Prevorsek

For a turbulent boundary layer, the total drag may be roughly estimated using Eqs. (6-184) and (6-185) for finite cylinders. Measured forces by Kwon and Prevorsek ]. Eng. Jnd., 101, 73-79 [1979]) are greater than predicted this way. 

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. 

Numerous studies of the structure and properties of drawn crystalline polymers have led to the microfibrillar model of fibrous morphology177 179 180. According to Peterlin 179) and Prevorsek et al. 180), the long and thin microfibrils are the basic elements of the fibrous structure. The microfibrils consist of alternating folded chain crystallites and amorphous regions. The axial connection between the crystallites is accomplished by intrafibrillar tie-molecules inside each microfibril and by inter-fibrillar tie-molecules between adjacent microfibrils. 

The other approach concerns the analysis of models of oriented crystalline polymers 7,8172). For the Peterlin-Prevorsek model, the expression for px can be represented as... 

FIG. 2.14 Structural model of a drawn polyester fibre, fibre axis vertical (after Prevorsek and Kwon, 1976 by permission of Marcel Dekker, Inc.). 

Percec V (Guest Editor) "Frontiers in Polymer Chemistry", Thematic Issue of Chemical Reviews 101 (2001) Issue 12. Peterlin A, J Phys Chem 75 (1971) 3921 J Polym Sci A-2, 7 (1969) 1151 IUPAC Symposium, Helsinki, 1972. Prevorsek DC and Kwon YD, J Macromol Sci, Phys B 12 (1976) 453. 

An interesting model of the possible structure of semi-crystalline yarns is that given by Prevorsek and Kwon (1976) and shown in Fig. 19.14. 

It consists of fibrils with a definite long period and thickness, consisting of crystallites characterised by a height of about 5 nm and a thickness of the order of 6 nm. Prevorsek supposes that along the fibrils a number of extended tie molecules" are present, this number being responsible for the strength at higher draw ratios the fraction of taut tie molecules increases. 

FIG. 19.14 Structure model of (A) nylon fibres (B) PETP fibres (fibre axis vertical) as suggested by Prevorsek and Kwon (1976). 

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