摘要:

We have extensively studied the nonequilibrium behavior of atactic polystyrenes in a broad range of temperatures. Depending on the conditions of the initial temperature (To) and the aging temperature (T), the shapes of isotherms plotted in the form of total fractional free volume (viz., δ) versus logarithmic aging time can be linear or nonlinear. At aTin the vicinity ofTg, the isotherms converge together. When theTislowered, the isotherms become increasingly nonlinear; eventually, these isotherms intersect one another at various points, forming a newly discovered crossing phenomenon. Differences in volume contraction persist throughout further aging, regardless of the equality in δ right at the moment of crossing. In a further case, the nonlinear isotherms between the two polystyrenes may be superposable by a simple δ shift, despite their substantial differences in δ. As none of these phenomena can be justified by kinetic arguments, this study seriously questions the validity of the existing kinetic theories as well as the existence of the extrapolated underlying equilibrium state.

ISSN:0022-2348    

DOI:10.1080/00222349108215448

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

In samples of isotactic polypropylene, the influence of the nucleation agent chinacridine permanent red E3B on the crystallization rate and content of α and β modifications and on the rate and mechanism of the β →αtransition was investigated using x-ray diffraction, DSC microcalorimetry, and electron microscopy. It was found that, though E3B increases the content of the β phase, it is also a very efficient nucleation agent for the α-phase crystallization. The β → α transition occurring above 140°C could be characterized by a gradual melting and annealing of the β phase, accompanied by a surface crystallization of the α phase on lateral faces of β lamellae by a mechanism similar to cross-branching of α lamellae.

ISSN:0022-2348    

DOI:10.1080/00222349108215449

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A graphical multiple-peak resolution method for wide-angle x-ray scattering (WAXS) patterns is described and used to estimate the apparent crystallinities relative to the entire sample (Xca) in polymethyl methacrylate-polyethylene oxide graft copolymers (PMMA-g-PEO) and their ionic complexes with LiClO4, KSCN and FeCl2. The crystallinities (Xcg) and crystallite sizes (L120) in the PEO graft component, alone, were estimated at the same time. A concept of “reduced degree of crystallinity (Rc)” and “critical PEO content” is proposed and applied as a yardstick of the effects of PMMA backbone chains and salts on the crystalline behavior of the PEO graft chains.XcaandXcgincrease with both the content and molecular weight of the PEO graft chains. The critical PEO content, below which PEO crystallinity is not seen, is about 23.7%. PMMA backbone chains can reduce the crystallization of the PEO graft chains at any given PEO content.XcaandXcgdiminish after complexing with the salts and the order of reducing crystallinity is: LiClO4 > KSCN > FeCl2; salts have two inverse effects, reduction and promotion, on the crystallization of the PEO graft chains. The salt effects depend on the salt concentrations and coordination of the cations. The crystallite sizes in the PEO graft chains are smaller than those in the PEO homopolymers but increase after complexing with salts. The PEO crystallite sizes are not noticeably affected by the content and molecular weight of the PEO graft chains but are noticeably affected by salt concentration.

ISSN:0022-2348    

DOI:10.1080/00222349108215450

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The methods of x-ray analysis, mechanical testing, and density measurements were used in an investigation of the connection between the quantity and geometrical dimensions of the side groups (statistically introduced into the linear polyethylene chain), on the one hand, and physicomechanical characteristics of the polymer system, on the other. It was shown that an increase in the number of “defects” of a relatively small size brings about “loosening” of the crystal lattice and the loss of the ability to crystallize of an increasingly large number of fragments of macromolecules, up to the complete disappearance of the crystalline phase, starting from a certain critical composition. The effects are the more pronounced, the larger the dimensions of the side groups. It appears that comparatively long pendants which are incorporated in the lattice virtually without distorting the latter do not cause any considerable changes in the degree of crystallinity of the polymer. The authors come to a conclusion that the “optimal” copolymer (i.e., showing a perceptible distortion of the crystal lattice with a relatively small loss of crystallinity) is that which contains 1 mol% of defective links, with the length of the branchings being 2–6 carbon atoms in the side chain.

ISSN:0022-2348    

DOI:10.1080/00222349108215451

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Uniaxially oriented films of linear medium-density polyethylene and samples subjected to high pressure during heating were investigated by methods of x-ray structure analysis, scanning electron and optical microscopy, dynamic mechanical testing, differential scanning calorimetry, and density measurements. It was found that the polymers can be readily oriented up to high drawing ratios and that they possess improved mechanical characteristics. The structure and morphology of such material appeared to be typical of superhigh-modulus polymers. Heating of the original sample under pressure above 4–5 kbar results in a phase transition of the orthorhombic crystal into the pseudohexagonal mesophase. The crystallinity of the cooled material amounts to almost 100%; the longitudinal dimensions of the crystallites are up to 100 run. The pressure-crystallized material is more perfect than the highly drawn material.

ISSN:0022-2348    

DOI:10.1080/00222349108215452

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor