In Situ NMR Measurement of Novel Silicone Elastomer Obtained by

Jan 28, 2014 - Macromolecules , 2014, 47 (3), pp 888–896. DOI: 10.1021/ma402291e. Publication Date (Web): ... Cite this:Macromolecules 47, 3, 888-89...
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In Situ NMR Measurement of Novel Silicone Elastomer Obtained by Cross-Linking of Silicones Having Phenylene Backbone and Hyperbranched Molecular Architectures Hiroki Uehara,*,† Masazumi Saitoh,† Ryosuke Morita,† Eiichi Akiyama,‡ and Takeshi Yamanobe† †

Division of Molecular Science, Faculty of Science and Technology, Gunma University, Kiryu, Gunma 376-8515, Japan Sagami Chemical Research Institute, 2743-1 Hayakawa, Ayase, Kanagawa 252-1193, Japan

‡

S Supporting Information *

ABSTRACT: Novel silicone elastomer was obtained by crosslinking reaction of thermal hydrosilylation reaction between poly(oxysilphenylenesiloxydisiloxane) having vinyl groups in side chain and SiH-terminated hyperbranched polycarbosiloxane. In situ measurements using solid-state 1H NMR were performed during the cross-linking reaction on heating. The results suggest that the cross-linking reaction was accelerated beyond 110 °C. Two overlapping reaction mechanisms reflect the characteristic molecular structures of poly(oxysilphenylenesiloxydisiloxane) having vinyl groups and SiH-terminated hyperbranched polycarbosiloxane. A possible structural model to satisfy both reaction mechanisms is proposed to characterize the development of the cross-linking reaction.

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INTRODUCTION Silicone materials including poly(dimethylsiloxane), PDMS, are macromolecules composed of the propagation of siloxane bonds in the main chain. They exhibit excellent chemical stability, viscoelasticity,1,2 hydrophobicity,3 and biocompatibility.4−7 Such characteristics in silicones are often superior to those of carbon-based polymers; thus, they are industrially applied as various functional materials. For example, silicones with a molecular weight (MW) below 2000 are liquid-like and are thus used for lubrication, low adhesion, and waterproofing.1,3,8−10 In contrast, cross-linked silicones with MW of 5000−10 000 are rubbery and thus are used as medical materials,11 e.g., bypass tubes,12,13 heart valves,14 and artificial breast fillers.15−19 Industrially, network structures of crosslinked silicones have excellent elasticity2 and gas permeability.20−24 Another characteristic of silicones is their ease of chemical modification because the siloxane unit can combine with various atoms and chemical groups, e.g., cyclic architectures25,26 and mesogens,27,28 for further improvement of the above physical properties. In particular, the insertion of a phenylene ring in the main chain can improve heat resistance of silicone.27 Such silicones are used as inflammable elastomers for aircrafts and automobiles.29,30 In this study, unique molecular designs were adopted as novel elastomer materials. One is poly[(tetramethyl-p-oxysilphenylenesiloxy-1,1,3,3-tetramethyldisiloxane)-ran-(tetramethyl-p-oxysilphenylenesiloxy-1,3-dimethyl-1,3-divinyldisiloxane)] (PSV) (Scheme 1). The phenylene group is introduced in the main chain of PSV, giving it molecular rigidity. Also, the vinyl groups are arranged at the side chain of PSV in order to increase cross-linking density. Another one is SiH-terminated hyperbranched polycarbosiloxane (SHP) as cross-linker agent, © 2014 American Chemical Society

which is synthesized according to Scheme 2. Conventionally, a linear polysiloxane having hydrosilyl groups (SiH-LPS) is well used as cross-linker agent for industrial silicone production. Two kinds of SiH-LPS are commercially available: one is SiHterminated linear PDMS, and another one is copolymer consist of dimethylsiloxane and methylsiloxane units. The former is not enough to be attributed to cross-linking because of only two hydrosilyl groups in a polymer molecule. The latter is able to introduce desired amount of hydrosilyl group, but the reactivity is low compared with the hydrosilyl group on the terminal position of polymer backbone. The SHP has a lot of highly reactive hydrosilyl groups as the terminal groups of branch chains to enhance cross-linking reaction of hydrosilylation reaction between vinyl group and hydrosilyl group. This cross-linking reaction between PSV and SHP induces multiple amorphous components, including starting reactants and cross-linked products. Unfortunately, few methods have been developed to analyze the amorphous structure of polymeric materials. Scattering methodologies (e.g., smallangle scattering using light, X-ray, or neutron beam) are candidates for such analysis, but these two-phase models are not appropriate for multiple amorphous phases having different molecular structures, as in this study. In contrast, solid-state 1H NMR measurement is a powerful tool for evaluating multiple amorphous phases, which are distinguishable by relaxation time (T2), indicating the molecular motion of each phase. Also, this measurement can estimate the component fraction of each phase. Our recent solid-state 1H NMR measurements for Received: November 6, 2013 Revised: January 16, 2014 Published: January 28, 2014 888

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Scheme 1. Preparation of Poly[(tetramethyl-p-oxysilphenylenesiloxy-1,1,3,3-tetramethyldisiloxane)-ran-(tetramethyl-poxysilphenylenesiloxy-1,3-dimethyl-1,3-divinyldisiloxane)] (PSV)

Scheme 2. Preparation of SiH-Terminated Hyperbranch Polycarbosiloxane (SHP)

Synthesis. Poly[(tetramethyl-p-oxysilphenylenesiloxy-1,1,3,3-tetramethyldisiloxane)-ran-(tetramethyl-p-oxysilphenylenesiloxy-1,3dimethyl-1,3-divinyldisiloxane)] (PSV). A mixture of 1,4-bis(hydroxydimethylsilyl)benzene (1) (2.450 g, 10.82 mmol) and pyridine (1.9 mL, 24 mmol) in chloroform (21.5 mL) was cooled in liquid nitrogen bath under an inert atmosphere. 1,3-Dichloro-1,3dimethyl-1,3-divinyldisiloxane (0.20 mL, 0.96 mmol) and 1,3-dichloro1,1,3,3-tetramethyldisiloxane (1.9 mL, 9.7 mmol) were added to the mixture at −78 °C, which was stirred for 3 h as elevating temperature up to room temperature. Then a slight amount of 1 (0.050 g, 0.22 mmol) was added to the mixture, stirring for 13 h. A precipitate was filtered off after adding diethyl ether (30 mL). The filtrate was poured into an excess amount of methanol (1 L). The obtained viscous solid was dissolved in diethyl ether (6 mL), and this solution was poured into methanol (1 L) to afford PSV (3.156 g) as colorless waxy solid (82.6% yield). The a/b ratio of the composition determined by 1H NMR was 90/10 (%). 1H NMR, δ (250 MHz, CDCl3, ppm): 0.041 (s, Si−CH3), 0.308 (s, Si−CH3), 5.85 (m, −CHCH2), 7.540 (s, Ph). IR, ν (reflect, cm−1): 1257 (Si−CH3), 1064 (Si−O), 1030 (Si−O), 1018 (Si−O). Mn: 1.42 × 105, Mw·Mn−1 = 1.82. Glass transition temperature was observed at −79.8 °C on a DSC curve (second heating process). 1,1,3,5,5-Pentamethyl-3-vinyltrisiloxane.44 The mixture of chlorodimethylsilane (28.7 g, 303 mmol) and dichloromethylvivnylsilane (17.5 g, 124 mmol) was added slowly dropwise to a solution of diethyl ether (200 mL), sodium hydrogen carbonate (46.4 g; 552 mmol), and water (5.00 mL, 278 mmol) at 0 °C. The reaction mixture was stirred at room temperature for 1.5 h. After removing salts by a filtration, the filtrate was dried with anhydrous sodium sulfate. Triethylamine (7.42 g, 73.3 mmol) and N,N-dimethyl-4-aminopyridine (0.145 g, 1.19 mmol) were added to the obtained solution, and then chlorodimethylsilane (6.90 g, 72.9 mmol) was added slowly dropwise. After stirring for 1 h, salts were removed with Celite filtration. The filtrate was concentrated and purified by distillation under reduced pressure (55 °C/2.6 kPa) to afford 1,1,3,5,5-pentamethyl-3-vinyltrisiloxane (19.2 g) as a colorless liquid (70.2% yield). 1H NMR, δ (250 MHz, CDCl3, ppm): 0.14 (3H, s, SiCH3), 0.20 (12H, d, Si(CH3)2, J = 2.8 Hz), 4.72 (2H, sep, Si−H, J = 2.8 Hz), 5.73−6.08 (3H, m, −CH CH2). IR, ν (reflect, cm−1): 2962 (w, C−H), 2127 (m, Si−H), 1597 (w, CC), 1254 (m, Si−CH3), 1049 (s, Si−O−Si), 897 (s), 793 (s), 768 (s). MS, m/z (EI,): 220 (M)+, 205 (M − CH3)+, 191 (M − 2CH3)+, 177, 163, 147. SiH-Terminated Hyperbranched Polycarbosiloxane (SHP). Platinum (5 wt %) on activated carbon powder (17.7 mg; Pt: 4.54 μmol)

molten polyethylene (PE) revealed multiple amorphous components having different entanglement characteristics.31 This solid-state 1H NMR measurement has been applied for in situ analysis of structural change during heating of PE materials.32,33 Some previous studies have addressed solid-state 1H NMR,34−41 2H NMR,41 or 29Si NMR41 analysis of cross-linked PDMS. Recently, Kovermann et al.42 reported the real-time 1H NMR observation of network formation within semidilute silicone solution. In contrast, our in situ solid-state NMR measurement technique is applicable to cross-linking reaction during heating. In this study, the reaction process between PSV and SHP was traced by in situ 1H NMR measurement. The analyzed relaxation characteristics will enable us to predict the cross-linking mechanism between PSV and SHP.

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EXPERIMENTAL SECTION

Materials. Chloroform and triethylamine were distilled over calcium hydride just before use. Dichloromethylvinylsilane and 1,3dichloro-1,3-dimethyl-1,3-divinyldisiloxane were obtained from Acros Organics and Gelest, Inc., respectively. Chlorodimethylsilane, 1,3dichloro-1,1,3,3-tetramethyldisiloxane, and 4-N,N-dimethylaminopyridine were purchased from Tokyo Chemical Ind. Co., Ltd. Platinum (5 wt %) on activated carbon powder was brought from Wako Pure Chemical Ind., Ltd. Sodium hydrogen carbonate, anhydrous diethyl ether, anhydrous tetrahydrofuran (THF), and anhydrous pyridine were provided from Kanto Chemical Co., Inc. 1,4-Bis(hydroxydimethylsilyl)benzene was prepared according to our previous paper.43 Characterizations. 1H NMR spectra were recorded on a Bruker DPX-250 (250 MHz) or DRX-500 (500 MHz) NMR spectrometer. IR spectra were recorded with Horiba FT-720 equipped with SensIR technologies DuraSamplIR II. Number-average molecular weight (Mn) and weight-average molecular weight (Mw) were determined by a sizeexclusion chromatography (SEC) analysis with a Tosoh GPC-8020 system (detector: Tosoh RI-8020) by using THF as an eluent, equipped with four columns of TSKgek SuperH5000, 3000, 2000, and 1000. Standard polystyrenes (Tosoh) were used for calibration. Glass transition temperatures of the obtained polymers were determined by differential scanning calorimetry (DSC) (SII Nano Technology DSC6220, at heating rate of 10 °C min−1 under nitrogen flow of 30 mL min−1). 889

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and 1,1,3,5,5-pentamethyl-3-vinyltrisiloxane (5.00 g, 22.7 mmol) were mixed at 0 °C in inert gas and stirred at 50 °C for 15 h. After diluting the mixture with diethyl ether, the activated carbon powder was removed with Celite filtration. The filtrate was concentrated and poured into an excess amount of acetonitrile. The obtained precipitate was dried under vacuum to afford SHP (3.71 g) as a colorless and viscous oil (74.2% yield). 1H NMR, δ (500 MHz, CDCl3, ppm): 0.00− 0,20 (m, Si−CH3), 0.37−0.54 (m, CH2−CH2, 1.02−1.05 (m, CH3− CH2−Si), 4.72 (sep, Si−H). IR, ν (reflect, cm−1): 2958 (w, C−H), 2125 (w, Si−H), 1252 (m, Si−CH3), 1036 (s, Si−O), 903 (s), 768 (s). Mn: 2.94 × 103, Mw·Mn−1 = 1.77 (SEC chart showed that SHP was monodispersion polymer although the calculated molecular weight would be underestimated.). No vinyl group was detectable in SHP. Glass transition temperature was observed at −105 °C on a DSC curve (second heating process). The degree of branching of SHP could not be evaluated from the corresponding 1H NMR spectrum. Measurements. PSV and SHP were blended at 1:1 by weight composition, followed by homogeneous mixing with a spatula for 5 min at room temperature. Forty milligrams of this mixture was placed in an aluminum sample pan for thermal gravimetric analysis (TGA) and differential thermal analysis (DTA). Both gravity change and temperature difference between the sample and the standard were recorded during heating at 2 °C min−1 from 30 to 300 °C under nitrogen gas flow using a Rigaku TG8120. The standard was alumina powder with the same weight (40 mg). A DTA scan can detect the exothermic peak attributed to the cross-linking reaction. For solid-state NMR measurements, the 1:1 mixture of PSV and SHP was placed in a glass tube with a 5 mm radius and heated at 2 °C min−1 from 30 to 180 °C in a pulse NMR spectrometer (JEOL MU-25) operated at 25 MHz. During such heating, free induction decay (FID) was measured using the Carr−Purcell−Meiboom−Gill (CPMG) pulse sequence.45 The repetition time and 90° pulse width was 5 s and 2 μs, and four profiles were stacked. The time delay between PI pulses was 0.5 ms, and detection time scale was 0−250 ms. Before FID recording at the given temperature, the sample temperature was maintained for 5 min. It should be noted that the FID measurement performed with empty sample tube gives the zero intensity, corresponding to a noise level. Here, the repeating scans not only improve the sensitivity of FID curves but also elongate the measurement time periods, which misses the rapid polymerization process. Therefore, we adopted the rapid 4 scans in this study. Indeed, 32 and 64 scans are applied, but the fitting results are very similar to that for 4 scans. The sample heated up to 180 °C for NMR measurements was cooled to room temperature. The obtained rubbery material was weighted and then immersed in an excess amount of toluene at room temperature. After keeping in toluene for 24 h, the sample was taken out and dried in a vacuum. The weight loss was estimated from the difference between the initial and final weights.

Figure 1. TGA and DTA profiles for PSV/SHP blend.

Figure 2. Changes in FID profiles during heating for the PSV/SHP blend. (a) The whole time region was compared for some selected temperatures, and (b) the short time region is enlarged for all profiles recorded at every 10 °C.

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RESULTS TGA/DTA Measurements. TGA/DTA measurement was performed to investigate the cross-linking temperature of the PSV/SHP blend. Cross-linking between PSV and SHP is an exothermic hydrosilylation reaction. Therefore, the DTA profile enables us to estimate the reaction temperature. In contrast, the TGA profile suggests the degradation temperature of the obtained cross-linked silicone of PSV/SHP. Figure 1 plots the TGA and DTA profiles on heating for the PSV/SHP blend. The DTA profile exhibits a broad but apparent exothermic peak at temperatures ranging from 100 to 150 °C. Such broadness of the exothermic peak indicates that this cross-linking reaction gradually proceeds upon heating. In contrast, the TGA curve indicates that no weight is lost until 300 °C, which corresponds to the thermal degradation temperature of the cross-linked PSV/SHP blend. In Situ NMR Measurements during Heating. Solid-state 1 H NMR measurements of the PSV/SHP blend were

performed in order to detect changes in molecular motion during the cross-linking reaction. Figure 2 depicts the stacked FID profiles obtained at given temperatures upon heating of the PSV/SHP blend. A series of the profiles were recorded at every 10 °C. These profiles gradually change and overlap each other. Therefore, some representative temperatures were chosen in Figure 2a. Figure 2b enlarges the profile changes in the shorter time region. As measurement temperature increased from 30 to 100 °C, the FID slope gradually decreased. At higher temperature ranging from 110 to 180 °C, the FID curves shift downward due to the appearance of the component with a shorter relaxation time. Such restricted molecular motion is attributed to the cross-linked molecular structure. Here, the resultant sample in the NMR sample tube after heating was a rubbery solid. This result confirms that the cross-linking reaction occurs during heating of the PSV/SHP blend. 890

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addition, the other peak newly appears in the lower time region around 1 ms. These results indicate that three-component fitting is appropriate for the profiles obtained in the higher temperature region. In order to confirm these inverse Laplace transform analyses, two- and three-component fittings were performed for the profiles recorded in each temperature range. Figures 4a and 4b

The obtained FID was deconvoluted into different components for quantitative analysis of the cross-linking hydrosilylation reaction. Usually, 1H solid-state FID is an “Abragam function”.32 If molecular motion is present to an extent that is able to average out 1H−1H dipolar interaction, the FID will depict a liquid-like behavior, i.e., an exponential decay. Therefore, the FID of rubbery material recorded by solid-state 1 H NMR measurement can be represented by the following exponential decay:32 ⎛ −t ⎞ I(t ) = I0 exp⎜ ⎟ ⎝ T2 ⎠

(1)

where t is measurement time, I(t) is signal intensity at t, and T2 is spin−spin relaxation time. According to eq 1, the plot of log I(t) as a function of t should be linear. However, all the FID profiles in Figure 2 exhibit a downward curvature in such semilog plots, independent of temperature, indicating the coexistence of several components with different T2s. Therefore, the FID obtained at each temperature was deconvoluted using the above exponential function. Excel’s Solver was used to perform a nonlinear least-squares fit of eq 1 to the FID. For effective deconvolution, the number of components should be defined a priori. Here, introducing more components leads to better fitting of the deconvoluted profile to the observed one. However, each component requires a corresponding assignment; thus, fewer components are preferred for representing the molecular structure of each component. At least two components are required for reasonable fitting of the obtained series of the decay profiles even before cross-linking. The other component is created above the cross-linking temperature, which can be estimated from TGA measurements; thus, a three-component fitting is expected. It has been known that the inverse Laplace transform can effectively elucidate the number of component in multiexponential decays.46 We applied this method for a series of FID profiles in Figure 2. The profiles obtained by the inverse Laplace transform analyses are depicted in Figure 3. At 30 °C,

Figure 4. Comparison of decay-fitting results for the FID profile recorded at 30 °C for PSV/SHP. (a) Two- and (b) three-component fittings are compared. Inset figures indicate the deviation between fitting value and experiment value.

compare two- and three-component fittings for the PSV/SHP blend at 30 °C, assuming exponential decay as represented by eq 1. The deviation between observed and fitted FID profiles is plotted in the inset figure. Both fittings indicate similar deviation in the entire region of measurement time. Thus, a two-component fitting is sufficient to reproduce the observed FID profile of PSV/SHP below the cross-linking temperature. This is well coincident with the above inverse Laplace transform analysis depicted in Figure 3. Similar two- and three-component fittings are performed for the FID profile obtained at 130 °C (Figure 5a,b). The twocomponent fitting results in larger deviation (exceeding 10% of the observed data level) in the shorter time region less than 10 ms, as denoted by the dotted circle in Figure 5a. This result is quite different from the fitting result before heating in Figure 3, where both fittings had similar results. Therefore, a threecomponent fitting is preferred for the PSV/SHP blend at 130 °C, which is above the cross-linking temperature. This also agrees with the inverse Laplace transform analysis in Figure 3. Here, the newly appearing component 3 is introduced in the shorter time region, indicating reduced molecular motion. DTA measurement indicates that cross-linking is achieved at this temperature; therefore, the newly appearing component 3 corresponds to the cross-linked region. Two- and three-component fittings are also performed for the series of FID profiles obtained below and above the cross-

Figure 3. Inverse Laplace transform profiles for a series of the FID profile recorded during heating for PSV/SHP.

the main peak locates at 30 ms with a shoulder at 100 ms, indicating two components with different T2 values coexist. With increasing temperature until 100 °C, the peak position gradually shift into the longer time region (right side) with a rapid increase of shoulder peak. Beyond 110 °C, the main peak intensity decreases and shifts into the shorter time region (left side). Further, the shoulder peak at about 100 ms decreases. In 891

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Figure 5. Comparison of decay-fitting results for the FID profile recorded at 130 °C for PSV/SHP. (a) Two- and (b) three-component fittings are compared. Inset figures indicate the deviation between fitting value and experiment value. The dotted circle denotes the larger deviation exceeding 10% of the observed intensity level.

Figure 6. (a) Relaxation time, T2, and (b) fraction as a function of temperature for PSV/SHP blend. Components 1, 2, and 3 in Figure 4 correspond to mobile, intermediate, and rigid fractions. Rigid T2 change was enlarged in the inset figure in (a).

linking temperature of 110 °C. Figure 6 compares the analyzed T2 and component fraction as a function of measurement temperature. The component fraction was normalized by the I0 value of each profile. The lower value of T2 indicates restricted molecular motion, whereas the higher value corresponds to enhanced molecular motion. The T2 change with increasing temperature suggests that a mobile component 1 with a higher T2 value and an intermediate component 2 with a middle T2 value coexist below 100 °C, followed by the addition of rigid component 3 with the lower T2 value (43 ms) at 110 °C, corresponding to the cross-linked component. Below 100 °C, both of intermediate and mobile T2 increase with increasing temperature, indicating the gradual enhancement of chain mobility. In contrast, they decrease beyond 110 °C and become constant. Considering that the rigid component appears at this temperature, cross-linking reaction also restricts the chain mobility of the surrounding intermediate and mobile components. Figure 6b depicts changes in the component fractions for the PSV/SHP blend with increasing temperature. At room temperature before heating, 50% was obtained for the mobile and intermediate components. From 110 °C, the fraction of the mobile component rapidly decreased with increasing temperature. Further increasing of temperature above 140 °C resulted in gradual decreasing of the intermediate component. The rigid component attributed to the cross-linked region reached nearly 60%. These results suggest that the cross-linking reaction proceeds stepwise: the starting step from mobile to rigid and the delayed step from intermediate to rigid. The cross-linking reaction mechanisms of these different steps will be discussed later.

Structural Analysis of Synthesized Silicone Materials. Here, the temperature at which the rigid component appears is coincident with the beginning of the exotherm observed in the TGA heating profile, confirming that this rigid component is due to the cross-linked molecular structure. In contrast, mobile and intermediate components coexist even at room temperature before heating; thus, they are attributed to the original molecular architecture of pure PSV or SHP. Therefore, in situ 1 H NMR measurements were performed for PSV and SHP without blending. Comparison of the obtained FID characteristics of these pure reactants to those of the PSV/SHP blend may enable us to assign intermediate and mobile components to corresponding molecular structures. Figure 7 depicts the stacked FID profiles obtained during heating of pure PSV. Whole and short time regions were compared for a series of the profiles recorded at every 10 °C. With increasing temperature, increased molecular motion leads to gradual reduction of the profile slope. This result is coincident with that obtained for the PSV/SHP blend (Figure 2). All the obtained FID profiles exhibit a downward curvature, indicting the coexistence of different exponential components. Therefore, corresponding exponential deconvolution, similar to that applied to the above blend, was applied to these series of FID profiles. Here, two-component deconvolution was applied to the FID profiles of pure PSV. Three-component deconvolution was also applied; however, the deviation between the observed and fitted profiles was not effectively reduced, indicating that two-component deconvolution is preferable. Figures 8a and 8b depict the T2 and component fraction as a function of temperature for heating pure PSV. In Figure 8a, with increasing temperature, the T2 values of both components 892

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experiment, indicating that no cross-linking reaction occurred. Concerning the component fractions in Figure 8b, the lower T2 component is 3 times higher than the higher T2 component at room temperature. Therefore, the higher T2 component with enhanced molecular motion can be ascribed to the portion near the molecular ends. It is reasonable that the fraction of such molecular ends is lower than the content of the internal portion. In contrast, the lower T2 component with restricted molecular motion corresponds to the internal portion of the PSV. Comparing these results with Figure 6, the lower T2 value in Figure 8a is similar to that for the intermediate component in Figure 6a. Therefore, the intermediate component in Figure 6a can be attributed to the molecular motion of the internal portion of PSV. On the other hand, the higher T2 value of the portion near the molecular ends is less than T2 of the mobile component in Figure 6a. Since the viscosity of SHP is lower than that of PSV, there is a possibility of the contribution by SHP to T2 of the mobile component in Figure 6a. Figure 9 depicts the stacked FID profiles obtained during heating of pure SHP. With increased temperature, apparent

Figure 7. Changes in FID profiles during heating for pure PSV. (a) Whole time region was compared for some selected temperatures, and (b) short time region is enlarged for all profiles recorded at every 10 °C.

Figure 9. Changes in FID profiles during heating of SHP. (a) Whole time region was compared for some selected temperatures and (b) short time region is enlarged for all profiles recorded at every 10 °C.

instability due to profile noise is observable beyond 90 °C and enhanced beyond 110 °C. This noise is attributed to the radicals formed from the many hydroxyl groups within SHP. Profile deconvolution was applied to these series of FIDs of SHP as well as PSV, but noise formation prevented precise profile deconvolution above 110 °C. The resultant material was rubbery solid. Such solidification of the SHP sample after this heating experiment indicated that the cross-linking reaction occurs even for pure SHP. Radical formation beginning at 110 °C leads to such a cross-linking reaction. In contrast, no noise was generated when heating the PSV/SHP blend (Figure 2). This result suggests that the radicals formed from SHP were

Figure 8. (a) Relaxation time, T2, and (b) fraction as a function of temperature for pure PSV.

gradually increase, which indicates that the molecular motion of PSV is monotonically enhanced. Since there is no cross-linking agent, the mobility of PSV is activated by the thermal energy without any reaction. In fact, the sample appearance and characteristics were the same before and after this heating 893

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An FID profile recorded after cooling the cross-linked SHP material was also acquired at room temperature, as depicted in Figure S1. Fitting results give a combination of lower and higher T2 components. These T2 values (9.7 and 48.1 ms) correspond to those of the rigid and intermediate components for the PSV/SHP blend. This is good evidence that the rigid component corresponds to the cross-linked SHP portion in the blend.

expended by the cross-linking reaction with PSV, resulting in the elimination of radical noise. Below 100 °C, the radical noise seems not to affect very much the FID profile to prevent a deconvolution. These FID profiles could be fitted by two components with the higher and lower T2. Figures 10a and 10b depict the T2 and component

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DISCUSSION The cross-linking reaction between PSV and SHP is summarized here. Figure 11 depicts the schematic model based on the above results. First, changes in T2 were analyzed. T2 of the intermediate component (Figure 6a) for the PSV/ SHP blend is quite similar to that of the lower T2 (Figure 8a) for pure PSV. In contrast, T2 of the mobile component of the blend is much higher than that of the higher T2 component for pure PSV, indicating that the mobile component of the blend is also affected by SHP molecule. It is reasonable that T2 of SHP is much higher than that of PSV because SHP exhibits liquidlike fluidity. During heating of the PSV/SHP blend below 100 °C, T2 of both the intermediate and the mobile components gradually increases; however, the former T2 begins to decrease at 140 °C, which is not coincident with the continuous increase in the lower T2 for pure PSV in this temperature range. These results suggest that the molecular motion of the intermediate component of the blend is restricted because the rigid

Figure 10. (a) Relaxation time, T2, and (b) fraction as a function of temperature for pure SHP.

fraction of pure SHP in the temperature range from 30 to 100 °C. In Figure10a, the higher and lower T2 values at 30 °C are about 300 and 90 ms, respectively. As the temperature increases, both T2 values increase and the fraction of higher T2 component increases, which indicates that the molecular mobility of SHP is activated with temperature and the crosslinking reaction within SHP does not take place below 100 °C. Comparing Figure 10 with Figure 6, the lower T2 value of SHP is close to the mobile T2 value of the PSV/SHP blend. The higher T2 value in Figure 10a is 4 times higher than the mobile T2 value in Figure 6a. As indicated in Figure 8, the higher T2 component of PSV also contributes to the mobile component of the PSV/SHP blend. Therefore, the mobile component in the PSV/SHP blend in Figure 6 is attributed to the higher T2 component of PSV and both of the higher and lower T2 components in SHP. Here, the component fractions with the mobile and intermediate T2 in Figure 6b are about 50% at 30 °C. Considering that the proton fraction in SHP is lower by about 20% than that in PSV, it is reasonable to conclude that a part of PSV contributes to the mobile component in the PSV/ SHP blend. In addition, from the point of view of the fractional behavior, the increment of T2 value from 30 to 100 °C in Figure 6a is much larger than that in Figure 8a. This large increment of T2 value is attributed to SHP. Thus, the mobile component in PSV/SHP blend could be attributed to SHP and the end proton of PSV.

Figure 11. Schematic model of structural development during heating of the PSV/SHP blend system. 894

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reaction characteristics differ: the mobile component rapidly transforms into a rigid one at lower temperatures, whereas the intermediate component gradually cross-links at a higher temperature. These results indicate that the cross-linking of PSV/SHP blend involves stepwise reactions with different mechanisms: a reaction within the SHP component alone and that between PSV and aggregated SHP.

components are interconnected through the intermediate component. Here, the coincidence of T2 values for this intermediate component and the lower T2 component of pure PSV indicates that the internal portion of the PSV segment plays a role of tie molecules between the rigid components but is gradually integrated as the rigid component. Next, changes in component fraction during heating of the PSV/SHP blend are interpreted. In the initial state before heating, the blend is made up of 50% mobile components and 50% intermediate components. At 110 °C, where the crosslinking reaction is recognizable in the DTA/TGA measurements (Figure 1), a new rigid component appears as the fraction of the mobile component rapidly decreases (Figure 6). Such component changes are coincident with the radical formation of pure SHP (Figure 9), implying that self-crosslinking of SHP forms a rigid component in the early stage of the reaction process. In contrast, the fraction of the intermediate component begins to decrease gradually beyond 140 °C, corresponding to the gradual T2 decrease. Such gradual change for the intermediate component looks followed by the former rapid change for the rigid component. This implies that the aggregated SHP components are interconnected through the internal portion of PSV segments in the later stage of the reaction, which agrees with the above interpretation based on T2 change. Here, the self-cross-linking of SHP component is very fast due to the higher molecular motion of SHP fluid, but the interconnection between the aggregated SHP components gradually proceeds because the molecular motion of PSV segments connecting such aggregated SHP components gradually increases with increasing temperature. Thus, the latter step is rate-determining during heating. In summary, the cross-linking reaction of the PSV/SHP blend proceeds in stepwise: the reaction within the SHP component alone and that between PSV and aggregated SHP. These different reaction mechanisms overlap at the cross-linking temperature, as indicated by broad endotherm on DTA profile. Such reaction scheme indicates that the non-cross-linked molecules still remain even after heating. Indeed, the 15.8% weight loss was obtained when the resultant material after heating to 180 °C during above in situ NMR measurements was treated by toluene for 24 h. This indicates that 1/3 of PSV was unreacted even at 180 °C. As revealed in Figure 8b, the ratio of the intermediate and mobile components is 60:40 for pure PSV at 180 °C; thus, 10% of unreacted PSV is attributed to the mobile component even after cross-linking reaction, which is well coincident with that in Figure 6b. These results prove the reasonability of our assignments of these components, as depicted in Figure 11.

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ASSOCIATED CONTENT

S Supporting Information *

FID profile recorded for cross-linked SHP at room temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.U.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Industrial Technology Research Grant Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan and the Nakatani Foundation of Electronic Measuring Technology Advancement.

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REFERENCES

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CONCLUSIONS This study traced the cross-linking reaction between PSV and SHP by in situ solid-state 1H NMR measurement. Deconvolution analysis of the FID profiles during heating indicated two components: a mobile component with a longer T2 and an intermediate component with a middle T2 below 120 °C. However, an additional rigid component with a shorter T2 appears beyond 130 °C. The fraction of this rigid component increases with increasing temperature and approaches almost 60% at 180 °C, indicating that this component is attributed to cross-linked molecules. In contrast, mobile and intermediate components coexist even at room temperature before the crosslinking reaction and thus could be attributed to the original materials of PSV and SHP. However, their cross-linking 895

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