Thermal properties and liquid crystallinity of side-chain azobenzene copolymer containing pendant polyhedral oligomeric silsequioxanes

Thermal properties and liquid crystallinity of side-chain

azobenzene copolymer containing pendant polyhedral oligomeric

silsequioxanes

Xiao-Tao Wang•_{Ying-Kui Yang}•_{Zhi-Fang Yang}•

Xing-Ping Zhou• _{Yong-Gui Liao}•_{Chen-Chen Lv} •

Feng-Chih Chang•_{Xiao-Lin Xie}

Received: 4 June 2009 / Accepted: 24 March 2010 / Published online: 8 April 2010 Ó Akade´miai Kiado´, Budapest, Hungary 2010

Abstract Vinylated polyhedral oligomeric silsesquiox-ane (POSS-M) was prepared by the reaction of POSS containing amine groups with acrylic acid. Azobenzene liquid crystalline copolymer (LCP-POSS) was then syn-thesized with 6.0 mol% POSS-M and 94.0 mol% acrylate monomer containing azobenzene liquid crystalline moiety (Azo-M) by free-radical copolymerization. Homopolymer of Azo-M (LCP) was also synthesized under the same conditions. Their thermal properties and liquid crystallinity were characterized by Thermal gravimetric analysis (TG), differential scanning calorimetry (DSC), Wide-angle X-ray diffraction experiments (XRD) and polarized optical micrographs (POM). The results showed that LCP-POSS has higher thermal stability and glass transition tempera-ture than pure LCP due to the incorporation of the rigid cage-like POSS. Especially, LCP-POSS exhibits enantio-tropic smectic and nematic liquid crystalline behaviors, its smectic-nematic transition temperature (TSN) and

nematic-isotropic transition temperature (TNI) are higher

than those of pure LCP, which may promote and extend its applications on stimuli-responsive materials and devices.

Keywords Polyhedral oligomeric silsesquioxanes  Liquid crystalline polymers Azobenzene

Introduction

Polyhedral oligomeric silsesquioxane (POSS) is a unique modifier to high-performance materials due to its hybrid chemical composition with nanosized inorganic silicon and oxygen cage which is surrounded by organic substituents at the corner of silicon atom [1–3]. Since one or more of the substituents can react with organic precursors, while the remaining unreactive groups increase the compatibility of POSS with organic system, POSS molecules have been successfully incorporated into various polymers via copo-lymerization [4], grafting [5,6], or blending [7, 8]. Sub-sequently, it leads to the enhancements in thermal, mechanical, and dielectric properties, flame-retardation, and liquid crystal (LC) phase stability of thermoplastic, thermoset and liquid crystalline polymers [9–23]. Espe-cially, LC hybrids based on POSS have attracted consid-erable interests due to the integrated properties of the organic LC and inorganic silsesquioxane materials. Goodby [17–20] and Laine [21,22] groups investigated the liquid crystalline behaviors and structures of LC-POSS hybrids by appending mesogenic groups on silsesquioxane cores. Their results confirmed that the incorporation of mesogenic group on POSS cores improved the stability of the liquid crystalline phase, and promoted a tendency to form the LC phase even if the cores were irregu-larly substituted. Chujo and co-workers [23] synthesized X.-T. Wang Z.-F. Yang  X.-P. Zhou  Y.-G. Liao (&) 

C.-C. Lv X.-L. Xie (&)

Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

e-mail: [email protected] X.-L. Xie

e-mail: [email protected] Y.-K. Yang

Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, China

F.-C. Chang

Department of Applied Chemistry, National Chiao-Tung University, Hsin Chu, Taiwan, ROC

LC-POSS hybrids with functionalized silsesquioxanes as pendant groups by the free-radical copolymerization of methacrylate monomers containing phenyl LC moiety and POSS macromonomer with single vinyl group. The resulting hybrids with 10 mol% POSS moieties exhibited monotropic liquid crystallinity behavior during the heating cycle with wider temperature range for LC phase, higher glass-transition temperatures and thermal stability than those of LC homopolymer. The other hybrid polymers with more than 10 mol% POSS moiety show more and more enhanced thermal stability with increasing the content of POSS [23].

Azobenzene side-chain liquid crystalline polymers (LCPs) have attracted increasing attentions due to the photore-sponsive properties of azo-group and high optical anisot-ropy in liquid crystalline state [24]. Recently, our group have synthesized multiwalled carbon nanotubes (MWNTs) grafted by azo side-chain liquid crystalline polyurethanes (AzoPU-MWNTs) [25]. The thermal stability of the azo polyurethanes is enhanced by the incorporation of MWNTs. To the best of our knowledge, there is no report on azobenzene liquid crystalline copolymer containing POSS.

In this study, we synthesized an azobenzene side-chain liquid crystalline copolymer containing pendant POSS by a free-radical copolymerization of 6.0 mol% vinylated POSS macromonomer (POSS-M) and acrylate monomer con-taining azobenzene LC moiety (Azo-M), its thermal properties and liquid crystallinity were investigated. The confinement of the low content of POSS might enhance the thermal and the liquid crystal phase stability to some extent, which may promote and extend its applications on stimuli-responsive materials and devices.

Experimental section

Materials

Aminopropylisobutyl polyhedral oligomeric silsesquioxane (POSS) was obtained from Hybrid Plastics, Inc. Dimeth-ylformamide (DMF) was purified by vacuum distillation before use. High-purity azobis(isobutyronitrile) (AIBN) was recrystallized from 95% ethanol. Spectroscopic grade tetrahydrofuran (THF) and toluene were pre-dried by 4 A˚ molecular sieves, and distilled from sodium benzophenone ketyl immediately prior to use. N,N0 -dicyclohexylcarbodi-imide (DCC) and 4-dimethylaminopyridine (DMAP) were purchased from Chemical Reagent Co., Ltd, Jiangsu, China. Other chemicals were used as received from Sin-opharm Group Chemical Reagent Co., Ltd, Shanghai, China.

Measurements

Fourier transform infrared (FTIR) spectra were recorded on a Bruker Equinox 55 spectrometer with a disc of KBr.1H nuclear magnetic resonance (1H-NMR) spectra were car-ried out on a Bruker AV400 spectrometer using tetra-methylsilane (TMS) as an internal standard. Molecular weights were measured by Agilent 1100 gel permeation chromatography (GPC) with PS as the standard and THF as the eluent. Thermal gravimetric analysis (TG) was con-ducted on a TGA-7 Perkin-Elmer calorimeter under argon flow (20 mL/min) at a rate of 10 K/min. Differential scanning calorimetry (DSC) was carried out in an argon atmosphere with a PE DSC-7 at a rate of 20 K/min. The optical pictures of the samples in solvents were taken with a digital camera (Olympus C-4000 ZOOM). Wide-angle X-ray diffraction experiments (XRD) were carried out on powder with a Philips X0 Pert diffractometer with Cu Ka radiation 1.541 A˚ , scanning from 2h of 2°–40° with step size 0.02° and time per step 4 s.

Synthesis of azobenzene LC monomer

The azobenzene LC monomer, 6-(4-methoxy-40 -oxy-azo-benzene) hexyl methacrylate (Azo-M), was synthesized according to reference [26]. Yield 21.1%. IR (KBr, cm-1): 3100–3050 (t C–H, aromatic), 2980–2850 (t C–H, CH2,

and CH3), 1707 (t C=O), 1640 (t C=C), 1600–1450 (t

C=C, aromatic), 1450–1380 (d C–H), 1250–1040 (t C–O). 1H NMR (400 MHz, CDCl3, d, ppm): 7.7–7.95 (4H, Ar–

H), 6.90–7.0 (4H, Ar–H), 6.1 (2H, CH2=C–), 4.2 (2H,

CH2–OOC–), 4.0 (2H, ArOCH2), 3.9 (3H, ArOCH3), 1.2–

2.0 (11H, –CH2(CH2)4CH2–, CH3C(COO)).

Synthesis of POSS macromonomer (POSS-M)

Polyhedral oligomeric silsesquioxane macromonomer (POSS-M), acrylaminopropyl-heptaisobutyl polyhedral olig-omeric silsesquioxane, was synthesized by the condensation reaction of aminopropylisobutyl POSS and acrylic acid as shown in Scheme1.

In detail, POSS (100 mg, 0.114 mmol) and acrylic acid (10 mL) were dissolved in dried THF, followed by the addition of DCC (24 mg, 0.116 mmol) and DMAP (1 mg, 0.0088 mmol). The reaction mixture was stirred at 303 K

R=CH2CH(CH3)CH3 O Si O Si O Si O Si O Si O Si O Si O Si R R R R R R R O _O O O NH2 + OH O DCC/DMAP 30°C/24 h O Si O Si O Si O Si O Si O Si O Si O Si R R R R R R R O _O O O N H O

for 24 h. The product was then poured into excess distilled water under vigorous stirring. After filtration, the product was purified with distilled water again to remove excessive acrylic acid. The crude product was purified by column chromatography using dichloromethane as eluent to yield 70.1 mg. The final product was dried in a vacuum oven, giving 60.5 g of white powders. Yield 57.1%. IR (KBr, cm-1): 3300 (t N–H), 2980–2850 (t C–H, CH2, and CH3),

1633 (t C=O), 1577 (t C=C), 1450–1380(d C–H), 1250– 1040 (t C–O), 1109 (t Si–O–Si). 1H NMR (400 MHz, CDCl3, d, ppm): 6.2–6.4(1H, –NHOCH=CHH cis), 6.0–6.2 (1H, –NHOCH=CH2), 5.6–5.7 (1H, –NHOCH=CHH

trans), 5.5–5.6 (1H, –NHOCH=CH2), 3.2–3.4 (2H, –CHO

NHCH2–), 1.8–2.0 (7H, –CH2CH–), 1.6–1.7 (2H, –SiCH2

CH2–), 0.8–1.0 (42H, –CH2CH(CH3)2), 0.5–0.7 (16H,

–SiCH2–).

Synthesis of side-chain liquid crystalline azobenzene copolymer containing pendant POSS (LCP-POSS)

Liquid crystalline azobenzene copolymer containing pen-dant POSS (LCP-POSS) was synthesized by a conventional free-radical copolymerization of POSS-M and Azo-M, as shown in Scheme2.

In detail, a mixture of POSS-M (547 mg, 0.59 mmol), Azo-M (3.738 g, 9.44 mmol) and 20 mL dried THF was added in a 150 mL three-neck flask. The mixture was evacuated by nitrogen thrice, and subsequently stirred at 333 K. Then, AIBN (4 mol% based on the monomer) was added to the above mixture as an initiator, and the copo-lymerization continued for 48 h. The hybrid polymer was precipitated as a yellow solid in an excess of methanol, then purified thrice by re-dissolution in THF, re-precipi-tation in methanol, and dried under reduced pressure overnight at room temperature, giving 2.05 g of yellow powders. Yield 48.2%. IR (KBr, cm-1): 3300 (t N–H), 2980–2850 (t C–H, CH2, and CH3), 1727 (t C=O), 1600–

1450 (t C=C, aromatic), 1450–1380 (d C–H), 1250–1040 (t C–O), 1109 (t Si–O–Si).1H-NMR (400 MHz, CDCl3, d,

ppm): 8.0–8.1(–NHCO–), 7.7–7.95 (Ar–H), 6.90–7.0 (Ar– H), 4.0–4.2 (–CH2COO–, ArOCH2), 3.85–3.95 (ArOCH3),

2.9–3.0 (–NHCH2–), 2.6–2.8 (–CH2CONH–), 1.0–1.8

(–CH2–, –CH3), 0.5–0.7 (–SiCH2–).

For comparison, homopolymer of Azo-M (LCP) was also synthesized under the same conditions. IR (KBr, cm-1): 3100–3050 (t C–H, aromatic), 2980–2850 (t C–H, CH2, and CH3), 1707 (t C=O), 1600–1450 (t C=C,

aro-matic), 1450–1380 (d C–H), 1250–1040 (t C–O).1H NMR (400 MHz, CDCl3, d, ppm): 7.7–7.95 (4H, Ar–H), 6.90–

7.0 (4H, Ar–H), 4.0–4.2 (4H, –CH2OOC–, ArOCH2–),

3.85–3.95 (3H, ArOCH3), 1.0–1.8 (10H, –CH2–).

Results and discussion

Molecular weight and composition of LCP-POSS

Molecular weights of LCP and LCP-POSS were measured by GPC. Table1 summarizes the molecular weight and composition of LCP-POSS hybrid polymer. Apparently, when the feed molar ratio of POSS-M and Azo-M is 1:16, LCP-POSS has a slightly higher molecular weight than LCP, i.e., 13,580 and 11,670, respectively. The results may be related to their lower reactivity derived from the bulk-iness of the POSS macromonomer [23] and a radical deactivation by the azobenzene sidegroup, respectively [27]. Similar phenomena have been observed in the copolymers with different feed ratio of POSS-M and LC monomer, 6-[4-(40-cyanophenyl) phenoxy] hexyl methac-rylate [23]. Based on 1H-NMR spectrum of LCP-POSS (Fig.1b), molar ratio (x/y) of POSS and LC units in LCP-POSS can be calculated by the peak integration areas of the methylene protons connected with amide (k (2H, –COONHCH2–) 4.0–4.2 ppm) and phenyl protons (a (4H,

Ar–H) 7.7–7.95 ppm and b (4H, Ar–H) 6.90–7.0 ppm).

Thermal stability of LCP-POSS polymer

Figure2 shows TG and DTG curves of LCP and LCP-POSS. Similar to the previous report [28], decomposition of LCP shows three regimes, corresponding to the cleavage of N=N bonds in azobenzene side chains of LCP at 533– 643 K, the breakdown of ester bonds combined with

O O N N H3CO AIBN 65°C/72 h O O N N OCH3 O Si O Si O Si O Si O Si O Si O Si O Si R R R R R R R O O O O N H O x y R=CH2CH(CH3)CH3 O O POSS-M +

Scheme 2 Synthesis of LCP-POSS

Table 1 Molecular weight, composition, and TG results of LCP-POSS

Run Feed ratioa Mnb Mwc T10/K Tmax/K

LCP – 3,696 11,670 600 636

LCP-POSS 1:16 3,780 13,580 621 646 a _{Molar ratio of POSS-M to Azo-M monomers used in synthesis} b _{Number-average molecular weight by GPC}

aliphatic flexible spacers in side chains at 643–763 K, and the degradation of main chains of LCP at 763–1,053 K. Interestingly, LCP-POSS exhibits higher thermal stability than LCP even the molar content of POSS is only 6.0%. The 10% mass-loss temperature (T10) of LCP-POSS

increases to 621 K from 600 K of LCP, and the maximum mass-loss temperature (Tmax) of LCP-POSS also increases

to 646 K from 636 K of LCP. In other word, LCP-POSS were more thermally robust with the incorporation of the inorganic POSS nanoparticles.

LC behavior of LCP-POSS polymer

Figure3 shows the polarized optical micrographs (POM) of LCP and LCP-POSS at different temperatures. Similar to other azobenzene side-chain liquid crystalline copoly-mers with the flexible spacers (CH2)n (n [ 4) [29, 30],

LCP exhibits a nematic phase with a schlieren texture at 363 K (Fig.3a) and a smectic phase with a broken-fan texture at 338 K (Fig.3b). The incorporation of the rigid cage-like POSS does not change the liquid crystalline structure of LCP-POSS, only decreases the size of its liquid crystal domains in nematic (Fig.3c) and smectic (Fig.3d) phases due to the confinement of POSS on the formation of liquid crystal phases. From XRD patterns of POSS, LCP, and LCP-POSS shown in Fig.4, sharp diffraction peaks appear at 2h = 8.2, 11.0 and 19.4°, corresponding to d-spacings of 10.7, 8.0, and 4.6 A˚ for POSS, indicative of a structure with high crystallinity. The peak corresponding to a d-spacing of 10.7 A˚ is attributed to the size of POSS molecule, while the other two peaks are attributed to the rhombohedral crystal structure of POSS molecules [31]. However, only a broad peak at 2h = 20° appears for LCP, indicative of an amorphous structure of LCP at room 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 a b _d h i g f _k j l e m n o ppm c O O N N OCH3 O Si O Si O Si O Si O Si O Si O Si O Si R R R R R R R O O O O N H O x y R=CH2CH(CH3)CH3 m n o o O m l k f h j j i c eg ged a b b a d 1.0 2.0 3.0 4.0 5.0 6.0 i 5.50 6.00 a ppm f e gd c h b f e _{g d} O Si O Si O Si O Si O Si O Si O Si Si R H2 C R R R R R R O O O O C H2 H2 C N H C O R=CH2CH(CH3)CH3 a b c dH H H e f g a h i i C a b

Fig. 1 1_{H NMR spectra of POSS-M and LCP-POSS}

0 20 40 60 80 100 LCP-POSS Weight/ % Temperature/K LCP a 473 673 873 1073 –15 –10 –5 0 LCP-POSS LCP d W /d T Temperature/K b 473 673 873 1073

Fig. 2 TG (a) and DTG (b) curves of LCP and LCP-POSS

Fig. 3 Polarized optical micrographs of LCP at 363 K (a) and 338 K (b), LCP-POSS at 363 K (c), and 338 K (d)

temperature. For LCP-POSS, the weak broad peak at 2h = 11.8°, concerning POSS crystallinity may show the presence of small or disordered regions of POSS crystals [32]. The peak at 2h = 20° suggests that LCP-POSS is still amorphous as LCP at room temperature. The heating and cooling DSC curves of LCP and LCP-POSS are shown in Fig.5. Based on the POM and XRD results, the LCP exhibits a glass-transition temperature (Tg) at 323.9 K,

smectic–nematic transition temperature (TSN) at 341.7 K,

nematic–isotropic transition temperature (TNI) at 379.2 K

during heating. The glass-transition temperature was obtained from the inflection point. For LCP-POSS, the incorporation of 6.0 mol% POSS units into LCP increases Tg, TSNand TNIto 336.9, 347.3, and 386.9 K (increment of

5.6–13.0 K), respectively. Although all the increments of the thermal transition temperature are not much, LCP-POSS hybrid polymer exhibits an enantiotropic liquid crystalline behavior as LCP.

However, LC-POSS copolymer of 10 mol% POSS ma-cromonomer with single vinyl group and methacrylate monomers containing biphenyl LC moiety only exhibits monotropic liquid crystallinity during the heating cycle [23]. These suggest that the incorporation of POSS units (i.e., 6.0 mol%) into LCP does not destroy the liquid crystalline order of LCP-POSS, and increases Tgand the

liquid crystalline phase transition temperature (Ti). In this

case, the rigid POSS units physically restrict the motion of polymer segments, and the dipole–dipole interaction between POSS siloxane and the polar carbonyl of AzoM moiety also plays a positive role on the enhancements of Tg. The effect of POSS on Tg of the polymer has been

investigated in detail [33]. The phenomenon that the incorporation of POSS to LC system exhibited higher Ti

has been observed in LC-POSS system as well. Goodby et al. synthesized two organic–inorganic hybrid materials containing a central POSS core by reacting octavi-nylsilsesquioxane with a three or four aromatic rings liquid crystal [17]. Compared with the LC, the LC-POSS with four aromatic rings showed that a rise of isotropization temperature and smectic C phase temperature by 22 K (from 397.0 to 419.0 K and 53 K (from 333.2 to 386.2 K), respectively. The three aromatic rings LC-POSS exhibited 11.4 K higher isotropization temperature (from 312.3 to 323.7 K). And, an underlying smectic phase was observed from both LC-POSS materials. They attributed this pro-motion of the stability of the mesomorphic state to the incorporation of a suitably functionalized inorganic core. In addition, in this case, the slightly higher molecular weight of LCP-POSS might benefit the increase of Ti, which has

been described elsewhere [34].

Conclusions

Due to the confinement of the rigid cage-like POSS, LCP-POSS exhibits higher glass-transition temperature and thermal stability than LCP homopolymer. Especially, LCP-POSS exhibits enantiotropic smectic and nematic liquid crystalline behaviors, its smectic–nematic transition tem-perature (TSN) and nematic–isotropic transition temperature

(TNI) are higher than those of pure LCP. A further study

about the effect of POSS content in LCP-POSS copolymer on liquid crystallinity and thermal stability is in progress.

10 20 30 POSS Intensity/cps 2θ/ o LCP LCP-POSS

Fig. 4 X-ray diffraction curves of POSS, LCP, and LCP-POSS

LCP LCP-POSS Heat flow/mW Temperature/K Tg336.9 Tg323.9 386.9 347.3 341.7 379.2 a 273 293 313 333 353 373 393 LCP-POSS LCP Heat flow/mW Temperature/K b 378.5 337.0 373.1 333.9 273 293 313 333 353 373 393

Fig. 5 DSC heating and cooling curves of LCP and LCP-POSS at a rate of 20 K/min

Acknowledgements We are grateful for the financial support pro-vided by the Outstanding Youth Fund of the National Natural Science Foundation of China (50825301), Natural Science Foundation of Hubei Province (2009CDB257), and Open Fund of State Key Labo-ratory of Plastic Forming Simulation and Die and Mould Technology of HUST. We are also grateful for Analytical and Testing Center of Huazhong University of Science and Technology.

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