Thin polymer blends, phase behavior

This review has discussed the phase behavior of polymer blends and symmetric block copolymer melts in thin film geometry, considering mostly films confined between two symmetrical hard walls. Occasionally, also an antisymmetric boundary condition (i.e. one wall prefers component A while the other wall prefers component B) is studied. These boundary conditions sometimes approximate the physically most relevant case, namely a polymeric film on a solid substrate exposed to air or vacuum with a free, fiat surface (Fig. 1). The case where the film as a whole breaks up into droplets (Fig. 2) due to dewetting phenomena is not considered, however, nor did we deal with the formation of islands or holes or terraces in the case of ordered block copolymer films (Fig. 4b-d). 

The picture presented above is not complete as it neglects non-mean field behavior of polymer blends in the temperature range close to Tc [149]. The Ising model predicts phase diagrams of thin films, which are more depressed and more flattened than those yielded by mean field approach (as marked in Fig. 31d). Both effects were shown by Monte Carlo simulations performed by Rouault et al. [150]. In principle, critical regions of phase diagrams cannot be described merely by a cross-over from a three- to two-dimensional (for very thin films) situation. In addition, a cross-over from mean field to Ising behavior should also be considered [6,150]. 

Different polymer blends like PE (polyethylene)/PS (polystyrene) [10-11] and PMMA (polymethylmethacrylate)/PS [12-13] have been produced using supercritical C02-assisted extrusion. Fully intermeshing twin-screw extruders have been used in these studies. A decreased shear thinning behavior on dissolution of supercritical CO2 into blends was observed. The obtained reduction in viscosity ratio resulted in a finer dispersion of the minor phase, which is desirable to create a good polymer blend. The effect of supercritical CO2 on the dispersion of the minor phase for a PMMA/PS blend can be seen clearly in Fig. 12.5. 

I. Mitra, X. Li, S.L. Pesek, B. Makerenko, B.S. Lokitz, D. Uhrig, J.R Anker, R. Verduzco, G.E. Stein, Thin film phase behavior of bottlebrush/linear polymer blends. Macromolecules 47 (15) (2014) 5269-5276. 

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. The phase separation process can be controlled to effect desired morphologies. Under suitable conditions a film deposition process can lead to pattern replication. Demixing of polymer blends can lead to structure formation. The phase separation process can be characterized by the binodal and spinodal curves. UCST is the upper critical solution temperature, which is the temperature above which the blend constituents are completely miscible in each other in all proportions. LUST behavior is not found as often in systems other than among polymers. LUST is the lower critical solution temperature. This is the... 

Widmaier and Meyer [205] studied the structure of an ABA polystyrene-fc-isoprene block copolymer as a function of temperature by osmium tetroxide staining thin cast films. Hsiue and Yang [206] studied the morphology and properties of a-methylstyrene butadiene diblock copolymer films cast from several solvents. Films at a 0.1% concentration were cast on water and stained with 2% osmium tetroxide solution for 1 h. The microstructure was shown to differ for films cast from different solvents as there is a pol5mier-solvent interaction. Reich and Cohen [207] studied the phase separation of polymer blends in thin films and compared the behavior to that of the bulk material, as it is well known that phase... 

Liao Y, You J, Shi T, An L, Dutta PK. Phase behavior and dewetting for polymer blend films studied by in situ AFM and XPS from thin to ultratbin films. Langmuir 2007 23 11107-11. 

Confined crystallization is a phenomenon that occurs in droplet dispersions, polymer blends, block copolymers, and thin films. Confinement has many consequences on the nucleation and crystallization behavior. Among the most notorious are the production of fractionated crystallization and the possibility of isolating crystallizable phases whose nucleation may be very different heterogeneous, superficial, or homogeneous nucleation. In specific cases confinement can also lead to crystal modifications for polymorphic polymers. 

As described previously in Section 8.02.2.4, blending block copolymers with either nonselective or selective solvents alters the polymer phase behavior. Stated another way, the thermodynamically stable state of a solvent-swollen block copolymer is different from that of a neat block copolymer. In the case of solvent annealing, the swollen state may be maintained upon evaporation of the solvent. After solvent evaporation, the block copolymer is no longer in a thermodynamically stable state. However, as long as the Tg of the two blocks is sufficiently higher than the temperature at which the film is kept (typically, room temperature), there is little effective mobility of the polymer chains - the morphology is kinetically trapped. As discussed above, Tg s of polymer thin films can be altered from the bulk state strong interactions (attractive or unattractive) with the substrate (Section 8.02.2.3.4). " However, if the Tg is sufficiently above room temperature, little mobility would be expected even in the case of very thin films. 

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