Figure 1

Orthogonal orientations for solvation of MEH-PPV in a smectic LC. Top: schematic picture of the nematic (left) and ideal smectic (right) LC phase. In the nematic phase, the solvent molecules (indicated by cylinders) have an orientational order with an alignment of the main molecular axis parallel to the LC director. In the ideal smectic phase, the solvent molecules possess positional order along the director forming two-dimensional layers. Solvation of larger polymers (not scaled) is indicated by the blue and red chains. Bottom: $P$ histograms measured with excitation polarized parallel (a), (c) and orthogonal (b), (d) to the director. The insets show a typical fluorescence burst from a single polymer molecule diffusing through the excitation volume. Positive $P$ is consistent with parallel alignment to the director (blue chain). For orthogonal excitation in the smectic phase (d), the fluorescence transient reveals the presence of molecules with an opposite $P$. These molecules are orientated perpendicular to the director (red chain). The corresponding $P$ histogram shows that a small fraction ($\sim 10\%$) of polymer molecules is aligned perpendicular with a larger degree of disorder. The solid lines are theoretical fits to the histograms using a previously described anisotropic mean-field solvation model which also includes optical effects due to photon shot noise and the high numerical aperture of the objective [4].Reuse & Permissions

Figure 2

$P$ histograms using parallel (left) and orthogonal excitation (right). The temperature is varied from 28 to 37 and back to 28 °C (top to bottom) cycling between the smectic and nematic LC phases. The $P$ histograms on the right show a second distribution with negative $P$ values indicating polymer molecules aligned perpendicular to the director in the smectic phase (top and bottom), which is absent in the nematic phase (middle) and for a sample containing only 8CB (not shown). For parallel excitation (left, 488 nm, $1\mathrm{kW}/{\mathrm{cm}}^2$), the maximum of the $P$ histogram shifts to smaller values in the nematic compared to the smectic phase (see line for $P=0.8$). This is due to the increased stretching of the polymer chain with increasing order of the LC solvent molecules (see Fig. 3).Reuse & Permissions

Figure 3

Peak $P$ as a function of temperature. $P$ increases roughly linearly with temperature in the studied temperature range from 27 to 37 °C across the smectic-nematic phase transition. The lines are independent linear fits to $P$ in the smectic and nematic LC phases demonstrating the lack of a strongly discontinuous phase transition for $P$. The decrease of $P$ with increasing temperature can be explained by a decrease in conformational order of the polymer chain due to a lower solvation energy counteracting the intrinsic bending energy and conformational entropy of the chain. (The shown error bars are representative for all data points.)Reuse & Permissions

Figure 4

Graphical representation of the experimentally determined orientational probability distribution functions for the segments of the polymer molecules in the parallel (a) and perpendicular (b) solvation sites. The average molecular polarizations $P_{\mathrm{chain}}$ of the polymer chains are 0.75 and 0.4 for the parallel and perpendicular solvation site, respectively. (c),(d) Corresponding conformations of a 100-segment homopolymer generated by beads-on-a-chain simulations [2, 4]. The arrow indicates the direction of the LC director.Reuse & Permissions