Local phase separations

For PDMS-modified Si02 glasses, structural analysis shows that this hybrid material has some degree of localized phase separation of the PDMS component, even though OH-terminated PDMS can be successfully incorporated into the Si02 network by chemical bonding. The PDMS component can behave as an elastomeric phase because the glass-transition temperature of PDMS is far below room temperature.52 54... 

Phase separation during freezing and/or drying may provide a possible reconciliation for the two competing hypotheses to explain protein stabilization by excipients during lyophilization. If a protein phase separates from an excipient on a microscopic scale, the excipient will not afford mechanical protection even if it forms a glass. Likewise, failure of an excipient to hydrogen-bond to a protein could be considered a manifestation of local phase separation. Indeed, the exper-... 

The spectral parameters for component A again coincide with those of TEMPO in pure water ( a 48.3 MHz), i.e., this spin probe is located in a strongly hydrated, hydrophilic environment. The observed decrease of ai o by 3.7 MHz for species B ( hnal at 65 °C) is indicative of much more hydrophobic and less hydrated surroundings for these spin probes (comparable to chloroform or tert-hutyl alcohol [83]). At temperatures below the collapse temperature Tq, only the hydrophilic spectral component A is observed since all dendritic units are water-swollen. Above the critical temperature of 33 °C an increasing fraction of hydrophobic species B is observed with increasing temperature. The dehydration of the dendritic units thus leads to a local phase separation with the formation of hydrophobic cavities. Unlike in PNIPAAM hydrogels (Sect. 3.1), here hydrophobic regions are not observed below the macroscopic collapse temperature. 

Physical evidence supporting the Idea of solvent accumulation In amorphous regions due to crystallization Is discussed by Zachmann (23).) If this causes local saturation of the amorphous polymer, then continued crystallization would produce local phase separation, creating pockets of pure liquid solvent and a network of holes and cavities In the dried specimen. Accepting this, an Immediate conclusion Is that saturation of the amorphous component must precede complete crystallization for cavitation to develop. 

Figure 5.21 TEM images of poly(w-butyl methacrylate)-grafted MWCNTs with azido groups (MWCNT-Az-PnBMA) (a), MWCNTs grafted with both poly( -butyl methacrylate) and poly(ethylene glycol) brushes (MWNT-PnBMA-PEG) (b, c). (d) Cartoon for the local phase separation and assembly of amphiphilic polymer brushes into Janus polymer structures on CNTs as shown in (c) (marked by arrows), (e) Photograph of MWCNT-PnBMA-PEG dispersed in a mixed solvent of water (upper layer) and chloroform (bottom layer). Reprint with permission from Zhang et al...

The simple theory put forward by Meinhardt et al. accounts in a unified manner for both ripple phases and raft states in membranes. The prerequisites for the formation of such modulated phases is local phase separation (e.g., in the ripple case, between a liquid and a gel phase, or in the raft case, between a liquid disordered and a liquid ordered phase) and curvature stress in at least one of the two phases (typically the ordered one), resulting, e.g., from a size mismatch between head group and tails. In order to reproduce rippled states or rafts, coarsegrained simulation models must meet these criteria. This is often not the case. For example, the standard version of the popular MARTINI model does not have a ripple phase, because the low-temperature gel phase of saturated phospholipids is until ted. 

One alternative eiqilanation for the observed high conversimis is that the ingredients may have phase-separated within the film into domains of locally high concentration of reactant and DCA. However, we were unable to detect any evidence for phase separation of die ingredients in the film by electron microscopy. Furdiermore, the T, of the film (before irradiation) was reduced from 120 to 93 by the incorporation of the reactant, indicative of plasticization of the PMMA. These observations support the assunqition that the ingredients were randomly diqiersed. Another means to inhibit local phase separation of the reactant is to covalendy bond the conpound to the polymer, instead of dissolving the material in a polymer host. 

Figure 25.8 The free energy F(x) is concave upward so the system is stable to small fluctuations near the composition x = Xq. This is because f (xq) (o) is lower than the free energy (l/2)[F(x + Ax) + F x - Ax)] (°) of a locally phase-separated system. 

Tanaka H, Nishi T. Local phase separation at the growth front of a polymer spherulite during crystallization and nonlinear spherulitic growth in a polymer mixture with a phase diagram. Phys Rev A 1989 39 783-794. 

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