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Studies on the synthesis and reaction of silicone oxirane dendrimer and their thermal and rheological properties
European Polymer Journal 58 (2014) 115–124
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Studies on the synthesis and reaction of silicone oxirane dendrimer and their thermal and rheological properties Sangeeta Kandpal ⇑, A.K. Saxena Defence Materials and Stores Research and Development Establishment, G.T. Road, Kanpur 208013, India
a r t i c l e
i n f o
Article history: Received 20 March 2014 Received in revised form 30 May 2014 Accepted 10 June 2014 Available online 27 June 2014 Keywords: Dendrimers Curing Rheology TGA
a b s t r a c t A series of silicone core dendrimers bearing SiAH terminal group was synthesized by the reaction of tetraethoxysilane and diorganochlorosilane in high yields, which on reacting with allylglycidylether in presence of Speier’s catalyst yielded oxirane terminated dendrimers. These dendrimers were characterized using physico-chemical techniques viz; elemental analysis, Fourier transforms infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance spectroscopy (1H NMR spectroscopy). Molecular weight was determined by vapour pressure osmometry (VPO). The rheological studies were performed to study the curing behavior of oxirane dendrimer with triethylenetetraamine. Thermal properties of cured dendrimers were evaluated using thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC). Change in thermal properties of epoxy resin (LY556) on addition of dendrimers was also studied by TGA and DSC. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Since the pioneering work of well-defined, threedimensional structurally ordered macromolecules by Vögtle [1], Tomalia [2,3] and Newkome [4] interest in dendrimers and hyperbranched polymers has been increasing at an amazing rate as they exhibit distinct properties in comparison to their linear analogue [5–8]. Dendrimers are spheroid or globular nanostructures that are synthesized by the using the multistep interactive controlled reaction procedures like hydrosilylation, Grignard reactions, dehydrocoupling, alcoholysis and alkenylation reactions either by divergent or convergent manner [9,10]. From the application point of view dendrimers are expected to play a key role as enabling building blocks for nanotechnology during the 21st century [11] and deemed to serve as chemical sensor [12–14], catalyst ⇑ Corresponding author. Tel.: +91 0512 2451759; fax: +91 0512 2450404. E-mail addresses: [email protected] (S. Kandpal), arvsaxena @gmail.com (A.K. Saxena). http://dx.doi.org/10.1016/j.eurpolymj.2014.06.009 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved.
[15], molecular device [16,17], chemo-delivery [18,19], applications in coating, drug and gene delivery [20], macromolecular building block and supramolecular sciences [21,22] etc. As dendrimers are reported [23–25] to have better solubility in common solvents and less bulk viscosity as compared to equivalent molecular weight linear polymers, hence they may be also used more effectively as coupling agent [26], reactive diluents [27], effective curing reagent [28], coating substrates etc. and also in preparation of high char yield material for advanced composites [29]. The literature survey [1,19,30,31] further revealed that in comparison to organic dendrimers, the silicone dendrimers are scarcely studied though organosilanes and silicones have found numerous high tech applications [32–35] especially in the preparation of thermally stable and ecofriendly materials. Despite several important features, yet these dendrimers failed to make any commercial impact except PAMAMOS [36]. The reason for it may be attributed to the low yield of the products. Keeping in view of it and
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our continued interest in the area of synthesis of functional organosilanes, polysilanes, polycarbosilanes [37–39] and silicone dendrimers [40] it is considered worthwhile to synthesize some new organosilicone dendrimer bearing terminal epoxy groups in high yield and explore their application as an additive to epoxy resin to improve thermal properties. 2. Experimental 2.1. Materials All reactions involving air and/or moisture sensitive compounds were carried out in an N2-filled drybox or using Schlenk technique. THF (LR grade, Ranbaxy), hexane (LR grade, E. Merch) and dichloromethane (LR grade, E. Merck) were purified and dried before use as reported [41]. Tetraethoxysilane (TEOS) (98%, Aldrich), chlorodimethylsilane (98%, Aldrich), chloromethyl(phenyl)silane (>93%, Aldrich), vinyltriethoxysilane (98%, Aldrich), allylglycidylether (AGE) (99%, Aldrich), triethylene tetramine (TETA) (>97%, Aldrich) and epoxy resin LY556 (epoxy equivalent 72-180, Dow chemical’s) were used as such. Speier’s catalyst [42] was synthesized in laboratory. Ethyl alcohol (99.9%, SD Fine chemicals) and silica gel (60– 120 mesh LR grade, SD Fine chemicals) were used as such. 2.2. Syntheses of dendrimers 2.2.1. GoA dendrimer [Tetrakis(dimethylsiloxy)silane] A solution of TEOS (7.34 g, 0.03 mol), ethyl alcohol (6.57 g, 0.14 mol) and water (4.0 g, 0.21 mol) was taken in a three necked round bottom flask equipped with magnetic stirrer, thermometer, condenser, dropping funnel and stirred for 30 min. Into the above solution chlorodimethylsilane (16.67 g, 0.18 mol) was added drop wise within 3–4 h under stirring. Afterward, few drops (0.2 ml) of concentrated H2SO4 was added in the solution and mixture was further stirred for 15 min. Later on, the solution was allowed to settle down and both silicone and aqueous layer were separated using separating funnel. The silicone layer was washed with distilled water till it became neutral. The aqueous layer was extracted with petroleum ether thrice (3 20 ml) and mixed with silicone layer. The silicone layer was then washed with brine solution (50 ml) and kept on anhydrous sodium sulphate overnight and filtered. The filtrate was distilled on water bath to remove petroleum ether. The residue was distilled on oil bath to collect dendrimer GoA (C8H28O4Si5). Yield 28.0 g, 82.5%, b.p. 190 °C. FTIR (KBr): 1077 (ASiOSiA), 2136 (ASiH) cm1; 1 H NMR (400 MHz, CDCl3, d ppm): 0.22 (d, 24H, ASiACH3), 4.74 (m, 4H, ASiAH); elemental analysis (%) C 29.23, H 8.56, Si 42.74 (calcd. C 29.21, H 8.58 and Si 42.72 respectively). Mol wt. (VPO): 320.14 (calcd. 328.73). Similarly other reactions as mentioned below were carried out. 2.2.2. GoB dendrimer (SiAH terminated) Reaction of TEOS (8.8 g, 0.04 mol), ethyl alcohol (6.5 g, 0.08 mol), water (4.0 g, 0.22 mol) and chloromethyl
(phenyl)silane (31.3 g, 0.21 mol) was carried out as above to afford GoB dendrimer. Reaction mixture was distilled under vacuum (26 mbar at 64 °C) to remove unreacted reactants and disiloxanes (by-products of partial hydrolysis of chloromethyl(phenyl)silane. The pot residue was purified by column chromatography (silica gel as stationary phase and n-hexane as mobile phase) to afford colorless liquid GoB (C28H36O4Si5) dendrimer; Yield 18.7 g, 80%. FTIR (KBr): 1080 (ASiOSiA), 2139 (ASiH), 1456 (ASiAPh) cm1; 1H NMR (400 MHz, CDCl3, d ppm): 0.21 (d, 12H, ASiACH3), 4.74 (m, 4H, ASiAH), 7.1–7.3 (d, 20H, ASiAPh); elemental analysis (%) C 58.28, H 6.29, Si 24.31 (calcd. C 58.15, H 6.96 and Si 24.34 respectively). Mol wt. (VPO): 512.21 (calcd. 576.15). 2.2.3. GI dendrimer (SiAOC2H5 terminated) A solution of GoA dendrimer (11.7 g, 0.035 mol), and Speier’s catalyst (0.03 mol%) in dry THF (50 ml) was taken in a three necked round bottom flask (100 ml) fitted with a dropping funnel containing vinyltriethoxysilane, magnetic stirrer, thermometer, condenser and heated up to 60 °C in inert atmosphere. vinyltriethoxysilane (27.09 g, 0.142 mol) was gradually added drop wise in GoA solution within 30 min. afterwards the solution was heated for an additional 4 h and cooled to room temperature. The solution was distilled under reduced pressure (26 mbar at 64 °C) to remove unreacted vinyltriethoxysilane and solvent. The pot residue was column chromatographed (silica gel and n-hexane) to afford colourless and transparent GI (C40H100O16Si9) dendrimer; yield 30.14 g, 79%. FTIR (KBr): 1080 (ASiOSiA), 1254 (ASiCH3) cm1; 1H NMR (400 MHz, CDCl3, d ppm): 0.22 (s, 24H, ASiACH3), 0.31 (t, 16H, ASiACH2A), 3.75 (q, 24H, ASiAOACH2A), 1.57 (t, 36H, ASiAOACH2CH3); elemental analysis (%) C 44.07, H 9.17, Si 23.14 (calcd. C 43.95, H 9.02 and Si 23.19 respectively). Mol wt. (VPO): 1001.27 (calcd. 1089.98). 2.2.4. GIA dendrimer (dimethyl SiAH terminated) Reaction of GI dendrimer (7.69 g, 0.0071 mol) in ethyl alcohol (2.13 g, 0.028 mol), water (0.76 g, 0.0426 mol) and chlorodimethylsilane (8.061 g, 0.0852 mol) was carried out similarly as the procedure adopted for the synthesis of dendrimer G0A, which yielded colourless, transparent liquid dendrimer GIA (C40H124O16Si21), yield 9.27 g, 90%. FTIR (KBr): 1075 (ASiOSiA), 2136 (ASiH) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.24 (d, 72H, ASiACH3), 4.74 (m, 12H, ASiAH), 0.23 (s, 24H, ACH2ASiACH3), 0.64 (t, 16H, ASiACH2A); elemental analysis (%) C 33.10, H 8.61 Si 40.59 (calcd. C 33.06, H 8.23 and Si 40.64 respectively). Mol wt. (VPO): 1360.18 (calcd. 1451.19). 2.2.5. GIB dendrimer (methylphenyl SiAH terminated) Similarly as above, the reaction of GI dendrimer (15.2 g, 0.014 mol) in ethyl alcohol (4.2 g, 0.056 mol), water (1.4 g, 0.08 mol) and chloromethyl(phenyl)silane (26.52 g, 0.168 mol) was carried out to afford colourless, transparent liquid dendrimer GIB (C100H148O16Si21), Yield 27.0 g, 88%. FTIR (KBr): 1075 (ASiOSiA), 2139 (ASiH), 1457(ASiAPh) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.22 (m, 36H, ASiACH3), 4.74 (m, 12H, ASiAH), 0.23 (s, 24H, ACH2ASiACH3), 0.64 (t, 16H, ASiACH2A), 7.1–7.3 (d, 60H,
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ASiAPh); elemental analysis (%) C 54.69, H 6.79 Si 26.61 (calcd. C 54.69, H 6.78 and Si 26.66 respectively). Mol Wt. (VPO): 2085.48 (calcd. 2196.02). 2.2.6. GoAx dendrimer (Si-oxirane terminated) Hydrosilylation reaction of GoA dendrimer (8.84 g, 0.027 mol) with allylglycidyl ether (12.3 g, 0.11 mol) in presence of Speier’s catalyst (0.03 mol%) was carried out as above for the synthesis of GI dendrimer, after completion of reaction, the solvent and unreacted allylglycidylether was removed by vacuum distillation at 40 °C (20 mbar). The residue was purified by column chromatography (silica gel and n-hexane) to afford colorless, transparent liquid GoAx dendrimer (C32H68O12Si5). Yield 19.08 g, 89%. FTIR (KBr): 913 (epoxy ring, asy), 1080 (ASiOSiA), 3050 (epoxy ring, sym) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.07 (s, 24H, ASiACH3), 0.54 (t, 8H, ASiACH2A), 1.82 (m, 8H, ACH2ACH2ACH2A), 2.59 (d, 8H, epoxy ring, ACH2A, trans), 2.77 (d, 8H, epoxy ring, ACH2A, cis), 3.13 (m, 4H, epoxy ring, ACHA), 3.37 (d, 8H, AOACH2Aepoxy ring, cis), 3.71 (d, 8H, AOACH2Aepoxy ring, trans), 4.01 (t, 8H, ACH2ACH2AOA); elemental analysis (%) C 48.95, H 7.96, Si 17.83 (calcd. C 48.23, H 8.03 and Si 17.88 respectively). Mol wt. (VPO): 712.93 (calcd. 785.3). Similarly other reactions as mentioned below were carried out. 2.2.7. GoBx dendrimer (Si-oxirane terminated) Reaction of GoB dendrimer (4.18 g, 0.007 mol), Speier’s catalyst (0.03 mol%) and allyl glycidyl ether (3.4 g, 0.03 mol) yielded a colourless, transparent liquid dendrimer GoBx (C52H76O12Si5). Yield 6.20 g, 85%. FTIR (KBr): 913 (epoxy ring, asy), 1080 (ASiOSiA), 3050 (epoxy ring, sym) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.07 (s, 12H, ASiACH3), 7.1 (d, 20H, ASiAPh), 0.54(t, 8H, ASiACH2A), 1.82 (m, 8H, ACH2ACH2ACH2A), 2.59 (d, 8H, epoxy ring, ACH2A, trans), 2.77 (d, 8H, epoxy ring, ACH2A, cis), 3.13 (m, 4H, epoxy ring, ACHA), 3.37 (d, 8H, AOACH2Aepoxy ring, cis), 3.71 (d, 8H, AOACH2 Aepoxy ring, trans), 4.01 (t, 8H, ACH2ACH2AOA); elemental analysis (%) C 60.43, H 7.41, Si 13.54 (calcd. C 61.02, H 7.15 and Si 13.59 respectively). Mol wt. (VPO): 989.18 (calcd. 1032.42). 2.2.8. GIAx dendrimer (Si-oxirane terminated) Reaction of GIA dendrimer (6.0 g, 0.0041 mol), Speier’s catalyst (0.03 mol%) and allylglycidyl ether (6.75 g, 0.0592 mol) yielded a dendrimer GIAx (C112H244O40Si21) which was colourless and transparent liquid, yield 9.59 g, 85%. FTIR (KBr): 913 (epoxy ring, asy), 1080 (ASiOSiA), 3050 (epoxy ring, sym) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.07 (s, 96H, ASiACH3), 0.64 (t, 16H, ASiACH2A) 0.54 (t, 24H, ASiACH2A), 1.82 (m, 24H, ACH2ACH2ACH2A), 2.59 (d, 24H, epoxy ring, ACH2A, trans), 2.77 (d, 24H, epoxy ring, ACH2A, cis), 3.13 (m, 12H, epoxy ring, ACHA), 3.37 (d, 24H, AOACH2Aepoxy ring, cis), 3.71 (d, 24H, AOACH2 Aepoxy ring, trans), 4.01 (t, 24H, ACH2ACH2AOA); elemental analysis (%) C 47.68, H 8.72, Si 20.90 (calcd. C 48.02, H 8.69 and Si 20.91 respectively). Mol wt. (VPO): 2753.79 (calcd. 2820.91).
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2.2.9. GIBx dendrimer (Si-oxirane terminated) Reaction of GIB dendrimer (4.0 g, 0.0018 mol), Speier’s catalyst (0.03 mol%) and allyl glycidyl ether (2.5 g, 0.0218 mol) yielded a colorless, transparent liquid dendrimer GIBx (C172H268O40Si21). Yield 5.7 g, 90%. FTIR (KBr): 913 (epoxy ring, asy), 1080 (ASiOSiA), 3050 (epoxy ring, sym) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.07 (s, 60H, ASiACH3), 7.3 (d, 60H, ASiAPh) 0.64 (t, 16H, ASiACH2A) 0.54 (t, 24H, ASiACH2A), 1.82 (m, 24H, ACH2ACH2ACH2A), 2.59 (d, 24H, epoxy ring, ACH2AA, trans), 2.77 (d, 24H, epoxy ring, ACH2A, cis), 3.13 (m, 12H, epoxy ring, AACHA), 3.37 (d, 24H, AOACH2Aepoxy ring, cis), 3.71 (d, 24H, AOACH2 Aepoxy ring, trans), 4.01 (t, 24H, ACH2ACH2AOA); elemental analysis (%) C 57.94, H 7.57, Si 16.52 (calcd. C 57.32, H 7.95 and Si 16.54 respectively). Mol wt. (VPO): 3498.31 (calcd. 3565.74). (Schemes 1–3). 2.3. Curing of oxirane dendrimers The oxirane dendrimers (GoAx, GoBx, GIAx and GIBx) initially cured neat with stoichiometric amount of TETA and further there 1%, 2% and 3% amount were mixed homogeneously with epoxy resin (LY556) then cured with TETA in stoichiometric amount and kept at room temperature (28 °C) for 24 h. Afterwards the mixtures were heated for post curing at 50 °C for 0.5 h, 70 °C for 1 h, 85 °C for 2 h and 101oC for 2 h. The thermal properties i.e., DSC and TGA were carried out for post cured resin matrix (Scheme 4). 2.4. Equipment Fourier transform infrared spectroscopy (FT-IR) was used to characterize the functional group and monitoring the reaction progress of different dendrimers. Infrared spectra of the sample were recorded on a Perkin Elmer FT-IR spectrometer RX1 at room temperature using KBr pellets. The analysis was performed between 500 and 4000 cm1. 1 H nuclear magnetic resonance spectroscopy spectra were recorded at room temperature on a Bruker Avance 400 MHz NMR spectrometer. Samples (5–10 mg) for NMR analysis were dissolved in deuterated chloroform (CDCl3). Vario EL III CHNOS elemental analyzer was used for elemental analysis of dendrimers and silicone elemental analysis was carried out according to reported method [41]. Vapour pressure osmometre (VPO) was performed to determined molecular weight of dendrimers using a Knauer-7000 in toluene at 60 °C in a concentration range of 5–10 mg/ ml. and polystyrene in the range of 500– 3000 was used for calibration. Differential Scanning Calorimetry (DSC) analysis was performed on a TA Instruments Q 100 DSC. All measurements were conducted in crimped aluminium pans under a nitrogen atmosphere, at a purge gas flow rate of 50 ml/ min. samples were heated from 25 to 300 °C at a rate of 10 °C/min. samples were than cooled from 450 to 25 °C. Finally, the samples were heated for a second time at rate of 10 °C/min to 450 °C. Thermogravimetric analysis (TGA) was performed on a TA Instruments Hi-Res TGA 2950 thermogravimetric
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Scheme 1. Synthesis of oxirane dendrimers GoAx and GoBx, Where for GoAx, R = CH3 and for GoBx, R = C6H5.
analyser in a nitrogen atmosphere. Approximately 5– 10 mg of each sample was heated from 25 to 800 °C at a rate of 10 °C/min. Rheological analysis was performed on a AR-G2 rheometer from TA Instruments with 25 mm diameter disposable aluminium parallel plate. The curing cycle were performed in a dynamic oscillation mode at a frequency of 6.283 rad/ s, temperature ramp 3 °C/ min and a strain of 0.1%. 3. Results and discussion As the microenvironment of inside and outside of the dendrimer could be easily modified with a variety of organic functionalities hence these dendrimers may serve potential reagent as reactive diluents and thermal properties modifier of various organic resin to suit them for any designed high tech applications [43,44]. Hence in continuation of our previous study, in present work, we have synthesized SiAH terminated silicone dendrimer in high yield which on addition to allylglycidylether yielded oxirane terminated dendrimers. These oxirane terminated dendrimers have been used as additive to modulated curing and thermal behavior of epoxy resin. In our previous work we have reported the synthesis of Go SiAH silicone dendrimer where the yield was not satisfactory (69%) hence a new method has been adopted to synthesize Go dendrimer (SiAH terminated) in high yield (82–90%) using co-hydrolysis of tetraethoxysilane and chloromethyl(organo)silane. Synthesis of ethoxy terminated GI dendrimer was carried out by using hydrosilylation reactions of Go dendrimer with vinyltrieth-
oxysilane in THF solution under strict inert environment. Afterwards, the ethoxy terminated GI dendrimer was co-hydrolysized with chloromethyl(organo)silane to afford SiAH terminated GI dendrimers. These SiAH terminated dendrimers were further hyrosilylated with allylglycidylether to afford oxirane terminated dendrimers. These oxirane terminated dendrimers were cured with TETA and there curing behavior were studied by rheology. Thermal study of the cured resin matrix of dendrimers was also carried out using thermogravimetric analysis. The additive effects of these dendrimers on the thermal properties of epoxy resin (LY556) were also studied in detail. The characterization and thermal studies of these dendrimers and resin matrixes have been discussed below. 3.1. FTIR spectroscopy FT-IR spectroscopy provided valuable information about the reaction progress and the functional group attached on the dendrimers. In all the dendrimers an IR absorption peak appeared at 1080 cm1 which indicates the presence of m SiAOASi bond. The m SiAH absorption peak appeared at 2131 cm1 in dendrimers GoA, GoB, GIA and GIB. When the hydrosilylation reaction of dendrimer GoA with vinyltriethoxysilane was carried out to prepare GI dendrimer, the m SiAH peak at 2136 cm1 and m CH2@CH peak at 1600 cm1 disappeared which confirmed the addition of SiAH group on vinyl moiety. The FT-IR spectra of the reaction product GoAx, GoBx, GIx A and GIBx showed the appearance of characteristic absorption peak of oxirane ring at 913 cm1 and
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Scheme 2. Synthesis of SiAH functional dendrimer GIA and GIB, where R = CH3 for GIA and R = C6H5 for GIB.
3054 cm1 and the disappearance of m SiAH peak at 2136 cm1 which was very much prominent in dendrimers GoA, GoB, GIA and GIB respectively. Therefore, it may be inferred that hydrosilylation of allyl glycidyl ether with all SiAH terminated dendrimers took place. IR spectra of cured resin matrix showed the peak of unsymmetrical stretching of ether linkage at 1259 cm1 which showed the presence of ether linkage of allylglycidylether moiety due to crosslinking of allylglycidylether with TETA. The oxirane ring which appeared at 913 cm1 and 3054 cm1 disappeared and in the resin matrix secondary hydroxyl group appeared at 3439 cm1 due to the reaction of amino group of TETA with dendrimer and LY556. 3.2. NMR spectroscopy 1
H NMR spectra of dendrimers GoA, GoB, GIA and GIB respectively showed multiplet for d SiAH at 4.74– 4.85 ppm due to coupling with SiACH3 protons and
doublet for d SiACH3 at 0.22 ppm due to coupling with protons of SiAH group. The progress of hydrosilylation of the dendrimer GoA with vinyltriethoxysilane was confirmed by disappearance of d SiH proton at 4.74 ppm and d H2C@CH proton at 5– 6 ppm. Similarly the hydrosilylation of dendrimers GoA, GoB, GIA and GIB with allyl glycidyl ether was confirmed with the disappearance of d SiAH proton at 4.74 ppm and d H2C = CH at 5–6 ppm. The formation of dendrimers GoAx, GoBx, GIAx and GIBx were confirmed by characteristic signals of oxirane ring at d 2.59 ppm (CH2, trans), 2.77 ppm (CH2, cis) and 3.13 ppm (CH). The SiACH3 appeared as a singlet at 0.07 ppm whereas d SiACH2 appeared as a triplet at 1.17 ppm due to coupling with protons of the nearby ACH2 group. The signals of d ACH2 of the SiACH2ACH2ACH2 – group appeared as a multiplet at 1.82 ppm, which may be assigned to the coupling of protons of the ACH2 group with the protons present on either side carbon atom, d CH2, which joins the ether linkage to the oxirane ring
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Scheme 3. Synthesis of epoxy terminal dendrimer GIAx and GIBx, where R = CH3 for GIAx and R = C6H5 for GIBx.
appearing at 3.37 ppm (cis) and 3.71 ppm (trans), both appearing as triplets due to coupling with the proton present on the same carbon atom as well as protons attached to the next carbon atom of the oxirane ring. The d CH2 of the ACH2ACH2 AOA group appeared at 4.01 ppm as a triplet, which showed coupling with the protons of the next CH2 group. 3.3. Thermal analysis 3.3.1. DSC analysis In DSC study a broad exotherm was observed for all epoxy resins when cured with TETA in the temperature range of 60–178 °C. The characteristic curing temperatures are summarized in Table 1. As it is established that curing of epoxy resin proceeds by nucleophilic attack of amine on the electrophile oxirane ring, so it may be assumed that more oxirane centers will initiate the curing at low temperature with fast rate. Therefore, onset temperature of
exotherm (To) may be used as a criterion for evaluating the relative reactivity of various oxirane terminal dendrimers (1%) and LY556. From the results it is evident that the lowest To occurred with 1% GIAx and LY556 due to maximum cross linking cites and less bulky group at periphery. The glass transition of dendrimers and dendrimer modified resin (LY556) increased with the increase in silicone core which may be tentatively attributed to the fact that with the increase of crosslinking density Tg increases (Table 2). The glass transition temperature obtained by DSC analysis showed that the Tg of GoAx is lower and GIBx is higher among all dendrimers. 3.3.2. TGA analysis The TGA graph (Fig. 1) of the TETA cured neat dendrimers showed a single step degradation process which reflects the formation of uniform structure and single component system. The initial decomposition temperature (To), the temperature of maximum rate of mass loss (Tmax) and
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S. Kandpal, A.K. Saxena / European Polymer Journal 58 (2014) 115–124 O O R
Me Si O
R O
O
Si
O
Me
Me
Si O O
O Si
NH
NH O
NH
NH2
+
R
2
TETA
Si R
Me
Room temperature curing O
O
Dendrimer N
2 N
N HO O
Me R N N
O
O
2
OH
Me R
OH
Me
O Si
N
R
Si
O
Si O O Si
N
Si
N
2
R
N
Me
O
OH
N N
2 N
Cured dendrimer Scheme 4. Curing of dendrimer with triethylenetetraamine [TETA].
the extrapolated final decomposition temperature (Tef) in the range of 360–375 °C, 410–431 °C and 450–500 °C. Moreover, the char yields of resin matrix at 800 °C were about 29%. Table 3 summarizes the obtained TGA results of the cured product of neat epoxy and epoxy containing dendrimers with TETA. All the epoxy dendrimer composition showed an enhancement of onset temperature of decomposition 15 ± 5 °C with respect to neat epoxy system. And maximum degradation temperature (Tmax) of GIAx dendri-
mer showed a remarkable increase of +10 °C. Whereas varied amount (1–10%) of dendrimers did not show any remarkable variation. TGA results revealed that the char yield of cured dendrimers containing LY556 epoxy were 25–40% higher than neat cured LY556 (Table 3) in case of 1–3% addition and it will increase as we increases the percentage of dendrimers. This may be due to presence of siloxane (SiAOASi) linkage in different dendrimers and it shows a linear relation with dendrimers percentage, on increasing the dendri-
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Table 1 DSC results of Curing of neat LY556 and LY556 with dendimers with TETA.
a b c
Epoxy
Toa (°C)
Tpb (°C)
Tfc (°C)
LY556 1%GoAx + LY556 1% GIAx + LY556 1% GoBx + LY556 1% GIBx + LY556
66.41 61.77 60.77 63.40 64.66
101.79 98.91 96.53 100.84 101.60
173.11 177.81 173.54 166.93 173.92
To (onset of exotherm). Tp (temperature of peak position of exotherm). Tf (temperature of end of exotherm).
Table 2 TGA data of cured oxirane dendrimers. Dendrimers
GoAx GIAx GoBx GIBx
Decomposition temperature (°C) To
Tmax
Tef
375 372 365 360
408 410 424 431
456 451 462 500
Char yield (%)
LOI (%)
Tg (°C)
15.02 20.33 21.81 29.60
23.51 25.63 26.22 29.34
223.25 246.67 248.41 254.50
Fig. 1. Thermogravimetric analysis of cured dendrimers.
of methylphenyl group bearing dendrimers and also the char yield in the later case was higher. Conclusively the TGA studies of dendrimers modified LY556 resin showed that there was marginal yet distinct increase on onset temperature of degradation (To) in case of all epoxy composition but due to increase of dendrimers quantity from 1% to 10% had no significant effect which may be understood with the nullifying effect of addition of dendrimers in epoxy resin (LY556). The change in the value of max degradation temperature (Tmax) in case of addition of GIAx dendrimer was found maximum (+10 °C) which may be due to good synergistic effect of silicone and organic content present in the epoxy composition of this dendrimer. The char yield also increased in case a mixture of resins and it was more in case of GIBx invariably in all ratios. The results are consistent with neat dendrimer results for char yields. The LOI of dendrimer containing resin matrix also increased distinctly, yet marginally due
mer concentration in LY556, the char yield of cured products also increases. The limiting oxygen index (LOI) [45] value for neat dendrimer cured resins were found up to 29% (Table 2), while LOI value for LY556 with dendrimer cured resin shows increment of 10–13% in compare to neat LY556 (Table 3). The TGA results showed that the To and char yield of cured dendrimers was higher than cured LY556 which may be attributed to the ASiAOASiA backbone present in the dendrimers. Among dendrimers the dimethyl group dendrimers showed high To but the Teff was higher in case
Table 3 TGA data of cured hybrid resin matrix. SN
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
TETA cured epoxy resins
LY556 1% GoAx + LY556 1% GIAx + LY556 1% GoBx + LY556 1% GIBx + LY556 2% GoAx + LY556 2% GIAx + LY556 2% GoBx + LY556 2% GIBx + LY556 3% GoAx + LY556 3% GIAx + LY556 3% GoBx + LY556 3% GIBx + LY556 5% GoAx + LY556 5% GIAx + LY556 5% GoBx + LY556 5% GIBx + LY556 10% GoAx + LY556 10% GIAx + LY556 10% GoBx + LY556 10% GIBx + LY556
Decomposition temperature (°C) To
Tmax
Tef
341 348 362 356 354 350 363 350 348 351 363 356 353 352 364 350 353 354 365 352 353
366 367 375 366 363 366 377 364 365 366 375 365 365 367 378 367 368 367 376 368 368
414 416 421 407 402 405 418 403 410 406 422 409 420 408 419 404 421 410 419 408 422
Char yield (%)
LOI (%)
8.01 8.32 10.66 8.76 11.97 9.31 12.03 9.82 13.74 12.21 13.18 13.79 14.50 12.65 13.81 13.97 14.79 12.95 14.21 14.56 15.03
20.71 20.83 21.76 21.00 22.29 21.23 22.31 21.43 23.00 22.38 22.77 23.02 23.30 22.56 23.02 23.08 23.42 22.68 23.18 23.32 23.51
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Fig. 2. Plots of storage modulous against temperature for curing of dendrimers.
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cross-linking the viscoelastic properties of the dendrimers are changing and the viscous emulsion is becoming an elastic cured network. It is shown in Fig. 2 that the initial low value of complex viscosity of the system increases abruptly in the specific time range as the curing reaction proceeds. Graph reflects that initiation of curing has been started at 75 °C for GIAx, at 80 °C for G0Ax, 95 °C for GIBx and 110 °C for G0Bx. There is substantial increase in the value of storage modulus which shows an increase from 10 to 20 Pascal to 105 Pascal. Storage modulous and complex viscosity of G0Bx dendrimer increases slowly compare to other dendrimers which shows that the dendrimer is least reactive among all. It may be due to less crosslinking units and hinderance in its structure due to bulky phenyl groups. Conclusively, the rheological results of all dendrimers curing shows that dendrimer GIAx having maximum oxirane and dimethyl group at periphery, cured fast compare to remain dendrimers due to more crosslinking units and less hindrance at terminals while dendrimer G0Bx having minimum oxirane methylphenyl groups at periphery took maximum time for curing. 4. Conclusions
Fig. 3. Plots of complex viscosity against temperature for curing of dendrimers.
to ASiAOASiA linkage as it formed SiO2 on burning in air atmosphere, which acted as self extinguishing material. So, it may be expected that with the increase of ASiAOASiA groups containing dendrimers or their quantities may further enhance LOI and char yield of resin matrix.
3.4. Rheological analysis Rheological analysis has been used to study the curing process of epoxy resin [46–48] like polymers; epoxy resin is a viscoelastic material. During a curing process under continuous stresses or strains, its viscoelastic characteristics change; this is reflected in the variations of the viscosity. In the present work all oxirane dendrimers were mixed with hardener TETA in stoichiometric ratio, to study the change in storage (G0 ) modulous and complex viscosity with respect to temperature (Figs. 2 and 3). In rheological study, increase in temperature speeds up the crosslinking reaction and increases the velocity of the system, during
In this study the different generations SiAH terminated dendrimers were successfully synthesized in high yield via a new alternative modified method. Hydrosilylation of dendrimers was done with allyl glycidyl ether using Speier’s catalyst to prepare novel oxirane terminated dendrimers. Rheological study of curing behavior of oxirane dendrimers were carried out with TETA. And its result reflects that curing time is decrease as the number of oxirane unit increases in terminals and steric hinderance decreases in the system. Thermal studies of the cured product of dendrimer were carried out. These dendrimers were further used as a modifier for up gradation of thermal properties of commercial epoxy LY556. Enhancement in thermal behavior and flame retardency were observed after adding these dendrimers. It proves that these novel dendrimers can further used for modifier, diluents and in different High-Tech applications. Acknowledgements The authors wish to thank (1) Head and all scientist and technical officers of ACD, DMSRDE for necessary encouragement and providing laboratory facilities to facilitate the work. (2) Mr. Raghwesh Mishra, Scientist ACD for supporting in the synthesis of dendrimers and (3) CAF and Polymer Division, DMSRDE for thermal studies. References [1] Buhleier E, Wehner W, Vögtle F. Synthesis 1978;2:155–8. [2] Tomalia DA, Barker H, Dewald JR, Hall M, Kallos G, Martin S, et al. Polym J (Tokyo) 1985;17:117. [3] Tomalia DA, Barker H, Dewald JR, Hall M, Kallos G, Martin S, et al. Macromolecules 1986;19:2466–8. [4] Newkome GR, Yao ZQ, Baker GR, Gupta VK. J Org Chem 1985;50:2003–4.
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[5] Lu P, Paulasaari JK, Weber WP. Macromolecules 1996;29(27): 8583–6. [6] Harth EM, Hecht S, Helms B, Malmstrom EE, Fre´chet JMJ, Hawker CJ. J Am Chem Soc 2002;124:3926–38. [7] Miravet JF, Frèchet JMJ. Macromolecules 1998;31(11):3461–8. [8] Gong C, Frèchet JMJ. J Polym Sci Part A: Polym Chem 2000;38:2970–8. [9] Kim C, Kwark K, Song C. Appl Organometall Chem 2005;19:108. [10] Kim C, Jeong Y, Jung I. J Organometall Chem 1998;570:9. [11] Tomalia DA. Prog Polym Sci 2005;30(3–4):294. [12] Kim C, Park E, Song CK, Koo BW. Synth Met 2001;123(3):493–6. [13] Chen S, Yu Q, Li L, Boozer CL, Homola J, Yee S, et al. J Am Chem Soc 2002;124:3395–401. [14] Pugh VJ, Hu QS, Zuo X, Lewis FD, Pu L. J Org Chem 2001;66(18):6136–40. [15] Yamamoto K, Higuchi M, Shiki S, Tsuruta M, Chiba H. Nature 2002;415:509–11. [16] Nijhuis CA, Lu F, Knoll W, Huskens J, Reinhoudt DN. Langmuir 2005;21(7):7866. [17] Burn PL, Lo SC, Samuel IDW. Adv Mat 2007;19(13):1675. [18] Frey H, Hagg R. Rev Mol Biotec 2002;90(3–4):257–63. [19] Senthil Kumar M et al. IJSID 2012;2(1):37–49. [20] Khandare J, Kolhe P, Pillai O, Kannan S, Lieh-Lai M, Kannan RM. Bioconjugate Chem 2005;16(2):330–7. [21] Gao C, Yan D. Prog Polym Sci 2004;29(3):183. [22] Al-Jamal KT, Ramaswamy C, Florence AT. Adv Drug Deliv Rev 2005;57(15):2238. [23] Newkome GR, Moorefield CN, Vogtle F. Dendritic molecules: concepts, synthesis, perspectives. Weinheim: VCH; 1996. p. 261. [24] Dvornic PR, Owen MJ. Polym Preprints 2009;50(2):851. [25] Bischoff R, Cray SE. Prog Polym Sci 1999;24:185. [26] Liu M, Kono K, Frechet JMJ. J Control Release 2000;65(1–2):121.
[27] Xu DM, Zhang KD, Zhu XL. J App Polym Sci 2004;92(2):1018. [28] Wang SJ, Fan XD, Kong J, Liu YY. J App Polym Sci 2008;107(6):3812. [29] Abadie MJM, Voytekuans VY, Rusaunov AL. Iran Polym J 2006;15(1):65–77. [30] Vander Made AW, Van leeuwen PWN, Wilde DJC Brande RAC. Adv Mater 1993;5:466–8. [31] Chase PA, Gebbink RJMK, Koten GV. J Organometall Chem 2004;689:4016–54. [32] Choi GJ. J Ind Eng Chem 1997;3:223–8. [33] Ly HQ, Talor R, Day RJ, Heatly F. J Mater Sci 2001;36:4037–43. [34] Vladimir V. Semenov Russ Chem Rev 2011;80:313. [35] Chikawa H. Key Eng Mater 2007;352:59–64. [36] Dvornic PR. J Polym Sci Pt A: Polym Chem 2006;44(9):2755–73. [37] Gupta RK, Mishra R, Tiwari RK, Ranjan A, Saxena AK. Silicon 2011;3(1):27–35. [38] Nasim M, Saxena AK, Pande LM. Polyhedron 1998;7:2189. [39] Saxena K, Bisaria CS, Saxena AK. Appl Organometall Chem 2009;23:535–40. [40] Saxena K, Bisaria CS, Saxena AK. Appl Organometall Chem 2010;24:251–6. [41] Vogel AI. Practical organic chemistry. III ed. London: Longmans; 1971. [42] Speier JL, Webster JA, Barnes GH. J Am Chem Soc 1957;79:974. [43] Trivedi V. IJPRBS 2012;1(2):1–21. [44] Krempner C. Polymers 2012;4(1):408–47. [45] Krevelen DWV. Polymer 1975;16(8):615–20. [46] Laza JM, Julian CA, Larrauri E, Rodriguez M, Leon LM. Polymer 1999;40(1):35–45. [47] Thomus R, Yumei D, Yuelong H, Le Y, Moldenaers P, et al. Polymer 2008;49(1):278–94. [48] Ampudia J, Larrauri E, Gil EM, Rodriguez M, Leon LM. J Appl Polm Sci 1999;71(8):1239–45.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Studies on the synthesis and reaction of silicone oxirane dendrimer and their thermal and rheological properties Sangeeta Kandpal ⇑, A.K. Saxena Defence Materials and Stores Research and Development Establishment, G.T. Road, Kanpur 208013, India
a r t i c l e
i n f o
Article history: Received 20 March 2014 Received in revised form 30 May 2014 Accepted 10 June 2014 Available online 27 June 2014 Keywords: Dendrimers Curing Rheology TGA
a b s t r a c t A series of silicone core dendrimers bearing SiAH terminal group was synthesized by the reaction of tetraethoxysilane and diorganochlorosilane in high yields, which on reacting with allylglycidylether in presence of Speier’s catalyst yielded oxirane terminated dendrimers. These dendrimers were characterized using physico-chemical techniques viz; elemental analysis, Fourier transforms infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance spectroscopy (1H NMR spectroscopy). Molecular weight was determined by vapour pressure osmometry (VPO). The rheological studies were performed to study the curing behavior of oxirane dendrimer with triethylenetetraamine. Thermal properties of cured dendrimers were evaluated using thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC). Change in thermal properties of epoxy resin (LY556) on addition of dendrimers was also studied by TGA and DSC. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Since the pioneering work of well-defined, threedimensional structurally ordered macromolecules by Vögtle [1], Tomalia [2,3] and Newkome [4] interest in dendrimers and hyperbranched polymers has been increasing at an amazing rate as they exhibit distinct properties in comparison to their linear analogue [5–8]. Dendrimers are spheroid or globular nanostructures that are synthesized by the using the multistep interactive controlled reaction procedures like hydrosilylation, Grignard reactions, dehydrocoupling, alcoholysis and alkenylation reactions either by divergent or convergent manner [9,10]. From the application point of view dendrimers are expected to play a key role as enabling building blocks for nanotechnology during the 21st century [11] and deemed to serve as chemical sensor [12–14], catalyst ⇑ Corresponding author. Tel.: +91 0512 2451759; fax: +91 0512 2450404. E-mail addresses: [email protected] (S. Kandpal), arvsaxena @gmail.com (A.K. Saxena). http://dx.doi.org/10.1016/j.eurpolymj.2014.06.009 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved.
[15], molecular device [16,17], chemo-delivery [18,19], applications in coating, drug and gene delivery [20], macromolecular building block and supramolecular sciences [21,22] etc. As dendrimers are reported [23–25] to have better solubility in common solvents and less bulk viscosity as compared to equivalent molecular weight linear polymers, hence they may be also used more effectively as coupling agent [26], reactive diluents [27], effective curing reagent [28], coating substrates etc. and also in preparation of high char yield material for advanced composites [29]. The literature survey [1,19,30,31] further revealed that in comparison to organic dendrimers, the silicone dendrimers are scarcely studied though organosilanes and silicones have found numerous high tech applications [32–35] especially in the preparation of thermally stable and ecofriendly materials. Despite several important features, yet these dendrimers failed to make any commercial impact except PAMAMOS [36]. The reason for it may be attributed to the low yield of the products. Keeping in view of it and
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our continued interest in the area of synthesis of functional organosilanes, polysilanes, polycarbosilanes [37–39] and silicone dendrimers [40] it is considered worthwhile to synthesize some new organosilicone dendrimer bearing terminal epoxy groups in high yield and explore their application as an additive to epoxy resin to improve thermal properties. 2. Experimental 2.1. Materials All reactions involving air and/or moisture sensitive compounds were carried out in an N2-filled drybox or using Schlenk technique. THF (LR grade, Ranbaxy), hexane (LR grade, E. Merch) and dichloromethane (LR grade, E. Merck) were purified and dried before use as reported [41]. Tetraethoxysilane (TEOS) (98%, Aldrich), chlorodimethylsilane (98%, Aldrich), chloromethyl(phenyl)silane (>93%, Aldrich), vinyltriethoxysilane (98%, Aldrich), allylglycidylether (AGE) (99%, Aldrich), triethylene tetramine (TETA) (>97%, Aldrich) and epoxy resin LY556 (epoxy equivalent 72-180, Dow chemical’s) were used as such. Speier’s catalyst [42] was synthesized in laboratory. Ethyl alcohol (99.9%, SD Fine chemicals) and silica gel (60– 120 mesh LR grade, SD Fine chemicals) were used as such. 2.2. Syntheses of dendrimers 2.2.1. GoA dendrimer [Tetrakis(dimethylsiloxy)silane] A solution of TEOS (7.34 g, 0.03 mol), ethyl alcohol (6.57 g, 0.14 mol) and water (4.0 g, 0.21 mol) was taken in a three necked round bottom flask equipped with magnetic stirrer, thermometer, condenser, dropping funnel and stirred for 30 min. Into the above solution chlorodimethylsilane (16.67 g, 0.18 mol) was added drop wise within 3–4 h under stirring. Afterward, few drops (0.2 ml) of concentrated H2SO4 was added in the solution and mixture was further stirred for 15 min. Later on, the solution was allowed to settle down and both silicone and aqueous layer were separated using separating funnel. The silicone layer was washed with distilled water till it became neutral. The aqueous layer was extracted with petroleum ether thrice (3 20 ml) and mixed with silicone layer. The silicone layer was then washed with brine solution (50 ml) and kept on anhydrous sodium sulphate overnight and filtered. The filtrate was distilled on water bath to remove petroleum ether. The residue was distilled on oil bath to collect dendrimer GoA (C8H28O4Si5). Yield 28.0 g, 82.5%, b.p. 190 °C. FTIR (KBr): 1077 (ASiOSiA), 2136 (ASiH) cm1; 1 H NMR (400 MHz, CDCl3, d ppm): 0.22 (d, 24H, ASiACH3), 4.74 (m, 4H, ASiAH); elemental analysis (%) C 29.23, H 8.56, Si 42.74 (calcd. C 29.21, H 8.58 and Si 42.72 respectively). Mol wt. (VPO): 320.14 (calcd. 328.73). Similarly other reactions as mentioned below were carried out. 2.2.2. GoB dendrimer (SiAH terminated) Reaction of TEOS (8.8 g, 0.04 mol), ethyl alcohol (6.5 g, 0.08 mol), water (4.0 g, 0.22 mol) and chloromethyl
(phenyl)silane (31.3 g, 0.21 mol) was carried out as above to afford GoB dendrimer. Reaction mixture was distilled under vacuum (26 mbar at 64 °C) to remove unreacted reactants and disiloxanes (by-products of partial hydrolysis of chloromethyl(phenyl)silane. The pot residue was purified by column chromatography (silica gel as stationary phase and n-hexane as mobile phase) to afford colorless liquid GoB (C28H36O4Si5) dendrimer; Yield 18.7 g, 80%. FTIR (KBr): 1080 (ASiOSiA), 2139 (ASiH), 1456 (ASiAPh) cm1; 1H NMR (400 MHz, CDCl3, d ppm): 0.21 (d, 12H, ASiACH3), 4.74 (m, 4H, ASiAH), 7.1–7.3 (d, 20H, ASiAPh); elemental analysis (%) C 58.28, H 6.29, Si 24.31 (calcd. C 58.15, H 6.96 and Si 24.34 respectively). Mol wt. (VPO): 512.21 (calcd. 576.15). 2.2.3. GI dendrimer (SiAOC2H5 terminated) A solution of GoA dendrimer (11.7 g, 0.035 mol), and Speier’s catalyst (0.03 mol%) in dry THF (50 ml) was taken in a three necked round bottom flask (100 ml) fitted with a dropping funnel containing vinyltriethoxysilane, magnetic stirrer, thermometer, condenser and heated up to 60 °C in inert atmosphere. vinyltriethoxysilane (27.09 g, 0.142 mol) was gradually added drop wise in GoA solution within 30 min. afterwards the solution was heated for an additional 4 h and cooled to room temperature. The solution was distilled under reduced pressure (26 mbar at 64 °C) to remove unreacted vinyltriethoxysilane and solvent. The pot residue was column chromatographed (silica gel and n-hexane) to afford colourless and transparent GI (C40H100O16Si9) dendrimer; yield 30.14 g, 79%. FTIR (KBr): 1080 (ASiOSiA), 1254 (ASiCH3) cm1; 1H NMR (400 MHz, CDCl3, d ppm): 0.22 (s, 24H, ASiACH3), 0.31 (t, 16H, ASiACH2A), 3.75 (q, 24H, ASiAOACH2A), 1.57 (t, 36H, ASiAOACH2CH3); elemental analysis (%) C 44.07, H 9.17, Si 23.14 (calcd. C 43.95, H 9.02 and Si 23.19 respectively). Mol wt. (VPO): 1001.27 (calcd. 1089.98). 2.2.4. GIA dendrimer (dimethyl SiAH terminated) Reaction of GI dendrimer (7.69 g, 0.0071 mol) in ethyl alcohol (2.13 g, 0.028 mol), water (0.76 g, 0.0426 mol) and chlorodimethylsilane (8.061 g, 0.0852 mol) was carried out similarly as the procedure adopted for the synthesis of dendrimer G0A, which yielded colourless, transparent liquid dendrimer GIA (C40H124O16Si21), yield 9.27 g, 90%. FTIR (KBr): 1075 (ASiOSiA), 2136 (ASiH) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.24 (d, 72H, ASiACH3), 4.74 (m, 12H, ASiAH), 0.23 (s, 24H, ACH2ASiACH3), 0.64 (t, 16H, ASiACH2A); elemental analysis (%) C 33.10, H 8.61 Si 40.59 (calcd. C 33.06, H 8.23 and Si 40.64 respectively). Mol wt. (VPO): 1360.18 (calcd. 1451.19). 2.2.5. GIB dendrimer (methylphenyl SiAH terminated) Similarly as above, the reaction of GI dendrimer (15.2 g, 0.014 mol) in ethyl alcohol (4.2 g, 0.056 mol), water (1.4 g, 0.08 mol) and chloromethyl(phenyl)silane (26.52 g, 0.168 mol) was carried out to afford colourless, transparent liquid dendrimer GIB (C100H148O16Si21), Yield 27.0 g, 88%. FTIR (KBr): 1075 (ASiOSiA), 2139 (ASiH), 1457(ASiAPh) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.22 (m, 36H, ASiACH3), 4.74 (m, 12H, ASiAH), 0.23 (s, 24H, ACH2ASiACH3), 0.64 (t, 16H, ASiACH2A), 7.1–7.3 (d, 60H,
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ASiAPh); elemental analysis (%) C 54.69, H 6.79 Si 26.61 (calcd. C 54.69, H 6.78 and Si 26.66 respectively). Mol Wt. (VPO): 2085.48 (calcd. 2196.02). 2.2.6. GoAx dendrimer (Si-oxirane terminated) Hydrosilylation reaction of GoA dendrimer (8.84 g, 0.027 mol) with allylglycidyl ether (12.3 g, 0.11 mol) in presence of Speier’s catalyst (0.03 mol%) was carried out as above for the synthesis of GI dendrimer, after completion of reaction, the solvent and unreacted allylglycidylether was removed by vacuum distillation at 40 °C (20 mbar). The residue was purified by column chromatography (silica gel and n-hexane) to afford colorless, transparent liquid GoAx dendrimer (C32H68O12Si5). Yield 19.08 g, 89%. FTIR (KBr): 913 (epoxy ring, asy), 1080 (ASiOSiA), 3050 (epoxy ring, sym) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.07 (s, 24H, ASiACH3), 0.54 (t, 8H, ASiACH2A), 1.82 (m, 8H, ACH2ACH2ACH2A), 2.59 (d, 8H, epoxy ring, ACH2A, trans), 2.77 (d, 8H, epoxy ring, ACH2A, cis), 3.13 (m, 4H, epoxy ring, ACHA), 3.37 (d, 8H, AOACH2Aepoxy ring, cis), 3.71 (d, 8H, AOACH2Aepoxy ring, trans), 4.01 (t, 8H, ACH2ACH2AOA); elemental analysis (%) C 48.95, H 7.96, Si 17.83 (calcd. C 48.23, H 8.03 and Si 17.88 respectively). Mol wt. (VPO): 712.93 (calcd. 785.3). Similarly other reactions as mentioned below were carried out. 2.2.7. GoBx dendrimer (Si-oxirane terminated) Reaction of GoB dendrimer (4.18 g, 0.007 mol), Speier’s catalyst (0.03 mol%) and allyl glycidyl ether (3.4 g, 0.03 mol) yielded a colourless, transparent liquid dendrimer GoBx (C52H76O12Si5). Yield 6.20 g, 85%. FTIR (KBr): 913 (epoxy ring, asy), 1080 (ASiOSiA), 3050 (epoxy ring, sym) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.07 (s, 12H, ASiACH3), 7.1 (d, 20H, ASiAPh), 0.54(t, 8H, ASiACH2A), 1.82 (m, 8H, ACH2ACH2ACH2A), 2.59 (d, 8H, epoxy ring, ACH2A, trans), 2.77 (d, 8H, epoxy ring, ACH2A, cis), 3.13 (m, 4H, epoxy ring, ACHA), 3.37 (d, 8H, AOACH2Aepoxy ring, cis), 3.71 (d, 8H, AOACH2 Aepoxy ring, trans), 4.01 (t, 8H, ACH2ACH2AOA); elemental analysis (%) C 60.43, H 7.41, Si 13.54 (calcd. C 61.02, H 7.15 and Si 13.59 respectively). Mol wt. (VPO): 989.18 (calcd. 1032.42). 2.2.8. GIAx dendrimer (Si-oxirane terminated) Reaction of GIA dendrimer (6.0 g, 0.0041 mol), Speier’s catalyst (0.03 mol%) and allylglycidyl ether (6.75 g, 0.0592 mol) yielded a dendrimer GIAx (C112H244O40Si21) which was colourless and transparent liquid, yield 9.59 g, 85%. FTIR (KBr): 913 (epoxy ring, asy), 1080 (ASiOSiA), 3050 (epoxy ring, sym) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.07 (s, 96H, ASiACH3), 0.64 (t, 16H, ASiACH2A) 0.54 (t, 24H, ASiACH2A), 1.82 (m, 24H, ACH2ACH2ACH2A), 2.59 (d, 24H, epoxy ring, ACH2A, trans), 2.77 (d, 24H, epoxy ring, ACH2A, cis), 3.13 (m, 12H, epoxy ring, ACHA), 3.37 (d, 24H, AOACH2Aepoxy ring, cis), 3.71 (d, 24H, AOACH2 Aepoxy ring, trans), 4.01 (t, 24H, ACH2ACH2AOA); elemental analysis (%) C 47.68, H 8.72, Si 20.90 (calcd. C 48.02, H 8.69 and Si 20.91 respectively). Mol wt. (VPO): 2753.79 (calcd. 2820.91).
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2.2.9. GIBx dendrimer (Si-oxirane terminated) Reaction of GIB dendrimer (4.0 g, 0.0018 mol), Speier’s catalyst (0.03 mol%) and allyl glycidyl ether (2.5 g, 0.0218 mol) yielded a colorless, transparent liquid dendrimer GIBx (C172H268O40Si21). Yield 5.7 g, 90%. FTIR (KBr): 913 (epoxy ring, asy), 1080 (ASiOSiA), 3050 (epoxy ring, sym) cm1. 1H NMR (400 MHz, CDCl3, d ppm): 0.07 (s, 60H, ASiACH3), 7.3 (d, 60H, ASiAPh) 0.64 (t, 16H, ASiACH2A) 0.54 (t, 24H, ASiACH2A), 1.82 (m, 24H, ACH2ACH2ACH2A), 2.59 (d, 24H, epoxy ring, ACH2AA, trans), 2.77 (d, 24H, epoxy ring, ACH2A, cis), 3.13 (m, 12H, epoxy ring, AACHA), 3.37 (d, 24H, AOACH2Aepoxy ring, cis), 3.71 (d, 24H, AOACH2 Aepoxy ring, trans), 4.01 (t, 24H, ACH2ACH2AOA); elemental analysis (%) C 57.94, H 7.57, Si 16.52 (calcd. C 57.32, H 7.95 and Si 16.54 respectively). Mol wt. (VPO): 3498.31 (calcd. 3565.74). (Schemes 1–3). 2.3. Curing of oxirane dendrimers The oxirane dendrimers (GoAx, GoBx, GIAx and GIBx) initially cured neat with stoichiometric amount of TETA and further there 1%, 2% and 3% amount were mixed homogeneously with epoxy resin (LY556) then cured with TETA in stoichiometric amount and kept at room temperature (28 °C) for 24 h. Afterwards the mixtures were heated for post curing at 50 °C for 0.5 h, 70 °C for 1 h, 85 °C for 2 h and 101oC for 2 h. The thermal properties i.e., DSC and TGA were carried out for post cured resin matrix (Scheme 4). 2.4. Equipment Fourier transform infrared spectroscopy (FT-IR) was used to characterize the functional group and monitoring the reaction progress of different dendrimers. Infrared spectra of the sample were recorded on a Perkin Elmer FT-IR spectrometer RX1 at room temperature using KBr pellets. The analysis was performed between 500 and 4000 cm1. 1 H nuclear magnetic resonance spectroscopy spectra were recorded at room temperature on a Bruker Avance 400 MHz NMR spectrometer. Samples (5–10 mg) for NMR analysis were dissolved in deuterated chloroform (CDCl3). Vario EL III CHNOS elemental analyzer was used for elemental analysis of dendrimers and silicone elemental analysis was carried out according to reported method [41]. Vapour pressure osmometre (VPO) was performed to determined molecular weight of dendrimers using a Knauer-7000 in toluene at 60 °C in a concentration range of 5–10 mg/ ml. and polystyrene in the range of 500– 3000 was used for calibration. Differential Scanning Calorimetry (DSC) analysis was performed on a TA Instruments Q 100 DSC. All measurements were conducted in crimped aluminium pans under a nitrogen atmosphere, at a purge gas flow rate of 50 ml/ min. samples were heated from 25 to 300 °C at a rate of 10 °C/min. samples were than cooled from 450 to 25 °C. Finally, the samples were heated for a second time at rate of 10 °C/min to 450 °C. Thermogravimetric analysis (TGA) was performed on a TA Instruments Hi-Res TGA 2950 thermogravimetric
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Scheme 1. Synthesis of oxirane dendrimers GoAx and GoBx, Where for GoAx, R = CH3 and for GoBx, R = C6H5.
analyser in a nitrogen atmosphere. Approximately 5– 10 mg of each sample was heated from 25 to 800 °C at a rate of 10 °C/min. Rheological analysis was performed on a AR-G2 rheometer from TA Instruments with 25 mm diameter disposable aluminium parallel plate. The curing cycle were performed in a dynamic oscillation mode at a frequency of 6.283 rad/ s, temperature ramp 3 °C/ min and a strain of 0.1%. 3. Results and discussion As the microenvironment of inside and outside of the dendrimer could be easily modified with a variety of organic functionalities hence these dendrimers may serve potential reagent as reactive diluents and thermal properties modifier of various organic resin to suit them for any designed high tech applications [43,44]. Hence in continuation of our previous study, in present work, we have synthesized SiAH terminated silicone dendrimer in high yield which on addition to allylglycidylether yielded oxirane terminated dendrimers. These oxirane terminated dendrimers have been used as additive to modulated curing and thermal behavior of epoxy resin. In our previous work we have reported the synthesis of Go SiAH silicone dendrimer where the yield was not satisfactory (69%) hence a new method has been adopted to synthesize Go dendrimer (SiAH terminated) in high yield (82–90%) using co-hydrolysis of tetraethoxysilane and chloromethyl(organo)silane. Synthesis of ethoxy terminated GI dendrimer was carried out by using hydrosilylation reactions of Go dendrimer with vinyltrieth-
oxysilane in THF solution under strict inert environment. Afterwards, the ethoxy terminated GI dendrimer was co-hydrolysized with chloromethyl(organo)silane to afford SiAH terminated GI dendrimers. These SiAH terminated dendrimers were further hyrosilylated with allylglycidylether to afford oxirane terminated dendrimers. These oxirane terminated dendrimers were cured with TETA and there curing behavior were studied by rheology. Thermal study of the cured resin matrix of dendrimers was also carried out using thermogravimetric analysis. The additive effects of these dendrimers on the thermal properties of epoxy resin (LY556) were also studied in detail. The characterization and thermal studies of these dendrimers and resin matrixes have been discussed below. 3.1. FTIR spectroscopy FT-IR spectroscopy provided valuable information about the reaction progress and the functional group attached on the dendrimers. In all the dendrimers an IR absorption peak appeared at 1080 cm1 which indicates the presence of m SiAOASi bond. The m SiAH absorption peak appeared at 2131 cm1 in dendrimers GoA, GoB, GIA and GIB. When the hydrosilylation reaction of dendrimer GoA with vinyltriethoxysilane was carried out to prepare GI dendrimer, the m SiAH peak at 2136 cm1 and m CH2@CH peak at 1600 cm1 disappeared which confirmed the addition of SiAH group on vinyl moiety. The FT-IR spectra of the reaction product GoAx, GoBx, GIx A and GIBx showed the appearance of characteristic absorption peak of oxirane ring at 913 cm1 and
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Scheme 2. Synthesis of SiAH functional dendrimer GIA and GIB, where R = CH3 for GIA and R = C6H5 for GIB.
3054 cm1 and the disappearance of m SiAH peak at 2136 cm1 which was very much prominent in dendrimers GoA, GoB, GIA and GIB respectively. Therefore, it may be inferred that hydrosilylation of allyl glycidyl ether with all SiAH terminated dendrimers took place. IR spectra of cured resin matrix showed the peak of unsymmetrical stretching of ether linkage at 1259 cm1 which showed the presence of ether linkage of allylglycidylether moiety due to crosslinking of allylglycidylether with TETA. The oxirane ring which appeared at 913 cm1 and 3054 cm1 disappeared and in the resin matrix secondary hydroxyl group appeared at 3439 cm1 due to the reaction of amino group of TETA with dendrimer and LY556. 3.2. NMR spectroscopy 1
H NMR spectra of dendrimers GoA, GoB, GIA and GIB respectively showed multiplet for d SiAH at 4.74– 4.85 ppm due to coupling with SiACH3 protons and
doublet for d SiACH3 at 0.22 ppm due to coupling with protons of SiAH group. The progress of hydrosilylation of the dendrimer GoA with vinyltriethoxysilane was confirmed by disappearance of d SiH proton at 4.74 ppm and d H2C@CH proton at 5– 6 ppm. Similarly the hydrosilylation of dendrimers GoA, GoB, GIA and GIB with allyl glycidyl ether was confirmed with the disappearance of d SiAH proton at 4.74 ppm and d H2C = CH at 5–6 ppm. The formation of dendrimers GoAx, GoBx, GIAx and GIBx were confirmed by characteristic signals of oxirane ring at d 2.59 ppm (CH2, trans), 2.77 ppm (CH2, cis) and 3.13 ppm (CH). The SiACH3 appeared as a singlet at 0.07 ppm whereas d SiACH2 appeared as a triplet at 1.17 ppm due to coupling with protons of the nearby ACH2 group. The signals of d ACH2 of the SiACH2ACH2ACH2 – group appeared as a multiplet at 1.82 ppm, which may be assigned to the coupling of protons of the ACH2 group with the protons present on either side carbon atom, d CH2, which joins the ether linkage to the oxirane ring
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Scheme 3. Synthesis of epoxy terminal dendrimer GIAx and GIBx, where R = CH3 for GIAx and R = C6H5 for GIBx.
appearing at 3.37 ppm (cis) and 3.71 ppm (trans), both appearing as triplets due to coupling with the proton present on the same carbon atom as well as protons attached to the next carbon atom of the oxirane ring. The d CH2 of the ACH2ACH2 AOA group appeared at 4.01 ppm as a triplet, which showed coupling with the protons of the next CH2 group. 3.3. Thermal analysis 3.3.1. DSC analysis In DSC study a broad exotherm was observed for all epoxy resins when cured with TETA in the temperature range of 60–178 °C. The characteristic curing temperatures are summarized in Table 1. As it is established that curing of epoxy resin proceeds by nucleophilic attack of amine on the electrophile oxirane ring, so it may be assumed that more oxirane centers will initiate the curing at low temperature with fast rate. Therefore, onset temperature of
exotherm (To) may be used as a criterion for evaluating the relative reactivity of various oxirane terminal dendrimers (1%) and LY556. From the results it is evident that the lowest To occurred with 1% GIAx and LY556 due to maximum cross linking cites and less bulky group at periphery. The glass transition of dendrimers and dendrimer modified resin (LY556) increased with the increase in silicone core which may be tentatively attributed to the fact that with the increase of crosslinking density Tg increases (Table 2). The glass transition temperature obtained by DSC analysis showed that the Tg of GoAx is lower and GIBx is higher among all dendrimers. 3.3.2. TGA analysis The TGA graph (Fig. 1) of the TETA cured neat dendrimers showed a single step degradation process which reflects the formation of uniform structure and single component system. The initial decomposition temperature (To), the temperature of maximum rate of mass loss (Tmax) and
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S. Kandpal, A.K. Saxena / European Polymer Journal 58 (2014) 115–124 O O R
Me Si O
R O
O
Si
O
Me
Me
Si O O
O Si
NH
NH O
NH
NH2
+
R
2
TETA
Si R
Me
Room temperature curing O
O
Dendrimer N
2 N
N HO O
Me R N N
O
O
2
OH
Me R
OH
Me
O Si
N
R
Si
O
Si O O Si
N
Si
N
2
R
N
Me
O
OH
N N
2 N
Cured dendrimer Scheme 4. Curing of dendrimer with triethylenetetraamine [TETA].
the extrapolated final decomposition temperature (Tef) in the range of 360–375 °C, 410–431 °C and 450–500 °C. Moreover, the char yields of resin matrix at 800 °C were about 29%. Table 3 summarizes the obtained TGA results of the cured product of neat epoxy and epoxy containing dendrimers with TETA. All the epoxy dendrimer composition showed an enhancement of onset temperature of decomposition 15 ± 5 °C with respect to neat epoxy system. And maximum degradation temperature (Tmax) of GIAx dendri-
mer showed a remarkable increase of +10 °C. Whereas varied amount (1–10%) of dendrimers did not show any remarkable variation. TGA results revealed that the char yield of cured dendrimers containing LY556 epoxy were 25–40% higher than neat cured LY556 (Table 3) in case of 1–3% addition and it will increase as we increases the percentage of dendrimers. This may be due to presence of siloxane (SiAOASi) linkage in different dendrimers and it shows a linear relation with dendrimers percentage, on increasing the dendri-
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Table 1 DSC results of Curing of neat LY556 and LY556 with dendimers with TETA.
a b c
Epoxy
Toa (°C)
Tpb (°C)
Tfc (°C)
LY556 1%GoAx + LY556 1% GIAx + LY556 1% GoBx + LY556 1% GIBx + LY556
66.41 61.77 60.77 63.40 64.66
101.79 98.91 96.53 100.84 101.60
173.11 177.81 173.54 166.93 173.92
To (onset of exotherm). Tp (temperature of peak position of exotherm). Tf (temperature of end of exotherm).
Table 2 TGA data of cured oxirane dendrimers. Dendrimers
GoAx GIAx GoBx GIBx
Decomposition temperature (°C) To
Tmax
Tef
375 372 365 360
408 410 424 431
456 451 462 500
Char yield (%)
LOI (%)
Tg (°C)
15.02 20.33 21.81 29.60
23.51 25.63 26.22 29.34
223.25 246.67 248.41 254.50
Fig. 1. Thermogravimetric analysis of cured dendrimers.
of methylphenyl group bearing dendrimers and also the char yield in the later case was higher. Conclusively the TGA studies of dendrimers modified LY556 resin showed that there was marginal yet distinct increase on onset temperature of degradation (To) in case of all epoxy composition but due to increase of dendrimers quantity from 1% to 10% had no significant effect which may be understood with the nullifying effect of addition of dendrimers in epoxy resin (LY556). The change in the value of max degradation temperature (Tmax) in case of addition of GIAx dendrimer was found maximum (+10 °C) which may be due to good synergistic effect of silicone and organic content present in the epoxy composition of this dendrimer. The char yield also increased in case a mixture of resins and it was more in case of GIBx invariably in all ratios. The results are consistent with neat dendrimer results for char yields. The LOI of dendrimer containing resin matrix also increased distinctly, yet marginally due
mer concentration in LY556, the char yield of cured products also increases. The limiting oxygen index (LOI) [45] value for neat dendrimer cured resins were found up to 29% (Table 2), while LOI value for LY556 with dendrimer cured resin shows increment of 10–13% in compare to neat LY556 (Table 3). The TGA results showed that the To and char yield of cured dendrimers was higher than cured LY556 which may be attributed to the ASiAOASiA backbone present in the dendrimers. Among dendrimers the dimethyl group dendrimers showed high To but the Teff was higher in case
Table 3 TGA data of cured hybrid resin matrix. SN
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
TETA cured epoxy resins
LY556 1% GoAx + LY556 1% GIAx + LY556 1% GoBx + LY556 1% GIBx + LY556 2% GoAx + LY556 2% GIAx + LY556 2% GoBx + LY556 2% GIBx + LY556 3% GoAx + LY556 3% GIAx + LY556 3% GoBx + LY556 3% GIBx + LY556 5% GoAx + LY556 5% GIAx + LY556 5% GoBx + LY556 5% GIBx + LY556 10% GoAx + LY556 10% GIAx + LY556 10% GoBx + LY556 10% GIBx + LY556
Decomposition temperature (°C) To
Tmax
Tef
341 348 362 356 354 350 363 350 348 351 363 356 353 352 364 350 353 354 365 352 353
366 367 375 366 363 366 377 364 365 366 375 365 365 367 378 367 368 367 376 368 368
414 416 421 407 402 405 418 403 410 406 422 409 420 408 419 404 421 410 419 408 422
Char yield (%)
LOI (%)
8.01 8.32 10.66 8.76 11.97 9.31 12.03 9.82 13.74 12.21 13.18 13.79 14.50 12.65 13.81 13.97 14.79 12.95 14.21 14.56 15.03
20.71 20.83 21.76 21.00 22.29 21.23 22.31 21.43 23.00 22.38 22.77 23.02 23.30 22.56 23.02 23.08 23.42 22.68 23.18 23.32 23.51
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Fig. 2. Plots of storage modulous against temperature for curing of dendrimers.
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cross-linking the viscoelastic properties of the dendrimers are changing and the viscous emulsion is becoming an elastic cured network. It is shown in Fig. 2 that the initial low value of complex viscosity of the system increases abruptly in the specific time range as the curing reaction proceeds. Graph reflects that initiation of curing has been started at 75 °C for GIAx, at 80 °C for G0Ax, 95 °C for GIBx and 110 °C for G0Bx. There is substantial increase in the value of storage modulus which shows an increase from 10 to 20 Pascal to 105 Pascal. Storage modulous and complex viscosity of G0Bx dendrimer increases slowly compare to other dendrimers which shows that the dendrimer is least reactive among all. It may be due to less crosslinking units and hinderance in its structure due to bulky phenyl groups. Conclusively, the rheological results of all dendrimers curing shows that dendrimer GIAx having maximum oxirane and dimethyl group at periphery, cured fast compare to remain dendrimers due to more crosslinking units and less hindrance at terminals while dendrimer G0Bx having minimum oxirane methylphenyl groups at periphery took maximum time for curing. 4. Conclusions
Fig. 3. Plots of complex viscosity against temperature for curing of dendrimers.
to ASiAOASiA linkage as it formed SiO2 on burning in air atmosphere, which acted as self extinguishing material. So, it may be expected that with the increase of ASiAOASiA groups containing dendrimers or their quantities may further enhance LOI and char yield of resin matrix.
3.4. Rheological analysis Rheological analysis has been used to study the curing process of epoxy resin [46–48] like polymers; epoxy resin is a viscoelastic material. During a curing process under continuous stresses or strains, its viscoelastic characteristics change; this is reflected in the variations of the viscosity. In the present work all oxirane dendrimers were mixed with hardener TETA in stoichiometric ratio, to study the change in storage (G0 ) modulous and complex viscosity with respect to temperature (Figs. 2 and 3). In rheological study, increase in temperature speeds up the crosslinking reaction and increases the velocity of the system, during
In this study the different generations SiAH terminated dendrimers were successfully synthesized in high yield via a new alternative modified method. Hydrosilylation of dendrimers was done with allyl glycidyl ether using Speier’s catalyst to prepare novel oxirane terminated dendrimers. Rheological study of curing behavior of oxirane dendrimers were carried out with TETA. And its result reflects that curing time is decrease as the number of oxirane unit increases in terminals and steric hinderance decreases in the system. Thermal studies of the cured product of dendrimer were carried out. These dendrimers were further used as a modifier for up gradation of thermal properties of commercial epoxy LY556. Enhancement in thermal behavior and flame retardency were observed after adding these dendrimers. It proves that these novel dendrimers can further used for modifier, diluents and in different High-Tech applications. Acknowledgements The authors wish to thank (1) Head and all scientist and technical officers of ACD, DMSRDE for necessary encouragement and providing laboratory facilities to facilitate the work. (2) Mr. Raghwesh Mishra, Scientist ACD for supporting in the synthesis of dendrimers and (3) CAF and Polymer Division, DMSRDE for thermal studies. References [1] Buhleier E, Wehner W, Vögtle F. Synthesis 1978;2:155–8. [2] Tomalia DA, Barker H, Dewald JR, Hall M, Kallos G, Martin S, et al. Polym J (Tokyo) 1985;17:117. [3] Tomalia DA, Barker H, Dewald JR, Hall M, Kallos G, Martin S, et al. Macromolecules 1986;19:2466–8. [4] Newkome GR, Yao ZQ, Baker GR, Gupta VK. J Org Chem 1985;50:2003–4.
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