Soft domain

Styrene-Butadiene-Styrene Block Copolymers. Styrene blocks associate into domains that form hard regions. The midblock, which is normally butadiene, ethylene-butene, or isoprene blocks, forms the soft domains. Polystyrene domains serve as cross-links. 

To achieve low stress embedding material, low modulus material such as siUcones (elastomers or gels) and polyurethanes are usually used. Soft-domain elastomeric particles are usually incorporated into the hard (high modulus) materials such as epoxies and polyimides to reduce the stress of embedding materials. With the addition of the perfect particle size, distribution, and loading of soft domain particles, low stress epoxy mol ding compounds have been developed as excellent embedding materials for electronic appHcations. 

A detailed description of AA, BB, CC step-growth copolymerization with phase separation is an involved task. Generally, the system we are attempting to model is a polymerization which proceeds homogeneously until some critical point when phase separation occurs into what we will call hard and soft domains. Each chemical species present is assumed to distribute itself between the two phases at the instant of phase separation as dictated by equilibrium thermodynamics. The polymerization proceeds now in the separate domains, perhaps at differen-rates. The monomers continue to distribute themselves between the phases, according to thermodynamic dictates, insofar as the time scales of diffusion and reaction will allow. Newly-formed polymer goes to one or the other phase, also dictated by the thermodynamic preference of its built-in chain micro — architecture. 

Reason Frequently the arrangement of hard and soft domains is more perfect along the principal axis of the oriented material. 

Figure 8.10. The real two-phase system (a) and the transition into an ideal system (c) by removal of the density fluctuation background, Ipi, and of a transition layer of thickness dz between the hard and the soft domains. The elongated white region indicates a void...

Figure 8.38. Structural parameters of an ensemble of needle-shaped soft domains in a poly(ether ester) as a function of elongation . D (open circles) is the average needle diameter, a/D (filled circles) is the relative standard deviation of the needle-diameter distribution. Square symbols demonstrate the lateral compressibility of the soft needles during elongation... 

Therefore, the following melting model of a fatty acid monolayer could be suggested during increase of the temperature Solid domains — Soft domains — Two co-existed phases between domains and molten monolayer — completely molten monolayer, as shown in Figure 2. 

Solid domain Soft domain Two-phases co-existed Molten monolayer... 

Figure 15.9. 13C CPMAS NMR spectrum of humin extracted from a brown chernozem soil from Western Canada. The characteristic doublet in the unsubstituted aliphatic region is characteristic of methylene carbon (28-34 ppm) and shows the presence of both amorphous (soft) domains at 29 ppm and crystalline (rigid) domains at 33 ppm in soil humin. Reprinted from Simpson, M. I, and Johnson, R C. E. (2006). Identification of mobile aliphatic sorptive domains in soil humin by solid-state 13C nuclear magnetic resonance. Environ. Toxi. Chem. 25, 52-57, with permission from the Society of Environmental Toxicology and Chemistry.

The presence of hard and soft domains in segmented polyurethanes also has been confirmed by experimental results using pulsed NMR and low-frequency dielectric measurements. Assink (55) recently has shown that the nuclear-magnetic, free-induction decay of these thermoplastic elastomers consists of a fast Gaussian component attributable to the glassy hard domains and a slow exponential component associated with the rubbery domains. Furthermore, the NMR technique also can be used to determine the relative amounts of material in each domain. 

Copoly(ester ester)s belong to the family of thermoplastic elastomers (TPEs) and consist in general of thermo-reversible hard and elastic soft domains [11]. The copoly(ester ester) used here consists of 60% poly(butylene terephthalate), 35% poly(butylene adipate) and 5% 4,4 -methylenebis(phenyl isocyanate), and shows domain sizes of about 20 nm [12]. The material possesses a rubber plateau between the glass transition temperature of the mixed amorphous PBA/PBT phase (the PBT phase is semi-crystalline) at about -30°C and the melting point of the PBT at about 220°C. Due to the vulnerability of the amorphous PBA/PBT soft domains towards water attack [13] the PBT/PBA copoly(ester ester) is used here to study the existence of ESC of a chemical rather than a physical nature. For the sake of clarity it should be emphasized that no additives have been used in the copoly(ester ester) described here. 

Based on the above interpretation, the main effect of increased catalyst concentration 1s to produce more diffuse boundaries between the hard and the soft domains, since differences in T c among the samples are small, while differences in temperature dependency of the modulus are large as Indicated by Figure 6. It is interesting to observe in this figure also that all curves converge to a limiting modulus ratio value of about 3.0, G,(-30°C)/G,(70°C). 

Figure 1.7 The hard domains and soft domains of polyurethane elastomers...

A temperature dependence of k suggests that it increases with a rise in temperature for all the penetrants except j -xylene and bromobenzene. Furthermore, k appears to depend on structural characteristics of the penetrant molecules i.e., it decreases successively from benzene to mesitylene this decrease in k parallels the decrease in the values of sorption equilibrium. Similarly, for chlorobenzene to nitrobenzene via o-dichlorobenzene k decreases successively. Thus, it appears that k not only depends on the structural characteristics of the polymer and penetrant molecules, but also on solvent interactions with the polyurethane chains. At any rate, the greater tendency for non-Fickian coefficients in Equation (1) seen for 60°C, may reflect some aspect of swelling of interphase regions between the hard and soft domains. 

Commercial BC s are prepared from monomers that upon polymerization yield immiscible macromolecular blocks, one rigid and the other flexible, that separate into a two-phase system with rigid and soft domains. The concentration and molecular weights provide control of the size of the separated domains, thus morphology and the interconnection between the domains. The existence of a dispersed rigid phase in an elastomeric matrix is responsible for its thermoplastic elastomer behavior. For symmetric block copolymers, Leibler [1980] showed that a sufficient condition for microphase separation is (%abN) = 10.5, where binary thermody-... 

Microstructure is a key aspect for polymers in general, and for polyurethanes (PUs) in particular. The morphology of PUs is governed by the formation of hard and soft domains and their intercalation. Consequently, new microstructures can be developed using dendritic and hyperbranched (HB) polymers in PU systems. 

Unsaturated elastomers can be readily metallated with activated organolithium compounds in the presence of chelating diamines or alkoxides of potassium or sodium. For example, polyisoprene, polybutadiene, styrene-butadiene copolymers, and styrene-isoprene copolymers can be metallated with n-butyllithium TMEDA complexes (1/1 or 1/2 ratio) to form allylic or benzylic anions. The resulting allylic anion can be employed as an initiator site to grow certain branched or comb polymer species. These polymers can include polystyrene, which would form hard domains, or polybutadiene, which forms soft domains. 

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