HMPAM - Wiley Online Library

Macromolecular Rapid Communications

Communication

Shape Memory Hydrogel based on a Hydrophobically-Modified Polyacrylamide (HMPAM)/α-CD Mixture via a Host-Guest Approach Akram Yasin, Wanfu Zhou, Haiyang Yang,* Huazhen Li, Yin Chen, Xingyuan Zhang*

A novel thermally sensitive shape memory (SM) hydrogel is prepared by block copolymerization of a cationic surfactant monomer, dimethylhexadecyl[2-(dimethylamino)ethylmethacrylate]ammoniumbromide (C16DMAEMA), and acrylamide (AM) in the presence of α-cyclodextrin (α-CD) using N,N’-methylenebisacrylamide (MBA) as a crosslinker. XRD, solid state 13C NMR, and DSC measurements show that the crystalline domains, induced by the hydrogen bonds between α-CDs threaded on the hydrophobic units of the polymer chains through the host-guest approach, can reversibly melt and crystallize at different temperatures. Rheological measurements show that both the elastic modulus G’ and viscous modulus G’’ drastically change due to the formation and dissolution of the crystalline domains. These thermo-sensitive crystalline domains serve as reversible physical crosslinks, endowing the hydrogel with excellent SM properties. Cyclic experiments show that the hydrogel can recover to almost 100% of the deformation in each cycle and can be reused several times.

1. Introduction Shape memory polymers (SMPs) can rapidly change their shapes from a temporary shape back to their original (or permanent) shapes under an appropriate stimulus, such as temperature,[1] light,[2] electric field,[3] magnetic field,[4] pH,[5] or specific ions.[6–8] Among these stimulus sensitive SMPs, thermally sensitive SMP is a typical one that has

A. Yasin, Prof. H. Yang, Dr. H. Li, Y. Chen, Prof. X. Zhang CAS Key Laboratory of Soft Matter Chemistry, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China E-mail: [email protected]; [email protected] Dr. W. Zhou Oilfield Production Technology Institute, Daqing Oilfield Co. Ltd., Daqing 163514 , China Macromol. Rapid Commun. 2015, 36, 845−851 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

been widely studied and applied in industry. For thermoresponsive SMP, the shape of the SMP can be deformed to the temporary state initially at a temperature above its crystallization (Tc) or glass transition temperature (Tg).[9] After quenching SMP to a temperature below its Tc or Tg, the temporary shape of SMP is fixed because the segmental motion of the polymer chain is frozen. Once the SMP is heated to a temperature higher than its Tc or Tg, segmental motion of the polymer chain becomes possible and its original shape can be recovered spontaneously. The idea of a thermally sensitive SMP can also be applied to synthesize thermally sensitive shape memory gels that are based on hydrophobically-modified polymer. To date, a great deal of research has been published on this topic. Osada and co-workers[10–12] reported the shape memory behaviour of gels for the first time. Their water-swollen gel was synthesized by the radical

wileyonlinelibrary.com

DOI: 10.1002/marc.201400698

845


A. Yasin et al.

www.mrc-journal.de

copolymerization of n-octadecyl acrylate (C18) and acrylic acid (AA) in the presence of a crosslinker in ethanol. The thermally sensitive crystalline domains, introduced by the hydrophobic interaction between C18 chains, can reversibly melt and crystallize at different temperatures, endowing the gel with excellent shape memory properties. During the radical copolymerization, ethanol was selected mainly because n-octadecyl acrylate (C18) could not dissolve in water. Obviously, the use of an organic solvent may limit applications of the gel, for example in biomaterials. Recently, the groups of Bilici successfully synthesized this type of shape memory hydrogel in aqueous solution via micellar copolymerization.[13] Their wormlike micelles formed by sodium dodecyl sulfate (SDS) in the presence of NaCl enabled the solubilization of large amounts (35 mol% in the feed) of C18 in water. The wormlike micelles introduced significantly improved the copolymerization at a highly hydrophobic level to obtain a copolymer with a high C18 mole fraction without need for an organic solvent. Due to the thermally sensitive crystallization of C18, the hydrogel was endowed with shape memory properties. In addition to the thermally responsive shape memory hydrogels, other stimuli responsive shape memory hydrogels have also been synthesized and studied. Liu et al.[6,7] reported a new type of stimuli responsive shape memory hydrogel which was designed smartly by introducing an imidazole-zinc ion coordination interaction. This Zn2+driven shape memory hydrogel can recover its original shape when Zn2+ is removed with a chelating agent. In our previous study, we reported a Fe3+-driven shape memory hydrogel, which was developed by introducing the interaction between Fe3+ and phosphate groups of the polymer. The trick of replacing Zn2+ with Fe3+ is that Fe3+ can not only be removed by a chelating agent, but can also be reduced to Fe2+ by reducer. Considering that the Fe2+ has no interaction with phosphate groups, the Fe3+driven shape memory hydrogel can recover to its original shape when Fe3+ is either removed by chelating agent or reduced to Fe2+ by a reducer.[8] In the present study, we report a novel thermo-responsive shape memory hydrogel that was designed by introducing a hydrogen bonding interaction between α-CDs threaded on the hydrophobic units (C16) of polymer chains using the host-guest approach. It is already well known that the host-guest approach between α-CD and the hydrophobic units of polymer chains is versatile and can be used effectively to prepare a variety of highperformance materials. We have already shown that a novel thermo-responsive self-healing hydrogel can be developed by introducing the host-guest approach between α-CDs and hydrophobic units (C12) of polymer chains, requiring no special synthesis, no third competitive guest or harsh conditions.[14] In this study, we found

846

with astonishment that a novel thermo-responsive shape memory hydrogel could also be developed by introducing the host-guest approach between α-CDs and the hydrophobic units of polymer chains. It has already been reported[15–17] that the hydrogen bonds between α-CDs threading on polymer chains (PEG) by means of inclusion complexes can act as crosslinkers that bring PEG solution into the gel. In our experiment, the hydrogen bonding interaction between α-CDs threaded on the hydrophobic units (C16) of polymer chains by the host-guest approach was used to induce the formation of thermo-responsive crystalline domains, endowing the hydrogel with thermal sensitivity and shape memory properties.

2. Experimental Section 2.1. Synthesis of Dimethylhexadecyl[2-(dimethylamino) ethylmethacrylate]ammonium Bromide (C16DMAEMA) C16DMAEMA was synthesized according to a previously published method (Scheme S1, Supporting Information).[18–20] Briefly, a 250 mL three-necked round-bottom flask fitted with a dropping funnel and a reflux condenser was charged with 1-bromohexadecane (0.1 mol) in isopropanol (40 mL). The bath was heated to 70 °C under a nitrogen atmosphere. DMAEMA (0.11 mol) and 4-methoxyphenol (0.1 wt% relative to the DMAEMA mass) were dissolved in isopropanol (40 mL) and slowly added into the flask. After complete addition, the reaction was left to continue for several hours at 70 °C. After vacuum distillation and recrystallization in acetone, the crude product was obtained and dried under a vacuum for 2, then white dimethylhexadecyl[2-(dimethylamino) ethylmethacrylate]ammonium bromide (C16DMAEMA) powder was obtained in a yield of 85%. FTIR spectra were measured to confirm the formation of C16DMAEMA (Figure S1, Supporting Information).

2.2. Solution Copolymerization of AM/C16DMAEMA/ α-CD Hydrogels Using a one-pot method in aqueous solution, a macroscopic hydrogel constructed by the self-assembly of amphiphilic multiblock copolymers was synthesized through free-radical micelle copolymerization in the absence of surfactants. AM and C16DMAEMA with C16DMAEMA-to-AM ratios of 40/60, 30/70, and 20/80 (mol/mol) were dissolved completely in deionized water by agitation, and the total monomer concentration was controlled at 20 wt%. A saturated aqueous solution of α-CD was added to C16DMAEMA and AM aqueous solutions before the copolymerization started. α-CD was added to the mixture by varying the α-CD:C16DMAEMA feed ratio (1:5, 1:10, 1:20), and without α-CD as a blank control for comparison purposes. After the crosslinker MBA (0.5 wt%) and initiator V50 (0.5 wt%) were added to the solution, it was stirred until homogenous. Then, a portion of the solution was transferred between two glass plates (5 × 5 cm) separated by a 1 mm Teflon spacer for rheological measurements of G’ and G’’. For shape memory experiments, the

Macromol. Rapid Commun. 2015, 36, 845−851 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.MaterialsViews.com


Shape Memory Hydrogel based on a Hydrophobically-Modified Polyacrylamide (HMPAM)/α-CD Mixture . . .

www.mrc-journal.de

remaining solution was transferred into several plastic syringes with 5.0 mm internal diameters. Finally, micelle copolymerization was conducted at 40 °C for 3 h and hydrogels with excellent mechanical properties and shape memory properties were prepared. The shape memory effect was examined by a bending test.[9,13] A cylindrical hydrogel sample (diameter 5.0 mm, length about 6 cm) was folded at 60 °C and then cooled to room temperature to keep the deformation. The deformed hydrogel sample was immersed in a water bath at 25 °C and then heated to 70 °C. At each temperature T, the final angle θT was recorded after it was steady. The measurements of θT were conducted using a digital camera. The shape recovery ratio R was calculated as R = θT /180.

3. Results and Discussion 3.1. Effect of α-CD and C16 on the Properties of Hydrogels The complexation of α-CD with C16DMAEMA is the foundation of the shape memory ability in this study. To investigate the influence of α-CD and C16DMAEMA on the properties of the gels, a series of hydrogels with different concentrations of α-CD and C16DMAEMA was synthesized, as described previously. Amongst the gels we have investigated, the sample with 40 mol% C16 and α-CD with the mole ratio of α-CD:C16 = 1:5 exhibits excellent shape memory properties compared to the others. In Figure 1(a) and (b) are pictures of 40 mol% C16 gels. The gel in (a) contains no α-CD; by contrast, (b) contains 8 mol% of α-CD (its mole ratio is 1:5 to C16); (c) is a gel containing 20 mol% of C16 and α-CD in the ratio 1:5. Therefore, the influence of α-CD on the gel's shape memory performance is demonstrated clearly in Figure 1(a) and (b). The gel in Figure 1(a) is soft at room temperature and has no shape memory at all.

The gel in Figure 1(b) is tough at room temperature and changes to soft when heated to Tm. The gel in Figure 1(c) contains 20 mol% of C16 and α-CD, and is tougher than the gel in (a) but softer than that in (b). Gel (c) has such strong elasticity that it can be tied a knot and be stretched as shown in (d). In comparison, the gel in (b) is too tough to stretch, and is crisp at room temperature. It has been accepted that hydrophobicity and α-CD content play a key role in determining the mechanical properties of their associated crosslinked hydrogels.[18,19,21] However, it should be pointed out that additional C16DMAEMA and α-CD will show an adverse impact on the mechanical properties because of the formation of excess crystalline microzones. This concern is discussed further later on. When increasing the concentration of C16 and α-CD, the elasticity of the hydrogel increased significantly at first (C16 0–20 mol%, α-CD:C16 = 1:5), and then decreased because of the excess formation of the crystalline domains. 3.2. Structure of Hydrogels In order to investigate the structure of the gels, X-ray diffraction, NMR, and scanning electron microscopy (SEM) measurements were performed. Figure 2 shows the X-ray diffraction powder patterns of the freeze-dried gels with various mol ratios of α-CD to C16DMAEMA. For comparison, the X-ray diffraction diagram of the freeze-dried gel without α-CD and free α-CD are shown. All the gels in Figure 2 were formed from aqueous solutions containing 20 w/v% of monomer and 40 mol% of C16DMAEMA in the monomers. The structures of inclusion complexes of CDs with low molecular weight compounds can be classified into two groups: one is the “cage type” and the other is the “channel type”. The free α-CD showed characteristic diffraction peaks (2θ = 14.1°), and the hydrogels showed a

1000

CD

Intensity/cps

800

1

600

2

3

400

4 200

0 10

20

30

40

50

2

Figure 1. Hydrogels of (a) 40 mol% C16DMAEMA without α-CD, (b) 40 mol% C16DMAEMA with α-CD, α-CD:C16DMAEMA = 1:5, (c) 20 mol% C16DMAEMA with α-CD, α-CD:C16DMAEMA = 1:5, (d) knotted and stretched gel of (c).


Figure 2. XRD pattern of the hydrogels with 40 mol% C16DMAEMA in the 5–50° 2θ range with a scan rate of 1 ° min−1. Different ratios of α-CD:C16DMAEMA: (1) 1:5; (2) 1:10; (3) 1:20. (4) Without α-CD, and free α-CD.


847


A. Yasin et al.

www.mrc-journal.de

hydrogel. It was difficult to prepare hydrogel with a characteristic single peak (2θ = 21°). The crystal structures higher α-CD-C16 concentration because of the limited solof α-CD and α-CD–C16 form a cage-type structure and a ubility of α-CD-C16 in aqueous solution. The α-CD rings channel-type structure, respectively, which was revealed become hydrophobic because of the consumption of the by crystal X-ray analysis. The pattern of none-α-CDs gel hydroxyl groups, and interactions of α-CD-C16 complexes represents the crystallization via hydrophobic associaresult in crystallization. tion by C16 alkyl chains, hydrogels prepared from other In contrast with the traditional thermo-responsive alkyl chains like stearyl acrylate also have a single peak shape memory systems that form their crystalline strucat around 21° corresponding to a Bragg d-spacing.[13,22,23] ture only through hydrophobic alkyl chains.[10,13,28–30] and This Bragg d-spacing is typical for the paraffin-like hexagonal lattices formed by the packing of n-alkyl chains. thermo-induced sol-gel transitions by α-CD-PEG inclusion The gels exhibited a crystalline peak at 21° in all patterns complexes,[15,24] we successfully combined both of the and the peak at 14.1° disappeared. The patterns showed crystallization mechanisms above in this work. an increase in strength with an increase in concentration of α-CD, because a high content of CD can lead to larger 3.3. Shape Memory Properties amounts of channel-type crystalline structures in the gels. As described in other papers,[15,24–26] the X-ray diffraction The thermal behavior of hydrogels was investigated by DSC as well as by rheometry using oscillatory deformapattern showed a number of sharp reflection lines and one tion tests. The gel samples were subjected to heating and strong and sharp main reflection line at around 20 ° correcooling cycles between 5 and 80 °C, during which the sponding to the crystalline α-CD-PEG inclusion complexes. changes in the heat flux and in the dynamic moduli of gels This is attributed to the channel-type structure of the α-CD were monitored as a function of temperature. In Figure 3, and PEG complex. The X-ray diffraction powder patterns in the variations of the elastic modulus G’ and the viscous their works were almost similar to the result in Figure 2. modulus G’’ of a hydrogel sample formed at 40 mol% In fact, only 1 exhibited excellent shape memory propC16 with α-CD in the ratio of α-CD:C16DMAEMA = 1:5 erties. With increasing α-CD, the crystalline domains increased, and the shape memory properties were also are shown during the course of the heating and cooling enhanced accordingly. The sufficient formation of crystalperiods. DSC traces of the hydrogels are also shown in the line domains induced by α-CD is essential to the design figure by the dashed curves. The DSC curves reveal that the of the thermally sensitive shape memory hydrogels. hydrogel can melt and crystallize with a change in temSolid state 13C NMR spectra of the gels and free α-CD also perature. During the cooling of the gel sample from 80 to 5 °C, an exothermic peak appears at 30.0 °C, corresponding proved that a C16 chain is included in the cavities of the to the crystallization temperature Tc. During heating back α-CDs (Figure S2, Supporting Information). Corresponding to their mechanical property developup to 80 °C, the swollen gel melts, as evidenced from the ment, the morphological evolution of the gels was eviendothermic peak at 31.7 °C, corresponding to the melting dent because of the α-CD concentrations with fixed C16 temperature Tm. at 40 mol%. The microstructure of these hydrogels was Figure 3 also shows that, at the transition temperainvestigated using scanning electron microscopy (SEM) tures, both the elastic G’ and viscous moduli G’’ drastiafter cryo-drying (Figure S3, Supporting Information). If cally change due to the formation and dissolution of α-CD induces the formation of crystalline domains, the crosslink density of 6 G'&G''/ Pa 10 the hydrogels should increase in the DSC presence of α-CD. At the same time, the pore size within the polymeric matrix 5 should decrease. The experimental 10 results shown in Figure S3 in the Supporting Information thoroughly verify G' this assumption.[18,23,27] As pointed out 4 10 in the Introduction,[10] for comparison, the shape memory poly(C18-AA) gels G'' exhibited side chain crystallization when the C18 mole fraction was high 80 70 60 50 40 30 20 10 10 20 30 40 50 60 70 80 o o enough in ethanol, which played a role Temperature/ C Temperature/ C in fixing the temporarily deformed shape as the reversible phase, and the Figure 3. Viscoelastic behavior and DSC curve of the hydrogel with 40% C16 and α-CD obtained gel acted as a shape memory (α-CD:C16 = 1:5) during the cooling-heating cycle between 80 and 5 °C.

848




Macromolecular Rapid Communications www.mrc-journal.de

formed by α-CD-C16 blocks as switchable segments with a transition temperature to keep the temporary (spiral) shape. Cylindrical hydrogel samples were also stretched at 60 °C to three times their original length, and then the temporary shapes were held by cooling to room temperature. When the samples were immersed in water at 60 °C, their permanent shapes were recovered within a few seconds (movie 2, Supporting Information). To quantify the shape memory properties of the hydrogels, the shape recovery ratio R was calculated at various temperatures and the results are shown in Figure 4(b). The hydrogel sample exhibited a shape recovery ratio of 100% at or above 50 °C. The recovery ratio R is equal to zero below 25 °C, indicating that the temporary shape remains unchanged. It has been reported by Li et al.[15] that PEG can form a gel in solution in the presence of α-CD, due to the hydrogen bonding between α-CDs penetrating the PEG chains. In our experiment, α-CD can thread on the hydrophobic groups through a host-guest approach, Figure 4. (a) Images demonstrating the transition from the temporary shape (spiral) to as shown in Figure 5. If the concentrathe permanent shape (rod) for the hydrogel with 40 mol% C16. The recovery takes about 22 s after immersing the gel sample in a water bath at 60 °C. (b) Shape recovery ratio R tion of α-CD is sufficient, these α-CDs of the hydrogel shown as a function of temperature. adsorbed on the polymer chains can form crystalline domains through hydrogen bonding, and these crystalline domains act as the crystalline domains in the gel sample. The extent of physical crosslinks. In other similar works,[10,11,13,18,21] the change in the dynamic moduli of gels was strongly dependent on the hydrophobic level. Figure S4 in the the crystallization occurs by direct interaction between Supporting Information shows the viscoelastic behavior hydrophobic alkyl chains such as C12, C16, and C18. Howof the hydrogels formed with various amounts of C16 ever, in our system, α-CD-C16 inclusion complex chains during the course of the cooling-heating cycle. As the successfully result in crystallization instead of the alkyl hydrophobic level was increased from 10 to 40%, G’ also chains. increased. To date, shape memory hydrogels have been designed Figure 4 and the movie attached in the Supporting by either the coordination interaction between polymer Information demonstrate the shape memory behavior and metal ions, or the hydrophobic interaction between of a hydrogel sample with 40 mol% of C16DMAEMA the hydrophobic groups of polymer chains. Our experimental results have shown that shape memory can also and a mole ratio of 1:5 for α-CD to C16DMAEMA. The been designed by introducing hydrogen bonding between permanent shape of the sample is rodlike. After being α-CDs which are adsorbed on polymer chains through a heated to 60 °C, the gel turns soft and can be easily host-guest approach. deformed to a spiral shape (movie 1, Supporting Information). This temporary shape is retained by cooling the sample to room temperature. On immersion of the gel sample in a water bath at 60 °C, it returns to its 4. Conclusion original shape within 22 s. Its permanent (rod) shape is determined by the covalently crosslinked network In summary, we have synthesized a novel thermally structure in the hydrogel when crystalline domains are responsive shape memory hydrogel by introducing a



849


A. Yasin et al.

www.mrc-journal.de

Figure 5. Mechanism of the micelle polymerization and shape memory property.

hydrogen bonding interaction between α-CDs threaded on the hydrophobic units of polymer chains through a host-guest approach. This hydrogen bonding interaction results in the formation of thermally sensitive crystalline domains which can melt and crystallize reversibly with changing temperature. As a result, the hydrogel can be processed into temporary shapes as needed at the crystallization temperature (Tc) and is also able to recover its initial shape after being heated to the melting point (Tm). The hydrogel exhibits excellent shape memory properties, and cyclic experiments have shown that the hydrogel can recover almost 100% of the deformation in each cycle and can be re-used several times.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: This work was supported by the National Natural Science Foundation of China (Grant no.51273189), the Petro China Innovation Foundation (2012D-5006–0202) and the China postdoctoral Science Foundation funded project (No.2013M531513). Received: December 5, 2014; Revised: January 19, 2015; Published online: March 12, 2015; DOI: 10.1002/marc.201400698

850

Keywords: cyclodextrin; inclusion complexes; polyacrylamides; shape memory hydrogels; thermo-sensitive polymers

[1] C. Liu, H. Qin, P. T. Mather, J. Mater. Chem. 2007, 17, 1543. [2] A. Rose, Z. Zhu, C. F. Madigan, T. M. Swager, V. Bulovic, Nature 2005, 434, 876. [3] N. G. Sahoo, Y. C. Jung, N. S. Goo, J. W. Cho, Macromol. Mater. Eng. 2005, 290, 1049. [4] C. M. Yakacki, N. S. Satarkar, K. Gall, R. Likos, J. Z. Hilt, J. Appl. Polym. Sci. 2009, 112, 3166. [5] X. J. Han, Z. Q. Dong, M. M. Fan, Y. Liu, J. H. Li, Y. F. Wang, Q. J. Yuan, B. J. Li, S. Zhang, Macromol. Rapid Commun. 2012, 33, 1055. [6] Y. Han, T. Bai, Y. Liu, X. Zhai, W. Liu, Macromol. Rapid Commun. 2012, 33, 225. [7] W. Nan, W. Wang, H. Gao, W. Liu, Soft Matter 2013, 9, 132. [8] A. Yasin, H. Li, Z. Lu, S. U. Rehman, M. Siddiq, H. Yang, Soft Matter 2014, 10, 972. [9] G. Liu, X. Ding, Y. Cao, Z. Zheng, Y. Peng, Macromolecules 2004, 37, 2228. [10] Y. Osada, A. Matsuda, Nature 1995, 376, 219. [11] A. Matsuda, J. I. Sato, H. Yasunaga, Y. Osada, Macromolecules 1994, 27, 7695. [12] Y. Tanaka, Y. Kagami, A. Matsuda, Y. Osada, Macromolecules 1995, 28, 2574. [13] C. Bilici, O. Okay, Macromolecules 2013, 46, 3125. [14] X. Hao, W. Zhou, R. Yao, Y. Xie, S. U. Rehman, H. Yang, J. Mater. Chem. A 2013, 1, 14612. [15] J. Li, A. Harada, M. Kamachi, Polym. J. 1994, 26, 1019. [16] X. Liao, G. Chen, M. Jiang, Langmuir 2011, 27, 12650.




Macromolecular Rapid Communications www.mrc-journal.de

[17] K. Wei, J. Li, G. Chen, M. Jiang, ACS Macro Letters 2013, 2, 278. [18] K. Xu, H. An, C. Lu, Y. Tan, P. Li, P. Wang, Polymer 2013, 54, 5665. [19] H. An, C. Lu, P. Wang, W. Li, Y. Tan, K. Xu, C. Liu, Polym. Bull. 2011, 67, 141. [20] G. Tuin, F. Candau, R. Zana, Colloids Surf. A 1998, 131, 303. [21] P. Kujawa, J. M. Rosiak, J. Selb, F. Candau, Macromol. Chem. Phys. 2001, 202, 1384. [22] X. Lin, L. Chen, Y. Zhao, Z. Dong, J. Mater. Sci. 2010, 45, 2703. [23] S. Livshin, M. S. Silverstein, Macromolecules 2008, 41, 3930.


[24] A. Harada, M. Kamachi, Macromolecules 1990, 23, 2821. [25] J. Wu, C. Gao, Macromol. Chem. Phys. 2009, 210, 1697. [26] Y. Kuratomi, M. Osaki, Y. Takashima, H. Yamaguchi, A. Harada, Macromol. Rapid Commun. 2008, 29, 910. [27] E. A. Appel, F. Biedermann, U. Rauwald, S. T. Jones, J. M. Zayed, O. A. Scherman, J. Am. Chem. Soc. 2010, 132, 14251. [28] T. Miyazaki, K. Yamaoka, J. P. Gong, Y. Osada, Macromol. Rapid Commun. 2002, 23, 447. [29] C. Yuan, J. Guo, F. Yan, Polymer 2014, 55, 3431. [30] K. Inomata, T. Terahama, R. Sekoguchi, T. Ito, H. Sugimoto, E. Nakanishi, Polymer 2012, 53, 3281.


851