Morphology, Dynamic Mechanical, Thermal, and Crystallization

Mar 17, 2010 - School of Chemical Sciences, Mahatma Gandhi UniVersity, Kottayam, 686560 Kerala, ... The phase morphology of the blends, investigated...
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Ind. Eng. Chem. Res. 2010, 49, 3873–3882

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Morphology, Dynamic Mechanical, Thermal, and Crystallization Behaviors of Poly(trimethylene terephthalate)/Polycarbonate Blends Indose Aravind,*,† Alain Boumod,‡ Yves Grohens,‡ and Sabu Thomas† School of Chemical Sciences, Mahatma Gandhi UniVersity, Kottayam, 686560 Kerala, India, and UniVersite´ de Bretagne-Sud, 56100 Lorient, France

The phase morphology, dynamic mechanical, thermal, and crystallization behaviors of poly(trimethylene terephthalate) (PTT)/polycarbonate (PC) blends were studied. The phase morphology of the blends, investigated as a function of the composition, indicated a two-phase structure. Dynamic mechanical thermal analysis (DMTA) studies revealed that there is a shift in tan δmax toward each other especially in higher PTT contents. This is attributed to the small number of transesterification reactions most likely to occur under the reaction conditions that are more pronounced at higher PTT content. Thermogravimetric analysis revealed that PTT is more susceptible to thermal degradation than PC. The blends with a higher PC content showed a higher degradation temperature. However, the blends with a higher PTT content exhibited the lowest initial degradation temperature, which might be ascribed to its higher extent of transesterification reaction. The addition of PC to PTT increases the activation energy of the blends, which indicates an improvement in the thermal stability. The crystallization behavior of the blends was analyzed by differential scanning calorimetry (DSC) and wideangle X-ray diffraction (WAXD). The DSC results showed that the crystallization behavior of PTT/PC blends was very much affected by the PC content. The onset and peak crystallization temperatures shifted to lower temperatures and the area of the crystallization exotherm decreased with an increase in the PC content. This suggests that the crystallization process of PTT was suppressed by the presence of PC. WAXD analysis revealed that that the intensity of the crystalline diffraction peaks of PTT decreased with an increase in the PC content in the blends. The amount of transesterification reactions occurring between PTT and PC is quantified by Fourier transform IR techniques; it is found that the amount of copolymer formed as a result of transesterification reactions is small, and hence this merely enhances the compatibility between the components. 1. Introduction Blending is a convenient route to time-efficient and costeffective upgrading of commodity resins together with the tailoring of such resins to specific performance profiles for the desired applications. Indeed, an increasing number of commercial polymer products is derived from blending two or more polymers to achieve a favorable balance of physical properties. In the last few decades, the continuous need for materials of high technological properties has stimulated scientists to develop new classes of polymers with special properties that are able to serve in applications where the usual materials fail. One of the most exciting and rapidly expanding branches of materials science is in the area of polymers. This could be achieved either by the synthesis of novel polymers or by the preparation of blends and alloys, representing mechanical mixtures of polymers with different chemical compositions and physical structures. Polymer blends are industrially important materials because of the fact that the blend properties can be tuned according to need. The proper selection and combination of polymeric components in a precise ratio can result in a blend with optimal properties for a specific application. This strategy is cheaper and less time-consuming than the development of new monomers and or new polymerization routes.1 Generally the properties improved by blending are impact, heat distortion, and processability. Because most of the polymers are thermodynamically immiscible, polymer blends are heterogeneous systems and * To whom correspondence should be addressed. E-mail: [email protected]. † Mahatma Gandhi University. ‡ Universite´ de Bretagne-Sud.

represent a two-phase morphology. The phase morphology developed during processing depends on the process parameters, intrinsic properties, and interfacial properties of the component polymers. The properties of such blend systems can be further modified by use of a third component, widely known as a compatibilizer, which stabilizes the phase morphology and improves the interfacial adhesion. The physical, dynamic mechanical, and thermal properties of immiscible polymer blends depend not only on the constituent polymers but also on the morphology of the blends.2,3 The important studies on polyester blends include poly(ethylene terephthalate)/Nylon 6,6,4–6 poly(ethylene terephthalate)/cellulose,7 poly(ethylene terephthalate)/polypropylene,9 poly(ethylene terephthalate)/Bisphenol A,10 poly(ethylene terephthalate)/ poly(butylene terephthalate),11 poly(ethylene terephthalate)/highdensity polyethylene,12 poly(ethylene terephthalate)/ethylene propylene diene monomer,13 poly(trimethylene terephthalate)/ polyamide-12,14 poly(trimethylene terephthalate)/ethylene propylene diene monomer,,15–17 poly(trimethylene terephthalate)/ LLDPE,18 poly(butylene terephthalate)/polycarbonate,19,20 poly(trimethylene terephthalate)/polypropylene,21 poly(ethylene terephthalate)/Bisphenol A polycarbonate/(E/nBA/GMA),22 poly(ethylene terephthalate)/dibutyl succinate functionalized polyethylene,23 poly(trimethylene terephthalate)/poly(hexamethylene isophthalamide) blends,24 poly(trimethylene terephthalate)/ poly(ethylene naphthalate),25 and poly(ethylene-2,6-naphthalate)/ poly(pentylene terephthalate).26 More recently, the mixing time effect on the thermal properties and phase morphology of melt-mixed blends of poly(trimethylene terephthalate) (PTT) with polycarbonate (PC) was studied by Chiu and Ting,27 and it was found that, with

10.1021/ie901767y  2010 American Chemical Society Published on Web 03/17/2010

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increasing mixing time, the miscibility of the blends was enhanced, owing mostly to the occurrence of a transesterification reaction. The miscibility and phase behavior of solution-cast blends of PTT with bisphenol A polycarbonate were studied by Lee and Woo.28 The solution-cast PTT/PC blends are inherently immiscible, and after annealing at 260 °C, the blends could become miscible because of the transesterification reaction. According to Yavari et al.,29 PTT/PC blends are partially miscible, and after annealing at 300 °C for 10 min, the blends changed to a miscible state through a transesterification reaction. From these investigations, it can be concluded that transesterification plays an important role in controlling the properties of PTT/PC blends. Therefore, the effects of transesterification reactions on the various properties of PTT/PC blends are of paramount importance and should be investigated in detail. In this study, we used Sorona, a new generation polymer from DuPont, which is basically poly(trimethylene terephthalate), made by the condensation polymerization of corn-derived 1,3propanediol and terephthalic acid. The new polymer has exceptional properties like softness, stretch with recovery, resilience, stain resistance, easy dyeability for fibers, and high air impermeability. PTT is semicrystalline, but PC is amorphous. PC has several advantages like thermal stability, toughness, transparency, etc. PTT/PC blends are of commercial interest because of their potential combination of impact strength, modulus, heat and chemical resistance, and abrasion resistance. The main aim of this work is to investigate the morphology and dynamic mechanical, crystallization, and thermal properties of melt-blended samples of PTT with PC as a function of the blend composition and to quantify the amount of transesterification reactions that occur under the experimental conditions. 2. Experimental Section 2.1. Materials. Poly(trimethylene terephthalate) (PTT) was supplied by DuPont Co., USA. The polycarbonate (PC) used was a product from LG Dow Chemical Co., Korea, with a melt flow rate of 30 g per 10 min (ASTM D1238, 300 °C, 1.2 kg). Blends were prepared in Haake mixer at 260 °C for 5 min with a rotor speed of 60 rpm. Before melt mixing, PTT and PC were dried under vacuum at 105 °C for at least 16 h to minimize the possibility of hydrolysis during the mixing. 2.2. Melt Blending. The blends were prepared in an internal mixer with a rotor speed of 60 rpm; the total mixing time was fixed as 5 min. First PTT was melted at a temperature of 260 °C, and then PC was added after 2 min. Blending was continued for 3 min more. The blends having PTT/PC concentrations of 90/10 to 10/90 were prepared by melt blending. The blends were compression-molded into sheets at 260 °C with a pressure of 20 kg cm-2 for 2 min. The blends are designated as PTT90, PTT80, etc., to PTT10, where the subscripts indicate the weight percent of PTT in the blend; i.e., PTT90 means 90% PTT and 10% PC. All samples were melted at 260 °C for 3 min in a vacuum melting oven to eliminate any previous thermal history. We used the same specimens with the very same thermal history for different analyses. 2.3. Phase Morphology Measurements. The morphology of the blends was analyzed using a scanning electron microscope (Jeol 5400, Tokyo, Japan). The samples for the morphology measurements were prepared by cryogenically fracturing the samples in liquid nitrogen. The size of the dispersed phase was analyzed by an image analysis technique. About 300 particles were considered for the diameter measurements. The numberand weight-average diameters were determined using the following equations:

Number-average diameter Dn )

∑ND ∑N i

i

(1)

i

Weight-average diameter Dw )

∑ND ∑ND

2

i

i

i

i

(2)

2.4. Dynamic Mechanical Analysis (DMA). PTT/PC blend samples of dimensions of 30 mm × 5 mm × 0.5 mm were used for testing. The dynamic mechanical analyzer was DMA 2980 from TA Instruments. Measurements were performed from 30 to 180 °C at a heating rate of 1 °C min-1 in tension film mode with a deformation amplitude of 5 µm and a frequency of 1 Hz. 2.5. Thermogravimetric Analysis (TGA). The thermal degradation studies of the blends were done on a Mettler TG 50. The samples were scanned from 30 to 800 °C at a heating rate of 10 °C min-1. From the TGA curves, the thermal degradation characteristics such as the onset of degradation (Ton), temperature at the maximum rate of degradation (Tmax), and temperatures at different weight losses have been calculated. In addition, the Horowitz-Metzger (HM) equation was used for computing the activation energy (Ea) for degradation. 2.6. Differential Scanning Calorimetry (DSC). The crystallization and melting behaviors of the samples were studied using a Mettler Toledo DSC 822 differential scanning calorimeter using the Star software for data collection and treatment. Calibration was done with indium and tin in the temperature range (-120 to +350 °C). Aluminum pans with holes were used, and the samples weight was approximately 10 mg. All samples were first heated to 250 °C for 2 min to get rid of the thermal history. The heating and cooling rates were 10 °C min under a nitrogen atmosphere. All of the temperatures measured at the peak maximum (Tc and Tm) are determined with an accuracy of less than (0.5 °C. Melting enthalpies were determined using constant integration limits. 2.7. Wide-Angle X-ray Diffraction (WAXD) Measurements. The WAXD instrument was a Bruker AXS Karlsruhe with Cu KR radiation (40 kV and 30 mA) and a wavelength of 1.542 Å. The scanning angle ranged from 2θ ) 3° to 41°, with a step scanning of 2° for 1 min. 3. Results and Discussion 3.1. Phase Morphology Analysis. The morphology development and stability of multiphase polymer melts is a complex function of the blend composition, interfacial characteristics, rheological properties, and shear conditions. The scanning electron microscopy (SEM) pictures showing the morphology of the PTT/PC blends are presented in Figure 1. It is clear from these pictures that, as the PC content in the PTT matrix increases, the particle size increases and beyond a certain limit of composition (30 wt %) both PTT and PC form a cocontinuous phase structure and, at 60 wt %, phase inversion occurs, where PC forms the matrix and PTT is the dispersed phase. The dispersed particle size (Dn and Dw) is presented in Figure 2. The difference in the particle size of dispersed PC and PTT phases for a given dispersed-phase concentration (e.g., PTT90 and PTT10) can be explained by considering the relative difference in their viscosities in the blend (see Figure 3). It should be noted that the less viscous component (PTT) forms finely dispersed particles in more viscous matrix (PC) because

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Figure 3. Complex viscosities of PTT and PC as a function of the frequency at 260 °C.

Figure 1. SEMs of PTT/PC blends.

Figure 4. Effect of the blend composition on the cocontinuity.

Figure 2. Effect of the PC concentration on Dn and Dw of PTT/PC blends.

of comparatively restricted diffusion effects on the coalescence of particles and increased shear stress resulting from the more viscous matrix phase. The fundamental reasons responsible for the unstable morphology are the unfavorable interactions at the interface between the components, which create a high interfacial energy and low interfacial thickness and which would, in turn, lead to poor interfacial adhesion between the phases that may result in premature failure of the interface upon stress transfer. Another aspect that deserves attention is the coalescence of the dispersed phase, which makes the dispersed-phase particles larger and nonuniform, leading to an unstable morphology. Here the morphological parameters showed that all blends are associated with two-phase unstable morphology owing to

the high interfacial tension and greater coalescence effects in the absence of favorable interactions at the interface between the phases. As the concentration of one phase in the blends increases, the incompatibility intensifies. The continuity of the dispersed phase is calculated by the solvent dissolution method. When PTT forms the matrix, the minor phase PC was extracted using a dichloromethane solvent. The continuity of the component is defined as the ratio of the difference of the weight of the component present initially and the calculated weight of the residual component after extraction to the weight of the component present initially. continuity of A (%) ) initial weight of the component - weight after extraction weight fraction of component A × initial weight of the component

(3) The results are summarized in Figure 4. When the percentage continuity of both of the components equals 100%, the morphology of the blend is considered to be cocontinuous. From Figure 4, it is evident that the continuity of the PC phase is close to 90% in PTT40 and above 90% in PTT50 blends. This suggests that PTT40 and PTT50 exhibit cocontinuous morphol-

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Figure 5. Variation of the storage modulus of PTT, PC, and their blends as a function of the temperature.

Figure 6. Effect of the blend ratio on the variation of the loss modulus as a function of the temperature.

ogy. For the other three blend compositions (PTT90, PTT80, and PTT70), the continuity is less than 65%, suggesting matrix/ droplet morphology. The sample after extraction did not break down (disintegrate) between 0 and 50 wt % of PC, and this indicates that the PTT phase is continuous in that range. 3.2. DMA. DMA has become a classical method for the determination of the blend miscibility because the height and position of the mechanical damping peaks are remarkably affected by the miscibility, intermolecular interaction, interface feature, and morphology. The dynamic mechanical properties of the blends are also affected by the composition, with particular emphasis on the amount of the minor phase composition. The dynamic mechanical properties such as the storage modulus (E′), loss modulus (E′′), and damping (tan δ) of the PTT/PC blends were evaluated from 30 to 180 °C and are presented in Figures 5-7. Each blend showed two separate glass transition relaxations corresponding to a PTTrich phase and a PC-rich phase, respectively.30,31 Figure 5 shows the variation of the storage modulus as a function of the temperature for PTT and PC homoploymers and their blends. PC has a higher value of the storage modulus than PTT in all temperature ranges except at higher temperature (above the Tg of PC), and the blends have values in between. As in the case of blend components, the storage modulus of

Figure 7. Effect of the blend ratio on the variation of tan δ as a function of the temperature.

the PTT/PC blends also decreases with an increase in the temperature. PTT shows a very sharp drop in the storage modulus in the temperature range from 45 to 85 °C, and PC shows a sharp drop from 145 to 169 °C, as shown in the figure. For the blends, a sharp drop in E′ is observed when the temperature is increased from 50 to 90 °C due to the presence of PTT, followed by another sharp drop in the storage modulus in the temperature range 140-164 °C due to the PC content. Because PC is an amorphous polymer, it tends to decrease the crystallinity of the blend system because of small interactions with PTT. Figure 6 shows the variation of the loss modulus as a function of the temperature for various blend compositions. These curves show two maxima corresponding to the glass transitions temperatures of PTT and PC. Figure 7 shows the variation of tan δ as a function of the temperature for the PTT/PC blends. The tan δ curve of PTT shows a peak at ∼70 °C due to the R transition arising from the onset of segmental motion. This corresponds to the glass transition temperature of PTT. PC shows a tan δ peak at ∼164 °C, which corresponds to its glass transition temperature. The reports say that generally, for an incompatible blend, the tan δ vs temperature curve shows the presence of two tan δ or damping peaks corresponding to the glass transition temperatures of the component polymers.32–35 For a highly compatible blend, the curve shows only a single peak between the transition temperatures of the component polymers,32 whereas broadening of the transition peak occurs in the case of a partially compatible system.35 In the case of compatible and partially compatible blends, the Tg’s are shifted to higher or lower temperatures as a function of the composition. The PTT/PC blends show two transition peaks corresponding to the glass transition temperatures of PTT and PC. When PC is added to PTT, there is a slight shifting of tan δmax of PTT and PC toward each other because of transesterification reactions taking place in the system under the experimental conditions including melt blending, compression molding, heat processing, DMA, etc. This shift is more pronounced in PTT90 and PTT80 blends, where the polyester (PTT) content is high and thereby the transesterification reaction rate is high. Starting from PTT with the components propylene (P) and terephthalate (T) in a (PT)n chain and PC with the components bisphenol A (B) and carbonate (C) in a (BC)m chain, the exchange reactions will generate a four-component polycondensate containing the components in a polymer

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Scheme 1. Components in the Exchange Reaction between PTT and PC

We have done extensive research on the transesterification phenomenon of PTT/PC annealed blends that was reported earlier.36 We noticed that a considerable amount of transesterification occurred in PTT/PC blends only after the sample was annealed for more than 1 h. The blends annealed for 2 h and more exhibited a homogeneous morphology. When annealing is extended, the original semicrystalline morphology becomes completely amorphous. Figure 9 compares the result obtained from a PTT80 blend annealed for 3 h with that of the samples used in DMA in the present study and the unannealed blend (pure blend). From Figures 8 and 9, we can see that the number of transesterification reactions occurring in the present case is small compared to that of the annealed blends. The presence of a new band at 1069 cm-1 indicates the occurrence of a transesterification reaction under the present reaction conditions compared to neat blends. A random copolymer formed as a result of a transesterification reaction between PTT and PC serves as a compatibilizer at the beginning stage. 3.2.1. Determination of the Apparent Weight Fractions of Components. In the DMA of PTT/PC blends, two glass transition regions are observed and also there is a shift in the

Scheme 2. Expected Chemical Structures of the Transesterification Products of PTT and PC

Table 1. Variation of the Temperature Corresponding to tan δmax of PTT and PC with the Blend Composition blend PTT PTT90 PTT80 PTT50 PTT40 PTT20 PC

temp corresponding to tan δmax of PTT (°C) 73 79 80 79 79 79

temp corresponding to tan δmax of PC (°C) 149 150 153 156 158 164

Figure 8. FT-IR used to quantify the number of transesterification reactions in PTT80 blends.

structure with a certain degree of randomness (Scheme 1). Also, Scheme 2 shows the expected chemical structures of the transesterification products of PTT and PC. The variation of the temperature corresponding to tan δmax of PTT and PC with the blend composition is shown in Table 1. The mid-IR spectra of PTT80 pure blends and the sample used for DMA after compression molding and heat processing are shown in Figure 8. The new small band present at 1069 cm-1, for the PTT80 blend used for DMA, beside the PTT and PC bands indicates the formation of the fully aromatic ester structure of the transesterification product, i.e., COO linked to two phenyl groups on each side as shown below (see also PTB and BTB in Scheme 2). This small band arises as a result of the C-O-C stretching vibration.

Figure 9. FT-IR used to compare the number of transesterification reactions in PTT80 blends with that of completely amorphous blends annealed for 3 h.

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Table 2. Apparent Weight Fractions (w) of PTT and PC in the PTT- and PC-Rich Phases PTT-rich phase blends PTT/PC 1 0.9 0.8 0.5 0.4 0.2 0

Tg1 (°C) 64 68 70.5 73 77 79

Tg2 (°C) 145 146 147 148 150 158

PC-rich phase

w1′

w2′

w1′′

w2′′

1.0000 0.9575 0.9308 0.9043 0.8617 0.8404

0.0425 0.0692 0.0957 0.1382 0.1596

0.1383 0.1277 0.1170 0.1064 0.0851

0.8617 0.8723 0.8830 0.8936 0.9149 1.0000

Tg’s of the individual components with the blend composition. Therefore, from these two glass transition temperatures of PTT and PC in different blends, we can estimate the apparent weight fractions of PTT and PC, which are dissolved in the PC- and PTT-rich phases, respectively. The apparent weight fractions of PC in the PTT- and PC-rich phases were determined by the following empirical equation (eq 4), which is often used to describe the dependence of Tg on the composition in random copolymers and plasticized systems.37,38 Tg ) w1Tg1 + w2Tg2

(4)

where Tg is the observed Tg of the copolymer, w1 is the weight fraction of the homopolymer 1 having Tg1, and w2 is the weight fraction of homopolymer 2 having Tg2. Equation 4 may be rearranged as w1′ )

Tg1b - Tg2 Tg1 - Tg2

(5)

where w1′ is the apparent weight fraction of polymer 1 in the polymer 1 rich phase, Tg1b is the observed Tg of polymer 1 in the blends, and Tg1 and Tg2 are the Tg’s of homopolymers 1 and 2, respectively.37 Applying the DMA results of Tg(PTT) and Tg(PC) in PTT/ PC blends, we have calculated the apparent weight fractions of PTT and PC in the PTT- and PC-rich phases, which are shown in Table 2. It is evident from the table that the apparent weight fractions of dissolved PTT in the PTT- and PC-rich phases decreased with an increase in the PC content. Similarly, the apparent weight fractions of PC in PC- and PTT-rich phases also decreased with an increase in the PTT content. This shift in Tg values and the corresponding decrease in the apparent weight fractions are attributed to the transesterification-reactioninduced miscibility of PTT and PC under the experimental conditions. Also, the apparent weight fractions calculated are very useful in the determination of the Flory-Huggins interaction parameter of the blend systems. 3.3. TGA Measurements. One of the most accepted methods for studying the thermal properties of polymeric materials is thermogravimetry. The integral (TGA) and derivative (DTG) thermogravimetric curves provide information about the nature and extent of degradation of the polymeric materials. The effect of the blend ratio on the thermograms (TGA) and derivative thermograms (DTG) of PTT/PC blends is given in Figure 10. A detailed evaluation of the thermograms is presented in Tables 3 and 4. Table 3 gives an idea about the effect of the blend ratio on the temperatures corresponding to different weight losses (viz., Ton ) onset of degradation, T30 ) temperature corresponding to 30 wt % degradation, and so on). It is seen from the table that PTT is more susceptible to degradation whereas PC shows maximum thermal stability. It should be noted that, in the PTT80

Figure 10. Effect of the blend ratio on the thermograms of PTT/PC blends: (a) TGA; (b) DTG. Table 3. Effect of the Blend Ratio on the Temperatures Corresponding to Different Percentage Weight Losses in PTT/PC Blends blend

Ton (°C)

T30 (°C)

T40 (°C)

T50 (°C)

PTT PTT90 PTT80 PTT50 PTT20 PC

330 312 308 320 338 432

377 386 386 385 412 496

383 392 392 395 439 502

387 397 397 409 467 508

Table 4. Effect of the Blend Ratio on the Tmax’s of PTT and PC in PTT/PC Blends blend

Tmax (°C)

PTT PTT90 PTT80 PTT70 PTT50 PTT30 PTT20 PC

391 397, 479 392, 472 391, 476 393, 475 387, 476 388, 487 507

blend, PTT is the matrix and, in the PTT50 blend, both PTT and PC form continuous phases. Thus, in PTT80 and PTT50 blends, the PTT phases are more susceptible to thermal degradation. On the other hand, in PTT20 blends, PC forms the

Ind. Eng. Chem. Res., Vol. 49, No. 8, 2010 Table 5. Effect of the Blend Ratio on the Activation Energy of PTT and PC (Calculated by the HM Method) in PTT/PC Blends blend

Ea (kJ mol-1)

PTT PTT90 PTT70 PTT50 PTT20 PC

148.3 134.2, 245.3 145.5, 263. 2 139.5, 256. 2 155.5, 270. 2 190.1

ln[ln(1 - R)-1] ) Ea[Eaθ/RTmax2]

Table 6. Crystallization and Melting Behaviors of PTT/PC Blends blend Tc (°C) Tm (°C) ∆Hc,n (J g-1) ∆Hf,n (J g-1) crystallinity (%) PTT PTT90 PTT80 PTT70 PTT50 PTT30 PTT20

matrix whereas PTT is the dispersed phase. As a result, the thermal degradation of PTT is suppressed because the PC matrix offers protection to the dispersed PTT domains. Table 4 displays the effect of the blend ratio on the Tmax’s of the blends. The Tmax’s of PTT and PC were found to be 391 and 507 °C, respectively. As the amount of PC is increased from 0 to 50 wt %, the Tmax value of PTT increases marginally, and above that, it decreases slightly. It is clear from the tables that, as the amount of PC in the blend is increased, the thermal stability increases, and the thermal stability of the blends depends on the phase morphology and the extent of improvement in the thermal stability depends on the type of morphology. As revealed from TGA, the blend with a higher PC content showed a higher degradation temperature. Further, it is worth noticing that the blends with higher PTT content exhibit the lowest initial degradation temperature among the blends examined, which might be ascribed to its higher extent of transesterification reaction. Activation Energy for Thermal Decomposition. The activation energy (Ea) for the decomposition of PTT and PC in PTT/PC blends was measured using the HM method.39 In HM, the activation energy (Ea) was calculated using the equation

(6)

where R is the decomposed fraction and is given as R ) Ci C/Ci - Cf, where C is the weight at the temperature chosen, Ci the weight at the initial temperature, Cf the weight at the final temperature, Tmax the temperature at the maximum rate of weight loss, and R the universal gas constant and θ is given by T Tmax. Kinetic plots were made with ln[ln(1 - R)-1] versus θ. From the slope of the plots, Ea was calculated. The effect of the blend ratio on the Ea values of PTT and PC is listed in Table 5. Ea’s of PTT and PC were found to be 148.3 and 190.1 kJ mol-1, respectively. It is seen that the addition of PC into PTT increases the activation energy of the blends. The blends are two-phase heterogeneous systems; we obtained two Ea values for each blend because of the degradation peaks of PTT and PC. The increase in Ea indicates that more energy is required for the major degradation step, which, in turn, implies an improvement in the thermal stability of the blends. Thus, it can be concluded that, as the amount of PC in the blend is increased, the thermal stability also increases. 3.4. DSC. The crystallization behaviors of PTT in PTT/PC blends are investigated. When a polymer crystallizes in immiscible matrixes such as in a polymer blend, various crystallization behaviors are possible depending on the component polymers, their compositions, the interfacial adhesion, the processing parameters, etc. Here PTT is semicrystalline while PC is an amorphous polymer. The effect of the blend ratio on the melting and crystallization parameters of PTT in PTT/PC blends is depicted in Table 6. The crystallization temperature (Tc) and melting temperature (Tm) of PTT are 170 and 228 °C, respectively. Figure 11 shows the DSC cooling scans of PTT/

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170 158 142 141

228 220 216 209 215 223 227

52.8 49.7 42.9 4.7

53.9 48.9 43.1 36.5 33.1 25.3 21.9

37.02 33.6 29.6 25.1 22.7 17.3 15.1

PC blends at 10 °C min-1. When PTT/PC blends were cooled from the melt, the crystallization exotherms of PTT were observed at ∼170-140 °C because of the PTT phase crystallizations. Although the two polymers are immiscible, the presence of one component appeared to influence the onset and peak crystallizations of the other component depending on the blend compositions. Changes in the Tc value with blend compositions showed that crystallizations in the PTT phase were affected by the presence of the other component, implying that there is some interaction between the components that affect the crystallization process.40 The DSC results showed that the crystallization behaviors of PTT/PC blends were severely affected by the PC content. With an increase in the PC content in the blend, the onset (Tci) and peak (Tc) crystallization temperatures shifted to lower temperatures. The area of the exotherm also decreased quickly with an increase in the PC content. These behaviors indicate that the presence of PC suppresses the crystallization process of PTT. The crystallization temperature (Tc) of the PTT phase is shifted from 170 to 141 °C upon the addition of 30 wt % PC and above which the crystallization peaks disappeared.41 The DSC cooling scans of the blends with greater than 50 wt % PTT content showed glass transitions of the PTT-rich phase at ∼56-75 °C (shown marked by arrows), in addition to crystallization exotherms. With an increase in the PC content in the blend, this is shifted to higher temperatures. However, when the amount of PC was above 70 wt %, no crystallization exotherms were seen, but the glass transitions of the PC-rich phase (shown marked by arrows) were exhibited. This is shifted to lower temperatures as the PTT content is increased. Therefore, from the variations of the two glass transitions of PTT and PC with the blend composition, we can conclude that the miscibility of PTT/PC blends can be correlated with the blend composition. The crystallization exotherms became very broad and indistinct when the PC content was greater than 20 wt %. These broad

Figure 11. DSC cooling scans of PTT/PC blends.

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% crystallinity ) (∆Hf /∆Hf°) × 100

Figure 12. DSC heating scans (second heating) of PTT/PC blends.

crystallization exotherms appeared to end at the glass transition temperatures of the PTT-rich phase for blends with 20-50 wt % PC content. This was due to the fact that PC severely restrained the mobility of the PTT molecules or segments, which led to much longer and more varied relaxation times. As a result, the PTT crystallization process took place over a wider temperature range. The PTT chain segments were frozen instantaneously at various crystallization stages as the temperature was decreased to the glass transition temperature. Finally, the crystallization of the PTT-rich phase was completely restricted above 70 wt % of PC. Figure 12 shows the DSC heating scans of PTT/PC blends at 10 °C min-1. The melting temperature (Tm) of PTT is also shifted from 228 to 209 °C as the PC content is increased to 30 wt %. Above 30 wt %, the Tm gradually increases. At higher temperatures of lower than Tm, the imperfect crystallites would experience recrystallization or reorganization processes to give rise to the final crystallites with similar perfection, and as a result, the peak melting temperatures exhibit little change with the blend composition. However, here in the blends with higher PTT content, transesterification reactions would occur, and as a result, Tm decreases up to 70 wt % of PTT. At high PC content (>30 wt %), the transesterification reaction rate is low, and hence an increase in Tm will occur. There is also a shift in the Tg value of PTT to higher temperature with an increase in the PC content, as is also clear from the heating curves. The melting endotherms also decreased with an increase in the PC content. The Tg value of PC is shifted to lower temperatures with an increase in the PTT content. Furthermore, a well-defined exotherm associated with PTT cold crystallization is identified at ∼200 °C, which is due to the reorganization (recrystallization) phenomenon of PTT chains.42 These observations also indicated that a small amount of transesterification reactions occurred between PTT and PC under the experimental conditions of DSC. The percentage crystallinity of PTT in the blend is obtained from the expression

(7)

where ∆Hf is the enthalpy of fusion obtained calorimetrically and ∆Hf° is the enthalpy of fusion of the 100% crystalline PTT. The values calculated are presented in Table 6. It can be seen that the percentage crystallinity of PTT is decreased with an increase in the PC content, which indicates that PC severely restricts crystallization of PTT in the blend. 3.5. WAXD. In WAXD measurements, PTT/PC blends of different composition were analyzed with a scanning angle ranging from 2θ ) 3 to 41°, with a step scanning of 2° for 1 min. Figure 13 shows the WAXD patterns of PTT/PC blends. The characteristic X-ray peaks for PTT were observed at the scattering angles 2θ of about 15.8°, 17.5°, 20.1°, 22.1°, 24.1, 25.2°, and 27.4°, corresponding to the reflection planes of (0 1 0), (0 1j 2), (0 1 2), (1 0 2j), (1 0 2), (1 1j 3), and (1 0 4j), respectively, indicating that PTT has a triclinic crystalline structure.43 However, PC gives only an amorphous halo in the WAXD spectrum, indicating that it is amorphous in nature. It can be seen that the intensity of the crystalline diffraction peaks of PTT is decreased with an increase in the PC content in the blends. The crystallinity values of the PTT/PC blends are shown in Table 7. It is evident from the table that the crystallinity of blends decreases with an increase in the PC content. However, the crystalline structure of PTT is unaffected by the second component in the blend. 4. Conclusions The phase morphology analysis of the PTT/PC blends indicated heterogeneous phase structures. The solvent dissolution method also supported the morphological observations from SEM. DMTA studies revealed that the storage modulus of the blends increased with an increase in the PC content and blends showed two glass transitions corresponding to the component phases, showing the incompatible nature of the blend. The tan δ curve indicated that there is a shift of the tan δ maxima toward each other at higher PTT contents. This is attributed to the fact that, at higher PTT content, the number of transesterification reactions that occurred under the experimental conditions was more. We have quantified the amount of transesterification reactions occurring between PTT and PC by a FT-IR technique, which revealed that the amount of copolymer formed as a result of transesterification reactions is small and hence it merely enhances the compatibility between the components. TGA revealed that PTT is more susceptible to thermal degradation whereas PC shows maximum thermal stability. As the amount of PC in PTT increases, the thermal stability of the blends increases, which shows that phase morphology has a definite role in determining the thermal stability. The blends with higher PTT content exhibit the lowest initial degradation temperature among the blends examined, which is due to its higher extent of transesterification reaction. It is seen that the addition of PC to PTT increases the activation energy (Ea) of the blends, which, in turn, indicates an improvement in the thermal stability of the blends. The DSC results showed that the crystallization behaviors of PTT/PC blends were very sensitive to the PC content. The area of the crystallization exotherm decreased with an increase

Table 7. Crystallinity Index Values of PTT/PC Blends Evaluated Using WAXD sample

PTT

PTT/PC 90/10

PTT/PC 70/30

PTT/PC 50/50

PTT/PC 40/60

PTT/PC 20/80

PC

crystallinity index (Rx)

0.630

0.513

0.300

0.192

0.117

0. 041

0

Ind. Eng. Chem. Res., Vol. 49, No. 8, 2010

Figure 13. WAXD patterns of PTT/PC blends.

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ReceiVed for reView November 8, 2009 ReVised manuscript receiVed February 22, 2010 Accepted February 26, 2010 IE901767Y