Photopolymerization of Acrylic Monomers Initiated by Modified Silica with 4,4 -Azo-bis (4-cyanopentanoic acid) chloride.

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1 Photopolymerization of Acrylic Monomers Initiated by Modified Silica with 4,4 -Azo-bis (4-cyanopentanoic acid) chloride. Kinetic aspects Marcel Popa a), Monica Arnautu a), Marc J. M. Abadie b) and Victor Bulacovschi a) a) Department of Macromolecular Chemistry, Technical University of Iasi, B-dul D. Mangeron 71, 66 Iasi, Romania, b) Laboratory of Polymer Science & Advanced rganic Materials/LEMP/MA, cc 1, Place Eugene Bataillon, 3495 Montpellier, Cedex 5, France, Tel: (33) , Fax: (33) Key words: colloidal silica, functionalization, photopolymerization, polymer grafting, kinetic parameters. Abstract Silica, modified by reaction of superficially -H groups with 4,4 -azo-bis (4- cyanopentanoic acid) chloride, was used to initiate grafting of the acrylic monomers onto particle surface. The grafting efficiency was studied as related to reaction temperature, type of the monomer, concentration of inorganic particles and reaction medium. The kinetic study and several photo-polymerisation parameters (e.g. reaction rate and conversion, the enthalpy and induction period, the time for reaching maximum conversion and percent of the monomer reacted at this moment) were also approached. Introduction Grafted polymers onto inorganic particles (e.g. colloidal silica) are largely used as modified fillers in high impact composite materials. They are prepared via radical active precursors such as monomer, transfer agent or initiator, previously grafted onto particle surface via a suitable coupling agent. Several routes of binding initiating groups onto silica surface have been reported and most of them refer to intermediate coupling of suitable agents [1-9]. Carlier et al. [1,11] describes grafting of vinylbenzyl chloride onto silica via silane coupling agent. 'Haver et al. [1,13] reports the modification of the inorganic material by "in situ" poly- and copolymerisation of polar and nonpolar monomers. 1

2 Materials and Methods The monomers, solvents and phosphorus pentachloride (Aldrich) were used after purification by common methods or as delivered. 4,4 -azo-bis(4-cyanopentanoic) acid was recrystallized from methanol. Colloidal silica ( m /g and 16 - particle size) was dried before use for 4 hrs at 1 C. Its characteristics are shown in Table 1. Table. 1 Characteristics of colloidal silica. BET Particle size Pore volume Pore size Silanol Group contents surface area (nm) (m /g) (cm 3 /g) (Å) [14] (mmol/g) Silica Preparation of 4,4 -azo-bis(4-cyanopentanoic) acid chloride (ACPAC) In a reaction vessel were charged 5.34 g PCl 5,.8 g ACPA and 1 ml benzene. The reaction was carried out for 4 hrs at 1 C under magnetically stirring and then, the benzene and PCl 3 were removed under low pressure. Grafting of azo-groups onto silica surface A charge of.5g ACPAC dissolved in 5 ml benzene, 1 g colloidal silica and 1 ml of pyridine (acid acceptor), was allowed to stand for 6 hrs at room temperature under magnetically stirring. Subsequently, the colloidal silica was filtered off, washed several times with methanol to remove the unreacted acid chloride and dried under vacuum for 4 hrs at room temperature. Photo-grafting of acrylic and methacrylic polymers onto silica surface. The reaction was carried out in a DSC 91 TA differential photo-calorimeter with power compensation (DPC), equipped with a 93 Du Pont illumination source ( W mercury lamp) of 4.5 mw/cm intensity at sample level. Analysis of two samples under identical experimental conditions may be performed at the same time. The method was largely described in literature [14-19]. The reactions were conducted in air or in nitrogen atmosphere supplied through a special device. To avoid mass losses by evaporation, samples were covered with a Mylar thin film (PET). Results and Discussions The introduction of azo-groups onto particles surface was achieved by reaction of ACPAC with the superficial silica H groups. By elemental analysis the ACPAC corresponds to a diacid chloride. Both products, (A) and (B), may decompose and initiate the photo-

3 polymerisation of vinyl monomers. ne should note that in the case of product (B), a mixture containing grafted and non-grafted polymer is formed. Si H + Cl C 3 3 ( ) C N N C ( ) C CN CN Cl Si Si 3 3 C ( ) C N N C ( C ) Si (A) or CN CN 3 3 Cl C ( ) C N N C ( ) C (B) CN CN Photo-polymerisation of acrylic monomers initiated by modified silica. The photo-polymerisation of DAEM, HDDA and TPGDA onto modified silica occurs by means of free radicals resulted from photo - decomposition of the azo- groups. Besides the grafted product one may expect the formation of the homopolymer also. Si R N N R 1 UV Si R. +. R 1 + N In connection with this process, many parameters have to be considered in assessing the grafting efficiency. 1. Influence of the temperature. The Figure 1 presents the evolution of the conversion and reaction rate versus time, for grafting of the TPGDA in presence of different amounts of modified silica. ne can easily observe that the temperature has a high influence upon the monomer conversion.. conversion,8,1 Si-CA, 1% TPGDA 6 C 55 C 45 C 3 C Si R. + n Si R R Fig. 1 Conversion and reaction rate variation versus time for the TPGDA photopolymerization, in presence of 1% Si-CA for different working temperatures.. Inlfuence of the concentration of modified silica. reaction rate (1/min),8,1 R n TPGDA, 1% Si-CA 6 C 55 C 45 C 3 C 3

4 In addition, it determines the decrease of the induction period and the time at which the maximum rate is reached, but is favourable for reaction enthalpy, rate constants and to amount of the monomer transformed at the maximum rate The Figure reflects the evolution of conversion and reaction rate versus the amount of modified silica (Si-CA) used for polymerisation of TPGDA at 6 C. Surprisingly, the conversion and reaction rate are lower as the concentration of the initiator increases, in spite of that the concentration of azo-groups is high in such conditions. ne reason might be the opacity of the silica to UV-radiation and the increase of the system viscosity at higher amounts of the initiator. conversion,8,1 TPGDA, 6 C 1% Si-CA % Si-CA 3% Si-CA reaction rate (1/min),8,1 TPGDA, 6 C 1 % Si - CA % Si - CA 3% Si - CA Fig. Conversion and reaction rate variation versus time for the TPGDA photopolymerization at 6 C, for different functionalized silica contents. 3. Influence of the monomer. The experimental results have showed for DAEM a low induction period (Table ) and a high reaction rate while for TPGDA were proper higher conversions and for DAEM the lowest one. It might be that the DAEM molecules having low dimensions penetrate easily into silica pores to the active centres, although the final conversion remains low. The following order of the rate constants could be established: DAEM TPGDA HDDA. Table. Kinetic parameters for photopolymerization of different acrylic monomers in presence of 3% functionalized Silica at 55 C. Monomer Enthalpy ccurrence of the Reacted at the Induction Rate (J/g) maximum peak (s) maximum peak (%) period (s) constant DAEM HDDA TPGDA

5 As concerns the induction period, reaction enthalpy and time for the maximum peak to appear, they obey the following order: TPGDA HDDA DAEM. The higher reactivity of the TPGDA was attributed to the mobility of the monomer molecule conferred by the ether type oxygen in its structure. Influence of the reaction medium. Since the oxygen is known to have an inhibiting effect, the photo-polymerisation xperiments were carried out under different environmental conditions. The Figure 3 presents the conversion and reaction rate for polymerisation of TPGDA at 3 C in presence of 1% functionalized silica (Si-CA) in nitrogen atmosphere with Mylar film protected sample, and in air with and without protected sample respectively. conversion,1 TPGDA, 3 C 1% Si-CA nitrogen, Mylar air, Mylar air, without Ml reaction rate (1/min),8,1 TPGDA, 3 C 1% Si-CA nitrogen, Mylar air, Mylar air, without Mylar Fig. 3. Conversion and reaction rate variation versus time for photopolymerization process of TPGDA in presence of 1% functionalized silica at 3 C, in air and nitrogen atmosphere. The experimental results assert that when no protection was provided, the conversion and reaction rate are extremely low. The best results are obtained in nitrogen atmosphere. By changing environmental conditions, was proved once again that the oxygen influences negatively the polymerisation process. Conclusions 1. The silica hydroxyl groups may react with ACPA acid chloride, introducing in this way the azo-groups onto its surface. So modified silica was used as photo-initiator for grafting acrylic monomers.. The polymerisation process is sensitive to temperature, monomer type and concentration of the initiator and reaction medium, which influence the conversion and reaction rate. 5

6 3. The temperature favourably influences the evolution of conversion and reaction rate. The reaction enthalpy, rate constant and conversion at the maximum peak rise with the temperature, while the induction period decreases. 4. The azo groups constitute the photo-initiator in this process. Since the silica is nontransparent to UV radiation, the increasing of its amount determines the decrease of the conversion and reaction rate. 5. The most reactive monomer was found to be DAEM, however its conversion is low, probably due to rapid deactivation of its reactive centres. The best results were obtained for TPGDA and HDDA monomers. 6. ne should work in an inert atmosphere to obtain good results. The medium results were obtained when the photo-polymerisation was carried out in air with Mylar film protected samples. References 1. N. Tsubokawa,, Y. Shirai, H. Tsuchida et S. Handa, J. of Polym. Sci. Part A: Polym. Chem., 3, 37 (1994).. N. Tsubokawa, H. Ishida, J. of Polym. Sci. Part A : Polym. Chem., 3, 41 (199). 3. Guyot, A. Revillon, E. Carlier, D. Leroux et C. le Deore, Makromol. Chem, Makromol. Symp., 7/71, 65 (1993). 4. Carlier, A. Guyot, A. Revillon, M. F. Llauro-Darricades et R. Petiaud, Reactive Polymers, 16, 41 (199). 5. M.J.M. Abadie, M. Popa, M. Zaharia-Arnautu, V. Bulacovschi and A. A. Popa, Eur. Polym. J., 36, ,. 6. S. W. Shang, J. W. Williams, K. J. M. Soderholm, J. of Mat. Sci., 9, 46 (1994). 7. S. S. Ray and M. Biswas, Mat. Research Bull., 33, (4), 533 (1998). 8. S. W. Shang, J. W. Williams, K. J. M. Soderholm, J. of Mat. Sci., 3, 433 (1995). 9. N. Tsubokawa, Y. Shirai, K. Hashimoto, Colloid Polym. Sci., 73, 11, E. Carlier, A. Guyot and A. Revillon, Reactive Polymers, 18, 167 (199). 11. E. Carlier, A, Guyot and A. Revillon, Reactive Polymers, 16, 115 (199). 1. J.. Haver, J. H. Harwell, L. R. Evans and W. H. Waddell, J. of Appl. Polym Sci., vol. 59, , W. H. Wadell, J. H. Haver, L. R. Evans and J. H. Harwell, J. of Appl. Polym. Sci., vol. 55, , Sato, Y. Kanbayashi, K. Kobayaschi and Y. Shimo, J. Catalysis, 1,34,

7 15. M. J.M. Abadie et M. Rouby, Eur. Polym. J., 3, (9), 151 (1994). 16. D. Wank, L. Carrera and M.J.M. Abadie, Eur. Polym. J., 9, (1), 1379 (1993). 17. M.J.M. Abadie, Euro. Coat. J., 7-8, 518 (1993). 18. M. Rouby and M.J.M. Abadie, Eur. Polym. J., 31, (3), 31 (1995). 19. W. K. Busfield, Polymer Handbook, third edition, J. Bandrup Ed., J. Wiley & Sons, New York (1989). 7

Figure 4.10 HPLC Chromatogram of the Carbazole-Phenoxy Based Methacrylate

Figure 4.10 HPLC Chromatogram of the Carbazole-Phenoxy Based Methacrylate The percent yield of the methacrylation was 85.2 %, with a purity of 98.2 % determined by HPLC (Figure 4.10). Elemental analysis gave excellent agreement to expected elemental ratios (Table 4.2). Disregarding