Nodular morphology

Dyn ic mechanical analysis was discussed in terms of the nodular morphology concept in crossllnked structures. Beta relaxations in all the cured resins were bimodal in appearance. But, vAiile MPD-cured resins shewed a maximum at 25 C with a smaller shoulder at -40 C, TDA and DAEB-cured resins had maxima at -40 C with a less significant peak at 25 C. For DAIPB and DATBB-cured resins the two peaks were approximately equal in magnitude. The two overlapping peaks at -40 and 25 C were attributed to crankshaft motions in the matrix and nodules. 

Table 3.2 shows the influence of the initial PEI concentration on the resulting particle size distribution. Both the size and the number of PEI domains increased in the sea-island morphology, whereas the size of thermoset nodule decreased and the number of nodules increased in the nodular morphology as the initial PEI concentration was increased. 

In the composition range showing the nodular morphology, the increase in the PEI concentration increased the viscosity of the system and the PEI phase volume, thus reducing the rate of coalescence of the epoxy nodules in the late stage of spinodal phase separation. Smaller epoxy nodules, therefore, were formed at higher PEI concentration. 

In the composition range where the nodular morphology was shown, because the matrix (PEI phase) contained relatively small amount of epoxy, the temperature effect on the viscosity was larger than the reaction effect. Thus, the viscosity of the PEI matrix in the nodular morphology became reduced as the cure temperature was increased, which made epoxy nodules coalesce more easily with each other. As the cure temperature is increased, the viscosity of the PEI-rich matrix decreases from 210 Pa.s at 150°C to 50 Pa.s at 190°C. 

Network defects in the form of unreacted groups serve as sites for crack initiation and propagation. When such defects are non-randomly distributed within the network a nodular morphology will be observed upon fracture or chemical etching of the bulk network. 

A non-random distribution of defects in the network, that produces the observed nodular morphologies in epoxies, results in weak paths in the network that favor crack propagation. 

There have been attempts to relate the assumed nodular morphology with the physical properties of networks (Labana et al., 1971). This point is important, because if crosslinked polymers are considered as homogeneous three-dimensional structures, their ultimate properties can be related to the properties of such a continuum. On the other hand, if they are inhomogeneous, the supramolecular structure shown in Fig. 7.1 provides a more fruitful approach to interpreting of macroscopic properties. 

Mijovic, J. Tsay, L.L. Correlations between dynamic mechanical properties and nodular morphology of cured epoxy resins. Polymer... 

Typical nodular morphology of the Mo—Ni alloy surface [35] is shown in Fig. 7.24a, while cross section (Fig. 7.24b) revealed the presence of large cracks in the electrodeposit [36]. Such behavior was characteristic for aU Mo—Ni alloys independently of the solution composition and applied current density. 

These qualitative deductions, which are based on considering a smooth surface, lead to a contradiction as the polymer surface-treated before metalization appears to be oxidized over an appreciable depth whilst the oxidized polymer layer left on the metal side after the rupture does not screen completely aluminum. Actually an AFM examination has shown that the surfaces obtained after rupture are not smooth. They present a nodular morphology with a rms roughness (measured on 2 x 2 pm ) in the range of 3-7 nm, i.e., of the order of the IMFP. Further analysis of the results would require modeling, nsing the kind of approach described below. 

Rico et al. have observed the influence of some of these variables on the epoxy/polyester blends morphology. Three different structures can be formed depending on modifier concentration, sea-island, dual-structure and nodular morphology. The former was observed to show a low modifier amount (until 6% in this case), the dual-structure to show intermediate amounts (9-12%) and the latter to show modifier amounts of up to 15%. 

There is a modifier critical concentration for each polymer blend. Up to this concentration, the modifier acts as a matrix, generating the nodular morphology [30]. 

Mei et al. [64] studied the effect of hydrophilic silica nanoparticles (SiOj) on the shape stability of crystallizable polybutylene terephthalate (PBT) fibrils in polystyrene (PS) mattix under quiescent and shear conditions. Upon the addition of only 0.05 wt% SiO nanopariicles, the shape stability of PBT fibrils during quiescent annealing increased noticeably due to the rapid crystalhzation in these filled PBT droplets. With increased nanoparticle content, an enhanced viscoelasticity of PBT droplets was observed, due to the addition of nanoparticles, which began to play a role in improving the shape stability of droplets. The nanoparticles were also found to suppress the development of nodular morphology on PBT fibrils under shear flow caused by heterogeneous crystalhzation. 

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