摘要:

We propose a new method for the investigation of molecular motion in polymeric solids. In a mechanical step-function experiment, we thermally stimulate the response to a constant stress. The high resolving power of this technique permits detailed study of the complex retardation modes observed in polymers. By using “fractional” loading programs it is possible to differentiate a discrete and a continuous distribution of retardation time. This technique allows us to predict the complex compliance in a very wide frequency range: 104-10−12Hz for experiments performed between liquid nitrogen temperature and 500°K. In low-density polyethylene, we have shown the existence of a discrete spectrum of mechanical retardation times which has the same fine structure as the spectrum of dielectric relaxation times obtained from the study of depolarization thermocurrent on the same sample. The predicted variation versus temperature and frequency of the loss compliance is compared with that of the dielectric loss factor.

ISSN:0022-2348    

DOI:10.1080/00222347708212208

出版商:Taylor & Francis Group    

年代:1977

数据来源: Taylor

摘要:

The direct reading dynamic viscoelastmoeter DDV-II (Rheovibron) has been completely automated to provide continuous measure of stress, strain, phase angle, temperature, and elongation of sample. The tension on the sample is automatically controlled to permit measurements under constant strain or to maintain sufficient tension to prevent sagging during the cycling operation. The sensitivity has been greatly increased, which permits extension of mechanical properties far beyond the Tg. More accurate locations of the relaxations are possible so that minor transitions can be detected over the temperature range of −160 to 250°C. The tan δ has been measured to ±0.001. Stress and strain can be maintained at constant values over an extended time. Data are processed on an off-line computer to give printout of storage modulus, loss modulus and tan δ as a function of temperature. Measurements have been made on many types of polymer films and fibers.

ISSN:0022-2348    

DOI:10.1080/00222347708212209

出版商:Taylor & Francis Group    

年代:1977

数据来源: Taylor

摘要:

A method is developed to analyze the distortion of stress waves resulting. from sinusoidal strains. This distortion can result from two effects: (a) changes in modulus as function of strain (or angle θ) during the cycle, and (b) changes in mechanical loss as function of strain (or angle θ) during the cycle. The determination of the relative contribution of these two effects requires the knowledge of specimen modulus and loss at each instant of the experiment. This can be achieved by superimposing a high frequency small amplitude wave on the high amplitude low frequency wave. This technique is used in the analysis of dynamic responses of nylon 6 fibers. The results show that the energy loss in extension is different from that in contraction. This indicates that, in addition to strain dependence of modulus and loss, the analysis of the hysteresis curve must account also for the reversible strain induced structural change. An analytical procedure is presented to treat the data, and the effects of strain amplitude and temperature on the type of strain-induced structural changes are discussed.

ISSN:0022-2348    

DOI:10.1080/00222347708212210

出版商:Taylor & Francis Group    

年代:1977

数据来源: Taylor

摘要:

Fillers have been reported to raise, have no effect upon, or to lower the glass transition temperature Tgof polymers. In those studies, comparisons have been made between filled and unfilled polymers having equal thermal histories. In the work report here, however, the thermal history (cooling rate) was also varied. Two systems, polystyrene-silica and epoxy-rubber were studied. The glass transitions were measured by using a differential scanning calorimeter and a method of analysis first described by Ellerstein and later elaborated upon by Flynn. The data could be represented by a linear plot of the log of cooling rate versus Tgor Tg−1. For the polystyrene system, the filled material had a larger negative slope for the plot of log cooling rate versus Tg−1so that after fast cooling the filled material had a lower Tgthan the unfilled material. However, when the cooling rate was lowered, the Tgof the filled and unfilled materials approached each other, and for very slow cooling (annealed samples) the filled material had a higher Tgthan the unfilled one. For the epoxy-rubber system, the rubber lowered the Tgby an amount which increased with cooling rate. Thus the extent and even the direction of the change in Tgwith filler is shown to be dependent upon thermal history. It is shown that the slope of the plot of log cooling rate versus Tg−1or Tgis related to an activation energy. A new simplified method of correcting Tgfor DSC instrument thermal lag is described.

ISSN:0022-2348    

DOI:10.1080/00222347708212211

出版商:Taylor & Francis Group    

年代:1977

数据来源: Taylor

摘要:

The testing and evaluation program leading to the certification of a selected batch of polystyrene as ICTA Certified Reference Material GM 754 is described. Defined points from the glass transition curve were obtained, first in a preliminary program, then in 24 laboratories with the use of ten kinds of apparatus. Data were obtained in four heating rate ranges, 4-6, 8-12, 16-24, and 30-50°C/min. The initial departure was rejected for certification because of its relative lack of reproducibility and the comparatively subjective nature of its determination. The two defined points, Tband Tc, had unweighted mean values of 104.4 and 107.5°C, respectively, with standard deviations of 1.5 and 1.7°C, respectively. Data from individual investigators had a mean standard deviation of 0.53 ± 0.28°C. Separation of the data by heating rate, sample holder configuration, temperature sensor location and combinations of these disclosed that the reproducibility under a given set of conditions warranted certification; that the apparent heating rate dependence of the measured point (sensor location) from the sample and the inherent time dependence of the glass transition lead to an intrinsic contribution to the apparent heating rate dependence.

ISSN:0022-2348    

DOI:10.1080/00222347708212212

出版商:Taylor & Francis Group    

年代:1977

数据来源: Taylor

摘要:

The ESR spin-probe technique with the use of the nitroxide probe 2, 2, 6, 6-tetramethyl-4-hydroxypiperidin-1-oxyl benzoate (BzONO), has been used to study Tgvariation in a series of polystyrenes of varying molecular weight and molecular weight distribution. Glass temperatures, as determined by ESR, for “monodisperse” polystyrenes vary smoothly as a function of molecular weight. From results on polystyrene binary blends it is shown that the technique is responsive toMnrather thanMw. Data obtained by ESR are shown to agree with conventional DSC measurements. Some advantages, limitations, and implications of the technique are also presented and discussed. The actual shapes of extrema separation vs. temperature plots are functions of molecular weight (becoming broader as molecular weight increases) and are discussed in terms of free volume effects.

ISSN:0022-2348    

DOI:10.1080/00222347708212213

出版商:Taylor & Francis Group    

年代:1977

数据来源: Taylor

摘要:

Hypersonic relaxation was studied in amorphous polyisobutylene (PIB), atactic polypropylene (PP), atactic poly(vinyl acetate) (PVA), atactic polystyrene (PS), and bisphenol-A polycarbonate (PC) by Brillouin scattering. The hypersonic loss tan δ was determined as a function of temperature. Only a single loss maximum was observed for the polymers studied in this work. The temperatures of maximum loss are 200°C at 4.95 GHz for PIB, 180°C at 4.11 GHz for PP, 200°C at 4.50 GHz for PVA, 240°C at 5.50 GHz for PS, and 280°C at 5.43 GHz for PC. The present results agree very well with the extrapolated secondary glass-rubber transition lines above the temperature where the primary and secondary lines have merged. The loss process is proposed to be local torsional motions of the polymer backbone which lead to structural relaxation of the fluid.

ISSN:0022-2348    

DOI:10.1080/00222347708212214

出版商:Taylor & Francis Group    

年代:1977

数据来源: Taylor

摘要:

Dipolar-decoupled, natural abundance, cross-polarization13C NMR spectra of polycarbonate have been obtained at 22.6 MHz, both with and without 3-kHz mechanical spinning at the magic angle. The 100-Hz resolution achieved by spinning produces spectra as detailed as those of the same polymer in solution. The13C rotating frame relaxation times (at 30 kHz) for individual lines of polycarbonate are dominated by spin-lattice processes and so contain information about slow motions. Analysis of these slow motions emphasizes the dynamic heterogeneity of the glassy state.

ISSN:0022-2348    

DOI:10.1080/00222347708212215

出版商:Taylor & Francis Group    

年代:1977

数据来源: Taylor