Sorption and diffusion of low-molecular-weight compounds in polymer composites based on high-density polyethylene and poly(methyl methacrylate)

910

L . I . LOPATINAet aL REFERENCES

1. M. I. BESSONOV, M. M. KOTON, V. V. KUDRYAVTSEV and L. A. LAIUS, Poliimidy-klass termostoikikh polimerov (Polyimides - A Class of Thermally Stable Polymers). 328 pp., Leningrad, 1983 2. K. N. VLASOVA, A. V. SAMOKHVALOVA and G. A. RUZHENTSEVA, Plast. massy, 11, 56, 1971 3. N. A. ADROVA, M. M. KOTON and L. K. PROKHOROVA, Vysokomol. soyed. BI7: 409, 1975 (Not translated in Polymer Sci. U.S.S.R.) 4. A. N. KRASOVSKII, T. T. REDROVA and V. B. ALESOVSKII, Dokl. Akad. Nauk SSSR 268: 1412, 1983 5. N. M. EMANUEL' and D. G. KNORRE, Kurs khimicheskoi kinetiki (A Course of Chemical Kinetics). 463 pp.. Moscow, 1984

PolymerScienceU.S.S.R. Vol.29, No. 4, pp. 910-915, 1987 Printed in Poland

0032-3950/87 $10.00+.00 © 1988PergamonPress plc

SORPTION AND DIFFUSION OF LOW-MOLECULAR-WEIGHT C O M P O U N D S IN POLYMER COMPOSITES BASED ON HIGH-DENSITY POLYETHYLENE AND POLY(METHYL METHACRYLATE) * L. I. LOPATINA, M. S. ARZHAKOV, A. L. VOLYNSKII and N. F. BAKEYEV M. V. Lomonosov Moscow State University (Reeived 23 September 1985)

Sorption and diffusion of low-molecular-weight compounds was studied in high-density polyethylene deformed in heptane and also in polymer composites on its basis. Thermodynamic parameters characterizing the sorption have been calculated for high-density polyethylene defolmed in heptane and for its composites with poly(methyl methacrylate). The relationship between structural transitions in polyethylene deformed in a liquid and the sorption characteristics is discussed. AN ORIGINALm e t h o d has been developed [1 ] for the p r e p a r a t i o n o f p o l y m e r composites f r o m high-density polyethylene ( H D P E ) a n d poly(methyl methacrylate) ( P M M A ) , whereby H D P E is d r a w n in the methacrylic m o n o m e r which is subsequently p o l y m e r ized. Uniaxial extension o f a H D P E film in a compatible liquid strongly raises the p o l y m e r swelling capacity [1, 2], so that a substantial a m o u n t of the m o n o m e r can be i n c o r p o rated. The resulting p o l y m e r composites represent two-phase systems consisting of tWO i n c o m p a t i b l e polymers. * Vysokomol. soyed. A29: No. 4, 823-827, 1987.

Sorption and diffusion of low-molecular-weight compounds

911

In order to elucidate the structure of such systems a n d to clarify the t h e r m o d y n a m i c s of mixing, we have studied sorption a n d diffusion of low-molecular-weight substances in H D P E / P M M A composites p r e p a r e d by p o l y m e r i z a t i o n of M M A in the p o l y m e r matrix. This a p p r o a c h can yield valuable i n f o r m a t i o n on the structure o f a m o r p h o u s regions in the crystalline p o l y m e r a n d also of the second polymeric c o m p o n e n t , since sorption characteristics are k n o w n to be very sensitive to changes in p o l y m e r structure. The details of methodology, the properties of the starting extruded HDPE, and the composition of the prepared composites have been already published [1]; results of X-ray analysis and mechanical parameters of the composites can be found in [1, 3]. The sorption experiments were carried out in an apparatus of McBain type; the sensitivity of the employed quartz spirals was 3"2 rag/ram and 0"6 mg/mm, respectively. The sorption cell was thermostated and evacuated (10 -2 to 10 -3 mmHg). Carbon tetrachloride and acetone were used alternatively as sorbates; the former is a selective solvent for HDPE since it swells polyethylene to a considerable degree and does not interact with PMMA, while acetone does not swell HDPE but dissolves PMMA. The change in polymer mass during sorption was determined from the elongation of the quartz spiral, measured by means of a cathetometer KM-6. The kinetics of sorption was followed in a stepwise manner, increasing the vapour pressure after establishment of sorption equilibrium. The thermodynamic functions were calculated according to [4]. The reported changes of thermodynamic functions refer to the amorphous phase in HDPE in the case of sorption of CC14 and to the weight fraction of PMMA for sorption of acetone vapour. The degree of crystallinity was calculated for HDPE and for each composite from the calorimetric data registered with the instrument DuPont 1090. Degree of crystallinity of HDPE and the content of PMMA in the composites is plotted in Fig. 1 as a function of HDPE elongation. The sorption behaviour of the composites wa s compared with that of a model system, represented by HDPE elongated in heptane and annealed under isometric conditions at 100°C for 1 hr.

Typical sorption isotherms of CCI4 are shown in Fig. 2 for HDPE and for HDPE/ /PMMA composites prepared by polymerization of MMA in the polymer matrix.

~rle/k9 ~o,%

x,% 2

60

l

~

3

60

o

2

20

20 t__

I

100

300

FIG. 1

2t,%

0.2

O.6

P/P~

I l'O

FIG. 2

FIG. 1. Content of PMMA (~) in the composite (l) and the degree of crystallinity (x) of pure HDPE deformed in heptane and of HDPE in the composite with PMMA (2), plotted against the draw ratio 2 of HDPE in either heptane or MMA. F~G. 2. Sorption isotherms of CCI4 vapour in the original HDPE (1), in HDPE defored to 200% in heptane (2), and in the HDPE/PMMA composite (3) prepared by elongation in MMA to 2=200%.

912

L.I. LOPATINAet

al.

In all instances the extent of CC14 sorption in the amorphous regions of HDPE and of acetone in PMMA was considerable. The shape of the isotherms was different between samples prepared under various conditions. The data on sorption of CCl4 in the investigated polymers show that the sorption capacity and the free energy of mixing are substantially different between HDPE elongated in a plasticizing liquid and the prepared composites; for example, the amount of CC14 sorbed into the amorphous regions of HDPE in the composite is almost twice as high as in the amorphous regions of the original, non-deformed HDPE or of HDPE deformed in heptane. The sorption is obviously very sensitive to polymer structure, and one must consider the character of structural rearrangements that take place in HDPE during deformation in the liquid monomer and during the subsequent polymerization. The following mechanism of deformation of HDPE in a liquid medium has been proposed [5]: HDPE with lamellar crystallites oriented perpendicularly to the extrusion direction was elongated along the machine direction, because in this case the liquid is "sucked in". (The liquid is not absorbed when the polymer is elongated along the direction parallel with the orientation of the lamellar crystallites.) Deformation of extruded HDPE proceeds as irregular unfolding of lamellae. A fraction of polymer originally in the lamellae thus becomes a part of the intercrystalline amorphous phase. Deformation of this type is accompanied by considerable absorption of the low-molecular-weight liquid, which continues up to some 200 ~ elongation, where a distorion of lamellae leads to recrystallization resulting in a formation of fibrillar structure; further incorporation of the liquid into HDPE stops at this point. Let us now consider from this perspective the observed changes in sorption characteristics of HDPE deformed in heptane and subsequently annealed under isometric conditions. Figures 3 (curve 1) and 4 show the fi'ee energy, enthalpy, and entropy of mixing for sorption of CC1, as a function of extension ratio of HDPE in heptane. We see a marked gain in free energy of mixing, essentially independent of elongation. However, the detailed mechanism responsible for this gain in free energy depends on polymer structure. The enthalpic component dominates in the original, non-deformed polymer. This can be attributed to the fact that the elastic deformation in swelling of the amorphous phase in a crystalline polymer is governed not by entropy but by enthalpy [6]. Moreover, because the specimens were crystallized from melt, the tie chains in the amorphous regions between the neighbouring crystallites are strained, so that the amorphous phase is less densely packed; one may reasonably assume that sorption of CC14 in these regions will result in a non-additive increase in density, a process accompanied by a gain in enthalpy and a loss in entropy. Following an elongation to less than 200 ~, the amorphous regions in HDPE grow at the expense of crystallites. Recrystallization takes place during the subsequent annealing of this material and the overall degree of crystallinity is then the same as in the original polymer (Fig. 1), although the two materials differ in the structure of the amorphous phase: the original structure developed as a result of crystallization from melt cannot be fully restored in a material annealed at a temperature below the melting temperature of HDPE. Consequently, the amorphous regions in such matelials are

Sorption and diffusion of low-molecular-weight compounds

913

less strained and the more relaxed chains resemble more or less those of a true rubber. Swelling is then governed by a positive entropy of mixing. In the region of larger deformations (300 to 4 0 0 ~ ) recrystallization leads to the formation and development of fibrillar structure, where the amorphous regions between the crystallites inside and outside the fibrils after the annealing are again strained; it is then natural that the enthalpic contribution to the free energy of mixing becomes once more decisive. The thermodynamic characteristics of sorption are thus seen to correlate with the structural transitions in the deformed polymer. tt

4

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o

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I 300 F]o. 3

,,1,%

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I

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FIG. 4

FIG. 3. Free energy of mixing (AG) of CC14in HDPE, plotted as a function of draw ratio 2 of HDPE in heptane or in MMA. HDPE samples elongated in heptane (1) and the HDPE/PMMA composite (2). Here and in Fig. 4 and 5 I4"2=0'95. F~o. 4 Entropy (1) and enthalpy of mixing from sorption of CCL vapour as a function of the draw ratio for HDPE elongated in heptane. Thermodynamic parameters of sorption change markedly when a second polymer, incompatible with HDPE, is present in the intercrystalline regions. As shown in Fig. 3 (cu,'ve 2) and Fig. 5, in this case the driving force of sorption is exclusively enthalpic, regardless of the extent o f polymer elongation. In the region of small draw ratios (below 200 ~ ) the contribution of the entropic term increases but slightly, never attains positive values, and thus cannot be decisive for the direction of the process. The enthalpic term increases rapidly in the region of large deformations. The obtained results can be explained as follows. A system containing a second polymeric component, highly dispersed in the intercrystalline space, is apparently rather unfavourable from the thermodynamic point of view, and can be stable only due to the high modulus of the crystalline structure of the matrix polymer or because of the existence of a mixed transition phase; its presence has been proved in [1]. Thermodynamic instability of the system has been demonstrated by annealing experiments: when the composite is annealed above the melting pint Tm of HDPE, the system separates into coarse aggregates of the incompatible polymer components. The amorphous regions in such a composite are naturally extremely strained and their structure is very loose, so that for reasons explained above sorption of a low-molecular-weight

914

L. I, LOPATINA et al.

compound in this phase leads to a substantial gain in enthalpy. The highly developed interfacial surface area represents an additional factor which lowers the entropy of sorption, since adsorption of the sorbate molecules on the surface also leads to a decrease of the entropy of mixing. The assumed highly strained character of the two coexisting, incompatible polymers in the composite is confirmed by experinaents with sorption of acetone, a compound which practically does not interact with H D P E but is a good solvent for PMMA, as demonstrated by its sorption in pure linear PMMA: starting from p/p, =0.3 sorption equilibrium could not be attained since the polymer dissolved and it was impossible to measure the whole sorption isotherm. On the other hand, P M M A incorporated into the H D P E matrix behaves like a crosslinked polymer capable only of limited swelling. The PMMA. phase in the composite is also strained, as documented by high values of the enthalpy of mixing and negative values of entropy, but its structure is essentially independent of H D P E elongation; this is not surprising since IR spectroscopy has shown t h a t - i n contrast to H D P E - P M M A is not oriented in the composite regardless of the draw ratio of HDPE. 0 2

-•-2

log /9 -7

I

t'~

07 ~

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x

03

-3

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IO0

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300

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FIG. 6

FIG. 5. Dependence of etnhalpy AH (1) and entropy AS (2) of mixing on the draw ratio 2 for HDPE elongated in MMA; from sorption of CC14 in HDPE/PMMA composites. Fro. 6. Effect of the draw ratio 2 of HDPE elongated in heptane or in MMA on diffusion of CC14 in HDPE elongated in heptane (1) and in HDPE/PMMA composite (2), and also on diffusion of acetone in the composite (3). The results are further supported by data on sorption kinetics. Figure 6 shows the diffusion coefficients of CCI~ (curve 2) and of acetone (curve 3) in a H D P E composite. The diffusion coefficient of CCI,~ drops to one tenth when P M M A is incorporated into the H D P E matrix, apparently because of the presence of a dispersed impermeable phase in the amorphous regions; similar lowering in the rate of diffusional transport through a polymer by means of an impermeable heterogeneity has been frequently observed [7]. The substantial reduclion of the rate of acetone diffusion observed for the composites characterized by draw ratio above 200~o may be attributed to the for-

Sorption and diffusion of low-molecular-weight compounds

915

mation in these composites of fibrillar structure, less permeable to small molecules of the sorbate than the original lamellar structure [8], and possibly hindering the access of acetone molecules to the P M M A phase. We have shown that thermodynamic parameters characterizing sorption of low-molecular-weight compounds in polymer composites can be used to elucidate structural rearrangements which take place in detbrmed polymers. The driving force of CC14 sorption in H D P E deformed in heptane changes its character in dependence on the extent of elongation of the polymer: in non-deformed H D P E it is almost entirely enthalpie in the region of elongations below 200 ~ the entropic term pre vails, while the contribution of enthalpy again begins to dominate at still larger elongations. In contrast to pure H D P E , sorption of a liquid by H D P E phase in the composite is governed essentially by enthalpy regardless of the extent of initial elongation. Translated by M. KUBiN REFERENCES

1. A. L, VOLYNSKII, A. Sh. SHTANCHAYEV and N. F. BAKEYEV, Vysokomol. soyed. A26: 2374, 1984 (Translated in Polymer Sci. U.S.S.R. 26:11, 2654, 1984) 2. 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) 3. A. L. VOLYNSKII, A. Sh. SHTANCHAYEV and N. F. BAKEYEV, Vysokomol. soyed. A27: 831, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 4, 933, 1985) 4. A. A. TAGER, Fizikokhimiya polimerov (Physical Chemistry of Polymers). p. 300, Moscow, 1978 5. A. L. VOLYNSKII, A. Sh. SHTANCHAYEV and N. F. BAKEYEV, Vysokomol. soyed. A26: 2445, 1984 (Translated in Polymer Sci. U.S.S.R. 26: ll, 2741, 1984) 6. Yu. K. GODOVSKII, Teplofizika polimerov (Thermophysics of Polymers). p. 171, Moscow, 1982 7. W. B. HOPFENBERG and D. R. PAUL, In: Polymer Blends (Eds. D. R. Paul, S. Newman), p. 503, Moscow, 1981 8. A. PETERLIN, Pure Appl. Chem. 39:239,1974