Elastic deformation polymer glasses

Properties of composites obtained by template poly condensation of urea and formaldehyde in the presence of poly(acrylic acid) were described by Papisov et al. Products of template polycondensation obtained for 1 1 ratio of template to monomers are typical glasses, but elastic deformation up to 50% at 90°C is quite remarkable. This behavior is quite different from composites polyacrylic acid-urea-formaldehyde polymer obtained by conventional methods. Introduction of polyacrylic acid to the reacting system of urea-formaldehyde, even in a very small quantity (2-5%) leads to fibrilization of the product structure. Materials obtained have a high compressive strength (30-100 kg/cm ). Further polycondensation of the excess of urea and formaldehyde results in fibrillar structure composites. Structure and properties of such composites can be widely varied by changes in initial composition and reaction conditions. 

We are in the presence of an entropic deformation. Elasticity results from chain conformational changes due to stretching rather than from changes of intermolecular distances as in polymer glasses, metals, and other classic materials. 

The situation for amorphous linear polymers is sketched in Fig. 2.8a. If a polymeric glass is heated, it will begin to soften in the neighbourhood of the glass-rubber transition temperature (Tg) and become quite rubbery. On further heating the elastic behaviour diminishes, but it is only at temperatures more than 50° above the glass-rubber transition temperature that a shear stress will cause viscous flow to predominate over elastic deformation. 

Despite the new insights achieved with the SFA, there are also severe problems and limits. It is difficult and slow to operate, only mica and few other materials [42, 43] provide suitable substrates, elastic deformation has to be taken into account. With the invention of a bimorph surface force apparatus, known as MAS IF, some of these Kmitations could be overcome [44, 45]. In this case the force between two glass or polymer spheres of typically few millimeter diameter is measured. One sphere is therefore attached to a piezoelectric bimorph that serves as a force sensor. 

SMP based on miscible blends of semicrystalline polymer/amorphous polymer was reported by the Mather research group, which included semicrystalline polymer/amorphous polymer such as polylactide (PLA)/poly vinylacetate (PVAc) blend [21,22], poly(vinylidene fluoride) (PVDF)/PVAc blend [23], and PVDF/polymethyl methacrylate (PMMA) blend [23]. These polymer blends are completely miscible at all compositions with a single, sharp glass transition temperature, while crystallization of PLA or PVDF is partially maintained and the degree of crystallinity, which controls the rubbery stiffness and the elasticity, can be tuned by the blend ratios. Tg of the blends are the critical temperatures for triggering shape recovery, while the crystalline phase of the semicrystalline PLA and PVDF serves well as a physical cross-linking site for elastic deformation above Tg, while still below T ,. 

If one looks closer, using a microscope, at the point where the polymer film is detaching from the oxide surface, at the micrometer level (Fig. 3.9(b)), then it is evident that there is a crack traveling along the interface between the polymer and the glass. This crack is the mechanism by which the polymer material detaches from the surface. All brittle adhesive joints fail by cracking. This is a mechanism which involves elastic deformations and creation of new surfaces. It can be analyzed by the energy balance theory described in Cluster 7. 

A common method of testing materials for resistance to indentation in which a diammid or steel ball, under pressure, is used to pierce the test specimen. Cross-linked polymers having glass transitimi temperatures below the room temperature that exhibit highly elastic deformation and have high elongatirai. 

Fig. 8.11. Elastic deformation of a polymer above the glass temperature. The molecules are straightened between the entanglement points...

This type of modeling is also required for new products, entering the market. Flexible displays are nowadays quite appealing, both due to their flexibility and the robustness of the cell. From fracture mechanical point of view, the base polymer substrate has certainly a higher failure strain than a glass substrate. The limiting strain for the substrate can be related to the onset to non-elastic deformations or to brittle failure behavior, thus the presence of large cracks. 

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