Berghmans’ point

Fig. 4 Schematic phase diagrams of a polymer solution showing LL phase separation with UCST behavior. Curve s is the spinodal, curve b is the binodal, and curve g is the glass transition temperature as a function of polymer concentration. BP indicates the Berghmans point, (a) LL phase separation is the only thermodynamic transformation of the system [17,25, 36]. (b) Curve c shows the crystallization temperature of a polymer fully miscible in a solvent as a function of concentration in the solution [17, 25], The LL phase coexistence curve (combined with vitrification) is a (classical) metastable process that lies beneath the crystallization curve c. In route 1, a polymer solution is supercooled at ALj, and the only active process is polymer crystallization. In route 2, the initially homogeneous solution is supercooled to a larger undercooling than namely AL2. Crystallization may compete either with LL phase separation when reaching point C, or LL phase separation coupled with vitrification when reaching point D. At C, crystallization may take place in the polymer-rich phase. At D, both LL phase separation and crystallization may become arrested by vitrification...

Fig. 4 represents the dependence of the glass transition temperature Tg, on the polymer concentration. Because Tg of the pure polymer decreases by addition of the solvent, curve g intersects the binodal at the point BP, named Berghmans point after Hugo Berghmans [36]. 

Fig. 1 Possible phase diagrams for polymers showing either (a) UCST (e.g. PS in cyclohexane) or (b) LCST type 11 (e.g. PiPAAm in aqueous medium) phase separation behaviour. Tdem is the demixing temperature, Tq is the theta temperature, and Tbp is the temperature corresponding to the Berghmans point [76], For both polymers, Tg in their solid state is weU above Tdsm- For this UCST-type polymer, Tg cannot be lower than Tbp. At temperatures below Tbp, the polymer is frozen in, and phase morphology is preserved [77]. For the LCST-type polymer shown, partial vitrification takes place at Tbp < T < To [78]...

The interception of these two curves is known as Berghmans point (BP) and defined as the point where the liquid-liquid phase separation binodal line is intercepted by the vitrification curve (9,10) see Figure 4.4. When the vitrification curve intercepts the binodal line, the development of ordinary tie lines is inhibited because molecular diffusion in the glassy state is far slower than in the liquid state see Figure 4.4. 

VA1 Vanhee, S., Kiepen, F., Brinkmann, D., Borchard, W., Koningsveld, R., and Berghmans, H., The system methylcyclohexane/polystyrene. Experimental critical curves, cloud-point and spinodal isopleths, and their description with a semi-phenomenological treatment, Makromol. Chem. Phys., 195, 759, 1994. 

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