MATERIALE PLASTICE ♦ 47♦ Nr. 4 ♦ 2010 http://www,revmaterialeplastice.ro 481 Study of Some Aromatic Polyimides Containing Methylene Units ION SAVA 1* , STEFAN CHISCA 1 , MARIA BRUMA 1 , GABRIELA LISA 2 1 ”Petru Poni” Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A, Iasi 700487, Romania 2 “Gh.Asachi” Technical University Iasi, Department of Natural and Synthetic Polymers, Bd. Mangeron 71, Iasi 700050, Romania Physical properties, such as thermal and dielectric characteristics, of some aromatic polyimides which contain methylene units have been investigated. The results indicated that the polymers were highly thermostable with decomposition temperature over 450°C and low dielectric constants. The thermogravimetrical values processed by differential method Freeman-Caroll showed that the activation energies and reaction orders are lower in air than in nitrogen. Keywords: methylene units, polyimides, thermal characterization, activation energy, dielectric constants Polyimides are an important class of high-performance polymers. Excellent mechanical and electrical properties as well as high-temperature stability make these polymers suitable for applications in the aerospace industry as high- performance composites and in the electronics industry as high-temperature coatings [1-4]. However, difficulty in processability has slowed the widespread use of these polymers by making them expensive and limiting their applicability. They are distinguished from other high performance polymers by the solubility of poly(amidic acid) precursor form which can be cast into uniform films and quantitatively conversed to polyimide structure. Most aromatic polyimides encounter processing difficulty due to their infusibilities and poor solubilities in organic solvents. Such difficulty is due to strong interchain forces, inherent macromolecular rigidity, or semicrystallinity [5-9]. Many studies have attempted to enhance their processabilities and solubilities either by introducing bulky groups or flexible chains, nonsymmetric, alicyclic or nonlinear moieties into the polymer backbones or by attaching bulky side groups [10-14]. Polyimides are the most thermally stable polymers due to their high glass transition and high decomposition temperatures and are useful in long-term service at high temperatures. Therefore, studying the physical properties of polyimides is of both scientific greatness and engineering applied importance. This paper presents the physical properties of two series of polyimides derived from aromatic diamines containing flexible methylene bridges such as 4,4’-diamino- diphenylmethane or 3,3’-dimethyl-4,4’-diaminodiphenyl- methane, and three different aromatic dianhydride 4,4’- isopropylidene-diphenoxy)bis(phthalic anhydride), benzophenontetracarboxylic dianhydride or hexa- fluoroisopropylidendiphthalic dianhydride. Experimental part The polymers investigated in this study have been obtained by methods previously reported and the detailed descriptions have been presented elsewhere [15]. The structure of these polyimides is presented in scheme 1. The polyimide films were obtained by casting the polyamidic acid solution 10-14% in dimethylacetamide (DMA) onto glass plates and drying at 60°C over 4 h to evaporate the solvent. The subsequent heating of the precursor films at 100, 150, 200, and 250°C consecutively (for 1 h at each temperature) resulted in a final polyimide film. * email: isava@icmpp.ro; Tel: 40-232 217 454 Ia: Ar = - O-C 6 H 4 - C(CH 3 ) 2 - C 6 H 4 - O -; X = H ; Ib: Ar = CO; X = H ; Ic: Ar = C(CF 3 ) 2 ; X = H ; Id: Ar = - O-C 6 H 4 - C(CH 3 ) 2 - C 6 H 4 - O -; X = CH 3 Ie: Ar = CO; X = CH 3 If: Ar = C(CF 3 ) 2 ; X = CH 3 Measurements FTIR spectra were recorded with a FT-IR VERTEX 70 (Bruker Optics Company), with a resolution of 0.5 cm -1 . Thermogravimetric analysis (TGA) was performed in air and nitrogen flow (20 cm 3 min -1 ) at a heating rate of 10°C/min from 25 to 900°C with a Mettler Toledo model TGA/SDTA 851. The initial mass of the samples was 3-5 mg. Differential scanning calorimetry (DSC) analysis was performed using a Mettler Toledo DSC 1 (Mettler Toledo, Switzerland) operating with version 9.1 of STAR e software, in the range of 25-350°C. The samples (2–4 mg) were encapsulated in aluminium pans having pierced lids to allow escape of volatiles. The heating rates of 10°C min –1 and nitrogen purge at 100 mL min –1 were employed. The mid- point of the inflection curve resulting from the typical second heating cycle was assigned as the T g of polymers. The mechanical properties of the polymer films were determined by stress-strain measurements at room temperature on an Instron Single Column Systems tensile testing machine (model 3345) equipped with a 5 kN load cell and activate grips, which prevented the slippage of the sample before break. The cross head speed was 50 mm min -1 . Dielectric spectroscopy measurements of the polymer films were performed using an experimental set-up from Novocontrol at room temperature and various frequencies (in the range of 10 -2 - 10 6 Hz). The system was equipped with an Alpha high resolution dielectric analyzer, the impedance analyzer HP 4191A and temperature controller Quatro version 4.0. Scheme 1. The structure of the aromatic polyimides I