Stabilization of bulk aggregation state in semiconducting polymer solutions

Chi Chung Hua; Chih Jung Lin; Yu Ho Wen; Show An Chen

doi:10.1007/s10965-010-9476-3

Journal of Polymer Research

July 2011, Volume 18, Issue 4, pp 793–800 | Cite as

Stabilization of bulk aggregation state in semiconducting polymer solutions

Authors
Authors and affiliations

Chi Chung HuaEmail author
Chih Jung Lin
Yu Ho Wen
Show An Chen

Original Paper

First Online: 28 July 2010

Received: 01 April 2010

Accepted: 15 July 2010

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3 Citations

Abstract

Stabilization of the aggregation state in semiconducting conjugated polymer solutions during sample preparation or flow processing, such as spin coating or ink-jet printing, is crucial to warrant controllable thin-film properties for applications with polymer-based light emitting diodes (PLED). Flow-turbidity measurements on the solutions made of a widely studied conjugated polymer revealed, for the first time, that flow could result in instant and/or persistent changes in the bulk aggregation state. Current observations, nevertheless, suggested that a combination of moderately high polymer concentration and intermediately poor solvent quality can substantially stabilize the aggregation state at the stages of sample preparation, prolonged storage, and flow processing.

Keywords

Aggregate Flow-turbidity Semiconducting polymer solution

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Notes

Acknowledgement

This paper is dedicated to the memory of Prof. Wunshain Fann. The support from the National Science Council of ROC under Grant No. 98-2752-E-007-002-PAE is gratefully acknowledged.

Appendix

The Mie theory for analyzing the light-turbidity data involves the following set of equations [16, 17]:

$$ {\tau_{\rm{T}}} = - \frac{1}{L}\ln \left( {\frac{I}{{{I_0}}}} \right) = - \frac{1}{L}\ln (T) $$

(A1)

$$ {\tau_{\rm{T}}} = N{C_{\rm{sca}}} $$

(A2)

$$ {C_{\rm{sca}}} = {Q_{\rm{sca}}}\pi R_{\rm{Mie}}^2 $$

(A3)

$$ {Q_{\rm{sca}}} = \left( {\frac{2}{{{\alpha^2}}}} \right)\sum\limits_{i = 1}^\infty {\left( {2i + 1} \right)\left\{ {{{\left| {{a_i}} \right|}^2} + {{\left| {{b_i}} \right|}^2}} \right\}} $$

(A4)

$$ \alpha = \frac{{2\pi {R_{\rm{Mie}}}}}{\lambda } $$

(A5)

$$ m = \frac{{{n_{\rm{p}}}}}{n} $$

(A6)

$$ {a_i} = \frac{{{\psi_i}\left( \alpha \right)\psi_{_i}^\prime \left( {m\alpha } \right) - m{\psi_i}\left( {m\alpha } \right)\psi_{_i}^\prime \left( \alpha \right)}}{{{\zeta_i}\left( \alpha \right)\psi_{_i}^\prime \left( {m\alpha } \right) - m{\psi_i}\left( {m\alpha } \right)\zeta_{_i}^\prime \left( \alpha \right)}} $$

(A7)

$$ {b_i} = \frac{{m{\psi_i}\left( \alpha \right)\psi_{_n}^\prime \left( {m\alpha } \right) - {\psi_i}\left( {m\alpha } \right)\psi_{_i}^\prime \left( \alpha \right)}}{{m{\zeta_i}\left( \alpha \right)\psi_{_i}^\prime \left( {m\alpha } \right) - {\psi_i}\left( {m\alpha } \right)\zeta_{_i}^\prime \left( \alpha \right)}} $$

(A8)

The symbols have the following significance: τ _T is the light turbidity, L the path length of light passing through the sample, N the number of scattering centers per unit volume, C _sca the total light energy scattered by one spherical particle, Q _sca the scattering efficiency, R _Mie the Mie radius, ψ _i(x) the Riccati-Bessel function, ς _i(x) the Hankel function, λ the wavelength of incident light in the medium (=λ ₀/n), n _p the refractive index of the scattering particle, n the refractive index of the solvent medium.

Using the above relations, the specific turbidity has been derived to be [18,19]

$$ \left( {\frac{{{\tau_{\rm{T}}}}}{c}} \right) = \left( {\frac{{3\pi }}{{2\lambda {\rho_{\rm{p}}}}}} \right)\left( {\frac{{{Q_{\rm{sca}}}}}{\alpha }} \right){,} $$

(A9)

where ρ _p is particle density, and c (g/ml) is the particle concentration. Equations A1 to A9 can be utilized to compute the specific turbidity as a function of Mie radius, as shown in Fig. 6. In estimating the refractive index of MEH-PPV aggregates, the Gladstone-Dale equation was employed [16]:

$$ \left( {m - 1} \right)n = \left( {{n_0} - n} \right)\phi $$

(A10)

where n ₀ (=1.882) is the refractive index of MEH-PPV at solid state, n (=1.486) is the refractive index of DOP, and ϕ is the mean volume fraction of MEH-PPV within an aggregate cluster. To obtain a rough estimate of ϕ, we have utilized the fact that the solvent quality of MEH-PPV/DOP is similar to that of MEH-PPV/toluene [12], so that the known monomeric density for the latter system [3] may be employed to obtain an estimate of ϕ = 0.248. For simplicity, the present analysis ignored possible concentration dependences of ϕ. Accordingly, m in Eqs. A6 and A10 is found to be 1.066

Fig. 6
A computation result of the specific turbidity as a function of Mie radius

.

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Authors and Affiliations

Chi Chung Hua
- 1
Email author
Chih Jung Lin
- 1
Yu Ho Wen
- 1
Show An Chen
- 2

1.Department of Chemical EngineeringNational Chung Cheng UniversityChia YiRepublic of China
2.Department of Chemical EngineeringNational Tsing Hua UniversityHsin ChuRepublic of China

Stabilization of bulk aggregation state in semiconducting polymer solutions

Abstract

Keywords

Notes

Acknowledgement

References

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