Mechanical behavior of high impact polystyrene based on SB copolymers as a function of synthesis conditions: Part II

e-Polymers 2008, no. 015 http://www.e-polymers.org

ISSN 1618-7229 Mechanical behavior of high impact polystyrene based on SB copolymers as a function of synthesis conditions: Part II Ramón Díaz de León,1* Graciela Morales,1 Pablo Acuña,1 Florentino Soriano1

1*Centro de Investigación en Química Aplicada. Blvd. Enrique Reyna 140; C.P.25253, Saltillo, Coahuila, México; fax: +52 844 4389830; e-mail: [email protected] (Received: 3 July, 2007; Published: 29 January, 2008)

Abstract: The mechanical properties of different high impact polystyrene (HIPS) were determined by means of impact and tensile tests. Impact strength (IS), yield stress (σy) and Young’s modulus (E) were evaluated as a function of the rubber phase morphological features such as type and particle size (Dp), volume fraction (Φ) and interparticle distance (IPD). In order to evaluate the changes produced in the rubber phase, the initiator, chain transfer agent (CTA), and type and concentration of SB copolymer were varied during the synthesis of HIPS. Transmission electron microscopy was used to analyze the materials’ morphology and the data obtained reveal that the Dp and Φ increases mainly with an increase in the PB content and/or with the use of CTA and decreases with an increase in initiator concentration. In addition, E and yield stress diminishes when increasing the dimensions of the rubber phase (Dp and Φ), contrary to the IS. The IPD/Dp ratio also indicates the dependency of the mechanical properties with respect to the rubber phase features. An increase in IPD/Dp provokes an increases in E and σy, whereas IS diminishes.

Introduction The addition of rubber particles into polymer matrices is widely used as a means to increase the toughness of polymeric materials [1, 2]. When a rubber phase is incorporated into fragile polymeric matrices, a marked increase in the plastic deformation is produced improving the mechanical properties. In the case of high impact polystyrene (HIPS), many reports deal with the effect of different morphological and physical parameters of the dispersed rubber phase on the mechanical properties [3-7] where the most important parameters to be considered are the type and size of rubber particles, the volume fraction of rubber phase, and the interfacial adhesion between the rubber particle phase and the PS matrix [8-11]. On the other hand, these parameters can be modified as a consequence of variations in the polymerization conditions that include the initiator and chain transfer concentration, the rubber type and concentration and the stirring rate, among others. It is important to mention that most of these studies involve polybutadiene as the rubber phase and only a few papers have described these effects with SB [12-14]. Moreover, when several block copolymers are used as modifiers, the studies demonstrate that the relative composition and molecular weight of block copolymer

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and the corresponding homopolymers play a very important role in the optimization of the physical properties in these materials [15-17]. If the mentioned morphological and physical parameters of the rubber phase can be controlled, HIPS with better mechanical performance will be developed, yielding a wide spectrum of different reinforced materials for different applications, where the reinforcement is related with the theory of multiple crazing, which establishes that during the deformation process a great amount of energy is dissipated due to the generation and termination of crazes [18] which is not necessarily the predominant mechanism in HIPS [19]. These crazes initiate at the polystyrene-rubber interface within the two phase particle, not at the particle-matrix interface [20] and subsequently propagate through the polystyrene matrix. The present work focuses on studying the effect of changes in the structure of the rubber phase, as a consequence of variations in the polymerization conditions, such as: initiator and chain transfer agent concentration, rubber type (SB’s with different wt-% of PB content) and concentration, on the morphological features and the repercussion on impact strength and tensile properties of HIPS. Results and Discussion Morphological features of the rubber phase: Particle diameter, volume fraction and IPD/Dn ratio -Influence of the precursor copolymer SB From TEM micrographs, it is observed (Fig.1, HIPS-1) that when using SB’s with 70 wt-% of PB, the particle size of the dispersed rubber phase remains around 0.2 µm and is mainly of the capsule type. The particle size increases slightly when using SB with 80 wt-% of PB (Fig. 2, HIPS-5). Finally, when using SB with 90 wt-% of PB (Fig. 3, HIPS 9) the particle size increases markedly, and the morphology changes to type salami in which it reaches more than 1 µm. Although all rubber phase particles contain occluded polystyrene, the salami type particles are the ones that contain the larger amounts of occluded polystyrene. Therefore, for a given content of SB in HIPS, the volume fraction of the rubber phase particles in HIPS will be larger for the salami type particles than for the capsule type. It is important to mention that the changes observed in the particle size and morphology, when increasing the PB content in the precursor copolymer SB, are a consequence of the lower stabilization between the PS matrix and the rubber particles due to a higher interfacial tension provoked essentially by the higher PB content present in the system. -Influence of the initiator concentration [I] On the other hand, independent of the system evaluated, an increase in initiator concentration decreases the size particle (shown in Fig. 1: HIPS-1 and HIPS-2; Fig. 2: HIPS-5 and HIPS-6; Fig. 3: HIPS-9 and HIPS-10 ), due to an increase in the amount of graft PS, as it can be observed from the values of grafting degree (Table 1). A larger amount of graft PS increases the stabilization between both phases (the rubber and PS phases) yielding a decrease in the dimensions of rubber phase such as in Φ and nD (Fig. 4 and 5).



-Influence of the SB concentration [SB] With respect to the effect of rubber concentration, employing copolymers PS/PB: 30/70 and 20/80, an increase in the SB concentration gives rise to a decrease in the size of the core-shell particles (shown in Fig. 1: HIPS-1 and HIPS-3; Fig. 2: HIPS-5 and HIPS-7) due to the ability of block copolymers to act as surfactants in blends of heterogeneous polymers [21]. It is well known that block copolymers tend to accumulate in the interface in blends of immiscible polymers (in this case; PS and rubber) only when the polymers to be stabilized are of the same nature than the corresponding blocks of the block copolymer, and the system is stabilized due to a decrease in the interfacial tension. Above a critical value of block copolymer concentration (i.e. 9 wt-%), an increase in the SB concentration makes this the more viable way to accumulate in the interface; through the increase in the superficial area of the disperse phase which can be achieved through a decrease in the particle size. On the contrary, when the copolymer PS/PB: 10/90 is used, an increase in its concentration suppresses the surfactant effect and in this case the particle size increases. -Influence of the chain transfer agent concentration [CTA] On the other hand, the use of CTA has marked effects on the size and structure of the particles (Figs. 1, 2 and 3).

Fig. 1. TEM micrographs of HIPS showing the effect of varying the concentration of initiator (0.10 to 0.15 wt-%) in HIPS-1 and HIPS-2, concentration of SB-A (8 to 12 wt-%) in HIPS-1 and HIPS-3 and concentration of chain transfer agent (0.0 to 0.10 wt-%) in HIPS-1 and HIPS-4 on the morphological features for the system 30/70.

Fig. 2. TEM micrographs of HIPS showing the effect of varying the concentration of initiator (0.10 to 0.15 wt-%) in HIPS-5 and HIPS-6, concentration of SB-B (8 to 12 wt-%) in HIPS-5 and HIPS-7 and concentration of chain transfer agent (0.0 to 0.10 wt- %) in HIPS-5 and HIPS-8 on the morphological features for the system 20/80.



Fig. 3. TEM micrographs of HIPS showing the effect of varying the concentration of initiator (0.10 to 0.15 wt-%) in HIPS-9 and HIPS-10, concentration of SB-C (8 to 12 wt-%) in HIPS-10 and HIPS-11 and concentration of chain transfer agent (0.0 to 0.15 wt-%) in HIPS-10 and HIPS-12 on the morphological features for the system 10/90. Tab. 1. Morphological parameters corresponding to the synthesized HIPS.

Material nD a

(nm) Φ,

b IPD/ nD c Iad

(1/μm) Ne

(μm3) GDf

(%)

HIPS-1 180 0.254 0.569 239.6 83.31 285.5

HIPS-2 80 0.160 0.833 285.5 596.84 206.1

HIPS-3 137 0.200 0.701 206.1 148.55 114.8

HIPS-4 260 0.328 0.440 114.8 35.72 257.5

HIPS-5 240 0.281 0.518 257.5 38.82 274.0

HIPS-6 233 0.192 0.732 274.0 29.04 199.8

HIPS-7 215 0.302 0.482 199.8 58.04 171.5

HIPS-8 872 0.427 0.318 171.5 1.23 250.8

HIPS-9 698 0.576 0.193 250.8 3.24 341.5

HIPS-10 447 0.425 0.322 341.5 9.09 288.8

HIPS-11 1209 0.755 0.090 288.8 0.82 258.8

HIPS-12 1050 0.623 0.163 258.8 1.03 285.5 a

nD : Number average particle diameter determined by TEM; bΦ: volume fraction determined by TEM, in HIPS-6 volume fraction was determined through Eq. (3); IPD: interparticle distance determined through Eq. (4); dIa: Interfacial area per unit volume in the blend determined through Eq. (4); eN: Particle number per unit volume determined through Eq. (6). fGD: grafting degree determined through Eq. (2).

For the system 30/70 the core shell particles obtained in the absence of CTA has a considerably lower size than those obtained with CTA. In the case of system 20/80 the presence of CTA yields not only larger particles but also the morphology changes from core shell to salami at high CTA concentration (i.e. 0.1 wt-%). Finally, for system



10/90 the effect of CTA is more evident at a concentration of 0.15 wt-% where the particles reach 2000 μm. The morphological changes observed in the presence of CTA are associated mainly to the modification of the interfacial tension provoked by changes in the stabilization between phases, which exclusively depends on the amount of graft PS. Therefore, as the amount of graft PS tends to decrease in the presence of CTA, the lower the values of grafting degree (Table 1: HIPS-1 and HIPS-4; HIPS-5 and HIPS-8; HIPS-10 and HIPS 12) and the interfacial tension and the particle size increases.

Fig. 4. Effect of A) [Initiator]0, B) [SB]0 and C) [CTA]0 on the particle size of HIPS synthesized using copolymers with a PS/PB composition of 30/70 (▲), 20/80 ( ) and 10/90 ( ). In A) all HIPS were synthesized using [SB]0 = 8 wt %. In B) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [Initiator]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed. In C) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [ATC]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed.

Fig. 5. Effect of A) [Initiator]0, B) [SB]0 and C) [CTA]0 on the volume fraction of HIPS synthesized using copolymers with a PS/PB composition of 30/70 (▲), 20/80 ( ) and 10/90 ( ). In A) all HIPS were synthesized using [SB]0 = 8 wt %. In B) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [Initiator]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed. In C) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [ATC]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed.



Fig. 6. Effect of A) [Initiator]0, B) [SB]0 and C) [CTA]0 on IPD/ nD of HIPS synthesized using copolymers with a PS/PB composition of 30/70 (▲), 20/80 ( ) and 10/90 ( ). In A) all the HIPS were synthesized using [SB]0 = 8 wt %. In B) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [Initiator]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed. In C) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [ATC]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed.

Fig. 7. Interfacial area as a function of IPD/ nD ratio, for the different synthesized HIPS. Moreover, the variations observed in the IPD/ nD ratio (Fig. 6) are essentially attributed to the interfacial area per unit volume according to Fig. 7, where the greater the IPD/ nD values, the greater the Ia. In this sense, the IPD/ nD ratio increases with the initiator concentration due to a large population of lower size particles with higher interfacial area. When increasing SB concentration, only the HIPS synthesized with the 30/70 SB copolymer, allows the increase in the IPD/ nD ratio, again, as a consequence of the lower size particles generated. In the case of the HIPS synthesized through the use of SB copolymers with a composition PS/PB: 20/80 and 10/90, an increase in their concentration, decreases the IPD/ nD ratio. On the other



hand, all the HIPS synthesized using CTA show lower values of IPD/ nD due to a lower number of particles that have a lower interfacial area. Impact and tensile properties The Fig. 8A shows the general trends of impact strength as a function of the different synthesis conditions for the evaluated systems. It is observed that for samples containing SB with 70 and 80 wt-% of PB, the impact strength decreases very slightly with the increase in initiator concentration as a consequence of the decrease in the

nD and Φ. Whereas in the case of HIPS containing SB with 90 wt-% of PB, the impact strength increases at the highest initiator concentration evaluated, where the particle size is larger (shown in Fig. 3: HIPS-9 and HIPS-10). Although the toughening efficiency of the rubber phase is a function mainly of the rubber particle size [22], it must be taken into account that the impact strength in HIPS is attributed to the energy required to form an extensive craze network so that the improvement in the impact strength is a consequence of the generation and efficient termination of crazes, another relevant factor that can determine the toughness in these materials is the compatibility between the phases and/or the interfacial interaction. In this sense, it is assumed that the higher the initiator concentration, the interfacial interaction is more pronounced due to a larger extent of PS grafts onto the SB as it can be inferred from the grafting degree values shown in Table 1: HIPS-1 and HIPS-2; HIPS-5, HIPS-9 and HIPS-10.

Fig. 8. Effect of A) [Initiator]0, B) [SB]0 and C) [CTA]0 on the impact strength of HIPS synthesized using copolymers with a PS/PB composition of 30/70 (▲), 20/80 ( ) and 10/90 ( ). In A) all HIPS were synthesized using [SB]0 = 8 wt %. In B) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [Initiator]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed. In C) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [ATC]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed. On the other hand, also from Fig. 8A, the impact strength increases much more noticeably with the wt-% PB in the SB –independent of the initiator concentration used- due to the modification of the morphological structure and to the increase in the rubber particle size that promotes and improves the absorption of energy for plastic deformation. It can be then concluded from Fig. 8A that as the wt-% of PB in



the SB increases, the initiator concentration that promotes though slightly a better impact strength which increases from 0.10 wt-% for HIPS containing SB with 70 and 80 wt-% of PB to 0.15 wt-% for HIPS containing SB with 90 wt-% of PB. It means that as the wt-% of PB in the SB increases, a larger amount of initiator is needed to provoke a better interfacial interaction between the phases that increases the impact strength. Fig. 8B presents the variation of impact strength of HIPS as a function of SB concentration where the impact strength decreases slightly as the SB concentration increases from 8 to 12 wt-%, for HIPS containing SB with 70 and 80 wt-% of PB due to a decrease in the rubber particle size (shown in Fig. 1: HIPS-1 and HIPS-3; Fig. 2: HIPS-5 and HIPS-7). The smaller particles are unable to initiate nucleation of crazes and offer a poor efficiency to terminate craze propagation so that the impact strength decreases. On the contrary, when using SB with 90 wt-% of PB, the impact strength increases markedly from 86 J/m, for HIPS with 8 wt-% of SB, to 221 J/m for HIPS with 12 wt-% of SB. The increase in these values is closely related to the increase in particle size (as shown in Fig. 3: HIPS-10 and HIPS-11), which according to the multiple craze theory, the larger the rubber phase particles (up to a limit, of course), the more able are these particles to participate in the nucleation and deactivation of crazes.

Fig. 9. Effect of the A) [Initiator]0, B) [SB]0 and C) [CTA]0 on the yield stress of HIPS synthesized using copolymers with a PS/PB composition of 30/70 (▲), 20/80 ( ) and 10/90 ( ). In A) all HIPS were synthesized using [SB]0 = 8 wt %. In B) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [Initiator]0 = 0.10 wt % and for HIPS with 10/90 ( ), [Initiator]0 = 0.15 wt % was employed. In C) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [ATC]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed. The variation of impact strength with CTA is observed in Fig. 8C where the impact strength increases very slightly with the chain transfer agent concentration when using SB with 70 wt-% of PB, however when using SB with 80 and 90 wt-% of PB, it increases very noticeably with the chain transfer agent concentration. In both situations the increase in impact strength is mainly attributed to morphological changes. Considering that the efficiency of the rubber particles in initiating and stabilizing the crazes depends on their size and internal morphology; it is evident that in the former case a very slight increase in the rubber particle size is observed



(shown in Fig. 1: HIPS-1 and HIPS-4) whereas in the later case a very marked change is observed not only in size but also in the morphology of the rubber particles from capsule and salami to salami type (shown in Fig. 2: HIPS-5 and HIPS-8; Fig. 3: HIPS-10 and HIPS-12). The variation in the tensile properties as a function of synthesis conditions is shown in Figs. 9 and 10 where it can be observed that the Young’s modulus (E) and the yield stress (σy) increase slightly with initiator concentration. This behavior can be explained taking into account that when HIPS is under deformation, the rubber particles suffer an elongation in the tension direction. Moreover, in the particles equatorial zone a high stress concentration is accumulated as a consequence by one hand, of the triaxial state in the disperse phase due to a differential thermal contraction between the rubber and the PS matrix phase (thermal expansion coefficients: 66*10-5 cm3/cm3/ºC and 25*10-5 cm3/cm3/ºC for the PB and PS, respectively) and on the other hand, the different Poisson ratios between the rubber particles and the matrix (0.5 and 0.35 for the PB and PS, respectively) that reflects the material response to a form and volume change. These events yield an interfacial matrix-particle separation that according with Schmitt [23] is the origin from which the crazes are formed. If it is considered that when initiator concentration increases the amount of graft PS increases, both the Poisson ratio and the rubber thermal expansion coefficient tend to be similar to that of the PS matrix and the interfacial adhesion increases. So that it is evident that as the larger the adhesion of the particles to the matrix and lower the particle stress concentration are, a larger stress is needed for interfacial separation and crazes formation, yielding materials with higher Young’s modulus and yield stress.

Fig. 10. Effect of A) [Initiator]0, B) [SB]0 and C) [CTA]0 on the Young’s modulus of HIPS synthesized using copolymers with a PS/PB composition of 30/70 (▲), 20/80 ( ) and 10/90 ( ). In A) all HIPS were synthesized using [SB]0 = 8 wt %. In B) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [Initiator]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed. In C) HIPS with 30/70 (▲) and 20/80 ( ) were synthesized using [ATC]0 = 0.10 wt % and for HIPS with 10/90 ( ), a [Initiator]0 = 0.15 wt % was employed. On the other hand, the Young’s modulus and the yield stress decrease with SB (composition PS/PB 10/90) and CTA concentration as a consequence of a noticeable increase in the volume fraction and in the particle size.



Considering that in the case of SB copolymers with a PS/PB composition of 30/70 and 20/80, the particle size decreases as the SB concentration increases to 12 wt-% (see Fig. 1: HIPS-1 and HIPS-3, Fig. 2: HIPS-5 and HIPS-7), it would be expected that the yield stress would also decrease due to the lower efficiency of the smaller particles to initiate craze formation. Nevertheless, according to Spiegelberg [24], the deformation in this kind of materials can be increased even though the craze mechanism formation is not well established.

Fig. 11. Behavior of the impact strength as a function of A) size particle, B) volume fraction, C) IPD/ nD ratio and D) N for the different synthesized HIPS. The full line is simply a guide to illustrate the tendency. In this case the rubber phase plasticizer’ effect is the main mechanism responsible for the decrease in the plastic resistance of the vitreous phase because this plasticization improves the propagation of the formed crazes. So that in the case of the HIPS synthesized with SB copolymers with a PS/PB composition of 30/70 and 20/80, the increase in the deformation as the rubber concentration increases can be attributed principally to a plasticizer effect. Figs. 11, 12 and 13 show the impact strength, yield stress and Young’s modulus as a function of different morphological features of the disperse phase: A) particle size, B) volume fraction and C) IPD/ nD ratio and D) number of particles per unit volume for the different synthesized HIPS. General trends can be observed for the variations in IS, σy and E, as a function of nD and Φ. Also IPD/ nD and N reveal a good correlation with the mechanical properties. Both, the impact strength and the yield stress present two noticeable performance zones. In zone I, the impact strength decreases



considerably with the increase in IPD/ nD and then in the zone II presents a slighter decrease, contrary to the behavior observed for the yield stress.

Fig. 12. Behavior of the yield stress as a function of A) size particle, B) volume fraction, C) IPD/ nD ratio and D) N for the different synthesized HIPS. The full line is simply a guide to illustrate the tendency.

Fig. 13. Behavior of the Young’s modulus as a function of A) size particle, B) volume fraction, C) IPD/ nD ratio and D) N for the different synthesized HIPS. The full line is simply a guide to illustrate the tendency.



These behaviors can be attributed to the fact that in the first zone the rubber particles are essentially of the salami type with multiple occlusions which suffer more marked changes in their size compared to the capsule type particles present in the second zone. The salami type morphology has an average size of 850-1200 nm meanwhile the capsule type particles present sizes between 180-260 nm. So that a slight decrease in particle size in the fist zone greatly diminishes their reinforcement capacity. In the second zone, the slight variations in impact strength and yield stress are consequences of a population of particles incapable of craze nucleation and growth. The behavior of the impact strength and the yield stress as a function of N is also an evidence of the antagonist dependency of these two properties. Impact strength drastically decreases as N increases, and vice versa.

With respect to the Young’s modulus it presents a linear dependency with nD , IPD/ nD and Φ. Nevertheless, an increase in N till a value close to 50, the modulus increase noticeably and then it remains practically constant. Conclusions It was found that when using SB with 70 wt-% of PB for the synthesis of HIPS, the particle size of the dispersed rubber phase, as well as the impact strength and the tensile properties of HIPS are slightly modified when varying the initiator, chain transfer agent or the SB concentration. When using SB’s with 80 or 90 wt-% of PB, the results were different. The particle size of the dispersed rubber phase slightly decreases when increasing the initiator concentration, and increases markedly when increasing the CTA, the SB or the PB concentration. With respect to the impact and tensile properties; it was found that the Young’s modulus changes slightly along the study, but the impact strength and the yield stress were noticeably dependant on the size of the rubber phase particles. The larger the particle size, the greater the impact strength and the lower the yield stress. That is, any variation in the initiator or CTA concentration during polymerization or in the SB or wt-% of PB in the final HIPS that increase the size of the rubber phase particles will increase the impact strength but will decrease the yield stress of HIPS. Experimental part

Materials The styrene/butadiene (SB) graded block copolymers [i.e.: (butadiene)-(butadiene→styrene)-(styrene)] were prepared by anionic solution polymerization. The anionic solution polymerization was carried out using a solvent/monomer ratio of 6.5, purified monomers and n-butyl lithium as initiator at 1.9 M in cyclohexane. The cyclohexane and the corresponding amounts of monomers for each copolymer compositions were fed to the reactor and when the temperature of 60 °C was reached, the first block of pure polybutadiene was formed due to the butadiene reactivity. Then a graded block of butadiene/styrene was formed, this graded block was initially rich in butadiene and as the polymerization takes place it became rich in polystyrene. Finally, at the last step of the copolymerization reaction, the polystyrene block was formed.



The characterization of the graded block copolymers (SB) used in the HIPS synthesis is summarized in Table 2. Styrene monomer was from Poliformas Plásticas, México, and used as received. Benzoyl peroxide initiator (BPO) was from Promotores y Catalizadores Orgánicos de México. T-dodecyl-mercaptane chain transfer agent (CTA) and polyvinyl alcohol (Mw = 124-186 kg/mol and hydrolysis grade 87-89%) were acquired from Aldrich and used as received, sodium chloride from Química Dinámica and Nonyl phenol from General Electric Specialty Chemicals. Tab. 2. Characteristics of the SB's used in the synthesis of HIPS.

Composition of PS/PB [wt %]

Mpa) of SB [kg/mol]

Total PS [wt-%]

PS in the block [wt-%]

Mpa) of the PS block [kg/mol]

SB-A:30/70 232 32.5 25.5 37 SB-B:20/80 344 21.5 16.0 29 SB-C:10/90 399 10.2 6.5 N.D.

a)Molecular Weight at the peak of the Gel Permeation Chromatography plot; ND = not determined. High Impact Polystyrene Synthesis In order to evaluate the effect of the initiator, SB copolymer and chain transfer agent (CTA) concentration, several HIPS were synthetized employing the initial reagent concentrations shown in Table 3. The initial reagent concentrations were established taking into account previous experimental data where the influence of reaction conditions on morphology type and size, as well on mechanical properties has been evaluated [25]. In this sense when 8 wt-% of SB copolymer with different PS/PB composition were used, it was demonstrated that the initiator concentration and the chain transfer agent concentration that give rise to morphologies that provide a better mechanical response has the following order: PS/PB:[I]:[CTA] wt-%: 30/70– 20/80: 0.10:0.10; 10/90:0.15:0.15. The HIPS were prepared by a bulk-suspension process as follows: in 1 gallon capacity stainless steel reactor, with an anchor-turbine stirrer and at room temperature: i) a fixed amount (8 wt-%) of graded block copolymer with respect to monomer was dissolved into styrene at a stirring rate of 20 rpm until the total dissolution of the copolymer; ii) benzoyl peroxide (BPO) was added, and a stirred bulk prepolymerization stage at 40 rpm and 80 °C occured until the monomer conversion was close to 30-35%; iii) during the finishing stage 0.1 wt-% of ter-butyl perbenzoate (TBPB) and a suspension medium were added, and the polymerization was carried out to total monomer conversion. The suspension medium involved water (2 L), polyvinyl alcohol (1.7 g), sodium chloride (1.8 g), and nonyl phenol (0.66 g). Processing of HIPS After the synthesis and following the procedure described in a previous paper [26], a master batch of each HIPS plus antioxidants and stabilizers was prepared. Thereafter, each masterbatch was extruded and pelletized with the rest of the corresponding HIPS and finally, all different HIPS samples were injection moulded



under the following experimental conditions: injection temperature = 240 °C, injection pressure = 130 bar and injection rate = 50 s, to obtain the specimens for several analysis. Tab. 3. Initial reagent concentrations used in the HIPS synthesis.

Material Precursor Copolymer

[SB]0 (wt-%)

[I]0 (wt-%)

[CTA]0 (wt-%)

[MI]0* (wt-%)

HIPS-1 SB-A 8 0.10 - -

HIPS-2 SB-A 8 0.15 - -

HIPS-3 SB-A 12 0.10 - 6

HIPS-4 SB-A 8 0.10 0.10 -

HIPS-5 SB-B 8 0.10 - -

HIPS-6 SB-B 8 0.15 - -

HIPS-7 SB-B 12 0.10 - 6

HIPS-8 SB-B 8 0.10 0.10 -

HIPS-9 SB-C 8 0.10 - -

HIPS-10 SB-C 8 0.15 - -

HIPS-11 SB-C 12 0.15 - 6

HIPS-12 SB-C 8 0.15 0.15 - []0: Initial concentration; SB: SB copolymer; I: Initiator; CTA: Chain transfer agent; MO: Mineral Oil. *: the mineral oil is considered only like aid of process and not like synthesis variable

Characterization -Physico-chemical characterization The gel (insoluble) fraction was isolated from the soluble fraction (free PS) dissolving 1 g of HIPS sample in 25 mL of toluene using the centrifugation technique under the following conditions: 45 min. at 20,000 rev/min at -20°C. The soluble fraction was precipitated from methanol and both fractions were dried under vacuum at 50 °C to constant weight and gravimetrically calculated. -Morphological characterization Transmission Electron Microscopy examination was performed in a JEOL TEM at 10kV on samples cut with a LEICA ULTRACUT ultra microtome and treated with osmium tetra oxide. Grafting degree (GD) The grafting degree (GD), defined as the ratio between the mass of graft PS branches and the original SB fraction in the recipe, was calculated as described below [27]:



SBGelPSgraft −= (1)

100SB

PSGD graft ×⎥

⎦

⎤⎢⎣

⎡= (2)

where PSgraft is the amount of PS chemically and physically bonded to the SB, Gel and SB are the weight fractions of toluene insoluble and original SB, respectively. Number-average particle diameter (Dn) and volume fraction (Φ) The apparent number-average particle diameter was estimated with the aid of an image analyzer, Image Pro 3.0 program. For each HIPS, several TEM micrographs were analyzed and more than 100 particles were measured. On the other hand, since the area fraction of the rubber particles in TEM micrographs can be used as a volume fraction with an error not greater than 5% [28], we directly adopted the latter. Because the volume fraction of the disperse phase in some HIPS samples was not possible to evaluate from TEM (i.e HIPS-2), the volume fraction was obtained from the modified Kerner’s equation [29]:

( )( ) ⎥

⎦

⎤⎢⎣

⎡ΦΦ

υ−υ−

+=cd

c

PSHIPS 571151

E1

E1

(3)

Where EHIPS = 2.22 GPa = initial Young’s modulus of the HIPS estimated from the tensile analysis at 25 ºC [30], EPS = 3.19 GPa = initial Young’s modulus of the PS matrix, cν is the PS Poisson ratio = 0.35, dν = is the rubber Poisson ratio = 0.5, Φ = volume fraction of the rubber phase and cΦ = volume fraction of the PS phase [30]. Interparticle distance (IPD).

The IPD can be determined from nD and Φ through the following expression [31]:

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡−⎟

⎟⎠

⎞⎜⎜⎝

⎛

Φ= n

3/13n DDIPD (4)

Interfacial area (Ia)

The interfacial area (Ia) per unit volume in a blend containing a volume fraction Φd with spherical particles of radius r in a glassy phase is equal to [32]:

r3IaΦ

= (5)

Particle number per unit volume (N) The particle number per unit volume can be estimate through equation 6 [33]:

334 r

NπΦ

=

Where Φ is the volume fraction of rubber phase, r is the radius of particles. (6)



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Mechanical behavior of high impact polystyrene based on SB copolymers as a function of synthesis conditions: Part II