Final morphology

In preliminary tests, melt mixed blends of PP and LCP were processed at six different temperatures (Tcyi 230, 240, 250, 260, 270, and 280°C) with a Brabender Plasti-Corder PLE 651 laboratory single-screw extruder. The measured melt temperatures were about 10°C higher than the cylinder temperatures (Tcyi). The objective was to study the influence of temperature on the size and shape of the dispersed LCP phase. Two different polypropylenes were used to ascertain the effect of the viscosity of the matrix on the final morphology. Different draw ratios were obtained by varying the speed of the take-up machine. 

Fig. 53 Different possible scenarios during formation of knitting pattern morphology. Upper row. PEB is microphase separated, PS and PMMA are still dissolved in chloroform lower row. final morphology after removal of solvent. From [166], Copyright 2001 American Chemical Society... 

In the time interval between phase inversion and gelation of the polystyrene continuous phase, the final morphological features such as size average and size distribution of elastomer domains become fixed. Since these morphological changes affect properties such as modulus and impact resistance, the characteristics of the system at and just after phase inversion and before gelation demand the closest scrutiny. The open time interval was found to decrease as the polyester prepolymer content increases, probably because higher polystyrene conversions are required for the system to reach suitable phase inversion conditions. 

Based on Eqs. (42) and (43), the development of a narrow or bimodal size distribution can be qualitatively explained without the detailed knowledge of the real phase diagram nor the exact dependency of the diffusion constant as a function of time. The final morphology depends mainly on the extent of reaction at which the metastable region is entered and the difference between ( )p and ( )o, as discussed below. 

These phenomena cannot be treated Independently, Consequently, the morphology of IPN s is often at a quasi-equlllbrlum state determined by a balance among the several kinetic factors [ l Therefore, in order to understand the domain formation process in IPN s, we should take into consideration the route taken to the final morphology as well as the chemical and physical properties of each constituent. 

In order to understand the domain formation process, an investigation of the Intermediate stages before formation of the final morphology is required. There are several different ways to prepare such intermediate materials [3,A2,A3], see Figure 9. The characteristic domain dimensions of PB/PS IPN s are compared in Figures 10 and 11 [3,12,A1]. 

Whatever the precursor, the formation of an intermediate solid phase was always observed. It can be inferred from X-ray diffraction (Fig. 9.2.7) and infrared spectroscopy that this intermediate phase shows a lamellar, incompletely ordered structure (turbostratic structure) built up with parallel and equidistant sheets like those involved in the lamellar structure of the well-crystallized hydroxides Ni(OH)2 or Co(OH)2, these sheets are disoriented with intercalation of polyol molecules and partial substitution of hydroxide ions by alkoxy ions (29). The dissolution of this solid phase, which acts as a reservoir for the M(I1) solvated species, controls the concentration of these species and then plays a significant role in the control of the nucleation of the metal particles and therefore of their final morphological characteristics. For instance, starting from cobalt or nickel hydroxide as precursor in ethylene glycol, the reaction proceeds according to the following scheme (8) ... 

The final morphology of specimens cured at different curing temperatures and composition was observed by SEM. Fractured surfaces of postcured specimens prepared in liquid nitrogen, were etched with methylene chloride before examining by SEM. 

In PNCs, the details of molecular structure and dynamics in the periphery of the nanoparticles (for example, within the lamellar gallery or at the interface) is quite difficult to establish by regular experimental techniques. The inability to monitor the thermodynamics and kinetics of the molecular interactions between the different constituents that determine the structural evolution and final morphology of the materials hinders progress in this field. This is probably the domain where there is an increasing need for computer modeling and simulations. 

Modifier miscibility plays an important role in this preparation. On the one hand, modifiers must be miscible with the reactive system on the other hand, they must phase-separate during cure. Final morphologies are influenced by the phase-separation conditions. 

When the modifier is a high-Tg thermoplastic, a postcure step at T > Tg M is necessary to complete the reaction in the (3-phase and to develop the final morphology of the blend. 

Independently of the mechanism by which phase separation is produced, final morphologies depend primarily on the location of the trajectory. Trajectory 1 leads to a random dispersion of modifier-rich particles in the thermoset-rich matrix, and trajectory 3 leads to the opposite situation. In a composition region located close to the critical point (trajectory 2), bicontinuous structures may be obtained. 

The use of block copolymers, with each one of the blocks exhibiting miscibility with each one of the phases increases the adhesion level but also promotes dramatic changes in the final morphologies (Girard-Reydet et al., 1999). 

The use of preformed nonmiscible TP powders enables the TP load to be increased in the final material while keeping a continuous thermoset matrix. The final morphology is more easily controlled than in the in-situ phase-separation process. 

FIGURE 1.19. Schematic representation of a hierarchic pattern formation in by an electric field. First, the top polymer layer is destabilized, in similarity to Fig. 1.9, leaving the lower layer essentially undisturbed. In a secondary process, the polymer of the lower layer is drawn upward along the outside of the primary polymer structure, leading to the final morphology, in which the the polymer from the lower layer has formed a mantle around the initial polymer structure. From [41]. 

The behavior of chemical phase-separated blends in the bulk after thermal quenching into the unstable region of the phase diagram is variable. In the bulk, the concentration fluctuations that govern the phase-separation process are random. As a result, the final morphology consists of mutually interconnected domain structures rich in a given blend component that coarsen slowly with time. 

Fig. 11. Scanning electron micrographs (a-d) shown sequential stages in the early part of the adhesion process for mouse fibroblasts from initial contact with a surface to the assumption of a more or less final morphology. The cytoskeleton has the ability to change cell shape quickly and an individual cell may pass from the initial spherical form to the final flattened one in a few minutes. The initial adhesion process at the points of contact between cell and surface is also very rapid but there are subsequent changes at the adhesion sites affecting the nature and strength of the bonds which may continue for many hours. These can be studied by TIRF microscopy...

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