1. IntroductionPolyimides (PI) are a class of high-performancepolymers possessing the cyclic imide and aromaticgroups in the main chains. The most familiar poly-imide is widely known as Kapton, developed by DuPont in the 1960s. After the marketing of this poly-imide, many others were commercially developedwith some modified properties. They have gainedconsiderable importance in advanced areas such asmicroelectronics, aerospace and separation tech-nologies owing to their outstanding properties interms of thermal stability, mechanical properties,and solvent resistance, coupled with relatively lowpermittivity and dielectric losses up to very hightemperatures . In order to facilitate progress ofPIs in their application fields, much effort has beenspent to further improve their properties. This canbeen achieved through the formation of in situ gen-
erated inorganic particles to produce organic-inor-ganic (O/I) hybrid materials .The O/I systems have been recognized as a newclass of high performance materials owing to theunique opportunity that these systems provide tocombine the excellent properties of inorganicglasses, such as high modulus, thermal stability andlow coefficient of thermal expansion, with the duc-tility and low temperature processing characteris-tics of organic polymers. When the two phases areinterconnected and the domain sizes of thesephases approach those of supramolecular entities,these materials are known as nanostructured mate-rials or phase-interconnected nanocomposites.Hybrid materials offer a large potential for applica-tions in a variety of advanced technologies, eitheras structural materials , including their use asmatrices for high performance composites [12, 13],
*Corresponding author, e-mail: [email protected] BME-PT
Polyimide/silica hybrids via the sol-gel route:High performance materials for the new technologicalchallenges
G. Ragosta*, P. Musto
National Research Council of Italy, Institute of Chemistry and Technology of Polymers (ICTP), Via Campi Flegrei 34,Olivetti Buildings, 80078 Pozzuoli, NA, Italy
Received 24 February 2009; accepted in revised form 16 April 2009
Abstract. The present review article describes in detail the state-of-the-art of organic-inorganic hybrid materials based onpolyimide/silica components. The article is divided in three parts. In the first the basic processing route for the preparationof these systems is described, i.e. the sol-gel technique, along with the strategies developed to control the final morphology.In the second part the curing characteristics, the dynamic-mechanical and the mechanical and fracture properties of hybridswith different morphologies are reviewed. Finally, the more technologically relevant applications devised for these highperformance materials are discussed.
Keywords: nanocomposites, hybrid materials, polyimide, silica, sol-gel
eXPRESS Polymer Letters Vol.3, No.7 (2009) 413428Available online at www.expresspolymlett.comDOI: 10.3144/expresspolymlett.2009.51
or as functional materials  such as catalyst sup-ports  and microelectronic devices . Thesol-gel route represents the preferred way for thesynthesis of polyimide based hybrids. The classicalsol-gel process consists in a two step hydrolysis-con-densation reaction, starting with a metal alkoxidesM(OR)4, typically tetraethoxisilane Si(OCH2CH3)4or titanium isopropoxide Ti[OCH(CH3)2]4, to pro-duce hydroxyl groups followed by the polyconden-sation of the hydroxyl groups and residual alkoxygroups to form a three-dimensional network [5,1721].Polyimides are particularly suited for this type ofprocess because they can be obtained from polyamicacid precursors, which are soluble in hygroscopicsolvents and can, therefore, tolerate the addition ofwater necessary to bring about the hydrolysis of themetal alkoxide. Moreover, the outstanding thermalstability of polyimides allows the hybrids to bepost-cured at very high temperatures (300350C)making it possible the development of a very denseinorganic network without inducing appreciabledegradation of the organic phase.Polyimide/silica hybrids made by the sol-gel tech-nique are the subject of this review, in which theapproaches used for a close control of their finalmorphology, the effects of the silica phase on thecuring process of polyimide precursors, thedynamic-mechanical and the mechanical and frac-ture properties that these materials can display, andtheir possible applications will be presented anddiscussed.
2. Processing and morphology
A typical procedure for the preparation of PI/silicahybrids via the sol-gel route, involves the followingsteps:i) The polyamic acid (PAA) (PI precursor) is
formed by a polyaddition reaction of a dianhy-dride [e.g., pyromellitic anhydride (PMDA),3,3,4,4-biphenyltetracarboxylic dianhydride(BPDA), or 2,2-bis(3,4-dicarboxy-phenyl)-hexa-fluoropropane dianhydride (6FDA)] with adiamine [e.g., 4,4-oxydianiline (ODA), orp-phenylen diamine (PPA)] in a common sol-vent [e.g., dimethylacetamide (DMAc), orN-methyl pyrolidone (NMP)]. The reaction andchemical structures of monomers are reported inFigure 1.
ii) The silica precursor [e.g., tetraethoxysilane(TEOS) or tetramethoxysilane (TMOS) isadded to the PAA solution, and the hydrolysisand polycondensation is carried out using anappropriate catalyst.
iii) The PAA/silica solution is film cast by dryingthe solvent, and then the film is cured by suc-cessive heating treatments up to 300C. Theheating induces the imidization reaction to con-vert PAA to PI and the crosslinking of the
Ragosta and Musto eXPRESS Polymer Letters Vol.3, No.7 (2009) 413428
Figure 1. Formation of polyamic acids, the precursor ofpolyimides
Figure 2. Sol-gel process for the production of a poly-imide/silica hybrid and the structures of somealkoxisilanes
siloxane component to form a silica network.This route is outlined in Figure 2 where thestructures of some silanes discussed in thepresent review are also reported.
The types of reactions involved in the preparationof PI/silica hybrids allow to exert a considerablecontrol on the final morphology by preventing pre-mature phase separation. This is achievable by opti-mizing the processing parameters (temperature,catalyst and nature of the solvent) and by promot-ing reactions or interactions between the two com-ponents, either directly or via a coupling agent.Various methods for effectively introducing a silicaphase into PI matrices have been reported by Nandiet al. [17, 22], Morikawa et al. [7, 11], Wang et al., Mascia et al. [8, 13], Schmidt and Wolter [24,25] among other. Given below is an account ofsome of the approaches used.In earlier studies, large sized silica domains weregenerated with inhomogeneous distribution withinpolyimide matrices. This led to brittle materialswith poor mechanical properties. In order to mini-mize the size of the inorganic particles and to pre-vent their agglomeration, Nandi and co-workersprepared polyimide-silica hybrids via the site iso-lation method [17, 22]. This method consisted ofpre-binding an alkoxide precursor to a polyamicacid, forming a carboxylate group, and subse-quently curing the system to imidize the polyamicacid and to drive the sol-gel reactions. Homoge-neously dispersed silica particles of nanometerdimensions were observed in the polyimide matrix,particularly at low silica content, although micron-sized particles were evident at higher silica concen-trations (> 40 wt%). However, this approach hadthe drawback that the prebinding of the alkoxideprecursor prevented a complete imidization of thepolyamic acid.Morikawa et al. [7, 26] produced PI/silica hybridsby mixing TEOS solutions and a polyamic acid,obtained from the reaction of PMDA and ODA.Except for systems with low silica content(
100 nm, while for the non-compatibilized system,the average diameter of the silica particles variesfrom 0.8 to 1.0 m. These morphologies are theresult of a typical phase separation by spinodaldecomposition [30, 31] differing from each other inthe extent of phase connectivity. The way throughwhich the coupling agent induces and controls theevolution of the morphology in these hybrid sys-
tems was attributed to interactions, possibly byhydrogen bonding, taking place between the epoxygroups of GOTMS and the acid groups of the poly-imide precursor. These interactions delay the onsetof phase separation by spinodal decomposition,which results in a higher viscosity of the solutionand in a reduction of the rate of particle growth [5,8, 32]. It was found that a threshold value of theGOTMS/TEOS molar ratio exists for the formationof co-continuous morphologies [8, 32]. This thresh-old value was found to change with the overall sil-ica content in the hybrid. For a silica concentrationof 22.3 wt%, calculated theoretically assumingcomplete conversion of TEOS and GOTMS to sil-ica , the threshold GOTM/TEOS molar ratio islocated between 0.06 and 0.08. Figure 4 shows theSEM micrographs of hybrids having 22.3 wt% ofsilica and GOTMS contents below and above thethreshold value . For GOTMS concentrationslower than the limiting value, the hybrids display(Figures 4a and 4b) a particulate morphology likethat of Figures 3a, but with smaller particles sizes.The dimensions of these particles decrease fromabout 0.61 to 0.32 m with increasing the amountof GOTMS. The micrograph of Figure 4c, on theother hand, illustrates the morphology obtainedwith a GOTMS content slightly above the thresholdvalue. This morphology is close to that of Figure 3bfor which a higher GOTMS concentration wasused. TEM examinations for co-continuous struc-tures revealed additional morphological features. Inparticular, it was found that increasing the amountof GOTMS above the threshold value had the effectof reducing the sizes of the silica domains