All-Acrylic Multigraft Copolymers: Effect of Side Chain Molecular

Article pubs.acs.org/IECR

All-Acrylic Multigraft Copolymers: Effect of Side Chain Molecular Weight and Volume Fraction on Mechanical Behavior Andrew Goodwin,† Weiyu Wang,† Nam-Goo Kang,† Yangyang Wang,‡ Kunlun Hong,‡ and Jimmy Mays*,†,§ †

Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡

S Supporting Information *

ABSTRACT: We present the synthesis of poly(n-butyl acrylate)-g-poly(methyl methacrylate) (PnBA-g-PMMA) multigraft copolymers via a grafting-through (macromonomer) approach. The synthesis was performed using two controlled polymerization techniques. The PMMA macromonomer was obtained by high-vacuum anionic polymerization followed by the copolymerization of n-butyl acrylate and PMMA macromonomer using reversible addition−fragmentation chain transfer (RAFT) polymerization to yield the desired all-acrylic multigraft structures. The PnBA-g-PMMA multigraft structures exhibit randomly spaced branch points with various PMMA contents, ranging from 15 to 40 vol %, allowing an investigation into how physical properties vary with differences in the number of branch points and molecular weight of grafted side chains. The determination of molecular weight and polydispersity indices of both the PMMA macromonomer and the graft copolymers was carried out using size exclusion chromatography with triple detection, and the structural characteristics of both the macromonomer and PnBA-g-PMMA graft materials were characterized by 1H and 13C NMR. Matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry was employed for monitoring the macromonomer synthesis. Thermal characteristics of the materials were analyzed using differential scanning calorimetry and thermogravimetric analysis. The mechanical performance of the graft materials was characterized by rheology and dynamic mechanical analysis, revealing that samples with PMMA content of 25−40 vol % exhibit superior elastomeric properties as compared to materials containing short PMMA side chains or 400% strain), with the ability to tune the stress values required to achieve a desired strain by over 3 orders of magnitude, and to increase the extent of physical cross-linking between the glassy domains by controlling the molecular weight of the PMMA macromonomer. This class of materials shows promise for use as highly tunable TPEs.

styrene block copolymers with acrylics improves the weatherability and optical properties, making this soft elastomer extremely desirable in certain applications.25,27 Additionally, it was found that the performance of these all-acrylic materials as TPEs is directly related to the extent of phase separation between the two polymer segments and the average molecular weight between chain entanglement of the rubbery phase, both of which are less suitable for dissipating deformation stress when compared to SIS and SBS triblock copolymers.19,20,22 Jang and co-workers demonstrated that changing the casting solvent between chloroform, toluene, and THF that the elastic modulus and elongation at break can be modified because each solvent produced different nanophase structures and disordered regions.27 Consequently, it is this higher degree of phaseblending between the hard and soft domains of all-acrylic TPEs that leads to their significantly lower force versus displacement curves as compared to SIS rubbers.17,25 The molecular construction of graft copolymers may be carried out by three general methods: grafting-to, grafting-from, and grafting-through.28 Each synthetic strategy has its own advantages and disadvantages for the synthesis and characterization of both the final branched structure and the linear backbone and side chain precursors. The grafting-through, or conventional macromonomer approach, relies on the formation of oligo- or polymeric chains endowed with a polymerizable headgroup. Following this methodology, the side chains are first covalently bound to a polymerizable moiety and then undergo copolymerization with a second monomer to introduce branch point junctions with spacing reflecting the reactivity ratios of the two monomers. The grafting-through approach allows for characterization of both the side chains and backbone separately and results in well-defined graft architectures; however, it does require consideration of important synthetic factors like solubility and steric complications associated with incorporating macromonomers. The use of PMMA macromonomers to synthesize comb, bottlebrush, and graft architectures has been well documented.29−31 An unusual macromonomer approach for the synthesis of TPEs has been demonstrated by Mays and co-workers on the basis of a step-growth polymerization using chlorosilane linking agents to produce regularly spaced tri-, tetra-, and hexafunctional multigraft SI copolymers.32,33 This approach was extended to produce more advanced comb and comb-on-comb architectures by Hadjichristidis and co-workers34−36 via an in situ approach by substitution reactions of one, two, or more polymer chains with chlorosilane groups present on styrene

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EXPERIMENTAL SECTION Materials. Methyl methacrylate (Sigma-Aldrich, >99%), nbutyl acrylate (Sigma-Aldrich, >99%), tetrahydrofuran (THF, Sigma-Aldrich, ≥99%), 1,1-diphenylethylene (DPE, SigmaAldrich, >99%), benzene (Sigma-Aldrich, ≥ 99.9), and 1(tert-butyldimethylsiloxy)-3-butyllithium (tBDMS-Li, FMC lithium) were all purified according to standards required for anionic polymerization as previously reported.37,38 2,2-Azobis(isobutyronitrile) (AIBN, Aldrich 90%) was recrystallized before use and the S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate chain transfer agent was synthesized B

DOI: 10.1021/acs.iecr.5b02560 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Recipes Used in Synthesis of PnBA-g-PMMA Multigraft Copolymers

multigraft copolymers sample ida MG MG MG MG MG MG MG MG MG

5.3-4.9-14.4 5.3-2.0-16.7 5.3-5.4-18.3 5.3-9.2-25.7 11.7-2.6-16.0 11.7-5.3-22.2 11.7-3.6-27.7 11.7-6.1-34.0 11.7-3.7-38.1

graft PMMA Mnb (kg/mol)

monomerc (mmol)

macromonomerd × 102 (mmol)

CTAe × 102 (mmol)

initiatorf × 103 (mmol)

Mn,Theorg (kg/mol)

Mnh (kg/mol)

PDIi

5.3

20.92 10.46 13.95 10.46 27.90 20.93 10.46 13.95 10.46

6.70 4.70 5.28 7.15 3.55 4.05 3.35 3.97 4.28

1.50 3.50 0.75 0.75 1.60 0.75 1.50 0.75 3.50

4.00 4.00 2.00 2.00 4.00 2.00 4.00 2.00 4.00

179 38.3 238 179 223 358 89.4 238 38.3

111 29.5 95.4 93.3 127 179 78.2 93.9 54.1

1.59 1.64 1.52 1.55 2.04 2.38 1.49 1.78 1.59

11.7

a Sample identification MG n-m-o. bNumber-average molecular weight for MM-PMMA by SEC. cn-Butyl acrylate monomer. dPMMA macromonomer. eChain transfer agent. fAIBN initiator. gTheoretical number-average molecular weight of PnBA only, excludes PMMA side chains. h Number-average molecular weight by SEC of MG sample. iPolydispersity indices for MG samples by SEC.

Scheme 2. Synthetic Route to Poly(n-butyl acrylate-g-methyl methacrylate) Multigraft Copolymers

following the procedure previously published by Lai et al.39 tertButylammonium fluoride (Sigma-Aldrich, 1.0 M in THF) was used as received. Triethanolamine (Sigma-Aldrich, 98%) and acryloyl chloride (Sigma-Aldrich, ≥97%) were distilled over CaH2, stored over activated molecular sieves, and purged with Ar prior to use. Synthesis of PMMA Macromonomer. All anionic polymerizations were carried out in sealed, all-glass apparatuses using standard high-vacuum techniques.37,38 All reagents in ampules including MMA, lithium chloride, DPE, and the protected lithium initiator were attached to a hand blown reactor and introduced in the appropriate order after purging the reactor with an alkyllithium washing solution. The polymerization was performed in dry THF in a −78 °C acetone/dry ice bath for 1 h. A typical reaction would consist of ∼40 mmol of MMA, with the concentration of initiator being altered to target the desired molecular weight, to produce around 4 g of PMMA macromonomer per polymerization. Both DPE and LiCl were used in excess to the concentration of the tBDMS-Li, with stoichiometric ratios of 1:1.5 and 1:10, respectively. The synthesis procedure for hydroxyl-terminated PMMA was performed by the simple desilylation reaction with excess tetrabutylammonium fluoride in dry THF for 18 h, as shown in Scheme 1.40−42 The final step in the PMMA macromonomer synthesis utilized the nucleophilic addition/ elimination reaction between acryloyl chloride, added in slight excess dropwise via syringe, and hydroxyl-terminated PMMA in the presence of triethylamine in dry THF. The number-average molecular weights (Mn) of the PMMA macromonomers were determined to be 5.3 and 11.7 kg/mol by SEC. Synthesis of All-Acrylic Graft Copolymers. To target desired backbone molecular weights, allow for better branch point incorporation by reducing the polymerization kinetics, and to inhibit undesired branching resulting from chain transfer to polymer, the controlled radical polymerization technique of

RAFT was used to synthesize the all-acrylic multigraft copolymers, opposed to the classic free-radical approach. Branching resulting from chain transfer to polymer would produce multigraft materials containing disperse PnBA side chains, along with PMMA side chains, which would present difficulties in accurately characterizing the materials structure and composition. The copolymerization was performed by dissolving PMMA macromonomer, n-butyl acrylate, AIBN, and CTA in benzene (amounts shown in Table 1) in a single-neck round-bottom flask equipped with a single side arm with a stopcock and male glass joint. The solution was placed on the high-vacuum line and subjected to three freeze/thaw cycles. After the last freeze/thaw cycle, the mixture was brought to room temperature and placed under positive Ar pressure before being removed from the vacuum line. The sealed flask was then put into an oil bath at 75 °C and stirred vigorously to initiate the polymerization. The reaction time was usually between 36− 48 h and terminated by introducing a small amount of methanol and cooling the solution in an ice water bath for 5 min. The graft copolymers were precipitated into cold methanol twice to remove any small molecules and unreacted low molecular weight PMMA macromonomer. In general, the conversion of macromonomer using the grafting-through technique produced conversions >75%, which were determined by NMR. Scheme 2 shows the synthetic procedure based upon RAFT copolymerization. The general nomenclature for these materials is MG-n-m-o, where MG stands for multigraft, “n” represents the PMMA side chain molecular weight, “m” represents the average number of branch points per sample, and “o” represents the PMMA volume fraction.

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CHARACTERIZATION SEC. SEC was carried out at 40 °C using a Polymer Laboratories GPC-120 unit equipped with a Precision C


Industrial & Engineering Chemistry Research

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Detectors PD2040 (two-angle static light scattering detector), a Precision Detectors PD2000DLS (dynamic light scattering detector), Viscotek 220 differential viscometer, and a Polymer Laboratories differential refractometer. The column set employed was Polymer Laboratories PLgel; 7.5 × 300 mm; 10 μm; 500, 103, 10E5, and 10E6 Å. The calibration range was 600−7 500 000 g/mol using either PS or PMMA standards. The mobile phase was THF at a flow rate of 1.0 mL/min. 1 H- and 13C NMR. 1H and 13C NMR was carried out on a Varian Mercury 500 instrument. Samples were dissolved in deuterated chloroform (CDCl3). MALDI-TOF MS. The MALDI-TOF mass spectra were obtained on a Bruker Autoflex II model smart-beam instrument equipped with a nitrogen laser (λ = 337 nm). The matrix was trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB, >99%, Fluka). The ratio of matrix to sample was 10:1 in mg/mL, with sodium trifluoroacetate used as the cation source. DSC. A TA Instruments Q-1000 differential scanning calorimeter (DSC) was used from −80 to +150 °C at a heating rate of 10 °C/min with a 2 min isothermal hold at the maximum and minimum temperatures. Reported glass transition temperatures are those measured on the second of three heating scans. TGA. The thermal stability of the multigraft copolymers was examined using a TA Instruments Q-50 TGA. A 10−20 mg sample was placed in the platinum pan and equilibrated at 30 °C. The temperature was ramped at a rate of 10 °C/min over a temperature range of 30−600 °C under a nitrogen atmosphere. AFM. Morphological measurements were performed using a Multimode Scanning Probe microsope (SPM) in tapping mode on sample cross sections obtained from microtoming. Then 100 mg samples were prepared by dissolution of copolymer in 1 mL of toluene and slowly evaporating off the solvent over 5 days. The samples were then placed in a vacuum oven for 48 h before heating. The heating rate for sample annealing was as follows: 25−80 °C for 24 h, 80−135 °C 24 h, held at 135 °C for 72 h. The temperature was then reduced in the same manner and kept at room temperature under vacuum until the samples were microtomed and measured. To characterize sample cross sections, all samples were microtomed into thickness of 500 nm at −50 °C with a Leica Ultramicrotome EM UC7 and transferred onto SPM sample mounting disks with diameter of 12 mm. DMA. The mechanical properties were investigated on a TA Instruments Q-800 dynamic mechanical analyzer equipped with a single cantilever clamp. The controlled force experiments were run at 25 °C to observe stress/strain behavior and the temperature ramp/frequency sweep experiments were run at 0.5 Hz over a temperature range of −80 to +150 °C. The samples were cast into thin films by dissolving 1.5 g of MGCP in 5 mL of toluene. The toluene was allowed to slowly evaporate over a 5 day period, and then the samples were dried for 5 days in a vacuum oven at room temperature. Rheology. To evaluate their linear viscoelastic properties, small amplitude oscillatory shear measurements of the branched polymers were carried out on a Hybrid Rheometer 2 from TA Instruments. Polymer samples were analyzed using 3 mm and 20 mm parallel plates at low and high temperatures, respectively. The temperature was controlled by an Environmental Test Chamber with nitrogen as the gas source.

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RESULTS AND DISCUSSION

Characterization of PMMA Macromonomers. Poly(methyl methacrylate) macromonomer samples were synthesized by high-vacuum living anionic polymerization using the silyl-protected initiator in THF at −78 °C. The deprotection reaction to obtain hydroxyl-terminated PMMA was performed using the procedure described by Sivaram et al.40−42 The final PMMA macromonomer structures were produced by treating hydroxyl-terminated PMMA with acryloyl chloride, resulting in a polymerizable alkyl group at the α-end and proton termination at the ω-end. Both the 5.3 and 11.7 kg/mol macromonomers were carefully characterized by a combination of SEC and NMR. Figure 1 shows the SEC curve of the longer chain PMMA macromonomer sample, exhibiting a symmetrical peak corresponding to a Mn of 11.7 kg/mol with a PDI of 1.04.

Figure 1. SEC elugram of the longer chain MM-PMMA sample. The figure depicts the RI detector signal with a corresponding Mn = 11.7 kg/mol and PDI = 1.04.

The 1H NMR spectrum in Figure 2 shows the presence of the vinyl proton peaks of the acryloyl group (Ha, Ha′, and Hb

Figure 2. 1H NMR of the 11.7 kg/mol poly(methyl methacrylate) macromonomer. The enlarged spectrum highlights the proton signals of the polymerizable vinyl group.

vinyl proton signals between 5.75 and 6.25 ppm). Additionally, the characteristic signals from PMMA are present at 3.53, 1.80, and 0.80−1.00 ppm that corresponds to the methoxy protons, methyl protons, and CH2 backbone proton signals, respectively. The silyl-protected PMMA, hydroxyl-terminated PMMA, and final PMMA macromonomer 1H and 13C NMR overlays can be found in Figures S1 and S2 of the Supporting Information. D


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Industrial & Engineering Chemistry Research Further confirmation of the α-terminal vinyl group is found in the MALDI-TOF spectrum seen in Figure 3. To obtain a

for use in the copolymerization to construct all-acrylic multigraft copolymers. Characterization of All-Acrylic Graft Copolymers. The molecular weight and molecular weight distributions of the graft copolymers were obtained by SEC equipped with light scattering, viscometry, and RI detectors and can be viewed in Table 2. The SEC curve in Figure 4 shows the purified

Figure 4. SEC elugram of MG 11.7-6.1-34.0, from Table 2, the 11.7 kg/mol MM-PMMA peak (dashed line) is shown to depict the large difference in elution volume between the MG and macromonomer samples. Figure 3. MALDI-TOF mass spectrum of a low molecular weight poly(methyl methacrylate) macromonomer. The enlarged portion shows the peaks corresponding to n = 18, n = 19, and n = 20.

multigraft copolymer and the PMMA macromonomer peak to show that there is no residual unreacted PMMA macromonomer present in the sample. The graft copolymer peaks show a unimodal distribution with PDIs between 1.5 and 2.4 for all of the samples. The broad PDIs are a result of both the RAFT polymerization technique, which often yields polymers with PDIs between 1.2 and 1.5 for linear homopolymers, and an inherent consequence of the macromonomer approach to produce branched materials, where the addition of one branch junction produces a significant change in the overall molecular weight. This also explains the fact that the PDIs for the MG samples composed of the longer PMMA side chains are generally broader than those with the shorter PMMA graft side chains (Table 2). Additionally, a representative GPC of the crude and purified multigraft product can be seen in Figure S4 in the Supporting Information. The compositions of the graft copolymers were measured by 1 H NMR, as shown in Figure 5. The spectrum allows for the integration of the PMMA macronomer methoxy proton signal (3.53 ppm) with the β-CH2− proton signal of the PnBA butylpendent group (3.97 ppm) allowing for the calculation of the average number of branch points and the volume fraction of

well-resolved MALDI spectrum, a low molecular weight PMMA sample (Mn = 4.1 kg/mol determined by SEC) was synthesized; however, this sample was not used in any of the subsequent MG reactions. A representative spectrum shows the monoisotopic peak value of 2219.74 m/z, the corresponding 19-mer [307.17(C21H23O2) + 19 × 101.12(C5H9O2) + Na(22.98) − 31.02(OCH3)] labeled peak has a calculated monoisotopic mass of 2220.41 g/mol. The calculated mass of the major peak includes the macromonomer−DPE headgroup, the methyl methacrylate monomer repeat units, the Na+ proton source used to promote ionization, and the loss of 31.02 g/mol, which corresponds to the cyclization and extraction of the pendent methoxy-group located on the terminal monomer unit, previously reported in the MALDI-TOF analysis of PMMA.40,43 Again, MALDI-TOF spectra of silyl-protected, hydroxyl-terminated, and final PMMA macromonomer can be seen in Figure S3 of the Supporting Information. These results confirm the successful synthesis of the PMMA macromonomer Table 2. Composition of PnBA-g-PMMA Multigraft Copolymers

multigraft copolymer sample no. 1 2 3 4 5 6 7 8 9

sample id MG MG MG MG MG MG MG MG MG

a

5.3-4.9-14.4 5.3-2.0-16.7 5.3-5.4-18.3 5.3-9.2-25.7 11.7-2.6-16.0 11.7-5.3-22.2 11.7-3.6-27.7 11.7-6.1-34.0 11.7-3.7-38.1

graft chain

Mnb 5.3

11.7

(kg/mol)

Mnc

(kg/mol) 111 29.5 95.4 93.3 127 171 78.2 93.9 54.1

d

Mp (kg/mol)

PDIe

no.f

volume percentg (%)

168 58.1 139 153 175 237 119 151 76.4

1.59 1.64 1.52 1.55 2.04 2.38 1.49 1.78 1.59

4.9 2.0 5.4 9.2 2.6 5.3 3.6 6.1 3.7

14.4 16.7 18.3 25.7 16.0 22.2 27.7 34.0 38.1

a

Sample identification MG n-m-o. bNumber-average molecular weight of PMMA side chains calculated by SEC. cNumber-average molecular weight of MG sample calculated by SEC. dMaximum peak molecular weight of MG sample calculated by SEC. ePolydispersity indices for MG sample calculated by SEC. fAverage number of branch points per MG chain calculated using 1H NMR and the Mp calculated by SEC. gAverage PMMA volume percent per MG chain calculated using 1H NMR. E


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two-step process beginning at 295 °C and losing around 25% of its weight, with the remaining 75% weight loss occurring over the temperature range 320−405 °C. The linear PnBA displayed single-step decomposition over a temperature range 270−405 °C. The graft copolymer TGA thermogram in Figure 6 shows a two-step thermal decay process showing only 10−15 wt % being lost in the range 260−335 °C and the complete weight loss over the range 335−450 °C. Additionally, DSC was employed to determine the Tg of each precursor material along with the observed Tg’s of the branched materials. The Tg’s were as expected for the linear PnBA and linear PMMA macromonomer samples with each of them displaying a single, sharp Tg with a midpoint of −50 and +105 °C respectively. The DSC curves for the graft copolymers display a similar low Tg around −45 °C; however, the PMMA Tg is masked in most of the samples having high molecular weight backbones (>100 kg/mol) and containing less than 30 vol % PMMA. In Figure 7 the high-Tg curve is visible for

Figure 5. 1H NMR spectrum of MG 11.7-6.1-34.0 (red) and the 11.7 kg/mol PMMA macromonomer (blue). Enlarged portion depicts the disappearance of the vinyl headgroup monomer signal associated with the PMMA macromonomer.

each acrylic monomer. Furthermore, the disappearance of any −CH2− vinyl signals (5.75−6.25 ppm) confirms that the sample is free of both unreacted nBA monomer and PMMA macromonomer. Thermal properties of the both the graft copolymers and linear precursors were investigated using both TGA and DSC to determine the Tg’s and degradation temperatures. Figure 6 shows the TGA thermograms of various MG samples, PMMA macromonomer, and a linear sample of PnBA prepared by RAFT with a molecular weight and PDI similar to those of the backbones of the graft samples (Mn = 160 kg/mol and PDI = 1.48). The decomposition of the PMMA macromonomer is a

Figure 7. DSC thermogram of MG 11.7-3.7-38.1. The enlarged section is the first derivative versus temperature plot of the multigraft copolymer sample (bottom) and the 11.7 kg/mol PMMA macromonomer sample (top) to highlight the presence of the high-Tg material.

sample MG 11.7-3.7-38.1, which corresponds to 38% by volume of PMMA with a lower molecular weight backbone (Mn = 54.1 kg/mol) and large molecular weight PMMA side chains (11.7 kg/mol). As expected, the Tg’s for the PMMA segments are much broader than for the PMMA macromonomer precursor and the midpoint is slightly shifted to lower temperature (99.1 °C). The zoomed in portion of Figure 7 is the first derivative of the heat flow plotted against temperature, where the change in the slope can be viewed more easily and depicts a change similar to that of the linear PMMA macromonomer precursor. This result is in agreement with the findings published by Mijovic and co-workers along with more recent work carried out on PI-g-PS copolymers with comb architectures.7,10 Results such as these are found when the hard PS domains reside in different, poorly ordered

Figure 6. TGA thermograms and the first derivative weight change versus temperature for 11.7 kg/mol PMMA macromonomer, linear PnBA, and various multigraft copolymer samples. The enlarged section depicts the two-step decomposition of the PMMA macromonomer and multigraft copolymer samples with the loss in weight increasing with PMMA content regardless of side chain molecular weight. F


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Industrial & Engineering Chemistry Research microphase-separated domains. Moreover, polydispersity of the graft copolymers can lead to the dissolution of the short randomly spaced PMMA segments into the soft PnBA phase, effectively masking the high-Tg material similar to the case of linear block copolymers.7,44 The physical properties of five branched copolymer samples with similar overall molecular weights and volume fractions ranging from 15% to 35%, but differing in the PMMA molecular weight side chain and number of branch point junctions, were characterized using DMA to obtain stress/strain curves and storage and loss modulus as a function of temperature using controlled force and temperature ramp/ frequency sweep experiments. Previous works have shown using styrene and isoprene multigraft copolymers that architectural heterogeneity does effect the morphology of the graft copolymers and thus influences the mechanical properties of the material; however, the authors show that the number of branch point junctions and branch point functionality are much more impactful on enhancing the mechanical properties.13 According to their results, we should suspect the bulk mechanical behavior for the all-acrylic system to be less influenced by branch point placement and heavily dominated by the number of branch points and the volume faction of each component. Figure 8 shows the stress versus strain curves of

Figure 9. Stress at 400% strain (top) and elastic modulus (bottom) versus volume percent of PMMA for materials incorporating 5.3 kg/ mol (red) and 11.7 kg/mol (blue) graft side chains (Table 2). The dotted line is to illustrate that the materials composed of the longer graft PMMA chains undergo less deformation at lower PMMA volume fractions than the multigraft copolymer samples with more of the shorter PMMA side chains.

DMA instrument (polymers were elongated to the maximum allowed by the instrument or the films of the tougher materials began to slip out of the clamps at larger displacements). Indepth tensile testing of these materials is currently underway and will be published later. The storage modulus, loss modulus, and tan δ over a temperature range from −80 to +175 °C are shown in Figure 10. One interesting feature is that by using DMA, which is a

Figure 8. Stress versus strain curves for various multigraft samples performed using DMA. The volume percent of PMMA is indicated for each sample, and the multigraft copolymer samples composed of the shorter 5.3 kg/mol graft PMMA chains are labeled.

the five samples and depicts a large difference in the observed stress values of each sample below 500 strain % with one noticeable trend being that the samples synthesized using the higher (11.7 kg/mol) molecular weight side chain, regardless of PMMA volume percent, exhibited higher strength than the graft samples incorporating 5.3 kg/mol PMMA side chains. This result indicates that the higher molecular weight side chains provide superior elastic properties by increasing the degree of tethering to the glassy domains and by slightly increasing the domain sizes of the dispersed hard phases, which effectively strengthens the physical cross-linking within the material.14,45 The same trend is seen in Figure 9 where the elastic modulus and stress at 400% strain versus PMMA volume percent shows superior elastomeric properties with the most important factor being the use of the higher molecular weight side chains, followed by increasing the PMMA volume percent. It is important to note that these strain percent values are not the strain at break of the material, but the limitations of the

Figure 10. Storage modulus (black), loss modulus (blue), and tan δ (red) for MG 11.7-5.3-22.2.

much more sensitive technique for determining thermal transition temperatures, the T g of the rubbery PnBA component is observed around −25 °C by the loss modulus peak, with a second small transition around 110 °C. This result reflects the influence of a slow, controlled annealing process that leads to a greater number of glassy domains, while also presenting soft segregated phase boundaries that will introduce an intermediate Tg similar to those recently reported.7,24,26 Additionally, the storage modulus (E′) versus temperature plot of multiple MG samples (Figure 11) depicts that at low temperatures all the materials exhibit similar behaviors; however, beginning at intermediate temperatures, the short graft PMMA material undergoes a greater deformation when G


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The dynamic mechanical spectra at three representative temperatures: −35, +30, and +150 °C are shown in Figure 13

Figure 11. Storage modulus of various MG samples composed of both short and long graft side chains over a temperature range of −85 to +175 °C.

exposed to milder forces. At high temperatures the mechanical failure of the MG material composed of the 5.3 kg/mol chains happens promptly after Tg of the PMMA is reached, whereas the materials containing the longer graft PMMA chains do not exhibit this behavior until temperatures above 150 °C. Again, increasing the molecular weight of the PMMA side chains and increasing the volume percent of the hard PMMA material leads to better elastic and mechanical properties over a larger temperature range, especially when the high-Tg material is reached, when compared to materials having lower molecular weight side chains and lower PMMA volume fractions. In addition to the results obtained using DMA, small amplitude oscillatory shear measurements were also carried out on a rotational rheometer to further evaluate the mechanical behavior of the branched materials. The Cole−Cole-plot (Figure 12) shows that all the samples exhibit thermo-

Figure 13. Storage (G′) and loss (G′′) modulus versus frequency of MG samples ranging 16-to-38 PMMA volume percent of both low and high molecular weight side chains. In the figure the different samples are colored and labeled with the G′ being the filled points and the G″ being the open scatter points. There is only one multigraft copolymer incorporating the 5.3 kg/mol PMMA side chains (red), and all the others incorporate the 11.7 kg/mol PMMA side chains.

to further illustrate the mechanical behavior of the MG samples. At low temperatures, the mechanical behavior is dominated by the branched polymer’s Tg. The storage modulus for the polymers with the long PMMA side chains increases with increasing PMMA content and the short PMMA side chain with roughly 26 volume percent PMMA appears to have the highest storage modulus of all the samples, at a fixed frequency. This can be explained by the fact that the short PMMA graft side chain sample has more branches than the other samples, which could more effectively enhance the overall Tg of the copolymer. At high temperatures, the mechanical behavior is affected by a number of factors, including molecular weight, Tg, and the degree of phase separation. The short PMMA graft chain MG sample becomes more liquid-like, with the onset occurring over intermediate temperatures, in agreement with what was observed by DMA in Figure 11. This suggests that the

Figure 12. Cole−Cole plot of various MG samples composed of both short and long graft PMMA side chains. The arrows in the plot indicate the level of the plateau modulus.

rheological complexity as seen by the intermediate- and hightemperature curves, which do not collapse onto a single curve. Also, the graft copolymers with the 11.7 kg/mol PMMA side chains display a more pronounced rubbery plateau and the materials stiffness in this regime increases with PMMA content. H


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NMR, SEC, DSC, and TGA. The synthetic method employed is shown to be well-suited for producing high yields of advanced all-acrylic multiblock copolymer architectures. Characterization of the multigraft copolymer samples using DMA, AFM, and rheology provided insight into their morphology and structure/property relationships. Samples with PMMA content of 25−40 vol % and incorporating the higher molecular weight PMMA side chains exhibit good elastomeric properties, whereas materials with shorter PMMA side chains or 100 kg/mol, containing multiple, randomly spaced, branch point junctions. The PMMA macromonomers and linear PnBA were characterized alongside various multigraft copolymer samples using I


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