ORIGINAL PAPER Ariyanan Mani á Subramanian Tamil Selvan Kanala Lakshminarasimha Phani Solid state structural aspects of electrochemically prepared poly (p -phenylene) thin ®lms ± crystalline order and spherulite morphology Received: 29 July 1997 / Accepted: 27 October 1997 Abstract The electrochemical polymerization of benzene via the microemulsion approach yields highly crystalline and anisotropic ``spherulitic'' polyparaphenylene (PPP) thin ®lms. The crystalline order and the origin of spherulite morphology are discussed. Key words Polyparaphenylene (PPP) á Crystalline order á Spherulites Introduction Micellar and microemulsion media are being widely employed for the synthesis of polymer microstructures [1]. Microemulsions are microheterogeneous media en- abling higher solubility and the possibility of the surfactant monolayers acting as ``templates'' for synthesizing various organic and inorganic microstruc- tures. As proposed by Rusling [2], a few monolayers of the surfactant are formed on the electrode surface with well-de®ned microstructures, and these could serve as ``templates'' for the electrochemical reactivity. We have earlier reported the possibility of electrosynthesizing polyparaphenylene from an oil-in-water type micro- emulsion medium stabilized by sodium dodecylsulfate surfactant. The crystalline order and the evolution of morphology during electropolymerization were reported by us in a preliminary note [3]. There is a widespread realization that polymerization in such organized media provides an exciting possibility of controlling the poly- mer microstructures. As materials science seeks control of structure and properties (short- or long-range order), polymerization in anisotropic phases such as micelles is sure to take on increasing importance [4]. During recent years, conjugated polymers have be- come attractive materials for electroluminescent devices and photoconductors [5±9]. Among various candidate polymers, polyparaphenylene (PPP) and its substituted analogues are recognized for their good stability and conductivity, rendering them suitable for the above- mentioned device applications. The signi®cance of controlling the polymer morphology has also been highlighted in several publications [10±13]. In conjugated polymers, the electron- as well as ion- transport properties strongly depend on their solid-state structural aspects, particularly crystalline order and morphology. For example, electrical conductivity is largely controlled by interchain charge transport, which mainly depends on the alignment of the macromolecular chains in the ®lm matrix, and the rate of doping and dedoping depend on polymer morphology [14]. Al- though many reports are available in the literature on the synthesis and properties of PPP [15±17], the aspects concerning its crystalline characteristics and their rela- tion to its morphology have not been dealt with. In this paper, we report crystallographic details and morpho- logical features of PPP thin ®lms deposited by electro- chemical oxidation of benzene by potentiodynamic cycling. Experimental PPP thin ®lms were synthesized by a method similar to our earlier procedure [9±10]. The benzene monomer was solubilized in conc. H 2 SO 4 (18 M) using Triton X-100 (14.8 mM), thus yielding an oil- in-water type microemulsion, and the optimum benzene:H 2 SO 4 ratio of 2:5, corresponding to 70 ml of microemulsion, was directly used as an electrolysis medium. The PPP ®lms were grown on in- dium-tin-oxide (ITO) glass (1 cm 2 ) by cycling between )0.21 and + 0.91, V vs Hg/Hg 2 SO 4 /1 M H 2 SO 4 (MSE) with a scan rate of 100 mV s )1 at ambient temperature. The ®lm thickness (t) values are expressed in terms of the anodic charge consumed in the cyclic potentiodynamic growth pattern (mC cm )2 ), i.e., the ®rst 20 cycles correspond to 2.35 mC cm )2 , 12 cycles to 1.38 mC cm )2 , and 23±40 cycles to higher thickness values in the range of 5.75 > t > 2.35 mC cm )2 . The PPP ®lms after electrochemical J Solid State Electrochem (1998) 2: 242±246 Ó Springer-Verlag 1998 A. Mani á S.T. Selvan á K.L. Phani (&) Central Electrochemical Research Institute, Karaikudi ± 630 006, India Tel.: +91-04565-22322; Fax: +91-04565-22088; e-mail: cecrik@cscecri.ren.nic.in