Glass transition temperatures of butyl acrylatemethyl methacrylate copolymers

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1 See discussions, stats, and author profiles for this publication at: Glass transition temperatures of butyl acrylatemethyl methacrylate copolymers Article in Journal of Polymer Science Part B Polymer Physics September 1999 Impact Factor: 3.83 DOI: /(sici) ( )37:17<2512::aid-polb22>3.0.co;2-2 CITATIONS 21 READS 1,238 3 authors, including: Marta Fernández-García Spanish National Research Council 161 PUBLICATIONS 2,219 CITATIONS Rocío Cuervo-Rodríguez Complutense University of Madrid 33 PUBLICATIONS 256 CITATIONS SEE PROFILE SEE PROFILE Available from: Marta Fernández-García Retrieved on: 11 May 2016

2 Glass Transition Temperatures of Butyl Acrylate Methyl Methacrylate Copolymers M. FERNÁNDEZ GARCíA, R. CUERVO RODRIGUEZ, E. L. MADRUGA Instituto de Ciencia y Tecnología de Polímeros (C.S.I.C.), Juan de la Cierva Madrid, Spain Received 24 February 1999; revised 5 May 1999; accepted 10 May 1999 ABSTRACT: The glass transition temperatures T g of butyl acrylate methyl methacrylate copolymers obtained by free radical polymerization in 3 and 5 mol/l benzene solution have been measured using differential scanning calorimetry (DSC) and the values have been correlated using Johnston s equation with inter-intramolecular copolymer structure. From the data calculated with copolymer prepared at low conversion, the variation of glass transition temperature with copolymer conversion has been theoretically predicted John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: , 1999 Keywords: butyl acrylate methyl methacrylate copolymers; glass transition temperature; conversion INTRODUCTION It is well known that polymer properties are controlled by molecular properties such as molecular weight; molecular weight distribution; chemical composition and stereochemical sequence of copolymer chains; degree of crosslinking; and so forth, which, in turn, are a reflection of the kinetic history of the reactions occurring during their formation. A typical free radical copolymerization shows a gradual drift in copolymer composition over the course of copolymerization. The copolymer composition drift stems from the different monomer reactivities to combine with growing polymer radicals. Hence, the most reactive comonomer is depleted faster than the other comonomer, causing the product to become gradually enriched in the less reactive comonomer as the reaction progresses. Thus, the final polymer material is a combination of many individual copolymers with Correspondence to: E. L. Madruga Journal of Polymer Science: Part B: Polymer Physics, Vol. 37, (1999) 1999 John Wiley & Sons, Inc. CCC /99/ compositions, microstructures, and properties that differ from each other. Polymer glass transition temperature, representing the molecular mobility of the polymer chains, is an important phenomenon that influences the material properties and potential applications of a given polymer. Various structural characteristics (e.g., chain stiffness and intermolecular forces) influence the glass transition temperatures. The mobility of polymer chains depends on the possibility of rotation around the backbone carbon carbon bonds. This itself is determined by the structure of the monomer units. Taking into account that the monomer disposition in a copolymer chain is determined by kinetic events, it is indispensable to consider not only intermolecular microstructure (average and cumulative chemical composition), but also intramolecular microstructure (sequence distribution), since these parameters play an important role in understanding the relation between molecular structure and properties. In previous works, 1,2 we have studied the kinetics of copolymerization in benzene of methyl methacrylate with butyl acrylate using a total 2512

3 T g s OF BA MMA COPOLYMERS 2513 monomer concentration of 3 and 5 mol L 1 at low conversions and of 3 mol L 1 at high conversions. Composition studies have shown that the monomer reactivity ratios values were slightly dependent on monomer concentration used in their preparation. 1 In addition, copolymer composition measured as a function of conversion has demonstrated that the Mayo Lewis terminal model adequately describes the copolymer composition over the whole conversion range. 2 In this study, the determination of glass transition temperatures of BA MMA copolymers was made considering its dependency not only with the copolymer composition but also with the microstructure. Moreover, it was desired to relate the results obtained for glass transition temperatures at low conversion with those obtained throughout the course of polymerization. EXPERIMENTAL Materials The monomers butyl acrylate (BA) (Merck) and methyl methacrylate (MMA) (Merck) were purified by conventional procedures Azobutyronirile (AIBN) (Fluka) was purified by successive crystallization from methanol. Benzene (Merck) for analysis was used without any further purification. Copolymerization Copolymers were prepared by free radical polymerization of mixtures of both monomers with different compositions in benzene at 50 C. At low conversion, the total concentration of monomers was 3 and 5 mol L 1 ; meanwhile, at high conversion it was 3 mol L 1, using three different compositions of BA/MMA: 30/70, 50/50, and 70/30. In all cases, the concentration of initiator was mol L 1. The copolymer samples were isolated after the polymerization time desired, by pouring the reaction mixture into methanol. The precipitated samples were filtered off, washed, and dried at reduced pressure until constant weight was attained. Copolymer Composition 1 H NMR spectroscopy was used to determine copolymer composition. Spectra were recorded at room temperature on 8% solutions in deuterochloroform by using a Varian Gemini spectrometer operating at 200 MHz. Copolymer composition was determined by the method of Grassie et al. 4 Copolymer Glass Transition Temperatures Glass transition temperatures were measured using a Differential Scanning Calorimeter, Perkin Elmer DSC/TA7DX, PC series with a water-circulating system for temperatures over ambient, and a Perkin Elmer DSC-2 Data Station 3700 with an Intracooler for low temperatures. The temperature scale was calibrated from the melting point of high purity chemicals (lauric and stearic acids and indium). Samples ( 10 mg) weighed to mg with an electronic autobalance (Perkin Elmer AD4) were scanned at 10 /min under dry nitrogen (20 cm 3 min 1 ). The actual value for the glass transition temperature T g was estimated as the temperature at the midpoint of the line drawn between the temperature at the intersection of the initial tangent with the tangent drawn through the point of inflection of the trace and the temperature at the intersection of the tangent drawn through the point of inflection with the final tangent. The current value was the average for several measurements realized for each composition. The values estimated according to this criterion, when they are compared with those obtained following other procedures, might be apparently higher. In our case, this was also due in part to the heating rate employed (10 /min). RESULTS AND DISCUSSION The kinetics of copolymerization of methyl methacrylate with butyl acrylate, at 3 and 5 mol L 1 at low conversions and at 3 mol L 1 at high conversions, have been described by our group. 1,2 Composition studies have shown that the monomer reactivity ratios were slightly different. The results are depicted in Table I. Differences in monomer reactivity ratios mean that depending on the global monomer concentration used, copolymers synthesized with the same monomer feed composition have different copolymer compositions. 1 In this case, the same feed composition provokes small variations in copolymer composition. Besides, copolymer composition as a function of conversion is adequately described over

4 2514 FERNÁNDEZ GARCÍA, CUERVO RODRIGUEZ, AND MADRUGA Table I. Monomers Reactivity Ratios for Both Total Monomer Concentration [M] (mol L 1 ) r BA r MMA the whole conversion range using the Mayo Lewis terminal model and the obtained monomer reactivity ratios. 2 Table II. Methyl Methacrylate Molar Fractions in the Feed (f MMA ) and in the Copolymer (F MMA ) and Values for the Glass Transition Temperatures of BA MMA Copolymers at Low Conversion and [M] 3 mol L 1 f MMA F MMA T g (K) The glass transition temperatures for copolymers synthesized at low conversion, for both feed concentrations used, are collected in Tables II and III. Its representation as a function of methyl methacrylate molar fraction in the copolymer F MMA is depicted in Figure 1. The curves correspond to the best fit to the experimental data. As can be seen, the variation in composition is so small that the measured glass transition temperature could be less than the limits of the experimental accuracy. However, the glass transition temperature increases with the molar fraction of MMA in the copolymer chain. As described earlier, 2 copolymers at high conversion are performed at 3 mol L 1 with three different compositions of BA/MMA: 30/70, 50/50, and 70/30. These results are exhibited in Tables IV, V, and VI, respectively. Figure 2 shows the glass transition temperature of these three copol- Table III. Methyl Methacrylate Molar Fractions in the Feed (f MMA ) and in the Copolymer (F MMA ) and Values for the Glass Transition Temperatures of BA MMA Copolymers at Low Conversion and [M] 5 mol L 1 f MMA F MMA T g (K)

5 T g s OF BA MMA COPOLYMERS 2515 Table V. Methyl Methacrylate Molar Fractions in the Copolymer (F MMA ), Conversion (%), and Values for the Glass Transition Temperatures of BA MMA Copolymers with 50/50 in Feed Composition of BA/ MMA at [M] 3 mol L 1 Time (min) Conversion T g (%) F MMA (K) Figure 1. Glass transition temperature versus methyl methacrylate molar fraction for BA MMA copolymers obtained at low conversion and [M] 3 and 5 mol L 1. ymer series as a function of cumulative methyl methacrylate molar fraction in the copolymer chain, along with the experimental curve shown in Figure 1 for copolymers at 3 mol L 1. Experimental deviations from the T g behavior at low conversion are found. This means that it is necessary to know the formation mechanisms Table IV. Methyl Methacrylate Molar Fractions in the Copolymer (F MMA ), Conversion (%), and Values for the Glass Transition Temperatures of BA MMA Copolymers with 30/70 in Feed Composition of BA/ MMA at [M] 3 mol L 1 Time (min) Conversion T g (%) F MMA (K) throughout the copolymerization process, to correlate the thermal behavior with the overall structure of a copolymer. Originally, the glass transition temperatures of copolymers were described by simple additive relations, 5,6 based on free volume theories 6 8 ; thermodynamic theories, 5 which did not take into consideration the sequence distribution of the monomer units; and the effect of their compatibil- Table VI. Methyl Methacrylate Molar Fractions in the Copolymer (F MMA ), Conversion (%), and Values for the Glass Transition Temperatures of BA MMA Copolymers with 70/30 in Feed Composition of BA/ MMA at [M] 3 mol L 1 Time (min) Conversion T g (%) F MMA (K)

6 2516 FERNÁNDEZ GARCÍA, CUERVO RODRIGUEZ, AND MADRUGA third relation, developed by Couchman, 11 is based on mixed-system entropy and was also able to predict composition-dependent glass transition temperatures for a variety of systems. Among all of these, the ones derived by Johnston, Barton, or Couchman, which correlate T g to the dyad distribution in the instantaneous copolymer molecules, have better agreement with experimental T g s. 16,17 In this work, we use Johnston s equation, which is based in the free volume concept and the inter-intramolecular composition of the copolymer. The description of Johnston s model follows. Johnston s equation 9 assumes that M 1 M 1 and M 1 M 2,orM 2 M 1 and M 2 M 2 dyads have their own glass transition temperature, with the overall T g of a copolymer described by the following expression: Figure 2. Glass transition temperature versus cumulative methyl methacrylate molar fraction for BA MMA copolymers obtained over the whole range of conversions. ity on steric and energetic interactions. The free volume theory developed by Fox and Flory 7 suggests that the glass transition temperature occurs when the free or unoccupied volume of the material reaches a constant value and does not decrease further as the material is cooled below its T g. A thermodynamic theory, proposed by Gibbs and DiMarzio, 5 is based on the change of material configurational entropy as a function of temperature. At equilibrium, it postulates that the configurational entropy Sc is zero at the glass transition. However, these linear relationships often failed to predict accurate glass transition temperatures of copolymers, since they neglected the effect of the chemical nature and organization of the monomers on the mobility of a polymer chain. 9 Several models, therefore, were proposed 9 11 that differentiated between homo- (A A, B B) and heterolinkages (A B), recognizing the significant effect of monomer arrangement on glass transition temperature, such that both negative and positive deviations from the linearity may be predicted. The relations proposed by Barton 10 ; Uematsu and Honda 12 ; Hirooka and Kato 13 ; Furukawa 14 ; and Suzuki et al. 15 may be considered as extensions of the Gibbs DiMarzio 5 relation, whereas the approach by Johnston 9 is based on the Fox equation. 7 A 1 T g w 1P 11 T g11 w 2P 22 T g22 w 1P 12 w 2 P 21 T g12 (1) in which w 1 and w 2 are the weight fractions of monomeric units in the main chain; P 11,P 12,P 21, and P 22 are the probabilities of having various linkages, which can be calculated by using the monomer feed composition and the monomer reactivity ratios 18 ; T g11 and T g22 are the glass transitions of the respective homopolymers; and T g12 is the supposed glass transition for the alternating sequence M 1 M 2 or M 2 M 1. To apply Johnston s theory, it is necessary to determine the glass transition temperature of a strictly alternating copolymer T g12. In this case, T g12 for BA MMA copolymers is unknown, but it can be calculated from our own experimental values: T g of PMMA, T g of PBA (T g11 and T g22, respectively), and T g s of a series of copolymers obtained at low conversion. A linearized form of eq. (1) is used to determine T g12. In Figure 3a and b,as it can be observed, the experimental data for 3 and 5 mol L 1 produce a very nearly straight line, with the T g value of and K, respectively. Using the T g12 value found and Johnston s equation, the curves of Figure 4a and b were drawn, which display the dependence of T g on methyl methacrylate weight molar fraction in the copolymer for both total concentrations of monomers. A good agreement between experimental and theoretical values is found, indicating that Johnston s equation and the terminal model of

7 T g s OF BA MMA COPOLYMERS 2517 the T g of a series of copolymers of varied composition, did not always correspond with that of the chemically synthesized alternating copolymer. This deviation depends on the type of T g -composition relationship of the statistical copolymer. T g of a pure alternating copolymer should be higher, lower, or similar to that T g estimated from T g - sequence distribution when the T g -composition curve for statistical copolymer is convex, concave, or linear, respectively. Figure 3. Plot of linearized Johnston s equation for BA MMA copolymers obtained at low conversion. (a) [M] 3 mol L 1, (b) [M] 5 mol L 1. Mayo and Lewis through reactivity ratios may be used to describe the dependence between experimental glass transition temperature of BA MMA copolymers and their sequence distribution. It is worthwhile to note that Figures 4a and 4b are both superimposable, because the small differences in copolymer composition and in sequence distribution are insufficient to create remarkable differences in the glass transition temperature. On the other hand, Hirooka et al. 13 have observed that the T g for dyads T g12, calculated from Figure 4. Glass transition temperature versus methyl methacrylate weight molar fraction for BA MMA copolymers obtained at low conversion. (a) [M] 3 mol L 1, (b) [M] 5 mol L 1.

8 2518 FERNÁNDEZ GARCÍA, CUERVO RODRIGUEZ, AND MADRUGA Figure 5. Left hand: Instantaneous ( F MMA, F BA ) and cumulative copolymer composition ( F MMA, F BA ). Right hand: DSC thermograms at different conversions for copolymers with f MMA 0.7. Conversion (%): (a) 13.5, (b) 21.4, (c) 36.7, (d) 56.5, (e) 81. Tonelli 19 has used the conformational entropy as a characterizing parameter for the polymer intramolecular chain flexibility. The production of positive, negative, or no deviation from bulk additive (namely T g12, estimated from T g -sequence distribution behavior) is observed when the conformational entropy for a given copolymer chain is lower, higher, or similar, respectively, to the weighed sum of entropies calculated for the constituent homopolymer chains. Moreover, the T g of the polymer is related to the chain flexibility and this parameter is, to a large extent, a reflection of the rotational barrier about the bond linking two monomer units. In this work, the average T g is K, with the understanding that it is the average of the homopolymer glass transition temperatures corresponding to both concentrations. The values of T g12 obtained using Johnston s equation (289.5 and K) are lower than the T g average; these values correspond to respective equimolecular random copolymers, which indicated that this Figure 6. Glass transition temperature for BA MMA copolymers obtained in [M] 3 mol L 1 benzene solution with different feed composition over the whole conversion range.

9 T g s OF BA MMA COPOLYMERS 2519 system has a heterolink stiffness of lower or higher flexibility than the average of the homopolymer links. In addition to this, a previous study has demonstrated that the MA MMA copolymers, 16 obtained under the same conditions, behave similarily. The T g12 for this copolymer is K (r MA 0.42 and r MMA 2.36), which is slightly higher than for BA MMA copolymers, because of less flexibility in the MA MMA chain. It is apparent that the glass transition temperature depends on the chemical structure, and it decreases with lowering chain length as a result of higher flexibility and increasing molecular motion. 20 Likewise, the average T g of MA MMA copolymers 16 is higher (T g K) than the value obtained from experimental data. This has been seen in other systems. 20,21 Another purpose of this work is to know whether the results obtained from the analysis of copolymer data prepared at low conversion can be employed to predict the variation of T g with the conversion. To do this, it is necessary to assume that any macromolecule in the copolymer sample has its own glass transition behavior. The objective of the simulation is to compute the overall mixture behavior of copolymer macromolecules produced through the process. Variation of T g with conversion has been approximated by using eq. (1) by a step function, in which copolymerization theory allows us to derive instantaneous copolymer composition and microstructure. These values and the parent homopolymer s T g can give the T g for each step, which is then accumulated over the conversion interval to yield integrated values. The left-hand side of Figure 5 shows the instantaneous and cumulative copolymer compositions as a function of conversion for 70 : 30 :: MMA : BA copolymers. As can be observed, the average MMA molar fraction in the copolymer chain decreases as the conversion levels increase. At the same time, the variation of instantaneous copolymer chain composition yields remarkable increases for conversions higher than 0.5. Therefore, the chemical heterogeneity of copolymer samples rises. This fact can be related to the traces of DSC thermograms for 70 : 30 :: MMA : BA copolymers obtained at different conversion levels, which are represented on the right-hand side of Figure 5. It is worth pointing out that each copolymer sample has a single value of T g, which decreases as the conversion level increases. Besides, the transition range broadens with a rise in the conversion level. The T g depression is related with the overall BA molar fraction in copolymer chain increment as the conversion increases, whereas the glass transition width might be explained by considering that the chemical heterogeneity increases when the conversion does. In this way, the existence of heterogeneity in block copolymers or in certain blend composition is manifested by a broadening of the glass transition. 22 Although similar behavior has been observed for all copolymers performed at high conversion, no definitive statement can be offered on the width of glass transition. The experimental data, gathered together with the theoretically estimated curves, are depicted in Figure 6, where good agreement can be observed. This means that not only does the BA MMA system follow the terminal model, but also that Johnston s equation for the glass transition temperature of copolymer is able to explain the dependence between the glass transition temperature and both copolymer structure and conversion. In other words, the apparent glass transition temperature calculated, namely T g12, adequately fits the T g variation of the copolymers at different conversions in a wide range of monomer feed compositions. In conclusion, this approach could be useful to understand and predict the glass transition temperature of random copolymers over the whole conversion range. This could lead to an ability to select the appropriated conditions in the copolymerization process to synthesize a copolymer with desired structure and properties. This research has been supported by The Comisión Interministerial de Ciencia y Tecnología (CICYT), (MAT97-682). The authors thank Professor Dr. Fernández Martín from the Instituto del Frio (CSIC, Madrid) for his advice and for allowing us to use the equipment located at his Institute. REFERENCES AND NOTES 1. Madruga, E. L.; Fernández García, M. Macromol Chem Phys 1996, 197, Fernández García, M.; Madruga, E. L. J Polym Sci Part A: Polym Chem 1997, 35, Stickler, M. Makromol Chem Macromol Symp 1987, 10/11, Grassie, N.; Torrance, B. J. D.; Fortune, J. D.; Gemmell, J. D. Polymer 1965, 6, Gibbs, J. H.; DiMarzio, E. A. J Polym Sci Part A: Polym Chem 1963, A1, 1417

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