Epoxy morphology

Morphologies are generally characterized by size and concentration of dis-persed-phase particles. Figures 8.7-8.10 give some examples of morphologies obtained in the case of modified epoxies. Morphologies are controlled by many factors, such as miscibility, modifier concentration, temperature, reaction rate, and presence of an emulsifier, etc. (Williams et al., 1997). 

Studies of the particle—epoxy interface and particle composition have been helphil in understanding the mbber-particle formation in epoxy resins (306). Based on extensive dynamic mechanical studies of epoxy resin cure, a mechanism was proposed for the development of a heterophase morphology in mbber-modifted epoxy resins (307). Other functionalized mbbers, such as amine-terminated butadiene—acrylonitrile copolymers (308) and -butyl acrylate—acryhc acid copolymers (309), have been used for toughening epoxy resins. 

Mechanical properties of mbber-modifted epoxy resins depend on the extent of mbber-phase separation and on the morphological features of the mbber phase. Dissolved mbber causes plastic deformation and necking at low strains, but does not result in impact toughening. The presence of mbber particles is a necessary but not sufficient condition for achieving impact resistance. Optimum properties are obtained with materials comprising both dissolved and phase-separated mbber (305). 

Fig. 2. Morphology model of a core-shell, rubber-toughened epoxy adhesive.

Urethane structural adhesives have a morphology that is inverse to the toughened epoxy just described. The urethanes have a rubber continuous phase, with glass transition temperatures of approximately —50°C. This phase is referred to as the .soft segment . Often, a discontinuous plastic phase forms within the soft segment, and that plastic phase may even be partially crystalline. This is referred to as the hard segment . A representation of the morphology is shown in Fig. 3 [34]. 

Due to the chemical structure, fimctionality and composition of their constituents, ordinary variations of the processing conditions as well as the quality of the raw materials have been observed to lead to changes of the fine structure which are often responsible of lower ageing resistence of the polymer. The data reported here correlate the morphology of some widely utilized epoxy matrices to the informations that can be obtained from the study of the sorption behaviours. 

When a polymer film is exposed to a gas or vapour at one side and to vacuum or low pressure at the other, the mechanism generally accepted for the penetrant transport is an activated solution-diffusion model. The gas dissolved in the film surface diffuses through the film by a series of activated steps and evaporates at the lower pressure side. It is clear that both solubility and diffusivity are involved and that the polymer molecular and morphological features will affect the penetrant transport behaviour. Some of the chemical and morphological modification that have been observed for some epoxy-water systems to induce changes of the solubility and diffusivity will be briefly reviewed. 

Water molecules combine the tendency to cluster, craze and plasticize the epoxy matrices with the characteristic of easily diffusion in the polymer1 10). The morphology of the thermoset may be adversaly influenced by the presence of the sorbed moisture. The diffusion of the water in glassy polymers able to link the penetrant molecules is, therefore, characterized by various mechanisms of sorption which may be isolated giving useful information on the polymer fine structure. 

This study reveals the need for separate investigative tools for quantitatively characterizing the influence of manufacturing defects and chemical characteristics on the hygrothermal fatigue response and morphology of epoxy thermosets. 

W. A. Romanchick, J. E. Sohn, and J. F. Geibel Synthesis, Morphology, and Thermal Stability of Elastomer-Modified Epoxy Resin, in ACS Symposium Series 221 — Epoxy Resin Chemistry II, R. S. Bauer (ed.), American Chemical Society, Washington DC, 1982, pp. 85-118. 

Siloxane containing interpenetrating networks (IPN) have also been synthesized and some properties were reported 59,354 356>. However, they have not received much attention. Preparation and characterization of IPNs based on PDMS-polystyrene 354), PDMS-poly(methyl methacrylate) 354), polysiloxane-epoxy systems 355) and PDMS-polyurethane 356) were described. These materials all displayed two-phase morphologies, but only minor improvements were obtained over the physical and mechanical properties of the parent materials. This may be due to the difficulties encountered in controlling the structure and morphology of these IPN systems. Siloxane modified polyamide, polyester, polyolefin and various polyurethane based IPN materials are commercially available 59). Incorporation of siloxanes into these systems was reported to increase the hydrolytic stability, surface release, electrical properties of the base polymers and also to reduce the surface wear and friction due to the lubricating action of PDMS chains 59). 

Other reports on the morphology and mechanical behavior of organosiloxane containing copolymeric systems include polyurethanes 201 202), aliphatic 185, 86) and aromatic117,195> polyesters, polycarbonates 233 236>, polyhydroxyethers69,311, siloxane zwitterionomers 294 295) and epoxy networks 115>. All of these systems display two phase morphologies and composition dependent mechanical properties, as expected. 

Literature search shows that epoxy-based nanocomposites have been prepared by many researchers [34-38]. Becker et al. have prepared nanocomposites based on various high-functionahty epoxies. The mechanical, thermal, and morphological properties were also investigated thoroughly [39 3]. The cure characteristics, effects of various compatibilizers, thermodynamic properties, and preparation methods [16,17,44 9] have also been reported. ENR contains a reactive epoxy group. ENR-organoclay nanocomposites were investigated by Teh et al. [50-52]. 

Morphology of the cured samples was analyzed by SEM of the fractured samples etched with tetrohydrofuran (THE), which is a solvent for the rubber. Figure 11.24 shows the fracture surfaces of the PWE and PNE specimens. Whereas the PWE fracture surface presents an essentially homogeneous surface with only a few small voids present, small yet uniformly distributed cavities are seen in PNE samples. The PWE morphology is consistent with the high degree of intermolecular link between rubber and epoxy macromolecules. The PNE morphology indicates incomplete reaction between epoxy and rubber. 

Instead of the familiar sequence of morphologies, a broad multiphase window centred at relatively high concentrations (ca. 50-70% block copolymer) truncates the ordered lamellar regime. At higher epoxy concentrations wormlike micelles and eventually vesicles at the lowest compositions are observed. Worm-like micelles are found over a broad composition range (Fig. 67). This morphology is rare in block copolymer/homopolymer blends [202] but is commonly encountered in the case of surfactant solutions [203] and mixtures of block copolymers with water and other low molecular weight diluents [204,205]. 

Fig. 67 Schematic of phase behaviour for blend of novolac epoxy resin with nearly symmetric poly(methyl acrylate-co-glycidylmelhacrylate)-0-polyisoprene. Ordered L can be swollen with up to about 30% of resin before macroscopic phase separation occurs, producing heterogeneous morphologies containing various amounts of L, C, worm-like micelles and pristine epoxy. At lower concentrations, disordered worm-like micelles transform into vesicles in dilute limit. According to [201]. Copyright 2003 Wiley...

The three TS-1 catalysts with similar Ti contents have cuboidal morphology with comparable particle sizes of 0.2-0.3 pum (as shown in SEM pictures, Fig. 53). The EPR spectra of the samples in contact with aqueous H202 (46%) (Fig. 54) indicate that the ratio of the A to B superoxo species in various TS-1 samples increases in the order TS-1 (fluoride) < TS-1 (with anatase) < TS-1 (without anatase). Catalytic activity for phenol hydroxylation and allyl alcohol epoxi-dation (Table LIII) was found to parallel the A/B ratio of the oxo-Ti species (TS-1 (fluoride) < TS-1 (with anatase) < TS-1 (without anatase)). 

Selection Criteria for the Preparation of Solvent-Modified and Macroporous Epoxy Networks with Tailored Morphologies Prepared via CIPS. 

Phase Separation Mechanism in Hexane-Epoxy Systems. . Influence of Reaction Parameters on the Morphology of Cyclohexane-Modified Epoxy Networks Prepared via CIPS... 

One particularly interesting system is the epoxy 2,6-dimethyl-4-heptanone as up to 40 wt % of this solvent can be easily mixed together with the epoxy precursors to generate a phase separation process. This allows one to verify experimentally the possible morphologies which were predicted based on the schematic phase diagram at concentrations below the phase inversion (see Fig. 7). Shown... 

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