摘要:

The morphological changes accompanying the dissolution of polyethylene single crystals in solvent have been correlated with the temperature of annealing. By this method, the solution temperatures of crystals of finite thicknesses were accurately determined. Based on these results, a surface free energy of 58 ergs/cm2was calculated for the fold surface of polyethylene lamellae crystallized at 80°C from dilute solution. In addition, several novel morphological features were observed at intermediate stages of crystal dissolution which may aid in the understanding of folded‐chain crystallization.

ISSN:0021-8979    

DOI:10.1063/1.1713099

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The internal morphology of isothermally bulk‐crystallized fractions, and of whole polymer, Marlex 50 polyethylene is revealed by fracture experiments. Three distinctly different types of lamellae are then observable if one uses suitable electron microscopic techniques: (1) regular or type I lamellae which are similar in appearance to solution‐grown lamellae, (2) narrow or type II lamellae which have finite widths, and (3) extended‐chain or type III lamellae which have step heights approximately equal to fully extended chain lengths calculated from viscosity‐average molecular weights. The type I and II lamellae appear to consist of folded chains. The step heights of the type I and type II lamellae are shown to increase from about 150 to 350 Å with increasing crystallization temperature. The widths of the type II or narrow lamellae vary directly with temperature from 350 to 800 Å. However, the step heights of the type III lamellae, which range from 150 to 1050 Å, are essentially independent of temperature but do vary with molecular weight. The melting points of these different types of lamellae are shown to be consistent with presently existing theories and the relationship between morphological parameters of these lamellae and reported low‐angle x‐ray periodicities are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1713100

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The influence of the particle factor and lattice factor on the positions of the small‐angle reflections of polymer crystalline samples is investigated. Since a single‐crystal cake or a polymer solid crystallized from melt supposedly consists of stacks of nearly parallel plate‐like lamellae, one may describe the scattering behavior of polymer crystals based on a periodic step function with fluctuating periods (linear paracrystal). In this model the particle factor as well as the lattice factor may shift the scattering maxima out of the positions given by the Bragg equation. Good agreement with the observation that the first order leads to a larger spacing than the second one may be obtained by introduction of an unsymmetrical distribution function for the lamellar thickness.

ISSN:0021-8979    

DOI:10.1063/1.1713101

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Annealing of single crystals and of single‐crystal cakes of polyethylene leads to an increase in density because the growth of crystal thickness with time reduces the area of the density deficient surface regions. When the same process is applied to the isothermal crystallization and to the annealing of bulk samples, one has to consider the long period distribution of the crystals due to their varying age and thermal history; then one is able to reproduce sufficiently well the experimental data. In particular, starting with inhomogeneous athermal nucleation and uniform three‐dimensional growth (n= 3), one obtains a higher initial value of the Avrami exponent (n′ = 3.7), i.e., a faster density increase as a consequence of the relatively rapid decrease of the surface mass defect contribution to the density of the crystals in the first stage of their thickness growth. The extremely slow reduction of this contribution with growing age of the crystals also explains the long tail of secondary crystallization and annealing curves where the crystallinity, although slowly approaching, never reaches unity. The early drop of crystallinity below the calculated values and particularly the observed transition from primary to secondary crystallization, however, being much gentler than predicted by the theory, have to be explained by additional effects, i.e., mainly by impurity rejections during crystallization by negative pressure relaxation developing in areas completely blocked by growing spherulites and by tied molecules partly included in two different crystals so that the intervening loops from pure geometrical reasons are prevented from further crystallization.

ISSN:0021-8979    

DOI:10.1063/1.1713102

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The course of isothermal crystallization of a series of high molecular weight polyethylene oxide copolymers was studied with a microdensity balance. The comonomers, butylene and styrene oxides, ranged from 0.2 to 4 mole%; several examples of different polymerization conditions were also studied. The consequence of adding noncrystallizing components to a presumably linear polymer was observed in both melting point depression and the shape of the crystallization isotherm. From the melting points obtained on the experimental copolymers, the heat of fusion appeared to be lowered by the addition of noncrystallizable sequences. Inasmuch as the shape of the crystallization isotherm is changed when different amounts of comonomers are present in a random copolymer, an estimate can be made of the amount of noncrystallizing polymer acting like a block from the crystallization kinetics data and the melting point depression.

ISSN:0021-8979    

DOI:10.1063/1.1713103

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

For many polymers an effective glass transitionTg′can be defined via a change in slope of log&sgr;[&OHgr;−1cm−1] vsT−1[°K−1]. Measurements for dry cellulose acetate (CA), previously doped by soaking (48 h) in 0.1Malkali‐chloride solutions, indicate thatTg′depends on ion and polymer properties. A model based on fluctuation theory and ``free volume'' concepts is developed. The main assumptions are: (1) ions partially fill void space reducing the available local free volume; and (2) reduction in local free volume about an ion can be accounted for in the statistical expression⟨(V−V0)2⟩av=RTV0/Bby subtracting an effective ion volumeVifrom the total volume (per mole)Vof a reference aggregation of particles of most probable volumeV0. (Bis the bulk modulus,Rthe gas constant.) With simplified distribution functions, we obtained the expressionsTg≈B&dgr;2/6RV0andTg′=Tg−2a(&dgr;/m)m2Wi+am2Wi2, wherea = B/6V0R, Wiis ionic volume (per mole) from x‐ray measurements on crystals,m = Vi/Wi, and &dgr; is a measure of free volume. For CA, the best fit isTg′≈Tg−0.3Wi+0.14Wi2corresponding to a fractional free volumefg≈&dgr;/V0, which may be as much as 12%. BelowTg′, activation energies for conduction by alkali‐chloride‐doped CA were found to be given byEb[kcal / mole]≈ 15+0.7Wi≈16.1+2.45(1024&agr;), where &agr;[cm3/ion] is the electronic polarizability of the ion. AboveTg′, the energyEawas approximately 29 kcal/mole, independent of the ion. Alternative interpretations in terms of internal pressure and polarization effects are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1713104

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Equations for the apparent heat capacity in the glass‐transition interval as functions of temperature, heating rate, and thermal history have been developed and programmed for computation. The hole theory of liquids was used as basis for the analysis of the glass transition. Experimental information was derived from dynamic differential thermal analysis, DDTA, on polystyrene.The maximum of the apparent heat capacities found experimentally agrees with the theory. The peak temperaturesTmcan be expressed over four decades of heating rates by logq = A′ − B/Tm, whereqis the heating rate,A′ is an approximate constant, andBis the activation energy for hole formation. Higher cooling rates lead to higher activation energies on subsequent heating, indicating the need to recognize a hole size distribution.The minimum in the heat capacity that precedes the maximum on heating through the glass‐transition interval could be detected on quenched samples. Mathematical expressions for the minimum temperature and magnitude were developed.The temperature of ``half‐freezing'' on cooling, equivalent to Tool's ``fictive temperature,'' was found experimentally to occur at constantq·&tgr;, where &tgr; is the relaxation time (q·&tgr; = 6.6° for polystyrene). From the ``half‐freezing'' temperatures as a function of cooling rate one can determine the properties of a ``mean hole.'' For polystyrene the activation energy of the mean hole is 157 600 cal/mole of holes.

ISSN:0021-8979    

DOI:10.1063/1.1713105

出版商:AIP    

年代:1964

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1713011

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Spontaneous and stimulated emission from junctions in diodes made of silicon‐doped, melt‐grown GaAsxP1−xalloys have been studied. Variations of the emission spectra as a function of current and of composition ranging up to 40 mole % GaP were examined both at room temperature and 77°K.

ISSN:0021-8979    

DOI:10.1063/1.1713012

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The xenon‐filled arc discharge tube has found wide applications as an optical pumping source for ruby lasers. An experimental study of the shock waves in the flashtube at laser pumping energies and their effect on tube deterioration has been made. The plasma discharge was investigated by taking high‐speed image converter and streak photographs of the discharge and by measuring the relative shock amplitude at the tube walls with a piezoelectric transducer. The velocity of the luminous boundary of the plasma discharge was proportional to the input energy in the range from 60 to 500 J and varied from 90 to 900 m/sec. The luminous boundary velocity remained constant for pressures from 100 to 1500 Torr. The relative shock amplitude measured by the electromechanical transducer was independent of the tube diameter up to 12 mm, increased linearly with the input energy, and increased with gas pressure. Shock data were correlated with efficiency measurements to determine optimum tube and operational parameters. The microscopic effects of the shock wave upon the tube wall indicated that prestressing in compression the exterior surfaces of the glass wall would yield a stronger flashtube capable of higher intensities.

ISSN:0021-8979    

DOI:10.1063/1.1713013

出版商:AIP    

年代:1964

数据来源: AIP