Latex is absolutely right. TEFLON is renowned for having very little "memory". In other words, when it is subjected to a stress, it deforms into another shape and fails to return to its original shape once the stress is relieved. That is why pure TEFLON gaskets, while a good idea, don't work and can't be relied upon.
Another point that goes against your idea is that you probably can't come up with some technique or method to build in the necessary features of a typical pressure vessel: nozzles, heads, and seams that require some type of "welding" that can resist the stress imposed by the 2 atmospheres of pressure. Glue just won't do the job. So how do you fuse or join all the necessary components together?
Although your idea is simplistic and simply won't work, as a student this type of question can take on good learnings for you that you may take into your future profession: There are a lot of practical points associated with any engineering problem and their resolution depends on the engineer being able to take the time, concentrate on these details and have ready answers or options prior to implementing the project with time and expenses. Being able to extrude a cylindrical, TEFLON piece is not enough to qualify it as a means to build a complete, safe, and operable pressure vessel. Before being converted into a pressure vessel, you must allow for the normal, required extra items such as a means to install heads on each end, install usable nozzles for inlet, outlet, inspection, temperature, pressure readings, as well as a supporting skirt or legs. Otherwise, the vessel will essentially be useless.
How do you reach the first level of practical know-how that Latexman has revealed? You do your engineering leg work by researching the properties of the basic material in this case and find out all you can about its characteristics. Sometimes this can be done by coming here to our Forum and getting it from an expert like Latexman - which can save you a lot of wasted time and effort. So, in that respect you have done the correct thing.