Showing papers on "Glass transition published in 2014"
TL;DR: In this article, the authors discuss recent experiments that directly measure mobility at or near the surface of glassy polymers and indicate that enhanced mobility near the free surface can exceed bulk mobility by several orders of magnitude and extend for several nanometers into the bulk polymer.
Abstract: The past 20 years have seen a substantial effort to understand dynamics and the glass transition in thin polymer films. In this Perspective, we consider developments in this field and offer a consistent interpretation of some major findings. We discuss recent experiments that directly measure mobility at or near the surface of glassy polymers. These experiments indicate that enhanced mobility near the free surface can exceed bulk mobility by several orders of magnitude and extend for several nanometers into the bulk polymer. Enhanced mobility near the free surface allows a qualitative understanding of many of the observations of a reduced glass transition temperature Tg in thin films. For thin films, knowledge of Tg by itself is less useful than for bulk materials. Because of this, new experimental methods that directly measure important material properties are being developed.
437 citations
TL;DR: In this article, the effects of modified boron nitride (h-BN) content on the thermal and insulating properties of epoxy/BN composites were investigated, and it was found that incorporation of h-BN particles in the epoxy matrix significantly enhanced the thermal conductivity of the composites.
Abstract: Hexagonal boron nitride (h-BN) microparticles, modified by surface coupling agent 3-aminopropyl triethoxy silane (APTES), were used to fabricate thermally conductive epoxy/BN composites, and the effects of modified-BN content on the thermal and insulating properties were investigated. It was found that incorporation of h-BN particles in the epoxy matrix significantly enhanced the thermal conductivity of the composites. With 30 wt% modified-BN loading, the thermal conductivity of the composites was 1.178 W m−1 K−1, 6.14 times higher than that of the neat epoxy. Fabricated epoxy/BN composites exhibited improved thermal stability, storage modulus, and glass transition temperature with increased BN content. The composites also possessed excellent electrical insulation properties. These results revealed that epoxy/BN composites are promising as efficient heat-releasing materials for thermal management and microelectronic encapsulation.
258 citations
TL;DR: In this article, the static and dynamic properties of poly(2-vinylpyridine)/silica nanocomposites are investigated by temperature modulated differential scanning calorimetry, broadband dielectric spectroscopy (BDS), small-angle X-ray scattering (SAXS), and transmission electron microscopy.
Abstract: The static and dynamic properties of poly(2-vinylpyridine)/silica nanocomposites are investigated by temperature modulated differential scanning calorimetry, broadband dielectric spectroscopy (BDS), small-angle X-ray scattering (SAXS), and transmission electron microscopy. Both BDS and SAXS detect the existence of an interfacial polymer layer on the surface of nanoparticles. The results show that whereas the calorimetric glass transition temperature varies only weakly with nanoparticle loading, the segmental mobility of the polymer interfacial layer is slower than the bulk polymer by 2 orders of magnitude. Detailed analysis of BDS and SAXS data reveal that the interfacial layer has a thickness of 4–6 nm irrespective of the nanoparticle concentration. These results demonstrate that in contrast to some recent articles on polymer nanocomposites, the interfacial polymer layer is by no means a “dead layer”. However, its existence might provide some explanation for controversies surrounding the dynamics of poly...
241 citations
TL;DR: In this article, the main and secondary relaxations of metallic glasses are analyzed and the correlation between the atomic mobility and the thermo-mechanical treatments in metallic glasses is analyzed.
228 citations
TL;DR: The experiments and model provide a measure of surface mobility in a simple geometry where confinement and substrate effects are negligible and fine control of the glassy rheology is of key interest to nanolithography among numerous other applications.
Abstract: Thin polymer films have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature Tg. Above Tg the entire film flows, whereas below Tg only the near-surface region responds to the excess interfacial energy. An analytical thin-film model for flow limited to the free surface region shows excellent agreement with sub-Tg data. The system transitions from whole-film flow to surface localized flow over a narrow temperature region near the bulk Tg. The experiments and model provide a measure of surface mobility in a simple geometry where confinement and substrate effects are negligible. This fine control of the glassy rheology is of key interest to nanolithography among numerous other applications.
208 citations
TL;DR: Biological assay results corroborated that the fibrous PLMC scaffolds were cytocompatible by supporting osteoblast adhesion and proliferation, and functionally promoted biomineralization-relevant alkaline phosphatase expression and mineral deposition.
Abstract: Multifunctional fibrous scaffolds, which combine the capabilities of biomimicry to the native tissue architecture and shape memory effect (SME), are highly promising for the realization of functional tissue-engineered products with minimally invasive surgical implantation possibility In this study, fibrous scaffolds of biodegradable poly(d,l-lactide-co-trimethylene carbonate) (denoted as PDLLA-co-TMC, or PLMC) with shape memory properties were fabricated by electrospinning Morphology, thermal and mechanical properties as well as SME of the resultant fibrous structure were characterized using different techniques And rat calvarial osteoblasts were cultured on the fibrous PLMC scaffolds to assess their suitability for bone tissue engineering It is found that by varying the monomer ratio of DLLA:TMC from 5:5 to 9:1, fineness of the resultant PLMC fibers was attenuated from ca 1500 down to 680 nm This also allowed for readily modulating the glass transition temperature Tg (ie, the switching temperature for actuating shape recovery) of the fibrous PLMC to fall between 192 and 442 °C, a temperature range relevant for biomedical applications in the human body The PLMC fibers exhibited excellent shape memory properties with shape recovery ratios of Rr > 94% and shape fixity ratios of Rf > 98%, and macroscopically demonstrated a fast shape recovery (∼10 s at 39 °C) in the pre-deformed configurations Biological assay results corroborated that the fibrous PLMC scaffolds were cytocompatible by supporting osteoblast adhesion and proliferation, and functionally promoted biomineralization-relevant alkaline phosphatase expression and mineral deposition We envision the wide applicability of using the SME-capable biomimetic scaffolds for achieving enhanced efficacy in repairing various bone defects (eg, as implants for healing bone screw holes or as barrier membranes for guided bone regeneration)
208 citations
TL;DR: In this paper, the authors investigate how dynamic behavior of functional poly(n-butyl acrylate) melts and cross-linked networks is influenced by different hydrogen-bonding side groups.
Abstract: Polymers containing hydrogen-bonding side groups (HBGs) can form transient supramolecular networks that exhibit technologically useful viscoelastic properties. Here, we investigate how dynamic behavior of functional poly(n-butyl acrylate) melts and cross-linked networks is influenced by different HBGs. Random copolymers containing weak and strong HBGs were synthesized and subjected to thermal and dynamic mechanical analysis. The glass transition temperature (Tg) increased nearly linearly with the HBG concentration, and this effect was similar for both weak and strong binding groups. Copolymers containing weak HBGs behaved as unentangled melts and exhibited higher storage and loss modulus with increasing amounts of binding group. In contrast, copolymers containing strong HBGs behaved like entangled networks. Flow activation energies increased linearly with comonomer content; and, for weak hydrogen-bonding groups, they depended only on the departure from Tg. Similar behavior was observed in cross-linked fil...
187 citations
TL;DR: The authors reviewed the features and mechanisms of β-relaxations, which are intrinsic and universal to supercooled liquids and glasses, and demonstrate their importance in understanding many crucial unresolved issues in glassy physics and materials science, including glass transition phenomena, mechanical properties, shearbanding dynamics and deformation mechanisms, diffusion and the breakdown of the Stokes-Einstein relation as well as crystallization and stability of glasses.
Abstract: Focusing on metallic glasses as model systems, we review the features and mechanisms of the β-relaxations, which are intrinsic and universal to supercooled liquids and glasses, and demonstrate their importance in understanding many crucial unresolved issues in glassy physics and materials science, including glass transition phenomena, mechanical properties, shear-banding dynamics and deformation mechanisms, diffusion and the breakdown of the Stokes-Einstein relation as well as crystallization and stability of glasses. We illustrate that it is an attractive prospect to incorporate these insights into the design of new glassy materials with extraordinary properties. We also outline important questions regarding the nature of β-relaxations and highlight some emerging research directions in this still-evolving field.
179 citations
TL;DR: In this paper, the thermal and mechanical properties of graphene/epoxy nanocomposites using molecular dynamics (MD) simulation were investigated, where three different formats of graphene were incorporated in an epoxy matrix to form the graphene/polyurethane composites, and the results showed that the local density in the vicinity of the graphene is relatively high.
Abstract: This study aims to investigate the thermal and mechanical properties of graphene/epoxy nanocomposites using molecular dynamics (MD) simulation. Three different formats of graphene: graphene flakes, intercalated graphene and intercalated graphene oxide, were incorporated respectively in an epoxy matrix to form the graphene/epoxy nanocomposites. The mechanical properties of the graphene/epoxy nanocomposites, including Young’s modulus (E), glass transition temperature (Tg) and coefficient of thermal expansion (CTE), in terms of three different formats of graphene, were characterized in this study. In addition to the mechanical properties, the influences of graphene on the density distribution of epoxy polymers in the nanocomposites were also examined. The results showed that the local density in the vicinity of the graphene is relatively high, and then progressively decreases to the bulk value in regions further away from the interface. On the other hand, for the mechanical and thermal properties, the nanocomposites with intercalated graphene exhibit a higher Young’s modulus, a higher glass transition temperature and a lower thermal expansion coefficient than do those with graphene flakes. This is because the intercalated graphene can lead to a high amount of high density polymer in the nanocomposites, and thus enhance the overall properties of the nanocomposites. In addition, the interacted graphene oxide provides the best reinforcement of the three systems of nanocomposites. Based on the calculation of interaction energy, it appears that the oxide modification of the graphene surface can effectively lead to the high interaction energy, such that the graphene oxide can demonstrate a relatively high reinforcing efficiency.
174 citations
TL;DR: In this paper, the authors investigated the role change of Na+ cations from network modifier to charge compensator in the presence of Al3+ ions and showed that the configuration entropy of the configurational entropy at the glass transition temperature is strongly nonlinear, with sharp increases and decreases depending on the Al/(Al+Na) ratio.
173 citations
TL;DR: It is demonstrated from a study of several metallic glass-forming liquids that the rate of this structural ordering as a function of temperature correlates with the kinetic fragility of the liquid, demonstrating a structural basis for fragility.
Abstract: Glass fragility characterises the rate of change in viscosity as a liquid is cooled towards the glass transition temperature. Here, the authors demonstrate via neutron diffraction that the rate at which structural ordering occurs within the melt correlates with kinetic fragility in a range of metallic glass samples.
TL;DR: In this paper, a molecular-level understanding of dynamics in imidazolium-based ionomers with different counterions and side chain lengths was investigated using X-ray scattering, oscillatory shear, and dielectric relaxation spectroscopy (DRS).
Abstract: A molecular-level understanding of dynamics in imidazolium-based ionomers with different counterions and side chain lengths was investigated using X-ray scattering, oscillatory shear, and dielectric relaxation spectroscopy (DRS). Variations of the counterion size and side chain length lead to changes in glass transition temperature (Tg), extent of ionic aggregation, and dielectric constant, with consequences for ion transport. A physical model of electrode polarization is used to determine the number density of simultaneously conducting ions and their mobility. Imidazolium-based ionomers with larger counterion and longer side chain have lower Tg, resulting in higher ionic conductivity and mobility. The ionic mobility is coupled to ion motions that are directly measured as a second segmental process in DRS, as these are observed to share the same Vogel temperature. Time–temperature superposition (tTS) was applied to create linear viscoelasticity master curves and to investigate the delay in chain motion re...
TL;DR: In this article, a series of rigid, bulky triptycene-based diamine monomers were designed, synthesized, and subsequently incorporated into the backbone of polyimides via polycondensation with 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) to obtain a series polyimide membranes with high fractional free volume.
Abstract: Robust polymer membranes that are highly permeable and selective are desired for energy efficient gas separation processes. In this study, a series of rigid, bulky triptycene-based diamine monomers were designed, synthesized, and subsequently incorporated into the backbone of polyimides via polycondensation with 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) to obtain a series of polyimide membranes with high fractional free volume. These triptycene-containing polyimides with systematic variations in their chemical structure demonstrate the viability of the ‘tunable’ fractional free volume by introducing various substituents onto the polymer backbone. All the polyimides synthesized exhibited film-forming high molecular weight, high solubility, and excellent thermal properties, with glass transition temperatures ranging from 280 °C to 300 °C and thermal stability up to 500 °C. Compared to other classes of glassy polymers, these triptycene-polyimides had high combinations of permeability and selectivity, suggesting that a favorable free volume size distribution in these triptycene polyimides was induced by the unique chain packing mechanism of triptycene units. The correlation between gas transport properties and the polymer chemical structure was also investigated. Altering the size of the substituents neighboring the triptycene units provides greater opportunity to fine-tune the fractional free volume and free volume size distribution in the polymer, which in turn can change the transport properties effectively to meet various separation needs. It is expected that additional design modifications made by exploiting the chemistry versatility of the triptycene moiety and by selectively adding other components may improve these membranes to break the gas permeability–selectivity trade-off barrier.
TL;DR: In this article, a method for the preparation of PLA bio-nanocomposites containing cellulose nanocrystals (CNCs) is reported, which enhances interfacial adhesion and dispersion of CNCs into PLA matrix.
TL;DR: In this paper, the effect of polymer crystallization on ion transport is decoupled by designing and fabricating a model polymer single crystal electrolyte system with controlled crystal structure, size, crystallinity, and orientation.
Abstract: Polymer electrolytes have attracted intensive attention due to their potential applications in all-solid-state lithium batteries. Ion conduction in this system is generally considered to be confined in the amorphous polymer/ion phase, where segmental relaxation of the polymer above glass transition temperature facilitates ion transport. In this article, we show quantitatively that the effect of polymer crystallization on ion transport is twofold: structural (tortuosity) and dynamic (tethered chain confinement). We decouple these two effects by designing and fabricating a model polymer single crystal electrolyte system with controlled crystal structure, size, crystallinity, and orientation. Ion conduction is confined within the chain fold region and guided by the crystalline lamellae. We show that, at low content, due to the tortuosity effect, the in-plane conductivity is 2000 times greater than through-plane one. Contradictory to the general view, the dynamic effect is negligible at moderate ion contents....
TL;DR: In this paper, the mesoporous metal organic framework (MOF) Fe(BTC) in a Matrimid®-PI matrix was characterized in terms of their carbon dioxide (CO2) and methane (CH4) separation performance at low and high pressures.
TL;DR: In this paper, the authors have shown that ZnO acts as a network modifier with BiO6 octahedral structural units for glass samples with x = 5, 10, and 15.
TL;DR: In this article, a covalently modified 3-nm thick graphene platelets (GnPs) by the reaction between the GnPs' epoxide groups and the end-amine groups of a commercial long-chain surfactant (M w ǫ = 2000), compounded the modified platelets with a model polymer epoxy, and investigated the structure and properties of both m -GNPs and their epoxy composites.
TL;DR: In this article, three different homopolymers (polystyrene, poly(2-vinylpyridine), and poly(methyl methacrylate), which have similar inherent stiffness and bulk glass transition temperature (Tg), but have different affinities with Si substrates, were chosen as models.
Abstract: Thermal annealing is one of the most indispensable polymer fabrication processes and plays essential roles in controlling morphologies and properties of polymeric materials. We here report that thermal annealing also facilitates polymer adsorption from the melt on planar silicon (Si) substrates, resulting in the formation of a high-density polymer nanolayer with flattened chain confirmations. Three different homopolymers (polystyrene, poly(2-vinylpyridine), and poly(methyl methacrylate)), which have similar inherent stiffness and bulk glass transition temperature (Tg), but have different affinities with Si substrates, were chosen as models. Spin-cast films (∼50 nm in thickness) with the three polymers were prepared on cleaned Si substrates and then placed in a vacuum oven set at a temperature far above the bulk Tg. In order to monitor the polymer adsorption process at the solid-polymer melt interface during thermal annealing, we used the protocol that combines vitrification of the annealed films (via rapi...
TL;DR: In this article, reduced graphene oxide (RGO)-encapsulated SiO2 hybrids (SiO2@RGO) were fabricated from the thermal reduction of electrostatically assembled SiO 2@GO hybrids.
Abstract: Reduced graphene oxide (RGO)-encapsulated SiO2 hybrids (SiO2@RGO) were fabricated from the thermal reduction of electrostatically assembled SiO2@GO hybrids. Then, epoxy composites, filled with SiO2, SiO2@GO and SiO2@RGO hybrids, were prepared by a solvent-free curing process, and their thermal, dielectric and thermo-mechanical properties were investigated and compared. In the SiO2@RGO/epoxy composites, the mono-dispersed SiO2 nanoparticles are firmly embedded in the thin layer of RGO nanosheets, forming unique core–shell nanostructures that effectively prevent the aggregation of RGO nanosheets in the polymer matrix, construct conductive pathways at the particle–polymer interface and afford the epoxy composites with outstanding thermo-mechanical properties. The dielectric properties of the SiO2@RGO/epoxy composites exhibit a typical percolation transition near 0.174 vol% for RGO (20 wt% of SiO2@RGO hybrids), where the dielectric constant could reach 77.23 at 1 kHz, which is 22 times that of the neat epoxy resin. Upon further increase of the loading content, the gradual contact of the filler particles leads to the formation of interfacial continuous conductive networks, and both the thermal conductivity and dielectric constant of the composites show a dramatic increase. With a filler loading of 40 wt% SiO2@RGO (0.373 vol% for RGO), a thermal conductivity of 0.452 W m−1 K−1 is obtained, which is two times larger than that of neat epoxy. In addition, SiO2@RGO/epoxy composites reveal significantly decreased coefficient of thermal expansion (CTE) and increased glass transition temperature (Tg). We believe this special core–shell SiO2@RGO structure, with its inner mechanically enhanced inorganic particles and outer interfacial conductive phase, could make full use of the enhancement effect of different components and thus endow the polymer composites with outstanding properties overall.
TL;DR: In this article, the authors proposed a phase diagram model to predict the structural units of glass with mutually miscible or immiscible phases based on infrared spectroscopy, Raman and nuclear magnetic resonance (NMR) measurements.
TL;DR: In this paper, the effect of the curing temperature on the physical and mechanical properties of three structural adhesives was investigated using an in-house developed apparatus and the results showed that the strength and stiffness of the adhesive increase as the cure temperature increases and the T g is higher than the cure temperatu...
Abstract: This paper describes the influence of the curing temperature on the physical and mechanical properties of three structural adhesives. This work was undertaken to improve the understanding of the effect of curing temperature in the glass transition temperature, T g , and stiffness of epoxy adhesives. The mechanical properties (Young's modulus and yield strength) of the adhesives were measured in bulk specimens. T g was measured by a dynamic mechanical analysis using an in-house developed apparatus. The curing process was the same for all tests, consisting of a curing stage followed by a post cure stage. The initial stage was performed at different temperatures. T g and the mechanical properties was found to vary as a function of the cure temperature of the adhesive. When cured below the cure temperature, T cure , at which the T g of the fully cured network, T g ∞, is achieved, the strength and stiffness of the adhesive increase as the cure temperature increases and the T g is higher than the cure temperatu...
TL;DR: In this review, the peculiarities of the thermodynamics and dynamics of glass-forming polymers are discussed, with particular emphasis on those topics currently the subject of debate.
Abstract: The fate of matter when decreasing the temperature at constant pressure is that of passing from gas to liquid and, subsequently, from liquid to crystal. However, a class of materials can exist in an amorphous phase below the melting temperature. On cooling such materials, a glass is formed; that is, a material with the rigidity of a solid but exhibiting no long-range order. The study of the thermodynamics and dynamics of glass-forming systems is the subject of continuous research. Within the wide variety of glass formers, an important sub-class is represented by glass forming polymers. The presence of chain connectivity and, in some cases, conformational disorder are unfavourable factors from the point of view of crystallization. Furthermore, many of them, such as amorphous thermoplastics, thermosets and rubbers, are widely employed in many applications. In this review, the peculiarities of the thermodynamics and dynamics of glass-forming polymers are discussed, with particular emphasis on those topics currently the subject of debate. In particular, the following aspects will be reviewed in the present work: (i) the connection between the pronounced slowing down of glassy dynamics on cooling towards the glass transition temperature (Tg) and the thermodynamics; and, (ii) the fate of the dynamics and thermodynamics below Tg. Both aspects are reviewed in light of the possible presence of a singularity at a finite temperature with diverging relaxation time and zero configurational entropy. In this context, the specificity of glass-forming polymers is emphasized.
TL;DR: In this paper, the relationship between glass structure and tensile behavior across a wide range of structural states was investigated using ion irradiation of thermoplastically molded Pt57.5Cu14.3Ni5.7P22.5 metallic glass nanowires.
TL;DR: In this paper, the authors investigated charge transport and structural dynamics in low molecular weight and polymerized 1-vinyl-3-pentylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids (ILs).
Abstract: Charge transport and structural dynamics in low molecular weight and polymerized 1-vinyl-3-pentylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids (ILs) are investigated by a combination of broadband dielectric spectroscopy, dynamic mechanical spectroscopy and differential scanning calorimetry. While the dc conductivity and fluidity exhibit practically identical temperature dependence for the non-polymerized IL, a significant decoupling of ionic conduction from structural dynamics is observed for the polymerized IL. In addition, the dc conductivity of the polymerized IL exceeds that of its molecular counterpart by four orders of magnitude at their respective calorimetric glass transition temperatures. This is attributed to the unusually high mobility of the anions especially at lower temperatures when the structural dynamics is significantly slowed down. A simple physical explanation of the possible origin of the remarkable decoupling of ionic conductivity from structural dynamics is proposed.
TL;DR: In this paper, the nucleation efficiency of multiwall carbon nanotubes (MWCNT) in poly(e-caprolactone) (PCL) was tested for a wide range of temperatures and cooling rates and compared to the efficiency of homogeneously formed nuclei.
TL;DR: In this article, the authors reported the dramatic deviation of glass transition behaviors of poly(methyl methacrylate) (PMMA) confined in cylindrical nanopores with diameter significantly larger than chain's radius of gyration (Rg).
Abstract: Recently, confinement of polymers with different geometries has become a research hotspot. Here, we report the dramatic deviation of glass transition behaviors of poly(methyl methacrylate) (PMMA) confined in cylindrical nanopores with diameter significantly larger than chain’s radius of gyration (Rg). Fast cooling a PMMA melt in the nanopores results in a glass with one single glass transition temperature (Tg). But two distinct Tgs are detected after slow cooling the melt. The deviation in Tg could be as large as 45 K. This phenomenon is interpreted by a two-layer model. During vitrification under slow cooling two distinct layers are formed: a strongly constrained interfacial layer showing an increased Tg as compared to that of the bulk polymer and a core with a decreased Tg. By thermal annealing experiments, we find that these two Tgs are inherently correlated. In addition, the deviation of Tg for PMMA confined in nanopores reveals a dependence on molecular weight.
TL;DR: In this article, the effect of the addition of graphene on the glass transition temperature (Tg) of polymers was investigated, first with poly(methyl methacrylate) and then with an extensive literature review.
Abstract: The effect of the addition of graphene on the glass transition temperature (Tg) of polymers was investigated, first with poly(methyl methacrylate) and then with an extensive literature review. Isotactic (i-PMMA) and atactic PMMA (a-PMMA) were blended with pristine graphene (PG) and thermally reduced graphene (TRG). A Tg increase was found for a-PMMA nanocomposites made via in situ polymerization with TRG but not when a-PMMA was solvent blended with TRG. However, a Tg increase was found for TRG solvent blended into i-PMMA and a smaller increase for PG with i-PMMA. Nearly all the increase occurred at the lowest loading, 0.25 wt %, with little change at increased graphene concentration. Tg increases due to interfacial interactions between matrix polymers and fillers. Physical blending such as solvent processes cannot provide enough interaction at the interfaces, whereas chemical blending processes such as in situ polymerization can yield strong covalent bonds. However, i-PMMA molecules can align on graphene ...
TL;DR: This work shows that to quantitatively understand stability at a microscopic level, the presence of weakly interacting pairs of particles must be included, and predicts various nontrivial scaling behavior of the elasticity and vibrational properties of colloidal glasses that can be tested experimentally.
Abstract: How a liquid becomes rigid at the glass transition is a central problem in condensed matter physics. In many scenarios of the glass transition, liquids go through a critical temperature below which minima of free energy appear. However, even in the simplest glass, hard spheres, what confers mechanical stability at large density is highly debated. In this work we show that to quantitatively understand stability at a microscopic level, the presence of weakly interacting pairs of particles must be included. This approach allows us to predict various nontrivial scaling behavior of the elasticity and vibrational properties of colloidal glasses that can be tested experimentally. It also gives a spatial interpretation to recent, exact calculations in infinite dimensions.
TL;DR: In this article, the authors derived the detailed expression of stress relaxation time, which reveals an Arrhenius type dependency of material relaxation behavior on the applied temperature, and determined the energy barrier for the BERs in different networks.
Abstract: Thermally malleable polymers which undergo covalent bond exchange reactions (BERs) have been shown to be able to rearrange their network topology at high temperatures without impairing the network integrity. At low temperatures, the BERs are so sluggish that the materials behave like traditional thermosetting polymers. In this paper, we demonstrated that the temperature dependent BER rate could be tuned by adjusting the stoichiometry of monomers. As the ratio of hard segments in the epoxy thermoset network is increased, the material's glass transition temperature (Tg) is increased, with a corresponding increase in the temperature required to achieve a given stress relaxation rate. The material stress relaxation behavior was studied from both a theoretical and experimental point of view. Based on the kinetics of BERs, we derived the detailed expression of stress relaxation time, which reveals an Arrhenius type dependency of material relaxation behavior on the applied temperature. Subsequently, from the experimental stress relaxation curves, we determined the energy barrier for the BERs in different networks. With the Tg being elevated from 30.3 °C to 63.0 °C, the BER energy barrier is linearly increased from 68.2 kJ mol−1 to 97.3 kJ mol−1. Such a correlation between these two thermomechanical behaviors provides an additional design parameter (beyond catalyst choice) which can aid in achieving highly tunable service conditions for practical engineering applications of thermally malleable thermosets.