Polymer Glass Transition Research Articles

Surface Engineering of Polymeric Colloidal Crystals by Temperature - Pressure Annealing.

Polymer colloidal crystals (PCCs) have been widely explored as acoustic and optical metamaterials and as templates for nanolithography. However, fabrication impurities and fragility of the self-assembled structures are critical bottlenecks for the device's efficiency and applications. We have demonstrated that temperature-assisted pressure [ annealing results in the mechanical strengthening of PCCs, which improves with the annealing temperature. Here, the enhancement of elastic properties and morphological features of self-assembled PCC's is evaluated using Brillouin light scattering and scanning electron microscopy. The pressure-induced effects on the vibrational modes of PCCs are also illustrated at temperatures well below the polymer glass transition. While the PCCs colloid constituents display reversibility, the PCC material is strongly irreversible in the performed thermodynamic cycle. The effective elastic modulus enhances from 0.7GPa for the pristine sample to 0.8GPa, solely by pressure annealing at room temperature. [ annealing at higher temperatures leads to a maximum effective elastic modulus of 1.7GPa, more than twice the value in the pristine sample. Above a cross-over pressure, 725bar at 348K), the PCCs respond elastically and, hence, reversibly to pressure changes.

Morphological Characterization and Molecular Dynamics Investigation of In-Situ Silica Nanoparticle-Reinforced Poly(Dimethylsiloxane) as a New Approach to Fouling-Release Coatings

To explore a better understanding of the features of poly(dimethylsiloxane) (PDMS)/silica nanocomposites as fouling-release coatings, a series of PDMS networks filled with silica nanoparticles synthesized in-situ was investigated using small angle x-ray scattering (SAXS) and dynamical mechanical analysis (DMA). Three hydroxyl-terminated (HT) PDMS with different molecular weights, 18,000, 49,000, and 79,000 g/mole, were considered for preparing the nanocomposites at the various weight ratios: 1, 3, and 5:1 of HT-PDMS to tetraethyl orthosilicate (TEOS) as a precursor to the in-situ formation of the nano-silica particles. Two different TEOS were used regarding their degree of polymerization: monomeric type (n∼1) and oligomeric type (n∼5). The small-scale structure of the silica domains was examined using SAXS while scanning electron microscopy (SEM) was used to investigate the large-scale structures of the silica domains. The SAXS data showed a bicontinuous array of aggregates. In addition, the glass transition and molecular dynamics in the nanocomposites were investigated using DMA. The sample including 20 wt% silica, exhibited two tan δ peaks. One was related to the usual polymer glass transition, while the other, occurring at a higher temperature, was attributed to the glass transitions associated with polymer chains in proximity to silica domains. The mobility of these polymer chains was diminished due to their interactions with the surface of the silica particles. This result demonstrated the existence of two types of physical and chemical interactions between the silica aggregates and PDMS chains.

Chemical analysis and performance evaluation of ClearCorrect® aligners as received and after intraoral use: Implications for durability, aesthetics, and patient safety

BackgroundOrthodontic treatment with transparent aligners is popular with patients. Any alteration of the plastic material, as subjected to the oral environment, could influence the treatment's durability, the aligner's aesthetic appearance, and the patient's safety. PurposeThis study concerns the physicochemical properties of ClearCorrect® aligners before and after intraoral use, focusing on transparency, surface topography, leachable, polymer glass transition temperature, and viscoelastic properties. MethodsAligners were collected after two weeks of intraoral use. Unused samples were obtained from the manufacturers. Transparency was measured by UV–visible spectroscopy. Chemical modifications were studied using infrared and Raman spectroscopies. Thermal degradation, glass transition (Tg), and storage modulus (E') were characterized by thermal analysis (DSC, TGA, DMA). Surface morphology and roughness were studied thanks to SEM and AFM. Aligners were immersed in water-based solutions to identify and quantify organic leachable by HPLC chromatography and trace elements by atomic absorption spectroscopy. ResultsClearCorrect® aligners have a three-layer structure (outer PETG/inner PU layers). Slight chemical alterations occurred after aging. There was also no significant evolution in Tg and thermal degradation temperatures and only a minimal evolution of E'. Surface and transparency alterations occurred. A difference in organic compound and trace element release levels between new and used aligners was evidenced, suggesting an intraoral release during use. SignificanceIntra-oral aging mainly impacts the aligner transparency and surface. The leachable study suggests significant ingestion of organic and non-organic compounds by the patient: investigations are needed to assess the impact of the long-term use of trays on patient health.

Self‐Consistent Thermodynamics and Dynamics during Polymer Glass Transitions

AbstractThe mechanisms behind the polymer glass transition, though not fully elucidated, have garnered significant attention. The Locally Correlated Lattice (LCL) theory [R. P. White, J. E. Lipson, Macromolecules 2016, 49, 3987] provides a novel perspective on free volume and establishes the relationship between polymer glass transition and free volume. In this study, we present a dynamic theoretical model based on the thermodynamic LCL theory, focusing on the kinetic evolution of free volume during polymer glass transition. Our analysis simultaneously examines the segmental diffusion coefficient, dynamic correlation length, free volume, and isobaric heat capacity. The results indicate that, upon reaching a critical temperature, heterogeneous dynamics and singular thermodynamics emerge concurrently. This study establishes a comprehensive correlation between thermodynamic singularities and dynamic inhomogeneities, offering valuable insights into the polymer glass transition process.

Experimental Investigation of Processing Temperature Effect on Adhesive Bond Strength Between Engineering Thermoplastics in the Plastic Injection Molding Process

Abstract Multicomponent injection molding industry is experiencing a growth due to its ability to reduce production costs and streamline processes. However, compared to single injection, multicomponent injection molding introduces interface regions where multiple engineering polymers meet. Consequently, it is essential to comprehend and enhance the adhesive bonding strength properties of these polymers. This study investigates the adhesive bond strength of polymer–polymer multimaterial molding using two-shot bi-injection and overmolding techniques. The research also emphasizes the influence of injection molding process parameters of mold temperature and melt temperature on the adhesive bond strength of polycarbonate (PC), polycarbonate–acrylonitrile butadiene styrene (PC–ABS), acrylonitrile butadiene styrene (ABS), and styrene ethylene butadiene styrene (SEBS). Tensile strength results revealed that the bi-injection method yields the highest interface strength, approximately 10 MPa lower than the reference value for single-material hard–hard plastics. Results from overmolded samples for both injection sequences are presented, indicating that material with low melting temperature was found to be the first injected part for better adhesion strength. Empirical equations for estimating adhesion strength were derived as a function of interface temperature obtained from CAE numerical simulations and polymer glass transition temperatures. The proposed equation achieved R2 values greater than 0.96. This empirically derived equation will serve as a guide for multi-injection manufacturing processes.

A General Approach to Activate Second-Scale Room Temperature Photoluminescence in Organic Small Molecules.

Organic small molecules that exhibit second-scale phosphorescence at room temperature are of interest for potential applications in sensing, anticounterfeiting, and bioimaging. However, such materials systems are uncommon-requiring millisecond to second-scale triplet lifetimes, efficient intersystem crossing, and slow rates of nonradiative recombination. Here, a simple and scalable approach is demonstrated to activate long-lived phosphorescence in a wide variety of molecules by suspending them in rigid polymer hosts and annealing them above the polymer's glass transition temperature. This process produces submicron aggregates of the chromophore, which suppresses intramolecular motion that leads to nonradiative recombination and minimizes triplet-triplet annihilation that quenches phosphorescence in larger aggregates. In some cases, evidence of excimer-mediated intersystem crossing that enhances triplet generation in aggregated chromophores is found. In short, this approach circumvents the current design rules for long-lived phosphors, which will streamline their discovery and development.

New Insights into the Application of Biocompatible (Un)Modified TiO2 and TiO2-ZrO2 Oxide Fillers in Light-Curing Materials.

A novel UV-light-curable poly(ethylene glycol) diacrylate matrix composite material with unmodified and methacryloxyl-grafted TiO2 and TiO2-ZrO2 systems was developed and tested as a potential coating material for medical components. The main goal of the research was to evaluate how the addition of (un)modified inorganic oxide fillers affects the properties of the composition (viscosity, UV/Vis spectra), the kinetics of photocuring (photo-DSC), and the morphological (SEM), physicochemical, and thermal properties (DSC, TGA) of the resulting composites. The applied filler functionalization process decreased their polarity and changed their size, BET surface area, and pore volume, which influenced the viscosity and kinetics of the photocurable system. In addition, the addition of synthesized fillers reduced the polymer's glass transition temperature and increased its thermal stability. It was also observed that additional UV irradiation of the tested composite changed its surface, resulting in hydrophobic properties (with the addition of 7 wt.% filler, an increase in the contact angle by more than 45% was observed).

Active Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) Films Containing Phenolic Compounds with Different Molecular Structures.

To obtain more sustainable and active food packaging materials, PHBV films containing 5% wt. of phenolic compounds with different molecular structures (ferulic acid, vanillin, and catechin) and proved antioxidant and antimicrobial properties were obtained by melt blending and compression molding. These were characterized by their structural, mechanical, barrier, and optical properties, as well as the polymer crystallization, thermal stability, and component migration in different food simulants. Phenolic compounds were homogenously integrated within the polymer matrix, affecting the film properties differently. Ferulic acid, and mainly catechin, had an anti-plasticizing effect (increasing the polymer glass transition temperature), decreasing the film extensibility and the resistance to breaking, with slight changes in the elastic modulus. In contrast, vanillin provoked a plasticizing effect, decreasing the elastic modulus without notable changes in the film extensibility while increasing the water vapor permeability. All phenolic compounds, mainly catechin, improved the oxygen barrier capacity of PHBV films and interfered with the polymer crystallization, reducing the melting point and crystallinity degree. The thermal stability of the material was little affected by the incorporation of phenols. The migration of passive components of the different PHBV films was lower than the overall migration limit in every simulant. Phenolic compounds were released to a different extent depending on their thermo-sensitivity, which affected their final content in the film, their bonding forces in the polymer matrix, and the simulant polarity. Their effective release in real foods will determine their active action for food preservation. Catechin was the best preserved, while ferulic acid was the most released.

The role of monomer geometry on the properties of polyolefins: A numerical study

AbstractWe explore the role of monomer geometry on the structural, dynamic, and thermodynamic properties of polyolefis by employing all‐atom molecular dynamic simulations. Specifically, we compare properties of atactic polyolefins in the molten state including polypropylene (aPP), a short‐chain branched polymer: poy(1‐hexene) (aPH), and a polymer having cyclic olefins: poly(vinyl cyclobutane) (aPVCB). We find polymers having the same chain mass and atom composition (hydrocarbon‐based molecules), but having different monomer architecture differ strongly in material properties. In particular, the polymer glass transition () and bulk modulus () show higher values for aPVCB in comparison to aPP and aPH. This increase is caused by having the carbon atoms in a cyclic structure, making aPVCB achieve higher mass and energy densities. By contrast, adding linear short side chains to polymer backbones causes a reduction in and , since side chains make backbones displace each other reducing their packing and thus their mass and energy densities. More broadly, our numerical results suggest that the incorporation of VCB monomers to linear polyolefins will enhance their properties, opening the possibility for designing a new set of materials.

Efficient Prediction of Polymer Glass Transition Temperatures through Machine Learning Methods

The glass transition temperature (Tg) plays a crucial role in defining polymer properties. Despite the widespread use of machine learning for material design and property prediction, there are still challenges concerning the interpretability and model performance when predicting Tg. In this study, Simplified Molecular Input Line Entry System strings are utilised to encode the polymer structure, which are then transformed into molecular descriptors for analytical training and prediction of Tg using Artificial Neural Network and Random Forest models. Meticulous hyperparameter tuning of the Random Forest model was performed, resulting in reasonable Tg predictions. This methodology forges a connection between polymer structure and Tg, opening up new avenues for research in the field of polymers.

Glass Transition and Structure of Organic Polymers from All-Atom Molecular Simulations

Glass Transition and Structure of Organic Polymers from All-Atom Molecular Simulations

Viscoelastic phenomena in methylcellulose aqueous systems: Application of fractional calculus

Fractional calculus models can potentially describe the viscoelastic phenomena in soft solids. Nevertheless, their successful application is limited. This paper explored the potential of using fractional calculus models to describe the viscoelastic properties of soft solids, focusing on methylcellulose aqueous systems. Methylcellulose is an important food additive, and it is known for its complex rheological behaviors, including thermogelation, which still puzzle rheologists. Through dynamic mechanical analysis and fractional rheology, we demonstrated that fractional calculus described the frequency- and temperature-dependent rheology of methylcellulose. This paper also showcased how including one springpot could potentially replace numerous spring-dashpot arrangements. Our findings using fractional calculus suggested that the thermogelation of methylcellulose involves the cooperative mobility of polymer chains and can be described as a process analogous to the glass transition in polymers. This study highlighted the power of combining fractional calculus and rheology to understand complex viscoelastic phenomena in soft solids.

Maximizing the electrochemical performance of supercapacitor electrodes from plastic waste

The management of the increasing volume of plastic waste has become a key challenge for society. A promising strategy now consists in the transformation of plastic waste into high-value materials that can be utilized in energy storage devices such as batteries and supercapacitors. In this study, we demonstrate a two-step procedure, involving pyrolysis, followed by chemical activation that will convert common plastic waste into activated carbons (ACs). This technique makes ACs suitable for supercapacitor electrode materials. Further, the electrochemical performance of ACs is outstanding in terms of capacitance, energy density, and cycling stability. Besides the well-established parameters, including a specific surface area and micropore volume, we found that other critical factors such as polymer glass transition temperature, polymer-activating agent miscibility, activating agent (K2CO3):AC ratio, and AC water dispersion stability also play a crucial role in determining the supercapacitors performance. Controlling these parameters, we obtained ACs as supercapacitor electrodes from a range of plastic waste materials with a competitive electrochemical performance. Specifically, the ACs exhibited a specific capacitance of 220 F g−1 (at a current density of 1 A g−1), energy and power densities of 61.1 Wh kg−1 and 36.9 kW kg−1, respectively, and excellent cycling stability (95 % retention after 30,000 cycles). Our findings provide a pathway towards transforming plastic waste into valuable electrode materials for supercapacitors.

Temperature influence on MBS latex aggregate morphology

This paper aims to better understand the impact of process conditions on the morphological properties of MBS (Methacrylate Butadiene Styrene) core-shell latex aggregates during an aggregation process. Laboratory scale experiments were performed in a stirred reactor following a standard industrial procedure including a first destabilization step by adding acid at a moderate temperature followed by a second heating step. The size and shape distribution of the aggregates as well as their fractal dimension were measured by laser diffraction and image analysis. The experimental data were analyzed in terms of number and volume distributions to obtain information on the entire population, from primary nanoparticles to aggregates several orders of magnitude larger. The main new finding of this work concerns the influence of the aggregation temperature on the size, shape and structure of latex aggregates. Indeed, the closer the temperature is to the glass transition of the MBS shell polymer, the better the agglomeration and the aggregates formed tend to be larger and more circular.

PLA-PEG forming worm-like nanoparticles despite unfavorable packing parameter: Formation mechanism, thermal stability and potential for cell internalization

Most nanoparticles produced for drug delivery purposes are spherical. However, the literature suggests that elongated particles are advantageous, notably in terms of cellular uptake. Thus, we synthesized biocompatible polylactide-b-poly(ethylene glycol) (PLA-PEG) polymers bearing carboxylate moieties, and used them to formulate worm-like nanoparticles by a simple emulsion-evaporation process. Worm-like nanoparticles with variable aspect ratio were obtained by simply adjusting the molar mass of the PLA block: the shorter the molar mass of the PLA block, the more elongated the particles. As PLA molar mass decreased from 80,000 g/mol to 13,000 g/mol, the proportion of worm-like nanoparticles increased from 0 to 46%, in contradiction with the usual behavior of block polymers based on their packing parameter. To explain this unusual phenomenon, we hypothesized the shape arises from a combination of steric and electrostatic repulsions between PEG chains bearing a carboxylate moiety present at the dichloromethane-water interface during the evaporation process. Worm-like particles turned out to be unstable when incubated at 37 °C, above polymer glass transition temperature. Indeed, above Tg, a Plateau-Rayleigh instability occurs, leading to the division of the worm-like particles into spheres. However, this instability was slow enough to assess worm-like particles uptake by murine macrophages. A slight but significant increase of internalization was observed for worm-like particles, compared to their spherical counterparts, confirming the interest of developing biocompatible anisotropic nanoparticles for pharmaceutical applications such as drug delivery.

Colloidal Clusters and Networks Formed by Oppositely Charged Nanoparticles with Varying Stiffnesses.

Colloidal clusters and gels are ubiquitous in science and technology. Particle softness has a strong effect on interparticle interactions; however, our understanding of the role of this factor in the formation of colloidal clusters and gels is only beginning to evolve. Here, we report the results of experimental and simulation studies of the impact of particle softness on the assembly of clusters and networks from mixtures of oppositely charged polymer nanoparticles (NPs). Experiments were performed below or above the polymer glass transition temperature, at which the interaction potential and adhesive forces between the NPs were significantly varied. Hard NPs assembled in fractal clusters that subsequently organized in a kinetically arrested colloidal gel, while soft NPs formed dense precipitating aggregates, due to the NP deformation and the decreased interparticle distance. Importantly, interactions of hard and soft NPs led to the formation of discrete precipitating NP aggregates at a relatively low volume fraction of soft NPs. A phenomenological model was developed for interactions of oppositely charged NPs with varying softnesses. The experimental results were in agreement with molecular dynamics simulations based on the model. This work provides insight on interparticle interactions before, during, and after the formation of hard-hard, hard-soft, and soft-soft contacts and has impact for numerous applications of reversible colloidal gels, including their use as inks for additive manufacturing.

Toward a practical method for measuring glass transition in polymers with low-frequency Raman spectroscopy

Glass transition temperature is one of the most important characteristics to describe the behavior of polymeric materials. When a material goes through glass transition, conformational entropy increases, which affects the phonon density of states. Amorphous materials invariably display low-frequency Raman features related to the phonon density of states resulting in a broad disorder band below 100 cm−1. This band includes the Boson peak and a shoulder, which is dominated by the van Hove peak, and quasi-elastic Rayleigh scattering also contributes to the signal. The temperature dependence of the ratio of the integrated intensity in proximity of the Boson peak to that of the van Hove peak shows a kink near the glass transition temperature as determined by differential scanning calorimetry. Careful analysis of the Raman spectra confirms that this is related to a change in the phonon density of states at the transition temperature. This makes low-frequency Raman a promising technique for thermal characterization of polymers because not only is this technique chemically agnostic and contactless but also it requires neither intensity calibration nor deconvolution nor chemometric analysis.

Second order nonlinear optical properties of poled films containing azobenzenes tailored with azulen-1-yl-pyridine

Azo compounds, which are highly versatile molecules, have attracted a considerable attention both for their fundamental properties and for practical applications, especially in photonics. The conjugated azo-aromatic systems containing 4-(R-azulen-1-yl)-2,6-dimethyl-pyridine were investigated for their nonlinear optical properties. These molecules were embedded in polymethyl methacrylate (PMMA) matrix, and the obtained guest-host systems were processed into good optical quality thin film by spin coating technique. The dipolar moments of dissolved in PMMA molecules were oriented by applying a high DC electric field at a temperature close to the polymer glass transition temperature. The second - order nonlinear optical (NLO) properties of poled films were studied by the optical second harmonic generation technique (SHG). The poling kinetics, studied by in situ SHG as well as the measured second-order NLO susceptibilities of poled films are reported and discussed.

Automatic Multiscale Method of Building up a Cross-linked Polymer Reaction System: Bridging SMILES to the Multiscale Molecular Dynamics Simulation

An automatic method is introduced to generate the initial configuration and input file from SMILES for multiscale molecular dynamics (MD) simulation of cross-linked polymer reaction systems. Inputs are a modified version of SMILES of all the components and conditions of coarse-grained (CG) and all-atom (AA) simulations. The overall process comprises the following steps: (1) Modified SMILES inputs of all the components are converted to 3-dimensional coordinates of molecular structures. (2) Molecular structures are mapped to the coarse-grained scale, followed by a CG reaction simulation. (3) CG beads are backmapped to the atomic scale after the CG reaction. (4) An AA productive run is finally performed to analyze volume shrinkage, glass transition, and atomic detail of network structure. The method is applied to two common epoxy resin reactions, that is, the cross-linking process of DGEVA (diglycidyl ether of vanillyl alcohol) and DHAVA (dihydroxyaminopropane of vanillyl alcohol) and that of DGEBA (diglycidyl ether of bisphenol A) and DETA (diethylenetriamine). These components form network structures after the CG cross-linking reaction and are then backmapped to calculate properties in the atomic scale. The result demonstrates that the method can accurately predict volume shrinkage, glass transition, and all-atom structure of cross-linked polymers. The method bridges from SMILES to MD simulation trajectories in an automatic way, which shortens the time of building up cross-linked polymer reaction model and suitable for high-throughput computations.

An extended shoving model for dynamic fluctuation of glass transition in amorphous polymer towards cooperative shape-memory effect

Phase transition theory is a powerful approach to study shape-memory effect (SME) and dynamic relaxation behaviour in amorphous shape-memory polymers (SMPs) undergoing glass transition processes. However, previous studies are mostly limited to building-up of experimental models, mainly due to poor understanding of dynamic fluctuation of glass transition. In this study, the dynamic fluctuation of glass transition in the amorphous SMPs, whose thermodynamic SME is governed by the phase transition theory, was investigated using a shoving model. A constitutive relationship between potential energy and cooperatively rearranging region (CRR) volume is firstly developed to explore the working principle of phase transition in the thermodynamic SME of amorphous SMPs, based on the shoving model and Adam–Gibbs domain size model. The effect of CRR volume on the phase transition is further formulated using the strain function, and then employed to develop a constitutive stress–strain equation based on the extended Maxwell model. The working principles of cooperative SME and dynamic relaxation behaviours have been explored and discussed. Finally, the effectiveness of the analytical results obtained using the proposed model has been verified using the experimental results of amorphous SMPs reported in literature.