- Email: [email protected]
Formation of a buried density grating on thermal erasure of azobenzene polymer surface gratings
Colloids and Surfaces A: Physicochemical and Engineering Aspects 198– 200 (2002) 31 – 36 www.elsevier.com/locate/colsurfa
Formation of a buried density grating on thermal erasure of azobenzene polymer surface gratings Th. Geue a,*, O. Henneberg a, J. Grenzer a, U. Pietsch a, A. Natansohn b, P. Rochon c, K. Finkelstein d a
Institute of Physics, Uni6ersity of Potsdam, P.O. Box 601553, D-14415 Potsdam, Germany b Department of Chemistry, Queen’s Uni6ersity, Kingston, Ont., Canada K7L 3N6 c Department of Physics, Royal Military College, Kingston, Ont., Canada K7K 5L0 d CHESS, 275 Wilson Laboratory, Cornell Uni6ersity, Ithaca, NY 14853, USA Received 30 August 2000; accepted 22 January 2001
Abstract The transformation of a lateral surface relief grating inscribed on a polymer film containing azobenzene moieties into a density grating of equal spacing buried under a smooth surface was found for the polymer poly{(4-nitrophenyl)[4-[[2-(methacryloyloxy)-ethyl]ethylamino]phenyl]diazene} (pDR1M) using an X-ray scattering experiment. Annealing a polymer sample pre-inscribed with a surface grating for several hours above the glass transition temperature creates a nearly sinusoidal lateral density difference up to about 10%. The new state is stable between room temperature and the decomposition temperature of the original polymer when the formation was performed under the influence of VIS light. Growth of liquid-crystalline aggregates is proposed as a most probable explanation for the process. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface grating; Azobenzene; Polymer films PACS numbers: 61.10.Kw; 61.44.e; 68.35.Rh; 68.65. + g; 42.25.Fx
1. Introduction It is well-known that a sinusoidal relief pattern can be inscribed on the surface of amorphous polymer films containing azobenzene moieties [1,2]. This is done by exposing the sample to a periodic holographic pattern obtained using polarised light at a wavelength of about 500 nm. The * Corresponding author.
physical origin of the behaviour is still not well established. It is known however that the same light induces trans –cis photoisomerization of the azobenzene chromophores and subsequent molecular movement seen by the appearance of birefringence and dichroism in the film. A number of mechanisms have been proposed to explain the massive displacement that occurs even at low laser powers [3–6]. In an attempt to understand the processes involved, we have recently studied the thermal erasure of the surface relief gratings [7].
0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 0 9 1 1 - 6
32
Th. Geue et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 31–36
During these investigations we found that the surface grating is transformed into a weak density grating below the sample surface when the sample is heated at temperatures just below the glass transition temperature (Tg) of the initial polymer material [8]. This type of behaviour implies modification of the material even during the initial
surface grating formation. The geometric structure of this state and the kinetics of the process are not clear. It is assumed to be a more densely packed polymer induced by direct shear flow or a liquid crystalline arrangement of polymer side groups. In the present study it is found that when the sample is heated above Tg, the surface relief gratings disappear, but that in the case of the poly{(4nitrophenyl)[4-[[2-(methacryloyloxy)-ethyl]ethylamino]phenyl]diazene} (pDR1M, Tg = 129 °C, see Fig. 1) polymer, the density grating starts growing. A high-temperature treatment above Tg (T\ 140 °C) of several hours creates a lateral density difference of about 10%. This requires substantial material modification. The formation of this new density grating not only raises interesting questions on the fundamental processes but also invites consideration of new applications for the material [8].
2. Experimental
Fig. 1. Side-chain polymer pDR1M.
Fig. 2. Experimental set-up for X-ray/VIS experiment.
The sample consists of a polymer film 400 nm thick (pDR1M, Tg = 129 °C) [9] that has been spin coated onto a clean glass substrate. Surface relief gratings were inscribed onto the polymer films using the holographic interference pattern produced by contra-circularly polarised beams obtained from the argon ion laser operating at the wavelength of 488 nm and at a power density of 150 mW cm − 2 (Fig. 2). A three-minute inscription time resulted in a grating depth of about 170 nm. The spacing period was selected to be about D= 1.33 mm. We then performed time and temperature resolved measurements of the surface gratings by simultaneous illumination with coherent synchrotron source X-rays under grazing angles (uX = 0.1388/0.144 nm) and visible light (uVIS = 660/632 nm) at higher angles of incidence (up to 75°). This allows the investigation of the grating on different length scales. Due to refraction effects, the penetration depth of monochromatic X-rays within the samples can be tuned down to about 50 nm which gives mainly information from the near surface region of the sample. The X-ray experiments carried out are easily
Th. Geue et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 31–36
Fig. 3. Structure of reciprocal space for surface gratings and resulting PSD curves.
described in terms of reciprocal space geometry. The incoming monochromatic X-ray plane wave hits the sample at an angle of incidence hi and will be scattered to an angle of exit hf. Within the scattering plane of our experiment the components qx, qz of the reciprocal scattering vector q are constructed by adding the vectors of incoming and outgoing beam are determined by 2y q6 z = ( sin2 hi −sin2 hc + sin2 hf −sin2 hc) u :
2y (hi + hf) u
2y y q6 x = (cos hf −cos hi) : (h 2i −h 2f ) u u
(1) (2)
using small angles hi, hf. hc is the critical angle of total external reflection. In the case of a lateral structure grating, side peaks appear (Fig. 3), spaced by Dqx =2y/Deff while Deff =D/sin .
3. Results and discussion We earlier reported on thermal erasure of the surface gratings next to Tg [7]. Up to now, it had been assumed that the surface relief pattern was
33
erased when the polymer becomes an isotropic melt when heated close to or above its glass transition temperature Tg. However, in the case of the amorphous polar azobenzene side-chain polymer (pDR1M) a new lateral density pattern was found after erasure of the surface pattern upon long-term annealing at a temperature above Tg [8]. In order to understand the process of transformation from a surface into density pattern we performed temperature-resolved coherent X-ray and VIS light scattering measurements. Typical X-ray results are illustrated by the qx -scans across grating side peaks measured at qz = 0.05 A, − 1 (Fig. 4). Upon increasing the temperature to Tg (square) the X-ray grating peak intensities increase to a maximum while the VIS signal (first diffracted order) almost vanishes. The AFM inspection reveals the complete erasure of the surface structure (not shown here). At T\ Tg the X-ray grating peak intensities go down to zero (down triangle) but upon further heating at a temperature above Tg (up triangle) a completely new type of grating develops under the former surface grating (not in the other areas of the spin cast film!) representing a modulated density distribution with exactly the same period and position the previous surface grating had (circle). Our initial estimate from the X-ray scattering experiment gave a density difference of about 10% between the buried grating and the remaining bulk polymer. In the present investigations we determined the time dependence of density grating formation at different depths below the surface. To do this, the sample was held at 160 °C for 14 h. An X-ray experiment done after the formation of the density grating gives a qx –qz -reciprocal space map (Fig. 5) that has a larger number of grating peaks than the initial surface grating. These are produced by a highly correlated density modulation within the pDR1M film. After erasure of the surface relief grating, the sample surface appears to be smooth therefore, the new grating that appears has to be formed entirely in the bulk of the polymer. Instead of a surface relief with an amplitude d, an intrinsic density grating is formed that the VIS refractive index varies nearly sinusoidally as n(x)=Dn sin(2yx/D). From the diffraction efficiency of the VIS diffraction we
34
Th. Geue et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 31–36
estimate that Dn/n :10%. This is a large amount and requires a fundamental modification of the material into a new state. The new state can be formed without the influence of UV – VIS light, indicating that the trans– cis isomerisation of the azobenzene is not required. Up to now this new
phenomenon was observed in pDR1M only, a polymer that has donor–acceptor groups on the azobenzene. The most probable mechanism for the formation of the density grating seems to be the formation of a liquid crystalline or crystalline phase within the gratings. It has been demon-
Fig. 4. qx -scans for in-situ density grating formation.
Fig. 5. qx –qz -map for a density grating with D =1.33 mm after cooling down to room temperature.
Th. Geue et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 31–36
35
Fig. 6. Model description for the density grating formation process.
strated [10] that the azobenzene groups acquire a certain preferred orientation during the inscription process, depending on the laser polarisation. Under the inscription conditions used here (Fig. 6), the azobenzene groups may be oriented almost vertically in the peaks (1) and almost flat along the troughs (2) at the end of the inscription pro-
cess. When heating starts, the flow direction of the azobenzene groups from the peaks may push them into the material’s bulk, thus creating a phase of higher density and higher orientation of the azobenzene side groups (3). Further heating after the surface grating has been flattened, may serve as some kind of a nucleation process that
36
Th. Geue et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 31–36
allows the formation of an ordered phase in the place where the peaks were (4). At the trench sites the molecular organisation of the material would remain similar to the initial polymer film. This would create a density grating of the same period as the original surface grating. The temperature must be above the glass transition temperature of the initial film, in order to allow flow and significant self-organisation. pDR1M is not known to form crystalline or liquid crystalline phases, mainly because of the relatively short spacer between the rigid rod group (azobenzene) and the main polymer chain. However, the substituted azobenzene in pDR1M is a known mesogen and does form polymer liquid crystals when its motion can be decoupled from the main chain motion via longer spacers. The appearance of an ordered phase in polymers is always associated with heating above Tg, thus providing the motion required for alignment. We have also observed that the density grating formation was more thermally stable if it was created under illumination of red laser light (632 nm). This light is not highly absorbed by the material but it affects the growth rate and the final density of the new state. On the other hand, the absence of red probing light led to a collapse of the density grating in a second heating cycle. We have no explanation for this behaviour at this time. It could imply that organised phases (liquid crystalline phases?) may be induced not only thermally or in solution (as with the classic thermotropic and lyotropic liquid crystal polymers), but also by using light under certain circumstances. While the case of photoinduced LC-isotropic transitions [11] and
vice versa [12] is well documented, this would be the first instance — to our knowledge — when an ordered phase has been photoinduced in a polymer material with no known thermotropic liquid crystalline behaviour.
Acknowledgements The authors thank A. Pucher and W. Regenstein (University of Potsdam) for technical assistance and the DFG (grant Pi 217/17-1) for financial support.
References [1] P. Rochon, E. Batalla, A. Natansohn, Appl. Phys. Lett. 66 (1995) 136. [2] D.Y. Kim, S.K. Tripathy, L. Li, J. Kumar, Appl. Phys. Lett. 66 (1995) 1166. [3] C.L. Barrett, P.L. Rochon, A.L. Natansohn, J. Chem. Phys. 109 (1998) 1505. [4] T.G. Pedersen, P.M. Johansen, N.C. Holme, P.S. Ramanujam, S. Hvilsted, Phys. Rev. Lett. 80 (1998) 89. [5] N.K. Viswanathan, S. Balasubramanian, L. Li, J. Kumar, S. Tripathy, J. Phys. Chem. B 102 (1998) 6064. [6] P. Lefin, C. Fiorini, J.-M. Nunzi, Pure Appl. Opt. 7 (1998) 71. [7] T. Geue, M. Schultz, J. Grenzer, U. Pietsch, A. Natansohn, P. Rochon, J. Appl. Phys. 87 (11) (2000) 7712. [8] U. Pietsch, P. Rochon, A. Natansohn, Adv. Mater. 12 (15) (2000) 1129. [9] M.S. Ho, A. Natansohn, P. Rochon, Macromolecules 28 (1995) 6124. [10] F. Lagugne´ Labarthet, T. Buffeteau, C. Sourisseau, J. Phys. Chem. B 102 (1998) 5754. [11] T. Ikeda, et al., Macromolecules 23 (1990) 42. [12] T. Geue, et al., Mol. Cryst. Liq. Cryst. 329 (1999) 113.
Formation of a buried density grating on thermal erasure of azobenzene polymer surface gratings Th. Geue a,*, O. Henneberg a, J. Grenzer a, U. Pietsch a, A. Natansohn b, P. Rochon c, K. Finkelstein d a
Institute of Physics, Uni6ersity of Potsdam, P.O. Box 601553, D-14415 Potsdam, Germany b Department of Chemistry, Queen’s Uni6ersity, Kingston, Ont., Canada K7L 3N6 c Department of Physics, Royal Military College, Kingston, Ont., Canada K7K 5L0 d CHESS, 275 Wilson Laboratory, Cornell Uni6ersity, Ithaca, NY 14853, USA Received 30 August 2000; accepted 22 January 2001
Abstract The transformation of a lateral surface relief grating inscribed on a polymer film containing azobenzene moieties into a density grating of equal spacing buried under a smooth surface was found for the polymer poly{(4-nitrophenyl)[4-[[2-(methacryloyloxy)-ethyl]ethylamino]phenyl]diazene} (pDR1M) using an X-ray scattering experiment. Annealing a polymer sample pre-inscribed with a surface grating for several hours above the glass transition temperature creates a nearly sinusoidal lateral density difference up to about 10%. The new state is stable between room temperature and the decomposition temperature of the original polymer when the formation was performed under the influence of VIS light. Growth of liquid-crystalline aggregates is proposed as a most probable explanation for the process. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface grating; Azobenzene; Polymer films PACS numbers: 61.10.Kw; 61.44.e; 68.35.Rh; 68.65. + g; 42.25.Fx
1. Introduction It is well-known that a sinusoidal relief pattern can be inscribed on the surface of amorphous polymer films containing azobenzene moieties [1,2]. This is done by exposing the sample to a periodic holographic pattern obtained using polarised light at a wavelength of about 500 nm. The * Corresponding author.
physical origin of the behaviour is still not well established. It is known however that the same light induces trans –cis photoisomerization of the azobenzene chromophores and subsequent molecular movement seen by the appearance of birefringence and dichroism in the film. A number of mechanisms have been proposed to explain the massive displacement that occurs even at low laser powers [3–6]. In an attempt to understand the processes involved, we have recently studied the thermal erasure of the surface relief gratings [7].
0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 0 9 1 1 - 6
32
Th. Geue et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 31–36
During these investigations we found that the surface grating is transformed into a weak density grating below the sample surface when the sample is heated at temperatures just below the glass transition temperature (Tg) of the initial polymer material [8]. This type of behaviour implies modification of the material even during the initial
surface grating formation. The geometric structure of this state and the kinetics of the process are not clear. It is assumed to be a more densely packed polymer induced by direct shear flow or a liquid crystalline arrangement of polymer side groups. In the present study it is found that when the sample is heated above Tg, the surface relief gratings disappear, but that in the case of the poly{(4nitrophenyl)[4-[[2-(methacryloyloxy)-ethyl]ethylamino]phenyl]diazene} (pDR1M, Tg = 129 °C, see Fig. 1) polymer, the density grating starts growing. A high-temperature treatment above Tg (T\ 140 °C) of several hours creates a lateral density difference of about 10%. This requires substantial material modification. The formation of this new density grating not only raises interesting questions on the fundamental processes but also invites consideration of new applications for the material [8].
2. Experimental
Fig. 1. Side-chain polymer pDR1M.
Fig. 2. Experimental set-up for X-ray/VIS experiment.
The sample consists of a polymer film 400 nm thick (pDR1M, Tg = 129 °C) [9] that has been spin coated onto a clean glass substrate. Surface relief gratings were inscribed onto the polymer films using the holographic interference pattern produced by contra-circularly polarised beams obtained from the argon ion laser operating at the wavelength of 488 nm and at a power density of 150 mW cm − 2 (Fig. 2). A three-minute inscription time resulted in a grating depth of about 170 nm. The spacing period was selected to be about D= 1.33 mm. We then performed time and temperature resolved measurements of the surface gratings by simultaneous illumination with coherent synchrotron source X-rays under grazing angles (uX = 0.1388/0.144 nm) and visible light (uVIS = 660/632 nm) at higher angles of incidence (up to 75°). This allows the investigation of the grating on different length scales. Due to refraction effects, the penetration depth of monochromatic X-rays within the samples can be tuned down to about 50 nm which gives mainly information from the near surface region of the sample. The X-ray experiments carried out are easily
Th. Geue et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 31–36
Fig. 3. Structure of reciprocal space for surface gratings and resulting PSD curves.
described in terms of reciprocal space geometry. The incoming monochromatic X-ray plane wave hits the sample at an angle of incidence hi and will be scattered to an angle of exit hf. Within the scattering plane of our experiment the components qx, qz of the reciprocal scattering vector q are constructed by adding the vectors of incoming and outgoing beam are determined by 2y q6 z = ( sin2 hi −sin2 hc + sin2 hf −sin2 hc) u :
2y (hi + hf) u
2y y q6 x = (cos hf −cos hi) : (h 2i −h 2f ) u u
(1) (2)
using small angles hi, hf. hc is the critical angle of total external reflection. In the case of a lateral structure grating, side peaks appear (Fig. 3), spaced by Dqx =2y/Deff while Deff =D/sin .
3. Results and discussion We earlier reported on thermal erasure of the surface gratings next to Tg [7]. Up to now, it had been assumed that the surface relief pattern was
33
erased when the polymer becomes an isotropic melt when heated close to or above its glass transition temperature Tg. However, in the case of the amorphous polar azobenzene side-chain polymer (pDR1M) a new lateral density pattern was found after erasure of the surface pattern upon long-term annealing at a temperature above Tg [8]. In order to understand the process of transformation from a surface into density pattern we performed temperature-resolved coherent X-ray and VIS light scattering measurements. Typical X-ray results are illustrated by the qx -scans across grating side peaks measured at qz = 0.05 A, − 1 (Fig. 4). Upon increasing the temperature to Tg (square) the X-ray grating peak intensities increase to a maximum while the VIS signal (first diffracted order) almost vanishes. The AFM inspection reveals the complete erasure of the surface structure (not shown here). At T\ Tg the X-ray grating peak intensities go down to zero (down triangle) but upon further heating at a temperature above Tg (up triangle) a completely new type of grating develops under the former surface grating (not in the other areas of the spin cast film!) representing a modulated density distribution with exactly the same period and position the previous surface grating had (circle). Our initial estimate from the X-ray scattering experiment gave a density difference of about 10% between the buried grating and the remaining bulk polymer. In the present investigations we determined the time dependence of density grating formation at different depths below the surface. To do this, the sample was held at 160 °C for 14 h. An X-ray experiment done after the formation of the density grating gives a qx –qz -reciprocal space map (Fig. 5) that has a larger number of grating peaks than the initial surface grating. These are produced by a highly correlated density modulation within the pDR1M film. After erasure of the surface relief grating, the sample surface appears to be smooth therefore, the new grating that appears has to be formed entirely in the bulk of the polymer. Instead of a surface relief with an amplitude d, an intrinsic density grating is formed that the VIS refractive index varies nearly sinusoidally as n(x)=Dn sin(2yx/D). From the diffraction efficiency of the VIS diffraction we
34
Th. Geue et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 31–36
estimate that Dn/n :10%. This is a large amount and requires a fundamental modification of the material into a new state. The new state can be formed without the influence of UV – VIS light, indicating that the trans– cis isomerisation of the azobenzene is not required. Up to now this new
phenomenon was observed in pDR1M only, a polymer that has donor–acceptor groups on the azobenzene. The most probable mechanism for the formation of the density grating seems to be the formation of a liquid crystalline or crystalline phase within the gratings. It has been demon-
Fig. 4. qx -scans for in-situ density grating formation.
Fig. 5. qx –qz -map for a density grating with D =1.33 mm after cooling down to room temperature.
Th. Geue et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 31–36
35
Fig. 6. Model description for the density grating formation process.
strated [10] that the azobenzene groups acquire a certain preferred orientation during the inscription process, depending on the laser polarisation. Under the inscription conditions used here (Fig. 6), the azobenzene groups may be oriented almost vertically in the peaks (1) and almost flat along the troughs (2) at the end of the inscription pro-
cess. When heating starts, the flow direction of the azobenzene groups from the peaks may push them into the material’s bulk, thus creating a phase of higher density and higher orientation of the azobenzene side groups (3). Further heating after the surface grating has been flattened, may serve as some kind of a nucleation process that
36
Th. Geue et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 198–200 (2002) 31–36
allows the formation of an ordered phase in the place where the peaks were (4). At the trench sites the molecular organisation of the material would remain similar to the initial polymer film. This would create a density grating of the same period as the original surface grating. The temperature must be above the glass transition temperature of the initial film, in order to allow flow and significant self-organisation. pDR1M is not known to form crystalline or liquid crystalline phases, mainly because of the relatively short spacer between the rigid rod group (azobenzene) and the main polymer chain. However, the substituted azobenzene in pDR1M is a known mesogen and does form polymer liquid crystals when its motion can be decoupled from the main chain motion via longer spacers. The appearance of an ordered phase in polymers is always associated with heating above Tg, thus providing the motion required for alignment. We have also observed that the density grating formation was more thermally stable if it was created under illumination of red laser light (632 nm). This light is not highly absorbed by the material but it affects the growth rate and the final density of the new state. On the other hand, the absence of red probing light led to a collapse of the density grating in a second heating cycle. We have no explanation for this behaviour at this time. It could imply that organised phases (liquid crystalline phases?) may be induced not only thermally or in solution (as with the classic thermotropic and lyotropic liquid crystal polymers), but also by using light under certain circumstances. While the case of photoinduced LC-isotropic transitions [11] and
vice versa [12] is well documented, this would be the first instance — to our knowledge — when an ordered phase has been photoinduced in a polymer material with no known thermotropic liquid crystalline behaviour.
Acknowledgements The authors thank A. Pucher and W. Regenstein (University of Potsdam) for technical assistance and the DFG (grant Pi 217/17-1) for financial support.
References [1] P. Rochon, E. Batalla, A. Natansohn, Appl. Phys. Lett. 66 (1995) 136. [2] D.Y. Kim, S.K. Tripathy, L. Li, J. Kumar, Appl. Phys. Lett. 66 (1995) 1166. [3] C.L. Barrett, P.L. Rochon, A.L. Natansohn, J. Chem. Phys. 109 (1998) 1505. [4] T.G. Pedersen, P.M. Johansen, N.C. Holme, P.S. Ramanujam, S. Hvilsted, Phys. Rev. Lett. 80 (1998) 89. [5] N.K. Viswanathan, S. Balasubramanian, L. Li, J. Kumar, S. Tripathy, J. Phys. Chem. B 102 (1998) 6064. [6] P. Lefin, C. Fiorini, J.-M. Nunzi, Pure Appl. Opt. 7 (1998) 71. [7] T. Geue, M. Schultz, J. Grenzer, U. Pietsch, A. Natansohn, P. Rochon, J. Appl. Phys. 87 (11) (2000) 7712. [8] U. Pietsch, P. Rochon, A. Natansohn, Adv. Mater. 12 (15) (2000) 1129. [9] M.S. Ho, A. Natansohn, P. Rochon, Macromolecules 28 (1995) 6124. [10] F. Lagugne´ Labarthet, T. Buffeteau, C. Sourisseau, J. Phys. Chem. B 102 (1998) 5754. [11] T. Ikeda, et al., Macromolecules 23 (1990) 42. [12] T. Geue, et al., Mol. Cryst. Liq. Cryst. 329 (1999) 113.