Mechanism of action of adsorption active media on deformation of glassy polyethylene terephthalate

MECHANISM OF ACTION OF ADSORPTION ACTIVE MEDIA ON DEFORMATION OF GLASSY POLYETHYLENE TEREPHTHALATE* A. G. ALESKEItOV, A. L. VOLYNSKII and N. F. BAKEYEV M. V. Lomonosov State Ur~iversity, Moscow (Received 25 December 1975) Elongation of glassy ur~oriented PET1 ) was examined in eight organic liquids r~ot causing swellir~g at room temperature. I t was shown that PETP samples deformed in liquid media show considerable reversible deformations at room temperature, the exteIlt of these deformations depending on the adsorption active r~editml used. Results were compared with mechanical test results of PETP in the same media and ar~ in- verse relatio1~ was established between the limit of forced elasticity and reversible de- formation in these media. I t is assumed that the correlation four~4 is due to surface effects, which deper~d on properties of the highly dispersed material of micro-cracks; these cracks are formed and increase in size durir~g elongation of glassy polymers iu adsorption active media. :LIQUID media may have a decisive effect on mechanical properties of solid poly- mers [1-3]. This effect is explained in most studies by micro-cracking caused by solvents (solvent crazing) [4-6]. The mechanism of formation and extension of specific micro-cracks in solid polymers i. ~ related by some authors [7, 8] to the local plasticization of polymers, while others believe that they are first of all due to the variation of inter-phase surface energy at the polymer-medium boundary [9-11]. Attempts have been made to describe micro-cracking by combining both factors [5, 6, 12]. While the effect of plasticization on mechanical properties of glassy polymers has been studied in fair detail [13], the adsorption effect of liq- uid media on polymers is far from being clear. The understanding of this prob- lem is hindered by the absence of a suitable method for evaluating adsorption interaction between a solid polymer and liquid; it is therefore impossible to predict at the present time the effect of an adsorption active medium. Using amorphous PETP an attempt is made in this study to evaluate the ad- sorption interaction between a solid polymer and liquid medium with the help of ideas concerning the structure of micro-cracks formed and propagating during the elongation of polymers below glass temperature T~ in liquid media [2]. Results of this evaluation were compared with mechanical test results of PETP in many adsorption active media. * Vysokomol. soyed, hlS: No. 9, 2121-2127, 1976. 2427 2d28 A. G. ALESF.EROV et al. Industrial films of amorphous unoriented PETP 250 ~m thick were used in this study. Samples for mechanical tests were made in the form of reversible blades, the size of the operating part being 5 × 15 ram. To examine reversible deformations, the samples were elongated, using manual mobile clamps in various liquid media, to given degrees of deforma- tion, they wore then freed and allowed to stay in the same media for 200 hr. The dimensions of samples were then measured and contraction determined. Curves of elongation of PETP in liquid media were obtained by methods previously described [14]. Swelling of PETP in liquid media was determined after keeping the sample in eonf~act with a liquid medium for 720 hr at room temperature. Some properties of liquid media selected for investigation are tabulated. PROPERTIES OF ADSORPTION ACTIvw. MEDIA z~ (v), : Medium ~, %* •, opt [15] kg/mma Triethylamino Heptano CC1, Butyl iodide n-Propanol Formamide Ethylene glycol Oloic acid 1.0 1.1 2.3 2.0 1.7 0.5 0 0.6 0.394 (15) 0-44 (15) 0.965 (20) 2.52 (15) 3.76 (20) 26.0 (15) 38.8 (20) 0.75 0.50 0.80 0.23 0.53 0.80 1.28 1.03 A very important problem for studies of this kind is the selection of active me- dia, since their action on polymers may vary. As indicated, liquid media can be divided approximately to two large groups as regards physical effects on solid polymers: 1) liquids penetrating into the volume of the polymer and causing swelling and plasticization (absorption interaction); 2) liquids interacting with the surface layer of the polymer, which cannot pene- trate into its volume, but reduce the surface energy of the polymer (adsorption interaction). Indirect information concerning the adsorption interaction of the polymer and liquid medium may be obtained from results of measuring contact wetting angles [16]. In this case the result obtained could characterize inter-phase surface ten- sion at the polymer-medium boundary. However, the accurate surface tension at the polymer-air boundary has to be known. There are no direct methods available at the present time for determining the surface tension of solid polymers. For the majority of surface-active liquids, which normally satisfactorily wet the poly- mer [17], the marginal wetting angle is close to zero and experimental determina- tion is very difficult. The evaluation of the type of interaction between a solid polymer and liquid medium is very complicated because of the ability of polymers to swell and conse- * Values of swelling of PETP after heating in corresponding liquid media at 60 in 30hr. * The temperature (°C) at which viscosity was measured Is shown in brackets. $ Reduction of the limit of forced elasticity af.,. of PETP during deformation in cor- responding liquid media on changing the rate of elongation from 1.6 to 0.13 mm/min. Deformation of glassy polyethylene terophthalato 2429 quently, plasticize. The low rate of diffusion in solid polymers often hinders the simple determination of the mechanism of action of the liquid medium. For exam- ple, detailed examination shows that alcohols of aliphatic series which are not solvents for PS and PMMA in the general sense can, nevertheless cause consider- able swelling of these polymers and markedly reduce the value of Tg [8]. The consid- erable importance of effects of plasticization in elongation of glassy polymers in active media, causing micro-cracking is emphasized in many studies. In earlier studies [18-20] Kambour derived a clear correlation between the reduction of T of the polymer and deformation, where micro-cracking begins; it appeared unim- portant whether the active medium acts from the surface of the sample, or is added to the polymer beforehand. In order to characterize the possibility of solution (or swelling) of the polymer in a low molecular weight liquid, the solubility parameter 5 is normally used. However, this parameter only predicts solubility satisfactorily for non-polar polymer-solvent vapours. In polar liquids, particularly in liquids prone to hydro- gen combination solubility does not correlate with the value of 5. For example, it follows from our measurements that amorphous PETP which has a value of 5~10.7 [1], does not swell fully at room temperature in n-oetanol which has a similar solubility parameter (5~--10.3 [21]). At the same time benzyl alcohol (5~ 12.1 [21]) and benzonitrile (5~8.4 [22]) which have a 5 value noticeably dif- ferent from PETP cause the polymer to swell considerably (11.7 and 23.5 wt.%, respectively) which is accompanied by intensive crystallization. Considering the empirical parameter of combining hydrogen [21] is of little use for describing the mechanical behaviour of glassy polymers in liquid media. There are, evidently~ many liquids which cannot cause a marked swelling of a given polymer and their effect on mechanical behaviour seems to be fundamen- tally different. For example, the addition to PMMA of only 1.2~o polyethylsil- oxane liquid (PES-5) results in micro-phase separation of the system therefore, any further addition of PES-5 to the polymer ceases to influence the value of Tg. Nevertheless, the addition of PES-5 continues to have a strong effect on the me- chanical behaviour of the polymer, which is due to its intensive micro-cracking. Evidently the liquid medium acts on the polymer by an adsorption mechanism in this case [23, 24]. There is therefore no criterion available at the present time by which the type of interaction between the solid polymer and low molecular weight liquid could be simply evaluated; attempts to classify liquid media according to their effect on mechanical characteristics of polymers [21, 25] are qualitative descriptive in nature. Liquid media were selected in this study so as to minimize the effect of plasti- cization. Maximum adsorption effects and particularly, adsorption reduction of strength (Rebinder effect) can be expected in those cases, when the active medium cannot dissolve or penetrate into the volume of the adjoining phase, but interacts with the inter-phase surface layer [16]. PETP film samples were therefore kept in 2430. A. G. ALESKEROV et al. contact with a number of the 35 organic liquids for 720 hr at room temperature. The extent of swelling was then determined gravimetrically. Several liquid media were selected by this method (Table) which do not cause swelling of PETP. It was noted previously that the extent of swelling also depends on the rate of diffusion of the liquid medium into the polymer which increases suddenly with an increase of temperature. The liquid media selected were therefore kept for 30 hr in contact with PETP samples near Tg at 60 °, after which the extent of swelling was again determined. It appeared (Table) that swelling remained very slight ,( ~ 20/o) even in this case. In liquid media thus selected PETP was subjected to deformation and rever- sible deformations studied by the methods previously described. The results of these experiments are shown in Fig. 1. I t appeared that on being kept in liquid media PETP samples undergo considerable reversible deformations, which are not typical of polymer glass their extent depending on the type of liquid. Contrac- tion varies between 37~o for heptane and 17~/o for butyl iodide for the media used. I t is natural to assume that the effect observed is due to the varying abilities of liquid media selected to reduce the surface energy of PETP. During deformation of PETP in an adsorption active medium specific micro-cracks are formed in it, filled with an oriented, highly dispersed material [2, 26]. The highly developed sur- face of this material is stabilized by the adsorption layer of the liquid, in which .deformation was carried out. According to the type of liquid used, adsorption at the interface and therefore, the reduction of surface energy will naturally be different. After relieving stress the area of the highly developed inter-phase sur- :face begins to decrease and this depends on the reduction of inter-phase surface energy of the polymer. Macroscopically, this type of partial coagulation of a highly dispersed polymer is manifested in the contraction of PETP samples in liquid media. Figure 1 clearly shows that in the interval studied contraction is inde- pendent of the extent of elongation of the polymer. At the same time it is obvious that the overall area of highly developed PETP surface continuously increases in proportion to elongation in an adsorption active medium [27]. Consequently, the reduction of the highly developed surface area in each of the media selected continues up to a given level of specific area of interfaces. Interphase surface energy of the polymer determines the interphase surface area stable under these conditions (for unit volume of the microcraek). Thus, the inter- action of a highly dispersed material of the micro-crack with a liquid adsorption active medium determines to a large extent the mechanical behaviour of the poly- mer as a whole. The absolute value of the contraction observed is closely related to the interphase surface energy of the polymer and may be a criterion for eva- luation. The greater the extent to which the liquid reduces the surface energy of ~he polymer, the more stable the highly dispersed material of the micro-crack .and the less considerable the contraction observed and vice versa. I f the criterion selected for the evaluation of interphase surface energy of the polymer is true, it should correlate with the deformation-strength properties of Deformation of glassy polyethylene tercphthalato 2431 PETP since, as noted previously, polymer deformation under these conditions is first of all due to surface adso:ption effects resulting in specific microcracks filled with a highly dispersed material. Figure 2 shows results of this comparison as a relation of the relative reduction of the limit of forced elasticity in medium am, compared with the limit of forced elasticity a t obtained during elongation in air C% ]dl tk A o i o • × x ,x × 0 "> o2 ~3 "4 ,,5 . f i 07 -3' r ] i J l(tl] 2gU •, % Fro. 1. Relation between reversible deformation e and the degree of elongation ~t of PETP in heptane (1), formamide (2), oleie acid (3), ethylene glycol (4), propanol (5), butyl iodide (6), CCl~ (7) and triethylamine (8). according to contraction observed in these media. It is clearly shown that except for two media (ethylene glycol and oleic acid) all points are satisfactorily situated on a straight line and contraction is inversely proportional to the relative reduction of the limit of forced elasticity. I t is obvious that the contraction detected in this study is of non-entropy type and therefore, does not involve the plasticization of the polymer, since the most considerable contraction is observed precisely for the medium (heptane), in which the limit of forced elasticity decreases the least, while the plasticization effect is evaluated from the reduction of af. e. The value of af. e. is very closely related to micro-cracking and properties of the highly dispersed material of micro-cracks. The energy used for elongation of a glassy polymer in an adsorption active medium is not only used for macromolecular orientation inside individual fibrils of micro-cracks, but also for the formation of highly developed interphase surfaces and their stabilization. It is natural from this point of view that we ob- serve an inverse dependence of the reduction of % e. of PETP on contraction in an adsorption active medium, i.e. on the value characterizing interphase surface tension. As noted previously (Fig. 2), two of the media studied stand out of the system described. First, the high viscosity of oleic acid and ethylene glycol should be noted, compared with other media used (Table). In studies by Marshall et al., [28-30] 432 A. G. A I~s~Bov et aL it was shown that according to the decomposition mechanism tl~e rate of extension of micro-cracks in glassy polymers is determined by the flow of liquid through the porous structure of the micro-crack towards the top. We assume [31] that since deformation of PETP in a liquid adsorption active medium is due to the formation o'~e.- o'~ ,, Ioo % O'£e ' look- 60- z~O 20 \ • \ • \ I I 20 ~? \ \ \ \ I \1 8O ~,% Fzo. 2. Relation between the relative reduction of the limit of forced elasticity of PETP and contraction in the s~me media (see Fig. 1 for notations}. and extension of micro-cracks, the rate of penetration of the liquid to the top may have a decisive effect on the mechanical behaviour of the polymer as a whole. In other words, mechanical properties of the polymer under these conditions are determined by the ratio of rates of deformation and viscous flow of the liquid to the top of the growing micro-crack. This ratio can be easily adjusted by chang- ing the rate of polymer elongation in the medium. It follows from tabulated re- sults that on changing the rate of elongation from 1.6 to 0.13 mm/min, the effect of highly viscous liquid media is much more marked than for media of low visco- sity, which confirms the kinetic nature of the effect observed. It is obvious that at fairly low rates of elongation, when the rate of medium penetration will not limit the rate of extension of the micro-crack, the effect of oleic acid and ethylene gly- col will not differ from the general relation shown in Fig. 2. Let us examine some further features of the relation shown in this Figure. On extrapolating it to zero contraction, we obtain the relative reduction of af. e. ~100~. This means that if a liquid could be found which is able to reduce mar- kedly the value of at. e., in this liquid the structure of the micro-crack would be so stable that the interphase surface area would not show any reduction and contrac- Deformation of glassy polyethylene terephthalate 2433 tion would be zero. Extrapolat ion of this relation to zero reduction of af. e. gives a contract ion value close to 100°/o . In other words, if a highly dispersed structure of the micro-crack could be formed during elongation in air (or in a medium which reduces the surface tension of the polymer to the same extent), after relieving stress, the polymer would reduce its interphase surface so vigorously that contrac- t ion would be ~ 100~, i.e. deformation would become practically reversible. This ease is observed in practice in experiments, when the medium is removed from the volume of the micro-crack after elongation [27]. Natural ly, contraction in this case independent of the type of adsorption active medium used, since the final state of the material is only determined by surface tension at the polymer-a i r inter- face. As shown previously [31], contraction under these conditions reaches 75-90 ~o, which correlates satisfactori ly with data shown in Fig. 2. Exper imental results suggest that the contraction of glassy polymers in liquid media enables interphase surface energy to be defined at the po lymer-medium interface. This contraction shows a clear correlation with the reduction of the li- mit of forced elasticity of PETP in deformation in adsorption active media. The correlation suggests that deformation of glassy polymers in liquid adsorptio~ active media is determined by processes of micro-cracking and depends on the properties of the highly dispersed'material of micro-cracks. Translated by E. SEMERE REFERENCES 1. Yu. S. ZUYEV, Razrusheniye polimerov pod deistviyem agressivnykh sred (Polymer Decomposition by the Action of Corrosive Media). Izd. "Khimiya", 1972 2. R. P. KAMBOUR, Macromolec. Rev. 7: 1, 1973 3. A. N. TYNNYI, Proehnost' i razrusheniye polimerov pod deistviyem zhidkikh sred (Strength and Decomposition of Polymers by the Action of Liquid Media). Izd. "Naukova dumka", 1975 4. N. BROWN, ft. Polymer Sci., Polymer Phys. (Ed.) 11: 2090, 1973 5. N. BROWN and S. FISHER, J. Polymer Sei., Polymer Phys. (Ed.) 13: 1315, 1975 6. H. G. OLF and A. PETERLINE, J. Polymer Sei. Polymer Phys. (Ed.) 12: 2209, 1974 7. B. MAXWELL and L. F. RAHM, Industr. and Engng. Chem. 41: 1988, 1949 8. B. L. EARL, R. I. LONERAGAN, J. N. T. JOHNES and M. CROOK, Polymer Eng~lg. and Sci. 13: 390, 1973 9. S. FISHER and N. BROWN, J. Appl. Phys. 44: 4322, 1973 10. Ye. A. SINEVICH, R. P. OGORODOV and N. F. BAKEYEV, Dokt. AN SSSR 212: 1383~ 1973 11. M. PARRISH and N. BROWN, J. Maeromolee. Sei: E8: 655, 1973 12. E. N. ANDREWS and L. BEWAN, Polymer 13: 337, 1973 13. A. V. YEFIMOV, Dissertation, 1975 14. A. L. VOLYNSKII, V. D. SMIRNOV, P. H. STOCHES, V. I. GERASIMOV, A. G. ALESKE- ROV and N. F. BAKEYEV, Vysokomol. soyed. A18: 940, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 4, 1074, 1976) 15. A. VAISBERGER, E. PROSKAUER, Dzh. RIDDIKH and E. TUPS, Organieheskiye- rastvoriteli (Organic Solvents), Izd. inostr, lit., 1958 16. S. S. VOYUTSKH, Kurs kolloidnoi khimii (Course on Colloid Chemistry). Izd. "Khimiya", 1964 2434 A. ~T. OTmm~r et al. 17. I. NARISAWA, J. Polymer Sci. 10, A-2: 1789, 1972 118. G. A. BERNIER and R. P. KAMBOUR, Macromolecules 1: 393, 1968 19. R. P. KA1KBOUR, E. E. ROMAGOSA and C. L. GRUNER, Macromoleeules 5: 335, 1972 20. R. P. KAMBOUR, G. A. BERNIER and E. E. ROMAGOSA, J. Polymer Sci., Polymer Phys. (Ed.) 11: 1879, 1973 21. R. I. VINCENT and S. RAHA, Polymer 13: 283, 1972 22. J. D. CROWLEY, G. S. TEOQUE and J. W. LOWE, Farbe und Lack, No. 2, 120, 1967 23. A. Ye. SKOROBOGATOVA, Dissertation, 1975 ~24. A. L. VOLYNSKII, A. V. YEFIMOV, A. A. RYZHKOVA and N. F. BAKEYEV, Vysoko- mol. soyed. A17: 500, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 3, 576, 1975) 25. N. V. PERTSOV and N. I. IVANOV, Fiziko-khimich. mekhanika materialov, No. 3, 56, 1974 26. D. G. LEGRAND, R. P. KAMBOUR and W. R. HAAF, J. Polymer Sci. 10, A-2: 1565, 1972 27. A. L. VOLYNSKII and N. F. BAKEYEV, Vysokomol. soyed. A17: 1610, 1975 (Translated in Polymer Sei. U.S.S.R. 17: 7, 1855, 1975) 28. G-. P. MARSHALL and J. G. WILLIAMS, J. Appl. Polymer Sci. 17: 987, 1973 29. J. G. WILLIAMS, G. P. MARSHALL, I. GRAHAM and E. L. ZICHY, Pure and Appl. Chem. 39: 275, 1974 30. J. G. WILLIAMS and G. P. MARSHALL, Prec. Roy. Soc. A342: 55, 1975 31, A. L. VOLYNSKII, A. G. ALESKEROV, T. Ye. GROKHOVSKAYA and N. F. BAKEYEV, Vysokomol. soyed. A18: No. 8, 1976 (Translated in Polymer Sei. U.S.S.R. 18: 8, 1976) METHODS OF INVESTIGATION USE OF THE METHOD OF MEASURING THE ABSOLUTE INTENSITY OF SMALL ANGLE X-RAY SCATTERING FOR THE STUDY OF THE STRUCTURE OF AMORPHOUS REGIONS IN ORIENTED POLYETHYLENE FILMS* A. N. 0ZERIN, "YU. A. ZUBOV, V. I. SE]';I-KttOVA, S. N. CHV~U~ and N. F . BAKE¥~.V L. Ya. Karpov Scientific Research Inst i tute of Physical Chemistry (Received 20 February 1976) Methods of calibrating standards were compared, in order to determine pr imary X - ray intensity. A study was made of the possible use of methods of measuring the intensity of meridional small angle X-ray scattering and measuring scattering in abso- lute units, in order to evaluate the structural variation of amorphous regions in an- nealing and elastic deformation of oriented PE films. *Zc-ysokomol. soyed. AIS: No. 9, 2128-2134, 1976.