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Journal of Chemical Physics / Volume 133 / Issue 4 / ARTICLES / Polymers and Complex Systems

J. Chem. Phys. 133, 044901 (2010); doi:10.1063/1.3436517 (11 pages)

Second-harmonic generation in conjugated polymer films: A sensitive probe of how bulk polymer crystallinity changes with spin speed

Ian M. Craig, Christopher J. Tassone, Sarah H. Tolbert, and Benjamin J. Schwartz

Department of Chemistry and Biochemistry, UCLA, Los Angeles, California 90095-1569, USA

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(Received 22 March 2010; accepted 5 May 2010; published online 22 July 2010)

In this paper, we examine the second-harmonic generation (SHG) from spin-cast films of the conjugated polymer poly(2-methoxy-5-(2′-ethylhexyloxy)para-phenylenevinylene) (MEH-PPV). We find that the SHG intensity depends strongly on the speed used to spin cast the films. Two-dimensional grazing incidence x-ray diffraction (XRD) experiments show that the bulk crystallinity of the MEH-PPV films varies in the same way with spin speed as the SHG intensity. This strongly suggests that instead of being interface specific, the second-harmonic signal from conjugated polymer films is dominated by the crystalline domains in the bulk. The nonmonotonic dependence of both the SHG intensity and the degree of MEH-PPV crystallinity results from a competition between the shear forces and the solvent evaporation rate during spin coating, which produces a maximum degree of crystallinity for MEH-PPV films spin cast at around 1400 rpm. We also use XRD to show that thermal annealing produces MEH-PPV films with a single degree of bulk crystallinity, independent of how they were originally cast. This allows us to model the angle- and thickness-dependent SHG from annealed MEH-PPV films with a single polarizability tensor. We find that the SHG from MEH-PPV films fits best to a bulk-allowed electric quadrupole mechanism, consistent with the bulk SHG seen in other π-stacked aromatic molecules. Thus, rather than providing information about conjugated polymer interfaces, SHG can be used as a sensitive probe of the local degree of crystallinity in the bulk of conjugated polymer films.

© 2010 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. EXPERIMENTAL
  3. RESULTS AND DISCUSSION
    1. The basic nature of SHG from thin MEH-PPV films
    2. The relationship between spin speed and local polymer crystallinity
    3. The relationship between spin speed and SHG in MEH-PPV thin films
    4. The effects of thermal annealing on MEH-PPV structure and SHG
      1. How thermal annealing changes the crystallinity of MEH-PPV films
      2. The thickness dependence of the SHG intensity from annealed MEH-PPV films
  4. CONCLUSIONS

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KEYWORDS and PACS

Keywords

annealing, conducting polymers, optical films, optical harmonic generation, polymer films, spin coating, X-ray diffraction

PACS

  • 78.66.Qn

    Polymers; organic compounds

  • 42.79.Wc

    Optical coatings

  • 81.40.Gh

    Other heat and thermomechanical treatments

  • 42.65.Ky

    Frequency conversion; harmonic generation, including higher-order harmonic generation

ARTICLE DATA

Permalink
http://link.aip.org/link/doi/10.1063/1.3436517

PUBLICATION DATA

ISSN:

0021-9606 (print)  
1089-7690 (online)

Publisher:

$abstract.society.value is a Member of CrossRef American Institute of Physics

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