analogue and with the wider structural chemistry of phenylsilsesquioxane cage and ladder architectures. The properties of composite materials based on polyhedral silsesquioxanes therefore depend jointly on the physical and chemical characteristics of the substituents attached to the cage and on the quantity of cage introduced into the polymer matrix.
Dispersion Morphology, Glass Transition Temperature, and Mechanical Properties
How the cages sit inside the matrix is equally decisive. Acting as polymeric fillers, POSS molecules may be dispersed at the molecular level, associate into aggregates, or organize into a crystalline or an amorphous phase, and the state adopted depends on the loading and on the substituents the cage carries. Molecular dispersion places isolated inorganic cores between chain segments, whereas aggregation produces discrete POSS-rich domains with interfaces of their own. These morphologies are not interchangeable, because they decide whether a given cage restricts the mobility of the surrounding chains or lubricates their motion, and the thermal and mechanical response of the composite follows from that choice rather than from the composition alone.
Fig. 2. Dispersion states available to POSS cages in a polymer matrix. (a) Cages dispersed at the molecular level between chain segments, (b) cages associated into aggregates and (c) cages organized into an ordered crystalline domain. The state that is adopted determines whether the cage restricts chain mobility and raises the glass transition temperature or promotes chain slippage and lowers it.
Since the cages are dimensionally similar to the segments of polymer chains, they are able to restrict the motion of macromolecules and to raise the glass transition temperature (Tg). The behavior recorded for non-reactive cages is nevertheless twofold. The glass transition temperature may fall when POSS molecules are dispersed at the molecular level and promote slippage of polymer chains past one another, or it may change very little or rise when compatibility between the cage and the polymer is poor and separate POSS domains are formed. Related plasticizing effects appear during processing, where the addition of spherical POSS molecules can lower the viscosity of the polymer and reduce polymerization shrinkage, the latter being valuable in formulations that are cured in place. Across most reported systems, hybrid polymer materials containing POSS display improved mechanical properties.
The application range of hybrid polymers containing POSS derivatives and of their nanocomposites is correspondingly wide. It covers elastomers, thermoplastics, self-healing materials, biodegradable polymers, thermosetting resins, dendrimers, holographic recording media, resists for electron-beam lithography, dental restorative composites and materials intended for medical use, among them leaflets for synthetic heart valves and scaffolds for tissue engineering. Work on POSS-based poly(1-haloacetylene)s obtained by solid-state polymerization extends the same design logic to conjugated backbones. Beyond bulk reinforcement, POSS units also act as interfacial agents that increase the miscibility of polymers that do not otherwise mix, and building appropriate cages into the structure of polystyrene and poly(methyl methacrylate) improves the compatibility of that blend.
Taken together, these observations define POSS not as a conventional filler but as a molecular building block whose inorganic core delivers thermal and mechanical benefit while its eight organic arms are tuned to the matrix that surrounds it. The polypropylene data quantify the benefit in its simplest form, 49 °C of additional thermal stability from a phenyl-substituted cage in a commodity polyolefin, and the breadth of the reported applications indicates that the same principle transfers across chemistries from elastomers to biomedical devices. The outstanding challenge is control of dispersion, since morphology mediates between the molecular structure of the cage and every macroscopic property of the composite, and predictive rules connecting the choice of substituent to the final state of dispersion would convert an empirical practice into a design discipline.
DOI: 10.1016/j.polymer.2005.06.121
Full text: Polymer → Publisher
Background reading: D. B. Cordes, P. D. Lickiss and F. Rataboul, “Recent Developments in the Chemistry of Cubic Polyhedral Oligosilsesquioxanes,” Chemical Reviews, 2010, 110, 2081–2173. DOI: 10.1021/cr900201r
S. Li, G. P. Simon and J. G. Matisons, “The Effect of Incorporation of POSS Units on Polymer Blend Compatibility,” Journal of Applied Polymer Science, 2010, 115, 1153–1159. DOI: 10.1002/app.31225
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