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Biodegradable plastics can make an important contribution to the struggle against increasing environmental pollution through plastics. However, biodegradability is a material property that is influenced by many factors. This review provides an overview of the main environmental conditions in which biodegradation takes place and then presents the degradability of numerous polymers. Polylactide (PLA), which is already available on an industrial scale, and the polyhydroxyalkanoates polyhydroxybutyrate (PHB) and polyhydroxybutyrate-co-valerate (PHBV), which are among the few plastics that have been proven to degrade in seawater, will be discussed in detail, followed by a summary of the degradability of further petroleum-, cellulose-, starch-, protein- and CO2-based biopolymers and some naturally occurring polymers.

In the present study, we investigated the galvanomagnetic transport properties of polycrystalline AgxSe thin films with silver excess in the range from x=1.5 to 18. The results prove that the silver excess controls the transition from linear magnetoresistance (MR) behavior to the quadratic ordinary MR and the temperature for the metal–semiconductor transition. Analyzing the MR effect by Kohler’s rule and comparing the results with the field-free resistivity we observe for 2<x<2.3 a steep rise of the product of mean free path and electron concentration (λ·n2∕3). We interpret this result as a consequence of the percolation of nanoscale silver networks within the semiconducting matrix, i.e., as a consequence of the two-phase character of the system.

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The magnetoresistance (MR) effect of the low-temperature phase of silver selenide ͑␣-Ag 2+␦ Se͒ is measured as a function of composition. Very small composition variations in the order of ⌬␦ =10 −6 are achieved by coulometric titration and can be performed simultaneously during the MR measurement. A homogeneous Ag 2+␦ Se shows an ordinary magnetoresistance (OMR) effect, which can be well described by the two-band model. For silver selenide with a heterogenous silver excess, we found quite a different MR behavior. Up to a minor silver excess of 1 ϫ 10 −4 Ͻ ␦ Ͻ 1 ϫ 10 −2 , a saturating negative MR effect, a linear positive MR effect, or a superposition of both can be measured. The microstructure of the silver-rich Ag 2+␦ Se determines its complicated MR behavior. A heterogeneous silver selenide with a larger silver excess ͑␦ Ͼ 10 −2 ͒ shows again an OMR effect.

In the nonstoichiometric low-temperature phase of silver selenide a very small silver excess within the semiconducting silver selenide matrix in the order of 0.01% is sufficient to generate a linear magnetoresistance ͑LMR͒ of more than 300% at 5 T, which does not saturate at fields up to 60 T. Different theoretical models have been proposed to explain this unusual magnetoresistance ͑MR͒ behavior, among them a random resistor network consisting of four-terminal resistor units. According to this model the LMR and the crossover field from linear to quadratic behavior are primarily controlled by both the spatial distribution of the charge-carrier mobility and its average value, being essentially functions of the local and average compositions. Here we report measurements on silver-rich thin Ag x Se films with a thickness between 20 nm and 2 m, which show an increasing average mobility in conjunction with an enhanced MR for increasing film thickness. We found a linear scaling between the size of the transverse LMR and the crossover field, as predicted by the theory. For films thinner than about 100 nm the MR with field directed in the sample plane shows a breakdown of the LMR, revealing the physical length scale of the inhomegeneities in thin Ag x Se devices.In any class of conducting material the relative change in resistance ⌬ / in a magnetic field usually is far from linear and saturates at high fields. As exceptions some polycrystalline metals such as potassium or indium show a nonsaturating linear magnetoresistance ͑MR͒ ͑LMR͒, caused by macroscopic voids or inhomogeneities. This effect is rather small and the linear behavior is only found at large fields of about 1 T, 1,2 so these materials did not stimulate any thoughts of magnetic sensor applications. This changed when a large and nonsaturating LMR was found in Ag x Se and Ag x Te with linear characteristics down to 10 −4 T, leading to intense research activities. [3][4][5][6][7][8] The first approach for the explanation of this unusual behavior can be found already several years ago in the theory of Stroud, 9 who calculated the resistance of a two-phase mixture by the resistance of its single components. At certain volume fractions of the components the increase in resistance does not saturate with increasing magnetic field but is linear. [10][11][12] Other calculations show that the MR also depends strongly on the shape of the conducting inclusions and therewith on the microstructure of the compound. 13-15 Above the percolation threshold, when conducting inclusions in an insulating matrix form a continuous network, the magnetoresistance is asymptotically proportional to the magnetic field. 16 Another theoretical explanation was proposed by Abrikosov 17 as a quantum magnetoresistance for a twocomponent system with a zero band gap of the semiconducting matrix.An alternative classical approach reduced on the spatial distribution of the charge-carrier mobility and its average value was presented by Parish and Littlewood ͑PL͒. 18,19 They found that in a two-dimens...