Surface Properties of Hyper-Cross-Linked Polymeric Resins Using

Article pubs.acs.org/JPCC

Surface Properties of Hyper-Cross-Linked Polymeric Resins Using Inverse Gas Chromatography: Effect of Post-Cross-Linking Solvents Lijuan Jia,† Xiaofei Song,† Jian Wu,† and Chao Long*,†,‡ †

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, China ‡ Yancheng Environmental Protection Technology and Engineering Research Institute, Nanjing University, 888 Yingbin Road, Yancheng 22400, China ABSTRACT: Two series of hyper-cross-linked polymeric resins (hyper-resins) were prepared via post-cross-linking of low-cross-linked macroporous poly(vinylbenzyl chloride-divinylbenzene) (poly(CDVB)) or poly(styrene-divinylbenzene) (poly(DVB)) swollen in the solution of 1,2-dichloroethane or nitrobenzene, respectively. Inverse gas chromatography was used to investigate the effect of post-cross-linking solvents on the surface chemistry of hyper-resins. The dispersive component of the surface free energy (γds ) and surface acid−base character of hyper-resins were estimated using the retention time of different nonpolar and polar probes at the infinite dilution region. Results showed that the values of γds were basically identical for the hyper-resins prepared using the same precursor; the acidity and basicity constants KA and KD confirmed that the surface of four hyper-resins were Lewis amphoteric with predominantly basic character, but the strength of the surface acidic and basic sites of hyper-resins synthesized using nitrobenzene as post-cross-linking solvent were slightly higher than that of hyper-resins synthesized using 1,2dichloroethane as post-cross-linking solvent. In addition, characterization of the pore structure determined by N2 gas adsorption at 77 K revealed that the effect of the post-cross-linking solvents on the porous structure of hyper-resins was minimal.

1. INTRODUCTION During the last few decades, porous polymeric resins have been emerging as a potential alternative to activated carbon as an adsorbent because of its controllable pore structure, stable physical, chemical properties, as well as regenerability on site.1 Among the permanently porous polymeric resins, “Davankovtype” hyper-cross-linked polymeric resins (hyper-resins) represent a class of predominantly microporous organic materials with an extremely high specific surface area (>1000 m2/g)2,3 and have been used as sorbents for organic vapors,4−7 for the recovery of organic compounds in water,1 and in chromatography.8,9 Generally, Davankov-type hyper-resins are prepared by an extensive post-cross-linking of very lightly cross-linked polystyrene or chloromethylated styrene-divinylbenzene (VBC-DVB) copolymers dissolved or swollen in the presence of a thermodynamically “good” solvent and a Friedel−Crafts catalyst.10 Therefore, swelling behavior is very important for the reaction on the polymer matrix. In thermodynamically “good” solvents, the side chains of the polymer have enough extension and mobility and the cross-linking bridges can easily form.11 The porous structure and surface chemistry of the adsorbent are two key factors affecting its adsorption behaviors. It is wellknown that the post-cross-linking solvents have significant influences on the nanopore structure of the polymeric adsorbent.12−15 Taking into account the hydrophobicity of the matrix of poly(styrene-divinylbenzene) material, hyper© 2015 American Chemical Society

resins should possess an absolute hydrophobic surface. Therefore, little attention was paid to investigate the effect of post-cross-linking solvents on surface chemistry of hyper-resins. However, it is surprising that neutral nonmodified hyper-crosslinked polystyrene displayed wettability by water,16 which was due to the existence of oxygen-containing functional groups on its surface.17 It is believed that the formation of oxygencontaining functional groups can be attributed to oxidation of chloromethyl groups by post-cross-linking solvent nitrobenzene.18,19 Some studies argued that the formation of surface functional groups was due to hydrolysis of chloromethyl groups.20,21 Therefore, it is of great interest for preparing the specific adsorbent to understand the effect of post-cross-linking solvents on its surface chemistry in addition to its porous structure. The nitrobenzene and 1,2-dichloroethane are the most widely used solvents in manufacturing commercial hyperresins.22−25 However, to our knowledge, there are few studies comparing the difference of salient properties of the two kinds of polymers, especially surface chemistry, prepared using nitrobenzene and 1,2-dichloroethane as post-cross-linking solvents. In the present study, two series of hyper-resins were prepared by post-cross-linking of poly(vinylbenzyl chloridedivinylbenzene) (poly(CDVB)) or poly(styrene-divinylbenReceived: April 29, 2015 Published: September 1, 2015 21404

DOI: 10.1021/acs.jpcc.5b07110 J. Phys. Chem. C 2015, 119, 21404−21412

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The Journal of Physical Chemistry C Scheme 1. Synthetic Procedures of Hyper-Resins

(DCE) for 8 h under N2 atmosphere in a 500 mL three-neck round-bottomed flask. Then, 10 g of ferric chloride was added to the mixtures. The mixtures were stirred at 60 rpm for 6 h at 353 K. The obtained samples were termed poly(CDVB)-NB or poly(CDVB)-DCE, respectively. Two other hyper-resins (poly(DVB)-DCE and poly(DVB)NB) were synthesized by suspension polymerization of styrene and divinylbenzene followed by a Friedel−Crafts-type postcross-linking (procedure 2). First, the St-DVB copolymer with cross-linking density of 8% was synthesized as described in ref 30 then chrolomethylated using chrolomethyl methyl ester as the reaction reagent and anhydrous ferric chloride as catalyst. The obtained chloromethylated St-DVB copolymers (50 g) were swollen in 250 g of nitrobenzene or 1,2-dichloroethane (DCE) for 8 h under N2 atmosphere in a 500 mL three-neck round-bottomed flask. Then, 10 g of ferric chloride was added to the mixtures. The mixtures were stirred at 60 rpm for 6 h at 353 K. All the final sample beads were washed with hot water then extracted with ethanol in a Soxhlet for 12 h, and finally dried in a vacuum oven at 353 K for 24 h. The porous texture of the four hyper-resins was determined by N2 isotherms data at 77 K, using a gas adsorption analyzer ASAP 2020 (Micromeritics Instrument Co., US). Their specific surface area (SBET), micropore volume (Vmicro), and mesopore volume (Vmeso) were calculated from the N2 isotherm data at 77 K by Brunauer−Emmett−Teller (BET), Dubinin−Astakov (DA), and Barrett−Joyner−Halena (BJH) methods, respectively. The surface properties of the four resins were evaluated by the method of Boehm31 and Fourier transfrom infrared (FTIR) spectroscopy. 2.3. IGC Experiments. 2.3.1. Apparatus and Procedure. IGC experiments were carried out with a gas chromatography (Shimadzu GC2014, Japan) equipped with a flame ionization detector (FID). The injector and the detector were stabilized in the GC system at 453 and 523 K. The column of the measurement was carried out in the temperature range of 403− 463 K under a nitrogen flow rate of 35 mL/min. Examined

zene) (poly(DVB)) swollen in the 1,2-dichloroethane or nitrobenzene, respectively. The main objectives of this study were to investigate the effect of post-cross-linking solvents on surface chemistry of hyper-resins. The surface chemistry measurements, including the dispersion component of surface energy and acid−base parameters on the four samples, were performed using inverse gas chromatography at infinite dilution (IGC-ID) and at column temperatures of 403, 423, 443, and 463. It has been proven that IGC-ID is a straightforward and very sensitive technique for the characterization of polymers with very low concentration of surface groups.26−28 Compared with the traditional methods such as infrared spectroscopy and Bohem titration, IGC could afford an accurate result with less sample and shorter time.

2. MATERIALS AND METHODS 2.1. Materials. Vinylbenzyl chloride (VBC) and styrene (St) were purchased from Sigma-Aldrich Chemical Co., China. Divinylbenzene (DVB, 63% grade) was purchased from Shandong Dongda Chemical Industry (Group) Company, China. Toluene, benzoperoxide, gelatin, sodium chloride, sodium carbonate, magnesium sulfate, methylthionine chloride, nitrobenzene, 1,2-dichloroethane, alkanes (n-pentane, n-hexane, n-heptane, and n-octane), ethyl acetate, acetone, and chloroform were supplied by Nanjing Chemical Reagent Company, China (>99.5% purity). 2.2. Synthesis of Hyper-Cross-Linked Polymeric Resins. Hyper-resins were prepared using two different synthetic procedures (shown in Scheme 1). In procedure 1, the hyper-resins were synthesized by suspension polymerization of vinylbenzyl chloride and divinylbenzene followed by a Friedel−Crafts-type post-cross-linking. First, low-cross-linked VBC-DVB copolymer was synthesized by suspension polymerization; the synthetic process was described in detail in a previous paper.29 The next step is the synthesis of hyper-crosslinked polymeric adsorbents. The precursor resin beads (50 g) were swollen in 250 g of nitrobenzene or 1,2-dichloroethane 21405


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The Journal of Physical Chemistry C

Figure 1. Adsorption−desorption isotherms of N2 on four hyper-resins at 77 K.

contributions, corresponding to the dispersion and specific interactions, respectively:34

solid polymeric resins (1 g) were dried at 383 K and placed in the chromatographic stainless steel column (50 cm length, 3 mm i.d.). Experiments were carried out at infinite dilution region, injecting less than 0.2 μL of the gaseous probes using a 1 μL syringe. For each measurement, at least three repeated injections were taken, obtaining reproducible results. The experimental parameter measured in IGC is the retention time of the probes, which can be converted to the net specific retention volume (Vg) by the following relationship:32 Vg = Fj

(t R − tM) ⎛ P0 − Pw ⎞⎛ T ⎞ ⎜ ⎟⎜ ⎟ m ⎝ P0 ⎠⎝ Tmeter ⎠

γs = γsd + γss

The dispersive component of the surface energy of the solid, γds , which is due to London forces and is unspecific for all adsorbates, is determined using Dorris and Gray’s method with the retention times of n-alkanes,35 given by γsd =

2 ⎡ ⎤ 3 ⎢ (pi /p0 ) − 1 ⎥ 3 2 ⎢⎣ (pi /p0 ) − 1 ⎥⎦

ΔGCH2 2 4N 2αCH2 2γCH

(4)

2

where N is Avogadro’s number, αCH2 the area occupied by a CH2 group (0.06 nm2), and γCH2 (mJ/m2) the surface energy of a solid consisting only of CH2 groups, which is a function of temperature (K):36

(1)

where tR is the retention time (min) and tM is the retention time of nonadsorbing marker (hold up time), estimated by Grobler−Balizs method33 using the retention time of methane. F is the flow rate of carrier gas, P0 the outlet column pressure, Pw the vapor pressure of water at the flowmeter temperature (Pa), T the column temperature, and Tmeter the ambient temperature (K). j is the James−Martin factor for the correction of gas compressibility when the column inlet (Pi) and outlet (P0) pressures are different and it is given by j=

(3)

γCH = 35.6 + 0.058(293 − T ) 2

(5)

The increment of adsorption energy corresponding to methylene group, ΔGCH2, could be calculated from the standard free energy of adsorption at infinite dilution, ΔGads (kJ/mol), which can be expressed as37 ΔGads = −RT ln Vg + C

(2)

(6)

The parameter, C, is a constant related to the standard states. Then, ΔGCH2 may be calculated from

2.3.2. Standard Surface Free Energy: Dispersive and Specific Components. As in the case of the free energy of adsorption, the surface free energy of the adsorbent, γs (mJ/ m2), may be split into dispersion, γsd, and specific, γss,

ΔGCH2 = −RT ln 21406

Vg(n) Vg(n + 1)

(7) DOI: 10.1021/acs.jpcc.5b07110 J. Phys. Chem. C 2015, 119, 21404−21412

Article

The Journal of Physical Chemistry C where Vg(n) and Vg(n+1) are the specific retention volumes of four consecutive n-alkanes having n and (n + 1) carbon atoms, respectively. ΔGCH2 is independent of the chosen reference state of the adsorbed molecule. Thus, at constant temperature, for a series of alkane probes, a plot of RT ln Vn versus the number of carbon atoms should give a straight line; the slope of this line is equal to a single methylene group’s free energy of adsorption, ΔGCH2. For a given adsorbate, the free energy of adsorption, ΔGads, is the sum of energies of adsorption attributed to dispersive and specific interactions. The standard free energy of adsorption takes into account the standard free energy of adsorption of polar solutes on solid surfaces, namely, the dispersive contribution, ΔGd, and the specific contribution, ΔGsp:37 ΔGads = ΔGd + ΔGsp

for all four hyper-resins. According to IUPAC classification, all the hyper-resins were typical of adsorbents with a predominantly microporous structure, as the majority of pore-filling occurred at relative pressures below 0.1. For poly(CDVB)-DCE and poly(CDVB)-NB resins synthesized according to procedure 1, the remarkable hysteresis loops indicated the existence of mesoporous structures; there were not obvious hysteresis loops for poly(DVB)-DCE and poly(DVB)-NB resins synthesized according to procedure 2, suggesting a smaller mesopore volume. However, it is worth noting that two kinds of hyperresins prepared using the same synthetic procedure presented similar pore structures. Table 1 lists the porous parameters of Table 1. Pore Structure and the Content of Oxygen Surface Groups on Four Hyper-Resins

(8) adsorbent

sp

The specific component of surface free energy (ΔG ), which can provide information about chemical surface functionalities, is closely related with specific interactions such as acid−base interactions, hydrogen bonding between a particular polar adsorbate and the solid surface, calculated from eq 9: ΔGSP = −RT ln(Vg /Vgref )

(9)

where Vg is the net retention volume of a polar probe and Vref g is the net retention volume of a nonpolar probe (n-alkane) at the same molar deformation polarization (PD). 2.3.3. Acid−Base Nature of the Surfaces. To characterize the Lewis acid−base properties of a solid surface, IGC using extra polar probes is needed. The specific interactions of polar probes with the solid substrates38−41 are characterized by their donor (DN) and acceptor (AN*) numbers. The concept of donor−acceptor interactions is an extension of the Lewis acid− base reactions, dealing with coordinate bonds which are formed by sharing a pair of electrons between donor and acceptor species.41 The specific enthalpy of adsorption (ΔHsp) can be obtained by plotting ΔGsp/T vs 1/T using eq 10. ΔGsp = ΔH sp − T ΔS sp

(10)

Given ΔH of the various polar molecules, the acidic constant KA and basic constant KD, the two constants characterizing the solid substrate, are determined using the following relationship: (11)

KA and KD characterize the degree of Lewis acidity (electron acceptors) and the degree of Lewis basicity (electron donors) of solid surface, respectively. To compute KA and KD, a plot of ΔHsp/AN* versus DN/AN* yields a straight line with slope KA and intercept KD (eq 12). The ratio KD/KA describes the character of the sorbent surface (acidic or basic). When KD/KA > 1, the surface is considered to be basic, while for KD/KA < 1 the surface is considered to be acidic. −ΔH sp DN = KA + KD AN* AN*

SBET (m2/g) Vmicro (mL/g) Vmeso (mL/g)

1347.4 0.49 0.93

-COOH (mmol/g) -COOR (mmol/g) −OH (mmol/g) CO(mmol/ g) total acidic groups (mmol/g)

0.28

poly(CDVB)NB

Pore Structure 1248.2 0.49 0.93 Surface Chemistry 0.27

poly(DVB)DCE

poly(DVB)NB

1379.5 0.52 0.49

1213.9 0.53 0.48

0.19

0.21

nd

nd

nd

nd

0.17

0.20

0.15

0.19

0.05

0.05

0.03

0.07

0.50

0.52

0.37

0.47

the four resins. It can be clearly seen that the hyper-resins possessed similar micropore and mesopore volumes as well as BET surface area, no matter that they were synthesized according to procedure 1 or procedure 2, indicating that the nitrobenzene or 1,2-dichloroethane as post-cross-linking solvents had a negligible influence on pore structure of hyper-cross-linked resins. The possible reason is that the nitrobenzene and 1,2-dichloroethane have basically identical solubility parameters (10.0 and 9.8 (cal/cm3)0.5, respectively),42 which can provide useful information for the judicious selection of the appropriate solvent, termed a thermodynamically good solvent.43 Therefore, the precursors low-cross-linking poly(CDVB) and poly(DVB) had similar swelling behavior in the solution of nitrobenzene or 1,2-dichloroethane, resulting in hyper-resins with similar porous structure. These results are consistent with Zheng’s result that different post-cross-linking solvents (including nitrobenzene, chlorobenzene, and odichlorobenzene) with similar solubility parameters (10.0, 9.5, and 10.0(cal/cm3)0.5, respectively) had little effect on the porous structure of hyper-resins.19 In addition, the type and quantity of surface groups were evaluated using the Boehm method. It is worth noting that there was only a slight difference in the total number of groups between two hyperresins synthesized using procedure 1 or procedure 2 (seen in Table 1). To further understand the surface chemistry of four resins, inverse gas chromatography experiments were performed. 3.2. Dispersive Component of the Surface Free Energy. Specific retention volumes of probes, Vg, were calculated in the temperature range of 403−463 K using eq

SP

−ΔH sp = AN*·KD + DN·KA

poly(CDVB)DCE

(12)

3. RESULTS AND DISCUSSION 3.1. Textural Characterization. It is observed in Figure 1 that the nitrogen uptake increased sharply with the increment of relative pressure at lower relative pressure (P/P0) below 0.05 21407


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Figure 2. Plot of RT ln Vg vs carbon number in alkane probes for the adsorption on four hyper-resins.

1. Figure 2 shows that RT ln Vg and the number of carbon atoms have a good liner relationship. The slopes of the linear functions given in Figure 2 represent the increment in ΔGCH2. The dispersive component of the surface energy (γds ) was calculated through eq 4 and is shown in Figure 3. It was observed that the γds value decreased with temperature, which had also been observed in polymers,44 carbon black,45 and other adsorbents. This decrease was attributed to the entropic contribution to the surface free energy change. Noticeably, the dispersive components of surface free energy of the poly(CDVB)-DCE and poly(CDVB)-NB over the temperature

range examined were basically identical; the same result was found for poly(DVB)-DCE and poly(DVB)-NB with a maximum deviation of less than 7.1%, which could be attributed to the similar porous parameters of hyper-resins synthesized using nitrobenzene or 1,2-dichloroethane as the post-cross-linking solvent. The results suggest that the effect of the post-cross-linking solvent (nitrobenzene or 1,2-dichloroethane) on the microstructure (i.e., pore volume and porous distribution) of hyper-cross-linked resins was minimal. 3.3. Acid−Base Nature of the Surfaces. Adsorption of nalkanes took place through dispersive interactions, thus providing information related to microcosmic porous structure; however, polar probes were needed to determine the acid−base character of the polymer surfaces. Generally, the surface free energy of solids consisted of the dispersive component and the specific component. The specific component contained all the polar forces, such as dipole− dipole and acid−base interactions (or electron acceptor−donor effects). In fact, it is usually assumed that the specific contribution of the adsorption of polar probes are actually acid−base interactions only, because acid−base interactions involved much higher energies than dipole−dipole.38 Specific interactions (ΔGsp) were determined at a given temperature from the differences of adsorption free energy between the polar probes and the reference line composed with data obtained from the elution of n-alkanes. An example of such a calculation is depicted in Figure 4, where the alkane line corresponds to the series from C5 to C8. The contribution of specific interactions to the free energy of adsorption, ΔGsp, of polar probes was determined according to Figure 4. The straight reference line defined the London dispersive interactions, while the polar probes lay above this line,

Figure 3. Dispersive component of the surface energy of four hyperresins at various temperatures. 21408


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Figure 4. Specific interaction parameter based on PD at 403 K on four resins.

Table 2. Variation of ΔGsp with Temperature and ΔHsp and ΔSsp Values of Polar Probes on Four Resins −ΔGsp (kJ/mol) adsorbents

polar probes

403 K

423 K

443 K

463 K

−ΔHsp (kJ/mol)

ΔSsp (J/mol K)

poly(CDVB)-DCE

acetone ethyl acetate chloroform acetone ethyl acetate chloroform acetone ethyl acetate chloroform acetone ethyl acetate chloroform

5.60 4.87 3.90 5.62 5.30 4.04 6.25 5.49 4.08 6.19 5.51 3.99

5.69 4.82 3.82 5.49 4.76 3.70 6.20 5.29 4.04 6.00 5.24 3.83

5.42 4.39 3.67 5.26 4.46 3.55 6.02 5.07 4.01 5.92 5.06 3.65

5.13 4.10 3.62 5.11 4.23 3.37 5.89 4.79 3.97 5.69 4.76 3.46

8.98 10.35 5.86 9.12 12.43 8.42 8.79 10.19 4.78 9.55 10.41 4.91

8.12 13.40 4.87 8.66 17.89 10.98 6.22 11.62 1.75 8.34 12.16 2.20

poly(CDVB)-NB

poly(DVB)-DCE

poly(DVB)-NB

With the values shown in Table 3 extracted from the literature39,40 and eq 12, we could calculate KA and KD of four hyper-resins (Table 4). Herein, values of −ΔHsp, together with KA and KD, are used as parameters to compare the acid−base character of four hyper-resins. For four hyper-resins, the amphoteric probes (ethyl acetate, acetone) presented the higher value of −ΔHsp among all probes, and the lower value of −ΔHsp was observed for the acidic probe (chloroform). The results indicated that the surface of the polymers was amphoteric. On the other hand, the KD/KA values of the four hyper-resins were 1.67, 1.79, 1.25, and 1.42, confirming that the four hyper-resins surface had a basic character. The basic sites in the surface of four hyper-resins were identified with the oxygen atoms in the carboxylic, carbonyl, and hydroxyl groups as electron donors; the Lewis acidic sites were identified with

indicating that acid−base interactions were present. The free energy change of specific interactions, ΔGsp, between polymeric resins and polar probes at different temperatures is given in Table 2. It was observed that the strong polar probe of acetone showed higher values of −ΔGsp than ethyl acetate and chloroform, which was attributed to the strong electron acceptor−donor interaction between carbonyl group of acetone and carboxyl and hydroxyl in the surface of the polymers. The specific components of the enthalpy of adsorption, ΔHsp, of the polar probes on polymers were calculated from the slope of −ΔGsp/T versus 1/T according to eq 10. The plot of −ΔGsp/T versus 1/T is shown in Figure 5, and the slope of the lines correspond to the value of −ΔHsp for a particular probe onto the polymer surface. The values obtained for −ΔHsp are also presented in Table 2. 21409


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Figure 5. Determination of the specific component of the enthalpy of adsorption and the entropy of adsorption of the polar probes on four resins.

chloromethyl groups.20,21 Therefore, when nitrobenzene was used as post-cross-linking solvent, the surface functional groups could be formed by oxidation of chloromethyl groups by nitrobenzene18 besides the hydrolysis of chloromethyl groups; because 1,2-dichloroethane has no oxidation, the hydrolysis of chloromethyl groups was the only possible reason for the formation of surface functional groups when it was used as post-cross-linking solvent. The more surface acidic and basic sites might lead to the greater strength force for polar probes. Therefore, it can be found in Table 2 that poly(CDVB)-NB and poly(DVB)-NB display a value of −ΔHsp for three polar probes slightly larger than that of poly(CDVB)-DCE and poly(DVB)-DCE; however, the difference of −ΔHsp between two resins was much lower and varied from 7.1% to 13.6%.

Table 3. Physical Constants for Probes Used in IGC Experiments acetone ethyl acetate chloroform

AN* (kJ/mol)

DN (kJ/mol)

10.5 6.3 22.7

71.4 71.6 0

Table 4. Acceptor (KA) and Donor (KD) Interactions Parameters of Four Adsorbents

KA KD KA + KD KD/KA

poly(CDVB)DCE

poly(CDVB)NB

poly(DVB)DCE

poly(DVB)NB

0.12 0.20 0.32 1.67

0.14 0.25 0.39 1.79

0.12 0.15 0.27 1.25

0.12 0.17 0.29 1.42

4. CONCLUSIONS The effect of nitrobenzene and 1,2-dichloroethane as postcross-linking solvents on pore structure and surface properties of hyper-resins has been successfully examined by means of N2 adsorption at 77 K and inverse gas chromatography at infinite dilution (IGC-ID), respectively. The post-cross-linking solvent had little influence on micropores and mesopores volume as well as the BET surface area of hyper-resins prepared using the same synthesis procedure. Similarly, the strength of the surface acidic and basic sites of hyper-resins synthesized using NB as post-cross-linking solvent were only slightly higher than that of hyper-resins synthesized using DCE as post-cross-linking solvent. In addition, nitrobenzene exhibits higher toxicity than 1,2dichloroethane. Furthermore, compared with 1,2-dichloroethane, nitrobenzene has a higher boiling point, which would increase the difficulty and cost of solvent recovery. Hence,

the hydrogen atoms in the hydroxyl end-group and in the carboxylic end-group. Compared with the hyper-resins synthesized using DCE as post-cross-linking solvent, the slightly larger values of both KA and KD were observed for resins synthesized using NB as postcross-linking solvent, whether procedure 1 or procedure 2 was adopted. This result indicated that the strength of the surface acidic and basic sites of hyper-resins synthesized using NB as post-cross-linking solvent was slightly higher than that of hyperresins synthesized using DCE as post-cross-linking solvent, which was consistent with the results of Boehm titration (shown in Table 1). The possible reason is as follows: As stated in the introduction, the formation of surface functional groups may be attributed to oxidation of chloromethyl groups by oxygen and solvent during the synthesis18,19 and hydrolysis of 21410


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although the post-cross-linking solvents (nitrobenzene and 1,2dichloroethane) had a negligible influence on porous structure and surface properties of hyper-resins, 1,2-dichloroethane maybe a better option as post-cross-linking solvent for preparing hyper-resins based on environmental and economic considerations.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 25 89680380. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Surface Properties of Hyper-Cross-Linked Polymeric Resins Using

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