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Thermal conductivity of Nylon 46, Nylon 66 and Nylon 610 characterized by Flash DSC measurement
Journal Pre-proof Thermal Conductivity of Nylon 46, Nylon 66 and Nylon 610 Characterized by Flash DSC Measurement Kefeng Xie, Yucheng He, Jun Cai, Wenbing Hu
PII:
S0040-6031(19)30851-2
DOI:
https://doi.org/10.1016/j.tca.2019.178445
Reference:
TCA 178445
To appear in:
Thermochimica Acta
Received Date:
19 September 2019
Revised Date:
3 November 2019
Accepted Date:
4 November 2019
Please cite this article as: Xie K, He Y, Cai J, Hu W, Thermal Conductivity of Nylon 46, Nylon 66 and Nylon 610 Characterized by Flash DSC Measurement, Thermochimica Acta (2019), doi: https://doi.org/10.1016/j.tca.2019.178445
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Thermal Conductivity of Nylon 46, Nylon 66 and Nylon 610 Characterized by Flash DSC Measurement
Kefeng Xie, Yucheng He, Jun Cai, Wenbing Hu*
Department of Polymer Science and Engineering, State Key Laboratory of Coordinate Chemistry,
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School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China
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*E-mail：[email protected]
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Graphical abstract
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Highlights:
1 The results are consistent with the results of conventional approaches; 2 The general applicability of Flash DSC to the thermal conductivity of bulk materials was verified;
2 Higher hydrogen‐bonding density favors higher thermal conductivity.
Abstract We recently developed Flash DSC application to characterize the cross-plane thermal
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conductivity of polyethylene thin films (Thermochimica Acta 2019, 677, 21-25). We hereby applied this method to measure the thermal conductivities of Nylon 46, Nylon
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66 and Nylon 610 after their complete crystallization at 156.6 °C, i.e. the temperature around the melting points of Indium particles. The melting point differences of two
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Indium particles separately placed on the top of the Nylon samples and on the reference cell exhibit a linear dependence of the heating rates, demonstrating the Fourier’s heat
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conduction law. From the slope, we further derived the thermal conductivity after we measured the specific heat capacity via conventional DSC and measured the sample
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thickness via the laser confocal fluorescence microscope. The obtained thermal conductivities of three Nylon samples increase with their hydrogen-bonding densities, which are consistent with conventional experimental approaches. The observations
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validated the general application of our method for the measurement of thermal conductivity of bulk materials.
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Keywords: Thermal conductivity, Flash DSC, Nylons, Hydrogen-bonding
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1. Introduction Fast-scan chip-calorimetry has been developed in recent years [1]. The chip sensor uses a minute sample mass to effectively reduce the thermal lag [2] and thus allows a rapid heating up to 1×106 K s-1 [3,4], which brings many promised advantages and expands the potential application of DSC measurement [5]. In 2010, Mettler-Toledo company adopted the ceramic-substrate chip sensor MultiSTAR USF1 (XI-400) to make the first
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commercialized power-compensated fast-scanning chip-calorimeter Flash DSC 1 [6,7,8]. The apparatus of Flash DSC can use the mechanical refrigerator for the effective
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range of measurement from -100 to 450 °C, for the maximum heating rate 40,000 K/s and the maximum cooling rate -4,000 K s-1 [9]. Flash DSC has been widely applied to
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characterize various aspects of crystallization in polymer materials [10], such as the low-temperature crystal nucleation [11,12], crystallization kinetics [13,14,15,16,17],
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crystal melting [18,19] and crystal annealing [20,21]. Most recently, the thermal lags between the top and the bottom of the regularly shaped samples were characterized by
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the melting points of two Indium particles separately placed on the sample top and the cell surface without sample [22]. The melting point differences of Indium particles between the sample top and the reference cell were found to exhibit a good linear
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relationship with the heating rates, demonstrating the Fourier’s heat conduction law while the slope k provided the thermal conductivity on the cross-plane direction of polyethylene thin films [23]. He et al. have derived the cross-plane thermal conductivity of thin films [23], as given by
λs=ρs×cs×dc2/k
(1)
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where λs is the cross-plane thermal conductivity, ρs is the sample density, cs is the
specific heat capacity, dc is the film thickness. Compared to other conventional steady-
state approaches [24,25] and even to the transient approaches [26,27,28,29], the advantages of this Flash DSC measurement are obvious: small and directional sample size for free temporal and spatial selection inside solid or viscous bulk samples. In this report, we continue to use this approach to characterize thermal conductivity of series polyamides with various hydrogen-bonding densities crystallized at the temperature 3
around the melting point of Indium particles. Our results will demonstrate and thus validate the general application of this method in the bulk materials. Thermal conductivity of bulk materials mainly relays on two kinds of heat carriers: the electron in the conductive matter, and the phonon in the non-conductive matter. In principle, each phonon can carry more heat than each electron, and the free path of phonon determines the thermal conductivity [30]. Therefore, while diamond holding all carbon-carbon covalent bonds exhibits the highest thermal conductivity around 2000
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W m-1 K-1 [31], polymer materials holding also carbon-carbon covalent bonds along the chain show only around 0.2 W m-1 K-1, ten thousand times less than in the case of
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diamond, due to phonon free path reduced seriously by phonon dispersion and
incomplete lattice [32]. For enhancing the thermal conductivity of polymer materials,
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the free path of phonon can be extended by either the orientational order of polymer chains or the inter-chain interactions. Recently, the stretching of polyethylene thin films
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raises thermal conductivity to 62 W m-1 K-1, higher than in case of many metals, while polyethylene nanofibers hold 104 W m-1 K-1 and its single crystals can theoretically
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reach up to 237 W m-1 K-1 [33]. On the other hand, a network of thermal conductive interchain bonds can likewise enhance the thermal conductivity in amorphous polymer blends [34]. More specifically, strong hydrogen-bonding benefits the thermal
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conductivity of engineering polymers, because it serves as “soft grips” to restrict the torsional motion of polymer chains [35]. In this sense, the enhancement of thermal conductivity can be tuned by changing the density of hydrogen bonding. Hereby, we employed our Flash DSC measurement to characterize the thermal conductivities of
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Nylon 46, Nylon 66 and Nylon 610 after crystallized at 156.6 °C, and to prove that their thermal conductivity enhances with the higher hydrogen-bonding density.
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2. Materials and Methods 2.1 Materials The sample of Nylon 46 was kindly supplied by DSM Company, with the density 1.180 g cm-3 and the melting point 297 °C. The sample of Nylon 66 was purchased from Sigma Aldrich Company, with the density 1.140 g cm-3 and the melting point 264 °C.
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The sample of Nylon 610 was kindly supplied by Shandong Guangyin New Materials Company, with the density 1.080 g cm-3 and the melting point 229 °C. The Indium
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particles were supplied by Mettler-Toledo Company as the standard for the temperature
2.2 Flash DSC Measurement
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calibration of DSC measurement.
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Flash DSC 1 (Mettler-Toledo, Switzerland) equipped with optical microscope was used with Huber TC100 intracooler and the inert atmosphere of Nitrogen gas at a purging
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rate of 50 mL min-1. The chip-sensor was pre-conditioned according to the instruction. Under an optical microscope with a certain magnification, the small samples were cut into thin film shapes of tiny pieces with almost uniform thickness by an anatomical
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knife. Then, the prepared samples were transferred to the sample cell of the sensor, which has been covered by a very thin layer of silicone oil for a good thermal contact and a protection of the sensor upon the removal of the sample. We neglected the additional thermal lag caused by this very thin layer of silicone oil in the thermal
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conductivity measurement. Finally, Flash DSC 1 executed the programmed heating processes for the observation of the melting temperatures of Indium particles placed separately on the top of the samples and on the reference cell, as shown in Fig. 1. Since the sizes of Indium particles were relatively small and their thermal conductivities were relatively high, we neglected the thermal lag of Indium particles in the thermal conductivity measurement.
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Indium particle 1 Film sample
Indium particle 2
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Chip-sensor with twin cells
Sample cell
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Reference cell
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Figure 1. Schematic illustration and photographs of two indium particles separately placed on the top surface of the regular-shaped thin film sample in the sample cell and on the surface of the
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reference cell.
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2.3 Conventional DSC Measurement Conventional DSC experiments were carried out by using DSC 1/700 (Mettler-Toledo AG, Switzerland) with FT100-MT internal cooler (Julabo AG, Germany) and purging inert Nitrogen gas (50 mL min-1). The temperature was calibrated by Indium standard.
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The specific heat capacities of samples were measured by the standard three-segment method referring to the standard Sapphire with known specific heat capacity [36] at the temperature around the melting point of Indium.
2.4 Laser Confocal Fluorescence Microscope Measurement Laser confocal fluorescence microscope LSM 710 (Zeiss, Germany) was used in the 6
experiment, with 20-times objective lens and 10-nm Z-step motor drive for measurement. The thickness of Nylon samples was calculated by the vertical height difference of focusing between the chip-sensor platform and the fluorescent Rhodamine B casted via ethanol solvent on the top of the sample, respectively, as illustrated in Fig. 2. The emission wavelength of fluorescence observation was 543 nm. 10-nm steps in
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the adjustment of focus were accurate enough in comparison to the sample thickness.
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Figure 2. Images of focusing on the chip-sensor platform (left) under laser confocal
fluorescence microscope and on Rhodamine B (right, red bright region in the center) spread on
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the top of the thin film sample. The emission wavelength of fluorescence observation was 543 nm.
3. Results and discussion
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3.1 Preparation samples with identical thermal histories We used Flash DSC 1 to compare the crystallization kinetics of three Nylon samples at 156.6 °C. The temperature 156.6 °C was chosen because it was the onset melting temperature of Indium particles observed in the reference cell, indicating the beginning of thermal conduction that we designed to study upon heating scanning. The temperature was used to calibrate the instrument temperature according to the standard protocol for DSC measurement. We first heated Nylon 46, Nylon 66 and Nylon 610 samples to 360 °C with the 7
heating rate 3000 K s-1, and then held for 2 seconds to erase their thermal histories and to ensure a perfect thermal contact of the melted samples on the sample cell. After that, the samples were cooled at – 3000 K s-1 to 156.6 °C and were stayed for various periods before cooling (-4000 K s-1) to -50 °C and heating (4000 K s-1) back in order to observe the saturation of melting peaks that reflects the completion of isothermal crystallization. The temperature protocol and the melting curves of three samples are shown in Fig. 3. One can see that Nylon 46 crystallizes with the saturation time around 0.3 s when no
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more cold crystallization occurs, Nylon 66 needs about 1.5 s to complete crystallization, and Nylon 610 with the saturation time around 10 s. We made the total period of 1000
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s to ensure the complete isothermal crystallization in each sample used for the subsequent measurement of thermal conductivity.
Heating rate 4000 K/s
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Cooling rate -4000 K/s -50 °C
Time
-8
-1
5×10 JK
Nylon 46
Crystallization time 10 s 7s 4s 2s 1s 0.3 s 0.1 s 0.04 s 0.02 s 0.014 s 0.012 s 0.007 s
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Cooling rate Heating rate -3000 K/s 3000 K/s 156.6 °C, 0-1000 s
Endo up
-1
360 °C
Temperature
Apparent heat capacity / J K
0.2 s
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b.
a.
0.004 s 0s
Heating at 4000 K/s
0
50
100
150
200
250
300
Temperature / °C
-1
Crystallization time
Nylon 66
-1
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Endo up
45 s
Apparent heat capacity / J K
2×10 JK
30 s 15 s
5s 1.5 s 0.7 s 0.5 s 0.3 s 0.2 s 0.1 s 0.04 s 0.01 s 0s
Heating at 4000 K/s
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Apparent heat capacity / J K
-1
-7
0
50
100
150
200
250
Temperature / °C
300
Endo up
d.
c.
-7
2×10 JK
Nylon 610
Crystallization time 90 s 60 s 30 s 10 s 5s 2s 1.7 s 1.4 s 1s 0.8 s 0.5 s 0.2 s 0.1 s 0s
Heating at 4000 K/s
0
-1
50
100
150
200
250
Temperature / °C
Figure 3. (a) Illustration of temperature program for isothermal crystallization and subsequent melting of three samples. (b-d) Heating curves of three samples after isothermal crystallization with various periods at 156.6 °C as labeled.
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3.2 Measurement of thermal lags After three Nylon samples were prepared with identical crystallization histories, we placed two Indium particles of similar masses separately on the top of the samples in the sample cell and on the center surface of the reference cell in the chip-sensor, as shown in Fig. 1. The melting points of the two Indium particles were then measured with the temperature scanning programs shown in Fig. 4a. The differences of melting
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points of two Indium particles were compared at different heating rates from 10 to 120 K s-1.
Heating rate 3000 K/s
Heating rate 10, 20, 30, …, 120 K/s
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-100 °C
-1
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Cooling rate -3000 K/s
-6
2×10 J K
Nylon 46
Time
-1
Heating rate 10 K s 20
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30
160
50 60 70 80 90 100 120
In. surf.
In. ref.
150
40
170
180
Temperature / °C
d.
Endo up
Nylon 66
-1
-1
Apparent Heat Capacity / J K
-6
2×10 JK
160
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Endo up
Apparent heat capacity / J K
-1
c.
40 50 60 70 80 90 100 120
170
180
190
Temperature / °C
-6
5×10 J K
-1
Nylon 610 -1
Heating rate 10 K s
30 40 50 60 70 80 90 100 120 In. surf.
In. ref.
30
20
150
190
20
In. surf.
In. ref.
150
-1
Heating rate 10 K s
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2s
200 °C
Temperature
Apparent heat capacity / J K
-1
a.
Endo up
b.
160
170
180
190
Temperature / °C
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Figure 4. (a) Illustration of the temperature-scanning program for the thermal lag measurements. (b-d) Heating curves of apparent heat capacity of Nylon 46, Nylon 66 and Nylon 610 samples holding two Indium particles separately placed on the sample top and on the reference cell under various heating rates as labeled. The peaks are the melting of two Indium particles.
The heating curves of two Indium particles at various heating rates are shown in Fig. 4b-4d. One can see that when heated at a rate of 10 K s-1, all the Indium particles 9
in the reference sides begin to melt at 156.6 °C, which are not much sensitive to the heating rates. The other Indium particles placed on the sample top begin to melt around 164 °C, which exhibit the melting peaks shifting and lowering with the increase of heating rates. The melting point was read at the onset temperature of melting peaks. The onset temperature was defined as the intersection between the baseline and the extrapolation from the highest tangent of the low-temperature flank of the melting peak. Figure 5 summarizes the onset temperature differences between
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two melting peaks of Indium particles at various heating rates. One can see that there is a good linear relationship between the melting point differences and the heating
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rates, for the Indium particles on all three samples. The linear relationship reflects that the cross-plane heat flux in the film sample conforms to the Fourier’s heat conduction
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law. The thermal conductivity can thus be derived from the slope, with the specific
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heat capacity and the sample film thickness according to Eqn. (1).
Nylon 66
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20
Fitting slope: 7.75×10-2 s
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Nylon 610 Nylon 46
4.95×10-2 s
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Melting point difference /K
24
5.22×10-2 s
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8
20
40
60
80
100
120
140
-1
Heating rate /K s
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0
Figure 5. Melting point differences of two Indium particles at various heating rates.
Data were averaged over three independent measurements, with error bars. The melting points were read at the onset temperatures of melting peaks.
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3.3 Calculation of thermal conductivity In order to eliminate the systematic errors caused by the instrument itself and the measuring environment, we used the indirect three-segment method with standard sapphire as reference to measure the specific heat capacity of Nylon samples. The heat flux values of Nylon samples, standard sapphire and blank crucible were measured by conventional DSC with the same aluminum crucibles at the heating rate
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of 20 K min-1, and the temperature range of 100-200 °C. It was necessary to set the isothermal program for 5 min at the beginning of temperature range 100 °C and the
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end 200 °C, respectively, so as to obtain the stable heat flux signals. Thus one could
obtain the stable and repeatable baselines by multiple scanning with blank crucible to
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determine the contribution of heat capacity of crucible material. Since the heat flux is the product of sample mass, heating rate and specific heat capacity, one could obtain
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the specific heat capacity of Nylon samples by the calibration of standard sapphire after subtracted from the baselines of the blank crucible at the temperature of
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156.6 °C where the Indium particles on the reference pan began to melt. The calculation results of specific heat capacity of Nylon 46, Nylon 66 and Nylon 610 after three time repeating are separately 1800.1±1.4, 1863.2±0.6, 2571.0±3.6 J Kg-1
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K-1.
The film thickness were measured by using laser confocal fluorescence microscope allowing separate focusing on the cell surface and the sample top as monitored by the fluorescent Rhodamine B, as shown in Fig. 2. The measurement results for the film thickness of Nylon 46, Nylon 66 and Nylon 610 after the above three-time repeating
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are (849±1.05)10-7, (998±2.14)10-7 and (642±1.60)10-7 m, respectively. According to Eqn. (1), the Fourier’s heat conduction law gave the thermal
conductivities of Nylon 46, Nylon 66 and Nylon 610 as summarized in Fig. 6. We compared the measurement results to the literature reports on Nylon 46 [37], Nylon 66 [38] and Nylon 610 [39] at the similar temperatures, and found a satisfying consistence. In addition, the observations confirmed that a higher hydrogen-bonding density results in a higher thermal conductivity. 11
Flash DSC measurement Literature reports 0.28
Nylon 46
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Nylon 66
0.24
Nylon 610 0.20 100
110
120
130
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-1 -1
Thermal conductivity /Wm K
0.32
140
150
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Amide units per 1000 backbone atoms
Figure 6. Thermal conductivities versus amide unit density on the backbone chains of three Nylon
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samples in Flash DSC measurements. The results are averaged over three repeating experiments.
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The literature results measured by other methods are also shown for comparison.
4. Conclusion
We employed Flash DSC measurement to characterize the thermal conductivity of three samples, i.e., Nylon 46, Nylon 66 and Nylon 610. The linear relationship between the
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heating rates and the melting point differences between two Indium particles at the top of Nylon samples and on the blank reference cell follows well Fourier's heat conduction law. The slope thus characterizes the thermal conductivity once one obtained the specific heat capacity via the three-segment method of conventional DSC measurement as well as the sample thickness via the laser confocal fluorescence microscope. The obtained thermal conductivities demonstrated a higher thermal conductivity due to a higher hydrogen-bonding density. Their quantities are consistent with the conventional 12
approaches of thermal conductivity on the same kinds of samples, which validates the general application of Flash DSC method for the thermal conductivity of bulk solid materials.
No conflict interests The authors claimed no conflict interests in this paper!
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Acknowledgements
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This work was supported by the National Natural Science Foundation of China (Grant
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(IRT1252) and the CAS Interdisciplinary Team.
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No. 21973042), the Program for Changjiang Scholars and Innovative Research Teams
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PII:
S0040-6031(19)30851-2
DOI:
https://doi.org/10.1016/j.tca.2019.178445
Reference:
TCA 178445
To appear in:
Thermochimica Acta
Received Date:
19 September 2019
Revised Date:
3 November 2019
Accepted Date:
4 November 2019
Please cite this article as: Xie K, He Y, Cai J, Hu W, Thermal Conductivity of Nylon 46, Nylon 66 and Nylon 610 Characterized by Flash DSC Measurement, Thermochimica Acta (2019), doi: https://doi.org/10.1016/j.tca.2019.178445
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Thermal Conductivity of Nylon 46, Nylon 66 and Nylon 610 Characterized by Flash DSC Measurement
Kefeng Xie, Yucheng He, Jun Cai, Wenbing Hu*
Department of Polymer Science and Engineering, State Key Laboratory of Coordinate Chemistry,
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School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China
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*E-mail：[email protected]
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Graphical abstract
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Highlights:
1 The results are consistent with the results of conventional approaches; 2 The general applicability of Flash DSC to the thermal conductivity of bulk materials was verified;
2 Higher hydrogen‐bonding density favors higher thermal conductivity.
Abstract We recently developed Flash DSC application to characterize the cross-plane thermal
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conductivity of polyethylene thin films (Thermochimica Acta 2019, 677, 21-25). We hereby applied this method to measure the thermal conductivities of Nylon 46, Nylon
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66 and Nylon 610 after their complete crystallization at 156.6 °C, i.e. the temperature around the melting points of Indium particles. The melting point differences of two
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Indium particles separately placed on the top of the Nylon samples and on the reference cell exhibit a linear dependence of the heating rates, demonstrating the Fourier’s heat
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conduction law. From the slope, we further derived the thermal conductivity after we measured the specific heat capacity via conventional DSC and measured the sample
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thickness via the laser confocal fluorescence microscope. The obtained thermal conductivities of three Nylon samples increase with their hydrogen-bonding densities, which are consistent with conventional experimental approaches. The observations
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validated the general application of our method for the measurement of thermal conductivity of bulk materials.
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Keywords: Thermal conductivity, Flash DSC, Nylons, Hydrogen-bonding
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1. Introduction Fast-scan chip-calorimetry has been developed in recent years [1]. The chip sensor uses a minute sample mass to effectively reduce the thermal lag [2] and thus allows a rapid heating up to 1×106 K s-1 [3,4], which brings many promised advantages and expands the potential application of DSC measurement [5]. In 2010, Mettler-Toledo company adopted the ceramic-substrate chip sensor MultiSTAR USF1 (XI-400) to make the first
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commercialized power-compensated fast-scanning chip-calorimeter Flash DSC 1 [6,7,8]. The apparatus of Flash DSC can use the mechanical refrigerator for the effective
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range of measurement from -100 to 450 °C, for the maximum heating rate 40,000 K/s and the maximum cooling rate -4,000 K s-1 [9]. Flash DSC has been widely applied to
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characterize various aspects of crystallization in polymer materials [10], such as the low-temperature crystal nucleation [11,12], crystallization kinetics [13,14,15,16,17],
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crystal melting [18,19] and crystal annealing [20,21]. Most recently, the thermal lags between the top and the bottom of the regularly shaped samples were characterized by
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the melting points of two Indium particles separately placed on the sample top and the cell surface without sample [22]. The melting point differences of Indium particles between the sample top and the reference cell were found to exhibit a good linear
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relationship with the heating rates, demonstrating the Fourier’s heat conduction law while the slope k provided the thermal conductivity on the cross-plane direction of polyethylene thin films [23]. He et al. have derived the cross-plane thermal conductivity of thin films [23], as given by
λs=ρs×cs×dc2/k
(1)
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where λs is the cross-plane thermal conductivity, ρs is the sample density, cs is the
specific heat capacity, dc is the film thickness. Compared to other conventional steady-
state approaches [24,25] and even to the transient approaches [26,27,28,29], the advantages of this Flash DSC measurement are obvious: small and directional sample size for free temporal and spatial selection inside solid or viscous bulk samples. In this report, we continue to use this approach to characterize thermal conductivity of series polyamides with various hydrogen-bonding densities crystallized at the temperature 3
around the melting point of Indium particles. Our results will demonstrate and thus validate the general application of this method in the bulk materials. Thermal conductivity of bulk materials mainly relays on two kinds of heat carriers: the electron in the conductive matter, and the phonon in the non-conductive matter. In principle, each phonon can carry more heat than each electron, and the free path of phonon determines the thermal conductivity [30]. Therefore, while diamond holding all carbon-carbon covalent bonds exhibits the highest thermal conductivity around 2000
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W m-1 K-1 [31], polymer materials holding also carbon-carbon covalent bonds along the chain show only around 0.2 W m-1 K-1, ten thousand times less than in the case of
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diamond, due to phonon free path reduced seriously by phonon dispersion and
incomplete lattice [32]. For enhancing the thermal conductivity of polymer materials,
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the free path of phonon can be extended by either the orientational order of polymer chains or the inter-chain interactions. Recently, the stretching of polyethylene thin films
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raises thermal conductivity to 62 W m-1 K-1, higher than in case of many metals, while polyethylene nanofibers hold 104 W m-1 K-1 and its single crystals can theoretically
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reach up to 237 W m-1 K-1 [33]. On the other hand, a network of thermal conductive interchain bonds can likewise enhance the thermal conductivity in amorphous polymer blends [34]. More specifically, strong hydrogen-bonding benefits the thermal
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conductivity of engineering polymers, because it serves as “soft grips” to restrict the torsional motion of polymer chains [35]. In this sense, the enhancement of thermal conductivity can be tuned by changing the density of hydrogen bonding. Hereby, we employed our Flash DSC measurement to characterize the thermal conductivities of
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Nylon 46, Nylon 66 and Nylon 610 after crystallized at 156.6 °C, and to prove that their thermal conductivity enhances with the higher hydrogen-bonding density.
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2. Materials and Methods 2.1 Materials The sample of Nylon 46 was kindly supplied by DSM Company, with the density 1.180 g cm-3 and the melting point 297 °C. The sample of Nylon 66 was purchased from Sigma Aldrich Company, with the density 1.140 g cm-3 and the melting point 264 °C.
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The sample of Nylon 610 was kindly supplied by Shandong Guangyin New Materials Company, with the density 1.080 g cm-3 and the melting point 229 °C. The Indium
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particles were supplied by Mettler-Toledo Company as the standard for the temperature
2.2 Flash DSC Measurement
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calibration of DSC measurement.
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Flash DSC 1 (Mettler-Toledo, Switzerland) equipped with optical microscope was used with Huber TC100 intracooler and the inert atmosphere of Nitrogen gas at a purging
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rate of 50 mL min-1. The chip-sensor was pre-conditioned according to the instruction. Under an optical microscope with a certain magnification, the small samples were cut into thin film shapes of tiny pieces with almost uniform thickness by an anatomical
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knife. Then, the prepared samples were transferred to the sample cell of the sensor, which has been covered by a very thin layer of silicone oil for a good thermal contact and a protection of the sensor upon the removal of the sample. We neglected the additional thermal lag caused by this very thin layer of silicone oil in the thermal
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conductivity measurement. Finally, Flash DSC 1 executed the programmed heating processes for the observation of the melting temperatures of Indium particles placed separately on the top of the samples and on the reference cell, as shown in Fig. 1. Since the sizes of Indium particles were relatively small and their thermal conductivities were relatively high, we neglected the thermal lag of Indium particles in the thermal conductivity measurement.
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Indium particle 1 Film sample
Indium particle 2
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Chip-sensor with twin cells
Sample cell
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Reference cell
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Figure 1. Schematic illustration and photographs of two indium particles separately placed on the top surface of the regular-shaped thin film sample in the sample cell and on the surface of the
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reference cell.
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2.3 Conventional DSC Measurement Conventional DSC experiments were carried out by using DSC 1/700 (Mettler-Toledo AG, Switzerland) with FT100-MT internal cooler (Julabo AG, Germany) and purging inert Nitrogen gas (50 mL min-1). The temperature was calibrated by Indium standard.
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The specific heat capacities of samples were measured by the standard three-segment method referring to the standard Sapphire with known specific heat capacity [36] at the temperature around the melting point of Indium.
2.4 Laser Confocal Fluorescence Microscope Measurement Laser confocal fluorescence microscope LSM 710 (Zeiss, Germany) was used in the 6
experiment, with 20-times objective lens and 10-nm Z-step motor drive for measurement. The thickness of Nylon samples was calculated by the vertical height difference of focusing between the chip-sensor platform and the fluorescent Rhodamine B casted via ethanol solvent on the top of the sample, respectively, as illustrated in Fig. 2. The emission wavelength of fluorescence observation was 543 nm. 10-nm steps in
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the adjustment of focus were accurate enough in comparison to the sample thickness.
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Figure 2. Images of focusing on the chip-sensor platform (left) under laser confocal
fluorescence microscope and on Rhodamine B (right, red bright region in the center) spread on
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the top of the thin film sample. The emission wavelength of fluorescence observation was 543 nm.
3. Results and discussion
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3.1 Preparation samples with identical thermal histories We used Flash DSC 1 to compare the crystallization kinetics of three Nylon samples at 156.6 °C. The temperature 156.6 °C was chosen because it was the onset melting temperature of Indium particles observed in the reference cell, indicating the beginning of thermal conduction that we designed to study upon heating scanning. The temperature was used to calibrate the instrument temperature according to the standard protocol for DSC measurement. We first heated Nylon 46, Nylon 66 and Nylon 610 samples to 360 °C with the 7
heating rate 3000 K s-1, and then held for 2 seconds to erase their thermal histories and to ensure a perfect thermal contact of the melted samples on the sample cell. After that, the samples were cooled at – 3000 K s-1 to 156.6 °C and were stayed for various periods before cooling (-4000 K s-1) to -50 °C and heating (4000 K s-1) back in order to observe the saturation of melting peaks that reflects the completion of isothermal crystallization. The temperature protocol and the melting curves of three samples are shown in Fig. 3. One can see that Nylon 46 crystallizes with the saturation time around 0.3 s when no
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more cold crystallization occurs, Nylon 66 needs about 1.5 s to complete crystallization, and Nylon 610 with the saturation time around 10 s. We made the total period of 1000
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s to ensure the complete isothermal crystallization in each sample used for the subsequent measurement of thermal conductivity.
Heating rate 4000 K/s
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Cooling rate -4000 K/s -50 °C
Time
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-1
5×10 JK
Nylon 46
Crystallization time 10 s 7s 4s 2s 1s 0.3 s 0.1 s 0.04 s 0.02 s 0.014 s 0.012 s 0.007 s
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Cooling rate Heating rate -3000 K/s 3000 K/s 156.6 °C, 0-1000 s
Endo up
-1
360 °C
Temperature
Apparent heat capacity / J K
0.2 s
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b.
a.
0.004 s 0s
Heating at 4000 K/s
0
50
100
150
200
250
300
Temperature / °C
-1
Crystallization time
Nylon 66
-1
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Endo up
45 s
Apparent heat capacity / J K
2×10 JK
30 s 15 s
5s 1.5 s 0.7 s 0.5 s 0.3 s 0.2 s 0.1 s 0.04 s 0.01 s 0s
Heating at 4000 K/s
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Apparent heat capacity / J K
-1
-7
0
50
100
150
200
250
Temperature / °C
300
Endo up
d.
c.
-7
2×10 JK
Nylon 610
Crystallization time 90 s 60 s 30 s 10 s 5s 2s 1.7 s 1.4 s 1s 0.8 s 0.5 s 0.2 s 0.1 s 0s
Heating at 4000 K/s
0
-1
50
100
150
200
250
Temperature / °C
Figure 3. (a) Illustration of temperature program for isothermal crystallization and subsequent melting of three samples. (b-d) Heating curves of three samples after isothermal crystallization with various periods at 156.6 °C as labeled.
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3.2 Measurement of thermal lags After three Nylon samples were prepared with identical crystallization histories, we placed two Indium particles of similar masses separately on the top of the samples in the sample cell and on the center surface of the reference cell in the chip-sensor, as shown in Fig. 1. The melting points of the two Indium particles were then measured with the temperature scanning programs shown in Fig. 4a. The differences of melting
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points of two Indium particles were compared at different heating rates from 10 to 120 K s-1.
Heating rate 3000 K/s
Heating rate 10, 20, 30, …, 120 K/s
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-100 °C
-1
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Cooling rate -3000 K/s
-6
2×10 J K
Nylon 46
Time
-1
Heating rate 10 K s 20
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30
160
50 60 70 80 90 100 120
In. surf.
In. ref.
150
40
170
180
Temperature / °C
d.
Endo up
Nylon 66
-1
-1
Apparent Heat Capacity / J K
-6
2×10 JK
160
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Endo up
Apparent heat capacity / J K
-1
c.
40 50 60 70 80 90 100 120
170
180
190
Temperature / °C
-6
5×10 J K
-1
Nylon 610 -1
Heating rate 10 K s
30 40 50 60 70 80 90 100 120 In. surf.
In. ref.
30
20
150
190
20
In. surf.
In. ref.
150
-1
Heating rate 10 K s
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2s
200 °C
Temperature
Apparent heat capacity / J K
-1
a.
Endo up
b.
160
170
180
190
Temperature / °C
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Figure 4. (a) Illustration of the temperature-scanning program for the thermal lag measurements. (b-d) Heating curves of apparent heat capacity of Nylon 46, Nylon 66 and Nylon 610 samples holding two Indium particles separately placed on the sample top and on the reference cell under various heating rates as labeled. The peaks are the melting of two Indium particles.
The heating curves of two Indium particles at various heating rates are shown in Fig. 4b-4d. One can see that when heated at a rate of 10 K s-1, all the Indium particles 9
in the reference sides begin to melt at 156.6 °C, which are not much sensitive to the heating rates. The other Indium particles placed on the sample top begin to melt around 164 °C, which exhibit the melting peaks shifting and lowering with the increase of heating rates. The melting point was read at the onset temperature of melting peaks. The onset temperature was defined as the intersection between the baseline and the extrapolation from the highest tangent of the low-temperature flank of the melting peak. Figure 5 summarizes the onset temperature differences between
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two melting peaks of Indium particles at various heating rates. One can see that there is a good linear relationship between the melting point differences and the heating
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rates, for the Indium particles on all three samples. The linear relationship reflects that the cross-plane heat flux in the film sample conforms to the Fourier’s heat conduction
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law. The thermal conductivity can thus be derived from the slope, with the specific
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heat capacity and the sample film thickness according to Eqn. (1).
Nylon 66
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20
Fitting slope: 7.75×10-2 s
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Nylon 610 Nylon 46
4.95×10-2 s
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Melting point difference /K
24
5.22×10-2 s
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8
20
40
60
80
100
120
140
-1
Heating rate /K s
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0
Figure 5. Melting point differences of two Indium particles at various heating rates.
Data were averaged over three independent measurements, with error bars. The melting points were read at the onset temperatures of melting peaks.
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3.3 Calculation of thermal conductivity In order to eliminate the systematic errors caused by the instrument itself and the measuring environment, we used the indirect three-segment method with standard sapphire as reference to measure the specific heat capacity of Nylon samples. The heat flux values of Nylon samples, standard sapphire and blank crucible were measured by conventional DSC with the same aluminum crucibles at the heating rate
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of 20 K min-1, and the temperature range of 100-200 °C. It was necessary to set the isothermal program for 5 min at the beginning of temperature range 100 °C and the
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end 200 °C, respectively, so as to obtain the stable heat flux signals. Thus one could
obtain the stable and repeatable baselines by multiple scanning with blank crucible to
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determine the contribution of heat capacity of crucible material. Since the heat flux is the product of sample mass, heating rate and specific heat capacity, one could obtain
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the specific heat capacity of Nylon samples by the calibration of standard sapphire after subtracted from the baselines of the blank crucible at the temperature of
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156.6 °C where the Indium particles on the reference pan began to melt. The calculation results of specific heat capacity of Nylon 46, Nylon 66 and Nylon 610 after three time repeating are separately 1800.1±1.4, 1863.2±0.6, 2571.0±3.6 J Kg-1
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K-1.
The film thickness were measured by using laser confocal fluorescence microscope allowing separate focusing on the cell surface and the sample top as monitored by the fluorescent Rhodamine B, as shown in Fig. 2. The measurement results for the film thickness of Nylon 46, Nylon 66 and Nylon 610 after the above three-time repeating
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are (849±1.05)10-7, (998±2.14)10-7 and (642±1.60)10-7 m, respectively. According to Eqn. (1), the Fourier’s heat conduction law gave the thermal
conductivities of Nylon 46, Nylon 66 and Nylon 610 as summarized in Fig. 6. We compared the measurement results to the literature reports on Nylon 46 [37], Nylon 66 [38] and Nylon 610 [39] at the similar temperatures, and found a satisfying consistence. In addition, the observations confirmed that a higher hydrogen-bonding density results in a higher thermal conductivity. 11
Flash DSC measurement Literature reports 0.28
Nylon 46
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Nylon 66
0.24
Nylon 610 0.20 100
110
120
130
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-1 -1
Thermal conductivity /Wm K
0.32
140
150
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Amide units per 1000 backbone atoms
Figure 6. Thermal conductivities versus amide unit density on the backbone chains of three Nylon
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samples in Flash DSC measurements. The results are averaged over three repeating experiments.
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The literature results measured by other methods are also shown for comparison.
4. Conclusion
We employed Flash DSC measurement to characterize the thermal conductivity of three samples, i.e., Nylon 46, Nylon 66 and Nylon 610. The linear relationship between the
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heating rates and the melting point differences between two Indium particles at the top of Nylon samples and on the blank reference cell follows well Fourier's heat conduction law. The slope thus characterizes the thermal conductivity once one obtained the specific heat capacity via the three-segment method of conventional DSC measurement as well as the sample thickness via the laser confocal fluorescence microscope. The obtained thermal conductivities demonstrated a higher thermal conductivity due to a higher hydrogen-bonding density. Their quantities are consistent with the conventional 12
approaches of thermal conductivity on the same kinds of samples, which validates the general application of Flash DSC method for the thermal conductivity of bulk solid materials.
No conflict interests The authors claimed no conflict interests in this paper!
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Acknowledgements
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This work was supported by the National Natural Science Foundation of China (Grant
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(IRT1252) and the CAS Interdisciplinary Team.
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No. 21973042), the Program for Changjiang Scholars and Innovative Research Teams
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