News Reader.info

ABSTRACT The phase change temperature and enthalpy change as a function of polystyrene (PS) concentration in dimethylformamide through a dynamic heating and quenching process were

investigated. Cold crystallization, freezing and melting phenomena in a 10 wt% PS solution were all observed. Cold crystallization and melting phenomena were still observed in a 20 wt%

solution. In a 30 wt% solution, all three phenomena disappeared without any solvent enthalpy changes, e.g., enthalpy changes at the melting temperature. The disappearance of both the melting

temperature and the melting enthalpy change indicated that all polymer and solvent molecules in the 30 wt% solution existed only in the amorphous phase without any phase changes despite

repeated heating and quenching processes. Thus, our results can provide a new approach for gelation through enthalpy changes and can be applied in the fabrication of porous membranes with a

narrow distribution. SIMILAR CONTENT BEING VIEWED BY OTHERS GYROID NANOSTRUCTURED SOFT MEMBRANES FORMED BY CONTROLLING THE DEGREE OF CROSSLINKING POLYMERIZATION OF BICONTINUOUS CUBIC

LIQUID-CRYSTALLINE MONOMERS Article 27 October 2020 MISCIBILITY AND TERNARY DIAGRAM OF AQUEOUS POLYVINYL ALCOHOLS WITH DIFFERENT DEGREES OF SAPONIFICATION Article Open access 31 May 2023

STRUCTURES AND PROPERTIES OF TEMPERATURE-RESPONSIVE GELS WITH HOMOGENEOUS NETWORKS PREPARED BY PHOTOGELATION OF FOUR-ARMED STAR-SHAPED POLY(_N_-ISOPROPYLACRYLAMIDE) Article 24 December 2024

INTRODUCTION Polymer chains can be completely extended in a very dilute solution when they are dissolved in an appropriate solvent. A single polymer chain is usually in a coil form in

solution due to balanced interactions with the solvent and the polymer itself1. Molecular interactions with solvents and polymers have typical properties that can be applied to many

industrial processes, and they play important roles in applications, such as membrane casting and formation2,3,4,5,6, battery separator preparation7,8,9, and polymer synthesis10,11, as well

as in determining solubility parameters12,13,14,15. Recently, a 30 wt% polystyrene (PS) solution in dimethylformamide (DMF) was studied in an ultralow temperature region by Samitsu et al16.

While preparing mesoporous polymer nanofiber networks by flash freezing the polymer solution, the authors observed a cold crystallization temperature (Tc) at − 85 ~ − 65 °C. The change in Tc

with polymer concentration suggested that the crystallization of the solvent in the polymer-rich phase was limited. Additionally, the molecular interaction between polymer and solvent is

closely related to the gelation of the polymer solution17,18,19,20 and has been studied in thermal insulation21, thermoreversible gels22,23,24 and size-controlled microgels25. Gelation can

occur by physical linking or by chemical linking between the chains, resulting in a single macroscopic molecule where the gel point is defined and the solution loses fluidity and suddenly

becomes very viscous26. The gelation of a polymer solution has been studied mainly by various gel formation mechanisms, such as the attraction between molecules as a function of

temperature27, the formation of organometallic complexes26, and the attractive interaction between a few hydrophobic groups28,29. The polymer and solvent that constitute the polymer solution

have their own melting temperature (Tm) and enthalpy change (ΔHm). When the polymer in the polymer solution is the main component, the polymer’s Tm is observed, the solvent’s Tm is not

observed, and the glass transition temperature (Tg) of the polymer varies with the polymer type and the affinity of the solvent30. In this study, the polymer was not considered alone, but

the polymer solution in which the solvent is the main component was the focus. We studied the phenomena of the solvent in solution in which both the enthalpy change (ΔHc) at the Tc and the

enthalpy change (ΔHm) at the Tm of the solvent upon heating became zero, and thus, the Tm and Tc of the solvent disappeared. In this regard, this thermal behavior is discussed further in

comparison with conventional gelation. To understand the characteristics of the solvent itself in polymer solutions, the Tc and Tm of DMF were measured through a dynamic cooling and heating

scan process. The Tc, Tm, ΔHc, and ΔHm of the solvent in the polymer solution consisting of PS and DMF were measured and compared. In addition, a new approach for understanding the gelation

phenomena was investigated by examining Tm and ΔHm by varying the concentration of the polymer solution. Further studies were carried out in solutions of PS and _N_-methyl-2-pyrrolidone

(NMP), polyethersulfone (PES) and DMF, and PES and NMP, as shown in the Supplementary Information. Based on the experimental results, we constructed a schematic diagram showing the molecular

states between the polymer chains and solvent molecules at different polymer concentrations as a function of temperature. RESULTS PHASE CHANGE BEHAVIOR OF SOLVENTS A dynamic quenching and

heating scan process by differential scanning calorimetry (DSC) is shown schematically in Scheme 1. The DSC measurement procedure was carried out by a sequential temperature process, as

shown in Scheme 1, in which the solvent or polymer solution was quenched at room temperature, the temperature was decreased to an ultralow temperature and held at − 170 °C for 5 min to

stabilize the solution, and then the solution was heated to room temperature and quenched again. The supercooled solvent molecules that were not partially crystallized and existed in the

frozen amorphous state became active as the temperature increased, resulting in a cold crystallization phenomenon below Tm. Then, during the heating scan, all previously formed crystals were

melted at the Tm, and when they were cooled again, crystallization reversibly occurred at Tf. At Tf, Tc, and Tm, corresponding enthalpy changes (ΔHs) and changes in the degree of

crystallization (x) occurred because phase changes took place in which the crystalline and amorphous phases reversibly changed. First, to observe the crystallization phenomenon of the pure

solvent, Tf and Tm were measured by changing the heating and cooling rates according to Scheme 1, as shown in Fig. 1. Note that steps ① quenching and ③ quenching in Scheme 1 were performed

at the same cooling rate. The DSC graph was plotted with the data measured during steps ② heating and ③ quenching shown in Scheme 1. Here, the data in ① quenching were not used to eliminate

the thermal history. The method for determining Tf and Tm in the DSC thermogram is described in detail in the Methods section, where ΔHm was calculated by integrating the peak area in the

DSC thermogram. Here, the DSC thermograms of Figs. 1 and 2 are plotted and were compared by shifting them vertically for a clear comparison. Cooling the DMF at room temperature with varying

rates of − 70 K/min, − 10 K/min and − 1 K/min, Tf appeared at − 113 °C, − 96 °C and − 83 °C, respectively, and Tm was observed at − 60 ± 10 °C. Thus, Tf was greatly affected by the cooling

rate, and the measured ΔHf was − 94 J/g with − 1 K/min and − 121.6 J/g with − 10 K/min. The value of ΔHm was determined to be 128 J/g. When the cooling rate was − 10 K/min, a small

difference was observed in the value of ΔHf (− 122 J/g), which is the enthalpy change in pure DMF molecules from an amorphous liquid state to a crystalline state, and ΔHm (128 J/g) is

assumed due to the DSC measurement error. The literature values for the Tm and ΔHm of DMF are 212.9 K (− 60.1 °C) and 122.4 J/g, respectively31. CRYSTALLIZATION BEHAVIOR OF POLYMER SOLUTIONS

The experimental results of the polymer solutions of PS and DMF according to Scheme 1 are shown in Fig. 2. The temperature and enthalpy change behaviors were investigated when the PS

concentrations increased to 10, 15, 20, 24 and 30 wt%. The DSC graph was plotted with data measured during steps ② heating (10 K/min) and ③ quenching (− 70 K/min), as shown in Scheme 1. When

the polymer solution was supercooled to below Tf, the solvent tended to nucleate and crystallize to reduce the Gibbs free energy32,33,34. As shown in Fig. 2a, a 10 wt% PS solution (PS/DMF

(10/90)) was quenched at − 70 K/min and then heated at 10 K/min to increase the mobility of amorphous state DMF molecules, and at point A (Tc), cold crystallization occurred, resulting in an

exothermic peak (ΔHc). When the temperature was increased to melting point B (Tm) of the solvent, the solvent became a liquid, and the crystalline phase of the molecules disappeared and

became an amorphous liquid phase. At this time, ΔHm appeared as an endothermic peak. Considering the general phase change, the following Eq. (1) can be given, since the enthalpy changes are

state functions. $$\Delta {\text{H}}_{{\text{c}}} + \Delta {\text{H}}_{{\text{m}}} + \Delta {\text{H}}_{{\text{f}}} = \, 0.$$ (1) In Scheme 1, the sum of ΔHf due to the phase change at Tf in

step ③ quenching, the ΔHc produced by cold crystallization in step ② heating, and the ΔHm during melting in step ② heating are “0”. Here, if we represent xf and xc as the degree of

crystallization at Tf and Tc, respectively, and if xm is the degree of crystallization change at Tm to the amorphous phase, then xm = xf + xc, since all crystals disappear at Tm. When the

solution was quenched at all concentrations in Fig. 2, the DSC heat flow showed a distinct curve from − 110 to − 150 °C. This phenomenon shows the mechanical characteristics of the DSC

process. That is, when the solution was quenched to approximately − 170 °C with a fast cooling rate of − 70 K/min, heat was applied to match this cooling rate, and the curve appeared to show

an unstable baseline, which was due to equipment problems. Therefore, it was difficult to accurately measure ΔHf in the ultralow temperature region below − 100 °C, and the value of ΔHf was

replaced by the value calculated by Eq. (1). Since there were changes in ΔHf and ΔHm due to the effect of the polymer on the Tf and Tm of the solution, ΔHf,D and ΔHm,D were introduced to

represent these quantitative values. ΔHm,D is the difference at Tm between the calculated enthalpy change and the experimentally measured enthalpy change when the solvent weight fraction

excluding the polymer in the solution is considered as pure solvent and calculated. Thus, the values ΔHm,D and ΔHf,D can be a factor in determining the polymer influence on the pure solvent.

Equations (2) and (3) show the calculation methods of ΔHf,D and ΔHm,D, respectively. At the Tf of the pure solvent, a freezing enthalpy change value of − 122 J/g was used, similar to the

literature value, and at the Tm of the pure solvent, an experimental enthalpy change value of 128 J/g was used. $$\Delta {\text{H}}_{{{\text{f}},{\text{D}}}} = { 122 }* \phi_{1} + \Delta

{\text{H}}_{{\text{f}}} \left( {{\text{J}}/{\text{g}}} \right),$$ (2) $$\Delta {\text{H}}_{{{\text{m}},{\text{D}}}} = { 128 }* \phi_{1} - \Delta {\text{H}}_{{\text{m}}} \left(

{{\text{J}}/{\text{g}}} \right),$$ (3) where \({\phi }_{1}\) is the weight fraction of DMF in the polymer solution. Table 1 shows the measurement values in Figs. 1 and 2. In Fig. 2a, the

PS/DMF (10/90) solution showed the Tc and Tm at − 116 °C and − 54 °C, respectively. Considering the Tf values of the pure DMF solvent to be where Tc was not seen (Fig. 1 and Table 1) and

considering the ΔHc value of − 42 J/g of the PS/DMF (10/90) solution, some pure solvent molecules formed crystallites, and the other molecules remained in the amorphous frozen phase,

resulting in cold crystallization during the heating scan. For the PS/DMF (10/90) solution in Table 1, ΔHf,D was 49 J/g and ΔHm,D was 12 J/g because the number of DMF crystallites in the

solution produced with PS interference was less than that produced in the pure DMF solution. That is, the hindered crystallization seemed to be due to steric hindrance as well as the

molecular interaction between the polymer and solvent35. The experimental measurements were reproducible, as shown in Supplementary Fig. 9. In Fig. 2b, when the PS concentration was 15 wt%,

Tc and Tm appeared at − 108 °C with ΔHc equal to − 48 J/g and − 54 °C with ΔHm equal to 87 J/g, respectively. In solutions with 10 wt% and 15 wt% PS, the ΔHf,D values were 49 J/g and 65 J/g,

respectively. Comparing the enthalpy change values between the 15 wt% and 10 wt% PS solutions, at the higher polymer concentration, fewer solvent crystallites were formed in the quenching

process due to the influence of the polymer interaction. A similar phenomenon was observed for ΔHm,D, i.e., a value of 12 J/g for the 10 wt% PS solution and a value of 21 J/g for the 15 wt%

PS solution. In Fig. 2c, when 20 wt% PS was used, Tc and Tm were − 90 °C and − 57 °C, respectively, with similar absolute values of ΔHc (− 62 J/g) and ΔHm (61 J/g). According to Eq. (1), ΔHf

approached 0 J/g, indicating that the solid DMF that was frozen under Tf was almost a crystallite-free amorphous phase. The xc generated by cold crystallization at Tc can be estimated from

ΔHc, which was equivalent to the degree of crystallization (xm) that disappeared at Tm. In Fig. 2d, the behavior of the 24 wt% PS solution tended to be the same as that of the 20 wt%

solution, except that the absolute values of ΔHc and ΔHm were further reduced. In Fig. 2e, the 30 wt% PS solution did not show any unique peaks, implying that there was no physical change

during the DSC scan. When quenched or heated, the solution contained a sufficient amount of PS compared to the amount of DMF in the solution, which resulted in a sharp increase in viscosity,

reduced diffusion, increased physical interactions between molecules, and no crystallite formation. Additionally, no cold crystallization occurred. This phenomenon observed in a 30 wt% PS

solution indicates that the state of the polymer and the solvent in the solution was only the amorphous phase without a phase change, even if the cooling and heating processes were repeated,

meaning that they were in an amorphous frozen phase at low temperatures and an amorphous liquid phase at high temperatures. The solution in this state must have 100% molecular interaction

between the polymer and the solvent, making it similar to the gelation phenomenon. In this regard, comparative experiments were carried out. Typically, gelation experiments are performed by

rapidly increasing the viscosity21. In a simple experiment, as shown in Supplementary Fig. 6, the physical gelation was determined by taking into account the degree of flow when the PS/DMF

(30/70) solution was inverted after 24 h of storage at − 5 °C and − 20 °C. In Supplementary Fig. 6, the PS/DMF (30/70) solution flowed well, even when it was inverted. In contrast, the

PES/DMF (30/70) solution did not flow. In Supplementary Fig. 1, the ΔHc and ΔHm of the PES/DMF (30/70) solution became 0, and Tc and Tm disappeared. This phenomenon was the same as that of

the PS/DMF (30/70) solution. Figure 3 shows the measurement results by a Brookfield viscometer, revealing a sudden change in viscosity. In both cases, there was a sharp increase in

viscosity, but the viscosity of the PS/DMF (30/70) solution was 1,334.5 cP, and the solution could flow. On the other hand, the viscosity of the PES/DMF (30/70) solution was higher than the

maximum measurable value of 9,999 cP of the equipment used, and the PES/DMF (30/70) solution did not flow. In this experiment, PS/DMF (30/70) and PES/DMF (30/70) appeared to have lost the Tc

and Tm, and the enthalpy changes, ΔHc and ΔHm, were both zero. Thus, it was questionable whether gelation had occurred in the PS/DMF (30/70) solution. ENTHALPY CHANGE BEHAVIOR OF POLYMER

SOLUTIONS Figure 4 shows the enthalpy change trend in Table 1 and Fig. 2. In Fig. 4a, |ΔHc| increased and then began to decrease after reaching ΔHm, which monotonously decreased. In Fig. 4b,

|ΔHf|, calculated by Eq. (1), decreased monotonically, and ΔHf,D and ΔHm,D increased. The two values reflect the degree of differences between the states in which the polymer and solvent

are not supposed to interact with each other and the actual state in which the polymer interacts with the solvent. Thus, the influence between the polymer chains and the solvent molecules

could be estimated. For the PS/DMF (20/80) solution, ΔHf was 0, and |ΔHc| and ΔHm had the same value. In the PS/DMF (30/70) solution, the interaction between the solvent and polymer was

maximized so that all Tc, Tm, ΔHc and ΔHm parameters disappeared. SCHEMATIC DIAGRAM OF PS/DMF SOLUTIONS BASED ON DSC MEASUREMENTS As described above, Fig. 5 shows the enthalpy change

graphically according to the PS concentrations of the crystalline and amorphous phases due to the phase change in the polymer chain and solvent molecules in the PS/DMF solution. The small

boxes in Fig. 5 show the phases of the polymer and solvent molecules on the basis of the enthalpy change and degree of crystallization. In Fig. 5a, the solvent molecules are in the amorphous

state at room temperature to temperatures higher than Tm, and the molecules are represented as ‘ ’. When quenched at − 70 K/min from room temperature to the supercooled region, almost all

of the solvent molecules are expected to freeze at Tf, indicating solvent crystals as ‘ ’. When heated at 10 K/min, it can be seen that the solvent crystal molecules return to an amorphous

molecular state ( ) at Tm, which is supported by the close values of ΔHf and ΔHm of pure DMF listed in Table 1. In a 10 wt% PS solution (Fig. 5b), the polymer chain ( ) and solvent molecules

( ) are homogeneously mixed in the amorphous liquid state at room temperature and distributed as shown in Fig. 5b. When quenched, both frozen amorphous solvent molecules ( ) and crystalline

solvent molecules ( ) coexist below Tf. During the heating scan of the quenched solution, the degree of crystallization (xc) of amorphous solvent molecules ( ) increases due to cold

crystallization. At Tm, the solvent molecules ( , ) in which the two phases coexist are converted to the liquid amorphous state ( ). In Table 1, the number of solvent crystallites formed at

Tc in Fig. 5b in the PS/DMF (10/90) solution can be estimated from ΔHc (− 42 J/g), and ΔHm was 103 J/g. According to Eq. (1), ΔHf can be estimated to be approximately − 61 J/g. The molecular

arrangement of some amorphous solvent molecules ( ) changed to crystallite solvent molecules ( ) as they passed through Tc during the quenching and heating scan cycle, as shown in the small

box in Fig. 5b. As shown in Fig. 5c, during the quenching and heating scan of the 20 wt% PS solution, crystallite solvent molecules ( ) were produced at Tc, and they changed into the

amorphous phase ( ) at Tm. In Table 1, the values of ΔHc and ΔHm were − 62 J/g and 61 J/g, respectively, and their absolute values were almost identical. Based on this result, solvent

crystallization did not occur during quenching up to − 170 °C, and no solvent crystallites were formed or existed as a frozen amorphous phase ( ), which would have been generated only by

cold crystallization. At room temperature to above Tm, the solution existed as a liquid amorphous phase ( ), so the molecular arrangements of the PS/DMF (20/80) solution at − 170 °C and room

temperature were the same. In Fig. 5d, the 30 wt% PS solution existed as a homogeneous solution () at room temperature, similar to the 10 wt% and 20 wt% PS solutions, but the enthalpy

change (ΔH) due to the phase change during quenching and heating did not appear, and both Tc and Tm disappeared. In this solution, the phase of the solvent molecules affected by the polymer

chain was the same during the following scan cycle, i.e., ① at room temperature, ② quenching below Tf, ③ heating above Tc, and ④ returning to room temperature. The results explained that the

liquid phases were frozen and then liquefied as an amorphous phase without crystallization. A strong interaction between the polymer and the solvent is necessary to break down the

crystallites of the polymer and to dissolve it36. On the other hand, the interaction or steric hindrance of the polymer mediates the crystallite formation in the solvent or breaks down the

solvent crystallites. As a result, below Tf, the frozen amorphous phase was completely formed under certain conditions; despite the repeated cooling and heating cycles, crystallization could

not occur. Thus, no amorphous phase to crystal phase change could be observed, but a liquid to solid state change could be observed under certain conditions. Both of these phenomena were

found to occur simultaneously in a 30 wt% PS solution. APPLICATIONS OF ENTHALPY CHANGES IN POLYMER SOLUTIONS PORE SIZE DISTRIBUTION OF MESOPOROUS MATERIALS In Fig. 6a,b, the cross-sectional

scanning electron microscopy (SEM) images of samples prepared with the PS/DMF (22/78) solution without annealing were obtained at magnifications of 50,000 and 300,000, respectively. Figure 

6c,d show samples after 24 h of annealing that were maintained at − 80 °C for 24 h. In Supplementary Fig. 5, the average pore sizes measured by a surface area analyzer (Micromeritics,

ASAP2420) were 16 nm and 20 nm, and the average pore size increased due to the growth of solvent DMF crystallites during the annealing treatment. The strut structures of PS open cells showed

that the strut structures of the samples formed after 24 h of annealing were thicker than those without annealing. In Supplementary Fig. 5, the Brunauer–Emmett–Teller (BET) surface area was

measured and found to be 281 m2/g and 236 m2/g, respectively. Thus, the annealed sample accelerated nucleation and growth of solvent crystallites, resulting in smaller surface areas due to

larger open cell pore sizes and thicker struts compared to samples with no annealing, and mesoporous materials with narrower pore size distributions were obtained, as shown in Fig. 6 and

Supplementary Fig. 5. A NEW APPROACH FOR GELATION Figure 5 shows the crystallization of the solvent molecules and the change in the amorphous phase. The presence of the frozen amorphous

phase of solvent molecules was determined by the effect of the polymer concentration on mediating solvent crystallization. In other words, the interaction between solvent and polymer was

measured, which might explain the gelling phenomenon. For example, neither the PS/DMF (30/70) solution nor the PES/DMF (30/70) solution showed Tc and Tm, and consequently, the enthalpy

changes were all zero. It was concluded that both solutions were gelled. By applying another method to verify the gelation, in Supplementary Fig. 6, unlike the gelled PES/DMF (30/70)

solutions, flow was observed for PS/DMF (30/70) solutions. Then, conditions were applied to prevent gelation of the PS/DMF (30/70) solution, in contrast to the ΔHm test method. Thus, the

phenomena in which ΔHc, ΔHm and ΔHf are all zero might be proposed as an alternative and new approach for gelation. DISCUSSION The phase change temperature and enthalpy change behavior of

polymer solutions in the ultralow temperature region were studied. As the cooling scan rate was increased, the Tf of pure DMF changed to − 83 °C, − 96 °C and − 113 °C, but Tm was

approximately − 61 °C with a ΔHm of 128 J/g, which are close to the literature values. The phase change temperature of the solvent DMF with the cooling and heating scan cycles was confirmed.

The PS/DMF solution was quantitatively analyzed for Tc, Tm, ΔHc and ΔHm by repeating the cooling and heating processes. When the PS concentrations increased to 10 wt%, 15 wt%, 20 wt% and 24

wt%, Tc shifted to − 116 °C, − 108 °C, − 90 °C, and − 75 °C, respectively, and the ΔHc values were − 42 J/g, − 48 J/g, − 62 J/g, and − 18 J/g, respectively, with their absolute values

increasing and decreasing; ΔHm simply decreased to 103 J/g, 87 J/g, 61 J/g, and 20 J/g, respectively. Especially in the PS/DMF (30/70) solution, Tc, Tm, ΔHc and ΔHm disappeared. The increase

in the ΔHm,D value with increasing polymer concentration was in good agreement with the fact that increasing the molecular interactions or affinity between polymer and solvent mediates the

formation of solvent crystallites. As the polymer concentration increased, |ΔHc| ≈ ΔHm and ΔHf ≈ 0 for PS/DMF (20/80). For PS/DMF (30/70), Tc did not appear, Tm did not appear, and ΔHc = ΔHm

 = 0. By adjusting the polymer concentration, the pore size could be controlled. That is, after annealing at Tc, the average pore size was changed from 16 to 20 nm through the quenching and

heating process of the concentrated polymer solution. We found that the PS/DMF (30/70) solution showed a rapid increase in viscosity due to the strong interaction between the solvent and

polymer; however, flow was observed, and the enthalpy change approached zero. METHODS PREPARATION OF THE POLYMER SOLUTION PS (Mn = 240,000 g/mol) was supplied by Kumho Petrochemical Ltd. and

PES (Mn = 75,000 g/mol) was obtained from BASF Ultrason 6020P. DMF (99.0%) and NMP (99.5%) as solvents were purchased from Samchun Chemical Co. PS and PES were dried at 50 °C and 100 °C for

24 h in a vacuum oven to remove moisture completely. Casting solutions were prepared by dissolving PS and PES in DMF and NMP, respectively, at the desired concentration for 3 h at room

temperature and stirring at 100 °C for 12 h. Finally, the solution was kept at room temperature for 24 h to remove air bubbles. DSC MEASUREMENTS The quenching and heating data of the polymer

solution were measured by a DSC instrument (NETSCH, Polyma 214) equipped with a liquid nitrogen cooler. Aluminum pans were used and tested in a closed state. A DSC sample mass of

approximately 10–20 mg was transferred using a micropipette. For the DSC measurements, as shown in Scheme 1, Figs. 1 and 2, the circled numbers ①, ②, and ③ represent the sequence of

processes. Specifically, the solution was first cooled to − 170 °C by ① quenching (− 70 K/min) and stabilized for 5 min, heated to room temperature by ② heating (10 K/min), followed by ③

quenching (− 70 K/min) again to − 170 °C. To eliminate the thermal history of the solution, ① quenching was ignored, and the ② heating and ③ quenching results were used, as shown in Figs. 1

and 2. PREPARATION OF POROUS MATERIALS Porous material preparation experiments were carried out in a glove box in a controlled manner under a nitrogen atmosphere. The prepared polymer

solutions were dropped on polyethylene terephthalate (PET) films and cast using a casting knife. The cast solution was quenched with liquid nitrogen, and the quenched casting solution was

subjected to solvent exchange using nonsolvent methanol in a cryogenic freezer. Porous materials were obtained by drying at 50 °C using a vacuum oven to remove the residual solvent

completely. SEM OBSERVATIONS The morphology of porous materials from the polymer solution (Supplementary Fig. 5) was visualized with a Hitachi FE-SEM SU8220 Field-Emission Scanning Electron

Microscope at the Western Seoul Center, Korea Basic Science Institute. The film sample was completely dried in a vacuum oven at 50 °C and cut using liquid nitrogen. To observe the cross

section of the sample, the sample was adhered to the SEM sample holder using carbon adhesive tape and then sputtered at 30-s intervals using an osmium coater (vacuum device, HPC-30). Cross

sections of the coated samples were observed at magnifications of 50,000 and 300,000. GAS ADSORPTION MEASUREMENTS The pore size distribution was measured using a volumetric gas adsorption

apparatus (Micromeritics, ASAP2420) to confirm the pore size control of the porous materials using a polymer solution (Fig. 6 and Supplementary Fig. 6). One hundred milligrams of the dried

sample was filled in a glass sample tube and pretreated at 50 °C to remove the adsorbed substances on the sample for 24 h. Nitrogen adsorption isotherms were measured at 77 K, and the pore

distribution was determined using the BET method. REFERENCES * Su, W. F. _Principles of Polymer Design and Synthesis Polymer Size and Polymer Solutions_ (Springer, Berlin, 2013). Book 

Google Scholar  * Caicedo-Casso, E. _et al._ A rheometry method to assess the evaporation-induced mechanical strength development of polymer solutions used for membrane applications. _J.

Appl. Polym. Sci._136, 47038 (2019). Article  Google Scholar  * Zhao, J., Chong, J. Y., Shi, L. & Wang, R. Explorations of combined nonsolvent and thermally induced phase separation

(N-TIPS) method for fabrication novel PVDF hollow fiber membranes using mixed diluents. _J. Membr. Sci._572, 210–222 (2019). Article  CAS  Google Scholar  * Kurada, K. V. & De, S. Role

of thermodynamic and kinetic interaction of poly(vinylidene fluoride) with various solvents for tuning phase inversion membranes. _Polym. Eng. Sci._58, 1062–1073 (2017). Article  Google

Scholar  * Sadeghi, A., Nazem, H., Rezakazemi, M. & Shirazian, S. Predictive construction of phase diagram of ternary solutions containing polymer/solvent/nonsolvent using modified

Flory-Huggins model. _J. Mol. Liq._263, 282–287 (2018). Article  CAS  Google Scholar  * Sadrzadeh, M. & Bhattacharjee, S. Rational design of phase inversion membranes by tailoring

thermodynamics and kinetics of casting solution using polymer additives. _J. Membr. Sci._441, 31–44 (2013). Article  CAS  Google Scholar  * Lu, W. _et al._ Advanced porous PBI membranes with

tunable performance induced by the polymer-solvent interaction for flow battery application. _Energy Storage Mater._10, 40–47 (2018). Article  Google Scholar  * Bae, J. Y. _et al._ Polar

polymer-solvent interaction derived favorable interphase for stable lithium metal batteries. _Energy Environ. Sci._12, 3319–3327 (2019). Article  CAS  Google Scholar  * Djian, D., Alloin,

F., Martinet, S. & Lignier, H. Macroporous poly(vinylidene fluoride) membrane as a separator for lithium-ion batteries with high charge rate capacity. _J. Power Sources_187, 575–580

(2009). Article  ADS  CAS  Google Scholar  * Wenning, C., Barbe, S., Achten, D., Schmidt, A. M. & Leimenstoll, M. C. Prediction of initial miscibility for ternary polyurethane reaction

mixtures on basis of solubility parameters and Flory-Huggins theory. _Macromol. Chem. Phys._219, 1700544 (2018). Article  Google Scholar  * Benamer, S., Mahlous, M., Boukrif, A., Mansouri,

B. & Youcef, S. L. Synthesis and characterisation of hydrogels based on poly(vinyl pyrrolidone). _Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact Mater. Atoms._248, 284–290

(2006). Article  ADS  CAS  Google Scholar  * Mazinani, S., Darvishmanesh, S., Ehsanzadeh, A. & van der Bruggen, B. Phase separation analysis of Extem/solvent/non-solvent systems and

relation with membrane morphology. _J. Membr. Sci._526, 301–314 (2017). Article  CAS  Google Scholar  * Moradihamedani, P., Ibrahim, N. A., Yunus, W. M. Z. W. & Yusof, N. A. Separation

of CO2 from CH4 by pure PSF and PSF/PVP blend membranes: effects of types of nonsolvent, solvent, and PVP concentration. _J. Appl. Polym. Sci._130, 1139–1147 (2013). Article  CAS  Google

Scholar  * Ferrell, W. H., Kushner, D. I. & Hickner, M. A. Investigation of polymer-solvent interactions in poly(styrene sulfonate) thin films. _J. Polym. Sci. Pt. B-Polym. Phys._55,

1365–1372 (2017). Article  ADS  CAS  Google Scholar  * Emerson, J. A., Toolan, D. T. W., Howse, J. R., Furst, E. M. & Epps, T. H. Determination of solvent-polymer and polymer-polymer

Flory-Huggins interaction parameters for poly(3-hexylthiophene) via solvent vapor swelling. _Macromolecules_46, 6533–6540 (2013). Article  ADS  CAS  Google Scholar  * Samitsu, S. _et al._

Flash freezing route to mesoporous polymer nanofibre networks. _Nat. Commun._4, 1–7 (2013). Article  Google Scholar  * Tan, H. M., Moet, A., Hiltner, A. & Baer, E. Thermoreversible

gelation of atactic polystyrene solutions. _Macromolecules_16, 28–34 (1983). Article  ADS  CAS  Google Scholar  * Hong, P. D. & Huang, H. T. Effect of polymer-solvent interaction on

gelation of polyvinyl chloride solutions. _Eur. Polym. J._35, 2155–2164 (1999). Article  CAS  Google Scholar  * Hong, P. D., Chou, C. M. & Chuang, W. T. Effects of mixed solvent on

gelation of poly(vinyl alcohol) solutions. _J. Appl. Polym. Sci._79, 1113–1120 (2001). Article  CAS  Google Scholar  * Li, S. G., van den Boomgaard, T., Smolders, C. A. & Strathmann, H.

Physical gelation of amorphous polymers in a mixture of solvent and nonsolvent. _Macromolecules_29, 2029–2053 (1996). Article  Google Scholar  * Samitsu, S. Thermally stable mesoporous

poly(ether sulfone) monoliths with nanofiber network structures. _Macromolecules_51, 151–160 (2018). Article  ADS  CAS  Google Scholar  * Kumar, S. K. & Douglas, J. F. Gelation in

physically associating polymer solutions. _Phys. Rev. Lett._87, 188301 (2001). Article  ADS  Google Scholar  * Kobayashi, K., Huang, C. I. & Lodge, T. P. Thermoreversible gelation of

aqueous methylcellulose solutions. _Macromolecules_32, 7070–7077 (1999). Article  ADS  CAS  Google Scholar  * Semenov, A. N. & Rubinstein, M. Thermoreversible gelation in solutions of

associative polymers. 1. Statics. _Macromolecules_31, 1373–1385 (1998). Article  ADS  CAS  Google Scholar  * Omari, A., Chauveteau, G. & Tabary, R. Gelation of polymer solutions under

shear flow. _Colloids Surf. A-Physicochem. Eng. Asp._225, 37–48 (2003). Article  CAS  Google Scholar  * Gulrez, S. K. H., Al-Assaf, S. & Phillips, G. O. _Progress in Molecular and

Environmental Bioengineering: From Analysis and Modeling to Technology Applications_ (Books on Demand, Norderstedt, 2011). Google Scholar  * de Carvalho, W. & Djabourov, M. Physical

gelation under shear for gelatin gels. _Rheol. Acta_36, 591–609 (1997). Article  Google Scholar  * Aubry, T. & Moan, M. Rheological behavior of a hydrophobically associating water

soluble polymer. _J. Rheol._38, 1681–1692 (1994). Article  ADS  CAS  Google Scholar  * Tanaka, F. & Koga, T. Intramolecular and intermolecular association in thermoreversible gelation of

hydrophobically modified associating polymers. _Comput. Theor. Polym. Sci._10, 259–267 (2000). Article  CAS  Google Scholar  * van Krevelen, D. W. & te Nijenhuis, K. _Properties of

Polymers: Their Correlation with Chemical Structure; Their Numerical Estimation and Prediction from Additive Group Contributions_ (Elsevier, Amsterdam, 2009). Book  Google Scholar  *

Smirnova, N. N., Tsvetkova, L. Y., Bykova, T. A. & Marcus, Y. Thermodynamic properties of N,N-dimethylformamide and N,N-dimethylacetamide. _J. Chem. Thermodyn._39, 1508–1513 (2007).

Article  CAS  Google Scholar  * Wang, B., Ji, J., Chen, C. & Li, K. Porous membranes prepared by a combined crystallisation and diffusion (CCD) method: Study on formation mechanisms. _J.

Membr. Sci._548, 136–148 (2018). Article  CAS  Google Scholar  * Franks, F., Mathias, S. F. & Trafford, K. The nucleation of ice in undercooled water and aqueous polymer solutions.

_Colloids Surf._11, 275–285 (1984). Article  CAS  Google Scholar  * Hollomon, J. H. & Turnbull, D. Nucleation. _Prog. Metal Phys._4, 333–388 (1953). Article  ADS  CAS  Google Scholar  *

Wang, B., Ji, J. & Li, K. Crystal nuclei templated nanostructured membranes prepared by solvent crystallization and polymer migration. _Nat. Commun._7, 1–8 (2016). ADS  Google Scholar  *

Łabudzińska, A. & Ziabicki, A. Effect of composition and gelation conditions on structural changes accompanying the gelation of PAN, PVA and gelatin solutions. _Colloid Polym. Sci._243,

21–27 (1971). Google Scholar  Download references ACKNOWLEDGEMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT

(No. NRF-2018R1D1A1B07047567). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Department of Polymer Science and Engineering, Chungnam National University, 99 Daehak-Ro, Yuseong-Gu, Deajeon,

34134, Republic of Korea Mi Rae Kim, Kang Ho Cheon & Kee Yoon Lee * Western Seoul Center, Korea Basic Science Institute, 150 Bugahyeon-Ro, Seodaemun-Gu, Seoul, 03759, Republic of Korea

Hee Jung Park * SepraTek, 730 Gyejok-Ro, Daedeok-Gu, Daejeon, 34396, Republic of Korea Choong Kyun Yeom Authors * Mi Rae Kim View author publications You can also search for this author

inPubMed Google Scholar * Hee Jung Park View author publications You can also search for this author inPubMed Google Scholar * Kang Ho Cheon View author publications You can also search for

this author inPubMed Google Scholar * Choong Kyun Yeom View author publications You can also search for this author inPubMed Google Scholar * Kee Yoon Lee View author publications You can

also search for this author inPubMed Google Scholar CONTRIBUTIONS All authors contributed to this research works. M.R.K. designed and carried out experiments such as DSC, membrane

fabrication, and gelation. H.J.P. analyzed the experimental results and participated in manuscript preparation. K.H.C. conducted a basic experimental investigation. C.K.Y. analyzed the

experimental results and K.Y.L. was responsible for the overall progress of the project. CORRESPONDING AUTHOR Correspondence to Kee Yoon Lee. ETHICS DECLARATIONS COMPETING INTERESTS The

authors declare no competing interests. ADDITIONAL INFORMATION PUBLISHER'S NOTE Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional

affiliations. SUPPLEMENTARY INFORMATION SUPPLEMENTARY INFORMATION. RIGHTS AND PERMISSIONS OPEN ACCESS This article is licensed under a Creative Commons Attribution 4.0 International License,

which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a

link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license,

unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory

regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit

http://creativecommons.org/licenses/by/4.0/. Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Kim, M.R., Park, H.J., Cheon, K.H. _et al._ Novel behavior in a polymer solution:

the disappearance of the melting temperature (Tm) and enthalpy change (ΔHm) of the solvent. _Sci Rep_ 10, 13348 (2020). https://doi.org/10.1038/s41598-020-70331-4 Download citation *

Received: 13 December 2019 * Accepted: 21 July 2020 * Published: 07 August 2020 * DOI: https://doi.org/10.1038/s41598-020-70331-4 SHARE THIS ARTICLE Anyone you share the following link with

will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard Provided by the Springer Nature SharedIt

content-sharing initiative