Annealing of Ultrathin Polyethylene Films from Aqueous Dispersions

نویسندگان

  • Qiong Tong
  • Marina Krumova
  • Stefan Mecking
چکیده

Introduction Thin and ultrathin crystalline polymer films are of fundamental scientific and practical interest, e.g. in terms of their mechanical properties or of crystallization in a confined environment. A common route to ultrathin crystalline polymer films is spin-coating from solution, often at high temperature, which involves formation of crystalline structures during solvent evaporation. Recently, we have found that ultrathin crystalline polyethylene films can be prepared at room temperature by evaporation of aqueous dispersions (I) of polyethylene nanocrystals. The latter possess a lamella thickness of 6 nm with a thin amorphous layer of 1 nm on the lamella surface, and a lateral pseudo-diameter of ca. 12 nm. That is, although the particles are semicrystalline, the minor amorphous portion is located entirely at the surface, due to their single lamella nature. This enhances interactions between the original particles in (ultrathin) films. By variation of the catalyst utilized in the preparation of the dispersions, nanoparticles of similar overall size can also be prepared which are composed not of linear but of branched polyethylene (ca. 50 methyl branches / 1000 carbon atoms), and are much less crystalline. These dispersions (II) are of interest here for comparative purposes. Orientation of crystals in semicrystalline polymer films and the effect of annealing has been studies intensely. Surface-induced reorientation has been reported for drawn polyethylene films, at temperatures below the bulk melting point. Annealing at 124°C induced orientation with the (110) face parallel to the free film surface, in addition to the fibre texture along the drawing direction in the as-drawn film. Melting of crystals with lattice defects below the bulk Tm, and migration of amorphous phases during annealing were suggested as possible mechanisms of this process. It has also been reported that annealing on single crystalline polyethylene results in thickening of the crystals and reorientation of the lamellae. We report preliminary studies of the annealing of the aforementioned ultrathin films formed from nanocrystal dispersions.

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تاریخ انتشار 2008