Structural evolution of as-casting and quenched poly(9,9-di-n-hexyl-2,7-fluorene) (PFH) in heating process were quantitatively examined with in-situ small/wide angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC).
After removal of background scattering from fractal-like matrix structure, SAXS profiles obtained during programmed heating of a quenched PFH glass from 30 to 280 □C at 10 □C/min can be interpreted with a similar sequence of events previously observed in cold crystallization of poly(9,9-di-n-octyl-2,7-fluorene) (PFO). Specifically, the nanograin evolution of PFH involves five stages: (1) the frozen-in stage below 110 □C, (2) the nucleation of prolate-like nanograins with a constant radius of gyration Rg=3.7 nm between 125 and 130 □C, (3) the growth of nanograin size to Rg=6.0 nm concomitant with the emergence and development of WAXS-determined crystallinity (Xc,WAXS) from 133 to 160 □C, (4) the coalescence of the oblate nanocrsytals into a greater size of Rg=9.0 nm upon further heating up to 210 □C, and (5) the gradual melting of thinner nanograins, leaving thick crystals of Rg=12.0 nm and wider inter-grain spacing d with decreasing crystallinity just before final melting near 260 □C.
We have also investigated via model fitting the phase transformation process in solution-cast poly(9,9-di-n-hexyl-2,7-fluorene) (PFH) upon heating from the as-cast state (rich in beta phase) up to final melting. There emerged alpha nuclei is mainly owing to the transformation from beta phase in beta nanograins melting stage (189~200 □C). There was then some extent of coalescence of the □ nanograins. The SAXS/WAXS profiles exhibited little changes during the subsequent isothermal annealing at 200 □C. The delayed nucleation and the activation of growth and coalescence processes only after melting of beta phase suggest that alpha nuclei were formed within the beta nanograins. Results indicate that the beta-to-alpha transition completed quickly in the range of 189 to 200 □C (ca. 60 s at a heating rate of 10 □C/min), with significant decreases in Qinv and Io.

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