Thermal stability of poly(methyl methacrylate-co-butyl acrylate) and poly(styrene-co-butyl acrylate) polymers.
Thermal stability of polymeric materials is an important physical property which has led to many applications. The properties may be profoundly affected by the presence of particular sequences of comonomers (1) as well as of quite small proportions of additives. Comonomer sequences such as methacrylic acid or anhydride, as well as acrylic acid lead to the increased thermal stability of copolymers (1-4).
Addition of N-methylolacrylamide (NMA) in a polymer system can lead to crosslinking with more or less improved thermal stability of copolymers (5-7). In this investigation, the influence of acrylic acid and/or N-methylolacrylamide on the improvement of stability of a series of copolymers of methyl methacrylate (MMA) or styrene (S) with butyl acrylate (BA) has been studied. The above copolymers are widely used in all types of paint formulations: interior and exterior; semigloss and gloss; and primers and topcoats.
The thermal properties of investigated copolymer systems were measured by the programmed thermogravimetry. Kinetic parameters were determined by using MacCallum-Tanner's approach (8).
Copolymers of poly(methyl methacrylate-co-butyl acrylate), (MMA-BA) and poly(styrene-co-butyl acrylate) (S-BA) with an addition of acrylic acid (AA) and/or N-methylolacrylamide (NMA) as a crosslinking agent were prepared by the free radical emulsion polymerization. Copolymerization was performed at 50 [degrees] C, with ammonium persulfate as an initiator, following the procedure described in literature (9). All used specimens were commercial products and were not specially purified. The initial composition of investigated systems is presented in Table 1.
Thermogravimetric (TGA) and differential thermogravimetric (DTG) curves for the studied copolymer systems were obtained by using TGA V4 DuPont 2000 instrument in the temperature range of 50-450 [degrees] C. Samples (average size 2.5 mg) were degraded under the nitrogen flow of 50 ml/min, at the heating rate of 10 [degrees] C/min.
The detailed analysis of particular copolymer systems based on methyl methacrylate and/or styrene with added acrylic acid (M1, S1 in Table 1) and N-methylolacrylamide (M2, S3 in Table 1) were made in order to show how small amounts of comonomers can [TABULAR DATA FOR TABLE 1 OMITTED] change the mechanisms of thermal degradation. The kinetic parameters of degradation were measured at various heating rates of 5, 10 and 20 [degrees] C/min under a continuous flow of nitrogen.
Differential Scanning Calorimetry
A Perkin-Elmer differential scanning calorimeter (model DSC-7) was used for determination of glass transition temperatures, in the temperature range from -60 to 160 [degrees] C, at a heating rate of 20 [degrees] C/min, in a nitrogen flow.
RESULTS AND DISCUSSION
The TGA and DTG curves of the investigated copolymers: poly(methyl methacrylate-co-butyl acrylate) and poly(styrene-co-butyl acrylate) of various composition, obtained at a heating rate of 10 [degrees] C/min are shown in Figs. 1-4, respectively.
The values of glass transition temperature are presented in Table 1. TGA curves for both copolymer series show the existence of a one-step degradation mechanism above T9 in a relatively narrow temperature range from 350-410 [degrees] C/min [ILLUSTRATION FOR FIGURE 1 AND 3 OMITTED].
Initial decomposition temperature and DTG maximum temperatures corresponding to the maximum degradation rate are presented in Table 2. The shift of DTG maximum to higher temperatures as shown in Fig. 2 is related to the improvement in stability of copolymers with methyl methacrylate (system M0), caused by the addition of a small amount of acrylic acid and/or N-methylolacrylamide (systems M1-M4). [TABULAR DATA FOR TABLE 2 OMITTED] The highest stability has copolymer M4 which contains 4 mass % of NMA with 2 mass % of AA.
On the contrary, the addition of AA and NMA in copolymer systems with styrene, TG curves and DTG maximum were lowered as well as thermal stability, especially when higher amounts of crosslinking agent were added (3.7 mass% of NMA and 3.3 mass% of AA) (Table 1 and [ILLUSTRATION FOR FIGURES 3, 4 OMITTED]). The initial decomposition temperature of copolymer systems with styrene sequences was higher, but the addition of NMA and specially AA (system S5 in Fig. 4) led to the deterioration of initial properties. According to the literature (10, 11), polystyrene (PS) is thermally more stable than poly(methyl methacrylate) (PMMA). The initial decomposition temperature of PS is 328.4 [degrees] C while the initial decomposition temperature of PMMA has a value of 237 [degrees] C (Table 2).
Copolymerization of methyl methacrylate with butyl acrylate leads to an increase of thermal stability in comparison with the thermal stability of homopolymer PMMA: the initial decomposition temperature of copolymer MMA-BA (system M0) is 337 [degrees] C while the initial decomposition temperature of PMMA is 237 [degrees] C (Table 2). The increased thermal stability of copolymer M0 could be due to the blocking action of BA sequences, which unlike the PMMA chain does not depolymerize below 300 [degrees] C. The thermal stabilities of MMA-BA copolymers with NMA and/or AA have also been found to be higher than those of homopolymer PMMA. Addition of a small amount of crosslinking agent in the presence of AA, which acts as an acid catalyst, causes a progressive shift of DTG maxima from 380 [degrees] C for MMA-BA (system M0) to 386 [degrees] C for system M4 with 2mass% AA and 4mass% NMA. The presence of crosslinking structures is found to have a significant stabilizing effect on the MMA-BA copolymer chain.
On the other hand, thermal stabilities of S-BA copolymers which also decompose in a one-step mechanism are only slightly higher than those of homopolymer PS. The small amount of NMA and/or AA did not change or even lower the thermal stability of copolymers with styrene. The butyl acrylate (BA) sequences (system S0) only slightly change the degradation behavior (initial decomposition temperature of system S0 is 344 [degrees] C compared with 328.4 [degrees] C for PS) (Table 2).
Kinetic parameters such as activation energy ([E.sub.a]), pre-exponential factor (A) and reaction order, were evaluated from programmed thermogravimetric curves [ILLUSTRATION FOR FIGURE 5 OMITTED] obtained experimentally by using a multiple heating rate procedure. Kinetic parameters provide a quantitative measure of the thermal stability.
The methods employed give mean values of the parameters which control the degradation. In the present study, the integral method using the MacCallum and Tanner procedure (8) has been chosen. MacCallum and Tanner proposed the complete integrated rate equation (1), as follows:
logF(C) = log(A[E.sub.a]/[Beta]R) - [0.48[E.sub.a].sup.044]
- (0.45 + 0.22[E.sub.a])/T [multiplied by] [10.sup.-3] (1)
where F(C) is a mathematical expression that depends upon the order of the reaction and the amount of unreacted residue; [E.sub.a] is activation energy; A is pre-exponential factor; T is absolute temperature (K); R is gas constant (8.314 kJ/mol) and [Beta] the heating rate (dT/dt, K/min). F(C) has been evaluated by MacCallum [TABULAR DATA FOR TABLE 3 OMITTED] and Tanner (8) according to the reaction order, n. For n = 1, where the activation energy is independent of the heating rate, the authors gave the expression:
F(C) = ln[1/(1 - C)] (2)
The term (1-C) is the fractional conversion value, which according to Mazon-Arechederra and Barrales-Rienda (12), can be evaluated by the equation (3):
(1 - C) = (1 - W/[W.sub.o]) (3)
where [W.sub.o] is the initial weight of the sample and W is the weight consumed at temperature T. If log ln [1/(1-C)] is plotted against 1/T, a series of straight lines is obtained for each of the heating rates used if the reaction is of the first order.
The primary thermograms of four copolymer systems [ILLUSTRATION FOR FIGURE 5 OMITTED], obtained at different heating rates show the typical sigmoidal form characteristic of a great number of polymers (12).
The investigated copolymers degrade continuously in one stage at low and high heating rates. This proves that no other preferred or competitive and simultaneous process occurs at high temperatures. The thermograms of (M1), (M2), (S1) and (S3) and DTG curves of some copolymers are shown in Figs. 5 and 6. Evidently, curves of thermal degradation and the DTG maxima are shifted towards higher temperatures with an increasing heating rate, which is in accordance with kinetic theory (13). Numerical data of the initial decomposition temperature and DTG maxima at different heating rates are shown in Table 3. Assuming that the degradation reaction follows the first order rate law i.e. n = 1, Eq 2 has been applied for the calculation of the activation energy of copolymers (M1) and (S1), which contain a small quantity of acrylic acid, and copolymers (M2) and (S3), which contain N-methylolacrylamide. This approach is valid when the plot of log ln [1/(1-C)] against 1/T gives a straight line.
From the plot log ln [1/(1-C)] vs. 1/T the slope and intercept of straight lines (in Fig. 7) provide data which allow the calculation of kinetic parameters; activation energy, [E.sub.a], and pre-exponential factor, A. Numerical values of the kinetic parameter of copolymers [TABULAR DATA FOR TABLE 4 OMITTED] (M1), (M2), (S1) and (S3) are given in Table 4. The linear relationship obtained in Fig. 7 indicates the validity of the first order assumption. The straight lines in Fig. 7 show the linear fitting of experimental results, and correlation coefficients vary between 0.994 and 0.999. The plot of activation energy against heating rate in Fig. 8 emphasizes that there is no appreciable dependence of activation energy on the heating rate. According to Grassie and Farish (14), mechanism of the thermal degradation of polystyrene consists of chain homolysis and disproportionation below 300 [degrees] C. Above 300 [degrees] C, the formation of monomer (about 45%), dimer and smaller amounts of higher oligomers prevail, thus leading to the decrease of molecular mass. In copolymer S-BA, BA sequences did not change the degradation mechanism. The thermal degradation of PMMA yields almost 100% of monomer (15).
The increase in thermal stability of MMA-BA copolymers could be explained by the stabilizing influence of BA sequences. The results show that the crosslinking reaction after addition of small amount of NMA, initiated by AA, leads to the improved thermal stability. Parameters [E.sub.a] and A are increased by the addition of NMA and/or AA depend on the amount of crosslinking agent.
Thermal degradation of poly(methyl methacrylate-co-butyl acrylate) and poly(styrene-co-butyl acrylate) takes place in a relatively narrow temperature range of 350-410 [degrees] C.
Butyl acrylate sequences increased the thermal stability of copolymers much more in the case of MMA-BA copolymers compared to those of the homopolymers, due to the stabilizing effect of BA units. The presence of small amounts of crosslinking agent increased the thermal stability of copolymers. This effect was more pronounced in MMA-BA copolymers in combination with AA, which obviously acted as an acidic catalyst.
The reactions of thermal degradation were found to be of the first order under the experimental conditions described herein. It was shown that MacCallum and Tanner's procedure based of calculation of kinetic parameters of thermal degradation is a very convenient method for the calculation of thermal stability of copolymers.
This work was supported by The Ministry of Science and Technology of the Republic of Croatia.
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|Title Annotation:||5th International Conference on Polymer Characterization|
|Author:||Leskovac, Mirela; Kovacevic, Vera; Fles, Dragutin; Hace, Drago|
|Publication:||Polymer Engineering and Science|
|Date:||Mar 1, 1999|
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