The Effect of Even-Odd Methylene Spacer Groups on the Thermal Stability of Epoxy-Amine Polymers.
Summary: A series of some novel epoxy-amine polymers was successfully synthesized by using DGEBA and aromatic diamines as starting materials. The cured free standing flexible polymer films were characterized by FTIR, DSC, TGA, and WAXD. The values of Tg decreased as the length of the central methylene spacers increased. This decrease in Tg values occurred in a zig-zag fashion affirming an even-odd effect. Epoxy-amine polymers containing even number of methylene spacers showed remarkably higher Tg values as compared to the polymers containing an odd number of methylene spacers. TGA analysis revealed that epoxy-amine polymers exhibited thermal stability upto 340 C. The length of central methylene spacers showed no significant detrimental effect on the degradation temperatures of the polymers. XRD data of epoxy-amine polymers showed their amorphous nature.
Key words: Even-odd methylene groups, Thermal stability, Epoxy, Amine-epoxy, Polymer, Matrix.
Epoxy resins are being used widely for the fabrication of high performance materials [1-3]. Owing to excellent chemical resistance, superior electrical and mechanical properties [4, 5], their area of applications broadens to laminates and castings, engineering composites, adhesives, electronic encapsulations, and surface coatings [6-8]. The most widely explored epoxy resin is the diglycidyl ether of bisphenol-A (DGEBA) due to its low cost and crosslinking properties. However, the properties and performance of DGEBA are not compatible with more challenging areas of electronics and aerospace industry due to their relatively low thermal stability and brittleness [9-13]. Thus, it is crucial to tailor such epoxy resins which may broaden their applications and meet the demands of some specific areas like electronics, adhesives, construction, and matrix materials in aerospace composites .
Over the years, several researches reported procedures with little alteration to further improve the properties and performance of epoxy resins [15-23]. For instance, Park et al. reported the siloxane containing diamine as curing agent for DGEBA. Their study showed the increase in thermal stability as a function of the siloxane containing diamine contents . Curing of DGEBA with maleic anhydride was reported by Jain et al. the increased thermal stability in terms of char yield was observed . Geeta et al. made another effort to improve thermal stability of DGEBA by curing with aromatic diamine containing imide linkages . However, a study dealing with the effect of even-odd aliphatic groups in the diamine hardner is not well reported in the literature.
In the present study, a series of novel epoxy- amine blends was prepared. The main objective was to explore the effect of even-odd alkyl groups and ether linkages in the hardener on the thermal stability of epoxy resin. For this purpose, p-aminophenoxy alkanes having alkyl groups with even and odd number carbon chains and flexible ether linkages were condensed with commercially available DGEBA. At first, p-nitrophenoxy alkanes were synhesized by the Williamson etherification of p- nitrophenol with corresponding dibromoalkanes . These p-nitrophenoxy alkanes were hydrogenated to p-aminophenoxy alkanes by hydrazine and Pd/C catalyzed reduction in a good yield.The influence of incorporated long flexible alkyl chains and ether linkages on the processability and thermal stability of epoxy amine thermosets was observd by FTIR spectroscopy, DSC, TGA, and X-ray diffraction pattern.
The thermal behavior of resulting epoxy- amine blends showed that the incorporation of even number flexible structures directly into the backbone furnished the thermosets with greater thermal stability as compared to odd number alkyl groups.
Diglycidyl ether of bisphenol-A (DGEBA D.E.R.TM 332 liquid epoxy resins (epoxide equivalent mass 174 g/eq) was purchased from DOW Chemical Co. Dibromoalkanes, with an alkyl chain length from 5-11 carbon atoms, were purchased from Alfa Aesar. p-Nitrophenol, palladium charcoal (Pd/C), hydrazine monohydrate (64-65%), N,N'-dimethylformamide (DMF) were purchased from Sigma Aldrich.
Fourier Transform Infrared (FTIR) Spectroscopy
Structural characterization of epoxy-amine polymers was complied by FTIR spectroscopy. Spectral analysis of free standing flexible films of epoxy-amine thermosets was carried out at 25C using Thermo Scientific Nicolet 6700 ATR-FTIR Spectrometer over the range 4000-400cm-1.
Differential Scanning Calorimetry (DSC)
The glass transition temperature of fully cured exoxy-amine polymers was determined by STARe SW 9.0 DSC analyzer. A sealed aluminum pan under an inert atmosphere of nitrogen was used to heat the specimen by ramping the instrument at 10C/min from room temperature to 300C.
Thermogravimetric (TG) Analysis
TGA thermograms were recorded on TGA analyzer STARe SW 9.0. The decomposition profile was acquired by heating the pre-dried samples in the temperature range from 25C to 800C at a rate 10C/ min under nitrogen atmosphere.
Wide angle X-ray diffraction pattern (WAXD)
The amorphous nature of epoxy-amine polymers was determined by X-ray diffraction studies using 3040/60 X' Pert PRO diffractometer.
The synthesis of p-aminophenoxy alkanes was carried out in two steps, as depicted in Fig. 1. In the first step various p-nitrophenoxyalkanes were synthesized according to a given procedure .
A 250 mL two-necked flask equipped with a condenser, thermometer and magnetic stirrer was charged with a reation mixture of p-nitrophenol (8.0 g, 57 mmol), anhydrous potassium carbonate (7.86 g, 57 mmol), DMF (50 mL) and toluene (30 ml). The reaction mixture was stirred at room temperature (about 30 C) for one hour under an inert atmosphere of nitrogen. Afterwards, the corresponding dibromoalkane (28.5 mmol) was added dropwise over a period of 30 and the mixture was refluxed for 12 h at 120 C. The conversion of reactants into the product was monitored by TLC (n-hexane : ethyl acetate. 1:4). The complete consumption of reactants was followed by cooling the reaction mixture to room temperature and then precipitation of the yellow colored product in distilled water. After being washed repeatedly with water, the product was collected by filtration and was recrystallized from ethanol.
In the second step, the reduction of p- nitrophenoxy alkanes to p-aminophenoxy alkanes  was carried out by taking p-nitrophenoxy alkanes (3 mmol), 10% Pd/C (0.03 g) in 80 mL of ethanol in a 250 mL two-necked round bottom flask. The solution/suspension was heated to boiling temperature. Hydrazinium monohydrate (3.5 mL) was added dropwise to the boiling reaction mixture over a period of 1 hour. The reaction mixture was refluxed for three hours after which the catalyst was filtered off from the colorless hot diamine solution. The p-aminophenoxy alkanes were obtained by precipitation in distilled water, followed by drying and recrystallization in the smallest possible volume of ethanol.
A very simple and facile nomenclature was coined to differentiate between various epoxy-amine polymers, and it will be utilized thoroughly herein, as given in Table-1 and 2. The first two letters of each code i.e. EA' stand for epoxy-amine polymer, preceded by a number' representing the alkyl chain length derived from the domain part of epoxy-amine polymer. The epoxy-amine films were fabricated by blending the calculated amounts of the diamine and epoxy monomers (Fig. 2). The ratio of amine- hydrogen and epoxide group was adjusted to be 1.0 in all samples, i.e (CNH/CEp-1.0) . The synthesized diamines were used to cure DGEBA. The preparation of homogeneous mixture of epoxy and amine monomers was ensured by vigorously stirring for 30 mins at 80 C. The trapped air or bubbles were removed by keeping the mixture under vacuum for 10-20 min. The degassed homogenous mixture was then poured into rectangular shaped teflon coated preheated (100C) moulds to obtain epoxy-amine films.
The moulds were carefully transferred on a leveled surface inside an oven pre-maintained at 100 C. The flexible free standing epoxy-amine films were obtained by curing at 100C for 5 h and post cured at 150C for 3 h.
Results and Discussion
The diamine monomers, i.e. p- aminophenoxy alkanes, were synthesized in a two step reaction as given in Fig. 1. In the first step the p- nitrophenoxy alkanes were synthesized by Williamson etherification reaction of p-nitrophenol with six different dibromoalkanes (1,5- dibromopentane, 1,6-dibromohexane, 1,8- dibromooctane, 1,9-dibromononane, 1,10- dibromodecane) to give the corresponding dintro compounds. In the second step these dinitro compounds were reduced to the respective diamine monomers by Pd/C catalyzed reduction process using hydrazine monohydrate in refluxing ethanol as solvent. The synthesis of all dinitro and diamino compounds were confirmed by their melting points which were found according to the reported literature.
Synthesis and Characterization of Epoxy-Amine Polymers
A series of epoxy-amine polymers were prepared by curing DGEBA with the synthesized p- aminophenoxy alkanes. To avoid deterioration of properties of the final polymer by any residual monomer functionalities, the stoichoiometric amounts of diamine monomers and epoxy resin were used to ensure complete curing process. The curing was carried out by heating the epoxy-amine blends at 100C for 5 h followed by heating at 150C for 3 h. Physical data of polymer networks is given in Table-1.
Table-1: Physical data of epoxy-amine polymer.
###EA -6###Yellowish brown###F, T
###EA -8###Yellow###F, T
###EA -9###Yellowish brown###F, T
###EA -10###Red###F, T
###EA -11###Yellow###F, T
FTIR Spectroscopic Analysis
The structure of epoxy-amine polymers was elucidated by FTIR spectroscopy. The evidence for the complete curing of epoxy resin by the diamine hardner comes from the absense of absorption band at about 910 cm-1 in the FTIR spectrum of epoxy- amine polymers (Fig. 3). This absorption band is characteristic of epoxide ring of DGEBA. In the FTIR spectrum, the absorption band due to stretching vibrations of hydroxyl groups, produced as a result of the opening of the epoxide ring by the amine groups, appeared at 3400-3200 cm-1. It is a measure of the extent of the epoxy-amine reaction. Thus, FTIR data indicated the proper formation of the cross-linked network within the epoxy-amine polymer.
Thermal stability of epoxy amine polymers was evaluated by means of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).
Differential Scanning Calorimetry (DSC)
DSC analysis revealed that rapid cooling from 300 C to room temperature produced predominantly amorphous samples so that Tg of the polymers could be easily read in the second heating trace of DSC. The values of Tg of the epoxy-amine polymers are given in Table-2. The Tg values of epoxy-amine polymers were found to be dependent on the structure of diamines used in the curing process. These values decreased as the polymer structure become more and more flexible with an increase in the the length of central methylene spacers in the diamines. Table-2 shows that the values of Tg of epoxy-amine polymers varied in the range of 106-125C (EP-5 to EP-11). However, this decline in the the Tg values is not linear, rather it is in a zigzag fashion revealing the even-odd effect as given in Fig. 4.
Epoxy amine polymers with odd number of methylene spacers have lower Tg values than polymers with an even number of methylene spacers which suggest that odd number methylene spacers in the polymer backbone decreased the intermolecular interactions leading to a decreased Tg value. In contrast to this, epoxy amine polymers with even number methylene spacers increased the intermolecular interactions due to strong chain interaction leading to an increased Tg value. The Tg order corresponded to the decreasing order of chain stiffness. The relatively lower value of Tg for EP-11 can be explained in terms of greater flexibility due to the odd and the largest number of methylene spacers in its diamine moiety. Whereas, the highest Tg value associated with EA-8 can be attributed to the presence of an even and smaller number of methylene spacers in its diamine moiety. Reduction in the number of methylene spacers in diamines leads to enhanced stiffness of polymer backbone.
Table-2: Thermal properties of epoxy amine polymers.
Codes###Td C###T5 C###T10 C###Tmax C Char yield (%) TgC
Fig. 5 shows the TGA thermograms of epoxy-amine polymers. As obvious from Fig. 5, the loss of mass occured in only one step. Thermogravimetric analysis showed no significant weight loss upto 340C. The thermal parameters which include, the temperature of initial degradation Td, temperature at 5% weight loss T5, temperature at 10% weight loss T10, temperature at the maximum rate of degradation Tmax and the char yields of the polymers are listed in Table-2. The values of Td were determined from the onset of TGA thermograms realized before and after the sudden drop at the start of degradation. Its values varied between 335-349C. The 5% and 10 % weight loss temperatures which are the two main criteria in determining the thermal stability were in the range of 342-349 and 346-353C respectively. The values of Tmax were observed from 395-402 C. As the Td and T5 values of these thermosets are all above 330C which indicates that they all exhibit good thermal stability.
It was also observed that the length of the central methylene spacers imposes no prominent effect on the degradation temperatures of these polymers. The residual weight retentions at 800 C in the nitrogen atmosphere were quite low in the range of 12-14%.
X-ray Diffraction Analysis
The crystallinity of the epoxy amine polymers was examined by WAXD analysis with 2 ranging from 10-70. Fig. 6 shows the representative X-ray diffractograms of epoxy amine polymers. These polymers exhibited amorphous behavior as is obvious from the presence of broad peak in the diffractograms. This amorphous behavior was attributed to the presence of flexible central methylene spacers and ether linkages which induced looser chain packing and revealed a larger decrease in the crystallinity of the epoxy-amine polymers.
This study presented the synthesis of a series of some novel epoxy-amine polymers by the polymerization of DGEBA with the aromatic diamines bearing long alkyl chains and ether linkages. The alkyl chains imparted flexibility to free standing polymer films while ether linkages maintained the thermal stability upto 340C. The resulting epoxy- amine polymers were characterized by FTIR, DSC, TGA and XRD analysis. The novel epoxy-amine polymers were obtained with fairly high Tg values and excellent thermal stability. The length of the central spacer groups imposed no prominent effect on the degradation temperatures of these polymers. DSC of epoxy-amine thermosets showed that an increase in the number of methylene spacers increased the flexibility leading to a decrease in the values of Tg in a zig-zag fashion revealing an even-odd effect. The XRD data of polymers showed amorphous nature due to the presence of flexible central methylene spacers.
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|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Feb 28, 2015|
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