J Stefan Institute NMR of Demixing and Phase

J. Stefan Institute NMR of Demixing and Phase Separation in Liquid Crystals Boštjan ZALAR Andrija LEBAR Blaž ZUPANČIČ Slobodan ŽUMER Daniele FINOTELLO

Nanophase segregation 7 ABtrans t=0 J. Stefan Institute “Photocontrolled nanophase segregation in a liquid-crystal solvent”, 7 ABtrans t Y. Lansac M. A. Glaser, N. A. Clark, O. D. Lavrentovich Nature 398, 54, (1999) 7 ABcis t=0 7 ABcis t

Separation of components UV illumination Nematic Smectic J. Stefan Institute Separation

NMR of photoizomerization trans-7 AB 7 AB- d 2 J. Stefan Institute 7 AB - d 2/8 CB Cis -7 AB UV thermal population decay time-dependant order parameters

Nanosegregation - NMR arguments § linear dependence d(S) in the absence of UV light § lncrease in d on UV illumination much larger than expected from the direct d-S coupling J. Stefan Institute

Transparent nematic phase ? J. Yamamoto and H. Tanaka Nature 409, 321 -325 (2001) J. Stefan Institute

DDAB/water in 5 CB J. Stefan Institute Emulsion of inverse nanomicelles in liquid crystal H 20 + DDAB (didodecyl dimethyl ammonium bromide) + 5 CB (pentylcyanobiphenyl) nematic liquid crystal TNI = 308 K [(H 2 O)0. 15 DDAB 0. 85]0. 1[5 CB]0. 9: ~4 nm ~8 nm

Microemulsion behavior J. Stefan Institute Irreversible seggregation of components Top: micelle-rich region Bottom: micelle-poor region

Molecular configuration below TNI Transparent nematic ~ few nm << optical Phase separation cmic(I) >> cmic(N) J. Stefan Institute

Micelles/LC selective deuteration J. Stefan Institute D 2 O micelles I L Transparent nematic or phase separation -d 2 deuterated 5 CB /2 N I N L Phase separation L Transparent nematic

Impact of diffusion on NMR J. Stefan Institute Diffusion of LC molecules among nonoriented nanosized nematic domains: (t) - “motional averaging” Criterion: D=D/ 2 S Q /3 Q=270 k. Hz Bulk 5 CB at TNI: Dbulk = 7. 5 x 10 -11 m 2 s-1 D/ 2 = 750 k. Hz 5 CB confined to 10 nm: D Dbulk/10 D (m 2 s-1): 10 -14 10 -13 10 -12 10 -10 fwhm 500 Hz S=0. 7 =10 nm D (k. Hz): 10 -11 0. 1 1 10 100 k. Hz fwhm=( T 2)-1 S 2 D-1 1000 100 H 2 O/DDAB/5 CB T 2 – spin-spin relaxation time No anomaly in T 2 detected below TNI S 0 at T<TNI in TN phase

1 D deuteron NMR imaging J. Stefan Institute c 5 CB , 1 -cmic min max I I I N N B=B 0 + Gz z 2 -15 mm + N 4. 5 mm - L Gz = 0 Gz 0 L L L Measurement of concentration gradient

Bulk 5 CB Deuteron NMR, L=58. 34 MHz, Gz 0=0. 22 T/m J. Stefan Institute

J. Stefan Institute =0. 15 microemulsion Cooling rate 2 K/h, Gz=0, l=7 mm Intensity (arb. units) Deuteration TNI=295 K virgin sample TNI=299 K subsequent runs ? Phase separation ? large samples small samples - L (k. Hz) -z +z

=0. 15 microemulsion, c 5 CB profile Deuteration Cooling rate 2 K/h, Gz=0. 08 T/m, l=7 mm c 5 CB Scenario J. Stefan Institute -z - L (k. Hz) +z

=0. 15 microemulsion, cmic profile Deuteration Cooling rate 2 K/h, Gz=0. 08 T/m, l=15 mm cmic Scenario J. Stefan Institute -z - L (k. Hz) +z

Parameters determined via NMR I N Sharp doublet observed at all temperatures below TNI nematic domains with homogeneous S J. Stefan Institute I

Microemulsions-conclusions n J. Stefan Institute phase separation of inverse micelles n nematic phase (almost pure nematogen) coexists with micelle-rich isotropic phase gravity-driven global phase separation in bulk samples transparent nematic phase not detected

Photoizomerization-driven phase separation in 7 AB UV on UV off J. Stefan Institute

NMR spectra director distribution Isotropic phase J. Stefan Institute Nematic phase B 0 L Cylindrical distribution of n Isotropic distribution of n /2 B 0 /2 L /2 L n “powder” pattern L

Determination of Q • Power-spectrum frozen 7 AB 3 Q J. Stefan Institute

2 H NMR – bulk Φ dependence • wide lines • additional nematic doublet (cis) • determination of Φ at the N+I → N transition J. Stefan Institute

Strong UV-absorption Ill-conditioned measurements in bulk! J. Stefan Institute

NMR in Anopore-confined system • optimal photoisomerization – – – • thin flat samples (60 μm) uniform illumination homogeneous sample large surface-to-volume ratio – • J. Stefan Institute resolvable surface-ordering effects angular dependence – determination of director configuration

In-situ photoisomerization NMR B 0 J. Stefan Institute fiberoptic guide NMR coil collimator 7 AB confined to Anopore UV beam

2 H • NMR – 7 AB in Anopore splitting of the central line – surface induced order S(r) J. Stefan Institute

Theory • Flory-Huggins isotropic mixing • Maier-Saupe nematic ordering • Landau-de Gennes surface induced ordering J. Stefan Institute

Nematic ordering • comparison between MS and Ld. G models – MS expansion – Ld. G analogy J. Stefan Institute

Cis-ordering • coupling through mixed term • small impact on trans ordering – no cis-contribution to nematic free energy: J. Stefan Institute

Surface-induced ordering • concentration dependence • minimization by Euler-Lagrange S(r) J. Stefan Institute

Phase separation • • • Fmix + Fnem Fs ignored because of low S determination of coexistence region – bitangent J. Stefan Institute

2 H • NMR experimental results determination of concentration – relaxation according to Arrhenius Bulk trans OP Surface OP J. Stefan Institute

Phase diagram • nematic (N), isotropic (I) and coexistence region (N+I) • Fitting parameters J. Stefan Institute

Isonematic lines • relationship between T and φ effective temperature J. Stefan Institute

Conclusions • • • 2 H J. Stefan Institute NMR determination of T- phase diagram of 7 AB confined system provides for measurements of order parameters in both N and I phases in coexistence region, phase separation was observed, with constant bulk and surface nematic OP