Polymer-rich domains

In those cases, there was no reported evidence of pronounced lateral structure or separation. This is in marked contrast to the study by Fleming et al. [77], who reported micron-sized domains of surfactant rich ordered aggregates inter-dispersed by disordered ill-defined polymer rich domains. Whether this is a feature of a hydrophobic surface or a highly non-equilibrium condition is not clear. 

Phase separation is another method that can be employed for the preparation of nanoscale sttucmres and is frequently used to prepare three-dimensional tissue-engineering scaffolds. Phase separation of a polymer solution can produce a polymer-rich domain and a solvent-rich domain and this morphology can be fixed by quenching under low temperamre conditions. 

For the polystyrene/cyclohexanol system, = 1-0. This means that the polymer-rich domains will have to travel a composition distance that is equal to that of the solvent-rich domains in order to reach its binodal composition (symmetric case in Fig. 1.4.4). However, if the polymer composition asymmetry ratio is equal to about 2 (such as in Fig. 1.4.5), then half of the polymer-rich domains is believed to migrate to adjacent domains in order for the rest of the polymer-rich domains to continue to approach the binodal composition (Cahn, 1961). Since there is equal competition for polymer-rich material from every domain, then the position of the resulting holes (or cells) will be in a regular lattice position. Continued growth of structure should be based on the belief that the domains that are eaten up are those that are contiguous to the most number of polymer-rich domains. Also, as implied by the presence of distinct dominant frequencies for spinodal decomposition mechanism, the disappearance of contiguous polymer-rich domains should occur uniformly in space. 

Disappearance of polymer-rich domains is beginning to be seen from the figure even at the early stages of the phase separation of the reactive system, although there is no evidence of this happening with MAA and water in the reactive system. 

What is the effect of the reaction exotherm with this thermodynamic condition of phase separation above the LCST for the FRRPP process Here and at this point, the author will start to integrate concepts and experimental results into a cohesive and plausible picture. As shown in a ternary composition-temperature reaction trajectory plot (Fig. 2.1.11), when the polymer-rich domains are formed (Point B), its local temperature would increase due to the reaction exotherm. This places the reactive polymer-rich domains at Point B in the vertical temperature projection of Fig. 2.1.11 where diffusional fluxes drop to zero due to the approach of the system toward the spinodal curve at a higher temperature T2. Note that in Section 1.2, it was shown that all component mutual diffusion coefficients are zero at the spinodal. Then, the polymer-rich domains attain the binodal composition at C and at the higher temperature T2. If new chains are initiated and propagated at T2, then the reaction trajectory can go on, until a certain ceiling temperature is reached. 

At A and A , the system phase separates until the polymer-rich phase reaches B and B . If the polymer-rich domains undergo gel effect, their temperature will increase to C. 

It seems to be evident that the cutoff value of -1,000 for Ch is supported experimentally and could then be used to quantitatively characterize the necessary and sufficient condition for the occurrence of the FRRPP process. A final note here is that this criterion corresponds to strict FRRPP process, wherein all aspects of polymerization control occur with a flat temperature profile. If the flat temperature profile is to be relaxed, then the cutoff value of Ch will have to be less than —1,000 but below zero. This will reach even smaller polymer-rich domain particle sizes in Fig. 2.5.2. 

First-stage polymerization of styrene in ether (33.4 g styrene, 0.200 g ether, 0.34 g V-65, or AIBN in a 300-ml Parr reactor system) was carried out at 80°C up to five times initiator half-life. Then, the reactor fluid was withdrawn through a 1/8-in. copper tube that is immersed in ice-water bath. The cold reactor fluid was collected into a 1000-ml glass reactor that contains 400 ml distilled water and 12 g acrylic acid (AA). The mixture was continuously mixed at room temperature for at least 2 h in order to soak-in the AA monomer into the polymer-rich domains. Then, the... 

Liquid crystal and polymer dispersions are fabricated using thermally-induced phase separation (TIPS), solvent-induced phase separation (SIPS), or Polymerization-induced phase separation (PIPSX/I)- For TIPS, a homogeneous mixture of a low-molecular weight liquid crystal and thermoplastic polymer is cooled below the critical phase separation temperature to induce phase separation into liquid crystal rich and polymer rich domains. The morphological properties (domain size, number of domains per unit volume, and the composition of the domains) depend primarily on the choice of liquid crystal and thermoplastic polymer, the initial weight fraction of liquid crystal in die initial mixture, and the rate of cooling. 

The reactor pressure was found to increase with increasing both the concentration of VC and conversion up to 70-80% [158] (see Fig. 10). This was ascribed to the increase of the mole fraction of VC in vapor and polymer particle phase. Beyond the critical conversion (70-80%), the reactor pressure abruptly decreases which is due to the onset of the gel effect (depletion of VC monomer droplets). With decreasing VC concentration, the critical conversion is shifted to higher conversion. The pressure increase of VC was ascribed to the exotherm which is very often observed in polymerizations of acrylates. The gel effect, which is operative in polyVC/BA particles, increases the pressure of VC due to the temperature increase in the polymer-rich domains. 

For practical use of PDLC, dispersions are mainly prepared by mixing a small amount of misdble monomers with liquid crystals and photo-polymerizing [1], since polymer and liquid crystals tend to be immiscible. As the polymerization evolves, the system undergoes phase separations into a liquid crystalline phase rich in liquid crystals and an isotropic phase rich in polymers. Before the system reaches equilibrium states, however, the polymerization freezes the system into a crosslinked network of polymer-rich domains. Then, the morphologies of the PDLC involve interplay among three kinetic processes polymerization, phase separation, and phase ordering. These phase separation dynamics have been simulated by some aulhors [105-108]. 

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