Autoaccelerating degradation

An autoaccelerated degradation kinetics seems to be the best approach toward these requirements provided the rate constant can be modified according to needs by modifications of the polymer. This has been demonstrated by data shoim in Fig. 11. 

The conversion-time curves appear to be very similar to the shape typical of emulsion polymerization, i.e., an S-shaped curve is attributed to the autoacceleration caused by the gel effect (Smith-Ewart 3 kinetics, n>>l). The rate of polymerization-conversion dependence is described by a curve with two rate maxima. The decrease in the rate after passing through the first maximum is ascribed to the decrease of the monomer concentration in particles. Particle nucleation ends between 40 and 60% conversion, beyond the second rate maximum. This is explained by the presence of coemulsifier which stabilizes the monomer droplets against diffusive degradation. 

Thus, let us denote the general features of PI, PPQ and SPPQ thermo-oxidation. The first important likeness of the degradation behaviour is O2 absorption at rather low temperatures (200 °C). At temperatures 250 °C or higher, when it becomes possible to search O2 absorption kinetics in real time, polymers display equal oxidation types independently of temperature - the three-stage kinetics, sequentially described by laws of the first and the zero orders with future autoacceleration. 

In papers [39,40] it was shown, that the cause of transition from autodecelerated regime of polymers thermooxidative degradation to autoaccelerated one is a structural factor, namely, the increase of fractal dimension of macromolecular coil in melt. We have all reasons to assume, that this factor will be decisive in the considered case as well. The fractal dimension D of the heptylbenzoate fractal-like molecule can be determined with the help of the Eq. (18) of Chapter 2. In Table 2 the values according to the data of paper [26], the values D, calculated according to the Eq. (18) of Chapter 2 and the type of the curve for transesterification... 

These types of oxidative degradation reaction are classically represented in terms of two autoaccelerating, interacting cycles which may be interrupted at a variety of points. However, many standard stabilisers are too volatile and/or too unstable to operate well at PEEK processing temperatures. 

The absence of observable weight loss together with the exponential increase in carboxyl end group content implies an autoaccelerated, random degradation process occurring throughout the bulk of the sample. The simultaneous occurrence of surface errosion cannot be ruled out but its contribution to the overall process is too small to become observable at the surface to volume ratios of our implants. 

The decomposition of hydroperoxides creates more radicals than are initially formed by initiation, resulting in an autoacceleration of the degradation process (chain branching). Additionally, initiation by hydroperoxides is autoregenerated in the sense that, overall, the same quantity of peroxides is recovered at the end of the decomposition cyde. 

Polymer hydroperoxides play a major role in the photo-oxidative degradation. They play an evident role in the initiation process however, they are the main product of the oxidation chain reaction. Since they are photolytically unstable, they may produce chain branching, leading to autoacceleration of the oxidation. 

The photo-oxidation of polypropylene is characterized by an induction period in which oxygen uptake (whether measured directly or by carbonyl index) occurs at a very low rate, followed by autoacceleration to a steady rate of oxidation. Measurements of mechanical properties and polymer molecular weight [2187] have shown that extensive degradation occurs in the induction period and the useful life extends little beyond this point [1443]. The surface of photo-oxidized polypropylene is highly degraded to a depth of 0.5 ym during the induction period, leading to microcracks and loss of mechanical properties [390]. 

The rate of hydrolytic degradation of poly DL-lactic acid was studied in a variety of media at 25 C and 37 C. The rate was linear in liquid media, but showed autoacceleration in moist air. The rate was found to be sensitive to factors which increased chain mobility... 

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