Non-equilibrium Phenomena in Confined Soft Matter by Simone Napolitano

Author:Simone Napolitano
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

6.4.4 Are the Long Range Perturbations Modified in the Presence of a Solvent?

In parallel with thermal annealing, solvent annealing has been widely used as an alternative to process supported thin polymer films. Especially, the use of solvent vapor annealing has been shown to have a great advantage in controlling orientations of block copolymer thin films on solid surfaces [132]. However, an unsolved question arises: Are the long-range perturbations associated with the adsorbed layer facilitated or diluted in the presence of solvent molecules? To answer this question, using a high-pressure neutron reflectivity (NR) technique [133], we investigated the effect of CO2 solvent annealing on the interdiffusive motion at the chemically identical polymer/polymer interface as a function of the distance from the substrate interface [134]. We used PS because (i) a wide variety of monodisperse hydrogenated and deuterated PS are commercially available; (ii) the long-range perturbations on Si substrates have been characterized in melts [16, 17, 46]; (iii) the protocol to prepare the adsorbed layer has been established [48, 52]; (iv) the swelling [133, 135–138] and interdiffusion [139, 140] behavior of polymer thin films in CO2 has been reported. It is known that sorption of CO2 molecules into polymers plays a role as a diluent or plasticizer for glassy polymers by significantly lowering the glass transition temperature (T g) and hence enhancing chain mobility [141–144]. At the same time, our previous studies have shown that the excess sorption of CO2 molecules is induced within the narrow temperature and pressure regimes near the critical point of CO2, known as the “density fluctuation ridge” [145], which results in the anomalous swelling of polymer films [133] and enhanced interdiffusion at the hydrogenated PS (h-PS)/deuterated PS (d-PS) interface even at temperatures below the bulk T g [140].

Figure 6.11 summarizes the effective segment diffusion constant D eff values for the h-PS (M w = 650 kDa)/d-PS (M w = 334 kDa) bilayers as a function of the initial film thickness of the bottom h-PS spin-cast films scaled by R g (R g is the radius of polymer gyration of the h-PS), while the thickness of the d-PS overlayers was fixed to 3R g. We chose the ridge condition of T = 36 °C and P = 8.2 MPa (i.e., the density fluctuation ridge condition) for all the interdiffusion experiments to maximize the plasticization effect of CO2 [140]. The details of high-pressure NR experiments performed on the NG-7 horizontal reflectometer at the National Institute of Standards and Technology, Center for Neutron Research have been described elsewhere [133]. Based on the root-mean-square (RMS) roughness (σ) between the two layers, D eff values were calculated. The diffusive motion gets extremely slow when the distance of the h-PS/d-PS interface is less than 1R g from the substrate. This is due to the formation of an adsorbed PS nanolayer with 0.6R g thick on the HF-etched Si substrate during the CO2 annealing [134]. Hence, the adsorbed segments do interdiffuse with the free chains in CO2, but the dynamics remains hindered due to the presence of the segment-solid contacts.

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