Crystallization and Morphology of Triple Crystalline Polyethylene-b-poly(ethylene oxide)-b-poly(ε-caprolactone) PE-b-PEO-b-PCL Triblock Terpolymers

The morphology and crystallization behavior of two triblock terpolymers of polymethylene, equivalent to polyethylene (PE), poly (ethylene oxide) (PEO), and poly (ε-caprolactone) (PCL) are studied: PE227.1-b-PEO4615.1-b-PCL3210.4 (T1) and PE379.5-b-PEO348.8-b-PCL297.6 (T2) (superscripts give number average molecular weights in kg/mol and subscripts composition in wt %). The three blocks are potentially crystallizable, and the triple crystalline nature of the samples is investigated. Polyhomologation (C1 polymerization), ring-opening polymerization, and catalyst-switch strategies were combined to synthesize the triblock terpolymers. In addition, the corresponding PE-b-PEO diblock copolymers and PE homopolymers were also analyzed. The crystallization sequence of the blocks was determined via three independent but complementary techniques: differential scanning calorimetry (DSC), in situ SAXS/WAXS (small angle X-ray scattering/wide angle X-ray scattering), and polarized light optical microscopy (PLOM). The two terpolymers (T1 and T2) are weakly phase segregated in the melt according to SAXS. DSC and WAXS results demonstrate that in both triblock terpolymers the crystallization process starts with the PE block, continues with the PCL block, and ends with the PEO block. Hence triple crystalline materials are obtained. The crystallization of the PCL and the PEO block is coincident (i.e., it overlaps); however, WAXS and PLOM experiments can identify both transitions. In addition, PLOM shows a spherulitic morphology for the PE homopolymer and the T1 precursor diblock copolymer, while the other systems appear as non-spherulitic or microspherulitic at the last stage of the crystallization process. The complicated crystallization of tricrystalline triblock terpolymers can only be fully grasped when DSC, WAXS, and PLOM experiments are combined. This knowledge is fundamental to tailor the properties of these complex but fascinating materials.

Still, few works have been published using the apolar PE block as one of the crystallizable blocks in triblock terpolymers. Müller et al. [96] analyzed the crystalline behavior and morphology of PE-b-PEO-b-PLLA and PE-b-PCL-b-PLLA triblock terpolymers employing different cooling rates. DSC, WAXS, and PLOM techniques were used to confirm the triple crystalline character of the copolymers. They concluded that there is no change in the sequential crystallization for the PE 21 2.6 -b-PEO 32 4.0 -b-PLLA 47 5.9 triblock terpolymer using 1 or 20 • C/min, since the sequence remains the following: the PE block crystallizes first, then the PLLA block, and finally the PEO block. However, the crystallization sequence changed in the PE 21 7.1 -b-PCL 12 4.2 -b-PLLA 67 23.0 triblock terpolymer, since when using 20 • C/min as cooling rate, the crystallization begins with the PE block. In contrast, at 1 • C/min the PLLA is the first block to crystallize. PLOM experiments showed that this variation in the crystallization sequence affects the final morphology, so the cooling rate is a factor that can be used to tune the final properties.
In the present work, the triple crystalline nature of PE-b-PEO-b-PCL triblock terpolymers is analyzed, varying molecular weight and block content. The corresponding PE-b-PEO diblock copolymers and PE homopolymers are also investigated. Samples were synthesized by combining polyhomologation and catalyst-switch strategies. We study the influence of molecular weight and block composition on the crystallization of these triblock terpolymers, consisting in an apolar (PE) and two polar blocks, PEO (biocompatible) and PCL (biodegradable). The study employs differential scanning calorimetry (DSC), in situ small-angle and wide-angle X-ray scattering (SAXS/WAXS), and polarized light optical microscopy (PLOM). Understanding the crystalline behavior and the analysis of the morphology is essential to tune crystallinity and obtain novel materials with enhanced properties.

Materials
All reagents used for the synthesis of the triblock terpolymers were purchased from Merck KGaA (Darmstadt, Germany). Two different "catalyst switch" strategies were used in the synthesis of the tricrystalline terpolymers poly (ethylene)-b-poly (ethylene oxide)-b-poly (ε-caprolactone) (PE-b-PEO-b-PCL). First, the polyhomologation of dimethylsulfoxonium methylide was performed to synthesize a hydroxyl-terminated polyethylene (PE-OH) macroinitiator [101]. Then, the strong phospazene base t-BuP 4 was employed as the catalyst to promote the ring-opening polymerization (ROP) of ethylene oxide (EO) to obtain PE-b-PEO, followed by the addition of diphenyl phosphate (DPP) to neutralize t-BuP 4 . For the ROP of ε-caprolactone (CL), two different catalysts were used, Sn(Oct) 2 for T1 (organic/metal catalyst-switch), and phosphazene base t-BuP 2 for T2 (organic/organic catalyst-switch). These catalyst switch-strategies were applied to avoid as many possible side-reactions during the ROP of CL in toluene at 80 • C (Scheme 1) [102]. Table 1 shows the molecular weights of each of the blocks of the synthesized triblock terpolymers. The subscript numbers represent composition in wt %, and superscripts indicate M n values of each block in kg/mol. The polyethylene block precursors are not 100% linear because of possible side reactions and monomer purity issues. NMR measurements indicate that the PE block of T1 (see Table 1) contains 0.32% propyl side groups and 3% methyl groups, and that of T2 contains 0.45% propyl side groups and 2% methyl groups. Different melting points are obtained because of this variation in microstructure, since the T m value of PE 7.1 is 129.7 • C, while that of PE 9.5 is 117 • C (see Table S3), as the latter contains a higher amount of short-chain branches. employed as the catalyst to promote the ring-opening polymerization (ROP) of ethylen oxide (EO) to obtain PE-b-PEO, followed by the addition of diphenyl phosphate (DPP) t neutralize t-BuP4. For the ROP of ε-caprolactone (CL), two different catalysts were used Sn(Oct)2 for T1 (organic/metal catalyst-switch), and phosphazene base t-BuP2 for T (organic/organic catalyst-switch). These catalyst switch-strategies were applied to avoi as many possible side-reactions during the ROP of CL in toluene at 80 °C (Scheme 1) [102 Table 1 shows the molecular weights of each of the blocks of the synthesized triblock terpolymers. The subscript numbers represent composition in wt %, and superscripts indicate Mn values of each block in kg/mol. The polyethylene block precursors are not 100% linear because of possible side reactions and monomer purity issues. NMR measurements indicate that the PE block of T1 (see Table 1) contains 0.32% propyl side groups and 3% methyl groups, and that of T2 contains 0.45% propyl side groups and 2% methyl groups. Different melting points are obtained because of this variation in microstructure, since the Tm value of PE 7.1 is 129.7 °C, while that of PE 9.5 is 117 °C (see Table S3), as the latter contains a higher amount of short-chain branches. The formation of double crystalline copolymers and triple crystalline terpolymers was confirmed by differential scanning calorimetry (DSC), polarized light optical microscopy (PLOM), and X-ray diffraction (SAXS/WAXS).

Differential Scanning Calorimetry (DSC)
Non-isothermal DSC experiments were carried out with a Perkin Elmer DSC Pyris 1 (Perkin Elmer, Norwalk, USA) equipped with a refrigerated cooling system (Intracooler 2P). Indium and tin standards were used for the calibration of the equipment. Aluminum pans with about 3 mg of sample were tested using ultra-high quality nitrogen atmosphere.
A temperature range between 0 and 160 °C and 20 °C/min as cooling and heating rates were employed in non-isothermal DSC experiments. The samples are kept for 3 min 30 °C above the peak melting temperature of the block showing the highest melting temperature to erase the thermal history of the samples. They are then cooled down at 20 °C/min keeping them 1 min at low temperatures, and finally heating up also at 20 °C/min until the block at the highest temperature melts.

Small-Angle and Wide-Angle X-ray Scattering (SAXS/WAXS)
Simultaneous in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) experiments were performed at the ALBA Synchrotron facility in Barcelona (Spain), beamline BL11-NCD. A Linkam THMS600 (Linkam, Surrey, UK) hot stage coupled to a liquid nitrogen cooling system was used to cool and heat the samples, which were previously placed into glassy capillaries. The same thermal protocol adopted in the non-isothermal DSC experiments was used to get the SAXS/WAXS patterns, in which a The formation of double crystalline copolymers and triple crystalline terpolymers was confirmed by differential scanning calorimetry (DSC), polarized light optical microscopy (PLOM), and X-ray diffraction (SAXS/WAXS).

Differential Scanning Calorimetry (DSC)
Non-isothermal DSC experiments were carried out with a Perkin Elmer DSC Pyris 1 (Perkin Elmer, Norwalk, USA) equipped with a refrigerated cooling system (Intracooler 2P). Indium and tin standards were used for the calibration of the equipment. Aluminum pans with about 3 mg of sample were tested using ultra-high quality nitrogen atmosphere.
A temperature range between 0 and 160 • C and 20 • C/min as cooling and heating rates were employed in non-isothermal DSC experiments. The samples are kept for 3 min 30 • C above the peak melting temperature of the block showing the highest melting temperature to erase the thermal history of the samples. They are then cooled down at 20 • C/min keeping them 1 min at low temperatures, and finally heating up also at 20 • C/min until the block at the highest temperature melts.

Small-Angle and Wide-Angle X-ray Scattering (SAXS/WAXS)
Simultaneous in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) experiments were performed at the ALBA Synchrotron facility in Barcelona (Barcelona, Spain), beamline BL11-NCD. A Linkam THMS600 (Linkam, Surrey, UK) hot stage coupled to a liquid nitrogen cooling system was used to cool and heat the samples, which were previously placed into glassy capillaries. The same thermal protocol adopted in the non-isothermal DSC experiments was used to get the SAXS/WAXS patterns, in which crystallization and melting of the samples are followed, thus obtaining comparable results by the two different techniques.
The X-ray energy source was 12.4 keV (λ = 1.03 Å). For the SAXS setup, the distance between the sample and the detector (ADSC Q315r detector, Poway, CA, USA, with a resolution of 3070 × 3070 pixels, pixel size of 102 µm 2 ) was 6463 mm with a tilt angle of 0 • . Calibration was performed with silver behenate. Regarding WAXS configuration, a distance of 132.6 mm was used between the sample and the detector, with a tilt angle of 21.2 • . Chromium (III) oxide (Rayonix LX255-HS detector, Evanston, IL, USA, with a resolution of 1920 × 5760 pixels, pixel size of 44 µm 2 ) was employed for calibration. Scattering intensity as a function of scattering vector, q = 4πsinθλ −1 data are obtained, where λ is the X-ray wavelength, and 2θ is the scattering angle.

Polarized Light Optical Microscopy (PLOM)
An Olympus BX51 polarized light optical microscope (Olympus, Tokyo, Japan) was used to follow the morphological changes occurring within the samples while cooled and heated at a constant rate of 20 • C/min. For accurate temperature control, a Linkam THMS600 (Linkam, Surrey, UK) hot stage with liquid nitrogen was used. Micrographs were recorded by an Olympus SC50 camera (Olympus, Tokyo, Japan). First, a glass slide in which samples are melted is used, with a glass coverslip, and then, 20 • C/min as cooling and heating rates are employed. Morphological variations that occur during the application of this constant rate are recorded as micrographs in which crystallization and melting of each of the blocks can be followed.
Furthermore, the software ImageJ [103] was used to analyze the micrographs by measuring transmitted light intensities. The increase in light intensity detected refers to the increase in crystal content of a certain sample since crystallization of one component has started. Crystallization of this component can be followed by the increase in intensity by decreasing temperature, and the temperature range at which crystallization of a specific block occurs can be determined. In order to detect intensity changes, the whole micrographs are considered as "region of interest". Thus, all superstructures that can be formed during the cooling scans contribute to this analysis. So, the entire crystallization process is followed by analyzing intensity changes as a function of temperature, and the crystallization temperature of a particular block of the diblock copolymers and triblock terpolymers can be determined.

Results and Discussion
3.1. Small-Angle X-ray Scattering (SAXS) SAXS measurements are useful to study not only the phase segregation in the melt but also if the phase segregation is kept when the block components crystallize or if crystallization destroys it by breaking out the phase structure of the melt. Figure 1 shows the SAXS patterns of the homopolymer PE 7.1 , the diblock copolymer PE 32 7.1 -b-PEO 68 (Figure 1a,b), there is no phase segregation in the melt, as evidenced by the lack of scattering peaks in the molten state. The broad peak that appears at lower temperatures corresponds to the diffraction from crystalline lamellar stacks in the formed superstructures (i.e., spherulites or axialites).

REVIEW
6 of 23 phase structure established by phase segregation in the melt was destroyed by the breakout.  Figure 2 shows SAXS patterns of the homopolymer PE 9.5 , the diblock copolymer PE52 9.5 -b-PEO48 8.81 , and the triblock terpolymer PE37 9.5 -b-PEO34 8.8 -b-PCL29 7.6 (T2) at the indicated temperatures reached upon cooling. In this case, the behavior of the homopolymer  The diblock copolymer PE52 9.5 -b-PEO48 8.81 (Figure 2b) and the triblock terpolymer (Figure 2c) are phase segregated in the melt, with possible However, there is weak phase segregation for the PE 22 32 10.4 (T1) triblock terpolymer (Figure 1c) since there is a broad scattering peak in the melt, which disappears as crystallization breaks out when the first block upon cooling from the melt starts to crystallize (i.e., the PE block). This behavior is evidenced by the shift in q values between the reflection in the melt and the weaker reflection at room temperature, which appears at lower q values. The broad peak at room temperature corresponds to the average long period of the lamellae formed during the crystallization process because the phase structure established by phase segregation in the melt was destroyed by the break-out. Figure 2 shows SAXS patterns of the homopolymer PE 9.5 , the diblock copolymer PE 52 9.5 -b-PEO 48 8.81 , and the triblock terpolymer PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2) at the indicated temperatures reached upon cooling. In this case, the behavior of the homopolymer PE 9.5 ( Figure 2a) is the same as for the homopolymer PE 7.1 ( Figure 1a) explained above, not showing any phase segregation in the melt, as expected for a homopolymer.
The diblock copolymer PE 52 9.5 -b-PEO 48 8.81 (Figure 2b) and the triblock terpolymer PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2) (Figure 2c) are phase segregated in the melt, with possible lamellar and interpenetrated morphologies, respectively, although more detailed analysis of the scattering curves would be needed to ascertain the exact melt morphology. The clear scattering peaks in the molten state in these two materials corroborate the phase segregation behavior; however, their phase segregation is weak, since when the first block crystallizes upon cooling, i.e., the PE block at 100 • C, the phase structure is destroyed, the one generated by phase segregation in the melt, as deduced by the change in q values and intensities of the scattering peaks.  Figure 2 shows SAXS patterns of the homopolymer PE 9.5 , the diblock copolymer PE52 9.5 -b-PEO48 8.81 , and the triblock terpolymer PE37 9.5 -b-PEO34 8.8 -b-PCL29 7.6 (T2) at the indicated temperatures reached upon cooling. In this case, the behavior of the homopolymer PE 9.5 (Figure 2a) is the same as for the homopolymer PE 7.1 ( Figure 1a) explained above, not showing any phase segregation in the melt, as expected for a homopolymer. The diblock copolymer PE52 9.5 -b-PEO48 8.81 (Figure 2b) and the triblock terpolymer PE37 9.5 -b-PEO34 8.8 -b-PCL29 7.6 (T2) (Figure 2c) are phase segregated in the melt, with possible lamellar and interpenetrated morphologies, respectively, although more detailed analysis of the scattering curves would be needed to ascertain the exact melt morphology. The clear scattering peaks in the molten state in these two materials corroborate the phase segregation behavior; however, their phase segregation is weak, since when the first block crystallizes upon cooling, i.e., the PE block at 100 °C, the phase structure is destroyed, the one generated by phase segregation in the melt, as deduced by the change in q values and intensities of the scattering peaks. (T2) at the indicated temperatures.
One way to predict the segregation strength in linear diblock copolymers is by multiplying the Flory-Huggins interaction parameter (χ) (evaluated at the interest temperature in the melt) by N (the total degree of polymerization). The estimation becomes more difficult in the case of triblock terpolymers. Different behaviors can be predicted depending on the segregation strength values. Values equal or lower to 10 indicates miscibility in the melt, between 10 and 30 weak phase segregation, between 30 and 50 intermediate segregation, and if values are higher than 50, the systems are strongly segregated. A rough approximation for each pair of blocks is reported in Table S1 (see Supporting Information), using the solubility parameters of PE, PEO, and PCL from the literature [60,104]. In this case, the predicted values suggest that at least the diblock copolymers should be strongly segregated, but the experimental SAXS findings indicate miscibility for PE 32 7.1 -b-PEO 68 15.1 and weak segregation for the PE 52 9.5 -b-PEO 48 8.8 . As the dominant behavior during crystallization is that of break out, the final morphology is that of crystalline lamellae arranged in superstructures like axialites or spherulites. Therefore, we will not explore in detail the morphology of the materials in the melt, as it is destroyed upon crystallization.

Non-Isothermal Crystallization by DSC
DSC cooling and heating scans of the homopolymers, diblock copolymers, and triblock terpolymers of the two systems (Table 1) are discussed in this section. In addition, all data obtained are collected in Tables S2-S4 (Supporting Information). Figure 3 shows the cooling (A) and heating (B) DSC scans for the PE 7.1 homopolymer, PE 32 7.1 -b-PEO 68 15.1 diblock copolymer, and PE 22 7.1 -b-PEO 46 15.1 -b-PCL 32 10.4 (T1) triblock terpolymer. The crystallization peak of each block (T c ) has been assigned using WAXS data collected under identical conditions at the synchrotron (shown and described below). The same color code is used throughout this work to highlight the crystallization and melting of the different blocks (blue for PCL, red for PEO, and violet for PE). The sharp exotherm ( Figure 3A(a)) and subsequent endotherm ( Figure 3B(a)) of the neat PE 7.1 precursor is a consequence of its linear character (synthesized by polyhomologation). content and cooling from a homogenous melt are not the best scenarios to enhance cr tallization. The overlapped melting peak at the lowest temperature for the triblock terp ymer (Figure 3Bc) corresponds to the PEO (red) and the PCL (blue) blocks (an estimat of the crystallinity values is provided in Table S4), whereas the melting at the high temperatures occurs for the PE block crystals, although its crystallinity degree is only (Table S4) of its 32 wt % block content in the terpolymer.  Figure 4 shows the cooling and heating scans of the PE 9.5 homopolymer, the PE52 9. PEO48 8.8 diblock copolymer, and the PE37 9.5 -b-PEO34 8.8 -b-PCL29 7.6 (T2) triblock terpolym The crystallization and melting transitions of the blocks in these samples (Figure 4Acfollow the same trend described before in Figure 3, but with some differences due to phase behavior of the materials. The crystallization of PE 9.5 homopolymer (Figure 4Aa) occurs in a single and sha transition. For the PE52 9.5 -b-PEO48 8.8 diblock copolymer (Figure 4Ab), the crystallization the PE block (violet) occurs at high temperatures, followed by the crystallization of PEO block (red) at lower temperatures. Note that the PE block crystallizes in a uniq crystallization step in this diblock copolymer, not in a fractionated way as in the previo diblock copolymer discussed before (Figure 3Ab). The difference remains in the ph behavior in the melt, on the one hand, since this diblock copolymer shows weak ph segregation (as evidenced by SAXS experiments shown in Figure 2b), and the fact of be segregated in the melt enhances the crystallization ability of the PE block. In addition, PE block content is higher in this copolymer (52 wt %) with a higher molecular wei (9500 vs. 7100 g/mol). So, higher PE content and cooling from a segregated melt, do largely hinder its crystallization, showing a crystallization enthalpy of 81 J/g (Table S2 The crystallization sequence in the PE37 9.5 -b-PEO34 8.8 -b-PCL29 7.6 (T2) triblock terpo mer is the same as the one explained in the previous triblock terpolymer (T1) ( Figure 3A first the PE block (violet), and then the PCL (blue) and PEO (red) blocks. Although, a In the PE 32 7.1 -b-PEO 68 15.1 diblock copolymer, PE (violet arrow) is the first block crystallizing upon cooling from the melt, and then the crystallization of the PEO block (red arrow) occurs (Figure 3Ab). The crystallization of the PE block does not occur in a unique step since three exothermic crystallization peaks appear for the PE block crystallization: at 118 • C, 82 • C, and 79 • C. This evidences that the PE block crystallizes in a fractionated way, which means that several crystallization exotherms appear at lower temperatures instead of a single crystallization exotherm corresponding to the PE block's bulk crystallization temperature. Note that as shown in Figure 1b, this diblock copolymer shows miscibility in the melt, and as crystallization occurs from a homogeneous melt, as well as only having 32 wt % of PE block content and a relatively low molecular weight, the crystallization of the PE block is somehow hindered, as evidenced by its crystallization enthalpy value of 22 J/g (Table S2). However, the sharp crystallization exotherm of the PEO block and the high block content (68 wt %) suggest its high crystallization ability, as the enthalpy for the PEO is 177 J/g (Table S2).
The crystallization in the PE 22 32 10.4 (T1) triblock terpolymer ( Figure 3A(c)) starts with the PE block (violet arrow). In this case, the PE block content is low (22 wt %), and a very small crystallization exotherm is observed in the cooling scan (14 J/g) (Table S2). Crystallization continues with the PCL block (blue arrow) and the PEO block (red arrow). Although the crystallization peaks of the PEO and the PCL blocks are overlapped, WAXS results below demonstrate that the PCL block crystallizes some degrees above the PEO block ( Figure 5c). As we are not able to distinguish between both transitions, an estimation of the crystallization enthalpies is reported in Table S2 by employing block content for the calculations. Figure 3B shows the subsequent heating scans with the endothermic melting peaks (T m ) for each sample; data are collected in Table S3. The homopolymer PE 7.1 (Figure 3B(a)) shows a crystallinity value of 75% (Table S4), as expected, observing the sharp melting transition. For the diblock copolymer ( Figure 3B(b)), melting starts with the PEO block (red) with a crystallinity value of 85%; and it continues with the PE block melting (violet), with a crystallinity value of only 7% (Table S4), because as previously mentioned, small Polymers 2021, 13, 3133 9 of 25 block content and cooling from a homogenous melt are not the best scenarios to enhance crystallization. The overlapped melting peak at the lowest temperature for the triblock terpolymer ( Figure 3B(c)) corresponds to the PEO (red) and the PCL (blue) blocks (an estimation of the crystallinity values is provided in Table S4), whereas the melting at the highest temperatures occurs for the PE block crystals, although its crystallinity degree is only 5% (Table S4) of its 32 wt % block content in the terpolymer. Figure 4 shows the cooling and heating scans of the PE 9.5 homopolymer, the PE 52 9.5 -b-PEO 48 8.8 diblock copolymer, and the PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2) triblock terpolymer. The crystallization and melting transitions of the blocks in these samples ( Figure 4A(c)-B(c)) follow the same trend described before in Figure 3, but with some differences due to the phase behavior of the materials.
in this case, there is an overlap of the crystallization peaks of the PCL and PEO bloc WAXS measurements show () that the PEO block crystallizes a few degrees lower th the PCL block; and estimations of the enthalpies are provided in Table S2.
The subsequent heating scans are shown in Figure 4B. The homopolymer PE 9.5 in F ure 4Ba shows a clear melting transition and a crystallinity value of 55% (Table S4). In case of the PE52 9.5 -b-PEO48 8.8 diblock copolymer (Figure 4Bb), the melting starts with PEO block (red) and ends with the PE block (violet). As previously mentioned, segre tion in the melt and higher PE content enhance its crystallization, and thus, a clear a sharp melting transition with a crystallinity value of 27% is obtained (Table S4). Fina the PE37 9.5 -b-PEO34 8.8 -b-PCL29 7.6 (T2) triblock terpolymer follows the same trend as in triblock terpolymer T1 (Figure 3Bc): melting of the PEO (red) and PCL (blue) blocks oc with a difference of some degrees, although not enough to distinguish between both D melting transitions (demonstrated by WAXS experiments in Figures S3c-S4c); and m ing of the PE block showing a higher crystallinity degree (44%) ( Table S3).

In Situ Wide Angle X-Ray Scattering (WAXS) Real-Time Synchrotron Results
The crystallization of each block in the WAXS patterns is identified by analyzing crystal planes indexing for the PE, PCL, and PEO blocks reported in Table [29,32,38,60,64,66,92,105,106]. In addition, normalized intensity measurements as a fu tion of temperature upon cooling from the melt (at 20 °C/min) are provided, confirm the samples' double and triple crystalline nature.
As shown in Figure 5, all blocks are able to crystallize, as demonstrated by the pr ence of their characteristic scattering peaks at certain q values, pointed out with the col we are employing throughout the whole work.
The PE 7.1 homopolymer crystallization starts at 118 °C (Figure 5a), as its characteri scattering peak at 15.4 nm −1 (violet arrow) corresponding to the (110) crystallograp plane appears at this temperature. Cooling down the sample, at 16.9 nm −1 , the other sc tering peak of the (200) plane confirms PE crystallization. In addition, the normaliz WAXS intensity calculation as a function of temperature for the PE110 (15.4 nm −1 ) reflect in Figure 6a confirms the crystallization of the PE block by the sharp increase of the int sity. The crystallization of PE 9.5 homopolymer ( Figure 4A(a)) occurs in a single and sharp transition. For the PE 52 9.5 -b-PEO 48 8.8 diblock copolymer ( Figure 4A(b)), the crystallization of the PE block (violet) occurs at high temperatures, followed by the crystallization of the PEO block (red) at lower temperatures. Note that the PE block crystallizes in a unique crystallization step in this diblock copolymer, not in a fractionated way as in the previous diblock copolymer discussed before ( Figure 3A(b)). The difference remains in the phase behavior in the melt, on the one hand, since this diblock copolymer shows weak phase segregation (as evidenced by SAXS experiments shown in Figure 2b), and the fact of being segregated in the melt enhances the crystallization ability of the PE block. In addition, the PE block content is higher in this copolymer (52 wt %) with a higher molecular weight (9500 vs. 7100 g/mol). So, higher PE content and cooling from a segregated melt, do not largely hinder its crystallization, showing a crystallization enthalpy of 81 J/g (Table S2).
The crystallization sequence in the PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2) triblock terpolymer is the same as the one explained in the previous triblock terpolymer (T1) (Figure 3A(c)): first the PE block (violet), and then the PCL (blue) and PEO (red) blocks. Although, also in this case, there is an overlap of the crystallization peaks of the PCL and PEO blocks, WAXS measurements show () that the PEO block crystallizes a few degrees lower than the PCL block; and estimations of the enthalpies are provided in Table S2.
The subsequent heating scans are shown in Figure 4B. The homopolymer PE 9.5 in Figure 4B(a) shows a clear melting transition and a crystallinity value of 55% (Table S4). In the case of the PE 52 9.5 -b-PEO 48 8.8 diblock copolymer ( Figure 4B(b)), the melting starts with the PEO block (red) and ends with the PE block (violet). As previously mentioned, segregation in the melt and higher PE content enhance its crystallization, and thus, a clear and sharp melting transition with a crystallinity value of 27% is obtained (Table S4). Finally, the PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2) triblock terpolymer follows the same trend as in the triblock terpolymer T1 (Figure 3B(c)): melting of the PEO (red) and PCL (blue) blocks occur with a difference of some degrees, although not enough to distinguish between both DSC melting transitions (demonstrated by WAXS experiments in Figures S3(c)-S4(c)); and melting of the PE block showing a higher crystallinity degree (44%) ( Table S3).

In Situ Wide Angle X-ray Scattering (WAXS) Real-Time Synchrotron Results
The crystallization of each block in the WAXS patterns is identified by analyzing the crystal planes indexing for the PE, PCL, and PEO blocks reported in Table S5 [29,32,38,60,64,66,92,105,106]. In addition, normalized intensity measurements as a function of temperature upon cooling from the melt (at 20 • C/min) are provided, confirming the samples' double and triple crystalline nature.
As shown in Figure 5, all blocks are able to crystallize, as demonstrated by the presence of their characteristic scattering peaks at certain q values, pointed out with the colors we are employing throughout the whole work.
The PE 7.1 homopolymer crystallization starts at 118 • C (Figure 5a), as its characteristic scattering peak at 15.4 nm −1 (violet arrow) corresponding to the (110) crystallographic plane appears at this temperature. Cooling down the sample, at 16.9 nm −1 , the other scattering peak of the (200) plane confirms PE crystallization. In addition, the normalized WAXS intensity calculation as a function of temperature for the PE 110 (15.4 nm −1 ) reflection in Figure 6a confirms the crystallization of the PE block by the sharp increase of the intensity.
tures, 34 °C, the PEO block (red arrows) starts to crystallize with (032)/(112)/(132)/(212) reflections at 13.8 and 16.4 nm −1 , respectively. Althoug lization of these two blocks is clear, the normalized WAXS intensities calcul 6b, show this sequential crystallization by analyzing separately the uniq peaks of the PEO120 (13.8 nm −1 ) and the PE110 (15.4 nm −1 ). At high temperatu sity starts to increase at 118 °C due to PE crystallization, and the second inc also corresponds to PE, because as reported in Figure 3Ab, PE crystallizes in Figure 5c corresponds to the PE22 7.1 -b-PEO46 15.1 -b-PCL32 10.4 (T1) triblock t this case, the crystallization sequence starts with the PE crystallization (vio evidenced by the PE110 reflection at 82 °C and the PE200 reflection at 70 °C. this crystallization temperature low for the PE block, but as discussed previo 3Ac, the PE content is low (22 wt %) and the crystallization enthalpy is 14 block that crystallizes is the PCL block (blue arrows). At 42 °C, the PCL PCL111 (15.6 nm −1 ), and PCL200 (16.7 nm −1 ) reflections prove the presence of PC tals. The last block to crystallize upon cooling from the melt is the PEO block The presence of its scattering peak at 13.8 nm −1 corresponding to the (120) cry plane at 32 °C confirms the crystallization. At lower temperatures, the other peak of PEO (16.4 nm −1 ) appears at 30 °C corresponding to the (032/112/1 (Figure 5c).
The normalized intensities are analyzed to detect the exact temperature of the blocks crystallizes (Figure 6c). The joint reflections of PE110 (15.4 nm (15.5 nm −1 ) are used to determine their crystallization temperature ranges. T change in intensity at 82 °C confirms PE crystallization (violet), barely not the low content of the PE block in the terpolymer (22 wt %). Then, the sharp °C indicates the crystallization of the PCL block (blue). The single PEO120 ( flection (along with the other PE and PCL reflections) confirms its crysta sharp increase in intensity.   Figure 5b shows that the first block to crystallize, during cooling from the melt, in the PE 32 7.1 -b-PEO 68 15.1 diblock copolymer is PE at 118 • C (violet arrows) with its scattering peaks at 15.4 and 16.9 nm −1 (reflections (110) and (200), respectively). At lower temperatures, 34 • C, the PEO block (red arrows) starts to crystallize with its (120) and (032)/(112)/(132)/(212) reflections at 13.8 and 16.4 nm −1 , respectively. Although the crystallization of these two blocks is clear, the normalized WAXS intensities calculated in Figure 6b, show this sequential crystallization by analyzing separately the unique scattering peaks of the PEO 120 (13.8 nm −1 ) and the PE 110 (15.4 nm −1 ). At high temperatures, the intensity starts to increase at 118 • C due to PE crystallization, and the second increase at 82 • C also corresponds to PE, because as reported in Figure 3A(b), PE crystallizes in two steps. Figure 5c corresponds to the PE 22 7.1 -b-PEO 46 15.1 -b-PCL 32 10.4 (T1) triblock terpolymer. In this case, the crystallization sequence starts with the PE crystallization (violet arrows), as evidenced by the PE 110 reflection at 82 • C and the PE 200 reflection at 70 • C. One may find this crystallization temperature low for the PE block, but as discussed previously in Figure 3A(c), the PE content is low (22 wt %) and the crystallization enthalpy is 14 J/g. The next block that crystallizes is the PCL block (blue arrows). At 42 • C, the PCL 110 (15.5 nm −1 ), PCL 111 (15.6 nm −1 ), and PCL 200 (16.7 nm −1 ) reflections prove the presence of PCL block crystals. The last block to crystallize upon cooling from the melt is the PEO block (red arrows). The presence of its scattering peak at 13.8 nm −1 corresponding to the (120) crystallographic plane at 32 • C confirms the crystallization. At lower temperatures, the other characteristic peak of PEO (16.4 nm −1 ) appears at 30 • C corresponding to the (032/112/132/212) plane (Figure 5c).   and PCL 110 (15.5 nm −1 )) with colored data points and lines indicating crystallization of the corresponding blocks. Empty data points correspond to the molten state.
The normalized intensities are analyzed to detect the exact temperature at which each of the blocks crystallizes (Figure 6c). The joint reflections of PE 110 (15.4 nm −1 ) and PCL 110 (15.5 nm −1 ) are used to determine their crystallization temperature ranges. The first slight change in intensity at 82 • C confirms PE crystallization (violet), barely noticeable due to the low content of the PE block in the terpolymer (22 wt %). Then, the sharp increase at 42 • C indicates the crystallization of the PCL block (blue). The single PEO 120 (13.8 nm −1 ) reflection (along with the other PE and PCL reflections) confirms its crystallization by a sharp increase in intensity.
Similarly, in Figures 7 and 8, WAXS patterns upon cooling the melt (at 20 • C/min) and the normalized intensity measurements confirm crystallization of all blocks in the other set of samples listed in Table 1: the homopolymer PE 9.5 , the diblock copolymer PE 52 9.5 -b-PEO 48 8.8 , and the triblock terpolymer PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2). triblock terpolymer discussed above (Figure 5c): the PE block first (viole °C ( (110) Figure 8c demonstrate the c the three blocks by analyzing the joint reflection that the three blocks s between 16.4 and 16.9 nm −1 . Note that as the PE content is higher in this t mer (T2) (37 wt % vs. 22 wt %), the increase in intensity is clearer than triblock terpolymer (T1), in which it was very low (Figure 6c).    and (PEO 032 (16.4 nm −1 )) with colored data points and lines indicating crystallization of the corresponding blocks. Empty data points correspond to the molten state.
In this case, Figure 7a shows that the crystallization of the homopolymer PE 9.5 starts at 112 • C (PE 110 at 15.4 nm −1 ), and the second scattering peak appears at 100 • C, PE 200 (16.9 nm −1 ) (see violet arrows). Figure 8a shows the broad temperature range at which PE crystallizes since a plateau is not reached until approximately 60 • C, determining this way that PE crystallizes in between 112 and 60 • C.
Continuing with Figure 7b, the first reflection at 103 • C ((110) reflection at 15.4 nm −1 ) corresponds to the PE block, along with the (200) reflection (16.9 nm −1 ) at 100 • C (see violet arrows). The second block to crystallize in this diblock copolymer at 39 • C is the PEO block (red arrows), identified due to the presence of the (120) reflection at 13.8 nm −1 and ((032)/(112)/(132)/(212) reflections at 16.4 nm −1 . Once again, normalized intensities in Figure 8b confirm the temperature ranges at which both the PE and the PEO blocks start to crystallize due to the sharp increase in the intensity of the corresponding peaks.
To conclude, Figure 7c shows the WAXS patterns for the PE 37  . In addition, the normalized intensities shown in Figure 8c demonstrate the crystallization of the three blocks by analyzing the joint reflection that the three blocks show at q values between 16.4 and 16.9 nm −1 . Note that as the PE content is higher in this triblock terpolymer (T2) (37 wt% vs. 22 wt%), the increase in intensity is clearer than in the previous triblock terpolymer (T1), in which it was very low (Figure 6c). In addition, to confirm the crystallization of every single block in the cooling scans, results for the subsequent heating scans are shown in the Supporting Information. Figures S1-S4 report WAXS diffraction patterns and normalized intensity measurements of both triblock terpolymers here analyzed (T1 and T2).

Polarized Light Optical Microscopy (PLOM) Observations
PLOM was employed to follow crystallization of the blocks and to give evidence of the final morphology. Micrographs taken at room temperature (after cooling the samples at 20 • C/min) are shown in Figures 9-12. ers 2021, 13, x FOR PEER REVIEW (PEO032 (16.4 nm −1 )) with colored data points and lines indicating crystalliza ing blocks. Empty data points correspond to the molten state.
In addition, to confirm the crystallization of every single block results for the subsequent heating scans are shown in the Supporting S1-S4 report WAXS diffraction patterns and normalized intensity m triblock terpolymers here analyzed (T1 and T2).

Polarized Light Optical Microscopy (PLOM) Observations
PLOM was employed to follow crystallization of the blocks and the final morphology. Micrographs taken at room temperature (after at 20 °C/min) are shown in Figures 9-12.   Figure 10, in which the whole cooling process a lowed. Figure 10a indicates that the sample at 120 °C is in the molten °C (Figure 10b), the first block to start to crystallize is the PE block and barely observable microspherulites. Due to this difficulty, ligh ments as a function of temperature were measured since slight chan crographs can be better detected. Figure 11 shows all intensity changes that occur during the cool ple. Curve a of Figure 11 shows the increase in intensity related to the PE block, which crystallizes until saturation at 80 °C. Going ba second block to crystallize is the PCL block at 40 °C. A slight change micrograph, but the difference in intensity in curve b of Figure 11 con crystallization. Finally, Figure 10d,e shows the crystallization of th PLOM was employed to follow crystallization of the blocks and to give evid the final morphology. Micrographs taken at room temperature (after cooling the s at 20 °C/min) are shown in Figures 9-12. Figure 9a corresponds to the homopolymer PE 7.1 , showing very small spherul Figure 9b, the PE32 7.1 -b-PEO68 15.1 diblock copolymer shows large spherulites charac of PEO. According to the evidence gathered in the previous sections, the PE block c lizes first, probably forming microspherulites that are later engulfed by the much PEO block spherulites.  Figure 10, in which the whole cooling process at 20 °C/min w lowed. Figure 10a indicates that the sample at 120 °C is in the molten state. Coolin °C (Figure 10b), the first block to start to crystallize is the PE block, forming very and barely observable microspherulites. Due to this difficulty, light intensity me ments as a function of temperature were measured since slight changes in the PLO crographs can be better detected. Figure 11 shows all intensity changes that occur during the cooling scan of th ple. Curve a of Figure 11 shows the increase in intensity related to the crystalliza the PE block, which crystallizes until saturation at 80 °C. Going back to Figure 1 second block to crystallize is the PCL block at 40 °C. A slight change is appreciable micrograph, but the difference in intensity in curve b of Figure 11 confirms the PCL crystallization. Finally, Figure 10d,e shows the crystallization of the PEO block, corresponds to the sharp increase in intensity in curve c of Figure 11. Due to the cr zation of the three blocks, a triple crystalline block copolymer is obtained.  The micrographs taken during the subsequent heating of this PE22 7.1 -b-PEO46 15.1 -b PCL32 10.4 (T1) triblock terpolymer are provided in Figure S5 in the SI, along with the normalized intensity calculations as a function of temperature also in the SI ( Figure S6). These graphs show the melting of all blocks, demonstrating the triple crystalline behavior of the sample. In addition, all PLOM observations match very well with DSC ( Figure 3) and WAXS (Figures 5 and 6) results previously discussed.
Regarding the second system listed in Table 1, the same PLOM observations were performed in order to compare the crystalline behavior of both series of samples. Figure  12 shows the PLOM micrographs at 25 °C of the precursors of the PE37 9.5 -b-PEO34 8.8 -b-PCL29 7.6 (T2) triblock terpolymer after cooling the samples at a constant rate of 20 °C/min.     Figure 13 shows the cooling process employing as cooling ra triblock terpolymer PE37 9.5 -b-PEO34 8.8 -b-PCL29 7.6 (T2). As indicated in F the sample is melted. Decreasing temperature to 110 °C (Figure 13b the micrograph indicates that the crystallization of the PE block oc Figure 13c shows that all PE has crystallized until saturation at 50 challenging to notice meaningful changes in the micrographs, so the calculations as a function of temperature are provided in Figure 14. intensity shows the crystallization of the PE block (curve a of Figur slight increase in intensity corresponds to the crystallization of the P Figure 14), also shown in Figure 13d Figure 9b, the PE 32 7.1 -b-PEO 68 15.1 diblock copolymer shows large spherulites characteristic of PEO. According to the evidence gathered in the previous sections, the PE block crystallizes first, probably forming microspherulites that are later engulfed by the much larger PEO block spherulites.
The triple crystalline morphology of the PE 22 7.1 -b-PEO 46 15.1 -b-PCL 32 10.4 (T1) triblock terpolymer is shown in Figure 10, in which the whole cooling process at 20 • C/min was followed. Figure 10a indicates that the sample at 120 • C is in the molten state. Cooling to 80 • C (Figure 10b), the first block to start to crystallize is the PE block, forming very small and barely observable microspherulites. Due to this difficulty, light intensity measurements as a function of temperature were measured since slight changes in the PLOM micrographs can be better detected. Figure 11 shows all intensity changes that occur during the cooling scan of this sample. Curve a of Figure 11 shows the increase in intensity related to the crystallization of the PE block, which crystallizes until saturation at 80 • C. Going back to Figure 10c, the second block to crystallize is the PCL block at 40 • C. A slight change is appreciable in this micrograph, but the difference in intensity in curve b of Figure 11 confirms the PCL block crystallization. Finally, Figure 10d,e shows the crystallization of the PEO block, which corresponds to the sharp increase in intensity in curve c of Figure 11. Due to the crystallization of the three blocks, a triple crystalline block copolymer is obtained.
The micrographs taken during the subsequent heating of this PE 22 7.1 -b-PEO 46 15.1 -b PCL 32 10.4 (T1) triblock terpolymer are provided in Figure S5 in the SI, along with the normalized intensity calculations as a function of temperature also in the SI ( Figure S6). These graphs show the melting of all blocks, demonstrating the triple crystalline behavior of the sample. In addition, all PLOM observations match very well with DSC ( Figure 3) and WAXS (Figures 5 and 6) results previously discussed.
Regarding the second system listed in Table 1, the same PLOM observations were performed in order to compare the crystalline behavior of both series of samples. Figure 12 shows the PLOM micrographs at 25 • C of the precursors of the PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2) triblock terpolymer after cooling the samples at a constant rate of 20 • C/min. Figure 12a corresponds to the PE 9.5 homopolymer, in which very small PE spherulites can be observed. The micrograph in Figure 12b, on the contrary, refers to the PE 52 9.5b-PEO 48 8.8 diblock copolymer. Although there are no clear PEO spherulites, it shows a double crystalline morphology at room temperature. Figure 13 shows the cooling process employing as cooling rate 20 • C/min for the triblock terpolymer PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2). As indicated in Figure 13a, at 118 • C, the sample is melted. Decreasing temperature to 110 • C (Figure 13b), a slight change in the micrograph indicates that the crystallization of the PE block occurred. In addition, Figure 13c shows that all PE has crystallized until saturation at 50 • C. Once again, it is challenging to notice meaningful changes in the micrographs, so the normalized intensity calculations as a function of temperature are provided in Figure 14. The first increase in intensity shows the crystallization of the PE block (curve a of Figure 14). The following slight increase in intensity corresponds to the crystallization of the PCL block (curve b of Figure 14), also shown in Figure 13d at 40 • C. Cooling down the sample, the last block to crystallize is the PEO block (Figure 13e,f), and its crystallization continues until saturation is obtained at approximately 0 • C (Figure 13g). Curve c in Figure 14 indicates that the crystallization of the PEO block starts at around 28 • C and continues with further decreases in temperature.
the sample is melted. Decreasing temperature to 110 °C (Figure 13b), a slight change in the micrograph indicates that the crystallization of the PE block occurred. In addition, Figure 13c shows that all PE has crystallized until saturation at 50 °C. Once again, it is challenging to notice meaningful changes in the micrographs, so the normalized intensity calculations as a function of temperature are provided in Figure 14. The first increase in intensity shows the crystallization of the PE block (curve a of Figure 14). The following slight increase in intensity corresponds to the crystallization of the PCL block (curve b of Figure 14), also shown in Figure 13d at 40 °C. Cooling down the sample, the last block to crystallize is the PEO block (Figure 13e,f), and its crystallization continues until saturation is obtained at approximately 0 °C (Figure 13g). Curve c in Figure 14 indicates that the crystallization of the PEO block starts at around 28 °C and continues with further decreases in temperature.  Figures S7 and S8 in the SI provide the subsequent heating scan and the normalized intensity measurements of the PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2) triblock terpolymer, respectively. The discussed results agree well with DSC ( Figure 4) and WAXS (Figures 7 and 8) according to the evidences discussed above. ers 2021, 13, x FOR PEER REVIEW Figure 14. PLOM intensity measurement from data in Figure 13 as a func cooling from the melt (20 °C/min), indicating crystallization of the follo the PCL block, and (c) the PEO block for the triblock terpolymer PE37 9 Colored data points and lines (violet for PE, blue for PCL, and red for PE the crystallization. Empty data points correspond to the molten state of t Figures S7 and S8 in the SI provide the subsequent heating s intensity measurements of the PE37 9.5 -b-PEO34 8.8 -b-PCL29 7.6 (T2) trib tively. The discussed results agree well with DSC ( Figure 4) and according to the evidences discussed above.

Conclusions
The main objective of this study is the analysis of the morpho of triblock terpolymers with three potentially crystallizable block polar PEO (biocompatible), and PCL (biodegradable) blocks, as w ing precursors. Although adding a third block to diblock copol more challenging, it was possible to ascertain the crystallization blocks following the crystallization process by three compleme WAXS, and PLOM.
The aim of comparing two triblock terpolymers, PE22 7.1 -b-PEO PE37 9.5 -b-PEO34 8.8 -b-PCL29 7.6 (T2), was to determine the effect of com weight on the properties. Regarding melt miscibility, both triblo T2) show weak phase segregation, and the microstructure present when crystallization of the first block starts (PE crystallization). F Figure 14. PLOM intensity measurement from data in Figure 13 as a function of temperature during cooling from the melt (20 • C/min), indicating crystallization of the following: (a) the PE block, (b) the PCL block, and (c) the PEO block for the triblock terpolymer PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2). Colored data points and lines (violet for PE, blue for PCL, and red for PEO) are employed to follow the crystallization. Empty data points correspond to the molten state of the sample.

Conclusions
The main objective of this study is the analysis of the morphology and crystallization of triblock terpolymers with three potentially crystallizable blocks: the apolar PE and the polar PEO (biocompatible), and PCL (biodegradable) blocks, as well as their corresponding precursors. Although adding a third block to diblock copolymers makes the study more challenging, it was possible to ascertain the crystallization sequence of each of the blocks following the crystallization process by three complementary techniques: DSC, WAXS, and PLOM.
The aim of comparing two triblock terpolymers, PE 22 7.1 -b-PEO 46 15.1 -b-PCL 32 10.4 (T1) and PE 37 9.5 -b-PEO 34 8.8 -b-PCL 29 7.6 (T2), was to determine the effect of composition and molecular weight on the properties. Regarding melt miscibility, both triblock terpolymers (T1 and T2) show weak phase segregation, and the microstructure present in the melt is destroyed when crystallization of the first block starts (PE crystallization). Furthermore, the crystallization of the three blocks upon cooling from the melt employing 20 • C/min as cooling rate in both triblock terpolymers is identified. The crystallization sequence resulted as follows: the PE block crystallized first, followed by the PCL block and finally by the PEO block, as evidenced by DSC, in situ WAXS experiments, and PLOM observations with light intensity calculations.
The crystalline behavior of both triblock terpolymers (T1 and T2) is very similar regardless of the molecular weight and composition. However, for their corresponding diblock copolymer precursors, the effect of the PE block content and the molecular weight is significant. The PE 32 7.1 -b-PEO 68 15.1 diblock copolymer is melt miscible, and the PE block crystallization is hindered due to its low content (32 wt%). Nevertheless, in the PE 52 9.5b-PEO 48 8.8 diblock copolymer, the PE block crystallization is enhanced due to its higher content (52 wt%) and phase segregated nature in the melt.