Effect of Substrate Interaction on Thermodynamics of Prefreezing

Prefreezing refers to the abrupt formation of a crystalline layer at the melt-solid interface above the melting temperature, and is an equilibrium phenomenon.¹ Phenomenological theory of prefreezing² predicts that the transition temperature T_max depends primarily on the difference of the interfacial energies Δγ = γ_sub,melt – (γ_sub,cry + γ_cry,melt), whereas the minimum jump of thickness l_min at T_max is controlled by the ratio (γ_sub,cry + γ_cry,melt)/γ_sub,melt. To test these predictions, we performed in situ AFM experiments on various polymer-substrate systems. The results for polyethylene (PE) on a molybdenum disulfide (MoS₂) substrate evidence a much higher T_max than on graphite, caused by a larger value of Δγ of PE-MoS₂. In case of poly(ε-caprolactone) (PCL), where direct measurements of the prefrozen layer thickness are possible, T_max of the prefrozen PCL on MoS₂ remains the same as on graphite, whereas l_min decreases to a smaller value. With these findings, we confirm that, firstly, Δγ is the driving force for prefreezing and, secondly, T_max and l_min are independent.

1. Tariq et al. Macromolecules 2019, 52, 9140-9148
2. Dolynchuk et al. J. Phys. Chem. Lett. 2019, 10, 1942-1946