Neutron reflection polymers

Kanaya T, Miyazaki T, Watanabe H, Nishida K, Yamano H, Tasaki S, Bucknall D (2003) Annealing effects on thickness of polyst5uene thin films as studied by neutron reflectivity. Polymer 44(14) 3769-3773... 

The polymer concentration profile has been measured by small-angle neutron scattering from polymers adsorbed onto colloidal particles [70,71] or porous media [72] and from flat surfaces with neutron reflectivity [73] and optical reflectometry [74]. The fraction of segments bound to the solid surface is nicely revealed in NMR studies [75], infrared spectroscopy [76], and electron spin resonance [77]. An example of the concentration profile obtained by inverting neutron scattering measurements appears in Fig. XI-7, showing a typical surface volume fraction of 0.25 and layer thickness of 10-15 nm. The profile decays rapidly and monotonically but does not exhibit power-law scaling [70]. 

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

Other technique—for example, dynamic secondary ion mass spectrometry or forward recoil spectrometry—that rely on mass differences can use the same type of substitution to provide contrast. However, for hydrocarbon materials these methods attain a depth resolution of approximately 13 nm and 80 nm, respectively. For many problems in complex fluids and in polymers this resolution is too poor to extract critical information. Consequently, neutron reflectivity substantially extends the depth resolution capabilities of these methods and has led, in recent years, to key information not accessible by the other techniques. 

The high depth resolution, nondestructive nature of thermal neutrons, and availability of deuterium substituted materials has brought about a proliferation in the use of neutron reflectivity in material, polymer, and biological sciences. In response to this high demand, reflectivity equipment is now available at all major neutron facilities throughout the country, be they reactor or spallation sources. 

The toughness of interfaces between immiscible amorphous polymers without any coupling agent has been the subject of a number of recent studies [15-18]. The width of a polymer/polymer interface is known to be controlled by the Flory-Huggins interaction parameter x between the two polymers. The value of x between a random copolymer and a homopolymer can be adjusted by changing the copolymer composition, so the main experimental protocol has been to measure the interface toughness between a copolymer and a homopolymer as a function of copolymer composition. In addition, the interface width has been measured by neutron reflection. Four different experimental systems have been used, all containing styrene. Schnell et al. studied PS joined to random copolymers of styrene with bromostyrene and styrene with paramethyl styrene [17,18]. Benkoski et al. joined polystyrene to a random copolymer of styrene with vinyl pyridine (PS/PS-r-PVP) [16], whilst Brown joined PMMA to a random copolymer of styrene with methacrylate (PMMA/PS-r-PMMA) [15]. The results of the latter study are shown in Fig. 9. 

Significant experimental support for these relations and the minor chain repta-tion model has been obtained from neutron reflection and SIMS experiments using specially deuterated polymers... 

Fig. 2.59 Neutron reflectivity profiles for a PS-riPMMA symmetric diblock copolymer (Mw = 29.7kgmor ) film of total thickness 5232 A (Menelle et al. 1992). Experiments were performed on samples annealed at the temperatures shown. The solid lines were computed using the scattering length density profiles shown in the insets, which show that the surface induces lamellar order even above the bulk ODT 157 8°C (the air-polymer interface is located at z - 0). 

Concentration profiles of PS graft chains were studied by a neutron reflection method [101]. Graft chains consisted of end-functionalized deuterated polystyrene. Although the measurement was not performed under the true equilibrium conformation, the observed metastable state was in good agreement with that predicted from the SCF theory. The kinetics of the penetration of graft chains into the polymer matrix was also investigated. 

As with all supramolecular structures, one of the most important issues is whether a direct relationship between the structure of a material and its function or properties can be established. In the following, some examples of polymer systems which show such a correlation will be discussed. The materials addressed will include block copolymers, polyalkylthiophenes and a multilayer system based on the self-assembly of polyelectrolytes. Detailed studies on the electrochemical properties of redox-active polymers, based on poly(vinyl pyridine) modified with pendent osmium polypyridyl moieties, have shown that electrochemical, neutron reflectivity and electrochemical quartz crystal microbalance measurements can yield detailed information about the structural aspects of thin layers of these materials. 

Structural Assessment of Redox Polymers using Neutron Reflectivity... 

Figure 4.29 Scattering length density profiles for an [Os(bpy)2(PVP)ioCl]Cl film in (a) perchloric acid, and (b) p-toluene sulfonic acid at different pff levels. From R.W. Wilson, R. Cubitt, A. Glidle, A.R. ffillman, P.M. Saville and J.G. Vos, A neutron reflectivity study of [Os(bpy)2(PVP)ioCl]+ polymer film modified electrodes effect of pff and counterion, /. Electrochem. Soc., 145,1454-1461 (1998). Reproduced by permission of The Electrochemical Society, Inc...

Figure 4.32 Structure of the stratified film of the polymer as obtained from neutron reflectivity experiments shown in terms of volume fractions distances are normal to the substrate surface [(PSS-h7-PAH)3-PSS-d7-PAH]. Above 900 A, the distribution of water associated with the different polyelectrolytes is indicated. Reprinted with permission from M. Losche, J. Smitt, G. Decher, W.G. Bouwman and K. Kjaer, Macromolecules, 31, 8893 (1998). Copyright (1998) American Chemical Society...

In the last 10-15 years, neutron reflectometry has been developed into a powerful technique for the study of surface and interfacial structure, and has been extensively applied to the study of surfactant and polymer adsorption and to determine the structure of a variety of thin films [14, 16]. Neutron reflectivity is particularly powerful in the study of organic systems, in that hydrogen/deu-terium isotopic substitution can be used to manipulate the refractive index distribution without substantially altering the chemistry. Hence, specific components can be made visible or invisible by refractive index matching. This has, for example, been extensively exploited in studying surfactant adsorption at the air-solution interface [17]. In this chapter, we focus on the application of neutron reflectometry to probe surfactant adsorption at the solid-solution interface. 

Our approach to this problem involves a detailed mechanistic study of model systems, in order to identify the (electro)chemical parameters and the physicochemical processes of importance. This approach takes advantage of one of the major developments in electrochemical science over the last two decades, namely the simultaneous application of /ton-electrochemical techniques to study interfaces maintained under electrochemical control [3-5]. In general terms, spectroscopic methods have provided insight into the detailed structure at a variety of levels, from atomic to morphological, of surface-bound films. Other in situ methods, such as ellipsometry [6], neutron reflectivity [7] and the electrochemical quartz crystal microbalance (EQCM) [8-10], have provided insight into the overall penetration of mobile species (ions, solvent and other small molecules) into polymer films, along with spatial distributions of these mobile species and of the polymer itself. Of these techniques, the one upon which we rely directly here is the EQCM, whose operation and capability we now briefly review. 

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