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Graft copolymers as elastomeric fibers—I synthesis of the graft copolymers
International Journal of Applied Radiation and Isotopes, 1975, Vol. 26, pp. 159-168. Pergamon Press. Printed in Northern Ireland
Graft Copolymers as Elastomeric Fibers I Synthesis of the Graft Copolymers J. L. W I L L I A M S , D. K. W O O D S and V. S T A N N E T T * Camille Dreyfus Laboratory, Research Triangle Institute, Post Office Box 12194, Research Triangle Park, North Carolina 27709, U.S.A. and S. B. S E L L O and C. V. S T E V E N S J. P. Stevens & Co., Inc. Research & Devdopment Dept., 141 Lanza Avenue, Garfield, N.J. 07026, U.S.A.
(Received 19 August 1974) Pre-irradiation grafting, mainly with an ethyl acrylate emulsion system, has been used to produee wool and cellulosic fibers having el~tomerie properties. Comparative studies using selected chemically-initiated systems were also carried out. The pre-irradiation grafting-time curves were found to be auto-accelerative and grafting levels in excess of 3000 per cent could be achieved. Circa 1000 per cent graft was required to obtain fibers with high "rubber-like" elasticity. Grafting rates were found to increase substantially with temperature reducing the grafting times necessary to achieve high elasticity to the order of minutes at 35oc compared to 20 hr at ambient temperatures. Reduction of the level of grafting required to obtain elastomeHc properties is clearly of interest, and it was found that when rayon fibers containing low levels of graft (below 100 per cent) were subjected to post-treatments with reagents capable of decrystallizing cellulose, the fibers, which heretofore exhibited no significant elastomeric properties, became highly elastic. INTRODUCTION IN AN earlier study, a) it was reported that wool could be rendered highly elastic by a preirradiation grafting technique. Subsequent work has shown that similar results could be obtained with cotton and rayon, ~2'8) however, these systems required approximately 1200 per cent graft to achieve high elasticity. Interesting work along similar lines with cellulosic fibers has also been presented by Nv.GIsni et al. (4~ and by NAKAMURA et al.~5'6) I n all of these earlier investigations high levels of grafting were required to achieve rubber-like elastic properties, Details of the grafting processes in the case of wool and rayon have been examined and will be discussed below. I n particular, careful attention has been given to the possibility of reducing the degree of grafting necessary to achieve these highly elastic properties. I n this regard, a posttreatment of the grafted rayon fiber has been found to render a moderately grafted fiber highly elastic. Details of this post-treatment and * Present address: Dept of Chemical Engineering, North Carolina State University, Raleigh, N.C. 27607.
its ultimate influence on the mechanical properties will be discussed both here and in the following p a p e r J ~) EXPERIMENTAL A continuous filament "semi-duU" rayon was used in this work. All samples were extracted with methanol and rinsed several times in distilled water prior to grafting. A material balance study indicated that there was less than 1 per cent weight loss during the extraction procedure. Repeated extractions did not result in any change in weight. A Beltsville wool (56s grade), having an average fiber diameter of 22 #m, was also used in this work. Prior to grafting the wool was extracted with ethanol and diethyl ether (8 hr each).
Grafting via pre-irradiation After extraction, samples were dried in a v a c u u m oven for 16 hr at 60°C, wound onto glass bobbins and weighed. Each sample was then transferred to a pre-irradiation grafting tube and evacuated at approximately 10- s T o r t for 16 hr. T h e tubes were then sealed a n d 159
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J . L . Williams, D. K. Woods, V. Stannett, S. B. Sello and C. V. Stevens
placed in a cobalt-60 gamma source for irradiation. Following irradiation, the respective solutions were placed in a side-arm tube which was separated from the sample tube by means of a glass break-seal. Each solution was then degassed by three freeze-thaw cycles and then added to the rayon sample. The samples were then placed in a constant temperature bath for various periods of time, with slight agitation to maintain the emulsion. T h e emulsion-yarn weight ratio was approximately 300. At the completion of grafting, the sample tubes were opened and the samples washed with tap water, extracted with chloroform or acetone for 16 hr. T h e samples were dried in a vacuum oven and weighed. I t was found that the extraction procedure of yarn samples could be facilitated considerably if the yarn was rewound onto a porous spool (e.g. a "hair curler") prior to extraction. When this procedure was not followed, evidence of small nodules of homopolymers could at times be found on the extracted samples. When the procedure was followed, no such imperfections could be found on microscopic examination. Grafting via chemical initiation Geric-ion method. Work with chemical initiator systems has been carried out using a polymerization apparatus constructed with four parts through which samples could be placed and removed after various time intervals. A central port was fitted with a gas dispersion tube through which dry nitrogen could be bubbled into the monomer solution. Each port was covered with a serum cap to prevent back diffusion of oxygen into the system. T h e needle on the syringe containing the initiator solution was inserted through one serum cap allowing nitrogen to bubble up through the initiator. After the solutions were flushed with nitrogen for a prescribed time (usually 30 man) the initiator was added. Samples were removed after various periods of time and extracted as described above. T h e graft emulsion contained 2.3-3.1 ~o ethyiacrylate, 0"27-1-8~o Ce(NHd)a(NOa)e and 0-63~o HNOa at an emulsion-yarn weight ratio of approximately 3100. Gra~ng of ion-exchange rayon via FeS+-HaOs initiation. Phosphorylated rayon yarn containing 1-3 ~o P was prepared by padding small yarn
skeins with 3"5, 7.0, or 10"6 ~ phosphoramide based on weight of yarn, drying at 65°C, and curing for 2 rain at 200°C in a forced draft oven. Samples were washed in dilute Na~COs, neutralized, rinsed and dried. T h e grafting method of G A L L A G H E R t8) W a s then followed. Silver-ion method.(°~ Emulsions of 5, 10, or 20~o ethylacrylate, 0 . 0 0 5 ~ ammonium persulfate, and 1.0 ~o Triton X-405 were used for the grafting step at an emulsion-yarn weight ratio of approximately 300 after deposition of colloidal silver in the yarn. Post-decrystallization Grafted samples, wound on a U-shaped glass rod, were exposed to 68 ~o aqueous ZnC12 solution for 20 rain at 68°C or 70 ~o ZnCls for 30 min at 20°C. T h e y were rinsed with water, neutralized, and dried under reduced pressure over a mixture of CaSOd-CaCI s. RESULTS
AND D I S C U S S I O N
Preirradiation grafting to wool Prior work on the mutual irradiation grafting of vinyl monomers to wool ~1°) has shown that to achieve substantial grafting the substrate must be suitably swollen in order to facilitate the diffusion of monomer to the actively growing sites. Under such conditions of swelling, grafting proceeds readily with the extent of grafting determined mainly by the exact swelling conditions employed and the reactivity of the monomer. Certain monomers, such as the acrylics, do not lend themselves readily to the mutual technique of grafting due to their high sensitivity to radiation. In order to graft such monomers, a preirradiation grafting technique has been used which does not require direct irradiation of the monomer solution. Swelling conditions, however, can be varied in much the same manner as with the mutual technique. In the early part of this study, it was found that when grafting experiments with ethyl acrylate were carried out under swelling conditions with water, very substantial grafting yields resulted (Fig. 1). At the high levels of graft (ca. I000 per cent) the wool samples were highly elastic and possessed good recovery propertles. When experiments were repeated incorporating a mutual solvent, only low levels of
Graft copol.yra~s as elastomeric fibresnI 7000
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Experiments carried out to determine the exact influence of reaction time on the grafting yield using various amounts of water in the m o n o m e r solution indicate (Table 1) that grafting yields continue to increase with contact time for a given m o n o m e r concentration. Also, it is evident that the grafting yields increase with increasing m o n o m e r concentrations of up to 80yo. T h e samples containing the higher amounts of graft polymer in all cases possessed highly elastic properties. Visual examination of the grafted samples, however, indicated that grafting was not uniform throughout the sample due to the immiscibility of water and ethyl
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Fro. 1. Preirradiation grafting of ethyl acrylate to wool at 25°C (monomer concentration = 60%, dose = 4.8 Mrads at 0.1 Mrads/hr). graft were obtained (Fig. 2), and the resulting samples did not possess elastic properties. U n doubtedly, the inclusion of dioxane or acetone in the grafting solution reduces the activity of water in regards to its swelling characteristics for wool. As a consequence, the grafting yields level off at considerably lower values than in the absence of a mutual solvent.
% Water
Dose (Mrads)
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10 20 40 80 33 33 33 67 67 67
4-0 4.0 4.0 4.0 4.2 4.2 4.2 4.3 4.3 4.3
46.0 46.0 46.O 46.0 3 21 67.5 23.0 44.8 68.8
30.2 5476.2 2276.3 645.7 22.3 97.9 784.9 32.0 234.4 3216-8
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FIo. 2. Effect of water as a swelling agent on the prelrradlatlon grafting o£ ethyl acrylate to wool at 25°C (0-18% H~O, 30% monomer and 52% dioxane; 0 - - 3 5 % H=O, 15% monomer and 50% acetone, d o s e = 4-0 Mrads at 0.I Mrad/hr).
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J. L. Williams, D. K. Woods, IT. Stannett, S. B. Sello and C. V. Stevens
acrylate. These results indicated that a welldispersed grafting system was needed which would allow maximum swelling by water. Therefore, subsequent experiments were tried using an emulsifier to disperse the monomer in the water phase. Several of the resulting grafting curves using a nonionic polyoxyethylene type emulsifier are shown in Fig. 3. At this particular ethyl acrylate concentration, the grafting yields clearly increase as the emulsifier concentration decreases from 3.2 to 0.8 ~o. I t is also evident that these curves do not reach a plateau grafting level as is normal with preirradiation grafting, but continue to increase with time. Closer examination of the grafting curves indicates that there exists a sharp break in the grafting curve, where the initial rate suddenly changes to a considerably faster rate. Examination of the grafted samples taken from the two regions of the grafting curve indicates that there is also a large difference in the elastic properties of the samples taken from the two regions of the curve. I n general, samples taken from above the rough transition point possess very good elastic 7000
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FIo. 3. Effect of emulsifier concentration on the preirradiation grafting of ethyl acrylate to wool at 25°C. ( A - - 0 . 8 % Triton X-100, 33% water; O--1"6% Triton X-100, 33% water; 0-3.2% Triton X-100, 33% water; O---No emulsifier Dose -----4.8 Mrads at 0-1 Mrad/hr).
properties, while those taken from this point were not greatly changed. Experiments were carried out using different surfactants to determine if the grafting reaction might be sensitive to the type of emulsifier used. The results of these experiments using sodium lauryl sulfate (anionic), Triton X-100 (nonionic), and Triton X-405 (nonionic) indicate that a high level of grafting can be obtained with each type of surfactant examined. Studies of the influence of monomer concentration in the emulsion using both Triton X-100 and Triton X-405 were performed. The main chemical difference between Triton X-100 and X-405 is in the molecular weight of the polyoxyethylene side-chain, that of the latter emulsifier being approximately fourfold higher. Preirradiation grafting curves using Triton X-405 (Fig. 4) indicates that grafting yields for a given contact time increase with increasing monomer concentration over the range studied. Similar results were found with Triton X-100; however, grafting yields at a given monomer concentration were considerably lower than when using the higher molecular weight Triton X-405. In addition, these curves indicate that the grafting rates continue to decrease with increasing amounts of water. As a consequence, the time corresponding to the rough transition points are shifted to higher values as the monomer concentration is decreased. Also, the abruptness of the transition decreases with decreasing monomer concentration. Beyond the transition point, a small change in time makes a large difference in the grafting level because of the high grafting rate in this region. I n order to lower the grafting rate for a given monomer concentration and thus achieve more experimental control over the grafting level, a portion of the water was replaced with methanol, which is a somewhat poorer swelling agent for wool. Surprisingly, the graft rate dropped tremendously upon the addition of 23 methanol to the grafting solution (Fig. 5) in spite of the increased solubilization of the monomer. There was no longer a transition point in the grafting curves and the grafting rates continued to decrease with time. Even small amounts of methanol added to the system, exhibited pronounced effects on the rate of grafting (Table 2). Low levels of grafting were
Craft co#olym~s as elastomericflbres--I
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FIo. 4. Effect of monomer concentration on the preirradiation grafting of ethyl acrylate to wool using 3.2% Triton X-405 at 25°C (0---60% monomer; [2---87% monomer; A--20% monomer. Dose = 4.8 Mrads at 0.1 Mrad/hr). which has been shown previously also for the mutual grafting of styrene to wool. tl°) I t is also possible that chain transfer to the methanol to give the mobile .CH=OH radical is responsible for this result. Such a radical could diffuse out of the polymer matrix and eventually terminate in the liquid phase. Several attempts were made to digest away the wool backbone by sodium hypochlorite treatments and to measure the molecular weight of the grafted poly(acrylates). Upon isolation of the grafted side chains however, it was observed that they were insoluble in usual solvents for such acrylic polymers, indicating that network formation had taken place during grafting.
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Fxo. 5. Effect of methanol on the preirradiation grafting of ethyl acrylate to wool at 25°C ([7-0% MeOH; 0---23.0% MeOH; 0---6"3% MeOH; and A--1.6% MeOH. All suspensions contained 60% monomer. Dose = 4.8 Mrads at 0.1 Mrad/hr). obtained, in the presence of a small percentage of methanol even after prolonged reaction times. Evidently, these small concentrations of methanol in the emulsion system are capable of severely reducing the efficiency of water as a swelling agent. This again illustrates the unique properties of water in promoting the grafting reaction,
Pre-irradiation grafting to cellulosicfibers Although cellulose fibers are extremely different chemically and morphologically from wool fibers, both can be characterized by a fibril-type structure embedded in a less ordered matrlx.ma2~ Since the grafting reaction predominately takes place in these less ordered regions, tla) experiments were designed to attempt to obtain elastic properties with cellulosic fibers similar to those obtained with wool. Also, the possibility of having a continuous filament elastic fiber presented itself in the case of regenerated cellulose. T h e preirradiation grafting of ethyl acrylate to regenerated cellulose was found to proceed in
164
J. L. Williams, D. K. Woods, IT. Stannett, S. B. Sello and C. V. Stevens
TABLE 2. The influence of methanol on the preirradiation grafting of ethyl acrylate to wool from emulsion Swelling conditions % Methanol
% Water
50.0 50.0 50-0 10"0 10-0 2.7 2.7 0
50'0 50.0 50.0 90.0 90.0 97-3 97.3 100
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Reaction time (hr)
% Graft
2'3 4.8 4.8 4.6 4-6 4.1 4.1 4.1
24.5 52.0 96.5 52.0 71.0 47.0 142.0 50.0
21.52 19-07 18.24 6-1 28'2 27-0 53.2 2290.0
60% monomer concentration. Dose rate: 0.1 Mrad/hr. a similar fashion as that to wool. After approximately 20 hr reaction time at 25°C, the grafting rate, rather suddenly, accelerates in m u c h the same m a n n e r as found with wool (Fig. 6). Again examination of the grafted samples above the ./
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rough transition point in the grafting curve indicates they are highly elastic while those below this inflection point are essentially inelastic. T h e times corresponding to the rough transition point are quite long for practical purposes and it was thought that, due to the diffusion controlled nature of the reaction, these times might be considerably reduced by simply increasing the temperature• I n Fig. 6 it can also be seen that grafting rates do increase over the range of 25-55°C, and the times necessary to achieve grafting levels required for high elasticity are reduced from 20 hr at 25°C to approximately 40 min at 35°C, a 30-fold decrease. A dramatic increase in initial grafting rate occurs between 25 and 35°C. Further experimentation indicated that this abrupt change occurs at about 32°C. Experiments have been carried out at temperatures up to 75°C, but grafting rates do not increase substantially beyond 55°C and the degree of homopolymer formation begins to increase at the higher temperatures for these m o n o m e r concentrations. This homopolymer formation is evidenced by a decrease in the grafting rates due to premature reduction in the m o n o m e r concentration in the grafting solution and by visible evidence of occluded homopolymer on the grafted rayons. T h e optim u m temperature to achieve high grafting rates with m i n i m u m h o m o p o l y m e r formation is around 45°C. T h e influence of total dose on the grafting rates a n d thus the overall grafting time is quite
GraJ~ ¢opolymers as elastomericfibres--I pronounced, as can be seen by the results presented in Fig. 7. The initial grafting rate is increased approximately tenfold when the total dose is increased by a similar factor from 0.45 Mrads to 4-8 Mrads. However, beyond a total dose of approximately 6.0 Mrads, the grafting rates are not increased considerably. Samples taken at the higher total doses appear to possess very good elastic properties in comparison to samples grafted to similar levels at lower doses; perhaps higher total doses assure a greater uniformity of radical population in the sample. The radical buildup in rayon prior to exposure to monomer solution was measured by electron spin resonance techniques, and it is shown (Fig. 8) that the radical concentration continues to increase up to a dose of circa 6.0 Mrads. Beyond this point, the radical concentration is essentially stationary. Any further increases in grafting level beyond the steadystate point must then be explained in terms of 14.3 MRADS, O 4.8 MRAOS- ZI O . J *0 (45"C) 1400
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chain cleavage, e.g. by increasing accessibility. The incorporation of a small percentage of bifunctional monomer in the grafting solution offered the possibility of varying the type and degree of elasticity of the modified rayon, i.e. a small degree of cross-linking could increase both the modulus and breaking tenacity. The use of N-methylol acrylamide as cross-linking monomer gave the best results from both the viewpoint of properties and grafting behavior. N-methylol acrylamide (MAA) is very soluble in the water phase and not in the ethyl acrylate, and the grafting curves paralleled the grafting behavior exhibited by pure ethyl acrylate. Grafting yields drop considerably with increased concentration of N-methylol acrylamide (Fig. 9). This reduction in grafting is due mainly to the reduced ethyl acrylate concentration in the monomer feed and not to an inhibiting effect of MAA, as inferred from the fact that grafting yields in the absence of MAA continue to increase with increasing concentrations of ethyl acrylate over the entire range studied (Fig. 10). Examination, however, of the MAA containing grafting emulsions indicated that, at high MAA concentrations, considerable cross-linking did occur in emulsion.
Chemically-initiated grafting to ¢ellulosic fibers In addition to the irradiation grafting studies, a typical chemical initiation method was also explored• Work here was centered around the eerie ion method, using eerie ammonium nitrate as the initiator. This method relies on the presence of hydroxyl groups on the substrate and is therefore readily applicable to rayon and other cellulosics, i.e.
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J. L. Williams, D. K. Woods, V. Stannett, S. B. Sello and C. V. Stevens
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tion dominates and grafting yields drop. Results are also quite irreproducible at the higher initiator concentrations due, undoubtedly, to the heavy homopolymerization at these concentrations. In an attempt to improve the strength properties of the grafted fiber, acrylonitrile was added to the ethyl acrylate grafting system as a
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it can be seen that grafting yields continue to increase with increasing initiator concentration up to about 2.0 raM. When higher initiator concentrations are employed, homopolymeriza-
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zinc chloride solution and subsequent rinsing in distilled water. As a result of the post-decrystallization procedure, such sample was found to have 450 per cent elongation at break, compared to only 15 per cent elongation for the original grafted fiber, uS) Accompanying this increase in elongation is a large decrease in tenacity. Attempts to render chemically-grafted rayons highly elastic at low grafting levels by postdecrystallization were not successful. The differences between the results yielded by the two grafting systems are also discussed in Part II. In conclusion, it can be stated that an irradiation grafting process has been developed whereby fibers, films, or fabrics can be rendered highly elastic with elongations approaching those of spandex materials. Conventional fibers such as wool, cotton and rayon are extremely well suited for this method for modification. The most significant part of this study is the finding that high rubber-like elasticity can be achieved at very low grafting levels ( < 70 per cent) if the lightly grafted fibers are subjected to a postdecrystallization treatment. In terms of practical applications, the process offers many unique 100% EA 600
comonomer. In Fig.12, it is evident that the presence of acrylonitrile considerably decreased the overall grafting rate.
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Post-decrystallization of graft copolymers A full account of the mechanical properties and morphology of the grafted fibers is presented in Part I I of this work. (7) However, it is worthwhile to briefly mention at this point the importance of the post-decrystallization treatment. Examination of the properties of the grafted fibers indicate that circa 1000 per cent graft is generally required to achieve highly elastic properties. At this grafting level, both irradiation and chemical grafting methods are applicable. It has been found however, that if the pre-irradiated, grafted fiber is post-decrystallized by swelling in a suitable solvent, high elasticity can be achieved at far lower grafting levels. In fact, rayon fibers with less than 70 per cent polyethylacrylate graft via the preirradiation technique become highly elastic following a post-decrystallization treatment in 70 ~o aqueous
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168
J. L. Williams, D. K. Woo~, V. Stannett, S. B. Sello and C. V. Stevens
possibilities of rendering a preformed material or fiber assemblies totally elastic or of imparting local or uni-directional stretch properties to a structure. REFERENCES I. (a) WILLIAMSJ. L. and STANNETTV., Text. Res. J. 38, 1065 (1968); (b) WILLL~MSJ. L., WooDs D. K., STAm~'r V., ROLVANL. G., SBLLOS. B. and STEVENSC. V. Textile. Res. J. 43, 205 (1973). 2. WILLIAMSJ. L. and STAI~mTTV. Polymer Lett. 8, 711 (1970). 3. WILLIAMSJ. L. and STAm~TT V. I doEC Product R d~ D 11, 211 (1972). 4. NEOISHXM., NAICAMURAY. and Hosox M. J. appl. Polymer Sci. 12, 1209 (1968). 5. NAX~AMURAY:, I-I~oJOSAO. and ARTHURJ. C. J. appl. Polymer S~. 14, 789 (1970).
6. NAKAMU~ Y., HINOJOSAO. and ARTHURJ. C. J. appl. Polymer Sci. 14, 929 (1970). 7. WILl.JAMSJ. L., STANNETTV., ROLDAN L. G., SELLO S. B. and STEVENS C. V. Int. J. appl. Radiat. Isotopes 26, 169 (1975). 8. GALLAOHRRD. M. Text. Res. J. 40, 621 (1970). 9. HOROWXTZC. U.S. Patent 3,376,168 (1968). 10. STANNETTV., ARAtaK., GERVASXJ. and McI~sKY S. J. Polymer Scl. A-2, 3763 (1965). 11. ALEXANDER P. and HuDsON R. Wool: Its Chemistry and Physics. Reinhold, NewYork (1954). 12. MAw_.~YR. Nature, Lond. 204, 1155 (1964). 13. TRmP W., MOORE A., I)EGRuY I. and ROLLINS M. Text. Res. J. 30, 140 (1960). 14. SCHWAB E., ST~SETT V., RAKowrrz D. and MAORA~ J. Tappi 5, 390 (1962). 15. WmLIAMSJ. L. and STA~NETTV. Polymer Lett. 10, 665 (1972).
Graft Copolymers as Elastomeric Fibers I Synthesis of the Graft Copolymers J. L. W I L L I A M S , D. K. W O O D S and V. S T A N N E T T * Camille Dreyfus Laboratory, Research Triangle Institute, Post Office Box 12194, Research Triangle Park, North Carolina 27709, U.S.A. and S. B. S E L L O and C. V. S T E V E N S J. P. Stevens & Co., Inc. Research & Devdopment Dept., 141 Lanza Avenue, Garfield, N.J. 07026, U.S.A.
(Received 19 August 1974) Pre-irradiation grafting, mainly with an ethyl acrylate emulsion system, has been used to produee wool and cellulosic fibers having el~tomerie properties. Comparative studies using selected chemically-initiated systems were also carried out. The pre-irradiation grafting-time curves were found to be auto-accelerative and grafting levels in excess of 3000 per cent could be achieved. Circa 1000 per cent graft was required to obtain fibers with high "rubber-like" elasticity. Grafting rates were found to increase substantially with temperature reducing the grafting times necessary to achieve high elasticity to the order of minutes at 35oc compared to 20 hr at ambient temperatures. Reduction of the level of grafting required to obtain elastomeHc properties is clearly of interest, and it was found that when rayon fibers containing low levels of graft (below 100 per cent) were subjected to post-treatments with reagents capable of decrystallizing cellulose, the fibers, which heretofore exhibited no significant elastomeric properties, became highly elastic. INTRODUCTION IN AN earlier study, a) it was reported that wool could be rendered highly elastic by a preirradiation grafting technique. Subsequent work has shown that similar results could be obtained with cotton and rayon, ~2'8) however, these systems required approximately 1200 per cent graft to achieve high elasticity. Interesting work along similar lines with cellulosic fibers has also been presented by Nv.GIsni et al. (4~ and by NAKAMURA et al.~5'6) I n all of these earlier investigations high levels of grafting were required to achieve rubber-like elastic properties, Details of the grafting processes in the case of wool and rayon have been examined and will be discussed below. I n particular, careful attention has been given to the possibility of reducing the degree of grafting necessary to achieve these highly elastic properties. I n this regard, a posttreatment of the grafted rayon fiber has been found to render a moderately grafted fiber highly elastic. Details of this post-treatment and * Present address: Dept of Chemical Engineering, North Carolina State University, Raleigh, N.C. 27607.
its ultimate influence on the mechanical properties will be discussed both here and in the following p a p e r J ~) EXPERIMENTAL A continuous filament "semi-duU" rayon was used in this work. All samples were extracted with methanol and rinsed several times in distilled water prior to grafting. A material balance study indicated that there was less than 1 per cent weight loss during the extraction procedure. Repeated extractions did not result in any change in weight. A Beltsville wool (56s grade), having an average fiber diameter of 22 #m, was also used in this work. Prior to grafting the wool was extracted with ethanol and diethyl ether (8 hr each).
Grafting via pre-irradiation After extraction, samples were dried in a v a c u u m oven for 16 hr at 60°C, wound onto glass bobbins and weighed. Each sample was then transferred to a pre-irradiation grafting tube and evacuated at approximately 10- s T o r t for 16 hr. T h e tubes were then sealed a n d 159
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J . L . Williams, D. K. Woods, V. Stannett, S. B. Sello and C. V. Stevens
placed in a cobalt-60 gamma source for irradiation. Following irradiation, the respective solutions were placed in a side-arm tube which was separated from the sample tube by means of a glass break-seal. Each solution was then degassed by three freeze-thaw cycles and then added to the rayon sample. The samples were then placed in a constant temperature bath for various periods of time, with slight agitation to maintain the emulsion. T h e emulsion-yarn weight ratio was approximately 300. At the completion of grafting, the sample tubes were opened and the samples washed with tap water, extracted with chloroform or acetone for 16 hr. T h e samples were dried in a vacuum oven and weighed. I t was found that the extraction procedure of yarn samples could be facilitated considerably if the yarn was rewound onto a porous spool (e.g. a "hair curler") prior to extraction. When this procedure was not followed, evidence of small nodules of homopolymers could at times be found on the extracted samples. When the procedure was followed, no such imperfections could be found on microscopic examination. Grafting via chemical initiation Geric-ion method. Work with chemical initiator systems has been carried out using a polymerization apparatus constructed with four parts through which samples could be placed and removed after various time intervals. A central port was fitted with a gas dispersion tube through which dry nitrogen could be bubbled into the monomer solution. Each port was covered with a serum cap to prevent back diffusion of oxygen into the system. T h e needle on the syringe containing the initiator solution was inserted through one serum cap allowing nitrogen to bubble up through the initiator. After the solutions were flushed with nitrogen for a prescribed time (usually 30 man) the initiator was added. Samples were removed after various periods of time and extracted as described above. T h e graft emulsion contained 2.3-3.1 ~o ethyiacrylate, 0"27-1-8~o Ce(NHd)a(NOa)e and 0-63~o HNOa at an emulsion-yarn weight ratio of approximately 3100. Gra~ng of ion-exchange rayon via FeS+-HaOs initiation. Phosphorylated rayon yarn containing 1-3 ~o P was prepared by padding small yarn
skeins with 3"5, 7.0, or 10"6 ~ phosphoramide based on weight of yarn, drying at 65°C, and curing for 2 rain at 200°C in a forced draft oven. Samples were washed in dilute Na~COs, neutralized, rinsed and dried. T h e grafting method of G A L L A G H E R t8) W a s then followed. Silver-ion method.(°~ Emulsions of 5, 10, or 20~o ethylacrylate, 0 . 0 0 5 ~ ammonium persulfate, and 1.0 ~o Triton X-405 were used for the grafting step at an emulsion-yarn weight ratio of approximately 300 after deposition of colloidal silver in the yarn. Post-decrystallization Grafted samples, wound on a U-shaped glass rod, were exposed to 68 ~o aqueous ZnC12 solution for 20 rain at 68°C or 70 ~o ZnCls for 30 min at 20°C. T h e y were rinsed with water, neutralized, and dried under reduced pressure over a mixture of CaSOd-CaCI s. RESULTS
AND D I S C U S S I O N
Preirradiation grafting to wool Prior work on the mutual irradiation grafting of vinyl monomers to wool ~1°) has shown that to achieve substantial grafting the substrate must be suitably swollen in order to facilitate the diffusion of monomer to the actively growing sites. Under such conditions of swelling, grafting proceeds readily with the extent of grafting determined mainly by the exact swelling conditions employed and the reactivity of the monomer. Certain monomers, such as the acrylics, do not lend themselves readily to the mutual technique of grafting due to their high sensitivity to radiation. In order to graft such monomers, a preirradiation grafting technique has been used which does not require direct irradiation of the monomer solution. Swelling conditions, however, can be varied in much the same manner as with the mutual technique. In the early part of this study, it was found that when grafting experiments with ethyl acrylate were carried out under swelling conditions with water, very substantial grafting yields resulted (Fig. 1). At the high levels of graft (ca. I000 per cent) the wool samples were highly elastic and possessed good recovery propertles. When experiments were repeated incorporating a mutual solvent, only low levels of
Graft copol.yra~s as elastomeric fibresnI 7000
161
Experiments carried out to determine the exact influence of reaction time on the grafting yield using various amounts of water in the m o n o m e r solution indicate (Table 1) that grafting yields continue to increase with contact time for a given m o n o m e r concentration. Also, it is evident that the grafting yields increase with increasing m o n o m e r concentrations of up to 80yo. T h e samples containing the higher amounts of graft polymer in all cases possessed highly elastic properties. Visual examination of the grafted samples, however, indicated that grafting was not uniform throughout the sample due to the immiscibility of water and ethyl
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Fro. 1. Preirradiation grafting of ethyl acrylate to wool at 25°C (monomer concentration = 60%, dose = 4.8 Mrads at 0.1 Mrads/hr). graft were obtained (Fig. 2), and the resulting samples did not possess elastic properties. U n doubtedly, the inclusion of dioxane or acetone in the grafting solution reduces the activity of water in regards to its swelling characteristics for wool. As a consequence, the grafting yields level off at considerably lower values than in the absence of a mutual solvent.
% Water
Dose (Mrads)
Reaction time (hr)
% Graft
10 20 40 80 33 33 33 67 67 67
4-0 4.0 4.0 4.0 4.2 4.2 4.2 4.3 4.3 4.3
46.0 46.0 46.O 46.0 3 21 67.5 23.0 44.8 68.8
30.2 5476.2 2276.3 645.7 22.3 97.9 784.9 32.0 234.4 3216-8
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FIo. 2. Effect of water as a swelling agent on the prelrradlatlon grafting o£ ethyl acrylate to wool at 25°C (0-18% H~O, 30% monomer and 52% dioxane; 0 - - 3 5 % H=O, 15% monomer and 50% acetone, d o s e = 4-0 Mrads at 0.I Mrad/hr).
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J. L. Williams, D. K. Woods, IT. Stannett, S. B. Sello and C. V. Stevens
acrylate. These results indicated that a welldispersed grafting system was needed which would allow maximum swelling by water. Therefore, subsequent experiments were tried using an emulsifier to disperse the monomer in the water phase. Several of the resulting grafting curves using a nonionic polyoxyethylene type emulsifier are shown in Fig. 3. At this particular ethyl acrylate concentration, the grafting yields clearly increase as the emulsifier concentration decreases from 3.2 to 0.8 ~o. I t is also evident that these curves do not reach a plateau grafting level as is normal with preirradiation grafting, but continue to increase with time. Closer examination of the grafting curves indicates that there exists a sharp break in the grafting curve, where the initial rate suddenly changes to a considerably faster rate. Examination of the grafted samples taken from the two regions of the grafting curve indicates that there is also a large difference in the elastic properties of the samples taken from the two regions of the curve. I n general, samples taken from above the rough transition point possess very good elastic 7000
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FIo. 3. Effect of emulsifier concentration on the preirradiation grafting of ethyl acrylate to wool at 25°C. ( A - - 0 . 8 % Triton X-100, 33% water; O--1"6% Triton X-100, 33% water; 0-3.2% Triton X-100, 33% water; O---No emulsifier Dose -----4.8 Mrads at 0-1 Mrad/hr).
properties, while those taken from this point were not greatly changed. Experiments were carried out using different surfactants to determine if the grafting reaction might be sensitive to the type of emulsifier used. The results of these experiments using sodium lauryl sulfate (anionic), Triton X-100 (nonionic), and Triton X-405 (nonionic) indicate that a high level of grafting can be obtained with each type of surfactant examined. Studies of the influence of monomer concentration in the emulsion using both Triton X-100 and Triton X-405 were performed. The main chemical difference between Triton X-100 and X-405 is in the molecular weight of the polyoxyethylene side-chain, that of the latter emulsifier being approximately fourfold higher. Preirradiation grafting curves using Triton X-405 (Fig. 4) indicates that grafting yields for a given contact time increase with increasing monomer concentration over the range studied. Similar results were found with Triton X-100; however, grafting yields at a given monomer concentration were considerably lower than when using the higher molecular weight Triton X-405. In addition, these curves indicate that the grafting rates continue to decrease with increasing amounts of water. As a consequence, the time corresponding to the rough transition points are shifted to higher values as the monomer concentration is decreased. Also, the abruptness of the transition decreases with decreasing monomer concentration. Beyond the transition point, a small change in time makes a large difference in the grafting level because of the high grafting rate in this region. I n order to lower the grafting rate for a given monomer concentration and thus achieve more experimental control over the grafting level, a portion of the water was replaced with methanol, which is a somewhat poorer swelling agent for wool. Surprisingly, the graft rate dropped tremendously upon the addition of 23 methanol to the grafting solution (Fig. 5) in spite of the increased solubilization of the monomer. There was no longer a transition point in the grafting curves and the grafting rates continued to decrease with time. Even small amounts of methanol added to the system, exhibited pronounced effects on the rate of grafting (Table 2). Low levels of grafting were
Craft co#olym~s as elastomericflbres--I
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Fxo. 5. Effect of methanol on the preirradiation grafting of ethyl acrylate to wool at 25°C ([7-0% MeOH; 0---23.0% MeOH; 0---6"3% MeOH; and A--1.6% MeOH. All suspensions contained 60% monomer. Dose = 4.8 Mrads at 0.1 Mrad/hr). obtained, in the presence of a small percentage of methanol even after prolonged reaction times. Evidently, these small concentrations of methanol in the emulsion system are capable of severely reducing the efficiency of water as a swelling agent. This again illustrates the unique properties of water in promoting the grafting reaction,
Pre-irradiation grafting to cellulosicfibers Although cellulose fibers are extremely different chemically and morphologically from wool fibers, both can be characterized by a fibril-type structure embedded in a less ordered matrlx.ma2~ Since the grafting reaction predominately takes place in these less ordered regions, tla) experiments were designed to attempt to obtain elastic properties with cellulosic fibers similar to those obtained with wool. Also, the possibility of having a continuous filament elastic fiber presented itself in the case of regenerated cellulose. T h e preirradiation grafting of ethyl acrylate to regenerated cellulose was found to proceed in
164
J. L. Williams, D. K. Woods, IT. Stannett, S. B. Sello and C. V. Stevens
TABLE 2. The influence of methanol on the preirradiation grafting of ethyl acrylate to wool from emulsion Swelling conditions % Methanol
% Water
50.0 50.0 50-0 10"0 10-0 2.7 2.7 0
50'0 50.0 50.0 90.0 90.0 97-3 97.3 100
Dose (Mrads)
Reaction time (hr)
% Graft
2'3 4.8 4.8 4.6 4-6 4.1 4.1 4.1
24.5 52.0 96.5 52.0 71.0 47.0 142.0 50.0
21.52 19-07 18.24 6-1 28'2 27-0 53.2 2290.0
60% monomer concentration. Dose rate: 0.1 Mrad/hr. a similar fashion as that to wool. After approximately 20 hr reaction time at 25°C, the grafting rate, rather suddenly, accelerates in m u c h the same m a n n e r as found with wool (Fig. 6). Again examination of the grafted samples above the ./
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rough transition point in the grafting curve indicates they are highly elastic while those below this inflection point are essentially inelastic. T h e times corresponding to the rough transition point are quite long for practical purposes and it was thought that, due to the diffusion controlled nature of the reaction, these times might be considerably reduced by simply increasing the temperature• I n Fig. 6 it can also be seen that grafting rates do increase over the range of 25-55°C, and the times necessary to achieve grafting levels required for high elasticity are reduced from 20 hr at 25°C to approximately 40 min at 35°C, a 30-fold decrease. A dramatic increase in initial grafting rate occurs between 25 and 35°C. Further experimentation indicated that this abrupt change occurs at about 32°C. Experiments have been carried out at temperatures up to 75°C, but grafting rates do not increase substantially beyond 55°C and the degree of homopolymer formation begins to increase at the higher temperatures for these m o n o m e r concentrations. This homopolymer formation is evidenced by a decrease in the grafting rates due to premature reduction in the m o n o m e r concentration in the grafting solution and by visible evidence of occluded homopolymer on the grafted rayons. T h e optim u m temperature to achieve high grafting rates with m i n i m u m h o m o p o l y m e r formation is around 45°C. T h e influence of total dose on the grafting rates a n d thus the overall grafting time is quite
GraJ~ ¢opolymers as elastomericfibres--I pronounced, as can be seen by the results presented in Fig. 7. The initial grafting rate is increased approximately tenfold when the total dose is increased by a similar factor from 0.45 Mrads to 4-8 Mrads. However, beyond a total dose of approximately 6.0 Mrads, the grafting rates are not increased considerably. Samples taken at the higher total doses appear to possess very good elastic properties in comparison to samples grafted to similar levels at lower doses; perhaps higher total doses assure a greater uniformity of radical population in the sample. The radical buildup in rayon prior to exposure to monomer solution was measured by electron spin resonance techniques, and it is shown (Fig. 8) that the radical concentration continues to increase up to a dose of circa 6.0 Mrads. Beyond this point, the radical concentration is essentially stationary. Any further increases in grafting level beyond the steadystate point must then be explained in terms of 14.3 MRADS, O 4.8 MRAOS- ZI O . J *0 (45"C) 1400
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chain cleavage, e.g. by increasing accessibility. The incorporation of a small percentage of bifunctional monomer in the grafting solution offered the possibility of varying the type and degree of elasticity of the modified rayon, i.e. a small degree of cross-linking could increase both the modulus and breaking tenacity. The use of N-methylol acrylamide as cross-linking monomer gave the best results from both the viewpoint of properties and grafting behavior. N-methylol acrylamide (MAA) is very soluble in the water phase and not in the ethyl acrylate, and the grafting curves paralleled the grafting behavior exhibited by pure ethyl acrylate. Grafting yields drop considerably with increased concentration of N-methylol acrylamide (Fig. 9). This reduction in grafting is due mainly to the reduced ethyl acrylate concentration in the monomer feed and not to an inhibiting effect of MAA, as inferred from the fact that grafting yields in the absence of MAA continue to increase with increasing concentrations of ethyl acrylate over the entire range studied (Fig. 10). Examination, however, of the MAA containing grafting emulsions indicated that, at high MAA concentrations, considerable cross-linking did occur in emulsion.
Chemically-initiated grafting to ¢ellulosic fibers In addition to the irradiation grafting studies, a typical chemical initiation method was also explored• Work here was centered around the eerie ion method, using eerie ammonium nitrate as the initiator. This method relies on the presence of hydroxyl groups on the substrate and is therefore readily applicable to rayon and other cellulosics, i.e.
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J. L. Williams, D. K. Woods, V. Stannett, S. B. Sello and C. V. Stevens
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tion dominates and grafting yields drop. Results are also quite irreproducible at the higher initiator concentrations due, undoubtedly, to the heavy homopolymerization at these concentrations. In an attempt to improve the strength properties of the grafted fiber, acrylonitrile was added to the ethyl acrylate grafting system as a
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it can be seen that grafting yields continue to increase with increasing initiator concentration up to about 2.0 raM. When higher initiator concentrations are employed, homopolymeriza-
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zinc chloride solution and subsequent rinsing in distilled water. As a result of the post-decrystallization procedure, such sample was found to have 450 per cent elongation at break, compared to only 15 per cent elongation for the original grafted fiber, uS) Accompanying this increase in elongation is a large decrease in tenacity. Attempts to render chemically-grafted rayons highly elastic at low grafting levels by postdecrystallization were not successful. The differences between the results yielded by the two grafting systems are also discussed in Part II. In conclusion, it can be stated that an irradiation grafting process has been developed whereby fibers, films, or fabrics can be rendered highly elastic with elongations approaching those of spandex materials. Conventional fibers such as wool, cotton and rayon are extremely well suited for this method for modification. The most significant part of this study is the finding that high rubber-like elasticity can be achieved at very low grafting levels ( < 70 per cent) if the lightly grafted fibers are subjected to a postdecrystallization treatment. In terms of practical applications, the process offers many unique 100% EA 600
comonomer. In Fig.12, it is evident that the presence of acrylonitrile considerably decreased the overall grafting rate.
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Post-decrystallization of graft copolymers A full account of the mechanical properties and morphology of the grafted fibers is presented in Part I I of this work. (7) However, it is worthwhile to briefly mention at this point the importance of the post-decrystallization treatment. Examination of the properties of the grafted fibers indicate that circa 1000 per cent graft is generally required to achieve highly elastic properties. At this grafting level, both irradiation and chemical grafting methods are applicable. It has been found however, that if the pre-irradiated, grafted fiber is post-decrystallized by swelling in a suitable solvent, high elasticity can be achieved at far lower grafting levels. In fact, rayon fibers with less than 70 per cent polyethylacrylate graft via the preirradiation technique become highly elastic following a post-decrystallization treatment in 70 ~o aqueous
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168
J. L. Williams, D. K. Woo~, V. Stannett, S. B. Sello and C. V. Stevens
possibilities of rendering a preformed material or fiber assemblies totally elastic or of imparting local or uni-directional stretch properties to a structure. REFERENCES I. (a) WILLIAMSJ. L. and STANNETTV., Text. Res. J. 38, 1065 (1968); (b) WILLL~MSJ. L., WooDs D. K., STAm~'r V., ROLVANL. G., SBLLOS. B. and STEVENSC. V. Textile. Res. J. 43, 205 (1973). 2. WILLIAMSJ. L. and STAI~mTTV. Polymer Lett. 8, 711 (1970). 3. WILLIAMSJ. L. and STAm~TT V. I doEC Product R d~ D 11, 211 (1972). 4. NEOISHXM., NAICAMURAY. and Hosox M. J. appl. Polymer Sci. 12, 1209 (1968). 5. NAX~AMURAY:, I-I~oJOSAO. and ARTHURJ. C. J. appl. Polymer S~. 14, 789 (1970).
6. NAKAMU~ Y., HINOJOSAO. and ARTHURJ. C. J. appl. Polymer Sci. 14, 929 (1970). 7. WILl.JAMSJ. L., STANNETTV., ROLDAN L. G., SELLO S. B. and STEVENS C. V. Int. J. appl. Radiat. Isotopes 26, 169 (1975). 8. GALLAOHRRD. M. Text. Res. J. 40, 621 (1970). 9. HOROWXTZC. U.S. Patent 3,376,168 (1968). 10. STANNETTV., ARAtaK., GERVASXJ. and McI~sKY S. J. Polymer Scl. A-2, 3763 (1965). 11. ALEXANDER P. and HuDsON R. Wool: Its Chemistry and Physics. Reinhold, NewYork (1954). 12. MAw_.~YR. Nature, Lond. 204, 1155 (1964). 13. TRmP W., MOORE A., I)EGRuY I. and ROLLINS M. Text. Res. J. 30, 140 (1960). 14. SCHWAB E., ST~SETT V., RAKowrrz D. and MAORA~ J. Tappi 5, 390 (1962). 15. WmLIAMSJ. L. and STA~NETTV. Polymer Lett. 10, 665 (1972).