Mechanical properties degradation

These fillers are soft and do not dramatically affect mechanical properties. PTFE loadings commonly range from 5 to 20% the others are usually 5% or less. Higher loadings can cause mechanical degradation. 

However, despite this lack of a basic understanding of the electrochemistry of these materials, much progress has been made in characterizing polymerization mechanisms, degradation processes, transport properties, and the mediation of the electrochemistry of species in solution. These advances have facilitated the development of numerous applications of conducting polymers, and so it can be anticipated that interest in their electrochemistry will remain high. 

In all types of PHAs, P4HB is of the most interest because it was used in the degradable scaffold that resulted in the first successful demonstration of a tissue-engineered tri-leaflet heart valve in a sheep animal model. Its copolymers with PHB and polyhydroxyoctanoate (PHO) are also promising in tissue engineering because of their nontoxic degradation products, stability in tissue culmre media, and the potential to tailor the mechanical and degradation properties to match soft tissue. 

In order to obtain solutions with the desired flow properties, shear-induced degradation should be avoided. From mechanical degradation experiments it has been shown that chain scission occurs when all coupling points are loose and the discrete chains are subjected to the velocity field. Simple considerations lead to the assumption that this is obtained when y) is equal to T sp(c-[r ]) (Fig. 18). The critical shear rate can then easily be evaluated [22]. 

Viscoelastic properties have been discussed in relation to molar mass, concentration, solvent quality and shear rate. Considering the molecular models presented here, it is possible to describe the flow characteristics of dilute and semi-dilute solutions, as well as in simple shear flow, independent of the molar mass, concentration and thermodynamic quality of the solvent. The derivations can be extended to finite shear, i.e. it is possible to evaluate T) as a function of the shear rate. Furthermore it is now possible to approximate the critical conditions (critical shear rate, critical rate of elongation) at which the onset of mechanical degradation occurs. With these findings it is therefore possible to tune the flow features of a polymeric solution so that it exhibits the desired behaviour under the respective deposit conditions. 

Mechanical and Degradation Properties. Studies characterizing the mechanical properties of these highly crosslinked materials indicate properties that are intermediate between those of cortical and trabecular bone. Table I summarizes these results along with the mechanical properties of bone. 

Poly(esters) (Table 11.2) are the first class of polymers discussed, as they are the most widely investigated of all of the polymer families for oral protein delivery. Poly(esters) used for oral drug delivery have primarily been biodegradable polymers (Figure 11.1). Biodegradation is the primary delivery mechanism for poly(ester) polymers used for protein and peptide delivery. The degradation properties of poly(esters) are dependent on the monomers used to produce the poly(ester). Several poly(esters) are discussed in detail in the following sections. 

In turbulent flow, the macromolecules are also believed to experience burst-induced elongation. However, it has to date not been possible to formulate structure-property relationships which allow an ad hoc prediction of the onset of mechanical degradation and its influence on drag reduction. 

However, the long range effectiveness of polymer additives remains, due to the mechanical degradation, a hitherto unsolved problem. By application of the above-mentioned theoretical approaches and the influence of laminar and elongational flow on polymer stability described in Sect. 6.3.4, it seems possible to retain the flow features over a longer period. It is therefore necessary to reinforce investigations which enable a more quantitative description of turbulent flow, so that in the future structure-property relationships can be established which permit a correlation of the microscopic structure of the macromolecules with the observed flow phenomena. 

The tacticity of PLA influences the physical properties of the polymer, including the degree of crystallinity which impacts both thermo-mechanical performance and degradation properties. Heterotactic PLA is amorphous, whereas isotactic PLA (poly(AA-lactide) or poly (55-lac tide)) is crystalline with a melting point of 170-180°C [26]. The co-crystallization of poly (RR-lactide) and poly(55-lactide) results in the formation of a stereocomplex of PLA, which actually shows an elevated, and highly desirable, melting point at 220-230°C. Another interesting possibility is the formation of stereoblock PLA, by polymerization of rac-lactide, which can show enhanced properties compared to isotactic PLA and is more easily prepared than stereocomplex PLA [21]. 

The main purpose of these process analysis activities is to evaluate goal achievement in terms of polymer product properties. For example, the residence time as well as the deformation history of a single particle is an indication for the polymers thermal and mechanical degradation. 

R.J. Farris, "The Stress-Strain Behavior of Mechanically Degradable Polymers," In POLYMER NETWORKS STRUCTURAL AND MECHANICAL PROPERTIES, ed. A.J. Chompff and S. Newman, pp. 341-394, Plenum, New York, 1971. 

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