How can you make a flexible polymer more rigid?

At the design stage adding molecules/monomers with more rings than chains will make the polymer more rigid. If this option is not available, adding fiber or particles or fillers is the usual technique.

You can make a stiff polymer more flexible by adding plasticizers, but going the other way is more difficult. Technically speaking the polymer itself isn't changing, but the material overall will be more rigid than the polymer matrix due to the fiber support.