Thermal behaviour of polyethylene-block-poly(methyl methacrylate) block copolymer: Effect of multiple heating and cooling rates versus mathematical artefact

A new non-isothermal Avrami-Erofeev crystallization model combined with an appropriate calculation algorithm ratifies the invariance of activation energy, and offers a different opinion about the concept of variable/instantaneous activation energy. The melting and crystallization behaviours of a polyethylene-block-poly(methyl methacrylate) (PE-b-PMMA) diblock copolymer and a PE homopolymer were investigated using multiple heating and cooling rate differential scanning calorimetry (DSC) experiments, and modelling of the crystallization kinetics and lamellar thickness distribution. This new model was first validated applying literature and experimental data. The model-predicted morphology (n = 3.2) closely matched the spherulitic morphology (n = 3), which was determined using polarized optical microscopy. For each experimental cooling rate, the model predicted diblock copolymer crystallinity that well matched the entire DSC crystallinity curve, notably for an Avrami-Erofeev index of n = 2; and apparent crystallization activation energy that hardly varied with the cooling rates used, relative crystallinity (α), and crystallization temperature or time. This disfavours the concept of variable activation energy. The use of the right crystallization model and parameter estimation algorithm is important for addressing the mathematical artefact. Under non-isothermal cooling, the PE-b-PMMA diblock copolymer, as per the model prediction, crystallized without confinement (n ≠ 1), preserving the cylindrical structure. From the characteristic shapes of the crystallization function f(α(T)) versus 1/T and crystallization rate versus α plots, the resulting T_cmax and narrow α_max range can guide the search for an appropriate crystallization model. The overall treatment illustrated in this study is not restricted to a PE homopolymer and a PE-b-isotactic PMMA block copolymer. It can be generally applied to crystalline homopolymers and copolymers (alternating, random and block), as well as their blends. The block copolymers and blends can be crystalline-amorphous as well as crystalline-crystalline.