Synthesis and properties of perfluorocarbon chain terminated poly(ether sulfone)

Abstract

Perfluorocarbon groups or related compounds are usually used to modify polymer materials because of their low surface energy properties. In this paper, a novel perfluorocarbon-containing compound was synthesized and used as a capping agent for the preparation of perfluorocarbon chain terminated poly(ether sulfone). The regulation of the fluorine content was achieved by controlling the molecular weight. FTIR and ¹H NMR spectra were used to confirm the existence of perfluorocarbon terminated groups in the polymer chains. In addition, the ratios of F/C were acquired by integral area calculation from the ¹H NMR spectrum. XPS characterization showed that the surface migration phenomenon of fluorine became apparent along with the increase of the F/C ratio. Further testing found that the introduction of perfluorinated end groups into the PES chains had little influence on the polymers' mechanical and thermal performance, by contrast, their surface energy decreased efficiently from approximately 40 mJ m⁻² to 16 mJ m⁻² with the F/C ratio on the surface increased from 0 to 0.556, which could develop PES as a low surface energy material for industrial applications.

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1. Introduction

Poly(ether sulfone) (PES) is a class of special engineering plastics which has excellent chemical stability, heat resistance, mechanical properties and creep characteristics.^1–4 Based on the above advantages, PES materials can be widely used as coatings, water treatment agents, and ultrafiltration membranes.^5–8 In recent years, blending modified fluorine-containing PES composites with enhanced properties have also been processed into a wide range of self-lubricating mechanical parts and non-stick coatings in some special working environments.^9,10 These materials are widely studied and applied due to their simply preparation method and excellent properties such as water-repellent ability, corrosion resistance and friction properties.^11–13 One kind of PES resin (BASF corporation, Ultrason KR 4113) blending with polytetrafluoroethylene (PTFE) has been achieved supply on the market over the past few years, and the blend show excellent sliding wear and friction properties which are detected by different professional inspection agency.¹⁴

Although blending modification has many benefits, the deficiency of above is the high-content demand of fluorine resins, about 10–30 wt%, which causes a serious problem to interface compatibility. Typically as a result, the thermal and mechanical performance of blending composites deteriorate significantly compared with the pristine PES. It is found that the tensile strength (92.1 MPa) of PES starts to decrease with the introduction of PTFE, and while the mass fraction of PTFE is 15%, tensile strength of composites has deteriorated massively as is 65.8 MPa.¹⁵ While such unwanted consequence can be avoided by adding the solubilizer into the material, this will undoubtedly make the preparation process complicatedly and may increase the cost. In this premise, many chemical attempts have been made to prepare exceptional fluorine-containing PES or similar materials. Kim et al. reported highly fluorinated poly(arylene ether sulfone) (FPAESO) designed from low loss optical waveguide materials containing ethynyl end group as thermal crosslinkable groups. The birefringence of copolymers was 0.0021–0.0025, and it also had high initial decomposition temperature which was 479 °C.¹⁶ Another effective modification method is to introduce the large fluorine-containing side group into the polymer chain, for instance, poly(aryl ether) with 3′-trifluoromethyl phenyl group was reported to show an excellent thermal stability, good solubility, low water absorption, low dielectric constant and low refractive index all at the same time.¹⁷

Among a variety of chemical methods, end blocking method can be advantageous in such aspects as reducing costs, simplifying methods and improving performance. A great example of this is the one-step synthesis of waterborne blocked polyurethane, which has a good stability in the pH value of 3–4 by using NaHSO₃ as the blocking agent.¹⁸ In addition to the above-mentioned advantages, the end blocking method usually has little effects on molecule of terminated polymers without changing their main chain structures. Through the FTIR analysis, Chen et al. discovered that the chemical structure of polylmide end-capped with fluorinated phenylethynylaniline (3FPA-PI-50) had not changed before and after the curing, while the postcured polyimide showed a 5% weight loss temperature of 536 °C and T_g about 404 °C (DMA).¹⁹

Perfluorocarbon-containing compounds with aliphatic structures is a very special kind of fluoride.^20–23 Owing to outstanding long perfluoroalkyl chains characteristics, polymers modified by them typically have stable and low surface energy, due to –CF₃ groups in long carbon chains can accumulate closely on the polymer surface.^24,25 However, it is rare in modifying PES by perfluorocarbon functional groups, even though they have been thoroughly studied and used as mechanical material and membrane in polymer modify fields.^26,27

In this study, a novel perfluorocarbon-containing compound was designed and synthesized as capping agent to modify PES by using chemical method. Hydroxyl-terminated poly(ether sulfone) (PES-OH) and perfluorocarbon chains terminated poly(ether sulfone) (PES-F) with different molecular weights were prepared, analysed and contrasted. The characterizations of polymers focused on the chemical structure, mechanical and thermal properties, respectively. What's more, the relationships between fluorine contents and surface energy of the film were further evaluated through a combination of F/C ratio, static contact angle, and XPS analysis.

2. Materials and methods

2.1 Materials

Fluorobenzene (C₆H₅F; 98%), aluminium trichloride anhydrous (AlCl₃; 99%), tetramethylene sulfone (TMS; 97%), N,N-dimethylacetamide (DMAc; 99%), 4,4′-sulfonyldiphenol (99%), sodium carbonate anhydrous (Na₂CO₃; 99.8%) were purchased from Shanghai Aladdin Chemistry Co., Ltd. Bis(4-fluorophenyl) sulfone (99%) was obtained by Jiangxi Renming Pharmaceutical Chemical Co., Ltd. Toluene (99.5%) and ethylene glycol (EG; 99.7%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Perfluorooctanoyl fluoride (97%) was purchased from Sigma-Aldrichin Reagent Co., Ltd. Poly(ether sulfone) (commercial and plastic grade PES; T_g = 225 °C; η = 0.44 dL g⁻¹) was purchased from Jida Special Plastic Co., Ltd. All of solid reagent had been dried for 12 h at 80 °C under vacuum.

2.2 Preparation and characterization of capping agent, PES-OH and PES-F

2.2.1 Preparation of capping agent, p-perfluorooctanoyl fluorobenzene. 14.67 g (0.11 mol) of AlCl₃ and 43 ml of fluorobenzene were added into a 250 ml flask equipped with a mechanical stirrer, a dropping funnel and a thermometer. Then 43.25 g (0.1 mol) of perfluorooctanoyl fluoride mixed with 50 ml of fluorobenzene were dropped into the reactor continuing for 1 h at −10 to 0 °C. The reaction mixture was stirred at room temperature for another 5 h, and poured into dilute hydrochloric acid (1%, mass concentration) under ice cooling. After the colour of lower stirring layer changed from brown-red to colourless, the product was washed by sodium bicarbonate solution (2%, mass concentration), water and DMSO each for twice. At last, collected the fraction at 110–112 °C under 0.01 MPa by vacuum distillation. The capping agent was a colourless transparency liquid. Yield: 81%, mp: −1 to 0 °C (obtained by DSC measurement).

2.2.2 Preparation of hydroxyl-terminated PES (PES-OH). 6.36 g (0.025 mol) of bis(4-fluorophenyl) sulfone, 4,4′-sulfonyldiphenol (excess mole fraction were 2%, 3%, 4% and 6%, respectively), 2.92 g (0.0275 mol) of Na₂CO₃, 15 ml of toluene and 30 ml of TMS were added together into a 100 ml flask equipped with a mechanical stirrer, a dropping funnel and a thermometer. The reaction mixture was heated to 150 °C for 2 h to remove the water by co-boiling. After distilling off excess toluene, the reaction temperature was maintained to 200 °C for 3 h to finish the polymerization. Finally, the viscous reaction solution was poured into deionized water and white strip solids precipitated. After mechanical crushing, hydroxyl-terminated PES (PES-OH) powders were washed with deionized water and anhydrous ethanol under the nitrogen protection, and dried at 80 °C in a vacuum oven. In order to facilitate the description of polymers, the products are named as PES_2%-OH, PES_3%-OH, PES_4%-OH and PES_6%-OH according to the excess fractions of 4,4′-sulfonyldiphenol.

2.2.3 Preparation of perfluorocarbon chains terminated PES (PES-F). PES-OH was prepared by the similar process as Section 2.2.2 at first. When polymerization of PES-OH finished, the polymer solution was cooled to 160 °C and the capping agent was added to terminate PES-OH for 1 h. Then the capping reaction was continued for 3 h at 200 °C. The purification method of products was the same as PES-OH, and the corresponding products were named as PES_2%-F, PES_3%-F, PES_4%-F and PES_6%-F.

The synthesis of capping agent and polymers are shown in Fig. 1.

Fig. 1 Synthesis of capping agent and polymers.

2.2.4 Inherent viscosity and molecular weight comparison of PES-OH and PES-F. The inherent viscosity of polymers was measured by Ubbelohde viscometer. PES-OH and PES-F were dissolved in DMAc at concentration of 0.5 g dL⁻¹. The pure solvent flow time (t₀) and the polymer solution flow time (t) were recorded, and then relative viscosity (η) was calculated according to the formula

η = (1/C) × ln(t/t₀).

The polymer molecular weights (M_n) were tested at 80 °C by GPC (Polymer PL-GPC 220), using DMF (HPLC) as the mobile phase with a flow rate of 1.0 ml min⁻¹.

2.2.5 Characterization of capping agent, PES-OH and PES-F. The FTIR spectra of PES-OH and PES-F were recorded with potassium bromide (KBr) discs by Nicolet Impat 410. The ¹H NMR (500 MHz) spectra of capping agent, PES-OH and PES-F were recorded on a Bruker AcanceIII500 (Bruker Co.) using CDCl₃ or DMSO-d₆.

2.2.6 Thermal and mechanical tests of PES-OH and PES-F. The glass transition temperature (T_g) of the polymers were determined by using a Q2000 DSC (TA Instruments Co.) under a heating rate of 10 °C min⁻¹ from 50 °C up to 300 °C in nitrogen. The thermal stability of PES-OH and PES-F were analysed using a Pyris 1 TGA thermal analyser system under the heating rate of 5 °C min⁻¹ in air atmospheres. PES-OH and PES-F mechanical testing splines were prepared by injection molding, and tested by SHIMADZU AG-1 universal testing machine. The injection temperature and pressure was 340 °C and 720 Pa, and the mould temperature was 180 °C.

2.2.7 Preparation and tests of PES-OH and PES-F films. The polymer films were prepared by solution casting method in DMAc at a concentration of 1 g/10 ml. Then DMAc solvent was removed following the steps of 60 °C/12 h, 80 °C/2 h, 100 °C/2 h, 120 °C/2 h under atmospheric conditions, respectively, and 60 °C/6 h under vacuum finally. After all the films removed from the glass plates, their static contact angles on air-contacting surface were tested by JC2000C2 contact angle goniometer (Shanghai Digital Technology Co., Ltd.) with deionized water and EG in the means of hanging drop method. Then the surface energy was calculated by Owens method. Fluorine content (at%) on the film surface was tested by ESCALAB 250 X-ray photoelectron spectroscopy. All the test objects were the air-contacting surface of films in the same preparation process.

3. Results and discussion

3.1 Synthesis of capping agent and polymers

A novel perfluorocarbon-containing compound, (p-perfluorooctanoyl fluorobenzene), was synthesized at a high yield above 80% by Friedel–Crafts reaction. ¹H NMR spectra of the capping agent is shown in Fig. 2. The peaks at 7.22 ppm and 8.12 ppm are corresponding to the only two chemical shifts of hydrogen atoms in the benzene ring, indicating that the capping agent was successfully synthesized.

Fig. 2 ¹H NMR spectra of the capping agent.

PES-OH with various molecular weights were synthesized by controlling molar ratio of two monomers, and through the next capping reaction, PES-Fs with different end group content were prepared. As shown in Table 1, with the mole excess fraction of 4,4′-sulfonyldiphenol increased from 2% to 6%, the viscosity of PES-OH reduces from 0.55 dL g⁻¹ to 0.22 dL g⁻¹, corresponding to the M_n (tested by GPC) from 46 100 to 21 300. After the capping reaction, by comparison with PES-OH, PES_3%-F and PES_6%-F show the closest polymerization degree except inherent viscosity (η) and molecular weight (M_n), which due to its high content and molecular mass of perfluorocarbon end groups. These results prove that the molecular weight and the end group content of PES have been effectively regulated by adjusting the feeding ratio of monomers. The characterizations of inherent viscosity and molecular weights indicate that there are almost not any other effects on chain segment, which confirm that the fine-tuned for blocking reaction is effective and reasonable. As shown in Table 2, it is noteworthy that PES_3%-OH and PES_3%-F both show comparable inherent viscosity 0.41 dL g⁻¹ and 0.43 dL g⁻¹ respectively, with the commercial PES (η = 0.44 dL g⁻¹), which could be used as engineering plastics.

Table 1 Comparison of inherent viscosity (η), molecular weight (M_n), glass transition temperature (T_g) and other basic dates

Sample	η^a (dL g^−1)	Mn^b (×10^3)	PDI^b	Tg^c (°C)	^d	Mn^d (×10^3)
a η is measured at a polymer concentration of 0.5 g dL^−1 in DMAC at 25 ± 0.1 °C.b Mn and PDI are tested by GPC.c Tg is tested by DSC.d and Mn are acquired by integral area calculation from ^1H NMR spectrum.
PES2%-OH/F	0.55/0.53	46.1/48.7	1.55/1.83	235/230	54.3/49.2	13.10/12.36
PES3%-OH/F	0.41/0.43	37.1/38.9	1.61/1.65	230/221	45.3/44.9	10.98/11.36
PES4%-OH/F	0.32/0.31	33.0/31.1	1.63/1.62	223/215	40.9/36.0	9.96/9.30
PES6%-OH/F	0.22/0.26	21.3/25.5	1.65/1.72	221/210	27.6/27.3	6.87/7.28

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Table 2 Mechanical properties of polymers

Sample	η (dL g^−1)	Tensile strength (MPa)	Young's modulus (GPa)	Elongation at break (%)
PES3%-OH	0.41	84.2	2.90	9
PES3%-F	0.43	82.6	2.90	9
PES (commercial)	0.44	89.7	3.07	13

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3.2 Characterization of polymers and end-group determination

FTIR and ¹H NMR spectra (500 MHz) are used to verify the existence of perfluorocarbon end groups and calculate the fluorine content. FTIR spectra of PES-OH and PES-F are shown in Fig. 3. In comparison with PES-OH, PES-F shows a new peak at 1700 cm⁻¹ in FTIR spectra, corresponding to the absorption of carbonyl groups, which increases with the end group and fluorine contents after normalization with the peak intensity of –SO₂– at 1150 cm⁻¹ as the benchmark.

Fig. 3 FTIR spectra of polymers.

¹H NMR spectra of PES_6%-OH and PES_6%-F are shown in Fig. 4, and it is clearly consistent with the polymers structure. The peaks at 7.90 ppm, 7.73 ppm and 6.87 ppm observed in spectra are the three different chemical shifts of PES_6%-OH end group. After the capping reaction, these peaks have disappeared, while two kinds of methyl signals at 8.13 ppm and 7.39 ppm appeared in addition to the aromatic signals, which are the chemical shifts of corresponding protons on perfluorocarbon end group. These new peaks prove that perfluorocarbon group have been introduced into the molecular chain successfully. Fluorine contents are also represented by F/C ratio, acquired by integral area calculation at the corresponding position, as summarized in Table 4. The F/C ratios of PES_2%-F, PES_3%-F, PES_4%-F and PES_6%-F are 0.049, 0.053, 0.066 and 0.086, respectively. All the results of ¹H NMR agree with the content changes in FTIR spectra, which further prove that the little effect of the blocking reaction on the molecular structure of PES backbone.

Fig. 4 ¹H NMR spectra of PES_6%-OH and PES_6%-F.

3.3 Effects of thermal and mechanical properties

In this part, to explore the effects of introducing the perfluorocarbon group into the end of PES molecular chain, commercial PES, PES_3%-OH and PES_3%-F were selected for comparative study (as shown in Table 2). The first reason is their inherent viscosity data exhibit the high molecular weights of these polymers, which is a basic requirement for achieving the authentic thermalstability and mechanical properties of the polymers. The second is that compare with PES_2%-OH and PES_2%-F, the effects of PES_3%-OH and PES_3%-F should be more obviously due to their higher content of end-capping group. In addition, only the polymers with similar inherent viscosity can be conducted an effective contrastive study, so the inherent viscosity of commercial PES with 0.44 dL g⁻¹ was adopted.

The thermal properties of the typical PES_3%-OH, PES_3%-F and commercial PES were evaluated via TGA at the same heating rate under air atmospheres, as shown in Fig. 5. As we can see, due to the instability of hydroxyl end groups in hot atmosphere condition, initial decomposition temperature of PES_3%-OH is lower than that of PES_3%-F and commercial PES, which is 410 °C. Although the decomposition temperature of PES-F is similar with that of commercial PES, PES_3%-F has a narrow decomposition range from about 450 °C to 625 °C, which indicates its decomposition rate is significantly higher than that of commercial PES, this may due to the end group structure of long carbon chain can't keep enough stability at the high temperature in air atmosphere, even though –CF₃ group has good thermal stability. However, this does not influence actual use of PES-F. Above all, PES_3%-F still exhibited excellent thermalstability at practically the same initial decomposition temperature as commercial PES with no remarked weight loss up to 450 °C.

Fig. 5 TGA curves of polymers in air at scanning rate of 5 °C min⁻¹.

The glass transition temperature (T_g) values of all the PES-OH were observed in the range of 221–235 °C by differential scanning calorimetry (DSC). As summarized in Table 1, all the T_g values of PES-OH are higher than PES-F obviously, due to the existence of hydrogen bond among the PES-OH chains, which can improve the molecular forces of PES-OH. The above results were similar with previous research for modifying polymer by perfluorocarbon group.²⁸

The mechanical properties of PES_3%-OH, PES_3%-F and commercial PES are summarized in Table 2. Due to the end-capping modifications with –OH and perfluorocarbon groups on PES, all the mechanical property data of PES_3%-OH and PES_3%-F are slight lower than the data of commercial PES, and the tensile strength of PES_3%-F is slightly lower than that of PES_3%-OH. Even so, the experimental results showed that PES_3%-F also possesses the potential application in structural material field as commercial PES.

3.4 Relationships between fluorine contents and surface energy of polymers

It is known that element enrichment phenomenon is a very important characteristic of fluorine-containing materials. Surface energy theory reveals that component parts (elements or groups) with lower surface energy of polymer film trend to transfer to the air-contacting surface, and the analysis of surface energy and content of elements are the most commonly means for studying this phenomenon.^29–31 Based on this premise, the deionized water and EG contact angles of films was measured in order to analysis the change of surface energy, as shown in Fig. 6. The average values of contact angle which calculated from three parallel samples are recorded as θ₁ and θ₂ in Table 3. Taking PES_6%-OH and PES_6%-F for example, the water contact angles of PES_6%-F are greater than that of PES_6%-OH, as well as the expect of EG contact angle results. The surface energy values were calculated with two liquids by Owens method. It is worth noting that the contact angels and surface energy results of PES-OH have not shown apparent regularity, especially for that of PES_4%-OH. It may due to the low content of –OH end group and testing accuracy. What's more, some research showed that migration of –OH group could also happen on certain conditions, which caused a change in contact angle on the air-contacting surface of the film.³² Above all, these results show that perfluorocarbon groups can reduce the surface energy effectively, and along with the increasing of mole excess fraction of 4,4′-sulfonyldiphenol from 2% to 6%, the surface energy of PES-F reduces from 28.2 mJ m⁻² down to 16.3 mJ m⁻².

Fig. 6 Water/EG contact angle of films containing commercial PES (a/g), PES_3%-OH (b/h), PES_2%-F (c/i), PES_3%-F (d/j), PES_4%-F (e/k) and PES_6%-F (f/l).

Table 3 Static contact angles and surface energy calculation results of PES-OH and PES-F. The contact angles were recorded after calculating an average of three measurements

Solvent	H2O	(CH2OH)2	Surface energy/mJ m^−2
Contact angle/°	θ1	θ2
PES2%-OH/F	82.0/82.8	45.8/59.0	40.7/28.2
PES3%-OH/F	80.0/86.6	43.7/65.0	40.8/24.8
PES4%-OH/F	81.3/92.2	44.1/70.1	41.8/22.8
PES6%-OH/F	78.8/100.1	42.1/82.0	41.3/16.3
PES (commercial)	82.4	46.4	40.4

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For further investigation on this appearance, the surface compositions of PES-OH and PES-F films were characterized carefully by XPS analysis on air-contacting surfaces. The XPS curves of PES_6%-OH and PES_6%-F films are presented in Fig. 7. It is observed that the peak at 689 eV represents of fluorine element, and no signal of fluorine element can be detected when running XPS to the PES_6%-OH film.

Fig. 7 XPS curves of PES_6%-OH and PES_6%-F.

In addition, migration of fluorine can normally be studied by analyzing element ratio³³ by comparing with ¹H NMR and XPS integrated results, and the corresponding calculation methods are summarized in Table 4, the ratios of fluorine and carbon (F/C) on the film surface are significantly higher than the normal rates in the polymers. The ratios between the F/C values on surface and in bulk are also used to indicate the degree of fluorine migration, which is denoted by the letter Q. It is found that as the ratio of F/C on the surface obviously changed from 0.125 to 0.556, the value of Q dramatically increased from 2.551 to 6.465. These results also suggest that the enrichment degree of F element can be increased on PES film surface by increasing the content of end-capping perfluorocarbon groups.

Table 4 Ratios of fluorine and carbon (F/C) and degree of fluorine migration (Q)

Sample	F/C^a	F/C^b	Q[(F/C^b)/(F/C^a)]
a Ratios of F/C in PES-F ontology are calculated through two of same end groups and the average degree of polymerization in Table 1.b Ratios of F/C on the surface of PES-F films, obtained by XPS tests. All the F/C ratios are calculated through the element atom content. The value of Q is the ratio between F/C^b and F/C^a, which indicate the degree of fluorine migration.
PES2%-F	0.049	0.125	2.551
PES3%-F	0.053	0.211	3.981
PES4%-F	0.066	0.365	5.530
PES6%-F	0.086	0.556	6.465

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Above all, the surface migration of F element leads to the reduction of film surface tension which show a rapidly and efficiently change of contact angles.

4. Conclusions

In conclusion, a series of perfluorocarbon end-capped PES with various fluorine contents were successfully synthesized by a simple reaction method. Our study found that the perfluorocarbons of modified PES can migrate to the PES film surface spontaneously. And the test results indicated without incurring excessive the mechanical and thermal properties loss, only introducing a few content of perfluorocarbons into the end of PES chains can effectively reduce the surface energy of PES film, make these modified PES materials have the excellent properties on non-hydrophilic and non-lipophilic. Hence, we have reasons to believe that it is a kind of promising material with great potential in applications, such as coating, membranes and friction fields.

Acknowledgements

This work was supported by China Jilin Provincial Science & Technology Department (Grant 20150203001GX). The study was being funded as part of corresponding base foundation, valuable advices and assistance were given by some researchers from Engineering Research Center of High Performance Plastic at Chinese Jilin University, and we would thank our laboratory members for their generous help particularly.

Notes and references

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