Article pubs.acs.org/Macromolecules
Crystal Nucleation in Glassy Poly(L‑lactic acid)† René Androsch*,‡ and Maria Laura Di Lorenzo§ ‡
Center of Engineering Sciences, Martin-Luther-University Halle-Wittenberg, 06099 Halle/Saale, Germany Istituto di Chimica e Tecnologia dei Polimeri (CNR), c/o Comprensorio Olivetti, Via Campi Flegrei, 34, 80078 Pozzuoli (NA), Italy
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ABSTRACT: Aging of glassy poly(L-lactic acid) (PLLA) allows formation of crystal nuclei which enhances/accelerates subsequent crystallization at temperatures above the glass transition. The effects of the time and temperature of aging on nuclei formation have quantitatively been probed by analysis of isothermal crystallization at 393 K, using fast scanning chip calorimetry and polarizing optical microscopy. Crystal nuclei begin to form on aging the glass of PLLA at 343 K after about 101 s. The time of nuclei formation increases exponentially with decreasing temperature, so that aging at 323 K requires a minimum time of 104 s, and the extrapolated time for generation of nuclei at 295 K is about 108 s. The agingcontrolled increase of the nuclei density in glassy PLLA leads to a distinct decrease of the half-time of crystallization. The halftime of crystallization of nonaged PLLA at 393 K is about 600 s and decreases to less than half of this value due to aging at 343 K for a period of only 103 s. Nuclei formation on aging the glass of PLLA is connected with a tremendous decrease of the size of spherulites which develop upon subsequent cold-crystallization. The detection of formation of crystal nuclei in glassy PLLA is discussed in the framework of prior analyses of the effect of the crystallization pathway on structure and properties of crystallizable polymers.
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minimum half-time of crystallization is higher than 20 min.1 The maximum spherulite growth rate is around 5 μm min−1 at 390−400 K for PLLA of low D-unit content of less than 1%. It has been reported that the spherulite growth rate shows a second maximum around 385 K which is related to the change of the crystal structure.13−16 Since melt-crystallization of PLLA is rather slow, for commercial uses modification of the neat material is required to obtain semicrystalline products after processing including injection-molding, blow-molding, or extrusion which typically involve fast cooling of the melt below the glass transition temperature (Tg) of 320−330 K. Typically, the overall crystallization rate of PLLA is increased by the addition of heterogeneous nucleators or plasticizers.17 The addition of heterogeneous nucleators leads to an increase of the number of nucleation sites for the crystallization process and with that of the number of simultaneously growing crystals/spherulites at a predefined temperature/supercooling. For unmodified PLLA, quantitative information about the nucleation rate, as determined by analysis of the spherulite density, is available in a wide temperature range and suggests that the nucleation rate increases with increasing supercooling, to reach a plateau value around 360−370 K.1,16,18−20 There exist only few studies about crystal nucleation in PLLA at high supercooling of the melt.21−24 It has been shown for
INTRODUCTION Poly(L-lactic acid) (PLLA) is a thermoplastic aliphatic polyester which is produced from annually renewable resources and is commercially used as packaging material or for biomedical applications due to its compostability and biocompatibility/ bioresorbability, respectively. Application-relevant properties of PLLA like stiffness, clarity, or degradation/bioresorption kinetics depend on the semicrystalline structure which, in turn, is controlled by the chemical architecture of the macromolecule including the molar mass or percentage of Dunits and by the condition of crystallization.1−4 In the present work, we attempt to analyze the crystallization behavior of PLLA as a function of the condition of aging in the glassy state, which allows formation of crystal nuclei, promoting crystallization at elevated temperature. PLLA is polymorphic, that is, as a function of the conditions of crystallization different crystal structures form.5−12 Crystallization of the relaxed melt at temperatures above about 390 K leads to formation of α-crystals with two antiparallel aligned helical chain segments packed in an orthorhombic unit cell.5,6 At temperatures lower than 390 K, formation of α-crystals is replaced by formation of α′-crystals. The α′-form is considered as a conformationally disordered α-crystal with slightly increased lattice spacings and transforms upon heating into the stable α-form.11,12 The gross crystallization rate of PLLA is relatively low and exhibits a broad maximum around 375−380 K. The minimum reported half-time of crystallization of PLLA is about 2 min, which, however, increases with increasing D-unit concentration; for PLLA containing 3−4% D-units, the © XXXX American Chemical Society
Received: May 18, 2013 Revised: June 26, 2013
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nucleation/crystallization of poly(ε-caprolactone) (PCL),25,26 isotactic polypropylene (iPP),27−30 poly(butylene terephthalate) (PBT),31 or polyamide 11 (PA 11)32 revealed an increase of the nucleation rate at high supercooling, presumably related to a change of the nucleation mechanism. In conjunction with the proof of crystal nucleation below the glass transition temperature, that is, in the absence of cooperative segmental mobility, it has been speculated that homogeneous nucleation may be the dominant nucleation mechanism at low temperatures/high supercooling. The formation of a large number of nuclei at high supercooling is connected with a change of the crystal morphology and the semicrystalline superstructure. While crystallization at rather low supercooling of the melt is connected with the formation of laterally extended lamellae and large spherulites, at high supercooling lamellar crystal growth is hindered due to topological constraints and the spherulite size decreases; eventually, spherulitic crystal growth is fully suppressed and crystals are of nonlamellar shape,33−39 with tremendous effects on, e.g., mechanical or optical properties.40 Such approach of tailoring properties of PLLA has not intensely been followed yet which we intend to initiate with the present work focusing on analysis of the effect of aging on the nucleation/crystallization kinetics.
PLLA with 4.25% D-unit content that isothermal coldcrystallization is faster than melt-crystallization at identical temperature.21 For example, the half-time of melt-crystallization at 110 °C (383.15 K) was around 155 min, while it was decreased to 105 min on cold-crystallization after prior cooling the relaxed melt at a rate of 10 K min−1 to 25 °C (298.15 K) and immediate reheating at identical rate to 110 °C (383.15 K). The observed result has been explained by a difference of the nucleation density, that is, by additional nuclei formed during cooling to 25 °C (298.15 K). Variation of the minimum temperature before cold-crystallization suggested that the increase of the nuclei density saturates at the glass transition temperature of around 60 °C (333.15 K). The effect of the residence time below the cold-crystallization temperature has not been analyzed in this particular work.21 Besides the abovereported results, a cooling-rate dependence of the nucleation kinetics of PLLA has been reported.22 PLLA samples were cooled at different rates between 5 and 500 K min−1, which led to complete vitrification of the melt and subsequently coldcrystallized. It has been shown that cooling at moderate rates resulted in formation of a larger number of nuclei compared to samples cooled at higher rate. The influence of the residence time on the nucleation kinetics at two selected temperatures below and above Tg, at 326 and 346 K, respectively, was determined in ref 23. The nucleation kinetics was estimated from the shift of the cold-crystallization exotherm to lower temperatures on subsequent heating, which varied with the annealing time for both the investigated temperatures. Nucleation was overlapping with partial crystal growth upon annealing at 346 K, whereas annealing at 326 K resulted in less nucleation, which, however, was probed to occur in the glassy state, too. The number of nuclei formed during annealing below and above Tg was estimated from the differential scanning calorimetry (DSC) curves by modeling with the Avrami theory.23 In a further work, melt-quenched PLLA was aged below the glass transition temperature, and changes of structure of the initially fully amorphous specimen were monitored in situ by time-resolved infrared spectroscopy.24 The data provided evidence for local ordering and the formation of a mesophase. It has been suggested that the formed mesophase disorders in conjunction with the physical aging peak at the glass transition temperature while simultaneously enhancing cold-crystallization on continuation of heating. The above reports proof that nuclei/locally ordered structures develop at rather low temperature which increases the crystallization kinetics at elevated temperature. Consistent and quantitative information about the effects of time and temperature of aging on nuclei formation, however, are available only for limited temperatures/times, which is therefore the primary object of the present study. We attempt in particular to provide evidence that nuclei formation is not restricted to the temperature range above the glass transition temperature. In extension to prior studies on PLLA in this field,21−24 we varied systematically the aging conditions in the glassy state and analyzed the process of nuclei formation by the kinetics of crystallization and the number of spherulites evolving at elevated temperature. The experiments about the nucleation kinetics of PLLA at high supercooling of the melt including the glassy state are part of our research efforts to gain general knowledge about crystal nucleation in polymers and its impact to control the semicrystalline morphology and with that of ultimate properties. Investigation of the kinetics of
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EXPERIMENTAL SECTION
Materials. In the present study we used a commercial PLLA 4032D grade with a D-isomer concentration of 1.5% from Nature Works. The material was delivered in form of pellets which were directly used for preparation of specimens for analysis of the spherulitic superstructure and crystallization kinetics by polarizing optical microscopy and calorimetry, respectively, both as a function of the aging conditions in the glassy state. Instrumentation. Fast Scanning Chip Calorimetry (FSC). We used a power-compensation Mettler-Toledo Flash DSC 1 in conjunction with a Huber intracooler TC90. Specimens were obtained by cutting thin sections of 20 μm thickness from the pellet using a microtome. The lateral size of the thin section was then reduced to about 100 μm with the aid of a stereomicroscope. Before placing the specimens onto the FSC sensor, the latter was conditioned and temperature-corrected according the specification of the instrument provider. The furnace was permanently purged with dry nitrogen gas at a flow rate of 35 mL min−1. The sample mass of 445 ng, which is required to obtain specific heat capacity data and enthalpies of transitions, was determined by comparing the measured heat-capacity increment at the glass transition temperature on heating a fully amorphous sample with the expected value of 0.61 J g−1 K−1 available in the literature.41,42 Further details about the instrument/sensor are reported elsewhere.43 Note that there has been collected evidence that crystallization of sub-microgram specimens, as typically used in FSC experiments, seems not affected by the relatively large sample surfaceto-volume ratio or the FSC-sensor membrane support.44,45 The temperature−time profiles for analysis of aging and crystallization of PLLA are described below. Polarizing Optical Microscopy (POM). The spherulitic superstructure of specimens of different aging/crystallization history was monitored using a Motic BA410 polarizing optical microscope. Specimens were prepared by heating thin sections of 30 μm thickness between circular Plano microscope coverslips of 100 μm thickness each to a temperature of 473 K, using a hot stage. After a period of 3 min, the molten sample was quenched to the aging temperature, employing a second hot stage, and kept there for a predefined aging time. Subsequently, the sample was crystallized at a temperature of 393 K for a period of 10 min, using a Paar TTK temperature chamber. B
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RESULTS AND DISCUSSION Temperature Range of Aging. Figure 1 shows specific heat capacity data of PLLA as a function of temperature
has been proven in prior tests that aging at these conditions leads to enthalpy relaxation and crystal nucleation only, while growth of nuclei to crystals only occurs at temperatures higher than the glass transition.1,16,18−20 POM Analysis of the Nucleation Kinetics on Aging Glassy PLLA. Figure 2 shows POM images obtained on initially fully amorphous samples of PLLA, which were aged in the glassy state and then cold-crystallized at 393 K for a period of 10 min. The thermal history of the specimens is provided with the temperature−time profile shown in the bottom right part of the Figure. Images shown in the various rows were obtained on samples which were aged for constant periods of time of 2, 10, 30, 100, 500, and 1000 min (from top to bottom), and images shown in the various columns were obtained on samples aged at constant temperatures of 323, 328, 333, 338, and 343 K (from left to right). Aging at 323 K (left column) for periods of time less than about 100 min is not connected with formation of nuclei as it is concluded from the constant low number of spherulites growing at 393 K. Only if the aging time exceeds 100 min then we observed an increase of the spherulite density on crystallization at 393 K which we interpret by formation of nuclei at the aging temperature. With increasing aging temperature, an increased number of spherulites evolving at the crystallization temperature of 393 K is observed on aging for shorter periods of time. For example, aging at 333 K (center column) leads to a distinct increase of the nuclei density after 30 min while nuclei formation at 343 K is detected already after aging for 2 min (right column). It is emphasized at this
Figure 1. Specific heat capacity of PLLA as a function of temperature, measured on cooling the isotropic melt at a rate of 103 K s−1 (blue curve) and subsequent heating (red curve) at an identical rate.
obtained on cooling the relaxed melt (blue line) and immediate subsequent heating (red line) both at a rate of 103 K s−1. Crystallization on cooling is suppressed, and the supercooled liquid completely vitrifies at the glass transition temperature of about 335 K. On subsequent reheating, the glass transition is observed around 350 K, with the cooling/heating temperature hysteresis of the glass transition caused by the kinetics of devitrification the glass.46,47 Aging of fully amorphous PLLA has been performed in the gray-shaded temperatures range, that is, at temperatures between 323 and 343 K in the glassy state. It
Figure 2. POM images obtained on initially fully amorphous samples of PLLA, aged in the glassy state, and then cold-crystallized at 393 K for a period of 10 min. The thermal history of the specimens is provided with the temperature−time profile shown in the bottom right part of the figure. The scaling bar corresponds a distance of 100 μm. C
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Figure 3. POM images obtained on samples of PLLA, melt-crystallized at 393 K (left image), and cold-crystallized (right image) at identical temperature until completion of crystallization. Before cold-crystallization, the glass of PLLA was aged at 343 K for a period of 2 min. The scaling bar corresponds a distance of 200 μm.
occasion again that aging regardless the condition applied in this work did not result in crystal growth. POM images of samples which were cooled after aging to 295 K did not show spherulites and were featureless. For evaluation of the effect of aging the glass of PLLA on the spherulite density upon crystallization at 393 K we selected a crystallization time of 10 min (Figure 2). Extension of the crystallization time allows completion of spherulitic crystallization and demonstration of the effect of aging the glass of PLLA on the final semicrystalline morphology at the micrometer length scale. Figure 3 shows POM images of meltcrystallized PLLA (left image) and PLLA which was coldcrystallized at identical temperature of 393 K (right image) until crystallization was complete and a space-filling spherulitic superstructure was achieved. Before cold-crystallization, the equilibrium melt of PLLA has been quenched to 343 K, aged at identical temperature for 2 min, and then heated to 393 K. Melt-crystallization, that is, crystallization of the supercooled melt with the crystallization temperature directly approached by cooling the equilibrium melt, leads to formation of only few crystal nuclei which allows growth of rather large spherulites with a size of the order of magnitude of 100 μm or larger. In contrast, cold-crystallization at identical crystallization temperature of 393 K, after prior aging the glassy material at 343 K for a period of 2 min, is connected with formation of a distinctly larger number of spherulites and the evolution of a rather finespherulitic superstructure; we conclude that aging at 343 K for 2 min resulted in formation of additional nuclei compared to the nonaged sample. Crystallization Kinetics of Nonaged PLLA. Analysis of the temperature dependence of the crystallization kinetics of PLLA has been object of intense research in the past and reviewed recently.1 We re-evaluated the effect of supercooling on the crystallization rate of the particular PLLA grade used, in order to subsequently analyze the effect of aging the glass of PLLA on the crystallization kinetics at 393 K. Figure 4 shows a series of FSC heating scans performed after cold-crystallization of PLLA at 393 K for different periods of time between 100 s (blue curve) and 2 × 103 s (red curve). The relaxed melt was cooled at a rate of 103 K s−1 to 213 K, kept there for instrumental reasons for a period of 0.1 s, and reheated at identical rate to 393 K. A residence time of 100 s did not result in formation of crystals as is concluded from the absence of melting on subsequent heating (blue curve). Melting, and with that formation of crystals, is only observed if the crystallization time exceeds about 102 s (black curve). With increasing crystallization time, the enthalpy of melting on subsequent
Figure 4. Apparent specific heat capacity of PLLA as a function of temperature, obtained on heating after prior cold-crystallization at 393 K for different periods of time, as is indicated in the legend. The thermal history of the samples, prior to the heating scan, is described in the text. The dashed and dash-dotted lines are the liquid and solid heat capacities of PLLA.41
heating increases, to reach a maximum value of 50−60 J g−1 after crystallization for 103 s. Figure 5 is a plot of the enthalpy of crystallization as a function of the time of cold-crystallization (light-gray squares)
Figure 5. Enthalpy of crystallization of PLLA as a function of the time of cold-crystallization (light gray symbols) and melt-crystallization (dark-gray symbols) at 393 K. The dotted line at the crystallization time of 300 s serves for later explanation of specific aging experiments.
and melt-crystallization (dark-gray squares) at 393 K. Enthalpies of crystallization were determined from the FSC heating scans as were exemplarily shown in Figure 4. As a consequence of fast heating at 103 K s−1, cold-crystallization does not occur during heating; that is, the enthalpies of melting represent enthalpies of prior isothermal crystallization at 393 K. D
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The data of Figure 5 reveal that the kinetics of cold- and meltcrystallization at the particular conditions to approach the crystallization temperature are indifferent. We consider our data of being not in conflict with formerly performed similar experiments which proved a distinct effect of the coldcrystallization pathway on the crystallization kinetics.21,22 The residence time of the sample at temperatures lower than 393 K, in our experiments, was too short in order to permit formation of nuclei which would accelerate subsequent crystallization. Note again that the sample was cooled at 103 K s−1 to 213 K and immediately reheated to 393 K at an identical rate. The cold-crystallization experiments described in the literature employed slower cooling and heating rates on the approach of the crystallization temperature, which ultimately permitted formation of nuclei. The experimental approach of determination half-times of crystallization of PLLA by measurement of the enthalpy of melting after prior isothermal crystallization for different time, as demonstrated with Figures 4 and 5, has been used to evaluate the temperature dependence of crystallization kinetics. Figure 6 shows half-times of crystallization as a function of
Figure 7. Temperature−time program of FSC experiments applied for analysis of the effect of the aging temperature on the onset time of formation of crystal nuclei.
then the observed enthalpy of crystallization would not be a function of the aging conditions since crystallization always would be completed. Similarly, if the crystallization time is too short, then crystal growth may not occur regardless of the prior aging. The efficiency of the analysis of the formation of nuclei during isothermal aging the glass of PLLA using the measurement protocol shown in Figure 7 is demonstrated with the FSC heating scans shown in Figure 8. The blue curve
Figure 6. Half-time of crystallization of PLLA as a function of the temperature of crystallization.
temperature in the temperature range between 363 and 408 K. The data reveal the expected maximum of the crystallization rate around 390 K, with the observed minimum half-time of crystallization of 600−700 s being in agreement with earlier collected data obtained on the same PLLA grade.48−51 Following, the effects of the temperature and time of aging the glass of PLLA on the kinetics of nucleation and crystallization are discussed. Onset Time of Nuclei Formation on Aging the Glass of PLLA. The onset time of formation of crystal nuclei in glassy PLLA has been determined as a function of the aging temperature according to the measurement scheme shown in Figure 7. The sample was melted and then rapidly cooled to the aging temperature between 323 and 343 K at a rate of 103 K s−1. After aging for periods of time between 0 and 104 s the material was first cooled to 213 K at a rate of 103 K s−1 and then heated at identical rate to 393 K to allow crystallization for a period of time of 300 s. Finally, the enthalpy of crystallization was determined by analysis of the enthalpy of melting during subsequent fast heating at 103 K s−1. The crystallization time of 300 s at 393 K has been selected in order to efficiently detect differences of the crystallinity related to the formation of nuclei during aging. For example, if the crystallization time is too long,
Figure 8. Apparent specific heat capacity of PLLA as a function of temperature, obtained on heating after prior cold-crystallization at 393 K for 300 s, and aging at 343 K for different time between 0 and 104 s.
has been obtained on a nonaged sample after coldcrystallization at 393 K for 300 s. It shows the glass transition at around 350 K and a melting peak at 450 K; the enthalpy of melting is about 8 J g−1 as it is also given with the data of Figure 5 (see data point at the dotted vertical line). With increasing aging time at 343 K, the enthalpy of melting, and therefore enthalpy of isothermal crystallization for 300 s at 393 K increases, to reach a value of 45 J g−1 after aging for 104 s (red curve). It is emphasized again that the crystallization time at 393 K was kept constant in all experiments; that is, the increase of the enthalpy of crystallization is due to nuclei formation at the aging temperature only. Similar analyses have been performed after aging at temperatures of 323, 328, 333, and 338 K, with the enthalpy of crystallization at 393 K for 300 s plotted as a function of the time of aging in Figure 9. The horizontal line indicates the enthalpy of crystallization of 8 J g−1 of nonaged PLLA. Aging of E
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time of nuclei formation by POM, for example, inspection of the left column of the POM micrographs of samples aged at 323 K reveals an increase of the spherulite density after aging longer than 100 min but less or equal 500 min. This time range is provided in Figure 10 with the gray bars for all aging temperatures analyzed. Regarding the estimation of the onset time of nuclei formation by FSC, after aging at 343 K, we detected first deviation from the enthalpy of crystallization of 8 J g−1 of nonaged PLLA after 10 s, indicating the onset of nucleation. Similar evaluation has been done for the data sets obtained at different temperatures in Figure 9. The data observed permit an extrapolation of the aging behavior to 295 K such that first generation of nuclei may be expected after a dwell time of 107−108 s. Effect of Aging the Glass of PLLA on the Crystallization Rate. The gross crystallization rate of polymers, that is, the increase of the crystal fraction as a function of time, is controlled by both the number of growing nuclei/crystals and the crystal growth rate. The growth rate of individual crystals is independent of the number of growing nuclei/crystals and is therefore not expected to be influenced by the aging history. The growth rate of crystals typically is analyzed by monitoring the growth of spherulites, using optical microscopy. Though it is not in foreground of the present investigation, the optical micrographs of Figure 2 confirm that the spherulite growth rate is independent of the number of nuclei as it is concluded from the constant size of spherulites after crystallization at 393 K for a period of 10 min, independent of the aging history.13 For analysis of the effect of aging the glass of PLLA on the gross crystallization rate, we measured half-times of crystallization using the approach which has been explained with Figures 4 and 5. Samples of fully amorphous and glassy PLLA have been aged at different temperatures for a period of 103 s, before crystallization at 393 K. Interruption of the crystallization process and analysis of the enthalpy of melting during subsequent heating allowed determination of the enthalpy of crystallization as a function of the crystallization time, as is shown in Figure 11. Note that crystallization enthalpies have
Figure 9. Enthalpy of crystallization of PLLA at 393 K for 300 s as a function of the time of aging at different temperatures between 323 and 343 K, as indicated in the legend.
fully amorphous PLLA at 343 K for a period of 101 s or longer is connected with nuclei formation, as is straightforward concluded from the increase of the enthalpy of crystallization at unchanged crystallization conditions. With decreasing aging temperature, there is observed a shift of the onset time of nuclei formation to 102 s on aging at 333 K and almost 104 s on aging at 323 K. The data of Figure 9 are in qualitative agreement with the aging experiments analyzed by polarizing optical microscopy, presented in Figure 2. The optical micrographs of Figure 2 showed on aging at 323 K an increase of the nucleation density if the aging time exceeds 100 min (6 × 103 s), while on aging at 333 K the nucleation density increased if the aging time was exceeding 10 min (6 × 102 s). On aging at 343 K, an increased nucleation density was already observed at the shortest aging time of 2 min (120 s). Figure 10 shows the onset time of nuclei formation in glassy PLLA on log scale as a function of the aging temperature which
Figure 10. Onset time of nuclei formation as a function of the temperature of aging fully amorphous and glassy PLLA. Data were obtained from FSC experiments shown with Figures 7−9 (black squares) and by visual inspection of the POM micrographs of Figure 2 (gray bars).
Figure 11. Enthalpy of crystallization of PLLA as a function of the time of cold-crystallization at 393 K and as a function of the temperature of aging for 103 s, as is indicated in the legend.
also been collected at shorter crystallization times down to 100 s, which, however, are not shown since being zero. Aging at temperatures of 323 and 328 K for 103 s does not affect the overall crystallization rate since the conversion−time curve is unchanged compared to that of nonaged PLLA. This result is expected since with Figure 9 it has been shown that aging at 323 and 328 K requires a waiting time longer than 103 s before first development of nuclei. However, if the aging temperature
reveals a linear decrease with increasing temperature, being in accord with the classical crystal nucleation theory which predicts an exponential decrease of the nucleation rate with decreasing temperature.52−54 Figure 10 includes data extracted from the FSC experiments explained with Figures 7−9 (black squares) and data based on inspection of the POM micrographs of Figure 2 (gray bars). Regarding the estimation of the onset F
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which, at high supercooling, is counterbalanced/opposed by decreasing segmental mobility of the macromolecules. As a result of increasing mobility constraints of molecular segments at high supercooling, the temperature dependence of the nucleation rate passes through a maximum and decreases to zero at sufficiently low temperature. It is obvious that the data collected between 323 and 343 K in this work mirror the decrease of the nucleation rate with decreasing temperature; prior analyses focused on the temperature dependence of the nucleation rate at rather low supercooling, that is, on the temperature range in which the nucleation rate increases with supercooling. In extension to the few earlier studies about crystal nucleation of PLLA at high supercooling,21−24 we provided evidence that formation of crystal nuclei is not restricted to the temperature range above the glass transition temperature and that it also occurs in the glassy state without the requirement of large-amplitude cooperative segmental mobility. This observation is in line with formerly performed research about the kinetics of formation of crystal nuclei in vitrified but inherently crystallizable polymers.25,26,30,55 The data about the temperature dependence of the nucleation kinetics presented in Figure 10 allow an extrapolation of the rate of nuclei formation to ambient temperature. The observed characteristic time of nuclei formation of 107−108 s (103−104 days) indicates the importance of the present study since it may fall into the lifetime of PLLA products.56,57 It is emphasized that the reported characteristic times for nuclei formation, shown in Figure 10, hold only for the specific cooling history applied. We assume that with decreasing cooling rate on vitrification the melt of PLLA nuclei formation at a given temperature in the glassy state will occur faster. This assumption is justified with experimental evidence that formation of homogeneous nuclei in the glassy state requires a prior densification of the glass. Studies of the nucleation rate in glassy poly(ε-caprolactone)21 and polyamide 655 revealed that isothermal annealing of the glass first leads to a relaxation of the enthalpy to the value the liquid state, and only then crystal nuclei are forming. In other words, if PLLA is cooled at lower rate, though sufficiently fast to suppress crystallization, the glass transition will occur at lower temperature and the enthalpy relaxation at a predefined temperature will be completed faster, to shorten the time for nuclei formation. Accordingly, nuclei formation on aging of amorphous and glassy PLLA at ambient temperature will proceed distinctly faster. Though not experimentally proven yet, a decrease of the glass transition temperature by only 10 K may reduce the characteristic time for nuclei formation in the glassy state by more than 1 order of magnitude, as it is suggested by the data of Figure 10. The effect of the pathway of nucleation on the semicrystalline morphology of PLLA has been demonstrated with the POM micrographs of Figure 3. Crystal nucleation at high supercooling of the melt, or in the glassy state, leads to faster crystallization and formation of smaller spherulites at elevated temperature, with well-described effects of the spherulite morphology on ultimate engineering/application-relevant properties.58−60 Knowledge of the relation between the conditions of nucleation/crystallization, the corresponding semicrystalline structure, and final properties is indispensable to exploit the potential of this important engineering polymer. We believe that the aging experiments performed contribute to improved understanding of polymer nucleation and crystallization in general, however, are aware that continuation of
is further increased to 333, 338, and 343 K, then the gross crystallization rate increases, as it is seen by shift of the conversion−time curves toward shorter time. Figure 12 is a plot of the half-time of cold-crystallization of PLLA at 393 K as a function of the temperature of prior aging
Figure 12. Half-time of cold-crystallization of PLLA at 393 K as a function of the temperature of prior aging for 103 s.
in the glassy state for a period of 103 s (16.67 min). The horizontal line represents the half-time of crystallization of close to 700 s of nonaged PLLA, as it was discussed with Figures 4−6. Aging the glass of PLLA for 103 s is connected with a distinct increase of the gross crystallization rate at 393 K. While the increase of the crystallization rate is negligible on aging at temperatures lower about 320 K, aging at 343 K for 103 s reduces the half-time of crystallization to almost 1/3 of that of nonaged PLLA. On the basis of the knowledge that nuclei formation in the glassy state is a temperature- and timedependent process, which has been quantified with Figures 2 and 10, it is straightforward to assume that with increasing aging time similar reduction of the crystallization rate occurs as a result of aging at lower temperatures, including ambient temperature.
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CONCLUSIONS In the present work we analyzed the kinetics of formation of crystal nuclei in fully amorphous and glassy PLLA with low content of D-isomers of 1.5%. PLLA samples were aged below the glass transition temperature for different periods of time, and crystal nuclei formation was probed by subsequent analysis of the crystallization process at elevated temperature using fast scanning chip calorimetry (FSC) and polarizing optical microscopy (POM). FSC was applied to gain information about the effect of aging the glass of PLLA on the overall crystallization kinetics, while POM has been employed to obtain information about the nucleation density. The data observed using different instrumentation are consistent and provided quantitative information about the kinetics of formation of crystal nuclei on isothermal aging of the glass of PLLA. The kinetics of formation of crystal nuclei has been quantified by the time of aging required to observe an increase of the nucleation density and crystallization rate. There has been observed an increase of the onset time of formation of crystal nuclei with decreasing temperature of aging, which is in accord with the classical theory of nucleation.52−54 The classical nucleation theory predicts with increasing supercooling of the melt an exponential increase of the nucleation rate due to the increasing thermodynamic driving force for crystallization, G
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research is required to, e.g., quantify the effects of the coolingrate controlled variation of the dynamic glass transition temperature, the initial crystallinity, or D-isomer concentration on the kinetics of crystal nuclei formation in the glassy state. Regarding the presence of crystals in the glassy amorphous matrix, the nucleation kinetics may be affected due to an additional immobilization of amorphous chain segments. Such immobilized amorphous chain segments are part of the rigid amorphous fraction (RAF),61−63 which also has been detected in semicrystalline PLLA.3,12,41,42,64−66
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AUTHOR INFORMATION
Corresponding Author
*Phone +49 3461 46 3762; Fax +49 3461 46 3891; e-mail rene.
[email protected] (R.A.). Notes
The authors declare no competing financial interest.
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DEDICATION Dedicated to the memory of Professor Bernhard Wunderlich, our mentor, a great scientist, and teacher. †
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