Polymers mass loss

Figure 5 Fractional release of dextran ( ) and rhodamine ( ) from a disk of poly(MSA) as a function of the polymer mass loss. 

As a result of thermal destruction, owing to formation of volatile products polymer mass losses take place. Mass losses at 310 °C are 1%. The main stage of decay (up to 85%) ends at 487 °C and at 610 °C polymer completely bums out [191]. 

Time of temperature effect on the sample influences the process of PETP destruction. During short effect sharp decrease of polymer specific viscosity takes place while polymer mass does not change. Drop of viscosity value slows down during the increase of temperature effect time and polymer mass loss sharply increases. Viscosity change is caused by thermal destruction of macromolecule, and mass change - by the destruction of chain end groups [207]. 

Data of kinetics of mass loss at samples warming up in the air at 200°C are given in Figure 3.18, from which it is seen that introduction of hexaazocyclanes into PETP decreases mass loss of polymer. Since for the period of 5 hours unmodified PETP - fibre loses about 5%, while PETP - fibres, modified by hexaazocyclanes, lose weight much less under the same conditions. For example, the fibre, modified by the dye HC-2, loses 0,8% of the weight. As at differential - thermal analysis HC-3 and HC-4 additives cause the increase of the rate of polymer mass loss. 

Polymer Mass Loss Rate Fluorine to Carbon Ratio... 

A polymer mass loss rate of 10 kgh /area and an activation energy of 21.1 kJ/mol for atomic oxygen-exposed poly(methyl methacrylate) was reported by Whitaker et al. The mass loss rate was found to be directly related to the exposure area and to be independent of sample thickness. 

When water molecules co-exist with samples without any intermolecular interaction, the mass loss(TG) and vaporization peak (DTA) terminate at a temperature lower than 100 °C. In contrast, when water is tightly attached to polymers, mass loss is observed in two stages or multiple stages. Even if mass loss is observed in one stage on the TG curves, two or more peaks may be observed if the DTG curve (dni/dT) is deconvo-luted. DTA endotherms due to vaporization also show multiple peaks. 

The actual time required for poly-L-lactide implants to be completely absorbed is relatively long, and depends on polymer purity, processing conditions, implant site, and physical dimensions of the implant. For instance, 50—90 mg samples of radiolabeled poly-DL-lactide implanted in the abdominal walls of rats had an absorption time of 1.5 years with metaboHsm resulting primarily from respiratory excretion (24). In contrast, pure poly-L-lactide bone plates attached to sheep femora showed mechanical deterioration, but Httie evidence of significant mass loss even after four years (25). 

The second phase of polymer degradation is characterized by a decrease in the rate of chain scission (Fig. 19) and the onset of weight loss. Weight loss has been attributed to (1) the increased probability that chain scission of a low molecular weight polymer will produce a fragment small enough to diffuse out of the polymer bulk and (2) the breakup of the polymer mass to produce smaller particles with an increased probability of phagocytosis. The decrease in the rate of chain scission, as well as the increased brittleness of the polymer, is the result of an increase in the crystallinity of PCL,... 

It is important to distinguish between erosion and degradation. Erosion is mass loss from a bioerodible polymer and may be a consequence of polymer dissolution or degradation of the polymer backbone, followed by dissolution of the degradation products. Degradation typically occurs by hydrolysis of the polymer backbone, the kinetics of which is a function of the polymer chemistry. Thus, erosion is the sum of several elementary processes, one of which may be polymer degradation. 

Thermogravimetric analysis (TGA) has often been used to determine pyrolysis rates and activation energies (Ea). The technique is relatively fast, simple and convenient, and many experimental variables can be quickly examined. However for cellulose, as with most polymers, the kinetics of mass loss can be extremely complex (8 ) and isothermal experiments are often needed to separate and identify temperature effects (9. Also, the rate of mass loss should not be assumed to be related to the pyrolysis kinetic rate ( 6 ) since multiple competing reactions which result in different mass losses occur. Finally, kinetic rate values obtained from TGA can be dependent on the technique used to analyze the data. 

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