Study of the effect of external heating and internal temperature build-up during polymerization on the morphology of porous polymethacrylate adsorbent
Chan Yi Wei, Clarence M. Ongkudon, Tamar Kansil
Abstract. Modern day synthesis protocols of methacrylate monolithic polymer adsorbent are based on existing polymerization schemes without an in-depth understanding of the dynamics of pore structure and formation. This has resulted in ineffectiveness of polymer adsorbent thereby affecting final product recovery and purity, retention time, productivity and process economics. The problems magnified in monolith scaling-up where internal heat buildup resulting from external heating and high exothermic polymerization reaction was reflected in cracking of the adsorbent. We believe that through careful and precise control of the polymerization kinetics and parameters, it is possible to prepare macroporous methacrylate monolithic adsorbents with controlled pore structures despite being carried out in an unstirred mould. This research involved the study of the effect of scaling-up on pore morphology of monolith, in other words, porous polymethacrylate adsorbents that were prepared via bulk free radical polymerization process by imaging the porous morphology of polymethacrylate with scanning electron microscope.
Monolithic supports are novel developing technology with high potential, more so than conventional particulate supports. A lot of researches and developments have been conducted in the past decade to utilize monolithic supports as the stationary phase in chromatographic separation due to its scalable feasibilities and better hydrodynamics. The reason lies within the presence of interconnected macro pores in monolithic sorbents that allow convective transport mechanism instead of diffusion that features as the only mean of transport mechanism for particulate support. The monolith hydrodynamic property is predominated by convective transport mechanism, an important feature of a chromatography of larger molecules that are unable to penetrate into the internal structure of particulate support . Monolithic support also features lower pressure drop that varies with different pore structure orientations [2, 3]. Such feature allows for higher mobile phase flow rates to be applied which could enhance the separation efficiency. Despite the low absolute surface area, the increase in flow rate actually more than makes up for the lost capacity for larger molecules due to smaller specific surface area. The comparison of physical characteristics between monolithic supports and particulate supports extend much further than pore size alone .
Monolith is constructed in an unstirred mould that features significant lack of interfacial tension between an aqueous and an organic phase thus leading to large interconnected flow-through channels. In contrast, bead polymers prepared from identical polymerization mixtures but in a suspension polymerization process do not exhibit the same type of macroporous structure with large flow-through channels . Unstirred mould also results in poorer heat transfer, thus leading to formation of temperature gradient across the monolith sorbent with nuclei forming at different rates and porous channel forming at different sizes . This inherent issue magnifies during monolith scaling-up with obvious cracks observed during polymerization process. The key to achieving controlled macroporous structure is dependent on gaining control over the process kinetics within the unstirred mould (e.g. temperature of reaction) .
This work involved the use of scanning electron microscope to visualize the morphology of porous methacrylate monolithic polymer under different porogen concentrations (50%, 60% and 70%) and different scales (2ml and 150ml) which provided a better insight on the effect of scaling-up on pore morphology.
MATERIALS AND METHOD
The monolith was prepared via free radical co-polymerization of cross-linker EDMA and GMA as functional monomers. EDMA/GMA mixture was combined with an alcohol-based porogen solvent in the proportion of 35/15/50(GMA/EDMA/cyclohexanol) making a solution with a total volume of 160ml. AIBN (1% weight with respect to monomer) was added to initiate the polymerization reaction. The polymer mixture was sonicated for 20 minutes. The mixture of 2ml and 150ml were gently transferred into conical 0.8 cm ? 4 cm polypropylene column (BIORAD) and 5.0 cm x 10 cm Econo column (BIORAD) respectively sealed at the bottom end. The top end was sealed with a parafilm sheet and placed in a water bath for 3 h at 60oC. Same method was repeated for 21/9/70 and 28/12/60 (GMA/EDMA/cyclohexanol) mixture. For conical 0.8 cm x 4 cm polypropylene column, the polymer resin was washed with 400ml methanol at room temperature to remove all porogens and other soluble matters. The polymer was then washed with 200ml deionized water at room temperature to remove trapped air bubbles. Slightly different washing method for econo 5.0 cm x 10 cm, the polymer resin was extracted and placed in 1.0 L beaker filled with 600ml of methanol followed by placing it inside incubator shaker overnight under 140 rpm and 37oC. The next day, methanol was replaced with 600ml of deionised water under same incubation condition for 4 hours. For analysis of monolith morphology, the monolith was oven dried at 70a-¦C overnight and scanning electron microscopy was done at 15 kV using high resolution scanning electron microscope (Hitachi S-3400N, Japan) according to the manufacturer’s instructions.
RESULTS AND DISCUSSION
As can be observed from Fig. 1, both small scale and large scale porous polymethacrylate sorbents featured the increment of globules and pores size as the concentration of porogen was increased while monomer and cross-linked agent decreased. This phenomenon was due to the fact that an increase in the EDMA concentration led to the formation of more cross-linked nuclei and magnified by the presence of more functional monomer GMA consequently limiting their swelling which resulted in the concentration of the monomers in the swollen gel nuclei becoming lower than that in the solution. Hence, the chances of newly formed nuclei adsorbed by the macro pre-formed globules by coalescence of nuclei in abundance decreased greatly. The decline in local concentration of monomer decreased the size of the globules and thus contributing to the overall decrease in the pore size.
External heating and exothermic heat buildup associated with the construction of polymethacrylate sorbent also play a role in the pore formation. The rate of initiator decomposition and free radicals formation rely heavily on temperature. The rate of radicals formation declines significantly at lower temperature than at higher temperature which results in lesser number of nuclei formed per unit time. This allows the coalescence of many nuclei that result in formation of larger preglobules and larger pore size as well as delayed formation of monolith. The same is true for pore formation at higher temperature. High level of exothermic free radical copolymerization reaction and external heating contribute greatly to immense heat buildup within the polymerization mixture. Reaction that takes place in an unstirred mould could contribute to exothermic heat buildup to a certain degree. Hence, the relative differences in the rate of radicals formation, nuclei and pore sizes can be deduced by observing the results in Fig. 2. The effect of heat buildup was profoundly increased in 150ml volume, in which cracking occurred and the monolith was considered unreliable. It was presumed that the exothermic heat build-up led to pressure build-up which eventually forced the monolith structure to break apart.
Small Scale (2ml)
Large Scale (150ml)
FIGURE 1. Effect of both cross-linking agent and monomer concentration in the polymerization mixture on the surface morphology of methacrylate monolith. Polymerizations were carried out with a constant monomer ratio (EDMA/GMA) of 30/70; porogen concentrations of 50%, 60% and 70%; polymerization temperature of 60 a-¦C; AIBN concentration of 1% (w/w) of monomers. The SEM pictures show increased pores size with increased concentration of porogen in the polymerized feedstock. Microscopic analysis was performed at 15 kV.
FIGURE 2. The effect of exothermic heat associated with the construction of large scale (150ml) polymethacrylate monolithic column on the surface morphology of methacrylate monolith. Polymerizations were carried out with a constant monomer ratio (EDMA/GMA) of 30/70; porogen concentrations of 70%; polymerization temperature of 60 a-¦C; AIBN concentration of 1% (w/w) of monomers. The SEM pictures show heterogenous globules and pores size distribution due to instant heat buildup generated from external heating and high exothermic reaction associated with the construction of polymethacrylate monolith. Microscopic analysis was performed at 15 kV.
There were not many differences when we compared the polymethacrylate adsorbents of both small scale and large scale monolith from 50%, 60% and 70% porogen content in terms of globules and pore sizes (Fig. 1). However, the effect of exothermic heat buildup was evident (data not shown) in large-scale monolith and without a doubt contributed to heterogeneous pore size distribution across the adsorbent compared to small scale monolith as evident from Fig. 2. Thus, further analysis is required in characterizing the pore size from different sections of the adsorbent in order to obtain a conclusive summary of the effect of scaling-up on the pore size distribution.
We would like to thank UMS (University Malaysia Sabah) Research Priority Grant for funding this project that is essential in establishing the foundation for next step forward on the scaling up of monolithic adsorbent.