Phase Transition and Side-Chain Crystallization of Poly(methyl vinyl

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Phase Transition and Side-Chain Crystallization of Poly(methyl vinyl ether-alt-maleic anhydride)‑g‑Alkyl Alcohol Comb-like Polymers Jing Li,† Haixia Wang,† Lei Kong,‡ Yong Zhou,§ Shuqin Li,† and Haifeng Shi*,† †

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Tianjin Key Laboratory of Advanced Fibers and Energy Storage, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China ‡ Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, China § Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: A series of poly(methyl vinyl ether-alt-maleic anhydride)-galkyl alcohol (PVEM -Cn) comb-like polymers were synthesized by a simple esterification reaction between poly(methyl vinyl ether-alt-maleic anhydride) and n-alkyl alcohol with n = 14−26. Thermal property, side-chain crystallization, and phase transition behavior of PVEM-Cn comb-like polymers were investigated by 1H NMR,DSC, variable-temperature X-ray scattering and FTIR, as well as solid-state 13C CP/MAS NMR. Obvious sidechain crystallization behavior from the pended side chains onto the PVEM backbone is well confirmed. With the side-chain length increasing from 14 to 26, phase transition temperature and melting enthalpy both increase from 30.1 to 79.4 °C and from 17.1 to 55.0 kJ/mol, respectively, and the crystallizable carbon atoms (Nc) also vary from Nc = 5.6 to 17.7 accordingly. Regular trans conformation state first transforming into irregular trans conformational disordered one and then to gauche state is well demonstrated, and the chain packing manners of orthorhombic and hexagonal show a size-dependent phenomenon. Comparison of the phase transition behavior and chain packing structure for the single-component and the two-component polymer backbone comb-like polymers is approached. And, the influence of chemical junction on the side-chain crystallization behavior is also discussed from the aspect of molecular chain packing and crystallization behavior.

1. INTRODUCTION Comb-like polymers are an interesting topological polymer, and the characteristic hierarchical structure is regulated by polymeric backbone and side alkyl chains.1,2 Different from the linear polymers, comb-like polymers give remarkable characteristics such as the mesophase formation,3−5 the critical length requirement,6−12 and the varied chain packing mode.13−22 Considering the chain stacking and its assembled structure, they have been widely studied as a template to understand the crystallization habit and the ordered morphology of molecular chains.22−36 Typically, ordered-disordered phase transition and crystal evolution of comb-like polymers are well shown, and they can be regulated by two counterparts from polymer backbone and side alkyl chain.1,2 And, different states of molecular chains, especially for the pended side chains, induce a different crystalline structure and conformation state against the free ones. In our previous studies,18,31,32,34,35,37−47 comb-like polymers with the different polymer backbones and the varied side-chain length had been prepared, and their crystallization and phase transition are deeply characterized through the various techniques. Influenced by the rigidity of the polymer backbone, the packed crystal structure and the crystallizable side-chain length exhibit a strong dependence.18,34,35,39,46,47 In © XXXX American Chemical Society

the meantime, the rigid backbone requires a higher crystallizable carbon atom than that of the flexible one. For the rigid backbone such as poly(p-benzamide),34 poly(pphenylene terephthamide),18,44,48,49 polyaniline50−53 and polypyrrole,39 the critical requirement for crystallizable carbon atoms is at least 10, which is higher than that of the flexible one (at least 6) such as polyethylenimine31,33,35,45 and the polyethylene-like backbone.8,23,54 The incorporated side alkyl chains contribute the different crystalline packing manner in light of the influence of the polymer backbone. Regular ordered packing structure from a rigid backbone is hard to be seen because of the suppressed chain mobility and the reduced crystallinity of the side chain under the rigid-rod nanoconfined condition. In addition, the evolution of mesophase or metastable phase during the phase transition from the ordered to the disordered one provides a good tool to analyze the early stage of polymer crystallization.55−57 Moreover, the pended side alkyl chains onto the polymer backbone also present the different side-chain crystallites, such as orthorhombic phase (βO), hexagonal phase (αH), and monoclinic phase (βM), Received: August 29, 2018 Revised: October 15, 2018

A

DOI: 10.1021/acs.macromol.8b01856 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis Process of PVEM-Cn Comb-like Polymers

2. EXPERIMENTAL SECTION

showing a varied chain stacking structure for the nanoconfined molecular chains. Regulating the chemical junction between polymer backbone and side alkyl chains, i.e., -N-, -S-, -O-, -COO-, and -CONH-, side-chain crystallization ability and crystal structures show a difference. As the chemical junction becomes a larger one such as the -COO- group from poly(n-alkyl acrylate),23,54 only αH phase is found even when the temperature is downed to an extremely low value. This hints that the larger chemical junction impacts the side-chain crystal form. From the previous studies, the polymeric backbone usually is a single component, and the conformational adjustment is mainly influenced by the rigidity of the main chain and the side-chain length. In a recent study, taking poly(styrene-co-maleic anhydride) (SMA) 40 and poly(ethylene-graf t-maleic anhydride) (EMA)38 as polymer backbone, a series of comb-like polymeric phase change materials had been prepared via the esterification reaction with n-alkyl alcohol as the grafted side chains. These two types of polymer structures afford an obvious phase change character, showing an increased phase change temperature and enthalpy against the carbon atoms of side chains. Note that, however, SMAbased comb-like polymers exhibit a lowered thermal performance as compared with EMA-based ones. This is attributed to the varied conformational adjustment ability of the twocomponent backbone and the reduced mobility of nanoconfined alkyl chains. To the best of our knowledge, however, comb-like polymers with a two-component backbone have not been deeply investigated from the aspect of chain packing manner and side-chain crystallization. Therefore, on the basis of our studies of comb-like polymers containing single- or twocomponent polymeric backbone, poly(methyl vinyl ether-altmaleic anhydride) (PVEM) is used as a template to compare the crystallization and phase transition behavior of side-chain crystallites with the previous studies. Herein, in this paper, a series of PVEM-Cn comb-like polymers are synthesized through the esterification reaction between PVEM and n-alkyl alcohol with n changing from n = 14 to 26. Chain structure, side-chain crystallization, and phase transition of PVEM-Cn comb-like polymers are characterized by 1H nuclear magnetic resonance (1H NMR), differential scanning calorimetry (DSC), variable-temperature Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction scattering, as well as solid-state 13C CP/MAS NMR. In the meantime, the chain packing manner and phase transition of PVEM-Cn comb-like polymers is also compared with the previous results, and the influence of structure factor is further clarified.

2.1. Materials. Poly(methyl vinyl ether-alt-maleic anhydride) (PVEM, Mn = 80000) was purchased from Sigma-Aldrich Co. Ltd., and it was dried at 100 °C for 8 h before use. 1-Tetradecyl alcohol (97.7%, C14H29OH), 1-hexadecyl alcohol (99.6%, C16H33OH), 1octadecyl alcohol (97.7%, C 18H 37 OH), 1-docosanol (96.6%, C22H45OH), and 1-hexacosyl alcohol (97.5%, C26H53OH) were purchased from Tianjin Chemical Reagent Co. Ltd. and used without further purification. Tetrahydrofuran (THF, 99.7%) was purchased from Tianjin Guangfu Chemical Reagent Co. Ltd., and it was refluxed with sodium metal before use. Petroleum ether and p-toluene sulfonic acid (PTSA, 98%) were purchased from Alfa and used as received. 2.2. Synthesis of PVEM-Cn Comb-like Polymers. Taking PVEM as the template, the esterification reaction between PVEM and n-alkyl alcohol (n = 14, 16, 18, 22, or 26) is conducted. PVEM was first solved in THF, and then n-alkyl alcohol was added slowly under N2 protection. The reaction was conducted at 80 °C for 24 h with PTSA as a catalyst. The raw product was precipitated into distilled water and rinsed with petroleum ether for 4 times to remove the unreacted n-alkyl alcohol. The purified products were dried for 24 h in a vacuum oven. Scheme 1 presents the detailed reaction process of PVEM-Cn comb-like polymers. 2.3. Characterization. 1H Nuclear Magnetic Resonance Spectroscopy (1H NMR). 1H NMR measurement was carried out with a Bruker DMX 300 MHz NMR spectrometer at 25 °C. Deuterated acetone was used as the solvent and tetramethylsilane as an internal reference. Differential Scanning Calorimetry (DSC). DSC measurements were performed with NETZSCH DSC 200 F3 under N2 conditions. Temperature and enthalpy were calibrated with an indium standard. Samples (ca.10.0 mg) were heated to 120 °C and held for 3 min to remove the thermal history. Subsequently, the thermal cycle between −30 and 120 °C was performed at a scanning rate of 10 °C/min. The second cycle was used to analyze the thermal event of comb-like polymers. Variable-Temperature Fourier Transformed Infrared Spectroscopy (VT-FTIR). A Nicolet iS5 spectrometer was used to characterize the chain packing and conformational variations for PVEM-Cn comblike polymers. Spectra were collected in the range of 4000−500 cm−1 at a 2 cm−1 resolution and 64 scans. Temperature was changed from −30 and 140 °C. At each point, the sample was equilibrated for 5 min before measurements. In-Situ Variable-Temperature Wide- and Small-Angle X-Ray Scattering. WAXS and SAXS measurements were performed on a Xeuss 2.0 SAXS/WAXS system (Xenocs SA, France). X-ray radiation (wavelength = 1.5418 Å) was produced by means of the Cu Kα radiation generator (GeniX3D Cu ULD) at 50 Kv and 0.6 mA. Scattered signals were collected by a semiconductor detector (Pilatus 300 K, DECTRIS, Swiss). The one-dimensional intensity profiles were integrated from background corrected 2D WAXS patterns with an azimuthal angle range of 0−45°. Transmission geometry was adopted for in situ measurements. The temperature was controlled by a Linkam THMS600 hot stage (Linkam Scientific Instruments, UK). Specimens were heated from −30 to 120 °C at 10 °C/min. Specimens were held for 5 min at the selected temperature to stabilize the temperature, and then WAXS B

DOI: 10.1021/acs.macromol.8b01856 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules were obtained with 2 min exposure times and SAXS were obtained with 5 min exposure times. Solid-State 13C CP/MAS NMR Spectroscopy. Solid-state 13C CP/ MAS (cross-polarization/magic angle spinning) NMR experiment was performed on a Bruker AVANCE NEO 400 M spectrometer at 75.4 MHz, operated at magnetic field of 7.05 T and spinning speed of 14 kHz. Five μs 90° pulse was used for 13C NMR spectra, and the contact time for CP process was 1 ms at a recycle delay time of 2 s. Spectra were recorded in the temperature range of 40−120 °C. The chemical shift of adamantine (38.5 ppm) was used as the standard to calibrate the chemical shift.

3. RESULTS AND DISCUSSION 3.1. Chemical Structure. The molecular structure of PVEM-Cn comb-like polymers was analyzed by FTIR and 1H NMR. Figure 1 gives the FTIR spectra of PVEM-C18 comb-

Figure 2. 1H NMR spectrum of PVEM-C18 comb-like polymer.

Table 1. Degree of Substitution for PVEM-Cn Comb-like Polymers Samples

Degree of substitution

PVEM-C14 PVEM-C16 PVEM-C18 PVEM-C22 PVEM-C26

59.1% 61.5% 55.3% 44.7% 39.3%

3.2. Thermal Properties. Comb-like polymer containing longer alkyl side chains exhibits an obvious side-chain crystallization behavior regardless of the rigidity of the main chain and the type of chemical junctions. Figure 3 illustrates

Figure 1. FTIR spectra of PVEM, C18H37OH, and PVEM-C18 comblike polymer.

like polymer, C18H37OH and PVEM. For C18H37OH, a doublet band at 1463/1471 cm−1 and 730/720 cm−1 is shown, which is ascribed to the formed monoclinic crystal structure.58 A singlet band at 1467 and 720 cm−1 appears, indicating PVEM-C18 forms a different side-chain crystallite. In addition, the band disappearance of 1858 and 1784 cm−1 demonstrates the ringopening of maleic anhydride of PVEM, and new bands at 1734 and 1182 cm−1, originated from carboxyl groups and ester groups, confirm the successful grafting of C18 alkyl chains. This demonstrates that PVEM-C18 comb-like polymer has been prepared. To analyze the esterification process of PVEM-Cn comb-like polymer, 1H NMR is used (Figure 2). The chemical shift at 0.96, 1.33, and 3.54 ppm is assigned to CH3, intermediate CH2, and CH2 connected with a -COO- group, respectively.59−61 Moreover, the appearance of 4.08 ppm, characteristic of a -OH group from the PVEM backbone, further demonstrates the grafted C18 alkyl chains via the ring-opening of the maleic anhydride group. The degree of substitution (DS) of alkyl side chains onto the PVEM backbone is calculated through the area ratio of the proton of CH2 groups connected to the ester group (c, 3.54 ppm) and the -OH group (a, 4.08 ppm). The DS results are summarized in Table 1. It can be seen that only part of maleic anhydride groups has been replaced by the C18 alkyl chains. In addition, with the increased side-chain length, the DS gives a decreased phenomenon, which possibly is ascribed to the steric hindrance of longer alkyl side chains.

Figure 3. DSC curves of PVEM, PVEMA, and PVEM-Cn comb-like polymers at a scanning rate of 10 °C/min.

the thermal properties of PVEM-Cn comb-like polymers with n changing from 14 to 26 in the heating and cooling process. Details of phase transition temperature and enthalpy are summarized in Table 2. Before analyzing the thermal performance of PVEM-Cn comb-like polymers, PVEM and hydrolyzed PVEM (PVEMA) are first detected by the DSC method. No thermal event appears in the heating and cooling C

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Macromolecules Table 2. Calorimetric Data of PVEM-Cn Comb-like Polymers PVEM-C14 PVEM-C16 PVEM-C18 PVEM-C22 PVEM-C26

Tm (°C)

Tc (°C)

ΔHm (kJ/mol)

Xc

Nc

30.1 38.8 55.1 69.3 52.6/79.4

16.9 25.3 41.4 58.6 46.9/66.7

17.1 24.8 34.3 44.2 55.0

21.1 28.4 36.5 41.7 46.2

5.6 8.1 11.1 14.3 17.7

run, implying that PVEM is a noncrystalline material. On the contrary, PVEM-Cn comb-like polymers give an obvious thermal event, which is originated from the side-chain crystallites.23,62 With the increased side-chain length, the melting temperature (Tm), the freezing temperature (Tc) and the melting enthalpy (ΔHm) show a linear increment. Tm from 30.1 to 79.4 °C, Tc from 16.9 to 66.7 °C, and ΔHm from 17.1 to 55.0 kJ/mol are well shown. This demonstrates that the increased carbon atoms promote the side-chain crystallization ability and form an ordered crystal structure. This coincides with the previous studies that the longer side chains can easily form an ordered chain structure.31 Different from the other PVEM-Cn series, PVEM-C26 presents a shoulder peak at 52.6 or 46.9 °C in the heating or cooling process, respectively. This shoulder peak demonstrates the formation of regular packed side-chain crystalline structure. In our previous studies, Ndocosylated (PEI22C)33 and N-hexacosylated polyethylenimine (PEI26C)31 comb-like polymer also showed such a shoulder peak, which was ascribed to the transformation from orthorhombic to monoclinic crystal lattice. Against PEI22C and PEI26C comb-like polymer, PVEM-C22 gives a single phase transition temperature, while for PVEM-C26, two transitions appear, indicating that the order−disorder transition and the disorder−amorphous state exist.31,33−35 In the meantime, the flexible PEI backbone induces the methylene chains packing into the regular ordered structure easier than that of PVEM based on the difference in the conformation adjustment and the varied component of polymeric main chains. Additionally, the smaller chemical junction could also influence the packing manner of alkyl chains from the present comparison. To understand the relationship of Tm and the polymeric backbone, Figure 4 plots Tm against the number of carbon atoms per side chain. Combining with fatty alcohol and nalkane, comb-like polymers with the different polymer backbones give the varied Tm results with the increased sidechain length. The details are illustrated in Table 2. For the presented comb-like polymers, single- and double-component polymer backbone are classified, and Tm also shows a difference based on the varied polymer backbones and chemical junction. Interestingly, poly(1-alkyl ethylene) gives the maximum Tm value,8 which is ascribed to the regular chain packing and the directly connected side chains onto polyethylene-based backbone. However, for the other comb-like polymers, their Tm basically stays between fatty alcohol and nalkane. The chemical junction indicates an influence on the chain packing of alkyl chains. As compared with the singlecomponent polymer backbone, the double-component one shows a higher Tm and a larger spatial distance for the pended side alkyl chains. Meantime, from the results of EMA-Cn,38 SMA-Cn,40 and PVEM-Cn, the flexible EMA backbone allows much more CH2 groups entering into the side-chain crystallites, offering a high Tm value. For SMA-Cn and

Figure 4. Tm against the carbon atom number for poly(1-alkyl ethylene), poly(n-alkyl acrylate), PEI(n)C, SMA-Cn, EMA-Cn, and PVEM-Cn comb-like polymers.

PVEM-Cn, they show a similar polymer backbone, as shown in Scheme 2, and the SMA backbone is much more rigid than that of PVEM. Thus, Tm shows a decreased value due to the disturbance of branched groups such as benzene ring and -OCH3 on the chain packing. Additionally, although the branched -OCH3 group is relatively more flexible than that of benzene ring onto the SMA backbone, the Tm value of PVEMC14 and PVEM-C16 is lower than that of SMA-C14 and SMAC16. The details are not very clear, and some study needs to be done. In fact, the double-component polymer backbone hardly controls the distribution and the grafting degree of the side chain along the backbone, as compared with the singlecomponent one. For the isotactic poly(1-alkyl ethylene)8 and poly(n-alkyl acrylate),54 Tm is much higher than that of PVEMCn and EMA-Cn. Furthermore, the distribution of side chain along the polymer backbone and the tacticity also are demonstrated to have an important influence on the sidechain crystallization behavior and its chain packing mode. Subsequently, the melt enthalpy (ΔHm) against the carbon number (n) of alkyl side-chain is plotted in Figure 5 according to eq 1,34,35,39 where k is the contribution of each added CH2 group to enthalpy, and ΔHm,e is a constant reflecting the contribution of chain end to enthalpy. ΔHm = nk + ΔHm,e

(1)

Through the fitted result, the slope, k, is 3.1 kJ/mol·CH2. According to the previous studies,34,35,39,62 the slope k reflects the chain packing manners of alkyl groups. Typically, if the k value is around 3.99 or 4.2 kJ/mol·CH2, the alkyl side chains will assemble into a rhombic or triclinic phase. When the k value is close to 3.07 kJ/mol·CH2, the hexagonal phase (αH) is a major one. PVEM-Cn comb-like polymers should be packed into αH phase, which coincides with the FTIR result. That is to say, only a part of the pended side alkyl groups, which are away from the polymer backbone, take part in the formation of the crystalline structure. In the following, the crystallizable carbon atoms (Nc) and crystallinity (Xc) are calculated by eqs 2 and 3, respectively. These results are listed in Table 2.

D

Nc = ΔHm/k

(2)

Xc = Nc × 14.026/M unit

(3) DOI: 10.1021/acs.macromol.8b01856 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 2. Series of Comb-like Polymers with the Different Polymer Backbones and the Chemical Junctions

Figure 6. WAXD patterns of PVEM and PVEM-Cn comb-like polymers with the varied side-chain length.

Figure 5. Melt enthalpy of PVEM-Cn vs the carbon number of alkyl chains. The “■” values are experimental data, and the red line is a linear fit with a slope of 3.1 kJ/mol per CH2 group.

comb-like polymers at room temperature. PVEM-Cn comblike polymers illustrate an obvious crystalline structure, showing a sharp crystal diffraction peak at 21.5° against PVEM. This crystalline structure for PVEM-Cn comb-like polymers is mainly originated from the introduced alkyl side chains, assembling a side-chain crystallite between the PVEM backbones. Moreover, with the carbon atoms of side chain changing from n = 14 to 22, a singlet peak at 21.5° appears, characterizing αH phase packing.23 This is in agreement with the fitted slope k, demonstrating the formation of the αH phase for the pended alkyl chains. However, two diffraction peaks at 21.5 and 24° appear for PVEM-C26 comb-like polymer, supporting the formation of the orthorhombic (βO) form. Similarly, the double peaks in the DSC curve and the double bands at 719/730 cm−1 further prove this point. The sidechain length greatly influences the chain packing manners of PVEM-Cn and the form of side-chain crystallites. The longer the side-chain length is, the easier the packing structure of alkyl chains. Under the free conformation adjustment of polymer backbone, much more CH2 groups are allowed to pack into a crystalline state in a nanoscale domain. Figure 7 gives the variable-temperature wide-angle X-ray scattering (WAXS) results for PVEM-C26 with temperature changing from −30 to 100 °C. Clearly, two diffraction peaks at 21.7 and 24.3° appear, characteristic of βO form. Up to 65 °C, the peak at 24.3° gradually disappears, and only one peak at

As shown in Table 2, a linear increment for Nc and Xc is found, and Nc increases from 5.6 for PVEM-C14 to 17.7 for PVEM-C26. A minimum requirement for side-chain length in PVEM-Cn is 9 CH2, and this also implies the rigid backbone of PVEM. The side-chain crystallites show a varied Xc from 21.1% for PVEM-C14 to 46.2% for PVEM-C26. This further confirms that only a part of the pended side alkyl chains participates in the formation of crystal structure. Besides, the present Nc and Xc for PVEM-Cn comb-like polymers is smaller than that of PEI(n)C35 but is higher than that of PBA(n)C34 and PPyCn.39 This indicates that the double-component PVEM backbone is more flexible than the PBA and PPy backbone because of the polyethylene-based molecular backbone. Then, CH2 groups entering into the crystal lattice are higher, and the conformational adjustment ability of the PVEM main chain also promotes the crystallization of alkyl chains. In combination with our previous studies, a conclusion can be drawn that the conformational state of the polymer backbone exerts an important effect on the side-chain crystallites formed by the pended long alkyl side chains. 3.3. Chain Packing Structure. Comb-like polymers with different polymer backbones and side-chain lengths have varied chain packing manners. And, the side-chain crystal form is influenced by the structural parameters of comb-like polymers. Figure 6 presents the XRD patterns of PVEM and PVEM-Cn E

DOI: 10.1021/acs.macromol.8b01856 Macromolecules XXXX, XXX, XXX−XXX

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Figure 7. Variable-temperature WAXS profiles for PVEM-C26 in the heating process from −30 to 100 °C.

Figure 8. Variable-temperature SAXS patterns (a); the varied long periodicity for PVEM-C26 in the heating process from 30 to 120 °C (b).

21.4° remains, characteristic of αH, demonstrating the transition from βO to αH phase. This transition temperature keeps a good agreement with that of the DSC result, and the first endothermic peak is ascribed to the transition from βO to αH phase. Subsequently, the singlet peak at 21.4°, characteristic of the αH phase, begins to decrease with the increased temperature. It completely disappears when temperature is beyond 85 °C, demonstrating that the side-chain crystallite has entered into the amorphous state. That is to say, transition from αH phase to amorphous state is completed. Similarly, PEI26C31 and PEI22C33 both exhibit multiple phase transition from βO, βM, to αH phase, and then to amorphous state. This proves the flexible polymer backbone easily promotes the regular packing structure of the side alkyl groups, and the mesophase is the prerequisite phase before assembling a regular crystal form. Against PPy-C26,39 only the αH phase is seen even when the temperature is decreased to a lower temperature. This explains that the rigid polymer backbone prohibits the regular form of alkyl chains, and its conformation adjustment is harder than that of the flexible one. At the same time, on the basis of the nanosized crystal structure separated by the polymer backbones, the side-chain crystallites are highly dependent on the crystallizable carbon atoms and the conformation adjustment of the polymer backbone. Through in situ variable-temperature SAXS experiment, PVEM-C26 comb-like polymer is characterized with temperature changing from 30 to 120 °C in the heating process. Figure 8a presents the SAXS patterns, and Figure 8b gives the varied long periodicity of PVEM-C26 comb-like polymer. Only a scattering peak is shown, and it exhibits a linear temperaturedependence behavior. Before 60 °C, the long periodicity (d) changes little, similar to the qmax, showing a d-spacing at 7.16 nm, and up to 75 °C, the d-spacing decreases to 7.02 nm. Based on the DSC and WAXS results, the decreased d-spacing is mainly ascribed to the crystallite transition from βO to αH. Although the varied crystal lattice is shown, the protection of the PVEM backbone is stable, and then the d-spacing changes little. As temperature further increases to 80 °C, the d-spacing shows a minimum value at 6.68 nm. This explains the disappearance of the αH phase. The molten side alkyl chains between PVEM backbones induce the distortion of packing manner, and so the d-spacing shows a certain decrease. With temperature further increasing, d-spacing begins to increase due to the volume expansion of the fully molten side chain and the PVEM backbone. Up to 120 °C, d-spacing reaches a

maximum, 8.06 nm. However, note that, the PVEM backbone shows a good structure stability below 120 °C, and then the PVEM-Cn comb-like polymers keep a good form-stable state at the transition from βO to the αH phase and from the αH phase to the isotropic state for the side-chain crystallites. The slight varied d-spacing before 80 °C also demonstrates the different forms of βO and αH crystal lattice, and αH is thought a metastable phase before the ordered packing structure. In addition, when the temperature is 120 °C, which is far above the melting temperature of C26 side-chain crystallites, the q value also can be seen for PVEM-C26 comb-like polymer. This indicates the existence of microphase separation, which is ascribed to the bad compatibility between the residual hydrophilic COOH groups and the introduced hydrophobic alkyl chains. 3.4. Phase Transition and Conformational Behavior. To clarify the phase transition behavior and the conformation variations, variable-temperature FTIR experiments are performed for PVEM-C26 comb-like polymer. Figure 9 presents the rocking band of CH2 sequences for PVEM-C26 in the heating process with temperature changing from −30 to 140 °C. A doublet band at 719/731 cm−1 is shown, which is

Figure 9. Rocking band of CH2 sequences from PVEM-C26 comblike polymer in the heating process with temperature changing from −30 to 140 °C. F

DOI: 10.1021/acs.macromol.8b01856 Macromolecules XXXX, XXX, XXX−XXX

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Figure 10. Conformational band of CH2 sequences for PVEM-C26 comb-like polymer in the heating process with temperature changing from −30 to 140 °C (a); the enlargement spectra for the conformational band at the assigned temperature from 35 to 65 °C.

Figure 11. Combined phase transition process of rocking band (a, b) and bending band (c, d) from FTIR and DSC result for PVEM-C26 comblike polymer during the heating process.

Actually, PVEM-C18 comb-like polymer does not present βO crystallite even when the temperature is decreased to a lower one. This explains that the side-chain length greatly influences the chain packing manners. Conformational variations for PVEM-C26 comb-like polymer are presented in Figure 10a with temperature increasing from −30 to 140 °C. Obvious conformation changes are observed on the basis of the absorption band and its intensity (Figure 11c and d). Generally, the conformational ordered bands at 1473 and 1462 cm−1, characteristic of the trans band,31,34,35,39,47 exhibit a temperature-dependent variation. Below 40 °C, conformational ordered bands are stable, which is similar to the rocking band shown in Figure 9. This confirms that the βO phase is mainly composed by the trans conformational ordered bands, and two molecular chains are included in one crystal lattice. Once when the temperature is above 40 °C, as shown in Figure 10b, the enlargement spectra between 35 and 65 °C clearly demonstrate the conformation variation from the ordered to the disordered state. Moreover,

characteristic of βO phase formed by CH2 chains.31,47 This splitting of rocking band is contributed from the intermolecular interaction in a crystal lattice.63−65 With temperature increasing, the absorption intensity gradually decreases, and the band shows a downshift for 731 cm−1, followed by an upshift of 719 cm−1. Systematic variations for the rocking band are plotted in Figure 11a and b. Combined with the results of rocking band and DSC, four transition regions are marked. Region I is a stable ordered βO phase below 40 °C, and region II is ascribed to the crystal transition from βO to αH phase between 40 and 60 °C, which is in good agreement with that of DSC and X-ray results. From the DSC result of PVEM-C26, the first endothermic peak at 52.6 °C is originated from the crystal lattice transformation from βO to αH phase.66 The following region III between 60 and 90 °C is from αH phase to amorphous state, and the last region IV is the amorphous state. For the shown phase transition region in Figure 11a and b, the long C26 alkyl side chains onto the PVEM backbone give a more regular ordered packing manner than the short ones. G

DOI: 10.1021/acs.macromol.8b01856 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules the trans conformational ordered band begins to transform into the trans disordered one, followed by the disappearance of 1463 cm−1 and the appearance of a conformational disordered band at 1467 cm−1 and the gauche band at 1457 cm−1. However, as compared with the rocking band, a different process is seen. The rocking bands still are stable at the beginning of 40 °C marked by the blue line in Figure 9, while the trans conformational ordered band transforms to the trans conformational disordered one. These conformational variations are basically similar to C21H44 and low molecular weight PE glasses,67 constrained PE,68 and PEI(n)C series.31,33,35,45 This is to say, the conformational regular molecular chain gradually enters into the irregular one even though the crystal lattice still is in the βO phase. This indicates that the conformational band is sensitive to the chain packing and the structural variation and can be used as a structure probe. Up to 65 °C, both the rocking band and the conformational band complete their variations. Generally, before the regular crystal structure such as βO phase transforming into the irregular one, the conformational variation of molecular chains first undergoes, and subsequently the crystal lattice transformation follows. The changes of absorption band and its intensity of conformational band are plotted in Figure 11c and d. The same four-temperature region for the conformational band is observed, and they are highly in agreement with that of the DSC result of PVEM-C26. Correspondingly, from the combined results of DSC and conformational bands, the trans conformational ordered band (the regular packed crystal lattice) is first transformed to the trans conformational disordered one (the irregular one), and then to the amorphous state during the transition from βO to αH phase (region II) and from αH phase to amorphous state (region III). According to the early state of polymer crystallization, the mesophase is transient, and it is dependent upon the lamellae size. For comb-like polymers, regulating the side-chain length can realize the different chain packing and ordered crystal state. And, the αH mesophase is composed by the randomly arrayed trans and gauche sequences.69,70 Therefore, by controlling the side-chain length and the rigidity of polymer backbone, the size-dependent metastable crystal form is suitable to analyze the polymer crystallization with comb-like polymer as template. Solid-state NMR spectroscopy is a powerful technique to elucidate the conformation behavior of a molecular chain for polymeric materials, and here it is used to understand the transition process of PVEM-C26. According to the previous studies,32,46,71 the chemical shifts of 30, 33, and 34 ppm corresponds to the different conformation states for trans and gauche conformers in amorphous, orthorhombic, and triclinic forms, respectively. Figure 12 presents the variable-temperature 13C CP/MAS NMR spectra for PVEM-C26 comb-like polymer, and the peak assignments are labeled. Due to the experiment limitation, 13C CP/MAS NMR measurements are performed within temperature from 46 to 120 °C. Below 71 °C, only the trans conformation at 32.4 ppm exists, indicating that the βO crystalline form is stable. Note that, however, the transition of the βO phase is postponed due to the slow temperature exchanging with the NMR heater, as compared with FTIR results. When the temperature is above 71 °C, a minor peak at 29.4 ppm coexists with the main peak at 32.4 ppm, indicating that the trans conformation gradually transforms into the gauche one. At 83 °C, the polymethylene chains

Figure 12. Solid-state 13C CP/MAS NMR spectra of PVEM-C26 as a function of temperature.

undergo a fast transition between the trans and the gauche conformers. This temperature is in good coincide with that of DSC, X-ray, and FTIR results. With temperature further increasing to 120 °C, the trans conformation entirely transforms into the gauche conformation. In addition, the peaks characteristic of α-CH2 and β-CH2 groups still can be found, followed by a slight upfield shift. This possibly is attributed to the ordered−disordered transition behavior of side alkyl chains. The conformational transition of PVEM-C26 described by 13C CP/MAS NMR is similar to that of PEI20C,46 poly(α-alkyl β, L-aspartate)s,72 and poly(α-alkyl γglutamate)s.71 And, the combined characterization techniques such as DSC, X-ray, FTIR, and solid-state 13C NMR confirm the phase transition process undergoes a transformation from trans to gauche conformation, and then enters into the amorphous state. Therefore, the present study further supports that the formation and evolution of mesophase for comb-like polymer can be well regulated by controlling the polymer backbone, the side alkyl chain, and the chemical junction.

4. CONCLUSIONS A series of poly(methyl vinyl ether-alt-maleic anhydride)-galkyl alcohol comb-like polymers (PVEM-Cn, n = 14, 16, 18, 22, 26) are successfully prepared through the esterification process. The incorporated alkyl groups assemble the side-chain crystallites, and show an obvious thermal behavior for PVEMCn comb-like polymers. Their phase change temperature, Tm and Tc, and enthalpy, ΔHm, depend on the side-chain size, and the double-component polymer backbone exhibits a higher Tm than that of the single-component one. A minimum requirement for the crystallizable CH2 groups is 9, and the αH phase is a dominant crystal form for PVEM-Cn comb-like polymers. Various characterizations demonstrate the transition from βO to αH phase and from αH phase to amorphous state. The regular trans conformation state first transforms into trans conformational disordered state, and then to irregular trans− gauche coexisting state during the phase transition process, and this is beneficial to understand the evolution of metastable αH phase, especially for the chain packing of comb-like polymer and the early stage of polymer crystallization.

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*E-mail: [email protected] (H. Shi). H

DOI: 10.1021/acs.macromol.8b01856 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules ORCID

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Haifeng Shi: 0000-0001-8421-0002 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Grant Nos. 21875163 and 21404080), National Key R&D Program of China (Grant No. 2017YFB0309100), and the Key Project of Tianjin Municipal Natural Science Foundation (Grant No. 16JCZDJC37000). Also, Dr. G. Liu is thanked for his discussion on SAXS results (ICCAS). Dr. X. Wang and Dr. L. Shan are thanked for their help to conduct the solid-state 13C CP/MAS NMR experiments at Bruker (Beijing) Scientific Technology Co. Ltd.

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DOI: 10.1021/acs.macromol.8b01856 Macromolecules XXXX, XXX, XXX−XXX