Poly(ethylene imine) hybrids containing polyhedral oligomeric silsesquioxanes: Preparation, structure and properties

European Polymer Journal 44 (2008) 3946–3956

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Macromolecular Nanotechnology

Poly(ethylene imine) hybrids containing polyhedral oligomeric silsesquioxanes: Preparation, structure and properties Ke Zeng, Yonghong Liu, Sixun Zheng * Department of Polymer Science and Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China

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a r t i c l e

i n f o

Article history: Received 28 May 2008 Received in revised form 26 July 2008 Accepted 28 July 2008 Available online 6 August 2008

Keywords: Polyhedral oligomeric silsesquioxane Poly(ethylene imine) Organic–Inorganic hybrids Amphiphilicity Hydrogels Properties

a b s t r a c t Both octaglycidyletherpropyl polyhedral oligomeric silsesquioxane and hepta(3,3,3-trifluoropropyl)glycidyletherpropyl polyhedral oligomeric silsesquioxane were synthesized via the hydrosilylation reactions between octahydrosilsesquioxane [and/or hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane] and allyl glycidyl ether. The polyhedral oligomeric silsesquioxane (POSS) macromers were characterized by means of Fourier transform infrared and nuclear magnetic resonance spectroscopy. The inter-component macromolecular reactions between the POSS macromers and poly(ethylene imine) (PEI) were employed to prepare the POSS-containing organic–inorganic PEI hybrids. The inclusion of octaglycidyletherpropyl POSS into PEI results in the formation of the organic–inorganic hybrid networks whereas the introducing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS to PEI affords the linear POSS-grafted PEI copolymers. Differential scanning calorimetry and thermogravimetric analysis show that the POSS-containing PEI hybrids displayed increased glass transition temperatures (Tg’s) and enhanced thermal stability compared to the plain PEI. These PEI hybrid composites can be significantly swollen with water without dissolving, suggesting the formation of hydrogels. The PEI hydrogels containing octaglycidyletherpropyl POSS is in reality the chemically-crosslinked hydrogels whereas the those containing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS displayed the behavior of physical hydrogels. The formation of physical hydrogels is ascribed to the microphaseseparated morphology in the hybrids. In addition, the hybrids containing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS exhibited the typical amphiphilicity as evidenced by the increase in surface hydrophobilicity. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Poly(ethylene imine) (PEI) is an important water-soluble polymer and it is generally synthesized via cationic ring-opening polymerization of ethylene imine [1,2]. PEI has been widely used in paper industry [3,4]. In addition, PEI is a desirable candidate for polyelectrolyte [5]. In the fields of bio-medical application, PEI is a good candidate for gene delivery vector in vitro and in vivo transfection [6–8]. The crosslinked poly(ethylene imine) hydrogels can * Corresponding author. Tel.: +86 21 54743278; fax: 86 21 54741297. E-mail address: [email protected] (S. Zheng). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.07.049

display a response to environmental pH values. The widespread application of PEI motivates the structural modification of this polymer to obtain additional properties. For instance, PEI has been structurally modified to achieve amphiphilicity and double-hydrophilicity. The complexation of PEI with n-alkanoic acids (or long-chain alkyl halides) via comb-like structures has been employed to prepare ultrathin polymer films with well-defined molecular architecture [9–11]. The double-hydrophilicity of this polymer can be archived via the formation of poly(ethylene oxide)-block-poly(ethylene imine) block copolymers (PEOb-PEI), which can be used in formulations for delivery of oligonucleotides and as drug carriers for retinoic acid [6].

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Polyhedral oligomeric silsesquioxane (POSS) technology is becoming a main avenue to organic–inorganic nanocomposites due to the simplicity in processing and the excellent comprehensive properties of this class of hybrid materials [12–31]. A typical POSS molecule possesses the structure of cube-octameric frameworks represented by the formula (R8Si8O12) with an inorganic silica-like core (Si8O12) surrounded by eight organic corner groups, one or more of which is reactive or polymerizable. As a class of new nanosized building blocks, POSS molecules have been successfully incorporated into a variety of polymer systems via reactive blending, copolymerization and macromolecular reaction. Most of the previous studies have focused on the preparation of POSS-containing polymer nanocomposites, i.e., the thermomechanical properties of nanocomposites in bulk were taken in account. Recently, it is identified that the presence of POSS could also optimize the functional properties of polymer materials via the formation of specific morphological structures. For instance, Chen et al. [31], Huang et al. [32] and Chujo et al. [33] reported the modification of polyfluorenes via the formation of star-like polymers with POSS cores. Schiraldi et al. [34] ever used octamethacryloxylpropyl POSS as one of crosslinking agents of PNIPAM to promote the interactions between the crosslinking agent and silicate filler in the clay-containing poly(N-isopropylacrylamide) hydrogels. Zheng et al. [35] found that the temperature response of the POSS-crosslinked PNPAM hydrogels can be improved while an octafunctional POSS macromer was used as the crosslinking agent of PNIPAM hydrogels. However, such investigations remains largely unexplored vis-à-vis the studies on the improvement of thermomechanical properties of polymers through inclusion of POSS cages. To the best of our knowledge, there are few reports on POSS-modified PEI. In the present work, we report the investigations on the organic–inorganic PEI hybrids containing POSS. The POSS macromers used are octaglycidyletherpropyl POSS and hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS (Scheme 1). The inter-component macromolecular

A

reactions between PEI and the POSS macromers were employed to prepare the POSS-containing PEI hybrids. It is expected that the reaction of the octafunctional POSS with PEI will form the chemically-crosslinked networks whereas the reaction of the monofunctional POSS allow the preparation of the linear POSS-containing PEI hybrids. Owing to water-solubility of PEI and hydrophobicity of POSS component, it is of interest to investigate the behavior of hydrogels and amphiphilicity of the POSS-containing PEI hybrids. In this contribution, we will firstly report the synthesis and characterization of the POSS-containing PEI hybrids. Thereafter, the thermal properties, behaviors of hydrogels and amphiphilicity of the PEI hybrid were addressed. 2. Experimental

3,3,3-Trifluoropropyltrimethoxysilane [CF3CH2CH2Si(OMe)3] was purchased from Zhejiang ChemTechnology Co., China. Trichlorosilane (HSiCl3) was obtained from Shanghai Lingguang Chemical Co., China and used as received. Allyl glycidyl ether (AGE) was purchased from the Shanghai Regent Co., Ltd, China. Poly(ethylene imine) was purchased from Aldrich Co., USA and it has a quoted molecular weight of Mn = 10,000. Other reagents such as sodium, calcium hydride (CaH2), K2CO3, CaCl2 and NaOH were of chemically pure grade, obtained from Shanghai Reagent Co., China. The solvents such as tetrahydrofuran (THF), dichloromethane, methanol, petroleum ether (distillation range: 6090 °C) and triethylamine (TEA) were obtained from commercial resources. Before use, THF was refluxed over sodium and then distilled and stored in the presence of the molecular sieve of 4 Å. TEA was refluxed over CaH2 and then was purified with p-toluenesulfonyl chloride, followed by distillation. The platinum-containing Karstedt catalyst was prepared with chloroplatinic acid hexahydrate [H2PtCl66H2O] and 1,3-divinyltetramethylsiloxane [36].

B

O

O

O

O

O

O

O

F3C O

Si O

O

O O

O

Si

O

Si

O

O F3C O

Si

Si F3C O Si

O

O

O

F3C O

O

Si

O

O O Si O Si Si O Si O O O O

O

Si

CF3

O O O Si Si O Si O O O O

CF3

Si CF3

O

Scheme 1. Chemical structures of the POSS macromers: (A) Octaglycidylether POSS; (B) hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS.

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2.1. Materials

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2.2. Synthesis of POSS macromers The POSS macromers used in this work are octaglycidyletherpropyl polyhedral oligomeric silsesquioxane and hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS, respectively. The preparation of octaglycidyletherpropyl POSS has been previously reported [37] and herewith the synthetic procedure of the octafunctional POSS macromer is just described in brief. In the first step, the octahydrosilsesquioxane (H8Si8O12) was prepared via the hydrolysis and rearrangement of trichlorosilane. The hydrosilylation between octahydrosilsesquioxane and ally glycidyl ether were employed to obtain octaglycidyletherpropyl POSS. In order to synthesize hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS, hepta(3,3,3-trifluoropropyl) tricycloheptasiloxane trisodium silanolate [Na3O12Si7(C3 H4F3)7] was firstly prepared by following the method reported by Fukuda et al. [27]. Typically, (3,3,3-trifluoropropyl) trimethoxysilane (50 g, 0.23 mol), THF (250 ml), deionized water (5.25 g, 0.29 mol) and NaOH (3.95 g, 0.1 mol) were charged to a flask equipped with a condenser and a magnetic stirrer. The mixture was refluxed for 5 h and maintained at room temperature for additional 15 h. After the solvent and other volatile were removed by rotary evaporation, the white solids [i.e., Na3O12Si7(C3H4F3)7] (37.3 g) were afforded and dried at 40 °C in vacuo for 12 h with the yield of 98%. Hepta(3,3,3-trifluoropropyl) hydrosilsesquioxane was synthesis via the reaction between Na3O12Si7(C3H4F3)7 and trichlorosilane in the anhydrous THF. Typically, Na3O12 Si7(C3H4F3)7 (10.0 g, 8.8 mmol) and triethylamine (1.3 ml, 8.8 mmol) were charged to a 500 ml single-necked flask equipped with a magnetic stirrer, 200 ml anhydrous THF were added to the flask with vigorous stirring. The flask was immersed into an ice-water bath and trichlorosilane (1.43 g, 10.56 mmol) (dissolved in 20 ml anhydrous THF) were slowly dropped within 30 min. The mixture was maintained at 0 °C for 4 h and then at room temperature for 20 h. After that, the system was filtrated to remove the solids. The solvent and the volatile were removed via rotary evaporation at 40 °C to obtain white powder, which was further dispersed in 100 mL methanol. The insoluble parts were collected by filtration and then dried at 40 °C in a vacuum oven for 24 h and the product (7.3 g) was obtained with the yield of 76%. 29Si NMR (ppm): 68.6 ppm (O–Si–O), 58.3 ppm (O–Si–H). FTIR (cm1, KBr window): 1090 to 1000 (Si–O–Si), 2900 to 2850 (–CH2), 1120–1300 (–CF3), 2262 (Si–H). 1H NMR (ppm, acetone-d6): 4.30 (s, 1.0H, Si–H), 2.32 (m, 14.0H, SiCH2CH2CF3), 1.03 (m, 14.0H, SiCH2CH2CF3). Hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS was synthesized via the hydrosilylation between hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane and allyl glycidyl ether catalyzed by the Karstedt catalyst. The typical process was shown as follows. Both (CF3CH2 CH2)7Si8O12H (1.2 g, 1.095 mmol) and allyl glycidyl ether (0.188 g, 1.646 mmol) were charged into a round-bottom flask equipped with a magnetic stirrer. The flask was connected to a standard Schlenk line. The system was dried by exhausting-refilling process using highly pure nitrogen for three times. After that 10 ml anhydrous

THF and Karstedt catalyst were added using syringes. The reaction system was heated up to 60 °C, at which the reaction was performed for 48 h to insure the completion of the hydrosilylation reaction. After that, the excessive allyl glycidyl ether and the solvent were removed via rotary evaporation to afford the solid products, which were further washed using 50 ml methanol for three times. After dried in vacuo at 60 °C, 1.19 g product was obtained at the yield of 90%.1H NMR (ppm, CDCl3): 2.11 (m, 14.0H, SiCH2CH2CF3), 0.89 (m, 14.0H, SiCH2CH2CF3), 0.61 (t, 2.0H, SiCH2CH2CH2–), 1.61 (t, 2.0H, SiCH2CH2CH2–), 3.44 (t, 2.0H, SiCH2CH2CH2–), 3.73 and 3.72 (t, 2.0H, SiCH2CH2CH2–), 3.11 (1.0H, OCH2CH epoxide), 2.80 and 2.58 (2.0H, OCH2CH epoxide). 2.3. Preparation of POSS-containing PEI hybrids For the preparation of octaglycidyletherpropyl POSScontaining PEI hybrids, PEI was mixed with desired amount of octaglycidyletherpropyl POSS at 60 °C with vigorous stirring until transparent and homogenous mixture was obtained. The viscous mixture was poured into an aluminum foil, which was then transferred into a vacuum oven at 60 °C. The reaction was carried out in vacuo at 110 °C for 4 h to attain the complete reaction. The possible unreacted POSS macromer was eliminated via extraction using tetrahydrofuran for 4 h. The preparation of hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS-containing PEI hybrids is slightly different from that of octaglycidyletherpropyl POSS-containing PEI hybrids. In a typical experiment, PEI (1.80 g) and hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS (0.20 g) were charged to a flask equipped with a magnetic stirrer; 0.5 ml tetrahydrofuran was added to facilitate the mixing. The flask was emerged into an oil bath at 60 °C. Under the nitrogen atmosphere, the reaction was carried out at 60 °C for 5 h. The solvent was removed via rotary evaporation. After that, 10 ml methanol was added into the flask to dissolve the polymer and then filtration was used to remove any unreacted POSS monomer. The filtrate was collected and the solvent was removed to obtain 1.92 g transparent viscous liquid with the yield of 96%. The product was further dried in vacuo at 40 °C for 24 h before use. 2.4. Techniques and measurement 2.4.1. Nuclear magnetic resonance spectroscopy (NMR) The 1H and 29Si NMR measurements were carried out on a Varian Mercury Plus 400 MHz NMR spectrometer. For 1H NMR measurement, the samples were dissolved with deuterated acetone (acetone-d6) and the solutions were measured with tetramethylsilane (TMS) as the internal reference. The high-resolution solid-state 29Si NMR spectra were obtained using cross polarization (CP)/magic angle spinning (MAS) together with the high-power dipolar decoupling (DD) technique. The 90°pulse width of 4.1 ls was employed for free induction decay (FID) signal accumulation, and the cross polarization (CP) Hartmann-Hahn contact time was set at 3.5 ms for all the measurements. The rate of MAS was

K. Zeng et al. / European Polymer Journal 44 (2008) 3946–3956

2.4.2. Fourier transform infrared spectroscopy (FTIR) The FTIR measurements were conducted on a PerkinElmer Paragon 1000 Fourier transform spectrometer at room temperature (25 °C). The sample films were prepared by dissolving the polymers with methanol (5 wt.%) and the solutions were cast onto KBr windows. The residual solvent was removed in a vacuum oven at 60 °C for 2 h. All the specimens were sufficiently thin to be within a range where the Beer-Lambert law is obeyed. In all cases 64 scans at a resolution of 2 cm1 were used to record the spectra. 2.4.3. Measurement of swelling ratio The kinetics of swelling of hydrogels was gravimetrically measured at 25 °C. After wiping off water on the surface, the weight changes of gels were recorded at regular time intervals. Swelling ratio is defined as follows:

Swelling ratio ¼ ðW t  W d Þ=W d

ð1Þ

where Wt is the weight of the gels at regular time intervals and Wd is the dry weight of the gel. 2.4.4. Contact angle analysis The samples for surface contact angle measurement were prepared via spin-coating the solution of the POSS-containing PEI hybrids on cleaned silicon wafers. Typically, the POSS-containing PEI was dissolved in methanol to form 20 wt.% solutions; the solution was spin-coated at the speed of 2500 r/min on cleaned silicon wafers to form a flat and thin layer. The wafers were further dried in vacuo at 30 °C for 24 h. The flat free surfaces of the POSS-containing PEI were used for the measurement of contact angle. The static contact angle measurements with ultra-pure water and diiodomethane were carried out on a KH-01-2 contact angle measurement instrument (Beijing Kangsente Scietific Instruments Co., China) at room temperature. 2.4.5. Thermal analyses The DSC measurement was performed on a PerkinElmer Pyris-1 thermal analysis apparatus in a dry nitrogen atmosphere. The instrument was calibrated with standard indium. All samples (about 10 mg in weight) were heated from 70 to 100 °C and the thermograms were recorded using a heating rate of 20 °C/min. The glass transition temperatures (Tg’s) were taken as the midpoints of the capacity change. A Perkin-Elmer TGA7 thermal gravimetric analyzer was used to investigate the thermal stability of the nanocomposites. All the thermal analysis was conducted in nitrogen atmosphere from ambient temperature to 800 °C at the heating rate of 20 °C/min. The thermal degradation temperature was taken as the onset temperature at which 5 wt.% of weight loss occurs.

3. Results and discussion 3.1. Synthesis of POSS macromers The route of synthesis for hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS is shown in Scheme 2. The starting compound is 3,3,3-trifluoropropyltrimethoxysilane, which was hydrolyzed in the presence of NaOH to afford hepta(3,3,3-trifluoropropyl)tricycloheptasiloxane trisodium silanolate [Na3O12Si7(C3H4F3)7] by following the literature method reported by Fukuda et al. [27]. The trisodium silanolate was used for the so-called ‘‘corner-capping” reaction with trichlorosilane (HSiCl3) to obtain hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane. In the Fourier transform infrared spectrum (FTIR) of the product, two intense bands appeared at 2262 and 1119 cm1. These bands are charactersitic of Si–H and Si–O– bonds, indicating that the presence of Si–H and Si–O–Si moeities. The 29Si NMR spectroscopy shows that the cubic cage-like silsesquioxane structure was formed with this corner capping reaction [27]. In term of the ratio of integration intensity of Si–H proton to those of 3,3,3-trifluropropyl groups in 1H NMR spectrum, it is judged that hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane was successfully prepared [27]. The hydrosilylation between hepta(3,3,3-trifluoropropyl) hydrosilse squioxane and allyl glycidyl ether was employed to prepare hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS. Shown in Fig. 1 is the 1H NMR spectrum of hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS. In terms of the ratio of integration intensity of 3,3,3-trifluoropropyl to glycidyletherpropyl protons, it is judged that the monofunctional POSS macromer was successfully obtained. 3.2. Preparation of organic–inorganic hybrids of PEI containing POSS The organic–inorganic hybrids of poly(ethylene imine) containing POSS were prepared via the reactions between PEI and POSS macromers, which is depicted in Scheme 3. The preparation was involved with the reaction of epoxide groups of POSS macromers with amino groups of PEI and the organic–inorganic hybrids were obtained with the content of POSS up to 20 wt.%. It should be pointed out that for the compositions investigated for the preparation of the POSS-containing hybrids the amino groups of PEI were largely excessive with respect of the epoxide groups of the POSS macromers (See Table 1). Therefore, it is plausible to propose that the POSS macromers were fully reacted and bonded with PEI. The complete reaction between PEI and the POSS macromer was judged in terms of the disappearance of the stretching bands of epoxide group at 906 cm1 in the FTIR spectra as shown in Fig. 2. It is noted that the infrared bands virtually disappeared under the present reaction condition; in the mean time the intensity of the bands at 3438 cm1 appeared and its intensity increased with increasing the content of POSS. The new bands at 3438 cm1 are ascribed to the stretching vibration of hydroxyl groups resulting from the reaction of epoxide groups of the POSS macromers with amino groups of PEI. For the PEI hybrids containing octaglycidyletherpropyl

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4.0 KHz for measuring the spectra. The Hartmann-Hahn CP matching and dipolar decoupling field was 57 KHz. The chemical shifts of 29Si CP/MAS NMR spectra were determined by taking the silicon of solid Q8M8 relative to TMS as an external reference standard.

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R Si

CF3CH2CH2CH2 S i

O

O

O O O O Si R Si Si O R R O O O O

OCH 3

R Si

NaOH/H 2 O THF

OCH 3 OCH 3

O

Si

Na+3

Si

R

R THF HSiCl3

R

R Si

O

Si

O

O

O O RO Si O Si Si O Si R R O O O O

R

Si

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AGE THF,Karstedt R

R

Si

O O O O Si R Si O Si R R O O O O

Si

Si

R

O

O

R

Si

O

H Si

O

O

Si R

R = -CH2 C H2C F3 Scheme 2. Synthesis of hepta(3,3,3-trifluropropyl) glycidyletherpropyl POSS.

TMS CDCl3

c

R Si R

O

Si

O

O O O Si O Si R R Si O Si R O O O O Si

O

R

e d

f O

h g

O

b

2.37–3.0 ppm are assignable to the protons of methylene in PEI. In terms of the ratio of integration intensity of the POSS protons to those of PEI protons, it is judged that the content of POSS was identical with the feed ratio, indicating that the reaction between PEI and the POSS macromer occurred to completion.

a

Si R

3.3. Properties of organic–inorganic PEI hybrids R = -CH2-CH2-CF3

a

b

d

f eg hh

8

7

6

5

4

3

2

c

1

0

Chemical shift (ppm) Fig. 1. 1H NMR spectrum of hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS.

POSS, the octafunctional POSS macromer acts as the crosslinking agent and thus the chemically-crosslinked products of PEI were obtained. In contrast to the crosslinked PEI hybrids containing octaglycidyletherpropyl POSS, the reaction between PEI and hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS affords the POSS-grafted PEI copolymers. In this work, the PEI hybrids containing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS were prepared with the content of POSS up to 20 wt.%. Representatively shown in Fig. 3 is the 1H NMR spectrum of the organic–inorganic PEI hybrid containing 20 wt.% POSS together with the assignment of the spectrum. The resonance signals which appeared in 2.21 and 0.94 ppm are assignable to the protons of 3,3,3-trifluoropropyl groups of the POSS cages; the resonance signals in the range of

3.3.1. Thermal properties The POSS-containing PEI Hybrids were subjected to differential scanning calorimetry (DSC). The DSC curves are presented in Figs. 4 and 5. The control PEI displayed a glass transition at 51 °C. Compared to the plain PEI, the POSScontaining hybrids displayed enhanced Tg’s and the Tg’s of the hybrids increased with increasing the content of POSS. It is noted that with the identical content of POSS, the hybrids containing octaglycidyletherpropyl POSS gave the higher Tg’s than those containing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS. For the POSS-containing hybrids containing the octafunctional POSS, the increased Tg’s could be attributed to the formation of the organic– inorganic hybrid PEI networks. In the hybrid networks, the POSS moiety acts as the chemical crosslinking points, which restrict the motion of PEI chains and thus Tg’s of the materials increased. In addition, the increased Tg’s could be ascribed to the nanoreinforcement effect of POSS cages on polymer matrices. The dispersion of POSS cages at the nanometer level could significantly restrict the motions of macromolecular chains as shown in other POSS-reinforced polymers [12–36]. Returning to Fig. 5, it is seen that all the organic-inorganic hybrids containing the monofunctional POSS displayed single glass transition temperatures (Tg). The observed glass transition is assignable to the PEI matrix since the Tg’s of POSS component

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N-C H2 -CH 2

NH-CH 2 -CH 2

y

x C H2 -CH 2 -NH 2 Poly (ethylene imine)

POSS1

POSS2

Linear PEI-POSS hybrids

: Hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS

POSS2 : Octaglycidyletherpropyl POSS :

O

N

C H2

C H2

OH :

POSS Scheme 3. Synthesis of POSS-containing PEI hybrids.

Table 1 Molar ratio of epoxide to amino groups for POSS-containing hybrids POSS (wt.%)

5 7.5 10 15 20

Molar ratio of epoxide to amino groups PEI-octafunctional POSS hybrids

PEI-monoepoxide hybrids

1:74 1:43 1:35 1:22 1:16

1:535 1:394 1:253 1:159 1:113

is undetectable due to its high rigidity. The increased Tg’s could be associate with the following two aspects of factors. In the one hand, the POSS cages behave as the nanoreinforcement, which will reinforce the PEI matrix, i.e., the POSS component restricts the molecular motion of PEI chains and thus the Tg’s of the hybrids were increased. On the other hand, the increased Tg’s could be attributed to the specific morphology of the organic–inorganic PEI hybrids. Owing to the big difference in solubility between the organic (viz. PEI) and the inorganic components (viz. POSS) the grafted POSS moiety will be microphase-separated and exits in the hybrids in the form of microdomains. The microdomains will act as the physical crosslinking points and thus enhance the glass transition temperatures of the system. The formation of physical crosslinking

should be readily confirmed with the solubility tests using the corresponding solvents of the PEI and POSS components. It should be pointed out that the intermolecular specific interactions between PEI chains and POSS cages could also play an important role in enhancing Tg’s of the hybrid materials [38–40]. Thermogravimetric analysis (TGA) was applied to evaluate the thermal stability of the POSS-containing PEI hybrids. Shown in Figs. 6 and 7 are the TGA curves of the organic–inorganic hybrids, recorded in nitrogen atmosphere at 20 °C/min. Within the experimental temperature range, all the samples displayed similar degradation profiles, suggesting that the existence of POSS did not significantly alter the degradation mechanism of PEI matrix. Under the present condition, the initial decomposition temperature of the control PEI is about 355 °C and no char yield was obtained as expected. It is seen that the temperatures of thermal decomposition for the POSS-containing PEI copolymers are slightly lower than that of the control PEI due possibly to the inclusion of the hydroxyether structural units between PEI main chains and POSS cages. However, it is noted that the rates of degradation for the POSS-containing copolymers are lower than that of the control PEI. It is plausible to propose that mass loss from segmental decomposition via gaseous fragments could be suppressed by well-dispersed POSS cubes at the segment level. The similar results were also founded in fully exfoli-

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POSS1

Crosslinked PEI-POSS hybrids

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1106

POSS wt% 20

-1 oC

POSS wt%

15

-7 oC

910

3489

100

10

-14oC

3263

7.5

20

Endo

-19 oC

15

5

- 37 oC

10 7.5

0

-51oC

5

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0 -60

-40

-20

0

20

40

60

80

100

120

Temperature (oC ) 4000

3600

3200

2800

2400

2000

1600

1200

800

400

Fig. 4. DSC curves of the PEI modified by octaglycidyletherpropyl POSS.

Wavenumber (cm-1) Fig. 2. FTIR spectra of PEI modified by octaglycidyletherpropyl POSS.

the two hybrid systems are identical with the values calculated according to the content of POSS, indicating that the reactions between PEI and the POSS macromers were carried out to completion.

ated polymer-clay nanocomposites [41–43]. It is proposed that the POSS component will be transformed into ceramics (viz. SiO2) due to thermal oxidation and decomposition of silsesquioxane cages. For the PEI hybrids containing octaglycidyletherpropyl POSS, the thermal decomposition of the samples gives the ceramic yields to be 0.7, 1.8, 4.7, 11.3 and 13.2 wt.% for the hybrids containing 5, 7.5, 10, 15 and 20 wt.% of POSS, respectively. For the PEI hybrids containing hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS, the experimental values are 3.1, 5.0, 7.3 and 11.2 wt.% for the hybrids containing 5, 10, 15 and 20 wt.% POSS, respectively. The experimental SiO2 quantity for

3.3.2. Hydrogel behavior The above POSS-containing PEI hybrids were subjected to the solubility tests with water. It is noted that all the PEI hybrids can be significantly swollen with water but without dissolving, suggesting the formation of hydrogels. It is understandable for octaglycidyletherpropyl POSS-containing PEI hybrids to form the hydrogels since the chemical crosslinking occurred with the inter-component reaction between PEI and the POSS macromer. Therefore,

NH- CH2- CH2

x

N -C H 2-C H 2 C H2 -C H 2 -N H

y

N -C H2 -C H 2

z

C H2 -C H 2 -N H 2

HO

PEI

O

R

Si O Si O O O Si R O R Si OO Si R O O O Si O Si R R O

R Si

C D3OD

R=-CH2CH2CF3 a b b

6

5

4

3

a

2

1

0

Chemical shift (ppm) Fig. 3. 1H NMR spectrum of the PEI hybrids containing 20 wt.% hepta(3,3,3-trifluropropyl) glycidyletherpropyl POSS.

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K. Zeng et al. / European Polymer Journal 44 (2008) 3946–3956

100

POSS (wt%) 20 o

-45 C

80

15 o

Weight (%)

-46 C

10 o

Endo

-47 C

5

o

60

POSS wt%

40

-49 C

20 -51 C

20

0 -30

0

30

60

90

100

o

Weight ( %)

80

60

0

POSS wt% 20 15 10 7.5 5 0 100

200

300

400

500

600

700

800

Fig. 7. TGA curves of PEI modified by hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS.

out that lower swelling ratio for the hybrid containing 5 wt.% POSS than that containing 7.5 wt.% POSS is responsible for the lower crosslinking density of the hybrid. It is proposed that too low crosslinking density is not sustainable for the swollen crosslinked networks. In marked contrast of the octaglycidyletherpropyl POSS-containing PEI hybrids, the hydrogels resulting from PEI and hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS is a kind of physical hydrogels. This judgment is evidenced by the solubility tests by using some organic solvents such as methanol and ethanol. It was observed that all the POSS-containing PEI hybrids can be easily dissolved in methanol, indicating that no chemical crosslinking was

100

20

300

Temperature ( C)

Fig. 5. DSC curves of the PEI modified by hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS.

40

200

o

Temperature ( C)

8

400

500

600

700

800

Temperature (oC)

the POSS-containing PEI hydrogel is a chemically-crosslinked hydrogel. Shown in Fig. 8 are the plots of water uptake as functions of time in the swelling experiments for the dried octafunctional POSS-containing PEI hybrids. It is seen that the time for the hydrogels to attain equilibrium increased with increasing the content of POSS in the hybrids (or the crosslinking density). All the POSS-containing PEI hybrids can be swollen with water within the time of 1.5–9.0 h and attain the swelling ratios from six to seven times. It is seen that the highest swelling ratio of 7.0 was obtained for the hybrid containing 7.5 wt.% POSS. The swelling ratio decreased with increasing the content of POSS. This observation could be attributed to the increased crosslinking density of the networks. It should be pointed

Swelling ratio (g/g)

6 Fig. 6. TGA curves of PEI modified by octaglycidyletherpropyl POSS.

4

POSS wt% 5 7.5 10 15 20

2

0 0

250

500

750

1000

1250

1500

Time (min) Fig. 8. Plots of swelling ratio as functions of time for the PEI hydrogels crosslinked with octaglycidyletherpropyl POSS.

MACROMOLECULAR NANOTECHNOLOGY

-60

15 10 5 0

0

o

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K. Zeng et al. / European Polymer Journal 44 (2008) 3946–3956

Table 2 Static contact angles and surface free energy Surface free energy (mN  m1)

POSS content (wt.%)

Static contact angle hH2O

hCH2I2

cds

cps

cs

0 5 10 15 20

0 73.1 90.3 94.4 95.3

22.4 59.8 61.5 66.4 66.5

32.0 24.4 26.7 24.9 25.0

42.2 15.7 6.2 5.2 4.9

74.2 40.1 32.9 30.1 29.9

MACROMOLECULAR NANOTECHNOLOGY

Water: cds ¼ 21:8mN=m, cps ¼ 51:0mN=m. Diiodomethane: cds ¼ 48:5mN=m, cpS ¼ 2:3mN=m.

formed with the reaction between PEI and the monofunctional POSS macromer. Therefore, the formation of the hydrogels for the POSS-containing PEI hybrids suggests the presence of physical crosslinking points in the system. It is plausible to propose that the pendant POSS cages were aggregated and existed in the form of hydrophobic microdomains of POSS in the system since the hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS is immiscible with PEI chains. Nonetheless, the POSS microdomains can be easily dissolved with organic solvents such as methanol.

3.3.3. Surface properties Poly(ethylene imine) (PEI) is a water-soluble polymer. For the hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS-containing PEI hybrids, it is expected that the inclusion of the bulky and hydrophobic POSS cages as the side groups result in the enhancement of hydrophobicity of the polymer, i.e., the POSS-modified PEI possesses the amphiphilicity. The POSS portion combines the features of both organosilicon and organofluorine compounds and is of low free energy [27]. The enrichment of the POSS portion at the surface could occur upon incorporating POSS into organic polymer and thus the surface hydrophobicity (or dewettability) of materials will enhanced. The surface properties of the organic–inorganic nanocomposites were investigated in terms of the measurement of static contact angle. The static contact angles were measured with ultrapure water and diiodomethane as probe liquids, respectively, and the results are summarized in Table 2. Upon introducing the POSS into the system, the contact angles are significantly enhanced, which increased with increasing the content of POSS. For the hybrid containing 5 wt.% POSS the contact angle is 73.1° whereas the water contact angle attained to 96° for the hybrid containing 20 wt%

Fig. 9. Image of water droplet on the surface of (A) pure PEI; (B) 5 wt.% POSS content PEI; (C) 10 wt.% POSS content PEI; (D) 15 wt.% POSS content PEI; (E) 20 wt.% content PEI.

POSS as shown in Fig. 9. The increase in water contact angle indicates that the hydrophobicity of the materials was significantly enhanced, i.e., the surface free energy of the materials was reduced. The surface free energies of the hybrid nanocomposites containing different percentage of POSS were calculated according to the geometric mean model: [44,45]

cos h ¼

2



cL


1

cdL cds 2 þ cpL cps


12



1

ð2Þ

cs ¼ cds þ cps

ð3Þ

where h is contact angle and cL is the liquid surface tension; cpL and cdL are the polar and dispersive components of cL , respectively. According to the data of surface contact angles from water and diiodomethane, the calculated results of surface free energy were also incorporated into Table 2. The surface free energy of the control PEI is about 74.2 mN  m1. It is noted that the total surface free energies of the nanocomposites were diminished to 29.9 mN  m1 while the concentration of POSS is 20 wt.% as shown in Fig. 10. The non-polar component (i.e., cds ) seems to be less sensitive than the polar component (i.e., cps ) to the concentration of the POSS, suggesting that the inclusion of POSS increased the distribution of the non-polar groups (i.e., silsesquioxane cage) on the surface energy of materials. The silsesquioxane portion on the surface could behave as a screening agent and reduce the surface energy of the nanostructured thermosets. It should be pointed out that the formation of micro- or nanostructures at the surface also contributed to the enhancement of surface hydrophobicity of the materials [46–52]. 4. Conclusion Both octaglycidyletherpropyl POSS and hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS were synthesized 100

Surface energy (mN/m)

80

60

40

20

0

0

5

10

15

20

POSS (wt %) Fig. 10. Surface free energy of the PEI modified by hepta(3,3,3-trifluoropropyl) glycidyletherpropyl POSS.

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via the hydrosilylation reactions between octahydrosilsesquioxane [and/or hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane] and allyl glycidyl ether (AGE). The POSS macromers were characterized by means of Fourier transform infrared (FTIR) and nuclear magnetic resonance spectroscopy (NMR). The inter-component macromolecular reactions between the POSS macromers and poly(ethylene imine) (PEI) were employed to prepare the POSS-containing organic-inorganic PEI hybrids. The inclusion of octaglycidyletherpropyl POSS into PEI results in the formation of the organic–inorganic hybrid networks whereas the introducing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS to PEI afford the POSS-grafted PEI copolymers. Thermal analyses showed that the POSS-containing PEI hybrids displayed increased glass transition temperatures (Tg’s) and the enhanced thermal stability compared to the plain PEI. All the PEI hybrid composites can be significantly swollen with water without dissolving, suggesting the formation of hydrogels. The swelling of the hybrids containing the octafunctional POSS constitute the chemically-crosslinked hydrogels whereas that of the hybrids containing the monofunctional POSS displayed the behavior of a physical hydrogels. The formation of physical hydrogels is ascribed to the microphase-separated morphology in the hybrids. In addition, the hybrids containing the monofunctional POSS exhibited the typical amphiphilicity as evidenced by the increase in surface hydrophobic properties. Acknowledgments The financial support from Natural Science Foundation of China (No. 20774058) is acknowledged. This work is also supported by Shanghai Leading Academic Discipline Project (Project No.: B202). References [1] Nango M, Klotz LM. J Polym Sci A Polym Chem 1978;16:1265. [2] Kimura Y, Nango M, Kurokie N, Ihara Y, Klotz LM. J Polym Sci Polym Symp 1984;71:167. [3] Horn D, Linhart F. In: Roberts JC, editor. Paper chemistry. London: Blackie; 1991. p. 44. [4] Matsumoto K, Suganuma A, Kunui D. Powder Technol 1989;25:1. [5] Goethals EJ. Polymer amines and ammonium salts. New York: Pergamon Press Inc.; 1980. [6] Park MR, Han KO, Han IK, Cho MH, Nah JW, Choi YJ, et al. J Control Release 2005;105:367. [7] Godbey WT, Wu KK, Mikos AG. J Control Release 1999;60:149. [8] Wightman L, Kircheis R, Rössler V, Garotta S, Ruzick R, Kursa M, et al. J Gene Med 2001;3:362. [9] Thuenemann AF, General S. Langmuir 2000;16:9634. [10] Noeding G, Heitz W. Macromol Chem Phys 1998;199:1637; Ren S, Yang S, Zhao Y, Yu T, Xiao X. Surf Sci 2003;546:64. [11] Lichtenhan JD, Vu NQ, Carter JA, Gilman JW, Feher FJ. Macromolecules 1993;26:2141. [12] Haddad TS, Lichtenhan JD. J Inorg Organomet Polym 1995;5:237. [13] Lichtenhan JD, Otonari YA, Carr MJ. Macromolecules 1995;28:8435. [14] Zhang C, Laine RM. J Organomet Chem 1996;521:199. [15] Mantz RA, Jones PF, Chaffee KP, Lichtenhan JD, Gilman JW, Ismail IMK, et al. Chem Mater 1996;8:1250. [16] Haddad TS, Lichtenhan JD. Macromolecules 1996;29:7302. [17] Feher FJ, Wyndham KD, Baldwin RK, Soulivong D, Lichtenhan JD, Ziller JW. Chem Commun 1999:1289. [18] Li G, Wang L, Ni H, Pittman CU. J Inorg Organomet Polym 2001;11:123. [19] Abe Y, Gunji T. Prog Polym Sci 2004;29:149.

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