Preferential Formation of β-Form Crystals and Temperature

Oct 26, 2017 - At Tc = 120 °C, the 2L-4.4k-U SM-PLLA has a larger LP of 12.6 nm, compared to the value of 10.3 nm of 2L-4.4k precursor. ..... Synthes...
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Article Cite This: Macromolecules XXXX, XXX, XXX-XXX pubs.acs.org/Macromolecules

Preferential Formation of β‑Form Crystals and TemperatureDependent Polymorphic Structure in Supramolecular Poly(L‑lactic acid) Bonded by Multiple Hydrogen Bonds Jianna Bao, Xiaohua Chang, Qing Xie, Chengtao Yu, Guorong Shan, Yongzhong Bao, and Pengju Pan* State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Supramolecular polymers (SMPs) have quite different crystallization behavior from the conventional polymers, because of the confinement effects of supramolecular units. Crystallization of SMPs undergoes in a “confined” and “dynamic” manner. Herein, we selected the 2ureido-4[1H]-pyrimidione (UPy)-bonded poly(L-lactic acid) (PLLA) as a model SMP and investigated the crystallization kinetics, polymorphic crystalline structure and phase transition of supramolecular PLLAs (SM-PLLAs). SM-PLLAs were synthesized by the end functionalization of hydroxylterminated 2- and 3-arm PLLA precursors with different molecular weights. Crystallization rate and crystallinity of SM-PLLAs are strongly depressed in both nonisothermal and isothermal crystallizations, as compared to the nonfunctionalized PLLA precursors. Crystalline structure of SM-PLLAs is sensitive to the crystallization temperature (Tc). A low Tc (75−95 °C) facilitates the formation of metastable β crystals of PLLA in SM-PLLAs; while a high Tc (100−130 °C) favors the generation of α (or α′) crystals. The β crystals formed in SM-PLLAs transform into the more stable α crystals in the following heating process. We propose that the preferential formation of β crystals is ascribed to the “strong” confinements of UPy motifs and UPy−UPy interactions, which may exert an orientation and stretching effects to the linked PLLA chains during crystallization. This study has shed light on the unique “confined” and “dynamic” crystallization behavior of SMPs and also paved a way to obtain the PLLA β-form from SM-PLLA under the crystallization conditions free of pressure, stretching, and shearing.

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INTRODUCTION Supramolecular polymers (SMPs), in which the monomeric units or polymer segments are linked by reversible noncovalent interactions, have received growing interests in the scientific community.1,2 Incorporation of supramolecular groups brings useful and potentially tunable properties and functionalities to conventional polymers and is also a practical route to prepare the high-molecular-weight polymers3 and (pseudo)block copolymers.4−6 Multiple H-bonds (MHBs) have been extensively used as the building blocks to create SMPs, due to its strong association strengths and high sensitivity to external stimuli.7−9 2-Ureido-4[1H]-pyrimidinone (UPy) is a widely used MHB unit, and it can form the self-complementary quadruple H-bonds by dimerization.10 UPy has a high dimerization constant11 and is easy to synthesize and functionalize. Thus, it has been extensively used as a building block to prepare the SMPs with excellent properties.3,12−17 When the crystallizable polymer precursors (or oligomers) were functionalized by UPy to generate SMPs, the oligomer chains linked to UPy may be still crystallizable. Although the syntheses, physical properties and functions of SMPs have been extensively studied in the recent years, their crystallization kinetics and polymorphic crystalline structure are far from © XXXX American Chemical Society

being well understood. The crystallizable SMPs are expected to display a quite different crystallization behavior from the conventional polymers; they crystallize in a “confined” and “dynamic” manner. First, it has been well demonstrated that the UPy dimers18−20 and other associated MHB motifs21−24 can stack and self-aggregate into microdomains (or UPy crystals) via the lateral H-bonding interactions in SMPs; these nanometer-scaled microdomains or stacks may restrict the diffusion and mobility of linked polymer chains, due to the steric effects.23,24 This is similar to the confined crystallization of microphase-separated polymer systems such as block copolymers and miscible polymer blends.25 Besides, the strong UPy−UPy interactions also constraint the diffusion and movement of linked polymer segments in crystallization. Therefore, UPy stacking and interactions would have significant confinement effects on the crystallization of linked polymer chains, thus restricting the formation of crystalline lamellae and depressing the crystallization kinetics.23,26 Such physical confinement can also influence the crystalline structure of Received: August 7, 2017 Revised: October 16, 2017

A

DOI: 10.1021/acs.macromol.7b01705 Macromolecules XXXX, XXX, XXX−XXX

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behavior of SM-PLLAs under different confinement degrees such as the “strong” confinement at a low Tc and “weak” confinement at a high Tc. In this study, SM-PLLAs were prepared by the UPy end functionalization of 2 and 3-arm oligomeric PLLA precursors. Crystallization kinetics and polymorphic crystalline structure of SM-PLLAs with different UPy contents, PLLA oligomer lengths and architectures at different Tcs were systematically investigated. It is found that the SM-PLLAs with high UPy contents prefer to crystallizing in the β-form at a low Tc (i.e., “strong” confinement). This represents a novel method to directly obtain PLLA β-form from the cold or melt crystallization without using the external forces (e.g., stretching and shearing forces). The heating-included βto-α phase transition of SM-PLLAs was further investigated by the synchrotron-radiation wide-angle X-ray diffraction (WAXD) and time-resolved FTIR spectroscopy. Mechanism for the preferential formation of β-form in SM-PLLAs was also proposed.

polymorphic polymers by restricting the crystal growth or controlling the orientation of crystalline domains.27−29 Because the noncovalent bonds of SMPs are dynamic, reversible, and stimuli-responsive, a unique feature of SMPs is the equilibrium between associated and dissociated states, which is determined by the environmental conditions (e.g., temperature).3,21,30,31 For example, the UPy bonding strength decreases and the UPy dimers can dissociate with heating to the dissociation temperature (Td,UPy ∼ 80 °C).30,31 Besides, UPy stacks will melt when heating to the melting temperature of UPy crystals (Tm,UPy, typically 80−100 °C).18,19 As a result, the confinement degree of UPy motifs to crystallization will decrease as the crystallization temperature (Tc) increases, causing a unique temperature-sensitive (“dynamic”) crystallization behavior of SMPs. We arbitrarily call the confinement effects at high and low Tc as the “weak” and “strong” confinements, respectively. Because of the temperature-dependent changes of confinement degree and supramolecular interactions, crystalline structure of UPy-based SMPs would depend on the crystallization conditions (e.g., Tc). Accordingly, it is possible to control the crystalline structure of UPy-based SMPs by varying Tc; this may provide a novel approach to tune the physical properties and functions of SMPs in processing. In order to elucidate the unique “confined” and “dynamic” crystallization behavior of SMPs, we select the UPy-bonded supramolecular poly(L-lactic acid) (SM-PLLA) as a model SMP and compare the crystallization kinetics and polymorphic crystalline structure of SM-PLLAs with the conventional covalent PLLAs in this study. PLLA is a representative biobased and biodegradable polymorphic polymer. It can crystallize in different crystalline forms including α,32 α′,33−38 β,39−48 and γ49,50 forms, depending on the crystallization conditions. PLLA α-form is thermally stable and generally formed from the solution crystallization and melt, cold crystallizations at a high Tc (Tc > 120 °C). PLLA α′-form, also called as δ-form,38 is a disordered polymorph and generated in the melt and cold crystallizations at a low Tc (2.0 g/L. This stems from the strong self-complementary H-bonding interactions between UPy units and the dimerization of UPy terminals in solution above a critical concentration.10,54 Therefore, it is considered that the as-prepared UPy-terminated 2- and 3-arm PLLAs would form the linear and branched SM-PLLAs, respectively, as illustrated in Scheme 1. Crystallization Kinetics of Supramolecular PLLAs. Thermogravimetric analysis (TGA) and 1H NMR measurements were conducted to exclude the possibility of thermal decomposition of PLLA segments and terminal UPy units in the thermal treatment and crystallization process. As shown in Figures S9 and S10, the onset decomposition temperature of both SM-PLLAs and PLLA precursors are higher than 220 °C; 1 H NMR spectra of SM-PLLA do not change after thermal treatment at 180 °C for 2 min, which is the highest temperature used in the crystallization process. Crystallization and melting behavior of SM-PLLAs were investigated and compared with those of PLLA precursors via DSC and POM. Figure 2 shows the DSC thermograms

Table 2. Thermal Properties of SM-PLLAs and PLLA Precursors Obtained in Nonisothermal Cold Crystallization of Melt-Quenched Samples sample

Tg (°C)

Tcc (°C)

ΔHm,cc (J/g)

Tm (°C)

ΔHm (J/g)

2L-4.4k-U 2L-9.2k-U 2L-26.8k-U 3L-8.5k-U 2L-4.4k 2L-9.2k 2L-26.8k 3L-8.5k

58.7 61.3 63.7 63.5 49.1 51.9 58.8 56.1

− 124.0 117.6 − 92.8 90.9 101.8 116.8

0 41.8 47.4 0 49.3 51.2 50.0 45.6

− 152.4 170.1 − 138.0 154.5 171.7 147.7

0 45.8 49.3 0 53.6 72.1 61.6 50.5

the UPy end functionalization (Table 2). PLLA precursors with various MWs exhibit the crystallization exotherms at 90−117 °C and melting endotherms at 130−180 °C in the heating process (Figure 2B). Nevertheless, no obvious crystallization or melting peak is observed for 2L-4.4-U and 3L-8.5k-U SMPLLAs during heating (Figure 2A). Even though the SMPLLAs with longer PLLA precursors (i.e., 2L-U-9.2k, 2L-U26.8k) show the crystallization and melting peaks during heating; their Tcc’s are higher and ΔHcc’s, ΔHm’s are smaller than the corresponding values of PLLA precursors (Table 2). Tm’s of both linear SM-PLLA and PLLA precursor increase as the length of PLLA segment or precursor increases, similar to that observed in PLLA.34 Notably, we did not observe the melting peak of stacked UPy crystals in the DSC heating curves; this might due to the relatively low UPy content in our SM-PLLAs. Parts A and B of Figure 3 show the DSC thermograms of SM-PLLAs and PLLA precursors collected in isothermal cold crystallization at different Tc’s. Crystallization half-times (t1/2’s) were calculated from isothermal DSC thermograms, as elaborated in the Supporting Information and Figure S1.55 t1/2’s of SM-PLLAs and PLLA precursors are plotted as a

Figure 2. DSC thermograms recorded in the heating process of meltquenched SM-PLLAs and PLLA precursors: (A) SM-PLLAs; (B) PLLA precursors.

recorded in the heating process of melt-quenched SM-PLLAs and PLLA precursors. Thermal parameters including glass transition temperature (Tg), crystallization temperature (Tcc), and enthalpy (ΔHcc), melting temperature (Tm) and enthalpy (ΔHm) were calculated from these DSC curves, as listed in Table 2. An apparent increase of Tg is observed for PLLAs after

Figure 3. DSC results of SM-PLLAs and PLLA precursors recorded in isothermal cold crystallization at various Tc’s: (A) DSC curves of SMPLLAs; (B) DSC curves of PLLA precursors; (C) t1/2 ∼ Tc plot. D

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Melting Behavior of Supramolecular PLLAs. Melting behavior of SM-PLLAs and their PLLA precursors after isothermal crystallization were investigated by DSC, as shown in Figure 5. To quantitatively analyze the effects of UPy motifs

function of Tc in Figure 3C. As shown in Figure 3, the crystallization peaks of PLLAs become broad after UPy end functionalization and the introduction of UPy motifs causes a remarkable increase of t1/2, especially for the SM-PLLAs made from the PLLA precursors with lower MWs. t1/2’s of PLLA precursors range in 0.55−14.8 min at Tc = 80−120 °C; they increase to 4.9−181 min after UPy end functionalization. For the 2L-4.4k and 3L-8.5k precursors crystallized at 90 °C, their t1/2’s are 2.1 and 5.0 min, respectively, which increase to 180 and 150 min for the 2L-4.4k-U and 3L-8.5k-U SM-PLLAs. Moreover, 2L-4.4k-U and 3L-8.5k-U SM-PLLAs cannot finish crystallization even after annealing at 80 °C for 10 h. As shown in Figure 3C, the crystallization rate of linear SM-PLLA enhances as the length of PLLA segment increases, due to the decreased confinement effects of UPy groups. As shown in Figure 4, SM-PLLAs and PLLA precursors exhibit similar Maltese cross spherulites; yet the SM-PLLA

Figure 5. DSC heating curves of SM-PLLAs and PLLA precursors after isothermal cold crystallizations at different Tc’s: (A) 2L-4.4k-U; (B) 2L-4.4k; (C) 3L-8.5k-U; (D) 3L-8.5k. Figure 4. POM results of SM-PLLAs and PLLA precursors during melt crystallization at 120 °C. (A) Representative POM micrographs. (B) Plots of spherulitic radius against crystal growth time.

on thermal properties, Tm’s and degrees of crystallinity (Xc’s) of SM-PLLAs and PLLA precursors were estimated and plotted as a function of Tc in Figure 6. Tm was taken from the melting

spherulites grow much slower than its precursor during melt crystallization. 2L-4.4k and 3L-8.5k precursors show the large G values (12 and 8.5 μm/min) at Tc = 120 °C; they decrease by more than 1 order of magnitude after the UPy end functionalization and SMP formation (Figure 4B). All these DSC and POM results demonstrate that the UPy end functionalization and SMP formation drastically depress the crystallization rate and crystallizability of PLLAs. Similar phenomenon has been observed for the poly(ε-caprolactone) (PCL) end-functionalized by UPy and other MHB groups.19,23 The depressed crystallization of SM-PLLAs is caused by the (i) retardant effects of UPy−UPy interactions and (ii) confined effects of segregated UPy domains on the crystallization of PLLA segments. First, UPy motifs have much larger volume and strong H-bonding interactions; they can decrease the diffusion ability of linked PLLA segments in crystallization. On the other hand, the segregated and phase-separated UPy domains could have physical confinement effects and thus prohibit the regular crystallization of linked PLLA segments. As expected, the retardant effect of UPy to PLLA crystallization becomes less pronounced for the SM-PLLAs with longer PLLA segments (or lower UPy content). For SM-PLLAs made from the 2-arm PLLA precursors, their t1/2’s decrease and ΔHcc, ΔHm increase as the MW of PLLA precursor increases. t1/2 of 2L-9.2k-U SM-PLLA is relatively closed to that of 2L-9.2k precursor. Moreover, the 2L-9.2k-U and 3L-8.2k-U SM-PLLAs having the similar MWs of PLLA precursors show much different crystallization kinetics and crystallizabilities, proving that the retardant and confined effects of UPy on PLLA segments in the SM-PLLAs increase when the cross-linked structure is formed for the UPy-functionalized 3-arm PLLAs.

Figure 6. DSC results of SM-PLLAs and PLLA precursors derived from Figure 5: (A) Tm ∼ Tc plot; (B) Xc ∼ Tc plot.

peak located at lower temperature. Xc was calculated by comparing the measured ΔHm with the ΔHm of crystals with infinite large lamellae thickness (ΔHm0 = 93 J/g56), i.e., Xc = ΔHm/ΔHm0. Generally, Tm and Xc of all samples increase continuously with Tc, due to the formation of more perfect crystals at a higher Tc. The presence of UPy motifs drastically decreases the Tm and Xc of SM-PLLAs (Figure 6), because the confinement effects of supramolecular units prohibit the formation of ordered and prefect crystals. Similar results have been observed for the heteronucleobase-functionalized PCL.23 Xc’s of 2L-4.4k and 3L8.5k precursors range 52−57% and 49−52%, respectively; they decrease to 37−48% and 29−38% after the UPy end functionalization. In addition, Tm and Xc of SM-PLLAs exhibit more remarkable increases with Tc than those of PLLA E

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Macromolecules precursors; this is due to the decrease in UPy−UPy interactions with increasing Tc, as verified by the in situ FTIR results shown in Figure 1. UPy−UPy interactions are weaker at a higher Tc, leading to the smaller confinement effects on the crystallization of PLLA segments. This results in the more remarkable increases of Tm and Xc with Tc. Melting behavior of PLLAs strongly depends on the UPy end functionalization and Tc (Figure 5). The 2L-4.4k and 3L-8.5k precursors show dual melting endotherms after crystallization at 80−120 °C. Multiple melting behavior of LMW PLLAs has been clarified via FTIR spectroscopy.34 Because α crystals are generated in PLLA precursors at Tc ≥ 110 °C (as indicated by the following WAXD results), the dual melting behavior is attributed to the melt-crystallization mechanism. Because the disordered α′ crystals are formed in PLLA precursors crystallized at low Tc’s (80−100 °C), dual melting behavior also includes the contribution of α′-to-α phase transition. It has been demonstrated that the α′-to-α phase transition and direct melt of α′ crystals occur simultaneously in the heating process of LMW PLLAs.34 Therefore, no exotherms is observed in the heating process of 2L-4.4k and 3L-8.5k precursors crystallized at low Tc’s (80−100 °C), as shown in Figure 5B. In contrast, 2L-4.4k-U and 3L-8.5k-U SM-PLLAs exhibit more complicated melting behavior after crystallization under the same conditions; this is more obvious for the samples crystallized at 80−100 °C. For the 2L-4.4k-U and 3L-8.5k-U SM-PLLAs crystallized at 80−100 °C, an exothermic peak is observed during heating before the final melting, as indicated by Pexo in Figure 5A,C. As demonstrated by the following WAXD and FTIR results, SM-PLLAs crystallize in α′ and β crystals at 80−90 °C. Therefore, the exothermic peak Pexo stems from the phase transition (or melt recrystallization) from α′ or β crystals to α crystals occurred upon heating.34,36 The dominant melting peak located at ∼140 °C is assigned to the melts of α crystals formed in the isothermal crystallization or heating-induced phase transition. On the other hand, SMPLLAs form less prefect crystals than PLLA precursors because of the confinement effects of UPy motifs; this could lead to the more intensified melt recrystallization during heating. The melt-recrystallization of preexisted crystals can also cause the presence of exothermic peak (i.e., Pexo) before the final melting. Formation of β-Form and Temperature-Sensitive Crystalline Structure of Supramolecular PLLAs. In order to elucidate the effects of SMP formation on the polymorphic crystalline structure of PLLA, we investigated the crystalline structure of SM-PLLAs crystallized at different Tc’s (70−130 °C) by WAXD and FTIR spectroscopy and also compared the results with those of PLLA precursors (Figures 7 and 8). Table 3 lists the characteristic diffraction angle, corresponding lattice spacing (d), and assignment of diffraction index for the 2L-4.4kU SM-PLLA and 2L-4.4k precursor crystallized at low (90 °C) and high Tc’s (130 °C). As shown in Figure 7A, the 2L-4.4k precursor forms α′ and α crystals after crystallization at Tc ≤ 100 °C and Tc ≥ 110 °C, respectively, in good agreement with the reported results of PLLA homopolymers.33−36 The 2L-4.4k precursor crystallized at 75−130 °C exhibits a conformationalsensitive band at 921 cm−1 in the FTIR spectra (Figure 8A), corresponding to the combination of γ(CH3) and ν(C−COO) modes for the 103 helical chains in PLLA α′ and α crystals.33,51 Similar Tc-dependent WAXD and FTIR results are also observed for the 3L-8.5k and 2L-9.2k precursors. Interestingly, SM-PLLAs display the different Tc-dependent WAXD patterns and FTIR spectra from PLLA precursors. We

Figure 7. WAXD patterns of SM-PLLAs and PLLA precursors after isothermal cold crystallizations at different Tc’s: (A) 2L-4.4k; (B) 2L4.4k-U; (C) 3L-8.5k-U; (D) 2L-9.2k-U.

Figure 8. FTIR spectra in the 965−890 cm−1 wavenumber region for SM-PLLAs and PLLA precursors after isothermal cold crystallizations at different Tc’s: (A) 2L-4.4k; (B) 2L-4.4k-U; (C) 3L-8.5k-U; (D) 2L9.2k-U. The bands at 921 and 912 cm−1 are characteristic of α and β crystals, respectively.

first discuss the crystalline structure of SM-PLLAs generated at a low Tc (75−95 °C); the UPy motifs have “strong” confinement effects on the crystallization of linked PLLA segments under these conditions. As shown in Figure 7B,C,D, SM-PLLAs cannot crystallize completely after annealing at 70 °C for 10 h, because of the extremely slow crystallization under this condition. Two new diffraction peaks, located at around 2θ = 13.7° (d = 5.19 Å) and 15.7° (d = 4.53 Å) are observed in the WAXD patterns of SM-PLLAs (2L-4.4k-U, 3L-8.5k-U, and 2L9.2k-U) crystallized at a low Tc (75−95 °C). Also, for the SMF

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Table 3. Characteristic Diffraction Angle (2θ), Corresponding Lattice Spacing (d), and Assignment of Diffraction Index for 2L4.4k-U SM-PLLA and 2L-4.4k Precursor Cold-Crystallized at 90 and 130 °Ca 2L-4.4k-U SM-PLLA sample Tc = 90 °C

Tc = 130 °C a

2L-4.4k precursor

2θ (deg)

d (Å)

diffraction index

2θ (deg)

d (Å)

diffraction index

13.2 15.1 13.7 15.7 17.7 20.9 22.4 23.8 25.1 13.4 15.3

5.39 4.72 5.19 4.53 4.02 3.42 3.19 3.01 2.85 5.31 4.66

α′110/200 α′203 β200 β131 β221 β132 β222 β003 β023 α110/200 α203

13.2 (vs) 15.1 (s)

5.39 4.72

α′110/200 α′203

13.4 (vs) 15.3 (s)

5.31 4.66

α110/200 α203

(s) (m) (vs)b (s) (s) (m) (s) (m) (s) (vs) (s)

Wavelength of X-ray used is 0.124 nm. bKey: vs, very strong; s, strong; m, medium.

PLLAs crystallized at a low Tc (75−95 °C), the conformationsensitive band was observed at 912−919 cm−1 (Figure 8B,C,D), which was lower than that of α or α′ crystals at 921 cm−1. To clearly see the weak diffractions, the WAXD pattern of 2L-4.4k-U SM-PLLA crystallized at 90 °C was enlarged, as shown in Figure 9. Because the X-axis region in Figure 9 is

having the 31 helical chain conformation.44,45 Therefore, it is concluded that β crystals are also generated in the SM-PLLAs crystallized at a low Tc (75−95 °C), except for the α′ crystals. As shown in Figure S11, the in situ WAXD analysis indicates that the β and α′ crystals form simultaneously in SM-PLLA during the crystallization at a low Tc (75−95 °C). To further confirm the formation of β-form in SM-PLLAs, WAXD patterns of 3L-8.5K-U SM-PLLA melt-crystallized at various Tc’s were performed. Characteristic diffractions of β-form are also observed for the SM-PLLAs melt-crystallized at a low Tc of 80−90 °C (Figure S12). Therefore, it is considered that the formation of β-form is a general behavior for the SM-PLLAs crystallized at a low Tc, which is regardless of the crystallization manner (cold or melt crystallization) or the initial state of crystallization (melt or glassy state). Because the WAXD diffractions of α (or α′) and β forms overlap in 2θ = 12−15° region, we calculate the β-form fraction in the crystalline phase of SM-PLLAs (i.e., fβ) by peak splitting/ fitting, as illustrated in Figure S13. In the peak splitting, positions of two split peaks were kept at 2θ = 13.3° (α′ form, or 2θ = 13.4° of α form) and 13.7° (β form); intensities (or areas) of these split peaks reflected the relative fractions of α (or α′) and β forms. The fitted curves of all samples agree well with their original curves, having the correlation coefficient (R2) larger than 0.995 (Figure S13). Therefore, fβ can be calculated as44,47

Figure 9. Enlarged WAXD profile of 2L-4.4k-U SM-PLLA coldcrystallized at 90 °C. Lattice spacings corresponding to the characteristic diffraction planes are indicated in this figure.

much broader than that in Figure 7B, the two WAXD profiles show different baselines. Figure 9 exhibits several characteristic diffractions at 2θ = 17.7°, 20.9°, 22.4°, 23.8°, and 25.1°, besides the diffractions at 2θ = 13.2°, 13.7°, 15.1° and 15.7° (Table 3). In the case of 2L-4.4k-U and 3L-8.5k-U SM-PLLAs crystallized at 90 °C, the new diffractions at 2θ = 13.7° and 15.7° overwhelm the (110)/(200) (2θ = 13.2°) and (203) (2θ = 15.1°) diffractions of PLLA α′ crystals (Figure 7B,C) and the confirmation-sensitive band also shifts to ∼912 cm−1 (Figure 8B,C). These results imply that another polymorph having the different structure and chain conformation from the PLLA α or α′ polymorph is generated in the SM-PLLAs after crystallization at a low Tc (75−95 °C). In order to assign this new polymorph, WAXD patterns of SM-PLLAs crystallized at a low Tc (75−95 °C) are compared with all the reported polymorphs of PLLAs such as α,32 α′,33−35 α″,57 β,32,47 γ49,50 forms. The new diffractions observed for SMPLLAs crystallized at a low Tc (75−95 °C) agree well with the diffraction features of PLLA β-form, as indicated by the assignments of β-form diffraction peaks in Figures 7 and 9 and Table 3. The 912 cm−1 band is also a character of PLLA β-form

fβ =

Aβ(200) Aβ(200) + Aα

′ (110)/(200)orα(110)/(200)

(1)

where Aβ(200), Aα(110)/(200), and Aα′(110)/(200) are the peak areas of β200 (2θ = 13.7°), α110/200 (2θ = 13.4°), and α′110/200 (2θ = 13.3°) diffractions, respectively. Figure 10 shows the calculated fβ of SM-PLLAs crystallized at various Tc’s. Crystalline structure of SM-PLLAs crystallized at a low Tc (75−95 °C) is strongly influenced by the MW and architecture of PLLA precursor. As shown in Figure 10, the formation ability of β-form in SM-PLLAs is depressed significantly with increasing the MW or decreasing the UPy content and branch number of PLLA precursors. For the linear SM-PLLAs (made from 2-arm PLLA precursors) crystallized at 90 °C, the intensity of β200 diffraction decreases and that of α′110/200 diffraction increases; meanwhile the wavenumber of conformation-sensitive FTIR band shifts from 912 to ∼917 cm−1 as the MW of PLLA precursor increases from 4.4 to 9.2 kg/mol G

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Figure 10. Fractions of β crystals in the crystalline phase of SM-PLLAs crystallized at different Tc’s.

Figure 11. Lorenz-corrected SAXS profiles of 2L-4.4k-U SM-PLLA and 2L-4.4k precursor after isothermal cold crystallization at 90 and 120 °C.

(Figures 7 and 8). Accordingly, the 2L-9.2k-U SM-PLLA has much smaller fβ than 2L-4.4k-U after crystallization at low Tc’s (75−95 °C). Moreover, 2L-26.8k-U SM-PLLA shows the same Tc-dependent WAXD patterns as the nonfunctionalized PLLA precursors and does not generate β-form at all the Tc’s investigated (80−130 °C), as shown in Figure S14. Even though the 3L-8.5k-U SM-PLLA has the similar MW of PLLA precursor with 2L-9.2k-U, the former exhibits much higher fβ after crystallization at the same Tc (75−95 °C, Figure 10). This is attributable to the multiarmed architecture of PLLA precursor in 3L-8.5k-U, which leads to the formation of cross-linked SM-PLLA and thus increases the confinement effects of UPy motifs on the crystallization of linked PLLA segments. On the other hand, the SM-PLLA becomes noncrystallizable when the PLLA segments are too short. Therefore, there are the MW limits of PLLA segments that cannot crystallize or cannot form the β crystals in the SM-PLLAs. In order to attain these MW limits, we synthesized the other two SM-PLLAs, i.e., 2L-2.6k-U (Mn of each arm: 1.3 kg/mol) and 3L-28.0k-U (Mn of each arm: 9.3 kg/mol). As shown in Figure S15, the 2L-2.6kU SM-PLLA is almost not crystallized after cold crystallization at 90 °C for 10 h; yet the 2L-2.6k precursor has high crystallinity after crystallization under the same condition. As shown in Figure S16, only the α or α′ crystals are generated in the 3L-28.0k-U SM-PLLA crystallized at 80−130 °C, demonstrating that β-form cannot be formed in this SMPLLA. On the basis of these results, it is concluded that the Mn limits of each PLLA arm in SM-PLLA for forming the β crystals is 1.3−9.3 kg/mol. Since the UPy−UPy interactions are sensitive to temperature, the crystalline structure of SM-PLLAs is significantly influenced by Tc. At Tc ≥ 100 °C, all SM-PLLAs crystallize exclusively in α (or α′) form, which is the same as PLLA precursors. As shown in Figures 7 and 8 and Table 3, the SMPLLAs crystallized at Tc ≥ 100 °C show the characteristic WAXD diffractions of α (or α′) form and also the characteristic conformation-sensitive band at 921 cm−1, corresponding to the 103 chain conformation of PLLA α (or α′) form. fβ values of all SM-PLLAs decrease to zero as Tc increases to ≥100 °C (Figure 10). This indicates that the crystal modification of PLLA is not influenced by UPy−UPy interactions and SMP formation at a high Tc (100−130 °C). The lamellar structure of SM-PLLA was analyzed by SAXS. Figure 11 shows the Lorenz-corrected SAXS profiles (Iq2 ∼ q plot) of 2L-4.4k-U SM-PLLA and 2L-4.4k precursor after isothermal cold crystallization at 90 and 120 °C, in which I denotes the scattering intensity and q is the scattering vector. Long period (LP) was calculated by Bragg equation (LP = 2π/ qmax), where qmax corresponds to the peak top of Lorentzcorrected SAXS profile. At Tc = 120 °C, the 2L-4.4k-U SM-

PLLA has a larger LP of 12.6 nm, compared to the value of 10.3 nm of 2L-4.4k precursor. This suggests that the terminal UPy groups are segregated into the interlamellar region of PLLA crystals. Proposed Mechanism for Temperature-Sensitive Crystalline Formation in Supramolecular PLLAs. As described in the Introduction, PLLA β-form is a metastable phase and it is generally formed under high pressure, strong stretching, or shearing. Herein, we successfully attained the PLLA β-form just by cold or melt crystallizations of SM-PLLAs at a low Tc (75−95 °C). However, the β-form disappears in SM-PLLAs at Tc ≥ 100 °C. Figure 12 illustrates the mechanistic

Figure 12. Schematic illustration for Tc-sensitive crystalline structure of SM-PLLA.

explanations on the Tc-sensitive crystalline structure of SMPLLA. As illustrated in Figure 12A, the preferential formation of metastable PLLA β-form in SM-PLLA at a low Tc (75−95 °C) is due to the “strong” confinement effects of UPy motifs to the crystallization of linked PLLA chains. The confinement effects are originated from both the restrained chain mobility by strong UPy−UPy interactions and the spatial confinements caused by stacked UPy crystals. Although the Tm,UPy values reported in the literatures18,20,58 vary in the wide range for different SMPs, it is typically in the range of 80−100 °C.18,19 Our in situ FTIR results (Figure 1) have verified that the UPy− UPy H-bonds are partially destroyed with heating to ∼90 °C. It has been demonstrated from NMR and rheological analyses that the UPy−UPy H-bonds dissociate at ∼80 °C for the UPyfunctionalized poly(isoprene)30 and poly(alkyl acrylate).31 With combing our in situ FTIR data and the reported results, it is concluded that Tm,UPy and Td,UPy of our SM-PLLAs would be around 80−100 °C; this temperature range agrees with the H

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Macromolecules Tc range associating with the β/α′ structural change (90−100 °C) in isothermal crystallization. When Tc is around and smaller than the Td,UPy and Tm,UPy, UPy motifs are in the dimerized and stacked state. The strong UPy−UPy interactions and the presence of stacked UPy crystals will constrain the diffusion and folding of PLLA chains in crystallization; this may exert a local stretch or orientation to the linked PLLA chains. Therefore, during crystallization under the “strong” confinement (i.e., low Tc), the PLLA chains linked to UPy motifs can adopt a more extended conformation than those of the nonfunctionalized PLLA precursors. It has been reported that the chain orientation by stretching or shearing is essential to obtain the PLLA β-form.47 The local stretching effects of dimerized and stacked UPy motifs to the linked PLLA chains would facilitate the generation of β-form with the more extended chain conformation. It is expectable that the confinement effects of UPy motifs will depress as the UPy content decreases or the chain length between two UPy groups increases. Therefore, a high fraction of β-form is generated in the SM-PLLAs having lower MW or star-shaped precursor architecture (i.e., 2L-4.4k-U, 3L-8.5k-U), as seen in Figures 7, 8, and 10. The T c -dependent variation of SM-PLLA crystalline polymorph is correlated to the thermally sensitive and reversible nature of UPy−UPy bonds. At a high Tc (≥100 °C), most of the stacked and dimerized UPy dimers disassociate and melt, leading to the drastic decrease of interactions between PLLA segments, as illustrated in Figure 12B. This has been demonstrated by the remarkable decrease in 1670-1650 cm−1 band intensity of SM-PLLA with temperature at >90 °C (Figure 1) and the significant decrease of viscosity for the UPy-based SMPs at a high temperature.30,31 Therefore, the confinement effects of disassociated UPy groups on the crystallization of linked PLLA chains will be highly depressed at a high Tc (>100 °C). Under such “weak” confinement (i.e., high Tc), PLLA chains have relatively higher freedom, diffusion ability and thus form the common α (or α′) crystals, similar as the crystalline polymorph generated in PLLA precursor. Interestingly, the β-form fraction of SM-PLLAs exhibits a unique Tc dependence at Tc < 100 °C (Figures 7 and 10). It is surprising that the highest β-form fraction is attained at Tc = 90 °C; it decreases with Tc decreasing to 75 °C or increasing to 130 °C. The remarkable decrease of β-form fraction with Tc at Tc > 90 °C is originated from the decline of UPy−UPy interactions, as verified by the FTIR results shown in Figure 1. Exact reason for the decrease in β-form fraction with Tc decreasing from 90 to 75 °C is still unclear. As shown in Figure 1, the UPy−UPy interactions do not change significantly with temperature at Tc < 90 °C. Therefore, it is considered that the decrease of β-form fraction with Tc decreasing from 90 to 75 °C may be caused by the competing formation between α′ and β forms. It has been well documented that α′-form is kinetically favorable at a low Tc;36,59 the favorable formation of α′-form with decreasing Tc will decrease the β-form fraction. Thermally Induced β-to-α Phase Transition of Supramolecular PLLA. Because PLLA β-form was a metastable phase compared to its α counterpart, we further investigated the heating-induced crystalline phase transition of β-form in SM-PLLA by the temperature-variable synchrotron radiation WAXD. Figure 13 shows the temperature-dependent WAXD profiles enlarged in the different angle regions for the 2L-4.4kU SM-PLLA (Tc = 90 °C) collected upon heating. On the basis of these results, the intensity changes of α110/200 (α′110/200), β200

Figure 13. Temperature-variable WAXD patterns collected upon heating of 2L-4.4k-U SM-PLLA after cold crystallization at 90 °C: (A) WAXD patterns in 2θ = 12−15° region; (B) enlarged patterns in 2θ = 14−17° region.

diffractions and fβ,WAXD were calculated and plotted against temperature in Figure 14. The intensities (i.e., peak area) of

Figure 14. Temperature-variable WAXD results derived from Figure 13 for the 2L-4.4k-U SM-PLLA (Tc = 90 °C): (A) intensity changes of α110/200 (or α′110/200) and β200 diffractions during heating; (B) changes of β-form fraction during heating.

α110/200 (α′110/200) and β200 diffractions were evaluated by curve fitting (Figure S13). Before heating, the SM-PLLA mainly contains β crystals, with a small fraction of α′ crystals. As shown in Figures 13 and 14, the intensities of β and α′ diffractions slightly decrease but fβ keeps nearly constant with heating to 120 °C; this is ascribed to the melting of some unperfected crystals. Upon further heating of the sample from 120 to 140 °C, the characteristic diffraction peaks of β-form in SM-PLLA (e.g., β200 at 2θ = 13.7°, β131 at 2θ = 15.7°) disappear and those of the αform (α110/200 at 2θ = 13.4°, α203 at 2θ = 15.3°) gradually appear (Figure 13). The α110/200 (or α′110/200) intensity shows an obvious increase with heating the SM-PLLA from 120 to 140 °C, in contrast to the drastic decrease of β200 intensity and fβ,WAXD (Figure 14). Similar changes of WAXD patterns with temperature are also observed for the 3L-8.5k-U SM-PLLA (Tc = 90 °C) during heating (Figures S17 and S18). We emphasize that the decreased intensity of β200 diffraction is much larger than the increased intensity of α110/200 diffraction with heating the SM-PLLA at 120−140 °C (Figure 13A). This suggests that the β-form is thermally unstable and it partially transforms to the thermally stable α-form upon heating. Notably, the α110/200 diffraction observed at above 130 °C is stronger than the α′110/200 diffraction observed at 80 °C, meaning that the content of reformed α crystals is higher than that of initial α′ crystals. This further demonstrates that the β-form indeed transforms into α-form upon heating, rather than melting directly. The phase transition can take place through solid−solid transition I

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Figure 15. Temperature-variable FTIR results in the wavenumber range of 970−890 cm−1 for 3L-8.5k-U SM-PLLA (Tc = 90 °C) collected during heating (5 °C/min): (A) temperature-variable FTIR spectra collected in 90−150 °C (shown with a 4 °C interval); (B) intensity changes of 912 and 921 cm−1 during heating. The intensities were normalized by the maximum intensity of the 921 cm−1 band.

and melt-recrystallization mechanism; our present results cannot distinguish these two mechanisms. As shown in Figures 13 and 14A, the diffractions of α-form (α110/200 at 2θ = 13.4°, α203 at 2θ = 15.3°) disappears with further heating from 140 to 150 °C, attributable to the melting of α-form generated in the phase transition process. On the other hand, the α′110/200 and α′203 diffraction peaks shift from 13.0° to 13.2° and 14.9° to 15.1° during heating from 120 to 140 °C, respectively, indicating the occurrence of α′-toα phase transition. This coincides with the α′-to-α phase transition of PLLA occurred upon heating or annealing at elevated temperatures.34−36 The heating-induced α′-to-α transition of SM-PLLAs takes places in the similar temperature range as the β-to-α transition. Because of the lower MW of PLLA precursor and less perfect crystals formed in SM-PLLAs, the temperatures (∼130 °C) corresponding to β-to-α and α′to-α phase transitions are lower than those reported in the literatures.42,47 The β-to-α phase transition and melting process of SMPLLAs were also analyzed by temperature-variable FTIR spectroscopy. Figure 15A shows the temperature-variable FTIR spectra in the wavenumber range of 970−890 cm−1 for 3L-8.5k-U SM-PLLA (Tc = 90 °C) collected upon heating. Normalized intensity changes for the characteristic bands at 921 and 912 cm−1 were evaluated during heating, as shown in Figure 15B. The intensities (i.e., peak areas) of 921 and 912 cm−1 bands, which reflected the relative fractions of α (or α′) and β crystals, respectively, were evaluated by curve fitting, as illustrated in Figure S19. The corresponding FTIR results of 3L-8.5k precursor were also shown in Figure S20 for comparison. As shown in Figure 15A, the conformationsensitive band of SM-PLLA first shifts from 912 to 921 cm−1 upon heating to ∼130 °C and then disappears with further heating to ∼146 °C. Meanwhile, the intensity of 912 cm−1 band (characteristic of β-form) decreases upon heating from 114 to 130 °C; whereas that of 921 cm−1 band (characteristic of α or α′ form) increases in this temperature range. However, in the case of 3L-8.5k precursor, the wavenumber of conformationsensitive band keeps at ∼921 cm−1 and its band intensity decreases monotonically in the heating process (Figure S20). These FTIR results also confirm the occurrence of β-to-α phase transition in SM-PLLAs during the heating process.

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CONCLUSIONS

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ASSOCIATED CONTENT

In summary, the UPy-bonded SM-PLLAs exhibit the unique and different crystallization kinetics, polymorphic crystalline structure, and phase transition behavior from the conventional covalently bonded PLLAs. SM-PLLAs have much slower crystallization rate and lower crystallinity than the corresponding covalently bonded PLLA precursors, as caused by the confinement effects of H-bonding UPy motifs. Polymorphic crystalline structure of SM-PLLAs can be tuned by varying Tc or the degree of confinement. Nonfunctionalized PLLA precursors crystallize into α or α′ form at different Tc’s. Because of the thermally responsive nature of UPy−UPy interactions, crystalline structure of SM-PLLAs is sensitive to Tc. SM-PLLAs prefer to forming the metastable β crystals at a low Tc (75−95 °C, “strong” confinement), whereas the α (or α′) crystals at a high Tc (100−130 °C, “weak” confinement). At the same Tc (75−95 °C), the fraction of β-form decreases with increasing the MW or decreasing the arm number of PLLA precursor. Preferential formation of β crystals in SM-PLLAs at a low Tc is ascribed to “strong” confinement effects of UPy motifs, originated from both the restrained chain mobility by strong UPy−UPy interactions and the spatial confinements caused by stacked UPy crystals. The metastable β crystals in SM-PLLAs transform into the thermally stable α crystals in the heating process. This study can enable us to further understand the unique crystallization kinetics and polymorphic crystalline structure of SMPs and also provide the potential ways to tailor the physical properties and functions of SMPs in processing.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01705. Synthesis information, GPC, 1H NMR, TGA, viscosity results of hydroxyl and UPy-terminated PLLAs, and partial WAXD and FTIR data of SM-PLLAs and PLLA precursors (PDF) J

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-571-87951334. E-mail [email protected] (P.P.). ORCID

Guorong Shan: 0000-0001-5676-6310 Pengju Pan: 0000-0001-6924-5485 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (21674095, 21422406) and Natural Science Foundation of Zhejiang Province, China (LR16E030003). WAXD and SAXS were measured on the beamline BL16B1 of SSRF, China.

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DOI: 10.1021/acs.macromol.7b01705 Macromolecules XXXX, XXX, XXX−XXX