Polymorphism, structure, and chromism in poly(di-n-octylsilane) and poly(di-n-decylsilane)

Semicrystalline films and powders of poly(di-n-octylsilane) and poly(di-n-d ecylsilane) have been investigated using in situ quenching studies by X-ray scattering and UV-vis absorption in combination with detailed model calcul ations. These polymers displayed a surprisingly rich array of monotropic an d enantiotropic structures during thermal cycling. Poly(di-n-decylsilane) w as observed to have no less than five distinct ordered forms (crystalline o r semicrystalline) which varied in their respective chain structure, packin g, and optical properties. Of these only one appeared to be thermodynamical ly stable. One of the metastable polymorphic structures exhibited an unprec edented tripling of the basic unit cell structure. Poly(di-n-octylsilane) s amples also exhibited up to five distinct structural forms at temperatures below that of a 25 degreesC transition to the thermotropic mesophase. Three of these were clearly ordered while the other two are mesophases with feat ures suggestive of a less ordered state. Suggested structural models are us ed to document and identify the predominant side chain and main chain packi ng motifs in the various crystalline polymorphs.