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Investigation of radical polymerization kinetics of poly(ethylene glycol) methacrylate hydrogels via DSC and mechanistic or isoconversional models

  • Dimitris S. Achilias
  • Ioannis S. Tsagkalias
Article

Abstract

Monomers composed of a polymerizable methacrylate moiety connected to a short poly(ethylene) glycol (PEG) chain are versatile building-blocks for the preparation of “smart” biorelevant materials. Hydrogels based on these PEG methacrylates are a very important class of biomaterials with several applications. The radical polymerization kinetics of two such oligomers, namely poly(ethylene glycol) methacrylate (PEGMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) was investigated. Experimental polymerization rate and monomer conversion data were measured using DSC operating under non-isothermal conditions, at several constant heating rates, or isothermal, at different constant reaction temperatures. Isoconversional techniques were employed to estimate the variation of the polymerization effective activation energy as a function of the extent of reaction. It was found that isothermal and non-isothermal experiments results in similar trends of the activation energy, whereas by comparison of differential to integral isoconversional methods it was postulated that in non-isothermal polymerization experiments integral methods should not be used. From comparison of the two oligomers employed in the polymerization experiments, it was clear that the presence of the terminal hydroxyl group in PEGMA compared to the methoxy group in PEGMEMA leads to different conversion time profiles and activation energies. In particular, monomer–monomer association through hydroxyl groups results in initially lower activation energy of PEGMA. As polymerization proceeds, the existence of aggregated hydroxyl structures (···OH···OH···OH···) in the PEGMA macromolecular chains result in higher activation energies and a more abrupt increase in the conversion time curve.

Keywords

Polymerization kinetics DSC Isoconversional methods PEGMA PEGMEMA 

Introduction

Radical polymerization usually involves vinyl monomers with at least one double bond in their structure. Polymerization of these vinyl monomers is accompanied by a significant heat release due to the addition reaction to the double bond [1]. This polymerization enthalpy can be recorded by thermal analysis techniques, such as Differential Scanning Calorimetry (DSC) and converted to polymerization rate after some appropriate mathematical transformations [2]. DSC has long been proven a powerful technique for measuring the variation of polymerization rate as a function of time (isothermal mode) or temperature (non-isothermal mode). It offers the advantage of continuous recording of the variation of the reaction rate, which permits the estimation and identification of specific phenomena taking place during polymerization (such as diffusion controlled, besides chemical reactions). This is very significant, since in other techniques of measuring the polymerization conversion (e.g. gravimetry, FTIR, etc.) only discrete experimental data are collected. DSC measurements can be easily carried out in a variety of experimental conditions and monomer(s) chemical structure. The latter is very important especially when polymerization leading to crosslinked structures is investigated, where other techniques, requiring the dissolution of the polymer formed, significantly fail [3]. Use of DSC in the investigation of the isothermal bulk polymerization of a variety of monomers, including the well-studied methyl methacrylate (MMA), as well as the curing of several epoxy resins, has been reported in literature [1, 2, 3, 4, 5, 6]. Moreover, a number of papers have been published on modelling polymerization kinetics using detailed mechanistic models [7]. However, investigations on the simulation of polymerization kinetics of vinyl monomers using isoconversional methods are rather rare [8].

Monomers composed of a methacrylate moiety connected to a short poly(ethylene glycol) (PEG) chain are versatile building-blocks for the preparation of “smart” biorelevant materials [9]. Many of these monomers are commercial and can be easily polymerized by either anionic, free-radical, or controlled radical polymerization. Depending on the molecular structure of their monomer units, non linear PEG analogues can be insoluble in water, readily soluble up to 100 °C, or thermoresponsive. Thus, these polymers can be used for building a wide variety of modern materials such as biosensors, artificial tissues, smart gels for chromatography, injectable hydrogels, capsules for drug-release, antibacterial surfaces, or stationary phases for bioseparation [10, 11, 12, 13]. Although the applications of these polymeric hydrogels have been extensively considered, a detailed study on their polymerization kinetics has not been published so far.

Recently, the polymerization kinetics of 2-hydroxyethyl methacrylate (HEMA) has been extensively studied in our laboratory using DSC measurements and mechanistic or isoconversional models and compared to the corresponding of MMA [8, 14]. HEMA has a hydroxyl group in its molecule, which is responsible to hydrogen bonding and thus it presents high boiling point and hydrophilicity. PHEMA materials are used as hydrogels. The variation of the activation energy with the extent of reaction was estimated using isoconversional methods. As a step further here we investigate the polymerization kinetics of two oligomers, namely the poly(ethylene glycol) methacrylate and poly(ethylene glycol) methyl ether methacrylate in the presence of benzoylperoxide initiator. These oligomers include two parts in their molecular structure: a polymerizable methacrylate and a PEG side chain with a few number of repeated poly(ethylene oxide) units. Besides, depending on their structure may or may not have terminal hydroxyl groups. Polymerizations were carried out at several constant heating rates (non-isothermal conditions) and different constant reaction temperatures (isothermal conditions). Differential and integral isoconversional methods were employed and the results were compared. Differences in the reaction kinetics were attributed to the chemical structure of the oligomers used and the formation of side chemical bonds.

Experimental

Materials

The monomers used were poly(ethylene glycol) methacrylate (PEGMA) with average molecular weight 360, containing 500–800 ppm MEHQ as inhibitor and poly(ethylene glycol) methyl ether methacrylate (PEGMEMA), with average molecular weight 300, containing 100 ppm MEHQ as inhibitor (Scheme 1). Both were purchased from Sigma-Aldrich. According to the chemical structure of these oligomers and their average molecular weight, the number of ethylene oxide units –CH2CH2O– was estimated to be approximately equal to 6 and 4.5 for PEGMA and PEGMEMA, respectively. Therefore, the correct should be that the monomers used were oligo(ethylene glycol) methacrylate and oligo(ethylene glycol) methyl ether methacrylate. However, since the company provided these materials uses the term poly—instead of oligo—we decided to keep the names with the prefix poly. The free-radical initiator used was benzoyl peroxide (BPO) with a purity > 97%, provided by Fluka and purified by fractional recrystallization twice from methanol (Merck). All other chemicals used were of reagent grade.
Scheme 1

Chemical formula of the oligomers used as monomers in the polymerizations

Polymerization kinetics

Polymerization was investigated using the DSC, Diamond (from Perkin-Elmer) equipped with the Pyris software for windows. Indium was used for the enthalpy and temperature calibration of the instrument. Polymerizations were carried out under both isothermal and non-isothermal conditions. Isothermal polymerizations were carried out at constant reaction temperatures ranging from 70 to 90 °C, whereas in non-isothermal experiments, constant heating rates were used varying between 2.5 and 20 °C min−1. In the isothermal experiments, the reaction temperature was recorded and maintained constant (within ± 0.01 °C) during the whole conversion range. The samples were weighted (approximately 10 mg) sealed and placed into the appropriate position of the instrument. The reaction exotherm (in normalized values, W g−1) was recorded as a function of time or temperature. The rate of heat release (d(ΔΗ)/dt) measured by the DSC was directly converted into the overall reaction rate (dx/dt) using the following formula:
$$\frac{{{\text{d}}x}}{{{\text{d}}t}} = \frac{1}{{\Delta H_{\text{T}} }}\frac{{{\text{d}}(\Delta H)}}{{{\text{d}}t}}$$
(1)
where ΔHT denotes the total reaction enthalpy and x fractional conversion.

The polymerization enthalpy and conversion were calculated by integrating the area between the DSC thermograms and the baseline, which in the case of the isothermal experiments was established by extrapolation from the trace produced after complete polymerization (no change in the heat produced during the reaction). The total amount of heat released during non-isothermal experiments of PEGMEMA was in the vicinity of 177 J g−1, which multiplied by its average molecular weight leads to 53 kJ mol−1, whereas in non-isothermal polymerization of PEGMA it was around 154 J g−1, which multiplied by its average molecular weight results in 55.4 kJ mol−1. Both values were very close to what is proposed in literature for the standard heat of polymerization of a methacrylate double bond, i.e. 54.9 kJ mol−1 [2].

After the end of the polymerization the pans were weighted again and a negligible loss of monomer (less than 0.1 mg) was observed only in a few experiments.

Results

Isothermal polymerization

When performing kinetic computations on thermal analysis data, one question that arises is whether experiments will be performed under isothermal or non-isothermal conditions. According to the ICTAC report, isothermal experiments are carried out in a limited temperature range [15]. According also to our previous experience on such experiments [2] when polymerization is carried out at low temperatures, it may be difficult to reach complete conversion over a reasonable time period (sometimes it may last for a lot of hours). In contrast, in polymerizations at high temperatures, the heat-up time becomes comparable to the characteristic time of the process, which means a significant extent of conversion is reached before the isothermal regime sets in [3]. Usually, inhibitors included in the monomer molecules, if not removed, play the role of permitting the reaction to be recorded even when it is carried out at high temperatures. Nevertheless, it has been suggested that the best practice would be to perform at least one isothermal run in addition to a series of constant heating rates runs [15]. The isothermal run would be of assistance in selecting a proper reaction model. In fact, a combination of non-isothermal and isothermal experiments would be the best way to properly establish kinetic models [15]. A truly good model should simultaneously fit both types of runs with the same kinetic parameters.

In this investigation, isothermal experiments for both PEGMA and PEGMEMA were carried at temperatures ranging from 70 to 90 °C. Results on the variation of heat released with time appear in Fig. 1a. As it can be seen, polymerization at the highest temperature starts almost immediately and ends in 16 min, whereas at the lowest temperature (i.e. 70 °C) an induction period of almost 2 min was recorded, while the reaction last for more than 1 h. From these data, the reaction rate was estimated according to Eq. (1). Monomer double-bond conversion is further calculated by integrating Eq. (1) and is plotted as a function of time in Fig. 1b. Furthermore, since in most kinetic computations it is assumed that conversion should change in a relative extent, meaning from 0 to 100%, such variation was calculated and it is plotted in Fig. 1c.
Fig. 1

Effect of temperature on the variation with time of heat released (a) and conversion in absolute (b) or relative (c) values, during isothermal bulk radical polymerization of PEGMEMA

From an inspection of the curves presented in Fig. 1, the typical behaviour of a radical polymerization system with diffusion-controlled phenomena affecting reaction rate was observed in all experiments. A typical radical polymerization mechanism includes mainly three steps: decomposition of the initiator to primary radicals, reaction of these radicals with monomer molecules to initiate polymerization, propagation of the macro-radicals by reacting with several monomer molecules and termination of two macro-radicals to form the final polymer. At the initial polymerization stage, purely chemical kinetics controls the reaction. As polymerization proceeds, the reaction rate significantly increases with time accompanied by an increase in monomer conversion. This is the well-known auto-acceleration or gel-effect phenomenon and is attributed to the effect of diffusion-controlled phenomena on the termination reaction and the reduced mobility of live macro-radicals in order to find one another and react. Therefore, their concentration increases locally, leading to increased polymerization rates [7]. Afterwards, the reaction rate falls significantly and at very high conversions, beyond 80–90%, it tends asymptotically to zero. Polymerization almost stops before the full consumption of the monomer. At this point the glass transition temperature, Tg of the monomer-polymer mixture approaches reaction temperature, thus a glassy state appears and it corresponds to the well-known glass effect. This is attributed to the effect of diffusion-controlled phenomena on the propagation reaction and the reduced mobility of monomer molecules to find a macro-radical and react [7].

Simple free-radical polymerization kinetics using mechanistic modelling

As it was reported previously, free-radical polymerization proceeds through a three-step mechanism, including initiation (with kinetic rate constant for the initiator decomposition, kd and initiator efficiency, f, propagation (rate constant, kp) and termination (rate constant, kt). Assuming the steady-state hypothesis for the total radical concentration, the long-chain hypothesis and negligible chain transfer to monomer reactions, the polymerization rate, Rp, can be expressed as a function of monomer concentration, [M] and conversion, X, from:
$$R_{\text{p}} = \frac{{{\text{d}}[M]}}{{{\text{d}}t}} = - k_{\text{p}} [M][R^{ \cdot } ] = - k_{\text{p}} [M_{0} ](1 - X)\left( {\frac{{fk_{\text{d}} [I]}}{{k_{\text{t}} }}} \right)^{1/2}$$
(2)
which after some rearrangement and assuming that at short reaction times the initiator concentration does not change significantly (i.e. [I] = [I0]) leeds to:
$$\frac{{{\text{d}}X}}{{{\text{d}}t}} = k_{\text{p}} \left( {\frac{{fk_{\text{d}} }}{{k_{\text{t}} }}} \right)^{1/2} [I_{0} ]^{1/2} (1 - X) \cong k(1 - X)\quad {\text{with}}\quad k = k_{\text{p}} \left( {\frac{{fk_{\text{d}} }}{{k_{\text{t}} }}} \right)^{1/2} [I_{0} ]^{1/2}$$
(3)
where [M0] and [I0] denote the initial (at t = 0) concentrations of the monomer and initiator, respectively.
In order to quantify the effect of temperature on the reaction kinetics, Eq. (3) can be integrated assuming that all kinetic rate constants and initiator concentration and efficiency are constant, to yield an expression which directly correlates the monomer conversion with the observed overall kinetic rate coefficient, k. It should be noted that Eq. (4) is valid only at low degrees of monomer conversion:
$$- \ln (1 - X) = kt$$
(4)
The overall kinetic rate constant, k can be obtained from the slope of the initial linear part of the plot of − ln(1 − X) versus t. Such plots at conversion values in between 1 and 9% have been created and illustrated in Fig. 2. It was found that the experimental data fit very well to straight lines at all temperatures (R2 = 0.995–0.999). Thus, overall kinetic rate values were measured at different temperatures. Then assuming an Arrhenius expression, for the temperature dependence of the overall kinetic rate constant, k, the overall activation energy of the polymerization rate, Eeff can be estimated from the slope of ln(k) versus 1/T. The value estimated here for PEGMA was equal to 94.4 ± 3.6 kJ mol−1. This value is slightly larger than the corresponding estimated for PHEMA, i.e. 89 ± 3.1 kJ mol−1 (R2 = 0.997) [8]. Both monomers have a hydroxyl group, though the PEGMA monomer is larger than HEMA. This results in higher overall polymerization activation energy.
Fig. 2

Estimation of the overall polymerization kinetic rate constant from isothermal experiments at several temperatures

Isothermal polymerization with isoconversional methods

Furthermore, the variation of the effective overall activation energy with monomer conversion was estimated using isoconversional methods and particular the differential method of Friedman. Accordingly, the reaction rate dX/dt is written using a simple kinetic equation:
$$\frac{{{\text{d}}X}}{{{\text{d}}t}} = k{\kern 1pt} f(X)$$
(5)
assuming an Arrhenius-type expression for the kinetic rate constant, k, Eq. (5) can be written as:
$$\ln \left( {\frac{{{\text{d}}X}}{{{\text{d}}t}}} \right)_{\text{X}} = \ln \left( {A{\kern 1pt} f(X)} \right) - \frac{{E_{\text{X}} }}{R}\frac{1}{{T_{\text{X}} }}$$
(6)
By plotting the left-hand side of Eq. (6) as a function of 1/T at a specific conversion, the activation energy can be estimated. Such plots were created and final results are illustrated in Fig. 3.
Fig. 3

Variation of the overall activation energy with conversion during isothermal and non-isothermal polymerization of PEGMA

As it can be seen, the value estimated at 5% conversion is the same (i.e. 94.4 kJ mol−1) with that estimated previously using the − ln(1 − X) versus t plots. This was expected since these plots were created by taking the 1–9% conversion interval. Effective activation energy decreased with conversion from 95 to almost 84 kJ mol−1 until about 30% conversion, where afterwards it almost reaches a plateau at this value. The kinetics at the initial stages of polymerization (i.e. at conversions less than 30%) occurs in the kinetic mode, as normally is the case of all polymerization reactions [8]. It is frequently found that the largest value of the activation energy is estimated at conversion tending to zero. This, according to Vyazovkin [16], is because the activation energy of initiation, Ei, is typically larger than that of propagation, Ep and termination, Et reactions. Therefore, at the initial stages Ei dominates the overall activation energy. Afterwards, a steady-state regime becomes operative where E drops to the value estimated using Eq. (3), i.e. Eeff = Ep + (Ed − Et)/2. Using the values of Ep, Ed and Et calculated for PHEMA, i.e. 21.9, 143 and 5.6 kJ mol−1, respectively [14], Eff is estimated at 90.6 kJ mol−1. The steady-state value of nearly 84 kJ mol−1 reached here is slightly lower attributed to the bulkier monomer of PEGMA compared to HEMA. As polymerization progresses further at high monomer conversions, the rate control changes from a kinetic to diffusion regime as reflected by an increase in the activation energy. The increasing character of the dependence indicates that the rate is limited by the mobility of large molecules or long segments of the polymer chains [16].

Non-isothermal experiments

Furthermore, non-isothermal experiments were carried out at heating rates ranging from 2.5 to 20 °C min−1. Again heat flow versus temperature data were collected which after integration were transformed to conversion versus temperature curves shown for four heating rates in Fig. 4a. As it was expected, higher heating rates result in shifting of the curves at higher temperatures, as it is common for such experiments. Moreover, the reaction rate, dX/dt, was estimated form the heat flow data and is plotted as a function of the relative conversion in Fig. 4b. The maximum in the polymerization rate profile was observed at conversions ranging from 60 to 70%.
Fig. 4

Effect of heating rate on the variation of conversion with temperature (a) and polymerization rate with conversion (b) during non-isothermal bulk polymerization of PEGMEMA

Subsequently the variation of the effective activation energy with conversion can be estimated using some isoconversional method. Accordingly, in non-isothermal experiments, Eq. (5) is written as
$$\frac{{{\text{d}}X}}{{{\text{d}}t}} = \beta \frac{{{\text{d}}X}}{{{\text{d}}T}} = k{\kern 1pt} f(X)$$
(7)
Assuming an Arrhenius-type expression for the kinetic rate constant, k, Eq. (7) becomes
$$\ln \left( {\beta \frac{{{\text{d}}X}}{{{\text{d}}T}}} \right)_{\text{X}} = \ln \left( {A{\kern 1pt} f(X)} \right) - \frac{{E_{\text{X}} }}{R}\frac{1}{{T_{\text{X}} }}$$
(8)

This equation actually consist the differential method of Friedman, where EX can be estimated from the slope of the linear curves obtained after plotting the left-hand side of Eq. (8) versus 1/T.

In literature, often integral methods are used to interpret the data, since usually the differential term dX/dt is quite noisy. From several integral methods it was recommended to use in the computations the more accurate equation based on the Kissinger–Akahira–Sunose equation KAS equation:
$$\ln \left( {\frac{{\beta_{\text{i}} }}{{T_{\text{i}}^{2} }}} \right)_{\text{X}} = {\text{Const}} - \frac{{E_{\text{X}} }}{R}\frac{1}{{T_{\text{X,i}} }}$$
(9)

According to this method, the effective activation energy can be estimated from a plot of ln(β/T2) versus 1/T.

In this investigation, both the differential isoconversional method of Friedman and the KAS method were employed to estimate the variation of the effective activation energy with conversion. Comparative results are plotted in Fig. 5. It can be seen that the integral method has a systematic error of 20–30 kJ mol−1. Since in DSC experiments data on the reaction rate are measured and the polymerization is calculated using Eq. (1) it can be postulated that in non-isothermal polymerization experiments it is not recommended to use integral methods (as it has been done in the literature).
Fig. 5

Comparison of activation energies obtained from non-isothermal experiments and differential or integral methods for PEGMEMA

Furthermore, results from the differential non-isothermal variation of activation energy are compared to corresponding from isothermal data in Fig. 3. Similar values to those obtained from isothermal experiments were calculated. For PEGMA, in isothermal polymerizations, Eff varied from 95 to 80 kJ mol−1, whereas in non-isothermal it starts from 90 kJ mol−1 decreases to 70 kJ mol−1 and then increases again to 95 kJ mol−1. Data from non-isothermal experiments were lower than the corresponding from isothermal, at conversions less than 70%, whereas they are larger at higher. This could be a result of the higher mobility of the monomer molecules and macro-radicals inside the polymerizing mixture due to the increased reaction temperatures in non-isothermal experiments compared to the isothermal. In addition in both experiments a clear tendency to decrease at the initial conversions was observed, which is a result of the effect of diffusion-controlled phenomena.

Effect of the monomer chemical structure on radical polymerization kinetics

In this investigation two monomers were used with similar chemical structures (Scheme 1) resulting both to polymeric hydrogels. Their main difference was that PEGMA includes a terminal hydroxyl group which makes it more hydrophilic, whereas PEGMEMA is more hydrophobic with no such end group. The amount of heat released was recorded for both monomers (oligomers) polymerized under non-isothermal conditions at several heating rates and isothermal conditions at different temperatures and after integration, the variation of conversion with temperature or time is calculated and illustrated in Fig. 6a, b, respectively.
Fig. 6

Variation of monomer conversion with temperature during non-isotheral polymerization at different heating rates (a) or time during isothermal polymerization at different temperatures (b) of PEGMA and PEGMEMA

In non-isothermal experiments it was noticed that in PEGMA the reaction starts at 70 °C and ends near 93 °C at the lowest heating rate, whereas at the highest heating rate it starts and ends almost 25 °C higher. In the polymerization of PEGMEMA polymerization starts in the range from 70 to 80 °C, depending on the heating rate, and stops at much higher temperatures ranging from 100 to 140 °C. Although the monomer chemical structure is similar, significant differentiations were observed in the two reaction rate profiles. Thus, in the monomer molecule with the hydroxyl end-groups (PEGMA), the reaction seems to start later (especially at higher heating rates), while later increasing at a much more abrupt rate. Polymerization rise in PEGMEMA is more gradual. Similarly in the isothermal polymerization experiments at all temperatures polymerization of PEGMA starts and completes earlier compared to PEGMEMA. Higher conversion of PEGMA compared to PEGMEMA is equivalent to higher reaction rates of PHEMA compared to PEMA or PMMA reported in literature [14] and is attributed to higher initial propagation rate constant associated with the terminal hydroxyl groups and monomer/monomer or monomer/polymer association through hydrogen bonding (Fig. 7).
Fig. 7

Conversion versus time of PEGMA, PEGMEMA, PHEMA and PMMA polymerized under the same conditions, i.e. temperature 80 °C, initial initiator concentration 0.03 mol L−1. Experimental data for PHEMA from Ref. [8] and for PMMA from Ref. [2]

Trying to explain the experimental conversion versus temperature data, again isoconversional methods were employed to estimate the variation of the effective activation energy with the extent of conversion. Non-isothermal experimental data were used and results are illustrated in Fig. 8. Both activation energies start form a high value near 90 and 100 kJ mol−1 in PEGMA and PEGMEMA, respectively decreasing with conversion. However, the reduction in Eff, stops at a value of almost 70 kJ mol−1 and 20% conversion in PEGMA, whereas in PEGMEMA it continuous to decrease until 50 kJ mol−1 and conversion almost equal to 40%. Afterwards, the effective activation energy continuous to decrease at a lower rate in PEGMEMA, while it increases to 95 kJ mol−1 in the case of PEGMA. Results presented in Figs. 68 can be explained based on the structure of the monomer molecules and the phenomena taking place during polymerization. Initially, the lower activation energy of PEGMA at the start of polymerization is probably associated with lower activation energy of the propagation reaction. As it was reported previously, from the definition of the overall kinetic rate constant in Eq. (3), the effective activation energy is related to the individual activation energies of the elementary reactions, e.g. propagation (Ep), initiation (Ed) and termination (Et), according to: Eeff = Ep + 1/2(Ed − Et). The initiation decomposition activation energy is the same, since the same initiator was used and one can assume that Et would have similar values since it reflects the activation energy for the reaction of two macro-radicals. Then, differences in Eeff at the initial stages of polymerization are directly related to differences in the propagation reaction activation energy. Indeed, in two corresponding monomers, e.g. ethyl methacrylate (EMA) and hydroxyl-ethyl methacrylate (HEMA) it has been found in literature [17] that the activation energy of the propagation reaction was 23.4 and 21.9 kJ mol−1, respectively. The lower activation energy of the monomer bearing a hydroxyl end group (HEMA or PEGMA) is attributed to monomer–monomer association through Hydrogen bonding. The interpretation of these results can be carried out in terms of specific interactions and particularly the formation of intra- and inter-chain hydrogen bonds between the monomer and the polymer molecules. The monomer, HEMA or PEGMA, contains one hydroxyl (–OH) and one carbonyl (C=O) group on its molecule. The C=O group acts only as the proton acceptor, while the OH group acts as both the proton donor and acceptor [18]. Hydrogen bonding between the monomer hydroxyl group and carbonyl oxygen atom strengthens the positive partial charges at the carbonyl C atom and at the double bond, leading to a significant charge transfer in the transition state of propagation [19].
Fig. 8

Variation of the overall activation energy with relative conversion obtained from isoconversional methods during non-isothermal polymerization of PEGMA and PEGMEMA

Concerning the effect of diffusion-controlled phenomena on the variation of the effective activation energy, one can expect the EX dependence to decrease when the polymerization rate becomes limited by the diffusion of small molecules such as a monomer or a short segment of a polymer chain. An increasing dependence is expected when the process becomes determined by the diffusion of large molecules, such as the macromolecular chains [16]. Note that diffusion-control becomes operative long before the reaction system vitrifies. According to Vyazovkin [16], it is important to realize that the primary cause of a change from kinetic to diffusion control is an increase in the mixture viscosity that may or may not lead to vitrification. In Fig. 8, a sharp decrease in EX was observed for PEGMEMA at X < 0.3. Thus in this region for this system viscosity undergoes a significant decrease and a plateau is reached at low Ex values approximately equal to 50 kJ mol−1. A similar decrease to comparable values has been also reported in literature for an epoxy-amine system [16]. Note that no significant decrease in EX was observed in the same region for the isothermal case. This is because the effect is associated exclusively with a decrease in the viscosity, which under isotherm conditions it can only increase. Using the oligomer with the hydroxyl groups, i.e. PEGMA, this decrease of EX stops at 20% conversion, and a subsequent increase is observed. This secondary increase in PEGMA can be associated with specific interactions, since in the polymer, both OH···OH and C=O···HO types of hydrogen bonds can occur. Not only the dimer structure (OH···OH), but also the aggregate structure (···OH···OH···OH···) have been found in many systems, including liquid alcohols and solid polymers [18]. It has been found that 53.7% of the OH group on the PHEMA side chain terminal contributes to the OH···OH type of hydrogen-bond, while the remaining 47.3% are engaged in the OH···O=C type of hydrogen bond, at ambient temperature [18]. The formation of these bonds results in increasing activation energy during polymerization. No such phenomena appear in monomers without hydroxyl groups, such as PEGMEMA. So, that an increase in the activation energy was not observed.

Conclusions

The radical polymerization kinetics of two oligomers, namely PEGMA and PEGEMA having a polymerizable methacrylate moiety connected to a short polyethylene glycol chain was investigated. Experimental polymerization rate and conversion data were measured using DSC under non-isothermal, or isothermal conditions at different constant heating rates or reactions temperatures, respectively. Isoconversional techniques were employed to estimate the variation of the effective activation energy as a function of monomer conversion. It was found that isothermal and non-isothermal experiments results in similar trends of the activation energy, whereas by comparison of differential to integral isoconversional methods it can be postulated that in non-isothermal polymerization experiments it is not recommended using integral methods. From comparison of the two oligomer molecules employed in the polymerization experiments, it was clear that the presence of the terminal hydroxyl group in PEGMA compared to the methoxy group in PEGMEMA leads to different conversion time profiles and activation energies. In particular, monomer–monomer association through hydroxyl groups results in initially lower activation energy of PEGMA. As polymerization proceeds, the existence of aggregated hydroxyl structures (···OH···OH···OH···) in the PEGMA macromolecular chain result in higher activation energies and a more abrupt increase in the conversion time curve.

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Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2018

Authors and Affiliations

  1. 1.Laboratory of Polymer Chemistry and Technology, Department of ChemistryAristotle University of ThessalonikiThessaloníkiGreece

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