Introduction

Recently materials with unique properties that probably alter their properties with the response to their outer environment have been of great interest. These polymers and hydrogel materials were called environmental, responsive, stimuli-responsive, and smart [1,2,3,4,59]. It has been added to the supplanted material.

Preparation of (4-acetylphenyl acrylate) (APHA) [60]

It has been synthesized as described in reference [58]. It has been added to the supplanted material.

Preparation of vanillin acrylate or [4-formyl-2-methoxyphenylacrylate] (VA).

It has also been prepared, as reported in recent articles [60]. It has been added to the supplemented material.

Preparation of photo-cross-linker and adhesion

Dimethylmaleimidoacrylate (DMIA) photo-cross-linker

The method was used as described in the recent lecture [2, 61], discussed in detail in the supplemented material.

Preparation of [3-(3,4-dimethyl-2,5-dioxo-2,5-dihydro-pyrrol-yl)-propyl ester] adhesion (DMITAc),

It is discussed in detail in the supplemented material. The method was used as described in the recent lecture [2].

Fabrication of copolymers and terpolymers

Poly (SKA-co-DMIA)

Three round flasks 5 mol% (0.125 g, 0.383 mmol), 10 mol%, 0.325 g, (0.767 mmol), and 15 (0.375 g, 1.149 mmol) of (DMIA), respectively, 2.00 g (0.01 mol) SKA, and AIBN as initiator was dissolved in 50 mL 1,4-dioxane. They purged in argon for 15 min. and heated at 65 ℃ for 8 h. The polymer was extracted in diethyl ether, at − 50 ℃, a viscous oil material purified in THF.


1H-NMR (Chloroform-d), δ(ppm) = [1.28–1.44 (m, 6-H, j-2-CH3), 1.45–1.77(m, 3-H, f-CH, e-CH2), 1.91–2.03 (m, 6-H, a-2-CH3), 2.17–2.47 (m, 3-H, d-CH2, c–CH), 3.61–3.84 (m, 2-H, g-CH2), 3.92–4.18 (m, 2-H, i-CH2), 4.20–4.37 (m, 1-H, h-CH)].


FT-IR (KBr), ν (cm−1) = [2930–2976 (s) (CH, CH2 and CH3-Aliphatic), 1735–1747(s) (C = O, ester), 1655–1660 (s) (C = C maleimide).

Poly (NIPA-co-VA-co-APHA) terpolymer

100-mL round flasks, mixtures of 0.167, 0.334, 0.501 g corresponding to (5, 10, 15 mol%) of (APHA) were added to 10 mol% 0.362 g (VA), respectively, and (0.0176 mol) 2.00 g NIPA in 50 mL of 1,4-dioxane and 10–3 mol AIBN, and then purged in argon for 15 min. The reaction vessel was heated 8 h at 70 ℃. After cooling at room temperature and in a refrigerator, the polymer was precipitated in diethyl ether at − 60 ℃. The terpolymers were purified in THF. The product was dried under reduced pressure as a yellowish solid.


1H-NMR (Chloroform-d), δ(ppm) = 0.78–1.34 (m, 6-H, m-2-CH3), 1.42–2.45 (m, 9-H, c-h -CH2, -CH), 2.46–2.65 (m, 3-H, a-CH3), 3.64–3.72 (m, 3-H, j-CH3), 3.79–4.18 (m, 1-H, n–CH), 5.87–6.73 (m, 1-H, l-NH), 7.10–8.13 (m, 8-H, b, i-CH-Ar.), 9.85–10.07 (m, 1-H, k-HCO).


FT-IR (KBr), ν (cm−1) = 3365 (NH group), 2979–3070 (Aliphatic CH, CH2, and CH3), 1763 (s) (C = O in –COO ester group), 1692 (s) (HC = O, carbonyl aldehyde), 1571 (s) (C = O in CONH amide), 1105 (m) (CH3 in OCH3), 862 (m) (CH-Aromatic).

Preparation of hydrogel thin film

Fabrication of gel thin films based on poly (SKA-co-DMIA)

A slide of LaSFN9 glass (2 cm. H × 2 cm. W) was cleaned with pure ethanol and dried under reduced pressure. 45–50 nm of pure gold has been evaporated and then immersed in a solution of 5 mM of adhesion [3-(3,4-dimethyl-2,5-dioxo-2,5-dihydro-pyrrol-yl)-propyl ester (DMITAc)] 0.10 wt% poly (SKA-co-DMIA) polymer solution with (0.0005 wt%) thioxanthone in distilled cyclohexanone was spin-coated over LaSFN9 + DMITAc (2000 rpm). The coated slide has been exposed to UV irradiation through a UV lamp (300–430 nm). The cyclodimerization [2 + 2] between dimethyl maleimide groups and the photo-cross-linking is formed in Fig. 5 (8a).

Ring-opening of [2,2-dimethyl-1,3-dioxoylan-4-yl-methylacrylate (SKA) to 2,3-dihydroxypropyl acrylate] (DHPA)

100-mL flask, fixed with a reflux condenser, the spin-coated glass slide was prepared in step A and immersed in a mixture of [20 mL glacial acetic acid and 15 mL THF] and refluxed for 6 h at 90 ℃. Afterward, the slide was picked up and cleaned from THF, DI H2O, and dried using vacuum nitrogen. The ATR has measured the change of functional groups by the chemical modification on the surface; the (SPR/OW) has been used to determine the refractive index. The film thickness of the gel in the dry and swelling states was calculated and drawn in Fig. 6 (8b).


ATR-IR ν (cm−1) = 3870–4105 (s, br. multiple peaks) (OH), 3550–3760 (s, br. multiple peaks) (CH, CH2, and CH3 multiple peaks aliphatic), 1675–1745 (br. multiple peaks) (–C = O), 1560–1578 (s, multiple peaks) (–C = C–).

Surface grafting poly (DHPA-co-DMIA)-g-(NIPA-co-APHA-co-VA).

The coated slide by poly (DHPA-co-DMIA) gel has been immersed in a poly (NIPA-co-APHA-co-VA) in 30 mL CHCl3 dry. 0.2 g (1.16 mmol) of p-toluenesulfonic acid has also been added to 100-mL flask. They were refluxed at 80℃ for 4 h. They were cold, and 0.2 g of Na2CO3 was added with reflux for about 30 min. The gel was cleaned and dried, as discussed previously. It was chemically investigated by ATR and the film thickness by (SPR-OW).


ATR-IR ν (cm−1) = 3530–3720 (m, br. multiple peaks) (CH, CH2, and CH3 aliphatic), 2270–2575 (s) (O-C-O), 1675–1745 (br. multiple peaks) (-C = O), 1565–1595 (s, multiple peaks) (-C = C-), 720–830 (m) (CH-Ar.).

Swelling measurements

The film thickness for each layer and swelling properties has been scanned by surface plasmon resonance with an optical waveguide. The change in refractive indexes RI (θ) was used to intemperate the degree of swelling. Knoll et al. discussed the relationship between the waveguide and film thickness and demonstrated an indirect relation as the higher waveguide modes with lower incident angles and vice versa [53]. However, the plasmon minimum can be displaced from ~ 65.5° to ~ 68°, indicating the phase separation from the swollen to collapse. By simulations and Fresnel calculations, the relation of volume degree of swelling or refractive index with temperature was used to detect the phase separation temperature of the hydrogel.

Results and discussion

Fabrication of monomers

This work uses the preparation of three hydrophobic monomers with and without functionality as described in Scheme 1. [2,2-dimethyl-1,3-dioxolane-4-yl-methylacrylate] (sol-ketal acrylate) (SKA) has been synthesized in two steps, the first is the formation of isopropylideneglycerol (1) by the reaction of acetone and glycerol in acidic media the product was chemically evaluated. The 1H-NMR showed 2CH3 groups at δ = 1.27 and 1.34 ppm, and a broad peak at δ = 2.75 ppm for the OH group. The 13C-NMR and FT-IR have also been used in supplemented material. The final product [2,2-dimethyl-1,3-dioxoylan-4-yl-methylacrylate] compound (2) has been prepared by reacting (1) with acryloyl chloride and triethylamine. It was investigated by both 1H-NMR and 13C, representing the presence of 3-H of the vinyl group (–CH = CH2) at δ = 5.8, 6.0, and 6.3 ppm corresponding to δ = 109, 127, and 131 ppm. The FT-IR showed the essential peak for the C = C vinyl group at υ = 1660 cm−1. All data have been added to the supplemented material. For monomers (3) (APHA) and (4) (VA), they were prepared as mentioned in our recent articles [58]. They were investigated by 1H-NMR, 13C, and FT-IR and demonstrated good results with their chemical structures. These results have also been added to the supplemented material.

Scheme 1
scheme 1

Synthesis of SKA, APHA, and VA monomers

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Fabrication of polymers gel thin films

Here, we are concerned with fabricating a series of functional photo-cross-linked copolymers from SKA with different concentrations of DMIAm (6) and another functional thermos-responsive terpolymers series based on N-isopropylacrylamide. They have been implemented by free-radical polymerization in a solution using (AIBN) for the initiation process as Scheme 2. The chemical investigation occurred by the 1H-NMR and FT-IR, which agreed well with their chemical structures. The monomer concentrations after polymerization were calculated from the 1H-NMR integration of 6-H, 2-CH3 of SKA at δ = 1.28 to 1.44 ppm, at δ = 1.91–2.03 6-H of the cross-linker DMIAm, δ = 3.61–3.84 ppm 2-H of SKA and at δ = 3.92–4.18 ppm 2-H of SKA ring, as shown in Table 1 and Figs. 1, 3. NIPA, VA, and APHA terpolymers with different molar ratios of 5, 10, and 15 of APHA and 10 molar concentrations of VA (7) have also been prepared as illustrated in Scheme 2. The presence of ketone and aldehyde groups for each APHA and VA, respectively, encourages the formation of other chemical reactions; it will be discussed in the next step. 1H-NMR and FT-IR have investigated the chemical structures, exhibiting the absence of vinyl groups of monomers and an active aldehyde group, as shown in Figs. 2 and 3. The concentrations of each monomer were determined by the 1H-NMR integration of specific protons at δ = 4.18 ppm 1-H, CH for NIPA, and at δ = 2.56 ppm 3-H, CH3 APHA, and at δ = 10.07 ppm 1-H, CHO, VA as shown in Table 1. The FT-IR of solid polymers with dry KBr illustrated some specific peaks related to functional groups for polymer (6) at υ = 1747 cm−1 –C = O ester, 1660 cm−1 –C = C– cross-linker), while polymer (7) at υ = 1763 cm−1 –C = O, carbonyl ester, 1692 cm−1 –C = O, aldehyde, 1571 cm−1 –C = O amide as shown in Fig. 3.

Scheme 2
scheme 2

Fabrication of poly (SKA-co- DMIA) (6) and poly (NIPA-co-VA-co-APHA) (7)

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Table 1 The yield, the composition, the number average molecular weight, the dispersity, the glass transition temperature, and the phase separation temperature of poly(SKA-co-DMIA) and poly(NIPA-co-APHA-co-VA)
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Fig. 1
figure 1

1H-NMR spectra (Chloroform-d) of poly (SKA-co-DMIAm) (6)

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Fig. 2
figure 2

1H-NMR spectra (Chloroform-d) of poly (NIPA-co-VA-co-APHA) (7)

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Fig. 3
figure 3

FT-IR of monomers

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The process has been investigated by applying the surface to ZnSe ATR crystal (Fig. 4). The surface of the gel was exposed to the chemical reaction and ring-opening of SKA, and then ring closure with APHA and cross-linking. Figure 5 shows the formation of -OH groups at 3870–4105 (s) multiple peaks; otherwise, the disappearance of these peaks after cross-linking with APHA is shown in Fig. 6.

Fig. 4
figure 4

FT-IR of polymers

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Fig. 5
figure 5

ATR FT-IR of polymer 8b

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Fig. 6
figure 6

ATR FT-IR of polymer 8c

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Polymer characterizations

Size exclusion chromatography technique (SEC) or gel permeation chromatography (GPC) has been implemented to display the molecular weights (Mn) and dispersity (Ð) as well, in dimethylacetamide (DMA) as an eluent and polystyrene (PS) column; the concentration used 6 g/L. Figure 7 shows the relation between the weight average molecular weight with log [M]. It demonstrates one peak for each polymer, emphasizing polymer formation and the disappearance of impurities and small molecules like monomers [11]. Another observation pointed in the dispersity demonstrated a decrease in Ð by increasing the photo-cross-linker in the polymer (6) and VA and APHA for polymer (7) interpreted the free-radical polymerization and formation of copolymers and terpolymers for 6 and 7, respectively [11]. The glassy temperatures (Tg,s) were recorded as the second-order change in the heat flow using differential scanning calorimetry (DSC). The DSC thermograms represent decreasing in Tg − 23, − 28, and − 37 ℃ of polymers 6a-05, 6a-10, and 6a-15, respectively, due to the high hydrophobicity of both SKA and DMIAm in the polymer main chain, which decreases stiffness and increases the flexibility [59] Fig. 8. Alternatively, polymers 7a-05, 7a-10, and 7a-15 copolymerized with N-isopropylacrylamide have higher glass transition temperature Tg attributed to the amide group's presence, increasing the hydrophilicity in the polymer chain, further increasing stiffness, and decrease flexibility Fig. 9. The phase separation demonstrated in the lower critical solution temperature of temperature-responsive polymers 7a-05, 7a-10, and 7a-15 showed a lower value of Tc than N-isopropylacrylamide homopolymer. Figure 10 shows the relation between temperature and transmittance of polymers 7a-05, 7a-10, and 7a-15, and the Tc was determined at the inflection point exhibited 18 ℃, 15 ℃, and 13 ℃, respectively; the lowest Tc was detected with the higher concentration of APHA and VA that attributed to its higher hydrophobicity which faster the phase separation in solution [62].

Fig. 7
figure 7

GPC molecular weight of polymers (6a-c) and (7a-c)

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Fig. 8
figure 8

DSC thermogram of polymers 6a-5, 6a-10, and 6a-15

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Fig. 9
figure 9

DSC thermogram of polymers 7a-5-10, 7b-10-10 and 7c-1510

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Fig. 10
figure 10

The relation of transmittance with temperature for determination of LCST using UV–Vis spectroscopy

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Gel thin films

This study aims to modify the surface of the hydrophobic layer based on PSKA photo-cross-linked gel film. The modification process involved three steps; SPR/OWS described each step for dry and swollen states.

Formation of the hydrophobic gel layer

PSKA gel thin film was fabricated using a solution of poly (SKA-co-DMIA) (10 mol% of DMIA) dissolved in distilled cyclohexanone at (2000 rpm). The dry film was scanned by (SPR/OW). It demonstrated the absence of attenuated total reflection (ATR) and the presence of waveguide at 25°, indicating the homogenous film's formation. Fresnel calculations of the dry thickness showed 264 nm and 1.473 refractive index unit (RIU) in Table 2 and Fig. 11. The hydrophobic environment has been observed from the non-swelling of gel films measured via SPR/OW and using deionized water. The relation of the refractive indexes (RI) with temperature variations has occurred. Fresnel calculations were used to detect the RI for the volume degree of swelling (1/χp). Figure 12 shows the relationship between (1/χp) and RI with temperature, and the surface demonstrated unaffected by temperature.

Table 2 Film thickness, dielectric constants, and refractive indices of dry gel before and after grafting
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Fig. 11
figure 11

Dry hydrogel layers for the three steps via (SPR/OW)

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Fig. 12
figure 12

The phase transition temperature of the nonresponsive film (first step with SKA-hydrophobic layer) as the change in the volume degree of swelling or refractive index with temperatures

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Conversion to the hydrophilic gel layer

The hydrophobic gel layer has been optimized to a hydrophilic one by releasing the hydroxyl groups of SKA, as mentioned later. The chemical structure was investigated by ATR administrated logic results. The overall process has been schematically drawn in Scheme 3. SPR/OWS has investigated the surface for dry and swell film thickness. Fresnel calculations for the dry layer exhibited 250 nm (film thickness) and ~ 1.476 RIU with a waveguide at θ = 23, as shown in Fig. 11. The swelling was measured in deionized water. The scanning process has been performed with the change of temperature. An increase in (1/χp) from ~ 1.6 to ~ 1.7 corresponds to a decrease in RI from ~ 1.39 RIU to ~ 1.38 RIU by raising the temperature due to anisotropic behavior. Figure 13 illustrates the change in (1/χp) and RI with temperature as an independent and demonstrates the anisotropic behavior.

Scheme 3
scheme 3

The diagram represents the optimization of gel layers from nonresponsive to responsive

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Fig. 13
figure 13

The phase transition temperature of the nonresponsive film (second step with hydrophilic layer) as the change in the volume degree of swelling or refractive index with temperatures

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Formation of the temperature-responsive gel layer

The gel layer was converted to a hydrophilic in the previous step, and free hydroxyl groups were formed. Poly (NIPA-co-VA-co-APHA) (7b-10–10) was dissolved in cyclohexanone and then coated to the gel layer that was further chemically cross-linked, as discussed in the experimental section and described in Scheme 3.

The SPR/OWS scan for the dry state showed waveguide mode with ATR. Fresnel calculations for a single dry layer showed 320 nm (film thickness) and RI ~ 1.466 RIU, as shown in Fig. 11, indicating the additional thin film. After swelling in deionized water, the scanning of SPR with OW has been achieved, demonstrating the change of RI (θ) with temperature, as illustrated in Fig. 14. Some essential features have been recorded;

  1. A-

    The minimum line of plasmon was shifted from ~ 75 to ~ 77 by raising the temperature.

  2. B-

    Higher (1/χp) than detected in the previous step.

  3. C-

    The refractive index was lower than in the previous step.

Fig. 14
figure 14

Swelling of responsive layer (the third step) in water as the change of reflection intensity RI with (ϴ) with temperatures via SPR

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The phase separation temperature of the upper layer has been detected from the relation of (1/χp) or RI with temperature. It is difficult to see the change due to the hydrophobic layer covering and protecting the upper layer's sensitivity [49]. However, the higher the protection of the hydrophobic layer, the NIPA still has affection and demonstrated LCST (Tc) at approximately 20 ℃ at the inflection point, as shown in Figs. 14, 15.

Fig. 15
figure 15

The phase transition temperature of the thermo-responsive film (the third step after grafting) as the change in the volume degree of swelling or refractive index with temperatures

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Conclusion

In this study, new acrylate monomers were fabricated to act as functional and hydrophobic monomers. A series of copolymers and terpolymers were fabricated via free-radical polymerization. The copolymerization of the hydrophobic monomer, at high concentrations, with N-isopropylacrylamide resulted in lower phase transition temperatures. The photo-cross-linked sol-ketal acrylate (SKA) gel was formed over the gold substrate. Surface plasmon resonance/optical waveguide (SPR/OW) was used to measure the film thickness of the dry and swollen gels. The volume degree of swelling and refractive index showed no response to temperature variations. Furthermore, the surface of SKA was modified by ring-opening, forming hydroxyl groups to increase the hydrophilicity of the polymer gel. The free hydroxyl groups were grafted by poly (NIPA-co-VA-co-APHA). The modified gel surface showed thermo-responsive behavior, where the phase separation temperature was detected by SPR/OW. This study offers insights into the fabrication of thermo-responsive gels which can be used in bio-separation processes through the formation of gel vessels for attaching and releasing biomolecules.