Chapter 5
Morphology and Luminescence Properties of Poly(phenylenevinylene) and Poly(N-vinylpyrrolidone) Polyblends Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 9, 2018 at 15:12:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
King-Fu Lin, Lu-Kuen Chang, and Horng-Long Cheng Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China
Poly(vinyl pyrrolidone) (PVP) was mixed with poly(phenylene vinylene) (PPV) precursor (III) aqueous solution to prepare the PPV/PVP polyblends and its dilution effect on the luminescence properties of PPV was investigated. PVP is miscible with PPV precursor but immiscible with the transformed PPV. In PPV/PVP polyblends, PPV conjugated segments preserved their configuration of C symmetry but with less packing. The size of PPV phase is similar to the reported PPV crystallites and was decreased with the content of PVP. As to the photoluminescence (PL) properties, the energy gap to produce the excitons in the PPV conjugated chains was not changed by blending with PVP, whereas the PL intensity per mole of PPV conjugated units was increased. It was attributed to the dilution effect that decreased the non-radiative interchain's quenching of excitons. Similar results were also found for the electroluminescence (EL) properties of ITO/polyblend/Al light emitting diode (LED) device, except that the current to generate the excitons might leak to the PVP phase. As a result, the optimal content of PVP in polyblends to provide the best EL performance was 15~20 wt%. 2h
Poly(p-phenylene vinylene) (PPV) was the first reported polymer having electro luminescence (EL) properties (7), which made this material attractive in view of the potential applications in large-area visible emitting diode (LED). In the L E D devices incorporating PPV polymers, the injected positive and negative charges move through the conjugated chains under the influence of the applied electric field. Some of the charges annihilate one another in pairs and form a singlet exciton that decays to the ground state with a fluorescence emission, the process of which is called E L . The emitted E L spectrum is similar to that of the photoluminescence (PL) excited by ultraviolet (UV) light. Thus, the n* to n interband transition of excitons during emission has no different between E L and PL. However, the E L efficiency of PPV is rather low. Only up to 0.05% photons/electron was reported (7). Many efforts were aimed to improve the E L efficiency, such as (i) using multilayer L E D devices to enhance the E L through
© 1999 American Chemical Society Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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72 charge carrier confinement (2,3); (ii) minimizing the energy barriers between polymer and the L E D injection electrods (4,5); (iii) introducing side groups to the PPV units (6,7); (iv) copolymerizing PPV with other monomers (8,9); and (v) blending P P V with other polymers (10,11). The first two efforts were to increase the concentration of excitons, whereas the last two were to decrease the nonradiation decay of generated excitons. It has been indicated that one of the major non-radiation decay was through the migration of excitons to the quenching sites, which might be in intrachains or interchains of conjugated polymers (12). Several evidences have been reported for the intrachain's quenching. For examples, the photoluminescence was longer lived for less-conjugated PPV in time-resolved measurements of PL and it accounted for the higher E L efficiency of light-emitting devices made from less-conjugated materials (13). Wong et al. (14) also reported that the decay of PL was faster in more conjugated materials. Due to the fact that more-conjugated polymer chains contain more intrachain's quenching sites for excitons, blockcopolymers of PPV with non-conjugated segments showed much higher quantum yield than pure PPV in PL measurements (12). By the same token, the non-radiation interchain's quenching of PPV might be reduced by dilution of the conjugated polymer chains with non-conjugated polymers. It has been indicated that the dilution of poly(phenyl-p-phenylenevinylene) in a blend with polycarbonate leaded to the increase of radiation recombination (15). Significant enhancement of PL by isolation of extended conjugated polymer chains in the PPV-incorporated nanocomposites with a well-defined hexagonal architecture was also reported and has been attributed to the reduction of non-radiative interchain's quenching (16). In this study, we blended PPV (I) with poly(N-vinylpyrrolidone) (PVP) (II), a water-soluble non-conjugated polymer. The polyblends were prepared from the PPV precursor (III) aqueous solution mixed with various amounts of PVP. When their mixture was spin-coated on a glass plate and heated in vacuum to transform the PPV precursor into PPV, the converted PPV became immiscible with PVP. Since the non-radiative interchain's quenching of excitons depends on the characteristics of polyblends, we investigated the chemical structure and morphology of PPV/PVP polyblends first by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, transmission electron microscopy (TEM) and wide angle X-ray scattering (WAXS). Then, the effects of morphology on the PL and E L properties of PPV/PVP polyblends will be discussed. Experimental Sample Preparation. The preparation of PPV precursor is briefly shown in Figure 1. The synthesis of p-xylylene-bis(tetrahydrothiophenium chloride) monomer IV was according to the method of Lenz et al (17): 10 g dichloro-p-xylene (V, Tokyo Kasei) was dissolved in 150 mL methanol and then added with 10 mL tetrahydrothiophene (VI, Janssen Chimica) for reaction. After reacted at 50 °C for 20 h, the solution was concentrated and then added with 250 mL cold acetone (0 °C) to precipitate the monomer. A white monomer crystalline with m.p.=149-151 °C was obtained after filtrated, washed with cold acetone several times, and dried. The PPV precursor III was polymerized by the following method (18-20): 0.4 M monomer IV aqueous solution (20 mL) was mixed with 80 mL pentane and then cooled to 0-5 °C. The polymerization was carried out by further addition of 0.4 M sodium hydroxide (20 mL, already cooled to 0-5 °C ) under nitrogen and proceeded for 1 h. The reaction was terminated with 0.1 M HC1 to pH 7.
Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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c-(0)-
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*
MeOH, 50"C
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(miscible blend)
220vacuum
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Figure 1. The preparation procedure of PPV/PVP polyblends.
Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
74 The prepared PPV precursor solution after removed pentane was dialyzed in membrane (molecular weight cut-off: 3,500) against deionized water for at least a week. At this stage, the sodium ion content remained 3.57 ppm, measured by using a G B C 902 model atomic analyzer. Its PPV content measured from the thermal gravimetric analysis (TGA) was -0.25 wt% (27). Then, the aqueous solution was added with various amount of PVP II (Sigma, molecular weight=40,000) and mixed until homogeneous. The aqueous mixture to prepare the PPV/PVP blended samples for E L tests were spin-coated on an indium-tin oxide (ITO)-deposited glass plate, whereas those for PL, infrared and Raman spectroscopies, and W A X S tests were spin-coated on a regular glass plate. A l l the samples were vacuumed overnight at room temperature to remove water and then heat-treated in a high vacuum oven (
