Improved properties of PTFE composites filled with glass fiber modified by sol–gel method

PTFE/GF(glass fiber) composites are widely applied in high-frequency printed circuit board (PCB) substrate materials due to the excellent dielectric properties of PTFE and the low thermal expansion coefficient of GF. However, the poor interface compatibility between PTFE and GF affects the performance of the composite substrates. In this study, tetraethyl orthosilicate (TEOS) was used as the silicon source, and polydimethylsiloxane (PDMS) was the organic precursor to modify the surface of GF through the sol–gel method to promote the interface compatibility of GF and PTFE. The modified GF noted T-GF was filled in PTFE to prepare PTFE/T-GF composites. SEM, FTIR, XPS, and contact angle confirmed that organic–inorganic hybrids were successfully loaded on GF's surface. Moreover, compared with PTFE/GF and the conventional coupling agent modified GF filled PTFE composites, the PTFE/T-GF exhibited improved dielectric constant (2.305), decreased dielectric loss (9.08E−4), higher bending strength (21.45 MPa) and bending modulus (522 MPa), better thermal conductivity (0.268 W/m*K) and lower CTE (70 ppm/°C), making it has promising application as the substrate materials for high frequency PCB.


Introduction
The rapid development of 5G communication technology requires the printed circuit board (PCB) substrate as the carrier to transmit high-speed signal should possess stable dielectric constant and low loss in the high-frequency range [1]. Polytetrafluoroethylene (PTFE) has become one of the most competitive materials for PCB substrate in high frequency as it is the polymer with the best dielectric properties due to its symmetric molecular structure and extremely low polarizability [2]. However, the disadvantages of PTFE, such as the high linear coefficient of thermal expansion (CTE * 109 ppm/°C) and poor creep resistance, limit its applications [3]. One of approaches to decrease the high CTE of PTFE polymer is adding ceramic powders such as A1 2 O 3 [3], SiO 2 [4], TiO 2 [5], CaTiO 3 [6], MgSO 4 [7] or glass fiber (GF) to PTFE matrix [8,9]. The dielectric properties of PTFE composites are affected by the dielectric properties of the filler and its filling content. GF has a lower dielectric constant, and due to its large aspect ratio, it requires less filling content than ceramic fillers with spherical structure to achieve the purpose of reducing the CTE of the materials. Therefore, GF reinforced PTFE composite substrates is more appropriate for high frequency PCB.
However, the surface properties between PTFE and GF are incredibly different, leading to incompatibility at the interface, increasing the porosity of the composites and further affecting the dielectric and mechanical properties [10]. Therefore, the problem of interface compatibility between GF and PTFE needs to be completely resolved. By summarizing previous research, it can be found that there are usually two ways to improve the compatibility of PTFE and inorganic filler. One is to make the interface between PTFE and inorganic filler form a physical chimeric connection by increasing inorganic fillers' surface roughness. Han [11] introduced hollow silica microspheres with porous surfaces into PTFE, which improved the inorganic/organic interfacial adhesion, and obviously hindered the expansion of PTFE. But the pores induce air phase into the composites enlarges it's dielectric loss. The second is to treat the surface of inorganic filler with coupling agent [12][13][14] to make PTFE (SP * 12.7) and inorganic filler have similar Solubility Parameter (SP), thereby increasing the wettability of PTFE on the surface of inorganic filler and improving the interfacial adhesion [15]. Yao [16] used F8261 coupling agent to treat ceramic and GF's surface, made the water absorption of the composites reach 0.12%, and the dielectric loss decreased by 0.0024. Although studies have shown that coupling agents can improve the interface compatibility between PTFE and GF. However, the problem of high porosity still cannot be solved [17]. In addition, fluorine-containing silane coupling agents are harmful to the environment.
The sol-gel method [18][19][20][21][22] is a reliable method to coat organic-inorganic hybrid layers on the fiber surface to improve its compatibility with the polymer matrix. In our previous work [22], tetraethyl orthosilicate (TEOS) used as the silicon source connected the silane coupling agent KH550 and polypropylene PP-g-MAH (MPP) to modify the surface of PET fiber by sol-gel method. A layer of organic-inorganic hybrid film was formed on the surface of PET fiber. Moreover, the modified PET fiber filled PP composites showed outstanding interfacial and mechanical properties. Zhu et al. [23] also used TEOS as the silicon source and GPTMS as the silane coupling agent to modify PBO fiber. The formed SiO 2 nano-coating significantly improved the interface compatibility between PBO fiber and epoxy resin. Therefore, it may be useful to modify GF by forming a hybrid coating through the sol-gel method. In this study, inorganic layer from the hydrolysis and condensation of TEOS were designed to connect with the polydimethylsiloxane (PDMS) molecular chains through condensation reaction, which formed an organic-inorganic hybrid film on the surface of GF. In this way, not only is the surface roughness of GF increased without introducing holes, but also the improvement of SP on the GF surface is realized (PDMS has similar SP (14.9) to PTFE), which is expected to enhance interfacial bonding and suppress thermal expansion of composites. Moreover, the coupling agents modified GF, and untreated GF filled PTFE composites were also prepared for comparative analysis. The effects of different surface modifications of GF on the interface and dielectric properties between the fiber and the matrix were systematically studied.

Surface modification of GF
2.2.1.1 Preparation process of modified GF by combination of TEOS and PDMS The schematic illustration of sol-gel modification of GF is shown in Scheme 1. In our previous work [24], the ratio of TEOS to PDMS was 2:1, the organic-inorganic hybrid superhydrophobic layer can be successfully coated on the surface of PET fiber. Therefore, 15 wt.% TEOS and 7.5 wt.% PDMS, mass ratio to GF, were dissolved in ethanol under stirring for 6 h at room temperature, where the volume of ethanol is 20 times that of TEOS. Simultaneously, the 0.05 M NH 4 OH solution was added slowly until the pH was adjusted to 9 to accelerate the condensation reaction. Then GF washed with 1 M HCl solution was added into the modified solution under stirring for 12 h. Finally, the modified GF noted as T-GF was obtained after drying in the oven at 50°C for one day.

Preparation of modified GF with the coupling agents
For comparison, the conventional coupling agents modified GF was prepared.
K-GF modified with the mixture of KH550 and Z6124 was prepared by the following process. Firstly, at 1.2 wt.% KH550 and 0.3 wt.% Z6124, mass ratio to GF, were dissolved in ethanol and deionized water to obtain coupling agent solution, where the volume ratio of deionized water: coupling agent: anhydrous ethanol = 1:1:8. Then coupling agents were hydrolyzed at 55°C for 1 h, and the pH was maintained between 4 and 6 adjusted by acetic acid. Next, GF was added to the coupling agent solution and stirred continuously for 6 h. Finally, the mixture was dried in an oven at 80°C for 12 h.
F-GF modified with F8261 coupling agent was similarly prepared by the above method, and the amount of F8261 was fixed at 1.5 wt.% of GF.

Preparation of PTFE/GF composites
The PTFE and modified GF were first mixed in the mass ratio of 92:8 in a high-speed mixer under the speed of 2000 r/min for 5 min with ethanol as solvent, then filtrated 300 mesh sieve and dried at 90°C for 24 h. A high-speed grinder was used to smash the dried dough and got the PTFE/GF composite powders. Later, the powders were cold pressed at 15 MPa for 5 min, after that, the shaped rectangular sheets were sintered for 2 h under 370°C and finally PTFE/ GF composites were obtained.

Measurements
Fourier transform infrared (FTIR) spectra of untreated GF and modified GF were performed on Nicolet 6700 FTIR.
X-ray photoelectron (XPS) was used to analyze GF's surface characteristics on an ESCALAB 250Xi X-ray electron spectroscope (Thermo Fisher Co., USA) equipped with Al-Ka radiation as X-ray source.
The GF's contact angle was measured by the HARKE-SPACA contact angle measuring instrument according to the sessile drop method.
The dielectric constant and dielectric loss of the PTFE/GF composites were measured by the Agilent E8362B network analyzer using the strip-line resonance method. These dielectric properties were tested at the resonant frequency of 10 GHz.
PTFE/GF composites' bending properties were conducted by the three-point bending method on the universal mechanical machine (CMT4024-20KN, Shenzhen Sans Co., China). At least five specimens were tested for each type of composites.
The water absorption of the composites was measured according to IPC-TM-650 2.6.2.
The thermal conductivity was measured by the laser flash method (Netzsch, LFA447, Germany) on circular specimens.
The CTE was tested according to the standard of IPC-TM-650. Before testing, the samples were immersed in isopropyl alcohol and mixed for 20 s, then dried at 110°C for 1 h and cooled to room temperature.

Characterization of surface modification
The surface modification of GF can be verified by detecting functional groups on its surface by FTIR, as presented in Fig. 1. Compared to GF, the T-GF infrared spectrum appears three new peaks, the peak at 2962 cm -1 is attributed to the stretching vibration of C-H of TEOS and PDMS. The absorption bands at 1259 cm -1 and 800 cm -1 correspond to the C-O bond and Si-C bond's stretching vibration peak produced by TEOS and PDMS, respectively, proving that TEOS and PDMS have been loaded the surface of GF. In addition, in the range of 900 -1100 cm -1 , the band of Si -O -Si groups shifted to a higher peak wavenumber and became wider, confirming the condensation reaction between Si -OH on TEOS and terminal OH groups of PDMS formed a crosslinked structure.
In the spectrum of F-GF, two prominent characteristic peaks emerge at 2962 cm -1 and 2926 cm -1 , pertaining to CH 3 and CH 2 anti-symmetric stretching vibration, respectively, which originate from F8261. This means that GF loaded with F8261 silane coupling agent has been successfully manufactured. K-GF also appeared CH 2 CH 3 stretching vibration absorption peaks. In addition, an absorption peak emerged at 3100-3300 cm -1 , attributed to the stretching vibration peak of NH 2 in KH550, verifying that KH550 has been connected to the GF surface. The absorption peak at 1557 cm -1 corresponds to the stretching vibration peak of C = C of the benzene ring, and the double peaks at 737 cm -1 and 700 cm -1 are the out-of-plane bending peak of the unitary substituted C-H bond in the benzene ring, which confirms that Z6124 has been successfully loaded on the GF surface. XPS provides information on molecular structure and valence state, and also provides information on element composition and content of various materials for chemical research further to analyze the surfacemodified effect of GF through XPS. Figure 2 shows the XPS spectra of F-GF, T-GF, and K-GF. It could be seen that the F-GF appears a pronounced F1s peak, and the peak of N1s appears in the XPS spectrum of K-GF. Combined with the results of FTIR, the surface of GF was successfully modified by silane coupling agents, respectively. Figure 3 depicts the Si2p XPS spectra of GF and T-GF. As shown from the spectra, in addition to the fitting peaks at 103.2 eV and 102.36 eV (corresponding to the binding energy of silicon elements in SiO 2 and CaSiO 3 ), the surface of T-GF also shows the Si-O bond and Si-C bond, corresponding to the binding energy of 101.2 eV and 100.8 eV, respectively. The Si-O bond was formed by the hydrolytic condensation reaction of TEOS, while the Si-C bond was formed by the dehydration condensation of PDMS and TEOS, thus further illustrating that the hydrolytic condensation reaction of TEOS and PDMS has taken place successfully. Figure 4 shows the microscopic morphology of modified GF and GF. It is observed that the surfaces of unmodified GF and conventional coupling agent modified GF are smooth except for some GF debris. After sol-gel modification, the surface of T-GF exhibits a rough surface, which is associated with the organic-inorganic hybrid coating formed by the hydrolysis and condensation of TEOS and PDMS. To further analyze the coating distribution on the GF, the modified GF were subjected to EDS analysis. As shown in Fig. 4e, f, g, the N, F, and C elements are uniformly distributed on the surface of K-GF, F-GF and T-GF, respectively, indicating well-distributed coverage of the silane coupling agents and the organic-inorganic hybrid layer coating on the GF.
Their SP influences the compatibility of matrix and filler in the composites and fundamentally depends on the polarity of functional groups of the materials, characterized by surface energy. The water contact angle can reflect the material's surface energy, so the compatibility of modified GF with PTFE could be judged by measuring the modified GF's water contact angle. The surface energy of PTFE is minor at 18.5 mN/m. According to Young's formula [25], the larger the water contact angle of a material, the lower its surface energy, and the better its compatibility with PTFE. The contact angles of K-GF, F-GF, and T-GF are displayed in Fig. 5. The unmodified GF is fully waterabsorbing. But after surface modification, GF changed from hydroscopic to hydrophobic, and the contact angles are greater than 100°. The low surface energy of organic molecular chains is the main reason for the hydrophobicity of modified GF. T-GF displays the largest contact angle, reaching 117°, it was predicted that the compatibility of PTFE and T-GF would be dramatically improved.

The microstructure and properties of PTFE/GF composites
The interface is an important factor affecting the properties of composites. The two-phase interface in PTFE/GF could be observed in Fig. 6. Figure 6a shows that the interface adhesion between GF and PTFE matrix is weak for apparent gaps appear on the interface, resulting from the large surface energy difference between PTFE and GF. As shown in Fig. 6b, c, although the coupling agent treatment reduces GF and PTFE gaps to a certain extent, the modification effect is still limited. Figure 6d shows that the interface gaps between T-GF and PTFE almost disappear, and the surface of the T-GF and PTFE matrix form a better interface connection. The increase in surface roughness of GF increases its' specific surface area, and the coating of organic molecular chains leads to molecular entanglement between the fiber and the matrix. All these changes led to the improvement in the interfacial bonding The dielectric constant of the PCB substrate materials determines the signal transmission rate, and the dielectric loss affects the signal attenuation and is the key factor to produce heat loss. Therefore, the dielectric constant and dielectric loss are the essential performance characteristics of the PCB substrate material. Figure 7a shows the dielectric constant and dielectric loss of the PTFE/GF composites at 10 GHz. PTFE/GF, PTFE/T-GF and PTFE/K-GF display slightly lower dielectric constant and higher dielectric loss than PTFE/T-GF. As described above, the interface connection between GF, K-GF and F-GF and PTFE is weak. The air in the interfacial gap will reduce the dielectric constant of the materials. Otherwise, the gaps between two phases will lead to additional interface polarization, causing electrons or ions in the dielectric to gather at the interface, resulting in the increase of dielectric loss. Sol-gel modification grafts organic-inorganic hybrid on the surface of GF, which makes GF and PTFE have a good affinity, resulting in a strong PTFE/T-GF interface connection and a dense interface structure.
Thereby the dielectric loss of PTFE/T-GF is lower (tand = 9.08E -4 ) than those of untreated, and the coupling agents modified GF filled PTFE.
The dielectric properties of the composites are essentially the result of each component's dielectric properties and the interaction of the interface properties formed between them. Many theories have been used to predict the dielectric constant of the composites [26][27][28][29], the equations of these models were enclosed as following: Effective medium theory (EMT) [27] Modified Lichtenecker equation [29] log e c ¼ V 2 ð1 À nÞ logð Maxwell-Wagner equation [26] where e c , e 1 , and e 2 represents the dielectric constant of the composites, the matrix and fillers, respectively, V 2 is the fillers' volume fraction, m in Eq. (1) relates to a morphology factor, while n in Eq. (2) is a fitting parameter. Further, the parameter m in this paper is set to 0.17, which is the same as the PTFE/CNT composites reported by Lin [30]. Besides, the Modified Lichtenecker model matches well with experimental data when factor n is fixed at 0.28. Figure 7b shows the comparison of experimental permittivities and theoretical dielectric constant predicted by above mentioned models. Obviously, the permittivity variation trend of PTFE/T-GF with the increase of GF content is similar to the theoretical calculation value. Moreover, the modified Lichtenecker and EMT model fit well with the experiments, the percentage deviation is kept below 1%. Indicating that PTFE and T-GF in composites are closely bound to each other, which is consistent with the two-phase structure of the application conditions of the formulas, and confirming that the experimental data of dielectric constant is credible and predictable. However, the results calculated by the Maxwell-Wagner model are lower In many previous studies [13], Maxwell-Wagner model was only proved to be available at low working frequency, therefore, the high frequency test condition at 10 GHz is the main reason for the deviation. Otherwise, with the increase of GF filler content, the dielectric of PTFE/GF composites gradually deviates from the theoretical calculation value, which is attributed to the weak interface connection between GF and PTFE. Consequently, it further proves that the interfacial binding property of PTFE and GF plays a vital role in the composites' dielectric properties.
The bending strength and modulus are two important mechanical indicators of PCB substrate materials. The PCB plays a role in supporting circuit components, so bending resistance requirements are put forward [31]. The bending strength and modulus of PTFE/GF composites are represented in Fig. 8. The results show that the bending strength and modulus of the composites increase with enhancing the interface connection between GF and PTFE. So, the bending strength and bending modulus of PTFE/T-GF are the highest, 21.45 MPa and 522 MPa, respectively, 8% and 13% higher than those of PTFE/GF. As shown in Fig. 6d, coatings on GF led to the improvement in the filler-matrix compatibility. When the composite material is stressed, PTFE can transfer the stress to GF through the transition layer, resulting in the increase of the strength of composites [22,32].
The application environment of the circuit board is complex and changeable, especially in a humid environment. To keep the dielectric properties of the dielectric substrate, it is necessary to ensure it has low water absorption. Table 1 shows the effects of different surface treatment methods on the density, porosity and water absorption of the composites. The densities of GF and PTFE polymer are 2.76 and 2.15 g/cm 3 , respectively. It can be found that PTFE/ T-GF composites exhibit much lower moisture absorption and porosity than PTFE/GF, and also lower than PTFE/K-GF and PTFE/F-GF. The porosity of GF filled polymer composites mainly depends on the interface compatibility between the polymer and fillers. Moisture absorption is influenced by the porosity and relatively high porosity would cause higher moisture absorption. The improved interface compatibility between T-GF and PTFE makes the composite material display a lower porosity, thereby preventing water absorption and diffusion, and showing a very low moisture absorption. Generally, the theoretical density (q 0 ) was calculated by Eq. (4) and determined to be 2.188. The porosity was calculated by Eq. (5).
where q 1 , q 2 represents the density of the PTFE and GF fillers, respectively, V 1 and V 2 represents the matrix and fillers' volume fraction. The heating of the PCB often leads to a decrease in the substrate material's dielectric performance, thereby affecting signal transmission. Therefore, a suitable thermal conductivity is also an essential prerequisite for ensuring the PCB substrate's   reliability. The thermal conductivity of PTFE/GF composites are shown in Fig. 9a. As could be seen, comparing PTFE/K-GF and PTFE/F-GF, the thermal conductivity of PTFE/T-GF is significantly enhanced, reaching 0.268 W/m*K, which is 23% higher than that of PTFE/GF. The reason is that the interface between the filler and the matrix will cause the scattering of phonons. The larger the interface pores, the greater the scattering loss, and the worse the thermal conductivity [33]. The organic-inorganic hybrid layer formed by the sol-gel method on the surface of GF plays a role of bridging GF and PTFE. One end is bonded to the surface of GF by valence bonds, and the other end is compatible with the PTFE matrix, which reduces the porosity and improves the thermal conductivity of the composites. The effect of different filler content on the thermal conductivity is shown in Fig. 9b, but it is found that the influence of the filler content on the thermal conductivity of the PTFE/GF composites is weaker than that of the interface properties on the thermal conductivity. It is well known that the thermal conductivity of glass fiber (0.75 W/m*K) is higher than that of PTFE (0.2 W/m*K), but it is still not a good conductor of heat. As the filling content increases, the interface heat loss of the composite materials will also increase. Therefore, surface modification of GF is a relatively effective way to improve the thermal conductivity of composites.
For the sake of preventing the copper conductor path of PCB from breaking during the heating process, the CTE of the substrate should match that of the copper foil (18 ppm/°C) as much as possible. The thermal expansion curves of PTFE and GF reinforced composites are shown in Fig. 10a. The slope of the curve represents the CTE. The average CTE of PTFE and GF reinforced composites at 25-100°C is shown in Fig. 10b. The results show that the addition of GF filler can significantly reduce the CTE of the material. This is because GF dispersed in the PTFE matrix acts as a motion barrier to limit the thermal expansion of PTFE. Moreover, the CTE of composites decreases with the enhancement of the interface bonding force between GF and PTFE. At last, PTFE/T-GF maintains the lowest CTE, at 70 ppm/°C, which is 21% lower than that of PTFE/GF, indicating that the interface properties have a significant impact on the thermal properties of the composites.
The CTE of PTFE and its composites changes with temperature ( Fig. 10c, d) is obtained by deriving deformation. As shown in Fig. 10c, it could be observed that when the temperature is lower than 260°C, the CTE of PTFE remains in a relatively stable range. When the temperature exceeds 260°C, its CTE increases exponentially. According to previous studies [34,35], the molecular chains of PTFE gradually changed from a regular crystalline state to a stretched amorphous structure with the temperature increasing. Thereby the applicable range of PTFE should be lower than 260°C. The CTE curves within the range of 25 to 120°C are presented in Fig. 10d. The CTE of PTFE and its composites are maintained in a stable range. But at 30°C, the CTE of the PTFE composites increases significantly; because the PTFE crystals undergo crystalline relaxation at that temperature, causing the spiral of the chains to become irregularly entangled [36]. It is worth noting that the temperature point of the crystal relaxation is within Fig. 9 Thermal conductivity of a PTFE/GF composites and b PTFE/T-GF composites with 7, 8, 9 and 10 wt.% GF the usual temperature range, resulting in the volume of PTFE changing significantly and accordingly generating a certain impact on the application properties of PTFE and its composites.

Conclusion
The sol-gel method was used successfully to construct an organic-inorganic hybrids coating on the surface of GF, and then the modified GF noted T-GF was filled into PTFE to obtain PTFE/T-GF composites, which showed better performance than PTFE composites filled with GF modified by coupling agents, with a dielectric constant of 2.305, a dielectric loss of 9.08E -4 , a bending strength of 21.45 MPa, and a bending modulus of 522 MPa, the moisture absorption of 0.0167%, the thermal conductivity of 0.268 W/m*K, and the CTE of 70 ppm/°C. The performance improvement of PTFE/T-GF composites is attributed to the formation of an organic-inorganic hybrid layer on the surface of GF, which increases the surface roughness of GF and at the same time improves the SP on the surface of GF to make it more compatible with PTFE. The SEM micrograph of the composite material's cross-section shows that a dense interface structure is formed between T-GF and PTFE. The sol-gel surface modification provides a promising method to modify the surface of GF or other inorganic particles to improve the interface performance of composites and develops the application of PTFE-based PCB substrates.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or notfor-profit sectors.

Data Availability
The datasets used or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations
Conflicts of interest The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.