Effect of water on the glass transition temperature of poly(hydroxy methylene)

The effect of moisture on the glass transition temperature of poly(hydroxy methylene) was investigated by thermal analysis which entailed combinations of DTA and DSC with TGA techniques. As the moisture content increased, the glass transition temperature (T_g) decreased monotonically from a value (T_go) of 134°C for the dry polymer toward a finite value (T_gt) of about 39°C. This response of T_g to an increase in the moisture content (w) was adequately represented by the general relationship T_g = (ΔT_g)0 exp{–[ln(ΔT_g)₀]w/τw_l} + T_gl, recently developed for correlating T_g and moisture content of nylon 6 and and found generally applicable to hydrophilic polymers. With (ΔT_g)0 = 95, τ = 0.555, and w_l = 0.375, good agreement was found between experimental and calculated values for poly(hydroxy methylene).     

Crystallization kinetics of binary arsenic selenium chalcogenides

Binary As–Se glasses with different amounts of As content have been prepared and scanned with different heating rates (3 ≤ ψ ≤ 48 K min⁻¹) over temperatures ranging from 300 to 450 K. Both the glass transition temperature (T_gl) and the temperature peak of crystallization (T_pc) increase as a function of As and/or the ψ values. A good correlation between T_gl and T_pc has been investigated. The observed increase in T_gl and T_pc by increasing the ψ values or as a function of As is well discussed in accordance with Lasocka’s relationship and using the average coordination number A_cn, the viscosity at glass transition μ(T_gl) and the overall mean bond energy E. The glass transition E_gl and crystallization activation energies (E_gl and E_pc) were determined based on the changes in T_gl and T_pc values due to the heating rate (ψ). The increase in the values of E_gl and E_pc with increasing the As content is expected due to the increase in T_gl and T_pc values. The kinetic exponent n and the crystal dimensionality m have been determined.

     

Effect of structure on glass transition temperature of amine crosslinked epoxies

About 40 epoxide-amine network polymers with glass transition temperatures ranging from 0 to 232°C were investigated, about one-third being reported for the first time. The glass transition temperature T_gl of the corresponding linear copolymers was first calculated by use of an additivity law whose physical validity was carefully checked. Then the contribution of crosslink mers was determined, and various physical and empirical approaches of the effect of crosslinking on T_g were compared. The results gave evidence in favor of the DiMarzio approach. A predictive relationship based on these considerations is proposed.     

The effect of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water

The influence of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water has been investigated by dynamic mechanical methods. Radiation crosslinking by high-energy electrons was effective in preventing morphological changes during the measurement of the incremental change in heat capacity (ΔC_p) at T_g, which was performed by differential scanning calorimetry. The experimentally determined value of ΔC_p, when normalized to account for the crystalline phase, was found to deviate from a linear two-phase relation and was reduced further than would be expected based on this model. It is proposed that nylon 6 is best described by a three-phase model which consists of a crystalline domain, a wholly amorphous domain, and an “intercrystalline” region. The importance of this in explaining the relatively large depression of T_g by small quantities of water is illustrated by applying equations derived to account for the compositional dependence of T_g in polymerdiluent mixtures, based on a classical thermodynamic interpretation of the glass transition phenomenon.     

Water sorption in poly(ammonium sulfopropylbetaines). I. Differential scanning calorimetry

The hydration of four amorphous acrylic and methacrylic poly(zwitterions) bearing the ammonium sulfopropylbetaine function as a side-groups () was studied by differential scanning calorimetry over broad ranges of temperature (150-400 K) and water content (weight fraction W₁ < 0.5). Analyses were made of the first-order transitions and heat capacity of sorbed water, glass transition temperature (T_g) measurements. Nonfreezable bound water, about 7.7 ± 0.9 mol/monomeric unit, behaves as a single phase: Its mobility, fairly similar to that of bulk liquid water in viscoelastic systems at T > 250 K, decreases with temperature in the glassy systems, but never disappears, even at 185 K. The depression of the glass transition temperature of the hydrated polymers obeys Couchman's equation: T_g = Σ_i W_i ΔC_pi T_gi / Σ_gi W_iΔC_pi. Freezable bound water, about 6.7 ± 0.9 mole/monomeric unit, shows multipeak melting endotherms in the range 242–272 K. Because of their charged sites, the hydration process of the poly(zwitterions) appears more similar to that of poly(electrolytes) than to that of uncharged hydrophilic polymers. © 1992 John Wiley & Sons, Inc.     

Continuous measurement of dynamic tensile mechanical properties in polymer solids over a wide range of frequencies. III. Effect of absorbed water on nylon 6 at 23°c

The dynamic tensile mechanical properties (E′, E″, and tanδ) of nylon 6 have been studied over the frequency range 10^?25?10² H_z and water content up to 12.6 wt % at a constant temperature of 23°C. From the dispersion maps in the coordinates of frequency and water content, the relaxation behavior can be classified into three regions of water content: (A) dry to 2 wt %, (B) 2-5 wt % and (C) 5 wt % to wet. For region B, it is found that the logarithmic frequency shift Δ logf_α/Δx of the α dispersion per 1 wt % change of water content is 1.7. Taking into consideration that the change of glass transition temperature per 1 wt % change of water content Δ T_g/Δx is 3.7°C (according to Kettle), we find Δ logf_α/ΔT_g = 0.5. For regions A and C, such an evaluation cannot be made. The effect of absorbed water on the dynamic mechanical properties at 23°C is discussed in terms of two kinds of processes: (a) formation of water-amide hydrogen bonds with free amide groups and (b) scission of amide-amide hydrogen bonds.     

Thermal properties of freezing bound water restrained by sodium lignosulfonate-based polyurethane hydrogels

In order to develop a new functional product from lignin, sodium lignosulfonate (LS)-based polyurethane (LSPU) hydrogels were prepared from LS and hexamethylene diisocyanate (HDI) derivatives in water. Isocyanate/hydroxyl group ratio (NCO/OH ratio) was varied from 0.05 to 0.8 mol mol⁻¹, and water content (W_c = mass of water/mass of dry sample) of the obtained LSPU hydrogels was varied from 0 to 3.0 g g⁻¹. Phase transition behavior of hydrogels with various W_c’s was investigated by differential scanning calorimetry (DSC) and thermogravimetry (TG). In DSC heating curve of LSPU hydrogels, glass transition, cold crystallization, melting and liquid crystallization were observed. Cold crystallization, two melting peaks and variation of melting enthalpy indicate that three kinds of water, i.e., non-freezing water, freezing bound water and free water, exist in LSPU hydrogel. Glass transition temperature (T_g) decreased from 230 to 190 K in a W_c range where non-freezing water was formed in the hydrogel. T_g increased when freezing bound water was formed in the system. T_g leveled off in a W_c range where normal ice was formed. The effect of NCO/OH ratio on molecular motion of LSPU hydrogel is examined based on T_g and heat capacity difference at T_g (ΔC_p). Water vaporization curve measured by TG also indicates the presence of bound water which evaporates at a temperature higher than ca. 410 K. By atomic force microscopic observation, the size of molecular bundle of LSPU hydrogel is calculated and compared with that of LS-water system. By cross-linking, the height of molecular bundle decreased from ca. 3–1 nm and lignin molecules extend in a flat structure.

     

Moisture effects on the glass transition and the low temperature relaxations in semiaromatic polyamides

The influence of moisture absorption on the primary (glass) transition (T_a or T_g) and the low temperature relaxations of semiaromatic amorphous polyamides synthesized by isomeric aliphatic diamine and metha or para oriented phthalicdiacids has been investigated by means of differential scanning calorimeter (DSC) and dynamic mechanical thermal analyser (DMTA). The glass transition of semiaromatic polyamides was lowered due to the water absorption, and the β and the γ relaxations were as well. From the observed T_g and the difference in the heat capacity, the calculated T_g depression per 1 wt % water content was 12.3 K and the result was in good agreement with the experimental data. The depression of the glass transition may be expressed by the same manner as the plasticization of nylon 6 by water. The depressed β relaxation observed in the specimen containing a few percent of moisture was splitted into two transitions due to the reduction of water content, of which one was the elevation of the T_β and another was the simultaneous appearance of the T_γ, and then the single T_γ solely was observed for the completely dried specimen. The T_γ seemed to be merged into or not to be observed by the large and broad T_β transition when the sample was governed by a few percent of water, then it was emerged from the T_β due to water desorption. Thus, the T_β is believed to arise from the intermolecular hydrogen bonding between water molecules or between water and amide groups in wet polyamides. In addition, the γ relaxation originated from the peptide groups is attributable to the inter- and intramolecular hydrogen bonding between amide groups. © 1997 John Wiley & Sons, Inc. J Polyn Sci B: Polym Phys 35: 807–815, 1997     

Thermal and FTIR investigation of freeze-dried protein-excipient mixtures

The effect of excipients on the secondary structure of lyophilized proteins was studied through second-derivative Fourier transform infrared (FTIR) spectroscopic analysis. The glass transition temperature (T _g), denaturation temperature (T _d) and moisture content were determined by differential scanning calorimetry (DSC) and thermogravimetry (TG). T _g, T _d and the preservation of protein secondary structure were found to be dependent upon the type and amount of the excipient included in the formulation. Meanwhile, the lyophilized proteins easily adsorbed amounts of moisture during storage to reduce their T _gs and stability.     

The proper glass transition temperature of amorphous polymers on dynamic mechanical spectra

Glass transition is crucial to the thermal and dynamical properties of polymers. Thus, it is important to detect glass transition temperature (T _g) with a sensitive and proper method. Dynamic mechanical analysis (DMA) is one of the most frequently used methods to determine T _g due to its advantage of high sensibility. However, there is controversy in the past literatures to determine the proper glass transition temperature among three transition temperatures, i.e., T _g1, T _g2 and T _g3 in the dynamic mechanical spectra, which correspond to the temperature abscissa of intersect value of two tangent lines on storage modulus (E′), the peak of the loss modulus (E″) and the peak of the loss tangent (tan δ). In this work, these three transition temperatures were compared with the glass transition temperature determined by DSC (T _gDSC). Based on the discussion of different modes of molecular motion around the glass transition region, it is demonstrated that T _g1 and T _g2 have the same molecular mechanism as T _gDSC, i.e., local segmental motion which is enthalpic in nature and determines the proper glass transition temperature, while T _g3 is assigned to the transition temperature of entropic Rouse modes, thus cannot be used as the proper glass transition temperature.     

Positron annihilation lifetime studies of changes in free volume on cross-linking cis-polyisoprene,high-vinyl polybutadiene,and their miscible blends

Measurements of average free volume hole sizes, 〈v_f〉, and the fractional free volumes, f_ps, in vulcanized cis-polyisoprene (CPI), high-vinyl polybutadiene (HVBD), and their 50 : 50 blend were made via determination of orthopositronium annihilation lifetimes. The results are compared to corresponding data on the uncured materials. On crosslinking, 〈v_f〉 decreases in the rubbery state but remains essentially unchanged in the glass. This is consistent with the expectation that the crosslinks greatly restrict the thermal expansion of the chains above the glass transition temperature (T_g) but have less influence on the packing density in the glass. Scaling relationships between 〈v_f〉, f_ps, the thermal expansion coefficient α_f = df_ps/dt, and T_g are examined. We find that 〈v_f〉_g, the hole volume at T_g, and f_ps,g, the fractional free volume at T_g, each increase significantly with increasing T_g. This behavior is consistent with previous observations reported in the literature and has been interpreted as a manifestation of the kinetic character of the glass transition. High-T_g polymers need a larger free volume to pass into the liquid state. The change in expansion coefficient on passing from the glass to the liquid, Δα_f = α_f,l − α_f,g, increases slowly with T_g, as predicted by free volume theory. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2754–2770, 1999     

Glass transition and crystallization study of chalcogenide Se<Subscript>70</Subscript>Te<Subscript>15</Subscript>In<Subscript>15</Subscript> glass

Differential scanning calorimetry data at different heating rates (5, 10, 15 and 20 °C min⁻¹) of Se₇₀Te₁₅In₁₅ chalcogenide glass is reported and discussed. The crystallization mechanism is explained in terms of recent analyses developed for use under non-isothermal conditions. The value of Avrami exponent (n) indicates that the glassy Se₇₀Te₁₅In₁₅ alloy has three-dimensional growth. The average values of the activation energy for glass transition, E _g, and crystallization process, E _c, are (154.16 ± 4.1) kJ mol⁻¹ and (98.81 ± 18.1) kJ mol⁻¹, respectively. The ease of glass formation has also been studied. The reduced glass transition temperature (T _rg), Hruby’ parameter (K _gl) and fragility index (F _i) indicate that the prepared glass is obtained from a strong glass forming liquid.     

Glass transition of hydrated wheat gliadin powders

Modulated-temperature differential scanning calorimetric and dynamic mechanical analyses and dielectric spectroscopy were used to investigate the glass transition of hydrated wheat gliadin powders with moisture absorption ranged from 2.30 db%to 18.21 db%.Glass transition temperature(I_g)of dry wheat gliadin was estimated according to the Gordon Taylor equation.Structural heterogeneity at high degrees of hydration was revealed in dielectric temperature and frequency spectra.The activation energies(E_a)of the two relaxations were calculated from Arrhenius equation.     

Modulated differential scanning calorimetry

Modulated-temperature differential scanning calorimetry was used to measure the glass transition temperature,T _g, the heat capacity relaxation in the glassy state and the increment of heat capacity, C_p, in the glass transition region for several polymers. The differential of heat capacity with respect to temperature was used to analyseT _g and C_p simply and accurately. These measurements are not affected by complex thermal histories.     

Transition of polymers from rubbery elastic state to fluid state

On increasing the temperature of a polymer, the transition of the polymer from a rubbery elastic state to a fluid state could occur. The transition temperature is termed the fluid temperature of the polymer, T _f, which has a direct relationship with the polymer molecular weight. As one of polymer parameters, T _f is as important as the glass transition temperature of a polymer, T _g. Moreover, special attention to T _f should be paid for polymer processing. In research on the transition of a polymer from a rubbery elastic state to a fluid state, the concept of T _f would be more reasonable and more effective than the concept of T _l,l because it is neglected in the concept of T _l,l in that the molecular weight of a polymer may affect the transition of the polymer. In this paper the discussion on the fluid temperature involves the characters of polymers, such as the deformation—temperature curve, the temperature range of the rubbery state and the shear viscosity of polymer melt. From the viewpoint of the cohesional state of polymers, the transition of a polymer from a rubbery elastic state to a fluid state responds to destruction and construction of the cohesional entanglement network in the polymer. The relaxing network of polymer melt would be worthy to be considered as an object of study. __________ Translated from Huaxue Tongbao (Chemistry), 2008,71(3) (in Chinese)     

Thermal decomposition and glass transition of industrial hydrolysis lignin

Thermal properties of industrial hydrolysis lignin (HL) obtained from bio-ethanol production plants were investigated by thermogravimetry and differential scanning calorimetry. Thermal decomposition of HL was observed in two stages suggesting coexisting carbohydrates. Glass transition temperature (T _g) was observed in a temperature range from 248 to 363 K. T _g values were lower than that of other industrial lignins, such as kraft lignin or lignosulfate. Enthalpy relaxation was observed as sub-T _g, which is not as prominent as other industrial or laboratory scale isolated lignins. T _g of HL decreased in the presence of water and saturated at water content (W _c) of 0.18 (mass of water/mass of dry HL). The amount of bound water calculated from melting enthalpy of water and W _c was ca. 0.18. Thermal decomposition and molecular motion of as obtained industrial HL are affected by coexisting carbohydrates.     

Derivation of the Kissinger equation for non-isothermal glass transition peaks

A brief derivation of the Kissinger’s equation for analysis of experimental data of non-isothermal glass transition peaks based on the free volume model is given. This equation was applied successfully to Cu_0.3(SSe₂₀)_0.7 chalcogenide glass for different heating rates. For granted this model, the obtained glass transition activation energy, E _g must be constant throughout the whole glass transition temperature range. This required that T _g to be determined for three characteristic temperature points for each DSC curve.     

Water sorption and glass transition of freeze-dried camu-camu (myrciaria dubia (H.B.K.) Mc Vaugh) pulp

Differential scanning calorimetry (DSC) was used to determine phase transitions of freeze-dried camu-camu pulp in a wide range of moisture content. Samples were equilibrated at 25°C over saturated salt solutions in order to obtain water activities (a_w) between 0.11–0.90. Samples with a_w>0.90 were obtained by direct water addition. At the low and intermediate moisture content range, Gordon–Taylor model was able to predict the plasticizing effect of water. In samples, with a_w>0.90, the glass transition curve exhibited a discontinuity and T^’_g was practically constant (–58.8°C), representing the glass transition temperature of the maximally concentrated phase(T_g ).