[nish095]

How could the chemical structure of Polyethylene terepthalate (PETE) be changed to get something that is more rigid? (What would the repeat unit be)? And why? I know that HDPE and PVC materials are rigid.

[Hunter2]

Probably to use Polymerisation instead of Polycodensation. Have in shorter sequence the same unit, like in PE or PVC. Polyesters like Terephethalates to complicated built up already.

[Enthalpy]

Polyethylene terepthalate (PETP) is rigid. Its Yound modulus (3GPa) exceeds PE always and PVC often.
http://www.matweb.com/
If you read German, the best data handbook is Kunststofftabellen, by Bodo Carlowitz.

In fact, few polymers are stiffer than PETP - they belong to the expensive ones like PI, PEI, PAI...

One cheap method to stiffen polymers is to incorporate short graphite fibers called choppers. They still permit injection molding as well as maching, and are very efficient - more so than changing the polymer composition or production process.