Polymer blends guest-host

An alternative approach is to dissolve an efficient electroluminescent chro-mophore in an inert polymer host in a guest-host system or polymer blend. Unfortunately, phase separation and demixing often limits the amount of low-molar-mass electroluminescent chromophores that can be dissolved in polymer hosts. Therefore, the relative brightness is lower for such guest-host systems. This problem can be overcome by fixing the chromophore chemically to the polymer itself as a pendent group on a side-chain polymer separated by spacer units. Bulk phase separation is then impossible, although microphase separation may still take place. This is illustrated by the structures collated in Table 6.10 for some typical side-chain polymers 53-55. ... 

Previous experiments measuring fluorescence depolarization arising from excitation transport among chromophores on isolated guest colls in solid polymer blends demonstrated the feasibility of determining the relative size of individual chains in various host environments (18). The ability of these experiments to... 

FIGURE 7.3 Photoalignment of PLCPs under the influence of molecular cooperative motion in systems of copolymer or polymer blend (host/guest). 

Wendroff (1997) (2( ) found that as the emitting molecular material PSA in the blend of polynobomene (PN) with polyarylate Durel at the weight ratio 1 2, its EL spectrum is broad and nearly white as shown in Figure 8. This is a guest-host system with a chromophore as the guest molecule and a polymer blend as the host. PSA can form radical cations and is a hole transport material. Their PL spectra vary with host polymer, thus the host polymer blend can provide an emission of white light o y at a particular composition. 

Polymers, as a component of assemblies, polymer blends, alloys, and composites, have been studied by mixing them with different species. However, in these instances, the structures and properties are statistical in nature. Polymers have numerous structural potentials in the main chains and side chains. If polymers can be used as guest molecules, the resultant complexes could provide new structures and functions. Polymeric inclusion compounds are thought to be a typical example of nanoscale composites—molecular level composites made by bottom-up approaches. Urea, thiourea, and per-hydrotriphenylene have been extensively studied as host molecules for the formation of inclusion compounds with various polymers. 

The initial focus in dendrimer research was largely on their synthesis, but recently more importance has been given towards their functional aspects [23]. The successful blending of dendrimer chemistry with several contemporary themes such as host-guest chemistry [24], metallo-organic chemistry [25], luminescent materials [26], catalysis [20a], medicinal chemistry [18d] and polymers [27] has contributed enormously over the years to a rich chemistry with potential applications. As a detailed survey of this area is beyond the scope of this chapter, we will restrict ourselves to two topics involving these molecules (a) dendritic self-assembly [28] and (b) metallodendrimers [25,26]. 

Studies continue on the effects of a polymer host matrix on the excited-state properties of guest molecules. For example, the lifetime of excited singlet-state species may be greatly prolonged through restrictions of molecular motions (Gusten and Meisner, inter alia). Accurate information on the blend miscibility of polymers is provided by studies of excimer emission (Mikes et al.), and George et al. claim that the service life of many polymers can be predicted from their luminescence properties see also Martin. 

We also observed that the PC chains possess a preferred ability to form inclusion compounds with y-CD in solution, when competing with PMMA and PVAc. From the XH NMR spectrum of the coalesced 1 1 1 PC/PMMA/PVAc blend (not shown), the molar ratio of PC PMMA PVAc was determined to actually be 1.6 1 1.4 compared to the initial molar ratio of 1 24 24, respectively, used in solution to form their common y-CD-IC. Despite the initial 1 24 24 PC PMMA PVAc molar ratio in solution, the PC component in the coalesced PC/PMMA/PVAc blend is still prevalent over the PMMA and PVAc components, which indicates that there may be additional factors that govern the inclusion process from a multiguest system. We believe that this very strong preference of the host y-CD molecules for PC chains, rather than the other two possible guests, is due to their different hydrophobicities. Although the final molar ratio of the coalesced ternary blend can be somewhat controlled by modifying the initial molar ratio of polymers in their common solution, our eventual aim is to be able to adjust, as desired, the constituent polymer ratios in coalesced ternary blends. 

It is not necessary to use only dyes to take advantage of the energy transfer blends of two polymers can also be used as host-guest systems (Lee et al 2002). The guest molecules can be florescent or phosphorescent in nature. However, phosphorescent dyes based on Ir and Pt complexes have provided significantly higher efficiency of OLEDs because of their ability to emit from both singlet and triplet excitons of the host molecule (Kamata et al 2002),... 

The excimer fluorescence has been used to characterize the miscibility of the guest and host polymers [Xie et al., 1993]. Since the excimer forming site concentration depends on the extent of the guest polymer aggregation, the ratio of excimer to monomer fluorescence intensity, is related to blend miscibility. 

If the blend is miscible, the guest polymer dilutes the matrix of the host polymer. This lowers the excimer-forming site concentration, thereby leading to a decreased intensity of excimer emission. 

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