Multifunctional POSS Cyclic Carbonates and Non-Isocyanate

Article pubs.acs.org/Macromolecules

Multifunctional POSS Cyclic Carbonates and Non-Isocyanate Polyhydroxyurethane Hybrid Materials Hannes Blattmann and Rolf Mülhaupt* Institute for Macromolecular Chemistry, Freiburg Materials Research Center (FMF), Stefan-Meier Strasse 21, D-79104 Freiburg, Germany S Supporting Information *

ABSTRACT: The solvent-free chemical fixation of carbon dioxide with glycidyl ether functionalized polyhedral oligomeric silsesquioxanes (POSS) yields mono- and polydisperse multifunctional POSS cyclic carbonates. The silica equivalent content of POSS carbonates varies between 28.5 and 42.1 wt %, whereas 15.4−20.8 wt % carbon dioxide is incorporated. On cure with hexamethylenediamine and isophoronediamine, POSS carbonates and their blends with polyol-based cyclic carbonates produce organic/inorganic hybrid polyhydroxyurethane thermosets without requiring the need of isocyanate monomers. These blends prolong both gel and pot life time of NIPU formulations, thus allowing the preparation of tailored NIPU/POSS hybrid materials for casting and coating applications. Young’s modulus and tensile strength improve with increasing POSS content as a result of higher cross-link density and silica content. Submicron phases are formed during polymerization due to self-assembly and nanophase separation as was verified by scanning electron microscopy (SEM). Moreover, the incorporation of POSS significantly improves scratch resistance of optically transparent and colorless NIPU/POSS coatings as examined by the surface gloss and SEM images.

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INTRODUCTION

as additives and improved tear resistance, Young’s modulus, and even thermal stability.11 The ring-opening polyaddition of polyfunctional cyclic carbonates and amines affords non-isocyanate polyhydroxyurethanes (NIPU) as an eco-friendly and safe alternative to isocyanate- and phosgene-based PUs. This NIPU chemistry has been the subject of several reviews.12−14 Multifunctional cyclic carbonates are readily available by the chemical fixation of carbon dioxide with epoxides at 100−140 °C and 20−40 bar of CO2 pressure in the presence of catalytic amounts of tetrabutylammonium bromide (TBAB) or LiBr.15−18 Recently, Bivona et al. employed imidazolium functionalized POSS as an effective catalyst for the carbonation of epoxy resins.19 Multifunctional cyclic carbonates derived from renewable resources such as soy bean16,20 and linseed21 oil, epoxidized terpenes,17 and biobased polyol glycidyl ethers18 were employed in order to simultaneously improve glass temperature (Tg) and mechanical properties of bio-based NIPUs. Although six-membered cyclic carbonates are more reactive by a factor of 29−62, their synthesis is more demanding compared to five-membered cyclic carbonates, which limits the availability of suitable raw materials.22−24 The incorporation of electron-withdrawing groups like ether groups enhance the amine cure rate of cyclic carbonates, whereas sterically demanding substituents drastically

Polyhedral oligomeric silsesquioxanes (POSS), pioneered by Scott in 1946, comprise polymeric or cagelike organosilicons with the chemical formula (RSiO1.5)n, in which cages are denoted as RnTn, n = 6, 8, 10, 12, ...., and R represents functional organic or aliphatic groups attached to the periphery of the nanometerscaled molecular, silica-like core.1 The average POSS core diameter falls in the range of approximately 0.45−0.53 nm.2 The polarity and functionality of the organic substituents govern the POSS miscibility, dispersion, and self-assembly, which is the key to tailor a wide variety of nano- and microstructured organic/ inorganic hybrid materials.3 In general, POSS is readily prepared by the controlled hydrolysis of R1Si(OR2)3 compounds, in which R1 is alkyl with functional end groups and R2 is methyl or ethyl. Typically, the direct hydrolysis of R1Si(OR2)3 yields complex reaction mixtures containing different cage structures together with only partially condensed polysilsesquioxane cages.4−6 In the preferred two-step process the hydrolysis of either HSi(OR2)3 or HSiCl3 yields Si−H functional POSS, which is purified and alkylated with olefins through hydrosilylation in the presence of the Speier or Karstedt Pt catalysts.6−8 As molecular nanocomposites, POSS compounds combine the facile processing typical for thermosets with property profiles characteristic for ceramics such as high hardness, stiffness, abrasion resistance, thermal and oxidative stability, and inherent flame retardancy.3,9,10 In conventional polyurethane (PU) synthesis, multifunctional epoxy-functionalized POSS molecules were employed © XXXX American Chemical Society

Received: November 25, 2015 Revised: January 5, 2016

A

DOI: 10.1021/acs.macromol.5b02560 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Multifunctional POSS Cyclic Carbonates Derived from POSS Glycidyl Ethers by Chemical Carbon Dioxide Fixation (δ = 0: POSS-GC, 1, T8 Species Is Displayed; δ = 1: POSS-8GC, 2)

reduce the reactivity of the cyclic carbonates.17,18,25 Several groups have reported on the preparation of organic/inorganic NIPU hybrid materials. For example, thermoplastic NIPU hybrids were obtained by reacting diamines with difunctional cyclic carbonates derived from epoxy-terminated linear polysiloxanes.26,27 In addition, up to 4 wt % functional (nano)fillers such as Stöber silica and ZnO particles, surface-modified with 4((3-(trimethoxysilyl)propoxy)methyl)-1,3-dioxolan-2-one, were incorporated into NIPU thermosets and coatings.28−31 NIPU nanocomposite coatings exhibited substantially higher gloss as well as impact and abrasion resistance, while the stiffness and the tensile strength improved only marginally due to the rather low silica contents tolerated in NIPU formulations. In an alternative route, nanostructured polysiloxane/NIPU conetworks were prepared via the co-condensation of alkoxysilane groups with the NIPU’s hydroxyl groups and moisture either during or after NIPU cure.32−35 Such NIPU/polysiloxane hybrid materials with a higher silicate content exhibited improved mechanical properties, scratch resistance, adhesion, and environmental stability. Furthermore, aminopropyltrisalkoxylsilanes were hydrolyzed to produce branched polyfunctional siloxane amine oligomers which served as curing agents for the NIPU/ polysiloxane hybrid formation.36 In 2015, Liu et al. incorporated POSS into NIPU thermosets by curing gallic acid based cyclic carbonates with various diamines combined with the subsequent NIPU modification using epoxy-functionalized POSS in a solvent-assisted coating process.37 This POSS epoxy/NIPU hybrid coating formation accounted for substantial improvements of pencil hardness, water resistance, and thermal stability. However, in view of further applications it is highly desirable to develop polyfunctional POSS-based cyclic carbonate monomers to prepare solvent-free polyhydroxyurethanes. Herein, we report on the facile synthesis of novel multifunctional POSS cyclic carbonates and their cure with diamines to produce POSS/NIPU nanocomposites with variable POSS content as a new family of organic/inorganic NIPU hybrid materials and NIPU coatings. As illustrated in Scheme 1, the key reaction is the catalytic fixation of carbon dioxide via the carbonation of POSS glycidyl ethers. The influences of POSS cyclic carbonates on the thermal and mechanical properties as well as on the morphology were examined. Another objective of

this study was to develop transparent and scratch-resistant NIPU/POSS hybrids for coating applications.

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EXPERIMENTAL SECTION

Materials. Glycerol polyglycidyl ether (ipox CL 12, epoxy equiv 159 g equiv−1), trimethylolpropane polyglycidyl ether (ipox RD 20, epoxy equiv 136 g equiv−1), and pentaerythritol polyglycidyl ether (ipox CL 16, epoxy equiv 167 g equiv−1) were supplied by ipox chemicals GmbH. Octacglycidyloxypropyldimethylsilyl POSS (POSS-8G, EP0435, epoxy equiv 236 g equiv−1) and the POSS glycidyl ether (POSS-G, EP0409, epoxy equiv 167 g equiv−1), consisting of a mixture of POSS glycidyl ethers with different cage sizes, were purchased from Hybrid Plastics. Hexamethylenediamine was obtained from Alfa Aesar. Acetone, tetrabutylammonium bromide (TBAB), isophoronediamine, and phenol were purchased from Sigma-Aldrich. Carbon dioxide grade N45 was obtained by Air Liquide. Characterization. 1H and 13C NMR spectra were recorded in acetone-d6 or CDCl3 on a Bruker Avance II spectrometer (1H: 299.87 MHz, 13C: 75.40 MHz). The signal of acetone-d6 or CDCl3 was used as internal standard. Both the epoxy (EC) and carbonate content (CC) were determined using phenol as internal reference in 1H and 13C NMR measurements.38 Quantitative 13C NMR spectra were recorded using decoupled, inverse-gated NMR techniques with Cr(acac)3 to reduce relaxation times. The calculation of the carbonate content directly from the determined epoxy content, regarding the 100% epoxy conversion yielding cyclic carbonates, was in excellent agreement with the experimental content, as determined by NMR spectroscopy. The following equations were used for calculation:

EC =

CC =

mphenol × Iepoxide M phenol × Iphenol × mepoxide

× 1000 (1)

EC 1+

EC 1000

× 44.01

(2)

The FTIR measurements were conducted on a Bruker FTIR Vector 2200 spectrometer, equipped with a Goldengate unit, using the attenuated total reflectance (ATR) technique (30 scans per sample, 4 cm−1 resolution). MALDI-TOF experiments were performed using a Bruker Autoflex 3 spectrometer. All samples were measured in acetone using sodium trifluoroacetate and a trans-2-[3-(4-tert-butylphenyl)-2methyl-2-propenylidene]malononitrile (DCTB) matrix. The viscosities, pot life, and gelation times were measured on an Anton Paar Physica MCR 301 rheometer using plate−plate geometry with a diameter of 25 mm. Viscosities were measured by a frequency sweep experiment from 100 to 0.1 rad s−1 with 5 points per decade and 5% deformation at 20, 50, B


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Macromolecules

Figure 1. 1H NMR spectra (acetone-d6) of polydisperse POSS-GC (1, top) and monodisperse POSS-8GC (2, bottom). temperature from 50 to 650 °C under air. Additionally, swelling tests of NIPU materials were carried out storing cubical samples of tensile test specimen (150−200 mg) in water and toluene over a period of 14 days. The degree of swelling was calculated from the percentage of weight increase. Scanning electron microscopy was performed with a Quanta 250 FEG. The scratch resistance of the prepared coatings (thickness 0.5 mm) was investigated by measuring the coating’s surface gloss using an Erichsen Picogloss 650 MC (60°, statistical average of ten single measurements) before and after treating the coating surface with 200 double strokes of a 500 g movable metal block equipped with an abrasive silicon carbide/alumina nonwoven (3M Scotch-Brite CF-HP 7448 Ultra Fine, contact area 5 × 5 cm, thickness 0.5 cm) at 22 °C. For each sample, a new nonwoven layer was attached to the block. General Procedure of the Syntheses of Cyclic Carbonates. All cyclic carbonates were synthesized using procedures as previously reported by Fleischer et al.:18 The cagelike POSS glycidyl ether (EP0409, 30.0 g, 0.179 mol) was placed in a 200 mL stainless steel

and 80 °C. Pot life times were determined by reaching a viscosity of 10 Pa s. Gelation times were analyzed by the crossover of the storage (G′) and loss modulus (G″) during an oscillatory experiment with 10 rad s−1 and 5% deformation at 20, 50, and 80 °C. Glass transition temperature (Tg) measurements of cyclic carbonates were conducted on a DSC 204F1 Phoenix by Netzsch using a heating and cooling rate of 10 K min−1 in the temperature range between −100 and 100 °C. Glass transition temperatures of NIPUs were determined by dynamic mechanical analysis (DMA) performed on a DMA Q800 by TA Instruments from the maximum of the loss factor tan δ. DMA experiments were performed with 1 Hz and 0.1% strain amplitude, varying the temperature between −50 and 150 °C at a heating rate of 3 K min−1, following the equilibration at −50 °C for 5 min. The mechanical properties of NIPU materials were characterized at 23 °C on a Zwick Z005 (Ulm, Germany, ISO/DP 527) according to DIN-EN-ISO527. Thermogravimetric analysis was performed on a TGA 4000 from PerkinElmer using a heating rate of 10 K min−1 and varying the C


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Macromolecules

Table 1. Properties of Synthesized POSS-Based Cyclic Carbonates through Chemical Fixation of CO2 (30 bar) and 1 wt % TBAB at 100 °C within 24 h cyclic carbonate

ECa (mmol g−1)

POSS-Ge POSS-GC POSS-Gf POSS-G8C

6.0

CCa (mmol g−1)

w(CO2) (wt %)

4.7

20.8

3.6

15.4

w(SiO2) (wt %)

Tgb (°C)

Tdc (°C)

35.9 28.5 49.8 42.1

−59 −15 −75 −30

350 297 307 313

4.2

η20 °Cd (Pa s)

η50 °Cd (Pa s)

η80 °Cd (Pa s)

2339

62.7

6.8

158

8.5

1.5

Epoxy/carbonate content, quantitative 1H and 13C NMR spectroscopy. bDSC, −100 to 100 °C, 10 K min−1, second heating curve. cTGA, 10 K min−1, air. dRheology: oscillatory frequency sweep, 100−0.1 rad s−1, 5 points per decade, 5% deformation. eEP0409. fEP0435. a

reactor. Then tetrabutylammonium bromide (0.30 g, 1.0 wt %) and carbon dioxide (30 bar pressure) were added. The reaction mixture was stirred and heated at 100 °C for 24 h until no epoxy groups were detected by 1H NMR spectroscopy. At full conversion, a colorless viscous liquid was obtained in quantitative yield. After cooling to room temperature and degassing under reduced pressure, all cyclic carbonates were obtained in high yields and were used without further purification. The glycidyl ether-based POSS cyclic carbonate (POSS-GC, carbonate content 4.7 mmol g−1), containing a complex mixture of various POSS types, well-defined octaglycidyldimethylsilyl POSS carbonate (POSS8GC, carbonate content 3.6 mmol g−1), glycerol (GGC, carbonate content 4.9 mmol g−1), trimethylolpropane (TMPGC, carbonate content 5.6 mmol g−1), and pentaerythritol polyglycidyl ether-based cyclic carbonate (PGC, carbonate content 4.8 mmol g−1) were obtained as viscous liquids. A detailed structural analysis of the polyol-based cyclic carbonates is given by Fleischer et al.18 POSS Cyclic Carbonates Derived from POSS Glycidyl Ether Mixtures (POSS-GC, 1). Carbonate content: 4.7 mmol g−1. 1H NMR (299.87 MHz, acetone-d6): δ/ppm = 4.97 (1.00, 2-CH), 4.59 (1.02, 1CH2), 4.40 (1.02, 1-CH2), 3.49−3.85 (4.18, 3/4-CH2), 1.74 (2.10, 5CH2), 0.75 (2.00, 6-CH2). 13C NMR (75.40 MHz, acetone-d6): δ/ppm = 156.00 (−OCOO−), 76.53 (3-CH2), 74.15 (4-CH2), 70.69 (2-CH), 67.04 (1-CH2), 23.90 (5-CH2), 9.23 (6-CH2). νmax/cm−1 = 2929 (νCH2), 2870 (νCH2), 1786 (νCO), 1483 (δCH2), 1397, 1361, 1164 (νSi−O), 1101 (νSi−O), 1047 (νSi−O), 1082 (νSi−O). Octaglycidyldimethylsilyl POSS Cyclic Carbonate (POSS-8GC, 2). Carbonate content: 3.6 mmol g−1. 1H NMR (299.87 MHz, acetone-d6): δ/ppm = 4.96 (1.00, 2-CH), 4.60 (1.02, 1-CH2), 4.38 (1.02, 1-CH2), 3.73 (2.07, 3-CH2), 3.54 (2.07, 4-CH2), 1.69 (2.00, 5-CH2), 0.70 (1.97, 6-CH2), 0.23 (6.39, 7-CH3). 13C NMR (75.40 MHz, acetone-d6): δ/ ppm = 155.88 (−OCOO−), 76.53 (3-CH2), 75.06 (4-CH2), 70.84 (2CH), 66.98 (1-CH2), 24.00 (5-CH2), 14.47 (6-CH2), 0.39 (7-CH3). νmax/cm−1 = 2962 (νCH3), 2940 (νCH3), 2924 (νCH2), 2875 (νCH2), 1791 (νCO), 1481 (δCH2), 1397, 1364, 1253, 1167 (νSi−O), 1054 (νSi−O), 845 (νSi‑CH3). Glycerol Glycidyl Ether-Based Cyclic Carbonate (GGC, 3). Carbonate content: 4.9 mmol g−1. 1H NMR (299.87 MHz, CDCl3): δ/ppm = 4.83 (2-CH), 4.50 (1-CH2), 4.39 (1′-CH2), 3.44−4.0 (3/4CH2, 5-CH). νmax/cm−1 = 2960 (νCH2), 2916 (νCH2), 2874 (νCH2), 1785 (νCO), 1480 (δCH2), 1396 (δO−H), 1363, 1338, 1308, 1172, 1132, 1103, 1082, 1043 (νC−O). Trimethylolpropane Glycidyl Ether-Based Cyclic Carbonate (TMPGC, 4). Carbonate content: 5.6 mmol g−1. 1H NMR (299.87 MHz, CDCl3): δ/ppm = 4.82 (2-CH), 4.49 (1-CH2), 4.38 (1′-CH2), 3.25−4.0 (3/4-CH2), 1.36 (6-CH2), 0.82 (7-CH3). νmax/cm−1 = 2969 (νCH2), 2920 (νCH2), 2880 (νCH2), 1782 (νCO), 1482 (δCH2), 1386 (δO−H), 1360, 1335, 1305, 1168, 1132, 1103, 1043 (νC−O). Pentaerythritol Glycidyl Ether-Based Cyclic Carbonate (PGC, 5). Carbonate content: 4.8 mmol g−1. 1H NMR (299.87 MHz, CDCl3): δ/ ppm = 4.83 (2-CH), 4.50 (1-CH2), 4.41 (1′-CH2), 3.33−4.0 (3/4-CH2). νmax/cm−1 = 2960 (νCH2), 2918 (νCH2), 2873 (νCH2), 1785 (νCO), 1480 (δCH2), 1396 (δO−H), 1363, 1338, 1309, 1254, 1171, 1132, 1103, 1084, 1051 (νC−O). General Procedure of NIPU Formation and Preparation of NIPU Coatings. All NIPU samples were prepared according to the following procedure: The POSS cyclic carbonate (POSS-GC or POSS8GC) was mixed with the respective amount of GGC, TMPGC, or PGC at 60 °C. Then the amine curing agent (HMDA or IPDA) was added in

stoichiometric amount, taking into account the total carbonate content of the cyclic carbonate blend. The resulting mixture was mechanically stirred without further external heating for 20−60 s and poured into the mold. The NIPU coatings were prepared by casting the reaction mixture onto a glass surface using a doctor’s blade with an adjustable gap to produce coatings of 0.5 mm thickness. All samples were cured at 80 °C for 14 h and an additional 4 h at 100 °C.

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RESULTS AND DISCUSSION Mono- and Polydisperse POSS Cyclic Carbonates. As illustrated in Scheme 1, mono- and polydisperse POSS glycidyl ethers were carbonated with carbon dioxide in order to produce POSS cyclic carbonates. Typically, in the solvent-free carbonation process the POSS glycidyl ethers react with carbon dioxide at 30 bar pressure at 100 °C for the duration of 24 h in the presence of 1 wt % tetrabutylammonium bromide as catalyst. Regardless of their structure, complete conversion of the epoxy groups was achieved in this solvent-free carbonation of POSS glycidyl ethers. Therefore, both POSS cyclic carbonates were obtained as colorless viscous liquids in quantitative yield. The monodisperse POSS octaglycidyl ether (EP0435 from Hybrid Plastics), which exclusively contained T8 POSS, produced welldefined POSS cyclic carbonate (POSS-8GC, 2). In contrast, the POSS glycidyl ether (EP0409) consisted of a mixture of POSS having different cage sizes (T10 > T12 > T8) together with incompletely condensed POSS cages like T9 and T11 bearing silanol groups, as verified by MALDI-TOF analysis. Therefore, rather complex mixtures of polydisperse cyclic carbonates (POSS-GC, 1) were obtained. As it is apparent from Figure 1, the 1H NMR spectra of mono- and polydisperse POSS cyclic carbonates exhibited only the signals at 5.0, 4.6, and 4.4 ppm, typical for terminal cyclic carbonate groups as well as signals at 1.7 and 0.7 or 0.2 ppm that correspond to methylene and methyl groups. The carbonate content of polydisperse POSS-GC and monodisperse POSS-8GC was determined from their 1H and 13 C NMR spectra using phenol as internal reference to be 4.7 and 3.6 mmol g−1, respectively (see Table 1), which is equivalent to a complete functionalization of all silicon sides. For example, this corresponds to a carbon dioxide uptake of 20.8 wt % for POSSGC accompanied by incorporation of 28.5 wt % SiO2. Both POSS cyclic carbonates showed the IR absorption bands at 1790 cm−1 typical for the cyclic carbonate group and at 1160 cm−1 typical for the Si−O−Si stretching vibration of the polysilsesquioxane cage. The viscosities of the POSS-based cyclic carbonates were measured by oscillatory frequency sweep experiments at different temperatures (see Table 1). At room temperature, the viscosity of POSS-GC and POSS-8GC was determined to 2339 and 158 Pa s, respectively. At 80 °C, both POSS carbonates were low viscous carbonate resins with a viscosity of 6.8 (POSS-GC) and 1.5 Pa s (POSS-8GC). As a rule, the polydisperse POSS-GC exhibited a higher viscosity D


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Macromolecules

Scheme 2. Structures of Cyclic Carbonate Blend Components Combining POSS Cyclic Carbonates POSS-GC (1) or POSS-8GC (2) with Cyclic Carbonates Based on Glycerol (GGC, 3), Trimethylolpropane (TMPGC, 4), and Pentaerythritol (PGC, 5)

monodisperse POSS-8GC with IPDA at 80 °C, the gel time was 1270 and 1046 s, respectively, whereas the pot life times were in the range of 530−550 s. However, no gelation was achieved at ambient temperature or by smooth heating for the duration of 90 min at 50 °C, owing to the lower IPDA reactivity. In order to realize a better balance of processing and NIPU properties, POSS cyclic carbonates were used as reactive diluents and blend components in combination with biobased cyclic carbonates. Hence, the highly functional POSS carbonates with low viscosity were blended together with bio-based cyclic carbonates exhibiting lower carbonate functionality and higher viscosity. Both POSS carbonates were fully miscible with the cyclic carbonates based on glycerol (GGC, 3), trimethylolpropane (TMPGC, 4), and pentaerythritol (PGC, 5), which were readily available through carbonation of the corresponding glycidyl ethers.18 All blend components are displayed in Scheme 2. Both mono- and polydisperse POSS cyclic carbonates gave similar performance with respect to the gel time measured at different temperatures for the cure with HMDA as it is apparent in Figure 2. Owing to their higher carbonate functionality, the addition of POSS cyclic carbonates lowered the gel time of the corresponding blends drastically. For instance, if POSS cyclic carbonates were added to TMPGC (see Figure 2, top), the gel time at 80 °C was reduced from 725 to 160 s. Pot life times were in the range of 30−80 s. In order to examine the influence of temperature, the blends of POSS-GC and POSS-8GC with TMPGC were cured at different temperatures (see Figure 2, bottom). The gel times decreased with increasing temperature and were approximately 100 s at 80 °C, 220 s at 50 °C, and 610 s at 20 °C. As a consequence, the cyclic carbonates were blended further at room temperature and reacted at elevated temperature

compared to the monodisperse POSS-8GC. Most likely, this higher viscosity of POSS-GC is attributed to the presence of larger POSS cages as well as hydrogen bonds due to silanol groups. Interestingly, the POSS cyclic carbonate viscosities were substantially lower with respect to most other cyclic carbonates derived from conventional polyol glycidyl ethers.18 According to DSC measurements, POSS cyclic carbonates showed low glass transition with −15 °C for polydisperse POSS-GC and −30 °C for monodisperse POSS-8GC. As evidenced by thermogravimetric analysis (TGA) under air, the POSS cyclic carbonates decomposed at temperatures of 297−313 °C. Rheological Investigations of Pot Life and Gel Time. Gel and pot life times represent two key parameters for tailoring formulations suitable for NIPU preparation. The gel time characterizes the time required for network formation during cure. It corresponds to the crossover of storage (G′) and loss modulus (G″), as determined by rheological measurements. In contrast, the pot life time corresponds to the time during which the viscosity of the reaction mixture remains low enough, typically below 10 Pa s, to allow facile processing by casting and coating. Rheological experiments were performed on a rheometer with plate−plate geometry at 20, 50, and 80 °C with 10 rad s−1 and 5% deformation for the measurements of pot life and gel times. Upon curing polydisperse POSS-GC with hexamethylenediamine (HMDA) at 50 °C, the resulting pot life and gel times of 80 and 90 s, respectively, were very short. Clearly, the high carbonate functionality of the POSS cyclic carbonates accounted for the rapid network formation, which was accompanied by processing problems. Substituting HMDA for the considerably less reactive isophoronediamine (IPDA) substantially prolonged both pot life and gel times, thus facilitating processing. Hence, when curing POSS-GC or E


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Macromolecules

cryomicrotome. SEM images revealed the formation of uniformly dispersed spherical phases with an average diameter of 0.70 ± 0.07 μm (see Figure 3) when using a POSS-GC content

Figure 2. Gel time as a function of blend compositions and temperature for the gelation of POSS cyclic carbonate blends with GGC, TMPGC, or PGC (top) and of POSS cyclic carbonate/TMPGC blends (bottom) at different temperatures with HMDA as amine curing agent. Gel times were determined by rheological measurements (10 rad s−1, 5% deformation).

Figure 3. SEM images of NIPU hybrid materials prepared by HMDA cure of GGC containing 20 mol %/13.5 wt % (top) and 40 mol %/23.0 wt % POSS-GC (bottom). SEM samples were obtained from NIPU specimens for tensile testing using the cryomicrotome technique.

in order to affect rapid cure without encountering processing problems. Morphology of POSS/NIPU Hybrid Materials. In order to examine the influence of blend compositions on mechanical and morphological properties of organic/inorganic POSS/NIPU hybrid materials and the corresponding coatings, POSS-GC and POSS-8GC were blended together with GGC, TMPGC, or PGC and cured with HMDA at 80 °C for 14 h and postcured at 100 °C for 4 h. According to IR spectroscopic analyses, all NIPU/POSS hybrid systems gave full conversion of the cyclic carbonates with diamines to produce polyhydroxyurethane networks. In all cases, the IR absorption band corresponding to the cyclic carbonate group at 1790 cm−1 completely disappeared, accompanied by the appearance of the urethane band at 1695 cm−1. Side reactions as described by Besse et al. which would lead to the formation of urea bands at 1620 cm−1 or 2-oxazolidinone bands at 1745 cm−1 were not observed.39 Scanning electron microscopy (SEM) was used to examine the NIPU morphologies of GGC/POSS-GC/HMDA hybrid materials. In the absence of POSS cyclic carbonates and at low POSS cyclic carbonate content, no indication of phase separation was detected by SEM images of planar surfaces prepared with a

of 20 mol % (13.5 wt % POSS-GC, 3.8 wt % SiO2). Increasing the amount of POSS carbonate to 40 mol % (23.0 wt % POSS-GC, 6.6 wt % SiO2) afforded a significantly higher volume fraction of such spherical phases without affecting their average diameter of 0.68 ± 0.05 μm. In contrast to these findings, Liu et al. claimed that the use of epoxy-functionalized POSS in epoxy/NIPU hybrid thermosets yielded homogeneous phase systems.37 Although further research is required to elucidate the mechanisms responsible for this phase separation, it is highly likely that the very high functionality of POSS cyclic carbonates accounts for rapid cure and phase separation of POSS-GC/ HMDA within the GGC/POSS/HMDA matrix. As a consequence, the POSS molecules with a diameter in nanometer scale assemble to form much larger phases having diameters of several hundred nanometers. Furthermore, this SEM analysis showed that all uniformly dispersed submicron phases exhibited excellent interfacial adhesion, as expected for the covalent bond formation between POSS and the NIPU matrix. Mechanical and Thermal Properties of NIPU/POSS Hybrid Materials. The mechanical and thermal properties of F


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Macromolecules Table 2. Mechanical and Thermal Properties of NIPU and NIPU/POSS Hybrids cyclic carbonate

POSS cyclic carbonate

x(POSS)a (mol %)

w(POSS)a (wt %)

w(SiO2)b (wt %)

GGC GGC GGC GGC TMPGC TMPGC TMPGC TMPGC TMPGC TMPGC TMPGC TMPGC PGC PGC − − − −

− POSS-GC POSS-GC POSS-GC − POSS-GC POSS-8GC POSS-8GC POSS-8GC − POSS-GC POSS-GC − POSS-GC POSS-GC POSS-8GC POSS-GC POSS-8GC

0 20 40 60 0 20 20 40 60 0 40 60 0 20 100 100 100 100

0 13.5 23.0 30.1 0 14.6 18.2 29.8 37.9 0 22.2 28.8 0 13.2 78.5 82.7 71.4 76.5

0 3.8 6.6 8.6 0 4.2 7.7 12.6 16.0 0 6.3 8.2 0 3.8 24.0 26.1 23.2 25.6

amine

Young’s modulus (MPa)

tensile strength (MPa)

εbreak (%)

Tgc (°C)

Tdd (°C)

Qwatere (wt %)

Qtoluenee (wt %)

HMDA HMDA HMDA HMDA HMDA HMDA HMDA HMDA HMDA IPDA IPDA IPDA HMDA HMDA HMDA HMDA IPDA IPDA

31 ± 12 639 ± 8 1400 ± 300 25 ± 2 1600 ± 200 2800 ± 60 2240 ± 140 1610 ± 90 760 ± 90 3820 ± 50 4000 ± 160 3500 ± 30 2340 ± 130 3270 ± 130 −g −g 3150h 2650 ± 70

12.5 ± 0.8 26.6 ± 1.3 46.3 ± 1.3 8.4 ± 1.7 44 ± 11 72 ± 4 62 ± 4 49 ± 4 31 ± 2 14.9 ± 0.9 40 ± 30 15.2 ± 0.3 69 ± 4 87 ± 10 −g −g 12.6h 56 ± 12

99.8 ± 1.8 32 ± 9 3.4 ± 0.6 26 ± 3 2.86 ± 0.05 3.2 ± 0.2 2.9 ± 0.2 3.2 ± 0.2 5±2 0.41 ± 0.02 1.1 ± 0.9 0.41 ± 0.01 3.1 ± 0.7 3.2 ± 0.5 −g −g 0.4h 2.2 ± 0.6

32 41 50 34 50 51 47 53 50 95 91 91 56 61 49 41 106 88

n.d.f n.d.f n.d.f n.d.f 261 280 272 267 259 n.d.f n.d.f n.d.f n.d.f n.d.f 297 294 n.d.f 303

27 25 28 25 20 25 26 17 15 37 18 22 28 28 n.d.f 6 10 6

0.4 0.1 0.1 0 0 0.2 0.3 0.8 1.5 0 0.1 0.1 0 0.1 n.d.f 2.4 0.2 1.0

a Molar ratio with respect to carbonate blend, weight ratio with respect to total NIPU mass. bIncorporated SiO2 equivalent with respect to total NIPU mass. cDMA, 1 Hz, 0.1%, HMDA: −50 to 100 °C, IPDA: −50 to 150 °C, tan δ maximum. dTGA, 10 K min−1, 50−650 °C, air. eDegree of swelling after immersion of cubical samples of NIPU specimens for tensile testing for 14 days. fNot determined. gOnly DMA specimens were prepared due to short gelation times. hBrittleness of specimens.

Figure 4. Scratching device (left) and transparent, colorless NIPU/POSS hybrid coating consisting of POSS-GC/IPDA (22 °C, coating thickness: 0.5 mm).

Interestingly, elongation at break values was not affected and remained around 3%. The substitution of HMDA with IPDA did not only permit higher POSS concentrations but also afforded a much higher Young’s modulus of 4000 MPa in the case of TMPGC/POSS-GC/IPDA (40 mol %) and even increase the Tg from 95 to 106 °C if POSS-GC is directly cured with IPDA. It is noteworthy that this Tg value is among the highest for NIPU thermosets reported in the literature. The comparison of polydisperse POSS-GC with monodisperse POSS-8GC revealed that the higher polydispersity appeared to be beneficial for achieving improved mechanical properties. Most likely, this is attributed to its higher carbonate functionality, which affords higher cross-linking densities. According to the thermogravimetric analyses, the incorporation of POSS carbonates accounted for a marginally improved thermal stability, as evidenced by the slightly higher decomposition temperatures of 272−303 °C as compared to 261 °C for TMPGC/HMDA. Solvent Swelling of NIPU/POSS Hybrids. The solvent swelling of NIPU/POSS hybrids was examined by measuring the weight increase of cubical NIPU/POSS specimens immersed in

NIPU and NIPU/POSS hybrids are listed in Table 2 as a function of the blend compositions. The mechanical properties of GGC/POSS-GC blends cured with HMDA improved with increasing POSS-GC content. For example, the Young’s modulus increased from 31 (0 mol % POSS-GC) to 639 (20 mol %) and 1400 MPa (40 mol %) accompanied by an increase of the tensile strength from 12.5 to 46.3 MPa and a decrease of the elongation at break from 99.8 to 3.4%. On exceeding a POSS-GC content of 40 mol %, all following GGC-based NIPU/POSS samples were obtained as materials with aggravated mechanical properties due to short geltimesleading to gelation before a homogeneously mixture could be obtainedsimilar to the HMDA-cured POSS carbonates. These trends were also observed for TMPGC/ POSS-8GC blends. Therefore, the maximum amount of POSS carbonates was 20 mol % within the blends of TMPGC or PGC and 40 mol % with GGC. The Young’s moduli of TMPGC/ POSS-GC and PGC/POSS-GC blends cured with HMDA increased from 1600 to 2800 MPa and from 2340 to 3270 MPa, respectively, whereas the tensile strength was improved from 44 to 72 MPa (TMPGC) and from 69 to 87 MPa (PGC). G


Article

Macromolecules

Table 3. Scratch Resistance of Hybrid NIPU/POSS Coatings by Characterization of the Surface Gloss before and after Treatment with a Silicon Carbide/Aluminum Oxide Nonwoven gloss cyclic carbonate

x(POSS-GC) (mol %)

w(POSS-GC) (wt %)

w(SiO2) (wt %)

amine

before

after

%

TMPGC TMPGC TMPGC TMPGC

0 40 0 40 100

0 24.6 0 22.2 71.4

0 7.0 0 6.3 23.2

HMDA HMDA IPDA IPDA IPDA

114 ± 5 62 ± 2 107 ± 8 133 ± 6 100 ± 3

24 ± 2 53 ± 3 37 ± 4 75 ± 5 74 ± 7

21 85 35 56 74

water and toluene for the duration of 14 days at room temperature (see Table 2). In the case of conventional NIPUs, the presence of hydroxyl groups, formed during cure, accounts for considerable water uptake. Higher cross-link densities in NIPU thermosets are accompanied by higher hydroxyl group contents, thus enhancing water uptake. However, the incorporation of the high functional hydrophobic POSS-GC did not adversely affect water swelling. Most NIPU/POSS hybrids have similar water uptake ranging between 22 and 28 wt % when using HMDA as curing agent (see Table 2). Curing POSS-GC and POSS-8GC with the less polar IPDA gave a substantially lower water uptake of 6−10 wt %. Since all NIPU and NIPU/POSS hybrids comprise rather polar networks, it is not surprising that the toluene swelling (0−2.4 wt %) was very low. Scratch Resistance of NIPU/POSS Hybrid Coatings. In order to examine the influence of the incorporation of POSS cyclic carbonates on the scratch resistance, NIPU/POSS hybrid coatings were prepared by curing POSS-GC and TMPGC/ POSS-GC blends (40 mol %) with HMDA and IPDA. TMPGC, cured with either HMDA or IPDA, was chosen as the reference system. It is worth mentioning that all NIPU/POSS hybrid coatings were fully optically transparent, but only in the case of curing POSS-GC directly with IPDA also colorless (see Figure 4, right), whereas all polyol glycidyl ether-based NIPUs exhibited a yellowish color. In the herein conducted scratching tests, a highly abrasive nonwoven was attached to a metal block with a total mass of 500 g, which was then moved 200 times back and forth on the surface simulating a massive stress (see Figure 4, left) on the coating. The surface gloss of these transparent NIPU/POSS hybrid coatings, which is highly dependent on the surface condition, was examined before and after scratching. After scratching the coating surface by applying 200 double strokes, the surface gloss of both TMPGC reference systems drastically decreased to 21% for HMDA cure and 35% for IPDA cure (see Table 3). In sharp contrast, the incorporation of 40 mol % POSS-GC afforded a retention of the surface gloss after scratching of 85% if cured with HMDA (24.6 wt % POSS-GC, 7.0 wt % SiO2) and of 56% if IPDA (22.2 wt % POSS-GC, 6.3 wt % SiO2) was used as curing agent. Although only 56% gloss could be retained, this sample showed the highest absolute gloss values of all prepared NIPU/POSS hybrid coatings after scratching. Increasing the POSS and the corresponding silica content up to 71.4 and 23.2 wt %, respectively, by curing POSS-GC with IPDA, yielded the most promising system combining the coating’s colorlessness with high absolute and relative gloss values after the treatment with 200 double strokes. Additionally, the scratch resistance was optically proved and illustrated by SEM images, showing an obviously less damaged coating surface of IPDA cured POSS-GC compared to a TMPGC/HMDA coating (see Figure 5). These results clearly

indicate that the incorporation of POSS cyclic carbonates afforded significant improvements of the scratch resistance.

Figure 5. SEM images of scratched NIPU/POSS hybrid coatings after emulating massive stress by treatment with 200 double strokes of an abrasive nonwoven: TMPGC/HMDA (top) and POSS-GC/IPDA (bottom).

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CONCLUSION In this study we have succeeded in developing a highly versatile solvent-free synthesis of multifunctional POSS cyclic carbonates as intermediates for a new family of organic/inorganic, 100% non-isocyanate polyhydroxyurethane (NIPU) hybrid materials and coatings. Typically, carbonated POSS glycidyl ethers are colorless liquids obtained in quantitative yields by chemical fixation of carbon dioxide. While POSS-8GC is monodisperse and contains exclusively one octasilsesquioxane core and eight carbonate groups in its periphery, polydisperse POSS-GC comprises a blend of various oligosilsesquioxanes having different cage sizes as well as open POSS cages containing also silanol groups which altogether result in a higher carbonate H


Macromolecules

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functionality and viscosity. Among the state-of-the-art cyclic carbonate monomers, POSS carbonates are exceptional regarding the fixation of 15.4−20.8 wt % carbon dioxide as well as the incorporation of 28.5−42.1 wt % silica. Since they are fully miscible with bio-based carbonates derived from the glycidyl ethers of glycerol (GGC), trimethylolpropane (TMPGC), and pentaerythritol (PGC), they represent attractive reactive diluents and formulation components for NIPU/POSS hybrid thermosets with unique property profiles. Among NIPU/POSS hybrids, the blends of POSS-GC with GGC, TMPGC, or PGC cured with HMDA, afford substantially improved Young’s modulus and tensile strength without sacrificing the elongation at break. For example, blending GGC with up to 40 mol % POSS-GC increased the Young’s modulus from 31 to 1400 MPa (+4400%!). Substituting HMDA with the less reactive and more rigid IPDA enabled the incorporation of higher POSS concentrations. The resulting higher cross-link density together with the increased rigidity of the polymer backbone yielded NIPU/POSS hybrids with the highest stiffness and Tg values up to 106 °C. From the SEM analysis of the prepared NIPU/POSS hybrids, it is apparent that phase separation produces submicron phases with an average diameter around 700 nm, which is not affected by the POSS content within the margin of error. More research is required to explore the phase separation process and to achieve better insight and control of the nanostructure formation. One further aspect of our work was the preparation of NIPU/POSS hybrid coatings. The scratch resistance, as measured by comparing the surface gloss before and after emulating a massive stress by scratching the coating with an abrasive nonwoven, significantly improves when POSS-GC is incorporated into the NIPU coating. In addition, the incorporation of POSS in NIPU thermosets and coatings reduces water uptake without impairing the high resistance to organic solvents such as toluene and marginally improves the thermal stability. The development of NIPU hybrid materials and NIPU blends holds great prospects for tailoring advanced NIPU thermosets and coatings.

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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.5b02560. 1

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Article

H NMR, 13C NMR, FTIR, DSC, rheological experiments, TGA, solvent swelling, DMA, MALDI-TOF (PDF)

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by the Baden-Württemberg Stiftung for funding our work within the grünPUR project (BioMatS-020). We thank as well our coworkers Ralf Hanselmann, Ralf Thomann, Victor Hugo Pacheco Torres, Alfred Hasenhindl, Sascha Fischer, Klaus Hasis, Ion Lazar, Laura Burk, Vitalij Schimpf, and Marina Hagios for their enthusiastic support. I


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Multifunctional POSS Cyclic Carbonates and Non-Isocyanate

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