Synthesis and characterization of a novel amphiphilic copolymer capable as anti-biofouling coating...

Designed Monomers and Polymers 15 (2012) 159–174brill.nl/dmp

Synthesis and Characterization of Novel AmphiphilicAzo-Polymers Bearing Well-Defined

Oligo(Ethylene Glycol) Spacers

Tonatiuh García a, María del Pilar Carreón-Castro b, Adriana Gelover-Santiago a,

Patricia Ponce a,d, Margarita Romero c and Ernesto Rivera a,∗

a Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México,Circuito Exterior, Ciudad Universitaria, CP 04510, México DF, México

b Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México,Circuito Exterior, Ciudad Universitaria, CP 04510, México DF, México

c Facultad de Química, Universidad Nacional Autónoma de México, Circuito Exterior,Ciudad Universitaria, CP 04510, México DF, México

d Facultad de Ciencias Químicas, Universidad Juárez del Estado de Durango, Av. Veterinaria s/n,Circuito Universitario, CP 34120 Durango, Durango, México

AbstractFour novel azo-polymers bearing oligo(ethylene glycol) spacers were synthesized and characterized.Monomers were prepared by esterification of the RED-PEG dyes previously synthesized by us: N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(5-hydroxy-3-oxapentas-1-yl)amine (RED-PEG-2), N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(8-hydroxy-3,6-dioxaoctas-1-yl)amine (RED-PEG-3), N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(11-hydroxy-3,6,9-trioxaundecas-1-yl)amine (RED-PEG-4) and N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(17-hydroxy-3,6,9,12,15-pentaoxa-heptadecas-1-yl)amine (RED-PEG-6) with methacryloyl chloride. The obtained monomers (nPEGMANseries) were polymerized in the presence of AIBN in THF to give the corresponding azo-polymers(pnPEGMAN series). Also, Langmuir films were prepared with these polymers; the obtained monolay-ers exhibited good reversible behavior upon successive compression/expansion cycles.© Koninklijke Brill NV, Leiden, 2012

KeywordsAzo-polymers, amino–nitro-substituted azobenzene unit, oligo(ethylene glycol) spacers, Langmuir films

1. Introduction

Azopolymers are considered as highly versatile materials due to the photoin-duced motions, which occur on them when they are irradiated with laser polarized

* To whom correspondence should be addressed. Tel.: (52-55) 5622-4733; Fax: (52-55) 5616-1201; e-mail:[email protected]

© Koninklijke Brill NV, Leiden, 2012 DOI:10.1163/156855511X615047

160 T. García et al. / Designed Monomers and Polymers 15 (2012) 159–174

light [1]. Several reviews covering most of the implications of azobenzene in poly-mer structures have been published [1–4].

Several azopolymers have been synthesized and characterized for the develop-ment of novel materials bearing NLO properties, sensing properties, superior ther-mal properties, etc. For instance, second-order NLO properties of polymers con-taining 4-dialkylamino-4′-(alkylsulfonyl)azobenzenes have been recently reportedin the literature [5]. Concerning sensing properties, these were studied in pyrenecontaining azopolyurethanes by means of fluorescence spectroscopy [6]. On theother hand, the photophysical properties of azobenzene have been combined withthe thermal stability of inorganic moieties such as siloxanes (–Si–O–Si–) for thepreparation of new colored hybrid networks with enhanced thermal properties [7,8]. Another interesting example is the use of benzoxazines containing azobenzenein the synthesis of cross-linked polymer networks with high thermal stability [9].

In the last years, various azopolymers bearing amino–nitro-substituted azoben-zene units, such as those of the pnMAN series, have been synthesized and charac-terized [10]. In general, they exhibit maxima absorption wavelength close to thosereported for similar push-pull azo-compounds [11, 12]. In these materials, both J-and H-type aggregation have been observed in casted films [10].

In previous works, we reported the synthesis, characterization and optical prop-erties of a series of novel amphiphilic amino–nitro-substituted azobenzenes bear-ing end-capped oligo(ethylene glycol) side-chains (RED-PEGM series) [13–15].Azobenzene and poly(ethylene glycol) have been incorporated into various sophis-ticated systems such as co-polymers [16, 17], nanomaterials [18, 19], cellulosederivatives [20, 21] and cyclodextrin polymers [22, 23], in some cases formingsupramolecular complexes with interesting properties [24]. Poly(ethylene gly-col) moieties provide flexibility and water solubility to the systems to whichthey are incorporated. Furthermore, we published the synthesis and characteriza-tion of four novel azo-dyes bearing terminal hydroxyl groups (RED-PEG series):N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(5-hydroxy-3-oxapentas-1-yl)amine (RED-PEG-2), N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(8-hydroxy-3,6-dioxaoctas-1-yl)amine (RED-PEG-3), N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(11-hydroxy-3,6,9-trioxaundecas-1-yl)amine(RED-PEG-4) and N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(17-hydroxy-3,6,9,12,15-pentaoxaheptadecas-1-yl)amine (RED-PEG-6). These dyeswere incorporated into a low-density polyethylene (LDPE) matrix, using acry-loyl chloride as grafting agent and gamma radiation, in order to obtain a seriesof grafted azopolymer films containing oligo(ethylene glycol) segments (AC-g-PE-RED-PEG series). These materials showed high moisture sensitivity, giving rise tosolvatochromic effects upon exposure to steam, which make them good prospectsfor the elaboration of humidity sensors [25].

Herein, we report the synthesis and characterization of a new series of polymeth-acrylate-based azopolymers bearing well-defined oligo(ethylene glycol) spacers,named pnPEGMAN. According to this nomenclature, p means polymer, n indi-

T. García et al. / Designed Monomers and Polymers 15 (2012) 159–174 161

Figure 1. Structure of the pnMAN and pnPEGMAN polymers.

cates the number of ethylene glycol units in the spacer, PEG indicates the presenceof a short poly(ethylene glycol) segment, M means methacrylate and AN indicatesthe presence of an amino–nitro-substituted azobenzene unit. The structures of thepnPEGMAN and the pnMAN series are shown in Fig. 1. In this work, we synthe-sized four different pnPEGMAN polymers with n = 2,3,4 and 6. Besides, a modelpolymer containing Disperse Red-1 units (PMDR-1) was also prepared and used asreference [26]. The synthesis of the pnPEGMAN polymers is illustrated in Fig. 2(vide infra); their thermal and optical properties were studied. Due to their am-phiphilic behavior, pnPEGMAN polymers can easily form Langmuir films at theair–water interface.

2. Experimental

All the reagents used in the synthesis were purchased from Aldrich and used asreceived. RED-PEG-2, RED-PEG-3, RED-PEG-4 and RED-PEG-6 have been syn-thesized according to the method previously reported by us [25]. The preparationof the nPEGMAN monomers and the pnPEGMAN polymers is illustrated in Fig. 2(vide infra). The synthesis of the precursor RED-PEG dyes was achieved follow-ing the same method as used for the synthesis of the RED-PEGM dyes [13]. Thespectroscopic characterization of these compounds has been already reported by us[25].

https://www.researchgate.net/publication/231706907_Azo_Polymers_for_Reversible_Optical_Storage_10_Cooperative_Motion_of_Polar_Side_Groups_in_Amorphous_Polymers?el=1_x_8&enrichId=rgreq-f2bfda9cbb3b27cb5daea838fbdb5d84-XXX&enrichSource=Y292ZXJQYWdlOzI0MTY4NDc5ODtBUzoyMzE3NTA1ODg2OTQ1MjhAMTQzMjI2NTA1NDAzOA==


Figure 2. Synthesis of the pnPEGMAN polymers.

2.1. Synthesis of the nPEGMAN Monomers

RED-PEG dye (2.28 mmol) was dissolved in freshly distilled THF (12 ml) underargon atmosphere; then triethylamine (0.33 g, 3.32 mmol) was added with a syringeto the solution. The mixture was cooled in an ice bath and methacryloyl chloride(0.29 g, 2.76 mmol) dissolved in THF (4 ml) was added drop-wise via an additionfunnel. The reaction mixture was stirred for 24 h at room temperature. The resultingproduct was extracted with chloroform, dried with anhydrous MgSO4 and concen-trated at reduced pressure. Then, the crude product was purified by flash column


chromatography on silica gel, using an appropriate mixture of hexane and acetoneas eluent. Pure nPEGMAN monomers were obtained as dark red solids with goodyields. FT-IR spectra of all monomers are very similar. IR (KBr): ν = 3090 (s, C–Haromatic and vinylic), 2961 (s, CH2), 2924 (s, CH2 and CH3), 1727 (s, C=O), 1603(s, C=C aromatic), 1516 (s, NO2), 1447 (s, N=N), 1378, 1337 (s, C–O of the es-ther), 1261 (s, C–N), 1099 (s, O–CH2), 856 (out-of-plane, =CH2 vinylic) and 802(out-of-plane, =C–H aromatic) cm−1.

2.1.1. 2PEGMAN or(E)-2-(2-(Methyl(4-((4-Nitrophenyl)Diazenyl)Phenyl)Amino)Ethoxy)EthylMethacrylate

Yield: 80% (eluent: hexane/acetone 9:1). 1H-NMR (CDCl3, 400 MHz) (seeScheme 1): δ = 8.28 (d, 2H, J = 9.6 Hz, H4); 7.71 (d, 2H, J = 8.7 Hz, H3);7.67 (d, 2H, J = 8.1 Hz, H2); 6.68 (d, 2H, J = 9.3 Hz, H1); 6.02 (s, 2H, H5); 5.57(s, 1H, H6); 4.26 (t, 2H, J1 = 4.8 Hz, J2 = 4.8 Hz, COO–CH2); 3.74 (m, 14H, allOCH2 and NCH2); 3.19 (s, 3H, CH3–N); 1.87 (s, 3H, =C–CH3) ppm. 13C-NMR(CDCl3, 100 MHz) (see Scheme 1): δ = 186.3 (1C, C=O), 156.76 (1C, Ce), 152.54(1C, Ca), 147.40 (1C, Ch), 143.82 (1C, Cd), 136.12 (1C, C= of the methacrylate),126.05 (2C, Cc), 125.81 (1C, C= of the methacylate), 124.87 (2C, Cg), 122.59 (2C,Cf), 111.52 (2C, Cb), 72.49, 68.57 (2C, all OCH2), 63.76 (1C, CH2OCO), 52.18(1C, N–CH2), 39.32 (1C, CH3–N), 18.27 (CH3 of the methacrylate) ppm.

Scheme 1. Assignment of the signals for nPEGMAN monomers.


2.1.2. 3PEGMAN or(E)-2-(2-(2-(Methyl(4-((4-Nitrophenyl)Diazenyl)Phenyl)Amino)Ethoxy)Ethoxy)Ethyl Methacrylate

Yield: 77% (eluent: hexane/acetone 9:1) 1H-NMR (CDCl3, 400 MHz) (seeScheme 1): δ = 8.33 (d, 2H, J = 9 Hz, H4); 7.92 (d, 2H, J = 8.1 Hz, H3); 7.89 (d,2H, J = 8.4 Hz, H2); 6.78 (d, 2H, J = 9 Hz, H1); 6.13 (s, 1H, H5); 5.57 (s, 1H, H6);4.30 (t, 2H, J1 = 4.65 Hz, J2 = 4.5 Hz, COO–CH2); 3.64 (m, 10H, all OCH2 andNCH2); 3.14 (s, 3H, CH3–N); 1.94 (s, 3H, =C–CH3) ppm. 13C-NMR (CDCl3, 100MHz) (see Scheme 1): δ = 186.2 (1C, C=O), 156.75 (1C, Ce), 152.53 (1C, Ca),147.32 (1C, Ch), 143.81 (1C, Cd), 136.10 (1C, C= of the methacrylate), 126.06(2C, Cc), 125.78 (1C, C= of the methacylate), 124.64 (2C, Cg), 122.56 (2C, Cf),111.46 (2C, Cb), 70.81, 70.63, 69.18, 68.57 (4C, all OCH2), 63.77 (1C, CH2OCO),52.16 (1C, N–CH2), 39.35 (1C, CH3–N), 18.27 (CH3 of the methacrylate) ppm.

2.1.3. 4PEGMAN or(E)-2-(4-((4-Nitrophenyl)Diazenyl)Phenyl)-5,8,11-Trioxa-2-Azatridecan-13-YlMethacrylate

Yield: 82% (eluent: hexane/acetone 8:1). 1H-NMR (CDCl3, 400 MHz) (seeScheme 1): δ = 8.31 (d, 2H, J = 9.3 Hz, H4); 7.98 (dd, 4H, J = 8.7 Hz, H3 andH2); 6.83 (d, 2H, J = 9.3 Hz, H1); 6.12 (s, 1H, H5); 5.57 (s, 1H, H6); 4.30 (t, 2H,J1 = 4.8 Hz, J2 = 4.8 Hz, COO–CH2); 3.72, 3.63 (m, 14H, all OCH2 and NCH2);3.19 (s, 3H, CH3–N); 1.94 (s, 3H, =C–CH3) ppm. 13C-NMR (CDCl3, 100 MHz)(see Scheme 1): δ = 186.2 (1C, C=O), 156.82 (1C, Ce), 153.35 (1C, Ca), 147.17(1C, Ch), 143.01 (1C, Cd), 136.10 (1C, C= of the methacrylate), 127.36 (1C, CH2=of the methacylate), 125.71 (2C, Cc), 124.7 (2C, Cg), 122.17 (2C, Cf), 112.17 (2C,Cb), 72.39, 70.82, 70.59, 70.56, 70.22, 68.49 (6C, OCH2), 63.77 (1C, CH2OCO),52.53 (1C, N–CH2), 39.67 (1C, CH3–N), 18.28 (CH3 of the methacrylate) ppm.

2.1.4. 6PEGMAN or(E)-2-(4-((4-Nitrophenyl)Diazenyl)Phenyl)-5,8,11,14,17-Pentaoxa-2-Azanonadecan-19-Yl Methacrylate

Yield: 73% (eluent: hexane/acetone 8:1). 1H-NMR (CDCl3, 400 MHz) (seeScheme 1): δ = 8.33 (d, 2H, J = 9 Hz, H4); 7.93 (d, 2H, J = 9 Hz, H3); 7.90(d, 2H, J = 8.1 Hz, H2); 6.78 (d, 2H, J = 9.3 Hz, H1); 6.13 (s, 1H, H5); 5.57(s, 1H, H6); 4.30 (t, 2H, J1 = 4.95 Hz, J2 = 4.8 Hz, COO–CH2); 3.63 (m, 22H,all OCH2, NCH2); 3.15 (s, 3H, CH3–N); 1.94 (s, 3H, =C–CH3) ppm. 13C-NMR(CDCl3, 100 MHz) (see Scheme 1): δ = 186.2 (1C, C=O), 156.81 (1C, Ce), 153.34(1C, Ca), 147.19 (1C, Ch), 143.02 (1C, Cd), 136.10 (1C, C= of the methacylate),127.37 (1C, CH2= of the methacrylate), 125.72 (2C, Cc), 124.70 (2C, Cg), 122.18(2C, Cf), 112.19 (2C, Cb), 72.37, 71.09, 70.53, 70.39, 70.35, 70.29, 70.25, 70.01,68.27 (10C, all OCH2), 63.77 (1C, CH2OCO), 52.52 (1C, N–CH2), 39.65 (1C,CH3–N), 18.29 (CH3 of the methacrylate) ppm.


2.2. Synthesis of the pnPEGMAN Polymers

nPEGMAN monomer (1.36 mmol) was dissolved in dry THF (20 ml); AIBN (5%weight, 0.04 g) was added, and the reaction mixture was stirred for 48 h at 60◦C.After this time, the solution was poured into methanol in order to precipitate theproduct; the desired pnPEGMAN polymer was obtained as a dark red-purple solid.FT-IR spectra of all polymers are very similar. IR (KBr): ν = 3100 (s, C–H aro-matic), 2960 (s, CH2), 2871 (s, CH2 and CH3), 1726 (s, C=O), 1601 (s, NO2),1517 (s, C=C aromatic), 1376 (s, N=N), 1336 (s, C–O esther), 1269 (s, C–N),1101 (s, all O–CH2) and 858 (out-of-plane, =C–H aromatic) cm−1.

2.2.1. p2PEGMAN orPoly[(E)-2-(2-(Methyl(4-((4-Nitrophenyl)Diazenyl)Phenyl)Amino)Ethoxy)EthylMethacrylate]

Yield: 70%. 1H-NMR (CDCl3, 400 MHz) (see Scheme 2, broad signals): δ = 8.26(H4), 7.67 (H3), 7.65 (H2), 6.71 (H1), 4.02–3.69 (all OCH2 and N–CH2), 3.14(NCH3), 2.48 (CH3 of the backbone) and 2.25 (CH2 of the backbone) ppm.

Scheme 2. Assignment of the signals for pnPEGMAN polymers.


2.2.2. p3PEGMAN orPoly[(E)-2-(2-(2-(Methyl(4-((4-Nitrophenyl)Diazenyl)Phenyl)Amino)Ethoxy)Ethoxy)Ethyl Methacrylate]


2.2.3. p4PEGMAN orPoly[(E)-2-(4-((4-Nitrophenyl)Diazenyl)Phenyl)-5,8,11-Trioxa-2-Azatridecan-13-Yl Methacrylate]


2.2.4. p6PEGMAN orPoly[(E)-2-(4-((4-Nitrophenyl)Diazenyl)Phenyl)-5,8,11,14,17-Pentaoxa-2-Azanonadecan-19-Yl Methacrylate]

Yield: 60%. 1H-NMR (CDCl3, 400 MHz) (see Scheme 2, broad signals): δ = 8.28(H4), 7.88 (H3), 7.86 (H2), 6.74 (H1), 4.07–3.61 (all the OCH2 and N–CH2), 3.12(NCH3), 2.48 (CH3 of the backbone) and 2.25 (CH2 of the backbone) ppm.

2.3. Characterization of Monomers and Polymers

FT-IR spectra of all monomers and polymers were carried out on a Nicolet FT-IR 5 DXB spectrometer in KBr pressed pellets. 1H- and 13C-NMR spectra of themonomers and 1H-NMR polymers in CDCl3 solution were recorded at room tem-perature on a Bruker Avance 400 MHz spectrometer, operating at 400 and 100 MHzfor 1H and 13C, respectively. Thermal properties of the obtained azo-polymerswere studied by determining T10 (10% weight loss temperature), Tg (glass transi-tion temperature) and Tm (melting point). Thermogravimetric analysis (TGA) wasconducted on a Hi-Res TGA 2950 Instrument (from 20 to 550◦C) and differentialscanning calorimetry (DSC) was carried out in a DSC 2910 instrument (from 20 to200◦C), in both cases at a heating rate of 10◦C/min.

All azopolymers were dissolved in spectral quality solvents purchased fromAldrich, and their absorption spectra were recorded on a Varian Cary 1 Bio UV-Vis(model 8452A) spectrophotometer at room temperature, using 1 cm quartz cells.Absorption spectra of the polymers were also recorded in spin-coated films, usingthe same instrument. Molecular weights of the polymers, were determined by gel-permeation chromatography (GPC), using a Waters 2695 Instrument connected toa Diffraction Index Detector model Waters 2414. Measurements were carried outusing tetrahydrofurane (THF) as solvent against a polystyrene standard.

Langmuir films were prepared using a KSV-5000 system 3 (KSV Instruments);the isotherms were recorded on the water subphase at room temperature. Prior toexperiments, water was purified by the means of a Milli-Q system to reach a finalresistivity of 18.2 M�/cm. pnPEGMAN polymers were dissolved in chloroform


(HPLC grade) in order to obtain a solution with a concentration of 1 mg/ml. Ofthis solution 100 µl was spread on the water surface in order to form a monolayer,which was compressed symmetrically with a barrier speed of 10 mm/min. Hystere-sis studies were performed for all polymers under the same conditions used for thepreparation of Langmuir films.

3. Results and Discussion

3.1. Synthesis of Monomers and Polymers

nPEGMAN monomers and pnPEGMAN polymers were prepared according to thesynthetic sequence shown in Fig. 2.

The synthesis of the RED-PEG dyes, used as starting materials, was carried outaccording to a method previously reported by us [25]. In a general way, the RED-PEG dyes (n = 2,3,4,6) were esterified in the presence of metacryloyl chloride,using triethylamine as catalyst and anhydrous THF as solvent, to give the desirednPEGMAN monomers. Once obtained, these monomers were polymerized, usingAIBN as initiator in dry THF, via a radical mechanism, to give the correspondingpnPEGMAN polymers in good yields (60–70%).

3.2. Characterization of Monomers and Polymers

The obtained nPEGMAN monomers and pnPEGMAN polymers were fully char-acterized by FT-IR, 1H- and 13C-NMR spectroscopy. The most important signalswith their assignment have been described in the Experimental section. The struc-tures and spectra of the obtained monomers and polymers are very similar; theonly structural difference between the members of the series is the number of ethy-lene glycol units present in the spacer. For this reason, we selected 6PEGMANand p6PEGMAN as examples to describe the 1H- and 13C-NMR spectroscopy ofmonomers and polymers.

The FT-IR spectrum of the 6PEGMAN monomer shows the expected signalsfor all the functional groups present in the molecule: 3090 (s, C–H aromatic andvinylic) 2961 (s, CH2), 2924 (s, CH2 and CH3), 1727 (s, C=O), 1603 (s, C=Caromatic), 1516 (s, NO2), 1447 (s, N=N), 1378, 1337 (s, C–O of the ester), 1261 (s,C–N), 1099 (s, O–CH2), 856 (out-of-plane, =CH2 vinylic) and 802 (out-of-plane,=C–H aromatic) cm−1.

The 1H-NMR spectrum of 6PEGMAN in CDCl3 (Scheme 2) exhibits four sig-nals in the aromatic region at δ = 8.33,7.93,7.90 and 6.78 ppm, due to the H4,H3, H2 and H1 protons, respectively. Besides, a series of signals at 6.13 (H6), 5.57(H5), 4.30 (COO–CH2), 3.63 (all OCH2 and NCH2), 3.15 (CH3–N) and 1.94 ppm(CH3 group present in the methacrylate unit) is also observed. On the other hand,the 13C-NMR spectrum of 6PEGMAN in CDCl3 (Scheme 2) shows a signal at δ =186.20 ppm due to the carbonyl group of the metacrylate, followed by eight signalsin the aromatic zone at δ = 156.81,153.34,147.19,143.02,125.72,124.70,122.18


and 112.19 ppm, due to the eight types of carbons present in the azobenzene moi-ety. Among these, there are two intercalated signals at δ = 136.10 and 127.37 ppm,due to vinylic carbons H2C= and =C– of the methacrylate unit. In contrast, inthe aliphatic region we can observe 12 signals between δ = 72.37–63.77, due toall OCH2 present in the oligo(ethylene glycol) chain. Finally, there are three morepeaks at 52.52, 39.65 and 18.29 ppm, due to the N–CH2, N–CH3 and CH3 of themethacrylate, respectively.

p6PEGMAN polymer was also characterized by FT-IR and 1H- and 13C-NMRspectroscopy. The FT-IR spectrum of p6PEGMAN shows a band at 3100 cm−1 (s,C–H aromatic and vinylic) followed by two bands at 2960 and 2871 cm−1, due tothe methylene and methyl groups present in this polymer. Besides, we can observethree intense bands at 1726 (s, C=O), 1601 cm−1 (s, C=C aromatic) and 802 cm−1

(out-of-plane, =C–H aromatic). In addition, four less intense bands appear at 1517(s, NO2), 1376 (s, N=N), 1336 (s, C–O of the ester), 1269 (s, C–N) and 1101 (s, allO–CH2) cm−1, which confirm the presence of the other functional groups presentin the molecule.

The 1H-NMR spectrum of p6PEGMAN (Fig. 3, see Scheme 2) shows four broadsignals at δ = 8.28,7.88,7.86 and 6.74 ppm, due to the aromatic protons H4, H3,H2 and H1 of the azobenzene units. In the aliphatic region of the spectrum, we cannotice three broad signals between δ = 4.36–3.12 ppm, assigned to all hydrogenatoms present in the methylene groups (OCH2) of the oligo(ethylene glycol) spacersand in the alkyl amino groups (CH3–N and CH2–N). Besides, we can observe apeak at δ = 2.48 ppm, due to the methyl groups of the polymethacrylate backbone,followed by a broad signal at δ = 2.51–2.22 ppm, which can be attributed to allCH2 groups of the polymer backbone.

Molecular weights of the polymers were determined by GPC in THF againsta polystyrene standard; the results are summarized in Table 1. As we can notice,the pnPEGMAN polymers exhibit low molecular weights in the range of 4200–5800 g/mol; similar Mw and Mn values were reported for the pnMAN series byNatansohn and co-workers [10]. Polymerization of methacrylates bearing azoben-zene groups usually gives polymers with medium to low molecular weights [10].

3.3. Thermal Properties of the Polymers

The thermal properties of the obtained azopolymers were determined by TGA(from 20 to 550◦C) and DSC (from 20 to 200◦C), in both cases with a heatingrate of 10◦C/min; the results are summarized in Table 1. The TGA curves of thepnPEGMAN polymers are shown in Fig. 4. As we can see, these azopolymers showfairly good thermal stabilities with T10 values between 189 and 256◦C, exhibitingfast degradation in the range between 100 and 500◦C. Except for p3PEGMAN, thethermal stability tends to increase as the oligo(ethylene glycol) spacer length aug-ments. Concerning the glass transition temperature of the pnPEGMAN polymers,we can notice that they exhibit low Tg values around 20–42◦C, and melting pointsTm between 42 and 60◦C. From these results, we can conclude that the melting


Figure 3. 1H-NMR spectrum of p6PEGMAN in CDCl3 solution.

Table 1.Molecular weight, thermal and optical properties of pMDR1 and pnPEGMAN polymers

Polymer Mw (g/mol)a Mn (g/mol)a PDI Tg (◦C)b T10 (◦C)b Tm (◦C)b

pMDR1 5850 4500 1.30 13 194 54p2PEGMAN 4500 3350 1.35 42 189 60p3PEGMAN 4200 3100 1.36 20 150 52p4PEGMAN 4250 3200 1.33 27 256 49P6PEGMAN 5800 4350 1.34 31 228 42

a Determined in THF against a polystyrene standard.b Heating rate 10◦C/min.

point of these azopolymers decreases as the oligo(ethylene glycol) spacer lengthaugments.


Figure 4. TGA of the pnPEGMAN polymers.

Figure 5. Absorption spectra of the pnPEGMAN azopolymers in THF solution.

3.4. Optical Properties of the Polymers

The optical properties of the pnPEGMAN polymers and pMDR1 were studied byUV-Vis spectroscopy in solution and in cast film. The absorption spectra of thesepolymers in THF solution are shown in Fig. 5 and their optical properties are sum-marized in Table 2.

All azopolymers exhibit maximum absorption wavelength bands in the rangebetween λmax = 468–477 nm, due to the π–π∗ and n–π∗ transitions of the azoben-


Table 2.Optical properties of pMDR1 and pnPEGMAN polymers inTHF solution and in the solid state

Compound λmax (nm) Cut off (nm) in THF

THF Film

pMDR1 473 478 618p2PEGMAN 470 489 626p3PEGMAN 473 475 586p4PEGMAN 473 476 603p6PEGMAN 468 478 604

zene chromophores [1]. Precursor RED-PEG dyes show also maximum absorptionbands at about λmax = 471–480 nm, due to the same transitions [25] as other simi-lar push–pull aromatic systems [12]. In this case, since both RED-PEG compoundsand pnPEGMAN polymers contain amino–nitro-substituted azobenzene moietiesin their structure, they exhibit a total overlap of the π–π and n–π∗ bands in theirabsorption spectra, so that only one absorption band can be observed. This behavioris typical for donor–acceptor substituted azobenzenes, belonging to the ‘pseudos-tilbenes’ category, according to Rau’s classification [11].

On the other hand, it is worth to point out that the precursor RED-PEG-n dyes inTHF solution (λmax = 471–480 nm) exhibit red-shifted maximum absorption bandswith respect to those of the corresponding pnPEGMAN polymers (λmax = 468–477 nm). This indicates that RED-PEG compounds are more polar and possesshigher charge transfer character than their corresponding azopolymers. In contrast,p2PEGMAN and p3PEGMAN in THF solution show a discrete red-shifted shoul-der, which reveals the presence of traces of J-aggregates (head to tail) in thesepolymers [25–27]. The presence of aggregates in diluted solutions is a proof that in-tramolecular interactions between neighbor azobenzene chromophores take place,due to a coiling of the polymer backbone. The aggregation phenomenon is moreevident in cast film and can occur either in intramolecular or intermolecular way.Highly polar amino–nitro-substituted azobenzenes have a natural tendency to formpairs (aggregates) in order to reach certain electronic stability. In most of the cases,the amino group (donor) of one azobenzene unit interacts in a ‘face to face’ mannerwith the nitro group (acceptor) of a similar chromophore; this behavior was alsoobserved in polymers of the pnMAN series [28].

3.5. Preparation of Langmuir Films

Surface pressure versus mean molecular area isotherms for Langmuir films of thedifferent pnPEGMAN polymers were recorded at room temperature in the air–water interface and are illustrated in Fig. 6. As it could be expected, the differ-ences between the isotherm curves result from the variation of the oligo(ethyleneglycol) spacer length in the polymer series. Here, there is a significant influence



Figure 6. Surface pressure vs. molecular area, isotherms for Langmuir films of the differentpnPEGMAN polymers.

of the number (pair or not) of atoms in the spacer. Since the arrangement ofmolecules on the surface varies remarkably depending on the hydrogen bonds, allthe pnPEGMAN polymers exhibit a different isotherm behavior (�/A). For exam-ple, the surface pressure departs to different molecular areas are 3.4, 4.9, 5.6 and7.9 nm2/molecule, for p2PEGMAN, p3PEGMAN, p4PEGMAN and p6PEGMAN,respectively. As expected, an increase in the oligo(ethylene glycol) spacer lengthcauses an augment in the cross-sectional area per molecule (A0): 2.73, 3.32, 4.17and 5.34 nm2/molecule, respectively. It is also interesting to notice that the augmentof the spacer length provokes that the polymers display different isotherm phases.The hysteresis curves for p4PEGMAN and p6PEGMAN showed that the stabilityof the Langmuir films is favored as the spacer length increases, which allows anappropriate arrangement of the polymer on the water surface.

The formation of the Langmuir films on the air–water interface was monitoredby Brewster angle microscopy (BAM) at different surface pressures, where the filmtopography during the compression process of a pnPEGMAN polymer shows astable monolayer in all cases. The formation of a p2PEGMAN monolayer wasmonitored by BAM and the different images are shown in Fig. 7. At the begin-ning, we can see a high order of pre-compression in the film, where the surfacepressure is 0.2 mN/m. When the compression started (Fig. 7a), the appearance ofdomains was observed at 4 mN/m and 290 Å2. As the compression process con-tinued (Fig. 7b), the domains joined together to form a stable and homogeneousmonolayer until the surface pressure reached 33 mN/m (Fig. 7c). After mechani-cal collapse, some fractures can be observed in the previously formed monolayer.As we can see, pnPEGMAN polymers easily form Langmuir films and are suitableprospects for the future elaboration of Langmuir–Blodgett multilayer devices.


(a) (b) (c)

Figure 7. Topography of the Langmuir films, obtained by BAM imaging at different surface pressures,where the film topography during the compression of a pnPEGMAN, shows a stable monolayer in allthe cases.

4. Conclusions

A novel series of azopolymers (pnPEGMAN) containing amino–nitro-substitutedazobenzenes linked to a poly(methacrylate) backbone via a well-defined oligo(eth-ylene glycol) spacer was synthesized and characterized. The obtained pnPEGMANpolymers have molecular weights between 4200 and 5800 g/mol. All azopolymersexhibit a moderate thermal stability, displaying T10 values between 150 and 276◦Cand low glass transition temperatures Tg from 20 to 42◦C. Absorption spectra ofthe polymers exhibit absorption bands at λmax = 468–470 nm in solution and inthe range between λmax = 476–489 nm in cast films. The absorption spectrum ofp4PEGMAN in film shows the presence of H-aggregates, whereas the rest of thepolymers only exhibit traces of J-aggregates in the solid state. These polymers eas-ily form Langmuir films on the air-water interface and can be suitable for the futureelaboration of Langmuir–Blodgett multilayers.

Acknowledgements

We are grateful to Miguel Angel Canseco and Esteban Fregoso for their assistancewith FT-IR spectroscopy and Thermal analysis and to Gerardo Cedillo for his helpwith 1H- and 13C-NMR spectroscopy. We also thank Salvador López Morales forthe determination of the molecular weights of the polymers by GPC. This projectwas financially supported by PAPIIT-DGAPA (Projects IN105610, IN-118808).

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