Studying Polymer Surfaces and Interfaces with Sum Frequency

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Studying Polymer Surfaces and Interfaces with Sum Frequency Generation Vibrational Spectroscopy Xiaolin Lu, Chi Zhang, Nathan Ulrich, Minyu Xiao, Yong-Hao Ma, and Zhan Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04320 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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Analytical Chemistry

Studying Polymer Surfaces and Interfaces with Sum Frequency Generation Vibrational Spectroscopy

Xiaolin Lu*,1, Chi Zhang2, Nathan Ulrich,2 Minyu Xiao,2 Yong-Hao Ma,1 Zhan Chen*,2

1

State Key Laboratory of Bioelectronics, School of Biological Science & Medical

Engineering, Southeast University, Nanjing 210096, Jiangsu Province, P. R. China 2

Department of Chemistry, University of Michigan, 930 North University Avenue, Ann

Arbor, Michigan 48109, United States

Email Address: [email protected] and [email protected]

1

ACS Paragon Plus Environment


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1. Introduction Usually a surface or an interface is well defined with little diffusion from one surface or interface to the other, with the length scale being on the order of one to several molecules. The molecular interactions on a surface or at an interface are thus dominated by short-range interactions. Various properties, such as wettability, reactivity, biocompatibility, adhesion, and friction, etc., of a surface or interface are determined by the surface/interfacial structures at the molecular level (or the arrangement of the molecules). To improve the properties of a surface/interface, it is first necessary to characterize the molecular structures on the surface/interface

and

then

determine

the

structure–function

relationships

of

the

surface/interface. This review will summarize the recent progress in sum frequency generation (SFG) vibrational spectroscopic studies on molecular structures of polymer surfaces and interfaces. Many excellent results have been reported in a large amount of articles on such topics; it is impossible to cover all these studies in this review. Here we will first introduce the theoretical background of the SFG vibrational spectroscopy. We will then give a selective and systematic summary on the recent SFG studies on polymer surfaces in air, surface restructuring in water, and the buried polymer/solid interfaces such as polymer/metal, polymer/oxide, and polymer/polymer interfaces. SFG can provide unique results on polymer surfaces/interfaces such as molecular level structures, in situ capability of probing buried polymer/liquid and polymer/solid interfaces, as well as intrinsic sub-monolayer surface specificity. We will focus on such unique aspects of the SFG studies especially on buried polymer interfaces reported in the last three years in this review. Such studies developed SFG into a powerful spectroscopic tool for analytical chemistry.

With the development of modern surface science and technology in the past century, we are now able to study many aspects of a surface or interface using advanced 2


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surface/interfacial sensitive instrumentation.1 Among these techniques, spectroscopic methods have been shown to be powerful tools to elucidate molecular structures on surfaces or interfaces. Examples of such spectroscopic techniques include X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS), surface enhanced Raman scattering (SERS), and attenuated total reflectance (ATR)-FTIR spectroscopy, etc. These techniques have been employed to investigate surfaces and interfaces of a large number of materials, producing many important and valuable results. However, they do have some disadvantages, which limit their applications in the study of surfaces and interfaces. For example, some require high vacuum to operate (thus lack of the in situ analysis capability for many cases), need special substrates, have bulk signal confusion, or use large equipment (e.g., synchrotron) to perform the experiments. In the last thirty years, SFG has been developed into a powerful analytical tool to probe surface and interfacial molecular structures in situ. As a second order nonlinear optical process, the SFG process is forbidden for materials with inversion symmetry (under the electric-dipole approximation) but allowed at surfaces or interfaces where inversion symmetry is broken. Most bulk materials possess inversion symmetry, therefore under such circumstances SFG is an intrinsic surface/interfacial sensitive characterization technique with sub-monolayer sensitivity. It provides vibrational spectra for surfaces/interfaces from which the molecular structures can be elucidated. Currently, SFG has been widely utilized to study surfaces/interfaces involving a broad range of materials, including water, surfactants, selfassembled monolayers, ionic liquids, polymers, and biological materials, etc.2-22 A polymer molecule can have a large number of monomer units connected by chemical bonds.23 Polymers are widely used in many important applications/areas including structural materials with three dimensional architectures, paints and coatings for surface protection, electronic packaging materials to encapsulate chips, and skin-care creams for anti3



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ultraviolet radiation, etc. New polymer development and novel characterization of polymers are two important fields within the polymer science research. Especially it is crucial to characterize and understand the structure–property relationships of polymer materials, and extensive research has been performed to obtain such relationships. Polymer surface/interfacial properties play dominating roles in many polymer applications, such as biomedical implants, coatings, packaging materials, adhesives, lubricants, etc. Therefore it is crucial to understand the polymer surface/interfacial structure– property relationships in order to improve the polymer surface/interfacial properties. Furthermore, a trend in science and technology is the miniaturization of devices, which need the development of micro- or nano-materials to achieve more complex functions. For materials with dimensions on the order of microns or smaller, the ratio of the surface area to volume becomes very large, thus the surface greatly influences the material’s overall property. For example, the surface effect can substantially alter a polymer thin film’s properties, such as glass transition temperature (Tg),24-32 viscoelastic behavior,33,34 or chain conformation.35-37 Therefore it is important to study molecular structures of polymer surfaces and interfaces, which will be discussed in detail below.

2. SFG Background 2.1. SFG Process SFG vibrational spectroscopy has been developed based on the advancement of nonlinear optics38,39 and vibrational spectroscopy. The early research done by Shen et al. established the theoretical and experimental foundation for the development of the SFG research field.40-42 Extensive research has been performed by many research groups since then and detailed introduction of the SFG theory has been published in a large number of articles.2-20 4


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A schematic of the SFG experimental geometry (in the co-propagation mode) is shown in Figure 1 (left). A frequency-fixed visible (ω1) beam and a frequency-tunable infrared (ω2) beam temporally and spatially overlap at a surface or interface to generate a sum frequency (ω = ω1 + ω2) signal which can be measured by a detection system (e.g., monochromator/photo-multiplier tube (PMT)). When the IR frequency is tuned across a vibrational transition of the surface/interfacial molecules, the sum frequency signal intensity is resonantly enhanced. An SFG spectrum can be acquired by plotting the SFG signal intensity as a function of the IR frequency. A molecular dipole moment induced by the incoming photon(s) can be expressed as:21 ∙ : ⋮ ⋯

(1)

SFG corresponds to the second item in Eq. 1 as a second-order nonlinear optical process.

Figure 1 Schematic of SFG sample geometry in the reflection mode (left, β, β1, and β2 are the output angle of the sum frequency beam and the incident angles of the visible and infrared beams, respectively) and the energy level diagram for an SFG process (right).

The energy level diagram of an SFG process is shown in Figure 1 (right). SFG resonance is a combination of an IR absorption and an anti-Stokes Raman scattering process. Therefore,

the

transition

polarizability

describing

an

SFG

process,

termed

as

hyperpolarizability or 2nd-order polarizability, can mathematically be expressed in terms of the infrared transition dipole moment and Raman polarizability tensors,42,43 as shown in Eq. 2: 5



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−

∗ "#

! !

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(2)

where ε0 is the dielectric constant of the surrounding medium, ωq is the angular frequency of the qth vibrational mode, and Qq is the normal mode coordinate of the molecular vibration. The last two terms are the complex conjugate of Raman polarizability and infrared transition dipole moment components with respect to the normal coordinate of the qth vibration mode, respectively. The “abc” in the equation indicates a molecule-fixed coordinate system. Experimentally, all of the surface/interfacial molecules probed by SFG can participate in this optical process. Their overall molecular hyperpolarizability is the second-order nonlinear optical susceptibility (χ(2)) (shown in Eq. 3):2,42,43

χ$%& )* 〈$%& 〉 )* ∑,,〈(.̂ ∙ 0)12̂ ∙ 34153 ∙ ̂ 4〉 ()

(3)

Here Ns is the molecular number density (or surface coverage) of the molecules probed by SFG. The “ijk” stands for a lab or interface-fixed coordinate system. The correlation between the molecule-fixed system and the surface/interface-fixed system will be presented below. The brackets indicate the ensemble average for all the molecules. Then the overall induced dipole moment for an SFG process can be expressed in Eq. 4.2,40,43

7$8 (9) χ$%& : % (9 ) & (9 ) ()

(4)

(2 ) where χ ijk is a polar tensor, and an inversion operation will reverse its sign. For a

centrosymmetric material, an inversion operation does not change anything including the sign (2 ) of χ ijk . Therefore for centrosymmetric materials (or materials with inversion symmetry),

χ ijk(2 ) must be zero (Eq. 5). Most bulk materials possess inversion symmetry, SFG is therefore (2 ) forbidden as a result of χ ijk equaling zero. On a surface or at an interface, where inversion (2 ) symmetry is broken, SFG is allowed as a result of the nonzero χ ijk (Eq. 5). Therefore, SFG

6


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vibrational spectroscopy is intrinsically surface/interface sensitive due to the selection rule, not the limitation of the input or output light penetration depth.

0 ?@ABB C 0 ? ?@ABB

χ$%& : ()

(5)

SFG spectra are collected in a lab-fixed or interface-fixed coordinate system (“ijk” or “xyz” system). Vibrational modes are defined in a molecular coordinate system (“abc” system) based on the molecular symmetry.44 To deduce interfacial structure from SFG vibrational spectra, it is necessary to understand the transformation between the two systems.19,45

Figure 2 Schematic showing the transformation from the molecule-fixed coordinate (abc) system to the surface/interface-fixed coordinate (xyz) system using a methylene group with a C2v symmetry as an example. Euler angles (θ, ψ, ϕ) are defined for the coordinate transformation between the two systems, which facilitates the description of the orientation of the functional groups on a surface or at an interface.

Such transformations can be realized by rotations via three angles (Figure 2): the tilt angle (θ), twist angle (ψ) and azimuthal angle (ϕ).19,45 Each rotation here results in a

7



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transformation matrix so that there are three transformation matrices in total. Thus the mathematical expression for the three rotations is (z-y-z convention):19

D (E, F, ψ)

?@ E ?@ F ?@ ψ − @ E @ψ ?@ E ?@ F @ψ @ E ?@ψ G −@ E ?@F ?@ψ − ?@E@ψ −@E ?@F@ ψ cosE ?@ψ @F ?@ψ @θ@ψ

− ?@E ?@F @E@F K (6a) ?@F

The transformation for a transition dipole can then be expressed as

M ′ ′ LN ′P D(E, F, ψ) G ′K ′ O ′

(6b)

The transformation for a Raman polarizability can thus be express as

MM ′ MN ′ MO ′ ′ ′ ′ Q ′ ′ ′ L NM NN NO P D G ′ ′ ′K D (6c) ′ ′ ′ OM ′ ON ′ OO ′

According to the transformations of the transition dipole and Raman polarizability, it is feasible to deduce the transformation for the SFG hyperpolarizability, which will be discussed in detail later.

2.2 SFG Quantitative Analysis We can quantitatively analyze the molecular structure of a surface/interface from an SFG spectrum. Formula 7 shows the SFG output intensity in the reflection mode.21,46 Here ni(ωi) represents the refractive index of the incident medium at frequency ωi; while ω and β are the frequency and the reflected angle of the output beam, respectively. The angle β can be deduced from the momentum conservation of the input and output photons. I1(ω1) and I2(ω2) are the intensities of the two input beams with frequencies ω1 and ω2 (i.e. visible and infrared (2 ) beams). χ eff is the effective second-order nonlinear optical susceptibility measured using a

specific polarization combination for the input and output beams in an SFG experiment. 8


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R(9)∝

ST UV *W VX

U 8Y (Y )8Y (V )8Y ()

()

ZχW[[ Z R (9 )R (9 ) (7)

For example, we can experimentally measure the following four components of the ()

()

()

()

effective second-order nonlinear susceptibility, i.e. χW[[,**\ , χW[[,*\* , χW[[,\** , and χW[[,\\\ by collecting the ssp (s-polarized sum frequency signal, s-polarized visible input, and ppolarized IR input), sps, pss, and ppp spectra. However, such effective second-order nonlinear optical susceptibility components need to be converted to the second-order nonlinear optical susceptibility tensor components (χijk) defined in the lab-fixed (or surface/interface-fixed) coordinate (xyz) system. After that, the χijk tensor components can be correlated to the molecular hyperpolarizability tensor components; from such correlations molecular structures of a surface/interface can be analyzed. For an isotropic surface/interface, ()

χW[[ components are related to the lab-fixed χijk components in the following ways:46 χW[[,**\ ]NN (ω)]NN (9 )]OO (9 ) @ χNNO ()

(8a)

χW[[,*\^ ]NN (ω)]OO (9 )]NN (9 ) @ χONN ()

(8b)

χW[[,\*^ ]OO (ω)]NN (9 )]NN (9 ) @ χONN ()

()

χW[[,\\_

(8c)

−]MM (ω)]MM (9 )]OO (9 ) cos cos @ χMMO −]MM (ω)]OO (9 )]MM (9 ) cos @ ?@ χMOM

]OO (ω)]MM (9 )]MM (9 ) @ cos ?@ χOMM

]OO (ω)]OO (9 )]OO (9 ) sin sin @ χOOO ()

()

(8d)

()

()

Eqs. 8a to 8d link the four terms of χW[[,**\ , χW[[,*\* , χW[[,\** , and χW[[,\\\ to the lab-fixed susceptibility tensor components. The lab-fixed second-order nonlinear optical susceptibility is a polar tensor with 27 components. For an azimuthally isotropic surface/interface, there are only four independent non-zero second-order nonlinear susceptibility tensor components (χijk),42 as shown in Eq. 9:

χMMO χNNO , χMOM χNON , χOMM χONN , χOOO

(9)

9



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In Eqs. 8a-d, Lii’s (i = x, y, or z) are the local field correction terms known as “Fresnel factors”. β, β1 and β2 are the angles of the sum frequency signal beam, input visible and infrared beams, respectively, versus the surface normal. For a reflection-mode SFG process with respect to a single surface/interface sandwiched between two semi-infinite media, Lii’s can be expressed in Eqs. 10a-c.46 n′(ω) is the refractive index of the surface/interfacial layer. β and γ are the beam incidence angle and refracted angle, respectively. For more complex systems, for example, a thin film with a finite thickness sandwiched between two semi-infinite media, expression of the local field coefficients needs some modification, which have been discussed in detail in several articles47-53 and will not be presented here. Such analysis including the interference of the SFG signals contributed from two interfaces of a thin film will be discussed in later sections when presenting SFG studies on polymer/liquid and polymer/polymer interfaces.

]MM (9)

8Y ()b*c

8Y ()b*cd8V ()b*X

]NN (9) 8 ]OO (9) 8

8Y ()b*X ()b*Xd8 Y V ()b*c

8V ()b*X ()b*cd8 Y V ()b*X

(10a) (10b) 8 ()

∙ e 8Y′() f

(10c)

Therefore the non-zero second-order nonlinear susceptibility tensor components in the lab-fixed system can be deduced from the effective second-order nonlinear susceptibility tensor components measured using input/output beams with different polarization combinations. As we discussed above, the transformation between the lab-fixed coordinate system and the molecule-fixed coordinate system can be realized through a rotation operation. Eq. 6 shows the transformations for IR transition dipole and Raman polarizability. As we discussed above, SFG hyperpolarizability is a product of the IR dipole transition moment and Raman

polarizability,

we

therefore

can

deduce

the

transformations

from

the

hyperpolarizability components to the second-order nonlinear susceptibility tensor 10


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components (the measured χijk components) in the lab-fixed system through Euler angle transformations, which describe the orientation of molecules on surfaces or at interfaces. To quantify such orientation information, it is necessary to know the hyperpolarizability tensor components (βabc) of vibrational modes of the surface/interfacial molecule, which can be deduced via quantum chemical calculation. An alternative way to obtain βabc or relative ratios between different βabc components for a complex vibrational mode composed of multiple single bond vibrational modes is the bond additivity approach with the known parameters of these single bond vibrational modes.54-57

SFG has been extensively used to study orientations of various functional groups and molecules according to the transformations discussed above. According to the symmetry of the molecule or functional group, such transformations can be simplified. Taking the methyl functional group for example, here the relationships between the measured χijk components and hyperpolarizability tensor components (βabc) are presented below:46,58 For the symmetric C-H stretch: () () () gMMO,* gNNO,* ) h(1 >) < ?@F > −(1 − >) < ?@ l F >m

(11a)

gMOM,* gNON,* gOMM,* gONN,* ) (1 − >)h< ?@F > −< ?@ l F >m ()

()

()

()

()

gOOO,* ) h> < ?@F > (1 − >) < ?@ l F >m ()

()

(11c)

For the asymmetric C-H stretch:

gMMO,* gNNO,* − ) h< ?@F > −< ?@ l F >m ()

()

()

(11d)

gMOM,* gNON,* gOMM,* gONN,* ) < ?@ l F > ()

()

()

()

()

gOOO,* ) h< ?@F > −< ?@ l F >m ()

()

(11f)

11


(11e)

(11b)


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As presented above, “< >” means “ensemble average”. The Gaussian distribution has been used extensively to represent the orientation angle distribution.59,60 Using a Gaussian distribution, we have:

n (F ) o

Studying Polymer Surfaces and Interfaces with Sum Frequency

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