Curing Kinetics and Mechanical Properties of endo-Dicyclopentadiene

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Curing Kinetics and Mechanical Properties of EndoDicyclopentadiene Synthesized Using Different Grubbs' Catalysts Guang Yang, and Jong Keun Lee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie403285q • Publication Date (Web): 28 Jan 2014 Downloaded from http://pubs.acs.org on January 31, 2014

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Industrial & Engineering Chemistry Research

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Curing Kinetics and Mechanical Properties of Endo-Dicyclopentadiene Synthesized Using Different Grubbs' Catalysts

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Guang Yang and Jong Keun Lee*

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Department of Polymer Science and Engineering, Kumoh National Institute of Technology

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Gumi, 730-701, Republic of Korea

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ABSTRACT: Isothermal curing of endo-dicyclopentadiene (endo-DCPD) using the 1st and 2nd generation

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Grubbs’ catalysts as the polymerization initiators was studied by means of differential scanning calorimetry

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(DSC) according to model-free isoconversional and model-fitting approaches. It revealed that the two

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Grubbs’ catalysts exhibited apparent differences in ring-opening metathesis polymerization (ROMP) of

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endo-DCPD. The 2nd generation catalyst was more efficient in catalytic activity for overall ROMP of

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endo-DCPD than the 1st generation catalyst, as evident from the reaction rate and fractional conversion (α).

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The model-free isoconversional method showed similar dependence of the activation energy (Eα) on α for

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the two catalyst systems; however, the 2nd generation catalyst system involved a higher Eα up to α≈0.8

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above which the Eα value suddenly drops with increase of conversion. The model-fitting method of the

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ROMP was satisfactorily described by the decelerating reaction and the autocatalytic reaction mechanisms

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for the 1st and the 2nd generation catalyst systems, respectively. The effect of diffusion-control was

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incorporated into the model-fitting method to describe the ROMP reaction throughout the entire conversion.

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For fully cured samples (poly-DCPD), the 1st generation catalyst system has higher crosslinking density

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and lower tensile toughness relative to the 2nd generation. Dynamic mechanical behaviors were also

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different for poly-DCPD formed using the two catalysts.

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1. INTRODUCTION

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Dicyclopentadiene (DCPD) is an inexpensive monomer commercially derived from petrochemicals and is a clear, colorless liquid with a low viscosity. Polydicyclopentadiene (poly-DCPD) is formed through ring-opening metathesis polymerization (ROMP) of its monomer and exhibits excellent impact resistance, chemical corrosion resistance and high heat deflection temperature.1 A variety of transition-metal based metathesis catalysts have been used for ROMP of DCPD.2-6 The resulting polymers showed different in molecular structure, ranging from linear to highly crosslinked;2 therefore, the physical and chemical properties of the material formed are ultimately determined by the catalyst systems. DCPD contains two olefins: norbornene and cyclopentene. Both olefins are capable of binding to the catalyst to undergo metathesis. Nevertheless, the ring strain energy of the norbornene olefin is higher than that of the cyclopentene olefin.7 As a result, the norbornene olefin binds to the catalyst leading to metathesis more frequently than the cyclopentene olefin. To date, the reaction mechanism for endo-DCPD that is illustrated in Figure 1 has been currently accepted. It is known that the norbornene olefin undergoes ROMP to form a linear structure first during which little or no polymerization of the cyclopentene olefin occurs. The subsequent cyclopentene olefin undergoes ROMP once the norbornene olefin is depleted,

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leading to the formation of crosslinked polymer.8,9,10 Different crosslinking reaction mechanisms of DCPD due to an olefin addition have been reported.2,11

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1st generation

2nd generation

Figure 1. Reaction scheme of ROMP for endo-DCPD and the 1st and 2nd generation Grubbs' catalysts used in this work.

During processing of thermosetting materials, the curing kinetics are critical in determining the reaction mechanism and final properties of the polymers. The curing kinetics of the endo-DCPD system have been recently investigated by several researchers.12-14 Ng et al.12 investigated the rheokinetics of a DCPD-based RIM system and the effect of mixing and initial material temperature on the reaction kinetics; the Sestak-Berggren model (SB model) agreed well with the experimental data, which indicates that the SB model is well-suited to describe the reaction kinetics of the DCPD-based system. Kessler et al.13 analyzed the curing kinetics of endo-DCPD with different concentrations of Grubbs' catalyst and tested several reaction models. The model-free isoconversional method most reliably predicted the results over the range of heating rates studied. Our previous work14 investigated the dynamic curing kinetics of endo-DCPD with different Grubbs' catalysts using differential scanning calorimetry (DSC) and evaluated the kinetic parameters for the model-free isoconversional and model-fitting methods. The model-free isoconversional results suggested that endo-DCPD/Grubbs' catalyst systems have complicated reaction mechanisms. The models used showed good agreement; however, the model-free isoconversional method provided the best fit. Unfortunately, these studies did not directly reveal the reaction type and appropriate reaction models were obtained through trial-and-error. In the model-free isoconversional method, significant imprecision might 2


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be introduced due to the difficulty in determining the baseline and numerical differentiation.

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2. FUNDAMENTAL THEORY ON THE KINETICS OF THE CURING REACTION

The curing kinetics can be conveniently measured by an isothermal DSC technique. Although there are inevitable problems in isothermal experiments, e.g., a finite dynamic preheat time and limited temperature range, isothermal experimental data are sensitive to reaction details and enable more facile identification of the induction regions associated with the type of the reaction. This remains true as long as the time to heat the sample is much less than the duration of the reaction. In this case, the data from the isothermal run are very valuable for identifying a proper reaction model.15,16 In this work, the isothermal curing kinetics of endo-DCPD were analyzed using DSC and the resulting data were utilized to determine the kinetic parameters according to the model-free isoconversional and model-fitting methods. The two catalysts that were used to initiate the ROMP reaction of endo-DCPD include: the 1st generation Grubbs' catalyst, which possesses a high metathesis activity and is tolerant towards a wide range of functional groups,17-19 and the 2nd generation Grubbs' catalyst, which is more thermally stable and catalytically active.20-22 Also, this study evaluated the mechanical properties of fully cured poly-DCPD initiated by the 1st and 2nd generation Grubbs' catalysts.

The curing process of thermosetting systems is reflected by the DSC heat flow; thus, the fractional conversion (α) can be determined using Eq. (1), as follows: α

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(1)

where is the reaction enthalpy up to time t and is the total enthalpy. In most cases, the reaction rate (dα/dt) is parameterized into two main variables, i.e., temperature (T) and fractional conversion (α) according to the following relation:

88 89 90

(2)

where is the temperature-dependent rate constant and is the reaction model. often obeys the classic Arrhenius equation, as follows:

(3)

where A and E are the pre-exponential factor and activation energy, respectively, and R is the universal gas constant. Combining Eq. (2) and Eq. (3) provides:

(4)

Curing kinetics are often analyzed using model-free isoconversional kinetics, which obey the isoconversional principle, and model-fitting kinetics, which derive the kinetic parameters using a known reaction model. Hence, it is necessary to know the details of the reaction mechanisms before selecting a specific reaction model for model-fitting kinetics. 2.1. Model-free Isoconversional Method. All model-free isoconversional methods, without assuming or determining any particular expression of the reaction model, comply with the isoconversional principle that the reaction rate at a certain fractional conversion is merely a function of the temperature. Since A and f(α) are constants when α is constant, differentiating the logarithm of Eq. (4) with respect to yields 3



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/

elucidate the reaction mechanisms. Numerous isoconversional computation methods have been developed to determine a correlation of Eα with α, e.g., Friedman method,23 Flynn-Wall-Ozawa method,24,25 Kissinger-Akahira-Sunose method26 and Vyazovkin method.27-29 Among them, the Vyazovkin method is the most accurate and extensively applicable to arbitrary temperature programs. In the Vyazovkin method, a series of " values for each α are determined by minimization and iteration of the following equation, Eq. (6): +

#" $ $ '- ),'

%(" , ) *

!∆!

!

0

(6)

where the subscripts i and j denote the data from varied temperature programs, ∆ is the interval of conversion (∆ =0.025 in this work), and and ∆ are the curing times up to fractional conversions α and

∆, respectively.

2.2. Model-Fitting Method. Unlike model-free isoconversional methods, the reaction model f(α) that is related to the reaction mechanisms has to be selected in advance in order to estimate the kinetic parameters by matching the experimental data. Through thorough understanding of the reaction mechanisms in the model-free isoconversional method and knowing the characteristic conversion at which the maximum curing rate is reached, an adequate kinetic model can be assigned via the standard procedures presented in the literature.30,31 The simplest empirical model is the nth-order model, Eq. (7), as follows:

121

1

+

(7)

where n is the reaction order. The nth-order model is a decelerating kinetic model: its reaction rate is maximized in the beginning of the curing process and decreases continuously with fractional conversion.16 Sestak and Berggren32 introduced a two-parameter model (i.e., SB model) that accounts for autocatalytic effects; the equation, Eq. (8), is as follows:

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%" , '

%" , ' ≡ / !

111

122 123 124 125

(5)

The activation energy as a function of fractional conversion can thus be obtained and then used to

+

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!

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3 1

+

(8)

where m and n are reaction orders. The SB model is an example of the autocatalytic model whose maximum reaction rate typically appears around 20–40% conversion.33 There are two distinct stages in the isothermal curing of thermosets: chemically-controlled and diffusion-controlled.34 The time scale for the overall reaction is obtained as the sum of the time for chemical reaction and the time for diffusion of reactants. The reaction enters diffusion-controlled when the time scale of the diffusion of chemical reactants becomes longer than that of the chemical reaction. One significant disadvantage of the models, i.e., the nth-order and SB models, is unable to describe the later stage of cure that diffusion control may dominate the curing kinetics. In order to describe the curing kinetics along with the effects of diffusion, a diffusion factor (fd) was proposed as defined in Eq. (9),35,36 as follows: 4


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137 138 139 140 141

4567 89

(9)

where C is the diffusion constant, and : is the critical conversion that indicates the onset of diffusion control.

3. EXPERIMENTAL SECTION

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3.1. Materials. Endo-dicyclopentadiene monomer (endo-DCPD, 95%, stabilized with 100–200 ppm

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4-tert-butylcatechol, Acros Organics, Belgium) was used as received. To improve the dissolution kinetics,

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the 1st and 2nd generation Grubbs' catalysts (Sigma Aldrich, USA) were recrystallized to finer and more

145

soluble forms in dichloromethane (Sigma Aldrich, USA) prior to use in accordance with literature

146

procedures.37 Toluene, which was used to evaluate the swelling behavior of poly-DCPD, was purchased

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from Dae Jung, Korea.

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3.2. Uncured Sample Preparation and DSC Measurement. In a vial, endo-DCPD monomer was cooled to 15 oC in a water bath and mixed with recrystallized 1st or 2nd generation Grubbs' catalyst (catalyst concentration in endo-DCPD=3.0 mg/mL) under vigorous stirring until the mixture became homogeneous solution for 30 s. The vial was immediately placed in liquid nitrogen to prevent the DCPD monomer from curing before testing. All experiments in the curing kinetics study were performed using differential scanning calorimetry (DSC, 200 F3 Maia, Netzsch). A small amount of the frozen solution (~10 mg) was sealed in an aluminum DSC pan and placed in the furnace, which had been preheated to the desired reaction temperature. The isothermal curing process was monitored at 40, 60, 80, and 100 oC for the 1st generation catalyst system and 40, 50, 60, and 70 oC for the 2nd generation catalyst system under a dry nitrogen atmosphere; subsequently, a dynamic measurement from -50 to 250 oC at a heating rate of 10 oC /min was performed to determine the residual reaction enthalpy. 3.3. Cured Sample Preparation. Endo-DCPD monomer with recrystallized 1st or 2nd generation Grubbs' catalyst at the same catalyst concentration as a DSC sample were mixed homogeneously as quickly as possible under mechanical stirring in a water bath at 15 oC. The homogeneous solution was poured into a brass mold, covered with parafilm, and successively cured for 24 h at 25 oC, 2 h at 70 oC, and 1.5 h at 170 o

C. After cooling to room temperature and removing the cured specimens from the mold, they were

polished with abrasive papers one day after sample preparation to eliminate the residue from surface oxidation.38 3.4. Swelling Behavior. A crosslinked polymer soaked in a good solvent absorbs a certain amount of the solvent and swells rather than dissolving. In this work, swelling tests were performed to qualitatively evaluate the crosslinking density. Fully cured poly-DCPD was cut into small rectangular pieces (10 mm×5 mm×3 mm) and their initial weights (Wi) were determined. The samples were then immersed in toluene mixed with ethyl vinyl ether as a catalyst inhibitor (5:1 in weight) at room temperature and weighed after 24 h of swelling (W) at the absorption equilibrium. The percent swelling is defined in Eq. (10) below. % swelling

CC0 C0

D 100%

(10)

3.5. Tensile Test. Dog-bone-shaped cured specimens were used for the tensile tests (ASTM D638-V). Each measurement was performed at room temperature using a universal testing machine (UTM, 5



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AG-50KNX, Shimadzu) at a crosshead speed of 1 mm/min. The tensile strength, elongation at break and elastic modulus values presented are the average of more than four specimens. The fracture surface analysis was performed using field emission scanning electron microscopy (FE-SEM, JSM-6500F, Jeol). 3.6. Dynamic mechanical analysis. Rectangle specimens with dimension of about 35 mm×12 mm×3 mm were used for thermomechanical testing. Dynamic mechanical analysis (DMA) was performed using a TA Instruments DMA Q800. Sets of tests were run in a tension mode at a frequency of 1.0 Hz over the temperature range of 30–200 oC at a heating rate of 2 oC/min. Storage modulus and tan δ data as a function of temperature were obtained. 4. RESULTS AND DISCUSSION 4.1. Thermochemical Analysis. Figure 2 has the isothermal DSC curves of endo-DCPD/Grubbs' catalyst systems as a function of curing time at the curing temperatures of 40, 60, 80, and 100 oC for the 1st generation catalyst and 40, 50, 60, and 70 oC for the 2nd generation catalyst. Notice the obvious differences in the DSC curves between the 1st and 2nd generation catalyst systems. While all the DSC curves of the 1st generation catalyst system show a decreasing trend in the heat flow from the beginning of cure, those of the 2nd generation catalyst system comprise a single exothermic peak. The maximum heat flow values in the DSC curves of the 2nd generation catalyst system are larger than those in the DSC curves of the 1st generation catalyst system, which implies that the 2nd generation catalyst system is much faster in the overall ROMP reaction of endo-DCPD.

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Figure 2. The isothermal DSC scans of endo-DCPD with (a) the 1st generation and (b) the 2nd generation Grubbs' catalysts reacted at varying curing temperatures. Figure 3 displays dynamic DSC scans taken immediately after the isothermal cure showing the residual exotherm and the glass transition temperature (Tg). From Figures. 2 and 3, the isothermal reaction enthalpy (Hiso) and residual reaction enthalpy (Hres) were obtained by integrating the DSC heat flow curves, respectively. The conversion (αiso) during the isothermal reaction was calculated by Hiso/HT where HT=Hiso+Hres. The glass transition temperature (Tg) after the isothermal reaction was taken as the inflection point of the step-transition on the dynamic scans shown in Figure 3. All the thermal parameters obtained are listed in Table 1. The average HT values of curing are 352.1±10.8 J/g and 376.5±8.6 J/g for the 1st and 2nd generation catalyst systems, respectively. Note that Hiso (or αiso) and Tg increased with increasing curing 6


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temperature, which means that more DCPD monomer was converted during the isothermal curing process. In Table 1, the 1st generation catalyst results in a bigger αiso value than the 2nd generation catalyst at 40 oC; however, the opposite is true above 60 oC. This is attributed to the slower initial conversion of endo-DCPD in the 2nd generation catalyst system as will be discussed later.14,39 Although αiso increases with increasing curing temperature for both catalyst systems, the residual enthalpy in dynamic DSC tests indicates the incomplete reactions under these isothermal conditions.

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Figure 3. Dynamic DSC scans following the isothermal cure of endo-DCPD with (a) the 1st generation and (b) the 2nd generation Grubbs' catalysts. Table 1. Typical Parameters For the Isothermal Curing Reaction and the Subsequent Dynamic Curing Reaction

1st generation catalyst 2nd generation catalyst

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Temperature

Hiso

Hres

HT

(℃)

(J/g)

(J/g)

(J/g)

40

265.3

82.1

347.4

0.764

45.2

0.048

60

287.9

62.6

350.5

0.821

66.0

0.078

80

327.3

35.6

362.9

0.902

86.9

0.090

100

334.0

13.6

347.6

0.961

110.5

0.100

40

274.2

93.7

367.9

0.745

40.7

0.261

50

306.7

76.5

383.2

0.800

51.3

0.276

60

351.9

23.6

375.5

0.937

-

0.352

70

374.7

4.8

379.5

0.987

-

0.361

αiso

Tg (℃)

αp

"-" Not measurable The fractional conversion (α) versus reaction time curves shown in Figure 4 were determined by integrating the exothermic DSC peaks in Figures 2 and 3. All conversion curves for the 1st generation catalyst system at different curing temperatures show a similar feature: α increases rapidly with reaction time during the initial phase of the curing process, then continues to increase, but more slowly until it ultimately plateaus. However, the conversion curves for the 2nd generation catalyst system are sigmoidal: α increases relatively slowly during the early stage of the reaction, followed by much faster increase until it essentially reaches approximately a plateau that corresponds to the limit of conversion. The slower initial 7



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conversion of endo-DCPD has been observed when the 2nd generation catalyst is used in other reports.14,39 As mentioned before, endo-DCPD contains norbornene and cyclopentene olefins, which are capable of binding to the ruthenium center to progress the ROMP reaction. It is known that the ring strain energy of the norbornene olefin is greater than that of the cyclopentene olefin;7 therefore, the norbornene olefin is more reactive than the cyclopentene olefin in the presence of the catalyst. The 2nd generation catalyst more preferentially binds to olefins than the 1st generation catalyst.40,41 In this case, initiation of the ROMP of endo-DCPD by the 2nd generation catalyst is probably attenuated as a result of a greater affinity for the less reactive cyclopentene olefin, leading to the lower combination of the catalyst to the more reactive norbornene olefin.39

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Figure 4. Fractional conversion versus time for endo-DCPD with (a) the 1st generation and (b) the 2nd generation Grubbs' catalysts.

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Figure 5. Isothermal conversion rate as a function of the fractional conversion for endo-DCPD with (a) the 1st generation and (b) the 2nd generation Grubbs' catalysts. The curing rate versus fractional conversion curves that were obtained by differentiation of the fractional conversion relative to curing time are shown in Figure 5. The fractional conversion at which the reaction rate reaches its peak value (αp), as summarized in Table 1, increased with the applied reaction temperatures in both catalyst systems, which is attributed to the unavoidable finite dynamic preheat time.16 However, the αp values of the temperature programs investigated were ≤0.1 for the 1st generation catalyst system and 8


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0.2–0.4 for the 2nd generation catalyst system, which provides useful information for the selection of a qualified reaction model for each system. Notice that there is a shoulder between α=0.8 and 0.9 in the dα/dt versus α curves of the 2nd generation system reacted at 60 oC and 70 oC in Figure 5(b), which may be one or more heat-evolving processes involved other than ROMP.42 4.1.1. Model-Free Isoconversional Kinetics. The isothermal curing process of thermosetting polymers includes a series of simultaneously occurring steps as the fractional conversion increases; different chemical processes, liquid-rubber-solid phase transitions, and mass-transfer processes. It is necessary to fully elucidate the reaction mechanisms before selecting an appropriate model in a large number of reaction models. An isoconversional method that enables detection and treatment of multi-step processes is usually used to explore the mechanisms of the reaction and predict the curing kinetics by generating the activation energy as a function of the fractional conversion. In general, the activation energy depends on the reaction mechanism. In this study, the Vyazovkin method was employed to obtain the Eα-α correlations by minimizing and reapplying Eq. 6.

Figure 6. Activation energy as a function of the fractional conversion for endo-DCPD with the 1st and the 2nd generation Grubbs' catalysts. The dependence of the activation energy on α for endo-DCPD with the 1st and 2nd generation Grubbs' catalyst systems are compared in Figure 6. As is evident from this figure, Eα shows an upward trend with increased conversion for the endo-DCPD/1st generation Grubbs' catalyst system, especially when α>0.65. Similar results were reported by Kessler et al.13 who have studied the curing kinetics of endo-DCPD with three different concentrations of the 1st generation Grubbs' catalyst. In this study, Eα increases slowly up to α≈0.65, which is attributed to the formation of linear polymer chains through ROMP of norbornene olefins. Since the norbornene olefin is more reactive than the cyclopentene olefin as a result of higher ring strain energy, the ROMP of cyclopentene olefins is less likely to occur in this range. In isothermal experiments, the viscosity of the reaction system rises due to chain-extension, which causes the activation barriers that must be overcome to ensure the reaction to proceed. The accelerated growth of Eα beyond α≈0.65 is likely related to the ROMP of the less reactive cyclopentene olefins, which is initiated by the higher energy. In addition, metathesis of the cyclopentene olefins results in the formation of a crosslinked structure, which 9



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restricts the mobility of the polymer chains. These two factors are responsible for the increase of the activation energy. In the higher conversion range, i.e., 0.85–1.0, a steeper increase was observed, which implies that the isothermal curing was diffusion-controlled. In contrast, the Eα value of the endo-DCPD/2nd generation Grubbs' catalyst system is more complex. Relative to the 1st generation catalyst system, the 2nd generation catalyst system has a higher Eα up to α≈0.8 but a lower Eα in the final stage of cure. Initially, the plots of the 2nd generation catalyst system exhibit a rapidly increasing region, which may be associated with the aforementioned initial rate of the 2nd generation catalyst system. Subsequently, the plots show a similar tendency as those of the 1st generation catalyst system. The Eα initially slowly increases, then accelerates as α exceeds ~0.5. The acceleration stage in the 2nd generation catalyst system begins at a lower conversion, which suggests that the ROMP of the cyclopentene olefins proceeded earlier. This finding supports that the 2nd generation Grubbs' catalyst has a greater affinity for the cyclopentene olefins than the 1st generation catalyst. Interestingly, a significant drop, which was not observed in the 1st generation catalyst system, appears when α>0.8. The shoulders in the higher temperatures in Figure 5 appear to be related to the drop of Eα, which may be again due to reaction mechanisms other than the ROMP of the norbornene and cyclopentadiene olefins involved in the 2nd generation catalyst system. Finally, an additional rise in Eα is due to the vitrification occurring when the Tg reaches or exceeds the curing temperature, where a larger energy barrier is required for further reaction. 4.1.2. Model-Fitting Kinetics. The first and most important step in model-fitting kinetic analysis is to determine a suitable reaction model. The simulation and prediction of the curing process make sense as long as the first step is accomplished correctly. The isothermal run directly reflects the type of the reaction, which aids in the selection of a proper reaction model. As noted earlier, the increasing isothermal temperature enables the peaks of the curing rate to shift towards higher fractional conversions (see Table 1). However, all αp values for the 1st generation catalyst system under the investigated temperatures are below 0.1 and tend towards zero at lower temperatures. These are features of a decelerating-type reaction that the reaction rate is initially at its maximum and the change in αp with applied temperatures corresponds to the initial dynamic preheat time that is impossible to avoid, particularly with decelerating kinetics. Thus, an appropriate reaction model, i.e., nth-order model (Eq. (7)), can be adopted to fit the curing rate. The nonisothermal preheat period will be excluded in the simulation process because the obtained experimental data in this range cannot reflect real initial curing rates under the desired isothermal temperature. For the 2nd generation catalyst system, the reactions peak in the range of α=0.2–0.4, which indicates that the reaction is sigmoidal, and should follow SB model (Eq. (8)).16 It is worth noting that the autocatalytic characteristics may be due to the longer induction period caused by the preferential binding of the 2nd generation Grubbs' catalyst to cyclopentene olefins which leads to fewer bound to norbornene olefins relative to the 1st generation catalyst. The observed shoulders on the exothermal DSC curves (see Figure 5(b)) and large decline in Eα (see Figure 6) represent multiple reactions; therefore, the SB model is only employed when α < 0.8. Through comparative analysis of the reaction types, it is evident that the reaction mechanism of the DCPD system is dependent on the type of catalysts. By rearranging Eqs. (7) and (8) and taking the logarithm, we obtain their linear terms, as follows:

ln F G ln H nln1

α

10


(11)

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ln F G ln H mlnα H nln1

316 317 318 319 320 321 322 323 324

(12)

The kinetic parameters, i.e., k and n for the nth-order model and k, m and n for the SB model, can be determined using multiple linear regression techniques; the calculated values are summarized in Table 2. As can be seen, the reaction rate constant, k, and the reaction order, m, increase with operating temperature, whereas the degressive tendency of n with the experimental temperature is observed. It can be considered that the increase in the curing rate of the 2nd generation catalyst system, relative to the 1st generation catalyst system, is due to the higher values of k. Table 2. Calculated Kinetic Parameters for the nth-Order and SB Models Temperature

ln k -1

(℃)

(min )

m

n

Correlation coefficient

40

-2.746

—

1.691

0.999

1st generation

60

-1.626

—

1.560

0.998

catalyst

80

-0.396

—

1.497

0.995

100

0.534

—

1.400

0.994

40

-0.413

0.629

2.093

0.992

2 generation

50

1.073

0.695

1.822

0.996

catalyst

60

2.280

0.874

1.508

0.998

70

3.310

0.882

1.463

0.997

nd

325 326 327 328 329 330 331 332 333 334 335 336

α

Figure 7 shows Arrhenius plots of ln k versus 1/T for both catalyst systems, which were determined on the basis of the Arrhenius equation (Eq. (3)). The values of ln k vary linearly with 1/T and consequently the pre-exponential factor (A) and activation energy (Eα) can be calculated from the intercept and slope of the fitting lines, respectively. The 2nd generation catalyst system has higher activation energy, but higher pre-exponential factor ensures its faster curing rate. The comparison between the experimental reaction rate and the predicted reaction rate obtained from the nth-order model for the 1st generation catalyst system and the SB model for the 2nd generation catalyst system is displayed in Figure 8. This figure clearly presents that the predicted rates from the nth-order and SB models are in good agreement with the experimental data in the earlier stages of the reaction, whereas significant deflections can be observed in the later stages for all samples. The deflections imply that these two models are unable to describe the curing reaction during the diffusion-controlled stage.

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

337 338

Figure 7. Arrhenius plots of ln k versus 1/T for both catalyst systems

339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355

Figure 8. Comparison of the predictions from Eqs. (7) and (8) and the experimental data for endo-DCPD with (a) the 1st generation and (b) the 2nd generation Grubbs' catalysts. In order to apply two models to the entire curing reaction process, it is essential to account for a diffusion factor (Eq. (9)). The values of the diffusion factor, , were determined from the ratio of the experimental data to the predicted data, and as a function of the fractional conversion for two

catalyst systems are illustrated in Figure 9. For both catalyst systems, the diffusion factors are nearly constant (i.e., ~1.0) up to a certain conversion. Accordingly, chemically-control dominates the ROMP of olefins and diffusion has either no or a negligible influence on the reaction. Then, the diffusion gradually takes over the reaction resulting in a sharp downtrend with an accelerating rate in the -α curve until it reaches zero. Moreover, this downtrend moves to a higher conversion as the experimental temperature increases. The ascendant chemically-controlled stage occurs because the increased temperature postpones the effect of diffusion by depressing the viscosity of the reaction system and enhancing the mobility of the extended polymer chains. According to Eq. (9) and data in Figure 9, the critical conversion (: ) and diffusion parameter (C)

are determined through a nonlinear regression and listed in Table 3. The : increases with increase of 12


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356 357 358 359


isothermal curing temperature for both catalyst systems, indicating the delay of the onset of the diffusion-controlled stage as curing temperature increases. It is noticed that the : values are almost identical at curing temperature of 40 oC; however, the : for the 2nd generation system shows higher values than those in the 1st generation system above curing temperature of 60 oC.

360 361 362 363 364

Figure 9. Diffusion factor as a function of the fractional conversion for endo-DCPD with (a) the 1st generation and (b) the 2nd generation Grubbs' catalysts. Table 3. Critical Conversion (αc) and Diffusion Constant (C) Values for the Diffusion Factor JK L Temperature o

1st generation catalyst

2nd generation catalyst

( C)

40

60

80

100

40

50

60

70

αc

0.667

0.717

0.824

0.861

0.663

0.686

0.853

0.880

C

21.282

18.284

19.251

15.197

13.704

14.773

14.354

15.08

365

366 367 368 369 370 371 372

Figure 10. Comparison of the predictions from Eqs. (7) and (8) with and the experimental data for endo-DCPD with (a) the 1st generation and (b) the 2nd generation Grubbs' catalysts.

The isothermal reaction rate profiles of both catalyst systems predicted from the combination of the nth-order and SB models with a diffusion factor (modified nth-order and modified SB models) are shown in Figure 10. By comparison, the predicted and experimental lines are in good agreement, which supports that 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398

399 400 401

the modified models provide more reliable values for the simulation of the curing process of endo-DCPD/Grubbs' catalyst systems even in the diffusion-controlled regime. 4.2. Evaluation of Swelling Behavior and Tensile Properties. The swelling ratio of polymer in the steady state is a direct function of the extent of crosslinking involved in the sample. In this study, the swelling ratios of poly-DCPD were ~97 and ~362 % after 24 h of immersion for the 1st and 2nd generation catalyst systems, respectively. The 2nd generation catalyst system allows more solvent to penetrate into the sample relative to the 1st generation. The results imply that the 1st generation catalyst sample has a higher crosslinking density while the 2nd generation catalyst is less effective at producing crosslinks. It was reported that much less amounts of crosslinking reaction occurred in poly-DCPD with the 2nd generation Grubbs’ catalyst43, compared to that with the 1st generation catalyst44 when cyclopentene double bonds were measured using spectroscopic analyses. The difference in the crosslinking affects mechanical properties of both cured samples as will be discussed later. Tensile tests were performed to investigate the effect of the different catalysts on tensile properties in an endo-DCPD resin. The stress-strain curves of poly-DCPD initiated by the 1st and 2nd generation Grubbs' catalysts are presented in Figure 11. Both cured samples exhibit plastic deformation. The tensile strength increases rapidly up to the yield point, followed by necking with a local diminution in dimension within the gauge region of the samples; subsequently, it increases slightly again until the specimen fractures. Young's modulus (E), tensile stress at yielding (σy), strain to failure (εf) and tensile toughness (i.e., the area under the stress-strain curve) for both catalyst systems are given in Table 4. Poly-DCPD initiated with the 2nd generation catalyst has lower E and σy values, but noticeably higher εf, resulting in increased toughness. From literature,45-47 higher crosslinking density results in better packing of all chain fragments and potentially leads to a higher packing density. This is likely the reason that the 1st generation catalyst sample with a higher crosslinking density has larger E and σy values. The εf is rather sensitive to the degree of crosslinking. The more crosslinked a polymer is, the firmer the network becomes, which restricts the movement of the polymer chains. Therefore, the εf is smaller for the 1st generation catalyst sample.

Figure 11. Stress-strain curves for poly-DCPD initiated by the 1st and 2nd generation Grubbs' catalysts.

14


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402 403


Table 4. Results of Tensile Tests for Poly-DCPD Produced with the 1st and 2nd Generation Grubbs' Catalysts

1st generation catalyst nd 2 generation catalyst 404 405 406 407 408 409

E (GPa)

σy (MPa)

εf (%)

Toughness (MPa)

1.838 ± 0.071

64.75 ± 1.64

54.22 ± 2.64

28.27 ± 4.42

1.713 ± 0.039

61.49 ± 1.34

90.76 ± 5.30

45.61 ± 4.90

Figure 12 shows SEM images of the fracture surface of tensile specimens that were produced using the st

1 and 2nd generation catalysts. It is clear that the fracture surface of the pulled specimen generated with the 2nd generation catalyst is relatively rough, which indicates that the failure was accompanied with plastic deformation. However, the smooth fracture surface of the specimen catalyzed by the 1st generation catalyst reveals more brittle tensile behavior.

410 411 412 413 414 415 416 417 418 419 420 421 422 423

Figure 12. SEM images of the fracture surface of tensile specimens with (a) the 1st generation and (b) the 2nd generation Grubbs' catalysts 4.3. Viscoelastic Behavior. The viscoelastic behavior of poly-DCPD was studied using DMA. Figure 13 shows the storage modulus (E') and damping factor (tan δ) of poly-DCPD as a function of temperature at a heating rate of 2 ℃/min. As expected from the tensile tests, poly-DCPD initiated with the 2nd generation catalyst shows a lower storage modulus in the glassy region, compared to the 1st generation. The glass transition temperatures (Tg) corresponding to the peaks of the tan δ curves for the 1st and 2nd generation Grubbs' catalyst systems are 156.2 oC and 145.0 oC, respectively. This change in Tg can be explained in terms of the crosslinking density of poly-DCPD. In general, crosslinking stabilizes the entire polymer network and resists deformation; therefore, more energy or a higher temperature becomes imperative for the segmental motion of the polymer chains. The 1st generation catalyst sample with a higher crosslinking density reflects this.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452

Figure 13. storage modulus (E') and damping factor (tan δ) as a function of temperature for poly-DCPD initiated by the 1st and 2nd generation Grubbs' catalysts. Aside from the glass-rubbery transition, a broad shoulder between 70 and 130 ℃ below the Tg peak in the damping curve appears for the poly-DCPD initiated with the 1st generation catalyst, but is almost indiscernible for the 2nd generation catalyst system (see the inset in Figure 13.). A similar phenomenon for the DCPD-based system was observed by Constable.48 This shoulder is probably caused by inhomogeneous crosslinking, which affects the relaxation processes in polymer network. The inhomogeneity of crosslinked polymers have been reported in literature, but still in contention.49-51

5. CONCLUSIONS The effects of the two different Grubbs' catalysts (the 1st and 2nd generation) on the isothermal curing kinetics of endo-DCPD and the mechanical properties of fully cured poly-DCPD were investigated in this work. Relative to the 1st generation Grubbs’ catalyst system, the 2nd generation system showed a faster curing rate and larger limiting conversion at an isothermal curing temperature, indicating higher efficiency in ring-opening metathesis polymerization (ROMP). The isothermal curing process was analyzed with the model-free isoconversional and model-fitting methods using DSC. The activation energy (Eα) for the 1st generation catalyst systems with respect to fractional conversion (α) from the model-free isoconversional method exhibited an upward trend, especially above α≈0.65. As the reaction progresses to the diffusion-controlled region (α>0.85), the rising of Eα was further accelerated. However, the rapid increase of the Eα for the 2nd generation catalyst system appeared earlier from α≈0.5, indicating that the 2nd generation catalyst has a greater affinity to cyclopentene olefins. Unlike the 1st generation catalyst system, a sharp drop in α≈0.8–0.85 was observed for the 2nd generation catalyst system which is likely caused by the occurrence of reaction mechanisms in addition to the ROMP. According to the model-fitting analysis, the two catalyst systems were found to undergo different reaction mechanisms, i.e., decelerating (the nth-order model) and autocatalytic (the SB model) reactions for the 1st and 2nd generation catalyst systems, respectively. The models coupled with the diffusion factor term could predict the curing reaction over the entire conversion range. 16


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453 454 455 456 457


The swelling tests reflected that fully cured poly-DCPD initiated by the 1st generation catalyst has much higher crosslinking density than the sample initiated by the 2nd generation; therefore, this results in increased storage modulus and decreased tensile toughness in the glassy state. A higher crosslinking density in the 1st generation catalyst system also enables the Tg to shift to a higher temperature.

458

■ AUTHOR INFORMATION

459 460 461 462

Corresponding Author

463

■ ACKNOWLEDGMENTS

464 465 466

This research was supported by Basic Science Research Program through the National Research

467

■ REFERENCES

468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490

(1) Howe-Grant, M. Encyclopedia of Chemical Technology, 4th ed.; Wiley-Interscience: New York, 1996.

* Tel.: +82-54-4787686. Fax: +82-54-4787710. E-mail: [email protected] Notes The authors declare no competing financial interest.

Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0009510).

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reaction

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Curing Kinetics and Mechanical Properties of endo-Dicyclopentadiene

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