TPU Inclusion Complex Modified POM: Fabrication of High

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TPU Inclusion Complex Modified POM: Fabrication of High Performance POM Composites with both Excellent Stiffness-Toughness Balance and Thermostability Xin Zheng, Caixia Zhang, Chuntao Luo, Guanghua Tian, Lian Wang, and Yongjin Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04544 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 7, 2016

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Industrial & Engineering Chemistry Research

ABSTRACT The inclusion complex of thermoplastic polyurethane (TPU) and β-cyclodextrins (β-CD) with high TPU contents has been synthesized. The channel structure with large amount of uncovered TPU was confirmed by wide-angle X-ray diffraction (WAXD). High performance poly(oxymethylene) (POM) alloys were fabricated by simply melt mixing neat POM with the synthesized TPU inclusion complex (IC-TPU). The effects of the incorporation of IC-TPU on the structure and properties of POM have been investigated. Scanning electron microscope (SEM) results showed that IC-TPU was dispersed uniformly in POM matrix and there was robust interface between IC-TPU and POM matrix. Tensile tests results indicated the significant improvement in both strength and ductility of the IC-TPU modified POM as compared with neat POM. Moreover, the incorporation of IC-TPU resulted in the drastically enhanced thermal stability of POM. The initial degradation temperature increased as high as 40 °C with the addition of small amount of IC-TPU. The investigation indicated that the IC-TPU exhibited the novel structure with the soft shell (uncovered TPU) and hard core (β-CD covered TPU segments). Such “soft shell-hard core” structure improves not only the elongation at break but also the tensile strength of POM. The superior thermal stability was originated from the synergetic effects of the hydroxyl groups in β-CDs and the amino groups in TPU. The multifunctional effect of the IC-TPU opens the new avenue for the industrial application of POM.

Key words: Poly(oxymethylene) (POM); inclusion complex; stiffness-toughness balance; thermostability

ACS Paragon Plus Environment

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1. INTRODUCTION Strength and ductility are two critically important mechanical properties for polymers. Elastomers are effective toughening agents and usually act as dispersed phase in plastics matrix to improve the toughness of the plastics. 1-5

By simply blending plastics with an elastomer, typically 5 to 20% weight of matrix, it’s easy to get

improvement in elongation at break and impact strength. However, the incorporation of elastomers will inevitably sacrifice rigidity, strength, and modulus of the matrix plastics. Therefore, the investigation on the development of stiffness-toughness balanced polymers has attracted significant attentions. The strategy of using rigid particle toughening polymers was an extensive research topic as it could reinforce and toughen polymers simultaneously. Calcium carbonate (CaCO3) was an often used toughening agent. Fu 6 fabricated the toughened high-density polyethylene (HDPE) with a higher impact strength by incorporating phosphate modified CaCO3. Walter 7 reported that a kaolin-filled HDPE with optimized quality of coupling agent showed an improvement of the stiffness and toughness at the same time. A well-improved stiffness/strength/ toughness balance was achieved in Nylon 6/layered silicate nanocomposites processed with a twin-screw extruder by Kim et al.

8

Usually, stiffness and toughness of rigid particle filled composites are strongly depend on the

particle size polydispersity, interphase adhesion, matrix toughness as well as particle shape. 9 There were also some other routes to achieve the stiffness-toughness balance of composites. Haq et al.

10

fabricated the bio-

based unsaturated polyester (UPE)/ epoxidized methyl linseedate (EML) composites with good stiffnesstoughness balance by the incorporation of nanoclay. According to Li et al.,

11

poly (L-lactide acid)

PLLA/acrylonitrile-butadiene-styrene (ABS) blends exhibited a nice stiffness-toughness balance when the reactive styrene/acrylonitrile/glycidyl methacrylate copolymer (SAN-GMA) was used as the compatibilizer. On the other hand, the double incorporation of elastomers and rigid fillers has also been used as an effective route for high performance materials with good stiffness-toughness balance. Tjong et al.

12

found out that

montmorillonite (MMT)-reinforced polyamide-6 (PA6) can be melt-blended with maleated styrene/ethylene butylene/styrene elastomer (SEBS-g-MA) to maintain the stiffness-toughness balance. Wang

13

prepared

PA6/ethylene-propylene-diene copolymer grafted with maleic anhydride (EPDM-g-MA)/organoclay ternary nanocomposites using melt blending to simultaneously improve the toughness and stiffness of polyamide 6.


3

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Rösch

14

investigated the Polypropylene (PP)/PA-6/maleic-anhydride-grafted poly [styrene-b-(ethene-co-

butene-1)-b-styrene] (SEBS-g-MAH) blends and found that the rigid PA6 core and soft elastomer SEBS shell microparticles had contributed to high toughness and strength without sacrificing stiffness. According to their research, the stiffness/toughness balance was closely related to both volume fraction and stiffness of the elastomer shells. Long

15

had fabricated various PP/elastomer/filler ternary composites, in which the balance

between stiffness and toughness was obtained. He even presented three types of microstructures for thermoplastics/elastomer/rigid filler ternary systems: at the first situation, elastomer particles and filler are separated in the polymer matrix; second, there is core-shell microstructure of elastomer particles with filler core; and the third is a microstructure of mixed results of the former two. They believed that separated microstructure increased modulus and core-shell microstructure increased impact strength. Similar results were reported by Fu,

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a two-step processing method was used, in which a master batch was obtained by first

mixing TPU with CaCO3, then the master batch was melt-blended with POM to get rigid-tough balance material. Poly(oxymethylene) (POM), as one of the most important engineering thermoplastics, has a wide range of applications in electronics, precision instruments and automotive industry due to its high mechanical strength, low friction, excellent dimensional stability and abrasion resistance.

17-20

However, the brittleness at room

temperature together with the poor thermal stability are the two main shortcomings of POM and often exert great limitations for the applications. extending its application range.

24-27

21-23

Considerable efforts have been made on toughening POM and

Blending POM with an elastomer is an efficient way to improve the

toughness of POM. 28-30 Of all the elastomers used, thermoplastic polyurethane (TPU) was considered to be the best modifier for POM with great improvement in toughness. 31-34 TPU has good compatibility with POM due to the hydrogen bonding between the ether bonds of POM and the amino groups of TPU, however, the improvement in elongation of POM/TPU composites is at the expense of strength and modulus of POM matrix. 35-36

On the other hand, extensive investigations have also been carried out to enhance the thermostability of

POM.

37-40

Most of the stabilizers contain the crucial components of the formaldehyde absorbents, which are

nitrogenous compounds or compounds with hydroxyl groups. Melamine and dicyandiamide are two mainly used formaldehyde absorbents in industry. However, this kind of stabilizer with low molecular weight is easy


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to volatilize and migrate during mixing, leading to reduced stabilizing efficiency. Few investigations have been made on the synergistic effects of amino and hydroxyl groups. 41 In recent years, much attention have been given to supramolecular complexes due to their special “hostguest” structure.

42-43

The most commonly used host material is cyclodextrins (CD) . CDs are cyclic

oligosaccharides represented as shallow truncated cones, in which 6, 7 and 8 glucose units are linked by α-1, 4 glucosidic bonds and named as α-, β-, and γ-CD, respectively.

44

The hydrophobic cavities enable them to

include a variety of hydrophobic molecules via noncovalent interactions. Because of the long chain nature of polymers, polymer-CD-inclusion complexes (polymer-ICs) are practically channel type structure.

45-46

The

guest polymer chains are much restricted in the narrow channels created by CD lattice and the guest chains tend to form highly extended conformations.

47

Researchers have tried to use polymer-ICs as additives for

polymers in order to improve their properties, such as miscibility,

48

biodegradation,

49

and crystallization.

50

The normally immiscible PLLA/poly (ε-caprolactone) (PCL) blends could be intimately compatible when PLLA/PCL-α-CD IC were obtained and afterward washed with a solvent (H2O) for the α-CD host. 51 Dong et al.

52

prepared the poly (butylene succinate) inclusion complex (PBS-IC) with α-CD and they further found

that the PBS-IC could greatly accelerate the crystallization of PBS. So far, polymer-ICs have not been exploited for other applications in terms of the polymer modifications. Motivated by the modification investigations of POM and the novel structures and properties of polymer-ICs, we considered that the TPU-β-CD inclusion complex (IC-TPU) should be the multi-functional modifier for POM. By well controlling stoichiometric ratio, novel IC-TPU structure with rigid core of β-CD and included chain segment surrounded by soft shell of uncovered TPU was expected (as shown in Figure 1). Therefore, the hard core would strengthen POM matrix while the soft shell would toughen the matrix of POM with the good interface. Moreover, the hydroxyl groups of β-CD and amino groups of TPU can stabilize POM very well. Morphology and properties of POM composites with small amount of IC-TPU addition were investigated. It was found that high performance POM composites with simultaneously excellent stiffness-toughness balance and superior thermostability were achieved. It’s also the first time to use polymer-ICs as the multifunctional modification agents for engineering plastics.


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Uncovered TPU

Included chain segments and the channel structure of β-CD

Figure 1. The schematic structure of the designed TPU-β-CD inclusion complex.

2. EXPERIMENTAL SECTION Materials and Sample Preparation. The POM (MC 90) samples used in this work were kindly provided by Shenhua Co., Ltd., China. Melt flow index is 9.23 g/10 min. The weight-average molar mass and molecular polydispersity of the POM sample are Mw=174,300 g/mol and Mw/Mn=2.19. The thermoplastic polyurethane (TPU, WHT-1185) used in this study was purchased from Yantai Wanhua Polyurethanes Co. , Ltd. , China. The density of TPU is 1.19 g/cm3, tensile strength is 33 MPa and Tg is -35 °C. β-CD with purity≧98% was from Aladdin Industrial Inc.. N, N-Dimethylformamide (DMF) and methanol were analytical reagent which were bought from ShangHai LingFeng Chemical Reagent Co., Ltd., China. β-CD was recrystallized before using, all other materials were used as obtained from the suppliers. All materials were vacuum-dried at 80 °C for 12 h before using. IC-TPU was synthesized by blending β-CD with TPU in DMF. Briefly, β-CD (10 g) was dissolved in 60 mL of DMF in a 250 mL round bottom flask at room temperature, TPU (10 g) was dissolved in 100 mL DMF and then added dropwise to β-CD solution, the mixture was hold stand overnight at room temperature after stirred at 70 °C for 4 h. The IC-TPU was precipitated after the addition of excess amount of methanol (800 mL). After


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filtering the precipitation out, dried it under vacuum at 80 °C overnight. The obtained IC-TPU was clumped as whole and cut to small pieces for the further use. All composites were prepared using a mixer (Haake Polylab QC) with a twin screw at a rotation speed of 20 rpm at 190 °C for 2 min and then with rotation speed raised to 50 rpm for 5 min. After blending, all the samples were then hot-pressed at 190 °C under a 10 MPa pressure for 3 min to a film with a thickness of 500 µm, followed by quenched (cool-pressing) for 1.5 min at room temperature. The obtained films were used for the following characterization. Structural Characterization. A Siemens type-F X-ray diffractometer (XRD) with a nickel-filtered Cu KR radiation source (= 1. 54 Å) and voltage and current set to 30 kV and 20 mA, respectively, was operated at a scan rate of 2θ=1 °/min, between 2θ = 5 ° and 40 °, to obtain the film diffraction. Morphology of the samples was observed by field-emission scanning electron microscope (FESEM) . A Hitachi S-4800 SEM system was used for SEM measurements at an accelerating voltage of 10 kV. All the samples were fractured after immersion in liquid nitrogen for about 15 min. The sizes of the dispersed domains were counted and calculated by Nano Measurer 1.2 software. The phase structure of the composites was also observed directly using a transmission electron microscope (TEM) (Hitachi HT7700) operating at an acceleration voltage of 80 kV. The blend samples were ultramicrotomed at −120 °C to a section with a thickness of about 70 nm, and then treated with OsO4 overnight followed by RuO4 for 10 min in order to selectively stain the TPU phase and β-CD. A E-Sweep model Atomic Force Microscope (AFM) was used to image phase diagrams of trimming specimens in nanometer scales. The test was operated in tapping mode using Si3N4 cantilever with force constant of 2 N·m-1 (Nanosensors). In phase images, the darker region corresponding to lower modulus component while the brighter region relating to higher modulus component. Thermal Analysis. Thermal gravimetrical analysis (TGA) was done on TA Instruments TGA Q500. The sample was put into 40 µL platinum pan and heated from 30 to 600 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Dynamic mechanical analysis (DMA) was carried out with a TA Instruments Model Q800 apparatus in the Multi-Frequency-Strain mode. Dynamic loss (tan δ) was determined at a frequency of 5 Hz and a heating rate of 3 °C/min, as a function of temperature (from −100 to +190 °C).


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Differential scanning calorimetry (DSC) measurements were carried out on a DSC Q2000 (TA Instrument) in an inert atmosphere of nitrogen at a heating or cooling rate of 10 °C·min−1 with a temperature range from −20 to +200 °C. Mechanical Properties. Tensile tests were carried out according to the ASTMD 412−80 test method, using dumbbell-shaped samples punched out from the molded sheets. The tests were performed using a tensile testing machine (Instron, Model 5966) at a crosshead speed of 10 mm/min at 20 °C and 50% relative humidity. At least three specimens were tested for each sample. The total work of fracture (Wf) was used to evaluate the fracture toughness of these samples. Wf was determined by the expression: Wf = e/A, where e represents the energy of producing the crack, while A means crack’s area. The area under the strain-stress curve was calculated as the value of e by Origin Pro8.

3. RESULTS Formation of IC-TPU. We firstly prepared the IC-TPU with the weight ratio of 1:1 by using the DMF as the solvent. WAXD is widely used in the study of inclusion complex for the assembled structure. According to Harada, there were three types of ICs crystal structure: cage, channel, and layer. 53 The crystalline structures of neat TPU, IC-TPU and raw β-CD were studied using WAXD and the patterns were presented in Figure 2. Neat TPU is amorphous with only the amorphous halo in the WAXD profile. Major peaks at 9.2 °, 12.8 °, 13.5 ° and 17.8 ° indicated the cage structure of raw β-CD. 54 The fact that two diffraction peaks at 11.8 ° and 17.9 ° were observed indicated the formation of the channel structure between β-CD and TPU molecular chains.

55

Moreover, it’s clearly that the IC-TPU sample is not the simple mixture of cyclodextrin and TPU because ICTPU shows totally different diffraction peaks from the β-CD.


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Intensity(a.u.)


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NEAT TPU 210

17.9

002

11.8

IC-TPU 12.8 9.2

17.8 13.5

19.6 21.3 22.8 24.3

β -CD 36.0

5

10

15

20

25

30

35

40

2θ(°) Figure 2. Wide-angle X-ray diffraction patterns of neat TPU, IC-TPU and raw β-CD.

Morphology and Properties of POM/IC-TPU composites. The synthesized IC-TPU has been incorporated into POM by simply melt mixing. The structure and properties of the POM/IC-TPU have been systematically investigated. The comparison has been made on the composites of POM/IC-TPU and the simply ternary mixture of POM/β-CD/TPU. Morphology of POM/IC-TPU composites. Figure 3 shows the SEM images of fracture surface of the POM composites with incorporation of neat TPU, raw β-CD, IC-TPU and TPU/β-CD mixture, respectively. The additives incorporation loading was set to be 1 wt% for all samples. The typical “sea-island” morphology is observed in POM/TPU composites in Figure 3(a). According to statistics of 100 random domains of TPU measured by Nano Measurer 1.2, the domain size of TPU varies from 0.06 to 0.56 µm with the average size of 0.28 µm, the distance between neighboring particles is about 2.06 µm. It’s obvious that the particles are distributed uniformly in matrix with good interface between the TPU domains and the matrix. β-CD is incompatible with POM and a lot of β-CD agglomerates are observed with even 1 wt% loadings. The size of agglomerates ranges from micrometers to several hundred nanometers. The binary incorporation of TPU and β-CD shows a relative fine dispersion of TPU, as shown in Figure 3(c), while the very small amount of β-CD was not observed in this localized image. In consideration of the poor compatibility between POM matrix and the β-CD, we believe that even 0.5 wt% of β-CD would form aggregates. However, only very small domains


9

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with almost no β-CD aggregations were observed when the IC-TPU was incorporated in POM matrix. The morphological structure of POM/IC-TPU is very similar to that of the POM/TPU composites. This indicates that the prepared IC-TPU shows similar compatibility with POM as the neat TPU.

a

b

10µm

c

10µm

d

10µm

10µm

Figure 3. SEM images for fractured surface, (a) POM/TPU, 1 wt%, (b) POM/β-CD, 1 wt%, (c) POM/TPU/βCD, 0.5 wt%, 0.5 wt% and (d) POM/IC-TPU, 1 wt%. The scale bar is 10µm.

Mechanical Properties. The strain-stress curves of the prepared POM composites are shown in Figure 4. Table 1 summarized the main mechanical parameters of all samples. Neat POM exhibits the high strength and low ductility. With the addition of small amount of TPU (1 wt% in this work), the elongation at break of POM increases to about 80%, as compared with 47% of the neat POM. This means that TPU can improve the ductility of POM effectively. However, the yield strength of POM decreases with the addition of even only 1 wt% TPU. This is the typical behavior for the rubber toughened plastics. β-CD is a hydrophilic filler and it is incompatible with POM. The incorporation of β-CD leads to the slightly increased tensile strength, but at the


10


same time decreased the elongation at break greatly. The binary addition of both TPU and β-CD induces the good effects on both the tensile strength and ductility. The POM/β-CD/TPU ternary composites exhibit very similar mechanical performance as the neat POM. Surprisingly, the IC-TPU combines the strengthening effects of β-CD and toughening functions of TPU. The tensile strength and the elongation at break of the POM/ICTPU composites are 57.8 Mpa and 110%, respectively, which are much higher than those of the neat POM. From detailed statistics of the work of fracture, it’s obvious that with the addition of IC-TPU, the toughness of POM/IC-TPU is greatly improved, it would cost 1.4×1010 J·m-2 energy to develop and crack the sample, which is almost 6 times that of neat POM. This means that the POM/IC-TPU composites show perfect stiffness-toughness balance. This is very seldom reported previously. 60 50

Stress(MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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c

e

d

b a

40 30 20 10 0 0

10 20 30 40 50 60 70 80 90 100 110 120

Strain(%) Figure 4. Strain-stress curves for neat POM and composites: (a) neat POM, (b) POM/TPU, 1 wt%, (c) POM/βCD, 1 wt%, (d) POM/TPU/β-CD, 0.5 wt%, 0.5 wt%, and (e) POM/IC-TPU, 1 wt%.


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Table 1. Mechanical Properties of POM and POM composites. Yield strength

Elongation

Tensile strength

Work of fracture

/MPa

at break /%

at break/MPa

/J·m-2

NEAT POM

54.6±2.2

47.1±7.0

43.4±3.8

2.5×109

POM/TPU, 1 wt%

52.4±2.0

80.4±48.3

47.3±1.8

6.3×109

POM/β-CD, 1 wt%

55.2±0.8

20.7±1.5

53.6±1.0

5.8×108

POM/TPU/β-CD, 0.5 wt%, 0.5 wt%

55.3±2.0

41.1±5.9

50.7±2.7

1.9×109

POM/IC-TPU, 1 wt%

57.8±2.7

110.1±65.5

52.1±2.5

1.4×1010

Sample

Dynamic mechanical analysis of POM composites. DMA tests have been performed on neat POM and the POM composites. As shown in Figure 5, there are three relaxations in the DMA profile of neat POM and they are corresponding to the glass transition, interface relaxation and crystal relaxation, respectively.

56

The

relaxation at about -60 °C corresponds to the glass transition of POM in amorphous region. The broad peak at about -1.4 °C has been attributed to the interface relaxation between the crystalline and amorphous regions, while the relaxation in the crystalline regions locates at about 120 °C. For all composites, the Tg keeps almost constant, which means that the modifiers are not thermodynamically miscible with POM matrix, indicating no molecular chain entanglements between the POM matrix and the modifiers. However, for the composites with TPU (POM/TPU, POM/TPU/β-CD, and POM/IC-TPU), obvious shifts in the relaxation of the crystalline regions were observed and the relaxation peaks were also become sharper when compared with that of the neat POM. These results indicate that TPU affects the crystallization behaviors of POM. Specifically, the POM/ICTPU composites have lowest relaxation temperature in the crystalline region, which again indicates the good compatibility between IC-TPU and POM, and this help to improve mechanical properties as well. The distinctive relaxation temperature of POM/IC-TPU composition also convincingly proves that the IC-TPU was not the simple mixing of TPU and β-CD.


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f crystal relaxation e

Tanδ

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a c

0.1 Tg of POM

b d interface relaxation Tg of TPU

0.01 -100

-50

0

50

100

150

Temperature(°C) Figure 5. The loss tangent as a function of temperature with a frequency of 5 Hz for the samples of: (a) neat POM, (b) POM/TPU, 1 wt%, (c) POM/β-CD, 1 wt%, (d) POM/TPU/β-CD, 0.5 wt%, 0.5 wt%, (e) POM/ICTPU, 1 wt%, and (f) neat TPU.

DSC analysis. The effects of TPU, β-CD, and IC-TPU on the crystallization of POM were investigated by DSC using nonisothermal crystallization process. The crystallization temperature (Tc), crystallization enthalpy (∆Hc), melting temperature (Tm), melting enthalpy (∆Hm) and crystallinity (Xc) are summarized in Table 2. All samples were melted at 190 °C for 5 min to erase the thermal history residing in the samples. In Figure 6(a), the DSC cooling curves reveal the relative nucleation abilities. Dong et al.

57

successfully prepared inclusion

complexes between α-CD and poly-(ε-caprolactone) (PCL), poly(ethylene glycol) (PEG), poly(butylene succinate) (PBS) , and investigated the nucleation effects of these inclusion complexes on PCL, PEG, and PBS respectively, they found that PBS with the addition of the inclusion complex of PBS showed a much higher Tc and a narrower range of crystallization temperature, which indicated faster crystallization. In this work, it’s clear that the crystallization rate of neat POM is very fast. Upon cooling at 10 °C/min, the Tc value of POM, which is about 147.3 °C in the pure states, shifts to lower temperature with the addition of TPU, β-CD, and ICTPU, indicating the decreased crystallization rate of POM with the addition of the additives, which may attribute to interactions between POM matrix and dispersed phase. In Figure 6(b), melting temperature remains virtually unchanged. The crystallinity of POM in all samples was determined by equation as below:


13

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X

0

c

= ∆ H m (V m × ∆ H m) ×100% L

where ∆H0m is the theoretical melting enthalpy of POM with a value of 247 J/g, 58 Vm is the weight fraction of matrix in composites. It is seen that the crystallinity shows slightly decreasing with the incorporation of the additives, especially for those samples with the TPU additives. This again indicates the interactions between

Heat Flow Exo Up (W/g)

the POM and TPU. a

NEAT POM POM/TPU, 1% POM/β-CD, 1% POM/TPU/β-CD, 0.5%, 0.5% POM/IC-TPU, 1%

100

125

150

175

200

Temperature(°C)

Heat Flow Exo Up (W/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b NEAT POM POM/TPU, 1% POM/β-CD, 1% POM/TPU/β-CD, 0.5%, 0.5% POM/IC-TPU, 1%

125

150

175

200

Temperature(°C)

Figure 6. DSC first cooling (a) and second heating (after 5 min isothermal for erasing heat history) (b) curves of POM composites at a heating rate of 10 °C/min.


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Table 2. DSC results of POM and POM composites. Sample Name

Tm/°C

△Hm/J·g-1

Tc/°C

△Hc/J·g-1

Xc/%

NEAT POM

165.4

131.1

147.3

125.8

53.1

POM/TPU, 1 wt%

166.2

120.9

145.5

114.4

49.8

POM/β-CD, 1 wt%

165.8

126.4

144.1

117.4

52.0


166.1

123.6

145.2

117.7

50.9

POM/IC-TPU, 1 wt%

166.1

121.7

144.4

113.5

50.1

Thermal Stability. POM has very narrow processing temperature window due to the low thermal stability by the continuous deformaldehyde reaction, which limits the industrial applications. The thermogravimetry analysis (TGA) of the neat POM and the POM composites has been carried out and was shown in Figure 7. The corresponding parameters of the TGA analysis were summarized in Table 3. It is clear that TPU, β-CD and IC-TPU are all effective thermal stabilizers for POM. For neat POM, a sharp decomposition curve was observed with the maximum temperature of 327 °C because of the deformaldehyde reaction. By adding only tiny amount of TPU, the thermal stability was drastically improved and the maximum degradation temperature was 399 °C. It was reported that the amino group in TPU can not only form hydrogen bond with POM but also absorb formaldehyde (FA) released by POM.

59

Besides, TPU can neutralize the formic acid generated by

oxidation of FA because of its alkaline nature. On the other hand, with the addition of polyhydric β-CD, both the initial degradation temperature (T5%) and the maximum degradation temperature increase. It is expected that the hydrophobic cavity of β-CD may absorb the odor of FA gas and the hydroxyl groups can also catch the FA. Interestingly, POM/IC-TPU composites showed the further improvement in thermal stability when compared with the single addition of β-CD and TPU. The temperature at 5% weight loss (T5%) has increased by around 40 °C and T50% was 58 °C higher than that of neat POM. This means that the great synergistic effects have been achieved between the hydroxyl groups and amino groups by the inclusion of TPU in β-CD. Almost the same results have also been obtained in the POM/β-CD/TPU ternary composites. DTG curves gave information on temperature of maximum weight loss for each sample, as shown in Figure 6(b). Most significant shift was observed in POM/IC-TPU sample. Taken mechanical properties into consideration, it is


15

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clear that IC-TPU has multi-functional effects for POM and high performance POM composites were achieved with very tiny amount addition of IC-TPU.

100

TGA

Weight loss(%)

80 60 40 20 0 200

NEAT POM POM/TPU, 1% POM/β-CD, 1% POM/TPU/β -CD, 0.5%, 0.5% POM/IC-TPU, 1%

250

300

350

400

450

500

550

Temperature(°C) DTG

Deriv. Weight(%/°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NEAT POM POM/TPU, 1% POM/β-CD, 1% POM/TPU/β-CD, 0.5%, 0.5% POM/IC-TPU, 1%

200

250

300

350

400

450

500

550

Temperature(°C) Figure 7. The TGA and DTG curves for the degradation of neat POM; POM/TPU, 1 wt%; POM/β-CD, 1 wt%; POM/TPU/β-CD, 0.5 wt%, 0.5 wt%; POM/IC-TPU, 1 wt%. All samples were heated from 30 to 600 °C at a heating rate of 10 °C/min under nitrogen atmosphere.


16


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Page 16 of 26

Table 3. TGA parameters of POM and POM composites.

Sample Name

T5%/°C

T50%/°C

Tmax/°C

NEAT POM

310.5

338.9

326.9

POM/TPU, 1 wt%

344.0

391.0

399.0

POM/β-CD, 1 wt%

326.4

376.3

373.9


350.8

396.7

403.1

POM/IC-TPU, 1 wt%

351.3

396.1

403.5

4. DISCUSSION It is very interesting to find that the IC-TPU is a multi-functional additive for POM. Small amount of IC-TPU improves not only toughness but also strength of POM. The detailed morphology of the POM/IC-TPU composites has been investigated by TEM and AFM. Figure 8 shows the typical TEM images for the composite with 1 wt% IC-TPU. The specimens were stained by OsO4 overnight and followed by RuO4 for 10 min. Typical “sea-island” morphology has been observed with the IC-TPU homogeneously dispersed in POM matrix, which is consistent with SEM results in Figure 3. The detailed observation indicates that the IC-TPU domains are darker in the central part, indicating a core-shell structure of the domains. As schematically drawn in Figure 1, the IC-TPU was synthesized with the part of TPU chains free from β-CD inclusion. We considered that the dark core of the domains was the β-CD assembled aggregations, while the outer shell was the uncovered TPU. Such hierarchical structure of the composites was further confirmed by AFM, as shown in Figure 9. It is well known that the plastics with higher modulus show brighter images in the AFM phase diagram.

60

We observed a brighter POM matrix with the darker domains dispersed in the POM matrix. The

darker domains correspond to the soft elastomer phase of IC-TPU. The domain size is in good accord with that observed in SEM and TEM images. It’s noteworthy that some sparklets were also observed inside the dark elastomer phase, which means there were high modulus core composed of β-CDs aggregations encapsulated by


17

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uncovered TPU. It is convincingly demonstrated that IC-TPU forms “core-shell” structure in the POM matrix and hard β-CD cores are surrounded by the soft TPU shell.

500nm

200nm

Figure 8. TEM images of POM/IC-TPU, 1 wt%.

The structure of the POM/IC-TPU compositions was schematically diagramed in Figure 10, according to the morphological characterization. We attributed the excellent stiffness-toughness balance of the compositions to the “hard core- soft shell” structure of the domains. TPU has good compatibility with POM by the specific interaction between amino groups of TPU and ether groups of POM. Therefore, the good interface between the IC-TPU domains with the matrix can be achieved. The soft uncovered TPU shell improves the elongation at break by possible shear banding mechanism under the deformation stress. On the other hand, the very rigid βCD channel core provides the strengthening effects for the POM. Moreover, the IC-TPU contains both numerous amino and hydroxyl groups to catch formaldehyde, which improves the thermal stability significantly.


18


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Figure 9. AFM image of POM/IC-TPU, 1%. the image was used to illustrate dispersion of β-CDs in IC-TPU domains.

TPU domain POM Matrix IC-TPU domain (core-shell domain)

Figure 10. The schematic diagram of the core-shell structure of IC-TPU in POM matrix.

5. CONCLUSION Excellent stiffness-toughness balance POM alloys with superior thermostability have been fabricated by the incorporation of an inclusion complex based on TPU and β-CD. The precisely controlled stoichiometric ratio of TPU and β-CD during the synthesis ensures the large amount of uncover TPU of the inclusion complex. Therefore, the IC can not only perfectly dispersed in the POM matrix due to the interaction between TPU and


19

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POM, but also provide good compensation for the sacrificing of modulus by the toughening effects of TPU by the rigid core. At the same time, the combination of hydroxyl groups and the amino groups of the IC-TPU can stabilize the POM matrix at a much higher temperature. Such multi-functional additives for POM pave new avenue for the application of POM.

AUTHOR INFORMATION Corresponding Author *E-mail: (Y.L.) [email protected]. Fax: +86 571 28867899. Telephone: +86 57128867026. (L. W.) [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by Zhejiang Province Natural Science Foundation (LQ14B040004), the National Natural Science Foundation of China (21374027, 51403046), and Program for New Century Excellent Talents in University (NCET-13-0762).


20


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Page 25 of 26

For Table of Contents Use Only

TPU Inclusion Complex Modified POM: Fabrication of High Performance POM Composites with both Excellent Stiffness-Toughness Balance and Thermostability Xin Zheng 1, Caixia Zhang 2, Chuntao Luo 2, Guanghua Tian 2, Lian Wang1*, Yongjin Li1* 1

College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No.

16 Xuelin Rd., Hangzhou, 310036, People’ s Republic of China 2

Research and Development Center of Shenhua Ningxia Coal Group, YinChuan ， 750411,

People’s Republic of China

60 50

1% IC-TPU 1% TPU

Stress(MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Neat POM

40 30 TEM

AFM

20 200nm

10 0 0 10 20 30 40 50 60 70 80 90 100110120 130140

Elongation(%)


26

TPU Inclusion Complex Modified POM: Fabrication of High

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