Polyimide/Inorganic Composite - Interpenetrating Polymer Network for Stable Second-Order Nonlinear Optics
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JONG JENG*, JAYANT KUMAR*, AND SUKANT K. TRIPATHY* Center for Advanced Materials, Departments of Chemistry* and Physics**, University of Massachusetts Lowell, Lowell, MA 01854, USA ABSTRACT A new class of IPN system has been prepared and investigated. This IPN system combines the polybismaleinimide network and the NLO-active phenoxysilicon network. The secondorder NLO coefficients, d 33 , values of the samples range from 2.5 to 6.7 pm/V depending on the composition and the processing conditions. The temporal stability of the second-order nonlinearities for these samples at 110 'C varies from 47 to 88 % retention after 274 h. INTRODUCTION There has been tremendous interest in the development of second-order nonlinear optical (NLO) polymeric materials for photonics applications [l. A number of NLO polymers have been developed to exhibit large second-order NLO coefficients, d33 , comparable to those of the inorganic NLO materials which are currently in use in devices [2]. However, the major drawback of NLO polymers that prevents them from being employed in device applications is the decay of their electric field induced second-order optical nonlinearities. This decay is caused by the relaxation of the NLO chromophores from the induced acentric alignment to a random fashion. Numerous efforts have been made to minimize this decay through different approaches [3]. The search for a better material is still underway in order to make practical use of NLO polymers possible. Recently, we have introduced a new approach using a concept of an interpenetrating polymer network (IPN) to prepare a second-order NLO polymer [4]. This IPN system, with the hybrid properties of a high glass transition temperature (Tg), an extensively cross-linked network, and permanent entanglements, exhibited excellent temporal stability at elevated temperatures. Since Tg is associated with the main-chain mobility, an increase of Tg results in enhanced temporal stability [5]. Polyimides have been used as high Tg matrices to enhance the stability in several guest/host systems [5a, 6]. Thus, the introduction of a cross-linkable high Tg polyimide into an IPN structure is expected to further advance the Tg of the system and boost the stability of the nonlinearity of the IPN. In this paper, we report an investigation of a new NLO-active IPN system combining a thermoset polyimide and an NLO-active inorganic network. The polyimide network based on bismaleimide is formed by an addition polymerization process. The NLO-active inorganic 541 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
network which is a phenoxysilicon polymer [7] is formed by sequential hydrolysis and condensation reactions via a sol-gel process. The formation of both networks are controlled to proceed simultaneously. The preparation of the IPN, the characterization of the thermal and optical properties, and the temporal stability of the optical nonlinearity are discussed. EXPERIMENTAL Materials The NLO-active phenoxysilicon network is composed of an alkoxysilane dye (AS
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