Mostly impact strength will decrease with increasing crystallinity sincere there will be less number of chains in the amorphous region which will resist crack propagation in case of impact test (no chain mobility and slippage). But tensile strength will increase with increasing crystallinity upto certain point. During tensile test, sample breaks by both chain breakage and slippage, since crystals acts as a physical crosslinking points and leads to formation of interconnected network, chain slippage decreases with increasing crystallinity and thus requires more energy to break. So tensile strength will increase with increasing crystallinity. At extremely high crystallinity polymer will show lower strength. The reason is at low to medium amount of crystallinity, one chain will pass through a number of crystals thus will form a highly interconnected network but at high crystallinity one chain will pass through only few crystals therefore interconnected network will not exist and thus crack can propagate without breaking large number of chains therefore it will show lower strength.
To make something opaque, we need free electron to absorb light or interface (between amorphous/crystalline, water/air etc.) to reflect or refract light. As all electrons in diamond is used in forming covalent bond, it can not absorb light and also it is completely crystalline so there is no interface making it highly transparent.
Thanks. Can you please tell, is there any way that the Crystallization temperature of a polymer is higher than its Melting temperature? How are these two related?
The alpha form is more stable than beta form since beta form remain in a thermodynamically metastable state therefore alpha iPP shows higher melting temperature. To learn more please consult this paper "Orientation-induced crystallization of isotactic polypropylene, by Q. Liu et al. (2013).
Amorphous polymers (no or little crystalline) are transparent and crystalline polymers are opaque in nature. With increasing crystallinity transparency of a polymer decreases and opacity increases.
