Monomer phase transitions

E. V. Albano. Irreversible phase transitions in the dimer-monomer surface reaction process on fractal media. Phys Lett A 765 55-58, 1992. 

E. V. Albano. Critical behavior of the irreversible phase transitions of a dimer-monomer process on fractal media. J Phys A (Math Gen) 27 431-436, 1994. 

Fig. 30. DSC traces showing the phase transition of the model membrane in its monomeric and polymeric form. Note the difference in the enthalpies of the transition monomer AH = 56 J/g, polymer AH = 26 J/g...

An A-B diblock copolymer is a polymer consisting of a sequence of A-type monomers chemically joined to a sequence of B-type monomers. Even a small amount of incompatibility (difference in interactions) between monomers A and monomers B can induce phase transitions. However, A-homopolymer and B-homopolymer are chemically joined in a diblock therefore a system of diblocks cannot undergo a macroscopic phase separation. Instead a number of order-disorder phase transitions take place in the system between the isotropic phase and spatially ordered phases in which A-rich and B-rich domains, of the size of a diblock copolymer, are periodically arranged in lamellar, hexagonal, body-centered cubic (bcc), and the double gyroid structures. The covalent bond joining the blocks rests at the interface between A-rich and B-rich domains. 

Polymers designed with this technique have a number of important aspects in common with proteins. First of all, the transition from a liquid-like globule into a frozen state occurs as a first order phase transition. Further, the frozen state itself has an essential stability margin, which is determined by the design parameters. As in real proteins, neither a large variation of temperature or other environmental conditions, nor a mutational substitution of several monomers leads to any change in basic state conformation. In this respect the ability of sequence design to capture certain essential characteristics of proteins seems quite plausible. 

After polymerizing, the phase behavior changes dramatically. The phase transition temperatures return to values very close to those observed in the pure FLC. The interactions which lower the transitions in monomer/FLC systems are not significant in polymer/FLC systems. Similar results are observed in HDDA/FLC systems. The only notable exception is that the monomer saturation concentration occurs at a significantly lower concentration (5 wt%). 

The introduction of a polymer network into an FLC dramatically changes phase and electro-optic behavior. Upon addition of monomer to the FLC, the phase transitions decrease and after polymerization return to values close to that observed in the neat FLC. The phase behavior is similar for the amorphous monomers, HDD A and PPDA. The electro-optic properties, on the other hand, are highly dependent on the monomer used to form the polymer/FLC composite. The ferroelectric polarization decreases for both HDDA and PPDA/FLC systems, but the values for each show extremely different temperature dependence. Further evidence illustrating the different effects of each of the two polymers is found upon examining the polarization as both the temperature and LC phase of polymerization are changed. In PPDA systems the polarization remains fairly independent of the polymerization temperature. On the other hand, the polarization increases steadily as the polymerization temperature of HDDA systems is increased in the ordered LC phases. 

Fig. 32 a-d. Phase transition a from the extended coil b to a globule state as found by scaling analysis [83] c the transition is caused by lowering the surface pressure below a certain critical value TTc at which the fraction of adsorbed monomers 3=N2d/N undergo discrete changes d hereby, the depends critically on the side chain length... 

At CMC, micelles (aggregates of SD" with some counterions, such as Na+) are formed, and some Na+ ions are bound to these, which is also observed from conductivity data. In fact, these data analyses have shown that approximately 70% Na+ ions are bound to SD" ions in the micelle. The surface charge was estimated from conductivity measurements (Birdi, 2002). Therefore, the concentration of Na+ will be higher than SD" ions after CMC. A large number of reports are found in the literature, in which the transition from the monomer phase (before CMC) to the micellar phase (after CMC) have been analyzed. 

Nadkarni and Schultz (15) showed that the rate at which polymerization of trithiane is initiated by radiation is a maximum near 175° C and that entropy rate of change for the monomer is also a maximum near 175° C. Further, they established that trithiane undergoes a hitherto unknown phase transition at 194° C. 

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