In a previous study of solution-cast poly(9,9-di-n-hexyl-2,7-fluorene) (PFH) [Polymer 2012, 53, 3928], transition from solvent-induced β mesomorph to crystalline α form had been identified: upon heating from 120 °C, growth/coalescence of 2D-ordered β nanograins effectively suppressed the formation of α nanograins below 189 °C, above which transformation from β nanograins to thermodynamically favored α crystals was activated by partial melting of the β mesomorph. Here we report results of our attempt to clarify if similar processes exist in its close homologue, poly(9,9-di-n-octyl-2,7-fluorene) (PFO), which is well-known to have a similar (yet less ordered) β phase.
Structural evolution of β-rich PFO was monitored via simultaneous small/wide-angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC) during programmed heating of as-cast PFO specimen from 30 to 180 °C at 10 °C/min. After removal of background scattering from fractal-like matrix structure, SAXS profiles along with supporting WAXS observations can be interpreted with a similar sequence of events previously observed for as-cast PFH. Specifically, structural evolution of β-rich PFO above Tg involves four stages: (1) decreased lateral sixe of β nanograins with a minor change in ellipsoidal dimension (A, B) from (2.0 nm, 9.2 nm) to (2.1 nm, 8.2 nm) between 80 and 100 °C, (2) direct β-to-α transformation accompanied by emergence of α nuclei from the amorphous matrix, with a slight decrease in inter-particle distance from d = 28.4 to 26.0 nm, a concomitant change in ellipsoidal dimension changes from (A, B) = (2.1 nm, 8.2 nm) to (2.7 nm, 7.7 nm), and a significant increase in the SAXS invariant Qinv (signifying increased heterogeneity) from 100 to 110 °C, (3) growth of α nuclei resulting in increased ellipsoidal dimension from (A, B) = (2.7 nm, 8.5 nm) to (2.8 nm, 9.1 nm) with a concomitant DSC exotherm, and (4) partial melting/coalescence of α nanograins, leaving thick crystals of ellipsoidal dimension (A, B) = (3.9 nm, 14.5 nm) and wider inter-grain spacing d ≈ 40 nm before final melting near 145 °C. The distinct feature of the nanograin evolution process in β-rich PFO lies in Step 2, i.e., unlike the PFH case, the formation of α crystals in β-rich PFO is not limited to direct β-to-α transformation; nucleation and growth of α crystals from the amorphous matrix may also contribute significantly. This may probably be attributed to the less-ordered β mesomorphic structure in PFO as compared to the 2D-ordered β-packing in PFH.

In a previous study of solution-cast poly(9,9-di-n-hexyl-2,7-fluorene) (PFH) [Polymer 2012, 53, 3928], transition from solvent-induced β mesomorph to crystalline α form had been identified: upon heating from 120 °C, growth/coalescence of 2D-ordered β nanograins effectively suppressed the formation of α nanograins below 189 °C, above which transformation from β nanograins to thermodynamically favored α crystals was activated by partial melting of the β mesomorph. Here we report results of our attempt to clarify if similar processes exist in its close homologue, poly(9,9-di-n-octyl-2,7-fluorene) (PFO), which is well-known to have a similar (yet less ordered) β phase.
Structural evolution of β-rich PFO was monitored via simultaneous small/wide-angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC) during programmed heating of as-cast PFO specimen from 30 to 180 °C at 10 °C/min. After removal of background scattering from fractal-like matrix structure, SAXS profiles along with supporting WAXS observations can be interpreted with a similar sequence of events previously observed for as-cast PFH. Specifically, structural evolution of β-rich PFO above Tg involves four stages: (1) decreased lateral sixe of β nanograins with a minor change in ellipsoidal dimension (A, B) from (2.0 nm, 9.2 nm) to (2.1 nm, 8.2 nm) between 80 and 100 °C, (2) direct β-to-α transformation accompanied by emergence of α nuclei from the amorphous matrix, with a slight decrease in inter-particle distance from d = 28.4 to 26.0 nm, a concomitant change in ellipsoidal dimension changes from (A, B) = (2.1 nm, 8.2 nm) to (2.7 nm, 7.7 nm), and a significant increase in the SAXS invariant Qinv (signifying increased heterogeneity) from 100 to 110 °C, (3) growth of α nuclei resulting in increased ellipsoidal dimension from (A, B) = (2.7 nm, 8.5 nm) to (2.8 nm, 9.1 nm) with a concomitant DSC exotherm, and (4) partial melting/coalescence of α nanograins, leaving thick crystals of ellipsoidal dimension (A, B) = (3.9 nm, 14.5 nm) and wider inter-grain spacing d ≈ 40 nm before final melting near 145 °C. The distinct feature of the nanograin evolution process in β-rich PFO lies in Step 2, i.e., unlike the PFH case, the formation of α crystals in β-rich PFO is not limited to direct β-to-α transformation; nucleation and growth of α crystals from the amorphous matrix may also contribute significantly. This may probably be attributed to the less-ordered β mesomorphic structure in PFO as compared to the 2D-ordered β-packing in PFH.

ABSTRACT I
List of Figures III
1. Introduction 1
1.1. Background 1
1.2. PFO 2
1.3. PFH 2
2. Objectives and Approach 7
3. Experimental Details 8
3.1. Materials and Specimen Preparation 8
3.2. Instruments and experiment details 8
4. Results 10
4.1. Data Analysis 10
4.2. Phase transformation in β-rich PFO 11
4.3. In comparison to cold-crystallization of nematic phase 17
4.4. Phase-dependent optical absorption and photoexcited emission 21
5. Conclusion 22
References 24
Appendix A. Isothermal experiments 26

1. Chen, S. H.; Wu, Y. H.; Su, C. H.; Jeng, U.; Hsieh, C. C.; Su, A. C.; Chen, S. A. Macromolecules 2007; 40, 5353-5359.
2. Su, C. H.; Jeng, U.; Chen, S. H.; Lin, S. J.; Ou, Y. T.; Chuang, W. T.; Su, A. C. Macromolecules 2008; 41, 7630-7636.
3. Su, C. H.; Jeng, U.; Chen, S. H.; Lin, S. J.; Wu, W. R.; Chuang, W. T.; Tsai, J. C.; Su, A. C. Macromolecules 2009; 42, 6656-6664.
4. Wu, W. R.; Chuang, W. T.; Jeng, U. S.; Su, C. J.; Chen, S. H.; Chen, C. Y.; Su, C. H.; Su, A. C. Polymer 2012; 53, 3928-3936.
5. Chen, S. H.; Su, A. C.; Chen, S. A. Journal of Physical Chemistry B 2005; 109, 10067-10072.
6. Lu, H. H.; Liu, C. Y.; Chang, C. H.; Chen, S. A. Adv. Mater 2007; 19, 2574−2579.
7. Cadby, A. J.; Lane, P. A.; Mellor, H.; Martin, S. J.; Grell, M.; Giebeler, C.; Bradley, D. D. C.; Wohlgenannt, M.; An, C.; Vardeny, Z. V. Phys Rev B 2000; 62, 15604-15609.
8. Kilina, S.; Batista, E. R.; Yang, P.; Tretiak, S.; Saxena, A.; Martin, R. L.; Smith, D. L. ACS nano 2008; 2, 1381-1388.
9. Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. Adv Mater 1997; 9, 798-&.
10. Blondin, P.; Bouchard, J.; Beaupre, S.; Belletete, M.; Durocher, G.; Leclerc, M. Macromolecules 2000; 33, 5874-5879.
11. Kawana, S.; Durrell, M.; Lu, J.; Macdonald, J. E.; Grell, M.; Bradley, D. D. C.; Jukes, P. C.; Jones, R. A. L.; Bennett, S. L. Polymer 2002; 43, 1907-1913.
12. Teetsov, J.; Fox, M. A. J Mater Chem 1999; 9, 2117-2122.
13. Feigin, L. A.; Svergun, D. I. Structure Analysis by Small-Angle X-ray and Neutron Scattering; Plenum: New York, 1987; p 69.
14. Chen, S. H.; Chou, H. L.; Su, A. C.; Chen, S. A. Macromolecules 2004; 37, 6833-6838.
15. Chen, S. H.; Su, A. C.; Su, C. H.; Chen, S. A. Journal of Physical Chemistry B 2006; 110, 4007-4013.
16. Chen, S. H.; Su, A. C.; Chen, S. A. Macromolecules 2006; 39, 9143-9149.
17. Chen, S. H.; Su, A. C.; Su, C. H.; Chen, S. A. Macromolecules 2005; 38, 379-385.
18. Chuang, W. T.; Su, W. B.; Jeng, U. S.; Hong, P. D.; Su, C. J.; Su, C. H.; Huang, Y. C.; Laio, K. F.; Su, A. C. Macromolecules 2011; 44, 1140-1148.
19. Grell, M.; Bradley, D. D. C.; Long, X.; Chamberlain, T.; Inbasekaran, M.; Woo, E. P.; Soliman, M. Acta Polym 1998; 49, 439-444.
20. Grell, M.; Bradley, D. D. C.; Ungar, G.; Hill, J.; Whitehead, K. S. Macromolecules 1999; 32, 5810-5817.
21. Tseng, G. L.; Ruan, J. J.; Lan, Y. K.; Wang, W. Z.; Su, A. C. Macromolecules 2013; 46, 1820-1831