Effect of Initial Molecular Weight on the Structural Evolution of Polyamide 1012 during High-Temperature Thermal Treatments as Revealed by Successive Self-Nucleation and Annealing

Successive self-nucleation and annealing (SSA) can thermally fractionate semicrystalline polymers, thus detecting the characteristic structural features that lead to specific melting point distributions (corresponding to the lamellar thickness distribution in the sample). SSA has been employed to investigate the structural evolution of polyamide 1012 (PA1012) when it is submitted to thermal treatments in the solid state (below the melting point, Tm) as well as in the liquid state (above Tm) as a function of the molecular weight of the sample. Below Tm, mainly chemical cross-linking occurred, thus provoking the decrease in the melting enthalpy of the highest temperature SSA melting fraction. The structural evolution was limited primarily by crystallization, and the high crystallinity of the low-molecular-weight samples led to low cross-linking rates. Above Tm, the structural evolution was correlated to the melt viscosity and the end group concentration. The concentration and diffusion ability of end groups were inversely proportional to the initial melt viscosities; therefore, the linear chain growing rate increased with the decrease in initial melt viscosities. Cross-linking reaction rate increased with the increase in initial melt viscosities. This work has determined the structural evolution differences of PA1012 depending on different initial states. This work can provide guidance for studies of thermal structural evolution of all AABB-type polyamides and subsequent design of high-performance materials with various structures according to their application requirements.