Macro- and Nanoscopic Studies of Porous Polymer Swelling

Jun 21, 2017 - PALS provides an opportunity to recognize the sequence of solvent penetration into porous polymer particles. It allows in situ monitori...
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Macro- and Nanoscopic Studies of Porous Polymer Swelling Radosław Zaleski,† Patrycja Krasucka,‡ Krzysztof Skrzypiec,§ and Jacek Goworek*,‡ †

Institute of Physics, Department of Nuclear Methods, ‡Faculty of Chemistry, Department of Adsorption, and §Faculty of Chemistry, Analytical Laboratory, Maria Curie-Sklodowska University, 20-031 Lublin, Poland S Supporting Information *

ABSTRACT: A commercial Amberlite XAD7HP resin was investigated as a typical porous polymer which swells in tetraethoxysilane (TEOS). TEOS appears to be an extremely effective polymer swelling agent; thus, it serves as an easily assimilable silica source in polymer−silica composites. The present study discusses the application of light microscopy (LM) and positron annihilation lifetime spectroscopy (PALS) for the studies of macroscopic and microscopic features during the progress of polymer swelling. LM offers precise information on the swelling in the solvent vapor, especially for the well-defined, spherically shaped particles of the polymer. The swelling of a porous polymer consists of the swelling of pore walls and adsorption of the solvent on the internal surface of the walls. PALS provides an opportunity to recognize the sequence of solvent penetration into porous polymer particles. It allows in situ monitoring of the evolution of every free volume in the sample under study.

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INTRODUCTION Porous polymers are attracting more and more attention in the field of material science due to their chemical diversity and many potential applications in separation science,1 drug delivery systems,2,3 and technology.4 They are also components of the composites containing inorganic additives.5,6 The main feature of all polymeric materials is their capability to swell. The swelling of polymeric networks has been extensively studied during the past decades, both experimentally and theoretically.7−11 Several new models and modifications to the existing models have been developed to explain the swelling phenomena. These studies concern the interactions between a polymer network and a solvent with the assumption of the isotropic character of the swelling expansion. The aim of most these theories is to predict the macroscopic deformations of the specimen from the microscopic deformation of cross-linked network. The Flory−Rehner theory is widely used to determine cross-link density from swelling studies with various network models.12 Practically all the relevant investigations are devoted to the swelling of nonporous polymer where the volume of the swollen state is always the sum of volumes of both components, i.e., the polymer and solvent.13,14 The swelling of porous polymers is rarely discussed, and therefore more research is required to understand swelling in these systems. The problem is of great importance if it is taken into account that in all processes applied in separation science, such as adsorption methods or gas and liquid chromatography, polymers are highly porous. Their porosity estimated by conventional adsorption methods, such as nitrogen adsorption, mercury porosimetry, or X-ray diffraction, provides information concerning a dry polymer in the unswollen state and not at the conditions in © XXXX American Chemical Society

which it usually works. Porous swellable solids are also widely used as hosts for drugs in controlled drug delivery systems.2,15 An active substance is usually applied from a good solvent; subsequently, it is dried and used again in the release solution which is usually aqueous solution. The release is determined by swelling of the cross-linked polymer component and the character of pore system, which regulates the kinetic of the process and time of equilibration after swelling. The swelling of porous polymers is a particularly complex process because it is the result of two processes occurring simultaneously; the first one is the adsorption of a solvent on the polymer surface, and the second is the conventional swelling related to the diffusion of solvent molecules into the polymer matrix due to entropic forces. In the case of adsorption, the porosity of polymeric material is the dominant factor controlling the swelling kinetics. There is also a great difference in the swelling mechanism with regard to whether swelling occurs in a liquid solvent or a vapor.16 The initial uptake of the liquid solvent is almost instant. In this case, the swelling of a particle is proportional to the amount of the swelling polymer specimen. The penetration of the polymer matrix is fast, and the rate of swelling is controlled mainly by stress relaxation of polymer network until the stabilization of stretched chains. During swelling from gaseous phase, the solvent uptake proceeds in a different way. The first stage of swelling is subject to the rules of the simple physical adsorption of solvent molecules on a polymer surface. However, contrary to the classic adsorption on a rigid material, Received: April 24, 2017 Revised: June 8, 2017

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increase in the volume of the swelling sample. A custom-made stand allowed placing the vessel with TEOS below the container in such a way that it did not touch the weighing plate. Liquid TEOS was heated to 60 °C to force the flow of the TEOS vapor through the container. Nevertheless, the swelling was very slow and the system did not reach equilibrium even after ca. 900 h. All measurements were performed in air/TEOS vapor mixture. Conventionally, gravimetric or volumetric methods are used to characterize the swelling.22,23 Both methods take into account the relationship between the weight or volume of the swollen polymer and the weight or volume of the unswollen one. The degree of swelling, S%, may be expressed as Si% = (Qt − Q0)/Q0 × 100, where i = m, v for gravimetric and volumetric methods, respectively. Qt and Q0 are masses or volumes at time t and t = 0, respectively. One inconvenience connected with the gravimetric method is weighing of a swollen particle after the excess liquid has been separated. This problem becomes very important in the case of porous polymers, where part of a swelling agent is located in the pores. Drying such polymers without the evacuation of an unknown part of the liquid filling the pores is nearly impossible. The swelling solvent is kept inside the pores due to capillary forces and is easily removable from macropores, where these forces are relatively weak. A gravimetric approach is especially questionable when it comes to obtaining accurate data when volatile solvents are used. Such a problem does not apply to the volumetric method used for regularly shaped particles because the changes of their volume are easily measurable by the microscopic technique in the case when solvent is supplied from the gas phase. Characterization Methods. The morphology and external surface roughness of the initial polymer and the polymer swelled in TEOS were measured with the atomic force microscopy (AFM, NanoScope V, Veeco) using the SCANASIST-HR technique (silicon tip). A mechanical resistance experiment was performed with the use of Zwick Roell Z2.5 Materials Testing Machine. Single spheres of the XAD7HP polymer or polymer XAD7HP swelled in TEOS were compressed between two plates at a speed of 1 mm/min. The strain force vs compression was recorded. An ATR-FTIR spectra of XAD7HP polymer, liquid TEOS, and XAD7HP polymer swelled in TEOS were measured using a Nicolet 8700A (Thermo Scientific) spectrometer with a diamond crystal. The spectra were scanned from 4000 to 400 cm−1 at the resolution of 4 cm−1. In PALS experiment, a positron source (80 kBq of 22Na enclosed in the Kapton envelope) was placed in the middle of the sample container to ensure that all positrons annihilate in the sample. Two scintillation detectors with BaF2 crystals were placed on the sides of the sample container in a face-to-face geometry. A standard fast-slow delayed coincidence spectrometer was used for PALS measurements. The energy windows were ca. 130−600 and 800−1400 keV for the stop and start branches, respectively. Such wide windows assured the coincidence data rate ca. 600 cps as well as the registration of a possibly large fraction of three gamma annihilations, whose contribution to the annihilation of long-lived orthopositronium is considerable. The disadvantage of this setting was quite a wide resolution function, which had to be approximated by two Gaussian functions with FWHM = 270/730 ps and the relative contribution IGauss = 88/12%. The spectra were collected every 10 min, but they were summed over ca. 4.5, 11, or 22 h for the analysis, depending on the rate of their changes. This resulted in the spectra with ca. 9 × 106, 2 × 107, and 4 × 107 counts for time periods 0−64 h, 64−133 h, and over 133 h, respectively. The LT program24 was used for the data analysis. A reasonable accuracy (χ2 = 1.01−1.05) of the spectra fitting was obtained if five components with the dispersion of lifetimes in the longest-lived one were assumed. The components originate from the annihilation of parapositronium (p-Ps), unbound positrons, and three fractions of orthopositronium (o-Ps). The lifetime of the p-Ps component was fixed to its known vacuum value τ1 = 125 ps, which is usually preserved in organic solids. The lifetime of unbound positrons was stable (τ2 ≈ 417 ps) during the whole experiment.

the adsorption on the polymer surface is disturbed as the walls of a porous polymer can exhibit some permeability to the adsorbate. Undistorted adsorption could be estimated for the molecules which do not penetrate the polymer matrix, i.e., macromolecules. However, in this case, the adsorption would be disturbed by the sieve effect which maintains molecules separated from each other due to difference of their molecular size. In the present paper, we focus on the swelling process in the highly porous polymer, which gives us an opportunity to discuss the mechanism of a particle growth. Commercially available Amberlite XAD7HP was used as the polymer and tetraethoxysilane (TEOS) as the swelling agent. TEOS was chosen for this study because appears to be a good silica precursor in synthesis of silica−polymer composites presenting unique adsorption and structural properties.5,17,18 Amberlite XAD7HP is a moderately polar polymer, and therefore it is by far the most widely studied one.19 Perfectly spherical particles of this polymer permit precise determination of their dimensions by light microscopy (LM) during swelling. The LM method greatly facilitates the kinetic studies by providing swelling−time curves. However, although this approach is precise enough, it only provides the relation between time and the particle volume. For a more detailed quantitative description of the swelling process, simultaneous registration of the sample mass is desirable. This information is provided by the second experiment, in which the increase of the sample mass against time is registered. Finally, a new insight into polymer swelling mechanism on molecular level is achieved by application of positronium annihilation lifetime spectroscopy (PALS). This technique has been successfully utilized for study of hydrocarbon absorption on highly hydrophobic Amberlite XAD4.20

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EXPERIMENTAL SECTION

Materials. Amberlite XAD7HP (particle size 0.56−0.76 mm) (Rohm & Haas Co) and tetraethoxysilane (TEOS, 98%) were purchased from Sigma-Aldrich. Prior to the swelling experiment, Amberlite XAD7HP was washed with deionized water and dried at 80 °C under vacuum. Specific surface area SBET and total pore volume Vp derived from adsorption data of nitrogen at 77 K are 458 m2/g and 0.56 cm3/g, respectively.21 Adsorption−desorption isotherms and pore size distribution for polymer sample are shown in the Supporting Information. Swelling Experiment. Three different procedures were followed to perform the swelling experiments. First of all, the swelling of the polymer particles of varying diameter (chosen in range 0.62−0.72 mm) was examined using the optical microscope (inverted materials microscope, MA200 M, Nikon Co). To perform this, dry polymer beads were placed on a glass plate directly over liquid TEOS at RT and covered with an inverted Petri plate. In this case, self-generated vapor of TEOS was present around the sample. The change of the particle volume was registered every 10 s. Second, the gravimetric method was applied for swelling measurements. The mass change of the polymer beads placed in a desiccator filled with TEOS vapor at RT was registered. Weighing of the sample was performed with the use of a conventional microbalance Radwag AS/220/C/2 at a given time elapsed since the start of the swelling. During weighing, the sample was removed from TEOS vapor, which extended both the time of equilibration and total saturation of polymer with TEOS. Finally, the in situ gravimetric method was combined with the positron annihilation lifetime spectroscopy (PALS) experiment. The sample under study (0.4 g of XAD7) was placed on a microbalance in the container with multiple small holes at the top and bottom. The bottom plug of the container was loosely fit in order to allow an B

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RESULTS AND DISCUSSION Microscopic Studies and Gravimetry. Initially, the swelling of the polymer in TEOS was monitored by the light microscope LM. A set of spherically shaped polymer particles of different diameter (0.62, 0.70, and 0.72 mm) was chosen as a representative material. The diversification of particle dimensions was intended to eliminate any possible structural differences and inhomogeneities between the particles, while, additionally, enabling the possibility to test if the kinetic growth of the particles is independent of their dimensions. Figure S1 shows the optical images of the swelling particles taken every hour, and Figure 1 shows the time dependence of the swelling

tion). It is easy to observe that, initially, the change of the swelling degree vs the time is approximately linear. The time elapsed since the beginning of the experiment (i.e., exposition to TEOS vapors) to the equilibrium state is relatively long and exceeds 6 h. A similar experiment, in which the mass instead of volume was registered, lasted 336 h. Therefore, both curves were normalized to the same equilibration time, taking into account the limits represented by the zero starting point and the full saturation of polymer spheres indicated by the color change from matt white to glassy blue. As a result, we obtained the volume of polymer particles related to the mass of the sample at a given swelling stage. These data allow calculating the change of the sample density and solvent volume during the continuous uptake of TEOS. Amberlite XAD7HP, as a highly cross-linked polymer, is a rigid material. The change of color after TEOS uptake suggests that no empty mesopores are present in polymer beads and that the material changes its hardness. The best illustration of the latter effect is strain−compression curves for dry initial polymer and swollen one (Figure S2). The stress exerted by the polymer bead appears at much higher compression in the swollen polymer in comparison to the initial one. This indicates that TEOS introduction causes the softening of the polymer and its transformation from glassy to rubbery state. The analysis of the geometrical evolution of particles during the swelling is highly rewarding in the case of spherically shaped particles. For spherical particles, it is usually assumed that the volume ratio of the polymer sphere before swelling to volume of swollen sphere Qv,eq is

Figure 1. Volumetric swelling degree Sv% vs time of XAD7HP polymer in TEOS vapor.

ratio Sv% for Amberlite XAD7HP being in contact with the vapor of TEOS. The microscopic data are supplemented with a video presentation illustrating the continuous growth of particles and their visual transformations (Supporting Informa-

Figure 2. Schematic representation of various ways of swelling for different kinds of polymer pore network: thin pore walls, isotropic swelling (top) and thick pore walls, anisotropic initial swelling (bottom). Red spheres = polymer; blue area = TEOS. C

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Macromolecules Q v,eq =

Vpol

⎛ r ⎞3 V0 = ⎜⎜ 0 ⎟⎟ + Vsolv ⎝ req ⎠

The volume of the polymer matrix, Vpol, excluding pores, can be estimated as previously and represents the difference between the volume of the particle (V0) before swelling and the total volume of pores estimated from the nitrogen adsorption experiment (Vp,N2, see Figure S3):

(1)

where V0 is the volume of the initial particle, Vpol is the volume of the polymer matrix, and Vsolv is the volume of the solvent (Vsolv = msolv/ρsolv, where msolv and ρsolv are the mass and density of the solvent, respectively); req and r0 are particle radii in equilibrium and before swelling, respectively. For the porous polymer, swelling and the volume change of porous polymer particle are more complex. Swelling is determined in this case by the ratio of the volume of the polymer specimen to the pore volume, which reflects the average pore wall thickness. The swelling of a porous particle is presented schematically in Figure 2 for two models of porous polymer, i.e., a highly porous polymer, which is composed of thin pore walls and large pore volume, and a moderately porous one, which is composed of thick walls and small pore volume. Polymer network in the first case swells rapidly, and both the pore wall thickness and the pore volume increase markedly. In the second case, a vapor (or liquid) penetrating pore interior initially causes swelling of the surface layer of the polymer, which in turn may lead to total pore filling and blocking by solvent molecules. The reason for pore disappearance may be of dual character: (i) closing the pore with the swelling polymer or (ii) capillary condensation of solvent vapor in very narrow channels formed in situ due to swelling progress. Subsequently, even mesopores become narrower and finally disappear. This is an example of anisotropic swelling, when the glassy state of internal parts of pore walls prevents the particle from growing. In Figure 2, the fragments of the polymer which remain in a glassy-rigid state are spheres on a white background. The blue background marks the polymer parts which are transformed into a rubbery state. Local swelling causes the reduction of pore volume while the particle size remains constant in this case. For a real system, small increase at this stage of swelling also occurs because the external part of particle is exposed to solvent molecules permanently and swells. Thinner pore walls, also present in the sample due to structural heterogeneity of polymer, also swell in their whole volume. This contributes to the increase of pore size and, consequently, to the particle size. The swelling ratio is the same for both the length and thickness of the pore wall, regardless of the time.25 However, thicker walls swell more effectively and pore diameter is in this case further reduced. It may be assumed that for thick walls and a small pore diameter the volume of the pore is completely filled due to swelling, even at its initial stage. Simultaneously, for small pore diameters, capillary condensation will appear even at very low external pressure of the solvent. The consequences of pore filling via the condensation of liquid solvent in a porous polymer with thick walls are discussed in detail in refs 26 and 27. Generally, it may be assumed that the “response” of the particle of a porous polymer to the solvent, manifesting through the change of the particle’s volume, reflects in a certain way the thickness of the pore wall. The extent of swelling for porous polymer at a given stage of swelling may be expressed in the following form: Qv =

Vpol

V0 ⎛ r ⎞3 = ⎜ 0⎟ ⎝r⎠ + Vsolv + Vvoid

Vpol = V0 − Vp,N2

(3)

In these calculations, the accessibility of all the pores to nitrogen molecules was assumed. It is worth to note that the total pore volume Vp,N2 calculated from a single point adsorption on the adsorption isotherm at a relative pressure close to unity (p/p0 ∼ 1) is determined with high accuracy because it does not require any model assumption, and it is simply equal to the volume of the liquid adsorbate embedded inside the pores. The assumption that swelling of the polymer in liquid nitrogen is negligible seems to be quite reasonable. Indeed, in an independent experiment, when the polymer particles were wetted with liquid nitrogen, no noticeable swelling was observed, even regarding the heat expansion of polymer.21 To verify our initial data required in further discussion, the calculations of the density of the polymer skeleton were performed for the initial point of both mass and volume measurements (i.e., before swelling). The use of the nitrogen adsorption pore volume for Amberlite XAD7HP (Vp = 0.56 cm3/g) as well as the average diameter and the average mass of the studied set of particles result in the density of polymer walls being equal to 1.24 g/cm3. This is identical as the density reported for the same material in the relevant literature.28,29 The validity of eq 2 requires several conditions which should be fulfilled: (i) the condensation of the solvent occurs in pores; (ii) all voids are accessible for solvent molecules; (iii) Vvoids represents the solvent volume taking part in swelling. All these conditions may be fulfilled at an early stage of swelling when the solvent loading is small (no condensation occurs). The time course of Vvoid calculated from eq 2 for the system under study is shown in Figure 3. In the same figure, the volume of the absorbed solvent (Vsolv) and the difference between the initial pore volume and the solvent volume (Vp,N2 − Vsolv) as a function of time are plotted. The difference of both these volumes represents the hypothetical decrease of pore volume with the assumption that the polymer is not swelling.

(2)

Figure 3. Free volume Vvoid of XAD7HP calculated from eq 2 vs time for polymer beads swelling in TEOS: blue line with points; the volume of solvent uptake: solid red line; difference Vp,N2 − Vsolv: green dashed line.

where Vvoid is the volume of any space free of solvent and r is the particle radius at time t. D

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Figure 4. 3D AFM surface images of (a) XAD7HP polymer and (a′) XAD7HP swelled in TEOS and the surface roughness profiles of (b) XAD7HP and (b′) XAD7HP swelled in TEOS.

Interestingly, the initial segments of the Vvoid from eq 2 and (Vp,N2 − Vsolv) curves overlap, which suggests the “internal” swelling, i.e., swelling of pore walls on the expense of pore volume. The similarity of both curves indicates the intrapore swelling resulting in the consumption of the pore space, which corresponds to the anisotropic swelling (Figure 2). In this region, Vvoids decreases to 0.1 cm3/g. After 10 min, (Vp,N2 − Vsolv) values become negative, which has obviously no physical relevance. It indicates only that the volume of the solvent embedded in the polymer exceeds the initial volume of pores in the dry material. A further uptake of the solvent results in the swelling of the internal part of the walls also (i.e., swelling becomes isotropic) as well as the peripheral shell of a particle. Finally, the whole particle becomes swollen. This means that the free pore space practically disappears. Additionally, the lack of any internal polymer transformations during the uptake of solvent is assumed. It is worth to note that according to eq 2 for the constant volume of the particle at a simultaneous uptake of the solvent, Vvoid will be equal to the nitrogen volume diminished by the solvent volume, regardless of whether the solvent is located in the polymer network or condenses in micro- or mesopores. Capillary condensation (when present) would be a factor which diminishes the effect of pore and particle volume growth. Because of a serious assumptions introduced in eq 2, Figure 3 illustrates only some tendency in the changes of Vvoids in the initial stage of swelling. However, the analysis of the presented data clearly indicates that for thick pore walls it is possible that porous polymer becomes nonporous due to internal swelling, which is of great importance for systems containing the polymer where the permeability of gas and liquid media is required. After Vvoid minimum is reached, its value increases and finally stabilizes at a quite high value Vvoid = 0.3−0.4 cm3/g. The further change of Vvoid is much slower and consists in small decrease, which eventually has to lead to stabilization when the equilibrium is reached. Such an effect may be, at least in part,

the result of the contribution of volumetric effect related to polymer−TEOS interactions leading to the modification of their molar volumes. In the presented calculations, it was assumed that the partial specific volumes Vi,j* of the polymer and solvent are independent of their concentration and equal to the molar volumes Vmi,j of the pure component, (dV/dni)T,p,nj = Vmi, at constant temperature (T) and pressure (p) and constant number of moles (nj) of the second component in a polymer− solvent mixture. In consequence, the molar volume change on mixing is considered to be negligible. In other words, the standard molar volume additivity is preserved. In fact, the partial volume of the polymer is presumably slightly different in the polymer−TEOS composite than in the pure polymer and contains the contribution of both the intrinsic volume and volumetric effect due to polymer−solvent interactions. Thus, the calculated Vvoid represents not only the free space inside the pores but also the hardly predictable expansion of the polymer skeleton penetrated by TEOS as well as the expansion of the volume of TEOS in the polymer environment in comparison to its pure liquid form. During the absorption of TEOS, the polymer spheres become glassy, shiny and change their color from matt white to glassy blue (Figure S1). This suggests that empty pores are no longer present in polymer beads. In addition to the previously discussed changes of the molar volume, the observed effect may be explained by overestimation of the polymer particle diameter in the LM experiment. It may happen because the external, peripheral region of the particle contains large macropores which are within optical particle limits and the volume of these pores cannot be determined by any method based on meniscus curvature inside pores (the nitrogen method) or by diffraction methods. Suitable techniques for the visualization of the surface roughness of polymer particles in different state are atomic force microscopy (AFM) or profilometry. Indeed, AFM images shown in Figure 4 for initial Amberlite XAD7HP and the same polymer after swelling in TEOS provide the evidence of the E

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Macromolecules “blurry” and rough character of the polymer−air interface influencing the optical image and size of the particle. It follows form Figure 4 that swelling causes the diversification of surface species dimensions for the swollen polymer (a′, b′) in comparison to dry polymer samples (a, b). In a certain way, the direction of the void volume evolution presented in Figure 3 reflects the mechanism of the swelling of the polymer exposed to the solvent vapor. The Amberlite particles can swell significantly in comparison to their initial size when they are in contact with solvent molecules. Most likely, adsorption is the initial stage of swelling when most of the bulk of the polymer is in a glassy state. Obviously, in contrast to the conventional localized adsorption from gas phase on rigid materials, the molecules almost immediately diffuse into polymer phase. At the beginning of absorption, the amount of the solvent is too low and local swelling sites are too shallow to significantly change the total particle volume. However, the external surface of a particle reacts to the presence of the solvent vapor in a similar way as the pore surface, and consequently, the particle volume changes also during this stage. The further substantial increase of the particle volume, accompanied by increasing the void volume, starts probably when the solvent constitutes a continuous phase in the polymer network without “gaps” composed of the pure polymer. This appears for highly porous polymers at relatively low loading with the solvent. Possible transformations of TEOS inside polymer particles were tested using attenuated total reflectance (ATR) FT-IR technique. When comparing the spectra for initial Amberlite XAD7HP HP and after swelling followed by evacuation of TEOS (shown in Figure S4), the absence of characteristic bands for condensed silica species (νas Si−O−Si = 1200 cm−1) may be observed.30,31 Thus, it is quite reasonable to assume the stability of TEOS entrapped in polymer. Positron Annihilation Lifetime Spectroscopy Studies. It is interesting to investigate how the macroscopic geometrical effects of swelling are related to the microscopic transformations during the deformation of polymer network. Thus, an alternative study on swelling on the molecular level was performed using positronium annihilation lifetime spectroscopy (PALS), which allows looking into structural changes of a matter without destroying the sample. The great advantage of this method is the possibility to follow structural changes and phase transitions in situ, e.g., in the presence of a solvent vapor. This can be achieved by the properties of orthopositronium (o-Ps), which is the positron−electron bound state with parallel spins. This pseudoatom is formed mostly in nonconducting solids within the range of positrons, which are usually emitted from a β+ source (typically ca. 1 mm in organic materials). The intrinsic annihilation of o-Ps occurs with the relatively long lifetime of 142 ns. However, the o-Ps lifetime in matter can be greatly shortened due to the pick-off process, i.e., the annihilation of the positron with a foreign electron, which has the opposite spin. The pick-off probability depends on the size of the free volume in which o-Ps is trapped. Consequently, separate components characterized by different lifetimes are observed in positron annihilation lifetime spectra for fractions of o-Ps which annihilate in free volumes of distinctly different size. This enables the analysis of the evolution of free volumes with sizes specific to a solvent, solid network, and pores via the changes of respective spectra components. Additionally, in the studied system, o-Ps is quenched by oxygen in air, which shortens the o-Ps lifetimes.32−34 The shortening is significant for long lifetimes, but it is expected that the rate of quenching is

constant and does not change the shape of the observed dependencies. The positron annihilation lifetime spectra for the selected loadings of the polymer with TEOS are shown in Figure 5. The

Figure 5. Positron annihilation lifetime spectra of the XAD7HP sample for different loading with TEOS. The lines and filled areas represent the o-Ps components fitted to the spectra.

o-Ps components marked there are characterized by two main parameters: the lifetime (i.e., the slope, which depends on the free volume size) and intensity (i.e., the area, which reflects the concentration of free volumes). Before TEOS loading, there are three o-Ps components: short-, medium-, and long-lived, which can be ascribed to the polymer intrachain spaces, micropores, and mesopores, respectively. When the amount of the embedded TEOS increases, the share of the o-Ps components also changes. The most distinct change is the decrease of the intensity of the longest-lived component, which is connected with the successive elimination of the mesopores ranged within 2 and 50 nm. The medium-lived component, unlike others, clearly changes its lifetime. This indicates the change of its origin. When the amount of TEOS in the sample becomes significant (e.g., at Sm = 44%), the medium-lived component reflects annihilation in the liquid solvent. No component with the lifetime characteristic for micropores can be found, which indicates that the micropore concentration falls below the PALS detection threshold. The further increase of the intensity of the medium-lived component is the consequence of the increasing amount of TEOS. Initially, the intensity of shortlived component increases and predominates at moderate loadings, but it eventually decreases below the intensity of the TEOS related component. Thus, at high loadings, the o-Ps annihilation in TEOS predominates over the annihilation in the polymer and no pores at all can be detected. More precise information about the mechanism of swelling may be attained by a simultaneous analysis of the lifetimes (τi) F

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Figure 6. Lifetimes (τi), intensities (Ii), and dispersion (σ5, open symbols) of the o-Ps components as a function of the relative mass of TEOS Sm (%) loaded in XAD7HP.

In the second stage (Sm = 15−70%), the contribution of micropores to the medium-lived component (τ4, I4) is almost negligible. Instead, it represents annihilation in “bubbles”, which are “dug” by o-Ps in the liquid solvent.37 In consequence, I4 reflects the total volume of the liquid TEOS clusters, which are large enough to accommodate o-Ps bubble, i.e., V ≫ 0.3− 0.4 nm3. In contrast to the first stage, I4 shows linear and quite fast (0.15% I/%Sm) increase, which indicates a proportional change of this volume. The lifetime of this component is determined in a great part by the surface tension of the liquid. However, the decrease of τ4 from 5 to 3.5 ns is observed in this range. This is analogous to the relative pressure dependence of τ4 observed during the adsorption of n-heptane on silica.38 Therefore, it is probably the result of the increase of the partial pressure of TEOS vapor in the pores. Unlike I4, in the second stage, I5 continues to decrease at a constant rate. The transition between the stages is indicated by the slope change of the I 5 dependence, which is over 6 times slower in the second stage. It reflects the fact that this change is no longer related mostly to the suppression of the o-Ps migration but, instead, to the real depletion of free space in the mesopores. Quite surprising is the lack of any noticeable changes of either τ3 or I3 at this stage. It is expected that separate TEOS molecules penetrate the polymer skeleton, which should result in the modification of the size of intrachain free spaces as well as their concentration due to fragmentation. The most straightforward explanation is that no penetration of the polymer microspheres occurs at this stage, and the swelling of the whole particle is the result of a displacement of unswollen microspheres. Alternatively, the constant value of τ3 ≈ 2 ns could testify that both free spaces between polymer chains and the polymer chainTEOS molecule are very similar. Thus, the penetration of the polymer skeleton by TEOS molecules would not change τ3. In this case, the constant I3 would be a coincidental consequence of the relative character of intensities; i.e., a possible increase of

and intensities (Ii) plotted against weight % of TEOS (Sm) (Figure 6). The index i = 3, 4, 5 is ascribed to short-, medium-, and long-lived component, respectively. It should be noted that for the longest-lived component a log-normal dispersion of lifetimes with width σ5 was assumed. This implies that τ5 represents an average value of lifetime. Three stages with a different character of changes may be distinguished in the presented dependences. Initially, in the first stage (Sm < 15%), the intensities of both micro- and mesopores related components (I4 and I5) decrease to ca. half of their initial values. Simultaneously, the intensity of the component originating from polymer (I3) increases also ca. 2 times. Somewhat different are the changes of the lifetimes, where only τ4 decreases distinctly, while only small shortening of τ3 and lengthening of τ5 are observed. These pronounced changes are very likely the result of a quite subtle modification of the sample structure. Undoubtedly, TEOS is located mostly in micropores at this stage, which is confirmed by the decrease of their size (τ4) and concentration (I4). The other changes may be the consequence of the suppression of the o-Ps migration,35,36 i.e., the o-Ps fraction formed in the polymer intrachain spaces, which were open to micropores before swelling, can no longer migrate through micropores to mesopores. Such “sealing” of the polymer may be also the reason for the small shortening of τ3 due to the initial TEOS penetration of the largest intrachain spaces. However, such a slight change of τ3 can be also a result of the assumed approximations. Probably two components only are not enough to precisely reflect the possibly complicated lifetime distribution in the range of several nanoseconds in the base polymer. Therefore, τ3 can be artificially elongated to achieve the best fit. With the micropores being filled, the distribution becomes narrower and τ3 is no longer distorted and represents a typical polymer lifetime, which is characteristic of most cross-linked polymers. G

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Macromolecules I3 would be compensated by the same increase of competing I4 + I5. The third stage (Sm > 70%) is distinguished mainly by the step change of I3 from ca. 12% to 9%. Additionally, the changes of I4 and I5 gradually become slower there, to finally stabilize (I4 ≈ 18%, I5 = 0), but none of them changes stepwise as in the case of I3. Moreover, τ3 does not change between the second and third stage, either. Such a sudden change suggests an overcoming of some kind of barrier, and it could mark the onset of a deeper penetration of the polymer microspheres by TEOS but also the reaching of the maximum straightening of the polymer chains, above which any further introduction of TEOS molecules causes the collapse of the free volumes between them. Finally, it is worth to notice that τ4 = 3.2 ns observed for completely filled mesopores is somewhat smaller than 3.5 ns, which has been expected on the basis of the surface tension of TEOS.39 This indicates that a large part of the liquid TEOS is confined in nanometer-sized clusters which suppress the growth of an o-Ps bubble. The analysis of the lifetime distribution of the longest-lived component requires taking into account both the average lifetime and the dispersion of lifetimes. The average pore size, which facilitates the comprehension of the changes occurring in the sample, can be derived only from both of these parameters. The pore size distributions (PSDs), which can be calculated on the basis of these parameters,40 are even more convenient because they give more complete information about the pore sizes. The calculations were performed taking into account o-Ps quenching in the air.41 The log-normal shape, which is inherited from the analysis of the spectra, is often found in PSDs obtained by other methods. Nevertheless, such an a priori assumption can lead to distorted results in the case of other distribution shapes. Moreover, it is possible that PSDs obtained with this method are narrowed as a result of the pore sizes averaging caused by o-Ps migration before it annihilates.42 Fortunately, this effect seems to be suppressed in air due to shorter o-Ps lifetime (Figure S5). Therefore, it should be remembered that the PSDs obtained this way are only a rough approximation, which is suitable for discussing their relative changes only. The calculated PSDs (Figure 7) represent the TEOS-free volume in the mesopores. According to the previously proposed hypothesis, the initial decrease of the mesopore volume should not be taken into account as the real

change of the mesopore system. Instead, the decrease is caused by filling micropores, which are connected to mesopores. Therefore, the localization of micropores can be estimated from the first stage (Sm < 15%) of the PSD changes. The small mesopores (D < 5 nm) are affected by the changes in a clearly greater extent than the large ones (Figure S6). This confirms the expected pore structure, in which the micropores are located mostly on the junction of polymer microspheres and smoothly turn into small mesopores. The further PSD evolution (Sm > 15%) reflects the changes of the mesopore structure more directly. The difference between the shapes of the PSDs is better visible if they are normalized (Figure S7). It is necessary to describe the PSD shape using appropriate parameters before a qualitative discussion of their evolution during swelling can be presented. The area under a PSD (i.e., the volume, V), the pore size at the maximum of a PSD (Dmax), and the width of the distribution at a half of the maximum (FWHM) were chosen as these parameters (Figure 8).

Figure 8. Pore volume (V), size at maximum (Dmax), and the width of the distribution at a half of the maximum (FWHM) for the pore size distributions calculated from the parameters of the longest-lived o-Ps component as a function of the relative mass of TEOS Sm (%) loaded in XAD7HP.

The shapes of the mass dependence of V and fifth component are quite similar. This confirms that the intensity of component 5 is suitable to estimate the volume of free space left in the mesopores. More interesting relations were obtained in the case of Dmax and FWHM. The transition between the first and second stage is visible as sharp increase in FWHM at a nearly constant Dmax. Most likely, this indicates the change of the location where the majority of positronium is formed. In the first stage, o-Ps is formed mostly in micropores; subsequently, it can migrate to large mesopores only through small ones. Most of the migrating o-Ps annihilate in the smallest mesopores, where they arrive first, still in large numbers. Additionally, their annihilation rate is greater in the smaller pores than in the large ones. In the second stage, o-Ps is

Figure 7. Pore size distributions calculated from the parameters of the longest-lived o-Ps component for the selected Sm (%) of TEOS loaded in XAD7HP. H

DOI: 10.1021/acs.macromol.7b00820 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

pseudoatom as a probe (positronium). Moreover, the probe is randomly formed within the sample volume allowing access to closed free volumes. Positronium explores the large pores as well as the smallest free volumes between molecules. From PALS experiment, it follows that at high loading the continuity of solvent phase within polymer is likely to be achieved. At high solvent loading, its molecules are distributed along the chains of the polymer and the system transforms into a specific polymer−solvent binary mixture. It seems that almost half of o-Ps annihilating in mesopores is formed outside of them (e.g., in micropores). This fact should be taken into account if pore size distributions obtained by PALS are analyzed and compared.

formed on the surface of all mesopores equally, and annihilation in small pores no longer dominates. Still, a rapid change of the polymer structure when micropores are filled cannot be excluded. The analysis of PSDs allows noticing the transition between the second and the third stage, which was hardly visible in τ5 and σ5. Both Dmax and FWHM, which were stable or slowly increasing with TEOS load, respectively, start to decrease above Sm = 60−70%. This is in agreement with the interpretation that TEOS starts to penetrate the microspheres deeply, which causes their growth at the expense of mesopores.

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CONCLUSIONS Taking into account the nature of swelling, it is obvious that the amount of the solvent in a sample increases throughout the process. The liquid solvent alone is the component whose structural transformations seem to be negligible. The opposite is true for the polymer network, which is transformed during swelling due to the solvent distribution between the matrix chains. Initially, the solvent molecules are distributed randomly and, sparsely, sometimes as separate molecules. As swelling progresses, the creation of solvent clusters takes place. The clusters are initially separated, but they combine with each other, and eventually, they acquire properties similar to the bulk liquid solvent. The mutual permeability of the polymer and solvent causes the dilution of the polymer network and, finally, the formation of the continuous solvent phase. The subsequent expansion of the polymer network probably reaches its limit, which is supported by more than 2-fold increase of volume. This mechanism is particularly preferred in the case of porous polymers, where only walls of pores participate in swelling. In porous polymers exposed to a solvent vapor, adsorption and swelling occur simultaneously. At the beginning of the solvent uptake, local swelling is observed. At this stage of solvent absorption, the volume change of polymer particles is small. Subsequently, as an increasing amount of solvent is present in polymer network, the volume of a polymer particle increases rapidly. Optical microscopy is the most suitable technique for the study of the process, especially if regularly shaped particles are present. The polymer particles used in this study were almost perfect spheres. The macroscopic isotropy features, which are represented by polymer volume, reflect microscopic phase transitions during swelling. The monitoring of the swelling mechanism on the molecular level is possible with the use of PALS spectroscopy technique, which allows determining the existence of any free volumes in the sample. From the correlation between the macroscopic effects of swelling and positronium decay within polymer−solvent system, the location of the solvent within the polymer particle can be presumed. On this basis, it is possible to distinguish between the penetration of various elements of the polymer particle by the solvent. It was found that at high loading, of more than 100% of the solvent, a material whose pores are completely filled by the polymer matrix is produced. In micropores formed during the swelling at the expense of mesopores, capillary condensation takes place even at relatively low pressure of the solvent vapor. The standard methods, widely used in routine studies, such as low-temperature adsorption of nitrogen, mercury porosimetry, and thermoporosimetry method, entail several inherent problems in structural investigations due to the uncertainty regarding the pore size limits and the presence of closed pores inaccessible to adsorptives. PALS makes it possible to avoid these problems, which may be the source of errors because it uses the very small

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00820. Optical images of swelling XAD7HP polymer in TEOS vapor; strain−compression curves of XAD7HP polymer and polymer swelled in TEOS; nitrogen adsorption/ desorption data at 77 K of pure polymer XAD7HP; ATR-FTIR spectra of XAD7HP polymer, liquid TEOS, and polymer swelled in TEOS; pore size distributions calculated from the parameters of the longest-lived o-Ps component for pure XAD7HP obtained in air and vacuum; the volume of the pores with D < 5 nm (VD 5 nm (VD>5 nm), and their ratio (VD5 nm) for the pore size distributions calculated from the parameters of the longest-lived o-Ps component as a function of the relative mass of TEOS (Δm) loaded in XAD7HP; pore size distributions calculated from the parameters of the longest-lived o-Ps component for the selected Sm (%) of TEOS loaded in XAD7HP (PDF) Video S1 illustrating the continuous growth of particles and their visual transformations (MPG)

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AUTHOR INFORMATION

Corresponding Author

*(J.G.) E-mail [email protected]; Fax 48-815-333348; Tel 48-815-375-563. ORCID

Jacek Goworek: 0000-0003-4266-3470 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Contract POIG.02.01.00-06024/09 Center of Functional Nanomaterials). The authors thank Marek Gorgol and Artur Błażewicz from Department of Nuclear Methods, Institute of Physics, for their help in the construction of the sample and solvent holder used in the PALS measurement and Paweł Mergo and Grzegorz Wójcik from Department of Optical Fiber Technology, Faculty of Chemistry, for measurements of hardness of pure and swollen polymer. I

DOI: 10.1021/acs.macromol.7b00820 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b00820 Macromolecules XXXX, XXX, XXX−XXX