Mechanical Stability of Porous Copolymers by Positron Annihilation Lifetime Spectroscopy

A set of porous copolymer samples
(VP-TRIM, VP-DVB, and VP-DMN)
under mechanical stress up to 1.3 GPa was studied in situ using positron
annihilation lifetime spectroscopy. A few stages of porosity evolution
dissimilar in changes of the pore size distribution (PSD) were observed.
At low pressure, the maximum of PSD and the pore volume decreased,
indicating pore shrinkage. Simultaneously, the fast decrease in the
width of PSD shows that the size of the largest pores is mostly reduced.
In copolymers in which the number of relatively weak ester bonds is
significant, an additional stage exists at 40–70 MPa. In this
range, the trend of the changes in the width of PSD versus pressure
is reversed, indicating pore reorganization (probably, they connect
with each other). Above a certain pressure (0.2–0.5 GPa), both
the maximum and the width of PSD stabilize at minimal values, ca.
1–2 nm. These values as well as the stabilization pressure
are specific for each polymer. Pore collapse is observed in a high-pressure
range, where only the pore volume decreases. The pores are completely
closed at 0.8–1.3 GPa (depending on the polymer), but their
porosity is greatly recovered after pressure release. The size and
volume of the pores after porous polymer squeezing depend on the maximal
exerted pressure.