There are essential differences between polymer crystals and crystals of monomeric substances. From high polymers, macroscopic single crystals are scarcely formed; high polymers never crystallize completely but leave submicrocrystallites imbedded in amorphous matrix which is in “semicrystalline state”. The cause of this singular behaviour of high polymers has not been fully explained. The authors measured the crystallinity of polytrifluoroethylene of different molecular weights crystallized under most favourable conditions. The maximum crystallinity is found to increase linearly with the increase in molecular weight. In some extent, this is explained by assuming that the entanglement of polymer chains impedes the crystal development leaving amorphous regions and that the number of entanglements is proportional to the length of the chain. The cause of semi-crystalline formation of high polymers could thus be concluded as due to the entanglement of polymer chains.
Nuclear magnetic rosonance absorption measurements are reported for polymethyl methacrylate (PMMA), polymethyl acrylate (PMA), polyvinyl acetate (PVAc), polymethacrylic acid, poly-n-butyl methacrylate, and their monomers, in the temperature range from -170 to 100°C. It has recently been pointed out that methyl groups in PMMA are not active in re-orientating around C3 axis at room temperature in contrast with the ones in molecular crystals. Results obtained on PMMA, PMA, and PVAc confirm that all methyl groups are not in freely rotational state below the temperature at which the main line narrowing occurs. However, it should be noted that, in poly-methacrylic acid and poly-n-butyl methacrylate, the methyl groups seem to rotate fast enough at room temperature, and that in the crystalline state of methyl methacrylate and methyl acrylate monomers, the methyl group rotates freely even in the neighbourhood of -150°C. In each case of PMA and PVAc, the “anomalous narrowing” is found in lower temperature region. This phenomenon seems to be similar to the one that is found in PMCA. Observed line narrowing in PMA and PVAc is correlated satisfactorily with known dielectric and mechanical properties of the materials. Lastly, interpretations of the rotational states of groups in polymers are given.
Nuclear magnetic resonance of F19 in polytrifluorochloro-ethylene resin is studied in the range from -172°C to +205°C near the melting point. Line width transitions of this polymer are measured on two kinds of specimens, NST 300 and 240. The motions narrowing occurs above -30°C, and segmental motions seems to take place at about 70°C. Correlation time versus temperature curve shows a knick point at about 70°C, and two values of activation energy, 1.6 and 5.2 kcal/mole, are obtained at lower and higher temperatures respectively, corresponding with small scale intra-segmental motions and large scale main segmental motions in the second-order transition range respectively. Line width appears much broader with the lower molecular weight specimen (NST 240) than with the higher one (NST 300). This seems due to the higher degree of crystallization in lower molecular weight crystalline polymer. The second moment at rigid state is calculated with some crude assumptions in both cases of isotactic and syndiotactic arrangements. 8.0 and 6.9 gauss2 are obtained in both cases, the observed value being 7.7 gauss' at -172°C.
A study on elongation (ΔL/L)-temperature (T) relation under a small constant load has been made with sixteen species of polymer films, including melt-cast crystalline polymers (Teflon, poly-ethylene terephthalate, Nylon-6, polyethylene, etc.), melt-cast amorphous polymers (polystyrene, polyvinyl chloride, etc.), solution-cast crystalline polymers (polyvinyl alcohol, cellulose triacetate, etc.) and solution-cast amorphous polymers (polyvinyl acetate, cellulose diacetate). Below the melting region it is found that ΔL/L-T curves for solution-cast films show the same normal thermodynamic thermal changes, as are found in linear expansion regarding, for example, first and second order transitions and crystallization. But those for melt-cast films show the heat relaxation of internal stress caused by casting besides normal thermal changes. Data of expansion co-efficient and transition temperature are listed in a table.
Variation of extensional creep compliance (or its reciprocal) of polymethylmethacrylate and polystyrene with temperature is measured in their glasslike consistency by observing bendingcreep of beam-shaped specimens in the time range from 15 sec to 15min. In compliance-temperature diagram of each of the two polymers a characteristic break point is found, which is independent of creep time and coincides with those found in ultrasonic velocity-temperature diagram and in specific volume-temperature diagram and in specific volume-temperature diagram. The ratio of temperature coefficients of compliance, taken just above and just below the break point, agrees with that obtained from volume expansion measurement. It is, therefore, verified that the break point is completely independent of mechanical time-scale and that it is dominantly governed by an abrupt change in the density. In the case of polystyrene, another characteristic temperature, lower than the above-mentioned one, is found, below which relaxation effect is markedly depressed. The static experiment as employed in the present case could be used as a simple and direct method to explore the characteristic temperatures of high polymers which are defined as the break points in specfic volume-temperature diagram.
Upon rare occasions, viscoelasticity is required to be observed in low frequency region where _??__??_1cm-1. At present, however, analysis for observed values in such cases is not yet established. It is shown that the analysis can be performed graphically by using calculable plane wave approximation. A rheometer to which this approximation can be applied has been devised by way of experiment. This rheometer is provided with inner and outer cylindrical rotors retaining the merit of rotating type rheometer and gave satisfactory results.
Under cyclic stress variations of tension and compression, energy absorbed per cycle per unit volume is measured by a resonant-vibration method and numerical value of vibrational fatigue is obtained by using the formula for the energy absorbed at resonance by a single-degree-freedom system. By plotting observed data on log-log scale, it is found that the rate of absorption of the energy varies approximately linearly with the amount of stress. On the other hand, on log-log diagram, the rate of creep under a given load is known to vary approximately linearly with the amount of stress. Hence, the deformation-time characteristic of any given polymer can be obtained simply by measuring the rate of absorption of the energy at various stress amplitudes. In this experiment, the rate of creep is obtained from the rate of absorption of the energy using two polyethylene samples.
Dielectric proporties of several crystalline polymers are measured over wide ranges of frequency and temperature, and magnitudes of dielectric absorption determined. Transition temperatures estimated from temperature dependence of the magnitudes of absorption are compared with so-called second order transition temperatures of the crystalline polymers and it is shown that the transition temperatures from dielectric view-point coincide with those obtained in volume expansion experiments. As for the dielectric absorption, the magnitude of which is independent of temperature, the temperature at which the frequency corresponding to absorption maximum becomes 10-3c/s is defined as the subsidiary transition temperature. Ratio of transmission temperatures including the subsidiary ones to melting temperatures is compared with the outcome of so-called 2/3-rule. Plots of the transition temperatures versus the energy of activation in relaxation process gave one curve for six crystalline and additional two amorphous polymers.
Electrical conductivity of several synthetic resins at 15°C and 40°C is measured by the charac-teristics of potential decay after a set potential is applied. The potential decay is measured by a compensating voltage applied to a vibrating electrode placed over the resin. The potential decay curve is found to have a complex structure for each resin. From the decay constant of these curves, specific resistance and specific surface resistance are obtained. The surface resistance was found very large for some synthetic resins.
Various methods of estimating the cross-linking efficiency in irradiated polymers are surveyed with critical remarks. It is emphasized that we should take due account of molecular weight distribution when we are to estimate the cross-linking efficiency from the analysis of irradiation-gelation phenomena, since the gelation of polymer is much influenced by the molecular weight distribution. A method of analyzing the gelation of polymers with a general type distribution of molecular weight is proposed and applied to the experimental data of high density Polyethylene, Marlex-50, irradiated in vacuum by 60Co-γ rays in the authors' laboratory. It turns out that the theoretical curve coincides fairly well with the experimental one, without necessitating the breaking of chains as Charlesby did. The cross-linking efficiency Gc.l is found to be 1.35 at the gel point and 0.87 at the largest radiation dose, 75 Mega rep., in the experiment.
Experiments has been carried out on the irradiation of aqueous solutions of polyvinyl alcohol by gamma rays. For sufficient does of irradiation, polymer molecules in a solution cross-link with one another and form an insoluble gel. The change in degree of polymerization of the polymer with the absorbed dose and the minimum dose, Rg, required to produce a gel are measured as functions of concentration of polyvinyl alcohol. In dilute solutions below the critical concentration, gel formation does not occur even by prolonged irradiation. Some consideration are given on the phenomena. Since the effects of radiation on polyvinyl alcohol in a solution are considered as indirect, the energy absorbed by the solvent from the radiation is transfered to the energy of cross-linking between the polymer molecules. From the relation between Rg and the concentration of polymer, the energy per cross-linking absorbed from the water is estimated as 62 eV.