Spin diffusion polymer blends

Thus, for the investigation of buried polymer interfaces, several techniques with molecular resolution are also available. Recently NMR spin diffusion experiments [92] have also been applied to the analysis of a transition zone in polymer blends or crystals and even the diffusion and mobility of chains within this layer may be analyzed. There are still several other techniques used, such as radioactive tracer detection, forced Rayleigh scattering or fluorescence quenching, which also yield valuable information on specific aspects of buried interfaces. They all depend very critically on sample preparation and quality, and we will discuss this important aspect in the next section. 

A schematic illustration of how the relaxation process (Ti or Tlp) for H spins in a blend of polymers A and proceeds with spin diffusion (SD) is shown in Fig. 14. Here, we assume that (1) the 3H spins are divided into two species species A for polymer A and species for polymer B, and (2) both A and are characterized by their common relaxation times TA and , respectively. Suppose TA is much shorter than TB, and the whole spins are inverted by pulse. If spin diffusion between component polymers is slow, the spin system may reach a situation where all of the XH spins of polymer A are fully relaxed or up , while those of polymer are still down (Fig. 14 (1)). Spin diffusion tries to average this polarization gradient created by different T values, that is, to flip down the half of the XH spins in polymer (Fig. 14 (2)). The down spins of polymer A quickly flip up to create a polarization gradient again due to the short T of polymer A (Fig. 14 (3)), and again spin diffusion tries to average it, and so on. After all, both spin species eventually reach thermal equilibrium. When spin diffusion is much... 

Solid-state NMR has been used to examine compatibility in SAN copolymer blends with styrene-maleic anhydride copolymers [101]. Spin diffusion experiments indicate that the two polymers mix on a molecular scale, but the data suggest that there is no specific interaction between the nitrile group and the carbonyl groups in the maleic anhydride. This technique can provide some powerful chemical data that cannot be obtained by other methods. Unfortunately, the method requires the preparation of l3C-enriched polymers. 

As reviewed in the previous section, measurements of Ti and Tip can provide an estimation of the length scale of miscibility of polymer blends. Compared with such kinds of experiments, the results of the spin-diffusion experiments are more quantitative and straightforward. The accuracy of the results of spin-diffusion experiments relies, to a large extent, on the values of spin-diffusion coefficients (7)) employed in calculation of the constituent phase components. Despite efforts that have been made, there still lacks a suitably applicable method of directly measuring the spin-diffusion coefficients, at least for polymers. For rigid polymer below Tg, 0.8 nm /ms has been turned out to be a reliable value of spin-diffusion coefficient. The difficulty left then concerns how to determine the coefficient of the mobile phase, which is very sample dependent. Recently, through studies on diblock copolymers and blend samples with known domain sizes, Mellinger et al established empirical relations between the T2 and D as follows ... 

The miscibility of poly(4-vinylpyridine) (P4VP) with poly(4-vinylphenol) (PVPh) blends was investigated over a wide range of compositions by other techniques and high-resolution solid-state NMR. ° Relaxation times were studied as a function of blend composition. Ti(H) and Tip(H) results demonstrate that the spin diffusion can completely average out the entire relaxation process. It was also found that the intimate mixing of the polymer blends restricts the local chain mobility. 

In spin-diffusion studies it is possible to detect not only two but three domain sizes. The third domain can be considered the interface (i) between the other two domains, which can be different chemical species in a polymer blend or rigid crystalline (r) and mobile amorphous (m) material in a semicrystalline polymer. To illustrate this point, a mobility timescale is depicted in Fig. 7.2.25(a) and the simplified ID domain structure of PE underneath in (b). Rigid crystalline and mobile amorphous materials exhibit motion of chain segments with different correlation times Tc. The chains at the interface between both domains exhibit intermediate mobility. The exact ranges of correlation times in the individual domains depend on the particular choice of filters. Therefore, the values of domain sizes derived through spin-diffusion NMR also depend on the type of filters used. In particular, the interface is defined solely by the NMR experiment and can only be detected if the filters are properly chosen. 

Fig. 4.8, Schematic representation of the spin-diffusion process by a wave-front in (a) a compound consisting of different domains, e.g., a polymer blend (b) a regular structure with long-range order (e.g., a crystal) and (c) a microscopically disordered compound. The resonance frequency is encoded into the density of the filling pattern and simultaneously into the direction of the long elliptical axis, symbolizing that it can be determined either by the isotropic shift or the orientation of the shift tensor. Quasi-equilibrium is reached in (a), if the wave has extended over a typical domain size in (b) after the spin-diffusion wave has reached the next neighbors and in (c) after the wave has sampled all possible orientations, leading to the typical pattern for amorphous compounds discussed below.

As a final example of the use of proton NMR invoking spin diffusion to study miscibility of polymer blends, the use of CRAMPS to remove proton dipolar coupling in a blend of an aromatic poly(ether-imid) (PEI), and a poly(aryl-ether-ketone) (PEEK), with detection of the magnetization of the C in the blend under high resolution conditions is cited [51]. Here, detailed information on the chemical composition of the phases present, as inferred from high resolution NMR of C, is linked to typical sizes of domains as reflected in spin diffusion of proton magnetization. 

In this section 10.2, we review the various solid-state NMR methods used to investigate interpolymer interactions, molecular motion and the spatial structure of a polymer blend. An interaction between component polymers affects the chemical shifts and lineshapes (see Section 10.2.2.1) and the molecular motions of the component polymers (see Section 10.2.2.2). In Section 10.2.3.1, microheterogeneity from 2 to 50 nm is studied by measuring spin diffusion indirectly from its effects on H spin-lattice relaxation. The spin-diffusion processes can also be monitored by several methods based on the Goldman-Shen experiment [8] (see Section 10.2.3.2). Homonuclear and heteronuclear two-dimensional correlation experiments reveal how and to what extent component polymers interact with each other (see Section... 

Figure 10.8 shows a schematic illustration of how the Ti relaxation process for H spins in a blend of polymers A and B proceeds with spin diffusion (SD). Here, we assume that (1) H spins are divided into two species species A for polymer A and species B for polymer B, and (2) both A and B are characterized by their common relaxation times Tia and Tib, respectively. 

Characterization of the interfacial regions is important to understand the mechanical properties of incompatible polymer blends. As shown, in many heterogeneous blends, the simplifying assumption of the neglect of spin diffusion between domains is reconcilable with NMR observations. In other words, most of the NMR observables are not sensitive enough to appreciate the influences of the other domains. However, it is also true that the spins are interacting with each other via the interface. To study such interactions. 

If one of the component polymers has a third spin other than and H, several cross-relaxation experiments become possible. Since spin diffusion occurs a few 10 nm before the polarization decays, only a small amount of the spins near the interface must be detected for immiscible blends. Section... 

Note that we assume fast spin diffusion within each phase in the phase-separated blend (Model D in Fig. 10.18), i.e., each phase is homogeneously mixed from the spin-diffusion point of view. In other words, one Tip or Ti value is associated with one phase. As shown in Section 10.3.1, such heterogeneity of a blend manifests itself as multiexponential relaxation decay curves. For Model D, we expect a double-exponential decay for the respective spins of polymers A and B. For spin-locking Tip experiment, the two double-exponential decay curves are given for the two component polymers as... 

Solid-state NMR has been applied successfully in a few cases for the study of poly(imide) blends. It can be said, however, that the techniques arising from recent advances in the analysis of polymer blends, such as selection techniques based on multipulse methods, and improvements in the modelling methods of spin diffusion, have yet to be applied to the study of blends containing poly(imide)s. It is suggested that these techniques will have an important role to play in future studies of poly(imide) blends, particularly for blends such as impact-modified BMI resins. 

PMMA sufficiently close to the mixed phase that spin diffusion from it is important. Radial spin diffusion was assumed, with a sphere of intimately mixed phase of radius A and a shell of close PMMA of thickness L-A (up to a point of abutment with a neighbouring sphere). Figure 18.16 shows experimental data for the 60 40 PMMA/PVDF blend, which is fitted to A = 6 A, L = 12 A and 30% isolated PMMA. The same experiment was then employed [81], together with F- C H, F CP, to study the influence of PMMA tacticity on PMMA/PVDF miscibility, yielding evidence for a specific interaction between segments of the two polymers. The mixed PMMA/PVDF phase was determined to have a mean radius of 6-8 A, with some dependence on PMMA tacticity. Large differences were found in the... 

The relaxation time can provide information on the domain size in the solid polymer. If two component polymers, each of which has inherent relaxation times in its pure state, in the binary blends are intimately mixed and the domain sizes of the two components are small enough for effective spin diffusion during relaxation, both components exhibit the same relaxation time. When the domain sizes are larger, that is, the components... 

---