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though these results suggest that a compatible mixture exists at that con centration, this system is considered to be incompatible. At higher concentra tions (25% DMS), visible phase separa tion occurred and the surface was 100% DMS. We are currently studying a vari ety of incompatible and compatible blends, attempting to use ISS to com plement angle-dependent ESC A. Another application is in the area of surface-modified polymers (35-37). In a series of studies of different treat ments of PMMA, we used the higher surface sensitivity of ISS to develop specific models of orientation of modi fied functional groups. ISS results showed a great increase in surface oxy gen, yet ESCA results showed a de crease in oxidized carbon concentra tions (35, 36). These data were com pletely consistent with the adsorption of water vapor rather than permanent oxidation. Different treatments that promoted specific oxidation were then developed (37). Contact angle methods were also used to differentiate func tionality in the latter study. One out come of that work has been attempts to determine the relationships between ISS conformational sensitivity and contact angle techniques. Static SIMS
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Homopolymer s t r u c t u r e sensitivity. We will consider three different types of ions (Figure 2) generated in the stat ic SIMS experiment from polymers. They can be considered to be roughly divided by mass range (i.e., low, high, and in between), but more generally are divided by the mechanism of for mation (fragmentation and rearrange ment, desorption and cationization of oligomers, and simple bond breaking/ charge stabilization). The most complex of the three are low mass (i.e., < 300 amu) fragment ions, the original focus of much static SIMS polymer characterization. In the first applications of static SIMS to the analysis of polymer surfaces, low mass (0-150 amu) fragment ions (14, 15) were used to distinguish structure and isomerism in a series of methacrylate polymers with different side chain structures. The surfaces were distin guished primarily from positive ion spectra via their fragmentation pattern from the side chain, within a common pattern characteristic of the backbone. A significant result from these studies was the differentiation of isomeric structures of butyl methacrylate, which could not be distinguished by ESCA core-level analysis and by the analysis of slight reactive surface degradations of the tert-butyl methacrylate (38). Subsequent improvement in SIMS
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experimental conditions to lower sur face damage induced by the primary ion bombardment has allowed for the analysis of higher mass fragments that are related to the original structure of the polymer (15). In the course of these studies, the commonly accepted de scription of static or low-damage con ditions evolved from the original cur rent density (1 η A/cm 2 ) criterion based on sputter yields to preserve a surface for 1 h (10) to minimum ion dosages (e.g., 10~13 ions) (15, 20). In addition, other criteria for mini mal damage have been proposed, some of which are based on the theory that chemical and structural disruption re sult from inefficient dissipation of the primary charge input attributable to the inherent low conductivity of the polymer sample (16). The use of a pri mary atomic (neutral) beam was sug gested in this case to remove the input of charge to the sample, with similar spectral results as derived from prima ry ion beam analysis and better nega tive ion spectra (16). However, precise spatial and low current control of the atomic beam is much more difficult compared with an ion beam (16). It is instructive to compare results from the ISS work on nitrogen-con taining polymers discussed earlier with the typical conditions for static SIMS. In the ISS work, damage to the native polymer structure as assessed by changes in N/C signal intensities (Fig ure 8) was apparent after 2 min of irra diation at 2 nA/cm 2 (i.e., 1.5 X 10" 12 ions) with 2 kV He. This dosage is at or below static conditions, in part because the sputter yield of He is lower than that of the Ar or Xe primary ions used in a typical static SIMS experiment. The ion beam induced changes were not detectable in comparisons of ESCA spectra taken before and after analysis. Furthermore, simple sputtering theory at the beam currents used would not allow for significant ion beam depth profiling (10). Clearly in this case chemical modifi cation and damage of the surface of the polymer is occurring, and sputter-in duced consumption of the material is not necessarily involved. The problem of considering the higher damage cross sections in the determination of prima ry ion beam conditions for organic spe cies, as opposed to sputter yields, has been recognized by Benninghoven's group (17, 39). They have designed a TOF secondary ion mass spectrometer that incorporates a pulsed ion source of very low beam density (< 1 ρ A/cm 2 ). Lower dosages are possible because of the simultaneous detection of all ions from a pulse of primary ions in the TOF experiment. As conditions and instru-
