Amorphous polymers structure

The ability of a material to crystallise is determined by the regularity of its molecular structure. A regular structure is potentially capable of crystallinity whilst an irregular structure will tend to give amorphous polymers. Structural irregularities can occur in the following ways ... 

Cast film extrusion of polyolefins has been developed to obtain flexible films with a high level of transparency by freezing the amorphous polymer structure of the melt on a chill roll. Cast films are mono-oriented in extrusion direction. 

Aharoni has recently proposed a theory of flow based on entanglements between loops on the surface of collapsed coils (43, 228). The basic picture of amorphous polymer structure is almost certainly incorrect (see Section 2), and the derivation of viscosity is even more speculative than the others in this section. 

Historically most of the microscopic diffusion models were formulated for amorphous polymer structures and are based on concepts derived from diffusion in simple liquids. The amorphous polymers can often be regarded with good approximation as homogeneous and isotropic structures. The crystalline regions of the polymers are considered as impenetrable obstacles in the path of the diffusion process and sources of heterogeneous properties for the penetrant polymer system. The effect of crystallites on the mechanism of substance transport and diffusion in a semicrystalline polymer has often been analysed from the point of view of barrier property enhancement in polymer films (35,36). 

Then an important aspect is how precise the predicted D will be So far an agreement within one order of magnitude between an experiment and an atomistic simulation is considered to be a good achievement. For completely amorphous polymer structures and simple penetrants even better agreements have been reported in Tables 5-1 and 5-2. From the point of view of estimating the migration from polymeric materials used in the technical sector a prediction of D within the order of magnitude of the experimental one would be a result of certain practical use, see Chapter 15. The question is to what sophistication must be developed the computer simulation approach to meet this requirement also for the type of penetrant polymer systems which are usual in the named sector ... 

Figure 9.1 A typical presentation of an amorphous polymer structure. In reality, polymeric chains make many more interactions with each other, forming more like a hean structure, rather than being tightly packed.

Kotelyanskii M, Wagner N J and Paulaitis M E 1996 Building large amorphous polymer structures atomistic simulation of glassy polymers Macromolecules 29 8497- 506... 

Figure 16.18 Schematic DTA thermal curves for the totally amorphous polymer structure and the semicrystalline polymer structure shown in Fig. 16.17. Both show only the semicrystalhne polymer has a crystallization exotherm.

The cluster model assumes availability in the amorphous polymers structure of local order domains (clusters), consisting of several densely packed collinear segments of different macromolecules (amorphous analog of crystallite with stretched chains). These clusters are connected between themselves by tie chains, forming by virtue of this physical entanglements network, and are surrounded by loosely packed matrix, in which all fluctuation free volume is concentrated [107],... 

Kozlov, G. V Gazaev, M. A. Novikov, V. U. Mikitaev, A. K. Simulation of amorphous polymers structure as percolation cluster. Letter to Journal Engineering Physics, 1996, 22(16), 31-38. 

Chemically non-specific models of amorphous polymer structure... 

Figure 16.17 (a) Amorphous polymer structure, (b) Semicrystalline polymer structure with aligned chains in the crystalline regions, and random structure in the amorphous region. 

In contrast to amorphous polymers, structural details of lamellae can be imaged using electron diffraction contrast in TEM. In this case, unstained uitrathin sections or solution-cast films are used. Electron diffraction patterns allow structural analysis at the level of the crystalline unit cell, and they can be used to detect the crystal orientations of a polymer. An example with a sheaf-like morphology of PE with low molecular weight is shown in Eig. 2.3 by different modes in TEM. 

Kozlov, G. V, Beloshenko, V. A., Shogenov, V. N. (1999). An Amorphous Polymers Structural Relaxation Description within the Framewoiks of Cluster model Fiziko-Khi-micheskaya Mechanika Materialov, 35(5), 105-108. 

Kozlov, G. V, Gazaev, M. A., Novikov, V. U., Mddtaev, A. K. (1996). Simulation of Amorphous Polymers Structure as Percolation cluster. Ris ma v ZhTF, 22(16), 31-38. Serdyuk, V. D., Kosa, N., Kozlov, G. V. (1995). Simulation of Polymers Forced Elasticity Process with the Aid of Dislocation Analoques. Fizika i Tekhnika sokikh Davle-nii, 5(3), 37 2. 

The studies carried out earlier have shown that polymer film samples strength to a considerable extent is defined by growth parameters of stable crack in local deformation zone (ZD) at a notch tip [1-3], As it has been shown in Refs. [4, 5], the fiactal concept can be used successfully for the similar processes analysis. This concept is used particularly successfully for the relationships between fracture processes on different levels and subjecting fracture material microstructure derivation [5]. This problem is of the interest in one more respect. As it has been shown earlier, both amorphous polymers structure [7] and Griffith crack [4] are fractals. Therefore, the possibility to establish these objects fractal characteristics intercommunication appears. The authors of Refs. [8, 9] consider stable cracks in polyarylatesul-fone (PASF) film samples treatment as fractals and obtain intercommunication of this polymer structure characteristics with samples with sharp notch fracture parameters. 

As it is known [2, 12], within the frameworks of cluster model the elasticity modulus E value is defined by stiffness of amorphous polymers structure both components local order domains (clusters) and loosely packed matrix. In Fig. 13.3, the dependences E(v J are adduced, obtained for tensile tests three types with constant strain rate, with strain discontinuous change and on stress relaxation. As one can see, the dependences E(yJ are approximated by three parallel straight lines, cutting on the axis E loosely packed matrix elasticity modulus E different values. The greatest value E is obtained in tensile tests with constant strain rate, the least one - at strain discontinuous change and in tests on stress relaxation E = 0 [1]. 

And in eompletion of the present seetion let us note one more important feature of natural nanocomposites structure. In Refs. [24,25] the interfacial regions absence in amorphous glassy polymers, treated as natural nanocomposites, was shown. This means, that such nanocomposites structure represents a nanofiller (nanoclusters), immersed in matrix (loosely packed matrix of amorphous polymer structure), that is, unlike polymer nanocomposites with inoiganic nanofiller (artificial nanocomposites) they have only two structural components. 

At amorphous glassy polymers as natural nanocomposites treatment the estimation of filling degree or nanoclusters relative fraction (p j has an important significance. Therefore, the authors of Ref. [27] carried out the comparison of the indicated parameter estimation different methods, one of which is EPR-spectroscopy (the method of spin probes). The indicated method allows to study amorphous polymer structural heterogeneity, using radicals distribution character. As it is known [28], the method, based on the parameter - the ratio of spectrum extreme components total intensity to central component intensity-measurement is the simplest and most suitable method of nitroxyl radicals local concentrations determination. The value of dipole-dipole interaction is directly proportional to spin probes concentration C [29] ... 

Yech, G. S. (1979). The General Notions on Amorphous Polymers Structure. Local Order and Chain Conformation Degrees. N sokomolek.SoedA, 21 (Nil), 2433-2446. Perepechko, 1.1. (1978). Introduction in Physics of Polymers. Moscow, Khimiya, 312p. Kozlov, G. V, Zaikov, G. E. (2001). The Generalized Description of Local Order in Polymers. In Fractals and Local Order in Polymeric Materials. Kozlov, G. V., Zaikov, G. E., Ed., New York, Nova Science Pubhshers Inc. 55-63. 

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