Melt orientation macromolecules

Kaito, A., Iwakura, Y., Hatakeyama, K, Li, Y., (2007) Organization of Oriented Lamellar Structures in a Miscible Crystalline/Crystalline Polymer Blend rmder Uniaxial Compression Flow near the Melting Temperature, Macromolecules, Vol. 40, (February, 2007), pp. 2751-2759, ISSN 0024-9297... 

Zhang Q, Wang Y, Fu Q (2003) Shear-indnced change of exfoliation and orientation in polypro-pylene/montmorillonite nanocomposites. J Polym Sci B Polym Phys 41 1-10 Zhang WD, Shen L, Phang lY, Liu T (2004) Carbon nanotubes reinforced nylon-6 composite prepared by simple melt-compounding. Macromolecules 37 256-259 Zhao Y, Abdullayev E, Vasiliev A, Lvov YM (2013) Halloysile nanotubule clay for efficient water purification. J Coll Interface Sci 406 121-129... 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

Many authors studying the formation of ECC from melts and solutions suggested that preliminary unfolding and extension of macromolecules occurs. Keller and Maehin25 have shown that in all known cases (including such extreme variants as the crystallization of natural rubber under extension and a polyethylene melt under flow) the same initial process of linear nucleation occurs and fibrillar structures is formed by the macromolecu-lar chains oriented parallel to the fibrillar axes27. ... 

The formation of ECC is not only an extension of a portion of the macromolecule but also a mutual orientational ordering of these portions belonging to different molecules (intermolecular crystallization), as a result of which the structure of ECC is similar to that of a nematic liquid crystal. After the melt is supercooled below the melting temperature, the processes of mutual orientation related to the displacement of molecules virtually cannot occur because the viscosity of the system drastically increases and the chain mobility decreases. Hence, the state of one-dimensional orientational order should be already attained in the melt. During crystallization this ordering ensures the aggregation of extended portions to crystals of the ECC type fixed by intermolecular interactons on cooling. 

C. Ylitalo and G.G. Fuller, Temperature effects on the magnitude of orientational coupling interactions in polymer melts, Macromolecules, 24,5736 (1991). 

L. A. Archer and G. G. Fuller, Segment orientation in a quiescent block copolymer melts studied by Raman scattering, Macromolecules, 27,4359,1994. 

Abstract The discussion of relaxation and diffusion of macromolecules in very concentrated solutions and melts of polymers showed that the basic equations of macromolecular dynamics reflect the linear behaviour of a macromolecule among the other macromolecules, so that one can proceed further. Considering the non-linear effects of viscoelasticity, one have to take into account the local anisotropy of mobility of every particle of the chains, introduced in the basic dynamic equations of a macromolecule in Chapter 3, and induced anisotropy of the surrounding, which will be introduced in this chapter. In the spirit of mesoscopic theory we assume that the anisotropy is connected with the averaged orientation of segments of macromolecules, so that the equation of dynamics of the macromolecule retains its form. Eventually, the non-linear relaxation equations for two sets of internal variables are formulated. The first set of variables describes the form of the macromolecular coil - the conformational variables, the second one describes the internal stresses connected mainly with the orientation of segments. 

The set of internal variables is usually determined when considering a particular system in more detail. For concentrated solutions and melts of polymers, for example, a set of relaxation equation for internal variables were determined in the previous chapter. One can see that all the internal variables for the entangled systems are tensors of the second rank, while, to describe viscoelasticity of weakly entangled systems, one needs in a set of conformational variables xfk which characterise the deviations of the form and size of macromolecular coils from the equilibrium values. To describe behaviour of strongly entangled systems, one needs both in the set of conformational variables and in the other set of orientational variables w fc which are connected with the mean orientation of the segments of the macromolecules. 

H.M. Lauti Orientation of macromolecules and elastic deformations in polymer melts. Influence of molecular structure on the reptation of molecules. Progr. Colloid Polymer Sci. 75 (1987) 111-139... 

Aromatic polyamides are generally made by low-temperature reactions of aromatic diamines and aromatic diacid chlorides in special solvents such as a 1 3 molar mixture of hexamethylphosphoramide A-methylpyrrolidone, as in reaction (4-50). Intensive stirring is required to attain high molecular weights because the polymer precipitates. These macromolecules are very rigid and rodlike. They form oriented liquid crystalline arrays in solution and require little postspinning orientation to produce extremely strong and stiff fibers. The polymer would not be made in the melt because it is infusible. It must be synthesized and handled in solution, and this requires the use of reactive precursors. 

For both linear and star polymers, the above-described theories assume the motion of a single molecule in a frozen system. In polymers melts, it has been shown, essentially from the study of binary blends, that a self-consistent treatment of the relaxation is required. This leads to the concepts of "constraint release" whereby a loss of segmental orientation is permitted by the motion of surrounding species. Retraction (for linear and star polymers) as well as reptation may induce constraint release [16,17,18]. In the homopol5mier case, the main effect is to decrease the relaxation times by roughly a factor of 1.5 (xb) or 2 (xq). In the case of star polymers, the factor v is also decreased [15]. These effects are extensively discussed in other chapters of this book especially for binary mixtures. In our work, we have assumed that their influence would be of second order compared to the relaxation processes themselves. However, they may contribute to an unexpected relaxation of parts of macromolecules which are assumed not to be reached by relaxation motions (central parts of linear chains or branch point in star polymers). 

Liquid polymers (at ambient temperature) are in general macromolecules with a relatively low molecular weight, many of them being in fact oligomers. Some liquid polymers are utilized as synthetic oils. Certain polymers can form liquid crystals in other words they can have an ordered structure while being in liquid state (either melted or in a solution). The orientation of certain polymeric molecules in liquid state such that the properties of the material are anisotropic is possible. Polymer liquid crystals have practical applications, and solution of liquid crystal polymers can be used for extruding fibers that have a highly crystalline structure after solvent elimination. 

For this comparison, a melt-spinning process was chosen. Each special thermoplastic process influences the structure and thus the properties of the obtained polymer samples differently. This is particularly pronounced for fibers, since especially melt spinning is a process which makes extremely high demands on the deformation ability of the polymer melts at high deformation speeds. Particularly the tensile stress within the fiber formation zone is a very important factor to reach a high orientation of the macromolecules along the fiber axis and a stress-induced crystallization. This crystallization should be discussed in relation to PLA and PHB multifilaments, and at the same time the general property spectrum of these polymers should be represented. 

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