# Three-Dimensional-Printed Polymeric Cores for Methane Hydrate Enhanced Growth
## 4 Discussion  
As noted above, this work did not study nucleation. At the same time, it should be stressed that nucleation in such systems can occur at the water-gas, water-gas-metal, water-gas-sapphire (for example, see [53]), and water-gas-polymer interfaces with the gas-in-water concentration playing a vital role in the nucleation process [54]. The literature has repeatedly noted that hydrate nucleation at contact with a polymer is infrequent (for instance, on fluoroplastic and polycarbonate [55; 56; 57]). In most instances, nucleation undoubtfully prevailed at the water-gas or water-gas-metal interface (see Figures 4-9, Figures S3 and S5, and Videos S1-S9). As for the hydrate growth, Figure 10 summarizes the findings. This work revealed that the porous polymer insertions placed within the water volume could enhance methane hydrate formation (up to complete water-to-hydrate conversion for PLA sample compared to 4-5% for the water-gas system with no insertion; Table 3) with the type of polymer affecting the hydrate formation mode. All polymer cores except PolyFlex and PLA* promoted the hydrate growth. In the case of PolyFlex and PLA*, hydrate growth was observed only on the surfaces of water-gas and sapphire windows (Figure 10b). The formation of massive hydrate bodies both in the water volume and on the polymer cores' surface was not registered. Since the effective porosity differs slightly for PLA and PLA* samples (19.4 and 18.8%, Table 2), but the extrusion width differed by a factor of 2 (200 versus 400 \(\upmu\)m), it can be assumed that the main reason for the difference in the water-to-hydrate conversion for these samples (about 100 and 6%, Table 3) is the trajectory of connected pores (does not allow transfer between water and gas for PLA*). Large pores (6 and 8% of pores with a diameter of 2.8 and 4 mm; Figure S1) may also be an affecting factor. In the case of PolyFlex, the low value of effective porosity seems to hinder the transfer between water and gas. Figure 9: Methane hydrate formation from 2 mL of deionized water with ABS insertion; the blue arrow indicates the hydrate onset site. However, the polymer itself may also have some influence (for example, the adhesion energy of the hydrate to the surface of a given polyurethane). When it comes to the water volume effect, this is not an influencing factor if the polymer core does not affect the hydrate growth (compare the water-to-hydrate conversion for different water volumes with no insertion and with PLA*). The hydrate forms first on the water-gas surface and then slowly grows on the sapphire window's surface. If the samples sink in water, growth should proceed similarly to a system without a core. Models  
Figure 10: (**a**) The initial view of a system; (**b**) hydrate formation with PLA*, PolyFlex, and without a polymeric core; (**c**) hydrate formation with PLA, UltraX, ePC, and ABS (5 mL of water); (**d**) hydrate formation with ABS (2 mL of water); the red row line in subfigures (**b–d**) corresponds to the initial water level; (**e**) gas transfer through both upper and side surfaces for partial filling of porous space; (**f**) gas transfer through upper surface for complete filling of porous space; (**g**) growth of massive hydrate bodies from the cell’s walls (for PLA, ePC, UltraX) and blowing of hydrate crystals into the volume of water (for PLA, ePC, UltraX, and ABS); side arrows indicate the direction of hydrate bodies growth; the vertical ones show the motion of the core. made from ABS and ePC do not sink in water, even under methane pressure (Figure 10f). At the same time, with an increase in the filling of the porous volume with water from partial (with 2 mL of water) to complete (with 5 mL of water), the efficiency of hydrate growth decreases by two times (see Table 3). This suggests that the upper surface of the core and its ends make approximately the same contribution to the transfer of reagents between the aqueous and gas phases during hydrate growth.