A new thermodynamic derivation of the microhardness dependence on crystal thickness is developed. The approach makes use of a modified Clausius-Clapeyron equation to incorporate the effect of the finite size of polymer crystals. The derived equation describes the hardness depression due to finite thickness of the lamellae on the assumption that plastic deformation involves a partial melting of the polymer crystals. The present model approach offers an alternative view to the earlier concept of plastic deformation related to the energy dissipated during mechanical destruction of the crystals.

Grateful acknowledgment is due to the international joint research grant NEDO and to the DGICYT, Spain (Grant PB94-0049) for the support of this investigation.

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