Thermo-responsive rheological behavior of borinic acid polymer in dilute solution

Abstract

In comparison with other boron containing polymers, the investigations on the borinic acid containing polymers have been mostly ignored in spite of their easy synthesis. Our previous studies indicate a borinic acid polymer (PBA) holds promising potential as a new type of stimuli-responsive polymer. However, the property investigations were mainly based on turbidity changes. Here, we report the rheological property investigations of PBA solution. The PBA dilute solution (0.1 wt%) in DMSO exhibits a relatively high modulus (G′ of ∼60 Pa and G′′ of ∼15 Pa) at the frequency of 1 rad s⁻¹ at 25 °C. Temperature dependent rheology results reveal the thermo-responsive rheological behavior of the PBA dilute solution. Mechanism investigations indicate the coil–microgel transition of the PBA chains plays important roles in the thermo-responsive rheological behavior of the PBA dilute solution. This work therefore extends the field of thermo-responsive polymeric materials and opens up a new avenue in the applications of borinic acid containing polymers, where thermo-responsive rheology changes play an important role.

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Introduction

In the past decades, ever-increasing research interests have been attracted by the field of stimuli-responsive or “smart” polymers, which undergo reversible or irreversible significant property changes upon relatively small external stimuli such as pH, temperature, light irradiation, ion strength, electric and magnetic field, etc., or a combination of these.¹ Among these stimuli-responsive phenomena, thermo-responsive phenomena have been mostly investigated due to the easy modification of the temperature parameter, and they are commonly observed in nature as well. The dramatic conformational changes of peptides result in ultimately new physicochemical properties, due to temperature-induced helix-to-coil transition.² By taking clues from these natural phenomena, artificial thermo-responsive polymers (homopolymers, block copolymers and statistic copolymers with one, multiple and continuous thermo-transition points) have been designed and widely explored.³ Thermo-responsive behaviors of polymers are mostly investigated by the turbidity changes upon temperature variation, while other physical changes are rarely investigated, e.g. fluorescence.⁴ Rheological properties of polymer solution are very important for further applications of polymers.^5,6 Especially, the investigation of thermo-responsive rheological properties of polymer solution will expand the applications of thermo-responsive polymers and is therefore highly desirable for polymer scientists.

Boron-containing polymeric materials have become a promising functional and stimuli-responsive material in recent years, due to their outstanding optical, electronic, and sensory properties.^7,8 In comparison with tetracoordinate boron polymers,⁴ tricoordinate boron polymers are well-known as Lewis acids, attributed to vacant p orbitals. Tricoordinate boron polymers, especially conjugated boron polymers and triarylborane-decorated non-conjugated polymers, are therefore utilized for fluoride and cyanide anions detection, as well as neutral Lewis bases, accompanied with color and luminescence changes.⁹ The stability of these tricoordinate boron polymers can be increased by introducing bulk groups to the labile boron center. Boronic acid polymers and boroxole-functionalized polymers exhibit better air and moisture stability due to electron-donating oxygen-containing groups. Owing to the reversible binding to diols, such as sugars, etc., these boronic acid polymers and boroxole-functionalized polymers are attractive for applications in sugar-related disease diagnosis and therapy.^8,10

Compared with exclusive studies on boronic acid polymers, borinic acid polymers have been mostly ignored, even though borinic acid moieties can be easily synthesized, with enhanced Lewis acidity and excellent substrate binding efficiency.^11,12 Recently, we reported the synthesis of borinic acid homo polymer and block copolymers.¹³ These borinic acid containing polymers exhibit the potential as a new type of promising “smart” polymer in the areas of thermo-responsive (showing continuously tuneable upper critical solution temperature (UCST)), sensory (substrates detection, such as fluoride ion and acetylthiocholine chloride) and supramolecular materials (with poly Lewis base like poly(4-vinylpyridine)).¹³ Further in-depth investigation on the properties (especially rheological behavior) of borinic acid polymers will help us to understand this newly developed polymer. On the other hand, the thermo-responsive behavior of borinic acid polymers is based on turbidity changes. The investigation on the thermo-responsive rheological behavior of borinic acid polymers is therefore highly desirable to explore their new properties.

Herein, we report the thermo-responsive rheological properties of borinic acid polymer in dilute solution (0.1 wt%) with trace amount (1% v/v) of water compared with those ones obtained without water addition.

Experimental

The poly(styrylphenyl(tri-iso-propylphenyl)borinic acid) (PBA) homopolymer (M_n,GPC = 38 500 g mol⁻¹, Đ = 1.26; DP_PBA,GPC = 94; DP_PBA,NMR = 86)¹³ were prepared as reported previously by reversible addition–fragmentation chain transfer (RAFT) polymerization in analogy to literature procedures.¹⁴

The dynamic light scattering (DLS) measurements of particles were performed with a Malver Zetasizer Nano-ZS instrument, equipped with a 4 mW, 633 nm He–Ne laser and an avalanche photodiode detector at an angle of 173°. The dynamic light scattering (DLS) measurements of molecularly dissolved polymer was confirmed by ALV Goniometer Systems (ALV/CGS-3), equipped with a He–Ne laser (22 mW, 532 nm), two avalanche photo diode-based photon detectors and a ALV-5000/EPP correlator at a fixed scattering angle of 90°.

The rheological properties of the polymer solution (0.1 wt%) were determined with a Thermo Haake MARS rheometer using cone-plate geometry at desired temperature. The cone diameter was 35 mm, and the gap between the cone and the base plate was automatically set to 0.105 mm by program as temperature changes. The values of the strain amplitude and frequency were checked in order to ensure that all measurements were performed within the linear viscoelastic region. The frequency sweep data was collected in the frequency range from 0.1 to 10 rad s⁻¹ and with a controlled strain of 1%. The temperature dependence of the storage modulus (G′) and lose modulus (G′′) was tested at fixed frequency of 1.00 rad s⁻¹ and amplitude of 1%. For 25 °C–70 °C–25 °C cycle test, temperature was controlled with a ramping rate of 45 °C min⁻¹ and then keep at desired temperature for 300 seconds. For 20 °C to 70 °C test, temperature was controlled with a heating rate of 3 °C min⁻¹.

The temperature dependent ¹H NMR spectra were recorded on a Varian INOVA 500 MHz spectrometer in [D₆]DMSO with 1% (v/v) of D₂O at desired temperature from 25 °C to 65 °C.

Results and discussion

Rheology results

As a Lewis acid compound, Lewis base promotes dehydration of borinic acids to give oligomers that involve Lewis acid–base and also hydrogen bonding interactions (Fig. S1†).^12,15 To avoid the crosslinking via B–O–B bond formation due to dehydration, bulky tri-iso-propylphenyl groups are attached to boron center of borinic acid polymer so that intra and intermolecular hydrogen bonding will be the dominating interactions involved in further property investigations. The borinic acid containing polymer used in this work is therefore poly(styrylphenyl(tri-iso-propylphenyl)borinic acid) (PBA, Chart 1), as reported in previous work. Previous investigations indicate that PBA shows continuously tuneable UCST over a wide temperature range (from 20 to 100 °C) in DMSO with trace amounts of water (from ∼0 to 2.5%).¹³ However, this thermo-responsive property of PBA was based on turbidity changes.

Chart 1 Examples of structures of boron-containing polymers. (A) Tetracoordinate boron polymer, (B) conjugated triarylborane polymer, (C) triarylborane decorated non-conjugated polymer, (D) boroxole-functionalized polymer, (E) boronic acid polymer and (PBA) borinic acid polymer.

Rheological properties of polymers play significant roles in their further applications. The rheology tests were therefore carried out to characterize the PBA dilute solution. The frequency sweep of PBA dilute solution (0.1 wt%) with 1% (v/v) of water is shown in Fig. 1. It is clear that this PBA dilute solution shows a steady modulus in the sweeping range from 0.1 to 10 rad s⁻¹, with G′ of ∼60 Pa and G′′ of ∼15 Pa at a frequency of 1 rad s⁻¹ at 25 °C, which are relatively higher than most polymers with a concentration as low as 0.1 wt%.^5,16

Fig. 1 The frequency sweep of PBA dilute solution (0.1 wt% in DMSO with and without 1% (v/v) of H₂O) at 25 °C.

To try to explain the reason of the high modulus value of the PBA dilute solution, temperature dependence of its rheological properties was carried out. PBA solution performs a thermo-responsive rheological behavior, as shown in Fig. 2A. Reversible moduli appear when temperature changes between 25 °C and 70 °C, with steady G′ of ∼60 Pa and G′′ of ∼15 Pa at 25 °C and much lower G′ of ∼0.5 Pa and G′′ of ∼0.09 Pa at 70 °C respectively. To further verify the thermo-responsive rheological behavior of PBA dilute solution, continuous temperature dependence of rheological behavior of PBA solution was carried out, as shown in Fig. 2B. The moduli of PBA solution drop gradually with G′ from ∼65 Pa to ∼0.1 Pa and G′′ from ∼15 Pa to ∼0.05 Pa respectively, as temperature increasing from 20 °C to 70 °C. The moduli level off as temperature reaches ∼56 °C, where UCST_rheology is determined. In comparison, the moduli (G′ and G′′) of PBA solution in pure DMSO is relatively low (Fig. 1) and not thermo-responsive (Fig. 2).

Fig. 2 The thermo-responsive rheological behaviors of PBA solution (0.1 wt% in DMSO with and without 1% (v/v) of H₂O). (A) 25 °C–70 °C–25 °C cycle, (B) heating from 20 °C to 70 °C.

Mechanism study

PBA dilute solution exhibits clearly thermo-responsive rheological behavior from the above results. According to our previous reports,¹³ PBA solution exhibits a UCST_turbidity of 58 °C in DMSO with 1% (v/v) of water, which is similar to the UCST_rheology of 56 °C above. These thermo-responsive modulus changes are therefore assumed to be strictly correlated to the turbidity changes of PBA solution in DMSO with 1% (v/v) of water. To verify this hypothesis and investigate the relationship of thermo-responsive behavior based on rheology and turbidity, digital photos and DLS were used to characterize the difference of aggregates upon temperature changes (Fig. 3). Digital photos indicate obvious turbidity changes from milk-like to transparent solution as temperature increasing from 25 °C to 70 °C. And these results are confirmed by DLS results of great decrease of particle size from ∼1 μm at 25 °C to ∼4 nm at 70 °C, of which 1 μm at 25 °C is due to PBA aggregates of multiple polymer chains and 4 nm at 70 °C is attributed to molecularly dissolved PBA chains respectively.

Fig. 3 Digital photos and DLS results of PBA (0.1 wt%) in DMSO with 1% of H₂O at 25 °C (red line) and 70 °C (black line).

According to our previous experimental and simulation results, H₂O molecules play an important role in the thermo-responsive behaviour of PBA dilute solution. When temperature is lower than UCST, PBA chains are cross-linked with H₂O molecules (PBA microgel formation through BOH–H₂O–BOH hydrogen bonds formation), while PBA chains are molecularly dissolved (PBA coil formation through BOH–H₂O–BOH hydrogen bonds deformation) when temperature is higher than UCST.¹³ Meanwhile, gelation is a well-known strategy to increase the modulus of solution remarkably. The mechanism of thermo-responsive rheological behavior of PBA dilute solution should be therefore related to the coil–microgel transition occurred in PBA dilute solution upon temperature changes, as shown in Scheme 1. When temperature is lower than UCST, PBA microgels with a diameter of ∼1 μm are formed by microphase separation of PBA chains at this stage, resulting in high moduli (G′ of ∼60 Pa and G′′ of ∼15 Pa) of PBA dilute solution. When temperature is higher than UCST, PBA chains are molecularly dissolved with a diameter of 4 nm (confirmed by ALV Goniometer System, Fig. S2†), resulting in much reduced moduli (G′ of ∼0.5 Pa and G′′ of ∼0.09 Pa). This thermo-responsive rheological behavior of PBA solution is highly reversible, attributed to the high reversibility of BOH–H₂O–BOH hydrogen bonds upon temperature changes. The importance the cross-linker effect of H₂O molecules was confirmed by PBA in pure DMSO, where moduli of PBA dilute solution are low and not thermo-responsive (Fig. 1 and 2).

Scheme 1 The illustration of thermo-responsive coil–microgel transition of PBA solution with small amounts of water.

To confirm the coil–microgel transition mechanism of thermo-responsive rheological behavior of PBA dilute solution, temperature dependent ¹H NMR spectra were tested at different temperatures ranging from 65 °C to 25 °C (Fig. 4). The integration results of aromatic protons of PBA are listed in Table 1. PBA chains are molecularly dissolved at 65 °C because of higher temperature than its UCST. As temperature drops, all integrations of aromatic protons of PBA drop, indicating the micro-phase separation of PBA chains from solution. This phenomenon is consistent with modulus (Fig. 2B) changes of PBA solution, indicating that thermo-responsive modulus changes of PBA solution are related with its solubility changes.

Fig. 4 Temperature dependent ¹H NMR spectra of PBA in [D₆]DMSO with 1% of D₂O at different temperatures ranging from 65 °C, 55 °C, 45 °C, 35 °C to 25 °C, respectively.

Table 1 ¹H NMR integrations of aromatic protons of PBA in DMSO with 1% of H₂O at different temperatures^a

Temperature/°C	a–e	a	b + c	d	e
a The assignments of aromatic protons of PBA are the same as the ones in Fig. 4. All integrations were normalized by comparing with the integration of H2O.
65	0.79	0.18	0.27	0.11	0.23
55	0.77	0.18	0.26	0.10	0.23
45	0.69	0.17	0.23	0.07	0.22
35	0.62	0.16	0.20	0.06	0.20
25	0.51	0.13	0.16	0.05	0.17
Solubility decrease ratio	35%	28%	41%	55%	26%

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To investigate the detailed chemical structure of PBA during micro-phase separation, the detailed integrations of aromatic protons were calculated (Table 1). As temperature drops from 65 °C to 25 °C, the integration of entire aromatic protons (a–e) decreases 35%, while, the solubility decrease ratios of different aromatic protons are different (Table 1). Integrations of protons (b, c and d) decrease more than other protons as temperature drops, resulting in different decrease ratios of 28% for a, 41% for b–c, 55% for d and 26% for e, respectively. In comparison, small molecular borinic acid model compound, iodophenyl(triisopropylphenyl)borinic acid, was used to test the temperature dependence of solubility changes upon temperature changes and the results are shown in Fig. S3 and S4 and Table S1.† As temperature drops from 65 °C to 25 °C, the integration of all aromatic protons of model compound decreases 69% with a same decrease rate (Fig. S4 and Table S1†). Compared with 35% solubility decrease of entire aromatic protons and different decrease rate of each proton of PBA, the much higher solubility decrease of entire protons and the same solubility decrease rate of each proton of model compound, indicates that precipitation (macro-phase separation correspondingly) of model compound happens as temperature drops. The detailed temperature dependence of micro-phase separation of PBA solution is shown in Fig. 5. It is clear that the solubility of protons (a and e) decreases much slower than that of entire protons (a–e) and tiny difference can be observed between protons of a and e, which indicates protons of a and e locate in the shell of PBA microgel (Scheme 2). On the other hand, the solubility of protons (b–c) decreases much faster than entire protons (a–e) and that of proton d decreases fastest, which indicates protons of b and c locate in the outer core and proton of d locates in the inner core of PBA microgel (Scheme 2). The BOH shells are crosslinked by H₂O molecules as reported in previous work,¹³ which is consistent with the up field shift of protons (j) upon temperature decreases (Fig. 4), due to electron donating microdomain of BOH–H₂O–BOH hydrogen bonds. The reversible deformation–formation transition of BOH–H₂O–BOH hydrogen bonds causes the reversible coil–microgel transition of PBA molecules upon temperature changes, resulting in therefore thermo-responsive rheological behavior of PBA dilute solution.

Fig. 5 Solubility curves (based on ¹H NMR integration) of PBA aromatic protons in [D₆]DMSO with 1% of D₂O at different temperatures ranging from 65 °C, 55 °C, 45 °C, 35 °C to 25 °C, respectively. The assignments of aromatic protons of PBA are the same as the ones in Fig. 4.

Scheme 2 The illustration of chemical structure of PBA microgel solution formed with small amounts of water in DMSO.

Conclusions

All in all, we report the rheological property investigations of the PBA dilute solution in DMSO. Most interestingly, this PBA dilute solution is rheologically thermo-responsive upon temperature changes. In-depth investigation indicates that the cross-linker effect of water molecules with borinic acid moieties plays an important role in the coil–microgel transition mechanism of PBA, resulting in thermo-responsive rheological behavior of PBA dilute solution. This discovery therefore confirms that borinic acid containing polymer is a new type of “smart” polymer, and this work opens up a new avenue in the applications of borinic acid containing polymers, where thermo-responsive rheology changes play an important role.

Conflict of interest

The manuscript was written through contributions of all authors. The authors declare no competing financial interest.

Acknowledgements

We acknowledge funding support from NSFC 51503226 & 21404088, Shandong Provincial NSF ZR2015EQ018 & BS2014CL017, China University of Petroleum (East China) starting funding and “the Fundamental Research Funds for the Central Universities”.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18117a

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