Dielectric Properties for the Ring Opening Polymerisation of ε- Caprolactone

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vol. 493, pp doi: / Trans Tech Publications, Switzerland Dielectric Properties for the Ring Opening Polymerisation of ε- Caprolactone Mohd Johari Kamaruddin 1,a,Muhammad Abbas Ahmad Zaini 1,b, Anwar Johari 2,c and Tuan Amran Tuan Abdullah 2,d 1 Centre of Lipids Engineering & Applied Research (CLEAR), Faculty of Chemical Engineering, Universiti Teknologi Malaysia, UTM Johor Bahru, Johor, Malaysia. 2 Institute of Hydrogen Economy (IHE), Faculty of Chemical Engineering, Universiti Teknologi Malaysia, Skudai, Johor Bahru, Malaysia. a mjohari@cheme.utm.my, b abbas@cheme.utm.my, c anwar@cheme.utm.my, d tamran@cheme.utm.my Keywords: Dielectric properties, relaxation time, penetration depth, microwave assisted polymerisation and polymerisation of biodegradable polymer. Abstract A dielectric property study was performed across a wide range of frequencies and temperatures on ring opening polymerisation of ε-caprolactone system in order to relate quantitatively their dielectric properties to microwave heating mechanisms. An analysis of the results concluded that heating mechanism of the polymerisation mixtures in a microwave field was controlled by the dielectric properties of monomer, where the monomer was the major component (>90 % volume/volume) as well as the component with highest dielectric loss and dissipation factor. The penetration depth of mixtures at 2.45 GHz was noted to increase from ~0.58 cm (at 20 C) to ~3.3 cm (at 150 C). This small penetration depth limits the potential to achieve the successful scale up of a microwaveassisted polymerisation of ε-caprolactone in batch mode at 2.45 GHz. As a result, this will lead to inhomogeneous bulk temperature distribution within the polymerisation mixture and irreproducible chemistry. However, a fast heating rate based on a high value of dissipation factor and dielectric loss of the polymerisation mixtures shows potential to enable the reaction to be completed in a few seconds that may allow the polymerisation to be transferred to a continuous flow process. In so doing, small diameter tubular reactors can be employed hence removing this penetration depth issue. Thus, the polymerisation mixtures dielectric properties are worth to be considered to ensure the reliability and reproducibility of the microwave assisted synthesis of poly-ε-caprolactone at large scale production. 1.0 Introduction To date there are few reports of this type of dielectric property study in the field of microwave assisted polymerisation (MAP), especially in biodegradable ring opening polymerisation (ROP). There are small number of published papers [1-4] that included a description and discussion of the measurement and dielectric property data of MAP processes. The majority of the prior literatures on dielectric property studies have only focused on the monomer itself without considering other precursors such as catalysts, initiators and the concentration effects that resulted from the construction of the actual reactants mixture. For example in the ROP of ε- caprolactone (ε- cap), only the dielectric properties of ε-cap are available in literature [1-2] and that data is presented for a very limited temperature range (room temperature to 110 C). However, the typical ROP reaction temperature is 130 C and above, hence this data is of little real use. This is because the dielectric property data for materials have been shown significantly change and nonlinearly with temperature. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-04/03/16,18:38:40)

2 622 Advances in Applied Mechanics and Materials Only recently have a few researchers started to look in depth into how dielectric properties change with temperature and frequency and relate this to the empirical observation made upon microwave heated polymerisation progresses. Nakamura, Naghata et al. [5] studied the dielectric properties of a poly-condensation polymerisation of lactic acid. In this study, dielectric properties of lactic acid aqueous solution (reaction substrate), anhydrated lactic acid (intermediate product), oligo (lactic acid) (product) and water (by-product) were measured at 2.45 GHz as a function of temperature. However, this study was limited to the study of the individual components and did not consider how dielectric properties of the reaction mixture behaved during microwave heating. Consequently, the aim of this study was to quantify both the dielectric properties of ε- cap and the reaction mixtures within the microwave frequency range and temperature up to conventional reaction temperatures (150 C) by using coaxial probe techniques. 2.0 Experimental Materials The precursors, ε-cap (99 %), stannous octanoate, Sn(Oct)2) (96 %) and benzyl alcohol, BzOH (99 %) were purchased from Acros, Advocado and Sigma-Aldrich respectively. ROP of ε- cap was conducted in bulk with all reactants used without any further purification. The standard ROP procedure was as follows: 250 g (2.19 mol) of ε-cap was introduced into a 300 ml QuickFit flask. The required amounts of Sn(Oct)2 catalyst and BzOH to achieve the target degree of polymerisation (DP) then were added via syringes to the stirring ε-cap. For all experiments the [BzOH]:[Sn(Oct)2] ratio was kept constant at 1: For example to achieve a DP of 87, Sn(Oct)2 (5.00 ml 0.06 M Sn(Oct)2/toluene solution, 0.30 mmol) and BzOH (2.60 ml, mmol) were required. This solution was mechanically stirred about 3 min to ensure the mixture was homogeneous. The polymerisation mixture was then heated to the reaction temperature (150 C) by an oil bath with rigorous stirring was continued throughout the reaction. Measurements of dielectric properties The dielectric properties of the monomer and polymerisation mixtures for this study were measured using the coaxial probe technique. The coaxial probe measurement system involved the use of an Agilent 8753 ES VNA (30 khz 6 GHz), a cable and an open-ended coaxial probe. The probe was made of Inconel stainless steel and was capable of operating up to 250 C. The probe construction takes the form of a 7 mm coaxial line with an inner conductor of 3 mm diameter, which terminated to a radiating annular aperture with a metallic flange [6]. The coaxial probe method requires calibration prior to collection of the data to account for any systematic errors. The calibration involved the measurement of three different standards: (a) open line (air), (b) short circuit line (aluminium foil) and (c) a reference liquid of known dielectric response (methanol) [6, 7]. A detailed explanation of calibration procedures can be found in [8, 9]. The dielectric properties of ε-cap and polymerisation mixtures were measured by immersing the probe into the liquid samples. During the measurements, the S11 signal was collected by the VNA and the data stored on a dedicated PC [6]. The S11 data from the measured samples was then automatically transformed into ε and ε values software provided by National Physical Laboratory (NPL). The use of coaxial probe techniques for measurement of the dielectric properties of many materials at room and elevated temperatures has been well documented in the literature [5, 10-14] 3.0 Results and Discussion In order to get information on how the dielectric properties of different DP of the polymerisation mixtures behave in relation to their monomer, the tan δ of three polymerisation mixtures were plotted together with the tan δ of ε-cap. As can be seen in Figure 1(a), it is interesting to note that the tan δ of polymerisation mixtures of PCL DP 200, 87 and 20 shows a similar trend and nearly identical values with ε-cap for the frequency range GHz. Therefore,

3 Applied Mechanics and Materials Vol at ambient temperature (20 C), it shows that the dielectric properties of different PCL mixtures could behave like dielectric properties of ε-cap.the dielectric response of polymerisation mixtures with temperature change was assessed at a frequency of 2.45 GHz. The dielectric properties comparison between polymerisation mixtures and their monomer in expressions of tan δ are shown in Figure 1(b). From Figure 1(a) and (b), it can be seen that tan δ of polymerisation mixture, PCL DP 200 are identical with the dielectric properties of ε-cap. This similar behaviour also appears for a lower target DP, for example polymerisation mixture PCL DP 20 where the concentration of BzOH is 100 times higher than BzOH s concentration in polymerisation mixture PCL DP 200 (Figure 1(b)). Figure 1: (a). Plot of tan δ of different DPs of PCL mixtures and ε-cap versus frequency at 20 C. (b) Plot of tan δ of polymerisation mixtures and ε-cap versus temperature at 2.45 GHz. The maximum estimated standard deviations of the measured data are [tan δ ± 0.01]. The key finding of this survey is that the component that will dominate the dielectric properties of the polymerisation mixture at C is the monomer. Its response is the single most significant of all the precursors and the dielectric properties of the total mixture are shown to be almost identical to that of the monomer alone. This is not surprising as the quantity of all other components present in the mixture is typically greater or equal to 7 orders of magnitude less than that of the monomer, ε-cap. The similarity of the dielectric response of the polymerisation mixtures with monomer, as depicted in Figures 1(a) and 1(b), can be interpreted as follow: a) The dielectric response of the polymerisation mixtures during the progress of polymerisation may be dominated by the change of monomer s concentration as the monomer is converted into polymer. b) The heating rate and penetration depth (Dp) of the polymerisation mixtures under microwave field depends significantly on the dielectric properties of ε-cap. So further understanding of the monomer s dielectric properties behaviour through a dielectric relaxation analysis is necessary and this is discussed further in next section. Dielectric Relaxation Study of ε-caprolactone In order to investigate how the ε and ε of ε-cap change with frequency and temperature, a series of ε-cap heating experiments were conducted using a conventional oil bath as the heat source. A 300 ml round bottom flask containing about 200 ml ε-cap was introduced into an oil bath, heated to set point temperature with continuous rigorous stirring. The dielectric properties of ε-cap were measured using the coaxial probe technique once the sample had equilibrated to the set point temperature as determined by a thermocouple measurement. The dielectric properties of the ε-cap were assessed across the temperature range 20 C C at 10 C intervals. The analysis of the measured dielectric property data is typically executed by a curve D fitting technique based on one of the following models: Debye, Cole-Cole, Davidson-Cole and Havriliak- Negami [15]. In this paper the dielectric responses were fitted by the simplest model, Debye equation. The initial value of ε used for the fitting is 3 which is chosen slightly higher than thesquare of ε-cap s refractive index, n 2 at 20 C (2.14) [16]. This assumption

4 624 Advances in Applied Mechanics and Materials is based on observed ε for polar material being always higher than nd. The highest value of the measured dielectric constant at 0.1 GHz (the lowest frequency available) is used as the static permittivity (εs) due to the fact that ε values are nearly constant far below relaxation frequency and transition in ε values only occurs near the relaxation frequencies. The values of static permittivity (εs), very high frequency permittivity (ε ) and relaxation frequency at specific sample temperatures were substituted into the Debye equation [17-20]. Where, is static permittivity or dielectric constant at low frequencies, is dielectric constant at very high frequencies, is the angular frequency (ω=2πf, where f is frequency) (Hz) and is the dielectric relaxation time (τ=1/2πfc, where fc is relaxation frequency) (s). The ε and ε were calculated using the Debye equation. The measured dielectric response of ε-cap fits well with the Debye model with a small error of about (±5 %) in ε for temperatures between C. This indicates that the hypothesis is reliable and proves that ε-cap behaves as a single relaxation material. A relaxation model of ε-cap based on curve fitting of measured dielectric properties with the Debye equation, is shown in Figures 2(a) and (b). (a) (b) Figure 2: Plot of Debye relaxation model of (a) ε versus log frequency and (b) ε versus log frequency, for ε-cap at selected temperatures. From Figure 2(a), it is observed that ε reaches its maximum at the lowest frequency and temperature among the measurements performed in this study. The values of static permittivity (εs) reduce from 40 to 25 when temperature increases from 20 to 150 C. In addition, the values of the very high frequency permittivity (ε ) do not change with temperature and are constant at 6. This is in agreement with the assumption used and the results suggest that the distortion polarisation for ε- cap should be less dependent on temperature changes compared to εs and ε in the microwave region. The peak of ε" of ε-cap appears at 2.8 GHz (which is also the relaxation frequency, fc) at 20 C and its value is 17. Meanwhile, at 150 C the peak of ε emerges at 16 GHz. In general, the peak value of ε will be reduced, which due to it dependency on εs value as shown in Equation (1) and fc will shift to higher frequency when temperature rises (see Figure 2(b)). Furthermore, as the temperature is raised, the dispersion (ε vs. log f) and absorption (ε vs. log f) curves move to higher frequencies and most likely without a change in shape. Figure 2 also illustrates an important explanation to why ε-cap is a high loss material (ε =24.47, ε =16.63, tan δ=0.68) at 20 C and microwave frequency, 2.45 GHz and its tan δ decreases with increasing temperature as shown in Figure 1(b). This is due to the frequency of the input microwave energy, 2.45 GHz being close to the fc (2.8 GHz) where the maximum ε of ε- cap is observed. As temperature increases, the fc of the ε-cap shifts to higher frequencies which is considerably different to 2.45 GHz and the ε values at 2.45 GHz become lower. Therefore,

5 Applied Mechanics and Materials Vol the value of τ which is inversely related to frequency (τ = 1/2πfc) [21] will decrease as temperature increases. The values of τ based on the Debye relaxation model are 5.68 x seconds at 20 C and reduce to 9.95 x seconds at 150 C. The shorter τ of ε-cap at the higher temperature is related to the increasing of molecular collision and randomization rate during the heating process of ε-cap [2, 19, 22]. During the heating process, heat from the oil bath transfers to bulk ε-cap and provides energy to the ε-cap molecules, which makes the molecules more energetic as the temperature increases. The energetic molecules (dipoles) within the bulk move faster, more randomly, and consequently the collision and randomisation rate (change with time) of the dipoles becomes more rapid. By definition, the τ is the time required for dipoles to become 63 % oriented in an external electric field from their random condition or vice versa when the electric field is removed [21]. Therefore, at a higher temperature, with rapid collision and higher degree of randomization of molecules, the ε-cap dipoles require a shorter time to become 63 % oriented in the electric field than at a lower temperature. Penetration Depth of Microwave Power Flux into ε-caprolactone The penetration depth (Dp) is a very important parameter for ε-cap in microwave heating because it gives an immediate indication of the heat distribution within the bulk of ε-cap. In order for effective microwave heating to occur, the energy should penetrate as deeply as possible into the ε- cap. If this does not occur, then microwave heating is limited to the surface of the ε-cap sample. The values of Dp of ε-cap at microwave frequency, 2.45 GHz from ambient (20 C) to polymerisation temperature (150 C) were determined and shown in Figure 3. (a) (b) Figure 3: Plot of tan δ and Dp of ε-cap versus temperature at (a) 2.45 GHz and (b) 0.91 GHz. The maximum estimated standard deviations of the measured data are [tan δ ± 0.1]. From Figure 3(a), the tan δ of ε-cap at 2.45 GHz is maximum at about 0.67 at the lowest temperature measured (20 C) followed by a gradual decrease to 180 C. Meanwhile the Dp of ε- cap is at a minimum, of approximately 0.58 cm at 20 C and then increases with the temperature rise. The Dp of ε-cap at the polymerisation temperature, for example at 150 C is about 3.3 cm. This means that a reactor with a diameter of 6.6 cm can be utilised for the polymerisation of ε-cap for efficient microwave heating with the minimum temperature gradient inside the bulk of the ε-cap. This short Dp suggests that direct scale-up of microwave-assisted polymerisation in batch mode at frequency 2.45 GHz is limited due to the limitation of the microwave energy to penetrate deeply into the bulk of ε-cap. Scale-up of the process to larger volumes might benefit from use of a lower frequency [1] such as 0.91 GHz where Dp is about 20 cm at 150 C (see Figure 3(b)). However, microwave heating at 2.45 GHz could be benefited by the fast heating rate achievable (due to higher power density at higher frequency), which will make continuous flow of microwave polymerisation possible.

6 626 Advances in Applied Mechanics and Materials 4.0 Summary The dielectric properties of the ε-cap and different concentrations of polymerisation mixtures of PCL were measured using the coaxial probe techniques. PCL s monomer and polymerisation mixture are considered as dielectric heaters across the measured frequency and temperature range. A dielectric properties study of different target DPs of polymerisation mixture was undertaken at different temperatures and frequencies, revealed that the dielectric properties of a PCL polymerisation mixture system had similar values to its monomer, ε-cap. Therefore, the heating of the polymerisation mixture by using microwave energy is mostly dependent on the dielectric properties of ε-cap. In order to get a comprehensive view on how dielectric properties of the monomer change with temperature and frequency, the measured ε and ε were modelled by curve fitting technique using Debye relaxation model. The measured dielectric properties of ε-cap were well fitted by the relaxation model. Based on the relaxation model, important information that includes the relaxation time and frequency were determined. The value of relaxation time was ~5.68 x seconds at 20 C and this decreased to ~9.95 x seconds at the reaction temperature, 150 C. In addition, the Dp values of ε-cap at 2.45 GHz were calculated to be approximately 0.58 cm (20 C) increasing to 3.3 cm at 150 C, suggesting that direct scale-up microwave-assisted ROP of ε-cap in batch mode is may be limited. Nevertheless, the polymerisation system may still benefit from the fast heating rate based on high value of tan δ and ε of ε-cap, which could be achieved at a microwave frequency of 2.45 GHz. References [1] Hutcheon, R., J. Mouris, et al. (2000). Measurements of the high-temperature microwave ( MHz) complex dielectric constants of monomers ε-caprolactam and ε-caprolactone Microwaves:Theory and Application in Materials Processing V, Second World Congress on Microwave and Radio Frequency Procesing, Ceramic Transactions [2] Scola, D. A., X. M. Fang, et al. (2000). "Microwave syntheses of poly(epsilon-caprolactam-coepsilon-caprolactone)." Journal of Polymer Science Part a-polymer Chemistry 38(8): [3] Smith, A. D., E. Lester, et al. (2010). "Dielectric Properties of Free-Radical Polymerizations: Molecularly Symmetrical Initiators during Thermal Decomposition." Industrial & Engineering Chemistry Research 49(4): [4] Smith, A. D., E. H. Lester, et al. (2010). "Temperature Dependence of the Dielectric Properties of 2,2'-Azobis(2-methyl-butyronitrile) (AMBN)." Industrial & Engineering Chemistry Research 49(6): [5] Nakamura, T., R. Nagahata, et al. (2010). "In-situ measurement of microwave absorption properties at 2.45 GHz for the polycondensation of lactic acid." Polymer 51(2): [6] Kamaruddin, M. J., J. El Harfi, et al. (2011). "Continuous direct on-line reaction monitoring of a controlled polymerisation via dielectric measurement." Green Chemistry 13(5): [7] Agilent. (2006). "Application Note: Basics of Measuring the Dielectric Properties of Materials." [8] Pollitt, S., B. Clarke, et al. (2003). A Guide to characterisation of dielectric materials at RF and microwave frequencies. London, The Institute of Measurement and Control. [9] Kaatze (2007) Kaatze, U. (2007). "Reference liquids for the calibration of dielectric sensors and measurement instruments." Meas. Sci. Technol [10] Grantt, J. P., R. N. Clarke, et al. (1989). "A critical study of the openended coaxial line sensor technique for RF and microwave complex permittivity measurements" J. Phys. E: Sci. Instrum. 22:

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