Spectroscopic Characterization of Polymer Surfaces
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photoelectron spectroscopy (XPS)—also called electron spectroscopy for chemical analysis (ESC A). ESC A is now routinely used to obtain surface composition of polymers, and to follow processing steps and degradation chemistry. Advances in instrumentation 4 have driven many of these more sophisticated applications.12 Nevertheless, to improve the understanding of polymer-surface chemistry, more information is needed about surface structure with further sophistication, at a higher level of precision. For example, the knowledge of orientation and subsequent reactive availability of functional groups and of monomer ar-
rangement along a chain in copolymers and intrachain interactions is important. Macromolecular chain arrangement, termination, branching, and micromorphological information (i.e., domain size and distribution), molecular-weight distributions at or near the surface (in comparison with the average, bulk distribution), and higher degrees of spatial resolution in all three dimensions are all important in determining sophisticated surface structure-property relationships. To approach this level of structural and reactivity information at surfaces and interfaces, evolution of established methods and development of new methods must both be accomplished. Table I shows a comparison of the analytical characteristics of some surfacesensitive spectroscopic measurements for polymer analysis. For comparison, two methods not described further in this article, low energy ion scattering spectroscopy (ISS) and high resolution electron energy loss spectroscopy (HREELS) for vibrational spectroscopy at surfaces are also included. This article provides an overview of four methods of polymer-surface analysis: ESCA and secondary ion mass spectrometry (SIMS), both accomplished in UHV, and optical vibrational spectroscopic methods (infrared and Raman). Specific examples of how these have been developed and applied is presented, with a final section highlighting recent advances in polymer synthesis, design, and characterization using these methods.
Table 1. Characteristics of Some Spectroscopic Techniques Suitable for Studying Polymeric Materials. XPS (ESCA)
SIMS
IR and Raman
ISS
HREELS
Analysis Environment
High vacuum
Ambient
UHV
UHV
UHV
Resolution
0.6 eV
=1 cm" 1
0.1-1 amu
Variable
25-200 cm '
Elemental/Molecular Information
No H detection/ chemical shifts
???/ Functional group ID
All plus isotopes/ MW molecules, fragmentation
No H or He/ resolution limited
???/ Functional group ID
Detection Limit
% of monolayer
% in volume, 0.01 monolayer
ppm/ppb elemental, 0.01 monolayer molecular
% of monolayer
% of monolayer
Lateral Resolution
5 urn
=10 fim
10 nm (atomic), ??? molecular (ions)
None
None
Depth Sensitivity
>50 A
jam range
10 A?
3-5 A
3-60 A?
Sample Damage
Small, sometimes with unmonochromatized x-rays
None
High (needs study), lower with static conditions
High (needs study)
None
Major Outcomes
Elemental and chemical analysis, electronic structure
Molecular vibrations, functional
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