Influence of chromophore concentration and network structure on the photo-orientation properties of...

Influence of Chromophore Concentration and NetworkStructure on the Photo-Orientation Properties ofCrosslinked Epoxy-Based Azopolymers

R. FERNANDEZ,1 I. MONDRAGON,1 M. GALANTE,2 P. OYANGUREN2

1’Materials þ Technologies’ Group, Department of Chemical and Environmental Engineering, Polytechnic School,Universidad Paıs Vasco/Euskal Herriko Unibertsitatea, Plaza Europa 1, 20018 Donostia-San Sebastian, Spain

2Institute of Materials Science and Technology (INTEMA), University of Mar del Plata and National Research Council(CONICET), J. B. Justo 4302, 7600 Mar del Plata, Argentina

Received 2 December 2008; revised 24 February 2009; accepted 3 March 2009DOI: 10.1002/polb.21704Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: An azo prepolymer (TAZ) was synthesized by reaction between DisperseOrange-3 (DO3) and diglycidyl ether of bisphenol A (DGEBA). Selected amounts ofTAZ were blended with DGEBA and cured with an aliphatic diamine, either meta-xilylenediamine (MXDA) or polyetheramines (Jeffamine D series). The photoinducedanisotropy and optical storage properties in two series of crosslinked epoxy-based azopolymers with different architectures and chromophore contents (from 3 to 24 inweight) have been investigated. Measurements of the birefringence (Dn) inducedwith linearly polarized 488 nm light show that the Dn values increases with DO3weight fraction. Networks with the same chromophore concentration but differentbackbones exhibit similar levels of induced anisotropy under the same irradiationconditions. The remaining birefringence and relaxation times are influenced by themolecular weight between crosslinks Mc of networks. Higher Mc is reflected inshorter relaxation times and lower remaining birefringence. In terms of the level ofinduced dichroism measured on relaxed films, it was found an agreement with theremaining anisotropy. VVC 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47:

1004–1014, 2009

Keywords: azopolymer; epoxy networks; optical storage; photoinducedbirefringence

INTRODUCTION

Functionalized polymers with azobenzene chro-mophores covalently attached to the polymermain chain are versatile materials for construct-ing a variety of optical devices.1–3 Linearly polar-ized light is known to induce a reorientation ofazobenzene groups in the glassy state of amor-

phous and liquid-crystalline polymers. After anoptical irradiation for a sufficiently long time, thematerials are characterized by a preferentialchromophore orientation perpendicular to thepolarization direction of the light. This anisotropicmolecular reorientation induces both birefrin-gence and dichroism properties changing in thematerials. Such polymers have been consideredfor applications in the area of reversible opticalstorage.4 Materials that are suitable for suchapplications are required to display a high ther-mal and mechanical stability as well as photo-stability, defined as the ability of a polymer to

Journal of Polymer Science: Part B: Polymer Physics, Vol. 47, 1004–1014 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: P. Oyanguren (E-mail: [email protected])

1004

keep its photoinduced orientation. It is wellknown that epoxy networks are highly transpar-ent and have excellent mechanical and thermalproperties. There are only a few investigations onepoxy resins as photoresponsive polymers mainlyfocused on amorphous copolymers.5–11 We havepreviously studied the reaction kinetics and theoptical storage characteristics of two azobenzene-functionalized epoxy-based copolymers and ahomopolymer containing Disperse Orange-3, andhave contrasted the results with two guest–hostsystems.12–14 The question as far as applicationsin the area of optical storage are concerned arisesif a stable photo-orientation can be induced in thepolymeric form of the material and if the cross-links have any influence on the photokinetics ofthe light-induced process. Moreover, the azo groupcan be attached in different polymer backbones ei-ther in the main-chain or as a side chain, allowingthe mechanical and processing properties of con-ventional polymers to be combined with the opti-cal properties of azo groups.

In an effort to establish materials with thedesired optical storage properties, the photo-orientation induced via irradiation with linearlypolarized light has been investigated in a series ofcrosslinked epoxy-based azopolymers, each withdistinct differences in the molecular structure ofthe unit building blocks. All the epoxy-based net-works analyzed have the chromophore placed in abranch, also called crosslinked side chain.15 Wediscuss to what extent the polymer structure caninfluence chromophores phoro-orientation, andwe show that some polymeric epoxy systems pro-vide good results in optical storage and stability.Networks containing variable azobenzene unitsconcentration and different main-chain segmentsare compared and analyzed.

EXPERIMENTAL

Materials and Samples Preparation

An azo prepolymer (TAZ) was synthesized byreaction between Disperse Orange-3 (DO3,Aldrich) and diglycidyl ether of bisphenol A(DGEBA, DER 332, Dow).12,13 It was prepared ina stoichiometric ratio r ¼ eq DO3/eq DGEBA ¼0.5, to generate reaction products with epoxygroups in the extreme of chains. The resultingTAZ, having a chromophore concentration perweight relative to the polymer backbone of 26 wt%, was attached to networks via the epoxy groups.Selected amounts of TAZ were blended with

DGEBA and cured with a diamine, either meta-xilylenediamine (MXDA, Aldrich) or polyether-amines (Jeffamine D series, D230 and D400,Hunstman). The chemical structures of reactantsand TAZ are shown in Figure 1. The hardenerswere used at a stoichiometric ratio of epoxy toamino-hydrogen groups equal to 1. Curing reac-tion was carried out at 100 �C for MXDA and 140�C for the polyetheramines, for the time necessaryto obtain total conversion of the reactants.14

Two series of crosslinked epoxy-based azopoly-mers were synthesized. One of them, TAZ–DGEBA–MXDA, was prepared employing MXDAas curing agent and using variable TAZ content(3–24 wt % DO3). The other series, TAZ–DGEBA–D230/D400, was prepared from the polyether-amines using different compositions (D230/D400from 100/0 to 40/60). This series has a constantchromophore concentration equal to 13 wt % DO3.

Films were prepared by spin-coating using aP6700 spin-coater from 5 wt % reactive mixturesolutions in tetrahydrofuran, onto clean glassslides. Tetrahydrofuran (THF) was purchasedfrom Panreac and used without further purifica-tion. The reactive mixture was composed of TAZ,DGEBA, and the curing agent. The spinner cycleprogram was as follows: 1000 rpm for 30 sec, 4000rpm for 10 sec, and 8000 rpm for 20 sec. ResidualTHF was removed by evaporation at room tem-perature. Then, reactive mixture films were curedin an oven at the corresponding temperature,depending on the curing agent employed. Aftercuring, films were heated above the glass transi-tion temperature (Tg) of the network and subse-quently, stored at room temperature. Homogene-ous thin films were obtained by this procedure.All the films had comparable thickness values, onthe order of 700 (�100) nm.

Measurements

Optical storage experiments were carried out atroom temperature and under ambient conditions.The experimental setup used in the optical storageexperiments was similar to that previouslyreported.12 The optical birefringence was inducedin the film using a linearly polarized argon contin-uous laser operating at 488 nm (writing beam)with a polarization angle of 45� with respect to thepolarization direction of the probe beam. Thepower of the writing beam used in the experimentswas varied (between 2 and 20 mW) on a spot of0.4 mm2, to study the time and amplitude of theoptically induced birefringence. The change in the

PHOTOINDUCED ANISOTROPY OF AZOPOLYMERS 1005

Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

transmission of a low power He–Ne laser (readingbeam) operating at 632.8 nm, which passedthrough two crossed polarizers and the sample,was measured with a photodiode. Each photo-ori-entation experiment has been done on a differentpreviously nonirradiated spot so as to avoid irradi-ation history complications. The induced birefrin-gence was determined by measuring the probebeam transmission (T ¼ I/I0) according to:

Dn ¼ kpd

� �sin�1 I

I0

� �1=2

(1)

where k is the wavelength of the incident radia-tion, d is the film thickness, I0 is the incidentbeam intensity and I is the intensity after the sec-ond polarizer.

Differential scanning calorimetry (DSC) wasperformed using a DSC Mettler Toledo 822 device.The glass transition temperature, defined as theonset of the change in specific heat, was deter-mined under nitrogen flow at 10 �C/min.

UV–visible spectra were recorded in films on aMilton Roy Spectronic Genesys 5 spectrophotome-ter fitted with polarizing optics. The films were

Figure 1. Chemical structure of the epoxy monomer (DGEBA), amines (MXDA,Jeffamine D230 and D400), and TAZ.

1006 FERNANDEZ ET AL.


exposed to a linearly polarized argon laser beamat 488 nm (6 mW) for enough time to reach satu-ration, and UV–visible spectra of films, for lightpolarized parallel and perpendicular to the writ-ing polarization direction were recorded on therelaxed films. The dichroism ratio of the films, D,was calculated from the measured absorbance atmaximum wavelength (kmax) parallel (Ak) and per-pendicular (A\) to the electric vector of the inci-dent light (eq 2).

D ¼ A?Ak

(2)

The theoretical molecular weight betweencrosslinks Mc was calculated by assuming fullconversion by eq 3, previously employed by Lesserand Crawford.16

Mc ¼2 Me þ

P1fa¼2

Mfa

fa Ufa

� �P1

fa¼3 Ufa(3)

where Me is the epoxy equivalent weight of theresin, Mfa is the molecular weight of the fa-thfunctional amine, Ufa is the mol fraction of aminehydrogens provided by the fa-th functional amineand fa is the amine functionality.

RESULTS AND DISCUSSION

The studies involving the two series of crosslinkedepoxy-based azopolymers synthesized pointed to a

clear relation between the optical storage proper-ties and the structure of the network to which theazobenzene group is attached. Since the writingand relaxation times for azopolymers are usuallyin the range of a few seconds, and are basically in-herent to the mechanism of orientation, the fol-lowing discussion will be focused on the maximumand the residual fraction of the induced birefrin-gence. These two parameters are the most impor-tant for practical applications. Table 1 summa-rizes some optical storage features of films fromthe two series synthesized.

Typical UV–visible spectra of films for the twoseries are shown in Figure 2. Also shown for com-parison is the UV–visible spectrum of TAZ. Nota-bly, the peak maximum is the same for all samples,indicating that the electronic structure of the chro-mophore was not altered when the dye content waschanged and/or different matrices where chromo-phore was confined were used. These results can betaken as evidence of a negligible aggregation ofchromophores, since a significant shift would beexpected otherwise. More evidence of the absenceof chromophore mutual interaction is shown in theinset of this figure. The absorption coefficientdetermined at 480 nm is independent of the chro-mophore content in the films. Figure 2 also showsthe wavelength of the writing beam (488 nm),which is within the range of absorption of azoben-zene, and the wavelength of the reading beam (633nm), which is out of this range. All polymers had amaximum absorbance comprised between 476 and

Table I. Optical Storage Characteristics of Films from the Series of Azo-Networks TAZ–DGEBA–MXDA andTAZ–DGEBA–D230/D400

DO3(% w/w) Tg (�C)

Mc

(g/mol) Dn t80%WRITE (s)

% RemainingBirefringence

DichroismRatio

TAZ–DGEBA–MXDA3 110 472 0.0063 37.5 66 1.2435 106 537 0.0113 27.5 66 1.351

13 101 798 0.0207 9.8 66 1.3218 103 1070 0.0246 7.0 64 1.31124 103 1680 0.0294 7.8 61 1.258

TAZ–DGEBA–D230/D400D230/D400100/0 13 85 875 0.0221 7.2 58 1.30685/15 13 77 897 0.0223 8.1 57 1.29570/30 13 73 918 0.0234 7.3 56 1.29155/45 13 68 938 0.0192 4.6 56 1.27340/60 13 62 958 0.0200 4.3 55 1.251

Films were irradiated at 6 mW.



480 nm, very close to the argon laser wavelength.Thus, it is expected an efficient absorption of thewriting laser light. This fact could be reflected notonly in the level of birefringence reached, but alsoin the time necessary to achieve it.

The influence of the writing beam power hasalso been studied for the two series of polymers.Figure 3 shows the maximum intensity of trans-mitted probe beam for TAZ–DGEBA–D230 filmsat different writing beam powers. The intensity oftransmitted probe beam increased with the laserpower up to 6 mW, after which saturation wasreached. The saturation curve for TAZ–DGEBA–MXDA series was investigated with the same pro-cedure and similar results as those showed in Fig-ure 3 were obtained. Saturation was reached atapproximately 6 mW for all the polymers studied.

To understand the mobility of the azo chromo-phore in the polymer network and clarify theeffect of crosslinking on the orientation behavior,in situ birefringence measurements were per-formed during writing cycles (linearly polarizedpump on), relaxation cycles (pump off), and photo-induced erasing processes (circularly polarizedpump on). Figure 4(a,b) shows typical sequencesof inducing and erasing birefringence for TAZ and

TAZ–DGEBA–MXDA series. No transmission ofthe probe beam, which passed through twocrossed polarizers placed before and after the filmwas observed, indicating the random orientationof the chromophores. However, when the writing

Figure 2. UV–visible spectra of: (a) TAZ–DGEBA–MXDA, 13 wt % DO3, (b) TAZ–DGEBA–D230/D400 (40/60), (c) TAZ–DGEBA–D230, (d) TAZ–DGEBA–MXDA, 24 wt% DO3, (e) TAZ. Also marked in the figure are the wavelengths of the reading andwriting beams. The inset shows the UV–visible calibration curve of TAZ–DGEBA–MXDA series.

Figure 3. Extent of birefringence as a function ofthe power of the writing beam for TAZ–DGEBA–D230system.



beam was turned on at point A, the transmissionincreased and reached 80% of the saturationvalue in about 10 sec. The increase in transmis-sion was directly related to the induced birefrin-gence in the film. When the writing beam wasturned off at point B, birefringence levels fall offfrom the saturation level to a relaxed level (pointC). This was probably due to thermally activateddipole reorientation which would tend towardrandomization of the anisotropy. The inducedbirefringence could be optically erased by over-writing the test spot with a circularly polarizedlaser light that randomized the chromophores ori-entation, thereby eliminating the macroscopicdipole orientation. This writing–erasing cycle can

be repeated many times on the same spot on thepolymer film, achieving the same level of birefrin-gence at the same rate.

There are a few important parameters govern-ing the extent and rate of reorientation of the azo-benzene groups in an amorphous polymer film.These are the nature of the azo group, the azogroup content, and the polymer structure. All pol-ymers compared had a maximum absorbance veryclose to the writing laser wavelength (Fig. 2). Aswe discuss above, for both series it is expected anefficient absorption of the writing laser light. Theother parameters are analyzed in the followingsections.

Variable Chromophore Concentration

It is important to determine the relationshipbetween the maximum level of anisotropy thatcan be induced and the concentration of the chro-mophores in the polymeric material. Figure 5(a),illustrates the birefringence (Dn) of TAZ andTAZ–DGEBA–MXDA samples, as a function ofthe azo group content (by weight). There was anonlinear increase in the birefringence level asthe azo group content increased. Since Dn is dueto the anisotropy and photoinduced orientation ofthe azo units, a larger number of photoactivechromophores in the polymer chain generated ahigher birefringence. The absence of shift of theabsorption spectra of networks with variable DO3content (Fig. 2) suggests that it is not expecteddipolar interaction among dye groups in this net-work series. This means that the probability ofeach dipole being oriented is independent on theproximity of other dipoles. This should lead to alinear relationship between the chromophore con-centration and the attainable anisotropy. Theresults presented in Figure 5(a) may be related tothe increase in the writing beam absorption uponincreasing chromophore content, as reported byDhanabalan et al.17 The absorption increaseinduced a sample heating, which counteracted theprocess of orientation.

The level of maximum birefringence that canbe achieved was about 0.03 (for 24 wt % DO3),which is similar to previously published systems,such as the studies of the photo-orientation inmelamine networks containing DO3 reported byStracke et al.18 There, it was shown that the max-imum photoinduced birefringence obtained was0.034 for materials containing more than 50 wt %chromophore. In a series of papers, Rochonet al.19–21 demonstrated that for amorphous

Figure 4. Writing, relaxing, and erasing sequenceson: (a) TAZ film, and (b) TAZ–DGEBA–MXDA filmswith variable DO3 content. The power of the writingbeam is 6 mW.



polymers containing electron-donor substituentson the azobenzene group, the maximum variationin refractive index obtained was 0.08. Most stud-ies were performed on a Disperse Red 1-contain-ing polyacrylate with 85 wt % chromophore.Then, based on the reported values of birefrin-gence levels for amorphous polymers, the birefrin-gence level achieved in the present study is high.The level of Dn achieved for TAZ, shown in Figure5(a), is slightly lower than the ones obtained fornetworks with comparable chromophore content,indicating that there is no influence of crosslink-

ing on the maximum value attainable. It has to betaken into account that the saturated level of Dnis the result of a combination of two processes:photoselection, which increases birefringence, andthermal randomization, which decreases its level.These two processes reach an equilibrium statewhile illumination is on, and the preponderanceof one or the other determined the saturated level.The possibility for the azo chromophore to movearound is great for TAZ which has Tg ¼ 28 �C.Consequently, its birefringence level was lowerwhen comparing with networks.

It is important also to know how fast the bire-fringence can be induced in the film by the polar-ized laser. The time interval necessary to achieve80% of saturated birefringence, t80%

WRITE as afunction of chromophore concentration is shownin Figure 5(b). For comparison purpose, the corre-sponding values for TAZ are also incorporated(closed symbols). The results demonstrate thatthe crosslinks do not cause a slowing down of thekinetics of photo-orientation. On the contrary,similar writing times are observed for networkswhen comparing with TAZ. For an interpretationof this result we have to conclude that the photo-orientation of the azo chromophores is not influ-enced by the crosslinking itself but that itdepends rather on the local free volume distribu-tion around the chromophores. As the azo chromo-phore was not forming part of the bridgesbetween the polymer chains, it had a high mobil-ity in the free volume of the azonetwork. Similarresults were obtained for crosslinkable polyme-thacrylates containing azobenzene and acrylicgroups on separate side chains to maintain anadequate azo mobility in the networks.22 On theother hand, the order of magnitude of the time toreach the maximum birefringence is in agreementwith the ones reported for cast films containingpush–pull azo dyes.23 As shown in Figure 5(b), adecrease in the time necessary to reach the maxi-mum birefringence was observed with theincrease in DO3 content, independently of thepower of the writing beam employed. This behav-ior can be understood in terms of a thermal effect.As discussed above, the increase in the sampleabsorption induced a sample heating as morechromophore was incorporated into the polymer.This sample heating allowed a higher molecularmobility, which results in a faster writing process.In addition, writing dynamics was dependent onthe number of photons involved in the writingprocess. For a given DO3 concentration, writingtime was shorter when increasing the power ofthe writing beam.

Figure 5. (a) Maximum induced birefringence val-ues measured at 6 mW for TAZ–DGEBA–MXDA sys-tem; (b) time to achieve 80% of saturated birefrin-gence at two different powers of the writing beam,plotted as a function of the weight percentage of DO3in the networks. Also shown are experimental valuesfor TAZ (closed symbols).



When the writing laser beam was turned off,the birefringence decayed to a stable level within30 s for all the samples analyzed. Part of thisdecrease can be associated with thermal reorien-tation of some azobenzene groups.24 This thermalreorientation tends to restore the thermodynami-cally favored disordered state and probably it hap-pens because of some local heat associated withthe laser light which is dissipated through thefilm when the laser is turned off. The birefrin-gence conserved after relaxation for TAZ–DGEBA–MXDA series can be seen in Figure 6. Itis interesting to note a slight decrease in theremaining birefringence upon increasing the azoconcentration in the networks. This tendency isprobably related with the crosslinking density ofthe polymer matrix. Higher crosslinking restrainsthe mobility of oriented azo groups for randomiza-tion. The prepolymer TAZ exhibited the minimumremaining birefringence ¼ 50%. Thus, crosslink-ing plays an important role in the stability of thephotoinduced orientation.

Polymer Structural Effects

Recent studies on photo-orientation processes inamorphous polymers have addressed the role ofTg and polymer structural effects, including themain chain rigidity, the nature of the connectionof the chromophore to a rigid or semirigid or aflexible main chain, the free volume, the free vol-ume distribution, or a combination of these.25 The

polymer structure and Tg are not the only impor-tant parameters for polymers, but the molecularweight and its distribution are also important. Infact, the glass relaxation is characterized by Tg,which is affected by several factors including themolecular weight, swelling, and crosslinking. Aswas explained in the experimental section, all net-works were synthesized employing the same azo-prepolymer (TAZ) in which the chromophore istethered at both ends to the polymer chain. Theconnection of TAZ to network precursors is possi-ble thanks to the presence of epoxy groups in theextreme of chains. The difference between theazonetworks is the structure of the diamineemployed. The presence of aromatic rings in thebackbone of MXDA-cured samples leads to a lessflexible chain structure than for TAZ–DGEBA–D230/D400 system. In this section, we discuss towhat extent the polymer structure can influencechromophore photo-orientation in the series ofepoxy-based networks containing a constantconcentration of chromophore.

Figure 7 shows the saturated values of the pho-toinduced birefringence for TAZ–DGEBA–MXDA(13 wt % DO3) and TAZ–DGEBA–D230/D400 sys-tems against the molecular weight between cross-links of networks Mc. Networks with the samechromophore concentration but different back-bones exhibited similar levels of induced anisot-ropy under the same irradiation conditions. Thisis an indication that for the networks analyzed

Figure 6. Birefringence conserved after relaxationfor the TAZ–DGEBA–MXDA series plotted as a func-tion of DO3 weight fraction. The power of the writingbeam is 6 mW.

Figure 7. Maximum induced birefringence of net-works containing 13 wt % DO3 for TAZ–DGEBA–MXDA (open circles) and TAZ–DGEBA–D230/D400series (closed circles) measured at 6 mW, as a func-tion of the molecular weight between crosslinks.



here, the maximum induced birefringence is onlyfunction of the overall chromophore concentra-tion. These experimental results are consistentwith the ones reported by Takase et al.22 whencomparing maximum birefringence of azocontain-ing polymethacrylate copolymers before and aftercrosslinking. They found that the maximum valueachieved during the linearly polarized laser irra-diation is only a function on the azo content.

The writing process is associated with thetrans–cis–trans isomerization rates, quantumyields, and the local mobility of the azo group.Since the same chromophore is used in all the pol-ymers analyzed, and the energy absorbed by thepolymer films at the irradiation wavelength is thesame, it is expected that the writing processshould be affected by the molecular structure andTg of the polymer sample. During irradiation,t80%

WRITE values decreased as Mc increased (or Tg

decreased, as the relationship between Tg and Mc

for both types of networks is lineal for a constantDO3 content). These results are an indicationthat the trans isomers rotate more easily in poly-mers with lower Tg (see Table 1). Concerning thepolymer structural effects on the dynamics of thewriting process, we found previously that DO3-doped epoxy-based copolymers exhibit faster writ-ing response (five times) compared with copoly-mers based on epoxy resin containing the sameDO3 concentration (5 wt % DO3), attached cova-lently as a side chain.13 The writing dynamics ofthe azonetwork having 5 wt % DO3 content is 15

times slower when comparing with the guest–host system. That is to say, t80%

WRITE valuesincrease significantly because of the linkageeffect. This behavior is attributed to the restric-tion of the dynamics of the azo moiety by bondingcovalently to a polymer chain and/or the presenceof crosslinks between chains.

The influence of the next neighbor on the reor-ientation of an azo group is also provided by thelevel of relaxation after the writing laser is turnedoff. Figure 8 shows the remaining birefringence(curve a) as well as the time intervals necessaryto achieve 80% of remnant birefringence,t80%

RELAX, (curve b) as a function of the molecularweight between crosslinks of networks. It is worthto note that the movement of azo groups wasfacilitated in the polymers with higher molecularweight between crosslinks, which was reflected inshorter relaxation times and lower remainingbirefringence. This can be due to the smaller Tg

values (Table 1) of such polymer systems. Theglass transition temperature can be a guide topredict the relative size of the free volumes. Thefree volume in polymers is considered as one ofthe polymer matrix effects on polarized light-induced anisotropy, as the rotational motion oftrans isomers is considered to be restricted into asmall volume. Depending on the mobility of azo-benzene chains in matrices, different relaxationswere observed. The lower the Tg value, the higherthe molecular mobility and lower the remnantbirefringence. Similar results were obtained by

Figure 8. (a) Remaining birefringence and (b) relax-ation times of networks containing 13 wt % DO3 forTAZ–DGEBA–MXDA (open symbols) and TAZ–DGEBA–D230/D400 series (closed symbols) as a func-tion of the molecular weight between crosslinks.

Figure 9. UV–visible spectra of TAZ–DGEBA–MXDA (13 wt % DO3) (solid lines) and TAZ–DGEBA–D230 (dotted lines) for light polarized: (a) perpendicu-lar and (b) parallel to the writing direction.



Sekkat et al.25 when comparing polyurethanepolymers, confirming the influence of the polymerstructure on photo-orientation.

Typical examples of the optically induceddichroism are presented in Figure 9, where thepolarized UV–visible absorption spectra of TAZ–DGEBA–MXDA (13 wt % DO3) and TAZ–DGEBA–D230 films are shown. The absorptioncoefficient for light polarized parallel to the writ-ing direction was substantially less than that forlight polarized in the perpendicular direction. Inaddition, there appears to be no significant shapedifference between written films (parallel and per-pendicular) and unwritten ones [see Fig. 2(a,c)],indicating that the mechanism responsible for thedichroism is a reorientation of the trans azo mole-cule. The dichroic ratios for the different networksare given in Table 1. They were measured onrelaxed films and were evaluated from the maxi-mum absorbance recorded with polarized lightparallel and perpendicular to the writing direc-tion. As seen from their values, the level of opti-cally induced dichroism stable is in agreementwith the remaining anisotropy, as expected.

CONCLUSIONS

In this study we have investigated the photoin-duced birefringence and dichroism of azo chro-mophore in two series of crosslinked epoxy-basedazopolymers, each with distinct differences inthe molecular structure of the unit buildingblocks.

Measurements of the birefringence (Dn)induced with linearly polarized 488 nm light showthat the Dn values increases with chromophoreweight fraction. Networks with the same chromo-phore concentration but different backbones ex-hibit similar levels of induced anisotropy underthe same irradiation conditions. The remainingbirefringence and relaxation times are influencedby the molecular weight between crosslinks ofnetworks. Higher Mc is reflected in shorter relax-ation times and lower remaining birefringence. Interms of the level of induced dichroism measuredon relaxed films, it was found an agreement withthe remaining anisotropy.

Azobenzene containing epoxy networks are aclass of photosensitive materials which are char-acterized by a very high thermal, optical, and me-chanical stability. Furthermore, it has been dem-onstrated that the presence of crosslinks has nosignificant influence on the photo-orientation pro-

cess. These properties together with the high val-ues of photoinduced birefringence for low chromo-phore content, and high remaining birefringencefraction make these materials promising for opti-cal storage applications, waveguides, and secondharmonic generation.

This work was supported by a grant from University ofBasque Country (UPV/EHU), Grupo Consolidado IT-365-07, MAT2006-06331-MEC and inanoGUNE pro-jects. The authors also thank the financial support ofUniversity of Mar del Plata, CONICET, and ANPCyT(Argentina).

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