摘要:

Poly(ethylene oxide) fibrillar crystals were prepared from ethyl alcohol solutions by stirred crystallization in the temperature range 31°-37°C. Electron microscopy and calorimetric measurements show that the crystals consist of a central thread with extended chains and lamellar overgrowths with folded chains. The melting behavior of crystals prepared at the same temperature with stirring (fibrillar) and without stirring (lamellar) was compared and the results discussed in relation to their respective morphologies. The fibrillar crystals exhibited superheating which increased with the temperature of crystallization. This is related to the amount of material which crystallizes in the extended-chain central threads. The relatively narrow temperature range in which the fibrillar crystals form is related to the greater tendency of the highly flexible poly(ethylene oxide) chains to crystallize in the folded conformation. Molecular fractionation occurs during crystallization, and the plot of molecular weight of precipitated polymer against temperature of crystallization shows a minimum. This behavior is explained by postulating that in the formation of the fibrillar crystals there is a superposition of two crystallization processes with different fractionation effects.

ISSN:0022-2348    

DOI:10.1080/00222347008217121

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

A number of more-or-less distinctive crystalline morphologies have been shown to result from the annealing of polymer single crystals [1]. Among these morphological types are “picture frames,” “amoebas,” and “swiss cheese.” Generally, the lateral shape changes described by these terms accompany an increase in thickness. However, there is lacking in the literature an effort to correlate mechanistically the lateral shape change with the thickening process. In this paper an attempt is made to demonstrate how the picture frame morphology develops as the end state of a sequence of steps resulting in crystal thickening.

ISSN:0022-2348    

DOI:10.1080/00222347008217122

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

A theoretical treatment for the effect of thermal vibration on the X-ray scattering intensity for a helical polymer crystal is presented. The treatment involves assumptions that (1) the unit cell of the crystal contains only one polymer chain and (2) fluctuations of the atomic positions are small. The temperature factor of the intensity of Bragg reflections is expressed in terms of mean squares of fluctuations of helical parameters. The diffuse scattering is also considered in a special case.

ISSN:0022-2348    

DOI:10.1080/00222347008217123

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

The spin-lattice relaxation times (T1) of a series of branched polyethylenes has been measured between −175° and +100°C. An empirical separation of the relaxation attributed to the gamma process (centered at −26°C) permits investigation of the relaxation effects of methyl group reorientation and branch point motion. This motion of the branch points is related to the beta process observed by mechanical loss measurements and is found to be uncorrelated with motion of linear amorphous segments.

ISSN:0022-2348    

DOI:10.1080/00222347008217124

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

The present paper, dealing with nylons 66, 610, and 612, is part of a study on a series of polyamides as crystallized from solution. The lath-shaped crystallization products, aggregated in the form of sheaves, possess the characteristics of chain-folded single crystals. In particular, they can be collected in the form of oriented mats which display arced X-ray reflections both in the wide- and low-angle regions. This enables the fold surfaces to be indexed as 00l. In a given polymer the long spacing has a characteristic value and for different polymers it varies in the ratio of the monomer lengths. The measured layer thickness corresponds closely to four-unit cell lengths. The fact that this holds for all three polymers implies that the fold length is determined by the number of hydrogen bonds between the stems. In view of the small value of this figure (due to the large identity period) the long period can only be apportioned between straight stems and folds in a limited number of ways which enables some concrete suggestions to be made on the possible constitution of the fold itself. On annealing, the fold length increases preferentially by factors of 2. A mechanism with general implications for the refolding behavior of polymers is proposed.

ISSN:0022-2348    

DOI:10.1080/00222347008217125

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

One of the methods in use for measuring the kinetics of isothermal crystallization of polymers is the recording of the depolarization of plane-polarized light by a microscope specimen. A relation between this depolarization and the volume fraction crystallized has been derived taking into account the polycrystalline nature of the specimen and assuming uniaxially birefringent crystallites. Uniform, all-directional increase in size of the crystallites would cause the depolarization to follow an Avrami-type equation with the Avrami exponent one unit “too high.” For spherulitic growth the correct Avrami exponent is found on the assumption that the birefringent entities in the spherulites do not increase in size but only in number. The depolarization by a polycrystalline specimen of uniform particle size is proportional to both specimen thickness and particle size. The derived relations are tested by random number calculation and their limits of reliability are indicated accordingly.

ISSN:0022-2348    

DOI:10.1080/00222347008217126

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

Proton spin-lattice relaxation times of bisphenol-A polycarbonate, butyl rubber, and blends of the two polymers were studied at 18 Mc/sec in the temperature range 90°-450°K. The proton spin-lattice relaxation is primarily dipolar in each polymer, due to methyl group reorientation and to reorientation of chain segments. In a blend of bisphenol-A polycarbonate with 7 and 10 wt of butyl, a nonexponential decay of magnetization was observed in the temperature range 280°-380°K. This was explained by the existence of two spin temperatures in these blends, indicating that processes which bring about the equilibrium within the spin system are slow compared to the spin-lattice relaxation times of the two components of the blend.

ISSN:0022-2348    

DOI:10.1080/00222347008217127

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

The melt rheology of polysulfone was studied in steady shear and oscillatory shear flow. Even when melt viscosity data were corrected for the dependence of the glass transition temperature onMnthe viscosity molecular weight relation was not a simple power law over any appreciable range. The melt compliance is very low and reaches its maximum value at molecular weights > 2000. Since the molecular flexibility, measured as the size of the equivalent random segment of rubber elasticity theory, is similar to that of other common polymers, this suggests that the polar nature of polysulfone is contributing to the high “entanglement” density.

ISSN:0022-2348    

DOI:10.1080/00222347008217128

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

Shear modulus-temperature curves and swelling behavior of poly(dimethylsiloxane)-bisphenol-A polycarbonate block copolymers were studied. Two glass transitions corresponding to the silicone and polycarbonate phases are found. A correlation of the shear modulus between the two glass transitions and the swelling ratio of the rubbery phase is suggested.

ISSN:0022-2348    

DOI:10.1080/00222347008217129

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

A detailed study of high-pressure-crystallized poly(vinylidene fluoride) has indicated that a mixture of low-melting phase II and high-melting phase I is present, rather than a new crystalline phase (phase III) as originally suggested.

ISSN:0022-2348    

DOI:10.1080/00222347008217130

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor