Novel Polymer Aerogel toward High Dimensional ... - ACS Publications

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

A novel polymer aerogel towards high dimensional stability, mechanical property and fire safety Ke Shang, Jun-Chi Yang, Zhijie Cao, Wang Liao, Yu-Zhong Wang, and David A. Schiraldi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06096 • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 18, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

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

ACS Applied Materials & Interfaces

A novel polymer aerogel towards high dimensional stability, mechanical property and fire safety Ke Shang1, Jun-Chi Yang1, Zhi-Jie Cao1, Wang Liao1*, Yu-Zhong Wang1* and David A. Schiraldi2 1

Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China. 2 Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, USA

ABSTRACT: Inorganc silica-based aerogels, earliest and widely-used aerogels, have poorer mechanical properties than their organic substitutes, which are flammable. In this study, a novel polymeric aerogel with high strength, inherent flame retardancy and cost-effectiveness, which is based on poly(vinyl alcohol) (PVA) crosslinked with melamine-formaldehyde (MF), was prepared under aqueous condition with an eco-friendly freeze-drying and post-curing process. Combined with the additional rigid MF network and benifited from the resulting unique infrastructure of inter-crosslinked flexible PVA segments and rigid MF segments, PVA-based aerogels exibited a significantly decreased degradation rate, and sharply decreased peak heat release rate (PHRR) in cone calorimeter tests (by as much as 83%) compared with neat PVA. The polymer aerogels have a limiting oxygen index (LOI) as high as 36.5% and V-0 rating in UL-94 test. Furthermore, the aerogel samples exposured to harsh temperatures maintain their dimensions (< 10% change) and original mechanical strength and fire safety. Therefore, this work provide a novel

ACS Paragon Plus Environment


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

Page 2 of 32

stragegy for preparing pure organic polymeric aerogel materials with high mechanical strength, dimensional stability and fire safety. Keywords: aerogel; poly(vinyl alcohol); melamine-formaldehyde crosslinking; flame retardant; mechanical property

resin;

1. INTRODUCTION Aerogels were first reported by Kistler in 1931;1 the complexity of their preparation, delayed any meaningful development of these materials for many years, until the sol-gel method combined with supercritical extraction technology were effectively developed. Supercritical CO2 drying requires only modest temperatures, but relatively high operating pressures.2-5 In spite of this limitation, the development of aerogels has greatly widened the path for developing novel materials with high porosity, extremely low density, high internal surface area, low thermal conductivity and high acoustic resistance rates, meriting consideration for a broad range of applications6-15. NASA has explored silicon-based aerogels for aerospace missions, including hypervelocity particle capture, thermal insulation and cryogenic fluid containment.16 In these missions, aerogels have to bear extreme conditions, such as working at as high as 1000 °C, where they are mechanically fragile, will undergo a remarkable shrinking; these aerogels are not currently favored for use as thermal insulation in space.17 It is hence vital to design aerogels with highly thermal viability and dimensional stability for such strategic applications. Polymeric materials are well known for their simple synthesis and versatile functions.18-19 Meador et al. from NASA found that 1,3,5-benzenetricarbonyl


Page 3 of 32

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


trichloride could promote strong polyimide (PI) aerogels alike some prohibitively expensive crosslinkers. The specific moduli could be as high as ca. 400 m2/s2; these aerogels exhibited shrinkage of 30% in a week, however, at a high temperature environment of 150 °C.20 By introducing cellulose nanocrystals (CNF), the extent and rate of aerogel shrinkage could be significantly reduced.21 Cellulose is flammable, leading to oxidation of a PI aerogel matrix when exposed to sufficient heat and oxygen. The inherent flammabilities and dimensional instabilities of these lightweight materials still limit the usage of polymeric aerogels in rigorous environments, and the tedious supercritical drying process complicates their production. A more facile fabrication methodology for aerogels has been reported. Bandi et al. prepared nanoclay-reinforced poly(N-isopropyl acrylamide) aerogels with an oriented freeze and subsequent freeze-drying method.22 This method was further elaborated by Deville et al.23, which is at present a widely used, economic and eco-friendly way to prepare polymer-based aerogels, for applications such as supercapacitors,24 oil contaminant removal,25-26 drug delivery,27 antioxidants28 and superabsorbents29. For the fire safety consideration, Chen et al.

30

prepared poly(vinyl alcohol) (PVA) based

aerogels with nano silica, halloysite, montmorillonite (MMT) and laponite. Because of the migration of these nanofillers and formation of the protection layers, significantly increased char yields were observed.30 Higher fire resistance could be achieved by addition of some effective flame retardants, ammonium polyphosphate (APP) for example, but this system suffers from poor compatibility with PVA. By using piperazine-modified APP, Wang et al. reported increased mechanical strengths



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

in

PVA-MMT aerogels with increased amount of the flame retardant.31

Nonflammable alginate/clay aerogels were later realized by a facile post-crosslinked method which had enhanced LOIs even higher than 60%.32 Fire safety aerogels could also be fabricated with other biosourced molecules.33-34 Recently, cellulose composite aerogels prepared by Wicklein et al. showed not only good fire safety but extremely low thermal conductivity (15 mW/m•K) as well.35 As mentioned above, dimensional stability is another vital property for rigorous environments, such as those required for space exploration. To overcome the fragileness of the fire safety aerogels, PVA aerogels were reinforced by irradiation36-37 or divinylsulfone38. A conformal silica coating onto the aerogel framework proved to be a useful way to simultaneously improve the mechanical strength and fire safety.39 These polymer aerogels with atractive fire safety properties tend to be too mechanically weak/soft. To the best of our knowledge, no polymeric aerogel has fulfilled the requirements of satisfactory strength, dimensional stability, fire safty and cost efficiency. Abundant PVA and melamine-formaldehyde resins were selected to produce pre-gels in water, which were then converted to aerogels using an environmentally friendly freeze-drying process. A delicate post-crosslinking procedure was then applied to promote an infrastructure with inter-crosslinked flexible segments and rigid segments. These pure polymeric aerogels show inherently flame retardant and excellent dimensional stability, which are very promising for a wide range of applications. 2. EXPERIMENTAL SECTION


Page 4 of 32

Page 5 of 32

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


Materials: Poly (vinyl alcohol) (PVA) having a polymerization degree of 1000 or more and a saponification degree of 99 mol% or more (PVA-1799), melamine, formaldehyde (37 wt.% aqueous solution) and p-toluenesulfonic acid were supplied by Kelong Chemical Reagent Corporation (Chengdu, China). Sodium hydroxide (NaOH) was purchased from Rgent Chemical Reagent Corporation (Tianjin, China). All ingredients were used without further purification. Preparation of PVA aqueous solution and MF resin precursor: The preparation process of the aerogels is shown schematically in Scheme 1. A 5 wt% aqueous solution of PVA was prepared by dissolving 5 g PVA to 100 mL DI water at 90 °C under stirring for 8 h. The MF resin precursor was prepared as follows. 5 g melamine was mixed with 10 g formaldehyde solution (37%) with a magnetic stirrer at 85 °C. Several minutes later, as melamine was completely dissolved in formaldehyde solution, the solution became transparent. The pH of the solution was adjusted to 9.0 by 2.5 M NaOH solution to begin the methylolation reaction, which lasted for 30 min to prepare the MF resin precursor. Preparation of crosslinked PVA aerogels: The MF resin precursor was mixed with PVA solution at different mass ratios. The feeding mass ratios of PVA, melamine and formaldehyde solution were 1:1:2, 2:1:2 and 5:1:2. Then, pH of the mixed solution was adjusted to about 6.0 by p-toluenesulfonic acid solution. The mixed solution was poured into mould and frozen immediately with liquid nitrogen. The freeze-drying process was implemented at a VFD-1000 lyophilizer (Boyikang Co. Ltd., China) at -20 °C and less than 1 Pa for several days. The freeze-dried PVA aerogels were



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

crosslinked in a vacuum oven at 80 °C under the pressure of 0.9-1 bar for 0 h-7 d. The resulting sample is identified as PmMn-X, where P, M refers to PVA, melamine, m, n represents corresponding mass proportion, and X represents the crosslinking time, respectively. The detailed formulation is listed in Table 1. Characterization and Measurements. Fourier transform infrared spectroscopy (FTIR) spectra of samples were recorded on a Nicolet FTIR 170SX spectrometer over the wavenumber range from 500 to 4000 cm−1 using KBr pellets. Differential scanning calorimetry measurement (DSC) was conducted in a DSC (Q200, New Castle, DE) instrument, in which the resulting aerogels of approximately 5 mg in weight were encapsulated in aluminum pans and measured under nitrogen atmosphere. The densities were calculated from the mass and dimension measurements using mass measurements and digital calipers. Morphological microstructure of the samples was characterized with scanning electron microscopy (SEM, JEOL JSM 5900LV) at acceleration voltage of 20 kV. The observed cross-section was prepared by fracturing in liquid nitrogen. Compression testing was conducted using an electronic universal testing machine (CMT4104, SANS Co. Ltd., China), fitted with a 10 KN load cell, at a crosshead of 2 mm min-1. The dimension of all samples is about 20 mm in diameter and height. Thermogravimetric analysis was carried on a TG 209F1 (NETZSCH, Germany) thermogravimetric analyzer at a heating rate of 10 °C min-1 under N2.


Page 6 of 32

Page 7 of 32

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


The limiting oxygen index (LOI) values were tested on an HC-2C oxygen index meter (Jiangning, China) according to ASTM D 2863-2009 and the dimension of all samples is 120 mm × 10 mm × 10 mm. The vertical burning test (UL-94) was performed on a CZF-2instument (Jiangning, China) according to GT/T 8333-2008 and dimension of all samples is 125 mm × 10 mm × 10 mm. The combustion behaviors were measured by a cone calorimeter device (Fire Testing Technology). Samples with a size of 100 mm × 100 mm × 10 mm were tested under a heat flux of 50 kW m-2.

3. RESULTS AND DISCUSSION 3.1 Preparation of PVA-MF Aerogels The preparation processes for PVA-MF aerogels and the corresponding chemical reactions observed to take place are illustrated in Scheme 1. First, under the catalysis of an alkali, hydroxymethylation were initiated between melamine and formaldehyde to obtain melamine-formaldehyde resin.40-43 Molar ratio between melamine and formaldehyde and reaction time are the two major influential factors. Three main products, i.e. mono-hydroxymethylated melamine, di-hydroxymethylated melamine and tri-hydroxymethylated melamine, will form during the reaction. If the molar ratio between melamine and formaldehyde is less than 1:3, the main products would be mono-hydroxymethylated melamine and di-hydroxymethylated melamine. Meanwhile, condensation reaction between the two products may occur for prolonged reaction



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

Page 8 of 32

time. As a results, we set the molar ratio between melamine and formaldehyde as 1:3 and a reaction time of 30 min to ensure sufficient hydroxymethylation with negligible condensation.

Some

by-produces,

mono-hydroxymethylated

melamine

and

di-hydroxymethylated melamine, could still generate.

Scheme 1. Preparation of robust PVA aerogel with an inter-crosslinked infrastructure: (A) reaction equation; (B) mechanism diagram of the preparation; (C) intuitive representation of the mechanical peroperty.

As hydroxymethylated melamine is water soluble, it formed homogeneous aqueous solutions with PVA. The abundant hydrogen bonding between hydroxymethylated melamine and PVA promoted an even distribution of the small molecules along the polymer chains. A special crosslinking procedure was designed that the solution was subsequently freeze-dried and then followed with a post-crosslinking process in a vacuum oven. The degree of crosslinking could be better controlled in this manner. If a crosslinking process was conducted before the freeze-drying process, the resultant aerogels would possess numerous defects and their infrastructure would collapse during the freeze-drying procedure; this is probably one of the reasons for that MF


Page 9 of 32

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


aerogels were primerily prepared using the supercritical drying method.44-45 The crosslinking reaction between hydroxymethylated melamine and PVA takes place under heated and/or an acidic conditions; the pH value of a pre-freeze-drying solution is adjusted to 6.0. Aerogels prepared with various recipes and crosslinking time are summarized in Table 1. In contrast to the unmodified PVA aerogel, crosslinked PVA-MF aerogels do not dissolve in water. DSC results provide further surpport for successful crosslinking. A strong endothermic peak, which can be attributed to the movements PVA chains in the aerogel, can be observed at ca. 130 °C for P5M5-0h, Figure 1. After carrying out a 4-hour crosslinking process, the endothermic peak shifted to a higher temperature and also became weaker, indicating a decrease of mobility of the polymer chains. Further crosslinking led to no obvious peak in the DSC; based on these results, a crosslinking time of 24 h or longer were adopted for further study.

Table 1. Formulation and mechanical properties of the aerogels. Water Samples

Density PVA (g)

Melamine (g)

Formaldehyde (g)

Crosslinking time

Specific modulus Modulus (MPa)

-3

(mL)

(m 2/s2)

(g cm )

P5

100

5

-

-

-

0.0754

2.91

38.6

P5M1-24h

100

5

1

2

24h

0.0899

5.15

57.3

P5M2.5-24h

100

5

2.5

5

24h

0.109

7.02

64.4

P5M5-0h

100

5

5

10

0h

0.139

11.98

86.2

P5M5-4h

100

5

5

10

4h

0.132

17.58

133.2

P5M5-24h

100

5

5

10

24h

0.124

23.24

187.4

P5M5-7d

100

5

5

10

7d

0.122

23.56

193.1



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

Figure 1. DSC curves of the P5M5 samples with crosslinking time of 0 hour, 4 hours, 24 hours and 7 days.

Figure 2 shows the FTIR spectra of the aerogels. For neat PVA, the absorptions at 3430 and 1094 cm-1 represent the stretching vibration and bending vibration of secondary hydroxyl, respectively. For MF resin, the methylolation reaction had finished and hence the -NH2 groups were replaced by -NHCH2OH groups, and consequently, a broad peak appeared, which is assigned to the stretching vibration of -OH groups instead of twin peaks representing -NH2; a peak at 988.2 cm-1 indicates that the bending vibration of –OH is observed. After crosslinking for 24 h (P5M5-24h), the peak of C-O-C shifts to 1006 cm-1, indicating a successful crosslinking process.


Page 10 of 32

Page 11 of 32

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


Figure 2. FTIR spectra of the neat PVA, MF resin and PVA-MF aerogel.

3.2 Mechanical Properties The apparent mechanical properties of as prepared PVA-MF aerogels, i.e. densities, compressive modulus and specific modulus, are also summarized in Table 1. Neat PVA aerogels were prepared as control samples. Because PVA chains only possess hydrogen bonding, resultant PVA aerogels were insufficiently strong to maintain their shapes. During the freeze-drying, capillary stress made the aerogel shrink by 20.0% of their original volume. The P5M1 aerogel, a sample with a minor crosslinking with 1.0 wt% of MF, also shrunk to some degree after a 24-hour crosslinking, indicative of insufficient modification; these samples, therefore exhibited higher than predicted densities. For those samples without notable shrinkage, their densities increase with MF contents (Table 1). The compressive stress-strain curves of abovementioned aerogels are shown in Figure 3. The terminal point of each compressive test is 80.0% of the sample’s



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

original height. These compressive curves follow a typical deformation behavior of open honeycomb-like foams: a linear elastic behavior is shown at low strain, from which the compressive modulus could be calculated. At intermediate strains, a cell collapse-related stress reduction exhibits following with a plastic yielding plateau and finally stiffening at high strain range.46-47 In Table 1, specific moduli (defined as compressive modulus/density) of the aerogels are shown to increase with MF content. The value of P5M5-24h reached 187.4 m2/s2, the highest value among all present fire-safety aerogels.30-32,

35-37, 39, 48

Based on the related chemical reactions, the

infrastructure of the PVA-MF aerogels could be described in the following manner: MF crosslinks long PVA macromolecules and form a secondary network of their own. Simultaneously, flexible PVA segments reciprocally break up the brittleness of neat MF resin. This infrastructure, i.e. a crosslinked soft network inter-crosslink a hard network, significantly increases the strength of the aerogels (Scheme 1B). This kind of toughness could even bear step-on of an adult with a deformation (-0.3 %) in the measurement error (Scheme 1C). While the curves in Figure 3b reveal that the compression strength of the PVA-MF aerogels could be significantly improved with increase of the crosslinking time, the sample crosslinked for 7 days (P5M5-7d) became remarkably brittle so that many cracks appeared when strain was 20 % of its original height and the test was hence ceased. Even so, the compressive modulus of P5M5-24h is close to that of P5M5-7d, indicating that 24 hours is sufficient crosslinking time.


Page 12 of 32

Page 13 of 32

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


Figure 3. Compressive stress-strain curves of the aerogels. (a) Different formula: PVA 5.0 wt% (P5), PVA 5.0 wt%+MF 1.0 wt% (P5M1), PVA 5.0 wt%+MF 2.5 wt% (P5M2.5), PVA 5.0 wt%+MF 5.0 wt% (P5M5). Crosslinking time: 24h. (b) Different crosslinking time (0h, 4h, 24h and 7d) for P5M5 recipe.

3.3 Microstructure The visual microstructures of the PVA-MF aerogels were examined by SEM (Figure 4). Solution viscosity was found to be the decisive factor for the microstructure of an aerogel prepared with a freeze-drying method.49-50 Higher solution concentrations of polymer,

with correspondingly higher viscosities, lead to more compact

microstructures of the porous solids. The pores of neat PVA aerogel, whose solid concentration was 5.0 wt% in the precursor solution, followed the direction of ice crystal growth during freezing and adopted a layered architecture, reflecting its comparatively low solution viscosity. In contrast, the MF crosslinked aerogels adopted a more cellular, three dimensional network structure. Hydroxymethylated melamine interacted closely with PVA chains via hydrogen bonding and Van der



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

Waals' forces. After the crosslinking reaction, hydroxymethylated melamine reacted with PVA, leading to formation of crosslinked soft segments; reaction with hydroxymethylated melamine resulted in crosslinked rigid segments. More importantly, the two networks were inter-crosslinked. As a result, uniform pores formed and contributed to the excellent mechanical strengths observed. Because the crosslinking reaction occurred gradually with time, the resulting microstructure became more uniform. The strength of PVA aerogels were therefore significantly elevated after the combination and crosslinking of MF. Comparing with highly fire-safety PVA/MMT aerogel,30 the specific modulus increased 4.7-fold from 32.6 m2/s2 for PVA/MMT sample to 187.4 m2/s2 for PVA-MF aerogel.

Figure 4. SEM images of PVA and PVA-MF aerogels.

3.4 Thermal Stability


Page 14 of 32

Page 15 of 32

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


Figures 5 and 6 show thermogravimetric analysis (TGA) results of PVA-MF aerogels; resultant characteristic thermal degradation data are summarized in Table 2. A small peak at the beginning of all the curves is assigned to the loss of water absorbed in the air. For the samples with crosslinking times of less than 24 h, this process could further develop with increased temperature. The Td 5%, which indicates the onset of decomposition, decreased, therefore, in contrast to that of PVA aerogel. The increase of Td max and the remarkably reduced degradation rate at that temperature indicate improved thermal stability of the inter-crosslinked networks. To clarify the effect of crosslinking time, the TGA and corresponding DTG curves of P5M5 aerogels with different crosslinking time are summarized in Figure 6. For P5M5-0h and P5M5-4h, there is an obvious degradation at ca. 130 °C, which does not exist for the P5M5-24h and P5M5-7d samples. This phenomenon strongly suggests an uncompleted crosslinking reaction at a comparatively short reaction time, and release of water at ignition is hence possible.

Figure 5. TGA weight loss and corresponding DTG curves of pure PVA aerogels and PVA-MF aerogels with different contents of MF. Crosslinking time: 24 h.



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

Page 16 of 32

Figure 6. TGA weight loss and corresponding DTG curves for P5M5 aerogels with different crosslinking times.

Table 2. Characteristic TGA data of PVA-MF aerogels under N2 atmosphere. Samples

Td 5% (oC)

Td max (oC)

dW/dT (%/min)

Residue (%)

PVA

251.0

277.6

16.32

3.44

P5M1-24h

140.6

276.8

7.90

12.01

P5M2.5-24h

134.3

340.8

8.48

13.03

P5M5-24h

141.2

320.9

5.47

25.41

P5M5-0h

102.6

316.1

5.16

21.19

P5M5-4h

112.8

320.6

5.18

23.63

P5M5-7d

170.4

320.3

5.29

24.82

3.5 Combustion behavior The LOI tests and UL-94 tests were carried out to investigate the flammabilities of the samples, Table 3. Neat PVA aerogel is a highly flammable material with very low LOI value of 19.5% and with no UL-94 rating. The LOI values increased monotonically with MD addition. For the higher MF content samples, i.e. P5M2.5 and P5M5, the aerogels could reach V-0 rating with LOI > 32%.


Page 17 of 32

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


Table 3. The LOI and UL-94 test results of the PVA-based aerogels. Sample

LOI (%)

UL-94

PVA

19.5

NR

P5M1-24h

28.4

NR

P5M2.5-24h

32.7

V-0

P5M5-24h

36.5

V-0

Figure 7 shows that P5M5 samples with high LOI values in the range of 36-38%, but these values decrease with crosslinking time. Two reasons could be responsible for this phenomenon. First, melamine itself is well-known for the gas-phase flame-retardant effect. Second, the crosslinking and formation of melamine network could continue at a high temperature, during which the heat was taken away by the reaction and evaporation of water molecules. The first factor is believed to contribute more to the high LOI values, for the amount the melamine used; once a full crosslinking was achieved (≥ 24 h), the decreasing trend for LOI slowed.

Figure 7. LOI results of the P5M5 sample with different crosslinking time.



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

The combustion behaviors of the aerogels were further investigated by cone calorimetry. The 24 h-crosslinked PVA aerogels with different MF contents were measured under a heat flux of 50 kW/m2. The evolution of heat release rate and total heat release are exhibited in Figure 8 and corresponding key parameters are list in Table 4, including time to ignition (TTI), peak of heat release rate (PHRR), total heat release (THR), time to PHRR (TTPHRR), fire growth rate (FIGRA) and residue. For the neat PVA aerogel, the TTI is only 3 s, whereas for crosslinked PVA aerogels, i.e. P5M1, P5M2.5 and P5M5, TTI values increase to 11 s, 25 s and 60 s, respectively, indicating that the decomposition of MF segments hindered the spread of flame. The increase of MF segments also decreased the PHRR and FIGRA values, especially for the P5M5 aerogel, where the PHRR is only 60.9 kW/m2, 84% lower than that of the neat PVA aerogel. The FIGRA of P5M5 decreased to 0.4 W/s in dramatic contrast to that of the neat PVA aerogel (18.3 W/s). However, as the flame retardant of the aerogels is mainly due to the gas-phase flame retardant effect of melamine, all the materials almost burned completely and therefore the THR values were not significantly reduced.


Page 18 of 32

Page 19 of 32

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


Figure 8. The heat release rate (HRR, a) and total heat release (THR, b) plots of the PVA-MF resin aerogels crosslinked for 24 h under a heat flux of 50 kW/m2.

Table 4. Cone calorimetric data for PVA-based aerogels at a heat flux of 50 kW/m2. Sample

TTI

PHRR

THR 2

2

TTPHRR

FIGRA

Residue

(s)

(kW/m )

(MJ/m )

(s)

(W/s)

(%)

PVA

3

366.6

22.2

20

18.3

1.5

P5M1

11

244.6

13.4

55

4.4

7.3

P5M2.5

25

185.4

18.3

95

2.0

4.6

P5M5

60

60.9

12.8

155

0.4

3.5

To better understand the combustion behaviors of the aerogels, the residue microstructure of P5M5 and P5M1A1 were conducted by SEM (Figure 9). The residue of P5M5 adopts a char structure with bubble-like pores, which attributed to the gas released by MF and hence supported the primary gas phase flame retardant mechanism. Based on the abovementioned results, the fire safety of a material is indicated by two primary indexes, i.e. LOI and PHRR, and put in the figure with its mechanical strength (Figure 10 and Table S1). In this figure, double y-axis was designed for LOI (in sequence) and PHRR (in inverted sequence) to describe the fire



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

safety of a material, and specific modulus was along the x-axis to indicate the strength. A material with points closing to the upper right corner is stronger and consistent with higher fire safety. By comparing with the fire-safety polymeric aerogels reported in recently years, the PVA-MF aerogels, which have a special soft/hard segments inter-connected network, are obviously robust with remarkable fire safety.

Figure 9. SEM images of the residues of P5M5 after cone calorimeter tests.


Page 20 of 32

Page 21 of 32

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


Figure 10. A summary of aerogels with different strength and different fire safety.

3.6 Dimensional and Property Stability Dimensional and property stability are very important for the devices used for aerospace exploration. Silicon aerogels are inherently nonflammable but fragile. Newly developed, strong polymeric aerogels, as abovementioned, still underwent some shrinkage (from about 10% to 30%) and gave little consideration to their fire safety.[6-7] In this study, P5M5 aerogels were exposed in harsh conditions of -20 °C and 100 °C for a week; as Figure 11 and Table S2 shows, the sizes of the aerogels were fairly stable with shrinkages ca. 10%. For the strength and LOI, the later a key parameter for fire safety, were almost unchanged.



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

Figure 11. Dimensional stability and property stability of the P5M5 aerogels. The volume variation (a) are the sizes of aerogels before and after an exposure in -20 °C or 100 °C, and the values of modulus (black) and LOI (red) (b) before and after the same conditions.

4. Conclusions The preparation of inherently flame retardant aerogels based on poly(vinyl alcohol) and melamine-formaldehyde by a simple freeze-drying and routine curing process was demonstrated. The resulted aerogels adopted inter-crosslinked rigid and flexible segments infrastructure. The flame retardancy was improved by this structure and based on this novel structure, and the mechanical strength was significantly enhanced. In particular, the aerogels had excellent dimensional stability, which were confirmed by the tests in dissolution and hot/cold temperature conditions.

ASSOCIATED CONTENT Supporting Information


Page 22 of 32

Page 23 of 32

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


The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Summary of aerogels with different strength and different fire safety, change of dimensional, strength and fire safety stability of P5M5-24h aerogels.

AUTHOR INFORMATION Corresponding Author * Tel. & Fax: +86-28-85410755; E-mail: [email protected] (W. Liao); [email protected] (Y.-Z. Wang)

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 51320105011, 51121001 and 51603130) and Program for Changjiang Scholars and Innovative Research Team in University (IRT. 1026).

REFERENCES (1)

Kistler, S. S. Coherent Expanded Aerogels and Jellies. Nature 1931, 127,

741-741. (2)

Pierre, A. C.; Pajonk, G. M. Chemistry of Aerogels and Their Applications.

Chem. Rev. 2002, 102, 4243-4265. (3)

Xu, X.; Zhou, J.; Nagaraju, D. H.; Jiang, L.; Marinov, V. R.; Lubineau, G.;

Alshareef, H. N.; Oh, M. Flexible, Highly Graphitized Carbon Aerogels Based on



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

Bacterial Cellulose/Lignin: Catalyst-Free Synthesis and Its Application in Energy Storage Devices. Adv. Funct. Mater. 2015, 25, 3193-3202. (4)

Ziegler, C.; Klosz, S.; Borchardt, L.; Oschatz, M.; Kaskel, S.; Friedrich, M.;

Kriegel, R.; Keilhauer, T.; Armbrüster, M.; Eychmüller, A. Znpd/Zno Aerogels as Potential Catalytic Materials. Adv. Funct. Mater. 2016, 26, 1014-1020. (5)

Fricke, J.; Emmerling, A. Aerogels. Adv. Mater. 1991, 3, 504-506.

(6)

Bordjiba, T.; Mohamedi, M.; Dao, L. H. New Class of Carbon-Nanotube

Aerogel Electrodes for Electrochemical Power Sources. Adv. Mater. 2008, 20, 815-819. (7)

Fan, Y.; Ma, W.; Han, D.; Gan, S.; Dong, X.; Niu, L. Convenient Recycling of

3d Agx/Graphene Aerogels (X = Br, Cl) for Efficient Photocatalytic Degradation of Water Pollutants. Adv. Mater. 2015, 27, 3767-3773. (8)

Bi, H.; Yin, Z.; Cao, X.; Xie, X.; Tan, C.; Huang, X.; Chen, B.; Chen, F.; Yang,

Q.; Bu, X.; Lu, X.; Sun, L.; Zhang, H. Carbon Fiber Aerogel Made from Raw Cotton: A Novel, Efficient and Recyclable Sorbent for Oils and Organic Solvents. Adv. Mater. 2013, 25, 5916-5921. (9)

Zhao, Y.; Liu, J.; Hu, Y.; Cheng, H.; Hu, C.; Jiang, C.; Jiang, L.; Cao, A.; Qu, L.

Highly Compression-Tolerant Supercapacitor Based on Polypyrrole-Mediated Graphene Foam Electrodes. Adv. Mater. 2013, 25, 591-595. (10)

Yin, H.; Zhao, S.; Wan, J.; Tang, H.; Chang, L.; He, L.; Zhao, H.; Gao, Y.; Tang,

Z. Three-Dimensional Graphene/Metal Oxide Nanoparticle Hybrids for High-Performance Capacitive Deionization of Saline Water. Adv. Mater. 2013, 25,


Page 24 of 32

Page 25 of 32

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


6270-6276. (11)

Schiffres, S. N.; Kim, K. H.; Hu, L.; McGaughey, A. J. H.; Islam, M. F.; Malen,

J. A. Gas Diffusion, Energy Transport, and Thermal Accommodation in Single-Walled Carbon Nanotube Aerogels. Adv. Funct. Mater. 2012, 22, 5251-5258. (12)

Moon, I. K.; Yoon, S.; Chun, K. Y.; Oh, J. Highly Elastic and Conductive

N-Doped Monolithic Graphene Aerogels for Multifunctional Applications. Adv. Funct. Mater. 2015, 25, 6976-6984. (13)

Katsoulidis, A. P.; He, J.; Kanatzidis, M. G. Functional Monolithic Polymeric

Organic Framework Aerogel as Reducing and Hosting Media for Ag Nanoparticles and Application in Capturing of Iodine Vapors. Chem. Mater. 2012, 24, 1937-1943. (14)

Zu, G.; Shen, J.; Wang, W.; Zou, L.; Lian, Y.; Zhang, Z.; Liu, B.; Zhang, F.

Robust, Highly Thermally Stable, Core-Shell Nanostructured Metal Oxide Aerogels as High-Temperature Thermal Superinsulators, Adsorbents, and Catalysts. Chem. Mater. 2014, 26, 5761-5772. (15)

Zu, G.; Shen, J.; Zou, L.; Wang, W.; Lian, Y.; Zhang, Z.; Du, A.

Nanoengineering Super Heat-Resistant, Strong Alumina Aerogels. Chem. Mater. 2013, 25, 4757-4764. (16)

Jones, S. M. Aerogel: Space Exploration Applications. J. Sol-Gel Sci. Techn.

2006, 40, 351-357. (17)

Aegerter, M. A., Leventis, N., Koebel, M. M. Aerogel Handbook, New York,

Springer, 2011. (18)

Tan, C. B., Fung, B. M., Newman, J. K., Vu, C. Organic Aerogels with Very



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

High Impact Strength. Adv. Mater. 2001, 13, 644-646. (19)

Al-Muhtaseb, S. A., Ritter, J. A. Preparation and Properties of

Resorcinol-Formaldehyde Organic and Carbon Gels. Adv. Mater. 2003, 15, 101-114. (20)

Meador, M. A. B.; Aleman, C. R.; Hanson, K.; Ramirez, N.; Vivod, S. L.;

Wilmoth, N.; McCorkle, L. Polyimide Aerogels with Amide Cross-Links: A Low Cost Alternative for Mechanically Strong Polymer Aerogels. ACS Appl. Mater. Interfaces 2015, 7, 1240-1249. (21)

Nguyen, B. N.; Cudjoe, E.; Douglas, A.; Scheiman, D.; McCorkle, L.; Meador,

M. A. B.; Rowan, S. J. Polyimide Cellulose Nanocrystal Composite Aerogels. Macromolecules 2016, 49, 1692-1703. (22)

Bandi, S.; Bell, M.; Schiraldi, D. A. Temperature-Responsive Clay

Aerogel-Polymer Composites. Macromolecules 2005, 38, 9216-9220. (23)

Deville, S.; Saiz, E.; Nalla, R. K.; Tomsia, A. P. Freezing as a Path to Build

Complex Composites. Science 2006, 311, 515-518. (24)

Liu, R.; Wan, L.; Liu, S.; Pan, L.; Wu, D.; Zhao, D. An Interface-Induced

Co-Assembly Approach Towards Ordered Mesoporous Carbon/Graphene Aerogel for High-Performance Supercapacitors. Adv. Funct. Mater. 2015, 25, 526-533. (25)

Wang, J.; Zhao, D.; Shang, K.; Wang, Y. T.; Ye, D. D.; Kang, A. H.; Liao, W.;

Wang, Y. Z. Ultrasoft Gelatin Aerogels for Oil Contaminant Removal. J. Mater. Chem. A 2016, 4, 9381-9389. (26)

Mulyadi, A.; Zhang, Z.; Deng, Y. L. Fluorine-Free Oil Absorbents Made from

Cellulose Nanofibril Aerogels. ACS Appl. Mater. Interfaces 2016, 8, 2732-2740.


Page 26 of 32

Page 27 of 32

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


(27)

Zhao, J. Q.; Lu, C. H.; He, X.; Zhang, X. F.; Zhang, W.; Zhang, X. M.

Polyethylenimine-Grafted Cellulose Nanofibril Aerogels as Versatile Vehicles for Drug Delivery. ACS Appl. Mater. Interfaces 2015, 7, 2607-2615. (28)

Panzella, L.; Melone, L.; Pezzella, A.; Rossi, B.; Pastori, N.; Perfetti, M.;

D'Errico, G.; Punta, C.; d'Ischia, M. Surface-Functionalization of Nanostructured Cellulose Aerogels by Solid State Eumelanin Coating. Biomacromolecules 2016, 17, 564-571. (29)

Kettunen, M.; Silvennoinen, R. J.; Houbenov, N.; Nykänen, A.; Ruokolainen, J.;

Sainio, J.; Pore, V.; Kemell, M.; Ankerfors, M.; Lindström, T.; Ritala, M.; Ras, R. H. A.; Ikkala, O. Photoswitchable Superabsorbency Based on Nanocellulose Aerogels. Adv. Funct. Mater. 2011, 21, 510-517. (30)

Chen, H. B.; Wang, Y. Z.; Schiraldi, D. A. Preparation and Flammability of

Poly(Vinyl Alcohol) Composite Aerogels. ACS Appl. Mater. Interfaces 2014, 6, 6790-6796. (31)

Wang, Y. T.; Liao, S. F.; Shang, K.; Chen, M. J.; Huang, J. Q.; Wang, Y. Z.;

Schiraldi, D. A. Efficient Approach to Improving the Flame Retardancy of Poly(Vinyl Alcohol)/Clay Aerogels: Incorporating Piperazine-Modified Ammonium Polyphosphate. ACS Appl. Mater. Interfaces 2015, 7, 1780-1786. (32)

Shang, K.; Liao, W.; Wang, J.; Wang, Y. T.; Wang, Y. Z.; Schiraldi, D. A.

Nonflammable Alginate Nanocomposite Aerogels Prepared by a Simple Freeze-Drying and Post-Cross-Linking Method. ACS Appl. Mater. Interfaces 2016, 8, 643-650.



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

(33)

Lang, X. L.; Shang, K.; Wang, Y. Z.; Schiraldi, D. A. Low Flammability

Foam-Like Materials Based on Epoxy, Tannic Acid, and Sodium Montmorillonite Clay. Green Mater. 2015, 3, 43-51. (34)

Wang, T. W.; Sun, H.; Long, J. W.; Wang, Y. Z.; Schiraldi, D. Biobased

Poly(Furfuryl Alcohol)/Clay Aerogel Composite Prepared by a Freeze-Drying Process. ACS Sustain. Chem. Eng. 2016, 4, 2601-2605. (35)

Wicklein, B.; Kocjan, A.; Salazar-Alvarez, G.; Carosio, F.; Camino, G.;

Antonietti, M.; Bergstrom, L. Thermally Insulating and Fire-Retardant Lightweight Anisotropic Foams Based on Nanocellulose and Graphene Oxide. Nature Nanotech.2015, 10, 277-283. (36)

Chen, H. B.; Liu, B.; Huang, W.; Wang, J. S.; Zeng, G.; Wu, W. H.; Schiraldi, D.

A. Fabrication and Properties of Irradiation-Cross-Linked Poly(Vinyl Alcohol)/Clay Aerogel Composites. ACS Appl. Mater. Interfaces 2014, 6, 16227-16236. (37)

Chen, H. B.; Zhao, Y.; Shen, P.; Wang, J. S.; Huang, W.; Schiraldi, D. A.

Effects of Molecular Weight Upon Irradiation-Cross-Linked Poly(Vinyl Alcohol)/Clay Aerogel Properties. ACS Appl. Mater. Interfaces 2015, 7, 20208-20214. (38)

Chen, H. B.; Hollinger, E.; Wang, Y. Z.; Schiraldi, D. A. Facile Fabrication of

Poly(Vinyl Alcohol) Gels and Derivative Aerogels. Polymer 2014, 55, 380-384. (39)

Sun, H.; Schiraldi, D. A.; Chen, D. Y.; Wang, D. Q.; Sanchez-Soto, M. Tough

Polymer Aerogels Incorporating a Conformal Inorganic Coating for Low Flammability and Durable Hydrophobicity. ACS Appl. Mater. Interfaces 2016, 8, 13051-13057.


Page 28 of 32

Page 29 of 32

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


(40)

Blank, W. J. Reaction-Mechanism of Melamine Resins. J. Coatings Technol.

1979, 51, 61-70. (41)

Wohnsiedler, H. P. Polymerization in Melamine-Formaldehyde Molded Resins.

Ind. Eng. Chem. 1953, 45, 2307-2311. (42)

Zidar, J.; Pavlin, M.; Mihelic, I.; Ogorelec, P.; Plavec, J.; Mavri, J. Stability and

Reactivity of Progressively Methylolated Melamine Derivatives. Comput. Theor. Chem. 2013, 1006, 85-91. (43)

Ratzsch, M.; Bucka, H.; Ivanchev, S.; Pavlyuchenko, V.; Leitner, P.;

Primachenko, O. N. The Reaction Mechanism of the Transetherification and Crosslinking of Melamine Resins. Macromol. Symp. 2004, 217, 431-443. (44)

Nguyen, M. H.; Dao, L. H. Effects of Processing Variable on

Melamine-Formaldehyde Aerogel Formation. J. Non-Cryst. Solids 1998, 225, 51-57. (45)

Pekala, R. W.; Alviso, C. T.; Kong, F. M.; Hulsey, S. S. Aerogels Derived from

Multifunctional Organic Monomers. J. Non-Cryst. Solids 1992, 145, 90-98. (46)

Johnson, J. R.; Spikowski, J.; Schiraldi, D. A. Mineralization of Clay/Polymer

Aerogels: A Bioinspired Approach to Composite Reinforcement. ACS Appl. Mater. Interfaces 2009, 1, 1305-1309. (47)

Gawryla, M. D.; van den Berg, O.; Weder, C.; Schiraldi, D. A. Clay

Aerogel/Cellulose Whisker Nanocomposites: A Nanoscale Wattle and Daub. J. Mater. Chem. 2009, 19, 2118-2124. (48)

Wang, L.; Sanchez-Soto, M.; Maspoch, M. L. Polymer/Clay Aerogel

Composites with Flame Retardant Agents: Mechanical, Thermal and Fire Behavior.



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

Mater. Design 2013, 52, 609-614. (49)

Pojanavaraphan, T.; Liu, L.; Ceylan, D.; Okay, O.; Magaraphan, R.; Schiraldi,

D. A. Solution Cross-Linked Natural Rubber (Nr)/Clay Aerogel Composites. Macromolecules 2011, 44, 923-931. (50)

Gawryla, M. D.; Nezamzadeh, M.; Schiraldi, D. A. Foam-Like Materials

Produced from Abundant Natural Resources. Green Chem. 2008, 10, 1078-1081.


Page 30 of 32

Page 31 of 32

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


TOC



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

TOC 72x31mm (300 x 300 DPI)


Page 32 of 32

Novel Polymer Aerogel toward High Dimensional ... - ACS Publications

Recommend Documents