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Highly transparent thermoplastic elastomer from isotactic polypropylene and styrene/ethylene-butylene/styrene triblock copolymer: structure-property correlations.

INTRODUCTION

Silicone and polyvinyl chloride (PVC) polymers are widely used in medical and personal care products requiring good transparency (1). Silicone has excellent high temperature performance and chemical resistance. However, silicone is very expensive material. On the other hand, PVC is a low cost material with wide processing window. However, there is a growing recycling concern over PVC.

Polypropylene (PP) is one of the most familiar polymers used in industry and in daily life. Although PP exhibits superior mechanical properties, the poor transparency and brittleness of PP restricts its applications in the field of medical and personal care, where silicone and PVC are presently used (2). Therefore, it is of great importance to explore methods to increase the softness and transparency of PP. There are several ways to increase the transparency of PP (3), e.g., (1) to increase the fraction of amorphous phase, (2) to reduce the size of spherulites, and (3) to destroy spherulite structure by stretching. To increase the transparency, crystalline polymers are usually thermally quenched or crystallized by adding nucleating agents. Thermal quenching, however, reduces the dimensional stability of the polymers. Nucleation agents bring a contamination problem when the polymer is used as a container or as wrapping film for food, drugs, and so on. Consequently, it is very important to increase the transparency of PP without thermal quenching or nucleating agents. Therefore, enhancing the transparency of PP by increasing the fraction of amorphous phase could be an excellent choice among the various available options.

In literature, there are several reports which examine different TPEs based on PP especially for automotive and industrial applications (4-10). Various types of thermoplastic elastomers (TPEs) based on rubber-plastic blends (11-16) have been reported from our laboratory. However, a complete literature survey reveals that very limited researches have been pursued so far to develop transparent and soft PP by blending PP with different amorphous polymers. For most of the PP and amorphous polymer blend compositions, the resultant product is optically not fully transparent. Few studies have been reported in the patent literature that discusses the enhancement of the optical properties of PP by blending it with styrenic block copolymers (17). However, the improvement attained in the transparency is very modest (having haze value around 25%).

It is known that the clarity of compounds or blends is determined by multiple factors such as refractive index of component phases, internal microstructure, surface morphology, etc. Transparent TPEs can be made by blending two or more transparent components with closely matched refractive index. It can also be prepared with components having different refractive indices, if the morphology development is controlled in such a way to have the dimension of both the dispersed phase and continuous phases are below visible wavelength (18), (19). The present work focuses on the development of transparent clastomeric blends of PP with styrene-ethylene/butylene-styrene block copolymer (SEBS) with an approach to generate favorable properties by morphology design. Unique transparent TPE compositions were developed, having Shore-A hardness <90 and haze value as low as 6%, along with excellent percentage elongation and processability.

EXPERIMENTAL

Materials

Two grades of random copolymer of polypropylene (PP1 with MFI of 10 g/10 min and Mn of 3.5 X [10.sup.4] and PP2 with MFI of 2 g/min and Mn of 6.1 X [10.sup.4]) manufactured by Reliance Industries Limited, India, were used for the study. Two grades of SEBS [SEBS1 with MFI of 18 and 22% Styrene and SEBS2 with MFI of 4 and 14% Styrene] were procured from Kraton Polymers, USA. To stabilize the blends during extrusion and injection molding, a combination of two antioxidants (Irganox 1010 and Irgafos 168) was used. Both the antioxidants were procured from Ciba Specialty Chemicals India Pvt. Ltd., Mumbai, India.

Preparation of Blends

Four series of PP/SEBS blends in the composition of 0 to 70 wt% of SEBS were prepared by melt blending in a twin screw extruder (Berstorff ZE-25, l/d = 40) at a screw rpm of 190 and in the temperature range of 180 to 210[degrees]C at different zones. The details of the compositions and the PP/SEBS blend series prepared are summarized in Table 1. These granules were then injection molded in Windsor Injection molding machine (model SP180DD) to prepare samples for mechanical properties testing and in Arburg 320C Injection molding machine (model Allrounder 500-100) to prepare samples for optical properties testing.
TABLE 1. Composition of the PP/SEBS blends prepared.

                                     Series

                Material    TPA    TPB    TPC    TPD

                PP (MFI)    10      2     10      2

               SEBS (MFI)   18      4      4     18

Sl. No.  % PP   % SEBS

   1      90       10      TPA-1  TPB-1  TPC-1  TPD-1
   2      80       20      TPA-2  TPB-2  TPC-2  TPD-2
   3      70       30      TPA-3  TPB-3  TPC-3  TPD-3
   4      60       40      TPA-4  TPB-4  TPC-4  TPD-4
   5      50       50      TPA-5  TPB-5  TPC-5  TPD-5
   6      40       60      TPA-6  TPB-6  TPC-6  TPD-6
   7      30       70      TPA-7  TPB-7  TPC-7  TPD-7


Optical Properties

Haze Measurements. Optical properties such as haze, total transmission, and diffuse transmission of samples were measured in a 1-mm-thick injection molded plaques on ColorQuest II Hazemeter (HunterLab, USA) as per ASTM D1003-95 standard. In this study, the percentage haze values were calculated using Eq. 1 (20).

Haze (%) = [Diffuse Transmission/Total Transmission] x 100 (1)

where total transmission (TT) is the sum of the diffuse transmission (DT) and the direct transmission. Diffuse transmission is the portion of total transmitted light that is scattered or diffused by the irregularities within or at the surface of the material.

Refractive Index (RI) Value Measurements. The refractive index was measured using an Abbe Refractometer (Atago Co., USA) following the method proposed by Samuels (21).

Fourier Transform Infrared Spectroscopy. The percentage total transmission of infrared rays through the films of neat polymers and blends was measured with a Perkin-Elmer FTIR spectrophotometer. FTIR spectra on the films of thickness 0.3 mm were taken at room temperature (25[degrees]C). The samples were scanned from 4000 to 400 [cm.sup.-1] with a resolution of 4 [cm.sup.-1]. All spectra were reported after an average of 32 scans.

Crystallinity Studies

Measurement of % Crystallinity and Crystallite Size by Wide Angle X-ray Diffraction. Wide angle X-ray diffraction (WAXD) of the blends and the neat polymers were performed using PW 1820 goniometer (PHILIPS, Holland) at 20 kV and 20 mA with Cu k[alpha] ([lamda] = 0.154 nm) radiation source in the range of 10[degrees] to 60[degrees]. The scan rate was 3[degrees]/min. The samples were placed vertically in front of the X-ray source, perpendicular to the goniometer, which was fixed while the sample was rotating. The area under the crystalline and amorphous portions was determined in arbitrary units and the percent crystallinity was calculated using the following Eq. 2 (22).

Crystallinity (%) = [[[I.sub.C] x 100]/[[I.sub.C] + [I.sub.A]]] (2)

where [I.sub.C] and [I.sub.A] are the integrated intensity corresponding to the crystalline and amorphous phases, respectively. The crystallinity values reported are the average values based on three experiments per sample. The scan profile was fitted by using standard software, Origin 7.5[TM]. Scherer's equation (22) (Eq. 3) was used to estimate crystallites' thickness on the basis of the full width at half maximum ([[beta].sub.0]) of the corresponding WAXD peaks as:

[L.sub.hkl] = [[0.91 * [lambda]]/[[[beta].sub.0]Cos[theta]]] (3)

where [L.sub.hkl] represents the crystallite thickness in a direction perpendicular to that of the crystallographic plane identified by Miller indices hkl, [lambda] is the wavelength, [theta] is half the angle of diffraction (2[theta]), and 0.91 is a constant used.

Melt Flow Properties

The flow curves (viscosity vs. shear rate) were measured at 220[degrees]C by means of Monsanto Processability Tester (MPT) (barrel radius, 9.53 mm) which is a fully automated capillary rheometer. The entire barrel and the capillary assembly were electrically heated with a microprocessor-based temperature controller. The capillary used had a length to diameter ratio equal to 30 (length 30.0 mm; diameter 1.0 mm).

Morphological Characterization

Polarized Light Microscopy. Temperature-dependent PLM studies were carried out on an Olympus PLM, equipped with a Linkam THMS600 hotstage. Samples were initially heated above the melt temperature and cooled at a rate of 5[degrees]C/min, and were isothermally crystallized at the temperature where onset of crystallization was observed.

Transmission Electron Microscopy. Representative blend samples for TEM analysis were prepared by ultra-microtomy using a Leica Ultra cut UCT. Diamond knives from Diatome, with a cutting edge of 45[degrees] were used to get the cryosection of required thickness. Since these samples were elastomeric in nature, the sample temperature during ultra cryomicrotomy was kept constant at - 90[degrees]C (which was well below the glass transition temperature, [T.sub.g], of the blends), at which the samples existed in a hard glassy state, thus facilitating ultracryomicrot-omy. The cryosections of 80 nm thickness were collected and directly supported on a copper grid of 200-mesh size, followed with vapor staining with freshly prepared Ru[O.sub.4] solution for 6 min. TEM analysis was performed on an FET Tecnai 12, at an accelerating voltage of 100 kV.

Atomic Force Microscopy. For detecting surface phase morphology, scanning and analysis of the samples were carried out using a multimode atomic force microscope (AFM) with a nanoscope IIIa controller by Digital Instruments, Santa Barbara, CA. Phase images were taken for surface morphologies of different blends in air at ambient conditions (25[degrees]C temperature, 60% humidity) with a tapping mode Si probe (TESP) having spring constant (K) of 20-40 N/m.

Mechanical Properties

Tensile Testing. Tensile strength, elongation at yield, and elongation at break were measured as per ASTM D638 (23) standard in a universal tensile testing machine (UTM) (Lloyd Instrument, model LR10 K) at a crosshead speed 50 mm/min at 25[degrees]C. Injection molded specimens of gauge length 25 mm, width 6.35 mm, and thickness 3.4 mm were used for the measurements. The average of five results is reported here.

Tension Set. Tension set values were obtained after first, second, and third recycling of the blends. Tension set was measured at 25[degrees]C after stretching the sample for 10 min at 100% elongation according to ASTM D412-98 (24). The average of five results is reported here.

Hardness. Hardness of the blends was measured on the samples having minimum thickness of 6 mm using a Shore "A" and Shore "D" type durometer (25). The average of five results is reported here.

Dynamic Mechanical Analysis: Temperature Sweep Test

Temperature ramp test was carried out in a dynamic mechanical analyzer (DMAQ 800 from TA instruments), in the temperature range of -130 to 150[degrees]C for a few representative samples at a constant frequency of I Hz and at a constant strain of 0.1% in tension mode geometry. The sample dimension was around 15 mm X 6 mm X 1 mm (l X w X h). Data analysis was performed in the computer attached to the DMA machine using the DMA Q800 software. Storage modulus (E'), loss modulus (E"), and loss tangent (tan [delta]) were measured as a function of temperature for the entire representative samples under identical conditions. The temperature corresponding to tan [delta] peak was taken as the glass transition temperature ([T.sub.g]).

RESULTS AND DISCUSSION

Haze of Neat PP and PPISEBS Blends

Figure 1 shows the variation in haze with concentration of SEBS in all the four blends (TPA, TPB, TPC, and TPD) series. A gradual decrease in haze with increasing SEBS concentration is observed for TPA, TPC, and TPD blends with the lowest values at 70 wt% of SEBS. The haze values of the samples TPA-7, TPC-7, and TPD-7 are 6, 7, and 5, respectively. The sample TPD-7 has the lowest % haze value and exhibits very good transparency. In spite of having good transparency, mechanical properties of the sample TPD-7 are much inferior when compared to the mechanical properties of the samples TPA-7 and TPC-7 (discussed later). On comparing the samples TPA-7 and TPC-7, TPA-7 poses good transparency along with better mechanical properties. The mechanical properties of all the blend series have been discussed in detail in the later section.

[FIGURE 1 OMITTED]

On the other hand, the haze property of the TPB blend system does not follow the trend of the other blend systems. TPB blend series show an initial increase in haze with increase in SEBS content up to 50 wt%, followed by a decrease at further higher SEBS concentrations. The reason behind the anomalous behavior of TPB blend system has been explained by considering the MFI values of the blend components. Among all the blend systems studied, the blend components of the TPA family have the highest melt flow index values and the blend components of the TPB family have the lowest melt flow index values.

Therefore, it can be noted that the combination of high MFI PP with higher MFI SEBS results in a maximum decrease in the haze value in the TPA series. This is probably due to the better dispersion of the blend components in TPA series. On the other hand, combination of blend components with low MFI values, as in TPB series, does not seem to improve the haze properties. This can be attributed to the poor dispersion of the blend components in TPB series. Accordingly, the morphological studies (discussed later) provide evidences for the existence of finer and agglomerated morphology in the TPA and TPB series, respectively.

Figure 2 shows that the decrease in percentage haze value is directly proportional to the percentage decrease in the diffuse transmission in the TPA blend series. It is interesting to note that, in the visible region (380-780 nm), there is no significant change in the percentage total transmission values with the increase in the concentration of the SEBS content. The total transmission values of the neat PP and PP/SEBS blends in the infrared region (IR) (750 nm-1 mm) also suggest the same. The total transmission value of neat PP and PP/SEBS blends in the infrared (IR) and visible (Vis) regions are shown as inset of Fig. 2. Therefore, it can be seen that the % haze value is highly governed by the diffuse transmission values irrespective of any marginal increase or decrease in the total transmission values. The optical transparency of neat PP1 (MFI 10) and TPA-7 blend has been visually observed by placing the plaque of the injection-molded samples on a logo (Fig. 3a and b). It is clearly evident that the blend TPA-7 is much optically transparent than neat PP1.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Haze Modeling

From Fig. 2, an equation relating the haze values and the SEBS content in the TPA blends can be derived from the semi logarithmic plot between In haze (%) vs. SEBS (%) as shown in Fig. 4. Therefore, the equation relating the haze (H) values and the SEBS content ([C.sub.s]) in the blends can be written as:

[FIGURE 4 OMITTED]

ln H = 2.70 - 0.012 [C.sub.s] (4)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

Similarly, an equation relating the diffused transmission values and the SEBS content in the TPA blends has been derived from the semi logarithmic plot between In diffuse transmission (DT) (%) vs. SEBS (%) as shown in the inset of Fig. 4. Therefore, the equation relating the diffuse transmission values (DT) and the SEBS ([C.sub.s]) content in the blends can be written as:

ln DT = 2.55 - 0.012 [C.sub.s] (6)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

From Eqs. 5 and 7, it is seen that the haze and diffused transmission values decrease exponentially with the increase in the concentration of the SEBS content.

Crystallinity Studies

% Crystallinity From WAXD Measurements. Dependence of haze on blend compositions could be explained on the basis of % crystallinity, spherulite size, and phase morphology of the blends. The % crystallinity of neat PP samples and PP/SEBS blends of all four blend series have been calculated from the XRD curves by using Eq. 2. The variation in % crystallinity with SEBS concentration for all four blend series is presented in Fig. 5. A gradual decrease in % crystallinity has been observed with increasing SEBS content, except in TPB series. It is interesting to note that pattern of variation in % crystallinity is very similar to the pattern of variation of % haze with SEBS content, which clearly explains the strong relationship between the reduction in the % haze value and the reduction in % crystallinity values by the incorporation of SEBS copolymer.

[FIGURE 5 OMITTED]

Crystallite Size Through WAXD. The crystallite sizes of neat PP and PP/SEBS blend samples of TPA series have been calculated from the crystalline peaks of the XRD curves (Fig. 6a) using Eq. 3 and the values are plotted in Fig. 6b. It is surprising to see that there is no significant change in the crystallite size with the increase in the concentration of SEBS.

[FIGURE 6 OMITTED]

Melt Viscosity of Neat PP and PP/SEBS Blends (TPA Series)

At the temperature of blending (220[degrees]C), although the melt viscosity of neat PP (MFI 10) is lower when compared to neat SEBS (MFI 18) (see Fig. 7), there is not much difference. Therefore, one can expect very good mixing between the two blend components at the processing temperature. Accordingly, the morphological analysis of TPA blends (explained in later section) shows very fine and uniform dispersion SEBS in PP matrix.

[FIGURE 7 OMITTED]

Refractive Index

The transparency of a blend is determined by the RI of each ingredient and the resultant morphology (21). Ingredient selection for any transparent blend compound requires a careful match of the RI values. Careful study of the blend morphology and reduction of the difference of RI among the various ingredients will lead to a blend with improved transparency (3), (18). However, it is also known that transparent polymer blend can be made of polymers with different RI provided the dimension of the dispersed phase is below the visual wavelength (3), (18). In this work, the RI values of neat PP and neat SEBS of TPA blend system has been identified as 1.421 and 1.433. The RI values of both the blend components are quite close to each other. On the other hand, the RI values of the blend samples TPA-1, TPA-4, and TPA-7 are 1.429, 1.427, and 1.426, respectively. The RI values of the blends are almost similar to the RI values of the blend components (see Fig. 8). In the TPA blend samples, the dimension of the dispersed rubber phase is below the visual wave length (discussed in the forthcoming section), which has actually resulted in optically transparent PP/ SEBS (TPA) blends.

[FIGURE 8 OMITTED]

Morphological Characterizations

Figure 9a and b shows typical PLM images of selected samples from TPA and TPB series, with SEBS content of 10%, 40%, and 70%. A gradual decrease in spherulite size with increasing SEBS content has been observed in TPA series, reaching to lowest spherulite sizes at 70 wt% SEBS content. The reduction in spherulite size results from the geometric constraints imposed by the confining SEBS layer that restricts the spherulite growth (26). The spherulites with smaller dimensions will result in very limited scattering of the incident light. Accordingly, the transparencies of the TPA blend samples are superior when compared to neat PP. However, TPB series (Fig. 9b) depict an increase in spherulite size with increasing SEBS content till 40 wt%. Further increase in SEBS content results in finer and smaller spherulites. These trends correlate well with the variation in haze with SEBS content in both the series and indicate that along with the % crystallinity, spherulite size also have a decisive role in controlling the haze of a system.

[FIGURE 9 OMITTED]

Figure 10a-f shows the TEM micrographs of TPA and TPD series with high MFI SEBS at 10, 40, and 70 wt% at 16500X magnification. Figure 11a-f shows TEM micrographs of TPB and TPC series with low MFI SEBS at 10, 40, and 70 wt% at 16500X magnification. The dark phase in the TEM micrograph represents the rubbery phase and the white phase corresponds to PP. In high MFI SEBS blend series, i.e., in TPA and TPD series, finely dispersed nanorubber phase in continuous PP matrix is seen at 10 wt% of SEBS and the dimension of the dispersed phase is below the visual wave length. In TPA series, as PP is also having high flowing behavior, the morphologies generated at all the compositions are in the flow direction. At 40 wt% rubber, in TPA-4, a lamellar interpenetrating nanostructure in the flow direction is generated while a co-continuous interpenetrating nanostructure is depicted in TPD-4. At 70 wt% SEBS content, very fine co- continuous network is formed in TPA blend while phase inversion takes place with fine dispersed SEBS in TPD-7.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

The low MFI SEBS blend series, i.e., TPB and TPC shows poor dispersion of the domains and there is no evidence for the flow pattern. Also, more number of agglomerated dispersed rubber phase has been observed in TPB-4 sample having low MFT PP, which results in high haze value. Furthermore, the surface morphology of a representative blend sample has been analyzed by atomic force microscopy (AFM) at a magnification of 500 nm X 500 nm. Figure 12a and b shows the phase image of PP and the blend sample TPA-7. PP image shows the brighter crystalline phase as well as the darker amorphous phase. Very fine and uniform dispersion of the blend components are visibly seen in the AFM image of the blend sample TPA-7. The amorphous SEBS phase is in the range of ~10 nm which is also observed in the TEM image (Fig. 10c). The amorphous darker phase is more in the blend sample which is in accordance to our crystallinity studies and provides clear evidence for the low haze value of the TPA-7 blend.

[FIGURE 12 OMITTED]

Physicomechanical Properties

Table 2 shows the mechanical properties of PP control and PP/SEBS blend systems. With increasing weight percentage of the rubbery SEBS copolymer in the blend, the tensile strength, flexural modulus, and hardness values of all the blend systems gradually decrease with simultaneous increase in percent elongation. The blends containing more than 30 wt% SEBS content are much softer and have hardness values in Shore A range as due to increase in the rubber content and finer dispersion of SEBS (as discussed in the morphological studies). Shore-A value of 75-80 is achieved at 70 wt% of SEBS. Among all the blend systems, the TPA blend exhibits the combination of best mechanical and optical properties. This can be attributed to the good dispersion of the blend components as a result of their higher MFI values than that in TPB, TPC, and TPD blend systems.
TABLE 2. Hardness and tensile properties of PP/SEBS blend systems.

Samples      Hardness  Elongation at    Tensile         Flexural
                         break (%)      strength      modulus (MPa)
                                         (MPa)

PP 1           73 D    475 [+ or -]   32.5 [+ or -]   979 [+ or -] 35
                       30             0.6

PP 2           72 D    400 [+ or -]   33.7 [+ or -]  1005 [+ or -] 45
                       20             0.8

TPA-3 (30%)    60 D    610 [+ or -]   25.7 [+ or -]   424 [+ or -] 30
                       35             0.6

TPA-5 (50%)    80 A    615 [+ or -]   18.9 [+ or -]   124 [+ or -] 10
                       35             0.9

TPA-7 (70%)    75 A    660 [+ or -]   17.2 [+ or -]    67 [+ or -] 5
                       20             0.8

TPB-3 (30%)    58 D    555 [+ or -]   24.7 [+ or -]   290 [+ or -] 20
                       25             2.3

TPB-5 (50%)    83 A    515 [+ or -]   13.8 [+ or -]    60 [+ or -] 10
                       30             1.9

TPB-7 (70%)    75 A    520 [+ or -]   10 [+ or -]      38 [+ or -] 5
                       40             1.0

TPC-3 (30%)    56 D    610 [+ or -]   22.7 [+ or -]   294 [+ or -] 10
                       30             0.6

TPC-5 (50%)    85 A    680 [+ or -]   16.8 [+ or -]    62 [+ or -] 5
                       40             0.5

TPC-7 (70%)    78 A    680 [+ or -]   14.8 [+ or -]    42 [+ or -] 20
                       40             0.7

TPD-3 (30%)    58 D    480 [+ or -]   26.8 [+ or -]   372 [+ or -] 25
                       40             1.3

TPD-5 (50%)    85 A    480 [+ or -]   16 [+ or -]      90 [+ or -] 10
                       30             1.0

TPD-7 (70%)    78 A    490 [+ or -]   12.6 [+ or -]    51 [+ or -] 5
                       40             0.6


The tension set results of the samples TPA-1, TPA-4, and TPA-7 before and after three recycling is shown in Fig. 13. The tension set values of the blends TPA-4 and TPA-7 are lower than that of the blend TPA-1. This is due to higher amount of the rubbery inclusions in the TPA-4 and TPA-7 blends which will facilitate good elastic recovery. Moreover, the tension set values of the blends TPA-4 and TPA-7 marginally decrease after subsequent recycling. This can be attributed to the better dispersion of the blend components after continuous recycling. However, the tension set value of the blend TPA-1 does not decrease even after three recycling due to the lower concentration of the SEBS in the TPA-1 blend.

[FIGURE 13 OMITTED]

Dynamic Mechanical Properties

Dynamic mechanical properties of the neat SEBS1, neat PP1, and three representative blend samples of TPA blend system are reported in Fig. 14a and b. SEBS displays the following two transitions: the first near -26[degrees]C, due to the relaxation of the soft elastomeric ethylene-butylene segments and the second at around 115[degrees]C, due to the relaxation of the hard polystyrene domains (27), (28) (Fig. 14a). PP displays the following two transitions: the first near 5[degrees]C, which is the glass transition temperature of the PP and the second around 75[degrees]C, which can be due to the amorphous portions in the isotactic PP (28) (shown as the inset of Fig. 14a). The sample containing 10 weight percentage of SEBS (TPA-1) (shown as the inset of Fig. 14a) shows a transition at -38[degrees]C along with the transitions of polypropylene at 5 and 75[degrees]C. The transition at -38[degrees]C should be attributed to relaxation of the soft elastomeric ethylene-butylene segments of SEBS. With the increase in the concentration of SEBS to 40 wt% (TPA-4) (Fig. 14a), the transition of the elastomeric ethylene-butylene segments of SEBS appears at around -24[degrees]C and there is no evidence for the transitions of polypropylene. Moreover, a high temperature transition is identified around 115[degrees]C, which corresponds to the hard polystyrene domains of SEBS. With the further increase in the concentration of SEBS to 70 weight percentage (TPA-7) (Fig. 14a), the transition of the elastomeric ethylene-butylene segments of SEBS appears at around -25[degrees]C and the transitions of polypropylene at 5 and 75[degrees]C could not be identified. Another prominent transition corresponding to hard polystyrene domains of SEBS is identified at 115[degrees]C. It is interesting to note that for sample TPA-1 the transition of the ethylene-butylene segments of SEBS appears at -38[degrees]C. However, with the increase in the concentration of the SEBS (TPA-4 and TPA-7), the transition of the ethylene-butylene segment of SEBS appears around - 25[degrees]C. This shows that the low temperature loss peak of SEBS varies nonlinearly with blend compositions. Gupta and Purwar (29) have observed similar trend for SEBS/PP blends containing 5%, 10%, 15%, and 20% of SEBS content. They showed that the low temperature loss peak of SEBS varies nonlinearly with blend composition by about 10[degrees]C, showing a minimum around 15% of SEBS content. Bares (30) described the depression in [T.sub.g] for the dispersed thermoplastic elastomer to arise from the size of the dispersed phase domains as well as the interfacial effects at the domain boundaries. Therefore, it seems that, at lower weight percentage of SEBS (between 1 and 15%), the SEBS will be dispersed in the continuous PP matrix. The domain size of the dispersed SEBS phase will slowly increase with the increase in the concentration of the SEBS from 1 to 15%. Therefore, the effect of the size of the dispersed phase domains as well as the interfacial effects at the domain boundaries will decide the [T.sub.g] of the ethylene-butylene segment of SEBS. However, at higher percentage of SEBS (>20%), the morphology of the blend system will tend to change from dispersed morphology to co-continuous type and IPN type. Therefore, the factor determining the [T.sub.g] of the ethylene-butylene segment of SEBS is different. Similarly, in this work, the TEM micrograph of the blend TPA-1 (Fig. 10a) shows the dispersed SEBS phase in the PP matrix. Moreover, the TEM micrographs of the blends TPA-4 and TPA-7 displays co-continuous and IPN type of morphology. This could be the possible reason for the 10[degrees]C difference in the [T.sub.g] values of the ethylene-butylene segment of SEBS in the TPA-1 blend and [T.sub.g] values of the ethylene-butylene segment of SEBS in the TPA-4 and TPA-7 blends.

[FIGURE 14 OMITTED]

The storage modulus of PP (Fig. 14b) decreases on blending with the SEBS elastomer like the known behavior of other rubber modified polymers (29), (31). But the decrease in the storage modulus values is more prominent at higher weight of SEBS content.

CONCLUSIONS

From the present investigation, the following conclusions could be made:

1. Highly transparent thermoplastic elastomers (TPEs) have been developed from Isotactic-PP/SEBS blends having different melt flow index (MFI) values.

2. The blends containing PP and SEBS polymers of high MFI values (TPA Series) exhibit good optical and mechanical properties at 30/70 (PP/SEBS) blend ratio. However, the blends containing PP and SEBS polymers of low MFI and high MFI value combinations do not simultaneously exhibit both transparency and good mechanical properties.

3. The % crystallinity values of TPA series calculated from WAXD suggest the gradual decrease in the % crystallinity of neat PP by increasing the concentration of SEBS in the blend. Moreover, there is no significant change in the crystallite size of PP by the incorporation of SEBS block copolymer.

4. The melt viscosities of neat PP and neat SEBS polymer of TPA series are quite close to each other. This results in very fine dispersion of the blend components.

5. The RI values of PP and SEBS of TPA series are quite close to each other. Furthermore, by increasing the concentration of SEBS in the blend, the difference between the RI values of neat PP and neat SEBS gradually comes down. Accordingly, the haze value gradually reduces by increasing the concentration of SEBS in the blends.

6. The PLM pictures of the blends containing PP and SEBS polymers of TPA series show spherulites with very smaller dimensions. This results in very less scattering of the incident light and hence enhances the transparency of the blend.

7. The blends containing PP and SEBS polymers of TPA series exhibit very finer dispersion of the blend components in the TEM and AFM pictures of the blend samples. The finely dispersed blend components result in limited scattering of the incident light waves and finally enhance the transparency of the blend. However, very poor dispersion of the blend components has been observed in other blends.

REFERENCES

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Zubair Ahmad, (1) K. Dinesh Kumar, (1) Madhumita Saroop, (2) Nisha Preschilla, (2) Amit Biswas, (2) Jayesh R. Bellare, (3) Anil K. Bhowmick (1)

(1) Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India

(2) Polymer Research & Technology Centre, Reliance Industries Ltd., Mumbai 400071, India

(3) Department of Chemical Engineering, Indian Institute of Technology, Mumbai 400076, India

Correspondence to: A.K. Bhowmick: e-mail: anilkb@rtc.iitkgp.ernet.in

DOI 10.1002/pen.21540
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Author:Ahmad, Zubair; Kumar, K. Dinesh; Saroop, Madhumita; Preschilla, Nisha; Biswas, Amit; Bellare, Jayesh
Publication:Polymer Engineering and Science
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Geographic Code:9INDI
Date:Feb 1, 2010
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