Article pubs.acs.org/IECR
Phase Compatibilization through Chemical Exchange Reactions in Blends of Copolyesters with Poly(hydroxyether of bisphenol A) upon Annealing Chean-Cheng Su,*,† Shiang-Ching Wang,† Wan-Jing Chen,‡ and Li-Ting Lee§ †
Department of Chemical and Materials Engineering, National University of Kaohsiung, Number 700, Kaohsiung University Road, Nan-Tzu District, Kaohsiung 81148, Taiwan ‡ Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan § Department of Materials Science and Engineering, Feng Chia University, Number 100 Wenhwa Road, Seatwen, Taichung 40724, Taiwan ABSTRACT: Miscibility or compatibilization via transreactions in blends of one of two copolyesters, poly(butylene adipate-cobutylene terephthalate) [P(BA-co-BT)] or poly(butylene succinate-co-butylene terephthalate) [P(BS-co-BT)], with poly(hydroxy ether of bisphenol A) (phenoxy) were investigated. The P(BA-co-BT)/phenoxy blend exhibited a homogeneous phase and a composition-dependent glass transition temperature (Tg) without any heat annealing. The copolymer−polymer interaction parameter (χ12) for the P(BA-co-BT)/phenoxy blend was calculated from the melting-point-depression method to be −0.12. However, variation in the composition and structure of the copolyesters easily causes phase separation in copolyester/phenoxy blends. The P(BS-co-BT)/phenoxy blend had a phase morphology that could be homogenized only following annealing at high temperatures. As-blended P(BS-co-BT)/phenoxy (50/50 composition) exhibited immiscible phases with two distinct Tgs, but the initially phase-separated blends finally merged to form a homogeneous phase with a single Tg upon heating and annealing for 60 min at 280 °C. Chemical exchange reactions upon heat annealing of the P(BS-co-BT)/phenoxy blend caused phase homogenization.
1. INTRODUCTION Truly miscible or compatible polymer blends of a variety of polymers with different properties may offer a unique means of developing new polymer materials with flexible compositions of different constituents.1−6 Additionally, multipolymer blends offer flexibility and property balancing in the design of materials.7−9 In polymer blending, various factors influence the compatibility of polymers, including the solvent used,10 operating temperature,11 molecular weight of the polymers,12 and chemical structures of the molecules involved as well as their interaction among others.13 These factors inhibit or favor the miscibility and compatibility of mixed polymers. Miscibility and phase homogeneity in polymer blends can be enhanced by chemical interactions, van der Waals forces, and hydrogen bonding.14,15 Poly(hydroxyl ether of bisphenol A) (phenoxy) is an amorphous thermoplastic that comprises a linear chain that contains ether linkages in the backbone and pendant hydroxyl groups which exhibit a high impact resistance and favorable oxygen barrier properties. The pendant hydroxyl group in the repeating unit enables interaction with the proton-accepting functional group in the polymers. The two kinds of interactions that can increase the compatibility of phenoxy and other polymers in polymer blends are physical (hydrogen bonding) and chemical (transreaction).16−29 The miscibility of polymer blends that contain phenoxy typically arises from hydrogen bonding between the hydroxyl group of phenoxy and other groups of the other compounds, including the carbonyl group, amine group, sulfonyl group, and ether group. Phenoxy has © 2013 American Chemical Society
been known to form miscible blends with other polymers, such as polyesters,16,17 polyethers,18 poly(methyl methacrylate),19 poly(vinylpyrrolidone),20 polyamides,21 polysulfones,22 and various epoxy resins.23 Such blends exhibit a significantly positive deviation from a linear relationship between Tg and composition, which is attributed to the pronounced interaction between phenoxy and polymers. Furthermore, reactions in polymer blends are widely used both to improve their properties, such as compatibility, and to synthesize new polymeric materials with desired properties. The pendant hydroxyl groups of phenoxy can participate in the specific chemical interaction, transreaction, with polyesters.24−29 Those blends of phenoxy with polyesters that consist of poly(butylene terephthalate) (PBT)/phenoxy,24 poly(ethylene terephthalate) (PET)/phenoxy,25 poly(trimethylene terephthalate) (PTT)/ phenoxy,26,27 poly(butylene succinate-co-butylene adipate) [P(BS-co-BT)]/phenoxy,28 and poly(3-hydroxy butyrate)(PHB)/phenoxy29 are immiscible or partially miscible. However, the chemical interactions that occur following annealing at high temperatures promote miscibility. The occurrence of a transreaction in all such blends has been analyzed. The transreaction or exchange reactions significantly alter the primary structure of polymer chains; consequently, Received: Revised: Accepted: Published: 12587
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they change the final properties of the blends, potentially enhancing their compatibility. Petroleum-based synthetic biodegradable aliphatic−aromatic copolyesters such as poly(butylene adipate-co-butylene terephthalate) [P(BA-co-BT)], poly(butylene adipate-co-terephthalate) [P(BA-co-T)], and poly(butylene succinate-co-butylene terephthalate) [P(BS-co-BT)] are disposable or highly durable, depending on their composition and specific application.30−33 Aliphatic−aromatic copolyesters combine the biodegradable properties of aliphatic polyesters with the strength and thermal properties of aromatic polyesters. As is widely regarded, this class of biodegradable polymers provides an effective means of forming fully biodegradable polymers with property profiles that resemble those of commodity polymers, such as lowdensity polyethylene. To elucidate the physical properties of these copolyesters, which have great potential for further application, many studies have been conducted especially on the crystallization of P(BA-co-BT). Cranston et al.31 studied the crystalline structure and molecular mobility of P(BA-co-BT) that was crystallized from the melt by X-ray diffraction and solid-state 13C nuclear magnetic resonance (NMR). The above results suggest that the butylene terephthalate units are in both the crystalline and the amorphous region, whereas all of the butylene adipate units are in the amorphous region. Gan et al.32 discussed how the crystalline structure and biodegradation, thermal behavior, and spherulite morphologies are related over a wide range of copolymer compositions of P(BA-co-BT)s. Molding compositions that contain biodegradable aliphatic− aromatic copolyesters plus biodegradable aliphatic polyesters can be prepared. The biodegradable aliphatic polyesters that can be used as a blend component include linear aliphatic polyesters without chain extensions and those with chain extensions and/or branches. Particularly favored aliphatic polyesters include polyhydroxybutyrate and poly(lactic acid), branched aliphatic polyesters that are based on polyhydroxybutyrate, polyhydroxyvalerate, poly(hexamethylene glutarate), poly(hexamethylene adipate), poly(butylene adipate), poly(butylene succinate), poly(ethylene adipate), poly(ethylene glutarate), poly(diethylene adipate), poly(diethylene succinate), poly(hexamethylene succinate), and starch.30,33 Several of these blends are immiscible or only partially miscible and may require compatibilizers to increase their compatibility. Moreover, P(BA-co-BT) was also blended with other nonbiodegradable polymers, including poly(4-vinylphenol)2 and polypropylene.34 Blending with these polymers allowed us to modify the properties of P(BA-co-BT). In this study, the copolyesters of P(BA-co-BT) and P(BS-coBT) were used as model copolymers for blending with phenoxy to elucidate how aliphatic−polyester and aromatic−polyester segments affect the interactions, miscibility, thermal behavior, and morphology of blends in which they are components. Phenoxy is known to interact favorably with butylene acid (BA) segments, butylene succinate (BS) segments, and butylene terephthalate (BT) segments, but its interactions with these three units may vary. Relevant analyses were performed in this study to elucidate the effects of BA segments, BS segments, and BT segments on the thermal behavior, phase morphology, and miscibility of phenoxy/copolyester blends as well as the interactions within them.
(Germany), with the commercial name Ecoflex, Mw = 135 000 g/mol, Tg = −32 °C, Tm = 124 °C (measured using DSC), and density = 1.27 g/cm3. The relative molar fraction of the copolymer, as determined by 13C solid-state cross-polarization/ magic angle spinning nuclear magnetic resonance (13C solidstate CP/MAS NMR), is 56 mol % BA and 44 mol % BT.31 Poly(butylene succinate-co-butylene terephthalate) (P(BS-coBT)) copolymer was obtained from Du Pont Co. (USA). A preliminary estimate, made using the Fox equation, suggests that the copolymer contains 90 mol % of PBT, with Tg = 46 °C and Tm = 204 °C (measured using DSC). Poly(hydroxy ether of bisphenol A) (known as phenoxy) was purchased from Scientific Polymer Products (USA), with Mn = 23 000 g/mol, Mw = 80 000 g/mol, and Tg = 90 °C. The structures of the repeat units of P(BA-co-BT), P(BS-co-BT) and phenoxy are as follows.
2.2. Preparation of Blends. Binary blends of P(BA-coBT)/phenoxy and P(BS-co-BT)/phenoxy were prepared by melt blending in a Brabender Plasti-Corder at 230 °C that was purged with nitrogen, as the blends were stirred for 10 min; the designed amounts of P(BA-co-BT), P(BS-co-BT), and phenoxy were poured into the Brabender mixer simultaneously. The driving speed was 70 rpm, and the deviation in speed was maintained at 0% through digital feedback. After 10 min of blending, the mixture was removed from the chamber and allowed to cool to room temperature. The blend compositions of P(BA-co-BT)/phenoxy were 10/90, 20/80, 30/70, 40/60, 50/50, 60/40, 70/30, 80/20, and 90/10, and that of P(BS-coBT)/phenoxy was fixed at 50/50. These samples are designated as “as-blended”. As-blended materials were further held at 280 °C for different times (0−150 min) for evaluation of effects of heat annealing. These latter samples are designated as “heatannealed”. Additionally, the well-blended copolyesters/phenoxy molten mixtures were spread onto potassium bromide (KBr) pellets (for examination by Fourier transform infrared spectroscopy (FT-IR)) or onto glass slides (to be examined under an optical microscope). 2.3. Characterization. A Nikon Optiphot-2 polarizing optical microscope (POM) equipped with a charge-coupled device (CCD) digital camera and a Linkam THMS-600 microscopic heating stage with a TP-92 temperature programmer was used to observe the crystal morphology and transition of phase behavior of the samples. Thin and uniform films (10− 15 μm) of the specimens for investigations on the phase behavior of the blends were sandwiched between two glass slides. Blends were thermally analyzed, and their Tg, Tm, and crystallization rates were measured using a differential scanning
2. EXPERIMENTAL SECTION 2.1. Materials. Poly(butylene adipate-co-butylene terephthalate) [P(BA-co-BT)] was obtained from BASF AG 12588
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microscope was used to examine preliminarily the P(BA-coBT)/phenoxy blends, and samples were found to be plain and transparent with no discernible phase domains/boundaries at ×800 (for brevity, graphs not shown). Blends were also placed on a microscopy heating stage whose temperature was raised gradually to observe the cloud-point transition. The results thus obtained revealed that no cloud-point or lower critical solution temperature phenomenon was observed up to 300 °C. The DSC and OM results suggest that the blends herein with the various compositions are all miscible, with close mixing of the three polymeric segments of BA, BT, and hydroxy ether of bisphenol A in the blends. To analyze further the relationship between Tg and composition, the dependence of the Tg of these blends on their composition was fitted using models that are based on the thermodynamics of glass transition. As shown in Figure 2, the
calorimeter (DSC) (Perkin-Elmer PYRIS I) that was equipped with a mechanical intracooler under purging with dry nitrogen. The scanning rate was 10 °C/min. All samples were pressed into flat films with a mass of 3−5 mg to ensure good thermal conduction and temperature distribution. Samples underwent the following thermal cycles. Samples were heated rapidly from 0 to 280 °C, which temperature was held for various annealing times (ta), and then cooled at 10 °C/min to 0 °C. The annealing times of P(BA-co-BT)/phenoxy blends that were heated at 280 °C were 2, 4, and 6 min. Blends underwent various numbers of cycles (n). The total annealing time Σta(280 °C) ta(280 °C). n = Σta was varied from 2 (n = 1) to 18 min (n = 3). Fourier-transform infrared spectroscopy (FT-IR) (Magna560) spectroscopy was used to identify possible interactions between copolyester and phenoxy. All spectra were recorded at a resolution of 4 cm−1 with an accumulation of 64 scans in the range of 400−4000 cm−1. Thin films of the well-blended copolyesters/phenoxy molten mixtures were spread onto potassium bromide (KBr) pellets and hot pressed to the proper thickness. IR measurements were made of uniform KBrcast film (1−3 μm) samples at ambient temperatures.
3. RESULTS AND DISCUSSION 3.1. Miscibility of P(BA-co-BT)/Phenoxy Blends. A single Tg-based value was applied to identify the miscibility of the P(BA-co-BT)/phenoxy blends during DSC experiments. Figure 1 shows the DSC thermograms of P(BA-co-BT)/
Figure 2. Tg vs composition for P(BA-co-BT)/phenoxy blends.
effect of followed 1 = Tg
composition on Tg of P(BA-co-BT)/phenoxy blends the Fox equation35 ω1 ω + 2 Tg1 Tg2 (1)
where ωi is the weight fraction of component i and Tgi is its glass transition temperature. In Figure 2, the Fox equation describes well the experimental Tgs of the compositions of the blends, except for the middle compositions. Additionally, the correlation between Tg and the composition data for P(BA-coBT)/phenoxy blends was best fitted using the Gordon−Taylor equation35,36 Tg =
Tg1ω1 + kTg2ω2 ω1 + ω2
(2)
where k represents the ratio of the thermal expansion coefficients of P(BA-co-BT) and phenoxy. The Gordon−Taylor equation, with an adjustable parametric constant, seems to describe most closely the Tg data of the P(BA-co-BT)/phenoxy blends. The parameter k was found to be 0.6. Both of the above modeling equations for a well-mixed or homogeneous state can be used to describe the Tg−composition relationship in a P(BA-co-BT)/phenoxy binary system, indicating formation of a miscible state without an inhomogeneous domain or phase separation in the P(BA-co-BT)/phenoxy system. FT-IR characterization was performed to study the interactions in P(BA-co-BT)/phenoxy blends. Figure 3 shows the phenoxy hydroxyl stretching regions (3750−3150 cm−1) of
Figure 1. DSC traces for quenched P(BA-co-BT)/phenoxy blends.
phenoxy blends of various compositions. The thermograms reveal that the blends had a clear and single Tg that increased with the concentration of phenoxy in the blends. They suggest that P(BA-co-BT) can mix with phenoxy homogeneously to form a miscible blend. Additionally, crystallization of P(BA-coBT) was observed in the DSC thermograms of the blends with P(BA-co-BT)-rich compositions (>50 wt %). An optical 12589
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Figure 3. Phenoxy hydroxyl stretching regions of the FTIR spectra of the P(BA-co-BT)/phenoxy blends with various compositions.
the FTIR spectra of the miscible P(BA-co-BT)/phenoxy blends with various compositions. In the blends, IR absorbance peaks of the phenoxy −OH group are observed at 3578 and 3465 cm−1, corresponding to the free hydroxyl and self-bonding −OH (intramolecular hydrogen bonding), respectively. As the P(BA-co-BT) content in the blend increases, the intensity of the IR peak associated with self-bonding decreases, indicating a corresponding increase in the strength of the intermolecular hydrogen bonding between P(BA-co-BT) and phenoxy. A previous study noted that favorable interactions between the −OH and carbonyl groups may occur between PCL/PVPh37,38 and aliphatic polyesters/tannic acid (TA)39 blends. Weak hydrogen bonding is expected to be present between P(BA-coBT) (with carbonyl CO) and phenoxy (with hydroxyl −OH). Further evidence of favorable interactions can be obtained using the classical Flory−Huggins thermodynamic analysis, based on the depression of the melting point of a crystalline polymer for two interacting components in miscible mixtures.40 The equilibrium melting temperatures of P(BA-co-BT) in the P(BA-co-BT)/phenoxy blends were obtained by the classical Hoffman−Weeks method.41 In this procedure, the measured Tm of the specimens that were crystallized at Tc was plotted against Tc. Also, a linear extrapolation to the line Tm = Tc was performed, where the intercept yielded Tm° Tm = Tm°(1 − 1/γ ) + Tc/γ
Figure 4. DSC traces of neat P(BA-co-BT) and P(BA-co-BT)/phenoxy blends melt crystallized at various temperatures for 8 h: (a) neat P(BAco-BT) and (b) 90/10, (c) 80/20, and (d) 70/30 blend compositions.
(3)
where Tm° is the equilibrium melting temperature and γ = l/l* is the ratio of the lamellar thickness l at the time of melting to the thickness l* of the critical nucleus at Tc. To observe the equilibrium melting temperatures (Tm°) of P(BA-co-BT) in the blends, the specimens were first melted at 160 °C for 5 min to eliminate crystalline residues. Polymer samples were then quenched at a rate of 320 °C/min to the desired crystallization temperature (Tc), isothermally crystallized until complete crystallization, and finally scanned at a rate of 10 °C/min. Figure 4 shows the DSC traces for various compositions of P(BA-co-BT)/phenoxy blends melt crystallized at various temperatures (Tc on each trace) for 8 h. P(BA-co-BT)/phenoxy blends with four compositions (neat P(BA-co-BT), 90/10, 80/ 20, and 70/30) were used. The melting temperatures in Figure 4 were applied to determine the extrapolated melting temperature (equilibrium melting temperature). Figure 5a
Figure 5. P(BA-co-BT)/phenoxy blends melt crystallized at various Tcs: (a) Hoffman−Weeks plots and (b) Flory−Huggins plot for assessing interaction parameter.
shows Hoffman−Weeks plots, indicating that the equilibrium temperatures decrease as the amorphous phenoxy content in the blends increased. After the equilibrium melting temperatures of all of the blends had been obtained from the 12590
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blend, the size of the phase domain declined and eventually disappeared as the annealing time increased. After the blend had been annealed at 280 °C for 120 min (Figure 6f), its morphology was homogeneous, suggesting that the specific interaction between P(BS-co-BT) and phenoxy can improve miscibility in the P(BS-co-BT)/phenoxy blend at the annealing temperature. Figure 7 plots the effect of annealing time on the glasstransition behavior of the blend with composition P(BS-co-
Hoffman−Weeks extrapolation, the interaction parameter (χ12) between P(BA-co-BT) and phenoxy was calculated from the Flory−Huggins relationship as follows41 Rν2 1 1 − =− χ (1 − ϕ2)2 Tm Tm° ΔHf2ν1 12 (4) where Tm and Tm° are the equilibrium melting points of P(BAco-BT) in the polymer mixtures and the neat crystallizing polymer, respectively. The subscript “1” refers to the noncrystallizing polymer (phenoxy), and “2” refers to the crystallized polymer [P(BA-co-BT)]. ν1 and ν2 are the molar volumes of the repeat units of the noncrystallizing and crystallizing polymers, respectively. ΔHf2 is the heat of fusion of the fully crystalline polymer, and ϕ2 is the volume fraction of the crystallizing polymer. The interaction parameter χ12 was determined from the slope of the plot of the terms on the lefthand side of eq 2 were plotted against those on the righthand side of eq 2. The physical constants that were used in the calculations were ν1 = 242.4 cm3 mol−1, ν2 = 167.88 cm3 mol−1, ρ2 = 1.377 g cm−3, and ΔHf 2 = 5.32 kJ mol−1.42 Figure 5b shows the plot that is used to calculate χ12. The slope yielded χ12 = −0.12. The negative value of χ12 reveals that P(BA-co-BT)/phenoxy blend is a miscible system, a finding which corresponds to the characterization of thermal transition during DSC experiments. 3.2. Phase Behavior in P(BS-co-BT)/Phenoxy Blends. Figure 6 shows the optical micrographs (Figure 6a−f) of the
Figure 7. DSC traces of the P(BS-co-BT)/phenoxy = 50/50 blend annealed at 280 °C for various times: (a) 0, (b) 2, (c) 5, (d) 10, (e) 30, (f) 60, (g) 90, and (h) 150 min.
BT)/phenoxy = 50/50. DSC thermograms showed that the blends exhibited two T g s, T g,P(BS‑co‑BT)‑rich region and Tg,phenoxy‑rich region, for P(BS-co-BT) and phenoxy, respectively. Additionally, the P(BS-co-BT)-rich region had a crystallization temperature (Tc = 142 °C) and a melting temperature (Tm = 195 °C). The Tg of the P(BS-co-BT)-rich regions increased with annealing time at 280 °C, while that of the phenoxy-rich regions decreased. At t = 30 min, the two Tgs of the blend converged into a single Tg because the specified chemical interaction occurred between P(BS-co-BT) and phenoxy. After 60 min, the Tg of the blends remained constant and independent of the annealing time. In the later stages of the annealing, the breadth of the glass-transition range remained constant and independent of reaction time. Figure 8 plots Tg as a function of time for the P(BS-co-BT)/phenoxy = 50/50 blend during annealing at 280 °C. Apparently, the two Tgs for phenoxy and P(BS-co-BT) were 44 and 79 °C, respectively, in the early stage of annealing, and then these values converged to a single Tg during the annealing process. In polymer blends, the chemical interactions of charge transfer complexes and ionic interactions in the ionomers most commonly proceed between molecules or between the segments in one of the polymers. Despite their immiscibility in the as-blended state, condensation polymer blends (e.g., PBT/phenoxy,24 PET/phenoxy,25 PTT/phenoxy,26,27 P(BS-co-BT)/phenoxy,28 and PHB/phenoxy29) exhibit a transreaction upon mixing at high temperatures, causing the separated phases in the blends to become partially or wholly miscible. According to the literature, the alcoholysis reaction is the main transreaction of P(BA-co-BT)/ phenoxy blends in the annealing process. The transreaction
Figure 6. Optical micrographs of the P(BS-co-BT)/phenoxy = 50/50 blend annealed at 280 °C for various times: (a) 0, (b) 10, (c) 30, (d) 60, (e) 120, and (f) 150 min.
P(BS-co-BT)/phenoxy = 50/50 samples after they were heated at 280 °C for various times. In Figure 6a, as-blended P(BS-coBT)/phenoxy = 50/50 samples that contained phenoxy-rich components (dispersed phase) and a homogeneous phase with a P(BS-co-BT)-rich composition (continuous phase) exhibited phase separation. Within the dispersed domains (3−45 μm long), smaller particulate domains (less than 1 μm) were presented. According to the micrographs, in the immiscible 12591
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initially formed graft copolymers and then resulted in crosslinked copolymers that improve the miscibility of the blend. The results obtained reveal that exchange reactions cause a progressive homogenization of the initially immiscible blends. However, calorimetric analysis shows a clear decrease in the rate of crystallization of P(BS-co-BT). These results are attributed to formation of copolymers from the components of the blends due to exchange reactions. After 60 min of annealing at 280 °C, the exchange reactions between P(BS-coBT) and phenoxy had run almost to completion. 3.3. Chemical Exchange Reactions in P(BS-co-BT)/ Phenoxy Blends. An earlier study noted that favorable interchange reactions via the −OH and ester functional groups may proceed in aliphatic polyesters and phenoxy blends.24−29 Accordingly, the specific interactions, transreactions, or alcoholytic exchanges can increase the compatibility of phenoxy with P(BS-co-BT) in the polymer blends. The alcoholytic
Figure 8. Tg (onset values) vs time for the P(BS-co-BT)/phenoxy = 50/50 blend.
Scheme 1. Chemical Exchange Reactions in P(BS-co-BT) with Phenoxy
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exchanges between the dangling −OH in phenoxy and the ester functional group in the BS and BT segments of P(BS-co-BT) polymer chains in the heated phenoxy/P(BS-co-BT) blends initially resulted in formation of graft copolymers and then formed cross-linked copolymers at the end of the reaction, increasing the miscibility of the blend. Upon heating treatments, various chemical links form in the blends between the reacting phenoxy and P(BS-co-BT) molecules, including via exchange reactions between phenoxy−BS and phenoxy−BT segments. Exchange reaction products include hydroxylterminated P(BS-co-BT) (IA and IIA) and phenoxy-grafted P(BS-co-BT) (IB and IIB) linkages, whose mechanism is proposed in Scheme 1. Notably, the as-blended phenoxy/P(BSco-BT) blends dissolved in N,N-dimethylformamide and hexafluoro isopropyl alcohol. However, after an extended period of annealing (t > 60 min) at 280 °C, the blends were only swelling in the solvent. In the annealing process, alcoholytic exchange proceeded between P(BS-co-BT) and phenoxy, forming the cross-linked network at the end of the reaction. 3.4. Crystallization Behavior of Heat-Annealed Blends. The effect of progressive alcoholytic exchange on the crystallization behavior of P(BS-co-BT) was studied using DSC. Figure 9 shows DSC thermograms of the crystallization
time increased and/or the number of cycles of annealing to 280 °C increased. Table 1 lists the crystallization enthalpies (ΔHc) for P(BS-coBT)/phenoxy = 50/50 samples that had been annealed at 280 Table 1. Crystallization Enthalpies for P(BS-co-BT)/ Phenoxy = 50/50 Samples after Annealing at 280 °C for Different Times and Then Cooled in Cycles annealing times per cycle a
a
exothermic heat of crystallization (J/g)
2 min
4 min
6 min
ΔHn=1 ΔHn=2 ΔHn=3
13.4 6.5 1.0
9.3 4.6 0.4
5.6 1.8 0.05
Exothermic heat of crystallization of neat P(BS-co-BT) = 16.1 J/g.
°C for various times and then cooled (10 °C/min) in cycles. Clearly, the decrease in the degree of crystallization of P(BS-coBT) with increasing total annealing time (Σta) reveals the reduction of regular crystalline structures for P(BS-co-BT). The behavior that was expected as a consequence of the transreactions increased upon heat annealing owing to inhibition of crystallization by the exchange reactions. Pompe et al.43 suggested that the extent of the transreaction can be determined from the time regime of the thermal treatment in the melt blend. The decrease in the crystallization peak temperature measures the extent of exchange reactions. The effect of the number, n, of cycles on the rate of the exchange reaction is explained as follows. The exchange reaction proceeds between neighboring molecular chains of P(BS-coBT) and phenoxy. This process is diffusion controlled in the melt of the blend. The interruption of the annealing in the melt by cooling is related to a thermally stimulated process of P(BSco-BT) separation in the blend that is associated with P(BS-coBT) crystallization. The P(BS-co-BT) and phenoxy molecules become newly distributed with an increasing probability that the neighboring molecules of P(BS-co-BT) and phenoxy are present in the interface of P(BS-co-BT) crystallites and surrounding matrix. In the reactive semicrystalline/amorphous polymer blends, arrays of folded chains in the crystalline lamellae of P(BS-co-BT) were interrupted by the formed copolymers that were formed in the exchange reaction. These formed copolymers reduce the ordering of the polymer chain, subsequently reducing the degree of crystallization of P(BS-coBT) in the blends. The effect of the total annealing time on the copolymer content was investigated using the change of the nonisothermal crystallization temperature (Tc). Figure 10 plots Tc as a function of the total annealing time for the P(BS-co-BT)/ phenoxy = 50/50 sample. In this experiment, the annealing time ta varies from 0 to 6 min. The Tc decreases faster when n is higher for a given total annealing time. The results herein suggest that interchange reactions become stronger as the total annealing time increases. In the blends, the decrease of Tc upon heat annealing becomes larger as the annealing time increases, owing to inhibition of crystallization by a reduction of the lengths of P(BS-co-BT) segments. The behavior of Tc reflects the promotional exchange reaction that is caused by the new molecular order, which is connected with the crystallization process. In this study, the complete history of annealing of the samples is considered to estimate the extent of the exchange reaction.
Figure 9. DSC thermograms of the crystallization behavior of neat P(BS-co-BT) and P(BS-co-BT)/phenoxy = 50/50 samples after being annealed at 280 °C for various times and then cooled in cycles: (a) 2, (b) 4, and (c) 6 min.
behavior of P(BS-co-BT)/phenoxy = 50/50 samples that underwent n cycles (number of cycles per measuring series) of annealing between 0 and 280 °C. In Figure 9a, 9b, and 9c, the heating rate was 10 °C/min and the annealing times (ta) at 280 °C were 2, 4, and 6 min, respectively. In these figures, both the exothermic heat of crystallization and the crystallization temperature (Tc) of P(BS-co-BT) decreased as the annealing 12593
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Figure 10. Tc as a function of the total annealing time for the P(BS-coBT)/phenoxy = 50/50 sample.
4. CONCLUSIONS The as-blended P(BA-co-BT)/phenoxy mixture herein has a homogeneous phase. DSC analysis reveals that all components of the blend have composition-dependent glass-transition temperatures without any heat annealing. FT-IR reveals only weak interactions between copolyester and phenoxy segments, suggesting that although phenoxy possesses −OH and copolyester possesses CO, hydrogen bonding between phenoxy and copolyester does not necessarily occur. The interaction parameter (χ12), as determined by melting-pointdepression analysis, also reveals a weak strength of χ12 = −0.12. Unlike the P(BA-co-BT)/phenoxy blend, the P(BS-co-BT)/ phenoxy blend exhibits distinct phase separation with islands− sea domains. Again, the presence of −OH and CO groups in constituent polymers does not guarantee hydrogen bonding, which would provide miscibility. Although as-blended P(BS-coBT)/phenoxy (50/50 composition) exhibited immiscible phases with two distinct Tgs, the initially phase-separated blends could eventually merge into a homogeneous phase with a single Tg upon annealing at 280 °C for 60 min. Chemical exchange reactions upon heat annealing were likely to have caused phase homogenization in the P(BS-co-BT)/phenoxy blend. Results of this study demonstrate that only heat annealing can induce phase homogenization in the P(BS-coBT)/phenoxy blend, whereas P(BA-co-BT)/phenoxy blend exhibits physical miscibility. Furthermore, its P(BS-co-BT) and P(BA-co-BT) constituents differ by two methylene groups in the chemical repeat units. The latter copolymer also has a lower PBT mol %.
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*Tel.: +886 7 591-9338. Fax: +886 7 591-9277. E-mail: ccsu@ nuk.edu.tw. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was sponsored by the National Science Council of Taiwan under contract no. NSC 101-2221-E-390-006-MY2. The authors would like to thank Professor E. M. Woo of National Cheng Kung University, who kindly advised on this study. 12594
dx.doi.org/10.1021/ie401371y | Ind. Eng. Chem. Res. 2013, 52, 12587−12595
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