5If you are talking about “citations and references”, it's not an academic study but my feeling.
I am afraid it's not a very helpful feeling.
The only distinction between a stressed syllable and a non-stressed syllable in Portuguese is the length of the vowel.
That is not the case in English
I think vowels like /aɪ/ /eɪ/ /ɔː/ /aʊ/ /oʊ/ /(j)uː/ /iː/ /ɑː/ /ć/ /ɛ/ seldom appear in a non-stressed syllable.
In a completely unstressed syllable, many vowels emerge as /ə/, but most vowels retain their quality when there is secondary stress.
/ɪ/, if it's represented by the letter E, is usually pronounced /ɛ/ or /iː/ when it's stressed.
Can you give some examples of this?.
Student or Learner