摘要:

AbstractThe transmission of polarized light by films of crystalline polymers depends on the size, shape, orientation, anisotropy, and arrangement of the constituent crystallites. Its measurement as a function of the polarization direction of the incident light and that of the analyzing polarization filter characterizes the structure. This may be readily followed during the crystallization or deformation of the polymer. The transmission depends upon some of the same optical parameters as does the scattering and provides complementary information. The scattering, measured as a function of angle, is capable of providing more information than the transmission but its measurement during rapid processes is more difficult. The utility of these techniques is illustrated for the isothermal crystallization of oriented polyethylene films and for the stress‐induced crystallization of rubbe

ISSN:0449-2994    

DOI:10.1002/polc.5070180103

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractThis discussion of the variability of polymer x‐ray diffraction patterns tries to show that there is no simple definition of crystallinity when judged by x‐ray diffraction, and that x‐ray crystallinity may differ sharply from judgments of order by other techniques. It is recommended that a clear distinction be made between ameasurementobtained from a diffraction pattern and themodelwhich is used to interpret that measurement. Many precise and valuable measurements can be made of the varying aspects of the pattern. These measurements are perfectly valid and, if done in a consistent and verifiable manner, allow one to compare and relate samples having different treatment histories or different physical properties. These relationships are useful and meaningful as such. Difficulties arise from theinterpretationof these measurements. The interpretation involves an unfortunate combination of semantics, morphology, and mental models. It is shown that there is no unequivocable interpretation that completely satisfies everyone in this fast‐changing area of study. The cause of this dissatisfaction stems from the simple fact that we do not really know the exact arrangement of the polymer chain from one of its ends to the other. Thus, all models and all interpretations have no meaning beyond being useful concepts. Difficulty comes when we make the mistake of assuming that our models aretruepictures of the solid state of polymers. It is hoped that, in time, enough detailed measurements will be obtained so that generalities can be drawn and so that some of the dilemmas of interpretation can be r

ISSN:0449-2994    

DOI:10.1002/polc.5070180104

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

ISSN:0449-2994    

DOI:10.1002/polc.5070180105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractThe problems involved in the determination of the fraction of crystallinity by thermal measurements are discussed. It is shown that in certain cases specific heat measurements themselves can be used to estimate the fraction of crystallinity. In the case of specific heat data on single crystals of polyethylene obtained by Karasz and Hamblin the specific heats are a linear function of the reciprocal fold length and can be extrapolated to infinite fold length to obtain the specific heat of 100% crystalline polyethylene. This extrapolated value is compared with other estimates and is used to calculate the fraction of crystallinity of polyethylene of different forms. Within the experimental uncertainties it would appear that estimates of the crystallinity from calorimetric, x‐ray, dilatometric, and infrared measurements are all in substantial agreemen

ISSN:0449-2994    

DOI:10.1002/polc.5070180106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractIn a semicrystalline polymer chain, ifnis the total number of backbone units andais the number of nonlattice backbone units, then the per cent crystallinity is(n – a)/n. By an empirical approach it is found that ifais set equal to ca. 40 for branched polyethylenes andnis determined from a melting point‐fold length relationship that the calculated values of per cent crystallinity are in good agreement with experiment. It is also shown that 100/(n – a)is equivalent to the total number of branches per 100 methylenes for branched polyethylene. A sinusoidal random coil is postulated for the melt state in order to rationalize why chain folding with adjacent reentry should occur if as many as 40 methylenes per fold are noncrysta

ISSN:0449-2994    

DOI:10.1002/polc.5070180107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractThe droplet technique was utilized to obtain estimates of the isothermal rate of homogeneous crystal nucleation in highly supercooled melts of unfractionated linear polyethylene. In a typical experiment a cell containing a suspension of several hundred micron‐size spherical droplets of the polymer was placed on a microscope hot stage and quenched from a temperature well above the melting point of polyethylene to the desired crystallization temperature. When viewed between crossed polarizers, the droplets were initially invisible but, upon freezing, they appeared as visible birefringent spheres. The time dependence of the process was followed by time‐lapse photography. Half times (T1/2) for the freezing of those droplets remaining unfrozen by the time the crystallization temperature was attained were thus obtained at a series of undercoolings and related to the desired nucleation rate constantI(nuclei cm.−1sec.−1) by the equationI= In2/T1/2/v, wherevis the droplet volume. The temperature dependence ofI, as obtained by the above procedure, was analyzed in accord with current theories of homogeneous nucleation of chain‐folded polymer crystals. From this analysis, an estimate, 14,960 ergs3cm.−6was made of the quantity (%sG2%sGe), where%sGand%sGeare, respectively, the lateral and end‐surface free energies o

ISSN:0449-2994    

DOI:10.1002/polc.5070180108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractUsing dilatometric techniques, detailed investigations have been made of the crystallization and melting behavior of high molecular weight poly(tetramethylene oxide), low molecular weight poly(tetramethylene oxide) glycol, and a high molecular weight copolymer prepared by the chain extension of the glycol. The degree of crys‐tallinity is expressed as a function of time and temperature. Using fast melting rates, the apparent melting point of the bulk polymers has been found to depend on both the temperature of crystallization and the level of crystallization allowed to develop before melting. Extrapolation gave an estimated equilibrium melting point for high molecular weight homopolymer of about 51°C. The highest experimentally observed melting point was 43°C. Visual observations have been made on crystallization of these materials from dilute solution (⪕3%) in toluene. Under some experimental conditions crystallization was accompanied by gel formation while under other conditions two distinct and separable phases were formed. Measurements were made on the dissolution temperatures of solutions crystallized under different experimental condi

ISSN:0449-2994    

DOI:10.1002/polc.5070180109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractCrystallographic data show that many inorganic crystals are inorganic polymers in which chemical bonds extend across the boundaries of the crystallographic unit cell. Crystals with metallic bonds form space‐network (3‐dimensional) polymers, as do ionically bonded crystals unless there is a great disparity in ionic radii. Covalently bonded inorganic crystals may form space‐network, sheet (two‐dimensional), or linear polymers. Organic macromolecular crystals have only been fully characterized for those cases in which bonding is covalent and the structure is linear. Where ionic bonding is involved, organic macromolecules probably form “scrambled salt structures.” The relation between crystal structure and polymer structure for macromolecular crystals can be characterized by the ratio betweenc, the number of atoms per crystallographic unit cell, andp, the number of chain atoms per polymer repeat unit. The ratioc:pis 1:1 for many space‐network polymers, including most crystals of low symmetry. For linear polymersc:pis usually greater than 1:1. Factors contributing to the higher ratio are the number of chains passing through the unit cell and the number of substituent atoms that are attached to the polymer chain. In considering macromolecular crystals, the distinction between bond types (e.g., ionic versus covalent) is more significant than the distinction between chemical types (e.g., organic versus inorganic). Some of the most challenging problems exist in characterizing the crystals of macromolecules with intermediate bond type. Examples are gold iodide, where both covalent and metallic character must be recognized, and nylon, where hydrogen bonding

ISSN:0449-2994    

DOI:10.1002/polc.5070180110

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractStudies of single‐crystal deformation explain the small‐ and wide‐angle x‐ray observations of the cold drawing of PE, particularly the effects in the necking zone where the original spherulitic structure is transformed into a highly aligned fiber structure. Whole blocks of folded chains are broken out of the original crystals, which have already undergone a substantial deformation by chain tilting and slip, and incorporated in the new fiber structure. In variance with drawing of single crystals, the plastic deformation of bulk samples adjusts the long period of folded chain blocks to the temperature of drawing so that the obtained value is a function of the latter, but very nearly independent of the long period of the sample before drawing. The amorphous regions of the drawn polyethylene contain a great many highly strained tie molecules with a lower entropy and heat content than in a completely relaxed supercooled liquid. Melting experiments yield the heat content reduction which is a consequence of more extended chain conformation(intramolecular energy)and of closer chain packing(intermolecular energy). The entropy and enthalpy reduction explains the drastic decrease of sorption and diffusion of solvents first observed in wide‐line NMR experiment. Annealing lets the sample relax and return to thermodynamic equilibrium nearly instantaneously. The heat content, the diffusion, and sorption resume the values of the undrawn bulk material. The long period grows to the value which one would obtain if the original drawing were performed at the temperature of

ISSN:0449-2994    

DOI:10.1002/polc.5070180111

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractA thermodynamic theory of melting for linear stereoregular homopolymer systems with a molecular weight distribution has been derived. The homopolymer system is regarded as a eutectic multicomponent system. The components of the system are the molecules themselves. Under conditions of thermodynamic equilibrium the molecules of different lengths should crystallize as extended‐chain crystals. The thickness of the crystals equals the molecular length. During crystallization of stereoregular homopolymer systems the molecular weight fractionation is usually not completely established. This can be described by only one additional internal parameter in the theory. To test the theory, fractions of linear polyethylene as the simplest examples of regular homopolymers were investigated by means of DTA and low‐angle x‐ray diffraction methods. The maximum melting points, the molar mass fractions of crystals as a function of temperature, and the long periods depend on the data of the molecular weight distribution, as predicted by the theory. Both the theory and the experiment agree quantitatively provided that only extended‐chain crystals exist. Under normal pressure conditions molecular folding processes occur, beginning above a critical molecular length. This critical molecular length is determined by kinetic

ISSN:0449-2994    

DOI:10.1002/polc.5070180112

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY