- Email: [email protected]
Low-temperature crystallization of poly(butylene succinate)
Accepted Manuscript Low-temperature crystallization of poly(butylene succinate) Maria Laura Di Lorenzo, René Androsch, Maria Cristina Righetti PII: DOI: Reference:
S0014-3057(17)31096-0 http://dx.doi.org/10.1016/j.eurpolymj.2017.07.025 EPJ 7978
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
20 June 2017 13 July 2017 16 July 2017
Please cite this article as: Laura Di Lorenzo, M., Androsch, R., Cristina Righetti, M., Low-temperature crystallization of poly(butylene succinate), European Polymer Journal (2017), doi: http://dx.doi.org/10.1016/j.eurpolymj. 2017.07.025
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Low-temperature crystallization of poly(butylene succinate) Maria Laura Di Lorenzo *1, René Androsch2, Maria Cristina Righetti3 1
Institute for Polymers, Composites and Biomaterials, Consiglio Nazionale delle Ricerche (CNR-IPCB), Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy
2
Interdisciplinary Center of Transfer-oriented Research in Natural Sciences, Martin Luther University Halle-Wittenberg, 06099 Halle/Saale, Germany
3
Institute for Chemical-and Physical Processes, Consiglio Nazionale delle Ricerche (CNRIPCF), INSTM, UdR Pisa, Via G. Moruzzi 1, 56124 Pisa, Italy
Abstract Low-temperature crystallization of poly(butylene succinate) (PBS) was investigated by conventional and temperature-modulated differential scanning calorimetry (DSC and TMDSC). After completion of primary crystallization, cooling of semicrystalline PBS to temperatures near or even below the glass transition temperature (Tg) leads to additional crystal growth. Such crystallization below Tg is identified by detection of frequency- and time-dependent reversing heat-capacity in non-isothermal and quasi-isothermal TMDSC analyses, respectively. Moreover, crystallization in the glassy state causes a shift of Tg to higher temperatures, thus affecting the properties of the amorphous phase. Low-temperature crystallization of amorphous PBS was recently reported in the literature and linked to completion of enthalpy relaxation of the PBS glass. In the present study, low-temperature crystallization has been shown to occur also in semicrystalline PBS, with low temperature crystallization probably involving constrained amorphous segments already coupled with the crystals.
Keywords: Polymer crystallization, poly(butylene succinate), thermal analysis
*
Email: [email protected]
1
Introduction Poly(butylene succinate) (PBS) is a biodegradable aliphatic polyester, commercially available since 1993 [1]. It is a polymer of wide industrial interest, used for production of mulching films, compostable bags, nonwoven sheets and textiles, as well as catering products and foams [1]. PBS is a "green" polymer, generally synthesized through polycondensation of 1,4butanediol and succinic acid, with both monomers produced from short-term renewable sources [2, 3]. Succinic acid can be attained from carbohydrates through fermentation with consumption of carbon dioxide, advantageously contributing to carbon sequestration [4-9], and 1,4-butanediol can be produced through hydrogenation and reduction of succinic acid [10]. PBS can also be synthesized via a “greener” route like enzymatic catalysis, using a lipase catalyst, Candida Antarctica, physically adsorbed within a macroporous resin [11, 12]. Biodegradability and compostability also contribute to the attractiveness of PBS. Biodegradation of PBS chains is initiated by hydrolysis of ester bonds, leading to the formation of water-soluble fragments with a molar mass lower than 500 Da. These short PBS chain segments can be assimilated by microorganisms and finally changed into eco-friendly products, that is, carbon dioxide, water and biomass [13, 14]. Given the many interesting properties of PBS, as for example a melting point similar to low density polyethylene, tensile strength close to polypropylene, stiffness between low density and high density polyethylene [15],
and
the
variety
of
possible
applications,
including
the
production
of
biomedical/bioresorbable materials, biodegradable agricultural film, packaging, etc. [16-20], it can be predicted that PBS resins will constitute a large market in few years. PBS is a crystallizable polymer. As such, its properties strongly depend on the crystal fraction and morphology. Crystallization from the relaxed melt leads to growth of -crystals, which transform to -crystals upon application of stress [21, 22]. Both crystal modifications have a monoclinic unit cell that includes two chemical repeating units. For the -form, the cell dimensions are a = 0.523 nm, b = 0.912 nm, c (fiber axis) = 1.090 nm, and = 123.98°. The lattice constants of the unit cell of -crystals, instead, are a = 0.584 nm, b = 0.832 nm, c (fiber axis) = 1.186 nm, and = 131.68° [22]. The crystallization kinetics and crystal morphology of the -form were studied by several authors [23-33]. Crystal growth usually occurs via heterogeneous nucleation, but can also be initiated by homogeneous nucleation at temperatures below 7 °C [31]. Experimental data on both the nucleation and crystallization kinetics of PBS are available in a wide temperature range, from the glass transition temperature Tg around -30 to -35 °C to temperatures close to the melting point. The melting
2
behavior of PBS has also received attention, due to the presence of multiple endotherms, which arise from melting of crystals of low stability and subsequent recrystallization of the unstable melt occurring during heating [25, 27, 28, 34-38]. The equilibrium melting o
temperature, Tm , was estimated to be 127.5 °C if determined with the Hoffman-Weeks approach, and 146.5 °C if calculated with Gibbs-Thomson equation [24]. A rigid amorphous fraction (RAF) was also detected and quantified in PBS [30]. In semicrystalline polymers, the RAF consists of amorphous chain segments which are coupled with the crystals and exhibit therefore a lower mobility compared to the bulk amorphous phase, often called mobile amorphous fraction (MAF), at identical temperature. The MAF consists of amorphous chain portions that devitrify at Tg, while the glass transition of the RAF occurs at higher temperature. Semicrystalline PBS displays a broadening of the glass transition, which was ascribed to the presence of different degrees of constraint in the amorphous phase of semicrystalline PBS [30]. The observation of different degrees of constraint in the amorphous phase is not peculiar of PBS, having been reported also for poly(L-lactic acid) [39, 40]. Recent analyses revealed that amorphous PBS can crystallize at temperatures below Tg [31]. Depending on the annealing temperature and time below Tg, the enthalpy-recovery peak, observed on heating the annealed glass, can exhibit a shoulder or even a separate peak, which was ascribed to melting of small PBS crystals formed during annealing at low temperatures. Growth of a small fraction of PBS crystals of low thermal stability was assumed to occur upon annealing below Tg for a time sufficient to complete enthalpy relaxation, since completion of densification of the glass allowed formation of homogeneous crystal nuclei and their growth [31]. Crystallization at low temperatures, below Tg, has been reported to date only for amorphous polymers [41-46]. To our knowledge, no crystal growth below Tg of already partially crystallized polymers has been reported in the literature. In this manuscript, it is shown for the first time that in a PBS with a crystal fraction around 30%, crystal growth can occur at temperatures below Tg. These low-temperature processes, initially identified by fast scanning chip calorimetry (FSC) only on fully amorphous PBS [31], have now been investigated also for semicrystalline PBS, using conventional differential scanning calorimetry (DSC) and temperature-modulated differential scanning calorimetry (TMDSC). The latter permits to separate thermal processes of different kinetics, usually identified as "reversing" and "non-reversing" components of the heat flow. Reversing heat flow is
3
associated with heat exchange that follows temperature modulation, for example the glass transition or reversing crystallization and melting. Conversely, non-reversing heat flow is caused by non-reversible processes, like enthalpy recovery or irreversible melting and crystallization [47-49]. However, it has been shown that analysis of the reversing heat-flow signal can provide valuable information about the time-dependence of slow irreversible processes too, which otherwise cannot be detected by conventional DSC. DSC analysis of PBS subjected to specifically designed thermal histories, together with TMDSC measurements at various frequencies of temperature modulation, demonstrate that crystallization of PBS can occur at temperatures below the glass transition of the MAF, not only in fully amorphous, but also in semi-crystalline PBS. This confirms the initial finding of crystallization in PBS at low temperatures in the glassy state presented earlier [31]. Possible mechanisms of crystal growth below Tg and its relation with the RAF are also discussed.
Experimental part Material PBS Bionolle 1020MD was kindly received by Showa Denko K. K. (Japan). This polymer grade has a melt-flow index of 25.9 g (10 min)-1 (190 °C, 2.16 kg). To attain smooth and flat surfaces, in order to enhance the thermal contact with the DSC sample pan [50], the asreceived dry polymer pellets were compression-molded to films with a thickness of 200 µm using a Collin Hydraulic Laboratory Forming Press P 200 E. The compression-molded films were stored in an evacuated desiccator before analyses, to avoid moisture uptake of the hygroscopic PBS.
Instrumentation DSC and TMDSC experiments were conducted with a TA Instruments Q2000 Tzero DSC, equipped with a RCS cooling accessory. The instrument was calibrated in temperature and energy with a high-purity indium standard at the various heating rates used. Dry nitrogen was used as purge gas at a flow rate of 30 ml min–1 . Preliminary tests were conducted to optimize melting conditions before crystallization experiments, needed to completely erase the previous thermal-mechanical history, and at the same time to minimize thermal degradation of the polymer [50]. Melting PBS at 150 °C for 2
4
min was identified as the minimum temperature/time needed to destroy all traces of previous order. In order to obtain precise heat-capacity values, the experimentally measured heatflow-rate raw data were corrected for instrumental asymmetry by subtraction of a baseline, measured under identical conditions as the samples, including close match of the masses of the aluminum pans. The heat-flow rate data were then converted into specific apparent heat capacities by point-by-point calibration with sapphire [51]. The compression-molded film was used for DSC and TMDSC analyses. This film had a thickness of 200 m, which corresponds to about 4 mg when cut to fit into Tzero TA sample pans. A fresh sample was used for each DSC and TMDSC experiments, to minimize thermal degradation. All measurements were repeated three times to improve accuracy.
Thermal treatments PBS was heated from 30 to 150 °C at a rate of 30 K min-1, melted at 150 °C for 2 min, cooled to the selected isothermal crystallization temperature (Tc) of 90 °C at 30 K min-1 and maintained at Tc for 15 min, a time sufficient to ensure completion of the phase transition at this temperature, checked by the return of the heat-flow rate curve to its steady-state value. After isothermal crystallization, the polymer was cooled to -70 °C at the programmed rate of 10 K min-1, and then heated until complete melting. Additional experiments were conducted by cooling PBS samples, isothermally crystallized at Tc = 90 °C for 15 min, from 90 to -20 °C at 10 K min-1, followed by cooling to -70 °C at various rates ranging from 0.05 to 2 K min-1, and subsequent heating at 20 K min-1 until complete melting. TMDSC experiments were performed after isothermal crystallization at Tc = 90 °C for 15 min, followed by rapid cooling to -70 °C. Temperature modulation from -70 °C to 150 °C was conducted with a sinusoidal temperature oscillation with an amplitude (AT) of 0.5 K and modulation periods (p) from 60 to 150 s; the underlying heating rate was 2 K min-1. TMDSC raw data were analyzed by approximating the modulated heat-flow rate and sampletemperature signals with Fourier series, and by using the corresponding amplitudes of the first harmonics for the calculation of the reversing heat capacity cp,rev according to equation (1) [47-49, 52-54]: c p ,rev ( , T )
AHF (T ) K ( , T ) AT (T ) m
( 1)
In equation (1), is the frequency of temperature modulation (=2/p), m the mass of the sample, and AHF(T) and AT(T) the amplitudes of the modulated heat-flow rate and sample 5
temperature, respectively. The frequency- and temperature-dependent calibration factor, K(,T), determined by calibration with sapphire, was 1.05 for p=60 and 90 s, and 1.00 for p=120 and 150 s, respectively. The correctness of the calibration factor was confirmed by agreement of measured and expected solid and liquid specific heat capacity data [30]. Quasi-isothermal TMDSC measurements were also conducted. The sample, again, was first melted at 150 °C for 2 min, isothermally crystallized at 90 °C for 15 min, cooled to -70 °C at the nominal rate of 10 °C/min, and then heated at 20 K min-1 to the selected base temperature To for quasi-isothermal modulation using an amplitude of 0.5 K and a period of 60 s.
Results and Discussion Figure 1 illustrates the apparent specific heat capacity (cp,app) of PBS as a function of temperature, after isothermal crystallization at 90 °C for 15 min, followed by cooling to below Tg. Heat-capacity data were gained upon heating at 2 and 20 K min-1. The experimental data are compared with thermodynamic (baseline) cp-values of solid and liquid PBS, taken from Ref. [30]. An enlargement of the plots is presented in Figure 2. A number of thermal events can be discerned in the data of Figure 1, including the glass transition centered at around -34 °C (details are shown in Figure 2), a broad and weak endotherm that extends from 50 °C to 90 °C revealed by the experimental cp,app values higher that the thermodynamic heat capacity of liquid PBS, followed by multiple melting events in a temperature range between about 90 and 120 °C. The multiple melting behavior of PBS has been discussed in the literature and ascribed to partial melting/recrystallization occurring during heating [25, 27, 28, 34-38]. This is confirmed by the data shown in Figure 1, as heating at different rates affects the extent of annealing and recrystallization, causing a slight variation of peak positions and areas. Because of the complex melting behavior of PBS, the crystallinity was initially determined from the heat evolved during isothermal crystallization, by normalizing the measured heat of crystallization with the heat of crystallization of 100 % crystalline PBS, which is 210 J g-1 at 90 °C [27]. The crystal fraction (wC) after completion of isothermal crystallization is 0.28. Details of the apparent-heat-capacity curves in the glass-transition range are provided in Figure 2, which is an enlargement of the curves shown in Figure 1. At low temperature and after completion of devitrification of the MAF, the cp,app curves overlap, and from the experimentally observed heat-capacity step at Tg a mobile amorphous fraction (wMAF) of 0.35 6
was determined by comparison with the heat-capacity step of fully amorphous PBS [30]. With the knowledge of the crystallinity and of the MAF, it is possible to determine the rigid amorphous fraction (wRAF) of PBS at Tg according to equation (2): wRAF=1 - wC - wMAF
(2)
Eq. (2) leads to wRAF = 0.37, which confirms the establishment of sizable RAF upon crystallization of PBS, as also reported in Ref. [30]. Figure 2 shows also the thermodynamic heat capacity of PBS containing 28 % crystals and 37 % RAF, calculated taking into account the temperature-dependence of the solid cp (wC + wRAF) and of the liquid cp (wMAF) [54, 55]. Further information about the thermal events that occur upon heating was gained by TMDSC. Figure 3 presents the reversing specific heat capacity of PBS measured with a modulation amplitude of 0.5 K, an underlying heating rate of 2 K min-1, and various modulation frequencies, with an enlargement shown in the right part of the figure, to highlight transition details. Data were gained after isothermal crystallization at Tc = 90 °C for 15 min and cooling to below Tg, and are compared with the apparent specific heat capacity of PBS of identical thermal history, measured on linear heating at the same rate of 2 K min-1, without superimposed temperature modulation (DSC). At temperatures lower than the glass transition and higher than the melting, experimental data obtained by DSC and TMDSC agree well with the heat capacity of solid and liquid PBS, respectively. The apparent specific heat capacity of PBS, measured by DSC, starts to deviate from the heat capacity of solid PBS at around -40 °C during the glass transition. However, some unexpected results can be observed in Figure 3: the heat-capacity increase at Tg should not be affected by the modulation frequency, as it should only be dependent on the MAF that devitrifies at Tg [56-59]. The increase in the reversing heat capacity with decreasing frequency of the temperature oscillation suggests that, besides the glass transition, additional thermal events occur at Tg. An influence of the TMDSC modulation period on the reversing heat capacity is often reported in the analysis of polymer melting, where repeated partial melting, immediately followed by recrystallization, continuously occur on heating; in such case, the reversing heat capacity increases with the modulation period [47, 49, 60-69]. When polymer melting is analyzed in a TMDSC heating experiment, it overlaps with crystal reorganization/perfection or melt-recrystallization, leading to exothermic and endothermic contributions in the cooling and heating half-cycles, respectively. Both release and absorption of latent heat produce an increase in the modulated heat-flow rate amplitude, and, in turn, an increase of the reversing heat capacity [48, 49]. In other words, if the reversing heat capacity is higher than the 7
expected thermodynamic value of the heat capacity of a given structure/system, then this indicates additional endothermic and/or exothermic events occurring during modulation, suggesting repeated melting and crystallization during the temperature oscillation [68]. The frequency-dependence of the cp,rev, seen in Figure 3, indicates that in PBS melting and crystallization start at the glass transition of the MAF, i.e. that crystals grown below Tg start their melting and perfection/recrystallization in the Tg range. To gain more quantitative information about the reversing thermal events occurring in the glass-transition range, quasi-isothermal TMDSC analyses were conducted on PBS samples isothermally crystallized at 90 °C, then cooled to below Tg, followed by temperaturemodulation around various base temperatures for a pre-defined period of 4 hours. These quasi-isothermal TMDSC experiments allow monitoring irreversible processes that occur during prolonged modulation around the base temperatures, by following the variation of the reversing heat capacity with time [68]. Results of these quasi-isothermal TMDSC analyses are presented in Figure 4, which shows the change of the reversing heat capacity with time during quasi-isothermal annealing at different temperatures in the Tg range. Despite the experimental noise, the time-dependence of the reversing heat capacity is reproducible and significant [67, 69]. For all analyzed temperatures, the reversing heat capacity slowly decreases with time, with a more rapid decrease at higher annealing temperatures. Such a decrease of cp,rev can only be caused by irreversible events that take place during the quasi-isothermal analyses, as reported in the literature for a variety of semicrystalline polymers in the melting range [68]. cp,rev decreases more rapidly at higher temperatures, indicating a larger extent of the irreversible processes occurring with the increased temperature of analysis. Further DSC analyses were conducted to confirm crystallization of PBS at temperatures below Tg. The polymer was isothermally crystallized at Tc = 90 °C, cooled at 10 K min-1 from Tc to -20 °C, i.e., to just above Tg, followed by slow cooling from -20 to -70 °C at various rates, ranging from 0.05 to 2 K min-1. This was followed by immediate reheating the polymer at 20 K min-1. The influence of the rate of prior cooling on the glass transition of PBS, measured upon subsequent heating, is presented in Figure 5. A higher cooling rate results in a shift of Tg of the MAF to lower temperatures, as it varies from -29 °C, after cooling at 0.05 K min-1, to -34 °C measured after cooling at 1 K min-1. Further increase of the cooling rate above 1 K min-1 does not affect the glass transition and as the experimental data measured on subsequent heating practically overlap, as seen by comparison of the plots measured after cooling at 1 and 2 K min-1 in Figure 5.
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As a general rule, the glass transition of amorphous and semicrystalline polymers measured upon heating is affected by the difference between cooling and heating rates, as at parity of heating rate, Tg moves to higher temperatures with the increased rate of prior cooling [54]. Such a trend can be observed also in Figure 5, but only when PBS is cooled at slow rates, up to 1 K min-1. Increasing the cooling rate above 1 K min-1 does not affect the glass transition, as proven by the data shown in Figure 5. This indicates that additional processes contribute to the shift of Tg depending on the thermal history. In semicrystalline polymers, the glass transition temperature often varies with the increase of the crystal fraction, with a trend similar to that seen in Figure 5 [54]. Such variation is also reported in Ref. [31] after crystallization below Tg of initially amorphous PBS. This variation of Tg is also seen in the plots of Figure 5, which confirms additional crystallization at temperatures below Tg triggered by the slow cooling, because slower cooling implies longer residence time at low temperatures available for crystal growth. However, the data of Figure 5 also reveal that the heat-capacity step at Tg does not vary with the rate of prior cooling, which indicates negligible variation of the MAF with the changed thermal history. In other words, despite the shift in Tg, linked to a varied crystal fraction, there is not observed a variation of the liquid-/solid-phase ratio of the polymer at completion of the glass transition. This indicates that crystallization at low temperatures does not involve the amorphous chain segments that are part of the MAF, but only amorphous chain parts that do not mobilize at Tg, i.e. the RAF. Irreversible changes of structure in PBS at low temperatures are also disclosed by the data shown in Figures 3 and 4. The frequency-dependent TMDSC data of Figure 3 and the decrease of the reversing heat capacity obtained in quasi-isothermal TMDSC experiments, shown in Figure 4, indicate that reversing partial melting and crystallization start at the onset of Tg, which confirms the experimental finding in Ref. [31] that amorphous PBS can crystallize at temperatures below Tg. Since crystal growth after cooling to below Tg does not imply a variation of the fraction of the mobile amorphous chains, it is suggested that the PBS chain segments that crystallize at low temperatures are connected to the RAF coupled with the crystals: the chain segments that become ordered at low temperatures are confined in geometrically restricted areas, presumably at the amorphous/crystal interface, where short-range motions may lead to rearrangement of chain segments into a more ordered structure. The occurrence of short-range local motions in constrained amorphous regions has been proven for a variety of polymers [70]. It is likely that such short-range motions of the amorphous chain segments that are part of the RAF, induce local ordering, with crystal formation facilitated by coupling of rigid 9
amorphous segments with crystals. Following the mechanism of molecular nucleation proposed by Wunderlich [71], a polymer molecule must be nucleated itself before the rest of the molecule can add to the growing crystal. This implies that no energy barrier for molecular nucleation needs to be overcome, since the PBS chain portions that order at low temperatures and are part of the RAF, are already bonded to the crystals. Crystal formation in amorphous PBS at temperatures below Tg was first proposed by Schick and coworkers [31], who observed a small endothermic peak at around -5 ÷ -10 °C, i.e. immediately above Tg, after prolonged annealing of amorphous PBS below Tg, around -50 °C. This revealed growth of a small amount of crystals of low stability in amorphous PBS. In semicrystalline PBS, an even smaller amount of tiny crystals grow at low temperatures, as demonstrated by the DSC and TMDSC shown above, probably a too small amount to allow measurable changes in the overall crystal fraction, in a sample already containing 30% of crystals of larger thermal stability. In amorphous PBS, formation of small crystalline aggregates at temperatures below T g was rationalized taking into account that the densification of the glass and the enthalpy relaxation need to be completed before formation of crystal nuclei, in agreement with similar studies on initially amorphous polymers, including poly(lactic acid), poly(-caprolactone), polyamide 6, or isotactic polybutene-1 [42, 46, 72]. In the case of semicrystalline PBS, it is likely that chain segments already coupled with the crystals are involved in formation of crystals at temperatures below Tg. This notion is supported by the data shown in Figure 5 that demonstrate no variation in the heat-capacity step, i.e., no diminution of the mobile amorphous fraction caused by crystal growth, but only a slight shift of Tg to higher temperature, linked to crystal development below Tg. The hypothesis of crystal formation involving constrained amorphous chain segments coupled to crystals is supported by the continuous partial melting and recrystallization of PBS revealed by the frequency dependence of the reversing heat capacity, since molecular nucleation, which is a prerequisite for reversing melting, is guaranteed by the pre-existing crystal surfaces [68, 73–75]. This confirms the role of constrained coupled amorphous segments of crystal growth in PBS at temperatures below Tg.
Conclusions Poly(butylene succinate) displays an unusual crystallization behavior, with possible crystal development at very low temperatures, below the glass transition. This peculiarity was 10
initially evidenced upon annealing below Tg of the amorphous polymer, which was shown to lead to growth of tiny and defective PBS crystals that melt and recrystallize upon subsequent heating [31]. With the DSC and TMSC analyses detailed above, it is now proven that not only the amorphous polymer, but also the partially crystallized PBS can further crystallize at temperatures below Tg. The reversing heat capacity measured upon modulated heating displays a frequency dependence in the glass transition region, indicating partial melting and recrystallization of crystals necessarily grown below Tg. Crystal growth below Tg also affects the glass transition, which moves to higher temperatures with progressive crystal formation below Tg. It is likely that crystallization at low temperatures involves constrained amorphous chain segments coupled to crystals, i.e. part of the RAF, which transforms into ordered phase with the pre-existing crystal surfaces acting as nuclei for this further crystal growth at low temperatures, below Tg.
Acknowledgements The authors wish to thank Showa Denko K. K. (Japan) for kindly providing the poly(butylene succinate) used in this research.
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[36] M. E. Makhatha, S. Sinha Ray, J. Hato, A. S. Luyt, M Bousmina, Thermal and thermomechanical properties of poly(butylene succinate) nanocomposites. J. Nanosci. Nanotechnol. 8, 1-11 (2007). [37] X. Wang, J. Zhou, L. Li, Multiple melting behavior of poly(butylene succinate). Europ. Polym. J. 43, 3163–3170 (2007). [38] M. Garin, L. Tighzert, I. Vroman, S. Marinkovic, B. Estrine, The kinetics of poly(butylene succinate) synthesis and the influence of molar mass on its thermal properties. J. Appl. Polym. Sci., 131, 40639 (2014). [39] M.C. Righetti, D. Prevosto, E. Tombari, Time and temperature evolution of the rigid amorphous fraction and differently constrained amorphous fractions in PLLA. Macromol. Chem. Phys., 217 2013-2026 (2016). [40] N. Delpouve,A. Saiter, J. F. Mano, E. Dargent, Cooperative rearranging region size in semi-crystalline poly(L-lactic acid). Polymer 49, 3130–3135 (2008). [41] C. Schick, E. Zhuravlev, R. Androsch, A. Wurm, J. W. P. Schmelzer, Influence of thermal prehistory on crystal nucleation and growth in polymers (Chapter 1). In: Schmelzer JWP (ed) Glass, selected properties and crystallization. De Gruyter, Berlin, pp 1–94 (2014). [42] R. Androsch, C. Schick, J. W. P. Schmelzer, Sequence of enthalpy relaxation, homogeneous crystal nucleation and crystal growth in glassy polyamide 6. Europ. Polym. J., 53,100-108 (2014). [43] I. Stolte, R. Androsch, M. L. Di Lorenzo, C. Schick, Effect of aging the glass of isotactic polybutene-1 on form II nucleation and cold-crystallization. J. Phys. Chem. B 117, 15196-15203 (2013). [44] E. Zhuravlev, J. W. P. Schmelzer, A. Abyzov, V. Fokin, R. Androsch, C. Schick, Experimental test of Tammann’s nuclei development approach in crystallization of macromolecules. Cryst. Growth Des., 15, 786-789 (2015). [45] R. Androsch, M. L. Di Lorenzo, Crystal nucleation in glassy poly( L-lactic acid). Macromolecules, 46, 6048-6056 (2013). [46] R. Androsch, M. L. Di Lorenzo, Kinetics of crystals nucleation of poly( L-lactic acid). Polymer, 54, 6882-6885 (2013). [47] A. Wurm, M. Merzlyakov, C. Schick, Reversible melting probed by temperature modulated dynamic mechanical and calorimetric measurements. Coll. Polym. Sci., 276, 289-296 (1998).
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[48] M. L. Di Lorenzo, B. Wunderlich, Temperature-modulated calorimetry of the crystallization of polymers analyzed by measurements and model calculations. J. Therm. Anal. Calorim., 57, 459-472 (1999). [49] M. L. Di Lorenzo, B. Wunderlich, Melting of polymers by non-isothermal, temperaturemodulated calorimetry: analysis of various irreversible latent heat contributions to the reversing heat capacity. Thermochim. Acta, 405, 255-268 (2003). [50] S. Vyazovkin, K. Chrissafis, M. L. Di Lorenzo, N. Koga, M. Pijolat, B. Roduit, N. Sbirrazzuoli, J. J. Suñol, ICTAC Kinetics Committee recommendations for collecting experimental thermal analysis data for kinetic computations, Thermochim. Acta, 590, 1-23 (2014). [51] D. G. Archer, Thermodynamic properties of synthetic sapphire (α‐ Al2O3), standard reference material 720 and the effect of temperature‐ scale differences on thermodynamic properties. J. Phys. Chem. Ref. Data, 22, 1441-1453 (1993). [52] R. Androsch, B. Wunderlich, Temperature-modulated DSC using higher harmonics of the Fourier transform. Thermochim. Acta, 333, 27-32 (1999). [53] I. Moon, R. Androsch, B. Wunderlich, A calibration of the various heat-conduction paths for a heat-flux-type temperature-modulated DSC. Thermochim. Acta, 357–358, 285–291 (2000). [54] B. Wunderlich, Thermal Analysis, Academic Press, Inc., San Diego, CA (1990). [55] M. L. Di Lorenzo, M. C. Righetti, M. Cocca, B. Wunderlich, Coupling between crystal melting and rigid amorphous fraction mobilization in poly(ethylene terephthalate), Macromolecules, 43, 7689–7694 (2010). [56] A. Hensel, C. Schick, Relation between freezing-in due to linear cooling and the dynamic glass transition temperature by temperature-modulated DSC, J. Non-Cryst. Solids 235–237, 510–516 (1998). [57] S. Montserrat, Y. Calventus, J. M. Hutchinson, Effect of cooling rate and frequency on the calorimetric measurement of the glass transition. Polymer 46, 12181-12189 (2005). [58] J. M. Hutchinson, M. Ruddy, Thermal cycling of glasses. III. Upper peaks, J. Polym. Sci. Polym. Phys. Ed. 28, 2127-2163 (1990). [59] A. Boller, C. Schick, B. Wunderlich, Modulated differential scanning calorimetry in the glass transition region. Thermochim. Acta, 266, 97-111 (1995). [60] M. L. Di Lorenzo, The melting process and the rigid amorphous fraction of cis-1,4polybutadiene. Polymer 50, 578-584 (2009).
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[61] S. Sohn, A. Alizadeh, H. Marand, On the multiple melting behavior of bisphenol-A polycarbonate. Polymer, 41, 8879-8886 (2000). [62] H. Xu, S. Ince, P. Cebe, Development of the crystallinity and rigid amorphous fraction in cold-crystallized isotactic polystyrene. J. Polym. Sci. B: Polym. Phys. 41, 3026-3036 (2003). [63] H. Lu, P. Cebe, Heat capacity study of isotactic polystyrene: dual reversible crystal melting and relaxation of rigid amorphous fraction. Macromolecules 37, 2797-2806 (2004). [64] M. C. Righetti, E. Tombari, M. Angiuli, M. L. Di Lorenzo, Enthalpy-based determination of crystalline, mobile amorphous and rigid amorphous fractions in semicrystalline polymers poly(ethylene terephthalate). Thermochim. Acta 462, 15-24 (2007). [65] M. C. Righetti, E. Tombari, M. L. Di Lorenzo, Crystalline, mobile amorphous and rigid amorphous fractions in isotactic polystyrene. Europ. Polym. J., 44, 2659-2667 (2008). [66] M. C. Righetti, M. L. Di Lorenzo, Melting process of poly(butylene terephthalate) analyzed by temperature-modulated differential scanning calorimetry, J. Polym. Sci.: Part B: Polym. Phys., 42, 2191-2201 (2004). [67] B. Wunderlich, M. Pyda, J. Pak, R. Androsch, Measurement of heat capacity to gain information about time scales of molecular motion from pico to megaseconds, Thermochim. Acta, 377, 9-33 (2001) [68] B. Wunderlich, Reversible crystallization and the rigid–amorphous phase in semicrystalline macromolecules. Progr. Polym. Sci., 28, 383-450 (2003). [69] R. Androsch, B. Wunderlich, A study of annealing of poly(ethylene-co-octene) by temperature-modulated
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17
perfection, and molecular mobility. J. Polym. Sci.: Part B: Polym. Phys. 39, 2969-2981 (2001). [74] B. Wunderlich, Effect of decoupling of molecular segments, microscopic stress-transfer and confinement of the nanophases in semicrystalline polymers. Macromol. Rapid Commun., 26, 1521-1531 (2005). [75] W. Qiu, M. Pyda, E. Nowak-Pyda, A. Habenschuss, B. Wunderlich, Reversibility between glass and melting transitions of poly(oxyethylene). Macromolecules, 38, 8454-8467 (2005).
18
Apparent heat capacity [J/ (g K)]
15
-1
2 K min -1 20 K min
10
5
0 -50
0
50
100
Temperature (°C)
Apparent heat capacity [J/ (g K)]
Figure 1. Apparent specific heat capacity of PBS, measured upon heating at the indicated rates after isothermal crystallization at 90 °C for 15 min followed by fast cooling to -70 °C. Baseline heat capacities of liquid and solid PBS were taken from Ref. [30] and are shown as dotted lines.
2,5
-1
2 K min -1 20 K min
2,0
1,5
1,0 -40
-20
0
20
Temperature (°C)
19
Apparent heat capacity [J/ (g K)]
Figure 2. Enlargement of the experimental data of Figure 1, to show details in the glass transition range. Thermodynamic heat capacities of liquid and solid PBS were taken from Ref. [30] and are shown as dotted lines while the thermodynamic heat capacity of PBS containing 28 % crystals and 37 % RAF is shown as dashed line. 25
DSC p=60 s p=90 s p=120 s p=150 s
20
15
10
5
-50
0
50
100
Apparent heat capacity [J/ (g K)]
Temperature (°C)
2,0
1,5
1,0
-50
0
50
100
Temperature (°C)
Figure 3. Apparent specific heat capacity of PBS, measured upon linear heating at 20 K min-1 using conventional DSC (DSC), and reversing specific heat capacity from non-isothermal TMDSC, with different modulation periods (p) as indicated in the legend. Data were gained after isothermal crystallization at 90 °C for 15 min, followed by fast cooling to -70 °C. The left plot reports full scale data, the right plot is an enlargement around thermodynamic heat capacity lines. Thermodynamic heat capacities of liquid and solid PBS were taken from Ref. [30] and are shown as dotted lines while the thermodynamic heat capacity of PBS containing 28 % crystals and 37 % RAF is shown with the dashed line.
20
0.1 J/(g K)
Reversing heat capacity
-40 °C
-20 °C 0 °C
0
100
200
Time (min)
0.3 J/(g K)
Heat capacity
Figure 4. Reversing heat capacity of PBS, measured during quasi-isothermal temperature modulation at the indicated base temperatures of 0, 20, and 40 °C. The modulation period and amplitude were 60 s and 0.5 K, respectively. Data were gained after isothermal crystallization at 90 °C for 15 min, followed by fast cooling to -70 °C and subsequent heating at 20 K min-1 to the temperature chosen for the quasi-isothermal TMDSC analysis.
-1
0.05 K min -1 0.1 K min -1 0.2 K min -1 1 K min 2 K min
-50
-40
-30
-1
-20
Temperature (°C) Figure 5. Heat capacity of PBS, measured upon heating at 20 K min-1 after isothermal crystallization at 90 °C for 15 min, followed by cooling to -20 °C at 10 K min-1, then cooling to -70 °C at the indicated rates.
21
Low-temperature crystallization of poly(butylene succinate) Maria Laura Di Lorenzo *1, René Androsch2, Maria Cristina Righetti3
Apparent heat capacity [J/ (g K)]
Poly(butylene succinate) 1,6
Frequency-dependent excess heat capacity reveals melting of PBS crystals grown below Tg
1,4
1,2
DSC
1,0
p=60 s p=90 s p=120 s p=150 s
-40
-30
-20
Temperature (°C)
*
Email: [email protected]
22
Highlights
Sub-Tg crystallization of semicrystalline PBS is proven by TMDSC PBS crystals grown in the glass, melt and recrystallize at Tg Crystallization of PBS below Tg possibly involves part of the RAF
23
S0014-3057(17)31096-0 http://dx.doi.org/10.1016/j.eurpolymj.2017.07.025 EPJ 7978
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
20 June 2017 13 July 2017 16 July 2017
Please cite this article as: Laura Di Lorenzo, M., Androsch, R., Cristina Righetti, M., Low-temperature crystallization of poly(butylene succinate), European Polymer Journal (2017), doi: http://dx.doi.org/10.1016/j.eurpolymj. 2017.07.025
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Low-temperature crystallization of poly(butylene succinate) Maria Laura Di Lorenzo *1, René Androsch2, Maria Cristina Righetti3 1
Institute for Polymers, Composites and Biomaterials, Consiglio Nazionale delle Ricerche (CNR-IPCB), Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy
2
Interdisciplinary Center of Transfer-oriented Research in Natural Sciences, Martin Luther University Halle-Wittenberg, 06099 Halle/Saale, Germany
3
Institute for Chemical-and Physical Processes, Consiglio Nazionale delle Ricerche (CNRIPCF), INSTM, UdR Pisa, Via G. Moruzzi 1, 56124 Pisa, Italy
Abstract Low-temperature crystallization of poly(butylene succinate) (PBS) was investigated by conventional and temperature-modulated differential scanning calorimetry (DSC and TMDSC). After completion of primary crystallization, cooling of semicrystalline PBS to temperatures near or even below the glass transition temperature (Tg) leads to additional crystal growth. Such crystallization below Tg is identified by detection of frequency- and time-dependent reversing heat-capacity in non-isothermal and quasi-isothermal TMDSC analyses, respectively. Moreover, crystallization in the glassy state causes a shift of Tg to higher temperatures, thus affecting the properties of the amorphous phase. Low-temperature crystallization of amorphous PBS was recently reported in the literature and linked to completion of enthalpy relaxation of the PBS glass. In the present study, low-temperature crystallization has been shown to occur also in semicrystalline PBS, with low temperature crystallization probably involving constrained amorphous segments already coupled with the crystals.
Keywords: Polymer crystallization, poly(butylene succinate), thermal analysis
*
Email: [email protected]
1
Introduction Poly(butylene succinate) (PBS) is a biodegradable aliphatic polyester, commercially available since 1993 [1]. It is a polymer of wide industrial interest, used for production of mulching films, compostable bags, nonwoven sheets and textiles, as well as catering products and foams [1]. PBS is a "green" polymer, generally synthesized through polycondensation of 1,4butanediol and succinic acid, with both monomers produced from short-term renewable sources [2, 3]. Succinic acid can be attained from carbohydrates through fermentation with consumption of carbon dioxide, advantageously contributing to carbon sequestration [4-9], and 1,4-butanediol can be produced through hydrogenation and reduction of succinic acid [10]. PBS can also be synthesized via a “greener” route like enzymatic catalysis, using a lipase catalyst, Candida Antarctica, physically adsorbed within a macroporous resin [11, 12]. Biodegradability and compostability also contribute to the attractiveness of PBS. Biodegradation of PBS chains is initiated by hydrolysis of ester bonds, leading to the formation of water-soluble fragments with a molar mass lower than 500 Da. These short PBS chain segments can be assimilated by microorganisms and finally changed into eco-friendly products, that is, carbon dioxide, water and biomass [13, 14]. Given the many interesting properties of PBS, as for example a melting point similar to low density polyethylene, tensile strength close to polypropylene, stiffness between low density and high density polyethylene [15],
and
the
variety
of
possible
applications,
including
the
production
of
biomedical/bioresorbable materials, biodegradable agricultural film, packaging, etc. [16-20], it can be predicted that PBS resins will constitute a large market in few years. PBS is a crystallizable polymer. As such, its properties strongly depend on the crystal fraction and morphology. Crystallization from the relaxed melt leads to growth of -crystals, which transform to -crystals upon application of stress [21, 22]. Both crystal modifications have a monoclinic unit cell that includes two chemical repeating units. For the -form, the cell dimensions are a = 0.523 nm, b = 0.912 nm, c (fiber axis) = 1.090 nm, and = 123.98°. The lattice constants of the unit cell of -crystals, instead, are a = 0.584 nm, b = 0.832 nm, c (fiber axis) = 1.186 nm, and = 131.68° [22]. The crystallization kinetics and crystal morphology of the -form were studied by several authors [23-33]. Crystal growth usually occurs via heterogeneous nucleation, but can also be initiated by homogeneous nucleation at temperatures below 7 °C [31]. Experimental data on both the nucleation and crystallization kinetics of PBS are available in a wide temperature range, from the glass transition temperature Tg around -30 to -35 °C to temperatures close to the melting point. The melting
2
behavior of PBS has also received attention, due to the presence of multiple endotherms, which arise from melting of crystals of low stability and subsequent recrystallization of the unstable melt occurring during heating [25, 27, 28, 34-38]. The equilibrium melting o
temperature, Tm , was estimated to be 127.5 °C if determined with the Hoffman-Weeks approach, and 146.5 °C if calculated with Gibbs-Thomson equation [24]. A rigid amorphous fraction (RAF) was also detected and quantified in PBS [30]. In semicrystalline polymers, the RAF consists of amorphous chain segments which are coupled with the crystals and exhibit therefore a lower mobility compared to the bulk amorphous phase, often called mobile amorphous fraction (MAF), at identical temperature. The MAF consists of amorphous chain portions that devitrify at Tg, while the glass transition of the RAF occurs at higher temperature. Semicrystalline PBS displays a broadening of the glass transition, which was ascribed to the presence of different degrees of constraint in the amorphous phase of semicrystalline PBS [30]. The observation of different degrees of constraint in the amorphous phase is not peculiar of PBS, having been reported also for poly(L-lactic acid) [39, 40]. Recent analyses revealed that amorphous PBS can crystallize at temperatures below Tg [31]. Depending on the annealing temperature and time below Tg, the enthalpy-recovery peak, observed on heating the annealed glass, can exhibit a shoulder or even a separate peak, which was ascribed to melting of small PBS crystals formed during annealing at low temperatures. Growth of a small fraction of PBS crystals of low thermal stability was assumed to occur upon annealing below Tg for a time sufficient to complete enthalpy relaxation, since completion of densification of the glass allowed formation of homogeneous crystal nuclei and their growth [31]. Crystallization at low temperatures, below Tg, has been reported to date only for amorphous polymers [41-46]. To our knowledge, no crystal growth below Tg of already partially crystallized polymers has been reported in the literature. In this manuscript, it is shown for the first time that in a PBS with a crystal fraction around 30%, crystal growth can occur at temperatures below Tg. These low-temperature processes, initially identified by fast scanning chip calorimetry (FSC) only on fully amorphous PBS [31], have now been investigated also for semicrystalline PBS, using conventional differential scanning calorimetry (DSC) and temperature-modulated differential scanning calorimetry (TMDSC). The latter permits to separate thermal processes of different kinetics, usually identified as "reversing" and "non-reversing" components of the heat flow. Reversing heat flow is
3
associated with heat exchange that follows temperature modulation, for example the glass transition or reversing crystallization and melting. Conversely, non-reversing heat flow is caused by non-reversible processes, like enthalpy recovery or irreversible melting and crystallization [47-49]. However, it has been shown that analysis of the reversing heat-flow signal can provide valuable information about the time-dependence of slow irreversible processes too, which otherwise cannot be detected by conventional DSC. DSC analysis of PBS subjected to specifically designed thermal histories, together with TMDSC measurements at various frequencies of temperature modulation, demonstrate that crystallization of PBS can occur at temperatures below the glass transition of the MAF, not only in fully amorphous, but also in semi-crystalline PBS. This confirms the initial finding of crystallization in PBS at low temperatures in the glassy state presented earlier [31]. Possible mechanisms of crystal growth below Tg and its relation with the RAF are also discussed.
Experimental part Material PBS Bionolle 1020MD was kindly received by Showa Denko K. K. (Japan). This polymer grade has a melt-flow index of 25.9 g (10 min)-1 (190 °C, 2.16 kg). To attain smooth and flat surfaces, in order to enhance the thermal contact with the DSC sample pan [50], the asreceived dry polymer pellets were compression-molded to films with a thickness of 200 µm using a Collin Hydraulic Laboratory Forming Press P 200 E. The compression-molded films were stored in an evacuated desiccator before analyses, to avoid moisture uptake of the hygroscopic PBS.
Instrumentation DSC and TMDSC experiments were conducted with a TA Instruments Q2000 Tzero DSC, equipped with a RCS cooling accessory. The instrument was calibrated in temperature and energy with a high-purity indium standard at the various heating rates used. Dry nitrogen was used as purge gas at a flow rate of 30 ml min–1 . Preliminary tests were conducted to optimize melting conditions before crystallization experiments, needed to completely erase the previous thermal-mechanical history, and at the same time to minimize thermal degradation of the polymer [50]. Melting PBS at 150 °C for 2
4
min was identified as the minimum temperature/time needed to destroy all traces of previous order. In order to obtain precise heat-capacity values, the experimentally measured heatflow-rate raw data were corrected for instrumental asymmetry by subtraction of a baseline, measured under identical conditions as the samples, including close match of the masses of the aluminum pans. The heat-flow rate data were then converted into specific apparent heat capacities by point-by-point calibration with sapphire [51]. The compression-molded film was used for DSC and TMDSC analyses. This film had a thickness of 200 m, which corresponds to about 4 mg when cut to fit into Tzero TA sample pans. A fresh sample was used for each DSC and TMDSC experiments, to minimize thermal degradation. All measurements were repeated three times to improve accuracy.
Thermal treatments PBS was heated from 30 to 150 °C at a rate of 30 K min-1, melted at 150 °C for 2 min, cooled to the selected isothermal crystallization temperature (Tc) of 90 °C at 30 K min-1 and maintained at Tc for 15 min, a time sufficient to ensure completion of the phase transition at this temperature, checked by the return of the heat-flow rate curve to its steady-state value. After isothermal crystallization, the polymer was cooled to -70 °C at the programmed rate of 10 K min-1, and then heated until complete melting. Additional experiments were conducted by cooling PBS samples, isothermally crystallized at Tc = 90 °C for 15 min, from 90 to -20 °C at 10 K min-1, followed by cooling to -70 °C at various rates ranging from 0.05 to 2 K min-1, and subsequent heating at 20 K min-1 until complete melting. TMDSC experiments were performed after isothermal crystallization at Tc = 90 °C for 15 min, followed by rapid cooling to -70 °C. Temperature modulation from -70 °C to 150 °C was conducted with a sinusoidal temperature oscillation with an amplitude (AT) of 0.5 K and modulation periods (p) from 60 to 150 s; the underlying heating rate was 2 K min-1. TMDSC raw data were analyzed by approximating the modulated heat-flow rate and sampletemperature signals with Fourier series, and by using the corresponding amplitudes of the first harmonics for the calculation of the reversing heat capacity cp,rev according to equation (1) [47-49, 52-54]: c p ,rev ( , T )
AHF (T ) K ( , T ) AT (T ) m
( 1)
In equation (1), is the frequency of temperature modulation (=2/p), m the mass of the sample, and AHF(T) and AT(T) the amplitudes of the modulated heat-flow rate and sample 5
temperature, respectively. The frequency- and temperature-dependent calibration factor, K(,T), determined by calibration with sapphire, was 1.05 for p=60 and 90 s, and 1.00 for p=120 and 150 s, respectively. The correctness of the calibration factor was confirmed by agreement of measured and expected solid and liquid specific heat capacity data [30]. Quasi-isothermal TMDSC measurements were also conducted. The sample, again, was first melted at 150 °C for 2 min, isothermally crystallized at 90 °C for 15 min, cooled to -70 °C at the nominal rate of 10 °C/min, and then heated at 20 K min-1 to the selected base temperature To for quasi-isothermal modulation using an amplitude of 0.5 K and a period of 60 s.
Results and Discussion Figure 1 illustrates the apparent specific heat capacity (cp,app) of PBS as a function of temperature, after isothermal crystallization at 90 °C for 15 min, followed by cooling to below Tg. Heat-capacity data were gained upon heating at 2 and 20 K min-1. The experimental data are compared with thermodynamic (baseline) cp-values of solid and liquid PBS, taken from Ref. [30]. An enlargement of the plots is presented in Figure 2. A number of thermal events can be discerned in the data of Figure 1, including the glass transition centered at around -34 °C (details are shown in Figure 2), a broad and weak endotherm that extends from 50 °C to 90 °C revealed by the experimental cp,app values higher that the thermodynamic heat capacity of liquid PBS, followed by multiple melting events in a temperature range between about 90 and 120 °C. The multiple melting behavior of PBS has been discussed in the literature and ascribed to partial melting/recrystallization occurring during heating [25, 27, 28, 34-38]. This is confirmed by the data shown in Figure 1, as heating at different rates affects the extent of annealing and recrystallization, causing a slight variation of peak positions and areas. Because of the complex melting behavior of PBS, the crystallinity was initially determined from the heat evolved during isothermal crystallization, by normalizing the measured heat of crystallization with the heat of crystallization of 100 % crystalline PBS, which is 210 J g-1 at 90 °C [27]. The crystal fraction (wC) after completion of isothermal crystallization is 0.28. Details of the apparent-heat-capacity curves in the glass-transition range are provided in Figure 2, which is an enlargement of the curves shown in Figure 1. At low temperature and after completion of devitrification of the MAF, the cp,app curves overlap, and from the experimentally observed heat-capacity step at Tg a mobile amorphous fraction (wMAF) of 0.35 6
was determined by comparison with the heat-capacity step of fully amorphous PBS [30]. With the knowledge of the crystallinity and of the MAF, it is possible to determine the rigid amorphous fraction (wRAF) of PBS at Tg according to equation (2): wRAF=1 - wC - wMAF
(2)
Eq. (2) leads to wRAF = 0.37, which confirms the establishment of sizable RAF upon crystallization of PBS, as also reported in Ref. [30]. Figure 2 shows also the thermodynamic heat capacity of PBS containing 28 % crystals and 37 % RAF, calculated taking into account the temperature-dependence of the solid cp (wC + wRAF) and of the liquid cp (wMAF) [54, 55]. Further information about the thermal events that occur upon heating was gained by TMDSC. Figure 3 presents the reversing specific heat capacity of PBS measured with a modulation amplitude of 0.5 K, an underlying heating rate of 2 K min-1, and various modulation frequencies, with an enlargement shown in the right part of the figure, to highlight transition details. Data were gained after isothermal crystallization at Tc = 90 °C for 15 min and cooling to below Tg, and are compared with the apparent specific heat capacity of PBS of identical thermal history, measured on linear heating at the same rate of 2 K min-1, without superimposed temperature modulation (DSC). At temperatures lower than the glass transition and higher than the melting, experimental data obtained by DSC and TMDSC agree well with the heat capacity of solid and liquid PBS, respectively. The apparent specific heat capacity of PBS, measured by DSC, starts to deviate from the heat capacity of solid PBS at around -40 °C during the glass transition. However, some unexpected results can be observed in Figure 3: the heat-capacity increase at Tg should not be affected by the modulation frequency, as it should only be dependent on the MAF that devitrifies at Tg [56-59]. The increase in the reversing heat capacity with decreasing frequency of the temperature oscillation suggests that, besides the glass transition, additional thermal events occur at Tg. An influence of the TMDSC modulation period on the reversing heat capacity is often reported in the analysis of polymer melting, where repeated partial melting, immediately followed by recrystallization, continuously occur on heating; in such case, the reversing heat capacity increases with the modulation period [47, 49, 60-69]. When polymer melting is analyzed in a TMDSC heating experiment, it overlaps with crystal reorganization/perfection or melt-recrystallization, leading to exothermic and endothermic contributions in the cooling and heating half-cycles, respectively. Both release and absorption of latent heat produce an increase in the modulated heat-flow rate amplitude, and, in turn, an increase of the reversing heat capacity [48, 49]. In other words, if the reversing heat capacity is higher than the 7
expected thermodynamic value of the heat capacity of a given structure/system, then this indicates additional endothermic and/or exothermic events occurring during modulation, suggesting repeated melting and crystallization during the temperature oscillation [68]. The frequency-dependence of the cp,rev, seen in Figure 3, indicates that in PBS melting and crystallization start at the glass transition of the MAF, i.e. that crystals grown below Tg start their melting and perfection/recrystallization in the Tg range. To gain more quantitative information about the reversing thermal events occurring in the glass-transition range, quasi-isothermal TMDSC analyses were conducted on PBS samples isothermally crystallized at 90 °C, then cooled to below Tg, followed by temperaturemodulation around various base temperatures for a pre-defined period of 4 hours. These quasi-isothermal TMDSC experiments allow monitoring irreversible processes that occur during prolonged modulation around the base temperatures, by following the variation of the reversing heat capacity with time [68]. Results of these quasi-isothermal TMDSC analyses are presented in Figure 4, which shows the change of the reversing heat capacity with time during quasi-isothermal annealing at different temperatures in the Tg range. Despite the experimental noise, the time-dependence of the reversing heat capacity is reproducible and significant [67, 69]. For all analyzed temperatures, the reversing heat capacity slowly decreases with time, with a more rapid decrease at higher annealing temperatures. Such a decrease of cp,rev can only be caused by irreversible events that take place during the quasi-isothermal analyses, as reported in the literature for a variety of semicrystalline polymers in the melting range [68]. cp,rev decreases more rapidly at higher temperatures, indicating a larger extent of the irreversible processes occurring with the increased temperature of analysis. Further DSC analyses were conducted to confirm crystallization of PBS at temperatures below Tg. The polymer was isothermally crystallized at Tc = 90 °C, cooled at 10 K min-1 from Tc to -20 °C, i.e., to just above Tg, followed by slow cooling from -20 to -70 °C at various rates, ranging from 0.05 to 2 K min-1. This was followed by immediate reheating the polymer at 20 K min-1. The influence of the rate of prior cooling on the glass transition of PBS, measured upon subsequent heating, is presented in Figure 5. A higher cooling rate results in a shift of Tg of the MAF to lower temperatures, as it varies from -29 °C, after cooling at 0.05 K min-1, to -34 °C measured after cooling at 1 K min-1. Further increase of the cooling rate above 1 K min-1 does not affect the glass transition and as the experimental data measured on subsequent heating practically overlap, as seen by comparison of the plots measured after cooling at 1 and 2 K min-1 in Figure 5.
8
As a general rule, the glass transition of amorphous and semicrystalline polymers measured upon heating is affected by the difference between cooling and heating rates, as at parity of heating rate, Tg moves to higher temperatures with the increased rate of prior cooling [54]. Such a trend can be observed also in Figure 5, but only when PBS is cooled at slow rates, up to 1 K min-1. Increasing the cooling rate above 1 K min-1 does not affect the glass transition, as proven by the data shown in Figure 5. This indicates that additional processes contribute to the shift of Tg depending on the thermal history. In semicrystalline polymers, the glass transition temperature often varies with the increase of the crystal fraction, with a trend similar to that seen in Figure 5 [54]. Such variation is also reported in Ref. [31] after crystallization below Tg of initially amorphous PBS. This variation of Tg is also seen in the plots of Figure 5, which confirms additional crystallization at temperatures below Tg triggered by the slow cooling, because slower cooling implies longer residence time at low temperatures available for crystal growth. However, the data of Figure 5 also reveal that the heat-capacity step at Tg does not vary with the rate of prior cooling, which indicates negligible variation of the MAF with the changed thermal history. In other words, despite the shift in Tg, linked to a varied crystal fraction, there is not observed a variation of the liquid-/solid-phase ratio of the polymer at completion of the glass transition. This indicates that crystallization at low temperatures does not involve the amorphous chain segments that are part of the MAF, but only amorphous chain parts that do not mobilize at Tg, i.e. the RAF. Irreversible changes of structure in PBS at low temperatures are also disclosed by the data shown in Figures 3 and 4. The frequency-dependent TMDSC data of Figure 3 and the decrease of the reversing heat capacity obtained in quasi-isothermal TMDSC experiments, shown in Figure 4, indicate that reversing partial melting and crystallization start at the onset of Tg, which confirms the experimental finding in Ref. [31] that amorphous PBS can crystallize at temperatures below Tg. Since crystal growth after cooling to below Tg does not imply a variation of the fraction of the mobile amorphous chains, it is suggested that the PBS chain segments that crystallize at low temperatures are connected to the RAF coupled with the crystals: the chain segments that become ordered at low temperatures are confined in geometrically restricted areas, presumably at the amorphous/crystal interface, where short-range motions may lead to rearrangement of chain segments into a more ordered structure. The occurrence of short-range local motions in constrained amorphous regions has been proven for a variety of polymers [70]. It is likely that such short-range motions of the amorphous chain segments that are part of the RAF, induce local ordering, with crystal formation facilitated by coupling of rigid 9
amorphous segments with crystals. Following the mechanism of molecular nucleation proposed by Wunderlich [71], a polymer molecule must be nucleated itself before the rest of the molecule can add to the growing crystal. This implies that no energy barrier for molecular nucleation needs to be overcome, since the PBS chain portions that order at low temperatures and are part of the RAF, are already bonded to the crystals. Crystal formation in amorphous PBS at temperatures below Tg was first proposed by Schick and coworkers [31], who observed a small endothermic peak at around -5 ÷ -10 °C, i.e. immediately above Tg, after prolonged annealing of amorphous PBS below Tg, around -50 °C. This revealed growth of a small amount of crystals of low stability in amorphous PBS. In semicrystalline PBS, an even smaller amount of tiny crystals grow at low temperatures, as demonstrated by the DSC and TMDSC shown above, probably a too small amount to allow measurable changes in the overall crystal fraction, in a sample already containing 30% of crystals of larger thermal stability. In amorphous PBS, formation of small crystalline aggregates at temperatures below T g was rationalized taking into account that the densification of the glass and the enthalpy relaxation need to be completed before formation of crystal nuclei, in agreement with similar studies on initially amorphous polymers, including poly(lactic acid), poly(-caprolactone), polyamide 6, or isotactic polybutene-1 [42, 46, 72]. In the case of semicrystalline PBS, it is likely that chain segments already coupled with the crystals are involved in formation of crystals at temperatures below Tg. This notion is supported by the data shown in Figure 5 that demonstrate no variation in the heat-capacity step, i.e., no diminution of the mobile amorphous fraction caused by crystal growth, but only a slight shift of Tg to higher temperature, linked to crystal development below Tg. The hypothesis of crystal formation involving constrained amorphous chain segments coupled to crystals is supported by the continuous partial melting and recrystallization of PBS revealed by the frequency dependence of the reversing heat capacity, since molecular nucleation, which is a prerequisite for reversing melting, is guaranteed by the pre-existing crystal surfaces [68, 73–75]. This confirms the role of constrained coupled amorphous segments of crystal growth in PBS at temperatures below Tg.
Conclusions Poly(butylene succinate) displays an unusual crystallization behavior, with possible crystal development at very low temperatures, below the glass transition. This peculiarity was 10
initially evidenced upon annealing below Tg of the amorphous polymer, which was shown to lead to growth of tiny and defective PBS crystals that melt and recrystallize upon subsequent heating [31]. With the DSC and TMSC analyses detailed above, it is now proven that not only the amorphous polymer, but also the partially crystallized PBS can further crystallize at temperatures below Tg. The reversing heat capacity measured upon modulated heating displays a frequency dependence in the glass transition region, indicating partial melting and recrystallization of crystals necessarily grown below Tg. Crystal growth below Tg also affects the glass transition, which moves to higher temperatures with progressive crystal formation below Tg. It is likely that crystallization at low temperatures involves constrained amorphous chain segments coupled to crystals, i.e. part of the RAF, which transforms into ordered phase with the pre-existing crystal surfaces acting as nuclei for this further crystal growth at low temperatures, below Tg.
Acknowledgements The authors wish to thank Showa Denko K. K. (Japan) for kindly providing the poly(butylene succinate) used in this research.
11
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Apparent heat capacity [J/ (g K)]
15
-1
2 K min -1 20 K min
10
5
0 -50
0
50
100
Temperature (°C)
Apparent heat capacity [J/ (g K)]
Figure 1. Apparent specific heat capacity of PBS, measured upon heating at the indicated rates after isothermal crystallization at 90 °C for 15 min followed by fast cooling to -70 °C. Baseline heat capacities of liquid and solid PBS were taken from Ref. [30] and are shown as dotted lines.
2,5
-1
2 K min -1 20 K min
2,0
1,5
1,0 -40
-20
0
20
Temperature (°C)
19
Apparent heat capacity [J/ (g K)]
Figure 2. Enlargement of the experimental data of Figure 1, to show details in the glass transition range. Thermodynamic heat capacities of liquid and solid PBS were taken from Ref. [30] and are shown as dotted lines while the thermodynamic heat capacity of PBS containing 28 % crystals and 37 % RAF is shown as dashed line. 25
DSC p=60 s p=90 s p=120 s p=150 s
20
15
10
5
-50
0
50
100
Apparent heat capacity [J/ (g K)]
Temperature (°C)
2,0
1,5
1,0
-50
0
50
100
Temperature (°C)
Figure 3. Apparent specific heat capacity of PBS, measured upon linear heating at 20 K min-1 using conventional DSC (DSC), and reversing specific heat capacity from non-isothermal TMDSC, with different modulation periods (p) as indicated in the legend. Data were gained after isothermal crystallization at 90 °C for 15 min, followed by fast cooling to -70 °C. The left plot reports full scale data, the right plot is an enlargement around thermodynamic heat capacity lines. Thermodynamic heat capacities of liquid and solid PBS were taken from Ref. [30] and are shown as dotted lines while the thermodynamic heat capacity of PBS containing 28 % crystals and 37 % RAF is shown with the dashed line.
20
0.1 J/(g K)
Reversing heat capacity
-40 °C
-20 °C 0 °C
0
100
200
Time (min)
0.3 J/(g K)
Heat capacity
Figure 4. Reversing heat capacity of PBS, measured during quasi-isothermal temperature modulation at the indicated base temperatures of 0, 20, and 40 °C. The modulation period and amplitude were 60 s and 0.5 K, respectively. Data were gained after isothermal crystallization at 90 °C for 15 min, followed by fast cooling to -70 °C and subsequent heating at 20 K min-1 to the temperature chosen for the quasi-isothermal TMDSC analysis.
-1
0.05 K min -1 0.1 K min -1 0.2 K min -1 1 K min 2 K min
-50
-40
-30
-1
-20
Temperature (°C) Figure 5. Heat capacity of PBS, measured upon heating at 20 K min-1 after isothermal crystallization at 90 °C for 15 min, followed by cooling to -20 °C at 10 K min-1, then cooling to -70 °C at the indicated rates.
21
Low-temperature crystallization of poly(butylene succinate) Maria Laura Di Lorenzo *1, René Androsch2, Maria Cristina Righetti3
Apparent heat capacity [J/ (g K)]
Poly(butylene succinate) 1,6
Frequency-dependent excess heat capacity reveals melting of PBS crystals grown below Tg
1,4
1,2
DSC
1,0
p=60 s p=90 s p=120 s p=150 s
-40
-30
-20
Temperature (°C)
*
Email: [email protected]
22
Highlights
Sub-Tg crystallization of semicrystalline PBS is proven by TMDSC PBS crystals grown in the glass, melt and recrystallize at Tg Crystallization of PBS below Tg possibly involves part of the RAF
23