Trifunctional polyurethane networks

Figure 10. Tg versus ac for dry, trifunctional polyurethane networks, (26). Reaction systems MDI/LHT240 (system 3 of Figure 9), Mc° is 710 g/mol, v is 30, at various initial dilutions of reactants. - - is MDI/POP diol. Mrepeat = M0°.

Gel Point and Shear Modulus. Trifunctional and tetrafunc-tional polyurethane(25,26,28) and trifunctional polyester net-works (32) have been studied. The gelation data for the reaction systems forming the polyurethane networks were those discussed with reference to Figure 6 and Table II. 

The deviations from Gaussian stress-strain behaviour for the tetrafunctional polyurethane networks of Figure 9 are qualitatively similar to these found for the trifunctional polyester networks (Z5), and the error bars on the data points for systems 4 and 5 in Figure 9 indicate the resulting uncertainties in Mc/Mc. It is clear that such uncetainties do not mask the increases in Mc/Mc with amount of pre-gel intramolecular reaction. 

The reactants used to form the networks studied are generally of lower molar mass than those used by other workers to form perfect networks (e.g. (35)). However, the present results do indicate that very little pre-gel intramolecular reaction can have a marked effect on modulus. For example, for pr,c = 0.05, or ac = 0.58, with a trifunctional polyurethane-forming system of Me = 635g mol l, the modulus is reduced by a factor of five below that calculated on the basis of the small-strain(affine) behaviour of the perfect network. As a result, it is recommended that the effective absence of pre-gel intramolecular reaction in endlinking reactions to form perfect networks be confirmed by experiment rather than be assumed. 

The functionality of precursors varying between/ = 2 and/ = 6 is considered to be low (Figure 5.2). Polyurethane networks prepared from bifunctional telechelics and trifunctional triisocyanates, diepoxide (f = 2)-diamine (f = 4) systems, diepoxide if = 4)-cyclic anhydride (/ = 2) systems, phenol (/ = 3)-formalde-hyde if = 4) resins, or melamine (/ = 6)-formaldehyde (/ = 2) resins are in this category. 

It is shown that model, end-linked networks cannot be perfect networks. Simply from the mechanism of formation, post-gel intramolecular reaction must occur and some of this leads to the formation of inelastic loops. Data on the small-strain, shear moduli of trifunctional and tetrafunctional polyurethane networks from polyols of various molar masses, and the extents of reaction at gelation occurring during their formation are considered in more detail than hitherto. The networks, prepared in bulk and at various dilutions in solvent, show extents of reaction at gelation which indicate pre-gel intramolecular reaction and small-strain moduli which are lower than those expected for perfect network structures. From the systematic variations of moduli and gel points with dilution of preparation, it is deduced that the networks follow affine behaviour at small strains and that even in the limit of no pre-gel intramolecular reaction, the occurrence of post-gel intramolecular reaction means that network defects still occur. In addition, from the variation of defects with polyol molar mass it is demonstrated that defects will still persist in the limit of infinite molar mass. In this limit, theoretical arguments are used to define the minimal significant structures which must be considered for the definition of the properties and structures of real networks. 

Network Synthesis (4) Solid MDI was weighed into a flask and an equivalent amount of polyol added. The mixture was heated to about 40°C to dissolve the MDI. The mixture was then cooled to room temperature and degassed for several minutes under vacuum in order to remove dissolved air. Catalyst was then added and the contents of the flask mixed under vacuum to ensure uniformity and then poured into a mold. All operations were carried out in a dry glove bag to minimize reaction with atmospheric water. The cross-linking process was also carried out in dioxane solution at 70% volume fraction of solids. Polyurethane networks with different crosslink densities were prepared by varying the ratio of difunctional and trifunctional polyols. All samples were extracted with dioxane to remove unreacted and uncrosslinked materialbefore swelling. 

Experimental results on reactions forming tri- and tetrafunctional polyurethane and trifunctional polyester networks are discussed with particular consideration of intramolecular reaction and its effect on shear modulus of the networks formed at complete reaction. The amount of pre-gel intramolecular reaction is shown to be significant for non-linear polymerisations, even for reactions in bulk. Gel-points are delayed by an amount which depends on the dilution of a reaction system and the functionalities and chain structures of the reactants. Shear moduli are generally markedly lower than those expected for the perfect networks corresponding to the various reaction systems, and are shown empirically to be closely related to amounts of pre-gel intramolecular reaction. Deviations from Gaussian stress-strain behaviour are reported which relate to the low molar-mass of chains between junction points. 

Using this technique, a large variety of polyurethanes have been prepared from different vegetable oils. Natural polyols like castor oil (generally trifunctional) are directly reacted with diisocyanates to obtain branched polyurethanes, although it is difficult to control the reactivity. However, bifunctional castor oil can be polymerised with diisocyanates in the presence of suitable chain extenders and catalysts to produce polyurethanes in a more controlled manner (Fig. 6.4). A castor oil polyol-based polyurethane network can also be prepared from epoxy terminated polyurethane pre-polymer with 1,6-hexamethylene diamine. Epoxy terminated pre-polymer is obtained by the reaction of glycidol and isocyanate terminated polyurethane pre-polymer of castor oil polyol, poly(ethylene glycol) (PEG) and 1,6-hexamethylene diisocyanate. ... 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

Where a step polymerization is used, almost always it is for the first polymer synthesized in a sequential IPN. The reasons involve the slow diffusion into a pre-existing network of most monomers used in step polymerization, and the relatively high glass transition of step polymerized polymers. The latter reason is important because in order for diffusion and concomitant polymerization to occur rapidly, polymer network 1 should be above its glass transition at the temperature of polymerization of monomer mix 11. Table 6.2 presents glycerol as a simple trifunctional crosshnker for step polymerized materials, suitable for polyesters and polyurethanes. 

Chitin, one of the most abundantly available natural polysaccharides, can be readily modified by acetylation reactions to form chitosan (CHI) [37]. CHI was modified with the four-member oxetane (OXE), followed by cross-Hnking with trifunctional hexamethylene di-isocyanate (HDI) in the presence of polyethylene glycol to form heterogeneous cross-linked polyurethane (PUR) network. When there is mechanical damage of the network, self-repairing occurs upon exposure of the damaged area to UV radiation. While the reactions leading to network formation are shown... 

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