Glass Transition Point Tg Research Articles

Combined X-ray Reflectivity and Infrared Study of the Effect of Hydrogen Bonding of the OH Group on the Relaxation Behavior in Ultrathin Polyvinylphenol Films on SiO2.

Utilizing X-ray reflectivity and infrared reflection absorption spectroscopy (IR-RAS), we have investigated the thermal expansion and contraction of ultrathin polyvinylphenol (PVPh) films supported on a silicon (100) substrate capped with an amorphous SiO2 layer. Despite being known to form strong interactions with the SiO2 surface, the thin PVPh films showed a reduction in the glass-transition point Tg, similar to the behavior of polystyrene thin films deposited on SiO2. We explored the relationship between thermal expansivity and film thickness using well-annealed films and found that it decreases with film thickness in the range below twice the radius of gyration of a polymer chain (2Rg) in the glassy state. Thickness expansion in the glassy state and contraction in thickness at temperatures higher than Tg bulk (melt state) showed the presence of two competing relaxation processes. The reported negative thermal expansion in PVPh thin films, which was discovered to be one of the inherent properties, may have been caused by the fast relaxations that take place at the free polymer surface. IR-RAS was utilized to investigate the effect of thickness on hydrogen bonding in PVPh, and it was confirmed that with decreasing thickness, hydrogen bonding becomes weak, and the number of free OH groups increases. Therefore, thinner PVPh samples exhibit lower Tgs as an effect of easier molecular motions.

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.

The Effect of Disperse Fillers on the Thermomechanical Characteristics of Epoxy Polymers

The properties of filled polymer-composite materials based on epoxy oligomers are studied. The dependence of the thermomechanical parameters of the materials on the type and amount of a disperse filler in the composite is demonstrated. The following temperature parameters are determined: softening point Ts, glass-transition point Tg, and high elasticity point The. A strain inversion at T > The is detected. The effect of the type and amount of the filler on the properties, such as Brinnel hardness, compression rupture stress, and reversible deformation, at T > The is analyzed.

Interfacial leakage of elastomer seals at low temperatures

Interfacial leakage of air in hydrogenated nitrile butadiene rubber (HNBR) O-ring seals exposed to sub-ambient temperatures is studied. Flange-type fixtures with sealing surfaces produced by 3 different surface finish processes are used. When the seals are cooled down to temperatures below the elastomer glass transition point Tg of (−23 °C), an abrupt increase of air leakage (>10−2cm3/min) is observed. The effects of surface finish conditions, compression ratio, grease lubrication and additions of carbon black in the HNBR on the cold leakage are discussed. Persson's contact mechanics and effective medium leakage theory coupled with finite element analysis (FEA) of the HNBR seals are utilized to capture the changes in the contact area and pressure with cooling and predict the seal failure temperatures. The main cause for the cold seal failures is believed to be the detachment of the elastomer seals from their mating sealing parts due to the elastomer thermal contraction and the negligible recovery of the HNBR in cold environment. In addition, the adhesive rubber-substrate interface influences the detachment and seal failure.

Effects of the iodine incorporation on the structure and physical properties of Ge-Sb-Se chalcogenide glasses

(100-x)(Ge23Se67Sb10)-xI (x = 0, 3, 5, 7, 10 wt%) chalcohalide glasses were prepared by the traditional melt-quenched method. The structure and physical properties were analyzed by using XRD, TMA, FTIR and Vickers hardness tester. It was found that the glass structure trends to crystallization with increase of iodine content. The values of infrared transmittance decrease from 62% to 35% in the range of 2.5–16 μm with increasing iodine content. The density decreases from 4.62 ± 0.3 g cm−3 to 4.31 ± 0.3 g cm−3, the hardness decreases from 164.9 ± 0.5 kg mm−2 to 154.6 ± 0.5 kg mm−2. The glass transition point Tg decreases from 267 °C to 205 °C, while the thermal expansion coefficient increases with iodine content.

Dynamics and Geometry of Icosahedral Order in Liquid and Glassy Phases of Metallic Glasses

The geometrical properties of the icosahedral ordered structure formed in liquid and glassy phases of metallic glasses are investigated by using molecular dynamics simulations. We investigate the Zr-Cu alloy system as well as a simple model for binary alloys, in which we can change the atomic size ratio between alloying components. In both cases, we found the same nature of icosahedral order in liquid and glassy phases. The icosahedral clusters are observed in liquid phases as well as in glassy phases. As the temperature approaches to the glass transition point Tg, the density of the clusters rapidly grows and the icosahedral clusters begin to connect to each other and form a medium-range network structure. By investigating the geometry of connection between clusters in the icosahedral network, we found that the dominant connecting pattern is the one sharing seven atoms which forms a pentagonal bicap with five-fold symmetry. From a geometrical point of view, we can understand the mechanism of the formation and growth of the icosahedral order by using the Regge calculus, which is originally employed to formulate a theory of gravity. The Regge calculus tells us that the distortion energy of the pentagonal bicap could be decreased by introducing an atomic size difference between alloying elements and that the icosahedral network would be stabilized by a considerably large atomic size difference.

Dielectric relaxation in VDF60/Tr40 and VDF88/Te12 polar copolymers embedded in porous glass matrices

The dielectric properties of composite materials prepared by embedding P(VDF60/Tr40) and P(VDF88/Te12) polar copolymers in porous glass matrices with a mean flow-through pore diameter of around 320 nm were investigated in the temperature range 200–450 K. Strong dielectric relaxation, the characteristic time of which conformed to the Williams-Landel-Ferry law, was observed in the vicinity of glass transition point Tg of an amorphous fraction of polymer inclusions. An increase (≈10 K) in the Tg temperature of the amorphous fraction of incorporated polymeric materials was detected.

Critical Exponents of Photoinitiated Gelation at Different Light Intensities

A photo-differential scanning calorimetric (Photo-DSC) technique was used to study the photoinitiated radical polymerization of a 75% epoxy diacrylate (EA) and 25% tripropyleneglycoldiacrylate (TPGDA) mixture with 2-mercaptothioxanthone (TX-SH) as photoinitiator by using different light intensities. Photopolymerization reactions were carried out under identical conditions of temperature and initiator concentration. It was observed that all conversion curves during gelation at various UV light intensities present good sigmoidal behavior as predicted by the percolation model. Observations around the critical time, called the glass transition point (tg), taken for polymerization to reach the maximum rate (Rp max) show that the gel fraction exponents β obeyed the universal percolation picture. On the other hand, Rp max, t g, and final conversion values were found to be dependent on the UV light intensity.

Improvement on Injection Molded Products Appearance by Induction Heating Mold

In injection molding, surface defects such as weld-line and silver streaks are serious problems which trouble engineers. It is also known that these surface defects can be prevented by heating the mold cavity surface to above the resin melting point Tm or glass transition point Tg. In this study, we designed and made a rapid heating and cooling mold applying induction heating, which is capable of continuously heating and cooling the cavity surface rapidly. This mold has a cavity insert with coils and cooling channels arranged inside the mold base. Using the mold, heating and cooling molding and normal molding without performing heating and cooling were performed and effects on the external view and surface properties of rapidly heated and cooled molded products were investigated. Observation of the surface of the product molded using this mold showed that in normal molding, the formation of V-shaped weld-lines is seen on the molded product surface. On the other hand, when heating and cooling is performed in molding, no weld-lines are found, and compared to normal molding, a highly glossy molded product surface has been confirmed.

High Temperature Elastic Constants of Pd- and Zr-Based Bulk Metallic Glasses Studied by Electromagnetic Acoustic Resonance

Elastic constants Cij of Pd40Ni40P20 and Zr55Al10Ni5Cu30 bulk metallic glasses (BMGs) has been investigated by MHz frequency range electromagnetic acoustic resonance (EMAR) from ambient temperature to their glass transition point Tg. With increasing in temperature the BMGs show elastically plateau region in Cij(T) curves which is due to two competitive processes; elastic softening by anharmonicity and stiffening by structural relaxation. Significant elastic softening in Cij(T) curves was confirmed around Tg without any precursor phenomena. Our theoretical analysis revealed that elastic inhomogeneity can explain the characteristic elastic behaviors around Tg.

Temperature dependences of the density of borate glasses in equilibrium states at temperatures below the glass transition point

The kinetics of density relaxation during isothermal treatment of samples of vitreous boron oxide and sodium borate glasses (containing 15, 20, and 30 mol % Na2O) cooled from high temperatures and after heating of the samples stabilized at low temperatures is investigated over a wide range of temperatures below the glass transition point Tg. It is established that there exists a temperature Ts below which the density in the equilibrium state does not depend on the temperature, i.e., the temperature at which the supercooled liquid transforms into a new noncrystalline solid state. The distinguishing feature of this state is the absence of structural transformations after a change in the temperature. The temperatures of the transition of the supercooled liquid to the noncrystalline solid state and the ranges of lower temperatures at which the supercooled liquid can reach an equilibrium state within the limits of experimental error coincide with those previously obtained in the study of relaxation processes in these glasses by the small-angle X-ray scattering technique.

On the nature of the second-order optical nonlinearity of nanoinhomogeneous glasses in the Li2O-Nb2O5-SiO2 system

The submicroscopic structure of lithium niobium silicate glasses of the compositions 2xLiNbO3 · (1 − x)(Li2O · 2SiO2) (x = 0.40, 0.45, 0.50) and 30Li2O · 25Nb2O5 · 45SiO2 in the initial state and after heat treatment for different times at temperatures in the vicinity of the glass transition point T g are investigated using X-ray powder diffraction, small-angle neutron scattering (SANS), synchrotron small-angle X-ray scattering (SAXS), and electron microscopy. A nanostructure with inhomogeneities ∼40 A in size is formed in glasses at the initial stages of phase separation at temperatures in the range 600–670°C. This structure is responsible for the appearance of the second-order optical nonlinearity. The SANS, SAXS, and electron microscopic data on the inhomogeneity size are in good agreement with each other. According to the X-ray diffraction, SANS, and SAXS data, the ordering of the glass structure and the difference between the density of inhomogeneities and the density of the matrix increase in the course of heat treatment. At the initial stage of amorphous phase separation, the glass decomposes into regions enriched in SiO2 and regions with an increased content of lithium and niobium. An increase in the temperature or time of heat treatment results in the precipitation of LiNbO3 ferroelectric crystals. The results obtained allow us, for the first time, to make the inference that nanoscale changes in the glass structure lead to considerable changes (by one order of magnitude and more) in the quadratic optical nonlinearity, which can be controlled by heat treatment. The origin of the second-order optical nonlinearity is associated with both the nanosized modulations of the polarizability due to the inhomogeneous glass structure and the polarity of structural nanoinhomogeneities from which the LiNbO3 phase precipitates at the later stages of phase separation.

Thermal Effect of Phosphate Salt Additives on Poly(L‐Lactic Acid) and the Coordination Structure

Poly(L‐lactic acid) (PLLA) was mixed with a few weight percent of magnesium or calcium hydrogen phosphate to improve the thermal property. Calcium hydrogen phosphate as an additive to PLLA was found to be more effective in increasing the glass transition point (Tg) than magnesium hydrogen phosphate, which was investigated by the differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Studies on the coordination structure for the polymer complex (ca‐(plla)) confirmed by infrared (IR) spectroscopy and extended X‐ray absorption fine structure (EXAFS) indicated that calcium ion radius would be suitable for the coordination with carbonyl groups of polymer chains.

Can the glass transition in bulk polymers be modeled by percolation picture?

Recent observations (Eur. Phys. J. E 9, 135 (2002)) showed that the vitrification process, which sets in during the linear bulk methyl methacrylate (MMA) polymerization carried out below glass transition temperatures, can be modelled by static percolation picture. To generalize this observation for different kind of bulk linear or crosslinked polymers not enough data are present in the literature. To cover partly this deficit we studied the glass transition of MMA and styrene (Sty) crosslinking copolymerization in varying ratios of MMA and Sty. Both the fluorescence intensity I and the lifetime tau of pyrene (Py) used as a nanosecond in situ fluoroprobe were monitored during the gelation time. Both I and tau increase dramatically as a result of the reduced mobility of the probes trapped in the "glassy" regions, appearing near the glass transition point. The average size of the glassy regions just below, and the strength of the infinite network formed upon the connection of the glassy regions above the glass transition point tg obey power law relations. The data around tg were interpreted on the basis of the percolation theory and we observed that the corresponding exponents gamma and beta give static percolation values independent of the polymer composition.

Properties of pressure-sensitive adhesives based on styrene copolymers

Pressure-sensitive adhesives (PSA) represent a specific kind of adhesives which are able to form adhesive joints practically solely owing to physical interaction between adhesive and adherent. PSA must possess apparently conflicting properties. As the formation of an adhesive joint necessitates perfect contact at the molecular level, the mobility of adhesive molecules has to be sufficiently high, i.e., the adhesive must possess adequate viscoelastic properties. On the other hand, it is valid in general that the cohesive strength of the joint decreases with diminishing viscosity of adhesive. The perfect contact between adhesive and adherent is not only dependent on the viscoelastic properties of the adhesive but also on its ability to wet the surface of the adherent which is connected with interfacial tension at the boundary of the adhesive joint. It is therefore advisable for the adhesive to have the highest possible value of surface free energy. While the used rubber or elastomeric copolymer has decisive influence on elastic properties of PSA, the lowmolecular tackifier resins or tacky admixtures significantly influence the viscoelastic properties of PSA and its ability to wet the surface of adherent. The block copolymers of elastomers with styrene, natural rubber, random polymer butadiene-styrene rubber, butyl rubber or polyacrylate are polymers most widely used as PSA [1–3]. Besides polymer, further components of PSA are tackifiers, softeners and stabilizer [4–6]. As for these admixtures, the utilization properties of PSA are significantly affected by tackifiers, which have decisive influence on its adhesive properties. A good tackifier resin must have the following basic properties. It must be sufficiently compatible with fundamental polymer and have very low molecular weight when compared with the used elastomer, and its glass transition point must be higher than that of the polymer used [7]. The majority of applied tackifiers have the glass transition point Tg in the temperature region between +30 ◦C and +60 ◦C. There are two fundamental groups of tackifier’s resins that are most frequently used for the preparation of PSA, i.e., derivatives of colophony or aliphatic hydrocarbon resins. In this study we used the PSA modified by colophony resins and derivatives (Union Camp Chemicals, England; Spolchemie, Czech Republic) as well as hydrogenated poly (cyclopentadiene) Escorez 530 D (Shell, USA). The softeners differ from tackifiers, in particular, by the low value of their Tg. In this line we used polybutene oligomers (British Chemicals, England), polypropylene oligomers (Slovnaft, Slovak Republik). Butadiene-styrene (BS) rubber Kralex 1507 (Kaucuk Kralupy, Czech Republic) and carboxylated butadienestyrene rubber LBSK 70-48 (Zaklady Chemiczne, Poland) were used as base for PSA and statistical (styrene-2-ethyl hexyl acrylate) (St–EHA) copolymer containing 14 mass% of styrene (Synpo, Czech Republic) was used as a PSA modifier. The adhesive mixtures were prepared in a mixer at 413 K. The mixture (100 g) was agitated for 30 min. The mechanical work of adhesion [8] was measured by peeling the adhesive joint at 90 ◦ angle on a universal testing machine Instron 4301 by means of an aluminum-peeling ring. The measurements were carried out at a cross head speed of 50 mm/min. The tack adhesion was measured on a prototype device by the Probetack method by which the tractive force perpendicular to the bonded joint necessary for its disconnection was measured. The adhesive joint was formed by short and light touch—0.2 s; 2 kPa. The surface free energy of the polymers was measured by a goniometric method on a Contact Angle Meter Zeiss (Zeiss, Germany) by determining the contact angles of a set of testing liquids deposited on the polymeric surface or on the surface of the substrate. The interfacial tensions were calculated from the values of polar and dispersion component of surface free energy of the polymers [9, 10]. The results of measurement of adhesive properties of PSA on the base of BS copolymers are presented in Figs 1–5. Fig. 1 represents the dependence of tack adhesion of BS copolymer (Kralex 1507) on content of oligomeric high-viscosity polybutene Hyvis 200. The tack adhesion of the mixture of BS copolymer with oligomer modifier increases with butene concentration from 5 to 75 kPa which corresponds to 20 mass% of oligomer in the mixture with BS copolymer. After reaching the maximum at the presence of 21 mass% of oligomer in the mixture its tack adhesion decreased with further increase of polybutene content because of decreasing cohesive strength of the BS copolymer/polybutene mixture. The investigation of the relationship between tack adhesion and concentration of softeners in the mixture with BS copolymer (Fig. 2) led to equal conclusion. In these experiments naftenic oil, polypropylene oligomer and polybutene Napvis 3 were used as modifiers. The increase in tackiness with increasing

Glass Transition and Crystallization of Glasses Based on Rare-Earth Borates

The glass formation is investigated and the highest contents of rare-earth oxides are determined in glasses of the Ln2O3–B2O3 system in the composition region (Ln2O3)1 + x · 3B2O3 (Ln = La, Nd, Gd, Er, Dy, Yb, Lu; 0 ≤ x ≤ 0.8). It is shown that the total content of rare-earth oxides in the La2O3–Ln2O3–B2O3 ternary system can substantially exceed their highest content in the binary systems. The devitrification of glasses in the La2O3–Ln2O3–B2O3 system at temperatures above the glass transition point Tg is analyzed.

Crystallization and Nonlinear Optical Properties of Potassium Niobium Silicate Glasses

The crystallization of the xK2O · xNb2O5· (1 – 2x)SiO2(x= 0.167–0.250) glasses and glasses close in composition to K2O · Nb2O5· 4SiO2at the ratio K2O : Nb2O5≠ 1 is investigated. In high-silica glasses, the metastable phase separation followed by the bulk multiphase crystallization are observed at temperatures close to the glass transition point Tg. The nanostructured transparent glasses that exhibit the optical second harmonic generation (SHG) effect are formed at the early stages of phase separation. The surface crystallization of glasses with the precipitation of the KNbSi2O7noncentrosymmetric phase occurs at higher temperatures.

Studies of Deformed Poly(Methyl Methacrylate) Using Positron Annihilation Lifetime Spectroscopy

Structural rearrangements in glassy PMMA on polymer plastic deformation and recovery of residual plastic deformation in glassy state were studied by positron annihilation lifetime spectroscopy. Uniaxial compression was shown to be accompanied by a decrease in concentration of free volume microregions in disordered polymer regions, which were characterized by low packing density. Recovery of residual deformation at elevated temperatures but below glass transition point Tg proceeds without any noticeable changes in fractional content of free volume both in disordered and ordered polymer regions. The advantages of positron annihilation lifetime spectroscopy for studying microstructure and structural rearrangements in polymers were discussed.

A possible new kind of glass state: Pinned-Domain-Wall glass (PDWG)

The dielectric measurements at 1kHz indicated that the low temperature side of dielectric loss (D) peak goes to zero rapidly at about 250K in TGS single crystal with the decreasing of temperature, which manifests the feature of phase transition. This behavior was not observed in LATGS singe crystal of mono-domain. Therefore, the anomaly in TGS is closely related to the domain walls. To explain this anomaly, a possible new kind of glass: Pinned-Domain-Wall glass (PDWG) is introduced which is similar to pinned-vortex glass, and the point at which the dielectric loss (D) goes to zero is defined as glass transition point (Tg). The experimental results was explained based upon the model.

Calculation of impact transition temperature of low density polyethylene from shift factor via a free volume approach

A relationship between the impact transition temperatureTi and the stress concentration factorKs is derived. The relationship involves the temperature shift factoraT; in turn,aT depends on the free volume. In earlier work in this problem Zewi and Corneliussen [6] utilized the W-L-F equation. Here a more direct relationship betweenaT and the free volume is applied. Satisfactory values ofKs corresponding to givenTi are obtained for a wide temperature range; the range also includes temperatures below the glass transition pointTg. The opinion that a free volume exists between 0 K andTg is upheld.