摘要:

The variation of mesophase textures of ethyl-acetyl celluose/ dichloroacetic acid solution with temperature was studied by polarizing microscopy and small-angle light scattering. It was found that the degree of order of polymer chains was different in different mesophase zones. When the solution was heated at a speed larger than 5 K/min, the mesophase with a relatively low degree of order could first transform into the isotropic phase. If the solution was heated slowly enough, however, the mesophase aggregates could become smaller, but the degree of order of polymer chains in the mesophase did not decrease with increasing temperature. When the solution transformed from the isotropic state to the anisotropic one, ordered domains were first formed, and then the disk-like texture appeared.

ISSN:0022-2348    

DOI:10.1080/00222349008245768

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The reasons for cessation of drawing and for the local fracture processes occurring in linear polyethylene near ultimate draw ratios, λmax, have been studied by broad-line proton NMR and scanning electron microscopy. As λmaxwas reached during drawing, the sample whitened and kink bands similar to those observed at deformation of low molecular solids were formed. The kink bands were clearly seen in the micrographs. As evidenced by NMR, the segmental mobility of chains in the amorphous regions is almost completely suppressed because of a sharp increase of the orientation stress near λmax(though the temperature of drawing is close to the melting temperature). This phenomenon of mechanical vitrification is regarded as a fundamental reason for cessation of drawing and for transition to the “solid-state” fracture mechanism. Quantitative estimates are given for formation of submicrocracks arising at boundaries of kink bands.

ISSN:0022-2348    

DOI:10.1080/00222349008245769

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The crystallization of polyamide 6.12 was investigated by X-ray diffraction and infrared spectroscopy. Similar to polyamide 6.6, there exist a pseudohexagonal γ modification and a triclinic α modification. The characteristic IR absorption bands of each modification were identified and assigned. In the amorphous state, polyamide 6.12 additionally shows an absorption band at 1255 cm−l. The γ modification can be obtained by heating an amorphous sample above the glass transition temperature near 40°C and is stable only at high temperatures. Cooling of the γ modification immediately resultsinthe Brill transition into the α modification. It could be shown that the Brill-transition temperature is identical with the annealing or crystallization temperature of the Y modification. The difference of the 100 and 010 lattice plane distances measured at room temperature as well as the ratio of the IR absorbances between some a and y bands increase with increasing annealing temperature and can be used for the determination of the individual Brill-transition temperature of a sample.

ISSN:0022-2348    

DOI:10.1080/00222349008245770

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A study has been made of the influence of transesterification on the miscibility in binary blends of poly(ethylene terephthalate) and poly(bisphenol-A carbonate). The blends were melt mixed in the range 260–300°C and studied by differential scanning calorimetry, dynamic mechanic analysis, and by Fourier transform infrared spectrometry. It was found that copolymer produced by a transesterification reaction can enhance the miscibility of this system. The new compositions were uniquely identified by FTIR. Gel permeation chromatography showed that molecular weight decreases were not the origin of miscibility. The ester exchange reaction itself was found to be initiated by the residual catalyst in the commercial polymers selected for study. This has been shown by the absence of reaction after polymer purification by solution and reprecipitation prior to melt mixing.

ISSN:0022-2348    

DOI:10.1080/00222349008245771

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Copolymers made by ester exchange reaction have been obtained from poly(ethylene terephthalate) (PET)/poly(bis-phenol-A carbonate) (PC) blends during melt mixing. The copolyesters were isolated by thin-layer chromatography (TLC) and identified by infrared spectroscopy. It was found that the quantity of copolymer formed was increased by the temperature and duration of melt mixing. The PET/PC blend was found to react at 270°C within 10 min, as detected by TLC. After 60 min, the pure PC had disappeared. The miscibility of PET/PC blends was found to be markedly aided by the addition of as little as 2% of the copolymer isolated by TLC.

ISSN:0022-2348    

DOI:10.1080/00222349008245772

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Volume recovery in the temperature range of above Tgto below Tβhas been studied for two atactic polystyrene specimens (Mw= 210,000 and 300,000 g/mol). Observations clearly disclose the collapse of free volume even at temperatures well below Tβ. In an excess free volume sufficient environment, as the aging temperature is lowered through Tβ, the retardation of the initial volume recovery becomes progressively longer, forming a variation in the nonlinearity of the isotherms. Indeed, it is this variation that leads to the observation of the β transition in the V-T plot. The β transition is time dependent, but molecular weight independent. On the other hand, in addition to well-known molecular weight and time dependence, the glass transition has another important feature: The difference in thermal expansion coefficients above and below Tgincreases slightly with aging time. Thus, time cannot alter the existence of Tg. Consequently, this study favors thermodynamic theories for the formation of Tg.

ISSN:0022-2348    

DOI:10.1080/00222349008245773

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The chemical potential of a solute in a solid polymer includes contributions from solute-polymer chain conformations, Flory-Huggins type interaction, and elastic energy of swelling. Presence of impermeable and rigid crystallites in such systems is expected to affect all these contributions. Theoretical calculations have been performed to check the direct effects of constrained chain conformations in the amorphous domains in semicrystalline polymers. Experimental results are used to determine Flory-Huggins coefficient and elastic modulus. From all these, the primary effects are shown to be on the entropic part of the Flory-Huggins coefficient and an increase in the elastic modulus by one or two order of magnitude. Finally, these results are used to calculate the rates of solvent-induced crystallization to show that these rates can drop to negligible values as the amount of crystals formed rises. Thus, the actual degree of crystallization can lie well below the Flory-Yoon limit.

ISSN:0022-2348    

DOI:10.1080/00222349008245774

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

An original method of zone drawing of polymers at constant load and a procedure for the optimization of thermomechanical conditions (stress, temperature) are suggested, allowing high draw ratios and favorable strength values to be obtained. The temperature (or stress) range of necking has been determined in a nonisothermal and in an isothermal regime. The advantage of the method consists in that the increasing orientation in the neck starting from the initiation point and up to fracture allows the morphology and properties to be quickly examined, depending on the varying thermomechanical conditions in different regions of the neck. At a high temperature and low load the mechanism of oriented crystallization from melt becomes operative; in opposite cases, orientation of the solid state takes place. It is shown that in the nonisothermal regime an increased rate of heating allows extreme draw ratios (up to ca. 150) to be obtained, approximately twice as high as those obtained in the isothermal regime. This is interpreted as a gradual improvement of the oriented structure by recrystallization during extension and by a quick fixation on cooling below the zone.

ISSN:0022-2348    

DOI:10.1080/00222349008245775

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Excess free-volume trapping incurred by quenching has been quantified for atactic polystyrenes with different molecular weights. As the initial temperature (T0) is elevated from the glass transition (Tg), the increment in excess free-volume trapping first responds sharply, then becomes sluggish, and finally levels off. Molecular weight increase shifts the isochronal curves to a higher temperature region, which coincides with the isochronal change in glass transition temperature (Tgt)- As the isochronal curves are superposed to form a master curve, the mechanism of excess free-volume trapping is essentially controlled by T0, from which the conformational structures of molecules are determined prior to quenching, rather than by the final aging temperature. In order for this superposition to occur, conceivably, the content of excess free volume trapped in the glassy state has to be fairly constant throughout all temperatures below Tg. Consequently, this study does not support the existence of an underlying equilibrium extrapolated from the rubber state.

ISSN:0022-2348    

DOI:10.1080/00222349008245776

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Theoretical analysis of flexible-chain polymer crystallization was carried out over a wide temperature range from glass transition temperature Tgto melting temperature Tm. Temperature dependence of dynamic behavior manifesting in the decrease of crystallizing length with decreasing temperature was taken into account. The dependence of crystallizing length on temperature was obtained using chosen values of it at Tgand Tm. The crystallization with the formation of folded-chain crystals (type I) and uncoiled-chain crystals (type II) was considered. The analysis of thermodynamical favorability of both types of crystals with respect to temperature made it possible to obtain a flexible-chain crystallizing polymer phase diagram. This diagram shows the existence of two ranges where type II crystal formation is more favorable: a narrow range near Tmand the wider one near Tg, separated by the temperature range of crystallization with chain folding. Temperature dependences of type II crystals fraction in the system and their size were calculated. It is shown that the crystallization at considerable supercooling leads to the appearance of a great number of type II small crystals connected by tie chains. The system formed is characterized by a high degree of crystallite interconnection.

ISSN:0022-2348    

DOI:10.1080/00222349008245777

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor