Dependence of the Specific Heat in Glassy Polybutadiene on Thermal History

The specific heat of glass-forming polybutadiene in the glass transition region depends strongly on the thermal history of the sample. After cooling and waiting at a temperature T w < T g ( T g ≈ 178 K is the glass transition temperature) for a (constant) time t w , the peak value of the specific-heat anomaly at T g passes over a maximum as a function of T w at T w ≈ 169 K. The energy taken up by the sample when heating through T g after waiting at T w = 169 K increases with t w and can be described by a Kohlrausch law Δ Q ( t w ) ∼ {1 - exp [- ( t w /τ) β ]} with β ≈ 0.5 and τ = 100 min. Comparison with recent neutron-scattering data indicates that the different time scales of microscopic and macroscopic relaxations may be attributed to their different length scales.