Synthesis and Characterization of Trifluoroethoxy Polyphosphazenes

Feb 2, 2016 - Synthesis and Characterization of Trifluoroethoxy Polyphosphazenes Containing Polyhedral Oligomeric Silsesquioxane (POSS) Side Groups...
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Synthesis and Characterization of Trifluoroethoxy Polyphosphazenes Containing Polyhedral Oligomeric Silsesquioxane (POSS) Side Groups Cuiyan Tong,†,‡ Zhicheng Tian,† Chen Chen,† Zhongjing Li,† Tomasz Modzelewski,† and Harry R. Allcock*,† †

Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States Institute of Chemistry, Northeast Normal University, Changchun 130024, P. R. China

‡

ABSTRACT: We report the macromolecular substitution synthesis of a series of soluble polyphosphazenes with different molar percentages of cosubstituted trifluoroethoxy and aminopropylisobutyl polyhedral oligomeric silsesquioxane R7Si8O12(CH2)3NH (R = isobutyl) (POSS-NH) cage side groups. The structure−property relationships of the novel polymers were examined using 1H, 31P NMR, GPC, FTIR, DSC, and TGA techniques, and properties such as contact angles, solubility, materials character, glass transitions, thermal stability, and molecular weight were measured. The average polymer chain length declined steadily from ∼3860 repeating units when only trifluoroethoxy side groups were present to 1354 repeat units when 25 mol % of the side groups were POSS, 603 units when 55 mol % were present, and only 36 units when 82 mol % of the side groups were POSS. At the same time the glass transition temperatures rose from −62 to +30.5 °C as the POSS content was increased. Unlike other bulky cosubstituents reported recently, the introduction of the amino-POSS side groups yields soft, film-forming polymers (up to ∼25% POSS) or nonflexible materials with higher loadings but does not generate elastomeric properties. These properties reflect the steric hindrance limitations imposed by the bulky POSS units. When 25 mol % of the polymer side groups are POSS units, contact angles to n-hexadecane around 67° are generated. These values are higher than all the previously reported phosphazene fluoropolymers or Teflon.

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INTRODUCTION Hybrid inorganic−organic polymers, especially macromolecules with an inorganic backbone and organic side groups, have attracted increasing attention in recent years. In terms of different side groups and architectures, poly(organophosphazenes) are the largest class of inorganic−organic macromolecules,1 followed by silicon-containing polymers. Intriguing possibilities exist for combining polyphosphazene structures with organosilicon side groups.2,3 In this paper we examine the property changes that result from the linkage of polyhedral oligomeric silsesquioxane (POSS) side units to a polyphosphazene chain, making use of the macromolecular substitution approach in which both fluoro-organic and organosilicon side groups are linked to the polymer skeleton via chlorine replacement reactions:

Figure 1. Structures of POSS (T8).

organic polymers and can lead to dramatic improvements in polymer characteristics such as high thermal and chemical stability, enhanced mechanical behavior, flame retardancy, oxidation resistance, and high gas permeability as well as

POSS units possess a cubic rigid (T8) structure represented by the formula R8Si8O12, where the central inorganic core (Si8O12) is decorated with organic moieties (R) at each of the eight vertices (Figure 1). These structures are known to impart a number of favorable properties when linked to classical © XXXX American Chemical Society

Received: December 2, 2015 Revised: January 25, 2016

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DOI: 10.1021/acs.macromol.5b02624 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Route to Aminopropylisobutyl-POSS-pentachlorocyclotriphosphazene

phosphazene−organosiloxane polymers, a class that has received only minimal attention up to this point.2,3 In a recent related study, Park et al.29 reported a system in which two POSS cages were assembled by corner-capping around organic substituents linked to a polyphosphazene chain. These polymers are different in concept and properties from the new species to be discussed here and require a more complex and challenging synthesis sequence. Thus, in this paper, we report a two-step synthesis of a series of phosphazene polymers with different molar ratios of POSS units and trifluoroethoxy side groups with use of a typical functional POSS molecule, R7Si8O12(CH2)3NH2 (R = isobutyl) (POSS-NH2), as a chlorine-replacement nucleophile. The feasibility of the substitution process was first monitored by a model reaction of POSS-NH2 with the cyclic small molecule hexachlorocyclotriphosphazene, (NPCl2)3, in the presence of triethylamine as a hydrochloride acceptor. After the feasibility of this model reaction was established, the synthesis conditions were then used as a starting point for the high polymer reactions. Thus, poly(dichlorophosphazene) was allowed to react first with POSS-NH2 in the presence of triethylamine and subsequently with sodium trifluoroethoxide. As much as 82 mol % of the chlorine in poly(dichlorophosphazene) could be replaced by POSS units. The polymer properties such as solubility, thermal stability, crystallization, brittleness, and surface hydrophobicity, etc., change significantly as increasing ratios of POSS side groups are introduced. The new polyphosphazenes with POSS side groups provide an indication of some of the changes that may be expected from other hybrid polyphosphazenes with bulky cage-type side groups.

changes in dielectric behavior and thermal conductivity. Moreover, they can also change the crystallization behavior and surface wetting properties of polymers.4−7 POSScontaining polymers have also been studied for utilization in bio-related applications,8−11 optical products,12 liquid crystals,13 proton exchange membranes,14,15 low-dielectric constant materials,16,17 gas transport,18 antifouling, and self-cleaning surfaces.19−21 Polyphosphazenes also have special properties, such as fire resistance, low-temperature flexibility, and elasticity, combinations that are difficult or impossible to find in classical organic backbone polymers.1 Moreover, a large number of different side groups have been linked to the skeletal phosphorus atoms via macromolecular substitution reactions,22−24 and this has generated a wide variety of different and often unique materials that include elastomers, membranes, fire-resistant polymers, biomedical materials, polyelectrolytes,25 NLO materials,26 and so on.1,27 Perhaps the most unique advantage of polyphosphazenes is access to a macromolecular substitution synthesis in which organic side groups are linked to the inorganic skeleton by replacement of chlorine side atoms in poly(dichlorophosphazene) by a wide variety of organic groups, such as alkoxy, aryloxy, or amino units. Cosubstituted poly(organophosphazenes) are synthesized by the reactions of two or more different nucleophiles with poly(dichlorophosphazene) to yield polymers with tunable properties controlled by the side group ratios.1,23−28 Recently, we have shown that rigid, bulky side groups linked to a polyphosphazene skeleton participate in intermolecular side group associations with their counterparts on neighboring chains, reducing chain slippage under tension, and generating elastomeric behavior.23,24 The main challenge for the introduction of rigid bulky side groups via macromolecular substitution is to overcome possible steric hindrance limitations which not only restrict the number of bulky substituents that can be linked to a polyphosphazene chain but can also bring about chain cleavage if forcing reaction conditions must be used. In this study we have examined the use of POSS and trifluoroethoxy cosubstituents to determine the influence of the organosilicon moieties on the properties of a well-characterized poly(fluoroalkoxyphosphazene). The unusual shape, stability, and dimensions of POSS side groups provide an opportunity to obtain more information about the properties of hybrid

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RESULTS AND DISCUSSION Model Cyclic Trimer 12. The linkage of bulky side groups to a high polymer chain by macromolecular substitution can be challenging due to steric hindrance effects and possible side reactions. Thus, a prudent first step is to attempt the same reaction with the use of a small molecule model system. For polyphosphazenes a good model is provided by the cyclic trimeric chlorophosphazene, (NPCl2)3, although the trimeric ring structure is more rigid and the phosphorus atoms more exposed than in the high polymeric counterpart. Scheme 1 shows the replacement of one chlorine atom of hexachlorocyclotriphosphazene by POSS-NH2. This reaction is fast and is complete in only 4 h at room temperature. Figure 2 shows the B

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Figure 2. 31P NMR spectrum of aminopropylisobutyl-POSS-pentachlorocyclotriphosphazene (cyclic trimer 12).

Scheme 2. Synthetic Route to Polymers 1−11

31

added to replace the remaining chlorine atoms after the generation of aminopropylisobutyl-POSS-pentachlorocyclotriphosphazene, the (POSS-NH−) group was not displaced from the cyclic phosphazene by the trifluoroethoxy group. Synthesis of Polymers 2−11. Polymers 2−11, with different molar percentages of POSS and trifluoroethoxy side groups, were synthesized by a similar procedure, illustrated in Scheme 2. The chlorine atoms in poly(dichlorophosphazene) were replaced in a two-step procedure. First, specific ratios of the bulky POSS-NH2 reactant were allowed to interact with poly(dichlorophosphazene) in THF solvent in the presence of triethylamine. Excess sodium trifluoroethoxide was then added to replace the remaining chlorine atoms and yield the final

P NMR spectrum of aminopropylisobutyl-POSS-pentachlorocyclotriphosphazene. This 31P NMR spectrum of 12 is a classic A2B spectrum (A represents the phosphorus atoms that bear two chlorine atoms and B is the phosphorus atom which bears one chlorine and one aminopropylisobutyl-POSS (POSS-NH−) group). The actual shifts of A and B are 20.67 and 18.85 ppm, respectively, and the coupling constant of JAB is 47.45 Hz, which can be easily calculated from the A2B spin system. This differs from the 31P NMR spectrum of hexachlorocyclotriphosphazene which consists of one phosphorus peak at δ (ppm) = 20.0. The composition of the cyclic phosphazene (trimer 12) was confirmed by the MOF MASS spectrum which showed a signal at m/z = 1182.08. It should be noted that when excess sodium trifluoroethoxide was then C

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Macromolecules trifluoroethoxy-POSS-polyphosphazene products (polymers 2− 11). The progress of the reactions was monitored using 31P NMR spectroscopy. For example, in the case of polymer 3, the existence of both the trifluoroethoxy and POSS-NH− groups was evident from signals in the 1H NMR spectrum (Figure 3) and from the 31P NMR spectrum in Figure 4.

Figure 3. 1H NMR spectrum of polymer 3.

Thus, the 31P NMR spectrum of polymer 3 (Figure 4B) contained a peak at −7.61 ppm, which is readily distinguished from the PCl 2 signal at −18 ppm in poly(dichlorophosphazene) and the intermediate product of poly(dichlorophosphazene) substituted by the POSS-NH− group (Figure 4A). This 31P NMR spectrum and the ratio of the integration intensities in the 1H NMR spectra confirmed the structure of the polymer. Polymers 2−11 were all prepared with use of a similar procedure and were characterized by the same 31P and 1 H NMR techniques. The fact that the reactions of POSS-NH2 with both the small molecule trimer and with poly(dichlorophosphazene) were accomplished at room temperature illustrates the ease of introduction of the POSS units. It is worth noting that after the first step, when poly(dichlorophosphazene) is substituted by different amounts of POSSNH− groups, the subsequent introduction of trifluoroethoxy cosubstituents groups to replace all the remaining chlorine atoms is inhibited because of the steric hindrance and shielding generated by the POSS groups. For example, an elemental analysis of polymer 5 shows that 1.12% unreacted chlorine remained. However, the remaining chlorine atoms in polymers 2−11 were not sensitive to water and could represent hydrogen chloride molecules coordinated to the polymers.2,3 Furthermore, as discussed below, the sensitivity of this system to high dilutions in acidic media and to depolymerization during the synthesis process is an aspect that needs to be taken into account. Characterization and Polymer Properties. Fourier Transform Infrared Spectroscopy (FT-IR). The polymers were characterized by FTIR, and Figure 5 shows some of the spectra. Figure 5 shows strong and symmetric Si−O−Si stretching peaks at ∼1082 cm−1 in polymers 2−11. These correspond to the silsequioxane cage of POSS, and the peaks

Figure 4. 31P NMR of polymer 3 after the first step (A) and the second step (B).

Figure 5. FTIR spectra of some representative polymers.

become more prominent with the increasing content of POSS. This illustrates that the POSS structure survives the processing protocol. In addition, the peaks at 2963 cm−1, which correspond to the aliphatic C−H vibration, become larger with the increasing POSS content in the polymers. D

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Macromolecules Table 1. Characterization Data for Polymers 1−11 polymer

mol % POSS

solubility in hexane (1 wt %)

film-forming ability (THF)

dielectric permittivity at 25 °C

Tg (°C)

T1 (°C)

Td (°C, 5% loss)

Mw (kDa)

repeat units

PDI

WCAc (deg)

HCAd (deg)

yield (%)

1 2 3 4 5 6 7 8 9 10 11

0 6.6 14 25.8 32.2 44.8 54.7 61.2 70.6 76.0 82.1

insoluble insoluble insoluble insoluble soluble soluble soluble soluble soluble soluble soluble

soft films soft films soft films soft films stiff and brittle stiff and brittle brittle brittle −b −b −b

6.15 6.05 4.28 3.60 3.10 2.09 −b −b −b −b −b

−62.0 −50.8 −6.8 11.6 17.3 18.0 26.0 30.5 NAa NAa NAa

58.2 NAa NAa NAa NAa NAa NAa NAa 42.1 46.5 46.7

343 307 321 332 316 330 306 311 301 280 277

937 670 780 870 887 608 657 791 360 280 55

3856 1942 1696 1354 1196 654 603 664 269 197 36

2.1 2.3 2.1 2.5 1.3 2.9 2.4 2.7 1.6 1.6 1.3

104 94.2 94.4 95.3 97.3 99.0 108.1 113.3 −b −b −b

53.4 53.3 66.5 67.2 −b −b −b −b −b −b −b

75.3 67.0 70.0 52.8 61.5 58.1 42.9 44.9 44.6 55.9 54.5

a

A Tg/T1 transition was not detected by DSC. bThe data were not obtained. cContact angle to deionized water. dContact angle to n-hexadecane.

Furthermore, the peaks at 1253 and 958 cm−1 from the C−O symmetric vibration of the trifluoroethoxy group become progressively weaker as the POSS content increases.30 Thus, the 1H NMR and FTIR results positively identify the POSS chemical structure and confirm that this group can replace the chlorine atoms to form polymers 2−11. It is interesting that no peaks near the 3400 and 1650 cm−1 that would correspond to N−H hydrogen bonds could be detected, and this suggests that such N−H bonding is shielded by the bulky POSS groups. This is important since it matches the interpretation that the decrease in the molecular weight and other changes with the increasing POSS content are mainly due to the presence of the bulky POSS units and not to N−H bonding interactions, which will be discussed later. Film-Forming Properties. The increasing content of POSSNH2 units in polymers 2−11 alters the physical properties in the following ways (Table 1). First, the presence of up to ∼25 mol % POSS allows the polymers to remain flexible, filmforming, or leathery materials. Above this loading the polymers become increasingly inflexible, possibly due to steric restrictions to conformational motions or to the effects of decreased chain length. This effect reaches its extreme when 82 mol % of the side groups are POSS units and the material lacks the coherence to form films. This is almost certainly a consequence of the fact that the chain length has fallen to only 36 repeat units. Interestingly, polymers with up to ∼25 mol % POSS are insoluble in hydrocarbons such as hexane or n-hexadecane, thus reflecting the high fluorocarbon content, but they become soluble at higher loadings of POSS, which illustrates the increasing influence of the organosiloxane components. Molecular Weights and Degree of Polymerization. Gel permeation chromatography derived molecular weights of polymers 1−11 are listed in Table 1. Thus, the molecular weights of the polymers decrease with the increased molar percentages of POSS-NH− units. For polymers 1−8, the molecular weights are in the range of 600−1000 kDa, while for polymers 9 and 10 the values are as low as 200−400 kDa. For polymer 11 the molecular weight is only 55 kDa. The corresponding degrees of polymerization for all the polymers are shown in Table 1. Polymer 1 contains 3856 repeating units. Polymers 2−5 have repeat units in the range of 1000−2000. Polymers 6−10 have 100−700 repeat units. However, polymer 11 has only 36 repeat units. As mentioned, a possible reason for this decline in chain length is that the steric hindrance imposed by the POSS cage is much more significant than that of the

trifluoroethoxy group, and this favors depolymerization in solution during synthesis, even at room temperature. Glass and Mesophase Transition Temperatures. Figure 6 shows the DSC curves of pure poly[(bis-2,2,2-trifluoroethoxy)-

Figure 6. DSC traces of polymers.

phosphazene] (polymer 1) and the POSS-NH−/OCH2CF3 counterparts, with the Tg and other transitions listed in Table 1. For polymers 1−8, the data revealed a near linear increase in the Tg with an increase in the side chain POSS-NH content, ranged from −62.0 to +30.5 °C. A T1 transition was detected only for polymers 9−11.3,24 Several factors could account for these phenomena. With polymers 1−8, the bulky POSS side groups probably restrict torsion of the backbone bonds. Thus, the increased POSS content will raise the Tg even though the polyphosphazene backbone itself is a highly flexible platform. Moreover, when the POSS loading reaches a high level, packing of the POSS units could lead to pseudocrystallinity and a corresponding T1 transition. Thus, for polymer 1, the low Tg of −62.0 °C reflects the high flexiblility of the backbone as seen in poly(dichlorophosphazene), while the T1 transition at +58.2 °C is a consequence of the efficient packing of trifluoroethoxy groups. Thermal Stabilities. The thermal weight loss curves evaluated by TGA in nitrogen are shown in Figure 7 and Table 1. For all the polymers and the oligomer 11, the weight loss begins between 250 and 300 °C. However, this does not E

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increasing contact angles to water, which demonstrates an oleophobic enhancement with the increasing content of POSS. However, it should be pointed that polymers 3 and 4 show contact angles to n-hexadecane around 67°, which is higher than all the reported phosphazene fluoropolymers23 and Teflon.31

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EXPERIMENTAL SECTION

Reagents and Equipment. All the syntheses were performed using standard Schlenk-line techniques under a dry argon atmosphere. The reaction glassware was dried for 24 h in an oven at 120 °C before use. Tetrahydrofuran (THF) and triethylamine (TEA) were dried using solvent purification columns.32 Sodium hydride (NaH, 60% dispersion in mineral oil, Sigma-Aldrich) was stored in an inert atmosphere and was used as received. Trifluoroethanol (SigmaAldrich) was distilled over sodium metal and stored over 4A molecular sieves (EMD) in an argon atmosphere. Aminopropylisobutyl POSS (POSS-NH2) was obtained from Hybrid Plastics and was used as received. Dialysis of polymers was carried out using Spectra/Por molecular porous cellulose dialysis membranes (Spectrum Laboratories, Inc.) with molecular weight cutoffs of 12 000−14 000 Da. Hexachlorocyclotriphosphazene (Fushimi Chemical Company, Japan) was purified by recrystallization from hexane followed by vacuum sublimation. Poly(dichlorophosphazene) was synthesized by the BCl3catalyzed thermal polymerization of the purified hexachlorocyclotriphosphazene in evacuated Pyrex tubes at 220 °C. Any unreacted hexachlorocyclotriphosphazene was removed by vacuum sublimation.33 Structural Characterization. 1H and 31P NMR spectra were obtained with use of a Bruker AV-360 instrument operated at 360 and 145 MHz, respectively. 1H shifts were reported in ppm relative to tetramethylsilane at 0 ppm. 31P NMR shifts were reported in ppm relative to 85% H3PO4 at 0 ppm. 31P NMR spectra were protondecoupled. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were measured with a FTIR Bruker V70 spectrometer using polymer powder or polymer films directly. Molecular Weights and Distributions. The molecular weights of the polymers were estimated using a Hewlett-Packard HP 1100 gel permeation chromatograph (GPC) equipped with two Phenomenex Phenogel linear 10 cm columns and a Hewlett-Packard 1047A refractive index detector, eluted at a rate of 1.0 mL/min using a 0.1 wt % solution of tetra-n-butylammonium nitrate (Aldrich) in THF. The elution times were calibrated with polystyrene standards. Thermal Analysis. Glass transition temperatures were measured with a TA Instruments Q10 differential scanning calorimetry (DSC) unit with a heating rate of 10−20 °C/min and a sample size of 10−15 mg. Data analysis was by means of TA Instruments Universal Analysis 2000 software. Thermal volatilization traces were recorded from 50 to 800 °C using a PerkinElmer TGA 7 thermogravimetric analyzer at a heating rate of 40 °C/min with a sample size of 10−20 mg. Dry nitrogen was used as the purge gas with a flow rate of 50 mL/min for both instruments. Contact Angle Measurements. Advancing water contact angle data for the polymers were obtained using the Ramé-Hart automated goniometer/tensiometer (Succasunna, NJ) with DROPimage advanced v2.6 at room temperature. Smooth polymer films were prepared by casting a polymer solution from THF onto clean glass microscope slides (25 × 75 mm) followed by drying in vacuum before measurements. A deionized water droplet of 2 μL was used on the surface of the films, and digital images of the drop silhouette were recorded with a video camera. Each result was the average based on 10 measurements. Parallel measurements were taken with the use of nhexadecane droplets. Dielectric Analysis (DEA). The dielectric constants (ε) were determined by the capacitance method performed using a HP 4275A multifrequency LCR meter. The thin films were made by a standard solvent-casting technique using THF solvent on a Teflon substrate. An array of Ag dots was deposited onto the film surface by painting with a silver metal ink (66 wt % Ag) before measurements. The frequency

Figure 7. TGA thermograms of polymers 1−11 under a N2 atmosphere.

necessarily represent the thermal stability since depolymerization or decomposition at lower temperatures may yield products that are not volatile below 250 °C. Dielectric Analysis (DEA). The dielectric constants of polymers 1−6 are summarized in Table 1. These values decrease steadily with the increased loading of the POSS-NH− groups because the POSS-based solids possess what is essentially a nanoporous structure. This can effectively decrease the dielectric constant due to the increased free volume in these polymers.16,17,30 Specifically, the high porosity of the POSSpolyphosphazene-TFE systems result from the nanometer scale porous structure surrounding the POSS units, and this markedly reduces the dielectric constant. Surface Contact Angle Measurements. Because of the importance of surface properties in many fields such as biomaterials, lithographic patterning, and as mold-release materials in nanoimprint technology,19−21 it was of interest to obtain information on how the POSS units affect the surface characteristics of the polymers synthesized here. This property was studied by measurement of the surface contact angle to water or n-hexadecane of polymer films. The results are shown in Table 1. For polymers 2−8 the contact angles to water increased with the increasing POSS-NH- content from 94.2° (polymer 2) to 113.3° (polymer 8), which reflects the high hydrophobicity of the siloxane units.2 This is illustrated by the representative images for polymers 2 and 8 in Figure 8. Curiously, the water contact angle for 1 (104°) lies close to the values for polymer 7, which suggests that at higher loadings of POSS units the organosilicon species may be buried beneath the surface. Furthermore, the oleophilic data for the contact angles to n-hexadecane show the same increasing trend as the

Figure 8. Contact angle to water images of polymers 2 (94.2°) and 8 (113.3°). F

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Macromolecules used for the DEA experiments was 100 kHz, and the capacitance (ε) was monitored at 25 °C. The permittivity (ε) was calculated based on the equation

ε = Cd /Aε0

phosphazene polymers to form lower polymers at elevated temperatures or in dilute solution by ring−chain or ring−ring depolymerization.

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(1)

where ε is the dielectric constant, C is the capacitance, d is the thickness of the films, A is the area of top Ag electrode, and ε0 is the permittivity of free space.16 Synthesis of Model Compound N3P3Cl5(NH-POSS) (12). To a solution of hexachlorocyclotriphosphazene (0.50 g, 1.44 mmol) in THF (50 mL) was first added, with stirring, triethylamine (0.24 mL, 1.73 mmol), followed by dropwise addition of a solution of POSSNH2 (1.26 g, 1.44 mmol) in THF (50 mL). The chlorine replacement reaction was allowed to progress at 25 °C overnight while monitored by 31P NMR spectroscopy. 31P NMR (145 MHz, THF with D2O capillary as external standard) δ (ppm): 20.67 and 18.85 (A2B). MS (ESI+): 1182.08 m/z (M + H+). The NMR spectrum was consistent with a mono-POSS-substituted pentachloro cyclic trimer. Synthesis of Polymer 1. Poly(dichlorophosphazene) (1.00 g, 8.63 mmol) was dissolved in THF (100 mL) with stirring at 25 °C. To a stirred suspension of sodium hydride (0.76 g, 18.99 mmol) in THF (50 mL), trifluoroethanol (1.36 mL, 18.99 mmol) was added slowly to form a clear sodium trifluoroethoxide solution. The sodium trifluoroethoxide solution was then added dropwise to the polymer solution at 25 °C overnight while monitored by 31P NMR spectroscopy. The resulting polymer was purified by precipitation from concentrated THF into water (2 L) three times and hexane (2 L) twice to yield (after drying in vacuum) a white leathery material. 1H NMR (acetoned6) δ (ppm): 4.58 (s, −OCH2CF3, 2H). 31P NMR (acetone-d6) δ (ppm): −7.81 (s). The polymer characteristics are consistent with previously published results.34 Synthesis of Polymers 2−11. Polymers 2−11 were synthesized with use of a similar procedure. For example, for polymer 2, poly(dichlorophosphazene) (2.00 g, 17.26 mmol) was dissolved in THF (200 mL) with stirring at 25 °C. Meanwhile, POSS-NH2 (1.51 g, 1.73 mmol) was dissolved with stirring in THF (50 mL) in a separate Schlenk flask at 25 °C. Triethylamine (0.48 mL, 3.45 mmol) was then added slowly to the poly(dichlorophosphazene) solution followed by the POSS-NH2 solution, added dropwise at 25 °C overnight while monitored by 31P NMR spectroscopy. In a separate vessel, to a suspension of sodium hydride (1.38 g, 34.52 mmol) in THF (50 mL), trifluoroethanol (2.48 mL, 34.52 mmol) was added slowly at 25 °C to form a clear sodium trifluoroethoxide solution. After the POSS-NH− side group had been linked to the polymer backbone (as indicated by 31 P NMR), the sodium trifluoroethoxide solution was added dropwise to the polymer solution, and the mixture was stirred at 25 °C overnight while monitored by 31P NMR spectroscopy. The reaction mixture was then concentrated first by solvent evaporation and then precipitated into water (2 L) twice. The resultant product was dried under high vacuum and then dissolved in THF and dialyzed against 10% v/v methanol/THF for 3 days (change of solvent once a day) to complete the purification. After dialysis the solution was concentrated by a rotoevaporator to remove the solvent, and the resultant product was then dried under high vacuum to afford the final polymers.

AUTHOR INFORMATION

Corresponding Author

*(H.R.A.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Cuiyan Tong was supported by the China Scholarship Council (CSC) and Northeast Normal University. The authors thank Jinshan Guo for help with the FTIR tests and analyses, Qiyao Li for help with the surface contact angle measurements, and Lei Qin for assistance with the dielectric analysis experiments.

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REFERENCES

(1) Allcock, H. R. Chemistry and Applications of Polyphosphazenes; John Wiley & Sons, Inc.: Hoboken, NJ, 2003. (2) Allcock, H. R.; Coggio, W. D. Organosiloxyphosphazene Polymers: Synthesis via Aminosiloxane Reagents. Macromolecules 1990, 23, 1626−1635. (3) Allcock, H. R.; Kuharcik, S. E.; Nelson, C. J. Synthesis and Characterization of Aminoorganosiloxane-Bearing Polyphosphazenes: New Properties by the Elimination of Hydrogen Bonding. Macromolecules 1996, 29, 3686−3693. (4) Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T. Silsesquioxanes. Chem. Rev. 1995, 95, 1409−1430. (5) Li, G.; Wang, L.; Ni, H.; Pittman, C. U., Jr. Polyhedral Oligomeric Silsesquioxane (POSS) Polymers and Copolymers: A Review. J. Inorg. Organomet. Polym. 2001, 11, 123−154. (6) Kuo, S. W.; Chang, F. C. POSS Related Polymer Nanocomposites. Prog. Polym. Sci. 2011, 36, 1649−1696. (7) Tanaka, K.; Chujo, Y. Advanced Functional Materials Based on Polyhedral Oligomeric Silsesquioxane (POSS). J. Mater. Chem. 2012, 22, 1733−1746. (8) Tutak, M.; Dogan, M. Development of Bio-Active Polypropylene Fiber Containing QA-POSS Nanoparticles. Fibers Polym. 2015, 16, 2337−2342. (9) Lorenza Gardella, L.; Colonna, S.; Fina, A.; Monticelli, O. On Novel Bio-Hybrid System Based on PLA and POSS. Colloid Polym. Sci. 2014, 292, 3271−3278. (10) Jeon, J. J.; Tanaka, K.; Chujo, Y. Light-Driven Artificial Enzymes for Selective Oxidation of Guanosine Triphosphate Using WaterSoluble POSS Network Polymers. Org. Biomol. Chem. 2014, 12, 6500− 6506. (11) Tanaka, K.; Chujo, Y. Advanced Functional Materials Based on Polyhedral Oligomeric Silsesquioxane(POSS). J. Mater. Chem. 2012, 22, 1733−1746. (12) Peng, D.; Qin, W.; Wu, X. A Study on Resistance to Ultraviolet Radiation of POSS−TiO2/Epoxy Nanocomposites. Acta Astronaut. 2015, 111, 84−88. (13) Tanaka, K.; Ishiguro, F.; Jeon, J. H.; Hiraoka, T.; Chujo, Y. POSS Ionic Liquid Crystals. NPG Asia Mater. 2015, 7, e174. (14) Wu, Z.; Zhang, S.; Li, H.; Liang, Y.; Qi, Z.; Xu, Y.; Tang, Y.; Gong, C. Linear Sulfonated Polyimides Containing Polyhedral Oligomeric Silsesquioxane (POSS) in Main Chain for Proton Exchange Membranes. J. Power Sources 2015, 290, 42−52. (15) Zhang, J.; Chen, F.; Ma, X.; Guan, X.; Chen, D.; Hickner, M. A. Sulfonated Polymers Containing Polyhedral Oligomeric Silsesquioxane (POSS) Core for High Performance Proton Exchange Membranes. Int. J. Hydrogen Energy 2015, 40, 7135−7143. (16) Ke, F.; Zhang, C.; Guang, S.; Xu, H.; Lin, N. POSS-based Molecular Hybrids with Low Dielectric Constant: Effect of Chemical Structure and Molecular Architecture. J. Appl. Polym. Sci. 2015, DOI: 10.1002/APP.42292.

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CONCLUSIONS In this work, the first series of trifluoroethoxy-polyphosphazenes containing bulky cubic POSS-NH− cosubstituent groups have been synthesized. The glass transition temperatures of these polymers increase with the increasing content of POSS due to steric interference and inhibited backbone torsion. POSS loadings up to 25 mol % yield polymers with over 1000 repeating units and good film-forming properties. However, further increases in the POSS content brings about a progressive decrease in chain length and a deterioration in morphological properties. The molecular weight declines associated with higher POSS loadings are attributed to steric hindrance, which may amplify the sensitivity of some G

DOI: 10.1021/acs.macromol.5b02624 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02624 Macromolecules XXXX, XXX, XXX−XXX