Chen S H, Su A C, Chen S A
Institute of Materials Science and Engineering and Center for Nanoscience and Nanotechnology, National Sun Yat-sen University, Kaohsiung 804, Taiwan.
J Phys Chem B. 2005 May 26;109(20):10067-72. doi: 10.1021/jp044079w.
Selective formation of amorphous, nematic (N), and beta phases in poly(9,9-di-n-octyl-2,7-fluorene) (PFO) films was achieved via judicious choice of process parameters. Phase structure and film morphology were carefully examined by means of X-ray diffraction as well as electron microscopy. "Amorphous" thin films were obtained by quick evaporation of solvent. Slow solvent removal during film formation or extended treatment of the amorphous film with solvent vapor resulted in predominantly the beta phase, which corresponds to a frozen (due to decreased segmental mobility upon solvent removal) and intrinsically metastable state of transformation midway between a solvent-induced clathrate phase and the equilibrium crystalline order in the undiluted state. The frozen transformation process is reactivated upon an increase in temperature beyond 100 degrees C. Compared to the amorphous film, extended backbone conjugation in the beta phase is evidenced from the emergence of a characteristic absorption peak around 430 nm near the absorption edge. For films of frozen nematic order (obtained by quenching from the nematic state), the conjugation length is also greater than the amorphous films as revealed by an absorption shoulder around 420 nm. Well-behaved single-chromophore emission with single-mode phonon coupling was observed for the beta phase; in the case of nematic films, dual-mode phonon coupling must exist if single-chromophore emission is assumed. In comparison, the emission spectrum of the amorphous film of generally shorter conjugation lengths exhibited mixed characteristics of nematic and beta phases, implying the presence of minor populations of extended conjugation similar to those in nematic and beta phases, which are of biased weightings in the emission spectra. All these films consist of nanograins (ca. 10 nm in size) of collapsed chains; the films are therefore inherently inhomogeneous in this length scale. In combination with previous observations on the crystalline (alpha and alpha') forms, the phase behavior of PFO is then generally summarized in terms of relative thermodynamic stability.
通过谨慎选择工艺参数，在聚（9,9 - 二正辛基 - 2,7 - 芴）（PFO）薄膜中实现了非晶相、向列相（N）和β相的选择性形成。通过X射线衍射以及电子显微镜仔细研究了相结构和薄膜形态。“非晶”薄膜通过溶剂的快速蒸发获得。在薄膜形成过程中缓慢去除溶剂或用溶剂蒸汽对非晶薄膜进行长时间处理，主要得到β相，它对应于一种冻结状态（由于去除溶剂时链段迁移率降低），是在溶剂诱导的笼形相和未稀释状态下的平衡晶体有序之间的一种本质上亚稳的转变状态。当温度升高到100℃以上时，冻结的转变过程会重新激活。与非晶薄膜相比，β相中主链共轭的延长可从吸收边缘附近约430nm处出现的特征吸收峰得到证明。对于冻结向列有序的薄膜（通过从向列态淬火获得），如在约420nm处的吸收肩所揭示的，共轭长度也大于非晶薄膜。对于β相观察到了具有单模声子耦合的良好单发色团发射；在向列薄膜的情况下，如果假设存在单发色团发射，则必须存在双模声子耦合。相比之下，通常共轭长度较短的非晶薄膜的发射光谱表现出向列相和β相的混合特征，这意味着存在少量与向列相和β相中类似的延长共轭结构，它们在发射光谱中的权重不同。所有这些薄膜都由塌缩链的纳米颗粒（尺寸约为10nm）组成；因此，在这个长度尺度上，薄膜本质上是不均匀的。结合之前对晶体（α和α'）形式的观察，然后根据相对热力学稳定性对PFO的相行为进行了总体总结。
