Molecular mobility in epoxy polymers at different stages of curing in bulk and at the interface

Molecular mobility in e p o x y polymers 7. 8. 9. 10. 11.

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A. K. KURILENKO and L. B. ALEKSANDROVA, Khimich. volokna, No. 3, 65, 1965 H. J. BREIS, Fluorine Chemistry vol. 5, New Y o r k - L o n d o n , 1964 A. KOBAJASHI, J. P o l y m e r Sci. 51: 359, 1961 G. B. RATHMAN and F. A. BOVEY, J. Polymer Sci. 15: 544, 1955 V. Ya. KABANOV, N. I. KAZIMIROVA, A. A. NESTERENKO and V. I. SPITSYN, Vysokomo]. soyed. B10: 855, 1968 (Not translated in Polymer Sci. U.S.S.R.)

MOLECULAR MOBILITY IN EPOXY POLYMERS AT DIFFERENT STAGES OF CURING IN BULK AND AT THE INTERFACE* Yu. S. LIPATOV, F. G. FABULYAK, N. G. POPOVA and I. M. NOSALEVICH I n s t i t u t e of the Chemistry of H i g h Molecular Weight Compounds, Ukr.S.S.R. A c a d e m y of Sciences V. I. Lenin Polytechnic, K h a r k o v (Received 5 June 1970)

IT WAS established in earlier studies [1-6] that the properties of polymers at the interface with the solid change, in comparison with bulk properties. Investigations of dielectric relaxation [2,4-6] and spin-lattice relaxation of protons [3-6] of polymethylmethacrylate, polystyrene, styrene-methylmethacrylate copolymers, polyurethanes and the initial materials used for preparation, which are in thin layers on the surface of finely dispersed quartz particles, modified and unmodified aerosil and fluoroplastic show that the positions of maxima tan g and minimum T1 of existing relaxation processes change since the mobility of groups, chain segments and other larger structural elements are different at the interface. Using molecular mobility at the interface of these objects an explanation was given of the effect on the mobility variation of polymer chains of a vigorous interaction with the surface and of conformation effects at the interface. It was shown that this is mainly determined by a reduction in the number of conformations possible for the macromolecule at the interface and not by the vigorous interaction with the surface. It is of interest to examine the variation of molecular mobility during the formation of a three-dimensional polymer and the effect of the solid surface on the hardening process of binders. This paper seeks to examine molecular mobility of polymer chains in glass fibre reinforced plastics and elucidate the relation of relaxation processes (variations in molecular mobility of polymer chains) with the hardening process of binders in glass fibre reinforced plastics and the effect of solid surface on curing over a period of time. * Vysokomo]. soyed. A13: No. 11, 2601-2606, 1971.

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Yu. S. LzPATOV ~ ~,.

SAMPLES AND METHODS OF MEASUREMENTS

A study was made of relaxation by dielectric measurements of systems prepared from epoxypolymer ED-5, a polyethylene polyamide curing agent and a dioctyl sebacate plasticizer. The proportion of plasticizer was 15~ in all systems studied, the content of curing agent varied from 0.75 to 15~ and the curing process of binders of glass fibre reinforced plastics was examined over a period of time using specimens containing 10~/o curing agent. Both unfilled systems and systems reinforced with glass fabric were tested. The weight proportion of filling was 35.0%. Standard cells 33.3 mm in diameter were used for measurement, layers of the substance studied (100-500 ~) being arranged between the sides. The cell had a safety ring insulated from electrodes by a fluoroplastic ring. The measurements were carried out in vacuum. When studying the relaxation processes over a period of time the requisite number o f cells were filled with initial materials carefully mixed and the measurement was carried out after a certain curing period. I t should be noted that all relaxation processes examined take place at temperatures lower than room temperature and cooling the sample retarded curing. Dielectric losses were measured over the temperature range of 50---150 ° using a TR-9701 apparatus with a selective receiver type TT~1301 to carry out measurements in the frequency range of 50 e/s to 300 kc/s. The frequency pulse on the measuring bridge was supplied from an RC~oseillator type B1~-344. A more detailed description of methods of measurement and automatic thermostatic control was given in earlier papers [4, 5]. RESULTS AND DISCUSSION

Dependence of relaxation processes on the de~greeof curing. Relaxation processes were studied using glass fibre reinforced plastics, in which epoxide ED-5 containing 15 dioctyl sebacate as plasticizer, was used as binder. By adding various amounts of polyethylene polyamide as curing agent (from 0.7 to 15.0~ wt.) different degrees of crosslinking were achieved to simulate the varying extent of curing for a system which is in equilibrium. The temperature dependence of tan 5 for some of these (up to 8.3% curing agent) are shown in Fig. a. For samples containing a small amount of curing agent, i.e. less than 6-7~/o two relaxation processes were observed typical of amorphous linear polymers and oligomers [7-9]: at low temperatures--the dipole-group process (at --128 °) and a higher temperatures (--20-20°)--the dipole-segmental process, which is displaced towards higher temperatures when increasing the content of curing agent. A study of specimens containing more than 7~/o curing agent (Fig. la curves 6, 7; l~ig. lb, curves 1-4) showed no segmental mobility in the already formed crosslinked polymer structure in this frequency range, but two low temperature processes take place at --128 ° (at this temperature we observed previously a maximum loss for the system with a low curing agent content; Fig. la, curves 1-5) and relaxation at --45---72 °, probably due to the mobility of kinetic units

Molecular mobility in epoxy polymers

2929

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which are larger than those responsible for dipole-group motion, but smaller than the chain segments. These kinetic units are formed after epoxy resin is combined to form a crosslinked polymer, i.e. after the gel point. Their relaxation process is displaced towards lower temperatures with an increase in curing agent content in the specimens (Fig. lb, curves 1-4). This suggests that their dimension decreases with an increase in the degree of crosslinking the resin into a crosslinked polymer. We also observed two processes at low temperatures with crosslinked polyurethanes [6]. For samples filled with glass fibre a displacement was observed in the maximum of dipole-segmental losses towards higher temperature, as shown by Fig. la and by dependence of temperature displacement of segmental relaxation of samples with different curing agent contents on the per cent content (Fig. 2).

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FIo. 2. Dependence of the temperature of displacement on the content of curing agent: /.--filled systems; 2--unfilled systems, 3--for a process taking place beyond the gel point (unfilled systems). This indicates a limitation of polymer chain mobility by the filler surface [1-5]. An investigation of these systems shows the pattern of relaxation processes for different degrees of crosslinking. The real kinetic dependence of the variation of molecular mobility (process rate) with the time of formation of a crosslinked polymer can be derived from a study of relaxation processes on curing samples with a given curing agent content. Molecular mobility in different stages of curing in bulk and on the interface. Relaxation processes in different stages of curing, i.e. the formation of a crosslinked network of a glass fibre reinforced plastic binder were studied over a period of time using specimens containing 10% curing agent and 15% plasticizer (Fig. 3). Figure 3 shows that as curing proceeds, the process corresponding to the segmental mobility of chains is displaced to high temperatures, this being clearly seen up to the gel point (Figs. 3 and 4). It can be seen that the point of gel formation is reached for these systems in about 4-5 hours after addition of the curing agent (Fig. 4). After the point of gel formation dipole segmental losses disappear when measured at a frequency of 10 kc/s. Above the gel point a new process takes place at --50---71 °, probably due to the mobility of kinetic units which

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FIG. 3. Temperature dependences of tan 6 for unfilled (a) and filled systems (b), plotted at different time intervals after the beginning of hardening: a: 1P~'itheut curing agent; 2 p a l t e r 0.38, 2~0.75, dP1.76, 5~2"74, 6~3"27, 7~3.8, 8~4.435, 9~5-835, 10--17.25, 11--82.43 hr; b: /--without curing agent, 2--after 0"5, 3~1"4, 4--2.0, 6--3.0, 6~3.8, 7--4"77 hr.

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are larger t h a n those responsible for the mobility of groups and smaller than chain segments. The hard surface restricts the mobility of polymer chains and affects the kinetics of maeromolecular combination to form a three-dimensional network. This is seen in Fig. 4: curing takes place at a lower rate in filled samples (i.e. relaxation processes of chain segments take place at higher temperatures and

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FIG. 4. Dependence of variation of temperature displacement of maximum tan ~ on the time of crosslinking binders to form a three-dimensional polymer: /--unfilled systems; 2--unfilled systems with 35 % glass fabric; 3--for a process taking place after the gel point (unfilled systems). consequently, the temperature displacements are greater). Inhibition of curing is also shown b y the frequency dependences of tan 8; here segmental relaxation for filled samples was shown at lower frequencies. However, in view of the considerable scatter we do not quote the results in which we observed this tendency. F r o m the dependence of the variation of relaxation in the temperature scale on curing time (Fig. 4) it can be seen t h a t curing takes place in three stages, i.e. is characterized by three rate constants. From earlier hypotheses [10] it m a y be assumed t h a t in the first stage (1.0-1.5 hr) binder molecules are slightly crosslinked; in the second stage, large units and branches are formed (1.5-3-5 hr). These units are combined into a three-dimensional structure in the third stage (3-5-5.0 hr after the beginning of curing) (Fig. 4). Results show t h a t during curing glass fibre reinforced plastic binders molecular mobility of polymer chains changes according to the degree of crossli~ldng of the three-dimensional polymer. The filler surface limits mobility of polymer chains, which results in slower crosslinking in filled samples. For unfilled and filled systems curing or formation of a three-dimensional system (Fig. 4) takes

Molecular mobility in epoxy polymers

2933

place in three stages. Hence it follows t h a t kinetic reaction conditions and conditions of forming a three-dimensional structure are determined to a large e x t e n t by th e presence of the filler surface and v a r y continuously during the process. CONCLUSIONS

(1) A s tu d y was made of the dielectric relaxation in a system comprising epoxide ED-5, a polyethylene polyamide curing agent and a dioxyl sebacate plasticizer. I t was found that, during curing up to the point of gel formation, two relaxation processes t ake place, typical of simple amorphous linear polymers; a dipole-group process and a process related to t he mobility of chain segments. B e y o n d the gel point two low t e m p e r a t u r e processes t ake place: dipole-group process and a process due to t he mobility of larger kinetic units. (2) I t was shown t h a t when curing takes place on t he filler surface pol ym er chain mobility is accordingly restricted, the formation of t he three
l. Yu. S. LIPATOV, Fiziko-khimiya napolnennykh polimerov (Physico-Ohemistry of Filled Polymers). Izd. "Naukova dumka", 1967 2. Yu. S. LIPATOV, Vysokomol. soyed. 7: 1430, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 8, 1584, 1965) 3. Yu. S. LIPATOV and F. G. FABULYAK, Vysokomol. soyed. A10: 1605, 1968 (Translated in Polymer Sci. U.S.S.R. 18: 7, 1858, 1968) 4. Yu. S. LIPATOV and F. G. FABULYAK, Poverkhnostnye yavleniya v polimerakh (Surface Effects in Polymers). Izd. "Naukova d,,ml~", 1970 5. Yu. S. LIPATOV and F. {;. FABULYAK, Vysokomol. soyed. All: 724, 1969 (Translated in Polymer Sei. U.S.S.R. 11: 4, 800, 1969) 6. F. G. FABULYAK and Yu. S. LIPATOV, Vysokomol. soyed. A12: 738, 1970 (Translated in Polymer Sei. U.S.S.R. 12: 4, 831, 1970) 7. J. ISBrIDA, M. 1TOM and M. TAKAYANAG, J. Polymer Sci. AS: 87, 1965 8. G. IgErID, KoUoid-Z. 49: 200, 1964 9. I. M. ERLI]KH~N. P. APUKHTINA and L. Ya. RAPPOPORT, Prom-st', sintet, kauchuka, 1966 10. A. Ye. NESTEROV, T. E. LIPATOVA, ¥. K. IVASHCHENKO and Yu. S. LIPATOV, Vysokomol. soyed. B12: 150, 1970 (Not translated in Polymer Sci. U.S.S.R.)