Pole procedure

Figure 7. Schematic of thermal processing and poling procedure for a thermoset NLO polymer.

In all techniques the chromophore orientation is frozen by cooling the poled film to the room temperature under the applied external field (Fig. 24) or by thermal or photocrosslinking during the poling procedure. In the case of thermal crosslinking it is important to tightly control the poling temperature and to in-... 

The processing steps are as follows. First, deposit a film of one micron thickness of the material on a substrate by means of dip or spincoating. Then, apply an electric field at temperatures greater than the glass temperature. Finally, cool down the sample to room temperature under the presence of a d.c. electric field. The pole procedure is shown in Figure 6.31, where Tg and Tp are the polarization and glass temperatures, respectively. 

MV/m) than that of solid ferroelectrics (usually a few MV/m or smaller). The ferroelectric properties depend sensitively upon the details of sample preparation, for example the use of melt quenching or melt extrusion, the annealing temperature, or the details of the poling procedure. Polymer ferroelectrics are useful for soft transducers. 

The statistically non-centrosymmetric polar order of the chromophore units reached during the poling procedure is thermodynamically stable only with the electric field on. Once cooled down at room temperature and switched off the field, that polar order becomes metastable and it will relax to the more stable centrosymmetric arrangement, which is not NLO active. So, an other fundamental issue is to reach an high time stability of the polar order. Industrial standards for device testing require stability of NLO performances for 10 years at 85°C, that are quite severe. 

Historically, guest-host systems have been the first polymeric NLO active materials to be developed. In these systems a high Per chromophore is physically dispersed in a suitable, amorphous polymeric matrix. The non centrosymmetry is induced by the usual poling procedure. Polymer matrices most frequently used include poly(methylmethacrylate), PMMA, because of its excellent optical properties, as well as other amorphous polymers with high Tg as amorphous polycarbonate (APC), polyquinoline, polyimides. Many reports have dealt initially with guest-host systems containing DRl or DANS as the dispersed chromophore. 

P-type main-chain polymers, in which the direction of the dipole moments of the chromophores is locally parallel to the polymer chain, can be further divided in two classes polymers in which the sequence of dipoles is random and those in which the sequence is regular. Polymers of the first type are potentially less interesting, basically because the poling procedure is expected to be difficult for them. In fact, chromophores are picked at both ends to the chain and, moreover, their disposition along the chain is statistically centrosymmetric. Yet, for a polymer of this type, a fair SHG activity has been reported upon poling [87]. 

These polymers, in fact, have the advantage that, within each chain, the polar orientation of chromophores, given by the synthesis, cannot be altered because it is due to covalent bonds moreover, the poling procedure should be efficient in this case because of the increased dipolar moment of chain segments. 

The dc electric field (typically up to 100 200 V/p,m) is applied to the material at a temperature close to the polymer glass transition temperature (Tg) during the poling procedure. At such temperatures the molecule dipoles acquire a sufficient mobility to be oriented under the electric field effect. The achieved orientation is then frozen in by cooling the polymer to room temperature. 

After the discovery of significant piezoelectricity in polymers in the late 60s, they were immediately considered for applications in the early 70s. Interest faded in the early 80s due to problems in producing reproducible devices. After solving these initial problems by developing efficient poling procedures and suitable electrical contacting methods, piezoelectric polymers became a commercial success in many niche applications. Science progresses in waves, and currently, we face a very active research phase where piezoelectric and relaxor ferroelectric polymers are employed... 

A typical poling procedure was to apply an electric field of about 120 MV/m, in-aease the temperature to about 20QI C, k the sample at that temperature for 10 min, and follow by decreasing the temperature under the fold. 

Although 0-3 composites appear to be simple in structure, many factors are involved in the choice of polymer and filler materials, fabrication and poling procedures. A large research effort has been mounted to optimize these factors and make better 0-3 composites. Analysis of the design considerations and the approach to fabricating a sensitive 0-3 composite are elaborated on later. 

Liepins and co-workers reported that homopolymers, copolymers, and ter-polymers of tributyltin methacrylate, and trimethyltin methacrylate were made piezoelectric active in thin films by preferential dipole-orienting solvent or by poling procedures. For instance unpoled tributytin methacrylate copolymers exhibited d3 piezoelectric activity of 1.5 X10 C/N, while poled film exhibited g31 activity of 1.6 X10 Vm/N. This is good piezoelectric activity, and it is surprising that this work has not been followed up. These materials also showed good antifouling properties and paint formulation characteristics. ... 

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