Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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0 + 0 = 2: Changeover of Stability and Photopolymerization Kinetics for the Rotator Phase of Long-Chain Acrylate through the UltraAddition Effect in Binary Systems Miao Yao, Jun Nie, and Yong He* College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China
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S Supporting Information *
ABSTRACT: The stability and lowest existing temperature for the rotator phase of long-chain acrylate were improved remarkably simply through the ultra-addition effect of physical blending of two long-chain acrylates, which leads to a wider operation window and better rotator-state photopolymerization. Hexadecyl acrylate (HDA) and tetradecyl acrylate (TDA) were proved existing rotator phase like previously reported octadecyl acrylate (ODA). The binary rotator phase systems were constructed by mixing HDA or TDA with ODA and investigated in detail through thermal analysis, X-ray diffraction, and photopolymerization kinetics. The chain-reaction photopolymerization conversion of the binary system significantly increased to 60% from near 0 for pure acrylate, which realized “0 + 0 = 2”. The mechanism of such an ultra-addition effect was explained on the basis of X-ray diffraction data and calculation of the geometric model. The effect of difference in chain length between two components on this enhancement was studied, and a threshold value was found.
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rotations.15,16 The carbon−carbon double bonds of acrylates in rotator phases can approach each other through the rotation of molecules along its long axis, which is the key point that ensures the chain radical propagates successfully to achieve polymerization. Octadecyl acrylate exists in four solid states: rotator II (RII), rotator I (RI), orthorhombic crystal (Cort), and triclinic crystal (C tri ) phases. 14 The C tri phase can be obtained by crystallization from solution, while the RII, RI, and Cort phases appear in the order from molten sample during the cooling process. Although the high polymerization conversions of octadecyl acrylate could be achieved for both rotator phases, the RI phase (lower temperature rotator phase) was unstable and can transform into the Cort phase very quickly during the isothermal process,14 which results in a very narrow time window to perform effective photopolymerization under the temperature lower than the RII → RI transition point (10.88 °C). What is worse is that the Cort cannot transform back to
INTRODUCTION Crystalline-state photopolymerization has aroused widespread interest of scientists due to its advantages of region-specific, stereospecific, and structural controllability.1 Crystalline-state chain-reaction photopolymerization could find more application potential due to the versatility of product properties, although it is less studied than crystalline-state step-reaction photopolymerization based on [2 + 2] and [4 + 4] cycloaddition reactions.2−6 Crystalline-state chain-reaction photopolymerization was first reported by Restaino7 for acrylic salts irradiated by γ-rays; then, a series of works on crystallinestate photopolymerization irradiated by γ-rays or UV light were published, which primarily concentrated on monomers with long alkyl chains including octadecyl acrylate, octadecyl methacrylate, and octadecyl vinyl ether.8−13 However, a question for this kind of process is why do crystalline-state monomers possess so high a capability of molecular motion in photopolymerization? We recently proved that the polymerization of octadecyl acrylate actually occurs in the rotator phase, not in the crystalline phase.14 Rotator phases are a series of special condensed phases between fully ordered crystal and isotropic liquid, which can perform free or hindered © XXXX American Chemical Society
Received: June 13, 2018 Revised: July 16, 2018
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DOI: 10.1021/acs.macromol.8b01263 Macromolecules XXXX, XXX, XXX−XXX
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RESULTS AND DISCUSSION In binary system of n-alkanes reported, both components possess rotator phases, which ensure the homogeneity of mixture and decrease the transition temperature from rotator phase to crystalline phase (TR→C). In this work, we suppose to adopt three shorter chain length acrylates, hexadecyl acrylate, tetradecyl acrylate, and dodecyl acrylate mixed with octadecyl acrylate to construct binary systems. The existing of the rotator phase of octadecyl acrylate was revealed;14 thus, whether other three long-chain acrylates possess rotator phase should be confirmed first. Polymorphic Behavior Analysis of Hexadecyl, Tetradecyl, and Dodecyl Acrylates. The thermal analysis of hexadecyl and tetradecyl acrylates are shown in Figure 1.
the RI phase during heating process but changes to the RII phase directly at relatively higher temperature (22 °C). Enlarging the temperature window and enhancing the stability of the RI phase are necessary to realize the low-temperature rotator phase photopolymerization. For n-alkanes, adding one n-alkane into another n-alkane with approximate length of molecule chain can decrease the transition temperature from the rotator phase to the crystalline phase of the mixture, and the reason was explained by Mukherjee within Landau theory.17 For example, Nowak18 mixed two n-alkanes (C21H44 and C23H48) together and found that the transition temperature from the rotator phase to the crystalline phase (about 23 °C for the mixture of C21H44:C23H48 in a 79:21 molar ratio) was lower than that for pure n-alkane (about 32.5 °C for C21H44 and 40 °C for C23H48). Inspired by the above works, one might infer the binary long-chain acrylates systems to show a broad temperature region of the RI phase. Because the difference in number of gauche conformations between the two components of mixture may change the phase transition character from rotator phase to crystalline phase, which could stabilize the RI phase and supply a wider time window to achieve more effective rotator phase photopolymerization in low temperature. In this paper, we first verified the existence of rotator phases in hexadecyl and tetradecyl acrylate and then employed each of which constructed the binary systems with octadecyl acrylate. It was found that this strategy can improve the stability of RI phase remarkably, and the stabilizing mechanism was explained, which subsequently led to more effective photopolymerization. The effect of the chain length difference between the two components on phase behavior was also studied, and the regularity was worked out. This work is aiming to broaden the temperature range of rotator phase photopolymerization with a view to a deeper understanding of this system and widen the application in the field of photopolymerization under special situations or to achieve high-resolution manufacturing.
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Article
EXPERIMENTAL SECTION Figure 1. Differential scanning calorimeter analysis of hexadecyl, tetradecyl, and dodecyl acrylates. Scanning rate: 2 °C/min.
Materials. Dodecyl acrylate, tetradecyl acrylate, hexadecyl acrylate, and octadecyl acrylate were purchased from TCI (Tokyo, Japan). The type I free radical photoinitiator ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate (TPOL) was obtained from High-tech Insight Co. Ltd. (Beijing, China). All the chemicals were used without further purification. Characterization. Thermal analysis was carried out by differential scanning calorimeter (DSC) (Q2000, TA Instruments, New Castle, DE). The sample weight was about 5 mg, and the scanning rate was 2 °C/min. Photopolymerization was studied by using photo-DSC (Q2000, TA Instruments, New Castle, DE). Approximately 5 mg of sample (monomers and 1 wt % TPOL) was placed as a thin layer in a standard aluminum pan. The sample was irradiated by a 385 nm lightemitting diode (LED) light source (UVEC-4II, Lamplic Technology, Shenzhen, China), and the intensity was 6 mW/cm2. The conversions of photopolymerization were calculated according to the equation C = ΔHt/ΔH0theor, where ΔHt is the reaction heat evolved at time t and ΔH0theor is the theoretical heat for complete conversion. The ΔH0theor was calculated from the equation ΔH0theor = ( fΔHf)/M, where f is the functionality and M is the molecular weight, and for acrylate double bonds, the ΔHf is 80256 J/mol.19 Temperature-dependent X-ray diffraction experiments were performed on an X-ray diffractometer (TTRAX III, Rigaku, Japan) over a temperature range of −35 to 30 °C, using Cu Kα radiation (1.54 Å), power of 200 mA/40 kV, and rotating angle 2θ = 3°−40°.
Hexadecyl acrylate exhibits three solid phases according to differential scanning calorimeter (DSC) results, which are referred as α-, β-, and γ-phase. During cooling the molten sample down to −20 °C, a main exothermic peak is followed by a small and a medium exothermic peak, which represents the process of liquid → α, α → β, and β → γ, respectively. On the heating process, a medium endothermic peak is found followed by the main endothermic peak, which represents the γ-phase transformed into α-phase and then α-phase melted. The polymorphic behavior of hexadecyl acrylate is similar to that of octadecyl acrylate.9,12,14 The X-ray diffraction (XRD) patterns for the α-, β-, and γ-phase of hexadecyl acrylate are shown in Figure S1a, and three solid state were identified based on them, in which the α-, β-, and γ-phase of hexadecyl acrylate are rotator II (RII), rotator I (RI), and orthorhombic crystalline (Cort) phase, respectively. The detailed identification process was concluded in the Supporting Information. Tetradecyl acrylate was proved four solid phases through the DSC test, which are named the α-, β-, γ-, and δ-phase (Figure 1). Similar to hexadecyl acrylate, during the cooling of the B
DOI: 10.1021/acs.macromol.8b01263 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules molten tetradecyl acrylate to −50 °C, a main exothermic peak is followed by a small and a medium exothermic peak, which represents the processes of liquid → α, α → β, and β → γ, respectively. In the heating process, a broad exothermic peak appeared followed by a large endothermic peak, which means the γ-phase of tetradecyl acrylate first transformed into a lower energy phase (δ-phase) and then δ-phase melted. The α-, γ-, and δ-phases were also identified as RII, Cort, and Ctri phase, respectively, based on XRD patterns (Figure S1b). The βphase of tetradecyl acrylate failed to be identified because it was too unstable to capture the XRD pattern. The detailed identification process of tetradecyl acrylate is also included in the Supporting Information. The polymorphic behavior of dodecyl acrylate was similar to that of tetradecyl acrylate, which also exhibits four phases: α-, β-, γ, and δ-phase (Figure 1). Unfortunately, the α- and βphase were unstable and cannot be captured in XRD test to verify whether they were rotator phases. In addition, it was reported that for binary mixture of n-alkanes the mixtures are homogeneous if the longer chain length carbon number is ≤1.22 times the number of carbons in the shorter chain.20 The multiples are 1.13, 1.29, and 1.50 for the octadecyl mixed with hexadecyl, tetradecyl, and dodecyl, which implies that the mixture of octadecyl acrylate/dodecyl acrylate (ODA/DDA) may phase separate, and it was confirmed through thermal analysis. While the binary system of ODA/DDA cooled from the molten state, the ODA in the mixture first solidified followed by a small phase transition, which are considered as liquid → RII and RII → RI processes (Figure S2). The solidification and phase transition temperatures are lower than that of pure ODA because the existing liquid DDA dilutes the ODA and delays the solidification. Upon continued cooling, the DDA solidified at the temperature that pure DDA did, which confirmed that complete phase separation occurred in the mixture of ODA/DDA. Thus, dodecyl acrylate was abandoned to be studied in detail in phase analysis. In these three long-chain acrylates (C18, C16, and C14), decreasing the carbon number of alkyl groups can drop the melting point and TR→C, but the negative effect that the temperature range of rotator phase is narrower was also observed (even for tetradecyl acrylate, the RII → RI transition peak is overlapping with the RI → Cort transition peak), which is unacceptable. Thus, decreasing the TR→C and enlarging the temperature range of rotator phase cannot be realized simultaneously in pure long-chain acrylate systems, which makes the binary system to be the most logical candidate. Binary System of Octadecyl Acrylate/Hexadecyl Acrylate (ODA/HDA). The thermal analysis results of the binary systems of ODA/HDA with different ratios are summarized in Figure 2. All the mixtures possess a single melting point, which indicates that the mixtures are homogeneous and no phase separation existed. The mixtures possess three solid phases, inferred as RII, RI, and Cort phase, by analogizing with pure octadecyl acrylate, which will be confirmed in the following text. The RI → Cort transition points decreased indeed as expected, and the difference is about 1.4−10.7 °C with the content of hexadecyl acrylate increasing. What is surprising is that the Cort phase of mixtures can totally transform back to the RI phase during the heating process while the molar percentage of octadecyl acrylate was 20−70%, which exhibits the ultra-addition effect. The RI phase of a mixture with 50% molar percentage of octadecyl acrylate showed very good stability, and no change was observed even
Figure 2. Phase behavior of binary mixture of octadecyl acrylate/ hexadecyl acrylate in different proportions during the (a) cooling process and (b) heating process. The hollow points represent the peak temperature of the phase transition in the DSC test.
after 3 h isothermal process at −10 °C (Figure S3), while the RI phase of the pure octadecyl acrylate started transforming into the Cort phase in 4 min and almost completed in 30 min.14 For the mixture the molar percent of octadecyl acrylate is 10%, 80%, and 90%; the Cort phase can partially transform back to the RI phase, and the rest of the Cort phase directly transform to the RII phase as the pure octadecyl acrylate and hexadecyl acrylate. This is because that while the content of one component of mixture was too low, it is not enough to affect the whole system to appear as a mixture, but the rest just act as pure compounds. In mixture, that the existence of the Cort → RI transition point is lower than the temperature range of the RI phase is the key property leading to the ultra-addition effect which results in better stability of the RI phase (i.e., not to transform into the Cort phase during the isothermal process). In the thermal analysis, it was found that the enthalpy of the RI → Cort process of the binary system of ODA/HDA during cooling process was much lower than those of pure octadecyl acrylate and hexadecyl acrylate (Table 1). The low enthalpy means that the energy difference between the RI phase and Cort phase is small (5−9 J/g), which suggested that the phase structures of the Cort phase might be close to those of the RI phase in the binary system. The XRD pattern of the binary system of ODA/HDA (molar ratio is 5/5) is shown in Figure 3, and each phase possesses one group of diffraction peaks, further proving no phase separation existed. The area per molecule (as viewed along the chain axis) is calculated as 19.08 and 18.76 Å2 at −5 and −30 °C, which confirmed that the mixture was indeed in C
DOI: 10.1021/acs.macromol.8b01263 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Enthalpy of RI → Cort Process of Binary Mixture of Octadecyl Acrylate/Hexadecyl Acrylate (ODA/HDA)a ODA/HDA enthalpy (J/g)
0/10 31.28
1/9 5.97
2/8 8.55
3/7 8.32
4/6 7.38
5/5 7.27
6/4 7.68
7/3 7.68
8/2 8.54
9/1 14.09
10/0 27.23
a
The molar ratios of ODA/HDA are in the range from 0/10 to 10/0.
rotation of bond 1 just changes the position of atoms hydrogen of the methyl group but makes no sense on reducing the length of molecules. With the rotation of bond 2, the extended molecule bends at the end, the scale of the molecule along the x direction decreases, and the scale of the molecule along the y direction increases. While the rotation angle is 40°, the molecule length decreases about 0.18 Å (Figure 4c), and the scale of molecule perpendicular to the alkyl chain increases about 1.06 Å. This expansion based on the molecular scale restricts the shrinkage of lattice; thus, the Cort phase of mixture cannot be similar to that of pure octadecyl acrylate. The mechanism of this ultra-addition effect was concluded as follows: the difference of the molecular length between two components in mixture can bring a little larger spacing that allows the end of the molecule to deform more easily and then inhibits the shrinkage of lattice during the RI → Cort process, which results in the similar structure of the Cort phase as the RI phase. Because of this structural similarity, the energy of the Cort → RI process during heating process is small, and the Cort phase could be easily transformed into the RI phase in the heating process, which, to some extent, stabilizes the RI phase. The photopolymerization kinetics was studied using photoDSC, and the conversion of pure and mixed acrylates is shown in Figure 5. While directly cooled to 5 °C and immediately exposed to UV light, either pure acrylates or binary systems exhibited high conversions because they were all in rotator phases and did not have enough time to transform into the crystalline phase. With the content of octadecyl acrylate increasing, the conversion of the binary system decreased because adding octadecyl acrylate can make the system change from the RII to RI phase at 5 °C, and the RII phase possessed a relatively higher polymerization ability than the RI phase. Furthermore, the samples were cooled to the crystalline state and then heated to 5 °C to simulate the long isothermal process at 5 °C to study the effect of stability of the RI phase on photopolymerization. As shown in Figure 5, the conversions of the binary system are far higher than those of pure acrylates because the Cort of mixtures can transform back to the RI phase during the heating process, while pure acrylate does not. For the mixture that the molar percent of octadecyl acrylate are 20−70%, the conversion in the RI phase that heated from the Cort phase was close to that cooled from the RII phase, which suggests the RI phases that generated through two ways have similar abilities for polymerization. The rotator phase photopolymerization abilities at low temperature of each component of the binary system were significantly improved accompanied by a much wider time window from minutes to hours just through physical blending, which realized “0 + 0 = 2”. In this case, the conformational disorders at the end of chains, which were usually negatively regarded as defects in other works, were utilized positively. In the binary system of ODA/HDA, the difference in carbon number of the alkyl group between two components is two, and we inferred that the systems with higher difference values may also possess such an ultra-addition effect, but a threshold value should exist. Thus, the binary systems of octadecyl acrylate/tetradecyl acrylate and octadecyl acrylate/dodecyl
Figure 3. X-ray diffraction pattern of binary mixture of octadecyl acrylate/hexadecyl acrylate (molar ratio was 5/5).
the RI and Cort phase at −5 and −30 °C, respectively,14 and is consistent with the result of DSC. From the XRD pattern, it is obviously found that the phase structure of Cort is very close to that of RI. The phase structure calculated from XRD data are shown in Figure 4a, in which the structure parameters of the RI phase of pure octadecyl acrylate and mixture are similar (0.6% and 3.0% difference in y and z direction), and the major distinction lies in the Cort phase (23.6% and 17.6% difference in y and z direction). During the transition process from RI to Cort, molecules in mixture just need to expand a little bit (0.13 Å) along the z direction, while the molecules in pure octadecyl acrylate have to expand 1.52 Å and shrink 0.93 Å along z and y directions, respectively. It is clear that much lower enthalpies of the RI → Cort process of the binary system than that of pure octadecyl acrylate resulted from close structure between RI and Cort being the essence to the existence of the Cort → RI transition point during the heating process, which leads to a stable RI phase. Further, the reason that led to such a large difference of the Cort phase between pure acrylate and mixture is the next key point to explore for comprehension of the ultra-addition effect. While studying the XRD pattern of mixture in more detail, it was found that the diffraction peak that corresponds to the second order of spacing 29.60 Å (the thickness of one lattice layer) was close to that of pure octadecyl acrylate (29.78 Å)14 and much further away from that of pure hexadecyl acrylate (27.34 Å). Snyder21 reported the conformational disordering of the binary system of C30H62/C36H74 and observed chain disorder mostly in their ends. By the measurement in number of gauche bonds, they also found that the conformational disorder in the longer chain was about twice that of shorter chains. In our work, the spacing difference (0.18 Å) between the mixture (ODA/HDA = 5/5) and pure octadecyl acrylate indicated that the gauche conformation might exist at the end of the molecular chain as n-alkanes. Assuming that the acrylate groups of two components are aligned in the mixture because of electrostatic interaction,22 the last two C−C bonds at the end of octadecyl (named bond 1 and bond 2 in Figure 4b) may have a relatively high freedom of rotation ability due to the difference in chain length between the two different alkyls. The D
DOI: 10.1021/acs.macromol.8b01263 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. (a) Diagram of phase transition from RI phase (black) to Cort phase (yellow). (b) Conformation of octadecyl acrylate while bond 2 rotates. (c) Plot of molecular length vs rotation angle of bond 2.
acrylate are possible candidates, for which the difference values are four and six, respectively. However, it was confirmed above that octadecyl acrylate and dodecyl acrylate were immiscible, and complete phase separation occurred; thus, only the binary system of octadecyl acrylate/tetradecyl acrylate will be discussed in the following. Binary Mixture of Octadecyl Acrylate/Tetradecyl Acrylate (ODA/TDA). The polymorphic behavior of ODA/ TDA system (Figure 6) was similar to that of ODA/HDA, and the detailed characterization was concluded in the Supporting Information. All the mixtures possess two close solidification temperatures, but most of them showed one set of solid−solid transition points. It is suggested that the majority of the binary systems performed partial phase separation during the solidification process and then homogeneous solid−solid transition process in further cooling. The range of ODA/ TDA that possess the transition point of the Cort → RI process was 30−60%, which was narrower than that of the ODA/HDA system (20−70%). It is because of the relatively weaker compatibility between octadecyl acrylate and tetradecyl acrylate, which resulted from the higher difference in chain length. While the two components were not comparative
Figure 5. Conversion of photopolymerization at 5 °C of pure acrylates and binary mixture of octadecyl acrylate/hexadecyl acrylate with 1 wt % TPOL as photoinitiator. The light intensity was 6 mW/ cm2. The hollow squares represent the conversions of samples that directly cooled from molten to 5 °C. The solid squares represent the conversions of samples that first cooled from molten to crystalline state and then heated to 5 °C.
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DOI: 10.1021/acs.macromol.8b01263 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. Phase behavior of binary mixture of octadecyl acrylate/ tetradecyl acrylate during the (a) cooling process and (b) heating process.
Figure 7. (a) Conversion at −5 °C of binary mixture of octadecyl acrylate/hexadecyl acrylate (oh, black hollow and solid squares) and octadecyl acrylate/tetradecyl acrylate (ot, purple hollow and solid circles). The hollow ones represent the conversions of samples that directly cooled from molten to −5 °C, and the solid ones represent the conversions of samples that first cooled from molten to crystalline state and then heated to −5 °C. (b) Conversion of octadecyl acrylate/ tetradecyl acrylate (molar ratio is 3/7) under different temperatures. For all mixtures, 1 wt % TPOL was added as photoinitiator. The light intensity was 6 mW/cm2.
enough, it is hard to homogeneously disperse the small fraction of component in the large fraction one to “turn on” the ultraaddition effect. Nevertheless, the benefit of blending with tetradecyl acrylate (a relative short chain) was that a lower existing temperature of the rotator phase achieved −30 °C, while that of the ODA/HDA system could just reach −20 °C. The lower existing temperature of the rotator phase could supply a wider temperature range to perform photopolymerization. The photopolymerization conversion of the pure and binary systems is shown in Figure 7a. While the samples were directly cooled to −5 °C, both the tetradecyl acrylate and the mixtures are in the rotator phase and showed high conversions, but the pure octadecyl acrylate failed to polymerize because it had already transformed into the Cort phase at −5 °C. With the content of tetradecyl acrylate increasing, the system changes from Cort to RII via the RI phase; thus, the conversions of the binary mixtures increased. While the sample was first cooled to the crystalline state and then heated to −5 °C, the binary mixtures also showed far higher conversion than those of the pure acrylates. The regularity of the ODA/TDA system is similar to that of ODA/HDA, but the former showed higher polymerization conversion because the area per molecule of the RI phase of the former (19.33 Å2) was higher than that of the latter (19.08 Å2), which suggested that the molecules in the RI phase of the former possess higher motion ability. For the mixtures of ODA/TDA, while the content of octadecyl acrylate was 30%, the existing lowest temperature of the rotator phase could achieve −30 °C; thus, the regularity of photopolymerization conversion versus temperatures under far below room temperature was studied for answering the
question of whether the system always shows high polymerization conversion as long as it is in the rotator phase, no matter how low a temperature is reached. As shown in Figure 7b, the mixtures show obvious polymerization in the temperature range of −25 to −5 °C, and with the temperature decreasing, the conversion decreased. It means that although the rotator phases possess high ability for polymerization, the motion ability of molecules in rotator phases decreased and led to relatively low conversion in further lower temperatures. As mentioned above, the molecules in the RII phase possess a higher ability for polymerization than those in the RI phase. It is reasonable to infer that if the existing temperature of the RII phase can be enlarged to such a low temperature, too, the polymerization conversion might be higher than that in the RI phase at the same temperature. However, the RII → RI transition point did not decrease significantly as the RI → Cort transition point through the physical blending of two longchain acrylates; some new systems and methods need to be explored, and we will work on it in the future.
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CONCLUSION Besides the octadecyl acrylate that reported previously, hexadecyl acrylate and tetradecyl acrylate were also proved F
DOI: 10.1021/acs.macromol.8b01263 Macromolecules XXXX, XXX, XXX−XXX
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(3) Odani, T.; Okada, S.; Kabuto, C.; Kimura, T.; Shimada, S.; Matsuda, H.; Oikawa, H.; Matsumoto, A.; Nakanishi, H. Nakanishi. H. Solid-state reaction of crystals containing two kinds of polymerizable moieties of diene and diyne. Cryst. Growth Des. 2009, 9 (8), 3481−3487. (4) Medishetty, R.; Park, I.-H.; Lee, S. S.; Vittal, J. J. Solid-state polymerisation via [2 + 2] cycloaddition reaction involving coordination polymers. Chem. Commun. 2016, 52 (21), 3989−4001. (5) Li, M.; Schlüter, D.; Sakamoto, J. Solid-state photopolymerization of a shape-persistent macrocycle with two 1,8-diazaanthracene units in a single crystal. J. Am. Chem. Soc. 2012, 134, 11721−11725. (6) Kissel, P.; Murray, D. J.; Wulftange, W. J.; Catalano, V. J.; King, B. T. A nanoporous two-dimensional polymer by single-crystal-tosingle-crystal photopolymerization. Nat. Chem. 2014, 6, 774−778. (7) Restaino, A. J.; Mesrobian, R. B.; Morawetz, H.; Ballantine, D. S.; Dienes, G. J.; Metz, D. J. γ-Ray initiated polymerization of crystalline monomers. J. Am. Chem. Soc. 1956, 78 (13), 2939−2943. (8) Shibasaki, Y.; Nakahara, H.; Fukuda, K. Solid-state polymerization of long-chain vinyl compounds. I. Effect of molecular arrangement on polymerizability of octadecyl methacrylate. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 2387−2400. (9) Shibasaki, Y.; Fukuda, K. Solid-state polymerization of longchain vinyl compounds. II. Effect of molecular arrangement on polymerizability of octadecyl acrylate. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 2947−2959. (10) Shibasaki, Y. Solid-state polymerization of long-chain vinyl compounds. III. Mechanism of γ-ray-initiated postpolymerization in layered structures of octadecyl methacrylate and acrylate. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 1693−1709. (11) Shibasaki, Y.; Fukuda, K. Solid-state polymerization of longchain vinyl compounds. IV. Effects of chain length on the polymorphic behavior and postpolymerization of n-alkyl methacrylates. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 2437−2449. (12) Jian, Y.; He, Y.; Wang, J.; Yang, W. T.; Nie, J. Rapid solid-state photopolymerization of octadecyl acrylate: low shrinkage and insensitivity to oxygen. Polym. Int. 2013, 62, 1692−1697. (13) Jian, Y.; He, Y.; Wang, J.; Xu, B.; Yang, W.; Nie, J. Rapid photopolymerization of octadecyl methacrylate in the solid state. New J. Chem. 2013, 37, 444−450. (14) Yao, M.; Nie, J.; He, Y. Can chain-reaction polymerization of octadecyl acrylate occur in crystal? Macromolecules 2018, 51, 3731− 3737. (15) Muller, A. An X-ray investigation of normal paraffins near their melting points. Proc. R. Soc. London, Ser. A 1932, 138, 514−530. (16) Denicolò, I.; Doucet, J.; Craievich, A. F. X-ray study of the rotator phase of paraffins (III): Even-numbered paraffins C18H38, C20H42, C22H46, and C26H54. J. Chem. Phys. 1983, 78, 1465−1469. (17) Mukherjee, P. K. Landau model of the RII-RI-RV rotator phases in mixtures of alkanes. J. Chem. Phys. 2007, 127, 074901−1. (18) Nowak, M. J.; Severtson, S. J. Dynamic mechanical spectroscopy of plastic crystalline states in n-alkane systems. J. Mater. Sci. 2001, 36, 4159−4166. (19) Abadie, M.; Novikova, O.; Voytekunas, V. Y.; Syromyatnikov, V.; Kolendo, A. Y. Differential scanning photocalorimetry studies of 1,6-hexanediol diacrylate photopolymerization initiated by some organic azides. J. Appl. Polym. Sci. 2003, 90, 1096−1101. (20) Matheson, R. R., Jr.; Smith, P. A simple thermodynamic analysis of solid-solution formation in binary systems of homologous extended-chain alkanes. Polymer 1985, 26, 288−292. (21) Snyder, R. G.; Conti, G.; Strauss, H. L.; Dorset, D. L. Thermally-induced mixing in partially microphase segregated binary n-alkane crystals. J. Phys. Chem. 1993, 97, 7342−7350. (22) Brock, C. P.; Dunitz, J. D. Towards a grammar of crystal packing. Chem. Mater. 1994, 6, 1118−1127.
to exist in rotator phases. The binary systems of long-chain acrylates were constructed using each of them with octadecyl acrylate. The polymorphic behavior of mixtures was studied and concluded that the mixtures also possess rotator phases (RII and RI) and crystalline phase (Cort) as each individual. In binary systems, the RI phase was stable due to the ultraaddition effect, the temperature range of the RI phase was enlarged from 12 to 27 °C, and the lowest temperature of the RI phase was dropped to −30 °C from −10 °C compared with tetradecyl acrylate (the lowest temperature RI species). The mechanism is that the conformation disorder at the end of molecules, due to the difference in chain length between two components, could restrict the shrinkage of lattice during the transition process from RI to Cort and lead to the fact that the phase structure of Cort is close to that of RI, which finally stabilized the RI phase. The binary systems showed much higher photopolymerization conversion (50−70%) at the temperature that the conversions of all the pure acrylates are near 0, which realized “0 + 0 = 2”. The effect of difference of chain length between two components on stabilizing the RI phase was studied, and a threshold value (i.e., 4) was found because complete phase separation appeared in the binary system while this difference was 6. For long-chain acrylates, physical blending is an effective method to stabilize the rotator phase and broaden the temperature range of rotator phase photopolymerization with a view to the application in the field of low-temperature photopolymerization.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01263. Verification rotator phases in hexadecyl and tetradecyl acrylate, stability of the RI phase of octadecyl acrylate/ hexadecyl acrylate mixture, phase separation of octadecyl acrylate/dodecyl acrylate mixture, and polymorphic behavior of the octadecyl acrylate/tetradecyl acrylate system (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(Y.H.) E-mail:
[email protected]. ORCID
Jun Nie: 0000-0003-2698-1751 Yong He: 0000-0002-4689-966X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the National Key Research and Development Program of China (2017YFB0307800) and National Natural Science Foundation of China (51373015 and 51573011) for their financial support.
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REFERENCES
(1) Biradha, K.; Santra, R. Crystal engineering of topochemical solid state reactions. Chem. Soc. Rev. 2013, 42, 950−967. (2) Guo, F.; Martí-Rujas, J.; Pan, Z.; Hughes, C. E.; Harris, K. D. M. Direct structural understanding of a topochemical solid state photopolymerization reaction. J. Phys. Chem. C 2008, 112 (50), 19793−19796. G
DOI: 10.1021/acs.macromol.8b01263 Macromolecules XXXX, XXX, XXX−XXX
