Clarifying the Origin of Multiple Melting of Segmented Thermoplastic

Article pubs.acs.org/Macromolecules

Clarifying the Origin of Multiple Melting of Segmented Thermoplastic Polyurethanes by Fast Scanning Calorimetry J. Balko,† B. Fernández-d’Arlas,‡ E. Pöselt,§ R. Dabbous,∥ A. J. Müller,‡,⊥ and T. Thurn-Albrecht*,† †

Institute of Physics, Martin Luther University, von-Danckelmann-Platz 3, 06120 Halle/Saale, Germany POLYMAT Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, 20018 San Sebastián, Spain § BASF-Polyurethanes GmbH, E-PME/NL, Elastogranstraße 60, 49448 Lemförde, Germany ∥ BASF Schweiz AG, RAV/B, Mattenstrasse 22, 4058 Basel, Switzerland ⊥ IKERBASQUE, Basque Foundation for Science, Bilbao, Spain ‡

S Supporting Information *

ABSTRACT: Segmented thermoplastic polyurethanes (TPU) often show multiple endothermic signals during melting in conventional differential scanning calorimetry (DSC). Repeated reorganization during heating and separate crystal melting and phase mixing have been proposed as the origin of this phenomenon. We used fast scanning calorimetry on a series of TPUs with polyether soft segments and hard segments consisting of 4,4′-methylenediphenyl diisocyanate and 1,4-butanediol to differentiate between the two possibilities. Fast heating scans (>500 °C s−1) of isothermally crystallized samples exhibited only a single melting peak, while complementary experiments on quenched, fully amorphous samples showed a single glass transition step indicating a homogeneous mixed phase. These results prove that the multiple melting peaks observed in DSC are related to reorganization during heating, i.e., repeated melting and recrystallization. Phase separation during cooling of such TPUs is induced by crystallization. A comparison with conventional DSC scans shows the consistency of both calorimetric techniques in the limit of low heating rates.

1. INTRODUCTION Linear segmented thermoplastic polyurethanes (TPUs) are statistical multiblock copolymers with alternating “hard” and “soft” segments (more precisely blocks) along the backbone. At ambient conditions TPUs are usually microphase separated with domains of semicrystalline hard segments and usually amorphous soft segments with a glass transition temperature far below room temperature. Urethane groups in the hard segments provide either intracrystalline H-bonds or bonds between hard and soft segments if they are intermixed.1−3 Versatile synthetic routes for TPUs lead to an abundant number of materials with different chemical structures, showing attractive mechanical properties such as high elongation, tensile strength, and excellent abrasion and tear resistance.2−4 TPUs are widely used in diverse industrial fields such as clothing, automotive, coatings, and biomaterials.2,3 Although the mechanical properties of these materials are directly related to the phase-separated nanostructure, there is no agreement in the literature about the origin and the driving forces of the microphase formation. In the classical case of diblock copolymers microphase separation in the liquid state depends on the copolymer composition and the segregation strength χN. Here χ is the Flory−Huggins interaction parameter and N the degree of polymerization. Upon cooling across the disorder-to-order transition temperature TODT or microphase-separation temper© XXXX American Chemical Society

ature, monomers of the same species will aggregate, forming periodic phase-separated structures on a length scale of some ten nanometers.5 The microphase-separation transition for the more general case of amorphous multiblock copolymers was studied by Hadziioannou and Benoit6 using a mean-field approach. In addition to this well-understood case of demixing in the liquid state, crystallization of block copolymers containing crystallizable blocks can also cause phase separation and nanostructure formation. In this case, the resulting structure consists of a crystalline phase and an amorphous phase, which contains the amorphous block as well as uncrystallized crystallizable blocks. The relative location of crystallization temperature Tc and TODT determines the pathway for self-assembly.7−10 For Tc < TODT, cooling of a homogeneous melt leads to microphase separation in the liquid state possibly followed by crystallization of the crystallizable block. Contrarily, for TODT < Tc crystallization is the driving force for nanostructure formation.7,8 Experimentally, extensive work has been performed to elucidate the microphase-separated structure of TPUs by using small-angle11−15 and wide-angle X-ray scattering11,14,16−19 (SAXS, WAXS), transmission electron microscopy20,21 Received: April 27, 2017 Revised: September 6, 2017

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DOI: 10.1021/acs.macromol.7b00871 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. TPU Samples Studied in This Worka macrodiol name

type

Mw [g mol−1]

Tg [°C]

Mn [g/mol]

Mw [g/mol]

wt % HS

cp(T′) [J/(g K)]

m [ng]

PU-30 PU-43 PU-75 MDI-100

polyether (PTHF)

1000

−75

45000 36000 50000 3990

94000 79000 113000 9030

30 42.6 75 100

2.5 2.5 2.3 2.2

514 50 380 36

a The glass transition temperature Tg of the pure soft phase and the specific heat capacity cp(T′) were obtained by conventional DSC with 20 °C min−1.

(TEM), and differential scanning calorimetry13,14,22−25 (DSC). Usually, at room temperature TPUs are in a semicrystalline, phase-separated state with an interdomain distance of about 10−20 nm as deduced from SAXS.12−14,26 The microdomains have only short-range order, and in general, attempts to model the SAXS data remained ambiguous due to incomplete knowledge about the shape of the crystallites.22,26 The crystal structure is known, although the Bragg reflections in WAXS are difficult to detect for TPUs with low hard segment (HS) content due to the small size of the crystallites.14,19 WAXS investigations on stretched and annealed samples revealed in most cases a triclinic unit cell,16−18,27 but also other polymorphs have been reported depending on thermomechanical history.14,19,28 On a larger scale, spherulitic superstructures may develop as observed with optical microscopy and TEM.19,20 In DSC TPUs often show multiple endothermic signals during melting, and the different peaks were interpreted in terms of the phase behavior of the TPUs.11,22,23 The first melting endotherm at temperature T1 is often called “annealing endotherm” since it shifts with annealing or crystallization temperature, whereas the high-temperature melting endotherms (T2 and sometimes T3) show a more complex behavior. It is clear that at elevated temperatures TPUs undergo a transition from a phase-separated structure to a disordered melt, but the nature of the driving force responsible for phase separation is still debated.13,15,24 Chen et al. explained the annealing endotherm with an enthalpy relaxation of the amorphous hard segments25 whereas others attributed the different melting peaks to crystalline polymorphs.14,28 Koberstein et al. concluded that microphase mixing precedes the melting transition.13,29 In contrast, Saiani et al. attributed T1 to the melting of the ordered phase followed by microphase mixing.24,30 As discussed below in conjunction with Figure 2, this ambiguity cannot be easily resolved by combined SAXS/ WAXS investigations due to the above-mentioned weak signals of the Bragg reflections. In a recent publication,15 we analyzed the melting behavior of a series of TPUs with different compositions by SAXS and DSC. All samples feature a well-developed microphaseseparated nanostructure as shown by small-angle X-ray scattering and AFM. The results suggested that microphase mixing is induced by melting of the crystals alone. We interpreted the observed continuous change of the position of the SAXS signal during heating as ongoing melting and recrystallization over the whole DSC melting range. It is common experience that such reorganization processes can often be suppressed with the still relatively new technique of fast scanning calorimetry (FSC), allowing for rates of several thousand Kelvin per second during heating and cooling.31−33 This approach was recently used to analyze the complex melting behavior of a number of semicrystalline homopol-

ymers.34−37 Here, we apply fast scanning calorimetry to study the melting and crystallization behavior of selected TPUs. We show that it is indeed possible to suppress reorganization during heating and that for the samples analyzed here structure formation is solely driven by crystallization; i.e., there is no additional microphase separation in the liquid state.

2. EXPERIMENTAL SECTION Materials. The TPUs were prepared by BASF Polyurethanes GmbH (Lemförde, Germany) by a one-shot process. They consist of 4,4′-methylenediphenyl diisocyanate (MDI) and 1,4-butanediol (BD) as components of the urethane-rich hard blocks and a 1000 g mol−1 polyether (polytetrahydrofuran, PTHF produced by BASF, Mn = 990 g/mol, Mw/Mn = 2.28) as macrodiol, as the main component of the soft phase. The TPU casts have been ground to chips and subsequently injection molded to obtain uniform sheets for testing. Table 1 summarizes the molecular structure, sample codes, molecular weight, composition, heat capacity, and sample mass of the studied materials. TPUs were named referring to the MDI-BD hard segment content. The pure hard phase material MDI-100 consisting only of MDI and chain extender BD was synthesized with a ratio of 1:1.2. Wide-Angle X-ray Scattering (WAXS). WAXS experiments were performed in Bragg−Brentano geometry, using a PANalytical Empyrean diffractometer, equipped with a position sensitive detector (PIXcel-3D). The samples were placed on silicon zero background substrates. The measurements were performed at room temperature in air with nickel-filtered Cu Kα radiation (λ = 1.5418 Å). The diffractograms were recorded in a scan range of 4° < 2θ < 60° with a step size of Δ2θ = 0.105° and a counting time per step of 238 s. Intensity data are presented as a function of scattering vector q = 2π sin θ/λ. Conventional Differential Scanning Calorimetry (DSC). Conventional differential scanning calorimetry was carried out using a DSC 7 and a DSC 8500 from PerkinElmer equipped with intracooler accessories. The experiments were performed under a N2 atmosphere, and the equipment was calibrated using tin and indium. For the DSC 7 measurements background contributions to the signal were subtracted, resulting in measurements of the heat capacity cp(T). The thermal protocol for isothermal crystallization at different temperatures was as follows: (a) The thermal history was erased at 220 °C for 1 min. (b) Subsequently, the sample was cooled to the selected Tc at 150 °C min−1. (c) The temperature was held at the selected Tc for 30 min, and then the temperature was lowered to 30 °C at a rate of 150 °C min−1. (d) Finally, the sample was analyzed by heating with 20 °C min−1 to the melt. The thermal protocol used to analyze the impact of heating rate on the reorganization was as follows: (a) The thermal history was erased at 220 °C for 1 min. (b) The temperature was set to 80 °C at a rate of 150 °C min−1. (c) The temperature was kept at 80 °C for 5 min. (d) The sample was subsequently cooled to 30 °C at a rate of 150 °C min−1. (e) Finally, the samples were analyzed during heating scans at 12, 30, 70, 97, or 120 °C min−1. A mass compensation to avoid heat conductance lag on the observed temperature was done by using different sample weights for each experiment at different heating rates: 6.79, 2.65, 1.14, 0.89, and 0.66 mg for the previously indicated rates, B


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Macromolecules respectively. Test measurements consisting of repeated heating and cooling measurements were performed to make sure that degradation during the high-temperature treatment is negligible. Fast Scanning Calorimetry (FSC). The Flash DSC 1 from Mettler Toledo/Switzerland used in this work is a power compensated differential scanning calorimeter allowing typical scanning rates of βH = 1−10 000 °C s−1 during heating and βC = 1−5000 °C s−1 during cooling.32,38 Equipped with an intracooler (TC100MT, Huber), the accessible temperatures range from −95 to 420 °C with a temperature uncertainty of ±5 °C. Physically separated heaters for the general temperature program and the compensation power along with special control routines allow the instrument to apply the compensation power Pdiff either to the sample or to the reference side, which is advantageous in terms of signal-to-noise ratio and response time of the system.38 Each chip sensor (MultiSTAR UFS1 (XI-400, Xensor Integration/Netherlands)) was conditioned and corrected for temperature calibration before use according to Mettler Toledo procedures. Nitrogen was used as a purge gas. Sample Preparation and Evaluation. For WAXS measurements the injection-molded samples were annealed at 100 °C for 20 h. This annealing step is an often applied procedure to limit the changes in the microstructure of TPUs, when storing samples for long periods of time at ambient conditions. However, thermal history was erased for most experiments by heating to a temperature well above the melting points of the sample, unless specified in the text. The samples for the FSC studies were cut with a scalpel below a microscope, placed on the chip sensor, heated to a temperature above the melting point and cooled down again to ensure good thermal contact. The sample mass was determined from the heat capacity in the molten state at T′ = 200 °C obtained from FSC scans and the specific heat capacity cp(T′) measured with conventional DSC. The mass determination is described in detail elsewhere.33 The data shown and discussed below were obtained from four samples whose characteristics including sample mass are listed in Table 1. The large range of variation in sample mass is due to preparation as the sample with a mass of 50 ng was cut with a microtome. For comparison purposes, we studied a number of additional samples with different masses and the general results are independent of sample mass. If not stated otherwise, the FSC experiments were performed using isothermally crystallized samples for which the thermal program is depicted in Figure 1. The dwell time at T = 240 °C in the molten state

and the diffraction patterns depicted in Figure 2 illustrate the difficulties to study the relation of phase mixing and melting in

Figure 2. (a) Conventional DSC heating scans with 20 °C min−1 for PU-30 directly after annealing at 100 °C for 20 h (black) and after isothermal crystallization at Tc = 70 °C for tc = 30 min (gray, shifted by −0.5 J/(g °C)). The arrows indicate the crystallization temperatures. (b) Powder diffractograms at room temperature for annealed samples. The curves of PU-43, PU-75, and MDI-100 are shifted for clarity (40, 80, and 120 cps). The inset shows a SAXS pattern for an annealed PU30 sample (data from ref 15).

TPUs. The DSC heating scans for PU-30 after annealing or isothermal crystallization show a broad melting range with several pronounced melting peaks. Whereas the first melting peak temperature Tm (taken at the maximum) is about 18 °C higher than the crystallization temperature Tc (arrows in Figure 2a), the last melting peak endotherm is at much higher temperatures of about 155 °C. The melting enthalpy obtained by integration for the sample crystallized at Tc = 70 °C is ΔHm = 14.9 J/g and for the annealed sample ΔHm = 19.3 J/g, respectively. Thus, even TPUs with low hard segment content (HSC) have a significant amount of crystalline material. The multiple melting peak endotherms are well-known, but their assignment to structural changes is ambiguous.11,13,14,22−25,28−30 Although the samples contain crystalline entities that melt during heating, the WAXS patterns at room temperature for the annealed samples PU-30, PU-43, and PU75 (Figure 2b) show no Bragg reflections14 but only a broad halo around q ≈ 1.5 Å−1, presumably due to the small size of the crystals and the large background from the amorphous parts of the samples. The clear reflections of the pure hard phase

Figure 1. Thermal program for isothermal FSC crystallization experiments. was 1 s, and cooling rate during the step from T = 240 °C to Tc was βC ≥ 1000 °C s−1, which is sufficient to prevent crystallization even at large undercoolings. Glass transition temperatures Tg from the FSC measurements were determined using the midpoint method provided by the STARe Software from Mettler Toledo. Therein, the heat flow before and after the glass transition is extrapolated with tangents. The midpoint taken as the value for Tg is the intersection of the bisector and the heatflow curve. This evaluation is in good agreement with other methods to determine Tg, as for example with half of the step height at the heat capacity jump.

3. RESULTS AND DISCUSSION Difficulty To Detect the Driving Force for Phase Separation. The heating traces obtained by conventional DSC C


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°C is ΔHm = 15.6 J/g, which is in good agreement with the above DSC result. For Tc > 80 °C the PU-30 sample cannot fully crystallize in the given crystallization time. Accordingly, the step of the heat capacity at the glass transition of the remaining amorphous phase shifts slightly to higher temperatures and increases in height since more hard phase material remains dissolved.13 Similarly, the Tg also shifts to higher temperatures with increasing HS content but constant crystallization temperature (Figure 3b−d). For PU-75 and MDI-100 crystallized at Tc = 60−80 °C the samples remain amorphous since Tc < Tg. In this case, instead of a melting peak the heating trace shows a glass transition step and a peak due to enthalpy relaxation. To further support the finding of a single melting peak endotherm, we show in Figure 4 a heating scan for PU-43 with

material MDI-100 could be indexed according to the triclinic unit cell proposed in the literature.16−18 At room temperature, the PU samples are phase separated on a length scale of 10−20 nm as exemplarily shown by the SAXS pattern for PU-30 (inset of Figure 2b) with a peak at q ≈ 0.05 Å−1. However, the absence of crystalline WAXS signal precludes studying the relation of crystal melting and phase mixing directly by temperature-dependent combined SAXS/WAXS measurements. The same problem arises during cooling. As we will show in the following, FSC can be used as an alternative approach to address the interplay of melting/crystallization and phase separation in TPUs. Single Melting Endotherm Employing High Heating Rates. Figure 3 shows a series of FSC heating scans measured

Figure 4. Heating scan for PU-43 to Tmax = 350 °C with 1000 °C s−1 after isothermal crystallization at 110 °C for 30 min and cooling down to −90 °C. The micrograph shows the sample without optically visible degradation on the flash DSC sensor after the high temperature heating. Scale bar: 250 μm.

Figure 3. FSC heating scans after isothermal crystallization at 60 °C ≤ Tc ≤ 140 °C for tc = 30 min for (a) PU-30 with 500 °C s−1, (b) PU-43 with 1000 °C s−1, (c) PU-75 with 500 °C s−1, and (d) MDI-100 with 4000 °C s−1. Curves with different Tc are shifted for clarity. The y-axes of panels a−d are scaled differently.

a final temperature of 350 °C. Also in this extended temperature range no second melting endotherm was detected. The increase in heat flow at elevated temperatures is partially caused by the increasing heat losses to the surrounding. No signs of degradation were optically visible. We found the same results for PU-30. We conclude that the high heating rates available with FSC can indeed be used to suppress reorganizational processes taking place in our samples during a DSC heating scan. The crystalline structures evolved at Tc simply melt in one step during heating at Tm > Tc. However, rate-dependent superheating effects lead to a larger temperature difference ΔT = Tm − Tc ≈ 50−70 °C than found in conventional DSC measurements. As a next step we will analyze heating rate effects on the FSC measurements in some more detail. Superheating and Recrystallization during Slow Heating. Figure 5 shows heating scans measured with different heating rates for two different crystallization temperatures (Tc = 40 °C and Tc = 80 °C) and two selected samples, namely PU30 and PU-43. Two effects were observed: a shift of the melting peak toward lower temperatures with decreasing heating rates and in addition a change in the shape of the measured curves. The latter effect shows up most clearly in the upper part of Figure 5a (PU-30, Tc = 40 °C). While for βH = 1000 °C s−1 only a single melting peak is observed, an additional peak develops at higher temperatures Tre for smaller βH. We attribute this latter endotherm to the final melting of material recrystallized at Tm or slightly higher temperatures.34,35,41 For Tc = 80 °C the second endotherm appears as a shoulder at the high-temperature side of the melting peak at Tm and is less pronounced than for Tc = 40 °C. This can be explained with a temperature-dependent recrystallization rate. Similar results

after isothermal crystallization at different crystallization temperatures T c for the four samples with different compositions listed in Table 1. In contrast to conventional DSC, all the FSC scans with crystallization temperature above the glass transition temperature exhibit a single melting peak. As it is generally known for polymers, the melting temperature is always higher than the crystallization temperature. Two effects contribute here, namely experimental superheating effects, which are analyzed in more detail below, and the fundamental kinetics and thermodynamics of crystallization and melting, which are out of scope of the current paper.39,40 Table 2 shows the melting enthalpies obtained from integration of the heating scans shown in Figure 3. The melting enthalpy of PU-30 after crystallization at Tc = 60−80 Table 2. Melting Enthalpies Obtained from Integration of the FSC Heating Scans in Figure 3 ΔHm [J/g] Tc [°C]

PU-30

PU-43

PU-75

MDI-100

60 70 80 90 100 110 120 130 140

15.5 15.6 15.6 14.7 12.8 11.1 6.6 2.1 1.0

20.8 19.5 20.2 20.4 20.1 19.3 18.7 18.7 18.9

24.7 30.7 33.9 34.5 36.0 36.7

40.3 50.1 56.6 56.6 56.7 57.6 D


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sample, but this difference disappears in the extrapolation to low heating rates. In order to demonstrate the occurrence of recrystallization during heating in a more direct way, we performed the experiment shown in Figure 6. During a first heating scan

Figure 6. FSC thermograms with βH = 1000 °C s−1 for PU-43 isothermally crystallized at Tc = 60 °C for tc = 5 min. After the first heating above the melting transition the sample is kept at 150 °C for the waiting time tre = 60 s. The insets show heating scans after prolonged tre and the thermal program.

Figure 5. FSC heating scans with varying heating rate for (a) PU-30 and (b) PU-43 isothermally crystallized for tc = 5 min at Tc = 40 and 80 °C (dashed lines). For clarity, not all measured scans are shown. (c) Shift of the melting point for PU-30 (gray) and PU-43 (black) isothermally crystallized at Tc = 80 °C in linear and (d) doublelogarithmic representation with fits according to ΔT ∼ βxH (PU-30: x = 0.18; PU-43: x = 0.17).43

(black), the sample, here PU-30, is kept at 150 °C, i.e., above the melting peak for tre = 60 s to allow for recrystallization.41 There is no crystallization during a subsequent cooling scan (blue), however, during the following heating scan a melting peak at a temperature Tre above 150 °C is visible. This endotherm is caused by the melting of the crystals recrystallized at T = 150 °C. Prolonged waiting times tre lead to a further increase of the final melting point (see inset). In conclusion, we showed that the multiple endothermic signals during heating are caused by melting and recrystallization and are suppressed at higher heating rates. There is no evidence for separate transitions due to melting of the crystals and phase mixing of liquid microdomains. Consequently, during cooling from the melt the crystallization should induce the microphase separation. We will address the question of how this process shows up in FSC in the following section. Crystallization Induces Phase Separation. The cooling and subsequent heating scans depicted in Figure 7 demonstrate that all samples undergo a single glass transition if the scan starts at a high temperature in the mixed state and cooling rates are sufficiently high so that crystallization during cooling is suppressed. Here a cooling rate of 1000 °C s−1 was chosen. This value is far above the critical cooling rate βcr ≈ 5 °C s−1, above which crystallization is suppressed. We determined βcr beforehand by varying the cooling rate from the mixed state, followed by heating and evaluation of the total melting enthalpy.33,36,37 Obviously in this case the samples solidify by a transition from a liquid mixed phase to a glassy mixed phase. Consequently, the heating scan shows also a single glass transition step. The corresponding glass transition temperatures are given in Table 3. As expected for a mixture of two amorphous components, the Tg of the PU samples varies

were obtained for PU-43 as shown in Figure 5b. Note that the increasing background signal for smaller βH visible for all measurements is caused by a rate-independent contribution to the signal Pdiff. As the data are normalized with βH this contribution shows up as increasing background.42 The effect of the heating rate on the melting temperature is analyzed in Figure 5c where the shift of melting point ΔT with respect to the crystallization temperature is plotted as a function of βH. Superheating has been studied for a number of polymers, but its microscopic origin is still under discussion.44,45 On the basis of an approach by Toda et al.,43 we used an empirical description with a power law ΔT ∼ βxH (PU-30: x = 0.18; PU-43: x = 0.17) to determine an extrapolated value for a typical DSC rate. The fit and the extrapolation are shown in Figure 5d. We obtained ΔT = 14 °C for βH = 0.33 °C s−1 = 20 °C min−1, which is in good agreement with ΔT ≈ 18 °C resulting from DSC measurements. Therefore, it is valid to assign the first melting endotherm observed in DSC (see Figure 2a) to the melting of the originally developed structure at Tc. Note that the data presented in Figure 5a were obtained from a very thick sample with m = 514 ng, which allows to perform measurements with rather low heating rates. The large mass on the other hand causes a large ΔT due to thermal lag, i.e., heat conduction through the sample.43 We cross-checked the results using the microtomed sample PU-43 with m = 50 ng and D = 2.7 μm for which thermal lag is negligible.43 As expected, we found melting temperatures 5−10 °C lower than for the PU-30 E


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Figure 7. FSC cooling (blue) and subsequent heating scans (red) measured with 1000 °C s−1 for PU-30 (solid), PU-43 (dashed, shifted by 0.5 J/(g °C)), PU-75 (dotted, +1.5 J/(g °C)), and MDI-100 (dotdashed, +1.75 J/(g °C)).

Table 3. Glass Transition Temperatures Determined According to the Midpoint Method from the Cooling and Heating Scans in Figure 7a name

TCg [°C]

THg [°C]

PU-30 PU-43 PU-75 MDI-100

−9 13 68 79

−9 8 84 85

Figure 8. FSC cooling and heating scans of isothermally crystallized samples kept at Tc for 0.1 ≤ tc ≤ 2000 s. The thermal program is given in Figure 1. After being kept at Tc the sample is cooled down to −90 °C shown in the insets and reheated as shown in the main part of the figures. (a) PU-30 measured with β = 500 °C s−1 (Tc = 110 °C) and (b) PU-43 measured with β = 1000 °C s−1 (Tc = 140 °C).

a

A Gordon−Taylor analysis of the glass transition temperatures is available in the Supporting Information.

between the values for the pure macrodiol phase Tflex g ≅ −75 °C (see Table 1) and for MDI-100 TMDI ≅ 85 °C. The latter value g is in acceptable agreement with a value of 93 °C reported for a TPU with noncrystallizable hard segments.46 Having shown that during rapid cooling from high temperatures the samples remain in a mixed, amorphous state, we demonstrate in a next step by a similar experiment that crystallization and phase separation taking place at lower temperatures are directly connected. Figure 8 shows isothermal crystallization experiments with different crystallization times and subsequent rapid cooling and heating according to the scheme depicted in Figure 1. The crystallization temperature was chosen such that a full crystallization takes roughly 1 h, i.e., 110 °C for PU-30 and 140 °C for PU-43. For PU-30 substantial crystallization takes only place beyond about 50 s (gray lines), as we conclude from the fact that there is only a single glass transition during cooling followed by a single glass transition without melting endotherm during subsequent heating. The value for the glass transition temperature during cooling, TCg (tc = 50 s) = −4 °C, is slightly higher but still in reasonable agreement with the value given in Table 3. Thus, the sample is still in the mixed state. Note that if demixing had already occurred in the molten state, two glass transition steps should be visible. Here we found the opposite behavior: after crystallization at Tc for tc ≥ 50 s we observed a shift of Tg to

lower temperatures and a decrease of the step height along with an increasing melting endotherm. After isothermal crystallization for 2000 s we determined Tg = −24 °C, corresponding to a shift of −20 °C compared to the mixed phase. The data for PU-43 in Figure 8b yielded similar results with Tg(tc = 50 s) = 18 °C and Tg(tc = 2000 s) = −4 °C. These results together with the experiments described above clearly show phase separation and crystallization respectively phase mixing and melting are connected processes; i.e., phase separation and nanostructure formation of the investigated samples are driven by crystallization. In line with our results, only for a polyurethane sample with noncrystalline hard segments Ryan et al.46 reported the existence of two distinct glass transition steps along with an ODT at TODT ≈ 150 °C. Analysis of the First Melting Peak in Conventional DSC. We now bridge the gap between FSC and DSC in terms of heating rate to show the consistency of the results obtained by both methods. Analogous to the FSC experiments shown in Figures 3a and 5a, we performed DSC heating scans for PU-30 after isothermal crystallization. As depicted in Figure 9a, the first melting endotherm at Tm shifts linearly with Tc with an offset of about 20 °C. In contrast, the position of the broad melting endotherm at a higher temperature Tre is nearly constant although its double-peak structure changes somewhat. F


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Figure 9. Conventional DSC analysis of PU-30. (a) Heating curves at 20 °C min−1 of samples after isothermal crystallization at the indicated temperatures for 30 min and cooling to 30 °C. (b) Heating curves at the indicated heating rates, after isothermal crystallization during 5 min at Tc = 80 °C and cooling to 30 °C. (c) Shift of the melting point for PU-30 isothermally crystallized at Tc = 80 °C obtained with conventional DSC (squares) and FSC (circles, data from Figure 5d). The value of x in ΔT ∼ βxH for the DSC data is 0.33.

Because of the same dependence Tm (Tc) as in Figure 3a, we assign the first endotherm to the melting of the originally crystallized structure. On further heating the low rate of the DSC of βH = 20 °C min−1 = 0.33 °C s−1 allows the sample to recrystallize and remelt.41 By varying βH from 12 to 120 °C min−1 (Figure 9b), we observed the already described superheating effect for the first melting endotherm. Along with this shift of the first peak its height increases, most pronounced for the highest DSC rate βH = 120 °C min−1 = 2 °C s−1. Figure 9c shows the same analysis of the superheating effect as in Figure 5d together with the FSC data. The different slopes, i.e., power law exponents, are to be expected given the different thermal lag conditions. However, the fact that in the limit of low heating rates the ΔT determined with both methods coincide with each other proves the consistency of DSC and FSC measurements and our interpretation of the data.

extent, the mechanical properties.15 As the TPU chemistry is extremely versatile and as the competition between crystallization and microphase separation depends on several molecular parameters, we cannot exclude that in other polyurethanes a separate microphase separation transition preceding crystallization exists and determines the microstructure. Nevertheless, it should be possible to address this question also for other TPU systems using the approach illustrated here. Furthermore, given the straightforward interpretation of the melting signal observable in FSC, the method can be used in future studies to acquire more detailed information about crystallization of TPUs, e.g., to investigate the exact relation between crystallization and melting temperature or the effect of crystallization temperature and composition on melting enthalpy, i.e., on crystallinity.

4. CONCLUSIONS AND OUTLOOK In our experiments, we successfully used fast scanning calorimetry to clarify the origin of the complex melting signal of TPUs commonly observed in conventional DSC measurements. Heating at high rates suppresses reorganization effects, and the resulting FSC scans only show one single peak, which can clearly be related to melting of the original crystals formed during the preceding crystallization. High cooling rates, on the other hand, suppress crystallization during cooling scans. The glass transition observed during cooling therefore contains information about the phase state at high temperatures, at which the cooling scan was started. Here we could show that the systems under study are phase mixed at high temperatures and that crystallization starts directly from this phase mixed state. By performing series of experiments with different heating rates on the conventional DSC as well as on the FSC, we could demonstrate the consistency of the two methods. In conclusion, we could demonstrate that there is no separate microphase separation taking place in the liquid state at high temperatures for the TPUs studied here, which are based on polyether soft segments and hard segments of 4,4′-methylenediphenyl diisocyanate and 1,4-butanediol. In consequence, the two-phase microstructure of the materials forms during crystallization and is determined by the crystallization conditions. This mechanism is the reason for the strong effect of crystallization conditions like thermal treatment or nucleation control on the microstructure and, to a certain

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00871. Figure S1 (PDF)

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Corresponding Author

*E-mail: [email protected] (T.T.-A.). ORCID

A. J. Müller: 0000-0001-7009-7715 T. Thurn-Albrecht: 0000-0002-7618-0218 Present Addresses

J.B.: Fraunhofer Institute for Applied Polymer Research, Geiselbergstr. 69, 14476 Potsdam, Germany. B.F.A.: INAMAT-Institute for Advanced Materials, Universidad Pública de Navarra, 31006-Pamplona, Spain. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.B. and T.T.A. acknowlegde partial financial support from the DFG within the framework of the collaborative research center SFB TRR 102. All authors gratefully acknowledge financial support from BASF. The authors thank K. Herfurt and X. Li for technical help with DSC measurements. G


Article

Macromolecules

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Clarifying the Origin of Multiple Melting of Segmented Thermoplastic

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