Solution-Crystallization and Related Phenomena in 9,9-Dialkyl-Fluorene Polymers. I. Crystalline Polymer-Solvent Compound Formation for Poly(9,9-dioctylfluorene)

Polymer-solvent compound formation, occurring via co-crystallization of polymer chains and selected small-molecular species, is demonstrated for the conjugated polymer poly(9,9-dioctylfluorene) (PFO) and a range of organic solvents. The resulting crystallization and gelation processes in PFO solutions are studied by differential scanning calorimetry, with X-ray diffraction providing additional information on the resulting microstructure. It is shown that PFO-solvent compounds comprise an ultra-regular molecular-level arrangement of the semiconducting polymer host and small-molecular solvent guest. Crystals form following adoption of the planar-zigzag β-phase chain conformation, which, due to its geometry, creates periodic cavities that accommodate the ordered inclusion of solvent molecules of matching volume. The findings are formalized in terms of nonequilibrium temperature–composition phase diagrams. The potential applications of these compounds and the new functionalities that they might enable are also discussed. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1481–1491


FIGURE S3
Heating rate dependence of the DSC thermograms recorded for PFO gels in (a) dodecane and (b) hexadecane, with polymer concentrations c p = 60 and 56 wt % respectively.
Mixtures were crystallized at constant 2 °C min -1 rates and re-heated at the indicated rates.
Dashed red lines are a guide to the eye for the position of the low-temperature endotherms.
The relative contribution of the high-temperature endotherm is reduced when higher heating rates are used. This is understood to be due to kinetically-hindered recrystallization at high heating rates which in turn results in lower relative enthalpy of the high-temperature endotherm. Such behaviour is well-documented for sPS gels [3] as well as neat sPS. [4] 4

FIGURE S4
Photoluminescence (PL) spectrum of a slowly-crystallized c p = 0.3 PFOdodecane gel (solid black line) that has previously undergone repeated heating/cooling cycles in the DSC between -50 and 160 °C. Prior to PL measurement, the gel was pressed into a film at room-temperature using a bench-top hydraulic press (load = 10 tons) and subsequently desiccated to ensure that minimal amount of solvent remained in the film. Also shown is the PL spectrum of a drop-cast PFO thin film (dashed red line) for which the fraction of β-phase chain segments was estimated to be ∼30%. Excitation wavelength λ ex = 390 nm in both cases.
The two spectra are normalized by their S 1 -S 0 0-1 vibronic in order to correct for the effect of different degrees of self-absorption for the two films. Two features deserve particular mention: The PL spectrum of the gel is typical of PFO samples possessing a high fraction of chains segments in the β-phase conformation, with S 1 -S 0 vibronic peaks located at 440, 467 and 500 nm.
(ii) PL intensity at 535 nm (indicated by the arrow) is comparable for both samples.
For PFO, 535 nm is expected to be the spectral position of the peak in green emission due to oxidative degradation of 9,9-dioctylfluorene to 9-fluorenone moieties. [5,6] This indicates that the solution-crystallization behaviour of PFO investigated in this study is unlikely to be complicated by the additional presence of on-chain fluorenone defects.  The free-solvent contribution has been subtracted in each case (cf. Figure S5). The increase in intensity and sharpening of the reflection at d = 0.42 nm with reducing temperature is evident. 7

Estimating the cavity volume in the PFO-solvent compound from WAXD
data.

FIGURE S8
Schematic illustration of chain structure in a PFO-solvent compound (cf. Figure   4(b) in the main text).
The maximum length of a single cavity along the c-axis corresponds to 0.84 nm, i.e. the length of a F8 repeat unit. [7,8] The spacing of adjacent PFO chains along the a-axis was calculated from the (100) reflection in WAXD patterns recorded for as-prepared gels in dodecane, for which the stoichiometry of 1 : 1 was measured. As is evident from Figure S6, only a fraction of the length along the a-axis would be available for the placement of intercalated solvent due to the presence of the dioctyl side-chains; thus, for the estimation of cavity volume V c , it was arbitrarily scaled by 75%. The spacing of adjacent chains along the b-axis is estimated as 0.4 nm, calculated from the (020) reflection in the WAXD patterns recorded for the as-prepared PFO-dodecane gels. We note that these values closely correspond to the dimensions of a solvent-free β-phase crystal reported by Liu et al. [8] Calculating V c using these values yields: cooling rates were used.
Crystallization half-time is defined as the time elapsed from the onset of crystallization to the system reaching 50% degree of transformation, i.e. the value of relative crystallinity = 50%.
Thus, crystallization half-time is inversely proportional to crystallization rate. These preliminary results emphasise that judicious choice of solvent is necessary for controllable solution-processing for PFO, e.g. in situations when it is desirable to avoid the presence of βphase chain segments in the solid-state.