High-strength boehmite-acrylate composites for 3D

printing: reinforced filler-matrix interactions

Yanyang Han,a FuKe Wang,b Haimei Wang ,a Xiuling Jiao a,* and

Dairong Chena,*

aSchool of Chemistry & Chemical Engineering, Shandong University, Jinan 250100, P. R. China. Email:

jiaoxl@sdu.edu.cn; cdr@sdu.edu.cn

bPolymeric Materials Department, Institute of Materials Research and Engineering, Agency for Science,

Technology and Research (A*STAR), 2 Fusionopolis Way, #08-03 Innovis, Singapore 138634.

Experimental section

1. Materials. Aluminum nitrate nonahydrate and ammonia (25%) were

purchased from Sinopharm Chemical Reagent, 1,6-hexanediol diacrylate (HDDA,

90%) and poly(ethylene glycol) diacrylate (PEGDA, average Mn=600), Bisphenol

A glycerolate dimethacrylate(Bis-GMA), triethylene glycol dimethacrylate

(TEGDMA), 3-trimethoxysilylpropyl methacrylate (TMSPM) and β-carboxyethyl

acrylate (β-CEA) were purchased from Aladdin, and photoinitiator phenylbis

(2,4,6-trimethylbenzoyl)-phosphine oxide (97%) was purchased from Sigma-

aldrich. All materials were used as received without further purification.

2. Synthesis of boehmite nanowires. AlOOH nanowires were synthesized by

a hydrothermal process as reported with minor modification [45]. Typically, 15 g

of Al(NO)3·9H2O was dissolved in 30 mL of deionized water under vigorously

stirring to form a transparent solution. Then, ammonium hydroxide (25%) was

dropwise added to the solution until the pH of the reaction mixture was 4. The

lacteous precipitates were collected by centrifugation and redispersed in 30 mL

of deionized water. The resulting mixture was transferred into a 40 mL Teflon-

lined autoclave, which was then sealed and kept in an oven at 160 C. After 48h,

the autoclave was air-cooled to room temperature. The resultant product was

centrifuged and washed with deionized water. The final product was freeze-dried

for 10 h.

3. Surface modification of boehmite nanowires. To improve the dispersity

of boehmite nanowires in acrylates, the as-prepared boehmite nanowires were

functionalized by two strategies. β-carboxyethyl acrylate (β-CEA) was employed

as a linker between boehmite nanowires and polymer matrix. Typically, 0.3 g

boehmite nanowires were refluxed with 50 mL ethanol containing 1.44 g β-CEA

for 16 h. The final product was washed with copious amounts of ethanol and

water and then freeze-dried. Meanwhile, 3-Trimethoxysilylpropyl methacrylate

(TMSPM) modification was realized using similar strategies to those carried out

on silica surfaces. The boehmite nanowires (0.3 g) were added to the TMSPM

solution consisting of 1 mL TMSPM, 50 mL ethanol, and 9 mL DI water and

sonicated to get homogeneous solution. The solution was then refluxed for 16 h.

After the surface functionalization step, the solution became viscous and was

rinsed with copious amounts of ethanol and water and then freeze-dried.

4. Photopolymerization of boehmite-acrylate composites. The

appropriate amount of powders of neat, TMSPM and β-CEA modified AlOOH were

incorporated in acrylate resins by sonication. For UV curing, 1 wt% of

photoinitiator (PI) bis-(2,4,6-trimethylbenzoyl) phenylphosphine oxide was

dissolved in the resins by stirring in dark at room temperature overnight. The as-

obtained resins were cured by UV irradiation (irradiation intensity: 150 mW/cm -2)

and then thermal cured for further characterization.

5. 3D printing. The resins with boehmite nanowires and acrylates were

injected to the tray of 3D printer (Sprintray StarRAY, SP-DD230, LED light source,

405 nm). The geometry was designed and sliced by the software, and printing

procedure was accomplished layer-by-layer (slice thickness: 0.05 mm, exposure

time: 10 s). Post-UV curing was also conducted on the printed objects.

6. Characterization. The morphology of the as-prepared boehmite nanowires

was characterized by transmission electron microscopy (TEM, JEOL JEM-1011,

accelerating voltage: 100 kV). The X-ray diffraction (XRD) patterns of the

powder samples were collected on a Rigaku D/MAX 2200PC diffractometer with

a graphite monochromator and CuKα (λ=0.15418 nm) radiation. The infrared

spectra were examined on a Brüker ALPHA-T Fourier Transform Infrared (FT-IR)

spectrometer using the KBr pellet technique. Cross-sectional characterization of

the resins after fracture was accomplished by scanning electron microscopy

(SEM, HITACHI, SU8010). Thermalgravimetric analysis (TGA) was carried out on

a Netzsch STA449F3 Jupiter thermal gravimetric analyzer at a heating rate of

10.0 C/min under air atmosphere. Elemental analysis was conducted on Vario

EL CUBE (Elementar) to determine the carbon content in modified AlOOH

nanowires. Rheological properties of neat and composite acrylate resins were

characterized on a rheometer (Anton Paar MCR 302), shear rate: 0.01~100 s-1.

7. Mechanical testing. Three-point bending flexural tests were conducted on

standard specimen (L=50.8 mm, W=12.7 mm, ASTM D790) with Universal

Testing Machine HZ-1004A (Lixian Instrument), results were average of at least

five specimens.

Fig. S1. Characterization of boehmite nanowires: (a-b), TEM and SEM images of the boehmite

nanowires prepared by hydrothermal reaction; (c), XRD pattern of freeze-dried boehmite powder.

Fig. S2. Structural illustration of the as-involved reagents: photoreactive monomer (HDDA) and

modifiers (TMSPM and β-CEA). TMSPM was commonly used as coupling agents for the affinity of

its siloxy groups to nanoparticles and lipophilicity of its alkyl groups; while β-CEA was first

employed as surface modifier in this report. TMSPM was expected to undergo siloxy hydrolysis to

connect boehmite nanowires and acrylate matrix. However, the weak acid surface of boehmite

nanowires may lead to fast hydrolysis reaction and condensation of TMSPM, which resulted in

decrease of surface modification effects. And this was why we claimed that TMSPM was not

coupling agent for all purpose.

Fig. S3. Schematic illustration of grafting of β-CEA onto the surface of boehmite nanowires and co-

crosslink of β-CEA-AlOOH with acrylate matrix. The carboxyl groups of β-CEA interacted with

surface hydroxyls of boehmite nanowires to graft acrylate groups onto boehmite nanowires. And

consequently, better dispersion of boehmite nanowires and strong filler-matrix interaction were

obtained.

Thermal test.

The representative thermalgravimetric (TG) and the corresponding differential thermogravimetric

(DTG) curves were shown in Fig. S4. The peak of DTG curve illustrates the highest mass loss of

sample at corresponding temperature, in other words, decomposition of the sample. Accordingly, the

decomposition temperature of neat HDDA and composites with certain amount (2 wt%) of

nanofillers were obtained through DTG curves and listed in Table S1. A slight improvement of

decomposition temperature was found in both modified boehmite nanowire incorporated composites.

As aforementioned, the filler content in the tested samples was 2 wt%, thus, it was inferred that such

small incorporation amount would induce limited extent of improvement in thermal stability of

composites.

Fig. S4. TGA curves (a) and the corresponding DTG curves (b) of neat HDDA and HDDA composite

resins with neat and modified boehmite nanowires. Note that the amount of boehmite nanowires in

composite resins was 2 wt%.

Tab. S1. Peak temperature of neat HDDA and composite resinsa obtained from DTG curves.

Sample Tpeak/C

Neat HDDA 427

HDDA+neat AlOOH 426

HDDA+TMSPM-AlOOH 433

HDDA+β-CEA-AlOOH 437aNote that the mass content of boehmite nanowires in composites was kept at 2%.

Fig. S5. Rheological characterization of neat HDDA resin and composite resins with neat and

modified AlOOH nanowires.

Fig. S6. Dispersion stability of HDDA resins with β-CEA-AlOOH (labeled as β) and neat AlOOH

(labeled as N): a, after sonication; b, 4 h after sonication; c, 24 h after sonication. The amount of neat

and β-CEA modified boehmite nanowires were 2 wt% for both resins.

Fig. S7. Illustration of printing performance: (a) HDDA resin with β-CEA modified boehmite

nanowires (4 wt%) and (b) neat HDDA resin. The dimensions of printed Fullerene model was 5 cm*5 cm*4.7 cm. The exquisite and delicate structure of Fullerene made it difficult to

manufacture by traditional processes. The successful fabrication of Fullerene model confirmed the

excellent performance in DLP-based 3D printing.

Tab. S1 Relative peak intensity calculated from FTIR spectra.

Sample Relative peak intensity

1070 cm-1 3290 cm-1

Neat AlOOH 0.75 0.75

β-CEA-AlOOH 0.74 0.65

TMSPM-AlOOH 0.82 0.76

Fig. S8. The acidity of AlOOH-H2O dispersion.