Combined Effect of Humidity and Composition on the Molecular

Article pubs.acs.org/Macromolecules

Combined Effect of Humidity and Composition on the Molecular Mobilities of Poly(ε-caprolactone-ran-ε-caprolactam) Copolymers Dinorah Newman,† Estrella Laredo,*,† Alfredo Bello,† and Philippe Dubois‡ †

Physics Department, Universidad Simón Bolívar, Apartado 89000, Caracas 1080, Venezuela Laboratory of Polymers and Composite Materials, Center of Innovation and Research in Materials & Polymers (CIRMAP), University of Mons - UMONS, Place du Parc 20, Mons B-7000, Belgium

‡

ABSTRACT: Poly(ε-caprolactone-ran-ε-caprolactam) random copolymers, P(CLo-ran-CLa), were studied in a wide composition range (0 ≤ PCLo ≤ 55 wt %) and in the wet and dry states. Both crystalline and amorphous phases of the poly(ester amide)s were characterized, the first by wide-angle X-ray scattering and the latter by the thermally stimulated depolarization current technique. Only PCLa presented 3D order, i.e., γ* and α structures, in addition to increasing lamellar thicknesses as PCLo amount rises. The amorphous phase of wet PCLa shows additionally to moisture plasticization a loss of rigidity due to the presence of PCLo units, which affects both segmental and local mobilities. Two cooperative modes are caused by the intercalated water with two different bounds (loosely, tightly), and their behavior during the drying process is the result of two competing effects, i.e., the loss of moisture and the coexistence of the PCLo segments. In the dry state the miscible amorphous phase presents a single drastic composition-dependent segmental mobility.

I. INTRODUCTION

concentration range; the copolymers were found to be miscible by the absence of structure in the melt. By using thermomechanical analysis, the changes induced by the addition of PCLo, for monomer concentration ranging from 5 to 80%, on the local and segmental mechanical relaxations were studied.1,3,8 A single broad peak was observed in the dry copolymers, thus confirming its close statistical microstructure. The moisture absorption in neat polyamide amorphous zones is well documented, and the most important effect is the effective plasticization by water molecules, which intercalate between the hydrogen-bonded amide groups in the dry material. Puffr and Sebenda12 have proposed a model for the hydration mechanism in the amorphous regions of polyamides consisting of three water molecules which are sorbed on neighboring amide groups: a tightly bound water molecule when it is formed by a double H-bond between two CO groups and a loosely bound one where two water molecules form H-bonds with the NH and CO groups. Additionally, at high moisture concentrations water clusters might exist between the disordered chains. The effect of water sorption on the molecular dynamics of neat polyamides has been studied by a variety of techniques: dynamic mechanical thermal analysis (DMTA),13 broad band dielectric spectroscopy (BBDS),14,15 and thermally stimulated

The increasing tendency to replace synthetic polymers with excellent mechanical properties by more ambient friendly materials with better biodegradability and biocompatibility of the degradation products has caused a renewed interest for poly(ester amide)s (PEAs). Copolymerization of poly(εcaprolactam), PCLa, with a high resistance to biodegradation, with poly(ε-caprolactone), PCLo, a biodegradable polyester, gives rise to a family of polymeric materials with extended applications in the modern engineering plastics industry. Several procedures have been used for the synthesis of P(CLo-co-CLa): (a) anionic copolymerization with different initiators,1−5 (b) interfacial copolymerization,6 and (c) anionic ring-opening copolymerization.7,8 The final products were in most of the cases random copolymers with variable PCLo concentration ranges. The results of standard characterization of these materials (1H NMR, FTIR, DSC) were typical of random copolymers. The thermal behavior of the copolymer showed significant negative shifts in the melting and a single glass transition temperature, Tg, as the number of CLo structural units increases.1,9 More recently, P(CLo-co-CLa) copolymers were synthesized by ring-opening copolymerization of CLa and CLo according to an hydrolytic reaction mechanism.10,11 The characterization of the copolymer backbone distribution through 1H NMR spectroscopy for different copolymer compositions showed high yields and random distribution of the components repetitive units in a wide © 2014 American Chemical Society

Received: February 3, 2014 Revised: March 24, 2014 Published: March 28, 2014 2471

dx.doi.org/10.1021/ma500260e | Macromolecules 2014, 47, 2471−2478

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carried out on an X’Pert-Pro Panalytical automatic θ−θ spectrometer equipped with a fast data acquisition device. Ni-filtered Cu Kα wavelength was employed (λ = 1.5418 Å). The angular sweep was recorded from 5° to 70° in 2θ. The spectrum deconvolution was performed from 10° to 35°. Thermally Stimulated Depolarization Current (TSDC). TSDC is a powerful dielectric technique which is applied to polar materials and has been widely used for the study of thermally stimulated relaxations in a variety of insulators such as ionic crystals (alkali halides,18 fluorites,19 oxides20) and semiconductors.21 Its use in polymers characterization has been often reported, as the high resolution of the technique, due to its low equivalent frequency, allows the separation of relaxations related to different length scales. It has been often applied to the study of important homopolymers,22−24 the epoxy curing process,25 the blend miscibility,26 and the block copolymer confined mobility.27,28 The changes induced by the presence of moisture in neat polyamides has also been previously studied by TSDC.16 The starting TSDC step is the generation of a polarized sample by application of a static electric field at a temperature, Tp, where the dipolar species under study are mobile. The polarization time should be long enough to allow the dipoles to reach the ordered state. The system is then removed from statistical thermodynamic equilibrium by quenching the polarized sample to the starting temperature, generally LN2 temperature, where the relaxation times are very long as compared to the time scale of the experiment. The static electric field (Ep ≈ 1 MV/m) is removed and the sample is short-circuited until the current, measured with a Keathley 6517 A electrometer, is stable. Temperature is then raised at a linear heating rate (b ≈ 4 K/min), while the depolarization current, I(T), originated by the recuperation of equilibrium is recorded. The polarizing step is made with a gaseous N2 atmosphere in the cryostat, and the depolarization current due to the reorientation of the dipolar species is recorded in a pure He atmosphere. The samples were equilibrated in ambient conditions, i.e., relative humidities of about 80% at 298 K. This is what we will label as the wet state in this work. The absorbed humidities, hsample, were measured by the weight loss of each sample before and after the drying process; they are reported in Table 1 together with the percentage humidity referred to the polyamide fraction only, hPCLa; it can be seen that in the three PEAs hPCLa is almost constant (≈3.3 wt %) while in pure PCLa it reaches 4.5 wt %. We first poled the samples at room temperature, and their spectrum was recorded up to 300 K. The secondary and primary modes were recorded, and an appropriate choice of the following polarization temperatures could be made in order to isolate the less intense local modes from the much more intense α modes. Then, samples were poled at T ≈ 245 K (except for pure PCLo) to observe the changes on the local modes with composition and water content. After each low temperature run, the poling temperature was increased to follow the drying process effect on the main relaxations. Finally, the samples were annealed in an oven at 393 K in order to reach a dry state. It is to be noted that the dry P(CLo55-ran-CLa45) TSDC spectrum was not on a sample as efficiently dried as the other PEAs; its melting temperature being 365 K,11 the samples could not be handled properly to reload the measuring TSDC cell after the annealing in the oven.

depolarization currents (TSDC);16,17 the latter technique has the highest resolution power due to its low equivalent frequency ≈30 mHz. The dry PCLa samples presented in the BBDS spectrum a single α mode while the wet ones showed very clearly the presence of two segmental modes whose average relaxation time variations with temperature were described by Vogel−Fulcher−Tammann expressions.14 Also, the low-temperature zone was composed by two wide secondary relaxations, γ and β, with increasing temperature, which are very sensitive to the water content in both their temperature positions and intensities. These variations in the dielectric spectra were interpreted as the effect of the different sites for water molecules sorption in the amorphous regions of PCLa.14,16 Even though the plasticization of PCLa by dielectric techniques has been carefully studied, no similar results exist in the case of P(CLo-ran-CLa) copolymers, where the molecular dynamics have been followed by DMTA only. In this work on P(CLo-ran-CLa) in a wide monomer composition range, the changes in the chain mobilities at different scales triggered by the PCLo segments, together with the humidity linked to the PCLa segments, will be studied through the evolution of the dielectric relaxations observed by TSDC of the amorphous chains; the samples were followed either in the wet or dry states as well as during the drying process. The quantification of the amount of 3D ordered regions through wide-angle X-ray scattering (WAXS) will also allow a better view of the morphology of such an important material.

2. EXPERIMENTAL SECTION Materials. The random copolymer synthesis was carried out by hydrolytic ring-opening polymerization by adding simultaneously CLo and CLa comonomers in bulk, i.e., in the absence of solvent, using H3PO2 (50 wt % in water) as catalyst and Jeffamine M1000 (from Hunstman) as α-NH2 end-functionalized macroinitiator. Jeffamine consisted of random copolymers of ethylene oxide and propylene oxide with a high PEO content (PO/EO molar ratio = 1/16). Full details of the copolymerization procedure are given in previous works.10,11 Some characteristics of the copolymers studied here are reported in Table 1. The subscripts represent the nominal weight percentage of the CLo and CLa units, and Mn is the number-average of the molecular weight of the entire copolymer. It is to be noted that specifically for P(CLo36-ran-CLa64) the macroinitiator was Jeffamine ED2003, i.e., a α,ω-NH2-bifunctional macroinitiator. Wide-Angle X-ray Scattering (WAXS). WAXS experiments at ambient conditions on the copolymers and the homopolymers were

Table 1. Some Characteristics of the Studied Poly(ester amide)s abbreviation PCLo PCLa PCLo6-ranCLa94 PCLo36-ranCLa64 PCLo55-ranCLa45

Mna (g/mol)

Xcb (%)

Tm,α1 (Tm,αdry)c (K)

hsample (hPCLa)d (wt %)

50 000 18 500 18 400

46 8 28

280 (330) 265 (302)

0 4.5 (4.5) 3.0 (3.2)

64 600

40

254 (260)

2.3 (3.5)

6 500

44

221 (230)

1.6 (3.5)

3. RESULTS AND DISCUSSION WAXS Experiments on Wet Samples. In Figure 1, the WAXS spectra given by the homopolymers poly(ε-caprolactone), PCLo, and poly(ε-caprolactam), PCLa, and the three copolymers composition are displayed. Each spectrum has been vertically shifted to separate the traces. The homopolymer patterns are given as references and not as the starting materials as the synthesis of the PEAs was made from ring-opening copolymerization of CLo and CLa following an efficient hydrolytic reaction scheme. The areas under each trace are normalized to a same value to make the comparison more

a Molecular weights as in ref 11. bWeight crystallinity percentage of PCLa component from WAXS results. cTemperature of the maximum of the TSDC α1 mode and of the (αdry mode). dMoisture wt % calculated as the ratio of the sample weight loss referred to the weight of the dry sample or to the (PCLa weight in the dry sample).

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The spectrum analysis was made by performing a deconvolution in Pearson peaks with adjustable width, position, and intensity. The fitting was based in the sum of Bragg peaks and wide halos caused by the amorphous regions of the two components. The weight crystalline percentage of the PCLa component for each sample, XcPCLa, which variation with composition is represented in the inset of Figure 1, was estimated by adding the integral intensity of the Bragg peaks (α + γ) divided by the integral areas of the amorphous halo plus the Bragg peaks due to the PCLa component. The variation plotted in the inset of Figure 1 shows that PCLa crystallinity in the copolymers decreases when the amount of PCLa increases. As the PCLa segments with 3D order form a separate phase, the crystallinity should not change if the amorphous phases were immiscible. The observed effect shows the importance of the PCLo concentration in the copolymer significantly favoring the crystallization of the rigid PCLa units, which are now linked to segments that have a higher mobility being in the liquid state. An example of the analysis is given in Figure 2 for the P(CLo55-ran-CLa45) sample; most of the crystallized chains are

Figure 1. WAXS patterns (λ Cu Kα) recorded for the homopolymers and the indicated copolymers. The area under the curves has been normalized. The variation of the PCLa crystallinity percentage, XcPCLa, and approximation to lamellar thickness, S , is represented in the inset as a function of the nominal composition of the copolymers.

accurate. The crystalline structure of PCLo is orthorhombic with space group P212121, and lattice parameters a = 7.496 Å, b = 4.974 Å, and c = 17.297 Å.29 As it is well-known, PCLa occurs in more than one polymorphic form, namely α and γ depending on the thermal treatment and moisture sorption.30−32 The α form corresponds to a monoclinic cell with parameters a = 9.71 Å, b = 8.19 Å, c = 17.4 Å (fiber axis), and γ = 115° as determined by Malta et al.33 They also assigned the P21 space group to the crystalline α-structure of PCLa which crystal packing may be visualized as parallel sheets of H-bonded antiparallel chains, oriented in the same sense but alternatively displaced by 3.73 Å along the c-axis. Only two main reflections are observed at angles 2θ(200) = 20° and 2θ(020)(22̅ 0) = 23°; the calculated and observed intensities of the other reflections are negligible. The γ form is made of parallel chains with the amide groups twisted out of the CH2 plane which leads to a monoclinic structure with shorter parameters as compared to the α enantiomorph (a = 9.33 Å, b = 16.9 Å, c = 4.78 Å, and β =121°).34 The γ structure is characterized by an intense peak at 2θ = 21° and a less intense one corresponding to the (002) reflection at 2θ = 11°. A third polymorph, labeled as γ*, more disordered than the γ one, presents a pseudohexagonal structure35 with similar parameters as the γ form but less stable (a = b = 4.79 Å, c = 16.7 Å); it arises from fast cooling from the melt or from cold crystallization and is easily converted into the α form upon annealing. In this case the (002) line is less significant than before. The neat PCLa spectrum shown in Figure 1 is characteristic of the γ* structure described above. A single peak at 2θ = 21° and no significant line at 2θ = 11° indicates that the PCLa crystallized in the γ* structure rather than γ. When CLo is copolymerized with CLa only, the PCLa fraction crystallizes and both the α and traces of the γ* structure are observed. Starting with the P(CLo6-ran-CLa94) copolymer and as the PCLo concentration increases, the amount of γ* structure decreases. In the most PCLo-enriched copolymer the line at 21° in 2θ is so weak that its effect is to distort the expected valley shape trace between the two α lines.

Figure 2. Example of a deconvolution of the diffraction scan for P(CLo55-ran-CLa45) showing the contribution of the amorphous and crystalline regions of both components.

ordered according to the α structure of PCLa. The (200) reflection presents a remarkably reduced width at high PCLo concentration as seen in Figure 1. By using the Scherrer equation S=

λ Δ(2θ)hkl cos θ hkl

(1)

which is equivalent to the calculation of the resolving power of an optical grating, one can estimate the apparent lamellar thickness, S , in a direction perpendicular to the reflecting planes (hkl). Δ(2θ)hkl, expressed in radians, is the breadth of the line which is the width of a line with a rectangular profile and with the same maximum intensity and integral intensity. A lamellar thickness variation from 7 to 17 nm in a direction perpendicular to the (200) planes in α-PCLa was found as the copolymer is more PCLo enriched. The variation of the PCLa lamellar size as a function of copolymer composition is also drawn in the inset of Figure 1 (right axis). As the crystallinity increases with the amount of PCLo, the polyamide crystals become more perfect. The plasticization effect of the PCLo on the amorphous PCLa regions that will be demonstrated below explains the lamellar thickness growth as PCLo fraction increases. 2473


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Neat PCLa in the wet state gives a complex spectrum very sensitive to moisture.14,16 A weak γwCLa peak is located at 110 K followed by a βwCLa peak at about 172 K which appears as a shoulder in the TSDC trace. The latter relaxation is very intense in the wet state, and its position is closely related to the amount of sorbed water molecules. As humidity is lost, the β peak moves to higher temperatures, and it is better resolved as it is less masked by the response of water molecules. For h < 3% the βD peak in pure PCLa remains located around 195 K, and its intensity decreases until it becomes comparable to the γ mode for the rest of the drying process.16 In the PEAs, as the PCLo concentration increases, the lowest temperature relaxation in the wet state, γw, remains stable at about 110 K (see Figure 4), thus indicating that the

As a conclusion after the WAXS experiments, we have demonstrated that 3D order sets in only in the polyamide sequences while PCLo segments remain amorphous, but their coexistence contributes to the crystallization of the PCLa segments and to the lamellar perfection. The homogeneity of the amorphous phase of the PEAs at all compositions will be studied from the results obtained from the TSDC experiments described below. TSDC Results. a. Subglass Relaxations of the PEAs. The secondary relaxations originated by the short-range motions of the polar entities occurring when both components of the amorphous material are still in the vitreous state are wide multicomponent bands covering a temperature range from 80 to 180 K. These fine structure features are not usually seen by DMTA experiments where the secondary modes are often interpreted as a single relaxation. As the presence of moisture has a strong effect on the low-temperature spectrum of both neat PCLa and the poly(ester amide)s, the results on the wet state will be presented first and then the spectrum evolution during the drying procedure as successive runs are performed and finally the spectrum in the so-called dry state. Figure 3a

Figure 4. Variation of the temperature position of the maximum of the secondary modes in the wet (γw, βw) and dry states (γD) as a function of the fraction of PCLo in the amorphous PEA phases. The lines are drawn to guide the eye.

corresponding reorientation relaxation time is not significantly sensitive to the decrease of moisture amount or to the presence of PCLo. However, the γw peak intensity varies and is close to that of the PCLo for the two more PCLo-enriched copolymers when in the wet state. As for the βw peak of the PEAs in Figure 3a it is shown that the intensity and temperature of the maximum, TMβw, decrease as PCLo is added in the wet state. As the amount of water sorbed by the CLa segments in the copolymer is lower than in neat PCLa (see Table 1), a moderate shift to higher temperatures of the βw mode could be expected, as the easing effect caused by water molecules should be less significant. The observed opposite trend, a shift to lower temperatures, is an indication that a plasticization effect of the CLo on the PCLa segments is present even for these shortrange motions of the polar entities. The observed intensity loss of the βw relaxation with the amount of PCLo also confirms its origin as due to the reorientation mechanism of water−amide dipolar species. The water intercalation described by Puffr and Sebenda12 will affect the secondary relaxations as the dipolar moment of the polar species increased but due to the more voluminous reorienting units more free volume is needed which is not available at these low temperatures. The first cause will affect the intensity and the second the position in the temperature scale of the relaxation. The high rise observed in Figure 3a in the temperature range starting at about 180 K is due to the contributions of the primary relaxations low-temperature tail which, as we will see in the next section, are downshifted due to the combined plasticization effect of water and PCLo segments.

Figure 3. TSDC low-temperature relaxation modes in the PEAs: (a) in the wet state; (b) in the dry state. Tp= 245 K except for PCLo.

shows the initial spectra for the two wet homopolymers and the PEAs with three different compositions. The poling temperatures were 245 K except for neat PCLo, which was polarized at 160 K in order to avoid the overlap with its primary relaxation which is located at 207 K; it has been shown that the broad peak can be decomposed in two wide overlapping contributions, the γCLo and βCLo modes at 110 and 150 K, respectively; their average relaxation time and distribution have been previously reported.36 Also, the reorientation energies have been calculated by decomposing the wide spectrum in elementary Debye contributions following Arrhenius behavior with temperature; the average values of the activation energies were found to be EA,γPCLo = 0.33 eV and EA,βPCLo = 0.51 eV. These values were confirmed when BBDS experiments were carried out on neat PCLo. 2474


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between the wet and dry materials of the γ relaxation, as well as the decrease of the βw mode intensity as compared to neat PCLa. These effects start to be noted by adding a PCLo concentration as low as 6 wt %; however, at this low PClo content the disappearance of the β mode in the dry state is not yet observed. This behavior differences are attributed to the insertion of PCLo segments in the PCLa chain. b. Primary Relaxations of the PEAs. The dielectric manifestation of the glass−rubber transition is the α mode that occurs in the TSDC spectrum usually at about the same temperature as the Tg determined by DSC, when the heating rates are comparable. When more than one α mode is found in our sensitive experiment, the coexistence of different cooperative mobilities in the amorphous phase has been invoked to justify the splitting of the segmental mode, e.g., appearance of a rigid amorphous fraction at higher temperatures than the mobile one. The existence of multiple primary modes indicates different dynamics in the amorphous phase which can be due to different causes. For example, in miscible blends such as poly(carbonate) or poly(styrene-co-maleic anhydride) blended with PCLo displaying a wide difference in the homopolymers Tg’s, the existence of two α relaxations was quantitatively explained with the self-concentrations model37 after high-resolution TSDC experiments.26,38 In the case of the randomly copolymerized PEAs the amorphous phase has been found after DMTA experiments to be miscible in the dry state as only one primary relaxation was reported; the single Tg value followed rather approximately the Fox equation applicable to miscible systems.8 In the wet state it was found that two main mechanical relaxations were present, suggesting the coexistence of two noncrystalline phases with different plasticization degrees.3 No dielectric studies that have a higher resolution power are available in the literature on these PEAs. Effect of Moisture on PCLa Primary Relaxation. The previous TSDC study on neat PCLa16 showed for all water concentrations a complex α peak, which indicated a heterogeneous amorphous phase with a wide range of relaxation times and different segmental processes. The width and profile of the peaks varied as the moisture concentration decreased as each component moved at different rates along the temperature axis (see Figure 7 in ref 16). In the present case the detailed study of the PCLa during its drying process was necessary as it served as the starting point for the interpretation of the more complex PEA spectra. In Figure 6 the successive runs of neat PCLa in the wet state are plotted; the three primary relaxations observed shift in temperature and their profiles varied. As the moisture decreases, the first two peaks, α1 and α2, move to higher temperatures, but their maxima distance remains constant and equal to 12 ± 1 K. As the sample loses most of its moisture, the α2 relaxation overlaps the third peak, labeled as αdry(wet), while the α1 still keeps moving to higher temperatures. During the gradual loss of moisture the αdry(wet) relaxation slightly shifted to higher temperature. When the sample is thoroughly dried, the only surviving peak, αdry, corresponds to the onset of the cooperative mobility of a homogeneous amorphous phase in the absence of water, and successive runs do not modify neither its position nor its profile. In Figure 7, we have plotted the variation of the peak temperatures of the three primary relaxations observed in the high-temperature TSDC spectrum of neat PCLa. This spectrum evolution is consistent with the following interpretation: (a) In the wet state the amorphous regions are

The partial drying of the PEAs occurred during the vacuum stages necessary for the gas interchange in the cell and the heating programs (up to 350 K for the P(CLo6-ran-CLa94)) sample; the recorded TSDC curves for successive runs on P(CLo36-ran-CLa64) are plotted in Figure 5 as an example. It is

Figure 5. Changes in the TSDC spectrum of the P(CLo36-ran-CLa64) during its progressive loss of moisture, Tp = 245 K.

clearly seen that as in pure PCLa γ and β relaxation shift to higher temperatures, the intensity variation being opposite for these two relaxations. The γ intensity increases and becomes predominant as moisture is lost in both PCLa and PEA. The DMTA results on PEA reported by Bernásǩ ová et al.3 showed a single weak secondary peak, labeled as β, which intensity grows in the dry state. This is not confirmed by the TSDC results where two subglass modes are clearly observed, the γ peak being the predominant one in the dry state. The variation of βwCLa is not similar to what is reported for the PEAs studied here, the difference being in intensity and peak position variations during the drying process where the loss of moisture is accompanied by the disappearance of the βw mode for the most PCLo enriched PEA. Therefore, the βw relaxation is due to the short-range reorientation of complexes where the amide groups are linked to water molecules. When the dry state is reached, i.e., when no changes can be appreciated in the TSDC spectra from one run to the next or after drying the sample in the oven, the results for the lowtemperature relaxations of the copolymers are given in Figure 3b. As PCLo concentration increases, the γD mode intensity further increases, and its position shifts to lower temperatures as seen in Figure 4. The difference in temperature between the γw and the γD is less important in the copolymers than in pure PCLa. The plasticizing effect of the randomly distributed CLo units hinders the rigidization caused by the loss of moisture. Also, the profile change of the γD mode as compared to the γw, observed in Figure 3, indicates a narrower relaxation time distribution involved in these motions. The variety of the reorienting species is now limited to the CO and NH polar groups without the water molecules contribution. Consequently, the energy landscape surrounding the dipoles may show more uniformity than in the presence of water. Finally, all the facts gathered in the previous detailed study on the subglass relaxations originated by localized motions occurring in the vitreous state show that they are strongly affected by the combined effect of moisture and PCLo; the most remarkable effect is the reduction in the peak temperature difference 2475


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intensity of the ρ peak should be higher. Besides, the important current rise observed in the wet state at lower temperatures than in the dry state often overlaps the ρ peak. The conductivity rise of the sample is attributed to the free charges motion after the onset of the cooperative mobility of the macromolecular chain. The detrapping of these charge carriers occurs at higher temperatures in the dry state and allows the observation of the ρ peak by shifting the conductivity to higher temperatures. The homogeneity of the amorphous phase in the dry PCLa where a single cooperative mobility exists is thus demonstrated. This behavior completes the results reported by Bellinger et al.39 after DMTA studies on neat PCLa. Instead of finding three relaxation modes, they report only two, αwet and αdry; they attributed the first one to all the water−amide complexes, the loosely bound, the tightly bound, and the water clusters. In our case we have been able to separate the effect of the different complexes that plasticize the amorphous regions in different ways and give rise to two distinct relaxation modes, α1 and α2. Our TSDC results go one step further as the αwet in our case is resolved into two relaxation modes, showing a more complex heterogeneity of the amorphous regions of the PCLa with three main relaxations. Combined Effect of Composition and Moisture on Poly(ester amide)s. In Figure 8, we present the important

Figure 6. TSDC spectra of neat PCLa taken during the drying process. Tp = 350 K, except runs 17 and 19 which were polarized at 370 K.

Figure 7. Evolution of the position of the maximum of the TSDC peaks for PCLa during the drying process.

heterogeneous as there is a coexistence of two different moisture plasticizations plus a dry state, which result in three relaxation modes originated by the various possible intercalation of water molecules or their absence. The complexes formed with the loosely bound water originate the α2 relaxation mode as it is the first mode that is lost due to water evaporation, and it tends first to the αdry relaxation during the partial drying of the sample. The remaining mode, α1, is attributed to the onset of the cooperative mobility of tightly bound water dipoles. (b) As the drying procedure progresses, these peaks move to higher temperature because the segmental motions become more hindered by the moisture loss and their intensities decline as the dipoles in the dry state have a lower moment. The slight increase observed in Tm,αdry(wet) in Figure 7, when the sample is only partially dried, was also reported by Bellinger et al.39 after DMTA. (c) When all moisture is extracted from PCLa by annealing in the oven and the TSDC spectra remains unchanged after successive runs (11 to 19), it is assumed that a stable dry state is reached and a single segmental relaxation is recorded followed by a ρ peak. In the wet samples the ρ peak is probably hidden under the steep current rise recorded at high temperatures as shown in Figure 6. This peak is commonly observed in semicrystalline materials and caused by a Maxwell−Wagner−Sillars interfacial polarization due to the accumulation of free charges at the interfaces of the crystal lamellae. As the crystalline phase is more abundant in the dry than in the wet state (17% vs 8%), the

Figure 8. High-temperature spectra of the PEAs in the wet state with various compositions (Tp,PCLo = 207 K, Tp,PEAs = 298 K, and Tp,PCLa = 350 K). The current density curve for neat PCLo has been multiplied by 10.

changes in the wet state (first TSDC run) of the PEAs under experimentation as their composition varies. Again, a multicomponent high-temperature spectrum sweeping the interval between TgPCLo = 207 K and TgPCLa = 330 K is observed as the PCLa concentration increases. The same α1 and α2 modes observed in neat PCLa appear here and were attributed to the segmental mobility of the amorphous chains plasticized either by the tightly or the loosely bound water. The wet PEAs sample spectra (hPCLa ∼ 3.3%) shift to much lower temperatures than that of the pure PCLa (hPCLa = 4.5%) as the PCLo content increases. The shift of the α1 mode from the wet PCLa to the most PCLo-enriched copolymer reached −59 K as a result of the additional plasticization effect of the more flexible PCLo segments. The morphology of the amorphous phase is maintained; i.e., two segmental mobilities are present. It is to be noted that the third peak identified in pure PCLa, αdry, is not visible here as the temperature interval to be explored was chosen below room temperature, too low to 2476


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to PCLa, the primary mode in the dry state is considerably shifted to lower temperature, that is −28 K. As the concentration of PCLo increases, the position of the αdry relaxation continues its negative shift until it reaches a temperature of 230 K for the most PCLo enriched copolymer. Referring to the temperature difference among the wet and dry sample relaxation positions, it is less important at higher PCLo concentrations where the single segmental mode appears in the dry state at a temperature intermediate between the corresponding Tm,α1 and Tm,α2 of the wet state. This similarity among the two states results from the plasticization effect of the PCLo which becomes predominant over the rigidization caused by the loss of water molecules. The comparable mobilities in the dry and wet states in the presence of 36 or 55% of CLo units is accompanied by more intense relaxation modes as the plasticization effect increases, even though the amorphous phase is less abundant as the crystallinity also increases. This indicates that as in the case of the short-range dynamics, the number of dipoles contributing to the cooperative motion increases as the PEA is enriched in PCLo. The PCLo component besides plasticizing the PCLa segments supplies carbonyl groups, which contribute to the short and long-range dynamics of the macromolecular chain. Because of the competing effects of the PCLo units and the drying of the sample, the Fox equation valid for miscible blends is only approximately followed in our case as it is shown in Figure 10 when applied to the three modes recorded for each PEA from the wet to the dry states. Finally, we have shown that a material designed to improve the mechanical properties and biodegradability of its components will also show a significant decrease in the glass transition temperature as a function of composition, in either the wet or the dry state.

induce any moisture vaporization. The low polarization temperature was chosen to avoid the huge current rise recorded in the first run which in the wet PEAs started at room temperature. Even though the conductivity increase was present, it was subtracted from the experimental trace before drawing Figure 8. An exponential rise following an Arrhenius law was assumed as the free charges transport contribution dependence on 1/T. In Figure 9, the TSDC spectra representing the final stage of the dehydration process (after several TSDC runs and heating

Figure 9. High-temperature TSDC spectra of the PEAs in the dry state for various compositions. The arrows indicate the polarization temperatures of each PEA.

in oven at 393 K) are shown. The high temperature spectrum is now reduced to a single mode, αdry, representing the segmental mobility of a far more homogeneous miscible amorphous phase than before. Also, in the absence of moisture the shift to lower temperatures under the influence of composition alone (−100 K) is larger than in the wet state. The CLo segments which are in the liquid state above 207 K act as a macromolecular solvent for the more rigid CLa units and ease the onset of the cooperative mobility in the PEAs. The detailed variation of the temperature of the maxima of the primary modes as a function of the composition of the amorphous phases in the wet (Tm,α1 and Tm,α2) and dry (Tm,αdry) states is represented in Figure 10. It is readily seen that when a concentration in weight of PCLo as low as 6% is added

4. CONCLUSIONS The morphology of the copolymer samples is formed by crystalline PCLa regions with γ* and α structures immersed in a complex amorphous region, which allows different cooperative mobilities due to the diverse links of the water molecules and to the amounts of PCLo segments always present in a disordered state. The crystallinity of the copolymers is composition-dependent and reaches 44 wt % of PCLa for the most abundant PCLo copolymer, with more perfect αstructured crystals. The high resolution power of our dielectric technique allowed the separation of the two local mobilities, γ and β, originated by short-range motions of polar groups and to study the plasticization effect of water molecules, which forms different types of complexes in the presence of variable amounts of PCLo. The combined effect of water and PCLo reduces the large temperature differences between the two secondary modes in the wet state as well as the amount of reorienting water amide dipoles originating the βw relaxation which importance declines in the dry samples. Additionally, the γdry mode which corresponds to the shortest range local motions shifts −46 K as the polyester concentration increases. In the presence of moisture and CLo segments the hightemperature spectra are made of two primary relaxations due to plasticization by different attached water molecules (tightly or loosely bound) that converge to a single segmental mode in the dry state where water has been extracted after the heating scans. We have shown that when 6 wt % of PCLo is added, the reduction of the Tgs in the wet state is caused by the combined

Figure 10. Glass transition temperatures evolution (temperature of the maxima of the primary TSDC relaxations) in the dry and wet states as a function of the PCLo content in the amorphous phase of the sample. The lines correspond to the Fox equation drawn for each case. 2477


Macromolecules

Article

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effect of moisture and the polyester segments intercalated in the chain, while for more PCLo-enriched copolymers the mobility enhancement is caused solely by the addition of increasing concentrations of the polyester already in liquid state. The significant negative shift observed in the dry state of the copolymers for the segmental relaxation is to be attributed to the sole increasing plasticization effect of the CLo units. The molecular dynamics as seen by the dielectric responses of these PEAs are drastically affected by the moisture and PCLo contents, which show synergistic plasticization effects on the relaxation modes at different scale. The significant lowering of their glass transition temperatures has to be added to the known improvement of the biodegradation and mechanical properties of these random copolymers as compared to its components. Amorphous zones in the liquid state at ambient temperature in the copolymers might contribute to the known increase in the biodegradability of the copolymers.

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AUTHOR INFORMATION

Corresponding Author

*Tel +58 212 9063536; Fax +58 212 9063527; e-mail elaredo@ usb.ve (E.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are grateful to Prof. Rose Mary Michell for her assistance with sample molding and to Prof. Alejandro J. Müller for helpful discussions. The partial financial support of FONACIT is also acknowledged here.

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Combined Effect of Humidity and Composition on the Molecular

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