Supramolecular Thermotropic Liquid Crystalline Materials with Nematic Mesophase Based on Methylated...

Supramolecular Thermotropic Liquid Crystalline

Materials with Nematic Mesophase Based on

Methylated Hyperbranched Polyethylenimine and

Mesogenic Carboxylic Acida

Yu Chen,1,2 Zhong Shen,2 Lionel Gehringer,2 Holger Frey,*2 Salah-Eddine Stiriba*3

1 Department of Chemistry, Tianjin University, 300072 Tianjin, P.R. China2 Institut fur Organische Chemie, Johannes Gutenberg-Universitat, Duesbergweg 10-14, 55099 Mainz, GermanyFax: (þ49) 6131 3924078; E-mail: [email protected]

3 Instituto de Ciencia Molecular (ICMOL), Universidad de Valencia, Vicent Andres Estelles s/n, Burjassot, 46100 Valencia, SpainFax: (þ34) 96 3543949; E-mail: [email protected]

Received: September 7, 2005; Revised: October 31, 2005; Accepted: November 2, 2005; DOI: 10.1002/marc.200500628

Keywords: dendrimers; hyperbranched; mesogen carboxylic acid; nematic phase; polyethylenimine; SAXS; self-assembly;supramolecular liquid crystal

Introduction

The construction of supramolecular liquid crystalline (LC)

materials from polymeric backbones by self-assembly is a

topic of great interest in supramolecular chemistry.[1] This

is due to the intriguing properties displayed by these

Summary: Supramolecular interaction of fully methylatedhyperbranched polyethylenimines (PEI) with a mesogen-based carboxylic acid, 5-(p-cyanobiphenoxy)pentanoic acid,results in the formation of supramolecular complexesexhibiting thermotropic liquid crystalline (LC) mesophases.In contrast to the common smectic mesophases of mostdendritic LC polymers, nematic LC phases were observed.The complexation of PEI and the mesogen units is due toelectrostatic interaction between the carboxylate groups andthe ammonium end groups of PEI. LC properties wereinvestigated by a combination of differential scanningcalorimetry, polarizing light optical microscopy, and X-raydiffractometry.

Schematic illustration of the supramolecular assembly ofCBPA with PEIMe backbone.

Macromol. Rapid Commun. 2006, 27, 69–75 � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication DOI: 10.1002/marc.200500628 69

a : Supporting information for this article is available at thebottom of the article’s abstract page, which can be accessedfrom the journal’s homepage at http://www.mrc-journal.de, orfrom the author.

materials and potential applications, like the fabrication of

optical data-storage systems and non-linear optical materi-

als. The specific driving forces for the assembly of the LC

components via non-covalent interactions can be hydrogen

bonding,[2] ionic interaction,[3] or coordination complexa-

tion.[4] Compared with the conventional chemical synthesis

of functional supramolecular materials involving the

formation of covalent bonds, the use of self-assembly in

supramolecular chemistry offers several advantages:

straightforward synthetic methodology, convenient isola-

tion of the products, and often a low number of synthetic

steps. In addition, the resulting structures can be capable of

reversible adaptive rearrangement in response to any

changes in their environment (e.g., solvent or temperature).

Avariety of linear polymers has been used as a backbone,

combined with various low molecular compounds to form

supramolecular side-chain LC polymers.[2–4] However,

less progress has been achieved for branched polymer

topologies in supramolecular LC materials.[5] Compared

with linear polymers, dendrimers[6] and hyperbranched

polymers[7] display compact structures and possess lower

viscosity, which may represent an advantage for potential

applications in this field. Recently, Ujiie et al. have report-

ed the complexation of poly(amidoamine) dendrimers

(PAMAM) with primary amino-terminated groups with

alkanoic acids via proton-transfer reaction, affording ionic

LC dendrimers with smectic A phase as well as disordered

hexagonal columnar mesophases.[5a] In the same context,

ionic thermotropic supramolecular dendritic systems with

liquid crystal potential have been prepared by Serrano et al.

using the spontaneous assembly of carboxylic acids at the

surface of amino-terminated PPI as well as in combination

with PAMAM dendrimers.[5c] Furthermore, commercially

available hyperbranched polyethylenimines (PEIs) have

also been complexed with fatty acids,[5b] however, in

analogy to the ionic LC dendrimers, only lamellar meso-

phases were obtained. Very recently, Tsiourvas and fellow-

workers have reported the preparation of supramolecular

LC materials exhibiting smectic A phases, based on the

complexation of pyridinylated hyperbranched polyglycerol

and cholesterol-based carboxylic acids via hydrogen bond-

ing.[5d] However, due to the smectic A phases, the viscosity

of such dendrimers or hyperbranched polymers is usually

high, which is prohibitive for application. One strategy to

overcome this problem would be the use of less viscous

mesomorphic materials displaying nematic mesophases.[8]

In some recent works, we have been able to demonstrate low

solution viscosites for alkyl-substituted hyperbranched

polyglycerols, which is correlated to their compact

structure.[9] To date, only few macromolecular dendritic

architectures, where nematic phases are displayed, have

been reported.[10] Supramolecular thermotropic LC den-

drimers or hyperbranched polymers exhibiting nematic

mesophases have not been described. Here, we report the

formation of nematic mesophases in supramolecular

complexes prepared from the non-covalent interaction of

methylated hyperbranched (PEIMe)s with a mesogen-

based carboxylic acid, namely, 5-(p-cyanobiphenoxy)pen-

tanoic acid (CBPA).

Experimental Part

Materials

Hyperbranched polyethylenimine PEI10K (Mn ¼ 1�104 g �mol�1, Mw=Mn ¼ 2.5, NH2:NH:N¼ 33:40:27), 18-crown-6 (99%) and 40-hydroxy-4-biphenylcarbonitrile (97%)were purchased from Aldrich. PEI1.8K (Mn ¼ 1 800 g �mol�1,Mw=Mn ¼ 1.14, NH2:NH:N¼ 37:35:28) was purchased fromPolysciences, Inc. Aldehyde (37%), methyl-5-bromopentano-ate (97%), and sodium hydroxide (p.a) were purchased fromAcros. Formic acid (98–100%) was obtained from RiedelDe-Haen AG.

Synthesis of 5-(p-cyanobiphenoxy)pentanoic acid (CBPA)

A mixture of 18-crown-6 (0.51 g, 2 mmol), finely groundpotassium carbonate (6.50 g, 47 mmol), 40-hydroxy-4-biphenylcarbonitrile (5.02 g, 25.7 mmol), and methyl-5-bromopentanoate (6.50 g, 33.7 mmol) in acetone (80 ml) wasrefluxed for 16 h under vigorous stirring. The mixture wascooled and filtered. Subsequently, the solution was concen-trated in vacuum and recrystallized from ethanol, yieldingmethyl-5-(p-cyanobiphenoxy)pentanoate as a white solid.Yield: 7.50 g, 24.4 mmol (95%).

1H NMR (CDCl3): d¼ 7.69 (d, 2H, Ar–H), 7.64 (d, 2H,Ar–H), 7.53 (d, 2H, Ar–H), 6.98 (d, 2H, Ar–H), 4.02 (t, 2H,CH2CH2O), 3.69 (s, 3H, COOCH3), 2.37 (t, 2H, CH2CH2-COO), 1.84 (m, 2H, C2CH2O), 1.73 (m, 2H, CH2CH2COO).

This ester (6.3 g, 20 mmol) was added to a sodium hydroxidesolution (100 ml, 3 N). THF was then added until a homo-geneous suspension was formed. The reaction mixture wasstirred at room temperature for 4 d. After hydrolysis comple-tion, the suspension was neutralized with a solution ofhydrochloric acid (5 N) at 0 8C. The crude product wasobtained as a white precipitate, which was filtered off andwashed with diethyl ether and water. The crude materialwas recrystallized from ethanol to yield the pure acid. Yield:3.61 g, 60%.

1H NMR (d6-DMSO): d¼ 12.04 (br s, 1H, CH2COOH),7.86 (d, 2H, Ar–H), 7.82 (d, 2H, Ar–H), 7.68 (d, 2H, Ar–H),7.03 (d, 2H, Ar–H), 4.00 (t, 2H, CH2CH2O), 2.23 (t, 2H,CH2CH2COO), 1.73 (m, 2H, CH2CH2O), 1.56 (m, 2H,CH2CH2COO).

Synthesis of Fully Methylated PEI

A mixture of 40 ml formaldehyde (37%) and 40 ml formic acidwas stirred at room temperature for 1 h; afterwards a solution of2 g of PEI in 10 ml of deionized water was added dropwise. Themixture was gently refluxed overnight at 95 8C. After cooling atroom temperature, the volatile fractions were removed undervacuum. The resulting brown residue was treated with a

70 Y. Chen, Z. Shen, L. Gehringer, H. Frey, S.-E. Stiriba

Macromol. Rapid Commun. 2006, 27, 69–75 www.mrc-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

saturated sodium hydroxide aqueous solution until pH> 10,and subsequently extracted three times with chloroform. Theorganic phase was dried over anhydrous sodium sulfate,filtered, and evaporated to yield yellow oil. Then the productwas purified by dialysis against chloroform.

1H NMR (CDCl3): d¼ 2.19 (s, CH3N–), 2.21 (s, CH3N–),2.29–2.62 (br, ––CH2CH2N–).

13C NMR (CDCl3): d¼ 42.8, 42.9, 45.8, 52.9, 53.0, 55.9,56.0, 57.2, 57.5, 57.6–60.

Characterization

1H NMR spectra were recorded on a Bruker ARX 300spectrometer, operated at 300 MHz. DSC measurements werecarried out on a Perkin-Elmer 7 series thermal analysis systemin the temperature range �70 to 150 8C at heating rates of10 K �min�1. Melting points are extrapolated to heating rate 0.The melting point of indium (156 8C) was used for calibration.IR spectra were recorded on a Nicolet 5DXC FT-IR spectro-meter. Small-angle X-ray scattering (SAXS) measurements ofthe complex were performed in transmission geometry withBraun Single wire detector (40 kV and 30 mA) using Cu Karadiation (wavelength 0.1542 nm) at room temperature. Datawere recorded by a small angle Kratky compact camera. Wideangle X-ray diffraction (WAXD) of the materials was per-formed in transmission geometry with D500 Simens y/2yganiometer, with single point detector, secondary monochro-

mator, using Cu Ka radiation (wavelength 0.1542 nm) at roomtemperature, and the scan step was 0.028. The textures of themesophases were observed with a polarizing optical micro-scope (Leica ORTHOLUX II POL-BK with Leica DC 200)equipped with a Mettler FP52 hot-stage connected to a MettlerFP5 temperature control unit.

Results and Discussion

Commercially available hyperbranched PEIs containing

primary, secondary, and tertiary amine groups have been

directly mixed with long chain fatty acids and investigated

with respect to their thermal and LC properties, affording

lamellar mesophases.[5b] We were interested in the com-

bination with carboxylic acids comprising calamitic meso-

genic units. It is known that amidation reaction of PEIs with

fatty acids can occur at elevated temperatures (120–

150 8C).[11] Such thermal conditions may occur in the

characterization of the obtained supramolecular LC poly-

mers by means of DSC, TGA, and polarized optical

microscopy (POM). Amidation is undesired, since it would

complicate the characterization results. In order to avoid the

possible amidation side reaction, we transformed the

primary and secondary amines of PEI into tertiary amino-

groups. Two hyperbranched PEI samples with different

Scheme 1. Synthesis of methylated PEI (abbreviated PEIMe) from PEI and comparison of the respective1H NMR spectra.

Supramolecular Thermotropic Liquid Crystalline Materials with Nematic Mesophase Based . . . 71


molecular weight, namely PEI10K (Mn ¼ 104 g �mol�1,

Mw=Mn ¼ 2.5, NH2:NH:N¼ 33:40:27, respectively)

and PEI1.8K (Mn ¼ 1 800 g �mol�1, Mw=Mn ¼ 1.14,

NH2:NH:N¼ 37:35:28, respectively) were methylated in

presence of formaldehyde and formic acid in water as a

solvent to PEI10KMe and PEI1.8KMe derivatives, as

shown in Scheme 1, using a procedure similar to that

developed for the synthesis of tris(2-dimethylaminoethyl)-

amine.[12] The degree of methylation was monitored by 1H

NMR spectroscopy. New signals at 2.12 and 2.13 ppm are

due to the protons of the methyl groups in PEIMe. Full

methylation of the primary and secondary amine groups of

PEI is confirmed by the disappearance of the broad

resonances of primary and secondary amines at 1.67 ppm,

as well as from the integration ratio of methyl and methyl-

ene units.

The THF solution of the mesogen-based carboxylic acid

CBPA was added dropwise into the dichloromethane

solution of the obtained methylated PEIs, PEI10KMe and

PEI1.8KMe, and the molar ratio (x) of CBPA and amine

groups of PEIMe varied in the range of 0.1–0.7. The

mixture was shaken for 1 h and then slowly evaporated at

room temperature. After complete evaporation of the sol-

vents, the resulting complexes (PEI10KMe-CBPAx) and

(PEI1.8KMe-CBPAx) were thoroughly dried. All final

polyelectrolyte-mesogen surfactant complexes were vis-

cous and opaque, except for those prepared with a low

degree of complexation (x¼ 0.1).

The thermal behavior of all polymer complexes was

investigated by combining POM, DSC, and X-ray diffrac-

tion (SAXS and WAXS). The LC behavior of the

supramolecular complexes PEI10KMe-CBPAx and

PEI1.8KMe-CBPAx is evident from the direct observation

using POM, showing fluid, strongly birefringent domains,

which coalesced upon increasing the temperature (Figure

S1, supplementary materials). Except for the complexes

with low degree of complexation (x¼ 0.1), which were

found to be amorphous, all materials exhibit an LC phase at

ambient temperature.

DSC analysis of all complexes and their precursors has

been carried out (Table 1), confirming the results of the

POM observations. The glass transition temperature (Tg) of

each complex (PEIMe-CBPAx) was enhanced in compar-

ison to the methylated PEI, once the fraction of CBPA

component was increased in the complex; however, the Tg

values are still below room temperature, as illustrated in

Figure 1.

The mesogenic precursor CBPA exhibited only a mono-

tropic nematic mesophase on cooling from the isotropic

phase in the range of 103–123 8C, as evidenced by DSC

Table 1. Thermal transition data of PEI10KMe-CBPAx, PEI1.8KMe-CBPAx and CBPA obtained from DSC and POM analysis.

Complexa) Thermal transitions (8C) from DSC[DH (kJ �mol�1 mesogen)]b)

Thermal transitions (8C) from POMe)

Heating Cooling Heating Cooling

CBPA C 139[37.3] I I 121[-1.23] N 107[-31.4] C C 138 i I 123 N 103 CPEI10KMe-CBPA0.1 g -29 I –c) I IPEI10KMe-CBPA0.3 g -8 N 69[0.30] I I �14 gd) N 73 I i 63 NPEI10KMe-CBPA0.5 g -7 N 68[0.96] I I 64[-0.46] N -11 g N 95 I i 84 NPEI10KMe-CBPA0.7 g 1 N 81[2.84] I I 79[-0.27] N -4 g N 98 I i 90 NPEI1.8KMe-CBPA0.1 g -30 I –c) I IPEI1.8KMe-CBPA0.3 g -12 N 61 [0.25] I I �14 gd) N 58 I IPEI1.8KMe-CBPA0.5 g -4 N 60[0.29] I I 45[-0.17] N -7 g N 89 I I 80 NPEI1.8KMe-CBPA0.7 g 4 N 70[0.08] I I 63[-0.08] N -0.6 g N 97 I I 91 N

a) Nomenclature: PEIXMe-CBPAx: PEIMe, methylated hyperbranched polymer; X: Mn of PEI; CBPA: 5-( p-CBPA); x: molar ratio ofCBPA/amine groups of PEIMe.

b) Data were obtained from the second scan; C: crystalline, I: isotropic, N: nematic, g: glass.c) Not observed.d) Transition from isotropic to nematic phase was not observable.e) Heating and cooling rates 2 K �min�1.

Figure 1. Tg of PEI10KMe-CBPAx and PEI1.8KMe-CBPAx as afunction of the molar ratio of CBPA and tertiary amine groups ofPEIME polymers.



and POM. DSC traces of the PEI10KMe-CBPA and

PEI1.8KMe-CBPA complexes prepared with x¼ 0.3–0.7

ratio show a glass-to-mesophase transition and a meso-

phase-to-isotropic liquid transition. Both the transition

enthalpy and the fluid character of the mesophase observed

by POM suggest the presence of nematic order. However,

commonly nematic systems can only be supercooled to

about 1–2 degrees below Tn,i. It should be noted that

the considerable deviation between heating and cooling

(Table 1) is unusual for an isotropic-nematic transition.

Unfortunately, the textures observed by POM remained

unspecific, even after extended periods of annealing close to

the transition temperature. The isotropization temperatures

are PEIMe molecular weight dependent and are indeed

considerably lower than the melting point of pure CBPA.

For all complexes with x� 0.7, the melting endotherm of

pure CBPA was not observed by DSC and POM. This

implies that all CBPA molecules are complexed with the

PEIMe backbone. The complex PEIMe-CBPAx with an

excess of CBPA mesogen (x¼ 1.4) has also been prepared.

In this case, the DSC curves clearly show the melting point

of the CBPA precursor that is due to the expected phase

separation between the excess of free CBPA and the com-

plex PEIMe-CBPA (cf. supplementary material).

FT-IR analysis of the PEIMe-CBPAx complexes has also

been carried out (Figure 2). The band intensity of the cyano

group at 2 223 cm�1 was used as internal reference to

normalize the spectra. The peak at 1 707 cm�1 is character-

istic for the stretching bond frequency n (C O) originating

from self-assembly of the carboxylic acid groups of CPBA.

For the complexes with x¼ 0.1, the band at 1 707 cm�1

showed very low intensity, indicating that the hydrogen

bonds between the carboxylic acid groups of CBPA were

broken and proton transfer took place between the CBPA

carboxylic acid groups and the amine groups of PEIMe,

resulting in electrostatic interaction between PEIMe and

CBPA. With increasing fraction of CPBA a new band cen-

tered at 1 716 cm�1 gradually appeared, considerably

weaker than the band of the carboxylic acid groups of

CBPA at 1 707 cm�1. It should be noted that the frequency

n (C O) of free aliphatic carboxylic acids was usually in the

range of 1 740–1 760 cm�1,[13] thus, the band at 1 716 cm�1

in the complexes with x¼ 0.3–0.7 is assigned to the (C O)

frequency of CBPA carboxylic acids in the complex

structure PEI10KME-CBPAx (Figure 2).

It has been established by previous studies on the pro-

tonation titration of hyperbranched PEIs[14] that primary

amines are protonated first, followed by secondary amines,

and then tertiary ones due to the short-range electrostatic

repulsive interaction mechanism; moreover, full protona-

tion of PEIs can only be achieved when pH is below 2. In our

case, the molar ratio of CBPA to the amine groups of

PEIME is equal to the ratio of CBPA and amino groups

involved in the complexation between both components

(Figure 3).

Further evidence concerning the structure of the LC

phases was obtained from X-ray diffraction analysis of

PEIMe-CBPAx complexes (Figure 4).

The SAXS profile of the complex, PEI10KMe-CBPA0.5

was recorded at room temperature. No scattering peak

corresponding to the long period, i.e., long-range order can

be observed (supplementary material). This observation

rules out the formation of a smectic phase. In the WAXS

Figure 2. FT-IR spectra of PEI10KMe, CBPA, and PEI10KMe-CBPAx complexes.

Figure 3. Schematic illustration of the supramolecular assemblyof CBPA with PEIMe backbone.



profile of the complex, only a diffuse halo-centered at

2y¼ 208 (d¼ 4.4 A) is observed, which is typical for the

lateral distance between the mesogens. These values are in

agreement with literature reports on side-chain polymers

containing cyanobiphenyl mesogens.[15] In summary, these

results support the formation of a nematic LC phase by the

supramolecular complex of CBPA with PEIMe, in agree-

ment with the other experimental data.

Conclusion

Supramolecular complexation of methylated hyperbran-

ched PEIs PEI10KMe and PEI1.8KMe with mesogen-

containing carboxylic acid, CBPA, based on proton transfer

from the acid moieties to the tricoordinated amine-

containing PEIMe enables the formation of supramolecular

complexes exhibiting thermotropic LC phases. The struc-

tral analysis of the mesophases supports nematic order.

Nematic phases are unusual for dendritic LC-polymers and

offer intriguing potential for switching and further electro-

optical application.

Acknowledgements: The Ministerio de Educacion y Ciencia(Spain) is acknowledged for support to (S.-E.S.) through ‘‘Ramony Cajal’’ program. Juana Kirsten is acknowledged for help withPOM-measurements. The Alexander von Humboldt Foundation isalso gratefully acknowledged for support to (S.-E.S) through thefollow-up program.

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Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2005.

Supporting Information for Macromol. Rapid Commun. 2006, 27, 69.

Supramolecular Thermotropic Liquid Crystalline Materials with Nematic Mesophase Based on

Methylated Hyperbranched Polyethylenimine and Mesogenic Carboxylic Acid

Y. Chen, Z. Shen, L. Gehringer, H. Frey,* S.-E. Stiriba*

E-mail : [email protected]

Content:

Figure S1. Polarizing optical microscopy of (a) PEI1.8KMe-CBPA0.5 at 68℃ (heating) ; (b)

PEI10KMe-CBPA0.3 at 27℃ (upon cooling).

Figure S2. DSC thermograms of CBPA and PEI1.8KMe-CBPA complexes obtained in second cooling.

Figure S3. DSC thermograms of CBPA and PEI10KMe-CBPA complexes obtained in second cooling.

Figure S4. SAXS profile of the complex PEI10KMe-CBPA0.5, recorded at room temperature

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Figure S1. Polarizing optical microscopy of (a) PEI1.8KMe-CBPA0.5 at 68℃ (heating) ; (b) PEI10KMe-CBPA0.3 at 27℃ (upon cooling).

Figure S2. DSC thermograms of CBPA and PEI1.8KMe-CBPA complexes.

-40 -20 0 20 40 60 80 100 120 140

x=1.4

x=0.7

T (oC)

Exo

ther

m

CBPA

x=0.5

3

Figure S3. DSC thermograms of CBPA and PEI1.8KMe-CBPA complexes.

-40 -20 0 20 40 60 80 100 120 140

x=0.3

x=0.1

x=0.7

T (oC)

Exo

ther

m

x=0.5

Figure S4. SAXS profile of the complex PEI10KMe-CBPA0.5, recorded at room temperature

1 2 3 4 5 6 7 8 90

1000

2000

3000

4000

5000

Inte

nsity

(a.u

.)

2θθθθ(deg)

Supramolecular Thermotropic Liquid Crystalline Materials with Nematic Mesophase Based on Methylated...

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