Holographic grating recording in azobenzene

DOI: 10.2478/s11532-006-0003-7 Research article CEJC 4(2) 2006 266–284

Holographic grating recording in azobenzene functionalized polymers Anna Sobolewska1∗ , Andrzej Miniewicz1† , Eugenia Grabiec2 , Danuta Sek2 1

Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, 50-370 Wroclaw, Poland 2

Centre of Polymer Chemistry, Polish Academy of Sciences, 41-819 Zabrze, Poland

Received 12 August 2005; accepted 23 November 2005 Abstract: A group of 22 polymers have been synthesized to test their suitability for recording holographic gratings. Polyamides, polyimides, polyesters and their combinations were functionalized with pendant azobenzene groups containing single or double N=N. The polymers were studied using a standard degenerate two-wave mixing technique, which enables measurement of light-induced periodic modification of polymer refractive index and absorption coefficient by analysis of the diffracted light. Two qualitatively different configurations of the holographic polarization recording were used, s − s and s − p. The relationship between structural properties of polymer matrix and azobenzene groups and the holographic grating recording kinetics and light diffraction efficiency was investigated. c Versita Warsaw and Springer-Verlag Berlin Heidelberg. All rights reserved. Keywords: Azobenzene functionalized polymers, azo-polymers, refractive index, light scattering, holographic grating recording, polarization recording

1

Introduction

Polymers functionalized with an azobenzene chromophore have been of considerable interest in the last decade. Such polymers are potentially useful as materials for optical information storage, polarization holography, and photonic devices [1–9]. These materials’ optical properties can be modified by absorbed light due to efficient ∗ †

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photoisomerization of the -N=N- in azobenzene groups. Inducing their photoreorientation with polarized light using wavelengths within an absorption band of the azo group is well known [7–11]. Molecular reorientation, a consequence of the angular hole burning due to multiple trans-cis-trans photoisomerization cycles, leads to photoinduced birefringence and dichroism [7, 12, 13]. The reversible photoisomerization process can also lead to mass transport, resulting in the formation of surface relief gratings [4–6, 14–28]. The polymer mass redistribution induced by an interference pattern of two laser beams takes place well below the polymer’s glass transition temperature Tg . Numerous mechanisms [14–28] have been proposed to explain the origin of surface relief gratings in polymers functionalized with azobenzenes. They include thermal gradient mechanisms, asymmetric diffusion based on the creation of a concentration gradient [21–23], isomerization pressure [24, 25], mechanisms based on electromagnetic forces – mean field theory [18, 26], permittivity gradient theory [27] and gradient electric force [15, 19, 28]. Studying a range of polymers with the same chromophore under the same experimental conditions should shed some light on the relationship between polymer matrix and the magnitude of photoinduced optical anisotropy. This paper is organized as follows: first, we note the syntheses and structures of the polymers; next we briefly describe the experimental technique and discuss the kind of information that can be obtained. In the section devoted to the dynamics of recording diffraction gratings we present the kinetic results and a discussion of their relationship to polymer properties. These studies are designed to select the best polymer for light processing photonic devices in terms of response speed and light sensitivity.

2

Experimental

2.1 Synthesis and characterization of studied polymers In this section we present polymers functionalized with azobenzene units and suitable for film forming either by casting or spin-coating. First, three different azo compounds (later used as the pendant groups) were synthesized: 2,4-diamino-4’-nitroazobenzene (1), 2,4-diamino-4-azo-(4’-nitroazobenzene)benzene (2) and 2,4-dihydroxy-4’-nitroazobenzene (azo-diol) (3). The structures of these monomers are shown in Scheme 1. Details of these monomers’ syntheses and characterizations i.e. NMR, FTIR spectra and elemental analyses can be found in Ref. [29] for diamines (1) and (2), and in Ref. [30] for azo-diol (3). Next, four polymer families with the azobenzene units covalently attached were synthesized, including polyamides, polyesters, poly(ester-imide)s and poly(amide-imide)s. For the poly(amide-imide)s, we prepared polymers with azo groups placed in the diamidedianhydride moiety, where a diamine with the chromophore is bonded via the amide group, as well as those containing azodiamine bonded via the imide. The syntheses and characterizations (NMR, FTIR spectra and elemental analyses) have been


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H2N

NH2

N N H2N

NH2

HO

N

a)

OH

N

N

N

N

N

NO2

NO2

NO2

b)

c)

Scheme 1 Molecular structures of 2,4 - diamino - 4’ – nitroazobenzene (1), 2,4 - diamino - 4 - azo - (4’ - nitroazobenzene)benzene (2) and 2,4 - dihydroxy - 4’ - nitroazobenzene (3). reported: poly(ester-imide)s (1a-e) in Ref. [30], poly(amide-imide)s (2a-d) in Ref. [31], poly(amide-imide)s (3a-d) in Ref. [29], polyamides (4a-e) in Ref. [32], polyesters (5a-d) in Ref. [33]. The polymer structures are depicted in Table 1. Included are the glass transition temperatures Tg (measured by differential scanning calorimetry) and the reduced viscosities ηred (measured in NMP at 25 ◦ C using an Ubbelohde viscometer). Labeling of the polymers will simplify their discussion.

2.2 Optical characterization of polymers All polymers were prepared as thin films cast on glass plates. The film thickness was measured with a Dektak profilometer and ranged from 0.5 to 2 μm. The morphology was observed with an OLYMPUS BX60 optical microscope equipped with a CCD color camera. Most films (all poly(ester-imide)s, all poly(amide-imide)s, polyamides, polyester (5b)) were homogenous and of good optical quality, especially when cast from p-chlorophenol. For some polymers dimethylacetamide (DMA) was used as solvent. High optical film quality is indispensable for holographic recording . The absorbance spectra for films cast on glass plates were measured over the range 300 – 750 nm and are shown in Fig. 1. The absorption bands in the visible are assigned to strongly absorbing azobenzene groups with electron-donor and -acceptor substituents. Depending on the particular side group and main chain, the absorption edge ranges from ∼ 420 nm to 580 nm and the absorption bands are relatively broad. Irradiating the polymers with polarized laser light within 420 - 580 nm and measuring the photoinduced birefringence and linear dichroism allows investigation of the azobenzene chromophores’ reorientation mechanisms.



3.0

269

(1e) (4c)

2.5

Absorbance [a. u.]

(3b) 2.0 (2a) 1.5 (5b)

1.0 0.5

514.4 nm

0.0 300

400

500

600

700

800

W avelength [nm]

Fig. 1 Representative absorption spectra of azo functionalized polymers as films cast on glass. An arrow indicates the position of the 514.5 nm line used in recording. Curve labeling corresponds to that introduced in Table 1.

2.3 Degenerate two-wave mixing technique The photoinduced birefringence and dichroism are easily probed by light, providing these changes are spatially periodic, e.g. form a diffraction grating. The kinetics of recording holographic gratings was studied using standard degenerate two-wave mixing technique (DTWM). The excitation source was a cw Ar+ laser (Innova 90, Coherent) at 514.5 nm and 1.5 mm beam diameter. The experimental DTWM setup is shown in Fig. 2. mirror

I2

shutter

.s beamsplitter

T

I1

O2 retardation plate

or

+2 +1

.s p

detectors

0 0 polymer foil

–1 –2

laser

power meter

Fig. 2 Degenerate two-wave mixing experimental set-up with the possibility of s − s and s − p polarization grating recording and kinetics measurements, λ = 514.5 nm.


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This technique is particularly sensitive to any light-induced changes in the polymer film, as it is based on the light diffracting from the grating as it is formed. In this technique a linearly polarized beam from the laser source is split into two beams of the same polarization. These two beams, of intensities I1 and I2 , cross each other and form an interference field resulting in light intensity modulation: I(x) = I0 (1 + m cos(Kx)) √ where I0 = I1 + I2 , m is the modulation factor m = 2 I1 I2 /(I1 + I2 ) and K = 2π/Λ is the grating wave-vector [34]. The intersection angle θ between the beams determines the period Λ of the grating. Due to grating formation in the polymer films self-induced diffraction occurs, i.e. spots of first or higher diffraction orders become visible beyond the sample at far field. The self-induced diffraction arises from periodic modulation of refractive index and absorption coefficient in the bulk of the material, as well as possible surface periodic modification (i.e. a relief grating). Measurements of the temporal evolution of the first-order diffracted beam’s power during grating recording enable study of its growth kinetics and estimation of the diffraction efficiency η. The last is defined as the intensity ratio of the first diffracted beam I1,1dif f to the incident beam I1 : η = I1,1dif f /I1 . As a rule, in such materials mixed amplitude and phase gratings are formed. The diffraction efficiency of a phase grating is described by the square of the first order Bessel function J1 [30]: η = |J1 (Δφ)|2 where Δφ is the maximum phase retardation accumulated by a plane wave of wavelength λ transmitted through the system. For a refractive index grating induced within the bulk of a polymer layer of thickness d the maximum phase retardation can be approximated by: Δφbulk = 2πΔnd where λ nmax −nmin Δn is the refractive grating amplitude Δn = . The phase changes experienced 2 ef f by the incoming light can also result from surface relief: Δφrel = π n λ Δh , where nef f is the effective refractive index associated with an inhomogeneous layer (polymer + air) and is estimated using the Maxwell–Garnett approach [31] (here nef f ≈ 1.3), and Δh is the amplitude of surface modulation. Changes in optical absorption result in an amplimin tude grating, expressed by the absorption coefficient modulation Δα = αmax −α . The 2 diffraction efficiency of thin amplitude gratings can be calculated using a transmittance amplitude approach [30].

3

Results and discussion

The time dependence of photoinduced orientation in azobenzene polymers has been analysed by Sekkat and Dumont et al. [35, 36] within a theoretical model considering population changes in the trans and cis states. In this model polarized light induces a selective angular hole burning followed by angular diffusion during the direct trans-cis and reverse cis-trans photoisomerizations. However, this model does not take into account any specific interaction of the azobenzene group with the surrounding polymer, e.g. hydrogen bond formation, polymer chain mobility, or any other matrix physicochemical property that may influence the dynamics of the photoinduced molecular reorientations. Therefore, this study focused on collecting holographic recording data (kinetics of grating recording and diffraction efficiency) under the same experimental conditions for various



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polymer backbones having the same photochromic side-groups. By this we expect to draw conclusions on the role of the backbone and its influence on chromophore reorientation. In all measurements the input light power was kept constant P1 = P2 = 12.5 mW, corresponding to I1 = I2 = 790 mW/cm2 . The low pump irradiance level was chosen to avoid photochemical degradation. The grating period for all measured samples was Λ = 5.9 μm (yielding ∼ 170 lp/mm). Two distinct polarization schemes were employed: s − s when two incoming beams I1 and I2 were linearly polarized with the electric field vector perpendicular to the incident plane, and s − p when one of the polarizations was turned 90◦ with respect to the other. It is important to note that in the case of s − s polarization the light intensity distribution in the sample is x-dependent and has the form of a sinusoidal fringe pattern (cf. eq. 1) while in the case of s − p polarization there is no intensity modulation within the beam’s cross sectional area (I(x) = I0 = constant). However, in the latter case a smooth periodic modulation of polarization with period Λ occurs (i.e. + 45◦ linear is followed by right circular; next, – 45◦ linear is followed by left circular [12]). The inscription of reversible laser induced gratings without spatial intensity modulation is a straightforward proof that irreversible chemical reactions can be neglected. In most of these polymers we have observed efficient grating build-up for both s − s and s − p configurations. Using a LabMaster (Coherent) dual channel laser power meter we monitored the temporal evolution of the first-order diffraction power during the DTWM experiment. Examples of the kinetics of the two processes are shown in Fig. 3 for poly(amide-imide) (3d) and in Fig. 4 for poly(ester-imide) (1c), respectively.

1-order diffraction [ PW ]

24

18

s-p

12

s-s 6

0 0

30

60

90

Tim e [ m in ]

Fig. 3 An example of kinetics of grating recording in poly(amide-imide) (3d) with double N=N bond for s − p and s − s polarization recording. The value of the diffracted power as parameters approximately follows an exponential growth function with time constant τ and a saturation value of the diffracted power. This is typical for many photorefractive systems. The noise amplitude in the diffracted power significantly increases with time. This behavior can be rationalized as follows: during inscription a gradual amplitude increase


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60

40

s-s s-p

20

0 0

4

8

12

Tim e [ m in ]

Fig. 4 An example of kinetics of grating recording in poly(ester-imide) (1c) for s − p and s − s polarization recording. For the s − spolarization the noise free line starting at 7 min shows the grating erasure process i.e. light diffraction when only a single beam is present. in the refractive index grating occurs. This promotes a two-beam coupling process [37] in which power from one input beam is transferred to the other. The amount of light transferred between the beams depends on grating amplitude Δn and an instantaneous phase mismatch between the refractive index grating and the light interference pattern. The phase mismatch occurs because of tiny (∼ 100 nm amplitude) chaotic vibrations of the beam directing mirrors, and is inevitable without active stabilization. In situ reading of the same grating with another laser beam gives an almost noiseless curve (cf. Fig. 5) because for a single beam the beam-coupling cannot occur. Therefore, starting grating erasure by cutting off one of the beams (cf. Fig. 4) results in immediate removal of the noise due to the two-beam coupling. In Fig. 5 we show an example of grating recording and erasure in s − s polarization using 514.5 nm beams, together with a reading of the recorded grating by another beam at 532 nm. It is evident that the excessive noise is present only for the 514.5 nm beams and absent for that at 532 nm. The square root of diffracted power can be used to gain insight into the primary processes of light induced refractive index and absorption changes. The temporal dependencies of the square root of diffracted power signals were fitted using a double-exponential growth function: P1,1dif f (t) = A[1 − exp(−t/τ1r )] + B[1 − exp(−t/τ2r )] where τ1r and τ2r are the recording time constants of the fast and slow grating build-up processes contributing [38, 39]. The decomposition of typical temporal evolution of grating recording into fast and slow contribution is shown in Fig. 6. It was performed for each of the studied polymers and the results are gathered in Table 2. The fast process is due to the trans-cis-trans isomerization and the local mobility of azobenzene groups, which is determined by their size, the free volume around them, and the strength of their coupling to the polymer backbone. The slow process depends on the coupling between azobenzene and polymer segments, and the mobility of whole polymer



273

1st(514) and 1st(532) diff. [PW ]

60 50

1st order of recording beam 514.5 nm

40 30 20

1st order of reading beam 532 nm

10 0 0

200

400

600

800

Time [s]

Fig. 5 Example of grating recording process in azopolymer using degenerate two-wave mixing with 514.5 nm light (s − spolarization) and simultaneous reading with another beam at 532 nm.

3

(P1,1diff)

1/2

[PW ]

1/2

4

2

1

0 0

200

400

600

800

1000

1200

1400

Time [s]

Fig. 6 Curve fitting for obtaining time constants of diffraction grating build-up in a photochromic polymer. The contributing exponential growth functions have parameters: A = 1.18, τ r1 = 24 s and B= 2.16, τ r2 = 226 s. chains. Parameters A and B describe the contribution of these processes to diffraction. The grating build-up process cannot always be described by a simple biexponential growth function. Sometimes the diffraction power reaches a maximum then decreases. Examples are shown in Fig. 7. Such atypical behavior in some azobenzene functionalized polymers has been reported by Yaroshchuk et al. [40]. After long-time irradiation the initially induced optical anisotropy (Δn) converges to a smaller value, which was explained by a preferential out-of-plane molecular orientation of chromophores and polymer chains. Irradiation conditions were identical for all samples so one may assume that the differences in grating build-up dynamics for different polymers having the same pendant groups indicate the difficulty of chromophore reorientation in a rigid matrix. The reasons for these difficulties are: small free volume around photoisomers that hinders their


274


24

s-s

s-p

80

40

0

1-order diffraction [ P W ]


120

s-s 18

12

s-p

6

0 0

4

8

12

0

40

Time [ min ]

Time [ min ]

a)

b)

80

120

Fig. 7 Examples of a typical kinetics of grating build-up process during recording in poly(ester-imide) (1b) (a) and poly(amido-imide) (3c) (b). reorientation [24], and limited local mobility of azobenzene groups (determined by their size), the rigidity of the linkage between the chromophore group and the backbone, or the mobility of polymer chains at ambient temperature. The characteristic parameters of holographic grating recording, for both s − s and s − p polarization geometries, such as time constants, maximum diffracted power and diffraction efficiency, and film thickness for all the investigated functionalized polymers are gathered in 2. The constants τ1r and τ2r reflect the kinetics of grating build-up. We have noted that grating recording is generally faster for s − s polarization than s − p. This can be understood because the recording time constant depends on light intensity; i.e. the time constant is shorter when higher laser light intensities are used for inscription. Given the same intensities I1 = I2 , for s − s recording the intensity at a fringe center is equal to 4I1 , whereas for s − p recording it is only 2I1 and uniform. Faster grating build-up is expected for the s − s configuration. The heat caused by absorption also favors molecular reorientation. Thus, at a bright fringe center in the s − s configuration the molecular reorientation is much faster than anywhere for an s − p configuration. It is generally difficult to draw conclusions by correlating polymer structures with experimental grating growth time constants. The constants τ2r were generally significantly longer for polymers containing chromophores with double N=N groups compared with the same polymers having single N=N groups (cf. τ2r values for (3a) and (3b) polymers versus (3c) and (3d) polymers in Table 2). τ2r was chosen because it describes the freezing of elongated trans molecules perpendicular to the polarization plane of the incoming light. Longer molecules, containing two N=N groups instead of one, require a larger free cage for complete reorientation. Therefore more attempts (i.e. photoisomerization cycles) are needed to complete this angular hole burning (AHB) process [7, 35, 41]. When this is



275

completed, subsequent thermal relaxation of double N=N ordered groups will be much slower than relaxation of single N=N groups for the same polymer matrix. In polymers having double N=N groups (4c and 4d) grating saturation occurs for shorter inscription times using the s − p configuration than the s − s configuration. The opposite holds for most of the polymers having single N=N groups. Polymers with the same azo group have shorter time constants for polymers with longer aliphatic chains. This can be explained easily: a longer aliphatic chain is more flexible, leaving more freedom for chromophores to perform their trans- to-cis transitions. This is also correlated with the glass transition temperatures: the longer the aliphatic polymer chain, the lower Tg . Keeping the backbone and azo group the same, but changing the point of attachment (cf. chemical structures of (3a) and (2a), (3b) and (2b)) influences the recording time constants. The shortest response times were observed in poly(amide-imide)s (group 2) for both type of recording. Attaching the chromophore to the diamidedianhydride moiety (polymers (2a) and (2b)) shortens the time constants compared to attachment via the imide bond (polymers (3a) and (3b)). This suggests that polymer chains (2a) and (2b) are more flexible than (3a) and (3b), which seems confirmed by the latter’s higher Tg values. The molecular origin of this phenomenon may also be linked to the difficulty of hydrogen bond formation between polymer chains caused by azo groups pendant from the diamine chromophore. Amides and esters in the polymer chains of poly(amide-imide)s (2a-d) and poly(esterimide)s (1a-d) also impact the kinetics. With amides, the recording time constant is shorter than with esters (cf. τ r2 values for poly(amide-imide)s and poly(ester-imide)s in Table 2). This behavior is not clear at present because an ester bond is considered more flexible than an amide. Also unexpectedly, Tg of the poly(ester-imide) (1a) is higher than Tg of the poly(amide-imide) (2a). Additional research must be carried out to clarify this case. Determinations of bulk refractive index, absorption coefficient and relief grating amplitude are possible but require sophisticated techniques. These measurements are the subject of separate investigations and will be published elsewhere. At present we cannot interpret the diffraction efficiency η reliably. As mentioned above, η is a function of grating type, sample thickness and optical density. In the case of a pure phase grating one would normalize them by dividing by the squared thickness d2 , but when we consider azo-polymers, where contributions from absorption (amplitude grating) and relief are present, this procedure is not justified. Film optical quality is also important. Imperfections like crystallites, defects, inclusions, etc. having size comparable to or larger than the wavelength used prevent grating recording due to excessive scattering. This was why small diffraction efficiencies were observed for polymers (1d) and (2a), and measuring self-induced diffraction was troublesome for polymers (4e and 5a, c, d and e). For both for s − p and p − p polarization configurations (cf. Table 2) diffraction efficiency is a complex function of the particular mechanisms of grating formation and the


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dominant grating type, i.e. that giving the highest contribution to the diffraction signal. For different polymers the, diffraction efficiency η can be different both in magnitude and in ηs−s /ηs−p (which may be greater or less than unity). We believe that higher chain flexibility and mobility promote larger and faster build-up of population gratings (cis-trans redistribution). Oppositely, larger but slower build-up of polarization gratings (based on trans molecules frozen perpendicularly to the light E-vector plane) is expected for polymers with more rigid polymer chains. The latter is related to polymer glass transition temperature Tg , but not uniquely. On the microscopic scale the important parameters are: (i) position of the chromophore in the polymer chain, (ii) specific interactions like H-bond formation and (iii) environmental conditions allowing chromophores to perform their trans-to-cis transitions e.g. the free volume cage. Other parameters like polymer density and average molecular mass, as well as the optical quality of a layer, its thickness, and absorption at the recording wavelength may seriously influence the results. As pointed out by Onoet al. [42] the diffraction efficiency is usually lowered due to time averaging of the fringe intensity, which in our case might be low due to vibration of the optical components. Moreover, we cannot rule out the appearance of relief gratings, which depend on a light-induced mass diffusion process. It should be stressed that relief gratings’ contribution to diffraction efficiency might in some cases dominate bulk contributions. Evaluation of amplitude, phase, and relief grating contributions to observed diffraction efficiencies is not simple or straightforward [43]. It requires the application of the grating translation method [44–46] supplemented with Atomic Force Microscopy studies. The relief gratings of polymers: (1b), (2d), (3a-d) were studied using AFM. The stability of these holographically recorded gratings was at least 1/2 year (time between recording and AFM measurements; samples were kept at room temperature). Fig. 8 shows the relief grating inscribed in poly(amide-imide) (3b) film using the s − s configuration. The relief amplitude of 100 nm was one of the highest observed in this group of polymers. Grating inscription with the s − p configuration usually gives a very weak surface modulation but is frequently accompanied by a half-period structure. Many of these polymers do not exhibit stable relief gratings; some decay after days or weeks. During s−p polarization grating recording we noticed that the linearly polarized light beam is diffracted into at least three waves: a 0-th order wave which has the same polarization as the incident wave and ±1 order diffracted waves with polarizations orthogonal to the incident wave. This phenomenon does not depend on sample thickness. It has been observed in other materials and explained theoretically [47, 48]. It is illustrated in Fig. 9. The results for some polymers showed that the strength of the mixed refractive/amplitude diffraction grating strongly depends on the way it was recorded. The s − p recording results in a mainly refractive index grating, whereas s − s yields an absorption coefficient grating. It is of great importance to know which type of grating dominates (i.e. gives the largest contribution to light diffracted into first order). Because such a study requires many additional experiments it cannot be performed for such a large group of polymers.



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106,46 nm

a)

b)

Fig. 8 Relief surface grating observed by Atomic Force Microscope in poly(amide-imide) (3b) inscribed with 514.5 nm light, s − s polarization.

-2

-1

0

0

+1

+2

Fig. 9 Self-diffraction of light observed in the case of polarization s − p grating recording in photochromic polymers. Arrows show planes of light polarization observed in the far-field for various diffraction orders. This will be the subject of a forthcoming paper for selected polymers.

4

Conclusions

This work confirmed the possibility of recording holographic gratings in the group of 22 new functionalized polymers, providing the chromophore contains a suitable azobenzene group. Systematic measurements of the grating recording dynamics show that the structural units influence that process. The kind of chromophores (i.e. containing single or double N=N bonds), their place in the polymer chain, the length of aliphatic units in the polymer chains, and the presence of functional groups (amide and ester groups) all play a role. The presence of double N=N bonds or high Tg polymers slows the recording process. Tailoring the structures with azo-groups in different positions on the chains allows tuning the time constants of hologram recording and the durability of recorded gratings.


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After optimization of their optical quality and processability, materials investigated in this work could find applications in optical information storage, polarization holography and photonic devices.

Acknowledgment We thank dr F. Kajzar of CEA Sacaly for enabling us to perform AFM measurements within the POLONIUM program.

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282


Table 1 Chemical structures of polymers functionalized with azo-groups, their glass transition temperatures Tg and reduced viscosities ηred. Polymer

Chemical structure

Ar or R in main polymer chain H3C

Poly(ester-imide)s

185

0.19

156

0.15

177

0.31

136

0.15

165

0.98

CH2 H3C

CH3

H3C

b c

a ηred [dl/g]

CH3

a

1

Tg ◦ [ C]

CH3 CH2

O

O

C

C O

O

O

C

N Ar

N

C

N N

O

CH3

H3C

C

O C

H3C

O

H3C

CH3

CH3

n

d

H3C

NO2

CH3

e Poly(amide-imide)s (having azo-group in amide group)

H3C

177

CH2 H3C

b 2

O

O H

H O

C

C N

N C

c

N Ar

N

O H3C

H3C

H3C

O H

H O

C

C N Ar

N C

0.56

NDb

0.32

NDb

0.77

281

0.19

254

0.20

266

0.29

217

0.40

CH3

H3C

C

NO2

O

NDb

C

n

d

CH3 CH2

O

N N

O

0.60

CH3

H3C

C

a

Poly(amide-imide)s (azo-group in imide group)

CH3

a

N C O

CH3 CH3 CH2

C

C

CH3

H3C

O

O

CH3

H3C

N

CH3

n N N H3C

b

CH3 CH2

NO2

3

c

O

O H

H O

C

C

N C

N Ar

O C N

C

C

O

O

N

N

n N

H3C

CH3 CH2

H3C

N

d

CH3

N

H3C

NO2

CH3 CH2



283

Table 1 (continued) Chemical structures of polymers functionalized with azo-groups, their glass transition temperatures Tg and reduced viscosities ηred. Polymer a

Chemical structure H

H

O

N

N

C

Ar or R in main polymer chain

O C

R

Tg C]

[◦

a ηred [dl/g]

—(CH2 )4 —

156

0.13

—(CH2 )8 —

133

0.33

—(CH2 )4 —

179

0.15

—(CH2 )8 —

154

0.12

233

0.38

—(CH2 )4 —∗

61

0.15

—(CH2 )8 —∗

53

0.20

238

0.15

147

0.17

N N

Polyamides

b NO2

4

c

H

H

O

N

N

C

n

O R

C

(Ar) N N

d n

N N

∗

e NO2

a Polyesters

O

O C

b

R ( Ar)

N N

5

∗

c d

a b ∗

O O C

NO2

n

∗

Reduced viscosity of azo-polymers measured in NMP, concentration = 0.2 g/100 ml at temperature 25 ◦ C, ND – not detected by DSC method. films of poor optical quality not suitable for holographic recording.


284


Table 2 Characteristics of holographic grating recording experiment for s − s and s − p polarizations. τ1r and τ2r are recording time constants of fast and slow components of the P1,1dif f (t)curve, P1,1dif f – maximum of power diffracted into first diffraction order and η is diffraction efficiency, d – thickness of the polymer film. Light power P1 = 12.5 mW. τ1r [s]

τ2r [s]

1

a b c d e

25 26 7 6 24

– – 49 40 230

2

a b c d

0.3 0.85

Polymer

3

4

5

s−s P1,1dif f [μW] 62 84 36 8 11

s−p P1,1dif f [μW]

η [%]

d [μm]

η [%]

τ1r [s]

τ2r [s]

0.50 0.67 0.29 0.06 0.09

19 32 14 16 54

140 210 180 210 1250

79 87 33 7 30

0.63 0.70 0.26 0.06 0.24

1.5 3.9 1.1 0.5 1.7

0.01 0.02

27 28 61 67

9 29 41 153

0.07 0.23 0.33 1.22

0.6 0.8 0.9 1.6

410 470

62 84

0.50 0.67

1.9 3.6

4

– 1 4 3 No saturation 1510 195

1.56

2 4 5 6

a b

15 41

100 240

65 87

0.52 0.70

14 14

c 2 N=N d 2 N=N

11

1830

21

0.17

59

950

14

0.11

44

1220

20

0.16

1.2

a b c 2 N=N d 2 N=N

2 – 36

– – 1280

8 – 45

0.06 – 0.36

6 0.34 35

680 3 410

26 12 8

0.21 0.10 0.06

1.0 0.4 0.9

24

480

11

0.09

35

610

53

0.42

0.5

b

6

22

8

0.06

12

21

4

0.03

1.6

No saturation


3.1