Macromolecular Rapid Communications

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Fast Optical Healing of Crystalline Polymers Enabled by Gold Nanoparticles Hongji Zhang, Daniel Fortin, Hesheng Xia,* Yue Zhao*

A general method for very fast and efficient optical healing of crystalline polymers is reported. By loading a very small amount of gold nanoparticles (AuNPs) in either poly(ethylene oxide) (Tm ≈ 63 °C) or low-density polyethylene (Tm ≈ 103 °C), the heat released upon surface plasmon resonance (SPR) absorption of 532 nm light by AuNPs can melt crystallites in the interfacial region of two polymer pieces brought into contact; and the subsequent recrystallization of polymer chains on cooling merges the two pieces into one. The fracture strength of such repaired sample can reach the level of the undamaged polymer after 10 s laser exposure. Moreover, in addition to an ability of long-distance remote and spatially selective healing, the optical method also works for polymer samples immersed in water.

1. Introduction Polymers capable of self-repairing a mechanical damage (crack or fracture) autonomously or can be healed by the input of a stimulus represent a class of emerging “smart” materials. They have generated fast growing interest because self-healing can prolong the lifetime of a material, reduce the cost, and improve the safety. There are basically two approaches to achieving polymer self-healing without a stimulus. One consists in loading microcapsules filled with a reactive monomer in the polymer matrix. Upon propagation of a fracture, capsules can be broken and the monomer flows into the damaged region and polymerizes in the presence of a catalyst.[1–4] The second approach is to prepare supramolecular polymers whose chain structures

H. Zhang, Dr. D. Fortin, Prof. Y. Zhao Département de Chimie, Université de Sherbrooke, Sherbrooke, Québec, Canada, J1K 2R1 E-mail: [email protected] Prof. H. Xia State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan University, Chengdu 610065, China E-mail: [email protected]

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are built up with non-covalent bonds. After being broken, those dynamic bonds reform when fracture surfaces are in touch.[5–9] In the case of polymer healing triggered by stimuli such as heat,[10,11] pH,[12] and light,[6,13,14] to name only a few, exposure of the fracture to a stimulus must initiate formation of new chemical bonds to merge the fracture surfaces.[5–16] Except the use of microcapsules, for both autonomous and stimuli-triggered healing, and regardless of the stimulus used, efficient repair requires formation of new chemical bonds across the fracture surfaces and for this to happen, polymer chains must have sufficient mobility to diffuse and interact. This fundamental condition of chain mobility and new chemical bonds formation generally limits the self-healing ability only to soft polymers such as elastomers, hydrogels or low Tg polymers.[5–20] Some recent reports have addressed this challenge by designing stiff self-healable polymers.[21,22] It is easy to understand that without healing capsules, hard or stiff crystalline polymers lack the chain mobility for spontaneous self-healing at room temperature. The best one can achieve would be efficient stimuli-triggered healing. In this paper, we demonstrate a general approach to this end. We show that making use of the photothermal effect from surface plasmon resonance (SPR) of gold nanoparticles (AuNPs), very fast and efficient

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DOI: 10.1002/marc.201300640

Macromolecular Rapid Communications

Fast Optical Healing of Crystalline Polymers Enabled by Gold Nanoparticles . . .

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optical healing of crystalline polymers can be achieved based on a melting and recrystallization mechanism. Although the photothermal effect via SPR absorption of AuNPs has already been used for many photoresponsive polymers, investigated for crystalline polymers and their processing,[23,24] and a similar photothermal effect using carbon nanotubes was utilized for laser welding under the assistance of external applied pressure,[25] to our knowledge, the present study is the first demonstration of using the SPR photothermal effect to achieve fast optical healing of crystalline polymers with recovered initial mechanical strength while the required loading amount of AuNPs is much lower than that of carbon nanotubes.[25] It should also be noted that heat generated by magnetic nanoparticles loaded in polymers upon exposure to an alternating magnetic field has been much exploited to induce phase transitions and utilized for such functions as polymer self-healing[26,27] and shape memory.[28–31] As compared to the magnetic inductive heating, the photothermal effect resulting from laser light absorption of AuNPs has several advantages such as long-distance remote activation, much higher spatial selectivity of localized heating and the need of a much lower content of nanofillers in a given polymer matrix.

2. Results and Discussion To evaluate the optical healing efficiency, the merging of two separate pieces was utilized as the test of fracture repairing. The envisioned optical healing mechanism is schematically illustrated in Figure 1. A small amount of AuNPs is dispersed in a crystalline polymer; when light

Figure 1. Schematic illustration of the mechanism allowing for fast optical healing of crystalline polymers. The parallel lines denote crystallized polymer chains (amorphous regions omitted), while the coiled lines (in b) show melted chains in the fracture area exposed to laser. The black dots represent gold nanoparticles (AuNPs). (a) Two pieces of a crystalline polymer brought into contact; (b) the junction area (fracture surfaces) is exposed to laser resulting in local melting of polymer crystallites and chain interdiffusion; and (c) melted chains in the fracture area recrystallize after turning off the laser.

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with a wavelength near the SPR of AuNPs (about 530 nm) is applied to the interfacial region between two pieces brought into contact, the heat released by AuNPs upon light absorption can raise the local temperature to above the melting temperature Tm of the polymer so that the crystallites are melted and polymer chains in the melt can diffuse across the interface. When light is turned off, interdiffused polymer chains in the interfacial area are cooled and can recrystallize during the cooling, which leads to the repair the fracture. As shown below, the local temperature rise can be controlled by adjusting the amount of loaded AuNPs or light intensity, which makes the method, in principle, applicable to even very hard crystalline polymers with high Tm. We first investigated the approach using poly(ethylene oxide) (PEO) that is a crystalline polymer with a relatively low Tm. PEO was loaded with AuNPs at various low contents (from 0.0002 to 0.04 wt% relative to PEO) by mixing with citrate-stabilized AuNPs in an aqueous solution at room temperature. Film samples (0.45 mm in thickness) used for light-triggered self-healing tests were prepared by compression molding of the mixture at about 100 °C (experimental details in Supporting Information). DSC measurements show that the presence of such small amounts of AuNPs has very little effect on the crystallinity of PEO (76–79%) and its Tm (63–64 °C) (Figure S1, Supporting Information). And the absorption spectrum of a solution-cast thin film of PEO/AuNP displays an SPR band at 532 nm similar to that of AuNPs solubilized in water (Figure S2), indicating good dispersion of AuNPs in the PEO matrix (also confirmed by TEM observation, Figure S3). The results of light-triggered self-healing tests are shown in Figure 2. On the one hand, for visual observation, a piece of PEO containing only 0.0002 wt% AuNPs was cut into two (Figure 2a); the two halves were put into contact without pushing (Figure 2b), then a diode laser (532 nm, 11.4 W cm−2, beam diameter ≈3 mm) was applied to the interfacial area for 10 s (scanning the laser along the junction of the two surfaces) and the two halves appeared to merge into one single piece (Figure 2c). This film healed by 10 s light exposure could already sustain a load >8000 times its weight (Figure 2d), indicating very fast and effective optical healing. At such a low AuNP content, the transparent film looks almost the same as neat PEO (Figure S7). On the other hand, to further evaluate the optical healing efficiency, tensile tests were performed on the original and optically healed samples containing 0.04 wt% of AuNPs with varying laser irradiation times. After turning off the laser and before conducting the mechanical tests, the samples were left at room temperature for at least several minutes for thermal equilibrium in the exposed area. The results of one set of experiments are shown (Figure 2e). A substantial tensile strength developed after even 3 s of light exposure at an intensity of

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region containing smaller spherulites that bridge the ravine. This is clear evidence that PEO chains in the damaged region melt when exposed to laser and recrystallize upon cooling after turning off the laser. Thanks to the tunable size of the laser beam, the optical healing can be made to occur only where a crack is formed or damage is imposed, without compromising the morphology or property of the rest of the material. This spatially confined self-healing also prevents macroscopic flow due Figure 2. a–d) Photos showing fast optical healing of PEO/AuNP (0.0002 wt%) at room temperature in air: a) original film; b) cut into two halves; c) healed film after 10 s laser to melting of crystalline polymers. exposure on the junction of the fracture surfaces at an intensity of 11.4 W cm−2; d) healed Figure 3c shows the results of a control film (indicated by an arrow) under a load >8000 times its weight. e) Stress–strain curves test that provides more evidence of the of the original and optically healed films of PEO/AuNP (0.04 wt%) for various laser expoproposed mechanism. We measured sure times at the intensity of 2.2 W cm−2. the change in temperature for neat PEO and a PEO/AuNP (0.04 wt%) film upon 2.2 W cm−2. The healed polymer almost completely exposure to the laser beam by using a digital thermogained its original strength after 10 s of laser irradiation couple embedded deep enough into the PEO/AuNP film to (2.0 ± 0.3 MPa at break for the healed samples). We did eliminate the impact of laser beam on the thermocouple multiple tests and observed that a number of optically device. Without AuNPs, the laser exposure only raised the healed samples actually fractured outside the healed temperature by a few degrees, which, by no means, can junction region upon stretching. The used PEO sample melt crystalline PEO. With the small amount of AuNPs has a high crystallinity and is brittle, which accounts for the low strain at rupture. The above results show that the dispersion of a very small amount of AuNPs makes it possible to realize very fast optical healing of hard crystalline polymers. We conducted experiments that confirmed unambiguously the mechanism illustrated in Figure 1, that is, AuNP SPR absorption-induced melting of crystallized polymer chains under light exposure and their subsequent recrystallization when light is turned off is at the origin of the repairing phenomenon. Figure 3a shows the polarizing optical micrograph of a thin film (50 μm) of PEO/AuNP (0.04 wt%), the crystallites of PEO can be seen from the typical spherulites observable under crossed polarizers. The film was cut through by a razor blade near the middle, splitting the spherulites, and forming a ravine-shaped fracture. Figure 3b shows the image of the same Figure 3. a) Polarizing optical micrographs (POM) of a PEO/AuNP (0.04 wt%) film with a see-through damage made by razor blade cut; b) after localized optical healing by film with the fracture area exposed exposing the damaged area to laser (λ = 532 nm, 2.2 W cm−2) at room temperature in air. to the 532 nm laser at an intensity of c) Change in temperature over time for blank PEO and PEO/AuNP films upon exposure 2.2 W cm−2 at room temperature in air to laser in air and after turning off the laser. d) X-ray diffraction patterns of a PEO/AuNP for 10 s. The damaged film completely (0.04 wt%) sample recorded before exposure to laser, under laser exposure (0.9 W cm−2) healed through a visibly recrystallized and after turning off the laser.

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Fast Optical Healing of Crystalline Polymers Enabled by Gold Nanoparticles . . .

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added in the polymer, once the laser was turned on, the local polymer temperature increased quickly to above Tm of PEO leading to the melting of spherulites. As expected, the temperature rise is quicker and more important with increasing the laser intensity. Interestingly, with the two intensities investigated, when the laser was turned off, Figure 4. Photos and tensile tests showing light-triggered self-healing of LDPE/AuNP the temperature dropped quickly until (0.005 wt%) films (0.45 mm in thickness) at room temperature in air: a) original film; reaching about 45 °C where the temb) cut into two halves; c) healed film after 10 s laser exposure (532 nm, 7.7 W cm−2) on perature decrease became much slower the junction of the fracture surfaces; d) and e) the film healed by 10 s light exposure can resist twisting and stretching (200% strain); f) stress–strain curves of the original and (with even a slight increase followed by optically healed films for various laser exposure times. a plateau in the case of the 2.2 W cm−2 laser exposure). This slow-down in the temperature decease is clearly caused by the recrysshow that AuNPs are also well dispersed in LDPE and tallization of PEO chains that is an exothermic process. that the presence of up to 0.05 wt% AuNPs affects very This part of curve actually reflects the recrystallization little the thermal transition temperatures as well as the kinetics of PEO chains after turning off the laser, which is crystallinity of LDPE (Figure S8 and Figure S9). As seen in determined mainly by three parameters: the laser beam Figure 4, the optical healing works equally well with this intensity, the AuNP content, and the temperature of the high Tm crystalline polymer. Photos a-e show the cutting surrounding medium (air in the present case). Needless of an LDPE/AuNP (0.005 wt%) film, the optical healing to say that at the same laser intensity, a higher content and the ability of the healed film to stand twisting or of AuNPs results in a faster rise in temperature. Finally, stretching to 200% deformation without failure. The tenFigure 3d shows the X-ray diffractograms of a PEO/AuNP sile test results in Figure 4f also show the very fast healing (0.04 wt%) sample before laser exposure, under laser process for LDPE. At a laser intensity of 7.7 W cm−2, 10 s exposure (0.9 W cm−2) and after turning off the laser. The of laser exposure along the cut junction region was enough to allow the healed film to basically recover the highly crystalline nature of PEO in the initial film was initial elastic modulus, yield strength, tensile strength, indicated by the characteristic diffraction peaks at 2θ ≈ 19° and large deformation at rupture. It should be pointed and 23° (crystallinity ≈ 77%); upon laser exposure, only out that this SPR-based fast optical repairing works for an amorphous halo was observed indicating the melting crystalline polymers. Only with crystallite melting, the of PEO crystallites. After the laser was turned off, the chain interdiffusion at the interface is important and the recorded diffraction pattern indicated the recrystallisubsequent recrystallization effectively “binds” the two zation of PEO with a similar crystallinity (≈73%). More pieces. For amorphous polymers, under similar conditions evidence for the optically induced melting and the suband with AuNP-induced heating to ≈20 °C above Tg of the sequent recrystallization of PEO are presented in Supporting Information, including infrared spectral changes polymer, negligible healing effect could be observed in of a PEO/AuNP film recorded with the laser beam on and seconds due to limited chain interdiffusion and entangleoff (Figure S5) as well as polarizing photomicrographs ment across the interface. showing the growth of crystallites of PEO from the isoThe optical healing approach enabled by AuNPs tropic state in the composite film immediately after repvresents a robust method. In principle, one can apply turning off the laser (Figure S6). it to other crystalline polymers. Besides the remarkably quick healing capability, since laser beam can travel long In order to demonstrate the generality of the optical distance, the spatially selective optical healing can be healing approach for crystalline polymers, we further realized by having the laser source far from the sample to investigated the use of a low-density polyethylene sample be repaired. We did a successful test by placing the laser (LDPE, ρ = 0.915 g cm−3). As compared to PEO, LDPE has a 10 m away from a sample of PEO/AuNP. Obviously, this higher Tm of about 103 °C and a lower crystallinity (≈30%). feature is appealing for healing objects that are difficult or It can undergo large plastic deformation when subjected even impossible to reach. Moreover, as long as light beam to tensile tests. Thick films of LDPE/AuNP (0.45 mm can be delivered, the method can also be utilized to heal in thickness) were prepared by first dispersing dodean object located in an environment other than air. We canethiol-stabilized AuNPs into LDPE through toluene have tested this feature by performing an optical healing solution-mixing at above 100 °C to ensure the dissolution experiment on a thick film of LDPE/AuNP (0.05 wt%) of LDPE, followed by compression molding at about 150 immersed in water. The results show that it is possible °C. The absorption spectra and DSC measurement results

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to achieve optical healing for samples in water by using higher laser intensity (Figure S10).

3. Conclusions To summarize, we have demonstrated a method for very fast and efficient optical healing of crystalline polymers. The mechanism has been clearly established based on the melting of polymer crystallites in the fracture region induced by localized polymer heating due to SPR absorption of a small amount of AuNPs and the subsequent recrystallization on cooling. This optical healing method is applicable to a large number of crystalline polymers and is particularly useful for hard or stiff polymers, which differs from other approaches for light-enabled healing of polymers.[6,13,14,22] The use of AuNPs requires a much lower loading amount than other light-absorbing nanofillers.[25–27]

4. Experimental Section Materials, sample preparation, and more characterization results using DSC, UV–vis, X-ray diffraction, IR, TEM, and polarizing optical microscope are supplied in the Supporting Information.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: Y.Z. acknowledges the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and le Fonds québécois de la recherche sur la nature et les technologies of Québec (FQRNT). HZ thanks the support of China Scholarship Council (CSC). H.X. acknowledges the financial support of the major project of Ministry of Education of China (313036). Y.Z. is a member of the FQRNTfunded Center for Self-Assembled Chemical Structures (CSACS) and the Centre québécois sur les matériaux fonctionnels (CQMF). Received: August 20, 2013; Revised: October 6, 2013; Published online: October 29, 2013; DOI: 10.1002/marc.201300640

Keywords: gold nanoparticles; optical healing; nanocomposites; stimuli-responsive polymers

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Fast optical healing of crystalline polymers enabled by gold nanoparticles.

A general method for very fast and efficient optical healing of crystalline polymers is reported. By loading a very small amount of gold nanoparticles...
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