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Abstract
Sustainable architecture requires development of new materials with tailored optical, mechanical, and thermal properties to provide both aesthetic appeal and energy-saving functionalities. Polymers and polymer-based composites emerge as promising lightweight and conformable materials whose optical spectra can be engineered to achieve both goals. Here, we report on the development of new types of organic-inorganic films composed of ultrahigh molecular weight polyethylene with a variety of organic and inorganic nano- and micro-scale inclusions. The films simultaneously provide ultra-light weight, conformability, either visual coloring or transparency on demand, and passive thermal management via both conduction and radiation. The lightweight semi-crystalline polymer matrix yields thermal conductivity exceeding that of many metals, allowing for the lateral heat spreading and hot spots mitigation in the cases of partial illumination of films by sunlight. It also yields excellent broadband transparency, allowing for the opportunities to shape the spectral response of composite materials via targeted addition of inclusions with tailored optical spectra. We demonstrate a variety of dark- and bright-colored composite samples that exhibit reduced temperatures under direct illumination by sunlight, and outline strategies for materials design to further improve material performance.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For centuries, optics and photonics have been key enabling technologies in art, design, and architecture. Ancient and modern architects used light creatively to illuminate the interiors of buildings and places of worship. To quote Le Corbusier, “the history of architecture is the history of the struggle for light.” [1] In their struggle to enrich visible light with color, architects and designers also experimented with adding pigments, textures, and nanoparticles to construction materials. These experiments generated striking early examples of pigment and structural color formation, which we admire nowadays in historical stained-glass windows, glassware, and surface coatings.
However, the sunlight not only brings bright visual illumination and color to the building exteriors and interiors, but also radiative heat. About half of the solar energy is carried to the Earth surface by the ‘invisible’ infrared photons. Depending on the season and geographical location, this energy can either be used to warm the buildings or needs to be reflected to avoid overheating. Ideally, the two functions can be combined in one material or a single structural design concept, such as the one described by Socrates: ‘the sun penetrates the portico in winter, while in summer the path of the sun is … above the roof so that there is shade’. While structural architectural solutions have been used for centuries to achieve seasonal sunlight filtering, recent development of spectrally-selective nano-structured materials and meta-surfaces has opened opportunities to manage both visible light and radiative heat year-round with conventional window and rooftop designs [2–9].
The struggle for light, color, and heat management extends beyond architecture, and underlies engineering and design of vehicles, electronics, and wearables [2,10]. The advances in broadband photonic engineering of materials fuel emerging applications in solar-thermal energy harvesting [11–16], solar water treatment [17–19], and passive personal thermoregulation [20–27]. Lightweight, flexible, and durable polymer materials and organic-inorganic composites are increasingly replacing conventional natural materials across different sectors of economy, including new construction, historical buildings retrofitting, and design of new multi-functional wearables. Polymer-based materials provide cheap and easily scalable solutions for rooftop passive radiative cooling [28–32] and atmospheric water harvesting [33–35], and can be structured to engineer new types of woven and non-woven textiles for apparel, bedding, and tent manufacture [20–27].
Here, we report on the design, fabrication, and characterization of polymer-based organic-inorganic composites with tailored broadband spectral properties, and controllable thermal conductivity. We show that by varying the degree of crystallinity of ultrahigh molecular weight polyethylene (UHMWPE) and the composition of nanoparticle fillers, we can achieve simultaneous control over optical and thermal properties of the resulting composite material. We demonstrate several types of polymer composites engineered for a specific functionality via tailoring their optical spectra in a broad frequency range. These composites include: (i) UHMWPE films, which are highly transparent across the visible and the infrared frequencies and featuring anomalously high lateral thermal conductivity, (ii) dark-colored composite films combining high thermal conductivity with high reflectance across the invisible near-infrared part of the solar spectrum, (iii) composite materials exhibiting a variety of visual colors via selective absorptance and/or visible phosphorescence as well as heat spreading functionality owing to the high thermal conductivity. The new functionalities of the composite materials pave the way to their potential applications in architectural design, wearable technology, and renewable energy harvesting.
2. Broadband spectral engineering for visual colors and thermal regulation
New materials for architectural uses as well as for wearables and electronics should be engineered with an eye on their optical properties in a very broad spectral range. Figure 1 illustrates several important wavelength ranges that need to be included into any design. The transparency spectrum of the terrestrial atmosphere (Fig. 1(a)) shapes the solar spectrum reaching the surface of the Earth (Fig. 1(b), red) and allows the thermal radiation from the terrestrial objects (Fig. 1(b), orange) to reach the outer space, thus contributing to the overall thermal balance of our planet [36]. The spectral range between about 380 nm and 780 nm has special significance for humans, as this is the part of the spectrum the three color detectors in our eyes are sensitive to, making possible the trichromatic color vision. Figure 1(c) shows the standard observer color matching functions, which were introduced in 1931 by the International Commission on Illumination (CIE) [37], and can be used to identify the perceived visible color from an optical spectrum of incoming light that spectrally overlaps with these functions [38,39].
Fig. 1. Wavelength spectra of (a) atmospheric transparency, (b) total terrestrial solar irradiance (red), infrared thermal emittance of a blackbody at temperature of 310 K (orange), and (c) CIE standard observer color matching functions underlying the basis of the color formation in human vision.
Absorptance of the sunlight – either of the whole solar spectrum or a part of it – not only determines the materials perceived color, but also leads to its temperature increase. Likewise, high emittance of the material in the mid-infrared spectral range facilitates its cooling via thermal radiation [40,41]. In contrast, high reflectance in the infrared range leads to heat trapping in the material accompanied by its temperature increase, making such materials promising for the development of spectrally-selective absorbers for solar-thermal applications [11,42–44]. On the other hand, high transparency in the mid-IR range allows the thermal radiation from the objects covered by the material to pass through unimpeded, thus opening up opportunities for the design of passively-cooling wearables and convection-blocking covers for night-time and day-time radiative coolers [20,22,23,40,45]. Finally, thermal conductivity of the material determines the likelihood of the local hot spots formation due to the partial illumination by the sunlight that constantly moves throughout the day. Mitigation or prevention of such hots spots is a significant challenge for the urban designers, architects, and civil engineers [46,47].
3. Semi-crystalline UHMWPE films offer broadband transparency
We demonstrate that stretched semi-crystalline ultrahigh molecular weight polyethylene can be used as a multi-functional base material to form organic-inorganic composites with tailored optical and thermal properties. We fabricated UHMWPE films via solvent casting and uniaxial roll-to-roll mechanical stretching [48,49], and measured their spectra in both visible and infrared spectral ranges (see Methods). Uniaxially-drawn UHMWPE films with polymer fibers oriented along each other are known to exhibit crystalline behavior unusual for polymers. Polymers – including polyethylene – are typically amorphous materials, which may exhibit some crystalline domains depending on the process of their fabrication [48]. However, uniaxial drawing of UHMWPE films increases the average degree of crystallinity as well as the homogeneity of the crystalline regions of the polymer [48–51]. As the draw ratio is increased, the amorphous regions of the polymer become increasingly ordered and co-oriented, increasing the overall crystallinity of the film. We discuss below the important role the increased crystallinity of drawn films plays in their optical and infrared properties.
Figure 2(a) summarizes the spectra of total transmittance and total absorptance of the fabricated UHMWPE films with different draw ratios, including as-cast (undrawn, blue line), uniaxially stretched to elongate by 20 times (x20 drawn, teal line) and uniaxially stretched to elongate by 60 times (x60 drawn, red line). It can be seen that the transparency of the drawn (oriented) films improves across the whole visible and near-infrared ranges overlapping with the solar spectrum, while their absorptance is reduced (Figs. 2(a)-(b)). The increase in the film transparency is a result of two contributing factors, i.e., the film thickness reduction, and the increase of the material internal crystallinity. While the former factor contributes to the total transmittance boost across the whole spectrum shown in Fig. 2(a), the role of higher crystallinity is pronounced in the light transport through the film becoming increasingly ballistic rather than diffuse with the increase of the films drawing ratio (Fig. 2(c)).
Fig. 2. The spectra of total transmittance (a) and absorptance (b) of UHMWPE films of varying draw ratios, including as-cast (blue), drawn to elongate by 20 times (teal), and by 60 times (red). The transmittance spectrum of a window glass is shown for comparison in (a) as the gray line. The inset to (b) shows results of the differential scanning calorimetry (DSC) measurements of the films. (c) Haze parameter in the visible spectral range of the undrawn and drawn UHMWPE films of varying crystallinity and thickness as in (a,b). (e) Infrared emittance spectra of the UHMWPE films compared to that of a window glass.
Ballistic light transport means that the photons do not scatter while passing through the material, and continue propagation in the same direction as they exit. In turn, diffuse transmission is accompanied by the photons scattering by the internal structure in other directions than that of the light incidence, giving the material hazy appearance. Figure 2(c) shows that the contribution of the diffuse light in the visible light transport (i.e., haze parameter, see Methods) in undrawn films ranges from 94% at wavelength of 380 nm to 91% at λ=780 nm. This contribution reduces to 77% at 380 nm to 64% at 780 nm for x20 drawn film, and from 70% at 380 nm to 50% at 780 nm for x60 drawn film. As a result, oriented films become not only less opaque but also more transparent (as opposed to translucent) with the increase of their internal crystallinity (see also Figs. 3(a)-(c)). This makes them attractive for architectural and artistic applications where high direct transmittance of light is important. Furthermore, the control over the ratio of diffuse versus ballistic (direct) light transport via control of crystallinity makes UHMWPE an interesting material candidate for the films with variable and tunable total transparency and haze characteristics.
Fig. 3. (a-c) Photographs of the films of varying draw ratio and internal crystallinity, including undrawn (a), uniaxially drawn to elongate by 20 times (b) and by 60 times (c) UHMWPE films. (d-f) The corresponding SEM images of the UHMWPE films: undrawn (d), uniaxially drawn to elongate by 20 times (e) and by 60 times (f).
The inset to Fig. 2(b) shows typical results of the differential scanning calorimetry (DSC) measurements of the UHMWPE films of different drawing ratios (see Methods), with the melting points of films manifested as endothermic dips in the plots [52,53]. The higher melting point of the stretched films (∼141 oC) relative to that of the undrawn UHMWPE films (∼134 oC) indicates the presence of extended chain crystals caused by stretching [54]. Low-temperature (130–135◦C) UHMWPE melting is typically associated with sequential removal of single chain stems from the crystalline domains, while high-temperature melting (136–141◦C) occurs via complete breakdown of these domains. The latter melting mechanism is a signature of tightly-folded disentangled extended-chain crystalline domains in the material, while the former indicates presence of loose chain folds connecting chains in the amorphous and crystalline regions [54]. The degree of crystallinity of the stretched films increased over that of the undrawn ones (∼78%), and ranged between 86% and 96%, depending on the film draw ratio and the details of the casting and drawing process. Finally, the temperature bandwidth of the endothermic dips of 60x drawn films was reduced relative to the 20x drawn ones, suggesting higher homogeneity of their crystalline domains in size and internal structure.
The decrease of the film thickness and the increase of crystallinity of the films also translates to their decreased absorptance (and, by reciprocity, reduced thermal emittance) in the mid-infrared spectral range, as illustrated in Fig. 2(d). In the striking contrast to the infrared emittance spectrum of a window glass shown for comparison in Fig. 2(d) [55], the UHMWPE films exhibit low emittance (and high total transmittance) in the infrared. The emittance of UHMWPE films decreases with the decrease in the film thickness and the corresponding increase in the material crystallinity. The emittance dip in the glass spectrum stems from the momentum mismatch between the propagating photons and phonon-polariton modes in silica, which prevents light coupling to the polariton modes and results in its strong reflectance [44,55]. Increased crystallinity of UHMWPE films may reduce photon absorptance in the infrared spectral range due to the changes in the vibrational modes spectra in crystalline films [50,51,56,57]. The emergence and disappearance of non-crystalline peaks in the spectra of stretched PE films have been also observed in Raman scattering experiments [58,59].
Figures 3(a)-(f) show optical and the corresponding scanning electron microscopy images of the UHMWPE films with varying draw ratios. The material structure evolution from a non-oriented internal composition of the as-cast sample in Figs. 3(a), (d) to the anisotropic semi-crystalline structure of the drawn samples with oriented fibers in Figs. 3(b), (c), (e), and (f) can be easily observed. The fibers orientation changes not only the optical properties of the films as illustrated in Fig. 2 and Figs. 3(a)-(c), but also their mechanical and thermal transport properties [49,60]. Our previous work has revealed that the thermal conductivity values of UHMWPE films produced via extrusion and uniaxial drawing grow with the increased level of crystallinity [48]. The thermal conductivity of the extruded UHMWPE films elongated over 20 times exceeds that of the stainless steel (16 W/mK), and reaches high value of 60 W/mK for film elongation of 110x [48]. UHMWPE films fabricated via solvent-casting followed by uniaxial drawing exhibit slightly lower values of thermal conductivity, yet still reach the stainless-steel conductivity for drawing ratios above 60. The thermal conductivity values of other transparent materials used in art and architecture are much lower, including window glass (0.96 W/mK), quartz (3 W/mK), and other polymers such as acrylic glass (poly(methyl methacrylate): PMMA (0.19 W/mK) [61,62].
Previous research also revealed that uniaxially drawn semi-crystalline UHMWPE films exhibit very high tensile strength – ranging from 1 GPa for x20 drawn films to as high as 4.7 GPa for the films with the draw ratio of 120 [63,64], which is in perfect agreement with our observations of the films strength. Broadband transparency of the drawn UHMWPE films (see Figs. 2, 3) together with their unique thermal and mechanical properties confirmed by our study [48,60] make them attractive host media for engineering composite materials with the tailored absorptance/emittance spectra across the visible and the infrared ranges.
4. Visible coloring via selective absorption and phosphorescence
We achieved coloring of the transparent UHMWPE films by modifying their reflectance spectra in the visible spectral range through embedding dyes, phosphorescent pigments, and inorganic nanoparticles with large resonant scattering cross-sections (see Methods). Figure 4 shows several examples of the dark-colored films, including black films colored by adding CuO nanoparticles to the UHMWPE matrix (Fig. 4(a)), dark blue films produced by adding blue dyes (Fig. 4(b)), and silvery gray films that exhibit color owing to the strong light scattering and absorption by Si nanoparticles embedded in the films (Fig. 4(c)). To evaluate the performance of colored UHMWPE films for architectural and visual exterior design applications, we selected a range of samples in Fig. 4 (and Fig. 5 below) by their optical properties, i.e., by their degree of visual transparency and the intensity of perceived color. This allowed us to evaluate thermal performance of films with various inclusions and various thicknesses that exhibit similar visual effects. Figure 4(d) also shows a commercial black-painted paper (Canson XL Series, colored with a mixture of pigments and dyes) as the reference dark material.
Fig. 4. (a-c) Optical images of the UHMWPE films dark-colored by embedding (a) CuO nanoparticles (CuO black: 30 nm-sized particles, 20 wt% filling, film thickness 28 micron, x20 drawn), (b) a blue dye (10 wt% filling, film thickness 180 micron, undrawn), and (c) Si nanoparticles (Si gray: 100 nm-sized particles, 20 wt% filling, film thickness 54 micron, x20 drawn). (d) Optical image of a black-painted paper sample used for comparison as a black surface. (e,f) Total spectral reflectance (e) and absorptance (f) of the visible and near-infrared light by the dark-colored films shown in panels a-c (teal lines: CuO black, gray lines: Si gray, blue lines: blue dye). The corresponding spectra of the black paper are shown for comparison as the black lines.
Fig. 5. (a-d) Optical images of the UHMWPE films light-colored by embedding (a) yellow dye (10 wt% filling, film thickness 8 micron, x20 drawn), (b) red dye (10 wt% filling, film thickness 14 micron, x20 drawn), (c) phosphorescent green pigment (10 wt% filling, film thickness 120 micron, undrawn), and (d) TiO2 nanoparticles (TiO2 white: 20 nm-sized particles, 20 wt% filling, film thickness 43 micron, x20 drawn). (e) Optical image of an aluminum sample used for comparison as a highly solar-reflective surface. (f,g) Total spectral reflectance (f) and absorptance (g) of the visible and near-infrared light by the light-colored films shown in panels a-d (yellow lines: yellow dye, red lines: red dye, teal lines: green pigment, blue lines: TiO2 white). The corresponding spectra of the aluminum foil are shown for comparison as the gray lines.
The reflectance and absorptance spectra of these films shown in Figs. 4(e), (f) reveal the spectral mechanism of their visible color formation. While black paint exhibits uniformly high absorptance and low reflectance across the whole visible and the near-infrared spectral ranges, CuO black and Si gray films show 20-65% absorptance in the visible part of the spectrum, and become 40-50% transparent and about 40% reflective in the near-infrared. In turn, blue-dye colored film absorbs most of the visible light with the exception of the blue part of the spectrum, where it exhibits about 20% reflectance, yielding its visible blue coloring. The absorptance of the blue film drops in the near-infrared spectral range, but remains higher than that of the Si gray and CuO black samples shown in Figs. 4(a), (c) because of the larger thickness of the blue-dye colored sample in Fig. 4(b). Blue films drawn to higher draw ratios show significantly reduced near-infrared absorptance yet much lighter blue color and higher visible transparency than undrawn blue films.
The bright-colored films in a variety of colors have also been fabricated by adding red and yellow dyes, phosphorescent pigments and strongly-scattering nanoparticles to the polymer. Some examples of the light colors achievable in the resulting composites are shown in Figs. 5(a)-(d), while Fig. 5(e) shows a photo of an aluminum foil. Aluminum is a commonly used roof cover material, and was chosen for comparison of its optical and thermal properties with the light-colored films. Not surprisingly, the optical spectra of the light-colored films shown in Figs. 5(f)-(g) reveal higher reflectance over most of the solar spectral range than the dark-colored films shown in Fig. 4. The visual color formation results from the selectively stronger reflectance in the part of the visible spectrum corresponding to the red, yellow, or green colors, respectively. The green-colored films also exhibit visible phosphorescence when illuminated by the sunlight, which enhances their color. The white-colored film exhibits over 50% reflectance in the visible, and about 40% in the near-infrared part of the solar spectrum, while the aluminum foil is over 80% reflective across the whole range of solar light.
5. Opto-thermal performance under illumination: IR coloring & heat spreading
The differences in the reflectance and absorptance in the whole range of frequencies of the solar spectrum not only contribute to the color variability of the films, but also affect their temperatures under solar illumination. We compared the steady-state temperatures reached by the films under direct illumination by an artificial solar light with the flux equal to that of one sun (see Methods). The temperatures were recorded after the films reached the quasi-steady state resulting from the balance between heating due to solar absorptance and cooling owing to the thermal radiation and air convection. The resulting temperatures are shown in Fig. 6(a) for the dark-colored films and in Fig. 7(a) for the light-colored films, respectively.
Fig. 6. (a) Steady-state temperatures in excess of the ambient temperature of 22 oC reached by the dark-colored samples in Figs. 4(a)-(d) under continuous illumination by an artificial sunlight. (b) Infrared emittance of the dark-colored films compared to that of the black paint.
Fig. 7. (a) Steady-state temperatures in excess of the ambient temperature of 22 oC reached by the light-colored samples in Figs. 5(a)-(e) under continuous illumination by an artificial sunlight. (b) Infrared emittance of the light-colored films compared to that of the aluminum foil.
Figures 6(a) and 7(a) reveal that even the dark-colored films exhibited significantly lower temperatures than the black paper sample, while aluminum foil remained the coldest of the samples studied. The CuO film was about 9 oC cooler than the black paper, while the blue dye and the Si gray films exhibited temperatures 20 oC below the black reference. Red and yellow-colored films were 21°C colder than the black reference, while the green film was nearly 28 oC colder than the black paper, and even about 1 oC colder than the TiO2 white film despite exhibiting higher solar absorptance. This temperature reduction of the green film stems from the relatively high reflectance of the solar light and cooling via both thermal radiation in the mid-infrared and phosphorescence in the visible. Figures 6(b) and 7(b) compare the infrared absorptance spectra of colored UHMWPE films and the two reference samples. While the black paper was highly absorptive across the whole infrared spectrum, and aluminum foil was highly reflective, all the films exhibited high transparency and low absorptance, which decreased with the increase in the drawing ratio and the corresponding decrease in the film thickness. Most films drawn to elongate by at least 20 times (including TiO2 white, yellow dye, red dye, and CuO black) were shown to be highly transparent in the wavelength range around 10 micron.
Finally, in Fig. 8, we compare the opto-thermal performance of the samples under localized illumination by a green laser beam. Comparison of the optical and infrared images of the illuminated black paper sample (Figs. 8(a),(b)) revealed that the heating area is strongly localized to the illuminated spot as the low thermal conductivity of paper prevents heat from spreading efficiently along the sample surface. In contrast, the infrared image of the locally-illuminated Si gray sample (Fig. 8(c)) shows that the film’s high thermal conductivity promotes heat spreading along the sample, which in turn reduces the peak temperature inside the illuminated hot spot. Our measurements (see Methods) of the in-plane thermal conductivity of UHMWPE films with embedded nanoparticles show that addition of particles does not degrade the films conductivity values. This functionality of the high-conductivity plastic films offers useful applications in architecture and wearables to avoid local overheating and temperature-induced material damage.
Fig. 8. (a) An optical image of a black paper sample illuminated by a laser beam. (b) The corresponding infrared image of the same sample showing the spatial temperature distribution. (c) The infrared image of the Si gray film illuminated by the same laser beam revealing the heat spreading laterally along the film, reducing the hot spot temperature. (d) Comparison of the cost, weight and thermal conductivity of semi-crystalline UHMWPE films with the corresponding characteristics of other common materials.
6. Discussion and outlook
We fabricated and characterized a family of lightweight polymer composite films with tailored combination of optical, mechanical and thermal properties. Our data show that the ultra-high molecular weight polyethylene films provide mechanical (high strength and high conformity) and thermal (ultra-high thermal conductivity) functionalities as well as high total transparency across a broad spectral range. In contrast to other high-conductivity materials such as metals or carbon, semi-crystalline UHMWPE films provide light weight and low cost (Fig. 8(d)) together with the electromagnetic transparency for sunlight, human eye, thermal radiation, and wireless communications. Carefully selected organic and inorganic particles embedded into the films provide spectral selectivity by design in both the visible and the infrared parts of the electromagnetic spectrum. The spectral selectivity in the visible range gives the films a variety of colors, while the low absorptance in the near-infrared part of the solar spectrum allows for their significant temperature reductions under direct illumination by sunlight. At the same time, the high thermal conductivity of the pure and composite films enables them to achieve lateral heat spreading away from the locally heated area. This allows making use of the larger area for passive cooling through radiation and may help to avoid thermal damage of the material.
The reduced thermal emittance as well as the indoors experimental setting prevented our films from cooling themselves to or below the ambient temperatures under solar light. However, their high infrared transmittance can allow the objects and surfaces covered by the films to cool radiatively. This high infrared transparency makes the polyethylene films stand apart from other polymers [20], which are highly absorptive in this spectral range [29], and offers exciting applications in wearable technologies as well as in the design of tents, sun umbrellas, and skylights. Our data shows that the high infrared transparency of the films can be preserved even if they are modified with embedded particles to achieve visible coloring effects beyond while color demonstrated before [22,23,65]. This infrared transparency property should be also expected to manifest in polyethylene composites of other form factors, including the fiber form essential for making wearable textiles, which is a promising future direction of this work [20,66,67].
The polyethylene films and fibers can however be modified to exhibit high emittance in the infrared spectral range via micro- and nano-particle inclusions with tailored emittance in mid-infrared [9,68]. These can include silica (SiO2) [28,69], aluminum oxide Al2O3, graphite [70], and indium tin oxide (ITO) [71] particles, etc. Finally, temperature reduction of the films exhibiting color through a combination of selective absorptance and visible phosphorescence (e.g., green-pigment colored films) or luminescence emerged as a promising and yet-unexplored mechanism of passive daytime cooling via radiative channels. Further detailed studies will however be required to quantitatively evaluate the role of fluorescence and phosphorescence in the radiative cooling of polymer films and other materials.
Exposure to the environment and the sunlight inevitably leads to the material degradation, mostly caused by the autoxidation by UV light and by microorganisms’ growth [72,73]. However, it is well known in the recycling industry that polyethylene (and especially UHMWPE) is non-biodegradable under natural environmental conditions owing to its hydrophobic carbon backbone and high molecular weight [73–75]. Previous studies have shown that long-range structure and morphology of polyethylene plays an important role in its stability to microbial growth causing material degradation, with amorphous regions of the polymer shown to be more prone to microbial attack than crystalline ones [73]. As such, high crystallinity of the stretched UHMWPE films is expected to make them more protected from the microbial degradation than standard amorphous PE materials.
In turn, the discoloration of dyes and pigments caused by the UV light can be hindered by using UV attenuators embedded into the polymer matrix, which absorb and scatter solar UV radiation [72,76,77]. The most commonly used inorganic UV attenuators are ZnO and TiO2 nanoparticles, both of which we have already successfully embedded into the UHMWPE films. The effect of whitening of the material color associated with adding these attenuators can be reduced by using the nanoparticles of small sizes, which do not scatter visible light. Both, UV attenuators and light stabilizers, which inhibit the oxidation process by trapping UV-light-generated free radicals, are commercially available and routinely added to architectural coatings and plastics used in agriculture and horticulture [78–80]. Several commercial products based on different types of PE materials have been reported to be capable of resisting UV light and general atmospheric conditions for several years, including Tyvek (DuPont), Tivar (Quadrant), and Dock gangway systems (EZ Dock). Furthermore, the shrinkage of the stretched UHMWPE films caused by their solar heating is expected to be reduced relative to conventional amorphous plastics. The aligned polymer chains in stretched films are very densely packed (see Fig. 3), leaving little empty space available for their rearrangement occurring during the shrinkage process. Finally, low absorptance of the colorants used in preparation of composite films does not mask the UHMWPE material signatures in the near infrared spectral range, allowing for their proper sorting during the recycling process [81].
7. Methods
7.1 Fabrication of polymer and composite films
Polyethylene-based films were prepared through a combination of solvent-casing and uniaxial drawing [48]. Briefly, a polyethylene solution was prepared by adding ultrahigh molecular weight polyethylene (UHMWPE, molecular weight 3-5 ×106 g/mol) in powder form into decalin (Decahydronaphthalene, mixture of cis + trans, anhydrous, ≥99%) at a polymer concentration of 2wt%, and was heated to 145 °C for 24 hours in a silicone oil bath. Subsequently, the hot solution was cast directly onto a liquid-nitrogen-cooled glass substrate. The solvent was almost completely evaporated at ambient conditions for 3 days inside a fume hood. The as-cast films were then removed from the substrate and fed to a constant-force roll-to-roll drawing platform with the rolls heated to a temperature in the range of 170-200 °C [49]. The temperatures of the stretched films were lower than those of the rolls and did not reach the melting point of UHMWPE material. The draw ratios of the films were estimated as the ratios of the final to the initial film length, within 10% uncertainty. Additionally, colored PE-based films were fabricated by adding different disperse dyes (disperse blue, red, yellow dyes, Sigma Aldrich), phosphorescent pigments (strontium aluminate, Eu2+:SrAl2O4, Art ‘N Glow), and resonantly scattering and absorbing inorganic nanoparticles (Si, ZnO, TiO2, SiO2, CuO, etc, Sigma Aldrich) into the UHMWPE solution with Decalin. The amount of dyes and pigments added was 10-20wt% with respect to the amount of the UHMWPE powder.
7.2 Structural and spectral characterization
The optical properties (reflectance and transmittance) of the fabricated films were measured by a UV-Vis-NIR spectrophotometer (Agilent, Cary 5000) equipped with an integrating sphere. The diffuse transmittance was measured through a methodology based on the ASTM D-1003 standard for haze and luminous transmittance of transparent plastics. Each film and the reflectance standard were placed respectively in the transmission and reflection ports of the integrating sphere. Four readings were taken for each sample, including: (i) ${T_1}$, the incident light measured with the reflectance standard in position and no sample, (ii) ${T_2}$, the total light transmitted by the sample measured with the sample and the standard in their respective positions, (ii) ${T_3}$, the light scattered by the instrument measured with the sample and the standard removed from their respective positions, and (iv) ${T_4}$, the light scattered by the instrument and the sample measured with the sample in position and the standard removed. The total transmittance (${T_t}$) and the diffuse transmittance (${T_d}$) were calculated as: ${T_t} = \frac{{{T_2}}}{{{T_1}}}$ and ${T_d} = \frac{{{T_4} - {T_3}({{T_t}} )}}{{{T_1}}}$. The haze parameter was defined as the percentage of the diffuse light (${T_d})\; $in the total transmitted light (${T_t}$): $haze = \frac{{{T_d}}}{{{T_t}}} \times 100$. The IR transmittance was measured by an FT-IR spectrometer (Thermo Scientific, model 6700) accompanied with a diffuse gold integrating sphere (PIKE Technologies). The spectral measurements with the integrating sphere were used to measure the total reflectance (${R_t}$) and total transmittance (${T_t}$) of the films, while the total emittance was calculated as ${\varepsilon _t}$ = 1 − ${R_t}$ − ${T_t}$. The SEM images were obtained by using a high-resolution scanning electron microscope (SEM, Zeiss, model Merlin) operated at 2 kV. For SEM imaging, the samples were coated with a 50 nm-thick layer of carbon.
Differential scanning calorimetry (DSC) measurements were performed using a calorimeter model Discovery from TA Instruments. Samples of ∼1 mg were weighed with a precision balance and encapsulated in aluminum pans of known mass. An identical empty pan was used as a reference. Nitrogen was purged at a rate of 50 ml/min. Heating-cooling-heating cycles in the range from −40 to 160 °C were applied with heating and cooling rates of 10 °C/min.
7.3 Thermal characterization
Thermal conductivity of the as-cast and drawn films has been measured by previously developed and validated home-built differential steady-state platform, in which the sample is mounted between an electric heater and a temperature-controlled heat sink. The heater is maintained at a constant temperature close to the environmental temperature, while the heat sink temperature is varied. This enables a large signal-to-noise ratio, and allows both accurate measurements of samples with small thermal conductance and measurements with small temperature differences required for samples with high thermal conductance [48,82].
Thermal imaging of the samples has been performed by using FLIR ETS320 infrared camera, which has 320 × 240 infrared image resolution, temperature measurement accuracy of 2°C, and temperature differences resolution below 0.06°C. To illuminate the films, we used the Class AAA solar simulator (ScienceTech, SS-1.6 K) with a beam-down mirror to direct the artificial sunlight to the films. The solar simulator was calibrated to output a flux of 1000 Wm−2. To visualize the heat spreading functionality of the drawn films, we illuminated them with a beam from a single-mode LD-pumped solid state green laser with 532 nm center wavelength, beam diameter of 2 mm, and maximum power up to 300 mW (Opto Engine, MGL-F-532-1.5W). The samples were placed on top of the insulating foam covered by a white paper and the temperature was detected via thermocouples located between the underlying paper and the bottom surface of each film. The temperature readings were recorded via thermocouples after the films reached a quasi-steady state. The transient period characterized by the rapid temperature increase of the films ranged 2-5 minutes after the solar illumination was switched on. It should be noted that continuous changes in the ambient environment (especially under real outdoor conditions) may prevent the material from reaching steady state conditions and will lead to it exhibiting temperature variations following the changes in the solar illumination and/or wind conditions.
Funding
Combat Capabilities Development Command Soldier Center (6938384).
Acknowledgements
The authors thank Aaron Schmidt, Asegun Henry, Evelyn Wang, and Bikram Bhatia for useful discussions, advice, and equipment sharing.
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