Polymer structure modification plasticization

It is clearly a truism that for reducing the fire risk in the applications of plastics, their flammability should be diminished. This is achieved either by reactive flame-retardants incorporated during the preparation (polymerization, polyaddition, polycondensation) of the polymer or by additive flame-retardants admixed in the course of plastics processing. The flammability of plastics is sometimes reduced by surface protection. The most recent methods of reducing flammability are the modification of the macromolecular structure and the development of thermally resistant polymers (high-temperature plastics). 

For thermoplastic polymers, both ductile and brittle modes are possible, and many of these materials are capable of experiencing a ductile-to-brittle transition. Factors that favor brittle fracture are a reduction in temperature, an increase in strain rate, the presence of a sharp notch, an increase in specimen thickness, and any modification of the polymer structure that raises the glass transition temperature (T ) (see Section 15.14). Glassy thermoplastics are brittle below their glass transition temperatures. However, as the temperature is raised, they become ductile in the vicinity of their T s and experience plastic yielding prior to fracture. This behavior is demonstrated by the stress-strain characteristics of poly(methyl methacrylate) (PMMA) in Figure 15.3. At 4°C, PMMA is totally brittle, whereas at 60°C it becomes extremely ductile. 

Bourtoom and Chinnan (2008] studied the WVP of rice starch-chitosan blend films with different type and concentrations of plasticizer. They observed that the WVP increased from 5.45 to 8.68 gmm/m day kPafor sorbitol 14.53 to 28.73 gmm/m day kPa for glycerol and finally from 14.70 to 19.10 g mm/m day kPa for polyethylene glycol. This tendency was due to possible structural modifications of the polymer network and also because the network could become less dense as a result of increasing the mobility of the polymeric chains or by the free volume of the film. Also, the increase of WVP could be related to the hydrophilicity of the plasticizers because the presence of this material increases the concentration of polar residues in hydrocolloid based film... 

Modifications of the polymer structure can influence the photochemistry as well as the physical chemistry of the resist. Cinnamate groups that are attached pendant to rubbery polymers photolyze much faster than when they are attached to higher Tg, more rigid plastic backbones. It is thought that the cinnamate groups have more mobility in the rubbery matrix to move into the proper orientation for dimerization. ... 

A considerable amount of data has accumulated regarding the modification of lignins to engineering plastics. Unfortunately, the incorporation of various monomers and polymers, such as di- and polyvalent epoxyphenols, esters and isocyanates, in the lignin structure in most cases resulted in brittle or tarry materials whose properties designated them as potential adhesives, lacquers, dispersants and films, but not as structural materials (36-40). 

Such information offers an opportunity to study details of the fibrillation mechanism. The fibers formed by stretching the spherulitic polymer representing nothing other than ribbon formations plastically deformed and oriented towards the mechanical stress that is released by comparatively weak mutual interconditions existing in an earlier formation (Figure 3). This behavior points to the existence of some weak surfaces in the crystalline polymers. Elements of the super-molecular structure detached by action of the external mechanical forces can slide on the weak surfaces. Evidence for the strain-destruction relationship must come from studies of the modification of the contact surfaces of two neighboring spherulites under mechanical stress. 

The antiplasticization phenomenon is presumably common to all the polymers exhibiting a relatively strong (5 transition, well separated from the a transition. It has been observed for both linear (PVC, polycarbonate, poly-sulphones) and network polymers (amine-crosslinked epoxies). For the case of thermosets, the phenomenon may be a consequence of both internal (change of the network structure) and external (incorporation of miscible additives) modifications of the structure or the composition but it always seems to be a consequence of the plasticization, as shown in Fig. 11.7. 

The crystalline plastics (basic polymers) tend to have their molecules arranged in a relatively regular repeating structure such as polyethylene (PE) and polypropylene (PP). This behavior identifies its morphology that is the study of the physical form or structure of a material. They are usually translucent or opaque and generally have higher softening points than the amorphous plastics. They can be made transparent with chemical modification. Since commercially perfect crystalline polymers are not produced, they are identified technically as semicrystalline TPs. The crystalline TPs normally has up to 80% crystalline structure and the rest is amorphous. 

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