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Poly(hydroxybutyrate) and epichlorohydrin elastomers blends: Phase behavior and morphology
EUROPEAN POLYMER JOURNAL
European Polymer Journal 42 (2006) 602–614
www.elsevier.com/locate/europolj
Poly(hydroxybutyrate) and epichlorohydrin elastomers blends: Phase behavior and morphology Juliana Ariste´ia de Lima, Maria Isabel Felisberti
*
Instituto de Quı´mica, Universidade Estadual de Campinas, CP 6154, 13083-970 Campinas, SP, Brazil Received 19 May 2005; received in revised form 8 August 2005; accepted 12 September 2005 Available online 18 October 2005
Abstract This work studied blends of PHB with epichlorohydrin elastomers, the PEP homopolymer and its copolymer with ethylene oxide, ECO. PHB is a microbial polyester, which is accumulated intracellularly by a large number of microorganisms, presenting characteristics of biodegradability and biocompatibility. It presents a high degree of crystallinity, so is a quite brittle material, and may undergo degradation when is kept for a relatively short time at a temperature above its melting point, about 180 C. PEP and ECO are linear and amorphous elastomers, exhibit miscibility with many aliphatic polyesters and these elastomers have been used in various branches of technology, such as the automotive industry. The proposed systems combine a polymer with high crystallinity and biodegradability, PHB, with amorphous epichlorohydrin elastomers. Blends were prepared by casting from chloroform solution at different compositions (0, 20, 40, 50, 60, 80 and 100 wt% of PHB). The phase behavior of PHB/PEP and PHB/ECO blends were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and the morphology of the crystalline phase of PHB had been examined by optical microscopy. Blends of PHB/PEP and PHB/ECO have been described in literature as miscible. However, our results from the DSC and DMA show that PHB/PEP and PHB/ECO blends are immiscible. This behavior should be related to the molecular weight of polymers used in the present work, which is higher than the molecular weight of polymers used in the previous works. The crystallization kinetics of PHB is strongly influenced by the presence of the elastomeric phase. The degree of crystallinity of PHB/PEP blends decreases with an increase in the PEP content. PHB/ECO blends present degrees of crystallinity that can be considered nearly independent of the ECO content. Differences in the morphology of the crystalline phase were also observed, and these are attributed to the presence of elastomeric phase in the intraspherulitic zone. 2005 Elsevier Ltd. All rights reserved. Keywords: Blends; Miscibility; Morphology
1. Introduction
*
Corresponding author. Tel.: +55 19 37883130; fax: +55 19 37883028. E-mail address: [email protected] (M.I. Felisberti).
There have been many studies of polymer blends in recent years. The interest in novel structures and properties of blends still motivates extensive studies, as does the increasing importance in
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.09.004
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practical applications of new polymeric materials having a wide range of physicochemical properties, with behavior different from the homopolymers and copolymers [1]. An important and determinant aspect of the properties of a blend is the miscibility of their components. Miscibility in polymer blends is assigned to specific interactions between the polymer components, which usually gives rise to a negative free energy of mixing, in spite of the high molecular weight of the polymers. The most common interactions present in blends are: hydrogen bond, p-electrons and ionic and dipole interactions, and charge-transfer complexes [2]. Recently, the interest of polymer blends development has been concentrated on systems in which at least one of the components is crystallizable. Attention has focused on the various morphologies resulting from the crystallization of the melt-miscible phase of semicrystalline/amorphous blends. The components of the blends studied in the present work are a semicrystalline polyester, poly(3hydroxybutyrate) (PHB), and an amorphous rubbery component, poly(epichlorohydrin) (PEP) and its copolymer with ethylene oxide, poly(epichlorohydrin-co-ethylene oxide) (ECO). Optically active PHB is an aliphatic polyester produced via biosynthesis by bacterial fermentation. PHB is a thermoplastic with a high degree of crystallinity and a well-defined melting point (around 180 C). Unlike other thermoplastic polymers, such as poly(propylene) and poly(ethylene), PHB is biodegradable and biocompatible [3,4], presenting potential in applications such as medical materials, absorbable surgical sutures, matrices for drug delivery systems and as biodegradable molded plastics [5]. However on undercooling PHB crystallizes from the melt, giving rise to the formation of large spherulites with a brittle behavior. Furthermore, PHB is thermally unstable at temperatures close to the melting point and degrades through chain scission resulting in terminations of crotonic acid and vinyl groups [3]. Thus, PHB has two limitations in its use: a very narrow processability window, and a relatively low impact resistance due to its high degree of crystallinity. These drawbacks have hampered the utilization of PHB as a common plastic. Blends of PHB with other polymers can offer opportunities to extend and explore their many useful and interesting properties and to modify its undesirable properties. For example, PHB has been blended with poly(ethylene
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oxide) [3], poly(vinyl butyral) [6], poly(vinyl acetate) [7], poly(vinylphenol) [8], cellulose acetate butyrate [9], chitin and chitosan [10]. PEP and ECO are linear amorphous elastomers, exhibiting glass transition temperatures at 23 C and 41 C, respectively [1,11]. Epichlorohydrin elastomers exhibit miscibility with many aliphatic polyesters [12] and these elastomers have been used in various branches of technology, such as the automotive industry, in fuel, lubricating fluids, in air and vacuum hoses, etc. [13]. There are few published papers that deal with PEP and the majority of these publications were about its properties, characterization and chemical modification [14–17]. PEP has been studied in blends with poly(ethylene oxide) [1], poly(vinyl acetate) [12], poly(styrene-co-acrylonitrile) [18] and poly(methyl methacrylate) [19]. The miscibility of PHB/PEP and PHB/ECO blends has been studied [12,20–25] and the authors have found that these blends are miscible, but the polymers used to prepare the blends present different characteristic from those used in this work, as the molecular weight. In this work, the influence of the PEP and its copolymer with ethylene oxide, ECO, on the miscibility with PHB was evaluated by an investigation of the thermal behavior, the crystallization and the morphology as a function of blend composition. 2. Experimental 2.1. Polymer materials The polymers used in this work were commercial products whose properties are listed in Table 1. 2.2. Cast films Binary blends of PHB/PEP and PHB/ECO of different compositions were prepared by casting from 5% (w/v) chloroform solutions. The solvent was Table 1 Molecular weight (M w Þ and polydispersity M w =M n of the PHB, PEP and ECO used to obtain the blends Polymer Source Mw M w =M n a 1 a (g mol ) PHB PEP ECO (48 mol% epichlorohydrin) a
PHB Industrial 450,000 Zeon Chem. Inc. 900,000 Zeon Chem. Inc. 900,000
GPC in chloroform at 40 C.
– 3.3 5.0
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allowed to evaporate slowly at room temperature (about 25 C) for 48 h, followed by drying in vacuum at room temperature for 24 h, and at 90 C for 4 h. 2.3. Differential scanning calorimetry Differential scanning calorimetry (DSC) was carried out using a MDSC 2910 TA Instruments equipment. The experiments were performed according to the following program: • Initial temperature 80 C; • First heating: heating rate of 20 C min1 to 200 C; • Cooling: cooling rate of 20 C min1 to 80 C; • Second heating: heating rate of 20 C min1 to 200 C. The same experiments were repeated, using a maximum temperature of 215 C for PHB/ECO blends and 230 C for PHB/PEP blends. The results reported in this work correspond to the cooling and second heating runs. All DSC curves were normalized with respect to the sample mass. 2.4. Dynamic mechanical thermal analysis A DMTA V Rheometrics Scientific was used. A film of polymer samples (8.0 · 1.5 · 5.0 mm) was clamped in a frame for tension tests. A frequency of 1 Hz, amplitude of 0.03% and heating rate of 2 C min1 from 100 C to 200 C were applied. The storage modulus, loss modulus and loss factor or damping were recorded as a function of temperature. 2.5. Polarizing optical microscopy The morphologies of the PHB crystalline phase in pure samples and in the blends were observed with a Nikon E800 Polarizing Optical Microscope. Sample sandwiched between two thin glass slides was melted for 1 min on a hot plate preheated to 200 C for PHB, PHB/PEP blends and PHB/ECO blends. Then, it was quickly transferred to the hot plate which was maintained at a desired temperature, isothermal crystallization, at 60 C and 70 C, for 2 h. 3. Results and discussion Fig. 1 shows DSC curves corresponding to the second heating for the two systems of blends,
PHB/PEP and PHB/ECO, submitted to a maximum temperature of 200 C and 215 C, respectively. The curves show glass transitions around 0 C, an exothermic peak and an endothermic peak corresponding to crystallization and melting of PHB phase, respectively. The glass transition temperature (Tg; assumed as the temperature corresponding to the half width of the transition), crystallization and melting temperature (Tc and Tm), crystallization and melting enthalpies (DHc and DHm) normalized with respect to the PHB content in the blends and degree of crystallinity (Xc) taken from the DSC curves (Figs. 1, 6 and 7) are listed in Tables 2 and 3. 3.1. Glass transition behavior The homopolymer PHB presents Tg at 5 C and PEP at 23 C. The copolymer ECO presents glass transition temperature at 39 C. The DSC curve obtained for blend PHB/PEP with 20 wt% of PHB shows a single glass transition in the same temperature range observed for PEP (Fig. 1A). This suggests the existence of a rich elastomeric phase and at least another phase, whose glass transition could not be determined by DSC. The others blends compositions show clearly two glass transitions not shifted from those observed for the individual polymers, indicating the coexistence of an elastomeric phase (PEP) and a PHB phase. Possibly the viscosity of elastomer is related to the extension of the phase segregation during the evaporation of the solvent, showing differences in phase behavior of each blend. Paglia et al. [20] studied the crystallization and thermal behavior of PHB/PEP blends. The binary blends were prepared by casting from dichloromethane solution over a wide range of composition and analyzed by DSC and microscopy. The molecular weight ðM w Þ of PHB utilized was 150,000 g mol1 and M w of PEP was 700,000 g mol1. The thermal analysis of the blends showed a single glass transition, whose values fit the Fox equation. Moreover, the melting point of PHB decreases with the blending and the negative values of the interaction parameters (v12) of the PHB/PEP systems suggest that the two components can form a miscible blend in the amorphous phase which is thermodynamically stable above the equilibrium melting temperature. The comparison between the results from PagliaÕs work and from our work shows that the molecular weight of PHB and PEP affects the
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605 0% 20%
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0
15
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- 60
-45
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15
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Fig. 1. DSC curves of: (A) PHB/PEP blends submitted to heating up to 200 C and (B) PHB/ECO blends submitted to heating up to 215 C. Second heating at 20 C min1.
miscibility, as the thermodynamic of polymer solutions predicts. PHB/ECO blends show two glass transitions and the transition at high temperature is shifted to a lower temperature than the observed for pure PHB (Fig. 1B). The dependence of the Tg at high temperature on the composition suggests the existence of an elastomeric phase (ECO) and another phase that could be a mixture of PHB and ECO in PHB/ECO blends. The observed effect could be attributed to the presence of ethylene oxide segments in ECO chains, that should improve miscibility with PHB. The miscibility of PHB and poly(ethylene oxide)
has been described in the literature [7]. Only the blend with 80 wt% of PHB and 20 wt% of ECO seems to present a single glass transition. Zhang et al. [21] studied the miscibility, melting and crystallization behavior of PHB/ECO blends by DSC and optical microscopy. Blends of varying compositions were prepared by casting from chloroform solution using PHB with M w equal to 230,000 g mol1 and ECO with M w equal to 440,000 g mol1. They concluded that the blends are miscible over the whole composition range. The glass transition temperature behavior of blends could be described by the Kwei equation. Negative interaction parameters are
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Table 2 Thermal properties of PHB/PEP and PHB/ECO blends PHB/PEP
Tg (C) (from E00 · T)
Tg (C) (from tan d · T)
Tg (C) (from DSC) (Fig. 1A)
Tm (C) (from DSC) (Fig. 1A)
Tm (C) (from DSC) (Fig. 7)
DHm (J/g) (from DSC) (Fig. 7)
Xc (%) (from DSC) (Fig. 7)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
16 17; 16; 16; 16; 15; 15
8 12; 14; 15; 16; 14; 22
23 18 18; 18; 18; 17; 5
– 169 171 174 172 170 175
– 156 170 173 171 171 160
– 29 65 77 82 85 90
– 19 43 51 54 56 60
PHB/ECO
Tg (C) (from E00 · T)
Tg (C) (from tan d · T)
Tg (C) (from DSC) (Fig. 1B)
Tm (C) (from DSC) (Fig. 1B)
– –
DHm (J/g) (from DSC) (Fig. 1B)
Xc (%) (from DSC) (Fig. 1B)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
38 39; 40; 40; 39; 35; 15
32 35; 42; 40; 35; 34; 22
39 40; 42; 43; 46; 4 5
– 167 169 170 171 173 175
– – – – – – –
– 80 82 82 83 85 87
– 53 54 54 55 56 58
10 6 4 0 13
10 15 15 15 14
6 2 5 16 20
17 19 13 21 21
4 3 5 6
23 16 14 12
Table 3 Crystallization temperature on cooling (T cc ) from the melting state at 200 C and 215 C, crystallization temperature on second heating (T cH ), crystallization enthalpy on cooling (DH cc ), crystallization enthalpy during the second heating (DH cH ) and total crystallization enthalpy (DH ctotal ) PHB/PEP
T cc (C) (from Fig. 6)
DH cc (J/g) (from Fig. 6)
T cH (C) (from Fig. 1)
DH cH (J/g) (from Fig. 1)
DH ctotal (J/g)* (from Figs. 1 and 6)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
– – 70 71 72 86 76
– – 10 37 44 68 61
– 62 68 69 69 – –
– 20 16 4 5 – –
– 20 26 41 49 68 61
PHB/ECO
T cc (C) (from Fig. 6)
DH cc (J/g) (from Fig. 6)
T cH (C) (from Fig. 1)
DH cH (J/g) (from Fig. 1)
DH ctotal (J/g)* (from Figs. 1 and 6)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
– – – 56 58 59 52
– – – 8 10 11 18
– 81 55 57 55 48 59
– 77 48 31 19 27 50
– 77 48 39 29 38 68
Obtained from DSC curves. * It is the sum of the crystallization enthalpy during the cooling and second heating (cold crystallization).
obtained from melting point depression data. Crystallization of PHB from the melt and from the glassy state is affected by addition of ECO. The molecular weight of PHB and ECO used in ZhangÕs work are smaller than the molecular weight used in our work. The differences from ZhangÕs conclusions and our conclusions are attributed to the molecular weight effect.
The second glass transition of some blends was not possible to determine by DSC. Because of this, dynamic mechanical analysis, a sensitive technique to study polymer relaxation, was used. The samples analyzed by DMA possibly present thermal histories corresponding to the formation of the films by solvent evaporation and this condition guarantees
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the maximum crystallization of sample, while the thermal histories of the samples analyzed by DSC were erased during the first heating. In DMA, the glass transition temperature (Tg) was assumed to be the temperature corresponding to the maximum of the peaks in the E00 · T curves or the maximum of the peaks in the tan d · T curves. The glass transition temperatures obtained from E00 · T and tan d · T are presented in Table 2. Figs. 2 and 3 show the loss modulus curves (E00 · T) and damping curves (tan d · T) for PHB/ PEP blend containing 80 wt% of PEP and PHB/ ECO blend containing 20 wt% of ECO, respectively. A second peak or a shoulder on the peaks is not easy to find in the E00 · T curves, except for the blend containing 20 wt% of ECO, for which the peak is broad and asymmetric. The first and second derivative curves were used to determine the temperature corresponding to the shoulder. The analysis of the dependence of Tg on blend compositions reveal (Table 2) that the most of blends present a glass transition in the characteristic temperature range of the glass transition of the corresponding elastomer. The Tg of the second phase is strongly dependent on the thermal history of the samples. Thus, the higher Tg determined by DSC for PHB/PEP blends is close to PHB glass transition. On the other hand, the same glass transition temperature determined by DMA is shifted to lower temperature in comparison with PHB. The main
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difference between samples analyzed by DSC and DMA is, in the first case, that blends are submitted to heating up to 200 C and in second case, blends present characteristics imposed by the rate of solvent evaporation. PHB/PEP blends analyzed by DMA possibly did not achieve the thermodynamic equilibrium and the phase segregation was not complete. The opposite is observed for PHB/ECO blends. From these results, PHB/PEP and PHB/ECO blends are immiscible over the whole composition range studied. However, other reported works describe PHB/PEP and PHB/ECO blends as a miscible system [12,20–25]. Shafee [12] studied binary blends of PHB/PEP, prepared by casting from dichloromethane solution, as a function of blend composition and crystallization conditions, by dielectric relaxation spectroscopy. The molecular weight of PHB utilized was 105,000 g mol1 and PEP was 700,000 g mol1. The quenched samples were miscible over the whole composition range, exhibiting only one glass transition, which could be reasonably described as a function of composition by the Gordon–Taylor equation. Finelli et al. [24] used the highest molecular weight of PHB, M n 539,000 g mol1, reported to prepare blends with epichlorohydrin elastomers. PHB and PEP with M w equal to 700,000 g mol1 were dissolved in dichloromethane to obtain films by solvent evaporation. Purification of PEP was carried out through dissolution in dichloromethane to purify and remove
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Fig. 2. (A) Loss modulus (E00 ) and (B) tan d as a function of temperature: (s) PHB, () PEP and (D) PHB/PEP blend: 80 wt% PEP.
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00
Fig. 3. (A) Loss modulus (E ) and (B) tan d as a function of temperature: (s) PHB, () ECO and (D) PHB/ECO blend: 20 wt% ECO.
the impurities and gel fraction by filtration. The films were inserted between two aluminum plates and were compression molded by heating. PHB/PEP blends were investigated over the whole range composition by DSC, DMA and optical microscopy. Blends are miscible in the melt in all proportions. After melt quenching, PHB/PEP blends show a single glass transition that linearly changes with composition between the glass transition temperatures of the pure components. Loss modulus curves show a broad relaxation peak attributed to the glass transition, it increases with decreasing of PEP content. Fig. 4 shows the amorphous character of PEP and ECO by the catastrophic drop of the elastic storage modulus (E 0 ) at the glass transition (more than two orders of magnitude). It is interesting to compare the experimental values of E 0 as a function of blend compositions at a selected temperature above the Tg of both phases but lower than melting temperature of PHB, for example 75 C. Above the Tg of both phases of blends exist three phases: a liquid PHB, a crystalline PHB and an elastomeric PEP or ECO phase. Fig. 5 shows the experimental storage modulus (E 0 ) for the blends at 75 C as a function of pure polymer or blend compositions. PHB/PEP blends present relatively high storage moduli (108 Pa) up to 60 wt% of PEP. Notably, small amounts of crystalline PHB lead to an increase of the E 0 about two
or three orders of magnitude as compared with the elastomer modulus. This suggests that PHB is the continuous phase and PEP is the disperse phase in the blends. However, to the blend with 80 wt% of PEP the storage modulus decrease to approximately 106 Pa, the same order of magnitude of PEP. This fact suggests that the blend with 80 wt% of PEP the continuous phase is PEP and PHB is the disperse phase. The storage modulus curves (Fig. 4) present an increase of the value around 30 C, corresponding to the crystallization. This crystallization is more pronounced for PHB/PEP blends. Finelli et al. [24] also studied the dynamic mechanical properties of PHB/PEP blends and found that storage modulus at 75 C decreases monotonically with the increasing of PEP content in the blends. This result is typical of miscible blends and it reinforces the hypothesis that the blends in our study are really heterogeneous. PHB/ECO blends show storage modulus with the same order of magnitude as the PHB phase, thus the continuous phase is PHB and ECO is the dispersed phase for all compositions. 3.2. Melting behavior As shown in Fig. 1A, PHB cooled from the melted state at 200 C at a controlled cooling rate shows only one melting peak during the second
J.A. de Lima, M.I. Felisberti / European Polymer Journal 42 (2006) 602–614 10
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Fig. 4. Storage modulus (E 0 ) as a function of temperature for blends: (A) PHB/PEP and (B) PHB/ECO.
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E´(Pa) (75 °C)
10
8
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Elastomer content (wt. %) Fig. 5. Storage modulus (E 0 ) at 75 C as a function of elastomer content for blends: (s) PHB/PEP and (h) PHB/ECO.
heating run whereas the PHB/PEP blends exhibit an exothermic peak corresponding to PHB crystallization followed by the PHB melting peak. The apparent melting temperature (Tm) of PHB decreases slightly with increasing elastomeric content, from 175 C for pure PHB to 169 C for the blend with 20 wt% of PHB and 80 wt% of PEP. Similar behavior can be observed for PHB/ECO blends (Fig. 1B)
and Tm of pure PHB is the same for both cases, at 200 C and 215 C. A further useful indication of miscibility is the melting point depression of a crystalline polymer in the presence of an amorphous diluent. However, this effect has to be correlated to other miscibility evidence, since it has also been observed for immiscible blends when some morphological changes of the crystalline component occur [25–27]. For the blends of this work, the melting point depression can not be associated to miscibility because the blends are immiscible. The apparent Tm of the blends is thus, affected by morphological effects. There are many polymers that do not crystallize completely upon cooling, but when they are heated it is very common to observe crystallization at temperatures lower than the melting temperature. This process is known as cold crystallization. Normally PHB undergoes also cold crystallization [5], but in our case, when PHB is heated up to 200 C it crystallizes completely during the cooling and no cold crystallization is observed during the second heating (Fig. 1A). However, blends with PEP and ECO with similar thermal histories of pure PHB present cold crystallization at temperatures (T cH ) dependent on the elastomer characteristics and contents (Table 3). It means that the elastomers influence the PHB crystallization. The crystallization enthalpy corresponding to cold crystallization (DH cH ) of PHB in the PHB/
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PEP blends increases with increasing of PEP content (Table 3). But the enthalpy of fusion (Table 2) increases with decreasing of PEP content. It implies that a fraction of PHB crystallizes during the cooling. This tendency is also observed for PHB/ECO blends, however in this case the cold crystallization is more intense, indicating that the crystallization of PHB during the cooling is more hindered in presence of ECO than in presence of PEP. The molecular weight of PEP and ECO are very close, therefore the difference observed for the kinetic of crystallization should be related more to the molecular differences between elastomers than to the molecular weight. Fig. 6 shows the DSC curves for polymers and their blends corresponding to cooling from 200 C and 215 C. The crystallization temperature and enthalpy under cooling (T cc and DH cc , respectively) are shown in Table 3. The influence of PEP and ECO on the crystallization of PHB is possible to observe even when the blends are immiscible. For example, the blend with 60 wt% of PHB and 40 wt% of PEP (Fig. 6A) presents an expressive decrease of the intensity of the peak at crystallization, that is completed during the second heating (Fig. 1A). DSC curves corresponding to the second heating after controlled cooling from 230 C for PHB/PEP blends are shown in Fig. 7. The DSC curve for PHB shows an exothermic peak corresponding to cold crystallization followed by two endothermic
peaks, a shoulder at lower temperature and a main endothermic peak, which arises from the recrystallization process of PHB [21]. Pure PHB submitted to an initial heating to 230 C exhibits the melting temperature at lower temperature than PHB submitted to only 200 C or 215 C.
20% 40% 50% 60% 80%
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80
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Temperature (°C) Fig. 7. DSC curves of PHB/PEP blends submitted to heating up to 230 C. Second heating at 20 C min1.
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(B) PHB 100%
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,75
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150
0
50
100
150
Temperature (°C)
Fig. 6. Non-isothermal crystallization behavior for blends on controlled cooling: (A) from 200 C for PHB/PEP blends and (B) from 215 C for PHB/ECO, at a cooling rate of 20 C min1.
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It is attributed to the thermal degradation accompanied by a decrease in molecular weight [28–32]. The probability that the crystalline history of PHB is completely destroyed at 230 C is high. So, this fact associated to the thermal degradation of PHB, may be responsible for the non-crystallization during the controlled cooling because PHB only crystallized during the second heating cycle [33,34]. Table 2 shows the heat of fusion (DHm) and the degree of crystallinity (Xc), both normalized with respect to PHB contents in the blends with PEP or ECO. The Xc of PHB in the blends was calculated
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assuming that the DHm value of 100% crystalline PHB is 151 J/g [6]. The degree of crystallinity of PHB/PEP blends decreases with an increase in the PEP content. PHB/ECO blends present degrees of crystallinity that can be considered nearly independent of the ECO content. 3.3. Isothermal crystallization Fig. 8 shows the optical micrographs of pure PHB and PHB in the PHB/PEP blends submitted to isothermal crystallization at 60 C and 70 C.
Fig. 8. Polarizing optical micrographs of samples isothermally crystallized at 60 C and 70 C: pure PHB (A) and PHB/PEP blends (B) 20 wt% PEP, (C) 40 wt% PEP and (D) 60 wt% PEP.
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The size and the average size distribution of crystals, as well as, their shape and orientation determine the melt behavior of the material and depend exclusively on the conditions of nucleation and growth of crystals. The size of spherulites depends on the ratio between the nucleation rate and the growth rate of crystals, thus the kinetics of crystallization determines the morphology and the degree of crystallinity of the polymer. The micrographs of PHB and its blends show that PHB crystallizes in a spherulitic form. The spherulites show the familiar Maltese cross birrefringent pattern and exhibit concentric extinction bands. The texture of PHB spherulites becomes more open with increasing PEP content in the blends (Fig. 8). In addition, the band spacing becomes larger with the increase in non-crystallizable component concentration, even though the blends are immiscible. The number of spherulites in the PHB crystallized at 60 C is higher than to PHB crystallized at
70 C (Fig. 8A). It means that the nucleation rate is higher than growth rate at 60 C. For blends with PEP, the same effect is observed (Fig. 8B, C and D). Similarly to PHB/PEP blends the presence of ECO elastomer affects PHB crystallization (Fig. 9). Fig. 10 shows optical micrographs of blends containing 50 wt% of elastomer, PEP or ECO, crystallized isothermally at 70 C for 2 h. The texture of PHB spherulites is strongly influenced by the elastomer nature. The micrographs show a volume completely filled with the spherulites and no elastomeric phase was observed among the spherulites. This observation and the fact that the elastomers influence the crystallization kinetics and morphology of crystalline PHB suggest that the non-crystallized component is segregated in the intraspherulitic zones. PEP and ECO molecules diffuse away from the front of PHB crystallization at a rate not sufficient to let them move away from the spherulites.
Fig. 9. Polarizing optical micrographs of PHB/ECO blends isothermally crystallized at 60 C and 70 C: (A) 20 wt% ECO, (B) 40 wt% ECO and (C) 60 wt% ECO.
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Fig. 10. Polarizing optical micrographs of samples contaning 50 wt% of PHB isothermally crystallized at 70 C: (A) PHB/PEP and (B) PHB/ECO.
4. Conclusions The results from the DSC and DMA show that PHB/PEP and PHB/ECO blends are immiscible. The apparent melting temperature (Tm) of PHB in the blends decreases slightly with increasing elastomeric content and the melting point depression cannot be associated to miscibility, because the blends are immiscible. Thus, Tm of the blends are affected by morphological effects. There is an influence of PEP and ECO on the crystallization that occurs upon cooling of PHB, even when the blends are immiscible due to an expressive decrease of the intensity of the peak at crystallization, that is completed during the second heating. The degree of crystallinity of blends with PEP has been found to decrease with an increase in PEP content. PHB/ECO blends present degrees of crystallinity that can be considered nearly independent of the ECO content. The study of the morphology of blends shown that the presence of elastomer influences the ratio of the growth rate and the nucleation rate. The elastomer component, probably resides in the intraspherulitic zones. Acknowledgements The authors CNPq.
acknowledge
fellowship
from
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European Polymer Journal 42 (2006) 602–614
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Poly(hydroxybutyrate) and epichlorohydrin elastomers blends: Phase behavior and morphology Juliana Ariste´ia de Lima, Maria Isabel Felisberti
*
Instituto de Quı´mica, Universidade Estadual de Campinas, CP 6154, 13083-970 Campinas, SP, Brazil Received 19 May 2005; received in revised form 8 August 2005; accepted 12 September 2005 Available online 18 October 2005
Abstract This work studied blends of PHB with epichlorohydrin elastomers, the PEP homopolymer and its copolymer with ethylene oxide, ECO. PHB is a microbial polyester, which is accumulated intracellularly by a large number of microorganisms, presenting characteristics of biodegradability and biocompatibility. It presents a high degree of crystallinity, so is a quite brittle material, and may undergo degradation when is kept for a relatively short time at a temperature above its melting point, about 180 C. PEP and ECO are linear and amorphous elastomers, exhibit miscibility with many aliphatic polyesters and these elastomers have been used in various branches of technology, such as the automotive industry. The proposed systems combine a polymer with high crystallinity and biodegradability, PHB, with amorphous epichlorohydrin elastomers. Blends were prepared by casting from chloroform solution at different compositions (0, 20, 40, 50, 60, 80 and 100 wt% of PHB). The phase behavior of PHB/PEP and PHB/ECO blends were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and the morphology of the crystalline phase of PHB had been examined by optical microscopy. Blends of PHB/PEP and PHB/ECO have been described in literature as miscible. However, our results from the DSC and DMA show that PHB/PEP and PHB/ECO blends are immiscible. This behavior should be related to the molecular weight of polymers used in the present work, which is higher than the molecular weight of polymers used in the previous works. The crystallization kinetics of PHB is strongly influenced by the presence of the elastomeric phase. The degree of crystallinity of PHB/PEP blends decreases with an increase in the PEP content. PHB/ECO blends present degrees of crystallinity that can be considered nearly independent of the ECO content. Differences in the morphology of the crystalline phase were also observed, and these are attributed to the presence of elastomeric phase in the intraspherulitic zone. 2005 Elsevier Ltd. All rights reserved. Keywords: Blends; Miscibility; Morphology
1. Introduction
*
Corresponding author. Tel.: +55 19 37883130; fax: +55 19 37883028. E-mail address: [email protected] (M.I. Felisberti).
There have been many studies of polymer blends in recent years. The interest in novel structures and properties of blends still motivates extensive studies, as does the increasing importance in
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.09.004
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practical applications of new polymeric materials having a wide range of physicochemical properties, with behavior different from the homopolymers and copolymers [1]. An important and determinant aspect of the properties of a blend is the miscibility of their components. Miscibility in polymer blends is assigned to specific interactions between the polymer components, which usually gives rise to a negative free energy of mixing, in spite of the high molecular weight of the polymers. The most common interactions present in blends are: hydrogen bond, p-electrons and ionic and dipole interactions, and charge-transfer complexes [2]. Recently, the interest of polymer blends development has been concentrated on systems in which at least one of the components is crystallizable. Attention has focused on the various morphologies resulting from the crystallization of the melt-miscible phase of semicrystalline/amorphous blends. The components of the blends studied in the present work are a semicrystalline polyester, poly(3hydroxybutyrate) (PHB), and an amorphous rubbery component, poly(epichlorohydrin) (PEP) and its copolymer with ethylene oxide, poly(epichlorohydrin-co-ethylene oxide) (ECO). Optically active PHB is an aliphatic polyester produced via biosynthesis by bacterial fermentation. PHB is a thermoplastic with a high degree of crystallinity and a well-defined melting point (around 180 C). Unlike other thermoplastic polymers, such as poly(propylene) and poly(ethylene), PHB is biodegradable and biocompatible [3,4], presenting potential in applications such as medical materials, absorbable surgical sutures, matrices for drug delivery systems and as biodegradable molded plastics [5]. However on undercooling PHB crystallizes from the melt, giving rise to the formation of large spherulites with a brittle behavior. Furthermore, PHB is thermally unstable at temperatures close to the melting point and degrades through chain scission resulting in terminations of crotonic acid and vinyl groups [3]. Thus, PHB has two limitations in its use: a very narrow processability window, and a relatively low impact resistance due to its high degree of crystallinity. These drawbacks have hampered the utilization of PHB as a common plastic. Blends of PHB with other polymers can offer opportunities to extend and explore their many useful and interesting properties and to modify its undesirable properties. For example, PHB has been blended with poly(ethylene
603
oxide) [3], poly(vinyl butyral) [6], poly(vinyl acetate) [7], poly(vinylphenol) [8], cellulose acetate butyrate [9], chitin and chitosan [10]. PEP and ECO are linear amorphous elastomers, exhibiting glass transition temperatures at 23 C and 41 C, respectively [1,11]. Epichlorohydrin elastomers exhibit miscibility with many aliphatic polyesters [12] and these elastomers have been used in various branches of technology, such as the automotive industry, in fuel, lubricating fluids, in air and vacuum hoses, etc. [13]. There are few published papers that deal with PEP and the majority of these publications were about its properties, characterization and chemical modification [14–17]. PEP has been studied in blends with poly(ethylene oxide) [1], poly(vinyl acetate) [12], poly(styrene-co-acrylonitrile) [18] and poly(methyl methacrylate) [19]. The miscibility of PHB/PEP and PHB/ECO blends has been studied [12,20–25] and the authors have found that these blends are miscible, but the polymers used to prepare the blends present different characteristic from those used in this work, as the molecular weight. In this work, the influence of the PEP and its copolymer with ethylene oxide, ECO, on the miscibility with PHB was evaluated by an investigation of the thermal behavior, the crystallization and the morphology as a function of blend composition. 2. Experimental 2.1. Polymer materials The polymers used in this work were commercial products whose properties are listed in Table 1. 2.2. Cast films Binary blends of PHB/PEP and PHB/ECO of different compositions were prepared by casting from 5% (w/v) chloroform solutions. The solvent was Table 1 Molecular weight (M w Þ and polydispersity M w =M n of the PHB, PEP and ECO used to obtain the blends Polymer Source Mw M w =M n a 1 a (g mol ) PHB PEP ECO (48 mol% epichlorohydrin) a
PHB Industrial 450,000 Zeon Chem. Inc. 900,000 Zeon Chem. Inc. 900,000
GPC in chloroform at 40 C.
– 3.3 5.0
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allowed to evaporate slowly at room temperature (about 25 C) for 48 h, followed by drying in vacuum at room temperature for 24 h, and at 90 C for 4 h. 2.3. Differential scanning calorimetry Differential scanning calorimetry (DSC) was carried out using a MDSC 2910 TA Instruments equipment. The experiments were performed according to the following program: • Initial temperature 80 C; • First heating: heating rate of 20 C min1 to 200 C; • Cooling: cooling rate of 20 C min1 to 80 C; • Second heating: heating rate of 20 C min1 to 200 C. The same experiments were repeated, using a maximum temperature of 215 C for PHB/ECO blends and 230 C for PHB/PEP blends. The results reported in this work correspond to the cooling and second heating runs. All DSC curves were normalized with respect to the sample mass. 2.4. Dynamic mechanical thermal analysis A DMTA V Rheometrics Scientific was used. A film of polymer samples (8.0 · 1.5 · 5.0 mm) was clamped in a frame for tension tests. A frequency of 1 Hz, amplitude of 0.03% and heating rate of 2 C min1 from 100 C to 200 C were applied. The storage modulus, loss modulus and loss factor or damping were recorded as a function of temperature. 2.5. Polarizing optical microscopy The morphologies of the PHB crystalline phase in pure samples and in the blends were observed with a Nikon E800 Polarizing Optical Microscope. Sample sandwiched between two thin glass slides was melted for 1 min on a hot plate preheated to 200 C for PHB, PHB/PEP blends and PHB/ECO blends. Then, it was quickly transferred to the hot plate which was maintained at a desired temperature, isothermal crystallization, at 60 C and 70 C, for 2 h. 3. Results and discussion Fig. 1 shows DSC curves corresponding to the second heating for the two systems of blends,
PHB/PEP and PHB/ECO, submitted to a maximum temperature of 200 C and 215 C, respectively. The curves show glass transitions around 0 C, an exothermic peak and an endothermic peak corresponding to crystallization and melting of PHB phase, respectively. The glass transition temperature (Tg; assumed as the temperature corresponding to the half width of the transition), crystallization and melting temperature (Tc and Tm), crystallization and melting enthalpies (DHc and DHm) normalized with respect to the PHB content in the blends and degree of crystallinity (Xc) taken from the DSC curves (Figs. 1, 6 and 7) are listed in Tables 2 and 3. 3.1. Glass transition behavior The homopolymer PHB presents Tg at 5 C and PEP at 23 C. The copolymer ECO presents glass transition temperature at 39 C. The DSC curve obtained for blend PHB/PEP with 20 wt% of PHB shows a single glass transition in the same temperature range observed for PEP (Fig. 1A). This suggests the existence of a rich elastomeric phase and at least another phase, whose glass transition could not be determined by DSC. The others blends compositions show clearly two glass transitions not shifted from those observed for the individual polymers, indicating the coexistence of an elastomeric phase (PEP) and a PHB phase. Possibly the viscosity of elastomer is related to the extension of the phase segregation during the evaporation of the solvent, showing differences in phase behavior of each blend. Paglia et al. [20] studied the crystallization and thermal behavior of PHB/PEP blends. The binary blends were prepared by casting from dichloromethane solution over a wide range of composition and analyzed by DSC and microscopy. The molecular weight ðM w Þ of PHB utilized was 150,000 g mol1 and M w of PEP was 700,000 g mol1. The thermal analysis of the blends showed a single glass transition, whose values fit the Fox equation. Moreover, the melting point of PHB decreases with the blending and the negative values of the interaction parameters (v12) of the PHB/PEP systems suggest that the two components can form a miscible blend in the amorphous phase which is thermodynamically stable above the equilibrium melting temperature. The comparison between the results from PagliaÕs work and from our work shows that the molecular weight of PHB and PEP affects the
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605 0% 20%
0% 20% 40% 50%
40% 50%
60%
60% 80% .5
80%
0
PHB 100%
Exo
5
Exo
5
-3.0
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.50
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(B) 0
60
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-60
0
60
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180
240
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0% 20%
0% 5
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8 0%
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PHB 100%
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-15
0
15
30
45
Temperature (°C)
- 60
-45
-30
-15
0
15
30
45
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Fig. 1. DSC curves of: (A) PHB/PEP blends submitted to heating up to 200 C and (B) PHB/ECO blends submitted to heating up to 215 C. Second heating at 20 C min1.
miscibility, as the thermodynamic of polymer solutions predicts. PHB/ECO blends show two glass transitions and the transition at high temperature is shifted to a lower temperature than the observed for pure PHB (Fig. 1B). The dependence of the Tg at high temperature on the composition suggests the existence of an elastomeric phase (ECO) and another phase that could be a mixture of PHB and ECO in PHB/ECO blends. The observed effect could be attributed to the presence of ethylene oxide segments in ECO chains, that should improve miscibility with PHB. The miscibility of PHB and poly(ethylene oxide)
has been described in the literature [7]. Only the blend with 80 wt% of PHB and 20 wt% of ECO seems to present a single glass transition. Zhang et al. [21] studied the miscibility, melting and crystallization behavior of PHB/ECO blends by DSC and optical microscopy. Blends of varying compositions were prepared by casting from chloroform solution using PHB with M w equal to 230,000 g mol1 and ECO with M w equal to 440,000 g mol1. They concluded that the blends are miscible over the whole composition range. The glass transition temperature behavior of blends could be described by the Kwei equation. Negative interaction parameters are
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Table 2 Thermal properties of PHB/PEP and PHB/ECO blends PHB/PEP
Tg (C) (from E00 · T)
Tg (C) (from tan d · T)
Tg (C) (from DSC) (Fig. 1A)
Tm (C) (from DSC) (Fig. 1A)
Tm (C) (from DSC) (Fig. 7)
DHm (J/g) (from DSC) (Fig. 7)
Xc (%) (from DSC) (Fig. 7)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
16 17; 16; 16; 16; 15; 15
8 12; 14; 15; 16; 14; 22
23 18 18; 18; 18; 17; 5
– 169 171 174 172 170 175
– 156 170 173 171 171 160
– 29 65 77 82 85 90
– 19 43 51 54 56 60
PHB/ECO
Tg (C) (from E00 · T)
Tg (C) (from tan d · T)
Tg (C) (from DSC) (Fig. 1B)
Tm (C) (from DSC) (Fig. 1B)
– –
DHm (J/g) (from DSC) (Fig. 1B)
Xc (%) (from DSC) (Fig. 1B)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
38 39; 40; 40; 39; 35; 15
32 35; 42; 40; 35; 34; 22
39 40; 42; 43; 46; 4 5
– 167 169 170 171 173 175
– – – – – – –
– 80 82 82 83 85 87
– 53 54 54 55 56 58
10 6 4 0 13
10 15 15 15 14
6 2 5 16 20
17 19 13 21 21
4 3 5 6
23 16 14 12
Table 3 Crystallization temperature on cooling (T cc ) from the melting state at 200 C and 215 C, crystallization temperature on second heating (T cH ), crystallization enthalpy on cooling (DH cc ), crystallization enthalpy during the second heating (DH cH ) and total crystallization enthalpy (DH ctotal ) PHB/PEP
T cc (C) (from Fig. 6)
DH cc (J/g) (from Fig. 6)
T cH (C) (from Fig. 1)
DH cH (J/g) (from Fig. 1)
DH ctotal (J/g)* (from Figs. 1 and 6)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
– – 70 71 72 86 76
– – 10 37 44 68 61
– 62 68 69 69 – –
– 20 16 4 5 – –
– 20 26 41 49 68 61
PHB/ECO
T cc (C) (from Fig. 6)
DH cc (J/g) (from Fig. 6)
T cH (C) (from Fig. 1)
DH cH (J/g) (from Fig. 1)
DH ctotal (J/g)* (from Figs. 1 and 6)
0:100 20:80 40:60 50:50 60:40 80:20 100:0
– – – 56 58 59 52
– – – 8 10 11 18
– 81 55 57 55 48 59
– 77 48 31 19 27 50
– 77 48 39 29 38 68
Obtained from DSC curves. * It is the sum of the crystallization enthalpy during the cooling and second heating (cold crystallization).
obtained from melting point depression data. Crystallization of PHB from the melt and from the glassy state is affected by addition of ECO. The molecular weight of PHB and ECO used in ZhangÕs work are smaller than the molecular weight used in our work. The differences from ZhangÕs conclusions and our conclusions are attributed to the molecular weight effect.
The second glass transition of some blends was not possible to determine by DSC. Because of this, dynamic mechanical analysis, a sensitive technique to study polymer relaxation, was used. The samples analyzed by DMA possibly present thermal histories corresponding to the formation of the films by solvent evaporation and this condition guarantees
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the maximum crystallization of sample, while the thermal histories of the samples analyzed by DSC were erased during the first heating. In DMA, the glass transition temperature (Tg) was assumed to be the temperature corresponding to the maximum of the peaks in the E00 · T curves or the maximum of the peaks in the tan d · T curves. The glass transition temperatures obtained from E00 · T and tan d · T are presented in Table 2. Figs. 2 and 3 show the loss modulus curves (E00 · T) and damping curves (tan d · T) for PHB/ PEP blend containing 80 wt% of PEP and PHB/ ECO blend containing 20 wt% of ECO, respectively. A second peak or a shoulder on the peaks is not easy to find in the E00 · T curves, except for the blend containing 20 wt% of ECO, for which the peak is broad and asymmetric. The first and second derivative curves were used to determine the temperature corresponding to the shoulder. The analysis of the dependence of Tg on blend compositions reveal (Table 2) that the most of blends present a glass transition in the characteristic temperature range of the glass transition of the corresponding elastomer. The Tg of the second phase is strongly dependent on the thermal history of the samples. Thus, the higher Tg determined by DSC for PHB/PEP blends is close to PHB glass transition. On the other hand, the same glass transition temperature determined by DMA is shifted to lower temperature in comparison with PHB. The main
607
difference between samples analyzed by DSC and DMA is, in the first case, that blends are submitted to heating up to 200 C and in second case, blends present characteristics imposed by the rate of solvent evaporation. PHB/PEP blends analyzed by DMA possibly did not achieve the thermodynamic equilibrium and the phase segregation was not complete. The opposite is observed for PHB/ECO blends. From these results, PHB/PEP and PHB/ECO blends are immiscible over the whole composition range studied. However, other reported works describe PHB/PEP and PHB/ECO blends as a miscible system [12,20–25]. Shafee [12] studied binary blends of PHB/PEP, prepared by casting from dichloromethane solution, as a function of blend composition and crystallization conditions, by dielectric relaxation spectroscopy. The molecular weight of PHB utilized was 105,000 g mol1 and PEP was 700,000 g mol1. The quenched samples were miscible over the whole composition range, exhibiting only one glass transition, which could be reasonably described as a function of composition by the Gordon–Taylor equation. Finelli et al. [24] used the highest molecular weight of PHB, M n 539,000 g mol1, reported to prepare blends with epichlorohydrin elastomers. PHB and PEP with M w equal to 700,000 g mol1 were dissolved in dichloromethane to obtain films by solvent evaporation. Purification of PEP was carried out through dissolution in dichloromethane to purify and remove
1013
(A)
(B) 10
0
12
tan δ
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10
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10
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-1
10
-2
1010
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0
15
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Fig. 2. (A) Loss modulus (E00 ) and (B) tan d as a function of temperature: (s) PHB, () PEP and (D) PHB/PEP blend: 80 wt% PEP.
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(A)
(B)
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12
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tan δ
E" (Pa)
100
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10
9
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0
15
30
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45
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00
Fig. 3. (A) Loss modulus (E ) and (B) tan d as a function of temperature: (s) PHB, () ECO and (D) PHB/ECO blend: 20 wt% ECO.
the impurities and gel fraction by filtration. The films were inserted between two aluminum plates and were compression molded by heating. PHB/PEP blends were investigated over the whole range composition by DSC, DMA and optical microscopy. Blends are miscible in the melt in all proportions. After melt quenching, PHB/PEP blends show a single glass transition that linearly changes with composition between the glass transition temperatures of the pure components. Loss modulus curves show a broad relaxation peak attributed to the glass transition, it increases with decreasing of PEP content. Fig. 4 shows the amorphous character of PEP and ECO by the catastrophic drop of the elastic storage modulus (E 0 ) at the glass transition (more than two orders of magnitude). It is interesting to compare the experimental values of E 0 as a function of blend compositions at a selected temperature above the Tg of both phases but lower than melting temperature of PHB, for example 75 C. Above the Tg of both phases of blends exist three phases: a liquid PHB, a crystalline PHB and an elastomeric PEP or ECO phase. Fig. 5 shows the experimental storage modulus (E 0 ) for the blends at 75 C as a function of pure polymer or blend compositions. PHB/PEP blends present relatively high storage moduli (108 Pa) up to 60 wt% of PEP. Notably, small amounts of crystalline PHB lead to an increase of the E 0 about two
or three orders of magnitude as compared with the elastomer modulus. This suggests that PHB is the continuous phase and PEP is the disperse phase in the blends. However, to the blend with 80 wt% of PEP the storage modulus decrease to approximately 106 Pa, the same order of magnitude of PEP. This fact suggests that the blend with 80 wt% of PEP the continuous phase is PEP and PHB is the disperse phase. The storage modulus curves (Fig. 4) present an increase of the value around 30 C, corresponding to the crystallization. This crystallization is more pronounced for PHB/PEP blends. Finelli et al. [24] also studied the dynamic mechanical properties of PHB/PEP blends and found that storage modulus at 75 C decreases monotonically with the increasing of PEP content in the blends. This result is typical of miscible blends and it reinforces the hypothesis that the blends in our study are really heterogeneous. PHB/ECO blends show storage modulus with the same order of magnitude as the PHB phase, thus the continuous phase is PHB and ECO is the dispersed phase for all compositions. 3.2. Melting behavior As shown in Fig. 1A, PHB cooled from the melted state at 200 C at a controlled cooling rate shows only one melting peak during the second
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9
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200
Temperature (°C)
Fig. 4. Storage modulus (E 0 ) as a function of temperature for blends: (A) PHB/PEP and (B) PHB/ECO.
10
10
9
E´(Pa) (75 °C)
10
8
10
7
10
6
10
5
10
0
20
40
60
80
100
Elastomer content (wt. %) Fig. 5. Storage modulus (E 0 ) at 75 C as a function of elastomer content for blends: (s) PHB/PEP and (h) PHB/ECO.
heating run whereas the PHB/PEP blends exhibit an exothermic peak corresponding to PHB crystallization followed by the PHB melting peak. The apparent melting temperature (Tm) of PHB decreases slightly with increasing elastomeric content, from 175 C for pure PHB to 169 C for the blend with 20 wt% of PHB and 80 wt% of PEP. Similar behavior can be observed for PHB/ECO blends (Fig. 1B)
and Tm of pure PHB is the same for both cases, at 200 C and 215 C. A further useful indication of miscibility is the melting point depression of a crystalline polymer in the presence of an amorphous diluent. However, this effect has to be correlated to other miscibility evidence, since it has also been observed for immiscible blends when some morphological changes of the crystalline component occur [25–27]. For the blends of this work, the melting point depression can not be associated to miscibility because the blends are immiscible. The apparent Tm of the blends is thus, affected by morphological effects. There are many polymers that do not crystallize completely upon cooling, but when they are heated it is very common to observe crystallization at temperatures lower than the melting temperature. This process is known as cold crystallization. Normally PHB undergoes also cold crystallization [5], but in our case, when PHB is heated up to 200 C it crystallizes completely during the cooling and no cold crystallization is observed during the second heating (Fig. 1A). However, blends with PEP and ECO with similar thermal histories of pure PHB present cold crystallization at temperatures (T cH ) dependent on the elastomer characteristics and contents (Table 3). It means that the elastomers influence the PHB crystallization. The crystallization enthalpy corresponding to cold crystallization (DH cH ) of PHB in the PHB/
J.A. de Lima, M.I. Felisberti / European Polymer Journal 42 (2006) 602–614
PEP blends increases with increasing of PEP content (Table 3). But the enthalpy of fusion (Table 2) increases with decreasing of PEP content. It implies that a fraction of PHB crystallizes during the cooling. This tendency is also observed for PHB/ECO blends, however in this case the cold crystallization is more intense, indicating that the crystallization of PHB during the cooling is more hindered in presence of ECO than in presence of PEP. The molecular weight of PEP and ECO are very close, therefore the difference observed for the kinetic of crystallization should be related more to the molecular differences between elastomers than to the molecular weight. Fig. 6 shows the DSC curves for polymers and their blends corresponding to cooling from 200 C and 215 C. The crystallization temperature and enthalpy under cooling (T cc and DH cc , respectively) are shown in Table 3. The influence of PEP and ECO on the crystallization of PHB is possible to observe even when the blends are immiscible. For example, the blend with 60 wt% of PHB and 40 wt% of PEP (Fig. 6A) presents an expressive decrease of the intensity of the peak at crystallization, that is completed during the second heating (Fig. 1A). DSC curves corresponding to the second heating after controlled cooling from 230 C for PHB/PEP blends are shown in Fig. 7. The DSC curve for PHB shows an exothermic peak corresponding to cold crystallization followed by two endothermic
peaks, a shoulder at lower temperature and a main endothermic peak, which arises from the recrystallization process of PHB [21]. Pure PHB submitted to an initial heating to 230 C exhibits the melting temperature at lower temperature than PHB submitted to only 200 C or 215 C.
20% 40% 50% 60% 80%
Exo
610
PHB 100%
40
80
120
160
200
240
Temperature (°C) Fig. 7. DSC curves of PHB/PEP blends submitted to heating up to 230 C. Second heating at 20 C min1.
(A)
PHB 100%
(B) PHB 100%
80% 60%
50%
60%
Exo
Exo
80%
50%
,25
40% 20%
40% ,50
20%
0%
0%
,75
,00
0
50
100
Temperature (°C)
150
0
50
100
150
Temperature (°C)
Fig. 6. Non-isothermal crystallization behavior for blends on controlled cooling: (A) from 200 C for PHB/PEP blends and (B) from 215 C for PHB/ECO, at a cooling rate of 20 C min1.
J.A. de Lima, M.I. Felisberti / European Polymer Journal 42 (2006) 602–614
It is attributed to the thermal degradation accompanied by a decrease in molecular weight [28–32]. The probability that the crystalline history of PHB is completely destroyed at 230 C is high. So, this fact associated to the thermal degradation of PHB, may be responsible for the non-crystallization during the controlled cooling because PHB only crystallized during the second heating cycle [33,34]. Table 2 shows the heat of fusion (DHm) and the degree of crystallinity (Xc), both normalized with respect to PHB contents in the blends with PEP or ECO. The Xc of PHB in the blends was calculated
611
assuming that the DHm value of 100% crystalline PHB is 151 J/g [6]. The degree of crystallinity of PHB/PEP blends decreases with an increase in the PEP content. PHB/ECO blends present degrees of crystallinity that can be considered nearly independent of the ECO content. 3.3. Isothermal crystallization Fig. 8 shows the optical micrographs of pure PHB and PHB in the PHB/PEP blends submitted to isothermal crystallization at 60 C and 70 C.
Fig. 8. Polarizing optical micrographs of samples isothermally crystallized at 60 C and 70 C: pure PHB (A) and PHB/PEP blends (B) 20 wt% PEP, (C) 40 wt% PEP and (D) 60 wt% PEP.
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The size and the average size distribution of crystals, as well as, their shape and orientation determine the melt behavior of the material and depend exclusively on the conditions of nucleation and growth of crystals. The size of spherulites depends on the ratio between the nucleation rate and the growth rate of crystals, thus the kinetics of crystallization determines the morphology and the degree of crystallinity of the polymer. The micrographs of PHB and its blends show that PHB crystallizes in a spherulitic form. The spherulites show the familiar Maltese cross birrefringent pattern and exhibit concentric extinction bands. The texture of PHB spherulites becomes more open with increasing PEP content in the blends (Fig. 8). In addition, the band spacing becomes larger with the increase in non-crystallizable component concentration, even though the blends are immiscible. The number of spherulites in the PHB crystallized at 60 C is higher than to PHB crystallized at
70 C (Fig. 8A). It means that the nucleation rate is higher than growth rate at 60 C. For blends with PEP, the same effect is observed (Fig. 8B, C and D). Similarly to PHB/PEP blends the presence of ECO elastomer affects PHB crystallization (Fig. 9). Fig. 10 shows optical micrographs of blends containing 50 wt% of elastomer, PEP or ECO, crystallized isothermally at 70 C for 2 h. The texture of PHB spherulites is strongly influenced by the elastomer nature. The micrographs show a volume completely filled with the spherulites and no elastomeric phase was observed among the spherulites. This observation and the fact that the elastomers influence the crystallization kinetics and morphology of crystalline PHB suggest that the non-crystallized component is segregated in the intraspherulitic zones. PEP and ECO molecules diffuse away from the front of PHB crystallization at a rate not sufficient to let them move away from the spherulites.
Fig. 9. Polarizing optical micrographs of PHB/ECO blends isothermally crystallized at 60 C and 70 C: (A) 20 wt% ECO, (B) 40 wt% ECO and (C) 60 wt% ECO.
J.A. de Lima, M.I. Felisberti / European Polymer Journal 42 (2006) 602–614
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Fig. 10. Polarizing optical micrographs of samples contaning 50 wt% of PHB isothermally crystallized at 70 C: (A) PHB/PEP and (B) PHB/ECO.
4. Conclusions The results from the DSC and DMA show that PHB/PEP and PHB/ECO blends are immiscible. The apparent melting temperature (Tm) of PHB in the blends decreases slightly with increasing elastomeric content and the melting point depression cannot be associated to miscibility, because the blends are immiscible. Thus, Tm of the blends are affected by morphological effects. There is an influence of PEP and ECO on the crystallization that occurs upon cooling of PHB, even when the blends are immiscible due to an expressive decrease of the intensity of the peak at crystallization, that is completed during the second heating. The degree of crystallinity of blends with PEP has been found to decrease with an increase in PEP content. PHB/ECO blends present degrees of crystallinity that can be considered nearly independent of the ECO content. The study of the morphology of blends shown that the presence of elastomer influences the ratio of the growth rate and the nucleation rate. The elastomer component, probably resides in the intraspherulitic zones. Acknowledgements The authors CNPq.
acknowledge
fellowship
from
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