I do not know if this is the best, but you can try to dissolve the EVOH in an alcohol or an alcohol/water mixture at an elevated temperature. Alcohols that may be used include, without limitation, methanol, ethanol, n-propanol, and iso-propanol. Sufficient alcohol or alcohol/water is used to result in a dissolved EVOH concentration of about 1% to about 25% (w/w), preferably about 2% to about 20% and most preferably about 5% to about 15%. More you can find in patent "Method of purifying ethylene vinyl alcohol copolymers for use with implantable medical devices:, https://patents.justia.com/patent/8183337

I agree with @Przemyslaw Wachulak