Effect of conditions of synthesis of kinetics of structure-formation of polycaproamide during anionic adiabatic polymerization

l~olYmerScieuce U.S.S.R. VoI. 23, No. I0, pp. 2545-2554, 1 9 8 1 Printed in Poland

0032--8950/81/102545-10507.50/0 ~ 1982 Pergamon Press Ltd.

EFFECT OF CONDITIONS OF SYNTHESIS ON KINETICS OF STRUCTURE-FORMATION OF POLYCAPROAMIDE DURING ANIONIC ADIABATIC POLYMERIZATION* T. IV[.FRIYNZE, R. B. SKI~l~ll3¢~l~, Y1/. K. GODOVSKII, YA. V. GENI~, T. V. VOLKOVA, V. A. KOTEL'NI:KOY, V. V. KlYRASHEV, S. P. I)AVR'YAN and D. YA. TSWt~K~ A. N. l~esmeyanov Institute of Hetero-organic Compounds, U.S.S.R. Academy of Sci~.l~ces L. Ya. Karpov Scientific Research Institute of Physico-chemistry (Received 23 October 1980)

A study was made of the effect of initial temperature of synthesis on kinetics of structure-formation of polycaproamide during anionic activated .polymerization of e-caprolactam under adiabatic conditions. It was established that a solid e-caprolactam solution is formed in polycaproamide at the initial stage of the process. It was shown that morphology is formed via an intermediate stage, which is characterized by the formation of large spherulitic units. I x anionic activated polymerization of 8-caprolactam (AAPC) the structure of the polymer which determines properties of the finished product, is formed during synthesis. Large polycaproamide (PCA) ingots are prepared under conditioIL~, close to adiabatic conditions, which explains the interest in the study of adiabatic crystallization of PCA. The effect of conditions of s~lthesis on the structure of polymers obtained b y adiabatic [1] and non-isothermal [2, 3] AAPC was shown previously. A study was made in this paper of kinetics of structure formation, i.e. of the morphology o f products separated at various stages of adiabatic AAPC, together with a kinetic study of heat liberation. The study was carried out at initial temperatures o f synthesis T o in the interval of 110-150 ° which, as is known [1, 4], enables the process to be carried out under different conditions: with joint and separate polymerization and crystallization. A similar pattern of structure-formation of PCA was shown b y the authors using AAPC taking place at To-----135° [5]. A study is made in this paper of special features of the process, determined b y To, which is essential for understanding crystallization during AAPC as a whole. Investigations were ealwied out by methods of adiabatic thermometry, differential scanning calorimetry, X - r a y diffractometry and optical microscopy. * Vysokomol. soyed..A23: No. 10, 2342-2350, 1981. 2545

2546

T . M . FRU~Z~ eta/.

8-Caprolactam was polymerized using an adiabatic device by methods previously described [6]. The process was carried out at T,-~ 110, 125, 135, 140 and 150° in the presence of equimolecular quantities of catalyst (sodium-s-caprolactam) and activator (N-acetyl-ccaprolactam), equal to (~0355 mole/1. (0.35 mole/%). The reaction was stopped by rapid cooling of the reaction mixture using liquid nitrogen. Polymer yield was determined after extraction of the crushed reaction product with water. Heat effects of polyraerization and crystallization were separated as previously [6]. X-ray investigations were carried out using a DROIq-1 diffractometer and Cu Ka-radiation. Polymer morphology was studied by an MBI-6 microscope in continuous polarized light using sections 5-10' lma thick. Calorimetric investigations were carried out using a DSK-2 differential scarming calorimeter (Perkin-Elraer). The density of polymers was determined by hydrostatic weighing in a CC14-undecane system. Figure 1 shows kinetic curves of the overall increase of t e m p e r a t u r e A T a n d t e m p e r a t u r e increase as a result of crystallization zITcr a t different initial t e m p e r a t u r e s o f synthesis. I t can be seen f r o m the Figure t h a t the curves are S-shaped a n d consist of sections showing different rates of increasing t e m p e r a ture. A v a r i a t i o n in T Odoes n o t change the t y p e o f curve, b u t affects t h e e x t e n t a n d t h e rate o f increase of t e m p e r a t u r e a n d therefore, t h e c r y s t a l l i n i t y of p r o d u c t s formed. The m a x i m u m value of ATcr decreases with a n increase in T Oa n d in the

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FIO. 1. Kinetic curves of the overall increase in temperature and temperature increase as a result of crystallization during AAPC with T0= 110 (1, 1'), 125 (2, 2'), 135 (3, 3'}, 140 (4, 4'), 150° (5). Here and in Figs. 3, 5, 6 samples separated during the reaction at various stages of conversion are denoted by letters. FzG. 2. Dependence of the crystallinity of PCA on the time of synthesis at T0~-ll0 (1), 125 (2), 135 (3) and 140° (4).

Kinetics of structure-formation of polyeaproaraide

2547

case of AAPC with T 0 = l l 0 gives a value of 36.4°; with To=125-27-9°; with To=135-24-3 ° and with T0=140-20 °. At T 0 = l S 0 ° crystallization takes place on cooling the polymer and ATcr, fixed during synthesis, is 6-7 ° in all. Figure 2 shows crystallinity K of products separated during the reaction, calculated from thermometric results using the formula K = a c / = c o n v , where ~e is the degree of crystallization and =cony--the degree of conversion of the polymer. The value of ~c was calculated based on its proportionality to AT*. Values of ~e for samples with maximum A T e and =eonv~97~ were determined previously from experimental values of density. It can be seen from Fig. 2 that, according to results of adiabatic thermometry, the crystallinity of products is rather high already at the beginning of the process a n d is ~ 20~/o. For samples with a degree of conversion of over 8 0 ~ crystallinity I ~ ['el. Un.

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FIe. 3. X-ray diffraction curves of PCA with different degrees of conversion %on: a-- ,-caprolactam, b-- 1B sample (%on= 11"3%), e-- 1D' sample ~con= 52. 9~/o), d--lG sample (~con=97.0%) * Under adiabatic conditions of synthesis temperature increase is directly proportional to the amount of heat liberated [6].

2548

T . M . F,~UNzE et al.

was calculated from experimental values of density*. It shows satisfactory correlation with crystallinity obtained from thermometric results. T h e complex variation of crystallinity over a period of time at T 0 : 1 1 0 and 125 ° is determined b y a varying ratio of rapid polymerization and crystallization during the process, which wilt be dealt with in detail in the next report. Figure 3 shows X-ray diffraction curves of samples separated during the reaction at different degrees of conversion. The identical nature of crystallization for AAPC taking place at To-~ 110~135 ° is significant when analysing diffraction curves. Three stages of structure-formation may be noted in these eases: the initial stage which is characterized b y an amorphous halo corresponding to the polymer and b y a gradual reduction in the intensity of lines of the monomer {Fig. 3b); intermediate stage to which diffraction curves representing the total of diffraction curves of the monomer and crystalline polymer, correspond (PCA reflections are denoted by arrows (Fig. 3c)); the stage of completion of crystallization, to which diffraction curves correspond that are similar to diffraction curves of crystalline PCA (Fig. 3d). The duration of these stages is determined b y T o and is ~ 10 rain in the case of AAPC with T0~135 ° and ~ 3 0 rain in the case of T 0 = l l 0 ° for the first stage. The degree of conversion of polymers at this stage is ~ 30%. The duration of the second stage of structure-formation is 15 and 45-50 rain, respectively. The degree of conversion of polymers reaches 60-70%. In the case of AA1)C with T0=140 ° no first stage of structure-formation was noted b y X-ray and diffraction curves of samples isolated at the very beginning of the process (Fig. l, sample 4 A') are similar to those in Fig. 3c. Structureibrmation then takes place the same w a y as with T0=110-135 °. When comparing thermometric and X-ray results significance is attached to the discrepancy between the existence of heat effects of crystallization and the absence of crystalline reflections of the polymer for a nmnber of samples separated at the initial stage of crystallization. To explain the causes of this discrepancy, samples were evacuated at a temperature of 20 ° for 6-10 hr using 1.33 X 10 -4 Pa. Comparison of diffraction curves of samples before and after evacuation shows that after evacuation the intensity of the amorphous halo corresponding to the polymer increased considerably and reflections of crystalline PCA which had not been observed previously, appeared. Lines of the crystalline monomer were observed only on diffraction curves o f samples, in which residual monomer content after evacuation exceeded 40 wt. ~ , which represented over 10% of the initial weight of the sample. With a lower monomer content in evacuated samples no crystalline reflections were noted o f the monomer and diffraction curves were quite similar to those in Fig. 3d. This fact suggests t h a t a t the initial stage of the process up to 40~/o monomer (of * The density of samples with aeonv<80°/o could not be determined as a consequence o f swelling in flotation liquids.

Kinetics of structure-formation of polycaproamide

254~

the weight of the polymer formed) is entrained by the polymer during crystallization and is present in the form of a solid solution in the amorphous phase of the polymer. The formation of a solid solution of s-caprolactam in PCA may evidently be explained also by the absence of lines of cryst~a,lline monomer on diffraction curves of samples separated at the third stage of structure-formation, dm'ing which the content of the residual monomer decreases from ~ 40 to 3%.

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The assumption proposed is confirmed by results of differential-scanning calorimetry. Figure 4 shows thermograms of melting samples separated at the initial stage of crystallization and evacuation using the methods described. Characteristics of samples are tabulated. Thermograms of all samples examined showed peaks of melting of crystalline PCA, Qx, situated in the range of ~ 220 °. When studying thermograms of samples with a higher content of residual monomer (Table, samples 1B, 3A) a heat effect was observed during the first heating at 68 °, which corresponds to melting of crystalline e-caprolactam (Fig. 4a). Based

2550

T.M.

FRUNZE et ed.

on this heat effect and the specific heat of melting e-caprolactam (Qmelt==150"5 J/g), the proportion of crystalline (free) lactam was calculated. Then, from the difference of the overall content of residual monomer and free lactam the amount of monomer dissolved in PCA was determined. It can be seen from the Table that up to ~ 40 wt, ~/o e-caprolactam m a y dissolve in PCA, which shows satisfactory agreement with X-ray data. During the second heating the peak of the crystalline monomer is absent and the heat of melting the polymer Q2 increases in proportion to the amount of free lactam, which is evidence of its polymerization during heating. Polymerization takes place in a fairly wide temperature range, which is recorded according to the proportional deviation of the curve of heat power from the base line. T H E R M A L P R O P E R T I E S OF

PCA SAMPLES

Content of the Content in the sample after evamonomer, ~o cuation, ere Qmelt o f deter, the poly- T~* of ~°melt mer (I the poly- of the mined ecnple* from dissolpolymer of poly- of mono- heating), mer Qmelt of ved in mer mer J/g the mo- PCA nomer 1B 3A' 1C 3B' 4A"

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35-5 33-5 73.3 81.0 69-6

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-66.0 89.9 81.5 73"9

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212 215 217 217 219

29.0 24.0

39.5 42.5 26.7 19.0 30.4

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j/g

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141.3 145.9 142.5

15.9 16.5

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* According~ol~ig. 1.

Thermograms of samples containing less than 40 w t . % monomer (Table) are of special interest. There is no peak of crystalline monomer on these thermograms (Fig. 4b) b u t exothermie effects dependent on polymerization of the monomer dissolved in PCA are observed (Fig. 4b). A joint study of heat effects of melting and polymerization bearing in mind monomer content enables the heat of polymerization to be determined. Results of calculations are tabulated. There is satisfactory correlation between these results and those in the literature: Qp--15.2-16.5 kJ/mole, [7] and this is evidence of the fact that the exo-proeesses recorded are, in fact, due to polymerization of the lactam dissolved in PCA, this process taking place after melting the polymer. During repeated heating of the sample the peak which corresponds to polymerization of e-caprolaetam, is absent. With high monomer content, when both free lactam and lactam dissolved in PCA are present in the sample,-no clear exothermic peaks of polymerisation of combined monomer (after melting PCA) were observed. Partial solution of PCA

Kinetics of structure-formation of polycaproamide

25~i1

in the melt of free lactam takes place probably in this case and polymerization of the monomer dissolved in PCA occurs gradually, simultaneously with polymerization of the free lactam. Therefore, during AAPC accompanied b y crystallization of products formed the monomer is evidently present in two forms: in the form of a crystalline substance and in the form of a solid solution in PCA. The formation of solid solutions of a low molecular weight substance in the polymer was observed using the ~k~rrocene-PE system [8]. When studying the morphology of polymers synthesized during adiabatic .~kPC at different T o three sequential stages of structure-formation m a y be noted which are characterized b y a certain supermolecular structure of products separated. Quantitative changes in polymer morphology are shown in Fig. 5. I t can be seen that curves are identical for all To values examined. The variation of sphenflite diameter passes through the maximum in every case and then decreases evenly reaching average values of 20-25/~m. Dimensions of supermolecular formations at intermediate stages of structure-formation and the duration of these stages are determined b y To. d D/d~ ,jlm /min i

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-2 Fro. 5. Dependence of dimensions (a} and the rate of cxtension of spherulites (b) on the time of synthesis at T0=ll0 (1), 135 (2) and 140° (3). The first stage of crystallization (nucleation) corresponds to a section with a low rate of temperature increase on kinetic curves (Fig. 1) and the formation of an amorphous halo and the first signs of crystallization on X-ray diffraction curves. Separate spherical birefringence particles, 3-10/~m in size are the main structural elements of this stage, of which crystallinity is ~ 200/0 (Fig. 2). We

2552

T . M . F~U~ZE et ed.

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Fxo. 6. Optical microphotographs of PCA samples obtained at different To: a--sample 1D (¢¢~=25.2~o), b - - 4 B " (~o0n-----23"3%), c--lF(acon~-76'l~o), d--3C' (=oon=96"7%), e-- 1G (=ee==97"0~o).

Kinetics of structure-formation of polyoaproamide

2553

have previously [5] shown the dendritic form of these particles. As shown by Fig. 5b, the rate of extension of structural elements in the period of nucleation during AAPC taking place from T o = 110 °, is very low and remains constant for a long period of time. A number of new structural formations appears in the system dttring this period and their dimensions are practically unchanged (Figs. 5a, 6a). At T0~135 ° nucleation is accompanied by a certain increase in spherulites and at T0-----140° the particles formed begin to grow at once at a. high constant rate and their number remains almost unchanged (Figs. 5, 6b). The second stage of structure-formation is characterized by ~ higher rate of temperature increase (Fig. 1) and a rapid growth of polymer structures. Large, closely adjacent spherulites with dendritic centres previously formed (Fig. 6c) are the main structural elements of this stage in all cases examined. Many spherulites have two and more centres, which m a y evidently be observed in the case of aggregation of fine spherulites. This aggregation is particularly likely since the reaction mixture at this stage of crystallization is a non-equilibrium, multicomponent system consisting of a crystalline phase swollen in the residual monomer and in the polymer not yet crystallized. A loose type of spherulite is also observed [9] at intermediate stages of crystallization. Maximum dimensions of spherulites (Fig. 5a) depend on To, which is due to different rates of extension of polymer crystals. The third stage of the process (secondary crystallization) corresponds to the sloping section of kinetic curves (Fig. 1). This is the stage of slow preerystallization and improvement of supermolecular structure of polymers. As shown by microscopic investigations, narrow structures with a clearly marked central part (Fig. 6c) are converted into spherulites of ~ 20/ira (Fig. 6d) typical of PCA during secondary crystallization. This process is evidently carried out as a result of precrystallization of the amorphous material inside the spherulites, which reduces the dimensions of dendritic nuclei and increases the thickness of lamellae [5]. A fhm spherulitic structure is formed as a result, the crystallinity and homogeneity of which is determined by T O. The least homogeneous structure is formed during AAPC taking place at T 0 = l l 0 °. In this case (Fig. 6e) in addition to spherulites of 20-251tm, a number of fine formations ~ 1 0 / t m in size were observed in the polymer. These were noted in the morphology of samples separated during the reaction at a temperature higher than 190 °, i.e. during secondary crystallization. It is obvious that they appear in this period as a result of polymer recrystallization; imperfect spherulites previously formed from oligomer reaction products melt during this process and undergo repeated crystallization at higher temperatures. Investigations carried out enable regularities of adiabatic crystallization of PCA to be established during AAPC. They indicate that under adiabatic conditions the formation of supermoleeular structures is complex and takes place by crystallization of the polymer formed during polymerization of free (crystalline) monomer and a monomer present in the form of a solid solution in 1)CA. The

25~14

T . M . F B u m ~ et al.

m o r p h o l o g y o f P C A during synthesis undergoes considerable changes a n d is d e t e r m i n e d b y t h e initial t e m p e r a t u r e a n d d u r a t i o n of t h e reaction. S e c o n d a r y crystallization is o f considerable i m p o r t a n c e in t h e f o r m a t i o n o f m o r p h o l o g y , d u r i n g w h i c h s p h e r u l i t e s are b e c o m i n g p e r f e c t a n d a h o m o g e n e o u s fine spherulitic s t r u c t u r e is formed. Translated by E. S E ~ R ~ REFERENCES 1. J. SEBENDA, Z. PELZBAUER and Z. TOMJKA, Collect. Czeehosl. Chem. Commun. 28: No. 2, 310, 1963 2. T. M. FRUNZE, R. B. SHLEIFMAN, T. M. BABCHINITSER, V. V. KURASHEV and V. I. ZAITSEV, Vysokomol. soyed. A13: 1103, 1971 (Translated irl Polymer Sei. U.S.S.R. 13: 5, 1241, 1971) 3. T. M. FRUNZE, R. B. SHLEIFMAN, V. I. ZAITSEV, V. V. KURASHEV, T. M. BAB° CHINITSER and V. V. KORSKXK, Vysokomol. soyed. 14: 962, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 4, 1071, 1972) 4. P. WITTMER and G. GERRENS, Makromolek. Chem., 89: 27, 1965 5. T. M. FRUNSE, R. B. SI:I:LEIFMAN, E. M. BELAVTSEVA, Ja. GENIN, T. V. VOLKOVA, V. A. K O T ~ O V , L. G. RADCHENKO, S. P. DAVTJAN and V. V. KURASHEV, J. Polymer Sci. Polymer Phys. Ed. 18: No. 7, 1523, 1980 6. V. V. KORSHAK, T. M. FRUNZE, S. P. DAVTYAN, V. V. KURASHEV, T. V. VOLKOVA, V. V. KOTEL'NI.KOV and R. B. SHLEIFMAN, Vysokomol..~oyed. A21: 1960, 1979 (Trm~slated in Polymer Sei. TY.S.S.R. 21: 9, 2161, 1979) 7. O. WICHTERLE, J. TOMKA and J. SEBENDA, Collect. Cheehosl. Chem. Commun. 29: No. 3, 610, 1064, 8. A. Sh. CHERDOBAYEV and D. Ya. TSVANKIN, Vysokomol. soyed. AI9: 1237, 1977 (Translated in Polymer Sei. U.S.S.R. 19: 6, 1426, 1977) 9. T. KOMOTO, M. IGUCHI, H. KANETSUNA and T. KAWAI, Makromolek. Chem. ]~, 135, 145, 1970