Polymer crystallization, nucleation thermodynamics

Two types of phase equilibria are superimposed phase separation equilibria in amorphous and ciystalhne phases. The thermodynamic equilibrium of the system presented in Figure 6.2 corresponds to polymer crystallization. The amorphous phase is metastable. It can exist fi om the beginning of crystalline phase nucleation through subsequent crystallization imtil the equilibrium state for the crystalline polymer is reached. A system can degenerate into many simple systems for polymers that erystallize readily in dilute solutions where polymer segregates into separate crystallites. Polymers, capable of partial crystallization at the ejq)ense of the formation of local regular blocks of chains (e.g., PVC) can exist as a metastable amorphous-erystalhne system. In such cases, one phase repre-... 

Hu WB, Mathot VBF, Frenkel D (2003a) Lattice model study of the thermodynamic interplay of polymer crystallization and liquid-liquid demixing. J Chem Phys 118 10343-10348 Hu WB, Frenkel D, Mathot VBF (2003b) Sectorization of a lamellar polymer crystal studied by dynamic Monte Carlo simulations. Macromolecules 36 549-552 Hu WB, Frenkel D, Mathot VBF (2003c) Intramolecular nucleation model for polymer crystallization. Macromolecules 36 8178-8183... 

The interplay of phase separation and polymer crystallization in the multi-component systems influences not only the thermodynamics of phase transitions, but also their kinetics. This provides an opportunity to tune the complex morphology of multi-phase structures via the interplay. In the following, we further introduce three aspects of theoretical and simulation progresses enhanced phase separation in the blends containing crystallizable polymers accelerated crystal nucleation separately in the bulk phase of concentrated solutions, at interfaces of immiscible blends and of solutions, and in single-chain systems and interplay in diblock copolymers. In the end, we introduce the implication of interplay in understanding biological systems. 

Liquid interfaces are prevailing within the immiscible polymer blends and solutions. The effect of interfaces to polymer crystallization cannot be overlooked, not only because the practical system accumulates impurities at interfaces for heterogeneous crystal nucleation, but also because the thermodynamic conditions for crystal nucleation at interfaces are different from that in the bulk phase. The latter effect can be revealed by the theoretical phase diagrams for immiscible polymer blends, as... 

In the context of the above-listed topics, there are several questions still open and often controversially debated. They concern nucleation - Which conditions are necessary to nucleate the polymer crystallization process . The applicability of thermodynamic concepts - Do concepts of equilibrium thermodynamics such as melting and crystallization describe correctly the nonequilibrium metastable nature of polymer crystals Morphological... 

One of the specific features of these polymer systems is a low mobility of the macromolecules and correspondingly slow rates of the processes associated with phase transitions. This phenomenon is well known for the case of polymer crystallization in bulk and in solution. The rate of nucleation of the crystalline phase is often so slow that in practice one has to deal with the amorphous state of the polymer, although thermodynamic equilibrium corresponds to its crystallization. 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

A drastic departure from nucleation theory was made by Sadler [44] who proposed that the crystal surface was thermodynamically rough and a barrier term arises from the possible paths a polymer may take before crystallizing in a favourable configuration. His simulation and models have shown that this would give results consistent with experiments. The two-dimensional row model is not far removed from Point s initial nucleation barrier, and is practically identical to a model investigated by Dupire [35]. Further comparison between the two theories would be beneficial. 

---