Showing papers in "Journal of Polymer Science Part B in 1975"

Crystal deformation in aromatic polyesters

Abstract: A study has been carried out of the changes in the x-ray diffraction patterns which occur when oriented fibers or tapes of poly(trimethylene terephthalate) (3GT) and poly-(tetramethylene terephthalate) (4GT) are subjected to mechanical tensile stress. Although the polymers show very different behavior in detail, in both cases comparatively large reversible lattice strains are observed (∼ several %). The diffraction pattern of 3GT changes monotonically with increasing macroscopic strain, suggesting that the lattice responds immediately to the applied stress, and deforms as though it were a coiled spring. In 4GT, on the other hand, there is no detectable change in the x-ray diffraction pattern at low macroscopic strains, i.e., low values of the applied stress. At higher stresses, changes in the pattern occur which suggest a definite change in the crystal structure. Finally at the highest values of applied stress, the lattice deformations cease to increase. A preliminary discussion is presented of the relationship of these x-ray diffraction results to the mechanical stress–strain behavior.

Quantitative structural characterization of the melting behavior of isotactic polypropylene

Abstract: The melting behavior of restrained isotactic polypropylene fibers is examined quantitatively in terms of the influence the anisotropic structural state of the polymer has on the observed properties. Two endotherm peaks are observed to occur in some of the samples. The formation and location of the multiple peaks are determined by the orientation of the noncrystalline chains, and is independent of the fabrication path used to achieve that orientation. Above a certain minimum orientation of the noncrystalline chains, multiple endotherm peak formation occurs. The high-temperature endotherm (T2M) extrapolates to an ultimate melting point for fully oriented noncrystalline chains of 220°C, while the lower-temperature endotherm (T1M) extrapolates to an ultimate melting point of 185°C. Noncrystalline chain orientation influences the endotherm temperature through its changing configurational entropy. It is shown quantitatively that the noncrystalline polymer must be considered as plastically deformed, since rubber elasticity theory is not followed as predicted. The melting behavior of isothermally crystallized samples are also reported to further elucidate the nature of the observed endotherms.

Threshold fracture energies for elastomers

Abstract: Fracture energies have been measured for two types of polybutadiene elastomer, crosslinked to various extents and torn under various conditions. Threshold values, ranging from 40 to 80 jm−2, were observed for samples torn at extremely low rates of tearing, at high temperatures, and in the swollen state. These values were found to be independent of temperature and rate of tearing, and also of the degree of swelling and the nature of the swelling liquid, provided a correction is made for the reduced areal concentration of molecular chains. They decreased somewhat with increased crosslinking, in good agreement with theoretical predictions of Lake and Thomas.6 The results for the two elastomers were substantially the same. These observations are strikingly different from those made at normal temperatures and rates of tearing, when large differences in fracture energy were observed between the elastomers, and between samples crosslinked to different extents, due to energy losses from nonequilibrium deformation processes in addition to the energy required for fracture.

Deformational, swelling, and potentiometric behavior of ionized poly(methacrylic acid) gels. I. Theory

Abstract: By using the model of a randomly coiled chain, a relation is derived describing the equilibrium stress–strain behavior of variously ionized polyelectrolyte gels swollen in solutions of a uni–univalent salt. The effect of the concentration of bound counterions calculated on the basis of the cylindrical model and the effect of the change of length of the statistical chain segment with the change in ionization of the gel on stress–strain, swelling, and potentiometric equilibria is discussed.

Crystallinity and the effect of ionizing radiation in polyethylene. I. Crosslinking and the crystal core

Abstract: An extensive study has been carried out on the effect of ionizing radiation on polyethylene in the form of solution-grown single crystals to follow up preceding investigations which had indicated that the radiation-induced effects along the fold surfaces could be significantly different from those within the crystal interior. In this first part of the series, radiation-induced crosslinking was investigated in the case of ''crystal core'' material. This material, obtained by removal of the fold surface by oxidative degradation with ozone, consists of isolated foldfree crystal layers of dicarboxylic acids of uniform length, the length itself depending on the fold length of the starting crystal. This ''core material'' was irradiated by $gamma$ rays and the effect of crosslinking was followed by GPC analysis by recording the development of dimer, trimer, etc., peaks in the chromatograms. For a given dose, the fractional amount of crosslinked material is independent of the chain length. This, together with other effects described, provides unambiguous evidence that crosslinking occurs at the chain ends or, in other words, that there is no crosslinking within the interior of the paraffinoid lattice. Further, no permanent scission was observed to occur within the lattice, at least in amounts detectable by themore » present molecular weight measurements. The obvious significance of these effects for radiation studies on paraffinoid substances in general, beyond those of the present long-chain dicarboxylic acids, is discussed prior to utilizing them in the study of chain-folded polyethylene crystals in the following parts of the series. (auth)« less

Effects of annealing and isothermal crystallization upon crystalline forms of poly(vinylidene fluoride)

Abstract: Poly(vinylidene fluoride) exists in three crystalline forms. Optimum conditions for preparing form III were established by infrared spectroscopy, differential scanning calorimetry, and x-ray diffraction measurements. Form III is easily obtained by annealing mats of solution-grown crystals of form II at 175–185°C and is also preferentially formed by isothermal crystallization from the melt between 165 and 175°C. Below 165° crystallization of form II is favored. The melting point of form III is higher than that of form II.

The crystal structure of poly(tetramethylene terephthalate)

Abstract: The unit cell of poly(tetramethylene terephthalate) is triclinic with parameters a = 5.96 A, b = 4.83 A, c (fiber axis) = 11.62 A, α = 115.2. β = 99.9, and γ = 111.3°; space group P1, calculated crystalline density 1.41 g/cc. The plane of the benzene ring is found to be inclined by about 15° from the fiber axis, contributing to a shortening of the fiber period as compared to the period expected on the basis of analogy with other members of the terephthalate ester series. The remaining shortening of the fiber period occurs in the O°CH2°CH2segment of the chain. No abnormally short distances among neighboring chain atoms were observed. A typical texture pattern was found in specimens of this polymer that were cold rolled and subsequently annealed. In this texture the c axis of the unit cell is highly oriented in the rolling direction; the a and b axes of the unit cell are oriented preferentially so that the terephthalate residue lies as close as possible to the plane of rolling.

Persistent polarization in poly(vinylidene fluoride). II. Piezoelectricity of poly(vinylidene fluoride) thermoelectrets

Abstract: The piezoelectricity of PVDF thermoelect rets formed with vacuum-coated aluminum electrodes has been investigated in detail. The piezoelectricity depends on the β-form crystal structure of PVDF homopolymer and copolymers. However, the piezoelectricity is not attributed to the stress dependence of the spontaneous polarization of β-form crystals, but rather to the persistent polarization arising from trapped charges. The trapping mechanism is discussed.

Flow, high‐elastic (recoverable) deformation, and rupture of uncured high molecular weight linear polymers in uniaxial extension

Abstract: The behavior of narrow molecular weight distribution polymers has been investigated under uniaxial extension at constant deformation rate and at constant stress. It has been established that up to rupture these polymers behave as linear viscoelastic bodies. A detailed investigation of the rupture phenomenon has shown that the rupture of fluid polymers is due to their transition to the rubbery state at critical deformation rates, with the result that they disintegrate like quasi-cured rubbers. The effect of the temperature and the molecular weight on the critical conditions of rupture has been described in terms of viscoelastic relaxation.

Persistent polarization in poly(vinylidene fluoride). I. Surface charges and piezoelectricity of poly(vinylidene fluoride) thermoelectrets

Abstract: Persistent polarization in poly(vinylidene fluoride) (PVDF) thermoelectrets prepared under high electric field strengths has been studied by measurements of surface charge and piezoelectricity. An anomalous heterocharge appears after the normal homocharge disappears. A model is proposed to explain the surface charge phenomena; the anomalous heterocharge is expressed as a sum of a hidden homocharge and a hidden heterocharge. It is concluded that the anomalous heterocharge as well as the apparent homocharge are not responsible for the piezoelectricity of PVDF electrets. The piezoelectricity is shown to depend on the structure of the original PVDF films or the amount of β-form crystals. However, the piezoelectricity is not attributed to stress dependence of the spontaneous polarization in the β-form crystal of PVDF, but to the hidden polarization which bring about the anomalous heterocharge. The hidden polarizations are attributed to trapped charges.

Diluent effects on molecular motions and glass transition in polymers. I. Polystyrene–toluene

Abstract: The effects of diluent on molecular motions and glass transition in the polystyrene–toluene system was studied by means of dielectric, thermal, and NMR measurements. Three dielectric relaxations were observed between 80 and 400°K. On the basis of NMR measurements on solutions in toluene and in deuterated toluene, relaxation processes were assigned to segmental motions of polystyrene, rotations of toluene, and the local motions of polystyrene and toluene in order of appearance from the high-temperature side. The concentration dependence of the relaxation strength and of the activation energy for the primary relaxation (that at the highest temperature) show a step increment at about 50% by weight. The activation plots for the primary process were expressed by the Vogel–Tamman equation. With this equation, the temperatures at which the mean dielectric relaxation time becomes 100 sec is determined. This agrees well with the glass-transition temperature Tg and hence Tg in concentrated solution is expressed by in terms of the parameters A, B, and T0 of the Vogel–Tamman equation. The values of A and B are, respectively, about 12 and 0.65 and independent of the concentration. The physical meaning of these parameters is discussed.

The elastic free energy and the elastic equation of state: Elongation and swelling of polydimethylsiloxane networks

Abstract: The elastic free energy for a tetrafunctional network is here expressed as where λ1, etc., are the principal extension ratios relative to the state of reference (wherein 〈r2〉 = 〈r2〉0), and C1, C2, and m are arbitrary parameters. The free energy of a swollen system is taken to be the sum G = Gmix + Gel of Gel and the free energy of mixing . Stress-strain relations and the chemical potential μ1 are derived from G as functions of the elasticity parameters C1, C2, and m, and of the thermodynamic interaction parameter χ or of . The dilation of the semi-open system subject to deformation when exposed to diluent at fixed activity is derived as the sum of the dilation at fixed composition and the dilation due to absorption of diluent. Experiments are reported on the dependence of the equilibrium retractive force on elongation for cross-linked polydimethylsiloxanes (PDMS) exposed to benzene or hexamethyldisiloxane vapor at regulated activities. Volume fractions of samples covered the range v2 = 1.00 to v2 ≈ 0.30. With the choice of m = ½ the elastic behavior of a given polymer is well represented by one combination of values for C1 and C2 at all dilutions by either diluent. The dependence of the Mooney-Rivlin (C2) term on volume is thus established, at least for PDMS, and the scope of the semi-empirical free energy expression and its consequents is greatly enlarged. Values of χ deduced from the equilibrium swelling of the unstrained networks exposed to benzene at various activities are in excellent agreement with those obtained previously from vapor pressures and osmotic pressures on linear PDMS. The results of Allen, Kirkham, Padget, and Price on the elastic behavior of natural rubber are discussed, with particular reference to the coefficients of dilation with elongation which they determined at fixed composition. The present results lend strong support to the principle of additivity of the free energies attributable to the network and to the bulk liquid system, respectively. This principle is fundamental to the analysis of rubber elasticity.

Dynamics of macromolecular chains. II. Orientation relaxation generated by elementary three-bond motions and notion of an independent kinetic segment

Abstract: In the preceding paper, general equations were established for the motions of chains confined to a tetrahedral lattice. In the present paper, bond orientation correlation and autocorrelation functions are explicitly calculated for the case where only three-bond elementary motions are considered. Effects due to the chain end are analyzed and the relaxation time distribution function is established. The expressions obtained reflect the influence of the chain structure. Finally, to characterize the dynamic behavior of chains in orientation relaxation experiments, the notion of an independent kinetic segment is proposed.

Heterogeneous nucleation of crystallization of high polymers from the melt. III. Nucleation kinetics and interfacial energies

Abstract: In the melt crystallization of isotactic polypropylene, poly(ethylene oxide) and poly(butene-1) in contact with substrates, the existence of a fixed number of nucleating sites on the substrate surfaces has been established. When these sites become active successively (the transient in the number of nuclei is long) during crystallization, pseudohomogeneous nucleation on the substrate occurs. Nucleation rates for poly(butene-1) and poly(ethylene oxide) on substrates and in bulk have been measured. These data can be used for comparing the nucleating ability of substrates. Estimates of the variation of bulk nucleation rates from one volume element to another as well as for repeated crystallization within a given volume element have been included. Finally, the temperature coefficients of heterogeneous nucleation rates have been combined with the temperature coefficient of spherulitic growth rate of poly(butene-1), to yield values of the interfacial energy parameters appearing in the theory of heterogeneous nucleation. The quantitative characterization of the nucleating ability of substrates by this method is an improvement over the mere use of nucleation densities or nucleation rates.

Superstructure in segmented polyether–urethanes

Abstract: The morphology of several series of segmented polyether–urethanes was studied. The “hard” segments contained urethane and urea linkages formed by 4,4′-dicyclohexylmethane diisocyanate (Hylene W) and selected aliphatic and aromatic monomeric diamines (DA). The “soft” segments were composed of oligomeric poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), or both PEO and PPO. For studying the composition–morphology relationships, the molecular weight and relative content of PEO, and the relative content of PPO were varied systematically. Different diamines were used as chain extenders. The methods of wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), polarizing microscopy, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) were employed in the investigation. The effects of PEO content on domain formation were very significant. Calculations based on a highly simplified model indicated that, for two adjacent molecules, if two hard segments are associated with each other, the probability for the association of the next two hard segments varies inversely with the third power of soft segment length. Copolymers composed of both POE and PPO displayed enhanced domain and anisotropic superstructure. The phenomenon was interpreted in terms of polymer incompatibility. The effects on morphology of different DA's as chain extenders were tentatively accounted for by the symmetry, hydrogen bonding, and rigidity of the hard segments as well as their incompatibility with the soft segments. The formation and deformation of superstructure were of particular interest. A model was proposed to account for the formation of the resultant anisotropic structure and mechanical properties.

ESR evidence for main‐chain scission produced by mechanical fracture of polymers at low temperature

Abstract: ESR spectra have been observed from several polymers (polyethylene, polypropylene, polymethylmethacrylate, polytetrafluoroethylene, and polybutadiene) which were mechanically fractured in vacuum at 77°K. The spectra permit identification of the free radicals produced in chain scission. It has been demonstrated for the polymers of the type such as polypropylene and poly(methyl methacrylate), that the mechanical fracture produces two different scission radicals in pairs, namely, The results indicate convincingly that microscopic scission of polymer chains is caused by macroscopic destruction of polymeric material. The critical molecular size for mechanical rupture of polyethylene chains was experimentally determined to be a degree of polymerization between 70 and 100. This size agrees fairly well with the value predicted from a theoretical model.

Deformational, swelling, and potentiometric behavior of ionized poly(methacrylic acid) gels. II. Experimental results

Abstract: The deformational, swelling, and potentiometric behavior of poly(methacrylic acid) gels was measured as a function of the degree of crosslinking, ionic strength, and degree of ionization. The comparison of the stress–strain behavior with theoretical relations derived in the preceding part has shown that the relations are valid only if an increase is assumed in the number of monomeric units in the statistical chain segment with increasing degree of neutralization of the gel. This dependence is affected by the salt content in the swelling solution and is also dependent on the activity coefficient of counterions. The pK0 values for an undissociated gel approach pK0 = 5.0 and increase somewhat with degree of neutralization. The swelling equilibria are in qualitative agreement with theoretical assumptions; their quantitative agreement depends on the activity coefficient of counterions.

Effect of molecular weight on the crystalline structure of polytetrafluoroethylene as-polymerized

Abstract: Melting and crystallization behavior of polytetrafluoroethylene as polymerized in emulsion and suspension is shown to depend on molecular weight. DSC heating curves for virgin PTFE with low molecular weight below 3 × 105 have a single peak, whereas curves for higher molecular weight samples have double peaks. With increasing heating rate the areas of higher melting peaks become larger than the lower melting peaks. The morphology of polymer exhibiting double melting peaks is mainly folded ribbons or granular particles. The phenomenon of double melting is explained on the basis of two different crystalline states which correspond to the “fold regions” and the “linear segments” in a folded ribbon. The melting temperature of virgin PTFE is almost constant at ca. 330°C for molecular weights below 1 × 106, and rises as the molecular weight increases above 1 × 106. The heat of melting of virgin PTFE is nearly independent of molecular weight. On the basis of these results, we propose a model for melting and crystallization of low and high molecular weight PTFE and for the crystal structure.

Crystallinity and the effect of ionizing radiation in polyethylene. II. Crosslinking in chain‐folded single crystals

Abstract: In order to ascertain whether the crystal core of irradiated polyethylene single crystals is free from crosslinks—as would follow from the results of Part I—the fold surface of irradiated single crystals was shaved off with ozone and the resulting product examined by GPC for molecular weight. It was found that on ozone degradation the entire material, which had been insolubilized by radiation-induced crosslinking, has become soluble hence became available for the GPC analysis. The chromatograms displayed the same peaked development with degradation as the unirradiated crystals leading eventually to single traverse dicarboxylic acids. This proves the absence of crosslinks within the crystal interior of the material as examined by GPC. The appearance of some additional low molecular weight material, is attributed to scission at the radiation-induced double bonds due to ozone which eliminates the possibility of the existence of crosslinks within the lattice such as might provide scission sites for ozone. The conclusion could therefore be reached that, to the extent assessable by our GPC test, there are no radiation-induced crosslinks within the crystal lattice, hence the crosslinks produced must be entirely confined to the fold surface region of the crystals.

The dynamic mechanical behavior of ultra‐high modulus linear polyethylenes

Abstract: The dynamic mechanical properties of a number of ultra-highly drawn polyethylenes have been studied over a wide range of temperature. It is shown that the materials possess low temperature Young's moduli as high as 1.6 Mbar, a figure which approaches the theoretical and experimental values for the c-axis crystalline modulus of this polymer. The α and γ relaxation processes are still clearly discernible even at highest drawn ratios (ca. 35) and a quantitative analysis of the results, using structural data obtained from broad line nuclear magnetic resonance (NMR) measurements, suggests that the data are consistent with a modified series model.

Transient and steady-state water vapor permeation through polymer films

Abstract: Water vapor transport properties for the polymers Kapton H11 and Parylene C were determined over a temperature range of 20 to 55°C. Activation energies and entropies for permeation as well as partial molar free energies, heats, and entropies of dilution were calculated for water vapor concentrations ranging from 3 × 10−6 to 1 × 10−3 mole H2O per cm3 of polymer. Mylar A was tested to extend the available data for partial molar heats and entropies of dilution and to compare permeation and diffusion results with the corresponding values in the literature. Diffusion coefficients were measured using the time-lag technique of Barrer but employing a modified test apparatus. Equilibrium sorption isotherms at 30°C were obtained for Mylar A and Kapton H with a Cahn microbalance. The ratios of the permeability to diffusion coefficients as measured from time-lag experiments agreed with solubility coefficients within 3% for Mylar A and within 12% for Kapton H. Both polymers obeyed Henry's law. The results were interpreted in light of polymer polarity and morphology.

Deformation and microstructure of extended‐chain polydiacetylene crystals

Abstract: The mechanical properties (Young's modulus, ultimate tensile strength, deformation processes) of extended-chain polydiacetylene crystals are investigated. The properties observed are similar to those of metal and ceramic whiskers. The elastic modulus is strain-dependent and the ultimate tensile strength increases with decreasing crystal size. The maximum tensile strength observed was 1700 Nmm−2. The ultimate tensile strength seems to be controlled by the presence of a small number of defects near the surface at which fracture nucleates. Irreversible deformation of the crystals was observed to occur by crack propagation normal and parallel to the direction of the macromolecules. The observed mechanical behavior corresponds to exceptionally high per-chain properties. The per-chain modulus obtained for these crystals is nearly as high as that of diamond. A chain-aligned polyethylene fiber with the same per-chain mechanical properties would have an ultimate strength as high as 0.9 × 104 Nmm−2.

Heterogeneous nucleation of crystallization of high polymers from the melt. II. Aspects of transcrystallinity and nucleation density

Abstract: In the initial stage of the development of transcrystallinity, nuclei appear sporadically on the substrate. The growth rate and melting temperature of the transcrystalline region are found to be the same as those of spherulites nucleated in the bulk of the polymer. Nucleation densities ns at the interface, and nb in bulk, for the crystallization of isotactic polypropylene, poly(ethylene oxide), and poly(butene-1) in contact with various substrates, have been measured by counting the number of spherulites generated. Despite variations in the results from various causes, the quantities ns and ns/nb are useful parameters for characterizing the nucleating ability of various substrates.