Independent Variation of Transition Temperature and Prefrozen Layer Thickness at the Prefreezing Transition

Abstract

Prefreezing is a first-order equilibrium phase transition at the melt− solid interface that results in an abrupt formation of a nanoscopic crystalline layer at the interface above the bulk melting temperature. A recently developed phenomenological theory predicts that the prefreezing transition temperature T max depends primarily on the difference of the interfacial free energies γ sub,melt − (γ sub,cry + γ cry,melt ), whereas the minimum jump of thickness at T max or minimum order parameter l min depends on the ratio γ γ γ + sub,cry cry,melt sub,melt. As such, T max and l min can vary independently. To test these predictions, we present in situ atomic force microscopy measurements of poly(ϵ-caprolactone) (PCL) prefrozen on a MoS 2 substrate and compare the results with those of our previous study of PCL prefrozen on graphite as well as polyethylene prefrozen on the same two substrates. The experiments show that T max of the prefrozen PCL on MoS 2 remains nearly the same as on graphite, whereas l min clearly decreases, thereby indicating that T max and l min are indeed independent of each other. The quantitative analysis of these experimental findings is consistent with the theory. With these results, we conclude a first series of experiments aimed at an experimental exploration of the theoretically predicted variations of the prefreezing phenomenon.

Polycaprolactone Adsorption and Nucleation onto Graphite Nanoplates for Highly Flexible, Thermally Conductive, and Thermomechanically Stiff Nanopapers

“…Recently, T m values as high as 80 and 84 °C have been reported by Thurn-Albrecht et al , They prepared thin PCL films (by solution in toluene, and then using spin coating on the selected surface) on freshly cleaved highly oriented pyrolytic graphite (HOPG) or onto molybdenum disulfide (MoS 2 ), and then, by atomic force microscopy measurements, they detected that the PCL is totally molten only at 84 °C (HOPG) and 80 °C (MoS 2 ). This behavior was attributed to the epitaxial crystallization of the PCL on the HOPG induced by pre-freezing.…”

“…The pre-frozen layer disappears at 155 and 124 °C for MoS 2 and HOPG, respectively. 83 , 86 Another possible explanation for peak D may involve a strongly adsorbed layer that might force the PCL crystals to crystallize in their extended chain conformation. For all these scenarios, the annealing of the PCL is indeed not expected, either because it is strongly bonded or because it is already in its extended conformation, explaining the absence of fractionation in peaks C and D.…”

Heterogeneous Crystal Nucleation from the Melt in Polyethylene Oxide Droplets on Graphite: Kinetics and Microscopic Structure

“…Note that when γ sm > γ sc + γ cm , the contact angle θ becomes zero and f (0) = 0 (Equation (4)), implying that the energy barrier for heterogeneous nucleation (∆G * het ) vanishes. Under this condition, crystallization can even take place above T m via prefreezing [3,[5][6][7][8][9], the other phenomenon of interface-induced crystallization, which is beyond the framework of classical nucleation theory. For heterogeneous nucleation, θ has a value 0 < θ < 180°and the function 0 < f (θ) < 1.…”

“…In general, a solid substrate can induce the crystallization of liquids either by heterogeneous nucleation [1,2] or by prefreezing [3,4]. Prefreezing, i.e., the formation of a stable crystalline layer at the substrate interface above the melting temperature (T m ) of the material, is an equilibrium phenomenon and does not require a nucleation event [5][6][7][8][9]. By contrast, heterogeneous nucleation is an activated process taking place at a finite supercooling below T m [10].…”

“…While epitaxy has been known as an important parameter that significantly increases the nucleation rate [12][13][14], recent in situ atomic force microscopy (AFM) studies showed the occurrence of prefreezing in such epitaxial systems [5][6][7][8]. Nevertheless, in the case of heterogeneous nucleation, the effects of the substrate in enhancing the nucleation kinetics are seen as an increase in the density of nucleation events [15] and, thus, as an increase in the crystallization temperature compared to homogeneous nucleation [16,17].…”