Investigation of gelation in poly(hexadecyl acrylate) solutions

Gelation in poly(hexadecyl acrylate) solutions

2939

8. V. A. TOCHIN and D. N. SAPOZ IKOV,Vysokomol. soyed. A16: 605, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 3, 699, 1974) 9. A. S. MICHAELS and H. J. BIXLER, J. Polymer Sci. 50: 393, 413, 1961 10. V. G. NIKOL'SKII and G. I. BURKOV, Khimiya vysokikh cnergii 5: 416, 1971 11. Ya. S. LEBEDEV, Kinetika i kataliz 8: 245, 1967 12. R. M. BERRER, Diffusion in Solids, 1948 13. T. F. SCHATZKI, Polymer Preprints 6: 646, 1964

INVESTIGATION OF GELATION IN POLY(HEXADECYL ACRYLATE) SOLUTIONS* A. I. ARTYUKHOV, T. I. Bo~isovA, L. L. BURSHTEI~, D. A. DMrr~tOCHEI~KO and V. A. SHEVELEV High Polymer Institute,U.S.S.R. Academy of Sciences (Received 23 December 1974)

A study was made of characteristic effects of thermally reversible gelation on the dielectric properties, electrical conductivity, proton magnetic relaxation properties and self diffusion in the system polydecadeeyl acrylate-n-decane with differing ratios of the components. It is shown that the effects in question are attributable to changes in the mobility of the kinetic units of the macromolecules, ion impurities, and in the rotational and translational mobility of solvent molecules during macromolecular network formation. UI~D~.R certain conditions solutions of a number of polymers are capable of crosslinking with the formation of a thermally reversible physical network with gel transitions. The latter m a y result from the formation of stable intermolecular bonds (e.g. H bonds, as in polyvinyl alcohol [1, 2]) or from the tendency of macromolecules to form supermolecular structures. Thermally reversible gelation mechanisms are only partially understood as yet where polymer solutions are concerned, with uncertainly remaining both as to the role of different types of polymer and solvent and as to kinetic aspects of the formation (breakdown) of gel structures. Homologues of poly(alkyl acrylates) and poly(alkyl methacrylates) are in the category of polymers t h a t form gels [3]. Strong interaction between side groups is characteristic of these polymers in the solid phase, and results in their crystallization [4-7]. Our aim in the present work was to examine the mechanism of the thermally reversible transition gel-solution of the system poly(hexadecyl acrylate (PA-16)* Vysokomol. soyed. A17: No. 11, 2552-2557, 1975.

A. I. ~_RTYUKHOV(~t al.

2940

n-decane by analysing the mobility of polymer and solvent molecules by the dielectric loss method, and by polarization, electrical conductivity and NMR investigations. The synthesis of PA-16 was described in Ref. [5]. The tangent of the dielectric loss angle (tan 5) was measured with a.e. bridges at frequencies o f f : 1-103 Hz. The temperature dependences of volume resistivity pv were plotted with the aid of an EK6-7 tera-ohmmeter using a double coordinated recording apparatus, the heating (cooling) the specimens at the rate of 1-2 deg/min; the voltage for a specimen was 10 V.

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FIG. 1. Temperature dependences of ~' (a) and t a n ~ (b) at a field frequency of 1 Hz for •solutions of PA-16 in deeane with PA-16 concentrations (wt.~o) of: 0 (1), 11 (2), 40 (3) a n d 70 (4). The pointers show the direction of change in temperature. \

FIG. 2. Temperature dependences of t a n ~ (1-4) and e' (1"-4') for a solution of PA-16 in deeane (2-6 wt. ~/o PA-16) at frequencies of 1 (1, 1'; 4, 4'), 11 (2, 2') and 800 Hz (3, 3') obtained b y heating (1, 1', 2, 2', 3, 3') and cooling the solutions (4, 4').

Gelation in poly(hexadeeyl acrylate) solutions

2941

The spin-lattice relaxation time T1 was measured by a plused method with the aid of a nuclear magnetic relaxometer. A 90 ° pulse sequence was used. The coei~eient of serf diffusion of the solvent D was determined by the spin-echo technique [8] with a magnetic field gradient of 1.3 gauss. We also analysed the magnitude of nuclear induction signal amplitude A. All the measurements were carried out in hermetic ampoules (NMR) or in cells, in a nitrogen atmosphere. The temperature measurement interval was 30-100 °. PA-16 solutions were prepared at 80-100°; these were kept for 1 to 2 hr, and then cooled at tho rate of 1 to 2 deg/min. ;,°C ~o,' 20

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FIG. 3. Temperature dependence of gelation vs. concentration of the solution of PA-16 in decade based on results of DTA [9] (1); tan 8ma~ and log pv ,nn (2); log T1 (3); A/A* (4) and log D (5). F i g u r e 1 shows the t e m p e r a t u r e dependences of t a n 5 and e' for the s y s t e m P A - 1 6 - d e c a n e a t 1 Hz. At t e m p e r a t u r e s of 15-22 ° near the gel-solution t r a n s i t i o n t e m p e r a t u r e Tgel the t a n 5-~q~(T) curve goes t h r o u g h a sharp m a x i m u m , t h e height of which is largely d e p e n d e n t on the r a t e at which the t e m p e r a t u r e o f a specimen changes, the height rising as t h e latter t e m p e r a t u r e is increased. At c--~const (c being the wt. a m o u n t of PA-16 in decane) the position of t a n 5max on the t e m p e r a t u r e scale is not a f u n c t i o n o f field f r e q u e n c y (Fig. 2), b u t its v a l u e falls as t h e reciprocal of f. The t e m p e r a t u r e o f the m a x i m u m Tmax for t a n 5, where f ~ const, practically coincides as a function of c o n c e n t r a t i o n with t h e gel-solution t r a n s i t i o n t e m p e r a t u r e d e t e r m i n e d b y D T A [9] (Fig. 3). A t t h e t i m e t a n 5 is passing t h r o u g h a m a x i m u m t h e r e is a simultaneous rise in e', the scale of which is directly proportional to p o l y m e r concentration. A b o v e the gel melting p o i n t T~e1values of t a n 5 are the reciprocal of field frequency, a n d m a y be a t t r i b u t e d to losses due to electrical c o n d u c t i v i t y (Fig. 2). T e m p e r a t u r e curves of t a n 5 a n d e' p l o t t e d for the heating a n d cooling o f specimens do n o t coincide. I n the case of cooling the region of t a n 5max is enlarged as a result of the low t e m p e r a t u r e b r a n c h being displaced towards lower t e m p e r a t u r e s (Fig. 2), a n d displacement of the region of the infiexion towards lower t e m p e r a t u r e appears o n t h e ~'-~a (T) curve. On c o m p a r i n g t h e observed dependences of e' a n d t a n 5 with corresponding d a t a for p u r e solvent, it can be said with confidence t h a t breaks in the m o n o t o n i c

2942

A.I. A R ~ O V

et al.

character of the curves are due to the presence of dissolved polymer molecules in the system. At the same time the fact that /'max and Tgei coincide within the range of composition means that the dielectric effects are related to gel:solution transition.

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Fro, 4. Temperature dependences ef log pv for the systems P A - l ~ e c ~ e with PA-16 concentrations of 5 (11 and 70 w t . ~ (2). A rise in ~' and tan Jmax at the moment the gel melts points to the polarization of polar groups which are possessed solely b y polymer molecules in the system of interest. This leads, in the case of gel-solution transition, to a change in the rotational motion of macrochain kinetic elements including the polar portion of the side pendant. To determine the possible contribution of ionic conductivity to tan Jmax in the vicinity of Tgel we investigated the temperature dependence of log p~ (Fig. 4). I t was found that at Tgel curves of log pv-=q~(T) undergo transition through a sharp minimum. On determining the corresponding value of tan J b y use of the formula tan J : 1.8 × 1012/f.~'p~ [10], it was found that the contribution made b y electrical conductivity is fairly substantial, and that the observed values of tan ¢fmaxare to a significant extent due to an anomalous increase in currents of continuous conductivity at the moment of gel-solution transition. The current carriers in the systems appeared to be ion impurities introduced together with polymer. Clearly the number of free ions in a system cannot be increased as a result of gel transition, nor can the charge on the ions be thereby increased. To account for the marked reduction in Pv, and the consequent rises in electrical conductivity and in tan j at Tael, it follows that one need only assume an anomalous time dependent reduction in the microscopical viscosity of a system. Evidence of the time dependence of crosslinking and viscosity comes to light in a widening of the transitional region of gelation during the cooling of a system,

Gelation in poly(hexadecyl acrylate) solutions

2943

as well as in the fact t h a t in the region of TgeI extremal values of t a n 5 and p~ are dependent on the rate of change in temperature. The period of time during which viscosity, at a n y rate localized viscosity, remains considerably below true solution viscosity is apparently determined by the duration of the state where no three dimensional network has as yet been formed, but where supermolecular formations are drawing closer together, and are thereby creating micro volumes of solvent in which no macromolecules are present.

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FIG. 5. Plots of log T1 (1-5) and A / A * (1", 3', 4') vs. reciprocal temperature in PA-16decano systems with polymer concentrations of 20 (1, 1'), 30 (2), 40 (3, 3'), 70 (4, 4') and 100 wt.% PA-16 (5). FIG. 6. Plots of log D vs. reciprocal temperature in PA-16-decane systems with polymer concentrations of 0 (1), 1 (2), 10 (3), 20 (4), 30 (5) and 40 wt.~/o (6). The rotational and translational motion of solvent molecules in the process of gel formation was investigated by NMR. The spin-lattice relaxation time (T1) of the solvent is for the most part inversely proportional to the rotational diffusion correlation time. The intramolecular contribu~tion to the value of 1 / T 1 is related to rotational motion, and for normal \

2044

A. I. A~T~HOV et al.

alkanes amounts to ~ 70% [12]. Figure 5 shows log T 1 vs. lIT plots characterizing change in the rotational motion of decane. With low concentrations of polymer the behaviour of a PA-16~teeane system, whether in the state of a solution or a gel, is that of a magnetically homogeneous system, i.e. spin-lattice relaxation can be satisfactorily described b y a single relaxation time constant, T1, which is assignable in tote to solvent protons. A reduction in T 1 on traversing the gelation temperature is therefore evidence of a longer correlation time for solvent molecules, and consequently points to increased hindrance of rotational motion of solvent molecules. In the case of 30-40~o concentrations the relaxation takes on a nonexponential to polymer owing to the fact that the induction signal component relating to polymer protons becomes appreciable, but it is quite possible to separate the relaxation curve of the solvent. With a 70% polymer concentration it is difficult to separate the latter curve. In this case determination of T1 was based on the initial slope of the total relaxation curve, which in fact gives a mean weighted value of the relaxation time for both the proton-containing components of the system of interest, with the contribution of the polymeric component predominating. The dependence of log T1 on 1/T for PA-16 containing no solvent shows that transition occurs at the moment the crystalline structure melts at 38 ° [13]. With the introduction of solvent the region of the transition is displaced towards low temperatures and becomes less prominent. The observed transition m a y therefore be attributed to a change in the order of side-chains, and is consequently related to mobility of the macromolecules, and of solvent linked to polymer. For the systems with a polymer concentration of 20% or more a change in the amplitude of the free nuclear induction signal observed after the single 90 ° pulse was detected at the gelation temperature. Figure 5 shows temperature dependences of the signal amplitude referred to its value at 20 ° A/A * for systems with differing concentrations of polymer. The marked reduction in signal amplitude at Tgel is attributable to the reduced mobility of macromolecules forming a network, and to the reduced mobility of solvent linked to polymer. As a result spin-spin relaxation times for protons entering the latter macromoleeules become close to the time for non-sensitivity of the relaxometer detector ( ~ 20/~ sec). This means that a portion of the signal is lost, and the observed portion relates mainly to free solvent protons. As the concentration rises, A/A* changes considerably not only on account of the larger number of polymer protons, b u t also on account of the larger amount of solvent linked to macromoleeules or immobilized in forming network cells. Figure 6 shows temperature dependences of the coefficient of progressive selfdiffusion of the solvent D measured b y the spin-echo method for differing polymer concentrations. Whereas D values for a 1% solution coincide with the values for pure solvent, D is reduced as polymer concentration rises. A rise in the average degree of inhibition of translational and rotational motion in a solution accompanying a rise in concentration points to a larger proportion of solvent that is

Gelation in poly(hexadecyl acrylate) solutions

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solvating macromolecules and is consequently less mobile, but which is fairly rapidly interchanged (in a time shorter t h a n TR) with the remainder of the solvent. At the point of gel formation the coefficient of self-diffusion of the solvent is greatly reduced, the degree of reduction being proportional to polymer concentration. The relative change in D exceeds the corresponding change in T 1. In view of this one may assume t h a t during the formation of a network translational motion o f the solvent is inhibited to a greater extent than the rotational motion. Gelation temperatures based on the data in Figs. 5 and 6 are in good agreement with those determined by DTA and on the basis of the dielectric measurements (Fig. 3). In the region of 1-30% concentrations of PA-16 in decane Tgel is practically independent of the ratio of components in the system, but with higher concentrations of polymer a monotonic rise in TgeLis observed. The smooth character of the composition dependence of Tgel in the case of 30-100% concentrations of polymer points to a gradual improvement in intra- and intermolecular packing, the final form of which resembles the crystalline structure of a block polymer. Further evidence of the presence of structures similar to the crystalline packing of block polymers in the gels with a >t 23% PA-16 concentration is ~seen in the shoulder appearing in the low temperature branch of the tan J ~ (T) plot (Fig. 1). This m a y be compared with the region of tan (~maxin PA-16 ( ~ 0°C) t h a t may be attributed to polarization processes due to transition from double layes to single layer packing of PA-16 molecules in the vicinity of the melting point [13]. Thus, the results obtained by investigation of temperature dependences of characteristics of dielectric-and nuclear magnetic relaxation, volume resistivity a n d self-diffusion coefficients show that gel formation in a PA-16-decane system is accompanied by a reduction in the mobility of polymer molecules, and likewise b y reduced translational and rotational mobility of non-solvated solvent. Translated by R. J. A. HENDRY REFERENCES 1. L. Z. ROGOVINA, G. L. SLONIMSKII, L. S. GEMBITSKII, Ye. A. SEROVA, V. A.

2. 3. 4.

5. 6.

GRIGOR'EVA and Ye. N. GUBENKOVA, Vysokomol. soyed. A15: 1256, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 6, 1411, 1973) L. N. VERKHOTINA, L. S. GEMBITSKHand Ye. N. GL~ENKOVA, Vysokomol. soyed. A15: 1350, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 6, 1516, 1973) N. A. PLATE and V. P. SHIBAYEV, Vysokornol. soyed. A13: 410, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 2, 466, 1971) V. P. SIIIBAYEV, R. V. TAL'ROZE, B. S. PETRUKHIN and N. A. PLATE, Vysokomol. soyed. BI3: 4, 1971 (Not translated in Polymer Sci. U.S.S.R.) V. P. SIY[RAYEV, B. S. PETRUKIIIN, Yu. A. ZUBOV, N. A. PLATE and V. A. KhRGIN, Vysokomol. soyed. A10: 216, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 1, 258, 1968) V. N. TSVETKOV, I. N. SHTENNIKOVA, Ye. V. KORNEYEVA G. F. PIROGOVA, L. HA_RDI and K. NITRAI, Vysokomol. soyed. All: 345, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 2, 392, 1969)

2946

T. M. Bn~sa~,~,~ et al.

7. R. V. TAL'ROZE, Yu. K. OVCHINNIKOV, L. A. SHTEINBERG, V. P. SHII~AYEV and N. A. PLATE, Vysokomol. soyed. B15: 289, 1973 (Not translated in Polymer Sci.

U.S.S.R.) 8. 9. 10. 11.

N. Y. CARR and E. M. PURCELL, Phys. Rev. 94: 630, 1954 R. V. TAL'ROZE, Dissertation, 1973 B. V. HAMON, :proc. Inst. Electr. Engrs. 90: 151, 1952 G. L. SLONIMSKII, V. B. TOLSTOGUZOV and D. B. IZYUMOV, Vysokomol. soyed. B12: 160, 1970 (Not translated in Polymer Sci. U.S.S.R.) 12. $. E. ANDERSON and W. P. SLICHTER, J. :Phys. Chem. 69: 3099, 1965 ] 3. T. I. BORISOVA, L. L. BIRSHTEIN, V. A. SHEVELEV, V. P. SHIBAYEV and N. A. PLATE, Vysokomol. soyed. AI3: 2332, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 10, 2619, 1971)

STUDY OF CONFORMATION CHARACTERISTICS OF BLOCK COPOLYMER MOLECULES IN SOLUTION BY THE MONTE CARLO METHOD* T. M. BIRSHTEr~¢,A. M. SKVORTSOVand A. A, S~mx_~ I n s t i t u t e of High Molecular Weight Compounds, U.S.S.R. Academy of Sciences

(Received 23 December 1974) The behaviour in solution of a block copolymer molecule containing two blocks of the same length was simulated. A study was made of the effect of hetero-interactions on the dimensions of the entire chain and its separate blocks and on the average n u m b e r of homo- and hereto-contacts. I t was shown that when polymer components are incompatible the blocks are segregated as is the case with homopolymer chain segments in a good solvent. I n the region where hetero-contaets are favourable from a n energy point of view the blocks are displaced, accompanied b y a sudden decrease in the dimensions of the entire chain and a slight reduction of individual block dimensions. The behaviour of a block copolymcr molecule on changing temperature was analysed and it was shown t h a t a sudden change m a y take place in the dimensions of the entire chain. Results were compared with experimental information concerning the temperature dependence of the intrinsic viscosity of block copolymers.

~OPERTIES of copolymers containing components of different chemical natures depend both on the general composition and the sequence of components in the chain, i.e. on primary polymer structure. Block copolymers, of which, the m01ecules contain elongated blocks of various components are of particular interest as regards structural characteristics and combination of properties. * Vysokomol. soyed. A17: No. 11, 2558-2565, 1975.