Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Differences between Isotropic and Self-Nucleated PCL Melts Detected by Dielectric Experiments L. Sangroniz,† R. G. Alamo,‡ D. Cavallo,§ A. Santamaría,† A. J. Müller,*,†,∥ and A. Alegría*,⊥ †
POLYMAT and Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, 20018, Donostia-San Sebastián, Spain ‡ Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer St., Tallahassee, Florida 32310-6046, United States § Department of Chemistry and Industrial Chemistry, University of Genova, Genova, Italy ∥ IKERBASQUE, Basque Foundation for Science, Bilbao, Spain ⊥ Departamento de Física de Materiales, University of the Basque Country UPV/EHU and Centro de Física de Materiales (CFM) (CSIC-UPV/EHU) - Materials Physics Center (MPC), Paseo Manuel de Lardizabal 5, 20018 San Sebastián, Spain S Supporting Information *
ABSTRACT: Melt memory effects on polymer crystallization are commonly reported in the literature, even when they are not completely understood. In particular, the exact nature of the melt heterogeneities that cause an enhanced nucleation (i.e., the “self-nuclei”) is unknown. This is partly due to sensitivity limitations of the experimental techniques employed to study melt memory. In this work, the melt memory effect of semicrystalline polymers is studied for the first time by dielectric measurements. Polycaprolactones of two different molecular weights have been investigated. Isotropic or selfnucleated melt states are obtained, at a given experimental temperature, by cooling from the isotropic melt or heating from the semicrystalline solid, respectively. A detectable decrease in electrical permittivity is obtained for a self-nucleated melt, consistent with the presence of molecular dipoles with restricted mobility in the case of samples displaying crystalline memory. The volume fraction of repeating units involved in the formation of self-nuclei is estimated to be lower than 0.4%. The relative difference in dielectric permittivity between self-nucleated and isotropic melt state shows excellent correlation with rheological measurements that detect an increase in Newtonian viscosity and with the enhancement of nucleation density, measured by DSC. Each of these measured parameters showed a different sensitivity to the presence of self-nuclei, which is linked both to their nature and to the features of the specific measurements. It is suggested that the relatively strong memory effect displayed by PCL, which can be evidenced by different techniques, is related to the presence of weak intermolecular hydrogen-bonding interactions.
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INTRODUCTION The overall crystallization of a semicrystalline polymer that is cooled from the melt comprises primary nucleation and crystal growth. Primary nucleation is triggered by heterogeneities in bulk polymers, but it can also start from self-nuclei. The self-nucleation (SN) technique in polymer science was first introduced by Blundell et al.1 in their pioneering studies on single crystals. Fillon et al.2 applied similar concepts to study SN of melt-crystallized isotactic polypropylene by differential scanning calorimetry (DSC). The SN of polymeric materials employing DSC has been recently reviewed by Michell et al.3 The SN thermal protocol first creates a standard crystalline state, and then the sample is heated to partially melt the polymer in order to produce self-nuclei. Therefore, nucleation density can be significantly increased by the generated selfnuclei. If self-nuclei are constituted by crystal fragments surviving from partial melting, they have the ability to produce epitaxial © XXXX American Chemical Society
nucleation. However, it has been demonstrated that self-nuclei can also be produced by completely melting the existing polymeric crystals.3,4 In those cases, neither the chosen temperature nor the time is enough to produce an isotropic melt. Hence, self-nuclei may be constituted by regions of the melt where chain conformations still retain some of the orientational memory they had inside the crystals. Such crystalline memory effects induce nucleation upon cooling from nonisotropic or self-nucleated melts.3−6 The standard procedure to perform self-nucleation by DSC has been described earlier.3 The method consists of five steps performed at constant scan rates as can be seen in Figure 1a: (1) Heating to a temperature high enough to erase crystalline memory (in a given time) and produce an isotropic melt Received: April 3, 2018 Revised: April 26, 2018
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DOI: 10.1021/acs.macromol.8b00708 Macromolecules XXXX, XXX, XXX−XXX
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have been considered in Figure 1b. In the high temperature range within domain II, the melt retains some residual chain orientation or crystalline memory (see High Ts, DII cartoon in Figure 1b). In the low temperature range within domain II, in addition to regions of residual chain orientation, small fragments of crystals remain that cannot be annealed during the time spent at Ts (see Low Ts, DII cartoon in Figure 1b). In domain III the polymer partially melts, while the unmolten crystals thicken (see DIII cartoon in Figure 1b) by annealing during the 5 min that the material remains at Ts. An example of a polymer whose self-nucleation domains can be described by the scheme presented in Figure 1b is poly(εcaprolactone).6 Figure 1c is a plot of the standard melting endotherm of one of the PCL samples employed in this work, where the vertical solid lines indicate the borders in between the different self-nucleation domains. Figure 1c shows that PCL is characterized by a large crystalline memory. The melting process according to the DSC trace is finished by 59 °C. However, PCL can be self-nucleated until 66 °C, as indicated by the increase in Tc values (the Tc values are shown plotted on the right-hand side y-axis of Figure 1c, while the x-axis has been used to plot Ts temperatures). This is the reason why a dotted vertical line has been included in Figure 1c. This dotted line divides domain II into two temperature regions: a high temperature one (High Ts DII in Figure 1b), where crystalline memory prevails (and the nature of self-nuclei is unknown) and a low temperature one (Low Ts DII in Figure 1b), where crystal fragments act as self-nuclei. Self-nucleation (SN) and melt memory effects have been widely studied for different polymers and copolymers in the literature.1−4,9−11 SN has been useful to identify liquid−liquid phase separation in melts of broadly distributed ethylene copolymers,11 to study isothermal crystallization kinetics of polymers with slow crystallization rates,3,4 to study fractionated crystallization,9 and to favor the formation of a determined crystalline phase in the case of polymorphic polymers.12,13 The exact nature of the active self-nuclei in the high temperature region of domain II is still unknown. The nucleation effects caused by such self-nuclei are usually referred to in the literature as crystalline memory effects. The reason for their unknown nature is that it is very difficult to detect differences between isotropic and self-nucleated melts. There are several hypotheses in the literature about the origin of melt memory effects. Lorenzo et al.4 considered that self-nuclei in the high temperature region within domain II originate from residual orientation of the chains that were inside the crystal. Other authors state that self-nuclei originate from remnants of crystal fragments.2,14 According to Reid et al.10 and Luo et al., 15 melt memory originates from heterogeneities in the distribution of topological constraints and melt entanglements, whereas for Muthukumar et al.,16 a self-nucleated melt is an inhomogeneous metastable melt state.4,10,15 DSC and FTIR can indirectly detect the differences between domain I and domain II, as they can both measure the induction time and crystallization temperatures upon cooling from an isotropic melt or from a self-nucleated melt.1−4,17,18 Proton NMR studies have been performed on polypropylene and polyethylene to unravel the effect of crystalline memory.19−22 Results of spin−spin relaxation performed in the melt have been interpreted as indicative of the presence of a certain fraction of high-segmental-density region, remarkably dependent on thermal history and correlating with enhanced
Figure 1. (a) Schematic representation of the standard self-nucleation protocol. (b) Self-nucleation domains. (c) Standard self-nucleation data for one of the PCL samples employed in this work (with Mw = 26 kg/mol). The peak crystallization temperatures (Tc) (plotted on the right-hand side y-axis and represented by diamonds) obtained in the self-nucleation experiments are superimposed on the standard heating scan by employing the temperature x-axis as Ts values. The vertical lines indicate the transitions between the different domains. The dotted vertical blue line separates the domain II into two regions: the low temperature region (Low Ts, DII in part b) and the high temperature region (High Ts, DII in part b) of domain II (see text).
(typically heating to 25−30 °C above the peak melting temperature for 5 min is usually enough). (2) Cooling from the isotropic melt to a minimum temperature (at which the crystallization is complete) to produce crystallization in a “standard state” (i.e., well-known thermal history). The peak crystallization temperature registered during cooling is the standard crystallization temperature (or standard Tc), and it is a function of the density of heterogeneities present in the sample. (3) Heating from the chosen minimum temperature up to a selected SN temperature (denoted Ts) and then held at this Ts temperature for 5 min. (4) Cooling from Ts down to the chosen minimum temperature. (5) Heating from the minimum temperature up to the maximum melting temperature established in step 1. By examining the DSC traces during cooling and subsequent heating scans, three different self-nucleation domains can be defined.2,3 They critically depend on step 3, described above. Domain I or isotropic melt domain occurs if Ts is too high, and the sample completely melts into an isotropic melt. The polymer is in domain II or self-nucleation domain when Ts is high enough to melt most of the polymer crystals or to completely melt the polymer but without erasing its crystalline memory, and the sample is only self-nucleated. If Ts is low enough to only produce partial melting, the sample will selfnucleate and, additionally, the unmolten crystals will be annealed, and the polymer will be in domain III or selfnucleation and annealing domain. Figure 1b presents a schematic representation of the different domains.7 In domain I the polymer chains form isotropic random coils or a homogeneous melt (see the cartoon for DI in Figure 1b). In domain II, the melt is self-nucleated, and two possible cases B
DOI: 10.1021/acs.macromol.8b00708 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules nucleation upon recrystallization.19,20 On the other hand, more recent experiments conducted by Saalwächter et al. on syndiotactic polypropylene excluded any difference between melts with and without memory on the basis of a variety of NMR experiments.22 As such, no conclusive result on the detection of residual melt order leading to the self-nucleation effect by NMR has yet been obtained. No differences have been found between domains I and II by X-ray scattering,10 optical microscopy,12 or atomic force microscopy.14 Recently, some of us have studied the differences between isotropic and self-nucleated melts employing rheological techniques.5,6 In the case of PP-ran-PE random copolymers with a major content of PP, no differences were found between the isotropic melt state and the self-nucleated melt in the Newtonian viscosity or in the relaxation time. However, the isotropic melt showed a thermorheologically simple behavior, whereas the self-nucleated melt was thermorheologically complex. The breakdown of frequency−temperature superposition was noticed in the rubbery region, but not in the flow or terminal region. According to these results, self-nuclei in PPran-PE random copolymers consisted of chain clusters of the previous crystalline structure, rather than small crystal fragments.5 In a recent study6 with PCL homopolymers, rheological techniques were capable of detecting more efficiently the differences between the two melt states. Three PCLs with different molecular weights were studied, and the results revealed that the Newtonian viscosity and the entanglement modulus were much higher for the self-nucleated melt than for isotropic melt. In agreement with the results obtained for PPran-PE copolymers, the self-nucleated melts of PCLs were thermorheologically complex as opposed to the thermorheologically simple response obtained for isotropic melts. Dielectric relaxation spectroscopy is an experimental technique that is very sensitive to the behavior of molecular dipole moment fluctuations in insulating materials and particularly in polymers.23 The PCL repeating unit has a relatively high dipole moment, and consequently, if the dipole moment fluctuations are affected by the structures present in the self-nucleated melt, measurable differences in dielectric permittivity between PCL isotropic melt and PCL selfnucleated melt could be expected. It should be noted that dielectric spectroscopy has been already applied to the study of polymer crystallization in general24,25 and also specifically to PCL. Of particular interest in this context are the observations by Schick et al.26−28 and Ezquerra et al.,29 who distinctly detected changes in the permittivity at low frequency and in the extent of the dielectric α-relaxation in the supercooled melt, well before any crystallinity was detected by WAXD, SAXS, or DSC. The results were interpreted as an evidence of preordering phenomena occurring prior to crystallization due to either the formation of internal surfaces26−28 or a “pinning” of the chains in the amorphous phase, affecting the melt dynamics.29 Despite the exceptional sensitivity demonstrated by the technique, no studies of the dielectric properties of a polymer self-nucleated melt have been reported so far. In this work, the dielectric permittivity of isotropic and selfnucleated melts is studied for the first time to determine if this technique is able to distinguish between the two melt states. This will also allow a direct comparison of the results obtained in this work with DSC and rheological measurements, which
can clearly distinguished between self-nucleated melts (domain II) and isotropic melts (domain I), as shown in our previous work.6
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EXPERIMENTAL PART
Materials. Two poly(ε-caprolactone)s with different molecular weights have been studied. One sample was purchased from SigmaAldrich (polymer grade 440752) with a Mw= 26 kg/mol (Mw/Mn = 1.53), and the other one was provided by Solvay (CAPA 6800) with a molecular weight Mw = 195 kg/mol (Mw/Mn = 1.76). The molecular weights were measured by size exclusion chromatography (SEC) in a Waters 717 autosampler and calculated in reference to polystyrene standards. The mobile phase used was tetrahydrofuran at a 1 mL/min flow rate and at 35 °C. Differential Scanning Calorimetric Measurements. Differential scanning calorimetry (DSC) measurements were carried out using a PerkinElmer 8500. About 4 mg of PCL sample was encapsulated in a DSC aluminum pan, and the measurements were performed under ultrapure nitrogen flow. The equipment was calibrated with indium. The SN procedure was performed following two different protocols. The first procedure applied is the standard self-nucleation protocol, described in the Introduction of this work or elsewhere in the literature.3 The standard SN protocol was performed as follows: (1) erasure of the previous thermal history by maintaining the sample at 90 °C for 5 min, (2) creation of the crystalline standard state by cooling down the sample at 10 °C/min until 0 °C, the sample is maintained at this temperature for 5 min, (3) heating at 10 °C/min up to a Ts temperature, at which the sample is kept for 5 min, (4) cooling down to 0 °C at 10 °C/min, and (5) final melting of the sample by heating at 10 °C/min up to 90 °C. The second SN procedure applied to the samples mimics the thermal protocol applied to the samples during the dielectric measurements. It will be described below, and it is represented in Figure 2. Dielectric Measurements. The sample capacitor consisted in a film sample of 0.1 mm thickness that filled the space between two gold-coated disks of 10 mm diameter. Narrow Teflon ribbons were used to maintain the thickness of the sample capacitor after melting. Once in the dielectric cell the sample capacitor was heated to 100 °C and maintained at this temperature for 1 h to remove any trace of solvent or humidity from the sample prior to the dielectric experiments. Sample temperature was controlled within ±0.01 K using a Novocontrol Quatro cryostat that operates with a continuous nitrogen-jet flow. The frequency-dependent capacitance of the sample was determined by a broadband and high-resolution dielectric spectrometer, Novocontrol Alpha. Two kinds of experiments ware performed. First of all, the permittivity of the sample during crystallization and melting was analyzed. The sample was cooled down from 100 °C at 1 °C/min, i.e., from the isotropic melt state (domain I), down to −70 °C, and then subsequently heated to 90 °C. In addition, in order to study accurately the differences between isotropic melt state and self-nucleated melt state, two procedures including isothermal steps during heating were implemented. The first protocol, shown in Figure 2a, generates an isotropic melt (in domain I), and the second one, shown in Figure 2b, produces a self-nucleated melt (in domain II). For the measurements in the isotropic melt state (domain I) the sample is cooled down from 90 °C to the first measurement temperature (Figure 2a), and no crystallization occurs. In the case of the self-nucleated melt (domain II), the sample is first crystallized by cooling down from 90 to 10 °C, and after maintaining the sample at 10 °C for 10 min the sample is heated to the first measurement temperature (Figure 2b). The dielectric experiment is repeated by taking the permittivity measurements for 5 min at each temperature (every 2 degrees) as indicated in the scheme shown in Figure 2b. Since the procedure employed in the dielectric measurement is not the same as that used to define the self-nucleation domains, the DSC C
DOI: 10.1021/acs.macromol.8b00708 Macromolecules XXXX, XXX, XXX−XXX
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temperatures needed for the thermal protocol to be applied during those measurements had to be determined by DSC. Hence, we first applied the standard SN protocol and then compared it with the new self-nucleation protocol, designed to simulate as accurately as possible the thermal history applied during the dielectric spectroscopy measurements (Figure 2b). The SN behavior of PCL has been studied in detail before by some of us and reported in a recent publication for similar PCL samples.6 The DSC heating and cooling scans during the standard self-nucleation experiments are shown in Figures S1a and S1b for the low molecular weight PCL sample in the Supporting Information. For self-nucleation temperatures (Ts) above 68 °C, the sample is in domain I, as Tc does not vary (i.e., a constant peak temperature crystallization temperature of 37 °C was obtained) with Ts in this high temperature region. Therefore, heating the sample above 68 °C produces an isotropic melt (i.e., domain I), since the crystalline memory is erased. For temperatures below 68 °C, but higher than 56 °C, the sample is in domain II or in the self-nucleation domain, as the crystallization temperature in this Ts region increases in comparison to the standard crystallization temperature obtained in domain I. Finally, for Ts temperatures equal to 56 °C or below, the sample is in domain III, or in the selfnucleation and annealing domain. This can be detected by the additional melting peak that appears at 59−60 °C in Figure S1b and corresponds to the melting of annealed crystals. Figure 1c shows the crystallization temperatures obtained at different Ts temperatures superimposed on the standard PCL melting endotherm in order to represent the location of each domain. Domain II has been divided with a dotted vertical line that separates the low temperature range, where crystal fragments are the main cause of the self-nucleation effect (as the material has not finished melting and crystal remnants remain), from the high temperature range. In the high temperature range within domain II, there are no crystal fragments left, as melting has finished and no latent heat of fusion is recorded. In the high temperature range within domain II (which extends up to 8 °C above the end of melting), the crystalline memory of the material is related to the nonisotropic nature of the melt, as will be discussed below. The results are consistent with those recently reported by some of us.6 Figure 3 shows the results of applying the new SN protocol (data points are represented with gray circles) designed to reproduce the thermal history applied during dielectric measurements (Figure 2b). The crystallization temperatures are between 1 and 2 degrees higher in comparison with those measured using the standard SN procedure (data plotted with red diamonds in Figure 3), but this value is within the error of the measurements, considering the intrinsic calorimetric uncertainties (within 0.5−1 °C) and the fact that the sample is submitted to a very long thermal treatment. The most important feature is that the transition temperatures between different domains are approximately the same regardless of the SN procedure employed. In Table 1, the transition temperatures between domains for the two PCL samples employed here (i.e., low-Mw PCL and high-Mw PCL) are shown. In the case of the high-Mw PCL the transition temperature between DIII/DII changes slightly with the SN procedure employed, but in any case the differences are small (the plot is shown in Figure S2).
Figure 2. Scheme of the thermal treatment employed to study the dielectric properties of (a) the isotropic melt and (b) self-nucleated melt states. The main difference between (a) and (b) is in the first steps of the thermal protocol. By cooling from the isotropic melt (90 °C) to 58 °C directly in protocol (a), the sample is always in the isotropic melt state (domain I), as it cannot crystallize at 58 °C or higher temperatures. In protocol (b), by cooling first from 90 °C down to 10 °C, the sample is allowed to crystallize during cooling from the melt; hence, when it is heated to 58 °C (a temperature within domain II), the PCL semicrystalline sample is converted to a self-nucleated melt. experiments were also performed using the thermal procedure shown in Figure 2 to allow a direct comparison. Rheological Measurements. The rheological properties were measured employing an Anton Paar MCR 101 rheometer with parallel plates (diameter 25 mm). Small-amplitude oscillatory shear measurements were performed in the linear viscoelastic regime to obtain the storage, G′, and loss, G″, moduli. Frequency sweeps were performed from high to low frequencies for the sample cooled down from the melt (isotropic melt in domain I) and for the sample heated from the solid state (self-nucleated melt in domain II). The procedure was very similar to the one used to perform dielectric measurements, with two not relevant differences: (a) The initial temperature is 74 °C instead of 90 °C to avoid possible degradation, because it is not possible to perform measurements under nitrogen flow in the rheometer. (b) The sample is maintained at each temperature for 10 min: 5 min to achieve thermal equilibrium and 5 min to perform the frequency sweep. The corresponding Newtonian viscosities, η0, of the samples at different temperatures were determined from
η0 = lim η′ ω→ 0
(1)
where η′ is the real part of the complex viscosity, obtained from the loss modulus G″, as η′ = G″/ω.
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RESULTS AND DISCUSSION Differential Scanning Calorimetry Measurements. Before performing the dielectric relaxation experiments, the D
DOI: 10.1021/acs.macromol.8b00708 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. Results of crystallization temperature (Tc) obtained after applying the standard protocol (diamonds in red) and after applying the same SN protocol used for dielectric measurements (circles in gray) are shown. Data for the low-Mw PCL sample (with Mw = 26 kg/ mol). The peak crystallization temperatures (Tc) (plotted on the righthand side y-axis) obtained in the self-nucleation experiments are superimposed on the standard heating scan by employing the temperature x-axis as Ts values. The vertical lines indicate the transitions between the different domains. The dotted vertical blue line separates the domain II into two regions: the low temperature region (where crystal fragments remain) and the high temperature region (where all crystals are molten) of domain II.
Figure 4. Temperature dependence of the dielectric permittivity of high-Mw PCL determined at frequencies where interfacial/electrode polarization phenomena can be neglected (employing scanning rates of 1 °C/min). The inset shows the frequency range where these phenomena are relevant. The vertical lines correspond to the Tc and Tm (peak values) obtained by DSC (employing scanning rates of 10 °C/min).
Table 1. Transition Temperatures (in °C) between Domains for Low- and High-Molecular-Weight PCL Using Two Different DSC SN Procedures: the Standard SN Procedure and the Thermal Protocol Represented in Figure 2 Designed To Perform the Dielectric Measurements sample
SN procedure
DIII/DII
DII/DI
low Mw low Mw high Mw high Mw
standard similar to dielectric standard similar to dielectric
56 56 58 56
66 66 66 66
During subsequent heating of the recrystallized PCL sample, the temperature dependence of the dielectric permittivity remains similar at lower temperatures, although ε′ values are lower (the sample crystallinity is larger). Upon further heating, there is a fast increase at around 57 °C due to the melting of PCL crystals. After melting (see Figure 4), the cooling and heating curves tend to superimpose, as would be expected. However, a closer look at this high temperature melting range shows some differences that seem to vanish at the highest temperatures. Two distinct frequencies (namely 105 and 5 × 105 Hz) were selected for Figure 4 to confirm that in this range interfacial polarization phenomena are not an issue and also to evidence when frequency-independent permittivity values are obtained (at around 105 Hz). We found that this is not the case only below room temperature, so the values of ε′ determined in this frequency range can be safely connected with the orientational polarization; i.e., they correspond to the static permittivity values of PCL, and they are not influenced by interfacial polarization phenomena detectable at much lower frequencies. Figure 5 presents a more detailed comparison of the high temperature behavior. Here we have used the data at 90 °C for normalization with the attempt of compensating any minor effect on the ε′ values related to possible changes in the sample capacitor geometry during the cooling−heating cycle, where crystallization and melting events take place. A noticeable difference between the cooling and heating traces is detected up to about 70 °C. To confirm this result and trying to remove possible artifacts related to temperature lags, or any other similar effect associated with measurements during continuous temperature variation, isothermal experiments were conducted. In the isothermal experiments, as explained in the Experimental Part and described in Figure 2, the sample temperature was increased by 2 K steps and the dielectric permittivity recorded for 5 min at each temperature after temperature stabilization. This kind of experiment has been
Dielectric Measurements. Figure 4 shows how the dielectric permittivity, determined at around 105 Hz, changes with temperature during cooling at 1 K/min from the isotropic melt state (in domain I) down to −70 °C and during the subsequent heating at the same rate. PCL has permanent dipole moments in the repeating unit that contribute to the orientational polarization. Upon cooling from the isotropic melt, Figure 4 shows that first there is a monotonous increase of ε′ that reflects the expected increase of the orientational polarization with decreasing temperature due to the reduction of the thermal energy that acts against dipole orientation in the field direction. Upon further cooling, a sudden drop of ε′ is observed at around 37 °C which is caused by PCL crystallization. At room temperature PCL is in a semicrystalline state and therefore molecular mobility is restricted, but the remaining mobile molecular dipoles, associated with the amorphous regions, fluctuate still relatively fast. The corresponding orientational polarizability is moderate, and the dielectric permittivity is independent of frequency in the range around 105 Hz (see inset in Figure 4). At much lower frequencies, the dielectric permittivity increases due to interfacial polarization phenomena related to the trapping of ionic impurities at the interfaces of the semicrystalline material and/or at the electrodes.23 Further cooling below room temperature yields a smoother decrease of ε′ due to the combination of increasing crystallinity and slowing down of the remaining mobile dipoles. E
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nucleated melt, domain II) results in a melt state with slightly lower dielectric permittivity values than those of the isotropic melt for temperatures in the range 58−70 °C. The ε′ values determined using both treatments only coincide, within uncertainties, at higher temperatures when domain I is reached during the second treatment. Furthermore, similar experiments were performed on a PCL sample of lower molecular weight, and the results obtained were similar (see Figure 6). From the comparison, it found that the effect is not much modified by decreasing the molecular weight of PCL. The previous dielectric results are in qualitative agreement with the presence of some “restricted” molecular dipoles in the PCL sample with crystalline memory. After heating the sample to temperatures within domain I, an isotropic melt is obtained. By increasing the temperature above the melting point of semicrystalline PCL, the “restricted” dipoles (responsible for the crystalline memory of the material in domain II) progressively unlock and therefore contribute to reach the whole orientational polarization of the melt. To obtain a direct comparison between present dielectric results and those obtained with other techniques as previously reported in the literature, we have performed DSC and rheology experiments using the same temperature protocol used for the isothermal dielectric experiments. Moreover, we have defined for each kind of experiment a suitable normalized quantity for a direct comparison. The results are found in Figure 7 (see also Figure S3). For the comparison with rheology experiments a normalized viscosity change was
Figure 5. Zoom-in of the temperature dependence of the normalized dielectric permittivity (εN) of PCL after normalization by the value measured at 90 °C. Arrows indicate the direction of temperature variation during measurement.
performed in PCL samples subjected to two different thermal treatments: one, shown in Figure 2a, in which the melted sample is cooled directly to 58 °C and the crystallization is avoided (isotropic melt in domain I) and the other, shown in Figure 2b, in which the sample is first allowed to crystallize by cooling to 10 °C and afterward heated above the melting temperature of PCL (i.e., 58 °C). According to the DSC data, just after this latter treatment, the sample will be in domain II (self-nucleated melt state) until the crossover to domain I, at temperatures above 66 °C. The results of these dielectric experiments are shown in Figure 6, where the temperature dependence of the normalized permittivity differences (ΔεN) between the isotropic and the self-nucleated melts is presented. By means of these experiments, we confirmed that melting semicrystalline PCL just above its melting point (i.e., self-
defined as Δ log η0 =
log η0,SNM − log η0,IM log η0,IM (90 °C)
where the subscripts
SNM and IM refer respectively to the self-nucleated and isotropic melts. Figure 7 shows the temperature dependence of the normalized differences between the Newtonian viscosity obtained with the two types of experiments explained in Figure 2: the isotropic melt state (domain I) and the selfnucleated melt state (domain II). The Newtonian viscosity is higher for the self-nucleated melt than for the isotropic melt, as far as the temperature remains below 70 °C. This results from the presence of self-nuclei which lead to specific interactions between chains. This hinders the diffusion of the polymer chains in the terminal zone as we reported previously.6 Therefore, it can be considered that self-nuclei bring extra physical entanglements which cause this rheological behavior. Both dielectric and rheological results agree in showing a measurable difference in the range 58−70 °C, which vanishes at higher temperatures following nearly the same law. Interestingly, the same kind of behavior is obtained for the difference in crystallization temperatures (see Figure S3). The results indicate that the rheological properties are more sensitive to the differences between the self-nucleated melt state and the isotropic one in comparison with dielectric measurements. As stated before, the self-nuclei provide extra entanglements which result from specific interactions between chains. Rheological properties are very sensitive to this kind of interaction; however, in the case of permittivity, this technique is not sensitive to entanglements in PCL since the PCL chain does not have a net dipole moment along the chain contour and only segmental scale fluctuations are relevant. Consequently, the effects detected in the dielectric permittivity have to be connected to the small fraction of PCL repeating units involved. To have a quantitative estimation of the fraction
Figure 6. Differences between the dielectric permittivity values (ΔεN) of PCL (high Mw, circles, and low Mw, squares) determined isothermally on samples subjected to different thermal treatments (following protocols of Figures 2a and 2b). Error bars are estimated as an average of all the deviations observed in the measurements. The vertical lines indicate the transitions between the different domains obtained from Figure 3. The dotted vertical blue line (also obtained from Figure 3) separates the domain II into two regions: the low temperature region (where crystal fragments remain) and the high temperature region (where all crystals are molten) of domain II. F
DOI: 10.1021/acs.macromol.8b00708 Macromolecules XXXX, XXX, XXX−XXX
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observed in repetitive experiments. The very low values of the volume fraction involved in the “restricted” structures (i.e.,
