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24 p3ht - poly(3-hexylthiophene)Figure 2.9.: P3HT ideal s-crystal (left), P3HT bulk relaxed at low temperature(center) and P3HT bulk after a room temperatureannealing (right).1.62 Å −1 decreases in the crystalline bulks as a result ofthe tilt of thiophene rings with respect to the x direction.2.4 p3ht surfacesThe equilibrium P3HT crystal can be cut across the π −π or the interdigitation directions, obtaining a 010 or 100surface, respectively. The surfaces equilibrated at 1 K andat room temperatures are shown in Figure 2.11.The surface energy in the two cases has been evaluatedand it is 0.008 J/m 2 larger for the 010 surface. This is consistentwith the larger cohesion in the π − π cut with respectto interdigitation. At low temperature, we note a sizabledifference in the order of the two surfaces, resulting in thedifferent S z peaks (green and red in Figure 2.12 bottom). Inparticular, the peak of the 010 surface is higher than that ofthe 100. This depends on the fact that 100 surface (havingflexible hexyl terminating groups) gives rise to a shrinkingof the underlying π channels.On the other hand, at room temperature the 010 surface(s-foil terminated) gives rise to a sizable microstructureevolution characterized by an increase of disorder (seeFigure 2.11 top right). The corresponding order parameter(green curves in Figure 2.12) lowers in all the directions.Once more, the different behavior can be attributed to thehigher excess energy induced by the 010 cut.2.5 nanocrystalline p3htThe polymer layers occurring in the hybrid interfacescan derive from the aggregation of nanocrystals previouslyformed during synthesis. The structure and the propertiesof a single nanocrystals have to be investigated in order tobetter understand the polymer morphology in real inter-