Il is shown tliat, at the yield stie^.s, glassy polymers exhibit viscous flow whioh is in agreement with the generalized theoiy of E\-ring. The study of the yield stress over a wide range of températures and straiu rates provides évidence on the secondary transi tions found by other methods. Froni our nieasurements we conclude that every second ary transition correspontls to tiie libération of one of the degrees of freedom of a segment of the main chain.

No abstract

The tensile yield stress is compared with the uniaxial compression yield stress for specimens of polycarbonate tested at constant strain rate from -120 to 4-120~ The strain rate dependence of the tensile yield stress is also studied from -140 to + 20~ The yield behaviour is described by a modification of the Eyring theory of nonNewtonian viscosity where the deformation is supposed to involve two different flow processes.The validity of a yield criterion previously established by one of us, is checked throughout the range of temperatures explored; the proposed equation gives a good fit to the data provided it is applied separately to each flow process.Then the present investigation, combining the yield condition with the modified Eyring theory, provides a formalism which agrees with the data and accurately renders the temperature, strain rate and pressure dependence of the yield stress of polycarbonate. IntroductionWe have previously shown [1] that the tensile yield stress of polycarbonate (PC), increases linearly with the logarithm of strain rate at constant temperatuIe and decreases linearly with temperature at constant strain rate, over a large range of strain rates and temperatures (.from + 20 to 140~ in the glassy range. This yield behaviour may be fairly well described by the formalism of the Eyring theory of nonNewtonian viscosity [2] where deformation is a rate process.We intend now: (1) to check throughout this range the validity of the yield criterion one of us has previously established [3,4].(2) to extend to lower temperatures the range explored in order to determine whether at lower temperatures, the Eyring model in terms of a single activated flow process is still applicable to the yield behaviour of PC, and whether and how the yield criterion would fit the data at low temperatures.This proposed criterion is pressure, temperature and strain rate dependent and has been established for an arbitrary state of stress. 176

This paper isconcerned with the effect of thermal pre-treatments (below the glass transition) on the yield stress and on the enthalpy change of polycarbonate. A treatment is proposed which accounts quantitatively for expérimental results. It relies both on free volume and activation concepts and makes use of classical équations and simple assumptions, chiefly that the same free energy is associated with the molecular motions giving rise to dé formation at yield and densification during annealing.

The strain-rate and température dependence o f yield o f po lycarbo nate in tensi o n, tensile creep and impact tests