Semiaromatic Polyamides Containing Carboxyl Unit - American

Article pubs.acs.org/IECR

Semiaromatic Polyamides Containing Carboxyl Unit: Synthesis and Properties Gang Zhang,*,† Guang-Ming Yan,† Ting Yu,‡ Jie-Hong Lu,† Xiang Huang,‡ Xiao-Jun Wang,† and Jie Yang*,†,‡ †

Institute of Materials Science & Technology, Analytical & Testing Center, Sichuan University, Chengdu 610064, People’s Republic of China ‡ State Key Laboratory of Polymer Materials Engineering (Sichuan University), Chengdu, 610065, People’s Republic of China ABSTRACT: In this work, 2-(bis(4-hydroxyphenyl)methyl)benzoic acid (BHPBA) was prepared by hydrolysis of phenolphthalein with addition of ferrous powder. Afterward, semiaromatic polyamides (SAPs) with varied carboxyl group content were prepared from BHPBA and the bisphenol 1,1-bis(4-hydroxyphenyl)-1-phenylethane (BHPPE) as well as 1,6-N,N′bis(4-fluorobenzamide)hexane (BFBH). It was found that they all exhibited glass transition temperatures (Tg’s) of 159.6−180.7 °C. The resultant copolymers could be hot compressed into films, which showed a failure strength of 60−70 MPa. Additionally, high storage moduli (1.0− 1.6 GPa) were observed at 150 °C, indicating the good thermal mechanical performance for these resultant semiaromatic polyamides (SAPs). Interestingly, such copolymers can be used as the compatibilizer of composites of PA6T/carbon fiber (CF) in the case of the carboxyl group in the main chain, which was beneficial for the improvement of their interfacial adhesion force.

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It is well-known that the ether unit (−O− or −S−) is very flexible, which can be introduced into the macromolecule main chain to enhance the processability. Good examples are polyarylene ether ketone,24 poly(arylene ether ether sulfone) (PAEES),25 poly(arylene ether amide) (PAEA),26 and polyarylene ether nitriles (PAEN). 27 All of them exhibit pronounced comprehensive physical properties. Additionally, the mechanical and thermal properties of SAPs could be further enhanced by formation of composites via incorporation of nanofillers in the case of some applications in extreme conditions. Recently, carbon fiber (CF) was largely used as reinforced materials for the formation of polymer composites because of the superior reinforcing effect and excellent thermomechanical properties.28 Therefore, CF reinforced composites were considered as an excellent candidate for high performance engineering applications.29,30 However, the mechanics of the composites is usually not as excellent as we expected because of the poor interfacial interaction force between the CF and polymer matrix. In this context, there is an urgent need to improve the interfacial mechanics between CF and SAPs. Therefore, the aim of this work is first introduce the flexible segments such as ether (−O−) and active unit carboxyl group into the polymer molecule chain of SAPs to enhance their processability, and then add the resultant resin containing

INTRODUCTION

Semiaromatic polyamides (SAPs), combining the advantages of aromatic polyamides, e.g., excellent thermal and mechanical properties, and aliphatic polyamides, e.g., good melt processability, have attracted a lot of attention and were well developed in the past two decades. Because of the excellent overall performance, they are widely applied in the fields of electric/ electronic products and automobile parts.1,2 However, quite a few SAPs, e.g., homopolymer of poly(nonamethylene terephthalamide) (PA9T) or copolymers of PA6T, are commercially available, even though they have a better comprehensive performance than aliphatic polyamides.3−10 The reason is that the thermal degradation of SAPs will occur if the temperature is higher than 340 °C, which is lower than their melting points, resulting in a challenge for the melt processing. For instance, some SAPs containing short aliphatic diamines (two to seven CH2’s), e.g., PA4T (Tm = 430 °C, Td = 350 °C) and PA6T (Tm = 370 °C, Td = 350 °C) are thought to be impractical for the melting process because they have been already thermally degraded below the melting point.11 Therefore, development of SAPs with novel properties and good processability are required. It is reported that the processability of SAPs could be improved by introduction of noncoplanar moieties, flexible segments, and unsymmetrical units into the macromolecule chain (as shown in Table 1),12−23 in which adding of flexible segments into SAPs backbones is found to be a pronounced approach to fabricating high performance processable polyamides. © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

May 15, 2017 July 28, 2017 August 2, 2017 August 2, 2017 DOI: 10.1021/acs.iecr.7b01998 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Thermal Properties of Common SAPs resin

Tg (°C)

Tm (°C)

Td (°C)

resin

Tg (°C)

Tm (°C)

Td (°C)

PA4T PA6T (homopolymer) PA9T PA10T PA12T

− − 126 125 103

430 370 305 315 295

350 350 464 472 429

PA18T SPAs (with naphthalene ring) SPAs (containing biphenylene group) SPAs (containing ester group) SPAs (containing bulky pendant units)

− − 300 110−140 270

245 300 − 219−260 −

− 495 510 − 500

Scheme 1. Synthesis Route of 2-(Bis(4-hydroxyphenyl)methyl)benzoic Acid (BHPBA)

Scheme 2. Synthesis Route of SAPs Copolymers (BHPBA-10, BHPBA-20, BHPBA-40, BHPBA-60, BHPBA-80, and BHPBA100)

the reaction solution was filtered, and the unreactive solid ferrous powder was removed. The liquor was acidified with concentrated hydrochloric acid (HCl), and then the white solid precipitated. To remove byproduct and residual salts, hot deionized water was used to wash the solid product several times. Then the crude monomer was dissolved in a NaHCO3 solution and precipitated in concentrated HCl again. After filtration and washing three times with hot deionized water, the final product was dried in a vacuum oven for 12 h (110 °C) to get pure monomer BHPBA. Yield: 89.6 g, 89.2%. Elemental analysis (%) (data in parentheses are predicted values): found: C, 74.87 (74.99); H, 5.08 (5.03). FT-IR (KBr, cm−1): 3427, 1228 (−OH), 3040 (C−H, benzene ring), 1684 (−COOH), 1601, 1509 (CC, benzene ring). NMR [400 MHz, DMSO-d6, ppm]: 6.382 (S, 1H, H1), 6.635−6.670 (d, 4H, H2), 6.760−6.781 (d, 4H, H3), 6.964−6.982 (d, 1H, H4), 7.256−7.296 (d, 1H, H5), 7.402− 7.443 (d, 1H, H6), 7.704−7.727 (d, 1H, H7), 9.230 (s, 2H, H8), 12.80 (s, 1H, H9). SAPs Synthesis. As shown in Scheme 2, BHPBA (6.4 g), BHPPE (23.2 g), potassium carbonate (22.1 g) BFBH (36.0 g), methylbenzene (20 mL), and NMP (160 mL) were added into a three-necked round-bottom flask. The reaction mixture was subsequently heated to 150−160 °C to remove the byproduct H2O. Afterward, the polymerization temperature of 190−200 °C was applied for 12 h. During this procedure the viscosity of the mixture got more and more large. Then white fibrous solids were achieved by pouring the polymerization mixture into cool water, which was further acidified with hydrochloric acid, and washed with hot water. After pulverization of the fibrous solids into powder, the powder was washed by deionized water and ethanol. Repeating the washing procedure three times, SAPs

carboxyl units into the composites of PA6T/CF as a compatibilizer to improve their interfacial binding strength for formation of the composites with the required enhanced mechanical properties. In this study, 2-(bis(4-hydroxyphenyl)methyl)benzoic acid (BHPBA) containing carboxyl group was prepared by hydrolysis of phenolphthalein.31 Then BHPBA and another bisphenol, 1,1-bis(4-hydroxyphenyl)-1-phenylethane (BHPPE), were conducted to react with 1,6-N,N′-bis(4-fluorobenzamide)hexane (BFBH) to prepare a series of SAPs containing different contents of carboxyl group. The effect of chemical structure on the physical performcance of the resultant SAPs was studied. Finally, the resultant SAPs were used as compatibility agents for the PA6T/CF composites and their properties were investigated.

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EXPERIMENTAL SECTION Materials. Potassium carbonate (K2CO3), ferrous powder, phenolphthalein (AR, KeLong Chemical Reagent Co.), Nmethyl-2-pyrrolidone (NMP; 99.8%), phenol, acetophenone, and other chemicals were purchased directly without any further treatment. PA6T (TOP3000, BASF), short carbon fiber (length 6 mm, gum free, Barnet (Shanghai) Trading Co. Ltd.), BHPPE, and BFBH were synthesized based on previously reported methods.23,32 Preparation of 2-(Bis(4-hydroxyphenyl)methyl)benzoic Acid (BHPBA). BHPBA was prepared as follows (Scheme 1): Phenolphthalein (100 g) and deionized water (2 L) were added into a 5000 mL three-necked flask combined with mechanical agitation, followed by addition of the ferrous powder (56 g) and NaOH (150 g). The reaction was maintained for 10 h (55−60 °C). During this procedure, the color of the mixture changed from purple to gray. Afterward, B

DOI: 10.1021/acs.iecr.7b01998 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

then fastened around a carbon fiber. Afterward, a testable droplet was formed by melting the PA6T fibers at 315 °C. After cooling to room temperature (RT), the microbond test sample was prepared completely. Similarly, the sample of BHPBA-20 was also prepared with the same method. The stretching speed of the microbond test is set at 0.02 μm/s. The apparent interfacial shear strength (IFSS, τapp) of the composites can be calculated with the formula35

were finally obtained, which were dried under vacuum for 10 h (80 °C), and named BHPBA-20 (yield 58.3 g, 94.6%). Using the same procedure, BHPBA-10, BHPBA-40, BHPBA60, BHPBA-80, and BHPBA-100 were prepared with adjustment of the contents of BHPBA and BHPPE. The yields of these copolymers were as follows: BHPBA-10 (95.1%), BHPBA-40 (93.4%), BHPBA-60 (93.5%), BHPBA-80 (92.1%), and BHPBA-100 (91.6%). Preparation of BHPBA-COOH/PA6T/CF Composites. Copolymer BHPBA-20 was selected as the model compatibilizer for the composites of PA6T/CF-30. The composites of PA6T/CF-30 (70/30, wt/wt) and BHPBA-20/PA6T/CF-30 (x/70/30, wt/wt/wt) was prepared as follows: The composites were prepared via a corotating twin-screw extruder (CTE-35). The processing temperatures of 10 heating zones and the die area were 235, 255, 275, 285, 290, 290, 290, 290, 285, 280, and 280 °C. The composites were injected with an injection machine (CJ150NC) at 300 °C. The BHPBA-20/ PA6T/CF-30 composites with BHPBA-20 content of x wt % (x = 1, 3, 5, 7, 9) were named 1% BHPBA-20/PA6T/CF-30, 3% BHPBA-20/PA6T/CF-30, 5% BHPBA-20/PA6T/CF-30, 7% BHPBA-20/PA6T/CF-30, and 9% BHPBA-20/PA6T/CF-30, respectively. Characterization. A Cannon-Ubbelohde viscometer was used to test the intrinsic viscosities of BHPBA-(10−100) at 30 ± 0.1 °C. Samples of 0.500 g were added into 100 mL of NMP to prepare a very dilute solution. The intrinsic viscosity values could be calculated by the formula ηint =

τapp =

Fmax πdf le

df is the fiber diameter, le is the fiber length which is embedded in the resin matrix, and Fmax is the maximum pull-out force. Both df and le were achieved by using an optical microscope together with Studio Measure software (EC imagine). The τapp values of these samples were calculated from at least 25 pull-out specimens to get statistical data.

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RESULTS AND DISCUSSION Chemical Structure of BHPBA. The FT-IR spectrum of monomer (Figure 1) shows the characteristic absorption of

2(ηsp − ln ηr ) C

where ηr = η/η0 and ηsp = η/η0 − 1. The molecular weight (Mn, number-average molecular weight; Mw, weight-average molecular weight; PDI, polydispersity index) of each of these copolymers was tested using gel permeation chromatography (GPC, Waters 1515) with DMF as mobile phase. An elemental analyzer (EURO EA-3000) was applied to determine the monomers. The chemical structures of the monomers and copolymer were measured by 1H nuclear magnetic resonance (1H NMR; Bruker-400 NMR) with DMSO-d6 as solvent and Fourier transform infrared (FT-IR) spectroscopy (Nexus670). The Tg’s and Td’s were investigated by differential scanning calorimetry (DSC; Netzsch DSC 200 PC) and thermogravimetric analysis (TGA; Q500), both with a heating rate of 10 °C/min under N2 atmosphere. For more property testing, first, a hot compressed film was fabricated from the polymer powder by a hydraulic press (YJAC) at 290 °C for 5 min under a pressure of 3 MPa. The mechanics of the samples was tested by a tensile test (Instron Corp. 4302) at ambient temperature. The width, length, and thickness of test samples were 5 cm, 0.3 cm, and 0.3 cm, respectively. Dynamic mechanics of the samples with the same size applied for tensile tests was determined by dynamic mechanical analysis (DMA; TA-Q800), operating from 50 to 250 °C (1 Hz, 5 °C/min). Rheological properties of SAPs were studied by a parallel plate rheometer (Bohlin Gemini 200) fitted with a parallel plate (diameter 2.5 cm), which was performed with temperature and time sweep tests (N2 atmosphere). The micromechanical performances of composites of PA6T/ CF and BHPBA-20/CF were investigated with the microbond test.33,34 First, the PA6T resin was pulled into fiber, which was

Figure 1. FT-IR spectrum of monomer (BHPBA).

−OH near 3200−3400 and 1220 cm−1. The absorption near 3200−3400 cm−1 is a strong and broad peak, which may be ascribed to the existence of the hydrogen bond between the molecules. The peak near 1690 cm−1 is the absorption of −COOH. Figure 2 displays the 1H NMR spectrum of BHPBA. Nine groups of proton signals that range from 6.382 to 12.800 ppm can be observed on the 1H NMR spectrum. The signal near 6.382 ppm was ascribed to the proton of tertiary carbon ). Comparing it to a common aliphatic chain, it shifted to ( a low field for the polarization effect of hydroxyl and carboxyl groups. The six groups of peaks ranging from 6.635 to 7.727 ppm are the proton signals of the benzene ring. The signals near 9.230 and 12.800 ppm were assigned to the active hydrogens of the hydroxyl (−OH) and carboxyl (−COOH) units. Combining the elemental analysis and FT-IR results indicates that BHPBA was successfully prepared as described in Scheme 1. Synthesis of Copolymers of BHPBA-(10−100). The polymerization reaction was carried out with a nucleophilic substitution method. The hydrogen of −COOH must be C


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Figure 2. 1H NMR spectrum of monomer (BHPBA).

suggests that the resultant SAPs had moderate molecular weights similar to previously reported results.19 Chemical Structures of BHPBA-(10−100). As shown in Figure 3, the characteristic absorption of −CONH− (N−H and CO) can be observed near 3330 and 1640 cm−1. The absorptions near 2930 and 2850 cm−1 were attributed to the methylene units. The peak near 1705 cm−1 is the absorption of carboxyl group (−COOH). Compared with the FT-IR spectrum of BHPBA, the wide absorption of the hydroxyl group (−OH) around 3200−3400 cm−1 has disappeared, and a new absorption around 1160 cm−1 has appeared ascribed to the absorption of the ether unit (−O−). This indicates the successful nucleophilic substitution reaction between the bisphenol and BFBH. As shown in the 1H NMR spectra of BHPBA-(10−100) (Figure 4), the signals of methylene unit (−CH2−) protons are near 1.436, 1.582, and 3.334 ppm, while the methyl group proton signal is a single peak and near 2.217 ppm. The proton signals of benzene ring are 7.1−8.5 ppm. Their chemical shifts are so close and multiple that they cannot be distinguished from each other. The signal near 12.993 ppm was assigned to the−COOH unit. It was also found that the proton signals of −COOH and tertiary carbon got more and more obvious by increasing the copolymerization content of BHPBA. Comparing with monomer BHPBA, the chemical shift of the copolymers was all toward low field. The main reason is the strong electrophilic effect of the introduction of amide group. The proton of the bisphenol is subjected to a large polarization effect, so the chemical shifts of these SAPs get larger. In combination with the data of FT-IR, it indicates the copolymers of SAPs were prepared as described in Scheme 2. Thermal Properties of BHPBA-(10−100). The thermal performances of copolymers BHPBA-(10−100) were tested by DSC and TGA. The Tg’s of BHPBA-10, BHPBA-20, BHPBA40, BHPBA-60, BHPBA-80, and BHPBA-100 were 159.6, 163.4, 168.2, 172, 176.4, and 180.7 °C, respectively (as depicted in Table 3 and Figure 5). They were 25−45 °C higher than those of copolymers of PA6T (Tg ≈ 135 °C) and PA9T (Tg = 126 °C). Increasing the content of BHPBA increased the Tg of the copolymer, whereby the copolymer BHPBA-100 showed the highest Tg value (180.7 °C) among this series. The reason could be that, with the addition of bisphenol that contained a carboxyl group, the hydrogen bond density got much larger, and then the interaction force between the polymer molecules

Table 2. Intrinsic Viscosity (ηint) and Molecular Weights (Mn, Mw, and PDI) of Copolymers (BHPBA-(10−100)) polymer

ηint (dL/g)

BHPBA-10 BHPBA-20 BHPBA-40 BHPBA-60 BHPBA-80 BHPBA-100

0.72 0.66 0.69 0.79 0.88 0.95

Mn 2.82 2.51 2.74 2.31 2.05 1.99

× × × × × ×

Mw 105 105 105 105 105 105

3.82 3.52 4.08 3.83 3.74 3.80

× × × × × ×

PDI 105 105 105 105 105 105

1.36 1.40 1.49 1.66 1.83 1.91

Figure 3. FT-IR spectra of SAPs (BHPBA-(10−100)).

balanced; otherwise, it may be the cause of the decomposition of solvent. Potassium carbonate was used as an alkali to neutralize the active hydrogens of the hydroxyl (−OH) and carboxyl (−COOH) units. The reaction temperature was first raised to 150−160 °C to form the organic salt of bisphenol and remove the formed water, which was beneficial for the reaction of nucleophilic substitution and got much higher molecular weight copolymers. After that, the reaction temperature was raised to 190−200 °C to further enlarge the polymer’s molecular weight. Table 2 describes the intrinsic viscosities (ηint) and molecular weights of the SAPs. The Mw values of BHPBA-(10−100) were in the range (3.52−4.08) × 105. This D


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Figure 4. 1H NMR spectra of SAPs (BHPBA-(10−100)).

Table 3. Thermal and Mechanical Performance of BHPBA-(10−100) and Commercial Product Such as PA6T and PA9T

a

polymer

Tg (°C)

T5% (°C)

char yield (%)

average failure strength (MPa)

BHPBA-10 BHPBA-20 BHPBA-40 BHPBA-60 BHPBA-80 BHPBA-100 PA6T PA9T

159.6 163.4 168.2 172 176.4 180.7 ∼135 126

407 400 387 372 364 351 385 464

2.8 8.2 11.4 12.4 19.8 16.5 − −

65 58 61 59 63 58 51 86

Young’s modulus (GPa) elongation at break (%) 2.1 1.9 1.7 1.4 1.4 1.6 1.56 2.7

7.3 14.3 14.5 9.3 9.1 6.0 4.2 4.2

E′ at 150 °Ca (GPa) 1.2 1.0 1.2 1.2 1.6 1.1 − −

Detected by DMA.

Figure 5. DSC curves of BHPBA-(10−100) in N2 (heating rate 10 °C/min).

Figure 6. TGA curves of BHPBA-(10−100) in N2 (heating rate 10 °C/min).

also became much larger, so the glass transition temperature of the copolymers showed an increasing trend with the higher

content of BHPBA. No melting endothermic peak can be found in the DSC curves, indicating that the obtained SAPs E


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Figure 7. DMA curves (storage modulus E′) of BHPBA-(10−100). Figure 10. Storage modulus and loss modulus curves versus temperature for BHPBA-10 and BHPBA-20.

Figure 8. Tan delta curves of BHPBA-(10−100). Figure 11. Plot of complex viscosities versus time for BHPBA-(10− 100).

750 °C in nitrogen atmosphere were about 0.2, 8.0, 11.1, 12.1, 19.4, and 16.2%, displaying an increasing trend with the higher content of BHPBA. They were much larger than those of copolymers of PA6T except for BHPBA-10, indicating that the thermal stabilities of these copolymers were good by introducing −O− and −COOH units into the macromolecule chain. Dynamic Mechanical Analysis of BHPBA-(10−100). DMA was carried out to determine the dynamic mechanics of the obtained SAPs. As shown in Figure 7, these SAPs copolymers showed storage modulus (E′) values of 1.3−1.9 GPa (50 °C), whereas they maintained about 75% storage modulus (1.0−1.5 GPa) at 150 °C, which indicates these SAPs have excellent thermal mechanical properties. The loss tangent can be used to calculate the Tg values of polymers. As shown in Figure 8, the Tg’s of these copolymers were about 177.6, 178.5, 180.2, 184.9, and 185.6 °C, respectively. The values were comparable with data obtained from the DSC characterization. Similarly, the Tg’s of the copolymers determined from DMA methods showed the same increasing trend as DSC measurement with increasing BHPBA content except for BHPBA-80 and BHPBA-100. Mechanical Properties. Table 3 summarizes the average failure strength of BHPBA-(10−100). The failure strengths of BHPBA-10, BHPBA-20, BHPBA-40, BHPBA-60, BHPBA-80,

Figure 9. Plot of complex viscosities versus temperature for BHPBA10 and BHPBA-20.

copolymers were amorphous, which was different from the commercial product such as PA6T and PA6ST.12 Figure 6 shows the thermal degradation curves in nitrogen atmosphere. The initial degradation temperatures (T5%) of BHPBA-(10−100) were in the range 351−407 °C; they are far higher than their Tg’s. The char yields of BHPBA-10, BHPBA20, BHPBA-40, BHPBA-60, BHPBA-80, and BHPBA-100 at F


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Industrial & Engineering Chemistry Research Table 4. Solvent Resistance of BHPBA-(10−100) and Nylon 6a copolymer

a

solvent

nylon 6

BHPBA-10

BHPBA-20

BHPBA-40

BHPBA-60

BHPBA-80

BHPBA-100

NMP DMSO CHCl3 1,4-dioxane acetone methylbenzene CHCl2CHCl2 + phenol H3PO4 CF3COOH formic acid hydrochloric acid (6 mol/L) concd H2SO4 NaOH (1 mol/L)

− − − − − − − + + + − + −

+ + − − − − − − + − − + −

+ + − − − − − − + − − + −

+ + − − − − − − + − − + −

+ + − − − − − + − + − − + −

+ + − − − − − + − + + − − + −

+ + − − − − − + + + − − + −

+, soluble at RT; + −, swelling; −, insoluble.

Figure 12. Apparent interfacial shear strength (τapp) of PA6T/CF and BHPBA-20/CF.

studied with a parallel plate rheometer. The temperature sweep was carried out to investigate the copolymer melt process window. As shown in Figure 9, the complex viscosities of BHPBA-10 and BHPBA-20 were found to be 1213−2489 Pa·s at different temperatures (280−320 °C). BHPBA-20 showed a more large complex viscosity than that of BHPBA-10, and the complex viscosities of the samples exhibited an increasing trend with higher content of BHPBA. The main reason is that the monomer BHPBA contains the strong polar carboxyl unit. In this case, with the increase of the content of BHPBA, the hydrogen bond and intermolecular force become stronger. It was also found that the melt viscosities of the samples increased rapidly when the content of BHPBA surpassed 20%, and the phenomenon of melt sharp expansion even appeared, so that the other samples’ data except for BHPBA-10 and BHPBA-20 could not be obtained. The melt loss modulus (G″) and storage modulus (G′) during the testing temperature were studied as shown in Figure 10. It was found that the curves of G′ and G″ could intersect in the test temperature range. It is well-known that if the melt loss modulus is larger than the storage modulus at a certain temperature, the polymer can be melt processed at this temperature in theory. Therefore, the copolymers (BHPBA-10 and BHPBA-20) are thought to be melt processable. The intersection point of G′ and G″ increased from 304.5 to 306.2 °C as could be observed with the content of BHPBA increasing from 10 to 20%, suggesting that a much higher melt processing

Scheme 3. Sketch of Compatibility Effect of BHPBA/CF Composites

and BHPBA-100 were 65, 58, 61, 59, 63, and 58 MPa, respectively. Those were similar to or even higher than the failure strength of commercial product of PA6T (Dupont product, grade FE18502 NC010, failure strength 51 MPa). The Young’s modulus and elongation at break of the resultant copolymers were in the ranges 1.4−2.1 GPa and 6.0−14.5%, respectively. This suggests that the obtained copolymers BHPBA-(10−100) have good mechanical properties. Rheological Properties of Copolymers BHPBA-(10− 100). The rheological properties of BHPBA-(10−100) were G


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Figure 13. SEM images of (a) microdroplets of PA6T/CF before debonding, (b) CF surface of PA6T/CF after debonding, (c) microdroplets of BHPBA-20/CF before debonding, and (d) CF surface of BHPBA-20/CF after debonding.

expanded for BHPBA-(40−100) during the test. This indicates that the melt stability of the copolymer is better with the lower content of BHPBA. The main reason is that the carboxyl group is not a very stable unit at high temperature; the thermal degradation reaction may occur during the melt process and thus increase the melt complex viscosity. Solubilities. The solubilities of BHPBA-(10−100) are listed in Table 4. These SAPs exhibited good solubility in NMP, DMSO, and so on. Their solubilities were found to be better than that of PA6T. However, they cannot dissolve in common solvents, e.g., toluene, acetone, snf formic acid. Compared with the aliphatic nylon, e.g., PA6, BHPBA-(10−100) showed better solvent resistance. Interfacial Shear Strength (τapp) of Composites of CF and Polymer Matrix. The most important factor for stress transfer of composites was the interfacial adhesion force between the polymer and fiber. Usually, the adhesion strength of the composites was studied with the “microbond test” through analysis of the apparent interfacial shear strength (τapp). Based on the above results, the semiaromatic polyamide BHPBA-20 was selected as the model material. For comparison, PA6T (TOP3000, BASF) was also used for the microbond test. The individual τapp values between PA6T and BHPBA-20 with CF are shown in Figure 12. The τapp values of PA6T and BHPBA with CF were 38.5 and 44.8 MPa, respectively. This indicates that BHPBA had an improved interfacial strength compared with PA6T due to the existence of a larger number of

Figure 14. Failure strength of the composites PA6T/CF (70/30) without and with different contents of modified semiaromatic polyamide compatibilizer BHPBA-20.

temperature could be applied for the copolymer with higher BHPBA content. To study the melt stabilities of BHPBA-10 and BHPBA-20, a time sweep (at 310 °C, 1 Hz) was conducted to calculate the results. As shown in Figure 11, the complex viscosities of BHPBA-10 and BHPBA-20 were kept almost unchanging from the beginning to the end of the test process while it was sharply H


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Industrial & Engineering Chemistry Research −COOH groups in the polymer chain. There are a lot of functional groups such as −C−OH and epoxy units on the surface of CF,36 and thus some earlier literature about modification of polymer chemical structure to enhance the interfacial interaction between CF and polymer resin had been reported.37,38 Here, the chemically modified semiaromatic polyamide BHPBA-20 can react with −OH and other reactive units on the surface of the CF (Scheme 3), which is helpful for improving interfacial interaction. Morphology of CF and Microbonds before and after Debonding Test. Figure 13 shows the SEM images of samples of PA6T/CF and BHPBA-20/CF before and after the debonding test. As shown as Figure 13, much more residue polymer matrix can be found on the surface of CF in the composites of BHPBA-20/CF than that of PA6T/CF composites. This indicates that BHPBA-20 has a powerful interaction force with CF. These above results were consistent with the data of τapp of composites of PA6T/CF and BHPBA20/CF. Mechanical Properties of Composites PA6T/CF (70/ 30). From the above experiments, BHPBA-20 had high potential to be used as the compatibilizer for the composites PA6T/CF (70/30) to improve their mechanical properties. As shown in Figure 14, it could be found that the compatibilizer BHPBA-20 had a distinguished effect on the mechanics of CF reinforced PA6T composites. Increasing the content of the compatibilizer BHPBA-20, the failure strength of reinforced PA6T/CF composite first increased. When the content of BHPBA-20 was 5 wt % in the PA6T/CF composite, the maximum failure strength of 58.9 MPa for PA6T/CF composite could achieved. In other words, the failure strength of composite PA6T/CF maximally increased by 54.2% compared with the pure composite PA6T/CF (38.2 MPa) with the addition of BHPBA-20, which was due to the introduction of larger number of carboxyl units in the polymer main chain. Here, the bridging effect of −COOH in BHPBA-20 between the PA6T/CF composites was better when compared with pure PA6T/CF composites because of the good physical compatibility effect between PA6T and the compatibilizer (the structure and thermal properties of BHPBA-20 were analogous to those of PA6T), and the strong covalent cooperation between CF and compatibilizer. It was clearly demonstrated that the synthesized semiaromatic polyamide with larger number of carboxyl units can be used as an efficient compatibilizer for the PA/CF composite systems. While the failure strength decreased when the BHPBA-20 content was larger than 5 wt %, that may be caused by the small amount of thermal degradation of carboxyl group in the composites with the condition of high content of BHPBA-20.

found that a part of the resultant copolymers BHPBA-20 can be applied to the compatibilizer for the composites of PA6T/CF, and played a notable role in improving the mechanics of the composites that the traditional SAPs could not match. Therefore, these SAPs containing carboxyl units can be potential candidates as coupling agents for special engineering plastics.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Gang Zhang: 0000-0002-0195-1209 Xiao-Jun Wang: 0000-0001-8049-5020 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the following funds: OYSFSCU (2015SCU04A25) and NSFC (21304060). REFERENCES

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CONCLUSIONS The bisphenol containing carboxyl unit monomer BHPBA was prepared using a facile hydrolysis reaction. It was conducted to prepare a kind of modified SAPs [BHPBA-(10−100)] with the nucleophilic polymerization method. The initial thermal degradation temperature was found to decrease with increasing the copolymerization ratio of BHPBA, while the glass transition temperature and complex viscosity of these SAPs gradully increased. These resultant copolymers were found to have good processability (including melt and solution processing except for BHPBA-(40−100)), thermal properties, and mechanical properties. Using this procedure, a series of SAPs exhibiting pronounced performance can be fabricated. Additionally, it was I


Article

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Semiaromatic Polyamides Containing Carboxyl Unit - American

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