Volume phase transition of electron beam cross-linked thermo-responsive PVME nanogels in the presence and absence of nanoparticles: with a view toward rheology and interactions

Abstract

We investigate the effect of nanoparticles and radiation dose on interactions in the PVME-based nanogel system and its phase behavior (swelling/deswelling behavior and phase separation mechanism) by rheological and FTIR measurements. The volume phase transition temperature of pure and hybrid nanogels are obtained by an isochronal temperature sweep test. Phase contrast optical microscopy is employed to investigate morphological evaluation. It is found that frustrating the mobility of polymer chains in nanogels by the addition of nanosilica or increasing the radiation dose increases the deswelling temperature, and also slows down the kinetics of phase separation. The enhanced dynamic asymmetry in the presence of nanoparticles leads to transition of the phase separation mechanism from nucleation and growth (NG) to viscoelastic phase separation (VPS). We investigate the linear and transient rheological behaviors of pure and hybrid nanogels synthesized with different radiation doses in the homogenous and phase-separated regions. Finally, the conformity of the prediction of emulsion models and rheological behavior of phase-separated nanogels is investigated.

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1. Introduction

Hydrogels (nano, micro and macroscopic hydrogels) are an important class of materials with tremendous application potentials in biology and pharmaceutical sciences.^1–11 Poly(vinyl methyl ether), PVME, is a water-soluble polymer that exhibits a lower critical solution temperature (LCST) of about 35 °C (close to the body temperature)^12–15 in aqueous solutions. This makes hydrogels (water-swollen, covalently cross-linked polymer networks) based on this polymer suitable for drug delivery systems.⁵ PVME aqueous solutions can be cross-linked with a high energy radiation to form temperature sensitive hydrogels.^3,8,10,11 This process is solvent and additive free³ yielding hydrogels suitable for biological applications.⁵ At low polymer concentrations, C, (C < C^*, threshold concentration for molecular overlap) and temperatures lower than LCST, macromolecules are well separated. Therefore, irradiation by an appropriate dose of energy leads to cross-linking of each macromolecule separately and formation of nanosized gels.^1,3,11 However, irradiation at temperatures higher than the LCST in the non-homogeneous state results in globular microgel particles. Therefore, the main factors which play a crucial role in the synthesis of different kinds of hydrogels are (i) radiation dose, (ii) temperature of solution, and (iii) concentration of polymer in the system.

High-energy radiation of an aqueous solution produces reactive radicals (H˙ and OH˙) by the radiolysis of water. These radicals are able to attack the polymer chains and abstract hydrogen atoms, resulting in the transfer of the radical center to the polymer chain.¹⁶ In dilute solutions, where the distances between the macromolecules are relatively high (as in our study), the recombination of the formed radicals results mainly in intramolecular cross-links^11,16 and formation of nanogels. Increasing the radiation dose increases the concentration of the induced radicals on each macromolecule and nanogel particles with decreased dimensions will be synthesized.³

In temperature sensitive hydrogels, by increasing the temperature above the LCST, volume of the system will significantly decrease.^6,11,17 In other words, deswelling or volume phase transition occurs.^17,18 In the PVME gel systems (hydrogels, micro and nanogels), the phase transition or deswelling of the PVME gel/water system with increasing temperature above LCST is due to the transition of hydrophilic interactions to hydrophobic ones in PVME chains.^{12–14,19–21} Below the LCST, hydrophilic ether oxygens of PVME chains stabilize the aqueous solution by forming hydrogen bonds with water molecules.^13,20,22 In addition, water molecules perch around the methyl groups along the polymer chains and hydrate the hydrophobic methyl groups. With an increase of temperature, arrangement of water around PVME chains changes: methyl groups initially become dehydrated, and then the hydrogen bonds between water molecules and macromolecules break. In other words, the hydrophobic interactions increase monotonously during heating and overcome the hydrophilic interactions at the LCST. Gou et al.¹⁴ demonstrated that the main chain of PVME (methane groups) dehydrates before the methyl side chains during the phase separation. The temperature sensitive gels can be used in medicine and pharmacy for drug delivery applications.^5,7 For such purposes, it is crucial to understand how the different parameters (such as temperature, radiation dose, and addition of nanofiller) affect the phase behavior and kinetics of deswelling.

Recently, the effect of nanoparticles on phase behaviour of gel systems has attracted considerable attention.^2,17 Van durme et al.²³ observed that the kinetics of deswelling for thermo-responsive hydrogels was increased by the introduction of silica nanoparticles. Hou et al.¹⁸ demonstrated that incorporation of gold nanoparticles inside the NiPAM, poly(N-isopropylacrylamide), microgels results in a decrease of microgel size in the swollen state and increase of the volume phase transition temperature of the microgels. They showed that the presence of nanoparticles increases the rigidity of polymer chains, which increases the volume phase transition temperature of the microgels. However, the effect of nanoparticles on interactions, kinetics and mechanism of phase separation in nanogels has not been investigated in the literature.

Rheology is a sensitive tool to study the bulk structure of polymeric systems in a molecular scale, which makes it suitable to study the gel systems. Senff and Richtering^24,25 observed that rheological behavior of the globular microgels at low cross-link density is similar to the rheological behavior of the linear polymer solutions; however, at high cross-link densities the globular microgels behave rheologically like hard sphere suspensions. A similar result was observed by Omari et al.²⁶ Tan et al.²⁷ observed an excellent agreement between experimental viscosity data (relative viscosity vs. effective volume fraction) of the NiPAM microgels and a semi-empirical model (modified Krieger–Dougherty) and a common master curve was obtained. However, there is no study in the literature on the theoretical and experimental investigations of the correlation between the evolution of phase-separating morphologies and corresponding linear rheological behavior in nanogels.

The aim of this work is to investigate the effect of nanoparticles and radiation dose on interactions, and phase behavior of PVME-based nanogels by rheological and FTIR measurements. An attempt was made to correlate the evolution of phase-separating morphologies with the corresponding rheological behavior. To the best of our knowledge, there is no study in the literature on the effect of nanoparticles and radiation dose on phase-separating morphologies of nanogels and their corresponding rheological behaviors. For the first time, an attempt was made to obtain the deswelling temperature of nanogels by rheology.

2. Experimental section

2.1. Materials

Hydrophilic fumed silica nanoparticles (Aerosil 200) were supplied by Degussa Corporation. The nanoparticles are aggregates of spherical particles with an average primary size of 12 nm, a density of 2.2 g cm⁻³ and a specific surface area equal to 200 m² g⁻¹. These colloidal particles are prepared by flame hydrolysis of silicon tetrachloride. Silanol groups are generated on the fumed silica surface. Irreversible aggregates, linked by bonds are formed during the fabrication process.²⁸ Two different grades of poly(vinyl methyl ether), PVME, with different molecular weights were used in this study: i – the commercial grade Lutanol M40, supplied by BASF Co. ii – PVME with lower molecular weight was obtained as a 50 wt% aqueous solution from Aldrich Chemical Co. The basic characteristics of PVME polymers are listed in Table 1. Deionized water was used as solvent, which was purified by distillation and subsequent passing through the Nanopure II system (Barnstead, final specific resistance > 17 MΩ cm).

Table 1 The basic characteristics of PVME polymers used in this study

	Mw (g mol^−1)	Mn (g mol^−1)	Tg (°C)	Supplier
PVME	60 500	20 500	−31	Aldrich Chemical
PVME	110 000	64 000	−32	BASF

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2.2. Sample preparation

2.2.1. Aqueous solution preparation for nanogel and phase diagram. To prepare the nanogel, a certain amount of PVME (from both kinds of PVME shown in Table 1, as will be explained) was dissolved in deionized water to obtain aqueous solution with the concentration, C, of 0.3 wt% (C < C* ≈ 0.5 wt% (ref. 8 and 11)). However, to obtain the phase diagram of irradiated PVME solution, aqueous solutions with a wide range of polymer weight fraction (0.3 to 80 wt%) were prepared by overnight stirring of the PVME at room temperature. Prior to the irradiation, the solutions were filtered through 0.45 μm filters. In the case of hybrid samples nanosilica with the amount of 0.05 wt% of polymer was dispersed separately by sonication in deionized water for 10 minutes at room temperature; then the dispersion was mixed with the polymer solution.

It should be noted that dilute polymer solutions containing nanosilica (solutions for preparation of nanogels) which are prepared by the above mentioned method are almost unstable; that is, nanoparticles aggregate and settle down in the bottle. In hybrid solutions with low polymer concentrations, the free polymer chains in the solution cause concentration fluctuations between nanoparticles which induce attractive interactions between them.²⁹ However, in concentrated polymer solutions, concentration fluctuations are negligible due to the high concentration of free chains in the solution. Therefore, the solution remains stable.^29,30

To stabilize dilute hybrid solutions (which are used for preparation of nanogels), we applied the concept of the steric stabilization mechanism. In polymer solution containing nanosilica, some polymer chains anchor to the surface of nanoparticles due to the favorable interaction. If the length of graft chains are longer than free chains, the solution becomes stable.³⁰ To have such a condition, at first, we prepared solutions containing nanoparticles with high molecular weight PVME (provided by BASF) as explained above (the amount of polymer used in this mixture is 90% of the total polymer used to prepare nanogel). Then, the polymer solution prepared by the low molecular weight PVME was mixed with the hybrid solution (the amount of polymer used in this mixture is 10% of the total polymer used to prepare the nanogel). This procedure yields a stable hybrid solution.

2.2.2. Radiation. Before irradiation it is necessary to discharge oxygen from the bottles containing the samples.^1,3 For this purpose, nitrogen was blowing for about 4 hours through the solution under stirring. After degassing, to minimize the rate of oxygen uptake, the bottles were sealed with PARAFILM. Subsequently, the samples were irradiated with a linear accelerator ELV-2 (Department of Physics, Amirkabir University of Technology) under irradiation conditions of: electron energy, 1.0 MeV; doses, 20 and 40 KGy;^8,16,31 solution temperature, 30 °C. At this concentration and temperature (C < C* and T < LCST), the macromolecules are well separated in which irradiation leads to formation of nanogels.^1,8,11

2.3. Methods

2.3.1. Rheological measurements. Rheological tests were carried out using an Anton Paar MCR-501 rheometer with cone and plate geometry with a cone angle of about 1° and 25 mm diameter and a gap of 59 microns at the rim. Temperature control of ±0.1 °C was achieved with a Peltier system under nitrogen atmosphere.

The following small-amplitude oscillatory shear experiments were done in the linear viscoelastic region, as was verified by preliminary amplitude sweep tests: (i) isochronal dynamic temperature sweep experiments at a fixed frequency of 0.05 Hz, which was low enough to be in the linear region in agreement with literature data,^32–34 and a constant heating rate (0.5 °C min⁻¹), from 25 °C in homogeneous state to 50 °C in phase-separated state at a certain strain (1%) in order to detect the phase separation temperature. (ii) Isothermal dynamic frequency sweep experiments at a fixed strain of 1% and temperatures of 30 and 40 °C in the swelling and the phase-separated states, respectively.

2.3.2. Optical microscopy (OM). For optical microscopy observations, a small amount of PVME solution or nanogel was placed between a microscope slide and a cover glass, separated by a spacer (thickness 10 μm). The phase contrast optical microscopy (Leica DMRX) was used to investigate morphological changes at a constant temperature of 40 °C. Samples were placed in a hot stage sample holder (Linkam LTS350), which was controlled by a hot stage controller (Linkam CI 94). A CCD camera mounted directly on the microscope allowed the recording of the evolution of phase-separating morphology in real time.

2.3.3. FT-IR measurements. Fourier transform infrared (FT-IR) spectroscopy of the samples was performed by a Nicolet-IR100 spectrometer to evaluate the interactions in the system. The freezed-dried samples were dispersed in deionized water and placed at 4 °C for a week to ensure sufficient swelling. Then, the samples were prepared by sealing aqueous nanogel solution between two GaF₂ tablets, which had no absorption bands in the MIR region. The spectrum was obtained from 400 to 4000 cm⁻¹ and with a resolution of 4 cm⁻¹. Spectroscopy of the samples was performed at two temperatures of 30 and 40 °C in homogeneous and phase-separated states, respectively. For convenience of comparison, the FTIR spectra were normalized and major vibration bands were associated with chemical groups. In order to normalize the infrared spectra, we used the 1074 cm⁻¹ band, assigned to the O–H stretching vibrations.¹³

2.3.4. Dynamic light scattering (DLS). The hydrodynamic radius, transversal diffusion coefficient and the size distribution of the pure and hybrid nanogels were measured by dynamic light scattering method with a Brookhaven Instruments setup (90 plus) consisting of a He/Ne laser (632.8 nm). Measurements were done using triple distilled deionized water as solvent. The samples were filtered through filters of 0.45 μm pore size before the experiments. In dynamic light scattering, the intensity time correlation function g₂(q,t) can be determined by Siegert relation:³⁵

g₂(q,t) = A(1 + β|g₁(q,t)|²) (1)

where t is the decay time, A the measured base line, β the coherence factor, q = (4πn/λ₀)sin(θ/2) the scattering vector, and g₁(q,t) the normalized first order electric field correlation function that is related to the measured relaxation rate Γ as follows:³⁵

The line width distribution, G(Γ), can be obtained from the Laplace inversion of g₁(q,t) using CONTIN procedure.³⁶ For a pure diffusion relaxation, the extrapolation of q → 0 and c_p → 0 (c_p polymer concentration) led to the transversal diffusion coefficient D, which is related to the hydrodynamic radius R_h by the Stokes–Einstein equation:³⁷

where K_B, T and η are the Boltzmann constant, the absolute temperature and the viscosity of the solvent, respectively. To compare the size of the particles with DLS measurements, no extrapolation to the above situations was done, and the apparent hydrodynamic radius of the particles gained by the DLS measurements directly with an experimental error of about 10% were used at θ = 90°. Here, the apparent hydrodynamic radius of the particles was used without any further calculations.

3. Results and discussions

3.1. Volume phase transition temperature

3.1.1. Rheology. In order to investigate the effect of nanoparticles and irradiation dose on volume phase transition temperature of PVME nanogels (0.3 wt% PVME), isochronal dynamic temperature sweep experiments were performed as explained in the Experimental section and the results are shown in Fig. 1. The temperature sweep curve of the PVME aqueous solution (blank sample with no radiation and free of particles) is provided in the ESI (Fig. S1†).

Fig. 1 Isochronal dynamic temperature sweep of storage modulus for (a) non-hybrid nanogel irradiated at 20 KGy, (b) non-hybrid nanogel irradiated at 40 KGy, (c) hybrid nanogel irradiated at 20 KGy, and (d) hybrid nanogel irradiated at 40 KGy. The rheologically determined volume phase transition temperatures are: (a) T = 36.9 °C, (b) T = 37.9 °C, (c) T = 37.4 °C and (d) T = 38.4 °C which are marked with arrows in the figure. The solid lines represent the slope of Gʹ curve before and after phase separation temperature and are guides to the eye.

It can be seen that at low temperatures in the homogeneous region storage modulus, Gʹ, increases slowly with a temperature rise for all the samples; however, near a critical temperature, Gʹ rapidly increases which is shown by the intersection of two slopes in each curve. The temperature at which Gʹ rapidly increases is assigned as volume phase transition (deswelling) or binodal temperature. The slight increase in Gʹ in the homogeneous region with temperature rise in the vicinity of phase separation can be attributed to the slight increase of inter and also intra-molecular attractive interactions in the nanogel particles. Heyes and Brańka³⁸ demonstrated that at temperatures well below the deswelling transition of microgels, there is a soft pair repulsive and weak attractive interaction between the particles. However, at temperatures above the deswelling transition temperature the interaction potential becomes strongly attractive; and thus the collapsed microgels aggregate. Therefore, in the vicinity of phase separation temperature the pair nanogel interactions start to change from soft repulsive to soft attractive interaction and also a slight dehydration of nanogels occurs by a temperature rise leading to a mild increase in the elastic modulus. As temperature exceeds the volume phase transition temperature, the nanogels fully collapse and interaction between them becomes strongly attractive leading to complete aggregation of collapsed nanogels (interpenetration of nanogels). This results in the formation of dynamic domains rich in the elastic PVME component, causing an upturn in Gʹ. Moreover, during the phase separation interface comes into play, by introducing a supplementary elasticity in the system due to the deformation and the shape recovery of the formed domains. Fig. 2 schematically shows the described evolution of pure and hybrid nanogels by increasing the temperature. The variations of interactions in the nanogels by temperature rise are studied in the next section by FT-IR measurements.

Fig. 2 Schematic representation of phase separation of nanogels (increasing inter and also intra-molecular attractive interactions) with temperature rise for: (a) pure nanogels: (a1) well below phase transition temperature, (a2) in the vicinity of phase transition temperature, and (a3) above the phase transition temperature; (b) hybrid nanogels: (b1) well below phase transition temperature, (b2) in the vicinity of phase transition temperature, and (b3) above the phase transition temperature.

As seen in Fig. 1, the addition of nanosilica slightly increases the volume phase transition temperature of nanogels. Interaction of PVME with nanosilica restricts the mobility of the macromolecules and increases their rigidity; hence, higher temperature is necessary to achieve the nanogel aggregation.¹⁸ PVME macromolecules anchor to the nanoparticles surface³⁹ leading to physical cross-links between polymer chains and nanoparticles. The interaction between the A200 nanoparticles and PVME molecules occurs through hydrogen bonding between the ether oxygen of PVME molecules and isolated silanol groups on the silica surface. As Fig. 1 shows, increasing the energy dose of radiation also increases volume phase transition temperature. Increasing the radiation dose increases the intramolecular bonds and cross-linking density³ (because of the low concentration of the aqueous solution the amount of the intermolecular cross-links is negligible). Therefore, mobility of the macromolecules decreases and nanogels aggregate at higher temperatures.

It should be noted that determination of the phase separation temperature from the dynamic temperature sweep experiment may involve an error in the estimation of slopes. To be sure of the data accuracy obtained from Fig. 1 we also used another method to determine the phase separation temperature which is provided in the ESI in Section S2.†

3.1.2. FT-IR measurements. To understand the effect of deswelling and addition of nanosilica on interactions in nanogels, FT-IR analysis was carried out for pure and hybrid nanogels. The FT-IR spectra of the blank sample with no radiation and free of particles are provided in the ESI in Section S1.†

Fig. 3a shows the results for pure nanogels irradiated at 40 KGy at homogeneous (30 °C) and non-homogeneous (40 °C) states. It can be seen that during volume phase transition, the characteristic IR bands of the PVME polymer, 2800–3050 cm⁻¹ and 1050–1150 cm⁻¹ which correspond to the C–H and C–O stretching bands respectively,¹³ were shifted. A blue shift (transition of an IR band to the higher wave numbers) in the C–O stretching IR bands at 1050 to 1150 cm⁻¹ and 1068 to 1088 cm⁻¹ can be seen due to the dehydration of the C–O groups. This means a breakdown of the hydrogen bond between ether oxygen and water. A red shift (transition of an IR band to the lower wave numbers) in C–H stretching bands at 2800 to 3000 cm⁻¹ and 2819 to 2816 cm⁻¹ can be seen in Fig. 3a due to the dehydration of methyl groups. The observed band shifts indicate the dehydration of PVME chains upon heating.^13,14

Fig. 3 FT-IR spectra of: (a) non-hybrid nanogel irradiated at 40 KGy at temperatures of 30 and 40 °C, (b) pure and hybrid nanogels irradiated at 40 KGy at temperature of 30 °C, (c) pure and hybrid nanogels irradiated at 40 KGy at temperature of 40 °C and, (d) non-hybrid nanogels irradiated at 20 and 40 KGy at temperature of 30 °C.

Fig. 3b and c shows the effect of nanosilica on the FT-IR spectra of the nanogels irradiated at 40 KGy at homogenous and non-homogenous states, respectively. Fig. 3b shows that in the homogeneous region in the presence of nanoparticles, the C–O stretching band exhibits a slight blue shift compared with the non-hybrid one, indicating interaction of C–O groups of polymer chains with silica nanoparticles. At the non-homogeneous state (Fig. 3c), a slight red shift of the C–H stretching band in the case of the hybrid nanogel compared to the non-hybrid nanogel indicates that C–H groups suffer higher dehydration in the presence of nanoparticles.¹⁸ By the addition of nanoparticles, associations of C–O groups of PVME with water molecules break and change to the interaction of C–O groups with free silanol groups on the surface of nanoparticles (further investigation on the pair interactions is provided by the frequency dependence of the complex viscosity in the ESI in Section S4†). This greater dehydration indicates that hybrid nanogels experience a full collapse after heating above the volume phase transition.¹⁸

Fig. 3d indicates FT-IR spectra of the non-hybrid nanogels irradiated at different doses, at temperature of 30 °C. A blue shift at the C–O bands with increasing radiation dose indicates more interchain interactions between C–O groups along the polymer chains.

In order to study the effect of nanosilica on the phase diagram of the irradiated water/PVME system, solutions with different PVME concentrations in the presence and absence of nanoparticles were prepared and irradiated by 20 KGy electron beam radiation. Fig. 4 shows the phase diagrams of the irradiated PVME solutions with and without nanoparticles obtained by rheological measurements, which show a type III phase separation behavior⁴⁰ similar to the non-irradiated PVME aqueous solution. Type III phase behavior is characterized by the occurrence of two lower critical points in the demixing curve and one zero limiting critical concentration.^40–42

Fig. 4 (a) Phase diagram of irradiated PVME solutions in the presence and absence of nanoparticles, (b) the phase diagram in the range of % wt PVME <1% is zoomed in.

There is a good agreement with this rheologically (isochronal temperature sweep of elastic modulus) determined phase diagram and the ones reported in the literature for PVME solutions determined by DSC and turbidity temperature measurements.^21,40 Interestingly, nanoparticles increase the phase separation temperature of nanogels (up to 1 wt% of polymer); however, phase separation temperature of hydrogels (higher concentrations of polymer) decreases in the presence of nanoparticles. This occurred due to the different roles of nanoparticles in each situation. In hydrogels, hydrophilic nanoparticles act as nano sized water reservoirs in the system²³ reducing the characteristic diffusion length of water in the macromolecules. Therefore, water molecules can be transported faster as compared to the non-hybrid hydrogels. However, because of high dilution of the solution in the case of nanogels, the characteristic diffusion length of water in the polymer chains does not change in the presence of nanoparticles. In nanogels, increasing the rigidity of the macromolecules in the presence of nanoparticles plays a major role in the volume phase transition. The less mobility of the polymer chains induces higher phase transition temperature.

3.2. Morphological observations

In order to study the effect of nanoparticle and radiation dose on morphology development of nanogels in the two-phase region of the phase diagram (non-homogeneous state), optical microscopy observations were carried out at constant temperature of 40 °C.

Fig. 5 shows the phase separation evolution of the non-irradiated 0.3 wt% PVME aqueous solution and irradiated ones at 20 and 40 KGy. As can be seen, all the samples phase separate by the nucleation and growth mechanism. In the early stages of phase separation, PVME-rich domains nucleate which grow in size with time.

Fig. 5 Optical micrographs indicating time evolution of phase-separating (aggregation) morphologies for: (a) non-irradiated 0.3 wt% of PVME aqueous solution: (a1) 3 min, (a2) 6 min, (a3) 9 min and (a4) 12 min; (b) non-hybrid nanogel prepared by 20 KGy electron beam radiation: (b1) 3 min, (b2) 6 min, (b3) 9 min and (b4) 12 min; (c) non-nanogel prepared by 40 KGy electron beam radiation: (c1) 3 min, (c2) 6 min, (c3) 9 min and (c4) 12 min. All the scale bars correspond to 50 μm.

Fig. 5 shows that in the phase-separated nanogels, the PVME-rich droplets size is significantly smaller compared to the phase-separated non-irradiated PVME solution. This indicates that radiation considerably decreases the phase separation kinetics. Cross-links along the macromolecules partially restrict the mobility of the polymer chains, which slows down coarsening of the domains. Therefore, the kinetics of phase separation for the sample irradiated at 20 KGy (Fig. 5b) is faster than the nanogel irradiated at 40 KGy (Fig. 5c) (larger PVME-rich domains at corresponding phase separation times). It can be seen that in the irradiated samples, droplets shapes deviate from spherical form.

Fig. 6 shows the phase separation evolution of the nanogels containing nanoparticles. The major water-rich phase nucleates and grows in the minor PVME-rich matrix phase, accompanied by volume shrinkage of the matrix that leads to the formation of a PVME-rich network structure. In the later stage, a morphological transition from a network structure to disperse-matrix morphology occurs where the initial matrix phase becomes the disperse phase at later stages of phase separation (phase inversion). This behavior is a characteristic of viscoelastic phase separation which takes place in the mixtures having largely different viscoelastic properties (dynamically asymmetric mixtures).^34,43,44 In other words, transition of the thermodynamically-driven phase separation mechanism (NG) to VPS occurs in the presence of nanoparticles.

Fig. 6 Optical micrographs indicating time evolution of phase-separating (aggregation) morphologies of hybrid nanogels irradiated at different radiation doses: (a) 20 KGy: (a1) 2 min, (a2) 4 min, (a3) 6 min, and (a4) 1 h; and (b) 40 KGy: (b1) 2 min, (b2) 4 min, (b3) 6 min and (b4) 1 h. All the black scale bars correspond to 50 μm and the white ones correspond to 10 μm.

Dynamic asymmetry which comes from a large difference in mobility between the component molecules can be induced by a large difference in the glass transition temperature (T_g) or a large size difference. The dynamic asymmetry between the water-rich and PVME-rich phases can be characterized theoretically by a dynamic asymmetry parameter ξ which is given as:

where τ₁ and τ₁ are the relaxation times of the slow and fast components, respectively (here PVME-rich and water-rich phases, respectively).

In dynamically asymmetric mixtures, domain growth induces self-generated stresses in the slower component during the phase separation. The resultant stresses mainly cancel the stress originating from the surface tension, thereby preserving the continuity of the more elastic phase even if it is the minor phase. During the late stage of VPS, when the volume of each phase approaches the thermodynamically favorable state, domain growth slows down, and thus, weakens the resulting self-generated stress fields. Consequently, the interfacial forces start to play a dominant role competing with the elastic forces. As a result, the network structure becomes unstable in the reduced stress fields, and the interconnectivity of the minor phase breaks up leading to droplet-matrix morphology where the total free energy is reduced.

Occurrence of phase separation through VPS indicates that the dynamics of PVME chains is considerably frustrated in the presence of nanoparticles (as will be shown by rheological measurements) leading to enhanced dynamic asymmetry. This confirms that PVME chains are attached to the surface of nanoparticles. The interaction between the A200 nanoparticles and PVME molecules occurs through the hydrogen bonding between the ether oxygen of PVME molecules and isolated silanol groups on the silica surface. Nanoparticles act as physical cross-links in addition to cross-links made by radiation. These excess cross-links cause a considerable decrease of dynamics of polymer chains, and thus enhancement of dynamic asymmetry.

Simulations^45,46 show that the polymer chains are stretched and widened in the filled systems due to their tangential orientation to the particle surface and a significant increase in the percentage of trans conformation in the vicinity of nanoparticles. The adsorbed macromolecular chains result in reduction of mobility near the nanoparticle surface and on average each polymer chain acquires an extra stretching energy. This energy, called “entropic surface tension”, is given by:⁴⁷

h ∝ 3R_p²/2Nr₀² (5)

where R_p is the radius of the nanoparticle, N is the degree of polymerization and r₀ is the radius of the polymer monomer.

Therefore, we propose the morphology of the nanogels in the presence of nanosilica as shown in Fig. 7. The observed behavior confirms the steric stabilization mechanism of nanogels in the homogeneous region explained in the Experimental section. As far as we know, this is the first time that the VPS mechanism is reported in the nanogels.

Fig. 7 Schematic representation of morphology of nanogels in the presence of nanosilica.

In drug delivery applications, network structures can provide more controlled release of drugs.^48,49 Since VPS can produce a percolated network structure in the nanogels, phase separation through this mechanism for drug release is quite preferred.

3.3. Rheological analysis

3.3.1. Dynamic frequency sweep test.
a) Homogeneous state. Fig. 8 shows the frequency sweep behavior of storage and loss modulus for non-hybrid and hybrid nanogels irradiated at 20 and 40 KGy in the homogeneous region at 30 °C.

Fig. 8 Dynamic moduli of non-hybrid (a and b) and hybrid (c and d) nanogels in the homogeneous region at 30 °C for: (a, c) nanogels irradiated at 20 KGy and (b, d) nanogels irradiated at 40 KGy.

Non-hybrid nanogels (Fig. 8a and b) exhibit liquid-like behavior (Maxwellian behavior), and the loss modulus is higher than the storage modulus in the low frequency region with the scaling relations as G′ ∝ ω² and G′′ ∝ ω. The inverse of frequency at the cross over point of storage and loss modulus indicates the relaxation time of the polymer chains.⁵⁰ As can be seen, the relaxation time of pure nanogels increases with an increase of radiation dose due to the higher cross-linking density.

In the presence of silica nanoparticles, nanogels exhibit rheologically solid-like behavior (Gʹ > G′′) (Fig. 8c and d), with a large deviation from Maxwellian behavior. In the case of hybrid nanogel irradiated at 40 KGy (Fig. 8d) Gʹ becomes nearly independent of frequency. This indicates that long-range relaxation of PVME chains is considerably restrained in the presence of nanoparticles, and thus, the dynamic asymmetry is enhanced.

Similar rheological behavior is observed for hard sphere colloidal suspension by Omari et al.²⁶ Due to the very low concentration of the nanosilica used in this study, a continuous network of nanoparticles cannot be formed (as will be shown by DLS measurements) to affect the rheological behavior.

To confirm that during the preparation of hybrid nanogels no network of nanoparticles is formed, DLS measurements were carried out for the aqueous solution of nanoparticles (without polymer), and also for hybrid nanogels irradiated at 40 KGy at a temperature of 30 °C. In the aqueous solution of nanoparticles used for DLS measurements, the concentration of nanoparticles is the same as the ones used for the preparation of hybrid nanogels. DLS measurements determine the size distribution of the total particles and the mean particle–particle distance. Fig. 9 shows the apparent size distribution of silica nanoparticles and nanogel particles in the aqueous solution and hybrid nanogels respectively obtained by DLS measurements. Fig. 9a shows that in the absence of polymer, the apparent average hydrodynamic diameter of the silica nanoparticles in the aqueous solution is about 52 nm. The finite size of the particles obtained by DLS measurements indicates that nanoparticles are free and the network of nanoparticles (the size of which would be infinite) is not formed. Based on Fig. 9b and using the relations mentioned in the Experimental section, the transversal diffusion coefficient of the nanoparticles is calculated as 8.46 × 10⁻⁹ cm² s⁻¹. It should be noted that for a network of nanoparticles, the diffusion coefficient is infinite.⁵¹

Fig. 9 Size distribution of the: (a) nanoparticles in aqueous solution of nanosilica, and (b) hybrid nanogel irradiated at 40 KGy.

In the following we show that the distance of nanoparticles in the aqueous solution is very large; therefore, after the addition of PVME, polymer chains are not able to bridge between them to form a network of nanoparticles to affect the observed rheological behavior. The distance between two aggregates of silica nanoparticles in the aqueous solution is determined by the following equation:^52,53

where φ_s is volume fraction of nanosilica, φ_m is the maximum random packing volume fraction (φ_m = 0.638),⁵² h is half of the average interparticle distance, and α is the average radius of the silica nanoparticles obtained from Fig. 9a (52 nm). Eqn (6) yields the apparent average inter-particle distance of silica particles 2h = 227 nm. This value is much higher than the radius of gyration of PVME chains (R_g ∼ 6 nm), indicating that at low concentrations of PVME (C < C*) macromolecules cannot bridge between aggregates of silica nanoparticles to form a continuous network. In this context, the dispersion morphology or structure of nanoparticles in the hybrid nanogels is provided by FE-SEM micrographs in the ESI in Fig. S6.†

b) Non-homogeneous state. We investigated the dynamic frequency sweep behavior of phase-separated hybrid and non-hybrid nanogels at 40 °C. Fig. 10a and b shows variations of dynamic moduli with frequency for non-hybrid nanogels in the late stage of phase separation (after 11 minutes from the onset of phase separation) for which morphology is the droplet-matrix.

Fig. 10 Dynamic moduli of non-hybrid nanogels (a and b) after 11 minutes phase separation at 40 °C irradiated at: (a) 20 KGy and (b) 40 KGy. Hybrid nanogels (c and d) after 4 minutes phase separation at 40 °C (network structure) irradiated at: (c) 20 KGy, (d) 40 KGy. Hybrid nanogels (e and f) after 1 h phase separation at 40 °C (after phase inversion, droplet-matrix morphology) irradiated at: (e) 20 KGy, (f) 40 KGy.

For droplet-matrix morphology, development of Gʹ at low frequencies is mainly related to the interfacial tension, which in turn largely depends on the shape deformation and variation of interfacial area of the droplets formed during phase separation, leading to the scaling relation of G′ ∝ ω², G′′ ∝ ω.^54,55 Fig. 10a and b shows that for non-hybrid nanogels irradiated at 20 and 40 KGy at late stages of phase separation, there is a deviation from the characteristic scaling relation of the droplet-matrix morphology at low frequencies. This deviation can be due to the nonspherical shape of the dispersed phase as observed by optical microscopy images.

Fig. 10c and d shows dynamic moduli as a function of frequency for hybrid nanogels irradiated with 20 and 40 KGy electron beam. It can be seen that after 4 min annealing, the dependence of Gʹ and G′′ on ω is weak at low frequencies with a large deviation from terminal behavior. This large deviation from terminal behavior can be attributed to the formation of a PVME-rich percolating network induced by the VPS mechanism that hinders the flow. Self-generated stresses due to VPS leads to volume shrinking of the PVME-rich phase by time; and hence, interconnectivity of the network structure decreased until breaking up into disconnected domains.³⁸ Loss of network interconnectivity induced a remarkable decrease of dynamic moduli at low frequencies as can be seen in Fig. 10e and f.

3.3.2. Damping factor vs. frequency.
a) Non-hybrid nanogels. The tan δ vs. angular frequency can be used as a tool to differentiate the liquid-like behavior from the solid-like one. At low frequencies, the slope of the tan δ curve against frequency is negative for the systems with liquid-like behavior and positive for the systems with solid-like behavior.⁵⁶ Fig. 11 shows the tan δ vs. angular frequency for non-hybrid nanogels at homogeneous (30 °C) and non-homogeneous states (40 °C, after 11 minutes from the onset of phase separation which results in a droplet-matrix morphology). All the curves show a negative slope at low frequencies indicating liquid-like behavior.

Fig. 11 Plots of tan δ vs. angular frequency for non-hybrid nanogels (a and b) irradiated at 20 KGy for: (a) homogeneous state, 30 °C and (b) non-homogeneous state, 40 °C (after 11 minutes from onset of phase separation); (c and d) non-hybrid nanogels irradiated at 40 KGy for (c) homogeneous state, 30 °C and (d) non-homogeneous state, 40 °C (after 11 minutes from the onset of phase separation).

b) Hybrid nanogels. Fig. 12 shows the tan δ vs. angular frequency for hybrid nanogels irradiated with different doses at homogeneous state (30 °C) and non-homogeneous state (40 °C, after 4 minutes annealing when the morphology is network-like structure (b and e), and after 1 hour from onset of phase separation which results in a droplet-matrix morphology (c and f)). At homogeneous state (a and d) and in the non-homogenous state in the early stages of phase separation (b and e) the samples exhibit solid-like behavior. For the former, this behavior is attributed to the restriction of the macromolecules mobility due to the polymer–particle interactions and for the later it is attributed to the self-generated stresses induced by VPS which lead to the formation of a network structure of the PVME-rich phase.³⁸ After phase inversion, the network structure breaks into droplet-matrix morphology and a negative slope at low frequencies is observed (Fig. 12c) demonstrating a liquid-like behavior.

Fig. 12 Plots of tan δ vs. angular frequency for hybrid nanogels irradiated at: i – 20 KGy at temperature of: (a) 30 °C (homogeneous state), (b) 40 °C, after 4 minutes phase separation for which morphology is network structure, and (c) 40 °C, after 1 h phase separation with droplet-matrix morphology; ii – 40 KGy at temperature of: (d) 30 °C (homogeneous state), (e) 40 °C, after 4 minutes phase separation for which morphology is network structure, and (f) 40 °C, after 1 h phase separation with droplet-matrix morphology.

3.3.3. Modeling linear viscoelastic behavior. In this section, we study the rheological behavior of the phase-separated nanogel systems using emulsion models. Palierne's emulsion model⁵⁷ predicts the rheological behavior of viscoelastic emulsions with droplet-matrix morphology in the linear viscoelastic range. When the droplets polydispersity is less than 2.3, Palierne's emulsion model can be written as follows:wherewhere indexes “m” and “d” refer to the matrix and droplet phase, respectively; α is the interfacial tension in the system, R is the volume average radius of the droplet phase and φ_d is the volume fraction of the dispersed phase.

Since the phase-separating nanogel with composition φ is in the two-phase region of the phase diagram at 40 °C, it consists of PVME-rich and water-rich domains for which the composition of each domain (φʹ and φ′′) can be determined by the tie line³⁸ of the phase diagram at 40 °C. Then, the volume fraction of the dispersed phase, φ_d is calculated by a conservation equation as follows:⁵⁸

φ = φ_dφʹ + (1 − φ_d)φ′′ (9)

This equation can be rearranged to obtain the volume fraction of the dispersed phase as follows:

According to the phase diagram at 40 °C, the solvent-rich region of the phase diagram is nearly superimposed to the solvent axis (φ_PVME = 0) and the volume fraction of the polymer-rich phase is about 0.89. Thus, the matrix phase is considered as pure water, which is a Newtonian fluid.

In order to use Palierne's model, the interfacial tension, α, is required. The interfacial tension of PVME-rich and water-rich phases is calculated from rheological measurements based on relaxation time of the interface. For this purpose, the non-hybrid nanogel irradiated at 20 KGy and phase-separated for 11 min at 40 °C was chosen (Fig. 10a). Relaxation time (τ) is obtained by two methods: i – the inverse of frequency at the crossover of storage and loss modulus⁵⁰ in Fig. 10a which gives τ = 0.66 s. ii – using Tschoegle equation (eqn (11)) and plotting the weighted relaxation spectrum, τH(τ) versus τ (ref. 59) (Fig. 13) yields τ = 0.6 s.

Fig. 13 Weighted relaxation spectra of the non-hybrid nanogel irradiated at 20 KGy after 11 min phase separation at 40 °C.

The relaxation time of the interface for droplet-matrix morphology can be expressed by the following equation given by Palierne's model:⁵⁷

where k is the viscosity ratio of the dispersed phase to the matrix. By substituting the relaxation times obtained from crossover of storage and loss modulus into eqn (12), the interfacial tension of the phase-separated domains in non-hybrid nanogels was calculated as 0.021 mN m⁻¹. Using the relaxation times obtained from Fig. 13, the interfacial tension in the hybrid system is obtained as 0.03 mN m⁻¹.

Fig. 14 shows a comparison between the prediction of Palierne's model and experimental results for non-hybrid nanogels irradiated at 20 KGy after 11 minutes from the beginning of the phase separation.

Fig. 14 Comparison between experimental storage modulus and predictions of Palierne's model for non-hybrid nanogels at 40 °C irradiated at 20 KGy.

As can be seen, there is a large difference between the prediction of Palierne's model and the experimental results. Similar results are also observed for the nanogels irradiated at 40 KGy energy beam (not shown here). This may be due to a large difference between the complex shear modulus of dispersed and matrix phases (PVME-rich and water-rich phases respectively) which significantly reduces the contribution of (α/R) to complex modulus in Palierne's model. Consequently, the sensitivity of the model toward interfacial tension value is too low to be capable of predicting reasonable results and only very large values of α give the best fit of experimental data to the model. Jafari et al. observed similar behavior in the case of polymer blends.⁶⁰

Gramesphacher and Meissner⁶¹ developed a theoretical model (G–M model) for an emulsion by combination of Choi and Schowalter's work⁶² and a linear mixing rule. It considers the complex shear modulus of a blend as a combination of the contribution of the shear moduli of phases along with contribution of the interface. In other words, they expressed that in addition to the relaxation spectra of the pure phases in a mixture under shear, an additional relaxation time that corresponds to the relaxation time of the interface should be considered. They obtained the following equations for the storage and loss shear moduli of a blend:

where in these equations:

K  =  η_d/η_m (19)

where η_d and η_m are the Newtonian viscosity of the dispersed phase and matrix phase of the mixture, respectively.

Fig. 15 shows a comparison of the prediction of the Gramespacher model and experimental results for non-hybrid nanogels after 11 minutes from the beginning of the phase separation and hybrid nanogels after 1 hour phase separation at 40 °C. For the hybrid nanogel after about 1 h annealing, breaking up of the structure leads to formation of a droplet-matrix morphology; hence, Palierne's model can be applied. It can be seen that there is a good agreement between experimental results and prediction of the Gramespacher model. It should be mentioned that the better applicability of Gramespacher's model to describe the storage modulus in comparison to Palierne's model refers to the approach on which the model is based. Gramespacher's model considers that blend dynamic moduli are controlled by three contributions, i.e., two phases and an interface separately. Thus, it takes the role of the interface more into account.

Fig. 15 Comparison between experimental storage modulus and predictions of Gramespacher's model for non-hybrid (a) and hybrid (b) nanogels at 40 °C ((a) after 11 minutes from the beginning of the phase separation and (b) after 1 hour from the beginning of the phase separation) irradiated at 20 KGy.

4. Conclusion

We investigated the effect of nanoparticles and radiation dose on interaction and phase behavior (swelling/deswelling behavior and phase separation mechanism) of nanogels by rheological and FTIR measurements. Rheological behavior of non-hybrid and hybrid nanogels prepared with different radiation doses were investigated in the homogenous and non-homogenous regions of the phase diagram. It was found that frustrating the mobility of polymer chains in nanogels by the addition of nanosilica or increasing radiation dose increases the deswelling temperature and also decreases the kinetics of phase separation. The addition of nanoparticles considerably decreased the dynamics of polymer chains in nanogels leading to rheologically solid-like behavior as in hard sphere colloidal suspension. In the non-homogeneous state, the addition of nanosilica increased dynamic asymmetry leading to transition of the nucleation and growth mechanism to the viscoelastic phase separation mechanism. Linear rheological behavior of nanogels consistently correlated well with the evolution of the corresponding phase-separating morphologies. The percolation of the PVME-rich phase during the VPS mechanism in hybrid nanogels induced rheologically solid-like behavior. Rheological behavior of phase-separated nanogels was compared with prediction of Palierne and Gramespacher's models. The former predicted much lower than experimental results which was attributed to the large difference between the components viscoelastic behaviors, while the latter's prediction was acceptable and close to the experimental results.

Acknowledgements

We thank Mrs. Tahereh Samaee Yekta for her assistance in carrying out the morphological observations of this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21021f

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