Atomic force microscopy visualization of morphology changes resulting fromthe phase transitions in poly(di-n-alkylsiloxane)s: Poly(diethylsiloxane)

The results of visualization of morphology and nanostructure of polydiethyl siloxane (PDES) are presented. The study was performed in a temperature ran ge, which covers melting, crystallization, mesophase formation, and isotrop ization of PDES, on polymer samples of different molecular weights and diff erent thicknesses. The morphological identification of alpha (2)- and beta (2)-polymorphs of PDES was further extended by demonstrating, for the first time, that the alpha (2)- and beta (2)-crystalline polymorphs transform to two morphologically incompatible alpha -mesophase and beta -mesophase. Dom ains, which are several microns in size, are typical morphologic patterns f or the alpha -phase material, whereas large lamellae (from a few to tens of microns in length and several hundreds of nanometers in width) are typical structures of the beta -phase PDES. The above polymorphs and amorphous pol ymers have appeared in different ratios in the samples depending on their p reparation (deposition way, thermal history) and thickness. Shearing of PDE S into thin film on Si substrate induced the formation of the mesomorphic b eta -phase structures, which are embedded in amorphous material. Crystalliz ation of the amorphous PDES occurred at much lower temperatures due to its constrained geometry. The width of the beta -phase lamellae in the crystall ine and mesomorphic state as measured from AFM images correlates with the l ength of the extended PDES chains in the crystalline state. Sublamellae str uctures were revealed by AFM imaging the mesomorphic lamellae with differen t tip-sample forces. Each lamella has a skeleton formed of 10-15 nm thick l inear structures, which are separated by 40-50 nm and tapered at the ends t o form a single entity. This skeleton is wrapped with numerous 0.8 nm thick layers, which are most likely formed of partially ordered domains of exten ded chains. Crystallization of the beta -mesomorphic lamellae leads to more ordered and stiffer top layers of lamellae. In some cases individual lamel lae are broken into small crystalline blocks, and in other cases crystallin e blocks incorporate two neighboring lamellae. The domains of the mesomorph ic alpha -phase are less organized, and upon crystallization large domains are broken into smaller blocks due to shrinkage. This is accompanied by sur face roughening. Some of the blocks exhibit periodic surface structures wit h a repeat distance of 50-60 nm. At least partial chain extension in the al pha -phase PDES is suggested.