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Effect of chemical crosslinking on the processes of microcracking of polyethylene in a liquid medium
Polymer Science U.S.S.R. Vol. 30, No. 5, pp. 984-991, 1988 Printed in Poland
0032-3950188 $10.00+ .00 © 1989 Pergamon Press pie
EFFECT OF CHEMICAL CROSSLINKING O N THE PROCESSES OF MICROCRACKING OF POLYETHYLENE IN A LIQUID MEDIUM* A. V. YEFIMOV, N. N. VALIOTTI, V. I. DAKIN,
A. N. OZERINand N. F. BAKEYEV Lomonosov State University, Moscow Karpov Physicochemical Research Institute
(Received 20 November 1986) The character of the development of strain and also the structure of the microcracks formed on stretching HDPE in a liquid medium are determined by the parameters of the spatial network initiated in the polymer by irradiation. The concentration of fibrillar material in the microcracks increases while the degree of stretching of the polymer in the fibrils decreases as the density of the nodes of the spatial network of HDPE rises. At high degrees of crosslinking the contribution of microcracking to the total strain of HDPE stretched in a liquid medium decreases. -
ON LOADING glassy and crystalline polymers specific microcracks (crazes) may develop. Unlike genuine fracture cracks the crazes are filled with highly disperse polymeric material. The walls of the microcracks join the strands of the polymer oriented in the direction of stretching with a thickness of the order of hundreds of Angstroms. The formation of such microcracks plays an important role in the processes of strain and fracture of polymeric materials [1-3]. In this connexion of great importance is information on the structure of the crazes and the processes of their generation and growth. Microcracks occurs particularly actively on strain of polymers in contact with adsorption-active media. In the present work we investigate the effect of the formation of a spatial network on the processes of strain of the crystalline HDPE polymer in a liquid medium. The structure of the microcracks formed on strain of crosslinked HDPE samples is characterized. We used isotropic samples of HDPE 400-700 pm thick with Mw=2"23 x l0 s. The degree of crystallinity of the initial polymeric sample according to DSC data was 71%. The network structure of HDPE was produced by irradiation at room temperature, source 6°Co. After irradiation the samples were annealed for 5 hr at 115°C. The conditions of irradiation are given in reference [4]. Calculation of the Mc between the chemical nodes in the irradiated samples was based on the formula M,=pRT[tan ~t, where tan ~¢is the tangent of the angle of slope of the straight line portion of the dependence of equilibrium stress on 2 - 2 z (2 is the degree of stretching) determined at 145°C with the Instron instrument. At doess D = 5, 10, 25 and 50 Mrad Mc x 10 -3 amounts respectively to 15, 3, 2.4 and 1.3-1.6. To this corresponds the content of gel fraction I0, 60, 67 and 77%. The first signs of gelation for the HDPE sample studied are observed on irradiation with a dose 5 Mrad. With increase in the radiation dose Mc falls. According to the DSC data the degree * Vysokomol. soyed. A30: No. 5, 963-968, 1988.
~84
Processes of microcracking of polyethylene in liquid medium
985
of crystallinity changes little for D< 50 Mrad. In further work the structural investigations were carried out in the main on HDPE samples irradiated with D= 10 and 50 Mrad. The structural changes in the HDPE samples stretched in a liquid medium were characterized by the small angle X-ray scatter method. The small angle photodiffraction patterns were obtained with the apparatus described in reference [5] using point collimation. The structure of stretched HDPE was also studied with the KRM-I small angle chamber with slit collimation of the beam. The angular resolution was 4'. The PE samples for the structural investigations were prepared as follows: the polymer samples were stretched in hexane at 30°C and strain rate 1 ram/rain to a defined degree of elongation, the stretched samples were then transferred to acetone and dried, the size of the sample being fixed. For the unirradiated and crosslinked H D P E samples stretched in hexane characteristic patterns of small-angle X-ray scatter were observed due to the formation of microcracks. The diffraction pattern on point collimation of the beam of the H D P E sample stretched in a liquid medium consisted of an intense meridional reflexion and a "line" reflexion extended in the equatorial direction (Fig. 1). Meridional scatter is
6 tn~"
d
2 6
/8 z llT-~ng.min e FIG. 1 FIG. 2 FlG. 1. Scheme of small angle reltexions and the structure of the HDPE sample stretched in a liquid medium. Direction of stretching shown by arrows. FlG. 2. Distribution of the intensity of X-ray scatter at small angles for the initial HDPE sample (1) and the sample stretched in hexane, filmed, in the equatorial direction (2). caused by the microcracks of width x arranged mostly perpendicular to the direction of stretching. The equatorial scatter is due to the system of fibrils disconnected in space and oriented in the direction of stretching and joining the opposite walls of the microcracks. The equatorial distributions of the intensity of small angle X-ray scatter obtained with use of slit collimation of the beam coupled with the data on change in the specific volume of H D P E on stretching in hexane were used to determine a number of parameters characterizing the structure of the microcracks. The function of intensity on filming with a slit chamber in the equatorial direction is formed by the superposing of scatter from the structural elements of two types: m i c r o c r a c k s - m e r i d i o n a l reflexion and the fibrils filling the m i c r o c r a c k s - e q u a t o r i a !
A.V. YErIMOVet al.
986
"line". Analysis of the diffraction patterns shows that on filming in the equatorial direction the meridional component makes an important contribution to the intensity of the scatter recorded in the region of small angles ~0<10-15'. To evaluate the characteristics of the fibrils joining the walls of the microcracks from the curve of equatorial scatter we subtracted the scatter due to the meridional component and then analysed the difference scatter. The components were separated in the Guinier coordinates In I-f02 assuming that the total scatter is made up additively of the corresponding components. Noting that the meridional component does not make an important contribution to the totai curve of scatter at fo> 10-15' it was considered that the curve with large slope in Fig. 2 reflects the contribution of this component to the equatorial scatter. Using the value of the invariant of the curve of equatorial scatter measured in absolute units Q and also the data on change in the specific volume of the polymer stretched in a liquid medium, we determined the volumetric concentration of the fibrils in the microcracks c oo
do Q = -2n ~ f I(~o) ~d~0= (/t,1)% ( 1 - e)(vo + ,dv)(1-c) 0
Here Vo is the specific volume of the non-strained HDPE; zlv is change in the specific volume on stretching; I=I' (~o)/lev'A, I' (q~) is the intensity of scatter measured in absolute units; v' is the scattering volume; 1e is the scattering capacity of the electron; A is the attenuation coefficient. The size of the volume of the fibrils related to the mass of the strained HDPE sample was evaluated from the relation C ~
vf vf +Av . . . .
The value of the invariant of the scatter curve was determined by graphical integration of the dependence of ar~ on ~. The technique of measuring the specific volume of the polymer on stretching in a liquid medium was described earlier [6]. The degree of stretching of the polymer in the fibrils of the microcracks was estimated from the equa12 + ,dr* where 2 is the degree of stretching of the polymeric sample in hexane. ti°n 2 f = e ( 1 -~'o) The dimensions of the fibrillar elements filling the microcracks were determined by the Porod method. In reference [7] it is shown that the scatter from a system of fibrils situated parallel to each other and separated by voids on filming with a slit chamber in the equatorial direction is described in the region of sufficiently large ~0by the relation I ~ k / ~ 3, where k is a coefficient proportional to the area of the surface of the fibrils. It turns out that for non-crosslinked and crosslinked HDPE samples stretched in hexane the product I~ 3 in the region of sufficiently large angles ~ is roughly constant,
Processes of microcracking of polyethylene in liquid medium
987
The size of the area of the surface of the fibrils related to the nuiss of the straincd H D P E sample was determined from the equation S
2 :k /l (~q) ,~.p\
vo /
where k=Iq) a in the region of sufficiently large angles. The diameter of the fibrils was found from the equation d=4t,~/S. The mean distance a bztween the centres of the fibrils was evaluated on the assumption of hexagonal packing of the fibrils in the microcrack from the relation a = di' 1.21,]~. The strain curves of the initial and crosslinked H D P E samples stretched in air and in hexane are presented in Fig. 3. It will be seen that the formation of a spatial network in the polymer leads to appreciable increase in o'~ and stress in the region of steady development of strain c~. On stretching of the H D P E sample in contact with hexane fall in o'~ and cTs is observed. The efficacy of the liqu.id medium evaluated from the relative fall in these characteristics diminishes as the density of crosslinking increases. Strain in air of the initial and also crosslinked H D P E samples is accompanied by neck forniation. It was earlier shown [8] that strain of non-crosslinked PE in hexane largely comes about through generation and subsequent development of specific microcracks. A characteristic sign of such a type of strain is growth of the specific volume of the polymer. Data on the dependence of the increment of specific volume Av/vo on the relative elongation of the sample may be used to evaluate the contribution of crazing to total strain. In general, elongation of the polymeric sample constitutes the sum of two terms: crazing and shear strain. A measure of the contribution of the microcracking process to the total deformation may be the slope of the curve Av)'o-~,. It will be seen (Fig. 4) that for non-crosslinked H D P E stretched in hexane the dependence of Av/vo on e is well approximated by a straight line with the tangent of the angle of slope 0.5-0.6. This means that in the region of elongations studied (up to 200 ~o) the contribution of microcracking to the total stcain of H D P E comes to 55-60~. Deformation of the crosslinked H D P E samples in hexane is also accompanied by the formation of crazes. In turns out that for the samples of H D P E irradiated with D = 10 Mrad, in the region e < 100% the relative role of each mechanism of strain changes little as compared with the non-crosslinkcd polymer. For such samples the contribution of microcracking to total deformation amounts to 45-50 ~. On stretching in hexane of the H D P E samples irradiated with a dose 50 Mrad shear strain p c e d o m i n a t c s - t h e contribution of microcracking to total deformation does not exceed 20--25 ~/o with a tendency for a neck to tbrm in such samples. Let us take a closer look at the stru.ctt, ral changes on deformation of the H D P E samples in hexane. From scanning electron microscopy on stretching of unirradiated H D P E samples in hexane (in the region of elongations roughly corresponding to a ~ of the polymer a large number of microcracks mostly aranged perpendicular to the direction of stretching and filled with oriented polymeric material appears. The width of the microcracks (linear size in the direction of stretching) is tenths of a micron but
988
A.V.
YEFIMOV et at.
the length'reaches several microns. The microcracks alternate with little defmmed regions of the non-porous polymer having much the same size. Schematically the structure of the HDPE sample stretched in a liquid medium is indicated in Fig. 1. From electron microscopy in the region of elongations exceeding a,c of the polymer strain of the HOPE is accompanied by increase in the width of the microcracks with insignificant change in their concentration. The formation of microcracks sharply raises the intensity of small angle X-ray scatter (Fig. 2). As noted, the small angle diffraction patterns of the HOPE samples stretched in a liquid medium are characterized by a meridional reflexion and also by a line reflexion extended in the equatorial direction. Scatter in line form is due to the system of fibrils disconnected in space and joining the walls of the microcracks. STRUCTURAL PARAMETERS OF THE MICROCRACKS FORMED ON DEFORMATION OF THE INITIAL AND CROSSLINKED
D, Mrad
0 0 0
e, % c, % 100 13 200 13 250 12
2t
6rim
10 11 13
10 12 11.5
HOPE
SAMPLES IN HEXANE
a, nm D, Mrad I e, %
23 27 27
10 10 50
100 200 100
c, %
At
6 nm
a, nm
21 23 32
7 8 5.5
9 l0 13
16.5 17.5 19
From the scatter curves recorded in the equatorial direction and also the data on change in the specific volume of the polymer for the HOPE samples stretched in hexane to different degrees of relative elongation, we determined the structural parameters of the microcracks (Fig. 5, Table). It was found that the volume and area of the surface of the fibrils increase approximately in proportion to e whereas the concentration of the fibrils in the microcracks and their diameter amounting to ~ 10 nm remain almost constant in the range of relative elongations studied. The degree of stretching of the polymer in the fibrils of the microcracks changes from 7 to 13 with change in e from 60 to 25070. These values are close to the degree of stretching of HOPE in the neck determined on stretching the polymer in air at 30°C. The value of the degree of stretching of the polymer in the neck changes from 7-5 to 12 with change in e from 200 to
40070. Since the quantity of polymeric material passing into the microcracks and also the area of the surface of the fibrils increase in proportion to the relative elongation of HOPE it may be assumed that the process of growth of the microcracks accompanied by drawing out of polymeric material from their walls. At the boundaries of the microcracks a highly localized process of transformation of the lamellar structure of the polymer to fibrillar takes place. Together with growth of*;he microcracks on stretching of HOPE in a liquid medium there is shear strain of the polymeric material situated between the microcracks. This process occurs without significant change in the specific volume of the material and together with microcracking determines the reaction of the crystalline polymer to the external influence in presence of a liquid medium. The structural rearrangement on stretching the crosslinked HOPE samples in a liquid medium follow a similar pattern, Onl_y the relationship between crazing and
Processes of microcracking of polyethylene in liquid medium
989
shear strain for the H D P E samples characterized by high density of thc nodes of the spatial network changes. The small angle scatter method was used to characterize the structure of the microcracks formed on deformation of the crosslinked H DPE samples in hexane (Tablc). It was found that the main feature of the structure of the crosslinked H D P E samples is the higher concentration of fibritlar material in the microcracks. This corresponds to a lower degree of stretching of the polymer in the fibrils of the microcracks formed in crosslinked H D P E samples as compared with the starting polymer. The diameter of the fibrils filling the microcracks virtually does not depend on the degree of crosslinking and the mean distance between fibrils decreases with rise in D. The 2r values determined for the crosslinked H D P E samples are comparable with the values of the degree of stretching of the crosslinked polymeric samples in the neck. The value of the degree of stretching in the neck for D = 10 Mrad is 5-6 and for D = 5 0 Mrad 4-5. It may be assumed that the limitation of the degree of stretching of the polymer in the fibrils of the microcracks of the crosslinked samples is connected with the presence of a spatial network in irradiated HDPE. The 2f values determined from small angle Xray scatter were compared with the calculated values of the maximum degree of stretching of the H D P E network characterized by a definite value of M,.. In the isotropic state the nodes of the H D P E network are separated by the distance d=0.57 M °'5 (with lhe condition that the segments of the macromolecules between nodes arc in the
20 ~,,~*~___._________._.__~ 3 lo
" --
I
1
I00
__
200
I
3OO
30 ~ . . ~ _ _ _ _ _ 5 ..~ !J
20
08-
Oq
I0
H-a I
.............
I
FIG. 3 FIG. 4 Fl(;. 3. Strain curves of HDPE irradiated with a dose of 0 (1, 2), 10 (3, 4) and 50 (5, 6) Mrad in air (l, 3, 5) and hexane (2, 4, 6). F!G. 4. Dependence of the increment of specific volume on the relative strain at D=0 (1), 10 (2) and 50 (3) Mrad for samples stretched in hexane.
990
A . V . YI~FIMOV et al.
-~60
0.2O'l
20
I00
200
e,%
FIc. 5. Change in the volume (1) and area of the surface (2) of the fibrils as a function of the elongation of HDPE in hexane. Gaussian coil conformation). The maximum degree of stretching of the network was determined from the relation ;tm,x= where l is the contour length of the completely
l/d,
extended segment of the chain between cross links
I=-~Io where Mo is the molar
mass of the monomer unit of HDPE and lo is the projection of the C - C bond on the axis of the chain. For D= 10 Mrad the value 2maxis 7"5 and for D=50 Mrad 5.3. These magnitudes agree with the 2f values determined from the small angle X-ray scatter. The presence of such a correlation suggests that the fall in the degree of stretching of the polymer in the fibrils of the microcracks is linked with the limited stretchability of the spatial network in the irradiated samples. It may also be concluded that the spatial network persists in the fibrils formed on microcracking of the crosslinked HDPE samples in a liquid medium. Thus, the structure of the microcracks formed on stretching HDPE in a liquid medium is determined by the parameters of the spatial network initiated in the polymer by irradiation. Another effect of the formation of the spatial network is change in the relationship between crazing and shear strain on stretching HDPE in a liquid medium. For sufficiently high degrees of crosslinking of HDPE shear strain becomes predominant on stretching the polymer in a liquid medium (Fig. 4). Change in the relationship of the strains of the two types may be explained as follows. The process of the formation and growth of crazes in a crosslinked polymer must be accompanied by local ruptures of the spatial network on formation of a highly developed surface of the fibrils disconnected in space. For a sufficiently high density of the nodes of the spatial network the energy of rupture of the chains on fibrillation makes a significant contribution to the work of formation of the crazes. At the same time the development of shear strain occurring without disturbances of the integrity of the material is apparently acompanied by slighter modification of the network structure of the polymer. This may hamper the process of crazing as compared with shear strain in the crosslinked polymeric samples.
Translatedby A. CROZy
[.~nk between gas permeability and structure of microporous films ot' PETP
991
REFERENCES 1. R. P. KAMBOUR, J. Polymer Sci. Macromolec. Rev. 7: 1, 1973 2. A. L. VOLYNSKII and N. F. BAKEYEV, Vysokodispersnoye oriyentirovannoye sostoyaniye polimerov (The Highly Disperse Oriented State of Polymers), Moscow, 1984 3. Crazing in Polymers. Advances in Polymer Sci. 52/53, 1983 4. Z. S. YEGOROVA, V. I. DAKIN and V. L. KARPOV, Vysokomol. soyed. A21: 9, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 9, 1979) 5. V. I. GERASIMOV and D. Ya. TSVANKIN, Pribl. tekn. eksp. 2, 1968 6. A. V. YEFIMOV, V. M. BULAYEV, A. N. OZERIN, A. V. REBROV, Yu. K. GODOVSKII and N. F. BAKEYEV, Vysokomol. soyed. A28: 1750, 1986 (Translated in Polymer Sci. U.S.S.R. 28: 8, 1951, 1986) 7. E. PARADES and E. W. FISCHER, Macromolec. Chem. 180: 2707, 1979 8. A. V. YEFIMOV, V. V. BONDAREV, P. V. KOZLOV and N. F. BAKEYEV, Vysokomol. soyed. A24: 1690, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 8, 1927, 1982)
Polymer Science U,S.S.R. Vot, 30, No. 5, pp, 991-999, 1988 Printed in Poland
0032-3950/88 $10.00+ .00
LINK BETWEEN GAS PERMEABILITY AND THE STRUCTURE OF M I C R O P O R O U S FILMS OF POLYETHYLENE TEREPHTHALATE STRETCHED IN ADSORPTION-ACTIVE MEDIA* YE. A. SINEVICH, M. S. ARZHAKOV, M. A. KRYKIN, S. F. TIMASHEV a n d N. F. BAKEYEV Karpov Physicoehemical Research Institute
(Received 24 November 1986) The results of measurements of gas permeability are used to model the structure of microporous PETP films obtained by stretching in liquid media. It is shown that the compacted external layers "sealing" the crazes on isothermal drying of the moist samples must contain a small number of through microhoIes strongly influencing gas permeability. MICROPOROUS p o l y m e r s o b t a i n e d by s t r e t c h i n g in p h y s i c a l l y active liquid m e d i a m a y be used in t h e processes o f m e m b r a n e s e p a r a t i o n o f liquids a n d gases [ l - 3 ] . On s t r e t c h i n g b e l o w the glass t r a n s i t i o n p o i n t the p o r o s i t y o f such m a t e r i a l s is g o v e r n e d by the presence in t h e m o f specific m i c r o c r a c k s (crazes) with a h i g h l y d e v e l o p e d i n n e r surface. A t t h e initial stages o f s t r e t c h i n g t h e n u m b e r a n d v o l u m e o f t h e crazes increases but in presence o f strains e a b o v e I 0 0 - 2 0 0 ~ t h e m i c r o p o r o u s s t r u c t u r e begins to c o l l a p s e * Vysokomol. soyed. A30: No. 5, 969-975, 1988.
0032-3950188 $10.00+ .00 © 1989 Pergamon Press pie
EFFECT OF CHEMICAL CROSSLINKING O N THE PROCESSES OF MICROCRACKING OF POLYETHYLENE IN A LIQUID MEDIUM* A. V. YEFIMOV, N. N. VALIOTTI, V. I. DAKIN,
A. N. OZERINand N. F. BAKEYEV Lomonosov State University, Moscow Karpov Physicochemical Research Institute
(Received 20 November 1986) The character of the development of strain and also the structure of the microcracks formed on stretching HDPE in a liquid medium are determined by the parameters of the spatial network initiated in the polymer by irradiation. The concentration of fibrillar material in the microcracks increases while the degree of stretching of the polymer in the fibrils decreases as the density of the nodes of the spatial network of HDPE rises. At high degrees of crosslinking the contribution of microcracking to the total strain of HDPE stretched in a liquid medium decreases. -
ON LOADING glassy and crystalline polymers specific microcracks (crazes) may develop. Unlike genuine fracture cracks the crazes are filled with highly disperse polymeric material. The walls of the microcracks join the strands of the polymer oriented in the direction of stretching with a thickness of the order of hundreds of Angstroms. The formation of such microcracks plays an important role in the processes of strain and fracture of polymeric materials [1-3]. In this connexion of great importance is information on the structure of the crazes and the processes of their generation and growth. Microcracks occurs particularly actively on strain of polymers in contact with adsorption-active media. In the present work we investigate the effect of the formation of a spatial network on the processes of strain of the crystalline HDPE polymer in a liquid medium. The structure of the microcracks formed on strain of crosslinked HDPE samples is characterized. We used isotropic samples of HDPE 400-700 pm thick with Mw=2"23 x l0 s. The degree of crystallinity of the initial polymeric sample according to DSC data was 71%. The network structure of HDPE was produced by irradiation at room temperature, source 6°Co. After irradiation the samples were annealed for 5 hr at 115°C. The conditions of irradiation are given in reference [4]. Calculation of the Mc between the chemical nodes in the irradiated samples was based on the formula M,=pRT[tan ~t, where tan ~¢is the tangent of the angle of slope of the straight line portion of the dependence of equilibrium stress on 2 - 2 z (2 is the degree of stretching) determined at 145°C with the Instron instrument. At doess D = 5, 10, 25 and 50 Mrad Mc x 10 -3 amounts respectively to 15, 3, 2.4 and 1.3-1.6. To this corresponds the content of gel fraction I0, 60, 67 and 77%. The first signs of gelation for the HDPE sample studied are observed on irradiation with a dose 5 Mrad. With increase in the radiation dose Mc falls. According to the DSC data the degree * Vysokomol. soyed. A30: No. 5, 963-968, 1988.
~84
Processes of microcracking of polyethylene in liquid medium
985
of crystallinity changes little for D< 50 Mrad. In further work the structural investigations were carried out in the main on HDPE samples irradiated with D= 10 and 50 Mrad. The structural changes in the HDPE samples stretched in a liquid medium were characterized by the small angle X-ray scatter method. The small angle photodiffraction patterns were obtained with the apparatus described in reference [5] using point collimation. The structure of stretched HDPE was also studied with the KRM-I small angle chamber with slit collimation of the beam. The angular resolution was 4'. The PE samples for the structural investigations were prepared as follows: the polymer samples were stretched in hexane at 30°C and strain rate 1 ram/rain to a defined degree of elongation, the stretched samples were then transferred to acetone and dried, the size of the sample being fixed. For the unirradiated and crosslinked H D P E samples stretched in hexane characteristic patterns of small-angle X-ray scatter were observed due to the formation of microcracks. The diffraction pattern on point collimation of the beam of the H D P E sample stretched in a liquid medium consisted of an intense meridional reflexion and a "line" reflexion extended in the equatorial direction (Fig. 1). Meridional scatter is
6 tn~"
d
2 6
/8 z llT-~ng.min e FIG. 1 FIG. 2 FlG. 1. Scheme of small angle reltexions and the structure of the HDPE sample stretched in a liquid medium. Direction of stretching shown by arrows. FlG. 2. Distribution of the intensity of X-ray scatter at small angles for the initial HDPE sample (1) and the sample stretched in hexane, filmed, in the equatorial direction (2). caused by the microcracks of width x arranged mostly perpendicular to the direction of stretching. The equatorial scatter is due to the system of fibrils disconnected in space and oriented in the direction of stretching and joining the opposite walls of the microcracks. The equatorial distributions of the intensity of small angle X-ray scatter obtained with use of slit collimation of the beam coupled with the data on change in the specific volume of H D P E on stretching in hexane were used to determine a number of parameters characterizing the structure of the microcracks. The function of intensity on filming with a slit chamber in the equatorial direction is formed by the superposing of scatter from the structural elements of two types: m i c r o c r a c k s - m e r i d i o n a l reflexion and the fibrils filling the m i c r o c r a c k s - e q u a t o r i a !
A.V. YErIMOVet al.
986
"line". Analysis of the diffraction patterns shows that on filming in the equatorial direction the meridional component makes an important contribution to the intensity of the scatter recorded in the region of small angles ~0<10-15'. To evaluate the characteristics of the fibrils joining the walls of the microcracks from the curve of equatorial scatter we subtracted the scatter due to the meridional component and then analysed the difference scatter. The components were separated in the Guinier coordinates In I-f02 assuming that the total scatter is made up additively of the corresponding components. Noting that the meridional component does not make an important contribution to the totai curve of scatter at fo> 10-15' it was considered that the curve with large slope in Fig. 2 reflects the contribution of this component to the equatorial scatter. Using the value of the invariant of the curve of equatorial scatter measured in absolute units Q and also the data on change in the specific volume of the polymer stretched in a liquid medium, we determined the volumetric concentration of the fibrils in the microcracks c oo
do Q = -2n ~ f I(~o) ~d~0= (/t,1)% ( 1 - e)(vo + ,dv)(1-c) 0
Here Vo is the specific volume of the non-strained HDPE; zlv is change in the specific volume on stretching; I=I' (~o)/lev'A, I' (q~) is the intensity of scatter measured in absolute units; v' is the scattering volume; 1e is the scattering capacity of the electron; A is the attenuation coefficient. The size of the volume of the fibrils related to the mass of the strained HDPE sample was evaluated from the relation C ~
vf vf +Av . . . .
The value of the invariant of the scatter curve was determined by graphical integration of the dependence of ar~ on ~. The technique of measuring the specific volume of the polymer on stretching in a liquid medium was described earlier [6]. The degree of stretching of the polymer in the fibrils of the microcracks was estimated from the equa12 + ,dr* where 2 is the degree of stretching of the polymeric sample in hexane. ti°n 2 f = e ( 1 -~'o) The dimensions of the fibrillar elements filling the microcracks were determined by the Porod method. In reference [7] it is shown that the scatter from a system of fibrils situated parallel to each other and separated by voids on filming with a slit chamber in the equatorial direction is described in the region of sufficiently large ~0by the relation I ~ k / ~ 3, where k is a coefficient proportional to the area of the surface of the fibrils. It turns out that for non-crosslinked and crosslinked HDPE samples stretched in hexane the product I~ 3 in the region of sufficiently large angles ~ is roughly constant,
Processes of microcracking of polyethylene in liquid medium
987
The size of the area of the surface of the fibrils related to the nuiss of the straincd H D P E sample was determined from the equation S
2 :k /l (~q) ,~.p\
vo /
where k=Iq) a in the region of sufficiently large angles. The diameter of the fibrils was found from the equation d=4t,~/S. The mean distance a bztween the centres of the fibrils was evaluated on the assumption of hexagonal packing of the fibrils in the microcrack from the relation a = di' 1.21,]~. The strain curves of the initial and crosslinked H D P E samples stretched in air and in hexane are presented in Fig. 3. It will be seen that the formation of a spatial network in the polymer leads to appreciable increase in o'~ and stress in the region of steady development of strain c~. On stretching of the H D P E sample in contact with hexane fall in o'~ and cTs is observed. The efficacy of the liqu.id medium evaluated from the relative fall in these characteristics diminishes as the density of crosslinking increases. Strain in air of the initial and also crosslinked H D P E samples is accompanied by neck forniation. It was earlier shown [8] that strain of non-crosslinked PE in hexane largely comes about through generation and subsequent development of specific microcracks. A characteristic sign of such a type of strain is growth of the specific volume of the polymer. Data on the dependence of the increment of specific volume Av/vo on the relative elongation of the sample may be used to evaluate the contribution of crazing to total strain. In general, elongation of the polymeric sample constitutes the sum of two terms: crazing and shear strain. A measure of the contribution of the microcracking process to the total deformation may be the slope of the curve Av)'o-~,. It will be seen (Fig. 4) that for non-crosslinked H D P E stretched in hexane the dependence of Av/vo on e is well approximated by a straight line with the tangent of the angle of slope 0.5-0.6. This means that in the region of elongations studied (up to 200 ~o) the contribution of microcracking to the total stcain of H D P E comes to 55-60~. Deformation of the crosslinked H D P E samples in hexane is also accompanied by the formation of crazes. In turns out that for the samples of H D P E irradiated with D = 10 Mrad, in the region e < 100% the relative role of each mechanism of strain changes little as compared with the non-crosslinkcd polymer. For such samples the contribution of microcracking to total deformation amounts to 45-50 ~. On stretching in hexane of the H D P E samples irradiated with a dose 50 Mrad shear strain p c e d o m i n a t c s - t h e contribution of microcracking to total deformation does not exceed 20--25 ~/o with a tendency for a neck to tbrm in such samples. Let us take a closer look at the stru.ctt, ral changes on deformation of the H D P E samples in hexane. From scanning electron microscopy on stretching of unirradiated H D P E samples in hexane (in the region of elongations roughly corresponding to a ~ of the polymer a large number of microcracks mostly aranged perpendicular to the direction of stretching and filled with oriented polymeric material appears. The width of the microcracks (linear size in the direction of stretching) is tenths of a micron but
988
A.V.
YEFIMOV et at.
the length'reaches several microns. The microcracks alternate with little defmmed regions of the non-porous polymer having much the same size. Schematically the structure of the HDPE sample stretched in a liquid medium is indicated in Fig. 1. From electron microscopy in the region of elongations exceeding a,c of the polymer strain of the HOPE is accompanied by increase in the width of the microcracks with insignificant change in their concentration. The formation of microcracks sharply raises the intensity of small angle X-ray scatter (Fig. 2). As noted, the small angle diffraction patterns of the HOPE samples stretched in a liquid medium are characterized by a meridional reflexion and also by a line reflexion extended in the equatorial direction. Scatter in line form is due to the system of fibrils disconnected in space and joining the walls of the microcracks. STRUCTURAL PARAMETERS OF THE MICROCRACKS FORMED ON DEFORMATION OF THE INITIAL AND CROSSLINKED
D, Mrad
0 0 0
e, % c, % 100 13 200 13 250 12
2t
6rim
10 11 13
10 12 11.5
HOPE
SAMPLES IN HEXANE
a, nm D, Mrad I e, %
23 27 27
10 10 50
100 200 100
c, %
At
6 nm
a, nm
21 23 32
7 8 5.5
9 l0 13
16.5 17.5 19
From the scatter curves recorded in the equatorial direction and also the data on change in the specific volume of the polymer for the HOPE samples stretched in hexane to different degrees of relative elongation, we determined the structural parameters of the microcracks (Fig. 5, Table). It was found that the volume and area of the surface of the fibrils increase approximately in proportion to e whereas the concentration of the fibrils in the microcracks and their diameter amounting to ~ 10 nm remain almost constant in the range of relative elongations studied. The degree of stretching of the polymer in the fibrils of the microcracks changes from 7 to 13 with change in e from 60 to 25070. These values are close to the degree of stretching of HOPE in the neck determined on stretching the polymer in air at 30°C. The value of the degree of stretching of the polymer in the neck changes from 7-5 to 12 with change in e from 200 to
40070. Since the quantity of polymeric material passing into the microcracks and also the area of the surface of the fibrils increase in proportion to the relative elongation of HOPE it may be assumed that the process of growth of the microcracks accompanied by drawing out of polymeric material from their walls. At the boundaries of the microcracks a highly localized process of transformation of the lamellar structure of the polymer to fibrillar takes place. Together with growth of*;he microcracks on stretching of HOPE in a liquid medium there is shear strain of the polymeric material situated between the microcracks. This process occurs without significant change in the specific volume of the material and together with microcracking determines the reaction of the crystalline polymer to the external influence in presence of a liquid medium. The structural rearrangement on stretching the crosslinked HOPE samples in a liquid medium follow a similar pattern, Onl_y the relationship between crazing and
Processes of microcracking of polyethylene in liquid medium
989
shear strain for the H D P E samples characterized by high density of thc nodes of the spatial network changes. The small angle scatter method was used to characterize the structure of the microcracks formed on deformation of the crosslinked H DPE samples in hexane (Tablc). It was found that the main feature of the structure of the crosslinked H D P E samples is the higher concentration of fibritlar material in the microcracks. This corresponds to a lower degree of stretching of the polymer in the fibrils of the microcracks formed in crosslinked H D P E samples as compared with the starting polymer. The diameter of the fibrils filling the microcracks virtually does not depend on the degree of crosslinking and the mean distance between fibrils decreases with rise in D. The 2r values determined for the crosslinked H D P E samples are comparable with the values of the degree of stretching of the crosslinked polymeric samples in the neck. The value of the degree of stretching in the neck for D = 10 Mrad is 5-6 and for D = 5 0 Mrad 4-5. It may be assumed that the limitation of the degree of stretching of the polymer in the fibrils of the microcracks of the crosslinked samples is connected with the presence of a spatial network in irradiated HDPE. The 2f values determined from small angle Xray scatter were compared with the calculated values of the maximum degree of stretching of the H D P E network characterized by a definite value of M,.. In the isotropic state the nodes of the H D P E network are separated by the distance d=0.57 M °'5 (with lhe condition that the segments of the macromolecules between nodes arc in the
20 ~,,~*~___._________._.__~ 3 lo
" --
I
1
I00
__
200
I
3OO
30 ~ . . ~ _ _ _ _ _ 5 ..~ !J
20
08-
Oq
I0
H-a I
.............
I
FIG. 3 FIG. 4 Fl(;. 3. Strain curves of HDPE irradiated with a dose of 0 (1, 2), 10 (3, 4) and 50 (5, 6) Mrad in air (l, 3, 5) and hexane (2, 4, 6). F!G. 4. Dependence of the increment of specific volume on the relative strain at D=0 (1), 10 (2) and 50 (3) Mrad for samples stretched in hexane.
990
A . V . YI~FIMOV et al.
-~60
0.2O'l
20
I00
200
e,%
FIc. 5. Change in the volume (1) and area of the surface (2) of the fibrils as a function of the elongation of HDPE in hexane. Gaussian coil conformation). The maximum degree of stretching of the network was determined from the relation ;tm,x= where l is the contour length of the completely
l/d,
extended segment of the chain between cross links
I=-~Io where Mo is the molar
mass of the monomer unit of HDPE and lo is the projection of the C - C bond on the axis of the chain. For D= 10 Mrad the value 2maxis 7"5 and for D=50 Mrad 5.3. These magnitudes agree with the 2f values determined from the small angle X-ray scatter. The presence of such a correlation suggests that the fall in the degree of stretching of the polymer in the fibrils of the microcracks is linked with the limited stretchability of the spatial network in the irradiated samples. It may also be concluded that the spatial network persists in the fibrils formed on microcracking of the crosslinked HDPE samples in a liquid medium. Thus, the structure of the microcracks formed on stretching HDPE in a liquid medium is determined by the parameters of the spatial network initiated in the polymer by irradiation. Another effect of the formation of the spatial network is change in the relationship between crazing and shear strain on stretching HDPE in a liquid medium. For sufficiently high degrees of crosslinking of HDPE shear strain becomes predominant on stretching the polymer in a liquid medium (Fig. 4). Change in the relationship of the strains of the two types may be explained as follows. The process of the formation and growth of crazes in a crosslinked polymer must be accompanied by local ruptures of the spatial network on formation of a highly developed surface of the fibrils disconnected in space. For a sufficiently high density of the nodes of the spatial network the energy of rupture of the chains on fibrillation makes a significant contribution to the work of formation of the crazes. At the same time the development of shear strain occurring without disturbances of the integrity of the material is apparently acompanied by slighter modification of the network structure of the polymer. This may hamper the process of crazing as compared with shear strain in the crosslinked polymeric samples.
Translatedby A. CROZy
[.~nk between gas permeability and structure of microporous films ot' PETP
991
REFERENCES 1. R. P. KAMBOUR, J. Polymer Sci. Macromolec. Rev. 7: 1, 1973 2. A. L. VOLYNSKII and N. F. BAKEYEV, Vysokodispersnoye oriyentirovannoye sostoyaniye polimerov (The Highly Disperse Oriented State of Polymers), Moscow, 1984 3. Crazing in Polymers. Advances in Polymer Sci. 52/53, 1983 4. Z. S. YEGOROVA, V. I. DAKIN and V. L. KARPOV, Vysokomol. soyed. A21: 9, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 9, 1979) 5. V. I. GERASIMOV and D. Ya. TSVANKIN, Pribl. tekn. eksp. 2, 1968 6. A. V. YEFIMOV, V. M. BULAYEV, A. N. OZERIN, A. V. REBROV, Yu. K. GODOVSKII and N. F. BAKEYEV, Vysokomol. soyed. A28: 1750, 1986 (Translated in Polymer Sci. U.S.S.R. 28: 8, 1951, 1986) 7. E. PARADES and E. W. FISCHER, Macromolec. Chem. 180: 2707, 1979 8. A. V. YEFIMOV, V. V. BONDAREV, P. V. KOZLOV and N. F. BAKEYEV, Vysokomol. soyed. A24: 1690, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 8, 1927, 1982)
Polymer Science U,S.S.R. Vot, 30, No. 5, pp, 991-999, 1988 Printed in Poland
0032-3950/88 $10.00+ .00
LINK BETWEEN GAS PERMEABILITY AND THE STRUCTURE OF M I C R O P O R O U S FILMS OF POLYETHYLENE TEREPHTHALATE STRETCHED IN ADSORPTION-ACTIVE MEDIA* YE. A. SINEVICH, M. S. ARZHAKOV, M. A. KRYKIN, S. F. TIMASHEV a n d N. F. BAKEYEV Karpov Physicoehemical Research Institute
(Received 24 November 1986) The results of measurements of gas permeability are used to model the structure of microporous PETP films obtained by stretching in liquid media. It is shown that the compacted external layers "sealing" the crazes on isothermal drying of the moist samples must contain a small number of through microhoIes strongly influencing gas permeability. MICROPOROUS p o l y m e r s o b t a i n e d by s t r e t c h i n g in p h y s i c a l l y active liquid m e d i a m a y be used in t h e processes o f m e m b r a n e s e p a r a t i o n o f liquids a n d gases [ l - 3 ] . On s t r e t c h i n g b e l o w the glass t r a n s i t i o n p o i n t the p o r o s i t y o f such m a t e r i a l s is g o v e r n e d by the presence in t h e m o f specific m i c r o c r a c k s (crazes) with a h i g h l y d e v e l o p e d i n n e r surface. A t t h e initial stages o f s t r e t c h i n g t h e n u m b e r a n d v o l u m e o f t h e crazes increases but in presence o f strains e a b o v e I 0 0 - 2 0 0 ~ t h e m i c r o p o r o u s s t r u c t u r e begins to c o l l a p s e * Vysokomol. soyed. A30: No. 5, 969-975, 1988.