Low Molecular Weight Glass Formers Research Articles

High throughput optimization procedure to characterize vitrification kinetics

We present VITRIFAST, a high throughput optimization procedure to characterize the vitrification kinetics based on calorimetric measurements. By analyzing the temperature dependence of specific heat capacity, the method determines the fictive temperature, Tf, and the enthalpy change during physical aging, ΔH, within only a few seconds. We tested VITRIFAST on the low molecular weight glass-former o−terphenyl (OTP) and on an archetypal glass forming polymer, polystyrene (PS), by analyzing the outcome of two classical sets of experiments. By means of fast scanning calorimetry (FSC), we characterized the vitrification kinetics in a wide range of cooling rates and the isothermal physical aging after vitrification at a given rate. In less than 3 minutes, our method could process 18 different calorimetric scans and provided values of Tf and ΔH in excellent agreement with those reported in the literature. VITRIFAST can be employed in the analysis of the temperature dependence of any type of second order derivative of free energy and represents a tremendous advance in the data analysis of calorimetric scans. The method is particularly helpful for fast scanning calorimetry users, considering the extremely large number of heat capacity scans recorded by this technique within a few minutes.

Breakdown of the isochronal structural (α) and secondary (JG β) exact superpositioning in probucol - A low molecular weight pharmaceutical

In this paper, Broadband Dielectric Spectroscopy (BDS) has been applied to study the molecular dynamics of the two active pharmaceutical ingredients (APIs), probucol (PRO) and droperidol (DRO), above and below the glass transition temperature, at varying thermodynamic conditions. We found that the structural (α)-relaxation process in both compounds is sensitive to compression. It was reflected in the high pressure coefficients of the glass transition temperature, dTg/dp = 227 K/GPa and dTg/dp = 427 K/GPa for DRO and PRO, respectively. In this context, it is worthwhile to emphasize that dTg/dp calculated for the latter API is the highest reported to date for the low molecular weight glass formers. Furthermore, the data collected upon squeezing samples revealed that the pressure dependence of the relaxation times of the Johari Goldstein (JG) β-process in PRO is weaker with respect to that of structural relaxation. Thus, a breakdown of exact superposition of both modes has been noted. A slightly different scenario was observed in DRO, where relaxation times of the secondary non-JG γ-process, obtained at different T and p, plotted versus Tg/T, collapsed forming a single curve. A breakdown of exact isochronal superpositioning of α- and JG β-relaxation times at varying thermodynamic conditions in PRO, that is weakly H-bonded low molecular weight glass former, is in accordance with the data reported for the strongly associating compounds (e.g., sorbitol). Moreover, recently a similar scenario has been observed for some polymers, i.e., 1,4-polybutadiene (PBD) (Ransom et al., 2018), poly(methyl methacrylate) (PMMA) (Casalini and Roland, 2013) and polyisoprene (PI) (Kołodziej et al., 2018). This phenomenon, which is consistent with the approximate invariance of the ratio of these two relaxation times to change of thermodynamics conditions (as predicted by the Coupling Model), was assigned to the increasing torsional rigidity of the macromolecule backbone or alternatively to the non-uniform responses to variations of temperature and pressure of the distributed modes composing the JG β-process, resulting in changes of the spectral shape and different fβ(p,T) obtained from the fitting procedure. One can suppose that due to the variation in the population of weak H-bonds at elevated pressure in PRO, the latter effect might be enhanced, contributing to the observed experimental finding. Finally, the data reported herein emphasize the impact of molecular aspects related to specific interactions on the correlation between α- and JG β-relaxation at varying T and p conditions.

Single Molecule Experiments Reveal the Dynamic Heterogeneity and Exchange Time Scales of Polystyrene near the Glass Transition

In a polymeric material near its glass transition temperature, segmental dynamics of a given spatial region may differ considerably from that of neighboring regions without apparent structural origin, analogous to the supercooled liquid state of low molecular weight glass formers. Given that the supercooled liquid state is a (metastable) equilibrium state, spatial variations in dynamics are expected to average out in time, consistent with ergodicity of the system. By probing the rotations of fluorescent guest molecules, local segmental dynamics of polystyrene was scrutinized molecule by molecule. Two perylene dicarboximide dyes were investigated as potential reporters, and one of these was found to report a substantial proportion of the dynamic heterogeneity of the host polystyrene. Using this probe, we demonstrate that the polystyrene system is ergodic and characterize time scales over which molecules experience changes to their dynamics. We identify a characteristic time scale of exchange much longer th...

Modeling Dynamics of Isotropic Dielectrics in a Laminar Heterogeneous Configuration

The predictive capabilities of models that satisfy the Weiner bounds and Hashin-Shtrikman (HS) bounds were studied for isotropic dielectrics in a laminar heterogeneous configuration oriented perpendicular to the electric field. The dynamics were investigated isothermally using broadband dielectric relaxation spectroscopy (DRS) in the frequency range of 1 MHz to 100 mHz. The molecules chosen for study were low molecular weight glass formers, glycerol, phenyl salicylate, imidazole, and dimethyl sulfoxide, and macromolecules, polymethylhydrosiloxane, polyvinylpyrrolidone-co-vinyl acetate, poly-dl-lactic acid, and poly l-lactic acid. It was found that none of the models were able to adequately predict in entirety the resultant dynamics. Of the models studied, the most successful were the HS upper bound (HSUB), the complementary universal Weiner equation (CWE), and the Lichtenecker model for the dimensional parameter, ζ = -1/2. The least successful models were the upper Weiner bound (UWB), the Neelakantaswamy, Turkman, and Sarkar (NTS) model for ζ = 1/2, and the Lichtenecker model for ζ = 1/2.

Segmental Dynamics of Dilute Poly(ethylene oxide) in Low and High Molecular Weight Glass-Formers

Deuterium NMR at Larmor frequencies of 15.6 and 76.7 MHz was used to study the segmental dynamics of dilute perdeuteriopoly(ethylene oxide) (d4PEO) in equilibrium miscible mixtures with four low molecular weight glass-formers: indomethacin, sucrose benzoate, o-terphenyl, and toluene. Results are compared with literature data for dilute PEO in two polymer hosts: poly(methyl methacrylate) (PMMA) and poly(vinyl acetate) (PVAc). The segmental dynamics of PEO are found to strongly correlate with the dynamics of both high and low molecular weight hosts. For dilute PEO in mixtures with PMMA, PVAc, and sucrose benzoate, the Lodge–McLeish model with a ϕself (self-concentration) of 0.35 gives a good fit to the dynamics of dilute PEO, similar to previously reported dilute polyisoprene blends and dilute polystyrene blends. A smaller ϕself is obtained for dilute PEO in indomethacin, consistent with the presence of a hydrogen-bonding effect. For PEO in o-terphenyl and toluene, the implementation of the Lodge–McLeish model utilized here fails to give quantitative fits for the dynamics of the dilute PEO chains; this result is obtained because the full temperature-dependent host dynamics are not utilized in the fitting procedure.

The temperature dependence of free volume in phenyl salicylate and its relation to structural dynamics: A positron annihilation lifetime and pressure-volume-temperature study

Positron annihilation lifetime spectroscopy (PALS) and pressure-volume-temperature (PVT) experiments were performed to characterize the temperature dependent microstructure of the hole free volume in the low molecular weight glass-former phenyl salicylate (salol). The PALS spectra were analyzed with the new routine LT9.0 and the volume distribution of subnanometer size holes characterized by its mean <v(h)> and standard deviation sigma(h) was calculated. Crystallization of the amorphous sample was observed in the temperature range above 250 K, which leads to a vanishing of the positronium formation. The positronium signal recovered after melting at 303 K. A combination of PALS with PVT data enabled us to calculate the specific density N(h)('), the specific volume V(f), and the fraction of holes f(h) in the amorphous state. From comparison with dielectric measurements in the temperature range above T(B)=265 K, it was found that the primary structural relaxation slows down with temperature, faster than the shrinkage of the hole free volume V(f) would predict, on the basis of the Cohen-Turnbull (CT) free volume theory. CT plots can be linearized by replacing V(f) of the CT theory by (V(f)-DeltaV), where DeltaV is a volume correction term. This was interpreted as indication that the lower wing of the hole size distribution contains holes too small to show a liquidlike behavior in their surroundings. Peculiarities of the relaxation behavior below T(B)=265 K and the possible validity of the Cohen-Grest free volume model are discussed.

Universal critical-like scaling of dynamic properties in symmetry-selected glass formers

Evidence for a possible general validity of the critical-like behavior of dielectric relaxation time or viscosity tau,eta proportional to (T-T(C))(-phi) with phi-->9 and T(C)<T(g) on approaching glass temperature (T(g)) is shown. This universal behavior is found in various systems where the vitrification is dominated by a selected element of symmetry. The supporting evidence was obtained on the basis of the distortion-sensitive, derivative-based analysis of tau(T) data for a rodlike liquid crystalline compound (E7), orientationally disordered crystals (plastic crystals), a colloidal nanofluid system, polymer melt (polystyrene), oligomeric liquid (EPON 828), and low molecular weight glass formers (glycerol, threitol, sorbitol, and 1-propanol). Results presented explain the puzzling experimental artifacts supporting the dynamical scaling model [R. H. Colby, Phys. Rev. E 61, 1783 (2000); B. M. Erwin, R. H. Colby, J. Non-Cryst. Solids 307-310, 225 (2002)]. It is suggested that spin-glass-like systems may be linked to the discussed pattern.

From Simple Liquid to Polymer Melt. Glassy and Polymer Dynamics Studied by Fast Field Cycling NMR Relaxometry: Low and High Molecular Weight Limit

Fast field cycling (FFC) NMR is applied to study the dispersion of the 1H spin−lattice relaxation in the low molecular weight glass-formers o-terphenyl, tristyrene, and oligomeric polybutadiene (PB, with M/gmol−1 = 355 and 466) over a broad temperature range (203−401 K). Differing from previous FFC NMR works, we analyze the relaxation data in the susceptibility form ω/T1(ω), and applying frequency−temperature superposition, master spectra are obtained covering up to 8 decades in frequency. In all cases solely the glassy dynamics (α-process) determines the relaxation behavior, and the Rouse unit is estimated to MR ≅ 500 g/mol. The time constant τα(T) in the range 10−11−10−6 s is extracted, which agrees well with those measured at the same time by dielectric spectroscopy. For the high molecular weight PB (M/gmol−1 = 56 500, 87 000, 314 000, and 817 000) pronounced polymer effects are observed at low frequencies (ωτα ≪ 1) which are isolated from the total spectrum by subtracting the “glass spectrum” as obtai...

“Self-concentration” effects on the dynamics of a polychlorinated biphenyl diluted in 1,4-polybutadiene

The mobility of isolated polychlorinated biphenyl (PCB54) in 1,4-polybutadiene (PB) has been investigated by means of broadband dielectric spectroscopy. The aim was to provide new insights about the effect of the environment on the dynamics of PCB54. The authors' results indicate that PCB54 structural dynamics is neither independent of the PB matrix nor slaved to the matrix itself. The authors interpret these results as a consequence of the limited size of cooperatively rearranging regions (CRRs) involved in PCB54 structural relaxation possessing an effective concentration different from the macroscopic one. This implies a non-negligible influence of "self-concentration," already proven for the component segmental dynamics in polymer blends, also in the relaxation of binary mixtures involving low molecular weight glass formers. This allowed the evaluation of the size of CRR, which was about 1 nm for PCB54 in PB. This means that the cooperativity extends over the first shell around PCB54 molecules.

Neutron reflectivity measurements of the translational motion of tris(naphthylbenzene) at the glass transition temperature

The translational dynamics of the low molecular weight glass-former tris(naphthylbenzene) have been studied on the length scale of a few nanometers at the glass transition temperature Tg. Neutron reflectivity was used to measure isotopic interdiffusion of multilayer samples created by physical vapor deposition. Deposition with the substrate held at Tg-6 K allows observation of dynamics characterizing the equilibrium supercooled liquid. The diffusion coefficient measured at q = 0.03 A(-1) was determined to be 1x10(-17) cm2/s at 342 K (Tg). The self-part of the intermediate scattering function I(s)(q,t) decays exponentially. Samples deposited well below Tg show a substantial thermal history effect during subsequent translational motion at Tg.

Self-Diffusion of Supercooled o-Terphenyl near the Glass Transition Temperature

Self-diffusion coefficients for the low molecular weight glass former o-terphenyl have been measured near Tg by isothermally desorbing thin film bilayers of deuterio and protio o-terphenyl in a vacuum chamber. We observe translational diffusion that is about 100 times faster at Tg + 3 K than the Stokes-Einstein prediction. Predictions from random first order transition theory and a dynamic facilitation approach are in reasonable agreement with our results; in these approaches, enhanced translational diffusion is associated with spatially heterogeneous dynamics. Self-diffusion controls crystallization in o-terphenyl for most of the supercooled liquid regime, but at temperatures below Tg + 10 K, the reported crystallization rate increases suddenly while the self-diffusion coefficient does not. This work and previous work on trisnaphthylbenzene both find a self-diffusion-controlled crystal growth regime and an enhancement in self-diffusion near Tg, suggesting that these phenomena are general characteristics of fragile low molecular weight glass formers. We discuss the width of the relaxation time distributions of o-terphenyl and trisnaphthylbenzene as they relate to the observation of enhanced translational diffusion.

Exchange rates of dynamic heterogeneities in a glass-forming liquid

Dynamic heterogeneities, i.e. the presence of molecules with different mobilities, have been established as one of the key features of the physics of supercooled liquids. Here we study in detail how the mobility of an individual molecule fluctuates with time. Our analysis is based on a time series of molecular dynamics simulations for a low molecular weight glass-former, propylene carbonate. We find that the variation of mobility with time of initially slow molecules significantly differs from that of initially fast molecules. We explicitly show the relation to the rate memory parameter which recently has been introduced to quantify the mobility fluctuations as observed via multidimensional NMR experiments. In this way qualitative agreement between simulation and experiment can be established although the time scales of simulation and NMR experiment differ by many orders of magnitude.

A Phenomenological Model of Volumetric and Free Volume Hole Properties in Supercooled Liquids: Theortho-Terphenyl Case

A phenomenological model linking the macroscopic volume from dilatometry with the microscopic free volume from positron annihilation lifetime spectroscopy is presented. The temperature dependences of both quantities enable to obtain the so-called initial temperature Ti at which dynamic free volume change sets in, the corresponding initial volume Vi, and subsequently, all free volume characteristics. Application of this model on the literature data of a typical low-molecular weight glass former, o-terphenyl, leads to the findings that (i) Ti ≅ TK the Kauzmann temperature and (ii) a packing fraction of the extrapolated equilibrium liquid at Ti is close to the closest regular packing of hard spheres. The published viscosity−temperature data over 12 decades of magnitude are correlated with the free volume hole fraction fh, as determined from the PALS results, via the WLF−Doolittle type equation. These findings indicate that the orthopositronium (o-Ps) is able to detect the part of the total free volume distribution which is relevant for the viscosity over the measured temperature interval from Tg up to ∼ 1.5Tg. In addition, the change of the free volume hole behavior above Tr = 1.25Tg, lying in the vicinity of the crossover temperature Tc from the idealized version of mode-coupling theory, is critically analyzed within a modified bubble concept (the equilibrium interaction of o-Ps with the soft matrix).

On the origin of dynamic heterogeneities in glass-forming liquids

We employ classical Molecular Dynamics simulations to study the cooperative dynamics of two low molecular weight glass-formers, propylene carbonate and salol. The length scales of dynamic heterogeneities are estimated. After appropriate scaling both glass-formers display a similar temperature dependence of this length scale. Local structural properties like density, regularity, and potential energy are correlated with different local dynamical observables. We find that the dynamic heterogeneities are most strongly related to the local potential energy. To obtain an optimum correlation the local dynamics has to be characterized by the residence time.

Shear modulus and compliance in the range of the dynamic glass transition for metallic glasses

The dynamic shear modulus and compliance of the dynamic glass transition were investigated in two metallic glasses (Pd40Ni40P20 and Zr65Al7.5Cu17.5Ni10). Isothermal measurements over four frequency decades enable the determination of shape parameters for the proper glass transition. The real part of the dynamic shear compliance shows a step height of approximately a factor of 2.6 for Pd40Ni40P20, comparable to low molecular weight glass formers or the segmental mode as part of the glass transition in high molecular weight polymers. The frequency dependence of the imaginary part of the shear compliance at the high frequency side of the peak shows an exponent near the Andrade value of 1/3.

Spatial heterogeneity in glassy polystyrene detected by deuteron NMR relaxation

Using deuteron NMR, the dynamics of supercooled polystyrene-d3 was investigated near the calorimetric glass transition. At these temperatures non-exponential spin lattice relaxation is found, indicating the presence of spatial heterogeneity. With increasing temperature, structural relaxation becomes fast enough to average efficiently over different spatial environments, leading to exponential magnetization decays. A qualitative comparison with toluene as a representative of a low molecular weight glass former is carried out. Indications are found that in polystyrene the observed averaging process is more effective at Tg than it is in toluene.

Light scattering studies on long‐range density fluctuations in a glass‐forming polymer

AbstractStatic and dynamic light scattering measurements were performed on a glass‐forming polymer, poly(methyl‐p‐tolylsiloxane) (PMpTS; Tg=−17°C), in a temperature range of 30°C to 120°C. It was found that long‐range density fluctuations, so‐called clusters, are observed in this polymer. They are characterized by “excess” angular‐dependent isotropic scattered intensity and additional slow decay in the photon correlation function. These properties are generally observed in low molecular weight glass formers as well as in glass‐forming polymers. In addition, we found that the clusters completely disappear above about 90°C. The kinetics of the cluster growth was also studied by temperature jump experiments. During the cluster formation process after the temperature jump, we measured the angular‐dependent polarized scattered intensities, and the photon correlation function as a function of annealing time. We discuss the cluster growth kinetics based on these results, especially the time evolution of the correlation length ξOZ evaluated using the Ornstein‐Zernike formula, the isotropic scattering intensity IOZ(0) at q=0, and the correlation time τcl of the cluster decay. Finally, we briefly discuss some properties of the equilibrated clusters, which are well reproducible.

Universal properties of polymer networks

The use of thermodynamics of irreversible processes allows one to describe time dependent deformation phenomena of polymer networks. It is shown that linear relaxation modes are coupled to the network ("global level"). The constitutive van der Waals equation is characterized by a strain, strain-type and a strain-rate independent memory function. Stress-strain cycles in the glass-transition regime can be described.The dynamics of networks can be characterized by introducing the concept of conformons. Conformons are shown to be Bosons with a photon statistics. Conformons in equilibrium have a self-energy- and self-entropy-term. They can be excited or emitted. Conformons also decay spontaneously. In the stationary limit a broad conformon spectrum results which is typical for glass forming liquids (including polymers). Relaxation-and retardation-time spectra are shown to be related to the conformon spectrum. Mechanical and dielectric relaxation measurements (even of low molecular weight glass formers) are fairly well reproduced. The dynamical structure factors of the incoherent neutron scattering can also be computed.