Photochromic composites based on porous stretched polyethylene filled by nematic liquid crystal mixtures

Research Article

100

Received: 1 December 2008, Revised: 6 January 2009, Accepted: 13 January 2009, Published online in Wiley InterScience: 23 February 2009

(www.interscience.wiley.com) DOI: 10.1002/pat.1404

Photochromic composites based on porousstretched polyethylene filled by nematicliquid crystal mixturesy

Alexey Bobrovskya*, Valery Shibaeva, Galina Elyashevichb, Elena Rosovab,Alexey Shimkinc, Valery Shirinyanc and Kung-Lung Chengd

A number of the novel photochromic polyethylene (

Polym. Adv

PE)-based liquid crystal composites were prepared and studied.The oriented stretched porous polyethylene films were used as the polymer matrices. Commercial liquid crystalsdoped with new photochromic compounds were introduced into PE films and photo-optical properties of theobtained composites were investigated. It was shown that a director of nematic liquid crystals is highly oriented alongthe stretching axis of PE films resulting in noticeable linear dichroism of the PE composite films. New approaches forreversible or irreversible image recording on PE LC composites by UV irradiation were demonstrated. Copyright �2009 John Wiley & Sons, Ltd.

Keywords: liquid crystals; polymer composites; photochromism; dichroism; photopolymerization

* Correspondence to: A. Bobrovsky, Faculty of Chemistry, Moscow State Uni-versity, Leninskie gory, Moscow, 119991 Russia.E-mail: [email protected]

y This article was published online on 23 February 2009. Revisions to the originalversion were subsequently made by the author. This notice is included in theonline and print versions to indicate both have been corrected on 21 December2009.

a A. Bobrovsky, V. Shibaev

Faculty of Chemistry, Moscow State University, Leninskie gory, Moscow,

119991 Russia

b G. Elyashevich, E. Rosova

Institute of Macromolecular Compounds, Russian Academy of Sciences,

31 Bolshoy pr., 199004 Saint-Petersburg, Russia

c A. Shimkin, V. Shirinyan

N. D. Zelinsky Institute of Organic Chemistry, Leninsky prospect, 47, 119991

Moscow, Russia

d K.-L. Cheng

Material and Chemical Research Laboratories, Industrial Technology Research

Institute, 321 Kuang Fu Road, Section 2, Hsinchu, Taiwan

Contract/grant sponsor: Russian Foundation of Fundamental Research;

contract/grant number: 08-03-00481, 08-03-00865.

Contract/grant sponsors: Program COST-D35; Industrial Technology Research

Institute (Taiwan).

INTRODUCTION

Polymer liquid crystals composites or the so-called polymerdispersed liquid crystals (PDLCs) are very promising materialsfrom the practical applications’ point of view for optoelectronics,smart windows creation, etc. Strong and continuously growinginterest to PDLCs is associated with the possibility of combiningthe good mechanical properties of polymer films with uniqueoptical and electro-optical properties of liquid crystals in thesame single material.[1–9] In several recent papers, dye-doped andphotochromic PDLC materials based on amorphous and LCpolymer matrix were described.[10–12]

In our recent papers, we have described a novel generation ofcomposite materials based on nematic or cholesteric liquidcrystals doped with different dichroic or photochromic dyesembedded in the stretched porous polyethylene (PE) films.[13–17]

These films are characterized by highly oriented microporousstructure and their morphology is completely different withrespect to the previously studied PDLC systems produced by theconventional techniques, emulgation, or polymerization-inducedphase separation.[9] The sizes of pores in such stretched PE matrixwere varied between 50 and 500 nm, and overall porosity wasabout 50%.[14] We have determined that the director of nematicliquid crystals embedded in the porous polymer films is orientedalong the direction of the film stretching. Analysis of thepolarized absorbance spectra of the dichroic dyes dissolved inliquid crystal allowed one to calculate the dichroism values. Thesevalues are comparable with those of liquid crystals replaced inthe glass cells coated with rubbed polyimide that indicates a highdegree of liquid crystal orientation in the PE films.The porous PE films were also used as polymer matrices for

cholesteric liquid crystal mixtures with photovariable helixpitch.[15] For the induction of the cholesteric phase in acommercial nematic host the chiral-photochromic dopant based

. Technol. 2010, 21 100–112 Copyright � 200

on sorbide and cinnamic acid capable of E-Z isomerization underUV irradiation was dissolved. A merocianine-type substance wasadded into mixtures as a dichroic dye. Introduction of thedye-doped cholesteric mixture with a helix pitch higher than�300 nm in the porous PE film led to an almost completeuntwisting of the helical structure and a transition from acholesteric to an oriented nematic phase, as well as to an increasein birefringence and an appearance of dichroism. The lowering ofthe helix pitch in LC composite by the increase in chiral dopant

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PHOTOCHROMIC COMPOSITES BASED ON POROUS STRETCHED PE FILMS

concentration results in a reduction of the dichroism values. UVirradiation of polymer composite leading to an isomerization ofthe chiral dopant and helix untwisting induces a noticeablegradual growth of dichroism and birefringence.Recently, a novel approach for the creation of photopatternable

fluorescent PE-based liquid crystal composites was developed.[16]

LC photopolymerizable mixture containing the nematogenicdimethacrylate, cholesterol-containing acrylate, and perylene-containing methacrylate was introduced into the pores of

Table 1. Chemical structures of photochromic and dichroic dyes,SPPE based composites containing liquid crystals and these dyes

Name (or type) Formula

Photochromic diarylethene



Photochromic naphthopyrandye linked with mesogen

Photochromic diarylethenelinked with mesogen

Merocyanine dye

a Data for photoinduced form of photochrome.

Polym. Adv. Technol. 2010, 21 100–112 Copyright � 2009 John Wiley

stretched porous PE films and the optical–photooptical propertiesof such composites were studied. The polarized spectral data anddichroism calculations showed that the transition moment ofperylene side groups and liquid crystal director in the compositefilms are oriented along a stretching direction of PE film. UVirradiation induced the photopolymerization of LC mixture but didnot lead to significant decrease in the dichroism of perylenegroups; however, stabilized the LC structure in the compositefilms. A possibility of photopatterning of the PE-based

their absorbance maxima and maximal values of dichroism of

Abbreviation lmax(nm) D

SVZ0499 �500a

0.17

Sh160 530 0.07

Diaryl II 606a

0.42

Leo120 437a

0

Leo141 360a, 504

a0

ASh216a 532 0.55

& Sons, Ltd. www.interscience.wiley.com/journal/pat

101

Scheme 1. Synthesis of merocyanine dye ASh216a.

A. BOBROVSKY ET AL.

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composite filled with the LC photopolymerizable mixture wasdemonstrated.In this paper, using the technique developed in our previous

papers [13–17] we prepared and studied a number of PE compositefilms containing different types of photochromic and dichroicsubstances (Table 1). As the liquid crystal matrices MLC6816 andZLI1695 (mixtures of cyclohexane derivatives, Merck, Germany)were used.In the beginningwewill consider properties of LC PE composites

containing photochromic diarylethenes (SVZ0499, Sh160, andDiarylII) having a noticeable dichroism in the composite films. Thenwe will discuss the data concerning the combined photochromesLeo120 and Leo141 containing the phenylbenozate mesogenicfragments improve their solubility in the LC matrix. Incorporationof the mesogenic fragments with the photochrome moietyprovides a possibility of increasing a compatibility of dopant withthe LC matrix by one order of magnitude. Next part of the paper isdevoted to a study of irreversible photorecording usingmerocyanine dye ASh216a embedded in LC PE composite films.Finally, new type of photopolymerizable LC composites with stablephoto-optical characteristics will be discussed. The chemicalstructures and the main characteristics of photochromes anddichroic dyes are listed in Table 1.

EXPERIMENTAL PART

Synthesis of dichroic and photochromic dyes

Synthesis of dichroic dye ASh216a (2-[2-(1,3,3-Trimethyl-1,3-dihydro-2H-[1]benzothieno[3,2-b]pyrrol-2-yliden)ethylide-ne]-1-benzothiophen-3-one) (Scheme 1)

Benzo[b]thiophen-3(2H)-one hydrazone (2). p-Toluenesulfonicacid (0.86 g, 5mmol) and hydrazine hydrate 15ml (0.3mol) wereadded with stirring to a solution of benzothiophen-3-one (1) (9.6 g,0.05mol) in 15ml of ethanol. The solution immediately becamedark. The mixture was refluxed for 1 hr and during this period, thesolution became light. The reaction mixture was kept for 3–4hr atroom temperature, and the precipitate was filtered off and dried inair. Yield 5.8 g (71%). m.p. 110–1128C. 1H NMR, d: 3.91 (s, 2H, CH2);5.25 (br.s, 2H, NH2); 7.11 (m, 1H, Harom), 7.24–7.27 (m, 2H, 2Harom);7.65 (d, 1 H, Harom, J¼ 7.35Hz). MS, m/z: 164 [M]þ. Found (%): C,58.79; H, 4.93; N, 16.91; S, 19.07. C8H8N2S. Calculated (%): C, 58.51; H,4.91; N, 17.06; S, 19.53.

2,3,3-Tr imethylbenzo[b]thieno[3,2-b]pyrrolenine(3). 3-Methylbutan- 2-one (2.2ml, 20mmol) was added to asuspension of benzothiophen-3-one hydrazone 2 (3 g, 18mmol)in 30ml of anhydrous benzene. The reactionmixture was refluxedfor 15min during which the precipitate dissolved. Whilecontinuing refluxing, hydrogen chloride gas was passed throughthe solution, the solution darkened, and a solid precipitated. Thereaction mixture was refluxed for 1 hr and cooled to roomtemperature, the supply of hydrogen chloride was terminated,the precipitate was filtered off, and the filtrate was poured into200ml of water and extracted with 3� 50ml of ethyl acetate. Theorganic fractions were combined, washed with water, andconcentrated under reduced pressure. After solvent evapora-tion, the residue was chromatographed on silica gel using a 3: 1CH2Cl2 – ethyl acetate mixture as the eluent to give 3.5 g(41%) of 2,3,3-trimethylbenzo[b]thieno[3,2_b]pyrrolenine (3) m.p.86–888C. 1H NMR, d: 1.40 (s, 6 H, CMe2); 2.33 (s, 3 H, 5-Me); 7.30 (t, 1H, ABCD, Harom, part C, JCB¼ 7.4 Hz, JCD¼ 8.1 Hz); 7.43 (t, 1 H,

www.interscience.wiley.com/journal/pat Copyright � 2009 John

ABCD, Harom, part B, JBC¼ 7.4 Hz, JBA¼ 8.1 Hz); 7.81 (d, 1 H, ABCD,Harom, part D, JDC¼ 8.1 Hz); 8.07 (d, 1 H, ABCD, Harom, part A,JAB¼ 8.1 Hz). 13C NMR, d: 15.68 (CH3-C¼N); 23.27 (CH3)2); 54.68(Me2 – C); 120.97, 123.74, 123.78, 124.73, 130.38, 143.40, 143.48,150.44 (Carom.); 189.91 (C––N). MS, m/z: 215 [M]þ. Found (%): C,72.53; H, 6.38; N, 6.50. C13H13NS. Calculated (%): C, 72.52; H, 6.09;N, 6.51

1,2,3,3-Tetramethyl-3H-[1]benzothieno[3,2-b]pyrrol-1-ium trifluoromethanesulfo-nate (4).. The solution of 3H-pyrrole 3 (0.3 g, 1.4mmol) and methyl trifluoromethanesulfonate(0.16ml, 1.4mmol) in acetonitrile (3ml) was refluxed for 20min,then cooled, and evaporated. The crude solid was washed withTHF and dried. Yield 0.4 g (75%). 1H NMR (250MHz, DMSO-d6,248C): d¼ 1.64 (s, 6H, CMe2), 2.78 (s, 3H, Me), 4.27 (s, 3H, NMe), 7.55(t, 3JHH¼ 7.9Hz, 3JHH¼ 7.3Hz, 1H, Harom), 7.63 (t, 3JHH¼ 7.9Hz,3JHH¼ 7.3Hz, 1H, Harom), 8.21 (d, 3JHH¼ 7.9Hz, 1H, Harom), 8.27 (d,3JHH¼ 7.9Hz, 1H, Harom) ppm. C15H16F3NO3S2 (379.42): calcd. C47.48, H 4.25, N 3.69; found C 47.25, H 4.39, N 3.47.

1,2,2,3-Tetramethyl-2H-[1]benzothieno[3,2-b]pyrrol-1-ium trifluoromethanesulfo-nate (5). The suspension of thesalt 4 (0.2 g, 0.53mmol) in p-xylene (2ml) was refluxed for 2.5 hr.After cooling the solid was filtered off, washed with benzene, anddried in vacuum. Yield 0.12g (60%). 1H NMR (250MHz, DMSO-d6,248C): d¼ 1.56 (s, 6H, CMe2), 2.38 (s, 3H, Me), 3.90 (s, 3H, NMe), 7.64(t, 3JHH¼ 7.9Hz, 3JHH¼ 7.3Hz, 1H, Harom), 7.92 (t, 3JHH¼ 7.9Hz,3JHH¼ 7.3Hz, 1H, Harom), 8.09 (d, 3JHH¼ 7.9Hz, 1H, Harom), 8.49 (d,3JHH¼ 7.9Hz, 1H, Harom) ppm. C15H16F3NO3S2 (379.42): calcd. C47.48, H 4.25, N 3.69; found C 47.23, H 4.41, N 3.44.

2-[2-(1,3,3-Trimethyl-1,3-dihydro-2H-[1]benzothieno[3,2-b]pyrrol-2-yliden)ethylide-ne]-1-benzothiophen-3-one (ASh216a). To the solution of compound 4 in EtOH (5ml)aldehyde 6 (0.41 g, 2.3mmol), and triethylamine (0.35ml,2.2mmol) were added and the solution was refluxed for 1 hr.The reaction mixture was cooled, and the solid precipitated wasfiltered off, washed with EtOH, and dried. Filtrate was evaporated,

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and the crude product was purified by column chromatographyeluting with CHCl3/ethyl acetate 3:1. Total yield 0.48 g (53%), m.p.239–2418C. lmax (MeCN)¼ 538nm. 1H NMR (250MHz, DMSO-d6þCF3COOH, 248C): d¼ 1.84 (s, 6 H, CMe2), 4.14 (s, 3 H, NMe), 6.44(d, 3JHH¼ 15.3 Hz, 1 H, 12-H), 7.42 (t, 3JHH¼ 7.3 Hz, 3JHH¼ 7.9 Hz, 1H, 60-H), 7.49 (t, 3JHH¼ 7.3 Hz, 3JHH¼ 7.9 Hz, 1 H, 6-H), 7.57 (t,3JHH¼ 7.9 Hz, 3JHH¼ 7.3 Hz, 1 H, 7-H), 7.60 (t, 3JHH¼ 7.3 Hz,3JHH¼ 7.9 Hz, 1 H, 50-H), 7.86 (d, 3JHH¼ 7.9 Hz, 1 H, 40-H), 7.92(d, 3JHH¼ 7.9 Hz, 1 H, 70-H), 8.14 (d, 3JHH¼ 7.9 Hz, 1 H, 5-H), 8.26(d, 3JHH¼ 7.9 Hz, 1 H, 8-H), 8.50 (d, 3JHH¼ 15.3 Hz, 1 H, 13-H) ppm.13C NMR (125MHz, DMSO-d6þCF3COOH, 248C): d¼ 27.34(10-C, 11-C), 35.19 (9-C), 51.35 (3-C), 103.41 (12-C), 117.18(20-C), 120.91, 123.74, 124.16, 125.05, 125.10, 125.31, 125.58,125.73, 128.21, 131.32, 132.44, 137.41 (13-C), 138.10, 140.46,142.67, 167.54 (30-C), 178.78 (2-C) ppm. MS: m/z¼ 389 [Mþ].C23H19NOS2 (389.54): calcd. C 70.92, H 4.92, N 3.60; found C 70.51,H 5.17, N 3.87.

Synthesis of photochromic dyes Leo120 and Leo141 (Scheme 2)

4-[(4-Propoxybenzoyl)oxy]benzoic acid (10). To a solutionof acid 8 (0.026mol) in (CH2)Cl2 (50ml) SOCl2 (0.051mol) and 4drops of DMF were added and the reaction mixture was refluxedfor 2 hr. Then the reaction mixture was cooled to roomtemperature and the solvent was removed in vacuo, the residuewithout any additional purification was used on the next stage. 1HNMR spectrum, d5ppm: 1.06 (m, 3H, CH3), 1.85 (m, 2H, CH2), 4.02(m, 2H, CH2), 6.97 (d, J¼ 8.8, 2H, Harom), 8.06 (d, J¼ 8.8, 2H, Harom).To a solution of chloranhydride 9 (0.016mol) in pyridine (70ml)

4-hydroxybenzoic acid (0.032mol) was added and the reactionmixture was stirred at room temperature for 10 days. Then thereaction mixture was poured onto ice and the precipitate wasfiltered off and dried in air. Yield – 47%. 1H NMR spectrum, d ppm:0.99 (m, 3H, CH3), 1.75 (m, 2H, CH2), 4.05 (m, 2H, CH2), 7.13 (d,

Scheme 2. Synthesis of photochromi


J¼ 8.4, 2H, Harom), 7.38 (d, J¼ 8.4, 2H, Harom), 8.06 (m, 4H, Harom).Mass spectrum, m/z: Mþ (300).

4{[(3,3-diphenyl-3H-benzo[f]chromen-5-yl)oxy]carbonyl}phenyl-4-propoxybenzoate (11), Leo120. To a solution ofcompound 10 (0.57mmol) in CH2Cl2 (5ml) DCC (0.68mmol) andDMAP (0.57mmol) were added and the reaction mixture wasrefluxed for 1 hr. Then the reaction mixture was cooled to roomtemperature and the solution of chromene 10 in CH2Cl2 (5ml)was added for 15min. The reaction mixture was stirred at thistemperature for 48 hr. Solvent was removed in vacuo, the residuewas purified by chromatography. Yield – 26%. 1H NMR (300MHz,DMSO-d6) d¼ 1.09 (m, 3H, CH3), 1.88 (m, 2H, CH2), 4.04 (m, 2H,CH2), 6.29 (d, J¼ 9.9, 1H, CH), 7.01 (d, J¼ 8.8, 2H, HapoM), 7.24 (m,2H, HapoM), 7.29 (m, 5H, HapoM), 7.36 (m, 2H, Harom), 7.41 (m, 5H,Harom), 7.46 (m, 2H, Harom), 7.59 (s, 1H, Harom), 7.72 (d, J¼ 8.07, 1H,Harom), 7.99 (d, J¼ 8.4, 1H, Harom), 8.18 (d, J¼ 8.81, 2H, Harom), 8.37(d, J¼ 8.43, 2H, Harom). Mass spectrum, m/z: Mþ (632). C42H32O6

(632.699): calcd. C 79.73, H 5.10, found C 79.53, H 5.17.

4-({2-[3,4-bis(2,5-dimethyl-3-thienyl)-2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl]ethoxy}carbonyl) phenyl 4-propoxybenzoate), Leo141. To a solution of compound 10(0.57mmol) in CH2Cl2 (5ml) DCC (0.68mmol) and DMAP(0.57mmol) were added and the reaction mixture was refluxedfor 1 hr. Then the reaction mixture was cooled to roomtemperature and the solution of dihetarylethene 11 in CH2Cl2(5ml) was added for 15min. The reaction mixture was stirred atthis temperature for 20 hr. Solvent was removed in vacuo, theresidue was purified by chromatography. Yield – 37%. m.p.�1008C. 1H NMR (300MHz, DMSO-d6) d, ppm: 1.08 (m, 3H, CH3),1.89 (s, 8H, 2CH3, 1CH2), 2.41 (s, 6H, 2CH3), 4.04 (m, 4H, 2CH2), 4.55(m, 2H, CH2), 6.61 (s, 2H, HapoM), 6.99 (d, J¼ 9.17, 2H, HapoM), 7.3(m, 2H, HapoM), 8.12 (m, 4H, HapoM), Mass spectrum: m/z 642.

c compounds Leo120 and Leo141.


103

Figure 1. Schematic representation of the porous PE films preparation process.

Figure 2. Scheme of the sample structures formed at extrusion(a), annealing (b), and uniaxial extension (c) stages of the porous PE film

preparation.

A. BOBROVSKY ET AL.

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Characterization of photochromic compounds

NMR spectra (1H, 13C) were performed on Bruker DRX-500,AM-300, WM-250, or AC-200 spectrometers. Mass spectra wereobtained on a Kratos mass spectrometer (70 eV) with directsample injection into the ion source. Melting points weremeasured on a Boetius hot stage and were not corrected. Columnchromatography was performed using silica gel 60 (60–200mesh), and TLC analysis was conducted on silica gel 60 F254plates. Commercially available (Acros, Merck) reagents andsolvents were used. Chromatography products were purchasedfrom Merck. Molecular modeling of photochromic molecules wasperformed using HyperChemTM 6.01 program.

Polyethylene film characterization

Microporous films of PE were obtained from commerciallyavailable PE of high density linear HDPE (Mw¼ 1.7 105, Mw/Mn¼ 5–6, Tm¼ 1328C) according to paper [14]. Flat PE films wereformed by the melt extrusion of HDPE grades using Laboratoryextruder (‘‘Scamia,’’ France). During extrusion and stretchingprocesses the polymer films are deformed and a porous structurewith the pore size of about 50–650 nm is formed. The pores sizedistribution was measured by filtration porometry method as isshown elsewhere.[14]

Small and wide-angle X-ray scattering (SAXS and WAXS) wereused to characterize crystalline structure of PE films (DRON-2.0(Russia) diffractometer with CuKa radiation).Mechanical characteristics of the porous films were measured

using the stress-strain tests on a P-5 tensile test machine(‘‘Tochpribor,’’ Ivanovo, Russia).

Mixture preparation and study of phase behavior andoptical properties of LC PE composite films

Liquid crystalline mixtures were prepared by dissolving thecomponents in chloroform followed by the evaporation of thesolvent and drying in vacuum. PE composites were prepared bycoating of the film with LC mixtures and consequently byremoving excess LC mixture by filter paper. The weight portion ofthe LC mixture in PE composite films obtained by weighing wasabout 55%.Polarizing microscopy investigations were performed using a

LOMO P-112 polarization microscope; temperature of thesamples was controlled by a Mettler FP-80 hot stage.

Photo-optical investigations

Photoirradiation was performed using a special optical set upequipped with a DRSh-250 ultra-high pressure mercury lamp andHeNe laser (633 nm). Using the interference and glass filters alight with the definite wavelengths was selected. To prevent theheating of the samples due to the IR irradiation of the lamp, awater filter was used. To obtain the plane-parallel light beam, aquartz lens was applied. During the irradiation, the constanttemperature of the test samples was maintained using a Mettler


FP-80 heating unit. In the case of photopolymerization, in orderto avoid contact with air composite films were placed in glasscells filled with diethyleneglycol. The intensity of incident lightwas measured by a LaserMate-Q intensity meter (Coherent).The linearly polarized absorbance and emission spectra of the

film samples were studied with a TIDAS spectrometer (J&M)equipped with a rotating polarizer (Glan-Taylor prism controlledby computer program).Dichroism values for composite films were calculated accord-

ing to equation (1)

D¼ðAjj � A?ÞðAjj þ A?Þ

(1)

where Ajj is the absorbance at the preferred direction and A? isthe absorbance perpendicular to this direction.

RESULTS AND DISCUSSION

Preparation of the porous stretched PE films as the matricesfor LC PE composites

The process of the porous PE films preparation consists of fourstages: extrusion, annealing, uniaxial extension, and thermalfixation. It is controlled by the following parameters: ls – spindraw ratio, Tann – annealing temperature, e and v – degree andrate of uniaxial extension, Tf – temperature of thermal fixation(Fig. 1).Structure transformations at each stage of the processes are

schematically presented at Fig. 2. At the extrusion stage thehighly oriented crystalline structure consisting of the largeparallel lamellae (folded-chain crystals) disposed perpendicularlyto melt flow direction has been obtained. This structure is a resultof the melt extrusion at high rate of the flow and at crystallizationtemperatures much lower than the melting temperature of PE(Fig. 2a). Molecular chains in folded-chain lamellar crystals areoriented in a melt flow direction. These lamellae are connectedby some numbers of the ‘‘bridges’’ consisting of tie chains.After annealing of the PE films at high temperature close to the

melting point the sizes of the crystals considerably increase,number of the tie bridges decreases, and all of them becomestressed (Fig. 2b). Such samples are characterized by specific


Figure 3. Through pores sizes distribution for the stretched porousPE films.

Figure 4. Electron micrograph of the porous PE film used for LC com-

posites preparation. Direction of stretching is shown by arrow.

Table 2. Crystalline-structure parameters for three stages ofPE porous structure formation

Structure parametersExtrudedsamples

Annealedsamples

Porousfilms

Degree of crystallinity (%) 55 72 57Long period (nm) 20 35 —Size of lamellaea (nm) 11 25 —Lateral size of lamellae (nm) 20 30 18a Length along stretchingdirection.


1

combination of mechanical properties known as ‘‘hard elastic’’ones. These properties give a possibility to extend the annealedsamples in several times at room temperature [18].The deformation of these hard elastic structures is microscopi-

cally heterogeneous. Under an external stress (uniaxial extension)the individual lamellae connected by distant bridges of tie chainsbend and depart locally and, as a result, discontinuities (pores)appear in the interlamellar space between the tie points (Fig. 2c).When a hard elastic material is extended to the high ratios, thenumber and sizes of pores between lamellae increase, and theindividual pores become interconnected. The formation ofthe through-flow channels is observed providing throughpermeability for liquids. Deformation of the lamellae andappearance of some voids in semicrystalline polymers initiatethe formation of the microporous structure.Small and wide-angle X-ray scattering (SAXS and WAXS,

respectively) were used to characterize the crystalline structure ofthe extruded, annealed, and porous films. Crystalline-structureparameters for three stages of the micropores structureformation are presented in Table 2. Sizes of the lamellae (lengthof the fold in lamella), X-ray long period (a sum of the crystallinelamella and the amorphous region sizes in the orientationdirection), and X-ray degree of crystallinity (a ratio of the lamellasize to the long period) were measured. Intensive diffusescattering prevented an analysis of the discrete reflections onSAXS for the porous samples and, consequently, to determine thesmall-angle long period.As shown by X-ray results, the annealing (the second stage of

the process) of the original spun samples caused a substantialincrease in the lamellar thickness (see Table 2) due to theinvolvement of the tie chains from the amorphous phase into thecrystals. As a result, the sizes of crystals and the degree ofcrystallinity become considerably higher, the number of the tiechains sharply decreases; all the tie chains become stressed links,and the tie points are fixed [19].The formation of interlamellar pores at the third stage of

structure formation dramatically decreases an integrity of thecrystalline structure. Moving apart of lamellae leads to anincreasing of distance between them (increase in intercrystallineregions) and, consequently, diminishing the degree of crystal-linity (Table 2). Partial damage of the crystals at deformation atthis stage reveals a decrease in the lateral sizes of lamellae(Table 2, 4th line). The formed porous structure is stabilized by thethermal fixation at the fourth stage of the process. It is the resultof stress relaxation in the sample with the fixed lamellar sizes atan elevated temperature (Tf ).The following conditions have been used for the porous film

preparation. PE melt was extruded until extension ratio l¼ 21 at2008C. The extruded films were annealed at Tann¼ 1268C during


30min. The annealed samples were extended up to e¼ 175%, atstretching speed v¼ 480%/min These films were stabilized atTf¼ 1158C during 30min. Thickness of the obtained porous filmwas about 17� 2mm.The porous films are characterized by a very low density

(600–630 kg/m3) as compared with the values of density of theextruded (932 kg/m3) and annealed (958 kg/m3) samples. Overallporosity of the porous films determined by measuring densitywas 40–45%.Through pore size distribution for these samples measured by

the filtration porosimetry method [14] is presented in Fig. 3. Asclearly seen, the most probable sizes of the pores (maximum ofthe curve) are ca. 200 nm; the maximal size of through poreschanged between 500–600 nm.Figure 4 shows the scanning electron micrograph (SEM) of a

surface of the porous PE film. SEM pictures (SCAN, Jeol, Japan)demonstrate the strongly developed relief-like character of theporous film surface. This picture also gives an opportunity toestimate the sizes of open-cell and through pores. Using thesedata it is difficult to conclude correctly which of these pores arethrough or open cell. Nevertheless, one could suggest that theintensive black regions on the pictures correspond to the throughpores or to the very deep open cell pores.Micrographs show that the sample has a highly oriented

structure. The pores have an asymmetric shape, and the poresizes are smaller in the orientation direction than in theperpendicular direction. All tie fibrils (‘‘bridges’’) connectingthe edges of pores are extended in the orientation direction thatis the evidence of high orientation degree of the PE samples.


05

Figure 5. Absorbance spectra of the composite film with 1wt% ofSVZ0499 before (a) and after (b) blue light irradiation (405 nm, 10min).

Figure 6. Polar plot of corrected absorbance at 500 nm after irradiation

of the LC PE composite containing 1wt% of SVZ0499. This figure is

available in colour online at www.interscience.wiley.com/journal/pat

A. BOBROVSKY ET AL.

106

Using atomic force microscopy it was observed [20] that theheight of relief is equal to the hundreds of nanometers(0.7–0.8mm). The relief is formed by the lamella-like crystallinedetails connected by the fibrillar ties. The surface relief picture issimilar to the inner lamellar structure of the porous sample(scheme in Fig. 2c) but it has a larger scale. The scale of reliefgrows with the increase in orientation.Due to a strongly developed relief and porous structure the

samples are characterized by the very high specific surface –35–40m2/g (for comparison, a dense non-porous PE film has thisvalue equal to 6.3� 10�4m2/g). The relief surface is a veryimportant feature of the porous films obtained in the processbecause it provides a high adhesion of these samples to anyorganic and inorganic coatings – that gives a possibility of usingthem successfully in composite systems as a matrix or support.The films are characterized by the following mechanicalproperties: breaking strength – 100MPa, elastic modulus –660MPa, and break elongation – 85%.

LC PE composites containing different photochromicdiarylethenes possessing a high degree of orientation anddichroism

In order to obtain PE composite films characterized by anoticeable photochromism together with dichroic properties thethree new diarylethene-type compounds were synthesized(SVZ0499, Sh160, and Diaryl II). Three mixtures containing1wt% of these photochromes dissolved in nematic MLC6816were prepared.Introduction of LC mixtures into the porous PE films results in

an appearance of the strong birefringence; therewith theorientation of LC director completely coincides with thestretching direction of the PE films.Figure 5a, b demonstrate the absorbance spectra of the PE

composite films with the above-mentioned photochromeSVZ0499 (Table 1) before and after irradiation; Fig. 6 shows apolar plot of absorbance for the PE LC mixture containing thisphotochrome.Irradiation of the LC composite with light of wavelength

405 nm leads to the transformation of the film from yellow to redcolor (Fig. 5b). Two new strong absorbance peaks grow up duringthe irradiation. One of them has maximum in UV spectral regionat 360 nm, whereas, another one at 500 nm is responsible forcoloration. Color change is associated with electrocyclic photo-rearrangement and formation of closed isomer of photochromeSVZ0499 as is shown below:

Before and after irradiation, the composite films have a smalldichroism (0.17 at 500 nm) that is associated with a minor degreeof orientation of the photochromic molecules in LC media. Low


anisometry of dye molecules results in a poor orientation degreein LC matrix. Nevertheless, the small degree of dichroism isaccompanied by a significant changing of color that may be usedfor the creation of PE films with color-variation properties underthe light irradiation.


Figure 7. Fatigue resistance properties of PE composite film containingliquid crystal MLC6816 and 1wt% of Sh160 under cyclic irradiation by

405 nm (5min) and 546 nm (2min). Absorbance was measured at 530 nm

(absorbance of closed form of photochrome).

Figure 8. Absorbance spectra of LC PE composite film containing

photochrome Diaryl II before (a) and after (b) UV irradiation (313 nm,

10min).

Figure 9. Polar plot of corrected absorbance for PE film containing LC

mixture with photochrome Diaryl II at 606 nm after UV irradiation(313 nm, 10min). This figure is available in colour online at www.

interscience.wiley.com/journal/pat


1

In order to compare properties of the PE composite films withthe LC mixture containing the same photochrome, we haveprepared a conventional glass cell (prepared using polyimide-coated and unidirectionally rubbed glass substrates) andperformed light irradiation and polarized spectroscopy measure-ments. Qualitative results of the photo-optical study were thesame, but the value of dichroism in this case was significantlygreater (D¼ 0.27 at 500 nm). These results indicate that the qualityof orientation induced by PE fibrilles in pores is lower than analignment provided by rubbed polyimide coating of the glass cell.Films of PE composite containing 1wt% of another photo-

chrome of diarylethene series Sh160 have the similar pecularitiesof the photo-optical behavior. An absorbance maximum of theclosed form is at 530 nm. In this case, degree of coloration as wellas the dichroism values were very low (D530¼ 0.07). Probably, theeffect of low coloration is connectedwith small quantum yields ofring closing reaction or the low extinction coefficient of theclosed isomer.Despite the low dichroism of the two above-mentioned

photochromes, they demonstrated good cyclicity after irradiationby two different wavelengths. We have studied fatigue resistanceproperties of the composite films containing Sh160, i.e., theirstability to many cycles of ‘‘recording’’ and ‘‘erasing’’ of opticalinformation. ‘‘Recording’’ was realized by blue light irradiation(405nm) accompanied with an appearance of the closed formabsorbancewithmaximum at 530nm. ‘‘Erasing’’ was performed by aback photoreaction induced with the green light (546nm). As seenfrom Fig. 5, the fatigue resistance of the photochromic compositefilms is quite high that allows one to consider such composites asthe promising materials for the dynamic optical processing.The more impressive results have been obtained with Diaryl II

photochrome. PE composite films containing this photochromicdiarylethene having more extended aromatic ring structure(Diaryl II, Table 1) are characterized by more pronouncedcoloration with higher dichroism (0.42 at 606 nm) (Figs 8, 9).Irradiation of the colorless films by UV light (313 nm) leads to ringclosure of photochromic moieties associated with the strong bluecoloration. The high anisometry of photochrome moleculesdemonstrates a molecular model of Diaryl II (Fig. 10). Thephotoinduced color changes are reversible and irradiation bylight of HeNe laser (633 nm) allows one to recover the initialcolorless state.

Polym. Adv. Technol. 2010, 21 100–112 Copyright � 2009 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/pat

07

Figure 11. Fatigue resistance property of PE composite film containing

Diaryl II under repetitive UV and visible light irradiation.

Figure 10. Molecular model of photochrome Diaryl II and proposed LC

director orientation. This figure is available in colour online at www.interscience.wiley.com/journal/pat

A. BOBROVSKY ET AL.

108

The fatigue resistance of the photochromic composite films isquite high (Fig. 11) that allows one to consider such LCcomposites of PE as the promising materials for dynamic opticalprocessing.For demonstration of a photorecording utility of such

composite films the PE LC composite containing 1wt% of DiarylII film was irradiated through amask. Figure 12 shows an exampleof such photorecording. Blue regions correspond to theirradiated areas. However, it is noteworthy that such a recordedimage is unstable and after storage for several hours at roomtemperature becomes completely invisible due to the diffusionprocess of the closed colored form molecules inside the lowmolar mass liquid crystal mixture.In order to avoid this disadvantage we have developed a novel

approach based on using the photopolymerization of the LCmixture doped with the same photochrome followed by thepolymer network formation stabilizing the recorded image (seeSection ‘‘Photopolymerization as a stabilization method of

Figure 12. Photograph showing image of the recorded letters using

mask on LC PE composite films containing 1wt% of photochromeDiaryl II.Irradiation was performed by UV light (313 nm) during 10min. This figure

is available in colour online at www.interscience.wiley.com/journal/pat


photochromic LC composites due to polymer network for-mation’’).Thus, we have prepared a novel family of LC PE composites

with good photochromic properties which can be used for opticaldata recording and processing. In the case of the LC PEcomposites containing photochromic diarylethene SVZ0499 andDiaryl II the best coloration properties and noticeable dichroismwere discovered.

PE LC composites containing combined photochromeswith high solubility in LC matrix

One of the important disadvantages of photochromic diary-lethene described above deals with a limited solubility of thesephotochromes in the LC mixtures. In most of the cases,introduction of more than 1wt% of photochrome in the LCmixture is impossible due to a low photochrome solubilityresulting in the phase separation. In order to improve solubility, anovel series of combined photochromic compounds containing aphotoactive group linked with the mesogenic phenylbenzoatefragment responsible for compatibility with liquid crystals havebeen synthesized. The first substance, Leo120 comprisesphotochromic naphthopyran, whereas the second one,Leo141, has photochromic dye fragment of diarylethene series.Both compounds have the same mesogenic alkoxy phenyl-benzoate fragment which plays a role of solubilizer increasing thecompatibility of photochromic substance with the low-molar-mass nematic liquid crystals.

First, let us consider the photo-optical properties of LCcomposite films of PE containing Leo120. This dopant does notdisplay any liquid crystalline properties and presents anamorphous glassy substance at room temperature. Two LCmixtures containing different concentrations of this photo-chrome (1 and 10wt%) have been prepared.UV irradiation of PE composite films containing the low

concentration of photochrome does not lead to any visiblespectral changes. On the other hand, UV irradiation of PEcomposite with 10% of Leo120 allowed one to observe anoticeable photochromism and appearance of the strong new


Figure 13. (a) Spectra of LC PE composite film containing 10wt% of

Leo120 before and during UV irradiation (365 nm). Spectra were recorded

every 1min of irradiation. Light intensity was 0.5mW/cm2. (b) Kinetics ofcorrected absorbance change during UV irradiation.

Figure 14. Spectra of LC PE composite film containing 10wt% of Leo120before and during thermal decoloration process at 168C. Spectra were

recorded every 2min.


1

absorbance band with maximum at ca. 450 nm (Fig. 13a). Thiseffect is associated with a ring opening process and formation ofthe colored merocyanine form of photochrome as is shownbelow:

More deep color of this compound is associated with extendedp-conjugation in chromophore fragment in comparison with theclosed form. Figure 13b demonstrates kinetic curve of theabsorbance growth during UV irradiation.It is noteworthy that the photochrome Leo120 is characterized

by a very short lifetime of the colored form. Full decoloration isobserved after�5min irradiation at 168C (Fig. 14). Due to the fastdecoloration process the measurements of polarized absorbancespectra present some difficulties.Our experiments showed that the conversion of ring-opening

photoreaction strongly depends on the light intensity due to the


competing thermal bleaching process. Only irradiation of suchcomposites by light with a relatively strong intensity (at least0.5mW/cm2 or above) allowed one to detect the open coloredform. Nevertheless, such composites could be used for dynamicoptical recording and data processing.Now let us go to the consider the LC PE composites containing

the diarylethene combined dopant Leo141. The dopant does notalso form the liquid crystalline phase and melts in the range of�1008C. An increase in Leo141 concentration in the LC mixtureleads to the gradual decrease in clearing temperature of themixturedue to a bulky diarylethene substituent. Introduction of 10wt% ofdopant results in a drop of isotropization temperature by about 108.LC PE composites containing small concentration of this

photochrome (�1wt%) also have only poor photochromism,whereas the introduction of the higher concentration of Leo141allowed one to achieve the good photochromic properties(Fig. 15a). UV irradiation converts almost colorless film to the deeplycolored red one. As in the case of the photochrome with the samechromophore structure (SVZ0499) the two new strong absorbancepeaks grow up during the irradiation. One of them has maximum inUV spectral region at 360nm, whereas, another one at 500nm. Theclosed form of photochrome is also stable at room temperature.Irradiation of the colored films with green light (546nm) leads tothe back conversion to the colorless state (Fig. 15b).In all the cases, we did not find any evidence of linear dichroism

in the composite films. Such an observation can be explained onthe basis of consideration of the molecular model of photo-chrome and its possible orientation in the LC matrix (Fig. 16). Asclearly seen from the figure the transition moment ofphotochrome molecule is oriented at a large angle, close to908 with respect to the mesogenic fragment and possible LCdirector orientation. These factors result in an isotropicdistribution of the transition moment axes of photochromemolecules even in an anisotropic medium.Summarizing results obtained in these parts of the paper one

can conclude that the LC PE composites containing the combinedmesogen-containing photochromes with improved solubilitypresent a novel promising type of materials which can be used fordifferent photo-optical applications.

PE LC composites for irreversible optical photorecording

In order to create LC PE composite films having the irreversiblephotochromic properties the mixture of liquid crystal material


09

Figure 15. (a) Changes in the absorbance of LC PE composite film

containing mixture of MLC6816 with 10wt% of Leo141 before and duringirradiation by blue light. For irradiation, lines 405 and 436 nm of mercury

lamp were selected by glass filter (intensity was 1.2mW/cm2). Spectra

were recorded every 1min of irradiation. (b) Changes in the absorbance ofcomposite film during irradiation by green light. For irradiation, lines 546

and 577 nm of mercury lamp were selected by glass filter (intensity was

0.4mW/cm2).

Figure 17. Changes in absorbance spectra (a) and kinetics of absor-

bance decrease at 532 nm (b) during irradiation of composite film con-taining 1wt% ASh216a by nonfiltered light of Hg-lamp (�20mW/cm2).

A. BOBROVSKY ET AL.

110

ZLI1695 with merocyanine dye ASh216a (1wt%) has beenprepared (Table 1).It should be pointed out that this merocyanine dye does not

undergo cyclization photoreaction, nevertheless, a prolongedirradiation results in the strong photobleaching which can beused for photorecording. As seen from Fig. 17, the irradiation ofPE composite films with nonfiltered light of mercury lamp leads

Figure 16. Molecular model of photochrome Leo141 in closed form.This figure is available in colour online at www.interscience.wiley.com/

journal/pat

Figure 18. Photo of composite films (a) just after irradiation through the

mask (30min of nonfiltered Hg-lamp); (b) after 20 hr storage at roomtemperature. This figure is available in colour online at www.interscience.

wiley.com/journal/pat

www.interscience.wiley.com/journal/pat Copyright � 2009 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2010, 21 100–112

Figure 19. Polarized optical microphotograph showing permanent opti-cal photorecording. Film was irradiated through the mask by UV light

(365 nm) during 30 s followed by dissolution of nonpolymerized mono-

mers mixture in acetone. Thickness of the bright lines is 50mm. This figure

is available in colour online at www.interscience.wiley.com/journal/pat


1

to an irreversible photobleaching of ASh216a. About 1 hr ofirradiation completely removes color of the film.Figure 18 demonstrates the recorded image of Cyrillic letters

‘‘MSU’’ (Moscow State University). Unfortunately, the recordedimage is not stable due to the diffusion of dye molecules in thecomposite films (Fig. 18b). This disadvantage can be excludedusing photopolymerizable network-forming photochromic LCmixtures containing ASh216a and mesogenic mono-diacryaltes(see the Section ‘‘Photopolymerization as a stabilization methodof photochromic LC composites due to polymer networkformation’’). In this case the recorded image can be kept inthe irradiated films for several months without color changing.

Photopolymerization as a stabilization method ofphotochromic LC composites due to polymer networkformation

As we have discussed above one of the disadvantages of thephotochromic composites prepared by the filling of PE films withlow-molar-mass LC mixture is a temporal instability of recordedimage due to the free diffusion of the composite components. Thisfeature hinders to use them for the long-termoptical data storage. Inorder to solve this problem we have developed a new approach forthe stabilization of LC PE composites and prepared photopolymer-izable mixture containing nematogenic photopolymerizable dia-crylate RM257 (77.5wt%), photochromic diarylethene SVZ0499(0.5wt%), photoinitiator for UV light-range Irgacure651 (2wt%), andnematic liquid crystal mixture MLC6816 (20wt%) as plasticizingcomponent decreasing the fragility of polymerized films. Thechemical structures of all these components are shown below:

Phase transitions of the obtainedmixture demonstrate a ratherwide interval of LC nematic state: Cr 59–628C N 86–888C I. Despitecrystalline phase formation, this mixture can be easily overcooledand nematic phase can be kept at room temperature for a coupleof hours. This time is enough for LC PE composite preparation andphotopolymerization procedure.Photopolymerization was performed in argon atmosphere by

irradiation with UV light (365 nm). Even at low power of light(<0.5mW/cm2) the polymerization rate is quite enough for thenetwork formation after several seconds of irradiation.Photo-optical behavior of the obtained polymerized compo-

sites is very similar to the above discussed for PE films filled bymixtureMLC6816 with SVZ0499. Polarized absorbance spectra for


the composite film in colored red form revealed a noticeabledichroism. Dichroism calculated at 500 nm was about 0.36. Greenlight action allows one to recover initial colorless state of the film(with photochrome in open form).Most important advantages of the LC photopolymerized PE

composites are their stability in time (any evaporation oflow-molar-mass components is completely excluded). Irradiationthrough a mask allows one to record very stable image which canbe kept in darkness for very long time (years).Another possibility of photorecording on such composite films

is photopolymerization by irradiation through amask followed bydissolution of nonpolymerized monomers mixture in acetone.Figure 19 shows the polarized optical microphotographdemonstrating this type of permanent optical photorecording.Bright areas in the figure correspond to the highly birefringentpolymerized crosslinked LC network. This photorecording iscompletely irreversible and extremely stable for years.It should be noticed that the photopolymerizable photochromic

composites based on stretched porous PE films provide uniquepossibility for the creation of materials for dual permanent anderasable data storage and have no analogs in literature.

CONCLUSIONS

A systematic research on photochromic LC PEss-based compo-sites including design, synthesis of new photochromic, andhybrid mesogen-photochromic compounds, preparation ofporous PE films, elaboration of the methods of incorporationof photochrome-doped liquid crystals into the porous PE films,characterization and study of optical and photo-optical proper-ties of the obtained samples has been performed.Such new hybrid materials developed in our work can be

considered as the very promising photo-optical and electro-opticalactive media for potential applications in optical and optoelec-tronic devices, display technology, for information opticalrecording and creation of erasable memory materials. Based onthese materials a novel family of optical elements as polarizers,retarders, molecular shutters, and switches can be developed.

Acknowledgements

This research was supported by the Russian Foundation ofFundamental Research (08-03-00481, 08-03-00865), ProgramCOST-D35, and Contract with the Industrial Technology Research


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A. BOBROVSKY ET AL.

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Institute (Taiwan). We sincerely thank all these organizations fortheir financial support.

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Photochromic composites based on porous stretched polyethylene filled by nematic liquid crystal mixtures

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