Showing papers in "Journal of Polymer Science Part C: Polymer Symposia in 1972"

Structural changes of drawn polyacrylonitrile during annealing

Abstract: Fibers and solvent-cast films of polyacrylonitrile have been investigated by x-ray small-angle and wide-angle measurements and dynamic-mechanical analysis. It has been found that the structure of this polymer is better described by a two-phase model consisting of more and less ordered regions. By the aid of x-ray methods, transition temperatures have been observed; there are indications that the molecules within the ordered regions are responsible for motion mechanisms. By annealing, the colloidal structure and thereby the mechanical behavior is influenced.

Precision differential calorimetry and the heat of fusion of polyethylene

Abstract: A procedure is described for the analysis by computer of the output from a differential scanning calorimeter. Enthalpy calibration with a variety of materials confirms that alumina is excellent for this purpose. Both the crystallinity, xT, and the heat of fusion of polyethylene, ΔHT, vary with temperature and it is important that both are measured. The use of a room-temperature crystallinity with a high-temperature heat of fusion can be very misleading. Interfacial effects are shown to be negligible in most cases and the heat of fusion of perfectly crystalline polyethylene is given by ΔHT/xT. It is 309 Jg−1 at the melting point.

Morphological changes in nylon 6 and effect on mechanical properties. II. Dynamic mechanical properties

Abstract: Dynamic mechanical measurements were carried out on nylon 6 film treated with heat and swelling agents to produce changes in the crystalline morphology. The original smectic hexagonal crystal form was changed by heat treatment and phenol treatment to an unoriented and oriented α-crystalline form, respectively, and by I2/KI treatment to a preferentially oriented γ-crystalline form. The behavior of the αa mechanical loss peak located at about +70°C was investigated in most detail. This peak was independent of frequency on the low-temperature side of the peak but markedly frequency-dependent on the high-temperature side. The low-temperature side of the peak probably represents a thermodynamically controlled “melting” or bond-breaking process which frees molecular units to participate in a viscoelastic relaxation process which is the controlling factor in the high-temperature side of the peak. Frequency-temperature superposition of data from the high-temperature side of the peak gave shift factor values which showed two straight-line regions in the Arrhenius plot for the specimens containing some smectic hexagonal form, and a single straight line in the other cases. Activation energy values obtained were 30–50 kcal. The αa peak split into two components with an increase in absorbed phenol, indicating that two loss mechanisms corresponding to two noncrystalline phases (one amorphous and one more highly ordered or hindered) are involved.

Influence of thermal history on the melting behavior of isotactic polystyrene

Abstract: Multiple melting transitions of thermally crystallized isotactic polystyrene were studied. This behavior was attributed to a very rapid reorganization during the melting of the crystalline structure originally formed. From differential scanning calorimetry and optical microscopy it was shown that the rate of reorganization decreases with increasing reorganization temperature TR. This reorganization will only occur if its rate is larger or equal to the scanning rate. A TR value can be defined for each scanning rate and the resulting melting temperature TRM increases when the scanning rate decreases. This increase of TRM is also examined by annealing at different temperatures. A linear relationship between TRM and TR is observed and gives an extrapolated equilibrium melting point of 240°C; moreover the slope is smaller than for isothermal crystallization (TM versus TC). These phenomena are confirmed by optical microscopic observations.

Structural changes during deformation of high molecular weight and low molecular weight polyethylene

Abstract: Structural changes caused by cold drawing low and high molecular weight polyethylene (M = 7 × 104 and 1.5 × 106) were investigated by using wide-angle x-ray diffraction, light and electron microscopy. Results obtained from drawn samples subjected to stress and samples at zero stress, revealed important differences in the behavior of these two polymers. In contrast with the low molecular weight polyethylene, the high molecular weight samples were deformed homogeneously without neck formation; the orthorhombic lattice, when subjected to stress, is transformed into a monoclinic one, not only in the initial stage, but over the whole deformation range up to breaking. The spherulitic structure in both polymers changes into the fibrillar one, while the degree of orientation attainable for the low molecular weight polyethylene is higher than that for the high molecular weight sample. During deformation of the low molecular weight polymer, cracks form between fibrils, and the fibrils become separated. Such an effect has not been observed with the high molecular weight material. After the external stress has been removed, a considerable contraction and decrease in orientation characterize the high molecular weight polyethylene. The differences in behavior of the two types of polymers are discussed in terms of different structural models.

Some problems in radiation grafting to cellulose fibers.

Abstract: In the case of grafting by preirradiation method, the presence of air during preirradiation by gamma rays has practically no influence on the percent grafting, whereas the presence of water decreases the activity to a great extent. ESR studies of preirradiated cotton and rayon show that the active species for the grafting are not peroxides but free radicals even when the irradiation is carried out in air. The number of scissions of chemical bonds of the cellulose molecule by the irradiation is found to be practically independent of the degree of polymerization and microstructure of the initial cellulosic fiber; the presence of air also has no effect on the degradation. In the case of styrene grafting, breaking strength of single fibers remains substantially unchanged by the grafting, but the increase of denier by the grafting results in a drop of tenacity (g/den) of the single fibers. It is true that cellulosic fibers become hydrophobic by the grafting of styrene, but the tendency to absorb water calculated on the cellulose basis increases by the increasing percent graft. This may be the reason for the rather low wet tenacity of the styrene grafted cellulosic fibers.

A refractometric and gel sorption study of the system poly(vinyl alcohol)-water-n-propanol

Abstract: It has been found that direct measurement of selective sorption of solvent mixture on the loosely crosslinked gels with low polymer concentration would yield some information on the preferential solvation and also on the nature of the interactions. Experimental data obtained by refractometry and sorption measurements carried out with the system poly(vinyl alcohol)-water-n-propanol have shown the proposed way of approximation to be successful. Based on the experimental results a solvate model for this system was proposed in which the n-propanol is bound to the polymer chain by both hydrogen and hydrophobic bonds.

Morphology of glassy polymers

Abstract: A study has been made of the annealing process which takes place when glassy polymers are heated at temperatures near their glass transition temperatures. Low-angle x-ray diffraction measurements, extending previous work using wide-angle techniques, infrared, laser-Raman, and scanning electron microscopical studies reflect no difference between quenched and annealed polymer. Replica electron micrographs do however indicate some structural reorganization. Differential scanning calorimetry and stress-strain studies in conjunction with density measurements show that changes in all these occur on annealing but that at the present time conclusive evidence of local chain parallization is still awaited.

The crystallization of branched polymers. II. Substitutional solution of side chains

Abstract: The crystalline structure of bulk polyethylene containing short chain branches was investigated with the aid of small-angle and wide-angle x-ray scattering. The crystalline density obtained from measurements of the invariant, the crystallinity, and the overall density were compared with the crystalline density following from the lattice constants. Good agreement was found for linear and alkyl branched samples; however systematic differences were observed for copolymers containing ester and carboxylic acid side groups. These results are explained by assuming the side groups to enter into the crystalline lattice by substitutional solution rather than at interstitial positions. The chains around the side groups would have to show a kinked structure. This type of crystallization is in agreement with the principle of optimal space filling laid down by Kitaigorodskii [5].

Change of spherulitic structure in melt-crystallized polymer fibers and films during uniaxial stretching

Abstract: As revealed by electron-microscopic studies, the morphology of melt-crystallized polymer films and fibers subjected to the ultimate degree of orientation, is characterized by layer-like organization of lamellar crystals (paracrystalline structure). If the direction of the long axes of the macromolecules in crystallites coincides with the texture axis, the morphological orientation of paracrystalline layers is orthogonal to the stretching direction. Partial transformation of a spherulite into a “microneck” can lead to flattening of spherulites.

On the adsorption of a polymer chain at a boundary

Abstract: On the basis of a simple lattice model, the adsorption of a polymer chain on a surface is discussed, taking excluded volume effects (intrachain interactions) into account. Configurations of finite chains of n elements (self-avoiding walks) are enumerated exactly for n ≤ 13 on the simple cubic lattice. Some predictions concerning the asymptotic behavior of long chains are derived from these data by using extrapolation methods previously devised by Domb and Sykes for bulk properties.

Synthesis and graft copolymerization on some polysaccharides by anionic mechanism

Abstract: Some esters like metallated cellulose monoethyl malonate were used as macromolecular initiators for grafting of some vinyl monomers and the obtained products were characterized.

Graft copolymerization of ethyl acrylate with γ‐irradiated cotton cellulose I and II

Abstract: The graft copolymerization of ethyl acrylate from methanol-water solutions with γ-irradiated cotton cellulose I and II was investigated by electron spin resonsance spectroscopy and by the formation of cellulose-poly(ethyl acrylate) copolymer. The rate and extent of graft copolymerization reactions in these heterogeneous systems were dependent on conditions of storage of the irradiated celluloses prior to reaction, the composition of the methanol-water solutions, reaction time, agitation during the reaction, and cellulose crystal lattice type. Agitation of the heterogeneous system during reaction decreased both the rate and extent of copolymerization. From water the extent of copolymerization of ethyl acrylate with irradiated collulose I was greater than with cellulose II; from methanol the extent of copolymerization with irradiated cellulose II was greater than with cellulose I. From methanol(40 vol-%)-water(60 vol-%) maximum extent of copolymerization occurred with both irradiated celluloses; however, the extent of copolymerization was less with irradiated cellulose II than with cellulose I. The extent of copolymerization was directly related to the initial concentration of free radicals in the irradiated cellulose. However, a fraction of the radicals, about 10% of the maximum concentration in cellulose obtained at a dosage of 5.2 × 1019 eV/g, was apparently in the highly ordered regions of the cellulosic structure. These radicals were not scavenged and did not initiate copolymerization reactions.

Etudes spectroscopiques de transitions “helice‐pelote” de polypeptides en solution acide

Abstract: Infrared spectra of various polypeptides (poly-L-alanine, or PLA; poly-L-methionine or PLM; poly (γ - benzyl-L-glutamate), or PBLG) in CF3COOH-CHCl3 solutions have been investigated in regions characteristic of the peptide group (amide A, I and II) and of the acid solvent (vOH, vC=O). These spectra have been analyzed by comparison with the infrared spectrum of a model amide (N-methylacetamide). This analysis shows that by addition of acid the simple amide and the polypeptides exhibit at first a similar behavior by forming complexes by hydrogen bonding, i.e., >C=O…HOOCCF3. At higher acid concentrations, unlike the model amide, polypeptides are not protonated. Furthermore, the evolution of the spectra as a function of complexation clearly indicates the helix-random coil transition of these polymers and agrees well with optical rotatory dispersion measurements. Thus, steep transitions have been obtained for PBLG and PLM in a small range of acid concentration or of temperature. To the contrary, no discontinuity in the evolution of PLA spectra has been seen. This observation compares well with the slow transformation observed by optical rotatory dispersion (ORD). Finally, it is possible to compare at the level of the repeat unit the strength of the acid perturbation which triggers the transformation of the three polypeptides. This points out the important part played by the sidechains as stabilization forces of the helix and the cooperative aspect of the transitions.

Electron‐microscopic study of polymer mixtures

Abstract: The effects of several factors on the morphology of films cast from binary polymer mixtures dissolved in a single solvent have been studied by electron microscopy An elastomer-elastomer system (isoprene rubber and polyisobutene) was used The effects of the relative amounts of components, change of the molecular weight of one of the components, the history of the solution, and the rate of evaporation of solvent on the morphology of films prepared from polymer mixtures were studied It is inferred from the results that these factors affect the kinetics of the phase separation of polymer mixtures