Linear Viscoelasticity of Polymer Tethered Highly Grafted

Chapter 18

Downloaded by UNIV OF ARIZONA on January 11, 2013 | http://pubs.acs.org Publication Date: August 13, 2009 | doi: 10.1021/bk-2009-1023.ch018

Linear Viscoelasticity of Polymer Tethered Highly Grafted Nanoparticles Vivek Goel 1, Joanna Pietrasik 2, 3, Krzysztof Matyjaszewski 3, and Ramanan Krishnamoorti 1 1

Department of Chemical & Biomolecular Engineering, University of Houston, Houston, TX 77204. 2 Department of Chemistry, Technical University of Lodz, Institute of Polymer and Dye Technology, 90924 Lodz, Poland. 3 Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213.

The linear viscoelastic properties in the melt state of highly grafted polymers on spherical silica nanoparticles are probed using linear dynamic oscillatory measurements and linear stress relaxation measurements. While the pure silica tethered polymer nanocomposite exhibits solid-like response, the addition of a matched molecular weight free matrix homopolymer chains to this hybrid material, initially lowers the modulus and later changes the viscoelastic response to that of a liquid. These results are consistent with the breakdown of the ordered mesoscale structure, characteristic of the pure hybrid and the high hybrid concentration blends, by the addition of homopolymers with matched molecular weights.

Introduction Understanding the structure – processing – property correlations for polymers with dispersed nanoparticles remains a compelling area for research.1 Model nanoparticle dispersions of polymer – grafted nanoparticles, either as pure hybrid materials or as mixtures with homopolymers that are chemically similar to the grafted chains, have become possible due to advances in living radical polymerization methods and provide a good framework for developing a © 2009 American Chemical Society In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

257


258 better understanding of the structure – processing – property correlations for polymer nanocomposites in general.2, 3 Such hybrids, combining the behavior of linear polymer chains and hardsphere character of nanoparticles, are capable of bridging much of our understanding of colloidal dispersions and that of star polymers.4, 5 When the grafting density of the polymer chains is high enough that the crowding caused by the grafting results in significantly altered conformations of the polymer, significant new issues arise and are captured by the developments in “polymer brushes”. In this work we have studied such hybrid materials where linear polymers are attached to spherical silica nanoparticles and the polymers are grafted such that they are in the brush regime. Specifically in this work we report on the linear rheological properties of such hybrid materials under conditions where the polymer by itself would be considered a liquid and with a relaxation time significantly smaller than 1 sec. Previously, linear rheological measurements of other polymer hybrids (with no free polymer chains), including ones where the polymers have been attached to layered silicates, have indicated that with increased nanoparticle loading, at roughly the same grafting density, there is a transition from liquid-like to solidlike behavior.2, 6 This behavior has been attributed to the formation of a sample spanning percolative network that gets created with increased nanoparticle loading.7 On the other hand, star polymers have shown extremely interesting behavior when high-functionality star polymers are studied either as pure melts or as dispersions in linear chains. For instance, upon addition of linear chains in the stars a re-entrant gelation transition is observed in those dispersions.8 Interestingly, the hybrids studied here show some structural similarity with highly functional star polymers, most notably the formation of face centered cubic ordered crystalline order9 that has been hinted previously in some studies of star polymers.

Experimental Materials The monomer n-butyl acrylate (BA, Acros, 99%) was purified by filtration through a basic alumina column to remove inhibitors before the synthesis. The procedure for the synthesis of 1-(chlorodimethylsilyl)propyl 2-bromoisobutyrate and the subsequent functionalization of the silica (30 % wt. silica in methyl isobutyl ketone, diameter D = 16.5 nm, MIBK-ST, Nissan) was described previously.3 Bis(2-pyridylmethyl)octadecylamine (BPMODA) was synthesized according to methods described previously.10 CuBr (Aldrich, 99 %) was purified via several slurries in acetic acid followed by filtration and washing with methanol and ethyl ether, and stored under nitrogen before use. CuBr2 (Aldrich, 99.999 %), polyoxyethylene (20) oleyl ether (Brij 98, Aldrich), hexadecane (Aldrich), L-ascorbic acid (AA, Aldrich, 99 %), dimethyl-2,6dibromoheptanedioate (DMDBrHD, Aldrich, 97 %), N,N,N’,N’,N”-

In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.


259 pentamethyldiethylenetriamine (PMDETA, Aldrich, 99 %), anisole (Aldrich) and hydrofluoric acid (50 vol % HF, Acros) were used as received. Poly(n-butyl acrylate) brushes were synthesized by activators generated by electron transfer (AGET) ATRP of BA from 2-bromoisobutyrate functionalized silica particles in miniemulsion under conditions similar to those reported previously, BA: SiO2-Br: CuBr2: BPMODA: ascorbic acid = 600: 1: 0.5: 0.5: 0.2, temperature 80°C, hexadecane 5 wt % based on monomer, Brij 98 - 14 wt % solid content.11 In a case of synthesis of PBA homopolymer, the polymerization was carried out at 70 °C. DMDBrHD (56.6 μL, 0.26 mmol), PMDETA (54.4 μL, 0.26 mmol), BA (11 mL, 78 mmol), and anisole (1.1 mL) were added to a 25-mL Schlenk flask equipped with a magnetic stir bar. The flask was sealed, and the resulting solution was subjected to three freeze-pump-thaw cycles. After equilibration at room temperature, CuBr (37.3 mg, 0.26 mmol) was added to the solution under nitrogen flow and the flask was placed in preheated oil bath. Aliquots were removed by syringe in order to monitor molecular weight evolution. After a predetermined time, the flask was removed from the oil bath and opened to expose the catalyst to air. The polymerization solution was diluted with CHCl3 and passed over an alumina (activated neutral) column to remove the catalyst. Solvent was removed by rotary evaporation, and the polymer was isolated by precipitation into cold methanol. Characterization of Polymers Molecular weight (Mn) and molecular weight distribution (Mw/Mn) were determined by GPC (Waters, 717 plus), with THF as eluent at 1 mL/min (Waters, 515) and four columns (guard, 105 Å, 103 Å, 100 Å; Polymer Standards Services) in series. Toluene was used as internal standard. Calculations of molar mass were determined using PSS software using a calibration based on linear polystyrene standards. Polymers were analyzed after etching silica with HF. Table 1. ATRP of n-butyl acrylate (BA) from functionalized silica particles or from small molecular weight initiator Mn, SEC g/mol

Mw/Mn

# of tethered chains/particle

AGET

[BA]: [initiating site] 600: 1

53,500a

1.25

∼ 600

Normal

300: 1

55,000

1.23

-

Sample

initiation

SiO2-PBA50K PBA50K a

Polymers were analyzed after etching silica with HF.


260 Rheological Characterization Rheological measurements were conducted using a TA Instruments ARES rheometer with a torque transducer with an operating torque range of 0.2 – 2000 gf cm and normal force range of 2 – 2000 g. Dynamic oscillatory shear measurements in the linear range were performed using a set of 25 mm diameter parallel plates. Frequency sweeps were performed using small amplitude oscillatory shear to investigate the linear viscoelastic properties, characterized by the storage and the loss moduli (G’ and G” respectively). A sinusoidal strain of the form γ(t) = γ0sin(ωt) was applied, and the resultant shear stress σ(t) (= γ0 (G’Sin(ωt) + G”Cos(ωt))) was monitored. The complex viscosity (η*) (=


G" (ω) 2 + G ' (ω) 2 ω

=

G* ) is also reported, where G* is the complex modulus. ω

Data was collected over a temperature range of 30 °C to 80 °C, and reduced to a master-curve via the principle of time-temperature superposition, at the reference temperature of 30°C. Stress relaxation measurements were investigated at 30°C using a 25 mm diameter cone and plate geometry with a cone angle of 0.0998 rad. Specifically this cone and plate geometry was employed in order to produce a uniform shearing rate across the sample space as

• γ = ω / tan α

(1)

•

where γ is the shear rate (s-1), ω is the cone angular velocity and α is the cone angle. For stress relaxation, a step strain γo was applied at time t = 0, and the shear stress monitored as a function of time, with the modulus G(t) obtained as G(t) = σ(t) / γo.

Results and Discussions The linear viscoelastic behavior of the SiO2 – PBA hybrid was examined using small amplitude oscillatory strain measurements. Typical linear dynamic viscoelastic data for the hybrid materials, with no free polymer chains, is shown in Figure 1. The presence of a frequency independent plateau in G’, with the value of G’ exceeding that of G”, at low frequencies is observed in the timetemperature superposed mastercurves. We note that excellent time-temperature superpositioning is observed and that the temperature dependent frequency shift factors for the nanocomposites are virtually identical to that of the homopolymers of PBA. Corroboration of the long-time solid like response was also observed from stress relaxation measurements shown in Figure 2. In fact, using a two-point collocation method, the stress relaxation data were found to be in quantitative agreement with the linear dynamic moduli data.7 We note that while the longest relaxation time for a free homopolymer chain of length


261

2

G' & G" (dynes/cm )


corresponding to that of a single grafted chain is

Linear Viscoelasticity of Polymer Tethered Highly Grafted

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