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Necklace-shaped dimethylsiloxane (DMS) polymers bearing polyhedral oligomeric silsesquioxane (POSS) cages with “alternating modulated chain” arrangements were synthesized, by the polycondensation of a dimeric POSS precursor and dimethyl oligo-siloxane dichloride. Films of POSS-DMS polymers with “alternating modulated chain” arrangements were highly transparent. This observation is consistent with amorphous structures. Along with “constant chain” and “random chain” type polymers, three different chain arrangements of necklace-shaped polymers can now be produced. The physical properties of the three polymer types with constant POSS cage contents were compared. “Alternating modulated chain” type polymers had lower glass transition temperatures than the other two polymer types. This was attributed to the cooperative relaxation of long chain moieties that comprised half of the connecting chains. “Alternating modulated chain” type POSS-DMS polymers with average chain lengths of three to four exhibited heat resistances of over 470 °C in air.

The transport of small gases (H2, CO2, N2, O2) through a series of novel membranes based on necklace-shaped inorganic polymers (DMS@POSS), in which a polyhedral oligomeric silsesquioxane (POSS) cage unit and soft chains of oligo-dimethyl siloxane (DMS) were alternately connected, was investigated. The influence of the DMS chain length and crosslinking density of the DMS@POSS on membrane properties were studied. The membranes revealed characteristic structure-property relation towards both glass transition and gases transport. Specifically, clear dependence of properties from the length of DMS units (or overall siloxane content) was revealed. Gas transport properties, when compared to state-of-art polydimethylsiloxane and commercial silicone rubber, demonstrated significantly higher selectivity of DMS@POSS for carbon dioxide (in CO2/N2), hydrogen (in H2/N2) and oxygen (in O2/N2) but lowered permeability, proportional to the amount of POSS in the material. With a precise control over mechanical and thermal properties compared to conventional silicone rubbers, described materials could be considered as materials of choice in niche gas separation or other applications.