Polyurethanes macrodiols

The melt temperature of a polyurethane is important for processibiUty. Melting should occur well below the decomposition temperature. Below the glass-transition temperature the molecular motion is frozen, and the material is only able to undergo small-scale elastic deformations. For amorphous polyurethane elastomers, the T of the soft segment is ca —50 to —60 " C, whereas for the amorphous hard segment, T is in the 20—100°C range. The T and T of the mote common macrodiols used in the manufacture of TPU are Hsted in Table 2. 

Martin D.J., Warren L.A.P., Gunatillake P.A., McCarthy S.J., Meijs G.F., and Schindhelm, K. Polydimethylsiloxane/polyether-mixed macrodiol-based polyurethane elastomer. Biomaterials, 21, 1021, 2000. 

These structures are well defined by conducting the polymerization in the presence of appropriate mono- and difunctional reagents. They are of considerable interest for the preparation of segmented block copolymers.24,25 For instance, the fluorinated macrodiols 21 have already been used to prepare an interesting new series of partially fluorinated segmented polyurethanes,26 and we are investigating other novel polymers that can be prepared from these intermediates. 

The block lengths and the final polymer molecular weight are again determined by the details of the prepolymer synthesis and its subsequent polymerization. An often-used variation of the one-prepolymer method is to react the macrodiol with excess diisocyanate to form an isocyanate-terminated prepolymer. The latter is then chain-extended (i.e., increased in molecular weight) by reaction with a diol. The one- and two-prepolymer methods can in principle yield exactly the same final block copolymer. However, the dispersity of the polyurethane block length (m is an average value as are n and p) is usually narrower when the two-prepolymer method is used. 

In this case of three-monomer polyurethane synthesis, there is no thermodynamic driving force for phase separation. The formation of clusters is fully controlled by the initial composition of the system, the reactivity of functional groups, and the network formation history (one or two stages, macrodiol or triol reacted with diisocyanate first, etc.). 

In the case of Fig. 7.6a the cluster formation and the size distribution can be influenced not only by chemical reactions but also by partial miscibility of the substructures during reaction. Polyurethane networks prepared from polyolefin instead of polyester or polyether as macrodiol, can serve as an example. In this particular case an agglomeration of hard domains takes place in the pregel stage, produced by a thermodynamic driving force. 

The processes described in Table 2 present a peculiar interest in the working out of new materials as polyurethanes. These last polymers are very often based on macro diols coming from polyethers or polyesters, a-co functional polyolefins being relatively uncommon. Hence, Rhein and Ingham [139] prepared macrodiols by ozonization of polyisobutylene in CC14 at... 

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.)...

Polyurethanes with a few chromophoric hard segment units which do not show phase separation can be obtained by a polyaddition reaction of only a macrodiol and a diisocyanate in nearly stochiometric amount (slight excess of diisocyanate) together with a small amount of a difunctional chromophore (DAAB), counterbalancing the diisocyanate excess. The structure of such polyurethanes investigated here are given in Fig. 13. 

D. Martin, L. Warren, P. Gunatillake, S. McCarthy, G. Meijs, K, Schindhelm, Polydimethylsiloxane/polyether-mixed macrodiol-based polyurethane elastomers biostability. Biomaterials 21 (10) (2000) 1021-1029. 

A. Simmons, J. Hyvarinen, R. Odell, D. Martin, P. Gunatillake, K. Noble, L. Poole-Warren, Long-term in vivo biostability of poly(dimethylsiloxane)/poly(hexamethylene oxide) mixed macrodiol-based polyurethane elastomers. Biomaterials 25 (20) (2004) 4887-4900. 

R. Adhikari, PA. Gunatillake, S.J. McCarthy, G.F. Meijs, Effect of chain extender structure on the properties and morphology of polyurethanes based on H12MD1 and mixed macrodiols (PDMS-PHMO), J. Appl. Polym. Sci. 74 (12) (1999) 2979-2989. 

Polyurethane elastomers, prepared from (i) polyether macrodiols that contain a reduced number of ether linkages compared with PTMO [poly(tetramethylene oxide)],... 

Segmented polyurethane elastomers, prepared from diisocyanates, macrodiols and chain extenders, are frequently used in the construction of implantable medical devices, such as cardiac pacemakers, heart valves, catheters, and heart assist devices, because of their excellent mechanical properties and haemocompatibility. Polyether macrodiols, such as poly(tetramethylene oxide) (PTMO), are used to prepare polyurethanes for implant, since they offer an increased resistance to enzymatic hydrolysis compared with polyester-based polyurethanes. 

We have prepared novel polyurethane elastomers with modified soft segment chemistry that has been designed to enhance stability. The new tliermoplastic polyurethanes are based on conventional diisocyanates and chain extenders, but contain macrodiols with a reduced number of ether linkages when compared with PTMO. As part of the development process, improved synthetic procedures involving condensation polymerization were devised (8,9) to prepare the macrodiols, poly(hexamethylene oxide) 1, poly(octamethylene oxide) 2, and poly(decamethylene oxide) 3. 

All polyurethanes reported herein were prepared by two step solution polymerization (10), with a diisocyanate macrodiol chain extender ratio of 2.5 1.0 1.5. 

A series of pH-sensitive polyurethanes were synthesized using pH-sensitive macrodiol, shown above, and a tripeptide chain extender. The obtained polyurethane had a number average molecular weights of 4,500-6,200. The material cleaves in acid media (pH 4-6) and degrades in enzymatic solution. 

This medical grade TPE polyurethane is the reaction product of an aliphatic diisocyanate, a polycarbonate-based macrodiol, and a chain terminating low molecular weight diol (Tables 4.4, 4.12, and 4.13). 

A series of polyurethane-ureas (PUUs) has been synthesized by us, based on the aromatic diisocyanates DBDI or MDI. The materials were synthesized with two SS macrodiols, the polyether macrodiol PTMO or the polyester SS, PEA. 

The introduction of the UV-stabilizer in PUs was made by different ways. The first kind of stabilized polymer (a) was obtained by adding a solution of BHMBE in DMF (cone. = 4.34 g/100 ml) under stirring into the melted prepolymer. The solvent DMF was then removed by vacuum distillation, and the chain extender was added. A polyurethane containing 0.58% BHMBE was obtained (Fig. 4.60). A PU sample obtained by introducing the powdered UV-stabilizer in the macrodiol directly (forming a dispersed phase) presented poor mechanical properties because of their less homogeneous structure [64]. 

Numerous polymers have been proposed as shape-memory polymers (SMPs), and many of them are based on polyurethanes. This is because of the intrinsic versatility of segmented copolyurethane systems. By suitable choice of diisocyanate and macrodiol, a wide variation in properties may be obtained, allowing the possibility of tuning the shape-memory response to suit different applications. Usually they are phase-segregated materials. For example, a dispersed rigid phase (usually based on the diisocyanate) provides physical crosslinks, while the macrodiol provides a soft amorphous phase with low glass transition that provides the trigger temperature for shape recovery [63]. 

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