Segments hard, 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. 

The elasticity of thermoplastic polyurethane rubbers (which are also known as thermoplastic urethanes or TPUs) is a function of their morphology which comprises hard and soft phases. The hard phases consist of hydrogen bonded clusters of chain segments, which are linked by flexible chain segments that make up the soft phase. The hard blocks, which are the minor phase, exist as separate domains within a continuous matrix of the majority soft phase, as shown schematically in Fig. 25.9. 

Thermoplastic polyurethane elastomers are normally based on polyester prepolymers. The properties of these polymers can be systematically varied by tailoring the nature and ratio of the hard and soft segments. The stiffness of a polyurethane elastomer increases as the proportion of hard blocks increases. As the stiffness increases, the extensibility of the material decreases. 

Such an example has been recently worked out, combining a very hard block i.e. polypivalolactone (PPVL), with a thermostable elastomer segment i.e. polydimethyl-siloxane (PDMS), using the reactions sequence described in scheme 3. (One should note that the starting PDMS has been obtained through a previously described procedure (14), avoiding any unstable Si-O-C bond in the final produc t). 

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. 

SBS block copolymers differ structurally from the random copolymer of styrene and butadiene (SBR). Because styrene and butadiene blocks are incompatible, they form separate phases joined at the junctions where the various blocks are connected. This gives an elastomeric material where the butadiene blocks form the soft segments and the styrene blocks form hard blocks. 

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.)...

Although short-segment sequences are expected to be the most compatible, the 1/2/1 random copolymer with an average hard-block-sequence length of only two units does exhibit a phase-separated morphology—as reflected for the as-reacted sample in hard-segment crystal-... 

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