- Email: [email protected]
Self-Healing Timescale
8
Self-healing Timescale Strain (stress-free) relaxation in mechanically prestrained bone has a time constant of 75 s.1 It occurs by a reorganization of the proteoglycan-glycoprotein matrix between collagen fibers, which requires ionic interactions. Relaxation times of about 1 min were reported
Figure 8.1. (a) Surface topography and (b) cross-section profiles of the evolution of the wear track on epoxy resin containing 7% ionic liquid. [Adapted, by permission, from Saurin, N; Sanes, N; Carrion, F J; Bermudez, M D, RSC Adv., 6, 37258, 2016.]
80
Self-healing Timescale
for stress relaxation of the whole bone and tendon. This should not be confused with the healing of stress fractures, and macroscopic bone traumas, taking days to weeks to heal. The fast process provides spontaneous healing of plastically strained bone in walking or running. Figure 2.1 shows that autonomic sealhealing of poly(vinyl butyral) takes several hours at high relative humidity. Healing efficiency is attributed to the molecular Figure 8.2. Crack area vs. distance from top surface for motion at the surface, which contains a a concrete sample after 5 cycles (one cycle: sample sub- large amount of water owing to the surface merged in water at 20±1°C for 24 h, and dried in air at localization of vinyl alcohol.2 The water 20±1°C and 50±5% RH for 24 h.). [Adapted, by permisacts as a plasticizer.2 sion, from Fan, S; Li, M, Smart Mater. Struct., 24, 015021, 2015.] The healing time is one of the common methods of expression of healing timescale. Thermoplastics or fiber-reinforced composites exhibit the highest strengths (from tens to hundreds of MPa) and their healing times range between hours to a few days.3 The healing efficiency of vulcanized chloroprene rubber increased with healing time increase.4,5
Figure 8.3. Self-healing process of hydrogel: (a) two cylindrical hydrogels (b) hydrogels cut in half; (c) two semicircle hydrogels healed within 1 min; (d) healed hydrogel picked up with forceps. The details of the self-healing process were recorded by optical microscopy microimages: (c1) 0 s; (c2) 5 s; (c3) 10 s; (c4) 20 s; (c5) 60 s; (c6) 5 min. [Adapted, by permission, from Li, G; Wu, J; Wang, B; Yan, S; Zhang, K; Ding, J; Yin, J, Biomacromolecules, 16, 3508-18, 2015.]
Self-healing Timescale
81
The effect of the addition of 1-octyl-3-methylimidazolium to a conventional epoxy resin on self-healing rate has been studied.6 Figure 8.1 shows the evolution of the wear tracks and cross-section profiles along the healing time.6 The self-healing ability over time increases with increasing ionic liquid concentration, to reach values higher than a 90% after 22 hours, for the materials with 9 and 12 wt% ionic liquid.6 Concrete cracking and deterioration show that the healing process at the greater depths (inside the crack) takes a significantly longer time (Figure 8.2).7 The healing results from continued hydration and pozzolanic reactions.7 Supramolecular self-assembled hydrogels based on poly(l-glutamic acid) were observed during the self-healing process (Figure 8.3).8 After 60 s, the two pieces of hydrogel disks healed and obtained their original shape.8 The initial gap width of the fresh crack was 126.7 μm (Figure 8.3c1), which then narrowed down to 42.3 μm after 5 s (Figure 8.3c2).8 The crack disappeared fast within 10 s (Figure 8.3c3), and the injury healed completely within 60 s (Figure 8.3c4 and 8.3c5).8 Photographs also shows that stains quickly interpenetrate each other side.8 A hybrid supramolecular polymeric hydrogel was constructed via host-guest interaction of a host cyclodextrin polymer with a guest α-bromonaphthalene polymer and mixed with 6-thio-β-cyclodextrin modified gold nanoparticles.9 The hydrogel has a rapid selfhealing (1 min).9 The G’ and G” of the self-healed hydrogel attained their original levels after 1 hour.9 The self-healing of poly(butyl acrylate) copolymer containing comonomers with 2ureido-4[1 H]-pyrimidinone quadruple hydrogen bonding groups was investigated.10 Fractured films fully recovered their self-adhesion strength to 40, 81, and 100% in 10 s, 3 h, and 50 h, respectively.10 To accelerate the healing process, antimony pentafluoride was used as an instant hardener of epoxy after encapsulation the highly active antimony pentafluoride-ethanol complex in the hollow silica spheres.11 The epoxy material with the embedded healant capsules can be healed within a few seconds (Figure 8.4).11 Using impact strength data, it was found that the healing efficiency increases with curing time.11 The self-healing specimens can be safely stored for many months without losing their healing efficiency.11 In another development, the healing rate of a self-healing process subjected to low cycle dynamic stress was proposed to be regulated according to the rate of damage.12 This
Figure 8.4.(a) Infrared absorption of epoxide groups as a function of time from hardener addition (b) curing degree of epoxy calculated with the peak areas in (a). [Adapted, by permission, from Ye, X J; Zhang, J-L; Zhu, Y; Rong, M Z; Zhang, M Q; Song, Y X; Zhang, H-Z, ACS Appl. Mater. Interfaces, 6, 3661-73, 2014.]
82
Self-healing Timescale
Figure 8.5. (A-C) Time-lapse microscopy images and (D-F) simulated video snapshots showing the nanomotor propulsion and localization process with an H2O2 fuel concentration of 7.5% and average motor density of 16.3 motors per 100 x 100 μm2 at times (A) 0 s, (B) 150 s, and (C) 300 s from adding the fuel solution. (G-I) Timelapse microscopy images showing the influence of the motor speed, controlled by the fuel concentration, on the propulsion and localization process of multiple nanomotors. Fuel concentrations, (G) 2.5%, (H) 5.0%, and (I) 7.5%. (J-L) Simulated video snapshots showing the propulsion and localization process of multiple nanomotors moving at speeds corresponding to H2O2 fuel concentrations of (J) 2.5%, (K) 5.0%, and (L) 7.5%. Scale bar (A.C; G.I): 50 μm. [Adapted, by permission, from Li, J; Shklyaev, O E; Li, T; Liu, W; Shum, H; Rozen, I; Balazs, A C; Wang, J, Nano Lett., 15, 7077-83, 2015.]
is to limit damage from the propagating fault.12 Active self-healing takes advantage of sensing and added external energy to achieve the desired healing rate.12 The concept of active self-healing is tested by modeling.12 Figure 4.2 shows the principles of modeling.12 Binders with relaxation times on the order of 0.1 s gave the best cycling ability with 80% capacity maintained for over 175 cycles using large silicon particles (~0.9 μm).13 This is attributable to an improved balance between the viscoelastic stress relaxation in the binder and the stiffness needed to maintain mechanical integrity.13 The more crosslinked binder showed worse performance, confirming the need for liquid-like flow.13 A synthetic repair system where selfpropelled nanomotors autonomously seek and localize microscopic cracks (for explanations see Figure 4.1).14 The nanomotors exhibit isotropic self-propulsion at consistent average speeds over long time periods.14 The self-propulsion of the motors is combined with their Brownian reorientaFigure 8.6. The self-healing efficiency changes as a tion to behave as a random walk.14 The function of the healing time from 10 to 960 min. surface irregularities (e.g., cracks) disrupt [Adapted, by permission, from Fan, F; Szpunar, J, J. Appl. Polym. Sci., 132, 42135, 2015.] the random walk through physical interac-
Self-healing Timescale
83
tions and entrench the nanomotors in a potential well within the crack.14 The hydrophobic hemispheres of the nanomotors can adhere to these exposed hydrophobic crack surfaces.14 Figure 8.5 shows that the nanomotors quickly find their destinations in the crack.14
Figure 8.7.Typical SEM images showing the post-drying evolution of a droplet. The aging times are (a) 4 h, (b) 12 h, (c) 24 h, (d) 48 h, and (e) 10 days. The first column displays the lower right quadrant of the droplet, the second column gives a magnification of the center, and the last column shows a high magnification of the contact line (indicated in (e) by a dashed line). The tiny cracks are artifacts caused by the sputter-coating. [Adapted, by permission, from van der Kooij, H M; de Kool, M; van der Gucht, J; Sprakel, J, Langmuir, 31, 4419-28, 2015.]
84
Self-healing Timescale
Figure 8.8. Optical microscope images of self-healing behavior of scratched crosslinked polymer films with diarylbibenzofuranone linkages healed at 50°C for 12 h (top), at 40°C for 72 h (middle), and at 30°C for 72 h (bottom). Scale bars are 0.1 mm. [Adapted, by permission, from Imato, K; Takahara, A; Otsuka, H, Macromolecules, 48, 5632-9, 2015.]
Figure 8.6 shows that the healing efficiency of an industrial acrylic elastomer increases with the increase in the healing time.15 The hydrogen bonding connects the damaged surfaces and provides a bridge for chain diffusion across the cut interface.15 The chain diffusion process creates chain entanglements.15 Also, the increased temperature promotes healing efficiency because the mobility of molecular chains increases with the temperature.15 Figure 8.7 shows the development of morphology during film formation from polymer dispersion.16 In the beginning of the process, the smaller cracks self-heal while the largest cracks expand up to a width of ~15 μm.16 However, after 10 days of aging also the largest cracks disappear, and the film has no distinguishable features.16 Polyurethane crosslinked with diarylbibenzofuranone-based dynamic covalent linkages has self-healing rate affected by temperature (Figure 8.8).17 At 50°C, the scar disappeared after 12 h.17 The scar was almost undetectable after healing for 72 h at 40°C.17 The scratch remained evident even after healing for 72 h at 30°C.17 A numerical model of healing agent flow from an orifice has been developed using smoothed particle hydrodynamics to explore the flow characteristics of healing agent leaving a vascular network and infusing a damage site within a fiber-reinforced polymer composite.18 The model is able to reproduce experimental results for the discharge coefficient at the high Reynolds number limit and the power-law behavior for low Reynolds numbers.18 REFERENCES 1 2 3
Akbarzadeh, J; Puchegger, S; Stojanovic, A; Kirchner, H O K; Binder, W H; Bernstorff, S; Zioupos, P; Peterlik, H, Bioinspired, Biomimetic, Nanomater., 3, 3, 123-30, 2014. Arayachukiat, S; Doan, V A; Murakami, T; Nobukawa, S; Yamaguchi, M, J. Appl. Polym. Sci., 132, 42008, 2015. D’Elia, Eslava, S; Miranda, M; Georgiou, T K; Saiz, E, Sci. Reports, 6, 25059, 2016.
Self-healing Timescale
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
85
Xinag, H P; Rong, M Z; Zhang, M Q, ACS Sustainable Chem. Eng., 4, 2715-24, 2016. Zheng, J; Xiao, P; Liu, W; Zhang, J; Huang, Y; Chen, T, Macromol. Rapid Commun., 37, 265-70, 2016. Saurin, N; Sanes, N; Carrion, F J; Bermudez, M D, RSC Adv., 6, 37258, 2016. Fan, S; Li, M, Smart Mater. Struct., 24, 015021, 2015. Li, G; Wu, J; Wang, B; Yan, S; Zhang, K; Ding, J; Yin, J, Biomacromolecules, 16, 3508-18, 2015. Chen, L; Chen, H; Yao, X; Ma; X; Tian, H, Chem. Asian J, 10, 2352-5, 2015. Faghihnejad, A; Feldman, K E; Yu, J; Tirrell, M V; Israelachvili, J N; Hawker, C J; Kramer, E J; Zeng, H, Adv. Funct. Mater., 24, 2322-33, 2014. Ye, X J; Zhang, J-L; Zhu, Y; Rong, M Z; Zhang, M Q; Song, Y X; Zhang, H-Z, ACS Appl. Mater. Interfaces, 6, 3661-73, 2014. Kuponu, O S; Kadirkamanathan, V; Bhattacharya, B; Pope, S A, Smart Mater. Struct., 25, 055028, 2016. Lopez, J; Chen, Z; Wang, C; Andrews, S C; Cui, Y; Bao, Z, ACS Appl. Mater. Interfaces, 8, 2318-24, 2016. Li, J; Shklyaev, O E; Li, T; Liu, W; Shum, H; Rozen, I; Balazs, A C; Wang, J, Nano Lett., 15, 7077-83, 2015. Fan, F; Szpunar, J, J. Appl. Polym. Sci., 132, 42135, 2015. van der Kooij, H M; de Kool, M; van der Gucht, J; Sprakel, J, Langmuir, 31, 4419-28, 2015. Imato, K; Takahara, A; Otsuka, H, Macromolecules, 48, 5632-9, 2015. Hall, J; Qamar, I P S; Rendall, T C S; Trask, R S, Smart Mater. Struct., 24, 037002, 2015.
Self-healing Timescale Strain (stress-free) relaxation in mechanically prestrained bone has a time constant of 75 s.1 It occurs by a reorganization of the proteoglycan-glycoprotein matrix between collagen fibers, which requires ionic interactions. Relaxation times of about 1 min were reported
Figure 8.1. (a) Surface topography and (b) cross-section profiles of the evolution of the wear track on epoxy resin containing 7% ionic liquid. [Adapted, by permission, from Saurin, N; Sanes, N; Carrion, F J; Bermudez, M D, RSC Adv., 6, 37258, 2016.]
80
Self-healing Timescale
for stress relaxation of the whole bone and tendon. This should not be confused with the healing of stress fractures, and macroscopic bone traumas, taking days to weeks to heal. The fast process provides spontaneous healing of plastically strained bone in walking or running. Figure 2.1 shows that autonomic sealhealing of poly(vinyl butyral) takes several hours at high relative humidity. Healing efficiency is attributed to the molecular Figure 8.2. Crack area vs. distance from top surface for motion at the surface, which contains a a concrete sample after 5 cycles (one cycle: sample sub- large amount of water owing to the surface merged in water at 20±1°C for 24 h, and dried in air at localization of vinyl alcohol.2 The water 20±1°C and 50±5% RH for 24 h.). [Adapted, by permisacts as a plasticizer.2 sion, from Fan, S; Li, M, Smart Mater. Struct., 24, 015021, 2015.] The healing time is one of the common methods of expression of healing timescale. Thermoplastics or fiber-reinforced composites exhibit the highest strengths (from tens to hundreds of MPa) and their healing times range between hours to a few days.3 The healing efficiency of vulcanized chloroprene rubber increased with healing time increase.4,5
Figure 8.3. Self-healing process of hydrogel: (a) two cylindrical hydrogels (b) hydrogels cut in half; (c) two semicircle hydrogels healed within 1 min; (d) healed hydrogel picked up with forceps. The details of the self-healing process were recorded by optical microscopy microimages: (c1) 0 s; (c2) 5 s; (c3) 10 s; (c4) 20 s; (c5) 60 s; (c6) 5 min. [Adapted, by permission, from Li, G; Wu, J; Wang, B; Yan, S; Zhang, K; Ding, J; Yin, J, Biomacromolecules, 16, 3508-18, 2015.]
Self-healing Timescale
81
The effect of the addition of 1-octyl-3-methylimidazolium to a conventional epoxy resin on self-healing rate has been studied.6 Figure 8.1 shows the evolution of the wear tracks and cross-section profiles along the healing time.6 The self-healing ability over time increases with increasing ionic liquid concentration, to reach values higher than a 90% after 22 hours, for the materials with 9 and 12 wt% ionic liquid.6 Concrete cracking and deterioration show that the healing process at the greater depths (inside the crack) takes a significantly longer time (Figure 8.2).7 The healing results from continued hydration and pozzolanic reactions.7 Supramolecular self-assembled hydrogels based on poly(l-glutamic acid) were observed during the self-healing process (Figure 8.3).8 After 60 s, the two pieces of hydrogel disks healed and obtained their original shape.8 The initial gap width of the fresh crack was 126.7 μm (Figure 8.3c1), which then narrowed down to 42.3 μm after 5 s (Figure 8.3c2).8 The crack disappeared fast within 10 s (Figure 8.3c3), and the injury healed completely within 60 s (Figure 8.3c4 and 8.3c5).8 Photographs also shows that stains quickly interpenetrate each other side.8 A hybrid supramolecular polymeric hydrogel was constructed via host-guest interaction of a host cyclodextrin polymer with a guest α-bromonaphthalene polymer and mixed with 6-thio-β-cyclodextrin modified gold nanoparticles.9 The hydrogel has a rapid selfhealing (1 min).9 The G’ and G” of the self-healed hydrogel attained their original levels after 1 hour.9 The self-healing of poly(butyl acrylate) copolymer containing comonomers with 2ureido-4[1 H]-pyrimidinone quadruple hydrogen bonding groups was investigated.10 Fractured films fully recovered their self-adhesion strength to 40, 81, and 100% in 10 s, 3 h, and 50 h, respectively.10 To accelerate the healing process, antimony pentafluoride was used as an instant hardener of epoxy after encapsulation the highly active antimony pentafluoride-ethanol complex in the hollow silica spheres.11 The epoxy material with the embedded healant capsules can be healed within a few seconds (Figure 8.4).11 Using impact strength data, it was found that the healing efficiency increases with curing time.11 The self-healing specimens can be safely stored for many months without losing their healing efficiency.11 In another development, the healing rate of a self-healing process subjected to low cycle dynamic stress was proposed to be regulated according to the rate of damage.12 This
Figure 8.4.(a) Infrared absorption of epoxide groups as a function of time from hardener addition (b) curing degree of epoxy calculated with the peak areas in (a). [Adapted, by permission, from Ye, X J; Zhang, J-L; Zhu, Y; Rong, M Z; Zhang, M Q; Song, Y X; Zhang, H-Z, ACS Appl. Mater. Interfaces, 6, 3661-73, 2014.]
82
Self-healing Timescale
Figure 8.5. (A-C) Time-lapse microscopy images and (D-F) simulated video snapshots showing the nanomotor propulsion and localization process with an H2O2 fuel concentration of 7.5% and average motor density of 16.3 motors per 100 x 100 μm2 at times (A) 0 s, (B) 150 s, and (C) 300 s from adding the fuel solution. (G-I) Timelapse microscopy images showing the influence of the motor speed, controlled by the fuel concentration, on the propulsion and localization process of multiple nanomotors. Fuel concentrations, (G) 2.5%, (H) 5.0%, and (I) 7.5%. (J-L) Simulated video snapshots showing the propulsion and localization process of multiple nanomotors moving at speeds corresponding to H2O2 fuel concentrations of (J) 2.5%, (K) 5.0%, and (L) 7.5%. Scale bar (A.C; G.I): 50 μm. [Adapted, by permission, from Li, J; Shklyaev, O E; Li, T; Liu, W; Shum, H; Rozen, I; Balazs, A C; Wang, J, Nano Lett., 15, 7077-83, 2015.]
is to limit damage from the propagating fault.12 Active self-healing takes advantage of sensing and added external energy to achieve the desired healing rate.12 The concept of active self-healing is tested by modeling.12 Figure 4.2 shows the principles of modeling.12 Binders with relaxation times on the order of 0.1 s gave the best cycling ability with 80% capacity maintained for over 175 cycles using large silicon particles (~0.9 μm).13 This is attributable to an improved balance between the viscoelastic stress relaxation in the binder and the stiffness needed to maintain mechanical integrity.13 The more crosslinked binder showed worse performance, confirming the need for liquid-like flow.13 A synthetic repair system where selfpropelled nanomotors autonomously seek and localize microscopic cracks (for explanations see Figure 4.1).14 The nanomotors exhibit isotropic self-propulsion at consistent average speeds over long time periods.14 The self-propulsion of the motors is combined with their Brownian reorientaFigure 8.6. The self-healing efficiency changes as a tion to behave as a random walk.14 The function of the healing time from 10 to 960 min. surface irregularities (e.g., cracks) disrupt [Adapted, by permission, from Fan, F; Szpunar, J, J. Appl. Polym. Sci., 132, 42135, 2015.] the random walk through physical interac-
Self-healing Timescale
83
tions and entrench the nanomotors in a potential well within the crack.14 The hydrophobic hemispheres of the nanomotors can adhere to these exposed hydrophobic crack surfaces.14 Figure 8.5 shows that the nanomotors quickly find their destinations in the crack.14
Figure 8.7.Typical SEM images showing the post-drying evolution of a droplet. The aging times are (a) 4 h, (b) 12 h, (c) 24 h, (d) 48 h, and (e) 10 days. The first column displays the lower right quadrant of the droplet, the second column gives a magnification of the center, and the last column shows a high magnification of the contact line (indicated in (e) by a dashed line). The tiny cracks are artifacts caused by the sputter-coating. [Adapted, by permission, from van der Kooij, H M; de Kool, M; van der Gucht, J; Sprakel, J, Langmuir, 31, 4419-28, 2015.]
84
Self-healing Timescale
Figure 8.8. Optical microscope images of self-healing behavior of scratched crosslinked polymer films with diarylbibenzofuranone linkages healed at 50°C for 12 h (top), at 40°C for 72 h (middle), and at 30°C for 72 h (bottom). Scale bars are 0.1 mm. [Adapted, by permission, from Imato, K; Takahara, A; Otsuka, H, Macromolecules, 48, 5632-9, 2015.]
Figure 8.6 shows that the healing efficiency of an industrial acrylic elastomer increases with the increase in the healing time.15 The hydrogen bonding connects the damaged surfaces and provides a bridge for chain diffusion across the cut interface.15 The chain diffusion process creates chain entanglements.15 Also, the increased temperature promotes healing efficiency because the mobility of molecular chains increases with the temperature.15 Figure 8.7 shows the development of morphology during film formation from polymer dispersion.16 In the beginning of the process, the smaller cracks self-heal while the largest cracks expand up to a width of ~15 μm.16 However, after 10 days of aging also the largest cracks disappear, and the film has no distinguishable features.16 Polyurethane crosslinked with diarylbibenzofuranone-based dynamic covalent linkages has self-healing rate affected by temperature (Figure 8.8).17 At 50°C, the scar disappeared after 12 h.17 The scar was almost undetectable after healing for 72 h at 40°C.17 The scratch remained evident even after healing for 72 h at 30°C.17 A numerical model of healing agent flow from an orifice has been developed using smoothed particle hydrodynamics to explore the flow characteristics of healing agent leaving a vascular network and infusing a damage site within a fiber-reinforced polymer composite.18 The model is able to reproduce experimental results for the discharge coefficient at the high Reynolds number limit and the power-law behavior for low Reynolds numbers.18 REFERENCES 1 2 3
Akbarzadeh, J; Puchegger, S; Stojanovic, A; Kirchner, H O K; Binder, W H; Bernstorff, S; Zioupos, P; Peterlik, H, Bioinspired, Biomimetic, Nanomater., 3, 3, 123-30, 2014. Arayachukiat, S; Doan, V A; Murakami, T; Nobukawa, S; Yamaguchi, M, J. Appl. Polym. Sci., 132, 42008, 2015. D’Elia, Eslava, S; Miranda, M; Georgiou, T K; Saiz, E, Sci. Reports, 6, 25059, 2016.
Self-healing Timescale
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
85
Xinag, H P; Rong, M Z; Zhang, M Q, ACS Sustainable Chem. Eng., 4, 2715-24, 2016. Zheng, J; Xiao, P; Liu, W; Zhang, J; Huang, Y; Chen, T, Macromol. Rapid Commun., 37, 265-70, 2016. Saurin, N; Sanes, N; Carrion, F J; Bermudez, M D, RSC Adv., 6, 37258, 2016. Fan, S; Li, M, Smart Mater. Struct., 24, 015021, 2015. Li, G; Wu, J; Wang, B; Yan, S; Zhang, K; Ding, J; Yin, J, Biomacromolecules, 16, 3508-18, 2015. Chen, L; Chen, H; Yao, X; Ma; X; Tian, H, Chem. Asian J, 10, 2352-5, 2015. Faghihnejad, A; Feldman, K E; Yu, J; Tirrell, M V; Israelachvili, J N; Hawker, C J; Kramer, E J; Zeng, H, Adv. Funct. Mater., 24, 2322-33, 2014. Ye, X J; Zhang, J-L; Zhu, Y; Rong, M Z; Zhang, M Q; Song, Y X; Zhang, H-Z, ACS Appl. Mater. Interfaces, 6, 3661-73, 2014. Kuponu, O S; Kadirkamanathan, V; Bhattacharya, B; Pope, S A, Smart Mater. Struct., 25, 055028, 2016. Lopez, J; Chen, Z; Wang, C; Andrews, S C; Cui, Y; Bao, Z, ACS Appl. Mater. Interfaces, 8, 2318-24, 2016. Li, J; Shklyaev, O E; Li, T; Liu, W; Shum, H; Rozen, I; Balazs, A C; Wang, J, Nano Lett., 15, 7077-83, 2015. Fan, F; Szpunar, J, J. Appl. Polym. Sci., 132, 42135, 2015. van der Kooij, H M; de Kool, M; van der Gucht, J; Sprakel, J, Langmuir, 31, 4419-28, 2015. Imato, K; Takahara, A; Otsuka, H, Macromolecules, 48, 5632-9, 2015. Hall, J; Qamar, I P S; Rendall, T C S; Trask, R S, Smart Mater. Struct., 24, 037002, 2015.