Soft block

Step-Growth Gopolymerization. A sample of a block copolymer prepared by condensation polymerisation is shown in equation 30 (37). In this process, a prepolymer diol (HO—Z—OH) is capped with isocyanate end groups and chain extended with a low molecular-weight diol (HO—E—OH) to give a so-called segmented block copolymer, containing polyurethane hard blocks and O—Z—O soft blocks. 

It should also be pointed out that the Tg of the soft blocks, which consist of fairly short polymer chains, will be somewhat lower than for a corresponding homopolymer of high molecular weight, for the reasons given in Section 4.2. This effect may, however, be more than compensated by the loss of molecular freedom due to the presence of and interaction with the hard phase polymer present. 

The rigid and soft blocks used in polyesteredier diermoplastic elastomers (polyesteretlier TPEs) are typically PBT and poly(oxyteti amethylene) (PTMO), respectively, witii block number-average molar mass varying between 1000 and 3000. They are obtained by the melt reaction between dimethyl terephthalate, butanediol, and dihydroxy-terminated PTMO in the conditions typical of a PBT syndiesis. 

Scheme 4.8 Secondary structure in polyurethanes hard and soft block microdomains.

The representation of hard-block domain structure shown in Scheme 4.8, implying rigid, crystallike molecular order, can be misleading because hard blocks are, at best, microcrystalline (as are soft blocks). Although microcrystallinity can be readily obtained, it requires careful selection of raw materials,... 

Halogenonitroaromatic compound, 295 Hard-block domain structure, 218-219 Hard-block microdomain, 215 Hard-block/soft-block phase separation, 220... 

Recently, Tong et al. [195] have shown that in methyl methacrylate-b-alkyl acrylate-b-methyl methacrylate (MAM) and SIS-type triblock copolymers, the ultimate tensile strength is inversely proportional to the molecular weight between the chain entanglements in the middle soft block at comparable proportion of the outer block. 

PU elastomers contain alternative soft and hard segments, which separate into different phases. Hard domains play a role of cross-links, whereas soft blocks provide extensibility. Therefore, morphology and properties of PU are defined by relative amount of soft and hard segments. For example, at 70% concentration of soft segments, the material is described as a mbbery matrix with... 

FIGURE 20.9 Phase images of cryo-ultramicrotomed surfaces of PU samples with different content of soft blocks 70% (a-b), 50% (c-d). 

Experiments were conducted with a dual catalyst chain shuttling system in a continuous solution polymerization reactor. A series of ethylene-octene copolymers of similar melt index were produced with a composition of ca. 30% (by weight) hard and 70% soft blocks. The level of DEZ was systematically varied to study the effects of CSA ratio on polymer microstructure. 

The hard-soft block copolymer approach employed to produce segmental PUs (Section 7.6) has also been used with polyesters, with the hard block formed from 1,4-butadienediol and terephthalic acid while the soft block is provided by oligomeric (approximate molecular weight of 2000 Da) poly(tetramethylene glycol) and is sold under the trade name Hytrel. 

Today, new copolymers are making use of the hard-soft block strategy where the hard segment is a block portion as PE that readily crystallizes forming a physical cross-link. The soft segment consists of blocks formed from alpha-olefins, such as 1-butene, 1-hexene, and 1-octane, where the substituted alkane-arm discourages crystallization. 

A number of siloxane ABA block polymers have been formed. Copolymers of dimethyl-siloxane (A)-diphenylsiloxane (B)-dimethylsiloxane (A) have been synthesized by the sequential polymerization of reactants. The diphenylsiloxane block acts as a hard block and the more flexible dimethylsiloxane blocks act as soft blocks. 

Hard segments have also been introduced using p-bis(dimethylhydroxysilyl)benzene with the soft block again being dimethylsiloxane. 

Poly(oxytetramethylene), poly(tetrahydrofuran), may assume the state of a viscous oil, a wax, or a crystalline solid (melting range around 55 °C), depending on the molecular weight. Poly(tetrahydrofuran) telechelics prepared with two OH end groups and in molar masses of 500-4000 g/mol are used widely as soft block in segmented polyurethanes and polyesters (see Sect. 3.4.2.1). 

The microstructure of the soft block can be varied to produce SBS in which the middle block is from 100% 1,2 to 1 5% or a mixture of varied composition. The work of Halasa and coworkers (25) is presented in Table X to illustrate this point. 

Copolymer Block arrangement Soft block structure Hard block structure and content (AU/W)V x... 

Careful XPS analysis of a series of poly(dimethylsiloxane-urea-urethane) multiblock copolymers demonstrated that, as well as a surface layer of siloxane, there was a layer enriched in the hard block immediately beneath this. The thickness of both these layers depended on the molecular weights of the soft and hard block segments, respectively112. Annealing of these copolymers increased the thickness of both layers. The same authors have also shown that the thickness of these layers of hard and soft blocks could be modified by use of solvent mixtures which selectively precipitate the polar hard block during film formation by solvent casting113. 

At the end of the reaction the crosslinked PU contains hard and soft blocks ... 

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