4,4’-DBDI

A series of intermediates (Fig. 1.12) obtained by us was considered. They were synthesized with two identical functional groups situated in different rings 2,2 -DBDI, 2,4 -DBDI and 4,4 -DBDI as described elsewhere [45]. A study was made by us on the reactivity of 2,2 -, 2,4 -, and 4,4 -DBDI with n-butanol in benzene. 

In the case of 2,2 -DBDI, geometric effects were clearly evident. Reactivity was influenced by the steric effects wich lead to significant intramolecular catalytic activity. These effects were responsible for the whole reaction pattern. 

This is one of the reasons for which in the present book we refered only to the 4,4 -DBDI isomer. Another thing is that from our earlier studies, we could onclude that the utilization of this isomer resulted in much better mechanical propeties than in the case in which 2,2 -DBDI or 2,4 -DBDI were employed in the PUs synthesis. 

For comparison reasons and not only— as you will see, in the present book, considerable attention has been given to the aromatic isocyanate of variable geometry 4,4 -dibenzyl diisocyanate (DBDI) [59-63], which (as shown in the preface of this book), is a relatively novel diisocyanate, now available commercially. 

Fig. 1.8 Examples of commonly used aromatic and aliphatic diisocyanates all of them are rigid also included is the flexible 4,4 -dibenzyl diisocyanate (DBDI) (produced in our Romanian laboratory), in the extended linear anti and contorted syn forms.

Our research reported in this book is mainly focused on materials obtained with aromatic diisocyanates MDI and DBDI. To compare the DBDI materials, I have... 

The molar fraction (-FN) of unreacted diisocyanate was determined as a function of the reduced time t (Fig. 1.13) [44]. Due to the changes in the rate constants during reaction, both the initial and average rate-constant values were calculated. The rate constants of the initial and average stages of the process and the values of the rate-constant ratios were determined. These values corresponded to the reactions of the first and second NCO groups of the symmetrical DBDI isocyanates (2,2 and 4,4 ) and to those of the 4,4 -MDI reaction. Note that the reactivity values of 4,4 -DBDI... 

From the reactivity standpoint, 2,4 -DBDI was similar to 2,4 -TDI, although the selectivity of the NCO groups at the ortho and para positions was found to be lower than that of 2,4 -TDI. The former material has an important practical implication since it is a liquid with a low vapour pressure and therefore reduces the risk of inhalation. 

In a second series of investigations [44-46], Caraculacu et al., studied the reactivity of the functional groups 4,4 -DBDI. The rate constants of the reactions of diisocyanates with n-butanol were determined by using the IR spectophotometric method of Bailey [99]. The rate constants of the consecutive reactions were determined by the time ratio method developed by Frost and Pearson [100]. 

In the case of 4,4 -MDI and of 4,4 -DBDI, the activating influence of the isocyanate and urethane groups, respectively, was transmitted by an inductive mechanism through the methylene and ethylene groups. Simultaneously, the methylene and ethylene groups had a deactivating influence. The balance among these... 

By studying the reactivity of 4,4 -DBDI and comparing it with other conventional diisocyanates, it observed that the reactivity of 4,4 -DBDI was very similar to that of the diisocyanate 4,4 -MDI. 

For simplification reasons, in the present book, we denoted 4,4 -DBDI as DBDI, as long as this was the only isomer which we used in the synthesis of the materials that make the subject of this book. Also for simplification reasons we denoted 4,4 -MDIasMDI. 

Crystallinity has been observed in the soft phase when the macrodiol chain is long enough, and it is also sometimes present in the hard phase. The latter is usually limited to only a few percent for most HS structures when solidified from the melt, but there is one particular diisocyanate, 4,4 -dibenzyl diisocyanate (DBDI) that, in the presence of suitable chain extender, gives rise to significant degrees of crystallinity [60,61,135], and this is included and detailed in the present book. 

As reported by ourselves [60, 61, 135] and by Lyman and Gowerr [58, 202], the MDI molecule introduces the rigid -Ph-CH2 Ph- moiety in the elastomeric PU hard segments. In contrast when using DBDI, the specific -Ph-CH2-CH2-Ph-moiety introduces a variable geometry into the hard segments due to the possibility of internal rotation of this isocyanate around the -CH2-CH2- ethylene bridge. 

This leads to the appearance of both anti and syn rotational conformations, which coexist in the DBDI based PU macromolecules, (Fig. 2.4-2.6). As a result, in this latter case the PU macromolecules can adopt a more compact packing which enhances significantly the ability to order in crystalline structures involving predominantly the anti form [60]. Shown in Fig. 2.4 and 2.5 are the extended linear anti and contorted syn DBDI positions as compared to the conventional rigid 4,4-diphenylmethane diisocyanate (MDI) non-crystallizing (Fig. 2.6). 

Shown in Fig. 2.8 (a) and (b) are example microphotos of the two-phase structure for two analogous materials based on PTHF of molar mass 2 000 g/mol and chain extended with BDO. They differ only in the type of diisocyanate, the rigid MDI or DBDI displaying a variable geometry. Transmission optical microscopy standard method was used to investigate the microstructures. 

In the DBDI based polymers there was a clear indication from data that the physical origin of the flow stress must be relative displacement of the hydrogen-bonded HS [61, 135, 175], In deformed PUs, the anisotropy of the structures relative to the strain direction was clearly visible in the 2D images (Fig. 2.9). Samples were... 

Fig. 2.8 Example two-phase structures (a) MDI BDO PTHF (b) DBDI BDO PTHF at 1 000 yum magnification...

Strained to up to 300% levels of extension. Depending on the type of diisocyanate (crystallizing or not), they showed various degrees of anisotropy, with the highest values corresponding to the PTHF/DBDI based materials. The macroscopic deformation was present in the molecular levels that were probed with neutrons. 

There were tendencies to phase separation, with a characteristic length of ca 20 nm, and, when DBDI was employed with BG or EG, to crystallization of the hard phase through its effect of increasing the flow stress. 

The SEM investigation made by us clealy revealed the strong influence of the variable geometry of the diisocyanate DBDI on the formation of hard domains with a pronounced crystallinity, especially when EG or BDO were employed as chain... 

Fig. 2.11 Topographical and phase contrast AFM images for freeze fracture surfaces for two analogous materials they differ only in the type of diisocyanate, (a) (DBDI BDO PTHF) (b) MDI BDO PTHF. Courtesy by Mrs. I. Stoica, Institute of Macromolecular Chemistry, Petru Poni, Iasi, Romania...

The materials displayed a relatively coarse structure on 10 //m scale but which varied between the polymers. Fig. 2.12 displays the features of a DBDI based original sheet. At 0.1 mm magnification the arrow points to a particularly prominent area of the banding which is typical of this specimen. The most prominent feature is what is referred to as the coarse structure of about 10 yum in scale this is due... 

The most prominent feature is what is referred to as the coarse structure which is due to large extent of phases separation in the polymers derived from DBDI. As previously shown, the dibenzyl based polymers did not retract after rupture [204]. After step stretching of the materials, the presence of the dibenzyl structures gave a much rougher surface morphology than that observed in the MDI based polymers [204]. 

Several chemical etching techniques were tried on these materials. In Fig. 2.14, etched surfaces of MDI and DBDI based materials are shown. 

In the materials with DBDI (Fig. 2.14(b)) the overall etched texture is much flatter. The regions of chemical segregation are more or less equally attacked. The material with DBDI does not show the precipitated crystals. A more detailed SEM description on the morphology of MDI and DBDI based PUs and mixtures of them, is made in section 2.3.2.2. where the materials are characterized and compared from two perspectives (a) effect of the SS macrodiol nature (polyester or polyether) (b) effect of type, and number of diisocyanates (crystallizing or not) and their order of introduction in the reaction synthesis. 

A systematic study of PUs with HS of variable crystallinity was made by us [60, 61, 127, 135]. Two diisocyanates were considered the frequently employed MDI, and its close relation DBDI, that is of special interest because of its tendency to crystallize on cooling from the melt in the presence of some chain extenders [135]. The family of model PUs was synthesized for this work in the authors Romanian laboratory. They were all three-component systems combined in stoichiometric proportions, and consisting of (1) a diisocyanate—either MDI or DBDI (2) a SS macrodiol—PEA, PTHF, or PBA and (3) a small molecule diol as chain... 

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