Effect of Solvent Treatments on the. Mechanical Properties of Nylon 6. J. RUBIN" and R. D. ANDREWS,. Department of Chemistry and Chemical Engineering,.
Effect of Solvent Treatments on the Mechanical Properties of Nylon 6 J. RUBIN" and R. D. ANDREWS, Department of Chemistry and Chemical Engineering, Stevens Institute of Technology, Hoboken, N . J. 07030
The effects of absorbed solvents and chemical agents on the stress-strain and dynamic mechanical properties of nylon 6 film have been examined. The agents investigated were: water, benzyl alcohol, phenol and iodine. These materials produce changes in the crystalline structure as well as plasticization when they are absorbed. Repeated introduction and removal of phenol indicated that the change of structure takes place completely during the first absorption; after that, the effects of absorption are reversible. The pure plasticizer effect can therefore be observed by comparison of a sample containing plasticizer and a sample from which the plasticizer has been subsequently removed. The general effect of absorbed plasticizer (except for iodine) seems to depend primarily on the amount of plasticizer absorbed, and very little on its exact chemical nature. However, different agents can produce different crystalline forms. A method of analyzing the stress-strain curve is hypothesized based on the concept of a two-phase solid state structure consisting of a crystalline lattice imbedded in an amorphous matrix.
INTRODUCTION a means of gaining better insight into the relations between the crystal structure and morrelations between the crystal structure and morphology and the mechanical properties of a crystalline polymer, a study was carried out of the effects of solvent treatments on the mechanical behavior of a typical crystalline polymer, nylon 6. The changes of crystalline structure produced in this way were investigated in detail by the use of infrared and x-ray measurements; these results will be reported in a separate publication (1). The present paper will present the mechanical property data, which consists primarily of stress-strain data but also includes some dynamic mechanical measurements. Some relevant TGA data are also presented. The present work relates to a broader investigation of the plastic yield behavior of crystalline polymers and the effect of plasticizers on this yield behavior. The solvents or chemical agents whose effects were particularly studied here were water, benzyl
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*PIA Post-Doctoral Fellow at Stevens Institute 1966-67. Present Address: Plastics & Packaging Laboratory, Feltman Research Labs., Picatinny Arsenal, Dover, N. J. 07801.
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alcohol, phenol and iodine. The fact that water or humidity has a significant effect on the properties of nylon type polymers is well known. It would be expected that alcohols would have an effect somewhat similar to water, and investigation of a series of alcohols ( 2 ) indicated that benzyl alcohol was particularly strongly absorbed by nylon 6. The amount of benzyl alcohol absorbed was varied by mixing the alcohol with various amounts of cyclohexanol. Phenol has a very strong plasticizer action, and can even be a solvent for nylon. Some of the effects of a combination of phenol treatment and drawing on the structure and state of order in nylon 66 have been discussed in a recent paper by Sakuma and Rebenfeld ( 3 ) . Phenol is absorbed even more strongly than benzyl alcohol. Water is only mildly absorbed. Phenol and benzyl alcohol are less volatile plasticizers than water, and this adds to the convenience of their use. Iodine treatments were also included in this study because published reports (43) have indicated that iodine treatments produce interesting changes in the crystal structure of nylon polymers. Iodine was found to be very strongly absorbed. POLYMER ENGlNEERlNG AND SCIENCE, OCTOBER, 1968, Vol. 8, No. 4
Eflect of Solvent Treatments on the Mechanical Properties of Nylon 6
EXPERIMENTAL PROCEDURE The nylon 6 polymer used in this study was a commercial film obtained from the Allied Chemical Corp. through the courtesy of Mr. G. P. Koo. The film was 1 mil in thickness, and was designated by the trade name Capran 77C. The sdlvent treatments were carried out by immersing pieces of the film in suitable liquids (either pure liquids or solutions) at room temperature and measuring the amount of absorption by periodic measurements of the weight of the sample. To do this the film samples were removed from the liquids, dried and weighed by a standardized procedure. The film was treated in this way until equilibrium absorption had been attained. The achievement of equilibrium was felt to be desirable to assure that the absorbed material was distributed uniformly in the polymer. Strip samples were cut from these treated films with scissors. This was found to be preferable to cutting samples with a razor blade ( a procedure which was used previously) because it seemed to have less tendency to produce edge cracks in the samples; such cracks tended to produce premature fracture during stress-strain tests. Sample strips were always cut from the film in the machine direction. The phenol treatment was carried out using a 0.25 molar solution (approximately 1.5% by weight) in carbon tetrachloride. The amount of phenol absorbed from this solution was about 70% of the original sample weight. The weight gain obtained from treatment in cyclohexanol was only 0.4%, indicating essentially no absorption (as was true also for carbon tetrachloride). This is an interesting result in view of the great similarity in chemical structure and molecular size between phenol and cyclohexanol, and the fact that both should be capable of hydrogen bonding. The presence of the aromatic ring seems to greatly promote absorption. The weight gain from solutions of benzyl alcohol in cyclohexanol of 10 and 50 volume percent was 2.6 and 27% respectively. The weight gain from treatment in pure benzyl alcohol was 45%. The amount of water absorbed from soaking in liquid water was about 5 % , though it must be realized that the film already contained some moisture which had been absorbed from the air (usually about 2 % ) . These figures on amount of absorption are derived from experiments carried out in a separate study ( 2 ) using 5 mil Capran 77C film. The iodine treatments were carried out in a water solution of iodine and KI (1 molar concentration of I, and 2 molar concentration of KI). The equilibrium amount of iodine absorbed in this case was 260%. Phenol and the alcoholic solvents were removed by repeated extraction with acetone, which itself had no effect upon the film. The iodine was removed by two different methods (as indicated in Figure 6) : by immersion in a fairly concentrated water solution of sodium thiosulfate, and by immersion in acetone. The stress-strain tests were run at room temperature on a table model Instron machine, at an extension rate of 100% per min. The film samples POLYMER ENGINEERING AND SCIENCE, OCTOBER, 1968, Vol. 8, No. 4
used were 1 in. in width, with 1 in. length between sample clamps. Using such a square sample might seem somewhat questionable, because the sample would be subjected to some degree of biaxial stress, rather than pure tension. However, this point was checked using samples of different length/ width ratios, and it was found that the stress-strain curve for such a square sample did not differ greatly from the curve obtained for a “long” strip (large length/ width ratio), The dynamic mechanical measurements were carried out at the Bell Telephone Laboratories through the courtesy of Dr. S. Matsuoka and Mr. J. Daane. The apparatus used was the Vibron model DDV-I1 tester, manufactured in Japan. Measurements were made over the temperature range from room temperature down to -14OoC, at a single frequency ( 110 cycles/sec). The significant transitions could be seen in this temperature range, though a still broader temperature range might be desirable. Cooling was achieved by use of a liquid nitrogen spray, and temperatures below -140” are therefore potentially possible to attain. The samples used were strips of the 1 mil film 0.5 cm in width and 2.5 cm in overall length. Masking tape was put on the ends of the strip to aid in clamping, and the length of sample between clamps was about 2.0 cm. This instrument subjects the sample to a sinusoidal tensile strain which is well within the linear viscoelastic range, and tan 6 (which is equal to E”/E’) is read directly from the instrument. The thermogravimetric ( TGA ) measurements were also carried out at the Bell Telephone Laboratories, through the courtesy of Dr. W. L. Hawkins. A temperature scan from room temperature up to about 500°C was used, with a heating rate of 15OC/ min. The samples were heated under nitrogen atmosphere.
EXPERIMENTAL RESULTS AND DISCUSSION Stress-Strain Experiments The stress-strain curve at room temperature for a sample of the original untreated film is shown in Figure 1. A stress maximum or yield peak is seen in the early part of the curve, followed by a relatively flat region and finally a region of significantly increasing stress before final fracture (which takes place between 300 and 400% extension). Stressstrain curves showing essentially the effects of different amounts of absorbed water are given in Figure 2. It is seen that the stress-strain curve for a sample soaked in water until equilibrium absorption was reached shows a lower yield peak and lower stress level in the early part of the curve. The yield peak is also less pronounced. Drying in vacuum produces the opposite effect: the yield peak is significantly increased as is also true of the stress values immediately following. Soaking the sample in acetone produces an effect similar to that of vacuum drying, suggesting that the acetone treatment essentially is removing absorbed moisture from 303
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the sample. The effect is slightly less pronounced than that of vacuum drying, however. Another very interesting effect seen in Figure 2 is that, despite differences in the early part of the stress-strain curve, the curves all seem to come together and coincide in the final region of rising stress before fracture. This final region of rising stress also seems to extrapolate back smoothly to the origin, as indicated by the dotted line in the figure. This is also true for several of the other stress-strain curves measured, and suggests an interpretation of the stress-strain curve which will be discussed later. Effects of the treatments with benzyl alcohol (in varying concentrations) and phenol are shown in Figure 3. The effect of soaking in cyclohexanol is about the same as was produced by soaking in acetone, suggesting again a dehydration effect or possible extraction of low molecular weight components. Soaking in the 10% solution of benzyl alcohol in cyclohexanol produced an effect in the same direction although somewhat less in magnitude. The effect here is opposite to that expected from plasticization. Soaking in a SO% solution of benzyl alcohol in cyclohexanol, however, produces a definite plasticizing effect, as shown by a complete disappearance of the yield peak and a considerable lowering of the stress level in the early part of
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E$ect of Solvent Treatments on the Mechanical Properties of Nylon 6
the curve. It will be noted, however, that the latter part of the curve coincides with the other curves discussed previously. Soaking in pure benzyl alcohol produces an even more extreme plasticizing effect in the early part of the curve. The latter part of the curve shows a drop in stress level as well; but even in this case, the latter part of the curve can be extrapolated back smoothly to the origin. Treatment with phenol produces the most drastic effect of all. The film shows a considerable softening; there is no yield peak, the stress level is very low in the early part of the curve, and the sample breaks quickly (before the rising region of the stress-strain curve is attained). These absorbed materials can be removed again from the polymer and the effects of such a removal on the stress-strain curve are very interesting, as seen in Figure 4. The curves shown here correspond to the treatments which produced a marked plasticization effect and a disappearance of the yield peak. When these absorbed materials are removed, the stress levels in the early part of the curve.return very closely to the values observed for the untreated film. However, the yield peak does not return. The observation of a yield peak in these curves seems to be connected with the formation of a localized neck in the sample. In the curves in which no yield peak
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was observed, a corresponding lack of neck formation in the sample was found. It is therefore clear that some permanent change has taken place in the material as a result of the plasticizing treatment, which is separate from any effect of the plasticizer being present in the sample. Irl, order to investigate this effect in greater detail, phenol was introduced and removed from the sample three times in succession. The extraction of the phenol was accomplished by immersion of the sample in 1 molar aqueous sodium hydroxide solution. The stress-strain curves obtained at different stages of this process are shown in Figure 5. The curves with phenol absorbed in the sample seem essentially identical in all cases, and the same is true of the samples from which the phenol has been extracted. However, there is a major difference between the curve for the original polymer and the curve after the first phenol treatment and extraction. After extraction of the phenol, the modulus is increased (compared to the polymer with the phenol in) but the yield peak continues to be absent and the sample shows no neck formation during drawing, as noted in connection with Figure 4. It is evident that some major change in the polymer has taken place during the first phenol treatment. It is also clear that in studying the effects
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J . Rubin and R. C . Andrews
of plasticizer on mechanical properties, the proper comparison to make is not between the original polymer and the polymer with the plasticizer absorbed (as is usually done) but between the polymer with plasticizer absorbed and the polymer with plasticizer subsequently extracted. This allows the pure plasticizer effect to be observed, rather than a combination of the effects of plasticizer and structural change. The effects of iodine treatment are shown in Figure 6. A comparison of the original curve and the curve for the sample containing iodine (about 260% by weight) indicates that the iodine produces a definite plasticizer action. The yield peak is no longer present and the entire curve shows a lowering of stress level. Removal of the iodine brings the stress level in the early part of the curve back to about the original magnitude. However the yield peak does not return and the later part of the curve does not show a return to the original stress magnitude. The later part of the curves seem approximately parallel for the samples with iodine in and with iodine removed by sodium thiosulfate solution in water. The difference in general trend of the curve for the sample from which the iodine was removed by acetone is undoubtedly related to the fact that the sample showed a marked whitening
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and milkiness after removal of the iodine. This was probably because the removal of iodine by acetone was so rapid that a void structure was produced in the sample. Removal of the iodine by thiosulfate solution was apparently a less drastic process. Dynamic Mechanical Measurements Results of the dynamic mechanical measurements plotted as E', E" and tans vs temperature are shown in Figures 7, 8 and 9. These results do not present a complete survey of the plasticizer effect; the effects of absorbed water were primarily examined. Comparison of the real part of the dynamic modulus for the original material and for film which was dried in vacuum and soaked in water in Figure 7 does not show a regular shift of the curves with moisture content. The curves show the separation expected at the highest temperatures (in the neighborhood of room temperature) but come together very closely at lower temperatures and seem to coincide at about -30°C. A sample which was treated with phenol and the phenol removed shows a higher modulus than the other samples throughout the temperature range examined, which extended down to -140°C. Three transitions can be seen in this temperature range (note curves in Figure 8): one near room temperature, one at about -60°C and the other near -140°C. The transition near room temperature ( a ) is frequently specified as the glass transition, and the low temperature transition near -140" is the gamma transition which is seen in polyethylene and other polymers (such as nylon 6 ) containing ethylenic sequences in the backbone or side groups (6). The nature of the beta transition at -60° is less clear. A discussion of these transitions can be found in a recent book by McCrum and coauthors (7). From the results shown here it is evident that the position of these transitions (temperature of the loss peak) is significantly shifted by absorbed water. The shift in the room temperature transition seems greater than is the case for the -60" transition. The data are not complete enough for the -140" transition to show the magnitude of the shift for this transition. Some difficulties have been encountered previously at Bell Telephone Laboratories in obtaining reproducible dynamic mechanical data on nylon 6 in experiments above room temperature. The same was found to be true in some preliminary experiments above room temperature with our samples. This is probably due to absorption and desorption of moisture by the sample as the temperature is changed during the experiment. The good results presented in Figures 7-9 are therefore undoubtedly due to the fact that the samples were never heated above room temperature. In the work of Deeley et a1 ( 8 ) curves of tan 6 vs temperature for a dried nylon 66 sample showed no p peak, suggesting that the presence of this peak is related to the presence of absorbed water. However, Kawaguchi (9) studied a series of polyPOLYMER ENGlNEERlNG AND SCIENCE, OCTOBER, 1968, Vol. 8, No.4
E f e c t of Solvent Treatments on the Mechanical Properties of Nylon 6
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amides, all dried over Pz05for a month, and found that these dried nylons all exhibited the beta peak. Our results are thus in agreement with Kawaguchi’s, since we see the beta peak in our dried sample. Thermogravimetric Measurements Results of the TGA measurements on the original film and two treated samples are shown in Figure 10.
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The original film showed a weight loss of 2.7% between room temperature and 3OO0C, and most of this weight loss occurred at temperatures significantly above 100°C, indicating a very diffuse and elevated “boiling point” ( or vaporization temperature) for this absorbed water. The samples soaked in water showed a weight loss of only 3.7% in the same temperature range, which is not much greater than that of the original film. This low value may
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be a result of water loss from the sample while it is being prepared for the TGA analysis. Measurement of sample weight immediately after removal from water indicated a 4.7% weight gain, as compared to the original sample, and adding this to the 2.7% measured above would give a total water content for the soaked film of 7.40j0, which is a much more reasonable value. For some reason, the soaked sample showed the onset of weight loss well below 100°C, in contrast to the behavior of the original film. The phenol-treated sample showed a 38.5% weight loss between room temperature and 300OC. This took place gradually in the range of 70-240°C, again indicating a very diffuse vaporization temperature for the absorbed plasticizer. The boiling point of pure phenol is 181OC. ( I t is also of interest that the onset of high-temperature degradation takes place at a lower temperature in the sample containing phenol than in the samples containing absorbed water. ) A direct weighing experiment showed that the film after absorption of phenol had a weight increase of 70.5% (based on untreated sample weight), which would convert to a weight loss of 41.3% based on the treated sample weight. This agrees fairly well with the 38.5% value measured in the TGA experiment; some sublimation of phenol may have taken place during preparation of the sample for the TGA measurement, which would account for the small remaining discrepancy.
CONCLUSIONS The results presented here must be regarded as a preliminary investigation of the effects of solvent treatment on the mechanical properties of a crystalline polymer. However, three general conclusions 308
can be drawn from the present results, as follows. 1) The plasticizer effect of absorbed water, benzyl alcohol or phenol seems to correlate fairly directly with the amount of material absorbed, irrespective of its exact nature. The plasticizing effects therefore seem to be surprisingly independent of the chemical structure. The effects produced by absorbed iodine seem very similar in nature to those produced by the other agents (particularly benzyl alcohol), but a larger amount of iodine is required to produce an effect of the same magnitude. 2 ) In order to make correct measurements of the nature of the plasticizer effect, it is necessary to do the experiments in the proper way. When solvents or other plasticizers are introduced into a semicrystalline polymer, this often produces very significant changes in the nature and amount of crystallinity present. Therefore, when the behavior of an untreated sample is compared with the behavior of a sample in which plasticizer has been absorbed, the difference in behavior observed is actually due to two causes : the change in crystal structure produced by the introduction of the plasticizer, and the effect of the plasticizer on the behavior of the altered structure. We have found from experiments in which phenol was repeatedly introduced and removed, that the change in crystalline structure takes place entirely during the first introduction of the plasticizer. Therefore, in order to observe the pure plasticizer effect, it is necessary to compare a sample containing plasticizer with a sample in which the plasticizer has been introduced and then subsequently removed. It is probably not possible to study the original structure in both plasticized and unplasticized forms. 3) The present study has also suggested an inPOLYMER ENGINEERING AND SCIENCE, OCTOBER, 1968, Vol. 8, No. 4
Effect of Solvent Treatments on the Mechanical Properties of Nylon 6
terpretation of the stress-strain curve of nylon 6 in terms of the structure of the polymer. It seems possible to regard the stress-strain curve as being made up of two components. Component I is seen in essentially pure form in the latter part of the stressstrain curve, in which the stress is continuously rising. This region of the stress-strain curve can be extrapolated smoothly back to the origin as indicated in Figure 2, and this (partially extrapolated) curve will represent the contribution of this component of the polymer structure to the entire stress-strain curve. This component of the behavior appears to be related to the nature of the crystal form present. This curve shows a continuous monotonic increase with increasing strain. Superimposed on this Component I is a Component I1 which is observed at lower strains and which is progressively broken down by increasing stress or increasing strain. This is the component in which the yield peak is observed and therefore must represent the component of the physical structure in which the initial yield takes place. The value of the yield stress is therefore determined by the nature of this component. The pure plasticizer effect is probably localized in this part of the structure. If the structure of a semi-crystalline polymer is visualized as being a two-phase structure consisting of crystalline and amorphous components, Component I must be related to the crystalline component and Component I1 must be related to the amorphous component. The structure might be regarded as a crystalline lattice imbedded in an amorphous matrix. Although we arrived at this interpretation independently, we have discovered recently that a very closely related analysis of the stress-strain curve of nylon 6 in terms of the solid state structure was proposed in 1956 by Yumoto (10). The mild plasticizing effect of water shown in Figure 2 seems to produce no significant change in Component I. The water softens Component I1 and reduces the yield stress or stress required to break the corresponding physical structure down. In Figure 3 it is seen that only the benzyl alcohol and phenol produce any drastic alteration in Component I. The 50/50 mixture of benzyl alcohol and cyclohexanol produces no change in structure I, although it produces quite a drastic softening effect in structure 11. The reason for the lack of reappearance of the yield peak when the plasticizers are removed (see, e.g., Figure 4 ) is not yet clear. From our infrared and x-ray studies on these samples ( l ) ,it is observed that the treatments with benzyl alcohol and phenol convert the structure originally present in the film to an alpha crystalline form. Treatment with iodine, on the other hand,
POLYMER ENGINEERING AND SCIENCE, OCTOBER, 1968, Vol. 8, No. 4
converts the structure to a gamma crystalline form. The alternation in the high strain region of the curve produced by iodine treatment, as seen in Figure 6, is undoubtedly a result of the production of this gamma form. The high strain region corresponding to the gamma form appears to show a more gradual stress increase than that corresponding to the alpha form. Examination of the curve in Figure 6 for the sample treated with iodine and the iodine removed by thiosulfate shows a high strain region which does not extrapolate back to the origin. This is probably due to an altered physical structure (containing voids?) somewhat similar to that for the sample in which the iodine was removed by acetone. Further studies are needed to investigate these effects and interpretations in more detail. A very significant fact relevant to the interpretation of the stress-strain curves, which has not been examined here, is the reversibility and recoverability of the curves; this could be seen, for example, from measurements of stress-strain cycles. It would be desirable to follow the changes of crystalline form throughout the course of the stress-strain curve. It would also be desirable to carry out parallel studies involving mechanical measurements of other types, such as creep.
ACKNOWLEDGMENTS One of the authors (J.R.) would like to thank the Plastics Institute of America for the award of a post-doctoral fellowship during the academic year 1966-67. The authors would also like to express their appreciation to the U. S. Army Natick Laboratories for financial support of this work. REFERENCES 1. R. D. Andrews, J. Rubin and B. K. Tsai, unpublished data. 2. V. A. Lathiya, S. R. Shoney and R. D. Andrews, unpublished data. 3. Y. Sakuma and L. Rebenfeld. J . Appl. Polymer Sci. 10, 637 (1966). 4. S. Ueda and T. Kimura, Kobunshi Kagaku 15, 249 (1958). 5. M. Yoshida and M. Endo, Kogyo Kagaku Zasshi 59, 1074 (1956). 6. R. D. Andrews and T. J. Hammack, j . Polymer Sci. B3, 655,659 ( 1965). 7. N. G. McCrum, B. E. Read and G. Williams, Anelastic and Dielectric Effects in Polymeric Solids, John Wiley & Sons, New York (1967). 8. C. W. Deeley, A. E. Woodward and J. A. Sauer, J . Appl. Phys. 28, 1124 ( 1957). 9. T. Kawaguchi, J. Appl. Polymer Sci. 2, 56 (1959). 10. H. Yumoto, Bull. Chem. Soc. Japan 29, 141, 353 (1956).
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