## 3 Sample Environments
### Electromagnetic Fields  
Sample environments generating electric fields have not been extensively developed for use at neutron beamlines. AC fields that can be used to (partially) align relatively short rigid polymeric molecules in solution in order to perform fiber diffraction experiments have been used for biological molecules. The main problem with this approach is that the frequencies used should be such that the sample does not heat up and that the fields are high enough that they are not shielded by the salts in the buffer solution [86]. The main problem with using static electric fields is that the voltage required as function of sample thickness is considerable. When using too high a field, one risks electric breakdown damaging the sample. To induce domain alignment in thin films of symmetric diblock-copolymer of polystyrene and poly(methylmethacrylate), (PS-b-PMMA) fields of 40 V/\(\upmu\)m were used in a sample cell consisting of an aluminumized Kapton film used as an electrode and a similar electrode that is isolated from the sample by a thin layer of poly(dimethylsiloxane) [87]. The alignment process could be followed by time-resolved SANS experiments. As to be expected, the interplay between the sample thickness and the interfacial interactions between the electrode and the sample plays an important role. For thin samples, where the interfacial interactions are most important, the structure can evolve over periods of hours. On the other hand, for thicker films, complete alignment takes only minutes. Static electric-field-induced deformation of bulk poly(styrene-block-isoprene) lamellar phases, swollen in toluene or tetrahydrofuran, has been studied with SAXS and with SANS. Capacitor cells with 3-5 mm electrode gaps and typically 5 mm beam path length were used to apply up to 60 kV, reaching electric fields up to 12 kV/mm [88]. SeeDielectrophoresis uses the intrinsic polarizability of objects/molecules to be manipulated in a gradient or AC electric field. When this method is applied in combination with SANS, it is possible to follow the development of molecular ordering due to different field strengths and frequencies. A model system, polystyrene particles in H\({}_{2}\)O (diameter 195 nm and 530 nm), and an applied AC field formed colloidal crystals consistent with the space groups P6 mm and C2 mm [90]. A schematic overview of the cell is shown in Figure 11. The AC field was applied in transverse direction to the neutron beam and depending on the experiment, a range of 0 \(<\omega<\) 100 kHz and 0.62 \(<\) E\(<\)1.4 kV was used with a cell thickness in the neutron beam direction of 130 microns. The cell thickness was limited in this design due to the requirement of exposing the sample to a reasonably homogeneous AC field; a thicker cell would contain field gradients. Where the formation of colloidal crystals by the application of an AC electric field is maybe somewhat counterintuitive, the formation of oriented bundles of rather stiff fibrous molecules of the conjugated polymer poly(3-hexylthiophene) (P3 HT), with a persistence  
Figure 11: Top view of an electrophoresis cell used for on-line SANS experiments (panel (**a**)). The AC field is applied perpendicular to the beam direction and induces the formation of a colloidal crystal. Panel (**b**) shows a representative diffraction result. Figures not to scale (reproduced with permission from [90], The Royal Society of Chemistry, 2010). Figure 10: Electric field setup used for on-line SANS experiments [89]. Schematic drawing of experimental setup (**a**). Capacitor dedicated for small-angle neutron scattering experiments of polymer solutions (**b**). The capacitor consists of gold electrodes and its interior parts are mantled in Teflon. length of several nm, is less surprising [91]. The authors are also describing parallel optical experiments where the data were Fourier transformed to provide an estimate of the degree of orientation.