Structural Evolution of Poly(9,9-di-n-octyl-2,7-fluorene) during Mesophase-modulated Crystallization

王瑋澤(Wei-Ze Wang)

指導教授 ： 蘇安仲

國立清華大學/工學院/化學工程學系/碩士(2013年)

https://doi.org/10.6843/NTHU.2013.00361

摘要

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.

關鍵字

高分子 ； 結晶 ； 相轉換

並列摘要

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.

並列關鍵字

polymer ； crystallization ； phase transition

參考文獻

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