This is such a great thing to do, especially in this time where non GMO seeds are disappearing. Having a seed bank should be a great thing to have. However i believe glass should be the ideal container for this because of change of temperature, UV sunlight and high pressure some plastics will release particles such as BPA however PET bottle is considered safe.
If contained for few weeks I would say it's okay.
If contained for long time you should try glass.
RE: Reciclaje de plásticos al almacenar semillas para la próxima cosecha./ Recycling plastics when storing seeds for the next harvest.