Polymer structure modification initiators

Photodegradation of polymers (photoageing), involving chain scission and/or cross-linking, occurs by exposure to solar or artificial radiation and causes structural modifications, usually accompanied by a dramatic deterioration of the physical and mechanical properties of the polymer.954 Typically, radical intermediates are formed upon excitation, which initiate subsequent (dark) degradation. The presence of other species, such as oxygen, water, organic solvents or additives, and also mechanical stress and heat, may enhance the efficiency of these processes. 

Effects of ultrasounds in polymer chemistry are relatively less explored. Sonochemistry of polymers consists of three main fields the degradation and modification of polymers, the ultrasonically assisted synthesis of polymers and the determination of the polymer structure. Special attention has been devoted to ultrasound induced chain degradation [3, 4] and to the ultrasonically influenced preparation of anionic initiators and anionic polymerization [17]. 

McDonald et al found that the modification of an emulsion polymerization with a water-miscible alcohol and a hydrocarbon nonsolvent for the polymer can influence the morphology and enables the formation of monodisperse particles with a hollow structure or difiuse microvoids [58]. Both kinetic and thermodynamic aspects of the polymerization dictate particle morphology. Complete encapsulation of the hydrocarbon occurs, provided that a low molecular-weight polymer is formed initially in the process. Monodisp>erse hollow particles with diameters ranging from 0.2 to 1 pm were obtainable, and void fractions as high as 50% are feasible. 

A final consideration is that the Argon theory essentially regards yield as nucleation controlled, analogous to the stress-activated movement of dislocations in a crystal produced by the applied stress, aided by thermal fluctuations. The application of the Eyring theory, on the other hand, implies that yield is not concerned with the initiation of the deformation process, but only that the application of stress changes the rate of deformation until it equals imposed rate of change of strain. The Eyring approach is consistent with view that the deformation mechanisms are essentially present at zero stress, and are identical to those observed in linear viscoelastic measurements (site model analyses in Section 7.3.1). Here, a very low stress is applied merely to enable detection of the thermally activated process, without modification of the polymer structure. 

Abstract In this chapter, selected examples of sequential post-polymerization modifications are highlighted. Initially, we focus on side chain and chain end modifications in solution and at surface bounded polymers. Afterwards, the usage of this modifications as powerful tools in the synthesis of polymer structures such as graft and star polymers are discussed. 

Thus, the data of Figure 7.11 indicate that the ability of reinforcement of the nanocomposite is defined not actually by the anisotropy of form of the filler particles, but by the ability of the polymer matrix to reflect (reproduce) this anisotropy. In other words, the filler role comes to polymer matrix structure modification in comparison with the initial matrix polymer. A similar concept was used for description of the structure of polymer microcomposites [36, 37]. However, distinction of this general treatment consists in the fact that in the microcomposites case the bulk polymer matrix structure changes (its fractal dimension d is increased) [37] and in the nanocomposites case only the structure of the interfacial regions at the common condition d = const, changes [38]. 

The value of the deformed polymer consists of two parts the free volume of non-deformed polymer and the dilatational free volume, which is the consequence of the polymer density change in the deformation process [35]. The first component of the decrease in due to introduction of the epoxy polymer in HOPE (that is illustrated by a reduction in the permeability to gas) will require compensation at the expense of the second component increase, which should result in growth in the yield strain y [35]. This assumption is confirmed experimentally (see Figure 8.9). Within the frameworks of the yielding phonon concept [4, 19] this means that not only the parameters characterising the initial non-deformed polymer structure should be taken into account, but also its degree of modification in the deformation process, which can be realised with the help of the Griinesen parameter y. In the first approximation the first factor can be characterised with... 

Modification of alkyd resins with high proportions of silicones considerably reduces rates of attack, but the most spectacular extension of life is shown by fluorinated polymers such as polyvinylidene fluoride where erosion rates can be reduced to 0 -1 /tm/year. If this level of durability can be achieved an initial coating, if firmly adherent and free from any breaks, may often be expected to maintain protection over a metal substrate for the likely life of the structure. The considerably increased first cost, as compared with more conventional coatings, has to be balanced against the probable saving in maintenance costs or consequences of failure. 

The in situ bulk polymerization of vinyl monomers in PET and the graft polymerization of vinyl monomers to PET are potential useful tools for the chemical modification of this polymer. The distinction between in situ polymerization and graft polymerization is a relatively minor one, and from a practical point of view may be of no significance. In graft polymerization, the newly formed polymer is covalently bonded to a site on the host polymer (PET), while the in situ bulk polymerization of a vinyl monomer results in a polymer that is physically entraped in the PET. The vinyl polymerization in the PET is usually carried out in the presence of the swelling solvent, thereby maintaining the swollen PET structure during polymerization. The swollen structure allows the monomer to diffuse in sufficient quantities to react at the active centers that have been produced by chemical initiation (with AIBM) before termination takes place. 

Polymer modification at the macroscopic level (either as a material subjected to mechanical processing or as a running object) consists of initiating the destructive phenomena at microdefects—that is, at submicroscopic cracks, statistically distributed on the surface or within the body of the stressed material. These cracks become centers where a detachment of intermolecular bonds occurs. This process might be called a mechanical disaggregation, the opposite of aggregation, a term that expresses (in this context) the assembly of various structural elements into polymers. 

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