Study of free volume in high-vinyl polybutadiene/cis-polyisoprene blends using positron annihilation spectroscopy

Abstract

High‐Vinyl Polybutadiene (HVBD)/cis‐Polyisoprene (CPI) blends were characterized by Differential Scanning Calorimetry (DSC) and Positron Annihilation Lifetime Spectroscopy (PALS). A single DSC glass transition temperature Tg is observed, whose composition dependence strongly deviates from additivity, and shows an apparent cusp when the weight fraction of HVBD ≈ 0.75. The free‐volume hole size, Vh, and the scaled fractional free volume, hps/C, = I3Vh were determined by PALS from the orthopositronium (o‐Ps) intensities, I3, and lifetimes, τ3, over a temperature range encompassing Tg and the temperature at which “positronium bubble” formation occurs. In the glass, Vh and hps/C are smaller for CPI than for HVBD, but the thermal expansion coefficient for hole volume, αf, is larger in the melt for CPI than for HVBD; thus, an iso‐hole volume temperature occurs in these blends at Tiso ≈ −34°C. Above and below Tiso, Vh and hps/C each show a negative departure from additivity. A quantitative interpretation of the cusp in the composition dependence of Tg can be obtained, via a modified analysis of Kovacs, using free‐volume quantities from PALS, with the ratio of scaling constants CCPI/CHVBD as an adjustable parameter. At high temperatures, the positron bubble size is smaller in CPI than in HVBD. This agrees with the observation that the thermal expansivity of hole volume, and, hence the internal pressure are larger in the equilibrium melt of CPI. The effect of e+‐irradiation on the o‐Ps intensity was investigated. I3 decreases more rapidly in the melt as T → Tg, and then more slowly in the glass, suggesting that the effect is due to trapping of radical or ionic species which inhibit o‐Ps formation. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 861–871, 1998

Positron Annihilation Lifetime Studies of Free Volume in Heterogeneous Polymer Systems

“…Blends of a high-vinyl polybutadiene and cis-polyisoprene (HVBD/CPI) were characterized by differential scanning calorimetry (DSC) and PALS [Peng et al, 1998]. A single DSC glass transition temperature T g value was observed, defined as the temperature at which a maximum was observed in the temperature derivative of the heat flow, d(dH/dt)/dT, whose composition dependence, shown in Figure 12.3, deviates strongly from additivity and shows an apparent cusp when the weight fraction of HVBD ≈ 0.75.…”

Differential scanning calorimetric and free volume study of reactive compatibilization by EPM‐g‐MA of poly(trimethylene terephthalate)/EPDM blends

“…This shift reduces sometimes, as the weight percentage of one of the components was increased. Such blends were termed as partially miscible blends 24, 28. The DSC scans for the present blends (Figs.…”

Positron Annihilation Lifetime Study of High and Low Free Volume Glassy Polymers: Effects of Free Volume Sizes on the Permeability and Permselectivity

“…It has been assumed that intensities I 3 and I 4 can be used as the measures of the concentrations of free volume of different size. Indeed, an empirical equation with adjustable (scaling) parameter C (Å -3 ) has been proposed and widely used − for calculating the fractional free volume in rubbery and conventional glassy polymers that have three component PAL spectrum where v f3 (Å 3 ) is the volume of spherical FFV calculated using the radius of eq 4 found from τ 3 , I 3 is the intensity of the single Ps component of the PAL spectrum, and C is the empirically determined constant found by comparison of the PAL data with independent free volume values estimated via bulk volume−temperature data. ,, …”