Synthesis and characterization of side-chain liquid-crystalline block-copolymers containing laterally attached photoluminescent quinquephenyl units via ATRP

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Synthesis and characterization of side-chain liquid-crystalline block-copolymers

containing laterally attached photoluminescent quinquephenyl units via ATRP

Ling-Yung Wang, Kuang-Chieh Li, Hong-Cheu Lin

*

Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, Taiwan 30049, ROC

a r t i c l e

i n f o

Article history: Received 24 July 2009 Received in revised form 23 October 2009 Accepted 24 October 2009 Available online 10 November 2009 Keywords:

Side-chain liquid-crystalline polymer Atom transfer radical polymerization (ATRP) Block-copolymer

a b s t r a c t

A series of novel side-chain liquid-crystalline polymers (SCLCPs) consisting of laterally attached photo-luminescent p-quinquephenyl (QQP) pendants with different flexible terminal- and/or side-alkoxy chains were synthesized via atom transfer radical polymerization (ATRP). Homopolymers (HP1–HP3) and block-copolymers (PSP1–PSP3 and PEOP1–PEOP3), where QQP units were copolymerized with styrene or ethylene oxide monomers, possessed the number average molecular weights (Mn) of 8.7– 26.0 103with narrow PDI values of 1.08–1.26. Various characterization techniques of polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were used to investigate their mesomorphic properties, and all homopolymers and block-copolymers exhibited the nematic phase affected by the flexible terminal- and/or side-alkoxy chains of the conjugated rod-like pendants. In addition, the photophysical properties of these polymers were measured by UV–vis and photoluminescence (PL) spectroscopies, which showed blue PL emissions with rather high fluorescence quantum yields in solutions.

1. Introduction

Self-organization into a particular order is a general phenom-enon in nature. Block-copolymers (BCPs) present one of the most straightforward models to achieve self-organized nanostructures, which can lead to the formation of a variety of structures such as cylinders, lamellas, and spheres, etc[1]. There are several major classes of synthetic materials that can readily accomplish BCPs with various phase structures, such as amorphous (coil–coil) [2,3], crystalline–amorphous (or crystalline–crystalline) [4–6], and liquid-crystalline (LC) structures[7,8]. LC BCPs have been an active research target in creating different hierarchical structures with unique mesomorphic and mechanical behaviors for several past decades. Numerous LC BCPs have been focused on the LC molecular architectures, which can be divided into main-chain [9–11]and side-chain LC BCPs[12–23], as well as BCPs made of mesogenic jacketed LC polymers (MJLCPs)[24–30]. However, most researches of LC BCPs concentrated on ‘‘longitudinally linked’’ side-chain liquid-crystalline polymers (LCPs) forming a comb-like fashion whose termini of rigid rods in mesogenic units were attached to the polymer backbones through completely ﬂexible spacers[12–23]. Side-chain LCPs consisting of cyanophenyl [12–15], azobenzyl

[16–19], and (n-alkoxyphenyl)benzyl [20–23] moieties have been usually studied in different applications of mesomorphic, electro-optical, and microphase-separated structural properties, respectively. For example, Yu et al. revealed that well-deﬁned conjugated-LC block-copolymers with side-chain LCPs exhibited the properties that energy transfer from the LC mesogens to the conjugated oligomers[12]. Pugh et al. found that a series of poly-norbornenes (PNBEs) laterally attached side-chain LC polymers by using different symmetrically di-substituted mesogens exhibited the tilted layer structure of a smectic C (SC) phase at room

temperature and a nematic (N) phase at higher temperatures, which were characterized by polarizing optical microscopy (POM) and X-ray diffraction (XRD) experiments [22]. Based on these concepts, the subject of functional materials can be set more clearly to be explored for new types of block-copolymers with novel mesomorphic and optical properties in this work.

To investigate the structures of LC block-copolymers, many research groups have synthesized several varieties of living or controlled polymerization methods, such as cationic, ring-opening metathesis, and anionic living polymerizations[31–34]. Recently, the controlled (‘‘living’’) radical polymerizations of atom transfer radical polymerization (ATRP) and nitroxide-mediated radical polymerization have been used to synthesize side-chain LCPs[25– 37]. Because the ATRP method can be handled easily and applied to a wide number of monomers and lead to special polymer archi-tectures with narrow polydispersities [38–40], the successful *Corresponding author. Tel.: þ8863 5712121x55305; fax: þ8863 5724727.

E-mail addresses:[email protected],[email protected](H.-C. Lin).

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j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / p o l y m e r

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polymerization technique was chosen to develop new functional block-copolymers for this research. For instance, a new class of side-on LCP-containing block-copolymers with different hydro-philic/hydrophobic ratios were developed by Li et al. via ATRP method[65].

An approach to increase the rigidity of the polymers, particu-larly at high temperatures, was to introduce oligo-phenyl segments into the rigid cores of the polymer backbones[41–49]. Thus, the presence of p-quinquephenyl (QQP) chromophores in the polymer structures was of considerable interest for the creation of new promising multifunctional (including photoluminescent) materials. Besides, such chromophores bearing alkoxy-substituted side groups not only showed increased solubility and high modulus at high temperatures but also exhibited a different phase behavior [44]. For example, Zhou et al. reported that p-terphenyl groups with different symmetrical alkoxy terminal substituents exhibited different mesophases, regardless of the lengths of the tails [26]. Furthermore, because of the strong

p

–

p

interactions between the conjugated benzene rings favoring the self-organization of mole-cules, these types of chromophores possessed the multiple prop-erties of photoluminescence, thermally stable liquid crystallinity, and microphase-separated behavior in such systems[45–49].

Herein, we report the synthesis of novel photoluminescent monomers with laterally connected pendent QQP moieties substituted by different alkoxy groups at both ends and central sides, the ATRP polymerizations of LC homopolymers and their functional block-copolymers (Chart 1) with narrow polydispersities initiated by different ﬂexible macroinitiators, i.e., poly(ethylene oxide) (PEO) and polystyrene (PS). The phase transition and mesomorphic properties (investigated by POM, DSC, and temper-ature-variable XRD) as well as the UV–vis and PL properties of the rod-coil block-copolymers will be evaluated.

2. Experimental 2.1. Measurements

1_{H NMR spectra were recorded on a Varian Unity 300 MHz}

spectrometer using CDCl3and DMSO solvent. Elemental analyses

were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Transition temperatures were determined by differential scanning calorimetry (Perkin–Elmer Pyris 7) with a heating and cooling rate of 5_{C/min. Thermogravimetric analysis (TGA) was conducted with}

a TA instrument Q500 at a heating rate of 20_{C/min under nitrogen.}

Gel permeation chromatography (GPC) analysis was conducted on a Waters 1515 separation module using polystyrene as a standard

and THF as an eluent. UV–vis absorption spectra were recorded in dilute THF solutions (106_{M) on a HPG1103A spectrophotometer,}

and ﬂuorescence spectra were obtained on a Hitachi F-4500 spec-trophotometer, where the excitation wavelengths of PL were ca. 290 nm. Fluorescence quantum yields were determined by using a 9,10-diphenylanthracene standard (106M in cyclohexane,

F

F¼ 90%)[50]. Polymer solid ﬁlms were spin-coated (at 1000 rpm)

on quartz substrates for 1 min from THF solutions with a concen-tration of 10 mg/mL and dried under vacuum at 50_{C for 3 h. LC}

textures were studied via a polarizing optical microscope (POM, model Leica DMLP) coupled with a hot stage. Synchrotron powder X-ray diffraction (XRD) measurements were performed at beamline BL17A of the National Synchrotron Radiation Research Center (NSRRC), Taiwan, where the wavelength of X-ray was 1.33361 Å. The XRD data were collected using Mar345 image plate detector mounted orthogonal to the beam with sample-to-detector distance of 250 mm, and the diffraction signals were accumulated for 3 min. The powder samples were packed into a capillary tube and heated by a heat gun, whose temperature controller is programmable by a PC with a PID feedback system. The scattering angle theta was calibrated by a mixture of silver behenate and silicon. The self-assembling surface morphology was investigated by the tapping mode of atomic force microscopy (AFM) at room temperature using a Digital Instruments Nanoscope IIIa microscope, where the sample was prepared by dipping a 0.5 wt% copolymer solution (in toluene) onto a Si wafer and then annealed under vacuum at 170_{C for 2 h}

then cooling slowly to 100_{C for 1 day.}

2.2. Materials

Chemicals and solvents were reagent grades and purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Dichloro-methane, THF, and triethylamine were distilled to keep anhydrous before use. Catalyst copper (I) bromide was puriﬁed successively by stirring in acetic acid and ethanol, and then dried [51]. Styrene monomers were distilled under nitrogen over calcium hydride and 2,6-di(tert-butyl-4-methyl)phenol as an inhibitor before use. All of the other chemicals and solvents were used as received.

2.3. Synthesis of polymers

The synthetic routes of macroinitiator 14–15 are shown in Scheme S1. Homopolymers (HP1–HP3) and diblock-copolymers (PSP1–PSP3 and PEOP1–PEOP3) were synthesized by using the analogous procedure except for the utilization of different initiators and monomers via ATRP inScheme 1.

O O (CH2)6 O OR2 R₁O OR₁ O O m HP1 : R1 = C8H17 ; R2 = CH3 HP2 : R1 = C8H17 ; R2 = C8H17 HP3 : R1 = CH3 ; R2 = C8H17 O (CH2)6 O O 92 2 2 OR2 R1O OR1 2 2 PSP1 : R1 = C8H17 ; R2 = CH3 PSP2 : R1 = C8H17 ; R2 = C8H17 PSP3 : R1 = CH3 ; R2 = C8H17 PEOP1 : R1 = C8H17 ; R2 = CH3 PEOP2 : R1 = C8H17 ; R2 = C8H17 PEOP3 : R1 = CH3 ; R2 = C8H17 O O (CH2)6 O O O OCH2CH2 H3C 44 OR2 R1O OR1 2 2 m m

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2.3.1. Macroinitiator polystyrene-Br (14)

In a Schlenk ﬂask, 3.46 mg of N,N0_,N0_,N00

-pentamethyldiethylene-triamine (PMDETA, 0.02 mmol),14.3 mg of CuBr (0.1 mmol), and 5.0 g of styrene (48 mmol) were added and stirred for 30 min. 74 mg of 1-(1-bromoethyl)benzene (0.4 mmol) was added, and the mixture was immediately frozen in liquid nitrogen under vacuum. After several freeze-thaw cycles, the ﬂask was sealed under vacuum and put in an oil bath at 100_{C for 20 h. The content was dissolved in chloroform,}

and the chloroform solution was precipitated into methanol after being concentrated. The precipitation was repeated three times, and the ﬁnal product was dried at 50_{C under vacuum. Yield: 55%.}

Mn¼ 9716 g/mol and PDI (Mw/Mn) ¼ 1.22 (by GPC).

2.3.2. Macroinitiator PEO-Br (15)

A solution of 1.8 g (7.7 mmol) of 2-bromo-2-methylpropionyl chloride in 10 mL of dry THF was added to a mixture of 1.1 g (10 mmol) of triethylamine and 10 g (5 mmol) of poly(ethylene glycol) methyl ether (with Mn¼ 2000) in 30 mL of THF at 0C, and

then the mixture was stirred for 18 h. After the mixture was ﬁltered, half of the solvent was evaporated, and poly(ethylene glycol) macroinitiator was precipitated into cold ether. After dissolving in ethanol, the solution was stored in refrigerator to re-crystallize to yield a white solid. Yield: 55%.1H NMR (ppm, CDCl3),

d

: 1.94 (s, 6H),

3.38 (s, 3H), 3.54–3.76 (m, 174H), 4.33 (dd, 2H). 2.3.3. Polymerization of homopolymer HP1

7.2 mg (0.05 mmol) of CuBr, 7.4

m

L (9.8 mg, 0.05 mmol) of ethyl 2-bromoisobutyrate (EBriB), and 852 mg (1 mmol) of monomer M1 were mixed and ﬁlled with nitrogen. 21

m

L (17 mg, 0.1 mmol) of

PMDETA in 4 mL of THF was added through a syringe. The mixture was degassed three times using the freeze-pump-thaw procedure and sealed under vacuum. After stirring for 30 min at room temperature, the reaction mixture was placed in a preheated oil bath at 100_{C for 24 h. The solution was passed through a neutral}

Al2O3column with THF as an eluent to remove the catalyst. The

white ﬁltrate was concentrated under reduced pressure and re-precipitated twice into a mixed solvent of EA and methanol (EA/ methanol ¼ 1/1). The white product of polymer was collected by ﬁltration and dried under vacuum. Yield: 300 mg (35%). Mn¼ 10791 and PDI (Mw/Mn) ¼ 1.08 (by GPC).

2.3.3.1. HP2. Homopolymer HP2 was synthesized according to the procedure described for polymer HP1 except for the utilization of another monomer M2. Yield: 314 mg (33%). Mn¼ 10884 and PDI

(Mw/Mn) ¼ 1.08 (by GPC).

2.3.3.2. HP3. Homopolymer HP3 was synthesized according to the procedure described for polymer HP1 except for the utilization of another monomer M3. Yield: 241 mg (32%). Mn¼ 11358 and PDI

(Mw/Mn) ¼ 1.09 (by GPC).

2.3.4. Polymerization of block-copolymer PSP1

23 mg (0.16 mmol) of CuBr, 388 mg (0.04 mmol) of macro-initiator 14, and 853 mg (1 mmol) of monomer M1 were mixed and ﬁlled with nitrogen. 83.5

m

L (69.3 mg, 0.4 mmol) of PMDETA in 4 mL of THF was added through a syringe. The mixture was degassed three times using the freeze-pump-thaw procedure and sealed under vacuum. After stirring for 30 min at room

PEOP1 : R1 = C8H17 ; R2 = CH3 PEOP2 : R1 = C8H17 ; R2 = C8H17 PEOP3 : R1 = CH3 ; R2 = C8H17 CuBr / PMDETA THF / 100¢J O O (CH2)6 O OR2 R1O OR1 HP1 : R1 = C8H17 ; R2 = CH3 HP2 : R1 = C8H17 ; R2 = C8H17 HP3 : R1 = CH3 ; R2 = C8H17 92 Br O (CH2)6 O OR2 R1O OR1 O O (CH2)6 O OR2 R1O OR1 CuBr / PMDETA THF / 100¢J CuBr / PMDETA THF / 100¢J M1 M2 M3 14 15 O O Br OCH2CH2 H3C O O m O 92 O O OCH2CH2 44 H3C 44 O O Br PSP1 : R1 = C8H17 ; R2 = CH3 PSP2 : R1 = C8H17 ; R2 = C8H17 PSP3 : R1 = CH3 ; R2 = C8H17 m m

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temperature, the reaction mixture was placed in a preheated oil bath at 100_{C for 24 h. The solution was passed through a neutral}

Al2O3column with THF as an eluent to remove the catalyst. The

white ﬁltrate was concentrated under reduced pressure and re-precipitated twice into methanol. The white product of polymer was collected by ﬁltration and dried under vacuum. Yield: 307 mg (36%). Mn¼ 24380 and PDI (Mw/Mn) ¼ 1.26 (by GPC).

2.3.4.1. PSP2. Block-copolymer PSP2 was synthesized according to the procedure described for block-copolymer PSP1 except for the utilization of another monomer M2. Yield: 295 mg (31%). Mn¼ 26026 and PDI (Mw/Mn) ¼ 1.24 (by GPC).

2.3.4.2. PSP3. Block-copolymer PSP3 was synthesized according to the procedure described for polymer PSP1 except for the utilization of another monomer M3. Yield: 234 mg (31%). Mn¼ 24886 and PDI

(Mw/Mn) ¼ 1.21 (by GPC).

2.3.5. Polymerization of block-copolymer PEOP1

23 mg (0.16 mmol) of CuBr, 80 mg (0.04 mmol) of macroinitiator 15, and 853 mg (1 mmol) of monomer M1 were mixed and ﬁlled with nitrogen. 83.5

m

L (69.3 mg, 0.4 mmol) of PMDETA in 4 mL of THF was added through a syringe. The mixture was degassed three times using the freeze-pump-thaw procedure and sealed under vacuum. After stirring for 30 min at room temperature, the reaction mixture was placed in a preheated oil bath at 100_{C for 24 h. The}

solution was passed through a neutral Al2O3column with THF as an

eluent to remove the catalyst. The white ﬁltrate was concentrated under reduced pressure and re-precipitated twice into methanol. The white product of polymer was collected by ﬁltration and dried under vacuum. Yield: 264 mg (31%). Mn¼ 10083 and PDI (Mw/

Mn) ¼ 1.10 (by GPC).

2.3.5.1. PEOP2. Block-copolymer PEOP2 was synthesized accord-ing to the procedure described for polymer PEOP1 except for the utilization of another monomer M2. Yield: 275 mg (29%). Mn¼ 9884 and PDI (Mw/Mn) ¼ 1.12 (by GPC).

2.3.5.2. PEOP3. Block-copolymer PEOP3 was synthesized accord-ing to the procedure described for polymer PEOP1 except for the utilization of another monomer M3. Yield: 226 mg (30%). Mn¼ 8784 and PDI (Mw/Mn) ¼ 1.13 (by GPC).

3. Results and discussion 3.1. Synthesis and characterization

The synthetic routes of macroinitiators 14–15 and monomers M1–M3 with different alkoxy tail lengths are outlined inScheme S1. In this work, Suzuki coupling reaction was utilized to prepare the QQP (p-quinquephenyl) units with different alkoxy tail lengths symmetrically, which has been proven to be very successful in the synthesis of multi-aryl mesogenic systems by Hird et al.[52]ATRP has been proven to be a successful polymerization technique to prepare block-copolymers for a variety of monomers[35–38]. To investigate the properties of block-copolymers containing various LC blocks (composed of monomers M1–M3 with different ﬂexible chain lengths), macroinitiators 14 (with PS block)[53]and 15 (with PEO block)[54]were used to copolymerize with M1–M3 (bearing laterally attached QQP units) to produce LC diblock-copolymers (PSP1–PSP3 and PEOP1–PEOP3). In addition, homopolymers HP1–HP3 consisting of methacrylate monomers M1–M3 were prepared by using EBriB as an initiator. For all polymerization, polymers were prepared by using CuBr and PMDETA as the catalyst and ligand, and the initial monomer concentration [M]0¼ 0.25 M in

THF as the reaction conditions were all kept at 100_{C for 24 h,}

where the speciﬁc experimental conditions are also given in Scheme 1andTable 1.

The relative molecular weights and polydispersity index (PDI) values of the block-copolymers were determined by gel permeation chromatography (GPC) using PS standards, in which THF was used as an eluent. As shown inTable 1, all homopolymers (HP1–HP3) and diblock-copolymers (PSP1–PSP3 and PEOP1–PEOP3) had extended molecular weights (Mn¼ 8784–26026 g mol1) with

narrow PDI values (PDI ¼ 1.08–1.26). The Mnvalue of macroinitiator

14 obtained by GPC was 9716 g mol1_{with a PDI value of 1.28, and}

macroinitiator 15 was acquired from commercially available PEO (Mn¼ 2000 g mol1) with PDI ¼ 1.04. The repeat unit (m value) of

the mesogenic QQP block in each polymer can be estimated by subtraction of the ﬁrst block molecular weight from the ﬁnal molecular weight measured by GPC and NMR (developed by Yang et al.[65]), and their m values are listed inTable 1. Hence, the repeat units (m values) of the mesogenic QQP blocks in polymers are in the order of diblock-copolymers PSP1–PSP3 > homopolymers HP1– HP3 > diblock-copolymers PEOP1–PEOP3, and the molecular weights of all polymers also follow the same order of PSP1– PSP3 > HP1–HP3 > PEOP1–PEOP3.

In this work, all monomers and polymers had good solubility at ambient temperature in common organic solvents, such as THF, EA, and dichloromethane. Since the monomers had poor solubility in methanol, the specially prepared solvent (EA/methanol ¼ 1/1) was chosen to remove un-reacted monomers in the normal precipitation process. The chemical structures of the monomers and block-copolymers were conﬁrmed by1H NMR spectroscopy, and some representative1H NMR spectra of monomer (M1), homopolymer (HP1), and diblock-copolymers (PSP1 and PEOP1) are shown in Fig. S1 of the supporting information. The spectral assignments clearly supported the proposed structures: M1 showed character-istic proton peaks of the vinyl groups at 5.50–6.06 ppm (denoted as h and i inFig. S1(a)), the sharp and clearly separated proton reso-nances of monomers disappeared after polymerization inFig. S1(c– d), while broad and overlapped resonances of LC mesogens attached to polymeric backbones appeared at nearly the same positions. 3.2. Thermal properties and phase behavior

The thermal stability of the polymers (HP1–HP3, PSP1–PSP3 and PEOP1–PEOP3) was determined by thermogravimetric Table 1

Molecular weights and yields of ATRP results for homopolymers (HP1–HP3) and block-copolymers (PSP1–PSP3 and PEOP1–PEOP3).

Sample [M]0/[I]0/[C]0/[L]0a Mnb PDI (Mw/Mn)b m (repeat unit of QQP mesogens)c Yield (%) HP1 20/1/1/2 10791 1.08 13 35 HP2 20/1/1/2 10884 1.08 11 33 HP3 20/1/1/2 11358 1.09 15 32 PSP1 25/1/4/10 24380d _1.26 ₁₇ ₃₆ PSP2 25/1/4/10 26026d _1.24 ₁₇ ₃₁ PSP3 25/1/4/10 24886d _1.21 ₂₀ ₃₁ PEOP1 25/1/4/10 10083e _1.10 ₉ ₃₁ PEOP2 25/1/4/10 9884e _1.12 ₈ ₂₉ PEOP3 25/1/4/10 8784e _1.13 ₉ ₃₀

a_{Feed molar ratio; [M], monomer; [I], initiator; [C], catalyst; [L], ligand.} b _{Molecular weights and polydispersity index (PDI) values were measured by} GPC, using THF as an eluent, polystyrene as a standard. Mn, number average molecular weight; Mw, weight average molecular weight.

c _{m values are based on the number average molecular weights M}

nmeasured by GPC. Molecular weights of M1–M3 are 852, 950, and 754 g/mol, respectively.

d _{As measured by GPC, the number average molecular weight (M}

n) of the PS block (macroinitiator 14) was 9716 g/mol.

e _{As acquired from commercially available PEO, the number average molecular} weight (Mn) of the PEO block (macroinitiator 15) was 2000 g/mol.

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analysis (TGA) under nitrogen, which indicated that all polymers exhibited the degradation temperatures (Td) higher than 375C (5%

weight loss under nitrogen, as shown inTable 2). Generally, the Td

values of three series homopolymers and block-copolymers could be roughly differentiated by the side-chain molecular components. The Tdvalues of homopolymers (HP1–HP3) were slightly higher

than those of block-copolymers (PSP1–PSP3 and PEOP1–PEOP3) even if the molecular weights of PSP1–PSP3 were larger than those of HP1–HP3. This is due to the rigid molecular arrangements from the single side-chain moiety in homopolymer system. However, the molecular arrangement was dispersed slightly by the introduction of block moieties PS and PEO, and the Tdvalues of

block-copoly-mers PSP1–PSP3 and PEOP1–PEOP3 were reduced. To compare the Tdvalues in the identical series of polymers, polymers bearing

longer lateral-alkoxy chains (HP3, PSP3, and PEOP3) exhibited lower Tdvalues[64]. The phase and glass transition temperatures of

all polymers were characterized by polarizing optical microscopy (POM) and DSC traces (the seconding heating and ﬁrst cooling scans), which are illustrated inTable 2andFigs. S2 and S3 of the supporting information. The mesophases of diblock-copolymers were also conﬁrmed by POM, for instance, homopolymer HP1 displayed the nematic phase with dark threads by cooling the sample below the clearing temperature (TNI¼ 135C) as shown in

Fig. S3. All polymers illustrated the nematic phase which were adopted from the mesomorphic properties of QQP units. The ranges of the nematic phase for all polymers are in the order of polymer series 1 (polymers containing monomers M1) > 3 (poly-mers containing mono(poly-mers M3) > 2 (poly(poly-mers containing mono-mers M2), which could be realized by the trend of the similarity in rod-like shape for calamitic LC mesogens in monomers: M1 > M3 > M2. Hence, it can be realized that the long peripheral ﬂexible chains (–OC8H17) on both longitudinal termini and lateral

sides of the rigid pendent rods would interfere with the rod-like shape in the molecular architectures of M2 and its related polymer series 2 (i.e., HP2, PSP2, and PEOP2 containing M2), and thus to be detrimental to their mesophase and have the narrowest nematic LC ranges in all polymers. Furthermore, the mesomorphic properties of block-copolymers PSP1–PSP3 and PEOP1–PEOP3 were compared with those of homopolymers HP1–HP3. Due to the disturbance of the other non-mesogenic coil-blocks of PS and PEO was surveyed, and the mesophasic ranges of all polymers follows

the order of: homopolymers (HP1–HP3) > block-copolymers (PSP1–PSP3 with larger m values and longer QQP blocks) > block-copolymers (PEOP1–PEOP3 with smaller m values and shorter QQP blocks), respectively.

Glass transition temperatures (Tg) of polymer series 1 and 3

containing monomers M1 and M3 (except PEOP3) were easy to be identiﬁed in DSC results, which are in the range of 60–77_C.

However, glass transition temperatures of polymer series 2 con-taining monomer M2 are not detectable as the alkoxy groups on the terminal ends and lateral sides were all elongated to octyloxy (–OC8H17) chains, but sharper melting peaks were observed in this

series. Except for two endothermic transitions (including the respective crystal-nematic and nematic-isotropic phase transi-tions) observed in polymer series 2 containing monomer M2, all block-copolymers displayed a nematic-isotropic phase transition endotherm, which were similar to homopolymers (HP1–HP3) containing LC QQP units. Because of the smallest molecular weights and shortest QQP blocks of block-polymers PEOP1–PEOP3 in cor-responding polymers, the lowest isotropization temperatures were observed inTable 2. For example, TNIof PEOP1 (131.7C) was lower

than those of HP1 and PSP1 (151.4 and 139.4_{C, respectively).}

However, even though PS block-copolymers had much higher molecular weights and longer QQP blocks than homopolymers, block-copolymers PSP1–PSP3 had lower isotropization tempera-tures than homopolymers HP1–HP3. It could be owing to the decreased stability of the mesophases by connecting PS block in the micro-domain architectures of the diblock-copolymers. In addition, the lowest isotropization temperatures of polymer series 2 con-taining monomer M2 were observed in comparison with those of analogous polymers in series 1 and 3 (seeTable 2). Furthermore, an interesting point of the DSC results is that the enthalpies of both endothermic and exothermic transitions were similar and small. Compared with the conventional transition enthalpy changes of low ordered N, SA, or SCphase to isotropic phase, the DSC results in

Table 2are quite smaller[56,57]. It could be suggested that if the thermal behavior is correlated to an order–disorder transition attributed to the supramolecular structure change, such as block-copolymers in colloids, and the transition detected by DSC must be associated with a small change of molecular interactions (an enthalpy term)[58,59].

3.3. X-ray measurements

To further identify the phase behavior revealed by DSC and POM measurements, X-ray diffraction (XRD) experiments were carried out to give more mesomorphic properties of molecular packings. Fig. S4 of the supporting information shows the plots of X-ray intensity (I) versus angle (2

q

) for some examples of polymer series 2 (i.e., HP2, PSP2, and PEOP2). Hence, the wide-angle X-ray studies on polymer samples supported more deﬁnitive information about the nematic liquid-crystalline phase. Irrespective of chain archi-tectures in homopolymers HP2 or block-copolymers PSP2 and PEOP2, all polymers possessed almost identical XRD patterns, which were not affected by the coil-blocks of PS and PEO units in the block-copolymers PSP2 and PEOP2. Regardless of the lengths of mesogenic QQP units with either long tails (–OC8H17) or short tails

(–OCH3), the XRD results of all polymers showed a broad

wide-angle peak around 2

q

¼ 16–17 _{(correlated to d w 4.7 Å), which}

corresponded to the intermolecular distance between the meso-genic units. Moreover, different longitudinally linked side-chain liquid-crystalline block-copolymers in our previous report also showed a similar broad wide-angle peak around 2

q

¼ 16–17

(correlated to d w 4.5 Å) to illustrate the intermolecular distance between the mesogenic units of the smectgens.[14,63]This inter-esting result suggested that the compactness of lateral packing of Table 2

Phase behavior and thermal properties of homopolymers and block-copolymers.

Sample

Phase transitions (_C)a,b

(corresponding enthalpy changes, J g1₎

Tg(C)a Td(C)c Heating Cooling HP1 g 63.3 N 151.4 (0.8) I I 149.3 (1.0) N 63.3 391 HP2 Cr 97.5 (19.6) N 107.7 (1.0) I I 106.6 (1.2) N 62.4 (9.5) Cr nde ₃₉₁ HP3 g 60 N 128.7 (0.8) I I 127.4 (1.3) N 60.0 378 PSP1 g 65 N 139.4 (0.6) I I 138.2 (0.6) N 65.0 386 PSP2 Cr 90.2 (5.4) N 97.7 (0.7) I I 96.7 (0.8) N nde ₃₈₇ PSP3 g 77.3 N 126.5 (0.6) I I 123.9 (0.6) 77.3 380 PEOP1 g 65.4 N 131.7 (0.6) I I 130.9 (0.5) N 117.2 (0.2) Cr 65.4 379 PEOP2 Cr 93.4 (11.3) N 98.8 (0.6) I I 97.6 (0.7) N 62.4 (6.4) Cr nde ₃₈₁ PEOP3 Cr 95.0d_{N 104.9 (0.4) I} _{I 103.4 (0.4) N} _nde ₃₇₅

a_{Transition temperatures (}_{C) and enthalpies (in parentheses, J g}1_{) were} measured by DSC (at a heating and cooling rate of 5_C/min).

b _{Cr ¼ crystalline; N ¼ nematic phase; I ¼ isotropic phase.}

c _{Temperature (}_{C) at 5% weight loss measured by TGA at a heating rate of 20}_C/ min under nitrogen.

d _{Determined by POM.} e _{nd ¼ Not detected.}

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polymer backbones does not seriously affect this correlation length between mesogens in different mesophases.

Another important result is thatFig. S4demonstrates another moderate sharper peak at the low-angle region around 2

q

¼ 2–3

(correlated to d ¼ 25–38 Å). However, the nematic liquid-crystal-line phase of longitudinally linked side-chain liquid-crystalliquid-crystal-line copolymers in our previous reports did not show the low-angle peak[20,60]. These low-angle XRD peaks inFig. S4were related to the interchain packing distance of the polymer backbones induced by the jacket-like arrangements of mesogens in the nematic phase, and similar phenomena were also observed in the previously reported mesogenic jacketed LC polymers (MJLCPs) [61,62]. Therefore, the low-angle XRD peaks of homopolymers HP2 (2

q

¼ 2.39_{), block-copolymers PSP2 (2}

_q

_{¼ 2.21}_{), and PEOP2}

(2

q

¼ 2.30) inFig. S4had the d-spacing values in the sequence of HP2 < PEOP2 < PSP2, because the coil-blocks of PEO and PS units in block-copolymers PEOP2 and PSP2 would expand the interchain packing distance of the polymer backbones to different extents. Moreover, in contrast to the low-angle XRD peaks at 2

q

¼ 4.9–5.1

(correlated to d ¼ 18.0–17.2 Å) for MJLCPs reported in Refs.61 and 62, another evidence of the larger interchain packing distance (d ¼ 38–25 Å) of the polymer backbones in our polymers (HP2, PSP2, and PEOP2) were revealed owing to our longer spacers (i.e., CO–O–C6H12–O) between the polymer backbones and mesogenic

units. Furthermore, due to thermal expansion of the interchain

packing distance of the polymer backbones and the intermolecular distance between mesogenic units, both low- and wide-angle XRD peaks of homopolymer HP1 inFig. 1shifted slightly to smaller 2

q

angles (correlated to larger d-spacing values) upon heating. The relative orientation of side groups versus the backbones should determine the mesophasic structures of the polymers. In our systems, these side-chain polymers with laterally attached rigid rod-like mesogenic groups exhibited remarkable differences in phase behavior from conventional SCLCPs on the basis of ﬂexible or semiﬂexible backbones, suggesting the formation a nematic liquid-crystalline phase.

3.4. Optical properties

The UV–vis absorption and photoluminescence (PL) data of polymers HP1–HP3, PSP1–PSP3, and PEOP1–PEOP3 were measured in both solutions (ca. 1 106_{M in dilute solutions of}

good solvent THF) and solids, where the excitation wavelengths of PL were ca. 290 nm, and the photophysical properties are summarized inTable 3. As shown inFig. 2, HP1–HP3 possessed almost identical UV–vis absorption and PL spectra. Thus, two maxima were obtained in the absorption spectra: the one at ca. 332 nm was a typical peak for substituted quinquephenyl moieties [55,61], and the other one at ca. 287 nm was a superposition absorption band originated from the quinquephenyl unit. In the PL

2 4 6 8 10 12 14 16 18 20 22 24 Intensity (a. u. ) 2 theta (Deg.) 30 °C 115 °C 130 °C 145 °C

Fig. 1. X-ray diffraction (XRD) patterns of polymer HP1 at various temperatures during the heating scan.

Table 3

Absorption and PL emission spectral data of polymers in THF solutions and solid ﬁlms.

Sample Absorptionlabs(nm) PL emissionlmax(nm) F(Solution)b Solutiona _{Solid ﬁlm} _Solutiona _{Solid ﬁlm}

HP1 292, 332 292, 340 404 407 0.76 HP2 287, 332 287, 343 405 406 0.67 HP3 292, 333 292, 340 404 407 0.65 PSP1 286, 332 286, 339 404 408 0.72 PSP2 288, 333 288, 342 405 407 0.69 PSP3 288, 328 288, 339 405 407 0.68 PEOP1 288, 332 288, 341 404 407 0.68 PEOP2 287, 333 287, 341 404 406 0.51 PEOP3 287, 333 287, 342 404 406 0.70 a_{Absorption and PL emission spectra were measured in THF solutions at room} temperature, where the excitation wavelengths of PL were ca. 290 nm.

b_{Fluorescence quantum efﬁciencies of solutions were measured in THF, relative} to 9,10-diphenylanthracene (FPL¼ 0.90).

a

300 350 400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Absorbance (a. u.)

Wavelength (nm) HP1 HP2 HP3

b

400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Normalized PL Wavelength (nm) HP1 HP2 HP3

Fig. 2. Normalized (a) UV–vis absorption spectra and (b) PL spectra of homopolymers HP1–HP3 in THF solutions.

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spectra, a blue fluorescence maxima at ca. 404 nm and a shoulder at ca. 420 nm were observed, which belong to PL emission charac-teristics of the quinquephenyl moiety[55,61]. As shown inTable 3, all polymers showed blue PL emissions with rather high fluores-cence quantum yields in solutions.Fig. 3 shows the examples of UV–vis and PL spectra of homopolymer HP1 and diblock-copoly-mers PSP1 and PEOP1. Compared with the UV–vis absorption and PL spectra in THF solutions, the polymers exhibit minor red-shifted effects in solid films, because of the intermolecular

p

–

p

*

aggregation of the rigid cores in photoluminescent quinquephenyl blocks. In contrast to analogous homopolymers, most correspond-ing block-copolymers possessed UV–vis absorption maxima with 0–6 nm blue shifts in both solutions and solid ﬁlms, which might be associated with minor dilution effect of coil-blocks in block-copolymers, and much less blue shifts occurred in polymer series 2 due to the longer peripheral ﬂexible chains (–OC8H17) around

luminescent chromophore units of M2. Nevertheless, due to the same explanation of minor dilution effect of coil-blocks in block-copolymers, no obvious variations in PL emission maxima and ﬂuorescence quantum yields were achieved between analogous homopolymers and block-copolymers. In general, UV–vis absorp-tion and PL spectra of all polymers are quite similar, which are mainly due to the similar photoluminescent rigid cores of quin-quephenyl blocks (even with different ﬂexible alkoxy lengths).

3.5. AFM investigation of self-assembling nanostructures

In order to investigate the self-assembling property of the diblock-copolymers, the surface morphology was measured by AFM as shown inFig. 4. Due to the incompatibility of hydrophilic PEO segments and hydrophobic parts (five-ring rigid cores) in diblock-copolymers, the flexible PEO segments of polymer PEOP3 could be self-aggregated in toluene due to the insoluble parts of hydrophilic PEO segments. As shown in the AFM image, the morphological image presents individual spherical micelles and the core–shell model. The phenomenon of globularity protrusion was obtained and dispersed like the short uninterrupted serpentine pinnacle, which meant the rigid segment of mesogenic rods in PEOP3, and the high-profile (bright) area was indicative of the soft segments of PEO polymer chains (insoluble parts in toluene). As shown inFig. 4, the diameter of the aggregated observation in the AFM image was about 200 nm which is rather heterogeneous depending on the block composition. These results clearly demonstrate the effect of block-copolymer composition on the micellar dimensions. Although QQP chains compared with PEO coils are shorter, the molecular volume fraction of the rod blocks in monomers is larger, resulting in a larger number of aggregation for the micelles and hence self-assembled into larger micelles. These results indicate that the well-documented scaling laws established for block-copolymer micelles are in agreement with the core size

a

300 350 400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Absorption (a. u.)

Wavelength (nm)

UV-Vis (solutions) UV-Vis (solid films) PL (solutions) PL (solid films)

b

300 350 400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Absorption (a. u .) Wavelength (nm) UV-Vis (solutions) UV-Vis (solid films) PL (solutions) PL (solid films)

c

300 350 400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

PL intensity (a. u.)

A b s o rptio n ( a . u .) Wavelength (nm) UV-Vis (solutions) UV-Vis (solid films) PL (solutions) PL (solid films)

Fig. 3. (d) UV–vis absorption and (- - -) PL spectra of (a) HP1, (b) PSP1, and (c) PEOP1 in THF solutions and solid ﬁlms.

Fig. 4. AFM image of the self-assembled nanostructure of copolymer PEOP3 (from toluene).

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with respect to the total size, and the number of aggregation mainly depend on the degree of polymerization of the rod blocks. 4. Conclusions

In summary, a series of novel side-chain liquid-crystalline polymers (SCLCPs) were made of laterally attached photo-luminescent p-quinquephenyl (QQP) monomers containing ﬂex-ible terminal- and/or side-alkoxy chains via Suzuki coupling reactions in this research. Using monofunctional polystyrenes and poly(ethylene oxide)s as macroinitiators, QQP-based macromo-lecular architectures such as homopolymers and diblock-copoly-mers with 2.6 104 Mn 8.8 103 and narrow polydispersity

index (PDI) values (1.26 Mw/Mn 1.08) could be successfully

synthesized by atom transfer radical polymerization (ATRP). The nematic LC phase was further confirmed to exist in all homopoly-mers and copolyhomopoly-mers, which were influenced by the long periph-eral flexible chains (–OC8H17) on both longitudinal termini and

lateral sides of the rigid pendent rods. Thus, the ranges of the nematic phase for all polymers are in the order of polymer series 1 (polymers containing monomers M1) > 3 (polymers containing monomers M3) > 2 (polymers containing monomers M2), which could be comprehended by the degree of the similarity in rod-like shape for calamitic LC mesogens in monomers: M1 > M3 > M2. Furthermore, due to the disturbance of the other non-mesogenic coil-blocks of PS and PEO units, the mesophasic ranges of all polymers follows the order of: homopolymers (HP1–HP3) > block-copolymers (PSP1–PSP3 with larger m values and longer QQP blocks) > block-copolymers (PEOP1–PEOP3 with smaller m values and shorter QQP blocks), respectively. Besides mesomorphic properties, all polymers had almost identical maximum UV–vis absorption and blue PL emission wavelengths originated from their similar rod-like conjugated units in dilute solutions and solid ﬁlms, but a red-shifted effect in solid ﬁlms occurred due to intermolecular

p

–

p

*_{aggregation of the rigid cores. The nematic (N) phase was also}

proven by XRD patterns so that the low-angle XRD peaks (2

q

¼ 2– 3_{) were related to the interchain packing distance of the polymer}

backbones induced by the jacket-like arrangements of mesogens in the nematic phase. The wide-angle XRD peaks (2

q

¼ 16–17₎

illus-trated the intermolecular distance between the mesogenic units. Therefore, the mesomorphic and photophysical properties of the corresponding LC homopolymers and block-copolymers with laterally attached photoluminescent mesogenic units were ﬁrst explored here.

Acknowledgements

The powder XRD measurements are supplied by beamline BL17A (charged by Dr. Jey-Jau Lee) of the National Synchrotron Radiation Research Center (NSRRC), in Taiwan. The instruments of GPC measurements were provided by Prof. Kung-Hwa Wei (Dept. of Materials Science & Engineering, National Chiao Tung University). Appendix. Supplementary data

Supplementary data associated with this article can be found in the online version, atdoi:10.1016/j.polymer.2009.10.071.

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