Poly , volume relaxation

The effects of morphology (i.e., crystallization rate) (6,7, 8) on the mechanical properties of semicrystalline polymers has been studied without observation of a transition from ductile to brittle failure behavior in unoriented samples of similar crystallinity. Often variations in ductlity are observed as spherulite size is varied, but this is normally confounded with sizable changes in percent crystallinity. This report demonstrates that a semicrystalline polymer, poly(hexamethylene sebacate) (HMS) may exhibit either ductile or brittle behavior dependent upon thermal history in a manner not directly related to volume relaxation or percent crystallinity. 

If the step in temperature is larger than a couple of degrees Celsius, then the aging process is not governed by a linear spectrum. Figure 4-15 shows the volume relaxation of poly(vinyl acetate) at a temperature of after the sample had been equilibrated at initial temperatures Ti of 30°C and 40°C and then suddenly heated or cooled to Tf = 35°C. Note that for AT = 5°C, the volume relaxation is asymmetric the relaxation following a jump up in temperature is very slow, and it eventually accelerates. The opposite occurs for a downward jump in temperature. 

Figure 3. Volume relaxation in poly(vinylacetate) upon cooling from 40 C, through Tg to 30 C ( ), contraction upon cooling from 40 C, through Tg to 25 C holding for 90 hr and heating to 30 C (o), memory and upon cooling from 40 C, through Tg to 25 C holding until quasi-equilibrium is reached and heating to 30 C (—) expansion, (Adapted from ref. 24.)...

Enthalpy and Volume Relaxation of Poly-(methyl Methacrylate) Penslfied by Pressure... 

Figure 8.1 Volume relaxation curves for a downward and an upward quench near the glass-transition temperature. (The time axis is logarithmic, and the polymer is poly[vinyl acetate]). (From Kovacs, A.J., Glass transition in amorphous polymers a phenomenological study, Adv. Polym. Sci., 3, 394, 1963. With permission.)...

Fig. 5. Isothermal volume relaxation of poly(vinyl acetate) data of Kovacs (replotted after Ref 9) showing the results of memory experiments. Sample 1 shows the response at 30° C after a quench from 40° C. For samples 2,3, and 4, the material was initially quenched from 40°C and held at T for a time t before jumping up to Ta of 30°C. Sample 2 T = 10°C for = 160 h Sample 3 T = 15°C for = 140 h Sample 4 Ti = 25 °C for = 90 h.

Simulations using free volume theories were performed where the gravitational parameter ag was arbitrarily varied. Lower ag represents reduced gravity environment. Figure 2 shows the volume relaxation of poly (vinyl acetate) after sudden cooling from an initial Ti = 313 K to final Ti = 303 K for three different values of ag, where the curves A, B and C correspond to ag = 0.5 (A), 5.0 (B) and 15.0 (C) respectively and Vf = is the final volume at Tf = 303 K for ag = 0. We note that simulation results are qualitatively in agreement with experiments. An exact comparaison is difficult at this time because the simulation parameters for DRl and PMMA are not available in the literature and must be measured. 

Due to the universality in all glasses, physical aging can be theoretically explained in a straightforward way based on the free-volume concept. As proposed by Struik, This is the basic and rather obvious idea that the transport mobility of particles in a closely packed system is primarily determined by the degree of packing of the system or by its inverse measure, viz. the free volume [2]. The idea could date back to 1943 when Alfrey et al. proposed that the isothermal aging below Tg can be attributed to the diffusion of free volume holes from the interior of polymers into the surface [34]. This free volume diffusion model (FVDM) was developed by Curro et al. [35] to quantitatively analyze the volume relaxation experiments of poly(vinyl acetate) [36, 37]. The motion of free volume holes can be described by a diffusion equation ... 

Work by Kovacs [75] on the volume relaxation of poly(vinyl acetate) highlighted the nonlinearity in the kinetics of isothermal recovery processes, and that the distribution of relaxation times, t, was necessary if the memory effects in glasses were to be explained. These ideas are incorporated in some of the phenomenological models developed to describe the aging of a glass and met in earlier sections. 

The time-temperature superpositioning principle was applied f to the maximum in dielectric loss factors measured on poly(vinyl acetate). Data collected at different temperatures were shifted to match at Tg = 28 C. The shift factors for the frequency (in hertz) at the maximum were found to obey the WLF equation in the following form log co + 6.9 = [ 19.6(T -28)]/[42 (T - 28)]. Estimate the fractional free volume at Tg and a. for the free volume from these data. Recalling from Chap. 3 that the loss factor for the mechanical properties occurs at cor = 1, estimate the relaxation time for poly(vinyl acetate) at 40 and 28.5 C. 

Fig. 6.16 NSE relaxation curves obtained from a 16% volume fraction poly(fluorosilicone) gel in acetone using tbe bigb resolution NSE spectrometer INI5 at tbe ILL, Grenoble. Tbe existence of plateaus that represent tbe level of excess scattering from static inhomogenieties at low wave vector Q is clearly visible. Tbe decay rates times of tbe dynamic parts yield tbe collective diffusion coefficient of tbe gel. (Reprinted with permission from [291]. Copyright 2002 American Chemical Society)...

The non-aqueous system of spherical micelles of poly(styrene)(PS)-poly-(isoprene)(PI) in decane has been investigated by Farago et al. and Kanaya et al. [298,299]. The data were interpreted in terms of corona brush fluctuations that are described by a differential equation formulated by de Gennes for the breathing mode of tethered polymer chains on a surface [300]. A fair description of S(Q,t) with a minimum number of parameters could be achieved. Kanaya et al. [299] extended the investigation to a concentrated (30%, PI volume fraction) PS-PI micelle system and found a significant slowing down of the relaxation. The latter is explained by a reduction of osmotic compressibihty in the corona due to chain overlap. 

Kakizaki, M. and Hideshima, T., Effect of distribution of free volume on concentration dependence of dielectric relaxation in water mixtures with poly(ethylene glycol) and glucose, Jpn. J. Appl. Phys., Part 1, 1998, 37, 900. 

In this system the a relaxation can be analyzed by the symmetric equation of Fuoss-Kikwood and a new model which is similar to Havriliak- Negami equation used in the analysis of dielectric spectroscopy. According to the Tg values calculated for these systems, the free volume can be appropriately described by the free volume theory. The analysis of these families of poly(methacrylate)s allow to understand in a good way the effect of the structure and nature of the side chain on the viscoleastic behavior of polymers [33],... 

For polymers containing branched side chains Fig. 2.67 show the variation of the modulus E , E and the loss tangent for PDIPI and PDIBI in the temperature range under study. Two relaxations can be observed where the most prominent is the a relaxation associated to the glass transition as in the systems previously reported. Tg increases as the volume of the side chain increases. This result is in good agreement with that observed for the corresponding family of poly(methacrylate)s [242],... 

PTC) is a family of polymers whose thermal properties, Tg, unperturbed dimensions and partial specific volumes has been reported [248-252], The dielectric properties of these polymers were recently studied [253-256], Relaxational studies on poly(thiocarbonate)s are scarce, but on the contrary there is much information about relaxation processes of the analogues poly(carbonate)s (PC). Therefore the study of PTC is an interesting approach to get confidence about the motions responsible of the relaxational behavior of these polymers. 

The amount of radicals in carbon black filled rubbers decreases significantly upon extraction of free rubber with the aid of a solvent containing a free radical scavenger. The extraction nevertheless causes a substantial increase in the fraction of the T2 relaxation component with the decay time of about 0.02-0.03 ms [62], This increase is apparently caused by an increase in the total rubber-carbon black interfacial area per volume unit of the rubber due to the removal of free rubber. The T2 relaxation component with a short decay time is also observed in poly(dimethyl siloxane) (PDMS) filled with fumed silicas [88], whose particles contain a minor amount of paramagnetic impurities. Apparently, free radicals hardly influence the interpretation of NMR data obtained for carbon-black rubbers in any drastic way [62, 79]. 

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

The fluorescence intensity of quinoline derivatives has been found to increase dramatically with an increase in the molecular weight of the host polystyrene. " This is attributed to a decrease in the free volume in the polymer matrix restricting molecular rotation/motion of the fluorophore. Similar effects have been observed for juliodinemalononitrUe in different stereo-regular poly(methyl methacrylates), and temperature effects on the luminescence properties of indole and coumaric acid derivatives in different polymer matrices showed abrupt changes in emission intensity at temperatures which correspond to the onset of local relaxation processes in the polymer. ... 

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