Most of conjugated copolymers are semicrystalline. The crystal modification and crystalline morphology have an obvious effect on their charge transport. Therefore, controlling the crystal structures of conjugated copolymers is necessary for optimizing charge transport. Herein, large-area oriented poly [2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) crystalline films are produced by the epitaxial crystallization on oriented polyethylene (PE) substrates based on perfect two-dimensional lattice matching. Furthermore, the stacking modes of PCPDTBT crystals has been successfully controlled by epitaxy combined with chlorobenzene (CB) annealing or addition of 1,8-diiodooctane (DIO). PCPDTBT films spin-coated from CB solution on a PE substrate exist in both edge-on and face-on molecular chain orientations. The amount of edge-on stacking in CB-annealed PCPDTBT/PE films is much higher than that in the DIO-processed films. Moreover, the oriented PCPDTBT films exposed to CB vapor are confirmed to adopt the dimer crystal form. The addition of DIO can significantly improve the orientation extent of PCPDTBT main chains with a π-stacked form. The highly oriented films with a πstacked form demonstrate excellent carrier mobility (2.1 × 10 −2 cm 2 V −1 s −1 ) measured along the direction of PCPDTBT chains. The carrier mobility of DIO-processed PCPDTBT films along molecular chains is 76 times higher than that measured in the perpendicular direction of molecular chains. As a result, the ratio of anisotropic photoresponsivity of DIO-processed oriented PCPDTBT films can reach to 100. This work demonstrates a facile method to fabricate large-area and highly oriented PCPDTBT crystalline films with controlled crystal forms, which have potential applications in phototransistors.

In this work, a simple and efficient approach has been developed for preparing γ-PVDF films with a wide range of thickness based on the spontaneous diffusion of ionic liquids (ILs) into the α-PVDF matrix. It involves three steps: (i) preparation of α-PVDF films, (ii) surface coating of the resultant α-PVDF films with ionic liquids or quaternary ammonium solution, and (iii) annealing the surface modified PVDF thin film at a high temperature. In this way, the original α-PVDF film is then transferred into a γ-rich one through the well-known α−γ′ phase transition. The α−γ′ transition is found to be much faster and more efficient for high-temperature prepared samples. It has been further confirmed that the α−γ′ phase transition includes the induction and slow phase transition stages for low-temperature fabricated samples as well as the rapid and then slow transition stages for the samples prepared at 120 and 150 °C.

The polar phase of poly(vinylidene fluoride) (PVDF) is attractive for fabricating electroactive devices. However, the nonpolar α form is thermodynamically most accessible during the crystallization process. Here three types of aliphatic polyesters are chosen for blending with PVDF to explore their effect on inducing polar γ phase crystallization of PVDF. It is found that the ability of γ phase nucleation of PVDF enhances as the blend ratio of aliphatic polyesters and isothermal crystallization temperature increase. Poly(butylene succinate) (PBS) with highest melting temperature shows the highest ability for inducing γ phase crystallization. For example, the pure γ phase has been obtained under an appropriate blend ratio and isothermal crystallization temperature in PVDF/PBS blends. This is attributed to the hydrogen bonding interaction of H atoms in PVDF with carbonyls in PBS and the slow diffusion rate of PBS chains in PVDF. The addition of PBS lowers the folding surface free energy of the γ phase. This work offers a reference for selecting a suitable blending system to obtain pure γ phase PVDF and contribute to a better understanding of the relationship between intermolecular interaction and polymorphic phase selection.

Bent and faceted single crystals of poly(vinylidene fluoride) (PVDF) were prepared by isothermal crystallization in its blends with poly(butylene succinate) (PBS). After removing PBS by CHCl 3 washing, the remaining PVDF crystals were examined by optical, atomic force, and transmission electron microscopies and Fourier transform infrared spectroscopy. PVDF single crystals are attributed to α-phase and can reach hundreds of microns with two or more bent branches. The C-and X-shaped crystals are composed of bent basal lamellae, seen flat-on, with a great number of overgrowths. The overgrowths are lozenge-shaped, small-sized single crystals, and most of them grow along the C curves of either C-shaped or X-shaped crystals, keeping the same growth direction with the basal one. Consequently, the electron diffraction patterns taken everywhere of the whole crystal exhibit always a single crystal feature but change constantly with b-axis. These may be attributed to the unbalanced growth of four (110) faces and an inhomogeneous surface order or arrangement of loops and cilia. The lozenge-shaped, large-sized monolayer crystal can be observed upon decreasing the film thickness but without overgrowth, and its shape changes with crystallization temperature. These results can help to understand the surface structure of lamellae and its proliferation process profoundly.

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