PVDF (polyvinylidene difluoride) as material for active element of twisting-ball displays

Date

2014-05-15

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Abstract

Töö „PVDF (polülinüleendifluoriid) kasutamine twisting-ball tüüpi kuvari aktiivelemendi valmistamiseks“ eesmärgiks oli töötada välja tehnoloogilised protsessid selle materjali eriomaduste kasutamiseks, sealhulgas tema pinna keemiliseks modifitseerimiseks ja katmiseks anorgaanilise (alumiiniumoksiid) kihiga. E- paber (valgustpeegeldav kuvar) erineb oluliselt tavapärastest emiteerivatest ekraanidest - taustvalgusega vedelkristallkuvarid ( LCD) , plasmapaneelid (PDP) ja valgusdioodekraanid. E-paber (LED) muudab oma värvust ja peegeldab tagasi välisvalgust nagu tavaline paber. Et puudub vajadus valguse tekitamise järele, on nad äärmiselt energiasäästlikud. See suurendab kaasaskantavate seadmete (e-lugerid) aku eluiga, sest energiatarve staatilise pildi kuvamisel on null, energiat kulub ainulty pildi vahetamisel või video esitamisel ( ekraanid on bistabiilsed ) . Sellised seadmed on odavad, tugevad, vastupidavad. Neid on võimalik valmistada plastkilest, kasutades roll- to-roll tehnoloogiat ja olla seetõttu painduvad Twisting-ball tehnoloogia („Gyricon“), üks võimalusi e-paberi valmistamiseks, leiutati Xeroxi uurimiskeskuses Palo Alto Research Center ( PARC ). Selline ekraan koosneb õhukesest läbipaistvast silikoonkilest, milles paiknevad kahevärvilised sfäärilised polariseeritud osakesed. Iga pall kujutab endast elektrilist dipooli (on valmistatud elektreetsest materjalist, mis on püsimagneti elektriline analoog) ja asub kileõõnsuses, miks on täidetud dielektrilise vedelikuga. Kui ekraani juhtpinge on konstantne või null, „kleepub“ palli elektrostaatuilise jõu toimel õõnsuse seina külge. Juhtpinge polaarsuse muutmisel hakkab pall liikuma süvendi vastasseina poole. Palli mikroasümmeetriad põhjustavad tema telje kõrvalekaldumise ja tekib pöörav jõumoment. Osake pöördub teistpidi ja vahetab vaataja poole suunatud külge. Sellist ekraani on lihtne valmistada, ta on odav ja energiasäästlik ning silmasõbralik, sest sarnaneb tavalisele trükitud paberile. Algse Xerox Gyriconi aktiivelement ( pallid ) ekraanid olid valmistatud erinevatesti vahadest või polüetüleenist ja elektriliselt laetud suhteliselt stohhastilise protsessi tulemusena, mistõttu osakeste laengud erinesid üksteisest. Tänu kasutavate materjalide madalatele elektreetsetele omadustele tuli kasutada kõrgeid juhtpingeid, mis oli suureks probleemiks miniatuursete kantavate seadmete valmistamisel. 1990ndatel otsustas Xerox katkestada sellealase teadus-ja arendustegevuse . Autor tegi ettepaneku kasutada aktiivelemendi materjalina polüvinüleendifluoriidi, PVDF on üks tugevamaid ja stabiilsemaid elekteete tänu sellele, et polariseerimisel toimub kristallstruktuuri muutumine ja polariseeritud kristalli (β-faas)siseenergia on madalam polariseerimata kristalli omast (α-faas). Kuid PVDF on keemiliselt inertne materjal ja on ka äärmiselt hüdrofoobne, tema pinna värvimine ja katmine on praktiliselt võimatu . Nii sai tema kasutamine e-paberi jms rakendustes võimalikuks alles vastava pinnatöötlemise viisi leiutamise järel. Selleks modifitseeritakse pinda esmalt keemiliselt (kasvatatakse külge imeõhuke polüstüreeni kiht), et muuta see hüdrofiilseks, seejärel sadestatakse pinnale suhteliselt paks läbipaistmatu (valge) alumiiniumoksiidi kiht. Leidsime, et soovitavaid tulemusi võib saavutada, kasutades ATRP reaktsiooni (atom transfer radical polymerization ). See lubab polümeeri koostises olevatele halogeeniahelatele liita erinevaid monomeere. Kasutades vaskkatalüsatorit, mis koosnes CuCI ja CuCl2 kompleksist TRENiga (tris(2-aminoeüül)amiin) õnnestus PVDF-i pinnal tekitada hüdrofiilne polüstüreenikiht. Reaktsioonitingimuste (kontsentratsioonid, temperatuur9 muutmisega oli kihi omadusi võimalik soovitavas suunas muuta. Osakeste heade optiliste omaduste (läbipaistmatus, peegeldumisvõime) saavutamiseks oli vajalik osakeste katmine ühtlase alumiiniumoksiidi kihiga. See oli ka töö üks kriitilisemaid etappe. Perfektse tulemuse saavutasime, kui tekitasime harjaselises sadestatud polüstüreenikihis kristallisatsioonitsentrid trimetüülalumiiniumi ja vee reageerimisel mng hiljem sadestasime neile sol-gel meetodil paksu oksiidikihi analoogselt alumiiniumoksiidi monodisperssete mikrososakeste valmistamisele. Selleks sadestasime pinnale aeglaselt alumiiniumoksiidi alumiiniumi soolade lahusest, kontrollides lahuse pH-d ja seega ka sadestamiskiirust karbamiidi termilise lagundamise abil. Hiljem polariseeriti alumiiniumoksiidiga kaetud osakesed koroonalahenduse abil, olles eelnevalt kinnitanud nad kleepkilele. Seejärel värviti pallikeste üks poolkera mustaks. Osakesed segati vedela kahekomponendilise silikooniga, formeeriti kile ja see kuivatati. Õõnsused osakeste pöörlemise võimaldamiseks saadi kile töötlemisel heksaaniga. Saadud kile paigutati kahe elektroodi vahele (vaatajapoolne elektrood läbipaistev, indium-tinaoksiidiga kaetud klaas) ja uuriti kuvari mudeli käitumist. Töötati välja ka osakese käitumist kirjeldav matemaatiline mudel (diferentsiaalvõrrandite süsteem), mis lahendati Runge-Kutta meetodil, kasutades programmi MATHLAB. Tulemusena tõestati uuritava materjali sobivus antud tüüpi kuvari valmistamiseks ja töötati välja tehnoloogiline järgnevus.
E-paper (reflective) displays are fundamentally different from the conventional emissive flat panel displays as backlight liquid crystal displays (LCD), plasma display panels (PDP) and light emitting diode (LED) displays in the following way: they reflect ambient light lake paper and are extremely energy efficient. This increases battery lifetime in portable devices, as their power consumption, when displaying a static image, is zero. They only require power when changing the image or presenting video (bi-stability). Therefore, these devices could be low cost, robust, durable and flexible, and can be (eventually) made on plastics using roll-to-roll manufacturing methods. A twisting-ball display is a kind of electrophoretic information display invented at the Xerox Palo Alto Research Center (PARC) and called “Gyricon”. This kind of display consists of a thin layer of transparent silicone where bichromal spherical polarised particles (“balls”) are dispersed. Each ball is an electrical dipole and is placed in the cavity film filled with dielectric fluid. The diameter of the cavity is 10–30% greater than the diameter of the balls. When the control voltage is constant or zero, the ball is “glued” to the wall of the cavity. When the polarity of the control voltage is changed, the ball begins to move towards the opposite wall of the cavity. Microscopic asymmetries and the “rolling effect” cause a deviation of the axis of the electrical dipole from the direction of the electrical field. Then an electrostatic torque appears and causes the ball to rotate and different coloured hemisphere will be exposed to the viewer. The displays based on such physical principles are highly bi-stable, robust, easy to manufacture and have very low power consumption as the image is being formed using ambient light, similar to conventional printed paper. The active elements (balls) of Xerox Gyricon displays were made of different waxes or polyethylene and were charged by a stochastic process that caused each particle to have a slightly different charge. Because of this the dipole charges of the balls were low and unequal and the required control voltages were high. Therefore, the quality of the image was insufficient for market needs. In the 1990s Xerox discontinued Gyricon R&D activities. The author of this work suggested an alternative method for preparing the bichromal polarised particles, made of polyvinylidene difluoride (PVDF). This material is an electret with an extremely high residual electric field. During the polarisation process the charged –CF and -CH groups align themselves in the crystal structure of the dielectric material following the direction of the electrostatic field, producing a permanent electrostatic bias. Therefore, it was suggested that the electret properties of this material could be used for preparation of the active elements for the twisting ball display. However, as PVDF is chemically inert material and extremely hydrophobic, coloring and covering of its surface is impossible. Its usage in this kind of applications wasn't possible until efficient process of chemical modification of PVDF surface with polystyrene layer and thereafter coating with a non-transparent alumina layer was proposed. As a relatively thick layer of inorganic material was needed for the “twisting ball” display elements, a new approach for directed alumina deposition was invented, starting from alumina formation from interior of the polystyrene layer. It was found that the ATRP (atom transfer radical polymerization) reaction can be used for chemical functionalisation and grafting of spherical PVDF particles with polystyrene. The copper catalyst, consisting of mixture of CuCl and CuCl2 in complex with TREN was used for the initiation of the process. The rate of the grafting process as well as the properties of the polystyrene layer could be governed through variation of the ratio of copper salts used, but also by variation of the reaction time. These results were in agreement with the general mechanism of ATRP reactions, and provided a good possibility for optimisation of the surface functionalisation technology. In addition another possibility for PVDF surface grafting was discovered, probably including the base-catalysed carbanion formation and the following anionic polymerisation of styrene on the surface of the particles. To improve the opto-mechanical properties of PVDF particles it was important to obtain uniform coating of the particles by alumina layer. This objective was reached by developing the two-step alumina deposition procedure, where the preliminary functionalisation of the PVDF surface with polystyrene brushes was the most critical step. The presence of this polystyrene layer was used for the initial deposition of alumna particles into the polystyrene structure by using hydrolysis of trimethyl aluminium, followed by the controlled alumina precipitation procedure from water solution. Afterwards the alumina-covered particles were polarised, their electrostatic bias was generated by using the corona charge. One hemisphere of the alumina coated particles was coloured and thereafter, the PVDF “balls” used for building of the model display element, to demonstrate voltage controlled movement of the particles. The particles were deposited in transparent liquid silicone mix, the film was formed and cured. The cavities for enabling the balls’ rotation were formed by processing the film in pure hexane. Then the film was placed between two control electrodes. One of these electrodes was transparent glass plate covered with ITO (indium tin oxide). The behaviour of the particles in this model display element was compared with the theoretical model. In our study we demonstrated the possibility of using PVDF as a material of the active element for twisting-ball displays. The process of fictionalization and covering the PVDF particles' surface, and poling and colouring the particles was proposed and tested in the case of a functional model of the display element. Finally, a mathematical model that described the behaviour of an elementary cell of the twisting-ball display was introduced that gave a theoretical insight into the ball shift and rotation movements, and allowed description of the dependence between the luminance and rotation time in the case of different physical parameters of the active element (poled ball). Taken together, the study confirmed that PVDF might be a proper material for the preparation of active elements of a twisting-ball display

Description

Väitekirja elektrooniline versioon ei sisalda publikatsioone.

Keywords

materjaliteadus, kuvarid, e-paber, polüvinüleendifluoriid, materials science, displays, electronic paper, polyvinylidene difluoride

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