Peter Avakian

Cooperative relaxations in amorphous polymers studied by thermally stimulated current depolarization

Abstract Thermally stimulated current depolarization (t.s.c.) was used to study the relaxations in amorphous polymers including poly (ethyl methacrylate) (PEMA), poly (methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and polyarylate (PAR) over temperature ranges covering the β and α (glass transition) regions. A.c. dielectric was used to obtain activation energies (Ea) for PS and PC to verify the accuracy of those values determined by the t.s.c. thermal sampling method. At temperatures below the glass transition (Tg) the values of Ea were found to agree with those predicted using an activated states equation with a zero activation entropy. This is evidence of the localized, non-cooperative nature of the low temperature secondary β relaxations which are found to be characterized by a continuous variation of activation energies as a function of temperature. The measured values of Ea depart from the zero activation entropy curve and exhibit a prominent maximum at Tg. This behaviour is known to be due to an enhanced degree of cooperativity of segmental relaxations near Tg. The results indicate that the main advantage of the thermal sampling method is the high sensitivity and high temperature resolution for cooperative relaxations. For the polymers studied here, only PEMA and PMMA show a substantial population of cooperative relaxations more than 60°C below Tg. This is tentatively explained in terms of structural heterogeneity due to variable tacticity in the methacrylates. Compensation of the t.s.c. relaxation spectra plotted in Arrhenius or Eyring plots was found for all polymers to differing degrees. Some discussion of compensation is made in terms of independently measured values of the coefficient of thermal expansion.

A.C. dielectric and TSC studies of constrained amorphous motions in flexible polymers including poly(oxymethylene) and miscible blends

Poly(oxymethylene) ( POM ) and its miscible blends were studied by multifrequency A.C. dielectric and thermally stimulated currents (TSC). The blends contained small amounts of either poly(vinyl phenol), which is a high glass transition (T g diluent, or a styrene-co-hydroxy styrene oligomeric low T g diluent. The variation of the 10°C β transition with blend composition proves that it is the glass transition, and that the -70°C y transition is a local motion. Dielectrically the P transition is very weak in pure POM even in fast-quenched samples. The TSC thermal sampling method also detected two cooperative transitions, and β, in POM and its blends, and was used to directly resolve the transition into low and high activation energy components. If one considers the contribution of exclusion of the diluents from the crystal lamellae, it is shown that the blends behave like typical amorphous blends as a function of concentration. The effect of crystals on amorphous motions is examined in light of comparison with van Krevelens 37 predictions of an amorphous T g , and the transitions in POM are contrasted with those for other semicrystalline polymers.

Cooperative relaxations in semicrystalline fluoropolymers studied by thermally stimulated currents and ac dielectric

Semicrystalline fluoropolymers including poly(tetrafluoroethylene) (PTFE), a 8 mol % hexafluoropropylene (HFP)/92% TFE random copolymer (FEP), and poly(vinyl fluoride) (PVF) were studied using thermally stimulated current depolarization (TSC), ac dielectric, and other thermal analysis techniques. The TSC thermal sampling (TS) technique is emphasized here for the detection of broad and weak “cooperative” relaxations with all three of the polymers studied exhibiting two cooperative (i.e., relatively high apparent activation energy) transitions. The well-studied low-temperature γ relaxation in PTFE at ca. −100°C is characterized by this method as well as the γ relaxation in the less crystalline FEP sample. Higher temperature cooperative glass transitions, associated with constrained noncrystalline regions, are found at ca. 100°C in PTFE and ca. 80°C in FEP at TSC frequencies. Comparisons with relaxation studies of linear polyethylene are made, and the effects of crystallinity on the various transitions are discussed. The unique characterization by the TSC-TS technique in the detection of multiple “cooperative” relaxations, even in the case of overlapping transitions, is emphasized here. An example is the low-temperature relaxation in FEP. Two cooperative transitions were detected in PVF. The higher temperature one at ca. 45°C is the glass transition, as is well known in the literature. More information is needed to confirm the molecular origin and the effects of crystallinity and chemical structure on the low-temperature cooperative transition in PVF.

Copolymer modification of nylon-6,6 with 2-methylpentamethylenediamine

Polyamides were prepared in which part or all of the hexamethylenediamine (HMD) in nylon-6,6 was replaced by its isomer, 2-methylpentamethylenediamine (MPMD). The intermediate compositions are considered random copolymers between the two homopolymers (66 and MPMD-6). The crystalline properties of these polymers were studied by differential scanning calorimetry, X-ray diffraction and optical microscopy. It was concluded that the two kinds of units, 66 and MPMD-6, do not co-crystallize. In copolymers containing large fractions of HMD, the MPMD units are excluded from the crystals. With appropriate thermal treatment, MPMD-6 can crystallize with two different unit cells and a variety of spherulitic morphologies. The two crystal forms can be distinguished by their crystallization, melting and optical properties. The crystallization kinetics of the copolymers are slower than those of the homopolymers, which is associated with the slower rate of nucleation formation rather than the transport ability. This is consistent with the dielectric results, which showed that the glass transition temperatures of the different copolymers are similar. It was found that MPMD-6 can be quenched into an amorphous state. The properties of the amorphous portions were further investigated through dielectric relaxations. Although, the incorporation of MPMD tends to shift the dielectric gamma relaxation towards higher temperatures, the total effects of variations on chemical structure were much smaller than those which pertain to the crystalline phase.

Studies of phase separated copolymer blends using thermally stimulated currents and dielectric spectroscopy

Abstract Thermally stimulated currents (t.s.c.s) and a.c. dielectric spectroscopy were used to study poly(butylene terephthalate) (PBT) and blends of PBT with segmented copoly(ester-ether) elastomers. The high sensitivity of t.s.c. and a.c. dielectric techniques as compared to d.s.c. was useful for the study of these blends where the elastomer phase was present at only 20%. The techniques together cover a frequency range of ∼5 × 10−3–105 Hz allowing detailed characterization of amorphous relaxations in these phase separated blends. Two separate glass transitions for the PBT/copoly(ester-ether) blends were observed, unchanged from the original pure components, indicating that they are immiscible and that ester exchange during processing was minimal. Peak assignments are made using a new analysis scheme by comparing activation energies (Ea) determined by the t.s.c. thermal sampling technique with predicted values using the activated states equation. With this method all values of Ea agree with the zero activation entropy prediction except those measured in the vicinity of glass transitions.

Thermal Properties of the Polyamide from 2-Methylpentamethylenediamine and Dodecanedioic Acid

The polyamide from 2-methylpentamethylenediamine and dodecanedioic acid (MPMD-12) has been studied by differential scanning calorimetry, dynamic mechanical analysis, dielectric analysis, X-ray crystallography, and simultaneous small and wide-angle X-ray scattering using synchrotron radiation. The polymer exhibits polymorphism which is shown to be associated with the incorporation of the branched diamine. At relatively low temperatures, the crystal structure is similar to the gamma form which has been found in many other polyamides. At higher temperatures, a new delta form appears in which the diamine moiety adopts a bent conformation. In this form, the chains follow a zig-zag pattern with two chemical repeats units per crystallographic repeat with a shortening of about 10% along the c-axis.