Respective Glass Transition Temperatures Research Articles

Influence of chemical structure on glass transition temperatures of poly(propylene terephthalate) and poly(propylene isophthalate)

AbstractPoly(propylene terephthalate)Systematic IUPAC name: poly(oxypropyleneoxyterephthaloyl). (PPTP) and poly(propylene isophthalate)Systematic IUPAC name: poly(oxypropyleneoxyisophthaloyl). (PPIP) were prepared by melt polycondensation. Different fractions with number‐average molar masses in the range 5 000 − 20 000 g·mol−1 were obtained and their respective glass transition temperatures (Tg) determined by calorimetry. The solubility parameters of the polymers were obtained by viscosity measurements in different solvents and were found to be 9,4 and 9,5 cal1/2·cm−3/2 (19,2·103 and 19,4·103 J1/2·m−3/2) for PPIP and PPTP, respectively. The glass transition temperatures were compared with those reported for analogous polyesters and it was shown that intramolecular interactions highly influence the Tg of these polyesters, whereas the effect of intermolecular interactions seems to be less important.

Intermolecular interaction in polymer alloys as studied by crack healing

AbstractThe “method of crack healing” permits measuring the mechanical effects resulting from an interdiffusion of chain molecules across an interface. Having shown that in highly cross‐linked ruptured styrene‐acrylonitrile (SAN) a two‐stage healing mechanism is active, a similar phenomenon was expected in blends of polystyrene (PS) with a more flexible component such as poly(vinyl methyl ether) (PVME), or with a more rigid component such as poly(−2,6 dimethyl 1,4 phenylene oxide) (PPO). However, in both cases crack healing at a temperature 10 K above the respective glass transition temperature (Tg) was linear with the healing time to the power of 1/4 and slower than with pure PS. This unexpected observation is explained by analysis of the thermal and viscoelastic behavior of these blends and by the different contribution to the stress intensity factor resulting from the entanglements formed by the interdiffusing molecules.

Evidence for the defect origin of states which control photoelectronic behavior of amorphous chalcogenides

The extraordinary sensitivity of xerographic measurements to fluctuations in bulk space charge has made it possible to resolve clearly, changes in the population of deep states (traps and thermal emission centers) which occur during the isothermal relaxation (physical ageing) of selenium and selenium-rich Se: Te: As alloy films below their respective glass transition temperatures T g. An intimate connection between population of deep defects and the structural state of the defect-containing glass is thus demonstrated. The energy distribution of deep states has been xerographically mapped and found to remain invariant during relaxation while the total number of defects varies systematically with annealing temperature T A. Isothermal relaxation toward a stable but temperature dependent defect population is observed during annealing at T A when T g-T A is not too large. The latter provides direct evidence that these chalcogenides are thermodynamic defect systems.

New epoxy resins. I. The stability of epoxy–trialkoxyboroxines triaryloxyboroxine system

AbstractA polymer with high aromaticity and/or cyclic ring structures chain backbone usually has high heat, thermal, and flame resistance. Two diglycidyl ethers of bisphenols were prepared from 4,4′ isopropylidenediphenol (DGEBA) and 9,9‐bis(4‐hydroxyphenyl) fluorene (DGEBF) for evaluation. Four boroxines—trimethoxyboroxine (TMB), triethoxyboroxine (TEB), triisopropoxyboroxine (TIPB) and triphenoxyboroxine (TPB)—were used as the curing agents. DGEBA and DGEBF cured with various boroxines indicate that the trend for their respective glass transition temperature (Tg's), degradation temperatures (Td's), and gel fractions are TMB‐cured epoxy ≈ TEB‐cured epoxy < TIPB cured epoxy < TPB cured epoxy. The DGEBF system usually has a higher Tg, Td, gel fraction, oxygen index (OI), and char yield than the related DGEBA system. DGEBF/DGEBA (80/20 mol ratio) shows a synergistic effect in regard to char formation. This effect exists not only in the copolymer system but also in blended homopolymers of the separately cured resins. A modified mechanism for the polymerization of phenyl glycidyl ether (PGE) with TMB has been proposed.

Dielectric studies of As-Se glasses

Dielectric constants and loss tangents of As-Se glasses have been measured between 300 K and the respective glass transition temperatures and between 1 kHz and 20 kHz. The variation of dielectric constants has been interpreted in terms of both heteropolarity of bonds and average bond energies employing a chemically ordered network model. Various contributions to total molar polarizations have been estimated. Rapid rise of loss tangent in the vicinity of glass transitions has been interpreted in terms of rapid increase; of d.c. conductivity.

Dielectric spectroscopy of a-Se and some a-Se : As alloys

Dielectric activity is observed in a-Se and in some a-Se : As alloy films in the neighborhood of their respective glass transition temperatures. When measurements are made at fixed frequency in the temperature scan mode, enthalpy relaxation and volume hysteresis effects are readily identified. Relaxation behavior conforms to the phenomenological WLF equations and for a:Se C1=10.4 and C2=20.5 K. The free volume fraction is 0.042. Arsenic addition to a-Se significantly broadens the relaxation distribution function and increases the relaxation oscillator strength. The effective incremental dipolar contribution per As atom is 0.6 D despite the very small Pauling ionicity of the Se : As bond. The latter is consistent with the formation of a polar pyramidal complex at each incorporated As site. Sub-Tg relaxation processes are not observed in a-Se. Dielectric activity in homopolar a-Se could reflect contributions from polar chemical impurities, deformation induced dipoles, or dipoles associated with native charged defect pairs. It is the latter two which are the most likely candidates. In both cases estimates of the unit dipole strength and density will vary depending on calculational models.

Plastic deformation in polyimides, with new implications on the theory of plastic deformation of glassy polymers

Abstract The plastic deformation behaviour of a series of glassy aromatic polyimides, derivatives of resorcinol, hydroquinone, oxydiphenyl, and pyromellitic acid, have been investigated between room temperature and a temperature 20 K below their respective glass transition temperature. The results are analysed by means of a theory due to Argon (1973) for plastic flow in glassy polymers, and show that, as the spacing between natural hinges on polymer molecules increases with increasing chemical complexity, the thermally activated complex grows in size, and plastic deformation becomes less local.

The effect of excess thermodynamic properties versus structure formation on the physical properties of glassy polymers

Abstract The dynamic mechanical properties of glassy polymers, such as atactic polystyrene and poly(ethylene terephthalate), have been followed as a function of the excess enthalpy during isothermal annealing at selected temperatures below their respective glass transition temperatures. For glassy polymers prepared from crystallizable as well as noncrystallizable polymers, the changes in modulus parallel the extent of enthalpy relaxation that occurs during the annealing period as a result of the nonequilibrium nature of the glassy state, i.e., the modulus increases with time and approaches a limiting value that increases with decreasing temperature. These results suggest that the observed changes in the dynamic mechanical moduli are associated with changes in the excess thermodynamic properties of the polymeric glasses, and not with structure formation other than the normal liquidlike packing of the chain segments. Thus the physical properties for corresponding equilibrium glassy states can be determined ...

Biaxial stretching of heat-softened plastic sheets using an inflation technique

Numerical solutions to the axially symmetric, membrane-inflation problem are presented based on a new form of the strain energy function. The calculated axial profiles, extension-ratio distributions and thickness distributions are compared to experimental values obtained with high-impact polystyrene and cellulose acetate butyrate processed above their respective glass-transition temperatures. Principal curvatures and stress resultants are also presented from the numerical calculations.

Sorption kinetics and equilibria in annealed glassy polyblends

AbstractThe sorption kinetics and equilibria of n‐hexane in glassy polyblends of polystyrene and poly(2,6 dimethyl‐1,4 phenylene oxide) were studied as a function of annealing conditions. Cast film samples were annealed 20°C above their respective glass transition temperatures for two hours and twenty‐four hours. The rate of relaxation‐controlled (Case II) sorption of n‐hexane in these films was reduced markedly consequent to annealing. The effect of annealing on the sorption kinetics and the independently determined film densities was more pronounced for the poly(phenylene oxide)‐rich samples. Although sorption rates were reduced by as much as a factor of 100, the sorption equilibrium was insignificantly affected by annealing. Super Case II transport was observed for the slow absorbing annealed samples whereas the more rapid sorption in the unannealed samples followed ideal Case II kinetics. The more pronounced effects of annealing for the poly(phenylene oxide)‐rich samples on sorption rates and film densities were explained by considering the increasing difference between the film Tg and the drying temperature used in the original film preparation for the poly(phenylene oxide)‐rich samples. These results suggest that glassy polymers, cast and dried well below their glass transition temperatures, will be subject to large long‐term reductions in absorption rates and specific volume. Moreover, residual, excess free volume significantly affects relaxation‐controlled absorption of vapors in partially annealed glassy polymers.

Viscous Liquids and the Glass Transition. III. Secondary Relaxations in Aliphatic Alcohols and Other Nonrigid Molecules

The dielectric permittivity and the dielectric loss factor of 5-methyl-3, 4-methyl-3 and 3-methyl-3-heptanol, n- and iso-butanol, 1,2-propanediol, dimethyl and diethyl phthallate, and 3-methylpentane have been measured from 50 to 105 Hz and from −196 to about 20°C above their respective glass transition temperatures. The glass transition temperature Tg of these substances, several more isomeric octanols, and 1-phenyl-1-propanol have been measured by differential thermal analysis. All substances except for 3-methylpentane and iso-butanol show either a well-defined secondary relaxation peak in tanδ, or a clear indication of the presence of a secondary relaxation below their Tg's. Arrhenius plots for the α-relaxation process of the isomeric octanols are linear with an activation energy of 16–18 kcal/mole, while for other substances they are nonlinear with the activation energy changing from 30 to 70 kcal/mole. The Arrhenius plots for the secondary relaxations are linear and have an activation energy of 4–8 kcal/mole. It is pointed out that the presence of a spectrum of relaxation times in liquids near Tg is not necessarily concomitant with non-Arrhenius behavior. It is concluded that the presence of secondary relaxations should be considered as a characteristic property of the liquid in or near the glassy state, and do not require specific intramolecular mechanisms for their existance.

Viscous Liquids and the Glass Transition. II. Secondary Relaxations in Glasses of Rigid Molecules

The dielectric loss factor and dielectric permittivity of 8–16 mol% solutions of chlorobenzene, o-dichlorobenzene, and 1-chloronaphthalene in cis-decalin; 50–60 mol% mixtures of pyridine with chlorobenzene, bromobenzene, 1-chloronaphthalene, and toluene; 50–60 mol% mixtures of tetrahydrofuran with bromobenzene and 1-chloronaphthalene; the pure liquids cis-decalin, o-terphenyl, iso-propylbenzene, propylene carbonate; and two fused salt systems, 45 mol% Ca(NO3)2–KNO3 mixture and Ca(NO3)2·4H2O have been measured from 50 Hz to 1 × 105 Hz from − 196° in the vitreous state to about 30° above their respective glass transition temperatures. The Tg's of the organic glasses have been measured by DTA. With the exception of propylene carbonate, all glasses show the presence of one secondary relaxation between − 196° and their respective Tg's either as a peak or shoulder in a tanδ–temperature plot at a single frequency, or in the dielectric loss spectrum. Arrhenius plots of the frequency of maximum loss against temperature in the main relaxation region for all systems are nonlinear, with the activation energy at the lowest temperature of our measurements ranging from 55 kcal/mol to 70 kcal/mol. The Arrhenius plots in the secondary relaxation region are linear and have activation energies between 5 and 12 kcal/mol. These glasses, most of which are composed of rigid molecules, show a remarkable similarity in their dielectric behavior to amorphous polymers. The results confirm the prediction made by one of the authors that the occurence of secondary relaxations is an intrinsic property of the glassy state.

Multiple Transitions in Polyalkyl Methacrylates

A series of polyalkyl methacrylates, two hydroxylated methacrylates, polymethoxy ethyl methacrylate, and polymethyl acrylate have been studied by linear expansion and torsion-pendulum measurements, in the temperature range from −180°C to just above the respective glass transition temperatures. The length-temperature data (L-T) were analyzed by a least-squares differentiation procedure, based on moving arcs, to give directly the thermal expansion coefficients as a function of temperature. The L-T measurements suggest the existence of two Tgg transitions (Tg−100<Tgg<Tg) in the polymers with the more flexible side chains, and one transition for polymers with bulky side groups. There is also evidence of a third Tgg transition at about −120° to −90°C, which may be associated with adsorbed water molecules. In similar fashion to that of Tg, the location of the Tgg transitions decreases upon increasing the length of the n-alkyl side chain. The hydroxylated polymers have higher Tg's than the corresponding alkyl samples, and have thermal expansion coefficients only about one-half as large, both features being a result of interchain hydrogen bonds. Torsion-pendulum measurements indicate only one clear-cut Tgg transition region; hence, it seems that the two Tgg transitions observed in the expansion studies cannot be unequivocally resolved by dynamic measurements. Expansion coefficients of the n-alkyl series suggest the retention of some excess free volume below Tg for side chains larger than ethyl. Substitution of 3α′g″, the expansion coefficient below our second Tgg transition, into the Simha-Boyer equation (αl-αg) Tg gives a value close to their empirical constant. This supports their suggestion that polymers with pronounced subgroup motions exist in an iso-free volume state below the subgroup transition temperature.

Structure and properties of polyamides

Sorption and desorption isotherms of water at 27 °C have been determined for a series ofα-substituted nylons 3 (methyl to butyl and phenyl derivatives), which at room temperature are deep below the respective glass transition temperatures. It follows from the rate of establishment of sorption equilibria as well as from sudden changes of the sorption isotherms, that in all samples, without respect to their different structure, a change of the mechanism takes place when vapour activity passes through a value of about 0.25. From the point of view of the amount of water (C1) sorbed on the accessible amide group with water activity equal 1 (accessibility having been determined by means of deuteriation), the methyl, dimethyl and phenyl derivative sorb like non-substituted nylons 2 to 6 (C1=3/2), the other samples with longer aliphatic substituents sorb like nylon 8 and higher nylons (C1=1/2).

The effect of hydrogen bonds on the dynamic mechanical properties of glassy polymethacrylates from 77[ddot

Abstract The role of hydrogen bonds in the dynamic mechanical properties of a series of polymethacrylates was investigated from 77[ddot]K to their respective glass transition temperatures. An automatic, resonancetype acoustic spectrometer was designed for this purpose. It was found that the presence of hydrogen bonds increases the glassy moduli, the primary glass transition temperatures, and the low-temperature gamma transition temperatures. The internal friction peaks of hydrogen-bonding polymers are also broader. Similar effects were noted in polymers containing other types of polar bonds. A series of poly(butyl methacrylate) isomers which possess similar intermolecular cohesive bonding were also studied. With the exception of poly(butyl methacrylate), similar properties were exhibited by these isomers. These results were examined in the light of both the “volume of action” hypothesis of Hoff, Robinson, and Wilbourn and the “bond loosening” hypothesis of Andrews and Hammack. The internal friction as a f...

Viscoelastic Behavior of Copolymers of Methyl Methacrylate and n-Butyl Methacrylate and Glass Polishing by Use of Them

In the first part of the work, shear creep compliance is measured for six kinds of copolymers of methyl methacrylate and n-butyl methacrylate above the respective glass transition temperatures. Shift factors and retardation spectra are calculated therefrom. The molecular weight between entanglement points which is obtained from the maximum of spectrum increases with the increase in n-butyl methacrylate fraction. This effect is interpreted in terms of the plasticizer effect of side chains. The maxima of the spectra of the copolymers are proportional to the square root of the corresponding retardation times if the spectra are reduced to the corresponding state in which the wedge portions of the spectra coincide with each other. In the second part, the results by Orioka who polished optical glass pieces by use of the copolymers are semi-quantitatively explained assuming that the surface of the glass polisher is heated by friction up to the temperature at which the shear creep compliance J(t) at t=polishing time is above 10-7cm2/dyn.

Transitions in polymers by nuclear magnetic resonance¨ a study ¨

AbstractThe principles of nuclear magnetic resonance, NMR, are briefly reviewed and the use of proton resonance for the study of segmental motion in solid high polymers is outlined. Studies of the temperature dependence of NMR spectra have shown that both completely amorphous and partially crystalline polymers exhibit NMR line narrowing processes at temperatures near the respective glass transition temperature due to increased segmental motion. The presence of small molecules, such as H2O or D2O in the polyamides, or a change of tacticity to a greater isotatic content, such as for poly(methyl methacrylate), is found to lower this line narrowing process on the temperature scale. Heating of polyethylene single crystal aggregates at temperatures somewhat below the melting point also leads to a decrease in temperature for this line narrowing process, presumeably due to the introduction of defects in the polymer crystals. In the ⩽ 77°K to 200°K region numerous polymers having methyl and methyl group containing side chains are found to exhibit line width and second moment dispersions, attributed to methyl group rotation. If the methyl group is attached to a phenyl or ester side chain, methyl rotation takes place at 77° K and below.