- Email: [email protected]
Toughening PMMA with fillers containing polymer brushes synthesized via atom transfer radical polymerization (ATRP)
Accepted Manuscript Toughening PMMA with fillers containing polymer brushes synthesized via atom transfer radical polymerization (ATRP) Joshua M. Kubiak, Jiajun Yan, Joanna Pietrasik, Krzysztof Matyjaszewski PII:
S0032-3861(17)30380-4
DOI:
10.1016/j.polymer.2017.04.012
Reference:
JPOL 19592
To appear in:
Polymer
Received Date: 16 January 2017 Revised Date:
2 April 2017
Accepted Date: 4 April 2017
Please cite this article as: Kubiak JM, Yan J, Pietrasik J, Matyjaszewski K, Toughening PMMA with fillers containing polymer brushes synthesized via atom transfer radical polymerization (ATRP), Polymer (2017), doi: 10.1016/j.polymer.2017.04.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
RI PT
Toughening PMMA with Fillers Containing Polymer Brushes Synthesized via Atom Transfer
SC
Radical Polymerization (ATRP)
a
M AN U
Joshua M. Kubiaka,1 , Jiajun Yana, Joanna Pietrasikb, Krzysztof Matyjaszewskia,* Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States b
Institute of Polymer and Dye Technology, Lodz University of Technology, Stefanowskiego
1
TE D
12/16, 90-924 Lodz, Poland
Present Address: Department of Materials Science and Engineering, Massachusetts Institute of
EP
Technology, Cambridge, Massachusetts 02139, United States
AC C
KEYWORDS: PMMA, toughening filler, star polymer, particle brush, ATRP
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
GRAPHIC ABSTRACT
ACCEPTED MANUSCRIPT
RI PT
Toughening PMMA with Particle Brushes and Star Polymers Synthesized via Atom Transfer Radical
SC
Polymerization (ATRP)
a
M AN U
Joshua M. Kubiaka,1 , Jiajun Yana, Joanna Pietrasikb, Krzysztof Matyjaszewskia,* Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States b
Institute of Polymer and Dye Technology, Lodz University of Technology, Stefanowskiego
1
TE D
12/16, 90-924 Lodz, Poland
Present Address: Department of Materials Science and Engineering, Massachusetts Institute of
EP
Technology, Cambridge, Massachusetts 02139, United States
AC C
KEYWORDS: PMMA, toughening filler, star polymer, particle brush, ATRP
ABSTRACT
Hybrid particle brushes and star polymers with a core-shell architecture were prepared
via atom transfer radical polymerization (ATRP) and incorporated into bulk poly(methyl methacrylate) (PMMA) in order to improve the toughness and ductility of glassy, transparent PMMA. Mechanical and optical characterization demonstrated an improvement in both the
1
ACCEPTED MANUSCRIPT
modulus and the toughness of the reinforced materials without significantly compromising the
AC C
EP
TE D
M AN U
SC
RI PT
optical transparency.
2
ACCEPTED MANUSCRIPT
INTRODUCTION Poly(methyl methacrylate) (PMMA) has a glass transition temperature (Tg) of 105 °C and has been broadly utilized as a substitute for glass owing to its outstanding transparency, high
RI PT
modulus, relatively low cost, and ease of processing. However, the pure glassy PMMA is brittle and tends to fail by cracking and chain scission when stressed.[1] Brittle failure is often avoided by the incorporation of a small fraction of filler particles. Inorganic materials, such as alumina,
SC
have been studied extensively and shown to significantly increase toughness.[2] Anisotropic fillers such as carbon nanotubes have also been exploited to significantly augment the
M AN U
mechanical properties of PMMA.[3] A promising class of fillers are rubber particles which can form an interfacial bond with the bulk plastic and absorb energy during fracture via multiple mechanisms. Engineered rubber particles for use as impact modifiers are commercially available from companies, including Arkema and Dow. Because glassy polymers, such as PMMA, are
TE D
often used in applications requiring high optical transparency, care must be taken so that the incorporation of a filler material does not lead to excessive light scattering in the material which would reduce optical transparency and cause the filled material to be ill-suited as a glass
EP
substitute.[4]
Engineered rubber filler particles are typically made with a core-shell architecture in which the
AC C
core is designed to undergo cavitation and induce shear yielding to dissipate energy. The shell both improves filler distribution and allows transfer of stress from the matrix to the filler. Such behavior has been observed in polycarbonate,[5] polyester resin,[6] and polystyrene matrices.[7] The core is commonly composed of poly(n-butyl acrylate) or polybutadiene which are soft materials that can absorb stress and reduce cracking; the shell is typically similar to or the same as the bulk material to promote dispersion and avoid aggregation
3
ACCEPTED MANUSCRIPT
Because of PMMA’s utility as a glass substitute, toughening of PMMA has garnered a significant level of attention. Commercially available toughened PMMA or Plexiglass™ products are used in durable goods including automotive parts, lighting, and medical supplies.
RI PT
PMMA can be toughened with low Tg inclusions, which increase fracture and bulk toughness by different mechanisms, primarily crazing, cavitation,[8] and shearing.[9] The prevalence of a particular mechanism is sensitive to stress, loading speed, and ambient conditions. Additionally,
SC
the effectiveness of toughening fillers is known to be size dependent. Smaller particles are more effective at initiating shear deformation and crazing, while larger particles sustain propagation of
M AN U
crazing by absorbing more energy.[10, 11] The interface between the shell and matrix is also critical and is made more coherent by increasing the molecular weight of the shell polymer. The use of an interfacial layer is particularly important for stress reduction at high strain rates such as those experienced during impact.[12]
TE D
Synthesis of core-shell filler materials has been conducted by a variety of methods. A twostage emulsion synthesis is frequently employed in which the core is synthesized in the presence of a crosslinking agent followed by swelling with the shell monomer and a second
EP
polymerization.[6] In this free-radical method, particle size is controlled by adjusting the size of emulsion droplets and monomer concentrations within the droplets. Emulsion polymerization has
AC C
been used to create gradient[13] and multi-layer rubber particles,[14] and their performance has shown that the internal structure of the particle influences toughening ability. Control over filler architecture can also be provided by the use of controlled radical polymerization (CRP) techniques including atom transfer radical polymerization (ATRP)[15-17] and reversible addition-fragmentation chain-transfer (RAFT) polymerization.[18] Core-shell fillers have been synthesized by grafting polymer chains from functionalized organic
4
ACCEPTED MANUSCRIPT
materials,[19] and multilayer particle brushes have been prepared from inorganic core particles.[20-23] Incorporation of neat, hard particles typically decreases ductility and toughness. However, polymer grafted particles avoid this drawback providing improvements in ductility,
RI PT
modulus, and strength when used as filler materials for glassy polymers.[24] In addition, fully organic core-shell particles have been synthesized by the preparation of polymer stars with multi-block arms.[25] Rubber toughening particles have also been prepared by emulsion CRP of
SC
block copolymers.[26] The use of a controlled polymerization method, such as ATRP, has significant benefits. These methods provide precise control over the polymer chain length which
M AN U
allows the refractive index of the filler particles to be matched to the refractive index of the matrix. Index matching avoids scattering, and the composite materials maintain a glass-like transparency.[27] Furthermore, previous work has demonstrated the existence of a critical length for the polymer chains of a particle brush beyond which the chains can entangle providing a
TE D
coherent interaction between particles and the matrix.[28] It is anticipated that controlling entanglement of a filler material with the matrix will allow tuning of the strength of the interface between the filler and the matrix. In this paper, both multi-layer particle brushes and block-
EP
copolymer stars are investigated as filler materials for the toughening of PMMA. An azeotropic copolymer poly(styrene-co-acrylonitrile) (PSAN) was used as outer layer for both star polymer
AC C
and particle brush fillers, to achieve desirable compatibility due to the enthalpic interaction.[29] EXPERIMENTAL SECTION Materials. Butyl acrylate (BA, 99%, Acros), styrene (S, 99%, Aldrich), and acrylonitrile (AN, 99%, Aldrich) were passed through a column of basic alumina to remove inhibitor before using. 2,2’-Azobisisobutyronitrile (AIBN, 98%, Aldrich), copper(II) bromide (CuBr2, 99.9%, Aldrich), tris(2-(dimethylamino)ethyl)amine (Me6TREN, 99%, Alfa), tin(II) 2-ethylhexanoate (Sn(EH)2,
5
ACCEPTED MANUSCRIPT
92.5-100%, Aldrich), methyl 2-bromopropionate (MBP, 98%, Aldrich), diethylene glycol diacrylate (DEGDA, 75%, Aldrich), copper(I) bromide (CuBr, 99%, Aldrich), N,N,N',N'',N''pentamethyldiethylenetriamine (PMDETA, 99%, Aldrich) were used as received. Microbeads of
RI PT
poly(methyl methacrylate) (75,000 g/mol, 200 µm) were purchased from Polysciences. A slurry of silica nanoparticles (15 nm) in methyl isobutyl ketone (MIBK-ST) was kindly donated by Nissan Chemical.
SC
Instrumentation. The apparent number-average molecular weights (Mn,app) and dispersity (Mw/Mn) were measured by size exclusion chromatography (SEC). The SEC was conducted with
M AN U
a Waters 515 pump and Waters 2414 differential refractometer using PSS columns (SDV 105, 103, 500 Å) with THF as eluent at 35 °C and at a flow rate of 1 mL/min. Linear PMMA standards were used for calibration. Monomer conversions were monitored with 1H nuclear magnetic resonance (1H NMR) spectroscopy performed on a Bruker Avance 300 MHz
TE D
spectroscope with CDCl3 as the solvent. Organic content by mass was evaluated using a TA instruments TGA 2950. Free radical bulk polymerization reactions were performed in cells composed of a 3mm thick PDMS gasket (McMaster Carr) sandwiched between 3”x5” glass
EP
slides backed by stainless steel plates and compressed with spring clamps. Dog bone samples were prepared using a Rabbit RL 1290 80W laser cutter. Tensile testing was performed on an
AC C
Instron 4400R tensile frame at a rate of 10 mm/min using a Type V specimen as described in ASTM D638-10. Optical microscopy was performed on a Lecia DM750M light microscope. The hydrodynamic size distributions of the fillers in THF were measured using dynamic light scattering (DLS) on a Malvern Zetasizer Nano ZS particle analyzer. Initiator Functionalization of Silica. Silica nanoparticles were modified with ATRP initiators according to previously reported procedures.[30, 31]
6
ACCEPTED MANUSCRIPT
Surface initiated ATRP of BA from Silica. In order to graft BA from initiator functionalized silica, 27 mL of filtered BA, 0.5 g of functionalized silica, 4.2 mg of CuBr2, and 35 µL of Me6TREN were dissolved in 27 mL of anisole in a clean 100 mL Schlenk flask. The flask was
RI PT
sealed, and nitrogen was bubbled through the solution for 1 hour. A solution of 51 mg/mL Sn(EH)2 in anisole was prepared separately in a 20 mL glass vial and bubbled with nitrogen for 20 minutes. A 0.1 mL aliquot of the tin solution was injected into the Schlenk flask to reduce a
SC
fraction of the catalyst complex to the activator state, and the flask was heated to 70 °C in an oil bath until 10-20% conversion of the monomer was measured by NMR. The reaction was stopped
M AN U
by exposure to air and dilution with THF, and the product was isolated and purified by dialysis against methanol.
Chain extension of SiO2-g-PBA with S/AN. 2.2 g of SiO2-g-PBA was combined with 7.8 mL of filtered St, 2.7 mL of filtered AN, 2.0 mg of CuBr2, 24 µL of Me6TREN, 32 mL of DMF, and
TE D
63 mL of anisole in a 200 mL Schlenk flask,. The flask was sealed and cooled in a salted ice bath. Dry N2 was bubbled through the solution for 1 hour. A solution of 92 mg/mL Sn(EH)2 in anisole was prepared in a 20 mL glass vial and bubbled with nitrogen for 20 min. A 0.4 mL
EP
aliquot of the tin solution was injected into the Schlenk flask, and the reaction was bubbled with
AC C
dry N2 for an additional 10 minutes while in the ice bath. The flask was then heated to 60 °C in an oil bath for approximately 3 hours or until 5-10% of the monomer was converted to polymer. The reaction was stopped by exposure to air and dilution with THF and the product was isolated and purified by dialysis in methanol. Synthesis of PBA Star Polymer. A PBA star polymer was prepared using an in-situ core-first method.[32, 33] The core was prepared by copolymerization of 0.69 mL of DEGDA cross-linker with 0.2 mL of MBP, in the presence of 0.83 mL of PMDETA, 20 mg of CuBr2, and 9.0 mL of
7
ACCEPTED MANUSCRIPT
dimethylformamide (DMF) in a Schlenk flask. The flask was sealed, and the solution was bubbled with dry N2 for 20 minutes before being frozen with liquid nitrogen. The flask was opened, and 0.12 g of CuBr was added. After being resealed, the flask was purged with dry N2
RI PT
for 1 hour before being thawed. The reaction mixture was heated to 80 °C in an oil bath for 1 hour. Next, the arms were prepared by addition of 77 mL of degassed, filtered BA to the flask, and the “grafting-from” polymerization reaction was allowed to proceed at 80 °C for 47 hours.
M AN U
by precipitation in cold hexane-acetone 9:1.
SC
The reaction was stopped by exposure to air and dilution with THF, and the product was isolated
Extension of SiO2-g-PBA with S/AN. In a 200 mL Schlenk flask, 2.2 g of SiO2-g-PBA was combined with 7.8 mL of filtered St, 2.7 mL of filtered AN, 2.0 mg of CuBr2, 24 µL of Me6TREN, 32 mL of DMF, and 63 mL of anisole. The flask was sealed and cooled in a salted ice bath. Dry N2 was bubbled through the solution for 1 hour. In a 20 mL glass vial, a solution of
TE D
92 mg/mL Sn(EH)2 in anisole was prepared and bubbled with nitrogen for 20 min. A 0.4 mL aliquot of the tin solution was injected into the Schlenk flask, and the reaction was bubbled with dry N2 for an additional 10 minutes while in the ice bath. The flask was then heated to 60 °C in
EP
an oil bath for approximately 3 hours or until 5-10% of the monomer was converted to polymer.
AC C
The reaction was stopped by exposure to air and dilution with THF and the product was isolated and purified by dialysis in methanol. Chain extension of the PBA Star with S/AN. A similar procedure to that employed for chain extension from PBA particle brushes was used. Preparation of PMMA samples. To produce plastic sample suitable for testing physical properties, MMA was polymerized in a mold using a free radical method with and without a
8
ACCEPTED MANUSCRIPT
filler material. A solution of 20 vol % PMMA microbeads in filtered MMA was prepared by dissolving 7.6 g of microbeads in 40 mL of MMA overnight. Once homogenous, the desired wt % of filler material was added to a 12 mL aliquot of the MMA solution in a 20 mL vial, and the
RI PT
filler was dispersed by combination of vortex mixing and sonication. Once a homogenous mixture was attained, 0.25 wt % (relative to MMA and PMMA) AIBN was added to each 12 mL aliquot. Each solution was stirred for 5 minutes and degassed under mechanical vacuum. A
SC
syringe was used to fill each mold to approximately 85% full (~11.5 mL). The molds were placed in a 50 °C water bath for 48 hours. The temperature of the bath was then raised to 95 °C
RESULTS AND DISCUSSION
M AN U
for 1 hour. Molds were allowed to cool before samples were removed and rinsed with water.
Three PBA-b-PSAN star polymer samples were prepared using the two-step, core-first ATRP
TE D
synthesis shown in Scheme 1. In addition, a SiO2-g-(PBA-b-PSAN) particle brush material was prepared using the two-step ATRP process shown in Scheme 2. The specifications of these materials were determined by SEC and TGA and are detailed in Table 1. In the star polymer
EP
samples (S1, S2, S3), the length of the PBA core block remained constant, while the length of
AC C
the PSAN interface block was varied. The hybrid particle brush material (PB1) was prepared by grafting from 15 nm silica nanoparticles. Scheme 1. Preparation of PBA-b-PSAN star polymers.
9
ACCEPTED MANUSCRIPT
RI PT
Scheme 2. Preparation SiO2-g-(PBA-b-PSAN) particle brushes.
SC
Table 1. PMMA filler materials prepared by ATRP
Sample
architecture
Mn a (PBA)
S1
star
10 500
25 800
S2
star
10 500
S3
star
PB1
particle brush
# chainsb
Dhc
neffd
12
288±5
1.53
20 600
12
214±7
1.52
10 500
18 600
12
205±4
1.52
20 800
37 900
400
252±2
1.49
M AN U
Mn (PBA-bPSAN)
a
b
TE D
SEC number-average molecular weight of polymer. Determined by SEC-MALLS for star polymers, by TGA for the particle brush filler. c Z-averaged hydrodynamic diameter measured in THF by DLS. d Effective refractive index at 589 nm estimated by the Maxwell-Garnett model.
EP
Slabs of cast PMMA containing varying weight fractions of the synthesized fillers were prepared using conventional free radical polymerization. Then, dog-bone shaped samples were
AC C
machined via laser cutting from the composite materials. Optical clarity was generally high in samples containing the star polymer fillers. However, some opacity was noticeable in samples with higher filler loadings and filler S3. The transparency of a nanocomposite material depends highly on the matching of the refractive indices of the filter and the matrix.[27] The refractive indices of the fillers were estimated based on the Maxwell-Garnett model and listed in Table 1. =
(1 +
3 1−
)
10
ACCEPTED MANUSCRIPT
In this equation, neff is the effective refractive index estimated by the Maxwell-Garnett model; nshell and ncore are the refractive indices of the shell and core component at 589 nm, respectively; ϕcore = vcore/(vcore+vshell) is the volume fraction of the core; and x = (ncore2-nshell2)/(ncore2+2nshell2).
RI PT
The calculated neff of PB1 matches with the refractive index of PMMA (1.489 @589 nm), which agrees with the measured high transparency of the samples (see Figure 1). However, the refractive indices of all three star polymers are slightly off, leading to some scattering. The
SC
reason for the limited transparency of the sample with S3 could be ascribed to the short PSAN outer layer, which exposed more of the PBA block to the PMMA matrix. Consequently, the star
M AN U
polymer filler may have aggregated into larger objects inside the matrix. Images of the remaining slabs after laser cutting and their corresponding transmittance spectra over the near-UV-visible
AC C
EP
TE D
range are shown in Figure 1.
Figure 1. Optical clarity of PMMA sample slabs after removal of dog-bone cut-outs. Two dog-bone samples from each slab were tested in tension to failure. True strain and true stress values were calculated, and ultimate strength as well as elongation at break were recorded. In addition, mechanical toughness, or the energy absorbed during deformation to failure, was calculated by middle Riemann sum of the area under the stress-strain curves. The tensile
11
ACCEPTED MANUSCRIPT
properties are summarized in Table 2, where the wt% filler indicates the entire mass of either the particle brush or the star polymer incorporated into a given sample.
RI PT
Table 2. Mechanical properties of filled and unfilled tensile testing samples.a
%wt filler
energy of fracture (MJ/m3)b
strength (MPa)c
elongation at break (%)d
average transmittance (%)e
-
0
6.1 (± 0.1)
77 (± 1)
14 (± 0.2)
89.3
S1
4
7.1 (± 1.0)
78 (± 2)
15 (± 5)
82.3
S2
7
7.7 (± 0.1)
81 (± 0.1)
16 (± 0.1)
72.9
S3
4
9.4 (±0.1)
87 (± 0.3)
18 (± 0.1)
54.6
8
10.3 (± 1.2)
81 (± 1)
19 (± 2)
45.6
4
6.5 (± 1.1)
80 (± 3)
14 (± 2)
86.7
7
10.3 (± 2.3)
84 (± 5)
19 (± 3)
75.8
Tabulated values are averages of two measurements. b Calculated from area under stressstrain curve. c Maximum sustained true stress. d True strain at failure. e Calculated from the UVvis transmittance spectra between 400 – 760 nm (Figure 1).
TE D
a
M AN U
PB1
SC
filler
Every sample except S1 and PB1 exhibited significant increases in both toughness and strength compared to unfilled PMMA. Elongation at break was also significantly increased for
AC C
EP
these samples (Table 2, Figure 2).
Figure 2. Effect of filler loading on toughness during tensile tests.
12
ACCEPTED MANUSCRIPT
For the particle brush filled samples, toughness and elongation increased significantly with filler loading. The lack of toughening at low loadings of PB1 may be the result of the filler particles providing crack initiation sites while not having a high enough particle concentration to
RI PT
slow crack propagation, especially considering the larger size and mass of the PB1 particles. Considering the star polymer filled samples, S3, which has the shortest PSAN block of the star polymer samples, produced a filled material that exhibited higher toughness than S1 and S2
SC
when comparing similar filler fractions. The results indicate that, although a shell layer is needed to act as an interface, increasing the shell thickness above some minimum molecular weight
M AN U
necessary for a coherent interface is not beneficial. It is noted that a larger number of particles is incorporated for a given weight fraction of filler as the interfacial block size (and therefore the star polymer mass) decreases, which may also contribute to the improved performance. At higher loadings, S3 and PB1 filled samples demonstrated similar levels of toughness.
TE D
Following tensile testing to failure, the fracture surfaces were imaged with an optical microscope (See Figure 3). Fracture planes are visible in the unfilled sample, indicating quick propagation of the crack from an initiation site outward across the sample. The cleavage planes
EP
begin to diminish in appearance at 4 wt % filler loading. Samples with 7-8 wt % filler loading
AC C
show surface regions which are rough and pitted, is indicative of macroscopically ductile failure.
13
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 3. Optical fractography of failed dog-bone samples. All images taken at 50x; scale bars
M AN U
are 500 microns. CONCLUSIONS
Star shaped filler materials having a soft core and an outer layer compatible with the target matrix were prepared via ATRP in addition to a silica based hybrid brush particle. By embedding
TE D
low weight fractions of the prepared materials into a PMMA matrix, the composite material’s toughness and strength were improved while optical clarity was preserved with minimal haze formation. Tensile testing data was further corroborated by optical fractography[34] which
EP
showed that a change in macroscopic fracture mechanism was induced after filler materials were introduced. Higher loadings of the fillers improved the toughness of the PMMA samples, while
AC C
only slightly affecting the samples’ transparency and maintaining or increasing sample strength. A thin, compatibilizing shell layer was found to be critical to improving the toughness. The increases in toughness exhibited by the studeied materials are modest compared to those demonstrated for more traditional rubber particle fillers. However, the use of ATRP to generate core shell materials provides unique design handles allowing easy control of the composition of the energy absorbing core as well as the interfacial shell so that the filler can be altered for use in
14
ACCEPTED MANUSCRIPT
different materials under different loading conditions. Furthermore, hazing can likely be eliminated by adjusting the filler composition to match the refractive index of the matrix polymer. These materials are a step toward developing optimized, low Tg, toughening filler
RI PT
materials for the preparation of transparent PMMA plastics which could be utilized as glass substitutes to provide additional safety in architectural or automotive applications.
SC
AUTHOR INFORMATION
*Email: [email protected] Author Contributions
M AN U
Corresponding Author
The manuscript was written through contributions of all authors. All authors have given approval
ACKNOWLEDGMENT
TE D
to the final version of the manuscript
EP
We acknowledge Nissan Chemical for their generous donation of silica nanoparticles. We thank the NSF (DMR 1501324) and National Science Centre, Poland (via Grant UMO-
AC C
2014/14/A/ST5/00204). J.Y. acknowledges the support from the Richard King Mellon Foundation Presidential Fellowship. J.M.K. acknowledges support from the Charpie Scholarship at Carnegie Mellon University.
REFERENCES [1] M. Ishikawa, H. Ogawa, I. Narisawa, Brittle fracture in glassy polymers, Journal of Macromolecular Science, Part B: Physics 19(3) (1981) 421-443.
15
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[2] B.J. Ash, D.F. Rogers, C.J. Wiegand, L.S. Schadler, R.W. Siegel, B.C. Benicewicz, T. Apple, Mechanical properties of Al2O3/polymethylmethacrylate nanocomposites, Polym. Compos. 23(6) (2002) 1014-1025. [3] J. Zeng, B. Saltysiak, W. Johnson, D.A. Schiraldi, S. Kumar, Processing and properties of poly (methyl methacrylate)/carbon nano fiber composites, Composites Part B: Engineering 35(2) (2004) 173-178. [4] E.T. Kopesky, G.H. McKinley, R.E. Cohen, Toughened poly (methyl methacrylate) nanocomposites by incorporating polyhedral oligomeric silsesquioxanes, Polymer 47(1) (2006) 299-309. [5] D.S. Parker, H. Sue, J. Huang, A. Yee, Toughening mechanisms in core-shell rubber modified polycarbonate, Polymer 31(12) (1990) 2267-2277. [6] Y.J. Huang, J.H. Wu, J.G. Liang, M.W. Hsu, J.K. Ma, Toughening of unsaturated polyester resins with core–shell rubbers, J. Appl. Polym. Sci. 107(2) (2008) 939-950. [7] C. Zhou, S. Wu, B. Yang, Y. Gao, G. Wu, H. Zhang, Toughening Polystyrene by Core-shell Rubber Particles: Analysis of the Internal Structure and Properties, Polymers & Polymer Composites 23(5) (2015) 317. [8] D. Ayre, C. Bucknall, Particle cavitation in rubber-toughened PMMA: experimental testing of the energy-balance criterion, Polymer 39(20) (1998) 4785-4791. [9] M. Todo, K. Takahashi, P.-Y. Ben Jar, P. Beguelin, Toughening Mechanisms of Rubber Toughened PMMA, JSME International Journal Series A Solid Mechanics and Material Engineering 42(4) (1999) 585-591. [10] C. Wrotecki, P. Heim, P. Gaillard, Rubber toughening of poly (methyl methacrylate). Part I: Effect of the size and hard layer composition of the rubber particles, Polymer Engineering & Science 31(4) (1991) 213-217. [11] K. Cho, J. Yang, C.E. Park, The effect of rubber particle size on toughening behaviour of rubber-modified poly (methyl methacrylate) with different test methods, Polymer 39(14) (1998) 3073-3081. [12] K. Cho, J. Yang, C.E. Park, The effect of interfacial adhesion on toughening behaviour of rubber modified poly (methyl methacrylate), Polymer 38(20) (1997) 5161-5167. [13] J.-S. Chung, K.-R. Choi, J.-P. Wu, C.-S. Han, C.-H. Lee, Effect of Poly (butyl acrylate)Poly (methyl methacrylate) Rubber Particle Texture on the Toughening Behavior of Poly (methyl methacrylate), Korea Polymer Journal 9(2) (2001) 122-128. [14] P. Lovell, J. McDonald, D. Saunders, M. Sherratt, R. Young, Multiple-phase Tougheningparticle Morphology: Effects on the Properties of Rubber-toughened Poly (methyl methacrylate), American Chemical Society, Washington, DC (United States), 1993. [15] J.S. Wang, K. Matyjaszewski, Controlled radical polymerization - atom-transfer radical polymerization in the presence of transition-metal complexes, J. Am. Chem. Soc. 117(20) (1995) 5614-5615. [16] K. Matyjaszewski, J. Xia, Atom transfer radical polymerization, Chem. Rev. 101(9) (2001) 2921-2990. [17] K. Matyjaszewski, Atom transfer radical polymerization (ATRP): current status and future perspectives, Macromolecules 45(10) (2012) 4015-4039. [18] J. Chiefari, Y.K. Chong, F. Ercole, J. Krstina, J. Jeffery, T.P.T. Le, R.T.A. Mayadunne, G.F. Meijs, C.L. Moad, G. Moad, E. Rizzardo, S.H. Thang, Living Free-Radical Polymerization by Reversible Addition−Fragmentation Chain Transfer: The RAFT Process, Macromolecules 31(16) (1998) 5559-5562.
16
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[19] T. Shah, C. Gupta, R.L. Ferebee, M.R. Bockstaller, N.R. Washburn, Extraordinary toughening and strengthening effect in polymer nanocomposites using lignin-based fillers synthesized by ATRP, Polymer 72 (2015) 406-412. [20] J. Pyun, S. Jia, T. Kowalewski, G.D. Patterson, K. Matyjaszewski, Synthesis and Characterization of Organic/Inorganic Hybrid Nanoparticles: Kinetics of Surface-Initiated Atom Transfer Radical Polymerization and Morphology of Hybrid Nanoparticle Ultrathin Films, Macromolecules 36(14) (2003) 5094-5104. [21] C.M. Hui, J. Pietrasik, M. Schmitt, C. Mahoney, J. Choi, M.R. Bockstaller, K. Matyjaszewski, Surface-Initiated Polymerization as an Enabling Tool for Multifunctional (Nano)Engineered Hybrid Materials, Chemistry of Materials 26(1) (2013) 745-762. [22] R. Barbey, L. Lavanant, D. Paripovic, N. Schüwer, C. Sugnaux, S. Tugulu, H.-A. Klok, Polymer brushes via surface-initiated controlled radical polymerization: synthesis, characterization, properties, and applications, Chem. Rev. 109(11) (2009) 5437-5527. [23] A. Khabibullin, E. Mastan, K. Matyjaszewski, S. Zhu, Surface-Initiated Atom Transfer Radical Polymerization, Adv. Polym. Sci. 270 (2016) 29-76. [24] D. Maillard, S.K. Kumar, B. Fragneaud, J.W. Kysar, A. Rungta, B.C. Benicewicz, H. Deng, L.C. Brinson, J.F. Douglas, Mechanical properties of thin glassy polymer films filled with spherical polymer-grafted nanoparticles, Nano Lett. 12(8) (2012) 3909-3914. [25] H. Gao, K. Matyjaszewski, Modular Approaches to Star and Miktoarm Star Polymers by ATRP of Cross‐Linkers, Macromolecular symposia, Wiley Online Library, 2010, pp. 12-16. [26] Y. Zhu, X. Gao, Y. Luo, Core–shell particles of poly (methyl methacrylate)‐block‐poly (n‐butyl acrylate) synthesized via reversible addition–fragmentation chain‐transfer emulsion polymerization and the polymer's application in toughening polycarbonate, J. Appl. Polym. Sci. 133(1) (2016). [27] L. Bombalski, H. Dong, J. Listak, K. Matyjaszewsk, M.R. Bockstaller, Null‐Scattering Hybrid Particles Using Controlled Radical Polymerization, Adv. Mater. 19(24) (2007) 44864490. [28] J. Choi, C.M. Hui, J. Pietrasik, H. Dong, K. Matyjaszewski, M.R. Bockstaller, Toughening fragile matter: mechanical properties of particle solids assembled from polymer-grafted hybrid particles synthesized by ATRP, Soft Matter 8(15) (2012) 4072-4082. [29] S. Ojha, A. Dang, C.M. Hui, C. Mahoney, K. Matyjaszewski, M.R. Bockstaller, Strategies for the synthesis of thermoplastic polymer nanocomposite materials with high inorganic filling fraction, Langmuir 29(28) (2013) 8989-8996. [30] J. Yan, X. Pan, M. Schmitt, Z. Wang, M.R. Bockstaller, K. Matyjaszewski, Enhancing Initiation Efficiency in Metal-Free Surface-Initiated Atom Transfer Radical Polymerization (SIATRP), ACS Macro Lett. 5 (2016) 661-665. [31] J. Yan, T. Kristufek, M. Schmitt, Z. Wang, G. Xie, A. Dang, C.M. Hui, J. Pietrasik, M.R. Bockstaller, K. Matyjaszewski, Matrix-free Particle Brush System with Bimodal Molecular Weight Distribution Prepared by SI-ATRP, Macromolecules 48(22) (2015) 8208-8218. [32] J. Kamada, K. Koynov, C. Corten, A. Juhari, J.A. Yoon, M.W. Urban, A.C. Balazs, K. Matyjaszewski, Redox Responsive Behavior of Thiol/Disulfide-Functionalized Star Polymers Synthesized via Atom Transfer Radical Polymerization, Macromolecules 43(9) (2010) 41334139. [33] H. Gao, K. Matyjaszewski, Synthesis of Star Polymers by A New “Core-First” Method: Sequential Polymerization of Cross-Linker and Monomer, Macromolecules 41(4) (2008) 11181125.
17
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[34] S. Gong, S. Bandyopadhyay, Fracture properties and fracture surface morphologies in rubber-PMMA composites, J. Mater. Eng. Perform. 16(5) (2007) 607-613.
18
ACCEPTED MANUSCRIPT
Star-shaped tri-layer fillers for PMMA were prepared via ATRP. Mechanical strength and toughness of the filled materials was improved.
AC C
EP
TE D
M AN U
SC
RI PT
Transparency of the filled materials was retained.
S0032-3861(17)30380-4
DOI:
10.1016/j.polymer.2017.04.012
Reference:
JPOL 19592
To appear in:
Polymer
Received Date: 16 January 2017 Revised Date:
2 April 2017
Accepted Date: 4 April 2017
Please cite this article as: Kubiak JM, Yan J, Pietrasik J, Matyjaszewski K, Toughening PMMA with fillers containing polymer brushes synthesized via atom transfer radical polymerization (ATRP), Polymer (2017), doi: 10.1016/j.polymer.2017.04.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
RI PT
Toughening PMMA with Fillers Containing Polymer Brushes Synthesized via Atom Transfer
SC
Radical Polymerization (ATRP)
a
M AN U
Joshua M. Kubiaka,1 , Jiajun Yana, Joanna Pietrasikb, Krzysztof Matyjaszewskia,* Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States b
Institute of Polymer and Dye Technology, Lodz University of Technology, Stefanowskiego
1
TE D
12/16, 90-924 Lodz, Poland
Present Address: Department of Materials Science and Engineering, Massachusetts Institute of
EP
Technology, Cambridge, Massachusetts 02139, United States
AC C
KEYWORDS: PMMA, toughening filler, star polymer, particle brush, ATRP
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
GRAPHIC ABSTRACT
ACCEPTED MANUSCRIPT
RI PT
Toughening PMMA with Particle Brushes and Star Polymers Synthesized via Atom Transfer Radical
SC
Polymerization (ATRP)
a
M AN U
Joshua M. Kubiaka,1 , Jiajun Yana, Joanna Pietrasikb, Krzysztof Matyjaszewskia,* Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States b
Institute of Polymer and Dye Technology, Lodz University of Technology, Stefanowskiego
1
TE D
12/16, 90-924 Lodz, Poland
Present Address: Department of Materials Science and Engineering, Massachusetts Institute of
EP
Technology, Cambridge, Massachusetts 02139, United States
AC C
KEYWORDS: PMMA, toughening filler, star polymer, particle brush, ATRP
ABSTRACT
Hybrid particle brushes and star polymers with a core-shell architecture were prepared
via atom transfer radical polymerization (ATRP) and incorporated into bulk poly(methyl methacrylate) (PMMA) in order to improve the toughness and ductility of glassy, transparent PMMA. Mechanical and optical characterization demonstrated an improvement in both the
1
ACCEPTED MANUSCRIPT
modulus and the toughness of the reinforced materials without significantly compromising the
AC C
EP
TE D
M AN U
SC
RI PT
optical transparency.
2
ACCEPTED MANUSCRIPT
INTRODUCTION Poly(methyl methacrylate) (PMMA) has a glass transition temperature (Tg) of 105 °C and has been broadly utilized as a substitute for glass owing to its outstanding transparency, high
RI PT
modulus, relatively low cost, and ease of processing. However, the pure glassy PMMA is brittle and tends to fail by cracking and chain scission when stressed.[1] Brittle failure is often avoided by the incorporation of a small fraction of filler particles. Inorganic materials, such as alumina,
SC
have been studied extensively and shown to significantly increase toughness.[2] Anisotropic fillers such as carbon nanotubes have also been exploited to significantly augment the
M AN U
mechanical properties of PMMA.[3] A promising class of fillers are rubber particles which can form an interfacial bond with the bulk plastic and absorb energy during fracture via multiple mechanisms. Engineered rubber particles for use as impact modifiers are commercially available from companies, including Arkema and Dow. Because glassy polymers, such as PMMA, are
TE D
often used in applications requiring high optical transparency, care must be taken so that the incorporation of a filler material does not lead to excessive light scattering in the material which would reduce optical transparency and cause the filled material to be ill-suited as a glass
EP
substitute.[4]
Engineered rubber filler particles are typically made with a core-shell architecture in which the
AC C
core is designed to undergo cavitation and induce shear yielding to dissipate energy. The shell both improves filler distribution and allows transfer of stress from the matrix to the filler. Such behavior has been observed in polycarbonate,[5] polyester resin,[6] and polystyrene matrices.[7] The core is commonly composed of poly(n-butyl acrylate) or polybutadiene which are soft materials that can absorb stress and reduce cracking; the shell is typically similar to or the same as the bulk material to promote dispersion and avoid aggregation
3
ACCEPTED MANUSCRIPT
Because of PMMA’s utility as a glass substitute, toughening of PMMA has garnered a significant level of attention. Commercially available toughened PMMA or Plexiglass™ products are used in durable goods including automotive parts, lighting, and medical supplies.
RI PT
PMMA can be toughened with low Tg inclusions, which increase fracture and bulk toughness by different mechanisms, primarily crazing, cavitation,[8] and shearing.[9] The prevalence of a particular mechanism is sensitive to stress, loading speed, and ambient conditions. Additionally,
SC
the effectiveness of toughening fillers is known to be size dependent. Smaller particles are more effective at initiating shear deformation and crazing, while larger particles sustain propagation of
M AN U
crazing by absorbing more energy.[10, 11] The interface between the shell and matrix is also critical and is made more coherent by increasing the molecular weight of the shell polymer. The use of an interfacial layer is particularly important for stress reduction at high strain rates such as those experienced during impact.[12]
TE D
Synthesis of core-shell filler materials has been conducted by a variety of methods. A twostage emulsion synthesis is frequently employed in which the core is synthesized in the presence of a crosslinking agent followed by swelling with the shell monomer and a second
EP
polymerization.[6] In this free-radical method, particle size is controlled by adjusting the size of emulsion droplets and monomer concentrations within the droplets. Emulsion polymerization has
AC C
been used to create gradient[13] and multi-layer rubber particles,[14] and their performance has shown that the internal structure of the particle influences toughening ability. Control over filler architecture can also be provided by the use of controlled radical polymerization (CRP) techniques including atom transfer radical polymerization (ATRP)[15-17] and reversible addition-fragmentation chain-transfer (RAFT) polymerization.[18] Core-shell fillers have been synthesized by grafting polymer chains from functionalized organic
4
ACCEPTED MANUSCRIPT
materials,[19] and multilayer particle brushes have been prepared from inorganic core particles.[20-23] Incorporation of neat, hard particles typically decreases ductility and toughness. However, polymer grafted particles avoid this drawback providing improvements in ductility,
RI PT
modulus, and strength when used as filler materials for glassy polymers.[24] In addition, fully organic core-shell particles have been synthesized by the preparation of polymer stars with multi-block arms.[25] Rubber toughening particles have also been prepared by emulsion CRP of
SC
block copolymers.[26] The use of a controlled polymerization method, such as ATRP, has significant benefits. These methods provide precise control over the polymer chain length which
M AN U
allows the refractive index of the filler particles to be matched to the refractive index of the matrix. Index matching avoids scattering, and the composite materials maintain a glass-like transparency.[27] Furthermore, previous work has demonstrated the existence of a critical length for the polymer chains of a particle brush beyond which the chains can entangle providing a
TE D
coherent interaction between particles and the matrix.[28] It is anticipated that controlling entanglement of a filler material with the matrix will allow tuning of the strength of the interface between the filler and the matrix. In this paper, both multi-layer particle brushes and block-
EP
copolymer stars are investigated as filler materials for the toughening of PMMA. An azeotropic copolymer poly(styrene-co-acrylonitrile) (PSAN) was used as outer layer for both star polymer
AC C
and particle brush fillers, to achieve desirable compatibility due to the enthalpic interaction.[29] EXPERIMENTAL SECTION Materials. Butyl acrylate (BA, 99%, Acros), styrene (S, 99%, Aldrich), and acrylonitrile (AN, 99%, Aldrich) were passed through a column of basic alumina to remove inhibitor before using. 2,2’-Azobisisobutyronitrile (AIBN, 98%, Aldrich), copper(II) bromide (CuBr2, 99.9%, Aldrich), tris(2-(dimethylamino)ethyl)amine (Me6TREN, 99%, Alfa), tin(II) 2-ethylhexanoate (Sn(EH)2,
5
ACCEPTED MANUSCRIPT
92.5-100%, Aldrich), methyl 2-bromopropionate (MBP, 98%, Aldrich), diethylene glycol diacrylate (DEGDA, 75%, Aldrich), copper(I) bromide (CuBr, 99%, Aldrich), N,N,N',N'',N''pentamethyldiethylenetriamine (PMDETA, 99%, Aldrich) were used as received. Microbeads of
RI PT
poly(methyl methacrylate) (75,000 g/mol, 200 µm) were purchased from Polysciences. A slurry of silica nanoparticles (15 nm) in methyl isobutyl ketone (MIBK-ST) was kindly donated by Nissan Chemical.
SC
Instrumentation. The apparent number-average molecular weights (Mn,app) and dispersity (Mw/Mn) were measured by size exclusion chromatography (SEC). The SEC was conducted with
M AN U
a Waters 515 pump and Waters 2414 differential refractometer using PSS columns (SDV 105, 103, 500 Å) with THF as eluent at 35 °C and at a flow rate of 1 mL/min. Linear PMMA standards were used for calibration. Monomer conversions were monitored with 1H nuclear magnetic resonance (1H NMR) spectroscopy performed on a Bruker Avance 300 MHz
TE D
spectroscope with CDCl3 as the solvent. Organic content by mass was evaluated using a TA instruments TGA 2950. Free radical bulk polymerization reactions were performed in cells composed of a 3mm thick PDMS gasket (McMaster Carr) sandwiched between 3”x5” glass
EP
slides backed by stainless steel plates and compressed with spring clamps. Dog bone samples were prepared using a Rabbit RL 1290 80W laser cutter. Tensile testing was performed on an
AC C
Instron 4400R tensile frame at a rate of 10 mm/min using a Type V specimen as described in ASTM D638-10. Optical microscopy was performed on a Lecia DM750M light microscope. The hydrodynamic size distributions of the fillers in THF were measured using dynamic light scattering (DLS) on a Malvern Zetasizer Nano ZS particle analyzer. Initiator Functionalization of Silica. Silica nanoparticles were modified with ATRP initiators according to previously reported procedures.[30, 31]
6
ACCEPTED MANUSCRIPT
Surface initiated ATRP of BA from Silica. In order to graft BA from initiator functionalized silica, 27 mL of filtered BA, 0.5 g of functionalized silica, 4.2 mg of CuBr2, and 35 µL of Me6TREN were dissolved in 27 mL of anisole in a clean 100 mL Schlenk flask. The flask was
RI PT
sealed, and nitrogen was bubbled through the solution for 1 hour. A solution of 51 mg/mL Sn(EH)2 in anisole was prepared separately in a 20 mL glass vial and bubbled with nitrogen for 20 minutes. A 0.1 mL aliquot of the tin solution was injected into the Schlenk flask to reduce a
SC
fraction of the catalyst complex to the activator state, and the flask was heated to 70 °C in an oil bath until 10-20% conversion of the monomer was measured by NMR. The reaction was stopped
M AN U
by exposure to air and dilution with THF, and the product was isolated and purified by dialysis against methanol.
Chain extension of SiO2-g-PBA with S/AN. 2.2 g of SiO2-g-PBA was combined with 7.8 mL of filtered St, 2.7 mL of filtered AN, 2.0 mg of CuBr2, 24 µL of Me6TREN, 32 mL of DMF, and
TE D
63 mL of anisole in a 200 mL Schlenk flask,. The flask was sealed and cooled in a salted ice bath. Dry N2 was bubbled through the solution for 1 hour. A solution of 92 mg/mL Sn(EH)2 in anisole was prepared in a 20 mL glass vial and bubbled with nitrogen for 20 min. A 0.4 mL
EP
aliquot of the tin solution was injected into the Schlenk flask, and the reaction was bubbled with
AC C
dry N2 for an additional 10 minutes while in the ice bath. The flask was then heated to 60 °C in an oil bath for approximately 3 hours or until 5-10% of the monomer was converted to polymer. The reaction was stopped by exposure to air and dilution with THF and the product was isolated and purified by dialysis in methanol. Synthesis of PBA Star Polymer. A PBA star polymer was prepared using an in-situ core-first method.[32, 33] The core was prepared by copolymerization of 0.69 mL of DEGDA cross-linker with 0.2 mL of MBP, in the presence of 0.83 mL of PMDETA, 20 mg of CuBr2, and 9.0 mL of
7
ACCEPTED MANUSCRIPT
dimethylformamide (DMF) in a Schlenk flask. The flask was sealed, and the solution was bubbled with dry N2 for 20 minutes before being frozen with liquid nitrogen. The flask was opened, and 0.12 g of CuBr was added. After being resealed, the flask was purged with dry N2
RI PT
for 1 hour before being thawed. The reaction mixture was heated to 80 °C in an oil bath for 1 hour. Next, the arms were prepared by addition of 77 mL of degassed, filtered BA to the flask, and the “grafting-from” polymerization reaction was allowed to proceed at 80 °C for 47 hours.
M AN U
by precipitation in cold hexane-acetone 9:1.
SC
The reaction was stopped by exposure to air and dilution with THF, and the product was isolated
Extension of SiO2-g-PBA with S/AN. In a 200 mL Schlenk flask, 2.2 g of SiO2-g-PBA was combined with 7.8 mL of filtered St, 2.7 mL of filtered AN, 2.0 mg of CuBr2, 24 µL of Me6TREN, 32 mL of DMF, and 63 mL of anisole. The flask was sealed and cooled in a salted ice bath. Dry N2 was bubbled through the solution for 1 hour. In a 20 mL glass vial, a solution of
TE D
92 mg/mL Sn(EH)2 in anisole was prepared and bubbled with nitrogen for 20 min. A 0.4 mL aliquot of the tin solution was injected into the Schlenk flask, and the reaction was bubbled with dry N2 for an additional 10 minutes while in the ice bath. The flask was then heated to 60 °C in
EP
an oil bath for approximately 3 hours or until 5-10% of the monomer was converted to polymer.
AC C
The reaction was stopped by exposure to air and dilution with THF and the product was isolated and purified by dialysis in methanol. Chain extension of the PBA Star with S/AN. A similar procedure to that employed for chain extension from PBA particle brushes was used. Preparation of PMMA samples. To produce plastic sample suitable for testing physical properties, MMA was polymerized in a mold using a free radical method with and without a
8
ACCEPTED MANUSCRIPT
filler material. A solution of 20 vol % PMMA microbeads in filtered MMA was prepared by dissolving 7.6 g of microbeads in 40 mL of MMA overnight. Once homogenous, the desired wt % of filler material was added to a 12 mL aliquot of the MMA solution in a 20 mL vial, and the
RI PT
filler was dispersed by combination of vortex mixing and sonication. Once a homogenous mixture was attained, 0.25 wt % (relative to MMA and PMMA) AIBN was added to each 12 mL aliquot. Each solution was stirred for 5 minutes and degassed under mechanical vacuum. A
SC
syringe was used to fill each mold to approximately 85% full (~11.5 mL). The molds were placed in a 50 °C water bath for 48 hours. The temperature of the bath was then raised to 95 °C
RESULTS AND DISCUSSION
M AN U
for 1 hour. Molds were allowed to cool before samples were removed and rinsed with water.
Three PBA-b-PSAN star polymer samples were prepared using the two-step, core-first ATRP
TE D
synthesis shown in Scheme 1. In addition, a SiO2-g-(PBA-b-PSAN) particle brush material was prepared using the two-step ATRP process shown in Scheme 2. The specifications of these materials were determined by SEC and TGA and are detailed in Table 1. In the star polymer
EP
samples (S1, S2, S3), the length of the PBA core block remained constant, while the length of
AC C
the PSAN interface block was varied. The hybrid particle brush material (PB1) was prepared by grafting from 15 nm silica nanoparticles. Scheme 1. Preparation of PBA-b-PSAN star polymers.
9
ACCEPTED MANUSCRIPT
RI PT
Scheme 2. Preparation SiO2-g-(PBA-b-PSAN) particle brushes.
SC
Table 1. PMMA filler materials prepared by ATRP
Sample
architecture
Mn a (PBA)
S1
star
10 500
25 800
S2
star
10 500
S3
star
PB1
particle brush
# chainsb
Dhc
neffd
12
288±5
1.53
20 600
12
214±7
1.52
10 500
18 600
12
205±4
1.52
20 800
37 900
400
252±2
1.49
M AN U
Mn (PBA-bPSAN)
a
b
TE D
SEC number-average molecular weight of polymer. Determined by SEC-MALLS for star polymers, by TGA for the particle brush filler. c Z-averaged hydrodynamic diameter measured in THF by DLS. d Effective refractive index at 589 nm estimated by the Maxwell-Garnett model.
EP
Slabs of cast PMMA containing varying weight fractions of the synthesized fillers were prepared using conventional free radical polymerization. Then, dog-bone shaped samples were
AC C
machined via laser cutting from the composite materials. Optical clarity was generally high in samples containing the star polymer fillers. However, some opacity was noticeable in samples with higher filler loadings and filler S3. The transparency of a nanocomposite material depends highly on the matching of the refractive indices of the filter and the matrix.[27] The refractive indices of the fillers were estimated based on the Maxwell-Garnett model and listed in Table 1. =
(1 +
3 1−
)
10
ACCEPTED MANUSCRIPT
In this equation, neff is the effective refractive index estimated by the Maxwell-Garnett model; nshell and ncore are the refractive indices of the shell and core component at 589 nm, respectively; ϕcore = vcore/(vcore+vshell) is the volume fraction of the core; and x = (ncore2-nshell2)/(ncore2+2nshell2).
RI PT
The calculated neff of PB1 matches with the refractive index of PMMA (1.489 @589 nm), which agrees with the measured high transparency of the samples (see Figure 1). However, the refractive indices of all three star polymers are slightly off, leading to some scattering. The
SC
reason for the limited transparency of the sample with S3 could be ascribed to the short PSAN outer layer, which exposed more of the PBA block to the PMMA matrix. Consequently, the star
M AN U
polymer filler may have aggregated into larger objects inside the matrix. Images of the remaining slabs after laser cutting and their corresponding transmittance spectra over the near-UV-visible
AC C
EP
TE D
range are shown in Figure 1.
Figure 1. Optical clarity of PMMA sample slabs after removal of dog-bone cut-outs. Two dog-bone samples from each slab were tested in tension to failure. True strain and true stress values were calculated, and ultimate strength as well as elongation at break were recorded. In addition, mechanical toughness, or the energy absorbed during deformation to failure, was calculated by middle Riemann sum of the area under the stress-strain curves. The tensile
11
ACCEPTED MANUSCRIPT
properties are summarized in Table 2, where the wt% filler indicates the entire mass of either the particle brush or the star polymer incorporated into a given sample.
RI PT
Table 2. Mechanical properties of filled and unfilled tensile testing samples.a
%wt filler
energy of fracture (MJ/m3)b
strength (MPa)c
elongation at break (%)d
average transmittance (%)e
-
0
6.1 (± 0.1)
77 (± 1)
14 (± 0.2)
89.3
S1
4
7.1 (± 1.0)
78 (± 2)
15 (± 5)
82.3
S2
7
7.7 (± 0.1)
81 (± 0.1)
16 (± 0.1)
72.9
S3
4
9.4 (±0.1)
87 (± 0.3)
18 (± 0.1)
54.6
8
10.3 (± 1.2)
81 (± 1)
19 (± 2)
45.6
4
6.5 (± 1.1)
80 (± 3)
14 (± 2)
86.7
7
10.3 (± 2.3)
84 (± 5)
19 (± 3)
75.8
Tabulated values are averages of two measurements. b Calculated from area under stressstrain curve. c Maximum sustained true stress. d True strain at failure. e Calculated from the UVvis transmittance spectra between 400 – 760 nm (Figure 1).
TE D
a
M AN U
PB1
SC
filler
Every sample except S1 and PB1 exhibited significant increases in both toughness and strength compared to unfilled PMMA. Elongation at break was also significantly increased for
AC C
EP
these samples (Table 2, Figure 2).
Figure 2. Effect of filler loading on toughness during tensile tests.
12
ACCEPTED MANUSCRIPT
For the particle brush filled samples, toughness and elongation increased significantly with filler loading. The lack of toughening at low loadings of PB1 may be the result of the filler particles providing crack initiation sites while not having a high enough particle concentration to
RI PT
slow crack propagation, especially considering the larger size and mass of the PB1 particles. Considering the star polymer filled samples, S3, which has the shortest PSAN block of the star polymer samples, produced a filled material that exhibited higher toughness than S1 and S2
SC
when comparing similar filler fractions. The results indicate that, although a shell layer is needed to act as an interface, increasing the shell thickness above some minimum molecular weight
M AN U
necessary for a coherent interface is not beneficial. It is noted that a larger number of particles is incorporated for a given weight fraction of filler as the interfacial block size (and therefore the star polymer mass) decreases, which may also contribute to the improved performance. At higher loadings, S3 and PB1 filled samples demonstrated similar levels of toughness.
TE D
Following tensile testing to failure, the fracture surfaces were imaged with an optical microscope (See Figure 3). Fracture planes are visible in the unfilled sample, indicating quick propagation of the crack from an initiation site outward across the sample. The cleavage planes
EP
begin to diminish in appearance at 4 wt % filler loading. Samples with 7-8 wt % filler loading
AC C
show surface regions which are rough and pitted, is indicative of macroscopically ductile failure.
13
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 3. Optical fractography of failed dog-bone samples. All images taken at 50x; scale bars
M AN U
are 500 microns. CONCLUSIONS
Star shaped filler materials having a soft core and an outer layer compatible with the target matrix were prepared via ATRP in addition to a silica based hybrid brush particle. By embedding
TE D
low weight fractions of the prepared materials into a PMMA matrix, the composite material’s toughness and strength were improved while optical clarity was preserved with minimal haze formation. Tensile testing data was further corroborated by optical fractography[34] which
EP
showed that a change in macroscopic fracture mechanism was induced after filler materials were introduced. Higher loadings of the fillers improved the toughness of the PMMA samples, while
AC C
only slightly affecting the samples’ transparency and maintaining or increasing sample strength. A thin, compatibilizing shell layer was found to be critical to improving the toughness. The increases in toughness exhibited by the studeied materials are modest compared to those demonstrated for more traditional rubber particle fillers. However, the use of ATRP to generate core shell materials provides unique design handles allowing easy control of the composition of the energy absorbing core as well as the interfacial shell so that the filler can be altered for use in
14
ACCEPTED MANUSCRIPT
different materials under different loading conditions. Furthermore, hazing can likely be eliminated by adjusting the filler composition to match the refractive index of the matrix polymer. These materials are a step toward developing optimized, low Tg, toughening filler
RI PT
materials for the preparation of transparent PMMA plastics which could be utilized as glass substitutes to provide additional safety in architectural or automotive applications.
SC
AUTHOR INFORMATION
*Email: [email protected] Author Contributions
M AN U
Corresponding Author
The manuscript was written through contributions of all authors. All authors have given approval
ACKNOWLEDGMENT
TE D
to the final version of the manuscript
EP
We acknowledge Nissan Chemical for their generous donation of silica nanoparticles. We thank the NSF (DMR 1501324) and National Science Centre, Poland (via Grant UMO-
AC C
2014/14/A/ST5/00204). J.Y. acknowledges the support from the Richard King Mellon Foundation Presidential Fellowship. J.M.K. acknowledges support from the Charpie Scholarship at Carnegie Mellon University.
REFERENCES [1] M. Ishikawa, H. Ogawa, I. Narisawa, Brittle fracture in glassy polymers, Journal of Macromolecular Science, Part B: Physics 19(3) (1981) 421-443.
15
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[2] B.J. Ash, D.F. Rogers, C.J. Wiegand, L.S. Schadler, R.W. Siegel, B.C. Benicewicz, T. Apple, Mechanical properties of Al2O3/polymethylmethacrylate nanocomposites, Polym. Compos. 23(6) (2002) 1014-1025. [3] J. Zeng, B. Saltysiak, W. Johnson, D.A. Schiraldi, S. Kumar, Processing and properties of poly (methyl methacrylate)/carbon nano fiber composites, Composites Part B: Engineering 35(2) (2004) 173-178. [4] E.T. Kopesky, G.H. McKinley, R.E. Cohen, Toughened poly (methyl methacrylate) nanocomposites by incorporating polyhedral oligomeric silsesquioxanes, Polymer 47(1) (2006) 299-309. [5] D.S. Parker, H. Sue, J. Huang, A. Yee, Toughening mechanisms in core-shell rubber modified polycarbonate, Polymer 31(12) (1990) 2267-2277. [6] Y.J. Huang, J.H. Wu, J.G. Liang, M.W. Hsu, J.K. Ma, Toughening of unsaturated polyester resins with core–shell rubbers, J. Appl. Polym. Sci. 107(2) (2008) 939-950. [7] C. Zhou, S. Wu, B. Yang, Y. Gao, G. Wu, H. Zhang, Toughening Polystyrene by Core-shell Rubber Particles: Analysis of the Internal Structure and Properties, Polymers & Polymer Composites 23(5) (2015) 317. [8] D. Ayre, C. Bucknall, Particle cavitation in rubber-toughened PMMA: experimental testing of the energy-balance criterion, Polymer 39(20) (1998) 4785-4791. [9] M. Todo, K. Takahashi, P.-Y. Ben Jar, P. Beguelin, Toughening Mechanisms of Rubber Toughened PMMA, JSME International Journal Series A Solid Mechanics and Material Engineering 42(4) (1999) 585-591. [10] C. Wrotecki, P. Heim, P. Gaillard, Rubber toughening of poly (methyl methacrylate). Part I: Effect of the size and hard layer composition of the rubber particles, Polymer Engineering & Science 31(4) (1991) 213-217. [11] K. Cho, J. Yang, C.E. Park, The effect of rubber particle size on toughening behaviour of rubber-modified poly (methyl methacrylate) with different test methods, Polymer 39(14) (1998) 3073-3081. [12] K. Cho, J. Yang, C.E. Park, The effect of interfacial adhesion on toughening behaviour of rubber modified poly (methyl methacrylate), Polymer 38(20) (1997) 5161-5167. [13] J.-S. Chung, K.-R. Choi, J.-P. Wu, C.-S. Han, C.-H. Lee, Effect of Poly (butyl acrylate)Poly (methyl methacrylate) Rubber Particle Texture on the Toughening Behavior of Poly (methyl methacrylate), Korea Polymer Journal 9(2) (2001) 122-128. [14] P. Lovell, J. McDonald, D. Saunders, M. Sherratt, R. Young, Multiple-phase Tougheningparticle Morphology: Effects on the Properties of Rubber-toughened Poly (methyl methacrylate), American Chemical Society, Washington, DC (United States), 1993. [15] J.S. Wang, K. Matyjaszewski, Controlled radical polymerization - atom-transfer radical polymerization in the presence of transition-metal complexes, J. Am. Chem. Soc. 117(20) (1995) 5614-5615. [16] K. Matyjaszewski, J. Xia, Atom transfer radical polymerization, Chem. Rev. 101(9) (2001) 2921-2990. [17] K. Matyjaszewski, Atom transfer radical polymerization (ATRP): current status and future perspectives, Macromolecules 45(10) (2012) 4015-4039. [18] J. Chiefari, Y.K. Chong, F. Ercole, J. Krstina, J. Jeffery, T.P.T. Le, R.T.A. Mayadunne, G.F. Meijs, C.L. Moad, G. Moad, E. Rizzardo, S.H. Thang, Living Free-Radical Polymerization by Reversible Addition−Fragmentation Chain Transfer: The RAFT Process, Macromolecules 31(16) (1998) 5559-5562.
16
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[19] T. Shah, C. Gupta, R.L. Ferebee, M.R. Bockstaller, N.R. Washburn, Extraordinary toughening and strengthening effect in polymer nanocomposites using lignin-based fillers synthesized by ATRP, Polymer 72 (2015) 406-412. [20] J. Pyun, S. Jia, T. Kowalewski, G.D. Patterson, K. Matyjaszewski, Synthesis and Characterization of Organic/Inorganic Hybrid Nanoparticles: Kinetics of Surface-Initiated Atom Transfer Radical Polymerization and Morphology of Hybrid Nanoparticle Ultrathin Films, Macromolecules 36(14) (2003) 5094-5104. [21] C.M. Hui, J. Pietrasik, M. Schmitt, C. Mahoney, J. Choi, M.R. Bockstaller, K. Matyjaszewski, Surface-Initiated Polymerization as an Enabling Tool for Multifunctional (Nano)Engineered Hybrid Materials, Chemistry of Materials 26(1) (2013) 745-762. [22] R. Barbey, L. Lavanant, D. Paripovic, N. Schüwer, C. Sugnaux, S. Tugulu, H.-A. Klok, Polymer brushes via surface-initiated controlled radical polymerization: synthesis, characterization, properties, and applications, Chem. Rev. 109(11) (2009) 5437-5527. [23] A. Khabibullin, E. Mastan, K. Matyjaszewski, S. Zhu, Surface-Initiated Atom Transfer Radical Polymerization, Adv. Polym. Sci. 270 (2016) 29-76. [24] D. Maillard, S.K. Kumar, B. Fragneaud, J.W. Kysar, A. Rungta, B.C. Benicewicz, H. Deng, L.C. Brinson, J.F. Douglas, Mechanical properties of thin glassy polymer films filled with spherical polymer-grafted nanoparticles, Nano Lett. 12(8) (2012) 3909-3914. [25] H. Gao, K. Matyjaszewski, Modular Approaches to Star and Miktoarm Star Polymers by ATRP of Cross‐Linkers, Macromolecular symposia, Wiley Online Library, 2010, pp. 12-16. [26] Y. Zhu, X. Gao, Y. Luo, Core–shell particles of poly (methyl methacrylate)‐block‐poly (n‐butyl acrylate) synthesized via reversible addition–fragmentation chain‐transfer emulsion polymerization and the polymer's application in toughening polycarbonate, J. Appl. Polym. Sci. 133(1) (2016). [27] L. Bombalski, H. Dong, J. Listak, K. Matyjaszewsk, M.R. Bockstaller, Null‐Scattering Hybrid Particles Using Controlled Radical Polymerization, Adv. Mater. 19(24) (2007) 44864490. [28] J. Choi, C.M. Hui, J. Pietrasik, H. Dong, K. Matyjaszewski, M.R. Bockstaller, Toughening fragile matter: mechanical properties of particle solids assembled from polymer-grafted hybrid particles synthesized by ATRP, Soft Matter 8(15) (2012) 4072-4082. [29] S. Ojha, A. Dang, C.M. Hui, C. Mahoney, K. Matyjaszewski, M.R. Bockstaller, Strategies for the synthesis of thermoplastic polymer nanocomposite materials with high inorganic filling fraction, Langmuir 29(28) (2013) 8989-8996. [30] J. Yan, X. Pan, M. Schmitt, Z. Wang, M.R. Bockstaller, K. Matyjaszewski, Enhancing Initiation Efficiency in Metal-Free Surface-Initiated Atom Transfer Radical Polymerization (SIATRP), ACS Macro Lett. 5 (2016) 661-665. [31] J. Yan, T. Kristufek, M. Schmitt, Z. Wang, G. Xie, A. Dang, C.M. Hui, J. Pietrasik, M.R. Bockstaller, K. Matyjaszewski, Matrix-free Particle Brush System with Bimodal Molecular Weight Distribution Prepared by SI-ATRP, Macromolecules 48(22) (2015) 8208-8218. [32] J. Kamada, K. Koynov, C. Corten, A. Juhari, J.A. Yoon, M.W. Urban, A.C. Balazs, K. Matyjaszewski, Redox Responsive Behavior of Thiol/Disulfide-Functionalized Star Polymers Synthesized via Atom Transfer Radical Polymerization, Macromolecules 43(9) (2010) 41334139. [33] H. Gao, K. Matyjaszewski, Synthesis of Star Polymers by A New “Core-First” Method: Sequential Polymerization of Cross-Linker and Monomer, Macromolecules 41(4) (2008) 11181125.
17
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[34] S. Gong, S. Bandyopadhyay, Fracture properties and fracture surface morphologies in rubber-PMMA composites, J. Mater. Eng. Perform. 16(5) (2007) 607-613.
18
ACCEPTED MANUSCRIPT
Star-shaped tri-layer fillers for PMMA were prepared via ATRP. Mechanical strength and toughness of the filled materials was improved.
AC C
EP
TE D
M AN U
SC
RI PT
Transparency of the filled materials was retained.