Guest-Host-Type Polymers

Quantum mechanical analysis based on a simple two-level model [22] and bond-order alternation (BOA) principle exploiting aromaticity [23] have worked surprisingly well in providing useful structure/property relationships for the design of chromophores with ever improving molecular hyperpolarizability. Table 1 provides some representative examples with improved molecular optical nonlinearity developed over the past decade. It has been shown that very large nonlinearities 

The molecular orientation in the poled polymers is thermodynamically unstable and quickly decays in low-Tg polymers such as PMMA, resulting in a greatly reduced nonlinearity. However, if the Tg of the polymer is roughly 150-200 °C above the ultimate operating temperature, decay of the orientation would be negligible over the device lifetime. Many kinds of NLO chromophores were incorpo- 

The experimental value of r, , also agrees fairly well with the predicted value of 48 pm that was calculated from by using a two-level model suggested by Katz and Singer et al. [33], after accounting for dispersion effects from both the EFISH and electro-optic measurements. The r, , is expressed as  

Here a) is the optical frequency used in the EFISH measurement, co is the frequency adopted in the electro-optic measurement, and cOq is the transition energy between the ground state and first excited state of the chromophore. 

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The dashed line represents the data for the guest/host type samples reported by StShelin et al An arrow indicates the temperature at 110 °C, at which no decay of the SH coefficient was observed after a period of 168 h. The error bar for the data point at 130 °C is indicative of the large deviations in x values at lower temperatures. The results show that the relaxation behavior of the IPN is significandy different from those of the guest/host type uncrosslinked polymers. The stability of the d33 coefficient for the IPN samples at low temperatures as well as temperatures close toTg is substantially superior to the guest/host type samples. 

In the case of LC polymers, the polymeric matrix performs as a host, while the guest is a dye, whose molecules are elongated in shape, and the absorption oscillator is parallel (or perpendicular) to the big axis of the molecule 65,163-165>. The experiments investigating guest-host effect in nematic polymers with dichroic dyes covalently attached to the polymer 163) (type I) and mechanically incorporated65) (type II) reveal the possibility to obtain regulated color indicators (see page 60). 

Fig. 1. Schematic representation of different types of functionalized polymers guest-host system(a), side-chain polymer (b), main-chain polymer (c), photo-and thermally crosslinking polymer...

Zhou, Y., L. Shaojun, and Y. Cheng. 2003. Poling properties of guest-host polymer films of X-type nonlinear optical chromophores. Synth Met 147 519-1520. 

Progressing from guest-host systems (chromophores dissolved in a polymer matrix) to sidechain polymer systems (chromophores chemically attached to a polymer backbone), one arrives at another class of materials where the dipole is actually part of the polymer backbone, i.e., mainchain polymer systems. In fact, there are some reports on mainchain chromophoric copolymers in which the chromophores are attached head to tail. Figure 1 summarizes the four types of polarized organic materials. 

The use of chromophores covalently bonded to the polymer chain (NLO polymer) is a natural evolution of guest-host systems. In this case the polymer is obtained from a monomer (or a co-monomer) which is a chromophore too. There are clearly different choices for the way the chromophore is oriented with respect to the polymer chain. One possibility is to put the chromophore unit as a side pendant of the polymer chain (side-chain NLO polymer). This, in turn, can be accomplished by connecting the chromophore to the chain through suitable spacer groups, for instance conforma-tionally flexible polymethylenic units (polymer type I) in these polymers, no atoms of the chromophore unit, between the donor and acceptor groups, formally belong to the polymer chain. 

The layered guest-host structures that can also develop undergo a structural evolution more reminiscent of the intercalation process in 2D layered materials (e.g., graphites or silicates). In this case, quasi-2D galleries open up between stacked sheets of the polymers chain as shown in Fig. 25.8. This type of ordering has often been reported in structural studies [84-89] using molecular dopants. Systems showing evidence of layered structures include iodine-doped PA [86] and AsF.s-doped PPV [87]. These layered structures may occur either alone or in combination with channel formations [85,89]. 

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