Showing papers on "Glass transition published in 2017"

Photoswitching of glass transition temperatures of azobenzene-containing polymers induces reversible solid-to-liquid transitions

Abstract: The development of polymers with switchable glass transition temperatures (Tg) can address scientific challenges such as the healing of cracks in high-Tg polymers and the processing of hard polymers at room temperature without using plasticizing solvents. Here, we demonstrate that light can switch the Tg of azobenzene-containing polymers (azopolymers) and induce reversible solid-to-liquid transitions of the polymers. The azobenzene groups in the polymers exhibit reversible cis-trans photoisomerization abilities. Trans azopolymers are solids with Tg above room temperature, whereas cis azopolymers are liquids with Tg below room temperature. Because of the photoinduced solid-to-liquid transitions of these polymers, light can reduce the surface roughness of azopolymer films by almost 600%, repeatedly heal cracks in azopolymers, and control the adhesion of azopolymers for transfer printing. The photoswitching of Tg provides a new strategy for designing healable polymers with high Tg and allows for control over the mechanical properties of polymers with high spatiotemporal resolution.

Polymer Composite with Improved Thermal Conductivity by Constructing a Hierarchically Ordered Three-Dimensional Interconnected Network of BN.

Abstract: In this work, we report a fabrication of epoxy resin/ordered three-dimensional boron nitride (3D-BN) network composites through combination of ice-templating self-assembly and infiltration methods. The polymer composites possess much higher thermal conductivity up to 4.42 W m–1 K–1 at relatively low loading 34 vol % than that of random distribution composites (1.81 W m–1 K–1 for epoxy/random 3D-BN composites, 1.16 W m–1 K–1 for epoxy/random BN composites) and exhibit a high glass transition temperature (178.9–229.2 °C) and dimensional stability (22.7 ppm/K). We attribute the increased thermal conductivity to the unique oriented 3D-BN thermally conducive network, in which the much higher thermal conductivity along the in-plane direction of BN microplatelets is most useful. This study paves the way for thermally conductive polymer composites used as thermal interface materials for next-generation electronic packaging and 3D integration circuits.

Glass transition of polymers in bulk, confined geometries, and near interfaces

Abstract: When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass - a solid lacking long-range order. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.

The glassy state of matter: Its definition and ultimate fate

Abstract: The objective of this communication is to clarify the meanings of solid and liquid, to dwell on the ultimate fate of glass in the limit of infinitely long time, and to propose a modern, improved definition of glass. We review the four characteristic states of matter related to vitrification: the stable equilibrium liquid (L), the metastable supercooled liquid (SCL), the unstable nonequilibrium glass (G), and the stable crystal (C). We also discuss some relevant terms and phenomena, including glass transition, crystallization, non-crystalline, amorphous, solid, and frozen. We review several previously published definitions of glass and finally propose an improved definition in two alternative forms. The first improved definition is: “Glass is a nonequilibrium , non-crystalline state of matter that appears solid on a short time scale but continuously relaxes towards the liquid state.” This is an intuitive description for the general public and young students. An alternative, more detailed definition to be understood and used by advanced students, researchers, and professors is: “Glass is a nonequilibrium , non-crystalline condensed state of matter that exhibits a glass transition. The structure of glasses is similar to that of their parent supercooled liquids (SCL) , and they spontaneously relax toward the SCL state. Their ultimate fate , in the limit of infinite time, is to crystallize.” This definition is for experts who understand the meaning of glass transition.

Lithium Ion Conducting Poly(ethylene oxide)-Based Solid Electrolytes Containing Active or Passive Ceramic Nanoparticles

Abstract: Polymer-based solid electrolytes containing ceramic nanoparticles are attractive alternatives to liquid electrolytes for high-energy density Li batteries. In this study, three different types of fillers have been dispersed in poly(ethylene) oxide (PEO) polymer matrices, and the effects on the resulting ionic conductivity of the nanocomposites have been examined. In this respect, the efficacy of one active, liquid-feed flame spray pyrolysis synthesized amorphous Li1.3Al0.3Ti1.7(PO4)3 (LATP), and two passive filler materials, TiO2 and fumed silica nanoparticles, are compared. Nanocomposite electrolytes are prepared with up to 20 wt % particle loadings. PEO/LiClO4 with 10 wt % LATP nanoparticles exhibits an ionic conductivity of 1.70 × 10–4 S·cm–1 at 20 °C, the highest among the surveyed systems, despite exhibiting comparable or higher degrees of crystallinity and glass transition temperatures than the systems containing passive fillers. The ionic conductivity of the composites with LATP nanoparticles exceed...

Effect of Selective Contacts on the Thermal Stability of Perovskite Solar Cells.

Abstract: Thermal stability of CH3NH3PbI3 (MAPbI3)-based perovskite solar cells was investigated for normal structure including the mesoporous TiO2 layer and spiro-MeOTAD and the inverted structure with PCBM and NiO. MAPbI3 was found to be intrinsically stable from 85 °C to 120 °C in the absence of moisture. However, fast degradation was observed for the encapsulated device including spiro-MeOTAD upon thermal stress at 85 °C. Photoluminescence (PL) intensity and the time constant for charge separation increased with thermal exposure time, which is indicative of inhibition of charge separation from MAPbI3 into spiro-MeOTAD. A full recovery of photovoltaic performance was observed for the 85 °C-aged device after renewal with fresh spiro-MeOTAD, which clearly indicates that thermal instability of the normal structured device is mainly due to spiro-MeOTAD, and MAPbI3 is proved to be thermally stable. Spiro-MeOTAD with additives was crystallized at 85 °C due to a low glass transition temperature, and hole mobility was s...

Diffusive dynamics during the high-to-low density transition in amorphous ice

Abstract: Water exists in high- and low-density amorphous ice forms (HDA and LDA), which could correspond to the glassy states of high- (HDL) and low-density liquid (LDL) in the metastable part of the phase diagram. However, the nature of both the glass transition and the high-to-low-density transition are debated and new experimental evidence is needed. Here we combine wide-angle X-ray scattering (WAXS) with X-ray photon-correlation spectroscopy (XPCS) in the small-angle X-ray scattering (SAXS) geometry to probe both the structural and dynamical properties during the high-to-low-density transition in amorphous ice at 1 bar. By analyzing the structure factor and the radial distribution function, the coexistence of two structurally distinct domains is observed at T = 125 K. XPCS probes the dynamics in momentum space, which in the SAXS geometry reflects structural relaxation on the nanometer length scale. The dynamics of HDA are characterized by a slow component with a large time constant, arising from viscoelastic relaxation and stress release from nanometer-sized heterogeneities. Above 110 K a faster, strongly temperature-dependent component appears, with momentum transfer dependence pointing toward nanoscale diffusion. This dynamical component slows down after transition into the low-density form at 130 K, but remains diffusive. The diffusive character of both the high- and low-density forms is discussed among different interpretations and the results are most consistent with the hypothesis of a liquid–liquid transition in the ultraviscous regime.

Synthesis, thermal and structural properties of pure TeO2 glass and zinc-tellurite glasses

Abstract: We have synthesized pure TeO2 glass and glasses in the systems xZnO − (1 − x)TeO2 (0 ≤ x ≤ 0.50) and yAl2O3 − (1 − y)TeO2 (0 ≤ y ≤ 0.03) by melting in Pt crucibles, and measured their glass transition temperature (Tg), density (ρ) and Raman spectra to correlate glass properties with structure. For pure TeO2 glass, synthesized using our newly developed intermittent quenching technique, we find onset- and midpoint-Tg at 301.1 and 306.7 °C and ρ = 5.62 g/cm3, in clear disagreement with TeO2 glass melted in alumina crucible for which we find Tg ≈ 380 °C and ρ = 4.86 g/cm3. This latter method, used frequently in the literature, was shown by Raman spectroscopy to introduce Al2O3 in the tellurite matrix which becomes cross-linked by Te-O-Al bridges, resulting in the increase of Tg and decrease of ρ. Raman spectroscopy showed also that doping TeO2 with ZnO or Al2O3 causes the progressive conversion of TeO4 trigonal bipyramids to TeO3 + 1 polyhedra with two terminal oxygens, and then to TeO3 trigonal pyramids with three terminal oxygens. This structural transformation is reflected in the composition dependence of the volume per mole TeO2 evaluated from density data. The ZnO-dependence of this parameter is described by two linear parts with an inflection point at x = 0.25, which indicates an increasing rate of forming terminal Te O bonds at higher ZnO contents. The Tg was found to increase with ZnO and Al2O3 contents and this was attributed to the glass-forming ability of both oxides, while density was found to decrease due mainly to replacement of the heavier TeO2 by the lighter ZnO and Al2O3. The results of this study are discussed with reference to previous works on TeO2 and zinc-tellurite glasses.

50th Anniversary Perspective: Challenges in the Dynamics and Kinetics of Glass-Forming Polymers

Abstract: The phenomenology of the glass transition and the associated behavior in the near liquid and glassy states are detailed, including the cooling rate dependence of the glass transition, Kovacs’ three signatures of structural recovery, and enthalpy overshoots. Dynamics in the liquid regime just above Tg and the associated temperature dependences are also covered since this behavior is important to understanding the glassy dynamics. The current models of structural recovery and their shortcomings are presented. A number of important unanswered questions are discussed, including how the relaxation time in the glassy state depends on structure, the relationship between the evolution of different properties, the resolution of the Kauzmann paradox, and the behavior of the equilibrium relaxation time below Tg. New experimental approaches are needed to make breakthroughs, such as two that are described: one involving 20 Ma amber to test whether the Vogel temperature dependence continues for the equilibrium state be...

Recyclable high-performance epoxy based on transesterification reaction

Abstract: Self-healing thermoset epoxy based on dynamic covalent bond chemistry has been developed in the past several years, which warrants the creation of recyclable epoxy. However, the existing systems produce epoxy that has lower strength, stiffness, and glass transition temperature, making them unsuitable for load-bearing structures. In this study, we developed a new recyclable thermoset epoxy through solid form recycling. The epoxy has strength, stiffness, and glass transition temperature similar to those found in conventional thermoset epoxy. The effect of healing temperature, healing time, healing pressure, and powder size on the healing efficiency was experimentally investigated. It was found that the healing efficiency is as high as 88.1%, and the epoxy can be recycled more than one time.

Development of Antimicrobial Packaging Film Made from Poly(Lactic Acid) Incorporating Titanium Dioxide and Silver Nanoparticles.

Abstract: Polylactide (PLA)/nano-TiO₂ and PLA/nano-TiO₂/nano-Ag blends films were prepared by a solvent volatilization method. Compared to pure PLA film, the nano-blend films have low water vapor permeability (WVP) and a poor transparency. With the increase of the NPs in the PLA, the tensile strength (TS) and elastic modulus (EM) decreased, while the elongation at break (e) increased. SEM analysis indicated a rougher cross-section of the nano-blend films. According to the FTIR analysis, no new chemical bonds were formed in the nano-blend films. By using DSC to examine the crystallization and melting behavior, the result shows that the NPs have no effect on the glass transition (Tg) and melting temperature (Tm), but they caused an increase on the cold crystallization (Tc) and crystallinity (Xc). TGA results show that the addition of nanoparticles significantly improved the thermal stability. The PLA nano-blend films show a good antimicrobial activity against. E. coli and Listeria monocytogenes. Most important, we carried out migration tests, and verified that the release of NPs from the nano-blend films was within the standard limits.

Resilience of Malic Acid Natural Deep Eutectic Solvent Nanostructure to Solidification and Hydration.

Abstract: Little is presently known about the unique nanostructure of deep eutectic solvents (DES). The order of the liquid–solid phase transition is contended and whether DES-water mixtures are merely aqueous solutions, or have properties dominated by the eutectic pair, is unclear. Here, we unambiguously show the structure of choline chloride-malic acid (malicine) as a liquid, and also in solid and hydrated forms, using neutron total scattering on D/H isotope-substituted samples, and quasi-elastic neutron scattering (QENS). Data were refined using empirical potential structure refinement. We show evidence for a stoichiometric complex ion cluster in the disordered liquid, with strong choline–chloride bonding and a hydrogen bond donor (HBD) contribution. The 1:1 eutectic stoichiometry makes these ionic domains more well-defined, with less HBD clustering than seen previously for reline. There is minimal structural difference for the solidified material, demonstrating that this DES solidification is a glass transition...

Relationship between local structure and relaxation in out-of-equilibrium glassy systems.

Abstract: The dynamical glass transition is typically taken to be the temperature at which a glassy liquid is no longer able to equilibrate on experimental timescales. Consequently, the physical properties of these systems just above or below the dynamical glass transition, such as viscosity, can change by many orders of magnitude over long periods of time following external perturbation. During this progress toward equilibrium, glassy systems exhibit a history dependence that has complicated their study. In previous work, we bridged the gap between structure and dynamics in glassy liquids above their dynamical glass transition temperatures by introducing a scalar field called “softness,” a quantity obtained using machine-learning methods. Softness is designed to capture the hidden patterns in relative particle positions that correlate strongly with dynamical rearrangements of particle positions. Here we show that the out-of-equilibrium behavior of a model glass-forming system can be understood in terms of softness. To do this we first demonstrate that the evolution of behavior following a temperature quench is a primarily structural phenomenon: The structure changes considerably, but the relationship between structure and dynamics remains invariant. We then show that the relaxation time can be robustly computed from structure as quantified by softness, with the same relation holding both in equilibrium and as the system ages. Together, these results show that the history dependence of the relaxation time in glasses requires knowledge only of the softness in addition to the usual state variables.

Investigation of structural, thermal properties and shielding parameters for multicomponent borate glasses for gamma and neutron radiation shielding applications

Abstract: Multicomponent borate glasses with the chemical composition (60 − x) B2O3–10 Bi2O3–10 Al2O3–10 ZnO–10 Li2O–(x) Dy2O3 or Tb4O7 (x = 0.5 mol%), and (60 − x − y) B2O3–10 Bi2O3–10 Al2O3–10 ZnO–10 Li2O–(x) Dy2O3–(y) Tb4O7 (x = 0.25, 0.5, 0.75, 1.0, 1.5, and 2.0 mol%, y = 0.5 mol%) have been fabricated by a conventional melt-quenching technique and were characterized by X-ray diffraction (XRD), Attenuated Total reflectance-Fourier transform Infrared (ATR-FTIR) spectroscopy, Raman spectroscopy, thermo-gravimetric analysis (TGA), and differential scanning calorimetry (DSC). Also, the radiation shielding parameters such as mass attenuation coefficient (μ/ρ), half value layer (HVL), mean free path (MFP) and exposure buildup factor (EBF) values were explored within the energy range 0.015 MeV–15 MeV using both XCOM and MCNPX code to determine the penetration of gamma and neutron radiations in the prepared glasses. The main BO3, BO4, BiO6, and ZnO4 structural units and AlOAl bonds were confirmed by ATR-FTIR and Raman spectroscopy. Weight loss, and the glass transition (Tg), onset crystallization (Tx), and crystallization (Tc) temperatures were determined from TGA and DSC measurements, respectively. The stability of the glass against crystallization (ΔT) is varied within the temperature range 114–135 °C for the studied glasses. In addition, the shielding parameters like the (μ/ρ) values investigated using both MCNPX Monte Carlo and XCOM software are in good agreement with each other. The (μ/ρ) values calculated using XCOM software were used to evaluate the HVL and MFP in the photon energy range 0.015 MeV–15 MeV. It is found that all the synthesized glasses possess better shielding properties than ordinary concrete, zinc oxide soda lime silica glass and lead zinc phosphate glass indicating the high potentiality of the prepared glasses to be utilized as radiation shielding materials.

Preparation and characterization of gel polymer electrolytes using poly(ionic liquids) and high lithium salt concentration ionic liquids

Abstract: Polymerized ionic liquids or poly(ionic liquids) (polyILs) have been considered as promising hosts for fabrication of gel polymer electrolytes (GPEs) containing ionic liquids. In this work, a novel GPE based on a polyIL, poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PDADMA TFSI), and a high lithium-concentration phosphonium ionic liquid, trimethyl(isobutyl)phosphonium bis(fluorosulfonyl)imide (P111i4FSI), is prepared. The composition-dependent behaviour of the GPEs is investigated by differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS) and solid-state nuclear magnetic resonance (solid-state NMR). The effects of Al2O3 nano-particles on the polymer electrolyte properties are also discussed. It is shown that the introduction of high lithium-concentration ionic liquids into the polyIL can effectively decrease the glass transition temperature (Tg) of the resulting GPE, leading to improved ion dynamics and higher ionic conductivity. The Al2O3 nano-particles effectively enhanced the mechanical stability of the GPEs. Most importantly, although adding PDADMA TFSI to the ionic liquids decreases the diffusion coefficient of both Li+ and anions, a greater decrease in the anion diffusion is observed, resulting in a higher Li+ transport number (as evaluated by NMR) than that seen in the original ILs. Finally, a highly conductive free-standing GPE membrane is fabricated, and extremely stable lithium symmetrical cell performance is demonstrated.

NH4NO3 as charge carrier contributor in glycerolized potato starch-methyl cellulose blend-based polymer electrolyte and the application in electrochemical double-layer capacitor

Abstract: Potato starch (PS)-methyl cellulose (MC) blend solid biopolymer electrolytes infused with ammonium nitrate (NH4NO3) and glycerol as plasticizer are made via the solution cast technique Fourier transform infrared (FTIR) spectroscopy indicates that NH4NO3 has interacted with the polymer blend host The addition of 40 wt% glycerol in the highest conducting plasticizer free electrolyte has improved the conductivity to the order of ∼10−3 S cm−1 The thermal stability of the electrolytes is identified by thermogravimetric analysis (TGA) Result from X-ray diffraction (XRD) analysis shows that the electrolyte with maximum conductivity value has the lowest degree of crystallinity Differential scanning calorimetry (DSC) analysis reveals that the highest conducting plasticized electrolyte possesses the lowest glass transition temperature (T g) of −275 °C Conductivity trend is further verified by dielectric analysis Transference numbers of ion (t ion) and electron (t e) for the highest conducting electrolyte are identified to be 098 and 002, respectively, confirming that ions are the dominant charge carriers Linear sweep voltammetry (LSV) evaluates that the potential window for the electrolyte is 188 V The internal resistance of the electrochemical double-layer capacitor (EDLC) is between 29 and 64 Ω From the charged-discharged measurement, the value of C s is 31 F g−1 The EDLC is stable over 1000 cycles

Predicting the glass transition temperature and viscosity of secondary organic material using molecular composition

Abstract: . Secondary organic aerosol (SOA) accounts for a large fraction of submicron particles in the atmosphere. SOA can occur in amorphous solid or semi-solid phase states depending on chemical composition, relative humidity (RH), and temperature. The phase transition between amorphous solid and semi-solid states occurs at the glass transition temperature ( Tg) . We have recently developed a method to estimate Tg of pure compounds containing carbon, hydrogen, and oxygen atoms (CHO compounds) with molar mass less than 450 g mol −1 based on their molar mass and atomic O : C ratio. In this study, we refine and extend this method for CH and CHO compounds with molar mass up to ∼ 1100 g mol −1 using the number of carbon, hydrogen, and oxygen atoms. We predict viscosity from the Tg -scaled Arrhenius plot of fragility (viscosity vs. Tg∕T) as a function of the fragility parameter D . We compiled D values of organic compounds from the literature and found that D approaches a lower limit of ∼ 10 ( ± 1.7) as the molar mass increases. We estimated the viscosity of α -pinene and isoprene SOA as a function of RH by accounting for the hygroscopic growth of SOA and applying the Gordon–Taylor mixing rule, reproducing previously published experimental measurements very well. Sensitivity studies were conducted to evaluate impacts of Tg , D , the hygroscopicity parameter ( κ) , and the Gordon–Taylor constant on viscosity predictions. The viscosity of toluene SOA was predicted using the elemental composition obtained by high-resolution mass spectrometry (HRMS), resulting in a good agreement with the measured viscosity. We also estimated the viscosity of biomass burning particles using the chemical composition measured by HRMS with two different ionization techniques: electrospray ionization (ESI) and atmospheric pressure photoionization (APPI). Due to differences in detected organic compounds and signal intensity, predicted viscosities at low RH based on ESI and APPI measurements differ by 2–5 orders of magnitude. Complementary measurements of viscosity and chemical composition are desired to further constrain RH-dependent viscosity in future studies.

Structure-property correlations in TiO 2 -Bi 2 O 3 -B 2 O 3 -TeO 2 glasses

Abstract: Tellurite and borotellurite glasses containing Bi 2 O 3 and TiO 2 were prepared and structure-property correlations were carried out by density measurements, X-ray diffraction, Differential Scanning Calorimetry (DSC), Raman, FTIR, UV–visible and 11 B MAS-NMR studies Titanium-tellurite and titanium bismuth tellurite glasses require high melt-cooling rates and were prepared by splat quenching On adding B 2 O 3 , the glass forming ability (GFA) enhances considerably and bulk glasses could be synthesized at lower quenching rates The density of glasses shows a direct correlation with molecular mass of the constituents UV–visible studies were used to determine the optical band gap and refractive index Raman studies found that the co-ordination number of tellurium ions with oxygen (N Te O ) remains constant with variation in TiO 2 molar content, while the incorporation of B 2 O 3 and Bi 2 O 3 decreases N Te O DSC studies show that the glass transition temperature (T g ) increases with B 2 O 3 and TiO 2 concentrations and that T g correlates well with bond enthalpy of the metal oxides 11 B MAS-NMR studies found that the co-ordination number of boron with oxygen (N B O ) decreases with increase in B 2 O 3 content while increasing TiO 2 and Bi 2 O 3 concentration does not significantly modify N B O

Thiol-acrylate main-chain liquid-crystalline elastomers with tunable thermomechanical properties and actuation strain

Abstract: The purpose of this study was to investigate the influence of cross-linking on the thermomechanical behavior of liquid-crystalline elastomers (LCEs). Main-chain LCE networks were synthesized via a thiol-acrylate Michael addition reaction. The robust nature of this reaction allowed for tailoring of the behavior of the LCEs by varying the concentration and functionality of the cross-linker. The isotropic rubbery modulus, glass transition temperature, and strain-to-failure showed strong dependence on cross-linker concentration and ranged from 0.9 MPa, 3 °C, and 105% to 3.2 MPa, 25 °C, and 853%, respectively. The isotropic transition temperature (Ti) was shown to be influenced by the functionality of the cross-linker, ranging from 70 °C to 80 °C for tri- and tetra-functional cross-linkers. The magnitude of actuation can be tailored by controlling the amount of cross-linker and applied stress. Actuation increased with increased applied stress and decreased with greater amounts of cross-linking. The maximum strain actuation achieved was 296% under 100 kPa of bias stress, which resulted in work capacity of 296 kJ/m3 for the lowest cross-linked networks. Overall, the experimental results provide a fundamental insight linking thermomechanical properties and actuation to a homogenous polydomain nematic LCE networks with order parameters of 0.80 when stretched. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 157–168

Irreversible Adsorption Erases the Free Surface Effect on the Tg of Supported Films of Poly(4-tert-butylstyrene)

Abstract: When cooled at constant rate, a 25 nm thin film of poly(4-tert-butylstyrene) vitrifies 50 K lower than in bulk. This record sets the largest depression in thermal glass transition temperature (Tg) ever observed upon confinement at the nanoscale level. Same as for other supported polymer layers, this reduction in Tg has been attributed to the presence of a free surface, the ensemble of molecules at the interface with air remaining in the liquid state also at temperatures well below bulk Tg. Here, we verify that such tremendous shifts can be erased upon prolonged annealing in the liquid state, hinting at a metastable nature of confinement effects. We demonstrate that the recovery of bulk behavior and the manifestation of the free surface are enslaved to the kinetics of irreversible adsorption of chains on the supporting substrate.

Intrinsic low dielectric constant polyimides: relationship between molecular structure and dielectric properties

Abstract: Low dielectric organic materials have been studied extensively, however, the relationship and regularity between the molecular structure and the dielectric properties is still not clear and barely reported. In this study, we report the synthesis of three polyimides (PPy6F, mBPPy6F and mTPPy6F) containing the same diphenylpyridine core structure but different side-chains with varied number of benzene rings. The polyimides PPy6F, mBPPy6F and mTPPy6F which contain one, two and three benzene rings in the pendant group, respectively, show intrinsic dielectric constant k values of 2.81 (PPy6F), 2.61 (mBPPy6F) and 2.44 (mTPPy6F). The reason for the reduced k value with increasing number of benzene rings in the pendant group is discussed through the investigation of morphology, density and water absorption properties of these polyimides. This discovery provides us with a new and easy strategy to obtain lower dielectric polymers. Polyimide mTPPy6F shows excellent comprehensive properties, with a high glass transition temperature (Tg) of 342 °C, a 5 wt% loss temperature (Td,5%) of 551 °C, tensile strength of 105 MPa, low moisture absorption of only 0.61% and good solubility in common organic solvents.