Josef Bartoš

Free volume factor in supercooled liquid dynamics

The positronium (o-Ps) annihilation lifetime parameters, i.e., the mean o-Ps lifetime, s3, and the relative o-Ps intensity, I3, were measured for a series of molecular and structurally simple polymer glass formers over wide temperature ranges by means of positron annihilation lifetime spectroscopy (PALS). In addition to the most pronounced effect defining the PALS glass temperature, T PALS g , further two groups of the characteristic bend temperatures in the liquid state, one at T lq b1 ffi 1:15–1:4T PALS g and the other at T lq b2 ffi 1:3–1:75T PALS g were found. These characteristic temperatures were compared with certain crossover temperatures from both empirical ðTg; Tm; TB; TA; Tcoll; TMÞ and theoretical ðTcrÞ analyses of the thermodynamic and numerous dynamic data and their coincidences were discussed. Decrease in I3–T plots for glycerol and cis-trans-1,4-polybutadiene can be related to the fast motions on the nanosecond time scale as detected by neutron scattering or dielectric spectroscopy. The corresponding free volume hole characteristics, i.e., the mean free volume hole size, Rh, and the free volume hole fraction, fh, were determined by using a quantum-mechanical model of o-Ps in spherical hole or a model of volumetric and free volume hole properties, respectively. Consequently, the role of the free volume hole fraction from the PALS data in the dynamics of some supercooled liquids was established by analyses of various expressions containing the free volume fraction such as the Doolittle-type equation or the Macedo–Litovitz ansatz. 2002 Elsevier Science B.V. All rights reserved.

Free volume microstructure of tetramethylpolycarbonate at low temperatures studied by positron annihilation lifetime spectroscopy: a comparison with polycarbonate

The annihilation of positrons in amorphous tetramethylpolycarbonate (TMPC) has been investigated in the temperature range from 30 to 300 K. The dependences of the mean lifetime of the ortho-positronium τ3 and its relative intensity I3 on temperature can be described by the empirical relations: τ3 = 2.08(1 + 4.8 × 10−4 T) (ns) and I3 = 20.2(1 + 7.9 × 10−4T) (%). These dependencies are interpreted within the framework of the microstructural free volume concept. The comparison with the polycarbonate (PC) reveals that both values τ3 and I3 are larger for TMPC, indicating larger free volume realized through higher number (concentration) of larger free volume entities. The temperature dependencies of τ3 and I3 for TMPC exhibit a different character and a weaker dependence on the temperature. This different behaviour of the free volume is due to a different packing efficiency of chains and a different dominant mechanism of the free volume generation, as well as a decrease in the flip dynamics of the phenyl rings. The implications of different free volume microstructures on some mechanical properties (e.g. relaxation and fracture) as well as on the transport properties of gases in TMPC and PC are discussed.

Positron annihilation and relaxation dynamics from dielectric spectroscopy and nuclear magnetic resonance: cis-trans-1,4-poly(butadiene).

We report a joint analysis of positron annihilation lifetime spectroscopy (PALS), dielectric spectroscopy (BDS), and nuclear magnetic resonance (NMR) on cis-trans-1,4-poly(butadiene) (c-t-1,4-PBD). Phenomenological analysis of the orthopositronium lifetime τ(3)-T dependence by linear fitting reveals four characteristic PALS temperatures: T(b1)(G)=0.63T(g)(PALS), T(g)(PALS), T(b1)(L)=1.22T(g)(PALS), and T(b2)(L)=1.52T(g)(PALS). Slight bend effects in the glassy and supercooled liquid states are related to the fast or slow secondary β process, from neutron scattering, respectively, the latter being connected with the trans-isomers. In addition, the first bend effect in the supercooled liquid coincides with a deviation of the slow effective secondary β(eff) relaxation related to the cis-isomers from low-T Arrhenius behavior to non-Arrhenius one and correlates with the onset of the primary α process from BDS. The second plateau effect in the liquid state occurs when τ(3) becomes commensurable with the structural relaxation time τ(α)(T(b2)). It is also approximately related to its crossover from non-Arrhenius to Arrhenius regime in the combined BDS and NMR data. Finally, the combined BDS and NMR structural relaxation data, when analyzed in terms of the two-order parameter (TOP) model, suggest the influence of solidlike domains on both the annihilation behavior and the local and segmental chain mobility in the supercooled liquid. All these findings indicate the influence of the dynamic heterogeneity in both the primary and secondary relaxations due to the cis-trans isomerism in c-t-1,4-PBD and their impact into the PALS response.

Free volume aspects of the strong-fragile classification of polymer liquids

Abstract The strong–fragile classification or, alternatively, the cooperativity concept of polymer liquid dynamics was tested from a free volume point of view by positron annihilation lifetime spectroscopy. For a series of six polymers we found that the expansivity of the mean free volume hole size correlates with the fragility as well as with the non-exponentiality. These tendencies support the original interpretational proposals.

Positron annihilation and relaxation dynamics from dielectric spectroscopy: poly(vinylmethylether)

We report on the temperature dependence of the lifetime of the ortho-positronium (o-Ps), τ(3), annihilation in amorphous polymer poly(vinylmethylether) (PVME) from positron annihilation lifetime spectroscopy (PALS). We show that the behavior of τ(3)(T) can be divided into five regions, each of them having a linear temperature dependence, and that the crossover PALS temperatures situated at T(b1)(G), 0.76T(g)(PALS), T(b1)(L) = 1.14T(g)(PALS) and T(b2)(L) = 1.37T(G)(PALS), and marking the discontinuity in the free volume microstructure are related to various dynamic features from neutron scattering (NS) and broadband dielectric spectroscopy (BDS). First, a slight change in the PALS response in the glassy PVME at T(b1)(G) is related to the onset of the excess wing in an apparent correspondence with the fast secondary β motion from NS. A further slight bend in the liquid state at T(b1)(L) is related to a high-frequency tail of the primary α process as well as to the slow secondary β relaxation from BDS. In addition, it lies also in the vicinity of the crossover temperature, T(B)(βKWW), in the spectral dispersion of the primary α process, indicating a connection of the change in the o-Ps lifetime with the variation in the width of the primary α relaxation times distribution. Finally, the τ(3) value at T(b2)(L) is close to the mean relaxation time of the primary α process, τ(α), in coincidence with the crossover in the secondary effective β process between two regimes in the liquid PVME. All these relationships point to very close connections between the PALS response and the dynamic behavior of PVME, which can be explained in terms of the temperature dependence of the probability function of the liquid-like and the solid-like domains, as obtained from the two-order parameter (TOP) model description of the liquid to glass transition in glass-formers.

Positron annihilation response and broadband dielectric spectroscopy: Salol

A phenomenological analysis of the ortho-positronium (o-Ps) annihilation from positron annihilation lifetime spectroscopy (PALS) and the dynamics from broadband dielectric spectroscopy (BDS) are reported on a small molecular glass former of intermediate H-bonding and fragility: salol. The dielectric spectra extend over a very broad frequency range of about 2 × 10−2−3.5 × 1011 Hz, providing information on the α-relaxation, the secondary relaxation giving rise to the excess wing, and the shallow high-frequency minimum in the micro- to milli-meter wave range. A number of empirical correlations between the o-Ps lifetime, τ3(T), and the various spectral and relaxation features have been observed. Thus, the phenomenological evaluation of the τ3(T) dependence of the PALS response of the amorphous sample reveals three characteristic PALS temperatures: TgPALS, Tb1L = 1.15TgPALS and Tb2L = 1.25TgPALS, which are discussed in relation to similar findings for some typical small molecular vdW- and H-bonded glass formers. A slighter change of the slope at Tb1L appears to be related to the transition from excess wing to the primary α-process-dominated behavior, with the secondary process dominating in the deeply supercooled liquid state below Tb1L. The high-temperature plateau effect in the τ3(T) plot occurs at Tb2L and agrees with the characteristic Stickel temperature, TBST, marking a qualitative change of the primary α process, but it does not follow the relation Tb2L < Tα [τ3(Tb2) < τα]. Both effects at Tb1L and Tb2L correlate with two crossovers in the spectral shape and related non-exponentiality parameter of the structural relaxation, βKWW. Finally, the application of the two-order parameter (TOP) model to the structural relaxation as represented by the primary α relaxation times from BDS leads to the characteristic TOP temperature, Tmc, close to Tb1 from PALS. Within this model the phenomenological interpretation is offered based on changes in the probability of occurrence of solid-like and liquid-like domains to explain the dynamic as well as PALS responses. In summary, all the empirical correlations support further very close connections between the PALS response and the dielectric relaxation behavior in small molecule glass formers.

Free volume from the positron annihilation lifetime spectroscopy method and its relationships with the various microscopic and macroscopic dynamic properties of ortho-terphenyl

A phenomenological model linking the macroscopic volumetric behaviour found from dilatometry to the microscopic free-volume hole properties found from positron annihilation lifetime spectroscopy is applied to a set of dynamic properties of a fragile low-molecular-weight glass former: ortho-terphenyl. A number of correlations between the free-volume hole fractions derived from this model and the various dynamic properties such as viscosity, self-rotational diffusion, and tracer translational diffusion can be established over the temperature range from the glass-liquid transition temperature up to via a Williams-Landel-Ferry (WLF)-Doolittle-type equation.

Molecular probe dynamics and free volume in organic glass-formers and their relationships to structural relaxation: 1-propanol.

A joint study of the rotational dynamics and free volume in amorphous 1-propanol (1-PrOH) as a prototypical monohydroxy alcohol by electron spin resonance (ESR) or positron annihilation lifetime spectroscopy (PALS), respectively, is reported. The dynamic parameters of the molecular spin probe 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and the annihilation ones of the atomic ortho-positronium (o-Ps) probe as a function of temperature are compared. A number of coincidences between various effects in the ESR and PALS responses at the corresponding characteristic ESR and PALS temperatures were found suggesting a common origin of the underlying dynamic processes that were identified using viscosity (VISC) in terms of the two-order parameter (TOP) model and broadband dielectric spectroscopy (BDS) data.

Study of the influence of physical ageing and rejuvenation on macroradical reactivity in polycarbonate

Abstract Amorphous polycarbonate with various mechanical and/or thermal histories was investigated via macroradical decay in the temperature interval between −30°C and 100°C by e.p.r. spectroscopy. The decay of end macroradicals was found to take place at low temperatures more rapidly in anisotropic drawn samples than in isotropic materials. The rate constants of both samples show breaks at ∼20°C and 70°C and coincide at higher temperatures. After heating in the region below the glass transition temperature, the order of the rate constants is maintained but the decay is more rapid in aged samples compared to unaged materials. The concentration and kinetic characteristics are correlated with previous small-angle X-ray scattering results and the dynamic-mechanical relaxation results presented here and interpreted in terms of changes in free volume characteristics after cold deformation and/or annealing of polycarbonate.

Relationships between Positron Lifetime and Dynamics of Polymers

In this work positron annihilation lifetime spectroscopy (PALS) is employed to study the ortho-Positronium lifetime parameters τ3>, σ3 and I3 in two structurally simple amorphous polymers 1,2-poly(butadiene) and cis-1,4-poly(isoprene). The ortho-Positronium mean lifetime parameter τ3> is compared with dynamics data from broadband dielectric spectroscopy (BDS) and electron spin resonance (ESR) experiments. Coincidences of characteristic temperatures from PALS with BDS and ESR experiments show close relationships between positron lifetime and dynamics of polymers.