Synthesis,characterization and thermal degradation kinetics of photoresponsive graft copolymers

Abstract

A macroinitiator was prepared by copolymerization of styrene (St) with 7-methacryloyloxy-4-chloromethylcoumarone (MAOCMC). Grafting studies of coumarone with methyl methylacrylate were carried out in the presence of the macroinitiator poly(7-methacryloyloxy-4-chloromethylcoumarone-co-styrene) and with the catalyst of copper(I) bromide/2,2′-bipyridyne at 110°C. The activation energy valuations of graft copolymers acquired by Coats–Redfern, Tang and Flynn–Wall–Ozawa methods were designated to be 212.69, 214.44 and 223.57 kJ mol⁻¹, respectively. For the outcomes were compared with these valuation differential methods and discrepant integral were used. In terms of experiential outcomes, the reaction mechanism was a dimensional diffusion (Dn) deceleration type in the transformation range worked.

Keywords

Thermogravimetric analysis (TGA)coumarone activation energy graft copolymerization atom transfer radical polymerization (ATRP)

Introduction

(Meth)Acrylic copolymers have reached foremost significance in a variety of ways in industrial practices.¹ ^– ³ For instance, copolymers comprising altered coumarone are used in the manufacture of superb fluorescent properties with high photoluminescence (PL) quantum efficiency, superb photostableness and ample spectrum range.^4,5

They are widely used in the areas of polymer science, medicine, laser dyes and biology.⁶ Furthermore, copolymerization is beneficial and a significant way to advance novice materials. Copolymerization modifies both intermolecular and the intramolecular powers executed among polymer fragments. Hence, certain properties, such as, the glass transition temperature and the procedural decomposition temperatures with respect to thermal degradation may diversify within ample limits.⁷ Because of the compatibilization ingredients in polymer blends, thermoplastic elastomers, surfactants, adhesives and adjuvants in high-impact materials, graft copolymers have been used in industrial practices and widely worked.⁸ Grafting reactions have been used along with atom transfer radical polymerization to generate diversified graft copolymers in which the polymer involves a pendent group with atom transfer radical polymerization (ATRP) initiator functionality, like benzylic halides, α-halocarbonyls and aromatic sulphonyl chlorides.⁹ Activated carbon–halogen (C-X) bond with many of the organic compounds can be used qua ATRP initiators. Allyl,¹⁰ adenosine and uridine,¹¹ thiol,¹² acid¹³ and hydroxyl¹⁴ groups have been accomplishedly introduced into polymer ends by cautiously selecting the matching initiators. Some novice functional initiators were also offered by discrepant research groups. Bipyrridine,¹⁵ oxazoline,¹⁶ pyrrole,¹⁷ norbornenyl,¹⁸ aldehyde,¹⁹ sole-walled carbon nanotubes,²⁰ pyrene²¹ and N,N-dimethylaniline²² were used as functional initiators to prepare end-capped polymers. However, they were seldom considered as light sensitive initiators. Thermal degradation works for polymers are required since many practices rely on their thermal stability. Owing to its frugality and the knowledge provided by an ordinary thermogram, thermogravimetric analysis is a technique broadly used.²³ Knowledge about the kinetics of decomposition and the mechanism function may be acquired and the kinetic data acquired from thermogravimetric analysis may be used as criteria for the choice of a polymer with suitable experiential procedures. The kinetic parameters of degradation process, like activation energies, the rate fixed, Arrhenius pre-exponential factors and reaction orders can be evaluated in light of data registered from thermograms.

In this article, a photosensitive ATRP initiator and graft copolymers were prepared using ATRP method. The evident activation energy (E_a) and the kinetics of the thermal degradation mechanism of polymers were investigated in discrepant heating rates.

Experiential section

Materials

Before use, styrene (St) and methyl methacrylate (MMA) were distilled under vacuum. 2,2′-bipyridyne (bpy), copper(I) bromide (CuBr), 2,2′-azobisisobutyronitrile, tetrahydrofuran (THF), dichloromethane and ethanol were used without further purification.

Instrumentation

Infrared spectra were registered on a Perkin–Elmer Spectrum One Fourier transform infrared spectroscopy (FTIR; Waltham, Massachusetts, USA) spectrometer. Proton nuclear magnetic resonance (¹H NMR) spectra was acquired on a 400 MHz Bruker AVIII 400 machines (Billerica, Massachusetts, USA), using tetramethylsilane as an interior standard and deuterated chloroform as the solvent. Ultraviolet–visible (UV–vis) spectra were registered using a Shimadzu spectrophotometer (Japan). Thermal stability studies were carried out on a Shimadzu thermal gravimetric analyzer (TGA)-50 thermobalance under nitrogen (N₂) flow with a heating rate of 10°C min⁻¹.

Synthesis of macroinitiator poly(7-methacryloyloxy-4-chloromethylcoumarone-co-styrene) (P(MAOCMC-co-St))

Synthesis of poly(7-methacryloyloxy-4-chloromethylcoumarone-co-styrene) (P(MAOCMC-co-St)), was carried out by free radical copolymerization. MAOCMC²⁴ in the copolymer was 5% (by mole) lean upon peak integrations at 4.57 ppm (–CH₂–Cl) and 6.46–7.25 ppm (aromatic protons) in the ¹H NMR spectrum. The copolymer was refined and dried, dissolved in dichloromethane, and reprecipitated into ethanol. Then, the copolymer was dried under vacuum at 40°C for 24 h.

Synthesis of graft copolymers by ATRP

Atom transfer radical graft copolymerizations were carried out in a 25-mL flask equipped with a condenser. The necessary amount of the macroinitiator P(MAOCMC-co-St) was dissolved in dichloromethane, and the following were then added to the solution: the suitable monomer (methyl methacrylate), macroinitiator, the catalyst 2,2′-bipyridyl and CuBr at a mole ratio of 100:1:2:1, respectively. The solution contained similar molar amounts of group MAOCMC and CuBr. Argon gas was moved through the solution for 15 min. The polymerization flask was permitted to react at 110°C for the required time, after which, the graft copolymer was precipitated into slender acidic ethanol. The graft copolymers, poly((7-methacryloyloxy-4-chloromethylcoumarone-co-styrene)-g-(methylmethacrylate)) P[(MAOCMC-co-St)-g-MMA], were cleansed from dichloromethane solution by reprecipitating with slender acidic ethanol.

Thermal decomposition kinetics

The practice of dynamic TGA methods keeps huge procurement as a tool for untying the mechanisms of chemical and physical process that happen throughout polymer degradation. Isothermal decomposition reactions of solid state can be denoted as follows:

\frac{d α}{d T} = \frac{A}{β} e^{- E / R T} f (α)

where α is the level of transformation, A is the pre-exponential factor (min⁻¹), T is the absolute temperature (K), R is the gas fixed (8.314 J mol⁻¹ K⁻¹), f (α) and E is the activation energy (kJ mol⁻¹) is a function rely on the reaction mechanism. The rearranging of equation (1) and the integration of both sides of the equation gives rise to the following denotation:

g (α) = \int_{0}^{α_{p}} \frac{d α}{f α} = \frac{A}{β} \int_{0}^{T_{p}} e^{- E / R T} d T

where g(α) is the integral function of transformation. In polymers, the degradation process conforms to either a retardation function or a sigmoidal function and T_p corresponds peak temperature and α_p is a level of transformation at peak temperature. Table 1 indicates that discrepant denotations of g(α) for the dissonant solid-state mechanisms.^25
–27

Table 1.

Algebraic expressions for g(α) for the most frequently used mechanisms of the solid-state processes.

Symbol	g(α)	Solid-state process
Sigmoidal curves
A₂	[−ln(1 − α)]^1/2	Nucleation and growth [Avrami equation (1)]
A₃	[−ln(1 − α)]^1/3	Nucleation and growth [Avrami equation (2)]
A₄	[−ln(1 − α)]^1/4	Nucleation and growth [Avrami equation (3)]
Deceleration curves
R₁	α	Phase-boundary-controlled reaction (one-dimensional movement)
R₂	[1 − (1 − α)^1/2]	Phase-boundary-controlled reaction (contraction area)
R₃	[1 − (1 − α)^1/3]	Phase-boundary-controlled reaction (contraction volume)
D₁	α ²	One-dimensional diffusion
D₂	(1 − α) ln(1 − α) + α	Two-dimensional diffusion
D₃	[1 − (1 − α)^1/3]²	Three-dimensional diffusion (Jander equation)
D₄	(1 − 2/3α) − (1 − α)^2/3	Three-dimensional diffusion (Ginstling–Brounshtein equation)
F₂	1/(1 − α)	Random nucleation with two nuclei on the individual particle
F₃	1/(1 − α)²	Random nucleation with three nuclei on the individual particle

Flynn–Wall–Ozawa method

This method which can designate the E_a without knowledge of the reaction order was derived from the integral method. It is used to designate the E_a for serving transformation valuations.^28,29 Along with the Doyle approximation,³⁰ equation (2) may then be integrated to serve the following in a logarithmic form:

log β = log [\frac{A E}{g (α) R}] - 2.315 - \frac{0.457 E}{R T}

The E_a for dissonant transformation valuations can be conjectured from a plot of ln β versus 1000/T.

Tang method

With an approximation formula used and with the logarithms of the side taken for the solution of equation (2), the following equation can be acquired:

ln (\frac{β}{T^{1.894661}}) = ln [\frac{A E}{R g (α)}] + 3.635041 - 1.894661 ln E - \frac{1.001450 E}{R T}

Plots of 1/T versus ln(β /T^1.894661) yield a group of straightaway lines. The E_a can be acquired from the slope of −1.001450E/R of the regression line.³¹

Coats–Redfern method

An asymptotic approach for the resolution of equation (2) uses the Coats–Redfern method:³²

ln \frac{g (α)}{T^{2}} = ln \frac{A R}{β E} - \frac{E}{R T}

The E_a for each degradation process listed in Table 1 can be designated from a plot of 1000/T versus ln g(α).

Results and discussion

Synthesis and characterization of the macroinitiator

In this study, in order to carry out one-armed grafting on P(MAOCMC-co-St), resulting in P[(MAOCMC-co-St)-g-MMA] (Figure 1). Consequently, it was found that the chloro group in the macroinitiator was to be in ratio of 5% (by mole). The macroinitiator for ATRP, P(MAOCMC-co-St), was acquired by copolimerization. Characteristic bands of the phenyl in styrene units at 3050 and 3075 cm⁻¹ (=C–H stretching vibration), at 1602 cm⁻¹ (C=C stretching vibration) and 756 and 700 cm⁻¹ (=C–H out-of-plane bending of phenyl ring) were observed. Figure 2 indicates that the FTIR spectra of P(MAOCMC-co-St) has a characteristic absorbance at 1737 cm⁻¹ (C=O stretching vibration), 3073 (–CH stretching of the aromatic ring), 2986 (–CH₃), 1737 (broad, C=O in ester), 1614 (C=C stretching of the aromatic ring), 1230 (asymmetric C–O–C) and 1142 (symmetric C–O–C). 1454 cm⁻¹ and 1496 cm⁻¹ were assigned to typical C–C vibrations of the polystyrene chain.³³

Figure 1.

Synthesis of macroinitiator and graft copolymers.

Figure 2.

The FTIR spectra of (a) P(MAOCMC-co-St), (b) P[(MAOCMC-co-St)-g-MMA] after 30 h of grafting and (c) P[(MAOCMC-co-St)-g-MMA] after 60 h of grafting. FTIR: Fourier transform infrared spectroscopy; P(MAOCMC-co-St): poly(7-methacryloyloxy-4-chloromethylcoumarone-co-styrene); P[(MAOCMC-co-St)-g-MMA]: poly((7-methacryloyloxy-4-chloromethylcoumarone-co-styrene)-g-(methylmethacrylate)).

Synthesis and characterization of the graft copolymers

The macroinitiator, P(MAOCMC-co-St), was used to initiate ATRP of MMA in the presence of the catalyst CuBr/bpy at 110°C in THF (1 mL per 0.1 g of the macroinitiator). The molar ratio of chloromethyl group (CH₂–Cl):CuBr:bpy used in all grafting reactions was 1:1:2. The carbonyl band at 1737 cm⁻¹ (C=O) of the macroinitiator shifted to 1731 cm⁻¹ (stretching vibration of C=O in MMA units) after a grafting of 30 h and 60 h (Figure 2). ¹H NMR spectrum of P[(MAOCMC-co-St)-g-MMA] after 30 h and 60 h of reaction indicates characteristic signals at 1.80 ppm (–CH₂–C in MMA units), 3.70 ppm (–O–CH₃ in MMA units), and 1.0 and 0.85 ppm (CH₃ protons on the MMA chain), which gained intensity over time. The other assignments are presented in detail in Figure 3.

Figure 3.

The ¹H NMR spectra of (a) P(MAOCMC-co-St), (b) P[(MAOCMC-co-St)-g-MMA] after 30 h of grafting and (c) P[(MAOCMC-co-St)-g-MMA] after 60 h of grafting. ¹H NMR: proton nuclear magnetic resonance; P(MAOCMC-co-St): poly(7-methacryloyloxy-4-chloromethylcoumarone-co-styrene); P[(MAOCMC-co-St)-g-MMA]: poly((7-methacryloyloxy-4-chloromethylcoumarone-co-styrene)-g-(methylmethacrylate)).

UV–vis spectra of the P(MAOCMC-co-St) and P[(MAOCMC-co-St)-g-MMA] after 30 h and 60 h of reaction indicates UV–vis absorption at 320 nm. The UV–vis absorption of graft polymers (λ_max = 320 nm) decreased after grafting reactions. The other assignments are presented in detail in Figure 4.

Figure 4.

The UV–vis spectra of (a) P(MAOCMC-co-St), (b) P[(MAOCMC-co-St)-g-MMA] after 30 h of grafting and (c) P[(MAOCMC-co-St)-g-MMA] after 60 h of grafting. UV–vis: ultraviolet–visible; P(MAOCMC-co-St): poly(7-methacryloyloxy-4-chloromethylcoumarone-co-styrene); P[(MAOCMC-co-St)-g-MMA]: poly((7-methacryloyloxy-4-chloromethylcoumarone-co-styrene)-g-(methylmethacrylate)).

Thermal decomposition kinetics

Thermal decomposition curves of the graft copolymers and macroinitiator were carried out at discrepant heating rates of 5, 15, 25, and 35°C min⁻¹. After complete degradation, the initial decomposition temperature, decomposition temperature at 50% weight loss, weight loss (%) at 300°C and 350°C and residual mass at 500°C were designated from these curves and are presented in Tables 2 to 4.

Table 2.

TGA data of macroinitiator at different heating rates.

Reaction rate (°C min⁻¹)	T _i	T_50% (°C)	Weight loss at 300°C (%)	Weight loss at 350°C (%)	Residue at 500°C (%)
5	220.9	373.3	8	29	13.13
15	233.2	391.8	6	25	14.92
25	241.4	398.3	5	22	12.93
35	246.9	405.4	4	20	12.33

TGA: thermogravimetric analysis; T_i: initial decomposition temperature (°C); T_50%: temperature at 50% decomposition.

Table 3.

TGA data of graft copolymer (30 h) at different heating rates.

Reaction rate (°C min⁻¹)	T _i	T_50% (°C)	Weight loss at 300°C (%)	Weight loss at 350°C (%)	Residue at 500°C (%)
5	170.3	382.4	7	20	6.1
15	199.7	396.5	6	14	9.1
25	193.3	410.1	6	12	7.1
35	207.4	418.9	5	10	7.1

TGA: thermogravimetric analysis. T_i: initial decomposition temperature (°C); T_50%: temperature at 50% decomposition.

Table 4.

TGA data of graft copolymer (60 h) at different heating rates.

Reaction rate (°C min⁻¹)	T _i	T_50% (°C)	Weight loss at 300°C (%)	Weight loss at 350°C (%)	Residue at 500°C (%)
5	202.7	378.3	8	22	3.5
15	206.4	394.2	7	13	6.9
25	210.1	398.9	5	12	3.7
35	223.8	403.6	5	11	4.5

TGA: thermogravimetric analysis; T_i: initial decomposition temperature (°C); T_50%: temperature at 50% decomposition.

These curves indicate that, for graft copolymer (60 h) at 500°C, the residue decreased 3.5% with a 5°C min⁻¹ heating rate. The 10°C min⁻¹ intervals between measurements were chosen to avoid the imbrication of inflection point temperatures.²⁷ The E_a can be designated with the Flynn–Wall–Ozawa (FWO) method (equation (3)) from a linear suiting of 1000/T versus log β at discrepant transformations. Since this equation was derived from the Doyle approach, just transformation valuations in the range of 5–20% can be used. For this study, we used the transformation valuations of 3, 5, 7, 9, 12, 15 and 18%. Figures 5 to 7 indicate that the suiting straightaway lines are almost parallel, thus indicating the applicability of this method to our polymer in the transformation range worked.³⁴ Tables 5 to 7 indicate that the activation energies corresponding to the discrepant transformations conjectured with the FWO method. A mean value of 309.69 (macroinitiator), 307.34 (graft copolymer 30 h) and 213.44 (graft copolymer 60 h) kJ mol⁻¹ were found from these valuations.

Figure 5.

FWO method applied to the experimental data (3–18%) of macroinitiator. FWO: Flynn–Wall–Ozawa.

Figure 6.

FWO method applied to the experimental data (3–18%) of graft copolymer (30 h). FWO: Flynn–Wall–Ozawa.

Figure 7.

FWO method applied to the experimental data (3–18%) of graft copolymer (60 h). FWO: Flynn–Wall–Ozawa.

Table 5.

E_a values of macroinitiator obtained with the FWO and Tang methods.

FWO method			Tang method
α (%)	E_a (kJ mol^m1)	R	E_a (kJ mol⁻¹)	R
3	149.50	0.9942	148.07	0.9946
5	189.11	0.9950	165.61	0.9873
7	217.83	0.9989	229.21	0.9817
9	254.25	0.9940	257.48	0.9937
12	349.29	0.8933	356.67	0.8844
15	503.93	0.9297	517.13	0.9228
18	503.93	0.9297	456.67	0.8977
Mean	309.69		304.40

E_a: activation energy; FWO: Flynn–Wall–Ozawa; α: level of transformation.

Table 6.

E_a values of graft copolymer (30 h) obtained with the FWO and Tang methods.

FWO method			Tang method
α (%)	E_a (kJ mol⁻¹)	R	E_a (kJ mol⁻¹)	R
3	503.93	0.9297	524.61	0.9334
5	393.32	0.9919	318.84	0.9952
7	503.93	0.9297	517.96	0.9253
9	222.85	0.9731	238.11	0.9316
12	167.91	0.9297	166.28	0.9219
15	188.65	0.9584	187.48	0.9548
18	170.82	0.9831	168.26	0.9805
Mean	307.34		303.07

E_a: activation energy; FWO: Flynn–Wall–Ozawa; α: level of transformation.

Table 7.

E_a values of graft copolymer (60 h) obtained with the FWO and Tang methods.

FWO method			Tang method
α (%)	E_a (kJ mol⁻¹)	R	E_a (kJ mol⁻¹)	R
3	503.93	0.9297	522.95	0.9493
5	150.08	0.9411	188.55	0.9043
7	134.44	0.9905	131.66	0.9905
9	161.18	0.9511	164.94	0.9830
12	146.26	0.9247	150.35	0.9674
15	184.10	0.9622	183.15	0.9586
18	214.12	0.9745	223.39	0.9739
Mean	213.44		223.57

E_a: activation energy; FWO: Flynn–Wall–Ozawa; α: level of transformation.

For the detection of E_a, some other isotransformation method used in this study was the Tang method. So far, as this method (using equation (4)), the E_a can be conjectured from a plot of 1000/T versus ln (β/T^1.894661) suit to a straightaway line. Figures 8 to 10 indicate the suiting straightaway lines designated by the Tang method applied to the experiential data at diversified transformation valuations in the range of 3–18%.

Figure 8.

Tang method applied to the experimental data (3–18%) of macroinitiator.

Figure 9.

Tang method applied to the experimental data (3–18%) of graft copolymer (30 h).

Figure 10.

Tang method applied to the experimental data (3–18%) of graft copolymer (60 h).

The conjectured outcomes are summarized in Tables 5 to 7. The mean value of the E_a was very contiguous to the valuation acquired with the FWO method.

In comparison to other methods, Tang and FWO methods are advantageous, because they don’t require previous knowledge of the reaction mechanism for designating the E_a. These methods have been used by some authors, for control of a mechanism of thermodegradation models.³⁵

The E_a for each g(α) function listed in Table 1 was offered by Coats and Redfern using equation (5). These valuations were acquired at fixed heating rates from the suiting of 1000/T ln [g (α)/T²] plots. In this study, we used similar transformation valuations. Tables 8 and 9 indicate correlations for transformations in the range of 3–18% at fixed heating rates of 5, 15, 25 and 35°C min⁻¹ and E_a. At all the heating rates, these tables indicate that, the E_a are in better agreement with those acquired using the FWO method that corresponded to a Dn type mechanism.

Table 8.

E_a values of graft copolymer (60 h) obtained with the CR method.^a

	5°C min⁻¹		15°C min⁻¹
Mechanism	E_a (kJ mol⁻¹)	R	E_a (kJ mol⁻¹)	R
A₂	31.84	0.9982	53.87	0.9958
A₃	20.95	0.9982	35.50	0.9958
A₄	15.87	0.9982	26.93	0.9958
R₁	101.26	0.9965	103.42	0.9965
R₂	62.41	0.9983	105.60	0.9962
R₃	62.85	0.9983	106.35	0.9960
D₁	122.23	0.9984	206.85	0.9965
D₂	123.94	0.9984	209.72	0.9963
D₃	123.08	0.9983	212.69	0.9960
D₄	124.52	0.9983	210.71	0.9962
F₂	5.29	0.9719	8.9	0.9585
F₃	10.58	0.9719	17.81	0.9585

E_a: activation energy; CR: Coats–Redfern.

^aE_a values obtained with the CR method for solid-state processes at heating rates of 5 and 15°C min⁻¹.

Table 9.

E_a values of graft copolymer (60 h) obtained with the CR method.^a

	25°C min⁻¹		35°C min⁻¹
Mechanism	E_a (kJ mol⁻¹)	R	E_a (kJ mol⁻¹)	R
A₂	51.38	0.9833	53.87	0.9958
A₃	33.92	0.9833	35.50	0.9958
A₄	33.92	0.9833	26.93	0.9958
R₁	98.65	0.9830	103.42	0.9965
R₂	100.75	0.9832	105.60	0.9962
R₃	101.48	0.9832	106.35	0.9960
D₁	197.30	0.9830	206.85	0.9965
D₂	200.09	0.9831	209.72	0.9963
D₃	202.96	0.9832	212.69	0.9960
D₄	201.04	0.9831	210.71	0.9962
F₂	8.61	0.9722	8.9	0.9585
F₃	17.22	0.9722	17.81	0.9585

E_a: activation energy; CR: Coats–Redfern.

^aE_a values obtained with the CR method for solid-state processes at heating rates of 25°C and 35°C min⁻¹.

Conclusions

The ATRP macroinitiator was prepared by copolymerization of St with MAOCMC. The grafting of the macroinitiator with MMA was carried out at 110°C. The FTIR and ¹H NMR data confirmed the structures of all the graft copolymers and the starting materials. The thermal stability of the graft copolymers (30 h and 60 h) indicated higher thermal stability. The thermal degradation kinetics of the polymers was also investigated with diversified methods of TGA. The E_a valuations of the graft copolymer acquired by Tang, FWO and CR methods were designated to be 223.57, 214.44 and 212.69 kJ/mole, respectively. The analysis of the outcomes acquired using CR method indicated that the degradation mechanism of graft copolymer under N₂ atmosphere is a dimensional diffusion mechanism in transformation range worked. It is noted that a diffusion type kinetic model is proposed here. This kinetic model is very rare in polymer degradation studies and has been reported in the studies that thermal degradation of polyethylene and polyphenol which have a diffusion kinetic model.^36,37

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors wish to thank FUBAP-1792 for financial support of this project.

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