Microheterogeneity polymer blends

It is a routine SFM experiment to investigate the heterogeneous structure of polymer blends and composites containing micrometer sized domains [69]. A less trivial problem is to resolve and characterise the features on the nanometer scale (around 10 nm), which are comparable to the tip size and the contact area. Typical systems, which demonstrate microheterogeneous structures, are block copolymers consisting of chemically different and physically incompatible blocks, e.g. A and B. As a result of the interconnectivity of the blocks, block copolymers undergo microphase separation, where the size of the microdomains is restricted to the molecular dimensions. One can distinguish between AB diblock copolymers and triblock copolymers (ABA and ABC). 

Multicomponent polymeric materials with microheterogeneous mophologies include a number of polymer blends and block copolymers, however, an especially easy way to bring about the desired morphology is through interpenetrating polymer networks. Several papers in the symposium are concerned with IPN s and related materials. 

Figure 1. Generalized mechanical loss (tan S) and modulus behavior for different types of polymer blends. Case 1 (dashed-dotted line), miscible case 2 (dashed line), limited miscibility case 3 (dotted line), microheterogeneous case 4 (solid line), heterogeneous. (Reproduced with permission from Ref. 5. Copyright 1979 Academic Press.)...

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... 

It is noteworthy that homogeneity at fairly fine level is necessary for optimum performance, but some degree of microheterogeneity is usually desirable to preserve the individual properties of respective polymer components (Hess et al. 1993). Note that nearly aU commercial polymer blends (with the notable exception of the PVDF/PMMA blend) are immiscible. One tends to study miscibility not so much as to develop single-phase commercial blends, but mainly to design better compatibilizers and compatibilization strategies. 

Theory on Toughness of Microheterogeneous Two-Phase Polymer-Polymer Blends... 

The miscibility of some polymer blends is of considerable technological importance although, fundamentally, the reasons for the miscibility are not completely understood. Polystyrene (PS) and poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) is one such system. 2D correlation studies have been made on a blend of 80% of the former and 20% of the latter by Palmer et al. [31]. The results suggest a different dynamic behaviour for the PS and PPO portions of the blend, depsite their compatibility, with the PS chains responding to the perturbing force faster than those of PPO. Some asynchronous cross peaks develop between the constituents, indicating the possible existence of submolecular level microheterogeneity. 

Most of the IPNs and SINs so formed are phase separated, as are most polymer blends. For IPNs and SINs, however, if gelation of both networks precedes phase separation, the crosslinks will tend to hold the phases together and limit their sizes, for.ming a microheterogeneous morphology. However, if phase separation precedes gelation, then the crosslinks will tend to keep the phases apart, and gross phase separation will occur with concomitant loss of physical and mechanical properties. 

Dielectric relaxation studies have been applied to polymer blends to determine the extent of concentration fluctuations (microheterogenous structure). These measurements are conducted by measurement of e" versus frequency at different temperatures. Broadening of the resultant curves relative to the imblended controls demonstrates increasing concentration fluctu-... 

The major field of application of functionalized PO blends and functionalized homo-polyolefins is compatibilization of blends of condensation polymers. The use of functionalized PO blends, in place of homopolyolefins, has a number of advantages. First, by ratio variation of PO in the blend, it is possible to change the degree of microheterogeneity of the final product this fact is of importance for impact strength (67). Second, the selective nature of grafting can be useful in controlling the functionalization power of one or another component in PO blends. 

Addition of AN to a level of 40% (NBR-40) destroys the phase boundaries entirely, resulting in the microheterogeneous system shown in Figure 3.10. The phase domains ( 100 A) shown in Figure 3.10 are clearly smaller than the polymer molecules themselves, yet the material is not totally compatible. Only a few cases are known in which the phase division in blends is so fine such cases include the IPN s discussed in Chapter 8 and the poly(2,6-dimethyl phenylene oxide)/polystyrene blend described in Section 9.7.1. 

Nonlinearity of the dependence on the blend content could be explained, to a certain extent, with structural changes in the polymer melt, as a result of which the intermolecular interaction is weakened, which undoubtedly also influence the relaxation processes in the polymer. The kinetics of the diffusion of low-molecular compounds and permeability of gases depends on the microheterogeneous stracture of the investigated PVC-CPE blends as well as on the amount of the microvoids on the interfacial boundary area, locked there during thermoplastic mixing of PVC and CPE. Different mass transfer mechanisms can govern the diffusion process in various stractural areas of the PVC-CPE blend. 

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