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Polyvinyl acetate with cellulose dinitrate holograms
Optical Materials 28 (2006) 342–349 www.elsevier.com/locate/optmat
Polyvinyl acetate with cellulose dinitrate holograms S. Toxqui-Lo´pez, A. Olivares-Pe´rez *, I. Fuentes-Tapia Instituto Nacional de Astrofı´sica Optica y Electro´nica, A.P. 51 y 216, C.P. 72000 Puebla, Pue., Mexico Received 17 November 2004; accepted 3 January 2005 Available online 3 March 2005
Abstract A new recording material with a high diffraction efficiency of around 90% has the possibility to be applied for replication of computer phase holograms. This material is polyvinyl acetate with cellulose dinitrate, both substances are doped, and it is easy to apply for any substrate without the need for a dark room and critical conditions. This is because the absorption spectra of polyvinyl acetate with cellulose dinitrate is localized in the IR region k = 7.43 lm. We recorded on this material by applying heat by friction pressure on the surface of a binary mask (hologram) by lithographic techniques. 2005 Elsevier B.V. All rights reserved. Keywords: Polyvinyl acetate; Polymers; Cellulose nitrate; Holograms
1. Introduction Cellulose nitrate, known to most people as ÔcelluloidÕ, was the first plastic to have achieved real success. Credit for the invention goes to the British inventor, Alexander Parkes [1], who displayed his material (which he called Parkesine) at the Great International Exhibition in London, 1862. Among other things, he saw his material as a substitute for the increasingly scarce materials ivory and tortoiseshell. To exploit his invention, in 1866 the Parkesine Company, under the direction of the Merriam family and their British Xylonite Company Limited, so that the material (renamed Xylonite by then) began to achieve commercial success. However, credit goes to the Hyatt brothers in America for inventing a material, which they called ÔcelluloidÕ. Through the unlikely work of developing a substitute for ivory, for making billiard balls, they devised a process for manufacturers using a cellulose nitrate composition. In their patent of 1870 they described
*
Corresponding author. Fax: +52 222 247 2940. E-mail address: [email protected] (A. Olivares-Pe´rez).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.01.022
the all-important discovery of the solvent action of camphor on cellulose nitrate [1]. Celluloid, made of cellulose nitrate and camphor, and produced in 1870, was the first synthetic plastic material on the American market. Cellulose is usually derived from cotton, and camphor is added as a plasticizer. The resin is not soluble in turpentine or mineral oil and is liquified by very strong solvents, such as acetone, or esters, such as ethyl acetate. On liquefaction, it can be thinned with toluene. The material becomes very yellow and brittle with age. When employed as an ingredient in picture varnish, its acetone content will dissolve dried oil paint films [1]. On the other hand, the synthesis and patenting of vinyl acetate monomer by Dr Fritz Klatte in 1913, in Germany, provided the foundation for many valuable and now essential plastic products. He found that the catalyzed reaction of acetylene with acetic acid gave a readily polymerizable low boiling liquid (vinyl acetate) to yield a potential range of dense solid materials. These are now often referred to as PVAc or PVA polymers [2]. Klatte and others found that PVAc was compatible with other polymers and plasticizers which could give valuable adhesives and coatings for cellulose and textile
S. Toxqui-Lo´pez et al. / Optical Materials 28 (2006) 342–349 OCH 2NO 2 O (
O O
HO ONO 2
OCH 2NO 2
ONO 2 O HO
343
O
) n
HO
OCH 2NO 2
ONO 2
Fig. 1. Cellulose dinitrate (NC) monomer.
products. From ca.1930, many companies manufactured a range of products such as PVA for liquid solutions and emulsions and hot melt adhesives and paints. From cellulose nitrate (NT) with polyvinyl acetate (PVAc) both of which are synthetic substances, doped with solvents such as acetone and chloroform, we constructed a material with the ability to record images and phase holograms generated by computers [3,4] by a thermal process induced by friction. The absorption spectrum of the polyvinyl acetate with cellulose nitrate is localized in the IR region k = 7.43 lm. This material showed an anaerobic response when applied on a binary mask (acetate kodalith film base) constructed by lithographic techniques [5]. The holographic replication is good and the diffraction efficiency is high, around 90% with phase refraction index modulation. An important characteristic of this material is that it is independent of the developed process; it only corresponds to the cured polymer process. These properties are attractive because it opens up new possibilities of acquiring new parameters of hologram replication as new technology, and controls the phase from material determinates only to control the curing process, induced by temperature by friction applied on the surface of the mask [6].
2. Cellulose nitrate (nitrocellulose NT) This is made from a series of esters of cellulose with up to three nitrate (NO3) groups per monosaccharide unit by the action of concentrated nitric acid on cellulose (for example, cotton waste) in the presence of concentrated sulphuric acid. Fully nitrated cellulose (gun cotton) is explosive, but esters with fewer nitrate groups were once used for making lacquers, rayon, and plastics, such as colored and photographic films, until they were replaced by the nonflammable cellulose acetate. Celluloid is based on cellulose nitrate [7,8]. Cellulose nitrate is also a thermoplastic. Its flammability, the fact that it causes severe deterioration of the materials it supposedly protects (because of the generation of oxides of nitrogen which yield nitric acid in the presence of water), and the fact that it cannot be manufactured with a thickness down to 0.001 in., has precluded its use in preservation work [8,9]. The thermoplastic property derived from nitrate (NO3) groups contained in cellulose, catalyse the pro-
duction of the curing process on adhesive materials as common as cellulose dinitrate shown Fig. 1.
3. Polyvinyl acetate This is vinyl resin and is one of the clear, water-white, thermoplastic synthetic resins produced from its monomer by emulsion polymerization. Polyvinyl acetate, abbreviated as PVAc (see Fig. 2) has the advantage over the other resinous adhesives in that it is available in the form of an emulsion that is readily diluted with water, is easily applied, and is safe to use because it contains no flammable solvents. In addition, there is no need to use preservatives or fungicides because it does not deteriorate quickly and is unaffected by mold or fungi. The emulsion slowly hydrolyzes, however, and should not be stored for more than one or two years before use. Freezing also destroys the emulsion; therefore, precautions must be taken to avoid exposing it to temperatures near or below the freezing point [10,11]. PVAc is not an ideal molding plastic, so the development of many economic and attractive alternatives rapidly ensued based on the ability of vinyl acetate to co-polymerize with many other monomers. Hence, copolymers with vinyl chloride, acrylic monomers, styrene, ethylene and others gave a great range of molding, coating, sheeting, adhesives, insulating materials, etc.
4. Sensitivity hypothesis Our polymer mixture is activated by heat induced by pressure and for anaerobic reaction, permitting a good replication of the image or holograms as changes of zones of crystallized polymer (exposed) and amorphous
H3C
CH3
n O
O
O
O
O CH3
O CH3
CH3
Fig. 2. Polyvinyl acetate (PVAc): structure between brackets is the monomer of vinyl acetate.
S. Toxqui-Lo´pez et al. / Optical Materials 28 (2006) 342–349
344
zones (unexposed). Anaerobic phenomena (without oxygen) by induction correspond to the process of removing oxygen molecules when we apply pressure on the surface from soft holograms with an acetate base assembling on
+ N
O
R
the mixed material polymer (our mask). The metal ions necessary to start the curing process are obtained from silver ions Ag+. Although after the developing process the silver is an Ag0 metal, this is ionized again by the
O O
R
_ O
+ N
_ O
O
(a) H2 C
( OCH 2NO 2 O (
O
OHO
HO
O
O
+
n ONO 2
O
C CH3
(b)
OCH 2NO 2
(
O
n
n
C
CH3
O C
O
HC
Cl
CH 2
ONO 2
H
O O
)
O
OCH 2NO 2
n HC
O
O
H
CH 3
covalent links
Cl
CH 2
H
Cl
H
Cl
Cl
hydogen bridges
Cl OCH 2NO 2
O
OHO
O
O
O H
OCH 2NO 2
ONO 2
O
(
)
O
OCH 2NO 2
ONO 2
n
O
O
HC
C
O
HC
CH3
ONO 2
H n
n
CO(CH3)2
O
OHO
ONO 2
CCl3H
OCH 2NO 2
ONO 2
O
) n
O
) HO
OCH 2NO 2
ONO 2
CH
OCH 2NO 2
ONO 2
O
C
CH3
CH2
CH 2
Cl
Cl
H
Cl
(c)
H
Cl
Cl
Cl
CH2
H3C
C O
O
CH
OCH 2NO 2
OCH 2NO 2
ONO 2
O (
A g+ (-)
n
O
OHO
O
(-) A g+
ONO 2
O
O
)
O
n
(-) A g+
ONO 2
OCH 2NO 2
H2C
H3 C
(d)
C
OH
O
H2C
CH
H3 C
n
C
OH
O
CH
n
Fig. 3. (a) Dipolar resonance derived from cellulose nitrate. (b) Molecular description of the structures involucrate in the mixture, cellulose dinitrate with PVAc diluted with acetone and chloroform solvents. (c) Our emulsion is a liquid with amorphous consistency, which is the result of mixing organic and inorganic compounds. (d) Polymeric crystalline network as consequence of the curing process after appling thermal energy by pressure and friction, with silver halogen ion through photographic emulsion.
S. Toxqui-Lo´pez et al. / Optical Materials 28 (2006) 342–349
action of the nitrate molecule contained in cellulose nitrate [12]. Conventionally, the bonds in the glucosidic molecule R–O–NO2 (nitro group linked to the ring carbon atom) where R is the three chair conjugate glucose structure R = (C24H40 nO20 n) together with the group (NO3)n, where n is the nitration degree, normally between 11.2% and 12.2% to get cellulose dinitrate regularly used to produce some adhesives [13]. One basic reactivity concept from these molecules is their feasibility to generate a dipole due to resonance present in the cellulose nitrate molecule in the nitro group and can be seen in Fig. 3a [14]. It is stabilized by the presence of the metallic ion in the Ag+ mask, it starts curing local process to record the image. Our emulsion can be prepared using the polyvinyl acetate PVAc as a matrix, represented in Fig. 3b. By applying an algebraic structure, we show the complex mixture between the cellulose dinitrate (NC) with polyvinyl acetate (PVAc), diluted with acetone and chloroform solvent. Forming an amorphous solution (Fig. 3c), the hydroxyl radical from NC forms hydrogen bridges with the oxygen molecule of the PVAc at the carbonyl group, and this in turn unite with the carbon trichloride through hydrogen bridges between the chlorine and methyl hydrogen of the PVAc. When a small amount of heat energy is applied, the carbon trichloride changes to the molecular structure as in (3d) and it forms a long polymeric crystalline network as in (3c), and in the unexposed regions, it does not
7
6
NN
Absorbance (a.u)
5
NB
345
change the structural form of the amorphous polymeric network. In Fig. 3d we can see that the hydrogen atom from the hydroxyl group is displaced and incorporated in to the PVAc structure. This is due to the induction of a polarization charge that exists between carbonyl connections of the double bond modifying for a simple bond. As a result of the applied temperature in the amorphous solution, a resonant group of nitrates with double links, start a charge induction to produce a polarization, with partial negative ( ) ion, which is stabilized by the presence of silver (+) ion through a photographic emulsion (our mask) Fig. 3d. Our hologram is recorded in a high contrast commercial kodalith negative film that contains halogens salts such as Ag+Br from Kodak company [15]. The thermal process corresponds to the heat applied to the material by friction when we recorded the hologram. Pressure as a parameter is essential to start the thermal curing and anaerobic reaction to modulate the material by more and less amorphous and crystallized located zones [16]. 4.1. Basic concepts Reactions that harden some polymers can be brought about by any chemical, physical or catalytic reaction. Depending on the type of reaction, these polymers may be single- or two-component commonly as in adhesives. Single-component reaction polymers: These are adhesives that, depending on their type, react to environment humidity, UV radiation or ambient oxygen (aerobic adhesives) or on the exclusion of air, e.g. using metal ions (anaerobic adhesives). Pressure sensitive polymers remain adhesive permanently. They are used wherever the assembly is only temporary and subsequent separation is required. Pressure sensitive polymers are usually applied in the form of adhesive films [16]. As organic materials, polyvinyl acetate (PVAc) is most commonly used as a thermoplastic polymer resin.
4
3
Table 1 Functional groups from our polymer film, after induction heat
2
Functional groups
m (cm 1) Reported [4]
m (cm 1) Results (after inducing heat)
C–O–C
1140–1210
1184 1180
CH3–C @ O
1175–1375
1180 1181
C@O
1750–1375
1754.62
NO2 C-nitrate
1390–1300 1600–1530 1750–1735 1870–1650
1377.05 1345.57 1531 1502–1599
1
0 2000
1500
1000
Wavenumbers
500
(cm-1)
Fig. 4. Infrared spectrum of the PVAc with cellulose dinitrate subsequent to heat application by friction process. The main peak is 7.43 lm.
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346
5. Results 90
Diffraction Efficiency (%)
5.1. IR analysis 80
70
60
50
40
30 0
10
20
30
40
50
60
Friction time (s)
Fig. 5. Diffraction efficiency of gratings, made by doping PVAc with cellulose dinitrate (b spline fit).
Thermoplastic resins are polymers in which the monomeric units are linked together to form two dimensional linear chains that are soluble in a range of solvents. They remain permanently fusible and soluble; some thermoplastic resins may form insoluble, infusible resins after long exposure to light or heat. Such exposure may cause formation of chemical bonds or links, referred to as cross linking, which become established among linear chains to form three-dimensional networks characteristic of thermosetting resins [7].
For our mixes of PVAc with cellulose dinitrate, we obtain a polymer gel–liquid, for which the thermal sensitivity is high (see Fig. 4). Since these are nitrate films based they are extremely flammable. By adjusting the ratio of nitric and sulfuric acids, products of different degrees of nitration can be obtained [8]. For PVAc with cellulose dinitrate, the IR absorbance spectrum of the resultant polymer film after heat induction by friction process is shown in Fig. 4. In the IR spectrum of the (Fig. 4) NN corresponds to the heat induced on the dark areas of the mask an NB belongs to clear areas. Localized regions dark and clear areas to form the binary image that form our computer hologram mask. Dark zones contain a high concentration of metallic silver Ag0 than the clear zones. Nitrate group activates again the silver Ag+ to start the thermal anaerobic polymerization process in our emulsion. Fig. 4 shows an important response at k = 7.43 lm, that corresponds to 1345 cm 1, which indicates stretching vibrations between the ring and the nitrate group, nitro O2N–O–. These events start the polymerization of our emulsion, recording a phase image. On analysis of the vibration, we observed that the absorption bands are almost conserved; we noticed new bands with a strong intensity in regard to the other ones that correspond to the nitrate group in the region 1750–1735, and they belong to the stretching vibrations
Fig. 6. (a) Microphotograph of a sinusoidal grating recorded in our doped polymer and (b) diffraction pattern (a).
S. Toxqui-Lo´pez et al. / Optical Materials 28 (2006) 342–349
between the nitrogen and the oxygen from nitrate group –NO2. 1345 corresponds to the main absorbance peak in Fig. 4, which signifies stretching between the between the ring and the nitrate group nitro O2N–O–, appearing as a 1502–1509 tension asymmetric vibration from the O–NO2 bond in the R–O–NO2 molecule nitrate group linked carbonyl radical (Table 1). 5.2. Modulation analysis For a period of 35 s with heat induced by friction time, we replicate computer hologram gratings obtaining a high diffraction efficiency of around 90.1% which corresponds 37 C, showing 26 diffracted orders. Therefore in Fig. 5 the diffraction efficiency was measured collecting all the diffracted orders produced by the gratings without zero order, divided by the incident light. We show that the maximum number of diffracted orders not only corresponds to a higher efficiency value but, it also corresponds to the points which showed a diffraction efficiency of the order of 49%.
347
It is worth emphasizing that to obtain a fast hologram with this material with a diffraction efficiency of the order of 47.6%, we need only 1 s as the recording time, and in reality this material is fast in absorbing heat energy. However the material does not show linear behaviour, see Fig. 5, and it is difficult to get a standardized parameter for fast holograms, for which the humidity of the environment is very important for recording holograms in the material and the optimum exposure time can be increased. The recording time increases with exposure time and it decreasing start to 50 s after 60 s the films show, a diffuse degradation. For this reason, we cannot have a good replication. Fig. 6a is a microphotograph of a sinusoidal grating in the material, that is polyvinyl acetate doped with cellulose dinitrate, with an amplification of 25·. Fig. 6b shows a diffraction pattern with many diffracted orders. This response can only be caused for phase gratings (Fig. 6a); like the material it is not revealed for watery chemical agents and it does not remove material, therefore we assumed that the modulation is by refraction index.
Fig. 7. (a) Binary image, (b) computer Fourier hologram and (c) computer reconstruction.
348
S. Toxqui-Lo´pez et al. / Optical Materials 28 (2006) 342–349
6. Conclusions
Fig. 8. (a) Fourier hologram microphotograph with a magnification of 100·. (b) Fourier hologram reconstruction showing two conjugate images.
Fig. 7a shows the computer binary image of PVAc + Nc. These abbreviations t correspond to polyvinyl acetate (PVAc) doped with cellulose dinitrate (Nc), created with 300 for 300 pixels and (b) shows a Fourier computer hologram generated with 256 gray levels, and (c) represents the numeric reconstruction of the computer hologram image in 7b. This reconstruction altered its histogram to visualize the numeric reconstruction better. Fig. 8a shows the hologram microstructure from the image in Fig. 7b recorded on the polyvinyl acetate doped with cellulose dinitrate. The microphotograph shows the material modulation with the holographic code transferred by lithographic techniques for heat induction by applying the friction technique. The circular aperture is produced by the eyepiece of the microscope; the magnification was of the order of 100·. Fig. 8b shows the photograph of the reconstructed hologram, projected on a dark paper 2 m from the hologram, showing a pair of conjugate images characteristic of Fourier holograms. This image shows a good diffraction efficiency and presents a low noise.
We obtained adaptable acrylic resin composed mainly of PVAc and cellulose dinitrate, when doped with acetone and chloroform solvents, producing at environmental temperatures an excellent thermo-sensitive material, activated only by heat induced by a friction process. The diffraction efficiency is high of the order of 90.1%, with low noise. Our polymer mixture is activated by heat induced by pressure and for anaerobic reactions, permitting a good replication of the image or holograms as changes of zones of crystallized polymer (exposed) and amorphous zones (unexposed). Anaerobic phenomena start when we apply pressure on the surface from a soft hologram with acetate base assembling on the mixed material polymer (our mask). Our mask soft hologram is recorded with a high contrast commercial kodalith negative film that contains halogen salts such as Ag+Br from Kodak company. The thermal process corresponds to the heat applied to the material by friction when recording the hologram. Pressure as a parameter is essential for starting the thermal curing and anaerobic reaction to modulate the material by more and less amorphous and crystallized located zones. This emulsion can be used to copy or replicate holograms or images, by applying lithographic techniques. Furthermore, the polymer material opens up possibilities to design elements of phase diffraction. One of the advantages of using this material is that it does not require any technique developed to get a phase hologram.
References [1] R.P. Gwinn, P.B. Norton, R. McHenry, The New Ensiclopædia Britannica, 15th Ed., vol. 9, Encilopædia Britannica, Inc., Chicago USA, 1993, p. 159. [2] Plastics Historical Society. Available from:. [3] J.L. Jua´rez-Pe´rez, A. Olivares-Pe´rez, L.R. Berriel-Valdos, Appl. Opt. 36 (1997) 7437. [4] Joseph W. Goodman, Introduction to Fourier Optics, second ed., McGraw-Hill International Editions, New York, 1996, p. 295 (Chapter 9). [5] A. Olivares-Pe´rez, Opt. Eng. 32 (1993) 1257. [6] B. Pinto-Iguanero, A. Olivares-Pe´rez, I. Fuentes-Tapia, Opt. Mater. 20 (2002) 225. [7] S.P. Koob, The Conservator 6 (1982) 31. [8] S.M. Allen, E.L. Thomas, first ed., John Wiler & Sons, Inc., New York, 1999, p. 56 (Chapter 2). [9] L. Philips, Nature 165 (1950) 564. [10] K.G. Budinski, M.K. Budinski, Engineering Materials, sixth ed., Prentice Hall, NJ, 1999, p. 81 (Chapter 3). [11] Miall Laurence Mackenzie, D.W.A. Sharp, A New Dictionary of Chemistry, fourth ed., Wiley, New York, 1968. [12] D.T. Clark, Polymer 22 (1981) 1112.
S. Toxqui-Lo´pez et al. / Optical Materials 28 (2006) 342–349 [13] B.J. Kim, Hsin-Neng, F.-J. Tai, CERL Technical report 99/45, US Army Corps of Engineers, Construction Engineering Research Laboratories USA, 1999, p. 14. [14] A.F. Cotton, W. Geoffrey, Advanced Inorganic Chemistry, ninth ed., Editorial Limusa, S.A. de C.V. Mexico, 2001, p. 501 (Chapter 13).
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[15] Technical Information Instruction Sheet, TI1105, Kodalith Ortho Films 2556,6556, Type 3, Copyright 2001 Kodak Polychrome Graphics, Norwalk, CT 06851 USA. Available from:. [16] G. Gelernter, L.C. Browning, S.R. Harris, C.M. Mason, J. Phys. Chem. 60 (1956) 1260.
Polyvinyl acetate with cellulose dinitrate holograms S. Toxqui-Lo´pez, A. Olivares-Pe´rez *, I. Fuentes-Tapia Instituto Nacional de Astrofı´sica Optica y Electro´nica, A.P. 51 y 216, C.P. 72000 Puebla, Pue., Mexico Received 17 November 2004; accepted 3 January 2005 Available online 3 March 2005
Abstract A new recording material with a high diffraction efficiency of around 90% has the possibility to be applied for replication of computer phase holograms. This material is polyvinyl acetate with cellulose dinitrate, both substances are doped, and it is easy to apply for any substrate without the need for a dark room and critical conditions. This is because the absorption spectra of polyvinyl acetate with cellulose dinitrate is localized in the IR region k = 7.43 lm. We recorded on this material by applying heat by friction pressure on the surface of a binary mask (hologram) by lithographic techniques. 2005 Elsevier B.V. All rights reserved. Keywords: Polyvinyl acetate; Polymers; Cellulose nitrate; Holograms
1. Introduction Cellulose nitrate, known to most people as ÔcelluloidÕ, was the first plastic to have achieved real success. Credit for the invention goes to the British inventor, Alexander Parkes [1], who displayed his material (which he called Parkesine) at the Great International Exhibition in London, 1862. Among other things, he saw his material as a substitute for the increasingly scarce materials ivory and tortoiseshell. To exploit his invention, in 1866 the Parkesine Company, under the direction of the Merriam family and their British Xylonite Company Limited, so that the material (renamed Xylonite by then) began to achieve commercial success. However, credit goes to the Hyatt brothers in America for inventing a material, which they called ÔcelluloidÕ. Through the unlikely work of developing a substitute for ivory, for making billiard balls, they devised a process for manufacturers using a cellulose nitrate composition. In their patent of 1870 they described
*
Corresponding author. Fax: +52 222 247 2940. E-mail address: [email protected] (A. Olivares-Pe´rez).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.01.022
the all-important discovery of the solvent action of camphor on cellulose nitrate [1]. Celluloid, made of cellulose nitrate and camphor, and produced in 1870, was the first synthetic plastic material on the American market. Cellulose is usually derived from cotton, and camphor is added as a plasticizer. The resin is not soluble in turpentine or mineral oil and is liquified by very strong solvents, such as acetone, or esters, such as ethyl acetate. On liquefaction, it can be thinned with toluene. The material becomes very yellow and brittle with age. When employed as an ingredient in picture varnish, its acetone content will dissolve dried oil paint films [1]. On the other hand, the synthesis and patenting of vinyl acetate monomer by Dr Fritz Klatte in 1913, in Germany, provided the foundation for many valuable and now essential plastic products. He found that the catalyzed reaction of acetylene with acetic acid gave a readily polymerizable low boiling liquid (vinyl acetate) to yield a potential range of dense solid materials. These are now often referred to as PVAc or PVA polymers [2]. Klatte and others found that PVAc was compatible with other polymers and plasticizers which could give valuable adhesives and coatings for cellulose and textile
S. Toxqui-Lo´pez et al. / Optical Materials 28 (2006) 342–349 OCH 2NO 2 O (
O O
HO ONO 2
OCH 2NO 2
ONO 2 O HO
343
O
) n
HO
OCH 2NO 2
ONO 2
Fig. 1. Cellulose dinitrate (NC) monomer.
products. From ca.1930, many companies manufactured a range of products such as PVA for liquid solutions and emulsions and hot melt adhesives and paints. From cellulose nitrate (NT) with polyvinyl acetate (PVAc) both of which are synthetic substances, doped with solvents such as acetone and chloroform, we constructed a material with the ability to record images and phase holograms generated by computers [3,4] by a thermal process induced by friction. The absorption spectrum of the polyvinyl acetate with cellulose nitrate is localized in the IR region k = 7.43 lm. This material showed an anaerobic response when applied on a binary mask (acetate kodalith film base) constructed by lithographic techniques [5]. The holographic replication is good and the diffraction efficiency is high, around 90% with phase refraction index modulation. An important characteristic of this material is that it is independent of the developed process; it only corresponds to the cured polymer process. These properties are attractive because it opens up new possibilities of acquiring new parameters of hologram replication as new technology, and controls the phase from material determinates only to control the curing process, induced by temperature by friction applied on the surface of the mask [6].
2. Cellulose nitrate (nitrocellulose NT) This is made from a series of esters of cellulose with up to three nitrate (NO3) groups per monosaccharide unit by the action of concentrated nitric acid on cellulose (for example, cotton waste) in the presence of concentrated sulphuric acid. Fully nitrated cellulose (gun cotton) is explosive, but esters with fewer nitrate groups were once used for making lacquers, rayon, and plastics, such as colored and photographic films, until they were replaced by the nonflammable cellulose acetate. Celluloid is based on cellulose nitrate [7,8]. Cellulose nitrate is also a thermoplastic. Its flammability, the fact that it causes severe deterioration of the materials it supposedly protects (because of the generation of oxides of nitrogen which yield nitric acid in the presence of water), and the fact that it cannot be manufactured with a thickness down to 0.001 in., has precluded its use in preservation work [8,9]. The thermoplastic property derived from nitrate (NO3) groups contained in cellulose, catalyse the pro-
duction of the curing process on adhesive materials as common as cellulose dinitrate shown Fig. 1.
3. Polyvinyl acetate This is vinyl resin and is one of the clear, water-white, thermoplastic synthetic resins produced from its monomer by emulsion polymerization. Polyvinyl acetate, abbreviated as PVAc (see Fig. 2) has the advantage over the other resinous adhesives in that it is available in the form of an emulsion that is readily diluted with water, is easily applied, and is safe to use because it contains no flammable solvents. In addition, there is no need to use preservatives or fungicides because it does not deteriorate quickly and is unaffected by mold or fungi. The emulsion slowly hydrolyzes, however, and should not be stored for more than one or two years before use. Freezing also destroys the emulsion; therefore, precautions must be taken to avoid exposing it to temperatures near or below the freezing point [10,11]. PVAc is not an ideal molding plastic, so the development of many economic and attractive alternatives rapidly ensued based on the ability of vinyl acetate to co-polymerize with many other monomers. Hence, copolymers with vinyl chloride, acrylic monomers, styrene, ethylene and others gave a great range of molding, coating, sheeting, adhesives, insulating materials, etc.
4. Sensitivity hypothesis Our polymer mixture is activated by heat induced by pressure and for anaerobic reaction, permitting a good replication of the image or holograms as changes of zones of crystallized polymer (exposed) and amorphous
H3C
CH3
n O
O
O
O
O CH3
O CH3
CH3
Fig. 2. Polyvinyl acetate (PVAc): structure between brackets is the monomer of vinyl acetate.
S. Toxqui-Lo´pez et al. / Optical Materials 28 (2006) 342–349
344
zones (unexposed). Anaerobic phenomena (without oxygen) by induction correspond to the process of removing oxygen molecules when we apply pressure on the surface from soft holograms with an acetate base assembling on
+ N
O
R
the mixed material polymer (our mask). The metal ions necessary to start the curing process are obtained from silver ions Ag+. Although after the developing process the silver is an Ag0 metal, this is ionized again by the
O O
R
_ O
+ N
_ O
O
(a) H2 C
( OCH 2NO 2 O (
O
OHO
HO
O
O
+
n ONO 2
O
C CH3
(b)
OCH 2NO 2
(
O
n
n
C
CH3
O C
O
HC
Cl
CH 2
ONO 2
H
O O
)
O
OCH 2NO 2
n HC
O
O
H
CH 3
covalent links
Cl
CH 2
H
Cl
H
Cl
Cl
hydogen bridges
Cl OCH 2NO 2
O
OHO
O
O
O H
OCH 2NO 2
ONO 2
O
(
)
O
OCH 2NO 2
ONO 2
n
O
O
HC
C
O
HC
CH3
ONO 2
H n
n
CO(CH3)2
O
OHO
ONO 2
CCl3H
OCH 2NO 2
ONO 2
O
) n
O
) HO
OCH 2NO 2
ONO 2
CH
OCH 2NO 2
ONO 2
O
C
CH3
CH2
CH 2
Cl
Cl
H
Cl
(c)
H
Cl
Cl
Cl
CH2
H3C
C O
O
CH
OCH 2NO 2
OCH 2NO 2
ONO 2
O (
A g+ (-)
n
O
OHO
O
(-) A g+
ONO 2
O
O
)
O
n
(-) A g+
ONO 2
OCH 2NO 2
H2C
H3 C
(d)
C
OH
O
H2C
CH
H3 C
n
C
OH
O
CH
n
Fig. 3. (a) Dipolar resonance derived from cellulose nitrate. (b) Molecular description of the structures involucrate in the mixture, cellulose dinitrate with PVAc diluted with acetone and chloroform solvents. (c) Our emulsion is a liquid with amorphous consistency, which is the result of mixing organic and inorganic compounds. (d) Polymeric crystalline network as consequence of the curing process after appling thermal energy by pressure and friction, with silver halogen ion through photographic emulsion.
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action of the nitrate molecule contained in cellulose nitrate [12]. Conventionally, the bonds in the glucosidic molecule R–O–NO2 (nitro group linked to the ring carbon atom) where R is the three chair conjugate glucose structure R = (C24H40 nO20 n) together with the group (NO3)n, where n is the nitration degree, normally between 11.2% and 12.2% to get cellulose dinitrate regularly used to produce some adhesives [13]. One basic reactivity concept from these molecules is their feasibility to generate a dipole due to resonance present in the cellulose nitrate molecule in the nitro group and can be seen in Fig. 3a [14]. It is stabilized by the presence of the metallic ion in the Ag+ mask, it starts curing local process to record the image. Our emulsion can be prepared using the polyvinyl acetate PVAc as a matrix, represented in Fig. 3b. By applying an algebraic structure, we show the complex mixture between the cellulose dinitrate (NC) with polyvinyl acetate (PVAc), diluted with acetone and chloroform solvent. Forming an amorphous solution (Fig. 3c), the hydroxyl radical from NC forms hydrogen bridges with the oxygen molecule of the PVAc at the carbonyl group, and this in turn unite with the carbon trichloride through hydrogen bridges between the chlorine and methyl hydrogen of the PVAc. When a small amount of heat energy is applied, the carbon trichloride changes to the molecular structure as in (3d) and it forms a long polymeric crystalline network as in (3c), and in the unexposed regions, it does not
7
6
NN
Absorbance (a.u)
5
NB
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change the structural form of the amorphous polymeric network. In Fig. 3d we can see that the hydrogen atom from the hydroxyl group is displaced and incorporated in to the PVAc structure. This is due to the induction of a polarization charge that exists between carbonyl connections of the double bond modifying for a simple bond. As a result of the applied temperature in the amorphous solution, a resonant group of nitrates with double links, start a charge induction to produce a polarization, with partial negative ( ) ion, which is stabilized by the presence of silver (+) ion through a photographic emulsion (our mask) Fig. 3d. Our hologram is recorded in a high contrast commercial kodalith negative film that contains halogens salts such as Ag+Br from Kodak company [15]. The thermal process corresponds to the heat applied to the material by friction when we recorded the hologram. Pressure as a parameter is essential to start the thermal curing and anaerobic reaction to modulate the material by more and less amorphous and crystallized located zones [16]. 4.1. Basic concepts Reactions that harden some polymers can be brought about by any chemical, physical or catalytic reaction. Depending on the type of reaction, these polymers may be single- or two-component commonly as in adhesives. Single-component reaction polymers: These are adhesives that, depending on their type, react to environment humidity, UV radiation or ambient oxygen (aerobic adhesives) or on the exclusion of air, e.g. using metal ions (anaerobic adhesives). Pressure sensitive polymers remain adhesive permanently. They are used wherever the assembly is only temporary and subsequent separation is required. Pressure sensitive polymers are usually applied in the form of adhesive films [16]. As organic materials, polyvinyl acetate (PVAc) is most commonly used as a thermoplastic polymer resin.
4
3
Table 1 Functional groups from our polymer film, after induction heat
2
Functional groups
m (cm 1) Reported [4]
m (cm 1) Results (after inducing heat)
C–O–C
1140–1210
1184 1180
CH3–C @ O
1175–1375
1180 1181
C@O
1750–1375
1754.62
NO2 C-nitrate
1390–1300 1600–1530 1750–1735 1870–1650
1377.05 1345.57 1531 1502–1599
1
0 2000
1500
1000
Wavenumbers
500
(cm-1)
Fig. 4. Infrared spectrum of the PVAc with cellulose dinitrate subsequent to heat application by friction process. The main peak is 7.43 lm.
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5. Results 90
Diffraction Efficiency (%)
5.1. IR analysis 80
70
60
50
40
30 0
10
20
30
40
50
60
Friction time (s)
Fig. 5. Diffraction efficiency of gratings, made by doping PVAc with cellulose dinitrate (b spline fit).
Thermoplastic resins are polymers in which the monomeric units are linked together to form two dimensional linear chains that are soluble in a range of solvents. They remain permanently fusible and soluble; some thermoplastic resins may form insoluble, infusible resins after long exposure to light or heat. Such exposure may cause formation of chemical bonds or links, referred to as cross linking, which become established among linear chains to form three-dimensional networks characteristic of thermosetting resins [7].
For our mixes of PVAc with cellulose dinitrate, we obtain a polymer gel–liquid, for which the thermal sensitivity is high (see Fig. 4). Since these are nitrate films based they are extremely flammable. By adjusting the ratio of nitric and sulfuric acids, products of different degrees of nitration can be obtained [8]. For PVAc with cellulose dinitrate, the IR absorbance spectrum of the resultant polymer film after heat induction by friction process is shown in Fig. 4. In the IR spectrum of the (Fig. 4) NN corresponds to the heat induced on the dark areas of the mask an NB belongs to clear areas. Localized regions dark and clear areas to form the binary image that form our computer hologram mask. Dark zones contain a high concentration of metallic silver Ag0 than the clear zones. Nitrate group activates again the silver Ag+ to start the thermal anaerobic polymerization process in our emulsion. Fig. 4 shows an important response at k = 7.43 lm, that corresponds to 1345 cm 1, which indicates stretching vibrations between the ring and the nitrate group, nitro O2N–O–. These events start the polymerization of our emulsion, recording a phase image. On analysis of the vibration, we observed that the absorption bands are almost conserved; we noticed new bands with a strong intensity in regard to the other ones that correspond to the nitrate group in the region 1750–1735, and they belong to the stretching vibrations
Fig. 6. (a) Microphotograph of a sinusoidal grating recorded in our doped polymer and (b) diffraction pattern (a).
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between the nitrogen and the oxygen from nitrate group –NO2. 1345 corresponds to the main absorbance peak in Fig. 4, which signifies stretching between the between the ring and the nitrate group nitro O2N–O–, appearing as a 1502–1509 tension asymmetric vibration from the O–NO2 bond in the R–O–NO2 molecule nitrate group linked carbonyl radical (Table 1). 5.2. Modulation analysis For a period of 35 s with heat induced by friction time, we replicate computer hologram gratings obtaining a high diffraction efficiency of around 90.1% which corresponds 37 C, showing 26 diffracted orders. Therefore in Fig. 5 the diffraction efficiency was measured collecting all the diffracted orders produced by the gratings without zero order, divided by the incident light. We show that the maximum number of diffracted orders not only corresponds to a higher efficiency value but, it also corresponds to the points which showed a diffraction efficiency of the order of 49%.
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It is worth emphasizing that to obtain a fast hologram with this material with a diffraction efficiency of the order of 47.6%, we need only 1 s as the recording time, and in reality this material is fast in absorbing heat energy. However the material does not show linear behaviour, see Fig. 5, and it is difficult to get a standardized parameter for fast holograms, for which the humidity of the environment is very important for recording holograms in the material and the optimum exposure time can be increased. The recording time increases with exposure time and it decreasing start to 50 s after 60 s the films show, a diffuse degradation. For this reason, we cannot have a good replication. Fig. 6a is a microphotograph of a sinusoidal grating in the material, that is polyvinyl acetate doped with cellulose dinitrate, with an amplification of 25·. Fig. 6b shows a diffraction pattern with many diffracted orders. This response can only be caused for phase gratings (Fig. 6a); like the material it is not revealed for watery chemical agents and it does not remove material, therefore we assumed that the modulation is by refraction index.
Fig. 7. (a) Binary image, (b) computer Fourier hologram and (c) computer reconstruction.
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6. Conclusions
Fig. 8. (a) Fourier hologram microphotograph with a magnification of 100·. (b) Fourier hologram reconstruction showing two conjugate images.
Fig. 7a shows the computer binary image of PVAc + Nc. These abbreviations t correspond to polyvinyl acetate (PVAc) doped with cellulose dinitrate (Nc), created with 300 for 300 pixels and (b) shows a Fourier computer hologram generated with 256 gray levels, and (c) represents the numeric reconstruction of the computer hologram image in 7b. This reconstruction altered its histogram to visualize the numeric reconstruction better. Fig. 8a shows the hologram microstructure from the image in Fig. 7b recorded on the polyvinyl acetate doped with cellulose dinitrate. The microphotograph shows the material modulation with the holographic code transferred by lithographic techniques for heat induction by applying the friction technique. The circular aperture is produced by the eyepiece of the microscope; the magnification was of the order of 100·. Fig. 8b shows the photograph of the reconstructed hologram, projected on a dark paper 2 m from the hologram, showing a pair of conjugate images characteristic of Fourier holograms. This image shows a good diffraction efficiency and presents a low noise.
We obtained adaptable acrylic resin composed mainly of PVAc and cellulose dinitrate, when doped with acetone and chloroform solvents, producing at environmental temperatures an excellent thermo-sensitive material, activated only by heat induced by a friction process. The diffraction efficiency is high of the order of 90.1%, with low noise. Our polymer mixture is activated by heat induced by pressure and for anaerobic reactions, permitting a good replication of the image or holograms as changes of zones of crystallized polymer (exposed) and amorphous zones (unexposed). Anaerobic phenomena start when we apply pressure on the surface from a soft hologram with acetate base assembling on the mixed material polymer (our mask). Our mask soft hologram is recorded with a high contrast commercial kodalith negative film that contains halogen salts such as Ag+Br from Kodak company. The thermal process corresponds to the heat applied to the material by friction when recording the hologram. Pressure as a parameter is essential for starting the thermal curing and anaerobic reaction to modulate the material by more and less amorphous and crystallized located zones. This emulsion can be used to copy or replicate holograms or images, by applying lithographic techniques. Furthermore, the polymer material opens up possibilities to design elements of phase diffraction. One of the advantages of using this material is that it does not require any technique developed to get a phase hologram.
References [1] R.P. Gwinn, P.B. Norton, R. McHenry, The New Ensiclopædia Britannica, 15th Ed., vol. 9, Encilopædia Britannica, Inc., Chicago USA, 1993, p. 159. [2] Plastics Historical Society. Available from:
S. Toxqui-Lo´pez et al. / Optical Materials 28 (2006) 342–349 [13] B.J. Kim, Hsin-Neng, F.-J. Tai, CERL Technical report 99/45, US Army Corps of Engineers, Construction Engineering Research Laboratories USA, 1999, p. 14. [14] A.F. Cotton, W. Geoffrey, Advanced Inorganic Chemistry, ninth ed., Editorial Limusa, S.A. de C.V. Mexico, 2001, p. 501 (Chapter 13).
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[15] Technical Information Instruction Sheet, TI1105, Kodalith Ortho Films 2556,6556, Type 3, Copyright 2001 Kodak Polychrome Graphics, Norwalk, CT 06851 USA. Available from: