Polymer Oriented state

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. 

These two different approaches for attaining an oriented state in flexible-chain and rigid-chain polymers indicate that the fundamental property of macromolecules - their flexibility - is of great importance to the orientation processes. However, the mechanism of the transition into the oriented state and the properties of highly oriented systems exhibit many features characteristic of both rigid- and flexible-chain polymers. 

Anisotropy of Physical Properties as the Main Feature of the Oriented State of Polymers... 

The processes of ordering in polymer systems consisting of linear polymers are related, at least on one level of supermolecular organization, to the development of a predominant localization of macromolecules (or their parts) along some directions the orientation axes, i.e. to the transition of the system into the oriented state. The most simple and most widely spread type of polymer orientation is the uniaxial orientation, i.e. the one-dimensional orientation in the direction of the axes of macromolecules. 

Usually, the transition of polymer systems into the oriented state occurs as a result of deformation e.g. upon exposure to external stress. When the polymers undergo deformation both the macromolecule as a whole and its parts (segments) can undergo orientation. The rates of these orientation processes are very different and, hence, the orienting forces affect first of all the orientation of chain segments and subsequently that of a chain molecule as a whole. However, by varying the extension velocity and the temperature, only the overall orientation process may predominate, thus extension of all chains occurs in a single act. 

Hence, the transition of a polymer system into the oriented state is a result of the competition of two fundamental properties of a polymer molecule (1) its inherent anisotropy which is the main reason for the ability of polymer systems to form an oriented phase and (2) its flexibility which favours coiling of a long molecule. The result of this competition is determined by the chemical nature of the molecule however, kinetic hindrance can prevent the transition into the oriented state. 

Hence, the main aim of the technological process in obtaining fibres from flexible-chain polymers is to extend flexible-chain molecules and to fix their oriented state by subsequent crystallization. The filaments obtained by this method exhibit a fibrillar structure and high tenacity, because the structure of the filament is similar to that of fibres prepared from rigid-chain polymers (for a detailed thermodynamic treatment of orientation processes in polymer solutions and the thermokinetic analysis of jet-fibre transition in longitudinal solution flow see monograph3. ... 

One of the main methods for improving the mechanical properties of linear polymers is their drawing that can be uniaxial (fibres), biaxial (films), planar symmetrical (films-membranes) etc. As a result of polymer deformation, the system changes into the oriented state fixed by crystallization. 

A further increase in extension leads to irreversible changes which immediately precede the transition of the polymer into the oriented state. During this transition, the spherulites undergo considerable structural changes and are thus converted qualitatively into different structural elements i.e. macrofibrils4). After a certain critical elongation has been attained, the initial crystallites collapse and melt and a new oriented structure is formed in which the c axes of crystals are oriented in the direction of extension. 

The transition into the oriented state is accompanied by the formation of a neck , a sharp and abrupt local constriction of the sample, in which the extent of orientation and the degree of extension are mudh higher than in the rest of the polymer. After the neck has been formed, further orientation of the sample occurs by spreading of the neck to the entire length of the polymer. When the sample is extended after passing into the oriented state, it undergoes further deformation and at some critical extension it breaks. 

It should be noted that the relative accessibility of the transition into the oriented state observed for polymers of various rigidity under appropriate conditions is due to the internal anisotropy of macromolecules caused by their chain structure (see Sect. 1 of this paper and monographs2 3 ). 

Polymer crystallization under flow or under highly oriented states is of prime importance in industrial polymer processing. We expected that the crystallization would be highly accelerated when the initial amorphous chains were highly orientated. Therefore, we dared to use a realistic molecular model of... 

We assume the system under consideration to be a single domain. Then the orientational state of the system can be specified by the order parameter tensor S defined by Eq. (63), The time evolution of S is governed by the kinetic equation, Eq. (64), along with Eqs. (62) and (65). This kinetic equation tells us that the orientational state in the rodlike polymer system in an external flow field is determined by the term F related to the mean-field potential Vscf and by the term G arising from the external flow field. These two terms control the orientation state in a complex manner as explained below. 

Solutions of rigid polymer molecules (e.g., poly-/)-phenylene terephthalate) may also exhibit extrudate swelling because they too are entropy elastic, molecules exit the capillary in a fairly oriented state and become randomly oriented downstream. 

If a sample of an amorphous polymer is heated to a temperature above its glass transition point and then subjected to a tensile stress, the molecules will tend to align themselves in the general direction of the stress. If the mass is then cooled below its transition temperature while the molecule is still under stress, the molecules will become frozen in an oriented state. Such an orientation can have significant effects on the properties of the polymer mass. The polymer is thus anisotropic. 

Isotropic solutions exhibit a monotonic increase in shear viscosity with increasing concentration. The viscosity increases to a maximum when the isotropic to anisotropic transition is approached. Upon formation of the anisotropic phase, the viscosity begins to decrease, after which the viscosity increases strongly as the concentration continues to increase (Fig. 6). In the isotropic state, the hydrodynamic volume is large because of the random polymer orientation. This restricts the polymer diffusivity and causes an increase in viscosity. In the anisotropic phase, the aligned polymer leads to a small hydrodynamic volume and a decrease in viscosity as rotational diffusion is much easier with a net orientation. 

If < Iq, it becomes the case of the discrete picture and jointed-rods are applicable. If > lo, however, the elastically discrete polymer evolves into a worm chain, i.e., an elastically homogeneous chain. The orientation change can then take place over a length comparable to Iq. With this continuum of allowed orientational states, the worm chain accurately represents the angular entropy of the real polymer. 

A fiber will remain in the oriented state at room temperature indefinitely however, if heated to an elevated temperature in the absence of an external stress, the fiber will revert to the essentially random arrangement of the unoriented crystalline polymer. The crystallites themselves are destroyed by melting. 

This chapter discusses the independent determination of parameters for a cluster entanglement network enabling the accurate description of the experimental data on the molecular orientation in polymethyl methacrylate. This confirms the validity of the earlier proposed structural model for the polymer amorphous state [1, 2]. 

If a polymer sample is subjected to a mechanical stress or to an electric field the structure responds, i.e. relaxes, in such a way as to reach an equilibrium under the stress or the field. This response generally involves some rearrangement of the structural units of the polymer, i.e. changes in the proportions of the various conformational or orientational states of its molecules, and it is the nature of these changes and the extent to which they take place that determine the elastic modulus or dielectric constant of the polymer. In the rest of this section and in section 5.7.3 it is assumed that the equilibrium ratio of the numbers of molecules in a particular pair of states between which jumps take place is actually influenced by the stress or electric field. This is so for many such pairs of states, but not for all pairs (see sections 7.6.1 and 9.2.5). 

Characterization of polymer orientation is most often accomplished via X-ray techniques which are suited to crystalline and paracrystalline regions (i-d). However, semicrystalline polymers present a complex system of crystalline, amorphous, and intermediate pluses ( -d) and complete characterization of semicrystalline polymers can only be achieved by application of a variety of techniques sensitive to particular aspects of orientation. As discussed by Desper (4), one must determine the degree of orientation of the individual phases in semicrystalline polymers in order to develop an understanding of structure-property relationships. Although the amorphous regions of oriented and unoriented semicrystalline polymers are primarily responsible for the environmental stress cracking behaviour and transport properties of the polymers, few techniques are available to examine the state of the amorphous material at the submicroscopic level. 

It has become increasingly evident that there is much to be gained from a detailed understanding of the structure and properties of polymers in the oriented state. This book reflects the growth of interest in this area of polymer science and attempts to give the reader an up to date view of the present position. The individual chapters are for the most part self-contained, and cover a very wide range of topics. It is intended that each of them should serve the dual purpose of an expository introduction to the subject and a topical review of recent research. 

At about 55 % the slope of the Ago curve shows a sudden change. This indicates the occurrence of a polymer-solvent compound with 6 molecules acetone per glucose-unit, in conformity with X-ray investigations It is to be noted in this connection, that the entropy of mixing in the system cellulose nitrate-acetone becomes negative in the limit of very low concentrations of acetone. This is explained by Schulz on the assumption that the solvent molecules are absorbed by the cellulose nitrate on to localised sites (perhaps in an orientated state), to which one would have to attribute a lower entropy. A behaviour of this type is, of course, not accounted for in Huggins and Flory s theory of the entropy of mixing. 

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