Chapter 9
Combined Atomic Force Microscopy and Sum Frequency Generation Vibrational Spectroscopy Studies of Polyolefins and Hydrogels at Interfaces AricOpdahland Gabor A. Somorjai Department of Chemistry, University of California, and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
AFM and sum frequency generation (SFG) vibrational spectroscopy experiments provide complementary information that can be used to relate the morphology and mechanical properties of a polymer interface to the molecular structure of the interface. The application of the two techniques to the study of polymer interface structure is presented, focusing on surface segregation and wetting behavior of polyolefin blends and on the interface structure and mechanical behavior of hydrogels exposed to various hydration conditions.
Introduction This chapter emphasizes the incorporation of sum frequency generation (SFG) vibrational spectroscopy to A F M studies of polymer behavior at interfaces. In recent years, AFM has been proven as a versatile technique that can be used to map the surface morphology/topography and to probe the
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113 mechanical properties of polymer interfaces in a variety of environments. In spite of this versatility, one of the major limitations of AFM is the lack of direct measurement of the molecular structure of the interface. Although chemical information can sometimes be inferred from the interface mechanical properties (elasticity, adhesion), or by experiments where the AFM tip is coated with a specific chemical functionality, AFM experiments in general lack chemical specificity. SFG spectroscopy experiments fill this void by providing information regarding the molecular structure of the interface. SFG surface vibrational spectroscopy is an optical technique that is highly sensitive to the chemical composition, orientation, and ordering of molecular groups at an interface. Like AFM., SFG spectroscopy experiments can be designed to probe polymer/air and polymer/liquid interfaces, making this a powerful combination of experimental techniques for in situ studies of interface phenomenon. Two sets of experiments that highlight the application of both AFM and SFG spectroscopy to the study of polymer interface behavior are presented in this chapter. The first focuses on surface segregation and wetting behavior of polyolefin copolymers and blends. In that example, chemical information obtained by SFG spectroscopy is connected to lateral morphology measurements made of the interface by AFM. The second example focuses on the interface behavior of hydrogels. That example highlights the versatility of both techniques for studying polymer/liquid interfaces and in connecting mechanical measurements of the interface made by AFM to chemical measurements made by SFG spectroscopy.
Experimental
SFG surface vibrational spectroscopy Excellent descriptions of SFG surface vibrational spectroscopy have been published by Shen ' and by Hirose . The application of SFG surface vibrational spectroscopy to the study of polymer interfaces has recently been reviewed. In our experiments, SFG vibrational spectra of polymer/air and polymer/liquid interfaces were obtained by overlapping two laser beams at the interface and measuring the light generated from the interface at the sum frequency in the reflected direction. One of the input fields is a tunable infrared beam that is resonant with one or more vibrational modes of the species at an interface. The picosecond laser and OPG/OPA system we have used to generate 1
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2,3,4
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the visible beam (dty, 532 nm) and the tunable infrared beam (flfc, 2000 cm to 4000 cm") has been described elsewhere. The interface specificity of SFG spectroscopy is a result of selection rules obtained under the electric-dipole approximation. The sum frequency signal, I(ω ), is proportional to the square of the nonlinear susceptibility of the material being measured, %b\ a 27 component tensor (Eq. 1). Under the electric-dipole approximation, the 27 components of jj/^are equal to zero for centrosymmetric materials. Materials that are centrosymmetric, or that are randomly oriented in the bulk, are not expected to generate large sum frequency signals. However, if a material assumes a preferred orientation at an interface - then symmetry is broken in the interface plane, and some of the components of jj^may be non zero. Measurement of is specifically sensitive to this type of ordering at an interface. 1
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The vibrationally resonant contribution to the nonlinear susceptibility, s{^R))
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mode belonging to one of the molecular groups at the interface (û) ). The molecular hyperpolarizability, ci , can be related to the product of the dynamic dipole and polarizabilities of a vibrational mode. Thus, the mode must be both IR and Raman active in order to be measured. The measured strength, Â »fora q
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