Synthesis of Structure-Controlled Polyborosiloxanes and Investigation

Dec 2, 2016 - A series of polyborosiloxanes (PBSs) was synthesized by mixing hydroxy-terminated polydimethylsiloxanes (PDMS) and boric acid (BA) in ...
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Synthesis of Structure-Controlled Polyborosiloxanes and Investigation on Their Viscoelastic Response to Molecular Mass of Polydimethylsiloxane Triggered by Both Chemical and Physical Interactions Miao Tang,† Wentao Wang,† Donghua Xu,*,‡ and Zhigang Wang*,† †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China S Supporting Information *

ABSTRACT: A series of polyborosiloxanes (PBSs) was synthesized by mixing hydroxy-terminated polydimethylsiloxanes (PDMS) and boric acid (BA) in toluene at 120 °C. The molecular masses of selected PDMS precursors were in a wide range, covering from below up to far above the critical entanglement molecular mass of PDMS. The reaction kinetics was followed by using Fourier transform infrared (FTIR) spectroscopy. Unreacted BA was removed from raw PBSs after the reactions. The influence of molecular mass of PDMS precursors on the rheological property of PBSs was explored by dynamic oscillatory frequency sweeps. The results showed that the plateau elastic moduli of PBSs were highly dependent on the molecular mass of PDMS precursors. The plateau elastic moduli of PBSs decreased at first and then increased with increasing molecular mass of PDMS precursors. PBS1 and PBS2 prepared from unentangled PDMS precursors showed sufficient fits by using the two-mode Maxwell model, whereas PBS3 to PBS6 prepared from highly entangled PDMS precursors showed obvious deviations from the two-mode Maxwell model. It could be concluded that the changing trend of plateau elastic modulus of PBSs versus molecular mass of PDMS precursors was determined by the number density of supramolecular interactions (Si−O:B weak bonding and hydrogen-bonding of the end groups Si−O−B(OH)2) and the number density of topological entanglements.

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INTRODUCTION Polyborosiloxanes (PBSs) represent an important branch of polydimethylsiloxane (PDMS) derivatives. PBSs have attracted considerable industrial and academic interests over the past few years due to their thermodynamically stable backbone,1 which have been widely used as the components for self-adhering heat-stable and flame-retardant materials,2,3 high-temperature resistance adhesives and coatings,4−7 and as precursors for hightemperature stability and chemical resistance fibers and ceramics.8−13 Abundant borono (Si−O−B(OH)2) end groups of PBSs allow formation of reversible physical cross-links through hydrogen-bonding.14 Therefore, PBSs exhibit a viscous fluid behavior under long time scales and a rigid mechanical behavior under a rapid extensional strain. The fascinating viscoelastic property, high mobility of siloxane backbone, and reversible intermolecular interactions make PBSs be applied as impact-resistant protective materials.15,16 The rheological properties of PBSs are highly sensitive to their molecular structures. The influence of molecular mass on the viscoelastic property of PBSs is an interesting subject. Seetapan et al.17 studied the viscoelastic behaviors of raw PBSs (unreacted boric acid, BA remained in PBSs), which were © XXXX American Chemical Society

prepared by mixing hydroxy-terminated PDMS precursors and BA at 120 °C. Note that PDMS was largely employed as a precursor of elastomeric materials because environmentally friendly PDMS showed an interesting combination of physical properties, such as low glass transition temperature, high gas permeability, low dielectric constant, and excellent biocompatibility.18−22 They observed that the elastic moduli of raw PBSs decreased at first and then remained almost unchanged with increasing molecular mass of PDMS precursors.17 Liu et al. recently mixed a high molecular mass trimethylsiloxy-terminated PDMS and BA at 200 °C.23 PDMS chains underwent random chain scissions at 200 °C and then the chain ends were modified by polar, hydrogenbonding moieties from BA.23 Since the chain scission degree increased with increasing reaction time at 200 °C, the molecular mass of PBSs decreased accordingly. However, the storage moduli of PBSs increased with reaction time, which was Received: October 2, 2016 Revised: November 19, 2016 Accepted: November 21, 2016

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Characterization and Measurements. GPC measurements were conducted at 30 °C using Agilent 1260 produced by Agilent Technologies Company with an RID detector connected to a PL-Gel mixed DGPC column. The eluent for GPC measurements was tetrahydrofuran (THF) of HPLC grade, and the standards used for calibration were monodisperse polystyrenes. Fourier transform infrared (FTIR) spectra in the spectral range from 4000 to 400 cm−1 were recorded on Thermo Nicolet 6700 using the attenuated total reflectance (ATR) mode. The samples for FTIR measurement were prepared as KBr pellets. Rheological measurements were carried out at 25 °C on an ARES G2 Rheometer (TA Instruments) with a 25 mm parallel-plate geometry. Dynamic strain sweeps were performed to determine the linear strain range. Dynamic oscillatory frequency sweeps from 0.1 to 500 rad/s were performed with appropriate strain in the linear regime. Steady shear measurements for PDMS precursors were performed at 25 °C in the shear rate range from 10−3 to 103 s−1. Synthesis of Polyborosiloxanes (PBSs). To prepare PBSs, BA was first dispersed in toluene followed by addition of the PDMS precursor with the stoichiometric ratio, r = 1 (the mass content ratio of hydroxyl groups of BA to those of PDMS precursor). For example, to prepare PBS1, BA of 26.6 mg was dispersed in toluene of 100 mL, and then PDMS1 of 5.00 g (r = 1) was added in the toluene. The mixture was stirred at room temperature for 2 h and then was heated to 120 °C with further stirring. Subsequently, the mixture was held at 120 °C for 48 h to allow further reaction. The resulting water during the reaction could form an azeotrope with toluene, and water was removed from the reaction system with a Dean−Stark trap, because water could suppress the condensation reaction and the formed Si−O−B bonds were susceptible to water. After the reaction, toluene was removed under reduced pressure at 60 °C. The reaction kinetics for PBSs in the mixture was followed up by collecting the intermediate products at different reaction time. The PBS samples obtained from the reactions of PDMS1, PDMS2, PDMS3, PDMS4, PDMS5, and PDMS6 with BA were denoted as raw PBS1, PBS2, PBS3, PBS4, PBS5, and PBS6, respectively. Raw PBSs were subtransparent. The purified PBS samples from raw PBSs were denoted as PBSs. In order to remove unreacted BA, raw PBSs were dissolved in dry n-hexane to form solutions, which were filtered through PVDF membranes with 0.22 μm apertures, similar to the procedure applied in a previous report.23 Colorless transparent PBSs were obtained after evaporation of n-hexane at 60 °C and PBSs were further dried under vacuum at 60 °C for 24 h.

attributed to the increasing effect of hydrogen-bonding among the modified chain ends.23 It was considered that BA could cause PDMS chain random scissions at above 150 °C.14,23 Therefore, to synthesize PBSs with well-defined molecular structures (specific molecular masses) from PDMS precursors at higher temperatures (>150 °C) would not be ideal. In this study, we considered to employ a solvent to reduce the viscosities of the reactive system, which was particularly helpful for the PDMS precursors with higher molecular masses. Thus, a low reaction temperature could be applied. With this consideration, a series of PBSs was synthesized by mixing hydroxy-terminated PDMS precursors and BA in the solvent toluene and the reactions were performed at the temperature of 120 °C. A further refining step was applied to remove unreacted BA. Thereafter, the influences of molecular mass in a wide range for selected PDMS precursors on the rheological property of PBSs were explored.

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MATERIALS AND METHODS Materials. Hydroxy-terminated PDMS precursors with kinematic viscosity values (specified by the supplier) of 65, 90−150, 750, 1800−2200, 18 000, and 50 000 cSt were purchased from the Sigma-Aldrich Company and are named as PDMS1, PDMS2, PDMS3, PDMS4, PDMS5, and PDMS6, respectively. The values of zero shear viscosity, η0 were measured by using a rheometer. The number-average molecular mass, Mn, weight-average molecular mass, Mw, and polydispersity index, PDI (PDI = Mw/Mn) of the six PDMS precursors were determined by using gel permeation chromatography (GPC). The GPC traces of PDMS precursors can be found in Figure S1 of the Supporting Information. The −OH contents can be calculated according to the number-average molecular masses, Mn of the PDMS precursors. All the obtained parameters are listed in Table 1. According to the GPC results, Table 1. Zero Shear Viscosity, Molecular Mass, Polydispersity Index, and −OH Content for PDMS Precursors sample code

η0 (Pa·s)a

Mn (kg/mol)b

Mw (kg/mol)b

PDIb

PDMS1 PDMS2 PDMS3 PDMS4 PDMS5 PDMS6

0.06 0.08 0.8 3.5 18.7 77.6

7.7 8.6 30 53 85 117

9.5 11 49 76 125 199

1.2 1.3 1.6 1.4 1.5 1.7

−OH content (mol/g) 2.6 2.3 6.7 3.8 2.4 1.7

× × × × × ×

10−4 10−4 10−5 10−5 10−5 10−5

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a

Zero shear viscosities were obtained from steady shear measurements at 25 °C. bDetermined by GPC at 30 °C.

RESULTS AND DISCUSSION Synthesis of PBSs and Reaction Kinetics. In order to examine the condensation reaction of PDMS and BA in toluene at 120 °C, the reaction kinetics was followed up by using FTIR spectroscopy. Figure 1a shows the FTIR spectra of raw PBS1 samples taken out from the reactive system at different reaction time. The spectra of raw PBSs are normalized according to the absorption band at 1260 cm−1, which is assigned to the Si(CH3)2 group that hardly changes during reaction.23 Quantitative analysis was performed by taking the characteristic absorption band at 1260 cm−1 as an internal standard. It can be found that a weak band appears initially at 1340 cm−1, illustrating the formation of Si−O−B bonds as a result of mutual condensation of OH groups linked to silicon and boron. Furthermore, the band intensity of Si−O−B bonds for raw

four PDMS precursors (PDMS3 to PDMS6) have molecular masses beyond the critical entanglement molecular mass of PDMS (Me = 12 kg/mol).24,25 The critical molecular mass, Mc for PDMS precursors is 24.5 kg/mol.25−28 Figure S2 shows that the change of zero-shear viscosity, η0 versus Mw of the entangled PDMS precursors (PDMS3 to PDMS6) is in reasonable agreement with the power law, η0 ∼ Mw3.4.29,30 Boric acid provided by the Sigma-Aldrich Company (USA) was ground, sieved via a 100-mesh sieve aluminum screen, and then was dried at 120 °C for 24 h prior to use. n-Hexane was dried by using calcium hydride prior to use. All other reagents were used as received. B

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Figure 2. FTIR spectra of (a) PDMS1, (b) raw PBS1, and (c) PBS1.

PBS1, indicating that unreacted BA had been removed when raw PBS1 was purified to PBS1. A similar phenomenon was reported in the literature.23 The absorption band at 3700 cm−1 is assigned to Si−OH groups for PDMS1,14 which is absent for both raw PBS1 and PBS1. Rheological Properties of PBSs. BA modification on a PDMS precursor has a large impact on the rheology of the PBS system. The rheological property of PBSs has been explored in detail in this study, with that of PDMS precursors measured as well for the comparison purpose. Figure 3 shows the frequency dependence of storage modulus, G′ and loss modulus, G′′ for PDMS precursors and PBSs as measured at 25 °C. The G′ and G′′ values for PBSs as shown in Figure 3b are much higher than that for PDMS precursors as shown in Figure 3a, which was also reported in other studies.17,23,31 PBSs demonstrate a solidlike behavior at high frequencies, as seen from that the G′ values are higher than G′′ values at high frequencies (Figure 3b), while all PDMS precursors exhibit a liquid-like behavior in the measured frequency range, as seen from that all the G′ values are lower than G′′ values (Figure 3a).32,33 Note that the influences of the stoichiometric ratio, r on the rheological properties of PBSs were examined, and the typical result for PBS4 is shown in Figure S4. It can be seen that the storage modulus, G′ of PBS4 increases with increasing r. At the stoichiometric ratio, r of 0.5 PBS4 exhibits a viscous behavior in the measured angular frequency range. When r increases to 2, the rheological response of PBS4 shows a similar trend as that obtained at r of 1, reflecting by the close plateau elastic moduli, Ge. Therefore, in this study, we chose the stoichiometric ratio, r of 1 to synthesize PBSs. PBSs enable the measurements of G′ and G′′ over an appropriate frequency range to obtain a clear onset of the plateau elastic modulus zone. The G′ and G′′ curves shown in Figure 3b can be shifted horizontally to provide clear comparison by avoiding overlap and the shifted G′ and G′′ curves are shown in Figure 3c. The experimental plateau elastic moduli, Ge,exp for PBSs can be obtained from the curves shown in Figure 3c by using the “MIN method”:34

Figure 1. (a) FTIR spectra of raw PBS1 at different reaction time and (b) change of intensity ratio for the absorption bands at 1340 and 1260 cm−1 (I1340/I1260) with reaction time for raw PBS1.

PBSs exhibits a dramatic enhancement as the reaction proceeds (inset in Figure 1a). Figure 1b shows the change of intensity ratio for the absorption bands at 1340 and 1260 cm−1 (I1340/ I1260) with reaction time. The values of I1340/I1260 begin to increase markedly during the first 12 h and then keep about constant, indicating that the reaction can be finished after 12 h. The FTIR result for raw PBS6 is shown in Figure S3. The similar reaction kinetics can be found for raw PBS6. Therefore, the reactions for preparing of raw PBSs in this study were performed at 120 °C for 48 h to allow a complete condensation reaction of the OH groups of PDMS and BA. It is noted that for the synthesis of PBSs, the stoichiometric ratio, r, of 1 was chosen in this study because the previous study had demonstrated that r ∼ 1 was the optimum ratio for all the PDMS precursors to form the complete networks of unimodal PBS.17 Figure 2 shows typical FTIR spectra for PDMS1, raw PBS1 and PBS1. For these three samples, the absorption band at 1084 cm−1 is attributed to symmetric stretching vibration of Si−O−Si bonds, while the absorption bands at 2905−2960 cm−1 are due to asymmetric stretching vibration of C−H bonds of the methyl group. Compared with the FTIR spectrum of PDMS1, a characteristic absorption band at 1340 cm−1 appears in the FTIR spectrum of raw PBS1, which is due to symmetric stretching vibration of Si−O−B bonds. Meanwhile, the absorption band located at 3189 cm−1 for raw PBS1 is attributed to B−O−H moieties.14,23 The FTIR spectrum of PBS1 demonstrates that the absorption band of B−O−H moieties located at 3189 cm−1 for raw PBS1 cannot be seen for

Ge,exp = G′(ω)tanδ→ min

(1)

where tan δ is the damping factor, and Ge,exp is the G′ value at the minimum of tan δ. The Ge,exp values for PBS1 to PBS6 are 243, 129, 73, 71, 91, and 112 kPa, respectively. It can be seen from Figure 3c that the Ge,exp values for PBSs exhibit an initial decreasing trend with increasing molecular mass of PDMS C

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side in our studied molecular mass range.17 The mechanism for the above phenomenon will be presented in the last section. To well understand the relaxation behavior of PBSs, the experimental results in Figure 3b are further fitted by using the two-mode Maxwell model expressed by eqs 2 and 3:36 G′ =

Ge,fitω 2τe 2 1 + ω 2τe 2

G ′′ =

Ge,fitωτe 1+

ω 2τe 2

+

G b,fitω 2τb 2 1 + ω 2τb 2

+

(2)

G b,fitωτb 1 + ω 2τb 2

(3)

where Ge,fit and τe represent the plateau elastic modulus and the relaxation time for the slow relaxed network structure, respectively, and Gb,fit and τb represent the elastic modulus and the relaxation time for the fast relaxed supramolecular structure, respectively. In order to demonstrate the contribution of the fast relaxed supramolecular structure on rheological characteristics, the van Gurp-Palmen plots are showed in Figure S5. The minima of |G*| values in the van Gurp-Palmen plots correspond to the plateau elastic moduli. The fast relaxed supramolecular structure can have an influence on the slope in the terminal region for the van Gurp-Palmen plot.37 PBS1 and PBS2 with the lower molecular masses of PDMS precursors show the higher slopes in the terminal region, corresponding to their fast relaxed supramolecular structures. A similar result for supramolecular epoxide-terminated polyethers was reported by Miao et al.38 Figure 4 shows the experimental and fitting results for PBS1 and PBS6 for comparison, because they show much different viscoelastic properties with their PDMS precursors in the two ranges of molecular mass, i.e., Me, respectively.17,39 The fitting results for PBS2 to PBS5 are provided in Figure S6. On the basis of the fitting, the τe values for PBS1 to PBS6 are 1.1, 0.95, 0.56, 0.43, 1.3, and 1.6 s, respectively, which are close

Figure 3. Changes of storage modulus, G′ and loss modulus, G′′ versus angular frequency, ω as measured at 25 °C for (a) PDMS precursors, (b) PBSs with no curve shifts, and (c) PBSs with curve shifts. In panel c, the G′ and G′′ curves are shifted horizontally for providing clear comparison and avoiding overlap. The horizontal shift factors applied for PBS1, PBS2, PBS3, PBS5, and PBS6 are 10−9, 10−6, 10−3, 103, and 106, respectively.

precursors (from PBS1 to PBS4) and an increasing trend with further increasing molecular mass of PDMS precursors (from PBS4 to PBS6). The characteristic relaxation time, τc is the inverse of the crossover frequency, ωc, which can be obtained from the crossover point of the G′ and G′′ curves as shown in Figure 3c.35 The τc values for PBS1 to PBS6 are 1.08, 0.91, 0.70, 0.41, 1.45, and 1.72 s, respectively, consistent with the changing trend for Ge,exp with molecular mass of PDMS precursors. This result is partially consistent with that reported by Seetapan et al. if considering that their PBSs were prepared from PDMS precursors with weight-average molecular masses ranging from 7.5 to 84 kg/mol, which only stayed in the low molecular mass

Figure 4. Experimental and fitted storage modulus, G′ and loss modulus, G′′ versus angular frequency, ω, for (a) PBS1 and (b) PBS6 measured at 25 °C. Solid red lines are the fitted results for PBSs by using the two-mode Maxwell model. D

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Industrial & Engineering Chemistry Research to the corresponding τc values, and the Ge,fit values for PBS1 to PBS6 are 240, 121, 66, 55, 75, and 100 kPa, respectively. Again, the Ge,fit values are also close to the Ge,exp values. The above results are correlated with the slow relaxed network structures in PBSs. For the fast relaxed supramolecular structure, the τb values for PBS1 to PBS6 are 0.00050, 0.0010, 0.0012, 0.0012, 0.0013, and 0.0011 s, respectively, which are much lower than the corresponding τe values, and the Gb values for PBS1 to PBS6 are 54, 29, 51, 56, 58, and 60 kPa, respectively, indicating less influence of the supramolecular structure. Furthermore, it can be seen that the linear viscoelastic behavior for PBS1 and PBS2 can be sufficiently fitted by using the two-mode Maxwell model. However, large deviations from the fitting of the twomode Maxwell model are found at frequencies at above 1 rad/s for PBS3 to PBS6 where the elastic behavior dominates (G′ > G′′). It is considered that such deviations are attributed to the existence of some additional relaxation processes for PBS3 to PBS6, which deserve further study. It is apparently seen that the Ge,fit values are close to Ge,exp values for PBS1 and PBS2, while Ge,fit values are lower than Ge,exp values for PBS3 to PBS6. It is thought that the differences between Ge,fit and Ge,exp for PBS3 to PBS6 are attributed to additional relaxation processes possibly due to the coupling of supramolecular structure and topological entanglements of longer PDMS chains,17 because the molecular masses, Mn for PDMS3 to PDMS6 are all above the critical entanglement molecular mass, Me for PDMS, while the Mn values for PDMS1 and PDMS2 are much lower than Me for PDMS.24 The continuous relaxation spectra, H(λ) for PBSs can be derived from the result in Figure 3b by using the Trios Software (TA Instruments) according to eqs 4 and 5: G′(ω) =

⎡ ω 2τ 2 ⎤ H (λ )⎢ ⎥ d(ln λ) −∞ ⎣ 1 + ω 2τ 2 ⎦

∫

∞

∞

G ′′(ω) =

structures and physical interactions in PBSs as synthesized from PDMS precursors and BA can be proposed, which are illustrated in Scheme 1. It should be noted that the obtained PBSs could dissolve in n-hexane; thus, PBSs should be supramolecular elastomers, which are formed not only by the covalent bonds but also by noncovalent physical interactions. In PBSs, the formed Si−O−B bonds may act as branched points, while the supramolecular interactions can be hydrogen-bonding of the Si−O−B(OH)2 end groups23 and the Si−O:B weak bonding as reported by Li et al.31 Nevertheless, for PBSs synthesized from the entangled PDMS precursors, the topological entanglements also can be treated as physical cross-linking points, which can also enhance the elastic modulus of PBSs and influence the rheological properties of PBSs. In Scheme 1, the chemical structures and physical interactions for PBS1 and PBS6 are drawn as typical examples. After the chemical reactions, the formed Si−O−B bonds act as branched points in both the chemical structures of PBS1 and PBS6. Meanwhile, the hydrogen-bonding of the Si−O− B(OH)2 end groups and Si−O:B weak bonding formed between boron and oxygen in PBSs also exist in both structures. Because the molecular mass of PDMS1 is lower than the critical entanglement molecular mass of PDMS, PBS1 is more likely to have hydrogen-bonding and Si−O:B weak bonding in its structure (Scheme 1, PBS1). On the contrary, the molecular mass of PDMS6 is much higher than the critical entanglement molecular mass of PDMS, leading to the topological entanglements in PBS6. It is considered that the changing trend of plateau elastic modulus of PBSs versus molecular mass of PDMS precursors is determined by the number density of supramolecular interactions (hydrogen-bonding of the Si−O−B(OH)2 end groups and Si−O:B weak bonding) and the number density of topological entanglements. In order to compare the influences of reactive groups and topological entanglements in PBSs, Figure 6 shows the change of plateau elastic modulus, Ge,exp of PBSs as a function of −OH content of corresponding PDMS precursors. The change of Ge,exp with the −OH content for PBS1 and PBS2 can be linearly fitted with a straight red line as shown in Figure 6, for which the Mn values of PDMS1 and PDMS2 are below the critical entanglement molecular mass of PDMS. The change of Ge,exp with further decreasing −OH content for PBS3 to PBS6 does not follow the linear relationship. Instead, it appears with an increasing trend, indicating that some other factors have contributed to Ge,exp of PBSs, for which the Mn values of PDMS precursors are above the critical entanglement molecular mass of PDMS. It is thought that the number density of topological entanglements takes effect for the PBSs prepared using PDMS precursors with higher Mn values. For the case of an end-linked PBS system in this study, a phenomenological model developed by Langley43 and Dossin and Graessley44 as expressed in eq 6 can be applied here.17

⎡

(4)

⎤

∫−∞ H(λ)⎢⎣ 1 +ωτω2τ 2 ⎥⎦ d(ln λ)

(5)

The weighed relaxation spectra, λ*H(λ) versus time λ can be obtained by referring to the previous studies,40−42 and the result for PBSs is shown in Figure 5. It can be seen that the relaxation peak position for PBSs shifts to the short time side from PBS1 to PBS4 and then shifts opposite to the long time side from PBS4 to PBS6. This trend is again consistent with the changing trend for the characteristic relaxation time, τc. Influence of Molecular Mass of PDMS Precursors. On the basis of the above experimental results, the chemical

Ge,exp = A

ρRT + G N0Te Mn

(6)

where ρ is density for PDMS precursor, R is gas constant, T is absolute temperature (Kelvin), G0N is plateau modulus of uncross-linked high molecular mass polymer melt. The prefactor A is equal to 1 − 2h/ϕ, where h is an empirical parameter ranging between 0 (affine model) and 1 (phantom model) and ϕ is a functionality of cross-linker or the number for branches originating from a cross-linking site. Te is the

Figure 5. Weighed relaxation spectra, H(λ)*λ versus time λ for PBSs. E

DOI: 10.1021/acs.iecr.6b03823 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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appropriately determine the influences of molecular mass of PDMS precursors on the viscoelastic responses of PBSs. Further study in our group might focus on the individual influence of each type of physical interactions for a well-defined PBS system.

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CONCLUSIONS PBSs were synthesized through modification of hydroxyterminated poly(dimethylsiloxane) (PDMS) precursors of different molecular masses with boric acid (BA) mixed in toluene at 120 °C. As evidenced by FTIR spectroscopy, the Si− O−B groups were formed and the reaction was almost complete after 12 h in solution. The rheological measurements showed obvious increases of storage and loss moduli for PBSs as compared with PDMS precursors due to formation of hydrogen-bonding of the Si−O−B(OH)2 end groups and Si− O:B weak bonding and/or the topological entanglements. As the molecular mass of PDMS precursors increased, the elastic moduli of PBSs first decreased and then increased. The changing trend of elastic modulus for PBSs versus molecular mass of PDMS precursors was determined by the number densities of formed supramolecular interactions (hydrogenbonding of the Si−O−B(OH)2 end groups and Si−O:B weak bonding) and the number density of topological entanglements. The results suggested that the reversible physical interactions modulated by molecular mass of PDMS precursors could be an optional simple way to tailor the network structures and the viscoelastic properties of PBSs, which could be practically applied in the industry.

Figure 6. Change of plateau elastic modulus, Ge,exp of PBSs versus −OH content of corresponding PDMS precursors.

trapping factor, i.e., the proportion of the maximum concentration of the topological entanglements that contribute to the elastic modulus. For the end-linked PBS system in the study, ϕ is 3, which is close to the value obtained from the phantom prediction.17 The reported value of h is 0.95.39 The G0N value for PDMS precursors at the measurement temperature of 298 K is about 0.2 MPa.45 For the PDMS precursors (PDMS1 and PDMS2) with Mn lower than Me, the Ge,exp values of PBSs have no relationship with the topological entanglements, whereas for the PDMS precursors (PDMS3 to PDMS6) with Mn higher than Me, the obtained values of Te for PBS3 to PBS6 are 0.22, 0.27, 0.40, and 0.52, respectively. The increasing values of Te from PBS3 to PBS6 indicate that the elastic contribution of topological entanglements becomes larger and larger. The low topological entanglements in PBS3 and PBS4 are apparently insufficient to compensate the decrease of Ge,exp due to the decreasing −OH content. For PBS5 and PBS6, the topological entanglements become dominant due to further increased chain lengths, leading to the upturn in Ge,exp.24,42 This further explains why the plateau elastic moduli of PBSs increase as the molecular mass of PDMS precursors further increases in the late stage. In this work, the unreacted BA was removed after synthesis of PBSs and the rheological property test could

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03823. GPC curves of PDMS precursors; change of zero shear viscosity, η0 as a function of Mw for PDMS precursors; reaction kinetics of raw PBS6; changes of storage modulus, G′ and loss modulus, G′′ versus angular frequency, ω for PBS4 prepared with different stoichiometric ratio, r; changes of phase angle, δ versus F

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complex modulus, |G*| for PBSs; and the experimental and fitting results for PBS2 to PBS5 using the two-mode Maxwell model (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86 0551-63607703. Fax: +86 0551-63607703. Email: [email protected]. *E-mail: [email protected]. ORCID

Zhigang Wang: 0000-0002-6090-3274 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.W. acknowledges financial support from the National Basic Research Program of China (Grant No. 2015CB351903) and the National Science Foundation of China (Grant No. 51473155). The project is also supported by the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. D.X. acknowledges financial support from the National Science Foundation of China (Grant No. 21274152).

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DOI: 10.1021/acs.iecr.6b03823 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.6b03823 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX