Rubbing-induced anisotropy of long alkyl side chains at polyimide surfaces Himali Jayathilake Department of chemistry Wayne State University Detroit, MI

Polymer surfaces used for LC alignment LCD Liquid Crystal Polymer coating Polymer surface alignment Better LC alignment

Rubbing induced anisotropy of polymer LC  Backbone alignment  Side alkyl chain alignment Mechanical rubbing cause: Rubbing

Experiment CaF2 Substrate Polymer is spin coated and rubbed  Unrubbed Polymer  Strongly Rubbed Polymer  Weakly Rubbed Polymer Samples n ® Nissan Chemicals

 Surface selectivity (no bulk- phase contribution)  Sub-monolayer sensitivity  Accessibility to buried interfaces  Sensitivity to molecular structure ( Vibrational Spectroscopy)  Ultra fast dynamics fs time-resolved experiments  Orientation of molecules at interfaces E(  1 +  2 )  s (2) E(  2 ) E(  1 ) Sum Frequency Generation (SFG)

Sum Frequency Generation 2 nd order nonlinear susceptibility Broad-Band Vibrational SFG Broad-band IR pulse + Spectrally narrow vis pulse   SFG =  IR +  vis S()S() IR  IR vis  vis + vis IR SFG |v=0  |v=1   IR  vis  SFG

Molecular Orientation Orientation angle (θ) Angle between Z axis and C 3 axis Azimuthal angle (  ) Angle between X axis and rubbing direction  Rubbing Direction vis IR P S y x z ψ SFG Plane of Incidence=XZ C 3 axis α

Unrubbed polymer  - Azimuthal angle Rubbing Direction vis IR SFG  Plane of Incidence=XZ H.D. Jayathilake, M.H. Zhu, C. Rosenblatt, A.N. Bordenyuk, C. Weeraman, and A.V. Benderskii, J. Chem. Phys. 125, (2006) ® Nissan Chemicals =0o=0o  =90 o  =180 o r+r+ r + FR

Strongly Rubbed Polymer H.D. Jayathilake, M.H. Zhu, C. Rosenblatt, A.N. Bordenyuk, C. Weeraman, and A.V. Benderskii, J. Chem. Phys. 125, (2006) Intensity of CH3 symmetric stretch (r + ) =0o=0o  =90 o  =180 o Unrubbed Polymer  =360 o  =225 o  =180 o  =90 o  =0o =0o r+r+

Weakly Rubbed Polymer Intensity of CH3 symmetric stretch (r + ) z Rubbing Direction vis IR SFG  Plane of Incidence=XZ H.D. Jayathilake, M.H. Zhu, C. Rosenblatt, A.N. Bordenyuk, C. Weeraman, and A.V. Benderskii, J. Chem. Phys. 125, (2006)  =0 o  =240 o  =180 o  =90 o  =360 o r+r+

Spectral Fitting Second order non linear susceptibility: B j = Amplitude Γ j = Lorentzian line width ω j = Transition Frequency A NR =Non resonant contribution Intensity of SFG signal:

Molecular Origin Surface coverage Euler Matrices f ( θ, φ,  ) Molecular Hyperpolarizability x  Rubbing Direction vis IR P S y x z ψ SFG Plane of Incidence=XZ C 3v axis φ

Molecular Orientation For CH 3 (C 3v ), terminal group For PPP (SFG, vis, IR) Polarization; Gaussian Distribution Functions

Weakly Rubbed Polymer Strongly Rubbed Polymer (a) θ 0 =15 o σ ψ =80 o (b) θ 0 =50 o σ ψ = 80 o (c) θ 0 =30 o σ ψ =80 o (a) θ 0 =45 o σ ψ =30 o (b) θ 0 =30 o σ ψ =30 o (c) θ 0 =15 o σ ψ =30 o CH3 Symmetric stretch H.D. Jayathilake, M.H. Zhu, C. Rosenblatt, A.N. Bordenyuk, C. Weeraman, and A.V. Benderskii, J. Chem. Phys. 125, (2006)

Molecular Orientation Rubbing direction σψ σψ  Sampleθ0θ0 σθσθ σψσψ Weakly rubbed 30 o ±10 o <15 o 80 o ±10 o Strongly rubbed 30 o ±10 o <10 o 30 o ±10 o

Summary  Rubbing induces considerable anisotropy to the side alkyl chains in polyimide  Long alkyl chains tend to induce azimuthal anisotropy  Orientation analysis shows a chain tilt of 30 o ±10 o for both strongly rubbed and weakly rubbed polymers  Weaker rubbing cause broader azimuthal distribution

Acknowledgements The Group Advisor: Prof. Alex Benderskii Dr. Andrey Bordenyuk Dr. Igor V. Stiopkin Champika Weeraman Achani Yatawara Fadel Y. Shalhout Adib J. Samin Collaborators Prof. Charles Rosenblatt Minhua Zhu Dr. Ichiro Kobayashi (Nissan Chemical Industries Ltd) WSU start-up grant WSU research grant ACS-PRF NSF CAREER Funding

Experimental setup IR output: 3-8  m fs 350 cm -1 bandwidth 4-5  J/pulse Narrowed vis pulse: Bandwidth 2.4 cm -1 Chirp controlled Fs oscillator Regenerative amplifier 2-pass amplifier OPA with DFM 800 nm 40 nm bandwidth 803 nm 26 nm bandwidth 40 fs 2 mJ/pulse, 1 kHz Optical delay stages Sample Monochromator LN 2 -cooled CCD Reflection geometry 0.1  m precision Vis pulse shaping

Fitting Results #Sample  (deg) Vibr. mode ω j (cm -1 ) B j (a.u.)Γ j (cm -1 ) 1 strongly rubbed 0 d+d r+r d+d r+r d+d r+r d+d r+r d+d r+r weakly rubbed 0 r+r r+r r+r r+r r+r

SFG Tensor and Molecular Orientation Intensity of P polarized SFG signal Visible and IR pulses SFG P S P S X Y Z X Y Z α α α α α For PPP (SFG-Vis-IR) Polarization

Gauche Defects Weakly rubbed polymerStrongly rubbed polymer  More pronounced d + vibrational modes in strongly rubbed polymer d+d+  Stronger rubbing cause more Gauche Defects d + FR d-d-

all-trans gauche defect µ µ Strongly rubbed Polymer Gauche defects (a) θ 0 =15 o σ ψ =30 o (b) θ 0 =65 o σ ψ = 30 o (c) θ 0 =50 o σ ψ =30 o

2 nd order Nonlinear Optical Spectroscopy Polarization  Surface selectivity:  (2) = 0 in isotropic media 2 nd order term: Apply inversion operation î: in isotropic media Polarization

Four layer model (thin film) 2 (n 2 ) 1 (n 1 ) Interfacial layer (n / ) 3 (n 3 ) Fresnel Factors M. B. Feller, W. Chen, and Y. R. Shen, Physical Review A 43 (12), 6778 (1991). h

Refractive index for IR  Refractive index become complex when a material absorbs radiation

Refractive index Absorption and Emission Refraction

Kramers-Kronig Relations (1) (2) P = Principal value of the integral Peiponen, K.E., et al., Kramers-Kronig relations and sum rules of negative refractive index media. Europian Physical Journal B, (1): p