Simultaneous improvement of mechanical and fire-safety properties of

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Applications of Polymer, Composite, and Coating Materials

Simultaneous improvement of mechanical and fire-safety properties of polymer composites with phosphonate-loaded MOF additives Xiao-Lin Qi, Dong-Dong Zhou, Jing Zhang, Shuang Hu, Maciej Haranczyk, and De-Yi Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02357 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 1, 2019

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ACS Applied Materials & Interfaces

Simultaneous Improvement of Mechanical and Fire-Safety Properties of Polymer Composites with Phosphonate-Loaded MOF Additives Xiao-Lin Qia, Dong-Dong Zhou,b Jing Zhanga, Shuang Hua, Maciej Haranczyka,*, DeYi Wanga,* a IMDEA b

Materials Institute, C/Eric Kandel, 2, Getafe, Madrid, 28906, Spain

School of chemistry and chemical engeneering, Sun Yat-sen University, 510275,

Guagzhou, China

AUTHOR INFORMATION ORCID: Xiao-Lin QI: 0000-0002-7117-1320 Dong-Dong Zhou: 0000-0003-1105-8702 Maciej Haranczyk: 0000-0001-7146-9568 Corresponding Author Email: [email protected] [email protected]

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Abstract Flame retardant (FR) additives are commonly used to improve fire-safety of synthetic polymers, which are widely employed in manufactured consumer goods. An incorporation of a FR in a polymer typically leads to deterioration of its mechanical properties. It also manifests itself in a non-negligible volatile organic compounds (VOCs) release, which in turn increases environmental risks carried by both the application and disposal of the corresponding consumer goods. Herein, we present a hierarchical strategy for the design of composite materials, which ensures simultaneous improvement of both mechanical and fire-safety properties of polymers while limiting the VOC release. Our strategy employs porous metal organic framework (MOF) particles to provide a multifunctional interface between the FR molecules and the polymer. Specifically, we demonstrate that the particles of environmentally friendly HKUST-1 MOF can be infused by a modern FR - dimethyl methylphosphonate (DMMP), and then embedded into widely used unsaturated polyester. The DMMPHKUST-1 additive endows the resulting composite material with improved processibility, flame retardancy and mechanical properties. Single-crystal X-ray diffraction, thermogravimetric analysis and computational modeling of the additive suggests the complete pore filling of HKUST-1 with DMMP molecules being bound to the open metal sites of the MOF.

Keywords: Metal-Organic Framework, Flame Retardancy, Hybridization of MOF and Polymers, Open Metal Site, Porous Coordination Polymer, Unsaturated Polyester,


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Introduction Synthetic polymers are indispensable to the modern world. They constitute key components of omnipresent manufactured consumer goods. Although widespread in their applications, the suitability of a particular synthetic polymer to a specific product or application is determined by its mechanical, chemical and physical properties, e.g. stiffness, flammability. Many of these properties can be tuned by incorporation of additives.1-2 In particular, polymers’ additives that improve their flame retardancy and mechanical properties without increasing health and environmental risks are in high demand. In recent years, phosphorus-containing molecular flame retardants (FRs) have gained importance while replacing their chlorine or bromine-substituted predecessors.3 Nevertheless, the development of fire-safe polymers by incorporation of FR additives still poses a significant challenge. Firstly, high dosages of the FR additives are often required to achieve the desired fire-safety levels, at which the deterioration of thermal stability and mechanical properties of the corresponding composites are commonly observed. Secondly, the FR molecules are not sufficiently immobilized in the composite structure, and their migration onto the polymer surface leads to the release of volatile organic compounds (VOCs).4 Metal-Organic Framework (MOF) particles have been prototyped as polymer additives. The resulting composite materials have been tested in a number of applications exploiting the material porosity, e.g. gas separation, nanofiltration, capture of VOCs, and have been shown to exhibit better performance than those of their individual components.5-8 It has been also recognized that MOFs themselves exhibit some flame retardant properties as the metal centers within their structure can promote char formation during the combustion of the composite. However, the fire resistant effect of MOFs is limited9-10 with respect to molecular FRs which can also offer more 3 ACS Paragon Plus Environment


fire suppression mechanism by quenching the combustion reactions and/or forming a non-flammable layer on the burning material.11-14 Conveniently, due to their intrinsic porosity, such molecular flame retardants can be incorporated into MOFs as functional guest molecules.15-18 Herein, we present the synthesis and characterization of hierarchical FR-MOFpolymer composites, in which FR-loaded MOF additive offer significant improvement of the polymer’s mechanical and flame retardancy properties. Our work involves thermoset unsaturated polyester (UP) as a polymer matrix (Scheme 1),19-21 which incorporates MOF particles encapsulating FR. Highly porous and environmentally friendly HKUST-1 (1) is used as the host MOF,22 functionalized by encapsulating dimethyl methylphosphonate (DMMP), which is a widely used modern FR. By comparing the thermal stability, fire resistant behavior, and mechanical properties, it is found that the DMMP-loaded 1 improves both the flame retardancy and mechanical properties of the composites with respect to the guest-free 1, while also offering a good processibility and a negligible VOC release. We refer to our approach as hierarchical due the arrangement of FR in ranks as opposed to a direct molecular dispersion within the polymer, i.e. in our approach, DMMP is encapsulated inside MOF particles, and furthermore, due to presence of open metal sites (OMS) in HKUST-1, it is further organized into groups of strongly OMS-bound and pore filling molecules. Our work highlights new opportunities for improving the performance, environmental friendliness and likely other properties of polymer composites by using the outlined hierarchical design strategies involving porous MOF particles loaded with property enhancing guest molecules.


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Scheme 1. Encapsulating FR into MOF and UP composite preparation.

Results and discussion Structure analysis The crystal structure of HKUST-1 (1) is characterized by the largest cavity diameter of ca. 13.5 Å and the pore limiting diameter of ca. 6.75 Å. The approximate kinetic diameter of DMMP, based on typical bond lengths, is around 6 Å. Therefore, DMMP can penetrate the cavities of 1 as guest molecules. In fact, the volume available to the DMMP molecules can be characterized by the Probe Occupiable volume (POV),23-24 which amounts 0.65 cm3/g, as calculated by our Zeo++ tool,

25

and

corresponds to a void fraction of 57%. POV represents the entire volume that can be occupied by a spherical probe of a given radius. It is different (i.e. larger value) from a commonly used accessible volume (AV), which corresponds to the volume available to the center of the spherical probe. Algorithmic details of POV calculation were presented in Ref 23. The integrity of the host framework during our experiment is confirmed by identical powder X-Ray diffraction patterns of DMMP loaded 1 (1·FR), the corresponding UP composite (1·FR19/UP, the number in the subscript represent the percentage of additives), and as-synthesized 1 (1·guest), indicating that the original 5 ACS Paragon Plus Environment


structure of the host framework kept integral after the FR loading and the combination with polymer matrix (Fig. S1). Thermogravimetric analysis (TGA) sheds light on the encapsulation of DMMP within the pore system. In the case of the mixture of 1 and free DMMP liquid, the TGA weight losing curve before 100 °C is almost identical to the single DMMP while the weight loss the sample 1·FR goes much slower and lasts steadily even after the boiling point (b.p.) of DMMP (180 °C). This remarkable difference in TGA curves suggest that the 1·FR sample has DMMP molecules incorporated inside 1’s pores as guest molecules instead of being adsorbed on the particle surface. The observed slower weight loosing trend in 1·FR implies the strong delaying-release of DMMP caused by the pore system in 1.

60

Mixed DMMP & 1

40

Guest-free 1

20 0

(a)

DMMP 100

200

300

400

500

Temperature / °C

600

700

700 600

1·FR

500

(b)

400 300 200 100 0

20

40

60

80

Time/min

100

120

Temperature/°C

1·FR

80

100 90 80 70 60 50 40 30 20 10 0

Mass (%)

100

Weight(%)

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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0

(c) Fig. 1 (a) Control release of DMMP by 1 confirmed by TGA; (b) removing DMMP by heating 1·FR at 195 °C (15 °C higher than its b.p.) recorded by TGA, (c) EDS results of Cu, O, and P distribution in 1·FR and its residue after being insulated at 300 °C for half an hour

To determine the encapsulation amount of DMMP, the mass change of powder of 1 before and after loading DMMP was recorded which resulted in an encapsulation amount of ca. 41 wt% (table S1). This value corresponds well with an estimate based on POV and the DMMP liquid density of 1150.7 g/cm3 at T = 303 K, which would lead to


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loading of 42.7 wt% (assuming crystal density of 0.879 g/cm3). To study the effects of the DMMP confinement inside 1’s pores that could affect the DMMP liquid density in the above calculations, we used Grand Canonical Monte Carlo simulations to predict the DMMP loading at T = 303 K and pressure of 1 atm. The simulation has indicated that DMMP fills entire available free volume with its density roughly corresponding to the liquid density. The predicted loadings amount to 0.8 g DMMP/1 g of 1 or 44%. In order to determine whether all the DMMP loaded into the host framework can be released before the decomposition of 1, which happens at the temperature 100 °C higher than the boiling point (b.p.) of DMMP (Fig. 1a). Nevertheless, the TGA results indicate that the amount of DMMP removed before the thermal decomposition of 1 limits up to 33% (11% less than the predicted loading). The same amount was observed in the case of 1·FR sample thermally insulated at 195 °C (15 °C higher than b.p. of DMMP) until no weight loss could be observed (Fig. 1b). This observable partial irreversible adsorption of DMMP may be assigned to the interactions between –P=O subgroup of DMMP and dehydrated CuII, which comprise an unsaturated open metal site.26-28 As shown in photographs within Fig. 1a, after being heated at 300 °C, the residue of 1 and 1·FR show remarkable distinctions. The intumescent residue of 1·FR arises from the reaction of the irreversibly adsorbed DMMP and the framework of 1, which generates phosphocarbonaceous compounds. The irreversible adsorption of DMMP is furtherly confirmed by the elemental distribution scan (EDS) of 1·FR and the residue insulated at 300 °C for half an hour (Fig. 1c). The evenly distributed phosphorus can still be observed after 1·FR was heated 110 °C higher than the b.p. of DMMP, which indicates the strong interaction of DMMP with the framework of 1 during the heating process.29



Fig. 2 ORTEP plot [40% probability ellipsoids] of the asymmetric unit and coordination environment (ball and stick) in 1·FR and the possible location of DMMP (violet molecules) in the pore system of 1

To confirm the interaction of DMMP with the framework of 1, single-crystal diffraction data of 1·FR was collected (Table S2). The encapsulation of DMMP didn’t change the space group of the host framework (Fm-3m). After FR adsorption, each CuII ion adopts a pyramidal coordination geometry, in which the axial coordinative site is partially occupied by the oxygen atom of the phosphonate group in DMMP (Fig. 2), leaving H2O as the rest coordinative occupier. The distance between the CuII and phosphate oxygen atom (2.21(5) Å) suggests a coordinative interaction between the copper dimmer and DMMP molecule. The occupying ratio (ca. 30%) of DMMP corresponds well with its irreversible adsorption amount calculated from TGA, giving out a formula of 1·FR as [Cu3(BTC)2(DMMP)0.91(H2O)2.09]·(DMMP)2.48. Looking over the geometry of the cage in 1 and DMMP structure, we believe that the incomplete occupation of the axial coordinative sites of CuII by DMMP is mainly because of the larger guest molecule size and relatively narrow space around the copper dimmer. Taking a look at the results of single-crystal structure refinement, atomic displacement parameters of terminal carbon atoms in DMMP are larger than the same type of atoms in the host framework, which should be caused by the flexibility of DMMP molecules and the intrinsic high symmetry of the host framework, so that the electron cloud 8 ACS Paragon Plus Environment

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density of highly disordered methyl and methoxy groups of DMMP are distributed around the phosphorus atom. Despite the latter, considering the well refined phosphonate oxygen atom, the coordination between DMMP and 1 are reliably proved. According to the analysis above, 1·FR can be considered as a hierarchically functionalized MOF because of the simultaneous existence of open metal site coordination and pore-capsulation within DMMP and 1, which can possibly improve both flame retardancy and mechanical properties of the UP composites in different ways.

The processibility of UP composites As shown in Table S3, composites with different ratios of components have been prepared, including 1·FRx/UP and 1x/UP (x = 10 or 19, representing the corresponding additive percentage, Table S3). PXRD patterns indicate that the structure of 1 maintains well after being embedded into UP, even after months (Fig. S1). At the same time, the composites prepared by physical blend of DMMP, activated 1 and UP (FR0.41x/10.59x/UP) are also made for the comparison with 1·FRx/UP. With the same formulation but different interaction between DMMP and 1, the effectivity of our hierarchical functionalization strategy could be illustrated in the fire behavior and mechanical characterizations. We compared the processibility of 1, 1·FR and the traditional FR, i.e. DMMP, under the same dosage. With the curing reagent recommended by the manufacturer of the UP resin, 1·FRx/UP can be well cured, i.e. the specimen shows smooth surface, with no bubble-release observed, and with a negligible odor of VOC composing the resin. The good compactness and distribution of 1·FR of in composite can be observed in images of Scanning Electron Microscopy (SEM, Fig. 3a, S2a, S2b). In contrary, 1x/UP and FR0.41x/10.59x/UP show more bubble inside the cured specimen (Fig. 3b, 3c, S2c, 2d), which should arise from the gas released from the guest-free 1 during the exothermic 9 ACS Paragon Plus Environment


curing process. On the other hand, at 10 wt% DMMP dosage, an under-cured specimen was obtained. Even with a much lengthened curing time, the specimen can still bend and exhibit a very sticky surface as well as a strong odor of styrene (one of the main volatile component of the UP resin). With 19 wt% of DMMP, the specimen is fully liquidized because of the plasticization effect of DMMP.30 These observations indicate the superiority in processibility of 1·FRx/UP, while the guest-free 1 and DMMP exhibit an opposite trend as additives for UP.

Fig. 3 SEM images of cross sections of (a) 1·FR19/UP, (b) 119/UP and (c) FR7.8/111.2/UP at 100 times magnification

Thermal stability and fire behaviors of UP composites All the composites exhibit one-step decompositions, which start at different temperatures depending on the composition (Fig. S3, Table S4). Compared with neat UP, thermal decomposition of all the additive-modified composites is brought forward in different extent. Among all the composites, 1·FR19/UP has the highest thermal stability while 119/UP shows the lowest initial decomposition temperature. Similar trend is shown in limiting oxygen index (LOI), which reflect the minimum oxygen content sustaining the burning of samples (Fig. 4a). The addition of 1 remarkably reduces the LOI from 20.5 (for neat UP) to 20.0 (for 110/UP) and 19.4 (for 119/UP), respectively. The earlier decomposition of the composites can be explained with the CuII ions which accelerated the decomposition of molecular chains of polymer because of the catalytic interaction of OMS and unsaturated covalent bonds in UP matrix.

31-32

After being further functionalized by DMMP, 1·FR influenced the LOI of

composites in quite an opposite way. 1·FR10/UP and 1·FR19/UP exhibit increased LOI 10 ACS Paragon Plus Environment

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values up to 22.5 and 24.4 respectively. These results imply the synergetic flame retardant effect between 1 and DMMP. To prove this assumption, more tests are furtherly carried out. Table S5 demonstrates that a single addition of DMMP improves the LOI of UP. On the other side, the physically blended composite FR7.8/111.2/UP does not exhibit the LOI value in between ones of FR7.8/UP and 111.2/UP but rather exhibits a greater improvement, which proves the synergetic effect of DMMP and 1. Additionally, the higher LOI of 1·FR19/UP than FR7.8/111.2/UP should be attributed to the different interactions between CuII atoms and their surrounding molecules. Considering that there are still large amount of copper ions in the composites, the accelaration of polymer decomposition caused by OMS should be restrained by the coordination between phosphonate groups and unsaturated CuII dimmers (scheme 1). 26

24.4 24

LOI (%)

22.5 22

20.5 20

20.0

19.4

18 16

(a)

P P UP 1 10/UP 1 19/UP R 10/U R 19/U F F 1· 1·

140

1000

1·FR19/UP 1·FR10/UP 119/UP 110/UP UP

100 80

1·FR19/UP 1·FR10/UP 119/UP 110/UP UP

800

HRR (kW/m²)

120

THR (MJ/m²)

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


60 40

600 400 200

20 0

(b)

0

0

50

100 150 200 250 300 350 400

(c) 0

50

100

150

200

250

Time (s) Time (s) Fig. 4 (a) LOI, (b) THR, and (c) HRR results of UP composites

300

350

Table 1 Summarization of Cone calorimetry results of UP and composites Samples TTI pHRR THR at SPRmax EHC Residue at (s) (kW/m2) 300 s (m²/s) (MJ/kg) 350 s (%) (MJ/m²) 11 ACS Paragon Plus Environment

400


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UP

27±1

946±2

121±9

0.321

24.2

5.4

110/UP

15±1

650±18

108±1

0.296

24.2

8.7

119/UP

11±1

592±8

105±5

0.219

22.0

17.3

1·FR10/UP

18±1

450±5

69±1

0.226

18.0

8.6

1·FR19/UP

19±1

270±2

67±1

0.189

16.4

22.1

To monitor the fire behaviors in detail, the cone calorimeter test was also carried out (Fig. 4b, 4c, S4-S8, Table 1, Table S6), in which the properties of heat release and smoke produce were characterized. By adding 1 and 1·FR, although the time to ignition (TTI) of composites are shortened, the peak of heat release rates (pHRR) and total heat release (THR) decreased in varying degrees compared with neat UP (Fig. 4), meaning that the heat release during the combustion is reduced. As shown in Fig. 4b, 1·FR produces a much lower THR and a longer TTI than 1. Both 1·FR10/UP and 1·FR19/UP similarly reduce the THR by ca. 45% with respect to UP (Table 1). Different from THR, the pHRR showed a strong relevance to the formulation of composites. In 1·FR19/UP, an obvious platform is observed in the HRR curve and the pHRR reduced by 72% compared with neat UP and 40% compared with 1·FR10/UP. While the 1x/UP had the pHRR reduced no more than 17%. Although many traditional phosphorous contained FR works effectively in reducing the THR and HRR, the high CO and smoke release may constitute a fatal disadvantage in a real application.33 According to the total smoke produce (TSP) curves, the guest-free 1 does no help in aspect of smoke suppression. 1·FR10/UP shows good performance in smoke suppression (lowest TSP among all samples in our work), and COPmax and SPRmax of 1·FR19/UP are lower than those of UP and all the other samples simultaneously containing DMMP and 1 (Fig. S6-S8, Table S6). These results suggest that 1·FR has some effects in smoke suppression and toxicity reduction. It is worth pointing out that the physical-blend-prepared samples FR0.41x/10.59x/UP present similar results of HRR and THR which should be the result of 12 ACS Paragon Plus Environment

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the synergetic effect of MOF and DMMP. However, they show worse performances in smoke and CO suppression and shorter TTI than 1·FRx/UP at the same formulation. This should be explained by the same reason of the similar trend observed in LOI test (Fig. S4-S8, Table S6). The acceleration of polymer decomposition by OMS and the synergetic flame retardant effect of DMMP and 1 both contribute to the result The results of TGA, LOI and remarkably shortened TTI all suggest that the decomposition of molecular chains in 1x/UP is significantly accelerated,34-35 which should ascribe to coordination-unsaturated CuII ions in 1.31-32 They catalysed the decomposing reactions of unsaturated covalent bonds in UP matrix. The effective heat of combustion (EHC) reflects the extent of complete combustion taken place during the measurement of cone calorimetry. The very close EHC of 1x/UP and UP suggest that no more incomplete combustion of 1x/UP happens in gaseous phase compared with UP. In another words, the main flame retardant mechanism of 1 should be attributed to the condensed phase mechanism, in which the char residue formed serves as an insulation layer hindering the heat release during the combustion. Although no remarkable distinction can be observed in the morphology of the char residue (Fig. S9), the Raman spectra show that the signal of G-band (diffusion band around 1580 cm-1) corresponding to the C-C vibrations in aromatic structures and D-band (diffusion band around 1350 cm-1) related to sp3 hybridized C atoms can be clearly identified in all the residue samples. Among all the composites, 1·FR19/UP has the highest peak intensity ratio of D-band/G-band (ID/IG), meaning that more turbostratic carbon structures growing among polyaromatic layers are formed during the combustion, giving out stronger char layer which plays important roles in fire resistance process. Nevertheless, the char layer formed in 1x/UP is not strong enough to prevent the dispersion of soot particles, so that the corresponding SPR and TSP values remain high.36 By the lowered EHC and high



amount of char residue, mechanism of both gaseous and condensed phase in 1·FRx/UP are suggested. The radical capture from the gaseous phase should be attributed to the phosphorus contained DMMP in 1·FR, which also reflected in the higher CO produce.37 The polyphosphoric acid and phospho-carbonaceous layer formed during the combustion furtherly help in the reduction of THR and HRR, and smoke suppression (Fig. S9). It should be noted that, under the same additive ratio, 1·FRx/UP exhibit very closing char residue amount with 1x/UP, although the content of 1 is only 59% of that in 1x/UP, meaning that the DMMP has unnegligible contributions to the condensed phase formation during the combustion. As reported in many previous cases, DMMP mainly worked as a gaseous phase flame retardant.37 Therefore, we can conclude that through the interaction with MOF, the flame retardant mechanism of DMMP has been regulated into both ways, which is apparently more effective in fire resistance. Considering that there is still large amount of copper ions in 1·FRx/UP, the accelaration of polymer decomposition caused by OMS should be restrained by the coordination between phosphonate groups and unsaturated CuII dimmers (scheme 2). In another words, the hierarchical functionalization presented in this work not only overcomes the negative affects to the thermal stability of the polymer matrix, but also gives rise to a stronger synergetic flame retardant effect between 1 and DMMP.

Scheme 2 Schematic illustration of different interactions of OMS contained 1 and functionalized 1·FR with the UP matrix


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Mechanical properties To further compare the influence of the additives on the mechanical properties of UP composites, the tensile, impact tests and dynamical mechanical analysis (DMA) were conducted (Fig. 5, Table 2). As shown in Table S7, guest-free 1 increased storage modulus E' modestly possibly because of the rigidity.38 Nevertheless, the two peaks in the loss factor curve imply that there could be more than one glass transition temperature (Tg) in 1x/UP composites (Fig. S11), suggesting that the composites may not undergo a curation homogeneously, which restricts the real utility of composites. This should be attributed to the large amount of active CuII ions contained in the composite. In many cases, low amount of Cu salts or complexes act as promoters in the curing reactions of UP,39 while the excess dosage will cause the explosive polymerization and the permanent under cure of polymers.40-41 At the same time, the possibility that some short branches of molecular chains in UP might insert into pores on the surface of 1 cannot be excluded. Together with those bubbles produced during the curing process (fig. S2c), all these factors exacerbate the inhomogeneity of composites. As a result, a fierce deteriorate of and tensile strength () (42% decreased),

 (57% reduced) and Efrac (47% lowered) were got at high dosage of 1 (119/UP, Fig. 5, Table S3). As expected, when the activity of CuII ions was restrained through being saturated by DMMP, the negative impact toward the polymerization can be diminished. Apart from that, both the organic ligand in 1 and hydrocarbon groups of DMMP molecules exposed on the outer surface of HKUST-1 endow the 1·FR particles stronger interfacial interactions with the polymer matrix in comparison with guest-free 1, which brings about a good affinity of 1·FR with the UP matrix. Therefore, the movement of polymer molecular chains is restricted in 1·FRx/UP, which is reflected by increased Tg (more 15 ACS Paragon Plus Environment


than 12 °C, Fig. S11, Table S7). On the other side, in 1·FR10/UP and 1·FR19/UP, the

 has been increased by 58% and 53%, and  showed the simultaneous increase of 43 and 83%, respectively (Fig. 5, Table 2, S7).42 Additionally, at high dosage, no deterioration of impact strength (Efrac) can be observed (5.0 kJ/m2 for 1·FR19/UP and 4.9 kJ/m2 for UP), implying that 1·FR might be a load-bearing component in UP matrix. The significantly increased Tg and tensile strength imply the elimination of the plasticization effect of DMMP, which should be attributed to the encapsulation by MOF. Together with the rigidity and polymer affinity of the 1·FR, improved performances in tensile and impact tests were got in 1·FRx/UP. All the results mentioned above indicate that our hierarchical functionalization strategy works effectively not only in improving the fire safety, but also ameliorating mechanical properties of polymer composites (Scheme 2). The mechanical properties of FR0.41x/10.59x/UP are affected by promotor effect of OMS in 1, bubbles produced during the curing process, and plasticization effect DMMP simultaneously. The corresponding   and Efrac finally turned out to be in the middle of 1·FRx/UP and 1x/UP. These results are in agreement with those of DMA (Table S7, Fig. S11). 1·FR10/UP

32 28

110/UP

1·FR19/UP

24 20

UP

16 12

119/UP

8 4

(a)

Impact Strength / kJ·m-2

6

36

Tensile strength/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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4.9

4.6

5 4 3

3.3 2.6

2 1

0

1

2 3 4 Enlogation at break(%)

5.0

P P P P P (b) U 1 10/U 1 19/UFR10/U·FR19/U ·

5

1

1

Fig. 5 (a) Tensile properties and (b) impact strength of UP and composites


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Table 2 Results of mechanical properties of UP composites Sample

Tensile strength (MPa)

Tensile Strain (%)

Impact strength (kJ/m2)

Young's modulus (MPa)

Tg (°C)

UP

20.5 ± 1.5

2.3 ± 0.9

4.9±0.4

2491 ± 370

80

110/UP

25.5 ± 2.4

1.6 ± 0.3

3.3±0.6

2473 ± 200

78

119/UP

11.9 ± 5.7

1.0 ± 0.8

2.6±0.7

2705 ± 8

79

1·FR10/UP

30.7 ± 3.5

2.6 ± 0.6

4.6±0.4

2311 ± 179

94

1·FR19/UP

31.2 ± 1.9

4.2 ± 1.0

5.0±0.4

2513 ± 143

92

Conclusions In conclusion, we have presented a hierarchical strategy to improve mechanical and fire safety of composite materials by incorporation of guest-molecule loaded MOF particles. In this work, a modern, phosphorous-based flame retardant DMMP and an environmental friendly MOF HKUST-1 with open metal sites were employed to assemble a polymer additive 1·FR, with which polymer composites with remarkably improved processibility have been prepared. This hierarchical strategy and the selection of components allowed that not only the flame retardant mechanism of the composite is regulated, but also the affinity of the additive to the polymer matrix is strengthened, resulted in more effective flame retardancy and mechanical reinforcement of the composite. The presented hierarchical strategy for developing high performance composite materials by incorporating MOF particles loaded with property enhancing molecules can be adopted to other applications by selection of polymer-compatible MOFs as well as property barring MOF-guest composite.

Sample Preparation Methods Materials



Page 18 of 24

Unsaturated polyester (UP, NORSODYNE® H 15239 TAE) was supplied by Polynt S.A., Spain with density: 1.09 g·cm˗3. Commercialized Cu(NO3)2·3H2O, benzene-1,3,5tricarboxylic acid (H3BTC), methanol and N,N'-dimethylformamide, dimethyl methylphosphonate (DMMP) were used directly without further purification

Sample preparation Synthesis of HKUST-1 (1·guest, [Cu3(BTC)2(H2O)3]·guest): The microcrystal sample was synthesized according to the method reported before. Cu(NO3)2·3H2O (10.5 g), benzene-1,3,5-tricarboxylic acid (5.5 g), methanol (60.0 mL) and N,N'dimethylformamide (60.0 mL) were stirred well in a 200.0 mL Teflon-lined solvothermal reactor with stainless steel shell and heated at 120 °C for 30 hours. Blue crystallites were collected as precipitate and washed with methanol for 2 ~ 3 times. Activated MOF 1: The as-synthesized 1·g was heated under vacuum at 170 °C for 5 hours to remove all the guest molecules in the pore system. Preparation

of

FR

loaded

MOF

powder

1·FR

([Cu3(BTC)2(DMMP)0.91(H2O)2.09]·(DMMP)2.48): the powder of 1 was dipped into DMMP for 3 days to make sure the maximum amount of DMMP is loaded into the pore system of HKUST-1. Afterwards, the powder was washed with acetone quickly to remove all the FR adhering on the particle surface and dried at room temperature. UP composites with FR loaded MOF as additives 1·FRx/UP (x = 10 or 19): Firstly, the solid filler 1·FR (15.00 g or 28.50 g) were mixed with the UP resin (132.0 g or 118.5 g) by means of a triple-roll mix (EXAKT 80E) for 0.5 h. The mixture was transferred onto magnetic stirring apparatus and the curing agent POLYCAT34 (3.0 g) was blended into the mixture and then stirred until a homogenous suspension was obtained. The suspension was casted into the polytetrafluoroethylene molds immediately. The cure process was at room temperature for 24 hours. 18 ACS Paragon Plus Environment

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UP composites with activated MOF as the additive 1x/UP (x = 10 or 19): The powder of 1 (15.00 g or 28.50 g) was quickly added into UP resin (132.0 g or 118.5 g) and well mixed by a triple-roll mix (EXAKT 80E) for 0.5 h before being transferred onto magnetic stirring apparatus. Then the curing agent POLYCAT34 (3.0 g) was blended into the mixture and then stirred until a homogenous suspension was obtained. The curation was processed under room temperature for 24 hours. UP composites with physically blended 1 and DMMP: FR0.41x/10.59x/UP (x = 10 or 19): The powder of 1 (8.50 g or 16.80 g) was quickly added into UP resin (132.0 g or 118.5 g) and well mixed by a triple-roll mix (EXAKT80E) for 0.5 h before being transferred onto magnetic stirring apparatus. Then DMMP (6.15 g or 11.70 g) and curing agent POLYCAT34 (3.0 g) were blended into the mixture and then stirred until a homogenous suspension was obtained. The curation was processed under room temperature for 24 hours.

Data Availability The X-ray crystallographic data for 1·FR have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition number CCDC 1883092. These data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk. All other relevant data supporting the findings of this study are available from the corresponding authors on request.

Author Contributions X.-L. Qi and D.-Y. Wang conceptualized and plan the presented research. X.-L. Qi performed material synthesis and processing. X.-L. Qi, J. Zhang, S. Hu and D.-D. Zhou performed experimental material characterization while M.H. performed computer modeling. All authors wrote and revise the manuscript. 19 ACS Paragon Plus Environment


Acknowledgements This work is supported by the “EU-H2020 Marie Sklodowska-Curie Individual Fellowship (MSC-IF, 7052365)”. Furthermore, M.H. was supported by the Spanish Ministry of Economy and Competitiveness (RYC-2013-13949). We thank the staff of the BL17B/BL18U/BL19U1/BL19U2/BL01B beamlines at the National Center for Protein Sciences Shanghai and Shanghai Synchrotron Radiation Facility, for assistance in collecting the single crystal diffraction data of 1·FR. We acknowledge the computational resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Competing Interests No competing interests are declared by the authors

Supporting Information Characterization methods, PXRD patterns of composites, details of crystal data and structure refinements, TGA, cone calorimetry, DMA, mechanical characterization results and Raman spectra are supplied as Supporting Information.

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Simultaneous improvement of mechanical and fire-safety properties of

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