Bulk Glass Transition Temperature Research Articles

Nanoscopically Confined 1,4-Polybutadiene Melts: Exploring Confinement by Alumina Nanorod and Nanopore Systems.

We present molecular dynamics simulations of a chemically realistic model of 1,4-polybutadiene (PBD) in contact with curved alumina surfaces. We contrast the behavior of PBD infiltrated into alumina pores with a curvature radius of about three times the radius of gyration of the chains to its behavior next to a melt dispersed alumina rod of equal absolute curvature. These confinement types represent situations occurring in polymer melts loaded with nanoparticles due to nanoparticle aggregation. While there are observable differences in structure and dynamics due to the different types of geometric confinement, the main effects stem from the strong attraction of PBD to the alumina surfaces. This strong attraction leads to a deformation of the chains in contact to the surfaces. We focus on temperatures well above the bulk glass transition temperature, but even at these high temperatures, the layers next to the alumina surfaces show glass-like relaxation behavior. We analyze the signature of this glassy behavior for neutron scattering or nuclear magnetic resonances experiments.

Statistical aspects of diffusive adhesion between polymers with drastically different molecular mobility: Weibull’s and normal probability analysis

ABSTRACT A comprehensive statistical analysis of the distributions of the lap-shear strength (σ) originated during the contact of the specimens of dissimilar miscible entangled amorphous polymers with drastically different bulk glass transition temperatures (T g b) has been carried out in the vicinity of the T g b of one polymer but well below the T g b of the other. For this purpose, polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) were chosen. To form mechanically durable PS-PPO adhesive joints (AJ) via the chain interdiffusion mechanism, the specimens of PS (T g b = 103°C) and PPO (T g b = 216°C) were kept in contact for a period of time (t) of 2 min to 96 h at a constant bonding temperature (T) in the vicinity of the PS T g b but, at the same time, well below (even by 126°C) the PPO T g b. The AJs formed were shear fractured in tension at ambient temperature. Totally, 21 σ datasets measured have been analyzed using Weibull’s and normal probability models. By using proposed two dimensionless statistical parameters, the reciprocal Weibull’s modulus (1/m) and the standard deviation (SD) reduced to the average σ value (σ av), SD/σ av, the data scatter estimated in the two methods have been compared.

Two glass transitions in substrate-supported polymer ultrathin films: A molecular dynamics simulation

Broadening of glass transition and even two glass transitions in polymer ultrathin films were repeatedly reported in experiments. The glass transition and dynamic gradient in substrate-supported polymer films are studied by performing molecular dynamics simulations in this work. Two glass transitions are observed in ultrathin films with sufficiently strong attraction of the substrate. Due to the strong attraction of the substrate and spatial confinement of the ultrathin film, the adsorbed chains adopt a flattened conformation with slow dynamics, resulting in a pronounced dynamic gradient across the ultrathin film. The dynamics of chains near the free surface are suppressed by the adsorbed chain as the tail of a few of adsorbed chains is longer than the film thickness, therefore two glass transitions occur at higher temperatures than the bulk glass transition temperature. However, only a single glass transition is observed when the substrate attraction weakens or the film thickness increases.

Measurement of the depth-dependent local dynamics in thin polymer films through rejuvenation of ultrastable glasses

The depth dependence of structural relaxation dynamics is a key part of understanding thin glassy films. Despite this importance and decades of research, a method to provide this information has proved elusive. We measure the isothermal rejuvenation of stable glass films of poly(styrene), and demonstrate that the propagation of the front responsible for the transformation to a supercooled-liquid state serves as a highly localized probe of the local dynamics of the supercooled liquid. We use this connection to probe the depth-dependent relaxation rate with nanometric precision for a series of polystyrene films over a range of temperatures near the bulk glass transition temperature. The analysis shows the spatial extent of enhanced surface mobility and reveals the existence of an unexpected large dynamical length scale in the system. The results are compared with the cooperative-string model for glassy dynamics. The data reveals that the film-thickness dependence of whole film properties arises mainly from the volume fraction of the near-surface region. While the dynamics farthest from the free surface shows the expected bulk-like temperature dependence, the dynamics in the near-surface region shows very little dependence on temperature. This technique can be used in a broad range of thin film materials to gain previously unattainable information about localized structural relaxation.

Gas phase product evolution during high temperature pyrolysis of PTFE: Development of ReaxFF simulation protocol

The formation of products of incomplete destruction (PIDs) from fluoropolymer incineration is poorly understood and it is imperative to environmental impact studies. The lack of analytical standards limits the experimental approaches targeting product analysis. To navigate this challenge, computational modeling of the thermal degradation of fluoropolymers provides simulated product distributions. However, it is essential to benchmark reactive forcefields to accurately simulate fluoropolymer pyrolysis. The present work describes a protocol to perform accurate simulations of the thermal degradation of fluoropolymers to probe the PIDs. The ReaxFF force field was applied to reproduce the experimental bulk density and glass transition temperature of polytetrafluoroethylene (PTFE). The benchmarked methodology developed has been extended to provide simulated product distributions and mechanistic insights which are in excellent agreement with primary literature. On the basis of our simulated data, we observe a degradation mechanism that proceeds through three primary steps: 1) initiation of random backbone cleavage, 2) C2F4 unzipping through β–scission (as opposed to CF2 unzipping), and 3) secondary product formation. An extension of the developed protocol has the potential to simulate the thermal degradation of non-polymeric per- and polyfluoroalkyl substances (PFASs) in addition to long-chain fluoropolymers.

On the temperature dependence of enzymatic degradation of poly(ethylene terephthalate)

Enzymatic recycling of poly(ethylene terephthalate), PET, has attracted significant attention in recent years. Temperature is a governing factor in the enzymatic degradation of PET, influencing simultaneously the catalytic activity and thermal stability of enzymes, as well as the biodegradability of PET materials from many perspectives. Here we present a detailed examination of the complex and mutual effects of temperature on the degradation of low-crystallinity PET (LC-PET, 7.6%) and high-crystallinity PET (HC-PET, 30%) microparticles using the WCCG variant of the leaf-branch compost cutinase (LCC). The degradation velocity apparently increases exponentially with increasing temperature at temperatures below 65 °C. Arrhenius plots show a sudden reduction in activation energy at temperatures higher than 40 °C, suggesting the onset of the surface glass transition of PET particles. This is more than 20 °C lower than the bulk glass transition temperature, underscoring the interfacial catalytic nature of enzymatic PET degradation. WCCG undergoes substantial conformational changes upon thermal incubation at temperatures ranging from 50 to 70 °C and exhibits enhanced activity, owing to the increased intrinsic catalytic activity and improved adsorption on PET surface. Further increasing the temperature leads to the inactivation of enzymes alongside the rapid recrystallization of amorphous PET, impeding the enzymatic degradation. These findings offer a detailed mechanistic understanding of the temperature dependence of the enzymatic degradation of PET, and may have implications for the engineering of more powerful PET hydrolases and the selection of favorable conditions for industrially-related recycling processes.

In Situ Real-Time Atomic Force Microscopy Observations of Chain Mobility at Polymer/Water Interfaces of Poly(methyl methacrylate), Poly(2-hydroxyethyl methacrylate), and Poly(2-methoxyethyl methacrylate) Films in Water.

Polymer materials are widely used in water or in contact with an aqueous environment. However, evaluating the chain mobility, a crucial parameter, at a polymer-water interface is challenging. In this study, we, for the first time, observed poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), and poly(2-methoxyethyl methacrylate) (PMEMA) film surfaces in water via in situ real-time atomic force microscopy (AFM) in tapping mode and quantified the chain mobility. The average displacement between adjacent images (nm/8.75 min) was evaluated using particle image velocimetry. The displacement of PMMA, which has a high bulk glass-transition temperature (Tg) (108 °C) and exhibits limited water absorption, was low both in air (0.54 nm/8.75 min) and water (0.86), while PHEMA, which has a high bulk Tg (99 °C) and exhibits high water absorption, exhibited low mobility in air (0.40) but two orders of magnitude higher mobility in water (60). PMEMA, which has a low bulk Tg (14 °C) and exhibits limited water absorption, already started to move in air (4.5), and its mobility moderately increased in water (20). These behaviors were reasonable, considering the bulk Tg and water absorption characteristics of the polymers. Further, the chain mobility in water was compared with that of dried samples at high temperatures in air. The mobility of PMMA, PHEMA, and PMEMA in water corresponded to that of the dried samples observed in air below the surface Tg (97 °C) for PMMA, at ∼125 °C for PHEMA, and at ∼35 °C for PMEMA. In situ real-time AFM analysis of polymer materials in water is an effective method for evaluating the chain mobility at the polymer/water interface.

Surface Glass Transition Temperature Region of Diarylethene Films Determined by Nano-Marangoni Effect.

For the last two decades, research has addressed whether the glass transition temperature and the molecular motions on the surface of organic films are significantly different from those inside the bulk glasses. It is reported that the surface of the photochromic diarylethene film prepared by vacuum deposition has fluidity and the vacuum deposition of small amount of rubrene molecules induces surface tension fluctuations, generating dents due to the Marangoni flow in nanoscale. The depth of the dents increases in proportion to these radii for the colorless diarylethene film with a bulk glass transition temperature (Tg) close to room temperature. On the other hand, in the colored diarylethene obtained by UV irradiation to the colorless film, the depth becomes constant at a certain level. The Tg distribution in the depth direction is clarified based on an analysis of the dent depth. By approximating the obtained Tg depth distribution with an exponential function, the outermost surface Tg is about 100 K lower than the bulk Tg in the case of photoisomerized diarylethene.

Evolution of Statistical Strength during the Contact of Amorphous Polymer Specimens below the Glass Transition Temperature: Influence of Chain Length.

A comprehensive study of the statistical distribution of the auto-adhesion lap-shear strength (σ) of amorphous polymer-polymer interfaces using various types of statistical tests and models is a useful approach aimed at a better understanding of the mechanisms of the self-healing interface. In the present work, this approach has been applied, for the first time, to a temperature (T) range below the bulk glass transition temperature (Tgbulk). The interest of this T range consists in a very limited or even frozen translational segmental motion giving little or no chance for adhesion to occur. To clarify this issue, the two identical samples of entangled amorphous polystyrene (PS) with a molecular weight (M) of 105 g/mol or 106 g/mol were kept in contact at T = Tgbulk - 33 °C for one day. The as-self-bonded PS-PS auto-adhesive joints (AJ) of PSs differing in M by an order of magnitude were fractured at ambient temperature, and their σ distributions were analyzed using the Weibull model, the quantile-quantile plots, the normality tests, and the Gaussian distribution. It has been shown that the Weibull model most correctly describes the σ statistical distributions of the two self-bonded PS-PS AJs with different M due to the joints' brittleness. The values of the Weibull modulus (a statistical parameter) m = 2.40 and 1.89 calculated for PSs with M = 105 and 106 g/mol, respectively, were rather close, indicating that the chain length has a minor effect on the σ data scatter. The Gaussian distribution has been found to be less appropriate for this purpose, though all the normality tests performed have predicted the correctness of the normal distribution for these PS-PS interfaces.

Surface Enhancement of Crystal Nucleation in Amorphous Acetaminophen

Crystal nucleation rates have been measured in the bulk and at the surface of acetaminophen melt. Of its six known polymorphs, Form III nucleates in the temperature range investigated (290–333 K), both in the bulk and at the surface. Nucleation is the fastest near 318 K (about 20 K above the bulk glass transition temperature Tg) at a rate of 3 × 106 s–1 m–3 in the bulk and 200 s–1 m–2 on the surface. On the per-molecule basis, surface nucleation outpaces bulk nucleation by 5 orders of magnitude, highlighting the importance of the liquid/vapor interface in crystal nucleation. In contrast to its strong effect on nucleation, the free surface has a weak effect on crystal growth. The observed nucleation kinetics is well described by the Classical Nucleation Theory (CNT) if the crystal growth rate is used to represent the kinetic barrier. Our finding is relevant for understanding the physical stability of amorphous materials for applications in drug delivery and electronics.

Optical constants of the as-prepared and annealed (Se80Te20)94Ag6/PMMA thin films

Thin films of (Se80Te20)94Ag6/PMMA have been deposited on glass substrate by thermal co-evaporation technique. Melt quenching approach has been adopted to prepared bulk (Se80Te20)94Ag6 chalcogenide glass. Glass transition temperature of bulk (Se80Te20)94Ag6 glass and poly (methyl methacrylate) (PMMA) polymer have been found out by differential scanning calorimetry (DSC) at a heating rate of 10 K/min. Thin films were annealed above glass transition temperatures of glass (332 K) and polymer (340 K) at two different temperatures 343 K and 365 K. The presence of different phases in annealed film was confirmed by X-ray diffraction studies. Transmission spectra of as-deposited and annealed films were obtained by Uv-Vis spectroscopy in the wavelength range of 350-900 nm. To analyze refractive index dispersion Wemple-Didomenico model have been used. Optical parameters viz refractive index (n), extinction coefficient (k), real and imaginary dielectric constants (ε′ and ε′′) and optical band gap (Eg) are calculated by using optical transmittance. The increasing behavior of the optical band gap with increase in annealing temperature is explained on the basis of density of state model.

Collective Nanoparticle Dynamics Associated with Bridging Network Formation in Model Polymer Nanocomposites.

The addition of nanoparticles (NPs) to polymers is a powerful method to improve the mechanical and other properties of macromolecular materials. Such hybrid polymer-particle systems are also rich in fundamental soft matter physics. Among several factors contributing to mechanical reinforcement, a polymer-mediated NP network is considered to be the most important in polymer nanocomposites (PNCs). Here, we present an integrated experimental-theoretical study of the collective NP dynamics in model PNCs using X-ray photon correlation spectroscopy and microscopic statistical mechanics theory. Silica NPs dispersed in unentangled or entangled poly(2-vinylpyridine) matrices over a range of NP loadings are used. Static collective structure factors of the NP subsystems at temperatures above the bulk glass transition temperature reveal the formation of a network-like microstructure via polymer-mediated bridges at high NP loadings above the percolation threshold. The NP collective relaxation times are up to 3 orders of magnitude longer than the self-diffusion limit of isolated NPs and display a rich dependence with observation wavevector and NP loading. A mode-coupling theory dynamical analysis that incorporates the static polymer-mediated bridging structure and collective motions of NPs is performed. It captures well both the observed scattering wavevector and NP loading dependences of the collective NP dynamics in the unentangled polymer matrix, with modest quantitative deviations emerging for the entangled PNC samples. Additionally, we identify an unusual and weak temperature dependence of collective NP dynamics, in qualitative contrast with the mechanical response. Hence, the present study has revealed key aspects of the collective motions of NPs connected by polymer bridges in contact with a viscous adsorbing polymer medium and identifies some outstanding remaining challenges for the theoretical understanding of these complex soft materials.

Exploiting maltodextrin and whey protein isolate macromolecules as carriers for the development of freeze dried honey powder

This study was aimed at development of freeze drying powders from three types of honeys viz. Robinia pseudoacacia (RSA), Plectranthus rugosus (PR) and multifloral honey (MF) from Kashmir Himalayas of India using whey protein isolate (WPI) and maltodextrin (MD) as drying carriers. The powders were evaluated for physical, morphological and structural characterization. Results exhibited that powders with MD as drying carrier showed significantly higher water activity (aw), hygroscopicity, loose bed porosity (ɛL), tapped bed porosity (ɛT) and mean particle diameter (p < 0.05). While powders produced with WPI as carrier showed significantly higher bulk loose density (DL), bulk tapped density (DT) and glass transition temperature (Tg) (p < 0.05). Colour parameters L* and a* were affected by carrier used however b* was affected by type of honey. Morphological properties (SEM) showed that powders with MD as carrier were more “liquefied”, and agglomerated.FTIR analysis confirmed that there were some additional peaks in powders containing WPI (1500–1600 cm−1)and MD (1076 to 953 cm−1) as carriers exhibited interactions between honey components and carrier materials used.XRD analysis showed that powders with MD as carrier presented amorphous and partially crystalline materials. Thus the results concluded that WPI had higher potential as a carrier for production of honey powders using freeze drying in comparison to MD.

Statistical strength of a self-bonded incompatible polymer–polymer interface

The thick bulk films of amorphous polystyrene (PS) and poly(ethylene terephthalate) (PET) were held in contact for various periods of healing time (t) of 5 min to 15 h at 3 constant healing temperatures T = 74 °C, 89 °C, and 108 °C ranging from below to above the bulk glass transition temperatures of the 2 polymers involved. As a result, the PS–PET single lap-shear adhesive joints which can bear the mechanical load were formed. As-formed PS–PET adhesive joints (or self-bonded asymmetric incompatible PS–PET interfaces) were shear-fractured in tension, and their lap-shear strength (σ) developed due to physical self-healing of the interfaces was measured as a function of T and t. The distributions of the measured σ values (totally, 16 data sets) have been analyzed in the framework of the Weibull statistical model, and the Weibull parameters (Weibull modulus and scale factor) were estimated and compared with those reported recently for other partially self-healed interfaces of amorphous and semi-crystalline polymers. The Weibull behavior of the polymer–polymer interfaces differing in the chain chemical structure, compatibility, crystallizability, and crystallinity has been discussed.

Dynamic phase transitions in freestanding polymer thin films

After more than two decades of study, many fundamental questions remain unanswered about the dynamics of glass-forming materials confined to thin films. Experiments and simulations indicate that free interfaces enhance dynamics over length scales larger than molecular sizes, and this effect strengthens at lower temperatures. The nature of the influence of interfaces, however, remains a point of significant debate. In this work, we explore the properties of the nonequilibrium phase transition in dynamics that occurs in trajectory space between high- and low-mobility basins in a set of model polymer freestanding films. In thick films, the film-averaged mobility transition is broader than the bulk mobility transition, while in thin films it is a variant of the bulk result shifted toward a higher bias. Plotting this transition's local coexistence points against the distance from the films' surface shows thick films have surface and film-center transitions, while thin films practically have a single transition throughout the film. These observations are reminiscent of thermodynamic capillary condensation of a vapor-liquid phase between parallel plates, suggesting they constitute a demonstration of such an effect in a trajectory phase transition in the dynamics of a structural glass former. Moreover, this transition bears similarities to several experiments exhibiting anomalous behavior in the glass transition upon reducing film thickness below a material-dependent onset, including the broadening of the glass transition and the homogenization of surface and bulk glass transition temperatures.

Weibull statistics of the lap-shear strength of a symmetric interface of amorphous poly(ethylene terephthalate)

For the first time, the distribution of the lap-shear strength (σ) developed upon the contact of two amorphous bulk samples of poly(ethylene terephthalate) (PET) in the surroundings of the PET bulk glass transition temperature has been analysed as a function of healing temperature and healing time by using a Weibull statistical model. The Weibull parameters, Weibull modulus (m) and scale factor (σ0) of partially self-healed symmetric amorphous PET–PET interface have been estimated by analysing some tens of the lap-shear strength experimental data sets. It has been found that the σ0 values are very close to the corresponding mean σ values averaged for each test series, meaning that the Weibull analysis gives physically meaningful results. It has been shown that the majority of the σ experimental data sets can be described properly with the standard Weibull distribution function proposed for brittle solids. The m values for the PET–PET interface have been compared with those reported recently for the polymer–polymer interfaces of non-crystallizable amorphous polymers. The impact of the potential ability of an amorphous polymer to crystallize on the scatter in the measured interface lap-shear strength has been discussed.

Unusual thickness relaxation of spin-coated polystyrene ultrathin films in the glassy state

Relaxation in the thickness of ultrathin polystyrene films (thickness < 7 nm) on two different solid substrates is investigated at various temperatures by X-ray reflectivity. A thickness relaxation (i.e., ultraslow increase in thickness) is found even at room temperature, at which point any relaxation would hardly be expected because it is lower than the bulk glass transition temperature by at least 70 °C. At room temperature, the thickness relaxation depends on the annealing time and annealing temperature even upon annealing in the rubbery state. The relaxation time of the films formed on a Si–OH substrate is found to be larger than those deposited on a SiO2 substrate, and decreases with increasing temperature. Whereas, the slow increase in thickness also observed at temperatures above Tg, indicates that some of the molecular chains were not in an equilibrium state, which might be due to a persistent, highly strained interfacial layer. Almost no thickness relaxation is observed at temperatures close to the glass transition point Tg, which would suggest that the Tg of ultrathin polystyrene films is determined by the competition between slow relaxation in the interfacial layer and fast relaxation originated in the free surface region. The results demonstrate that the relaxation and glass transition behavior of confined thin films are influenced by residual stress in the substrate interface region.

Influence of Longer Range Transfer of Vapor Interface Modified Caging Constraints on the Spatially Heterogeneous Dynamics of Glass-Forming Liquids

Based on the Elastically Collective Nonlinear Langevin Equation theory of bulk relaxation in glass-forming liquids, and our recent ideas of how interface-nucleated modification of caging constraints are spatially transferred into the interior of a thick film, we present a force-based theory for dynamical gradients in thick films with one vapor interface that includes collective elasticity effects. Quantitative applications to the foundational hard sphere fluid and polymer melts of diverse fragilities are presented. We predict a roughly exponential spatial variation of total activation barrier which is non-perturbatively modified to ~10 particle diameters into film. This leads to prediction of a reduced alpha relaxation time gradient of a double exponential form characterized by a nearly constant spatial decay length, in qualitative accord with simulations. These spatially heterogeneous changes of the temperature variation of the alpha time result in a large and long-range gradient of the local glass transition temperature. An average interfacial layer thickness relevant to ensemble-averaged dielectric and other experiments is also computed. It is predicted to be rather large at the bulk glass transition temperature, decreasing roughly linearly with heating. Spatially inhomogeneous power law decoupling of the alpha relaxation time from its bulk value is predicted, with an effective exponent that decays to zero with distance from the free surface in a nearly exponential manner, trends which are in qualitative accord with recent simulations. This behavior and the double exponential alpha time gradient are related and can be viewed as consequences of an effective quasi-universal factorization of the total barrier in films into the product of its bulk temperature dependent value times a function solely of location in the film.

The deformation process of amorphous polymers and adhesive lap-shear joints formed thereof as studied by the emission of charged particles in high-vacuum technique

The emission of charged particles in high vacuum (at a pressure of 10−5 Pa) has been investigated during the deformation process of the monolithic bulk samples of amorphous polystyrene (PS) and poly(2,6-dimethyl-p-phenylene ether) (PPE), and of homo- and hetero-adhesive single lap-shear joints PPE–PPE and PS–PPE, respectively. The adhesive joints were formed by holding in contact the two bulk samples of PPE or the samples of PS and PPE at a small contact pressure of 0.03 MPa at the self-healing temperatures well below (by 100 K or even lower) of the PPE bulk glass transition temperature. It has been shown that the mechanoemission of charged particles has occurred at the moment of fracture (time interval < 1 ms) both of the PS and PPE bulk samples and of the PPE–PPE and PS–PPE adhesive joints and continued after the adhesive joint fracture. It has been found that, upon the adhesive joints fracture, the emission of negative ions has not been observed while electrons have been emitted. The fracture mechanisms of the bulk samples and of adhesive joints have been discussed. An equation relating the concentration of the broken chemical bonds to the intensity of the charged particles has been proposed.

Comparison of AFM Nanoindentation and Gold Nanoparticle Embedding Techniques for Measuring the Properties of Polymer Thin Films.

The surfaces of polymer and interfaces between polymer and inorganic particles are of particular importance for the properties of polymers and composites. However, the determination of the properties of surfaces and interfaces poses many challenges due to their extremely small dimensions. Herein, polystyrene and polymethyl methacrylate thin film on silicon wafer was used as a model system for the measurement of the properties of the polymer near free surface and at the polymer-solid interface. Two different methods, i.e., nanoindentation using atomic force microscopy (AFM) and the gold nanoparticle embedding technique, were used for these measurements. The results showed the elastic modulus of PS near the free surface determined by nanoindentation was lower than the bulk value. Based on contact mechanics analysis, nanoparticle embedding also revealed the existence of a lower-modulus, non-glassy layer near the free surface at temperatures below the bulk glass transition temperature (Tg). However, near the polymer-solid interface, the AFM nanoindentation method is not applicable due to the geometry confinement effect. On the other hand, the nanoparticle embedding technique can still correctly reflect the interactions between the polymer and the substrate when compared to the ellipsometry results.