Relationship between melt viscosity and dielectric relaxation time for a series of epoxide oligomers

Melt viscosity has been investigated for a series of bisphenol-A type epoxide oligomers with different weight-average mol wts (M?_w), ranging from 388 to 2640. The temperature dependence of the melt viscosity is described by the Williams–Landel–Ferry (WLF) equation. The melt viscosity η is correlated with both the direct current (dc) conductivity σ and the dielectric relaxation time τ. The two relationships between these three properties, σ·η^κ = const (0.63 ≦ κ ≦ 1.12) and η/τ^? = const (0.73 ≦ ? ≦ 1.06), are experimentally derived. Both exponents, κ and ?, depend on the M?_w of the oligomer. The lower M?_w oligomer has the larger value of κ. The κ value is close to unity for the low M?_w oligomer, which agrees with Walden's rule, σ·.η = const, applicable to most low mol wt liquids. The ? value is near unity for the epoxide oligomer with higher M?_w than 2000, which means that the melt viscosity is proportional to the dielectric relaxation time. The low M?_w oligomer (M?_w < 2000), on the other hand, has a smaller value of ? below unity. The result indicates that the melt viscosity is not proportional to the dielectric relaxation time for the low M?_w epoxide oligomer, whose dielectric α-relaxation is not governed by the Debye equation. © 1993 John Wiley & Sons, Inc.     

Dielectric properties above the glass transition for a series of epoxide prepolymers

Dielectric properties above the glass transition have been investigated for a series of bisphenol-A type epoxide prepolymers (388 ≤ M?_w ≤ 2640). Dielectric measurements were performed over a frequency range of 50 Hz–1 MHz using a vertical parallel plate cell which was constructed in the laboratory. The dielectric α-relaxation for each prepolymer fits the empirical model of the Havriliak–Negami equation. The temperature dependence of the dielectric relaxation time τ is described by the Williams–Landel–Ferry (WLF) equation as well as that of the direct current conductivity σ, which can be measured using the same cell. The relationship between τ and σ, σ^· τ^m = const, is derived from experimental results. The exponent m, which depends on the molecular weight of the prepolymer, is considered to correspond to the ratio of the segmental mobility to ionic mobility. The dielectric loss ε″ can be used as an indicator of the direct current conduction in the temperatures where the ionic component in ε″ becomes much larger than the dipole one.     

Thickness-dependent glass transition temperature of hydroxyethyl cellulose thin films based on dielectric properties

Hydroxyethyl cellulose (HEC) thin films with a molecular weight of 720,000 g/mol deposited by thermal evaporation in a thickness range of 250, 500, and 750 nm were measured in a frequency range of 1–10⁵ Hz and a temperature range of 233–373 K for dielectric characterization with increments of 10 K. Dielectric results were used to derive and evaluate the glass transition temperature and ductility, which are essential parameters for structural analysis. Results showed that the thickness of HEC thin films was an effective parameter on dielectric and structural properties. Because of the increasing thickness, the dielectric constant has values between 22 and 143 at 1 kHz, and glass transition temperature and ductility change between 211–175 K and 15–20, respectively. Based on the literature and the compatible results, the main effect of these variations could be dead layers and voids in structure. The effect of the dead layer gave an important idea about the adjustability of mechanical properties of HEC thin films depending on the thickness. In this way, it would be possible to use these thin films deposited from HEC with 720,000 g/mol molecular weight, especially in drug delivery, electrophoresis technologies, biomedical devices, and coverage applications.     

用差示扫描量热法测定橡胶填充油的玻璃化转变温度

采用差示扫描量热法(DSC)测定了不同组成的橡胶填充油的玻璃化转变温度(Tg),考察了操作条件对Tg的影响,并分析了测试方法的精密度和重复性.结果表明,试样用量和升温速率对测试结果均影响不大,综合考虑试样用量宜为5～20 mg,升温速率为20℃/min;DSC测试具有良好的精密度和重复性.     

Cure kinetics,glass transition temperature development,and dielectric spectroscopy of a low temperature cure epoxy/amine system

This article reports a study of the chemical cure kinetics and the development of glass transition temperature of a low temperature (40°C) curing epoxy system (MY 750/HY 5922). Differential scanning calorimetry, temperature modulated differential scanning calorimetry, and dielectric spectroscopy were utilized to characterize the curing reaction and the development of the cross‐linking network. A phenomenological model based on a double autocatalytic chemical kinetics expression was developed to simulate the cure kinetics behavior of the system, while the dependence of the glass transition temperature on the degree of cure was found to be described adequately by the Di Benedetto equation. The resulting cure kinetics showed good agreement with the experimental data under both dynamic and isothermal heating conditions with an average error in reaction rate of less than 2 × 10^?3 min^?1. A comparison of the dielectric response of the resin with cure kinetics showed a close correspondence between the imaginary impedance maximum and the calorimetric progress of reaction. Thus, it is demonstrated that cure kinetics modeling and monitoring procedures developed for aerospace grade epoxies are fully applicable to the study of low temperature curing epoxy resins. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Cure kinetics,gelation, and glass transition of a bisphenol F epoxide

Phenomenological cure kinetic relations for a commercially available amine‐cured epoxy were developed independently using differential scanning calorimetry (DSC) in isothermal and dynamic (constant heating rate) modes and using numerical optimization techniques. The resulting model was found to be independent of the method used to collect reaction rate data. The model was modified at high temperature to account for an observed deviation from Arrhenius behavior. The cure dependence of the glass transition of the material was also investigated with DSC. The gel point as measured by parallel plate rheometry was found to occur at a degree of cure of 0.71, a value significantly advanced from a simple Flory prediction based on the functionality of the cross‐linking agent.     

Rheological analysis of polyvinyl butyral near the glass transition temperature

In many polymer manufacturing operations, the material is processed near the glass transition temperature (T_g). Examples are thermoforming, blow molding, film blowing, hot embossing, forging, plastic welding, and de‐airing in safety glass lamination. In these processes, solid‐like behaviors such as strain hardening and yielding play important roles. These material properties cause the material to flow (or deform) in a way that substantially differs from a polymer melt. In order to understand the flow behavior near the T_g, polyvinyl butyral (PVB), a rubbery polymer used in safety glass lamination, was studied in this work. The flow properties of the polymer above the T_g were characterized by using both shear and elongational rheometers, and a tensile tester. The measured flow properties were described by a viscoelastic constitutive model.     

Thermoforming of a PMMA transparency near glass transition temperature

To simulate the thermoforming of a transparency, constitutive equations are proposed for the nonlinear viscoelastic behavior of poly(methyl methacrylate) near glass transition temperature, which include large deformations. In a first step, they are fitted on a set of uniaxial tension‐relaxation tests at various strain levels and strain rates. In a second step, their implementation in a finite element code is performed. Finally, the thermoforming of a transparency at a constant and uniform temperature is simulated and compared with experimental results. POLYM. ENG. SCI., 50:2004–2012, 2010. © 2010 Society of Plastics Engineers     

Determination of the glass transition temperature of polymer/layered silicate nanocomposites from positron annihilation lifetime measurements

The glass transitions of acrylonitrile-butadiene rubber (NBR)/organoclay nanocomposites with various silicate contents were investigated using positron annihilation lifetime spectroscopy (PALS). The nanocomposites were prepared through melt intercalation of NBR with various concentrations of organoclay (OC30B) modified with the organic modifier, methyl tallow bis(2-hydroxyethyl) quaternary ammonium (MT2EtOH), i.e., Cloisite^® 30B. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) measurements of the NBR/OC30B nanocomposites showed that the NBR chains were intercalated between the silicate layers, thereby increasing the gallery heights of the organosilicates. The glass transition temperature of NBR was determined using differential scanning calorimetry (DSC). However, it seemed to be very difficult to clearly resolve the very small differences in T_gs caused from various loading of nanosized silicate in NBR/OC30B nanocomposites. Hence, we performed positron annihilation lifetime spectroscopy (PALS) on NBR/OC30B nanocomposites containing various amounts of OC30B (1-10 wt%). Significant changes in the temperature dependencies of free volume parameters (i.e., lifetimes and intensities) were observed at the transition temperature, T_g,PALS, and the T_g,PALS values were found to increase with increasing organoclay content in the samples. These observations are consistent with PALS having a higher sensitivity in the detection of very small changes in free volume properties. The present findings thus highlight the usefulness of PALS for studying phase transition phenomena in polymeric materials with nanoscale structural variations.     

Low glass transition temperature aliphatic polyurethanes

Summary The present paper deals with the single step syntheses of a few aliphatic polyurethanes using some simple glycols like ethylene, propylene, 1,3- and 1,4-butylene glycols and two different bis(chloromethyl) compounds viz., 1,4-bis(chloromethyl)-2,5-dimethyl benzene (I), and 1,5-bis(chloromethyl)-2,4-dimethyl benzene (II). The glass transition temperatures, T_g, of these polymers were determined using dilatometric techniques and they ranged from –12 to –48°C. The polyurethanes derived from 1,4-butylene and ethylene glycols were amorphous gums with T_g well below –30°C.     

粉末涂料的玻璃化温度

重点讨论了粉末涂料玻璃化温度、玻璃化转变理论,并对玻璃化温度与粉末涂料稳定性、熔融黏度、热应力、分子量、化学结构和聚合物的关系做了全面的阐述。     

The glass transition temperature of star-shaped polystyrenes

The glass transition temperatures of linear and four- and six-branched regular star polystyrenes are measured by penetration, differential scanning calorimetry, and a density gradient technique. The results of the three methods show that the glass transition temperature depends on the concentration of chain ends in the polymer sample. An attempt is made to assess the influence of the presence of the branch point on the glass transition temperature using a series of four-branched polystyrenes containing varying amounts of isoprene near the branch point. However, any effect of the central branch point on the glass transition temperature is obscured by plastification of polystyrene by polyisoprene.     

Relationships between glass transition temperature and conversion

Summary The Couchman approach to the expression of the compositional variation of glass transition temperature (or the Di-Benedetto equation) can be used to evaluate the effect of molecular weight on T_g for a stepwise linear polymer. An expression for the K_L constant in the Fox-Flory equation is proposed. It is also possible to estimate the T_gL corresponding to the hypothetical linear structure defined in a three-dimensional network. For different epoxy-diamine systems, we have found a good concordance with the results obtained previously using an additive law.     

Permanent deformation in armos fibre at a temperature below the glass transition temperature

The character of accumulation of permanent deformation of Armos fiber as a function of the temperature was established. The important change in the character of ɛ_per.(_ass.) in the temperature region of 80–100°C can be attributed to breaking of hydrogen bonds of the “polymer-water-polymer’ type. The special character of accumulation of permanent deformation is observed in dried Armos fibre. This is hypothetically explained by the effect of two mechanisms: the absence of hydrogen bonds in the initial stage of deformation and their formation in subsequent deformation as a result of convergence of the molecules. Deceased St. Petersburg State University of Technology and Design. Translated fromKhimicheskie Volokna, No. 3, pp. 36–38, May–June, 1999.     

The glass transition temperature of a filled binder by nuclear magnetic resonance and thermal mechanical analysis

The glass transition temperatures of a binder and filled binder were determined by wide line proton nuclear magnetic resonance and by thermal mechanical analysis. The NMR study indicated only one transition temperature for the binder and three transition temperatures for the filled binder. TMA data gave two transition temperatures each for the binder and for the filled binder. These transition temperatures in the filled binder were interpreted as representing two phases: one for the filler-oriented binder and one for the bulk phase. A comparison of the transition temperatures of the binder and filled binder indicates that the transition temperature of the former is raised by the presence of the filler. It was also found that the presence of water lowered the transition temperature of the filled binder used in this work.     

Mobility and glass transition temperature of polymer nanospheres

Recent studies have illustrated a decoupling between cooperative segmental mobility and the glass transition temperature (T_g) of thin polymer films and nanocomposites. Here, we use dielectric spectroscopy to probe the cooperative segmental mobility and capacitive dilatometry to determine the T_g of films of polystyrene nanospheres with diameters (d) less than 400 nm. We find that both capacitive dilatometry and calorimetry revealed nearly identical suppressions in T_g as the size of the nanospheres was reduced. While T_g was impacted by confinement, in the range 130 nm ≤ d ≤ 400 nm, in stark contrast, the cooperative segmental mobility, i.e., the peak position of the α-relaxation process was not. Furthermore, when d ≤ 200 nm, an additional molecular relaxation process, not observed in bulk, was present. We interpret these findings as evidence of a decoupling between T_g and cooperative segmental mobility in nanospheres. That is, the latter may be impacted by confinement under conditions in which the former is not.