Maleic Anhydride Copolymer

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CHEM. RES. CHINESE U. 2007, 23 (6) , 726-732 Article ID 1005-9040( 2007) 46-726-07

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Morphology and Mechanical Properties of Nylon 6/PBT Blends Compatibilized with Styrene/Maleic Anhydride Copolymer * QIN Shu-haolpz,YU Jie' , ZHENG Qiang'.'* , HE M i d and ZHU Hong' 1. Departnent of Polymer Science and Engineering, Zhejiang University, Hangzhou 3 10027 , P. R. China ; 2. National Engineering Research Center for Compounding and Modtjkatwn of Polymeric Materials, Guiyang 550025 , P. R. China Received Feb. 12, 2007 The mechanical properties and dynamic mechanical properties of blends composed of Nylon 6 and p l y ( butylenes terephthalate) ( PBT) , with s p n d m a l e i c anhydride( SMA) as compatibilizer, were studied. The observation on the morphologies of the etched surfaces of the cryogenically fractured specimens v i a scanning electron microscopy( SEM) demonstrated that in the compatibilized Nylon 6/PBT blends, there exists a finer and more uniform dispersion induced by the in-situ interfacial chemical reactions during the preparation than that in the corresponding uncompatibilized blends. On the other hand, the overall mechanical properties of the compatibilized blends could be remarkably improved compared with those of the uncompatibilized ones. Moreover, increasing the amount of the compatibilizer SMA leads to a more efficient dispersion of the PBT phase in Nylon 6/PBT blends. Furthermore, there exists an optimum level of SMA added to achieve the maximum mechanical properties. As far as the mechanism of this reactive compatibilization is concerned, the enhanced interfacial adhesion is necessary to obtain improved dispersion, stable phase morphology, and better mechanical properties. Keywords Nylon4 ; PBT; Morphology; Dynamic mechanical property

Introduction There have been some approaches employed to enhance the compatibility of the polymer blends, among which the formation of block or graft copolymers at the domain interface during processing by in-situ reaction of functional groups, i. e. , reactive compatibilization, has been considered to be an effective approach[''21. A polymer containing some functional p u p s like -OH, -COOH, and/or -NH, at the chain ends can react with another polymer bearing a reactive component such as maleic anhydride, acrylic acid, or epoxide. A compatibilized Nylon 6/PBT alloy offers certain advantages as the stiffness ( modulus) and strength could be increased, depending on the composition ratio of the main constituents. On the other hand, the presence of PBT reduce not only the cost but also the moisture sensitivity of the final product. Kim et al. [31 studied the improvement of the toughness for PBT/Nylon 6 blends compatibilized by ethylenelvinyl acetate-g-maleic anhydride( EVA-g-MAH) . Kamal et al. [41 reported that poly ( ethylene terephthalate ) ( PET)/Nylon 66 blends are brittle even though the individual component is ductile failure, and owed the brittleness to the poor interfacial adhesion in those blends. Huang et al. [51 found

that the epoxide end groups are able to react with the end groups of Nylon 66 and PBT in-situ to form a certain amount of the desired epoxy-co-PBT-co-Nylon 66 copolymer at the melt. Watanable et al. ['I reported that the blends of PBT and Nylon 6 are incompatible and pointed out that the compatibility between this pair of polymers could be enhanced via the addition of a copolymer, poly ( styrene-maleic anhydride) ( SMA ) , because SMA can react with both the amine end p u p s of Nylon 6 and hydroxyl end groups of PBT to form copolymers using the in-situ reactive extrusion. It is also assumed that in-situ copolymer can improve the interfacial adhesion between Nylon 6 and PBT and can control the dispersed-phase particle size, and consequently, may affect the strength and toughness of the blends. To our knowledge, few articles of the systematic investigation on the in-situ reactive compatibilization of the Nylon 6/PBT blends by SMA have been reported to date. This study is focused on the current industrial applications of compatibilized blends consisting of Nylon 6 with poly ( butylenes terephthalate) ( PBT) We attempted to probe the relationship between the morphology and mechanical properties of Nylon 6/PBT blends compatibilized by SMA copolymer.

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* Supported by the Natural Science Funds of Guizhou Province, China( No. GY-2005-3036) and the National Basic Research Program of China( No. 2005CB623802).

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To whom correspondence should be addressed. E-mail: zhengqiang@zju. edu. cn

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Experimental 1 Materials and Sample Preparation Nylon 6 ( 1013B) was a commercial product of Ube Co. , Japan. A random SMA copolymer containing 24% ( mass fraction) maleic anhydride from Sinopec Shanghai Petrochemical Corp. of China was used as the compatibilizer. PBT( S3130) was purchased from Engineering Plastics Plant of Yihua Group Corp. of China, contained 0.02 moVkg of carboxylic groups at its chain ends. The preparation was conducted as follow. ( 1 ) Samples were prepared by extrusion in a twin-screw corotating extruder ( L/D = 40, D = 40 mm ) at 250 “c and a screw speed of 100 r/min unless otherwise specified. All the materials were dried in a vacuum oven at 80 “c for at least 24 h before melt-mixing. ( 2 ) The specimens for mechanical properties were prepared by injection molding using a injection molding machine (CJ-80, Chende Plastics Machinery Co. , Ltd) . The temperatures of cylinders 1 , 2 , 3 and nozzle were set at 220, 230, 240, and 250 ‘i:, respectively.

2 Measurements of Properties The dynamic mechanical analysis( DMA) was conducted using a TA Q800 DMA( TA Instruments, USA) at a frequency of 1 Hz and a heating rate of 10 ‘i:/min in a temperature range of 0-175 “c The low-temperature measurement was performed in a stream of dry air cooled with liquid N,, and the high-temperature measurement was carried out in a stream of dry N,. Izod impact tests were performed using a Z B C 4 Impact Pendulum ( Shenzhen SANS, China) under ambient conditions according to IS0 179196 standard. Tensile tests ( IS0 527-1/93) were carried out at a crosshead speed of 50 mm/min with an Instron 4302 universal tensile machine ( Instron , USA) . Experimental errors were calculated from five specimens for impact and tensile tests. The average deviations based on five specimens were of 5 % . On the basis of the results of solubility tests, it has been found that SMA is soluble in boiling toluene, whereas Nylon 6 and PBT are insoluble. Nylon 6/PBT/ SMA ( mass ratio, 70: 30 :9 ) and Nylon 6/PBT ( mass ratio, 70: 30) blends were extracted with toluene. A total of approximately 30 g of each of the two blends was added respectively to 250 mL of toluene and stirred at 140 ‘i: for five days. The suspensions were filtered off and the clear solutions obtained were precipitated with acetone, and consequently dried in a vacuum oven until constant weight of the samples reached. FI’IR spectra were recorded on a Nicolet 710 FTIR spectm-

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meter( Nicolet Instrument Corporation, United States) to confirm the grafting reaction. The observation of the blends with the aid of scanning electron microscopy ( SEM ) was conducted using a KYKY-2800 microscope ( KYKY Technology Development Ltd. , China) at an accelerating voltage of 25 keV. Samples for observing surface morphology were obtained from cryogenically fractured molded plaques as well as from the fractured Izod specimens. As the PBT phase of the blends was preferentially hydrolyzed by potassium hydroxide ( KOH ) , the morphology of the blends was obtained by etching the surface PBT with a 10% solution of KOH in ethanol at room temperature, followed by careful washing of the surface with water. After being dried in a vacuum oven at room temperature, the surface of the specimens was coated with Au and used for the test. A semi-automatic digital image analysis technique (Image Pro-plus software) was employed to determine the effective average diameters of the particles ( d, ) based on SEM photomicrographs. For the accurate statistical samplings, more than 200 particles from the multiple SEM photomicrographs were analyzed. Thermal treatment was carried out at 100, 120, 140 and 160 ‘i: for 6 h in an oven.

Results and Discussion 1 Characterization of i n - s h Compatibilization In general, a reactive compatibilizer is a copolymer containing reactive functional groups reactable with one or both of the blend components to form a block or a graft copolymer, and the in-situ copolymer tends to reside at the interface to compatibilize the components in the blend. The major chemical reaction responsible for this reactive cornpatibilized system involves the anhydride groups of SMA, the terminal m i n e groups of Nylon 6 , and the hydroxyl groups of PBT. The reaction between amine and carboxyl end groups would take place at the interface of compatibilized Nylon 66/PBT blendst3’ , whereas the grafted maleic anhydride reacts with both the hydroxyl end p u p s of PBT”] and the amine end p u p s of Nylon 6[8-111.On the basis of the order of the reaction reported by Zhubanov[I2I and Flory[l3I, it is assumed that in the presence of MAH, the order of the reaction rate of Nylon 6 and SMA is similar to that of PBT and SMA. Hence, the in situ copolymer is preferentially located at the interface to act as an effective compatibilizer. Fig. 1 gives the FI’IR spectra of the SMA separated from Nylon 6/PBT/SMA( mass ratio, 70: 30: 9) ternary blend by extraction in boiling toluene. Some characte-

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ristic peaks of the polyamides appear at 3300, 1638, and 1542 cm-' , which are assigned to N-H, C =0 and the combination of N-H deformation and C-N stretch, respectively. Moreover, the characteristic peak representing the phthalate groups of PBT also appears at 1106 cm-' , which is found only for PBT, suggesting that a number of Nylon and PBT chains have been bounded to the SMA chains to form a copolymer acting as an in-situ compatibilizer. 0.5

ratio, 70: 3 0 ) Nylon 6/PBT blends. It can be found that there exists two-phase morphology in the uncompatibilized blend as cryogenically fractured [ Fig. 2 ( A ) ] , whereas the phase contrast disappears in the compatibilized blend [ Fig. 2 ( B ) ] . This is probably because of the strong interfacial bonding in the compatibilized system.

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Fig. 1 FIlR spectra of SMA separated from Nylon 61 PBT/SMA( mass ratio 70:30: 9) ternary blend through extraction in boiling toluene

2 Morphologies It is well accepted that the addition of a suitably selected compatibilizer to an uncompatible( immiscible) system should ( 1 ) reduce the interfacial energy of the phases, ( 2 ) permit a finer dispersion during mixing, (3) provide a measure of stability against gross phase segregation, and ( 4 ) result in an improved interfacial adhe~ion"~].Fig. 2 shows the nonetched SEM micrographs of uncompatibilized and compatibilized( mass

Fig. 2 SEM micrographs of uncompatibilized and cornpatibillzed blends( non-etched) (A) m(Nylon6):m(PBT) =70:30; (B) m(Nylon6):m(PBT):m(SMA) =70:30:3.

Fig. 3 presents a comparison of SEM images of uncompatibilized and compatibilized blends containing compatibilizer SMA with a wide amount of addition ranging from 3 to 12 phr ( parts per hundreds of resin). After the KOH etching of the cryogenically fractured specimen for the compatibilized systems, the two-phase

Fig.3 SEM micrographs of uncompatibihed and cornpatibillzed blends (A) m(Nylon6):m(PBT) =70:30; (B) m(Nylon6):m(PBT):m(SMA) =70:30:3; (C) m(Nylon6):m(PBT):m(SMA) =70:30:6; (D) m(Nylon6):m(PBT):m(SMA) =70:30:9; (E) m(Nylon6):m(PBT):m(SMA) =70:30:12.

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morphology could be clearly observed. Voids or cavities in Fig. 3 may correspond to the KOH-extracted PBT domains. The size of the dispersed PBT phase decreases with the addition of the compatibilizer, and the reason for the reduction of particle size with the addition of SMA is ascribed to the reduction in interfacial tension between dispersed PBT phase and Nylon 6 matrix. Fig. 4 demonstrates the average domain size of the compatibilized blends as a function of the amount of compatibilizer added. The average diameter of domains for the uncompatibilized blend is 1.55 pm. On the other hand, the addition of 3 phr SMA in the compatibilized blend causes a reduction of 82% in domain size, i. e. , the average diameter of domains in this system is 0. 27 pm. No discernible change of domain size could be found in the case of further addition of SMA. There exists a minimum in domain size for the system containing 9 phr SMA. However, further addition of the compatibilizer results in an increase in the domain size. The equilibrium amount of addition of SMA at which the domain size leveled off can be considered as the so-called “critical micelle concentration( CMC) ”at which micelle form. The CMC could be estimated by the intersection of the straight lines at the low and high concentration region^"^*'^^ . Here, the calculated CMC value for SMA is about 3 phr. It is worth noting that CMC value represents the critical amount of compatibilizer required to saturate the unit volume of the interface. The increase in domain size above CMC may be because of the formation of the micelle of the compatibilizer in the continuous Nylon 6 phase. As the micelle formation starts, some of the compatibilizer at the interface has left the interface already, leading to an increase in domain

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dispersion in PBT phase.

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Particle s i z e / p

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Cumulative distribution plots of dispersed-phase particle sizes of UncompatibIliZed and compatibilized blends m(PA6): m(PBT):m(MPC) =70:30:0(-0-), , 70: 30: 3( -0-) , 70: 30: 6( -A-) 70: 30: 9( -v-) , 70: 30: 12( --n-).

3 Mechanical Properties In general, mechanical modification for a polymeric material usually results in a contradictory situation, i. e. , when the strength increases, the toughness decreases and vice versa. It is relatively rare to have strength and toughness enhanced simultaneously in any type of polymer modification. This general trend has also been frequently observed in many compatibilized blends , relative to their uncompatibilized counterparts. However, it is interesting to note that both strength and toughness could be improved in this compatibilized Nylon 6/PBT system. Fig. 6 presents the tensile strength and tensile modulus at room temperature for the Nylon 6/PBT/ SMA ternary blends with different amounts of SMA added. The tensile strength gradually increases with the increase in the amount of SMA, and the presence of 9 phr SMA is enough to make the sample approach the maximum of tensile strength. On the other hand, the trend in the tensile modulus is similar to that in the tensile strength, and the results of the tensile elongation shown in Fig. 7 are somewhat mxe. complicate.

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Relationship between SMA amount and average dispersed-phase particle size

Fig.5 depicts the cumulative distribution plots of the dispersed-phase particle sizes of the uncompatibilized and compatibilized blends. Obviously, in all cases, SMA effectively diminishes the size of PBT particles dispersed in Nylon 6 matrix and makes better

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Fig. 6 Relationships between S M A amount and tensile strength or tensile modulus a. Tensile strength; b. tensile modulus.

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The tensile elongation dramatically increases upon the addition of small amounts of SMA, and the maximum value appears in case of addition of 3 phr SMA. It should be emphasized that the increase of the amount of SMA above 3 phr is detrimental to tensile elongation.

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Flg. 7 Relationship between SMA content and tensile elongation at break

Fig. 8 shows the variation of impact strength for the compatibilized Nylon 6/PBT/blends containing different amounts of SMA. It is obvious that neither the uncompatibilized nor the compatibilized Nylon 6/PBT blends is brittle with low notched impact strength because both Nylon 6 and PBT matrices are notch sensitive. As the concentration of compatibilizer increases, the unnotched impact strength increases up to 140 kJ/mZ and remains the same level over a broad range of addition of SMA (3-9 phr) and then drops. More-over, the notched impact strength shows a slight maximum at 9 phr compatibilizer and is the lowest for the uncompatibilized blend.

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SMA amounvphr

Fig.8 Ef?& of SMA amount on impact strength of Nylon 6/ABS/SMA blends a. Notched impact strength; b. unnotched impact strength.

A compatibilized system, in general, has finer domain size and greater interfacial adhesion than the corresponding uncompatibilized blends. The addition of compatibilizer SMA results in improved compatibility between Nylon 6 and PBT, reflecting an improvement of the mechanical properties for the blends. The improvement in mechanical property is attributed to the anchoring of in-situ formed copolymer molecules along the interface. "he Nylon 6 and PBT segments of the

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copolymer at the interface intimately mix with the respective Nylon 6 and PBT phases. Hence, it can be affirmed that SMA is a competent reactive compatibilizer for Nylon 6/PBT blends on the basis of drastic improvement in their mechanical properties. The above information clearly demonstrates that an excessive concentration of compatibilizer is detrimental to the dispersed-phase particle size and resultant mechanical properties. Even some possible causes for this trend have been considered; few comprehensive and thorough explain for this effect has been put forward to date. Aiming at these aspects, we try to propose two factors that could be involved in. Doubtless, other factors may be operative also. ( 1 ) The reaction between components in the blend and SMA reaches a maximum around a certain amount of SMA, and excess amounts of it are not involved in its reaction with components in the blend and seem unfavorable for compatibilization between the two components of the blend. (2) The dispersion of SMA in the Nylon 6 and PBT matrix certainly induces the variation of the individual intrinsic properties of components in the blend, which may increase or decrease the intrinsic properties of matrix , depending on the system. In the case of a detrimental effect, the compatibilized blends may improve or deteriorate mechanical properties, depending on the competition between the advantages because of better adhesion( and dispersion) , and the disadvantages because of the loss of inherent properties of components in the blends. The expected improvement of properties induced by the enhancement of the interfacial adhesion may be partially offset because of the deterioration of the intrinsic properties of the matrix. 4 Dynamic Mechanical Analysis( DMA) It is widely accepted that relaxation behavior measured via DMA provides a useful means of studying the miscibility of polymer blends. Fig. 9 ( A) depicts the variation of dynamic storage moduli( G') of the Nylon 6/PBT ( mass ratio 70 : 30 ) blends compatibilized with SMA, in which, G' gradually increases with the increase in the amount of SMA added at lower temperatures, e. g. , below the glass transition temperature of SMA. On the other hand, at high temperatures, the G' values of compatibilized blends are lower than that of the uncompatibilized blend. As far as the modification of Nylon 6/PBT with SMA is concerned, the in-situ copolymer increases the interfacial interaction between Nylon 6 and PBT. We believe that the increased interaction at the interface is attributed to the fact that the

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addition of SMA results in an increase in G'. Meanwhile, the dispersion of SMA in the Nylon 6 matrix may increase G' of Nylon 6 matrix. This can explain 020

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why G' decreases above the glass transition temperature of SMA.

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Fig. 9 Effects of temperature(0-175 T)on dynamic storage modulus(G') ( A ) , dynamic loss tangent( taaS) values( B) and dynamic loss moduli(G") ( C) of u n c o m p a t i b i and compatibilized blends (-) rn(Nylon6):m(PBT) =70:30;(-----) rn(Nylon6):rn(PBT):rn(SMA) =70:30:3;(...*..) rn(Nylon6):m(PBT):rn(SMA) = 70:30:6;(-.-.--) rn(Nylon6):rn(PBT): rn(SMA) =70:30:9;(-..-..-) rn(Nylon6):rn(PBT):m(SMA) =70:30:12.

Fig. 9 ( B) presents the variation of dynamic loss tangent ( t a d ) as a function of temperature for the uncompatibilized and compatibilized blends. It is seen that the compatibilized blends also show the same behavior in the t a d - T curve as uncompatibilized one, i. e. , there exist two maxima corresponding to the glass transition temperatures of PBT and Nylon 6. This phenomenon indicates that the compatibilization could hardly alter the level of miscibility ; in other words, it seems difficult for a compatibilizer to promote molecular level miscibility of components in the blends. This is in agreement with the statement by who has suggested that if two polymers are far from being miscible, no copolymer ( compatibilizer ) is likely to make them form a single-phase system. In a completely immiscible system, the main role of the compatibilizer is to act as an interfacial agent. Moreover, at lower temperatures, the t a d values decrease with the addition of SMA, and all the compatibilized systems show lower values of t a d than the unmodified blends. At intermediate temperatures, the tan8 values of those blends dramatically increase upon the addition of small amounts of SMA, and maximum value appears at the addition of approximately 3 phr SMA. However, with further addition of SMA, tans decreases from the level of blend containing the 3 phr SMA. When the amount of SMA added approaches to 9 phr, tan6 values of the compatibilized blends become lower than those of unmodified blends. It has been reported that the addition of SMA is detrimental to the crystallinity of Nylon 61201. Accordingly, taking the decrease of crystals number in Nylon 6 acting as " physical crosslinking" point in the Nylon 6 matrix into consideration, it is assumed that the friction of amorphous phase in Nylon 6 matrix increases. Besides, when the amount of SMA is above

3 phr( CMC ) , the dispersed domain ( < 0.3 p m ) of SMA and in-situ copolymer micelle in the Nylon 6 matrix also act as physical cross linking point, leading to

a decrease in the friction of amorphous phase in the Nylon 6 matrix. As far as the compatibilized blends are concerned, the variation of t a d values because of the friction of amorphous phase may increase or decrease, depending on the competition between the two aforesaid factors. The Nylon 6 glass transition behaviors of the compatibilized blends are somewhat different from those of uncompatibilized blends, namely, the corresponding peaks broaden and substantially shift toward higher temperatures. The shape of the glass transition peak of PBT in the compatibilized one remains the same as that in the uncompatibilized, whereas it slightly shifts to a higher temperature with the increase in the amount of SMA. All the compatibilized samples present higher values of tan8 than uncompatibilized samples at higher temperatures. As the amount of SMA added is above 3 phr, there appears a sharp glass transition peak of SMA for the compatibilized blends, indicating free SMA copolymer having been dissolved in the matrix. It is noted that the dissolved SMA is responsible for the substantially higher Nylon 6 glass transition temperature observed. Fig. 9 ( C ) gives the relationship between dynamic loss modulus( G") and temperature for Nylon 6/PBT/SMA blends containing different amounts of SMA. It is obvious that G" shows a similar trend as that of t a d curves, as a function of temperature.

Coarsening of Phase Morphology upon a Thermal Treatment

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Fig. 10( A) illustrates the influences of thermal treatment at 100, 120, 140, and 160 T for 6 h on the phase morphology for m ( Nylon 6 ) : m ( PBT) :

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m( SMA) = 70: 30: 9 blend and the uncompatibilized blend. It is obvious that thermal treatment for the uncompatibilized blend can result in a considerable phase coarsening of the blend, and the dispersed-phase size dramatically increases with the increase in thermal

30 60 90 120 150 180 Thermal treatment temperature/'(:

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treatment temperature. In particular, the dispersedphase size shows an abrupt increase at 160 T . However, no clear sign of phase coarsening appears for the compatibilized blend.

40 80 120 160 Heat treatment temperature/'(:

40 80 120 160 Heat treatment temperature/'(:

Fig. 10 Etfects of thermal treatment temperature OD the disprsed-phaseparticle size( A), tensile strength( B) and impact strength( C) 4.

m( Nylon 6 ) : m( PBT) =70: 30; b. m( Nylon 6 ) : m( PBT): m( SMA) =70:30: 9.

Fig. 10( B ) presents the influences of thermal treatment on the tensile strengths of uncompatibilized and compatibilized samples. The tensile strength of the sample can be progressively improved by the increase of thermal treatment temperature. Compared to that of the uncompatibilized blend, the improvement of the tensile strength is more remarkable for the compatibilized one. Fig. 10( C ) illustrates the influences of thermal treatment on the impact strengths of uncompatibilized and compatibilized blends. It can be observed that the impact strength gradually decreases with the increase in the thermal treatment temperature up to 140 "c, , and then the impact strength of both the two blends show a sharp decrease. As for uncompatibilized blend, growth in the size of droplets dispersed in a matrix was already reported for polymer blends subjected to thermal treatment[2'-w . However, the presence of compatibilizer SMA can reduce interfacial tension and result in the reduction of coalescence in compatibilized blends. It is suggested that the reduced level of coalescence in compatibilized blends is dealt with the increase in steric hindrance at the interface as the dispersed particles become covalently bonded to the matrix. The mobility of the interface is a critical parameter for the coalescence of the two dispersed particles. In view of our experimental observation, for the compatibilized blends, there exists a finer phase morphology, which is stable when subjected to thermal treatment.

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