Novel Hydroquinone Diacetate based copolyesters: A detailed kinetic investigation

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1 Novel Hydroquinone Diacetate based copolyesters: A detailed kinetic investigation Adam Al-Mulla* Chemical Engineering Department, Kuwait University, P.O. Box 5969, 36 Safat, Kuwait, ABSTRACT Thermotropic liquid crystalline polymers have been developed using Hydroquinone diacetate (HQDA), Terephthalic acid (TA) and Adipic acid (AA). The kinetic evaluation and thermal characterization of this system was carried out. The composition chosen was HQDA mole%/ (TA + AA) (5 + 5) mole% such systems have never been synthesized before. This system is easy to process due to the liquid crystalline nature of the copolyester. The end use application of these materials could range from development of films, coatings and other engineering applications. Kinetic investigations on the synthesis of the novel copolymer systems are studied. Four different reaction temperatures (5,, 5 and 3 C) have been tried in this kinetic investigation. Two different catalysts, Sodium acetate and Zinc acetate ( mole%) were used as the polymerization catalysts. Uncatalyzed experiments were also carried out at the above mentioned temperatures. The thermal characterization of the synthesized polymers was carried out using differential scanning calorimeter (DSC) to evaluate the thermal properties. Key Words: Hydroquinone diacetate based copolymers, condensation polymers, reaction kinetics. *adamalmulla@kuc.kuniv.edu.kw

2 LIST OF SYMBOLS AA = Adipic acid TA = Terephthalic acid HQDA = Hydroquinone diacetate k = rate constant lit mol - min - k = rate constant lit mol - min - k 3 = rate constant lit mol - min - k 4 = rate constant lit mol - min - A = hydroquinone diacetate B = terephthalic acid C = adipic acid p = fractional conversion DSC = differential scanning calorimeter N = total number of molecules N o = total number of structural units present X n = number-average degree of polymerization c = concentration (moles/lit) M ie = amount of acetic acid collected by experiment (moles) M ic = amount of acetic acid collected theoretically (moles) ϑ o = initial moles of monomer molecules (moles) ϑ = number of moles t o = dead time (min) F = objective function R u = uncatalyzed reaction rate (mol lit - min - ) k u = uncatalyzed reaction rate constant (mol - lit - min - ) R c = catalyzed reaction rate (mol - L - min - ) k c = catalyzed reaction rate constant (lit mole - min - ) E = activation energy(kj/mole)

3 INTRODUCTION Liquid crystalline polymers, has become the subject of scientific research owing to their unique complexity of properties, structure, and practical applications. Melt polyesterification of dicarboxylic acids with diols is one of the frequently studied systems. Initially, the studies of poly esterification of dicarboxylic acids, eg. adipic acid, with diols, eg. diethylene glycol (Flory 939), served as models for the understanding of the esterification kinetics. However the combination of different reagents has enabled the production of polyesters with varying physical and chemical properties, but only few studies have been reported on the precise kinetics of mixtures of different carboxylic acids and diols and the kinetics of side reactions taking place simultaneously with polyesterification. Paateno, et al. [994] have presented a paper on the development of kinetic model for main and side reactions in the polyesterification of dicarboxylic acids with diols. In this work the kinetics of maleic acid and phthalic acid with propylene glycol are considered. The objective of their work was to study the polymerization kinetics for esterification, isomerization and double bond saturation. Liquid crystalline copolymers based on poly (ethylene terephthalate) (PET) and acetoxybenzoic acid (ABA) were developed by Al-Haddad and Mathew [997] to build ultra high strength materials. Transesterification reactions between PET and ABA were conducted using melt polymerization technique to understand the transesterification kinetics of a phase segregated system. Phase segregation was noted in these systems. A variety of high temperature liquid crystalline spacers have been developed by Al- Adwani et al. [3] using,4 naphthalene dicarboxylic acid and hydroquinone diacetate. Kinetic studies on the melt polymerization of these systems have been reported. The performance of two transesterification catalysts were also estimated for these systems. They have modeled the homopolymerization reaction utilizing four different rate constants and twelve differential equations. In another work Shaban and Mathew [996] have developed copolyesters of poly (ethylene terephthalate) hydroquinone diacetate and terephthalic acid. Three different compositions of PET/HQDA and TA were synthesized. They assumed second order kinetics and computed the moles of acetic acid produced. The rate constant k under phase separation conditions was determined. Al-Haddad et al. [999] used melt polymerization technique to synthesize poly 4 (oxybenzoate) using propion oxybenzoic acid as the monomer. Their objective was to carry out catalytic synthesis of the 4-oxybenzoate polymer and study its kinetic analysis in detail. The principal aim of this paper is to develop new thermotropic copolyesters and study the (oligomerization) reaction kinetics involved in the synthesis of these materials. The monomers are HQDA (Al-Haddad (), Lodha et al. (997, Navarro et al. (997), Okazaki et al. (), and Yerli et al. (, )] adipic acid (AA) (Navarro et al (997)), and Terephthalic acid, TA (Al-Haddad et al. ()). The polymers are expected to have the following structure: 3

4 O O O O HO C CH 4 C O O C C OH n The presence of AA as a spacer is expected to decline the melting point of the polyesters formed. These polymers will have a wide range of applications especially as a better substitute for poly (ethylene terephthalate) and poly (butylene terephthalate). The aim of the present work is to determine the kinetic parameters for the catalyzed and uncatalyzed copolyesterification reactions. The thermal properties of the polymers ae evaluate using DSC. MATERIALS Hydroquinone diacetate (HQDA), with chemical formula C H O 4 and molecular weight 94 was purchased from Aldrich Chemical Company, U.K. It had a purity of 99% and a melting point of C. The monomer TA (99%) chemical formula C 8 H 6 O 4 and molecular weight of 66 was purchased from Aldrich Chemical Company, U.K., and used as received. It had a melting point greater than 3 C. Adipic acid (AA) chemical formula HOOC(CH ) 4 COOH molecular weight 46 was purchased from M/s Aldrich Chemical Company, U.K., and used as received. It had a melting point of 5 C. Sodium acetate and zinc acetate of 99.99% purity were purchased from Glaxo fine chemicals, Bombay, India.. mole percent of the catalyst was used for all the catalyzed polymerization reactions. REACTOR AND REACTION DETAILS A cylindrical shaped reactor made of HASTELLOY-B was used for the experiments. It had 7 mm length and an internal diameter of 6 mm. A schematic diagram of the reactor is shown in Figure. The reactor had four ports for charging / stirring the reactants, nitrogen gas inlet, side product collection, and vacuum measurement. It could be maintained isothermally at any temperature between ambient and 4 C. The temperature could be maintained to an accuracy of ±. C. A provision for measuring the side product was made, as shown in Figure. The reactants in the pure form were charged into the reactor without further purification by opening the main flange. The experimentally weighed quantity of HQDA mole% / TA+AA (5 + 5) mole%, were 7.7,.85 and.43 grams, respectively. In all experiments an inert gas (N ) were bubbled through the reaction mixture both to avoid oxidation and to remove the by product, which is acetic acid. These reactions were carried out in the range 5-3 C. In the present experimental conditions, degradation reactions were low. 4

5 THERMAL CHARACTERIZATION The thermal characterization was done on a Mettler 3, DSC apparatus interfaced with a thermal analysis station. Around milligrams of the polymer was preweighed and loaded into a tared aluminum crucible. The crucible was introduced into a furnace which was heated at a specified heating rate and the analysis monitored online on the screen. A standard inhibit program was used to analyze the thermal changes occurring in the polymer. Kinetic Models Considered for the Present System Simple Second Order Textbook Equation (Model ) The goal of chemical kinetic measurements for well-stirred mixtures is to validate a particular functional form of the rate law and determine numerical values for one or more rate constants that appear in the rate law. Equation is a rate law, or rate equation in differential form. The second order rate equation for the formation of a polyester can be given as; d[ COOH ] = k [ COOH ][OH] () dt If the concentration of COOH = OH = c then dc = k c () dt Introducing the concept of fractional conversion the second order equation becomes c k t = + const. (3) p Equation (3) modified in terms of fractional conversion, i.e. = kco t where p = p fractional conversion was applied to the copolymerization data of HQDA, TA and AA. Simple Second Order Kinetic Model (Model ) The rate constant k is evaluated by comparing the measured moles of side product M ie (e.g. acetic acid) evolved as a function of time, with the values M ic calculated using the present model. It may be shown that the moles of side product generated is equal to the moles of monomer that have reacted to yield oligomers. Thus: ϑo ϑ = (4) + k ϑ o t M ie = ϑ o ϑ (5) 5

6 Substituting for ϑ from equation (4) yields: ϑo k t M ic = (6) + ϑok t where ϑ o is the initial moles of side product molecules and ϑ is the number of moles, of side product generated as a function of time, t. In practice, in polymerization reactions, there is a delay (dead time) at the start of the experiment before any noticeable amount of side product is collected. This is due to the reactor geometry. This dead time t o is also a function of initial reaction rate, catalyst activity and the reaction temperature. In order to take this factor into account, equation (6) was modified to yield: ϑo k ( t to ) M ic = (7) + ϑo k ( t to ) In this analysis, Solver, which is a built-in function in Microsoft Excel 97, was used to calculate the best values of k and t o from the experimental data. The following default values of dead time ( min), number of iterations () and precision limit ( -5 ) were employed in the calculations, which were based on Newton s method. Equation (7), was tried for the experimental data and Solver, was used to calculate the best values of k from the experimental data. For model () the objective function was defined as the sum of the percentage absolute deviations between experimentally measured moles of acetic acid M ie as a function of time and the corresponding computed values M ic. m M ie M ic F = (8) i= M ie Here m is the total number of data points. Equation (8) provides an equal weightage to all experimental points from the start to the completion of the reaction, thereby facilitating the best fit and correctly assessed optimal values of k. Initial estimate of k was provided to solver which then performed a fixed number of trials ( per variable) in which the values of the constant were changed until the objective function attained least possible value. RESULTS AND DISCUSSION Formation of the copolyesters of hydroquinone diacetate, terephthalic acid and adipic acid can be represented by reaction indicated in Fig. (). The reaction possibility of HQDA towards TA or AA depends on the flexibility, bulkiness of the molecule and ease of availability of the functional groups for the reaction. Figures (3) - (5) indicate the comparison of experimental data points against a linear fitting obtained using Excel, for uncatalysed HQDA + TA + AA. In these figures the mole ratio of TA:AA is varied. These figures indicate that with an increase in TA 6

7 concentration at a given temperature the DP, ( ), of the oligomers is found to p decrease. It is also found that DP is higher for higher mole ratios of AA. Figures (6) - (8) (plots obtained for sodium acetate catalyzed HQDA + TA + AA reactions) indicate a decrease in the DP as the TA content increases for a given temperature. A comparison of the uncatalyzed (Figures 3 5) and catalyzed reactions (Figures 6 8) indicate that sodium acetate helps in increasing the DP. As seen from the versus time plots (Figures 3 8), no proper match is found between the p experimental data points and equation, = k c o t, for the uncatalyzed and catalyzed p reactions especially at higher temperatures. The plots 3-8 indicates that the fitting of the data deviate from the experimental points as the temperature increases. Figure 9 indicates the effect of catalyst type at a typical temperature of 3 C for the (TA/AA, 6:4) system. The fit between the experimental point and the standard second order equation ( = k c o t) (model ) is not good. Two typical second order plots illustrating the p effect of composition variation (TA : AA) for uncatalysed and catalyzed at different temperature 3 C is indicated in Figures and. The model is not found to fit the data well. Higher DP is obtained for polymers with greater mole ratio of AA. Table (model ) indicate rate constants calculated using the model. The % error obtained for these plots using excel (linear regression) is indicated in Table. Since the fit between the experimental and predicted values did not match, the model and the rate constant values obtained are questionable. Figure indicates a typical plot showing the match obtained between calculated values using equation () of model and the uncatalyzed experimental data points. Figure 3 indicates the plots obtained for sodium acetate catalyzed HQDA +( TA + AA) (4: 6)) reactions. No proper match is observed between theoretical and experimental data points. A comparison of the uncatalyzed (Figure ) and catalyzed experiments (Figure 3) indicate that sodium acetate helps in increasing the formation of the oligomers. This is seen by the increase in moles of acetic acid. Figure 4 shows the effect of using different kinds of catalysts (Sodium acetate versus zinc acetate) on the formation of acetic acid versus time at 3 C and TA : AA equals 5: 5. As seen from this figure, model does not fit the data well (high error). Figure 5 indicates the effect of composition at a typical temperature of 3 C for the uncatalyzed reaction. Higher mole ratio of acetic acid is obtained for polymers with higher quantity of AA. The percentage error, i.e., the deviation between the experimental and theoretical data points is indicated in Table. The error is greater than 5% for majority of the plots. 7

8 POLYMER CHARACTERIZATION HQDA, TA and AA were acetone extracted and dried before carrying out the reactions. The melting points of HQDA, TA and AA are, 3 and 5 C respectively. Figure 6 indicates the thermogram obtained for HQDA, TA and AA where the TA to AA composition is 5 : 5 mole ratio. This figure indicates the effect of catalyst type on the thermal property of the polymer. The figure also indicates that the oligomers for the uncatalysed and catalyzed reactions melt in the range of 3 to 35 C. The area of the peak at 3 C for sodium acetate catalyzed reactions is found to be more followed by the uncatalyzed reaction and the zinc acetate catalyzed reaction. The less area of the thermogram could possibly indicate the formation of low molecular weight oligomers. Figure 7 is the thermogram for TA : AA, 6 : 4 mole ratio composition. The zinc and no catalyst polymers are found to melt around 3 C while the using sodium acetate catalyst is found to melt around 33 C. This possibly indicates that the oligomers formed could be some what molecular weight distribution. Figure 8 corresponds to the composition TA:AA, 4 : 6. The melting point of the polymers are found to be around 3 C. The melting curve of these polymers are found to exhibit peaks with less area compared to TA:AA, 5 : 5 and 6 : 4 indicating a lower molecular weight distribution. CONCLUSION Synthesis of novel, hydroquinone diacetate (HQDA), terephthalic acid (TA) and adipic acid (AA) based copolyesters have been carried out. Three compositions of these copolyesters have been developed; i) HQDA mole%/(ta + AA) (4 + 6) mole% ii) HQDA mole %/ (TA + AA) (5 + 5) mole% and iii) HQDA mole%/(ta + AA) (6 + 4) mole%. Two different polyesterification catalysts, ( mole%) sodium acetate and zinc acetate, were employed as the transesterification catalysts for this novel system. Reaction between HQDA, TA, and AA were carried out to evaluate the kinetic parameters for this copolyesterification reaction over the temperature range of 5-3 C. The reactions were assumed to follow second order kinetics. Three different rate models have been tried. In model, was plotted as a function of time. A new p second order model (model ) taking into consideration the amount of acetic acid generated was developed and tried. Both models did not fit the data well. Another second order kinetic model (MATLAB based) (model 3) was developed and tried for the three compositions. It had four rate constants (k, k, k 3 and k 4 ). The rate constants k and k indicates the depletion of terephthalic acid and adipic acid. k 3 indicates the depletion of dimers and k 4 indicates the consumption of trimers. Majority of the k 4 values for the three compositions are found to be zero compared to the values of k, k and k 3. This indicates that the reactivity of triad oligomers are much lower compared to dimers. The energy of activation for the rate constant, k, for all the three compositions is indicated. Sodium acetate is found to be a 8

9 good catalyst based on activation energy values. The values of E for the two systems TA:AA, 4:6 and 5:5 is determined. The activation energy values for sodium acetate catalyzed reactions for E and E is lower than uncatalyzed reactions. Among the three different models tried the MATLAB model was considered a better model to use based on the least error values. The polymers were thermally characterized using DSC. The oligomers formed for all the three compositions are found to melt in the range 3 C and 33 C. ACKNOWLEDGEMENT The author would like to thank the research administration of Kuwait University for providing funds from the project EC /6 for carrying out this research work. The author would also like to thank Dr. Johnson Mathew for his technical support. 9

10 Bibliography - Flory, P. J. Am. Chem. Soc. 939, 6, Paateno, E.; Nashi, K.; Salmi. T.; Still, M.; Nyholm, P., Immonen, K. Chem. Eng. Sci. 994, 49, Al-Haddad, A.; Mathew, J. J. of Reactive and Functional Polymers 997, 3, Solaro, R.; Talamelli, P.; Carbonaro, L.; Chiellini, E. Macromolecular Chemistry & Physics 3. 4, Al-Adwani, H.; Bishara, A.; Shaban, H. Journal of applied polymer science 3, 89, Shaban, H.I. ; Mathew, J. Journal of applied polymer science 996, 6, Al-Haddad, A.; Mathew, J.; Al-Kamel A.; Nagdi, M. Journal of Applied Polymer Science 999, 7, Al-Haddad, A.; Mathew, J.; Kendari, H. European Polymer Journal, 38, Lodha, A.; Ghadge, R.S.; Ponrathnam, S. Polymer 997, 38, Navarro, E.; Subriana, J.A.; Puliggali, J. Polymer 997, 38, Okazaki, M.; Hayakawa, T.; Ueda, M.; Taekuchi, K.; Asai, M. Journal of Polymer Science, Part A: Polymer Chemistry, 39: Yerli Kaya, Z; Aksoy, S; Bayramli, E. Journal of Applied Polymer Science, 85, Yerli Kaya, Z.; Aksoy, S.; Bayramli, E. Journal of Polymer Science, Part A: Polymer Chemistry, 39,

11 Table I. Table indicating values of rate constants for the copolyesterification reaction of HQDA + TA + AA (calculated using as a function of time) (model ) p Serial # Composition Temp Catalyst Rate Constant (moles) HQDA: (TA + AA) ( C ) Type k (lit mol - min - ) % Error : (4+6) 5 NaAc NaAc NaAc NaAc ZnAc : (5+ 5) 5 NaAc NaAc NaAc NaAc ZnAc : (6+ 4) 5 NaAc NaAc NaAc NaAc ZnAc (-) = Uncatalyzed reaction NaAC = Sodium Acetate ZnAc = Zinc Acetate

12 Table II. Table indicating values of rate constants for the copolyesterification reaction of HQDA + TA + AA using Model Serial # Composition (moles) HQDA : (TA +A) Rate Constant k (lit mol - min - ) Temp. ( C) Catalyst Type Error% : (4 + 6) 5 NaAc NaAc NaAc NaAc ZnAc : (5 + 5) 5 NaAc NaAc NaAc NaAc ZnAc : (6 + 4) 5 NaAc NaAc NaAc NaAc ZnAc (-) = Uncatalyzed reaction NaAC = Sodium Acetate ZnAc = Zinc Acetate

13 Figure 3: Second order plot showing the effect of temperature /(-p) Model /(-p) Model for uncatalyzed reactions for HQDA mol %/(TA/AA) (4:6) mole % composition Figure 4: Second order plot showing the effect of temperature for uncatalyzed reactions forhqda mol%/ (TA/AA) (5:5) mole % composition /(-P) Model /(-p) Model Figure 5: Second order plot showing the effect of temperature for uncatalyzed reactions for HQDA mol%/(ta/aa) (6:4) mole % composition Figure 6: Second order plot illustrating the effect of temperature for sodium acetate catalyzed reactions for HQDA mol % / (TA/AA) (4:6) mole % composition /(-p) /(-p). 3 5 Model 5. Model Figure 7: Second order plot illustrating the effect of temperature for sodium acetate catalyzed reactions for HQDA mol % / (TA/AA) (5:5) mole % composition Figure 8: Second order plot illustrating the effect of temperature for sodium acetate catalyzed reactions for HQDA mol% / (TA/AA) (6:4) mole % composition /(-p). Sodium Acetate Zinc Acetate Model /(-p).5 5:5 4:6 6:4 Model Figure 9: Second order plot indicating the effect of catalyst type (mole%) at 3 C for HQDA mol % / (TA/AA) (6:4) mole % composition Figure : Second order plot illustrating the effect of composition (TA:AA) for uncatalyzed reactions at the highest temperature (3 C) /(-p) 3.5 5:5 3: Moles of acetic acid formed Model.5 :3 Model Time Figure : Plot showing the effect of temperature for uncatalyzed reactions for (HQDA/TA/AA) (5:3:) mole % composition Figure : Second order plot illustrating the effect of composition (TA:AA) for sodium acetate catalyzed reactions at the highest temperature (3 C)

14 Figure 3: Plot showing the effect of temperature for.4... Moles of acetic acid formed Model Moles of acetic acid formed NaAc ZnAc Model Time Time Figure 4: Plot showing the effect of catalyst type ( mole%) at 3 C for (HQDA/TA/AA) (5:5:5) mole Sodium Acetate catalyzed reactions for (HQDA/TA/AA) (5::3) mole % composition % composition.4.. Moles of acetic acid formed :6 5:5 6:4 Model Figure 5: Plot showing the effect of composition for(ta/aa) uncatalyzed reactions at the highest temperature (3 C) Figure 6: Thermogram indicating the effect of catalysts for HQDA mole%/(ta:aa) (5:5) mole% composition Figure 7: Thermogram indicating the effect of catalysts for HQDA mole%/(ta:aa) (6:4) mole% composition Figure 8: Thermogram indicating the effect of catalysts for HQDA mole%/(ta+aa) (4:6) mole% composition

Polymer chemistry Prof. Dibakar Dhara Department of Chemistry Indian Institute of Technology, Kharagpur. Lecture - 4 Step-growth Polymerization

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