Dynamics and morphology of phase-separated separated polymers using liquid crystal templates

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1 electronic-liquid Crystal Crystal Presentations Dynamics and morphology of phase-separated separated polymers using liquid crystal templates Liang iang-chy Chien Liquid Crystal Institute, Kent State University Kent, Ohio 44242, USA

2 Photo-embossed polymers in liquid crystals SEM images electronic-liquid Crystal Crystal Presentations

3 Switchable PSLC gratings 1-D 2-D electronic-liquid Crystal Crystal Presentations 1-D blazed grating 0V 0.9V/μm 2.0V/μm Kim, Opt. Exp Kang, Apl 2007

4 Flexible Displays Flexible Displays Encapsulated cholesteric reflective LCDs on flexible substrates: Polarizing optical microscope image of the ChLC droplets in the thin film coating. electronic-liquid Crystal Crystal Presentations Schneider, et al. SID 2005, IDRC 2006

5 electronic-liquid Crystal Crystal Presentations Outline Effects of liquid crystalline phases on polymerization Formation of polymer networks in liquid crystals Application of liquid crystal templated polymer systems

6 Liquid Crystal and Polymer Composites Polymer Dispersed LC (PDLC): large concentration of pre-polymer (>10~90%) dissolved in LC Miscibility of heterogeneous species (Entropy effect) controls phase separation Polymerization kinetics controls phase-separated polymer structure and droplet morphology Polymer stabilizes LC (PSLC): Small concentration of pre-polymer (<10 wt%) dissolved in LC LC host aligns pre-polymer before polymerization Polymerization takes place in liquid crystal phase Resulting polymer or network mimics the structure of LC electronic-liquid Crystal Presentations

7 No distortion F = 0 Additive in liquid crystal host Free energy associated to elastic distortion: twist distortion F > 0 splay distortion F > 0 Ordering prior to polymerization for mesogenic monomer: Distinct defect structure determined by the nature of the phase S = ±1 S = ±1/2 bend distortion F > 0 For isotropic inclusions (monomer, oligomer, polymer), the hetero species tend to stay in defect area, or area prone to have defect to minimize the energy. Distortion is unfavorable! Thermal dynamics of liquid crystal determines that the liquid crystal and monomer system have to change its conformation to minimize free energy. electronic-liquid Crystal Presentations

8 electronic-liquid Crystal Crystal Presentations Method of liquid crystal alignment Coating glass substrate with a thin layer of polymer (polyimide) Rub coated surface with velvet-coated cylinder in one direction to create grooved surface Assemble two treated glass substrate face to face and separated with spacer rods/beads: a cell is made for homogeneous alignment Fill the cell with mixture of liquid crystal/pre-polymer

9 Liquid crystal alignment via rubbing Spin coating and rubbing PI: A polyimide is deposited and thermally cured via flexographic printing or spin-coating. The surface is rubbed in one uniform direction to form groves on the surface of polyimide layers, which forcing the LC director to following the rubbing direction. The rubbing process also generates the so called pre-tilt angle. The polyimide chemistry determines the magnitude of the pre-tilt. Rubbing aligns main chain and side chain (functional groups) of polymer films. The value of pretilt angle depends on the strength of rubbing and molecular structure of alignment layers. H O N HO O N O O O H CF 3 CF 3 N C OH O Coating Curing at 250~300 o C O O N CF 3 CF 3 C electronic-liquid Crystal Presentations Review: J. Cognard, Mol. Cryst. Liq. Cryst., 78, suppl. 1, 1 (1981). Seo, et al. SID 93 Digest, 953 (1993).

10 Phase separation of polymer in LCs ξ ~ kt ( φ ) η Rx 1 3 Where ξ is the size of nodular beads, k is the Boltzman constant, T is the temperature of polymerization, η is the viscosity of the media, Rx is the rate of crosslinking reaction, φ is the concentration of polymer R x α γ ~ [ M ],( φ I ),([ PI ] d ) 9 2 λ Rajaram, et al., Chem. Mater. 8, 2451 (1996) Evolution of polymer network morphology electronic-liquid Crystal Crystal Presentations

11 electronic-liquid Crystal Crystal Presentations Example of materials and preparation Example of materials and preparation Mesogenic diacrylate monomer: 1-10% RM257 (Merck) O O O O O O O O O Photoinitiator (PI): 1-4% Irgcure 651 C Liquid crystal: 89-98% BDH13739 Dissolve the three component in dichomethane O O K 70 N 126 I OCH3 C OCH3 IRG651 Evaporate solvent under vacuum, leaving only the blend of reactive polymer and liquid crystal

12 Fill this mixture into a cell: 15 μm, PI2555 (rubbed for planar alignment) UV irradiation in a UV chamber: Accurate control over polymerization condition: R p [ M ] ( φi ) ([ PI] d ) UV intensity, Concentration of components: monomer concentration, photoinitiator concentration; Curing temperature; Mesophase of the liquid crystal host = k p UV light Photo detector Sample After UV irradiation, cells were studied under polarizing microscopy. copy. Temperatur e controller To study polymer morphology by SEM, liquid crystal was removed: 20% dichlomethane,, 80% hexane, low temperature solvent evaporation (dry ice for top view, liquid nitrogen for cross section view) electronic-liquid Crystal Presentations

13 electronic-liquid Crystal Crystal Presentations Concentration dependence of polymer morphology 200 μm BDH13739, 6% monomer 200 μm BDH13739, 6% monomer P P n A A 1 um 200 μm BDH13739, 10%monomer, SEM 20 μm BDH13739, 4%monomer, SEM

14 As oligomers form and increase in size, solubility of oligomers in liquid crystal host decrease and induce phase separation Anisotropic and viscosity of liquid crystal induce different diffusing rate Minimization of free energy in this system leads to anisotropic aggregation of polymer: anisotropic phase separation F elas = F surface F mix K = r F = F elas + F sur + F mix r r ( n) + K 22( n ( n)) + K33( n ( n)) = Wθ sin ( θ θ0) + W 2 2 φ sin 2 θ sin 2 r ( φ φ ) electronic-liquid Crystal Presentations 0 r

15 electronic-liquid Crystal Crystal Presentations Dependent of morphology on rate of phase separation distance/μm polymer concerntration distance/μm photoinitiator concerntration UV intensity/mw/cm 2 distance/μm distance/μm temprature

16 electronic-liquid Crystal Crystal Presentations F elas Director field effect - gradient in elastic deformation - F = F elas + F diel + F sur + F mix LC monomer Periodic distortion of the LC host induces a spatial variation of F elas Reactive monomer molecules located at the less deformed regions x

17 LC Controlled Phase Separation Spatially ordered polymer walls with equal distance depend on order of the host and phase separation conditions. Higher monomer concentration results in smaller distance between polymer walls; or smaller droplet. Anisotropic phase separation depends on rate of phase separation. Distance between polymer walls increases functionality and concentration of monomer. electronic-liquid Crystal Presentations

18 Other Examples Other Examples electronic-liquid Crystal Crystal Presentations LCs: ~94.75 % Nematic (BL006) Reactive Monomer: RM257 ~ 5% CH3 CH2=CHCO2(CH2)3O CO2 O2C O(CH2)3O2CCH=CH2 Photoinitiator: ~.25 % Irgacure 651 O C OCH3 C OCH3 hv Cell: 10 μm cell gap, antiparallel rubbed PI or PI for homeotropic alignment UV: 4 mw/cm 2 or mw/cm 2

19 Light-induced anisotropic polymer densification Initiator Monomer LC R p UV (strong) Anisotropic polymer densification I I th 0 d d c = k [ M ] ( φi ) ([ PI] d ) λ p φi If there is anisotropy in the system, due to anisotropic absorption of incident light, modification should be introduced to Kp. Thus we have Kp // and Kp, denoting direction parallel and perpendicular to LC director: anisotropic polymerization electronic-liquid Crystal Presentations

20 electronic-liquid Crystal Crystal Presentations Effect of UV on morphology Strong UV or short λ UV (E=hc/λ) Planar/homeotropic alignment: Strong UV and short λ UV : network formed at the UV illuminated surface Kang, et al., Macromolecules 35, 9372 (2002).

21 electronic-liquid Crystal Crystal Presentations Initiator Monomer LC Liquid crystal-controlled anisotropic polymer densification Anisotropic polymer densification in bulk UV (weak) I I th 0 d Low intensity or long λ of UV yields a homogeneous polymerization across the sample thickness

22 electronic-liquid Crystal Crystal Presentations LC controlled polymer morphology Weak UV or long λuv Planar/homeotropic alignment: weak λuv: polymer network formed across the cell

23 Polymerization of LC thiol-enes Step-growth polymerization propagated by a free radical mechanism Important parameters: Rp = k p[ M ] ( φi ) ([ PI] d ) Molecular design of LC thio-ene Monomer purity Stoichiometry Mesogenic behavior Orientational order HS Thiol-ene Rigid liquid crystalline part electronic-liquid Crystal Presentations

24 electronic-liquid Crystal Crystal Presentations Mechanism of Thiol-ene polymerization Inititation I 2 I I + HSR IH + SR Propagation Termination m m n R = m n-1 m m n n n

25 Composition of materials THE4b4 (%) IRG651 (%) E31 (%) R1011 (%) 8CB (%) Nematic (1) Nematic (2) Smectic (1) Smectic (2) Cholesteric (1) Cholesteric (2) electronic-liquid Crystal Presentations

26 electronic-liquid Crystal Crystal Presentations Polymer formed in nematic (1) POM photos, after removal of liquid crystal. After removal of liquid crystal, polymer has some alignment along the rubbing direction. The morphology of polymer. The morphology of polymer. RD P A

27 Polymer formed in nematic (2) RD P electronic-liquid Crystal Crystal Presentations R.D. The morphology of polymer. The morphology of polymer. A

28 Polymer formed in smectic A (1) After polymerized, the polymer in smectic Liquid crystal. The morphology of polymer (40 o angle and top (insert) view). P RD A After removal of liquid crystal, the morphology of polymer (40 o angle view). electronic-liquid Crystal Crystal Presentations The morphology of polymer (the edge of cell of 40 o angle view).

29 electronic-liquid Crystal Crystal Presentations Polymer formed in smectic A (2) The morphology of polymer from top view. The morphology of polymer (the edge of cell at 20 o angle view). The morphology of polymer (20 o angle view). The morphology of polymer (the edge of cell at 40 o angle view).

30 Combined Maier-Saupe and Flory-Huggins model F Concept: Free energy of mixture can be reduced when monomer selectively replaces LC in regions of higher (or lower) gradient in director orientation based on a combination of the Flory-Huggins/Maier-Saupe (FHMS) model Illustrative example: A cholesteric LC host distorted by an electric field E applied perpendicular to the twist axis (z) 2 M L dφ Δ ε 2 0(, ) M L M L sin B M ln M + q c c E k T c L clln M + L 2 φ dz 8π cm cl K2 ( c, c ) ( c, c ) 2 c c c c = + φ, c M, and c L are the director twist angle, monomer, and LC concentrations of the mixture. K 2, q 0, and Δε are the twist elasticity, natural helical wavevector, and dielectric anisotropy. Energy Density erg cm APL 72, 885, (1998); Liq. Cryst., 28, 637 (2001); APL 78, 3782 (2001) u S.-W. Kang dissertation (2002) electronic-liquid Crystal Presentations

31 Formation of a cholesteric 1D pattern E V The cholesteric helices, original perpendicular to the substrates, were rotated to parallel to the substrates to induce a periodic 1D pattern by a small bias voltage ~ 0.2 V/μm. Subsequent UV polymerization (UV light: unpolarized 365 nm (0.01 W/cm 2 for 40 min) stabilizes the 1D pattern. R/ D P 10 μm A electronic-liquid Crystal Crystal Presentations

32 electronic-liquid Crystal Crystal Presentations Polymer formed in cholesteric (1) Fingerprint 1D texture before polymerization. After removal of liquid crystal, form some alignment along the rubbing direction. The morphology of polymer. The morphology of polymer.

33 electronic-liquid Crystal Crystal Presentations Polymer formed in cholesteric (2) After removal of liquid crystal, polymer aligns somewhat along rubbing direction. The polymer aligns with the rubbing direction at the edge. The morphology of polymer. The morphology of polymer at the edge.

34 electronic-liquid Crystal Crystal Presentations Templating of conductive polymers Templating of conductive polymers Background: Shirakawa et al [Synthetic Metals,117, 1, (2001); Science 282, 1683 (1998)] found oriented fibril morphology in polyacetylene films formed on homeotropic nematic surface. Conductivity anisotropy of 300:1. Monomer introduced in gaseous state. Catalyst R 1 C C R 2 C C * * R 1 R2 n

35 Templating of CPs in ordered liquid crystals F d/p = 1 A cholesteric LC host distorted by an electric field E applied perpendicular to the twist axis (z) 2 M L dφ Δ ε 2 0(, ) M L M L sin B M ln M + q c c E k T c L clln M + L 2 φ dz 8π cm cl K2 ( c, c ) ( c, c ) 2 c c c c = + φ, c M, and c L are the director twist angle, monomer, and LC concentrations of the mixture. K 2, q 0, and Δε are the twist elasticity, natural helical wavevector, and dielectric anisotropy. 1D pattern forming state of a cholesteric LC: V on electronic-liquid Crystal Presentations APL 72, 885, (1998); Liq. Cryst., 28, 637 (2001); APL 78, 3782 (2001)

36 Polymer morphology in cholesteric POM image SEM image Bottom substrate 25 μm electronic-liquid Crystal Crystal Presentations

37 SEM images of templated PHDs electronic-liquid Crystal Crystal Presentations A,B = hydrogen (a-d); ethyl (e) Adv. Funct. Mater. 14, (2004).

38 electronic-liquid Crystal Crystal Presentations PEDOTs formed in nematic LCs SEM images no alignment (BDH 13739) some alignment (E31) (E44)

39 electronic-liquid Crystal Crystal Presentations no alignment PEDOTs formed in a SmA LC (8CB) (high temperature) some alignment (low temperature)

40 electronic-liquid Crystal Crystal Presentations PEDOTs formed in a lyotropic nematic LC some alignment SEM images some alignment lyotropic host: Cromolyn Sodium (DSCG 16% in H 2 O) 1 μm

41 electronic-liquid Crystal Crystal Presentations PEDOT morphology on smectic A-templated A polymer network Before EDOT polymerization After EDOT polymerization R S ~400 ohm/ square σ=~3.5 S/cm d~8μm

42 Summary We have demonstrated using Liquid Crystals as tools for Sofl Lithography! Thermal dynamics of liquid crystal determines that the liquid crystal and monomer system have to change LC conformation to minimize free energy. The templated polymers grow in the direction following the rubbing as a result of minimization of free energy of the system. The rate of phase separation determines the final polymer morphology. The thio-ene based polymers produce striking morphologies such as cauliflower or mustache-like in nematic, dumpling-like like in smectic, and bowtie-like in cholesteric, controlled by the diffusion limit during the fast gelation process. electronic-liquid Crystal Presentations

43 Team Everything: Carmen Otilia Catanescu, Shin-Woong Kang, Sang-Hwa Kim, Lanfang Li, Xiaoli Zhou (LCI/CPIP/KSU) Thio-ene monomer: Sylvia van den Tillaart, Cees Bastiaansen, Hans Wilderbeek and Dick Broer (Eindhoven University of Technology and Philips Research Laboratories, Eindhoven, The Netherlands) Acknowledgement AFOSR, NSF-ALCOM electronic-liquid Crystal Presentations