Urethane segments

Siloxane-urethane segmented copolymers, which have very good mechanical, fatigue and surface properties 370,377 or their blends with conventional polyurethane-(ureas) have been successfully used in the production of blood pumps, intra-aortic balloons and artificial hearts 200,332,370,376,377). 

The thicker lines represent the sequences of hard urethane segments, and the clusters of these effectively act as crosslinks, making the material act as a conventional elastomer. When the temperature is raised high enough, the clusters disassociate and the material can be made to flow when subsequently cooled, the clusters can reform and the material again exhibits elastomeric properties. Thus these materials show elastomeric behaviour at room temperature, but can be processed as thermoplastics. Hence the name of the material class - thermoplastic elastomers. 

It is important to recognize that these correlations only apply to a specific polymer and, as discussed above, will be sensitive to changes in the polymer crystallinity, the inclusion of filler, and the exact chemical composition. The sensitivity of solubility in polydimethylsiloxane to the filler content has been noted (14 15) and the correlation in Table III for PDMS applies ony to the unfilled fluid. The crystallinity of many polymers depends on their molecular weight, and may change if the polymer is subject to biodegradation. The solubility parameter, i.e. the polarity, of polyurethanes, is sensitive to the nature and ratio of the ether (or ester) and urethane segments. 

This reaction is often employed in the production of flexible urethane foams, which are frequently block copolymers of polyether or polyester segments joined to polyurea segments. (The polyester or polyether segments terminate in urethane segments resulting from reaction of polyether or polyester hydroxyl end groups with the isocyanate.)... 

Multiple methyl substituents in TMDI inhibit intermolecular associations of the urethane segments derived from these building blocks and hence promote flexibility in the resulting PURs. An interesting heterodifunctional isocyanate is Dow s isocyanatoethyl methacrylate (IEM)... 

Polyurethane elastomers derive their elastomeric properties from phase separation of the hard and soft copolymer segments, such that the hard (urethane) segment domains serve as crosslinks between the amorphous soft segment domains, which are usually polyesters or polyethers. We are interested in the systems in which the hard segments are prepared from diphenyl-methane 4,4 -diisocyanate (MDI) with a linear diol as the chain extender ... 

The superior characteristics of polyester polyol based polyurethanes are explained by a better crystalline structure [1, 7] in the urethane segment, compared to the majority of poly ether polyols which are amorphous [except polytetrahydrofuran (PTHF)], due to the superior secondary forces between the polyester chains [8] and also due to a superior thermal and fire resistance, compared to polyether polyol based polyurethanes. Polyester-based polyurethanes (flexible foams, coatings), have a superior solvent resistance compared to the polyether-based polyurethanes [8]. 

Zhuang, H., Gardella, J.A. Jr., and Hercules, D.M., Determination of the Distribution of Poly(dimethylsiloxane) Segment Lengths at the Surface of Poly[(dim-ethylsiloxane)-urethane]-Segmented Copolymers by Time-of-Flight Secondary Ion Mass Spectrometry, Macromolecules, 30, 1153 (1997). 

Pike, J.K., Ho, T., Wynne, K.J. Water-induced surface rearrangements of poly(dimethylsi-loxane—urea—urethane) segmented block copolymers. Chem. Mater. 8, 856-860 (1996)... 

The applicability of infrared dichroism to the study of PUs has been determined by the availability of at least one weU-characterized absorption band in each type of domain. It has been shown [67, 140, 364] that the soft domains are comprised of prepolymer (polyether or polyester) segments, while the hard domains contain urethane segments. The N-H stretching which is located in the urethane linkage characterizes the orientation of the higher modulus hard domains. For the DBDI polymers, we have shown that it appears at 3 320 cm [67]. Similarly, the asymmetric C-H stretching absorption was determined. It appears at 2 940 cm and was used to describe the orientation of prepolymer segments which comprise the rubbery soft domains. 

So far in this chapter on sequencing, two techniques have predominated NMR spectroscopy and Py-GC. An exception is the work of Zhuang and co-workers [67] who used ToF-SIMS in their study of the distribution of polydimethylsiloxane (PDMS) segment lengths at the surface of PDMS-urethane-segmented copolymers. Their aim was to establish whether, at the copolymer surface, the distribution of segment or chain lengths is different from that in the bulk of the polymer. 

The hard segments are relatively stable towards thermal oxidation. Degradation of the urethane segment typically does not occur below its thermolytic decomposition temperature. Aromatic isocyanates are generally less susceptible to thermal degradation than aliphatic isocyanates [1]. 

A typical procedure followed in the preparation of siloxane-urethane segmented copolymers is as follows Calculated amounts of HMDI and PDMS oligomer are introduced into the reaction flask, stirred and heated up to about 60°C. This mixture is not miscible. However, when 0,5 mL of 1.0% DBTDL solution in toluene is added, the mixture turns clear in about one minute indicating a fairly fast reaction between PDMS and HMDI. There is also a dramatic increase in the reaction temperature from 60 to about 90°C, typical for very exothermic urethane formation reaction. Prepolymer formation is followed by FTIR, monitoring the disappearance of the broad hycfroxyl peak centered around 3300 cm Prepolymer obtained is then diluted with THF to about 50% solids and heated to reflux temperature of 64.5 C. Chain extender, BD, is dissolved in THF and added dropwise into the reaction mixture. As the system became viscous as a result of the chain extension reaction, it is diluted with THF and DMF to prevent the premature precipitation of the copolymer formed, which is indicated by the formation of a cloudy solution. Reaction was continued until the complete disappearance of sharp isocyanate peak around 2250 cm in the FTIR... 

Reaction scheme followed in the preparation of PDMS-urethane segmented... 

Table I gives the compositions of silicone-urethane and pob ether-urethane segmented copolymers prepared in this study.

Table II. Characteristics of Siloxane-Urethane and Polyether-Urethane Segmented Copolymers...

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