Amorphous orientation

Noncrystalline domains in fibers are not stmctureless, but the stmctural organization of the polymer chains or chain segments is difficult to evaluate, just as it is difficult to evaluate the stmcture of Hquids. No direct methods are available, but various combinations of physicochemical methods such as x-ray diffraction, birefringence, density, mechanical response, and thermal behavior, have been used to deduce physical quantities that can be used to describe the stmcture of the noncrystalline domains. Among these quantities are the amorphous orientation function and the amorphous density, which can be related to some of the important physical properties of fibers. 

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 amorphous orientation is considered a very important parameter of the microstructure of the fiber. It has a quantitative and qualitative effect on the fiber de-formability when mechanical forces are involved. It significantly influences the fatigue strength and sorptive properties (water, dyes), as well as transport phenomena inside the fiber (migration of electric charge carriers, diffusion of liquid). The importance of the amorphous phase makes its quantification essential. Indirect and direct methods currently are used for the quantitative assessment of the amorphous phase. 

The values of the amorphous orientation index in the form of Hermans function of orientation if a), determined by the authors for PET fibers are listed in Table 6. The differences in the values of fa quoted in Table 6 and referring to particular investigation methods can result from the fact that in some methods the orientation of the amorphous and mesophase are considered jointly. Consequently, in such a case the values of fa will be overrated. 

Overall orientation is understood as the joint arrangement of all the structural elements of the crystalline phase and noncrystalline part of the fiber in relation to the geometrical axis of the fiber. In its essence, the overall orientation of PET fibers, as a result of the crystalline and amorphous orientation, will be characterized by smaller values of the quantitative index of orientation than for the crystalline phase and by greater ones for the amorphous phase. 

Table 6 Amorphous Orientation Function (fa) of Differently Drawn PET Fibers...

Draw ratio Density of the amorphous material da) (g/cm-" ) Amorphous orientation function fa) Crystallite length Oc) (nm) Long period (L) (nm) Degree of crystallinity (X=>) Substructure parameter (A) Axial elastic modulus ... 

Fibers of a diversified draw ratio in the range 2.0-5.2 X were considered, determining the following parameters of their fine structure the crystalline and amorphous orientation functions,and/a, degree of crystallinity, X, and critical dissolution time (CDS) in seconds. The results obtained are listed in Table 11. 

Annealing temperature rc) Annealing time (min) Birefringence (An) Anid Volume crystallinity (%) TTM fraction Critical dissolve time (s) Amorphous orientation function (/ )... 

Amorphous orientation average Crystalline orientation average Nuclear spin number Scattered intensity Scattered intensity Transmitted intensity... 

Another way to divide the overall orientation is to recognise the distinction between crystalline material which is all in the trans conformation, and amorphous material which is both trans and gauche. By combining infra-red and X-ray orientation data, together with the determination of the proportion of crystalline material, the amorphous orientation average fa = gauche = 0. which is seen from the above results to be very reasonable,... 

The amorphous orientation can also be obtained, following conventional procedures, by combining X-ray and refractive index measurements, assuming the additivity... 

Fig. 9. Plot of amorphous orientation average fa, obtained from birefringence, etc., versus f, obtained from im etc- Triangles, draw temperature 80 °C circles draw temperature 85 °C open symbols, single-stage draw full symbols, two-stage draw. Reproduced from Polymer by permission of the publishers, Butterworth Co (Publishers) Ltd. (C)...

A comparison of the amorphous orientation averages fa obtained by these two different routes is shown in Fig. 9, and provides some support for the validity of these procedures. 

In the uniaxially oriented sheets of PET, it has been concluded that the Young s modulus in the draw direction does not correlate with the amorphous orientation fa or with xa "VP2(0)> 1r as might have been expected on the Prevorsek model37). There is, however, an excellent correlation between the modulus and x,rans,rans as shown in Fig. 15. It has therefore been concluded 29) that the modulus in drawn PET depends primarily on the molecular chains which are in the extended trans conformation, irrespective of whether these chains are in a crystalline or amorphous environment. It appears that in the glassy state such trans sequences could act to reinforce the structure much as fibres in a fibre composite. 

The sound velocity in a fiber, and the sonic modulus calculated therefrom, are related to molecular orientation (De Vries ). As shown by Moseley ), the sonic modulus is independent of the crystallinity at temperatures well below the T (which means that the inter- and intramolecular force constants controlling fiber stiffness are not measurably different for crystalline and amorphous regions at these temperatures). An orientation parameter a, calculated from the sonic modulus, is therefore taken as a measure for the average orientation of all molecules in the sample, regardless of the degree of crystallinity. The parameter is called the total orientation , as contrasted to crystalline and amorphous orientation, from X-ray data. 

Amorphous orientation decreases in both the cases. In free annealed sample the decrease is greater due to shrinkage allowed. 

The use of X-rays has been used in rare cases to determine amorphous orientation. See Samuels (57) or Wilchinsky (79). To do this... 

We see if one orientation function can be determined such as the crystalline orientation function by X-ray diffraction, then by sonic techniques the amorphous orientation can be calculated by Eq. (58). Samuels (57, 58) has made such measurements and finds reasonable values for amorphous orientation. 

X-ray Diffraction any moment can Combines well with other techniques as dichroism or birefringence. Dynamic techniques have been developed Considerable data reduction. Amorphous orientation measurements are not strongly sensitive. Must know crystalline structure 20,000 la, 12, 76, 78,79... 

Read, B. E., Stein, R. S. Polarized infrared studies of amorphous orientation in polyethylene and some ethylene copolymers. Macromol, I, 116 (1968). 

In spite of the above diversity of oriented crystalline morphologies, Samuels has shown that the structural state can sometimes be adequately characterized by the crystalline and amorphous orientation factors (37). For polypropylene samples prepared with different draw ratios, draw temperatures, shrinkage temperatures, etc., simple property correlation with these two orientation factors was observed ".. . these results suggest that different fabrication processes are simply different paths along which the sample is moved to equivalent structural states. Thus, general structure-property correlations are achieved by concentrating on the final structural state of the sample and not on the path by which the state was reached." Where applicable, this Is a most useful approach however, when radically different fabrication processes and radically different morphologies are compared, the definition of "structural state" must include more subtle features than the crystalline and amorphous orientation factors. 

Station (61) has pointed out that, in many cases, is quite dependent on the method of measurement and therefore caution must be taken in relying too strongly on quantitative interpretation. Other methods of obtaining a measure of amorphous orientation will be discussed later on. 

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