In crystalline materials the addition of particles works to impede dislocation motion and strengthen the material. However, for many semi-crystalline polymers particle fillers weaken the material. It has been suggested that for particles to have a toughening effect in polymers the interparticle matrix ligament thickness must be smaller than a certain threshold. Crystalline polymers polypropylene and polyethylene display particle strengthening. Plastics are viscoelastic materials meaning that under applied stress, their deformation increases with time (creep). The elastic properties of plastics are therefore distinguished according to the time scale of the testing to short-time behavior (such as tensile test which lasts minutes), shock loading, the behavior under long-term and static loading, as well as the vibration-induced stress. Crystalline polymers are usually opaque because of light scattering on the numerous boundaries between the crystalline and amorphous regions. The density of such boundaries is lower and  consequentially, the transparency is higher - either for low (amorphous polymer) or high (crystalline) degree of crystallinity. For example, atactic polypropylene is usually amorphous and transparent while syndiotactic polypropylene, which has crystallinity ~50%, is opaque. Crystallinity also affects dyeing of polymers: crystalline polymers are more difficult to stain than amorphous ones because the dye molecules penetrate through amorphous regions with greater ease.