Phenyl Polyhedral Oligomeric Silsesquioxane (POSS)-containing Polymers: Structure And Properties

Organic-inorganic nanocomposites have attracted considerable interest during the past few decades because they can exhibit unusual combinations of properties originating from the synergism of organic and inorganic components. Polyhedral oligomeric silsesquioxane (POSS) reagents, monomers, and polymers are emerging as a new chemical technology for nanoreinforced organic-inorganic hybrids. Polyhedral oligomeric silsesquioxanes (POSS) are a class of important nanosized cage-like molecules with a formula of [RSiO3/2]n, n = 6 ~12, where R can be various types of organic groups, one (or more) of which is reactive or polymerizable.In POSS technology, a diversity of polymerizable (or reactive) POSS macromers have been employed to copolymerize with organic monomer via the formation of covalent bonds between organic and inorganic components, in order to afford organic-inorganic nanostructured composites. The incorporation of the POSS nanocluster cages into polymeric materials can result in dramatic improvements in polymer properties, including mechanical properties, thermal stability, flame retardation, surface hydrophobicity, and melting and crystallization behavior etc.1. A Novel Photocrosslinkable Polyhedral Oligomeric Silsesquioxane and Its Nanocomposites with Poly(vinyl cinnamate) A novel photosensitive octacinnamamidophenyl polyhedral oligomeric silsesquioxane (OcapPOSS) was synthesized via the reaction between cinnamoyl chloride and octaaminophenyl polyhedral oligomeric silsesquioxane (OapPOSS). By initiating in situ UV photocrosslinking reaction of PVCIN in the presence of the photosensitive OcapPOSS, II is noted that the photocrosslinking rates of the PVCIN/POSS hybrids were slightly lower than those of the precursors, which could be ascribed to the nanoreinforcement of POSS cages on the polymer matrix. The glass transition temperatures (Tgs) of the nanocomposites were significantly higher than that of the control PVCIN. All the PVCIN nanocomposites containing POSS displayed the improved thermal stability. The high char yields of PVCIN nanocomposites also suggest improved flame retardence. It is plausible to propose that the improvement in weight retention is ascribed to the POSS constituent, which participated in the formation of the homogeneous hybrid network.2. Epoxy Resin Containing Polyphenylsilsesquioxane: Preparation, Morphology, and Thermomechanical PropertiesPolyphenylsilsesquioxanes (PSSQ) was used to incorporate into epoxy resin to prepare the organic-inorganic composites and two strategies were employed to afford the composites with different morphology. It is seen that the phase separation induced by polymerization occurred in the physical blending systems. However, the nanostructured composites were obtained while a catalytic amount Aluminum triacetylacetonate [Al(acac)3] was added to mediate the reaction between PPSQ and diglycidyl ether of bisphenol A (DGEBA). It is the inter-component reaction that significantly suppresses the phase separation on the micrometer scale. The organic-inorganic composites with different morphology displayed quite different thermomechanical properties. The nanostructured composites possessed higher glass transition temperatures (Tg's) with the same loading of PPSQ although the inter-component reaction between PPSQ and DGEBA reduced the crosslinking density of epoxy matrix. It is the nanoreinforcement of PPSQ domains that afford the enhanced dynamic storage modulus for the nanostructured composites in comparison with the phase-separated composites with the PPSQ content less than 15 wt%. The organic-inorganic composites displayed the improved thermal stability and flame retardance.3. Supramolecular Inclusion Complexation of Polyhedral Oligomeric Silsesquioxane Capped Poly(e-caprolactone) with a-CyclodextrinThe supramolecular inclusion complexes involving polyhedral oligomeric silsesquioxane (POSS)-capped poly(ε-caprolactone) (PCL) andα-cyclodextrin (α-CD) were investigated. The POSS-terminated PCL with various molecular weights were prepared via the ring opening polymerization ofε-caprolactone with 3-hydroxypropylheptaphenyl POSS as an initiator. The POSS-capped PCLs were used to prepare the supramolecular inclusion complexes (ICs) withα-CD. Due to the presence of the bulky group (viz. POSS), the inclusion complexation betweenα-CD and the POSS-capped PCL is only carried out with single end of PCL chain threading inside the cavity ofα-CD. The results indicate that the organic-inorganic ICs have a channel-type crystalline structure. It is noted that the stoichiometry of the organic-inorganic ICs is quite dependent on the molecular weights of the POSS-capped PCL. When the PCL chains are shorter, the values of the stoichiometry are higher than 1:1 (CL unit :α-CD in mol). The deceased efficiency of inclusion complexation could be attributed to the lower mobility of the bulky POSS group, which restricted the motion of PCL chain attached to silsesquioxane cage. This effect is pronounced with decreasing the length of the PCL chains.4. Melting and Crystallization Behavior of Polyhedral Oligomeric Silsesquioxane (POSS)-terminated Poly(ε-caprolactone)We investigated the melting and crystallization behavior of POSS-capped PCL with various lengths of PCL chains. The novel organic-inorganic association result in the significant alterations in the melting and crystallization behavior of PCL. The POSS-capped PCL displayed the enhanced equilibrium melting points compared to the control PCL. Both the overall crystallization rate and the spherulitic growth rate of the POSS-terminated PCLs increased with increasing the concentration of POSS (or with decreasing length of PCL chain in the hybrids). It is found that the folding free energy of surface for the POSS-terminated PCLs decreased with increasing the concentration of POSS. These results could be ascribed to the effect of the favorable heterogeneous nucleation of POSS terminal groups, which accelerates the process of crystallization.5. Self-organized Thermosets from Epoxy Resin and Amphiphilic POSS-capped Poly( -caprolactone) MixturesPolyhedral oligomeric silsesquioxane (POSS)-capped PCL was synthesized via ring-open polymerization ofε-caprolactone with 3-hydroypropylheptaphenyl POSS as the initiator. The novel organic-inorganic amphiphile was used to incorporate into epoxy resin to prepare the nanostructured thermosets. It is found that depending on the concentration of the POSS-capped PCL in the composites, the POSS microdomains displayed the structures from spherical, wormlike to lamellar nanoobjects. The formation of the nanostructures in the epoxy thermosets was addressed on the basis of the miscibility and phase behavior of the sub-components (viz. POSS and PCL chains) of the organic-inorganic amphiphilic macromolecule with epoxy resin after and before curing reaction. It is judged that the formation of the nanostructures in the organic-inorganic hybrid composites follows the mechanism of self-assembly. The organic-inorganic nanocomposites displayed a significant enhancement in surface hydrophobicity as well as reduction in surface free energy. The improvement in surface properties was ascribed to the enrichment of POSS moiety on the surface of the nanostructured thermosets.