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(a and b) SEM of PU 1 prior to degradation and at 2 months, (c and d) SEM of PU 2 prior to degradation and at 2 months, (e and f) SEM of PU 3 prior to degradation and at 2 months, and (g and f) SEM of PU 4 prior to degradation and at 2 months.
SEM: scanning electron microscope.
