Star-shaped POSS diblock copolymers and their self-assembled films

Abstract

Novel 16-arm, star-shaped POSS-containing diblock copolymers are first synthesized by methylmethacrylate (MMA) and methacrylisobutyl polyhedral oligomeric silsesquioxane (MA-POSS) using a cetylfunctional initiator of octakis(dibromoethyl) POSS (POSS-(Br)₁₆). Three well-defined copolymers of s-POSS-PMMA_277.3-b-P(MA-POSS)_{5.8,16.4,25.4} are discussed. The introduction of P(MA-POSS) could provide the copolymer with excellent hydrophobic/oleophobic performance and thermal stability. Although the size of core–shell micelles of s-POSS-PMMA_277.3-b-P(MA-POSS)_5.8–25.4 does not increase linearly with increasing MA-POSS content due to different self-assembly behaviors and steric effects, surface roughness (0.44–1.41 nm) and water–hexadecane contact angles (108°/50°–120°/58°) of films, as well as thermal stability (T_d = 350–380 °C, T_g = 112–125 °C) and storage modulus (842–1600 MPa) of copolymers increased linearly. The effect of solvents on self-assembled micelles and films indicates that 340–370 nm core–shell micelles, 330–370 nm sun-like stretching micelles and 180–200 nm three-layer-structured micelles are formed in tetrahydrofuran (THF), chloroform (CHCl₃) and butanone (MEK) solution, respectively. The lowest surface free energy (17.48 m Nm⁻¹) is produced by film casting from THF solution due to the highest surface roughness (1.12 nm) and Si content (6.01%). While, the lowest water absorptive (Δm = 3800 ng cm⁻²) and viscoelastic (ΔD/Δf = −0.36) film is produced by CHCl₃ solution, the film casting from MEK solution exhibits the highest water absorption (Δm = 6500 ng cm⁻²) and viscoelasticity (ΔD/Δf = −0.15). This is the first example of a 16-arm, star-shaped POSS diblock copolymers, and can be used as solvent-dependent coatings.

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1 Introduction

Polyhedral oligomeric silsesquioxane (POSS) with well-defined cube-like structures has received much attention as nanoscale building blocks to form hybrid materials.^1,2 Normally, POSS contains a silsesquioxane cage with the formula (SiO_1.5)_n (n = 8–14) and n organic functional groups originating at each Si atom on a cage vertex.^3,4 Therefore, functionality may vary from one organic group to another,^5–8 and will lead the reactive combination of POSS into polymers designed as branched,^9,10 star-shaped,^11,12 or core–shell^13,14 structures to obtain hydrophobicity or high mechanical and thermal properties.^15–18

In fact, POSS cube can be incorporated into the polymeric system by blending,^19,20 grafting,^21,22 cross-linking^23,24 and copolymerization.^18,25,26 Up to now, the synthesis of POSS-containing polymers has mostly focused on styryl-POSS, methacrylate-POSS, norbornyl-POSS, vinyl-POSS, epoxy-POSS, phenolic-POSS, benzoxazine-POSS, amine-POSS and hydroxyl-POSS.^{12,23,26–29} Shiao-Wei Kuo has reviewed methods for synthesizing POSS compounds and discussed the use of both mono- and multi-functional POSS monomers to develop thermoplastic and thermosetting polymers.² Comparatively, multi-functional POSS can form branched dendrimers, hyperbranched or star-shaped polymers, and thereby could significantly improve the properties of polymers.^9–12 For the synthesis of various POSS-based star polymers, both “grafting-to” and “grafting-from” approaches have been used as the common synthesis methods.³² Up to now, the highest number of arms in star-shaped POSS-containing polymer occurs in eight-arm polymers obtained by the hydrosilylation reaction between silyl hydride-functionalized polystyrene and octavinyl POSS,³¹ or by bimolecular nonlinear polymerization, utilizing either platinum-catalyzed hydrosilylation orsilanol condensation reactions⁹ or by anionic synthesis via the addition of poly(styryl)lithium to octavinyl POSS in benzene.¹¹ In the case of using difunctional monomers, the overall molecular structure and number of arms are often not well-defined and polydispersity is often relatively large (PDI = 1.9).⁹ Therefore, the preparation of uniform star polymers with molecular precision still remains a challenge.

With improvement in controlled polymerization techniques, multi-functional initiators for either an “arm-first” or “core-first” strategy have been developed toward this aim.^11–13 However, these methods often suffer either from prolonged reaction time and incomplete reaction due to steric hindrance of the coupling reaction in the “arm-first” strategy, or from the unequal initiation efficiency and poor control over polymer chain uniformity in the “core-first” strategy. While, the controlled polymerization of ATRP is a suitable technique for obtaining POSS block copolymers using multiple-arm as multi-functional initiators.^33–35 W. Yuan has reported a series of novel 8-arm star-shaped hybrid polymers POSS-(P(MEO₂MA-co-OEGMA))₈ of P(2-(2-methoxyethoxy)ethylmethacrylate)-co-oligo(ethyleneglycol)methacrylate(P(MEO₂MA-co-OEGMA)) synthesized using octafunctional POSS-(Cl)₈ initiator via ATRP (PDI = 1.3–1.4).¹³ W. Wang used a POSS core as an initiator to polymerize methyl methacrylate for the preparation of a star-shaped POSS/PMMA-(Cl)₈ macroinitiator for the ATRP of styrene to obtain AB block copolymers, forming a star-shaped structure with a “core” of POSS and an “arm” of polymer as PDI = 1.61–1.66.³⁰ It can be seen that PDI for POSS-(Cl)₈ initiator was higher than the signal initiator (PDI < 1.1). This led us to consider the 16-arm initiator. Because multi-initiator of ATRP for POSS-based star-shaped polymer, both 8-arm and 16-arm have structural symmetry, it is expected that the higher branch and grafting density in the 16-arm are much more uniform and balanced than the 8-arm, which will lead to higher initiating efficiency of every active point in the 16-arm initiator for ATRP and maybe a higher PDI for POSS-based star-shaped polymer than the 8-arm initiator if the molecular weights similar. However, to our best knowledge, no study has been reported on 16-arm, star-shaped diblock copolymer based on POSS obtained by ATRP techniques, and also there has been no previous research concerning the properties of their self-assembled films used as high-performance films.

Herein, the synthesis of 16-arm star-shaped POSS-containing diblock copolymer by ATRP techniques is reported. Octakis (dibromoethyl) polyhedral oligomeric silsesquioxanes (POSS-(Br)₁₆) is used as the initiator, poly(methylmethacrylate, PMMA) as the first segment, and poly(methacrylisobutyl POSS, P(MA-POSS)) as the end block. The chemical structure and molecular weight of s-POSS-PMMA_m-b-P(MA-POSS)_n are characterized by ¹HNMR and SEC. The effect of both MA-POSS content and solvents on the self-assembly of s-POSS-PMMA_m-b-P(MA-POSS)_n in tetrahydrofuran (THF), chloroform (CHCl₃) and butanone (MEK) solutions are compared by TEM and DLS. The surface properties of films, i.e., roughness, viscoelasticity and hydrophobicity are investigated by atomic force microscopy (AFM), quartz crystal microbalance with dissipation (QCM-D) static contact angle (SCA), respectively. The mechanical and thermal stability of s-POSS-PMMA_m-b-P(MA-POSS)_n are obtained by TGA, DSC and DMA.

2 Experimental

2.1 Materials

Octakis(dibromoethyl) polyhedral oligomeric silsesquioxanes (POSS-(Br)₁₆, Mw = 1911 g mol⁻¹) and methacrylisobutyl POSS (MA-POSS, Mw = 947.3 g mol⁻¹) were purchased from Hybrid Plastics Co. (USA) and used as received. Methyl methacrylate (MMA, 99 wt%) (Aldrich) was rinsed by 5 wt% NaOH aqueous solution and ion-free water until the rinsed water reached a pH of 7, which was followed by drying over CaH₂ for 24 h and distilling under reduced pressure to remove the inhibitor before use. Toluene and tetrahydrofuran (THF) were stirred over CaH₂ for 12 h at room temperature, and then distilled under reduced pressure prior to use. Cuprous chloride (CuCl) was purified according to the method of White Sides.³⁶ N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA, 97%) was supplied by Aldrich and used without further purification. All the other solvents were used as-received without any purification.

2.2 Preparation of s-POSS-PMMA_m-b-P(MA-POSS)_n by ATRP

After 0.4185 g CuCl (4.185 mmol) and a dry magnetic stirrer were added to a vacuumed Schlenk tube by N₂, the mixture of POSS-(Br)₁₆ (0.5 g, 0.2616 mmol), MMA (8.3704 g, 83.70 mmol), PMDETA (0.9710 g, 4.185 mmol) and toluene (15 g) was introduced to the tube under N₂ atmosphere (Table 1). This recipe was selected for the formation of continuous and stable films by sufficient molecular weight of PMMA and suitable MA-POSS content in the next step for obtaining the final products. Reaction started at 80 °C and continued for 24 h in an oil bath, as shown in Scheme 1. The left catalyst was removed by passing the copolymer solution through an alumina column using THF as the solvent, and the excess solvent was removed under reduced pressure. After the colorless solution was reprecipitated into methanol and dried in a vacuum oven overnight, the white powder s-POSS-PMMA_m was obtained. Yield: 82%.

Table 1 Detailed polymerization recipes for block copolymer s-POSS-PMMA_m-b-P(MA-POSS)_n

Samples	POSS-(Br)16/g	MMA/g	s-POSS-PMMAm/g	MA-POSS/g	CuCl/g	PMDETA/g	Toluene/g
Sample 1	0.5	8.3704	—	—	0.4185	0.9710	15
Sample 2	—	—	0.5	0.1296	0.0137	0.02381	2
Sample 3	—	—	0.5	0.2593	0.0137	0.02381	2
Sample 4	—	—	0.5	0.5185	0.0137	0.02381	2

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Scheme 1 Synthesis of s-POSS-PMMA_m-b-P(MA-POSS)_n.

After s-POSS-PMMA_m was dissolved in toluene in a Schlenk tube, a mixture of MA-POSS, CuCl and PMDETA was charged under N₂ atmosphere. Then, the reaction was permitted to continue for 24 h at 110 °C in an oil bath, as shown in Scheme 1. The left catalyst and excess solvent were removed in the same manner as above. The resulting star-shaped copolymer s-POSS-PMMA_m-b-P(MA-POSS)_n (Samples 2–4) were obtained in a yield of 73–78%. Detailed polymerization recipes are listed in Table 1.

Films of s-POSS-PMMA_m-b-P(MA-POSS)_n were prepared by casting the solutions onto a glass substrate and then drying it at ambient temperature.

2.3 Characterization

Proton nuclear magnetic resonance (¹H-NMR) measurement was performed on a Bruker AV-500 spectrometer using CDCl₃ as the solvent and tetramethylsilane (TMS) as the internal reference. The molecular weight of the samples were determined on a DAWN EOS size exclusion chromatography (SEC) (Wyatt Technology, USA). The eluent of THF (containing 0.01 molL⁻¹ LiCl at 25 °C) was used at a flow rate of 0.5 mL min⁻¹. Molecular weight was calibrated by polystyrene standards.

Transmission electron microscopy (TEM) was used to investigate the morphology of the self-assembled s-POSS-PMMA_m-b-P(MA-POSS)_n in THF, CHCl₃ and MEK solutions. Measurements were conducted on a JEM-3010 at an acceleration voltage of 100 kV. Samples were prepared by drop-casting micelle solutions onto carbon-coated copper grids, and then air-drying at room temperature before measurement. Dynamic light scattering (DLS) analysis was used to obtain the aggregates of samples in THF, CHCl₃ and MEK solutions (0.01 g mL⁻¹ sample solutions) using a MALVERN Nano ZS 90 (Malvern Instruments, U.K.) equipped with an He–Ne laser (λ = 632.8 nm). Scattering data were recorded at 25 ± 0.1 °C in the backscattering mode at a scattering angle of 2θ = 173°.

Atomic force microscopy (AFM) for characterizing the surface topographies and roughness of the obtained film was performed on a NT-MDT new Solver-Next at 38–42% R.H. Tip information: radius: <10 nm, cantilever length: 90 ± 5 mm; width: 40 ± 3 mm; thickness: 2.0 ± 0.5 mm, resonant frequency: 330 kHz, force constant: 48 N m⁻¹. X-ray photoelectron spectroscopy (XPS) measurement was processed on the air-exposed film surface by an AXIS ULTRA (England, KRATOS ANALYTICAL Ltd) using an Al mono Kα X-ray source (1486.6 eV) operated at 150 W. Overview scans were obtained with a pass energy of 160 eV and acquisition times of 220 s. Static contact angle (SCAs) measurement for deionized water and hexadecane on the air-exposed surfaces of the films were conducted on a JY-82 contact angle goniometer (Testing Machine Co. Ltd. China) by the sessile drop method with a microsyringe at 25 °C. The surface free energy was calculated by the water contact angles.³⁷

Q-Sense E1 Quartz crystal microbalance with dissipation monitoring (QCM-D, Sweden) for water adsorption of film surfaces was measured at 25 °C by AT-cut piezoelectric quartz crystals covered with gold with a fundamental frequency of 5 MHz and a diameter of 14 mm. The films were prepared by dropping 0.2 μL copolymer solutions (1 wt%) on the surfaces of quartz crystals and drying in a vacuum oven at 30 °C for 12 h. Δf and ΔD were recorded at 15 MHz with air as the baseline. Δm was calculated by Δm = KΔf (ref. 38) according to K = −5.9 ng Hz⁻¹ cm⁻².³⁹

Thermogravimetry (TGA) was performed under N₂ atmosphere with a 10 °C min⁻¹ temperature rise at 700 °C using TGA analyzer (STA449C Jupiter from NETZSCH). Differential scanning calorimeter (DSC) thermo-scans of the samples were recorded on a NETZCH DSC-200 apparatus. To eliminate thermal history, all the data were collected during a second heating run at a scanning rate of 10 °C min⁻¹ from −50 °C to 200 °C under a dry nitrogen atmosphere after heating the sample from 20 °C to 100 °C at 10 °C min⁻¹ and then rapidly cooling to −50 °C using liquid nitrogen. Dynamic mechanical analysis (DMA) was carried out using a DMA Q800 instrument with tension mode. Sample films were cut into 12 mm × 8 mm × 0.0200 mm (length × width × thickness), equilibrated for 5 min at 20 °C, and then heated to 200 °C at a constant heating rate of 5 °C min⁻¹ under nitrogen and a frequency of 1 Hz.

3 Results and discussion

3.1 Synthesis of s-POSS-PMMA_m-b-P(MA-POSS)_n

The star-shaped diblock copolymers s-POSS-PMMA_m-b-P(MA-POSS)_n were synthesized via ATRP using an POSS-(Br)₁₆ nanocage to initiate MMA and MA-POSS. ¹H-NMR spectrum for both s-POSS-PMMA and s-POSS-PMMA_m-b-P(MA-POSS)_n is shown in Fig. 1. Compared with δ_H of POSS-(Br)₁₆ (Fig. 1a) at 3.65 ppm (b) for Si–CH–, 3.8 and 4.05 ppm (a² and a¹) for –CH₂–, the chemical structure of s-POSS-PMMA_m (Fig. 1b) was realized by δ_H (ppm) at 3.60 (a) for –OCH₃ in PMMA, 1.86 (b) for Si–CH– in POSS, 0.85 (c) for –CH₃ in PMMA, and 1.56 (d) for –CH₂– in PMMA. The typical δ_H of s-POSS-PMMA_m-b-P(MA-POSS)_n (Fig. 1c) at 3.82 ppm (a) for –O–CH₂– in P(MA-POSS) and at 3.60 ppm (b) for –OCH₃ in PMMA, together with disappearing δ_H at 5.5 ppm and 6.2 ppm (a) for =CH₂ in MA-POSS (Fig. 1d), has confirmed the as-designed diblock structure of s-POSS-PMMA-b-P(MA-POSS).

Fig. 1 ¹H-NMR spectra of s-POSS-PMMA_m and s-POSS-PMMA_m-b-P(MA-POSS)_n.

Furthermore, the molecular weight (Mw) was determined by the SEC plot in Fig. 2 and Table 2. Mw of 29 230 g mol⁻¹ for Sample 1 (s-POSS-PMMAm), 34 690, 44 840 and 53 310 g mol⁻¹ for samples.

Fig. 2 SEC curves of s-POSS-PMMA_m-b-P(MA-POSS)_n.

Table 2 Molecular weight of s-POSS-PMMA_m-b-P(MA-POSS)_n

Samples	Copolymers/SEC (theoretical)*	Mw (g mol^−1)	Mn (g mol^−1)	PDI (Mw/Mn)
Sample 1	s-POSS-PMMA277.3(320)	29 230	23 190	1.261
Sample 2	s-POSS-PMMA277.3-b-P(MA-POSS)5.8(8)	34 690	24 990	1.388
Sample 3	s-POSS-PMMA277.3-b-P(MA-POSS)16.5(16)	44 840	34 450	1.302
Sample 4	s-POSS-PMMA277.3-b-P(MA-POSS)25.4(32)	53 310	37 910	1.406

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Samples 2–4 with the corresponding molecular weight distribution (PDI) of 1.261, 1.388, 1.302 and 1.406, respectively, indicated typical ATRP polymerization. Compared with 8-arm star-shaped hybrid with “core” of POSS and “arms” of the polymer obtained by POSS-(Cl)₈ initiator via ATRP as PDI = 1.3–1.4 (ref. 13) and PDI = 1.61–1.66,³⁰ these PDI (1.261–1.406) from POSS-(Br)₁₆ initiator for star-shaped hybrid in this paper are not higher than POSS-(Cl)₈ initiator. This suggests that 16-arm POSS-core star-shaped hybrid with higher branches are able to obtain a uniform grafting density copolymer in the ATRP approach. Therefore, the degree of m in Sample 1 of s-POSS-PMMA_m was evaluated to be 277.3 (theoretical m is 320 by calculation), and the degrees of n in Samples 2–4 of s-POSS-PMMA_277.3-b-P(MA-POSS)_n were evaluated to be 5.8, 16.4, 25.4 (theoretical n are 8, 16, 32, calculated by adding the ratios of the monomers and initiators). These results could suggest that the average number of P(MA-POSS) arms attached to one s-POSS-b-PMMA nanocage were 5.8, 16.4 and 25.4 in Samples 2–4.

3.2 Micelles and film surface self-assembled by s-POSS-PMMA_277.3-b-P(MA-POSS)_5.8–25.4

Considering the different solubilities of POSS, PMMA and P(MA-POSS) segments in THF solution, the self-assembly of Samples 1–4 with different content of P(MA-POSS) is shown in Fig. 3. Sample 1 of s-POSS-PMMA (Fig. 3a) shows 150–200 nm core–shell micelles, which corresponds to 136.3 nm aggregates in the DLS curve (with a few 7.1 nm unimers) (Fig. 3e). The micelles were formed by POSS core (the black region in the center) and PMMA shell (the light region in the edge) due to the much better solubility of PMMA than POSS, which is explained by the enlarged pattern in the schematic diagram (Fig. 3a).

Fig. 3 TEM morphology of micelles for Samples 1–4 (a–d) in THF solution and the corresponding DLS curves (e).

However, with the introduction of MA-POSS and increasing MA-POSS content (15.7–45.2 wt%), the size of self-assembled core–shell micelles did not increase linearly. For Sample 2 of s-POSS-PMMA_277.3-b-P(MA-POSS)_5.8, since only 5.8/16 branches were successfully occupied by P(MA-POSS) segments, the low solubility of the P(MA-POSS) segment and POSS-(Br)₁₆ initiator were aggregated into the inner core (dark region), and the better solubility of the PMMA segment occupied by P(MA-POSS) was also pulled back into the core but the other unoccupied PMMA segment stretched outside (the whitish region), as shown in the scheme (Fig. 3b). Therefore, 140–170 nm core–shell micelles are formed as similar size of 172.6 nm (82%) in the DLS curve (Fig. 3e). This indicates that the introduction of P(MA-POSS) segment into the copolymer does not increase the size of micelles. While, for Sample 3 of s-POSS-PMMA_277.3-b-P(MA-POSS)_16.5, 340–360 nm core–shell micelles were formed with 40 nm compact shell (Fig. 3c), which were further confirmed by 311.3 nm aggregates in the DLS curve (Fig. 3e). Since the arms were occupied at 16.5/16 (almost all the occupied PMMA arms were held by P(MA-POSS)), the inner core was formed by large amounts of P(MA-POSS) together with POSS-(Br)₁₆ so that the bigger core was produced (Fig. 3c) to increase the size of the micelles. For Sample 4 of s-POSS-PMMA_277.3-b-P(MA-POSS)_25.4 with large amounts of MA-POSS (25.4/16), as shown in Fig. 3d, 200–220 nm micelles with inlaid-dark-spot morphology were confirmed as 227.0 nm by the DLS curve in Fig. 3e. Since the flexibility of the copolymer and the movement of s-POSS-PMMA_277.3-b-P(MA-POSS)_25.4 were restricted by heavy P(MA-POSS)_25.4, the smaller aggregates in Sample 4 were quickly gathered to limit the formation of larger aggregates. Thus, the size of the micelles was smaller than that of Sample 3.

These self-assembled results reveal that Samples 2–4 of s-POSS-PMMA_277.3-b-P(MA-POSS)_5.8–25.4 are mainly self-assembled as POSS/P(MA-POSS) core and PMMA shell due to the low solubility of inorganic POSS/P(MA-POSS) core and better solubility of PMMA segments in THF solution. These results are similar to 8-arm star-shaped POSS-containing copolymer, as previously reported by W. Yuan.¹³ These results show that POSS-(P(MEO₂MA-co-OEGMA))₈ with an inorganic POSS core and eight organic P(MEO₂MA-co-OEGMA) arms can assemble into micelles in water because hydrophilic P(MEO₂MA-co-OEGMA) arms mainly in the shell of the micelles and hydrophobic POSS cores mainly in the core of the micelles. Therefore, it is possible to suggest that the self-assembly behavior of different armed POSS-containing polymer are similar, even though they are of different grafting density.

The effect of these self-assembled micelles on the surface morphology of casted films is characterized by AFM in Fig. 4. Sample 1 displays a smooth surface (Fig. 4a, Ra = 0.367 nm) with the rough curve floating between −0.6 nm and 0.6 nm (Table 3). However, Samples 2–4 show ordered convexes and heaves on the film surface with Ra = 0.438 nm, 1.12 nm and 1.41 nm for surface roughness, respectively (Fig. 4b–d). This is because P(MA-POSS) segment tends to migrate onto the film surface to form the convexes on increasing the roughness, which has been proven by previously reported studies.⁴⁰ Therefore, SCAs and surface free energies calculated in Table 3 indicate that the films of Samples 1–4 show obvious hydrophobicity (108–120° water contact angles), sufficient oleophobicity (28–58° hexadecane contact angles) and low surface free energy (16.16–24.46 mN m⁻¹), and SCAs increased with increasing MA-POSS content due to improved surface roughness.

Fig. 4 AFM images of film surface for Samples 1–4 (a–d) casting from THF solution.

Table 3 Micelles and the properties of film surface

Samples	Sample 1	Sample 2	Sample 3	Sample 4	Sample 3	Sample 3
a Stands for unimers.
Solvent	THF	THF	THF	THF	CHCl3	MEK
Micelle/nm (TEM)	150–200	140–170	340–360	200–220	330–370	180–220
Morphology (TEM)	Core–shell	Core–shell	Core–shell	Multi-core–shell	Sun-like stretching	Core–multi-shell
Micelle/nm (DLS)	136.3/7.1^a	172.6/9.7	311.3/26.88	227.0/40.26	407.2/11.7	206.5/8.44
Roughness/nm (Ra)	0.37	0.44	1.12	1.41	0.291	0.57
Rough curve/nm	−0.6 to 0.6	−0.7 to 0.9	−2.0 to 3.4	−2.2 to 3.5	−0.6 to 0.8	−0.1 to 1.2
SCAs/° (water–hex)	112/28	108/50	114/54	120/58	104/22	100/20
γ/m Nm^−1	24.46	19.11	17.48	16.16	25.94	26.74
Δm ng cm^−2	7900	—	4600	—	3800	6500
ΔD/Δf	−0.075	—	−0.19	—	−0.36	−0.15

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3.3 Thermal/mechanical properties of s-POSS-PMMA_277.3-b-P(MA-POSS)_5.8–25.4

The thermal stability of s-POSS-PMMA_277.3-b-P(MA-POSS)_5.8–25.4 was investigated by TGA at a thermal degradability ranging of 25–800 °C (Fig. 5). POSS-(Br)₁₆ and s-POSS-PMMA are used for comparison. POSS-(Br)₁₆ presents two degradations at 300–400 °C and 400–550 °C (Fig. 5a), which results from –CHBr–CH₂Br (with 45 wt% weight loss) and the skeleton of Si–O–Si (with 27 wt% weight loss). However, the first degradation temperature for Samples 1–4 occurred at 235 °C (Sample 1), 250 °C (Sample 2), 255 °C (Sample 3) and 270 °C (Sample 4) due to thermal degradability of the side chains of PMMA, and the second degradation occurred at 350 °C, 365 °C, 375 °C and 380 °C, for Samples 1–4, respectively, were due to the backbone of P(MA-POSS) and the skeleton of Si–O–Si. The final remainders increased with increasing MA-POSS content. Therefore, MA-POSS obviously improved thermal degradability. On the other hand, thermal degradability curves of Samples 1–4 were also confirmed by mass loss rate in the DTG curves in Fig. 5b. Sample 1 presents two separate peaks at 300 °C and 400 °C but these two peaks tend to blend together in Samples 2–4 because of the better compatibility of P(MA-POSS) and PMMA (Fig. 5b). In addition, DSC measurement in Fig. 5c also indicated that glass transition temperatures (T_g) for Samples 1–4 were located at 105 °C, 112 °C, 118 °C and 125 °C, respectively. These results not only confirm the successful synthesis of s-POSS-PMMA-b-P(MA-POSS) copolymer but also prove that compatibility becomes better with increasing MA-POSS.

Fig. 5 TGA curves (a), DTG (b) and DSC thermograms (c) of Sample 1–4.

Furthermore, the dynamic mechanical properties of Samples 1–4 were studied by DMA for storage modulus (Fig. 6a) and loss factor (Fig. 6b). With increasing MA-POSS content, the storage modulus of samples obviously increased from 842 MPa for Sample 1 to 1160, 1420 and 1600 MPa for Samples 2–4, respectively (Fig. 6a), showing that MA-POSS could limit the movement of segments and improve storage modulus. Meanwhile, the starting softening temperature of Samples 1–4 in Fig. 6b was also increased at 104 °C, 109 °C, 115 °C and 125 °C with increasing MA-POSS content, and the loss factor was highly consistent with T_g values in the DSC curves (Fig. 5c).

Fig. 6 Storage modulus (a) and Tan delta of mechanical tensile stress (b) for the films.

3.4 Effect of solvents on micelles and film surface

To understand the effect of solvents on the self-assembled micelles of copolymers, THF, CHCl₃ and MEK were selected based on the fact that MEK has the best solubility and CHCl₃ has the worst. Sample 3 of s-POSS-PMMA_277.3-b-P(MA-POSS)_16.5 was selected as the discussed example, based on the results of self-assembled micelles and films by the above mentioned comparative discussion for Samples 2–4. Compared with 360 nm core–shell micelles in THF solution discussed above (Fig. 3c), Sample 3 forms 330–370 nm sun-like stretching micelles in CHCl₃ solution (Fig. 7a), which is a slightly smaller than 407.2 nm in DLS curve due to PMMA stretching shell caused by its better solubility (Fig. 7c). However, in MEK solution (Fig. 7b), Sample 3 showed 200 nm three-layer-structured micelles as black core, dark shell and black crown, corresponding to 206.5 nm micelles in the DLS curve (Fig. 7c). The core and the crown are formed by P(MA-POSS) segment, and the shell was formed by PMMA segment.

Fig. 7 TEM morphology of micelles in CHCl₃ (a), MEK (b) and corresponding DLS curves (c), chemical composition of films by XPS (d), and AFM images of film surface casting from CHCl₃ (e) and MEK (f) solutions for Sample 3.

Compared with the micelles in THF (Fig. 3c, 360 nm core–shell micelles), CHCl₃ (Fig. 7a, 330–370 nm sun-like stretching micelles) and MEK (Fig. 7b, 200 nm three-layer-structured micelles) solutions, the different morphology is attributed to the solubility, flexibility and movement of the different segments. In THF and CHCl₃, the better solubility, flexibility and movement of PMMA segments were intertwined tightly into the shell but P(MA-POSS) segments were gathered in the inner core. Whereas, in MEK, because the heavy P(MA-POSS) segments were easy to aggregate but the POSS initiator did not move easily into the inner core due to space effects, the three-layer-structured micelles were formed, as shown in the scheme in Fig. 7b.

Although XPS analysis for Sample 3 revealed that P(MA-POSS) segments in the three solvents of THF, CHCl₃ and MEK easily migrated to the film surface, due to higher Si content on the film surface (4.62–6.01%) than the powder (2.75%) (Fig. 7d), THF solution was the best solvent for the migration of P(MA-POSS) segments to obtain a higher Si content surface (6.01%) than films casted from CHCl₃ and MEK (4.83% and 4.62%). In fact, this strong migration will result in increasing the surface roughness of the film. Therefore, compared with convexes/heaves on the surface with 30 nm roughness casting from THF solution (Fig. 4c, Ra = 1.12 nm), the film casted from CHCl₃ and MEK solutions presented a rather smooth surface as 5 nm roughness in Fig. 7e (Ra = 0.29 nm) and Fig. 7f (Ra = 0.57 nm).

For the surface wettability of these films, Table 3 shows that the film casted from the THF solution had higher water–hexadecane contact angles (114°/54°) and lower surface free energy (17.48 mN m⁻¹) than the film casted from CHCl₃ (104°/22°, 25.94 mN m⁻¹) and MEK (100°/20°, 26.74 mN m⁻¹) solutions. This is not only because 1.12 nm Ra casting from THF solution (Fig. 4c) was much higher than casting from CHCl₃ and from MEK, but also because THF film had much higher Si content (6.01%) than the other two films (4.83% and 4.62%). This corresponds to the results that increasing P(MA-POSS) segment tends to form higher roughness due to the migration of P(MA-POSS) onto the film surface for Samples 2–4 (Table 3). Therefore, although the roughness of film casted from MEK (0.57 nm) was higher than that from CHCl₃ (0.29 nm), the Si content on the film surface casted from CHCl₃ (4.83%) was higher than MEK (4.62%), the SCA value of CHCl₃-casting film was higher than the MEK-casting film. These results suggest that the higher roughness and higher Si content on the film surface significantly contributed to high contact angles.

3.5 Surface water adsorption of films

Surface water adsorption of films (Sample 3) is evaluated by QCM-D measurement in Fig. 8. Δf in the adsorption curves is used to indicate the adsorbed amounts of probe liquids, ΔD/Δf at the end of water absorption is used to indicate the viscoelasticity of the adsorbed layer (the higher ΔD/Δf value indicating the softer adsorbed layer) and calculated Δm is used to express the amount of adsorption. The QCM-D result for Sample 1 (s-POSS-PMMA, THF) is used for comparison. There are two equilibrium processes of Δf and ΔD for s-POSS-PMMA film in Fig. 8a. The 5–50 min process shows that the film could provide obvious resistance to water adsorption as the stable structure. However, the process of increasing Δf and increasing ΔD at 50–65 min shows that the structure of film collapsed and reconstructed to reach another equilibrium at 65 min. For Sample 3 casting from THF solution in Fig. 8b with water absorption after air equilibrium, a seeking balance process of water adsorption and viscoelasticity of the adsorbed layer is found at 9–12 min by a dynamic changing process of increasing ΔD with decreasing Δf until an exceeding equilibrium process is maintained due to the formation of a stable structure. The amount of water absorption in Sample 3 (Δm = 4600 ng cm⁻²) and the viscoelasticity (ΔD/Δf = −0.19) is much lower than in Sample 1 (Δm = 7900 ng cm⁻², ΔD/Δf = −0.075), which proves that the introduction of P(MA-POSS) into the copolymer in Sample 3 could provide the film obvious resistance to water adsorption and maintain a stable film than Sample 1 after water absorption. For the film casted from CHCl₃ solution (Sample 3, Fig. 8c), the water adsorption and viscoelasticity of the adsorbed layer reaches equilibrium at 30 min with Δm = 3800 ng cm⁻² and ΔD/Δf = −0.36, showing the least water adsorption and the lowest viscoelasticity. In Fig. 8d, for the film casted from MEK solution (Sample 3), a dynamic-seeking balance process occurs at 8–60 min with water adsorption increasing with decreasing Δf. The water adsorption (Δm = 6500 ng cm⁻²) and viscoelasticity (ΔD/Δf = −0.15) of the adsorbed layer is higher than the film casted from CHCl₃ (Δm = 3800 ng cm⁻² and ΔD/Δf = −0.36) but lower than the film casted from THF solution (Δm = 4600 ng cm⁻², ΔD/Δf = −0.19). Comparatively, the time required by the films to reach equilibrium after water absorption are 12 min, 30 min and 60 min, respectively. The film casted from CHCl₃ solution gives the lowest water adsorption and the lowest wettability state (Δm = 3800 ng cm⁻² and ΔD/Δf = −0.36) but a lesser stability of the film after water absorption. Whereas, the film casted from MEK solution is the worst for the highest water adsorption (Δm = 6500 ng cm⁻²) and viscoelasticity (ΔD/Δf = −0.15).

Fig. 8 QCM-D data of Δf, ΔD, ΔD/Δf and Δm on the surface of Sample 1 in THF (a), Sample 3 in THF (b), CHCl₃ (c), MEK (d) solution.

4 Conclusions

Star-shaped POSS diblock copolymers of s-POSS-PMMA_277.3-b-P(MA-POSS)_{5.8,16.4,25.4} are synthesized by using 16-arm POSS (POSS-(Br)₁₆) to initiate MMA and PA-POSS. The effect of MA-POSS content and solvents on the properties of star-shaped diblock copolymers and the self-assembled films is conducted. MA-POSS could obviously improve the surface roughness and water–hexadecane contact angles of the films, and the thermal stability and storage modulus of the copolymers. This 16-arm, star-shaped POSS diblock copolymer can be used as a solvent-dependent coating material based on the properties of the effect of solvents on the self-assembled films. Since different micelles are formed in THF, CHCl₃ and MEK solutions, different surface properties are obtained because THF could produce a film with the highest surface roughness and Si content, and therefore the lowest surface free energy; CHCl₃ could provide the film with the lowest water absorption and viscoelasticity but MEK enables the production of the highest water absorptive and viscoelastic film.

Acknowledgements

This work has been financially supported by the National Basic Research Program of China (973 Program, no. 2012CB720904), by the National Natural Science Foundation of China (NSFC Grants 51373133, 51073126, 21171138), the State Administration of Cultural Heritage (20110128) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (SRFDP no. 20130201110033). The authors also wish to express their gratitude to the MOE Key Laboratory for Non-equilibrium Condensed Matter and Quantum Engineering of Xi'an Jiaotong University.

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