Comparison of Theory with Experimental Data for Nanoclay-Filled

Article pubs.acs.org/IECR

Comparison of Theory with Experimental Data for Nanoclay-Filled TPU/PP Blend Murugasamy Kannan,*,† Suggu Bhagawan,† Sabu Thomas,‡ and Kuruvilla Joseph§ †

Department of Chemical Engineering, Amrita Vishwa Vidyapeetham, Coimbatore-641112, India School of Chemical Sciences, Mahatma Gandhi University, Kottayam-686 560, India § Indian Institute of Space Science and Technology, Thiruvananthapuram-695 547, India ‡

S Supporting Information *

ABSTRACT: The mechanical properties of thermoplastic polyurethane (TPU)/polypropylene (PP) blends were investigated with special reference to the effect of type of polyurethane (ester- or ether-based), blend ratio, compatibilizer, and sequence addition of nanoclay. Tensile strength, stress at different elongations, flexural modulus, and abrasion resistance were analyzed and correlated with morphology. Blends of nanoclay filled thermoplastic polyurethane (TPU)/polypropylene (PP) of various compositions were evaluated by dynamic mechanical properties such as storage modulus (E′), loss modulus (E″), and dissipation factor (tan δ), at a frequency of 10 Hz over a temperature range from −100 to 200 °C. Finally different theoretical models were used to compare the experimental results with theoretical predictions.

1. INTRODUCTION Thermoplastic elastomers (TPEs) are materials that combine the excellent processing characteristics of thermoplastics at high temperature and wide range of physical properties of elastomers at service temperature.1 During the last decades, a considerable amount of work has been reported on TPE blends. Coran and Patel2−4 published a series of articles on rubber/thermoplastic blends and attempted to correlate the physical properties with the characteristics of elastomer and thermoplastic components. Campbell and co-workers5 investgated the physical properties for natural rubber (NR)/ polyethylene (PE) and NR/polypropylene (PP) blends. Fischer6 reported on the properties of ethyylene propylene diene monomer (EPDM)/PP thermoplastic blends with partial cross-linking. Recently, many researchers evaluated the mechanical properties of different blends,7−14 including blends based on polyethylene15−19 and ethyl vinyl acetate.20,21 Bhattacharyya et al.22 evaluated the compatibilizing efficiency of polyamide 6/ethylene vinyl acetate blends upon ionomer incorporation by morphological and mechanical property studies. The compatibilization effect of maleic anhydride modified compatbilizers also was evaluated. Thermoplastic polyurethane (TPU) continues to play an important role within the rapidly growing family of thermoplastic elastomers and its application can be found in almost all industrial branches such as engineering materials, coatings, adhesives, and films. TPUs are systems with microphase-separated domains composed of relatively flexible soft segments (SSs) and stiff hard segments (HSs). The SS consists mainly of polyethers or polyesters, while the HS is composed of the diisocyanate component and the so-called chain ‘‘extender’’, a lowmolecular-weight diol. The HSs are able to aggregate, forming separate microdomains with a regular crystallite-like structure consisting of chain segments parallelly aligned. The TPU domains are stabilized by hydrogen bonds between the NH © 2012 American Chemical Society

group as donor and the carbonyl of another urethane group as acceptor.23 Glassy or crystalline HS domains act as physical cross-links and reinforcing fillers, while the SS phase causes the flexibility. Increased interest in blending TPU with other polymers has been caused by the necessity of handling the problems resulted from specific structures of TPU macromolecules, interphase interactions, and microphase transformations. Polypropylene (PP), a semicrystalline polyolefin plastomer, is quite an outstanding polymer material with respect to its wide property spectrum performance, in particular its ease of processing, good chemical resistance, rigidity and heat resistance, and relatively low cost. To improve low PP impact toughness at low temperatures, the common practice is to incorporate elastomers into PP. According to abovementioned facts, the idea of selecting TPU/PP blends was to improve lack of properties of one TPU or PP polymer in a complementary way by blending with other PP or TPU components. In the present study, ester and ether TPU based TPU and PP blends were prepared by melt blending techniques and the mechanical and dynamic mechanical properties were studied. As the system is incompatible, technological compatibilization was sought by the addition of maleic anhydride modified polypropylene. The effect of the compatibilizer on the mechanical properties of the blends was analyzed. 70/30 TPU/PP blend has been chosen for these studies to retain the main properties of TPUs like elastomeric property, abrasion resistance, damping, and low temperature flexibility. The main purpose of the DMA study is to analyze the dynamic mechanical properties of nanoclay filled TPU/PP blends. The properties including storage modulus (E′), loss modulus (E″), Received: Revised: Accepted: Published: 13379

February 29, 2012 September 13, 2012 September 23, 2012 September 23, 2012 dx.doi.org/10.1021/ie3005397 | Ind. Eng. Chem. Res. 2012, 51, 13379−13392

Industrial & Engineering Chemistry Research

Article

and dissipation factor (tan δ) were investigated as a function of temperature and frequency.24−30 The parameters of interest in this study are the respective loss factor peak heights and peak locations to assess the blend miscibility and phase continuity. This study mainly focused on the effect of nanoclay (sequence of addition) in TPU/PP blend with compatibilizer at fixed TPU/PP blend ratio in ester- and ether-based TPU.

Table 1. Sequence I and II of Blend Compositions Sequence I TPU(nano) (wt %)a

blend

ester-TPU(nano)/PP 70 ester-TPU(nano)/PP/MA-g-PP 70 ether-TPU(nano)/PP 70 ether-TPU(nano)/PP/MA-g-PP 70 Sequence II

2. EXPERIMENTAL SECTION 2.1. Materials. Ester-TPU (385 S) and ether-TPU (KU2− 8600) were supplied by Bayer (TPU) India Ltd., Chennai. The MFI values of 385 S and KU2−8600E are 10 g/10 min and 11 g/min respectively (190 °C/2.16 kg). The hardness values of 385S and KU2−8600E are Shore A 85 and 84 respectively. PP (MA 1100) was supplied by Reliance Industries Ltd., Jamnagar, India. The MFI value of PP (MA 1100) is 11 g/10 min (230 °C/2.16 kg). MA-g-PP compatibilizer was purchased from Pluss Polymers, New Delhi. The compatibilizer is a maleic anhydride functionalized polypropylene (MA-g-PP), containing 1 wt % of maleic anhydride. The MFI value of MA-g-PP compatibilizer is 12 g/10 min (190 °C/2.16 kg).The organoclay used in this study [Cloisite 10 A (C 10A)] was obtained from Southern Clay Products, USA. It is a Na + montmorillonite, chemically modified with dimethyl benzyl hydrogenated tallow quaternary ammonium ions (N + 2MBHT), where N + denotes quaternary ammonium ions and HT denotes hydrogenated tallow. HT is made of approximately 65% C18H37, 30% C16H33, and 5% C14H29. Cation exchange capacity is 125 mequiv./100 g of clay. 2.2. Preparation of Nanoclay-Filled TPU/PP Blends. Ester-TPU/C10A nanocomposites were prepared by melt blending TPU and nanoclay using twin screw extruder. Three weight percent nanoclay was used. The ester-TPU pellets were dried at 100 °C for 4 h. The nanoclay was dried at 100 °C for 12 h in vacuum oven. The dried pellets were fed into a corotating twin screw extruder (Berstrof ZE 25) and the temperature of the die zone was maintained at 190 °C. The extrudate was received as strands and cut into small granules for melt blending with PP. Later, the granules of nanocomposites, along with pellets of PP and MA-g-PP were predried at 100 °C for 4 h in a vacuum oven. Blending was done in the corotating twin screw extruder at a die zone temperature of 190 °C and the extrudate was again obtained in the form of strands. Blends of different compositions shown in Table 1 were prepared by the method described above. Neat ester -TPU/PP blends were prepared in the ratio of 70/30. Sequence I. TPU/C10A nanocomposites were prepared and subsequently blended with PP using MA-g-PP as compatibilizer. Three wt% of the nanoclay was used. The ester- TPU pellets were dried at 100 °C for 4 h. The nanoclay was dried at 100 °C for 12 h in a vacuum oven. The dried pellets were fed into a corotating twin screw extruder and the temperature of the die zone was maintained at 190 °C. The extrudate was received as strands, which were cut into small granules for melt blending with PP. The granules of nanocomposites, along with pellets of PP and MA-g-PP were predried at 100 °C for 4 h in a vacuum oven. Blending was done in the same co-rotating twin screw extruder with a die zone temperature of 190 °C and the extrudate was again obtained in the form of strands. Neat TPU/PP blends were prepared in the ratio of 70/30.

a

MA-g-PP (wt %)

30 25 30 25

blend

TPU (wt %)

PP (wt %)

ester-TPU/PP/MA-g-PP ester-TPU/PP-nano ester-TPU/PP-nano/MA-g-PP ether-TPU/PP/MA-g-PP ether-TPU/PP-nano ether-TPU/PP-nano/MA-g-PP

70 70 70 70 70 70

25

b

PP (wt %)

PP-nano (wt %)

5 5 MA-g-PP (wt %) 5

30b 25c 25 30b 2c

5 5 5

c

3% nano content. 7.0% nanocontent. 8.4% nano content.

Sequence II. Nanoclay was added to PP by similar extruder melt blending and subsequently melt blended with TPU using MA-g-PP as compatibilizer (Table 1). Ether-TPU based samples were also prepared by a similar method. 2.3. Scanning Electron Microscopy (SEM). The phase morphology was studied using cryogenically fractured samples. By immersing the samples in liquid nitrogen, a brittle fracture was obtained avoiding large deformations in the surface to be examined. These samples were immersed in xylene solutions to remove the PP from TPU base. This setup was kept in oil bath at 80 °C for 3 h. Then the sample was taken out and dried in oven at 100 °C in order to remove excess xylene. Thus chemical etching of the sample was obtained. Morphological studies were performed on the chemically etched and gold sputtered samples using JEOL (JSM-5800) scanning electron microscope. 2.4. Transmission Electron Microscopy (TEM). The samples for TEM analysis were prepared by ultracryomicrotomy with a Leica Ultracut UCT (Leica Mikrosystems GmbH, Vienna, Austria). Freshly sharpened glass knives with cutting edges of 45 °C were used to obtain cryosections of 100−120 nm thickness. Because these samples were elastomeric in nature, the sample and glass knife temperatures during ultracryomicrotomy were kept constant at −75 and −85 °C, respectively [these temperatures were well below the glass transition temperature (Tg values) of PUs]. The cryosections were collected individually in a sucrose solution and directly supported on a copper grid of 300 mesh size. Microscopy was performed with a JEOL JEM 2000 TEM instrument (Japan), operating at an accelerating voltage of 120 kV. 2.5. X-ray Diffraction (XRD). The change in gallery spacing of silicate layers in the blend nanocomposites was determined on an X-ray diffractometer (Panalytical X-pert PRO, Netherlands) using Cu (λ = 1.5406 Å) as the radiation source. The samples were scanned at a rate of 3° /min (the increment step was 0.01° and the step time was 0.2 s) at room temperature for 2θ values starting from 1 to 10°. The dspacings were calculated using Bragg’s equation (nλ= 2dsin θ). 2.6. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectroscopy was performed for the prepared nanocomposites and blend nanocomposites in the ATR mode for convenience of measurement. A Thermo Nicolet (Avatar 370) 13380

dx.doi.org/10.1021/ie3005397 | Ind. Eng. Chem. Res. 2012, 51, 13379−13392


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Figure 1. Ester-based TPU blend TPU (nano)/PP/MA-g-PP (70/25/5): (a) 1% nano, (b) 3% nano, (c) 5% nano (nano content in TPU).

concentration regions, the CMC value for MA-g-PP is 5% for this TPU/PP blend.30 SEM micrographs of chemically etched (PP material was removed by xylene at 80 °C) nanoclay (1, 3, and 5%)-filled ester- and ether-based TPU/PP/MA-g-PP [70(nano)/25/5] blend systems are shown in Figures 1 and 2. It is observed that the size of dispersed PP particle was considerably reduced in the ester −TPU (3% nano)/PP/MA-g-PP (Figure 1b) system. For nanocomposite SEM sample preparation, the PP portion was removed by chemical etching method which gave very fine holes. In the case of compatibilized blends, addition of 3% nanoclay reduced the domain size to the maximum extent and attained maximum interfacial area/unit volume. Beyond 3% concentration, increase in size of dispersed particle was due to inefficiency of nanoclay dispersion, which caused miscelle (see Figure S1 in the Supporting Information). 3.1.2. Influence of Sequence of Nanoclay Addition on Phase Morphology. SEM micrographs of the ester- and etherbased TPU/PP(nano)/MA-g-PP, and TPU(nano)/PP/MA-gPP blend systems are shown in Figure S2 (Supporting Information). It is observed that the size of dispersed PP particle was considerably reduced in TPU(nano)/PP/MA-gPP. Compared to sequence II blend nanocomposites, sequence I blends showed better miscibility as confirmed by SEM analysis. The clear difference between sequence I and sequence II is that in the case of sequence I the effect of nanoclay is fully utilized by changing the surface tension of the TPU hard segment.31 This change in surface tension favored better dispersion of PP in TPU material; also, hydroxyl group of

spectrometer was used. Measurements were done in the spectral range 4000−400 cm−1 at a resolution of 0.9 cm−1. 2.7. Mechanical Properties. Tensile tests were carried out as per ASTM D 638 on a universal testing machine (International Equipments, Mumbai, India) at a crosshead speed of 200 mm/min. The dumbbell specimen sizes were: 165 × 12.8 × 3.2 mm3. The tensile tests were carried out up to break and the stress required for break and 20, 100, and 200% elongation were recorded. Injection molded specimens were used for this test. Izod impact test was carried out as per ASTM D 638. Abrasion resistance for the samples was measured according to DIN 53516, using specimens from injection molded test specimens. The flexural test was determined as per ASTM D 790. 2.8. Dynamic Mechanical Analysis (DMA). Dynamic mechanical measurements were performed for the blend nanocomposites (60 × 13 × 3.5 mm3) using a Netzsch DMA 242 instrument provided with a dual cantilever. Analysis was done in the temperature range from −100 to 200 °C at a frequency of 1,10,20 Hz and a heating rate of 5 °C/min.

3. RESULTS AND DISCUSSION 3.1. Morphology. 3.1.1. Influence of Nanoclay Addition on Compatibilized Blend Phase Morphology. The equilibrium concentration at which the domain size levels off can be considered as the critical micelle concentration (CMC) of compatibilizer ie the concentration at which micelles are formed. The CMC has been estimated in a separate study by the intersection of the straight lines at the low and high 13381



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Figure 2. Ether-based TPU blend TPU (nano)/PP/MA-g-PP (70/25/5): (a) 1% nano (b) 3% nano, (c) 5% nano (nano content in TPU).

3.2. XRD Studies. X-ray diffraction (XRD) is an extremely useful tool to study the structure and morphology of polymer nanocomposites. Since the layer spacing increases due to intercalation of polymer chains between the layers, the process can be monitored by XRD.41−44 The interlayer spacing of Cloisite 10A is 1.92 nm before compounding. For the uncompatibilized system, the XRD pattern exhibited no significant increase in interlayer spacing (see Figure S5 in the Supporting Information). This indicated that the polymer material did not intercalate into the interlayer, even in the organically modified nanoclay. In contrast, for compatibilized systems, the XRD peaks were shifted to lower angles, indicating the increase in interlayer spacing by the intercalation of polymer. 3.2.1. Compatibilizer Effect on Nanoclay Dispersion in TPU/PP (70/30) Blends. The XRD diagram of nanoclay filled compatibilized ester- TPU(nano)/PP/MA-g-PP blend is shown in Figure S5 (Supporting Information). The nanoclay peak is absent in ester-TPU(nano)/PP/MA indicating good dispersion; it appears that the compatibilizer MA-g-PP aided the dispersion of nanoclay in the blend. Thus, it is confirmed that the nanoclay dispersion improved in the presence of compatibilizer. This is because maleic anhydride undergoes chemical reaction with the urethane groups in the TPU hard segments and also forms hydrogen bonds with the silicate layers of C10A. XRD results are in accordance with TEM observations. 3.2.2. Influence of Sequence of Nanoclay Addition in Compatibilized Blend Morphology. The diffraction characteristics of compatibilized ester and ether based TPU (nano)/PP

silicate layers formed from hydrogen bonding with the carbonyl group of TPU and maleic anhydride of compatibilizer. Compared to the ether-TPU-based blend nanocomposites, the ester-TPU blends showed better compatibility as confirmed by SEM analysis. The clear difference between ester-TPU and ether-TPU is that ester-TPU had carbonyl groups both in the polyol segments as well as the TPU hard segments. This has led to more hydrogen bonding in the blend system thus improving the compatibility. 3.1.3. TEM Investigation. It is well-known that no single characterization method can adequately describe the state of clay dispersion in the nanocomposites. The supporting analyses are necessary and especially TEM has proved to be quite effective.32−40 When MA-g-PP was not introduced into the system, only intercalated structure was obtained, and many larger aggregates exist in the matrix (Figures S3a and S4a, Supporting Information). When the compatibilizer was introduced in the system, the nanoclay dispersion improved. The TEM image in Figure S3b (Supporting Information) showed exfoliated and well-dispersed nanoclay particles. In view of this, a true nanocomposites was produced in the case of ester-TPU(nano)/PP/MA-g-PP and 3 wt % nanoclay loading. The TEM image of the composite showed that individual clay layers were well-dispersed in the polymer. These results are well-correlated with the XRD results where almost no peaks were observed for ester-TPU(nano)/PP/MA-g-PP nanocomposites. TEM images of nanocomposite of ether-TPUs are shown in Figure S4b (Supporting Information) ; the degree of dispersion of clay is observed to be poorer than that in esterTPU nanocomposites. 13382



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Figure 3. Stress−strain curves for (a) uncompatibilized ester-TPU/PP blends and (b) uncompatibilized ether-TPU/PP blends.

Table 2. Mechanical Properties of Uncompatibilized Ester-TPU-Based Blends stress at % strain (MPa) blend composition TPU/PP TPU/PP TPU/PP TPU/PP TPU/PP a

(100/0) (70/30) (50/50) (30/70) (0/100)

impact strength Izod (J M−1)a

hardness shore D

NF NF NF 38 29

42 52 56 58 62

tensile modulus (MPa) 90 253 613 1020 1450

± ± ± ± ±

5 5 5 5 5

20%

100%

± ± ± ±

3.9 ± 0.05 5.7 ± 0.05 14.8 ± 0.05 24.8 ± 0.05 32.6 ± 0.05 (at break)

3.0 5.3 12.9 22.9

0.05 0.05 0.05 0.05

200% 4.3 6.4 15.9 25.2

± ± ± ±

0.05 0.05 0.05 0.05

NF, no failure.

Table 3. Mechanical Properties of Uncompatibilized Ether-TPU-Based Blends stress at % strain (MPa) blend composition TPU/PP TPU/PP TPU/PP TPU/PP TPU/PP a

(100/0) (70/30) (50/50) (30/70) (0/100)

impact strength Izod (J M‑1)a

hardness shore D

NF NF NF 40 29

42 53 57 59 62

tensile modulus (MPa) 90 274 643 1055 1450

± ± ± ± ±

5 5 5 5 5

20%

100%

± ± ± ± ±

± ± ± ±

3.4 5.7 15.1 25.1 32.6

0.05 4.0 0.05 6.6 0.05 16.2 0.05 27.6 0.05 (at break)

0.05 0.05 0.05 0.05

200% 4.3 6.7 16.3 27.2

± ± ± ±

0.05 0.05 0.05 0.05

NF, no failure.

grip occurred at very high strain levels (≥1100%) and the specimens could not be broken. Hence the large strain data were not considered to be very reliable and not discussed here. Stresses at 20, 100, and 200% strain were recorded to avoid tensile strength variation at break. The differences in the deformation characteristics of the blends under an applied load are evident from the figure. Addition of noncrystalline elastomer in small amount to semicrystalline PP changed the nature of the curve considerably. In general, the stress−strain curves of PP and PP-rich blends showed linear elastic region followed by yielding and necking in the inelastic region, which is typical of plastics. The addition of more TPU considerably reduces the modulus and eliminates the necking tendency of PP. The phase change in morphology is also reflected in the nature of the stress−strain curves. The mechanical properties of uncompatibilized blends are given in Tables 2 and 3. In the case of PP-rich blend, the curves exhibit typical plastic behavior. In the case of TPU-rich blends, no necking was observed, no exact initiation and termination of the distinct regions as explained in the representative stress−strain curves. It was also observed that upon the addition of TPU the strain increased and the stress decreased. The PP showed the highest modulus where as TPU showed the lowest modulus and all the blends exhibited intermediate values. These phase changes are evident in morphology studies. From the figures, it is clear that the composition had a marked effect on the stress−strain curve behavior.

blends are shown in Figures S5 and S6 (Supporting Information). In TPU/PP (nano)/ MA, a peak appeared at 2θ ∼4.8° with reduced intensity as compared to neat nanoclay. This is an indication that the nonpolar nonpolar PP chains have been incorporated in the gallery space because of their affinity for the nanoclay. The peak was much weakened in esterTPU(nano)/PP/MA indicating nearly exfoliation dispersion; thus the compatibilizer MA-g-PP aided the dispersion of nanoclay in the blend . As mentioned in the previous section, TEM images were taken and the results showed similar trend as XRD results. In the section on TEM analysis, effects of nanoclay dispersion are discussed with blend morphology of these blends. Compared to the ether-TPU based blend nanocomposites, the ester-TPU blends showed better compatibility as confirmed by XRD analysis. This observation may be due to the formation of more hydrogen bonding in ester-TPUbased nanocomposites. 3.3. FTIR Analysis. Details are given in the Supporting Information. 3.4. Mechanical Properties. 3.4.1. Tensile Properties of Uncompatibilized Blends. Plots showing the stress−strain behavior of TPU/PP blends of different composition and virgin components are presented in Figure 3. Tensile strength and elongation at break could not be measured since most specimens did not break even at the maximum strain the machine could impose. Also, sample slippage problem from grip increased at large strains. A slip between the specimen and 13383



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3.4.2. Comparison of Experimental Data of Tensile Properties with Theory. Mechanical properties are widely suited for analysis of multicomponent composites, through a comparison of experimental results and prediction based on various models. The application of various composite models gives insight into the properties of individual components. It also helps to check the assumptions regarding structure, mechanism and properties of the interface. Several theories have been proposed to predict the tensile properties in terms of various parameters. These theories can be classified into two categories: (i) based on composition and (ii) based on morphology. Most of these theories assume perfect adhesion between the phases and a macroscopically homogeneous and isotropic sample. The different models examined to predict the mechanical behavior of the blends include the parallel, series, Halpin−Tsai, Takayanagi, Kerner, and Kunori models. The highest upper bound parallel model (Voigt prediction) is given by the rule of mixtures. The application of these models requires the knowledge of the experimental mechanical behavior of the pure component polymers TPU and PP. The two simple models are the so-called parallel and series models, which should represent the upper and lower bounds of the tensile strength predictions. The highest upper bound parallel model is given by the rule of mixtures as follows E U = E1ϕ1 + E2ϕ2

This model has also been successfully applied by several researchers to systems of polymer composites. It was reported that this model was also useful in determining the properties of polymer blends that contain both continuous and discontinuous phases. Takayanagi proposed a series-parallel model48,49 in which the concept of percolation was introduced. It is a phenomenological model consisting of mixing rule between two simple models involving connection in series (Reuss prediction) or in parallel (Voigt prediction) of the components. According to this model, E = (1 − λ)E1 + λ[(1 − ϕ)/E1 + (ϕ/E2)]−1

E1 is the property of the matrix phase, E2 is the property of the dispersed phase, and ϕ is the volume fraction of the dispersed phase and is related to the degree of series-parallel coupling. The degree of parallel coupling of the model can be expressed by %parallel = [ϕ(1 − λ)/(1 − ϕλ)] × 100 50

(1)

σb = σm(1 − Ad )

σb = σm(1 − Ad ) + σdAd

(2)

(E1/E2) − 1 (E1/E2) + Ai

(8)

where σm and σd are the tensile strength of the matrix and dispersed phase, respectively. If the fracture propagates mainly through the interface, eq 8 may be modified as σb = σm(1 − ϕd)2/3 + σdϕd 2/3

(9)

And, if the fracture propagates through the matrix, then the equation becomes σb = σm(1 − ϕd) + σdϕd

(10)

Here ϕd is the volume fraction of the dispersed phase. This equation is the same as the parallel model equation. Another important model for perfect adhesion is the Kerner equation.52 Kerner derived a theory for a matrix with spherical inclusions, when the system is isotropic and the adhesion between the two phases is perfect. The upper and lower bounds of this two phase polymer materials are given by eq 11 ⎡ ϕ E /[(7 − 5v )E + (8 − 10v )E ] + ϕ /15(1 − v ) ⎤ d m m m d m m ⎥ E b = Em ⎢ d ⎢⎣ ϕdEm /[(7 − 5vm)Em + (8 − 10vm)Ed] + ϕm /15(1 − vm) ⎥⎦

(3)

where B1 =

(7)

where σm is the tensile strength of the matrix and Ad represents the area of the fraction occupied by the dispersed phase in a transversal cross-section. Kunori and Geil51 assumed that when strong adhesive force exists between the blend components, the dispersed phase would contribute to the strength of the blend. Therefore, eq 7 may be modified as follows

In these models EU is any mechanical property of the blend in the upper bound parallel model and EL the moduli of the blend in the series model. E1 and E2 are the mechanical properties of components 1 and 2, respectively; ϕ1 and ϕ2 are their corresponding volume fractions. For both these models, no morphology is required, but strain or stress can be continuous across the interface, and Poisson’s ratio is the same for both phases. Halpin−Tsai and porosity models successfully represent the mechanical behavior of the blends. The model results allow a physical interpretation of the role of the dispersed phase in terms of the aspect ratio and of the stress concentration factors associated with the dispersed particles. According to Halpin− Tsai equation46,47 (1 + A1B1ϕ2) E1 = E (1 − B1ϕ2)

(6)

51

According to Nielsen and Kunori and Geil, tensile failure of a blend is the result of the adhesion failure between the blend components. When there is no adhesive force between the blend components, the tensile strength of the blend (σb) may be written as,

This model is applicable to materials in which the components are connected parallel to one another so that the applied stress lengthens each component to the same extent. In the lowestlower bound series model, the blend components are arranged in series (Reuss prediction) perpendicular to the direction of the applied force. The modulus prediction is given by the inverse rule of mixtures 1/E L = ϕ1/E1 + ϕ2 /E2

(5)

(11)

where, E is a mechanical property, ν is the Poisson’s ration of the matrix, and ϕ is the volume fraction. The subscripts, m, d and b stand for the matrix, dispersed phase, and blend respectively. The properties of an uncompatible blend usually are in between the upper bound parallel model (MU) and the lower bound series model (ML). According to Coran’s equation

(4)

In the Halpin−Tsai equation, subscripts 1 and 2 refer to the continuous and dispersed phases, respectively. The constant Ai is defined by the morphology of the system. For elastomer domains dispersed in a continuous hard matrix, Ai is 0.66. Instead, if the hard material forms the dispersed phase, Ai is 1.5. 13384



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Figure 4. Theoretical model for uncompatibilized (a) ester and (b) ether-TPU blends (tensile strength).

Figure 5. Theoretical model for uncompatibilized (a) ester and (b) ether-TPU blends (tensile modulus).

Figure 6. Stress−strain curves for (a) ester-TPU blends (compatibilized) and (b) ether-TPU blends (compatibilized)

M = f (MU − ML) + M

ϕ1

(12)

1+ε

where f can vary between zero and unity. The value of f is given by f = V n H(nVS + 1)

(

E1 E

)

−1

ϕ2

+ 1−ε

( EE

2

)

−1

=1 (14)

ε = [ 2(4 − 5ν)]/[15(1 − ν)], where ν is the Poisson’s ratio of the blend composites. The Budiansky model is found to fit more with the experimental data. Figures 4 and 5 show comparison of theoretical and experimental data for tensile strength and tensile moduli of ester- and ether-TPU/PP blends as functions of volume fractions of PP. The predictions of these theories have been made with E1 = 3.9 for ester-TPU, E1 = 4.0 for ether-TPU and E2 = 32.6 MPa for the tensile strength of TPU and PP respectively. The predictions of these theories also have been

(13)

where n contains the aspects of phase morphology. VH and VS are the volume fractions of hard phase and soft phase, respectively. Budiansky model assumes that the dispersed phase becomes the continuous one at some point in the mid composition range. According to this model 13385



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Table 4. Mechanical Properties of Compatibilized Ester-TPU-Based Blends stress at % strain (MPa) blend composition

impact strength Izod (J M−1)

hardness shore D

TPU/PP (70/30) TPU/PP/MA (70/25/5) TPU/PP(nano)/MA (70/25/5) TPU(nano)/PP/MA (70/25/5)

NF NF NF NF

52 53 53 55

tensile modulus (MPa) 253 298 356 430

± ± ± ±

5 5 5 5

20% 5.3 5.7 7.7 7.9

± ± ± ±

100%

0.05 0.05 0.05 0.05

± ± ± ±

5.7 6.7 8.0 10.1

0.05 0.05 0.05 0.05

200% 6.4 6.9 8.4 10.3

± ± ± ±

0.05 0.05 0.05 0.05

Table 5. Mechanical Properties of Compatibilized Ether-TPU-Based Blends stress at % strain (MPa) blend composition

impact strength Izod (J M−1)

hardness shore D

TPU/PP (70/30) TPU/PP/MA (70/25/5) TPU/PP(nano)/MA (70/25/5) TPU(nano)/PP/MA (70/25/5)

NF NF NF NF

52 53 53 54

made with E1 = 90 (both ester- and ether-TPU) and E2 = 1450 MPa for the tensile modulus of TPU and PP, respectively. In the experimental curve, there is only marginal increase in tensile strength with the addition of up to 30 wt % of PP to TPU. A very large increase in tensile strength beyond phase inversion, around 80 wt % can be observed. In immiscible blends, the tensile strength usually depends on the particle size of the dispersed phase. Smaller and more uniformly distributed particles are more effective in initiating crazes and terminating before they develop into catastrophic sizes. The lower values for the tensile strength in this blend system up to 30% PP may be due to poor interfacial adhesion between the dispersed PP phase and the continuous TPU matrix. The poor interfacial adhesion causes premature failure as a result of the usual crackopening mechanism. The experimental values of the tensile strength are not close to any of the aforementioned models. The experimental values lie between the series and parallel models predictions. The moderate fit indicated the absence of any favorable interaction between the TPU and PP phases. 3.4.3. Mechanical Properties of Compatibilized Blends with Nanoclay Addition. The effect of compatibilizers on the mechanical properties of immiscible polymer blends is shown in Figure 6. The uncompatibilized TPU/PP(70/30) blend has low tensile strength and exhibits considerable improvement by the addition of compatibilizer. The increase in tensile strength is due to the increase in interfacial adhesion between TPU and PP. This observation complements the other studies discussed in other sections. The experimental results for compatibilized blends are given in Tables 4 and 5. The stress required for 20, 100, and 200% strain improved with compatibilization, which is also attributed to improved interfacial interactions. Tensile tests indicate that ester-TPU materials have better tensile properties than ether-TPU materials. Compatibilized ester- and ether-TPU blend nanocomposites exhibited higher stresses at the respective elongations than the uncompatibilized blends. The tensile test results substantiate the compatibilization offered by MA-g-PP as seen in the other results. Ester-TPU (nano)/PP/MA exhibited the best tensile properties as shown in the Tables 4 and 5. Nanoclay filled thermoplastic polyurethane (TPU)/polypropylene (PP) blends compatibilized with maleic anhydride grafted polypropylene (MA-g-PP) have been studied with emphasis on sequence of nanoclay addition. In sequence I [ TPU(nano)/PP/MA-g-PP], nanoclay was first added to TPU and this nano composite was blended with PP, using MA-g-PP

tensile modulus (MPa) 274 310 367 436

± ± ± ±

5 5 5 5

20% 5.7 6.1 7.1 7.5

± ± ± ±

0.05 0.05 0.05 0.05

100% 6.6 6.9 7.8 9.5

± ± ± ±

0.05 0.05 0.05 0.05

200% 6.7 7.2 8.3 9.8

± ± ± ±

0.05 0.05 0.05 0.05

as compatibilizer. In the case of sequence II [TPU/PP(nano)/ MA-g-PP], nanoclay was added first to PP and blended with TPU, using MA-g-PP as compatibilizer. The results indicated that sequence I imparted greater compatibility to the polymers and better nanoclay dispersion than sequence II. Overall the TPU (nano)/PP/MA-g-PP blend system showed better dispersion than TPU/PP(nano)/MA-g-PP. Nanoclay was used to reduce the surface energy of the TPU hard segments and made them more compatible with the nonpolar PP. More miscible blends have been obtained by using MA-g-PP as the compatibilizer. Better dispersion of organoclay was due to two reasons. First, maleic anhydride formed hydrogen bonds with the hydroxyl groups of the silicate layers. Second, there is a possible chemical reaction between maleic anhydride and the urethane linkages in the TPU hard segments. Compared to the sequence II blend nanocomposites, the sequence I blends showed better miscibility as confirmed by tensile test. The clear difference between sequence I and sequence II is that in sequence I nanoclay is fully utilized by changing the surface tension of the TPU hard segment. This change in surface tension favored better dispersion of PP in TPU material, and a hydroxyl group of silicate layers formed by hydrogen bonding with the carbonyl group of TPU and maleic anhydride moieties. 3.4.4. Other Mechanical Properties. 3.4.4.1. Flexural Modulus. Flexural modulus is a measure of the stiffness of the material. The flexural modulus is represented by the slope of the initial straight-line portion of the stress−strain curve and is calculated by dividing the change in stress by the corresponding change in strain. Flexural modulus values shown in figures (see Figures S11 and S12 in the Supporting Information) indicate that compatibilization improved the material stiffness. 3.4.4.2. Abrasion Resistance. Abrasion resistance data for ester-TPU and ether-TPU blend nanocomposites were studied (Table S1, Supporting Information). It is observed that esterTPU (nano)/PP/MA offered the best abrasion resistance. The addition of compatibilizer and nanoclay led to chemical reaction and increased hydrogen bonding in the blend. Because of this, interface adhesion improved in the blend system and gave better resistance toward rate of removal of material. 3.5. DMA Analysis. 3.5.1. Uncompatibilized Blends. Figure 7 shows the wide temperature range DMA measurements [ log storage modulus, loss modulus and loss tangent factor (tan δ)] for ester-TPU/PP blends at 10 Hz. TPU 13386



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damping is also low because molecular segments are very free to move about and there is very little resistance for flow. Thus, when the segments are either frozen in or are free to move, damping is low. The Tg due to TPU is seen at −60 °C. There is change in Tg value due to TPU phase, on the addition of PP into TPU. The Tg due to TPU transitions obtained from E″max and tan δmax are given in Tables S2 and S3 (Supporting Information) for ester − and ether- based TPU blends respectively. There is considerable change in Tg value due to TPU phase, on the addition of PP into TPU. Panels a and b in Figure 7 show the temperature dependence of log storage and loss modulus of the uncompatibilized samples from −100 to 200 °C. The effects of temperature on tan δ values of the blends are evident in Figure 7c. At very low temperature modulus of the blends is high. The modulus decreased with increase in temperature and finally levels off at high temperature. The storage and loss moduli values of TPU are high at low temperature and decreased sharply in the region of −60 °C indicating distinct transition from glassy to rubbery region. TPU rich blend [TPU/PP(70/30)] also showed same trend. At low temperature, the molecules are frozen in and exhibit very high modulus. Some secondary relaxations occur after the glassy transitions. TPU exhibited very low modulus in the rubbery plateau. The modulus values finally level off at higher temperatures. The decrease in modulus above Tg is more pronounced in PP rich blends. Around the glass transition temperature of TPU the blends are in the glassy state and the modulus is high. The E′ intensity nearly followed the composition. Thus it can be concluded that the stiffness of the samples decreased with TPU content. A high modulus observed in [TPU/PP(70/30)] was mainly due to major contribution from PP. A sharp increase in damping value in [TPU/PP(70/30)] blend was due to the major contribution of TPU phase. It is observed that at room temperature, the storage and loss moduli of PP are greater than those for TPU due to the high brittleness of PP because of its high Tg. Thus the unmodified TPU/PP shows typical behavior for immiscible blends. The chain mobility of the polymer can be understood from the area under the loss modulus versus temperature curve.53 As expected, loss modulus increased up to transition zone, until they reached a maximum and then decreased with temperature. The damping behavior of the blends increased with increase in concentration of TPU. The same trend was observed in etherTPU/PP blends (see Figure S13 in the Supporting Information). McGrum and colleagues54 demonstrated that the tan δ curve of PP exhibited three relaxations localized in the vicinity of −80 (γ), 10 (β), and 108 °C (α). However DMA analysis (Figure 7c) did not show any separate tan δ peak corresponding to the β relaxation [glass transition temperature of PP (Tg)], but more definite tan δ peak was observed corresponding to Tg at approximately at 108 °C. The tan δ value at around 108 °C, corresponding to the α transition (Tα), is believed to be the result of molecular motions which resist the softening effect of the applied heat. Whereas Tg reflects mobility within the amorphous regions, Tα dictates the onset of segmental motion within the crystalline regions.55−57 Clear tan δ peak was observed corresponding to the relaxation for the TPU/PP blend composition of 0/100 and 30/70, but not in the case of 50/50, 70/30 and 0/100. The tan δ peak corresponding Tg for TPU/PP (0/100) and (30/70) blend composition are given in

Figure 7. (a) Log storage modulus of ester-TPU-based uncompatibilized blends. (b) Loss modulus of ester-TPU-based uncompatibilized blends. (c) Tan δ of ester-TPU-based uncompatibilized blends.

exhibited the minimum and intermediate values for the compatibilized blends. At low temperature, since the polymers are in the glassy state, effect of temperature on modulus improvement could not be observed for all compositions. The highest damping value is seen for TPU. Damping value decreased upon incorporation of PP, as seen in the figure and has higher loss tangent values. The damping is low below Tg because the chain segments are frozen in. Below Tg, the deformations are thus mainly elastic and molecular slip resulting in viscous flow is low. As temperature increases, damping goes through a maximum near Tg, in the transition region and then a minimum in the rubbery region. Above Tg where rubbery region exists, the 13387



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Figure 8. TPU-based blends theoretical model of (a) ester-based blends, (b) ether-based blends as a function of volume fraction of PP at 10 °C; TPU/PP (70/30) theoretical model of (c) ester-based blends, (d) ether-based blends as a function of temperature.

the applied force. The modulus prediction is given by the inverse rule of mixtures discussed in eq 2. For both these models, no morphology is required, but strain or stress can be continuous across the interface, and Poisson’s ratio is the same for both phases. Kerner52 proposed expressions for gross bulk and shear moduli of multi component systems for spherical particulate filled isotropic composites. Mazich et al.,58,59 Bandyopadhyay et al.,60 George et al.61 have applied these models for predicting dynamic properties of rubber−rubber blends. Oommen et al.62 modeled the viscoelastic properties of natural rubber/poly(methy1 methacrylate) blends (NR/PMMA). Kerner model is described in eq 11. This equation was found to represent dynamic data for a variety of systems of the type soft inclusion/ hard matrix reasonably well. Poisson ratio for homopolymers was assumed to be 0.48 for TPU and 0.40 for PP.

Tables S2 and S3 (Supporting Information). Increase of TPU portion eliminates the β relaxation in the blend system. 3.5.2. Comparison of Experimental Data of DMA Analysis with Theory. Various models exist for predicting elastic properties of polymeric materials. Different models like parallel, series, Halpin−Tsai, Kerner, Davies, and Budiansky were used to predict the mechanical behavior of the blends. The two simple models are the so-called parallel and series models, which could represent the upper and lower bounds of the tensile strength predictions. The highest upper bound parallel model is given by the rule of mixtures discussed in eq 1. This model is applicable to materials in which the components are connected parallel to one another so that the applied stress elongates each component to the same extent. In the lowestlower bound series model, the blend components are arranged in series (Reuss prediction) perpendicular to the direction of 13388



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Panels a and b in Figure 8 gives a comparison of theoretical and experimental values of storage modulus of the ester- and ether-TPU based blends as a function of volume fraction of PP at 10 °C and frequency of 10 Hz . The experimental values are in between series and parallel models. The predictions of these theories have been made with E1 = 293 for ester, E1 = 331 for ether and E2 = 6028 MPa for the storage modulus for TPU and PP, respectively. Panels c and d in Figure 8 show comparison of the variation in theoretical and experimental curves of the storage modulus of ester- and ether-TPU/PP (70/30) for different temperatures. 3.5.3. Effect of Compatibilization with Nanoclay. Preparation of ester- and ether-TPU/C10A nanocomposites by direct melt blending, using 3 wt % Cloisite 10A (organically modified montmorillonite clay) as the nanoscale reinforcement.63−66 Nanocomposites with PP using MA-g-PP as the compatibilizer were prepared by melt blending. Low-temperature DMA measurements on a few selected samples of the compatibilized blends were carried out at a frequency of 10 Hz. The effect of temperature on log storage modulus, loss modulus, and tan δ of MA-g-PP compatibilized blends are shown in Figure 9 for ester-TPU/PP blends. A single Tg for the blends is an indication of miscibility, whereas two separate Tg values indicate immiscibility of the system. Experimental evidence of miscibility is often found when a single and a sharp glass transition temperature, Tg, is observed in between the Tg values of the individual components. In the glass transition region of linear polymers, the storage modulus usually decreased by three to 4 orders of magnitude over a temperature range of 20−30 °C. Also, in the glass transition region, E″ and tan δ go through maxima. The quantities E″ and tan δ can be utilized in mechanical damping applications to reduce mechanical vibrations and noise emission. Tg from E″max and tan δmax values are given in Table 6. Similar to the uncompatibilized blends, the compatibilized blends show a decrease in modulus with temperature. The dynamic mechanical parameters (log storage modulus) for the various blend nanocomposites are plotted as a function of temperature, at a constant frequency of 10 Hz (Figure 9a). The storage moduli at −50, 0, 20, and 30 °C are shown in Tables S4 and S5 (Supporting Information). The storage modulus at −40 °C of ester-TPU/PP blends, without nanoclay but compatibilized with the same weight percent MA-g-PP, prepared under identical conditions, is reported to be 7.23 GPa.30 Thus, a 3 wt % C10A reinforcement increased the storage modulus of the blend by ∼40%. At room temperature, the storage modulus value of the ester-TPU (nano)/ PP/MA blend nanocomposite is more than that of the other blends. It is possible that well-dispersed clay platelets resulted in an increase of storage modulus. The dissipation factor (tan δ) of the materials is presented as a function of temperature in Figure 9c. The tan δ peak is associated with the soft segment glass transition temperature and the peak positions are given in Table S5 (see the Supporting Information). The temperature corresponding to the tan δ peak increases in the order: ester-TPU/PP< esterTPU/PP/MA-g-PP < ester-TPU(nano)/PP/MA. This is attributed to the increasing order of dispersion for the blends. In DMA testing, one way of calculating Tg is based on tan δ. Each tan δ peak temperature indicates the Tg of TPU soft segment for that particular composition. A shift in this peak indicates an effect on the dispersion level of blend system. The temperature corresponding to the tan δ peak increases in the

Figure 9. (a) Log storage modulus of compatibilized ester-TPU based blends. (b) Loss modulus of compatibilized ester-TPU-based blends. (c) Tan δ of compatibilized ester-TPU based blends.

Table 6. Tg of Compatibilized Ester TPU/PP (70/30) Blends ester

13389

ether

material composition

Tg from loss modulus (°C)

Tg from tan δ (°C)

Tg from loss modulus (°C)

Tg from tan δ (°C)

TPU/PP 70/30 TPU/PP/MA-gpp TPU/PP(nano)/ MA-g-PP TPU(nano)/PP/ MA-g-PP

−34 −26

−22 −15

−39 −34

−25 −18

−24

−11

−32

−14

−19

−7

−27

−10



Article

segments. This may lead to more extensive hydrogen bonding in the blend system thus improving the miscibility. The effect of temperature on log storage modulus, loss modulus and tan δ of MA-g-PP compatibilized blends are shown in Figure S14 (Supporting Information) for ether-TPU/PP blends. 3.5.4. Effect of Sequence of Nanoclay Addition on TPU/PP Blend. In sequence I, TPU/C10A nanocomposites were prepared and subsequently blended with PP using MA-g-PP as compatibilizer. In sequence II, nanoclay was added to PP by similar extruder melt blending and subsequently melt blended with TPU using MA-g-PP as compatibilizer. Compared to sequence II blend nanocomposites, the sequence I shows better miscibility (see Figure S15 and Table S6 in the Supporting Information). Unlike in sequence II, the effect of nanoclay is fully utilized for changing the surface tension of TPU hard segment in sequence I. The hydroxyl group of silicate layers forms hydrogen bonding with the carbonyl group of TPU and maleic anhydride moieties leading to reduction in surface tension of TPU hard segment. This change in surface tension favors better dispersion of PP in TPU material, and also the hydroxyl group of the silicate layers forms hydrogen bonds with the carbonyl groups of TPU and maleic anhydride moieties. In the case of sequence II, the dispersion and the reaction of nanoclay with TPU was reduced because nanoclay was first added to PP.

order: TPU/PP < TPU/PP/MA-g-PP < TPU/PP(nano)/MAg-PP < TPU (nano)/PP/MA-g-PP (−22 °C < −15 °C< −11 °C < −7 °C for ester and −25 °C < −18 °C< −14 °C < −10 °C for ether). The same order was observed in DSC experiment also (−61.5 °C < −59.3 °C < - 53.5 °C). The Tg may have different numerical values but it follows the same order. DMA and DSC experiments are based on different concepts and hence the numerical values do not match. Ether based blends also shows the same trend with less shift in Tg values. All the blends showed a sharp decrease in E′ around −60 °C. The MA-g-PP compatibilized blends showed an increase in storage and loss modulus at 5% compatibilizer loading compared to the uncompatibilized counterpart TPU/PP (70/ 30) at the same temperature. The higher value for storage modulus observed at this composition can be due to the increased interaction between TPU and PP phases upon the addition of compatibilizer. Thus it is important to note that there is a sharp increase in modulus with very small amount of the compatibilizer followed by a marginal decrease, as the copolymer content is increased above the equilibrium concentration. Thus addition of compatibilizer makes the blend technologically compatible, but molecular level miscibility is not achieved. At higher loading of compatibilizer, modulus value decreases. This is due to the formation of agglomerates of the compatibilizer molecule. There is a change in Tg values due to TPU on adding MA-gPP to the blend system. The two-step curves in the figure for the blends are due to two-phase morphology indicating immiscibility. Thus the transitions due to TPU and PP phases indicate that addition of compatibilizers does not make the system single phase. Compatibilizer addition did not promote molecular level miscibility. This observation is in agreement with conclusions made by Paul37 that if two polymers are far from being miscible, then no compatibilizer is likely to make the system single phase. In a completely immiscible system, the main role of the compatibilizer is to act as an interfacial agent. The Tg corresponding to TPU transition is shifted to higher temperature upon compatibilization. MA-g-PP with nanoclay compatibilized blends showed maximum shift. Nanoclay reinforcement, besides giving substantial increase in modulus and tensile strength, also functions as a surface modifier for TPU hard segments. The reduced interfacial tension between the thermoplastic polyurethane and the polypropylene due to the incorporation of nanoclay gives better compatible blends. Compatibilization effect is further improved by introducing functionalized PP (MA-g-PP) in the nanoclay containing blends. At room temperature, the storage modulus value of the ester-TPU blend nanocomposite is more than that of the ether-TPU blend nanocomposite. The high polarity difference between thermoplastic polyurethane and polypropylene limits the miscibility of their blends. Nanoclay was used to reduce the surface energy of the TPU hard segments and makes them more compatible with the nonpolar PP. More miscible blends have been obtained by using MA-g-PP as the compatibilizer. Better dispersion of organoclay may be attributed to two reasons. First, maleic anhydride forms hydrogen bonds with the hydroxyl groups of the silicate layers. Second, there is a possible chemical reaction between maleic anhydride and the urethane linkages in the TPU hard segments. Compared to the etherTPU based blend nanocomposites, the ester-TPU blends show better miscibility as confirmed by DMA analysis. Ester-TPU has carbonyl groups both in the polyol segments and TPU hard

4. CONCLUSION Investigation of mechanical and dynamic mechanical properties of thermoplastic elastomers from TPU/PP blends was carried out with special reference to the effects of nanoclay addition, sequence of nanoclay addition, blend ratio and compatibilization. Nanoclay addition and compatibilizer were key factors that determined the mechanical and dynamic mechanical properties of the blends. The observed properties were correlated with morphology of the blends. Different theoretical models were used to fit the experimental mechanical properties due to high interfacial adhesion and compatibility. Compatibilization of the blends using MA-g-PP and nanoclay improved the mechanical properties. Based on the improvement in mechanical properties on compatibilization MA-g-PP with nanoclay was found to be superior to other compositions. Very specifically, storage modulus, loss modulus, and tan δ were carefully measured for the systems and changes could be observed in these values for the systems compatibilized with MA-g-PP and nanoclay incorporation. There is an optimum concentration for the compatibilizer (5% MA-g-PP) which gave maximum property improvement. Ester-TPU(nano)/PP/MA blend nanocomposite was found to have very good overall performance and is a potential candidate for cost-effective, high strength applications of thermoplastic polyurethane. This is because maleic anhydride undergoes chemical reaction with the urethane groups in the TPU hard segments and also forms hydrogen bonds with the silicate layers of C10A. Ester-TPU with carbonyl groups both in the polyol segments and urethane hard segments has more extensive hydrogen bonding than ether-TPU. Various composite models were used to fit the experimental viscoelastic data. The Budiansky model was found to closely fit the experimental data. 13390



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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S6, Figures S1−S15, and FTIR analysis (from section 3.3.). This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*Tel: + 91 9600396427; +91 422 2685000. Fax: +91 422 2656274. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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