ИСТИНА

Zone annealing is a well-known process of refining crystalline materials. A typical installation (Figure 1) includes heater and cooler units that create a thermal gradient slowly moving along the sample surface. Such treatment can be applied to polymers inducing their directional crystallization (Lovinger et al., 1976; Schweicher et al., 2011; Toda et al., 2012) or creating highly ordered microstructures in block copolymer films (Hashimoto et al., 1999; Angelescu et al., 2007). The so-called cold zone annealing (Karim et al., 2007 to 2016) implies that a copolymer is not heated above its order-disorder transition temperature, which can be important to avoid degradation. The effect of ordering was demonstrated for a single PS–PMMA diblock copolymer system, in which shorter PMMA blocks formed cylindrical domains in the PS matrix. In our study [1] we apply this technique to PS–PB–PS triblock and PS–P4VP diblock copolymers, which also yield cylindrical morphology in thin films spin-coated onto silica substrate. PS–PB–PS copolymer revealed a broad temperature range between Tg (–93 °С for PB matrix and 63.5 °С for PS cylinders) and TODT (178 °С by DSC), in which minor phase PS blocks possess high mobility in the low-viscosity PB matrix in the microphase-separated state. Zone annealing at different peak temperatures Tmax applied to the films of various thicknesses below and above 100 nm modified the initial morphology of mixed horizontal and vertical cylinders (Figure 2, top row) but a distinct ordering was not observed. The thinnest film was stable up to 160 °С, which indicates the effective increase of Tg for chains contacting with the substrate. Annealing at the temperature close to TODT led to an appearance of thickened sections without microstructure at the film surface. Further increase in Tmax resulted in a gradual fading of the microstructure even at flat sections of the film surface. For thicker initial films zone annealing led to an increase of the vertical cylinders fraction, which can be explained by the combination of short annealing and rapid heating. Since terminal PS blocks of any triblock chain can belong to different cylindrical domains, the spatial rearrangement of cylinders and hence microstructure ordering can be hampered. PS–P4VP copolymer with Tg of 100 °С for PS matrix and 153 °C for P4VP minor phase formed vertical cylinders after annealing under dioxane vapor. The film surface remained apparently stable in the course of zone annealing at Tmax well below Tg for PS blocks. However, at Tmax = 95 °С clear changes in the microstructure were detected (Figure 2, bottom row). One can suppose that the matrix amorphization destroyed cylinders by transforming them into micelles of random shape and size. At the same time the interfacial area between PS and P4VP domains at the free film surface was considerably increased. After zone annealing at Tmax = 150 °С almost whole film surface was filled with P4VP blocks, which indicated their lower surface energy at that temperature. Increasing Tmax also led to the growth of the height difference at the AFM images of a given size. Thus, it seems that PS–PMMA diblock copolymer constitutes a rather unique system regarding applicability of the cold zone annealing technique to increase its degree of ordering. 1 A.S. Merekalov, Y.I. Derikov, A.A. Ezhov, E.N. Govorun, Y.V. Kudryavtsev Polym. Sci., Ser A 2018, in press The study was supported by the Russian Foundation for Basic Research (Project 16-03-00531)