Macromolecular chain modification

Finally it is instructive to present the resulting macromolecular chain modifications (MWD) and their effect on the rate-dependent viscosity during viscracking, as shown in... 

The chemical modification techniques refer to the treatments used to modify the chemical compositions of polymer surfaces. Those can also be divided into two categories modification by direct chemical reaction with a given solution (wet treatment) and modification by covalent bonding of suitable macromolecular chains to the polymer surface (grafting). Among these techniques, surface grafting has been widely used to modify the surface of PDMS. 

Chemical modification of polymers (J.) still remains a field of continuously increasing importance in macromolecular chemistry. In spite of its high diversification, it may be divided into 2 distinct but complementary main research lines a) the fundamental study of the chemical reactivity of macromolecular chains b) the synthesis of new homopolymers and copolymers, and the functionalization of linear or crosslinked polymers. Some of these facets have been reviewed in the last years (2-6), and the purpose of this presentation is to illustrate a number of characteristic topics both from fundamental and applied points of view, through some literature data and through our own studies on nucleophilic substitution of polymethylmethacrylate (PMMA). 

Being essentially a surface technique, XPS valence band spectra also allows to monitor modifications occurring at the polymer surface during adsorption, reactions, of degradation... Very few contributions are, up to now, dealing with such studies ( ). The most direct use of the technique is actually a comparison of the core and valence photoelectron line intensities to deduce informations about the surface and the in-depth composition of the polymer, as well as about the orientation of the macromolecular chain at the surface boundary. 

The last two theories described (chemical and thermodynamic) are intimately linked together because both of them induce a modification of the chemical composition at the surface. On the one hand, this modification can change the thermodynamic parameters (wettability) of the surface. On the other, changes in chemical composition influence the chemical adhesion established between the adherend and the adhesive layer. Numerous treatments are available for surface modification with coronas [8], plasmas [9, 10], lasers [11, 12], ion-assisted reactions [13], or coupling agents [14, 15]. All these treatments do not only change the chemical composition they can also affect the roughness, the orientation of macromolecular chains, and the mechanical behavior. 

The most effective method of modification of linear organosilicon polymers is the insertion of various elements or groups with a different chemical nature into the structure of the macromolecular chain. As a result of the insertion in the dimethylsiloxane chain of different fragments there are changes in the physical chemical properties [5]. The insertion of the cyclic fragments in the main linear dimethylsiloxane chain hinder the chain transfer reactions, which proceed with the release of D -type (where D = Me2SiO) cycles during the thermal depolymerisation, that raises the thermal-oxidative stability of the polymers [6]. 

The deformation of charged rubbers is very sensible to the temperature modification from two reasons. On one side, the elastomer chains that adhered to the surface of the charge particles are detached with the increase of temperature and, on the other side, the existence of a cage , made by elastomer macromolecular chains, immobilised around a polymer particle, which diminishes its thickness concomitantly with the increase of temperature. 

By UV irradiation the stable trans form passes into cis form, which is reflected in modification of some important properties of the system, such as the increase of hydrophobicity, the decay of viscosity, and the weakness of intermolecular interactions. On the initial stage, due to the trans conformation of the azobenzene groups, the dye is disposed alongside of the extended macromolecular chains the cis form, obtained by irradiation, limits the dye-polymer interactions and poly(methacrylic acid) adopts a coiled conformation. 

Chemical modification refers to the modification process in which chemical reactions occur between the backbone, branched chain, and side chain of a macro-molecular chain. The modification principle depends mainly on the structural changes of the main chain, branched chain, or side chain. Chemical modifications include copolymerization between different monomers, grafting reaction of macromolecular chains, crosslinking reaction within macromolecular chains, functional group reactions on the macromolecular chains, and so forth. For example. 

The presence of a sharply defined interface between a polymer solution and a solid wall leads to important modifications in the local polymer concentration with respect to the bulk concentration. These variations can be positive or negative depending on the sign of the interaction between the solid wall and the macromolecular chains immersed in the solvent. Attractive forces lead to adsorbed layers while repulsive forces lead to depletion layers. Due to their connection with important technological applications such as adhesives, protective coatings, microlithography, emulsion stabilizers, adsorbed polymer layers have been the subject of extensive theoretical studies. ... 

Based on values from Table 1 one may conclude that UVA radiations are capable of generating macromolecular chain scissions in a former stage, which may lead to initiation of important chemical modifications via radical intermediates formation [3, 4]. 

Photoizomerization is another type of reversibile stmctural modification supported by the macromolecular chains during exposure to light [8, 13]. 

Currently surface grafting is widely used by the research community to develop low-fouling composite membranes. In this method, grafted macromolecular chains are covalently bonded to the membrane surface. Surface grafting may be initiated by ultraviolet (UV)-irradiation, chemically, and by plasma or enzymatic treatment of the membrane surface. Choice of the specific graft polymerization technique depends on the chemical structure of the membrane and the desired characteristics after surface modification. 

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