Prediction of glass transition temperatures of plasticized polymers

The classical free volume theory is modified semi-empirically to derive an expression useful for predicting the glass transition temperatures (T_g) for compatible polymer blends. This equation produces values of T_g in better agreement than those from the Kelley-Bueche equation with the dilatometric data for T_g of a number of plasticized polymers. The improvement is primarily attributed to the consideration of intermolecular interactions in the present investigation.     

The glass transition temperature of nonstoichiometric epoxy–amine networks

Glass transition temperatures (T_g) of nonstoichiometric epoxy-amine networks based on the diglycidylether of bisphenol A (DGEBA), are analyzed in terms of the network structure. In most cases reasonable predictions of T_g can be made using an empirical equation reported by L. E. Nielsen together with the experimental T_g value of the stoichiometric network and statistical calculations of the concentration of elastic chains. It is stated that in these rigid networks the concentration of elastic chains is the main structural factor associated to the variations of T_g with stoichiometry. For flexible networks based on the diglycidylether of butanediol (DGEBD), the effect of elastic chains on the T_g value is much less significant.     

Thermal and dynamic mechanical investigations on poly[bis(benzimidazobenzisoquinolinones)]

A series of new semi-ladder poly[bis(benzimidazobenzisoquinolinones)], obtained by the polycondensation of dinaphthalene dianhydrides and aromatic tetraamines was investigated by TG, DSC and DMA methods. The influence of polymer structure on the thermal behaviour of the poly[bis(benzimidazobenzisoquinolinones)] was examined. The polymers were found to be thermally stable with T_d > 723 K in air and T_g ranging from 585 to 701 K. A good agreement was obtained in T_g values measured by DSC and DMA methods. It was found that some cross-linking processes occurred at temperatures above T_g. Some of the isothermal ageing curves were used to find the activation energies of isothermal cross-linking and decomposition.     

Theoretical calculations of glass transition temperatures of plasticized polymers

Based on free volume, an equation has been derived enabling calculation of glass transition temperature (T_g) of plasticized polymers, knowing the plasticizer content, the thermal expansion coefficients and values of T_g of the polymer and plasticizer. Determined and calculated T_g are in satisfactory agreement up to the plasticizer concentration which dissolves the polymer.     

Prediction of the glass transition temperature for compatible polymer blends of varying compositions

Compatible polymer blends have been found to have widespread commercial applications. The simplest criterion for judging polymer—polymer miscibility in the solid state is the glass transition temperature (T_g), which can vary widely according to blend composition for a compatible system.Recently, an equation which predicts the T_g of intimate mixtures of compatible polymers has been derived, based on classical thermodynamics. Only a knowledge of the T_g and heat capacity increment (ΔC_p) of each pure component is required to predict the T_g at any composition.In this paper, the validity of this entropy-based relationship is investigated for a variety of commercial compatible polymer blends, including some based on poly(vinyl chloride), polystyrene, and poly(2,6-dimethyl-,4-phenylene oxide). The T_g and ΔC_p of each pure component are measured with a Perkin-Elmer DSC-2 differential scanning calorimeter, are predicted glass transition temperatures are compared with those observed experimentally.     

The kinetics of phase transitions in vitreous chalcogenide semiconductors As10.2Se89.8 and As9Se90Bi in early stage of physical ageing process

The kinetics of glass transition in selenide glasses As_10.2Se_89.8 and As₉Se₉₀Bi in early stage of physical ageing process has been investigated by parallel differential scanning calorimetry (DSC) and exoelectron emission (EEE). It has been found that the glass transition process occurring in investigated glasses is evidenced by peaks on EEE intensity and DSC curves. Admixture of bismuth causes a distinct lowering of the temperature of glass transitions process both in the surface layer and in the volume. The addition of Bi causes a decrease in the value of the activation energy for glass transition process in both the volume and in the surface layer, thus reducing the thermal stability of investigated glasses. Physical ageing in Se-rich chalcogenide glasses leads to a significant increase of endothermic peak area A, temperature of glass transition T _g and decrease of the activation energy value E. All these effects are strongly dependent on glass composition.     

Is adhesion between amorphous polymers sensitive to the bulk glass transition?

Two bulk samples of one and the same or of different amorphous polymers were brought into contact and held for a chosen period of time at a constant healing temperature (T) over the interval of T from below the bulk glass transition temperature (T _g ^bulk) by ~50 °C to above T _g ^bulk by ~10 °C. As formed adhesive joints were shear-fractured in tension at room temperature, and lap-shear strength (σ) was measured as a function of T. It has been found that σ develops with T as logσ?~?1/T both at symmetric and asymmetric interfaces of polystyrene, poly (methyl methacrylate) and poly (2,6-dimethyl-1,4-phenylene oxide). This behaviour implies that there is no discontinuity in the evolution of σ when going through T _g ^bulk, and that this process is controlled by one and the same diffusion mechanism both below and above T _g ^bulk. The results obtained indicate that the contact layer of the polymers investigated is in the viscoelastic state at T well below T _g ^bulk and support the concept of a decrease in the T _g of a near-surface layer with respect to T _g ^bulk.     

Determination of ultra-low glass transition temperature via differential scanning calorimetry

A novel method was developed to determine the ultra-low glass transition temperature (T_g) of materials through physical blending via differential scanning calorimetry. According to the Fox equation for polymer blends, a blend of two fully compatible polymers has only one T_g. The single T_g is a function of the T_gs of the two simple polymers. Thus, the ultra-low T_g of one material can be obtained from the T_gs of another polymer and their blends. The error of T_g measurements depends on the measurement error of the T_gs for the blends and another polymer. The method was successfully applied to determine the T_gs of acetyl tributyl citrate (ATBC), tributyl citrate (TBC) and poly(ethylene glycol)s (PEG)s with different molecular weights. The T_gs for ATBC, TBC, PEG-4000 and PEG-800 were ?57.0 °C, ?62.7 °C, ?76.6 °C and ?83.1 °C, respectively. For all the samples, the standard deviation of measurements was less than 3.3 °C, and the absolute error of measurements was theoretically not more than 5.3 °C. These results indicate that this method has acceptable precision and accuracy.     

Characterization of isomeric elastomers using thermal analysis

Many of the isomers of polybutadiene and polyisoprene elastomers can be characterized by thermal analysis.T _g is sensitive to side chain units (1,2 or 3,4 structure) for both polymers. Crystallinity measurements can characterizecis andtrans isomers. DMA and DEA master curves provide an idea of the heterogeneity of the chain units from the width of the loss factor curves in theT _g region. Thermal and thermooxidative degradation, as followed by DSC and DTG, can differentiate specific natural and synthetic isomers of polyisoprenes in raw and vulcanized states.     

In-situ changes of thermo-mechanical properties of poly(lactic acid) film immersed in alcohol solutions

Poly (lactic acid) (PLA) has properties suitable for many applications. However, PLA's properties are affected by environmental conditions. In this study, the glass-rubber transition temperatures (T_g) of PLA films were measured during immersion (i.e., in-situ) in pure alcohols and alcohol aqueous solutions using a dynamic mechanical analysis technique. The T_g of PLA decreased when immersed in alcohols. For pure aliphatic alcohols, the T_g reduction became smaller as the number of carbons (C1–C10) in the alcohol main chains increased. The Fox equation and the Hansen solubility parameters (HSP)/Flory-Huggins (FH) model were used to explain the T_g reduction. The relationships explained the interactions between PLA and pure alcohols with small molecules (C1–C8), but bigger pure alcohols (C9–C10) did not fit the prediction. The chemical isomerism in pure propanol (i.e., propan-1-ol and propan-2-ol) did not affect the T_g reduction. The T_g reduction in propan-2-ol aqueous solutions was concentration dependent although the partition coefficients based on the HSP and the FH parameters did not fit this relationship. The in-situ immersion of PLA in alcohol solutions could be used to evaluate the change in T_g from the T_g of dry PLA.     

The effect of water on the physicochemical and mechanical properties of gelatin

Strips of gelatin have been prepared by extrusion at different water contents varying from 20 to 50% H₂O (dry weight basis, d.w.b.). The processes of subsequent hydration or dehydration of these strips were followed by dynamic mechanical thermal analysis (DMTA), wide-angle X-ray diffraction and NMR relaxation measurements. A comparison of the calculated dependence of theT _g of gelatin (T _g anhydrous, 200?C) on water content (using the Ten Brinke and Karasz equation) with experimental results derived from DMTA showed that freshly extruded material followed the theoretical plot below 25% H₂O (d.w.b.), but at higher water contents, the7 _g deviated positively, probably due in part to the effect of delayed re-equilibration of water content after thawing of separated ice crystals. The experimental results determined after storage for one week fell on a different line, with aT _g of 145?C for anhydrous gelatin Possibly, theT _g is elevated by crystallization — a view supported by the WAXS spectra. The NMR relaxation results also showed a profound mobilization of the gelatin protons at water contents greater than 25% d.w.b.     

Thermal stability and lifetime estimates of a high temperature epoxy by Tg reduction

Thermal degradation of a high temperature epoxy network is studied in terms glass transition temperature (T_g) reduction over a temperature window encompassing the T_g of the network. The T_g is shown to decrease as the network is thermally aged at elevated temperatures in air and in argon. The duration of the aging experiments is extended to long time such that the absolute T_g reduction approaches a long time reduction plateau. Degradation is dominated by non-oxidative pyrolysis with a small contribution from diffusion limited thermal oxidative degradation at the surface. A time–temperature superposition is constructed from the extent of T_g reduction of samples aged in air and the thermal shift factors are shown to have Arrhenius scaling behavior. An activation energy is extracted that agrees with previous activation energy measurements derived from other property measurements of the same network aged under similar conditions. The agreement of the activation energy with past results shows that T_g reduction is controlled by the same degradation mechanism and may be used as an observable for lifetime estimates when thermal degradation is pyrolytic in nature. The extent of T_g reduction is modeled with an autocatalytic rate expression and compared to previous property measurements to show the difference in sensitivity of observable material properties on degradation.     

Inert gas permeation through homopolymer membranes

New data are reported for the permeation of inert gases through polyethylene, polytetrafluoroethylene, and silicone and natural rubbers. Additional data are compiled from the literature. The relative solubilities of these gases are practically insensitive to chemical variations in the homopolymer. Hence variations in structure at the glass transition (T_g) and melting (T_m) temperatures that affect diffusion also unambiguously affect permcation. Consequently an equivalence results between permeation at a given temperature for different polymers and permeation at different temperatures for a given polymer. Although the diffusion coefficient changes continuously with temperature, the Arrhenius parameters D_o and E_d apparently change discontinuously at T_g and T_m. Their magnitudes and variations with atomic weight reach maxima at about T_g. These data indicate a dependence of the classical correlation between D_o and E_d on polymer properties. A perturbed diameter for the permeant, specific for each polymer, is proposed for correlating the D_o and E_d data. This correlation makes the changes observed at T_g and T_m more perceptible.     

Über die bestimmung von isomerisierungsenthalpien und -entropien mit der IR-intensitätsmethode

Among the many sources of error which may affect the results for the enthalpy ΔH and entropy ΔS of isomerization determined by the IR-intensity method, the influence of the temperature dependence of the absorptivities of the reference bands, α_g(T) and α_t(T), is studied theoretically. Based on the general case I, the following three special cases II–IV are examined, in which increasingly stronger constraints are imposed on the compound-specific functions α_g(T) and α_T(T): II. Constant ratio α_g(T)/α_T(T), III. α_g(T) and α_T(T) both temperature independent and different, IV. α_g(T) and α_T(T) both temperature independent and equal. For each case the formulae for the evaluation of ΔH and ΔS as well as the kind and the number of the quantities to be measured are given. Moreover, in the cases I and II new methods for the analysis of the measured data are suggested. In each case the plots of the normalized band intensities $\text{A}{}_{\mathrm{g}}$ and $\text{A}{}_{\mathrm{t}}$ and the temperature T versus each other in different two-dimensional diagrams have been examined to find out whether one of the simpler special cases is applicable for the respective pair of reference bands. It is shown that these plots are not a suitable tool because of the relatively poor accuracy and reproducibility of measured IR-band intensities obtainable up to now.     

Glass transition temperatures in binary polymer blends

Knowledge of the glass transition temperatures (T_gs) as function of composition reflects miscibility (or lack of it) and is decisive for virtually all properties of polymer‐based materials. In this article, we analyze single blend‐average and effective T_gs of miscible polymer blends in full concentration ranges. Shortcomings of the extant equations are discussed to support the need for an alternative. Focusing on the deviation from a linear relationship, defined as ΔT_g = T_g ? φ₁T_g,1 ? φ₂T_g,2 (where φ_i and T_g,i are, respectively, the weight fraction and the T_g of the i‐th component), a recently proposed equation for the blend T_g as a function of composition is tested extensively. This equation is simple; a quadratic polynomial centered around 2φ₁ ? 1 = 0 is defined to represent deviations from linearity, and up to three parameters are used. The number of parameters needed to describe the experimental data, along with their magnitude and sign, provide a measure of the system complexity. For most binary polymer systems tested, the results obtained with the new equation are better than those attained from existing T_g equations. The key parameter of the equation a₀ is related to parameters commonly used to represent intersegmental interactions and miscibility in binary polymer blends. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 80–95, 2008     

A study of thermal–mechanical properties of an automotive coating exposed to natural and simulated bird droppings

Biological resistance of coatings can be regarded as one of the main properties in automotive industries. This study aims to investigate the effects of biological materials on the mechanical performance of an automotive clear coat. To this end, two acrylic melamine clear coats containing different melamine cross-linker contents were used. In addition, biological resistance of these clear coats were studied at two different ageing processes including pre-ageing and post-ageing which involve various hot-cold, humid shockings and UV radiation of sunlight. By the aid of optical microscopy, micro Vickers and DMTA analyses, different optical and mechanical properties such as micro hardness, T _g, cross-linking density and storage modulus were studied. Results revealed an inverse impact of both biological materials to decrease the clear coats mechanical attributes. In addition, a complicated effect of ageing conditions was observed for both clear coats exposed to these materials. It was shown that the coating having a higher mechanical properties and T _g even resulted in a lower biological resistance.     

DSC investigation of interchange reactions in the melt of different polyesters

The course of a randomization process in the melt of poly(ethylene terephthalate) and other polyesters during melt-blending was observed by determining changes in T_g, T_c and T_m measured by DSC. the melting point of isothermic annealed copolyesters is the most advantageous criterion for determining the degree of randomization process. On the other hand, T_g and T_c are influenced by further processes taking place during the melt-blending of two kinds of homopolyesters, e.g. the changes of molecular mass and molecular mass distribution etc.     

Mechanical and thermal properties of perfluoroalkyl ethyl methacrylate-methyl methacrylate statistical copolymers synthesized in supercritical carbon dioxide

Poly(methyl methacrylate-ran-perfluoroalkyl ethyl methacrylate) copolymers having varying perfluoroalkyl ethyl methacrylate ester (Zonyl-TM) contents were synthesized in supercritical CO₂. Complete amorphous structures of the copolymers were verified by XRD. Young's modulus (Y_mod) of the copolymers was decreased linearly from 1.57 to 1.08 GPa and T_g values from 102 to 77 °C with the increase of Zonyl-TM content. A linear relationship between the Y_mod and the T_g values of the copolymer was also found. The increase of the large branched pendant groups resulted in the increase of the free volume and a corresponding decrease in Y_mod and T_g of the copolymers. A good fit was found when the Schneider equation was used. Negative deviation from the Gordon-Taylor equation was observed when Zonyl-TM content was lower than 14% due to rapid increase in free volume and then a positive deviation was found due to the dipole-dipole interactions between the methyl ester and fluoroalkyl ester groups.     

Low-temperature behaviour of water in nafion membranes

Proton NMR measurements of T₁, T₁π and T₂ in “Nafion” perfluorosulphonate membranes, together with neutron- scattering and dielectric data, show that the aqueous phase in Nafion solidifies at a glass transformation whose temperature T_g is 168 K in water-saturated acid membranes. T_g is higher for the salt forms. Mo¨ssbauer measurements on Eu³⁺ Nafion confirm that the cations are present in an aqueous phase with T_g ≈ 220 K.