Long periods in slow‐cooled linear and branched polyethylene: Part II

It is shown that the long periods L in slow‐cooled polyethylene materials obey the general law L = L0 + αrw, where rw is the weight average dimension of the coil before crystallization, and L0 is a parameter of the order of lc, the crystalline core thickness, which increases as the cooling rate V decreases. α is a parameter independent of M and V but decreasing with the number of long‐chain branches per molecule. The two terms in the above relation are, respectively, the contributions of crystalline and amorphous layers. For cooling rates from 800°C/min to 0.2°C/min, it is shown that the temperature Tc of crystallization is constant; hence the change of morphology (long period, crystalline core thickness, crystallinity) cannot be explained by supercooling. The increase in long period and crystallite thickness in slow‐cooled materials with decreasing cooling rate is interpreted in terms of annealing of the crystallized materials between the crystallization temperature Tc and the secondary transition temperature Tαc. Crystallization proceeds by a two‐step process of solidification and annealing. During the annealing stage, the mobility of the chains in the crystalline phase is due to defects; the kinetics of thickening is then governed by the mobility (or nucleation) of the defects appearing above Tαc. In the proposed model of crystallization, the assumption that the energy of activation is proportional to Tαc explains the observed laws L ≡ lc ≡ log ta, where the annealing time ta is equal to (Tc − Tαc)/V. The model applies also to polymers crystallized from the melt and subsequently annealed.