​In a CD polystyrene case, the outer, transparent portion is more fragile than the black part. What causes this difference?

Fragility is understood from the point of the material 's behavior in the vicinity of glass transition, in which liquid transform into an amorphous solid. The fragility indicator is traditionally obtained through measuring the relaxation time of the supermelted liquid for an increment of temperature near the glass transition. This value is up to twice as high for long-chain polymers (molecular weight above 200,000) than for short-chain polymers (580). This unexpected anomaly in the classical theoretical framework is explained by the fact that the longest chains introduce a "rigidity" which make certain configurations impossible.

On the other hand, when the fragility indicator is estimated based on a more macroscopic quantity such as viscosity, this anomaly disappears and the usual description of the glass transition phenomenon holds true. The length of the polymer chains is henceforth an important parameter to be taken into account. In particular, it allows for a better understanding of the differences in the resistance of polystyrene materials, such as CD cases: to be transparent, the polymer must be more amorphous and comprised of longer chains.