The physical properties of bisphenol-a-based epoxy resins during and after curing. II. Creep behavior above and below the glass transition temperature

The creep behavior of a series of fully cured epoxy resins with different crosslink densities was determined from the glassy compliance level to the equilibrium compliance J_e at temperatures above T_g and at the glassy level below T_g during spontaneous densification at four aging temperatures, 4,4-diamino diphenyl sulfone DDS was used to crosslink the epoxy resins. The shear creep compliance curves J(t) obtained with materials at equilibrium densities near and above T_g were compared at their respective T_gs. T_gs from 101 to 205°C were observed for the epoxies which were based on the diglycidyl ether of bisphenol A. Creep rates were found to be the same at short times, and equilibrium compliances J_e were close to the predictions of the kinetic theory of rubberlike elasticity. Time scale shift factors determined during physical aging were reduced to T_g. At compliances below 2 × 10^?10 cm²/dyn, Andrade creep, where J(t) is a linear function of the cube root of creep time, was observed. The time to reach an equilibrium volume at T_g was found to be longer for the epoxy resin with lower crosslink densities. The increase of density during curing is illustrated for the epoxy resin with the highest crosslink density.     

Positron annihilation in amine-cured epoxy polymers—pressure dependence

Positron annihilation spectroscopy has been used to study free volume in an arnine-cured epoxy as a function of external pressure at temperatures above and below the glass transition temperature. The observed ortho-positronium lifetime τ₃ and formation probability I₃ decreased with increasing pressure. The decrease in τ₃ is interpreted in terms of a corresponding decrease in average free-volume hole size over the range from 0.135 to 0.045 nm³. The fractional free-volume and the free-volume compressibility in the epoxy are calculated as functions of pressure at 100°C.     

Radiothermoluminescence. I. Radiothermoluminescence for investigation of thermal and radiation-induced crosslinking of rubber mixtures

Phase heterogeneity of vulcanizates based on cis-polybutadiene (cis-Europrene) and SBR (Europrene 1500) blends obtained by radiation, thermal, two-stage radiation-thermal, and two-stage thermal-radiation processes was investigated by the radiothermoluminescence method. Unvulcanized blends of these polymers are found to be heterogeneous, exhibiting two glass transition temperatures T_g, which coincide with the values for the initial components. Following vulcanization, the difference between the two T_g values for a polyblend decreases. The decrease is more distinct for polyblends crosslinked at high temperatures. When the vulcanization temperature exceeds 140°C and the crosslink density (or 1/M_c) exceeds 5 × 10^?5–1 × 10^?4 mole/cm³, the glass temperature ranges of the crosslinked blend are practically superimposed. Such crosslinked mixtures should be considered pseudohomogeneous in phase.     

Free volumes in nematic and smectic liquid-crystalline polymers probed by positron annihilation

Free volumes in thermotropic side-chain liquid-crystalline polymers were probed by positron annihilation technique. Lifetime spectra of positrons were measured in the temperature range between 130 and −60°C in cooling. For a nematic liquid-crystalline polymer (polyacrylate), the lifetime of ortho-positronium (τ₃) was decreased with decreasing temperature above the glass transition temperature (T_g, 21°C) with larger temperature coefficient than that below T_g. The intensity of ortho-positronium (I₃) was constant above T_g. These facts mean that the size of the free-volume holes decreased with the decreasing the temperature but the concentration was almost constant in nematic phase. For a smectic liquid-crystalline polymer (poly(p-methylstyrene) derivative), a discontinuous decrease in the value of τ₃ and that of I₃ were observed at 107°C, which was the transition temperature from smectic to crystalline phase. Such discontinuous changes were not observed for the polyacrylate specimen. This difference was considered to be attributed to the higher-ordered structure of the smectic phase. © 1996 John Wiley & Sons, Inc.     

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 linear rheological responses of wedge‐type polymers

The linear rheological responses of a series of specially designed wedge‐type polymers synthesized by the polymerization of large molecular weight monomers have been measured. These wedge polymers contained large side groups which contained three flexible branch chains per polymer chain unit. The master curves for these polymers were obtained by time temperature superposition of dynamic data at different temperatures from the terminal flow regime to well below the glass transition temperature, T_g. While these polymers maintained a behavior similar to that of linear polymers, the influence of the large side group structure lead to low entanglement densities and extremely low rubbery plateau modulus values, being near to 13 kPa. The viscosity molecular weight dependence was also somewhat higher than that normally observed for linear polymers, tending toward a power law near to 4.2 rather than the typical 3.4 found in entangled linear chains. The glassy modulus of these branched polymers is also found to be extremely low, being less than 100 MPa at T_g ?60 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 899–906     

Synergism and multiple mechanical relaxations observed in ternary systems based on benzoxazine,epoxy, and phenolic resins

The synergism in the glass‐transition temperature (T_g) of ternary systems based on benzoxazine (B), epoxy (E), and phenolic (P) resins is reported. The systems show the maximum T_g up to about 180 °C in BEP541 (B/E/P = 5/4/1). Adding a small fraction of phenolic resin enhances the crosslink density and, therefore, the T_g in the copolymers of benzoxazine and epoxy resins. To obtain the ultimate T_g in the ternary systems, 6–10 wt % phenolic resin is needed. The molecular rigidity from benzoxazine and the improved crosslink density from epoxy contribute to the synergistic behavior. The mechanical relaxation spectra of the fully cured ternary systems in a temperature range of −140 to 350 °C show four types of relaxation transitions: γ transition at −80 to −60 °C, β transition at 60–80 °C, α₁ transition at 135–190 °C, and α₂ transition at 290–300 °C. The partially cured specimens show an additional loss peak that is frequency‐independent as a result of the further curing process of the materials. The ternary systems have a potential use as electronic packaging molding compounds as well as other highly filled systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1687–1698, 2000     

Synthesis and characterization of mesogen‐jacketed liquid‐crystal polymers based on 2,5‐bis(4′‐alkoxyphenyl)styrene

A series of novel mesogen‐jacketed liquid‐crystal polymers, poly[2,5‐bis(4′‐alkoxyphenyl)‐styrene] (P‐n, n = 1–11), were prepared via free‐radical polymerization of newly synthesized monomers, 2,5‐bis(4′‐alkoxyphenyl)styrene (M‐n, n = 1–11). The influence of the alkoxy tail length on the liquid‐crystalline behaviors of the monomers and the polymers was investigated with differential scanning calorimetry (DSC), thermogravimetry, polarized optical microscopy (POM), and wide‐angle X‐ray diffraction (WAXD). The monomers with n = 1–4, 9, and 11 were monotropic nematic liquid crystals. All other monomers exhibited enantiotropic nematic properties. Their melting points (T_m's) decreased first as n increased to 6, after which T_m increased slightly at longer spacer lengths. The isotropic–nematic transition temperatures decreased regularly with increasing n values in an odd–even way. The glass‐transition temperatures (T_g's) of the polymers first decreased as the tail lengths increased and then leveled off when n ≥ 7. All polymers were thermally stable and entered the mesophase at a temperature above T_g. Upon further heating, no mesophase‐to‐isotropic melt transition was observed before the polymers decomposed. WAXD studies indicated that an irreversible order–order transition for the polymers with short tails (n ≤ 5) and a reversible order–order transition for those with elongated tails (n ≥ 6) occurred at a temperature much higher than T_g. However, such a transition could not be identified by POM and could be detected by DSC only on heating scans for the polymers with long tails (n ≥ 7). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1454–1464, 2003     

A relationship between macroradical decay kinetics and α-segmental dynamics in glassy amorphous polymers

For a series of five amorphous polymers with a broad range of T_g values the kinetics of macroradical decay was measured by ESR technique and evaluated by the second-order kinetic model. It was found that the temperature T_tr of the transition between two regions of different reactivity in free radical decay reaction agrees quite well with the temperature parameter T₀ of the Vogel-Fulcher-Tamman-Hesse (VFTH) equation for α-segmental dynamics. This parameter represents the onset of α-segmental mobility in glassy state below T_g. A nontraditional way of the estimation of T₀ values for α-segmental dynamics through study of the macroradical decay in glassy state of amorphous polymers has been suggested. © 1996 John Wiley & Sons, Inc.     

Thiol–ene polymerizations using imide‐based monomers

New diene and dithiol monomers, based on aromatic imides such as benzophenone‐3,3′,4,4′‐tetracarboxylic diimide were synthesized and used in thiol‐ene polymerizations which yield poly(imide‐co‐thioether)s. These linear polymers exhibit limited solubility in various organic solvents. The molecular weights of the polymers were found to decrease with increasing imide content. The glass transition temperature (T_g) of these polymers is dependent on imide content, with T_g values ranging from ?55 °C (with no imide) up to 13 °C (with 70% imide). These thermal property improvements are due to the H‐bonding and rigidity of the aromatic imide moieties. Thermal degradation, as studied by thermogravimetric analysis, was not significantly different to the nonimide containing thiol‐ene polymers made using trimethyloylpropane diallyl ether and 3,5‐dioxa‐1,8‐dithiooctane. It is expected that such monomers may lead to increased glass transition temperatures in other thiol‐ene polymer systems as these normally exhibit low glass transition temperatures. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4637–4642     

The temperature dependence of the viscoelastic softening and terminal dispersions of linear amorphous polymers

Thermorheological simplicity is shown to hold for poly(vinyl acetate) in the temperature range extending from T_g + 25°C to T_g + 80°C. Between T_g and T_g + 25°C the softening (glass to rubberlike) viscoelastic dispersion exhibits time-scale shift factors a_T different from those of the terminal (rubberlike to steady-state) dispersion. The a_T values calculated from zero-shear viscosities coincide with those from the terminal dispersion in the temperature range 60–154°C (T_g ? 35°C). The a_T shifts obtained from the response in the terminal dispersion can be fitted to the Williams, Landel, and Ferry equation over the entire temperature range 42–154°C. The a_T obtained from the softening dispersion is shown to exhibit a different functionality. An empirical modification of the Doolittle equation yields a very flexible relation which can be fitted to some a_Ts which cannot be represented by the usual Doolittle free-volume expression.     

New soybean oil–styrene–divinylbenzene thermosetting copolymers. II. Dynamic mechanical properties

A variety of new polymeric materials ranging from soft rubbers to hard, tough, and brittle plastics were prepared from the cationic copolymerization of regular soybean oil, low saturation soybean oil (LoSatSoy oil), or conjugated LoSatSoy oil with styrene and divinylbenzene initiated by boron trifluoride diethyl etherate (BF₃ · OEt₂) or related modified initiators. The relationship between the dynamic mechanical properties of the various polymers obtained and the stoichiometry, the types of soybean oils and crosslinking agents, and the different modified initiators was investigated. The room‐temperature storage moduli ranged from 6 × 10⁶ to 2 × 10⁹ Pa, whereas the single glass‐transition temperatures (T_g) varied from approximately 0 to 105 °C. These properties were comparable to those of commercially available rubbery materials and conventional plastics. The crosslinking densities of the new polymers were largely dependent on the concentration of the crosslinking agent and the type of soybean oil employed and varied from 74 to 4 × 10⁴ mol/m³. The T_g increased and the intensity of the loss factor decreased irregularly with an increase in the logarithmic crosslinking densities of the polymers. Empirical equations were established to describe the effect of crosslinking on the loss factor in these new polymeric materials. The polymers based on conjugated LoSatSoy oil, styrene, and divinylbenzene possessed the highest room‐temperature moduli and T_g 's. These new soybean oil polymers appear promising as replacements for petroleum‐based polymeric materials. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2721–2738, 2000     

Dielectric relaxation and glass transition temperature of copolymers

Dielectric relaxation measurements were made on methyl methacrylate—styrene and methyl methacrylate–p-chlorostyrene copolymers at temperatures higher than the glass transition temperature T_g. It was found that the temperature dependence of the relaxation time can be described satisfactorily by an expression derived recently for chain motion in amorphous polymers. The temperature T_g obtained from the expression agrees well with that determined by differential thermal analysis.     

Phenylethynyl-pendant polyphenylquinoxalines curable by an intramolecular cycloaddition reaction

Polyphenylquinoxalines containing 2,2′-bis(phenylethynyl)diphenylene moieties along the polymer backbone have been synthesized. As anticipated, these polymers were found to undergo a novel curing reaction consisting of an intramolecular cycloaddition (IMC) of pendant groups to a dibenzoanthracene backbone structure. The IMC reduces chain mobility, and the fused ring structure increases the glass transition temperature of the polymer. The potential of this approach to curing high-temperature polymers was demonstrated in the processing of one such polymer having an initial T_g of 215°C. Curing at 245°C with no evolution of volatiles produced a T_g of 365°C. This very significant increase in potential use temperature via a volatiles-free IMC cure provides promise for a tough phenyiquinoxaline resin system which can be used to fabricate reinforced composites that have use temperatures far exceeding processing temperatures.     

Tuning the LCST of poly(2‐cyclopropyl‐2‐oxazoline) via gradient copolymerization with 2‐ethyl‐2‐oxazoline

Poly(2‐propyl‐oxazoline)s can be prepared by living cationic ring‐opening polymerization of 2‐oxazolines and represent an emerging class of biocompatible polymers exhibiting a lower critical solution temperature in aqueous solution close to body temperature. However, their usability is limited by the irreversibility of the transition due to isothermal crystallization in case of poly(2‐isopropyl‐2‐oxazoline) and the rather low glass transition temperatures (T_g < 45 °C) of poly(2‐n‐propyl‐2‐oxazoline)‐based polymers. The copolymerization of 2‐cyclopropyl‐2‐oxazoline and 2‐ethyl‐2‐oxazoline presented herein yields gradient copolymers whose cloud point temperatures can be accurately tuned over a broad temperature range by simple variation of the composition. Surprisingly, all copolymers reveal lower T_gs than the corresponding homopolymers ascribed to suppression of interchain interactions. However, it is noteworthy that the copolymers still have T_gs > 45 °C, enabling convenient storage in the fridge for future biomedical formulations. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3118–3122     

Dynamic mechanical relaxation effects in some poly(arylene sulfones)

Poly-4,4′-oxydiphenylenesulfonyl and poly-4,4′-methylenediphenylenesulfonyl were synthesized by an electrophilic substitution polymerization of the arylene monosulfonyl chloride monomers. The glass-transition temperatures T_g of these polymers were determined by calorimetric and dynamic mechanical measurements, and the number-average molecular weights were determined by vapor-pressure osmometry. Both polymers were found to have the same T_g at equivalent molecular weight; the limiting value at high molecular weight is 238°C. Both polymers have two dynamic mechanical relaxation peaks at temperatures far below T_g. One is in the neighborhood of 0°C, and the other is at ?110°C. Plausible origins for these relaxations, and the absence of any near 0°C in poly(4,4′-isopropylidenediphenylene-co-4,4′-sulfonyldiphenylene dioxide), are discussed.     

Pressure–volume–temperature behavior of two polycyanurate networks

The pressure–volume–temperature (PVT) behavior was studied for two polycyanurate networks having different crosslink densities using a pressurizable dilatometer. The samples were studied at temperatures ranging from 60 to 180 °C and at pressures up to 170 MPa to yield PVT data in both rubbery and glassy states. The Tait equation is found to well describe the isobaric temperature scan and isothermal pressure scan data. The thermal expansion coefficients, instantaneous bulk moduli, and thermal pressure coefficients are extracted from the data and their dependence on crosslink density is examined. The time‐dependent viscoelastic bulk modulus (K(t)) is also calculated in the vicinity of the α‐relaxation from previously published pressure relaxation experimental data, and the strength and shape of the dispersion are found to be independent of crosslink density. The limiting bulk moduli depend strongly on temperature with those of the more loosely crosslinked sample being lower at a given temperature and pressure, although at T_g(P), the limiting moduli of the more loosely crosslinked sample are slightly higher than those of the more highly crosslinked sample. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Thermal behavior of ferroelectric Polyamide 11 in relation to pyroelectric properties

The pyroelectric properties of oriented thin films of ferroelectric Polyamide 11 have been studied in the temperature range of −100°C up to +140°C. The temperature dependence of the experimental pyroelectric coefficient has been analyzed. Three changes of slope of the pyroelectric coefficient are observed at −20, +50, and +100°C. The origin of the lower temperature event has not yet been defined. The upper transition is attributed to chain movements in crystalline regions, and more precisely, to a crystalline phase transition. The intermediate event is close to the glass transition temperature T_g observed by DSC. It is attributed to the manifestation of the glass transition. Below T_g, the variations of the pyroelectric coefficient are very small. For higher temperatures, it increases rapidly, attesting to a major contribution of secondary pyroelectricity and dimensional effects above T_g. The breaking of hydrogen bonds occurring at the glass transition temperature observed on DSC thermograms does not affect pyroelectric properties. Pyroelectric properties are mildly reduced after annealing at temperatures up to +140°C. A comparative study of oriented ferroelectric films prepared by quenching from the melt and nonoriented slowly cooled samples has been carried out by means of DSC. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 715–723, 1999     

Enhancing the glass‐transition temperature of polyimide copolymers containing 2,2′‐bipyridine units by the coordination of nickel malenonitriledithiolate

Polyimide copolymers containing 2,2′‐bipyridine were synthesized and characterized. The glass‐transition temperatures (T_g's) of the polymers ranged from 260 to 300 °C. In contrast to most known organic chromophore‐containing polyimides, the polyimide copolymers in this study showed elevated T_g's (270–320 °C) after coordination with nickel malenonitriledithiolate inorganic chromophores. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 498–503, 2000     

Glass transition temperatures of several fluorine-containing polymers

The glass transition temperatures T_g of several fluorine-containing polymers were determined by use of the differential scanning calorimeter. Values between ?3 and 230°C were obtained. In polymers of α-olefins, T_g increases with the fluorine content of the backbone and the length of the n-perfluoroalkyl branch. In styrene polymers T_g also is higher if the backbone contains fluorine but nearly the same T_g's are found for polymers with phenyl and pentafluorophenyl groups. Saturated polymers of perfluoro-α,ω-dienes have lower T_g's than polyperfluoro-α-olefins. The T_g's of chloroperfluoropolymers are higher than those of perfluoropolymers. Polyperfluoropentadiene-1,3 has the lowest T_g of the polymers examined. Polyperfluoropentadiene-1,3 forms by 1,4-addition.