Polymer molecular orientation

The physical properties, in particular, the modulus, depend on the degree to which the polymer chains lie along a particular direction. In the case of fiber spinning, the degree to which the chains lie along the fiber axis determines the stiffness and strength of the fiber. In this section we define molecular orientation and briefly describe how it is determined. 

To quantify the degree of orientation, we define orientation functions. To do this we need to know what the unit cell is. For polyethylene (PE), the unit cell is orthorhombic in which the three crystallographic axes, a, b, and c, are mutually perpendicular. The orientation functions are 

The angles are the angles each crystallographic axis makes with the z axis (which is the stretch direction). The values cos pi, where i = a, b, or c, are evaluated as follows  

FIGURE 5 5 Effect of isotactic polypropylene film extension on the wide-angle X-ray diffraction patterns. (Reprinted with permission of the publisher from Samuels, 1974, p. 27.) 

For the amorphous regions, we can use sonic waves or birefringence to determine orientation. (Certainly there are several methods, but our intention is not to review all these nor to say which is best.) To use birefringence we must know the intrinsic birefringence, AiV°, which is either calculated or measured on a perfectly oriented sample. The birelfingence, AAf, is related to the stress field through the stress/optic law. In particular, the law reads  

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This chapter is concerned with the definition and evaluation of molecular anisotropy in liquid crystal polymers. Molecular orientation is a central theme of the liquid crystal state however, its definition requires considerable care. The basic mathematical relationships for the description of molecular anisotropy are reviewed together with an assessment of the various orienting units ranging from the molecular envelope to particular covalent bonds. The different methods of measuring orientation parameters for liquid crystal polymer samples are compared. Particular emphasis is placed upon X-ray and electron scattering procedures which are able to provide structural as well as orientational information. The experimental methods are highlighted through the use of several examples. 

In many practical applications of synthetic polymers molecular orientation is produced by the fabrication process to give improved properties, especially with regard to stiffness and strength. Well known examples are textile fibres such as Terylene or nylon, polypropylene packaging films and polyester bottles. Most natural materials such as silk and cotton fibres, muscle and bone also show significant molecular orientation. All these synthetic and natural materials are anisotropic, i.e. their properties are different in different directions. 

The manner in which the polymer molecules are oriented in a part will have a major effect on the impact behavior of the polymer. Molecular orientation introduced into drawn films and fibers may give extra strength and toughness over the isotropic material (28). However, such directional orientation of polymer molecules can be very fatal in a molded part since the impact stresses are usually multiaxial. The impact strength is always higher in the direction of flow. 

Flow processes iaside the spinneret are governed by shear viscosity and shear rate. PET is a non-Newtonian elastic fluid. Spinning filament tension and molecular orientation depend on polymer temperature and viscosity, spinneret capillary diameter and length, spin speed, rate of filament cooling, inertia, and air drag (69,70). These variables combine to attenuate the fiber and orient and sometimes crystallize the molecular chains (71). 

Tensile Properties. Tensile properties of nylon-6 and nylon-6,6 yams shown in Table 1 are a function of polymer molecular weight, fiber spinning speed, quenching rate, and draw ratio. The degree of crystallinity and crystal and amorphous orientation obtained by modifying elements of the melt-spinning process have been related to the tenacity of nylon fiber (23,27). 

The molecular orientation of the polymer in a fabricated specimen can significantly alter the stress—strain data as compared with the data obtained for an isotropic specimen, eg, one obtained by compression mol ding. For example, tensile strengths as high as 120 MPa (18,000 psi) have been reported for PS films and fibers (8). PS tensile strengths below 14 MPa (2000 psi) have been obtained in the direction perpendicular to the flow. 

It should be pointed out that the view of the glass transition temperature described above is not universally accepted. In essence the concept that at the glass transition temperature the polymers have a certain molecular orientation time is an iso-elastic approach while other theories are based on iso-viscous. 

As previously stated, molecular orientation occurs during melt processing of polymers. On removal of the deforming stresses the molecules start to coil up again but the process may not go to equilibrium before the polymer cools to below its Tg. This leads to residual orientation (frozen-in strain) and corresponding frozen-in stresses. 

A characteristic feature of thermoplastics shaped by melt processing operations is that on cooling after shaping many molecules become frozen in an oriented conformation. Such a conformation is unnatural to the polymer molecule, which continually strives to take up a randomly coiled state. If the molecules were unfrozen a stress would be required to maintain their oriented conformation. Another way of looking at this is to consider that there is a frozen-in stress corresponding to a frozen-in strain due to molecular orientation. 

The absence of crystallisation gives polymers with low mould shrinkage. Molecular orientation. 

Lattice models have the advantage that a number of very clever Monte Carlo moves have been developed for lattice polymers, which do not always carry over to continuum models very easily. For example, Nelson et al. use an algorithm which attempts to move vacancies rather than monomers [120], and thus allows one to simulate the dense cores of micelles very efficiently. This concept cannot be applied to off-lattice models in a straightforward way. On the other hand, a number of problems cannot be treated adequately on a lattice, especially those related to molecular orientations and nematic order. For this reason, chain models in continuous space are attracting growing interest. 

Thermodynamics of Crystallization of Flexible-Chain Polymers Under Conditions of Molecular Orientation. 217... 

In principle, it is possible to obtain ECC in the absence of molecular orientation if the crystallization is carried out very slowly at high temperatures close to the melting temperature. Thus, Mandelkern obtained polyethylene crystals similar to ECC in their thermodynamic characteristics by a 40 days crystallization of an isotropic melt28. These experiments also characterize one of the possible paths of the generation of order in polymers order through fluctuations 29 (see below). 

We carried out thermodynamic studies on the crystallization from melts of flexible-chain polymers uniaxially stretched at various degrees of molecular orientation in the melt and studied the effect of the stretching stress on thermodynamic parameters such as degree of... 

Many papers deal with the crystallization of polymer melts and solutions under the conditions of molecular orientation achieved by the methods described above. Various physical methods have been used in these investigations electron microscopy, X-ray diffraction, birefringence, differential scanning calorimetry, etc. As a result, the properties of these systems have been described in detail and definite conclusions concerning their structure have been drawn (e.g.4 13 19,39,52)). 

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