Reaction-induced liquid crystalline polybenzoxazine thermosets using aryl ester bonds as cross-linkages

ABSTRACT The increasing tendency to develop integration in electronic devices lead to the demand for thermally conductive polymeric materials. We offered a simple synthetic route to thermally conductive polybenzoxazine thermosets bearing liquid crystalline structures without the addition of any filler. Benzoxazine monomer of BZ-COOH was synthesized via Mannich reaction of p-hydroxy benzoic acid, p-formaldehyde, and dodecyl amine. After ring-opening polymerization of BZ-COOH, the obtained benzoxazine oligomer of OBZ-COOH was converted into acetoxy-bearing benzoxazine oligomer of OBZ-AC. Through self-crosslinking reaction of acetoxy group and carboxyl group in OBZ-AC using Zn(CH3COO)2 as a catalyst, the polybenzoxazine thermosets of OBZ-PES were produced. The liquid crystalline behavior of OBZ-AC was studied by POM, SAXS, and DSC. The results show that OBZ-AC formed the LC mesophase during isothermal and non-isothermal curing. The LC mesophase displays a layered arrangement of the smectic C phase. The formation mechanism is esterification reaction-induced. Owing to the formation of liquid crystalline structure, the TC of OBZ-PES is found to be 0.284 W/m. K, which is 21% and 16% greater than those of OBZ-COOH and OBZ-AC, respectively. In addition, the OBZ-PES shows excellent thermal performance. Its Tg(G”) is up to 269°C. Its 5% weight loss temperature (Td5) and 10% weight loss temperature (Td10) are 383°C and 410°C, respectively. GRAPHICAL ABSTRACT


Introduction
The electronic devices in integrated circuits have a trend towards higher power and simultaneously with lighter, thinner, and smaller size [1].The size and performance enhancement of devices results in the generation of large amount of heat.This excessive heat causes a lot of potential problems such as malfunctions, reduced durability, and even device explosions.It has recently become one of the issues limiting the progress of lightemitting diode (LED) and integrated circuits.Therefore, the polymer materials with superior thermal conductivity (TC) and elevated heatresistance are required for the efficient removal of heat from the devices during operation [2,3].
However, most polymeric materials are considered as thermal insulators because of their ultra-low thermally conductive coefficient value.Traditionally, one route to improve the thermal conductivity of polymer is the incorporation of high thermally conductive inorganic particles into polymeric matrix such as boron nitride (BN), alumina (Al 2 O 3 ), and carbon nanotubes (CNTs) CONTACT Zaijun Lu z.lu@sdu.edu.cnSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2023.2272763.[4].Although high thermal conductivity is achieved, there is a need for excessive addition of filler (>30 vol %), which offers a challenge to mechanical and processing features of polymer composites [5,6].Another most effective route is the incorporation of crystalline or liquid crystalline structure into the polymer.Reactive liquid crystal molecules, LC acrylate monomers [7], LC epoxy monomers [8], and LC thiol-ene monomers [9] are the utmost outstanding basic units for preparing liquid crystalline polymers.
Recently, a novel class of thermosetting phenolic resin named benzoxazine resin has been found that has very high heatresistance, high chemical resistance, low dielectric constant, flame retardance, molecular design flexibility, and low cost [10].It also displays excellent dimensional stability and undergoes ring-opening polymerization without the release of any by-product [11][12][13].It is actively developed as high potential electronic materials, adhesives, precision machinery components, carbon fibre reinforced plastic matrices, and aerospace materials [14][15][16][17].For example, Takeichi et al. reported that the polymers comprising benzoxazine rings in the main chain display outstanding mechanical features and high thermal stability [18].However, polybenzoxazines have low thermal conductivity like other traditional resins.
To improve the intrinsic thermal conductivity of benzoxazine resin, many chemists have explored the liquid crystal benzoxazines.For example, Ishida et al. [19] reported the first synthesis and the phase behavior of LC benzoxazines containing a cyanobiphenyl mesogen.Kawauchi et al. [20] synthesized LC benzoxazine monomers containing ester and azomethine linkages in the mesogenic unit and aliphatic tails.Ito et al. [21] reported an increase in LC temperature range by introducing more rigid biphenyl groups instead of phenyl groups into the mesogenic moiety.Despite some successes, those LC benzoxazine monomers lose their liquid crystal behaviour after polymerization.It might be due to the rigid chain structure and cross-linking structures of polybenzoxazines [22].In view of this, Liu et al. [23] proposed an indirect method to synthesize the LC structures via interpenetrating network formation of liquid crystalline epoxy resin and benzoxazine resin.
More recently our group [24] reported the LC polybenzoxazine thermosets using cholesterol-based mesogen side groups.It might be caused by cholesterol's very strong ability to form liquid crystalline structures [25].This inspired us to incorporate an aromatic ester component into polybenzoxazine network, considering that polyarylates are a very typical liquid crystalline polymer and have the powerful ability to form liquid crystalline structures.Herein, we explore the LC polybenzoxazine thermosets using aryl ester bonds as cross-linkages.Specifically, the benzoxazine monomer bearing carboxyl group (BZ-COOH) occurred the ring-opening polymerization to yield benzoxazine oligomer (OBZ-COOH).Subsequently, the OH of OBZ-COOH was replaced by active acetoxy group via acetylation reaction.Finally, acetoxy group reacted with the carboxyl group of OBZ-AC to yield aryl ester cross-linkages.We have also studied the formation mechanism of liquid crystalline phase during non-isothermal and isothermal curing and furthermore their heat conductivity properties.

Synthesis of benzoxazine oligomer bearing carboxylic group (OBZ-COOH)
To a 100 mL flask, BZ-COOH (0.90 g, 2.6 mmol), p-tertbutyl phenol (0.10 g, 10 wt.%), and tetrahydrofuran (5 mL) were added.The p-tert-butyl phenol is used as an initiator for ring-opening polymerization.The mixture was poured in an aluminum mold and heated in an oven at about 60°C for 12 h and at 150°C for 3 h, respectively, according to literature [27].After cooling to room temperature, the crude product was dissolved in n-hexane and then precipitated in methanol to remove by-products according to literature [28].The precipitates are filtered and dried overnight in vacuum.An orange product was obtained (yield 60%).

Self-crosslinking of OBZ-AC (OBZ-PES)
To a 50 mL of flask, OBZ-AC (1.74 g, 4 mmol), Zn (CH 3 COO) 2 (1 wt.%), and DMAc (5 mL) were added and refluxed (under inert atmosphere with mechanical stirring) at 140°C for 3 h.Then, the mixture was poured into aluminum mold and dried overnight at 80°C.The cross-linking reaction was carried out in an air-circulating programmed oven by heating at 160°C, 180°C, 200°C, 220°C and 240°C for 2 h at each temperature.

Characterization
Size Exclusion Chromatography (SEC) measurement was performed using a 515 liquid chromatograph equipped with 3 styragel columns and 2414 refractive index detector (Waters, USA) at 40°C in THF (1 mL min −1 ) as eluent.A 10 mg sample was dissolved into 1 mL of THF.
1 H NMR spectra were carried out on an Avance-400 MHZ NMR spectrometer (Bruker, Switzerland) at room temperature using dimethyl sulphoxide-d 6 as deuterated solvent.The 10 mg solid sample was dissolved into 1 mL of solvent.
Solid-state 13 C (150.03 MHz) CP MAS NMR spectra were performed on a Bruker NEO 600 MHz spectrometer which was equipped with a 14.04 T magnet using Bruker 3.2 mm MAS probe.The 20-30 mg of powder sample was packed into 3.2 mm ZrO 2 rotor and spun at 15 kHz for MAS experiments.
FTIR spectroscopy was performed on a Tensor II FTIR spectrometer (Bruker, Switzerland) from 4000 cm −1 to 400 cm −1 with a resolution of 4 cm −1 .Samples were constrained into KBr pellets for analysis.The 5-10 mg samples were pressed into KBr pellets.
An OLYMPUS BX-51 polarising optical microscope (POM) equipped with a LINKAMTHMSE 600 hot stage and Q-imaging Micropublisher 5.0 RTV (CCD) camera was performed to record LC textures of samples.The heating/cooling rate was set at 10°C min −1 .The sample was sandwiched between glass slides.The dimension of the sample used was 5 × 5 × 0.1 mm 3 .
A SAXSess MC2 high-flux small-angle X-ray scattering instrument (Anton Paar, Austria with Ni-filtered Cu Kα radiation (λ = 0.154 nm) operated at 40 kV and 50 mA was performed to record small-angle X-ray scattering (SAXS) profiles.The measurement temperature was kept at 25°C controlled by a standard temperature control unit (Anton Paar TCS 120) connected to the SAXSess.The sample is 27.8 cm away from the detector.The dimension of the sample used was 5 × 5 × 0.1 mm 3 .
DSC was carried out by using DSC 822 e instrument (Mettler-Toledo, Switzerland) at a heating rate of 10°C min −1 from 50°C to 300°C under nitrogen atmosphere at a flow rate of 50 mL min −1 .The weight of the sample used was 5-10 mg.
Dynamic mechanical analysis (DMA) was carried out on Mettler-Toledo DMA/SDTA 861 e instrument (Mettler-Toledo, CH).Using a solution casting method, the OBZ-AC solution is poured into a rectangular mold of 50 × 100 mm 2 size.The curing temperatures were 120°C, 140°C, 160°C, 180°C and 200°C, respectively.The cured sample was smoothed on the sandpaper and cut into dimensions of about 5.0 × 5.0 × 0.8 mm 3 .The sample was heated from 30°C to 300°C.The measurement used shear mode with a heating rate of 3°C min −1 .
Thermogravimetric analysis (TGA) was obtained using a TGA/DSC STAR e instrument (Mettler-Toledo, CH) in nitrogen atmosphere.The heating rate was kept at 10°C min −1 and the gas flow rate was 100 mL min −1 .The weight of the sample used was 5-10 mg.
Laser thermal conductivity was measured using (Nano-Flash apparatus LFA 457 NETZSCH, Germany) under a nitrogen atmosphere at a flow rate of 80 mL min −1 .Using the solution casting method, the OBZ-AC solution is poured into the rectangular mold of 50 × 100 mm 2 .The curing temperature is 120°C, 140°C, 160°C, 180°C and 200°C, respectively.The cured sample was smoothed on the sandpaper and cut into dimensions of about (10 × 10 × 1 mm 3 ).

Synthesis and characterization of polybenzoxazine thermosets bearing aryl ester bonds (OBZ-PES)
Scheme 1 illustrates the synthetic route to polybenzoxazine thermosets bearing aryl ester bonds (OBZ-PES).Firstly, para-hydroxy benzoic acid (HBA) was reacted with dodecyl amine (DDA) and p-formaldehyde in 1:1:2 ratio by Mannich reaction to synthesize BZ-COOH monomer.Subsequently, the BZ-COOH carried out thermal ringopening polymerization to yield the oligomer of OBZ-COOH.The phenolic group of OBZ-COOH was converted into acetoxy group (OBZ-AC) by employing acetic anhydride under the catalysis of sulphuric acid.The acetoxy group is considered as an attractive activator for the formation of aryl ester bond.Finally, polybenzoxazine thermosets were formed by self-crosslinking reaction of acetoxy groups with COOH groups of OBZ-AC.This cross-linking reaction was speed up by using Zn(CH 3 COO) 2 as a catalyst [29] Figure 1 shows the SEC chromatograms of BZ-COOH, OBZ-COOH and OBZ-AC.As compared to BZ-  COOH, the SEC trace of OBZ-COOH shifts to higher molecular weight region, indicating the ring-opening polymerization of benzoxazine monomer and the formation of benzoxazine oligomer.In addition, the elution time of OBZ-AC is shorter than that of OBZ-COOH, indicating the successful acetylation of OBZ-COOH oligomer.The number average molecular weight (Mn) and polydispersity index (PDI) of OBZ-COOH and OBZ-AC are 1390 g/mol and 1550 g/mol, and 1.49 and 1.78, respectively (Calculated from MALDI-TOF MS).
Figure 2 shows the 1 H NMR spectra of (A) BZ-COOH, (B) OBZ-COOH, and (C) OBZ-AC.In Figure 2 ).This suggests the successful synthesis of the benzoxazine monomer of BZ-COOH.The peaks from 6.77 ppm (g) to 7.66 ppm (f) are ascribed to the aromatic protons [30].
After ring-opening polymerization, the peaks at 4.91 ppm (b) and 3.99 ppm (a) in Figure 2 In Figure 2(c), the new peak at 1.77 ppm (i) is attributed to the methyl protons of acetoxy group.Based on the integral values of the peak at 1.77 ppm (i) and peak at 0.84 (d), the integrated intensity ratio is 3.06:2.91,which is in good agreement with the theoretical value of 3:3 (see S3).
Figure 3 shows the FTIR spectra of (a) BZ-COOH, (b) OBZ-COOH, (c) OBZ-AC, and (d) OBZ-PES.In Figure 3(a), the peak at 925 cm −1 corresponds to the oxazine ring.In Figure 3(b), the peak at 925 cm −1 corresponding to the oxazine ring nearly disappears, and meanwhile the intensity of the peak at 1465 cm −1  corresponding to the tetrasubstituted benzene ring increases.The appearance of a new peak at 3525 cm −1 corresponds to the phenolic hydroxyl group of benzoxazine oligomer [31,32].All these findings suggest the ring-opening polymerization of the oxazine ring.
In Figure 3(c), the disappearance of the phenolic hydroxy peak at 3525 cm −1 and the appearance of C=O peaks of the acetoxy group at 1763 cm −1 indicates the successful acetylation reaction.Moreover, in Figure 3(d), the crosslinking/esterification reaction of the OBZ-AC is indicated by the disappearance of the peak at 1763 cm −1 due to carbonyl groups of acetates and the appearance of new peak at 1735 cm −1 due to carbonyl groups of aromatic esters (see Figure 3(d)).

Curing behavior of OBZ-AC
Figure 4 shows the DSC thermogram of OBZ-AC.T onset and T max of the exothermic peak are 160°C and 263°C, respectively.We think that the exothermic peak is attributed to the occurrence of an esterificationcrosslinking reaction.
The esterification is further studied by a thermogravimetric analyser (TGA) at a heating rate of 10°C min −1 (see Figure S4).A 23% weight loss was observed at 263°C.On the assumption that only the side product of acetic acid evaporated during curing, the yield of the ester bonds is calculated according to the formula [33].
where P is the weight loss percent, W 1 and W 2 are the relative molecular weights of acetic acid and OBZ-AC structure unit, respectively.The calculated yield of the ester bond at 263°C is 90.6%.
The solid-state 13 C NMR spectrum of OBZ-PES also proved the formation of aromatic ester bond (see S5).The new peak at 174.82 ppm (f) [34] is ascribed to carbon resonance of aromatic ester bond, indicating the occurrence of an esterification reaction.
Figure 5 shows the POM images of OBZ-AC at different temperatures upon heating at 10°C min −1 .Upon heating at 160°C, the appearance of black image indicates an amorphous phase.At 180°C, the birefringent texture appears, indicating the formation of LC mesophase.The bright region of LC birefringent texture increases with the increase in temperature.Above 260°C, there is no more increase in the birefringent texture, indicating that a further increase in temperature does not promote any enhancement in LC mesophase because of completion of esterification reaction.It is noted that the LC birefringent texture still exists after the temperature decreases to room temperature (RT).

Liquid crystalline behavior of OBZ-AC during isothermal curing
Figure 6 shows the POM images of OBZ-AC during isothermal curing at 270°C for different times.After curing for 1 min and 3 min, no LC birefringent image is present, indicating the isotropic phase.However, as the reaction continues for 6 min, the layered pattern appears clearly (see Figure 6 Moreover, we can also observe a continuous increase in the layered region after 17 min (see Figure 6(d)).A small increase in birefringent texture is observed after curing for 30 min while the changes hardly happened after curing for 60 min (see Figure 6(e,f)).We think that the observed birefringent texture was attributed to the orderly arrangement of aromatic ester molecular segments.As the esterification reaction proceeds, the length of aromatic ester structural units in polymer chain becomes longer and longer.This is beneficial for the orderly arrangement of rigid molecular segments.
Figure 7 shows the SAXS profiles of OBZ-AC (a) before and (b) after curing at 270°C for 60 min.In spectra Figure 7(a), no clear peak can be observed because of the amorphous nature.After curing, two types of peaks can be seen in Figure 7(b).The broad peaks at q 1 = 0.9 nm −1 , q 3 = 12.36 nm −1 and q 4 = 14.22 nm −1 correspond to the short-range-ordered layered arrangement [35,36] and the sharp peak at q 2 = 6.7 nm −1 corresponds to the long-range-ordered layered arrangement, confirming the smectic C phase structure.Furthermore, based on Bragg's equation (d = 2π/q), the calculated interlayer spacing is 2.3 nm [37,38].
Based on the above experimental results, we think that the formation mechanism of liquid crystalline mesophase is a reaction-induced process.At the initial stage of curing, the OBZ-AC oligomer is in an amorphous state.As the intermolecular esterification reaction goes on, more and more aryl ester bonds are produced.The rigid aryl ester units aggregate into a locally ordered layered structure, forming a liquid crystalline mesophase.POM images appear to have bright patterns.Therefore, the LC formation of OBZ-PES is mainly induced by chemical reaction.Furthermore, the LC regions increase till completion of the esterification reaction.Finally, the LC mesophase is fixed by the self-cross-linking network (see Scheme 2).

Thermal properties of OBZ-PES
Figure 8 shows the thermal conductivities (TCs) of OBZ-COOH, OBZ-AC and OBZ-PES.The TC of OBZ-PES is as high as 0.284 W/m•K, which is 21% and 16% greater than those of OBZ-COOH and OBZ-AC, respectively.The high thermal conductivity of OBZ-PES is caused by its liquid crystalline mesophase.This ordered structure facilitates its heat conduction.
Figure 9 displays DMA curves of cured OBZ-PES.From the specific peaks in G" and tan δ curves, the glass transition temperatures are at 269°C and 290°C, respectively.Generally, the rigidity of polymeric chains and  the crosslinking density of polymers can significantly influence Tg values of thermosets [39].We think that the occurrence of high crosslinking densities and rigid aryl ester units in OBZ-PES leads to high Tgs.
Figure 10 shows TGA curves of OBZ-COOH, OBZ-AC, and OBZ-PES.The 5% weight loss temperature (T d5 ) and 10% weight loss temperature (T d10 ) for OBZ-PES are as high as 383°C and 410°C, respectively.This is due to its high crosslinking density and high aromatic ester bond energy.In addition, the char yield of OBZ-PES was 49%, which is much higher than those of OBZ-COOH (16%) and OBZ-AC (17%).It is clearly observed    that the aryl ester cross-linkage caused the outstanding enhancement in thermal properties.All data are listed in Table 1.

Conclusion
We have successfully synthesized the polybenzoxazine thermosets using aryl ester bonds as cross-linkages.The benzoxazine monomer was synthesized by Mannich reaction of p-hydroxy benzoic acid, dodecyl amine and, p-formaldehyde in 1:1:2 ratio.The oligomer of OBZ-COOH was yielded via thermal ring-opening polymerization.The acetylation of phenolic group produced the oligomer of OBZ-AC.After self-crosslinking, polybenzoxazine thermosets of OBZ-PES were produced.
The LC mesophase of cured OBZ-PES is confirmed by POM images and SAXS profiles.The LC mesophase displays the layered arrangement of smectic C phase.The formation mechanism is induced by the esterification reaction.
Owing to the formation of liquid crystalline structure, the TC of OBZ-PES is found to be 0.284 W/m•K, which is 21% and 16% higher than those of OBZ-COOH and OBZ-AC, respectively.In addition, the OBZ-PES shows excellent thermal performance.Its T g(G") reaches 269°C.Its 5% weight loss temperature (T d5 ) and 10% weight loss temperature (T d10 ) for OBZ-PES are 383°C and 410°C, respectively.
Overall, we demonstrate a simple and feasible method for the preparation of LC polybenzoxazine thermosets with high thermally conductive and heatresistant performances.
(a), the characteristic resonance peaks of Ar-CH 2 -N (a) and O-CH 2 -N (b) are present at 3.99 ppm and 4.91 ppm, respectively, indicating the formation of an oxazine ring.The peak at 0.84 ppm (d) is ascribed to the protons of -CH 3 in n-dodecyl amine.Their integral ratios are 1.97:1.98:3.09,which are consistent with theoretical values of 2:2:3 (see S1 (b) corresponding to O-CH 2 -N disappear completely, and a new peak at 3.55 ppm appears, indicating the successful ring-opening polymerization of BZ-COOH.The integrated intensity of ratio of peaks at 0.84 ppm (d) and 3.55 ppm (a) is 3.00:3.99(see S2), which is in good agreement with the theoretical yield of 3:4.
Figure6shows the POM images of OBZ-AC during isothermal curing at 270°C for different times.After curing for 1 min and 3 min, no LC birefringent image is present, indicating the isotropic phase.However, as the reaction continues for 6 min, the layered pattern appears clearly (see Figure6(c)), indicating the formation of LC mesophase.It is further confirmed by fluidity upon pressing.Moreover, we can also observe a continuous increase in the layered region after 17 min (see Figure6(d)).A small increase in birefringent texture is observed after curing for 30 min while the changes hardly happened after curing for 60 min (see Figure6(e,f)).We think that the observed birefringent texture was attributed to the orderly arrangement of aromatic ester molecular segments.As the esterification reaction proceeds, the length of aromatic ester structural units in polymer chain becomes longer and longer.This is beneficial for the orderly arrangement of rigid molecular segments.Figure7shows the SAXS profiles of OBZ-AC (a) before and (b) after curing at 270°C for 60 min.In spectra Figure7(a), no clear peak can be observed because of the amorphous nature.After curing, two types of peaks can be seen in Figure7(b).The broad peaks at q 1 = 0.9 nm −1 , q 3 = 12.36 nm −1 and q 4 = 14.22 nm −1 correspond to the short-range-ordered layered arrangement[35,36] and the sharp peak at