Lea
Gemmer
a,
Qiwei
Hu
bc,
Bart-Jan
Niebuur
d,
Tobias
Kraus
de,
Bizan N.
Balzer
bcf and
Markus
Gallei
*ag
aChair in Polymer Chemistry, Universität des Saarlandes, Campus Saarbrücken C4 2, 66123 Saarbrücken, Germany. E-mail: markus.gallei@uni-saarland.de
bInstitute of Physical Chemistry, University of Freiburg, Albertstr. 21, 79104 Freiburg, Germany
cCluster of Excellence livMatS @ FIT-Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany
dINM – Leibniz-Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany
eColloid and Interface Chemistry, Universität des Saarlandes, Campus D2 2, 66123 Saarbrücken, Germany
fFreiburg Materials Research Center (FMF), University of Freiburg, Stefan-Meier-Str. 21, 79104 Freiburg, Germany
gSaarene, Saarland Center for Energy Materials and Sustainability, Campus Saarbrücken C4 2, 66123 Saarbrücken, Germany
First published on 14th June 2022
Functional amphiphilic block copolymers (BCPs) are versatile, smart, and promising materials that are often used as soft templates in nanoscience. BCPs generally feature the capability of microphase-separation leading to various interesting morphologies at the nanometer length scale. Materials derived from BCPs can be converted into porous structures while retaining the underlying morphology of the matrix material. Here, a convenient and scalable approach for the fabrication of porous functional polyvinylpyridines (P2VP) is introduced. The BCP polyisoprene-block-P2VP (PI-b-P2VP) is obtained via sequential anionic polymerization of the respective monomers and used to form either BCP films in the bulk state or a soft template in a composite with cellulose fibers. Cross-linking of the BCPs with 1,4-diiodobutane is conducted and subsequently PI domains are selectively degraded inside the materials using ozone, while preserving the porous and tailor-made P2VP nanostructure. Insights into the feasibility of the herein presented strategy is supported by various polymer characterization methods comprising nuclear magnetic resonance (NMR), size exclusion chromatography (SEC), and differential scanning calorimetry (DSC). The resulting bulk- and composite materials are investigated regarding their morphology and pore formation by scanning electron microscopy (SEM), atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS). Furthermore, chemical conversions were examined by energy dispersive X-ray spectroscopy (EDS), attenuated total reflection Fourier-transformation infrared spectroscopy (ATR-FTIR) and water contact angle (WCA) measurements. By this convenient strategy the fabrication of functional porous P2VP in the bulk state and also within sustainable cellulose composite materials is shown, paving the synthetic strategy for the generation of a new family of stimuli-responsive sustainable materials.
The self-assembly of BCP and the formation of microphase-separated structures relies on many different parameters, and the χ of the underlying block segments is one of the most important.32–34 Amphiphilic BCPs typically tend to feature higher χ parameters and therefore exhibit microphase separation with well-defined structures.35 Additionally, the microstructures depend on the volume fractions of the block segments and the domain sizes correlate with molar masses.33,34 It is well-known that also the parameters for preparing the BCP films have a major influence on phase separation behavior. For PI-b-P2VP, an increasing phase expansion of P2VP-phase was observed when shifting from non-polar and more PI-selective solvent like tetrachloromethane (Hildebrandt and Hansen total solubility parameter of δT = 17.8 MPa0.5) to more polar and P2VP-selective solvent like THF (δT = 20.3 MPa0.5) or 1,4-dioxane (δT = 20.5 MPa0.5).36,37 In literature Hildebrandt and Hansen solubility parameters δT of 16.6 MPa0.5 for 1,4-cis-PI36,38 and multiple parameters for P2VP can be found, namely 19.8 MPa0.5, 21.3 MPa0.5, and 21.7 MPa0.5 (pyridine).5,36,37,39 As a result, a non-selective solvent should have a Hildebrandt and Hansen solubility parameter between 18.2 and 19.2 MPa0.5, as for example chloroform (δT = 19.0 MPa0.5). Though BCPs are frequently used as soft templates in various procedures, the sensitivity of morphology regarding solvents and swelling agents can cause complications. Free-standing or supported mesoporous functional materials can be fabricated from BCP containing a sacrificial block segment, which can be removed upon thermal or chemical treatment, while the matrix block segment remains unaffected.40 Chemical degradation of sacrificial segments from BCPs demands sufficient material transport and adequate chemical resistance, and therefore the degradation conditions for the sacrificial block segment have to be matched with care. Moreover, to avoid collapse of the matrix polymer during the degradation procedure and to maintain the templated pore structure, cross-linking of the residual domains is a common practice.16,40–43 Vinylpyridines can be cross-linked using 1,4-diiodobutane (DIB) from solution or from the vapor phase.16,41,42 Ozone is a small molecule that is capable of cleaving double bonds into various soluble carboxylic products. Since most polydienes contain double bonds in their polymeric backbone they can be degraded into smaller fragments by ozone treatment and washed away using suitable solvents.44 As a result, mesoporous BCP-templated materials with high pore order can be obtained.8,31 However, to the best of our knowledge, ozone-mediated PI degradation with PI rendering the minor block segment as a spherical BCP morphology has not been investigated in more detail yet, and there are no investigations on polydiene-based BCP templates as coatings within a cellulose fiber matrix. These insights would pave the way to a new route for the preparation of porous cellulose-based materials, stimuli-responsive coatings and smart membranes. Additionally, the hierarchical pore design and BCP coating at the cellulose surface should provide a direct and technologically straight-forward control over the wettability and functionality for sustainable microfluidics or sensing applications. For this purpose, within the present study amphiphilic BCPs, consisting of PI-b-P2VP with a high content of unsaturated moieties within the polyisoprene backbone, were synthesized via sequential anionic polymerization and investigated with respect to their self-assembly to a microphase-separated structure. Cross-linking with DIB was investigated and protocols for the generation of porous structures by ozone treatment were established. Finally, cellulose fiber materials were combined with the PI-b-P2VP-system to gain insights into the influence on cross-linking, pore formation and stability of BCP inside a cellulose fiber substrate.
SEC PI (vs. PS): Mn = 12500 g mol−1; Mw = 13
800 g mol−1; Đ = 1.11.
SEC PI-b-P2VP (vs. PS): Mn = 39600 g mol−1; Mw = 42
900 g mol−1; Đ = 1.08.
SEC-MALLS: Mw = 62501 g mol−1 (refractive index increment dn/dc: 0.140).
1
H NMR (300 MHz, 300 K, CDCl3, δ in ppm): 8.4–8.1 (pyr-H 2VP, 1H); 7.3–7.0 (pyr-H 2VP, 1H); 6.9–6.6 (pyr-H 2VP, 1H); 6.5–6.2 (pyr-H 2VP, 1H); 5.1 (CH-1,4-PI, 1H); 4.7 (
CH2 3,4-PI, 2H); 2.7 (P2VP, bb); 2.3 (P2VP, bb); 2.0 (PI, bb); 1.8 (P2VP, bb); 1.7–1.6 (PI, bb); 1.3–0.9 (P2VP, bb).
![]() | ||
Fig. 1 Exemplary characterization of PI28-b-P2VP7263 using (a) SEC to determine relative molar masses (vs. PS-standard) and molar mass distributions, (b) thermograms by DSC measurements to determine glass transition temperatures, and (c) 1H NMR spectrum in CDCl3 to determine chemical composition of BCP and microstructure composition of polyisoprene moieties. For evaluated data see Table 1. |
Polymer | M SEC | Đ | mol%1,4-PId | mol%PIe | Φ PI | T g, PI | T g, P2VP |
---|---|---|---|---|---|---|---|
a Molar mass Mn in g mol−1 determined by SEC (PS standards, THF). b Molar mass Mw in g mol−1 determined by MALLS in SEC in THF. c Đ values determined by SEC (PS standards, THF). d Molar fractions of 1,4-PI microstructure in PI-segment in % determined from 1H NMR. e Molar fractions of PI segments in BCP in % calculated using molar fractions determined from 1H NMR (see Fig. 1). f Volume fractions of PI segments in % estimated from weight fractions using polymer densities (0.903 g cm−3 for PI48 and 1.153 g cm−3 for P2VP49). g T g in °C determined by DSC measurement. | |||||||
PI | 12![]() |
1.11 | 93 | ||||
PI28-b-P2VP7263 | 62![]() |
1.08 | 28 | 24 | −77 | 92 | |
PI | 95 | ||||||
PI35-b-P2VP65186 | 185![]() |
1.15 | 35 | 31 | −78 | 99 |
The exemplary molar mass distribution of PI as first block and corresponding PI28-b-P2VP7263 BCP is shown in Fig. 1, exhibiting a monomodal and narrow molar mass distribution both for the PI homopolymer precursor as well as the final BCP (1.11 and 1.08, respectively). The Tg of the PI segment is −77.6 °C, which is in accordance with a PI featuring a high content of 1,4-linkages.25 The 1H NMR spectrum of the BCP (Fig. 1) reveals the expected signals, while signals at 4.8 and 4.7 ppm correspond to the vinylic protons in the 3,4-PI unit and the signal at 5.1 ppm corresponds to the olefinic proton in the 1,4-PI unit. Signals marked in orange in the range from 6.1 to 7.3 ppm correspond to aromatic protons of 2VP (3 H, H3) and the signal at 8.2 ppm (1 H, H4) refers to the proton of 2VP-repeating unit next to the nitrogen atom. The composition of PI and the BCP were calculated from the relative integrals of H1, H2 and H4, summarized in Table 1.
![]() | ||
Fig. 2 Morphological analysis of films derived from PI28-b-P2VP7263. (a) Cross-sectional SEM images of untreated bulk polymer, (b) cross-sectional SEM images of DIB-crosslinked bulk polymer, (c) area within 2 μm from surface of bulk morphology of DIB-crosslinked PI28-b-P2VP7263 and (d) overview image of the border area. (e) SAXS pattern of untreated PI28-b-P2VP7263 bulk polymer. The lines are the calculated theoretical model given by eqn (1) (grey) and its individual contributions from a BCC paracrystal (red) and disordered spheres (blue). (f) Untreated PI28-b-P2VP7263 bulk polymer compared to DIB stabilized sample. |
Fig. 2e shows the SAXS pattern obtained for PI28-b-P2VP7263 before DIB treatment. Besides a prominent primary Bragg peak at ∼0.022 Å−1, several shallow secondary peaks are present, indicating a well-ordered structure. The SEM images recorded for the untreated film (Fig. 2a) suggest a spherical morphology that may be well-ordered. As a qualitative examination of the peak positions did not allow for an unambiguous determination of the ordering state, the SAXS pattern was compared with a theoretical model. The positions of the secondary Bragg peaks at 0.045 and 0.055 Å−1 match those expected for a body centered cubic (BCC) structure. However, an additional contribution from disordered spheres is required to satisfactorily describe the full scattering pattern. A combination of BCC and disordered spheres have been observed previously in BCP morphologies.50–52 Theoretically, the scattering pattern of such a mixed structure can be described by:
I(q) = SBCC(q)PS(q) + SHS(q)PS(q) + Ibkg | (1) |
Here, PS(q) is a sphere form factor with a Gaussian size distribution that depends on the radius of the spheres, RS, and the width of the distribution, σ.53SBCC(q) denotes the structure factor describing a BCC paracrystal,54,55 which is a function of the nearest neighbor distance a. The disordered arrangement of spheres was described by a hard-sphere structure factor, SHS(q).56 It is defined by the hard sphere radius, RHS, i.e., half of the center-to-center distance between the spheres. Ibkg accounts for background scattering.
The model captures all features of the scattering curve, which suggest that it qualitatively describes the structure of the sample. The analysis yields a sphere radius of 11–12 nm, and a nearest neighbor distance between spheres of 32–36 nm, which is in good agreement with the results from SEM. Slight deviations between the model and the scattering pattern point to small quantitative differences or the presence of minor amounts of additional structures that are not considered by the model.
Fig. 2f compares the scattering patterns of PI28-b-P2VP7263 before and after DIB treatment. The overall appearance of both scattering patterns is very similar, which implies that the DIB treatment did not lead to morphology changes in the sample. In comparison to the untreated sample, the DIB-treated sample has slightly broader Bragg peaks, which indicates a weaker long-range order. Furthermore, no signs of hexagonally packed cylinders (HPC), as observed by SEM (Fig. 2d), were found by SAXS. Therefore, this is most likely a surface effect only.
According to common microphase separation theory, volume fractions of PI between 21 and 31% in a PI-b-P2VP BCP (Table 1) should result in a morphology composed of PI cylinders in a P2VP matrix.32,33 When operating at BCP compositions close to phase transition, film casting conditions can cause changes in the observed morphology. The microphase separation is influenced by many factors, most importantly solvent selectivity, the presence of traces of water or underlying substrates like glass or paper.35 The sample shown in Fig. 2 clearly exhibits a spherical morphology. The deviation indicates that the P2VP phase is favored during casting and annealing process. As stated in the introduction, solubility parameters are essential when considering a solvent for a film casting process. Chloroform featuring a Hildebrandt and Hansen total solubility parameter of δT = 19.0 MPa0.5 can be considered as non-selective solvent. TGA of the polymer, however, exhibit mass losses of 10 wt% between 90 °C and 160 °C, which can be attributed to some water content (see Fig. S3†). However, polvinylpyridines feature polar and hydrophilic moieties that can adsorb water.57 To remove the bound water elevated temperatures are necessary, which can cause degrading or crosslinking processes in the polymer. We therefore avoided extensive drying procedures. In BCPs featuring a polar and hydrophilic segment those traces of water can shift the equilibrium morphology significantly,33,35 and according to analytical data this is the case in the present work. It is assumed that morphological changes occurred at the surface because of the DIB-mediated cross-linking reaction leading to a cylindrical morphology. Therefore, the cross-linking procedure was investigated in more detail and will be discussed further.
In literature, effective DIB-crosslinking is only described in films with a thickness below 60 μm,41,42 in solution,16 or in highly porous systems.58 In the present study, BCP films with a thickness up to 1 mm were used. It is assumed that cross-linking protocols could only affect the film's surface, because DIB is considered a bulky molecule with heavy iodide atoms that is introduced via vapor phase at atmospheric pressure. During the cross-linking procedure, iodide was incorporated into the polymer (chemical structure see Scheme S1†) and therefore EDS-data should give information on the locations of cross-linking reactions. In Fig. 3 cross-sectional SEM images of bulk sample PI28-b-P2VP7263, EDS-point-spectra and locations thereof are shown. Additionally, EDS-mapping of iodine in the cross-section is displayed with relative iodine concentration depicted as red dots. In locations c and d more iodine was found than in location e (Fig. 3c: 9.6 at% N and 2.4 at% I; Fig. 3d: 10.2 at% N and 1.8 at% I; Fig. 3e: 9.6 at% N and 0.4 at% I) and further away from the surface (Fig. 3f) no iodine could be detected. Mean values and standard deviations of these measurements can be found in the ESI (Table S1†), as well as a plot of iodine concentration in dependence of the distance from the surface (Fig. S4†). Considering the EDS-mapping (Fig. 3b) as well as the plot (Fig. S4†) it is self-evident that there is a gradient from higher iodine concentrations of 2 at% within 3 μm from the surface to no iodine content at 25 μm perpendicular to the surface, which can be directly correlated to the cross-linking density. Using the atomic ratios of iodine and nitrogen it can be estimated that within 3 μm from the surface (locations c and d) 25% of vinylpyridine units were quaternized with DIB and therefore covalently cross-linked (details and calculation in Table S2†).
Locations c and d cover the area in which a cylindrical morphology is observed, as visible in Fig. 2. At spots e and f (8–10 μm and 25 μm from surface, respectively) solely spherical morphology is found. As a result, it can be stated that cross-linking is highly efficient in surface-near areas, considering the protocol used. There are two possible mechanisms for the morphological change at the surface: due to the change in chemical structure, the Flory–Huggins interaction parameter χ was changed by increasing the polarity of the 2VP moieties. Also, the volume fraction of the P2VP domains was affected by cross-linking. Moreover, DIB is expected to additionally soften the P2VP-phase during cross-linking and therefore enabling the rearrangement from spheres to cylinders. As a conclusion to these findings, the shift in morphology from spheres to cylinders in surface-near areas can be attributed to the reaction of the polymer with DIB, while the spherical morphology in the bulk state is preserved.
The untreated and cross-linked BCP films were furthermore analyzed by ATR-FTIR spectroscopy in order to detect characteristic changes within the chemical structure attributed to the reaction of pyridinic moieties with DIB. The corresponding chemical structure is given in the ESI (Scheme S1†). Peaks found at 1589 and 1567 cm−1 can be attributed to vibrations of pyridine rings and peaks at 1472 and 1433 cm−1 correspond to CH2-deformation vibrations of the polymer backbone. Therefore, these two double-peaks are characteristic signals for P2VP and are explicitly shown in Fig. 4b (partial spectrum Fig. 4a, PI28-b-P2VP7263, blue). The peak at 743 cm−1 (dominant signal, Fig. 4a) corresponds to aromatic C–H bonds. Changes after crosslinking with DIB are dominant in transmission spectrum at 1626 cm−1, 1509 cm−1 and 1450 cm−1 (Fig. 4b, PI28-b-P2VP7263 + DIB, gray curve). The two additional transmission peaks at 1626 and 1509 cm−1 are known to be characteristic for quaternized pyridium,41,57,59 whereas the signal at 1450 cm−1 can be attributed to additional CH2-groups that were introduced with the cross-linking agent.59 As a result, the iodine found in EDS measurements can be associated with quaternized pyridinium moieties.
As a conclusion, the BCP in the bulk state self-assembled to a mixture of BCC and non-ordered spheres, whereas cross-linking with DIB shifted the morphology to cylinders in surface-near areas. As a next step, bulk samples were treated with ozone and the resulting materials were further investigated and will be discussed with respect to morphology and pore sizes.
For pore analysis, cross-sections were investigated by SEM, AFM and SAXS as obtained by freeze-fracturing of the BCP films. Bulk samples derived from PI28-b-P2VP7263 (termed B, numbers: duration of ozone-treatment in minutes) and DIB-stabilized bulk samples (termed Bs, numbers: duration of ozone-treatment in minutes) exhibited spherical pores with diameters between 15 and 18 nm according to SEM images and between 11 and 12 nm according to AFM measurements.
In Fig. 5, the topographies of the samples B90, Bs90, B10 and Bs10b are shown (for Bs10a see Fig. S5†). Fig. 5a–d show the aforementioned spherical pores derived from ozonolytic degradation of former PI domains with pore diameters inscribed. Close to the surface (Fig. 5e and f) the highly ordered cylindrical domains were retained and converted to hollow cylinders in a P2VP matrix. Diameters were determined using a visual image evaluation program (ImageJ) to measure 130 to 200 pores of the images shown in Fig. 5 and Fig. S5,† corresponding statistics are compiled in Table 2. The samples exhibit pore sizes between 15 and 18 nm, which is within the error values (±3) and therefore, no changes regarding the morphology with respect to varying ozonolysis protocols could be observed. However, sphere diameters determined from cross-sectional topography are assumed to be statistically distributed, since spheres will unlikely be exactly cut into two halves. All investigated film samples featuring a thickness of 1 mm revealed pores throughout the entire cross-sections.
Sample | Stabilizeda | t /min | Pore-ø SEMc/nm | Pore-ø AFMd/nm |
---|---|---|---|---|
a 24 h treatment with DIB vapor. b Time span of ozone treatment. c Measured visually using Image-J on at least 132 pores. d Obtained from AFM image processed with Gwyddion Free SPM analysis software as twice the maximum inscribed radius rm (Fig. 6). e This sample was not washed with water. | ||||
B90 | No | 90 | 15 ± 3 | — |
Bs90 | Yes | 90 | 18 ± 3 | 10.7 ± 0.9 |
B10 | No | 10 | 17 ± 2 | 11.1 ± 1.4 |
Bs10ae | Yes | 10 | 15 ± 2 | 10.6 ± 0.9 |
Bs10b | Yes | 10 | 16 ± 3 | 12.4 ± 2.2 |
AFM imaging revealed a porous surface structure for all samples (Bs90, B10, Bs10a and Bs10b, see Fig. 6), wherein the pore size is characterized by the maximum inscribed radius rm (see Table 2). As can be concluded from the height scale of Fig. 6b and d, Bs10b shows a flatter and more homogeneous surface than B10. The pore sizes of øB10 = (11.1 ± 1.4) nm and øBs10b = (12.4 ± 2.2) nm do not differ much. Bs10a was stabilized and underwent an ozonolysis for 10 minutes and also shows a similar pore size as B10 (øBs10a = (10.6 ± 0.9) nm). At the same time, with an ozonolysis for a longer time duration leads to a similar pore size of øBs90 = (10.7 ± 0.9) nm. The diameters of the pores of all samples from AFM data are smaller than those from SEM data, which could arise from cantilever tip broadening.62 In summary, the morphology is retained for stabilization with DIB and ozonolysis. An overview of samples examined in this chapter and results from SEM and AFM are compiled in Table 2.
SAXS measurements of the samples B90, B10, Bs90 and Bs10 are shown in Fig. 7 in comparison to corresponding samples before ozone treatment. The non-stabilized bulk-sample and resulting porous materials (Fig. 7a) are compared to stabilized bulk-sample and resulting porous materials (Fig. 7b). All samples exhibit the same characteristic peaks as well as the same overall appearance as the non-treated sample. Therefore, SAXS confirms the results from SEM and AFM.
Besides morphological and topographical analysis, conversion of functional groups was investigated by ATR-FTIR spectroscopy (Fig. S6†). The 2VP moieties stayed intact and are clearly detectible at 1587 and 1433 cm−1, as well as the characteristic peaks for pyridinium after DIB-crosslinking at 1626 and 1509 cm−1. After ozone treatment another signal occurs at 1729 cm−1, which corresponds to CO valence vibrations of carboxylic acids, aldehydes and ketones. That results from ozonolytic degradation of double bonds. Carboxylic derivatives are predominantly extracted from the pores by water, as can be seen in additional ATR-FTIR measurements shown in the ESI (Fig. S7†).
All experiments presented so far led to P2VP bulk material with hollow spheres, wherein the sphere sizes seemed to be independent of the ozone-treatment duration. Stabilizing the sample with DIB had no influence on the obtained morphology. Degradation of the PI in a sphere morphology still indicated a good diffusion of gases and degraded products of the PI segment through the P2VP matrix, while it retained its morphology. That underlines a high chemical stability of P2VP. Analysis of DIB-stabilized bulk samples indicated that accessibility and material transport can be an issue, since the penetration of the stabilizing agent is detectable for only a few micrometers. As long as the bulk substrates were relatively thick, stabilization was not necessary. To further highlight the application of the herein investigated ozone treatment and cross-linking protocols, these findings will be applied to a composite material consisting of cellulose fibers and amphiphilic BCP in order to gain access to a porous coating on a sustainable material.
SEM analysis of untreated composite materials reveals that the paper-fibers are fully covered with polymer, as can be concluded from Fig. 8a and b, while the macroporous fiber-structure is retained (Fig. S8†). SEM images of untreated filter paper and overview images can be found in the ESI in Fig. S8 and S9.† When examining higher magnifications, BCP microphase separation can be observed, while the structure varies between lamellar structure or lying cylinders (Fig. S11a†) and spheres or standing cylinders (Fig. S11b†). In Fig. S11b,† few single spheres or standing cylinders are open, which may be due to sample preparation or air inclusions inside the film, while major parts of the BCP represent a closed polymer film.
The as-prepared composite materials were treated with ozone for 10 minutes (Experimental section). While for the DIB-stabilized sample a dense polymer coating is retained (Cs10, Fig. 8c and d), the coating is mostly washed off in the non-stabilized sample while the remaining structure of the polymer matrix is mostly collapsed (C10, Fig. 8e and f). Consequently, cross-linking had a major influence on the samples’ stability when treating polymer-coated filter papers with ozone for only 10 minutes.
SEM images of the cross-sections of the untreated polymer-coating on cellulose substrates are shown in Fig. 9a and b. Fig. 9a gives indication on the microstructure of the cross-section of the coating. The thickness of the polymer layer varies between roughly 0.5 and 5 μm, as can be seen by comparing Fig. 9a and b. In Fig. 9b and c the border area between polymer and fiber is highlighted with arrows indicating the direction of the fibers. More SEM images of the cross-sections along with overview images can be found in the ESI in Fig. S12.† After 10 minutes of ozone treatment of sample Cs10 (Fig. 9c and d), a porous structure can be found. In particular, in Fig. 9d highly ordered domains of hexagonal hollow cylinders are visible, which is in agreement with the molar composition, i.e., PI content of 35% (see Table 1), according to BCP microphase separation theory. A combination of smaller and larger pore sizes on the nanometer scale could be observed in Fig. 9d, while the larger pores were already visible in the untreated samples (Fig. 9b and S11b†).
While in bulk non-stabilized samples could be treated with ozone for up to 90 min and were still intact (sample B90), non-stabilized coatings collapse already after 10 min of ozone treatment. To the contrary in DIB-stabilized samples (Cs10) highly meso- and macroporous composite materials could be obtained. It is assumed that the major difference between the two materials lies in the reduced film thickness while macropores are added. Meanwhile, the surface area increases and the effect of cross-linking with DIB is noticeable. EDS measurements were conducted on cross-sections of the cellulose composite material before treatment with ozone in order to investigate the limitation of cross-linking. In Fig. 10 exemplary EDS-point spectra are shown that were randomly collected in polymeric layers throughout the cross-section. It is noticeable that in all collected spectra characteristic emission energies for iodine can be detected. All locations of data points, mean values and standard deviations can be found in Fig. S14 and Table S3.†
As before, the chemical conversions in the process of coating, stabilizing and degrading PI is validated by ATR-FTIR measurements. In Fig. 11a and b IR-spectra of the untreated cellulose fiber substrate and the polymer-coated paper are shown. Characteristic signals of P2VP (compare to Fig. 4) are detected and highlighted in the partial spectrum (Fig. 11b). In Fig. 11c and d, the corresponding spectra of samples C10 and Cs10 are depicted. It is noticeable that P2VP-signals become less intensive, but are still clearly visible within both samples. In the DIB-stabilized sample Cs10 the characteristic signals for quaternization of pyridinium are visible as well as the signal for degrading products of PI (also compare to Fig. S6†). As a result, the chemical changing during the process of coating, stabilizing with DIB and PI degradation by ozone in the composite materials was followed and confirmed by ATR-FTIR measurements.
The WCA (Θ) is a measure of surface hydrophilicity that is commonly used for surface characterization, which can give good indications on chemical transformations at the surface and their connection to surface properties.46,65 In this work, static WCA is used to follow the procedure of coating paper (Θ = 0°) with a polymer, stabilization with DIB and ozonolytic degradation. Fig. S15a–c† reveals a decrease to smaller contact angles, i.e., an increase of hydrophilicity: the untreated polymer coating features a mean-WCA of 114° ± 7°, while the DIB-stabilized polymer coating exhibits a mean-WCA of 109° ± 7° and the ozone treated sample (Cs10) is most hydrophilic with a WCA of 102° ± 9°. Mean values and errors were calculated taking six single values into account (see Table S4†). The difference between untreated polymer-coating and DIB-stabilized coating is within the standard deviation and rather indicates a tendency, while the absence of hydrophobic PI domains due to ozonolytic degradation causes a decrease in WCA.
In this section, it has been shown that a thin BCP-coating on cellulose substrate could be successfully retained during ozone treatment, while degrading PI led to spherical and cylindrical mesopores in a P2VP matrix. These samples featured only a fraction of the films thickness (∼0.5 to 5 μm) of the bulk samples investigated before (∼1 mm) and therefore, cross-linking with DIB became essential for retaining the films stability.
The cross-section of sample Bs10b was studied upon its swelling behavior in aqueous solution by AFM imaging (Fig. 12b). As a reference, an image of the sample measured in air at 25 °C is included (Fig. 12a). The root mean square (RMS) roughness of the whole images (1 × 1 μm2) and more local areas (100 × 100 nm2, as indicated by the red masks) are calculated to account for the amount of swelling of the sample (Table 3). The RMS roughness of the Bs10b surface in H2O increases due to the swelling of P2VP. Also, the pores appear even closed, which leads to a lower number of observable pores (Fig. 12b).
Sample | Measurement condition | RMS roughness for the whole scanned area/nm | RMS roughness for the masked area/nm |
---|---|---|---|
Bs10b | Air | 3.5 | 1.5 |
H2O | 19.1 | 4.2 |
The stimuli-responsiveness of P2VP can also be observed via surface wettability. Therefore, WCA measurements were conducted on a polymer-coated and stabilized substrate versus the corresponding ozone-treated sample Cs10 with an acidic buffer solution at pH 4 (Fig. 12). After 1 minute, the droplet on sample Cs10 (Fig. 12d) exhibits a WCA of 86°, while the corresponding sample before ozone-treatment (Fig. 12c) exhibits a WCA of 111°. Hence, after ozonolytic treatment the wettability with aqueous acidic solution is increased, which can be attributed to the absence of the hydrophobic PI. Also, the acidic WCA of the ozone treated sample is smaller than the aqueous WCA (86° and 103°, respectively), which indicates that P2VP moieties in these materials are addressable by pH variations.
AFM images in water revealed that the P2VP matrix swells and WCA measurements showed that surface properties respond to change of pH value. In order to evaluate the possibility to use the herein presented cellulose based composite substrates as stimuli-responsive materials in future studies, the samples were stored in acid (HClaq., pH = 4.0) for 30 min, washed, dried and examined by SEM. Fig. 12e–h shows that the polymer coating is not delaminated from the cellulose fibers and therefore the macroporous fiber structure of the cellulose substrate is retained, as can be seen as well in Fig. S13.† Additionally, small open pores can be seen on the surface of the polymeric coating. The cylindrical pores (compare Fig. 12h to Fig. 9d) are less dominant and followingly changes in matrix structure cannot be excluded.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2py00562j |
This journal is © The Royal Society of Chemistry 2022 |