Thermal analysis of crystallization reveals subtle interactions in pristine graphene-polycaprolactone nanocomposites

Abstract

In this study, we investigate the melting behavior and crystallization of nanocomposites of poly(ε-caprolactone) (PCL), a biodegradable polymer, with pristine graphene, an economically feasible filler widely available from natural sources. Nanocomposites with pristine graphene loads between 0.01 and 5 wt% were prepared via solvent casting and primarily probed by Differential Scanning Calorimetry (DSC). Conventional DSC shows that the presence of graphene increases PCL crystallinity. Non-isothermal crystallization was studied using Mo's model, whereas other parameters as Activation Energy and Nucleation Activity were obtained. It is shown that graphene increases crystallization rates acting as nucleant, with no signatures of retardant effects. Compared with other classic nano-loads, like ad-hoc modified bentonite also analyzed for comparison, pristine graphene is more effective as nucleant, which indicates that it is better dispersed in PCL. Analysis by Self Successive Annealing (SSA), also carried out by DSC, reveals that graphene hinders the formation of crystals with lamellar thickness above 7.3 nm, as found in regular PCL. It may indicate that molecular interactions between PCL and pristine graphene disrupts the movement of polymeric chains, consequently limiting lamellar growth. Evidence of such interaction is found by Infrared and Raman spectroscopies that reveal broadening of the carbonyl peak of PCL and alteration of G and D' bands of graphene.