Hard segments structure

Hard Segments. The hard segments are the linear reaction products of the diisocyanate component and the third monomer type in the TPU elastomer recipe, the small glycol-chain-extender component. In Figure 5 we see a typical TPU hard-segment structure which is formed from MDI and 1,4-BDO (1,4-butanediol). 

Oprea, S. (2010). Dependence of fungal biodegradation of PEG/castor oil-based pol3nirethane elastomers on the hard-segment structure, Polvrn. Dearad. Stab.. 95, 2396-2404. 

Z.S. Petrovic, J. Ferguson, N. Hudson, I. Javni, M. Vranes, The effect of hard segment structure on rheological properties of solutions of segmented polyurethanes. Ear. Polym. J. 28 (6) (1992) 637-642. 

It is generally accepted that the structure and the chemical composition of polyester hard segments have influence on the rate of synthesis, molecular weight, crystallization rate, elastic recovery, as well as tensile and tear strength. The hard segments structure affects primarily mechanical and processing properties, while the structure of the soft segments influences physical and chemical properties. It is known that the poly(ether ester)s... 

While the sample PTMO-2000-MDI-31-CA displays the existence of both crystalline and melting temperature transitions, no such activity is observed in PTMO-2000-TDI-31-CA. The Tg of the soft segments in TDI-based materials is lower than that of MDI-based materials, suggesting better phase separation in the latter. It appears that the unsymmetric configurations of TDI units cause the the formation of random hard segment structures which inhibit, but not necessarily eliminate, the crystallization of soft segments under identical thermal histories. 

Ryan A J, Bergstrom T B, Willkomm W R and Macosko C W (1991) Thermal, mechanical, and fracture properties of copolyureas formed by reaction injection molding Effects of hard segment structure, J Appl Polym Sci 42 1023-1039. 

In the case of the polycrystalline polyester thermoplastic rubbers the simple domain theory does not seem to apply. With these rubbers it would appear that they contain spherulitic structures consisting of 4GT radial lamellae with inter-radial amorphous regions that are mixtures of PTMEG soft segments and noncrystalline hard segments. 

Urethane structural adhesives have a morphology that is inverse to the toughened epoxy just described. The urethanes have a rubber continuous phase, with glass transition temperatures of approximately —50°C. This phase is referred to as the .soft segment . Often, a discontinuous plastic phase forms within the soft segment, and that plastic phase may even be partially crystalline. This is referred to as the hard segment . A representation of the morphology is shown in Fig. 3 [34]. 

The structural effect of alkyl groups such as methyl, ethyl, and -butyl on the Rp is small. Alkyl 4-methyl-phenylcarbamate can be chosen as a model compound for the hard segment of poly(ether-urethane) (PEU). This group can initiate grafting reaction with Ce(IV) ion and the grafting site was proposed at the hard segment of PEU [3,15] as shown in Scheme (1). 

Table 17. Structure of the hard segments in poly(Aryl ether)-polydimethylsiloxane segmented copolymers...

Morphology of the anionically synthesized triblock copolymers of polyfp-methyl-styrene) and PDMS and their derivatives obtained by the selective chlorination of the hard segments were investigated by TEM 146). Samples with low PDMS content (12%) showed spherical domains of PDMS in a poly(p-methylstyrene) matrix. Samples with nearly equimolar composition showed a continuous lamellar morphology. In both cases the domain structure was very fine, indicating sharp interfaces. Domain sizes were estimated to be of the order of 50-300 A. 

Figure 1.14. Schematic structure ofTPEs (a) Copolymer with hard segments arranged in domains (b) compound of rubber particles dispersed in thermoplastic matrix...

These materials have two-phase structures based on hard segments of polystyrene and soft segments of butadiene for SBS, or isoprene for SIS, etc. 

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