Effect of Poly(Titanium Oxide) on the Viscoelastic and Thermophysical Properties of Interpenetrating Polymer Networks

: The inﬂuence of poly(titanium oxide) obtained using the sol-gel method in 2-hydroxyethyl methacrylate medium on the viscoelastic and thermophysical properties of interpenetrating polymer networks (IPNs) based on cross-linked polyurethane (PU) and poly(hydroxyethyl methacrylate) (PHEMA) was studied. It was found that both the initial (IPNs) and organo-inorganic interpenetrating polymer networks (OI IPNs) have a two-phase structure by using methods of dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The differential scanning calorimetry methods and scanning electron microscopy (SEM) showed that the presence of poly(titanium oxide) increases the compatibility of the components of IPNs. It was found that an increase in poly(titanium oxide) content leads to a decrease in the intensity of the relaxation maximum for PHEMA phase and an increase in the effective crosslinking density due to the partial grafting of the inorganic component to acrylate. It was shown that the topology of poly(titanium oxide) structure has a signiﬁcant effect on the relaxation behavior of OI IPNs samples. According to SEM, a uniform distribution of the inorganic component in the polymer matrix is observed without signiﬁcant aggregation. formal analysis, T.T. and A.I.P.; investigation, T.T.; resources, A.I.P.; data curation, A.I.P.; writing—original draft preparation, T.T. and A.I.P.; writing—review and editing, T.T. and A.I.P.; visualization, T.T.; supervision, A.I.P.; project administration, A.I.P.; funding acquisition, A.I.P.


Introduction
In the last decade, due to the high rates of development of various industries, there is a need to obtain materials with the desired set of new operational properties. Therefore, the development of new hybrid organic-inorganic nanocomposites is a topical area of polymer chemistry [1][2][3][4][5][6][7][8][9][10][11][12]. In this case, the concept of "hybrid" emphasizes the chemical nature of the interaction of system components [13]. Such materials demonstrate not only improved properties of the organic matrix, but also the appearance of new, specific properties that are not inherent to the organic component due to the presence of an inorganic one. Particular attention is drawn to organic-inorganic materials containing titanium, due to their potential application as photocatalyst and membrane, as well as in solar and fuel cells, biomedicine and in other areas, where their unique optical properties can be used [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. In particular, in [25], one of the most extensively studied polymers in biomedical applications, namely, PHEMA incorporated with TiO 2 nanoparticles and the appropriate bioactive behavior of such nanocomposite was investigated in detail. It is especially important to mention here that the formation of bone-like apatite, which is a necessary condition for a synthetic material to be considered bioactive [25]. The optically transparent TiO 2 particles incorporated in PHEMA thin films were prepared in [26], where their photocatalytic activity was successfully demonstrated. Another important result that needs to be mentioned is the synthesis of PHEMA hydrogels containing low concentration of TiO 2 nanoparticles, which show their potential use for cell implantation experiments in vivo [27]. The enhanced mechanical and thermal properties of TiO 2 nanocomposites reinforced with photo-resin via 3-dimensional printing were recently shown in [28]. Another interesting report [29] is about the excellent UV shielding properties of poly(methylmethacrylate) PMMA/oxide nanocomposites with different types of nanoparticles, which could be used in variety of applications such as sunscreens, aerospace, and several other fields related to UV photodegradation. It is important to note that the thermal and mechanical properties of PMMA-titania nanocomposites and their degradation behavior was discussed in detail in [30]. Note also that nanoporous anatase layers may become interesting for nonlinear optical applications [31].
It is also known from the literature that poly(titanium oxide) gels obtained using the sol-gel method have high photocatalytic activity [32]. They exhibit unique optical properties, i.e., UV-induced transition Ti 4+ ↔Ti 3+ . When light is absorbed in a semiconductor, electrons in the conduction band (CB) and positive holes in the valence band (VB) are formed. Light holes can rapidly move to the metal oxide network interface and initiate chemical reactions with environmental molecules. Heavier CB-electrons move on a shorter distance and then are trapped on Ti 4+ of the metal oxide network as Ti 3+ , which can also be situated at the interface. These trapped electrons can be visualized with a strong absorption in the UV-visible-nearIR spectral range with a maximum at~600 +200 −100 nm, which depends on the sample preparation conditions. The disadvantage of gel materials that limits their use is their mechanical instability, and the advantages for applications in photonics are their transparency and unique photosensitivity, therefore it is actually the choice of the polymer matrix to stabilize the gel while maintaining its structure and, accordingly, the sensitivity. The problem can be solved by creating the hybrid optically transparent organic-inorganic material, which combines valuable properties of organic and inorganic components.
In light of this problem, the synthesis of TiO 2 -containing organic-inorganic copolymers seems promising. In the previous research, the materials containing the nanostructured poly(titanium oxide) uniformly distributed over the volume of the polymer matrix were synthesized from the titanium alkoxides using the sol-gel method in 2-hydroxyethyl methacrylate (HEMA) medium [33][34][35]. The materials showed a photochromic transition Ti 4+ ↔Ti 3+ with a quantum yield >50%. In [36], a new polymeric material was developed, containing nanostructured poly(titanium oxide) in an organic copolymers of hydroxyethyl methacrylate and acrylonitrile and modified by F-content agent, which demonstrate photocatalytic activity in the decomposition reaction of organic pollutants and the self-cleaning effect. Recent studies have also shown the prospect of using such hybrid materials in the field of photonics, especially for 3D laser microstructure and optical information recording [37].
The sol-gel method is commonly used for obtaining the organic-inorganic composites with a uniform distribution of the inorganic nanodispersed phase even at the molecular level [38][39][40][41][42][43][44][45][46][47][48]. The important feature of this process is the ability to regulate the structure of the inorganic component in the organic matrix by controlling the conditions of the hydrolysis-condensation reactions.
It is also of interest to obtain organo-inorganic hybrid materials containing poly(titanium oxide) in which interpenetrating polymer networks (IPNs) act as an organic matrix, especially since research in this direction has not yet been carried out. Among the variety of composite materials, interpenetrating polymer networks attract particular attention due to the possibility of both modifying the properties of cross-linked polymers and obtaining new materials with a wide range of different properties [49,50]. Interpenetrating polymer networks are a combination of two or more polymers in networks where a partial interlacing on the molecular scale is present in the matrix. There is no covalent bonding and therefore the polymers cannot be separated unless the chemical bonds are broken [50]. This morphology of interpenetrating polymer networks can lead to synergistic properties of the initial components. However, for IPNs, as for most organic polymers, such disadvantages as low mechanical properties and thermal stability are inherent. To overcome these defects, these materials can be strengthened by the addition of an inorganic component. For these purposes, silica was used successfully [51,52].
When creating the new composite materials, it is important to study the characteristics that determine their phase structure, thermal modes of use, stability and mechanical strength, especially since the structure and properties of organic-inorganic nanocomposites are largely dependent on the content of the nanofiller and its distribution in the organic matrix.
This work aims to study the effect of poly(titanium oxide) obtained using the sol-gel method in HEMA medium by varying the content of inorganic component and a molar ratio of titanium isopropoxide to water on viscoelastic and thermophysical properties of OI IPNs.

Experimental
The following reagents were used in this study: Titanium (IV) isopropoxide ((Ti(OPr i ) 4 ), 97.0% purity, Sigma-Aldrich) was used without further purification. Organic-inorganic interpenetrating polymer networks are obtained on the basis of cross-linked polyurethane, poly(hydroxyethyl methacrylate) and poly(titanium oxide). Synthesis of poly(titanium oxide) was carried out by the hydrolytic polycondensation of Ti(OPr i ) 4 in the presence of hydrochloric acid in the medium of the initial component of IPNs-HEMA. The molar ratio of Ti(OPr i ) 4 /HEMA was 1/16; 1/12 and 1/8, which calculated as TiO 2 was 3.8; 5.1 and 7.4 wt% respectively. The hydrolysis of Ti(OPr i ) 4 was carried out at the rate of Ti(OPr i ) 4 /H 2 O = 1/1 and 1/2 mol. The reaction mixture was stirred vigorously for 3 h followed by the formation of poly(titanium oxide) for 48 h. Finally, transparent orange-colored liquid systems were obtained and evacuated at 40 • C with a residual pressure of 10-20 mmHg for removing the by-products of hydrolysis-condensation reactions; water and isopropyl alcohol.
The urethane component of OI IPNs was obtained in two stages. At the first stage macrodiisocyanate (MDI) was synthesized by the interaction of TDI and PPG with the ratio NCO/OH = 2/1. At the second stage, TMP as the cross-linking agent was added to MDI at the molar ratio MDI/TMP = 3/2. The reaction was carried out at 70 • C and with vigorous stirring for 15 min.
Furthermore, HEMA containing poly(titanium oxide) and AIBN initiator of radical polymerization with a concentration of 0.025 mol/L was added to the urethane component to form OI IPNs. The initial IPNs were obtained on the basis of PU and PHEMA without poly(titanium oxide) as described above. The ratio of PU/PHEMA components in the initial and OI IPNs was 30/70 wt%. For obtaining the films of IPNs/OI IPNs, the reaction mixture was poured into sealed forms, followed by polymerization at 60 • C (20 h) and 100 • C (3 h).
IR spectra of HEMA-containing poly(titanium oxide) were recorded on a Bruker Tensor-37 Fourier spectrometer (Bruker Optics, Ettlingen, Germany) in the frequency range 400-4000 cm −1 with a resolution of 4 cm −1 by applying the test material to KBr plates.
The viscoelastic properties of the initial IPNs and OI IPNs samples were studied by DMA (Dynamic Mechanical Analyzer Q 800, TA Instruments, New Castle, DE, USA). To measure the viscoelastic properties, polymer films with a size of 50 mm × 5 mm (0.2-0.5) mm were used. The measurements were performed in the tension mode at a frequency of 10 Hz, and the heating rate was 2.0 • C/min. The values of glass transition temperature (T g ) were determined by the position of the maximum of the tangent of mechanical losses (tanδ). The molecular weight of the chain segments between crosslinks (M c ) was calculated via the equation [53]: where ρ is the density of the polymer; R is the gas constant; T is the value of absolute temperature; and E ∞ is the value of the equilibrium elastic modulus. Thermophysical properties of the initial IPNs and OI IPNs were studied by DSC method using Differential Scanning Calorimeter Q 2000 (TA Instruments, New Castle, DE, USA) in a nitrogen atmosphere in the temperature range from 273 to 693 K and heating rate of 2.0 • C/min. Samples weighing 0.01-0.015 g were placed in aluminum capsules which were then sealed. The heating/cooling scan-mode was used. The midpoint of the endothermic transition on the curve of the temperature dependence of heat capacity (C p ) corresponded to T g value of the polymer.
The fraction of the interfacial region (1 − F) was calculated according to the simplified Fried approximation for partially compatible multicomponent polymer systems [54] based on DSC data: where: W 1 and W 2 are the mass fractions of the components in the mixture; ∆C p1 and ∆C p2 are the heat capacity increments of the phases that have released; and ∆C 0 p1 and ∆C 0 p2 are increments of heat capacity of individual polymers.
To analyze the morphology features of the investigated compositions, the film samples were split using liquefied nitrogen. The surface of the transverse cleavages was examined by scanning electron microscopy (SEM) using a JSM-35-C microscope (JEOL, Tokyo, Japan) at an accelerating voltage of 30 kV and an increase of x2000. To prevent the accumulation of surface charge and increase the contrast on the fractured surface a plasma deposition of a gold layer~4 nm thick was carried out using a Neo Coater installation (JEOL, Tokyo, Japan).

Results and Discussion
To understand the processes occurring at the stage of the formation of poly(titanium oxide) in HEMA medium with varying the molar ratio Ti(OPr i ) 4 /H 2 O, the IR spectra were studied (Figure 1). The appearance of a low-intensity absorption band at 624 cm −1 for curves 2 and 3 relative to 1 is associated with the vibrations of Ti-O-Ti-groups of poly(titanium oxide) [55,56]. An increase in the relative intensity of ν C=O groups at 1720 cm −1 is observed 1.5 times for curve 2 and 1.2 times for curve 3 relative to the spectrum of the initial HEMA (curve 1), and the appearance of a new band at 1522 cm    Thus, not only was the formation of poly(titanium oxide) in the HEMA medium during sol-gel synthesis established by using FTIR spectroscopy, but also the appearance of complex interactions by the donor-acceptor mechanism and the nucleophilic substitution reaction with the formation of poly(titanium oxide)-oxoethyl methacrylate. Additionally, it is worth noting that the amount of water that takes part in the hydrolysis of Ti(OPr i ) 4 directly affects both the presence of unreacted alkoxide groups and the structure of the resulting poly(titanium oxide) as a whole. In the middle of the twentieth century, R. Field and P. Kouv found that linear structure of poly(titanium oxide) is predominantly formed when Ti(OPr i ) 4 /H 2 O ≥ 1 ratio and branched one with a molar ratio of Ti(OPr i ) 4 /H 2 O < 1 (Figure 3) [60].
Dynamic mechanical analysis is one of the most widespread methods of studying the relaxation properties of polymer composites which allows the number of values (dynamic modulus, the tangent of mechanical losses, glass transition temperature, etc.) which have both fundamental and practical significance to be determined [53]. Therefore in this study, it was advisable to consider the influence of poly(titanium oxide) as well as its topology on the viscoelastic properties of OI IPNs. The temperature dependences of tanδ for IPNs and OI IPNs with the varying of the content of poly(titanium oxide) and the molar ratio of Ti(OPr i ) 4 /H 2 O are shown in Figure 4. The parameters of the relaxation transitions, the values of E ∞ and M c for the studied systems, are presented in Table 1.  Table 1. The T g at 128 • C corresponds to PHEMA [33] and the Tg at −5 • C corresponds to the PU polymer Figure 4a. Such temperature dependence tanδ indicates that IPNs is a two-phase system. Similar dependences of tanδ (T) are also observed for all samples (curves 2-5), which confirms their two-phase structure.
The study of the viscoelastic characteristics of OI IPNs showed that with an increase in poly(titanium oxide) content, there is a significant decrease in the relaxation transition height for the PHEMA phase (Figure 4a, curves 1-4, Table 1). The reduction in the intensity of the relaxation maximum with the increasing content of the inorganic component is often observed for the nanostructured hybrid systems. It is usually connected with the formation of the dense inorganic network that prevents the segmental mobility of the polymer chains [57,61]. In our case, the decrease in tanδ max2 is a result of blocking of the mobility of the significant fraction of the polymer segments (relaxators) involved in the relaxation transition for PHEMA phase. It may occur due to the partial formation of poly(titanium oxide)-oxoethyl methacrylate as a result of exchange reactions between non-hydrolyzed groups of poly(titanium oxide) and HEMA hydroxyl groups and also due to the formation of the complex interaction between Ti atoms and C=O-group of HEMA. The presence of such interactions is also supported by the fact that even a slight increase in poly(titanium oxide) content leads to a significant decrease in the M c (the molecular weight of the chain segments between crosslinks) value in OI IPNs (Table 1). This indicates a significant increase in the number of cross-links and/or topological entanglements in the organic-inorganic hybrid polymer system.
It is an interesting fact that the presence of the inorganic component practically does not change the value of T g2 (Table 1). Although in the classical sense, the increase in the cross-linking density is usually accompanied by the increase in the glass transition temperature and a corresponding decrease in the M c value. It can be assumed that poly(titanium oxide) in some way acts as a compatibilizer due to both the grafting of the inorganic component to the PHEMA macrochains, but also accidentally interacting with the urethane component due to highly active non-hydrolyzed alkoxide groups. According to the authors, this effect leads to the absence of an increase in the temperature of the acrylate component.
The comparison of the dependencies of tanδ(T) for OI IPNs-3 and OI IPNs-4 samples (Figure 4b) with the same content of poly(titanium oxide), but obtained at different molar ratios of Ti(OPr i ) 4 /H 2 O showed that the topology of inorganic component influences the phase structure of OI IPNs. This is evidenced by a change in the parameters of the relaxation transition for PHEMA phase, as well as a 2-fold decrease in the M c value for OI IPNs-4 compared to OI IPNs-3 sample (Table 1). Perhaps, the linear structure of poly(titanium oxide) which was formed at the molar ratio of Ti(OPr i ) 4 /H 2 O = 1/1, promotes an increase in the number of the grafting and donor-acceptor interactions. It increases the number of polymer segments with locked mobility and reduces the height of tanδ of PHEMA phase (Figure 4b). However, at the branched structure of poly(titanium oxide) which was obtained at the molar ratio Ti(OPr i ) 4 /H 2 O = 1/2, the number of such interactions may be limited due to the occurrence of steric hindrances and the deficiency of alkoxide groups. The comparison of the M c values for OI IPNs-3 and OI IPNs-4 samples ( Table 1) shows that the only change in the topology of poly(titanium oxide) structure from the branched to the linear leads to the significant increase in the cross-linking density in OI IPNs.
The results obtained by DSC method also confirm that both the initial IPNs and OI IPNs have a two-phase structure as evidenced by the presence of two heat capacity jumps (∆C p ) on the temperature dependence of C p = f(T). The glass transition temperature of PU (T g1 ) and PHEMA (T g2 ) were determined from C p = f(T) dependence (Table 2, Figure 5).   Table 2.
The results presented in Table 2 show that the introduction of poly(titanium oxide) in IPNs slightly increases the glass transition temperature of the PU phase, but the glass transition temperature of the PEMA phase is practically unchanged. The value of the heat capacity increment for the PU phase slightly increases relative to the initial IPNs with the increase in poly(titanium oxide) content, and it decreases for the PHEMA phase. It should be noted that the nonlinear nature of the change in the values of the heat capacity jump in the PU and PHEMA phases in OI IPNs samples with increasing Ti component. It can be assumed that poly(titanium oxide) not only increases the rigidity of the PHEMA phase due to the partial formation of poly(titanium oxide)-oxoethyl methacrylate, but also possibly acts as a compatibilizer which leads to the absence of increasing T g2 value of PHEMA component and a slight increase this parameter for PU component. The results obtained by DSC method confirm the data obtained by DMA method.
Micro-phase separation processes also occur with the formation of the interfacial region (IFR) between the components of IPNs. Each phase can be considered quasiequilibrium with the molecular level of mixing, but in general, these are systems where there is no deep penetration at the molecular level throughout the volume of the system.
The peculiarity of such systems is that for both IPNs and semi-interpenetrating polymer networks (semi-IPNs) IFR can be considered as a quasi-equal third phase. Its appearance is the result of a spinodal separation mechanism. Phase separation by the spinodal mechanism begins with the formation of a continuously interconnected periodic structure, which gradually shifts to droplet as a result of the decomposition of spinodal structures at the last stages of phase separation due to an increase in the interfacial tension [62,63].
The number of equations was proposed by various authors to evaluate IFR [53,64]. They were deduced on the assumption of additivity of heat capacity jumps in partially compatible systems. We used the simplified Fried approximation to calculate the share of IFR ((1 − F)) (2).
Thus, IFR is absent if (1 − F) = 0; all polymers are in IFR if (1 − F) = 1. The value of the share of the interfacial region calculated in such way is relative.
As is shown in Table 2, the proportion of the interfacial region increases in OI IPNs compared to initial IPNs with increasing of poly(titanium oxide) obtained at a molar ratio of Ti(OPr i ) 4 /H 2 O = 1/2, which is probably related to the growth of compatibility of IPNs components. This effect is probably due to the partial grafting of poly(titanium oxide) to both PHEMA and PU macrochains during their formation, and/or with a decrease in the phase separation rate at the initial stages of composite formation due to the catalytic effect of the titanium component on the urethane formation reaction and increasing the viscosity of the reaction medium as a whole.
The scanning electron microscopy was used for a more complete understanding of the structure and morphology of the obtained IPNs and OI IPNs containing poly(titanium oxide) synthesized in a HEMA medium.
SEM studies have shown sufficient differences in the morphology of initial IPNs and OI IPNs. The presence of characteristic morphology of the fractured piece of IPNs film, as well as the distribution of color shades in the obtained images (Figure 6a) may indicate phase separation of the components (PU and PHEMA) in the composite. However, the significant smoothing of the relief of the transverse cleavage surface of OI IPNs sample and the decrease in the color contrast level (Figure 6b) confirm that the introduction of poly(titanium oxide) into the polymer matrix significantly reduces the phase separation level of the hybrid composition.
The obtained results indirectly confirm the presence of the compatibilizing effect of poly(titanium oxide), as well as its influence on the relaxation and thermophysical behavior of the studied samples. Additionally, it is worth noting that the distribution of the inorganic component in the polymer matrix is quite uniform without significant aggregation, which may indicate the presence of the appropriate physicochemical interaction between them. This is especially important for the manufacture of appropriate fluorescent matrices based on TiO 2 , when the latter is doped with a luminescent impurity [65][66][67].

Conclusions
It was established by using DMA and DSC methods that the OI IPNs system is two-phase; however, the increase in the interfacial region share with the increase in poly(titanium oxide) content as well as their morphology according to SEM indicates that the inorganic component acts as a compatibilizer. It was shown by using the DMA method that the increase in poly(titanium oxide) content in OI IPNs leads to a decrease in the relaxation maximum intensity of PHEMA component and the reduction in the molecular weight of the chain segments between the cross-links. Such changes indicate an increase in the effective cross-linking density of OI IPNs due to the partial grafting of poly(titanium oxide) to PEMA. The decrease in the segmental mobility of the polymer chains due to the formation of poly(titanium oxide)-oxoethyl methacrylate is indicated by a decrease in the heat capacity increment of PHEMA component of OI IPNs relative to initial IPNs. It has been shown that the topology of poly(titanium oxide) structure significantly influences the relaxation behavior of the samples.