Nucleated nanocomposites of TPU–PDMS blends based on spherical nanohydroxyapatite

Abstract

The present investigation gives a profound insight into the preparation of nucleated nanocomposites of TPU–PDMS blends based on uniquely synthesized PPG-wrapped spherical nanohydroxyapatite (nHap). The spherical nHap at different doses of 1, 3, 5 and 7 phr in different blends of 90 : 10, 80 : 20, 70 : 30, 60 : 40, and 50 : 50 (TPU : PDMS) were prepared. Based on the mechanical properties of the nanocomposites as well as their scope in the field of biomedical applications, detailed studies were carried out for the 70 : 30 blend (T70P30) nanocomposites. The effect of nHap on the mechanical and thermal properties of the T70P30 blend has been thoroughly analysed by using a universal testing machine (UTM), dynamic mechanical analyser (DMA), differential scanning calorimetry (DSC) and a thermogravimetric analyser (TGA). The incorporation of nHap into the T70P30 blend is promoted by the enhanced nucleation effect and has shown a considerable improvement in the tensile strength, storage modulus and thermal stability over the neat blend. The enhancements in these properties are due to better dispersion, which has been confirmed by field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). DSC revealed the role of filler dosage in nucleation efficiency of nanocomposites. X-ray diffraction (XRD) studies were also carried out for the structural analysis and observed an increasing trend in crystallinity of the nanocomposite with filler dosage. The crystallinity, T_g and ΔH values of the nanocomposites with 7 phr nHap (T70P30H7) met the properties of pristine TPU.

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1. Introduction

Hydroxyapatite (Hap) particles are widely utilized in the fabrication of bone-like hybrid nanocomposites due to their structural and compositional similarity to minerals of natural bones.^1–4 In hard tissue applications, a variety of systems have been developed to mimic the specifically organized nanoscale structure of the bone which consists of collagenous fibers and mineralized apatite nanocrystals.^5–13 Among those, composites of apatite crystals and biocompatible polymers have received a great deal of attention in a sense that the composite system can provide the compositional benefits and preserve the structural and biological functions of the damaged hard tissues in a more efficient way closely similar to the natural system.^3,14–20 The major contributions of reported studies are based on rod shaped Hap nanocomposites.^10,21–24 Due to the dispersion problem of these nanocomposites, those studies were limited to lower doses of nanofillers.^19,25–27 One of the fascinating aspects of scientific significance in the nanoscale is the size and shape-dependent variation in properties of matter. Hence, by controlling the shape factor one can change the whole set of properties of nanocomposites. For spherical particles, since the interactions are the same in all directions, the collective effect and the resultant property of the assembly will be the same.²⁸ By modifying the synthesis conditions, spherical nanohydroxyapatite with suitable coatings can be prepared which will reduce the agglomeration tendency of the nanoparticles.^29–31

Incorporation of nHap in natural materials like collagen, gelatin etc. make superior biocompatible systems which possess inferior mechanical properties, whereas synthetic biocompatible polymer matrices exhibit comparatively good mechanical properties.^8,32,33 The effective selection of matrices as well as the modification of the existing materials expand the usage of nHap in biomedical world. Polydimethylsiloxane (PDMS) elastomers are considered as the most biocompatible synthetic polymers because of its excellent biostability, biocompatibility, physiological inertness, higher temperature resistance, and oxidative stability.³⁴ As mentioned in the case of natural polymers like collagen and gelatin, PDMS is also a very weak matrix in terms of its mechanical properties. An expanded search of other polymers with better mechanical properties for the incorporation of nHap reach in the doorsteps of thermoplastic polyurethane (TPU) which has a proven history of biomedical application along with very good mechanical properties and vastly studied as implants.^35–41 Hydroxyapatite based nanocomposites of TPU possess improved mechanical strength properties along with enhanced osteoconductivity.^24,42–44 The addition of more amounts of nanofillers in the polymer blend matrix, often causes filler particles agglomeration, instead of matrix–filler interaction except in the case of nucleated nanocomposites.

The most important characteristic features of the filler are its size, shape and its ability to act as nucleating agent during crystallization.⁴⁵ In the case of un-oriented polymer solutions, nucleation is initiated in a heterogeneous manner.⁴⁶ If the heterogeneities which start the nucleation process contain uniform nanoparticles in the dispersed phase, it can act as a semi crystalline polymer with numerous crystalline fragments. In the case of polymer blend nanocomposites, the phenomenon of nucleation has different aspects based on the individual polymers. For thermoplastic polyurethanes the crystallization process is quite a bit interesting due to the presence of crystalline as well as amorphous segments.⁴⁷ PDMS rubber shows entirely different cold crystallization phenomenon even in its amorphous state.^48,49 In TPU–PDMS blend nanocomposites the TPU hard segments possess very good self nucleation ability which are less affected by the nHap particles. The soft segments of TPU undergo nucleation and amorphous or poorly ordered regions of PDMS that follow reorganization by the action of nHap to form more stable crystal structure.⁵⁰

An extensive study in this area of nucleation may change the dimensions of entire hydroxyapatite nanocomposites field into a new era. This is possible by using an effective blend system as the matrix for nanoparticles. For an effective blend system, the PDMS and TPU are the most suitable and viable biocompatible synthetic polymers and the combination of which can solve the existing problems of agglomeration as well as inferior mechanical properties upto a particular extent. Even though this system has been initiated based on nHap nano-rods by melt mixing technique of nanocomposites, the poor dispersion limited its study for higher doses of nHap due to the lower percentage of PDMS.²⁶ For modifying this path towards highly filled nucleated nHap in TPU–PDMS blend system, solution mixing can act as the best technique and the PPG coated spherical Hap nanoparticles favors the nucleated nanocomposite preparation with proper dispersion. In this study, an attempt has been made to prepare nanocomposites of TPU–PDMS blends based on PPG-coated spherical nanohydroxyapatite by solution mixing technique and study the nucleation phenomenon.

2. Experimental

2.1. Materials

TEXIN RxT85A, an aromatic polyether-based TPU with specific gravity of 1.12 and a melt flow index of 4 g/10 min at 190 °C/8.7 kg was provided by Bayer Material Science (Chennai, Tamilnadu, India). Siloprene, a liquid grade PDMS was supplied by Momentive Specialty Chemicals Inc. (India). Calcium nitrate (Ca(NO₃)₂·4H₂O), diammonium hydrogen phosphate ((NH₄)₂HPO₄), and n-butanol were procured from Sigma Aldrich (Bengaluru, Karnataka, India). PPG of molecular weight 660 was purchased from Merck (Mumbai, Maharashtra, India). Solvents such as tetrahydrofuran (THF) and acetone are of analytical grades which were procured from Merck (India).

2.2. Methods

2.2.1. Synthesis of spherical PPG-coated nHap. The spherical nHap has been prepared according to the method proposed by Dedourkova et al.²⁹ and PPG modification of nHap has been carried out by modifying the procedure introduced by Selvakumar et al.²⁴ The pH values of the solutions of Ca(NO₃)₂·4H₂O and (NH₄)₂HPO₄, were adjusted to be more than 10 with ammonia solution. Then, 2 ml of PPG was added to Ca(NO₃)₂·4H₂O solution, and both the solutions were mixed with a stoichiometric ratio of calcium to phosphorus of 1.67, followed by stirring for 2 h at room temperature. The resultant precipitate was filtered and dried at 90 °C overnight. The following well-established reaction (1) explains the precipitation method of Hap crystal formation.

10Ca(NO₃)₂·4H₂O + 6NH₄H₂PO₄ + 2NH₄OH → Ca₁₀(PO₄)₂(OH)₂ + 8NH₄NO₃ + 12HNO₃ (1)

2.2.2. Preparation of TPU–PDMS blend nanocomposites. The TPU–PDMS blends were prepared in solutions at different blend ratios as follows: 4% (w/v) solutions of each TPU and PDMS rubber were prepared separately in tetrahydrofuran. The prepared solutions were mixed for 4 h at room temperature with the help of a magnetic stirrer. 1, 3, 5 & 7 wt% nHap dispersion in THF were made by sonication. These were mixed with the blend solutions and kept for 4 h at room temperature with magnetic stirrer. The mixture was then cast into a glass Petri dish and the resulting film was dried at 50 °C for 36 h followed by subsequent drying for 48 h under vacuum at the same temperature. The nanocomposites have been coded as TxPyHz, in which ‘T’, ‘P’ and ‘H’ denotes TPU, PDMS and nHap and ‘x’, ‘y’ and ‘z’ are the weight proportions of TPU, PDMS and nHap respectively.

3. Characterization and testing

3.1. Fourier transform infrared spectroscopy (FTIR)

In order to confirm the successful synthesis of nHap and to analyse the chemical interaction taking place in the nanocomposites, Fourier transform infrared (FTIR) spectroscopy studies were carried out. The FTIR spectroscopy studies were performed on a Bruker Equinox 55 spectrophotometer, at a resolution of 2 cm⁻¹, in the range of 4000–400 cm⁻¹, and 64 scans were averaged out for each spectrum.

3.2. Tensile properties

The tensile properties of the pristine TPU, blends and nanocomposites have been determined using dumbbell shaped specimens punched out from the tensile sheets using a hollow dumbbell cutting die (ASTM die C) and the tensile properties were measured as per ASTM D 412 using a Hounsfield H10KS universal testing machine at a crosshead speed of 500 mm min⁻¹ under ambient conditions. Five specimens of each sample have been tested and the average values have been reported.

3.3. Dynamic mechanical analysis (DMA)

Dynamic mechanical properties such as storage modulus and tan delta with respect to temperature were determined using a dynamic mechanical analyzer (DMA Q100) of TA instruments USA for virgin TPU, its blend and blend nanocomposites. The test was carried out using the dual cantilever mode at a constant frequency of 1 Hz and at amplitude of 20 μm in the temperature range from −145 °C to +130 °C at a scanning rate of 3 °C per minute.

3.4. X-ray diffraction studies (XRD)

The changes in percent crystallinity and crystal structure of nHap, blends and nanocomposites were calculated using Philips X-ray diffractometer (PW1710 X-ray diffractometer) using monochromatic CuKα radiation (wavelength 1.5418 Å) in the angular range of 10–80° (2θ) and at an operating voltage of 40 kV with a beam current of 20 mA.

3.5. Scanning electron microscopy (SEM)

A field emission scanning electron 348 microscope (FESEM), Germany (MERLIN) was used to study the phase morphology of the blend nanocomposites of TPU and PDMS rubber. The blends were subjected to cryo-fracture in liquid nitrogen and the PDMS phase was extracted preferentially from the specimens using toluene as the solvent. Before examination, the fracture surfaces were dried in a heating oven at 70 °C, brought down to room temperature in a desiccator and then sputter-coated with a thin layer of gold in a vacuum chamber. The specimens were subjected to scanning electron microscopy at 0° tilt angle.

3.6. Transmission electron microscopy (TEM)

The morphology of nHap and the dispersion of nHap in the blend matrix were analyzed with high-resolution transmission electron microscopy (HRTEM). The experiment was carried out in JEM 2100, JEOL high-resolution transmission electron microscope with lanthanum hexa-boride target, operating at 200 keV and with an average beam current of 116 μA. The samples of hydroxyapatite for TEM were prepared by dispersing a small amount of the sample in ethanol (10 mg/50 ml) and ultra sonicated for 30 minutes. Two drops of the resultant suspension were dropped on to a copper grid and it has been dried in desiccators before observing under the microscope. The dispersion and distribution of nHap in the blend matrix were observed through HRTEM analysis of the ultramicrotomed samples.

3.7. Differential scanning calorimetry (DSC)

The glass transition temperature melting behaviour and crystallization temperature of the virgin polymers, their blends and nanocomposites were determined using NETZSCH DSC 200F3, in the temperature range from −130 °C to +210 °C followed by a controlled cooling from +210 °C to −130 °C at a scan rate of 10 °C min⁻¹ in nitrogen atmosphere.

3.8. Thermogravimetric analysis (TGA)

Thermogravimetric analysis of the virgin polymers and their blends and nanocomposites were carried out using TGA Q500 of TA instruments, USA, in the temperature range from 30 °C to 800 °C at a heating rate of 25 °C per minute in nitrogen atmosphere.

4. Results & discussion

4.1. Characterization of synthesized spherical nHap

4.1.1. Fourier transform infrared spectroscopy (FTIR). Fig. 1(a) shows the FTIR spectrum of PPG-wrapped nHap crystal. The bands at 3571 and 631 cm⁻¹ are assigned to stretching mode and liberation mode, respectively, of the –OH groups. Bands at 1040, 946 and 569 cm⁻¹ are assigned to vibration of the phosphate group, –PO₄. The peak at 1040 cm⁻¹ is the triply degenerated vibration; 946 cm⁻¹ is the non-degenerated symmetric stretching mode, of the P–O bond of the phosphate group. The peak at 569 cm⁻¹ is assigned to a triply degenerated bending mode, of the O–P–O bond. The medium band at 1639 cm⁻¹ is assigned to adsorbed water. On the other hand, the peak assignments for the confirmation of PPG coated on to the surface of nHap is obtained from peaks for asymmetric CH stretching at 2964 cm⁻¹ and CH₃ symmetric deformation at 1259 cm⁻¹. The assigned bands are in good agreement with the literature values which confirms the successful formation of hydroxyapatite.²⁶

Fig. 1 (a) FTIR spectrum of spherical nHap, (b) XRD patterns of spherical nHap.

4.1.2. X-ray diffraction studies (XRD). The XRD pattern of the nHap crystal is shown in Fig. 1(b). For pure nHap crystal, the typical diffraction peaks of hexagonal Ca₁₀(PO₄)₆(OH)₂ can be seen, which can be indexed as the standard data (JCPDS no. 09-0432). The characteristic peaks at 2θ regions of 26, 29, 32–34, 40, and 46–54° indicate the crystalline nature of hydroxyapatite and it has been further cross checked with the literature.²⁶

In contrast, the peaks at the diffraction angles of 26, 32, 33 and 40 are attributed to (002), (211), (300) and (310) planes. The crystallite size has been calculated and presented using the reflections of various planes such as (002), (211), (300), (202), (310), (222), (312), (213) and (004) as given in Table 1.

Table 1 Crystallite size of nHap crystal with respect to various reflection planes

Reflection planes	2θ (degree)	β (radian)	d spacing (nm)	Crystallite size (nm)
(002)	25.92	0.1506	34.4	9.44
(211)	32.23	0.184	28.0	7.84
(300)	32.91	0.2676	26.5	5.40
(202)	34.09	0.2342	25.7	6.10
(310)	39.23	0.1673	22.5	8.79
(222)	46.66	0.1506	19.2	10.02
(312)	48.08	0.2342	18.9	6.40
(213)	49.50	0.1506	18.3	10.13
(004)	53.23	0.2007	17.3	7.77

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4.1.3. Transmission electron microscopy. The fine nanostructure of the prepared nHap has been studied by using HRTEM. Typical HRTEM images of nHap particles from different locations are taken for the morphology analysis. The representative image is shown in Fig. 2(a). From the figure it is evident that the obtained particles are in a spherical shape. The reaction temperature and pH are the major factors for the characteristic observation.³⁰ The high magnification HRTEM image of nHap given in Fig. 2(b) shows an average diameter of the particle as 15.93 nm.

Fig. 2 (a) and (b) HRTEM images nHap, (c) SAED of nHap, (d) overlapped image of SAED and XRD pattern (e) EDAX of nHap.

Selected area electron diffraction (SAED) study was conducted for verifying the crystallinity of PPG-coated spherical nHap. The SAED patterns of the synthesized nanoparticles are given in Fig. 2(c). From the figure, the crystalline nature of nHap got reconfirmed with that of XRD. The clear sharp rings correspond to crystalline planes of nHap that completely matches with the XRD data. This shows that the crystallinity is not affected even after PPG coating of the nanoparticles synthesized by modified method. The d-spacing of prepared nanoparticles obtained from XRD and SAED are similar and the overlapped image of XRD and SAED establishes the observation (Fig. 2(d)). For guaranteeing the molar ratio of nHap, TEM EDAX has been carried out. Fig. 2(e) exhibits the EDAX, which confirms the Ca to P ratio as 1.67, which corresponding to the hydroxyapatite.

4.2. Characterization of TPU–PDMS nHap nanocomposites

4.2.1. Preliminary selection of blend matrix for the preparation of nanocomposites based on tensile strength. The selection of blend matrix is an important consideration in the nanocomposite preparation for the effective utilization of the composites. The best blend matrix can be identified by analyzing the mechanical properties and which correlate with the phase morphology. Before diving down into deeper understanding, a preliminary study on mechanical properties of the nanocomposites has been carried out in order to select the most suitable blend matrix for the nHap based nanocomposites. PDMS rubber being a weak matrix has poor mechanical properties as compared to pristine TPU. However, agglomeration and poor state of dispersion are some of the most serious issues in TPU based nanocomposites at higher doses of nanofillers, which is expected to improve by introducing amorphous soft PDMS rubber phase in to the TPU matrix. Hence the dilution of the matrix containing stiffer crystalline hard segments takes place which can contain more nHap very easily. As expected, the addition of more amounts of PDMS decreases the tensile strength. The combination of PDMS as well as nHap in TPU has mutual benefits in the overall properties of the nanocomposites. Blend matrices with higher doses of nHap can go with higher proportion of PDMS without agglomeration as well as matrices with higher proportion of PDMS may impart better mechanical properties in the presence of higher doses of nHap. The nanocomposites based on different blend ratios with varying doses of nHap have been selected for the preliminary study. The TPU–PDMS blend ratios have been varied from 90 : 10 to 50 : 50 and the nHap has been introduced into the various blend ratios at different dosages of 1, 3, 5, and 7 phr. The mechanical properties of the developed nanocomposites and neat blends were determined by adopting standard methods. The tensile strength against the nHap proportion is given in Fig. 3. In the case of T90P10 blend nanocomposites, the tensile strength is increased upto a dosage of 5 phr of nHap and then it decreased due to possibly the agglomeration of the nanoparticles. In case of T80P20 blend nanocomposites, the tensile strength shows an increasing trend up to a dosage of 5 phr of nHp beyond which there is no change in the tensile properties may be due to the agglomeration tendency. The increase in strength properties is attributed to good interaction between nHap and the blend matrix. In T70P30 blend nanocomposites, the tensile strength increases proportionate with the nHap even beyond a dosage of 5 phr i.e. upto 7 phr studied. Such enormous improvement in tensile property of T70P30 based nanocomposites is due to the higher polymer–filler interaction. Even though the tensile strength increases with filler dosages in the case of T60P40 and T50P50 based nanocomposites, the tensile strength are lower due to the presence of higher proportion of PDMS rubber. The earlier literature reveals almost the same trend on mechanical properties of TPU–PDMS nanocomposites which contains higher proportions of PDMS.^51,52 The significant property improvement in T70P30 blend nanocomposites compared to the rest of the blends studied suggests that the T70P30 blend stand as the most appropriate and acceptable blend for study of the blend nanocomposites. Therefore, further in depth study on T70P30 based nanocomposites has been carried out.

Fig. 3 Tensile properties of TPU–PDMS nanocomposites.

4.3. Characterization of T70P30 nanocomposites

4.3.1. Mechanical properties. The stress–strain curves for the pristine TPU, T70P30 blend and blend nanocomposites are shown in Fig. 4. Neat TPU shows a tensile strength of 39 MPa and elongation at break of 650%, whereas in T70P30 blends show a tensile strength of 7 MPa due to the incorporation of 30 parts of PDMS rubber which is a weak matrix, having lower mechanical properties. However, with the introduction of nHap, the tensile strength of T70P30 blend nanocomposites is increased. The tensile strength of T70P30H1 nanocomposite is found to be 8 MPa with an elongation at break of 480%. In the case of T70P30H3 the tensile strength is increased by 50% than the pure blend. For the nanocomposites of T70P30H7 which contains 7 phr of nanofiller has very good mechanical properties exhibiting almost 100% improvement in tensile strength and it possesses an elongation at break of 520%. Even though the nanocomposites can bear more load at breaking point than the virgin blend, sliding of chains takes place while applying tensile force, so that it exhibits higher elongation. Here the physical interaction of nHap with blend matrix act as a bridging element which carries more load at breaking point and helps in linking the polymer chains, but this interaction is not enough to prevent the chains from sliding. The moduli of different nanocomposites with increasing dosage of filler increases in all the cases as shown in Table 2. A uniform dispersion of fillers in nanocomposites is one of the deciding factors of its ultimate strength properties. An improvement in tensile properties has been achieved by selecting solution mixing technique as well as choosing spherical nHap as the filler. The strong polymer filler interaction and a higher state of dispersion of individual nano-sized hydroxyapatite spheres naturally influence the effectiveness of applied load transfer across the interfaces.

Fig. 4 Stress–strain curves of T70P30 nanocomposites.

Table 2 Tensile properties of TPU–PDMS blend nanocomposites

Sample designation	TS (MPa)	EB%	Mod. at 100% (MPa)	Mod. at 200% (MPa)	Mod. at 300% (MPa)
T100P0	39.2 ± 0.8	650 ± 25	4.5 ± 0.3	7.5 ± 0.5	9.7 ± 0.1
T70P30	7.4 ± 0.5	450 ± 10	2.5 ± 0.1	3.3 ± 0.2	3.9 ± 0.4
T70P30H1	8.3 ± 0.7	480 ± 20	2.8 ± 0.2	3.5 ± 0.4	4.2 ± 0.4
T70P30H3	10.1 ± 0.8	520 ± 22	3.0 ± 0.2	3.8 ± 0.1	4.8 ± 0.3
T70P30H5	11.4 ± 1.0	550 ± 10	3.6 ± 0.3	4.0 ± 0.2	5.0 ± 0.3
T70P30H7	14.4 ± 0.5	600 ± 15	3.9 ± 0.4	4.2 ± 0.3	6.8 ± 0.1

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4.3.2. Dynamic mechanical analysis (DMA). DMA is widely accepted as one of the most helpful techniques for evaluating the effect of filler addition on to the polymer matrix. Fig. 5(a) and (b) displays the temperature dependency of tan δ and storage modulus (E′) of neat polymer blend and its nanocomposites over a temperature range of −130 °C to 130 °C. The temperature at the maximum of tan δ during the temperature sweep graph indicates the glass–rubber transition temperature (T_g) of the polymer matrix and the nanocomposites. In filled polymer systems, dynamic mechanical testing is often employed to study the effect of filler on the glass transition temperature and the dynamic properties of the polymers.⁵³ The tan δ values for different batches are given in Fig. 5(a) and in Table 3. In the case of pristine TPU, (i.e. T100P0) the tan δ peak is observed at −18 °C. After the incorporation of 30 parts of PDMS an additional peak at −120 °C appears corresponding to the T_g of the PDMS rubber in T70P30 blends. Here the tan δ peak corresponds to the TPU gets lowered and the peak temperature shifts to a value of −28 °C. With the incorporation of nHap in to the T70P30 blends, a special feature has been observed. For T70P30H1 blend nanocomposites, the T_g obtained is −25 °C and further addition of fillers in to the blend T70P30H3, T70P30H5 and in T70P30H7 raised the T_g's to −23 °C, −21 °C and −18 °C respectively. In the case of T70P30H7 blend nanocomposite, the T_g is increased by 10 °C and which is equivalent to T_g of neat TPU, which contains 30 parts of PDMS soft segments. Here the effect of flexible soft segments is nullified by the stiffening effect of nHap, that is why the T_g of T70P30H7 matched with the T_g of neat TPU. This shows the effective reinforcement of T70P30 blends by spherical nHap. With the addition of nHap, the chain stiffness improves tremendously due to the enhanced polymer filler interaction as reported in the literature.^53,54 The storage modulus of neat TPU, virgin T70P30 blend and the nanocomposites are given in Fig. 5(b) and the data are presented in Table 3.

Fig. 5 (a) and (b) tan δ and storage modulus of T70P30 nanocomposites.

Table 3 Dynamic mechanical properties of the TPU–PDMS blend nanocomposites

Sample designation	tan δ peak (I) (°C)	tan δ peak (II) (°C)	Storage modulus peak@30 (°C) (MPa)
T100P0	—	−18	40.0
T70P30	−120	−28	8.2
T70P30H1	−120	−25	9.4
T70P30H3	−120	−23	10.4
T70P30H5	−120	−21	14.2
T70P30H7	−120	−18	18.7

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The storage modulus of T70P30 blend is less than the neat TPU. For T70P30 blend nanocomposites the storage moduli increases with increase in filler doses in the blends. The trend for storage modulus at room temperature for the entire batches match with the tensile strength obtained from the mechanical properties which are given along with their tan δ peak values in Table 3.

4.3.3. Fourier transform infrared spectroscopy (FTIR). FTIR analysis of virgin TPU–PDMS blend and its nanocomposites has been carried out in order to find the chemical interaction taking place. With increase in dosage of filler, OH–NH interaction occurs which causes very slight broadening of OH stretch in the 3300 cm⁻¹ region as shown in Fig. 6(a). A significant change is observed in the fingerprint region (500–1000 cm⁻¹) of nanocomposites which is highlighted separately in Fig. 6(b). In the case of nanocomposites an additional peak of phosphate bond (PO₄³⁻) is appeared at 566 cm⁻¹ which was absent in the virgin blend. The increase in intensity of the peak corresponds to PO₄³⁻ is noticed with the dosage of filler. Even though, the chemical interaction due to nHap is negligible in nanocomposites, these observations confirm that the nHap particles are very well dispersed in TPU–PDMS blend (Fig. 7).

Fig. 6 (a) FTIR spectra of T70P30 nanocomposites and (b) FTIR spectra of T70P30 nanocomposites in the fingerprint region.

Fig. 7 XRD pattern of TPU and T70P30 nanocomposites.

4.3.4. X-ray diffraction (XRD). Structural morphology of the T70P30 blend nanocomposites has been studied using wide angle X-ray diffraction technique. It is one of the most predominantly used techniques for assessing the structural morphology of inorganic filler and the polymer nanocomposite. WAXD study helps to find out the state of dispersion of filler in the polymer matrix, which is utmost important in dictating the mechanical and thermal properties of polymer nanocomposites. In the case of T70P30 blend nanocomposites, the peak at 2θ = 20° corresponds to the crystalline peak of TPU hard segments and this peak intensity decreases with the incorporation of PDMS as well as nHap in to the matrix which proves the uniform dispersion of nanofiller in the matrix. With the incorporation of nHap, the crystals planes corresponding to hydroxyapatite appear in diffraction pattern. The characteristic diffraction pattern of crystalline peaks for (002) plane for nHap is found to be 2θ = 25.9° with a d-spacing of 3.45 Å in the inset of Fig. 6. The intensity of peaks of (211) plane is increased sharply with nHap content in the blend from T70P30H1 to T70P30H7. The other peaks at (310), (222), (213) and (004) planes are clearly visible in the nanocomposites and the peak intensity increases with the dosage of filler. The crystallinity of neat TPU, virgin blend, and blend nanocomposites are calculated from the area of amorphous and crystalline peaks which are given in Table 4. Due to the introduction of nHap, the degree of crystallinity of virgin blend has been increased from 10% to 12.9% with 7 phr of nHap. At 7 phr dosages of nHap in TPU–PDMS blend nanocomposites show an increased crystallinity equivalent to neat TPU which is 13% crystalline. The strong crystalline peak (211) at 2θ = 32.0° with d-spacing of 2.8 Å has a dominant role in increasing the crystallinity of the nanocomposites.

Table 4 Percentage crystallinity of TPU and T70P30 nanocomposites

Batches	% crystallinity
T100P0	13.0
T70P30	10.1
T70P30H1	10.7
T70P30H3	11.2
T70P30H5	12.2
T70P30H7	12.9

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4.3.5. Transmission electron microscopy. The state of nanoparticle dispersion in the polymer matrix is a powerful factor in determining ultimate properties of the nanocomposites. Uniform and homogeneous dispersion of nanoparticles can lead to the mechanical property enhancement of nanocomposites to a very high level. The HRTEM images of T70P30 nHap nanocomposites are given in Fig. 8 along with SAED images. In all the cases an increase in dose of the nanofiller increased the dispersion along with increased crystallinity as observed from the SAED pattern. In T70P30H1, some phase separated regions of nanocomposites causes the agglomeration of nHap during casting which is visible in Fig. 8(a). The reason behind the agglomeration of T70P30H1 composites is its inferior phase morphology than the other nanocomposites with more filler dosage. As compared to the reported work, it has been found that these nanocomposites do not show much agglomeration at higher dosages.²⁶ This has been confirmed from the solution blending technique so that the individual particles can easily penetrate in to the space in between swollen polymer chains and form uniform dispersion. The spherical size of nanoparticles plays an important aspect in the dispersion of the filler in the polymer matrix which ultimately imparting isotropic properties to the nanocomposites.

Fig. 8 HRTEM images and SAED patterns of (a) T70P30H1 (b) T70P30H3 (c) T70P30H5 (d) T70P30H7.

As seen from XRD, the addition of crystalline nHap into polymer blend matrix initiates orientation of the crystalline planes corresponding to nHap planes. From selected area diffraction pattern it is found that the addition of 1 phr nHap in to TPU–PDMS blend, some fused rings corresponding to (211) and (310) planes (refer Fig. 8) are observed. With the addition of 3 phr nHap the planes again got further aligned and oriented. Even we have started from a polymer blend with an amorphous part, at a dosage of 5 phr nHap, due to the nucleation of chains bright spots corresponding to the crystalline planes of nHap are appeared. This is an interesting fact that, the addition of more amounts of nanofiller is possible in a polymer matrix without agglomeration and where nanoparticles are very effectively utilised by the matrix. As a result of the best selection of blend matrix proportion, nanoparticle characteristics and method of blending imparts attractive properties to the nanocomposites.

In the case of T70P30H7 nanocomposites the crystalline peaks reach its highest point compared to all the other compositions which has been further confirmed from the SAED pattern as sharp rings representing the nHap crystalline planes.

4.3.6. Scanning electron microscopy. The phase morphology of the blends has been studied with the help of a SEM in order to understand the spatial distribution of phases within the blend matrix of TPU and PDMS (70 : 30) with various dosages of nHap. Fig. 9 shows the SEM images of virgin T70P30 blend and the four blend nanocomposites at different dosages of nHap. The PDMS rubber phase has been preferentially removed by etching in toluene solvent. The black vacuoles in the matrix indicate the PDMS rubber domains. In the case of TPU–PDMS blend nanocomposites after solvent etching three types of surfaces are formed. The most important and most visible one is based on TPU–PDMS interface. For virgin blend the extraction of PDMS is easy, and as a result, dark vacuoles are visible in the SEM images corresponding to the etched PDMS. Incorporated nHap in to the blend matrix results two types of action on the matrix. The nHap itself has good interaction with the matrix and these increases with the increase in filler loading. Also it favours the formation of dispersed PDMS domains in TPU matrix. Normally problem arises as phase separation or as coalescence of PDMS after casting of the polymer blend solutions. In the case of blend nanocomposites, the re-union of PDMS chains in the solution was prevented by the nHap and it acts as a physical barrier between dispersed polymer chains. In nanocomposites with very high dosages like T70P30H5 and T70P30H7, removal of PDMS is restricted by the nanoparticles. As a result, the etched PDMS proportion also gets reduced. Since the nanocomposites are prepared by solution mixing technique, a proper mixing of two matrices are ensured. The phase morphology of nanocomposites was found to be improved by the addition of nHap as observed in the case of compatibiliser. Here the nHap mimics the action of a compatibiliser.

Fig. 9 SEM images (a) T70P30 (b) T70P30H1 (c) T70P30H3 (d) T70P30H5 (e) T70P30H7 (f) FESEM image of TPU coated nHap surface of T70P30H7.

In the case of T70P30H7 nanocomposite, where the maximum filling of nHap has done, an extra surface is obtained at higher magnification where the nHap particles coated by TPU is clearly able to seen as in Fig. 9(f). In the case of TPU–PDMS blend nanocomposites, nHap acts as an interface in three cases. One is PDMS–nHap–PDMS phase, which directly goes to the solvent during etching. Second, the TPU–nHap–TPU phase which is not affected by the solvent and remains there. Third one is the visible nHap surface on TPU matrix after PDMS etching, is a clear evidence for the good dispersion of nanocomposites where PDMS–nHap–TPU interface was present. It exhibits the area where nHap separates the TPU and PDMS phases.

In very high magnification, nHap particles are appeared in the etched surfaces where PDMS–nHap interfaces were present. These were clearly observed in T70P30H1, T70P30H3, and T70P30H5 and in T70P30H7 as given in Fig. 10 at a magnification of 10⁵. With increase in dosage of filler the dispersed nanoparticles are more visible which confirmed that nHap particles are fully dispersed in TPU as well as PDMS phase. The phase morphology of TPU–PDMS nanocomposite after solvent etching can be clearly understood with the help of the schematic diagram (Fig. 11).

Fig. 10 FESEM images of PDMS etched out TPU–nHap surfaces of (a) T70P30H1 (b) T70P30H3 (c) T70P30H5 (d) T70P30H7.

Fig. 11 Schematic diagram of cryofractured nanocomposite surface after solvent etching.

4.3.7. Differential scanning calorimetry (DSC). The DSC thermogram after second heating for the neat TPU, virgin TPU–PDMS blend (70 : 30) and their nanocomposites with various doses of nHap are shown in Fig. 12(a). Blends of TPU and PDMS are found to be immiscible as they exhibit two glass transition temperatures with a wide margin. Neat PDMS reveals a T_g around −120 °C and stiffening occurs at a temperature of −50 °C whereas neat TPU exhibits two T_g's and two T_m's due to the presence of soft segments and hard segments. The T_g of soft segments of neat TPU is at around −50 °C and that of the hard segments shows a T_g around 30 °C. In T70P30 blend, due to the overlapping of brittleness temperature (T_b) of PDMS and T_g of soft segments of TPU at −50 °C, a sharp endothermic peak at this temperature is observed. Therefore the expected shift in T_g due to the incorporation of nHap at this temperature is not recognizable. The melting like transition due to soft segments (T_m) of T70P30 blend is observed at 116 °C. With the addition of nHap, T_m is shifted to 120 °C. The T_g due to hard segments of T70P30 blends decreases to 26 °C and the incorporation of nHap into this blend system does not change the T_g as well as T_m of hard segments significantly. From this observation one may conclude that addition of nHap is most affected for the soft segments than the hard segments and these soft segments are more responsible for the improvement in mechanical properties.

Fig. 12 (a) DSC thermograms for neat polymers and their blends nanocomposites after second heating cycle and (b) DSC thermograms for neat polymers and their blends nanocomposites after cooling cycle.

During the cooling cycle, crystallization of polymer chains occurs and as a result, an exothermic peak corresponding to the individual polymer appears. PDMS shows cold crystallization at a temperature (T_CC) of −89 °C. In the case of TPU, crystallization occurs in the hard segments as well as in soft segments as shown in Fig. 12(b). During the controlled cooling from 200 °C to −150 °C, the hard segment crystallizes at a temperature (T_C2) of 194 °C for neat TPU and the soft segment crystallization temperature (T_C1) occurs at 69.1 °C. For TPU–PDMS blends, the T_C1 got reduces to 61 °C and T_C2 shifts to 192 °C. For T70P30H1 T_C1 increases to 62.6 °C and T_C2 to 192.3 °C with the addition of nHap. This is presumed to be due to the nucleation of nHap in TPU–PDMS blend system. In all the nanocomposites, the crystallization temperatures as well as corresponding crystallization enthalpy get increased as depicted in Table 4. Interestingly, it is observed that in T70P30H7 nanocomposites, with 7 phr of nHap the T_C1 get increased by 8 °C than the virgin blends and matches well with the crystallization temperature of neat TPU (Fig. 13).

Fig. 13 Schematic representation of TPU–PDMS blend nanocomposite preparation based on spherical and rod shaped nanoparticles by solution and melt mixing techniques.

This is more clearly expressed in Fig. 14(a). From the plot of crystallisation enthalpies of hard segments and soft segments of TPU, it is evident that the nanocomposite addition plays an important role in the crystallisation of polymer chains. For the virgin T70P30 blend, the ΔH values of soft segments and hard segments are less than the pure TPU.

Fig. 14 (a) Crystallisation enthalpy of hard segments (HS) and soft segments (SS) of neat TPU, T70P30 blend, and nanocomposites, (b) nucleation efficiency of hard segments and soft segments of nanocomposite with various dosages of filler).

In the case of transitions based on hard segments, an increase in ΔH_c is observed upto T70P30H5 and then it remains almost same with nHap addition. The increment in ΔH_c corresponds to the hard segments due to the orientation of chains where the alignment of soft segments influence both sides of the hard segments which act as the driving force. The nHap may not play a direct role in crystallization of hard segments observed as in soft segments. It is certain that the transitions due to soft segments are most affected by nanohydroxyapatite incorporation and the enhanced rate of crystallization of nHap composites is due to the phenomenon of nucleation. Nucleation is the formation of either a new phase or new structure via self assembly or self organization of polymer chains.

TPU–PDMS blend nanocomposite based on rod shaped nHap has been previously studied by Jineesh et al. by melt mixing technique.²⁶ Due to the agglomeration tendency of the nanorods restricted the study beyond 2 phr of nHap. Hence, we have adopted the solution mixing technique, which is an entropy driven process compared to melt mixing technique. Thus the polymer chain orientation as well as the crystallisation properties are very good in solution blended nanocomposites. The schematic representation of TPU–PDMS blend nanocomposites prepared with spherical and rod shaped nHap by solution and melt mixing are shown in Fig. 12. In the case of spherical nanoparticles the single surface supports the nucleation process uniformly in solution blended nanocomposites and developed nanocomposites with isotropic properties. The chances of nucleation in nanocomposites based on nanorods is also possible, but it may result in anisotropy in nanocomposites. Here we got a reasonable improvement in properties due to the selection of best blend matrix, method of nanocomposite preparation and characteristics of nanofiller which helps in nucleation.

As a result of nucleation of nanocomposites, the crystallization enthalpy as well as the crystallization temperature has been improved than the neat blend. The crystallization enthalpy is more improved in the case of soft segments of blend matrix, which includes TPU soft segments and PDMS. With the incorporation of nHap in the neat blend, the crystallization temperature of individual segments increases which is more observed in the TPU segments. In the case of TPU–PDMS blend nanocomposites, orientation occurs in soft segments and hard segments of TPU which causes nucleation whereas in PDMS matrix, re-organisation as well as partial alignment of chains takes place. So the detailed study for the nucleation efficiency has been carried out for TPU matrix only.

4.3.7.1. Nucleation efficiency of nanocomposites. During controlled cooling of nanocomposites from 200 °C to −150 °C the molten chains flow and get oriented by the nHap spherical particles which make the T_c shift towards a higher temperature. Here nHap act as the initiative for the process of nucleation. The nucleation efficiency (NE) of the nanocomposites is calculated using the following eqn (2).⁵⁵

NE = (T_CNA − T_CP)/(T_CMP − T_CP) (2)

where, T_CNA is the peak crystallization temperature of the polymer blend nanocomposite with the nHap nucleating agent, T_CP is the peak crystallization temperature of the neat polymer blend T70P30 (without any nucleating agent), and T_CMP is the maximum crystallization temperature of the ideally self-nucleated neat TPU. NE is a convenient parameter for comparing the efficiency of different dosages of nucleating agents on individual polymer segments with respect to the unique property of every polymer. Here the NE of nHap on hard segments (HS) as well as soft segments (SS) of TPU is shown in Fig. 14(b) which is calculated using eqn (1). For hard segments, the NE increases from 8% to 16% when nHap vary from 1 phr to 3 phr and further addition of 5 phr and 7 phr of nHap raise the NE very slightly to 17.8% and 18% respectively which follows the crystallization enthalpy variation as in Fig. 14(a). In the case of soft segments, NE is proportional to the dosage of nHap which varies from 60% to 100%. This again confirms that the nucleation has a tremendous effect for improving the dynamical mechanical properties and crystallinity of the nanocomposites especially due to the soft segments. The interactions between the polymer and the nHap and excellent dispersion of the nanofiller help in nucleation of polymer chains. As a result of nucleation an increase in its crystallization temperature and rate of crystallization is observed. This effect is partly credited to the development of a larger number of crystallization nuclei. The interfacial interaction plays a critical role in the free energy of cluster formation and the rate of nucleation; the strong interaction increases the rate of nucleation. Nucleation efficiencies are related to bond energies between the nucleating agent and the polymeric crystals, and crystallographic mismatches between the substrate and the polymer (Table 5).^55–57

Table 5 TPU–PDMS blend nanocomposites

	PDMS segments		TPU soft segments		TPU hard segments
	TCC (°C)	ΔH (J g^−1)	TC1 (°C)	ΔH (J g^−1)	TC2 (°C)	ΔH (J g^−1)
T0P100	−89.4	19.79	—	—	—	—
T100P0	—	—	69.1	4.79	194.0	5.41
T70P30	−81.2	3.76	61.0	1.44	192.0	4.11
T70P30H1	−81.8	4.12	62.6	1.87	192.3	4.16
T70P30H3	−81.9	4.92	65.0	2.22	192.6	4.27
T70P30H5	−82.3	5.42	66.5	3.26	193.0	4.32
T70P30H7	−82.5	5.69	69.0	4.71	193.5	4.35

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4.3.8. Thermogravimetric analysis. Degradation of TPU–PDMS nHap nanocomposites occurs in three steps in which TPU soft segments degrade at 365 °C, hard segments degrade at 431 °C and PDMS degradation occurs at a high temperature of 532 °C in a single step.⁵⁸ The thermal stability of blend nanocomposites depends on the thermal stability of crystalline and amorphous phases. From Fig. 15, an important observation is obtained with the addition of nHap, the degradation temperature of PDMS and soft segment of TPU increases whereas hard segments degradation temperature is not affected. This indicates that the nHap incorporation has a positive role in thermal stability of the weak phases of the blend matrix than the hard phase. As observed in the case of nanocomposites of 80 : 20 blends and 90 : 10 blends which contain more amount of TPU (especially more percentage of hard segments) could not act as a good matrix in higher loading of nHap (refer Fig. 3), so that the tensile properties are decreased at 7 phr dosage due to the agglomeration tendency. In the presence of more soft segments for 70 : 30, 60 : 40, and 50 : 50 blends, the nHap supports the matrix even in higher loadings without any loss of mechanical properties. As the TPU proportion decreases nHap acceptance of the matrix increases due to the contribution of more amounts of soft segments which can interact with the nanofiller and so the temperature stability increases. In the case of TPU soft segments at onset degradation temperature, weight loss decreases with the addition of nHap (Table 6).

Fig. 15 TGA and DTG data of neat polymers and their blend nanocomposites.

Table 6 Degradation study of TPU–PDMS blend nanocomposites

Sample ID	Onset temperature of degradation (°C)			Weight loss (%)			Residue wt%
	First (TPU SS)	Second (TPU HS)	Third (PDMS)	First (TPU SS)	Second (TPU HS)	Third (PDMS)
T100P0	365	431	—	73.8	32.3	—	4.1
T0P100	532	—	—	56.0	—	—	1.1
T70P30	364	431	576	82.1	56.0	16.0	4.3
T70P30H1	367	431	584	81.9	50.9	15.6	5.7
T70P30H3	369	431	591	81.0	51.0	17.0	7.3
T70P30H5	372	431	602	80.5	51.1	16.7	8.7
T70P30H7	374	431	611	80.1	51.2	18.1	10.5

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For TPU hard segments of nanocomposites, the weight loss at the onset temperature remains same with increase in filler loading and also owns lesser weight loss than the virgin blend. It indicates that the hard segments are more oriented in the case of nanocomposites than the virgin blend where the formation of crystalline phase may prevented by the well dispersed PDMS domains. The incorporation of nHap causes the alignment of alternately placed soft segments which acts as a reason for hard segment orientation. Thus the effect of nHap on thermal stability of hard segments occur maximum in 1 phr dosage and the variation in nHap dosage does not further improve the thermal stability of the hard segments.

5. Conclusions

Spherical PPG coated nanohydroxyapatite has been successfully synthesised by coprecipitation method and characterised by FTIR, HRTEM and XRD. TPU–PDMS blend ratio of 70 : 30 is found to be the most suitable matrix for evaluating the effect of spherical nHap in TPU–PDMS matrix. A systematic study has been performed to investigate the role of nHap in T70P30 blend matrix. The incorporation of spherical nHap with uniform dispersion in the TPU–PDMS matrix by solution mixing technique promotes nucleation which resulted in enhancement of mechanical, thermal and crystalline characteristics. According to the enthalpy of crystallisation and nucleation efficiency of nanocomposites, it is found that soft segments are most affected by the presence of nHap than the hard segments of TPU. These soft segments have been contributing in the upgradation of crystallisation phenomenon of T70P30 blend to pristine TPU by the addition of 7 phr nHap. Besides, the combination nHap and PDMS in TPU improved the thermal stability of the nanocomposites which can confirm an extended application temperature. The biocompatibility of the blend matrices together with the nanohydroxyapatite can assure a better performance in bone tissue engineering.

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