Surface nature of polymers

Surface nature of polymers D BRIGGS Migration of additives and low molecular weight fraction to the surface... 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

Wool [32] has considered the fractal nature of polymer-metal and of polymer-polymer surfaces. He argues that diffusion processes often lead to fractal interfaces. Although the concentration profile varies smoothly with the dimension of depth, the interface, considered in two or three dimensions is extremely rough [72]. Theoretical predictions, supported by practical measurements, suggest that the two-dimensional profile through such a surface is a self-similar fractal, that is one which appears similar at all scales of magnification. Interfaces of this kind can occur in polymer-polymer and in polymer-metal systems. 

A PP sample after ozonization in the presence of UV-irradiation becomes brittle after 8 hrs of exposure, whereas the same effect in ozone is noticeable after 50-60 hours.Degradation of polymer chain occurs as a result of decomposition of peroxy radicals. The oxidation rapidly reaches saturation, suggesting the surface nature of ozone and atomic oxygen against of PP as a consequence of limited diffusion of both oxygen species into the polymer. Ozone reacts with PP mainly on the surface since the reaction rate and the concentration of intermediate peroxy radicals are proportional to the surface area and not the weight of the polymer. It has been found that polyethylene is attacked only to a depth of 5-7 microns (45). 

Before applying the ideas summarised in the first section to polymer latices it is appropriate to consider the nature of polymer latex particles. We know, for example, that each particle is composed of a large number of polymer chains, with the chains having molecular weights in the range of about 105 to 107. Moreover, the particles themselves can be amorphous, crystalline, rubbery, glassy or monomer swollen, either extensively or minutely. It follows, therefore, that the properties of the system on drying depends directly on the physical state of the particles, for example, if the particles are soft, coalescence can occur to form a continuous film, whereas with hard particles their individuality is retained. The nature of the particle obtained is directly related to the preparative method employed and the surface properties are often determined by s-... 

The analytical depth profiling for these systems (e.g. the polystyrene data is shown in Figure 5) revealed that the reaction is essentially confined to the topmost monolayer of material ( ). This is entirely reasonable in terms of the plasma chemistry since the most prominent reactive species is atomic oxygen f ich is expected to have an extremely short mean free path in hydrocarbon polymers. This serves as a very good example of the powerful nature of XPS when applied to the study of the surface modification of polymers. 

Several other properties of copolymers that are important in specific applications have also been measured. The surface properties of polymers determine the nature of adhesives that will stick to a substrate, and the nature of solvents that will wet the surface. The surface energy of some styrene and acrylonitrile have been measured, and the surface is rich in polystyrene when the acrylonitrile content of the copolymer is below 50% [108]. 

Thorough colloidal/surface characterisation is fundamental to the success of research on polymer colloids. A wide range of complementary techniques are available for colloidal/surface characterisation of polymer colloids and access to several is necessary since no single technique can provide full characterisation. There is an ongoing need for experimental and theoretical work on improvements to existing methods and on development of new techniques to support the needs of research. Additionally, the necessary improvements in process modelling will naturally lead to a demand for advances in on-line analysis to support feedback loops for process control and manufacturing. Thus, further developments in on-line methods for measurement of particle... 

The nature of polymer chains in confined geometries, thin films, and structured surfaces for example, is of much current interest. Grazing incidence SANS (using the evanescent wave to control depth sensitivity) and transmission SANS on free standing thin films in combination with H/D labelling, are likely to make a vital contribution to our understanding of conformation in confined geometries. 

The surface charging of polymers depends on the nature and concentration of additives used in the production process. Thus, the same monomeric material can produce polymers with very different surface properties. The PZCs/IEPs of polymers are presented in Tables 3.2091-3.2103. 

Before long-term hemocompatibility can be expected for any material, the nature of polymer surface-protein interaction must be established in more detail the way in which the polymer surface alters itself (rotation of segments, side groups, chain refolding, etc.) in response to the protein species the way the protein is altered in conformation (if at all) upon adsorption by the surface and how this conformational change provokes platelet retention. Of course, longer-term ex vivo or in vivo studies also will be necessary. 

All polymer single crystals have the same general appearance. Under an electron microscope, they appear as thin, flat platelets that are 100 to 120 A thick and several microns in lateral dimensions. This lamellar nature of polymer single crystals has been found to be fundamental. Growth of the crystal normal to lamellar surface occurs by the formation of additional lamellae of the same thickness as the basal lamellae thick crystals are usually multilamellar. 

Alkali and acid treatments have also been used to modify surface properties of polymers sulfonated polyethylene films treated first with ethylenediamine and then with a terpolymer of vinyhdene chloride, acrylonitrile, and acrylic acid exhibited better clarity and scuff resistance and reduced permeabihty. Permanently amber-colored polyethylene containers suitable for storing light-sensitive compoimds have been produced by treating fluorosulfonated polyethylene with alkali. Poly(ethylene terephthalate) dipped into trichloroacetic/chromic acid mixture has improved adhesion to polyethylene and nylons. Antifogging lenses have been prepared by exposing polystyrene films to sulfonating conditions. Acid and alkali surface treatments have also been used to produce desired properties in polymethylmethacrylates, polyacrylonitrile, styrene-butadiene resins, polyisobutylene, and natural rubber. Surface halogenation of the diene polymers natural rubber and polyisobutylene resulted in increased adhesion to polar surfaces. 

The slope constant Ke is influenced by the chemical nature of the end groups, as shown in Figure 12-2. 7 is independent of the molar mass and the nature of the end groups. Typical surface tensions of polymer liquids of finite molar mass are given in Table 12-1. The surface tensions do notvary very much with temperature. 

The primary peculiarity of the system under consideration is the fact that increase of the quantity of surfactant does not always result in decrease of surface tension. Thus, the surface tension of polymer containing 1% of L-19 is higher than that of a polymer containing 0.05% or 0.5% of L-19. Additionally, increase of the surfactant content of the liquid oligomer always results in the decrease of the surface tension (naturally when the surfactant concentration in the system is lower than CCMF). 

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