ISSN:0022-233X    

DOI:10.1080/10601326908051824

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The relaxation behavior of polymers at low temperatures, as studied by dynamic mechanical, dielectric, and nuclear magnetic resonance methods, is reviewed. Modulus values at 4 and 77°K are tabulated for a variety of both crystalline and amorphous polymers and are compared to room temperature values as well as to theoretical values deduced from intermolecular force constants. Specific attention is given to polymers that show a δ-type relaxation. At low frequencies, this relaxation occurs in the temperature region below 100°K. The δ-processes are shown to arise from reorientational motion of side chain methyl groups, as in PMMA or PVA, side chain ethyl groups, as in PEMA or PVPr, and side chain phenyl or phenyl-substituted groups, as in PS or PmCIS. Linear polymers, such as PE and PTFE, that have no side chain groups exhibit no δ-type relaxation. Consideration is also given to the effect of various internal and external variables on the low-temperature relaxation behavior. These variables include chemical structure, hydrogen atom substitution, crystallinity, impurities, drawing, and past history. An attempt is made to correlate the dielectric and nuclear magnetic resonance data with the mechanical data. Log frequency versus reciprocal temperature plots are given for several polymers of different chemical structure, and activation energies are estimated.

ISSN:0022-233X    

DOI:10.1080/10601326908051825

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The existing measurements and theories of the low-temperature thermal properties, heat capacity, and thermal conductivity of polymers are reviewed with particular attention paid to the differences between partly crystalline and amorphous polymers. The most striking feature of the low-temperature heat capacity of polymers is that in the liquid helium temperature range the heat capacity does not depend upon the cube of the temperature as for other solids. Further, only well below 1°K does the heat capacity approach the value predicted on the basis of the sound velocity. This behavior indicates the presence of a small number of low-frequency modes of vibration in the frequency spectrum. The fact that such anomalous behavior seems linearly related to the crystallinity implies that this behavior is associated with the amorphous structure, perhaps with motions of pendent groups within cavities formed in the amorphous structure. The thermal conductivity of semicrystalline and amorphous polymers differs considerably. Semicrystalline polymers display a temperature dependence of the thermal conductivity similar to that obtained from highly imperfect crystals, the thermal conductivity having a maximum in the temperature range near 100°K which moves to lower temperatures and higher thermal conductivities as the crystallinity is increased. Amorphous polymers display a temperature dependence similar to that obtained for glasses with no maximum but a significant plateau region in the range between 5 and 15°K. The theoretical interpretation of the thermal conductivity of these materials is considered.

ISSN:0022-233X    

DOI:10.1080/10601326908051826

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

A series of polyvinyl alkyl ethers, with the ether side chain ranging in length from methyl to n-decyl, has been studied from ∼60°K to above the glass transition. A linear variable differential transformer was used to measure the linear expansion coefficient, α, complemented by mechanical loss measurements with a freely oscillating torsion pendulum and dielectric loss measurements. In addition to the glass transition, two low-temperature transitions have been observed in these systems. The first below Tg, Tgg(1), follows the same trend as the glass transition, i.e., as the side chain length increases the temperature at which the transition occurs decreases, until n-octyl, where side chain crystallization is manifested. The second transition below Tg, Tgg(2), occurs at ∼100°K irrespective of side chain length. Because of their analagous dependence on side chain length, Tgg(1) is thought to be similar to the glass transition, i.e., due to main chain motion. Δα' at Tgg(2) is of greater magnitude than Δα' at Tgg(1) and is thought to be a result of reorientation of the side chain.

ISSN:0022-233X    

DOI:10.1080/10601326908051827

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The dynamic mechanical properties of polystyrene/ethyl benzene, polymethyl methacrylate/water, polyethyl methacrylate/ethyl hexyl adipate, and poly-2-hydroxyethyl methacrylate/water were studied between 10 and 100°K. A free-decay resonance acoustic spectrometer was designed for this purpose. It was found that the presence of diluents causes the low-temperature loss peaks of the polyrners in question to shift to higher temperatures. This observation is compatible with the hypothesis that at very low temperatures, the diluent molecules are immobilized to occupy free space between adjacent polymer chains. The hindering action of these molecules on the kinetic motions of the side groups of the polymer thus renders greater thermal energy necessary to initiate the relaxation processes.

ISSN:0022-233X    

DOI:10.1080/10601326908051828

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

An experimental technique has been developed at McDonnell Douglas Corporation (MDC) that can be used to derive the shearing stress-strain distribution in an adhesive-bonded joint and therefore its shear modulus. The MDC technique uses a double-lap joint model instrumented with either photostress coatings or strain gauges. The technique was used to determine the effects of different bond thickness on the stress-strain behavior of an epoxy adhesive and the effect of a cryogenic environment on the stress-strain characteristics of a polyurethane adhesive.

ISSN:0022-233X    

DOI:10.1080/10601326908051829

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The effects of fluorine exposure between 1 and 2 atm pressure at ambient temperature and in liquid fluorine at cryogenic temperature on some properties of nitroso rubbers (the CF3NO/CF2= CF2copolymer and the CF3NO/CF2=CF2/ON(CF2)3COOH terpolymer systems and the compounded terpolymers) are described.

ISSN:0022-233X    

DOI:10.1080/10601326908051830

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Differential thermal analysis (DTA), rebound resilience, and tensile proerties of a polyurethane adhesive were measured at cryogenic temperatures. The experimental methods are described, and test results which aid in evaluating the polyurethane for use at low temperatures are discussed. The DTA thermogram reveals that the glass transition temperature (Tg) is 235°K. The resilience profile indicates a resilience minimum (Tr) at 270°K and a frequency of 3800 Hz, which is consistent with the Tgmeasured by DTA. The low resilience below Tr, caused by secondary low-temperature transitions, shows the high energy absorption capabilities of the polyurethane. Considerable plastic flow at 195°K (40°K below Tg) is evidenced in the results of the tensile tests. The results of the three tests indicate that the polyurethane adhesive will perform well at low temperatures. The test methods should also be useful for evaluating the low-temperature performance of new polymers.

ISSN:0022-233X    

DOI:10.1080/10601326908051831

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The results of an evaluation of polymers as cryogenic adhesives are presented. It was observed that polyether-based polyurethanes provided the most attractive capability as adhesives for use at liquid hydrogen temperatures.

ISSN:0022-233X    

DOI:10.1080/10601326908051832

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

ISSN:0022-233X    

DOI:10.1080/10601326908051833

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor