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Application of Ion Scattering Techniques to Characterize Polymer Surfaces and Interfaces

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Application of Ion Scattering Techniques to Characterize Polymer Surfaces and Interfaces Materials Science and Engineering R 38 (2002) 107±180 Application of ion scattering techniques to characterize polymer surfaces and interfaces Russell J. Compostoa,*, Russel M. Waltersb, Jan Genzerc aLaboratory for Research on the Structure of Matter, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104-6272, USA bDepartment of Chemical Engineering, University of Pennsylvania, Philadelphia, PA 19104-6272, USA cDepartment of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA Abstract Ion beam analysis techniques, particularly elastic recoil detection (ERD) also known as forward recoil spectrometry (Frcs) has proven to be a value tool to investigate polymer surfaces and interfaces. A review of ERD, related techniques and their impact on the field of polymer science is presented. The physics of the technique is described as well as the underlying principles of the interaction of ions with matter. Methods for optimization of ERD for polymer systems are also introduced, specifically techniques to improve the depth resolution and sensitivity. Details of the experimental setup and requirements are also laid out. After a discussion of ERD, strategies for the subsequent data analysis are described. The review ends with the breakthroughs in polymer science that ERD enabled in polymer diffusion, surfaces, interfaces, critical phenomena, and polymer modification. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion scattering; Polymer surface; Polymer interface; Elastic recoil detection; Forward recoil spectrometry; Depth profiling 1. Introduction Breakthroughs in polymer science typically correlate with the discovery and application of new experimental tools. One of the first examples was the prediction by P.J. Flory that the single chain conformation in a dense system (i.e. a polymer melt) is ideal and follows (nearly) Gaussian statistics [1]. In the Flory model, chains interpenetrate and have a radius of gyration that varies as a(N/6)1/2 where N and a are the segment number and size, respectively. Although polymer scientists now take this fundamental law for granted, over 20 years passed before this model was proven. By blending a dilute concentration of deuterated molecules with identical chains (natural abundance of hydrogen), small-angle neutron scattering (SANS) experiments were able to directly determine chain conformation [2±4]. SANS is now a standard technique in the polymer scientist's toolbox for studying the bulk thermodynamic properties of polymer mixtures and solutions. Today new experimental techniques continue to push the frontiers as demonstrated by the recent imaging of individual molecules using the scanning force microscope [5]. Relevant to this review, our understanding of polymer surfaces and interface problems of fundamental and technological importance has been greatly advanced by ion beam analysis IBA, techniques. When light ions at MeV energies are incident on a target, some ions transfer energy to lighter target nuclei in an elastic collision such that the target nucleus recoils and ejects from the * Corresponding author. Tel.: 1-215-985-1386; fax: 1-215-573-2128. E-mail address: [email protected] (R.J. Composto). 0927-796X/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S0927-796X(02)00009-8 108 R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180 target. By detecting the energy of the recoiling nuclei the depth-profile perpendicular to the surface of the target is measured. In ``cleaning up the tower of babel of acronyms (CUTBA) in IBA'', the consensus of the ion beam community was to call this technique elastic recoil detection (ERD) analysis [6]. Although forward recoil spectrometry (FRES) is the acronym most used by polymer scientists, it is time for the polymer community to adopt the ERD convention to avoid further confusion. For several reasons ERD is a natural technique to study polymer systems. Whereas polymers are predominantly comprised of carbon and hydrogen, many polymers are available in their deuterated analogs. For example, the same polybutadiene can be deuterated or hydrogenated to produce isotopic blends having identical values of N. Because N and a are typically 1000 and 5 AÊ , respectively, the natural length scale for many surface and interface phenomena is 100 AÊ , comparable to the depth resolution of ERD. Since no one technique can provide the necessary depth resolution, lateral resolution, sensitivity and quantification, physical scientist are increasingly using a combination of complementary depth profiling techniques to better understand surface and interfacial issues. For example, consider the enrichment of one component at the surface of a two component blend. ERD, a direct profiling technique, can be used to determine the surface excess independent of any models, whereas neutron reflectivity, a model dependent technique provides details about the shape of the profile. Because patterning is an area of increasing interest, techniques with good depth and lateral resolution will be needed in the future. Ideally, such techniques will allow scientists to address interface issues regardless of geometry (e.g. fibers). The most comprehensive guide to ERD is ``FRES'' by Tirira et al. [7]. This text includes a detailed analysis of ion interactions in solids, provides a review of cross-sections important in recoil analysis, and describes variations of ERD including conventional, time-of-flight (TOF) and coincidence techniques. A general background for ERD is provided in [8,9]. Several brief review articles covering ion beam analysis of polymers are also available [10±13]. This review is intended to educate polymer scientists about ion beam techniques, particularly ERD, and make ion beam users aware of breakthroughs in polymer science brought about by ion beam analysis. Thus, Sections 2 and 3 are dedicated to reviewing the fundamental interaction between ions and solids, and the basic principles of ion beam techniques with an emphasis on ERD. To help polymer scientist optimize ERD experiments, Section 4 describes depth resolution, sensitivity and beam damage. In Section 5, strategies for analyzing and simulating data are presented. Section 6 reviews related IBA techniques as well as complementary ones. To demonstrate the impact of ion scattering on polymer science, Section 7 presents selected case studies in diffusion, surfaces, interfaces, critical phenomena and polymer modification. Although these studies mainly focus on ERD, the utilization of other ion beam techniques such as nuclear reaction analysis (NRA) are also included. 2. Overview of ion beam analysis techniques 2.1. Introduction When a beam of high-energy, MeV, incident atoms strikes a surface, three main interactions are possible. An incident atom could strike a target atom on the surface of greater atomic number, say an incident 4He striking a target 12C. In this case, the incident atom undergoes an elastic collision and the 4He will be repelled back toward the source of the beam, or backscattered. This interaction is the basis of Rutherford backscattering spectrometry (RBS). In RBS, the energy of the backscattered R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180 109 incident ions are detected and the elemental composition of the sample can be determined. Another possible outcome occurs when the incident atom posses greater mass than the target atom, 4He striking 1H. In this case, after the collision event due to the physics of an elastic collision, the incident 4He atom cannot be backscattered, but will continue in the direction forward and the 1H will be recoiled and expelled from the target sample. This interaction is the basis of ERD. The energy of the expelled recoiled 1H atom is detected in the forward direction to determine the depth-profile of 1H. The third possibility is that the incident atoms penetrate the surface and simply lose energy via low impact collisions with electrons. Eventually, at some depth below the surface, the incident ion undergoes an elastic collision and then travels back out of the sample again losing some energy. This well-defined loss of energy as the incident atom travels through the sample provides the unique depth profiling capability of RBS. 2.2. Rutherford backscattering spectrometry (RBS) The basic principles of ion beam analysis of materials were discovered more than 90 years ago by Rutherford when he bombarded solid targets with alpha particles [14]. His experiments provided the foundation for RBS. A detailed description of RBS can be found in the literature [11,15±17]. Here, we restrict ourselves to a brief description. RBS can be used for elemental determination and to probe the depth-profile of heavier elements. In order for backscattering to occur, the target elements must posses a larger atomic mass than the incident ion. For polymer investigations, the usual incident ion is 4He, so any element larger than helium can be identified, although it does become increasingly more difficult to distinguish elements with very large atomic masses. Most elements of interest in polymer studies 12C, 14N, 16O, 19F, 31P, 32S and 35Cl, can be resolved using RBS or some variation of RBS such as glancing angle RBS. A schematic of a typical set up is shown in Fig. 1. The RBS experiment consists of a monoenergetic beam of ions typically produced by a tandem accelerator (cf. Fig. 17), accelerated to an energy, Ein,0, of a few MeV that are then focused on a sample. After colliding with a heavier atom in the sample, the projectile is backscattered with an energy Eout,0 and travels outward to a solid state Fig. 1. Experimental configuration of RBS. An accelerator provides high-energy light ions that strike a planar surface and are backscattered to a detector. The detector registers the energy of each backscattered ion. The entire beam line and sample chamber is under vacuum. 110 R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180 Fig. 2. Schematic representation of an elastic collision between a projectile of mass Mp and velocity vp, and energy, Ein and 0 0 a target mass, Mt, which is initially at rest.
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