INTRAMOLECULAR CLUSTERS AS BUILDING UNITS FOR POLYMER NANOOBJECTSстатья
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Аннотация:The way polymers crystallize from melt and solution is drastically different from that of small molecules and its molecular mechanism is still not understood despite over 50 years of the investigation [1]. The two dominant approaches Lauritzen-Hoffman surface nucleation theory and the Sadler-Gilmer entropic barrier model which (with a suitable choice of parameters) describe quantitatively such macroscopic properties as the temperature dependence of growth rate and crystal thickness (fold length) appear to be irreconcilable whereas the alternative unified theory is still absent. Despite the differences it is assumed in both mentioned theories that the molecule before crystallization is in the state of random coil. At the same time the statistical mechanical “bundle” model developed in [2] as well as the results of many computer simulations [3-5] despite their still simplified character indicate on a possibility of a quite different crystallization mechanism in which the ideas of coil to globule transition should be used. In these new models the transition from isotropic solution to crystal state proceeds through the intermediate states in which the polymer molecules have the conformations with multiple internal folds (bundles) or adopt the compact globule state which however has not still be observed for semicrystalline polymers. From this point of view the direct microscopic visualization of these intermediate states as well as the conformational states of single molecules of flexible semicrystalline polymers is highly desirable with the resolution of the order of several nm.
The first microscopic observations of a variety of new compact conformational states have been reported in [6]. The measurements have been conducted for a number of polyolefines at quick depositions from hot diluted solutions on inorganic substrates (mica, glass, HOPG). Both monomolecular particles and those of containing few molecules have been observed all having complex internal substructure. Fig.1 represents the AFM height image of a polyethylene (PE) sample deposited on mica by a procedure described in [6]. The specimen contains both heavily tangled coils and compact nanoobjects with uncommon morphology shown by arrows. The height section shown down the figure indicates on height of nanoobjects within 1-3 chain diameters what gives the estimations of their volume as corresponding to a single or few number of PE molecules inside. It is clearly seen that all nanoobjects consist of several seemingly independent blocks linked with each other in a complex way due to chain connectivity. High-achieved lateral resolution allows estimating the number of chain stems within such blocks to be of the order of ten (the inevitable broadening of lateral dimensions due to probe convolution should be taken into account). The length of blocks is close to 10 nm – a typical value for fold length adopted by folded polymer molecules in a solid state. The Fig.1 is a representative of a large amount of similar images obtained for another polymer grades in a wide range of preparation conditions thus this picture should be considered as regularity requiring adequate treatment.
Fig.1 The AFM height image (inverse view) of polyethylene grade (MW=13200, MN=11600) deposited on mica from 1 ppm ODCB solution at a temperature of 160OC. The arrows point on compact nanoparticles with complex structure of a blocky type. The inset below is enlarged image of the nanoparticle in the left upper corner.
We notice firstly that such treatment seems impossible on the base of a common viewpoint on a polymer growth from a solution implying the formation of continuous lamellar structures in crystal state or featureless globules without any internal structure. To our belief the probable answers to this and another relevant questions may be found in some new approaches to the old problem of polymer crystallization [1]. Particularly the “bundle” theory seems to be a suitable candidature for the explanation of blocky structure if we assume that the observed blocks are bundles composed of several local chain folds. It was suggested in [2] that a polymer crystallization in a solution is controlled by the metastable equilibrium at the formation and dissolution of intramolecular associations (bundles) among parallel stems due to the action of long range van der Waals forces between nonbonded chain segments. The number of stems in a bundle was estimated to be within 4 - 15 what is in a good agreement with the estimation made above for number of stems in a single block.
The different level of morphological hierarchy may be assumed in this model – from the simplest parallel packing of bundles at lamella growth (see the second report in this volume) to the complex packing morphology in nanoparticles such as represented in the Fig.1. Such packing is assumed to take place at the approaching of a polymer molecule to the substrate and its subsequent trapping by an attractive surface force field. The short time of sample preparation indicate also on the rapidity of this process. Some specific influence of the lateral periodicity of this force field due to crystal symmetry of the substrate may be supposed. We have found however that similar nanoparticles morphologies have been observed for different substrates what implies that the choice of any particular morphology is determined by the mechanism somewhat similar to that of proteins folding [7].
The investigation of this fundamental question requires future experimental and theoretical efforts. The results obtained in the present report indicate on a high capability of AFM which seem to be the sole microscopic tool at the investigation of very fine structural features of polymers.
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