Phase
behavior and crystal structure of a series of poly(2,3-diphenyl-5-alkyl-p-phenylene vinylene)
(denoted as DPn-PPV, where n represents the carbon number of the
alkyl side-chain, n = 6, 8, 10, 12)
were studied using differential scanning calorimetry, one- and two-dimensional
(1D and 2D) wide angle X-ray diffraction (WAXD), and selected area electron
diffraction (SAED). The experimental results reveal that DPn-PPV exhibits one crystalline phase at low temperatures. On the
basis of 2D WAXD and SAED patterns obtained from the oriented samples, the
crystal structures are determined to be orthorhombic for DP6-PPV and monoclinic
for DP8-PPV, DP10-PPV and DP12-PPV. To account for the unusually large unit
cell dimensions, we propose that the unit cell of DPn-PPV contains 4 chains (8 chemical repeat units). The complex crystal
structure can be attributed to the longitudinal and transverse offsets between
the neighboring chains, which shall be mainly due to the requirement of minimizing
the steric hindrance caused by the attached pendent groups and maximizing the π-π
interaction between the chains. The molecular packing scheme was simulated by using
Cerius2 software, of which the result agrees with the experimental
data. The polarized UV-vis absorption and polarized solid-state
photoluminescence (PL) property of these polymers was also investigated. The PL
spectra indicated that the light emitted from the oriented film was preferentially
polarized parallel to the shear direction, implying that DPn-PPV may potentially be useful in linearly polarized luminescence
devices.
