Introduction

Poly(arylene ether)s (PAEs) are a kind of important high-performance thermoplastics that have rigid aromatic rings and flexible ether linkage in their backbones. The typical PAEs, such as poly(arylene ether ketone)s (PAEKs) and poly(arylene ether sulfone)s (PAESs) exhibit outstanding thermal stability, good mechanical properties and excellent chemical resistance [1,2,3,4]. Due to above excellent combination of properties, the applications of PAEs in the aerospace, automobile, energy, medical care, electrical and optical industries had attracted significant attention in the last several decades [5,6,7,8,9,10]. However, most of these PAEs have some drawbacks, such as high melt processing temperature, high melt viscosity and poor solubility, and these drawbacks make themselves difficult to process and limit their application in various technological fields. For example, a good balance between the film-forming ability and corrosion resistance of polymer is required for membrane application, good melt stability and flowability of polymer are necessary for injection molding process. Therefore, In order to enlarge their application field, chemical modifications of these polymers are becoming essential and critical.

To solve this problem, the studies on improving their melt processability by the introduction of flexible moiety, such as alkyl [11, 12], thioether [13] and amide [14] into the polymer backbone, and raising the ratio of ether linkage to polar groups [3] have been reported. In addition, the effort to improve their solution processability by incorporation of cardo groups, such as phthalazine [15], spirodilactam [16], phenolphthalein [17], xanthene [18], fluorene [19], decahydronaphthalene [20] and isophorone [21] into the polymer chain, and the introduction of pendant trifluoromethyl [5], polychloro [22], cyano [23, 24], aryl [25,26,27] and alkyl [27,28,29] groups onto the polymer chain have been afforded.

It is well known that the introduction of flexible moiety into the polymer backbone can effectively soften the polymer backbone and lower the melt point of polymers, but their crystallization properties still result in the poor solubility. The incorporation of bulk groups into the polymer chain is an effective way to weaken the molecular regularity and improve their solubility, but it also leads to a great increase in the glass transition temperature, and requires a harsh melt processing environment. Besides, the earlier studies on chemical structure modification of PAEs were mostly focused on the relationship between thermal properties and their aggregated structures, while just a few studies about their rheological properties. Therefore, the aim of this work is to synthesize the poly(arylene ether)s with excellent thermal and mechanical properties, as well as good melt and solution processability, which is estimated by the testing of their rheological behavior and solubility. To achieve this aim, the cyclohexyl units with good thermal stability and moderate rigidity were introduced into the polymer backbone. The pendant cyclohexyl groups are expected to weaken the molecular packing and the interaction of chain so as to lower the crystallinity and melt viscosity of the polymers, and improve their processabilities.

In this paper, 4,4′-cyclohexane-1,1′-diyldiphenol (CHDP) was synthesized. The pendant cyclohexyl groups containing PAEs were prepared by the solution polycondensation of 4,4′-cyclohexane-1,1′-diyldiphenol (CHDP) and 4,4′- difluorobenzophenone (DFBP) (or 4,4′-difluorodiphenylsulfone (DFDPS)). The effects of pendant cyclohexyl units on their thermal, mechanical, rheological, optical properties and solubility were discussed in detail.

Experimental

Materials

4,4′-difluorodiphenylsulfone (DFDPS) (AR, Aladdin Reagent Company), 4,4′- difluorobenzophenone (DFBP) (poly grade, Sino-High(China) Co., Ltd.), potassium carbonate (AR, Chengdu Kelong Chemical Industry Company), methylbenzene (AR, Chengdu Kelong Chemical Industry Company), N-methyl-2-pyrrolidone (NMP) (99.5%, Jiangsu NanJing JinLong Chemical Industry Company), hydrochloric acid (HCl) (AR, Chengdu Kelong Chemical Industry Company), acetic acid (AR, Chengdu Kelong Chemical Industry Company), phenol (AR, Chengdu Kelong Chemical Industry Company), cyclohexanone (AR, Aladdin Reagent Company), mercaptoacetic acid (AR, Aladdin Reagent Company) were used without further purification. The other solvents and reagents were commercially obtained.

Measurements

Intrinsic viscosity analysis

The intrinsic viscosity (ηint) of the resultant polymers were measured by one-point method at 30 ± 0.1 °C with 0.500 g of polymer dissolved in 100 ml of NMP, and using a Cannon-Ubbelhode viscometer. The results were calculated with following equation:

$$ \eta \operatorname{int}=\frac{\sqrt{2\left({\eta}_{\mathrm{sp}}-\ln \kern0.5em \eta \kern0.5em \mathrm{r}\right)}}{C} $$
(1)

Where ηr = η/η0 and η sp = η/η0 ‐ 1.

Chemical structure analysis

The 1H–NMR spectra were recorded on a BRUKER-600 NMR spectrometer with deuterated dimethyl sulfoxide (DMSO-D6) or deuterated chlorofprm (CDCl3) as solvents. The FT-IR spectra were obtained by a NeXuS670 FT-IR instrument. The results of elemental analysis were obtained by an elemental analyzer (EURO EA-3000). The X-ray diffraction (XRD) was performed with Philips X’pert Pro MPD.

Thermal analysis

Thermal analysis of the resultant polymers were obtained by Thermogravimetric analysis (TGA, Q500) and Differential scanning calorimetry (DSC, NETZSCH DSC 200 PC) at a heating rate of 10 °C/min under nitrogen atmosphere.

Mechanical test

The stress-strain behavior of the polymer casting films were studied by the Instron Corporation 402. The speed of testing was 10 mm/min. Dynamic mechanical analysis (DMA) was performed on TA-Q800 apparatus operating in the tensile mode at air atmosphere with a heating rate of 5 °C/min and a frequency of 1 Hz .

Rheological test

The rheological properties of the resultant polymers were measured by using a parallel plate rheometer with 2.5 cm diameter stainless steel parallel plates. Temperature sweep (shear frequency: 1 Hz, shear strain: 2%, temperature scanning rate: 5 °C min−1) and time sweep (shear frequency: 1 Hz, shear strain: 2%) were performed under air atmosphere.

Experiment of solubility

The solubility of the resultant polymers in various solvents was tested at room temperature for 12 h and at the boiling of the solvents for 10 min.

Optical properties

The optical transmittance of the prepared polymers films was determined by UV-Visible spectroscopy (U-2310II). The samples were prepared by the solution-casting method to form a thin film, whose thickness is about 10–15 μm.

Monomer synthesis

4,4′-cyclohexane-1,1′-diyldiphenol (CHDP) (shown in scheme 1)

Phenol (37.64, 0.4 mol), mercaptoacetic acid (0.5 g), a mixture of hydrochloric acid and acetic acid (90 ml, 2:1 by volume) were added into a 250 ml three-necked flask. The mixture was stirred at 50 °C for 0.5 h. Then cyclohexanone (9.82 g, 0.1 mol) was added dropwise within 0.5 h, and the reaction was kept for 12 h. Afterwards, the mixture was filtered and washed 3 times with hot deionized water to remove excess phenol. Then, the crude product was recrystallized from methanol aqueous solution. Finally, the product was dried under vacuum at 50 °C for about 24 h. That afforded CHDP as white crystals (yield: 94.54%).

Polymer synthesis

Poly[(4,4′-cyclohexane-1,1′-diyldiphenol)-alt-(4,4′-difluorobenzophenone)] (CHPEK)

A typical polymerization was carried out as shown in Scheme 1. CHDP (26.8 g, 0.1 mol), DFBP (21.8 g, 0.1 mol), toluene (15 ml), potassium carbonate (27.6 g, 0.2 mol) and NMP (120 ml) were added into a 250 ml three-necked flask fitted with a mechanical stirrer, thermometer and nitrogen inlet. The mixture was heated to 160 °C for 1 h to remove the byproduct water. Next, the reaction temperature was raised up to 200 °C and kept for another 6 h. After the reaction was over, the reaction solution was poured into deionized water to obtained fibrous precipitate, and then the precipitate was crushed into powder and washed with hot deionized water several times. After dried at 100 °C for 12 h, 42.62 g (yield: 95.78%) of CHPEK was obtained, the inherent viscosity (ηint) was 0.49 dLg−1.

Scheme 1
scheme 1

Synthesis routes of CHDP、CHPEK and CHPES

Full size image

Poly[(4,4′-cyclohexane-1,1′-diyldiphenol)]-alt-(4,4′-difluorodiphenylsulfone)] (CHPES)

CHPES was prepared following a similar procedure as that of CHPEK. The yield and inherent viscosity (ηint) of CHPES were 97.38% and 0.97 dLg−1, respectively.

Results and discussion

Monomers

Synthesis and chemical structure of CHDP

The monomer CHDP was synthesized by condensation of phenol with cyclohexanone in a mixture acid solvent (as shown in Scheme 1). Its structure was confirmed by 1H–NMR and FT-IR spectra. The 1H–NMR spectrum of the monomer is shown in Fig. 1. The signal at 9.1 ppm was attributed to the units of -OH. While the signals at 7.0 and 6.6 ppm were assigned to the protons on benzene ring. The signals rang from 1.4 to 2.1 ppm were assigned to the cyclohexyl units. The ratio of corresponding integral curves was about 1:2:2:2:3, that was in good agreement with the calculated value. From the FT-IR spectrum of CHDP (Fig. 2), we could observe the characteristic absorption bands of -OH at 3200–3600 cm−1 and the absorption bands of C-O stretching vibration at 1250 cm−1. The bands at 2935 and 2852 cm−1 in the spectrum indicated the existence of cyclohexyl units. The bands at 1612, 1513 and 1442 cm−1 were for the C = C stretching vibrations of benzene ring. Besides, the absorption bands of C-H stretching vibration and out-plane bending vibration of benzene ring could be observed at 3044 and 820 cm−1, respectively. Combining the results of 1H–NMR and FT-IR, we comfirmed that CHDP was successfully synthesized as shown in Scheme 1.

Fig. 1
figure 1

1H–NMR spectrum of CHDP in DMSO-D6

Full size image
Fig. 2
figure 2

FT-IR spectra of CHDP, CHPEK and CHPES

Full size image

Polymers

Synthesis and chemical structure of CHPEK and CHPES

The synthetic route of polymers were shown in Scheme 1. The polymerizations were carried out with potassium carbonate as the base. The FT-IR, 1H–NMR spectra and elemental analysis were used to confirm the chemical structure of the resultant polymers. The FT-IR spectra (Fig. 2) of CHPEK and CHPES both showed the characteristic absorption bands of C-O stretching vibrations at 1238 cm−1. The bands at 2932 and 2857 cm−1 in the spectra indicated existence of cyclohexyl units. The absorption bands at 1592 and 1496 cm−1 were for C = C stretching vibrations of benzene ring. Besides, the C-H stretching vibrations and out-plane bending vibrations of benzene ring appeared at 3060 cm−1 and 827 cm−1, respectively. Comparing with the spectrum of CHDP, the characteristic absorption bands of O-H at 3200–3600 cm−1 had almost disappeared. Besides, the new absorption band at 1654 cm−1 of ketone group or the absorption band at 1324 cm−1 of sulfone group could be observed, respectively. The 1H–NMR spectrum of CHPEK is shown in Fig. 3. The signals at 7.8, 7.3 and 7.0 ppm were assigned to the aromatic protons. While the signals at 2.3, 1.6 and 1.5 ppm were attributed to the cyclohexyl units. The ratio of corresponding integral curves was about 2:2:4:2:2:1. The 1H–NMR spectrum of CHPES in Fig. 3 appeared familiar signals with a same intensity at 7.8, 7.3, 7.0, 6.9, 2.3, 1.6 and 1.5 ppm, indicated there are benzene ring and cyclohexyl units in this resultant polymer. The chemical composition of CHPEK and CHPES was measured by elemental analysis. As shown in Table 1, the test results are very close to the calculated values. When combining the results of FT-IR, 1H–NMR and elemental analysis, we can confirm that the polymerization proceeds as what we have descript in Scheme 1.

Fig. 3
figure 3

1H–NMR spectra of CHPEK and CHPES in CDCl3

Full size image
Table 1 Intrinsic viscosity (ηint), yield and elemental analysis of CHPEK and CHPES
Full size table

X-ray diffraction analysis of polymers

The structure of aggregating state of CHPEK and CHPES were estimated by X-ray diffraction analysis. As shown in Fig. 4, no crystalline peaks could be observed, only a Gaussian distribution appeared. It signifies that the polymers were amorphous. The amorphous structure of the polymers was mostly due to the existence of pendant cyclohexyl groups and strong polarity of sulfone or ketone groups that weakened the molecular regularity and inhibited the close packing of the molecular chains.

Fig. 4
figure 4

XRD patterns of CHPEK and CHPES

Full size image

Thermal properties

The thermal properties of CHPEK and CHPES were investigated by DSC (Fig. 5), and TGA (Fig. 6) (summarized in Table 2). As shown in Fig. 5, the glass transition temperatures of CHPEK and CHPES were 158 and 208 °C, respectively. It was much higher than that of bisphenol A polyketone (144 °C) [30] and bisphenol A polysulphone (187 ± 3 °C) [31], respectively. From the DSC curve, it could find that the resultant polymers had no endothermic melting peak. It also suggests the amorphous nature of polymers. This result was consistent with the results from XRD. Figure 6 showed the thermal degradation curves of CHPEK and CHPES. The 5% weight loss temperatures of CHPEK and CHPES in nitrogen were 484 and 442 °C, respectively, which was greatly higher than their glass transition temperature. Above results indicate that these polymers have good thermal stability and can be processed with melt method.

Fig. 5
figure 5

DSC curves of CHPEK and CHPES at a heating rate of 10 °C/min in N2

Full size image
Fig. 6
figure 6

TGA curves of CHPEK and CHPES at a heating rate of 10 °C/min in N2

Full size image
Table 2 Thermal and mechanical properties of CHPEK and CHPES
Full size table

Mechanical properties

The tensile strength of the casting films of polymers was tested by a universal testing machine (as shown in Table 2). The average tensile strength of CHPEK and CHPES was 81.0 and 97.7 MPa, respectively. And the Young’s modulus of CHPEK and CHPES were 2.2 and 2.3 GPa, respectively. Besides, the elongation at break of CHPEK and CHPES were both more than 5%. It suggests that the casting films of CHPEK and CHPES both have good mechanical properties. Furthermore, the thermal mechanical properties of polymers were characterized by dynamic thermomechanical analysis (DMA). From the tan delta curves of CHPEK and CHPES in Fig. 7, one obvious transition peak could be observed. Through the transition peak temperatures, we could obtain that the Tgs of CHPEK and CHPES were 180 and 234 °C, respectively. The Tgs are slightly higher than those tested by DSC. The slight differences are mainly due to the different responses of the polymers with these two measurements. Moveover, as the storage modulus curves shown in Fig. 8, CHPEK and CHPES showed high storage modulus of 1.7 and 1.9 GPa, respectively. Most importantly, the storage modules of CHPEK and CHPES still remained over 1.5 GPa even near the glass transition temperature. It indicates that CHPEK and CHPES both have good thermal mechanical performance.

Fig. 7
figure 7

The tan delta curves of CHPEK and CHPES

Full size image
Fig. 8
figure 8

The storage modulus curves of CHPEK and CHPES

Full size image

Rheological properties

The rheological properties of CHPEK and CHPES were studied with a parallel plate rheometer. In order to study the melt stability and melt processing temperature of the resultant polymers, a time and temperature sweep were investigated, respectively. As shown in Fig. 9, the complex viscosity of CHPEK were in the range of 219–1322 Pas from 260 °C to 310 °C, meanwhile the complex viscosity of CHPEK almost remained at 310 Pas during the whole testing time (0–1800 s at 260 °C). It suggests that CHPEK has good melt flowability and stability. Figure 10 showed a decrease in the complex viscosity of CHPES with temperature increase could be observed, and the complex viscosity of CHPES kept at the range of 2700–4600 Pas during the whole testing time (0–1800 s at 340 °C), it also indicates the good melt flowability and stability of CHPES. Comparing with poly(arylene ether ketone) resins, such as PEEK and PEKK, the melt processing window of CHPEK had a significant improve. Besides, for the classical poly(arylene ether sulfone) resins, 340 °C is a relatively low melt processing temperature. These results were mainly caused by the incorporation of pendant cyclohexyl groups weaken the molecular packing and interaction of chain.

Fig. 9
figure 9

The curves of complex viscosities versus temperature and time for CHPEK

Full size image
Fig. 10
figure 10

The curves of complex viscosity versus temperature and time for CHPES

Full size image

Solubility experiment

The results of the solubility experiments of CHPEK and CHPES were summarized in Table 3. According to Table 3, it was found that CHPEK and CHPES could be dissolved in various solvents, such as CHCl3, THF, NMP, DMAc, DMI and etc. It suggests that the resultant polymers have outstanding solubility, and can be processed by solution method. In particular, the solution processability of CHPEK was greatly improved when compared with traditional poly(arylene ether ketone) resins, such as PEEK, PEKK and PEEKK. It was attributed to the incorporation of pendant cyclohexyl groups caused its amorphous structure. On the other hand, CHPEK and CHPES were insoluble in acetone, ethanol, diethyl ether, alkali and acid solution (except concentrated sulfuric acid). It suggests that these polymers still keep moderate corrosion resistance.

Table 3 Solubility behavior of CHPEK and CHPES
Full size table

The optical properties of films

The optical properties of CHPEK and CHPES films were determined by UV-Visible spectroscopy. The cutoff wavelength (λcutoff) and optical transmittance at 400 nm (T400) and 450 nm (T450) of the prepared polymers films were summarized in Table 4. As shown in Fig. 11, the cutoff wavelength (λcutoff) of CHPEK and CHPES films were 337 and 302 nm, respectively. The transparencies of CHPEK and CHPES films measured at 400 and 450 nm were 71.31–81.95% and 81.31–83.46%, respectively. It indicates that the resultant poly(arylene ether)s can be applied as the candidate materials for the optical films, that need good transparent and thermal properties.

Table 4 Optical properties of CHPEK and CHPES
Full size table
Fig. 11
figure 11

UV-vis spectra of CHPEK and CHPES

Full size image

Conclusion

A series of poly(arylene ether)s with pendant cyclohexyl units were designed and synthesized in order to develop high performance PAEs with great melt and solution processability. The poly(arylene ether)s were prepared by solution polycondensation of 4,4′-cyclohexane-1,1′-diyldiphenol (CHDP) and 4,4′- difluorobenzophenone (DFBP) (or 4,4′-difluorodiphenylsulfone (DFDPS)). The resultant poly(arylene ether)s showed good mechanical and thermal properties. Their Young’s Mdoulus were found to be 2.2–2.3 GPa, tensile strengths were 81.0–97.7 MPa, storage modulus were over 1.5 GPa even near glass transition temperature. Moreover, these poly(arylene ether)s exhibited high glass transition temperatures (Tg) of 158–208 °C, and good thermal stability with 5% weight-loss temperature (T5%) of 442–484 °C. Most importantly, all poly(arylene ether)s showed good melt stabilities, low melt processing temperatures and outstanding solubilities, mostly due to the introduction of pendant cyclohexyl groups weakens the molecular packing and interaction of chain. It suggests that the resultant poly(arylene ether)s have excellent processability, and they can be processed by either melt or solution methods. In addition, their solution-casting films exhibited high optical transmittance (>80%) at 450 nm. Hence, the prepared poly(arylene ether)s with pendant cyclohexyl units could be applied as heat resistant, tough and transparent films.