Methylene-blue-sensitized dichromatedgelatin holograms with antihumidity polymer coatings

Wang Ketai, Guo Lurong, Zhu Jianhua, Zhang Weiping, Tang Jiyaueo, and Pang Lin

An efficient method of resisting humidity for methylene-blue-sensitized dichromated gelatin ~MBDCG!holograms is reported. The method uses a viscous liquid pure poly~styrene! solution with a low degreeof polymerization as the coating material. On the basis of this coating, the diffraction efficiency oftreated MBDCG holograms exposed to a high-humidity ~relative humidity of 80–85%! environment wasnot obviously changed during observation over 3 mon. © 1997 Optical Society of America

Key words: Dichromated gelatin, antihumidity, polymer membrane.

1. Introduction

Dichromated gelatin ~DCG! is a near-ideal recordingmedium for making phase holograms. But the en-vironmental stability of DCG holograms is still aproblem, preventing its large-scale commercial use.It is quite clear that the ambient humidity and tem-perature have very adverse effects on the stability ofthe diffraction efficiency of DCG holograms1–4 be-cause the amino groups and carboxyl groups of thegelatin5,6 are strongly absorbent with respect to hu-midity. Naik et al.2 proposed an antihumiditymethod of coating the films with some polymer solu-tions. The results showed that, after exposure to ahigh-humidity environment ~relative humidity RH 580%!, the diffraction efficiency of holograms with apolymer layer was stable within 5 h, and then itdecreased quickly. So these holograms did not meetthe need for an antihumidity property for DCG holo-grams.Our investigations have revealed the reason for

the poor antihumidity capacity of the coated holo-grams of Naik et al.2 The results show that therewere many holes in their polymer-coating layer thatwere produced by the evaporation solvent in thepolymer solution, and the coating could not fullyresist humidity. Therefore, in view of the fact thatmethylene-blue-sensitized dichromated gelatin~MBDCG! is highly sensitive to red light and to

The authors are with the Information Optics Institute, SichuanUniversity, Chengdu 610064, China.Received 23 September 1996; revisedmanuscript received 2 Jan-

uary 1997.0003-6935y97y143116-04$10.00y0© 1997 Optical Society of America

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increase its commercial value, we synthesized in ourlaboratories a viscous liquid of pure poly~styrene!with a low degree of polymerization for investigatingthe efficiency of antihumidity-coated MBDCG holo-grams. The results show bright prospects.

2. Experiment

A. Preparation of Methylene-Blue-SensitizedDichromated Gelatin Holograms

The procedure for making MBDCG holograms is out-lined in Table 1. The relevant data for our holo-grams are a thickness of the gelatin of 10 mm, awavelength of the recording light of 633 nm ~obtainedby use of a He–Ne laser!, an energy density of doublerecording light beams of 12 mJycm2, an interbeamangle of 36.8°, a spatial frequency of 1000 linesymm,and a hologram area of 2.0 cm2.

B. Measurement of the Diffraction Efficiency

The diffraction efficiency of the holograms was mea-sured by use of a photometer ~China Southwest In-stitute of Technology and Physics, Model LT-901! andthe formula I1yIt. Here, I1 is the intensity of light inthe first-order diffraction and It is the intensity of thedirectly transmitted light.

C. Synthesis of the Viscous Liquid Poly~styrene!

The method for fabricating the liquid poly~styrene! isas follows: Add refined styrene to a reactor that canbe sealed. Gradually raise the temperature to 110–120 °C and maintain it for approximately 30 min,then reduce to room temperature to stop polymeriza-tion. Store the solution below 4 °C.

Table 1. Procedure for Making MBDCG Holograms

Step DescriptionTemperature

~°C! Relative Humidity Time

1 Preparation of MBDCG platesA Soak 4.5 g gelatin in 120 mL deionized water 25 – 4 hB Heat the suspension in a thermostated water bath with stirring 50 – 10 minC Add 7.5 mL of 0.5% potassium dichromate solution to the sus-

pension while stirring50 – 5 min

D Add 1.5 mL of a 25% stock solution of 1,1,3,3-tetramethyl guani-dine ~TMG! that has been adjusted to pH 7.0 with glacial aceticacid

50 – 5 min

E Adjust the pH to 9.0–9.5 with a 25% TMG solution 50 – 10 minF Add 0.75 mL of a 0.5% methylene blue solution to the suspen-

sion while stirring50 – 2 min

G Pipette out 6 mL of the solution from step F and put it on a 7 cm3 30 cm optical quality glass plate of horizontal station

– – –

H Dry the coated plate 25 40–50% 24 h2 Exposure

Expose the plate to double red light beams of 633 nm ~from aHe–Ne laser!

3 Development of the platesA Soak in a solution of F5a 25 – 2 minB Wash in running tap water – – 8 minC Dehydrate in 70% isopropyl alcohol 25 – 3 minD Dehydrate in 95% isopropyl alcohol 25 – 3 minE Dehydrate in 100% isopropyl alcohol 25 – 3 minF Dry holograms with flowing hot air – – 2 min

aF5 is a developing solution created in our laboratory.

D. Coating of the Holograms

Put the holograms on a horizontal plane. Add theviscous liquid pure poly~styrene! to the MBDCGholograms ~0.1 mLycm2! by use of a thermosettingtechnique to realize films of the desired thickness.

3. Results and Discussion

A. Diffraction Efficiency and Sensitivity ofMethylene-Blue-Sensitized Dichromated Gelatin

According to the above procedure ~Table 1!, the dif-fraction efficiency of MBDCG holograms reached96% when the energy density of the recording lightbeams was 12 mJycm2.

B. Surface Analysis of Polymer-Solution Coatings

Naik et al.2 chose a poly~methyl methacrylate!~PMMA! polymer solution of a known quantity ofPMMA dissolved in 10 mL of xylene ~a solvent! astheir protective-coating material. We preparedfilms with their solution and ours and measured thesurface structures of the coating materials by elec-tron microscopy ~Fig. 1!. The pictures show manyholes in the coating surfaces @Fig. 1~a!#, but the holesin the poly~styrene! solution @Fig. 1~b!# are smallerand fewer than those seen for the PMMA–xylenecoating @Fig. 1~a!#. The holes resulted from the pro-cedure of evaporation of the solvent from the polymerfilms. So when the MBDCG holograms were ex-posed to a high-humidity environment ~RH 5 80%!,the long-term stabilization of the diffraction efficiency

was affected by the slow diffusion of water vaporthrough the polymer film.Curve d in Fig. 2 shows the relation of the diffrac-

tion efficiency to the storage time in high humidity.Figure 2 also illustrates that the diffraction efficiencyof DCG holograms can be maintained for only 5 h inhigh humidity.

C. Surface Analysis of an Initiator-Polymerized Coating

We prepared a viscous liquid PMMA solution with alow degree of polymerization by use of an initiator,benzoyl peroxide, and used the polymer as a coatingmaterial. Because the benzoyl peroxides decom-posed and produced gas, the surface still developedsome holes @Fig. 1~c!#. Although the holes are ob-viously fewer and smaller than those shown in Fig.1~a! and the stability of the initiator-polymerizedPMMA was better ~curve c, Fig. 2! than that of thepolymer–xylene solution ~Fig. 2, curve d!, it was notstable in the high-humidity environment for long.

D. Surface Analysis of a MonomerSelf-Thermopolymerized Coating

The stability of uncoated MBDCG holograms is verypoor in high-humidity environments ~80%! if no spe-cial precautions are taken. In the uncoated condi-tion their diffraction efficiency can be kept for onlyseveral minutes ~curve e, Fig. 2!.We used the viscous liquid poly~styrene! with a low

degree of polymerization as a protective-coating ma-terial. The diffraction efficiency of these MBDCGholograms showed better stability than did those

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Fig. 1. Electron-microscope photographs of the surface structures of films coated with different polymers: ~a! PMMA–xylene solution,~b! poly~styrene!–xylene solution, ~c! PMMA with a low degree of polymerization prepared with an initiator, ~d! poly~styrene! membraneprepared by self-thermopolymerization, which exhibits too high a degree of polymerization, and ~e! poly~styrene! membrane prepared byself-thermopolymerization, which exhibits the correct degree of polymerization.

Fig. 2. Real diffraction efficiency plotted versus the time of expo-sure to a high-humidity environment for MBDCG holograms withdifferent coatings: Curve a is for a poly~styrene! membrane withthe correct degree of polymerization that was prepared by self-thermopolymerization at RH 5 80%. Curve b is for a poly~styrene!membrane with the correct degree of polymerization prepared byself-thermopolymerization after soaking in water at 30 °C. Curve cis for a viscous liquid pure PMMA coating with a low degree ofpolymerization that was prepared with an initiator. Curve d is forthe polymer–xylene solution of Naik et al.2 at RH 5 80%. Curve eis for an uncoated holographic film.

3118 APPLIED OPTICS y Vol. 36, No. 14 y 10 May 1997

coated with the PMMA polymer solution in high hu-midity ~RH 5 80%!. A plot of the diffraction effi-ciency versus storage time in high humidity forpoly~styrene! are shown by curve a in Fig. 2. Thecoating is stable during observation. Its diffractionefficiency holds for 30 h, even if soaked in water at atemperature of 30 °C ~curve b, Fig. 2!. Under theelectron microscope, the surface of the poly~styrene!-coated MBDCG holograms is seen to be very dense@Fig. 1~e!#.Controlling the degree of polymerization of the

poly~styrene! is very important. This is because apoly~styrene! membrane of too high a degree of poly-merization in the glass surface will break during con-densation @Fig. 1~d!#.

4. Conclusions

With respect to environmental stability, MBDCG hasthe same properties as does DCG. This similarity isdetermined by their having the same structure ofgelatin molecule. When a viscous liquid of purepoly~styrene! is used as a coating, it can isolate holo-grams from humidity. In addition, it has not shown

an obvious disadvantage for holograms. So thismethod may have good prospects for application.

The authors appreciate the support of the NationalEducation Commission Doctoral Foundation and theNational Natural Science Foundation of China.

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3. L. R. Guo, C. M. Dai, and Y. K. Guo, “Anti-humidity dichro-mated gelatin holographic recording material,” in Computerand Optically Generated Holographic Optics, Vol. 4, I. Cindrichand S. H. Lee, eds., Proc. SPIE 1555, 293–296 ~1991!.

4. L. R. Guo, C. M. Dai, and T. Q. Cai, “Anti-humidity dichromatedgelatin and its sensitized characteristics,” in Soviet-ChineseJoint Seminar on Holography and Optical Information Process-ing, A. L. Mikaelian, ed., Proc. SPIE 1731, 166–168 ~1991!.

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