Modulus of spherulitic polyethylene

In most spherulitic polymers, touching spherulites occupy whole of the space. Their microstructure is too complex to be completely modelled, especially if there is twisting of lamellar stacks about spherulite radii. Consequently, models simplify the structure, and use composite micromechanics concepts. A stack of parallel lamellar crystals with interleaved amorphous layers (Fig. 3.20) has a similar geometry to a laminated rubber/metal spring (Fig. 4.1). The crystals have different Young s moduli E, Eb and E (Section 3.4.3), and different shear moduli when the 

The high shape factor of the amorphous layers means that the amorphous Young s modulus Eam will be close to the amorphous bulk modulus of 2 GPa. The tensile compliance in the 3 direction will be dominated by the amorphous Contribution, because Ec of the polyethylene crystal is 250 GPa. Hence, 

Although E3 is relatively high, the shear moduli G31 and G32 are very small, so the inter-lamellar layers will shear, if at all possible. 

When tensile stresses or shear stresses act in the 12 plane, there are uniform strain conditions in the composite laminate. Consequently, the tensile moduli Ej and E2 can be added in proportion to their phase volume fractions, using 

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Figure 4.21 Predicted Young s modulus of spherulitic polyethylene vs. crystallinity, for lamella of various aspect ratios, compared with experimental data (Guan X and Pitchumani R., Polym. Eng. Sd, 44, 433, 2004) John Wiley and Sons Inc. reprinted with permission...

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