Structural Control of Fully Condensed Polysilsesquioxanes

Article pubs.acs.org/Macromolecules

Structural Control of Fully Condensed Polysilsesquioxanes: Ladderlike vs Cage Structured Polyphenylsilsesquioxanes Seung-Sock Choi,† Albert S. Lee,† Seung Sang Hwang,†,‡ and Kyung-Youl Baek*,†,‡ †

Materials Architecturing Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea Nanomaterials Science and Engineering, University of Science and Technology, Daejeon 305-333, Korea

‡

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 29, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.macromol.5b01539

S Supporting Information *

ABSTRACT: Through fine-tuning of the myriad of reaction conditions for an aqueous base-catalyzed hydrolysis−polycondensation reaction, a facile synthesis of structurally controlled polyphenylsilsesquioxanes was developed. Mechanism and kinetic studies indicated that the condensation reaction proceeded through a T1 structured dimer, which was quantitatively and in situ formed through mild hydrolysis of a phenyltrimethoxysilane (PTMS) monomer, to give either the cage-structured polyhedral oligomeric silsesquioxanes (POSS) or the corresponding ladderlike silsesquioxane (LPSQ) with excellent yields. Ladderlike and POSS materials were selectively achieved at higher and lower initial concentrations of PTMS, respectively, and an in-depth spectroscopic analysis of both compounds clearly revealed their structural differences with different molecular weights.

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INTRODUCTION Polysilsesquioxanes (PSQs), of chemical formula [RSiO1.5]n, comprise a class of inorganic−organic hybrid materials that exhibit unique physical and chemical properties unrealized in purely organic or inorganic sources.1 These materials have shown a myriad of advantageous properties including excellent thermal stability, low dielectric constant, good mechanical properties, chemical resistance, and even biocompatibility.1,2 Of the three known structural classes of PSQsrandom branched sols, polyhedral oligomeric silsesquioxanes (POSS), and ladderlike polysilsesquioxanes (LPSQ)only POSS and LPSQs can be characterized to be of controlled structures, where POSS generally possess oligomeric three-dimensional cage structures while a hypothetical LPSQ is a polymeric analogue with a linear, double-strained siloxane backbone.3 Although POSS compounds have been rigorously investigated,4,5 relatively few reports of LPSQ have been reported, especially for perfect ladder structured polysilsesquioxanes with high molecular weight, because of its difficult synthesis in spite of their superior thermal stability, solubility, and improved film properties.6 Synthesis of ladder-structured polysilsesquioxanes was first reported in 1960 by Brown et al. using a phenyltrichlorosilane monomer in the presence of potassium hydroxide (KOH) catalyst at high temperatures (∼200 °C).7 While polymeric, highly condensed siloxane structures may have been obtained by Brown, the highly reactive trichlorosilanes used for hydrolysis and polycondensation most probably did not lead to linear siloxane structures as hinted by the broad 29Si NMR peaks, but rather branched and random structures which are © XXXX American Chemical Society

too complicated to illustrate on paper. As such, the structural feasibility of the obtained products by Brown has been widely debated and eventually disproved by Frey.8 Yamamoto also studied the acid-catalyzed hydrolysis of phenyltrichlorosilane,9 with insights into the formation of a ladderlike structured oligomeric silsesquioxanes, but difficulties in obtaining high molecular weights, and significant uncondensed silanol moieties have rendered this synthesis impractical for obtaining polymeric LPSQ materials. Nevertheless, since Brown’s hypothesis of an idealistic perfect ladder structure, LPSQs have been a goal many organosilicon scientists have been striving toward, as a polymeric analogue of cage structured POSS would allow for new applications and properties for which polysilsesquioxanes have been limited to as fillers or composites with organic polymers. More recently, Zhang et al. developed a unique synthetic method to prepare LPSQ with relatively high molecular weight using template alkoxysilane monomers.3,10 However, several cumbersome synthetic steps for preparing the template alkoxysilane monomers such as aromatic amine derivatives for π−π stacking and hydrogen-bonding interactions were required, and the templating moieties between the siloxane bonds in the product were removed to obtain LPSQ by additional steps such as thermal or acid treatments.10 A similar synthetic concept for rodlike LPSQs with hexagonally stacked structures has also been reported by Kaneko et al. using ionic Received: July 12, 2015 Revised: August 18, 2015

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DOI: 10.1021/acs.macromol.5b01539 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Syntheses of LPPSQ (4) and T12-Phenyl POSS (5) with K2CO3 in H2O/THF at 25 °C at Different Initial PTMS (1) Concentrations: (A) Higher [PTMS] for LPPSQ (4) and (B) Lower [PTMS] for T12-Phenyl POSS (5)

the in situ hydrolysis−polycondensation reaction with our previously reported T12-Phenyl POSS, where in-depth characterizations were also carried out to figure out their structural differences. Moreover, in this study, we report the first investigation of the size and the shape of LPPSQ in solution as characterized by small-angle X-ray scattering (SAXS).

interactions of alkoxysilane monomers; however, these materials were only soluble in water.11,12 Another approach for the synthesis of perfect ladder structures has also been examined by Unno by sequential condensations of dimeric species, but low molecular weight and the HPLC methods used for isolation of the products were unsuitable for scaled-up mass production.13 A common theme among the recent strategies of Zhang, Kaneko, and Unno for the synthesis of LPSQ materials has entailed the use of templating silanol moieties in such a manner that regioregular condensations may occur through hydrogenbonding interactions, ionic interactions, or fractionation of irregular structures. However, the possibility remains in that, within a conventional sol−gel reaction in which the hydrolysis− polycondensation equilibrium is dynamic, if an intermediary structure may form under conditions in which hydrolysis is fast and condensation polymerization occurs in a controlled manner, LPSQ may form in one-pot. In this study, we sought to reinvestigate the synthetic feasibility of LPSQs by a facile synthetic method utilizing less reactive phenyltrimethoxysilane (PTMS) monomer coupled with a mild base catalyst, K2CO3, in water/THF mixture solvent at room temperature to give either the preferential formation T12-Phenyl POSS14 or ladderlike structured polyphenylsilsesquioxane (LPPSQ) (Scheme 1). While our previous study examined the in situ formation of a dimeric, intermediary species for the formation of T12-Phenyl POSS, by incrementally increasing the initial monomer concentration (up to 8.9 M), fully condensed, high molecular weight polyphenylsilsesquioxanes15,16 were obtained which we ascertained to be of ladderlike structure through previously reported analytical methods. However, the structure and characterization of the ladder structure of polysilsesquioxanes has not been universally accepted yet in the academic community due to the lack of conclusive evidence to elucidate the linearity of a hypothetical high molecular weight LPSQ, which was often confused with that of POSS. Therefore, a new, simpler, one-pot reaction for the preparation of LPSQ materials as well as the investigation into the analysis of the molecular shape and size for LPSQ should be explored. In this study, we examined the synthetic feasibility of the obtained ladderlike poly(phenylsilsesquioxane) (LPPSQ) through monitoring of

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EXPERIMENTAL SECTION

Materials. Phenyltrimethoxysilane (PTMS) (Shin Etsu, 98%) was vacuum-distilled before use. Tetrahydrofuran (THF) (J.T. Baker, 99.8%) was distilled from metal sodium with benzophenone before use. Potassium carbonate (K2CO3) (Sam-Jun) was dried overnight under vacuum at 50 °C. Deuterium oxide (D2O) (Aldrich, 99.9 atom % D), tetrahydrofuran-d8 (THF-d8) (Aldrich, 99.9 atom % D), and dichloromethane (J.T. Baker, 99.8%) were used as received. Synthesis of Ladderlike Polyphenylsilsesquioxane (LPPSQ). In a 1 L round-bottomed flask, deionized water (24 g, 1.33 mol) and K2CO3 (0.2 g, 1.45 mmol) were charged and stirred for 10 min. Dry THF (40 g, 0.56 mol) was added and stirred for additional 30 min. Afterward, phenyltrimethoxysilane (PTMS) (79.32 g, 0.4 mol) was added dropwise via syringe under atmospheric N2, and the reaction solution was stirred for 36 h at room temperature. After 36 h, the crude was divided into two phase as colorless and white phases. Crude white viscous products were obtained by decantation of the colorless mixed solvent. Crude white viscous product were dissolved in dichloromethane (150 mL) and extracted with deionized water (200 mL) several times. Thee organic layers were collected and dried over MgSO4 overnight. The solution was filtered to remove MgSO4, and the volatiles removed by rotary evaporator to obtain LPPSQ. (49.1 g, yield = 95%). Hydrolysis and Polymerization Conversion Ratio and Time for Kinetics Using NMR. In a 50 mL round-bottomed flask, deuterium oxide (2.66 g, 0.133 mol) and K2CO3 (0.02 g, 0.15 mmol) were charged and stirred for 10 min. THF-d8 (4.49 g, 0.056 mmol) was added and stirred for an additional 30 min. PTMS (7.932 g, 0.04 mol) was added dropwise via syringe under atmospheric N2, and the reaction solution was stirred at room temperature. The sample products were removed from the reaction flask 0.5 mL per every hour, from reaction start time to polymerization end point. All samples were analyzed for reaction kinetics using 1H NMR, 29Si NMR, and FT-IR. Characterization. The weight-averaged molecular weight (Mw) and molecular weight distributions (Mw/Mn) of the polymers were measured by JASCO PU-2080 plus SEC system equipped with refractive index detector (RI-2031 plus) and a UV detector (λ = 254 B


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nm, UV-2075 plus) using THF as the mobile phase at 40 °C with a flow rate of 1 mL/min. The samples were separated through four columns (Shodex-GPC KF-802, KF-803, KF-804, KF-805). 1H NMR, 13 C NMR, and 29Si NMR spectra were recorded in CDCl3 at 25 °C on a Varian Unity INOVA (1H: 300 MHz; 13C NMR: 75 MHz; 29Si: 59.6 MHz). Solid state CP-MAS 29Si NMR spectra were obtained on a Varian Inova 400 MHz (29Si: 79.5 MHz) with a 7.5 mm CP-MAS probe in air at 298 K, with a contact time of 4 ms, spun at a spin rate of 10 kHz with an automated spinner. FT-IR spectra were measured using PerkinElmer FT-IR system (Spectrum-GX) using solvent-cast films on KBr pellets. Thermal gravimetric analysis (TGA) was performed by TA Instruments TGA 2950 under N2. The X-ray film diffraction and small-angle X-ray scattering (SAXS) were examined at the beamline 3C2, 10B1, and 4C1 of Pohang light source (PLS) in the Pohang Accelerator Laboratory (PAL). Simulation by the MM2 method was conducted by Chem3D Pro 12.0 software.

the reaction time increased, the methoxy proton peaks of PTMS (1) at δ 3.6 ppm gradually became smaller and almost completely disappeared after 4 h, while an additional peak at δ 3.3 ppm appeared. This additional peak was originated from CH3OD, a byproduct after hydrolysis of PTMS (1) with D2O. However, resultant PTMS hydrolysate (2) [PhSi(OD)3] was not detected probably due to strong interaction of the hydroxyl groups with D2O. This result indicated that PTMS (1) was quantitatively hydrolyzed to give the PTMS hydrolysate precursor (2) within 3−4 h. This was a unique result, as conventional sol−gel reactions with base catalysts do not quantatively hydrolyze,19 but rather condense during hydrolysis. Using this method, we were able to determine that the reaction rate orders for PTMS and H2O were 1 and 2.3, respectively, by measuring the initial reaction rates at various [PTMS] and [H2O] concentrations (Table 1), giving an overall

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RESULTS AND DISCUSSION The selection of catalyst began with an overview of base catalysts. As base-catalyzed sol−gel reactions often suffer from low rates of hydrolysis and high rates of condensations, several hydroxides such as LiOH, NaOH, and KOH were disregarded and less reactive carbonates was investigated. And while Brown’s selection of potassium hydroxide was understandable due to the use of the highly hydrolyzable phenyltrichlorosilane,7 the use of industrially feasible trialkoxysilane derivatives under KOH-catalyzed sol−gel reactions were unsuccessful in obtained fully hydrolyzed precursors. Therefore, investigation of carbonates led us to potassium carbonate (K2CO3), which gives a pH on par with the more corrosive base, potassium hydroxide, KOH, while providing a surplus of potassium ions which facilitate condensations between silanol groups, without stabilizing the siloxide anion, a common phenomenon encountered for sodium and lithium cations.17,18 The kinetic study of phenyltrimethoxysilane (PTMS) was first carried out to monitor the intermediary products formed during the initial stages of hydrolysis. Figures 1A and 1B−E show the 1H NMR spectra of PTMS (1) and the products obtained during the first 4 h, where the reaction was carried out in deuterated mixture solvents of D2O and THF-d8, such that the sample was directly received from the reaction solution. As

Table 1. Selected Sample Reaction Conditions for K2CO3Catalyzed Hydrolysis of PTMS at 25 °C run no.

[PTMS]

[H2O]

rate (M/s)

1 2 3 4 5

2.25 4.45 8.9 8.9 8.9

29.6 29.6 29.6 59.2 118.4

1.55 3.10 6.18 3.04 1.51

× × × × ×

10−4 10−4 10−4 10−3 10−3

reaction rate equation of rate = k[PTMS]1[H2O]2.3, where the reaction rate constant (k) was found to be (2.87 ± 0.14) × 10−8 M−2.3/s. The rate orders of 1 and 2.3 for [PTMS] and [H2O], respectively, offer insight into the factors that most greatly affect hydrolysis rate in these set of conditions. It is worthwhile to note that the hydrolysis rates for PTMS are significantly lower (∼2−3 orders) compared with acid-catalyzed hydrolysis of other trialkoxysilane derivatives reported previously,20,21 allowing for facile in situ monitoring using NMR and the ability to model the hydrolysis reaction as elementary pseudo-firstorder, which was not possible for fast acid-catalyzed hydrolysis reactions. Using the above reaction parameters for the hydrolysis of PTMS (1), the kinetic rate conversion plots were plotted as a function of time for various [PTMS]i. As shown in Figure 2, the hydrolysis of PTMS greatly varied depending on [PTMS]i concentration, and the conversion of PTMS (1) to the hydrolysate (2) increased linearly with increasing reaction time, as indicative of the pseudo-first-order rate kinetics. By increasing the [PTMS]i up to 13.3 M and decreasing [PTMS]i

Figure 1. 1H NMR spectra of PTMS (A) and the products (B−E) obtained for the first 4 h in D2O/THF-d8 at 25 °C with K2CO3 at [PTMS]i = 8.9 M. Obtained PTMS hydrolysate (2) was then polymerized by condensation reaction.

Figure 2. Kinetic rate plots for hydrolysis of PTMS at various initial PTMS concentrations. C


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thought to be of the PTMS hydrolysate (2) as shown in Figure 1E, but under careful review was assigned to the T1 structure of −Si−O−Si(OH) 2 R, 17 as the T 0 structure of RSi(OH) 3 generally gives a peak around −51 ppm. This result indicated that the product obtained after 4 h was not the PTMS hydrolysate (2) as we concluded in Figure 1 but rather was the dihydroxyphenylsiloxy dimer structure (Ph(OH)2−Si−O−Si− (OH)2Ph) (3).22 This unexpected result has been already theoretically explained by Kudo et al., who reported that the energy barriers of hydrolysis of trimethoxysilanes and one single condensation (dimerization) were almost equal, because T1 structured dimer was stabilized by substantial hydrogenbonding interaction.24 This indicated that the dihydroxyphenylsiloxy dimer (3) was almost simultaneously generated as PTMS hydrolyzed. After 8 h, this sharp peak (b) broadened and disappeared while additional broad peaks (c and d) newly appeared at −70 and −79 ppm (Figure 3A(iii)), which indicated the formation of T2 [Ph−Si(OSi−)2(OH)] and T3 [Ph−Si(OSi−)3] structures, respectively (T1 = 4%, T2 = 26%, T3 = 70%). This result indicated that the dihydroxyphenylsiloxy dimer (3) was mostly consumed by condensation reaction to give a phenylsilsesquioxane oligomer. After 13 h, no more T1 structure was found, and the T2 peak substantially decreased in Figure 3A(iv), which was almost not detected after 36 h in Figure 3A(v). After final purification steps, the 29Si NMR spectra of LPPSQ are shown in Figure 3A(vi). Obtained LPPSQ only showed a monomodal, somewhat broad peak centered at −80 ppm, with a width of 2.7 ppm. Such high fraction of T3 siloxane structure is generally seen only in welldefined POSS and highly regularly structured LPSQ, where the peak shape of T3 in LPSQ is broader than that of POSS due to its high molecular weight.10−16 The T3 peak width in 29Si NMR for LPSQs has been a critical analytical tool for the formation of ladderlike siloxane structures, usually reported as the full width at half-maximum (Δ1/2).25 The LPPSQ obtained in our study showed a Δ1/2 of 172 Hz actually very comparable with previous reports of LPPSQs synthesized by Zhang,3 which have a Δ1/2 of 149 Hz. Moreover, the solid-state CP-MAS 29Si NMR spectrum (Figure 3C) for LPPSQ showed an exceeding high degree of condensation (∼98%), with the T3 Si-Phenyl peak centered at −78 ppm having a Δ1/2 of 89 Hz, which was even lower than that synthesized by Zhang.3 Thus, the structure of the synthesized LPPSQ can be said to have a high degree of regularity. When the mechanism for formation of LPPSQ in Figure 3A was compared with T12-Phenyl POSS in Figure 3B, two distinct differences were observed. First, while the hydrolysis of PTMS to the dimer (Ph(OH)2−Si−O−Si−(OH)2Ph) (3) was identical, the manner in which further condensations proceeded was drastically different. For T12-Phenyl POSS, the condensations between dimeric species yielded T3 [Ph−Si(OSi−)3] structures directly as shown in Figure 3B(iii), indicating that most of the silanols derived from the dimer condensed all at once. However, for the formation of LPPSQ, the condensations between the dimer proceeded in a much more continuous manner, as condensations between dimers yielded oligomeric polysilsesquioxanes and, consequently, high molecular weight LPPSQ. As been previously reported,9,26 the dimer (3) condenses to form the T4-tetraol, or so-called “half-cage”, and subsequently may form ladder-oligomers under acid-catalyzed hydrolysis−condensation. However, the base-catalyzed system used in this study yielded high molecular weight ladderlike


down to 0.9 M, were able to show that the hydrolysis rate of PTMS was pseudo-first-order with respect to [PTMS]i over a wide range of concentrations. This is an important result as the complete and impeccable hydrolysis can be said to be a key prerequisite into the formation of the highly condensed Si−O− Si structures without defect silanol groups.22 Moreover, the critical [PTMS]i for which the preferential formation of LPPSQ was at or above 4.5 M, while that of T12-Phenyl POSS was below 4.5 M. Figures 3A(i−vi) and 3B(i−v) show the 29Si NMR spectra of PTMS (1) and evolution of the condensed products obtained at predetermined time intervals for the synthesis of LPPSQ and T12-Phenyl POSS, respectively. A sharp and single peak (a) at −54.8 ppm originating from PTMS (1) indicative of a T0 structure in Figure 3A(i) mostly shifted to −63.7 ppm (b) after 4 h in Figure 3A(ii). At first, this peak at −63.7 ppm was

Figure 3. 29Si NMR spectra of PTMS monomer and condensation intermediary products for the synthesis of (A) LPPSQ, (B) T12-Phenyl POSS at various times in D2O/THF-d8, and (C) solid state CP-MAS 29 Si NMR spectra for LPPSQ. D


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Macromolecules polysilsesquioxanes, most likely due to the surplus of potassium ions, which facilitate a high degree of condensation. FT-IR analysis has been also known as strong tool to evaluate well-defined siloxane structures such as POSS4,5,26 and LPSQs.13 LPSQ structures are known to show two absorption peaks at 1150 and 1050 cm−1 because of the asymmetrical horizontal (−Si−O−Si−) and vertical (−Si−O−Si−R) directions of the siloxane bond.13 Thus, as the molecular weight of LPSQ increases, the asymmetrical horizontal peak was expected to increase in intensity. Figure 4 shows FT-IR spectra of the


Figure 5. SEC curves of the obtained products from 13 to 72 h for LPPSQ (A−C) and T12-Phenyl POSS (D).

structure with no POSS or POSS-derived structures mixed within the product. The high molecular weight of LPPSQ was also supported by 1H NMR (Supporting Information Figure S2), as substantial differences in the aromatic phenyl groups were observed, while LPPSQ gave highly broadened peaks attributed to the high molecular weight and amorphous crystalline structure of the polymeric LPPSQ backbone, to be discussed later. The selection of solvent in sol−gel reactions is a critical component in inducing a hydrolysis−condensation equilibrium tailored toward the formation of highly condensed structures. Up until this point the solvent selected was THF, but solvents of varying polarity may either increase or decrease hydrolysis rates as shown for PTMS hydrolysis kinetic rate plots (Figure 6A). As expected, solvents of higher polarity or greater dielectric constant gave faster hydrolysis rates,19,20 ranging 1− 2 orders faster. However, the correlation with solvent polarity on molecular weight is not always proportional. While polar solvents give faster hydrolysis rates, the highest molecular weights for obtained LPPSQ were obtained when polar aprotic solvents were used. This can be attributed to the fact that protic solvents contribute to the sol−gel hydrolysis reaction by solvolysis, which can depreciate the condensation reaction. And as polycondensation proceeds, the condensed siloxane structures become more hydrophobic and become less soluble in polar media. Thus, highly polar solvents such as DMSO and DMF produce LPPSQs with somewhat lower molecular weight compared with less polar aprotic solvents such as THF. Moreover, the fact that LPPSQs are completely insoluble in these highly polar solvents renders steady and continuous polycondensations rather difficult, due to premature precipitation of incompletely condensed siloxane structures. For the case of nonpolar solvents such as nonsubstituted aromatics and chloroform, these solvents are immiscible with the aqueous media forming phase separated emulsion-like solvent mixtures, which lead to greatly decreased hydrolysis rates and subsequent lower molecular weights (Figure 6B). Ladderlike structured polysilsesquioxanes and POSS compounds also vary in their thermal properties. Figure 7A,B shows the TGA thermograms for LPPSQ and T12-Phenyl POSS, respectively. Both exhibit superior thermal properties with no weight loss at temperatures exceeding 380 °C, indicative of fully condensed siloxane structures with indistinguishable amounts of silanol groups which condense around 150 °C.19,20 Furthermore, the manner in which the degradation occurs is

Figure 4. FT-IR spectra of the obtained LPPSQs from 4 to 72 h.

LPPSQs as shown in Figure 3A. As the reaction time increased from 4 to 36 h, the shape of the absorption peak ranged from 960 to 1250 cm−1 became sharply separated to be bimodal (a and b) and the peak intensity at 1150 cm−1 (b) became higher than that at 1050 cm−1 (a). In addition, the characteristic absorption peaks (c and d) of the silanol group (−Si−OH) at 930 and 3500 cm−1 also disappeared as condensation progressed. The spectra for the product, LPPSQ (4) shown in Figure 4E, showed sharp and doubly split peaks at 1050 and 1150 cm−1, without any silanol groups at 3500 and 960 cm−1. Compared with the FT-IR spectra for T12-Phenyl POSS (Supporting Information Figure S1) which showed only one single peak, Si−O−Si vibration peak at 1100 cm−1, the doubly split Si−O−Si bond vibrations were indicative of high molecular weight. In addition, we monitored the SEC derived molecular weights of the intermediary structures at 13 and 36 h as well the final products LPPSQ and T12-Phenyl shown in Figure 5. While the weight-averaged molecular weight (Mw) of the intermediary products that were extracted from the reaction mixture before 13 h were too small to be distinguished from the eluent, the products extracted at 13 and 36 h in Figure 5A,B were found to have a Mw of 2.2K and 9.3K, respectively, showing steady molecular weight increase. This is indicative of slow and steady polycondensation, of which is a prerequisite for LPPSQ formation. The Mw of LPPSQ obtained after 72 h was found to be 15.3K with a polydispersity of 2.02 (Figure 5C), which is in stark contrast to T12-Phenyl POSS, which showed an Mw of 900 with narrow polydispersity (PDI = 1.1) in Figure 5D. While this molecular weight of T12-Phenyl POSS was different from the absolute molecular weight derived from MALDI-TOF (FW = 1550.26 g/mol)14 due to the standardization of our SEC system, obtained LPPSQ was found to have a fully polymeric E


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temperatures. Moreover, the initial degradation temperature in which 5 wt % is lost is about 60 °C higher for LPPSQ compared with T12-Phenyl POSS. This result is also in line with previous reports of LPPSQ materials. Also noteworthy is that LPPSQ retained its solubility after thermal treatments at 400 °C for several hours, which is a key distinguishing characteristic with other traditional methods of preparation of LPPSQ materials3,11−13 indicative of thermoplastic behavior, and that LPPSQ showed no discernible glass transition, melting, or crystallization temperature by DSC analysis.28 The bulk structures of LPPSQ and T12-Phenyl POSS were analyzed by X-ray diffraction (XRD) (Figure 8). For T12-Phenyl

Figure 6. (A) Kinetic rate plots for hydrolysis of PTMS at [PTMS]i = 8.9 M under various solvent conditions and (B) Mw of LPPSQs obtained under various solvent conditions.

Figure 8. XRD patterns for (A) T12-Phenyl POSS and (B) LPPSQ and their coating properties on glass (C) T12-Phenyl POSS and (D) LPPSQ before rubbing with fingers (left) and after (right).

POSS (Figure 8A), single crystal peaks with the peak at 7.2° assigned to the cage face4 (c) were found, but for LPPSQ (Figure 8B), two characteristic diffraction peaks appeared. The first sharp peak (a) at 7.3° (d1 = 12.1 Å) and the second diffusion peak (b) at 18.8° (d2 = 4.7 Å) were assigned to the intramolecular periodic chain-to-chain distance and the average thickness of the amorphous LPPSQ, respectively. This XRD result was also comparable with the spectrum of the previous LPPSQ studies.3,15,16 As coupled with the above TGA result, XRD patterns showed no change after treatments at temperatures exceeding 400 °C (Supporting Information Figure S3), which indicated that the obtained LPPSQ maintained its structure at such high temperature without defects. This temperature-independent structure can be said to be distinguished from other polymeric silsesquioxanes with random structures, as secondary condensations between silanol groups lead to curing and inevitable structural change.1,9,20,21 Compared to T12-Phenyl POSS, LPPSQ was found to be a noncrystalline, amorphous polymer. These differences in crystallinity were further shown by examining the coating

Figure 7. TGA thermograms for (A) LPPSQ and (B) T12-Phenyl POSS.

different. While the phenyl groups in POSS compounds are known to degrade in intermittent steps,27 the degradation of the phenyl groups in LPPSQ occurs all at once,3 more like a polymer. It is interesting to note that while alkyl-POSS compounds sublimate under high-temperature conditions, phenyl-POSS leave behind a higher residual silica and carbon content,27 which is contributed to the higher oxidative resistance of phenyl-POSS. This can also explain the residual weight difference between LPPSQ and T12-Phenyl POSS, as polymeric materials exhibit enhanced oxidative stability at high F


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Macromolecules properties of T12-Phenyl POSS and LPPSQ, as shown in Figure 8C,D, respectively. While LPPSQ was able to be coated as a transparent film at thickness ∼10 um via the drop-casting technique, T12-Phenyl POSS exhibited inhomogeneous coating layers due to the crystalline domains exhibiting similar sizes to the visible light wavelength, while the LPPSQ was completely amorphous.29 In evaluating the feasibility of the proposed ladderlike polyphenylsilsequioxane (LPPSQ) by 29Si NMR, 1H NMR, FTIR, TGA, and XRD methods up to this point, LPPSQ compares favorably to more recent reports of LPPSQ materials as the 29Si NMR T3 peak was shown to be narrow (solution 29Si NMR Δ1/2 = 172 Hz, solid state 29Si NMR Δ1/2 = 89 Hz), with indistinguishable amounts of uncondensed silanol groups. Also, SEC, FT-IR, TGA, and XRD analyses indicated that LPPSQ was of polymeric nature. However, none of these analytical tools were able to prove the linearity of LPPSQ. This led us to investigate the size and shape of LPPSQ in solution (0.1 mg/ mL in THF) by small-angle X-ray scattering (SAXS) at 25 °C, which to our knowledge is the first of its kind (Figure 9). We stipulated that solutions SAXS experiments would not only be able to prove the linearity of LPPSQ but also undoubtedly distinguish LPPSQ from POSS-based materials. And while the Zhang group has previously reported the helical nature of LPPSQ materials through 29Si NMR,30 SAXS analyses of the shape of LPPSQ can be said to be a more direct measurement of the molecular shape.31 The Guinier analysis of the obtained SAXS profiles gave a radius of gyration (Rg) of 14.2 Å as calculated from the equation I(q) = I(0) exp(−1/3)(Rg2q2) (Figure 9A).31,32 The shape of LPPSQ was first preliminarily estimated from the SAXS profile, q = 1/Rg, because the relationship of I(q) and q−α, where the value of α = 1, 2, and 4 is that of thin rod (1D), thin disk (2D), and sphere (3D) shapes, respectively.33 In this case, the α value was 0.997, indicating thin rod shape of LPPSQ, which was also confirmed by correlation of the form factor of a thin rod model (open circles in Figure 9A).34,35 Thin rod shape of a molecule was generally well-corresponded to Kratky plot, where I(q)q2 was plotted as a function of q; thus, the straight line of the scattering curve was interpreted as a rigid-rod-like structured molecule.34 Our experimental result with LPPSQ clearly showed directly proportioned to q−1 (Figure 9B), indicating that the obtained LPPSQ resided as a thin rodlike molecule in dilute solution. From these results, cross-sectional radius of gyration (Rc) and the radius (r) of the LPPSQ was calculated from the following equations: (1) I(q) = I(0) exp(−1/2)(Rc2q2) and (2) r = 21/2Rc,32 which gave Rc and r values of 9.5 and 13.3 Å, respectively. In addition, the length of the LPPSQ (L) was also calculated from the equation Rg2 = (r2/2) + (L2/12), showing a length (L) of 49.1 Å.33 The shape of LPPSQ assuming Mw = 15 000 was estimated by simulation using the MM2 method. Figure 9C shows the resulting simulated shape, which was long and helical structured with asymmetrical high aspect ratio. This simulated model gave a length (L) of 50.3 Å, which was nearly identical to the calculated experimental length of 49.1 Å, further supporting our claim that LPPSQ was of thin rod shape with helical structure. And while dilute solution SAXS results for T12-Pheyl POSS were unable to be obtained due to its size, the thin rod shape of LPPSQ was clearly differentiated from POSS materials in terms of its size and shape. This molecular conformation of LPPSQ was quite different from that of conventional random coil polymer such as polystyrene, etc., which indicated that the

Figure 9. Model fitting of the SAXS profiles of LPPSQ in THF at 25 °C: (A) thin rod model, (B) Kratky model, and (C) the most stable structure of LPPSQ by MM2 simulation.

polymer chain of LPPSQ was formed as rod shape with ladderlike siloxane structure.

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CONCLUSION A one-pot, facile synthesis of structurally controlled polyphenylsilsesquioxanes was developed. Mechanism and kinetic studies indicated that the condensation reaction proceeded through a T1 structured dimer, which was quantitatively and in situ formed through hydrolysis of a mild phenyltrimethoxysilane (PTMS) monomer, to give either the phenyl-substituted, 12membered, cage-structured polyhedral oligomeric silsesquioxanes (T12-Phenyl POSS) or the corresponding ladderlike polyphenylsilsesquioxanes (LPPSQ) with excellent yields. Ladderlike and POSS materials were selectively achieved at higher and lower initial concentrations of PTMS, respectively. An in-depth spectroscopic comparison of both compounds clearly revealed differences in structure, molecular weight, thermal properties, crystallinity, and coating properties. Furthermore, the linearity of LPPSQ analyzed by dilute solution small-angle SAXS method revealed that LPPSQ G


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Macromolecules

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formed as rod shape with ladderlike siloxane backbone structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01539. FTIR spectrum of T12-Phenyl POSS, 1H NMR spectra for LPPSQ and T12-Phenyl POSS, and XRD patterns for LPPSQ at various temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Author Downloaded by NANYANG TECHNOLOGICAL UNIV on August 29, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.macromol.5b01539

*E-mail [email protected] (K.-Y.B.). Author Contributions

S.-S.C. and A.S.L. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a grant from the Fundamental R&D for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea, and partially from a grant from the Materials Architecturing Research Center of Korea Institute of Science and Technology (KIST). Synchrotron solution SAXS and WAXD measurements were performed at the Pohang Light Source (PLS).

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ABBREVIATIONS LPSQ, ladderlike polysilsesquioxane; POSS, polyhedral oligomeric silsesquioxanes. REFERENCES

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Structural Control of Fully Condensed Polysilsesquioxanes

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