T∥ from the temperature dependence of carbon‐13 NMR correlation times τc

We demonstrate that carbon‐13 NMR average correlation times τc published by Dekmezian et al. as plots of log τc against reduced temperature, T/Tg, reach a visibly apparent asymptotic limit, (T/Tg)as, at about 1.21 for 9 amorphous polymers and 4 semicrystalline polymers; PIB and ethylene copolymers are exceptions. The average value of Tc/Tg is 1.21, where Tc is the carbon‐13 NMR collapse temperature reported by Axelson and Mandelkern. Log τc − T data can be linearized by the WLF equation or by the Vogel equation using a reference temperature, Tr ≡ Tc. Log f, versus T/Tg data can also be linearized by a hyperbolic equation method. All three methods yield an asymptotic frequency of 1 Hz or less, rationalizing why Tc (about 106 Hz) ≡ T‖ (f ≤ 1 Hz). Dielectric loss data analyzed by the Lobanov–Frenkel method confirm and define a T* (K) at a frequency of about 105−107 Hz at T‖ associated with local chain motion. A plot of Tc or T* against Tg (static) is linear, suggesting that Tc ≡ T* ≡ T‖. It is demonstrated that values of log f versus 1/T with f = 1/2πτc help to define the high‐frequency locus in relaxation maps for cis‐PI, PIB, and at‐PP. The T‖ transition thus manifests itself in any of three ways: motion of the entire macromolecule in low frequency (1–1000 Hz) dynamic mechanical tests, an increase with temperature in Cp and in the coefficient of expansion in static measurements, and a high‐frequency local response in NMR and dielectric loss data. All three aspects are associated with the disappearance of interchain segment‐segment contacts at T‖, i.e., the “segmental melting” of Lobanov and Frenkel. Carbon‐13 NMR is a powerful tool for investigating T‖ dependence on structure in amorphous and crystalline polymers but is less satisfactory with ethylene copolymers. A linear correlation of Tc or Tas against Tg is demonstrated, where Tas is the asymptotic temperature from log τc versus T plots. This correlation can be used to define Tg in crystalline polymers if their Tc is known.