Polyurethane networks density

For imperfect epoxy-amine or polyoxypropylene-urethane networks (Mc=103-10 ), the front factor, A, in the rubber elasticity theories was always higher than the phantom value which may be due to a contribution by trapped entanglements. The crosslinking density of the networks was controlled by excess amine or hydroxyl groups, respectively, or by addition of monoepoxide. The reduced equilibrium moduli (equal to the concentration of elastically active network chains) of epoxy networks were the same in dry and swollen states and fitted equally well the theory with chemical contribution and A 1 or the phantom network value of A and a trapped entanglement contribution due to the similar shape of both contributions. For polyurethane networks from polyoxypro-pylene triol (M=2700), A 2 if only the chemical contribution was considered which could be explained by a trapped entanglement contribution. 

To determine if either of these answers were correct, the properties of a series of polyurethane networks which contained photo labile disulfide bonds were studied (7). The creep curve of the s imple in the initial crosslink density state (comparable to Vq) was determined in the usual way. Then, after complete recovery, another creep experiment was started but, at time t during this run, the sample was subjected to ultraviolet irradiation which photochemlcally broke some of the network chains. 

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. 

Stress Relaxation Polyurethane networks were also polymerized in a mold with a cylindrical cavity. Uniform rings were cut from the cylinders and weighed. The cross sectional area was then derived from the sample diameter and polymer density and approximated 0.05 cm2. Stress relaxation was measured at several strains between 10 and 43% with an Instron tensile tester. During stress relaxation, the samples were immersed in dioxane and swelled to equilibrium. 

Inordinately low M may result from an anomaly associated with polyurethane networks (12). The persistence of hydrogen bonding in the swollen state may lead to high apparent crosslink density and low values of Mc. In addition, it is possible that the Gaussian approximation implicit in all our treatments is inadequate for an extensively swollen network. 

TABLE 6.1 Influence of the Reinforcing Component on Equilibrium Force of Breakdown from Steel of Polyurethane Coatings with Various Network Densities... 

The Q value in the majority of solvents is below 50% (Table 6.5.3). The maximal sorption was obtained for segmented polyurethane with lower physical network density. Preliminary strain leads to increase in plasticizer sorption. 

Telechelic polymers rank among the oldest designed precursors. The position of reactive groups at the ends of a sequence of repeating units makes it possible to incorporate various chemical structures into the network (polyether, polyester, polyamide, aliphatic, cycloaliphatic or aromatic hydrocarbon, etc.). The cross-linking density can be controlled by the length of precursor chain and functionality of the crosslinker, by molar ratio of functional groups, or by addition of a monofunctional component. Formation of elastically inactive loops is usually weak. Typical polyurethane systems composed of a macromolecular triol and a diisocyanate are statistically simple and when different theories listed above are... 

In most cases the Ta values are related directly to the lignin content of the network. Soft segment-free networks show more scatter than those containing alkylether components, and this may be attributed to the crosslink density of the materials. The method of soft segment incorporation, simple addition or covalently bonded, has a minimal impact on Ts. A comparison of the HDI and aromatic isocyanate-crosslinked networks, at a given lignin content, shows that the HDI-crosslinked polyurethanes generally have a lower Tg than the aromatic isocyanate-crosslinked networks. 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

Creep behavior of a single sample at three different crosslink densities is depicted in Figure 4, vq being the virgin network sample which was crossllnked by irradiating for 30 minutes to yield which was further Irradiated 30 minutes to yield V2. In the previous work on polyurethane samples, values obtained from equation 3 were used to determine the rubber elasticity vertical... 

The network polyurethane obtained by reacting a diisocyanate R(N=C=0)2 with pentaerythritol C(CH20H)4 contains, according to elemental analysis, 0.2% (w/w) nitrogen and has a density of 1.05 g/cm. Determine (a) polymer chain segment density in mol/cm, and (b) molar mass of chain segments between branch points. 

Polyurethane foam was made by stepwise polymerization of methylenediphenyl isocyanate CH2(C6H4NCO)2 and 2-hydroxymethyl-2-ethyl-l,3-propanediol in the mole ratio of 1 0.7. Calculate the weight fraction solubles and the total crosslink density in the network if the extent of reaction of the isocyanate is 90%. 

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