Howard W. Starkweather

Aspects of simple, non-cooperative relaxations

Abstract The lower limit for the activation energies of internal motions in polymers is defined by a relationship corresponding to an activation entropy of zero. We associate this characteristic with non-cooperative motions of molecular segments. While these motions are localized in space, they are very diffuse in frequency or time. Examples are side-group relaxations and certain motions within the crystalline regions. Local-mode γ relaxations of the main chain appear to involve a spectrum of motions in which the limiting low-temperature, high-frequency component has an activation entropy close to zero. Glass transitions characteristically have very large apparent activation energies and entropies. This is attributed to a spectrum of motions that is sampled differently at different temperatures. At temperatures well above Tg, the apparent activation energy again approaches a value corresponding to a zero activation entropy. For a wide variety of relaxations, the distribution of relaxation times corresponds to a distribution of activation enthalpies having a width at half-maximum of 5–6 kcal mol−1.

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.

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.