Thermal Switching of Thermoresponsive Polymer Aqueous Solutions

Letter Cite This: ACS Macro Lett. 2018, 7, 53−58

pubs.acs.org/macroletters

Thermal Switching of Thermoresponsive Polymer Aqueous Solutions Chen Li,†,‡ Yunwei Ma,†,‡ and Zhiting Tian*,†,§ †

Department of Mechanical Engineering, Virginia Tech, Blacksburg, Virginia 24061, United States Macromolecules Innovation Institute, Virginia Tech, Blacksburg, Virginia 24061, United States

§

S Supporting Information *

ABSTRACT: Thermal switches are of great importance to thermal management in a wide variety of applications. However, traditional thermal switches suffer from being large and having slow transition rates. To overcome these limitations, we took advantage of abrupt second-order phase transitions in thermoresponsive polymer aqueous solutions to enable fast thermal switching. While thermoresponsive polymers have been widely studied for biomedical applications, their thermal switching capability has not been studied. In this work, we used poly(N-isopropylacrylamide) (PNIPAM) as a model system to demonstrate abrupt thermal conductivity changes of thermoresponsive polymer aqueous solutions across their transition temperatures by using a powerful approach, the transient thermal grating technique, which has high sensitivity. We observed a thermal switching ratio up to 1.15 in dilute PNIPAM aqueous solutions (up to 0.025 g/mL) across the transition. This work may provide new opportunities to engineer thermal switches using second-order phase transitions of thermoresponsive polymer aqueous solutions or abrupt higher-order phase transitions in general. polymers: the first type exhibits a lower critical solution temperature (LCST),26 where the polymer becomes insoluble after heating; the second type possesses an upper critical solution temperature (UCST),27 where the polymer becomes soluble after heating. The conformation of a polymer results from the balance between the entropy of the chain and the enthalpy of the isomers.28 The free energy of mixing for a polymer and a solvent leads to different equilibrium phases at different temperatures.28 Poly(N-isopropylacrylamide) (PNIPAM)18,28−35 is the most studied thermoresponsive polymer because its LCST is 32 °C (Figure S1 in the Supporting Information (SI)), which is close to human body temperature and can be shifted by adding additives or surfactants.36 Thermal switches with transition temperatures near room temperature have countless potential applications such as thermal energy storage37 and smart building.38 Like other LCST polymers, a significant volume change with temperature has been found for PNIPAM.39 In this work, we report the first direct measurement of thermal conductivity change in PNIPAM aqueous solutions across the LCST using a powerful approach, the laser-induced transient thermal grating (TTG) technique. The results show an abrupt thermal conductivity drop across the transition temperature, in contrast to the previously anticipated trend.40 As a first study of the thermal switching capability of thermoresponsive polymer aqueous solutions, this work may shed light on the potential of using thermoresponsive polymer aqueous solutions or higher-order phase transitions for thermal switch applications.

T

hermal switches are of great importance to thermal management in a wide variety of applications, ranging from space and building technologies to energy storage systems. Although the concept of thermal switches was first devised in the early 1960s for use on the Mariner mission to the moon, their practical development is still relatively new.1−8 Most thermal switches are based on mechanically making/ breaking thermal contacts via complicated system designs9 or first-order phase transitions.3,10 These traditional thermal switches suffer from large sizes and slow transitions. Fabricating fast thermal switches with simple designs is appealing yet challenging, as it requires a drastic change in the thermal conductivity of the constitutive materials. Thermal switching ratios ranging from 1.5 to 3.2 have been achieved in composites.11,12 However, these thermal switches were based on first-order phase transitions, where latent heat costs additional energy and time for the transition to happen. In this work, we take advantage of abrupt second-order phase transitions in thermoresponsive polymer aqueous solutions that can respond to varying thermal environments in a timely manner to enable fast thermal switching. Thermoresponsive polymers are the most widely studied stimuli-responsive polymers whose physical and chemical properties can experience fast and significant changes due to slight variations in ambient environment.13−22 Thermoresponsive polymers exhibit a reversible volume change and sharp changes in solvation state23 upon a small temperature change at phase transitions. In addition to their biomedical applications, including drug delivery, gene delivery, and tissue engineering, these polymers also have applications in batteries24 and sensors.25 However, the thermal switching capability of thermoresponsive polymer aqueous solutions has not yet been studied. There are two main types of thermoresponsive © XXXX American Chemical Society

Received: November 29, 2017 Accepted: December 13, 2017

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DOI: 10.1021/acsmacrolett.7b00938 ACS Macro Lett. 2018, 7, 53−58

Letter

ACS Macro Letters We first used TTG to measure the thermal conductivity of water for calibration. Its thermal conductivity is 0.611 ± 0.006 W/mK at 25 °C, and 0.628 ± 0.003 W/mK at 36 °C, which is a 0.7% and 0.6% difference compared with the literature values41−45 at 25 and 36 °C, respectively. This gives us confidence in our measurements. We then measured different concentrations of PNIPAM aqueous solutions. Because of the limitations of the transmission geometry, we used dilute concentrations below 0.025 g/mL. The solubility of PNIPAM is higher than 0.025 g/mL, but after transition, the solutions are too cloudy to be measured in the transmission geometry (the signal intensity drops significantly after the transition at 0.025 g/mL, as shown in Figure S2 in SI). Key results are summarized in Figure 1. At room temperature, the PNIPAM aqueous solutions show a slightly lower

until reaching the LCST. Once the temperature reaches the LCST, all of the PNIPAM aqueous solutions, except the solution at the lowest concentration (0.001 g/mL), show a clear and sharp drop in thermal conductivity upon phase transition. Because it is a higher-order phase transition, fast thermal conductivity change was observed with a transition time on the order of milliseconds.48 Control experiments were carried out and included in SI to show that the measured change in thermal conductivity does not result from the increased light scattering above the LCST of PNIPAM aqueous solutions. The observed trend of decreasing thermal conductivity upon phase transition is counterintuitive if one considers the water phase only because (1) the thermal conductivity of water increases with temperature; (2) above LCST, more hydrogen bonds form between water molecules and the thermal conductivity of water phase is expected to further increase. This trend is also opposite from the previous claim40 that thermal conductivity of PNIPAM aqueous solutions increases as the temperature increases across the LCST. We propose a possible explanation for the observed sudden drop. Below the LCST, PNIPAM is water-soluble and forms predominantly intermolecular hydrogen bonds with water. Above the LCST, the hydrogen bonds between PNIPAM and water molecules break due to the gain in entropy associated with the release of bound water molecules,31,49 and the intramolecular hydrogen bonds with adjacent side groups become dominant.50,51 PNIPAM changes its hydrophilicity, shrinks in volume, and expels water after transition, as shown in Figure 2. Previous studies showed that hydrophilicity facilitates thermal transport at solid−liquid interfaces compared with hydrophobicity.52−54 Transition from a homogeneous phase into two separate phases increases thermal interface resistance and yields a lower effective thermal conductivity. 55 The thermal interface resistance needs to be taken into account in order to use the effective medium theory. Another possible reason for the decrease in thermal conductivity is that the shrinking of the PNIPAM after phase transition reduces the thermal con-

Figure 1. Thermal conductivity of PNIPAM aqueous solutions of different concentrations as a function of temperature.

thermal conductivity than that of water due to the lower thermal conductivity of water-soluble polymers than that of water.46,47 Among the dilute solutions, their thermal conductivities barely vary before the transition. As the temperature increases, the thermal conductivity of the PNIPAM aqueous solutions increases with temperature, the same as with water,

Figure 2. Schematics of the PNIPAM aqueous solution (a) below and (b) above LCST. 54

DOI: 10.1021/acsmacrolett.7b00938 ACS Macro Lett. 2018, 7, 53−58

ACS Macro Letters ductivity of the polymer phase and thus the overall thermal conductivity, although we expect this effect to be minor. The thermal switching ratio of PNIPAM aqueous solutions across the transition keeps increasing with increasing concentration as shown in Figure 3. Across the LCST, the thermal switching ratio reaches 1.15 at a concentration of 0.025 g/mL. An even larger ratio is expected at higher concentrations.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Thermal Conductivity Measurement: Different methods have been developed to measure thermal conductivity, each having advantages and disadvantages in bulk or thin films.56 The steady-state method and the transient hot-wire method57 can be used for bulk measurements. The 3ω method 58−62 and time-domain thermoreflectance (TDTR)63−68 method can be used for thin film measurements. Recently, the TTG measurement has drawn attention as a new optical, in-plane thermal conductivity measurement.69−74 This setup has advantages over conventional methods in that high space and time resolution are obtained with little sample preparation and simple data interpretation. Typically, two pump beams with the same phase and frequency interfere in the sample to create a spatially periodic temperature distribution called thermal grating (Figure S3 in SI).69 The decay of the thermal grating is detected by a diffracted probe beam, which can give the thermal diffusivity with high accuracy (see SI for the detailed thermal model analysis to extract thermal diffusivity). We used the transmission geometry in the TTG setup and enhanced the signals by heterodyne detection75 (schematic is plotted in Figure 4, and the actual experimental setup is shown in Figure S4 in SI). The phase mask diffracted about 75% of the beam energy into the two first ±1 diffraction maxima at both 532 nm (pump) and 514 nm (probe). We chose a mask spacing of 3.7 μm, resulting in a grating period of L = 1.25 μm. Since the PNIPAM solution is transparent and colorless, we added blue dye into the solution to absorb part of the pump beams in order to generate thermal gratings in the sample. We carefully confirmed that the effect of the dye on thermal diffusivity was minimal. The sample solution was placed in a 50 μm-thick cuvette. During the measurements, the concentration of dye was kept the same for different solutions, and a water bath was used to control the sample temperature (Figure S5 in SI). To obtain the actual sample temperature at the beam spot, we carefully conducted infrared (IR) camera measurements (Figure S6 in SI) and then considered laser heating effects as well as the temperature difference between the cuvette surface and the center of the solution (see detailed derivation in SI).

Figure 3. Thermal switching ratio of the PNIPAM aqueous solutions across the transition as a function of PNIPAM concentration.

In summary, we report the first study of the thermal conductivity change of thermoresponsive polymer aqueous solutions across the LCST using the transient thermal grating technique. We observed a thermal switching ratio up to 1.15 in dilute PNIPAM aqueous solutions of concentrations up to 0.025 g/mL across the LCST. This fast thermal switching behavior can be used to effectively control heat flow and temperature at a desirable level as the temperature changes. This is the first time a higher-order phase transition has been demonstrated to achieve fast thermal switching in one polymer aqueous solution and may shed light on a new and exciting mechanism for thermal switches without complex system design.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00938.

Figure 4. Optical arrangement for heterodyne detection of laser-induced gratings in transmission geometry. ND is a neutral density filter. 55

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Details of samples; experimental setup; analysis of the thermal transport and typical thermal decay curves; IR camera results and estimation of temperature rise; control experiments (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chen Li: 0000-0003-2336-9524 Author Contributions ‡

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by Z.T.’s startup fund from Virginia Polytechnic Institute and State University and 3M NonTenured Faculty Award. We acknowledge Prashant Singh for doing the IR camera measurements.

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REFERENCES

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