Nanobiocomposite Adhesion: Role of Graft Length and Temperature

Article pubs.acs.org/Biomac

Nanobiocomposite Adhesion: Role of Graft Length and Temperature in a Hybrid Biomimetic Approach Niklas Nordgren,*,† Linn Carlsson,† Hanna Blomberg,† Anna Carlmark,† Eva Malmström,† and Mark W. Rutland‡,§ †

Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden ‡ Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden § SP Chemistry, Materials and Surfaces, SP Technical Research Institute of Sweden, SE-114 86 Stockholm, Sweden ABSTRACT: Cellulose microspheres bearing poly(ε-caprolactone) grafts of different molecular weights were investigated to evaluate the effect of graft length on the interfacial properties. Surface force and friction measurements were performed using an atomic force microscope in colloidal probe mode. The maximum interaction distance and adhesion is dependent on the temperature and the time in contact via a diffusion controlled mechanism. The effects are highest for the longer grafts, and molecular weight thresholds were found to lie between 21 and 34 kDa at 25 °C and between 9 and 21 kDa at 40 °C. The interpenetration of the graft into a matrix leads to “hidden length” contributions to adhesion, analogous to those in natural biocomposites. The nanotribology results display Amontonian behavior, and the friction force at zero applied load is higher at the graft−graft interface than for a bare cellulose sphere interacting with the graft. These results clearly demonstrate the benefits of the grafted polymer layer on the adhesion, toughness, and resistance to shear in the design of cellulosic nanobiocomposites.

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alter the sliding friction between them.5−7 The technique has also been applied to studying bioadhesion mechanisms in, e.g., lobster gastroliths.8 As will be demonstrated, the biomimicry exploited here refers to composites such as teeth, bone, and shell, but applied to plant-based biopolymers. Moreover, the recent development of the AFM fiber probe technique9 has allowed for measurements between native biofibers such as wood10 and hair.11 In addition to the versatility of the colloidal probe technique, it enables swift data collection, which is advantageous when a large number of measurements are required. A range of different approaches for increasing the interfacial adherence between filler and matrix in cellulose-based composites have been proposed.12−16 Generally, adhesion can be promoted through modification of the cellulose surface by anchoring polymeric species either by physical,5,17 electrostatic,6 or covalent means.15 The covalent attachment can be performed using either of two techniques. The “grafting-to” technique relies on the chemisorption of a presynthesized polymer onto reactive anchorage points on the surface. The other is the “grafting-from” technique, where the polymerization is initiated directly from reactive groups at the cellulose

INTRODUCTION Cellulose is a prospective key to advanced materials innovation. The outstanding mechanical and lightweight properties of the cellulose fiber continuously spur the use of its hierarchical assemblies in the design of novel materials1 and applications.2 As the world’s most abundant polysaccharide, cellulose also provides a rich sustainable source to be drawn upon. When considering all of the benefits, it is evident that cellulose rivals traditional materials, such as metals and synthetic fibers, for use as fillers in lightweight nanocomposite materials.3 However, when cellulose is dispersed in polymeric matrices, the difference in surface energy between the components leads to poor adhesion, ultimately resulting in cracking and delamination. Moreover, as the size of the components is scaled down into the nanoscopic regime, the interfacial stresses in the composite become substantial. Therefore in order to design adequately robust nanointerfaces, the intermolecular forces of the system need to be identified, measured, and evaluated for optimization. The atomic force microscope (AFM) in colloidal-probe configuration4 allows for the direct quantification of surface interactions at the nanoscopic scale. Since the pioneering work in the 1990s, the colloidal-probe technique has been successfully applied to a range of different systems, including extensive studies of cellulosic materials. For example, physical adsorption of biopolymers has been shown to enhance adhesion between cellulose surfaces and at the same time © 2013 American Chemical Society

Received: November 18, 2012 Revised: February 20, 2013 Published: February 22, 2013 1003

dx.doi.org/10.1021/bm301790b | Biomacromolecules 2013, 14, 1003−1009

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Figure 1. Synthetic route for the grafting of PCL from cellulose microspheres (upper). Schematic showing cellulose microspheres attached to a sheet of mica (lower): neat cellulose sphere (a); cellulose spheres surface-grafted with PCL of Mw = 9 kDa (b); Mw = 21 kDa (c); Mw = 34 kDa (d); Mw = 55 kDa (e). Note that the components in the figure are drawn out of scale for clarity.

surface.18,19 The “grafting-from” technique has the advantage over the “grafting-to” technique in that a higher surface grafting density can be achieved.20 A recent study using the colloidalprobe technique demonstrated the benefits of grafted polymers on the dynamic adhesion relevant for cellulose nanocomposite design.15 In that work measurements were carried out using cellulose microspheres that have previously been extensively employed by some of the authors in a range of studies dealing with biopolymer interactions in aqueous media.5−7,17 The cellulose microspheres were first grafted with poly(εcaprolactone) (PCL) via ring-opening polymerization (ROP)21 and subsequently attached to the end of AFMcantilevers. Such a functionalized probe was then brought into intimate contact with spheres attached to a lower surface to produce a model of an interface between filler and matrix material in a nanocomposite. An increase in the overall adhesion and the interaction distance was observed between grafted spheres at room temperature when compared with the untreated cellulose spheres. This is ascribed to the fact that the polymer chains interact more favorably with each other (via entanglements) than with the bare cellulose. At higher temperature (60 °C) close to the melting point of the PCL,the observed effects increased dramatically. This is due to the increased mobility of the polymer graft leading to higher diffusion rates and more chain-entanglements across the interface. Thus, the PCL graft efficiently increases the compatibility at the nanoscopic interface. However, to fully understand this phenomenon, it is necessary to elucidate at which chain length the entanglement threshold occurs for these systems. The effects of varying the molecular weight of the graft

on the adhesive forces and interfacial toughness are of great interest and are the focus of the current paper. A powerful feature of ROP is the possibility to tailor the molecular weight of the surface graft. In the current study, ROP was used to produce a well-defined model system consisting of a series of cellulose microspheres exhibiting PCL grafts of varying molecular weights. In addition, for nanocomposite applications, not only the normal (adhesive) forces but also the lateral (frictional) forces are decisive for the mechanical properties of the final material. Hence, the colloidal probe technique is applied to investigate both the effects of the PCL graft on the adhesive and frictional behavior of the interface.

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MATERIALS AND METHODS

The surface graft polymerization of PCL from cellulose microspheres (approximate diameters of 10 μm produced from regenerated cellulose by the viscose process (Kanebo, Japan)) via ROP was performed following the procedure schematically presented in Figure 1 and described elsewhere.15 The reaction times were varied in order to obtain polymer grafts of different molecular weights. Successful covalent linkage of the PCL from the cellulose spheres (after thorough purification using Soxhlet extraction to remove any physiosorbed polymer)15 was confirmed by the occurrence of the carbonyl-peak in the attenuated total refelctance fourier transform infrared (ATRFTIR) spectra (Figure 2). The chain lengths of the PCL-grafts were calculated on the basis of the molecular weight of the free polymer formed in the bulk (analyzed using size exclusion chromatography (SEC) in tetrahydrofuran (THF)). The validity of this approach, using another controlled polymerization technique, has recently been confirmed for polymer-grafts on cellulose surfaces where the molecular weights of the cleaved graft-polymers compared with the bulk-species 1004


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sufficiently low for avoiding any capillary flooding effects to the adhesion.26 To ensure reproducibility, all measurements were repeated using different substrates for all the conditions. The effective interaction radius of curvature, R, between the two interacting cellulose spheres was calculated according to the expression below, which describes the reduced radius of interaction for two spheres of different radius (and can be thought of as the radius of an equivalent sphere interacting with a flat surface)27

R=

R1R 2 R1 + R 2

(1)

where R1 and R2 are the radii of the two cellulose spheres. Friction measurements were performed in air as a function of increasing and decreasing normal load at a scan size of 2 μm and a scan rate of 1 Hz.25

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RESULTS AND DISCUSSION The interaction behavior upon retraction between the grafted cellulose probe (PCL graft of Mw = 55 kDa) and the cellulose spheres exhibiting varying graft lengths is qualitatively illustrated by the force−distance profiles displayed in Figure 3. At short separations, the interaction with the neat cellulose

Figure 2. Normalized ATR-FTIR spectra: neat cellulose spheres (a); nellulose spheres surface-grafted with PCL of Mw = 9 kDa (b); Mw = 21 kDa (c); Mw = 34 kDa (d); Mw = 55 kDa (e). The molecular weights refer to the bulk values obtained using size exclusion chromatography. were found to be almost identical.22 No analogous cleaving study has yet been performed for ROP grafted polymers, so it cannot be guaranteed that this correlation is valid. Although different chemistry was employed in the atom transfer radical polymerization (ATRP) cleaving study, the agreement in that case is nonetheless reassuring for the wider applicability of the method. NMR spectra were recorded at 400 MHz on a Bruker AM 400 using CDCl3 as the solvent. The solvent signal was used as an internal standard. FTIR spectroscopy was conducted on a Perkin-Elmer Spectrum 2000 FTIR equipped with a MKII Golden Gate, Single Reflection ATR system from Specac Ltd., London, U.K. The spectra were normalized against a specific ATR crystal adsorption to enable a comparison between the polymer-grafted cellulose substrates.18 The molecular weights of the PCL samples were analyzed using SEC comprising Mw = 9 kDa (Đ = 1.2), Mw = 21 kDa (Đ = 1.3), Mw = 34 kDa (Đ = 1.5), and Mw = 55 kDa (Đ = 1.9). The measurements were performed using a TDA model 301 equipped with one or two GMHHR-M columns with TSK gel (Tosoh Biosep), a VE 5200 GPC autosampler, a VE 1121 GPC solvent pump, and a VE 5710 GPC degasser, from Viscotek Corp. THF was used as the mobile phase (1.0 mL/min). The measurement was performed at 35 °C. The SEC apparatus was calibrated with linear polystyrene standards, and toluene was used as the flow-rate marker. Colloidal-probe adhesion and friction measurements were performed using a MultiMode Picoforce AFM with a Nanscope IIIa controller (Veeco Instruments, USA). The PCL grafted cellulose spheres were attached to the end of a cantilever using a tiny amount of epoxy resin (Araldite Rapid, Casco). The rectangular tipless silica cantilevers used were of type CSC 12/NoAl (MikroMasch, Estonia) with the approximate dimensions length = 90 μm and breadth = 35 μm. The spring constants of the cantilevers were determined using the calibration software AFM TuneIT v2.5 (ForceIT, Sweden) based on hydrodynamic damping.23,24 The force measurements were performed according to the procedures described in an IUPAC report.25 The adhesion was measured between the upper functionalized cellulose probe (PCL graft of Mw = 55 kDa) and a lower sphere (attached to a freshly cleaved sheet of mica) of either neat cellulose or cellulose bearing PCL grafts of either Mw = 9 kDa, Mw = 21 kDa, Mw = 34 kDa, or Mw = 55 kDa (see Figure 1) after being in surface contact at varying times at a constant applied load (14 mN/m). Typical force measurements were conducted in air (20 °C, 40 and 60 °C) at a rate of 2 μm/s. The relative humidity was monitored during experiments and shown to lie between 30 and 40%, which is

Figure 3. Normalized force profiles upon retraction between a cellulose sphere grafted with PCL (Mw = 55 kDa) and a neat cellulose sphere (a); a cellulose sphere grafted with PCL of Mw = 9 kDa (b); Mw = 21 kDa (c); Mw = 34 kDa (d); Mw = 55 kDa (e). The arrows indicate the maximum pull-off distance. The inset shows a magnified part of a force curve between two cellulose spheres (both bearing PCL grafts of Mw = 55 kDa). The displayed force curves were obtained at 60 °C after being in contact for 100 s.

sphere is characterized by a force minimum with a distinctly sharp transition jump to zero force. This behavior is a result of instability when the cantilever spring constant overcomes the adhesion force between the cellulose surface and the polymer, i.e., characteristic of “classical thermodynamic work of adhesion”. The force minimum (i.e., pull-off force) may be used in the composite design process as a relative measure of the inherent affinity between dissimilar materials, for example, to facilitate the adequate choice of matrix polymer.28 However, when cellulose spheres bearing PCL grafts are employed as lower substrates, a deviation from the “classical” adhesion mechanism occurs. First, the magnitude of the work of adhesion can be seen to increase as a function of the molecular weight of the graft. The observed trend is likely due to an increase of the surface area accessible for polymer bridging and 1005


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interdigitation across the interface. The slightly bent shape seen in the force profiles close to the apparent contact region is a combined result of sphere deformation5 and nonlinearity in the photodetector29 due to the large adhesion. However, more interestingly, at farther separations, an additional long-range adhesive force mechanism is present. The observed “jagged” pattern (see inset of Figure 3) owes its origin to multiple disruptions of polymer bridges and chain-entanglements as the interface widens. This characteristic distant-dependent adhesion behavior has previously been observed for other biopolymer systems both in aqueous media6,7,17 and in air.8,15 The concept of the “pull-off force” (the depth of the minimum seen for the classical adhesion, i.e., as for the asymmetric case here) is thus insufficient to describe the adhesion for the more complex polymer−polymer-induced adhesion behavior present. Instead, a more appropriate parameter for comparing the systems can be obtained by integration of the force curves, where the area defined by the force curve at negative loads corresponds to the total work of adhesion. Accordingly, in Figure 4, the total work required to separate the surfaces at 60

linear, highly flexible, polymers starts when there are between 300 and 600 atoms in the main chain.31 For PCL, this translates to molecular weights between 4.9 and 9.8 kDa. The result shows the importance of graft-length on the adhesion and highlights the benefit of tailoring the interface for optimum material performance. The total work of adhesion provides a measure of the reversible work required to separate the surfaces. However, when discussing fracture resistance, this is not always the most useful measure; a large force applied over a short time, such as a shockwave resulting from a sharp blow, may cause local deformation of a few angstroms, sufficient to separate the contact. In nature this issue is solved by sacrificial bonds that provide a “hidden length” mechanism to promote toughness.8,32 Surfaces must be separated large distances against a restoring force before fracture can occur. Thus, another parameter that also should be considered is the maximum pull-off distance (i.e., the surface separation at which final detachment occurs), which is expected to be related to the ability of the chain to intercalate; such results are seen in Figure 5. There it can be seen that, as expected, the maximum pull-off

Figure 4. Work of adhesion as a function of contact time. The data displayed are between a cellulose sphere grafted with PCL (Mw = 55 kDa) and a neat cellulose sphere (●); a cellulose sphere grafted with PCL of Mw = 9 kDa (□); Mw = 21 kDa (△); Mw = 34 kDa (■); Mw = 55 kDa (○, data reproduced with permission from ref 15. Copyright 2009 American Chemical Society.) The displayed measurements were conducted at 60 °C.

Figure 5. Maximum pull-off distance (nm) as a function of molecular weight (Da) and time in contact (s) at 60 °C.

distance increases with increasing molecular weight of the graft, and also with increasing time in contact.30 This is due to the fact that the relaxation time of the polymer is finite and is limited by Rouse dynamics, otherwise known as “breathing modes”.33 In Figure 6, the maximum pull-off distance of the graft (Mw = 21 kDa) is displayed as a function of the square root of the contact time. The fact that the maximum pull-off distance scales linearly with t1/2 characterizes a diffusioncontrolled interaction,34 and has been observed previously for interfacial polymers.5,15,30,35 At longer times, a deviation from linearity occurs, manifested as a plateau region, indicating that the maximum effect is achieved, which is in agreement with observations that most of the interfacial toughness arise early in the annealing process.33,35 Moreover, the rate of diffusion is increased at the higher temperature due to increased chain mobility of the grafted species. When focusing on the longest time in contact (100 s), most relevant for nanobiocomposite production (see Figure 7), the dramatic effect of the molecular weight on the maximum

°C is displayed as a function of contact time for the different systems. The temperature is close to the melting point for PCL, which results in large adhesion between the grafted samples due to the high mobility of the interacting polymer chains. The work of adhesion clearly scales with both the molecular weight of the graft and time in contact. The time dependence has previously been proposed to be an effect of a diffusion-based adhesion mechanism.7,15,30 Similar behavior was also observed for the lower temperatures (20 °C and 40 °C); however, under those conditions both the range and the magnitude of the adhesion are considerably smaller due to the lower mobility of the polymer graft. Significant effects can be seen in Figure 4 for all chain lengths except the shortest (Mw = 9 kDa), which displays behavior almost identical to the interaction with the bare cellulose surface and indicates that, at the lowest molecular weight, the chain length is insufficient for interpenetration or entanglements to occur. The formation of entanglements for 1006


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Figure 6. Maximum pull-off distance (nm) as a function of t1/2 (s1/2) between a cellulose sphere grafted with PCL (Mw = 55 kDa) and another spheres with PCL of Mw = 21 kDa at 25 °C (blue) and 60 °C (red).

Figure 8. Friction measurements at 60 °C between a cellulose sphere grafted with PCL (Mw = 55 kDa) and a neat cellulose sphere (▲) and a cellulose sphere grafted with PCL of Mw = 55 kDa (■).

and the symmetric case display Amontonian behavior where the friction force is linearly dependent on the applied normal load. However, the friction force is nonzero at zero applied load, which is commonly observed in adhesive systems. Therefore the behavior follows the slightly modified version of Amonton’s law

FFriction = μFLoad + F0

(2)

where FFriction is the friction force, FLoad is the applied load, μ is the friction coefficient, and F0 is the friction force at zero applied load. During sliding friction, the effects of intercalation discussed earlier for the normal adhesive forces are minimized as long as the sliding rate is fast compared to the polymer relaxation time. The frictional behavior is thus unlikely to be as dependent on molecular weight and temperature as the adhesion is. Nonetheless, the friction coef f icient is always slightly higher for the symmetric case, probably due to the larger dissipation induced by viscous losses in the polymer−polymer interaction. Repeated measurements were also performed at each molecular weight using different sliding rates and temperatures. Although the exact ratio varied slightly, the qualitative difference seen in Figure 8 was preserved, irrespective of the different conditions used. A more interesting value to consider for composite applications is the friction force at zero applied load, F0 which provides an indication of the initial shear resistance at the filler−matrix interface. Figure 9 summarizes the values of F0 obtained from several friction measurements, such as those shown in Figure 8, at 25 °C, 40 °C, and 60 °C for both the asymmetric and symmetric systems. The variation seen for all conditions reflects the fact that cellulosic surfaces are rough at the nanoscale,5,36 hence a somewhat nonuniform interaction at the interface is expected. Therefore, to be able to adequately elucidate the interfacial behavior for cellulosic materials, a large number of repeated measurements are necessary. As seen from the results in Figures 8 and 9, the friction force at zero applied load tends to be larger for the symmetric system, demonstrating the benefits of the graft on the mechanical resistance to shear. Moreover, the frequency of higher F0 values for the polymer− polymer interaction is significantly higher at 60 °C. The highest

Figure 7. Maximum pull-off distance (nm) as a function of molecular weight (Da) at 25 °C, 40 °C, and 60 °C after being in contact for 100 s. Inset show schematic of the macromolecular induced adhesion mechanism. The molecular weight thresholds are indicated by the blue arrow (25 °C) and the orange arrow (40 and 60 °C).

interaction distance is evident. Moreover, molecular weight thresholds can be discerned with respect to the temperature used. At the lowest temperature (25 °C), the threshold is located between molecular weights of 21−34 kDa (blue arrow, Figure 7) where a significant difference in the interaction distance appears. This is highly interesting as it demonstrates the molecular weight range needed to achieve significant diffusion controlled toughening at ambient conditions. On the other hand, at raised temperature (40 °C and above) the observed molecular weight threshold needed to achieve toughening occurs at even shorter grafts, between molecular weights of 9−21 kDa (orange arrow, Figure 7). In addition to the normal adhesive forces the lateral, sliding frictional forces should also be considered in nanobiocomposite design. Figure 8 displays typical friction behavior at 60 °C between a cellulose sphere grafted with PCL (Mw = 55 kDa) interacting with either a neat cellulose sphere or a cellulose sphere grafted with PCL (Mw = 55 kDa). Both the asymmetric 1007


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asymmetric (graft-cellulose) case. However, more interestingly for nanobiocomposite applications is the fact that the friction force is nonzero at zero applied load, hence acting as initial resistance to shear. The role of the graft is to increase this resistance with the most prominent effects shown at 60 °C likely as a result of larger amorphous regions at the interface due to the melting of the crystalline phase in the PCL layer. The findings and observations made in this work have clearly demonstrated the benefits of the grafted layer as a compatibilizer between filler and matrix materials for nanobiocomposites. The tools to tailor, control, and accurately measure adhesion, toughness, and shear resistance at the nanoscale have been demonstrated with the potential to improve future materials design.

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Figure 9. Friction force at zero normal load F0 (nN) at 25 °C, 40 °C, and 60 °C between a cellulose sphere grafted with PCL (Mw = 55 kDa) and a neat cellulose sphere (open bars) and a cellulose sphere grafted with PCL of Mw = 55 kDa (closed bars). The bars each represent F0 (nN) values obtained from individual friction measurements.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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resistance to shear found for the symmetric case at 60 °C (close to the melting temperature of the PCL graft) is likely due to a combination of effects. The melting of the crystalline regions increases the mobility of the anchored polymer species so that the relaxation time becomes shorter, allowing limited penetration into the opposing matrix. As a result the growing accessible area of amorphous regions increase the interfacial adhesion, which is manifested in a higher friction force at zero applied load. Thus, the lateral force behavior correlates well with the trends observed for the normal forces discussed earlier. Additionally, the sliding ability at the interface may be affected by physical surface topography effects, which in turn depend heavily on the nature of the attached polymer layer and its proximal environment.5,6,17,37,38

ACKNOWLEDGMENTS The authors acknowledge financial support from the Swedish Foundation for Strategic Research (via Biomime, the Swedish Center for Biomimetic Fiber Engineering), and the Swedish Research Council FORMAS. M.W.R. thanks the Swedish Research Council (VR) for financial support. The AFM was financed by a grant from the Knut and Alice Wallenberg Foundation.

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REFERENCES

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CONCLUSIONS Unique experimental design applying animal based bioadhesion mechanisms to plant-based materials in combination with the versatile colloidal probe AFM technique has shed light on a range of macromolecular phenomena at the interface, all of importance for improved compatibility and increased stress transfer between matrix and filler in cellulose-based nanobiocomposites. By investigating the normal forces upon retraction between cellulose micro spheres grafted with PCL at various molecular weights, the effects of the graft length on the adhesion and toughening of the interface could be deduced. It was shown that both the magnitude and the range of the adhesion scale with the molecular weight of the graft and time in surface contact due to increased entanglements and interdigitation events. The observed long-range adhesive interaction reflects the ability of the polymeric fiber interface to promote toughness in a hybrid biomimetic manner and a useful parameter to describe this phenomenon is defined as the maximum pull-off distance. Further, molecular weight thresholds for interpenetration could be discerned for grafts between 21 and 34 kDa (25 °C) and at higher temperature (40 and 60 °C) for grafts between 9 and 21 kDa. The lateral (frictional) force interaction displays Amontonian behavior both at the cellulose-graft interface and between two grafted cellulose particles. The friction coefficient is slightly higher for the symmetric (graft−graft) case than for the 1008


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