AFM investigation of condensed states of polymer chains of polyolefinesстатья
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Аннотация:Condensates are compact states adopted by long polymer chains freely floating in a solution at changing some physical parameters ( temperature, quality of the solution or due to the action of some condensing agents) . In experimental studies up to now this term was used for a description of some conformations of rigid DNA chains, particularly rods and toroids [1]. Here we present the results of observation of similar morphologies for polyolefines such as polyethylene and isotactic polypropylene (iPP) by using atomic force microscopy (AFM).
The condensates were obtained by a deposition on mica from hot diluted polymer solutions (0.01-10 ppm) at temperatures of 80-160OC by using dipping or dropping procedures. Height and phase AFM images were recorded with a Nanoscope III multimode AFM (Digital Instruments) operated in air in a tapping mode with low vibrational amplitudes. Silicon cantilevers of different manufacturers with spring constants of 2-30 N/m were used.
Fig.1 represents height and phase images of compact iPP particles obtained at conditions indicated in figure capture. The particles are seen as objects of squared or rectangular shapes with a depression in a middle indicating on a toroidal topology of chain packing, the latter is more clearly pronounced on the phase image. Images of more complex shapes have several depressions (not shown). Rod type condensates ( see fig.1 b,d of the first report in this volume ) with a different length may be selected in a separate group due to characteristic pattern having two longitudinal strips on both their sides and one or more transversal subdivisions. Some condensates shapes are difficult to describe due to their complexity.
Condensates heights are within 0.7-2.5 nm (see inset in fig.1a) what equals to 1-5 of polymer chain diameters. The condensates volumes as measured from AFM images were mainly in the range of 200-600 nm3 what should be compared with the expected volume of a single iPP molecule of 90 nm3. This allows us to conclude that the particles seen are composed of few or single polymer molecules.
The comparison of the polyolefines condensates images such as in fig. 1 with these of obtained for DNA for years by electron microscopy studies and especially in some recent works made by AFM [2] reveals their striking morphological similarity. It should be noticed however that this similarity seems unexpected from the point of view of large difference in chemical structures and physical properties of DNA and polyolephines. The theories of coil to globules transitions predict toroid or rod morphology only for sufficiently “stiff “ polymer chains whereas for polymers of vynil type with “soft” chain backbones such as polyolefines structureless spherical globules are expected [3]. The reasons of such a high similarity is not cleared at present. Increased chain stiffness may be explained by a more high value than commonly accepted of the torsional energy barrier of trans- gauche- transitions. An alternative explanation would require that the observed shapes are exclusively the result of surface induced condensation due to unknown mechanism resulting to the increase of apparent chain stiffness.
1. L.C.Gosule and J.A.Shellman, Nature, 259, 3333-335, (1976).
2. Z.Lin, C.Wang, X.Feng, M.Liu, J.Li and C.Bai, Nucl. Asid Res., 26(13), 3228, (1998).
3. A.Yu. Grossberg and A.R. Khohlov, “Statistical Physics of macromolecules”, AIP series in polymers and complex materials, 1994.