The Molecular Interphase

In this section, we will consider homogeneous, symmetric elastomer joints for which we want to find the relationship between peel strength and density of bonds in the molecular interphase. 

We will now report the results of autohesion for homogeneous, symmetric joints of polyisoprene rubber (IR) and styrene-butadiene copolymer (SBR) both vulcanized by a sulfur-based system (Section 24.2.1), and of ethylene-propylene diene terpolymer (EPDM) crossHnked by an electron beam (Section 24.2.2). 

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The adhesive strength of post-crossHnked samples was evaluated by a 180° peel test which measured the force of separation at a constant peel rate (5 mmmin ) at room temperature. The results reported in Fig. 24.3 show the effect of contact time (ranging from 1 h up to one month) in the second step of joint formation at room temperature. Up to approximately 300 h of contact, no effect of time is seen. For longer times, the peel strength increases slightly. Although the scatter is important, this weak trend can be related to crosslinks formed by irradiation in the molecular interphase. This is confirmed by the fact that no separation is observed for these joints immersed in a good solvent such as cyclohexane, whereas spontaneous delamination is observed for shorter contact times. 

In principle, therefore, these valuable techniques can provide all of the information needed to specify the molecular structure of the electrode/electrolyte solution interphase, the dynamics of adsorption/... 

In general, sol-gel catalysts are heterogeneous materials employing solid-liquid or solid-gaseous interphases. A mobile and a stationary component penetrate each other at the molecular level with the catalytic species being well-defined, highly mobile and homogeneously distributed... 

Let us recall the micellar aqueous system, as this procedure is actually the basic one. The chemistry is based on fatty acids, that build micelles in higher pH ranges and vesicles at pH c. 8.0-8.5 (Hargreaves and Deamer, 1978a). The interest in fatty acids lies also in the fact that they are considered possible candidates for the first prebiotic membranes, as will be seen later on. The experimental apparatus is particularly simple, also a reminder of a possible prebiotic situation the water-insoluble ethyl caprylate is overlaid on an aqueous alkaline solution, so that at the macroscopic interphase there is an hydrolysis reaction that produces caprylate ions. The reaction is very slow, as shown in Figure 7.15, but eventually the critical micelle concentration (cmc) is reached in solution, and thus the first caprylate micelles are formed. Aqueous micelles can actually be seen as lipophylic spherical surfaces, to which the lipophylic ethyl caprylate (EC) avidly binds. The efficient molecular dispersion of EC on the micellar surface speeds up its hydrolysis, (a kind of physical micellar catalysis) and caprylate ions are rapidly formed. This results in the formation of more micelles. However, more micelles determine more binding of the water-insoluble EC, with the formation of more and more micelles a typical autocatalytic behavior. The increase in micelle population was directly monitored by fluorescence quenching techniques, as already used in the case of the... 

The ILs interact with surfaces and electrodes [23-25], and many more studies have been done that what we can cite. As one example, in situ Fourier-transform infrared reflection absorption spectroscopy (FT-IRAS) has been utilized to study the molecular structure of the electrified interphase between a l-ethyl-3-methylimidazolium tetrafluoroborate [C2Qlm][BF4] liquid and gold substrates [26]. Similar results have been obtained by surface-enhanced Raman scattering (SERS) for [C4Cilm][PFg] adsorbed on silver [24,27] and quartz [28]. 

Topographical features comprise a significant portion of the interphase. In general, the epoxy matrix will conform to the topographical features of the substrate down to the molecular dimensions of the resin molecule. Since most epoxies are applied as a liquid of moderate to low viscosity, intimate contact between epoxy and substrate is achieved. Two aspects of the topographical features of the substrate must be considered as to their effect on the interphase structure of the epoxy. 

Theory for blends of two homopolymers with block copolymers was developed by Noolandi and Hong (1982) using the SCF method. They considered a quaternary system with a diblock in a good solvent for two incompatible homopolymers. Calculation of density profiles revealed that the block copolymer tends to be selectively located at the interface, and that the homopolymer tends to be excluded from the interphase. This is illustrated by the representative density profiles in Fig. 6.37. The exclusion of homopolymer from the interphase was found to be enhanced by increasing the molecular weight of the block... 

The molecular structure of epoxy/metal interphases in the presence of an amino coupling agent was studied by Boerio and co-workers [28] by IR and by XPS. The formation of amide and imide groups in the interphase provided evidence of chemical reaction between the silane primer and the curing agent for epoxy resin. 

In all adhesive joints, the interfacial region between the adhesive and the substrate plays an important role in the transfer of stress from one adherend to another [8]. The initial strength and stability of the joint depend on the molecular structure of the interphase after processing and environmental exposure, respectively. Characterization of the molecular structure near the interface is essential to model and, subsequently, to maximize the performance of an adhesive system in a given environment. When deposited on a substrate, the silane primers have a finite thickness and constitute separate phases. If there is interaction between the primer and the adherend surface or adhesive, a new interphase region is formed. This interphase has a molecular structure different from the molecular structure of either of the two primary phases from which it is formed. Thus, it is essential to characterize these interphases thoroughly. 

Hoh et al. [4] used differential scanning calorimetry (DSC), FT-IR, and solid-state 13C-NMR to gain information about the epoxy/silane resin interphase. They used FT-IR to correlate the extent of reaction with the extent of interdiffusion as in the above studies. For both NMR and DSC studies, bulk models were used to study the molecular mobility of interfacial components. 

The characterizations discussed thus far do not involve direct investigation of the actual interphase region although failed surfaces have been analyzed to give indirect evidence for interdiffusion. In other studies, assumptions that the observed properties (extent of reaction, increased solvent resistance, molecular mobility) correlate with the extent of interdiffusion have been made. 

The fact that the thickness of the interphase estimated here stays unchanged at 34A in the molecular weight range of 30,000-100,000, while the mass fraction and thickness of amorphous phase change remarkably, is particularly meaningful. Flory et al. [6,7] anticipated in 1984 based on their lattice theory that the methylene chains that emerge from the basal plane of lamellar crys-... 

As can be seen from Table 4, two lower molecular weight samples actually comprise only the crystalline and noncrystalline interlamellar material, devoid of amorphous phase. On the other hand, the larger molecular weight samples comprise three phases the crystalline, amorphous, and crystalline-amorphous interphase in a similar fashion to the atmospheric-pressure crystallized samples. However, we note that the T2c s of the crystalline-amorphous interphase for two higher molecular weight samples is appreciably shorter than those of the atmospheric-pressure crystallized samples. This demonstrates that the molecular chain motion in the crystalline-amorphous interphase of these pressure-crystallized samples on a T2C time scale is more severely restricted. 

The half-widths of 37-39 and 78-88 Hz, respectively, for the crystalline and amorphous phases are significantly larger than 18 and 38 Hz for those of the bulk-crystallized linear polyethylene (cf. Table 1). This is caused by incorporation of minor ethyl branches. The molecular alignment in the crystalline phase is slightly disordered, and the molecular mobility in the amorphous phase will therefore be promoted. With broadening of the crystalline and amorphous resonances, the resonance of the interphase also widens in comparison to that of bulk-crystallized linear polyethylene samples. This shows that the molecular conformation is more widely distributed from partially ordered trans-rich, conformation to complete random conformation, characteristic as the transition phase from the crystalline to amorphous regions. 

Similar line shape analyses for the equilibrium spectra at different temperatures were performed. At room temperature, where the amorphous phase is in a glassy state, the determination of the elementary line shape of the amorphous component was a little difficult. However, excellent line-decomposition analysis was obtained by introducing a broader Lorentzian centered at the same chemical shift as at higher temperatures. The result at room temperature is shown in Fig. 26-(b). Here the nature of the component line shapes A and B of the crystalline and crystalline-amorphous interphases is similar to that in the spectrum at 87 °C. However, the component line shape for the amorphous component is quite different from that at 87 °C that is distributed over a very wide chemical shift range centered at the same chemical shift to that at higher temperatures. This reflects the glassy state of the amorphous phase. In the glassy state, the molecular conformation in the amorphous phase will be distributed over all permitted conformations stationary in time and randomly in space. The wide component line shape of the amorphous component obtained here at room temperature well represents this molecular nature of the amorphous phase. 

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