Miscible polymer blends, fluorescence

Effects of shear on miscible polymer blends in situ fluorescence studies. Macromolecules, 24 (19), 5451-5458. 

Evans, C.M. and Torkelson, J.M. (2012) Determining multiple component glass transition temperatures in miscible polymer blends Comparison of fluorescence spectroscopy and differential scanning calorimetry. Polymer, 53 (26), 6118-6124. 

These early studies demonstrated that excimer fluorescence is a useful addition to the battery of experimental tools available to study solid state polymer blends. However, the longer range goal of explaining the significance of the absolute value of R was not realized because there was insufficient companion information about the thermodynamics of the blends. The PS/PVME blend does not suffer from this limitation, and thus provides an excellent system for characterization of the photophysies under conditions for which miscibility or immiscibility are firmly established. In this section we examine results for PS/PVME as well as more recent work on dilute blends containing P2VN that are believed to be miscible. 

Excitation fluorescence is the principle of the fluorescence techniques used for studying polymer blends. The method comprises of three steps incorporation of an excimer, its excitation, and recording the excitation delay. The excimer can be an aromatic polymer component of the blend (viz., PS, poly(viny 1-dibenzyl), polyvinylnaphthalene, an aromatic group grafted onto the macromolecular chain, etc.), or it can be added as probe molecule (e.g., anthracene). There are three possibilities for the aromatic rings to form excimers intramolecular adjacent, intramolecular nonadjacent, and intermolecular types. Each of these types is sensitive to different aspects of the chain conformation and environment, thus, sensitive to blend miscibility effects. The most important of these for studies of polymer blends is the intermolecular, usually identified from concentration measurements (Wiruiik et al. 1988). 

Another method involves excimer fluorescence as a molecular probe see Section 2.9. The question may be raised as to whether polymer blends will become more miscible if the differences in their solubility parameters are reduced. Excimer fluorescence provides some evidence see Rgure 4.14 (52). Here, 0.2 wt.% of poly(2-vinyl naphthalene), P2VN, is dispersed in a series of poly (alkyl methacrylates). These include the following, which are identified in Figure 4.14 by acronym methyl, PMMA ethyl, PEMA n-propyl, PnPMA isopropyl, PiPMA n-butyl, PnBMA isobutyl, PiBMA . yec-butyl, PsBMA ferf-butyl, PtBMA phenyl, PPhMA, isobomyl, PiBoMA benzyl, PBzMA and cyclohexyl, PCMA. Two other host polymers were polystyrene, PS, and poly(vinyl acetate), PVAc. 

Semerak SN, Frank CW (1984) Excimer fluorescence as a molecular probe of polymer blend miscibility. 6. Effect of molecular-weight in blends of poly(2-vinylnaphthalene) with poly (methylmethacrylate). Macromolecules 17(6) 1148-1157. doi 10.1021/ma00136a008... 

Excimer fluorescence involving a complex (excited state-ground state) between adjacent or non-adjacent fluorescent units with the same polymer chain or intermolecular association between units on different chains can also be studied to assess phase behavior and the level of mixing in polymer blends. These studies generally involve the addition of low concentrations of aromatic polymers (capable of fluorescence) to non-fluorescent polymers. Excimer fluorescence is favored by phase separation, because the intermolecular associations of the fluorescent polymer will be shielded by the miscible non-fluorescent polymer, in which case monomer emission will be more dominant. This technique was developed and demonstrated initially by Frank and coworkers [333-335]. 

In order to determine whether energy migration makes a significant contribution to the photophysical behavior of P2VN and PS in dilute miscible blends, it is instructive to calculate the expected exdmer-to-monomer fluorescence quantum yield ratio in the absence of energy migration. To do so, it is first necessary to assume that intermolecular and non-adjacent intramolecular EFS are absent. In addition, the adjacent intramolecular EFS are assumed to be frozen into the aryl vinyl polymer and must be excited by direct absorption of a photon. Since the absorption spectrum of an EFS is no different from that of non-EFS chromophores, then the calculated fraction of rings within EFS is sufficient to determine the fluorescence ratio. 

We conclude that the difference between the experimental value and the no-transfer value of the fluorescence ratio of P2VN and PS is less in solution than in dilute miscible blends, because energy migration must compete with rotational processes in the generation of excimers in solution. This difference is also present when the effect of molecular weight on aryl vinyl polymers in solution and in dilute miscible blends is considered in the next section. 

The preceding studies on the configuration of aryl vinyl polymer chains in dilute, miscible blends and on the kinetics of phase separation in concentrated blends were based on the implicit assumption that the Initial solvent cast blend represented an equilibrium state. In the final section of this paper we explore this question with new data on the effect of the casting solvent on the fluorescence behavior of PS/PVME blends. Our objectives are to determine first whether the fluorescence observables are sensitive to differences in as-cast films and then to identify the true equilibrium state. 

The dependence of excitation transport on local chromophore concentration has been used to provide qualitative information on the characteristics of polymers in blends. Excimer fluorescence resulting from excitation transport has been employed to characterize polymer miscibility, phase separation and the kinetics of spinodal decomposition (1-31. Qualitative characterization of phase separation in blends (4.51 and the degree of chain entanglement as a function of sample preparation and history (6.71 has also been investigated through transport with trapping experiments. In these experiments one polymer in the blend contains donor chromophores and the second contains acceptors. Selective excitation of the former and detection of the latter provides a qualitative measure of interpenetration of the two components. 

The emission intensity of blends of polyimides gives a measure of the degree of miscibility of two polymers.401 For example in blends of the strongly fluorescent polyimide (32) and the non-fluorescent polyimide (34) added at the level of 30 mo %, the intensity of the emission of (32) is quenched by half, indicating that the formation of intermolecular charge transfer complexes within (32) is disturbed significantly by the added polyimide. 

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. 

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