1. Introduction
Amphiphilic block copolymers (BCP) attract research interest as template materials for the fabrication of various hybrid nanostructures, such as core–shell, yolk–shell particles, vesicles, porous membranes, or nanofibers, which found their application in various fields [
1,
2,
3,
4]. Solution self-assembly provides opportunities to regulate size, shape, morphology, or composition of block copolymer structures and subsequent functional materials [
5]. Here, along with the BCP fundamental characteristics, solvent properties, temperature, polymer concentration, or micellization kinetics can be explored to tune the characteristics of the micellar aggregates [
6].
Block copolymers consisting of poly(vinyl pyridine) segments in their structure are frequently used for the fabrication of various hybrid nanostructures, and polystyrene-block-poly(4-vinylpyridine) (PS-
b-P4VP) is probably one of the most explored polymers in this area [
7]. The ability of pyridine units to undergo hydrogen bonding and coordinate with electron-deficient species largely simplifies the functionalization process and diversifies synthetic strategies, which can be implemented to achieve desired materials. Micellization of symmetric and asymmetric PS-
b-P4VP BCP was studied in the past by several research groups, focusing on different aspects of micelle formation and morphological transformation processes [
8,
9,
10,
11,
12,
13,
14,
15].
Surprisingly, most of the literature reports related to PS-
b-P4VP micellar systems have focused on either symmetric or asymmetric BCP compositions with longer PS block where the solution self-assembly was studied in PS-selective conditions. The PS-
b-P4VP micellar systems comprising longer P4VP blocks are scarce and less explored [
12]. Such micelles comprising an inner PS core and outer P4VP corona can be obtained with the aid of P4VP-selective solvents, for instance, alcohols. Alcoholic PS-
b-P4VP micellar solutions are particularly attractive for the synthesis of various polymer-inorganic core–shell, yolk–shell particles, or hollows particles, by exploring advantages of sol-gel processes [
16,
17,
18]. Although the strong affinity of alcohols toward P4VP is known from the literature [
11], to the best of our knowledge, there has been no effort to understand the effect of solvent on the PS-
b-P4VP micellization process taking place in different alcohols.
In the present work, we investigated the micellar behavior of PS-b-P4VP block copolymers of different Mn comprising longer P4VP blocks in various alcohols. In particular, we endeavored to understand the effect of solvent and heat treatment on micellar behavior in different alcohols and correlate these results with the strength of solvent selectivity obtained from vapor-swelling experiments on PS and P4VP thin films.
2. Materials and Methods
Poly(styrene)-block-poly(4-vinylpyridine) (PS-
b-P4VP) block copolymers of varied molecular weight, as well as polystyrene (PS) and poly(4-vinylpyridine) (P4VP) homopolymers (HP), were purchased from Polymer Source Inc. (Dorval, QC, Canada) and used as received. Main characteristics of BCPs and HPs are summarized in
Table 1. Chloroform, toluene and methanol (Fisher Scientific GmbH, Schwerte, Germany), absolute ethanol and 1-butanol (VWR International GmbH, Dresden, Germany), 1-propanol and 1-pentanol (Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany), ammonium hydroxide solution (28%) (Fisher Scientific GmbH, Schwerte, Germany), and hydrogen peroxide (30%) (Merck, Darmstadt, Germany) were of analytical grade and used as received. Highly polished single-crystal silicon wafers ((100) orientation) with ca. 1.5 nm thick silicon oxide layer were purchased from Semiconductor Processing Co. and used as substrates for thin film preparation.
Dynamic light scattering (DLS). DLS experiments were carried out using Zetasizer Nano S (ZEN 1600, NIBS Technology, Malvern Instruments, Malvern, UK) equipped with 4 mW He-Ne-laser (632.8 nm, scattering angle 173°). The PS-
b-P4VP block copolymer was added in the solvent at a concentration of 0.2 mg/mL and stirred at room temperature for at least 70 h, to equilibrate the sample and obtain reproducible DLS results. The quartz cuvette containing 1 mL of BCP solution was placed in Z-sizer, and continuous DLS measurements were started immediately after temperature equilibration (60 s equilibration time). The interval between successive measurements was 2 min. For each measurement point, ten autocorrelation functions (10 s of data collection time per scan) were averaged and evaluated, using Dispersion Technology Software (DTS) appendant to Zetasizer Nano S. DTS includes cumulants analysis and multimodal size-distribution algorithm NNLS, which have been used for the calculation of hydrodynamic particle size and polydispersity index (PDI). The refractive index (RI) of polystyrene latex (n = 1.590) was used as material RI, whereas temperature-dependent values of solvent viscosity were determined form data available in the literature [
20].
Swelling experiments and ellipsometry measurements. Silicon wafers were precleaned by repeated sonication in dichloromethane, followed by stirring in a mixture of Milli-Q® water, H2O2, and NH4OH (4:1:1 v/v) for 1 h, at 80 °C. Wafers were thoroughly rinsed with Milli-Q® water and dried with nitrogen flow before being used as substrates for thin film deposition. PS and P4VP polymer films were deposited onto precleaned silicon wafers by spin coating from corresponding polymer solutions in chloroform. Film thickness was adjusted by tuning polymer concentration and rotation speed during spin coating. Time-resolved spectroscopic ellipsometry measurements were carried out at room temperature (23 ± 1 °C), in reflection mode, using a rotating compensator alpha-SE® spectroscopic ellipsometer (J.A. Woollam, Co. Inc., Linkoln, NE, USA). Before measurement of the swelling of polymer films in solvent vapor, optical constants of the specific solvent vapor in equilibrium were analyzed, using a bare Si wafer with a thermally grown SiO2 layer of 30 nm. Hence, a silicon wafer with a thermally grown SiO2 layer of 30 nm was placed in a quartz cuvette (fixed angle of incidence of 70°, TSL Spectrosil, Hellma, Muellheim, Germany), together with a small aluminum vessel containing 80 µL of solvent. The cuvette was closed with a weighted glass slide, and ellipsometric data were recorded with a time interval of ca. 0.2 min, until equilibrium conditions were reached. For fitting the optical dispersion of the solvent vapor, a model of silicon/SiO2 (optical dispersion of Si and SiO2 taken from the database), solvent adlayer (fixed Cauchy dispersion), and solvent vapor as ambient (Cauchy dispersion with A and B fitted) were used. The optical constants of the solvent in the liquid phase were measured with refractometry, at a digital multiple wavelength refractometer (DSR-L, Schmidt + Haensch GmbH & Co., Berlin, Germany). The ellipsometric investigations of polymer film swelling were performed in a similar way. Silicon wafers coated with PS or P4VP layer, with a thickness of ca. 50 nm, were placed in the quartz cuvette, along with 80 µL of solvent. The cuvette was closed with a glass slide, and ellipsometric data were recorded until stable plateau values of the film thickness were obtained. Experimental data were fitted with a Cauchy model for the swollen polymer film, using optical constants for solvent vapors at equilibrium. The swelling ratio (Q) was calculated by dividing polymer film thickness in the swollen state to the initial film thickness in a dry state. The maximal swelling ratio (Qmax) was calculated by averaging Q values at equilibrium over the time interval of ca. 5 min.
Scanning electron microscopy (SEM). SEM images were obtained with a Carl Zeiss ULTRA 55 scanning electron microscope (Carl Zeiss SMT, Jena, Germany) at 3 kV acceleration voltage, using an in-lens secondary electron (SE) detector. A drop of BCP alcoholic solution was placed onto a precleaned silicon wafer, and solvent was allowed to evaporate at ambient conditions. All samples were analyzed without any additional coating.
4. Discussion
We first discuss our results on vapor-swelling experiments in more details. The affinity of a given solvent toward a particular polymer can be estimated from the difference in solubility parameters of the components:
where
is the polymer–solvent interaction parameter,
is the molar volume of the solvent,
R is gas constant,
T is the absolute temperature, and
and
are the solubility parameters of solvent and polymer, respectively [
23]. The first term in Equation (1) represents enthalpic contributions, whereas the second term is a correction factor for the entropic contributions. The inverse relationship between
and
T explains well the effect of different temperatures on micellar size. This estimation method, however, is limited to particular polymer–solvent systems, since it does not account for polar- and hydrogen-bonding interactions between the components. Moreover, even for well-studied polymers, such as polystyrene, the solubility parameter values vary from each other [
7,
9,
24]. Alternatively, the relative energy difference (RED) concept has been used to evaluate the polymer–solvent interactions and accounts for polar- and hydrogen-bonding interactions:
where
,
,
, and
,
,
are the Hansen solubility parameters (HSP) of the polymer and the solvent, which represent dispersive, polar, and hydrogen bonding components, respectively, whereas
is the radius of the sphere enclosing good solvents [
25]. Though the RED approach can be implemented to the polar systems, its application is limited to the polymers with known HSP and
values. While for PS these data are available, we could find only one report providing HSP values for P4VP (see
Table A4 in
Appendix A) [
14]. Unfortunately, we were not able to track back the original source for these values. We also could not find
R0 value for P4VP, which restricts the possibility to implement RED method to any of P4VP/solvent systems. Most significant, both
and
RED (or
) values based on the available solubility parameter values do not match well with the experimental observation on particular polymer–solvent systems (see
Table A5 and the following discussion in
Appendix A). Therefore, swelling experiments on thin PS and P4VP films exposed to the vapors of different solvents were carried out to revel the differences in their affinity.
Figure 8a summarizes the averaged values of
Qmax obtained upon swelling of PS and P4VP in alcohol vapors, as well as in vapors of toluene and chloroform. As expected, in alcohol vapors, P4VP swells more than PS and
Qmax(P4VP) ranges from 3 to 4 for different alcohols, while
Qmax(PS) varies from 1.05 to 1.10 only. The situation turns opposite in toluene vapors, where the PS swelling dominates over P4VP. Since low
Mw alcohols and toluene are known as P4VP- and PS-selective solvents, respectively, these results are in full agreement with our expectations. Chloroform can solubilize both PS and P4VP, being often considered as a good solvent for both blocks. Thus, in chloroform vapors, swelling of both P4VP and PS was comparatively high, though P4VP swelling was higher as compared to PS.
We assume that after reaching a plateau level in the polymer swelling ratio, an equilibrium is maintained between solvents in the vapor phase, in the liquid phase, and in the polymer layer, and no significant solvent leakage out of the cell took place with time. In fact, the measured film thickness was stable within a time interval over multiple measurement points, suggesting that the solvent concentration in the vapor phase was constant and the equilibrium conditions were maintained. However, a direct comparison of the swelling results in vapors of different solvents is less realistic due to the difference in their vapor pressure (see
Table A3 in
Appendix A). Nevertheless, the comparison is valid for different polymers swollen in the same solvent if the same experimental conditions are maintained for both the polymers. Thus, we plotted
Qmax results as
Qmax(P4VP) versus
Qmax(PS), which are shown in
Figure 8b [
26]. The results on
Qmax obtained from swelling of PS and P4VP in toluene and chloroform vapors are included for comparison (
Figure 8b, inset). The “solvent neutrality line” defined by
Qmax(P4VP) =
Qmax(PS), which splits the graph into the P4VP-selective and PS-selective regions, is also shown. As expected, in the
Qmax–
Qmax plot, toluene locates in the PS-selective region, i.e., strong PS swelling and minor P4VP swelling. Next, the position of chloroform, which is known as a good solvent for both PS and P4VP [
11], is close to the neutral line (
Figure 8b, inset), though its affinities toward PS and P4VP seem to be slightly different. Finally, all alcohols are located in the P4VP-selective region and show rather minor differences in position along the
Qmax(PS) axis. Interestingly, three higher
Mw alcohols, 1-propanol, 1-butanol, and 1-pentanol, are located close to each other also along the
Qmax(P4VP) axis. Considering that, in these solvents, PS-b-P4VP demonstrated distinct similarity also in solution phase, we speculate that, at specified conditions, these three alcohols should be very close to each other in terms of their selectivity for PS/P4VP pair. Ethanol stands separately and shifts toward the P4VP-selective region, which must be the consequence of the greater polarity and higher vapor pressure of this solvent. In contrast, methanol is shifted toward smaller
Qmax(P4VP), i.e., lower affinity toward P4VP. Considering that saturated vapor pressure of methanol at 25 °C is more than twice that of ethanol, the very high polarity of methanol might be the reason for the reduced affinity toward P4VP, as compared to other alcohols (see
Table A3 in
Appendix A).
Coming back to the results on the solution behavior of PS-
b-P4VP in different alcohols, several aspects should be addressed in view of solvent vapor-swelling experiments. For a given BCP, which forms spherical micelles in selective solvents, there are two factors which define the hydrodynamic micellar size at specified experimental conditions. The first factor is the aggregation number (N
agg), which depends on the polymer/solvent Flory–Huggins interaction parameter, χ
P,S [
27]. The second factor is the conformation of polymer chains comprising the core and shell of the micelle, which also depends on the strength of interaction of each block with a particular solvent. In selective solvents, which are good for P4VP and poor for PS, like alcohols, PS-
b-P4VP micelles comprise collapsed PS core and swollen P4VP corona. As the affinity of the solvent toward the corona-forming block is reduced, chains will adopt less stretched conformation, which will lead to the lowering of hydrodynamic micelle size. On the other hand, if the solvent quality for the core-forming block increases, the micellar core will swell due to additional chain stretching and an increase of solvent fraction accommodated in the micellar core. This is valid for the range of selective solvents (or solvent mixtures) in which micelles are stable and do not disintegrate, as the solvent selectivity changes. The effect of solvent selectivity on N
agg and micellar size was reported by Park and co-workers for symmetric PS-
b-P4VP, in a series of ethanol/toluene mixtures with different solvent ratios [
11]. They showed that the hydrodynamic size, the core size, and the aggregation number of spherical PS-
b-P4VP micelles gradually decreased upon lowering the selectivity of the solvent mixture. The hard and large spherical micelles were formed at highly selective solvent mixtures, while the soft and small micelles were formed at less-selective solvent mixtures. Due to the asymmetric composition of PS-
b-P4VP BCP comprising longer P4VP block and preferential solvation of P4VP block by alcohols, we expected formation of star-like (or core-corona) micelles in these solvents. These micellar structures should adopt core–shell morphology upon drying. Indeed, the core–shell morphology of micellar structures present in higher
Mw alcohols can be clearly seen on the corresponding SEM images (
Figure 3a–c and
Figure 5e–h). The central part of the micelle appears brighter, which is due to the topographical contrast provided by the PS core. The outer part of the micelle appears darker due to the elemental contrast, which originates from a thin P4VP layer adhered to the silicon substrate and surrounding the micellar core. The situation, however, turns different in PS-
b-P4VP/ethanol and PS-b-P4VP/methanol systems. In both cases, relatively large and broadly distributed spherical particles, which, plausibly, belong to so-called large compound micelles, are predominantly visible, along with a minor fraction of star-like micelles. Moreover, the in PS-
b-P4VP/methanol system, there are still larger and irregularly shaped particles, which are aggregates or clusters of several individual particles. Nevertheless, the above results revealed for different PS-
b-P4VP/alcohol systems correlate with results obtained by Park and co-workers for symmetric PS-b-P4VP in strongly selective ethanol/toluene mixtures [
11]. For comparison, star-like micelles comprising P4VP core and PS corona were also found in the case of symmetric and PS-longer PS-
b-P4VP BCPs in PS-selective solvents, such as toluene or THF [
8,
11], while for PS-
b-P4VP with a longer P4VP block, vesicular aggregates were found instead [
12]. It should be pointed out that, by using the same PS-
b-P4VP BCP, smaller and uniform in size star-like micelles can be also obtained in ethanol and methanol if the block polymer is first solubilized in a common good solvent and then transferred to a P4VP-selective alcohol [
18,
21]. The above difference can be explained as follows. The molecular exchange between individual micelles in case of BCPs is very slow as compared to a classical surfactant micelle. If the solvent cannot efficiently solvate one of the blocks, this block will remain in the frozen (glassy) state, though the second block is well soluble. If the polymer is directly added to a highly selective solvent, i.e., without pre-dissolution in a good solvent, the initial aggregates will be preserved and additional heat treatment is required to disintegrate them. This situation is seen in the case of PS-
b-P4VP/methanol system, where the large aggregates dominate before the heating step. These observations are consistent with ellipsometry results for polymer swelling in methanol vapors. Despite the highest vapor pressure, the ability of methanol to swell PS and P4VP was the lowest among all alcohols studied. In contrast, dissolution of PS-b-P4VP in 1-propanol, 1-butanol, and 1-pentanol under stirring permits disintegration of aggregates and formation of spherical micelles already at RT. This clearly indicates that these solvents can better solvate PS block as compared to more polar methanol and ethanol, which is also in agreement with our expectation. Vapor swelling results also suggest that these three alcohols might be very close to each other in terms of their selectivity toward PS/P4VP pair.
The effect of BCP chain length on PS-b-P4VP micellar size and the effect of temperature are summarized in
Figure 9. Expectedly, with an increase of the BCP chain length, the hydrodynamic size of PS-b-P4VP micelles increases (
Figure 9a). Moreover, the hydrodynamic micellar size shows linear dependency versus the overall degree of polymerization (DP) of BCP, which is in accordance with scaling relation derived for the amphiphilic BCP micelles in selective solvents [
9]. The effect of temperature on hydrodynamic micellar size is shown in
Figure 9b. As can be seen, after successive heating/cooling cycles maintained at different temperatures, the final hydrodynamic micellar size gradually decreases and reaches the minimum values at the highest temperature. Performing experiments at higher temperatures was hardly possible because of enhanced solvent-evaporation effects. Considering the trend in the change of hydrodynamic particle size, one may expect that the further increase of temperature would lead to a plateau level where no further changes in micellar size will happen. However, this may not be the case because of following reasons. There are two processes which contribute to the changes in hydrodynamic size of BCP assemblies upon heating. The first process is the disintegration of PS-
b-P4VP aggregates, which is characterized by substantial decrease of Z-ave and PDI. The second process is the reduction of the aggregation number of BCP micelles upon heating, which also results in a decrease in micellar size. The latter is associated with temperature-dependent changes in the polymer–solvent interaction parameter. Heat treatment provides more favorable condition for the solvation of core-forming block (PS) at elevated temperatures. This will ultimately lead to a reduced number of polymer chains (i.e., PS blocks) which could be confined within the micellar core.