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

The results of visualization of morphology and nanostructure of polydiethylsiloxane (PDES) are presented. The study was performed in a temperature range, which covers melting, crystallization, mesophase formation, and isotropization of PDES, on polymer samples of different molecular weights and different thicknesses. The morphological identification of a2- and β 2 -polymorphs of PDES was further extended by demonstrating, for the first time, that the α 2 - and β 2 -crystalline polymorphs transform to two morphologically incompatible α-mesophase and β-mesophase. Domains, which are several microns in size, are typical morphologic patterns for the α-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 β-phase PDES. The above polymorphs and amorphous polymers have appeared in different ratios in the samples depending on their preparation (deposition way, thermal history) and thickness. Shearing of PDES into thin film on Si substrate induced the formation of the mesomorphic β-phase structures, which are embedded in amorphous material. Crystallization of the amorphous PDES occurred at much lower temperatures due to its constrained geometry. The width of the β-phase lamellae in the crystalline and mesomorphic state as measured from AFM images correlates with the length of the extended PDES chains in the crystalline state. Sublamellae structures were revealed by AFM imaging the mesomorphic lamellae with different tip-sample forces. Each lamella has a skeleton formed of 10-15 nm thick linear structures, which are separated by 40-50 nm and tapered at the ends to 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 extended chains. Crystallization of the β-mesomorphic lamellae leads to more ordered and stiffer top layers of lamellae. In some cases individual lamellae are broken into small crystalline blocks, and in other cases crystalline blocks incorporate two neighboring lamellae. The domains of the mesomorphic α-phase are less organized, and upon crystallization large domains are broken into smaller blocks due to shrinkage. This is accompanied by surface roughening. Some of the blocks exhibit periodic surface structures with a repeat distance of 50-60 nm. At least partial chain extension in the α-phase PDES is suggested.