No, they are pronounced not differently. Well, if you seek for the pronunciation it could vary if we compare the American and typical British English pronunciation. In American English "can" and "can't" and "cannot" ( meaning exactly the same) is with sound "a",which is pronounced as "a" in cat,hat,etc.( that is between "a" and "e"). While in British English the sound "a" is very near to the clear/ a/ as in Spanish, Italian languages, for example as in the word "bath" and then "can't " is pronounciated /'ka:nt/and "can" /'kan/. In brief, there is difference if we envisage American and British English.
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