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Tailoring the Morphology and Melting Points of Segmented Thermoplastic Polyurethanes by Self-Nucleation Borja Fernández-d’Arlas,† Jens Balko,‡ R. Peter Baumann,§ Elmar Pöselt,∥ Raphael Dabbous,⊥ Berend Eling,# Thomas Thurn-Albrecht,‡ and Alejandro J. Müller*,†,% †

POLYMAT and Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel Lardizábal 3, 20018, Donostia-San Sebastián, Spain ‡ Institut für Physik, Martin-Luther Universität, Von-Danckelmann-Platz 3, 06120 Halle/Saale, Germany § BASF SE, GMC/O, Carl-Bosch-Strasse 38, 67056 Ludwigshafen, Germany ∥ BASF -Polyurethanes GmbH, GMP/LT, Elastogranstrasse 60, 49448 Lemförde, Germany ⊥ BASF Schweiz AG, GMV/B, Mattenstrasse 22, 4058 Basel, Switzerland # BASF -Polyurethanes GmbH, GMP/LP, Elastogranstrasse 60, 49448 Lemförde, Germany % IKERBASQUE, Basque Foundation for Science, Bilbao, Spain S Supporting Information *

ABSTRACT: It is demonstrated that the melting behavior and the morphology of three segmented thermoplastic polyurethane elastomers (TPUs) can be tailored by applying self-nucleation (SN) procedures. The self-nucleating temperature ranges for each of the TPU have been first determined by differential scanning calorimetry (DSC), while their morphology was studied by polarized light optical microscopy (PLOM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS). When the samples are cooled at slow to moderate rates after SN, the crystallization temperature of the TPUs increases by up to 45 °C, when the samples are ideally self-nucleated. This large reduction in supercooling increases the melting points of the samples by approximately 20 °C. At the same time, SAXS and AFM experiments demonstrate the growth of thicker lamellae under these slow to moderate cooling conditions as compared to untreated samples. When ideally self-nucleated samples are rapidly quenched (e.g., at rates of 100 °C min−1 or larger) from their self-nucleation temperature to room temperature, the effects of SN described above on the morphology and melting points of the samples disappear for the TPUs that do not crystallize fast enough.

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phases: a urethane rich, commonly addressed as “hard blocks” (HB), and another comprised mainly by the macrodiol phase.1 Depending on the multiblock compositions, the self-assembly of these copolymers can be very diverse developing either crystalline or amorphous phases, in the urethane-rich or in the macrodiol-rich phase.2 The recent review by Yilgör et al. details all the critical parameters needed to design segmented polyurethanes and their effect on their morphology and properties.3 Thermal treatments can be developed to trigger different morphological changes in TPUs involving different degrees of phase separation4 and/or crystallization.5,6 These types of treatments usually involve either a simple annealing process for the first case or melting followed by isothermal crystallization for the second case.

INTRODUCTION Thermoplastic polyurethanes (TPU) possess a combination of attractive properties like high elongation and tensile strength, high toughness, excellent abrasion, and tear resistance. They are in general melt-processed and used in a wide range of products like footwear, wire and cable sheeting, hoses, tubing, films, and sheets. Segmented thermoplastic polyurethane elastomers (TPUs) can be considered as multiblock copolymers comprising a wide range of materials. Their molecular structure can be described as a statistical block copolymer by the following notation: −{[D-G]x-(D-M)y}n−, where D, G, and M correspond to the elements that define the structure of the diisocyanate, a short molecular weight glycol, and a medium molecular weight macrodiol, respectively. The subscripts n, x, and y correspond to the degree of polymerization of the block copolymer and the so-called hard and soft segments, respectively. All monomers are linked through urethane groups (−O−CO−NH−) which act as strong physical cross-linking agents, via hydrogen bonding, and promote phase separation into two differentiated © XXXX American Chemical Society

Received: July 15, 2016 Revised: September 23, 2016

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DOI: 10.1021/acs.macromol.6b01527 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

nature, as well as the influence of hard block content, on the self-nucleation behavior of different TPUs is studied. The SN process is investigated by differential scanning calorimetry (DSC), while its consequences on TPUs morphologies are evaluated by polarized light optical microscopy (PLOM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS). The results indicate that a significant manipulation of the morphology and melting points of the materials can be performed by carefully choosing the SN thermal conditioning. The cooling rate after SN was found to be a determinant factor for tailoring the morphology and thermal properties of the examined TPUs.

In this paper, a novel way to modify the structure−properties relationship of TPUs is developed based on inducing selfnucleation (SN) of the crystalline phase followed by cooling at different rates. Self-nucleation is a carefully designed thermal protocol that is able to produce a large number of self-nuclei in a crystallizable material, so that its nucleation density can be increased by several orders of magnitude.7−9 Additionally, SN triggers morphological changes as well as increases in the crystallization temperature from the melt of semicrystalline polymers7 and copolymers.10,11 Self-nuclei can be composed of crystal fragments or chain segments with residual orientation from previous crystallization (crystalline memory effects), and they are considered the best possible nucleating agents for any given polymer.12,13 The interested reader is referred to a recent review on self-nucleation and references therein.11 Self-nucleation is produced when a polymer with a standard thermal history (i.e., crystallized from the melt at a constant cooling rate) is subsequently heated to a preset self-nucleation temperature denoted Ts. The polymer is left at Ts for a constant time (typically 5 min), and then the material is cooled from Ts until it completes its crystallization. Finally, the sample is reheated to evaluate its melting behavior after the applied thermal treatment. Fillon et al.12 defined three self-nucleation Domains depending on the relative position of Ts values with respect to the melting temperature range of the polymer and the effects of the thermal treatment. Domain I (DI) or complete melting Domain occurs when Ts is high enough to completely erase the crystalline memory of the material, so that only high temperature resistant, heterogeneous nuclei remain in the sample. Therefore, in this Domain, the peak crystallization temperature of the polymer remains invariant with Ts. Domain II (DII) or exclusive SN Domain occurs when Ts is high enough to almost completely melt the polymer but low enough to produce self-nuclei. If any crystal fragments remain unmolten, they may act as self-nuclei, but they are not capable of thickening and no annealing can be detected in subsequent melting. Self-nuclei can also be composed of chains with residual orientation (i.e., produced by crystalline memory effects11). Therefore, DII is characterized by the production of self-nuclei. As Ts decreases, the number density of nuclei increases exponentially within DII with a concomitant rapid increase in the peak crystallization temperature as the material is cooled from Ts. Finally, Domain III (DIII) or “self-nucleation and annealing” Domain occurs when Ts temperatures are in the lowest range and can only cause partial melting of the crystal population. The remaining unmolten crystals will be annealed during the 5 min holding time at Ts. In this complex Domain, the sample undergoes both self-nucleation and annealing. The trend of Tc as Ts decreases in DIII is irregular but is generally characterized by an increase in Tc values that can later saturate. Despite the wide potential application of TPUs, at least to the authors’ knowledge, there are no previous reports where self-nucleation of TPUs and its effects on morphology and thermal properties have been explored in detail. One previous reference (ref 14) dealing with heterogeneous nucleation of TPUs reports that they have employed self-nucleation to calculate the nucleation efficiency of some nucleating agents; however, they do not provide any data about self-nucleation of the employed TPUs, and they do not quote the value of the ideal self-nucleation temperature needed for their efficiency calculations. In the present work, the influence of the macrodiol

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EXPERIMENTAL SECTION

Materials. The TPUs were prepared by BASF Polyurethanes GmbH (Lemfö rde, Germany). They consist of 4,4′-methylenediphenyl diisocyanate (MDI) and 1,4-butanediol (BD) as components of the urethane-rich hard blocks (HB) and either a polytetrahydrofuran (polyether) or an adipic polyester macrodiol, both with Mn ∼ 1000 g/mol and a polydispersity index of ∼2, as the main component of the soft phase. The raw materials have been produced by BASF, and the TPUs have been prepared by BASF Polyurethanes GmbH (Lemförde, Germany) by a one-shot process. The TPU casts have been ground to chips and subsequently injection molded to obtain uniform sheets for testing. Prior to measurement, the sheets have been annealed at 100 °C for 20 h. Table 1 gathers the molecular structure, sample codes, and composition of the studied TPU.

Table 1. Relevant Properties of the TPUs under Study macrodiol TPU code

type

Mn [g mol−1]

Tga [°C]

hard blocks (HB)

wt % HB

PUest33 PUeth30 PUeth43

polyester polyether polyether

1000 1000 1000

−73 −75 −75

MDI-BD MDI-BD MDI-BD

33 30 43

a

Obtained by DSC at 20 °C min−1.

TPUs were named referring to the macrodiol nature and the MDIBD hard segment content (i.e., PUest33: a polyester based TPU with 33 wt % of MDI-BD). DSC Self-Nucleation (SN) Studies. DSC studies were carried out in a Perking Elmer 8500 equipped with a PerkinElmer Intracooler 3 cooling accessory. The tests were performed under a N2 atmosphere, and the equipment was calibrated using indium and tin. In order to ascertain the SN temperature Domain borders, the following thermal protocols were applied to each sample (see Figure 1): (a) Erasure of thermal history by heating the sample to 30 °C above the TPU melting temperature and keeping the sample 1 min at that temperature. (b) Creation of the “standard” semicrystalline morphology by cooling from the melt to 10 °C at a rate of 20 °C min−1.

Figure 1. Scheme of a DSC thermal program for the self-nucleation experiments as performed in this study. B


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Macromolecules (c) Heating the sample from 10 °C to a chosen self-seeding temperature, denoted Ts, and keeping the sample at this Ts for 1 min. This unconventionally low period of time (generally 5 min is employed 11 ) was chosen to minimize sample degradation. Several tests were performed to check that this period of time (1 min) was sufficient to melt (DI), self-nucleate (DII), or self-nucleate and anneal (DIII) the samples depending on the applied Ts. (d) Subsequent cooling to 10 °C at 20 °C min−1, while the DSC cooling scan was recorded. A period of 1 min stabilization was applied at 10 °C. (e) Heating the sample from 10 °C to the melt at 20 °C min−1 while the DSC heating scan was recorded. The Domain borders from Domain I to Domain II can be easily determined by examining the cooling runs from the selected Ts temperatures. When samples are in Domain I, the peak crystallization temperature (upon cooling from Ts) does not vary when Ts is changed. If Ts values are progressively decreased, eventually, the peak crystallization upon cooling from a specific Ts will increase as compared with the standard crystallization temperature (or peak crystallization temperature in Domain I); then, the sample has crossed from Domain I into Domain II. The Domain border between Domain II and Domain III can be detected by the appearance of an annealing peak on the subsequent heating scan after SN. More details about selfnucleation experimental procedure can be found in a recent review.11 One can refine the temperature of the transition between Domains by repeating the SN experiments around the crossover regions using Ts temperatures separated by 1 °C (or even 0.5 °C). Influence of Cooling Rate from Ts (during SN, Step D) on Subsequent DSC Heating Scans. The SN procedure was repeated but varying the cooling rate employed in step (d). The cooling rate influence on the subsequent DSC melting scans (step (e) of the SN procedure) was analyzed by first heating the TPUs with a “standard” morphology to the selected Ts. Then the samples were kept for 1 min at the selected Ts before cooling them to 10 °C at the following rates: (a) ballistically, i.e., at nominal rate of ca. 160 °C min−1, and (b) at controlled cooling rates of 80, 60, 40, 20, 10, and 5 °C min−1. The samples were then analyzed by recording their heating scans at 20 °C min−1. Polarized Light Optical Microscopy (PLOM). The TPUs texture as a function of the different thermal treatments was monitored by PLOM using a Leica DFC320 polarized microscope equipped with two crossed polarizers, a red tint plate, and a Wild Leitz digital camera. The temperature control was achieved using a Mettler Toledo FP82HT hot stage coupled with a Mettler Toledo FP90 control console. A piece of each of the TPU samples was placed between glass slides and hot pressed at 250 °C during some seconds. With this procedure a fine contact between the TPU and the glass was achieved. The procedure followed to study the morphology as a function of different thermal treatments was as follows: (a) The TPU sample was melted in DI for 1 min. (b) Standard crystallization. The sample was cooled from DI (i.e., 220 °C for PUest30) to a Tc (i.e., 160 °C for PUest30) in order to monitor the morphology of developed during isothermal crystallization from the melt. (c) Erasure of thermal history in DI for 1 min. (d) Creation of a “standard morphology” by cooling from DI to room temperature at 20 °C min−1. (e) Heating to Ts (i.e., 192 °C for PUest30) at 20 °C min−1. (f) Self-nucleation at Ts for 1 min. (g) Cooling down from Ts to Tc at 20 °C min−1 and recording the evolution of the self-nucleated morphology. Small-Angle X-ray Scattering (SAXS). To probe the semicrystalline morphology, a laboratory setup from SAXSLAB (Copenhagen, Denmark) was used in transmission geometry. It consists of a microfocus source (50 kV, 1 mA), a focusing X-ray optics device (AXO, Germany), and a HighStar 2D-detector (Bruker). The optics provided a monochromatic beam of Cu Kα radiation (λ = 1.54 Å)

having a diameter of about 0.6 mm. All measurements were done under rough vacuum conditions of p = 10−1 mbar. The TPU samples were measured either at room temperature or during heating to the molten state. For the latter, aluminum discs with a central hole of 2 mm in diameter were used as sample holders which were mounted on a Linkam hot-stage. Heat conducting paste was used to ensure good thermal contact. SAXS patterns during heating were recorded at temperature steps of 20 °C for 40 °C ≤ T ≤ 100 °C and of 10 °C for 100 °C ≤ T ≤ Tmax with Tmax being a sample specific temperature above the final melting point. The effective heating rate at high temperatures T > 100 °C was estimated to be 7 °C min−1. The exposure times were 30 min at room temperature and 1 min at elevated temperatures. Applying Bragg’s law, the interdomain distance d = 2π/qmax was calculated from the position of the SAXS peak maximum, qmax, where q = 4π sin(θ)/λ. Especially for the measurements at higher temperatures it is important to take into account the background intensity for the determination of qmax. To this end we modeled the data by a superposition of a Gaussian and a background consisting of a power law and a constant. Atomic Force Microscopy (AFM). The bulk morphology of thermoplastic elastomer films was investigated using atomic force microscopy (AFM). The nanomechanical properties were investigated on a Dimension Icon AFM from Bruker AXS in PeakForce QNM mode. The PeakForce QNM AFM images were recorded using RTESPA-150 silicon cantilevers (k = 5 N m−1). AFM samples were ultra cryo-microtomed. In this way, the AFM tip probes the inner parts of the specimens giving morphological information about the bulk material. Sample Preparation for SAXS and AFM. The samples were obtained from rectangular plaques with 2 mm thickness produced by injection molding. All materials studied with SAXS and AFM were subjected to three different thermal treatments: SN for 1 min in DII followed by either (1) slow cooling to room temperature with 5 °C min−1 or (2) fast cooling with 100 °C min−1 and (3) annealing at 100 °C for 20 h. The SN treatments were done in the DSC to ensure precise temperature control. The annealing was performed in an oven, and it 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.

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RESULTS AND DISCUSSION Nonisothermal DSC Characterization. Enthalpies and temperatures of crystallization for the TPUs were determined during a “standard” cooling scan at 20 °C min−1 from the isotropic melt (DI). The TPUs glass transition temperatures, Tg, melting peaks, Tm, and melting enthalpies, ΔHm, were determined during the subsequent heating scan at 20 °C min−1. Table 2 summarizes the basic thermal data of the studied TPUs. It is interesting to note that the Tg values of the macrodiols corresponding to the three samples reported in Table 1 are very similar (in the range −73 to −75 °C). At the same time, two of the TPUs employed here have similar hard block contents (i.e., PUeth30 and PUest33); nevertheless, their Tg’s are very Table 2. Thermal Properties of Studied TPUs As Obtained by DSC at 20 °C min−1 crystallization

Domain borders

melting

TPU

Tc [°C]

ΔHc [J g−1]

Tg,flexa [°C]

PUest33 PUeth30 PUeth43

82 86 106

13 11 15

−12 −31 −19

Tm [°C]

ΔHm [J g−1]

DI− DII [°C]

DII− DIII [°C]

166 167 178/187

13 14 16

207 201 223

186 183 207

a

Tg,flex: this temperature is the glass transition temperature (inflection point) of the TPUs macrodiol-rich phases. C


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Macromolecules different (Table 2). This is an indication of different degrees of phase segregation between hard and soft blocks. The ether based sample has a much lower Tg value (Tg = −31 °C), indicative of a higher degree of phase segregation, as compared to the ester based sample (Tg = −12 °C). The lower Tg of PUeth30 as compared to PUest33 is characteristic of the higher tendency of polyether-based TPUs toward phase separation as compared to polyester-based TPUs.15−17 An important basic difference in the materials under study is the higher Tm and ΔHm of PUeth43 as compared to the other two TPUs. These higher values are due to the higher volume fraction of hard blocks (or weight percentage), which are the crystal forming parts of the chains. Another difference of PUeth43 and the other two TPUs is the presence of marked multiple melting endotherms. Multiple melting peaks have been previously reported for different high MDI-BD content TPUs.18−21 The origin of multiple melting peaks in high MD-BD content TPUs is under discussion,22−25 and its detailed study is outside the scope of the present work. The most important question regarding multiple endotherms is whether the highest temperature melting endotherm or shoulder is related to a phase mixing phenomenon.22,23 In the present case, SAXS, PLOM, and DSC results (to be presented below) indicate that the final endothermic transition (or highest temperature endotherm) corresponds to the fusion of crystals, as indicated by the simultaneous disappearance of birefringence in PLOM (see Figure S2) and SAXS maxima (see Figure 11 and its discussion below). Another evidence of the correspondence of the final endothermic transition to a melting process is given by the melting point dependence on annealing temperature observed in Domain III (i.e., the melting point of the annealed crystals is a linear function of the Ts temperature applied). Additionally, temperature-dependent SAXS results (to be presented below in Figure 11) indicate that as soon as the sample is above its final melting temperature (as determined by DSC), a single phase melt is observed. Self-Nucleation (SN) Study. Crystallization Behavior after SN. Self-nucleation tests involve, as explained in the Experimental Section, exposing the material to a complex thermal protocol including high temperatures for short times. After all final DSC runs, the thermal history of the material was erased once more, and the standard melting endotherm was measured again. Because of the fact that the peak melting point was always identical to that measured during the first standard DSC heating run, it was assumed that the degradation effect on the measurements was minimal. Figure 2 presents cooling curves from selected Ts temperatures after SN for the three studied TPUs. The DSC cooling scans are represented from top to bottom in each Figure 2 (a, b, and c) in decreasing Ts order. The DSC traces are color coded in the digital version of this paper. The red color is used to indicate Domain I, while blue is for Domain II and green for Domain III. Additionally, all curves are labeled with their corresponding Domains (DI, DII, and DIII). The first two scans of each Figure 2 (i.e., a, b, and c) are selected cooling scans from Domain I or complete melting Domain, so they represent cooling from isotropic melts. As expected, the peak crystallization temperature does not change from the first curve (at the top) to the second one, since both curves are in Domain I. As a result of self-nucleation, the peak crystallization temperature increases as nucleation density is increased in Domain II. DSC cooling scans in Domain II are curves 3−5

Figure 2. DSC cooling scans (20 °C min−1) from the indicated Ts for (a) PUest33, (b) PUeth30, and (c) PUeth43. Red curves: DI; blue curves: DII; green curves: DIII.

counting from the top (in blue color on the digital version of the article) in each one of the plots of Figure 2 (i.e., a, b, and c). Self-nucleation in Domain II occurs gradually as T s temperatures decrease, and it is characterized in Figure 2a (for PUest33) by the appearance of a high temperature exotherm that grows in size at the expense of the originally present exotherm that remains at lower temperatures, until at the lowest Ts temperatures in Domain II, it completely disappears (see the two arrows pointing to these two exotherms for DSC cooling scans in Domain II in Figure 2a). Figure 2b shows a somewhat similar behavior for PUeth30. On the other hand, Figure 2c presents the behavior of PUeth43, for which the original exotherm splits into three components as SN proceeds, until at the lowest temperatures within DII, the highest crystallization exotherm dominates. The very large increase in peak crystallization temperature or Tc observed in Domain II for all TPUs is remarkable. For the PUest33 the Tc varied from 82 °C when cooled from Ts = 220 °C (DI) to 128 °C when cooled from Ts = 192 °C (DII). This comprises an increase of about 46 °C in Tc when the sample is self-nucleated as compared to when it is not. In the case of PUeth30 the increment is even higher since it varies from 86 to 144 °C by changing the Ts from 220 to 190 °C, respectively. This is a variation of about 58 °C in Tc. In Figure 2c, the DSC cooling scans for PUeth43 are presented. It can be observed that the standard cooling curve in Domain I (i.e., cooling from the isotropic melt, where no SN occurs) has a Tc peak at 106 °C, 20 °C higher than that of PUeth30 in Domain I. This is an expected result since PUeth43 has a larger fraction of crystallizable hard blocks (HB), MDIBD, than PUeth30. This higher fraction of crystallizable HB D


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Macromolecules leads to longer uninterrupted crystallizable sequences in the multiblock like copolymer structure characteristic of TPUs. Hence, PUeth43, as already stated in Table 2, is characterized by the highest melting point and the highest crystallization temperature in Domain I among the three samples employed in this work. Self-nucleation of PUeth43 leads to a maximum Tc of 159 °C when cooled from Ts = 210 °C, representing an increase of 53 °C in Tc. The effects of SN on the subsequent melting will be analyzed in the next section. The increase in Tc values after ideal self-nucleation (i.e., when the nucleation is performed at the lowest possible Ts value within DII) is related to the number of active heterogeneities (or nucleation density) present in a given polymer before SN. Self-nucleation produces an enhancement in nucleation density until saturation occurs at values around 1012−1013 nuclei cm−3. Therefore, it is easy to understand how a polymer like high density polyethylene, which has an intrinsically high nucleation density (typically 109 nuclei cm−3), only exhibits increases of Tc values of just 2°−4°, while isotactic polypropylene (with 106 nuclei cm−3) can increase its Tc values by 30 °C when it is self-nucleated.7,11,12 Müller et al. reported that in the case of polystyrene-bpolycaprolactone (PS-b-PCL) block copolymers the peak crystallization temperature of PCL blocks increased almost 10 °C (from ≈37 to 47 °C).10 For neat PCL, Pérez-Camargo et al. found larger increases in Tc depending on its molecular weight.26 Poly(p-dioxanone), PPDX, also exhibits a 30 °C increase in Tc after having been ideally self-nucleated.27 Therefore, the largest increase in Tc produced by selfnucleation is 30 °C, according to previous literature. Figure 2 shows that in the case of the TPUs examined here the increases in Tc values can reach 50 °C. This result is probably a consequence of the low density of active nuclei that is present in these TPUs or a large energy barrier for nucleation. Future nucleation kinetics experiments and nucleation density determinations will be performed to study in depth the origin of this peculiar result. Final Melting Behavior after SN. Figure 3 presents the subsequent heating scans, after the SN treatment and cooling curves shown in Figure 2. The large changes in melting temperatures after SN in Domain II are remarkable and not entirely expected. In most cases, when a polymer is self-nucleated or a nucleating agent has been added to a polymer, the crystallization temperature increases significantly (depending on the efficiency of the nucleation), but its melting temperature either remains unchanged or increases very little. A typical example is PP, for which increases in Tc of 30 °C can be obtained, while Tm either does not change or increases by 1 or 2 deg.11,12 The metastability of polymer crystals is responsible for this disparity, as a very large change in Tc is needed to change Tm. This can be corroborated under isothermal conditions by the much lower slope than 45° exhibited by experimental isothermal crystallization data on a Hoffman− Weeks representation of Tm versus Tc.28 At equilibrium Tm = Tc and a 45° slope is expected. Away from equilibrium conditions, slopes lower than 45° indicate the degree of metastability that characterizes typically thin lamellar crystals.29 In nonisothermal conditions this difference between changes in Tc and changes in Tm is even larger. Figure 3 shows that the change in peak melting temperatures for all samples between Domain I and Domain II (after SN) are in the order of 20 °C. Even if these increases are smaller than

Figure 3. DSC heating scans (20 °C min−1) after cooling (20 °C min−1) from the indicated Ts for (a) PUest33, (b) PUeth30, and (c) PUeth43. Red curves: DI; blue curves: DII; and green curves: DIII.

those observed for Tc (of the order of 50 °C), they are still probably the largest increases in melting point ever reported for self-nucleated samples, as far as the authors are aware. As can be seen in Figure 3, the TPUs subsequent melting behavior after SN can vary depending on the macrodiol nature and HS content. In Figure S1 (included in the Supporting Information) cooling and subsequent heating traces after SN at a wider range of self-nucleation temperatures are gathered. Figure 3a shows that PUest33 does not exhibit a substantial increase in Tm after SN even though its Tc is notoriously increased, for example when Ts temperatures are in the range of 205−195 °C. Only at Ts < 195 °C the melting peak is shifted to higher temperatures (i.e., Tm increases from 168 °C in DI to 180 °C). On the other hand, in the case of PUeth30, as soon as the Ts falls inside DII temperature range, a large increase in Tm is observed, in parallel to the large variation in Tc. The Tm varies from 165 °C when samples are cooled from DI, to 183 °C when they are cooled from Ts = 190 °C (DII). Finally, in the case of PUeth43, the behavior is somewhat intermediate. Immediately after crossing the border from DI to DII (225−222 °C) in a SN experiment, different high melting temperature shoulders appear. These shoulders increase in intensity when SN is performed at lower temperatures within DII. After SN at Ts = 210 °C a melting endotherm with a peak at Tm = 198 °C and a shoulder at 206 °C is obtained. SN Impact on TPUs Morphology. The evolution of the morphology with SN was monitored by polarized light optical microscopy (PLOM). Using the PLOM technique coupled with a hot stage, it was determined that the “standard” morphology corresponds to small spherulites ( 20 nm observed for the slowly cooled samples do not directly correspond to the chemical microstructure of the polymers. If during crystallization at higher temperatures thicker crystals form and thinner crystals are unstable at this temperature, it is mandatory that the shorter HS sequences remain part of the soft, amorphous phase. Such an effect would lead to a simultaneous increase in dc and da and therefore to stronger increase in d, in the present work about a factor of 2. This model is consistent with previous assumptions about the morphology of TPUs formed at different annealing temperatures.30,31

annealing occurs, the SAXS maxima detected during heating have already disappeared. Similar results were found for the other two TPU samples employed here (see Figure S6). This suggests that self-nucleation experiments were initiated when the sample had a one phase melt structure with only self-nuclei present in the melt (whose presence cannot be detected by SAXS). The nature of the self-nuclei that cause self-nucleation is a subject of debate in the literature and is considered to originate from very small crystal fragments that survive melting (but cannot be annealed) or residual chain orientation arising from crystalline memory of the chains.11 A summary of the results from above measurements is given in Table 3. The melting enthalpies given for the injection molded/annealed samples are larger than those of the SN samples, in view of the long annealing time at 100 °C (20 h). The large variations of the interdomain distances after different thermal treatments can be rationalized as a consequence of the molecular structure of the TPUs as sketched in Figure 12. The length of hard and soft segments will in general follow a distribution which depends on the composition and also on the detailed reaction conditions. To estimate a value for a minimal long period, we therefore follow the result obtained by Koberstein19 that only hard segments containing at least 3−4 MDI units can crystallize. As proposed by Blackwell,32 we assume extended chain hard block sequences in the crystals, and we can estimate a minimal crystal thickness of dc = 5−7 nm. Here we used dc = n × c − dBD with the repeat distance c = 1.9 nm for one MDI and one BD33−36 and the length of one BD in all-trans conformation dBD = 0.6 nm. To take into account stoichiometry, we assume two PTHF macrodiols intercalated, each with MPTHP = 1000 g mol−1 and an end-to-end distance of R0 = 4.3 nm.37 Thus, the expected minimal interdomain distance would be dc(n) + √2R0 = 11−13 nm, which is consistent with the measured values d ≈ 10−11 nm. The crystal thickness might also somewhat shorter than the length of the extended sequence, as the material will most likely develop tilted chain crystals due the constant chain flux through the crystalline−amorphous interface.38

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CONCLUSIONS Self-nucleation of different TPUs has been reported for the first time in this work. The three self-nucleation Domains were characterized and the limits between them established. Selfnucleation is capable of increasing the nucleation density of the TPUs examined by more than 2 orders of magnitude. Under ideal self-nucleation conditions (i.e., employing Ts temperatures in the lowest temperature range of Domain II and cooling from Ts at 20 °C min−1), the samples are characterized by a reduced spherulitic size and crystallization temperatures increase up to 45 °C with respect to samples that are not self-nucleated. As a consequence of such large decrease in supercooling, melting points can increase up to 20 °C. SAXS and AFM showed consistent results that indicate strong changes in the semicrystalline morphology and an increase in crystal thickness that explains the increase in melting point displayed by the ideally self-nucleated samples. One important conclusion derived from this work is that ideal self-nucleation is only effective when the cooling rate applied after the self-nucleation treatment is low enough for a particular sample to undergo substantial overall nonisothermal crystallization at elevated temperatures. This conclusion has been demonstrated to be valid for PUest33 and PUeth30 in the cooling rate range employed in this work. It is speculated that the same behavior would be observed for PUeth43 if a high enough cooling rate is employed (i.e., higher than its average overall nonisothermal crystallization rate). K


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Macromolecules

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01527. Figure S1: self-nucleation experiments; cooling and subsequent heating DSC traces; Figure S2: spherulitic texture of a “standard” morphology as observed by PLOM; Figure S3: impact of self-nucleation/crystallization on TPUs morphology as studied by PLOM; Figure S4: domain borders as extracted from DSC selfnucleation analysis; Figure S5: impact of cooling rate on self-nucleated samples at different domains ranges; Figure S6: in-situ SAXS curves during heating (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (+34) 943018191 (A.J.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge fruitful discussion with Prof. A. Stribeck. We thank X. Li for her assistance in conducting some of the SAXS and DSC experiments. J.B. and T.T.A. acknowlegde partial financial support from the DFG within the framework of the collaborative research center SFB TRR 102. Authors gratefully acknowledge financial support from BASF.

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REFERENCES

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