Segments soft, blocks

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. 

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... 

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. 

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). 

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. 

PU systems with hard blocks made of piperazine, have been also studied [90, 91, 92], In systems deuterated in the segments of the soft blocks, 2H NMR results show that a fraction of the soft segments have a restricted mobility, due to the connectivity to hard blocks, rather than to the adsorption on those blocks acting as filler particles [93]. To study the interface in a specific way, systems with hard blocks selectively deuterated at different positions have been synthetised. A reduction of quadrupolar interaction according to the position was observed, corresponding to an increase of the mobility in hard segments on approaching the hard-soft interface. 

Soft blocks are composed of linear, dihydroxy poly ethers or polyesters with molecular weights between 600 and 3000. In a typical polymerization of a thermoplastic polyurethane elastomer, the macroglycol is end capped with the full amount of aromatic diisocyanate required in the final composition. Subsequently, the end-capped prepolymer and excess diisocyanate mixture reacts further with the required stoichiometric amount of monomeric diol to complete the reaction. The diol links the prepolymer segments together while excess diol and diisocyanate form short hard-block sements, leading to the (AB)n structure illustrated in Figure 1. Block lengths in (AB)n polymers are frequently much shorter than those in anionically synthesized ABA block copolymers. 

Figure 9. Plot of heat of fusion vs. weight fraction of hard segment for each random copolymer, assuming k > 1 or k > 6, where k is the number of diisocyanate (hard-segment) units betwen two consecutive macrodiol (soft-segment) units. (Peebles (30) calculation of hard, block-length distribution in segmented polyurethane block copolymer is applied.)...

The urethane elastomers are complex-segmented or block polymers. Soft,... 

The structure of the aromatic diamines used in these elastoplastics had only a minor effect on the copolymer physical properties, as was observed for thermoplastics. The ratio of hard blocks to soft blocks in these segmented polymers could be varied, but such variations will result in a wide range of properties. 

The microstructure of segmented block copoly(ether esters) composed of poly(tetramethylene oxide) (PTMO) soft blocks and poly(butylene terephtha-late) (PBT) hard blocks was investigated by a variety of analytical techniques, including C solid-state NMR. ... 

Fig. 9 Multiblock copolymers consisting of a poly(ethylene glycol) soft block and a tetrapeptide Ala-Gly-Ala-Gly, crystalline hard block in two variants a Templated system in which an aromatic hairpin turn is used to force parallel jS-sheet formation, b Non-templated system in which peptide segments are free to form parallel and/or antiparallel /1-sheets. Reprinted with permission from [43]. Copyright 2001 American Chemical Society...

In many phase-separating block copolymers (especially segmented multiblock copolymers such as polyurethanes where the blocks are usually short), lowering the soft block Mn increases the Tg of the soft phase because of the crosslink-like topological constraints imposed by hard phase domains. 

Linear strictly bifunctional oligomers of THF with molecular weights close to 1000 or 2000 and having two terminal OH groups are prepared in bulk with strong protonic acids (cf. Sect. 6.2). These oligomers are mostly used as soft segments in block copolymers made by polycondensation with diisocyanates or with 1,4-alkylene (ethylene, butylene) terephthalates. 

The copolymers which are the subject of this section are formed by the copolymerization of a prepolymer, which will ultimately form the soft block or segments, with one or more monomers which will form the hard blocks... 

The principle of formation of segmented or block copolymers (see Section 4.4.5) has also been applied to polyesters, with the hard segment formed from butanediol and terephthalic acid, and the soft segment provided by a hydroxyl-terminated polyether [polytetramethylene either glycol (PTMEG)] with molecular weight 600-3000. 

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