Time immiscible blends

In this example of model reactive polymer processing of two immiscible blend components, as with Example 11.1, we have three characteristic process times tD,, and the time to increase the interfacial area, all affecting the RME results. This example of stacked miscible layers is appealing because of the simple and direct connection between the interfacial layer and the stress required to stretch the multilayer sample. In Example 11.1 the initially segregated samples do create with time at 270°C an interfacial layer around each PET particulate, but the torsional dynamic steady deformation torques can not be simply related to the thickness of the interfacial layer, <5/. However, the initially segregated morphology of the powder samples of Example 11.1 are more representative of real particulate blend reaction systems. 

Time-temperature superposition works for homopolymers and miscible blends but not for immiscible blends, filled systems (e. g., glass fiber reinforced plastics) or reactive or unstable polymers. 

Although the number of miscible blends is rapidly being increased, immiscibility is generally the result when unlike polymers are mixed. Consequently, a great deal of research has been and is being done on ways to improve the mechanical properties of immiscible blends. A widely practiced approach at the present time is to connect the minor dispersed phase to the major continuous phase through a covalent bond. This approach can take several forms. The oldest and most basic is to... 

For partially miscible and immiscible blends, various domain/phase structures can be invoked. Unfortunately, the resonance position of a particular spin in each domain is not appreciably affected by its respective domain structure (see Section 10.2.2.1). Therefore, we cannot expect to observe highly resolved NMR resonances for different domains. Since relaxation times are different for each domain, a relaxation curve is observed to be a featureless multiexponential one and, in most of the cases, is too monotonous to include interdomain spin diffusion. Therefore, most of the experimental results have been explained by using the simplified picture of no interdomain spin diffusion and the observed multiexponential decay is fitted to a sum of exponential functions. Practically three exponentials are enough to realize the observed decay. Each relaxation time represents one domain, thus, only a few domains can be distinguished by one resonance line. Inevitably, the heterogeneous structures deduced from NMR relaxation experiments become simple. 

As exemplified in BPAPC/PET, annealing sometimes helps mixing of immiscible blends (homogenization). Figure 10.27 shows the CP/MAS NMR spectra of mechanically mixed d8-PS/PVME with (e-h) and without (a-d) heating at 403 K for 30 min [130]. At the shortest CP time of 0.5 ms... 

Effects of addition of a compatibilizing block copolymer, poly(styrene-b-methyl methacrylate), P(S-b-MMA) on the rheological behavior of an immiscible blend of PS with SAN were studied by dynamic mechanical spectroscopy [Gleisner et al., 1994]. Upon addition of the compatibilizer, the average diameter of PS particles decreased from d = 400 to 120 nm. The data were analyzed using weighted relaxation-time spectra. A modified emulsion model, originally proposed by Choi and Schowalter [1975], made it possible to correlate the particle size and the interfacial tension coefficient with the compatibilizer concentration. It was reported that the particle size reduction and the reduction of occur at different block-copolymer concentrations. 

In immiscible blends, the t-T principle does not hold. Eor immiscible amorphous blends it was postulated that two processes must be taken into account the t-T superposition, and the aging time [Maurer et al, 1985]. On the other hand, in immiscible blends, at the test temperature, the polymeric components are at different distance... 

At high dilution the morphology of an immiscible blend is controlled by the viscosity ratio, f, the capillarity number, K, and the reduced time, t, as defined in Eq 9.8. The interfacial and rheological properties enter into K, and t. As the concentration increases, the coalescence becomes increasingly important. This process is also controlled by the interphasial properties. 

Figure 14.5. DSC thermograms for aged polymer blends (a) polyvinylchloride/poly isopropyl methacrylate, immiscible blend, aged at a temperature of 60°C, and (b) polyvinyl chloride/polymethylmethacrylate, miscible blend, aged at 80°C. Time of aging, t in hours, is shown alongside each curve. Broken lines represent the un-aged samples for comparison.

Proton spin-temperature equilibration between the hard- and soft-segment-rich domains of the polyurethane elastomer on the order of 10-100 ms might be considered fast relative to a macroscopically phase-separated blend [26] or copolymer, but slow relative to a strongly interacting mixture [25]. This is reasonable for a microphase-separated material whose solid state morphology has been the subject of considerable theoretical and experimental research. Under fortuitous circumstances, intimate (near-neighbor) contact between dissimilar molecules in a mixture can be studied via direct measurement of proton spin diffusion in a two-dimensional application of the 1H-CRAM PS experiment (Combined Rotation And Multiple Pulse Spectroscopy). Belfiore et al. [17,25,31] have detected intermolecular dipolar communication in a hydrogen-bonded cocrystallized solid solution of poly(ethylene oxide) and resorcinol on the f00-/xs time scale, whereas Ernst and coworkers [26] report the absence of proton spin diffusion on the 100-ms time scale for an immiscible blend of polystyrene and poly(vinyl methyl ether), cast from chloroform. 

Often times, the performance of immiscible blends is worse than that of either of the component polymers. This is true of the strength and modulus of the immiscible polymer blends. In terms of temperature performance, it is largely dictated by the temperature behavior of the lower Tg component. This is because, in the case of immiscible blends, the Tg s of the two component polymers are observed at the same location as in the case of the unblended materials. 

Lee and Park [263] derived a more general constitutive equation for immiscible blends. which compared well with the dynamic shear data of PS/ LLD P E blends over a full range of frequency and composition. Their model included the dissipative time evolution of Qand qjj, written as functions of (i) the degree of total relaxation, (ii) the size relaxation strongly dependent on concentration, viz. a and (iii) the breakup and shape relaxation, assumed dependent on (1— )- The parameters contain adjustable constants and depend on concentration as well as on the deformation mode. The interfacial tension coefficient was assumed to be constant, independent of As before [249, 264], the constitutive equation was written in form of three functions dq /dt, and Oy. The model predicts well the dynamic moduli of... 

The behavior of pol5uner alloys and immiscible blends have generally the same complex behavior as rubber-modified systems. In the case of polystyrene/polyethylene blends (177), the crack growth rates of the blends were as much as 20 times lower than that of unmodified polystyrene. However, like the results in Reference 176, the results in Reference 177 show that... 

The lack of synchronous cross peaks between polystyrene and polyethylene bands indicates these polymers are reorienting independently of each other. Cross peaks appearing in the asynchronous spectrum (Figure 1-19) also verify the above conclusion. For an immiscible blend of polyethylene and polystyrene, where molecular-level interactions between the phase-separated components are absent, the time-dependent behavior of IR intensity fluctuations of one component of the sample... 

The time-temperature superposition principle, t-T, is not valid even in miscible blends, e.g., in PS/PVME, where the deviation was evident in tan5 vs. co plot. It was postulated that the number of couplings between the macromolecules varies with concentration and temperature. Thus, even in miscible blends, as either orT changes, the chain mobility is differently affected. Thus, the relaxation spectra of the polymeric components have different temperature dependencies, what make the t-T principle invalid. In immiscible blends, the t-T principle does not hold. Two processes must be taken into account the t-T superposition, and the aging time — at test temperatures, the polymeric components are at different distances from their respective Tg s, T - Tgi T - Tg2. 

Ellipsometry is a powerful tool [16] for measuring the interfacial thickness between two polymers, whether in the case of immiscible or miscible polymer blends. In the case of miscible blends, investigations of changes in interfadal thickness with time at a fixed temperature allow the calculation of mutual diffusion coefficients [22]. In contrast, for immiscible blends the Flory-Huggins interadion parameter x can be deduced by measuring the interfacial thickness in an equilibrium state, and using the theory of Helfand [41] and its extended version [42]. 

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