J. K. Gillham

A relationship between the glass transition temperature (Tg) and fractional conversion for thermosetting systems

An equation, based on thermodynamic considerations to relate the glass transition temperature, T g , to compositional variation of a polymer system, is adapted in this article for modeling the T g vs. fractional conversion (x) relationship of reactive thermosetting systems. Agreement between the adapted equation and experimental Tg vs. x data is found for several thermosetting crosslinking systems (i.e., epoxies and cyanate ester/polycyanurate) as well as for reactive thermosetting linear polymer systems (i.e., polyamic acid and esters to polyimides). The equation models the experimentally obtained T g vs. x behavior of thermosetting systems which include competing reactions. Agreement for widely varying molecular structures demonstrates the generality of the equation. The entire T g vs. x relationship can be predicted for a thermosetting material by using the T g vs. x equation and the values of the initial glass transition temperature, T g0 , the fully reacted system glass transition temperature, T gx , and the ratio of the change in specific heat from the liquid or rubbery state to the glassy state (Δc p ) at T g0 and T gx , Δc px /Δc P0 . The values of T go, T gx , and ΔC px /Δc p0 can be measured generally from two differential scanning calorimetric experiments.

The TBA torsion pendulum: a technique for characterizing the cure and properties of thermosetting systems†

Generalizations on the cure and properties of thermosetting polymers, which have stemmed from the development and application of the torsional braid analysis (TBA) technique, have been formulated in terms of cure–property relationships. They are the isothermal time–temperature–transformation (TTT) cure diagram, the continuous heating time–temperature–transformation (CHT) cure diagram, the conversion–temperature–property (TgTP) diagram, and the glass transition temperature (Tg) versus conversion relationship. The relationships may be used to design time–temperature cure paths, that exploit gelation and vitrification, to optimize cure processes and glassy state properties. Glassy state properties studied have included modulus, density and microcracking versus conversion, and the dynamics of submolecular motions as represented by physical ageing and transitions. Rubber modification and water absorption have also been investigated. This review emphasizes results obtained by exploiting the ability of the TBA technique to examine amorphous specimens in which material changes from liquid (or rubber) to glass, and vice versa. ©1997 SCI

The Tℓℓrelaxation of polystyrene

Abstract A review of recent work by the authors on a relaxation (Tgll) above the glass transition Tg of amorphous polystyrene is presented. On the basis of experimental data from torsional braid analysis (TBA), differential scanning calorimetry (DSC), and differential thermal analysis (DTA), and of an examination of the literature, it is suggested that Tll represents a molecularly-based transition. Its main features are summarized by the expressions: Tll ≃ 1.2 Tg and Tll ≃Tll(∞)—KllMn −1, where temperatures are in degrees Kelvin and Kll is a constant. Jt follows that if Tg is an isofree-volume state, then so is Tll Further, when Mn≃Mw and M for each component <MC (the critical molecular weight for chain entanglements) Tll is also an isoviscous state. Recent work shows Tll to be ubiquitous for the amorphous state of polymeric materials.

Isothermal physical aging of a fully cured epoxy—amine thermosetting system

The rate and effects of isothermal physical aging of a fully cured epoxyamine/ glass fiber composite specimen were studied for a wide range of isothermal aging temperatures (-180 to 200°C) using a freely oscillating torsion pendulum technique: torsional braid analysis (TBA). As assigned from the maxima in the mechanical loss vs. temperature, the glass transition temperature, T g , was 182°C (0.9 Hz ) , and the principal glassy-state secondary transition temperature, T β , was ≃ -30°C (1.9 Hz). Plots of the increase in the isothermal modulus and of the decrease in the isothermal mechanical loss were linear vs. log aging time; their slopes provided aging rates. It was found that the isothermal aging rate varies with isothermal aging temperature (T a ) and that there are two maxima in the aging rate vs. T a . A correlation presumably exists between the two maxima in the aging rate and the two transitions. This is not surprising since mechanical loss maxima (i.e., transitions ) and aging rate maxima both correspond to specific, localized, and restricted submolecular motions. Effects after isothermal physical aging were investigated vs. temperature in terms of change of modulus of the specimen. The effect of isothermal aging existed primarily in a narrow temperature region localized about T a . The majority of the isothermal aging effect can be eliminated by heating to temperatures above T a but below T g . Theoretical and practical implications of this observation are discussed.

Evolution of properties of an isocyanate/epoxy thermosetting system during cure: Continuous heating (CHT) and isothermal time—temperature—transformation (TTT) cure diagrams

The present article describes a methodology for examining the evolution of the properties vs. cure of a complex thermosetting isocyanate/epoxy reactive mixture which reacts through two consecutive but separable reaction regimes. The methodology is based on the use of the torsional braid analysis (TBA) technique and the continuous heating (CHT) and isothermal time-temperature-transformation (TTT) cure diagrams.

CHARACTERIZATION OF THERMOSETTING EPOXY SYSTEMS USING A TORSIONAL PENDULUM

An automated torsional pendulum has been used to investigate epoxy systems with respect to the effects of cure, transitions, morphology and active environments (water vapor and organic liquid). The findings include: 1) the thermosetting process is characterized by two transition temperatures (T gg and T g∞ ), 2) thermomechanical and optical data correlate through a phase inversion in a rubber-modified system, 3) gelation time can be used to control development of morphology, 4) a reversible low temperature transition [T H2O ca. -70°C (1.6 Hz)] is induced by exposure to water vapor, 5) the glass transition temperature is lowered in the presence of water vapor, and 6) thermomechanical spectra of partially plasticized specimens display the transition of two phases (unplasticized and plasticized) superimposed.

Review: Torsional braid analysis of polymers

Abstract A review of the present status of torsional braid analysis (TBA) includes discussion of instrumentation, specimen preparation, theory, and data reduction. The obtained thermomechanical spectra (∼1 cps) are discussed in terms of advantages which result from the use of small specimens. Applications include structure-property relationships, kinetics of crosslinking, and optimization of thermomechanical behavior in reactive systems.

Conversion-temperature-property relationships in thermosetting systems : Property hysteresis due to microcracking of an epoxy/amine thermoset-glass fiber composite

A single specimen of an epoxy/amine thermoset-glass fiber composite was examined, using a freely oscillating torsion pendulum operating at ∼ 1 Hz, for different conversions (as measured by T g ) from T g0 = 0°C to T gx = 184°C during cooling and heating temperature scans. T g was increased for successive pairs of scans by heating to higher and higher temperatures. The data were used in two ways:(i) vs. temperature for a fixed conversion to obtain transitions, modulus, and mechanical loss data, and (ii) by crossplotting to obtain isothermal values of the mechanical parameters vs. conversion (T g ). Hysteresis between cooling and subsequent heating data was observed in temperature scans of essentially ungelled material (T g < 70°C) and was attributed to spontaneous microcracking. Hysteresis was analyzed in terms of the following three parameters: T crack , the temperature corresponding to the onset of microcracking on cooling; T heal, the temperature at which the specimen heals on subsequent heating; and the difference between isothermal cooling and heating data vs. conversion. Results were incorporated into a more general conversion-temperature-property diagram which serves as a framework for relating transitions (relaxations) to macroscopic behavior.

Pyrolysis-Molecular Weight Chromatography of Polymers: A New Technique

Abstract A pyrolyzer with programming capability has been coupled in series with a thermal conductivity cell and a mass chromatograph. The thermal conductivity cell gives the flexibility for selective trapping of decomposition products, and also provides data that are complementary to thermogravimetric and differential thermal analyses. The mass chromatograph is composed of two gas chromatographs that are run parallel from a common injection port. Each chromatograph uses a different carrier gas and is equipped with a gas density balance detector. The instrument simplifies identification of mixtures of unknowns in directly providing molecular weights, absolute quantities, and gas chromatographic retention times of the constituents. In this respect, the present system is analogous to pyrolysisgas chromatography-mass spectrometry. Discussion of the technique and its application to the investigation of the thermal degradation of low-density polyethylene are presented.

Low-temperature relaxations in amorphous polyolefins

Abstract The dynamic mechanical relaxation behavior (1cps) of two series of amorphous polyolefins, ─(CH2)mC(CH3)2─ and ─(CH2)mC(CH3)(CH2 CH3)─ where m = 1, 2, 3 was investigated from 4.2°K to the glass transition. Most of the polymers show a damping maximum or pleateau in the 40 to 50°K region. Various mechanisms which have been suggested for cryogenic relaxations in amorphous polymers are considered as they might relate to the polyolefins. Two secondary relaxation processes above 80°K are distinguished. A relaxation at about 160°K (β) in the second and third member of each series is associated with restricted backbone motion. This process requires a certain degree of chain flexibility since it is not observed in the first member of each series. A lower Temperature process (γ) is observed in each member of the second series and is attributed to motion of the ethyl side group.