Dielectric Properties of Free-Radical ... - ACS Publications

Ind. Eng. Chem. Res. 2010, 49, 1703–1710

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Dielectric Properties of Free-Radical Polymerizations: Molecularly Symmetrical Initiators during Thermal Decomposition Alastair D. Smith,† Edward Lester,† Kristofer J. Thurecht,‡,§ Jaouad El Harfi,†,‡ Georgios Dimitrakis,† Sam W. Kingman,† John P. Robinson,*,† and Derek J. Irvine*,†,‡ National Centre for Industrial MicrowaVe Processing, Faculty of Engineering, Department of Chemical and EnVironmental Engineering, and School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, U.K.

The first dedicated study investigating how the dielectric properties of free-radical initiators vary with both temperature and time is reported herein. The materials tested were chosen because they are standard initiators that are widely used by both industry and academia. Initially, each initiator was subjected to thermogravimetric analysis under the same temperature/time profiles, which reflects the mass lost from the materials when subjected to conventional heating. By comparing these data to the dielectric properties, we found that both the melting and decomposition points are directly related to large changes in the dielectric properties of the sample. We then used these data to investigate the likelihood of electromagnetic fields at microwave frequencies being capable of exerting an effect on the molecular decomposition of these initiating species, through a polarization-relaxation mechanism, and whether this effect could potentially alter the mechanistic pathways taken in free-radical polymerization chemistry. Finally, we considered whether direct molecular-based effects of electromagnetic fields at microwave frequencies on such initating species are responsible for the empirical observations related to polymerization that have been reported in the literature, such as rate and molecularweight enhancements. Introduction The use of microwave energy as a heating source to drive chemical reactions has been well documented in recent years. Indeed, based on the empirical data that have been reported, many authors have then gone on to infer numerous potential advantages offered by the use of microwaves as an energy source.1 These include rapid bulk heating, good temperature homogeneity, increased rate, and/or functional-group selectivity. On the basis of these observations, some investigators have taken a further step and suggested that novel, the use microwave energy introduces nonthermal effects that occur at a molecularbond level.2 As a result of these general studies, the field of microwave-assisted polymerization has received an increasing level of study in recent years. Literature reports have included studies in the areas of step-growth,3 ring-opening,4 and freeradical polymerizations,5 and this has led to several comprehensive literature reviews appearing on the subject.2,6 With particular focus on free-radical polymerization (FRP) reactions, which is the polymerization field relevant to this study, there are a number of literature reports that have claimed to demonstrate real advantages from the use of microwave energy. However, the research carried out in this field has produced some contradictory findings. The first microwave-assisted radical polymerization was reported by Gourdenne et al. in 19797 and involved the crosslinking of unsaturated polyesters with styrene. The technique * To whom correspondence should be addressed. E-mail: derek.irvine@ nottingham.ac.uk (D.J.I.), [email protected]. (J.P.R.). Tel.: +44(0)115 951 4088 (D.J.I.), +44(0)115 951 4092 (J.P.R.). Fax: +44(0) 115 951 4115 (J.P.R.). † National Centre for Industrial Microwave Processing, Faculty of Engineering, Department of Chemical and Environmental Engineering. ‡ School of Chemistry. § New address: Australian Institute for Bioengineering and Nanotechnology and Centre for Magnetic Resonance, The University of Queensland, St. Lucia, QLD 4072, Australia.

was later taken further and was applied to the direct polymerization of styrene in the presence of various initiators. Related studies by Sitaram and Stoffer8,9 used a domestic microwave oven at 800 W with 2,2-azobis(isobutyronitrile) (AIBN), tert-butyl peroxybenzoate, and tert-amyl peroxybenzoate as initiators to conduct the polymerizations and compared the results to experiments carried out under conventional heating. Their reported results showed that an increase in the applied microwave power led to an increased reaction rate, although little experimental control over the reaction temperature was reported. However, the use of microwave energy was also claimed to lead to products with a lower polydispersity index (PDI) value. In other words, there was a reduction in the spread of polymer molecular weights produced. From this work, one of the key conclusions was that AIBN is a microwave-absorbing initiator. Madras and Karmore10 similarly reported the acceleration of methyl methacrylate (MMA) polymerization with benzoyl peroxide used as the initiator when microwave energy at a power level of 700 W was applied for 20 s. This work is thought to have been conducted in a domestic microwave oven in an open beaker with no direct stirring applied to the flask. However, the make and electromagnetic design principle of the oven were not definitively stated in the report. Furthermore, it was claimed that an equilibrium between polymerization and depolymerization was produced within the system; that this equilibrium was reached within 10 min of the reaction commencing; and that this phenomenon resulted in very similar polymer distributions being isolated for all of the test reactions, even though the reaction mixtures contained different monomer and initiator concentrations. A similar increase in the polymerization rate was observed with both MMA and styrene by Jacob et al.,11 who claimed to have cut the reaction time to less than one-half that of the thermally heated equivalents. Other vinyl monomers that have been studied in microwave-assisted homopolymerizations include

10.1021/ie901201h  2010 American Chemical Society Published on Web 01/08/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

(hydroxylethyl)methacrylate (HEMA),12 methyl acrylate (MA),11 (meth)acrylic acid (MAA),13 (meth)acrylamide,13 4-nitrophenyl acrylate,12 acrylonitrile,13,14 and vinyl acetate.13 In the case of HEMA, it was also shown to be possible to polymerize this monomer in the absence of a radical initiator.15 Additionally, even though the microwave reaction rate was reported to be much higher than that observed with conventional heating, the resultant polymer products isolated were found to be very similar in molecular weights and PDIs. In the investigations with MA, similar observations were made; specifically, a rate increase was observed using microwave over conventional heating, and again the product polymers where shown to exhibit very similar molecular weights and PDIs irrespective of the power that was applied (200, 300, or 500 W).11 In the case of (meth)acrylamide, it has been claimed that these monomers can be both synthesized and polymerized efficiently using microwaves. It was proposed that the acrylamide monomer was being synthesized from the corresponding MAA precursor and polymerized in a one-step process.16 However, it is not clear whether the polymerization of the MAA occurs first and is then followed by postfunctionalization to form the corresponding acrylamide or whether the MAA is first converted into the acrylamide and then subsequently polymerized. Nitrophenyl acrylate monomers also proved to be potential precursors for obtaining acrylamide products, with the postfunctionalization proposed to occur after the polymerization of the acrylate monomer.12 Similarly, the rates of polymerization of acrylonitrile and vinyl acetate were also reported to be vastly increased when conducted in a domestic microwave oven.13,14 Although the majority of these studies were conducted in the presence of a solvent, bulk copolymerization of HEMA and MMA have also been reported, where complete conversion was observed in less than 1 h, which is approximately one-half of the time required for the analogous conventionally heated processes.17,18 The final comonomer compositions of the microwave and conventional polymers were very similar, but higher molecular weights and lower PDIs were recorded for these polymers formed using microwave heating. Statistical copolymerization of vinyl monomers through free-radical polymerization has also been investigated.19,20 However, this reaction will not be discussed in great detail in this work, because the key observations investigated in this study are the reports of increased rates and improved PDIs for microwave-assisted free-radical polymerization. In most of these reports, the empirically detailed outcomes of the practical experimentation are not in doubt; that is, the observations are based on high-quality spectral analysis of the final products. However, in reality, the direct comparisons made between the thermal and microwave-based test reaction properties, such as kinetics, conversions, and temperatures, are far less reliable. This is because the reactions performed under microwave radiation were conducted under less rigorous conditions than those in the more well-practiced thermal case. For example, many studies on microwave effects have been probed in commercially available, domestic, multimode microwave apparatuses without considering what effects this particular equipment arrangement might have on the reaction system. However, such an approach does allow the reaction to be conducted under the influence of microwave energy, is relatively low in cost to set up, and has the additional benefit that a high number of observations can be made as a result of its simplistic design. In truth, the system parameters that can be investigated by these routes are all very high-level properties, such as the

detection of a rate enhancement, a conversion increase, and/or a change in molecular selectivity. Unfortunately, in most cases, this type of equipment design does not allow the true scientific/ engineering effects that lead to a specific observation to be isolated and identified. This is mainly because the experimental setup does not allow for the decoupling of the various factors that can influence the reaction, such as bulk thermal changes, bulk viscosity changes, reaction pressure differences, penetration depths, and so on. As discussed earlier, in the particular case of polymerization chemistry, the microwave effects that have been reported thus far include increased polymerization rates, molecular weight differences, and changes to reaction selectivity (principally PDI). However, when considering the specific case of free-radical polymerization (FRP), the published works focus on the overall alterations to the final material or overall process conditions, rather than investigating the fundamental individual process stages in order to determine which of these stages has produced the observed phenomena. As a result, many of these FRP-based literature publications have claimed the influence of nonthermal microwave effects to explain the results reported, without any further postulation on a possible mechanistic explanation of the effects detailed. Thus, at this point in time, it is still not clear where in the overall chemical process the microwave energy is introducing the changes observed, and it is therefore very difficult to suggest exactly how any influence is generated on a scientific basis. This, in turn, makes it much harder to reach a point at which one can begin to predict what the effects of applying microwave energy to a particular target reaction will be or how to design the chemistry and/or equipment to maximize the positive and minimize the negative effects of any such influence on a particular chemical mechanism. Unfortunately, this lack of mechanistic information inevitably leads to an inability to effectively scale up these processes to the commercial scale and, as a result, the risk that microwave-based chemistry could remain a laboratory-based curiosity rather than a potential commercial advantage. This article details the first stage in our planned program to break down FRP chemistry into its component parts and to systematically study how the microwave energy influences the mechanistic processes involved. In so doing, we aim to increase the level of understanding of the pathways by which microwave energy influences polymerization chemistry at a molecular level and therefore help to remove the barriers to effective scaleup of microwave-assisted polymerization. One of the main aims of the present study was to determine the dielectric properties of the initiators used in industrial polymerization reactions. This is of interest because, when a molecule or atom is exposed to an external alternating electric field, it is transformed into what is referred to as an “induced dipole”. This new dipole tries to align itself with the field and therefore becomes polarized. The magnitude of this polarization depends on the susceptibility of the material to undergo this transformation and is referred to as its polarizability. The polarizability of a species directly affects its dipolar moment and is intrinsically related to the permittivity of the material. In practice, polarizability is expressed in the form of a complex number and can be separated into various types depending on the origin of the induced dipole (ionic, dipolar, electronic, etc). As a result, permittivity (ε*) is also expressed as a complex number consisting of a real part (ε′) and an imaginary part (jε′′) that are related by ε* ) ε′ - jε′′. In practical terms, the real part is related to the ability of an electromagnetic wave to


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Experimental Section

Figure 1. Initiators used in dielectric study.

propagate within the material and therefore the ability of the material to store energy. To determine the dielectric properties of the initiators, a cavity resonator method was employed. The cavity has the ability to store electromagnetic energy. Therefore, when a dielectric material is inserted, it causes a perturbation of the electromagnetic field, which results in a small change in the stored energy of the system. The change in the stored energy depends on the complex permittivity of the dielectric material and manifests itself by means of change in value of the eigenmodes and quality factor of the resonant cavity. Thus, by monitoring the change in the parameters of the resonant cavity, the permittivity values of the dielectric material can be determined. For cylindrical resonators, the real ε′ and imaginary ε′′ parts of the permittivity can be expressed by means of the frequency shift δf/fo and change in the quality factor (1/Q1 - 1/Q0) of the system. The following equations define the parameters used for measurements conducted by the cavity resonator method used in study ε′ ) 1 + 2J21(x1,m)

(

ε′′ ) J21(x1,m)

δf Vc fo Vs

(1)

)

(2)

1 1 Vc Q1 Qo Vs

tan δ )

ε′′ ε′

(3)

where J1(x1,m) is the second order of the first kind root of the Bessel function, Vc is the volume of the cavity resonator, and Vs is the volume of the sample. An understanding of both the dielectric constant and loss is essential to achieve successful simulation of the behavior of particular systems under the influence of an electromagnetic field. Furthermore, this understanding can also be used to identify the chemical and physical mechanisms that are dominant in individual microwave-assisted synthesis reactions. The dielectric loss tangent (tan δ), defined as the ratio of the real and imaginary parts of the permittivity (eq 3), is particularly useful in this respect, as it allows the microwave-absorbing characteristics of individual components of the overall reaction system to be ascertained. However, one key factor that must be considered in all of these studies is that the dielectric properties are highly dependent on temperature. Hence, it is vital that these properties be measured throughout the full range of temperatures that will be attained during the reaction. In this respect, during this investigation, it was ensured that the temperature range investigated spanned (a) the full half-life temperature range, (b) the melting point, and (c) the decomposition point of the initiators studied. The initiators studied in this work are detailed in Figure 1.

(a) Determination of the Dielectric Properties of FRP Initiators during Decomposition. Thorough descriptions of the cavity perturbation technique used in this work, which is summarized below, have also been given by both Meredith21 and Lester et al.22 The equipment setup consists of a furnace with temperature control on the power input that is used to heat the sample to the required temperature. At the temperatures stipulated by the particular experimental design, the sample is lowered into the cavity, which is situated directly below it, and any perturbations in the resonant frequency created by the presence of the sample are then measured using a network analyzer. In a typical experiment, a sample of known volume, usually in the range of 50-150 mm3, is introduced into a quartz tube containing a quartz shelf. This tube has an internal diameter of 3 mm and is mounted vertically in an automated arm that lowers the sample into the cavity from the furnace above. The total time that the sample spends out of the furnace and in the cavity during this procedure is on the order of 3 s. Therefore, heat loss is considered to be negligible. In practice, the average time between sequential measurements is approximately 15 min. To ensure that any perturbations in the field are due solely to the effect of the sample, the resonant frequencies of the cavity containing the empty tube are measured as a reference, and as such, the frequency shift observed when the sample is added can be properly quantified. Each reported dielectric property value is expressed as the variation from the mean of four separate experiments. The volume change in the sample during the temperature sweep was quantified, and the data were corrected using eqs 1 and 2. (b) On-line Thermogravimetric Evaluation of Initiator Decomposition Behavior. The thermogravimetric analysis method used in this study was a manual procedure conducted during dielectric property measurement assessment. After a dielectric property measurement had been made at a specific temperature, the sample was removed from the cavity, and its mass was recorded on a four-figure analytical balance, before the sample was returned to the cavity apparatus. The returned sample was subsequently reintroduced into the furnace, and the heating ramp was continued. The significant difference between this method of thermogravimetric measurement and the offline method detailed below is that the sample is covered by an air-permeable “bung” that permits nitrogen to escape but keeps the remainder of the sample held within the tube. The advantage of this method is that a direct comparison of the actual mass loss and the theoretical loss of nitrogen upon decomposition during the dielectric property measurements can be made. (c) Off-line Thermogravimetric Evaluation of Initiator Decomposition Behavior. This method was used to define the decomposition related solely to thermal heating for comparison to the on-line method described above. Degradation thermograms were measured using a TA 2950 thermogravimetric analyzer. In a typical experiment, 5 mg of sample was heated from 25 to 200 °C at a rate of 0.33 °C/min, where the quoted accuracy in measurement is (0.1 mg. All experiments were conducted using nitrogen as a purge gas. (d) Sample Volume Estimation and Correction. The height of the sample was measured using a traveling microscope attached to a Vernier scale. This measurement was then used to calculate any volume change in the sample that was produced as a result of the thermal treatment of the material during the procedures detailed above. This measurement was then used to correct for any volume change when calculating the dielectric properties and tan δ values using eqs 1-3. Thus, any empirically

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Figure 2. General schemes for the decomposition of vazo and peroxy initiators.

obtained change recorded in tan δ will be as a result of either greater freedom of rotation given to an exisiting dipole or the generation of a new dipole. Reproducibility analysis was carried out by conducting three repeat measurements, and the error bars in subsequent figures represent the standard deviations from the mean values plotted. (e) Materials. All materials were used following purity and water-content assesments by NMR spectroscopy. 2,2-Azobis(2methylbutyronitrile) (AMBN), 2,2′-azobis(2-methylpropionitrile) (AIBN), and 2,2-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V70) were all purchased from Wako, and dicumyl peroxide (DCP) was obtained from Acros. Results and Discussion When FRP is conducted using conventional heating, it is wellaccepted that it is important to fully understand how the reaction conditions affect the efficiency of each of the different mechanistic steps to produce consistent and well-defined polymeric materials. Likewise, it will be equally important to identify how each mechanistic stage is being influenced by the microwave energy. Only once the correct mechanistic stage in the polymerization reaction cascade that is undergoing any specific “unique” effect due to the use of microwaves has been identified can the nature of any “microwave effect” be correctly attributed. In turn, this effect will have to be fully understood in order to achieve similar optimized, high-quantity, and repeatable material syntheses on all scales with microwave heating. Our initial investigations concentrated on examining the decomposition of a free-radical initiator to form two free-radical species. This “breakdown” takes the form of a homolytic scission of a specific bond in the molecule (Figure 2). One of the key influences that the specific molecular structure of an individual initiator has over the process is that it defines the temperature at which this bond-breaking event takes place. Furthermore, as already discussed, it can be postulated that microwave-induced changes in initiator decomposition temperatures/mechanisms can explain the empirical observations (e.g., high reaction rates) reported for microwave-heated FRP reactions.8,9 Hence, the first area that requires investigation is the influence that microwave energy has on this decomposition step. In FRP, two main families of thermal initiators are widely used: vazo and peroxy initiators. The general formulas for these species are shown in Figure 2. The type of bond being influenced by the reaction conditions is different between these two initiator types: a N-R bond in the former and an O-O bond in the latter. This study was carried out to establish the nature of any differences between the microwave- and conventional-heatinginspired decompositions of both vazo and peroxy initiators. This was done by measuring how a target molecular system interacts with an electromagnetic field in the cavity across a range of temperatures and frequencies. The study was conducted to identify differences (a) within each initiator family (i.e.,

comparing the stability of the same type of fragile bond within each initiator family) and (b) between the two initiator families (i.e., comparing the stability of the two differing types of fragile bonds to detect any difference in microwave energy susceptibility between these two bond types). The data in this article represent the specific investigation of a series of molecularly symmetrical initiators that exhibit a trend in the position of the melting point of the initiator relative to its decomposition point (see Table 1). Study of the Dielectric Loss Factor of Pure 2,2′-Azobis(isobutyronitrile) (AIBN). Figure 3 shows the loss tangent of pure AIBN from room temperature to 150 °C. The change in loss tangent with temperature is nonlinear, as the value starts at 0.005 at room temperature, peaks at 0.14 at 85-95 °C, and then decreases at subsequent higher temperatures. As a guideline, materials with a loss factor of less than 0.1 are not considered microwave-absorbent, so at temperatures up to 75 °C, pure AIBN is essentially microwave-transparent at 2.45 GHz.21 Although AIBN contains two cyano groups with inherent dipoles, there is no evidence in Figure 3 that these groups can be polarized. One potential explanation for this phenomenon could be that the physical form of AIBN is solid under these conditions, so that the potential molecular motions of individual functional groups within the material, and thus the potential responses of their dipoles, are more restricted and the groups are unable to be polarized by the alternating electric field. If the dipolar bonds are unable to move significantly, then little energy will be dissipated, and therefore, the loss factor will be low. In liquid form, it could be expected that the loss factor would increase as the molecular motion within AIBN became greater, allowing the functional-group induced dipoles to interact with the microwave field in a more significant way and thus dissipate microwave energy. An example of this behavior is the difference in loss factor exhibited between ice and water; at 2.45 GHz, the tan δ value of ice is 0.0009, whereas the tan δ value of water is 0.17, a difference of 2 orders of magnitude.21 The melting point of AIBN is 104 °C (see Table 1) and is indicated in Figure 3. It is clear, therefore, that the observed step change in loss factor of AIBN at 80 °C does not correspond to a melting-point-based transition. The 1-h half-life of AIBN is 82 °C (see Table 1),23 which corresponds to the increase in loss shown in Figure 3. It is thought that the step change is due to radical formation, which greatly improves the microwave-absorbing properties of the initiator and results in a high tan δ value. This could be because the unsymmetrical electron distribution on the radical center allows it to respond to the alternating electric field much more readily than the cyanide groups on the original AIBN molecule. This explanation is supported by the standard electron paramagnetic resonance (EPR) techniques that are used to study radical environments in organic chemistry, which are based on the differences in how the radical environment interacts with an electromagnetic field.24 Radical formation from vazo initiators also results in the evolution of N2; therefore, thermogravimetric analysis (TGA) studies were conducted to detect any mass loss as a function of temperature for AIBN and compare this with the dielectric data. The heating rate in the TGA was set at 0.33 °C/min, which was the same as that for the sample in the cavity perturbation experiments. Figure 4 shows combined TGA and loss tangent data for AIBN. The TGA data were measured by both the offline and on-line methods for this initiator. It is clear from the data that the two methods record a significant mass loss at the same temperature threshold and that this signal corresponds to


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Table 1. Half-Life and Melting Point Data for Initiators Used in This Study entry b

1 2c 3b 4b a

initiator

t1/2 10a (°C)

t1/2 1a (°C)

t1/2 0.1a (°C)

melting point (°C)

formula weight

AIBN V70 AMBN DCP

64 30 66 115

82 45 84 132

101 60 104 154

103–105 50–96 49–51 38–40

164.2 192.3 308.4 270.4

t1/2 X is the X-hour half-life decomposition temperature for the initiator. b Data obtained from ref 23. c Data obtained from ref 24.

Figure 3. Loss tangent of AIBN with temperature at 2.45 GHz. The 1-h half-life (1-h t1/2) and melting point (mp) data for AIBN are also indicated.

Figure 4. Change in loss tangent and sample weight measured by both on-line and off-line TGA techniques for AIBN.

the major increase in the loss tangent. This finding supports the conclusion that the significant increase in the loss tangent is related to the generation of radicals from thermal decomposition rather than a simple phase change. The key difference between the two methods of TGA assessment differ only at higher temperature, where the on-line method still records significant mass of sample whereas the off-line method drops to essentially zero mass. This difference is due to the use of a nitrogen purge gas system in the off-line technique, so that some components of the weight loss are due to evaporative losses of the sample in the TGA instrument or the physical removal of sample carried by the produced N2. Stoichiometrically, one would only expect 17% mass loss if all of the AIBN were to decompose. Thus, from this point forward, the on-line method was the principle TGA technique employed, as it was thought to be more representative of the sample mass remaining in the cavity and can thus explain why loss tangent data are still being recorded at elevated temperature whereas the off-line measurement would predict that no loss tangent results should be recorded because no sample appeared to be retained.

Explanations are now proposed for the behavior of the loss factor up to 90 °C. The data in Figure 3 show that, above 90 °C, the loss factor decreases with increasing temperature. This is likely to be due to radical decay at temperatures above 90 °C, which results in a decrease in the concentration of the microwave-absorbent radicals with time. However, at 120 °C, the half-life of AIBN is on the order of seconds (the highest quoted half-life temperature is 6 min at 101 °C23), so it would be expected that the decay should be extremely rapid. This is not evident in Figures 3 and 4, where the measurements from 90 to 150 °C were conducted over a period of several hours. Because of the difficulties of collecting data and representative samples at these elevated temperatures, a more systematic investigation of the radical decay was carried out by measuring the loss tangent of AIBN at a constant temperature with varying time, and the data were compared to spectroscopic analysis of the residual material that gave rise to the loss tangent data. This comparison will form the basis of a subsequent publication where the spectroscopic data can be dealt with in detail. Dielectric Loss of Pure 2,2′-Azobis(4-methoxy-2,4dimethylvaleronitrile) (V70). Figure 5 shows the dielectric loss factor of pure V70 from room temperature to 150 °C. The change in loss tangent with temperature is again observed to be highly nonlinear, changing from approximately 0.15 at room temperature to a plateau of 0.26 from approximately 70 to 105 °C and then decreasing as the temperature increases further. Figure 5 shows that V70 exhibits a two step rise: one from approximately 40 to 50 °C, and the other from 50 to 70 °C. This is in contrast to the single-step response for AIBN. The total temperature range over which the step rise is achieved is also noted to be much broader in the case of V70 (∼30 °C) compared to AIBN (∼15 °C). We believe that the contrasting dielectric traces for AIBN and V70 are due to differences in the physical properties between the two initiators across the temperature range. Table 1 contains the half-life and meltingpoint data for the initiators that formed the basis of this study,

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Figure 5. Loss tangent of V70 with temperature at 2.45 GHz. The 1-h half-life (1-h t1/2) and melting-point (mp) data for V70 are also indicated.

and the 1-h t1/2 and the melting-point data relating to V70 are included in Figure 5. Upon examination of the dielectric loss data with respect to these identified points, one see that V70 initially performs exactly as AIBN does. Specifically, there is little response from the solid initiator sample until the 1-h t1/2 value is approached. At this point, radical generation starts, and a rapid rise in the dielectric loss factor is observed. However, the quoted meltingpoint (mp) range for V70 is very broad (∼46 °C) and is also much closer to its 1-h t1/2 value (∼5 °C difference) when compared to that of AIBN, which has an mp range of ∼2 °C that is approximately 22 °C above its 1-h t1/2.23 Thus, with V70, as the 50 °C threshold is crossed, there is a change in the gradient, which is thought to be due to radical generation between 40 and 50 °C, followed by a combination of radical formation and physical form change above 50 °C. The quoted melting-point range for V70 is much broader than that for AIBN,25 so the change in tan δ occurs over a much broader temperature range. The plateau and reduced level of decay observed for V70 relative to AIBN might be due to the different radical stabilities produced by the decomposition process. As the carbon-centered radicals produced have different molecular structures, they will exhibit different stabilities under the same environmental conditions. This conclusion is supported by the TGA data, which exhibit a maximum weight loss in the same temperature region as the rapid rise in tan δ that occurs over a much broader temperature range than in the case of AIBN. We propose the following hypothesis for the influence of the physical form change of the initiator on the dieletric properties measured. The hypothesis is based on the example of the iceto-water transition and relates to the change from solid to liquid form resulting in a greater potential for molecular motion of individual functional groups within the material, thus allowing an increased response of the contained dipoles with the alternating electric field. Other than the azo group, the key functional group in both AIBN and V70 is the cyano moiety, which appears in the same molecular position to the relative NdN bond in both species. Thus, one might expect a rise in loss tangent as a result of transition across their set melting points. However, the size of any mp contribution to the dielectric properties will be ultimately dependent on such properties as (a) the formula weight of the test species, (b) the magnitude of the interaction between the individual dipole and the applied molecular field, (c) the number of complementary dipoles present per molecule, and (d) the viscosity of the material as it is heated above its mp. Similar trends were observed when AMBN was studied under the same conditions, as reported previously.26 AMBN’s 1-h t1/2

Figure 6. Loss tangent of DCP at 2.45 GHz. The melting point and 1-h half-life temperature also shown.

value is essentially the same as AIBN’s (see Table 1), but AMBN exhibits a sharp melting point that is well below its half-life temperature (49-51 °C; Table 1).25 Thus, as would be expected, two transitions were observed in the loss tangent data. In this case, the first is the mp-related change, and the second is related to radical generation at approximately the 1-h t1/2. Dielectric Properties of Pure Dicumylperoxide (DCP). The dielectric behavior of dicumylperoxide, a peroxy initiator, was studied and compared with those of AIBN and V70; these results are shown in Figure 6. The loss tangent of DCP follows a trend similar to that reported for the vazo initiators. Figure 6 shows two regions where the loss tangent increases. The first increase occurs between 30 and 40 °C, which corresponds to the mp of DCP (Table 1), similar to the low-temperature transition in AMBN. Again, no TGA weight loss is observed for this transition, supporting the conclusion that it is likely to be due to a transition in physical form and suggesting that this increase in interaction does not contribute to early radical generation with this particular peroxy initiator either. The second increase in loss factor in Figure 6 occurs in the 1-h half-life temperature range of the initiator. Radicals are formed at around 120 °C, resulting in an increase in loss factor due to the presence of the charge distribution on the radical. This does have an associated weight loss in the TGA data; in this case, it is thought to indicate the subsequent loss of CO from the R-O · radical decomposition product for a stable carbon-centered radical as is often seen with benzoyl peroxide (BPO). Above 140 °C, the loss factor starts to decrease with increasing temperature and corresponds to radical decay or a reduction in sample viscosity. Conclusions and Implications for Microwave-Assisted Polymerization This work presents the first steps in an attempt to understand the true effects of microwave on FRP reactions. However, these initial findings have already shed significant light on the likely mechanisms by which microwaves influence these polymerizations. Figure 7 summarizes the findings based on AIBN. Up to 75 °C, AIBN is essentially microwave-transparent, as shown in Figure 7. All vazo and peroxy species studies exhibit similar initial behaviors, but all have two components to their dielectric properties. The primary response (i.e., most significant increase in dielectric properties) is due to the generation of radicals, where the threshold temperatures for this transition depend on achieving the 1-h half-life temperature by conventional heating. The secondary response is related to a phase change in the initiator and is noted to occur at the melting point of the


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Therefore, we propose that the microwave-accelerated FRP reactions documented in the literature are caused by the selective heating of the radicals once they are formed by thermal routes. These initiating systems studied therefore cannot be regarded as microwave-susceptible or microwave-absorbent initiators. Thus, the use of microwaves with these species cannot be expected to either increase the radical flux at a particular temperature or reduce the temperature at which these materials decompose. However, because of the relatively high loss tangent of the systems once the radicals are formed, this hypothesis suggests that propagation might be far more likely to be influenced by microwave energy.

Figure 7. Microwave-absorbing nature of AIBN with temperature.

particular species. In the case of V70, these two key temperatures are so close that the dielectric transition becomes a complex composite of the two effects. In all cases, no TGA weight loss is recorded with these phase transitions, suggesting that this increase in dielectric response does not lead to the early (lower-temperature) generation of radicals. These are the transitions in the dielectric properties that are detected in the range of the initiators’ melting points. It is proposed that these increases are due to an increase in the freedom movement for the molecular dipoles as a result of the materials’ transition from solid to liquid. These transitions are all located on the experimental data plots close to a broken line marked with mp as a legend. The lack of TGA weight loss indicates that no radical-forming decomposition has taken place at the melting point, as no byproduct gas has been evolved. There will be a volume change associated with the phase change, but as detailed in the Experimental Section, this is measured and corrected for in the calculations of the dielectric properties. In the case of AIBN, there is no phase-change dielectric transition because AIBN reaches its decomposition temperature before reaching a melting point. However, in all cases, when the heat ramp reaches the 1-h half-life temperature, a significant weight loss is recorded in the TGA values. This mass change is related to a second, typically more extensive dielectric property increase. This increase is attributed to the fact that significant levels of conventional, thermally induced, radicalgenerating degradation have occurred in the sample. This results in the introduction of a new dipole into the medium and thus results in an increased interaction with the applied electric field. In this case, the TGA weight loss is associated with the evolution of gas from the sample as dictated by the mechanisms of homolytic decomposition. Again, in all cases, these weight-lossrelated transitions are clearly associated with the 1-h half-life temperatures in the experimental data, which are indicated by a broken line marked with the legend 1-h t1/2. Therefore, we conclude that, in microwave-assisted FRP experiments, which utilize molecularly symmetrical initiating species such as these, the decomposition of such species is highly unlikely to be influenced by the application of microwaves. Rather, the initiator decomposition will be induced by standard thermal heating. Thus, in a microwave environment, initiators with this molecular design will predominantly be heated indirectly, that is, from heating of the solvent or from heat transfer from pyrex glass walls. The small amount of internal molecular heating will not contribute to any other molecular effect and will be quickly dissipated to the surrounding mixture because of the small quantities of initiator present. Only when the initiator has decomposed will microwaves be absorbed to a significant enough degree to influence the overall chemical process.

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ReceiVed for reView July 29, 2009 ReVised manuscript receiVed December 9, 2009 Accepted December 14, 2009 IE901201H

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