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Hydroxyl terminated PEEK-toughened epoxy–amino novolac phthalonitrile blends – Synthesis, cure studies and adhesive properties
Polymer xxx (2014) 1e11
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Hydroxyl terminated PEEK-toughened epoxyeamino novolac phthalonitrile blends e Synthesis, cure studies and adhesive properties Dhanya Augustine a, K.P. Vijayalakshmi b, R. Sadhana b, Dona Mathew a, C.P. Reghunadhan Nair a, * a b
Polymers and Special Chemicals Group, Vikram Sarabhai Space Centre, Trivandrum 695022, Kerala, India Analytical, Spectroscopy and Ceramics Group, Vikram Sarabhai Space Centre, Trivandrum 695022, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 July 2014 Received in revised form 17 September 2014 Accepted 18 September 2014 Available online xxx
Self cure promoting, amine-containing novolacephthalonitrile (APN) resins of varying compositions were synthesized and characterized. APN possessing amine functionalities reduced the cure initiation temperature from 310 C (typical of pure phthalonitrile systems) to 180 C. It showed excellent thermal stability up to 420 C and high char residue of 77e79 %. Co-reaction of APN with diglicydyl ether of bisphenol A (DGEBA) led to a decrement in their thermal stability though improved their adhesive properties. Evidences were obtained for epoxyeamine, epoxyephthalonitrile and amineephthalonitrile reactions. The latter reactions led to formation of oxazoline, triazine and phthalocyanine groups in the network. These were rationalized by density functional theory studies on model compounds. The extents of epoxyeamine and epoxyephthaonitrile reactions were quantified. Introduction of hydroxyl terminated poly ether ether ketone (PEEK) reduced the brittleness of the blends and improved their lap shear strength. Toughening of epoxyeamino novolac phthalonitrile networks occurred through phase separation of PEEK segments in cured matrix. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Amineephthalonitrile Epoxyephthalonitrile Self-curing polymer
1. Introduction Phthalonitrile resins and their composites are well known for their attractive properties such as high glass transition temperature, retention of mechanical properties at elevated temperature, better storage life, low moisture absorptivity and excellent flame retardancy [1e4]. The major limitation for their wide spread application is the requirement of high curing temperature (>300 C) for a long duration [5e9]. Active hydrogen sources such as aromatic amines and phenols, metals, metallic salts and complexes are commonly employed as curing agents for phthalonitrile groups [10e14]. Rather than external curatives, curing groups present in integral part of molecules are desirable for easy curing of resin. Hydroxyl-catalyzed and amine-catalyzed self-curing phthalonitrile systems can undergo curing at relatively low temperatures [5,15e19]. However, these groups provide small processing windows. This can be alleviated by means of providing flexible linkages in the polymer [20e22].
* Corresponding author. Tel.: þ91 471 2565689; fax: þ91 471 2564096. E-mail address: [email protected] (C.P. Reghunadhan Nair).
Earlier studies on epoxy/phthalonitrile thermosets revealed improved processability compared to neat phthalonitrile polymers and superior thermal stability and mechanical properties for the modified systems with respect to neat epoxies [23,24]. Binary blends of biphenyl and oligomeric phthalonitriles with epoxy resin showed that phthalonitrileeepoxy copolymerization could be materialized either in the absence of curing agents at relatively high temperatures or in the presence of curative like aromatic amines [19,25]. Even though the epoxy/phthalonitrile blends enhanced the processability of phthalonitrile polymers, they are not free from inherent brittle nature. Blending the thermosets with thermoplastics has been reported as an effective method to reduce the brittleness [22]. Interpenetrating networks formed by the incorporation of poly arylene ether nitrile into epoxy/phthalonitirle thermosets enhanced their mechanical and thermal properties [26,27]. In the present study, we report the synthesis and characterization of a self-cure promoting, phthalonitrile thermoset with low cure temperature and short cure times. The studies include the synthesis, characterization, cure studies and the resulting crosslinked networks of a novel amine-functionalized phthalonitrile
http://dx.doi.org/10.1016/j.polymer.2014.09.042 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
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system based on novolac backbone and their blends with diglicydyl ether of bisphenol A (DGEBA). The paper also examines the influence of a high temperature stable thermoplastic toughening agent, poly ether ether ketone on their cure characteristics, morphological features, toughness and adhesive performance of the blend. 2. Experimental 2.1. Raw materials Phenol (Nice Chemicals, India), formalin (34e37%) (CDH, India), p-aminophenol (Sigma Aldrich, USA), 4-nitro phthalonitrile (Acros Organics, USA), Diglicydyl ether of Bisphenol A (DGEBA) (Huntsman, USA), 4,40 -Methylene dianiline (MDA) (Sigma Aldrich, USA) and N-methyl pyrrolidone (SRL, India) were used as received. Hydroxyl terminated poly ether ether ketone (PEEK) with Mn: 8000 was synthesized according to the already reported procedure [28]. 2.2. Characterization In the present work, FT-IR spectra were recorded using a Perkin Elmer spectrum GXA spectrophotometer in the range of 4000e400 cm1. Proton and 13C NMR were recorded using Bruker Avance NMR spectrometer (300 MHz) using TMS as reference in acetone-d6 solvent. Molecular weight characterizations were carried out by Gel Permeation Chromatography (GPC) using Waters Alliance 2690; employing polystyrene standards. Elemental analysis was performed using Perkin Elmer 2400 CHN Analyzer. Differential Scanning Calorimetry (DSC) was performed using a TA instrument DSC Q-20 at a heating rate 10 C min1 in nitrogen atmosphere and thermogravimetric analysis (TG) was carried out using TA Instruments SDT Q-600 thermogravimetric analyzer at a heating rate of 10 C min1 in nitrogen atmosphere. Microstructure of the blends was examined using a scanning electron microscope (SEM) from Carl Zeiss SMT EVO 50 scanning electron microscope (SEM) at 10 kV. Lap shear strength of the systems were evaluated at 30 C and 150 C using stainless steel substrates of grade according to the ASTM D 1002. Molecular modeling at B3LYP/6-31G(d,p) level of DFT method was carried out to explain the experimental data. 2.3. Synthesis of amino functionalized novolac (ANC) To a mixture of phenol (25 g, 0.27 mol), p-amino phenol (5 g, 0.045 mol) and oxalic acid (1.24 g), formalin (37% w/w in water, 14 mL, 0.17 mol) was added (phenol:formalin ratio ¼ 1:0.63) dropwise and the system stirred at 70 C for 12 h. After the reaction, the product was washed repeatedly with distilled water followed by removal of water by azeotropic distillation with toluene. The polymer obtained was dried under vacuum at 50e60 C for 6 h, yield~ 50%. The polymer was characterized by FT-IR, GPC and CHN analysis. 1 H NMR (500 MHz, DMSO-d6, d ppm): 9.3 (eNH2 and OH), 3.8 (eCH2e), 6.3 & 7 (AreH). 13C NMR (500 MHz, DMSO-d6, d ppm): 22 (AreCH2eAr), 155.78 and 150.72 (aromatic CeOeC), 132.63, 130.10, 129.01 and 128.01 (aromatic carbon). FT-IR (KBr, cm1): 3400 (eNH2), 3300 (eOH), 2917(eCH2e), 1600, 1550 and 1450 (AreH)
reaction, the mixture was filtered and the filtrate was precipitated into 90:10 water-methanol mixture. The precipitate was filtered, washed several times with methanol and dried in vacuum for 4 h, yield ~60%. By varying the p-amino phenol content, different compositions of amino novolac phthalonitrile oligomers e APN-1, APN-2 and APN-3 were synthesized and characterized by FT-IR, GPC, 13C NMR, elemental analysis, hydroxyl value, and DSC. 1 H NMR (500 MHz, DMSO-d6, d ppm): 9.3 (eNH2 and OH), 3.8 (eCH2e), 6.3 & 7 (AreH). 13C NMR (500 MHz, DMSO-d6, d ppm): 30 (AreCH2eAr), 155.78 and 150.72(Aromatic CeOeC), 132.63, 130.10, 129.01 and 128.01 (Aromatic carbon) 115.4 (CN). FT-IR (KBr, cm1): 3400(eNH2), 2234(eCN), 2917(eCH2e), 1600, 1550 and 1450 (AreH) 2.5. Polymerization of amino novolac phthalonitrile oligomers Amino novolac phthalonitrile oligomers were cured under nitrogen atmosphere as per the schedule: 150 C, 200 C and 250 C for 30 min each; 300 C for 1 h and 350 C for 4 h. The cured polymers (CAPN) were characterized by FT-IR and TG. 2.6. Preparation of amino novolac phthalonitrileeepoxy blend (APNeDGEBA) and MDAeDGEBA blend APNeDGEBA blend was formulated by mixing APN-2 and DGEBA (equivalent stoichiometry of amine and epoxy functionalities) in THF solvent under ambient conditions. The solvent was then evaporated and the mixture was vacuum dried at 50 C for 2 h. The APN/epoxy blends were cured according to the schedule: 120 C for 2 h, 150 C for 3 h and 180 C for 5 h. MDAeDGEBA blend (equivalent stoichiometry of amine and epoxy groups) for DSC analysis was prepared by solution blending and evaporating the solvent. 2.7. Preparation of APNeDGEBAePEEK blends APN and DGEBA were blended at an equivalent molar ratio of amine and epoxy groups in minimum quantity of THF. To this solution of APN and DGEBA, 10 weight % of PEEK was added, thoroughly mixed and vacuum dried to remove the solvent. Toughened blend was cured according to the same schedule adopted for curing of APNeDGEBA. Cured APNeDGEBAePEEK blend is referred to as CAPNeDGEBAePEEK. 2.8. Adhesion test Lap shear strength of the polymers was determined at room temperature and at 150 C using steel (SS 304) substrates. The steel substrates were washed with acetone and dried before use. The single-lap joints had an overlap area of 25 mm 25 mm. The test specimens were prepared by applying APNeDGEBA blend and their toughened system dissolved in minimum amount of THF and cured according to the afore-mentioned schedules. 3. Results and discussion 3.1. Synthesis and characterization of ANC and APN oligomers
2.4. Synthesis of amino novolac phthalonitrile oligomer (APN) Amino functionalized novolac (10 g) dissolved in 35 mL Nmethyl pyrrolidone was added to a three necked round bottom flask equipped with nitrogen inlet. Anhydrous K2CO3 (15 g) was added into the flask. To this mixture, 4-nitro phthalonitrile (10 g, 0.058 mol) dissolved in 20 mL NMP was added drop-wise and stirred continuously for 24 h at room temperature. After the
Amino functionalized novolac oligomers were synthesized by the reaction of phenol and p-amino phenol with formaldehyde in the presence of oxalic acid as shown in Scheme 1. Phenol to formaldehyde ratio was maintained as 1:0.63 to obtain very low molecular weight novolacs. Different compositions of amino functionalized novolac (ANC) oligomers are characterized by elemental analysis, FT-IR, 1H NMR,
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Ratio of protons in (OH þ NH2):AreH ¼ (0.03 3 þ 0.97 1): (0.03 3 þ 0.97 4) ¼ 1:4
Scheme 1. Synthesis of amino functionalized novolac (ANC).
Table 1 Details of Amino functionalized novolac (ANC) oligomer. Reference
Extent of p-amino phenol incorporated (mol%)a
ANC-1 ANC-2 ANC-3
3 11 24
a
From nitrogen content.
GPC and DSC. Details of the oligomers synthesized are compiled in Table 1. Typical FT-IR spectrum of ANC oligomer is shown in Fig. 1a which exhibits a broad peak centered around 3300 cm1, contributed by both OH and NH2 groups. Characteristic absorption peaks of eOH and eNH2 appeared as a broad signal at 9.3 ppm (Fig. 1b). Integral ratios of OH and NH2 with aromatic protons was calculated for ANC-1 and found to be 1:4. This matches with the theoretical values. Since these are low molecular weight oligomers repeat unit can be better represented by
Amino novolac phthalonitrile oligomers (APN) with varying degrees of amino and phthalonitrile functionalities were synthesized according to the reaction shown in Scheme 2. Reaction involved nucleophilic displacement of the eNO2 group in 4-nitro phthalonitrile by the phenolic functions of amino functionalized novolac in alkaline medium in the dry dipolar aprotic solvent, NMP. Near complete phthalonitrilation of free eOH groups in ANC oligomers were confirmed from elemental analysis. Extent of phthalonitrilation was estimated at around 95% (calculations are shown in Supporting information, S1). Composition of APN oligomers are shown in Table 2. The products were further characterized by GPC, FT-IR and 13C NMR analyses. Fig. 2a and b shows the molecular weight distribution patterns of ANC and APN oligomers. There is a proportionate increase in molecular weight on phthalonitrilation of ANC oligomers. The number average molecular weight (Mn) of ANC oligomers ranges from 1025 to 1180 (corrected using the MarkeHouwink constant for novolac as MNL ¼ 0.7962 M0.9999 where MNL and MST are ST molecular weight of novolac and styrene oligomers at a given elution time) whereas APN oligomers showed Mn in the range of 1500e1800 [29]. Comparison of FT-IR of ANC and APN in Fig. 3 showed the evidences for phthalonitrilation of the amino novolac, by the appearance of characteristic absorption of eCN peak at 2231 cm1 with concomitant diminution of absorption centered around 3300 cm1, due to eOH groups. Peak due to amino groups is observed at 3440 cm1. 13C NMR spectra were also consistent with the expected structure of APN oligomers (Fig. 4). Signals were observed at 30 ppm (AreCH2eAr), 155.78 and 150.72 (Aromatic CeOeC) and 115.4 (CN). As a reference material, novolac phthalonitrile oligomer (without amine functions) referred to as NPN (Fig. 5) was also synthesized according to the reported procedure [30].
3.2. APNeDGEBA and APNeDGEBAePEEK blends
Theoretical calculation for integral ratios of OH and NH2 with aromatic proton from their structure.
In the present study, the cure behavior of APNeDGEBA blend was monitored for an epoxyeamine stoichiometry of 1:1. The engineering thermoplastic, hydroxyl terminated polyether ether ketone (PEEK) was used to toughen the APNeDGEBA polymer
Fig. 1. a. Typical FT-IR spectrum of ANC. b. 1H NMR spectrum of ANC-1 oligomer.
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Scheme 2. Synthesis of amino novolac phthalonitrile oligomers (APN).
Table 2 Composition of APN oligomers. Reference
Extent of p-amino phenol incorporated (mol%)a
Amine:phthalonitrile (molar ratio)
APN-1 APN-2 APN-3
3 11 24
1:32 1:8 1:4
a
From nitrogen content.
systems [28,31]. The chemical structures of epoxy resins and hydroxyl terminated PEEK are shown in Fig. 6a and b respectively. 3.3. Cure characteristics 3.3.1. Curing of APN The amine functionality is reportedly the most suitable curing agent for the phthalonitrile systems [17,32,33]. In the present study, the progress of cure reaction was monitored by FT-IR and DSC techniques. The FT-IR spectra of uncured and cured APN system (CAPN) are shown in Fig. 7. For CAPN, there is diminution in intensity of nitrile peak (2231 cm1). However, under the cure conditions employed as already mentioned, complete disappearance of eCN peak could not be achieved in any of the compositions; due to the incomplete conversion of nitrile groups [32,34]. The cure conditions employed in the present study drew a maximum of only 16% of eCN functionalities into the cured network as quantitatively estimated by FT-IR using the peak at 1220 cm1 (ether linkage) as the internal reference peak. Quantitative estimation was done using the Equation (1).
a¼1
ACN;t AR;0 ACN;0 AR;t
(1)
Fig. 3. FT-IR spectra of ANC-2 and APN-2.
where a is the extent of nitrile conversion and A refers to the area under the absorption peak. ACN,0 and ACN,t are the area under eCN peak before and after cure conditions. Similarly, AR,0 and AR,t represents the reference peak area before and after cure reaction. From the FT-IR spectra of cured APN systems, it was evident that the intensity of peak centered around 3440 cm1 increased with the dimunition of nitrile peak. This is an indirect evidence for the formation of isoindoline and phthalocyanine moieties in the cured network as shown in Scheme 3. In addition, the characteristic peak due to NH bending of phthalocyanine rings appeared at 1010 cm1 in the cured polymers [35,36].
Fig. 2. a. SEC profiles of ANC oligomers. b. SEC profiles of APN oligomers.
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Fig. 4. Typical
5
13
C NMR spectrum of APN.
Fig. 5. Novolac phthalonitrile (NPN) oligomer.
The DSC scans of amine functionalized novolac phthalonitrile system and corresponding novolac phthalonitrile are shown in Fig. 8. A systematic lowering in cure initiation temperature, Ti was observed with the increase of amine moieties in the APN systems as shown in Fig. 9. From the graph, we clearly get an insight into the influence of amine functionalities on curing of phthalonitrile groups. Thus, novolac phthalonitrile oligomer (NPN), which contain no amine functional groups shows cure initiation temperature (Ti)
around 310 C against a Ti of 180 C for APN-3 which contain 24 mol % amine functionalities incorporated on the novolac backbone i.e. decrease of about 130 C could be achieved in Ti with incorporation of 24 mol% of amine functionalities. The cure reaction pathways of the phthalonitrile systems catalyzed by various amines are well established [15,34,37e39]. On heating the APN oligomers, the amine groups attack the nitrile functions in phthalonitrile moieties to form amidine intermediates [40]. The imine linkages in the amidine undergo an intramolecular attack with the nitrile group, leading to the formation of isoindoline structures. The isoindolines can further react with other phthalonitrile end-capping units to form tetrameric phthalocyanine networks [41]. The amidines can also proceed through an intermolecular reaction with other nitrile functionalities to form heterocyclic triazine rings [42]. Upon amine-mediated curing, phthalonitrile systems form a polymeric network comprised of the afore-mentioned heteroaromatic moieties. Mechanisms of formation of the heterocyclic intermediates are depicted in Scheme 3. These networks exhibit high Tg and impart rigidity to the cured APN systems which is manifested in their higher glass transition temperature. DSC analysis of cured APN sample did not show any glass transition upto 350 C. It was deemed that Tg of the cured system is
Fig. 6. Structure of (a) DGEBA (b) PEEK.
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Fig. 7. FT-IR spectra of APN-2 and CAPN-2.
Fig. 8. DSC curves of amino novolac phthalonitrile and novolac phthalonitrile oligomers.
above 350 C (attempts to perform DMA analysis failed as defectfree samples could not be cast). 3.3.2. Curing of APNeDGEBA blend In the case of cured APNeDGEBA blend, there is a significant reduction in intensity of nitrile absorption at 2230 cm1 without appreciable increase in the intensity of characteristic peaks corresponding to phthalocyanine, isoindoline moieties in the FT-IR spectra as given in Fig. 10. This is a clear indication of nitrile curing through an alternate cure mechanism, operating via epoxyeamine and epoxyephthalonitrile reactions. The epoxyeCN reaction results in oxazoline rings as shown in Scheme 4 [24,43e45]. 3.3.3. EpoxyeCN reaction: investigation using model compounds The cure reaction of DGEBA with APN is quite complex. Therefore, cure behavior of APNeDGEBA blends was investigated with the support of theoretical modeling and simulation and also by reactions employing model compounds. The feasibility of crosslinking of epoxy groups with nitrile and amine functions was theoretically modeled at B3LYP/6-31G(d,p) level of DFT using
Scheme 3. Proposed pathways for amine-mediated phthalonitrile polymerization.
Fig. 9. Influence of amine content on initial cure temperatures (Ti) of APN oligomers.
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aniline, glycidyl ether of p-cresol and phthalonitrile ether of pcresol (CrPN). The model compound CrPN was synthesized by nucelophilic substitution reaction between cresol and 4-nitro phthalonitrile in alkaline medium. The optimized structures of products of epoxyeamine and epoxyenitrile reaction pathways are presented in Fig. 11. The computed heat of reaction for the formation of oxazoline and epoxyeamine adduct are 150 kJ/mol and 96 kJ/mol and the activation barriers for their corresponding transition state are found to be 268 kJ/mol and 176 kJ/mol respectively (Fig. 12). Comparison of heat of reaction and activation energies for the two reactions suggests that reaction of glycidyl ether of p-cresol with CrPN or with aniline could be equally competitive and occur in the same temperature regime. The reactions among phthalonitrile, amine and epoxy groups were studied experimentally using phthalonitrile derivative of pcresol (CrPN), MDA and DGEBA as model compounds. FT-IR spectra were monitored to study the reaction between epoxy and phthalonitrile functionalities. The, FT-IR spectra of uncured sample of CrPN/DGEBA and CrPN/DGEBA heated at 200 C and 230 C each for 4 h are shown in Fig. 13. Fig. 10. FT-IR spectra of APN, CAPN and CAPNeDGEBA.
Scheme 4. Probable epoxyephthalonitrile/epoxyeamine cure reaction pathways.
Fig. 11. Energy minimized network structures (a) Oxazoline (b) Epoxyeamine adduct.
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Fig. 12. Transition state corresponding to curing of (a) CrPN/Glycidyl ether of p-cresol (b) Glycidyl ether of p-cresol/aniline (bond length in A unit).
Intensity of characteristic absorptions of nitrile (2231 cm1) and epoxy peaks (915 cm1) were reduced by the afore-mentioned heat treatments. Quantitative estimation by FT-IR was done by comparing absorption peaks of interest against the reference peak at 830 cm1, characteristic of aromatic para substitution. During heat treatment at 200 C, involvement of only about 1.5% of eCN groups was accomplished with the participation of minimal quantity of epoxy groups. Meanwhile, 44% of eCN groups was drawn into the network by the epoxy functions on heating at 230 C with concomitant reduction in the intensities of absorption by nitrile and epoxy groups. Earlier studies also have reported the crosslinking of nitrile functions with epoxy groups resulting in oxazoline rings above 200 C [25]. Simultaneously, the characteristic band due to oxazoline network at 1606 cm1 got intensified with the enhancement of curing temperature. For understanding further the reaction of epoxy groups with APN, a separate DSC analysis was carried out. In order to avoid the volatility related problems, a high melting amine i.e., diamino diphenyl methane (or methylene dianiline, MDA) was used. MDAeDGEBA blend was prepared by maintaining the same amine/ epoxy equivalence as in APNeDGEBA blend. DSC thermogram of MDAeDGEBA (Fig. 14) showed a broad exotherm initiating from 88 C and extending upto 200 C corresponding to the curing of epoxyeamine groups. DSC thermograms of APNeDGEBA blend showed a multistep polymerization reaction (Fig. 14). Compared to APN oligomers, two additional peaks were observed for APNeDGEBA blends. The first exotherm appears as a shoulder peak emerging at 87 C and is due to the epoxyeamine reaction. The
overlapped second and third exotherms peaked at 182 C and 205 C represent the epoxyenitrile and amineenitrile coreactions in blend. Thus, DSC pattern also obviously points to the involvement of an alternate cure mechanism, as indicated by FT-IR studies, in addition to the homopolymerisation of nitrile groups, facilitated by 1 and 2 amine groups. Quantitative estimation of the extent of epoxyeamine and epoxyephthalonitrile reactions in APNeDGEBA blends was done by comparing the heat of reaction evolved from epoxyeamine reaction in MDAeDGEBA blend. Extent of epoxyeamine reaction was 21% and epoxy groups available for reaction with nitrile groups was 79% (calculations are shown in Supporting information, S2). Experimental evidences corroborated by model compound studies established that the two cure exotherms in APNeDGEBA blends can be attributed to the crosslinking via epoxyephthalonitrile and epoxyeamine in the temperature regime of 90 Ce230 C. Thereafter, the reaction proceeds mainly by the secondary aminemediated polymerization of phthalonitrile groups.
Fig. 13. FT-IR spectra of CrPNeDGEBA and its cured forms.
Fig. 14. DSC scans of APNeDGEBA and MDAeDGEBA blend.
3.3.4. PEEK- toughened APNeDGEBA blend PEEK has been used as a toughening agent in epoxy matrix [46]. But this has not been attempted in phthalonitrile resin systems. In the present work, PEEK resin of molecular weight 8000 was used along with DGEBA blended APN resin.
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Fig. 15. DSC scan of PEEK toughened APNeDGEBA blend.
In the DSC thermogram of PEEK toughened APNeDGEBA system in Fig. 15, a dip at around 145 C corresponding to the glass tranisition temperature of PEEK was observed. A slight shift of the cure temperature towards higher temperature region is seen. This shift is caused probably by the decreased proximity of crosslinking functionalities by the introduction of PEEK in the matrix. Thus, the overall cured network structures of APN/epoxy blends comprises of epoxyeamine adduct, oxazoline structures from epoxyephthalonitrile curing (Scheme 4) and heterocyclic intermediates resulting from amine ephthalonitrile crosslinking (shown in Scheme 3). In the toughened blend, the free terminal eOH groups of PEEK can trigger phenol mediated phthalonitrile curing resulting in heterocyclic networks in addition to amineephthalonitrile reaction [30]. However, the incorporation of a comparatively high molecular weight PEEK reduced the overall crosslink density and toughened the system which was evident from their improved lap shear strength values (Section 3.6). The
Fig. 16. Thermograms of cured APN systems and their blends.
overall network formation involved in PEEK-toughened APN/epoxy curing is depicted in Scheme 5.
3.4. Thermal stability of APN and their blends APN, APN/epoxy and APN/epoxy/PEEK blends were cured following the cure schedule already discussed. The thermal stability of cured blends was determined by TG studies and thermograms obtained are presented in Fig. 16. In the case of CAPN, the initial decomposition temperatures were all above 420 C. It is evident that, with the increase in amine content, thermal stability and char residue slightly enhanced due to participation of more cyano groups in amine-mediated crosslinking reaction, resulting in thermally stable heterocyclic structures. Thus, cured
Scheme 5. Likely cured network structure of APN/epoxy/PEEK blends.
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Fig. 18. Lap shear strength for various polymers.
previously been observed in PEEK toughened epoxy systems, wherein precipitation of PEEK led to enhanced fracture toughness [46]. The blister-like appearances on the surface were almost absent for the PEEK blended systems apparently indicating a toughening effect. The evidence for toughness was obtained indirectly from the lap shear strength evaluation of the resins (discussion follows). 3.6. Evaluation of lap shear strength (LSS)
Fig. 17. SEM micrographs of cured samples of (a) APN (b) APNeDGEBA (c) APNe DGEBAePEEK.
amineephthalonitrile polymers retained 95% weight at 500 C and it exhibited high char yield around 78% at 900 C. However, on blending with the epoxy resins, the temperature corresponding to 5% weight loss got reduced to 330 C with simultaneous reduction in char residue at 900 C to around 64%. It was observed that, the thermal stability of APN got significantly decreased on blending with DGEBA. Co-cured networks formed via epoxyeamine and epoxyephthalonitrile reactions are readily susceptible to thermal dissociation compared to phthalocyanine and triazine networks formed via amineephthalonitrile crosslinking. Incorporation of PEEK showed no significant effect on thermal stability of APNeDGEBA blends. 3.5. Morphological studies Morphological features of the cured APN, cured blends of APNeDGEBA and APNeDGEBAePEEK are displayed in Fig. 17. Cured APN showed a rough surface with blister-like appearances, which may be attributed to a brittle matrix, a trait of phthalonitrile polymers. APNeDGEBA blend micrograph clearly depicts a smooth surface with a two-phase morphological texture where the dispersed phases (droplets) represent the epoxy component. Interestingly, on blending further with PEEK, phase separation increased to a significant extent. This kind of phase separation has
Owing to their highly polar nature and favorable substituents, these resins are expected to possess good adhesive characteristics. APN as such gave poor LSS properties due to their highly brittle nature leading to poor lap shear strength on SS coupons. The bond strength was significantly improved both at RT and at 150 C in the case of blends of APN with DGEBA (Fig. 18). Thus, APNeDGEBA blend showed 20 and 45% enhancement in LSS value at RT and 150 C compared to neat APN. Blending with DGEBA enabled alleviating the brittleness of the APN systems to some extent as manifested by their higher cohesive strength, further corroborated by the morphological studies. As expected, addition of PEEK further increased the LSS value, by 45 and 81% respectively at RT and 150 C mainly due to the reduced brittleness of the cured network. 4. Conclusions Incorporation of about 20% amine functionalities into novolac phthalonitrile oligomers resulted in considerable reduction of cure temperature without much compromise in their thermal properties. Epoxy functions in APN system also facilitated cure reactions through an alternate mechanism with a concomitant decrease in their thermal stability. Incorporation of PEEK reduced the brittle nature of APN systems, as manifested by the enhancement in the LSS value of APNeDGEBAePEEK system when compared to bare resin. Toughening by PEEK did not have any adverse effect on the thermal properties. Acknowledgments The authors thank Director, VSSC for granting permission to publish this work. One of the authors (DA) thanks ISRO for the research fellowship. They also thank Analytical and Spectroscopy
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Division and Material Characterization Division, VSSC for the support in characterization and testing of the materials. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2014.09.042.
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Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Hydroxyl terminated PEEK-toughened epoxyeamino novolac phthalonitrile blends e Synthesis, cure studies and adhesive properties Dhanya Augustine a, K.P. Vijayalakshmi b, R. Sadhana b, Dona Mathew a, C.P. Reghunadhan Nair a, * a b
Polymers and Special Chemicals Group, Vikram Sarabhai Space Centre, Trivandrum 695022, Kerala, India Analytical, Spectroscopy and Ceramics Group, Vikram Sarabhai Space Centre, Trivandrum 695022, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 July 2014 Received in revised form 17 September 2014 Accepted 18 September 2014 Available online xxx
Self cure promoting, amine-containing novolacephthalonitrile (APN) resins of varying compositions were synthesized and characterized. APN possessing amine functionalities reduced the cure initiation temperature from 310 C (typical of pure phthalonitrile systems) to 180 C. It showed excellent thermal stability up to 420 C and high char residue of 77e79 %. Co-reaction of APN with diglicydyl ether of bisphenol A (DGEBA) led to a decrement in their thermal stability though improved their adhesive properties. Evidences were obtained for epoxyeamine, epoxyephthalonitrile and amineephthalonitrile reactions. The latter reactions led to formation of oxazoline, triazine and phthalocyanine groups in the network. These were rationalized by density functional theory studies on model compounds. The extents of epoxyeamine and epoxyephthaonitrile reactions were quantified. Introduction of hydroxyl terminated poly ether ether ketone (PEEK) reduced the brittleness of the blends and improved their lap shear strength. Toughening of epoxyeamino novolac phthalonitrile networks occurred through phase separation of PEEK segments in cured matrix. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Amineephthalonitrile Epoxyephthalonitrile Self-curing polymer
1. Introduction Phthalonitrile resins and their composites are well known for their attractive properties such as high glass transition temperature, retention of mechanical properties at elevated temperature, better storage life, low moisture absorptivity and excellent flame retardancy [1e4]. The major limitation for their wide spread application is the requirement of high curing temperature (>300 C) for a long duration [5e9]. Active hydrogen sources such as aromatic amines and phenols, metals, metallic salts and complexes are commonly employed as curing agents for phthalonitrile groups [10e14]. Rather than external curatives, curing groups present in integral part of molecules are desirable for easy curing of resin. Hydroxyl-catalyzed and amine-catalyzed self-curing phthalonitrile systems can undergo curing at relatively low temperatures [5,15e19]. However, these groups provide small processing windows. This can be alleviated by means of providing flexible linkages in the polymer [20e22].
* Corresponding author. Tel.: þ91 471 2565689; fax: þ91 471 2564096. E-mail address: [email protected] (C.P. Reghunadhan Nair).
Earlier studies on epoxy/phthalonitrile thermosets revealed improved processability compared to neat phthalonitrile polymers and superior thermal stability and mechanical properties for the modified systems with respect to neat epoxies [23,24]. Binary blends of biphenyl and oligomeric phthalonitriles with epoxy resin showed that phthalonitrileeepoxy copolymerization could be materialized either in the absence of curing agents at relatively high temperatures or in the presence of curative like aromatic amines [19,25]. Even though the epoxy/phthalonitrile blends enhanced the processability of phthalonitrile polymers, they are not free from inherent brittle nature. Blending the thermosets with thermoplastics has been reported as an effective method to reduce the brittleness [22]. Interpenetrating networks formed by the incorporation of poly arylene ether nitrile into epoxy/phthalonitirle thermosets enhanced their mechanical and thermal properties [26,27]. In the present study, we report the synthesis and characterization of a self-cure promoting, phthalonitrile thermoset with low cure temperature and short cure times. The studies include the synthesis, characterization, cure studies and the resulting crosslinked networks of a novel amine-functionalized phthalonitrile
http://dx.doi.org/10.1016/j.polymer.2014.09.042 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
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system based on novolac backbone and their blends with diglicydyl ether of bisphenol A (DGEBA). The paper also examines the influence of a high temperature stable thermoplastic toughening agent, poly ether ether ketone on their cure characteristics, morphological features, toughness and adhesive performance of the blend. 2. Experimental 2.1. Raw materials Phenol (Nice Chemicals, India), formalin (34e37%) (CDH, India), p-aminophenol (Sigma Aldrich, USA), 4-nitro phthalonitrile (Acros Organics, USA), Diglicydyl ether of Bisphenol A (DGEBA) (Huntsman, USA), 4,40 -Methylene dianiline (MDA) (Sigma Aldrich, USA) and N-methyl pyrrolidone (SRL, India) were used as received. Hydroxyl terminated poly ether ether ketone (PEEK) with Mn: 8000 was synthesized according to the already reported procedure [28]. 2.2. Characterization In the present work, FT-IR spectra were recorded using a Perkin Elmer spectrum GXA spectrophotometer in the range of 4000e400 cm1. Proton and 13C NMR were recorded using Bruker Avance NMR spectrometer (300 MHz) using TMS as reference in acetone-d6 solvent. Molecular weight characterizations were carried out by Gel Permeation Chromatography (GPC) using Waters Alliance 2690; employing polystyrene standards. Elemental analysis was performed using Perkin Elmer 2400 CHN Analyzer. Differential Scanning Calorimetry (DSC) was performed using a TA instrument DSC Q-20 at a heating rate 10 C min1 in nitrogen atmosphere and thermogravimetric analysis (TG) was carried out using TA Instruments SDT Q-600 thermogravimetric analyzer at a heating rate of 10 C min1 in nitrogen atmosphere. Microstructure of the blends was examined using a scanning electron microscope (SEM) from Carl Zeiss SMT EVO 50 scanning electron microscope (SEM) at 10 kV. Lap shear strength of the systems were evaluated at 30 C and 150 C using stainless steel substrates of grade according to the ASTM D 1002. Molecular modeling at B3LYP/6-31G(d,p) level of DFT method was carried out to explain the experimental data. 2.3. Synthesis of amino functionalized novolac (ANC) To a mixture of phenol (25 g, 0.27 mol), p-amino phenol (5 g, 0.045 mol) and oxalic acid (1.24 g), formalin (37% w/w in water, 14 mL, 0.17 mol) was added (phenol:formalin ratio ¼ 1:0.63) dropwise and the system stirred at 70 C for 12 h. After the reaction, the product was washed repeatedly with distilled water followed by removal of water by azeotropic distillation with toluene. The polymer obtained was dried under vacuum at 50e60 C for 6 h, yield~ 50%. The polymer was characterized by FT-IR, GPC and CHN analysis. 1 H NMR (500 MHz, DMSO-d6, d ppm): 9.3 (eNH2 and OH), 3.8 (eCH2e), 6.3 & 7 (AreH). 13C NMR (500 MHz, DMSO-d6, d ppm): 22 (AreCH2eAr), 155.78 and 150.72 (aromatic CeOeC), 132.63, 130.10, 129.01 and 128.01 (aromatic carbon). FT-IR (KBr, cm1): 3400 (eNH2), 3300 (eOH), 2917(eCH2e), 1600, 1550 and 1450 (AreH)
reaction, the mixture was filtered and the filtrate was precipitated into 90:10 water-methanol mixture. The precipitate was filtered, washed several times with methanol and dried in vacuum for 4 h, yield ~60%. By varying the p-amino phenol content, different compositions of amino novolac phthalonitrile oligomers e APN-1, APN-2 and APN-3 were synthesized and characterized by FT-IR, GPC, 13C NMR, elemental analysis, hydroxyl value, and DSC. 1 H NMR (500 MHz, DMSO-d6, d ppm): 9.3 (eNH2 and OH), 3.8 (eCH2e), 6.3 & 7 (AreH). 13C NMR (500 MHz, DMSO-d6, d ppm): 30 (AreCH2eAr), 155.78 and 150.72(Aromatic CeOeC), 132.63, 130.10, 129.01 and 128.01 (Aromatic carbon) 115.4 (CN). FT-IR (KBr, cm1): 3400(eNH2), 2234(eCN), 2917(eCH2e), 1600, 1550 and 1450 (AreH) 2.5. Polymerization of amino novolac phthalonitrile oligomers Amino novolac phthalonitrile oligomers were cured under nitrogen atmosphere as per the schedule: 150 C, 200 C and 250 C for 30 min each; 300 C for 1 h and 350 C for 4 h. The cured polymers (CAPN) were characterized by FT-IR and TG. 2.6. Preparation of amino novolac phthalonitrileeepoxy blend (APNeDGEBA) and MDAeDGEBA blend APNeDGEBA blend was formulated by mixing APN-2 and DGEBA (equivalent stoichiometry of amine and epoxy functionalities) in THF solvent under ambient conditions. The solvent was then evaporated and the mixture was vacuum dried at 50 C for 2 h. The APN/epoxy blends were cured according to the schedule: 120 C for 2 h, 150 C for 3 h and 180 C for 5 h. MDAeDGEBA blend (equivalent stoichiometry of amine and epoxy groups) for DSC analysis was prepared by solution blending and evaporating the solvent. 2.7. Preparation of APNeDGEBAePEEK blends APN and DGEBA were blended at an equivalent molar ratio of amine and epoxy groups in minimum quantity of THF. To this solution of APN and DGEBA, 10 weight % of PEEK was added, thoroughly mixed and vacuum dried to remove the solvent. Toughened blend was cured according to the same schedule adopted for curing of APNeDGEBA. Cured APNeDGEBAePEEK blend is referred to as CAPNeDGEBAePEEK. 2.8. Adhesion test Lap shear strength of the polymers was determined at room temperature and at 150 C using steel (SS 304) substrates. The steel substrates were washed with acetone and dried before use. The single-lap joints had an overlap area of 25 mm 25 mm. The test specimens were prepared by applying APNeDGEBA blend and their toughened system dissolved in minimum amount of THF and cured according to the afore-mentioned schedules. 3. Results and discussion 3.1. Synthesis and characterization of ANC and APN oligomers
2.4. Synthesis of amino novolac phthalonitrile oligomer (APN) Amino functionalized novolac (10 g) dissolved in 35 mL Nmethyl pyrrolidone was added to a three necked round bottom flask equipped with nitrogen inlet. Anhydrous K2CO3 (15 g) was added into the flask. To this mixture, 4-nitro phthalonitrile (10 g, 0.058 mol) dissolved in 20 mL NMP was added drop-wise and stirred continuously for 24 h at room temperature. After the
Amino functionalized novolac oligomers were synthesized by the reaction of phenol and p-amino phenol with formaldehyde in the presence of oxalic acid as shown in Scheme 1. Phenol to formaldehyde ratio was maintained as 1:0.63 to obtain very low molecular weight novolacs. Different compositions of amino functionalized novolac (ANC) oligomers are characterized by elemental analysis, FT-IR, 1H NMR,
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3
Ratio of protons in (OH þ NH2):AreH ¼ (0.03 3 þ 0.97 1): (0.03 3 þ 0.97 4) ¼ 1:4
Scheme 1. Synthesis of amino functionalized novolac (ANC).
Table 1 Details of Amino functionalized novolac (ANC) oligomer. Reference
Extent of p-amino phenol incorporated (mol%)a
ANC-1 ANC-2 ANC-3
3 11 24
a
From nitrogen content.
GPC and DSC. Details of the oligomers synthesized are compiled in Table 1. Typical FT-IR spectrum of ANC oligomer is shown in Fig. 1a which exhibits a broad peak centered around 3300 cm1, contributed by both OH and NH2 groups. Characteristic absorption peaks of eOH and eNH2 appeared as a broad signal at 9.3 ppm (Fig. 1b). Integral ratios of OH and NH2 with aromatic protons was calculated for ANC-1 and found to be 1:4. This matches with the theoretical values. Since these are low molecular weight oligomers repeat unit can be better represented by
Amino novolac phthalonitrile oligomers (APN) with varying degrees of amino and phthalonitrile functionalities were synthesized according to the reaction shown in Scheme 2. Reaction involved nucleophilic displacement of the eNO2 group in 4-nitro phthalonitrile by the phenolic functions of amino functionalized novolac in alkaline medium in the dry dipolar aprotic solvent, NMP. Near complete phthalonitrilation of free eOH groups in ANC oligomers were confirmed from elemental analysis. Extent of phthalonitrilation was estimated at around 95% (calculations are shown in Supporting information, S1). Composition of APN oligomers are shown in Table 2. The products were further characterized by GPC, FT-IR and 13C NMR analyses. Fig. 2a and b shows the molecular weight distribution patterns of ANC and APN oligomers. There is a proportionate increase in molecular weight on phthalonitrilation of ANC oligomers. The number average molecular weight (Mn) of ANC oligomers ranges from 1025 to 1180 (corrected using the MarkeHouwink constant for novolac as MNL ¼ 0.7962 M0.9999 where MNL and MST are ST molecular weight of novolac and styrene oligomers at a given elution time) whereas APN oligomers showed Mn in the range of 1500e1800 [29]. Comparison of FT-IR of ANC and APN in Fig. 3 showed the evidences for phthalonitrilation of the amino novolac, by the appearance of characteristic absorption of eCN peak at 2231 cm1 with concomitant diminution of absorption centered around 3300 cm1, due to eOH groups. Peak due to amino groups is observed at 3440 cm1. 13C NMR spectra were also consistent with the expected structure of APN oligomers (Fig. 4). Signals were observed at 30 ppm (AreCH2eAr), 155.78 and 150.72 (Aromatic CeOeC) and 115.4 (CN). As a reference material, novolac phthalonitrile oligomer (without amine functions) referred to as NPN (Fig. 5) was also synthesized according to the reported procedure [30].
3.2. APNeDGEBA and APNeDGEBAePEEK blends
Theoretical calculation for integral ratios of OH and NH2 with aromatic proton from their structure.
In the present study, the cure behavior of APNeDGEBA blend was monitored for an epoxyeamine stoichiometry of 1:1. The engineering thermoplastic, hydroxyl terminated polyether ether ketone (PEEK) was used to toughen the APNeDGEBA polymer
Fig. 1. a. Typical FT-IR spectrum of ANC. b. 1H NMR spectrum of ANC-1 oligomer.
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Scheme 2. Synthesis of amino novolac phthalonitrile oligomers (APN).
Table 2 Composition of APN oligomers. Reference
Extent of p-amino phenol incorporated (mol%)a
Amine:phthalonitrile (molar ratio)
APN-1 APN-2 APN-3
3 11 24
1:32 1:8 1:4
a
From nitrogen content.
systems [28,31]. The chemical structures of epoxy resins and hydroxyl terminated PEEK are shown in Fig. 6a and b respectively. 3.3. Cure characteristics 3.3.1. Curing of APN The amine functionality is reportedly the most suitable curing agent for the phthalonitrile systems [17,32,33]. In the present study, the progress of cure reaction was monitored by FT-IR and DSC techniques. The FT-IR spectra of uncured and cured APN system (CAPN) are shown in Fig. 7. For CAPN, there is diminution in intensity of nitrile peak (2231 cm1). However, under the cure conditions employed as already mentioned, complete disappearance of eCN peak could not be achieved in any of the compositions; due to the incomplete conversion of nitrile groups [32,34]. The cure conditions employed in the present study drew a maximum of only 16% of eCN functionalities into the cured network as quantitatively estimated by FT-IR using the peak at 1220 cm1 (ether linkage) as the internal reference peak. Quantitative estimation was done using the Equation (1).
a¼1
ACN;t AR;0 ACN;0 AR;t
(1)
Fig. 3. FT-IR spectra of ANC-2 and APN-2.
where a is the extent of nitrile conversion and A refers to the area under the absorption peak. ACN,0 and ACN,t are the area under eCN peak before and after cure conditions. Similarly, AR,0 and AR,t represents the reference peak area before and after cure reaction. From the FT-IR spectra of cured APN systems, it was evident that the intensity of peak centered around 3440 cm1 increased with the dimunition of nitrile peak. This is an indirect evidence for the formation of isoindoline and phthalocyanine moieties in the cured network as shown in Scheme 3. In addition, the characteristic peak due to NH bending of phthalocyanine rings appeared at 1010 cm1 in the cured polymers [35,36].
Fig. 2. a. SEC profiles of ANC oligomers. b. SEC profiles of APN oligomers.
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Fig. 4. Typical
5
13
C NMR spectrum of APN.
Fig. 5. Novolac phthalonitrile (NPN) oligomer.
The DSC scans of amine functionalized novolac phthalonitrile system and corresponding novolac phthalonitrile are shown in Fig. 8. A systematic lowering in cure initiation temperature, Ti was observed with the increase of amine moieties in the APN systems as shown in Fig. 9. From the graph, we clearly get an insight into the influence of amine functionalities on curing of phthalonitrile groups. Thus, novolac phthalonitrile oligomer (NPN), which contain no amine functional groups shows cure initiation temperature (Ti)
around 310 C against a Ti of 180 C for APN-3 which contain 24 mol % amine functionalities incorporated on the novolac backbone i.e. decrease of about 130 C could be achieved in Ti with incorporation of 24 mol% of amine functionalities. The cure reaction pathways of the phthalonitrile systems catalyzed by various amines are well established [15,34,37e39]. On heating the APN oligomers, the amine groups attack the nitrile functions in phthalonitrile moieties to form amidine intermediates [40]. The imine linkages in the amidine undergo an intramolecular attack with the nitrile group, leading to the formation of isoindoline structures. The isoindolines can further react with other phthalonitrile end-capping units to form tetrameric phthalocyanine networks [41]. The amidines can also proceed through an intermolecular reaction with other nitrile functionalities to form heterocyclic triazine rings [42]. Upon amine-mediated curing, phthalonitrile systems form a polymeric network comprised of the afore-mentioned heteroaromatic moieties. Mechanisms of formation of the heterocyclic intermediates are depicted in Scheme 3. These networks exhibit high Tg and impart rigidity to the cured APN systems which is manifested in their higher glass transition temperature. DSC analysis of cured APN sample did not show any glass transition upto 350 C. It was deemed that Tg of the cured system is
Fig. 6. Structure of (a) DGEBA (b) PEEK.
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Fig. 7. FT-IR spectra of APN-2 and CAPN-2.
Fig. 8. DSC curves of amino novolac phthalonitrile and novolac phthalonitrile oligomers.
above 350 C (attempts to perform DMA analysis failed as defectfree samples could not be cast). 3.3.2. Curing of APNeDGEBA blend In the case of cured APNeDGEBA blend, there is a significant reduction in intensity of nitrile absorption at 2230 cm1 without appreciable increase in the intensity of characteristic peaks corresponding to phthalocyanine, isoindoline moieties in the FT-IR spectra as given in Fig. 10. This is a clear indication of nitrile curing through an alternate cure mechanism, operating via epoxyeamine and epoxyephthalonitrile reactions. The epoxyeCN reaction results in oxazoline rings as shown in Scheme 4 [24,43e45]. 3.3.3. EpoxyeCN reaction: investigation using model compounds The cure reaction of DGEBA with APN is quite complex. Therefore, cure behavior of APNeDGEBA blends was investigated with the support of theoretical modeling and simulation and also by reactions employing model compounds. The feasibility of crosslinking of epoxy groups with nitrile and amine functions was theoretically modeled at B3LYP/6-31G(d,p) level of DFT using
Scheme 3. Proposed pathways for amine-mediated phthalonitrile polymerization.
Fig. 9. Influence of amine content on initial cure temperatures (Ti) of APN oligomers.
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aniline, glycidyl ether of p-cresol and phthalonitrile ether of pcresol (CrPN). The model compound CrPN was synthesized by nucelophilic substitution reaction between cresol and 4-nitro phthalonitrile in alkaline medium. The optimized structures of products of epoxyeamine and epoxyenitrile reaction pathways are presented in Fig. 11. The computed heat of reaction for the formation of oxazoline and epoxyeamine adduct are 150 kJ/mol and 96 kJ/mol and the activation barriers for their corresponding transition state are found to be 268 kJ/mol and 176 kJ/mol respectively (Fig. 12). Comparison of heat of reaction and activation energies for the two reactions suggests that reaction of glycidyl ether of p-cresol with CrPN or with aniline could be equally competitive and occur in the same temperature regime. The reactions among phthalonitrile, amine and epoxy groups were studied experimentally using phthalonitrile derivative of pcresol (CrPN), MDA and DGEBA as model compounds. FT-IR spectra were monitored to study the reaction between epoxy and phthalonitrile functionalities. The, FT-IR spectra of uncured sample of CrPN/DGEBA and CrPN/DGEBA heated at 200 C and 230 C each for 4 h are shown in Fig. 13. Fig. 10. FT-IR spectra of APN, CAPN and CAPNeDGEBA.
Scheme 4. Probable epoxyephthalonitrile/epoxyeamine cure reaction pathways.
Fig. 11. Energy minimized network structures (a) Oxazoline (b) Epoxyeamine adduct.
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Fig. 12. Transition state corresponding to curing of (a) CrPN/Glycidyl ether of p-cresol (b) Glycidyl ether of p-cresol/aniline (bond length in A unit).
Intensity of characteristic absorptions of nitrile (2231 cm1) and epoxy peaks (915 cm1) were reduced by the afore-mentioned heat treatments. Quantitative estimation by FT-IR was done by comparing absorption peaks of interest against the reference peak at 830 cm1, characteristic of aromatic para substitution. During heat treatment at 200 C, involvement of only about 1.5% of eCN groups was accomplished with the participation of minimal quantity of epoxy groups. Meanwhile, 44% of eCN groups was drawn into the network by the epoxy functions on heating at 230 C with concomitant reduction in the intensities of absorption by nitrile and epoxy groups. Earlier studies also have reported the crosslinking of nitrile functions with epoxy groups resulting in oxazoline rings above 200 C [25]. Simultaneously, the characteristic band due to oxazoline network at 1606 cm1 got intensified with the enhancement of curing temperature. For understanding further the reaction of epoxy groups with APN, a separate DSC analysis was carried out. In order to avoid the volatility related problems, a high melting amine i.e., diamino diphenyl methane (or methylene dianiline, MDA) was used. MDAeDGEBA blend was prepared by maintaining the same amine/ epoxy equivalence as in APNeDGEBA blend. DSC thermogram of MDAeDGEBA (Fig. 14) showed a broad exotherm initiating from 88 C and extending upto 200 C corresponding to the curing of epoxyeamine groups. DSC thermograms of APNeDGEBA blend showed a multistep polymerization reaction (Fig. 14). Compared to APN oligomers, two additional peaks were observed for APNeDGEBA blends. The first exotherm appears as a shoulder peak emerging at 87 C and is due to the epoxyeamine reaction. The
overlapped second and third exotherms peaked at 182 C and 205 C represent the epoxyenitrile and amineenitrile coreactions in blend. Thus, DSC pattern also obviously points to the involvement of an alternate cure mechanism, as indicated by FT-IR studies, in addition to the homopolymerisation of nitrile groups, facilitated by 1 and 2 amine groups. Quantitative estimation of the extent of epoxyeamine and epoxyephthalonitrile reactions in APNeDGEBA blends was done by comparing the heat of reaction evolved from epoxyeamine reaction in MDAeDGEBA blend. Extent of epoxyeamine reaction was 21% and epoxy groups available for reaction with nitrile groups was 79% (calculations are shown in Supporting information, S2). Experimental evidences corroborated by model compound studies established that the two cure exotherms in APNeDGEBA blends can be attributed to the crosslinking via epoxyephthalonitrile and epoxyeamine in the temperature regime of 90 Ce230 C. Thereafter, the reaction proceeds mainly by the secondary aminemediated polymerization of phthalonitrile groups.
Fig. 13. FT-IR spectra of CrPNeDGEBA and its cured forms.
Fig. 14. DSC scans of APNeDGEBA and MDAeDGEBA blend.
3.3.4. PEEK- toughened APNeDGEBA blend PEEK has been used as a toughening agent in epoxy matrix [46]. But this has not been attempted in phthalonitrile resin systems. In the present work, PEEK resin of molecular weight 8000 was used along with DGEBA blended APN resin.
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Fig. 15. DSC scan of PEEK toughened APNeDGEBA blend.
In the DSC thermogram of PEEK toughened APNeDGEBA system in Fig. 15, a dip at around 145 C corresponding to the glass tranisition temperature of PEEK was observed. A slight shift of the cure temperature towards higher temperature region is seen. This shift is caused probably by the decreased proximity of crosslinking functionalities by the introduction of PEEK in the matrix. Thus, the overall cured network structures of APN/epoxy blends comprises of epoxyeamine adduct, oxazoline structures from epoxyephthalonitrile curing (Scheme 4) and heterocyclic intermediates resulting from amine ephthalonitrile crosslinking (shown in Scheme 3). In the toughened blend, the free terminal eOH groups of PEEK can trigger phenol mediated phthalonitrile curing resulting in heterocyclic networks in addition to amineephthalonitrile reaction [30]. However, the incorporation of a comparatively high molecular weight PEEK reduced the overall crosslink density and toughened the system which was evident from their improved lap shear strength values (Section 3.6). The
Fig. 16. Thermograms of cured APN systems and their blends.
overall network formation involved in PEEK-toughened APN/epoxy curing is depicted in Scheme 5.
3.4. Thermal stability of APN and their blends APN, APN/epoxy and APN/epoxy/PEEK blends were cured following the cure schedule already discussed. The thermal stability of cured blends was determined by TG studies and thermograms obtained are presented in Fig. 16. In the case of CAPN, the initial decomposition temperatures were all above 420 C. It is evident that, with the increase in amine content, thermal stability and char residue slightly enhanced due to participation of more cyano groups in amine-mediated crosslinking reaction, resulting in thermally stable heterocyclic structures. Thus, cured
Scheme 5. Likely cured network structure of APN/epoxy/PEEK blends.
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Fig. 18. Lap shear strength for various polymers.
previously been observed in PEEK toughened epoxy systems, wherein precipitation of PEEK led to enhanced fracture toughness [46]. The blister-like appearances on the surface were almost absent for the PEEK blended systems apparently indicating a toughening effect. The evidence for toughness was obtained indirectly from the lap shear strength evaluation of the resins (discussion follows). 3.6. Evaluation of lap shear strength (LSS)
Fig. 17. SEM micrographs of cured samples of (a) APN (b) APNeDGEBA (c) APNe DGEBAePEEK.
amineephthalonitrile polymers retained 95% weight at 500 C and it exhibited high char yield around 78% at 900 C. However, on blending with the epoxy resins, the temperature corresponding to 5% weight loss got reduced to 330 C with simultaneous reduction in char residue at 900 C to around 64%. It was observed that, the thermal stability of APN got significantly decreased on blending with DGEBA. Co-cured networks formed via epoxyeamine and epoxyephthalonitrile reactions are readily susceptible to thermal dissociation compared to phthalocyanine and triazine networks formed via amineephthalonitrile crosslinking. Incorporation of PEEK showed no significant effect on thermal stability of APNeDGEBA blends. 3.5. Morphological studies Morphological features of the cured APN, cured blends of APNeDGEBA and APNeDGEBAePEEK are displayed in Fig. 17. Cured APN showed a rough surface with blister-like appearances, which may be attributed to a brittle matrix, a trait of phthalonitrile polymers. APNeDGEBA blend micrograph clearly depicts a smooth surface with a two-phase morphological texture where the dispersed phases (droplets) represent the epoxy component. Interestingly, on blending further with PEEK, phase separation increased to a significant extent. This kind of phase separation has
Owing to their highly polar nature and favorable substituents, these resins are expected to possess good adhesive characteristics. APN as such gave poor LSS properties due to their highly brittle nature leading to poor lap shear strength on SS coupons. The bond strength was significantly improved both at RT and at 150 C in the case of blends of APN with DGEBA (Fig. 18). Thus, APNeDGEBA blend showed 20 and 45% enhancement in LSS value at RT and 150 C compared to neat APN. Blending with DGEBA enabled alleviating the brittleness of the APN systems to some extent as manifested by their higher cohesive strength, further corroborated by the morphological studies. As expected, addition of PEEK further increased the LSS value, by 45 and 81% respectively at RT and 150 C mainly due to the reduced brittleness of the cured network. 4. Conclusions Incorporation of about 20% amine functionalities into novolac phthalonitrile oligomers resulted in considerable reduction of cure temperature without much compromise in their thermal properties. Epoxy functions in APN system also facilitated cure reactions through an alternate mechanism with a concomitant decrease in their thermal stability. Incorporation of PEEK reduced the brittle nature of APN systems, as manifested by the enhancement in the LSS value of APNeDGEBAePEEK system when compared to bare resin. Toughening by PEEK did not have any adverse effect on the thermal properties. Acknowledgments The authors thank Director, VSSC for granting permission to publish this work. One of the authors (DA) thanks ISRO for the research fellowship. They also thank Analytical and Spectroscopy
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Division and Material Characterization Division, VSSC for the support in characterization and testing of the materials. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2014.09.042.
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Please cite this article in press as: Augustine D, et al., Hydroxyl terminated PEEK-toughened epoxyeamino novolac phthalonitrile blends e Synthesis, cure studies and adhesive properties, Polymer (2014), http://dx.doi.org/10.1016/j.polymer.2014.09.042