In the previous report, relationship between Young's modulus and birefringence for drawn and heat-treated PET fibres was discussed.
In this report, the mechanism of small deformation of PET fibres below glass transition temperature is discussed and a new theoretical approach between Young's modulus and molecular orientation is proposed under the following assumptions: a) A segment transforms in accordance with “affindeformation” theory: b) Intersegmental force during the affin transformation of the segment is proportional to the rotational angle of the segment.
Then, Young's moduli of PET fibres heat-treated at constant length after drawing and ones drawn after crystallization are measured and compared with the theory. The results obtained are as follows:
1. The rotational angle ω of a segment due to the deformation of a sample and Young's modulus E may be expressed by following equations; where υ is Poisson ratio of the sample; θ is the angle between the fiber axis and the segmental axis; e
z is tensile strain of the bulk sample;
E0 and
G0 are constants.
2. The relationship between Young's modulus and amorphous orientation factor may be represented in terms of the above equations. The values of
E0 and
G0 are 1700_??_1900kg/mm
2, 80kg/mm
2, respectively, for the samples drawn without heat-treatment, but these are a little smaller for the samples drawn after crystallization,
Young's modulus of drawn PET fibres depends mainly upon amorphous orientation and the effect of crystallinity is rather small.
3. The above results suggest that the force constant of the amorphous segment below glass transition temperature dose not much differ from that of the crystalline chain, but a part of intermolecular strain makes a series to intra-molecular strain and lowers much the Young's modulus of bulk sample; and that there may be more parts of fold structure in samples drawn after crystallization than in ones drawn without heat-treatment.
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