Conformational Regulation and Crystalline Manipulation of PLLA

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Conformational Regulation and Crystalline Manipulation of PLLA through a Self-Assembly Nucleator Chunhai Li, Shanshan Luo, Jianfeng Wang, Hong Wu, Shaoyun Guo, and Xi Zhang Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b00367 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 21, 2017

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Conformational Regulation and Crystalline Manipulation of PLLA through a Self-Assembly Nucleator Chunhai Li, Shanshan Luo, Jianfeng Wang, Hong Wu*, Shaoyun Guo*, Xi Zhang The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China

ABSTRACT: Self-assembly nucleators have been increasingly used to manipulate the crystallization of PLLA due to their strong intermolecular interaction with PLLA, while the molecular mechanism of such interaction is still unrevealed. In present work, one special self-assembly nucleator (TMC-300) with relatively high solubility in PLLA matrix, is chosen to investigate how the interaction works at molecular level to promote the crystallization of PLLA mainly through time-resolved spectroscopy. The results indicate that due to the dipole-dipole NH…O=C interaction between dissolved TMC-300 and PLLA, PLLA chains are transformed into gt conformer before TMC-300 phase-separating from PLLA melt, resulting in low energy barrier to pass for the following formation of PLLA α-crystal (α-crystal is consisted of gt conformer). Once the dissolved TMC-300 starts to self-assemble into frameworks upon cooling, the transformed PLLA chains with high population of gt conformer form the primary nuclei on the surface of such self-assembling TMC-300 frameworks. For the first time, not only the heterogeneous nucleation but also the conformational regulation of PLLA chains are proved to be responsible for the high efficiency of the self-assembly

* To whom correspondence should be addresses. (Prof. Wu, Email: [email protected], Fax: 86-028-85466077) * To whom correspondence should be addresses. (Prof. Guo, Email: [email protected], Fax: 86-028-85405135) 1

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nucleators (TMC-300) in promoting the crystallization of PLLA. Therefore, conformational regulation is proposed for crystalline manipulation of PLLA, and this work brings new insight on promoting the crystallization of PLLA even other polymers by regulating their molecular conformation. KEYWORDS： PLLA, Conformation, Self-assemble, Crystallization, Nucleation 1. INTRODUCTION Polylactide(PLLA) is a biodegradable polymer that can be produced from annually renewable resources1, 2. As a semicrystalline polymer, the physical properties of PLLA strongly depend on its crystalline morphology and crystallinity3-9. However, due to its rigid chains and the steric hindrance of the adjacent methyl side groups10-13, PLLA exhibits poor crystallization ability. Consequently, tremendous efforts have been made in the scientific community toward the crystalline manipulation of PLLA. As one effective method, the nucleators are usually used to promote the crystallization of PLLA. With appropriate nucleator, the crystallization rate can be significantly accelerated. Layered metal phosphonate14, montmorillonite15, talc16, 17 , modified cellulose nanocrystal18 and carbon nanotube19, other inorganic powders20, 21 have shown nucleating ability for PLLA. In recent years, self-assembly nucleators have been increasingly used to manipulate not only the crystallization rate but also the crystalline morphology of PLLA4, derivatives (TMC-328)7,

22, 23, 32

7, 22-31

. Among them, benzenetricarboxylamide

, tetramethylenedicarboxylic dibenzoylhydrazide

(TMC-306)24, 25, 33, octamethylenedicarboxylic dibenzoylhydrazide (TMC-300)34, 35 , N, N′-dicyclohexylterephthalamide(TMB-5)36, dibenzylidene sorbitol4, 2


27, 28

and

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aliphatic amides

29,

30

have exhibited high efficiency in manipulating the

crystallization of PLLA. Compared with the traditional nucleators, self-assembly nucleators can be partially dissolved in PLLA melt and self-assemble into special frameworks like needles, dendritic or fibrillar network4, 22-31, 37 upon cooling, and then these frameworks guide the crystallization of PLLA , resulting in sorts of crystalline morphology, such as shish-kebab-like22-24, dendritic-like37, and cone-like23 structure. Unlike traditional nucleators, self-assembly nucleators can be perfectly dispersed in PLLA matrix since they can be partially dissolved in PLLA melt before phase-separating from PLLA melt. In the literatures29, 32, 38, the mechanism for the high efficiency of self-assembly nucleators in promoting the crystallization of PLLA is generally proposed as (i) their well-dispersed in PLLA matrix and (ii) high heterogeneous nucleating efficiency resulted from the strong intermolecular interaction such as the hydrogen bond. Nevertheless, up to now, how the interaction works is still unrevealed. On the other hand, increasing researches indicate that before the crystallites emerge, polymer chains undergo conformational adjustment in a way favored by their crystallites39-41. From this perspective, the crystallization of polymers can be manipulated by regulating their molecular conformation. For examples, as the specific C−H···O＝C interactions between the paired stereoisomeric PLLA and PDLA chains, racemic (32/31) helical conformation starts to emerge in the early stage and then serve as nucleating sites upon cooling42. The polar groups of the lactic acid alter the conformation of nylon-6 and then lead to the transition of nylon-6 crystallizing from 3


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γ-form into the thermodynamically stable α-form43. Wei et al.44 reported that electrospinning procedure transforms the PEO molecular chains from a helix into a planar zigzag conformation, while Li et al.41 found that shear field could induce the conformational adjustment of PP molecules and then promote its crystallization. Since the conformation of polymer chains strongly depends on the intermolecular interactions45-47, the conformation of PLLA chains can be manipulated by adding the self-assembly nucleator due to their strong intermolecular interaction. Taking the high efficiency of self-assembly nucleators in promoting the crystallization of PLA into consideration, we deduce that not only the high heterogeneous nucleating efficiency generated with their perfect dispersion and strong intermolecular interaction, but also the effect of the self-assembly nucleator on the conformation of PLLA chains plays a key role in promoting the crystallization of PLLA. In order to verify our hypothesis, in present work, one special self-assembly nucleator (TMC-300) with proton donors (N-H) and relatively high solubility in PLA melt is chosen. We speculate that the presence of TMC-300 should alter the conformation of PLLA chains for two reasons, (i) the proton donors (N-H) in TMC-300 will interact with the proton acceptors (C=O) in PLLA and (ii) the relatively high solubility of TMC-300 in PLLA guarantees sufficient molecular contact of PLLA and TMC-300. By tracking the conformational changes of the PLLA chains before and after TMC-300 phase-separating from the PLLA matrix, how this intermolecular interaction works at molecular level to promote the crystallization of PLLA is revealed. Besides, the effects of the self-assembly behavior of TMC-300 in PLLA 4


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melt on the crystalline superstructures of PLLA are also investigated. 2. EXPERIMENTAL SECTION Materials. Commercially available PLLA, comprising 2% DLA (trade name 4032D), was purchased from Nature Works Co. (USA), whose weight- and number-average molecular weights were 2.23×105 and 1.06×105 g mol−1, respectively. Octamethylenedicarboxylic dibenzoylhydrazide (TMC-300) was provided by Shanxi Provincial Institute of Chemical Industry, China. The chemical structure of TMC-300 is given in Figure S1. Preparation of PLLA/TMC-300 Compounds. A HAAKE internal mixer (Thermo Scientific, USA) was used to prepare PLLA containing various amounts of TMC-300. The melt compounding was performed at 180℃ with a rotation rate of 60 rpm/min for 8 min. For the compounds with low TMC-300 content, a master-batch of PLLA containing 5 wt% TMC-300 was first prepared and then diluted with Neat PLLA to a desired percent. For convenience, the samples were denoted as PLLA-x, where x indicates the weight percentage of TMC-300. It is noted that both PLLA and TMC-300 were dried in a vacuum oven at 60 ℃ for 12 h before compounding. Polarized Optical Microscopy (POM). POM observation was performed on an Olympus BX51 microscopy (Olympus Co., Japan) equipped with a crossed polarizer and a video camera. Temperature of the specimens were controlled by a hot-stage (HCS 302, INSTEC). All samples were firstly inserted between two microscope coverslips and squeezed at 180 ℃ to obtain a slice with a thickness of around 20 μm. To construct composition/temperature diagrams for PLLA-x, the non-isothermal 5


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crystallization was employed. In detail, the as-prepared slices were transferred to a hot stage, the temperature was firstly heated to 210 ℃ for 3 min at the heating rate 5 ℃/min to completely erase any thermal history and then cooled to 60 ℃ at the cooling rate 5 ℃/min. Both the dissolving temperature of TMC-300 and the melting point of PLLA were recorded during heating, while the self-assemble temperature of TMC-300 and the crystallization temperature of PLLA were recorded upon cooling. For conditionally isothermal crystallization, as shown in the upper-right of Figure S5, the as-prepared slices were also held at 210 ℃ for 3 min and then cooled to a preset temperature to allow TMC-300 to has enough time to finish its self-assemble, and then further cooled to a selected temperature to the isothermal crystallization of PLLA, the cooling rate is 20 ℃/min. Time-Resolved Fourier Transform Infrared Spectroscopy (FTIR). PLLA film for the FTIR measurement was cast on a KBr window from a 2% (w/v) PLLA-x chloroform solution. Note that only opaque solution was observed because TMC-300 cannot be completely dissolved in chloroform for PLLA-x with the content of TMC-300 above 0.5 wt%. Therefore, in order to keep better dispersion of TMC-300 in the cast film, the opaque solution was firstly ultrasound and stirred for 20 min at 30 °C and then cast on a KBr window immediately. After the majority of the solvent had evaporated, the film was placed under vacuum at 60 °C for 24 hours to completely remove the residue solvent. The thickness of such prepared polymer film was about 15μm. The time-resolved FTIR spectra of PLLA-x, were recorded at a 4 cm-1 spectral resolution using a Nicolet-IS10 (Thermo Electron Co., USA) 6


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spectrometer by signal averaging 20 scans. Two pieces of microscope KBr windows, with no absorption bands in the middle IR region, were used to prepare the transmission cell. Variable-temperature spectra, controlled by a temperature control instrument including a programmed heating cell and a circulating water jacket cooling system, were collected during both the isothermal and non-isothermal crystallization. For the isothermal crystallization, each sample was held at 210 °C for 3 min to completely erase the thermal history and then cooled to 130 °C at the rate of 10 °C /min. For the non-isothermal crystallization, each sample was also held at 210 °C for 3 min and then cooled to 60 °C at 1°C/min. To identify the hydrogen bond in TMC-300, with the same thermal condition of non-isothermal crystallization, the variable-temperature spectra for TMC-300 were also collected. All the measurements were carried out with the protection of high-purity nitrogen and the baseline correcting process was performed using the automatic baseline correction of OMNIC 8.2 spectral collecting software (Thermo Fisher Scientific Inc., USA). Scanning Electronic Microscopy (SEM). A field-emission SEM (JSM-5900LV, Japan) was applied to observe the crystalline morphology. All the samples were etched by a water−methanol (1:2 by volume) mixture solution containing 0.025 mol /L sodium hydroxide for 12 h at 30 °C. Subsequently, the etched samples were ultrasound cleaned by distilled water and dried in a vacuum oven at 40 °C and sputter-coated with gold before observation. Differential Scanning Calorimetry (DSC) Measurement. DSC measurement was performed on a TA Q2000 DSC (TA Instruments, USA) under a nitrogen flow of 7


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50 mL min-1, which was calibrated by indium as the standard. The samples with a weight of about 8 mg were first heated to 210°C at the heating rate 5 °C/min for 3 min to eliminate thermal history and then cooled to the 60 °C at the rate of 5 °C/min. X-Ray Diffraction (XRD). X-ray diffraction (XRD) patterns were recorded using a X＇Pert Pro MPD (Philips, Netherlands) diffractometer with Cu-Kα radiation (40 kV and 40 mA). The scanning angle covered a range between 4ºand 40º. 3. RESULTS AND DISCUSSION 3.1 Phase Transition of TMC-300 in PLLA. Temperature/composition diagrams for heating (dissolving) and cooling (self-assemble) constructed from the POM and DSC data are shown in the left and right of the Figure 1, respectively. The resultant diagrams show that both the dissolving and self-assembly behavior of TMC-300 depend on its content. The dissolving temperature increases with increasing the content of TMC-300 from 0.5 wt% at 177.3 °C to 2wt % at 207 °C, however, it remains stable at about 210 °C once the content of TMC-300 exceeds 2 wt%. As for PLLA, its melting point keeps stable at 175 °C. Please note that only the homogenous liquid was observed above the dissolving temperature, indicating the high solubility of TMC-300 in PLLA (≥ 5 wt%) at liquid-liquid state. Upon cooling the homogenous liquid, TMC-300 with different contents self-assembles into diverse frameworks at different temperatures. Furthermore, the self-assemble temperature of TMC-300 increases sharply for the content regime from 0.5 wt% to 2 wt% while slowly from 2 wt% to 5 wt%. It is also noted that liquid-liquid phase-separation occurs before TMC-300 starts to self-assemble once its content exceeds 3%. In the case of 8


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TMC-300 content below 0.5 wt%, TMC-300 cannot phase-separate from PLLA matrix upon cooling, and both its dissolving and self-assemble temperatures are non-existent. As for the crystallization temperature of PLLA, it remains stable at 140 ℃ despite TMC-300 self-assembles above 140 ℃ when its content is from 1 wt% to 5 wt%. The crystallization of PLLA and the self-assemble of TMC-300 process simultaneously when its content is from 0.5 wt% to 1 wt%. Slight decrease in the crystallization temperature of the PLLA was observed in the eutectic regime when the TMC-300 content is below 0.5 wt%, indicating that the completely dissolved TMC-300 may act as plasticizer to impair the entanglement of PLLA chains48.

Figure 1. Dissolving (left) and self-assemble (right) temperature/composition diagrams of the PLLA/TMC-300. In the diagrams the symbols TMC-300 while the

refer to the PLLA, specially,

liquid-liquid phase separation; while

refer to the transition temperature of refers to transition temperature of

refer to experimental data obtained by optical microscopy

by DSC, The denotation T refers to TMC-300, P to PLLA, L to liquid, and S to solid.

9


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3.2 Manipulating the Crystalline Superstructures of PLLA through TMC-300. During the construction of temperature/composition diagrams, we found that upon cooling the homogenous liquid (Figure 1), TMC-300 self-assembles into diverse frameworks in PLLA melt at different temperature. It is well known that the crystalline superstructures of semicrystalline polymers largely depend on the morphology of nucleators7,

22-28, 48, 49

. So, in order to manipulate crystalline

superstructures of PLLA, the non-isothermal crystallization as well as the conditionally

isothermal

crystallization

was

performed

based

on

the

temperature/composition diagrams. The conditionally isothermal crystallization indicates that the sample was firstly cooled to selected temperature for the self-assemble of TMC-300 and then further cooled to another selected temperature for crystallization of PLLA. Nevertheless, as mentioned in the introduction, by adding a self-assembly nucleator, PLLA with many crystalline superstructures have been obtained7, 22-28, 48, 49. In order to avoid repetitions, the detailed influence of TMC-300 on the crystalline superstructures of PLLA is provided in Supporting Information, Section 1, only some interesting superstructures such as the star multi-arm-like, short fibril-like and sunflower-like superstructures are presented in detail here. For the conditionally isothermal crystallization PLLA-1, TMC-300 first self-assembles into star multi-arm frameworks at 170 ℃, and then the resultant arm serves as a shish to induce the growing of PLLA lamellae when the temperature is cooled to 150 ℃ (Figure 2(A1)). The nucleating sites on the surface of TMC-300 arm at such temperature (150 ℃) are relatively rare, inevitably, resulting in the initiated 10


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Figure 2. POM (A1) and SEM micrographs (A2, A3) of the crystalline superstructures of PLLA-1 (PLLA-1 was firstly cooled to 170 ℃ for 10 min for the self-assemble of TMC-300 and then cooled to 150 ℃ for the crystallization of PLLA, the cooling rate is 20℃/min, POM (A1) was taken at 150 ℃ for 3 min, for SEM observation, PLLA-1 was quench in ice water after150 ℃ for 3 min); POM (B1) and SEM micrograph (B2, B3) of the crystalline superstructures of PLLA-0.5 (PLLA-0.5 was cooled to 60 ℃ at cooling rate 5 ℃/min, POM (B1) was taken at 123.9 ℃ while the SEM micrographs were taken after the temperature cooled to 60 ℃); POM (C1) and SEM micrograph (C2, C3) of the crystalline superstructure of PLLA-0.3, PLLA-0.3 was firstly cooled to 130 ℃ for 25 min and then cooled to 125 ℃ for 28.5 min, the cooling rate 20 ℃/min; (Before cooling, all samples were first held at 210 ℃ for 3 min).

lamellae growing into calabash with a branched way, finally, the star multi-arm shish-calabash crystalline superstructure can be observed in PLLA-1(Figure 2(A1)). More interestingly, as revealed by SEM (Figure 2(A2) and (A3)), apart from the 11


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multi-arm shish-calabash, both the ring-banded structure and “valley” alternating “ridge” superstructure can also be observed. As the close distance of the adjacent arm at central of star multi-arm frameworks, the PLLA lamellae would encounter and overlap during the growing process. Consequently, the “ridge” is originated from the overlap of PLLA lamellae while the “valley” is originated from the long groove left after removing the TMC-300 arm during the etch process (the yellow bars depicted in SEM), and once the TMC-300 arm was removed, the primary lamellae forms the ring-banded structure. As for the non-isothermal crystallization of PLLA-0.5, TMC-300 starts to self-assemble into short fibril-like frameworks at 125.3 ℃, where sufficient nucleating sites are available on the surface of the frameworks, resulting in short fibril-like transcrystalline superstructure in PLLA-0.5 (Figure 2(B1)). Moreover, as revealed by SEM (Figure 2(B2) and (B3)), these short fibrils serve as shish to induce the epitaxial growth of the PLLA lamellae approximately orthogonal to their long axis, therefore, hybrid fibril-like transcrystalline superstructure with denser and regular lamellae strung on the TMC-300 short fibrils is formed. Such hybrid fibril-like crystalline superstructure was firstly reported by the elaborate works of Bai et al7, 23, 24. Most interestingly, as shown in Figure 2(C1), a sunflower-like superstructure with big PLLA spherulite serves as flower disc while the hybrid fibril-like transcrystalline superstructure acts as the petal. As shown in the Figure 2(C1) and (C2), at the boundary between the PLLA spherulite and its amorphous region, TMC-300 self-assembles into fibril with their axis orienting along the radial direction of PLLA spherulite, and then induce the epitaxial growth of the PLLA lamellae. One may argue 12


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why TMC-300 self-assembles at the boundary of PLLA spherulite. Actually, during the growing process of the PLLA spherulites, TMC-300 is enriched at the boundary because it is treated as an impurity continuously expel into the boundary, as shown in self-assembly temperature/composition diagram (Figure 1 (right)), once the content of TMC-300 increases to the critical value (about 0.75 wt% for 125 ℃), TMC-300 starts to self-assemble at the boundary. 3.3 Why TMC-300 Self-Assembles into Different Frameworks in PLLA Melt. The chemical structure of TMC-300 in Figure S1 shows the hydride units (with the proton donors N-H and the proton acceptors C＝O) in its molecules, indicating that strong hydrogen bond (H-bond) should exist in TMC-300. As the H-bond usually acts as driven force to build superstructures in self-assembly systems due to its specificity and directionality50, 51, the H-bond, if exist, should be responsible for the self-assembly behavior of TMC-300. To identify the H-bond in the TMC-300, the

Figure 3. Time-resolved spectra of TMC-300 in the range of 3460-2890 cm-1 and 1750-1530 cm-1 with the temperature from 210 ℃ to 60 ℃, the temperature process was first heat to 210 ℃ for 3 min and then cooled to 60 ℃ with the cooling rate 1 ℃/min. The right figures are the corresponding absorbance change of the characteristic bands. 13


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time-resolved FTIR which is highly sensitive to the H-bond52 was employed. As shown in Figure 3, there are three types of bands for N-H and C＝O in TMC-300 from 210 ℃ to 60 ℃, i.e., the free N-H (3383 cm-1) and the free C＝O (1681 cm-1), the weak H-bonded N-H (3296 cm-1) and C＝O (1667 cm-1) which are associated via the weak N-H…O＝C H-bond, and the strong H-bonded N-H (3200 cm-1) and C＝O (1600 cm-1 and 1574 cm-1) which are associated via the strong N-H…O＝C H-bond. It is clear that there is no obvious variation in the free groups (3383 cm-1 for N-H and1681 cm-1 for C＝O) and in weak associated H-bonded groups (3296 cm-1 for N-H and 1667 cm-1 for C＝O) over the temperature region from the 210 ℃ to 170 ℃. However, the absorbance of both free groups (N-H and C＝O) and the weak H-bonded groups decrease dramatically while that of the strong H-bonded groups increase noticeably over the temperature region from 170 ℃ to 150 ℃. Interestingly, at the temperature below 150 ℃, only the absorbance of the strong H-bonded groups can be observed, indicating that all the N-H groups are associated with the C＝O via strong H-bond. Actually, the intensity of H-bond for a given system generally increases with decreasing temperature because the enthalpy of H-bond formation is usually negative53, which is consistent with the results shown here. It is also noted that the double peak for the strong H-bonded C＝O (1600 cm-1 and 1574 cm-1) is caused by its intramolecular transition dipole coupling interactions of the neighbor C＝O in TMC-300 molecules. If two C＝O groups are connected by atoms with unshared electron pairs (N,O), even connected by two or more N-N or O-O bonds (shown in Figure S1, two C＝O groups are connected by N-N bond), mechanical coupling 14


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occurs and splits the absorbance peak of C＝O groups into a symmetric and an asymmetric bands, where the intensity of the asymmetric band is relatively higher than that of the symmetric band54, 55. 3.4. Mechanism for Crystalline Manipulation of PLLA. As aforementioned, self-assembly nucleators have been increasingly used to manipulate the crystallization due to their strong intermolecular interaction with PLLA, while how these interaction works to promote the crystallization of PLLA is still unrevealed. Analyzing the isothermal and non-isothermal crystallization dynamics of PLLA-x (Supporting Information, Section 2) indicate that PLLA crystallization with the presence of TMC-300 needs less conformational adjustment in nucleating period, which indirectly prove that the presence of TMC-300 transforms the conformation of PLLA chains into a way favored by PLLA crystal. As the proton donors (NH) in TMC-300 are expected to interact with the proton acceptors (C=O) in PLLA, to directly ascertain how the interaction transforms the conformation of PLLA chains and then promotes the crystallization of PLLA, the evolution of the infrared spectra for the absorbance of carbonyl (C=O) in PLLA is investigated during the crystallization. According to the rotational isomeric state (RIS) model developed by Tonelli et al56, four different conformers with different energy, i.e., the tt, tg, gt and gg conformer can be observed in PLLA molecules. Among them, the gt conformer with the lowest energy forms 103 helix, and the 103 helix is consisted of the most stable α-crystal of PLLA57. Figure 4(A1) displays the original spectra in the carbonyl stretching region of Neat PLLA recorded during the non-isothermal crystallization 15


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from 210 ℃ to 60 ℃ (α-crystal proved by Figure S9). It is clear that the peak of the carbonyl region narrows while its peak maximum intensity increases with decreasing the temperature. From the second derivative spectra (Figure 4(A3)), four contributions apparently located at 1777, 1769, 1758, and 1747 cm-1 can be resolved, which correspond to the tt, tg, gt and gg conformer, respectively56. Besides, compared with other conformers, the lowest energy of gt conformer leads to its high population even at the melt state of Neat PLLA58, 59, resulting in the peak maximum intensity of carbonyl stretching region in PLLA locating at the band of gt conformer (1758 cm-1 shown in Figure 4). On the other hand, this also indicates that the PLLA chains are highly rigid as its chain conformation is dominated by the lowest energy gt conformer. As the α-crystal of PLLA is consisted of 103 helix that built with the gt conformer57, the other conformers would transform into gt conformer when PLLA crystallizes in 103 helix, which correspond to the positive shift shown in the difference spectra: only the absorbance intensity of gt conformer at 1758 cm-1 increases while the intensity for other conformers decreases (Figure 4(A2)). Consequently, the narrowing of the original carbonyl stretching region (Figure 4(A1)) can be attributed to the increase of crystalline phase (gt conformer). Most interestingly, compared with the Neat PLLA, the PLLA-2 presents a different carbonyl stretching region. Firstly, it is clear that the half-width of the original carbonyl peak for PLLA-2 (Figure 4(B1)) is smaller than that of Neat PLLA (Figure 4(A1)). Secondly, as shown in the difference spectra (Figure 4(A2) and (B2)), the variation of the bands at around 1780 cm-1 and 1740 cm-1 in PLLA-2 from 210 ℃ to 16


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Figure 4. Time-resolved spectra for the carbonyl stretching region (1820-1690 cm-1) of Neat PLLA and PLLA-2 as a function of temperature from 210 ℃ to 60 ℃ (the cooling rate is 1℃/min); (A1), (B1): Origin infrared spectra for Neat PLLA and PLLA-2, respectively; (A2), (B2): difference spectra for Neat PLLA and PLLA-2, respectively (obtained by subtracting the initial spectrum at 210 ℃ from the consecutive spectra, if the value of difference spectra is positive, means its absorbance intensity increase, otherwise, decrease); (A3), (B3): second derivative for Neat PLLA and PLLA-2, respectively.

60 ℃ are noticeably lower than that in Neat PLLA, indicating that the molecular chains undergo less conformational adjustment in PLLA-2 during the crystallization, which is consistent with the crystallization dynamics of PLLA-x (Supporting Information, Section 2). Thirdly, it seems that the peak for 1769 cm-1 (tg conformer) in PLLA-2 is not as clear as the that in Neat PLLA in the second derivative (Figure 4(A3) and (B3)). All these differences indicate that the addition of TMC-300 transforms the conformation of PLLA chains and thereafter affects its crystallization. Figure 5(a) shows the changes of both half-width of the carbonyl stretching region and the absorbance of 103 helix at 921 cm-1 for Neat PLLA from 210 ℃ to 17


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60 ℃. Clearly, only when the α-crystal of PLLA starts emerging (signified by the dramatic increase of the band at 921 cm-1 for α-crystal) that the half-width of the carbonyl region decrease obviously. As mentioned above, other conformers would transform into gt conformer when PLLA crystallizes in 103 helix (α-crystal), hence, the half-width of carbonyl region decrease quickly due to the formation of α-crystal.

Figure 5. Evolution of half-width of the carbonyl stretching region (1820-1690 cm-1) and the absorbance variation of the 103 helix at 921cm-1 of Neat PLLA and PLLA-2. (a): Neat PLLA; (b): PLLA-2; (1600 cm-1 belongs the hydrogen bonded C=O in TMC-300 (Figure 3)).

Most interestingly, as shown in Figure 5(b), the half-width of the carbonyl region for PLLA-2 at 210 ℃ is 34.0 cm-1 while that for Neat PLLA at the same temperature is 37.2 cm-1. As the absence of blue or red shift in the carbonyl region for PLLA molecules that includes four contributions (tt, tg, gt and gg conformer), it can be concluded that the low half-width of the carbonyl region for PLLA-2 is ascribed to its high gt population, which directly proves that the dissolved TMC-300 transforms the conformation of PLLA chains into gt conformer even at melting state (210 ℃). Moreover, the half-width of the carbonyl region for PLLA-2 decrease slightly when TMC-300 starts to self-assemble (150 ℃ signified by the dramatic increase of the 18


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band at 1600 cm-1), indicating that the self-assembled TMC-300 can also transform the conformation of PLLA chains into gt conformer by promoting the formation of primary nuclei or 103 helix (built with gt conformer) on its surface. Similar to the Neat PLLA, only when the α-crystal emerges, the half-width of the carbonyl region decrease obviously. Figure S11(a) shows that PLLA-x with different TMC-300 contents have different nucleating periods, however, in the temperature rangeing from 210 ℃ to 140 ℃, all samples are being in nucleating period. To further prove that the TMC-300 promotes the formation of gt conformer even at the nucleating period, the difference spectra of the carbonyl region of the Neat PLLA and the typical PLLA-x are presented in Figure 6(a~d). The 1758 cm-1 is chosen as an indicative to the formation of gt conformer. It is clearly there is only slight increase of the band at 1758 cm-1 in Neat PLLA. Whereas, the band at 1758 cm-1 increase obviously with the increase on the content of TMC-300 (Figure 6(a~d)), indicating that the addition of TMC-300 promotes the transition of PLLA chains from other conformers into gt conformer.

Figure 6. Difference spectra for PLLA-x for non-isothermal crystallization from 210 ℃ to 140 ℃ 19


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(the cooling rate is 1℃/min); (a): Neat PLLA; (b): PLLA-0.5; (c): PLLA-1; (d): PLLA-2.

As the conformation of polymer chains strongly depend on the intermolecular interactions45-47, the high population of gt conformer in PLLA/TMC-300 is ascribed to the intermolecular interaction between the proton donors (N-H) in TMC-300 and the proton acceptors (O＝C) in PLLA. However, as the absence of the red shift in the spectra of the carbonyl stretching region (Figure 4), this interaction should not be the hydrogen bond. On the contrary, this interaction should be the intermolecular dipole-dipole N-H…O＝C interaction, which transforms the conformation of PLLA chains into gt conformer from melting state. As the α-crystal of PLLA is built with gt conformer, the transformed PLLA chains only need less conformational adjustment to crystallize and thereafter decreases the crystallization energy barrier. Taking the similar structure of these self-assembly nucleators (with proton donors, such as TMC-328, TMC-306, TMC-300, etc.) into consideration, conformational regulation of PLLA chains through intermolecular interaction should also exist in other self-assembly nucleators and PLLA system. However, it is difficult to investigate how such intermolecular interaction works to regulate the conformation of PLLA chains due to their relative low solubility in PLLA, and this should be the reason why the conformational regulation for other self-assembly nucleators towards PLLA has not yet been reported in the pioneering works. Given the different thermal history, slight difference in experimental results between the isothermal and non-isothermal crystallization are allowed. However, the non-isothermal crystallization starts from 210 ℃, at which the PLLA chains are 20


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absolutely amorphous. Most importantly, non-isothermal crystallization includes both the transition of TMC-300 from a melt to a crystalline frameworks and the transition of the PLLA chains from an amorphous state to a crystalline 103 helical structure. Therefore, the microscopic crystallization process was proposed from both the thermodynamic and dynamic point of view mainly based on the experimental results from non-isothermal crystallization. From the thermodynamic point of view, the occurrence of nucleation requires a smaller free energy of final nuclei (𝐺𝑁 ) than that of initial melt (𝐺𝑀 ) (Scheme 1(a)). Taking a folded-chain lamellar nucleus model60, the nucleation barrier at PLLA without nucleators ∆𝐺 ∗ = 32𝜎𝑒 𝜎 2 𝑇𝑚° /(∆𝐻∆𝑇𝜌𝑐 )2 , where σ and 𝜎𝑒 are specific surface free energies of lateral and end surfaces, ∆𝐻 and 𝜌𝑐 are melting enthalpy

Scheme 1. Schematic illustration of the free energies of different systems (a): Neat PLLA; (b): PLLA with the traditional nucleators; (c): PLLA with TMC-300. G is the free energy, where the subscripts M and N represent melt and nuclei, respectively; ΔG*, ΔGt*, ΔGo* is corresponding nucleation barrier for critical nuclei; ΔGt, represents the decrease of free energy by adding the traditional nucleators; ΔSc represents the entropic reduction of PLLA chains by increasing the gt conformer; T represents temperature. 21


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and density of crystal, and 𝑇𝑚° and ∆𝑇 = 𝑇𝑚° − 𝑇 are equilibrium melting temperature and supercooling, respectively. As shown in Scheme 1(a), to form primary nuclei, the neat PLLA chains need to pass the nucleation barrier, ∆𝐺 ∗ . However, as shown in Scheme 1(b), for PLLA/traditional nucleators, the nucleation energy barrier decreases to ∆𝐺𝑡∗ = ∆𝐺 ∗ − ∆𝐺𝑡 , where the ∆𝐺𝑡 is caused by the decrease of the lateral surface (σ) due to the chemical or physical interaction between traditional nucleators and PLLA chains. The decrease of nucleation energy barrier will promote the crystallization rate and this is well known as the heterogeneous nucleation. In the case of PLLA/TMC-300 system (Scheme 1(c)), apart from the heterogeneous nucleation generated from the self-assembled TMC-300, the free energy of PLLA melt will increase by a factor of T∆𝑆𝑐 , where the (∆𝑆𝑐 ) is entropic reduction of the PLLA melt caused by regulating the conformation of PLLA chains into gt conformer through the dipole-dipole NH…O=C interaction between the dissolved TMC-300 and PLLA. Consequently, the correspondingly nucleation barrier will further decrease to ∆𝐺𝑜∗ = ∆𝐺 ∗ − ∆𝐺𝑡 − T∆𝑆𝑐 . From the dynamic point of view (Supporting Information, Section 2), the illustration for the dynamic crystallization process is presented in Scheme 2. For Neat PLLA, in the nucleating period, the characteristic bands change according to 1456 cm-1 ＞ 1210 cm-1 ＞ 921 cm-1 (Figure S11(b)), indicating the conformational adjustment firstly starts with the CH3 interchain interaction (Scheme 2(b)), subsequently, as the CH3 interchain interaction force the molecular chains adjust their position to pack more closely, making it possible for the νas(C-O-C)+γas(CH3) 22


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interchain interaction (Scheme 2(c)). Driven by both the CH3 and νas(C-O-C)+γas(CH3) interchain interaction, more other conformers will be transformed into gt conformer and when gt conformer is successively connected to exceed a critical value, the 103 helix forms and the crystallites emerge (Scheme 2(d)). However, for the PLLA-2, in the nucleating period, the characteristic bands change according to 1456 cm-1＞ 921cm-1 ＞ 1210cm-1 (Figure S11(d)), indicating the totally different nucleating mechanism. Actually, PLLA chains in PLLA-2 undergone less conformational

Scheme 2. Schematic diagrams for microscopic process of PLLA-x. (a~d): Neat PLLA; (e~h): PLLA-2.

adjustment due to the high population of the gt conformer, which is caused by the dipole-dipole N-H…O＝C interaction between the TMC-300 and PLLA (Scheme 2(e) and (h)). Moreover, TMC-300 starts to self-assemble into crystalline frameworks to offer a fixed surface to absorb a layer of PLLA chains once the temperature is cooled to the phase-separating temperature of TMC-300, and driven by such dipole-dipole 23


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N-H…O＝C interaction, the pre-absorbed PLLA chains form the primary nuclei or 103 helix (Scheme 2(f)). When the temperature is further decreased, the outer layer molecules crystallize on the surface of the primary nuclei (Scheme 2(g)). 4 CONCLUSION In this paper, how the strong interaction between a self-assembly nucleator (TMC-300) and PLLA works at molecular level to promote the crystallization of PLLA is investigated. Based on experimental results and discussion, three main conclusions can be obtained (i) the crystalline superstructures of PLLA can be manipulated through TMC-300 which can self-assemble into diverse frameworks to induce the crystallization of PLLA; (ii) the dipole-dipole N-H…O＝C interaction between PLLA and dissolved TMC-300 transforms the conformation of PLLA chains into gt conformer consisting of PLLA α-crystal even at high temperature, resulting in the low energy barrier to pass for the following crystallization; (iii) for the first time, not only the heterogeneous nucleation but also the conformational regulation for PLLA molecules, are proved to be responsible for the high efficiency of a self-assembly nucleator (TMC-300) in promoting PLLA crystallization. Therefore, conformational regulation is proposed for crystalline manipulation of PLLA, and this work brings new insight on promoting the crystallization of PLLA even of other polymers by regulating their molecular conformation. Associated Content Supporting Information: chemical structure of TMC-300, DSC curves for PLLA/TMC-300, all POM and SEM for crystalline superstructure of PLLA/TMC-300, 24


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crystallization dynamics of PLLA/TMC-300. Acknowledgements Financial support of the National Natural Science Foundation of China (51273132, 51573118 and 51227802), Program for New Century Excellent Talents in University (NCET-13-0392), Sichuan Province Youth Science Fund (2015JQ0015) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-15R48) are gratefully acknowledged. Xiaomeng Zhang and Lichao Xia are acknowledged for their assistant in the FI-IR testing, Prof. Jiang Li and Dr. Rong Chen are acknowledged for their comments for revising this paper. REFERENCE (1) Rhim, J.-W.; Park, H.-M.; Ha, C.-S. Prog. Polym. Sci. 2013, 38, 1629-1652. (2) Nagarajan, V.; Mohanty, A. K.; Misra, M. ACS Sustainable Chem. Eng. 2016, 4, 2899-2916 (3) Ma, Piming.; Jiang, L.; Xu, P.; Dong, W.; Chen, M. Q.; Lemstra, P. J. Biomacromolecules 2015, 16, 3723-3729

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Crystalline morphology of PLLA/TMC-300 35x25mm (300 x 300 DPI)


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Conformational Regulation and Crystalline Manipulation of PLLA

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