Article pubs.acs.org/IECR
Branched Hydroxyl Modification of SBS Using Thiol-Ene Reaction and Its Subsequent Application in Modified Asphalt Xue-Kun Li,†,‡ Guo-Shun Chen,‡ Min-Wei Duan,‡ Wei-Cheng Yang,‡ Song-Chao Tang,† Ya-Dong Cao,§ and Yong Luo*,‡ †
Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ‡ State Key Laboratory of Polyolefin and Catalysis, Shanghai Research Institute of Chemical Industry Co., Ltd., Shanghai 200062, People’s Republic of China § Shanghai Urban Construction Nichireki Special Asphalt Co., Ltd., Shanghai 200231, People’s Republic of China S Supporting Information *
ABSTRACT: Based on the thiol-ene radical addition reaction, the branched hydroxyl functionalized modified styrene-butadiene-styrene triblock copolymer (SBS-g-OH) was prepared. According to chemical structure characterization, the order of reactivity proved that 1,2-vinyls had a priority to react with 2mercaptoethanol (MCH) at a low functionalization degree of SBS-g-OH. A novel reaction kinetics model was set up to describe the addition reaction between MCH and 1,2-vinyls; then the effect of reaction time, temperature, and initial concentration ratio of MCH to 1,2-vinyls on functionalization degree of SBS-g-OH were investigated. Subsequently the performances of modified asphalt were investigated, where the softening point, ductility, and needle penetration of SBS-g-OH modified asphalt were almost equal to that of SBS modified asphalt, while the dispersibility of polymer in asphalt and storage stability of modified asphalt had been improved obviously using SBS-g-OH with an appropriate functionalization degree.
1. INTRODUCTION Asphalt, which is used as a binder mixed with aggregates (sand, crushed rock, etc.) to prepare asphalt concrete, plays an important part in road construction.1 During the past few decades, along with the growth of the economy, road construction has been developing rapidly all over the world, especially in developing countries. However, due to increased traffic loading and volume, traditional asphalt pavement has expressed some disadvantages, e.g., rutting and cracking. In order to enhance the quality of asphalt pavement, more and more investigations pay attention to the modification of asphalt,2 where polymer modification is one of the most popular approaches.3,4 After incorporating polymers in asphalt, the modified asphalt will express some improved performance at high and low temperatures, such as better moisture resistance, longer fatigue life, higher stiffness, and stronger cracking resistance.5−7 Polymers used for modifying asphalt can be divided into two categories, i.e., plastomers and thermoplastic elastomers,8,9 compared to plastomers, thermoplastic elastomers can improve not only the softening temperature and stiffness but also the elasticity recovery capacity of modified asphalt.6,9−12 Polystyrene-block-polybutadiene-block-polystyrene triblock copolymer (SBS) is the most popular and widely used thermoplastic elastomer in polymer modification of asphalt.2,3 © 2017 American Chemical Society
The microstructure of SBS contains dispersed (polystyrene (PS)) and continuous (polybutadiene (PB)) phases, and the glass transition temperature (Tg) of PS is around 100 °C, while that of PB is around −95 °C,13 thus under ambient temperature, the glassy PS and the rubbery PB segments contribute to the strength and elasticity of SBS, respectively. When SBS is used to modify asphalt, various performances of asphalt can be improved obviously; additionally, the excellent glassy−rubbery property and microblock structure of SBS lead to sufficient swelling and uniform dispersing of SBS in the asphalt matrix8,14,15 and the van der Waals interaction between SBS and asphalt.16−18 However, both SBS and asphalt are heterogeneous colloid structures.10 Meanwhile, SBS is a kind of nonpolar copolymer,19 while the asphaltenes, which contain sulfur, nitrogen, and oxygen atoms, are strongly polar components in asphalt.20,21 Thus, there is polarity difference between SBS and asphalt, and the chemical structure, reactivity, density, molecular weight, and solubility as well, resulting in compatibility and storage stability between SBS and asphalt being unsatisfactory.22−24 Therefore, in spite of the reported Received: Revised: Accepted: Published: 10354
June 2, 2017 August 8, 2017 August 25, 2017 August 25, 2017 DOI: 10.1021/acs.iecr.7b02280 Ind. Eng. Chem. Res. 2017, 56, 10354−10365
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Industrial & Engineering Chemistry Research
(M̅ n ≈ 40 kg/mol, mPB/mPS = 1:8, c1,2‑vinyl = 97 mol %) at 70 °C for 24 h under a dry argon atmosphere. Their results expressed that nearly all of the 1,2-vinyls vanished after reaction, while the functionalization degree could not achieve 100% (typically 70− 80%), due to the cyclic side-reaction of two neighboring 1,2vinyls. Lotti et al. 41 investigated the effect of initial concentration ratio on conversion of the thiol-ene reaction at 70 °C for 4 h and 48 min (the half-life time of AIBN) by using N-acetyl-L-cysteine (NCys) as the modifying agent and liquid PB (M̅ n ≈ 4080 g/mol, c1,2‑vinyl/c1,4‑units = 91:9) as the substrate polymer. They found that 1,4-units did not participate in the reaction, and the conversion of 1,2-vinyls increased from 15% to 78% as the [NCys]/[1,2-vinyl] increased from 0.1 to 1. The cyclic byproducts could also be observed after reaction. Many similar studies were reported,37,42−44 but all of the used substrate polymers were PB or related derivates with a low molecular weight and high content of 1,2-vinyls. Another important theory should be noticed; that is, Serniuk et al.45 reported that the reactivity order of double bonds of PB in the thiol-ene reaction was 1,2-vinyl ≫ 1,4-cis > 1,4-trans, which could be considered as a kind of pseudo-selectivity, allowing the thiol would be only added to the 1,2-vinyls under a certain condition. Yu and Zheng46 modified SBS (M̅ n ≈ 17.8 kg/mol, mPB/mPS = 5.6:4.4, n1,2‑vinyls/n1,4‑units = 6.5:3.5) with 3mercaptopropyl-triethoxysilane (MPTES; nMPTES/nCC/nAIBN = 100:10:1) at 60 °C for 48 h; their results indicated that all double bonds, including 1,4-units, almost reacted through the thiol-ene reaction. When the initial concentrations of mercaptan and AIBN were sufficiently in excess, 1,4-units would also be modified along with 1,2-vinyls at 80 °C for 6 h, and the conversion of double bonds in the thiol-ene reaction was affected by the reaction time, temperature, initiation mode, the amount of initiator, and functionalized mercaptan.47 In summary, using SBS with a high molecular weight (>150 kg/mol) and low content of 1,2-vinyls (n1,4‑units/n1,2‑vinyl > 8.0:2.0) as a substrate polymer for the thiol-ene radical addition reaction has not been previously reported. In this work, a kind of commercially available SBS dedicated to modifying asphalt is selected for functionalized modification by the thiol-ene reaction, where 2-mercaptoethanol (MCH), AIBN, and 1,4dioxane are used as a modifying agent, an initiator, and a solvent, respectively. The chemical structure of SBS-g-OH will be characterized by Fourier transform-infrared spectroscopy (FT-IR) and 1H nuclear magnetic resonance spectroscopy (1H NMR); furthermore, the functionalization degree (FD), that is, the conversion of MCH, is determined by 1H NMR analysis. In order to obtain the elastic branched hydroxyl functionalized SBS (SBS-g-OH), which indicates that MCH is only added to 1,2-vinyls, but not 1,4-units of PB segments in SBS, a desired operating condition should be established; thus the effect of reaction time, temperature, and the initial concentration ratio of MCH to 1,2-vinyls on the thiol-ene reaction will be investigated. A novel reaction kinetics model will be subsequently proposed by introducing the Arrhenius equation and net consumption rate equation of mercaptan into an ordinary reaction rate equation, and then expressing the two frequency factors in the Arrhenius equations in terms of temperature and initial concentration ratio of MCH to 1,2vinyls, respectively. After the related parameters are obtained by least-squares estimation of the experimental data, this reaction kinetics model can be used to correlate and predict the effects of reaction time, temperature, and initial concentration of mercaptan on the thiol-(1,2-vinyl) radical addition reaction.
advantages of SBS modified asphalt, researchers have been paying more attention on how to overcome the unfavorable drawbacks. In order to improve the compatibility and storage stability of SBS modified asphalt, several approaches had been attempted, such as sulfur vulcanization, saturated or functionalized modification of SBS, and even adding antioxidants or hydrophobic clay minerals into modified asphalt, where the functionalized modification of SBS was the approach considered to have the most potential.3 By end- or branched functionalization, various polar functional groups could be introduced into the molecule chains of SBS, then, to decrease the polarity difference between SBS and asphalt. On the basis of the anionic polymerization, polar groups, such as carboxyl, hydroxyl, or amino, could be attached to the end of SBS backbone chains with the functionalized initiator or termination agent, or even both of them.25,26 Nevertheless, the reaction environment of anionic polymerization is extremely sensitive to impurities, and the preparation of the functionalized initiator is difficult somehow. Besides, the conditions of the end-capping procedure are relatively harsh; the above three reasons limit the popularization of end-functionalized modification. There are two reaction routes in branched functionalized modification of SBS: one is the substitution reaction of α-H(allylic hydrogen) associated with double bonds in PB segments,27−29 and the other is the addition reaction of double bonds in PB segments, which is saturated by grafting different functionalized monomers, e.g., maleic anhydride (MAH),29 acrylic acid (AA),28 methyl methacrylate (MMA),30 ethylene oxide (EO),31 4-maleimidobenzophenone (4-MBP),32 N-isopropylacrylamide (NIPAAM),33 and methylacrylic acid (MAA).34 Because of the isomerization effect of butadiene during polymerization, the double bonds of PB consist of three types, i.e., 1,2-vinyls and 1,4-units (1,4-cis, and 1,4-trans),35 where 1,4-units exist in the backbone chains of SBS, affecting the elasticity and flexibility, while the 1,2-vinyls are exposed to the branched chains of SBS, making them more sensitive to heat, oxidation, and ultraviolet.36 To enhance the polarity of SBS as well as maintain the elasticity and flexibility of SBS, what we desire is to make 1,2-vinyls functionalized modified only, that is, to keep 1,4-units from reacting with a modifying agent. However, all the above attempts showed a lack of selectivity, and the addition reaction occurred in both 1,2-vinyls and 1,4units of PB segments. The thiol-ene radical addition reaction is a type of wellknown click chemistry, due to excellent characteristics, including a highly efficient reaction of mercaptan with C−C double bonds, insensitivity to oxygen or water, mild reaction conditions, region-specificity, and stereospecificity, the ability to control the spatial structure of polymers, etc. The thiol-ene reaction has gradually been used in the field of polymer synthesis and modification.37 Accordingly, when functionalized mercaptan is used as a modifying agent, double bonds can be modified, and then the functional groups will be tethered to the polymer. Schlaad and co-workers38−40 indicated that polymers containing an abundant concentration of readily accessible ene groups were suitable for being modified by mercaptans with the thiol-ene reaction, especially for PB and related copolymers. Justynska et al.38 used 2,2-azobisisobutyronitrile (AIBN) as a free radical initiator and several functionalized mercaptans (containing carboxylic acid, amine, L-amino acid, and fluorocarbon) to modify 1,2-PB (M̅ n ≈ 1.3 − 3.5 kg/mol, c1,2‑vinyl = 92 − 97 mol %) and 1,2-PB-b-PS diblock copolymer 10355
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Figure 1. Thiol-ene reaction between SBS and MCH.
Finally, several SBS-g-OH products with different FDs are used to modify asphalt; the effects of FD on the dispersibility of polymer in asphalt, the performances of ductility, needle penetration, softening point, and the storage stability of modified asphalt will be investigated.
where kMCH is the net consumption rate constant of MCH. Here, a dimensionless ratio coefficient (A) is introduced to describe the relative concentration shift of MCH, that is, nt ,MCH At = n0,1,2 ‐ CC (4)
2. REACTION KINETICS MODEL OF BRANCHED HYDROXYL MODIFICATION OF SBS On the basis of the thiol-ene radical addition reaction, MCH can react with the double bonds of PB segments in SBS. Also, according to the reactivity order of double bonds,45 there are two cases presented for the thiol-ene reaction (see Figure 1). Case I represents MCH only reacting with the 1,2-vinyls (i.e., 1,2-CC), while case II is with both 1,2-vinyls and 1,4-units (i.e., 1,4-CC). In this work, we should not only obtain the branched hydroxyl modified SBS but also maintain the elasticity of SBS-gOH; therefore, we just consider case I. Accordingly, the reaction rate can be directly expressed by the ratio of residual concentration (ct,1,2‑CC) to initial concentration (c0,1,2‑CC) of 1,2-vinyls changed vs reaction time, which can be written as
and the initial concentration ratio of MCH to 1,2-vinyls is n0,MCH A0 = n0,1,2 ‐ CC (5)
r=−
On the other hand, in this work, the functionalization degree of SBS can be defined as nt ,SBS ‐ g ‐ OH FD = n0,1,2 ‐ CC (6) If there is no MCH that will react with 1,4-units but only 1,2vinyls, then nt ,SBS ‐ g ‐ OH n0,1,2 ‐ CC − nt ,1,2 ‐ CC = n0,1,2 ‐ CC n0,1,2 ‐ CC (7) and then nt ,1,2 ‐ CC
d(ct ,1,2 ‐ CC/c0,1,2 ‐ CC) dt N1
= k(ct ,1,2 ‐ CC/c0,1,2 ‐ CC) (ct ,MCH/c0,1,2 ‐ CC)
N2
n0,1,2 ‐ CC (1)
−
−
dt
= k(nt ,1,2 ‐ CC /n0,1,2 ‐ CC) (nt ,MCH /n0,1,2 ‐ CC)
N2
(2)
(10)
At 0
dA t = −kMCH At
∫0
t
dt
(11)
we obtain
dt
A t = A 0 exp( −kMCHt )
d(nt ,MCH /n0,1,2 ‐ CC)
(12)
while by integrating eq 9, we can obtain
dt
= kMCH(nt ,MCH /n0,1,2 ‐ CC)
dA t = kMCHA t N2 dt
∫A
d(ct ,MCH/c 0,1,2 ‐ CC) =−
(9)
When N1 = 1 and N2 = 1, by integrating eq 10
while the net consumption rate of MCH can be expressed by −
d(1 − FD) = k(1 − FD)N1 A t N2 dt
and
d(nt ,1,2 ‐ CC /n0,1,2 ‐ CC) N1
(8)
Considering the above description, eqs 2 and 3 can be respectively represented by
where k is the apparent reaction rate constant and N1 and N2 represent the reaction order of 1,2-vinyls and MCH, respectively. Since the reaction system is isotonic and isometric in the entire process, eq 1 can be written as r=−
= 1 − FD
N2
ln(1 − FD) = −A t kt
(3) 10356
(13) DOI: 10.1021/acs.iecr.7b02280 Ind. Eng. Chem. Res. 2017, 56, 10354−10365
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Industrial & Engineering Chemistry Research ⎛ E ⎞ k(T ) = (a + bT + cT 2) exp⎜ − a ⎟ ⎝ RT ⎠
then FD = 1 − exp( −A t kt )
(14)
Then the reaction kinetics model can be described by modifying the form of eqs 14 and 18, which are represented as
When N1 ≠ 1 and N2 ≠ 1, by integrating eqs 10 and 9, respectively, we can obtain A t 1 − N2 − A 01 − N2 = −kMCHt 1 − N2
⎡ ⎛ E ⎞⎤ FD = 1 − exp⎢ −A t (a + bT + cT 2) exp⎜ − a ⎟t ⎥ ⎝ RT ⎠ ⎦ ⎣
(15)
for N1 = 1 and N2 = 1
then A t = [(N2 − 1)kMCHt + A 01 − N2 ]1/1 − N2
and FD = 1 1/1 − N1 ⎡ ⎛ E ⎞⎤ − ⎢(N1 − 1)A t N2 (a + bT + cT 2) exp⎜ − a ⎟t ⎥ ⎝ RT ⎠ ⎦ ⎣ (17)
for N1 ≠ 1 and N2 ≠ 1
then N2
1/(1 − N1)
FD = 1 − [(N1 − 1)A t kt ]
(18)
3. EXPERIMENTAL SECTION 3.1. Materials. 1,4-Dioxane (AR grade, 99%), cyclohexane (AR grade, 99.5%), tetrahydrofuran (AR grade, 99.9%), anhydrous methanol (AR grade, 99.9%), and ethanol (Tech grade, 95%) were purchased from Chinasun Specialty Products Co., Ltd. MCH (AR grade, 99%) was obtained from Chengdu Aikeda Chemical Reagent Co., Ltd. Argon (UHP grade, 99.9%) was supplied from Hukang Gas Co., Ltd. SBS (M̅ n ≈ 155 kg/ mol, M̅ w/M̅ n = 1.06, mPB:mPS = 6.9:3.1, n1,4‑units:n1,2‑vinyls = 8.3:1.7) was obtained from Baling Petrochemical Co., Ltd. Asphalt (70#, fractions content are as follows, m(asphaltenes):m(saturates):m(aromatics):m(resins) = 12.36:17.96:42.64: 27.04) was supplied from Ssangyong Co., Ltd. The above reagents were used without any further purification. AIBN (AR grade, 99%) was supplied from Shanghai Zhanyun Chemical Co., Ltd., recrystallized from ethanol, and stored under a dry argon atmosphere. 3.2. Modification of SBS via Thiol-Ene Reaction. The branched hydroxyl functionalized modification of SBS was carried out in a three-neck flask equipped with a magnetic stirrer. SBS (10 g), 1,4-dioxane (200 g), and different amounts of MCH were placed into the stirring flask until SBS was fully dissolved in the solvent; then a corresponding amount of AIBN (nMCH/nAIBN = 100:1) was charged and dissolved at −5 °C for 10 min, making the flask full of argon by displacing air with argon thrice. Then, the flask was entirely immersed into a temperature-controlled oil bath to maintain a temperature accuracy of ± 0.1 °C. During the reaction, for each temperature, in a time range from 1 to 15 h, 3 mL of solution was taken out of the flask using a syringe every 2 h and dropped into 20 mL of anhydrous methanol to precipitate polymer from the solution. After being dried, the polymer was characterized with FT-IR and 1H NMR, while the residual concentration of MCH in the solvent was determined with liquid chromatography−mass spectrometry (LC-MS, Agilent, 6120). After reaction, the flask was cooled to room temperature; then most of the solvent was removed by distillation in a vacuum. The residual polymer solution was dispersed in 2 L of ethanol with stirring to obtain the polymer precipitates. After filtration, the polymer was redissolved in cyclohexane and reprecipitated with ethanol in order to remove residual solvent and unreacted
(19)
where k0,MCH is the pre-exponential factor. Ea,MCH is the apparent activation energy for the consumption of MCH. R is the gas constant, and T is the temperature in degrees Kelvin. Since the initial concentration of 1,2-vinyls in PB and the initial concentration ratio of MCH to initiator is fixed, then A0 is able to affect the consumption efficiency of MCH. Here, we assume the k0,MCH can only be related to A0 in the following equation: k 0,MCH(A 0) =
1 α + βA 0 + γA 0 2
(20)
where α, β, and γ are the model parameters; thus, eq 19 can be corrected to kMCH(A 0) =
1 α + βA 0 + γA 0
2
⎛ Ea,MCH ⎞ exp⎜ − ⎟ ⎝ RT ⎠
(21)
On the other hand, k can be expressed as ⎛ E ⎞ k = k 0 exp⎜ − a ⎟ ⎝ RT ⎠
(22)
where k0 is the frequency factor, which describes the effective collision frequency between MCH molecules and 1,2-vinyls. Meanwhile, it implies the probability of effective regular arrangement among SBS segments during their movements. Ea is the apparent activation energy for the thiol-(1,2-vinyl) radical addition reaction. Since temperature can directly affect the movement ability of MCH molecules and SBS segments, here we assume the k0 can be related to temperature in the following equation: k 0(T ) = a + bT + cT 2
(26)
In summary, to apply the reaction kinetics model effectively, 10 parameters (N1, N2, a, b, c, α, β, γ, Ea,MCH, and Ea) should be estimated by fitting the experimental data.
The well-known Arrhenius equation is used to describe the effect of temperature on the reaction rate constant; thus kMCH can be expressed as ⎛ Ea,MCH ⎞ kMCH = k 0,MCH exp⎜ − ⎟ ⎝ RT ⎠
(25)
(16)
and (1 − FD)1 − N1 = −A t N2 kt 1 − N1
(24)
(23)
where a, b, and c are the model parameters; thus, eq 22 can be corrected to 10357
DOI: 10.1021/acs.iecr.7b02280 Ind. Eng. Chem. Res. 2017, 56, 10354−10365
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Industrial & Engineering Chemistry Research MCH. After being dried under a vacuum at 50 °C for 12 h, the target product SBS-g-OH was obtained and sealed in zip-lock bags. 3.3. Characterization and Measurement of SBS-g-OH. The FT-IR characterization was conducted on a Fourier transform spectrometer (Thermo Scientific, Nicolet 6700). The copolymer samples were first dissolved in tetrahydrofuran with a concentration of 10 wt %, and the solutions were cast onto KBr windows. The solvent was evaporated at room temperature for 3 h and then put in a vacuum at 40 °C for 2 h to remove moisture in the samples. In all cases, 64 scans at a resolution of 2 cm−1 were used to precisely record the spectra, and the wavenumber ranged from 400 to 4000 cm−1. The 1H NMR characterization was carried out on an NMR spectrometer (Varian, VNMRS 600) at 25 °C. The samples were dissolved in deuterium chloroform (CDCl3) with a density of 150 mg/mL, and the 1H NMR spectra were obtained with tetramethylsilane (TMS) as the internal reference. The operating frequency was 400 MHz. The acquisition time was 3.5 s with a relaxation delay of 1 s. The scan time was 150, and the signal enhancement was 16. 3.4. Preparation and Determination of Polymer Modified Asphalt. Asphalt was charged into an aluminum pot and heated to 180 °C; then, SBS or SBS-g-OH (wpolymer = 5.5%) was added into the asphalt along with shearing at a speed of 4000 r/min for 4 h with a high speed shearing machine (Kinematica, PT-6100D). Then, the asphalt was transferred from the pot to a different sample container to determine the performance of the asphalt. The dispersibility of polymers into asphalt was characterized with a fluorescence microscope (Shanghai OIF, 10XB-PC), according to the ASTM specification.48 The performances of ductility (Du, 5 °C, ASTM D113), needle penetration (NP, 25 °C, ASTM D5), and softening point (Ts, ASTM D36) of asphalt were characterized with a ductility tester (Wuxi PIE, LYY-9A), a softening point tester (Shanghai CJGI, SYD-2806H), and a needle penetration tester (Wuxi PIE, WSY-026), respectively. Segregation degree (SD, ASTM D5982-6, D5976-6, D5841-5) could be characterized by determining the difference in softening points of modified asphalt samples taken from the top and bottom of cylindrical molds after they had been stored vertically at 163 °C in an oven for 48 h.
Figure 2. Comparison of 1H NMR spectra between SBS and SBS-gOH, where the (1,4-), (1,2-), and (-SR) represent the content of 1,4units, 1,2-vinyls, and −CH2SCH2CH2OH, respectively.
addition reaction. In contrast, the 1,4-units scarcely reacted with MCH under the present conditions, except for the situation at FD = 38.57%, along with the decrease of peaks at 5.0 and 5.6 ppm, that at 5.4 ppm also decreased slightly. On the other hand, the signals of resonance at 4.2, 3.7, 2.7, and 2.6 ppm, which were attributable to the characteristic hydrogen displacement peaks of MCH,38,49 appeared after the reaction. According to the above description, since the structure of PS will not change before and after the reaction, the displacement peaks of hydrogen in the benzene ring can therefore be used as the internal standard to calculate FD by estimating the peak area. Referring to the reported method,41,50 eq 6 can be expressed as
FD =
nt ,SBS ‐ g ‐ OH n0,1,2 ‐ CC
=
+ A2.6 ⎛ A4.2 + A3.7 + A2.7 ⎞ 2 ⎜ ⎟/ 4 ⎝ ⎠t
⎛ A5.6 + A5.0 ⎞ ⎜ 2 2 ⎟ / ⎝ ⎠0
(
(
A 7.0 + A 6.5 5 t
)
A 7.0 + A 6.5 5 0
)
(27)
where A7.0 and A6.5 are the areas of displacement peaks of hydrogen in the benzene ring, “0” indicates the original structure of SBS, i.e., before reaction, and “t” indicates the structure of SBS-g-OH, i.e., after reaction under different experimental conditions. A typical FT-IR spectrum of SBS and SBS-g-OH is shown in Figure 3. The broad peak at 3200−3400 cm−1 (the stretching vibration of hydroxyl, v(O−H)) appeared due to the molecular association caused by an intermolecular hydrogen bond, which indicated that hydroxyl generated after the reaction. The 1405 cm−1 bands (the stretching vibration of the C−S bond, v(C− S)) represented that the C−S bond was also generated. The band at 911 cm−1 was attributable to the out-of-plane bending vibration of 1,2-vinyls (δ(−CHCH−)1,2), obviously decreased after efq the reaction. Bands at 966 cm−1 and 753 cm−1 were attributable to the out-of-plane bending vibration of 1,4-units (1,4-trans (δ(−CHCH−) trans‑1,4 ) and 1,4-cis (δ(−CHCH−)cis‑1,4), respectively), scarcely changed except for a slight decrease appearing at δ(−CHCH−)cis‑1,4 of SBSg-OH-38, which indicated that a few 1,4-cis units had reacted with MCH at FD = 38.57%. This result was in good agreement with that of 1H NMR characterization. FT-IR and 1H NMR
4. RESULTS AND DISCUSSION 4.1. Reliability of Thiol-Ene Reaction Results. A typical 1 H NMR spectrum of SBS and SBS-g-OH is shown in Figure 2. Taking two modified SBS samples for examples, SBS-g-OH-24 (FD = 24.07%) and SBS-g-OH-38 (FD = 38.57%), the signals of resonance in the range of 1.0−2.5 ppm represent the protons of methylene and methine groups in the main chains of PS and PB blocks, as indicated by “a.” The peaks in the range of 6.2− 7.3 ppm are attributable to the protons of aromatic rings of PS blocks, as indicated by “c.” In addition, the signals of resonance in the range of 4.6−5.8 ppm are assignable to the protons of methylene and methine groups associated with double bonds in the PB segments, where the peaks at 5.0 and 5.6 ppm are assignable to the protons of methylene and methine groups in 1,2-vinyls, whereas the resonance at 5.4 ppm is assignable to the protons of the methine in 1,4-units. With the thiol-ene reaction, the signals of resonance at 5.0 and 5.6 ppm obviously decreased, especially for SBS-g-OH-38, whereas that at 5.4 ppm was nearly still discernible, which indicated that a few of the 1,2-vinyls had a priori undergone the thiol-ene radical 10358
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Figure 3. Comparison of FT-IR spectra between SBS and SBS-g-OH.
characterization indicated that SBS had been successfully modified via the thiol-ene radical addition reaction. The 1,2vinyls could a priori react with MCH; i.e., the branched hydroxyl functionalized SBS was successfully obtained. Due to the inability to observe the cyclic byproducts from the characterizations of FT-IR and 1H NMR, it was reasonable to believe that there were no neighboring 1,2-vinyls existing in the substrate SBS used in this work.39 4.2. Reaction Kinetics of Branched Hydroxyl Modification of SBS. The data used for fitting in this work were over the following ranges: t = 0−15 h, T = 50−80 °C, and A0 = 0.1−1. A nonlinear least-squares method, which was based on the Levenberg−Marquardt algorithm, was used to estimate these parameters. The results are shown in Table 1.
Figure 4. Model values of pre-exponential factor of net consumption rate constant of MCH vs A0 and of apparent reaction rate constant vs temperature for thiol-(1,2-vinyl) radical addition reaction.
rate of MCH to 1,2-vinyls. Therefore, in order to enhance the conversion of this reaction, increasing temperature was more efficient than increasing the initial concentration of MCH, which was why we kept the A0 less than 1. For the parameter N1 = 1 and N2 = 1, it was indicated that both the thiol-ene reaction between a low concentration of 1,2-vinyls and MCH and the net consumption of MCH in this reaction were much more similar to a first-order reaction. The apparent activation energy for the net consumption of MCH was almost equal to that of the addition reaction, which indicated that the thiol-ene radical addition reaction occurred along with the consumption of MCH. The experimental and model values of the relative concentration shift of MCH and functionalization degree at different temperatures and initial concentration ratios of MCH to 1,2-vinyls for different times are shown in Figure 5. At any temperature and initial concentration ratio of MCH to 1,2vinyls, all the relative concentration shifts of MCH decreased, while the functionalization degree increased with increasing time. The reaction kinetics used in this work were just to predict the amount of mercaptan reacted with 1,2-vinyls of PB segments in SBS. For A0 = 0.1, all FD values were below 5%. The model values agreed well with experimental data, which indicated that MCH was only added to 1,2-vinyls. For A0 = 0.5, the results were similar to those of A0 = 0.1, and all FD values were below 25%. However, for A0 = 1, the model values were gradually lower than experimental data as FD increased above 33%, considering the characterization of sample SBS-g-OH-38 (see Figures 2 and 3). When FD = 38.57%, the MCH no longer only reacted with 1,2-vinyls. A small amount of MCH had begun to react with 1,4-units, which was what we did not desire. If there too much mercaptan reacted with the 1,4-units, this kinetics model would not be suitable to predict the FD values (i.e., conversion). On the other hand, FD increased sharply at times below 10 h, while the growth trend of FD gradually became inconspicuous as the time increased above 10 h,
Table 1. Estimated Parameters for Reaction Kinetics Model. no.
parameter
value
1 2 3 4 5 6 7 8 9 10
N1 N2 a b c α β γ
1 1 13.391 −0.1018 0.002692 0.01088 0.006331 0.0009867 20545.2 J/mol 23805.6 J/mol
Ea,MCH Ea
The pre-exponential factor of the net consumption rate constant of MCH (k0,MCH) and apparent reaction rate constant (k0) could be calculated with eqs 20 and 23, respectively, and the results are shown in Figure 4. k0,MCH decreased with increasing initial concentration ratio of MCH to 1,2-vinyls(A0). As described earlier, the initial concentration of 1,2-vinyls in PB and the initial concentration ratio of MCH to initiator were fixed. Thus, the increasing initial concentration of MCH resulted in a decreased effective collision of MCH molecules with 1,2-vinyls; i.e., most of MCH molecules had little opportunity to collide with 1,2-vinyls, which decreased the relative consumption efficiency of MCH. k0 increased with increasing temperature; as temperature increased, the movement ability of both MCH molecules and rubbery PB segments was enhanced, which would accelerate the addition reaction 10359
DOI: 10.1021/acs.iecr.7b02280 Ind. Eng. Chem. Res. 2017, 56, 10354−10365
Article
Industrial & Engineering Chemistry Research
Figure 5. Relative concentration shift of MCH and functionalization degree vs time at different temperatures for different initial concentration ratios of MCH to 1,2-vinyls.
especially for the temperatures of 60, 70, and 80 °C. As we know, the half-life time of initiator AIBN was below 10 h at temperatures above 65 °C (e.g., thalf‑life, AIBN ≈ 10 h at 65 °C, and ≈ 5 h at 70 °C);41,51,52 thus, the relative lower concentration of AIBN led to a slower reaction rate. 4.2.1. Deviation Analysis for the Reaction Kinetics Model. In order to confirm the suitability, conformance, and effectiveness of this reaction kinetics model, it is necessary to carry out the residual analysis and standard deviation analysis for the experimental and model values. The results of residual calculation are shown in Table S1 of the Supporting Information, and the standard deviation (σFD) for FD values was calculated by
where δFD is the residual deviation between the experimental and model FD values. By comprehensively analyzing the residual and standard deviation between experimental and model values at different ranges of FD values, the FD boundary for the situation that MCH only reacts with 1,2-vinyls can be defined precisely, and the results are expressed in Table 2. It could be seen that the standard deviation for all the experimental FD values obtained in this work was 1.34 × 10−2, and the residual deviation ranged from 0.01 to 6.57%, while that in the FD range below 37% was 0.62 × 10−2 and 0.01−1.75%, respectively. Furthermore, at an FD below 33%, the standard deviation was 0.60 × 10−2, and the experimental values were evenly distributed on both sides of the model curves. That is, deviations between the experiment and model appeared to be random, and this reaction kinetics model provides an excellent description of the thiol-(1,2-vinyl) radical addition reaction.
N
σFD =
∑1 δ FD2 N−1
(28) 10360
DOI: 10.1021/acs.iecr.7b02280 Ind. Eng. Chem. Res. 2017, 56, 10354−10365
Article
Industrial & Engineering Chemistry Research
4.2.3. The Effect of Initial Concentration of MCH. By predicting FD values at A0 ranging from 0.1 to 10 with a reaction kinetics model, Figure 7 reveals the effect of the initial
Table 2. Residual and Standard Deviation for Different Range of FD Values FDexp range,a %
δFD range, %
σFD, ×10−2
