Novel miscible blends of poly(p-dioxanone) with poly(vinyl phenol)

European Polymer Journal 48 (2012) 1455–1465

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Novel miscible blends of poly(p-dioxanone) with poly(vinyl phenol) Natalia Hernandez-Montero a, Emilio Meaurio a,⇑, Khaled Elmiloudi b, Jose-Ramon Sarasua a,⇑ a University of the Basque Country (UPV/EHU), Department of Mining-Metallurgy Engineering & Materials Science, School of Engineering, Alameda de Urquijo s/n, 48013 Bilbao, Spain b Université Hassiba Benbouali, Faculté des Sciences et Sciences de l’Ingénieur, B.P. 151 Chlef, Algeria

a r t i c l e

i n f o

Article history: Received 23 February 2012 Received in revised form 24 April 2012 Accepted 16 May 2012 Available online 23 May 2012 Keywords: Poly(p-dioxanone) (PPDO) Poly(vinyl phenol) (PVPh) Blends DSC FTIR Interaction parameter

a b s t r a c t Poly(p-dioxanone) (PPDO) has been blended with poly(vinyl phenol) (PVPh) and the PPDO/ PVPh blends have been investigated using DSC, FTIR and POM. According to the single Tg criterion, miscibility has been found in the whole composition range for the blends obtained by solvent casting from dioxane solutions. The dependence of the Tg on composition shows negative deviation from the Fox equation. The interaction parameter, obtained from melting point depression analysis, v12 = 1.0, confirms a thermodynamically miscible blend. Specific interactions have been analyzed by FTIR. The OH stretching region of PVPh indicates that upon addition of PPDO the hydroxyl–hydroxyl autoassociation interactions are mainly replaced by hydroxyl–carbonyl interassociation contacts, in detrimental of the possible hydroxyl–ether interactions. The carbonyl stretching region of pure PPDO is sensitive to intramolecular ether-ester interactions occurring in the oxyethanoate structures (–O–CH2–CO–O–) present along the PPDO chain. The –O–CH2–CO–O– structure presents only two minimum energy conformations, trans and cis, resulting in two different absorptions in the C@O stretching region located respectively at about 1757 and 1732 cm1. Blending with PVPh promotes two new contributions red shifted by about 23 cm1 relative to the ‘‘free’’ C@O components. Finally, POM analysis shows that the addition of PVPh to PPDO significantly decreases the crystallization rate of PPDO. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Polymer blends have shown to be an excellent way of developing new materials, often exhibiting combinations of properties superior to either of the pure components. They represent a more cost-effective way of modifying polymer properties than chemical modification. Some characteristics such as mechanical properties and degradation behavior can be modified by a favorable choice of the second component of the blend. Thus, the final properties will depend not only on the chemical composition of the blend but also on its physical characteristics, such as glass transition temperature, crystallinity, and morphology,

⇑ Corresponding authors. Fax: +34 94 601 3930. E-mail addresses: [email protected] (E. Meaurio), jr.sarasua@ ehu.es (J.-R. Sarasua). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.05.009

which are a direct consequence of the miscibility between the components of the blend [1]. Poly(p-dioxanone) (PPDO) is a biodegradable polyester used in tissue engineering, bone fracture fixation and controlled drug delivery due to its excellent biodegradability, bioadsorbability, biocompatibility and mechanical flexibility. It has also received the approval of Food and Drug Administration (FDA) to be used as suture material [2–5]. The coexistence of ester and ether groups endows PPDO with the special features mentioned above [6]. The chemical structure is shown in Scheme 1. While PPDO possesses outstanding potential for use in general medical devices in the form of films, molded products, laminates, foams, nonwoven materials, adhesives and coatings, its high cost and relatively fast degradation rate have hindered development of commercial applications [7]. Blending with other polymers is a relatively simple and convenient way of modifying the properties of aliphatic polyesters. Therefore,

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CH2

CH n

O OH

CH2

CH2

O

CH2

C

O n

PVPh

PPDO Scheme 1. Chemical structures of PVPh and PPDO.

there is an increasing interest in blending PPDO with other polymers in order to modify the drawbacks mentioned above [5]. Miscibility of PPDO with some polymers has been evaluated by several groups. Pezzin et al. [3] studied blends of PPDO and poly(L-Lactide) (PLLA). The analysis suggested immiscibility between both polymers, but some compositions showed improvement in mechanical properties. The same behavior was found by Bai et al. [5], but in this case PPDO was blended with poly(D,L-Lactide) PDLLA. Furthermore, PPDO has been shown to be immiscible with poly(vinyl alcohol) (PVA) [7] and with poly(3-hydroxy butyrate) (PHB) [8]. In addition, blends of PPDO and poly(ethylene glycol) (PEG) have been prepared by Zheng et al. [9] to investigate their crystallization behavior. Poly(4-vinyl phenol) (PVPh) is an amorphous polymer with high glass transition temperature. The chemical structure is presented in Scheme 1. PVPh is miscible with various polymers such as poly(ethylene oxide) [10], poly (3-hydroxy butyrate) [11], poly(trimethylene terephthalate) [12], poly(vinylpyrrolidone) [13], and poly(L-lactide) [14]. The miscibility of polymer blends containing PVPh usually arises from the hydrogen bonding interaction between the hydroxyl group of PVPh and hydrogen bonding acceptor groups located in the blend partner, such as the carbonyl group [15]. PVPh can act as a proton donor that forms hydrogen bonds with proton acceptor polymers [16]. In the present study, the miscibility of PPDO/PVPh blends prepared by solvent casting has been investigated. The miscibility of the blends was determined by measuring their glass transition temperature (Tg) using Differential Scanning Calorimetry (DSC). The specific interactions occurring in the PPDO/PVPh blends have been analyzed using Fourier Transform Infrared (FTIR) Spectroscopy, and the polymer–polymer interaction parameter has been calculated according to the melting point depression method. Finally, Polarized Optical Microscopy (POM) was used to study the crystallinity of the blends.

characterized by Fourier Transform infrared spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC).

2.2. Blend preparation Solvent casting was used to prepare the PPDO/PVPh blends using 1,4-Dioxane as common solvent. Heating at temperatures above about 60 °C was found necessary to melt and dissolve PPDO in 1,4-Dioxanone, and castings were actually performed at 70 °C from polymer solutions (10 wt.%) covering the whole compositions range. The cast films were dried at 60 °C in a vacuum oven for 1 day to remove any residual solvent.

2.3. Differential scanning calorimetry The DSC measurements were performed with a DSC Q200 Modulated from TA Instruments. The samples were placed in aluminium pans with a pinhole in the lid. The weight of the samples varied between 5 and 10 mg. Two consecutive scans were performed, with a scan rate of 20 °C/min, up to 250 °C to ensure the complete melting of the sample. All runs were performed under nitrogen flow. Glass transition temperatures (Tg) were obtained in the second scan and were measured as mid-point values.

2.4. FTIR Infrared spectra of the blends were performed on a Nicolet AVATAR 370 Fourier transform infrared spectrophotometer (FTIR). The spectra were recorded at room temperature with 2 cm1 resolution, averaging 64 scans in the range 4000–450 cm1 Dioxane solutions containing the blends were cast on KBr pellets at 70 °C. Then the samples were vacuum-dried at 60 °C for 24 h. The absorbance of all the studied samples was within the absorbance range in which the Lambert–Beer law is obeyed.

2. Experimental section 2.1. Starting materials

2.5. Melting point depression

Poly(p-dioxanone) with a viscosity of 2.2 dl/g was supplied by Aldrich Chemical Cop. (Spain). Poly(4-vinyl phenol) with an average of molecular weight (Mw) of 25.000 and density of 1.16 g/mL at 25 °C was purchased from Aldrich Chemical Corp. (Spain). Both homopolymers were

For melting point depression studies, samples were allowed to crystallize isothermally until crystallization was complete and were then heated with a scan rate of 5 °C/min to obtain the melting point values for pure PPDO and for the PPDO/PVPh blends.

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2.6. POM

200

Tg ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ Linear behavior

Fox Gordon Taylor −−−− Kwei simplified

150

Temperature (ºC)

Polarized Optical Microscopy (POM) measurements were performed to investigate the effects of miscibility on crystallization, the spherulitic morphology and spherulitic growth rates of pure PPDO and its blends. By using a polarized light microscope (Leica, DMLM) equipped with a heating stage (Metler-FP90, Toledo); photographs were taken with a CCD camera. The samples were melted on a hot stage at 200 °C for 5 min. Then the blends were transferred quickly to another hot stage at a predetermined crystallization temperature (Tc). The spherulite morphology was recorded by taking the photomicrographs at regular intervals.

100

50

0

0.0

0.2

0.4

0.6

0.8

1.0

PPDO Weight Fraction

3. Results and discussion

Fig. 2. Tg vs composition for PPDO/PVPh blends. The straight line represents linear behavior. The curves represent the different fits to the Fox, Gordon-Taylor (j = 0.40) and simplified Kwei equations (q = 149).

3.1. DSC analysis DSC is a well-known technique to study the miscibility of polymer blends, based on the appearance of a single glass transition temperature (Tg) between those of the pure polymers [1,21]. Fig. 1 shows the second scan DSC traces obtained for the pure polymers and for different PPDO/ PVPh blends. The DSC curves of pure PPDO and PVPh show Tgs at 8 and 156 °C, respectively. The PPDO/PVPh blends exhibit a single glass transition temperature (Tg) intermediate between those of the pure polymers for all the blend compositions, suggesting the miscibility of the system in the domain size range 20–40 nm [17,18]. In addition, the strong melting endotherm of PPDO is suppressed in the blends with PVPh contents above 20 wt.%. Hence, the presence of PVPh hinders the crystallization of PPDO. The

dependence of Tg on the composition of the miscible PPDO/PVPh blends is illustrated in Fig. 2. Over the years, a number of theoretical and empirical equations have been offered to predict the composition dependence of the glass transition temperature of miscible blends [1]. The simplest equation assumes the aditivity of the transition temperatures:

T gb ¼ w1 T g1 þ w2 T g2

ð1Þ

where w1 and w2 are the weight fractions of components 1 and 2 respectively. Tg1 and Tg2 are the respective glass transitions of the pure components, and Tgb is the glass transition temperature of the blend. Many random copolymers and polymer blends fulfill the Fox equation, later extended to polymer blends [1,19]

T gb ¼ w1 =T g1 þ w2 =T g2 :

ð2Þ

Furthermore, the Gordon-Taylor equation has frequently been applied to account for the Tg-composition dependence [20]

Heat Flow (mW), EXO >

PVPh

-45

T gb ¼

20/80 40/60 50/50 60/40

PPDO

45

90

135

180

225

270

ð3Þ

where j is an adjusting parameter related to the degree of curvature of the Tg-composition curve. It has been proposed that j can be taken as a semi-qualitative measure of the strength of the intermolecular interactions between the polymer blend components [20–22]. When j = 1, the Gordon-Taylor expression simplifies to Eq. (1). Although these classical models have been successfully applied to several systems, they can be still unsuitable in systems with significant deviations from the linearity. In systems containing intermolecular specific interactions, such as hydrogen bonding or complexation, the most suitable relationship is the Kwei equation [14,23]

80/20

0

w1 T g1 þ jw2 T g2 ; w1 þ jw2

315

Temperature (ºC) Fig. 1. Second scan DSC curves for PPDO, PVPh, and PPDO/PVPh blends of different composition.

T gb ¼

w1 T g1 þ jw2 T g2 þ qw1 w2 : w1 þ jw2

ð4Þ

The first term on the right hand side of Eq. (4) is identical to the Gordon-Taylor equation, derived assuming entropy and/or volume additivity for the blend components.

N. Hernandez-Montero et al. / European Polymer Journal 48 (2012) 1455–1465

The second term corresponds to the strength of the hydrogen bonding interactions in the blend, reflecting a balance between the loss of autoassociation and the gain of intermolecular hydrogen bonds. The parameter q is related to the change in energy associated with the formation of a contact pair between different segments in the mixture [22–24]. In systems with a highly symmetric deviation from linearity, it is a common practice to set j = 1; fitting to a single parameter, with a simplified Kwei equation [13,14,25]:

T gb ¼ w1 T g1 þ w2 T g2 þ qw1 w2 :

PVPh 20/80 40/60 60/40

Absorbance

1458

80/20

ð5Þ

In addition to the composition dependence of the Tg for the PPDO/PVPh system, Fig. 2 also displays the fits to the equations discussed above. As can be seen, the experimental data lie below the Fox equation, suggesting that the free volume of the blends is larger than predicted assuming free volume additivity. The Gordon-Taylor and the simplified Kwei equations fit appropriately the experimental Tg values. Consequently, these equations were chosen to predict the glass transition behavior of the PPDO/PVPh blends. The value obtained for the adjustable parameter j from the fit of experimental data to the Gordon-Taylor equation is j = 0.40 which indicates that the intermolecular interaction between PPDO and PVPh is weaker than in other polymer blends [16]. In addition, the value obtained for the adjustable parameter q from the simplified Kwei equation was q = 149. This large negative value for q suggests again relatively weak specific interactions [14]. 3.2. FTIR analysis Infrared spectroscopy is a highly effective tool in the investigation of the specific interactions between polymers. Currently, both qualitative and quantitative characterization of these interactions can be found in the literature [26–29]. These studies show that the precise distribution of the different associated species depends upon the composition of the mixture, the temperature and the equilibrium constants describing both self- and interassociation. The chemical groups involved in hydrogen bonding in the PPDO/PVPh system are the hydroxyl group of PVPh and the carbonyl and ether groups of PPDO; therefore the OH and C@O absorption regions have been analyzed here to investigate the association behavior. Fig. 3 displays the hydroxyl stretching region for pure PVPh and PPDO/PVPh blends of different composition. The spectrum of pure PVPh shows a broad, complex band due to the addition of contributions arising from a wide number of different species. The shoulder observed at about 3535 cm1 has been attributed in the traditional literature to free OH groups, but recent investigations indicate that it should be assigned to hydroxyl groups interacting with aromatic rings (OHp interactions) [30]. The actually free OH stretching band has been claimed to occur at about 3600 cm1, but is undetectable in the spectrum of pure PVPh [31]. Hydroxyl–hydroxyl autoassociation occurs through in a wide range of different species including dimers, trimers and so on, resulting in a very broad band centered at about 3360 cm1 [12–14,16,22].

3700

3600

3500

3400

3300

3200

3100

Wavenumber (cm-1) Fig. 3. Hydroxyl stretching region for pure PVPh and PPDO/PVPh blends of different composition at room temperature.

Upon mixing with PPDO the maximum of the OH stretching band shifts to higher frequencies. The location of the hydrogen bonded OH stretching band indicates the strength of the HB interactions, in fact Coleman et al. [32] have used the frequency difference (Dm) between the hydrogen-bonded hydroxyl absorption and the free hydroxyl absorption to measure the average strength of the intermolecular interaction. The observed blue shift indicates that interassociation interactions formed upon addition of PPDO to PVPh are weaker than the initially existing OH  OH autoassociating interactions. This behavior is consistent with the negative deviation relative to the Fox equation observed for the dependence of the Tg on composition (Fig. 2). In addition, in the PPDO rich compositions, most of the OH groups of PVPh should be associated with acceptor groups in PPDO, and the location observed for the PPDO/PVPh 80/20 composition (3420 cm1) can be actually attributed to these interassociation interactions. This value is closer to the typical value observed in polyester/PVPh blends (3440 cm1) than to the typical value observed for polyether/PVPh blends (3320 cm1) [16,32–41]. This result suggests that in the PPDO/PVPh blends most of the interassociation takes place preferentially with the ester carbonyl (C@OH–O interactions) rather than with the ether oxygen (–O–H–O interactions). Besides the hydroxyl-stretching region, the carbonylstretching band is also sensitive to hydrogen bonding. Fig. 4 presents the carbonyl stretching region for pure PPDO and PPDO/PVPh blends. Since miscibility is an amorphous phase feature, the spectra in Fig. 4 have been recorded in completely amorphous films (notice the absence of narrow bands attributable to crystalline phases) by choosing the appropriate experimental conditions in each case. Particularly, the spectrum of PPDO has been recorded at 135 °C (above its melting temperature), while the spectra of the PPDO/PVPh blends have been obtained at room temperature after quenching from the melt. Since the C@O stretching band is almost insensitive to temperature [42], the spectra in Fig. 4 can be directly compared

N. Hernandez-Montero et al. / European Polymer Journal 48 (2012) 1455–1465

Absorbance

20/80

1800

40/60 60/40 80/20 PPDO

1780

1760

1740

1720

1700

1680

Wavenumber (cm-1) Fig. 4. Carbonyl stretching region for completely amorphous films of pure PPDO (discontinuous line) and PPDO/PVPh blends of different composition.

-1

1709cm 40/60 20/80 PPDO

1800

1785

1770

1755

1740

1725

1710

1695

1680

Wavenumber (cm-1) Fig. 5. Second derivative of the carbonyl stretching region for completely amorphous films of pure PPDO (discontinuous line) and PPDO/PVPh blends of different composition.

regardless of the different sample temperatures. In addition, Fig. 5 shows the second derivative spectra for selected compositions. As can be seen in Fig. 4, the spectrum of amorphous PPDO does not show the usual bell shaped single gaussian band; it is instead split in two components located at about 1757 and 1732 cm1 according to the second derivative spectrum (Fig. 5). This splitting has been thoroughly analyzed in a parallel paper [43], and its origin is briefly explained here. PPDO contains the oxyethanoate structure (-O-CH2-CO-O-), and high level ab initio calculations indicate that the negative partial charge on the ether oxygen exerts a noticeable effect on the electronic resonance of the ester group, modifying its conformational and spectral behavior [43]. Particularly, the potential energy surface (PES) of the -O-CH2-CO-O- structure presents only two

1459

minima corresponding to the cis and trans conformations, instead of the usual three minima (gauche± and trans). This behavior can be rationalized by inspecting the resonance contributors for an ester group within an oxyethanoate structure (see Scheme 2). Let us first recall that ether groups carry a partial negative charge because of the Oxygen’s electronegativity. When the oxyethanoate structure is in the trans conformation, the partial negative charge of the ether oxygen destabilizes the resonant contributors of the ester group because of the electrostatic repulsion between negative charges. This interaction increases the relative weight of the first trans resonant contributor, increasing the double bond character of the C@O bond, and shifting the C@O stretching band to higher frequencies. In case of the cis conformation, the partial negative charge on the ether oxygen provides an electrostatic stabilization of the resonant structures up to merging the two gauche± minima into a single cis minimum according to the ab initio calculations [43]. In this case, electronic resonance is favoured in the ester group, the double bond character of the C@O group is reduced, and the C@O stretching band is therefore shifted to lower frequencies. In summary, splitting in the oxyethanoate structures can be attributed to intramolecular ether-ester interactions, and the components located at about 1757 and 1732 cm1 in PPDO can be assigned to C@O groups located in oxyethanoate structures adopting the trans and cis conformations respectively [43]. Turning back to the analysis of the C@O stretching band in the PPDO/PVPh blends, Fig. 4 shows that the addition of PVPh to PPDO broadens and red shifts the C@O stretching peak, indicating the promotion of new absorption components located at lower wavenumbers. The spectrum is poorly resolved because of the large number of overlapped contributions, and the second derivative techniques used in Fig. 5 enhance the spectral resolution. Three peaks can be observed in the second derivative spectra of the blends, and, for simplicity, we will denote ‘‘cis C@O groups’’ and ‘‘trans C@O groups’’ to the C@O groups located in oxyethanoate structures in the cis and trans conformations respectively. The peak at higher wavenumbers in Fig. 5 can be assigned to ‘‘free’’ trans C@O groups; the intermediate peak results from the overlap of ‘‘free’’ cis C@O groups and C@OO–H hydrogen bonded trans C@O groups; and finally the peak at lower wavenumbers (at about 1709 cm1) can be attributed to C@OO–H hydrogen bonded cis C@O groups. The intermediate band, located at about 1732 cm1 in case of pure PPDO, shifts to higher wavenumbers with the addition of PVPh up to about 1735 cm1 for the PPDO/PVPh 20/80 blend, location that can be attributed to hydrogen bonded trans C@O groups, since most of the C@O groups should be associated for the PVPh rich compositions. In summary, the locations observed for the ‘‘free’’ trans and cis C@O groups are, respectively, 1757 and 1732 cm1. The locations corresponding to the interassociated (hydrogen bonded) trans and cis C@O groups are, respectively, 1735 and 1709 cm1. The red shifts observed for both the trans and cis C@O groups are nearly identical, about 23 cm1, and are in good agreement with the values reported for other polyester/PVPh blends.

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Scheme 2. Ester resonance contributors in compounds containing the oxyethanoate structure in the trans (top) and cis (botton) conformation. The first contributor is in both cases the most important, and the central one is the less important since it contains one atom with one unfilled octet (the carbonyl carbon).

3.3. Equilibrium melting temperature: melting point depression The addition of a miscible counterpart to a crystalline polymer results in the depression of the melting point of the crystalline component of the polymer blend, especially in systems establishing specific interactions. Hence the analysis of the melting point depression serves to asses the miscibility of the system and to obtain important information such as the polymer–polymer interaction parameter (v12) [1,17,44,45]. An immiscible or partially miscible blend typically does not show the depression of the melting point associated to the addition of a miscible counterpart. In addition, the melting point of a polymer is affected not only by the thermodynamic factors but also by morphological factors such as the lamellar thickness. Therefore, equilibrium melting temperatures should be considered to separate the morphological effects from the thermodynamic effects. The depression of the equilibrium melting point can be analyzed using the Nishi-Wang equation [46], originally based on the Flory–Huggins theory [45]:

T mb



   1 RV 2 ln/2 1 1 ð1  /2 Þ ¼ þ  T m DH V 1 m2 m2 m1  þ v12 ð1  /2 Þ2

ð6Þ

where T m and T mb are, respectively, the equilibrium melting points of the pure crystallizable component and of the blends, the subscripts 1 and 2 refer correspondingly to the amorphous and crystallizable polymers, R is the universal gas constant, DH° is the heat of fusion of the perfectly crystallizable polymer, V is the molar volume of the repeating units of the polymers, m is the degree of polymerization, / is the volume fraction and v12 is the polymer–polymer interaction parameter. When both m1 and m2 are large, for high molecular weight polymers, the related terms in Eq. (6) can be neglected and the interaction parameter v12 can be written as:



DH  V 1 1 1   T mb T m RV 2



¼v

2 12 /1 :

ð7Þ

The Hoffman-Weeks method has been used [46] to eliminate the morphological effect from the melting point depression and to determine the equilibrium melting

Heat Flow (mW), EXO >

1

temperatures for PPDO and its blends. In this method, pure PPDO and its blends are isothermally crystallized at different temperatures (see experimental part) and the Tm corresponding to each Tc is obtained in a subsequent scan. The equilibrium melting point (T m ) is obtained from the intersection of the Tm vs Tc lines with the Tm = Tc line, which implies the extrapolation to lamellae of infinite thickness. After the determination of the equilibrium melting points for the pure crystalline polymer (T m ) and its blends (T 0mb ), Eq. (7) can be applied to obtain v12. However, application of the Hoffman-Weeks method to PPDO (and its blends) is not straightforward because PPDO shows two melting endotherms, a higher melting peak (HTm) and a lower melting peak (LTm) (see Fig. 6). According to recent investigations on the melting behavior of PPDO [47–49], only LTm shows the expected dependence on Tc, while HTm is attributed to the recrystallization of PPDO during the isothermal crystallization process and is reported to appear at a constant temperature of 105 °C. Therefore, LTm is the recommended choice to perform melting point depression studies [47–49]. The values obtained for the equilibrium melting point from the Hoffman-Weeks plot (Fig. 7) are summarized in Table 1. The decrease in the equilibrium melting point

LTm

80

85

90

95

HTm 100

105

110

115

120

125

130

Temperature (ºC) Fig. 6. DSC melting curve of PPDO/PVPh 90/10 blend at Tc = 80 °C. LTm is the lower melting point and HTm is the higher melting point.

N. Hernandez-Montero et al. / European Polymer Journal 48 (2012) 1455–1465

that is usually attributed to a residual entropy effect neglected in the theoretical derivation [50]. The value obtained for v12 is 1.0, a moderately negative value that confirms a thermodynamically miscible blend. The interaction energy density B, provides a different way to express the Flory–Huggins interaction parameter. According to the Flory–Huggins theory, the quantity kTv12. (k is the Boltzmann constant; T, the absolute temperature) is the change in internal energy corresponding to the formation of one inter-association contact at the expense of one self-association contact [44]. B measures the change of energy corresponding to this process but referenced to one unit of volume, instead of to one molecule (or mol), and is defined according to:

120 115

Tm´ (ºC)

110 105 100 95 90

Tm = Tc

85 60

70

80

90

100

110

120

Tc (ºC)

B¼

Fig. 7. Hoffman-Weeks plots for PPDO/PVPh blends of different composition: (D) 100/0, () 95/05, (.) 90/10, (⁄) 85/15, and (s) 80/20.

Table 1 Equilibrium melting temperature (T m ) for pure PPDO and PPDO/PVPh Blends. PPDO wt.%

T m (°C)

100 95 90 85 80

113.8 112.2 109.9 108.7 107.6

obtained upon addition of PVPh to pure PPDO suggests the miscibility of the system. As can be seen, the addition of 20 wt.% PVPh decreases equilibrium melting point of PPDO by about 6 °C. Fig. 8 shows the Nishi-Wang plot for the PPDO/PVPh system calculated using the following parameters: V1 = 100 cm3, V2 = 74.2 cm3, and DH° = 141.18 J/g [2]. The fit of the experimental data does not pass through the origin as should be expected according to Eq. (7), behavior

ν 2µ (10

(1/(T°mb)-1/(Tm°)) Δ H2µ/R ν 1µ/

2

)

10.5 10.0

9.0

9.5

8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 1.0

1.5

2.0

2.5

3.0

3.5

1461

4.0

4.5

5.0

5.5

φ1 2(102) Fig. 8. Nishi-Wang plot for the equilibrium melting temperatures obtained for the PPDO/PVPh blends, from which x = 1.0 is obtained.

RT v12 Vr

ð8Þ

where Vr is a reference volume, usually defined in terms of the molar volume of the amorphous component in the mixture. The B value obtained for the PPDO/PVPh system is 32 J/cm3. In order to gain insight on the relative importance of the ether and ester groups in the miscibility behavior of the PPDO/PVPh system, we have compared the miscibility related physico-chemical parameters obtained in this work with those already reported for other polyether/PVPh and polyester/PVPh blends. Table 2 lists the interaction parameter, v12, the interaction energy density, B, and the location corresponding to the interassociated hydroxyl stretching band for the blends of PVPh with different polyethers and polyesters. As indicated by the particularly large red shifts observed for the OH stretching band (see Table 2), PVPh is known to establish exceptionally strong HB-s with the polyethers with higher ether densities, such as poly(methylene oxide) and poly(ethylene oxide) [35]. However, dilution of the ether groups hinders dramatically the miscibility of PVPh with the polyethers, and the maximum CH2/O ratio allowing miscibility is actually about 4/1 (see Table 2) [32–34]. The miscibility of PVPh with polyesters seems less dependent on chemical structure; PVPh forms miscible blends with polyester covering a wide range of ester-group densities, with HB strengths of similar magnitude according to the location of the OH stretching band [32,33]. There are few exceptions, such as the PVPh/ Polylactide system, which shows weaker interactions that have been related to group accesibility issues [14,37]. According to Table 2, polyethers with ether group densities similar to that of PPDO show v12 > 0 values when blended with PVPh [32–34]. However, polyesters with ester-group densities similar to that of PPDO (such as PCL [38] or PHB [11]) blended with PVPh show v12 values close to that of the PPDO/PVPh system. Hence, the energetic balance corresponding to the change of interactions occurring on going from the pure polymers to the blend in the PPDO/ PVPh system is close to that observed in polyester/PVPh blends with similar functional group densities, but not to the one expected for polyether/PVP blends (considering again similar functional group densities). The comparative analysis of our results also suggests that interassociation seems to take place preferentially with the ester groups in PPDO rather than with the ether groups.

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Table 2 Interaction parameter (v12), interaction energy density (B), equilibrium melting temperature (T m ) and location of the hydrogen bonded OH stretching band of st PVPh (mOH inter ) in several polymer blends with PVPh. Blend partnera

v12

B (J/cm3)

T m (°C)

1 st mOH inter (cm )

PPDO

1.0

32

114

3420

Polyethers

PEO [10,37,40] PVME, PVEE [37,38,16] PTHF [37,38,16] PVBE, PVIBE [37,38,16]

1.0 <0 0 >0

30 <0 0 >0

69

3200 3320 3320 

Polyesters

PLLA[14] PCL [42,43] PPL, PHB, PBA, PMMA, PVAc, PTT, PBT [11,12,16,37–46]

0.4 1.1 <0

16 29 <0

212 62

3500 3440 3440 ± 10

a Acronyms: PEO, poly(ethylene oxide); PVME, poly(vinyl methyl ether); PVEE, poly(vinyl ethyl ether); PTHF, poly(tetrahydrofuran); PVBE, poly(vinyl butyl ether); PVIBE, poly(vinyl isobutyl ether); PLLA, poly(L-lactide); PCL, (e-caprolactone); PPL, poly(b-propiolactone); PHB, poly[(R)-3-hydroxybutyrate]; PBA, poly(butylene adipate); PMMA, poly(methyl methacrylate); PVAc, poly(vinyl acetate); PTT, poly(trimethylene terephthalate); PBT, poly(butylene terephthalate).

3.4. Polarizing light microscopy In crystallizable miscible blends, the presence of an amorphous component can either increase or decrease the tendency to crystallize depending on the effect of the composition of the blend on its glass transition and on the equilibrium melting point of the crystallizable component. The type of segregation of the amorphous component, influenced by parameters such as crystallization conditions, chain microstructure, molecular weight and blend composition, determine to a large extent the crystalline morphology of a crystallizable component. Fig. 9 illustrates the morphology of neat PPDO and PPDO/PVPh blends obtained by Polarizing Light Microscopy (POM) at 65 °C. For pure PPDO the micrographs exhibited the typical spherulitic morphology with Maltese cross. The crystalline arrangements grow until the crystallization process is limited by the increase of the spherulites in the neighbourhood. The spherulitic growth without banding was reported by Adjelic et al. [51] and Sabino et al. [52]. On the other hand, Pezzin et al. [47] observed regular concentric rings (single banded) for PPDO crystallized at 65 °C. Furthermore, PPDO has also the ability of showing double banding morphology. This morphological change on PPDO crystallization has been widely discussed in the academic literature by numerous researchers as well as for several polyesters [52]. In summary, when PPDO is isothermally crystallized, it exhibits large banded spherulites with different morphologies depending on the crystallization temperature from single banded structures with a very clear Maltese cross to double banded spherulites. Banding changes coincide with a change in growth regime according to the kinetic interpretation of spherulitic growth rate data [53–55]. The POM micrographs reveal that PPDO crystallizes from the melt in the presence of PVPh up to the PPDO/ PVPh 80/20 composition. In case of the 80/20 composition, the blend did not show the spherulitic morphology with the Maltese crosses as observed for the rest of the compositions investigated. The spherulites become less perfect and smaller with the addition of PVPh to PPDO. Moreover, macroseparation was not observed between PPDO and PVPh, suggesting good miscibility. The space-filling morphology of the PPDO/PVPh spherulites indicates that PVPh

is rejected in the crystallization process as a noncrystallizable component. Hydrogen bonding is expected to drive the diffusion of the amorphous component (PVPh), that occurs primarily in the interlamellar and/or interfibrillar domains of the PPDO spherulites [10]. This kind of segregation was also found in the crystallization process of miscible blends with a pair of crystalline/amorphous components [15,17,56]. Fig. 10 shows the radius of the PPDO spherulites as a function of time and composition at 65 °C. The solid lines represent the best least-squares fit to the data. From the slopes of the straight lines at each composition the growth rate of PPDO spherulites has been estimated. It is well known that the spherulite growth rate decreases with the addition of the hydrogen bond donating polymer at a given Tc [57]. As can be seen in Fig. 10, the growth rate of the PPDO spherulites decreases with the increase of the PVPh content in the PPDO/PVPh blend. As expected, the presence in the blends of amorphous PVPh, which is a hydrogen bond donating polymer with high Tg, decreases the crystallization rate of PPDO significantly. This observation agrees with the behaviors reported for PEO/PVPh [56], PCL/PVPh [57] and PES/PPDO blends [6].

4. Conclusions PPDO and PVPh are completely miscible over the whole composition range according to the single Tg criterion. The dependence of Tg on composition shows negative deviation relative to the Fox law, indicating that the free volume in the blends exceeds the additivity rule. This behavior is usually associated to weaker intermolecular interactions in the blends compared to the autoassociation interactions in the pure polymers. Pure PPDO shows a double melting peak by DSC; the peak located at lower temperatures (LTm) is attributed to a ‘‘normal’’ crystallization process from the amorphous phase, while the one at higher temperatures (HTm) is attributed to a recrystallization process. Melting point depression analysis based on the former peak has been performed on the PPDO/PVPh blends to obtain the interaction parameter, v12 = 1.0. The negative value confirms the thermodynamic miscibility of the system and is similar to the value obtained in other polyester/

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5 min

10 min

25 min

5 min

10 min

45 min

5 min

10 min

105 min

5 min

25 min

80 min

PPDO

95/05

90/10

80/20

Fig. 9. Optical micrographs of spherulite of pure PPDO and blends of PPDO/PVPh crystallized from the melt.

240

PPDO G = 9.9 95% G = 5.5 90% G = 2.2 80% G = 0.5

Spherulite Radius (μ m)

200

160

120

80

40

0 0

20

40

60

80

100

120

Time (min) Fig. 10. Plot Spherulitic growth of PPDO/PVPh blends at Tc = 65 °C. Growth rate (G) in lm min1.

PVPh blends with similar ester group densities, such as the PCL/PVPh system.

The OH stretching region of pure PVPh shows a broad band centered at about 3360 cm-1, attributed to hydroxyl–hydroxyl autoassociation. Upon addition of PPDO, the maximum of the OH stretching band shifts to higher wavenumbers up to 3420 cm1, location assigned to hydroxyl groups hydrogen bonded with the carbonyl groups of PPDO. This analysis resembles the typical behavior observed in polyester/PVPh blends. Therefore, both the interaction parameter obtained from melt depression studies and the analysis of the OH stretching band suggest that interassociation takes place preferentially with the carbonyl group rather than with the ether group. The C@O stretching band of pure PPDO is split in two components located at about 1757 and 1732 cm1, attributed to ‘‘free’’ C@O groups located in oxyethanoate structures (-O-CH2-CO-O-) in the trans and cis conformation respectively. Splitting can be explained in terms of the intramolecular ether-ester interactions occurring in the oxyethanoate structures present in PPDO. Hydrogen bonding with the hydroxyl groups of PVPh results in the corresponding associated contributions, located at about 1735 and 1709 cm1 respectively. The red shift is similar for

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the two types of C@O groups (about 23 cm1), and is in good agreement with the values observed in other polyester/PVPh blends. The POM micrographs obtained during the isothermal crystallization of PPDO show the typical spherulitic morphology with Maltese crosses. Addition of PVPh slows down the crystallization rate of PPDO, and crystallization is suppressed in blends containing above 20 wt.% PVPh.

Acknowledgment The authors are thankful for funds from Basque Government, Department of Education, Universities and Research (GIC10/152-IT-334-10) and from Spanish Government, Ministry of Science and Innovation MICINN (BIO201021542-C02-01). N.H-M. thanks the pre-doctoral Grant received from University of the Basque Country.

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