Geomembranes are present at the market for example pretecting layers, sheets at lanfilling. They are obtained from HDPE (Eg Borealis-as I well remember) modified with carbon black. There are many papers in the literaure. How to prepare: by blending procedure.
Modern geotextiles use fibre reinforcement with special types of polyolefines. Others use additionally silicones. Another type is the water tight formulation of polymer concrete with fibre reinforcement. You first have to decide which path you will follow.