Molecular weight determination polyurethane

Polyurethane networks were prepared from polyoxypropylene (POP) triols(Union Carbide Niax Polyols) after removal of water by azeotropic distillation with benzene. For Niax LHT 240, the number-average molecular weight determined by VPO was 710 and the number-average functionality fn, calculated from Mjj and the content of OH groupSj determined by using excess phenyl isocyanate and titration of unreacted phenyl isocyanate with dibutylamine, was 2.78 the content of residual water was 0.02 wt.-%. For the Niax LG-56, 1 =2630, fn=2.78, and the content of H2O was 0.02wt.-%. The triols were reacted with recrystallized 4,4"-diphenylmethane diisocyanate in the presence of 0.002 wt.-% dibutyltin dilaurate under exclusion of moisture at 80 C for 7 days. The molar ratio r0H = [OH]/ [NCO] varied between 1.0 and 1.8. For dry samples, the stress-strain dependences were measured at 60 C in nitrogen atmosphere. The relaxation was sufficiently fast and no extrapolation to infinite time was necessary. 

The functionality / of low-molecular-weight functionalized prepolymers (e.g., polyether polyols used in the preparation of polyurethanes) is often determined from Eq. (4.31) by combining functional group analysis with another suitable method of molecular weight determination such as vapor phase osmometry. Note that in this respect the functional groups do not have to be end groups. 

Molecular Weight Determination. For polyurethanes, the number average molecular weight was estimated by vapor phase osomometry (Perkin-Elmer-Coleman Model 115) on a 2% polymer solution in dimethyl formamide. For polyurethaneureas, number average molecular weight was estimated by GPC with a dilute polymer solution (0.25% in DMF, which contains 0.05M LiBr to prevent aggregation). 

Combined MALDI-MS and ion exclusion chromatographic techniques. In most of these techniques described in the literature the MALDI is not directly coupled off-line with the ion exclusion column. An exception is that of the work of Esser and co-workers [281], in which the two units are interfaced via a robotic interface. This technique was applied to studies of PS, PMMA, and butyl(methacrylate-methylmethacrylate) copolymers. Mehl and co-workers [282] combined ion exclusion with MALDI-MS to provide accurate molecular weight determinations on polyether and polyester polyurethane soft blocks. 

Selective degradation reactions combined with MALDI analysis were applied for molecular weight determination of polyether and polyester polyurethane soft blocks. Size exclusion chromatography was combined with MALDI to provide accurate molecular weight determination. 44 refs. USA... 

Off-line coupling of HPLC with FD-MS has been used by several authors [118-121] for the determination of oligomers, oligomeric antioxidants (such as poly-TMDQ), ozonation and vulcanisation products. Pausch [122] reported on rubbers, cyclic polyurethane oligomers, as well as on the determination of the molecular weight distribution (up to 5300 Da) and oligomer analysis of polystyrene. Also the components of an aniline-acetone resin were deduced from FD-MS molecular weights [122]. 

The molecular weight distribution (MWD) of the linear polyurethanes were determined by GPC. The solvent used was THF and the instrument calibrated by narrow MWD polystyrenes. Polymer BPUla... 

Much work has been done on the incorporation of castor oil into polyurethane formulations, including flexible foams [64], rigid foams [65], and elastomers [66]. Castor oil derivatives have also been investigated, by the isolation of methyl ricinoleate from castor oil, in a fashion similar to that used for the preparation of biodiesel. The methyl ricinoleate is then transesterified to a synthetic triol, and the chain simultaneously extended by homo-polymerization to provide polyols of 1,000, 000 molecular weight. Polyurethane elastomers were then prepared by reaction with MDl. It was determined that lower hardness and tensile/elongation properties could be related to the formation of cyclization products that are common to polyester polyols, or could be due to monomer dehydration, which is a known side reaction of ricinoleic acid [67]. Both side reactions limit the growth of polyol molecular weight. 

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. 

Using model compounds, the surface composition of polyurethane materials of unknown con osition can be identified. Large differences in surface structure are observed for polyurethanes. Also, the surface structure is sensitive to extraction and cleaning procedures. Some of these changes may be related to processing additives in the commercial grade polyurethanes used or to low molecular weight polyurethanes. Future experiments will look at carefully synthesized polyetherurethanes of known composition to relate bulk and surface structure. Also, extracts will be further analyzed by ESCA, GPC and IR to determine their structure. 

Given the above information, it is usually possible to determine likely end-uses for the prepolymer. Commercial supphers are not anxious to divulge full characterisations. Generally, functionality and particularly molecular weight information are withheld. Conversely, for a known end-use the polyurethane synthetic chemist designs a suitable characterisation and formulation for the urethane prepolymer. 

To conclude, the common physico-chemical characteristics of oligo-polyols for polyurethanes determined by standard analytical methods are hydroxyl number, hydroxyl percentage, primary hydroxyl content, molecular weight, equivalent weight, molecular weight distribution, viscosity, specific gravity, acidity and colour (See Chapters 3.1-3.11). 

Polyurethanes were first suggested for use as biomaterials in 1967 [36]. Polyurethane materials have excellent mechanical properties, making them suitable for many different biomedical applications. Currently, a variety of polyurethanes are used in biomedical devices like coatings for catheters and pacemaker leads (Table A.2). The biocompatibility of biomedical polyurethanes appears to be determined by their purity i.e., the effectiveness of the removal from the polymer of catalyst residues and low molecular weight oligomers [37]. The surface properties of commercially available polyurethanes, which are critically important in determining biocompatibility, can vary considerably, even among lots of the same commercially available preparation [38]. 

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