The Free Volume and Its Recovery in Pressure‐Densified and CO₂‐Swollen Heterocyclic‐Ring‐Containing Fluoropolymers

Abstract

Changes in the free volume in CYTOP due to pressure densification and swelling with CO2 have been examined using PALS and PVT experiments. Employing the Simha‐Somcynsky equation of state the specific hole free, Vf, and occupied, Vocc, volumes were estimated. The change in the total volume due to the pre‐treatments occurs exclusively in the hole free volume and the relative change in Vf is one order of magnitude larger than in the total volume. Vocc shows no memory for the history of the polymer glass. The mean and width of the size distribution of subnanometre holes in the glassy state decrease upon densification and increase upon swelling. The volume changes which are frozen in the polymer glass begin to recover at temperatures distinctly below Tg.magnified image

Gas Pressure Induced Porosity of CYTOP Polymer. A~Positron Study

“…In the range from 25 MPa down to zero, the lifetime τ 4 , being the main characteristics of the natural free volumes in the polymer structure, is longer than in the run with rising pressure; the pressure introduces a permanent damage of the polymer structure. The same eect was reported by Dlubek et al [7]. It is interesting that the lifetime of the main natural o-Ps component, τ 4 , is almost insensitive to the applied pressure (at least in the range below 120 MPa; at 320 MPa it diminishes to 3.4 ns).…”

“…Like in many other polymers, swelling of free volumes after the exposure to CO 2 at the pressure up to 5 MPa was observed in the whole range of temperatures below the glass transition point [7].…”

“…[7,8], the CYTOP spectra without gas intrusion were analyzed assuming the presence of one component belonging to o-Ps but with the lifetime distribution of logGaussian shape. However, the MELT processing shows that the o-Ps lifetimes in the samples without previous treatment by high pressure represent not a single broad component, but two distinctly dierent, quasi-discrete ones, ≈ 1.5 ns and ≈ 4 ns.…”

Positronium as a probe of small free volumes in crystals, polymers and porous media

“…At various polymer treatments the majority of volume change occurs just in "free spaces". At gas swelling of the polycarbonate (see below) the total sample volume increases by 0.95%, while the free volume -by 14.3% [150] . The "hole fraction" h = V f /V (V f -the total free volume, V -the sample volume) lies in the range from 0.06 to 0.28.…”

“…The increase of the free volume holes can be made by exposing the sample to a gas atmosphere of several MPa at room temperature and then decreasing the pressure at very low rate (rapid depressurization leads to another interesting effect of polymer foaming). Particularly effective from this point of view is the sorption of CO 2 , swelling and plasticizing the polymer [168][169][170][171]. If a polymer is in its rubbery state, at application of high pressure of CO 2 the lifetime and intensity rise, but after gas evacuation these parameters return to their initial values.…”