Development of temperature-responsive polymeric gels with physical crosslinking due to intermolecular 𝜋–𝜋 interactions

Abstract

Poly(N-isopropylacrylamide) PNIPAAm was polymerized with co-monomers containing a biphenyl moiety to create a unique thermoresponsive physically crosslinked system due to the presence of pi-pi interactions between the biphenyl moieties. The biphenyl monomers used were 2-phenylphenol monoacrylate (2PPMA) and 4-phenylphenol monoacrylate (4PPMA). These monomers were utilized to synthesize a set of polymers with biphenyl monomer (2PPMA/4PPMA) content from 2.5 to 7.5 mole percent and with initiator concentrations from 0.1 and 1.0 weight percent. The resulting polymers were characterized by various techniques, such as gel permeation chromatography (GPC), swelling studies and mechanical testing. The decrease in the average molecular weight of the polymers due to the increase in the concentration of initiator was confirmed by GPC results. Swelling studies confirmed the expected temperature dependent swelling properties and explored the impact of the biphenyl comonomers. These studies indicated that with the increase in biphenyl comonomers, the physical crosslinking increases which leads to decrease in the swelling ratio. The results from the mechanical tests also depict the effect of the concentration of biphenyl comonomers. These physically crosslinked polymeric systems with their unique properties have potential applications spanning environmental remediation/sensing, biomedicine, etc.

Graphical Abstract:

Thermoresponsive physically crosslinked polymeric system was prepared by incorporating n-isopropylacrylamide and biphenyl comonomers. These polymers were characterized for their molecular weight, swelling properties and their compression modulus.

1. INTRODUCTION

Stimuli responsive materials respond to an external stimulus (i.e., heat, pH, magnetic field, electric field, light, ionic strength, etc.) by changing their configuration or physical properties.^1-3 Temperature responsive polymers, specifically poly(N-isopropylacrylamide) (PNIPAAm), are one of the most widely studied stimuli responsive materials.^4-8 PNIPAAm is responsive to temperature changes due to the sharp phase transition at its lower critical solution temperature (LCST) (~32°C). Below the LCST, PNIPAAm is hydrophilic in nature and therefore dissolves in water; as one increases the temperature (above LCST), it turns hydrophobic, precipitating out in an aqueous solution due to a coil-to-globule-to-aggregate transition. Studies have shown this transition is due to the interruption in hydrogen bonding as well as the increase in hydrophobic interaction between the polymer chains as temperature increases, which causes the polymer to precipitate in water.^9-18 Due to this temperature responsive property, polymers with NIPAAm have been used in various applications such as drug delivery, pollutant capture, tissue engineering, etc.^19-28

Multi-functional PNIPAAm systems based on copolymerization of PNIPAAm with various other functional comonomers have been widely used in the above-mentioned applications.^29-36 Copolymerizing NIPAAm with a comonomer leads to an additional functionality of the system as well as alteration of the LCST. In previous studies, it has been shown that incorporating a hydrophilic comonomer increases the LCST, whereas the incorporation of hydrophobic comonomers decreases it.^37-44

Crosslinked PNIPAAm forms a temperature responsive hydrogel, which exhibit greater swelling in water at temperatures below the LCST and minimal swelling at temperatures above the LCST.^45-49 Crosslinkers can be classified as either chemical crosslinkers or physical crosslinkers. Chemical crosslinkers form a covalent bond between NIPAAm chains in the system, and these bonds entail strong interactions and are suitable when an irreversible system is desired. Physical crosslinkers lead to potentially reversible interactions between the NIPAAm chains such as hydrogen bonding, hydrophobic interactions, ionic interactions, etc.^50-53 These physically crosslinked systems can be preferred for situations where the reversibility of the crosslinking is used for processing or application of the material. An example of a functional moiety that can act as a physical crosslinker is a biphenyl ring. Biphenyl groups are known to have mesogenic properties that can be used to form liquid crystalline polymers with nematic, smectic and cholesteric phases, and these rings can possess a physical interaction between them in the form of pi-pi stacking interactions (as shown in Figure 1) which allows intermolecular interactions between multiple chains of PNIPAAm resulting in a physically crosslinked structure.^54-57

Figure 1:

pi-pi interactions acting as a crosslinker between 2 PNIPAAm growing chains

In this study, we have synthesized a physically crosslinked system of PNIPAAm with previously developed comonomers that contain biphenyl moiety, 2-phenylphenol monoacrylate (2PPMA)/4-phenylphenol monoacrylate (4PPMA) (structures shown in Figure 2). Upon copolymerizing NIPAAm with 2PPMA/4PPMA, we expect the system to behave in a physically crosslinked manner in water due to the intermolecular pi-pi interactions shown in Figure 1 between the intermittent biphenyl comonomers. This report details the synthesis of these unique polymers as well as their characterization via gel permeation chromatography (GPC), swelling studies, and mechanical testing.

Figure 2:

Structures of the monomers used. (a) N-isopropylacrylamide (NIPAAm), (b) 2-phenylphenolmonoacrylate (2PPMA), (c) 4-phenylphenolmonoacrylate (4PPMA).

2. EXPERIMENTAL

2.1. Materials

The monomer N-isopropylacrylamide (NIPAAm) and initiator ammonium persulfate (APS) were purchased from Sigma-Aldrich Corporation (St. Louis USA). All organic solvents were purchased from Sigma-Aldrich and Fisher Scientific (Hampton USA). Chemicals were used as received and without any further purification. No commercial comonomers were used in this work; 2PPMA and 4PPMA were used as comonomers. Both 2PPMA and 4PPMA were prepared in house and have the biphenyl functionality necessary to provide non-covalent crosslinked network with NIPAAm.

2.2. Preparation of Non-Covalently Crosslinked Hydrogels

The comonomers 2PPMA and 4PPMA used in this report have been previously synthesized by our group and the synthesis procedure is reported in the literature. In brief, 2-phenylphenol and 4-phenylphenol were reacted with acryloyl chloride which resulted in a conversion of the hydroxyl group to acrylate group forming 2PPMA and 4PPMA respectively.⁵⁷

Three different ratios (2.5, 5.0 and 7.5 mol %) of the biphenyl comonomer (2PPMA and 4PPMA) with NIPAAm were polymerized through free radical polymerization. The initiator amount was also varied in the system (0.1 and 1.0 % of the total polymer mass). The reaction was carried out with dimethylsulfoxide (DMSO) as the solvent. For consistency, the total moles of monomers to the volume of DMSO was kept constant at 2.5 mmol/ml. Feed compositions are given in Table 1.

Table 1:

Feed compositions and the nomenclature of the polymeric films used

System	Comonomer	APS (wt. %)	NIPAAm (mole %)	Comonomer (mole %)	Name
NIPAAm:2PPMA	2PPMA	0.1	97.5	2.5	2PP 0.1_2.5
			95.0	5.0	2PP 0.1_5.0
			92.5	7.5	2PP 0.1_7.5
		1.0	97.5	2.5	2PP 1.0_2.5
			95.0	5.0	2PP 1.0_5.0
			92.5	7.5	2PP 1.0_7.5
NIPAAm:4PPMA	4PPMA	0.1	97.5	2.5	4PP 0.1_2.5
			95.0	5.0	4PP 0.1_5.0
			92.5	7.5	4PP 0.1_7.5
		1.0	97.5	2.5	4PP 1.0_2.5
			95.0	5.0	4PP 1.0_5.0
			92.5	7.5	4PP 1.0_7.5

For a typical system of 2PP_2.5_0.1, the total number of moles (NIPAAm and 2PPMA) used are 2.5 mmol. Accordingly, 275.4 mg of NIPAAm with 14 mg of 2PPMA was added to 1 ml DMSO and dissolved to prepare a uniform solution. An aqueous solution of initiator APS at 0.05 g/mL (0.5 g/mL for 1.0% APS systems) was added to the mixture to comprise of 0.1 wt.% of total combined weight of NIPAAm and 2PPMA. The reaction mixture was then pipetted into a shell vial (15mm × 45mm) and nitrogen was bubbled through the solution to promote an inert environment in the vial headspace. The vial was kept in an 80°C water bath for an hour. After removing it from the water bath, the viscous polymer solution was pipetted out drop by drop using a repetitive pipette (for a consistent droplet size) into a beaker containing DI water at 20°C. Upon addition to water, these droplets instantly precipitated out resulting in spherical beads. The distance between the tip of the pipette and the water surface in which the droplet was precipitated was kept approximately 13 cm to get a consistent shape of the bead. These polymeric beads were then washed with DI water for 48 hours to remove any unreacted monomer and after the wash process, half of the beads were stored in DI water in a 20 ml vial at 20°C until further use (labelled as Group 1) and the remaining beads were dried in a vacuum oven at 50°C and 6 inHg pressure overnight (labelled as Group 2).

2.3. Characterization of Polymers

2.3.1. Molecular Weight Measurements

GPC was used to determine the molecular weight of the synthesized polymers in a Shimadzu Prominence LC-20 AB HPLC system installed with a Waters 2410 refractive index detector. A Polargel-M 300x7.5 mm 8μm column (Agilent) was used for separation. The linear range of calibration for molecular weight determination was from 1,840 DA to 2,210,000 DA of polymethylmethacrylate (PMMA) standards (Agilent). A set of polymeric beads from Group 1 were dissolved in DMSO at 5-7 mg/ml and passed through the GPC for determination of the molecular weight. Five replicates were completed for each polymeric system and the values are reported with their respective standard deviation.

2.3.2. Temperature Responsive Swelling Studies

A set of 5 polymeric beads of each system from the Group 1 were examined for their swelling properties while varying the temperature. These beads were added to a 4 ml vial with DI water, and the vial was kept in a water bath, varying the temperature from 15°C to 50°C with increments of 5°C per 24 hrs. At each 5°C increment the swollen mass was measured. After reaching 50°C, the films were dried in a vacuum oven at 50°C and 6inHg pressure overnight, after which the dry weight could be recorded. The swelling ratio was given by:

$$Swellingratio(q)=\frac{M_{swollen}}{M_{dry}}$$

where M_swollen is the swollen mass of the bead at a particular temperature and M_dry is the dry mass of that bead. Five replicates were completed for each polymeric system and the values are reported with their respective standard deviation.

2.3.3. Reversible Swelling Studies

Pulsatile temperature change was used for reversible swelling studies. The gels from Group 1 were first added to a 4 ml vial with DI water and the vial was placed in a 15°C water bath and allowed to swell until they reached equilibrium (24 hours). At this point, they were measured for their equilibrium swelling ratio. After this, the vial was then quickly transferred to a 50°C water bath and allowed to deswell until they reached equilibrium (24 hours), and the swelling ratio was recorded. After this, they were again transferred back to the 15°C water bath. This cycle was repeated five times, measuring the swelling ratio at each step. After 5 cycles of pulsatile temperature change the beads were dried in a vacuum oven at 50°C and 6inHg pressure overnight, and the dried weight was recorded. The swelling ratio was given by the same relation defined above in Section 2.3.2. Five replicates were completed for each polymeric system and the values are reported with their respective standard deviation.

2.3.4. Kinetic Swelling Studies of Dried Polymers

A set of 5 dried polymeric beads of each system from the Group 2 were examined for their kinetic swelling properties at 15°C. These dried beads were added to a 4 ml vial with DI water, and the vial was kept in a water bath at 15°C, and the swelling ratio was measured at 24 hours interval for next 10 days. The films were then dried again in a vacuum oven at 50°C and 6inHg pressure overnight, after which the dry weight could be recorded. Five replicates were completed for each polymeric system and the values are reported with their respective standard deviation.

2.3.5. Mechanical Testing

The polymeric beads were evaluated for their compression modulus with unconfined compression testing using a BOSE ELF 3300 system without any initial preload. A set of 5 beads for each polymer from Group 1 were deformed at a rate of 0.005 mm/sec until the polymer fractured or was compressed to 1.5 mm. These experiments were carried at 20°C. The polymeric beads were spherical in nature and therefore the stress and strain data were calculated by the analysis described in these reports.^58-60 The compressive moduli correspond to the slope of stress vs. strain curve up to a strain of 0.2. Five replicates were completed for each polymeric system and the values are reported with their respective standard deviation.

3. RESULTS AND DISCUSSION

3.1. Non-Covalently Crosslinked Hydrogel Synthesis

The reaction of NIPAAm with the biphenyl comonomer in DMSO after the addition of APS took place in a nitrogen environment at 80°C. After the reaction, the solution was a viscous liquid which was then pipetted out and added drop by drop to DI water at 20°C. Upon contact with water, these drops precipitated out instantly forming a spherical polymer bead. The average diameter as well as the average mass of the swollen beads were measured after the washing step and the values are given in Table 2. The formation of spherical polymer beads leads to the conclusion that the system forms an insoluble solid in water, which was hypothesized to be a result of the pi-pi interactions between the biphenyl moieties that act as a non-covalent crosslinking site. A schematic of the non-covalently crosslinked polymeric hydrogels can be seen in Figure 3. The presence of these non-covalent interactions were also tested by performing their dissolution test in DMSO which is described in the Supporting Information.

Table 2:

Average diameter and Average mass of the swollen beads in water at 20°C.

Name	Average diameter (mm)	Average mass (mg)
2PP 0.1_2.5	3.93 ± 0.10	23.26 ± 1.42
2PP 0.1_5.0	3.42 ± 0.07	20.73 ± 0.60
2PP 0.1_7.5	3.39 ± 0.02	16.00 ± 0.60
2PP 1.0_2.5	4.38 ± 0.05	26.16 ± 1.04
2PP 1.0_5.0	3.50 ± 0.08	23.22 ± 0.4
2PP 1.0_7.5	3.34 ± 0.05	17.33 ± 0.72
4PP 0.1_2.5	3.81 ± 0.02	26.67 ± 2.23
4PP 0.1_5.0	3.49 ± 0.02	18.46 ± 1.07
4PP 0.1_7.5	3.35 ± 0.06	21.92 ± 2.00
4PP 1.0_2.5	4.40 ± 0.08	34.90 ± 1.61
4PP 1.0_5.0	3.50 ± 0.08	25.25 ± 0.55
4PP 1.0_7.5	3.33 ± 0.05	27.20 ± 1.41

Figure 3:

Schematics of physically crosslinked polymeric hydrogels. Black lines represent the PNIPAAm chains while the blue lines represent the biphenyl pi-pi interactions.

3.2. Characterization of Polymers

3.2.1. Molecular Weight Measurements

GPC was used to determine the molecular weight distribution of the polymers. All the values represented in Figure 5 and Table 2 indicate the polymer’s ‘PMMA-equivalent molar masses’. Figure 5 depicts weight-average molecular weights for synthesized polymers. As expected, the average molecular weight decreased with increasing APS content from 0.1 to 1.0 percent.⁶¹ Table 2 shows the weight-average molecular weight, number-average molecular weight and polydispersity index (PDI) for individual polymers. High PDI’s are obtained for the synthesized polymers which can be related to uncontrolled free radical polymerization which leads to formation of a broad range of molecular weights. ^62-65 It can also be seen from the figure that the weight-average molecular weight of the polymers decreases as the amount of biphenyl monomer in the feed composition increases, which is likely the result of biphenyl monomers acting as a chain transfer reagent leading to premature termination of growing chains.

Figure 5:

Weight average molecular weight of the polymers calculated by GPC. Data were plotted as mean ± standard deviation and 3 measurements were taken for each sample

3.2.2. Temperature Responsive Swelling Studies

The temperature responsive swelling studies of the polymers were conducted in water from 15°C to 50°C. The resulting swelling ratios can be seen in Figure 6. The differences between swelling ratios of 5.0 and 7.5 mole percent for each polymeric system can be visualized in Figure S2 of the Supporting Information. Figure 6 illustrate that these polymers exhibit greater swelling at lower temperatures, as expected with PNIPAAm-based systems.^49,66 The swelling ratio was found to decrease with increasing amount of the biphenyl monomer in the feed composition, which is expected due to the intermolecular non-covalent interactions that act as crosslinking sites.⁶⁷ Interestingly, the majority of systems with a higher initiator content (1.0% APS) swell more than the systems with a lower initiator content (0.1% APS). This indicates that fewer potential non-covalent interactions present in the shorter chains dominates the swelling response of these systems. It can also be seen from the graph that the 2PPMA system swells more than the 4PPMA system. It is hypothesized that this is a result of steric hinderance within the biphenyl group of 2PPMA molecules due to the presence of a bulky group at the ortho position. This prevents a strong intramolecular interaction to exist within the 2PPMA monomer as compared to 4PPMA, where the biphenyl groups are expected to be present in a coplanar structure leading to stronger intramolecular interactions and thus lesser swelling ratio. There might also be presence of some intrachain biphenyl interactions as compared to interchain which may lead to some looping as well.

Figure 6:

Temperature responsive swelling study of (a) 2PP 0.1 (NIPAAm:2PPMA system with 0.1 percent APS), (b) 2PP 1.0, (NIPAAm:2PPMA system with 1.0 percent APS) (c) 4PP 0.1, (NIPAAm:4PPMA system with 0.1 percent APS) (d) 4PP 1.0 (NIPAAm:4PPMA system with 1.0 percent APS). All the legends indicate feed composition of the polymer. Data were plotted as mean ± standard deviation and 5 measurements were taken for each sample.

3.2.3. Reversible Swelling Studies

Reversible swelling and deswelling properties were tested for the polymeric beads in this study and the results are shown in Figure 7. The differences between swelling ratios of 5.0 and 7.5 mole percent for each polymeric system can be visualized in Figure S3 of the Supporting Information. The beads were subjected to pulsatile temperature change from a high temperature of 50°C to a low temperature of 15°C. This pulsatile temperature change was repeated for 5 cycles, and the results indicate that beads showed a decrease in the reswelling ratios for the second and third cycle before reaching a stable reversible equilibrium. It is hypothesized that this can be attributed to two main effects: polymeric chains that are not actively participating in physical crosslinking being released from the polymer beads in the initial swelling/deswelling cycles and additional physical crosslinking being formed during the initial swelling/deswelling cycles. The release of polymer chains and associated loss of mass during the initial swelling/deswelling cycles was confirmed by comparing the initial dried mass and the final dried mass (see Supporting Information Table S1 and Figure S4). Since this mass loss does not account for all of the drop in swelling ratio, the additional decrease in swelling ratio is likely attributable to the formation of additional physical crosslinking during the initial swelling/deswelling cycles as the polymer chains are able to rearrange with biphenyl moieties having the opportunity to form additional physical crosslinking. In summary, during the pulsatile temperature change, the swelling ratio remained constant for the remaining cycles, once unattached polymeric chains were released, and physical crosslinking was maximized in the first few cycles.

Figure 7:

Reversible swelling studies of (a) 2PP 0.1 (NIPAAm:2PPMA system with 0.1 percent APS), (b) 2PP 1.0 (NIPAAm:2PPMA system with 1.0 percent APS), (c) 4PP 0.1 (NIPAAm:4PPMA system with 0.1 percent APS), (d) 4PP 1.0 (NIPAAm:4PPMA system with 1.0 percent APS). All the legends indicate feed composition of the polymer. Data were plotted as mean ± standard deviation and 5 measurements were taken for each sample

3.2.4. Kinetic Swelling Study of Dried Polymers

The kinetic swelling study of the dried polymers (Group 2) were studied at 15°C. The results are shown in Figure 8. As it can be seen from the results, the time it takes for the network to swell and reach equilibrium in water is around 8 days, once the polymers were dried. The 2PP_2.5_0.1 and the 2PP_2.5_1.0 systems were difficult to handle due to high swelling ratio after 6 and 7 days, respectively, and thus, their swelling ratio and their dried mass were not calculated after that point of time. At these low amounts of 2PPMA, the systems have very high swelling ratios at low temperature (15°C) and limited mechanical integrity.

Figure 8:

Kinetic swelling studies dried polymers (a) 2PP 0.1 (NIPAAm:2PPMA system with 0.1 percent APS), (b) 2PP 1.0 (NIPAAm:2PPMA system with 1.0 percent APS,) (c) 4PP 0.1 (NIPAAm:4PPMA system with 0.1 percent APS), (d) 4PP 1.0 (NIPAAm:4PPMA system with 1.0 percent APS). All the legends indicate feed composition of the polymer. Data were plotted as mean ± standard deviation and 5 measurements were taken for each sample

After 10 days, the gels were dried, and the final dried mass was compared to the initial dried mass (see Supporting Information Table S2 and Figure S5). The results indicate that there was not any significant mass loss during the kinetic swelling of these dried polymers, which indicates that the drying process assisted in forming a physically crosslinked network with maximized crosslinking sites. By comparing the equilibrium swelling ratio for this study to the equilibrium swelling ratio for the reversible swelling study (15°C for the fifth cycle), it is clear that the equilibrium swelling ratio for the kinetic swelling study is higher than the equilibrium swelling ratio for the reversible swelling study. This is likely attributed to the mass loss for the reversible swelling study, whereas there is not a significant mass loss during the kinetic swelling study of the dried polymers.

3.2.5. Mechanical Testing

The compressive moduli of the swollen beads are shown in Figure 9. A representative stress vs. strain curve can be found in the Supporting Information (See Figure S6). The system is swollen in water and therefore acts as a crosslinked gel. The results indicated there was a strong effect of biphenyl monomer concentration on the compression modulus. As expected, with increase in the amount of the biphenyl monomer in the feed composition, the non-covalent crosslinking sites increases resulting in the increase of compressive moduli. Interestingly, there isn’t much change in the compression modulus values between the 2PPMA and 4PPMA system which may likely be due to the concentration of biphenyl monomer being very low (2.5 to 7.5 %) in both systems to depict a significant difference in their compression modulus values.

Figure 9:

Mechanical testing of the (a) 2PP 0.1 (NIPAAm:2PPMA system with 0.1 percent APS), (b) 2PP 1.0 (NIPAAm:2PPMA system with 1.0 percent APS), (c) 4PP 0.1 (NIPAAm:4PPMA system with 0.1 percent APS), (d) 4PP 1.0 (NIPAAm:4PPMA system with 1.0 percent APS) to determine their compression modulus. Data were plotted as mean ± standard deviation and 5 measurements were taken for each sample

4. CONCLUSIONS

Temperature responsive non-covalently crosslinked polymers containing NIPAAm with varying biphenyl comonomer (2PPMA/4PPMA) amount and varying initiator content were successfully synthesized. The presence of non-covalently crosslinked interactions was determined by the ability of the gel to reversibly swell in water and dissolve in DMSO. The average molecular weight data illustrate that an increase in the amount of biphenyl monomers leads to a decrease in the molecular weight. The temperature responsiveness of the polymers, the effect of physical crosslinker and the effect of the comonomer was demonstrated by the swelling studies. Mechanical tests of the polymer beads indicated that an increase in amount of the comonomer leads to increase in the compression modulus of the synthesized polymer. These materials with unique properties are expected to have potential applications spanning environmental remediation/sensing, biomedicine, etc.

Table 3:

Number-average molecular weight, weight-average molecular weight as well as PDI of the NIPAAm:2PPMA/NIPAAm:4PPMA polymeric systems

Polymer	Comonomer	APS content	Comonomer mole per centage	Number- average molecular weight (g/mol)	Weight- average molecular weight (g/mol)	Dispersity (Đ)
PNIPAAm 0.1	-	0.1	0.0	493,300	1,406,000	2.85
PNIPAAm 1.0		1.0	0.0	400,200	1,090,000	2.72
2PP 0.1_2.5	2PPMA	0.1	2.5	122,800	374,100	3.05
2PP 0.1_5.0		0.1	5.0	68,300	267,000	3.91
2PP 0.1_7.5		0.1	7.5	74,200	267,000	3.60
2PP 1.0_2.5		1.0	2.5	82,100	296,100	3.61
2PP 1.0_5.0		1.0	5.0	63,400	188,400	2.97
2PP 1.0_7.5		1.0	7.5	41,800	149,000	3.56
4PP 0.1_2.5	4PPMA	0.1	2.5	267,400	550,700	2.06
4PP 0.1_5.0		0.1	5.0	186,800	467,300	2.50
4PP 0.1_7.5		0.1	7.5	143,700	414,200	2.88
4PP 1.0_2.5		1.0	2.5	256,600	414,300	1.61
4PP 1.0_5.0		1.0	5.0	146,700	339,000	2.31
4PP 1.0_7.5		1.0	7.5	94,600	279,100	2.95