Coffee Grounds to Multifunctional Quantum Dots: Extreme

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Coffee Grounds to Multifunctional Quantum Dots: Extreme Nanoenhancers of Polymer Biocomposites Huan Xu, Lan Xie, Jinlai Li, and Minna Hakkarainen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09401 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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ACS Applied Materials & Interfaces

Coffee Grounds to Multifunctional Quantum Dots: Extreme Nanoenhancers of Polymer Biocomposites Huan Xu,†,§ Lan Xie,‡ Jinlai Li,§ and Minna Hakkarainen*,†

†

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Stockholm

10044, Sweden

‡

Department of Polymer Materials and Engineering, College of Materials and Metallurgy, Guizhou

University, Guiyang 550025, China

§

ENN Group Co., Ltd., Langfang 065001, China

*Corresponding Author: [email protected] (M.H.)

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ABSTRACT: Central to the design and execution of nanocomposite strategies is the invention of polymer-affinitive and multifunctional nanoreinforcements amenable to economically viable processing. Here a microwave-assisted approach enabled gram-scale fabrication of polymer-affinitive luminescent quantum dots (QDs) from spent coffee grounds. The ultrasmall dimensions (approaching 20 nm), coupled with richness of diverse oxygen functional groups, conferred the zero-dimensional QDs with proper exfoliation and uniform dispersion in poly(L-lactic acid) (PLLA) matrix. The unique optical properties of QDs were inherited by PLLA nanocomposites, giving intensive luminescence and high visible transparency, as well as nearly 100% UV-blocking ratio in the full UV region at only 0.5 wt % QDs. The strong anchoring of PLLA chains at the nanoscale surfaces of QDs facilitated PLLA crystallization, which was accompanied by substantial improvements in thermomechanical and tensile properties. With 1 wt % QDs, for example, the storage modulus at 100 °C and tensile strength increased over 2500% and 69% compared to those of pure PLLA (4 MPa and 57.3 MPa), respectively. The QD-enabled energy-dissipating and flexibility-imparting mechanisms upon tensile deformation, including the generation of numerous shear bands, crazing and nanofibrils, gave an unusual combination of elasticity and extensibility for PLLA nanocomposites. This paves the way to biowaste-derived nanodots with high affinity to polymer for elegant implementation of distinct light management and extreme nanoreinforcements in an ecofriendly manner.

Keywords: quantum dots, polymer affinity, multifunctional bionanocomposites, interfacial interactions, toughening mechanism


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INTRODUCTION In the broad family of carbonaceous nanostructures, one-dimensional carbon nanotubes (CNTs) and two-dimensional (2D) graphene and graphene oxide (GO) nanosheets have been recognized as outstanding representatives because of their large specific surface areas, high mechanical strength and prominent photoelectrical performances.1 These carbonaceous nanostructures offer a wealth of possibilities to impart property enhancements and multifunctionality particularly in polymer nanocomposites.2−3 This methodology has been elaborated in recent contributions toward mechanically strong and electrically conductive nacre-like GO/chitosan papers,4 polymer-supported GO framework membranes for water purification,5 GO-enhanced biomineralization on polymer surfaces,6 and high-barrier polymer films with exfoliated GO nanosheets serving as “nano-barrier walls” to resist gas permeation.7−9 The property translation from individual nanostructures to the aggregative state on larger scales in host polymers (e.g., filler exfoliation and dispersion) depends largely on the interfacial interactions that are governed by the geometric dimensions and surface chemistry of the nanofiller candidates.10,11 This provides an incentive for chemical modification of nanofiller surfaces when attempting to create strong interfacial adhesion and uniform filler dispersion in the pursuit of high-performance composites.12,13 For instance, dopamine reduction of GO facilitated the interfacial stress transfer and thus mechanical elasticity for epoxy composites,14 whereas polymeric modification of GO exhibited high efficiency to tailor the crystallization and thermal behaviors of poly(vinyl alcohol).15 The specific technical demands and additional production costs arising from the functionalization process, nevertheless, lay down challenges for the realistic development of 3 ACS Paragon Plus Environment


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nanocomposites. It may be further thwarted by adverse environmental issues and potential cytotoxicity and/or ecotoxicity of conventional nanostructures like CNTs.16 This prompts a potential alternative strategy—shrinking the nanofiller dimensions in combination with controlled surface chemistry—to enhance the interfacial interactions with polymer matrices. The promise underlying this strategy has been demonstrated by our recent work, revealing two times higher adhesion force to poly(L-lactic acid) (PLLA) after structural transformation from 2D GO nanosheets to zero-dimensional (0D) GO quantum dots (GOQDs).17 In contrast to the easy agglomeration and low reinforcing efficacy of GO nanosheets, the dimensionality shrinkage and higher oxygenation conferred GOQDs with significantly facilitated exfoliation and dispersion in PLLA, accounting for multiple enhancements in mechanical and barrier properties and optical transparency. Given the ultralow dimensions to trigger quantum-confined excitations and oxygen-rich chromophores to absorb UV protons, it is moreover expected to create optical functionality for QD composites,18 including luminescent emissions,19 prominent blocking of UV radiation and high transmittance to visible light that are not attainable for carbon nanotube- or GO-filled composites.9 Realization of the potential of multifunctional QDs focuses on affordable and efficient synthesis of QDs with well-controlled dimensions and surface chemistry.20 Despite recent contributions revealing the viability of diverse precursors including pee,21 plant leaves,22 fresh grass,23 soy milk24 and cabbage,25 a widespread starting carbon source that suits low-cost and feasible production of QDs with high and reliable quality is of high importance.26 In this work, used coffee grounds were selected as the carbon source because of their abundance, wide geographical distribution and 4 ACS Paragon Plus Environment

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satisfactory economic benefits, offering a good choice of starting materials amenable to scaled-up fabrication of QDs.27 Equally desirable is the hollow cell structure naturally carried by coffee ground particles,28 together with the richness of functional aromatic compounds,29 enabling effective transformation to nanostructures through facile reaction methods.30 Particularly, an environmentally benign approach to aqueous fabrication of QDs from spent coffee grounds was recently established in our group (Figure 1), strategically involving (1) the application of microwave-assisted hydrothermal (MAH) technique to afford fast and efficient activation of coffee grounds to dot precursor composed of fine carbon sheets, and (2) the oxidation of dot precursors in dilute acid for cutting the carbon sheets into uniform QDs assembled by few-layer GO.31 We hypothesize that the low dimensions and high density of oxygen functional groups would confer QDs with high affinity to PLLA matrix,17 opening a straightforward pathway to simultaneous improvements of mechanical, optical and thermal properties of PLLA materials (Figure 1). This approach is further enhanced by the generally good biocompatibility and environmental friendliness of biomass derived QDs,32 offering biosafety suitable for biomedical applications, unattainable with conventional cytotoxic nanodots like DdSe and PbS.

Figure 1. Synthetic approach to QDs and multifunctional biocomposites. Schematic illustration of the hypothesis for the structural transformation from spent coffee grounds to dot precursor composed of fine carbon sheets and QDs assembled by few-layer GO, giving the possibility to fabricate low-cost yet multifunctional biocomposites by homogeneous incorporation of QDs into PLLA.



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EXPERIMENTAL SECTION Materials. The spent coffee grounds (Arvid Nordquist, Sweden) were used as the carbon source. PLLA under the trade name of 4032D, with a weight-average molecular weight of 11.9 × 104 g/mol and a number-average molecular weight of 6.6 × 104 g/mol, respectively, was procured from NatureWorks (USA). All other chemical reagents in analytical grade were obtained from VWR, Germany, and were used as received.

Preparation of QDs. The coffee grounds were transformed to the QD precursor following an efficient MAH strategy devised previously in our group for carbonization of biopolymers/biomass. For the sake of briefness, the preparation details are described in our recent work.31 The MAH reaction was conducted at a preset temperature of 180 °C and a pressure of 40 bar using a microwave apparatus (SynthWAVE, Milestone Inc., USA). The dot precursors were oxidized in 10% HNO3 (~20 mL), leading to light brown QD powder with a throughput of around 96% by calculating the mass weights of dot precursor and QDs.

Preparation of PLLA Composite Films. The solution coagulation method was used to prepare the PLLA composites containing 0.05 wt %, 0.1 wt %, 0.5 wt % and 1 wt % QDs, respectively named QD0.05, QD0.1, QD0.5 and QD1. Specifically, QDs ultrasonically dispersed in ethanol (1 mg/mL) were gently dropped into the stirred PLLA/dichloromethane solutions to the preset nanodot concentrations. The coagulated composites then gradually precipitated from the solutions. After complete drying, the coagulations were compression molded into films (~200 µm) at 200 °C under a fixed pressure of 5 MPa. Pure PLLA was subjected to the same processing to make a control sample.


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Transmission Electron Microscopy (TEM) Observation. TEM was used to image the morphology of QDs. Droplets of QD suspension in ethanol at concentrations of 0.5 mg/mL were deposited onto the lacey carbon film 400 mesh copper TEM grids (Ted Pella, Inc.) and allowed to dry in ambient conditions prior to TEM imaging (Hitachi, 100 kV).

Ultraviolet−Visible (UV−Vis) Spectroscopy. The transmittance and absorption spectra of compress-molded PLLA/QD films were recorded on a SHIMADZU UV-2550 spectrophotometer (Shimadzu Co., Japan).

Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD) Measurements. The crystalline morphology in the compression-molded PLLA/QD composite films was evaluated by 2D-WAXD. Specifically, 2D-WAXD measurements were performed on a home-made laboratory instrument (Bruker NanoStar, CuKα-radiation) in the Crystallography Lab, Department of Molecular Biology and Biotechnology, University of Sheffield. The X-ray beam with a wavelength of 0.154 nm was focused to a tiny area of 4 × 4 µm2, and the distance from sample to detector was fixed at 350 mm. The diffraction patterns were collected by an X-ray CCD detector with a resolution of 2300 × 2300 pixels (Model Mar345, Rayonix Co. Ltd., USA).

Polarized Optical Microscopy (POM). The crystalline morphology in PLLA/QD composite films after isothermal crystallization and the crazing behavior in tensile fractured composites were imaged on an Optiphot 2 microscope equipped with a Leica digital camera and crossed polarizers. The temperature protocol for isothermal crystallization is shown in Figure S1a. Specifically, the composite films after annealing at 200 °C for 5 min were cooled to the isothermal crystallization temperatures at 120, 130 and 140 °C for 60 min on a Mettler FP82HT heating stage. 7 ACS Paragon Plus Environment


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Atomic Force Microscopy (AFM) Topography Characterization. The lamellar textures in the isothermally crystallized PLLA/QD nanocomposites were imaged using a Nanoscope Multimode 8 (Bruker AXS, Santa Barbara, USA) with a type E piezoelectric scanner. Images were acquired in tapping mode using RTESP Si cantilevers (Bruker Probes, Camarillo, USA) with a typical spring constant of 40 N/m.

Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra of QD-filled PLLA nanocomposites were recorded on a PerkinElmer Spectrum 2000 spectrometer (PerkinElmer Instrument) with 16 scans at a resolution of 4 cm−1.

Dynamic Mechanical Analysis (DMA) Measurements. The thermomechanical properties were measured on a DMA Q800 (TA Instruments, America) in a multi-frequency tension mode, following the guidance of the ASTM standard D4065. It was operated with a frequency of 1 Hz, over a temperature range of 25−160 °C at a heating rate of 3 °C /min.

Differential Scanning Calorimeter (DSC) Characterization. Thermal behaviors of compression-molded PLLA/QD composites were monitored by a Mettler Toledo DSC 820 under the nitrogen atmosphere (50 mL/min). As schematically depicted in Figure S1b, the composites were steadily heated up to 200 °C and held at this temperature for 5 min to remove thermal history, and finally cooled down to 40 °C. The heating and cooling rates were set at 10 °C/min.

Thermal Gravimetry Analysis (TGA). Thermal stability of PLLA/QD nanocomposites was evaluated by TGA on a Mettler Toledo TGA/STDA 851e, heating from 40 to 600 °C at a rate of 10 °C/min under nitrogen atmosphere (50 mL/min).


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Mechanical Performances of PLLA/QD Nanocomposites. Following the ASTM standard D638, tensile properties of dumbbell-shaped specimens (thickness of 200 µm, width of 10 mm) were measured on an Instron universal test instrument (Model 5944, Instron Instruments, USA) with a load cell of 500 N at 23 °C and relative humidity of 50%. The crosshead speed was set at 5 mm/min and the gauge length was 20 mm. Quintuplicate measurements were conducted on discrete samples, and average values with standard deviations were presented.

Scanning Electronic Microscopy (SEM) Observation. An SE-4800 SEM (Hitachi, Japan), operating at an accelerated voltage of 1 kV, was used to image the top surfaces and fracture surfaces of composite films after tensile failure. Sputter coating of a 3.5 nm-thick gold layer was conducted on all samples prior to SEM observation.

RESULTS AND DISCUSSION The structural transformation from spent coffee grounds to dot precursor and QDs was examined by SEM and TEM observations (Figure S2 and S3), revealing a diameter range of 20−40 nm and ordered graphenic domains for the QDs, likely assembled by few-layer nanosized GO-type sheets.33 Thorough morphological observation and structural determination of the QDs are described in our recent work.31 The coffee-ground-derived QDs were moreover characterized by high affinity to polymer matrix, enabling fabrication of homogenous PLLA nanocomposites with high transparency (Figure 2a). Nanoscale mapping of the incorporated nanodots revealed excellent exfoliation of QDs in PLLA matrix even at the highest QD concentration, giving a uniform dispersion of nanodots with a highly narrowed size distribution centered at approximately 55 nm (Figure 2b and S5). The low dimensions 9 ACS Paragon Plus Environment


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of the well-exfoliated QDs rendered quantum-confined excitons essential for emissive traps. Upon UV excitation at 365 nm wavelength, the composite films emitted green luminesce, with increased intensities at increasing QD concentration (Figure 2c). This differs from the white emission observed for poly(vinyl alcohol) films (thickness of ~10 µm) containing coal-derived graphene quantum dots,19 suggesting possibilities to control the emission properties for polymer films by structural manipulation of nanodots.18 The UV absorption capability of QDs enabled excellent shielding efficiency for PLLA nanocomposite films in the UV region (250−400 nm), as indicated by the largely promoted absorption intensities and the transmittance depression of UV light (Figure 2d,e). The visible light (400−700 nm), however, passed through the composite films at high transmissions comparable to pure PLLA, especially when the QD concentration was below 0.1 wt % (Figure 2e). Despite considerable mechanical and barrier enhancements provided by CNTs and graphene nanosheets, development of such conventional polymer nanocomposites is to some extent dwarfed by the substantial gaps in providing high transmission to visible light for transparent packaging applications.9 The pursuit of high transparency for packaging-adaptive composites stimulates the interest of designing and fabricating optically transparent nanostructures (e.g., cellulose-derived nanofibrils).34 This also opens new horizons for diverse applications in energy and optoelectronic devices such as solar cells and light-emitting diodes.35 The biowaste-derived nanodots developed here feature effective light management, which is highly desirable for transparent packaging applications.36


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To highlight the UV-blocking efficiency achieved by QDs, we compared the UV-blocking ratios (UVR) based on the transmittances of QD−PLLA composite films at wavelengths ranging from 275 nm to 400 nm, following the below equation: UVR = (T0 − T)/T0 × 100% where T0 and T represent the transmittances of pure PLLA and QD-filled composite films at a specific wavelength, respectively.37 Figure 2f shows that an exceptionally high UVR level of nearly 100% was achieved for both QD0.5 and QD1, showing weak relation to the UV wavelengths. This contrasted the selective UV blocking within a narrow wavelength range around 280 nm observed for poly(vinyl alcohol) films containing surface-modified cellulose nanocrystals at concentrations of over 5 wt %.38 This distinction illustrates the advantages of 0D QDs, with dimensions far below the UV wavelength, in blocking UV light in the full wavelength range.36 The dimension dependence on UV-blocking performances was linked to the UV screening mechanism of organic nanofillers. Nanofillers with higher dimensionalities (e.g., 1D nanofibrils and 2D nanosheets) mainly adopt the reflection mode to decrease UV transmissions,39 whereas in the case of 0D QDs a series of UV absorption and energy transformation by chromophores such as carbonyl and phenol groups in QDs could be involved.31 This mechanism could be further enhanced by (1) large specific surface areas and high edge-to-surface ratios of QDs that permitted accommodation of rich functional groups at the surfaces and edges, and (2) a large volume of QD−PLLA interphase that readily reflected and scattered UV light.



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Figure 2. Optical properties of QD-filled PLLA composite films. (a) Digital images of 200 μm thick composite films. (b) TEM micrograph of QD1 showing exfoliation and uniform dispersion of QDs. (c) Photographs showing green luminescence emitted by QD-filled composites under 365 nm UV light. (d) UV−vis absorption and (e) transmittance spectra of composite films. The bright orange and purple backgrounds define the UV region (250−400 nm) and the visible region (400−700 nm), respectively. (f) Plots of UVR at a set of wavelengths from 275 nm to 400 nm in 25 nm increments. Note that the UVR measurements could have been affected by scattering and the maximum UV blocking effect was obtained with more scattering samples.

Efficient control over the crystalline morphology and lamellar structure for host polymer is of particular importance to evaluate the efficacy of nanoreinforcements, not only yielding insights into the fundamental changes in chain dynamics but also enabling gains in mechanical and thermomechanical properties that are critical to broaden PLLA application range.40,41 2D-WAXD patterns revealed that QDs remarkably facilitated crystallization of PLLA even during the compression molding process that features rapid cooling (Figure 3a). Due to the poor crystallization ability inherent to PLLA materials, pure PLLA films were almost fully amorphous exhibiting only weak halo. This contrasted the rich formation of α-form PLLA crystals in the presence of QDs, as 12 ACS Paragon Plus Environment

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indicated by the distinct diffraction rings assigned to the lattice planes (010), (200)/(110), (203) and (015). In addition the diffraction intensities showed a direct relation to the QD concentration. The QD-assisted nucleation for PLLA was examined by POM observation using an isothermal crystallization protocol (Figure 3b). The addition of QDs featuring high surface activity largely pushed up the nucleation density of PLLA, resulting in the formation of compact spherulites for QD1 even at high crystallization temperatures that suppressed the self-nucleation of PLLA.42−44 The increased density of PLLA lamellae particularly in the vicinity of QDs underlay the QD-promoted nucleation activity, as illustrated by the comparison of lamellar textures imaged by AFM for pure PLLA and QD1 (Figure 3c). It is apparent that the QD-enabled preferential formation of lamellae created spatial hindrance for the size propagation of adjacent lamellae, leading to the decline of lamella diameter from around 50 nm for pure PLLA to approximately 15 nm for QD1. An inspection of the amplitude and phase images of QD1 manifests that the nanosized QDs appeared to be embedded into PLLA lamellae and separated by neighboring amorphous domains. This indicates that the QDs exhibited high affinity to PLLA chains.



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Figure 3. QD-tailored crystallization of PLLA. (a) 2D-WAXD patterns of compression-molded composite films. (b) POM images of composite films isothermally crystallized at 130 °C. Scale bar denotes 100 μm for all images. (c) AFM mapping of lamellar textures in pure PLLA and QD1 crystallized at 130 °C. Two pairs of arrows point out the PLLA lamellae and QDs for QD1 in the amplitude and phase images, respectively. Scale bar denotes 200 nm for all 2D AFM images.

The interactions between QDs and PLLA chains were discerned by FTIR, DMA and DSC measurements, providing insights into the polar interactions and segmental mobility (Figure 4a−e). In the C−H stretching region of FTIR spectra (Figure 4a), the characteristic bands of pure PLLA assigned to the CH3 asymmetric stretching (2995 cm−1) and CH3 stretching modes (2946 cm−1) were gradually blue-shifted to 2998 cm−1 and 2947 cm−1 for QD1, respectively.7 This was accompanied by evident blue-shifts in the 1452 cm−1 band arising from the addition of QDs, offering evidence for the 14 ACS Paragon Plus Environment

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facilitated conformational ordering degree (Figure 4b).45,46 The blue-shifts probably resulted from the improper hydrogen bonds created between the C＝O/−OH groups in PLLA chains and the oxygen functional groups across the surfaces and at the edges of QDs.47 Emerging from the prominent peak at 1747 cm−1 (attributed to C＝O stretching vibrations in the chain backbone of PLLA), a side peak located at 1755 cm−1 was generated with increasing the QD concentration (Figure 4c). The distinct factor group splitting in the C＝O stretching region could be triggered by the QD-facilitated formation of perfectly flat crystals that stored the concentrated O−C＝O stretching vibrations.48 The blue-shifting hydrogen bonds established in a set of polar groups (Figure 4a,b), together with the band splitting (Figure 4c), afforded demonstration of strong interactions between PLLA chains and nanoscale surfaces of QDs. Additionally, the 2851 cm−1 and 1294 cm−1 bands attributed to QDs exhibited notable intensity gains with increasing QD loadings (Figure 4a,b). Figure 4d,e interprets the PLLA−QD interactions from the thermodynamic point of view. Displaying gradual decrease for the peak area under the tan δ curves (Figure 4d), the QD-filled nanocomposites were characterized by a higher amount of immobilized segmental motion compared to pure PLLA.49 In the glass-transition region, notable endothermic peaks in DSC curves were exclusively observed for the QD nanocomposites, in clear contrast to the gentle transitions occurring in pure PLLA (Figure 4e). The additional endotherm was in principle related to the melting of percolated polymer interphase that was closely anchored onto the surfaces of QDs. Overlapping the spectroscopic identification and the thermodynamic study, we envisage that a percolated domain of PLLA interphase was created around the surfaces of well-dispersed QDs (Figure 4f). The high surface activity of QDs to anchor surrounding PLLA chains likely originates from (1) high density of 15 ACS Paragon Plus Environment


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oxygen functional groups in QDs that are ready to create hydrogen bonding associations with the carbonyl and hydroxyl groups in PLLA chains, and (2) geometric constraints arising from small QDs that provide nanoscale surface roughness for mechanical interlocking.10 The interfacial confinement effects in the interphase domains can give rise to the distortion of chain conformations, as well as the reduction of interchain interactions.50 This assumption is supported by the direct observation of chain adsorption and ordering templated earlier by the surfaces of multi-walled CNTs, inducing a thin layer of unique β-phase of PLLA.51 Protruding from the QD-immobilized chains, the neighboring chain segments with weaker interaction constraints can fold into ordered lamellae with intercalation of free amorphous chains. Compared to those distributed in the bulk, the chain segments constrained at the QD surfaces are characterized by a lower motional degree of freedom. In addition to the explanation of intimate PLLA−QD interactions, our proposed interphase model is identical to the direct observation of QD-embedded lamellar textures (Figure 3c).


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Figure 4. Identification of QD−PLLA interacƟons. FTIR spectra of compression-molded composite films in the range of (a) 3050−2800 cm‒1, (b) 1500−1290 cm‒1 and (c) 1810−1690 cm‒1. (d) Plots of tan δ as a function of temperature from DMA results. The peak area under the curves was evaluated and assigned to the amount of reduced molecular motions during the glass transition. (e) DSC heating traces of composites in the glass-transition region. (f) Schematic description interpreting the formation of percolated interphase (green segments) around well-dispersed QDs (orange spheres). Note that this molecular model is not drawn in correct scale with the AFM observation.

The good exfoliation and proper dispersion of QDs in PLLA, coupled with strong interfacial interactions, are expected to confer property improvements for the nanocomposites. Figure 5a,b reveals the QD-enabled promotion of thermal stability for PLLA composites, as demonstrated by over 10 °C increments in both the decomposition onset and decomposition maximum rate temperatures (Tonset and Tmax) compared to pure PLLA (see Table S2 for detailed values). The property distinction could arise from (1) preferential absorption of heat by QDs with conjugated 17 ACS Paragon Plus Environment


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carbons and chromophores52 and (2) enhanced thermal resistance by dense PLLA crystals.53 Figure 5c suggests that the resistance to heat deflection was significantly improved for QD nanocomposites over a wide temperature range, particularly in the glass-transition and viscoelastic regions (50−125 °C). Although substantial loss of storage modulus in the range of 50−90 °C was recorded for the composites with QD contents below 0.1 wt % due to the rapid softening of PLLA matrix,34 the modulus of QD0.5 and QD1 shifted to a much higher level with values even comparable to those of stereocomplex PLA with high crystallinity degree.54 Comparison of storage modulus at 25 °C and 100 °C clearly envisages the QD-promoted modulus in direct relation to the filler concentration (Figure 5d). Specifically, the increment of storage modulus at 25 °C steadily climbed to 65% for QD1 (4692 MPa), compared to the lowest point for pure PLLA (2844 MPa). The modulus data at 100 °C was striking: a 25-fold increase of storage modulus occurred at only 1 wt % of QDs (102 MPa) compared to pure PLLA (4 MPa). In the realm of PLLA nanocomposites, high resistance to heat deflection is an important goal to remove the application constraints under high temperatures.55 The above results pointed to high-efficiency improvement of PLLA thermal properties provided by the QDs derived from coffee grounds, surpassing the 3-fold increase in storage modulus for PLLA at 70 °C after addition of 1 wt % surface modified cellulose nanocrystals,56 and a limited increase (87%) observed at 100 °C for 5 wt % pristine cellulose nanocrystal−PLLA.57


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Figure 5. Thermal properties of QD nanocomposites. (a) TGA curves of composite films heated from 200 °C to 600 °C. The inset shows the decomposition onset region in the range of 260−350 °C. (b) DTG curves extracted from the TGA data. The inset lists Tmax values. (c) Plots of storage modulus as a function of temperature for the composites. (d) Storage modulus recorded at 25 °C and 100 °C as a function of QD loading.

In addition to multiple enhancements of thermal stability and thermomechanical properties, QDs improved the mechanical response of PLLA upon tensile deformation. Several important features for QD-filled composites are distinguished from pure PLLA in the measured stress−strain curves (Figure 6a). Limited by the intrinsic brittleness of PLLA materials, pure PLLA was directly fractured at the yield point after only 6.4% extension. By contrast, brittle-to-ductile transition was triggered by addition of QDs at as low concentrations as 0.05 wt %, showing a plateau of plastic deformation beyond the yield point with an additional gain of 5−10% strain for QD0.05, QD0.1 and QD0.5. Optimization of the strength–ductility–toughness property profile by addition of QDs is well illustrated by the key tensile property criteria (Table S3). Compared to the lowest tensile strength and 19 ACS Paragon Plus Environment


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modulus of pure PLLA (57.3 MPa and 1290 MPa), the addition of 1 wt % QDs resulted in an increase of over 69% and 67%, respectively. This was accompanied by simultaneous increase of ductility, with increments ranging from 27% for QD1 to 160% for QD0.05 and QD0.1 compared to the lowest elongation at break of pure PLLA (6.4%). It moreover witnessed at least 1-fold increase of toughness for all the QD composites, reaching 306% rise with 0.1 wt % loading of QDs. To highlight the mechanical nanoreinforcements achieved by coffee-ground-derived QDs, we compared the tensile strength for PLLA nanocomposites reinforced by the QDs and some conventional nanofillers used in the literature (Figure 6b). In this comparison the QDs stand out with high reinforcing efficacy even at low loadings bellow 1 wt %, showing unique advantages related to the ease of processing and renewability for the biowaste-derived QDs.


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Figure 6. Unusual combination of strength and toughness for QD-filled PLLA. (a) Typical stress−strain curves for the composite films. (b) Property space for tensile strength versus filler concentration for PLLA nanocomposites reinforced by QDs prepared in this study and other common nanofillers including, GO and graphene nanosheets,9 CNTs,58 bacterial cellulose (BC)34 and cellulose nanowhiskers (CNWs).59 The tensile strength values for the mentioned nanocomposites other than QD composites were derived from the cited literature.

Multiscale deformation mechanisms underlying the mechanical robustness of QD reinforced PLLA composites were appraised from the structural evolution involving deformation-induced crazing, shear bands and nanofibrillation (Figure 7 and 8). On macroscale, considerable volume of characteristic stress whitening was created in the plastically deformed zones for the composites loaded with low QD concentrations from 0.05 wt % to 0.5 wt % (Figure 7a). For PLLA based materials, plastic deformation-induced stress whitening was generally limited to the plasticized systems.60 To understand the unexpected transition in QD reinforced PLLA nanocomposites, in-depth elucidation of the fine microstructures in the stress-whitening zones was proposed. Following the protocol depicted in Figure 7b, the surface topographies in the plastically deformed regions of QD0.05, QD0.1 and QD0.5 were directly imaged by POM (Figure 7c−h) and SEM (Figure 7i,j). In the front zone near the fracture, extremely compact crazing was generated in all the nanocomposites, displaying a gradually increased crazing density with the increase of QD concentration (Figure 7c,e,g). It implies an important contribution from QDs to facilitate the propagation of crazing: the QDs may hold together the highly oriented PLLA chains and promote the redistribution of newly formed voids in between.61 Upon deformation the PLLA crystals were stretched, unpacked and reorganized into tiny crystals—a process potentially homogenized by neighboring QDs, which were responsible for the structural evolution from elongated spherulites 21 ACS Paragon Plus Environment


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with normal birefringence to homogenized crystals with monotonous refringence (Figure S11).9,62 In addition to the compact crazing and homogenized crystals, Figure 7d,f,h shows that highly paralleled shear bands were formed transverse to the deformation direction at an angle of 60−80°, which indicates shear flow of PLLA chains along the deformation for PLLA chains. The shear bands are generally recognized as significant contributors to the increased elasticity of semicrystalline polymers.61 High-resolution SEM observation of the plastically deformed surface topography revealed that the crazing was essentially composed of oriented chain bundles constituting numerous wrinkles, while the shear bands originated from shear flow of polymer chains past one another (Figure 7i,j), in line with the classic theory basically established in polyolefins.61 This is distinguished from the catastrophic cracks or cavities that were found in incompatible blends60 or poorly cohered composites,63 offering further evidence for the generation of robust PLLA interphase that enabled high extensibility by arresting and localizing the penetrating stress around the nanoscale QD surfaces (Figure 4f).


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Figure 7. QD-enabled crazing behavior during tensile deformation. (a) Digital photo of tensile fractured composite films showing plastic deformation-induced stress whitening. (b) Photograph clearly showing the large area of crazing for QD0.1. The surface microstructures are imaged in two zones: (c, e, g) the fracture front, and (d, f, h, i) approximately 1 cm from the fracture. POM images showing the crazing microstructure for (c, d) QD0.05, (e, f) QD0.1 and (g, h) QD0.5. The elongated and homogenized crystals and shear bands are marked. Scale bar denotes 100 μm for all images. (i, j) SEM micrographs of QD0.1 showing the surface topography in the plastically deformed region. The deformation direction is horizontal in (b−j).

In contrast to the brittle mode of deformation characterized by smooth and localized breaking of fracture surface for pure PLLA (Figure S11), nanofibrillated deformation acted as the predominant fracture mode in QD-filled nanocomposites independent of nanodot loadings (Figure 8a). These fine nanofibrils were featured by (1) very high density, (2) highly ordered alignment along the deformation and (3) ultrasmall diameter as low as tens of nanometers. The nanofibrils were closely 23 ACS Paragon Plus Environment


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adhered to each other, enabling the creation of ultrafine ligaments tightly bridging the neighboring nanofibrils in concert increasing deformation. This was accompanied by the deformation-induced nanovoids enclosing the nanofibrils, as well as the tapered tips at the fracture ends of nanofibrils, both of which were in favor of energy dissipation by transferring and redistributing the penetrating stress.64 The addition of high-strength nanofillers is generally demonstrated to restrict the plastic deformation of host polymers, resulting frequently in sacrifice of ductility or toughness.4,65 The high deformability of QD−PLLA that allowed remarkable microstructure transformations is of particular interest, and the underlying mechanisms are interpreted by QD-enabled morphological features (Figure 8b). Providing the direct observations (Figure 3) and spectroscopic analyses (Figure 4), it is reasonable to assume that the well-dispersed QDs are closely interactional with surrounding amorphous chain segments which are partially folded into ordered lamellae, creating a three-dimensional network crosslinked by nanoscale QDs and crystalline domains of PLLA (left panel in Figure 8b). Within the initial elastic deformation, an extensive alignment of amorphous chains is immediately elicited by the applied stress, driving rapid orientation of lamellae blocks and formation of chain fibrils (middle panel in Figure 8b).65 The reorganization process is enhanced by a large number of load-bearing units—numerous QDs that inter-tangle the oriented fibrils, conferring high capability to arrest, localize and redistribute the penetrating stress.66 After the yield point, large deformation is accommodated by amorphization, realignment and nanofibrillation of the chain bundles, rendering the formation of longer, thinner and stronger nanofibrils. The high strength and elasticity arising from the oriented chains and nanocrystals endow the nanofibrils with high ability to 24 ACS Paragon Plus Environment

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respond to the stress during the plastic deformation, sustaining a constant level far higher than the yield strength of pure PLLA (Figure 6a). In addition to the extension of amorphous chains crosslinked by QDs and nanocrystals along the nanofibrils, the formation of nanovoids and ultrafine ligaments between the nanofibrils is of significance to deflect the crack tips and to impart energy dissipation so as to increase the resistance to stress deformation.67 The additional inelastic energy dissipation is potentially imparted by secondary chain motion in the interphase domains anchored at the nanoscale QD surfaces.68 These QD-enabled energy-dissipating and flexibility-imparting mechanisms work synergistically to significant enhancements in the resistance to stress penetration, evading the strength–ductility trade-off dilemma.41

Figure 8. Mechanisms of QD-promoted elasticity and plasticity for PLLA composites. (a) SEM micrographs showing the fracture surfaces of composites after tensile failure. Scale bars are 1 μm (top panel) and 500 nm (lower panel). (b) Schematic model suggesting the enhanced resistance to stress deformation by incorporation of QDs.

CONCLUSIONS 25 ACS Paragon Plus Environment


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Using waste coffee grounds as a sustainable carbon source, a reliable and efficient MAH approach was utilized to gram-scale fabrication of ultrasmall and luminescent QDs with high surface oxygenation, dimensional uniformity and structural integrity. The unique optical properties of QDs were directly inherited by the PLLA composites resulting in multifunction ranging from luminescent emissions to excellent UV shielding and high transmission to visible light. The nanoscale surfaces of well-dispersed QDs provided strong anchoring interactions for PLLA chains, accounting for the facilitated crystallization even under rapid cooling, as well as significant improvements in thermomechanical and tensile properties. Compared to pure PLLA, a 25-fold increase in the storage modulus at 100 °C was observed for QD1 (102 MPa), accompanied by nearly 70% increase in both tensile strength and modulus (96.9 MPa and 2170 MPa). Underlying this unexpected mechanical robustness upon tensile deformation were QD-enabled energy-dissipating and flexibility-imparting mechanisms, involving the formation of compact crazing, oriented shear bands and ultrafine nanofibrils that synergistically contributed to arrest, localize, redistribute and transfer the penetrating stress. This opens a low-cost and reliable pathway to biowaste-based and multifunctional 0D nanoreinforcements, thus empowering a new class of high-performance and environmentally benign nanocomposites. We envision that the unusual combination of mechanical robustness and light management underpin the immense potential for diverse applications of PLLA/QD composites, e.g., packaging of food and drug where high transparency and effective UV shielding are desired.

ASSOCIATED CONTENT Supporting Information 26 ACS Paragon Plus Environment

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The Supporting Information is available free of charge on the ACS Publications website at DOI: . Experimental details, SEM images of coffee grounds, dot precursor and QDs, TEM images and UV−vis absorption spectra of QDs, AFM images mapping the QD dispersion morphology in PLLA composites, photoluminescence spectra and 1D-WAXD intensity curves of QD-filled composite films, POM images of isothermally crystallized composites, FTIR spectra, DSC heating and cooling curves, TGA data and tensile property profile of compression-molded composites, POM images of QD0.5 showing the evolution of crystal structure during tensile deformation, and SEM micrographs of tensile fractured composites showing the surface topographies and fracture surfaces (PDF)

AUTHOR INFORMATION Corresponding Author *M.H., Email: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS L.X. acknowledges the financial support from the National Natural Science Foundation of China (21604016) and Scientific Research Project of Introduced Talents of Guizhou University (201627).



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The authors are deeply indebted to Karin H. Adolfsson for generous help with experimental operation and valuable discussions.

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