## 3 Results
### Characterization of Copolymers
Considering the differences in S\({}_{\text{SET}}\) of 4 VP copolymers, the higher value for poly(4 VP-_co_-TRIM) may result from the functionality of a crosslinker. The 14 DMB is a bifunctional monomer, whereas  
Figure 4: SEM images of 4 VP copolymers and poly(St-_co_-DVB). TRIM is trifunctional. Using a crosslink with a higher number of polymerizable methacrylategroups, the phase separation between the microgel and liquid phase is more efficient. The formed microgels are highly crosslinked, and they are not able to dissolve each other as well as in a porogen. Hence, they aggregate into small gel particles which are tightly packed into the final microsphere. When 14 DMB is used as a crosslinker, the created microgels are less crosslinked than those with TRIM, the phase separation is hindered, and bigger agglomerates with bigger pores between them are formed, as is visible in Figure 4.  
Taking into consideration the PSD presented in Figure 5a, it is visible that they are of bimodal character. The 4 VP copolymers are characterized by a broad PSD, whereas the PSD of poly(St-_co_-DVB) is narrower, which is convergent with the SEM images. The 4 VP copolymers reveal mesopores and macropores in their internal structure; poly(St-_co_-DVB) is mesoporous. All the studied materials possess the first maximum PSD at about 4 nm. It is a characteristic maximum reported earlier for crosslinked porous polymers synthesized via heterogeneous polymerization techniques [31]. This observation was attributed by [PERSON] [32] to a very regular structure of the polymer on the micro-level scale and probably corresponds to the pores between nuclei. Since the studied copolymers were synthesized to be used as sorbents for SPE columns, it is worth knowing their internal porous structure in the swollen state. For this purpose, the copolymeric beads were packed into chromatographic columns and ISEC experiments were performed. The ISEC method allows the determination of the pore size distribution of a sorbent in the range from micropores to macropores with the use of phthalates, alkylphenones, and polystyrene standards. It can be seen from Figure 5b that in the internal structure of all the studied sorbents, micropores are present. The maxima on the PSD-ISEC curves in the range of 1-2 nm are designated as micropores, whereas the maxima at 20-40 nm correspond to mesopores. The pore size distribution determined on the basis of the ISEC measurements is of a different character than the PSD curves obtained from LTNA. It is a typical observation for porous polymers [33; 34; 35]. Moreover, taking into account the data presented in Table 1, it should be noted that there are significant differences in pore volume for dry and swollen copolymers. Generally, the V\({}_{\text{P}}\) values determined by ISEC are higher than those obtained from LTNA, but the difference is greater for 4 VP copolymers than for poly(St-_co_-DVB). In addition, all studied copolymers in swollen states reveal the presence of micropores in their structure. The volume of the micropores (V\({}_{\text{micro}}\)) does not exceed 20% of the total pore volume of the 4 VP copolymers, but for poly(St-_co_-DVB), the micropore contribution is 35%. Two reasons for the difference in porosity of dry and swollen materials can be considered. Firstly, the micropores present in the structure of a dry copolymer are  
Figure 5: The PSD obtained for studied microspheres from: (**a**) the BJH method using desorption plot; (**b**) the ISEC measurements. inaccessible to nitrogen molecules [35]; moreover, it should be emphasized that the values of specific surface area and pore volume were calculated using the 16.2 A\({}^{2}\)A\({}^{2}\) surface of a nitrogen molecule, which is normally used for hydrophilic surfaces, whereas the studied copolymers are not a typical hydrophilic material [36]. Secondly, the internal structure of swollen copolymers is changed by the interactions of the polymeric network with THF.