The effect of the composition of poly (arylate-butadiene) block copolymer on the temperature-time characteristics of its thermal degradation

Polymer Science U.S.8.R. Vol. 24, No. 5, pp. 10~-10~, 1 ~ Printed in Poland

00S2-=N~t0600106?-06~JW@ C) 1988 Peripm~n Prem Ltd.

EFFF.,~ OF ~ COMPOSmON OF POLY (ARYLATE-BUTADIENE) BLOCK COPOLYM]~ ON T i ~ TEMPERATURE-TIME CHARACTERISTICS OF IT8 T]qE~RMAL DEGRADATION* YE. N. ZADOBI~A, G. YE. VISHNEVSKII, K. S. ISAYEV, YU. I. SAKUNENKO N. K. ~[rgITAYEVA and Yu. V. ZELENEV S. Ordzhonikld~e Aviation Institute Mining and Metallurgical Institute "Plastmassy" Prwiuction Deptartment A. N. Kosygin Textile Institute, Moscow

(Rec~it~ 19 iVot~nbev 1980) Polyocndensed polyblock copolymers have been studied, the maoromolecules of which are formed from thermodynamically incompatible arylate and butadiene blocks. This results in mieroseparation of the block copolymer (BCP) which results not in averaging of the properties of the homopolymers but in combination of different properties of the blocks. The phase segregation affects the process of thern~l degradation of the block copolymers which is characterized by 2 intervals of intensive decay. It was shown that phase inversion occurs at a 30~o polybutadiene block content. The thermodynamic incompatibility of the blocks of t~hepolymer systems being studied results in the formation of a defective transitional layer at the phase separation boundary where phase inversion can also occur. UP TO the present, the widest application found for polyarylates has been in a new class of polymers which possess valuable properties [1]. High fusion and decomposition temperatures make them suitable for manufacturing thermally stable polymeric products. Thus polyarylate films can be used for lengthy periods at 500°K and above, and rigid forms have a thermal stability of the order of 600°K. Polyarylates are also stable to the action of many chemical agents and have high dielectric properties whilst being good insulators. However, the presence in them of rigid chains rather limits their application. On account of this, most attention has been paid to po|ycondensed-type polyblock arylate copolymers in view of the possibilities of regulating their properties [2]. Generally the properties of copolymers (e.g. thermal stability) are intermediate with respect to the properties of the corresponding homopolymers [3]. The formation of block copolymer (BCP) macromolecules from blocks of differc u t chemical nature causes stratification as a result of tendencies (due to the chemical nature or the structure of the chain segments or blocks) to form thermally stable regions. The chemical ]inks between the heterogeneous sections of * Vysokomol. soyed. A2& No. 5, 948-952, 1982. 1067

I068 ~

Y~.. N. Z~u~om~xeta/.

the chains (blocks) of the macromolecules prevent such a microstratification of BCP and the formation of separate phases. The presence of microregions, forming homogeneous and thermodynamically close blocks explains th~ additivity of the BCP physical properties i.e. does not cause averaging of homopolymer properties, which form BCP but combination and summation of different polymer properties. For example, in BCP consisting of heterogeneous crystalline blocks, crystallization and melting of discrete blocks occurs separately. X-ray diagrams of BCP are a superimposition of diffraction pictures, corresponding to different homopolymers. The combination of homopo]ymer properties in BCP clearly appear in the thermomechanical curves and in the temperature relationships of mechanical and dielectric losses [4, 5]. Thus in the case of incompatible blocks, there is the characteristic presence of several glass temperatures (according to the number of homopolymers present) and a wider high elasticity range than found with homopolymers. By changing the mass of the blocks and their ratio within wide limits, BCP with given properties may be obtained. Specially interesting results are achieved by introducing into the combination a single BCP with flexible and rigid links: This sort of BCP has good mechanical properties, combined with thermal stability and may be used for thermally stable materials and coatings. Examples of BCP whose macromolecules consist of bent and rigid blocks, are poly(arylate-dimethylsiloxane) [6] and poly(arylate-butadiene) copolymers. The presence in the macromolecule of blocks of different chemical nature (arylate rigid and butadiene or dimethylsiloxane flexible ones) whose glass temperatures differ by 300-400°K, introduces thermodynamic incompatibility of polyblock elements, exhibited by a fundamental segregation of the blocks and the formation of heterophase structures [7]. Polyarylate-butadienes (PAB) were studied, in which the ratio of rigid and flexible blocks was varied. The BCPs were examined by thermogravimetry with a rate of temperature rise of 2.67°K/sec in a flowing inert atmosphere (nitrogen). The following ratios of butadiene to arylate blocks were taken: 10 : 90, 20 : 80, 40 : 60 and 50 : 50 wt. %. The original thermogravimetric curve was characterized by the presence of two inflexion points (Fig. 1.). A small weight loss (5%) was observed in the 423573°K temperature range which is evidently connected with desorption of moisture and decomposition of the least stable, deflective structures. The main weight loss was noted in the range 723-853°K. It is suggested that it is explained by rupture of the mainpolyblock chain. Figure 1 also shows the temperature-time characteristic of thermodegradation, which we used to study the influence of BCP composition on its thermal properties. The relation of the temperature of the initial intensive decomposition T~ on polybutadiene,~block content is shown in Fig. 2. Points are also given here, calculated by the method of [8], using known values of thermal stability in isothermal conditions for pure polyarylate (PAr) [9] and polybut~diene (PB) [8, 9].

Effect of composition of poly (~'~]lm,t~lmtadibne) block eopolymer

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The relationship shown in Fig. 2 (line 1) shows that the thermal stability of BCP, formed from thermodynamically incompatible blocks, is an additive function of the thermal stability of the corresponding homopolymers. The introduotion of a definite amount of flexible and rigid chain blocks permits preparation of materials with pre-determined properties. This means that by varying the ratio of oligomers, we can appreciably improve the proeessibility of materials, whilst the thermal stability remains practically at the previous level. r,K 800,

% f

50 70~ ×

I

TiI ~

~ ~T~ - - ~ FIG. 1

Tfl'f

L

50 100 ContvM" of PB , %

FIG. 2

Fro. 1. A typical thermogravimetric curve for P A B (Am--relative weight loss). 1~o. 2. Dependence of the temperature-time property of thermodegradation of P A B T~ (1) Tint. (2) and T I (3) on flexible chain P B block content. Crosses--values of T~ calculated for pure P A r and P B [8].

The results .observed confirm the analysis of the relation of the inflexion Tint. of the thermogravimetrie curve and the end of intensive decomposition T! (Fig. 2, lines 2 and 3 respectively). However, the literature data for Tint. and T! of pure PAr and PB at the heating rates used are not available. In accordance with the increase in PB phase content, the initial (T~I) and final (TI1) temperatures of the first weight loss range are changed. (Fig. 3) In contrast with the interval of intensive thermodegradation, the basic macrochain increase in flexible chain blocks does not cause a decrease but an increase in the temperature-time characteristics of the first thermodegradation range for PAB. In order to explain this "abnormal" effect of BCP composition, we will examine each of the responsible decomposition ranges, as they change the characteristic relaxation time Vr of the thermal degradation process, according to the decrease in proportions of rigid chain blocks in the BCP. In [10] we showed t h a t the process of thermodegradation of polymer systems, consisting of several sub-systems [11], heated a~ a constant rate b, has a relaxation character where the size of t h e response may be considered as time Zr, during which the m ~ .

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Y s . N. ZAno,um~ ~ od.

rapid polymer deemnposition sta~e rr=(T1-'T,)/b is completed. Figure 4 d~ows t h e change in characteristic relaxation time ~r in relation to the com~a~d~oa of PAB. Moreover, the value ~r was determinect for both thermal degradation tem-

pemt~e intervals, from the formulae ~=(T~--~',,)/b aud ~=(~/--T,)/b. II

'The minimum value r, corresponds to phase inversion.

T,K 600

-

o

2

~ sec

° 1

2~ qo0 Conten~"of PB, %

Fm. 3

Conten~ of PB, %

FIG. 4

FZG. 3. Dependence of the temperature-time property of thermodegT_~dationof PAB T~t (1) and T/, (2) on oligobutadiene content. F,o. 4. Dependence of characteristic response time rr of BCP with external thermal effects on rubber component content: ~r=(T/,--T~,)/b (1) and ~*=(Tf--T~)/b (2). The relation of rru (curve 2, Fig. 4) to PB content characterizes the mobility of the structural elements (blocks) of the main macrochains. I t occurs at a flexible chain content of ca. 3 0 ~ of PB blocks. This means that according to the degree of enrichment of BCP by PB blocks, the transfer occurs from PAr as the disperse medium to the PAr dispersed phase. Such a phase inversion was confn~ned by the electron microscopic study of the BOP PAB structure. This clearly showed that with a content of oligobutadiene of ~ 30~, the continuous phase wa~ the -~ylate portion. Then a s the oligobutadieae content was increased, phase inve~ion occured and wit& the matrices, a phase ~f rubbery components grew. As the dispersing medium develops consisting of PB or PAr and regardless of it, phase segregation and intermolecular reaction in each of the isolating microregions wilt be increased. This will cause retardation of the intermingled blocks of microchains and therefore an increase in the time range of rr~ for thermodegradation o f BCP. With a 30% content of PB blocks corresponding to the iaitial phase inversion, t h e intermolecula~ reaction is evidently disturbed to a small extenb, which exthe much quicker response of the system to external temperature e~eets.

Effect of eomposition of poly ( a ~ y ~ e n e )

blook eopolymer

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I t is interesth~ to note that a gimilar phase hxversion was observed [4] with poly(arylate-dlmethylsiloxane), which contains thermodym~micaUy incompatible rigid and flexible chain blocks, in a ratio of polyarylate to polydimethylsiloxane blocks of ~ 10 : 70. Moreover, a sharp increase of elastic modulus and of maximum density occurs (see reports on degradation [4, 12]) which may be connected with the appearance of a network of physical links in the polymer. The presence of a spatial network between the structural elements was confirmed by electron microscope studies. However, the type of thermal decomposition observed with PAB could not be explained solely by the emergence of an interlinking physical grid, with certain proportions of PAr blocks. Figure 3 shows that increasing PB block content causes an increase in the thermal effect and completion of the first weight loss period, which cannot be solely explained by the formation of intermolecular links. Evidently, the phase segregation of thermodynamically incompatible PAB blocks causes the formation of a defective transition layer at the phase separation boundary. In the case where PAr plays the role of matrix, the defect zone also appears to be filled with PAr. Migration towards PB just as to the continuous phase, attracts an enrichment of the transition layer by flexible PB blocks and therefore, displacement of the first weight loss period towards higher temperatures, since heterolinking [13] in the ease of bulky, rigid, arylate blocks is present to a rather higher degree than in low-bulk flexible PB blocks. This causes rather lower temperature-time characteristics for the thermodegradation of a transition layer with PB contents ~ 3 0 % . The nature of the relation of transfer time Trx in the new equilibrium_ state, due to the infieunee of an external, non-stationary high temperature field, to the flexible block content, responding to the defect (transition) layer (Fig. 4, curve 1) confirms the link between the thermal destruction of the defect phase and its composition. The large response time ~ for polymer systems is connected with saturation of the tra~Afer layer with rigid blocks. The enrichment of this layer with flexibly ii~ked polymer causes a sharp increase of mobility in the II transition layer and a decrease in zr. It is interesting that the intersection point of the ratios, reflecting the response of the main BCP mass (Fig. 4, curve 2) and its defect phase (Fig. 4, curve 1) is also connected with phase inversion. This point A corresponds approximately to a 3 3 ~ flexibly-Unked block content. At low PB contents in BCP, the mobility of elements of the transition layer is determb~ed on the basis of the rigid chain portion i.e. by the defective PAr links. As the PB content grows, the mobility in the defect phase does so, which may be connected with the increase in defective flexible blocks and their fragments being displaced from the transition layer. Thus it is obvious that the thermal properties of BCP, formed by thermodynamically incompatible blocks, are simply determined by the BCP composition and are an additive function of the properties of the corresponding homopolymers. Besides, in the preparation of polyblock copolymers with molecules

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YE. N. ZADOmNA eta/.

c o n s t r u c t e d f r o m blocks o f different chemical n a t u r e , it is necessary to consider t h e m i c r o s e g m e n t a t i o n o f i n c o m p a t i b l e phases a n d its effect on t h e t e m p e r a t u r e time characteristics o f the t h e r m a l d e g r a d a t i o n o f these block copolymers. Tran~slated by C. W. CAPP REFERENCES

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