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cellulose acetate butyrate blends
European Polymer Journal 37 (2001) 2091±2104
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Miscibility, crystallization and phase structure of poly(3-hydroxybutyrate)/cellulose acetate butyrate blends E. El-Shafee *, Gamal R. Saad, Sherif M. Fahmy Department of Chemistry, Faculty of Science, Cairo University, 12613 Giza, Egypt Received 4 July 2000; received in revised form 5 February 2001; accepted 20 March 2001
Abstract Blends of poly(3-hydroxybutyrate) (PHB) with cellulose acetate butyrate (CAB) were prepared by solution casting from chloroform solutions at dierent compositions. The miscibility, crystallization behavior and phase structure were investigated using dierential scanning calorimetry, optical microscopy and small-angle X-ray scattering (SAXS). CAB/ PHB blends were found to be miscible in the melt state as evidenced by the deduction of a single glass transition (Tg ) for each composition, a depression in the equilibrium melting point of PHB, and a marked reduction in the spherulites growth rate of PHB in the PHB/CAB blends. The Flory±Huggins interaction parameter v12 , obtained from melting point depression data, is composition dependent, and its value is always negative. The nucleation factor, Kg , was determined using Lauritzen±Homan model. The Kg values for the PHB in the blends are considerably lower than the Kg value in the pure homopolymer. The phase structure of the blend in the solid state as revealed by SAXS is characterized by the presence of a homogeneous amorphous phase situated mainly in the interlamellar regions of crystalline PHB and consisting of CAB molecules and uncrystalline PHB chains. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Poly(3-hydroxybutyrate); Cellulose acetate butyrate; Miscibility; Crystallization; Phase structure
1. Introduction Poly(3-hydroxybutyrate) (PHB) and its related alkanoate P(HAs) are microbially produced semicrystalline polymers and are currently the focus of intensive fundamental and applied research [1±4]. Biodegradability and biocompatibility are the main merits of these thermoplastic polymers [5], thus they have much potential in applications of medical materials such as absorbable surgical sutures, matrices for drug delivery systems and as biodegradable moulded plastics. However, the high production cost is still preventing wider scale applications of these materials. Furthermore PHB in particular is quite brittle, has a very low biodegradation rate [5], and is unstable in molten state with a narrow thermal processing window [6,7]. *
Corresponding author. E-mail address: [email protected] (E. El-Shafee).
In an attempt to improve the properties of PHB, copolymerization of 3-hydroxybutyrate with 3-hydroxyvalerate and/or 4-hydroxybutyrate using the microbial fermentation method [8±11] was developed. However, there are restrictions on the carbon sources, the kinds of bacteria and the biochemical mechanisms of biosynthesis [12]. Thus the improvement in properties is rather limited and the cost remains high. To alleviate these problems blending of PHB with a second polymeric component can oer opportunities to modify the physical properties, improve the processability and lower cost. A wide variety of polymers have been considered as a components in binary blends with PHB (see Ref. [13] and references cited therein), however, among those, only blends with poly(ethylene oxide) [14±18], poly(vinyl alcohol) [19,20], poly(L -lactide) [21], poly(D ,L -lactide) [22], poly(e-caprolactone) [23±25], poly(b-butyrolactone) [26±28], and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [29±32] and various synthetic PHB stereoisomer analogs [32±34] are totally
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 0 9 7 - 0
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biodegradable. Polysaccharides are natural polymers and also biodegradable. Blends of PHB with cellulose and starch derivatives, chitin and chitosan have been reported [35±39]. Scandola and co-workers [40±43] have investigated the miscibility, thermal and viscoelastic properties of PHB/cellulose acetate butyrate (CAB) [40,42,43] and cellulose acetate propionate (CAP) blends [40,43]. Recently, the phase behavior and crystallization kinetics of PHB±cellulose propionate blends have been studied [44]. It has been shown that the system is miscible over the entire composition range. It has been reported [41] that when CAB or CAP content in the blends is P 50 wt.%, the blends are stable homogeneous amorphous glasses, characterized by single compositiondependent glass transition temperature. When the CAB or CAP component 6 50 wt.%, PHB crystallizes upon room temperature storage and the blends are partially crystalline. Both DMTA and DSC measurements [40], shown the evidence of two types of mobility phenomena, the higher temperature one characterized by a Tg -composition dependence and the other one only slightly composition dependent. This article presents an investigation on the miscibility, crystallization, and morphology of PHB and CAB blends using dierential scanning calorimetry (DSC), polarized optical microscope and small angle X-ray scattering (SAXS). 2. Experimental 2.1. Materials Natural PHB was purchased from Copersucar (Brazil). The sample was puri®ed by dissolution in chloroform, ®ltered to remove the insoluble fraction and then precipitated in diethylether. CAB was kindly supplied by Serva Co. (Germany). The content of butyryl was 17%, of acetyl 38%, and of hydroxyl 1.3%. The degree of substitution (DS) of butyryl was 0.6, of acetyl 2.2 and of hydroxyl 0.2. Both were measured by proton n.m.r spectroscopy. The molecular weight and polydispersity data for both polymers were determined by gel permeation chromatography (GPC) using a Waters Associates (model 510) apparatus at room temperature with chloroform as a solvent. Polystyrene standard with low polydispersity (Polystandard series, Mainz) were used to establish the calibration curve. The weight average molecular weights (Mw ) and polydispersities of CAB and PHB are Mw 9:6 104 , Mw =Mn 3:33 and Mw 1:05 105 , Mw =Mn 3:37, respectively. 2.2. Preparations of blends The blends in the form of thin ®lms (0.1±0.15 mm) were prepared by conventional solution-casting tech-
nique from 5 wt.% chloroform solutions using a Petri dish as a casting surface. The ®lms were dried in vacuum and then stored at room temperature at least for two weeks to obtain equilibrium crystallinity prior to any measurements. 2.3. Measurements All the DSC measurements were conducted using a Polymer Laboratories (PL-DSC) Dierential Scanning Calorimeter. The instrument was calibrated for temperature and heat ¯ow using high-purity standard. Samples, as cast ®lms (3±5 mg) were heated from 30°C to 190°C with a heating rate 20°C/min. (Run I) and then maintained at 190°C for 2 min before fast cooling (80°C/ min) to 50°C. The samples were then re-heated to 190°C with a heating rate 20°C/min. (Run II). In the DSC thermograms (Run II), the temperature at halfheight of the corresponding heat capacity jump was de®ned as glass transition temperature (Tg ). The peak temperature at the exotherm is considered the crystallization temperature (Tc ) and the temperature at which the endotherm shows the peak is taken the melting temperature (Tm ). To obtain the observed melting temperature, Tm , as a function of the crystallization temperature, Tc , the samples were melted at 190°C for 2 min, rapidly quenched to the desired Tc , isothermally crystallized, and ®nally heated with a rate of 20°C/min. In order to minimize the risk of degradation and subsequent molecular weight decrease, a new sample was used for each crystallization temperature. Spherulitic growth rate (G) was studied on thin ®lms with a Zeiss Axioseep polarized microscope equipped with a Linkam hot stage. The samples were ®rst melted at 190°C on hot stage, then quenched to the assigned crystallization temperature and allowed to crystallize isothermally. During crystallization the growth of the spherulites was monitored as a function of time. The radial growth rate (G dr=dt) was calculated as a slope of the line obtained by plotting the spherulite radius against time. SAXS was performed on Oak Ridge National Laboratory's 10 m SAXS instruments, using a source-tosample distance of 3 m and sample-to-detector distance of 5 m; the X-ray generator was operated at 100 mA and 40 kV using CuKa radiation (0.1542 nm). A 0:2 0:2 m2 two-dimensional, position-sensitive detector was used with each virtual cell element 3 mm apart. The scattering intensity was stored in a 64 64 data array. Corrections were made for instrumental background, dark current due to cosmic radiation and electronic noise, and detector non-uniformity and eciency (via an Fe55 radioactive standard, which emits c-ray isotropically) on a cell-by-cell basis. The data were radially averaged and converted to an absolute dierential
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scattering cross-section by means of precalibrated secondary standards [45]. The Lorentz method is used to change the raw data from 3-d to 1-d for long period interpretation.
3. Results and discussion 3.1. Thermal and miscibility behavior Fig. 1 shows the DSC scans of pure PHB and its blends with CAB as obtained by casting. The melting endotherms in all samples investigated had a bimodal shape (Run I). By increasing the heating rate, the intensity of the melting peak appearing at higher temperature was reduced compared to that of melting peak at lower temperature. Organ and Barham [46] found that the melting peak appeared at lower temperature was attributed to the melting of the crystallites formed during sample preparation and the one at higher temperature was due to those crystallized during heating scans; however, the lower melting peak disappeared when the PHB was crystallized by quenching from the melt state to the crystallization temperature. The DSC thermograms of the melt-quenched samples (Run II) with PHB content P 50 wt.% are shown in Fig. 2. Only one glass transition temperature, comparable to
the Tg of pure PHB, was observed. This suggest that an amorphous PHB phase is present in quenched PHB/ CAB blend containing 50 wt.% or more of the bacterial polymer component. As commonly observed in melt quenched PHB [47], the glass transition is followed by an exothermic cold crystallization phenomenon, with a main peak that shifts to higher temperatures with an increasing CAB content in the blend. Fig. 3 compares Tg s of the pure CAB and its blends with low level of PHB component as obtained from Run II. Over this composition range, an endothermic baseline shift associated with the glass transition is observed for all of the samples. With increasing PHB content in the blend, the transition is seen to broaden quite markedly on the low temperature side. This behavior is particularly evident in the blend with 30 wt.% PHB, where the transition spans 50°C. However, the high temperature side of the speci®c heat step is quite well-de®ned and has indicated in Fig. 3 by drawing the intersection of the tangent to speci®c heat increment with the baseline above Tg . The intersection temperature decreases with increasing PHB content up to 30 wt.%. The temperatures of the calorimetric transitions of the PHB/CAB blend, taken from the curves of Figs. 2 and 3, are plotted in Fig. 4 as a function of composition. It is seen that at high levels of CAB there is strong dependence of the measured Tg on composition. The line drawn through these data points was calculated using the Fox equation [48]: 1 W1 W2 Tg Tg1 Tg2
Fig. 1. DSC thermograms of PHB and PHB/CAB blends as obtained by casting. PHB weight percent is indicated on the curves.
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1
where W denotes weight fraction and subscripts 1 and 2 indicate the two polymers. A number of equations, based on dierent theoretical grounds, have been proposed to describe the composition dependence of the glass transition in miscible polymer blends. However, due to its simple form and to lack of any adjustable parameters, Eq. (1) is the most commonly used. The fact that the Tg s of the blends containing 10±30 wt.% follows the predictions of the Fox equation is a good indication that the system is miscible over this composition range. However, as the amount of PHB in the system is increased, the variation of Tg is seen to level o quite noticeably. In fact blends containing more than 30 wt.% of PHB have nearly a constant glass transition temperature located in proximity to the Tg of PHB. This overall trend is in agreement with what has been reported previously for blends of PHAs with cellulose esters [41,42]. At ®rst glance, the DSC results suggest that the 30/70 PHB/CAB blend represents a kind of solubility limit of PHB in CAB in the melt from which the blends have been quenched. In this hypothesis, all of the PHB
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Fig. 2. DSC thermograms of PHB and PHB/CAB blends after melt quenching. PHB weight percent is indicated on the curves.
Fig. 3. DSC thermograms of CAB and its blends with low level of PHB after melt quenching. PHB weight percent is indicated on the curves.
component exceeding 30 wt.% should be present as a pure phase, in equilibrium with a constant composi-
tion (i.e. 30/70 PHB/CAB) mixture. However, previous studies have argued quite convincingly that, in fact,
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Fig. 4. Temperature of the transition of various PHB/CAB blends as a function of the PHB content, using data obtained from the DSC curves shown in Figs. 2 and 3: glass transition, crystallization temperature, melting temperature, Ð curve represents the Fox equation (see text for details).
these systems are miscible over the entire composition range [41±43]. Rather, the trend in Tg is ascribed to the presence of two mobilization processes in a homogeneous blend [42]. As pointed out by Buchanan et al. [49], although the two blend components are miscible in the amorphous phase, they can have quite dierent segmental mobilities, a re¯ection of the inherent dierences in their repeating units structure. Thus in the case of PHB/CAB blend, the apparent break in the Tg -composition curve is a consequence of the Tg behavior being dominated by segmental mobility of the PHB component at high to moderate levels of PHB, and that the CAB component at low levels of PHB. Consistent with the miscibility in the PHB/CAB system at high levels of PHB is the fact that no evidence was found for a second, high temperature Tg . Further, as will be shown later, these blends exhibit a signi®cant depression in equilibrium melting point. As far as crystallization of PHB is concerned, the cold crystallization of pure PHB is found at 61°C. For blends with PHB content more than 50 wt.%, cold crystallization peaks were observed in the DSC curves as shown in Fig. 2. The cold crystallization temperatures are signi®cantly dependent on blend composition as
shown in Fig. 4. Compared with that of pure PHB, the Tcc s of the blends shift to higher temperatures, indicating the diculty in crystallization of PHB in the blends. The heats of crystallization of the blends, DHc , and DHc = WPHB are plotted against CAB content in Fig. 5. The values of DHc and DHc =WPHB are dependent on the blend composition. These results con®rms the eect of CAB content on crystallization of PHB, especially when its content is relatively high. In Fig. 4, while the melting temperatures of PHB in the PHB/CAB blends decrease slightly with composition, the measured enthalpy of fusion decreased linearly as CAB content increased (see Fig. 5). The latter result implies that the PHB component attains the same level of crystallinity regardless of the amount of CAB. The crystallinity of PHB phase, v12 (PHB), and of the blend, v12 (blend), was calculated by using the following relations: vc blend
DHf 100; DHf0
vc PHB
vc blend 100 WPHB 2
where DHf0 is the thermodynamic enthalpy of fusion per gram of PHB (146.6 J g 1 ) [50,51]; DHf is the apparent
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Fig. 5. Enthalpy of fusion, DHf , and enthalpy of crystallization, DHc , versus composition for the PHB/CAB blend system.
enthalpy of fusion per gram of blend and WPHB is a weight fraction of PHB in the blend. The values vcr (PHB) and of vcr (blend) are listed in Table 1. As can be seen from Table 1, the crystallinity of PHB is only slightly in¯uenced by blend composition while, as expected, that of the blends decrease with the increase of CAB content. The maintenance of a constant level of PHB crystallinity upon blending has been reported previously for the system bacterial-atactic PHB [28]. The implication is that the CAB component is behaving simply as a polymeric diluent; hence the PHB chains must disentangle from the CAB segments as the process of crystallization proceeds. 3.2. Equilibrium melting point and interaction parameter The equilibrium melting temperature, Tm , which may be de®ned as the melting point of the in®nity large Table 1 Overall blend crystallinity vc (blend) and crystallinity of PHB phase vc (PHB) (from DSC run II) Blend composition
vc (blend) (%)
vc (PHB) (%)
PHB/CAB PHB/CAB PHB/CAB PHB/CAB PHB/CAB PHB/CAB
51.00 45.64 42.56 28.40 32.00 22.68
51.00 50.70 53.20 40.57 53.45 45.37
(100/00) (90/10) (80/20) (70/30) (60/40) (50/50)
lamella, can be derived by the following Homan± Weeks equation (Fig. 6) [52]: Tm
Tc 1 g
1 Tm g
3
where Tm is the observed melting temperature, g is the ratio of the initial to the ®nal lamellar thickness. Tm is determined from the graphs of Tm versus Tc , where Tm is the intercept node of Tm with the straight line Tm Tc . The values of Tm determined by this method are listed for each blend in Table 2 and plotted versus CAB content in Fig. 7. A value of Tm0 of 189°C was obtained for pure PHB. This value agrees quite well with value of 188°C obtained by Barham et al. [53]. From the data of Table 2 and the trend of Fig. 7, it emerges that the addition of CAB causes a substantial depression in the Tm value of PHB. This in itself con®rms that there is miscibility between the two components in the amorphous regions. One of the routes to study the miscibility of two polymers is based on the interpretation of the Flory± Huggins polymer±polymer interaction parameter, v12 , by means of melting point depression analysis of the crystalline polymer in the blend. The melting-point depression of the crystallizable polymer caused by addition of miscible diluent can be due to the decrease of the chemical potential. The expression that describes the dependence of the melting point depression due to only
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Fig. 6. Homan±Weeks plots for isothermally crystallized blends at dierent compositions.
Table 2 Equilibrium melting point (Tm ) as a function of blend composition Blend composition
Tm °C
PHB/CAB PHB/CAB PHB/CAB PHB/CAB PHB/CAB PHB/CAB PHB/CAB
189.5 187.5 186.5 185.0 182.5 181.5 178.5
(100/00) (90/10) (80/20) (70/30) (60/40) (50/50) (40/60)
thermodynamic eects on the blend's composition is given, according to the Flory±Huggins theory modi®ed by Nishi±Wang [54,55] is: 1 Tm
1 RV2 ln /2 1 Tm0 DH0 V1 m2 m2
1 /1 v12 /21 m1 4
where Tm and Tm0 are the equilibrium melting points of the blend and homopolymer, respectively, DH 0 is the heat of fusion for the 100% crystallizable component, V1 and V2 are the molar volumes of the repeating units of non-crystallizable and crystallizable component, re-
Fig. 7. Equilibrium melting point (Tm ) of PHB as a function of blend composition.
spectively and m1 , /1 and m2 , /2 are the degrees of polymerization and the volume fractions of the noncrystallizable and the crystallizable components. By rearranging the terms of Eq. (4) we obtain:
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DH 0 V1 RV2
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1 Tm
b v12 /21
1 Tm0
ln /2 m2
1 m2
1 /1 m1 5
If v12 is independent of the blend composition and if the melting-point depression is independent of the morphological eects, there will be a linear relationship between b and /21 . Thus the interaction parameter, v12 is obtained from the slope of the straight line. In order to calculate the left-hand side of Eq. (5) the following parameter values have been used: DH 0 3001 cal mol 1 , m2 (PHB) 362.7, m1 86:5. For V1 (molar volume of CAB) a value of 276 cm3 mol 1 was used. This value, experimentally determined by dilatometric method, corresponds to the molar volume at 180°C. For V2 the value of 75 cm3 mol 1 was used, which was calculated by using the amorphous density of PHB (1.15 g cm 3 ) [50,51]. The plot of Eq. (5) obtained by using the values of Tm0 (PHB) and Tm (blend) found experimentally by us (see Table 2) is shown in Fig. 8 for PHB/CAB blends. A line may interpolate the experimental points with an intercept at the origin of 5:85 10 2 and a slope of 0:62. The fact that this line does not pass through the origin can be accounted for by a composition dependence of v12 . The negative value of the v12 parameter supports the
fact that the blends are thermodynamically miscible in the melt. In the present work the equilibrium melting point temperature have been obtained by the use of the Homann±Weeks equation. By this procedure the melting temperatures are corrected for morphological eects. Painter et al. [56] proposed that the composition dependence of v12 results from the strong interactions between the dierent components. Certainly hydrogen bonding between the residual hydroxyl groups on CAB and the carbonyl group of PHB would be expected to enhance miscibility and can also be responsible for the strong dependence of v12 on the composition. 3.3. Spherulitic growth kinetic When the PHB±CAB blends were melted in the hot stage of the optical microscope, the melt appeared to be homogeneous for all compositions investigated. Spherulite growth was monitored at dierent crystallization temperatures, Tc s, in the range between 115°C and 140°C. Plots of the radius of the PHB spherulites against time for all blend compositions and Tc s explored results in straight lines. Hence, it may be concluded that CAB is being incorporated into the growing PHB spherulites [57]. The dependence of spherulite radial
Fig. 8. Melting-temperature depression for PHB/CAB blends. Plot of b versus /21 according to the Flory±Huggins's equation.
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Fig. 9. Radial growth rate, G, of PHB spherulite, as a function of crystallization temperature, Tc , for various blend composition.
growth rate, G, as a function of crystallization temperature, Tc , for pure PHB and PHB in blends with CAB is shown in Fig. 9. It can be seen that G decreases for both increasing Tc and increasing CAB content at constant Tc . As shown by Fig. 10, the amount of such depression in G is composition and temperature dependent. The depression in G is larger at high value of undercooling, while at very low undercooling it tends to vanish. According to Lauritzen±Homan (LH) model of polymer crystallization [57], the linear growth rate G of a chain-folded polymer crystal is: U Kg G G0 exp exp 6 R Tc T1 Tc DT f where G0 is the pre-exponential factor which comprises all temperature-independent factors, U and T1 Tg C are the Williams±Landel±Ferry (WLF) energy term and WLF temperature, respectively. Tc is the crystallization temperature, DT Tm0 Tc is the degree of undercooling, Tm0 is the equilibrium melting temperature. f is a correction term of the order of unity, usually represented by: f 2Tc = Tm0 Tc 7 and Kg is the nucleation constant and it can be expressed as:
Kg
nb0 rre Tm0 qkDhf
8
where r is the lateral surface free energy, re is the fold surface free energy, b0 is the layer thickness, q is the density, k is the Boltzmann constant and Dhf is the heat of fusion per unit weight of crystal at the equilibrium melting temperature Tm0 . In Eq. (8), according to Homan's theory [58], n identi®es a parameter whose values depends on the crystallization regime; its value is 4 for regimes I (low undercooling) and III (very high undercooling), and 2 for regime II (intermediate undercooling). Eq. (6) was re-written by Homan [59] to take into account crystalline polymer±amorphous diluent polymer blend: ln G
ln /2
ln G0
U
R Tc T1 Kg Tc DT f
0:2Tm0 ln /2 a DT 9
where /2 is the volume fraction of the crystalline polymer. The nucleation factor Kg , as de®ned in Eq. (8), represents the free energy necessary to form a nucleus of critical size. Its value is determined by plotting the
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Fig. 10. Radial growth rate, G, of PHB spherulites as a function of composition, at a given Tc . Tc 120°C, 125°C and 130°C, respectively. Table 3 Nucleation factor, Kg , for PHB and PHB±CAB blends Blend composition
Kg (K 2 )
PHB/CAB PHB/CAB PHB/CAB PHB/CAB
1:63 105 0:98 105 0:95 105 0:78 105
(100/00) (80/20) (60/40) (40/60)
cal/mol and T1 Tg 51:6 K [13] while the TgPHB values are the experimental values obtained by DSC. The values of Kg for pure PHB and PHB/CAB blends, as calculated from the slope of the lines of Fig. 11 are cited in Table 3. It can be observed that the Kg value of PHB is decreased by an amount of 66% for the 80/20 PHB/CAB blend, 58% for the 60/40 PHB/CAB blend and 48% for the 40/60 PHB/CAB blend. This means that the nucleation process of PHB is in¯uenced by the presence of CAB molecules acting as diluent. This is consistent with what was found by Martuscelli et al. [14] in the case of PHB/PEO blends. Fig. 11. Plot of the left-hand side of Eq. (9) against 1=Tc DT f for pure PHB and PHB/CAB blends.
3.4. Segregation phenomena in CAB/PHB blends
left-hand side of Eq. (9) versus 1=Tc DT f (see Fig. 11). In Eq. (9), the following values were used: U 1500
The liquid±solid phase separation occurring during the crystallization process of PHB in miscible PHB/CAB
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Table 4 Values of long period (L), for pure PHB and PHB/CAB blends Blend composition L A PHB/CAB PHB/CAB PHB/CAB PHB/CAB
(100/00) (80/20) (70/30) (50/50)
49.0 60.0 82.5 95.0
creasing CAB content. This mean that the inclusion of the CAB component causes the interlamellar spacing to become large with increasing content of that polymer. Similar results have been observed in atactic PHB/microbial PHB blends [28]. A better morphological appreciation of this observation follows from inspection of the experimental one-dimensional correlation functions obtained by the expression [60]: c r
Fig. 12. Desmeared and Lorentz-corrected SAXS curves for pure PHB and PHB/CAB blends isothermally crystallized at 30°C.
blends requires the segregation and diusion of CAB away from the crystalline nucleus. With respect to this point, measurements of SAXS patterns can provide experimental evidence on the presence or absence of interlamellar incorporation of CAB during the lamellar growth process in crystallizable CAB/PHB blends. A typical desmeared and Lorentz corrected curves for pure PHB and PHB/CAB blends cold crystallized at 30°C for 12 days are shown in Fig. 12. The scattering pro®les show the presence of a maximum, which is due to scattering from regular arrays of lamellae with a long spacing (L). The scattering vector, qm , at maximum intensity decrease with increasing CAB content. This is a ®rst indication for the interlamellar incorporation of CAB during the isothermal crystallization of the PHB/CAB blends. The L values were calculated by: L 2p=qm
10
where qm 4p sin h=k is the scattering vector associated with the maximum in scattering intensity. As cited in Table 4, the L value increases drastically with in-
1 p
R 1 I q cos qr dq 0 R 1 1 I q dq 0 p
11
The model used to describe the scattering from a lamellar structure is the lamellar pseudo-two-phase structure discussed by Vonk [61,62]. This structure is de®ned as consisting of alternating parallel crystalline and amorphous lamellae connected by transition layers, where the variation of the electron density is assumed to be linear. All correlation functions are presented in Fig. 13. It is seen that, the correlation functions broaden on addition of CAB while the maximum shifts towards higher r values. We have used the correlation function and the crystallinity values obtained by DSC to estimate the morphological parameters of the average thickness of the amorphous hAin , crystalline hCin , and the transition layer hEin , respectively [63]. The results are cited in Fig. 14. Over the composition range studied, the thickness of the crystalline layer of the blends remains constant to within experimental errors and slightly higher than that of pure PHB, while the values of hEin is similar for pure PHB and blends. This is not reasonable, as hCin should actually decrease with increasing CAB content. A possible cause of this observation is that as the concentration of PHB in the blend decreased, the overall intensity decreases and produced larger errors in the experimentally obtained correlation functions. More importantly, the interlamellar distance hAin increases with addition of CAB, indicating that the CAB is segregated in the PHB interlamellar region. The occurrence of interlamellar segregation in PHB/CAB blends can be fully understood if one considers the low diusivity of CAB. The high Tg and/or the negative values of the thermodynamic interaction parameter v12 of CAB with PHB will be responsible for the hindered diusion process.
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Fig. 13. One-dimensional correlation functions for pure PHB and PHB/CAB blends isothermally crystallized at 30°C.
Fig. 14. Average amorphous interlamellar thickness hAin , average crystalline thickness hCin and average interphase thickness hEin as a function of blend composition.
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4. Conclusion Our results have shown that, despite the unusual trends in the glass transition behaviour for blends of PHB mixed with CAB, the blends exhibit miscibility throughout the whole composition range. It has been shown that addition of CAB to PHB results in a depression of the equilibrium melting point of PHB. The degree of crystallinity, enthalpies of crystallization and fusion of PHB in the blend were dependent on the blend composition. The Flory±Huggins interaction parameter v12 of the two polymers in the melt was calculated using the Nishi±Wang equation. It was found that the interaction parameter, v12 , is negative. The study of the isothermal crystallization process shows that the presence of CAB causes a depression in the growth rate, G, of PHB spherulites. The amount of such depression in G is composition and temperature dependent. This trend is entirely consistent with the addition of a second miscible component. The nucleation factor Kg is determined by applying the Lauritzen±Homan theory. The Kg of PHB in the blends is observed to decrease as compared to pure PHB. These results indicates that the nucleation process is in¯uenced by the presence in the PHB melt of CAB acting as diluent. The phase structure of the cold crystallized blends was studied by SAXS. The long period distance and the amorphous interlamellar thickness increased with increasing CAB content in the blends. While the thickness of the crystalline lamella was quite constant for the blends at the detection limits used. Accordingly, the phase structure in the solid state is characterized by the presence of a homogeneous amorphous phase situated mainly in the interlamellar regions of the crystalline PHB and consisting of CAB molecules and uncrystalline PHB chains.
References [1] Holmes PA. In: Bassett DC, editor. Develoments in crystalline polymers, vol. 2. New York: Elsevier; 1998. [2] Anderson AJ, Dawes EA. Microbiol Rev 1990;54:450. [3] Doi Y. Microbial polyesters. New York: VCH Publishers Inc.; 1990. [4] Holland SJ, Jolly AM, Yasin M, Tighe BJ. Biomaterials 1987;8:289. [5] Holmes PA. Phys Technol 1985;16:32. [6] Grassi N, Murray EJ, Holmes PA. Polym Degrad Stab 1984;47:95. [7] Barham PJ, Kelller A. J Polym Sci Polym Phys Ed 1986;24:29. [8] Doi Y, Tamaki A, Kunioka M, Soga K. Appl Microbiol Biotechnol 1988;28:330. [9] Kamiya N, Yamamoto Y, Inoue Y, Chujo R, Doi Y. Macromolecules 1989;22:1676.
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[10] Inoue Y, Kamiya N, Yamamoto Y, Chuje R, Doi Y. Macromolecules 1989;22:3800. [11] Shimamura E, Kasuya K, Kobayashi G, Shiotani T, Shima Y, Doi Y. Macromolecules 1994;27:878. [12] Yasin M, Holland ST, Jolly AM, Tighe BJ. Biomaterials 1989;10:400. [13] Verhoogt H, Ramsay BA, Favis BD. Polymer 1994;35:5155. [14] Avella M, Martuscelli E. Polymer 1988;29:1731. [15] Avella M, Martuscelli E, Greco P. Polymer 1991;32: 1647. [16] Avella M, Martuscelli E, Raimo M. Polymer 1993;34:3234. [17] Yoon JS, Choi CS, Maing SJ, Choi HJ, Lee HS, Choi SJ. Eur Polym J 1993;29:1359. [18] Choi HJ, Park SH, Yoon JS, Lee HS, Choi SJ. Polym Engng Sci 1995;35:1636. [19] Azuma Y, Yoshie N, Sakurai M, Inoue Y, Chujo R. Polymer 1992;33:4763. [20] Yoshie N, Azuma Y, Sakurai M, Inoue Y. J Appl Polym Sci 1995;56:17. [21] Blumm E, Owen AJ. Polymer 1995;36:4077. [22] Zang LL, Xiong CD, Deng XM. Polymer 1996;37:235. [23] Gassner F, Owen AJ. Polymer 1994;35:2233. [24] Lisuardi A, Schoenberg A, Gada M, Gross RA, McCarthy SP. Polym Mater Sci Engng 1992;67:298. [25] Abe H, Doi Y, Kumagai Y. Macromolecules 1994;27:6012. [26] Pearce R, Jesudason J, Orts W, Marchessault RH, Bloembergen S. Polymer 1992;33:4647. [27] Pearce R, Brown GR, Marchessault RH. Polymer 1994;35:3984. [28] Pearce R, Marchessault RH. Polymer 1994;35:3990. [29] Organ SJ, Barham PJ. Polymer 1993;34:459. [30] Organ SJ. Polymer 1994;35:86. [31] Satoh H, Yoshie N, Inoue Y. Polymer 1994;35:286. [32] Abe H, Doi Y, Satkowski MM, Noda I. Macromolecules 1994;27:50. [33] Pearce R, Jeesudason J, Orts W, Marchessault RH. Polymer 1992;32:4647. [34] Kumagi Y, Doi Y. Macromol Chem Rapid Commun 1992;13:179. [35] He Y, Masuda T, Cao A, Yoshie N, Doi Y, Inoue Y. Polymer J 1999;31:184. [36] Finelli L, Scandola M, Sadoco P. Macromol Chem Phys 1998;199:695. [37] Ceccorulli G, Pizzoli M, Scandola M. Macromolecules 1993;26:6722. [38] Tomasi G, Scandola M. JMS Pure Appl Sci A 1995;32:671. [39] Ikeiima T, Yagi K, Inoue Y. Macromol Chem Phys 1999;20:413. [40] Scandola M, Ceccorulli G, Pizzoli M. Macromolecules 1992;25:6441. [41] Lotti N, Scandola M. Polym Bull 1992;29:407; Book of Abstract, 273, New Orleans, USA 1991. [42] Ceccorulli G, Pizzoli M, Scandola M. Macromolecules 1995;26:6722. [43] Pizzoli M, Scandola M, Ceccorulli G. Macromolecules 1994;27:4755. [44] Mackawa M, Pearce R, Martchessault RH, Manley RSJ. Polymer 1999;40:1501. [45] Russell G, Lin JS, Spooner S. J Appl Cryst 1988;21:629. [46] Organ SJ, Barham PJ. Polymer 1993;34:2169.
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[47] Scandola M, Pizzoli M, Ceccorulli G, Paoletti S, Navarini L. Int J Biol Macromol 1988. [48] Fox TG. Bull Am Phys Soc 1956;2:123. [49] Buchanan CM, Gedon SC, White AW, Wood MD. Macromolecules 1992;25:7373. [50] Barham PJ, Keller A, Otun EL, Holms PA. J Mater Sci 1984;19:2781. [51] Barham PJ, Keller A, Otun EL, Holms PA. J Mater Sci 1984;19:3826. [52] Homan JD, Weeks J. J Res Natl Bur Stand 1962;66A:13. [53] Organ SJ, Barham PJ. J Mater Sci 1991;26:1368. [54] Vachom C, Vasco M, Perrier M, Prudhomme J. Macromolecules 1993;26:4023.
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Miscibility, crystallization and phase structure of poly(3-hydroxybutyrate)/cellulose acetate butyrate blends E. El-Shafee *, Gamal R. Saad, Sherif M. Fahmy Department of Chemistry, Faculty of Science, Cairo University, 12613 Giza, Egypt Received 4 July 2000; received in revised form 5 February 2001; accepted 20 March 2001
Abstract Blends of poly(3-hydroxybutyrate) (PHB) with cellulose acetate butyrate (CAB) were prepared by solution casting from chloroform solutions at dierent compositions. The miscibility, crystallization behavior and phase structure were investigated using dierential scanning calorimetry, optical microscopy and small-angle X-ray scattering (SAXS). CAB/ PHB blends were found to be miscible in the melt state as evidenced by the deduction of a single glass transition (Tg ) for each composition, a depression in the equilibrium melting point of PHB, and a marked reduction in the spherulites growth rate of PHB in the PHB/CAB blends. The Flory±Huggins interaction parameter v12 , obtained from melting point depression data, is composition dependent, and its value is always negative. The nucleation factor, Kg , was determined using Lauritzen±Homan model. The Kg values for the PHB in the blends are considerably lower than the Kg value in the pure homopolymer. The phase structure of the blend in the solid state as revealed by SAXS is characterized by the presence of a homogeneous amorphous phase situated mainly in the interlamellar regions of crystalline PHB and consisting of CAB molecules and uncrystalline PHB chains. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Poly(3-hydroxybutyrate); Cellulose acetate butyrate; Miscibility; Crystallization; Phase structure
1. Introduction Poly(3-hydroxybutyrate) (PHB) and its related alkanoate P(HAs) are microbially produced semicrystalline polymers and are currently the focus of intensive fundamental and applied research [1±4]. Biodegradability and biocompatibility are the main merits of these thermoplastic polymers [5], thus they have much potential in applications of medical materials such as absorbable surgical sutures, matrices for drug delivery systems and as biodegradable moulded plastics. However, the high production cost is still preventing wider scale applications of these materials. Furthermore PHB in particular is quite brittle, has a very low biodegradation rate [5], and is unstable in molten state with a narrow thermal processing window [6,7]. *
Corresponding author. E-mail address: [email protected] (E. El-Shafee).
In an attempt to improve the properties of PHB, copolymerization of 3-hydroxybutyrate with 3-hydroxyvalerate and/or 4-hydroxybutyrate using the microbial fermentation method [8±11] was developed. However, there are restrictions on the carbon sources, the kinds of bacteria and the biochemical mechanisms of biosynthesis [12]. Thus the improvement in properties is rather limited and the cost remains high. To alleviate these problems blending of PHB with a second polymeric component can oer opportunities to modify the physical properties, improve the processability and lower cost. A wide variety of polymers have been considered as a components in binary blends with PHB (see Ref. [13] and references cited therein), however, among those, only blends with poly(ethylene oxide) [14±18], poly(vinyl alcohol) [19,20], poly(L -lactide) [21], poly(D ,L -lactide) [22], poly(e-caprolactone) [23±25], poly(b-butyrolactone) [26±28], and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [29±32] and various synthetic PHB stereoisomer analogs [32±34] are totally
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 0 9 7 - 0
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biodegradable. Polysaccharides are natural polymers and also biodegradable. Blends of PHB with cellulose and starch derivatives, chitin and chitosan have been reported [35±39]. Scandola and co-workers [40±43] have investigated the miscibility, thermal and viscoelastic properties of PHB/cellulose acetate butyrate (CAB) [40,42,43] and cellulose acetate propionate (CAP) blends [40,43]. Recently, the phase behavior and crystallization kinetics of PHB±cellulose propionate blends have been studied [44]. It has been shown that the system is miscible over the entire composition range. It has been reported [41] that when CAB or CAP content in the blends is P 50 wt.%, the blends are stable homogeneous amorphous glasses, characterized by single compositiondependent glass transition temperature. When the CAB or CAP component 6 50 wt.%, PHB crystallizes upon room temperature storage and the blends are partially crystalline. Both DMTA and DSC measurements [40], shown the evidence of two types of mobility phenomena, the higher temperature one characterized by a Tg -composition dependence and the other one only slightly composition dependent. This article presents an investigation on the miscibility, crystallization, and morphology of PHB and CAB blends using dierential scanning calorimetry (DSC), polarized optical microscope and small angle X-ray scattering (SAXS). 2. Experimental 2.1. Materials Natural PHB was purchased from Copersucar (Brazil). The sample was puri®ed by dissolution in chloroform, ®ltered to remove the insoluble fraction and then precipitated in diethylether. CAB was kindly supplied by Serva Co. (Germany). The content of butyryl was 17%, of acetyl 38%, and of hydroxyl 1.3%. The degree of substitution (DS) of butyryl was 0.6, of acetyl 2.2 and of hydroxyl 0.2. Both were measured by proton n.m.r spectroscopy. The molecular weight and polydispersity data for both polymers were determined by gel permeation chromatography (GPC) using a Waters Associates (model 510) apparatus at room temperature with chloroform as a solvent. Polystyrene standard with low polydispersity (Polystandard series, Mainz) were used to establish the calibration curve. The weight average molecular weights (Mw ) and polydispersities of CAB and PHB are Mw 9:6 104 , Mw =Mn 3:33 and Mw 1:05 105 , Mw =Mn 3:37, respectively. 2.2. Preparations of blends The blends in the form of thin ®lms (0.1±0.15 mm) were prepared by conventional solution-casting tech-
nique from 5 wt.% chloroform solutions using a Petri dish as a casting surface. The ®lms were dried in vacuum and then stored at room temperature at least for two weeks to obtain equilibrium crystallinity prior to any measurements. 2.3. Measurements All the DSC measurements were conducted using a Polymer Laboratories (PL-DSC) Dierential Scanning Calorimeter. The instrument was calibrated for temperature and heat ¯ow using high-purity standard. Samples, as cast ®lms (3±5 mg) were heated from 30°C to 190°C with a heating rate 20°C/min. (Run I) and then maintained at 190°C for 2 min before fast cooling (80°C/ min) to 50°C. The samples were then re-heated to 190°C with a heating rate 20°C/min. (Run II). In the DSC thermograms (Run II), the temperature at halfheight of the corresponding heat capacity jump was de®ned as glass transition temperature (Tg ). The peak temperature at the exotherm is considered the crystallization temperature (Tc ) and the temperature at which the endotherm shows the peak is taken the melting temperature (Tm ). To obtain the observed melting temperature, Tm , as a function of the crystallization temperature, Tc , the samples were melted at 190°C for 2 min, rapidly quenched to the desired Tc , isothermally crystallized, and ®nally heated with a rate of 20°C/min. In order to minimize the risk of degradation and subsequent molecular weight decrease, a new sample was used for each crystallization temperature. Spherulitic growth rate (G) was studied on thin ®lms with a Zeiss Axioseep polarized microscope equipped with a Linkam hot stage. The samples were ®rst melted at 190°C on hot stage, then quenched to the assigned crystallization temperature and allowed to crystallize isothermally. During crystallization the growth of the spherulites was monitored as a function of time. The radial growth rate (G dr=dt) was calculated as a slope of the line obtained by plotting the spherulite radius against time. SAXS was performed on Oak Ridge National Laboratory's 10 m SAXS instruments, using a source-tosample distance of 3 m and sample-to-detector distance of 5 m; the X-ray generator was operated at 100 mA and 40 kV using CuKa radiation (0.1542 nm). A 0:2 0:2 m2 two-dimensional, position-sensitive detector was used with each virtual cell element 3 mm apart. The scattering intensity was stored in a 64 64 data array. Corrections were made for instrumental background, dark current due to cosmic radiation and electronic noise, and detector non-uniformity and eciency (via an Fe55 radioactive standard, which emits c-ray isotropically) on a cell-by-cell basis. The data were radially averaged and converted to an absolute dierential
E. El-Shafee et al. / European Polymer Journal 37 (2001) 2091±2104
scattering cross-section by means of precalibrated secondary standards [45]. The Lorentz method is used to change the raw data from 3-d to 1-d for long period interpretation.
3. Results and discussion 3.1. Thermal and miscibility behavior Fig. 1 shows the DSC scans of pure PHB and its blends with CAB as obtained by casting. The melting endotherms in all samples investigated had a bimodal shape (Run I). By increasing the heating rate, the intensity of the melting peak appearing at higher temperature was reduced compared to that of melting peak at lower temperature. Organ and Barham [46] found that the melting peak appeared at lower temperature was attributed to the melting of the crystallites formed during sample preparation and the one at higher temperature was due to those crystallized during heating scans; however, the lower melting peak disappeared when the PHB was crystallized by quenching from the melt state to the crystallization temperature. The DSC thermograms of the melt-quenched samples (Run II) with PHB content P 50 wt.% are shown in Fig. 2. Only one glass transition temperature, comparable to
the Tg of pure PHB, was observed. This suggest that an amorphous PHB phase is present in quenched PHB/ CAB blend containing 50 wt.% or more of the bacterial polymer component. As commonly observed in melt quenched PHB [47], the glass transition is followed by an exothermic cold crystallization phenomenon, with a main peak that shifts to higher temperatures with an increasing CAB content in the blend. Fig. 3 compares Tg s of the pure CAB and its blends with low level of PHB component as obtained from Run II. Over this composition range, an endothermic baseline shift associated with the glass transition is observed for all of the samples. With increasing PHB content in the blend, the transition is seen to broaden quite markedly on the low temperature side. This behavior is particularly evident in the blend with 30 wt.% PHB, where the transition spans 50°C. However, the high temperature side of the speci®c heat step is quite well-de®ned and has indicated in Fig. 3 by drawing the intersection of the tangent to speci®c heat increment with the baseline above Tg . The intersection temperature decreases with increasing PHB content up to 30 wt.%. The temperatures of the calorimetric transitions of the PHB/CAB blend, taken from the curves of Figs. 2 and 3, are plotted in Fig. 4 as a function of composition. It is seen that at high levels of CAB there is strong dependence of the measured Tg on composition. The line drawn through these data points was calculated using the Fox equation [48]: 1 W1 W2 Tg Tg1 Tg2
Fig. 1. DSC thermograms of PHB and PHB/CAB blends as obtained by casting. PHB weight percent is indicated on the curves.
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1
where W denotes weight fraction and subscripts 1 and 2 indicate the two polymers. A number of equations, based on dierent theoretical grounds, have been proposed to describe the composition dependence of the glass transition in miscible polymer blends. However, due to its simple form and to lack of any adjustable parameters, Eq. (1) is the most commonly used. The fact that the Tg s of the blends containing 10±30 wt.% follows the predictions of the Fox equation is a good indication that the system is miscible over this composition range. However, as the amount of PHB in the system is increased, the variation of Tg is seen to level o quite noticeably. In fact blends containing more than 30 wt.% of PHB have nearly a constant glass transition temperature located in proximity to the Tg of PHB. This overall trend is in agreement with what has been reported previously for blends of PHAs with cellulose esters [41,42]. At ®rst glance, the DSC results suggest that the 30/70 PHB/CAB blend represents a kind of solubility limit of PHB in CAB in the melt from which the blends have been quenched. In this hypothesis, all of the PHB
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Fig. 2. DSC thermograms of PHB and PHB/CAB blends after melt quenching. PHB weight percent is indicated on the curves.
Fig. 3. DSC thermograms of CAB and its blends with low level of PHB after melt quenching. PHB weight percent is indicated on the curves.
component exceeding 30 wt.% should be present as a pure phase, in equilibrium with a constant composi-
tion (i.e. 30/70 PHB/CAB) mixture. However, previous studies have argued quite convincingly that, in fact,
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Fig. 4. Temperature of the transition of various PHB/CAB blends as a function of the PHB content, using data obtained from the DSC curves shown in Figs. 2 and 3: glass transition, crystallization temperature, melting temperature, Ð curve represents the Fox equation (see text for details).
these systems are miscible over the entire composition range [41±43]. Rather, the trend in Tg is ascribed to the presence of two mobilization processes in a homogeneous blend [42]. As pointed out by Buchanan et al. [49], although the two blend components are miscible in the amorphous phase, they can have quite dierent segmental mobilities, a re¯ection of the inherent dierences in their repeating units structure. Thus in the case of PHB/CAB blend, the apparent break in the Tg -composition curve is a consequence of the Tg behavior being dominated by segmental mobility of the PHB component at high to moderate levels of PHB, and that the CAB component at low levels of PHB. Consistent with the miscibility in the PHB/CAB system at high levels of PHB is the fact that no evidence was found for a second, high temperature Tg . Further, as will be shown later, these blends exhibit a signi®cant depression in equilibrium melting point. As far as crystallization of PHB is concerned, the cold crystallization of pure PHB is found at 61°C. For blends with PHB content more than 50 wt.%, cold crystallization peaks were observed in the DSC curves as shown in Fig. 2. The cold crystallization temperatures are signi®cantly dependent on blend composition as
shown in Fig. 4. Compared with that of pure PHB, the Tcc s of the blends shift to higher temperatures, indicating the diculty in crystallization of PHB in the blends. The heats of crystallization of the blends, DHc , and DHc = WPHB are plotted against CAB content in Fig. 5. The values of DHc and DHc =WPHB are dependent on the blend composition. These results con®rms the eect of CAB content on crystallization of PHB, especially when its content is relatively high. In Fig. 4, while the melting temperatures of PHB in the PHB/CAB blends decrease slightly with composition, the measured enthalpy of fusion decreased linearly as CAB content increased (see Fig. 5). The latter result implies that the PHB component attains the same level of crystallinity regardless of the amount of CAB. The crystallinity of PHB phase, v12 (PHB), and of the blend, v12 (blend), was calculated by using the following relations: vc blend
DHf 100; DHf0
vc PHB
vc blend 100 WPHB 2
where DHf0 is the thermodynamic enthalpy of fusion per gram of PHB (146.6 J g 1 ) [50,51]; DHf is the apparent
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Fig. 5. Enthalpy of fusion, DHf , and enthalpy of crystallization, DHc , versus composition for the PHB/CAB blend system.
enthalpy of fusion per gram of blend and WPHB is a weight fraction of PHB in the blend. The values vcr (PHB) and of vcr (blend) are listed in Table 1. As can be seen from Table 1, the crystallinity of PHB is only slightly in¯uenced by blend composition while, as expected, that of the blends decrease with the increase of CAB content. The maintenance of a constant level of PHB crystallinity upon blending has been reported previously for the system bacterial-atactic PHB [28]. The implication is that the CAB component is behaving simply as a polymeric diluent; hence the PHB chains must disentangle from the CAB segments as the process of crystallization proceeds. 3.2. Equilibrium melting point and interaction parameter The equilibrium melting temperature, Tm , which may be de®ned as the melting point of the in®nity large Table 1 Overall blend crystallinity vc (blend) and crystallinity of PHB phase vc (PHB) (from DSC run II) Blend composition
vc (blend) (%)
vc (PHB) (%)
PHB/CAB PHB/CAB PHB/CAB PHB/CAB PHB/CAB PHB/CAB
51.00 45.64 42.56 28.40 32.00 22.68
51.00 50.70 53.20 40.57 53.45 45.37
(100/00) (90/10) (80/20) (70/30) (60/40) (50/50)
lamella, can be derived by the following Homan± Weeks equation (Fig. 6) [52]: Tm
Tc 1 g
1 Tm g
3
where Tm is the observed melting temperature, g is the ratio of the initial to the ®nal lamellar thickness. Tm is determined from the graphs of Tm versus Tc , where Tm is the intercept node of Tm with the straight line Tm Tc . The values of Tm determined by this method are listed for each blend in Table 2 and plotted versus CAB content in Fig. 7. A value of Tm0 of 189°C was obtained for pure PHB. This value agrees quite well with value of 188°C obtained by Barham et al. [53]. From the data of Table 2 and the trend of Fig. 7, it emerges that the addition of CAB causes a substantial depression in the Tm value of PHB. This in itself con®rms that there is miscibility between the two components in the amorphous regions. One of the routes to study the miscibility of two polymers is based on the interpretation of the Flory± Huggins polymer±polymer interaction parameter, v12 , by means of melting point depression analysis of the crystalline polymer in the blend. The melting-point depression of the crystallizable polymer caused by addition of miscible diluent can be due to the decrease of the chemical potential. The expression that describes the dependence of the melting point depression due to only
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Fig. 6. Homan±Weeks plots for isothermally crystallized blends at dierent compositions.
Table 2 Equilibrium melting point (Tm ) as a function of blend composition Blend composition
Tm °C
PHB/CAB PHB/CAB PHB/CAB PHB/CAB PHB/CAB PHB/CAB PHB/CAB
189.5 187.5 186.5 185.0 182.5 181.5 178.5
(100/00) (90/10) (80/20) (70/30) (60/40) (50/50) (40/60)
thermodynamic eects on the blend's composition is given, according to the Flory±Huggins theory modi®ed by Nishi±Wang [54,55] is: 1 Tm
1 RV2 ln /2 1 Tm0 DH0 V1 m2 m2
1 /1 v12 /21 m1 4
where Tm and Tm0 are the equilibrium melting points of the blend and homopolymer, respectively, DH 0 is the heat of fusion for the 100% crystallizable component, V1 and V2 are the molar volumes of the repeating units of non-crystallizable and crystallizable component, re-
Fig. 7. Equilibrium melting point (Tm ) of PHB as a function of blend composition.
spectively and m1 , /1 and m2 , /2 are the degrees of polymerization and the volume fractions of the noncrystallizable and the crystallizable components. By rearranging the terms of Eq. (4) we obtain:
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DH 0 V1 RV2
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1 Tm
b v12 /21
1 Tm0
ln /2 m2
1 m2
1 /1 m1 5
If v12 is independent of the blend composition and if the melting-point depression is independent of the morphological eects, there will be a linear relationship between b and /21 . Thus the interaction parameter, v12 is obtained from the slope of the straight line. In order to calculate the left-hand side of Eq. (5) the following parameter values have been used: DH 0 3001 cal mol 1 , m2 (PHB) 362.7, m1 86:5. For V1 (molar volume of CAB) a value of 276 cm3 mol 1 was used. This value, experimentally determined by dilatometric method, corresponds to the molar volume at 180°C. For V2 the value of 75 cm3 mol 1 was used, which was calculated by using the amorphous density of PHB (1.15 g cm 3 ) [50,51]. The plot of Eq. (5) obtained by using the values of Tm0 (PHB) and Tm (blend) found experimentally by us (see Table 2) is shown in Fig. 8 for PHB/CAB blends. A line may interpolate the experimental points with an intercept at the origin of 5:85 10 2 and a slope of 0:62. The fact that this line does not pass through the origin can be accounted for by a composition dependence of v12 . The negative value of the v12 parameter supports the
fact that the blends are thermodynamically miscible in the melt. In the present work the equilibrium melting point temperature have been obtained by the use of the Homann±Weeks equation. By this procedure the melting temperatures are corrected for morphological eects. Painter et al. [56] proposed that the composition dependence of v12 results from the strong interactions between the dierent components. Certainly hydrogen bonding between the residual hydroxyl groups on CAB and the carbonyl group of PHB would be expected to enhance miscibility and can also be responsible for the strong dependence of v12 on the composition. 3.3. Spherulitic growth kinetic When the PHB±CAB blends were melted in the hot stage of the optical microscope, the melt appeared to be homogeneous for all compositions investigated. Spherulite growth was monitored at dierent crystallization temperatures, Tc s, in the range between 115°C and 140°C. Plots of the radius of the PHB spherulites against time for all blend compositions and Tc s explored results in straight lines. Hence, it may be concluded that CAB is being incorporated into the growing PHB spherulites [57]. The dependence of spherulite radial
Fig. 8. Melting-temperature depression for PHB/CAB blends. Plot of b versus /21 according to the Flory±Huggins's equation.
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Fig. 9. Radial growth rate, G, of PHB spherulite, as a function of crystallization temperature, Tc , for various blend composition.
growth rate, G, as a function of crystallization temperature, Tc , for pure PHB and PHB in blends with CAB is shown in Fig. 9. It can be seen that G decreases for both increasing Tc and increasing CAB content at constant Tc . As shown by Fig. 10, the amount of such depression in G is composition and temperature dependent. The depression in G is larger at high value of undercooling, while at very low undercooling it tends to vanish. According to Lauritzen±Homan (LH) model of polymer crystallization [57], the linear growth rate G of a chain-folded polymer crystal is: U Kg G G0 exp exp 6 R Tc T1 Tc DT f where G0 is the pre-exponential factor which comprises all temperature-independent factors, U and T1 Tg C are the Williams±Landel±Ferry (WLF) energy term and WLF temperature, respectively. Tc is the crystallization temperature, DT Tm0 Tc is the degree of undercooling, Tm0 is the equilibrium melting temperature. f is a correction term of the order of unity, usually represented by: f 2Tc = Tm0 Tc 7 and Kg is the nucleation constant and it can be expressed as:
Kg
nb0 rre Tm0 qkDhf
8
where r is the lateral surface free energy, re is the fold surface free energy, b0 is the layer thickness, q is the density, k is the Boltzmann constant and Dhf is the heat of fusion per unit weight of crystal at the equilibrium melting temperature Tm0 . In Eq. (8), according to Homan's theory [58], n identi®es a parameter whose values depends on the crystallization regime; its value is 4 for regimes I (low undercooling) and III (very high undercooling), and 2 for regime II (intermediate undercooling). Eq. (6) was re-written by Homan [59] to take into account crystalline polymer±amorphous diluent polymer blend: ln G
ln /2
ln G0
U
R Tc T1 Kg Tc DT f
0:2Tm0 ln /2 a DT 9
where /2 is the volume fraction of the crystalline polymer. The nucleation factor Kg , as de®ned in Eq. (8), represents the free energy necessary to form a nucleus of critical size. Its value is determined by plotting the
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Fig. 10. Radial growth rate, G, of PHB spherulites as a function of composition, at a given Tc . Tc 120°C, 125°C and 130°C, respectively. Table 3 Nucleation factor, Kg , for PHB and PHB±CAB blends Blend composition
Kg (K 2 )
PHB/CAB PHB/CAB PHB/CAB PHB/CAB
1:63 105 0:98 105 0:95 105 0:78 105
(100/00) (80/20) (60/40) (40/60)
cal/mol and T1 Tg 51:6 K [13] while the TgPHB values are the experimental values obtained by DSC. The values of Kg for pure PHB and PHB/CAB blends, as calculated from the slope of the lines of Fig. 11 are cited in Table 3. It can be observed that the Kg value of PHB is decreased by an amount of 66% for the 80/20 PHB/CAB blend, 58% for the 60/40 PHB/CAB blend and 48% for the 40/60 PHB/CAB blend. This means that the nucleation process of PHB is in¯uenced by the presence of CAB molecules acting as diluent. This is consistent with what was found by Martuscelli et al. [14] in the case of PHB/PEO blends. Fig. 11. Plot of the left-hand side of Eq. (9) against 1=Tc DT f for pure PHB and PHB/CAB blends.
3.4. Segregation phenomena in CAB/PHB blends
left-hand side of Eq. (9) versus 1=Tc DT f (see Fig. 11). In Eq. (9), the following values were used: U 1500
The liquid±solid phase separation occurring during the crystallization process of PHB in miscible PHB/CAB
E. El-Shafee et al. / European Polymer Journal 37 (2001) 2091±2104
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Table 4 Values of long period (L), for pure PHB and PHB/CAB blends Blend composition L A PHB/CAB PHB/CAB PHB/CAB PHB/CAB
(100/00) (80/20) (70/30) (50/50)
49.0 60.0 82.5 95.0
creasing CAB content. This mean that the inclusion of the CAB component causes the interlamellar spacing to become large with increasing content of that polymer. Similar results have been observed in atactic PHB/microbial PHB blends [28]. A better morphological appreciation of this observation follows from inspection of the experimental one-dimensional correlation functions obtained by the expression [60]: c r
Fig. 12. Desmeared and Lorentz-corrected SAXS curves for pure PHB and PHB/CAB blends isothermally crystallized at 30°C.
blends requires the segregation and diusion of CAB away from the crystalline nucleus. With respect to this point, measurements of SAXS patterns can provide experimental evidence on the presence or absence of interlamellar incorporation of CAB during the lamellar growth process in crystallizable CAB/PHB blends. A typical desmeared and Lorentz corrected curves for pure PHB and PHB/CAB blends cold crystallized at 30°C for 12 days are shown in Fig. 12. The scattering pro®les show the presence of a maximum, which is due to scattering from regular arrays of lamellae with a long spacing (L). The scattering vector, qm , at maximum intensity decrease with increasing CAB content. This is a ®rst indication for the interlamellar incorporation of CAB during the isothermal crystallization of the PHB/CAB blends. The L values were calculated by: L 2p=qm
10
where qm 4p sin h=k is the scattering vector associated with the maximum in scattering intensity. As cited in Table 4, the L value increases drastically with in-
1 p
R 1 I q cos qr dq 0 R 1 1 I q dq 0 p
11
The model used to describe the scattering from a lamellar structure is the lamellar pseudo-two-phase structure discussed by Vonk [61,62]. This structure is de®ned as consisting of alternating parallel crystalline and amorphous lamellae connected by transition layers, where the variation of the electron density is assumed to be linear. All correlation functions are presented in Fig. 13. It is seen that, the correlation functions broaden on addition of CAB while the maximum shifts towards higher r values. We have used the correlation function and the crystallinity values obtained by DSC to estimate the morphological parameters of the average thickness of the amorphous hAin , crystalline hCin , and the transition layer hEin , respectively [63]. The results are cited in Fig. 14. Over the composition range studied, the thickness of the crystalline layer of the blends remains constant to within experimental errors and slightly higher than that of pure PHB, while the values of hEin is similar for pure PHB and blends. This is not reasonable, as hCin should actually decrease with increasing CAB content. A possible cause of this observation is that as the concentration of PHB in the blend decreased, the overall intensity decreases and produced larger errors in the experimentally obtained correlation functions. More importantly, the interlamellar distance hAin increases with addition of CAB, indicating that the CAB is segregated in the PHB interlamellar region. The occurrence of interlamellar segregation in PHB/CAB blends can be fully understood if one considers the low diusivity of CAB. The high Tg and/or the negative values of the thermodynamic interaction parameter v12 of CAB with PHB will be responsible for the hindered diusion process.
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Fig. 13. One-dimensional correlation functions for pure PHB and PHB/CAB blends isothermally crystallized at 30°C.
Fig. 14. Average amorphous interlamellar thickness hAin , average crystalline thickness hCin and average interphase thickness hEin as a function of blend composition.
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4. Conclusion Our results have shown that, despite the unusual trends in the glass transition behaviour for blends of PHB mixed with CAB, the blends exhibit miscibility throughout the whole composition range. It has been shown that addition of CAB to PHB results in a depression of the equilibrium melting point of PHB. The degree of crystallinity, enthalpies of crystallization and fusion of PHB in the blend were dependent on the blend composition. The Flory±Huggins interaction parameter v12 of the two polymers in the melt was calculated using the Nishi±Wang equation. It was found that the interaction parameter, v12 , is negative. The study of the isothermal crystallization process shows that the presence of CAB causes a depression in the growth rate, G, of PHB spherulites. The amount of such depression in G is composition and temperature dependent. This trend is entirely consistent with the addition of a second miscible component. The nucleation factor Kg is determined by applying the Lauritzen±Homan theory. The Kg of PHB in the blends is observed to decrease as compared to pure PHB. These results indicates that the nucleation process is in¯uenced by the presence in the PHB melt of CAB acting as diluent. The phase structure of the cold crystallized blends was studied by SAXS. The long period distance and the amorphous interlamellar thickness increased with increasing CAB content in the blends. While the thickness of the crystalline lamella was quite constant for the blends at the detection limits used. Accordingly, the phase structure in the solid state is characterized by the presence of a homogeneous amorphous phase situated mainly in the interlamellar regions of the crystalline PHB and consisting of CAB molecules and uncrystalline PHB chains.
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