Side-Chain NLO Polymers

Another successful approach involves the cross-linking of the side chain NLO polymer, after poling, at multiple sites by a different type of polymerisation mechanism. Subsequent curing and hardening produces a lattice that locks in the poled dipole. . One such process is outlined in Figure 5.32. 

In PI-3a, PAP produces a relatively small polar order as opposed to the efficient poling achieved by PAP in true side-chain NLO polymers, such as PMMA or polyimides with Tgs in the 120-265°C range, which are flexibly tethered by DR 1-type chromophores. The molecular size of the diaryiiSne azo chromophore of PI-3a is substantially larger than that of the DRl-type mokcules in the polymers studied previously (see PI-3b in Figure 8.4), a feature that requires more free volume for chromophore movement thereby decreasing... 

A number of side-chain NLO polymers have been prepared by various polymerization meth-... 

One efficient way to increase the number density of NLO chromophores in a polymeric host, without crystallization, phase separation, and any concentration gradient, is to attach them as side chains of a polymer [15,18]. Also, the temporal stability of the poled structures of side-chain NLO polymers has been proven to be much better than that of the same guest-host system due to the higher glass transition temperature of the polymer. A number of poled side-chain NLO polymers (Fig 49.4) have employed for SHG measurements. Table 49.3 exhibits values of some side-chain NLO polymers [16-28]. 

Many of the side-chain NLO polymers, both copolymers and homopolymers, have been designed using a PMMA polymer skeleton as the backbone. This includes novel methacrylate polymers which contain a molecular-ionic NLO chromophore, A-akkylpyridinium salt, in the side chain [25,26]. The corona-poled polymer films of such polymers showed a larger x value of the homopolymer... 

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. 

Fig. 2.12 Synthetic procedure for a side-chain NLO polymer (type I), after [56]...

Fig. 2.13 Typical behaviour of 33 as a function of the chromophore molar content for side-chain NLO polymers (type 1) (From Persico et al. [53]. Reprinted with permission of John Wiley Sons, Inc)...

An other approach to the synthesis of type I side-chain NLO polymers is the post-fiinctionalization of a preformed polymer chain with a NLO chromophore (Fig. 2.16). Examples include post-functionalization by azo coupling, Knoevenagel condensation, Mitsunobu condensation between imide or carboxy functions of the polymer chain and alcohol functions of the chromophore [66-68]. This approach to NLO polymers has several advantages as the possibility of easy modulation of the chromophore content in the functionalized polymer and, in some cases, the possibility of using commercially available polymers as substrates for the post-functionalization. 

Fig. 2.16 Synthetic approach for type I side-chain NLO polymers (polyimide) by postfunctionalization, Mitsunobu reaction, from [67]...

In another relevant class of side-chain NLO polymer, the chromophore unit is attached to the chain as a side pendant without spacers (type n), in such a way that one atom of the chromophore does belong to the polymer chain generally that atom is the N donor of the chromophore (donor embedded polymers). 

This was accomplished by inducing a cross-linking reaction between the reactive pendant groups of a side-chain NLO polymer and the reactive end groups of the chro-mophore. Stable NLO response at 90°C for more than 2000 h was demonstrated after a small initial decay [92]. 

Jen a al. have developed dendronized polymeric NEO materials that have shown significantly improved poling efficiency by encapsulating chromophore with dendritic substituents that can electronically shield the core, ii-electrons, and form spherical molecular shapes." "" " Figure 6 illustrates different molecular architectures of dendronized side-chain NLO polymers with crosslinkers. The diverse selections of molecular architectures provide additional flexibility in the molecular engineering of high-performance polymeric NLO materials. Moreover, the unique nanoscale environment created by the shape and size, dielectric properties, and distribution of chromophores in crosslinkable polymers with dendrons and dendrimers can all play critical roles in maximizing the macroscopic EO properties of polymeric NEO materials. 

NLO chromophore 1=1 Crosslinkable moiety Figure 6 Molecular architectures of dendronized side-chain NLO polymers with crosslinkers. 

Side-chain NLO polymer with dendronized chromophore... 

Figure 10 Molecular structures of guest/host polymer 27, conventional side-chain NLO polymer 28, and dendronized side-chain NLO polymer 29.

To completely consume 12 mol.% of the anthracenyl group in PMMA-AMA, 25wt.% of active core chromophore contents can be theoretically fed for the formation of 51. The data of values and optimal poling temperatures for the polymer 50 are very reproducible and agree well with the analogous side-chain NLO polymers prepared by solution reaction. For example, the ra values of 50 and its analogue are 43 and 42 pm V, respectively. 

Figure 26 Molecular structure of guest chromophore and side-chain NLO polymer (denoted DR-1-co-PMMA) system.

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