Nanoscale Thermal Analysis of Multiphase Polymer Nanocomposites

Mar 27, 2012 - †Laboratory of Energy and Nanosciences and ‡Material Science and Engineering, Masdar Institute of Science and Technology, Abu Dhabi...
5 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Nanoscale Thermal Analysis of Multiphase Polymer Nanocomposites Tewfik Souier,†,‡,∥ Yarjan Abdul Samad,‡,∥ Boor Singh Lalia,‡ Raed Hashaikeh,*,‡ and Matteo Chiesa*,†,‡,§ †

Laboratory of Energy and Nanosciences and ‡Material Science and Engineering, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates § Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, United States S Supporting Information *

ABSTRACT: Understanding the structural and thermomechanical properties of mixed-phase polymeric materials and composites at the nanoscale is one of the key challenges in developing a new class of materials with improved structural properties. Here, we present a nondestructive method, local thermal analysis (LTA), based on a heated atomic force microscope cantilever, for the nanoscale characterization of structural and thermal properties of polymer composites. The technique allows measurement of the local melting point, the dehydratation temperature, and the thermal expansion. Moreover, by monitoring the indentation with the heated tip after the melting point, the LTA may be used for phase assignment in multilayered polymers. The measured melting temperatures are found to be reproducible and match satisfactorily with those obtained by differential scanning calorimetry. Finally, the LTA technique coupled with transmission electron microscopy allows us to obtain a more precise description of the nanostructure of a recently developed cellulose/PEO nanocomposite.

1. INTRODUCTION Thermomechanical properties of polymers, namely, stiffness and viscosity, are intimately related to the glass phase transition in polymeric systems.1−3 Thermal methods such as differential scanning calorimetry (DSC) and modulated temperature DSC (MDSC), thermogravimetric analysis (TGA), thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA) are well-established techniques for the measurements of the transition, melting, and degradation temperatures of polymers. It is often possible to identify and quantify materials with reference to their characteristic transition temperatures and thermal stability. However, a serious limitation of conventional thermal methods is that they give only a sample's averaged response and cannot provide information on specific features on or within the sample. A DSC measurement, for example, may indicate the presence of more than one phase, but the technique cannot generally give any information regarding the size or distribution of phases. Moreover, the same technique is insensitive to the variation in phase transition temperature that can occur at surfaces and interfaces in heterogeneous materials. Another drawback of these methods is their destructive nature due to the required heating of the entire sample mass. The structural characterization of polymers at the submicrometer scale usually relies on scanning and transmission electron microcopy (SEM/TEM). These methods are timeconsuming and require complex sample preparation with multiple steps and different types of advanced tools such as a sputter coater and focused ion beam (FIB). Moreover, the identification of the phase separated morphology in nonconductive polymers is sometimes very difficult due to lack of contrasts, and thus, it requires very skilled personnel. © 2012 American Chemical Society

During the past decade, much effort was put forth to develop techniques capable of detecting single phases and characterizing thermomechanical properties of thin films and nanostructures at the micrometer and submicrometer scale. The use of microthermal analysis,4 frequency-domain thermoreflectance method,5 scanning probes microscopy (SPM)-based techniques such as phase imaging,6,7 scanning thermal microscopy,8 scanning expansion microscopy,9,10 variable temperature nanoindentation,11,12 and temperature transition microscopy based on contact resonance13,14 exemplifies some of these techniques. In this paper, we utilize local nanothermal analysis (LTA) techniques based on atomic force microscopy (AFM) with a heated tip15 to measure the nanoscale thermal properties of a dual-phase polymeric material. The material is a cellulose/ polyethylene oxide (PEO) blend, named GelPEO, developed through a novel technique.16,17 GelPEO has demonstrated enhanced water absorption and retention capability even above the boiling temperature of water. Thus, it offers great watersaving potential and can have a great impact for agriculture applications in arid regions. Structural and thermal properties of GelPEO are determined by LTA, combining the high spatial resolution imaging capabilities of AFM with the ability of quantifying the thermal behavior of materials with a spatial resolution of sub-100 nm. Received: February 12, 2012 Revised: March 21, 2012 Published: March 27, 2012 8849

dx.doi.org/10.1021/jp301410e | J. Phys. Chem. C 2012, 116, 8849−8856

The Journal of Physical Chemistry C

Article

2. EXPERIMENTAL METHODS 2.1. LTA Technique. AFM experiments are performed on MFP-3D stand alone AFM microscope from Asylum Research. The local thermal analysis is performed using a Z-therm Cantilever holder and heated AFM tips “ThermaLevers” from Anasys Instruments (AN2-200, spring constant ∼0.5 N/m and estimated by thermal noise method) in contact mode AFM. These analyses are carried out using Z-therm software from Asylum Research. The AN2-200 tip consists of a U-shaped single crystal silicon cantilever (Figure 1), where the legs are

consists of holding the probe in contact with the substrate, applying a ramp of temperature (ramp of voltage), and measuring the evolution of the vertical tip deflection. The problem with this technique is that the load increases when the material begins to soften under the heated tip, resulting in variations of the measured transition/melting temperatures. Moreover, variation in the load induces variation in the tip− substrate contact area, making it difficult to quantify the thermomechanical properties. Unlike the conventional LTA method with deflection feedback off, we carry out our experiment using a constant deflection (feedback on). This method insures more reproducible data since the contact area between the tip and the sample is maintained constant.22 In this method, the vertical displacement of the tip is measured by monitoring the displacement of the AFM Z-piezo. When the tip is in contact with the surface, heating of the tip results in a local thermal expansion followed by local softening when the temperature reaches the glass transition and/or melting point. To maintain a constant deflection, the piezo moves to accommodate the movements of the heated tip due to local expansion/softening of the material. When the substrate temperature reaches the melting point locally, the tip penetrates into the sample with a constant force. The experiment is stopped once the penetration depth (indentation) reaches a specified trigged value. Unlike the conventional LTA method, this method allows one to control the penetration depth, preserving the tip-apex and minimizing the effect of contamination with the melted polymer. Besides the measurements of the vertical displacement of the tip, the probe resistance Rtip and the electrical dissipated power Ptip were also measured as an indication of the tip temperature TH and the thermal dissipation at the tip−sample contact, respectively. It is evident that controlling the size of the tip−sample contact is a crucial step for obtaining quantitative thermomechanical analysis. However, it was reported that heated cantilevers are subject to bending and twisting, which in turn causes changes in the deflection.18,23 This unwanted deflection, unless measured and quantified, will appear as a signal indistinguishable from the deflection caused by tip−sample interactions. Consequently, even with a constant deflection, the contact force might change during heating. To quantify these effects and take into account the induced deflection in the analysis, the free probe was heated in air at a certain distance from the substrate. The cantilever deflection was found to vary with the applied voltage up to 0.2 V, which corresponds to the difference in contact forces of 60 nN and of tip−sample contact diameter of more than 10 nm (see the Supporting Information). The deflection versus voltage curves were fitted with a parabolic function, and the resultant function was used to correct the deflection when the tip is in contact with the sample. Another advantage of heating the tip in air is the ability to clean the tip apex from contamination (melted polymer), mainly after the indentation. This technique of cleaning the tip and accounting for the parasitic deflection is performed prior to all LTA measurements. 2.3. Temperature Calibration. The temperature of the cantilever TH is controlled by the voltage applied to the tip as previously explained. Because of the difference in thermal impedance between the cantilever and the tip, the temperature of the tip may be different from the temperature of the cantilever. Here, we use the standard polymeric samples [polycaprolactone (PCL, Tm = 60 °C), high density poly-

Figure 1. Differential drive scheme used in this work. By driving the cantilever legs differentially, it is possible to control the potential at the tip and the current flowing through it separately with two different control voltages.

highly doped, while the bridge has a low doping concentration, creating a resistive heater within the current path. By applying a drive voltage between the legs, the current flows through the heater and induces a tip temperature increase by joule heating. At room temperature, the resistance of the probe is found to be 1 kΩ, and after heating, the resistance of the tip increases up to 3 kΩ, which corresponds to a tip temperature of about 350 °C. In the Z-therm software, the resistance of the tip is estimated by measuring the voltage Vtemp near the cantilever (as shown in Figure 1). Both the current I and the tip resistance Rtip are calculated using the following formulas: I=

Vd − Vtemp 2Vd = 2R ref + R tip R ref

R tip =

2Vtemp

I where Vd is the drive voltage applied to the “U” probe legs and Rref is a reference resistance of the differential drive circuit (2 kΩ). To probe the “thermomechanical” properties of the tip− surface junction, the temperature at the end of the tip is regulated by sweeping a DC voltage with a typical rate of 0.1 V/ s. This results in the variation of the tip resistance. 2.2. Improvements of the Method. There are several methods to perform nanoscale thermal analysis using LTA: (1) indentation at various temperatures, (2) shear modulation, (3) differential impedance measurement, and (4) thermally measured tip penetration.15 These methods are used to increase the accuracy of transition/melting temperature measurements. However, the most common method for LTA, in polymer and pharmaceutical materials, is the deflection measurement during the heating cycle.18−21 This method 8850

dx.doi.org/10.1021/jp301410e | J. Phys. Chem. C 2012, 116, 8849−8856

The Journal of Physical Chemistry C

Article

Figure 2. (a) Z-sensor vs drive voltage curves showing the thermal extension and melting point on standard calibrated samples. (b) Temperature calibration for the tip used in the experiments.

Figure 3. (a and b) AFM height and phase (in insert) maps on PEO and cellulose bulk material, respectively (the scale bar = 250 μm). (c and d) LTA measurements on PEO and cellulose, respectively. (e) Macroscopic thermal analysis using DSC.

ethylene (HDPE, Tm = 125 °C), polypropylene syndiotactic (PPS, Tm = 130 °C), polypropylene isotactic (PPI, Tm = 163 °C), and polyethylene terephthalate (PET, Tm = 260 °C)] with known melting temperatures to calibrate the temperature at the tip by a continuously changing voltage with a ∼0.1 V s−1 rate. This approach for calibrating the dynamic transfer function was recently demonstrated by Lee24,25 and was extensively used in LTA measurements on polymeric materials. The calibration curve employed in this work is shown in Figure 2. The temperature calibration curve is representative for very slow changes in temperature because the voltage change rate is quite small (∼0.1 V s−1).

images are included in the insert. The AFM scans are obtained in amplitude modulation AM mode using small amplitude and small set point to preserve the tip apex. The AFM maps on PEO reveal a nanofiber structure with a typical width of 40 nm. Such a nanostructure is confirmed by SEM imaging (results not shown). The AFM maps on cellulose show a typical nanostructure that consists of nano-objects with a diameter in the range of 50 nm. This nanostructure of regenerated cellulose is confirmed by TEM and seems to be related to the networked structure (Figure 5a,b). Figure 3c,d show the typical LTA measurements for PEO and networked cellulose (NC), respectively, obtained with a heating rate of 0.1 V/s (∼4 °C/s). As the temperature increases by increasing the driving voltage, the resistance of the tip increases. Concurrently, because of the high expansion coefficient, the materials expand locally (under the tip apex), inducing a vertical tip displacement of several hundred nanometers. Once the local temperature (at the interface tip−sample) reaches the melting/decomposition point, the Zpiezo retracts due to local softening of the substrate. At the melting point of the PEO (Figure 3a), the resistance of the tip lies between 1250 and 1290 Ω, which corresponds to a temperature of 60−70 °C (by using the calibration: section 2.3). The range of the measured melting temperatures at the

3. RESULTS AND DISCUSSION The thermal properties of bulk materials PEO and cellulose and the nanocomposite material GelPEO are characterized by LTA analysis at the nanoscale, and the results are compared to those obtained macroscopically by DSC. The DSC measurements are preformed using PerkinElmer DSC 4000 at a heating rate of 10 °C/min over the temperature range of 25−445 °C under nitrogen atmosphere. 3.1. Thermal Analysis of Bulk Materials. Figure 3a,b shows the height images obtained, by using AN2-200 tip, on PEO and cellulose, respectively. The corresponding phase 8851

dx.doi.org/10.1021/jp301410e | J. Phys. Chem. C 2012, 116, 8849−8856

The Journal of Physical Chemistry C

Article

Figure 4. (a) AFM height and phase (in insert) maps on composite GelPEO (the scale bar = 250 μm), (b) macroscopic thermal analysis with DSC, and (c) corresponding nanoscopic LTA measurements.

Figure 5. (a) Schematic description of the thermal dissipation in the heated probe-sample contact. (b) Tip-power vs tip-resistance curves on studied polymers used for quantification of the thermal dissipation.

total expansion of GelPEO is about 800 nm, twice the expansion of pure cellulose material. This result may be easily explained by the existence of PEO that increases the effective thermal expansion coefficient of the composite. However, unlike cellulose, two peaks are observed in the Z versus voltage curves: one at low voltage corresponding to probe resistance of 1143−1160 Ω (temperature of 40−45 °C) and a second at intermediate voltage and resistance 1394 Ω (temperature of 120 °C). In some cases, the second peak is not clearly observable. The first peak is observed at a temperature close but lower than the melting point of PEO measured by LTA and DSC in bulk PEO (section 3.1), whereas the second peak is observed at temperature of 120 °C, which could be related to either the existence of glass transition28 or the dehydration of water. However, the glass transition is not observed on pure cellulose, which supports the hypothesis of local dehydration. To investigate these hypotheses, macroscopic DSC measurements are conducted on the composite GelPEO. The DSC thermogram (heat flow vs temperature) on GelPEO reveals the existence of an endothermic peak at 60 °C, corresponding to melting of PEO, and an endothermic peak at about 150−200 °C, corresponding to dehydration. Indeed, the second peak disappears in the cooling and reheating cycles. We concluded that at least qualitatively, the LTA data on the composite sample match well with the DSC results. However, the melting temperature of PEO and the dehydration of bonded water temperature, in GelPEO, are underestimated using LTA probably due to the size effect induces by the nanostructure. This hypothesis will be discussed below.

nanoscale matches well with the macroscopic DSC data on bulk PEO. Indeed, the DSC thermogram (heat flow vs temperature) shows a melting peak of PEO around 55−60 °C (Figure 3e). On the other hand, the decomposition peak of cellulose shows a sharp indentation at higher voltage of 7 V. The probe resistance at the nanodecomposition lies between 2327 and 2344 Ω, which correspond to a substrate temperature of 315− 320 °C (by using a calibration method: section 2.3). These temperatures are found to be within the peak of cellulose decomposition measured macroscopically by DSC (Figure 3e). The second result revealed by nano-LTA is the difference of thermal expansion of the two materials. Indeed, the PEO expand locally by 320 nm difference between room temperature and 60−70 °C. In contrast, the cellulose expands only by 350 nm by increasing the temperature until 320 °C. This result is in good agreement with the ultralow expansion coefficient of cellulose26 (10−7 K−1) in comparison to PEO27 (1.2 × 10−4 K−1). 3.2. Thermal Analysis of Dual-Phases Material GelPEO. A brief description of the method of preparation of GelPEO is described in the Supporting Information. The AFM map (Figure 4a) reveals that the topography of GelPEO is close to the one on cellulose: the same nanosctruture is observed, but the size of nano-objects seems to be greater in the range of 100 nm. This observation of the nanostructure is consistent with the observation obtained by means of TEM at the same magnification (Figure 5a). In the case of the nanocomposite GelPEO, the global behavior on LTA measurement is also similar to the one obtained in bulk cellulose: the Z-position increases with increasing the driving voltage (due to material expansion), and the temperature reaches 320 °C at 7 V (Figure 4c). The 8852

dx.doi.org/10.1021/jp301410e | J. Phys. Chem. C 2012, 116, 8849−8856

The Journal of Physical Chemistry C

Article

Figure 6. (a and b) TEM images of GelPEO showing a typical nanostructure of networked regenerated cellulose with encapsulated PEO nanoparticles at different magnifications. The image size is = 1.6 μm in panel a and 250 nm in panel b. (d) LTA measurement on GelPEO showing the first peak of melting point of PEO and allowing measurement of the vertical size of PEO nanoparticle by measuring the maximum indentation after melting.

measured. Figure 5a shows the equivalent circuit of the heated probe−substrate system. The probe thermal resistance Rp is estimated to be in the range of 106 W/k. According to King,31 the thermal contact impedance is higher by 1 order of magnitude Rint ∼ 107 W/k. In the case of polymers, the substrate thermal resistance Rs is in order of ∼108 W/k. These resistances are sufficiently high to cause significant differences between the heater temperature TH and the interface/substrate temperature Tint, Ts. The calibration method described in the paper gives accrued data if the studied samples as well as the calibration samples have the same thermal dissipation; that is, the difference between TH and Ts is constant for all samples. This difference is controlled by the thermal dissipation between the tip and the sample and depends on (1) the substrate thermal conductivity (or thermal diffusivity), (2) the tip−sample contact area that depends on the substrate young modulus, (3) the applied force, and (4) the adhesion forces. Variation in the substrate thermal conductivity leads to a strong difference between TH and Ts.32,33 To describe to quantify the thermal dissipation, we utilize power vs resistance curves. Indeed, the probe resistance is an indication of the heater temperature, and the power dissipation is an indication of the difference TH − Ts. The calibration process leads to more accurate values if the polymers exhibit the same power vs resistance curves. The tip power Ptip is calculated using the following formula:

We point out the existence of the dehydration peak on GelPEO and its absence in the NC by using both DSC and LTA. This result indicates that the absorption of water or its retention is superior in GelPEO in comparison to cellulose material. This interesting property of GelPEO is related to its unique nanostructure. The importance of LTA to fully describe the nanostructure of GelPEO and its relation to its water retention capability is discussed below. 3.3. Effect of Thermal Dissipation on the Measured Substrate Temperature. As compared with the DSC method, the LTA technique is found to be a fast and nondestructive method to access the melting, transition, and decomposition temperatures of polymers. Moreover, it is sensitive to water content, allowing the local measurement of dehydration temperature of bonded (absorbed or adsorbed) water. Furthermore, by using a calibrated technique, the temperatures measured by LTA at nanoscale match well with those obtained by DSC at a macroscopic scale. This is a surprising result since the LTA was conducted in air, while the DSC was performed under a controlled nitrogen atmosphere. Despite all of the advantages of LTA measurements listed above, the standard method of calibration for the local substrate temperature (section 3.2) needs to be improved mainly to investigate nanostructured and multiphase polymers. Indeed, the melting temperature of PEO in bulk material is overestimated as compared to DSC results, while the same temperature is underestimated in the GelPEO nanocomposite. Moreover, the dehydration temperatures measured in GelPEO by LTA is low as compared to the one obtained by DSC. The variation of the thermal dissipation at the nanocontact tip− sample could be one the reasons behind these differences in the estimated substrate temperature. Thermal contact resistance at the tip−substrate junction and the thermal resistance of the substrate under the tip can vary from one sample to another. This variation in terms of thermal dissipation is a source of error in the estimated substrate temperature. Figure 5a shows the equivalent circuit of the heated probe− substrate system. The probe resistance, used for temperature calibration (Figure 2b), provides a mean for estimating the heater temperature TH. The temperature rise in the heater may be calibrated by various methods, and the relation Rtip vs TH can then be obtained.29,30,25 However, the local thermal properties of the material (melting/transition/decomposition temperatures) are dependent on the interface temperature Tint and/or the local substrate temperature Ts. These two temperatures are of greater interest but cannot be directly

Ptip = VtipI = R tipI 2 ⎛ ⎞2 2Vd ⎟⎟ Ptip = R tip⎜⎜ ⎝ 2R ref + R tip ⎠

(2)

The power−resistance curves are calculated on all standard polymers (Figure 5b). A closer look shows that the thermal dissipation is not the same for the standard calibration samples. This implies that the calibration method for LTA measurement induces error in the estimation of the melting point since even the standard calibration samples exhibit different thermal dissipation, due to differences in sample stiffness, tip−sample contact area, and sample thermal conductivity. The induced error in terms of melting point for calibration samples can reach 25 °C. A thermal dissipation exists between the probe tip and the substrate, necessitating further modeling to characterize the uncertainties pertaining to absolute measurements of the transition and melting temperatures. 3.4. LTA Method for Nanostructure Analysis. In this section, we discuss the use of the LTA as a complementary tool 8853

dx.doi.org/10.1021/jp301410e | J. Phys. Chem. C 2012, 116, 8849−8856

The Journal of Physical Chemistry C

Article

Figure 7. Schematic representation of GelPEO formation and nanostructure.

for nanostructure investigation of an example multiphases polymer nanocomposite. To characterize the nanostructure of GelPEO, the TEM sample was prepared by using a FIB lamella lift out and thinning method (a detailed description of this method is described elsewhere16). First, a layer of chromium is deposited on the surface of the sample to provide an SEM image while performing FIB milling. Then, a protection layer of silicon and another of platinum is deposited to protect the polymer structure during the ion-beam milling. A lamella of 3 × 20 × 3 μm3 is lifted out using a microprobe and then thinned down to a thickness of 150 nm, which is sufficient to give atomic resolution in TEM imaging of polymers. The image shown in Figure 6 is obtained using a low intensity beam to minimize artifacts induced by charging. Figure 6a shows a TEM image of 1.5 × 1.5 μm2, which reveals a similar nanostructure to that of GelPEO, which was obtained in 1 × 1 μm2 AFM imaging in tapping mode (Figure 4a) or even on pure cellulose (Figure 3a). It is evident that the TEM images are more resolved than the AFM ones (due to the limited lateral resolution of AFM as compared to TEM). At higher magnification, the TEM image (Figure 6b) reveals the GelPEO nanostructure, which consists of NC with encapsulated PEO nano-objects. The dark areas represent the bundles of regenerated cellulose, while the brighter areas represent the encapsulated highly amorphous PEO. The lateral size of PEO nano-objects lies between 20 and 60 nm. It is evident that the TEM imaging is an irreplaceable technique to probe the nanostructure of polymer nanocomposites. The TEM images show an enhanced resolution as compared to the AFM images. Moreover, it has higher lateral resolution (fraction of nm) as compared to LTA resolution, which is estimated at 10−20 nm in studies polymers (see the Supporting Information). However, the TEM requires a tedious sample preparation and great experience in both taking and interpreting the images. Moreover, it has also a limitation for depth analysis of the nanostructure. Indeed, we know the lateral size of the PEO nano-objects, but we do not have any idea

about their three-dimensional structure. The vertical size of encapsulated PEO can be obtained by LTA by controlling the tip penetration after melting. Thus, we can deduce the thickness of encapsulated PEO by monitoring the indentation depth after melting. At low indentation (70 nm), the LTA curves are comparable to the ones presented in Figure 4b. In this case, the heated tip first melts the PEO nano-objects and then reaches the underlying cellulose network. The thickness of PEO corresponds to the critical depth at which the tip reaches the cellulose. An example of LTA measurements at the critical indentation is shown in Figure 6c. A statistical analysis confirmed that the thickness of PEO nano-objects is in the order of 60 nm. This result confirms that the PEO nano-object has a spherical shape, and they represent nanosized PEO spherulites that grow from cellulose bundles. This interesting result reveals that the LTA measurements, if combined to controlled indentation technique, can lead to quantifying the size of different phases within polymer nanocomposites. Moreover, coupled to TEM analysis, it allows the drawing of a three-dimensional picture of the surface region of the composite polymers, important for further mechanistic studies. With the LTA technique, one can now explore individual phase thermal behavior at the nanoscale and relate this to the material identity, composition, and distribution at that scale. In the light of these findings, the schematic of the structure of GelPEO formed through aqueous PEO assisted regeneration of acid-soluble cellulose is proposed in Figure 6. The structure consists of a network of cellulose fibers/bundles. The interspace between these fibers is filled with PEO polymer spherulites. The LTA technique confirmed that the dimensions of these components are at the nanoscale level in 3D. At the macroscale, the GelPEO material has interesting hydrophobicity and hygroscopicity properties that can be related to the explored microstructure. The PEO inside the cellulose is stable and is not washed away with water even though PEO is water-soluble. The material has a high affinity to water, and it 8854

dx.doi.org/10.1021/jp301410e | J. Phys. Chem. C 2012, 116, 8849−8856

The Journal of Physical Chemistry C

Article

absorbs water 20 times of its weight.16 Indeed, the peak of dehydration of water observed at 150−180 °C is confirmed in the present study by the nano-LTA technique. Moreover, the LTA technique confirms the previous finding16 that the enhancement in water retention is higher in the case of GelPEO than in cellulose. Indeed, the local dehydration of water observed by LTA in GelPEO is absent in bulk cellulose sample. We believe that the unique nanostructure of this material is behind the enhanced water absorption and retention capability even above the boiling temperature of water. The high molecular weight and hydrophilic PEO, which is encapsulated within the NC, is believed to give the material these enhanced properties. Knowing now the exact nanostructure, the lower melting point (40−45 °C) of PEO in the composite GelPEO could be related to size effects (this assumption should be verified after taking into account the effect of the thermal dissipation). The measured melting temperature by LTA corresponds to a PEO nanoparticle with a typical size of 20−60 nm surrounded by a cellulose network. The result agrees with recent findings on the decrease of the melting point of PEO nanofilm by 7−10 K when the thickness is less than 200 nm (both the melting and the crystallization temperatures were measured by studying the dynamic of a silicon microcantilever coated with PEO nanofilms during the heating cycle).27 Moreover, the size effect on the same PEO nanofilms was not evidenced by using standard DSC measurements. The size effects on the measured transition and melting temperature were also evidenced by using LTA on thin polystyrene films.18,20 The size effect on PEO nano-objects evidenced by LTA is an interesting result but needs further investigation.

surface phenomena, and nanoscale confinement effects over length scales too small to be analyzed by other thermal techniques.

■

ASSOCIATED CONTENT

S Supporting Information *

Further details on the material preparation (section A), the estimation of the lateral resolution of the local thermal analysis technique (section B), and the measurements of bending deflection of the AFM probe due to heating (section C). This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +971 21 109152. Fax: +971 26 988121. E-mail: [email protected] (R.H.). Tel: +971 21 109144. Fax: +971 26 988121. E-mail: [email protected] (M.C.). Author Contributions ∥

These authors have contributed equally.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Maritsa Kissamitaki for the improvement and the design of Graphics and Jason Li and Amir Moshar from Asylum Research for the technical support.

■

REFERENCES

(1) Bailey, N. P.; Christensen, T.; Jakobsen, B.; Niss, K.; Olsen, N. B.; Pedersen, U. R.; Schroder, T. B.; Dyre, J. C. J. Phys.: Condens. Matter 2008, 20, 8. (2) Ao, Z.; Li., S. Nano Res. Lett. 2011, 6, 243. (3) Rong, M. Z.; Zhang, M. Q.; Zheng, Y. X.; Zeng, H. M. Polymer 2001, 42, 167−83. (4) Ficher, H. J. Therm. Anal. Calorim. 2008, 92, 625−630. (5) Schmidt, A. J.; Cheaito, R.; Chiesa, M. Rev. Sci. Instrum. 2009, 80, 094901. (6) Sergei, N. M.; Darrell, H. R. Annu. Rev. Mater. Sci. 1997, 27, 175− 222. (7) Nguyen, T.; Gu, X.; VanLandingham, M.; Giraud, M.; Rosset, R. D.; Ryntz, R.; Nguyen, D. Proceedings of the 24th Annual Meeting of the Adhesion Society 2001, 68−70. (8) Price, D. M.; Reading, M.; Hammiche, A.; Pollock, H. M. Int. J. Pharm. 1999, 192, 85−96. (9) Varesi, J.; Majumdar, A. Appl. Phys. Lett. 1998, 72, 37−39. (10) Hammiche, A.; Price, D. M.; Dupas, E.; Mills, G.; Kulik, A.; Reading, M.; Weaver, J. M. R.; Pollock, H. M. J. Microsc. 2000, 199, 180−90. (11) Rowland, H. D.; King, W. P.; Cross, G. L. W.; Pethica, J. B. ACS Nano 2008, 2, 419−428. (12) Cross, G. L. W.; Connell, B. S. O.; Pethica, J. B.; Rowland, H.; King, W. P. Rev. Sci. Instrum. 2008, 79, 13. (13) Killgore, J. P.; Yablon, D. G.; Tsou, A. H.; Gannepalli, A.; Yuya, P. A.; Turner, J. A.; Proksch, R.; Hurley, D. C. Langmuir 2011, 27, 13983−13987. (14) Nikiforov, M. P.; Gam, S.; Jesse, S.; Composto, R. J.; Kalinin, S. V. Macromolecules 2010, 43, 6724−6730. (15) Nelson, B. A.; King, W. P. Rev. Sci. Instrum. 2010, 78, 023702. (16) Asghar, A.; Abdul Samad, Y.; Hashaikeh, R. J. Appl. Polym. Sci. 2012, DOI: 10.1002/app.36303. (17) Hashaikeh, R.; Abushammala, H. Carbohydr. Polym. 2011, 83, 1088−1094. (18) Li, A. G.; Burggraf, L. W. Rev. Sci. Instrum. 2010, 81, 123707.

4. CONCLUSIONS The LTA technique was found to be a robust nondestructive method for characterizing the thermomechanical properties of polymeric nanocomposites. In this paper, improvements of the method have been achieved to increase the accuracy of sample temperature measurements, namely, glass and melting points. Moreover, when coupled with a constant indentation technique, the LTA method not only allows the assignment of the surface phase but also the quantifying of their size within a multiphase structure. This method, combined with TEM analysis, can lead to a three-dimensional structure of polymer nanocomposites. The validation of the method is demonstrated on a PEO-modified NC nanocomposite named GelPEO. It was found that the method not only allows the measurement of the melting point but also decomposition temperature of cellulose. Also, the dehydration of bonded water is measured, which is of great interest in terms of water absorption and retention capability of a material. The standard calibration method, by using a standard polymer substrate, is discussed and the accuracy of the melting point was found with an error of up to 20 °C. This error is due to the variation of the thermal dissipation from one sample and another, which is related to the tip−sample contact area as well as the thermal conductivity of the substrate. In this paper, we propose a method based on plotting the tip power vs tip resistance to access the thermal dissipation in the tip−sample system. We believe that an appropriate thermal modeling to fit these curves can lead to a better quantification of the local sample temperature. The ability to sample with nanoscale spatial resolution and precision enables the effective use of LTA for characterization of pharmaceuticals, composites, near8855

dx.doi.org/10.1021/jp301410e | J. Phys. Chem. C 2012, 116, 8849−8856

The Journal of Physical Chemistry C

Article

(19) Zhang, J.; Bunker, M.; Chen, X.; Parker, A. P.; Patel, N.; Roberts., C. J. Int. J. Pharm. 2009, 380, 170−173. (20) Maruf, S. H.; Ahn, D. U.; Greenberg, A. R.; Ding, Y. Polymer 2011, 52, 2643−2649. (21) Harding, L.; King, W. P.; Dai, X.; Craig, D. Q. M.; Reading, M. Pharm. Res. 2007, 24, 2048−2054. (22) Jesse, S.; Nikiforov, M. P.; Germinario, L. T.; Kalinin, S. V. Appl. Phys. Lett. 2008, 93, 073104. (23) Greiner, C.; Felts, J. R.; Dai, Z.; King, W. P.; Carpick, R. W. Nano Lett. 2010, 10, 4640−4645. (24) Lee, J.; King, W. P. IEEE Sens. J. 2008, 8, 1805−1806. (25) Lee, J.; Beechem, T.; Wright, T. L.; Nelson, B. A.; Graham, S.; King, W. P. J. Microelectromech. Syst. 2006, 15, 1644−55. (26) Nishino, T.; Matsuda, I.; Hirao, K. Macromolecules 2004, 37, 7683−7687. (27) Zhao, J.; Yin, X.; Shi, J.; Zhao, X.; Gutmann, J. S. J. Phys. Chem. C 2011, 115, 22347−22353. (28) Szczesniak, L.; Rachocki, A.; Tritt-Goc, J. Cellulose 2008, 15, 445−451. (29) Vettiger, P.; Cross, G.; Despont, M.; Drechsler, U.; Durig, U.; Gotsman, B.; Haberle, W.; Lantz, M. A.; Rothuizen, H. E.; Stutz, R.; Binnig, G. K. IEEE Trans. Nanotech. 2002, 1, 39−55. (30) Nelson, B. A.; King, W. P. Sens. Actuators, A 2007, 140, 51−59. (31) Nelson, B. A.; King, W. P. Nanoscale Microscale Thermophys. Eng. 2008, 12, 98−115. (32) Kim, K. J.; King, W. P. Appl. Therm. Eng. 2009, 29, 1631−1641. (33) Haeberle, W.; Pantea, M.; Hoerber, J. K. H. Ultramicroscopy 2006, 106, 678−686.

8856

dx.doi.org/10.1021/jp301410e | J. Phys. Chem. C 2012, 116, 8849−8856