|
ИСТИНА |
Войти в систему Регистрация |
Интеллектуальная Система Тематического Исследования НАукометрических данных |
||
The AFM Comparison of Conformations Acquired on Mica by Single Molecules of Polystyrene and Polyethyleneстатья
Информация о цитировании статьи получена из
Web of Science
Статья опубликована в журнале из списка Web of Science и/или Scopus
Дата последнего поиска статьи во внешних источниках: 3 июля 2019 г.
- Авторы: Prokhorov V.V., Nitta K.
- Журнал: PHYSICS OF LOW-DIMENSIONAL STRUCTURES
- Том: 3
- Год издания: 2003
- Первая страница: 233
- Последняя страница: 242
- Аннотация: It is well known that the semicrystalline polymers form basically the same lamellar crystals upon crystallization from diluted solution both at growth in its bulk and at the epitaxial growth on inorganic substrates [1]. In these crystals the long polymer molecules are folded many times in folds with the typical fold length of about 10 nm. Despite this fact is considered as a fundamental property of crystalline polymers and has been well investigated, there is still no theoretical consistency in the description of polymer crystallization. The dominant theory of Lauritzen and Hoffman [2] consider it as the two stage process: at first secondary nucleation of a single stem of approaching polymer coil on a surface of growing crystal takes place (this step is considered to be time limiting as it has energetic barrier of incorporation into crystal structure) and second after nucleation coil quickly folds with constant fold length determined by lamella thickness. This model has always been challenged by another theoretical approaches but being in fact the sole theory giving a rather satisfactory quantitative description of a number of experimental parameters it is still alive. The comparison of different theories having different microscopic mechanisms could be more easy if the microscopic pictures of polymer crystallization at the level of single molecules could be obtained. We have reported [3] on the AFM study on thin polymer deposits adsorbed on mica at elevated temperatures from very diluted solutions of crystalline polyolefins. Mono- or few- molecular particles observed at such conditions possess unexpectedly various types of morphology. The specimens which were often heterogeneous due to uneven solvent evaporation at drying contained also the lamellar objects as well as very thin polymer layers. The inspection of AFM images of these lamellae reveals a number of anomalous structural features, which haven’t been noted in a literature so far. Below the results for the lamellae of isotactic polypropylene (iPP) will be briefly discussed. 2. Experimental results and discussion. The polymer used is a commercial iPP grade with a number average molecular weight MN = 49000 and polydispersity Mw/Mn =5.3. The deposition has been done in accordance with [3]. Fig. 1 represents the AFM phase images of different lamellar structures obtained for the same specimen. The left image (a) shows thin isolated lamellae whereas the right image (b) refers shows the part of a dendrite surface. The isolated lamellae have several interesting features: firstly, they are striated with striae separated by 3-8 nm and aligned predominantly perpendicular to lamellae long axes and secondly, the lamellae have blocky structure with blocks formed by several adjacent parallel striae of approximately equal length. The lamellae height is very low (1-2 nm). These features indicate on great variations in the fold length (lamella width) within 5-15 nm. It is noteworthy to say that the striae images have been obtained by using the ultrasharp cantilever. The Fig.1b represents the phase image of the top surface of the dendrite. Its height is about 5 nm and the influence of mica surface field which is very probable for growth of lamellae of Fig.1a may be neglected in this case. Thin striae such as in Fig.1a can’t be resolved on this image but common probe instead of ultrasharp such as in case of Fig.1a has been used. The blocky structure is preserved however in this case also. The volume of blocks estimated from height images is approximately equal to the volume of single iPP molecule. The top of the dendrite may be characterized as a whole as a rough mosaic surface containing both the single lamellae or bunches of several adjacent lamellae growing in a parallel (not represented) and isolated blocks such as in Fig.1b. As a whole the AFM images provide new important and unexpected information. The picture is very different from that of observed in the “classical” lamellae in which no irregularities are supposed. The probable explanation of such crucial differences implies that some intermediate states have been observed in our specimens due to the rapidity of the used deposition procedure and very thin depositions. The fine structural features seen on our images may be “annealed” at more longer times of common sample preparations. To our belief the adequate interpretation of the observed picture may be based on some new approaches to polymer crystallization in a solution [4]. Particularly the bundle theory [5] is a good candidature for the explanation of the lamellae striation. The bundles are metastable aggregates of several chain folds appearing due to the attraction between nonbonded chain segments. The number of stems in a bundle was estimted in [5] within 4 – 15 what is in a good agreement with the estimations for a number of stems in a single stria. The small compact metastable clusters resembling bundles can be seen on the images of a sufficiently long polymer chain at a computer modeling of its crystallization from a solution [6]. The blocky structure seems to be a common feature of thin lamellae growing on the substrate (Fig.1a) and those of nucleated on preformed polymer surface (Fig.1b). We should stress that due to final probe curvature the AFM smoothes out the boundaries between adjacent blocks which are expected to be sharper than on AFM images. The natural explanation of this block structure may be done we suppose that polymer molecules crystallize as whole entities. Particularly single blocks in the Fig. 1b may be interpreted as single nucleated iPP molecules. The possibility of the polymer lamella growth of such type which is principally different from common mechanism of coil nucleation follows from the computer simulations [7] where the adsorption of polymer chain in unordered collapsed state on the surface of a polymer crystal has been considered. It has been shown that due to the action of attractive surface fields the globule conformation transforms to ordered multilayered structure with final incorporation into crystal structure. This issue is of a large fundamental importance and future researches will shed more lights. As a resume the AFM images of thin iPP deposits obtained by quick adsorption from diluted solutions demonstrate the high capability of this technique. The observation of new structural features, which haven’t been observed so far, arises the important questions on the mechanism of polymer crystallisation from solutions. The probable answers may be obtained from new theoretical approaches and further experimental efforts. _____________________ 1. B. Wunderlich, Macromolecular Physics, V1, (1973), Academic Press, New York. 2. J. D. Hoffman and R. L. Miller, Polymer 38, (1997), 3151. 3. V.V. Prokhorov, K.Nitta, Phys. Low-Dim. Struct., 3/4, (2001), 141. 4. M. Muthukumar, Eur. Phys. J. E 3, (2000), 199. 5. G. Allegra and S. V. Meille, Phys. Chem. Chem. Phys. 1, (1999), 5179. 6. M. Muthukumar, P. Welch, Polymer, 41, (2000), 8833. 7. T. Yamamoto, J. Chem. Phys., 109, (1998), 4638.
- Добавил в систему: Прохоров Валерий Васильевич