Studies on thermal degradation kinetics of thermal and UV cured N-(4-hydroxy phenyl) maleimide derivatives

Thermochimica Acta 575 (2014) 70–80

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Studies on thermal degradation kinetics of thermal and UV cured N-(4-hydroxy phenyl) maleimide derivatives G. Pitchaimari, C.T. Vijayakumar ∗ Department of Polymer Technology, Kamaraj College of Engineering and Technology, S.P.G.C. Nagar, K. Vellakulam (Post) 625 701, India

a r t i c l e

i n f o

Article history: Received 20 June 2013 Received in revised form 17 October 2013 Accepted 21 October 2013 Available online 29 October 2013 Keywords: Fuctionalized maleimide Thermal and photopolymerization Crosslinking TGA Model free kinetics

a b s t r a c t The results of the studies concerning the thermal and photo initiated copolymerization of N-(4acryloyloxy phenyl) maleimide (AX), N-(4-methacryloyloxy phenyl) maleimide (MAX) and N-(4-cyanato phenyl) maleimide (CNX) with N-vinyl-2-pyrrolidone (NVP) were presented. The structures of all the copolymers prepared were confirmed by FTIR studies. The thermogravimetric studies of both the thermal and UV cured materials indicated that the UV cured materials were comparatively thermally more stable than the thermally polymerized materials during thermal degradation. Of all the materials investigated, liquid composition having NVP and CNX cured by UV irradiation showed better thermal stability. The degradation kinetic studies using Flynn–Wall–Ozawa, Vyazovkin and Friedman methods showed that the activation energies (Ea) for the thermal degradation of polymeric materials cured by UV irradiation were slightly higher than the Ea values calculated for the thermally polymerized materials. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Bismaleimide resins are one of the important types of thermoset polymeric materials that have found wide applications ranging from microelectronics to aerospace due to their good processability, high glass transition temperature and high modulus. However, maleimide monomers are having rigid molecular structures and the five membered planar rings present in the chain hinder the rotation of the imide group around the backbone chain of the macromolecules [1–4]. The use of radiation curing has been shown to give faster cure rates. This includes the use of microwave [5–7], UV [8,9] and electron beam (E-beam) [10–12] curing for the formation of thin films from the thermosetting monomers and has been shown to give practical and effective advantages. The photo induced polymerization of electron donor/electron acceptor monomer systems has been extensively studied in the past few years, mainly the vinyl ether/maleimide combination [13]. These kinds of reactions show some interesting features, among them a very high cure speed and a no or low effect of the presence of oxygen is remarkable. Most of the UV-curable resins currently used are based on free radical polymerization, which proceeds extensively within a fraction of a second. The UV-curable prepolymers or oligomers are often complex structures based on, for example, epoxy acrylates, urethane

∗ Corresponding author. Tel.: +91 94420 57674. E-mail addresses: [email protected], [email protected] (C.T. Vijayakumar). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.10.020

acrylates, polyether urethane acrylates, polyester acrylates, polyether acrylates and acrylated oils. Moreover, use of vinyl ethers appear as an environment friendly alternative to the acrylate monomers which smell and cause skin and eye irritation. The structures and formulations of diluent monomers and prepolymers may be varied to suit a particular application and property requirement. The N-vinyl-2-pyrrolidone (NVP) monomer is commonly used as a reactive diluent in ultraviolet and E-beam curable polymers applied as inks, coatings or adhesives. NVP monomer is attracting much attention in various fields due to its good biocompatibility, low toxicity, good film forming and adhesive characteristics [14,15]. NVP based polymers find applications in plasma substitutes, soluble drug carriers, and UV curable bio adhesives [16] and it contains a highly polar amide group, which confirms its hydrophilic and polar properties, while the methylene and methane groups in the main and side chain confirm its hydrophobic property, which helps in the preparation of surface active polymers [17,18]. The maleimide polymerization could be initiated more effectively using an acylphosphine oxide photoinitiator (Lucirin – TPO) and was added to the monomer at a concentration of 1 wt%. This photoinitiator is known for its great efficiency and absorbs UV light up to 400 nm, i.e. a wavelength region where the maleimide monomer is completely transparent, thus preventing a detrimental radiation inner filter effect. In our previous investigations N-(4-hydroxy phenyl) maleimide (HPMI = X) is functionalized with acryloyl, methacryloyl, allyl, propargyl and cyanate groups and the structures of the materials are characterized by FTIR, 1 H NMR and 13 C NMR. Thermal curing behavior of the monomers and thermal stabilities of the polymers were studied using thermal analysis.

G. Pitchaimari, C.T. Vijayakumar / Thermochimica Acta 575 (2014) 70–80

Owing to the fact that acrylate, methacrylate and cyanate derivatives are widely used as monomers for photopolymerization, it was decided to use these new compounds for the development of photopolymeriazable compositions. This study presents thermal and UV polymerization of N(4-acryloyloxy phenyl) maleimide (AX), N-(4-methacryloyloxy phenyl) maleimide (MAX) and N-(4-cyanato phenyl) maleimide (CNX). These compounds are solids at room temperature so their polymerization by UV radiation needs a reactive diluent and for which N-vinyl-2-pyrrolidone (NVP) was chosen and was used as the comonomer. New liquid formulations (80%)AX/(20%)NVP = (DAX), (80%)MAX/(20%)NVP = (DMAX) and (80%)CNX/(20%)NVP = (DCNX) were prepared and the systems were cured by both thermal and UV methods. The photopolymerization by UV radiation leading to the curing of the liquid formulations was carried out with the use of radical photo initiator and the photopolymerization was carried out in a UV photo reactor. The thermal stability of the thermally cured materials poly AX (PAX), poly MAX (PMAX), poly CNX (PCNX), poly AX/NVP (TDAX), poly MAX/NVP (TDMAX), poly CNX/NVP (TDCNX) and UV cured materials ploy AX/NVP (UDAX), poly MAX/NVP (UDMAX) and poly CNX/NVP (UDCNX) materials were studied. It is shown that most kinetic models correctly fit the data, each providing a different value for the activation energy. Therefore it is not really possible to determine the correct activation energy from a single non-isothermal curve. Model free kinetic methods FWO and VYZ are used to determine the kinetic triplets using multiple heating rate programs. When a set of curves recorded under different heating schedules are used, the correct kinetic parameters can be derived and the kinetic triplet obtained from these methods for non-isothermal condition is highly uncertain and cannot be compared with the kinetic triplets obtained from isothermal condition [19,20]. Vyazovkin model free approach through use of isoconversion method leads to a trust worthy way of obtaining reliable and consistent kinetic information from non-isothermal data from TG studies. The variation of the activation energy with the extent of conversion helps to reveal the complexity of multiple reactions taking place during thermal degradation of materials [21–23]. Hence in the present study the apparent activation energy for thermal degradation of the polymers are obtained using three model free kinetic [Flynn–Wall–Ozawa (FWO), Vyazovkin (VYZ), Friedman (FRD)] methods. The results obtained are presented and discussed.

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Scheme 1. Synthesis of N-(4-hydroxy phenyl) maleimide and its derivatives.

stirred continuously for half an hour. The slurry was filtered and washed with ice cold acetone to remove the acetone soluble materials and dried. The yield was 98%. FT-IR (KBr disk): 3105 cm−1 ( OH), 3284 cm−1 ( NH), 1712 cm−1 (C O), 1601 cm−1 (aromatic ring) and 838 cm−1 (C C); 1 H NMR (DMSO-d6 ): 6.78 (C2 -H), 7.48 (C3 -H), 6.90 (C6 -H), 6.38 (C6 -H), 4.1 (phenolic OH), 7.54 ( NH), 8.86 (carboxylic OH). 13 C NMR (100 MHz, DMSO-d6 ): 150.08 (C1 ), 113.34 (C2 ), 123.56 (C3 ), 132.41 (C4 ), 161.24 (C5 ), 138.18 (C6 ), 132.35 (C7 ), 169.76 (C8 ) ppm. Experimental procedure for the synthesis and characterization of N-(4-hydroxy phenyl) maleimide (X), N-(4-acryloyloxy phenyl) maleimide (AX), N-(4-methacryloyloxy phenyl) maleimide (MAX) and N-(4-cyanato phenyl) maleimide (CNX) is already presented in the previous work [24,25]. 2.3. Preparation of liquid formulations

2. Experimental

The solubility of AX, MAX and CNX in NVP at room temperature was tested at different weight ratios (80:20, 60:40, 40:60, 20:80) and the highest solubility is noted when the ratio of material to NVP is about 20:80. The liquid formulations were prepared in a test tube by dissolving AX, MAX and CNX powders in NVP. All the formulations were brown transparent solutions and the components of the different liquid formulations are listed in Table 1.

2.1. Materials

2.4. Thermal curing of AX, MAX and CNX with NVP

Maleic anhydride was purchased from Central Drug House (P) Ltd. 4-Amino phenol, p-toluene sulphonic acid and cyanogen bromide were purchased from Loba Chemie Pvt. Ltd., Hydroquinone was obtained from Qualigens Fine Chemicals, Mumbai 400 025. Acrylic acid, acetone, benzoyl chloride, N,N -dimethyl formamide, methacrylic acid, tetrabutyl ammonium chloride, tetrahydrofuran, triethylamine and toluene were purchased from MERCK Specialist Pvt. Ltd., Azobisisobutyronitrile (AIBN, Ajax Chemicals Laboratory, India), phenyl bis (2,4,6-trimethyl benzoyl) phosphine oxide, 1vinyl-2-pyrrolidone were purchased from Sigma-Aldrich Pvt. Ltd. All the chemicals were used as received.

The materials NVP, DAX, DMAX and DCNX were taken in separate micro test tubes and the initiator azobisisobutyronitrile (AIBN) (1 wt%) was added. Heating slowly and increasing the temperature in steps lead to efficient polymerization of these liquid formulations. The polymerization temperature was chosen as 50 ◦ C and kept at this temperature for 15 min. Then the mixture was heated to 55 ◦ C and kept at this temperature for 15 min. Finally the polymerization was carried out at 60 ◦ C for 60 min. The hard masses were post cured at 90 ◦ C for 3 h. The polymers thus made were removed from the test tubes and stored in a desiccator for further analysis. 2.5. UV curing of AX, MAX and CNX with NVP

2.2. Synthesis of amic acid (AA) A solution of 4-amino phenol (53.5 g) in acetone (500 mL) was prepared. To this solution, 47 g of powdered maleic anhydride was added in portions. Yellow precipitate (Scheme 1) was formed and

The photo polymerization of the liquid formulations was carried out in a UV chamber (Heber Scientific, Chennai, India) having eight 30 W low pressure mercury vapor UV lamps (Philips, Holland) emitting the radiations at 254 nm (two), 365 nm (two) and 420 nm

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Table 1 The components of liquid formulations. S. No.

Systems

1 2 3 4 5 6 7

AX MAX CNX NVP DAX DMAX DCNX

Samples AX (%)

MAX (%)

CNX (%)

NVP (%)

Initiator (%)

100 – – – 20 – –

– 100 – – – 20 –

– – 100 – – – 20

– – – 100 80 80 80

– – – 1 1 1 1

(four). The materials NVP, DAX, DMAX and DCNX were taken in separate micro test tubes and the photoinitiator APO was added (1 wt%). All the materials were exposed to UV irradiation for 15 min and the materials were hard. A post curing of the UV polymerized materials was carried out at 90 ◦ C for 3 h. 2.6. Swelling experiment The swelling behavior of TNVP, UNVP, TDAX, UDAX, TDMAX, UDMAX, TDCNX and UDCNX were carried out in toluene at 30 ◦ C. The weights were measured using an electronic balance (Shimadzu AUX120, Japan) having an accuracy of 0.1 mg. Pre weighed polymers were immersed in toluene. After specific intervals of the time (4 h), the polymers were removed from the medium, the surface adhered liquid drops were wiped with blotting paper and the increase in weight was measured. The measurements were continued till the weights of the swellon polymers attained constant values. The swelling ratio (SR %) was calculated [26], using the following expression. SR (%) =

where W0 is the weight at initial, WT is the weight at the particular temperature, We is the weight at the end of the degradation process. 2.8.1. Flynn–Wall–Ozawa method (FWO) The FWO method is widely used for dynamic kinetic analysis and does not require any assumptions to be made about the conversiondependence [27]. The equation used for this method is given as follows: Ea =

−R  ln ˇ 1.052 (1/T )

where Ea is apparent activation energy, R is the gas constant, ˇ is the heating rate and T is the temperature. In this method, plots of ln ˇ versus 1/T give parallel lines for each ˛ value. The slope of these lines gives apparent activation energy, as per the expression. Slope = −0.4567

 Ea 

(6)

R

Weight of the swellon polymer − Weight of the dry polymer × 100(1) Weight of the dry polymer

2.7. Methods The FTIR spectra of the materials were recorded using Fourier Transform Infrared Spectrophotometer-8400S, Shimadzu, Japan by employing KBr disk technique. The thermogravimetric (TG) curves for all the cured materials TNVP, UNVP, TDAX, UDAX, TDMAX, UDMAX, TDCNX and UDCNX were recorded in a TA Instruments TGA Q50. The samples (nearly 4–5 mg) were taken in a platinum pan and heated from ambient to 700 ◦ C at different heating rates (10, 20 and 30 ◦ C min−1 ) in nitrogen atmosphere flow (balance purge = 40 mL min−1 and sample purge = 60 mL min−1 ).

2.8.2. Vyazovkin method (VYZ) The activation energy can be determined at any particular degree of conversion by finding the value of Ea in Eq. (7) for which the objective function ˝ is minimized [28]. ˝=

n n   I(E˛ , T˛i )ˇj i=1 j=1

With



T˛

I(E˛ , T˛i ) =

exp

The rate of solid-state reactions can be described as (2)

where d˛/dt is the rate of the reaction, k(T) is the rate constant, f(˛) is the reaction model. According to Arrhenius’s equation, the temperature-dependent rate constant, k(T) is defined as

 Ea 

k(T ) = A exp −

RT

(3)

where A is the pre-exponential factor, Ea is the apparent activation energy, R is the gas constant and T is the temperature. The reaction extent (˛) for the degradation reaction is shown in the following equation

W − W  T 0 W0 − We

 −E  ˛

RT

dT

(8)

Theory and application of model free kinetic approaches, starting from basic rate equations and ending in activation energy prediction is discussed in the literature [29].

2.8. Kinetic analysis

d˛ = k(T )f (˛) dt

(7)

I(E˛ , T˛i )ˇi

0

˛=

(5)

(4)

2.8.3. Friedman method (FRD) This is one of the differential methods used to calculate Ea and the equation is [30] ln

 d˛  dt

= ln(z) + n ln(1 − ˛) −

 Ea  RT

(9)

From the slope (−Ea/R) of the linear plot between ln (d˛/dt) vs. 1/T, the activation energy (Ea) of the system can be calculated. 3. Results and discussion 3.1. FTIR studies The structure of NVP and the prepared liquid formulations such as DAX, DMAX and DCNX and the polymers obtained

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Fig. 1. FTIR spectra of monomers and its Thermal and UV cured monomers.

from them are investigated using FTIR technique. The FTIR spectrum of NVP (Fig. 1) shows the presence of an absorption peak around 3560 cm−1 and can be ascribed to the C N group. The absorption peak at 1721 cm−1 is associated with the C O group and the methyl group shows an absorption peak at 2945 cm−1 . The band at 1419 cm−1 characteristic of C C stretching confirms the structure of the monomer. The absence of absorption band at 1419 cm−1 in the FTIR spectrum of TNVP and UNVP indicates the polymer formation. The FTIR spectra of DAX and DMAX

show the presence of an absorption band at 1604 cm−1 characteristic of maleimide C C stretching and absorptions band at 825 and 813 cm−1 region which correspond to the vinyl C C stretching vibration. The absorption band noted at 2970 cm−1 in MAX indicates CH3 stretching and the band noted at 1396 cm−1 corresponds to CH bending in methyl group. The absence of bands at 1604, 825 and 813 cm−1 in TDAX, TDMAX, UDAX and UDMAX proves the poly-merization of both the double bonds present in the maleimide ring and the vinyl group present in DAX and DMAX

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Fig. 2. Comparision of swelling studies for synthesized polymers TNVP, UNVP, TDMAX, UDMAX, TDAX, UDAX, TDCNX and UDCNX.

when the polymerization was carried out either thermally or photolytically. The appearance of new absorbance band at 1213 cm−1 in UDAX and UDMAX confirms the formation of cyclobutane rings during the UV curing of DAX and DMAX. Further the absence of absorption band at 1419 cm−1 in the FTIR spectrum of the polymers indicate the consumption of the C C present in the diluents NVP. The FTIR spectrum of the DCNX shows a band at 2245 cm−1 . The disappearance of absorption band at 2245 cm−1 and the appearance of new absorption bands at 1573 and 1396 cm−1 in the FTIR spectrum of the cured resin (TDCNX and UDCNX) confirms the formation of cyanurate rings during the thermal and photo curing of DCNX. During UV polymerization of DCNX, the appearance new band at 1213 cm−1 proves the formation of cyclobutane rings. Here also the disappearance of the band at 1604 and 1419 cm−1 proves the polymerization of the double bond present in the maleimide ring and the vinyl group present in NVP. 3.2. Swelling studies The results of swelling studies for TNVP, UNVP, TDAX, UDAX, TDMAX, UDMAX, TDCNX and UDCNX are shown in Fig. 2. Of all the polymers investigated the polymers UDCNX and TDCNX shows the lowest swelling ratio value (11% and 14%) and the highest swelling ratio values are noted in TNVP and UNVP (Fig. 2). Because during the polymerization reaction of DCNX, the cyanate functionalized material is prone to trimerize and form cyanurate ring which further enters in crosslinking reactions. Apart from the trimerization reaction of the cyanate groups, the maleimide double bonds entering into the cyclization reaction with the cyanate groups is highly probable. Due to these factors, TDCNX and UDCNX end up with highly crosslinked networks compared to the other materials investigated. Generally, the crosslinker concentration is directly related to the density of crosslinks in the polymers. At lower crosslinker concentration, the polymers may have lower crosslinking density and hence higher swelling capacity. Whereas the polymers formed with high crosslinker concentrations will possess higher crosslinking density causing a decrease in the distance between the crosslink points, there by lowering the swelling capacity [31]. 3.3. TG and DTG studies The TG and DTG curves for thermally (TNVP, TDAX, TDMAX and TDCNX) and UV cured materials (UNVP, UDAX, UDMAX and UDCNX) recorded at 10 ◦ C/min in nitrogen atmosphere are shown in Figs. 3 and 4 respectively and for clarity the TG and DTG curves

are shifted uniformly in the Y axis. The detailed observations made from the thermogravimetric data for the samples obtained at a heating rate of 20 ◦ C/min are presented in Table 2 and the results are discussed. The TG curves for TDCNX and UDCNX recorded at multiple heating rates (10, 20 and 30 ◦ C/min) in nitrogen atmosphere are shown in Fig. 5. All the TG curves are shifted to higher temperatures with increasing heating rates. The DTG curves of TNVP and UNVP show double stage degradation and TDAX, TDMAX, TDCNX, UDAX, UDMAX and UDCNX show overlapping multi stage degradations in which before the completion of one stage of degradation of the material the next stage of degradation begins and thus the degradation is complex. The addition of AX, MAX and CNX into NVP mainly affects the thermal degradation behavior of the polymer during the initial stages. The initial degradation temperatures of the TDAX, TDMAX, TDCNX, UDAX, UDMAX and UDCNX are decreased and the values are 214, 215, 252, 238, 247 and 255 ◦ C respectively compared to TNVP (370 ◦ C) and UNVP (375 ◦ C). Polymers obtained by thermally polymerizing the pure monomers AX, MAX and CNX show the highest degradation onset temperatures, 402, 373 and 341 ◦ C respectively. During the polymerization of AX and MAX, apart from the homopolymerization of the acrylate/methacrylate entities and the maleimide units, the reaction between the double bond present in the acrylate/methacrylate unit with the double bond present in the maleimide unit is inevitable. Hence it is reasonable to expect much complex structure (Scheme 2) for the thermally polymerized AX and MAX. Similarly the thermal polymerization of pure CNX will lead to a complex network structure having triazine units formed due to the trimerization of the OCN units and also structures resulting due to the polymerization of OCN group with the maleimide double bond. When NVP is added as diluent in AX, MAX and CNX, the systems become much more complex due to the participation of the vinyl group of NVP in the polymerization. Owing to the complicated structure attained due to the polymerization of different double bonds present in DAX, DMAX and DCNX and the lowering of the concentration of the AX, MAX and CNX in the NVP diluted systems leads to lesser cross links. Although the thermal degradation of these systems are much complex the thermal stabilities of TDAX, TDMAX, TDCNX, UDAX, UDMAX and UDCNX are poor. Such a decrease in degradation temperature has been reported by Fan et al. [32]. The thermal stability of the UV cured liquid based bismaleimide (BMI) – acryloylmorpholine (AMP) system was studied in detail. It was observed that the BMI-AMP polymer degraded at a much lower temperature (143 ◦ C) compared to pure poly AMP (289 ◦ C). The resulting lot shows that addition of BMI clearly decreased the degradation rate, which is perhaps due to the crosslinking network structure in the film which limited the diffusion rate of broken segments and also due to the high thermal stability of BMI itself. Among the various polymers, the UV cured polymers (UNVP, UDAX, UDMAX and UDCNX) degrade slightly at higher temperatures compared to the thermally cured polymers (TNVP, TDAX, TDMAX and TDCNX). In the case of TNVP and UNVP the major degradation temperature starts at around 370 and 375 ◦ C and the maxima are located at 429 and 428 ◦ C and the degradation ends at 475 and 473 ◦ C respectively and the char yield was noted for 4% and 3% at 500 ◦ C. Except for the slight change in the initial degradation temperature, indicating the structural similarity in TNVP and UNVP. The thermal degradation of the TDAX and UDAX is a multistep process in which the first degradation starts at around 214 and 238 ◦ C and this may correspond to the degradation of AX/NVP (TDAX and UDAX) copolymer units and the maxima are located at 330 and 329 ◦ C and the degradation ends at 351 and 342 ◦ C

G. Pitchaimari, C.T. Vijayakumar / Thermochimica Acta 575 (2014) 70–80

Scheme 2. The possible cure reactions of AX and NVP.

Fig. 3. TG (A) and DTG (B) traces of thermally cured materials at a heating rate of 10 ◦ C/min: (a) TNVP (b) TDAX (C) TDMAX (d) TDCNX.

75

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Fig. 4. TG (A) and DTG (B) traces of thermally cured materials at a heating rate of 10 ◦ C/min: (a) UNVP (b) UDAX (C) UDMAX (d) UDCNX.

Fig. 5. TG traces of thermally cured DAX (A) and UV cured DCNX (B) at a different heating rate.

respectively. The major degradation starts at around 360 and 369 ◦ C. It may be attributed to the decomposition of the polyNVP units in the system and the maxima are located at 430 ◦ C and the degradation ends at 526 and 497 ◦ C respectively. In the materials investigated, UV cured polymer is comparatively more thermally stable than the thermally cured material. This may be due to better crosslinking in the UV initiated polymerization of DAX. The copolymerization between maleimide and vinyl ether takes place via the electron donor/acceptor mechanism, in which maleimide acts as an electron acceptor and vinyl ether acts as an electron donor. The maleimide can absorb UV light and give an excited complex. The generation of the initiating radicals is assumed to proceed either

through the decay reactions from an excited state charge complex or through a hydrogen abstraction by the excited maleimide from a donor molecule such as an acrylate/methacrylate monomer then it can react with one another and form a highly crosslinked structure in the presence of UV irradiation [33,34]. Hence, it may cause to form a highly crosslinked polymers compared to the thermally cured polymers. It has also been reported that maleimide/vinyl ether systems can be cured rapidly upon exposure to UV or electron beam source, to form crosslink network [5,31]. Thalacker et al. [35] studied about the thermal stability of radiation curable materials. A comparison was made regarding the thermal stability of acrylate and methacrylate monomers cured by

Table 2 TG studies: the degradation parameters for the thermally and UV cured polymer samples at 20 ◦ C/min. Sample code

I

PAX PMAX PCNX TNVP TDAX TDMAX TDCNX UNVP UDAX UDMAX UDCNX

Residue at 500 ◦ C (%)

II ◦

◦

◦

◦

◦

◦

Onset ( C)

Tmax ( C)

Endset ( C)

Onset ( C)

Tmax ( C)

Endset ( C)

– – 341 – 214 215 252 – 238 247 255

– – 386 – 330 331 295 – 329 328 299

– – 433 – 351 367 335 – 342 353 337

402 373 442 370 360 375 372 375 369 370 362

425 427 515 429 430 430 428 428 430 429 429

635 592 683 475 526 505 482 473 497 483 481

43 32 58 4 17 11 6 3 17 12 12

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thermal, UV and E-beam irradiation. It was pointed out that the poly (methyl acrylate) undergoes a complex degradation and yields only traces of monomer, whereas poly(methyl methacrylate) undergoes predominantly depolymerization reaction yielding the monomer. A comparison of EB, UV and thermal cured trimethylolpropane triacrylates showed that the EB cured material was more thermally stable than UV and thermally cured trimethylolpropane triacrylates. Here in the present investigation, UDMAX is found to be thermally more stable than TDMAX. The first decomposition peak for TDMAX observed in the range 215–367 ◦ C with the maximum of weight loss at 331 ◦ C and the second decomposition stage is noted around 375–505 ◦ C with the maximum of weight loss at 430 ◦ C. In the UDMAX decomposition, the first decomposition is noted in the temperature region 247–353 ◦ C with the maximum of weight loss at 328 ◦ C and the second decomposition stage took place between 370 and 483 ◦ C with the maximum of weight loss at 429 ◦ C. In the case of thermal degradation of TDMAX and UDMAX, the first decomposition peak is associated with MAX/NVP complex and the second is attributed to the degradation of the polyNVP units in the system. The initial degradation of UDMAX and TDMAX are noted at 247 and 215 ◦ C respectively. Thus UDMAX is found to be comparatively more thermally stable than TDMAX. During the curing process, the three dimensional network resulting by UV irradiation may be different from that the one resulting by thermal polymerization. Park et al. [36] reported UV and thermal curing studies of bisphenol A based methacrylate, glycidyl methacrylate, acrylic acid and a trifunctional monomers. The UV cured polymers were found to have better thermal stability than the thermally polymerized materials and was attributed to the increased crosslinking density in the UV cured materials. Because of the presence of rigid aromatic triazine rings in TDCNX and UDCNX, the material starts to degrade at higher temperature (252 and 255 ◦ C) than the other materials investigated. During the cure of cyanate ester resins, there is an initial shrinkage [37]. However, unlike epoxies, cyanate esters cure via an intermolecular reaction to form thermally stable triazine rings. As a consequence of this trimerization reaction and the structure of the resultant network, a large amount of free volume is created as the cure proceeds to completion after gelation [38]. From Table 2, one can easily identify that among all the materials investigated, irrespective of the curing methods (Thermal and UV), TDCNX and UDCNX materials show better thermal stability than others because of the formation of highly crosslinked triazine network during the curing process. 3.4. Kinetic analysis The apparent activation energy for the degradation of all the polymers were determined by three different model free kinetic (FWO, VYZ and FRD) methods. The plots between Ea and the reaction extent (˛) of thermally cured materials (FWO method) are shown in Fig. 6(A). The plots between Ea and the reaction extent (˛) values for TDAX by the different kinetic methods (FWO, VYZ and FRD) are shown in Fig. 6(B). The apparent Ea values calculated for thermally and UV cured materials by the FWO and VYZ methods were the same, but the values obtained by the FRD method differed from those obtained by the other two methods. This is due to the way in which the apparent activation energy was calculated that is FWO and VYZ methods are integral methods whereas FRD is a differential method. The plots between the apparent activation energy (Ea) and reaction extent (˛) values for the UV cured materials by VYZ methods were shown in Fig. 7(A) and the plots between Ea and the reaction extent (˛) values for UVP-NVP by the different kinetic methods (FWO, VYZ and FRD) are shown in Fig. 7(B). The apparent activation energy for thermal degradation obtained using Vyazovkin method for thermally and UV cured materials is considered for the discussion.

77

Table 3 The determination for the regression coefficients (R2 ) corresponding to ˛ values. ˛ Values

Regression coefficients (R2 ) PCNX

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90

TDCNX

UDCNX

FWO

VYZ

FWO

VYZ

FWO

VYZ

0.930 0.793 0.786 0.970 0.996 0.996 0.998 0.999 0.999 0.998 0.999 0.997 0.999 0.999 0.992 0.999 0.998 0.998

0.999 0.998 0.999 0.999 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

0.973 0.987 0.846 0.986 0.899 0.996 0.998 0.999 0.999 0.998 0.999 0.997 0.999 0.999 0.999 0.992 0.998 0.998

0.996 0.999 0.999 0.990 0.981 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

0.889 0.983 0.875 0.976 0.987 0.744 0.944 0.976 0.995 0.998 0.999 0.998 0.999 0.999 0.997 0.999 0.998 0.999

0.999 1.000 0.999 0.999 0.999 0.999 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

The resulting regression coefficients showing the quality of the fits are included (Table 3) for the FWO and VYZ methods. In addition, Figs. 6(B) and 7(B) show a selection of the plots resulting from the fitting of the data to the different kinetic models, so that the quality of the fit is clearly illustrated. From Table 3, it can be seen that R2 values for FWO and VYZ range from 0.744 to 0.999 and 0.996 to 1.000 respectively. The Ea values calculated using VYZ method for all ˛ value are greater than the acceptable R2 value of 0.9, indicating good fitting and this is the case with all systems investigated. Therefore one can safely conclude that the Ea values calculated by using VYZ method are more reliable than the values obtained by using FWO method. According to the ICTAC Kinetics Committee recommendations, Vyazovkin et al. [39] stated that the differential method of Friedman is the most universal one because it is applicable to a wide variety of temperature programs. Unfortunately, this method is used rather rarely in actual kinetic analyses, whereas the most commonly used is the integral method of Ozawa–Flynn–Wall that has a very low accuracy and limited to linear heating rate conditions compared to the Vyazovkin method. This is found to be case in the present system. The apparent activation energy for the degradation of TNVP and UNVP varies from 49 to 137 kJ/mol and 63 to 148 kJ/mol respectively. Initially the apparent activation energy gradually increases upto the reaction extent level 0.45 for TNVP and then it was constant for further reaction extent levels. But in the case of UV cured polymer UNVP, the Ea values constantly increases from beginning to end of the reaction levels ˛ = 0.1–0.9 (63–148 kJ). This provides the clue that the thermally polymerized material is structurally different from that of the UV cured material. The degradation kinetics of all the materials investigated TDAX, TDMAX, TDCNX, UDAX, UDMAX and UDCNX show high Ea values when compared to the values for the TNVP and UNVP because the thermal and UV polymerization of NVP results in linear homopolymer. The degradation process of these materials may be easier and involves degradation steps requiring low energy. One such process is the ␤-elimination process in polyNVP (Scheme 3) leading to the release of pyrrolidone [40], which requires simple electron reorganization and not any bond breakage. Out of the thermal and UV polymerized NVPs, the UV cured NVP needs slightly higher activation energy than the thermally polymerized NVP indicating suitable structural differences between these materials resulting due to the initiation process adopted for the polymerization.

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Fig. 6. Comparison of apparent activation energy for degradation of thermally cured materials by VYZ method (A) and The plots between Ea and the reaction extent (␣) values for TDAX by the different kinetic methods (B).

Fig. 7. Comparison of apparent activation energy for degradation of UV cured materials by FWO method (A) and the plots between Ea and the reaction extent (˛) values for UNVP by the different kinetic methods (B).

Scheme 3. The possible degradation reaction of Poly NVP.

In the case of TDAX, the apparent activation energy values vary from 74 to 221 kJ/mol (˛ = 0.1–0.9). But UDAX shows gradual in the apparent activation energy values (128–141 kJ/mol) initially when ˛ = 0.1–0.2 and the value decreased upto the reaction extent level of 0.5. After reaction extent of 0.5, the Ea value increased steadily for higher values of ˛. The initial low Ea values were attributed to the existence of weak points in the polymer chains whereas the higher Ea values at the later stages of degradation was associated with high degrees of random scission of the main chain. Krongauz [41] reported similar results for the crosslink density

dependence of polymer degradation kinetics in photocrosslinked acrylates. The rotation of polymer segments is also consistent with low apparent activation energy of polyacrylate degradation and with an increase in activation energy with increase in crosslink density. Change from degradation due to rotation of polymer segments at low temperature to degradation through direct bond scission at higher temperatures may also explain increase in apparent activation energy with temperature. The apparent activation energy values for the degradation of TDMAX increased from 84 to 156 kJ/mol gradually with increasing extent of reaction (˛ = 0.1–0.9). The initial lower value of the activation energy is most likely associated with the initiation process that occurs at these weak links. As these weak links are consumed, the limiting step of degradation shifts toward the degradation initiated by random scission, which typically has greater activation energy. The very high values of Ea calculated at large extents of degradation explain the large amount of solid residue remained. An increase of Ea with ˛ has been also reported in literature for other types of polymers [42,43]. Achilias et al. [44] report similar result for the thermal degradation of light cured bisphenol A glycidyl dimethacrylate (Bis-GMA), bisphenol A ethoxylated dimethacrylate (Bis-EMA), urethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA). But in the case of UV polymerized DMAX (UDMAX) initially it need to very high Ea value is 179–150 kJ/mol (˛ = 0.1–0.35) compared to the thermally polymerized DMAX. After the reaction extent ˛ = 0.35, the apparent activation energy values increase gradually from 150 to 171 kJ/mol. So the UV polymerized materials need higher Ea value compared to the thermally polymerized materials. During

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4. Conclusions Liquid formulations NVP and DAX, DMAX and DCNX were prepared. These materials were cured by both thermal and UV methods. The structures of the monomers and polymers were confirmed by FTIR studies. The copolymerization of AX, MAX and CNX with NVP mainly affected the thermal degradation behavior of the material and the effect was explicit during the initial degradation stage. The thermal stabilities of copolymers formed by UV curing of the monomers were comparatively higher than the copolymers formed by thermal curing. The apparent activation energy (Ea) values for the thermal degradation of TDAX, UDAX, TDMAX, UDMAX, TDCNX and UDCNX were high compared to TNVP and UNVP. All the materials cured by UV irradiation showed higher Ea values when compared to the materials cured by thermal means. In all the thermosets investigated, irrespective of the polymerization method, the Ea value increased continuously as the ˛ value increased. Due to the formation of highly crosslinked network structures during the thermal degradation, the Ea increased as extent of degradation of the materials increased. Acknowledgements The authors would like to acknowledge the Board of Research in Nuclear Science (BRNS), Department of Atomic Energy (DAE), Government of India, BARC, Mumbai 400 085 for financially supporting this work under the Grant 2010/35/5/BRNS/1279 dated 11-08-2010. The authors wish to thank Lalit Varshney and K.S.S. Sharma of BARC for their constant encouragement and support. Sincere thanks to the Management and the Principal of Kamaraj College of Engineering and Technology, S.P.G.C. Nagar, K. Vellakulam Post 625 701, India for providing all the facilities to do this work. References

Scheme 4. The possible addition reactions for DMAX.

the UV polymerization of DMAX, the 2␲ + 2␲ cycloaddition reaction may occur between the polymerizable olefinic groups (Scheme 4). Such 2␲ + 2␲ reactions are forbidden during thermal polymerization of DMAX. Due to the variation in the polymerization behavior of DMAX due to the difference in initiation, the structure of the crosslinked polymer varies leading to variation in the activation energy for the degradation. The TDCNX shows increasing apparent activation energy value from 108 to 156 kJ/mol with increasing extent of reaction (˛) from 0.1 to 0.8 and the Ea value for the UDCNX shows increasing Ea values from 145 to 165 kJ/mol. Initially the apparent activation energy values show a slight decrease (from 145 to 120 kJ/mol) with increasing reaction extent values (˛ = 0.1–0.35). After that the Ea values gradually increase upto further reaction extent levels. In the curing process of DCNX, apart from the trimerization of the cyanate groups, the maleimide double bond entering in the cyclization reaction with the cyanate groups is highly probable. Hence the system (DCNX) is getting more and more complex than the other investigated materials. Due to the presence of more aromatic units and crosslinking, the polymerized cyanate functionalized material requires higher energy for degradation when compared to other materials investigated.

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