Miscible polymers having lower critical solution temperature

The instance we have considered here, that of a polymer in a poor solvent, results in an upper critical solution temperature (UCST) as shown in Figure 2.33. This occurs due to (a) decreased attractive forces between like molecules at higher temperatures and (b) increased solubility. For some systems, however, a decrease in solubility can occur, and the corresponding critical temperature is located at the minimum of the miscibility curve, resulting in a lower critical solution temperature (LCST). This situation is illustrated in Figure 2.34. 

The most basic question when considering a polymer blend concerns the thermodynamic miscibility. Many polymer pairs are now known to be miscible or partially miscible, and many have become commercially Important. Considerable attention has been focussed on the origins of miscibility and binary polymer/polymer phase diagrams. In the latter case, it has usually been observed that high molar mass polymer pairs showing partial miscibility usually exhibit phase diagrams with lower critical solution temperatures (LCST). 

Cloud-point curves or precipitation curves for different polymer-solvent systems have different shapes (Figs. 3.12 and 3.13). The maxima and minima on these curves indicate the upper critical solution temperature (UCST) and the lower critical solution temperature (LCST), respectively. As indicated in Figs. 3.12 and 3.13, the phase diagram of a polymer solution has two regions of limited miscibility (i) below UCST associated with the theta temperature (see Problem 3.16) and (ii) above LCST. 

Of all stimuli-responsive polymers, temperature-responsive polymers are the best known and most studied. Among those, polymers that exhibit a lower critical solution temperature (LOST) have found the widest applicability [14]. The LOST is a fascinating phenomenon found for various polymer solutions. Polymer solutions often exhibit both an LCST and an upper critical solution temperature (UCST). For the LCST, at temperatures below the LCST the polymer is completely miscible in the solvent, whereas at temperatures above the LCST a phase separation occurs. In fact, the most investigated temperature-responsive polymer featuring a LCST in water is poly(N-isopropylacrylamide) (pNlPAm). The LCST of pNlPAm is 32°C,... 

Polymer pairs miscible at room temperature that appear to have a lower critical solution temperature (LCST) above room temperature. These polymer pairs are also fisted in one of the earlier Tables, usually Table 21.1 or 21.2. 

Whereas ordinary mixtures of low-molecular liquids in case of demixing usually exhibit binodals having a maximum (the so-called upper critical solution temperature, UCST), cloud-point curves of polymer solutions show in the rule minimum behavior (characterized by the so-called lower critical solution temperature, LCST). However, a number of polymer solutions possess even two demixing regions, one with UCST-behavior at moderate temperatures and one at higher temperatures with LCST-behavior (often at temperatures near the critical temperature of the solvent). At least, there are systems exhibiting closed miscibility gaps. 

Miscibility in polymer blends has been studied by both theoreticians and experimentalists. The number of polymer blend systems that have been found to be thermodynamically miscible has increased in the past 20 years. Systems have also been found to exhibit the upper or lower critical solution temperatures. So complete miscibility is found only in limited temperature and composition ranges. A large number of polymer pairs form two-phase blends. This is consistent with the small entropy of mixing that can be expected of high polymers. These blends are characterized by opacity, distinct glass transition temperatures, and deteriorated mechanical properties. Some two-phase blends have been made into composites with improved mechanical properties. Often, incompatibility is the general rule, and miscibility or even partial miscibility is the exception. 

Theoretical models have predicted that any miscible polymer blend should exhibit a lower critical solution temperature (LCST) " and that the LCST should decrease as the molecular weight of... 

TABLE 6. POLYMER PAIRS MISCIBLE AT ROOM TEMPERATURE THAT APPEAR TO HAVE A LOWER CRITICAL SOLUTION TEMPERATURE (LCST) ABOVE ROOM TEMPERATURE... 

Where the solubility parameter rule is in error is for natural rubber and polybutadiene. The differential in solubility parameters is around 0.6 but the two polymers are immiscible. Polybutadiene grade IISRP 1207 and an oil extended polymer such as IISRP 1712 have a differential of less than 0.1 and in this case the two elastomers are nearly fully miscible between the lower and upper critical solution temperatures. The blended elastomers mechanical properties then become a function of the filler type, distribution, vulcanization system, and any processing aids present. 

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