Successive Self-nucleation and Annealing (SSA): Correct design of thermal protocol and applications

In this feature article we review the principles, applications and perspectives of SSA or Successive Self-nucleation and Annealing, a technique designed and implemented by Muller and coworkers in 1997. SSA is a thermal fractionation protocol designed to deconvolute Differential Scanning Calorimetry (DSC) melting endotherms into elementary components. It is particularly useful to fractionate polymers that incorporate defects in their linear crystallizable chains (e.g., branches, comonomers, crosslinks, stereo-defects or any other molecular defect that cannot enter the crystalline lattice). SSA is less sensitive to molecular weight distribution, although for low viscosity polyesters it has also been proven effective. SSA is capable of performing thermal fractionation with much better resolution and in faster times than previously available techniques, such as step crystallization. It is normally performed in the absence of solvent and in a conventional Differential Scanning Calorimeter (DSC). Correct design of the fractionation method must include a conscientious choice of fractionation window, fractionation time, scanning rate, sample mass and especially the first self-nucleation temperature (Ts) to be employed. This last variable is extremely important and is often overlooked in the literature. In order to choose the first Ts, self-nucleation studies have to be performed before SSA. SSA fractionation profiles where an arbitrary choice of the first Ts has been made are often misleading and report an erroneous distribution of chain defects. Nevertheless, when SSA experiments are carefully designed, they yield valuable information on defect distributions (like short chain branching or comonomer distribution), annealing capability, nanocomposite/matrix interactions, chain topology, degradation effects, among other interesting applications. Further improvements in technique implementation like high scanning rates or employing SAXS and TEM to correlate with SSA, may lead to a wider range of applications.