Some Early Investigations, and Problems Raised

Investigations of some of the elastic constants of uniaxially stretched polymers were made on nylon 66 and polyethylene terephthalate by 

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Wakelin et in 1955, on polyethylene terephthalate by Kawaguchi in 1957, and on regenerated cellulose by Shinohara and Tanzawa in 1957, and Wilson in 1960 but the first determination of all 5 constants was that of Raumann and Saunders on low density polyethylene in 1961 and 1962.  

The polyethylene was alkathene 7 of only 47% crystallinity, as measured by X-rays, and as can be seen from Figs. 1 and 2 there were several unexpected features sheets cut at 45° to the stretch direction showed the lowest stifihess ( 4 ) and shallow minima in extensional moduli ( o) and ( 90) occurred at low draw ratios for strips cut parallel with and perpendicular to the stretch direction also the torsional stiffness (I/S44) was very low by comparison with the other 4 stiffnesses. It was this low value which accounted for the observed behaviour of 45. The compliance at an angle 6 to the stretch direction is given by = Si, sin 04-S33COs 0+(2S,3- -S44)sin 0cos 0. For 6 = 45° this reduces to S45 =i[S,+533-1-25,3+544], and will be large if S44 p other compliances. 

A complete X-ray investigation of the samples was not performed, but the minima in moduli could not be attributed to a fall in crystallinity. 

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