Design and preparation of benzoxazine resin with high-frequency low

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Design and preparation of benzoxazine resin with highfrequency low dielectric constants and ultra-low dielectric losses Jiangbing Chen, Ming Zeng, Zijian Feng, Tao Pang, Yiwan Huang, and Qingyu Xu ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00083 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 9, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Polymer Materials

Design and Preparation of Benzoxazine Resin with High-frequency Low Dielectric Constants and Ultra-low Dielectric Losses

Jiangbing Chena, b, Ming Zenga, b*, Zijian Fengb, Tao Pangb, Yiwan Huangc, Qingyu Xub*

a

Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of

Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, P. R. China b

Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, P. R. China c

School of Materials and Chemical Engineering, Hubei University of Technology, Wuhan 430068, P. R. China

Keywords: main-chain benzoxazine，copolymer, high-frequency dielectric properties, glass transition temperature, curing behavior

*

Corresponding author.

E-mail address: [email protected] (M. Zeng). *

Corresponding author.

E-mail address: [email protected] (Q. Xu). 1

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ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

The new main-chain benzoxazine copolymer oligomers with bulky hydrocarbon end groups are firstly designed and synthesized. In particular, the aliphatic diamine based copolymers owning low dielectric constants (< 3) and ultra-low dielectric losses (< 0.005) under high frequencies, is suitable for applications in the field of high-frequency communications. Therefore, this work not only provides a facile and effective protocol to simultaneously obtain excellent high-frequency dielectric properties, and improved processing and thermal properties of benzoxazine resins, but also widens the scope of the design and synthesis of functional and high-performance thermosetting polymers.

2


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The integrated circuit (IC) board, which is fabricated with copper clad laminates (CCLs) and electronic components, plays a critical role in performing the electrical functions of most electrical devices. CCLs consist of a polymeric matrix reinforced with glass fabrics and copper layers. As the key base material, the polymeric matrix mainly determines the dielectric, thermal, and mechanical properties of CCLs.1,2 In pace with the rapid development of high speed and high frequency IC, the design and development of low dielectric polymers have recently attracted widespread attention to increase the data transmission volume and data transmission rate.3,4 The proposed polymers must satisfy a variety of stringent requirements such as low dielectric constant (k) and dielectric loss (f), good heat and chemical resistance, low moisture uptake, good adhesion with metals and semiconductors. Signal propagation loss (L) is proportional to f and square root of k as shown in Eq. (1):5 L= K

F  kf c

(1)

where K is a constant, F and c are frequency and light speed, respectively. Therefore, the reduction of f is more important for the reduction of propagation delay, crosstalk, and power dissipation. However, the conventional thermosetting resins cannot meet the requirements of dielectric properties (k < 3, f < 0.005) in high frequency range (Table S1). In an attempt to improve their performances, thermoset polymers modified with engineering plastics have been reported. Very recently, Lin reported that poly(2,6-dimethyl phenyl oxide) (PPO) containing benzoxazine resin could reach the high-frequency requirement of f value below 0.005 (1GHz), due to the main component of thermoplastic PPO.6 In this paper, we first report, to the best our knowledge, the pure thermosetting benzoxazine resins owning low k (< 3) and ultra-low f (< 0.005) values 3



under high frequencies (5 & 10 GHz). Polybenzoxazines are a new type of thermoset polymers being developed as an alternative to the traditional phenolics, bismaleimides, epoxies, and polyimides. Polybenzoxazines have the attractive advantages of near zero volumetric shrinkage/expansion upon polymerization, high glass transition temperature (Tg), high char yield, excellent mechanical properties, low water absorption, and excellent resistance to chemicals and UV light.7-12 In especial, good dielectric properties with a relatively low and stable k (about 3.5) and f (about 0.02) under high-frequency make benzoxazine a good candidate for the application in the electronics industry.13 The main-chain type benzoxazines containing oxazine rings in the main chain can be synthesized by the traditional Mannich polycondensation of bisphenol and diamine. Then main-chain polybenzoxazines can be derived from the ring-opening polymerization of prepared precursors.14-17 In comparison to the traditional polybenzoxazines, the thermal and mechanical properties of main-chain polybenzoxazines are enhanced. However, high molecular weight main-chain benzoxazine precursors can form the cross-linking network structure via partial ring-opening reaction of oxazine rings, resulting in poor solubility.18 Rich molecular design flexibility of benzoxazine allows a new and simple route to design and synthesize main-chain benzoxazine copolymers.19,20 A series of short-chain oligomers from bisphenol-F, aromatic diamine, and aniline (AN) with formaldehyde were synthesized by Ishida.19 It is worth noting that the prepared oligomers had low viscosity and easy processability, while retained the excellent thermal properties of main-chain benzoxazines. In our laboratory, the copolymer precursors were prepared from bisphenol A, AN, paraformaldehyde, and different types of diamines, including 4,4’-diaminodiphenylmethane (DAM) and 1,6-diaminohexane (DAH).21,22 4


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Although the optimized aromatic and aliphatic main-chain benzoxazine copolymers had the highest Tg and the minimum high-frequency k values, the f values were still not satisfactory. In the light of the chemical structure of p-tert-butylphenol, the bulky –C(CH3)3 groups would be beneficial to increase the free volume and decrease the molecular polarity of corresponding polybenzoxazine, resulting in the improvement of dielectric properties. However, the incorporation of saturated and bulky hydrocarbon groups often leads to the reduction of thermal performance as a cost. By taking advantages of dielectric and thermal properties of p-tert-butylphenol based benzoxazine and main-chain benzoxazine, respectively, the designed main-chain benzoxazine copolymer prepolymers with bulky hydrocarbon functionality as the terminal groups are synthesized in this investigation. The designed copolymer precursors are prepared from the Mannich-type condensation of bisphenol A, paraformaldehyde, diamine, and p-tert-butylphenol in ethanol/toluene heterogeneous solvent. The preparation method is based on our previous reports, and provided in the supporting information.21,22 It is expected that p-tert-butylphenol can react with NH2 groups of main-chain type benzoxazines to form the terminal oxazine groups. Therefore, the bulky hydrocarbon groups can be incorporated in the main-chain benzoxazine copolymer oligomers, resulting in a decrease in molecular weight and molecular polarity of precursors. Two types of copolymer prepolymers (Scheme 1(a)) are prepared based on aliphatic diamine DAH and aromatic diamine DAM, and coded as TAH and TAM, respectively. For comparison, the corresponding main-chain benzoxazines (Scheme 1(b)) and difunctional benzoxazines (Scheme 1(c)) are synthesized and coded as AH and TH based on DAH, and AM and TM based on DAM, respectively. 5



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The FTIR spectra of TAH and TAM are shown in Figures 1(a) and S4, respectively. The asymmetric stretching corresponding to C–N–C and C–O–C of oxazine rings are observed at 1180 and 1234 cm−1, respectively. The characteristic absorption at 1502 cm-1 is assigned to trisubstituted benzene ring. The peak at 944 cm-1 is mainly related to the oxazine ring, indicating that two kinds of copolymer prepolymers are obtained.23 Additionally, a strong absorption band around 3400 cm-1 of both AH and AM represents the existence of NH2 and OH groups for traditional main-chain benzoxazines. Because the proposed main-chain copolymer prepolymers are terminated with oxazine rings, there is no such band for both TAH and TAM. The chemical structures are further investigated by 1H-NMR. The NMR spectrum of TAH is presented in Figure 1(b). The characteristic peaks at 1.38 and 2.70 ppm assignable to the aliphatic diamine chain protons can be found. The multiplet in the range of 6.58-7.48 ppm is assigned to the protons of the aromatic ring. The methylene (-O-CH2-N-) and methylene (Ar-CH2-N-) of oxazine groups appear at 4.81 and 3.92 ppm, respectively, which confirms the oxazine

structure

of

AH.15

The

characteristic

peaks

of

oxazine

structure

for

p-tert-butylphenol/DAH type benzoxazine can be observed at about 4.81 and 3.99 ppm. Both 1

H-NMR and FTIR measurement results suggest that the designed aliphatic diamine based

copolymer oligomer is synthesized successfully. In Figure S5, NMR results also indicate that both aromatic diamine based main-chain benzoxazine (5.29, O−CH2−N, 4.53, N−CH2−Ar) and p-tert-butylphenol/DAM type benzoxazine (5.29, O−CH2−N, 4.59, N−CH2−Ar) coexist in the TAM oligomer. It is noteworthy that there is no hexahydro-s-triazine signal for both TAH and AH. However, the characteristic hexahydro-s-triazine resonance at 4.88 ppm is observed for both TAM and 6


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AM, resulting from the influence of main chain structure and heterogeneous solvent. The solvent effects on the reaction mechanism of copolymer precursors were systematically studied in the previous papers.21,22 There are two steps for the preparation of benzoxazine monomers or oligomers: the formation of a hexahydro-s-triazine network and the dissociation of the resulting hexahydro-s-triazine network by phenols.7 It was found that the polar solvent could not only reduce the hexahydro-s-triazine network formation rate, but also slow down the reaction rate of the intermediate product with bisphenol A for aromatic diamine based copolymer precursors.22 On the contrary, both relatively fast dissociation rate of the hexahydro-s-triazine network and fast cyclization reaction rate were obtained for aliphatic diamine based counterparts prepared in the polar/nonpolar heterogeneous solvents.21 As a result, the time of completing the polycondensation reaction for aliphatic diamine based system was shorter than that for aromatic diamine based system. The data of the molecular weight and polydispersity (PDI) of samples are shown in Table 1. The weight average molecular weight (Mw) of the aliphatic diamine based copolymer prepolymer is lower than that of the corresponding main-chain benzoxazine prepolymer, suggesting that p-tert-butylphenol has an effective terminal functionality to control the molecular weight. It is noted that TAH has a relatively appropriate PDI value (2.38). In addition, the Mw and PDI values of TAM oligomer are 1684 Da and 1.78, respectively. Theoretically, the most probable PDI value equals to 2 for the oligomers derived from polycondensation reactions.15 Therefore, it can be concluded that the optimized ethanol/toluene heterogeneous solvent is a suitable solvent for fabricating the designed aromatic and aliphatic diamine based copolymer precursors. 7



DSC thermograms of TAH and TAM are presented in Figures S7 and S8, respectively, and the summary of the curing parameter values is listed in Table 1. Tonset, Tpeak, and Toffset are the starting, maximum, ending positions of the curing peak, respectively. The enthalpy (△H) is defined as the integration of normalized exothermic peak area. Both TAH and TAM have a broad exothermic peak, implying that the ring opening polymerization of both main-chain benzoxazines and p-tert-butylphenol based benzoxazines occur simultaneously. The exothermic enthalpy values (> 220 J/g) of main-chain benzoxazines and their copolymer oligomers are higher than those of corresponding difunctional monomers, resulting from relatively high molecular weight and corresponding high concentration of ring-closed groups in the main chain. Meanwhile, copolymer oligomers have lower exothermic enthalpies than those of corresponding main-chain benzoxazines due to the decrease in molecular weight and chain length. In comparison with difunctional benzoxazines, Tpeak values of both TAH and TAM decrease. The phenomenon could be explained that phenolic groups have the catalytic influence on the oxazine ring opening, resulting from the phenolic groups of main-chain type benzoxazines, copolymer oligomers, and their corresponding side products.21 Moreover, copolymer prepolymers have relatively lower Mw when compared with those of corresponding main-chain type benzoxazines, and hence possess relatively lower Tpeak. DSC measurement is also conducted to confirm the complete cure of benzoxazines (Figures S9 and S10). Obviously, all of the exothermic peaks corresponding to the ring-opening polymerization of the oligomers disappear. The chemical structures of cured TAH and TAM films are further characterized by FTIR as shown in Figures S11 and S12, respectively. The characteristic oxazine absorption band at 944 cm-1 disappears, which shows the consumption of the oxazine rings.24,25 Both DSC 8


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and FTIR results confirm that the ring-opening polymerization reactions of prepared copolymer precursors are nearly complete. The thermo-mechanical properties for polybenzoxazines are studied by DMA. The loss factor (tan δ) results of TAH and TAM are shown in Figures 1(c) and S13, respectively. The glass transition temperatures are obtained from DMA thermograms and listed in Table 2. Both TAH and TAM show the only one peak, indicating good miscibility of components. Tg values corresponding to TH and TM films are measured to be around 146 and 187 oC, respectively. Compared with the Tg values of difunctional benzoxazine films, the Tg values of two kinds of copolymer films increase significantly. Tg corresponding to TAH is measured to be around 195 o

C and that to TAM at 205 oC. The obvious increases in Tg values are due to the positive effect

of relatively higher molecular weights of copolymer prepolymers. However, Tg values of copolymers are lower than those of pure main-chain polybenzoxazine. Despite the decreases in Tg values, both copolymers still exhibit higher glass transition temperatures than most of traditional difunctional benzoxazines. It is well known that the cross-linking density (ρ) of a thermoset polymer may be estimated from its elastic modulus in the rubbery region by using the following equation Eq. (2):26 E′

ρ=3∅RT

(2)

where ρ is the number of moles of network chains per unit volume of the cured polymer; ∅ is the front factor and equals to 1; E' is the storage modulus of the polymer at the Tg plus 40 oC (Figures 1(d) and S14); and R and T are the gas constant and the absolute temperature, respectively. However, the equation is strictly valid only for lightly cross-linked polymers and therefore is used only to qualitatively compare the level of cross-linking in these cured 9



thermosetting resins.27 As shown in Table 2, the cross-linking degrees increase significantly in the order of main-chain polybenzoxazines > copolymers > difuntional polymers. The thermal stability is investigated by TGA, and the results of TAH and TAM films are presented in Figures S15 and S16, respectively. Data related to the residues at 800 °C are summarized in Table 2. Expectedly, the char yields of TAH and TAM copolymer films are appreciably higher than those of corresponding difunctional TH and TM films. The increased char residues are due to the improved cross-linking density. Therefore, DMA and TGA measurement results indicate that relatively higher cross-linking degrees for copolymer films are considered to be crucial for the improvement of glass transition temperature and thermal stability. TMA experiment is performed in penetration mode in order to measure the expansion and contraction of the cured polymer films under compression as a function of temperature. The obtained data are used to determine the coefficients of thermal expansion (CTE) of polybenzoxazines from the slope of the measurement curves. The typical TMA plot of cured TAH film is shown in Figure S17. The CTE values are obtained from the TMA curves and summarized in Table 2. It is clear that CTE values of both aliphatic and aromatic diamine based copolymers are lower than those of corresponding difuctional benzoxazines and main-chain type benzoxazines, indicating that copolymers have better dimensional stability. The k and f values of polybenzoxazine films are studied at the microwave frequencies of 5 and 10 GHz, and the corresponding data are presented in Table 2. Both k and f values of aliphatic diamine based polybenzoxazine films are significantly lower than those of aromatic diamine counterparts, which can be explained by the positive effect of a less polar backbone for 10


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aliphatic diamine based benzoxazine films. It is interesting that both TAH and TAM copolymer films have the minimum k values among the corresponding aliphatic and aromatic diamine based benzoxazine films. In theory, the dielectric constant is associated with the polarization and polarizability of the electrons that form individual bonds in the polymer matrix.28 The possible reasons for this phenomenon can be explained by the following Debye equation (Eq. (3)):29 k − 1 4 u2 = N (ae + ad + ) k +2 3 3kbT

(3)

where kb is the Boltzmann constant, N is the number density of dipoles, ae is the electric polarization, ad is the distortion polarization, u is the orientation polarization related to the dipole moment, and T and k are the temperature and the dielectric constant, respectively. First, the designed copolymer oligomers are fabricated with terminal oxazine rings, leading to a decrease in polar functional groups. Thus relatively lower ae values can be obtained for both copolymers. Second, low polar and bulky –C(CH3)3 group is able to efficiently reduce the molecular polarity and molecular packing, and increase the free volume, resulting in the decrease of N, ae and ad values.30 Third, in comparison with the difunctional TH and TM films, the movements of copolymer chains for TAH and TAM films are further restricted by their relatively denser chemical cross-linking networks, which also cause the reduction of both N and ad values to some extent. In addition, u values are very small for all cured films, because the

anisotropy of amorphous polymer is tiny. Therefore, two types of main-chain benzoxazine copolymers containing bulky hydrocarbon groups have the advantages of high-frequency low dielectric constants. It is worth noting that TAH film possesses the ultra-low f values (0.0046, 5 GHz; 0.0047, 10 GHz), which is comparable to those of the difunctional p-tert-butylphenol 11



based benzoxazine film. The result further suggests the positive effect of the incorporation of low polar and bulky hydrocarbon groups. In conclusion, the designed two types of main-chain benzoxazine copolymers containing bulky hydrocarbon groups have the attractive advantages of high-frequency low dielectric constants and low dielectric losses, and the enhanced processing and thermal properties. In particular, TAH copolymer has the low dielectric constants (< 3) and ultra-low dielectric losses (< 0.005) under high frequencies, which is satisfied with the stringent requirement of high-frequency communications. Furthermore, the flexible and effective strategy should be generalizable to other thermosetting resins. Therefore, this work possibly provides a general strategy for the structural design and understanding of high-frequency low dielectric thermosetting polymers.

Supporting Information Available Experimental section (materials, preparation methods, and characterization methods); digital photos of cured films (Figure S1); FESEM photographs of surfaces (Figure S2) and fracture surfaces (Figure S3) of cured films; FTIR (Figure S4) and 1H-NMR (Figure S5) spectra of aromatic diamine based benzoxazine prepolymers; original 1H-NMR spectra of prepared samples (Figure S6); DSC thermograms of aliphatic diamine (Figure S7) and aromatic diamine (Figure S8) based benzoxazine prepolymers; DSC thermograms of cured aliphatic diamine (Figure S9) and aromatic diamine (Figure S10) based benzoxazine films; FTIR spectra of cured aliphatic diamine (Figure S11) and aromatic diamine (Figure S12) based polybenzoxazines; loss factor (tan δ) (Figure S13) and storage modulus (E') (Figure S14) results of aromatic diamine based ploybenzoxazines; TGA and DTG thermograms of aliphatic diamine (Figure 12


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S15) and aromatic diamine (Figure S16) based ploybenzoxazines; TMA plot of cured TAH film (Figure S17); conduction current density of copolymer films (Figure S18); rough comparison of dielectric properties of benzoxazine resins (Table S1); breakdown strength and Tg (DSC) data of cured benzoxazine polymers (Table S2).

Conflicts of interest There are no conflicts to declare.

Acknowledgments The authors thank the SRF for ROCS, State Education Ministry (SEM1341), PR China, Hubei Provincial Department of Education (XD2010037), and Engineering Research Center of Nano-Geomaterials of Ministry of Education (CUG2015).

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TOC/Abstract Graphic

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Scheme 1 Preparation of main-chain benzoxazine copolymer prepolymers (a), main-chain benzoxazines (b), and difunctional benzoxazines (c).

19



(a)

(b)

Transmittance (a.u.)

TAH

1234

TAH

TH

1180 AH

TH

1502

944

AH 4000

3500

(c)

3000

2500 2000 1500 -1 Wavenumber / cm

1.5

1000

9

500

8

7

6

(d) AH TH TAH

5 4 3 2 Chemical shift (ppm)

1

0

-1

AH TH TAH

250

200

E' / MPa

1.0 Tan δ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

0.5

150

100

50 0

0.0 100

125

150 175 200 o Temperature / C

Figure 1 FTIR (a) and

1

225

100

150

200

250

300

o

Temperature / C

H-NMR (b) spectra of aliphatic diamine based benzoxazine

prepolymers, and the loss factor (tan δ) (c) and storage modulus (E') (d) results of aliphatic diamine based ploybenzoxazines.

20


Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 The molecular weights and curing process parameters of benzoxazine prepolymers Samples

Mn

Mw

PDI

Tonset / oC

Tpeak / oC

Toffset / oC

△H / J/g

AH

1159

6451

5.57

188

254

281

252

TH

562.0

688.0

1.23

226

243

258

157

TAH

1152

2743

2.38

192

233

283

235

AM

1392

2143

1.54

223

260

315

233

TM

807.0

1133

1.40

245

260

272

171

TAM

945.0

1684

1.78

221

251

287

221

21



Page 22 of 22

Table 2 The thermal properties and high-frequency dielectric properties of polybenzoxazines Crosslink Samples

Tg /

oC

density /

Char / %

mol/m3

k

CTE / ppm/oC

5 GHz

f 10 GHz

5 GHz

10 GHz

AH

226

472

17.1

90.6

2.69±0.01 2.61±0.01 0.0051±0.0001 0.0053±0.0001

TH

146

76.9

9.30

92.9

2.52±0.01 2.54±0.01 0.0044±0.0001 0.0045±0.0001

TAH

195

256

15.4

64.6

2.36±0.01 2.26±0.01 0.0046±0.0001 0.0047±0.0001

AM

245

976

34.7

111

3.14±0.01 3.53±0.01 0.0282±0.0001 0.0243±0.0001

TM

187

5.10

28.8

60.5

2.88±0.01 2.82±0.01 0.0116±0.0001 0.0113±0.0001

TAM

205

121

32.5

52.3

2.77±0.01 2.66±0.01 0.0093±0.0001 0.0091±0.0001

22


Design and preparation of benzoxazine resin with high-frequency low

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