Article pubs.acs.org/Macromolecules
Crystallization and Homogeneous Nucleation Kinetics of Poly(εcaprolactone) (PCL) with Different Molar Masses Andreas Wurm,† Evgeny Zhuravlev,† Kathrin Eckstein,‡ Dieter Jehnichen,‡ Doris Pospiech,‡ R. Androsch,§ B. Wunderlich,∥,⊥ and Christoph Schick*,† †
Institute of Physics, University of Rostock, Wismarsche Str. 43-45, 18051 Rostock, Germany Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Str. 6, 01069 Dresden, Germany § Center for Engineering Sciences, Martin-Luther-University Halle-Wittenberg, 06099 Halle/S., Germany ∥ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States ⊥ Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡
ABSTRACT: The crystallization and nucleation kinetics of poly(ε-caprolactones) (PCL) with molar masses between 1.4 and 6.1 kDa and negligible number of heterogeneous nuclei has been investigated by differential fast scanning calorimetry (DFSC) applying scanning rates up to 100 000 K/s. The samples were synthesized by ring-opening polymerization and chemically characterized by NMR spectroscopy, size exclusion chromatography (SEC), and multiangle laser light scattering (MALLS). For the smallest molar mass the chain length is comparable with the crystal thickness measured with small-angle X-ray scattering (SAXS), and extended chain like crystals may be formed. Because of the molar mass distribution (PDI ≈ 2), these crystals have a significant noncrystalline interface yielding nearly the same crystallinity for all molar masses. The critical cooling rate to obtain amorphous samples is below 1000 K/s and only for the lowest molar mass increased to 2000 K/s. The same trend holds for the about 1 order of magnitude higher critical heating rate to keep the samples amorphous on heating and for the analysis of isothermal nucleation and crystallization kinetics at 202 K. The samples which were shown not to contain heterogeneous nuclei active at a heating rate of >18 000 K/s were used for a study of the nucleation activity of ordered structures formed on annealing at low temperature. The analysis of the change of the thus-produced amorphous polymer samples on annealing from 202 to 272 K for times varying by a factor of more than 108 (0.1 ms to 8.3 h) revealed new details about the ordering processes (nucleation, poor crystal formation, crystallization, cold crystallization, and crystal perfection) and the accompanying changes in glass transition of the remaining amorphous phase (formation of rigid amorphous phases, RAF).
1. INTRODUCTION Poly(ε-caprolactone) (PCL) is an important thermoplastic polymer with high impact strength and resistance against oils combined with full bio- and hydrolytic degradability.1 To these properties improved mechanical properties and thermostability can also be added when copolymerized with poly(urethane)s and included as segments in organic/inorganic hybrid materials with even.2 PCL is one of the rare polymers offering thermodynamic miscibility with several other polymers, for instance poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC), which opens the opportunity to prepare miscible blends and to use corresponding block copolymers as compatibilizers for immiscible blends.3 It can be easily processed from the melt due to its low melting temperature of about 343 K by means of extrusion, melt spinning, film blowing, and injection molding. Melt-processed products find applications as fully degradable composting bags, sutures, and biodegradable fibers. PCL is a semicrystalline material whose crystallinity and morphology determine the mechanical properties. Thus, the detailed knowledge of the crystallization process is critical to develop materials with tailored crystalline structure © 2012 American Chemical Society
and properties. The crystal structure of PCL has been analyzed by X-ray diffraction which revealed an orthorhombic unit cell with a fiber repeat distance of c = 1.705 nm.4,5 The unit cell contains two molecular chains with a planar zigzag conformation of the methyl groups and slightly tilted ester groups. The carbonyl groups of the two chains are shifted to each other by 3/14 c, and the density of the unit cell is 1.195 g/cm3.6 There is a large difference between crystallization of low molecular weight compounds and polymers. Whereas in low molecular weight compounds the crystallizing elements, the motifs, can be transported and placed rather independently into the crystal, the situation is different for polymers. Here we have to consider covalently bound units which form a long chain of micrometer length. The motifs are no longer separated and cannot move independently. Additionally, the mobility of the crystallizing segments is hindered due to entanglements, leading to a complicated situation on the local scale and Received: February 21, 2012 Revised: March 27, 2012 Published: April 20, 2012 3816
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introduced as rate-determining step of the secondary crystallization process. It was found in crystallization experiments on polyethylene22 that only molecule segments with a length multiple times the crystallization temperature dependent fold length participate in crystallization, ultimately leading to the conclusion that a molecular nucleus21 must be formed before being retained on the growing crystal. Though the concept of molecular nucleation still is not fully accepted and understood, progress in experimental validation, seemingly, has been achieved recently on analyzing reversible crystallization and melting at the lateral faces of crystals in numerous semicrystalline polymers.23
particularly at the interfaces. Polymer crystallization is therefore assumed, and in theories often described, to be a multistep process with many influencing aspects. It commonly results in a semicrystalline structure. Because of the chain structure, it is easy to understand that a process which is thermodynamically forced to increase local ordering but is geometrically hindered cannot proceed into a final equilibrium state. As the result, nonequilibrium structures with different characteristics are usually formed, which depend on temperature, pressure, shearing, and other parameters.7 It is generally accepted that ordering starts with a nucleation process from the supercooled state followed by growth. Nucleation can be either homogeneous or heterogeneous. Homogeneous nuclei are formed due to thermodynamic driving forces by the polymer chains themself, whereas heterogeneous nuclei are often supported on interfaces, usually on intentionally added foreign materials or often on impurities. Self-nucleation, as a third type of primary nucleation, is similar to heterogeneous nucleation.8 It is caused by residues of prior crystals not heated sufficiently high or long enough above the melting temperature to be sufficiently randomized. Information about the nucleation mechanism can be derived in most cases only indirectly due to the small size of the nuclei and the low contrast to the melt. It can be attained by analyzing the final structure or by following the crystallization process after nucleation. In calorimetry, the cold crystallization enthalpy on heating or at isothermal conditions at low temperature was found to be dependent on the number of previously formed nuclei.9−14 This observation was used as a basis for a preceding study of isothermal nucleation and crystallization in PCL, covering times from 10−4 to 105 s and temperatures from about 25 K below the glass transition up to 330 K, close to the equilibrium melting temperature. With the applied fast scanning calorimetry it was possible to follow the development of crystals at a preselected, constant temperature over 9 orders of magnitude in time and to gain information about homogeneous nucleation and crystallization kinetics.15 Polymer crystallization is commonly heterogeneously nucleated, and large structures like spherulites are growing at low undercooling. Only at sufficient fast cooling the nucleation activity of heterogeneities can be suppressed. These heterogeneities can be catalyst residues, processing enhancers, or stabilizers.8,16−21 Particularly, if cold crystallization enthalpy is used to quantify the number of active nuclei in a sample, such heterogeneities often result in some crystallization. This causes difficulties when trying to study pure homogeneous nucleation.15 In this paper we extend our previous study on nucleation and crystallization kinetics in PCL15 in two directions: (i) By ringopening polymerization (ROP) of ε-caprolactone and special purification of the obtained PCL, the number of heterogeneous nuclei was reduced compared to the technical polymer used previously. Consequently, the ordering processes leading from a purely amorphous melt to cold crystallization on heating at the chosen heating rates for analysis could be studied. This allows extension of the previous work regarding activity and stability of nuclei. (ii) PCL of different molar mass was synthesized and allowed in combination with a study of homogeneous nucleation an initial insight into the problem of molecular segregation on crystallization.8,21 In order to accommodate the experimental observation of segregation of molecules of different molar mass on crystallization, and to explain required supercooling even in the presence of selfnuclei, the concept of molecular nucleation has been
2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization of PCL with Different Molar Mass. 2.1. Materials. Diethylaluminum ethoxide (DEAE, 25%, in toluene, Aldrich), n-heptane (99%, p.a., Merck), calcium hydride (95%, Sigma-Aldrich), and HCl (1 m, Titrisol, Merck) were used as received. ε-Caprolactone (CL, 99%, Acros Organics) was distilled and stored over calcium hydride prior to use. Tetrahydrofuran (THF, 99.9%, Sigma-Aldrich) was refluxed with a mixture of sodium and potassium and distilled off prior use. Characterization. NMR spectra were recorded on a Bruker Avance III NMR spectrometer at 500 MHz (1H NMR) or 125 MHz (13C NMR) in deuterated chloroform (CDCl3) as solvent and with internal standard (δ(1H) = 7.26 ppm, δ(13C) = 77.0 ppm). The molar masses and molar mass distributions were determined by size exclusion chromatography (SEC) using THF as eluent, at a flow of 1 mL/min (HPLC pump, series 1200, Agilent Techn.), refractive index dn/dc (ETA-2020), and viscosity detection (Bures), and additionally multiangle laser light scattering (MALLS) detection (Wyatt). Relative molar masses were calculated employing narrow distributed of PMMA standards. Absolute molar masses were determined with productspecific calibration uses dn/dc = 0.065. Long periods of the semicrystalline lamellae stacks were derived from small-angle X-ray scattering (SAXS) experiments via Bragg’s law from intensity maximum in the scattering curve I = I(detector channel) under consideration of strong decline of scattering background due to high amount of particle scattering of PCL powder. Experiments were carried out at beamline A2 (HASYLAB at DESY Hamburg, Germany) with synchrotron radiation wavelength of λ = 0.150 nm. Intensity was registered by a linear detector mounted in ≈106 cm distance with 15 s accumulation time per single scan. Detector calibrations were arranged using suitable calibration samples (i.e., rat tail collagen fibers, and silver behenate). Polymerization. In a typical polymerization experiment, THF (40 mL) was condensed into the polymerization flask with magnetic stirrer, the appropriate amount of initiator DEAE was added via nitrogen-flushed syringe, and the flask was thermostated at 303 K. The polymerization started after addition of the monomer CL (4.0 g, 0.035 mol) via syringe and was performed under stirring for 2 h at 303 K. Then, the reaction was stopped by the 3-fold molar amount of HCl with respect to DEAE used. After stirring the mixture for further 60 min, the solution was precipitated into n-heptane (400 mL). The polymer was filtered off, washed several times with water until a pH = 7 was reached, again filtered, and finally dried at reduced pressure for 2 days at 323 K. Yields: 95.5−100%. 1H NMR (500 MHz, CDCl3, δ): 4.12 (q, H7), 4.05 (t, H1), 3.64 (t, H1′), 2.30 (t, H5), 1.65 (m, H4), 1.63 (m, H2), 1.38 (m, H3), 1.24 ppm (t, H8). 13C NMR (125 MHz, CDCl3, δ): 173.29 (C6), 63.93 (C1), 62.32 (C1′), 60.03 (C7), 33.94 (C5), 32.17 (C2′), 28.18 (C2), 25.36 (C3), 25.17 (C3′), 24.55 (C4′), 24.40 (C4), 14.08 ppm (C8). Assignment according to
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Figure 1. Thin film chip sensor XI-396 based on a thin free-standing SiNx film on a silicon frame and measuring area of 60 μm × 80 μm the center of the film. (a) Different photographs of the sensor. (b) Schematic cross section of the sample-loaded sensor (not to scale). (c) Photomicrograph of a sample loaded sensor XI-396 (left) and, for comparison, sensor UFS 1 (right).
Table 1. Average Molar Masses (Mn, Mw), Molar Mass Distribution (PDI), Corresponding Chain Lengths in Crystalline Conformation (lc), and Lamellar Thickness (dc) of PCL Samples under Investigation entry
polymer
Mn,calca (Da)
Mw,RIb (Da)
PDIRIb
Mn,MALLSc (Da)
Mn,NMRe (Da)
lcg (nm)
dch (nm)
1 2 3 4 5 6
PCL1.4K PCL1.6K PCL3.1K PCL5.3Kd PCL6.1K PCLFlukaf
1.0K 1.0K 3.0K 10.0K 10.0K
3.7K 4.0K 9.0K 19.0K 17.2K 19.6K
1.68 1.554 1.88 2.26 1.76 2.13
1.4K 1.6K 3.1K 5.3K 6.1K 5.8K
1.5K 1.5K 2.8K 4.9K 5.8K
10.5 12.0 23.2 39.6 45.6 43.4
6.5 7.1 6.7 7.6
a
Molar mass calculated from [CL]0/[DEAE]0. bMolar masses by SEC relative to PMMA standards. cMolar mass from MALLS detection. dReaction time: 16 h instead of the 2 h used for samples 1−3 and 6. eCalculated from 1H NMR signal intensities (Mn,NMR = (I(H1) + I(H7))/I(H1′)·114.1 g/ mol + 45.1 g/mol) with an estimated relative error of ±5%. fNot synthesized in this work, only for comparison: commercial sample, which was investigated by.15 gEstimated chain length in the crystalline conformation, based on a fiber repeat distance of 1.705 nm, involving two monomers along the helix axis.4 hLamellae thickness of the as-synthesized PCL estimated from crystallinity and long periods, measured by SAXS, assuming a two-phase lamellae stack model. temperature (extrapolated onset30) was determined at different heating rates and compared to the melting temperature at zero heating rate.24 Operating the DFSC with liquid nitrogen as the coolant allows linear cooling and heating below the glass transition of PCL at about 203 K and above the melting temperature at about 343 K, at rates up to 100 000 K/s. At low scanning rates for heat flow rate determination with the DFSC is limited to 100 K/s because of the reduced signal-to-noise ratio. To bridge the gap between the DFSC (>100 K/s) and conventional DSC (
