Premature phase separation

Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

Perhaps one of the most important effects of a low reaction rate and premature phase separation is the decrease in molecular weight of the material. This clearly must be avoided as it seriously decreases the tensile properties of the polymer. There exists therefore a balance between the degree of phase separation and organization that can be achieved and the maximum molecular weight. 

The rubber is segregated as big shapeless patches in a continuous nylon phase (the black domains in the micrograph). When the polymerization conditions are not optimum this can also occur in system 3. "Figure 2D" shows a polymer made according to this system where as a result of premature phase separation the polymer has segregated as big particles (about 3 p) and quite a few active groups have not reacted to form nylon blocks. The interfacial adhesion is very small and the mechanical properties of this polymer are equal to those of a polymer prepared according to system 1. 

It was noted in the previous section that the carboxyl end groups on the CTBN elastomer affected the final performance of the material as a toughener since these groups would co-react with the epoxy resin and facilitate stress transfer from the brittle matrix to the phase-separated elastomer. Without this adhesion the particles could debond prematurely, which would lead to poor dissipation of the energy of the growing crack. It has also been noted that excessive adhesion between an epoxy resin and a thermoplastic could be deleterious to the performance (Williams et al, 1997). The process of toughening of a thermoset... 

Nanocomposites with carbon nanotubes have been an area of considerable R D ever since the excellent electrical and mechanical properties of carbon nanotubes were demonstrated. However, attempts to prepare carbon nanotube RPs often result in phase separation of the CNT and polymer phases causing premature material failure. Researchers at Nomadic Inc. and Oklahoma State University developed a layer-by-layer (LBL) assembly process that permits preparing polyelectrolyte/CNT RP with a CNT loading greater than 50 wt%. The excellent mechanical properties of these materials can be improved further by additional chemical action crosslinking of the CNT and polymer phases and by parallel alignment of the CNTs. The LBL method has been used to prepare various types of RPs. 

The photoadd generator should i) have suffident radiation sensitivity to ensure adequate add generation for good resist sensitivity (for photochemical reactions a quantum yield > 0.1 is desirable), ii) be fi ee of metallic elements such as antimony or arsenic that are perceived to be device contaminants, iii) be fully compatible with the matrix resin to eliminate the possibility of phase separation, iv) be stable to at least 175 C to avoid premature thermal generation... 

There are several possible flow scheme variations involved for this process. It can operate as an independent unit or be used in conjunction with a thermal conversion unit (Figure 9-25). In this configuration, hydrogen and a vacuum residuum are introduced separately to the heater, and mixed at the entrance to the reactor. T o avoid thermal reactions and premature coking of the catalyst, temperatures are carefully controlled and conversion is limited to approximately 70% of the total projected conversion. The removal of sulfur, heptane-insoluble materials, and metals is accomplished in the reactor. The effluent from the reactor is directed to the hot separator. The overhead vapor phase is cooled, condensed, and the hydrogen separated from there is recycled to the reactor. 

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