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.…”

“…, with (f 2g − f 1g )/α f1 = 7.5) (dotted-dashed line), using free-volume parameters obtained from PALS data, with scaling constant ratio C 2 /C 1 = 0.67; the solid curve is the best fit to the Gordon-Taylor equation, with a fitting constant k = 0.36. (Adapted from Peng et al [1998].) It is clear that an accurate description of the composition dependence of T g , shown in Figure 12.3, cannot be achieved using the Fox equation [Eq.…”

“…Such a phenomenon appears to have been observed experimentally [Jamil and Jamieson, 1989;Chung et al, 1994] and seems to arise because, in a miscible blend, due to local concentration fluctuations, the segmental dynamics of two polymers in a miscible blend can have quite different temperature dependences, particularly when the component glass transition temperatures are very different [Chung et al, 1994;Lodge and McLeish, 2000;Kumar et al, 2007]. Associated with this local heterogeneity, one expects a distinct broadening of the glass transition near the cusp composition, which is indeed evident in DSC analysis of HVBD-rich blends [Peng et al, 1998]. …”

Application to Polymers

“…PAL has been used to study both miscible and immiscible polymer blends [41,61,[67][68][69][70], PAL results have shown both positive and negative deviations from additivity of free volume with blend composition. In the case of multi phase systems, PAL data analysis is complicated by the fact that Ps may diffuse between the different blend phases.…”

Free volume dilatation in polymers by ortho-positronium

“…[14][15][16][17][18][19][20][21][22] A major increase of slope at low temperatures reflects the transition of the polymer from a glassy to a rubbery state (at the glass transition temperature T g ) and is accompanied by an increase of the expansion coefficient of the macroscopic volume. At temperatures above approximately 1.2-1.6 × T g , a clear decrease in slope may be observed.…”