Influence of Copolymer Configuration on the Phase Behavior of

UniVersity of Massachusetts, Amherst, Massachusetts 01003. Charles W. Paul. National Starch and Chemical, Bridgewater, New Jersey 08807. ReceiVed: ...
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J. Phys. Chem. B 2006, 110, 2541-2548

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Influence of Copolymer Configuration on the Phase Behavior of Ternary Blends Young Gyu Jeong, Suriyakala Ramalingam, Jared Archer, and Shaw Ling Hsu* Polymer Science and Engineering Department and Material Research Science and Engineering Center, UniVersity of Massachusetts, Amherst, Massachusetts 01003

Charles W. Paul National Starch and Chemical, Bridgewater, New Jersey 08807 ReceiVed: October 21, 2005; In Final Form: December 7, 2005

The influence of copolymer configuration on the phase behavior of various ternary polymer blends containing a crystallizable polyester, a noncrystallizable polyether, and an acrylic random copolymer of different chain configuration was investigated. In these ternary blends, the acrylic random copolymer is typically added to control rheological properties at elevated temperatures. In fact, the acrylic random copolymers composed of various compositions of MMA and nBMA were found to have different miscibility with polyester as well as polyether, leading to substantially different phase behavior of ternary blends. Remarkable temperature dependence was also found. The mean-field Flory-Huggins theory for the free energy of mixing, extended to ternary polymer blends, was adopted for predicting phase diagrams where the exact spinodal and binodal boundaries could be calculated. Phase diagrams of ternary blends, predicted by the Flory-Huggins formulations and related calculations, were in good agreement with experimental phase diagrams. The differences observed in the rheological processes of various ternary blends with different acrylic copolymers were directly related to changes in miscibility, associated phase behavior, and chain configuration.

Introduction Polymer blend studies are important from both fundamental and practical perspectives.1-6 Since their macroscopic properties can be altered in a systematic fashion, these blends are utilized in a large array of applications. Normally the properties of interest such as mechanical modulus, energy absorption, diffusion barrier, adhesion, interfacial bonding, and surface texture, are directly correlated to the static or dynamic morphological features. The morphological features can be controlled by the degree of crystallinity or crystallite size in some blends and also by size, composition, and interconnectivity of phaseseparated structures depending on the blends used. In each case, the formation of these morphological features is dictated by mixing, phase separation, and crystallization processes, separately or collectively.7-10 Numerous studies, both theoretical and experimental, have been carried out on the thermodynamics of phase behavior, kinetics of phase separation, and resultant morphological features of various polymer mixtures.1-6 Miscibility behavior and morphological features associated with ternary blends are generally complex and have been mostly experimental in nature with a limited number of works on prediction of phase behavior.11-19 The fundamental aspects of the ternary polymer blends, which have substantial commercial significance as adhesives and coating materials, are seldom understood.18,20-23 These blends are usually composed of a crystalline polyester, a noncrystallizable polyether, and a high Tg acrylic random copolymer. It has been shown that the miscibility, associated phase behavior, and morphology development of these ternary blends all are quite sensitive to the components, composition, molecular * Author to whom correspondence should be addressed. Phone: 413577-1125. Fax: 413-545-0082. E-mail: [email protected].

weight, and functionality.18,20,21 Depending on the phase behavior and thermal history of the blend, a number of morphological developments and properties can be varied in terms of phase-separated domain size, crystallization kinetics, degree of crystallinity, and crystallite size.22 These morphological factors in turn affect the mechanical and thermal properties as well as the reaction kinetics in functionalized reactive ternary blends.23 Each component of polymer blends brings specific characteristics to enhance blend properties. In ternary blend systems which we previously studied, crystallizable polyesters such as poly(hexamethylene adipate) (PHMA) and poly(hexamethylene sebacate) (PHMS) improve mechanical properties through crystallization. The noncrystallizable poly(propylene glycol) (PPG) provides elasticity. The third component, an acrylic random copolymer, poly(methyl methacrylate-co-n-butyl methacrylate) [P(MMA-co-nBMA)], is known to possess a high glass transition temperature. Thus its role is associated with viscosity control and mechanical strength at elevated temperatures.18,22 Indeed, the addition of this acrylate component yields a fascinating complex multistep solidification process.24 It should also be noted that the polyesters in these blends crystallize. It is obvious that this crystallization process depends on the phase behavior of each component. It has been established that, with small changes in polyester structure, very different phase behavior and crystallization kinetics are observed.18 The acrylic component is important as it controls local chain dynamics as well as formation of crystallites.18,22 In the present study, we investigate miscibility and associated phase behavior of PHMA/PPG/P(MMA-co-nBMA) ternary blends containing acrylic random copolymers with different compositions of MMA and nBMA. The two types of acrylates studied are known to have different glass transition temperatures which can be attributed to the different free volumes associated

10.1021/jp0560359 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/21/2006

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Figure 1. Chemical structures of blend components: (A) PHMA; (B) PPG; and (C) P(MMA-co-nBMA).

with the two types of monomers. In addition, these two acrylates yielded significant differences in phase diagrams. For a ternary blend system, the triangular phase diagram changed systematically into the concave upward “moon” shape phase diagram with the growth of an island as temperature was lowered. The origin for such differences has not yet been established. Undoubtedly these changes in ternary blends can be accounted for by changes in the binary mixtures. The central focus of this study is to show how these changes occur. Using the FloryHuggins theory to calculate free energy of mixing of the three binary mixtures, we were able to deduce the ternary phase diagrams.18 In this case, changes in binary interactions must be related to the specific interactions of each MMA and nBMA segment with either the polyester or polyether. We could not find direct evidence characteristic of these specific interactions. Instead, we found that solubility parameters associated with the two types of monomers are consistent with the phase diagrams measured. The rheological properties of ternary blends containing different acrylic copolymers were characterized in terms of miscibility, associated phase behavior, and chain dynamics, which is crucial in controlling physical properties such as film forming behavior. Our results are reported here. Experimental Section Materials. Consistent with the previous study,18 PHMA, PPG, and P(MMA-co-nBMA) were adopted as components of ternary polymer blends. Their chemical structures are shown in Figure 1 and their chemical and physical properties are summarized in Table 1. Characterizing methods for molecular weight, molecular weight distribution, density, glass transition, and melting temperatures have been described in previous publications.18 Two different P(MMA-co-nBMA)s composed of different ratios of MMA and nBMA were used. The composition of P(MMA-co-nBMA)s was analyzed with 1H NMR spectroscopy. The relative intensity of the resonances of methyl and methylene groups attached to the carboxyl groups at 3.5 and 3.9 ppm was used to determine the molar ratio of MMA/nBMA in the acrylate copolymers. As a result, the copolymer compositions of acrylates were determined to be MMA/nBMA ) 48/ 52 and 75/25 by mol %, respectively. P(MMA-co-nBMA) copolymers with 48/52 and 75/25 of MMA/nBMA are referred to as Acrylate I and II, respectively. Preparation and Characterization of Blends. Binary and ternary blends with various compositions were prepared by melt-

Jeong et al. mixing at 120 °C. These blends were prepared in glass vials and placed in a temperature-controlled oven purged with nitrogen to minimize oxidative degradation. The blends were mixed periodically and allowed to equilibrate until the phase of the blends remained unchanged. The miscibility and phase behavior of blends were also examined by lowering temperatures from 120 to 5 °C. An optical microscope (Olympus Vanox) equipped with a digital camera and custom-designed temperature controller was used to determine the miscibility and phase behavior of binary and ternary blends at a given temperature. For the fully phase-separated binary blends, the composition of each phase was analyzed with 1H NMR and/or Fourier transform infrared (FT-IR) spectroscopy.18 1H NMR spectra of the samples in CDCl3 solution were recorded on a Bruker DPX300 spectrometer. FT-IR spectra were obtained with use of a Perkin-Elmer 2000 Fourier transform infrared spectrometer fitted with a wide-band MCT detector in the transmission mode at 4 cm-1 resolution. The binary blends phase separated at a constant temperature were quenched in liquid nitrogen to minimize the change in composition with lowering temperature. Each phase was then collected for compositional analysis. For the phase-separated PHMA/Acrylate binary blends, intensity of resonances between 0.7 and 1.1 ppm in 1H NMR spectra was used for calculating the Acrylate concentration and intensity of resonance at 2.35 ppm for the PHMA concentration. For the phase-separated PPG/Acrylate and PHMA/PPG binary blends, the composition of Acrylate and PHMA in each phase could be calculated by comparison of the intensity of the CdO stretching vibration (∼1735 cm-1) with that of methyl stretching (∼2950 cm-1), since the CdO stretching vibration in FT-IR spectra is the characteristic peak of Acrylate and PHMA. The experimentally calculated composition of each phase in phaseseparated binary blends was used for estimating binary interaction parameters of binary blends, which will be discussed in the following section. The viscosity of ternary blends was measured under smallamplitude oscillatory shear, using the cone and plate geometry (0.1 rad in cone angle, 25 mm in diameter, and 50 µm in gap) of the Advanced Rheometric Expansion System (ARES, Rheometric Scientific Inc.) operated by RSI Orchestrator (version 6.3.0) software. The frequency and applied strain were controlled at 1 rad/s and 1%, respectively. The blends were first melted at 120 °C for 3 min and then cooled to various crystallization temperatures from 35 to 45 °C. The viscosity change was monitored as a function of time at a given temperature. Results and Discussion Experimental phase diagrams of ternary blends composed of PHMA, PPG, and Acrylate I or Acrylate II at 120 °C are shown in Figure 2, where open, opaque, and solid symbols indicate clear one phase, turbid but phase separated, and fully phase separated in two phases, respectively. Comparison of the phase diagrams of ternary blends with different acrylic copolymers at 120 °C revealed that different acrylic random copolymers with different MMA/nBMA compositions substantially affect miscibility and phase behavior of ternary blends. As expected, this difference in phase diagrams of ternary blends stems from the fact that both Acrylates I and II have different miscibility with PPG and PHMA. PPG/Acrylate I binary blends were observed to be miscible at 120 °C for all blend composition, whereas PPG/Acrylate II blends were immiscible for a certain range of blend composition (70/30 to 80/20 PPG/Acrylate). The miscibility of PHMA/Acrylate I at 120 °C was also quite different from that of the PHMA/Acrylate II. PHMA/Acrylate

Phase Behavior of Ternary Blends

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TABLE 1: Characteristics of Raw Components in Binary and Ternary Blends blend component

Mw (g/mol)

Mw/Mn

PHMA PPG Acrylate Ia Acrylate IIb

2505 1906 35130 30900

1.59 1.01 1.75 1.64

a

Fsolid (g/cm3)

Fliquid (g/cm3)

Tm (°C)

Tg (°C)

Mrepeat (g/mol)

Nrepeat

Vrepeat (cm3/mol)

n

1.158

1.051 1.003

54.5

-61.0 -66.0 65.0 85.3

228 58 122 110.5

11 33 288 280

216.2 57.4 111.2 95.9

41 33 554 468

1.097 1.150

MMA/nBMA ) 48/52 by mol %. b MMA/nBMA ) 75/25 by mol %.

where R, T, ni, φi, and χij are the gas constant, temperature, degree of polymerization of component i, volume fraction of component i, and binary interaction parameter between components i and j, respectively. The first terms in parentheses of eqs 1 and 2 indicate the entropic contribution to mixing, and terms containing χij denote the enthalpic contribution. The binary interaction parameter, χij, in theory is considered to be inversely proportional to temperature and independent of composition and pressure.25,26 This theory is also assumed to be under the uncompressibility condition of φ1 + φ2 + φ3 ) 1. For all components, the degree of polymerization, n, which was normalized with the repeating unit volume of PPG, is listed in Table 1. Using eqs 1 and 2, we can predict the free energy of mixing for binary and ternary blends by determining the χij. To determine the binary interaction parameters χij’s, we utilized the fact that most binary blends are phase separated.18 For phase-separated blends in an equilibrium state, the partitioning of mass between the two coexisting phases allows the chemical potential to be used to determine the binary interaction parameter. Taking the first derivatives of eq 1 with respect to composition, the chemical potentials, ∆µ1I and ∆µ1II, of component 1 in coexisting phases I and II of the phase-separated blends can be obtained respectively as

Figure 2. Experimental phase diagrams of ternary blends at 120 °C: (A) PHMA/PPG/Acrylate I and (B) PHMA/PPG/Acrylate II.

I binary blends display phase-separated phases at a much wider range of blend composition in comparison to PHMA/Acrylate II blends. It is clear that different interactions in three different binary blends (PPG/Acrylate, PHMA/Acrylate, and PHMA/ PPG) are crucial factors affecting miscibility and associated phase behavior. To understand the phase behavior of PHMA/PPG/Acrylate ternary blends with different Acrylates, their phase diagrams were obtained based on the Flory-Huggins theory, extended to ternary polymer blends. The Flory-Huggins free energy of mixing, ∆Gmix, for binary and ternary blends is expressed by11,14,15,25,26

(

) )

∆Gmix φ1 lnφ1 φ2 lnφ2 ) + + χ12φ1φ2 RT n1 n2

(

(1)

φ1 lnφ1 φ2 lnφ2 φ3 lnφ3 ∆Gmix + + + χ12φ1φ2 + ) RT n1 n2 n3 χ13φ1φ3 + χ23φ2φ3 (2)

∆µI1 n1 ) lnφI1 - φI1 - φI2 + n1χ12(φI2)2 + 1 RT n2

(3)

n1 ∆µII1 ) lnφII1 - φII1 - φII2 + n1χ12(φII2 )2 + 1 RT n2

(4)

By equating eq 3 with eq 4, the binary interaction parameter, χ12, in terms of composition of the two coexisting phases can be simply obtained as

ln χ12 )

() φI1

φII1

+ (φII1 - φI1) +

n1 II (φ - φI2) n2 2

n1((φII2 )2 - (φI2)2)

(5)

Equation 5 allows for a direct determination of the interaction parameter via compositional analysis of each phase in the phaseseparated blends equilibrated at a given temperature. For the binary blends (PHMA/Acrylate I, PHMA/Acrylate II, PPG/ Acrylate II, and PHMA/PPG) phase separated at 120 °C, the composition of each phase was characterized by using 1H NMR and/or FT-IR spectroscopy, as explained in the Experimental Section. For each binary blend exhibiting phase separation at 120 °C, the binary interaction parameters, calculated experimentally at different compositions, were found to be extremely small. Thus the average value for each binary blend is summarized in Table 2. For the miscible PPG/Acrylate I blends at 120 °C, the binary interaction parameter χij must be lower than its critical interaction parameter, χcritical . The critical ij interaction parameter represents the largest value of χij below which a binary blend exhibits complete miscibility. The critical

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TABLE 2: Critical and Experimental Binary Interaction Parameters, χcritical and χexp, of Binary Blends

a

binary blend

χcritical

χexp (at 120 °C)

PHMA/PPG PHMA/Acrylate I PHMA/Acrylate II PPG/Acrylate I PPG/Acrylate II

0.053 0.020 0.024 0.023 0.019

0.0647 0.0385 0.0272 a 0.0233

χexp ) 0.0256 at 5 °C.

interaction parameter is given by

1 χcritical ) (ni-1/2 + nj-1/2)2 ij 2

(6)

The critical interaction parameter for each binary blend is also listed in Table 2. It should be mentioned that binary interaction parameters for all binary blends phase separated at 120 °C are higher than the critical interaction parameters, as seen in Table 2. This demonstrates that the binary interaction parameters obtained experimentally are outside the experimental errors. By using the binary interaction parameters determined experimentally at 120 °C, the spinodal and binodal conditions can be calculated. The spinodal conditions for binary and ternary blends are given as14,27

1 1 + - 2χ12 ) 0 n1φ1 n2(1 - φ1)

(

)(

)

(7)

1 1 1 1 + - 2χ13 + - 2χ23 n1φ1 n3φ3 n2φ2 n3φ3 2 1 + χ12 - χ23 - χ13 ) 0 (8) n3φ3

(

)

For binary blends, eq 7 can be solved directly. However, for ternary blends, eq 8 was numerically evaluated to determine the spinodal boundaries. If the left-hand side of eq 8 is less than 0, the blends with a given composition are unstable, otherwise stable or metastable. The binodal boundaries indicating the metastable region were calculated by using a numerical minimization procedure from previous studies.18,27,28 An initial composition from the spinodal region is allowed to phase separate and the combined free energy of the two phases weighted by volume fraction is compared to the free energy of the initial composition.

F(φI1,φI2,VI) ) [G1(φI1,φI2,VI)(VI) + G2(φI1,φI2,VI)(1 - VI)] - G0 (9) The function F (programmed in Mathcad) has the arguments φI1, φI2, and VI, which represent the volume fraction of component 1 in phase I, the volume fraction of component 2 in phase I, and the volume fraction of phase I, respectively. The volume fraction of component 3 and the composition and volume of phase II are obtained via mass balance. The functions G1 and G2 (also programmed in Mathcad) return the free energy of phases I and II, respectively. The function G0 returns the free energy of the initial composition. For the minimization routine, the arguments of F are iterated until F is brought to a minimum. The compositions at the minimized F correspond to the binodal points. Figure 3 illustrates free energy of mixing as a function of composition with the locations of the spinodal and binodal boundaries for the five different binary blends at 120 °C. These plots of free energy of mixing with composition also confirm

Figure 3. Free energy of mixing as a function of composition for binary blends: (A) PPG/Acrylate I and PPG/Acrylate II, (B) PHMA/Acrylate I and PHMA/Acrylate II, and (C) PHMA/PPG. The filled and open circles indicate spinodal and binodal points, respectively. The dotted lines represent the coexistence ones.

the experimental results that Acrylate I is more miscible with PPG and less miscible with PHMA than Acrylate II is. Spinodal and binodal boundaries for ternary polymer blends were calculated by using the experimentally determined binary interaction parameters χij and represented as phase diagrams of ternary polymer blends, as shown in Figure 4. These calculated phase diagrams based on the Flory-Huggins formulations match

Phase Behavior of Ternary Blends

Figure 4. Calculated phase diagrams of ternary blends at 120 °C: (A) PHMA/PPG/Acrylate I and (B) PHMA/PPG/Acrylate II. The gray region indicates the spinodal boundary. The filled and open circles indicate binodal and three-phase boundaries, respectively.

well with experimental data, as can be seen in Figures 2 and 4. Predicted phase diagrams of ternary blends support the fact that the compositional difference of acrylic random copolymers yields remarkably different miscibility and phase behavior of ternary blends, as shown in Figure 4. The temperature dependence of miscibility and phase behavior of ternary blends was another area of investigation. By lowering temperature from 120 to 5 °C, several miscible or turbid binary and ternary blends became turbid or fully phase separated. Figure 5 shows experimental phase diagrams at 120 and 5 °C for PHMA/PPG/Acrylate I ternary blends. As noted above, PPG/ Acrylate I binary blends are miscible from 120 to 20 °C through all the blend composition. However, at 5 °C, PPG/Acrylate I blends displayed phase separation at blend compositions of 70/ 30 and 80/20, as shown in Figure 6. From PPG/Acrylate I blends phase separated at 5 °C, the binary interaction parameter χ23 was calculated and listed in Table 2. Using the binary interaction parameters for the PPG/Acrylate I blend, free energies as a function of composition are represented in Figure 7, where the spinodal and binodal boundaries are also shown. From the plot of free energy with composition, PPG/Acrylate I blends at 5 °C are phase separated from 60/40 to 90/10 blend composition, which matches well with experimental results. Considering that the χij shows an inverse proportionality with respect to temperature in the Flory-Huggins theory, increasing values of χ23 correspond to decreasing temperature. Binodal and spinodal boundaries are formed when the χij is higher than . For PHMA/PPG and PHMA/Acrylate I binary blends, χcritical ij

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Figure 5. Experimental phase diagrams of PHMA/PPG/Acrylate I ternary blends at different temperatures: (A) 120 and (B) 5 °C.

Figure 6. Optical micrographs of the PPG/Acrylate I binary blend with 70/30 blend composition at 120 and 5 °C.

their binary interaction parameters, χ12 and χ13 did not change with decreasing temperature within experimental error. By using the χij values for all the blends at different temperatures, phase diagrams of PHMA/PPG/Acrylate I blends were predicted at 120 and 5 °C, as can be seen in Figure 8, parts A and B. For the phase diagram at 120 °C, we used an extrapolatory χ23 ) 0.02 for miscible PPG/Acrylate I blends, which is lower than χcritical ) 0.023. The phase diagram at 120 °C was found 23 not to change noticeably with use of the χ23 value. A phase diagram calculated with χ23 ) 0.03 of PPG/Acrylate I is also shown in Figure 8C, which is a phase diagram expected at temperatures below 5 °C. For PHMA/PPG/Acrylate I ternary blends, the phase diagrams predicted at different temperatures of 120 and 5 °C are in good agreement with experiment, as shown in Figures 5 and 8. In addition, the phase diagram of PHMA/PPG/Acrylate I at temperatures below 5 °C is remarkably similar to that of PHMA/PPG/Acrylate II at 120 °C, as can be seen in Figures 8C and 4B.

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Figure 7. Free energy of mixing as a function of composition for the PPG/Acrylate I binary blend at different temperatures. The binary interaction parameters χ of 0.02, 0.0256, and 0.03 correspond to the values at different temperatures of 120 °C and 5 °C and an arbitrary temperature below 5 °C, respectively. The filled and open circles indicate spinodal and binodal points. The dotted lines represent those which coexist.

The above results clearly indicate that configurational changes in random copolymer composition can have substantial effects on miscibility and phase behavior of binary and ternary blends. Acrylate I is totally miscible with PPG, while Acrylate II is immiscible with PPG. The changing phase diagrams as a function of temperature are extremely fascinating. The fact that one type can “evolve” into another is of interest from both fundamental and practical perspectives. These observations can be explained by examining the interactions between individual constituents. For high molecular weight polymers, the combinatorial entropy terms for mixing are small. Therefore, enthalpic contributions often dominate the free energy of mixing in polymeric systems. In general, to exhibit miscibility, there needs to be some degree of interaction between the polymers, resulting in a favorable (exothermic) heat of mixing. A large number of studies exist in the literature concerning miscible polymer blends with specific interactions between components. These specific interactions were easily characterized with the aid of infrared spectroscopy.3,15,29 In an attempt to investigate the difference in interaction strength between PPG/Acrylate I and PPG/Acrylate II binary blends or between PHMA/Acrylate I and PHMA/Acrylate II, FT-IR spectra of those blends with various compositions were obtained. However, any specific strong interaction such as hydrogen bonds between Acrylates and PPG or PHMA has not been observed in FT-IR spectra. In our previous study,30 spectroscopic evidence related to the interaction between poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) was found. That is not the case here. Despite considerable effort to analyze the infrared spectra obtained, no changes in either band position or intensity were observed. By digitally adding the individual components, i.e., the two acrylates with polyether, the spectra obtained are identical. No spectroscopic features were found which represent differences in changes in either specific interaction or chain conformation. In Figure 9, the infrared spectrum of the miscible 70/30 PPG/Acrylate I is compared to the immiscible PPG/Acrylate II blend. Therefore, to consider the segmental interaction between various components, we considered the solubility parameter, δ, of each component, which is calculated by using the group contribution theory.31 The

Figure 8. Calculated phase diagrams of PHMA/PPG/Acrylate I ternary blends at different temperatures: (A) 120 °C; (B) 5 °C; and (C) an arbitrary temperature below 5 °C. The gray region indicates the spinodal boundary. The filled and open circles indicate binodal and three-phase boundaries, respectively.

solubility parameter is defined as

δ ) xδd2 + δp2 + δh2

(10)

where δd, δp, and δh represent solubility parameters contributed by dispersion, dipole-dipole, and hydrogen bond interactions, respectively. The solubility parameters of acrylic random copolymers of MMA and nBMA could be estimated from the values of the pure homopolymers (δMMA and δnBMA)

Phase Behavior of Ternary Blends

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Figure 9. Infrared spectra of (a) Acrylate I, (b) Acrylate II, (c) PPG, (d) PPG/Acrylate I with 70/30 blend composition, and (e) PPG/Acrylate II with 70/30 blend composition. The dashed lines in parts d and e are infrared spectra of PPG/Acrylate I and II blends calculated arithmetically by using the spectra of Acrylate I, Acrylate II, and PPG.

TABLE 3: Solubility Parameters, δ, of Blend Components Calculated by Group Contribution Theorya blend component

δd

δp

δh

δ

PHMA PPG Acrylate I Acrylate II PMMA PnBMA

17.665 16.603 17.327 17.396 17.460 17.204

4.975 7.634 4.829 5.428 5.983 3.763

8.430 7.566 8.250 8.767 9.245 7.332

20.196 19.778 19.789 20.223 20.643 19.077

a

Units: J1/2/cm3/2.

by means of the Scott relationship32

δcopolymer ) φMMAδMMA + φnBMAδnBMA

(11)

The solubility parameters of all components are listed in Table 3. There is a widely accepted concept that like dissolves like. By comparing solubility parameters of components, the solubility parameter of Acrylate I is much closer to the parameter of PPG and less close to that of PHMA than Acrylate II. This fact is qualitatively consistent with the experimental result that Acrylate I is more miscible with PPG and less miscible with PHMA than is Acrylate II. This is also supported by the fact that PPG is totally miscible with PnBMA homopolymer, but immiscible with PMMA. As described above, it is clear that the two acrylates are the origin for the significantly different miscibility and phase behavior when blended with PHMA and PPG. Because of their different chain configurations and associated free volumes, these two acrylates also have considerably different glass transition temperatures. Therefore, the kinetics of morphology evolution of the ternary blends prepared with either Acrylate I or II are expected to be dissimilar. In fact, these differences are employed to control rheological properties at elevated temperatures. The viscosity change as a function of time and temperature for cooled (120 to ∼35 °C) ternary blends (PHMA/PPG/Acrylate ) 33/ 33/33 blend composition) containing Acrylate I or II are shown in Figure 10. The PHMA/PPG/Acrylate I blend displays a threestep solidification process (Figure 10A). In contrast, the PHMA/ PPG/Acrylate II blend exhibits a two-step process (Figure 10B). The time dependence of the changes is quite different. There is no question these changes are directly attributed to the different chain mobility of the acrylates and crystallization kinetics of the incorporated polyesters.

Figure 10. Time-dependent viscosity development of (A) PHMA/PPG/ Acryalte I with 33/33/33 blend composition and (B) PHMA/PPG/ Acrylate II with 33/33/33 blend composition under a cooling process from 120 °C to various temperatures of 35-45 °C. The solid lines indicate the temperature profiles.

At 120 °C, the 33/33/33 ternary blend with Acrylate I is an immiscible mixture (Figure 2A). The first increase in viscosity originates from the limited local chain mobility due to the presence of the acrylate. The subsequent increases (the two

2548 J. Phys. Chem. B, Vol. 110, No. 6, 2006 additional steps) are due to the different crystallization kinetics of the polyester-rich and polyester-poor phases in the blends. Unlike the previous case, at elevated temperatures the ternary mixtures incorporating Arylate II are totally miscible (Figure 2B). In this case the first increase is due to the high glass transition temperature of the acrylate. The subsequent rise is due to crystallization of PHMA in this blend. The difference in miscibility behavior of the two ternary blends with different acrylates induces the different compositional distribution in the domains and matrix of the phase-separated morphology and kinetics of their formation. It should be mentioned that the ultimate viscosity of the first step, which originates from the reduced segmental mobility by the presence of immobile acrylate, is much lower in the PHMA/ PPG/Acrylate I than PHMA/PPG/Acrylate II blend as shown in Figure 10. This is because the glass transition temperature of Acrylate I is far lower than that of Acrylate II. It can be concluded that the time- and temperature-dependent morphology evolution of ternary blends is not only associated with miscibility, phase behavior, and chain configuration, but also with the chain dynamics of blends by the presence of acrylate with different glass transition temperatures. The detailed mechanism of morphology evolution in binary and ternary blends will be reported in the near future. Conclusions The influence of acrylic random copolymer composition on miscibility and phase behavior of ternary blends containing PHMA, PPG, and P(MMA-co-nBMA) was investigated. Temperature-dependent phase behavior of ternary blends was also studied. Specific interactions and associated phase diagrams of ternary blends were found to be quite different, depending on the copolymer composition of P(MMA-co-nBMA). Acrylate I (MMA/nBMA ) 48/52 by mol %) was miscible with PPG at 120 °C, while Acrylate II (MMA/nBMA ) 75/25 by mol %) was immiscible with PPG. Also, Acrylate I was less miscible with PHMA in comparison with PHMA/Acrylate II binary blends. These differences in miscibility of binary blends yielded totally different miscibility and phase behavior of ternary blends. PPG/Acrylate I binary blends, miscible at 120 °C, became immiscible at 5 °C around 70/30 and 80/20 blend compositions. Accordingly, phase diagrams of PHMA/PPG/Acrylate I ternary blends substantially changed with decreasing temperature from 120 to 5 °C. The Flory-Huggins formulations, extended to ternary polymer blends, were adopted for predicting phase diagrams of PHMA/PPG/Acrylate blends using the experimentally determined binary interaction parameters χij’s for the phaseseparated binary blends. Phase diagrams predicted for ternary blends with different acrylic random copolymers match well with experiment. In addition, phase behavior and chain dynamics of ternary blends with different acrylate were found to be strongly correlated with their morphology evolution as a function of time and temperature.

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