ISSN:0021-8979    

DOI:10.1063/1.1721750

出版商:AIP    

年代:1954

数据来源: AIP

摘要:

The molecular characteristics which determine whether a long‐chain polymer will crystallize at all—and if so, how readily—are considered. The criterion for crystallization is geometrical regularity of molecular structure; but absolute regularity is not essential, for certain copolymers, as well as certain homopolymers with geometrically irregular structures, do crystallize; the requirement is that the different units which indiscriminately occupy corresponding positions in the structure must be not too different in size and shape (as in monomeric ``mixed crystals''); an approximation to regularity is sufficient.It is unlikely that quite regular molecules can be incapable of crystallizing; when such substances are reported as noncrystalline, it is more likely that they crystallize only with difficulty, and the right conditions have not been found. Reluctance to crystallize is associated with a small interval between relaxation transition (``second‐order transition'') and melting points; this interval appears to be influenced by the general shape of the molecule (though shape is not the only factor). The temperature range within which crystallization proceeds at an appreciable rate may be still further restricted if the chain repeat distance of the molecules is long; and this same factor may make crystallization at any temperature very slow.

ISSN:0021-8979    

DOI:10.1063/1.1721751

出版商:AIP    

年代:1954

数据来源: AIP

摘要:

This paper concerns an empirical linear relationship between the melting temperatureT1and the second‐order or glassy transition temperatureT2for partially crystalline polymers. Two lines can be drawn, one for symmetrical polymers, the other for unsymmetrical polymers, the latter being of lower slope. These families are only approximate. It is shown for a limited group of polymers, thatT2is proportional to the cohesive energy density of the polymer. SinceT1=&Dgr;H/&Dgr;S, it is suggested that the two families of curves arise because of differences in &Dgr;Swhich depends on symmetry and chain stiffness. It is shown thatT1andT2are linearly related with diluent as a common parameter.

ISSN:0021-8979    

DOI:10.1063/1.1721752

出版商:AIP    

年代:1954

数据来源: AIP

摘要:

The rates of crystallization of five polymers, poly‐(ethylene oxide), poly‐(decamethylene adipate), and three poly‐(N,N′‐sebacoyl piperazine)'s differing in molecular weight, have been observed dilatometrically at various temperatures ranging from 5.5 to 24° below the respective melting pointsTm. The crystallization isotherms are remarkably reproducible, and the previous thermal history above the melting point is without effect provided that melting is completed before the crystallization experiment is begun. The crystallization isotherms display induction periods. The very large temperature coefficients observed are uniform throughout the crystallization process, i.e., isotherms at different temperatures are superimposable by rescaling the time. It is inferred from this and other evidence that steady‐state nucleation and growth occur simultaneously throughout the process. The results are satisfactorily interpreted by applying conventional nucleation theory and assuming that nuclei grow at rates proportional to their surface areas. The treatment resembles that applied to crystallization in metals. Spherical and disk‐shaped nuclei are considered, but the data do not discriminate between them. An extension of the theory accounts for the maximum commonly observed in the crystallization rate as the temperature is decreased below the range of the present measurements.

ISSN:0021-8979    

DOI:10.1063/1.1721753

出版商:AIP    

年代:1954

数据来源: AIP

摘要:

The current x‐ray and infrared techniques for estimation of the crystalline‐amorphous ratio in polymers are surveyed. Representative data are assembled for some of the common crystallizing polymers. Advantages and limitations of the methods are pointed out.

ISSN:0021-8979    

DOI:10.1063/1.1721754

出版商:AIP    

年代:1954

数据来源: AIP

摘要:

The melting point, temperature for the onset of crystallization, and the isothermal rate of crystallization of polyethylene all decrease on increasing the extent of branching of the polymer chains. In the molecular weight range studied the melting point is independent of the molecular weight. The temperature for the onset of crystallization at any given rate of cooling appears to increase as the molecular weight is decreased. The isothermal rate of crystallization becomes much greater as the crystallization temperature is lowered, however, at a given temperature the rate is affected only slightly by a change in molecular weight and is primarily a function of the degree of branching.

ISSN:0021-8979    

DOI:10.1063/1.1721755

出版商:AIP    

年代:1954

数据来源: AIP

摘要:

Butadiene‐styrene copolymers contain the following units:trans‐1,4‐polybutadiene,cis‐1,4‐polybutadiene, 1,2‐polybutadiene, and bound styrene. Of these the first is the only type which has been shown to form crystals. By the use of published data on volume change arising from crystallization (largely due to Meyer) and on structure as a function of polymerization temperature (largely due to Hampton) it is found that crystallization in emulsion polymers has been observed only when thetrans‐1,4‐polybutadiene content has been greater than about 58 percent, becoming more complete the more this value is exceeded. From this observation one can then draw the following conclusions, all of which are in accordance with direct experimentation:(1) Crystallization is not observed if the polymerization temperature is above 60°C.(2) For polymerizations at 50°C a small amount (2 to 6 percent) of bound styrene inhibits crystallization completely.(3) For polymerizations at 5°C the limit is at about 15 to 18 percent bound styrene content.(4) At −40°C (the lowest polymerization temperature normally utilized) this limit is at about 30 percent bound styrene.

ISSN:0021-8979    

DOI:10.1063/1.1721756

出版商:AIP    

年代:1954

数据来源: AIP

摘要:

Spherically symmetric optical instruments, and their desirability for microwaves, are discussed. Application of Hamiltonian optics in analyzing the rays in the Luneberg lens has led to formula of a new lens that has spherical symmetry and a focus near its center. A cylindrical version of this new lens has been designed for operation in theTE10mode.

ISSN:0021-8979    

DOI:10.1063/1.1721757

出版商:AIP    

年代:1954

数据来源: AIP

摘要:

The well‐known relation between the total energy cross section of a scatterer and its forward scattered amplitude is extended to obtain an approximate transmission coefficient for a uniform planar distribution of parallel cylinders. An analogous reflection theorem for an arbitrary cylindrical boss on a perfectly reflecting plane is then derived; here the total cross ssction of the boss is related to the scattering amplitude in the specular direction of reflection of the plane. This is extended to obtain an approximate reflection coefficient for a uniform distribution of cylindrical bosses on a plane.

ISSN:0021-8979    

DOI:10.1063/1.1721758

出版商:AIP    

年代:1954

数据来源: AIP

摘要:

The diffusion equation is solved for transistors for (a) linear flow of minority carriers and (b) non‐uniform flow of minority carriers due to a metal contact applied to the base region. The current transport ratio &bgr; is calculated for both of these cases as a function of frequency.Frequency response characteristics of a transistor, as predicted by the one‐dimensional diffusion equation, give excellent results for junction transistors having base length‐to‐base width ratios greater than ten and having negligible surface recombination rates. For transistors where these conditions are not applicable, the two‐dimensional diffusion equation must be used in the calculation of &bgr;.

ISSN:0021-8979    

DOI:10.1063/1.1721759

出版商:AIP    

年代:1954

数据来源: AIP