So I'm working on an article about the imapact resistance of ophthalmic lens materials. The two main lens materials used are those of a hard resin called CR-39 and the other is polycarbonate. I have been looking for something that describes the difference in their ability to withstand certain degrees of impact. All I could find is that when CR-39, or other hard resins. are cured the molecules cross link. I'm assuming that this is what makes the lens very hard and also some what brittle. Where as polycarbonate molecules do not cross link and instead create long chains. And this is what gives the lens it's impact resistance and flexability. Am I correct in my assumptions?