Polybenzoxazine Precursors As Self-Healing Agents for Polysulfones

Article pubs.acs.org/Macromolecules

Polybenzoxazine Precursors As Self-Healing Agents for Polysulfones Omer Suat Taskin,† Baris Kiskan,*,† and Yusuf Yagci*,†,‡ †

Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey Center of Excellence for Advanced Materials Research (CEAMR) and Chemistry Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, 21589, Saudi Arabia

‡

S Supporting Information *

ABSTRACT: In this work, a novel self-healing system based on the use of polybenzoxazine precursor (PBP) as a healing additive is presented. PBP (Mn = 2300 g/mol, Mw/Mn = 2.6) is facilely synthesized in a reasonable yield by Mannich type polycondensation of bisphenol A, 1,6-diaminohexane with paraformaldehyde. The additive PBP faintly undergoes a thermal ring-opening reaction when contained in polysulfone (PSU) films. Thermal treatment at 160 °C enables PBP to chemically bind to PSU chains and form networks through the Friedel−Crafts reaction, demonstrating a novel self-healing behavior. The extent of the recovery was studied using a stress−elongation (%) test and found to be 55%. Thermal properties of the polybenzoxazine precursor and the healed sample were investigated.

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INTRODUCTION Thermosetting resins derived from 1,3-benzoxazines namely polybenzoxazines have gained considerable attention in recent years. Compared to conventional novolac and resol type resins, polybenzoxazines have superior properties. High glass transition temperatures, release of limited byproduct during curing, high char yield, and noncatalytic polymerization are important features of polybenzoxazines.1−3 Moreover, low water adsorption, a direct consequence of inter and intramolecular hydrogen bonding, and low shrinkage during polymerization, stemming from dominant intramolecular hydrogen bonds and the cyclic structure of the corresponding 1,3-benzoxazines, are also noteworthy features.1,4 Polybenzoxazines can be synthesized by thermally activated ring-opening polymerization of the corresponding 1,3-benzoxazines (Scheme 1). Their polymerization temperatures vary

Synthesis of 1,3-benzoxazine monomers can be considered an easy process and offers a vast variety of monomers since various commercially available phenols and primary amines can be used with formaldehyde (Scheme 2).9−14 Scheme 2. Synthesis of Monofunctional 1,3-Benzoxazines

For practical applications, benzoxazines have to be fabricated into thin solid films, which cannot be achieved with monomer derived resins. Moreover, the cured benzoxazines are highly brittle, especially in the case of basic monofunctional phenol− aniline type (P−a) polybenzoxazines. One way to surmount this processing disadvantage is to construct polybenzoxazine precursors which can readily be fabricated by condensation of diamines and diphenols in the presence of formaldehyde (Scheme 3).15−19 Several other polymerization techniques with designed monomers can also be used to improve the toughness

Scheme 1. Thermally Induced Ring-Opening Polymerization of a Bisbenzoxazine Monomer Yielding Polybenzoxazine

Scheme 3. Synthesis of a Polybenzoxazine Precursor Using Mannich Type Condensation

between ca. 180 and 250 °C, depending on the functionalities on the monomer. Even much lower polymerization temperatures were reported in special cases, e.g. 120 and 150 °C. The resultant polymer is a network comprised of phenolics and amine bridges as the structural motifs.5−8 © 2013 American Chemical Society

Received: September 16, 2013 Revised: November 7, 2013 Published: November 15, 2013 8773

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of polybenzoxazines.20−24 The incorporation of soft segments (e.g., aliphatic, ester, urethane, or siloxane moieties) into the main chain of the polybenzoxazine precursors provided materials possessing better toughness.25−29 However, in spite of the aforementioned advantages offered by the availability of a number of polybenzoxazine precursors, their potential for use in terms of fabrication of self-healing materials has not been recognized. The development of selfhealing polymers has received much research attention in the last two decades since prolonged service life for many materials is an important economic issue. In general, self-healing polymers can be classified into two major categories. Systems based on “autonomous” healing are fully self-contained and require no external intervention of any kind. Such systems generally contain encapsulated healing agents enclosed in microspheres, vascular hollows, or other suitable microcontainers.30 Damages like delamination, scratches, cracks, or cuts cause the release of the healing agents to perform the healing reactions. The alternative systems based on “stimuliresponsive” healing need external factors to carry out the healing process. Those systems are partially self-contained and healing capability is imparted by a specific design at the molecular level. External factors provide the energy required for bond breaking and bond-rebuilding, which is transferred by light, heat, electricity or ultrasound etc. to the system.31−37 Pericyclic reactions such as [4 + 4], [4 + 2], [3 + 2], and [2 + 2] additions were used abundantly for stimuli-responsive selfhealing processes.38 Moreover, thiol-disulfide exchange reactions were also employed for many systems.39,40 In general practice, thermal processes are suitable and effective for treatment of polymeric materials with a wide range of sample sizes and thickness, since for example light induction can only penetrate into the system to a few centimeter depths. The effort of developing a light-induced self-healing technique, however, has met with limited success. As a result, heat-assisted reactions have received much attention for designing self-healing materials. For such reactions, suitable functional groups are needed to be incorporated into the polymers either by monomer design or by postmodification of the polymers.34,38 However, in the case of commercially available polymers, bringing in such self-healing capacity generally increases the cost of the material to a level that it is not economically acceptable. Evidently, more cost friendly methods are required for commercial polymers. One of the major commercial polymers is polysulfone (PSU), and it is known to have good toughness and stability at high temperatures. Polysulfones show high strength and stiffness even at elevated temperatures, high continuous use and heat deflection temperatures, excellent resistance to hydrolysis by acids and bases, good dimensional stability, and the highest service temperature among all meltprocessable thermoplastics.41,42 Incorporation of self-healing ability to PSU without using monomer design and postpolymer modification methods can be considered as a valuable challenge taking into account usability and economic perspectives. We have recently demonstrated that polybenzoxazines and PSU are compatible and can be cast into free-standing films.43,44 Taking advantage of their compatibility and thermally activated ring-opening reactions of benzoxazine, in this work, we report the preparation of PBP and its utilization as an additive in self-healing of PSU.

Article

EXPERIMENTAL SECTION

Materials. Paraformaldehyde (Acros, 96%), bisphenol A (Aldrich, 97%), 1,6-diaminohexane (Aldrich, 98%), sodium hydroxide (Acros, >97%), sodium sulfate (Acros, 99%), toluene (Aldrich, 99%), chloroform (Alfa Aesar, 99,5%), methanol (Aldrich, 99%), diethyl ether (Carlo Erba, 99.8%), N, N-dimethylformamide (Merck, 99.8%) and polysulfone (Udel-26000, Mn = 26000). The reagents and solvents were used as received. Instruments. The FTIR spectra were recorded at Perkin-Elmer Spectrum One with an ATR Accessory (ZnSe, Pike Miracle Accessory) and cadmium telluride (MCT) detector. Resolution was 4 cm−1 and 24 scans with 0.2 cm/s scan speed. Curing of the films was monitored on a Perkin-Elmer Diamond differential scanning calorimetry (DSC) with a heating rate of 10 °C min−1 under nitrogen flow. Thermal gravimetric analysis (TGA) was performed on PerkinElmer Diamond TA/TGA with a heating rate of 10 °C min−1 under nitrogen flow. The gel permeation chromatography (GPC) measurements were carried out with an Agilent instrument (Model 1100) consisting of a pump, refractive index (RI), and ultraviolet (UV) detectors and four Waters Styragel columns (guard, HR 5E, HR 4E, HR 3, HR 2), (4.6-mm internal diameter, 300-mm length, packed with 5-lm particles). The effective molecular weight ranges are 2000−4 000 000, 50−100 000, 500−30 000, and 500−20 000 g/mol, respectively. THF and toluene were used as eluent at a flow rate of 0.3 mL/min at 30 °C and as an internal standard, respectively. The apparent molecular weights (Mn,GPC and Mw,GPC) and polydispersities (Mw/Mn) were determined with a calibration based on linear PS standards using PL Caliber Software from Polymer Laboratories. SEM samples were coated with gold of ∼50 nm thickness using a SC7640 sputter coater (Quorum Technologies, Newhaven, U.K.) under vacuum (13.33 Pa), 1.2 kV, and 50 mA at 25 °C. The surface morphology of coated samples was examined by scanning electron microscopy (SEM) with a Jeol JSM-5910 LV instrument at 20 kV. Uniaxial Elongation Measurements. The measurements were performed on polymeric film samples (4 mm diameter -50 mm length −0.8 mm thickness). Measurements were carried out using a Zwick Roell, 500 N test machine at 25 °C under the following conditions: cross-head speed = 50 mm/min, sample length between jaws = 20 mm. The tensile strength (from the initial cross section of 12.57 mm2) and percentage elongation at break were recorded. Synthesis of Polybenzoxazine Precursor (Poly(bisphenol− hexamine benzoxazine)) (PBP). Bisphenol A (22.8 g, 0.1 mol), 1,6diaminohexane (11.6 g, 0.1 mol), paraformaldehyde (12 g, 0.4 mol), and 250 mL of a toluene/ethanol (2/1, v/v) mixture of solvents were introduced into a round-bottom 500 mL glass flask equipped with a condenser, and a magnetic stirrer. The reaction mixture was refluxed for 6 h. The solvent was concentrated using a rotary evaporator and the polymer was precipitated by pouring into 500 mL methanol. The precipitate was filtered and washed by methanol. After drying in a vacuum oven, a yellow powder (61% yield) was obtained. The polybenzoxazine precursor was prepared from bisphenol A and 1,6diaminohexane; we refer to it as PBP. Preparation of Thermally Curable Polysulfone−Polybenzoxazine Films. For the mechanical tests PSU−PBP films were prepared by solvent casting method. Appropriate solutions containing 1 g of polysulfone (PSU) and 100 mg of polybenzoxazine precursor in 10 mL chloroform were allowed to cast on aluminum Petri dishes for 24 h at 45 °C. Thermally Activated Self-Healing Experiments. A PSU−PBP film (4 mm diameter, −50 mm length, −0.8 mm thickness) was cut in half, and the cut edges were reattached and fixed using glass slides. The sample was exposed to heat (160 °C) for 4 h using a temperaturecontrolled oven.

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RESULTS AND DISCUSSION A successful synthesis of polybenzoxazine precursor (poly(bisphenol−hexamine benzoxazine)) (PBP) as healing agent was performed according to the procedure reported by Lin et al. (Scheme 4).45 Mannich type polycondensation of bisphenol 8774

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Thus, when the PBP is simply dispersed into the PSU polymer matrix, upon heating a competing Friedel−Crafts reaction is expected to take place between the aromatics of PSU and the precursor. The chemical binding of the polybenzoxazine precursor to the PSU matrix from various points is favored by the activation of aromatic rings on the main-chain PSU by the aromatic ether structure of PSU against electrophilic aromatic substitutions, this possible reaction could take place and chemically bind. By this way, polybenzoxazine could act as bridges between PSU chains. Consequently a network forms (See Scheme 6).

Scheme 4. Synthesis of poly(bisphenol− hexaminebenzoxazine) (PBP)

A, 1,6-diaminohexane with paraformaldehyde yielded the precursor polymer (Mn = 2310 g/mol, Mw/Mn = 2.6). In order to prevent the gelation during synthesis, induced by triazine formation between formaldehyde and diamine, toluene:ethanol (2:1, v/v) solvent system was used. The structure of the PBP was confirmed by spectral analysis. The 1H NMR spectrum of the precursor presented in Figure S1, Supporting Information, exhibits the typical proton signals attributed to the oxazine ring at 4.81 and 3.91 ppm for N− CH2−O and Ar−CH2−N, respectively. Since the ring closure degree has a significant impact on the PBP properties and efficiency, it is determined using peak areas of oxazine ring and calculated as 88%. Moreover, it is well-known that 1,3-benzoxazines and related polymeric precursors can be cured by thermally activated ringopening polymerization and this can be monitored using differential scanning calorimetry (DSC) since the ring-opening is an exothermic process. The curing maximum of such process vary from 180 to 250 °C, depending on the functionalities present on the benzoxazines or related polymeric precursors. On the other hand, most of the 1,3-benzoxazines can still be cured at lower temperatures such as 150−160 °C using prolonged heating.15,18 Figure S2 shows the DSC profiles of PBP. The precursor is curable and the curing temperature is at 254 °C, which is in accordance with the literature value.29 The proposed healing effect of PBP stems from this curing reaction. As stated, curing of benzoxazines is a kind of ringopening polymerization reaction. The basis of this reaction relies on the benzoxazine ring structure, which has distorted semi-chair conformation. Therewith, the formed ring strain from this molecular structure assists this type of six-membered ring to undergo ring-opening reaction under thermal conditions with a mechanism having two main stages in brief. The first step is the heterolytic cleavage of the C−O bond of the oxazine yielding a carbocation. The second step is the attack of the corresponding carbocation to either the ortho or para position of the neighbor aromatic ring, which can be considered as a kind of Friedel−Crafts reaction (Scheme 5).

Scheme 6. Binding of PBP to PSU and Formation of Possible Hydrogen Bonds between Phenolic −OH Groups and Sulfone SO

Moreover, the physical interactions between polybenzoxazine and PSU chainsmainly through hydrogen bonding of hydroxyl and sulfone groupsmay also have a slight effect on the mixing of the PSU chains and the cured PBP (see Scheme 6). This type of hydrogen bonding was reported for hydroxydiphenylsulfones with the −OH group in the ortho position to the SO2 group.46 The frequency of SO2 symmetric vibration and −OH stretching vibration was found to be lower than that of the para-substituent analogue and observed at 1138 and 3292 cm−1, respectively. Furthermore, ring-opening of oxazine can be monitored using FT-IR (Figure 1).

Scheme 5. Summary of Polymerization Mechanism of 1,3Benzoxazines Figure 1. FT-IR spectra of PSU (a), thermally treated PBP added PSU (b), and thermally cured PBP (c).

The FT-IR absorption bands of the polybenzoxazine resins were previously reported in details.47,48 In Figure 1c, the important bands of infrared absorptions of polybenzoxazine resin observed at 932 and 1496 cm−1 can be attributed to the trisubstituted benzene ring and the peak at 1229 cm−1 is assigned to the asymmetric stretching of C−O−C group of oxazine ring. These peaks are also detected at the self-healed sample (Figure 1b). Moreover, phenolic −OH stretching 8775

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vibration is visible at 3328 and 3300 cm−1 for both cured PBP and self-healed sample, respectively. The shift at the −OH stretching band could be related to the hydrogen bonds between SO and −OH groups. A detailed view of the region below 1700 cm−1 of Figure 1 is included in Supporting Information as Figure S3. The self-healing of such a system stems from the collective effect of the above-mentioned reactions and interactions. In this context, it should be mentioned that the healing process can only be realized once at the event position due to the consumption of the healing agent by nonreversible Friedel− Crafts reactions. Experimentally, the self-healable films were prepared simply by mixing PSU and PBP (10:1 w/w) and the cut specimens were exposed to heat at 160 °C for 4 h in an oven and kept in close contact from the edges of the cut with pressure applied using a glass slip. Heat application starts the curing and concomitantly ensures the transport of PBP to the site of damage. Clearly, in such a system, chain mobility and entanglements are indispensable in order to encounter the polymerizer precursor with PSU. Moreover, in order to verify the contribution of such effects, a control experiment was conducted and no healing was observed for cut films of pristine PSU under the same experimental conditions. Healing of a polymeric material can be regarded as the recovery of a specific property. However, because polymeric materials usually exhibit a wide range of features that can be measured before and after the recovery of healed samples, it is rather difficult to assess the exact degree of the healing. Nevertheless, self-healing efficiency is generally thought as the ratio between the toughness of the virgin specimen and the healed specimen. Thus, the healing efficiency (η) can be quantified by the corresponding tensile test according to the following equation; where K is the toughness of the virgin and healed specimens.33,38,49

Figure 3. Photographs of cut specimen (a) and healed specimen (b).

Figure 4. DSC thermograms of PSU (a), PBP added PSU (b) and healed sample (c).

PSU, and the healed sample. As expected, pristine PSU did not display any exotherm between 160 and 300 °C. However PBP added PSU showed an exotherm with an onset at 221 °C, endset at 271 °C, and a Tg at 192 °C. The curing maximum was detected as 246 °C. The total amount of curing exotherm is 75.1 j/g. After curing of PBP added PSU, the second run did not exhibit any curing exotherm indicating the consumption of the healing agent and showed an endothermic peak at 187 °C may be related to evaporation of side-products formed during thermal treatment or unidentified rearrangement. Moreover, preservation of the sulfone groups of PSU under such thermal conditions was confirmed by detecting asymmetric and symmetric stretching vibration bands of SO at 1325 and 1148 cm−1 using FT-IR (See Figure 1). Thermo gravimetric analysis (TGA) was also employed in order to determine the decomposition characteristics of healed sample compared to pristine PSU. The TGA profiles of the samples are shown in Figure 5 and the results are tabulated in Table 1. Expectedly, the healed sample exhibited higher degradation temperatures of T5%, T10%, and char yields than its corresponding PSU since benzoxazine units generate crosslinked structure in the healed PSU polymer. Accordingly, higher char yield (30%) was obtained. In the case of the healed PSU, benzoxazine linkages hold the overall structure together until the Mannich base cleavage and phenol degradation temperatures of the benzoxazine units are reached. Hence, very high T5% and T%10 are observed.

η (%) = 100 × (K healed /K virgin)

Figure 2 shows the tensile strength of the virgin and healed specimens. The relatively lower tensile strength obtained with

Figure 2. Stress-elongation (%) behavior of virgin specimen (a), pristine PSU (b) and healed specimen (c).

the healed sample is expected as the healing efficiency is found be 55%. To visually demonstrate the recovery, Figure 3 and Figure S4 present the photographs and SEM pictures of the cut and healed specimens, respectively. In order to get more insight into the whole healing activity, the thermal processes involving ring-opening reactions and degradation after curing were investigated independently. Figure 4 shows the DSC profiles for PBP added PSU, pristine

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CONCLUSION An efficient self-healing method for PSUs based on the thermally activated ring-opening polymerization of benzoxazines with attractive features has been demonstrated for the first time. During thermal treatment, polybenzoxazines are 8776

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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Istanbul Technical University Research Fund and B.K. thanks the FABED (Fevzi Akkaya Scientific Activity Support Fund) for financial support by means of a Young Investigator Award.

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Figure 5. TGA thermograms and derivatives of pristine PSU (a, a ) and healed PSU (b, bder), respectively.

Table 1. Thermal Properties of the Virgin and Healed PSUa Samples sample

T5%b (°C)

T10%c (°C)

Tmaxe (°C)

Ycd (%)

pristine PSU healed PSU

157 393

185 511

538 538

22 30

a Curing was performed in TGA at 230 °C for 15 min under N2 stream (200 mL/min.). bT5%: The temperature for which the weight loss is 5% cT10%: The temperature for which the weight loss is 10% dYc: Char yields at 800 °C under nitrogen atmosphere. eTmax: The temperature for maximum weight loss.

chemically bound to PSU chains and act as bridging agents by generating networks. The chain mobility of both the polybenzoxazine precursor and the PSU chains are enough to transport the healing agent (PBP) through the damaged area below Tg of PSU. It was found that for a successful healing it is not necessary to reach the curing temperature of the polybenzoxazine precursor and the required reactions could take place at around 160 °C. The thermal degradation behavior of the healed PSU sample was also investigated and it was found that adding polybenzoxazine increases the overall thermal stability of the PSU film. Consequently, this work is very appealing due to its very simple design and self-healing property supported by convincing mechanical data. It is anticipated that the approach can be applied to other aromatic polymers having high or no melting points such as polyimides, polyphenylenes etc. On the other hand, it appears that solvent casting methodology applied may be a limitation for some industrial applications.

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectrum, DSC thermogram of PBP, FT-IR spectra of PBP, pristine PSU and healed sample, and SEM images of the healed specimen. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

* E-mail: (B.K.) [email protected]. * E-mail: (Y.Y.) [email protected]. 8777

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