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Investigation was made on the relation between the temperatures (Tmax) at which the maximum thermal stress was observed and the equilibrium thermal shrinkage in free state.The results obtained are as follows :1. A fiber contains frozen strains when the polymer molecules take the nonequilibrium configu ration in the production process.The thermal shrinkage of a fiber may be caused by disorienting effect due to breakdown of either crystallites or secondary bonds among the oriented molecular chains in the amorphous region.According to Boltzmann, the probability that a restrictive energy E.c is released, is proportionalSo, assuming that the equilibrium thermal shrinkage (%) in free state is owing to breakdown of these secondary bonds and crystallites, the following formula may be obta ined ;where, IT is the equilibrium thermal shrinkage (%) in free state at Tom.The first term on the right hand side of the above equation represents the frozen energy released at T < Tmax, and the second term is the frozen energy released at T > Tmex (°K). Assuming that the latter is larger than the former, the following results are obtained.

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